Although many studies have been conducted to investigate the separate effects of different factors such as temperature, reaction time, pH, or precursor ratio on the yield of the producti
LITERATURE REVIEW
Introduction to silver nanoparticles@graphene oxide (AgNPs@GO)
Plasmonic nanoparticles have received great attention from researchers thanks to their possession of several extraordinary properties, including their localized surface plasmon resonance (LSPR) and high toxicity toward microorganisms [11] Recently, many studies have been carried out to manipulate these properties for many medicinal and environmental applications, such as the detection of metallic ions and biomolecules, or the inhibition of the growth of dangerous bacteria [12], [13] Among several plasmonic nanoparticles, silver nanoparticles (AgNPs) have been regarded as one of the most efficacious metallic nanomaterials when compared to other counterparts with the same sizes AgNPs also possess several superior properties, including high surface area, inertness in harsh environment, ease of visualization by the naked eye, simplicity of fabrication by using inexpensive techniques, and a broad spectrum of antibacterial and fungicidal performance, making them ubiquitous as primary component for abundant of consumer products [14] According to previous studies, it has been reported that the performance of the AgNPs in different applications can be varied due to the alternation of many factors including their shape, size, morphology, pH medium, temperature, or reaction time [15] Therefore, selecting appropriate fabrication approaches, as well as exploring novel precursors and supporting agents is important to improve the properties of the AgNPs and expand their applications on a larger scale
In addition to the plasmonic nanoparticles, carbonaceous materials, especially graphene, have also been widely investigated as an effective platform for the stabilization of nanoparticles [16], [17] Graphene oxide (GO), a graphene derivative composing a honeycomb structure of carbons and abundant oxygen-containing functional groups, has been reported to possess several advantageous properties, comprising large surface area, stability, and high conductivity Herein, GO has been broadly utilized in a wide range of applications such as drug delivery, energy storage, antimicrobial, wound treatment, sensors, catalysis, etc Furthermore, GO can also strongly interact with nanoparticles via physical adsorption, electron transfer, π-π interactions, or electrostatic bonding with oxygen-containing functional groups (Figure 1.1), which makes it a suitable support to reduce the self-agglomeration and enhance the
3 physicochemical properties of the AgNPs Indeed, the AgNPs have been widely fabricated with the participation of GO, in which the resulting nanocomposites possess synergic properties of their precursors Additionally, the enhanced performance of the AgNPs-doped GO (AgNPs@GO) nanocomposite has been reported in many previous studies It has been indicated that the AgNPs@GO nanocomposite exhibits a superior colorimetric detection of metallic ions compared to the pristine AgNPs thanks to the ultra-large surface area of the GO, which effectively facilitates the adsorption and interaction between the ions and the AgNPs surface [18] Besides, the antibacterial performance of the AgNPs can be also significantly exalted, which is attributed to the large contact area and sharp edges of GO that can significantly damage the cell walls Several functional groups present in the GO skeleton such as epoxide (–O–), hydroxyl (–OH), carbonyl (–C=O), and carboxyl (–COOH) also act as antibacterial agents, which induce disruption of inner and outer cell membranes, leading to cell death [19]
Figure 1.1: Structure of (a) GO and (b) AgNPs@GO
Recently, different approaches have been employed for the fabrication of nanomaterials, which includes three main categories namely physical, chemical, and reduction process using plant sources (Figure 1.2) Each technique provides different advantages and drawbacks, which have been intensively mentioned in several studies Particularly, physical and chemical methods have been conducted in the early days of nanomaterials technology, which involve the utilization of common reducing agents such as sodium borohydride (NaBH4), hydrazine (N2H4), L-ascorbic acid, or thiourea to convert metallic cations into nanoparticles [20], [21] Despite the ease of operation, these techniques possess a lot of negative impacts on the environment and human health, which restrict their possibility for application on a larger scale On the other hand, some biological methods that involve the participation of environmentally friendly reducing a b
4 agents such as plant extracts or enzymes have received an ever-growing attraction due to their being free of by-products and the abundance of precursors Regarding AgNPs@GO nanocomposite, although several studies have been carried out with various applications and fabrication techniques, the detailed comparison between these aspects is still rare, which induces more challenges for many researchers to get excess to the studies regarding graphene oxide-based nanomaterials Thus, a thorough summarization of the AgNPs@GO synthesis methods and their recent applications is of paramount importance
Figure 1.2: Some popular synthesis approaches for nanoparticles
According to the aforemention discussion, this literature review will focus on the comparison between three different fabrication methods of the AgNPs@GO nanocomposite with the advantages and drawbacks resulting from each approach The individual and concurrent influences of different factors such as temperature, pH medium, or precursor ratios on the morphology and stability of the material were also reviewed in detail Therewithal, the performance of the synthesized AgNPs@GO nanocomposite from different studies in the applications of colorimetric detection, antimicrobial activity, and catalytic reduction was also thoroughly compared.
Fabrication methods of AgNPs@GO
According to previous studies, nanomaterials have been commonly fabricated via physical techniques, which involve the utilization of mechanical pressure, thermal effects, or electrical energies to gradually stimulate condensation, melting, or evaporation of the bulk precursors to nanoparticles [22] These processes are broadly
5 known as top-to-bottom fabrication approaches, which require fewer toxic chemicals or pollutants when compared to their chemical or biological counterparts For instance, the thermal vaporization process was employed for the fabrication of AgNPs [23] Specifically, the thermal evaporation of silver was carried out in the convective flow of inert gases such as helium in a preparation chamber, before being deposited on an inert substrate A physical vapor deposition technique was also conducted to produce AgNPs The process involves the vaporization of a silver piece, which was subsequently carried by an argon flow downstream to quench into the gas phase, eventually forming AgNPs [24] For the fabrication of the AgNPs@GO nanocomposite, the preparation process usually involves the deposition of the as-synthesized AgNPs onto the graphene oxide surface with the support of ultrasonication or vigorous stirring [25] Indeed, ultrasonication was effectively utilized for the embedment of Ag-ZnS nanoparticles onto the surface of the GO, in which the nanoparticles were injected directly into the GO suspension under ultrasonic conditions for 1 h [26] Some recent studies have also applied the pulsed laser ablation methods for the direct embedment of AgNPs onto the
GO layers In particular, a thick solid Ag target was fixed on a side wall of a cuvette containing an appropriate amount of GO The trace Ag was then ablated into AgNPs by using laser beams under vigorous stirring conditions [27] While this technique provides an effective approach for the fabrication of graphene oxide-based nanocomposite without the manipulation of toxic reducing agents, some notable drawbacks can be also observed such as the attenuation of the laser beam penetration due to the increase in the AgNPs concentration [28] The broad particle size distribution and low production rate are also notable disadvantages when applying this technique [29] Furthermore, the requirement of exorbitant equipment is also one of the key factors that obstruct the practical applications of these types of advanced techniques, especially in developing countries
Chemical synthesis methods can be regarded as the most popular approach for the preparation of nanomaterials due to their simplicity and ease of operation procedures These techniques involve the participation of reducing agents and additional stabilizers for the reduction of metallic precursors and the embedment of the resulting nanoparticles onto the substrates Typically, four main stages are included during a chemical synthesis
6 process, namely reduction, nucleation, growth, and stabilization [30] In particular, the silver precursors, commonly silver nitrate, are added into the GO suspension, in which the electrostatic interaction between the metallic ions and oxygen-containing groups of
GO occurs After that, the addition of reducing agents into the mixtures causes the generation from Ag(I) to Ag(0), followed by the growth of nanoparticle clusters Furthermore, the GO can not only act as a stabilizer but also as an additional reducing agent for the reduction of metallic cations [31] Recently, some well-known reducing agents for the fabrication of AgNPs include sodium borohydride, ascorbic acid, hydrazine, glucose, etc Different reducing agents can lead to the formation of the resulting nanocomposites with distinct morphological structures For instance, the reducing performance of different reducing agents was thoroughly compared by Dat et al., including L-ascorbic acid, hydrazine, trisodium citrate, and oxalic acid [20] The reduction processes of the Ag + using these compounds are indicated in Equations (1.1) – (1.7) Particularly, while the L-ascorbic acid sacrificed the hydroxyl groups for the donation of electrons to the Ag + to form AgNPs, the reduction process using hydrazine involves the formation of the Ag(N2H4) + complex, which can electrostatically interact with the N2H4 to generate AgNPs On the other hand, the synthesis of the AgNPs using trisodium citrate involves the electron transfer from the citrate ions to the Ag + , whereas the reduction using oxalic acid may involve the electrostatic interaction between the Ag + with the hydroxyl groups The study indicated that the prepared samples fabricated with the citrate salt and oxalic acid provided a negligible amount of AgNPs on the GO surface, while the material fabricated with L-ascorbic acid possessed the largest size of AgNPs when compared to the remaining reducing agents [21]
Ag + + N2H4 → Ag(N2H4) + (1.2) Ag(N2H4) + + N2H4 → Ag 0 + 5/6N2 + 4/3NH3 + NH4 + (1.3) 4Ag + + C6H5O7Na3 + 2H2O → 4Ag 0 + C6H5O7H3 + 3Na + + H + + O2↑ (1.4)
Ag(NH3) + + NH3 ⇌ Ag(NH3)2 + (1.6)
