LITERATURE REVIEWS
Overview of glucose, blood sugar, and diabetes mellitus
Glucose, with the molecular formula C6H12O6 and IUPAC name (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxylhexanal, is the most prevalent monosaccharide It exists in two ring forms, α-D-Glucose and β-D-Glucose, which serve as the primary energy source for cells in the body The structural chemical formulas of D-glucose are illustrated in Figure 1.1.
Figure 1.1: Structural chemical formulas of glucose (D-glucose) [25]
Blood glucose levels are primarily derived from carbohydrate metabolism in food, but can also be influenced by hormones like glucagon, adrenaline, cortisol, and growth hormone Insulin plays a crucial role in lowering blood glucose concentrations, whereas the other hormones work to increase them.
Growth hormone (GH) plays a crucial role in regulating glucose levels, as it decreases glucose concentration in the bloodstream In a healthy individual, glucose is filtered through the kidneys and fully reabsorbed by the renal tubules, preventing its presence in urine However, this reabsorption capacity is only effective when blood glucose levels are below 180 mg/dL (10 mM) Consequently, monitoring blood glucose is essential for identifying impaired glucose metabolism, particularly in diabetes management.
Diabetes is a leading cause of mortality worldwide, resulting from a disruption in the metabolism of carbohydrates, fats, and proteins due to insufficient insulin production by the pancreas This condition is characterized by consistently elevated blood sugar levels and is a significant contributor to severe health complications, including heart disease, blindness, and kidney failure.
Currently, the most common way to diagnose diabetes is by measuring blood glucose levels Specifically, fasting, postprandial blood glucose levels, and tolerance to this substance will be analyzed:
To diagnose diabetes, a fasting blood glucose level is measured, with a result exceeding 126 mg/dL indicating diabetes A result between 110 and 116 mg/dL signifies prediabetes, highlighting an increased risk for developing type 2 diabetes and its associated complications.
Measure glucose level after eating: If the glucose concentration is higher than 200 mg/dL with symptoms (excessive thirst, polyuria, and fatigue), diabetes is suspected
The oral glucose tolerance test (OGTT) is a diagnostic tool used by doctors to identify early cases of diabetes that may not be detected through standard blood tests During the test, a patient drinks a glucose solution, and their blood sugar levels are measured two hours later If the results indicate a blood glucose level exceeding 200 mg/dL, it confirms a diagnosis of type 2 diabetes.
Glucose sensors
Figure 1.2: Schematic representation of a biosensor [32]
Biosensors play a vital role in biomedical diagnostics, disease monitoring, and drug detection across various sectors These devices produce signals that correspond to the concentration of specific analytes during biological or chemical reactions In electrochemical biosensors, the output signal is generated through the specific binding or catalytic reactions involving biomaterials such as enzymes, antibodies, and DNA.
In recent years, electrochemical biosensors have seen significant advancements and applications across diverse fields, including drug discovery, food quality monitoring, and the detection of heavy metals in water systems.
1.2.2 Introduction of electrochemical glucose sensor
Electrochemical glucose sensors are capable of detecting a wide range of glucose levels through the monitoring of electrochemical signals, which are translated into corresponding glucose concentrations Personal glucometers primarily utilize amperometric detection based on enzyme reactions for continuous glucose monitoring Significant research efforts have focused on enhancing electrode structures, surface functionalization, and electrochemical analysis techniques to achieve more sensitive and selective measurements As a result, various glucose monitoring systems, including both noninvasive and direct blood glucose monitoring methods, have been developed, facilitating patient-friendly diabetes management.
Glucose sensors can be categorized into two main types: enzymatic and non-enzymatic, with the former further divided into three generations Enzymatic glucose biosensors, which utilize specific enzyme immobilization, offer excellent selectivity and sensitivity but face challenges such as pH and temperature instability, limiting their practical applications In contrast, non-enzymatic glucose sensors have gained popularity due to their high sensitivity, long-term stability, and cost-effectiveness These sensors operate by directly oxidizing or reducing glucose on the electrode surface, employing various materials including noble metals like platinum and gold, as well as transition metals such as copper, cobalt, and nickel, along with their oxides and hydroxides.
