Arabian Journal of Chemistry (2018) 11, 1134–1143 King Saud University Arabian Journal of Chemistry www.ksu.edu.sa www.sciencedirect.com ORIGINAL ARTICLE A label-free colorimetric sensor based on silver nanoparticles directed to hydrogen peroxide and glucose Nghia Duc Nguyen a, Tuan Van Nguyen a, Anh Duc Chu a, Hoang Vinh Tran a,*, Luyen Thi Tran a, Chinh Dang Huynh a a Department of Inorganic Chemistry, School of Chemical Engineering, Hanoi University of Science and Technology (HUST), 1st Dai Co Viet Road, Hanoi, Viet Nam Received November 2017; accepted 31 December 2017 Available online January 2018 KEYWORDS Graphene quantum dots; Silver nanoparticles; Hydrogen peroxide (H2O2), Glucose detection; Human urine; Colorimetric sensor Abstract A simple method has been developed for preparation of silver nanoparticles (AgNPs) based on the use of graphene quantum dots (GQDs) as a reducing agent and a stabilizer The synthesized nanocomposites consisting of silver nanoparticles and graphene quantum dots (AgNPs/ GQDs) has been characterized by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Ultraviolet–visible spectroscopy (UV–Vis), Fourier-Transform Infrared spectroscopy (FT-IR), Energy Dispersive X-ray spectroscopy (EDX) and Dynamic Light Scattering (DLS) Results indicate that monodisperse of AgNPs has been obtained with particles size ca $ 40 nm and specific plasmon peak of silver nanoparticles at 425 nm by UV–Vis spectrum Using AgNPs/ GQDs nanocomposite, we have constructed a colorimetric sensor for hydrogen peroxide (H2O2) and glucose sensors based on the use of AgNPs/GQDs as both probes: capture probe and signal probe The fabricated sensors perform good sensitivity and selectivity with a low detection limit of 162 nM and 30 lM for H2O2 and glucose sensing, respectively Moreover, the biosensors have been successfully applied to detect glucose concentrations in human urine Ó 2018 The Authors Production and hosting by Elsevier B.V on behalf of King Saud University This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Introduction * Corresponding author E-mail address: hoang.tranvinh@hust.edu.vn (H.V Tran) Peer review under responsibility of King Saud University Production and hosting by Elsevier Diabetes, as well known, is a serious health problem, which has been declared as a global epidemic by World Health Organization (WHO) owing to its unprecedented growth worldwide (Jia et al., 2015; Vashist, 2012) The glucose level in blood is used as a clinical indicator of diabetes (Su et al., 2012; Baghayeri et al., 2016; Lu et al., 2015; Ensafi et al., 2016; Gao et al., 2017) However, drawing blood from vein or fingertip causes discomfort and pricking sensation Compared with https://doi.org/10.1016/j.arabjc.2017.12.035 1878-5352 Ó 2018 The Authors Production and hosting by Elsevier B.V on behalf of King Saud University This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) A label-free colorimetric sensor based on silver nanoparticles blood, urine is another informative body fluid as well and more importantly, it can be obtained noninvasively The glucose level in urine is also a good indicator for preliminary screening of patients with high level diabetes or having renal glycosuria (Jia et al., 2015; Radhakumary and Sreenivasan, 2011) In order to avoid the inconveniences caused by drawing blood intravenously or by hand pricking, a preliminary screening of the patients with high level diabetes can be done instantly by checking their urine glucose levels When the concentration of glucose in urine is more than 500–1000 mg/L (2.8–5.6 mM), the urine test is positive (Su et al., 2012; Radhakumary and Sreenivasan, 2011; Fine, 1965; Lankelma et al., 2012; Urakami