2Ag(NH3)2 + + G–CHO + H2O ⇌ 2Ag 0 + 4NH3 + G–COOH + 2H + (1.7) Nonetheless, it is noting worth that the conventional chemical reduction approaches usually suffer from many drawbacks, such as low production rate, limited synthesis efficiency, a requirement of high temperature, and toxic catalysts [32], [33] Thus, the performance of this process has been enhanced via the support of many physical techniques, such as microwave, ultrasonication, and gamma irradiation [34], [35], [36] For instance, microwave-assisted fabrication of silver/reduced graphene oxide on cotton fabric was prepared, in which a GO suspension was mixed with a solution of silver nitrate while trisodium citrate was used as a reducing agent [37] The reaction was carried out in a microwave oven, which gave an average nanosize from 70 to 100 nm The chemical reduction method assisted by microwave irradiation and vigorous stirring can endow a uniform heat distribution, promoting the rapid reduction process [37] Moreover, with the support of ultrasonication, the nanocomposite of the GO and AgNPs can be also fabricated without the participation of additional reducing agents Specifically, according to the study of Noor et al., the complex of silver nitrate and ammonia solution was mixed with GO suspension under ultrasonication conditions The nanoparticle size of the resulting material possessed a quasi-spherical shape with an average size of approximately 12 nm, which exhibited great colorimetric detection of mercury ions [38] The formation mechanism of the AgNPs during this process not only relies on the GO but also the effect of ultrasound Particularly, the ultrasonic environment facilitates the effect influences of acoustic cavitation, resulting from the formation, growth, and collapse of several bubbles The explosion of these bubbles can promote the generation of “hotspots” areas, which significantly stimulate the dissociation of water molecules into hydroxyl (OH) radicals These reactive radicals can efficaciously participate in the reduction process of the metallic cations and increase the accessibility of more reactive sites for the reducing agents, gradually leading to the enhancement in the fabrication efficiency and preventing the tendency of self- agglomeration of the nanoparticle clusters [30] Besides, recent studies have also employed gamma beams as a physicochemical approach for the fabrication of the AgNPs@GO nanocomposite Some advantages of this technique comprise the provision of a reduction medium for the metallic precursors or fragmentation of bulk solutions into nanoscales [39] For instance, the heterogeneous nanocomposite of silver, gold, and
8 platinum nanoparticles was successfully embedded onto the layers of GO under inert conditions and of 60 Co γ-irradiation [39] The obtained nanomaterials possessed an even distribution of the AgNPs with an average size of 16.73 nm However, it should be also noted that during the irradiation of gamma beams, the GO could be potentially converted to a reduced form, which is attributable to the reduction of oxygen-containing functional groups by the hydrated electrons or hydroxyl radicals
1.2.3 Reduction method using plant extract
In recent years, environmentally friendly fabrication approaches have gained great attention from the scientific communities because of the increasing demand for protecting the ecosystems and minimization of contaminated precursors, by-products, and processing costs Especially, the reduction process using plant extract method is one of the most promising techniques, which involves the replacement of conventional synthetic chemicals by naturally occurring sources, primarily including plant extract, fungi, bacteria, or some bio-reactive compounds [40] Furthermore, these substances not only can act as reducing agents but also potential stabilizers for nanoparticles thanks to the aid of the abundance of functional groups that can electrostatically interact with the positively charged metals [41] Especially, the reducing agents derived from plant sources have become ubiquitous among various eco-friendly synthesis approaches The abundant biomolecular contains in these precursors usually includes carbohydrate, phenolic, terpene, hydrocarbon, and fatty acid, which can electrostatically interact with the metallic precursors, effectively preventing them from aggregation and strongly enhancing bioactivities Some typical plant sources containing the aforementioned compounds that have been widely utilized for the reduction of metallic precursors include Citrus grandis [42], Mangifera indica [30], Morus alba [43], Curcuma longa [44], as indicated in Figure 1.3 For example, Hai et al reported a synthesis of AgNPs using the Mangifera indica leaf extract The study indicated that the utilized extract possessed several biomolecules exhibiting strong reducing performance and binding effects, which can be efficaciously employed for the fabrication of AgNPs (Figure 1.4) The reduction process from Ag + to AgNPs utilizing biomolecules derived from
Mangifera indica leaf can be expressed in Figure 1.4 [30]
The additional advantage of these sources is the inheritance of the superior bioactivities of the nature-derived compounds, which can be potentially employed in a wide range of pharmaceutical and environmental applications without harming human health It is also noteworthy that although the research on the synthesis of the AgNPs@GO nanocomposite by using bacteria or fungi has not received great attraction, numerous studies have been carried out to prepare the bare AgNPs with the enhanced performance of the antimicrobial activity, catalysis, pollutant treatment, or colorimetric sensor For example, the bacterial strain Bacillus cereus from contaminated soil was collected for the fabrication of the AgNPs, in which the proteins or enzymes derived from the bacteria cell are responsible for the generation and stabilization of the nanoparticles [45] The study also indicates that the fabricated nanomaterials exhibited strong performance in the inhibition of both Gram-positive (Staphylococcus epidermidis and Staphylococcus aureus) and Gram-negative (Escherichia coli and Salmonella enterica, Porteus mirabilis) strains
Figure 1.3: Reducing agents derived from plant sources
On the other hand, the fabrication of the AgNPs@GO nanocomposite has been broadly carried out via the utilization of biomass waste as well as plant extracts For example, Jeronsia et al utilized the extract of Punica granatum peel for the preparation of the AgNPs@GO nanocomposite, which showed a uniform distribution of AgNPs with an average size of 35 nm [46] The materials also possessed better antibacterial performance against Gram-positive (Staphylococcus aureus and Bacillus subtilis) and Gram-negative (Klebsiella pneumoniae and Escherichia coli) strains when compared to the bare AgNPs In another study, Basiri et al also reported a synthesis procedure of the AgNPs decorated reduced GO using pine leaf extract, which could be effectively employed for the detection of dopamine and Cu 2+ [47] The result indicated the successful synthesis with evenly-distributed AgNPs on the graphene sheets, which possessed an average particle size of 3.7 nm Furthermore, the exploitation of plant sources is not only limited to the reduction of metallic ions but also potentially extended to the production of graphene precursors Similar to the aforementioned chemical reduction methods, different physical supporting techniques can be also employed to enhance the synthesis efficiency of the nanocomposites As described, a novel graphite source was obtained via the carbonization of potato peels for 2 h, while a phosphorized derivative of chitosan was used for the exfoliation of graphene sheets under the support of ultrasonication for 1 h Following that, the AgNPs were decorated onto the surface of the as-prepared graphene sheets by mixing with silver nitrate under microwave irradiation [48]
Despite the aforementioned outstanding properties, it is also noteworthy that the this approach has also possessed some notable challenges, which considerably obstruct its large-scale applications Particularly, the participation of several metabolites derived from natural precursors leads to a great demand for the investigation of specific reaction mechanisms to optimize the production process Additionally, notwithstanding the abundant availability of nature-derived precursors, exploiting these agents necessitates several additional steps for the treatment of the raw precursors, leading to the requirement for more time as well as equipment [49]
Figure 1.4: Reduction process from Ag + to AgNPs by biomolecules derived from Mangifera indica leaf extract [26]
Effects of different parameters on the synthesis process
In order to obtain a resulting nanocomposite with the desired shape, size, and physicochemical properties, several internal or external parameters must be thoroughly considered, as indicated in Figure 1.5 For instance, types of reactants (silver precursors and reducing agents) and their proportion in the reaction medium are major internal factors, which can be also affected by several surrounding conditions such as temperature, reaction time, or pH medium [50] The appropriate selection of these aforementioned parameters can dictate numerous properties of the resulting nanomaterials, including synthesis efficiency, particle size, and stability, as well as their potential performance in different applications To investigate the effects of these factors on the morphological properties of the resulting material, several advanced analytical techniques have been employed, such as ultraviolet-visible spectroscopy (UV-Vis), X- ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis (TGA), dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and energy
12 dispersive spectroscopy (EDS) [51] There have been several scientific articles or reviews concentrating on the separate discussion of the principles of the aforementioned analytical techniques Thus, this review will focus on the discussion of several parameters dictating the efficiency of the synthesis process, as well as the mention of relevant analytical methods to confirm the influences of these factors on the physicochemical properties of the prepared AgNPs@GO nanocomposite
Figure 1.5: Effects of different factors on the fabrication process of the AgNPs
Previous studies have indicated that the morphological structure of the nanomaterial, as well as the efficiency of the fabrication process, can be varied by adjusting the temperature of the reaction medium [52], [53] Particularly, it has been reported that the reaction rate of the synthesis of the AgNPs increases with the increase in temperature [54] The increase in the conversion rate of the metallic precursors to nanoparticles can be confirmed by the variation in the surface plasmon resonance (SPR), which is the characteristic property of AgNPs Thus, UV-Vis spectroscopy has been indispensably utilized for the clear observation of this phenomenon For example, the effect of the reaction temperature on the bio-fabrication of the AgNPs using the extract of Salvadora persica stem extract was evaluated via the modification in the SPR absorption spectra
[55] In detail, the increase in the reaction temperature from 25 to 120 o C leads to a significant increase in the absorption intensity, revealing the increasing concentration of the AgNPs The well-defined absorption peaks are also an indication of the presence of
13 spherical AgNPs, while the sharpness of these signals also revealed the small size of the nanoparticles [56] It should be also noted that the position of the maximum absorption wavelength can reveal the shape and size of different nano-metals, in which the characteristic excitation SPR vibration of the AgNPs lies within the region from 380 to
450 nm [57] Specifically, the effects of temperature on the shape and size of the AgNPs have been widely assessed via the evaluation of the UV-Vis spectra, in which the shift of absorption peaks to a lower and higher wavelength indicates the reduction and increment in the AgNPs size, respectively [58] Indeed, a study on fungal-based AgNPs also examined the effect of reaction temperature on the particle size, where the increase in the reaction temperature from 10 to 40 o C leads to a reduction in the maximum absorption wavelength from 451 to 405 nm [59] The shift in the absorption peaks was also in agreement with the change in the particle size determined from the TEM images The variation in particle size with the temperature can be explained that when the reaction temperature increases, the reaction rate also rises, which vitally stimulates the rapid reduction of Ag + cations to Ag(0), preventing the secondary reduction process on the surface of the pre-generated nuclei [60] However, it should be noted that the inappropriate increase in the reduction temperature may cause reverse effects Several studies also revealed that the increase in reaction temperature might increase the particle size, which can be elucidated by the theory of Oswald ripening In particular, although higher reaction temperature can stimulate the detachment of diminutive nanoparticles from the larger clusters, the supersaturation of the free molecules resulting from the diffusion of excessive nanoparticles can pull the free atoms aggregate together, leading to the appearance of large nanosize [61] As seen, a study on the fabrication of anisotropic AgNPs using sodium bis(2-ethylhexyl) sulfosuccinate, sodium borohydride, sodium citrate, and L-ascorbic acid also showed that the reaction temperature increment from 17 to 55 o C leads to the red-shift of absorption wavelengths, indicating the increase in particle size [62]