First generation of glucose sensor
The concept of the biosensor for measuring glucose was first proposed in
In 1962, Clark and Lyons from the Children’s Hospital of Cincinnati developed a glucose biosensor that featured an oxygen electrode, an inner semipermeable membrane, a thin layer of glucose oxidase (GOx), and an outer dialysis membrane This innovative design allowed for the immobilization of enzymes at an electrochemical detector, creating an enzyme electrode The biosensor's functionality was based on the principle that a decrease in measured oxygen concentration directly correlated with glucose concentration, as the reaction oxidized glucose to produce hydrogen peroxide (H2O2).
In the reaction outlined in Equation 1.2, hydrogen peroxide (H2O2) is consumed, resulting in the exclusive production of oxygen (O2), which is detected by the oxygen electrode The generated current signal is directly proportional to the quantity of H2O2 decomposed, thereby correlating with the glucose concentration in the solution.
Figure 1.3: Schematic drawing of the first-generation glucose sensor [47]
This type of glucose sensor offers rapid analysis with high selectivity and accuracy due to the specific enzyme glucose oxidase (GOx) However, its reliance on oxygen as an oxidizing agent introduces potential errors from low oxygen levels and raises concerns about the electrode's durability Additionally, the use of expensive oxygen electrodes limits its widespread adoption Furthermore, the inherent instability of enzymes, which can be easily influenced by environmental factors, compromises the sensor's long-term reliability.
Second generation of glucose sensor
Figure 1.4: Schematic drawing of the second-generation glucose sensor [47]
This sensor utilizes glucose oxidase (GOx) to convert glucose into gluconic acid, producing reduced GOx in the process The reduced form of GOx is then oxidized by a mediator compound, such as Ferrocene or Ferricyanide, which generates an electrical current Consequently, this current allows for the determination of glucose concentration.
Glucose + GOx(ox) → Gluconic acid + GOx(red) E.q 3.3 GOx(red) + 2M(ox) → GOx(ox) + 2M(red) + 2H + E.q 4.4
The development of second-generation glucose sensors has been significantly enhanced by the absence of oxygen, which prevents side reactions and improves their functionality compared to first-generation models These sensors are not only cost-effective but also demonstrate reliable performance However, they still depend on enzymes, leaving challenges related to measurement accuracy and durability unresolved.
Third generation of glucose sensor
Third-generation glucose biosensors are innovative, reagentless devices that enable direct electron transfer between the enzyme and the electrode, eliminating the need for toxic mediators Utilizing organic conducting materials based on charge-transfer complexes, these biosensors have facilitated the development of implantable, needle-type devices for continuous blood glucose monitoring Despite their advanced technology, third-generation sensors remain relatively rare, representing only 5% of the total enzyme electrochemical sensor market.
Figure 1.5: Schematic representation of a third-generation biosensor [47]
Four generation of glucose sensor
Non-enzymatic electrodes present a promising advancement in glucose sensing technology, potentially leading to the fourth generation of glucose oxidation analysis Unlike traditional methods that rely on sensitive enzymes, these electrodes directly oxidize glucose in the sample, simplifying the detection process and enhancing reliability.
The efficiency of enzymatic glucose electrooxidation is significantly influenced by the choice of electrode material, as glucose oxidation is a slow process that typically does not generate a noticeable faradaic current on most commercial electrodes, necessitating electrocatalytic methods This challenge is evident in scan rate-dependent voltammetry, which often fails to capture direct glucose oxidation, suggesting a non-diffusion-driven mechanism To address this, various nanostructures have been developed for non-enzymatic glucose electrochemical sensors that facilitate glucose oxidation or reduction directly on the electrode surface Among these materials, Ni(OH)₂ stands out as a promising candidate due to its low toxicity, high sensitivity to glucose, and affordability The remarkable electrochemical performance of Ni(OH)₂ in glucose detection is attributed to the Ni(OH)₂/NiOOH redox couple in alkaline solutions.
Nickel(II) hydroxide nanostructures
Nickel(II) hydroxide is a vital compound in physics and chemistry, widely utilized in engineering applications such as batteries and electrochemical sensors It plays a crucial role in the surface layers of nickel metal and nickel-based alloys, forming through electrochemical processes or corrosion Research on nickel hydroxides in the late 20th century primarily focused on these applications In the late 1960s, Bode et al introduced a straightforward method to explain the electrochemical oxidation of nickel hydroxides to nickel(III) oxyhydroxide, followed by reduction back to nickel(II) hydroxide Their system involves two phases of nickel hydroxide (α- and β-Ni(OH)₂) and two phases of the oxidized form (β- and γ-NiOOH), as illustrated in figure 1.6 Although the complete process is more complex, the original model, with minor adjustments like the γ-NiOOH to β-Ni(OH)₂ transition, effectively describes the reactions occurring at nickel hydroxide battery electrodes.