et al., 2008; Zhang et al., 2017) Considering its convenience, painlessness, and affordability, urine glucose monitoring should not be completely given up, especially in low-income regions (Su et al., 2012) The sensing of glucose is usually based on electrical signal or color change generated by the specific reaction of active species (e.g., glucose oxidase or phenylboronic acid) with glucose (Jia et al., 2015) Most glucose sensors have been structured based on natural enzymes (e.g., horseradish peroxidase (HRP)) Natural enzymes in organisms are proteins composing of hundreds of amino acids that can catalyze chemical reactions It has been widely applied in various fields because of their high substrate specificity and catalytic efficiency However, their catalytic activity can be easily affected by environmental conditions such as acidity, temperature and inhibitors Furthermore, high costs of preparation, purification and storage also restrict their widespread applications (Ding et al., 2016; Tran et al., 2018; Zhang et al., 2014; Xing et al., 2014) In the light of this, exploiting stable enzyme mimetics is an urgent need Nowadays, many nanomaterials with unique peroxidase-like activity have been discovered, including magnetic nanoparticles and its composite (Ding et al., 2016; Wei and Wang, 2008; Dong et al., 2012), cerium oxide nanoparticles (Zhao et al., 2015), silver nanoparticles (Tran et al., 2018), carbon-based nanomaterials (Wang et al., 2015; Nirala et al., 2015; Wang et al., 2016); exfoliated Co– Al layered double hydroxides (Co-Al ELDHs) (Chen et al., 2013) and manganese selenide nanoparticles (MnSe NPs) (Qiao et al., 2014) These nanostructured materials as peroxidase mimetics show unparalleled advantages of low cost and stability over natural enzymes (Ding et al., 2016) Among them, carbon-based nanomaterials, such as graphene/ graphene oxide; carbon nanotubes and graphene quantum dots are the most widely studied enzyme mimics (Shu and Tang, 2017) In this work, we synthesized nanocomposites consisting of silver nanoparticles and graphene quantum dots (AgNPs/ GQDs) by a simple and green method Using AgNPs/GQDs, we constructed a directed colorimetric method for the direct detection of hydrogen peroxide (H2O2) A colorimetric glucose sensor has been designed and developed based on combining with glucose oxidase (GOx) The fabricated sensors perform excellent sensitivity and selectivity for hydrogen peroxide and glucose sensing Moreover, the proposed sensor has been successfully applied to detect of glucose concentrations in human urine samples Based on the good performances, the proposed colorimetric glucose sensor becomes a great promising candidate for glucose level sensing as a without blood needing and needle-free approach 1135 Experimental 2.1 Chemical Citric acid (C6H8O7ÁH2O); urea ((NH2)2CO); ammonia (NH3) solution 28%wt.; acetic acid (CH3COOH) solution 99%wt.; sodium hydroxide (NaOH); silver nitrate (AgNO3); glucose; ascorbic acid; galactose; fructose; lactose; scructose; hydrogen peroxide solution 30% (H2O2); phosphate buffered saline tablets (PBS); and glucose oxidase (GOx) were purchased from Sigma Aldrich Human urine samples were collected from a local hospital 2.2 Synthesis of graphene quantum dots (GQDs) 3.44 g citric acid and 3.005 g urea were dissolved into 100 mL distilled (D.I) water The solution was transferred to an autoclave and heated at 160 °C for h Then, the mixture was centrifuged at 5000 rpm for 20 to remove the big carbon particles The supernatant containing graphene quantum dots (GQDs) was collected 2.3 Synthesis of silver nanoparticles (AgNPs) using GQDs as reducing reagent and stabilizer 100 mL of GQDs stock