Regarding the cases for the AgNPs@GO nanocomposite, previous pieces of literature have reported that thanks to the stabilizing performance of the GO surface, the morphological structure of the generated AgNPs can remain more stable regardless of temperature change when compared to the fabrication process without the GO Due to the strong electrostatic interaction between the Ag + and negatively charged GO, the
14 generated Ag nuclei from the reduction processes can rapidly adsorb onto the surface of
GO, which can efficaciously maintain the small size of nanoparticles without further agglomeration [63] A study on the fabrication of AgNPs decorated on GO by using a rotating packed bed reactor also investigated the effects of reaction temperature on the formation of the resulting AgNPs@GO [64] According to the XRD spectra, the increment in reaction temperature from 50 to 80 o C showed insignificant change in the characteristic diffraction peaks of the AgNPs, indicating the stability of the crystallinity of the nanoparticles in the resulting nanocomposite Nonetheless, it is also noteworthy that the excessive increase in reaction temperature still potentially leads to a considerable modification of the morphology of the resulting material A study on the fabrication of AgNPs@GO by using sodium citrate and sodium borohydride at pH 11 also evaluated the morphological change of the nanomaterial with the increase in the reaction temperature from 25 to 100 o C [65] The results showed that as temperature increased significantly, the absorption signals became broader, indicating an increase in the size of the AgNPs Additionally, the excessively increasing temperature can also promote the decomposition of many oxygen-containing functional groups on the GO sheets, eventually attenuating the dispersity of the AgNPs@GO nanocomposite in water[66] This phenomenon can be observed via the disappearance of characteristic absorption peaks of the functional groups in the XPS spectra [67] Furthermore, the modification in the GO structure can be observed via the XRD patterns, in which the dramatic increase in temperature can lead to a reduction of GO signals and an emergence of additional diffraction peaks of reduced GO (rGO) [68]
It has been broadly reported that pH is also one of the crucial parameters for an efficient synthesis process of nanocomposites Earlier studies have indicated that the variation in pH can modify the electrical charges of the precursors, gradually influencing the activity of reduction or stabilization of the nanoparticles [69] A study on the fabrication of the AgNPs using Pinus eldarica bark extract also investigated the effect of pH variation from 3 to 11 on the growth of the AgNPs The results indicated that the increase in pH value also raises the maximum SPR absorption at 430 nm Meanwhile, another study on the synthesis of AgNPs using Dragon Fruit peel extract also performed that the variation in pH from 3.35 to 5.35 led to a blue-shift in the absorption peak from
457 to 431 nm, revealing the reduction in particle size [70] These phenomena have been widely discussed in different studies, which can be explained that the increase in pH can facilitate deprotonation, which endows the AgNPs with strong repulsive force with each other, thereby obstructing the self-agglomeration of particle clusters [71] Contrarily, reducing pH in the reaction medium might lead to the increment in protons to the system, facilitating the neutralization, attenuating the energy barrier between the AgNPs, and thereby promoting rapid aggregation [72] Nevertheless, it should be notably considered that the excessive increase in pH also potentially causes different results Another study on synthesis of AgNPs using Neem leaf extract showed that the increase in pH from 9 to 13 led to a red shift of SPR absorption peaks, along with the increase in the signal intensity, which affirmed the increase in the concentration and size of the formed AgNPs [73] Therewithal, a study on the synthesis of AgNPs using locust bean gum (LBG) polysaccharide also reported that the excessive increment in pH to 12 caused the agglomeration between several particle clusters when compared to pH 10 [74] This tendency has been also claimed for in an alkaline environment, it is also possible for the
Ag + to be precipitated to AgOH, which prevention the further bio-reduction and destabilization of the AgNPs [75]
Similar trends can be also observed in the fabrication process of the AgNPs@GO nanocomposite However, the variation of pH also significantly affects the functional groups in the GO skeleton, thereby influencing the adsorption and formation of the AgNPs [76] Indeed, a study on the AgNPs@GO nanocomposite using boric acid (0.02 M) or sodium tetraborate (0.005 M) also indicated that the increase in pH from 7.4 to 9 dramatically promoted the formation of AgNPs [76] Particularly, increasing pH in the reaction medium endows more OH- for the solution, which promotes the deprotonation process for the oxygen-containing functional groups of the GO This phenomenon can efficaciously stimulate the adsorption of the Ag+ onto the surface of the GO via strong electrostatic interaction [77] Furthermore, it has been also reported that the increasing concentration of hydroxyl groups also reduces the redox potential of the electron donors, concurrently increasing the electron donating performance of the phenolic groups of the
GO, eventually stimulating the reduction of the silver precursors [76], [78]
During the fabrication of the AgNPs@GO nanocomposite, the control of reaction time is of paramount importance to obtain the desired shape and size of the AgNPs, as well as the stability of the nanocomposite In general, the prolonged reaction time stimulates the continual formation of AgNPs until reaching reaction equilibrium, which can be easily observed via the variation in the maximum absorption peaks For evidence, Dou et al reported a study on the fabrication of the AgNPs/GO nanocomposites using the, in which the effect of reaction time was also investigated [79] According to the UV-Vis spectra at different reaction stages, the intensity of the absorption peak increased with time, which remained unchanged at 8 h of reaction, indicating the complete reduction of AgNO3 to AgNPs Besides, it has been reported that the reaction time is also related to the shape and size of the AgNPs Bai et al reported the fabrication of AgNPs-manganese oxyhydroxide-GO nanocomposite via the modified silver mirror reaction, in which SEM images were utilized for the measurement of particle size at different reaction periods [80] Collecting from the results, the presence of Ag nuclei with the size of 5 to 10 nm could be observed at the reaction time of 3 h The reaction took place for the next 3 h with the appearance of larger AgNPs with the size of 10 to
20 nm After 24 h, some larger AgNPs were obtained apparently due to the aggregation of the particle clusters, which can be more easily observed at the reaction time of 36 h The aforementioned results indicated that the desired size of the AgNPs can be controlled by the appropriate reaction time According to these phenomena, the formation mechanism of the AgNPs has been also thoroughly discussed Although some inevitable controversies still exist, the formation, growth, and stabilization of the AgNPs can be divided into four main stages: the formation of Ag atoms during the reduction processes; the nucleation of AgNPs; the aggregation of larger particle clusters; and the stabilization of the AgNPs [30], [81] On the other hand, the prolonged reaction time also considerably affects the structure of the GO sheets, in which the continual interaction with the additional reducing agents can lead to the partial transformation of
GO into reduced forms Specifically, in a study on the synthesis of the reduced GO-AgAu bimetallic nanocomposite, the prolonged reduction by the extract of Cetraria Islandica (L.) Ach for 24 h led to the removal of the number of oxygen-containing functional groups, which was confirmed by the red-shift of the -* transition peak from
232 to 270 nm, along with the absence of the n-* transition at 300 nm representing the existence of C=O bondings [82]
1.3.4 Ratio between silver precursors and graphene oxide
Several studies have been carried out to evaluate the impacts of the amount of silver and graphene precursors on the fabrication process of the AgNPs@GO nanocomposite Typically, the addition of GO or other common supporting agents is responsible for the stabilization of the AgNPs, preventing the formed nanoparticles from aggregation The evidence for the forming and stabilizing tendency can be effectively verified via the UV- Vis As can be described, Zhu et al reported the fabrication of GO-AgNPs nanomaterials with high antibacterial activity [83] The research indicated that the combination of AgNPs onto the GO-containing environment led to a reduction in maximum absorption peak and an increase in full width at half maximum (FWHM) from
Colorimetric detection activity of the AgNPs@GO
The colorimetric sensing activity of AgNPs has been widely employed for the detection of various toxic chemicals, including metallic species or organic pollutants in aqueous solution The colorimetric sensing principles of the AgNPs-based nanomaterial can be different between various pollutant structures or the physicochemical properties of the fabricated AgNPs In summary, the main sensing mechanism of the AgNPs is related to the resonance effect as a result of the interaction between the AgNPs and the incident light It has been posted that the position and intensity of the localized surface plasmon resonance of the plasmonic nanoparticles can be influenced by the morphological structure or the local dielectric constant of the reaction medium [107] Thus, the interaction of the nanoparticles with extraneous organic or metallic species
23 can lead to a change in the shape and size of the nanoparticles, or the dielectric constant of the surrounding medium, consequently varying the position and intensity of the absorption signals Withal, ease of oxidation and the tendency of self-agglomeration significantly attenuate the performance of the pristine AgNPs Therefore, the participation of the GO can be regarded as an efficacious approach for the enhancement of the morphological properties, as well as the colorimetric sensing of the bare AgNPs The improvement of the detection performance can also be attributed to the strong adsorption of the pollutants onto the surface of the GO sheets via electrostatic interaction, π-π stacking, or hydrogen bonding owing to the presence of numerous oxygen-containing functional groups and the sp 2 -bonded carbonaceous structure, which effectively promote the interaction with the AgNPs [108]
In the past few years, AgNPs-based nanomaterials have been widely utilized for the detection of metallic traces in water reservoirs with a high degree of selectivity One of the most common metallic ions that have been widely detected by using AgNPs is mercury (Hg(II)) For example, Noor et al reported the utilization of the AgNPs modified with GO for the colorimetric detection of GO [109] In the study, the experimental procedure involved the addition of 5 μL of Hg(II) solution into 2 mL of GO-Ag suspension and resting for 2 min The detection activity was monitored via the change in the absorption spectra, while experiments for other control metallic ions were also carried out the investigate the selectivity of the prepared materials The result showed a conspicuous reduction in the absorption intensity of the AgNPs along with a blue-shift and gradual decolorization as the amount of the Hg(II) increased, while no considerable change in the intensity could be observed in the figure for other metallic ions, which corroborated the high selectivity of the synthesized GO-Ag nanocomposite towards Hg(II) It is worth noting that the detection capability of the materials can be explained by the difference in the redox potential between the Hg 2+ /Hg (0.85 V) and