Figure 1.6: A general scheme of the chemical and electrochemical processes that occur at a nickel hydroxide battery electrode
Today, a variety of materials are utilized in practical applications such as photocatalysis, electrocatalysis, supercapacitors, electrochromic devices, and electrochemical sensors Notably, nickel hydroxide-based electrochemical sensors have gained considerable attention, particularly in the development of non-enzymatic glucose sensors.
1.3.1 Electrochemical behaviours of Ni(OH) 2 toward glucose in alkaline
Ni(OH)2-based nanomaterials demonstrate exceptional electrocatalytic properties for electroactive molecules such as ethanol, amino acids, and glucose The electro-oxidation of glucose at Ni(OH)2 electrodes occurs in an alkaline medium, where glucose is oxidized by NiOOH species, converting them back to Ni(OH)2 This mechanism has spurred interest in developing non-enzymatic glucose sensors using Ni(OH)2 For example, Linlin Chen et al created a non-enzymatic glucose sensor utilizing Ni(OH)2 nanoplatelets on a glassy carbon electrode, achieving a sensitivity of 1342.2 µA mM−1 cm−2 They detailed the oxidation-reduction mechanism between Ni(OH)2 and glucose in alkaline conditions Additionally, Tong Yang et al reported on Ni(OH)2 nanocages synthesized through a precipitation method, which exhibited a sensitivity of 1106.9 µA mM−1 cm−2 towards glucose, further showcasing the potential of Ni(OH)2-based sensors.
Figure 1.7: Mechanism of oxidation-reduction electrochemical reaction between
Ni(OH) 2 and glucose in alkaline medium
Figure 1.8: A non-enzymatic glucose sensor based on Ni(OH) 2 nanoplatelet based on GCE and ECF [55]
1.3.2 Structure and characteristics of Ni(OH) 2 nanostructures
Figure 1.9: The crystal structure of β-Ni(OH) 2 [57]
The two known pseudopolymorphs of Ni(OH)2, α- and β-Ni(OH)2, were first identified by Bode et al The β-phase, which is isostructural with the mineral brucite (Mg(OH)2), occurs naturally as the mineral theophrastite and exhibits trigonal symmetry, leading to a non-orthogonal relationship between its a- and b-axes, with an angle γ of 120° The unit cell parameters for these materials, determined by X-ray diffraction (XRD) and neutron diffraction, are listed in Table 1.1, while Figure 1.11 provides an example XRD pattern, and Table 1.2 summarizes the relevant XRD parameters.
The α-Ni(OH)₂·xH₂O polymorph of nickel hydroxide features layers of β-Ni(OH)₂ aligned parallel to the crystallographic ab-plane, intercalated with water molecules The hydration level of this material ranges from 0.41 to 0.7, indicating its intrinsic hydration properties.
The formula for α-Ni(OH)₂ typically omits 11 water molecules, leading to a potentially misleading representation in figure 1.10 Unlike fixed positions, these intercalated water molecules can rotate and translate within the ab-plane If they were tightly packed with hydroxide ions, the c-parameter would be around 7.6 Å, whereas it is actually greater than or equal to 7.8 Å Water serves as an "amorphous glue" that binds the Ni(OH)₂ layers, resulting in minimal orientation between adjacent layers This random orientation is referred to as a "turbostratic" structure.