solution was diluted by mL of D.I water Then 0.1 M NaOH and M CH3COOH solutions were used to control pH of GQDs solutions from to 11 After that, 20 lL of 0.1 M AgNO3 solution was added into the GQDs solutions The mixtures were heated to 90 °C for h to complete reduction of silver cation (Ag+) to silver nanoparticles (AgNPs) process to form nanocomposites consisting of silver nanoparticle and graphene quantum dots (AgNPs/GQDs) as the results AgNPs/GQDs solutions then were cooled to room temperature (RT) and stored at °C for use 2.4 Characterization UV–Vis spectra were measured using Agilent 8453 UV–Vis spectrophotometer system with the wavelength in a range of 200–1200 nm Morphology and crystal structure of nanoparticles were characterized using Transmission Electron Microscopy (TEM: JEM1010 - JEOL) Particles size distribution was analysed by Dynamic Light Scattering (DLS) on the Nano Partica SZ-100 (HORIBA Scientific, Japan) XRD pattern of AgNPs/GQDs was measured using D8 ADVANCE - Bruker Chemical composition of samples was determined by JEOL Scanning Electron Microscope/Energy Dispersive X-ray (SEM/EDS) JSM-7600F Spectrometer 2.5 Direct detection of hydrogen peroxide 200 lL of H2O2 solutions with different concentrations was added into a 1.5 mL eppendorf Then, 1000 lL of AgNPs/ GQDs solution was added into the eppendorf and the mixture was stirred by vortex machine The mixture was then incubated at 40 °C in a water bath for 30 minutes Then the UV–Vis spectra of the solutions were recorded The optical densities at 425 nm (OD425) of the AgNPs/GQDs solution before and after addition of various H2O2 quantities were used 1136 N.D Nguyen et al to draw a calibration curve, i.e DA/A0 vs [H2O2] the following equation: DA A0 À Ac ð%Þ ¼ Á 100% A0 A0 ð1Þ Here, A0 and AC are OD425 of the AgNPs/GQDs solution before and after H2O2 addition, respectively 2.6 Detection of glucose 100 lL of glucose solutions with the different concentrations (from 0.5 mM to mM) in PBS buffer (pH = 7) were added into eppendorfs, then after, 100 lL of GOx (2 mg mLÀ1 in 0.001 M PBS solution) solution was added The solution was mixed and incubated in a 37 °C water bath for 30 Then, 1000 lL of the AgNPs/GQDs solution was added to the above eppendorfs Finally, the mixed solutions were incubated in a 40 °C water bath for 30 and then they were transferred to cuvettes for UV–Vis absorbance measurement and the optical density at wavelength of 425 nm was recorded The optical densities at 425 nm (OD425) of the AgNPs/GQDs solution before and after addition of various glucose quantities and GOx were used to draw a calibration curve, i.e DA/A0 (Eq (1)) vs Cglucose (here, DA = A0 À AC where A0 and AC are OD425 of the AgNPs/GQDs solution before and after adding the mixture of glucose and GOx, respectively) Results and discussions 3.1 Characterization of AgNPs/GQDs hybrid The simple method has been developed for the preparation of nanocomposites consisting of silver nanoparticles and graphene quantum dots (AgNPs/GQDs) First, the small sized graphene quantum dots (GQDs) with abundant oxygen containing functional groups have been synthesized by the hydrothermal method Then GQDs adsorbed Ag+ ions and reduced them into silver nanoparticles (AgNPs) without adding any reducing reagents, while the oxygen containing functional groups were partially removed from the GQDs Thus, GQDs were coated on the surfaces of the resultant AgNPs, leading to the formation of AgNPs/GQDs The residual oxygen-containing groups on the GQDs made the obtained AgNPs/GQDs be excellent dispersive and long-term stable in water (Tetsuka et al., 2012) The UV–Vis spectra of GQDs and AgNPs/GQDs solutions have been shown in Fig 1A As can be seen