Ag + /Ag (0.8 V), which facilitates the oxidation of the Ag(0) to Ag(I), resultingly reducing the absorption intensity in the UV-Vis spectra [30] Moreover, it has been also reported that the shift of the SPR signals to the left can be attributed to the formation of the anisotropic Ag-Hg amalgam [110] The reaction between the AgNPs and Hg(II) can be also demonstrated in Equation (1.9):
Ag n + Hg 2+ → Ag n−2 Hg + 2Ag + (1.9) Besides, the formation of the Ag-Hg amalgam, as well as the modification in the morphology of the AgNPs@GO nanocomposite after the reaction process with Hg(II) has been also thoroughly investigated for further understanding of the colorimetric detection mechanisms Kamali et al reported a synthesis process for the Ag@GO nanocomposite via the chemical reduction method using NaBH4 as a reducing agent [111] The Hg(II) colorimetric sensing of the resulting material possessed a good performance with the limit of detection (LOD) value of 338 nM Especially, the amalgamation of the Ag-Hg was thoroughly investigated via TEM images, XPS, and XRD patterns The results indicated a reduction in the AgNPs size after the addition of Hg(II), along with the absence of characteristic diffraction signals of the AgNPs that were substituted by the characteristic peaks of the Ag-Hg amalgam in the XRD patterns Additionally, the XPS spectra also confirmed the successful oxidation of the Ag(0) to Ag(I) via the conspicuous observation of the characteristic peaks at 366.8 and 372.8 eV for 3d5/2 and 3d3/2
For expansion, it has been reported that the colorimetric detection activity of the AgNPs or AgNPs-based nanocomposites has been also employed for the determination of the presence of hydrogen peroxide (H2O2) H2O2 has been regarded as a popular oxidizing agent and is well-known as a by-product of oxidase enzyme Moreover, H2O2 has been also ubiquitously utilized in a wide range of applications such as catalysis, pharmaceutical, clinical, chemical, and dye industries [112], [113] Nevertheless, it should be noted that the excessive release of H2O2 into the surrounding environment also poses a detrimental influence on human health, including cancer, diabetes, and cardiovascular diseases [114] Therein, several techniques for H2O2 quantification have been widely developed, and the utilization of the AgNPs doped GO-based support is also considered a promising approach thanks to the ease of operation, high performance, and low cost Mehata et al reported a synthesis of graphene quantum dots (GQDs) from
GO, which was doped with AgNPs to form GQDs@AgNPs nanohybrids [114] The resulting materials were utilized for the colorimetric detection of H2O2 and reported to possess high performance, in which the variation in the absorption intensity Additionally, Chen et al also reported the fabrication of GQDs from GO, which were combined with AgNPs to synthesize GQDs/AgNPs hybrid for the colorimetric detection
25 of H2O2 The study indicated a high sensing performance of the material with an LOD value of 33 nM within an H2O2 concentration range from 0.1 to 100 μM In general, the
H2O2 colorimetric detection mechanism can be attributed to the interaction between
H2O2 and the AgNPs on the graphene sheets Particularly, electrons of the reduced sites of the AgNPs can be transferred to the H2O2 molecules and vice versa, consequently leading to the formation of OH• and HO2• that can react with each other to generate O2 and H2O, whereas the AgNPs were also partially oxidized to Ag2O This phenomenon significantly reduces the concentration of the AgNPs, resultantly decreasing the absorption intensity of the AgNPs at 400 nm The reaction between the AgNPs and H2O2 can be also expressed via Equation (1.10):
In addition to the utilization of amalgamation for the detection of Hg(II), the AgNPs decorated graphene-based support has been also utilized as a colorimetric sensor for other metallic ions via the aggregation/anti-aggregation mechanisms [115] Particularly, the aggregation mechanism involves the interaction between the metallic ions and the graphene nanosheets, which leads to the formation of visible precipitates in the reaction medium, consequently, modifying the SPR bands of the as-prepared AgNPs@GO nanocomposite [116] On the other hand, the anti-aggregation counterpart involves the interaction of the analytes into the solution containing the precipitate as a result of the reaction between the AgNPs and an appropriate reactive agent, which significantly modifies the morphological properties of the AgNPs The observation of these phenomena via analytical techniques such as UV-Vis can help to determine the presence of the metallic ions in the aqueous solution [116] For instance, Linh et al reported colorimetric detection procedures for the cadmium ions (Cd(II)) using the hybrid silver– doped graphene oxide material (Ag/GO), which indicated a low LOD value of 10.15 mg/L in the Cd(II) concentration range from 0 to 200 mg/L [117] The material was fabrication using glucose as a reducing agent The detection mechanisms in the study were explained by the predominant interaction between the oxygen-containing functional group in the graphene basal plane and the Cd(II), followed by the formation of the GO-Cd-GO matrix thanks to the conjugated π-π systems, resultantly leading to the formation of precipitates Moreover, the presence of the AgNPs was also reported to be of paramount importance during the interaction process, which can be elucidated that
26 the AgNPs can stimulate the exfoliation of the GO sheets, which efficaciously provides more space for the binding with the Cd(II) Therewithal, the AgNPs-based nanocomposites synthesized via green-synthesis approaches were also used as a colorimetric sensor based on anti-aggregation mechanisms, which provided a great performance thanks to the participation of the biomolecules derived from the biological precursors Basiri et al introduced a colorimetric detection technique for copper ions (Cu(II)) by using green-fabricated rGO@AgNPs nanocomposites [47] The procedure involves the addition of dopamine (DA) into the suspension of the rGO@AgNPs, which was then mixed with the solution of Cu(II) with different concentrations The change in absorption ratio (A515/A405) was monitored by the UV-Vis spectra to evaluate the detection ability of the fabricated materials During the detection process, the AgNPs can vigorously react with the DA via π-π or hydrogen interaction thanks to the sp 2 - bonded structure and the presence of oxygen-functional groups on the graphene sheets, eventually forming precipitates in the reaction medium Meanwhile, the Cu(II) can also strongly react with the DA due to the presence of hydroxyl and amine groups that can interact with the Cu(II) via Cu-O or Cu-N bonding, resultingly leading to the removal of DA molecules from the surface of the rGO@AgNPs Furthermore, one of the notable advantages of this study involves the utilization of pine leaf extract for the fabrication of the material This endowed the number of phenolic functional groups derived from the extract, which not only contribute to the stabilization of the AgNPs but also provide more -OH groups for the reaction with DA molecule hydrogen bonding The study concluded the good detection performance of the nanocomposites, which obtained an LOD value of 9.8 nM
On the other hand, many previous studies have also concentrated on the utilization of the AgNPs doped GO nanocomposite for the detection of many types of organic compounds, contributing to the improvement of medicinal treatments and the qualification of food products For instance, Alex et al introduced a synthesis of the Ag-
GO nanocomposite using NaBH4 as a chemical reducing agent, which was then utilized for the colorimetric detection of malathion based on the acetylcholinesterase (AChE) inhibition-based sensing mechanism [118] It is noteworthy that malathion is one of the organophosphate pesticides that can negatively cause lymphoma in humans The structure of this compound can also inhibit the enzyme AChE, while the interaction
27 between the AChE and acetylthiocholine (ATCh) can cause the production of thiocholine (TCh), which can promote the aggregation of the AgNPs via Ag–SH bindings and electrostatic interactions Thus, the observation of the inhibition activity of the malathion in this precipitation process via the change in SPR bands of the AgNPs can help to determine the presence of these compounds in the aqueous solution The study indicated a low value of LOD of 0.01 pM, which is a competitive result when compared to other previous studies Besides, Loganathan et al also reported the fabrication of the GO-AgNPs nanocomposite using NaBH4 as a reducing agent, in which the resulting material was utilized for the colorimetric detection of saccharin (SAC), a common synthetic sweetener present in many sweeten foodstuffs [119] The study indicated the good detection performance of the fabricated nanocomposite with the LOD value of 0.7 mg/L The detection mechanism of the GO-AgNPs nanocomposite was also discussed, which involved the aggregation of the AgNPs by interacting with the SAC molecules via Ag-N bonding, whereas the interaction between GO nanosheets and SAC molecules was insignificantly observed On the other hand, Tran et al also combined AgNPs and GO to produce AgNPs@rGO via green synthesis methods using glucose as a green reducing agent, which was utilized for the colorimetric detection of glucose [120] The detection of glucose in the study was explained via the catalysis potential of the AgNPs@rGO in the oxidation process In particular, glucose was first mixed with the glucose oxidase (Gox) to produce gluconic acid and H2O2 After that, the resulting product reacted with AgNPs@rGO to form Ag + , which can convert 3,30,5,50- tetramethylbenzidine (TMB) into an oxidized form The oxidized TMB could be visually observed with a blue color, possessing the maximum absorbance at 655 nm Therefore, the observation of the solution around this wavelength can provide a good determination of the glucose in the solution Additionally, the graphene sheet with sp 2 - bonded structure acted as an electron shuttle between the donor (TMB) and the acceptor (Ag + ), effectively enhancing the redox processes
According to the aforementioned studies, the AgNPs@GO nanocomposite has been widely fabricated for the colorimetric detection of a variety of pollutants as well as organic molecules Based on different purposes as well as target reacting agents, the structure of the nanocomposite can be modified using different approaches, including modifying the morphology of the GO to its reduced forms or changing chemicals to
28 plant sources to manipulate the advantages of the green precursors, such as utilizing the abundance of oxygen-containing functional groups of the extract for the binding or redox process Several studies that utilized the AgNPs@GO nanocomposite or its derivatives for the colorimetric detection of different pollutants and organic molecules were summarized in Table 1.1
Table 1.1: Colorimetric detection performance for different pollutants using
Detection range Pollutants LOD Ref
7 rGO-Ag - 0 – 1300 ppb Hg 2+ 9.5 nM [123]
10 Ag/GO Glucose 0 -200 mg/L Cd 2+ 10.15 mg/L [117]
Detection range Pollutants LOD Ref
Domestic and international research and the urgency of the topic
Several domestic studies have been carried out for the green fabrication of AgNPs@GO using several plant sources, as indicated in Table 1.2 However, there has been no study concentrating on the fabrication of the AgNPs@GO nanocomposites from the M indica leaf extract and applying the prepared material in the colorimetric detection of H2O2
Table 1.2: Domestic studies related to green synthesis of AgNPs
2023 Facile Synthesis of Eco-Friendly
Silver@Graphene Oxide Nanocomposite for
Vietnam National University Ho Chi Minh City