Figure 1.10: The idealized crystal structure of α-Ni(OH) 2 ã xH 2 O [57]
Figure 1.11: X-ray diffraction patterns of Ni(OH) 2 films on Ni substrates collected using a Cu Kα X-ray source [57]
Table 1.1: Unit cell parameters for the two fundamental phases of Ni(OH) 2 β-Ni(OH) 2 β-Ni(OD) 2 α-Ni(OH) 2 ãxH 2 O
Space group /P3ml/No.164 /P3ml/No.162 α=β 90° 90°
Extended X-ray absorption fine structure (EXAFS) measurements show that there is a 0.05 Å contraction in the Ni–Ni distance, d Ni−Ni , in α-Ni(OH) 2 relative to β-Ni(OH)2 Therefore, the unit cell parameters for the idealized stoichiometry (3 Ni : 2 H 2 O) shown in figure 1.10 are a = b = 5.335 Å and c = 8.0 Å Note that the a-parameter depends on the way in which the unit cell is defined, which in turn, is determined by the degree of hydration d Ni−Ni , however, may be compared directly with the a-parameter of β-Ni(OH) 2 (table 1.1) Furthermore, the c-parameter is greater if the interlayer space contains anionic impurities A representative XRD pattern is shown in figure 1.11 and the XRD parameters are listed in table 1.3
Table 1.2: X-ray diffraction parameters of β-Ni(OH)2 Diffraction angles are listed for Cu Kα (λ = 1.542 Å) and Co Kα (λ = 1.789 Å) X-ray sources
Miller indices (hkl) d(Å) 2 CuKα (°) 2 CoKα (°)
Table 1.3: X-ray diffraction parameters of α-Ni(OH)2 calculated using the unit cell shown in figure 1.8 Diffraction angles are listed for Cu Kα (λ = 1.542 Å) and Co Kα (λ = 1.789 Å) X-ray sources
The vibrational properties of chemical compounds serve as unique "fingerprints" for identification and analytical characterization Key techniques for measuring vibrational spectra include IR spectroscopy, conventional Raman spectroscopy, and surface-enhanced Raman spectroscopy (SERS) These optical methods are user-friendly and widely accessible, although they typically focus on measuring optical vibration modes at zero-wave vector in solids This article will discuss the Raman-active vibrational modes of β-Ni(OH)2 and α-Ni(OH)2.
Raman-active vibrational modes of β-Ni(OH) 2 [57]:
The Raman spectrum of β-Ni(OH)₂, depicted in figure 1.12a, reveals four predicted Raman-active transitions based on factor group theory Three of these modes are confirmed at 310–318, 445–453, and 3581 cm⁻¹, while the position of the fourth mode has only recently been clarified Notably, the Raman-active lattice modes of brucite [Mg(OH)₂], which is isomorphic with β-Ni(OH)₂, occur at similar frequencies: 280, 443, 725, and 3652 cm⁻¹ A comprehensive theoretical analysis of brucite's vibrational modes has elucidated the atomic displacements for each mode, leading to the assignment of symmetries for the observed Raman features By drawing parallels with brucite, a Raman peak around 880 cm⁻¹ has been attributed to the final lattice mode of β-Ni(OH)₂, confirming the symmetries of all four Raman-active transitions.
Figure 1.12 Raman spectra of (a) β-Ni(OH) 2 , (b) α-Ni(OH) 2 and (c) nitrate- intercalated α-Ni(OH) 2 [57]
In addition to the four main Raman peaks, several additional modes arise from combinations, overtones, and stacking fault disorder A Raman band observed at 508–519 cm −1 shows variable intensity across samples, typically being more pronounced in less crystalline materials While it has been linked to a lattice mode, its frequency is significantly lower than expected for the E g mode, and its intensity fluctuates independently of other lattice modes Consequently, this band is attributed to the harmonic overtone of an acoustic vibrational mode around 250–270 cm −1, as evidenced by INS measurements.
A very weak Raman band is observed at 601 cm −1 which is not a predicted lattice mode This peak is taken to be the harmonic overtone of the E g transition at 306–318 cm −1
At intermediate frequencies, there is a very weak feature at approximately
The spectrum displays a prominent peak at 1600 cm−1, accompanied by a weaker peak around 1630 cm−1, which is attributed to the O−H bending of adsorbed or trapped H2O on the material's surface While the 1600 cm−1 peak resembles that of free water, its position suggests it represents the O−H bend associated with a minor degree of hydration in the structure, referred to as a libration mode.
A small amount of water in β-Ni(OH)₂ can enhance the electrochemical activity of battery electrodes This water is weakly associated with nickel cations and does not create hydrogen bonds with the lattice hydroxide, contributing to the performance of the material.
In Raman spectra, a prominent peak at 3601 cm −1 is often observed, which has been attributed to various factors including adsorbed water, crystal defects, impurities, surface hydroxyl groups, and the O−H stretch of the β-Ni(OH)2 crystal phase This peak is also linked to selective XRD line broadening, specifically affecting diffraction peaks related to the crystallographic c-axis It's important to note that this selective line broadening is not indicative of crystallite size, thus the Scherrer equation is not applicable in this context.
A notable Raman peak appears at 3688 cm −1, which is associated with the intensity of the peak at 3601 cm −1 However, it is important to note that the intensity of the 3688 cm −1 peak can also fluctuate independently.