in Fig 1A, curve b, the adsorption band at 425 nm is attributed to the characteristic surface plasmon absorption of AgNPs, while this absorption is not observed in the case of the control sample (solution containing only GQDs, without AgNO3), where no AgNPs are formed (Fig 1A, curve a) A shoulder at 357 nm in Fig 1A (curve b) can be attributed to the presence of GQDs in AgNPs/GQDs solution when comparing to UV–Vis spectra of GQDs (Fig 1A, curve a) Moreover, the synthesized AgNPs/GQDs have been characterized by DLS (Fig 1B), XRD (Fig 1C) and TEM (Fig 1D) DLS data (Fig 1B) have indicated that AgNPs/GQDs have particles size from 20 nm to 100 nm with mean size at 40 nm Besides that, TEM analysis (Fig 1D) shows that AgNPs/QGDs are spherical particles with particles size around 40 nm These data are in a highly agreement with the DLS results The XRD pattern of the AgNPs/GQDs (Fig 1C) shows three main characteristic peaks at 2h = 37.5°, 43.1° and 64.8° which match very well with those of the standard AgNPs (PCPDF card number 40,783) (Mamatha et al., 2017) with Miller indices (1 1), (2 0) and (2 0) Normally, X-ray diffraction of GQDs presents a weak broad peak (0 2) centered at 2h $ 22.7° which indicates the disordered stacking structures of graphene layers; however, this (0 2) peak is strongly depend on the degree of oxidation of GQDs because the attached hydroxyl, epoxy/ether, carbonyl and carboxylic acid groups can increase the interlayer spacing of GQDs (Tetsuka et al., 2012) In Fig 1C, no specific XRD peak of GQDs can be seen, possibly this peak is too weak and overlapped by the background signal The EDS spectra of GQDs (Fig 1E, curve a) showed the peaks of C, O and N, which were three major constituents of GQDs EDX spectra of AgNPs/GQDs hybrid (Fig 1E, curve b) presented new appearing peaks, corresponding to Ag The EDS spectra provided an evidence for silver metal forming by GQDs: strong peak values at 2.99 and 3.17 keV were due to forming of AgNPs These results confirmed that AgNPs were efficiently formed onto surface of GQDs Fig 1F shows FTIR spectra of GQDs (curve a) and AgNPs/GQDs hybrid (curve b) Fig 1F (curve a) shows that the bands at 3100–3500 cmÀ1 belong to t(OAH) and t(NAH), which is important to facilitate the hydrophilicity and stability of the GQDs in aqueous state The absorption bands at 1641 cmÀ1 is attributed to t(C ‚ O), demonstrating that carboxylic acid may be used as Ag+ binding site These peaks indicate that GQDs have abundance of amino (ANH2), carboxyl (ACOOH) and hydroxy (AOH) groups on their surface and edges responsible for the excellent hydrophilicity of GQDs Interestingly, compared with the GQDs, the absorption bands of the OAH group at 1064 cmÀ1 almost disappear in the FT-IR spectrum of the AgNPs/GQDs hybrid (Fig 1F, curve b) These results indicate that Ag+ can be reduced to form AgNPs by OAH groups on the GQD/AgNP hybrid’s surface, resulting in OAH groups being converted into –COOH groups after the reaction 3.2 Hydrogen peroxide detection 3.2.1 Spectrometric assay for hydrogen peroxide detection and effect of pH The colorimetric H2O2 sensor was constructed basing on the reaction of AgNPs with H2O2, which leaded the change of the color of AgNPs/GQDs solutions from yellowish to colorless, depending on H2O2 concentration As can be seen in Fig 2, the presence of AgNPs/GQDs in the solution results in a strong absorption band at 425 nm (Fig 2a to Fig g, curve (i)), corresponding to the yellowish color The optical density of the AgNPs/GQDs solution at 425 nm (OD425) decreases after addition 200 mL of H2O2 50 mM (Fig 2a to Fig g, curve (ii)), corresponding to the color changing of the solution from yellowish to colorless This result