2020 Reduced graphene oxide-wrapped silver nanoparticles for applications in ultrasensitive colorimetric detection of Cr(vi) ions and the carbaryl pesticide
2020 Silver nanoparticles-decorated reduced graphene oxide: A novel peroxidase-like activity nanomaterial for development of a colorimetric glucose biosensor
Hanoi University of Science and Technology
2019 Silver nanoparticles on graphene quantum dots as nanozyme for efficient H2O2 reduction in a glucose biosensor
Hanoi University of Science and
1.5.2 Previous studies in foreign countries
Several international studies have been carried out for the green fabrication of AgNPs@GO and their application in different environmental aspects, as indicated in Table 1.3 However, there has been no study concentrating on synergistic effects of precursorcontents on the particle size and H2O2 colorimetric detection performance of the prepared material
Table 1.3: International studies related to green synthesis of AgNPs
Essentiality, objectives, contents, research methods, novelty, and
Recently, the ever-growing emergence of water pollutants has led to several detrimental consequences for the environment as well as human health Especially, H2O2 is also considered one of the useful and ubiquitous agents for different industries, whose excessive content released to water sources can lead to numerous environmental and health-related issues Thus, many advances in technology have been adopted to monitor this molecule in water However, these techniques still possess numerous disadvantages such as high cost or requirement of complicated operating procedures Meanwhile, the utilization of nanomaterial, especially AgNPs@GO, has been regarded as an effective approach to simultaneously tackle these aforementioned problems with affordable prices as well as easy operational procedures
2021 An ultra-sensitive and selective AChE based colorimetric detection of malathion using silver nanoparticle-graphene oxide (Ag-GO) nanocomposite
2021 Colorimetric and “turn-on” fluorescence detection of saccharin using silver nanoparticles-graphene oxide composite
2019 Dual Activities of Nano Silver Embedded Reduced Graphene Oxide
Using Clove Leaf Extracts: Hg 2+ Sensing and Catalytic Degradation
2018 Green synthesis of reduced graphene oxide-Ag nanoparticles as a dual- responsive colorimetric platform for detection of dopamine and Cu 2+
2016 Amalgamation based optical and colorimetric sensing of mercury (II) ions with silver@graphene oxide nanocomposite materials
Nowadays, several methods have been employed for the fabrication of AgNPs@GO, including electrochemical, chemical co-precipitation, thermal decomposition, radiation, etc Despite some advantages such as high efficiency and rapid particle formation, these techniques also possess several drawbacks, including high-cost, manipulation of toxic chemicals, which exert detrimental influences on the environment and human health Furthermore, the adsorption of undesirable by-products onto the surface of the AgNPs can also result in many adverse effects on the performance of the AgNPs [130] Meanwhile, biosynthesis has been considered an economical, reliable, and eco-friendly method for the fabrication of green nanoparticles, and Mangifera indica (M indica) or mango has been considered one of the most ubiquitous tropical plants, which have been widely investigated in several bio-fabrication processes thanks to the presence of several bioactive metabolites Nonetheless, there are few studies conducted to synthesize AgNPs by M indica leaf extract and investigate the simultaneous effects of the precursor component on the particle size, which is also one of the important factors contributing to the performance of the nanoparticles
Successful synthesis of graphene oxide-based silver nanoparticles using bio- valorization M indica leaf extract at room temperature with great H2O2 colorimetric sensing of performance.
• Propose the appropriate conditions for the AgNPs@GO and conclude the simultaneous effect of extract concentration, the concentration of AgNO3, and the volume ratio between GO and AgNO3 (GO/AgNO3) on the AgNPs size on the GO
• Conclude the characterization of the AgNPs@GO
• Conclude the H2O2 colorimetric detection performance of the AgNPs@GO
Content 1: Investigation of the simultaneous effects of synthesis conditions on the
AgNPs@GO with M Indica leaves as a reducing agent;
Content 2: Characterization of the synthesized AgNPs@GO;
Content 3: Investigation of the H2O2 colorimetric sensing of the AgNPs@GO
The M indica leaf extract was synthesized with hot water as a solvent;
GO was synthesized via the improved Hummers’ method;
The AgNPs@GO were synthesized with the M indica extract as a reducing agent
The particle size of the AgNPs was calculated from SEM images under different resolutions and magnifications using the ImageJ software Besides, the obtained particle size from SEM images was also compared with the results calculated according to the UV-Vis spectroscopy that has been thoroughly suggested by Barbir et al [131] Briefly, the calculation process was based on the dependence of wavelength at maximum absorbance on the particles Meanwhile, Mie theory was utilized for the simulation of the correlation between the wavelength and particle size of the material, in which a polynomial function was plotted for the calculation of particle size, as indicated in Equation (1.13) [131]: y=-0.0209x 2 +17.457x+3620.7 (1.13) where y is the diameter of the AgNPs and x is the absorption peak wavelength
1.6.4.3 Investigation of the simultaneous effects of different synthesis conditions
The effects of three different synthesis parameters including extract concentration, the concentration of AgNO3, and the volume ratio between GO and AgNO3
(GO/AgNO3) on the particle size and H2O2 colorimetric sensing performance of the prepared material was thoroughly investigated via the response surface methodology (RSM) according to Box–Behnken model This model has been widely employed in the investigation of more than two parameters, which are classified as rotatable or nearly rotatable second-order designs Regarding the design of the three investigated factors, the Box–Behnken model can also be demonstrated via a cube containing a central point and middle point on the edges, as indicated in Figure 1.11:
In addition, the regression equation for the Box-Behnken model can be expressed via Equation (1.14)
+ ∑ ∑ β ij X i X j n j=2 n−1 i=1 j > 1 (1.14) where Y is the response variable, Xi and Xj is the variables for the independent parameters, 𝛽 𝑜 is the model constant, 𝛽 𝑖 is the first-order coefficient, 𝛽 𝑖𝑖 is the second- order coefficient, and 𝛽 𝑖𝑗 is the interaction coefficient
The assessment of experimental data and extraction of response surface was carried out using Design Expert software v.13.0 Additionally, the analysis of variance (ANOVA) was also employed the evaluate the simultaneous influences of the three parameters as well as the statistical significance of the model via the determination of coefficient of determination R 2 , F-value, and P-value
The characterization of the synthesized composite is measured thoroughly using the below methods:
Principle: When a crystal is bathed in X–rays, each atom of crystal within the path of an X-ray absorbs some of its energy and then reemits it in all directions The Bragg law states that when the X-ray is incident onto a crystal surface, its angle of incidence,
34 θ, will reflect with the same angle of scattering, θ and when the path difference is equal to a whole number, n, of wavelength, constructive interference will occur The mechanism of XRD is shown in Figure 1.6
Figure 1.7: Principle of XRD Application: XRD is used to study the diffraction peak of the AgNPs
Based on the position and the diffraction peak width in the XRD patterns, the average crystal diameter can be determined by the Scherrer formula in Equation (1.11): t = 0.9λ
B × cosθ (1.11) where t (nm) is crystalline size, λ (nm) is X-ray wavelength, B (rad) is Full Width at Half Maximum (FWHM), the width of the diffraction peak, in radians, at a height halfway between the background and the peak maximum, and θ is an angle of scattering beam
Principle: This method is utilized to provide images with high resolution of the surface of the nanomaterials based on the scanning of the sample using a narrow electron beam Specifically, the images of the sample are accomplished through the recording of emitted radiation from the interaction of the electron beam with the surface of the sample Meanwhile, the electron beams can penetrate the sample, which induces several scattering effects These interactions are different among different morphological structures, which can give distinct surface images for the samples The main principles for the SEM analysis are illustrated in Figure 1.7:
Figure 1.8: Principle of SEM Applications: SEM is used to observe the surface structure and determine the average size of the AgNPs
Principle: This method involves the X-ray irradiation to the sample, leading the escape of a number of electrons from the surface Simultaneously, due to the distinct characteristic binding energy of each element, the kinetic energy of the emitted electrons from the surface of the material can be measured to provide information regarding the elemental composition and electronic state of each element in the material (Figure 1.8)
Figure 1.9: Principle of XPS spectroscopy
Applications: XPS is used to investigate components and the electronic state of the elements in the materials
Principle: This method relies upon the inelastic scattering of photons, known as
Raman scattering The laser light interacts with molecular vibrations, photons, or other excitations in the system, resulting in the energy of the laser photons being shifted up or down The energy shift gives information about the vibrational modes in the system Specific energy is used to distinguish one atom from another, enabling the analysis of the structural composition of the sample A source of monochromatic light, usually from a laser in the visible, near-infrared, or near–utraviolet range, is used; sometimes, X-rays can also be used Scattering light is collected on a lens and passed through a noise filter or spectrometer to obtain the Raman spectrum The principle of Raman spectroscopy is shown in Figure 1.9:
Figure 1.10: Principle of Raman spectroscopy Application: Raman spectroscopy is used to determine D peak, G peak, defects in the structure of GO and 3D-GO/SA
1.6.4.5 Determination of the concentration of AgNPs
Reasonably, the colorimetric method via ultraviolet-visible spectroscopy (UV-Vis) is applied for the detection process The working principle of UV-Vis equipment is shown in Figure 1.10 This is a technique used to quantify the light that is absorbed by a sample, so it can determine the concentration of some colored solution The data is plotted as extinction as a function of wavelength According to Lambert-Beer law, the light absorbance of the sample is directly proportional to its concentration in a solution according to the relationship expressed in Equation (1.12):
37 where A is absorbance, ε is the molar absorptivity, and C is the concentration of the sample (cm) is the length of the path light must travel through the cuvette
Figure 1.11: Principle of UV-Vis
This study involves the eco-friendly fabrication of AgNPs@GO nanocomposite using the abundant source of M indica leaf extract as both a reducing agent and stabilizer at room temperature This not only minimizes the usage of energy and complicated equipment but also contributes to the utilization of biomass waste in agricultural areas, which can be employed in different environmental treatment applications, including colorimetric detection of H2O2 Furthermore, the simultaneous effects of different conditions on the resulting particle size and the colorimetric sensing performance of the material are also investigated using response surface methodology according to the Box – Behnken model, which has not been thoroughly conducted in previous studies
EXPERIMENTAL
Experiments
- Preparation of M indica leaf extract;
- Investigation of simultaneous effects of different synthesis conditions on the AgNPs@GO;
- Characterization of the suitable AgNPs@GO;