Raman-active vibrational modes of α-Ni(OH) 2 [57]:
The Raman spectra of α-Ni(OH)2 and nitrate-intercalated α-Ni(OH)2 reveal distinct lattice modes at 460 cm −1 and 495 cm −1, with weak second-order transition features appearing around 790 cm −1 and 1075 cm −1 While some studies have noted additional Raman-active lattice modes, these were not detected in recent analyses and are likely due to impurities from β-Ni(OH)2.
The structural disorder causes the internal O−H bending mode of lattice
EXPERIMENTS AND METHODS
Chemical and apparatus
All the chemical reagents used in the experiment were analytical grade and have a purity of 99.9%
Nickel(II) nitrate hexahydrate (Ni[NO3]2·6H2O) and D(+)-glucose were sourced from Merck KgaA in Darmstadt, Germany, while hexamethylenetetramine (HMTA, C6H12N4) was obtained from Tianjin Dengfeng Chemical Co., Ltd Other analytical-grade chemicals, including sodium hydroxide (NaOH), sodium chloride, l-ascorbic acid (AA), dopamine (DA), and citric acid monohydrate (CA), were used without purification Additionally, acetone from Xilong, China, and deionized water (DI) were employed in the experiments.
This thesis explores the use of nickel foam (NF) as a substrate for the direct growth of Ni(OH)2 nanostructures, specifically designed for battery cathode applications Nickel-based electrodes are favored for their affordability, ease of modification, and straightforward disassembly As depicted in Figure 2.1, the study utilizes a commercial nickel foam roll for battery cathode substrates, along with a nickel foam piece measuring 25×5×1 mm³.
NF electrodes are emerging as promising candidates for use in electrochemical sensors, particularly in glucose sensing applications Unlike traditional electrodes, which retain by-products on their surface after electrochemical measurements, NF electrodes allow for easy disposal of used portions This feature enhances the sensitivity and reliability of the sensors, making NF electrodes a valuable innovation in the field of electrochemical systems.
Figure 2.1: Images of the commercial nickel foam
To fabricate materials, we use some basic apparatus in the laboratory, including pipettes, analytical balance, ultrasonic vibrato, and a drying oven.
Ni(OH) 2 nanostructures fabrication
A nickel foam substrate measuring 25×5×1 mm³ was sonicated in acetone for 10 minutes to eliminate organic contaminants Following purification, the nickel foam substrate was immersed in a mixed solution containing 0.1 M Ni(NO₃)₂·6H₂O.
M HMTA was dissolved in deionized (DI) water and heated at 90 °C for varying durations of 10, 30, and 50 minutes Following the heating process, the substrate was collected, thoroughly washed with DI water, and then dried in a vacuum chamber Figure 2.2 depicts the experimental procedure used for material fabrication.
Figure 2.2: Experiment procedure for fabrication of materials.
Characterization of the morphologies and composition of the synthesized
The Scanning Electron Microscope (SEM), pioneered by Professor Dr Charles Oatlev in the 1950s, is one of three main types of electron microscopes Unlike optical microscopes that utilize light for imaging, SEMs employ electrons, which have significantly smaller wavelengths than light This allows SEMs to achieve a resolution that is 1000 times greater than that of traditional light microscopes.
Scanning Electron Microscopy (SEM) generates images by scanning a sample's surface with a focused beam of high-energy electrons The resolution of SEM images is influenced by the characteristics of the electron probe as well as the interactions between the electrons and the specimen.
The acceleration rate of incident electrons in scanning electron microscopy (SEM) significantly influences the sample analysis, as these electrons carry substantial kinetic energy When the electron beam interacts with the specimen, it generates secondary electrons with energies below 50 eV, where the emission efficiency is highly sensitive to the surface geometry, chemical characteristics, and bulk composition SEM's high-resolution capabilities make it ideal for examining nanomaterials, where nanoscale structural features are crucial for determining properties and functionalities The interaction of electrons with the sample atoms produces various signals that provide insights into the sample's texture, chemical composition, crystalline arrangement, and orientation Furthermore, SEM is effective in identifying surface fractures, inspecting surface contaminants, revealing spatial variations in chemical compositions, and characterizing crystalline structures.