is explained by the oxidation of AgNPs in the presence of H2O2 The standard potential of Ag+/Ag is lower than that of H2O2/H2O (E0Agỵ =Ag = 0.8 V < E0H2 O2 =H2 O = 1.77 V) in water at pH = The following reaction will occur (Eq (2)): ðGQDsÞAg0 þ H2 O2 ! ðGQDsÞAgþ þ 2HỒ ð2Þ A label-free colorimetric sensor based on silver nanoparticles 1137 0.8 10.0 (A) (B) 8.0 420 nm (a) (b) Frequency (%) Absorbance (A.U) 0.6 0.4 343 nm 357 nm 0.2 (b) 400 4.0 2.0 (a) 0.0 6.0 0.0 600 800 10 100 1000 Particle size (nm) Wavelength (nm) 120 (C) Intensity (A.U) (D) (111) 100 80 60 40 (200) (220) 20 20 30 40 50 60 70 2θ/ degree C (F) N cps (a.u) O (a) C Ag Ag NO Transmittance (%) (E) AgNPs/GQDs (b) (b) 4000 GQDs (a) Energy (keV) 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm ) Fig (A) UV–Vis spectra of (a) GQDs and (b) AgNPs/GQDs (Inset: color of the corresponding samples); (B) Particles size distribution of AgNPs/GQDs by DLS method; (C) XRD pattern of AgNPs/GQDs; (D) TEM image of AgNPs/GQDs; (E) EDX of (a) GQDs and (b) AgNPs/GQDs; (F) FT-IR of (a) GQDs and (b) AgNPs/GQDs Therefore, AgNPs in AgNPs/GQDs hybrid will be etched from Ag0 to Ag+ So the concentration of Ag0 will decrease, leading to the fading of the AgNPs/GQDs solution after add- ing H2O2 The above behaviour provides a potential for quantitative detection of H2O2 by measuring the decrease in the AgNPs surface plasmon resonance at 425 nm 1138 N.D Nguyen et al 1.0 (a) pH = 11 1.0 (b) pH = 0.8 (i) Absorbance (A.U) Absorbance (A.U) 0.8 ΔA 0.6 (ii) 0.4 0.2 0.0 (i) ΔA 0.6 (ii) 0.4 0.2 0.0 300 400 500 600 300 400 Wavelength (nm) 1.0 ΔA 0.6 (ii) 0.4 0.2 (i) 0.6 ΔA 0.4 0.2 (ii) 0.0 0.0 300 400 500 300 600 400 500 1.0 (f) pH = (e) pH = 0.8 (i) (i) Absorbance (A.U) Absorbance (A.U) 0.8 0.6 ΔA 0.4 (ii) 0.2 0.6 ΔA 0.4 (ii) 0.2 0.0 0.0 300 400 500 300 600 400 (g) 80 Δ A/A0 (%) Absorbance (A.U) pH = (i) 0.6 ΔA 0.4 (ii) 0.2 60 40 20 0.0 300 600 (h) 100 0.8 500 Wavelength (nm) Wavelength (nm) 1.0 600 Wavelength (nm) Wavelength (nm) 1.0 600 pH = 0.8 (i) Absorbance (A.U) Absorbance (A.U) 0.8 (d) 1.0 pH= 7.5 (c) 500 Wavelength (nm) 400 500 Wavelength (nm) 600 10 11 12 pH Fig UV–vis spectrum of AgNPs/GQDs solutions: (i) before and (ii) after addition of 50 lM hydrogen peroxide at room temperature and reaction time was 15 at different pH: (a) pH = 11, (b) pH = 9, (c) pH = 7.5, (d) pH = 7, (e) pH = 5, (f) pH = 4, (g) pH = 3; (h) summarization of effect of pH on response signal of hydrogen peroxide sensors based on AgNPs/GQDs A label-free colorimetric sensor based on silver nanoparticles 1139 UV–vis spectra of AgNPs/GQDs solutions at different pH values without (curve i) and with (curve ii) 50 mM H2O2 are shown in Fig 2a–g, which corresponding with pH from 11 to It can be seen, when H2O2 was adding, the adsorption at 425 nm (OD425) was decreased The decreasing of OD425 was strongly depended on pH of solution Fig h presents the summarizing of the effect of pH on response signal of H2O2 sensors based on AgNPs/GQDs solution, which was given on the graph DA/A0 vs pH (here, DA = A0 À AC where A0 and AC are OD425 of the AgNPs/GQDs solution before and after H2O2 addition, respectively) As can be seen in Fig h, the value of DA/A0 is higher in acid environment than that in base environment This result is explained by the disintegration of H2O2 in base environment according to the following equation (Eq (3)): (ca 96.61%) at pH = 7, so that following experiments will be performed in neutral environment 2H2 O2 ! 