V SUPERVISOR: Bui Van Tien, Ph.D.; Assoc Prof Nguyen Huu Hieu, Ph.D
Ho Chi Minh City, June 25, 2024
HEAD OF KEY CEPP LABORATORY
HEAD OF FACULTY OF CHEMICAL ENGINEERING
(Signature with full name) iii
Firstly, I would like to express my sincere gratitude to my mentor, Assoc Prof Nguyen Huu Hieu, Ph.D and Bui Van Tien, Ph.D., in the Faculty of Chemical Engineering and Faculty of Materials Technology of the Ho Chi Minh University of Technology – Vietnam National University, for their dedicated advice and guidance for almost six years in the laboratory with knowledge, experience, and many interesting scientific projects To the end of my journey at my university, they have shown great patience and guided me on the right path for my research Their invaluable guidance has effectively helped me complete my thesis proposal with a high willingness and effort
I am solely appreciative to all my colleagues in my adorable Key Laboratory of Chemical Engineering and Petroleum Processing (Key CEPP Lab) for their precise support and advice for my thesis at present and for all the projects I have joined for six years It has been a great experience for me to be a part of CEPP, as well as my secondary family
I am also very thankful to my parents and my sister, who always raise my motivation and confidence and encourage me throughout my work in the laboratory They all strengthen me and help me a lot whenever I face difficulties
Finally, I also express my gratitude to others who have shown interest in and supported my work
Ho Chi Minh City, June 25 th , 2024
Herein, the silver nanoparticles@graphene oxide (AgNPs@GO) nanocomposite was successfully fabricated using Mangifera indica (M indica) leaves extract as a reducing agent and stabilizer The simultaneous effects of different synthesis parameters, including the extract concentration, the concentration of AgNO3, and the volume ratio between GO and AgNO3 (GO/AgNO3), were investigated via response surface methodology according to the Box-Behnken model According to the experimental design results, the appropriate synthesis conditions were determined, in which the objective function is the particle size of the AgNPs decorated on the GO sheets Besides, the particle size of the AgNPs was determined using scanning electron microscopy (SEM) and Mie theory according to ultraviolet-visible spectroscopy (UV-Vis)
The characterizations of the prepared AgNPs@GO were investigated via advanced analytical methods, including ultraviolet-visible spectroscopy (UV-Vis), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy
Additionally, the H2O2 colorimetric detection performance of the material was also investigated with different concentrations of H2O2 using UV-Vis spectroscopy The correlation between the absorbance and the concentration of H2O2 was constructed to determine the limit of detections (LOD) and the limit of quantitation (LOQ)
The main content of this thesis is summarized in Figure 1
Figure 1: Graphical abstract of the thesis v
Trong nghiên cứu này, vật liệu nano bạc@ graphene oxide (AgNPs@GO) được tổng hợp sử dụng chất khử từ dịch chiết lá xoài (Mangifera indica) Ảnh hưởng đồng thời của các điều kiện tổng hợp đến quá trình hình thành AgNPs bao gồm thể tích dịch chiết, nồng độ AgNO3 ban đầu, và tỉ lệ thể tích giữa GO và AgNO3 đã được khảo sát thông qua quy hoạch thực nghiệm (QHTN) bằng phương pháp bề mặt đáp ứng, thí nghiệm được bố trí theo mô hình Box-Behnken Từ kết quả QHTN, điều kiện tổng hợp phù hợp được xác định với hàm mục tiêu là kích thước hạt AgNPs tạo thành trên các tấm GO Kích thước hạt AgNPs được xác định bằng ảnh kính hiển vi điện tử quét (SEM) và thuyết Mie dựa trên kết quả phân quang phổ hấp thụ tử ngoại – khả kiến (UV-Vis) Đặc trưng của vật liệu AgNPs@GO tổng hợp ở điều kiện phù hợp được phân tích bằng các phương pháp phân tích hiện đại như: Quang phổ hấp thụ tử ngoại – khả kiến (UV-Vis), nhiễu xạ tia X (XRD), kính hiển vi điện tử quét (SEM), phổ quang điện tử tia
Khả năng cảm biến so màu H2O2 của vật liệu được khảo sát với nhiều nồng độ H2O2 thông qua phổ UV-Vis Sự tương quan giữa độ hấp thu và nồng độ H2O2 được xây dựng nhằm xác định giới hạn phát hiện (LOD) và giới hạn định lượng của vật liệu (LOQ) Nội dung nghiên cứu của luận văn được tóm tắt ở Hình 1
Hình 1: Nội dung nghiên cứu của luận văn vi
COMMITMENT OF THE THESIS’ AUTHOR
I hereby declare that the work was originally implemented by the author and carried out under the instructions of Bui Van Tien, Ph.D and Assoc Prof Nguyen Huu Hieu, Ph.D in Ho Chi Minh City University of Technology – Vietnam National University
I confirm that this work is the result of my research and is solely my work All the contributions related to this thesis have been fully acknowledged I affirm that any formulation, idea, research, reasoning, or analysis borrowed from a third party is correctly and accurately cited in both techniques and the author’s rights
The author takes full responsibility for the work
Ho Chi Minh City, June 25 th , 2024
COMMITMENT OF THE THESIS’ AUTHOR vi
1.1 Introduction to silver nanoparticles@graphene oxide (AgNPs@GO) 2
1.2 Fabrication methods of AgNPs@GO 4
1.2.3 Reduction method using plant extract 8
1.3 Effects of different parameters on the synthesis process 11
1.3.4 Ratio between silver precursors and graphene oxide 17
1.4 Colorimetric detection activity of the AgNPs@GO 22
1.5 Domestic and international research and the urgency of the topic 29
1.5.2 Previous studies in foreign countries 30
1.6 Essentiality, objectives, contents, research methods, novelty, and contribution 30
2.1 Raw materials, chemicals, equipment, and research location 38
2.2.4 Investigation of simultaneous effects of different synthesis conditions on the AgNPs@GO 42
3.1 Simultaneous effects of synthesis conditions on the AgNPs@GO with M indica leaves as a reducing agent 45
3.3 H 2 O 2 colorimetric sensing performance of the AgNPs@GO 53
Figure 1.1: Structure of (a) GO and (b) AgNPs@GO 3
Figure 1.2: Some popular synthesis approaches for nanoparticles 4
Figure 1.3: Reducing agents derived from plant sources 9
Figure 1.4: Reduction process from Ag + to AgNPs by biomolecules derived from
Figure 1.5: Effects of different factors on the fabrication process of the AgNPs 12
Figure 1.8: Principle of XPS spectroscopy 35
Figure 1.9: Principle of Raman spectroscopy 36
Figure 1.10: Principle of UV-Vis 37
Figure 2.2: Preparation procedure for the extract 40
Figure 2.3: Synthesis procedure for the GO 41
Figure 2.4: Synthesis procedure for the AgNPs@GO 42
Figure 3.1: SEM images of AgNPS@GO samples prepared under different conditions (order of each image was denoted corresponding to Table 2.4) 46
Figure 3.2: UV-Vis spectra of AgNPS@GO prepared under different conditions (order of each image was denoted corresponding to Table 3.1) 49
Figure 3.3: Linear correlation between actual and predicted responses 49
Figure 3.4: Simultaneous effects of (a) concentrations of AgNO3 and extract, (b) extract and GO/AgNO3 ratio, and (c) concentration of AgNO3 and GO/AgNO3 ratio on particle size 50
Figure 3.5 XPS spectra of the AgNPs@GO; (a) survey, (b) C1s, (c) O1s, (d) Ag3d, (e) S2p, and (f) N1s spectrum 51
Figure 3.6: (a) XRD patterns and (b) Raman spectra of the AgNPs@GO 53 x
Table 1.1: Colorimetric detection performance for different pollutants using
Table 1.2: Domestic studies related to green synthesis of AgNPs 29
Table 1.3: International studies related to green synthesis of AgNPs 30
Table 2.3: Experimental parameters of the Box-Behnken model 42
Table 2.4: Design of experimental trials 43
Table 3.1: Box-Behnken design of three variables corresponding with a particle size as the response 46
Table 3.2: Analysis of variance (ANOVA) and descriptive statistics for the Box-
Table 3.3: Optimization of the particle size of silver nanoparticles-graphene oxide 48
Table 3.4: Comparison between particle size calculated from SEM images and UV-Vis spectra and the effects of particle size on H2O2 colorimetric detection performance 54
Table 3.5: Comparison of H2O2 colorimetric detection performance between different nanomaterials 55 xi
Hg(II) Mercury (II) ion
LSPR Localized surface plasmon resonance
UV-Vis Ultraviolet-visible spectroscopy
FTIR Fourier transform infrared spectroscopy
These days, the determination of hydrogen peroxide (H2O2) in water reservoirs has gained great attraction from researchers The excessive release of these species exerts many negative consequences on the environment and human health, such as aging, neurological disorders, or cellular destruction [1] Among several advances in technology, colorimetric sensing based on the localized surface plasmon resonance (LSPR) of plasmonic nanomaterial, especially AgNPs, has been considered an efficient method The AgNPs are clearly observed by naked eyes due to the flexible variation of surface plasmon resonance (SPR) bands with the change in sizes and particle distances, which is one of the advantageous properties of colorimetric sensing [2] Furthermore, with the participation of additional supporting agents such as GO, the detection performance of AgNPs can be effectively enhanced thanks to the improved stability and interaction with H2O2 molecules
In recent years, biosynthesis has been considered an economical and eco-friendly method for the fabrication of nanoparticles [3] Among numerous biomass sources,
Mangifera indica (M indica) or mango has been considered one of the most ubiquitous tropical plants, which can be applied in the fabrication of metal nanoparticles with uniformly distributed sizes due to the presence of numerous metabolites such as phenolic acids, benzophenones, and flavonoids [4] These compounds can also be responsible as stabilizing agents thanks to the electrostatic interaction between functional groups and nanoparticle surfaces [5], [6]
It has been reported that the physicochemical properties, as well as the performance of the nanomaterial, are greatly influenced by different preparation conditions Although many studies have been conducted to investigate the separate effects of different factors such as temperature, reaction time, pH, or precursor ratio on the yield of the production process of the AgNPs@GO, there is no research concentrating on the simultaneous impacts of the amounts of precursors added on the morphology, particle size, as well as the performance of the resulting nanocomposite in the H2O2 colorimetric detection [7], [8], [9], [10] Thus, this thesis is coming up with “Investigation of synthesis conditions for silver nanoparticles@graphene oxide using Mangifera indica leaf extract as a reducing agent for the application in colorimetric detection of H 2 O 2 ”
1.1 Introduction to silver nanoparticles@graphene oxide (AgNPs@GO)
Plasmonic nanoparticles have received great attention from researchers thanks to their possession of several extraordinary properties, including their localized surface plasmon resonance (LSPR) and high toxicity toward microorganisms [11] Recently, many studies have been carried out to manipulate these properties for many medicinal and environmental applications, such as the detection of metallic ions and biomolecules, or the inhibition of the growth of dangerous bacteria [12], [13] Among several plasmonic nanoparticles, silver nanoparticles (AgNPs) have been regarded as one of the most efficacious metallic nanomaterials when compared to other counterparts with the same sizes AgNPs also possess several superior properties, including high surface area, inertness in harsh environment, ease of visualization by the naked eye, simplicity of fabrication by using inexpensive techniques, and a broad spectrum of antibacterial and fungicidal performance, making them ubiquitous as primary component for abundant of consumer products [14] According to previous studies, it has been reported that the performance of the AgNPs in different applications can be varied due to the alternation of many factors including their shape, size, morphology, pH medium, temperature, or reaction time [15] Therefore, selecting appropriate fabrication approaches, as well as exploring novel precursors and supporting agents is important to improve the properties of the AgNPs and expand their applications on a larger scale