2H2 O ỵ O2 3ị Therefore, in base environment, the decreasing of the concentration of Ag0 according to Eq (1) is lower than that in acid environment The maximum value of DA/A0 was obtained 0.8 (A) [H2 O2 ] concentration (mM ) 0.7 Absorbance (A.U) 0.6 0.5 0.4 0.3 -3 0.5x10 -3 1x10 -3 5x10 -3 10x10 -3 20x10 -3 30x10 -3 40x10 -3 50x10 -3 100x10 0.2 3.3 Glucose detection 3.3.1 Sensitivity of the sensor When glucose and GOx are added into the solution containing AgNPs/GQDs, the following reaction will occur: ðgluconic acidÞ 0.0 300 400 500 600 Wavelength (nm) (B) m M 40 M 0.0 01 mM 0.000 mM 0.01 m 00 60 0.02 m M 0.0 3m M 80 Δ A/A0 (%) The UV–Vis spectra of samples containing different H2O2 concentrations are shown in Fig 3A When the concentration of H2O2 increases from 0.5 lM to 100 lM, the optical density of the AgNPs/GQDs solution at 425 nm (OD425) decreases from 0.6 (a.u) to 0.07 (a.u) It can be seen that, a shoulder at 357 nm appears more clearly which can be attributed to specific plasmon peak of free GQDs in solution This phenomenon can be explained following: when H2O2 is added, H2O2 will react with AgNPs (Eq (2)), therefore, GQDs from AgNPs/ GQDs will be released to free GQDs in solution Using OD425 as the recorded signal, the calibration curve of hydrogen peroxide detection was generated under optimum conditions has been shown in Fig 3B by DA/A0 vsÁH2O2 concentration (Eq (1)) In the calibration, the linear relationship of DA/A0 vsÁH2O2 concentration is in range from 0.5 lM to 50 lM with the regression equation DA/A0 = (1734 ± 72.58) CH2O2 (mM) + (2.74412 ± 1.79846) with R2 = 0.98615 Based on the calibration curve and the blank samples, the limit of H2O2 detection (LOD) of the sensor is estimated of to be 162 nM) Moreover, it is able to monitor the color changing of the AgNPs/GQDs solution by naked eyes in the case of immediate and qualitative H2O2 detection (Fig 3B, insert) Glucose þ O2 þ H2 O ! D-glucono-1; 5-lactone þ H2 O2 0.1 100 3.2.2 Sensitivity of the sensor 04 m M m 0.05 M 0.1 mM mM 20 0.00 0.02 0.04 0.06 0.08 0.10 [H2O2]/ mM Fig (A) UV–Vis spectra of hydrogen peroxide sensor with various H2O2 concentrations; (B) Calibration curve for H2O2 detection (inset: color of sensor with corresponding samples in (A)) Experimental conditions were described in the text ð4Þ After that, H2O2 is measured by using the fabricated colorimetric sensor based on AgNPs/GQDs Fig 4A shows the UV– Vis spectra of samples containing different glucose concentrations When the concentration of glucose increases from 0.5 mM to mM, the optical densities of the AgNPs/GQDs solution at 425 nm (OD425) are decreased from 0.61 (a.u) to 0.31 (a.u) The calibration curve of glucose detection is shown in Fig 4B by DA/A0 vs glucose concentration as mentioned above In the calibration (Fig 4B), the linear relationship of DA/A0 vs glucose concentration is in range from 0.5 mM to mM with the regression equation DA/A0 = (7.06087 ± 0.40925) Cglucose (mM) + (3.05473 ± 1.49321) with R2 = 0.97695 The limit of detection (LOD) was estimated to be 30 lM based on three times the standard deviation of the blank tests, which is comparable to those of the previously reported methods (Table 1) The linear range of the sensor is from 0.5 mM to mM and the LOD value (30 lM) is lower than the value of the concentration of glucose in a urine sample which is positive for diabetes (2.8–5.6 mM) Thus, the developed colorimetric glucose sensor has great potential for application to a daily glucose test In addition, the above results have indicated that the synthesized AgNPs/GQDs nanostructured material not only has catalytic efficiency as peroxidase mimetics but also shows unparalleled advantages of low cost and stability over natural enzymes N.D Nguyen et al 0.4 0.2 0.0 300 400 M m 8.0 Glucose concentration (mM) Absorbance (A.U) M mM m 3.5 m m M 3.0 mM m 2.0 mM (A) 1.0 0.6 M 1140 500 M 0.0 0.5 1.0 2.0 3.0 3.5 4.0 8.0 600 Wavelength (nm) 60 (B) Δ A/A0(%) 50 3.3.2 Selectivity of the sensor 40 30 20 10 0 Glucose concentration/ mM Fig (A) UV–Vis spectra of glucose sensor with various glucose concentrations (inset: color of sensor with various glucose concentrations); (B) Calibration curve for glucose detection Experimental conditions were described in the text Table Moreover, as can be seen in Fig 4A, when the concentration of glucose increases from 0.5 mM to mM, the shoulder at 357 nm which can be attributed to specific plasmon peak of free GQDs in solution appears more clearly This result is in good agreement with the result obtained in the case of H2O2 sensors (Fig 3A) This result is an important clue for suggestion of a glucose detection mechanism following two steps (Fig 5): In the first step, glucose is converted to D-glucono-1,5-lactone (which is named as gluconic acid) and H2O2 following Eq (4) Then, AgNPs/ GQDs are etched by H2O2 (Eq (2)) in the second step Therefore, GQDs from AgNPs/GQDs nanocomposite will be released to free GQDs in solution, leading to the appearance more clearly of the shoulder at 357 nm The above result is a new point in the comparison with previous work (Chen et al., 2014; Xia et al., 2013) In this work, the phenomenon of etching AgNPs and releasing GQDs can be seen by experimental results It is thanks to the excellent dispersive and the long-term stable in water of the synthesized AgNPs/GQDs nanocomposite The selectivity of the glucose sensor was tested by conducting the control experiments in the presence of glucose, galactose, lactose, sucrose and fructose at concentration of mM It can be seen in Fig 6A, a small decreasing of OD425 was found when galactose, lactose, sucrose and fructose were added When glucose was added, a strong decreasing of OD425 was obtained The DA/A0 values of sensor when using various saccharides at concentration of mM were summarized in Fig 6B It can be found that the DA/A0 values were 14.59% for presence of galactose; 11.27% for lactose, 17.34% for sucrose, 16.29% for fructose These values are lower than that of the solution containing glucose (DA/A0 = 54.76%) at least 3.16 times at the same concentration These results have indicated the excellent selectivity for glucose of the developed sensor Comparison with some reports based on label-free colorimetric methods for the detection of glucose Signal probes Enzyme immobilization Line range (mM) Limit of detection (lM) Actual samples Reference AuNPs coupled AgNPs AuNPs No GOx 50 Á 10À3–70 Á 10À3 0.056–0.5 27.7 Human serum Human urine AgNPs/GQDs AgNPs CexOy nanoparticles MnO2 nanoparticles AgNPs/GQDs DNA-embedded Au@Ag nanoparticles Au@Ag core–shell nanoparticles P(DMA-co-PBMA) copolymer and AuNPs GOx GOx GOx GOx GOx GOx 0.17 0.2 500 0.17 30 0.01 N/R Human serum Human serum Human serum Human urine Fetal bovine serum GOx 0.5 Á 10À3–0.4 Á 10À4–0.1 0.5–100 0.5 Á 10À3–50 Á 10À3 0.5–8 0.01 Á 10À3–0.2 Á 10À3; Á 10À3–100 Á 10À3 0.5 Á 10À3–0.4 Gao et al (2017) Radhakumary and Sreenivasan (2011) Chen et al (2014) Xia et al (2013) Ornatska et al (2011) Huang et al (2017) This work (Kang et al., 2015) 0.24 GOx N/R 50 Human urine or human serum N/R N/R-not reported (Zhang et al., 2016) (Li et al., 2011) A label-free colorimetric sensor based on silver nanoparticles 1141 STEP Glucose + O2 + H2O D-Glucono-1,5-Lactone + H2O2 GOx ( Gluconic Acid) STEP (a) (b) H 2O H2 O2 without glucose H 2O (a) H2O2 Reaction H2O2 H2O2 UV -V is spectra with glucose (b) H 2O GQDs AgNPs Ag+ Glucose Oxidase-GOx Fig Illustration of detection mechanism of proposed label free and reagentless colorimetric sensor for hydrogen peroxide and glucose using AgNPs/GQDs as capture probe and signal probe 3.3.3 Application of