In addition to the plasmonic nanoparticles, carbonaceous materials, especially graphene, have also been widely investigated as an effective platform for the stabilization of nanoparticles [16], [17] Graphene oxide (GO), a graphene derivative composing a honeycomb structure of carbons and abundant oxygen-containing functional groups, has been reported to possess several advantageous properties, comprising large surface area, stability, and high conductivity Herein, GO has been broadly utilized in a wide range of applications such as drug delivery, energy storage, antimicrobial, wound treatment, sensors, catalysis, etc Furthermore, GO can also strongly interact with nanoparticles via physical adsorption, electron transfer, π-π interactions, or electrostatic bonding with oxygen-containing functional groups (Figure 1.1), which makes it a suitable support to reduce the self-agglomeration and enhance the
RESULTS AND DISCUSSION
Simultaneous effects of synthesis conditions on the AgNPs@GO with M
The effects the concentration of the extract, concentration of AgNO3, and the ratio between the mass of GO and AgNO3 were investigated to determine the optimal condition for the fabrication of the AgNPs@GO Table 3.1 illustrates the variation in the aforementioned variables for the Box-Behnken design corresponding with a particle size as the response, which was calculated according to SEM images (Figure 3.1) All samples showed the presence of AgNPs with quasi-spherical shapes which were evenly distributed on the wrinkled surfaces This confirmed the appearance of the GO sheets, as well as the successful embedment of the AgNPs on the carbonaceous matrix The successful formation of the AgNPs on the GO structure was also confirmed via the UV-Vis spectra (Figure 3.2) around the region from 380 to 450 nm, which revealed the characteristic SPR oscillation of the AgNPs Meanwhile, Table 3.2 demonstrates the analysis of variance (ANOVA) and obtained statistics for the model According to the results, the designed model was appropriate and statistically significant due to a high F- value of 148.76 (p < 0.0001) Besides, according to the p-value determined from each variable, it can be concluded that all variables, including A, B, C, AB, AC, BC, A 2 , B 2 , and C 2 significantly affect the resulting particle size of the AgNPs According to Table 3.3, the correlation coefficient (R 2 ) of the model was also determined to be 0.9948, confirming that 99.48% of the variation in the particle size is affected by the independent variables, while only 0.52% of the variation may be attributed to undesirable errors Additionally, the values for predicted and adjusted R 2 were also calculated to be 0.9168 and 0.9881, which indicated the great compatibility of the model with experimental data This tendency can be also affirmed by the linear plot indicated in Figure 3.3, revealing great linearity between the actual and predicted results According to the obtained simulation model, the optimal conditions for the fabrication of the AgNPs@GO were theoretically determined to be 1.476 mg/mL of extract, 0.111 mg/mL of AgNO3, and the GO/AgNO3 ratio of 0.307 with an estimated particle size of 22.002 nm Additionally, a polynomial quadratic equation expressing the correlation between the response and variables of the model was also determined, as indicated in Equation (3.1)
Figure 3.1: SEM images of AgNPS@GO samples prepared under different conditions (order of each image was denoted corresponding to Table 2.4)
Table 3.1: Box-Behnken design of three variables corresponding with a particle size as the response
GO/AgNO 3 ratio (mg/mg)
GO/AgNO 3 ratio (mg/mg)
1 Table 3.2: Analysis of variance (ANOVA) and descriptive statistics for the
Table 3.3: Optimization of the particle size of silver nanoparticles-graphene oxide
The particle sizes calculated according to the SEM images were also compared to the approach proposed by Barbir et al [131], as indicated in Table 3.4 Meanwhile, the UV-Vis spectra of the materials prepared from different conditions are also demonstrated in Figure 3.2, which was utilized for the determination of the wavelength position and the absorbance of each sample According to the results, although notable deviation between the two calculation approaches can be conspicuously observed, it is noteworthy that the variation tendency of the particle size with the change in precursor content between these methods is similar, affirming the good reliability of the obtained experimental data Besides, the calculation from Mie theory (Equation (1.13)) solely depends on the variation in the wavelength at the maximum absorbance, which can also witness a considerable deviation due to the resolution limit of the spectrophotometer Furthermore, the deviation between the calculation approaches can be also due to the difference in ideal and actual surrounding conditions, in which the presence of a considerable amount of GO can also change the electrical conductivity and dielectric constant of the medium [133]
Figure 3.2: UV-Vis spectra of AgNPS@GO prepared under different conditions
(order of each image was denoted corresponding to Table 3.1)
Figure 3.3: Linear correlation between actual and predicted responses
Additionally, the simultaneous effects of different factors on the variation in particle size of the AgNPs were also evaluated according to the response surfaces Specifically, Figure 3.4a demonstrates the concurrent effect of the concentrations of AgNO3 and extract on the average particle size of the AgNPs According to the results, the increase in the extracted content leads to a gradual reduction in particle size This tendency can be explained by the increase in the extracted content providing a large amount of stabilizer, which can obstruct the self-agglomeration between the AgNPs clusters and ultimately reduce the average particle size These results also affirm the abundance of stabilizing contents present in the M indica extract, which contribute to the reduction in the size of AgNPs It has been also reported that the formation of AgNPs significantly
50 relies on the compositions of functional groups present in the extract, in which each component plays a distinct role in the reduction or stabilization of the AgNPs Particularly, the hydroxyl groups in several plant extracts have been reported to possess strong reduction activity from Ag(I) to Ag(0), while several carbonyl or amino acids can be responsible for the stabilization of the plasmonic particles via electrostatic interaction between these compounds and the AgNPs [134] On the other hand, Figure 3.4a also revealed that the particle size of the AgNPs increased with the dose of AgNO3 This tendency has been also explained in previous research Particularly, the increase in AgNO3 content can stimulate the frequency of collision between the AgNPs, eventually, facilitating the agglomeration between particle clusters [135]
Figure 3.4: Simultaneous effects of (a) concentrations of AgNO3 and extract, (b) extract and GO/AgNO3 ratio, and (c) concentration of AgNO3 and GO/AgNO3 ratio on particle size
Characteristics of AgNPs@GO
Besides, XPS was also utilized to investigate the elemental composition and valence state of the fabricated AgNPs@GO nanocomposite, as indicated in Figure 3.5 According to the results, the presence of S, C, Ag, N, and O can be conspicuously observed, confirming the successful embedment of the AgNPs onto GO sheets To be more specific, Figure 3.5b demonstrates the XPS C1s core level spectrum for the AgNPs@GO nanocomposite The result can be deconvoluted into several components, including the signals at the binding energy of 283, 285, and 287 eV, corresponding to the presence of characteristic functional groups of GO, including graphitic carbon (C=C), alcoholic (C−OH), and carbonyl groups (C=O), respectively [136], [137] Besides, strong signals 531 and 535 eV in the O1s spectrum (Figure 5c) also revealed the presence of C=O and C−OH functional groups, respectively [138] On the other hand, the high-resolution Ag profile indicated in Figure 3.5d showed two strong peaks at 345 and 373 eV, which can be ascribed to the Ag 3d5/2 and Ag 3d3/2 states in the structure of the prepared material The predominant presence of these two signals also revealed the presence of only the metallic state of Ag, corroborating the successful formation of AgNPs as well as the embedment of these particles onto GO nanosheets It is also noteworthy that the XPS spectra also showed an insignificant peak intensity of S2p and N1s, as demonstrated in Figure 3.5e, f These signals can be attributed to the presence of other phyto-components containing N and S in the extract [139]
Figure 3.5 XPS spectra of the AgNPs@GO; (a) survey, (b) C1s, (c) O1s, (d) Ag3d,
Besides, the crystalline structure of the resulting material was also evaluated via XRD patterns, as indicated in Figure 3.6a Specifically, the result for GO showed two strong diffraction peaks at 10 and 23 o , which can be ascribed to the plane (001) of the GO structure Meanwhile, a small peak at 24 o can be attributed to the restacking of the reduced form of the GO after the drying process of the GO suspension for further analysis stages Regarding the AgNPs@GO sample, several signals at 38.2, 44.2, 64.3, and 77.4 o are attributed to the presence of (111), (200), (220), and (311) lattice planes, respectively [140] These results were also in agreement with the face-centered cubic structure of silver (JCPDS No 89.3722), affirming the successful embedment of the AgNPs onto the GO sheets Meanwhile, the intensity of the diffraction peak at 10 o in the figure for the AgNP@GO also witnessed a significant reduction compared to that of the GO This tendency can be explained by the exfoliation of the GO sheets and the predominant peaks of the AgNPs that overwhelmed the characteristic peaks of the GO Another reason for the reduction in the GO peak may be also related to the partial reduction of the GO due to the presence of phytochemicals acting as reducing agents during the reaction, leading to the disappearance of some oxygen-containing functional groups in the GO structure Besides, the obtained XRD patterns also showed no additional signals, especially for the Ag2O, which confirmed the high purity and crystallinity of the as-prepared material In previous studies, the typical signals of Ag2O are reported to be located at the diffraction peaks of approximately 28, 32, 54, and 67 o , which can be ascribed to the appearance of the crystalline plan (110), (111), (220), and (222), respectively [141] Nonetheless, the XRD pattern for the prepared AgNPs@GO showed no noticeable signals for the aforementioned peaks, confirming the negligible presence of Ag2O in the as-prepared material Moreover, it has been reported that the use of the [Ag(NH3)2]OH complex as a silver source instead of bare AgNO3 also contributes to the generation of AgNPs with negligible formation of Ag2O [142]
Figure 3.6: (a) XRD patterns and (b) Raman spectra of the AgNPs@GO
Additionally, Raman spectroscopy was utilized to compare the graphitic structures of the GO and AgNPs@GO, as indicated in Figure 3.6b Specifically, the result for both samples showed two strong peaks at 1340 and 1605 cm -1 ascribed to the D and G bands, which represent the structural defects and sp 2 hybridization of the graphitic carbons, respectively [143] The intensity of these two characteristic peaks increased significantly after the embedment of AgNPs This phenomenon is attributable to the surface-enhanced Raman scattering (SERS) effect, which involves the electromagnetic enhancement resulting from the excitation of localized surface plasmons, along with the chemical enhancement due to the presence of charge-transfer complexes between the AgNPs and GO via physicochemical interactions [144] Furthermore, the degree of defect of the prepared sample can be also evaluated via the calculation of the ID/IG According to the result, ID/IG for the GO was 1.01, compared to 1.16 for that of the AgNPs@GO This can be ascribed to the attachment of the AgNPs onto electron-rich sites of the GO sheets, which reduced the amount of the sp 2 hybridized regions as well as enhanced the disorder effects.