the sensor for detection glucose in human urine sample (A) (B) Human urine sample was firstly diluted because human urine may contain many soluble salts and residues In our previous work on human urine samples (Tran et al., 2017), we have found that high diluted ratio gives better signal than low diluted ratio However, because of the limits of the LOD, the optimized dilution ratio was 1:4 The standard addition method was used to analyse glucose concentration in the human urine sample using the proposed sensor Fig 7A shows UV–Vis spectra the glucose sensor in presence of the diluted urine sample and the three spiked urine samples by adding glucose with concentration from mM to mM The calibration curve for determination of glucose concentration using the standard addition method with the human urine sample is described in Fig 7B From these data, glucose concentration in the human urine sample has been determined of 3.68 mM Therefore, it is able to conclude that the above human urine sample is of a diabetic patient This experimental result shows the great potential for application of the developed colorimetric glucose sensor based on a low-cost, blood-free and needle-free approach to daily glucose tests Conclusions Fig (A) UV–Vis spectra of the glucose sensor in presence of different saccharides at concentration of mM; (B) Corresponding of DA/A0 of (A) (inset: color of sensor with various saccharides) In this work, silver nanoparticles decorated graphene quantum dots carbon (AgNPs/GQDs) hybrids have been synthesized and characterized by DLS, XRD, FT-IR, EDX and TEM methods; and the results indicate that mono-dispersed AgNPs have been obtained with particles size ca $40 nm Using AgNPs/GQDs as capture probe and signal probe, a spectroscopy method has been developed for determination of hydrogen peroxide with a low detection limit of 162 nM of 1142 N.D Nguyen et al 0.6 Absorbance (A.U) 0.5 0.4 References (A) (i) (2i) (3i) (4i) (5i) (i) AgNPs/GQDs (2i) = (i) + diluted urine (3i) = (2i) + mM glucose (4i) = (2i) + mM glucose (5i) = (3i) + mM glucose 0.3 0.2 0.1 0.0 300 400 500 600 Wavelength (nm) 35 (B) Δ A/A0 (%) 30 25 20 15 10 0.92mM -1 [Additional glucose] (mM) Fig Method of standard addition for glucose detection in urine: (A): UV–Vis spectra of (i) control sample (without diluted urine addition); (2i) sensor in from (i) after addition of the diluted human urine sample; (3i)-(5i) spiked urine sample with various glucose concentration from 1, and mM, respectively (B) Calibration curve of standard addition for urine glucose detection Experimental conditions were described in the text H2O2 Combining with the use of glucose oxidase (GOx), a simple colorimetric method for selective and sensitive detection of glucose has also been fabricated The above sensors perform excellent sensitivity and selectivity with a low detection limit of 30 lM of glucose concentration In addition, the level of glucose in the real human urine sample can also be measured accurately by using the AgNPs/GQDs-based colorimetric sensor following the addition standard method Therefore, the proposed colorimetric glucose sensor has great potential for application to a daily glucose test based on a low-cost; blood-free and needle-free approach 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Actuators, B 218, 42–50 Ornatska, M., Sharpe, E., Andreescu, D., Andreescu, S., 2011 Paper bioassay based on ceria nanoparticles as colorimetric probes Anal Chem 83, 4273–4280 A label- free colorimetric. .. (h) summarization of effect of pH on response signal of hydrogen peroxide sensors based on AgNPs/GQDs A label- free colorimetric sensor based on silver nanoparticles 1139 UV–vis spectra of AgNPs/GQDs