H 2 O 2 colorimetric sensing performance of the AgNPs@GO
On the other hand, the change in particle size also significantly influences the colorimetric detection performance of the material, as indicated in Table 3.4 Particularly, the increase in particle size is likely to increase the LOD value, indicating the attenuation in the H2O2 sensing activity This tendency can be explained that the reduction in particle size means the specific surface area increases, which provides more active sites for the electrostatic interaction between the AgNPs and H2O2 Nonetheless,
54 it should be also noted that other factors regarding the concentration of the AgNPs can also play a crucial role in the detection activity of the nanocomposite, which can be confirmed by the reduction in LOD in some trials when the concentration of AgNO3 increases Furthermore, the appropriate amount of the GO also greatly affects the performance of detection, which can be ascribed to the increasing adsorption of the H2O2 thanks to the π – π stacking, hydrogen bonding, and electrostatic interaction of the GO that can also facilitate the interaction between AgNPs and H2O2 [145] According to previous studies, the mechanism for the H2O2 colorimetric detection of the AgNPs@GO has been also proposed To be more specific, the interaction between the AgNPs and
H2O2 can be efficiently promoted thanks to the presence of GO, which not only provide an ultra-high surface area for the adsorption of H2O2 as well as the contact between these mo)lecules and the AgNPs This interaction leads to a decrease in the concentration as well as the absorption intensity of the AgNPs in the solution Subsequently, the electron transfer from the reduced sites of the AgNPs to the H2O2 molecules leads to the formation of •OH, whereas the electrons can be also transported from the H2O2 molecules to the oxidized regions of the AgNPs to generate HO2• [132] As a result, the two aforementioned free radicals can also interact with each other to form O2 and H2O The reaction between the AgNPs and H2O2 can be also expressed via Equation (3.2):
Table 3.4: Comparison between particle size calculated from SEM images and UV-
Vis spectra and the effects of particle size on H2O2 colorimetric detection performance
Run Particle size according to SEM images (nm)
Particle size according to UV-Vis spectra (nm)
Run Particle size according to SEM images (nm)
Particle size according to UV-Vis spectra (nm)
Additionally, the H2O2 colorimetric detection performance of the AgNPs@GO prepared under the optimal conditions was also compared with the green-synthesized AgNPs-containing nanomaterials prepared from previous studies, as demonstrated in Table 3.5 Accordingly, the as-prepared nanocomposite showed a competitive detection performance when compared to other materials This can be ascribed to the even distribution of small-size AgNPs onto the GO nanosheets, which facilitates the strong interaction between the AgNPs and H2O2 molecules This result confirmed the good potential of the as-prepared materials in eco-friendly pollutant detection applications
Table 3.5: Comparison of H2O2 colorimetric detection performance between different nanomaterials
1 AgNPs@GO from Mangifera indica extract 1.652 This study
2 AgNPs@GO from Curcuma longa extract 0.48 [126]
3 AgNPs-rGO-carbon nanotube (MWCNT) 0.900 [146]
4 AgNPs from sugarcane leaves extract 30000 [147]
5 AgNPs from neem kernel extract 1.000 [148]
CONCLUSION AND SUGGESTION
Conclusion
In this study, the AgNPs@GO nanocomposite was successfully prepared using M indica leaves extract as a reducing agent and stabilizer The simultaneous effects of the extract concentration, the concentration of AgNO3, and GO/AgNO3 ratio were evaluated According to the analytical results, all investigated factors exerted significant and simultaneous influences on the resulting particle size Furthermore, the resulting particle sizes calculated using SEM images also showed a similar tendency compared to the value determined from Mie theory, indicating the reliability of the experimental data In addition, according to the Box-Behnken model, the optimal conditions for the fabrication of the AgNPs@GO were determined to be 1.476 mg/mL of extract, 0.111 mg/mL of AgNO3, and the GO/AgNO3 ratio of 0.307 with an estimated particle size of 22.002 nm
Besides, the characterization analysis approaches including SEM, XRD, XPS, and Raman also showed the successful reduction of the silver precursor to form AgNPs Particularly, while the SEM images confirmed the formation of quasi-spherical AgNPs that were uniformly distributed onto the GO nanosheets, the XRD also indicated the formation of crystalline structure of the material via the presence of several characteristic diffraction peaks of the AgNPs and GO Besides, the XPS spectra also corroborate sucessful embedment of the AgNPs onto GO sheets of thanks to the participation of several functional groups derived from the M indica extract and GO Besides, the Raman result also revealed the structural modification as well as enhancing disorder effect of the GO sheets after embeding the AgNPs
Additionally, the H2O2 detection activity of the prepared material also showed the optimal LOD value of 56.1817 μg/L, showing a competitive sensing performance when compared to the material prepared from previous studies Furthermore, the H2O2 detection performance was also reported to considerably depend on the variation in particle size, as well as the amount of the AgNPs and GO in the material structure, which
57 necessitates further assessment for the expansion of the applications of the material into larger scales.
Suggestion
The AgNPs@GO nanocomposite fabricated from eco-friendly reducing agents and stabilizers such as M indica leaf extract has shown an impressive performance in the applications of colorimetric detection of H2O2 in water Nonetheless, in order to extend the promising application of the as-synthesized materials on a larger scale, additional investigation should be carried out to optimize the colorimetric detection activity under several conditions, including temperature, pH, material dosage, etc Besides, it is also necessary to conduct experimental design with appropriate variable ranges using multiple models and more synthesis parameters to optimize the synthesis efficiency as well as the morphology of the materials Furthermore, beyond the H2O2 detection in an aqueous environment, the as-prepared AgNPs@GO can be also tested in other media, including metallic-ion-containing solutions or natural water sources, which is also crucial to thoroughly evaluate the stability and selectivity under harsh conditions Finally, further structural modification, including the embedment of metallic oxides, nitrogen, sulfur, or organic molecules can be also promising approaches to enhance the physicochemical properties of the materials, which can be applied in multifunctional applications, including colorimetric detection, catalysis, or antibacterial activity
During the time implementing this thesis, the research has been also published in the Journal of Diamond and Related Materials with the title “Phytosynthesis of silver nanoparticles@graphene oxide using Mangifera indica leaves extract at room temperature: The simultaneous effects of synthesis conditions on controlled particle size and colorimetric sensing of H2O2”, a paper on Colloids and Surfaces B: Biointerfaces with the title of “Phytosynthesis of silver nanoparticles using Mangifera indica leaves extract at room temperature: Formation mechanism, catalytic reduction, colorimetric sensing, and antimicrobial activity”, and another paper on Materials Today Sustainability with the title: “A review on the chemical and biological synthesis of silver nanoparticles@ graphene oxide nanocomposites: a comparison”
1 N.D Hai, N.M Dat, N.T.H Nam, H An, C.Q Cong, Q.T.T Huong, and N.H Hieu, “Phytosynthesis of silver nanoparticles@graphene oxide using Mangifera indica leaves extract at room temperature: The simultaneous effects of synthesis conditions on controlled particle size and colorimetric sensing of H2O2,” Diam Relat Mater, vol 138, p 110264, Oct 2023, doi: 10.1016/j.diamond.2023.110264, IF 4.1, Q2
2 N.D Hai, N.M Dat, N.T.H Nam, H An, L.T Tai, L.M Huong, C.Q Cong, N.T.H Giang, N.T Tinh, and N.H Hieu, “A review on the chemical and biological synthesis of silver nanoparticles@graphene oxide nanocomposites: A comparison,” Materials Today Sustainability, vol 24, p 100544, Dec 2023, doi: 10.1016/j.mtsust.2023.100544, IF 7.8, Q1
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