Scanning Electron Microscopy (SEM) is a powerful technique used to scan selected areas of a sample surface, capturing data from regions as wide as 1 cm down to 5 μm With magnification capabilities between 20 and 30,000 and a spatial resolution of 50–100 nm, SEM enables detailed imaging Additionally, SEM can analyze specific point locations on the sample, making it invaluable for qualitatively and semiquantitatively assessing chemical compositions through Energy Dispersive Spectroscopy (EDS), as well as examining crystalline arrangements and orientations using Electron Backscatter Diffraction (EBSD).
The principle of Energy-dispersive X-ray Spectroscopy (EDS)
The working principle of Energy Dispersive Spectroscopy (EDS) involves an electron beam interacting with the specimen surface, causing the electrons in the atoms to become excited As these excited electrons return to their ground state, they emit energy that can be measured and analyzed.
The Energy Dispersive Spectroscopy (EDS) technique utilizes characteristic X-rays emitted from a specimen to analyze its elemental composition, detecting elements ranging from carbon to uranium in concentrations as low as 1.0 wt% When combined with Scanning Electron Microscopy (SEM), EDS allows for precise area analysis based on the specimen's magnification A basic EDS system comprises three main components: an X-ray detector, a pulse processor that measures voltage pulses corresponding to X-ray energies, and a computer The X-ray detector captures the emitted X-rays, generating a minute current that is converted into a voltage pulse proportional to the X-ray's energy, which the computer then measures over time.
The histogram displays a range of X-ray energies measured during the analysis, allowing for the identification of elements within the specimen Since the X-rays produced by electron interactions do not cause any loss of material, the same samples can be analyzed multiple times This characteristic makes Scanning Electron Microscopy (SEM) a non-destructive analytical technique.
Field Emission Scanning Electron Microscopy
Figure 2.4: Field-emission scanning electron microscopy (FESEM) with energy- dispersive X-ray spectroscopy (Hitachi S-4800)
Field emission scanning electron microscopy (FESEM) provides high-resolution topographical and elemental analysis at magnifications ranging from 10 X to 300,000 X, featuring virtually unlimited depth of field Unlike conventional SEM, FESEM produces clearer images with minimal electrostatic distortion, achieving a spatial resolution as fine as 1 to 0.5 nm—offering an enhancement of three to six times.
FESEM offers significant advantages by effectively analyzing small area contamination spots at electron accelerating voltages compatible with Energy Dispersive Spectroscopy (EDS) This technique utilizes low-kinetic-energy electrons, allowing for reduced penetration and closer examination of the material's surface Additionally, FESEM produces high-quality, low-voltage images with minimal electrical charging of samples, enhancing the accuracy of surface analysis.
25 ranging from 0.5 to 30 kV) can be obtained using FESEM One of the striking features of FESEM is that insulating materials need not be coated with conducting materials [60]
In this thesis, the morphologies and composition of the synthesized materials were characterized by field-emission scanning electron microscopy (FESEM) with energy-dispersive X-ray spectroscopy (Hitachi S-4800)
Transmission electron microscopy (TEM) is the leading technique for characterizing nanomaterials, offering chemical information and images at atomic-scale resolution The method involves an electron beam passing through a thin specimen, resulting in elastically or inelastically scattered electrons TEM provides superior spatial resolution compared to scanning electron microscopy (SEM), allowing for detailed analysis of particle size, aggregation, and dispersion of nanomaterials It delivers precise information on morphology, composition, and crystallography through bright field and dark field imaging TEM techniques encompass imaging, spectroscopy, and diffraction, with their classification and principles illustrated in figures 2.5 and 2.6.
Figure 2.6: Working principle of TEM [62]
Transmission Electron Microscopy (TEM) operates on the principles of optical microscopy, utilizing photons instead of electrons and electromagnetic lenses in place of glass lenses, with images displayed on a screen rather than through an eyepiece Key benefits of TEM include its exceptional magnification capabilities and the ability to provide detailed information about the structures of compounds and elements However, TEM demands specialized maintenance and produces costly black-and-white images as its output.
TEM operates on the principles of optical microscopy, utilizing photons instead of electrons and electromagnetic lenses in place of glass lenses Images are displayed on a screen rather than through an eyepiece The key benefits of TEM are its exceptional magnification capabilities and the ability to deliver detailed information.
27 compound and element structures TEM requires special maintenance and expensive, black-and-white images are considered as output
High Resolution Transmission Electron Microscopy (HRTEM)
High-resolution transmission electron microscopy (HRTEM) utilizes a large objective aperture, allowing for phase-contrast imaging where direct transmission interferes with diffracted beams The image contrast is influenced by the relative phases of these beams, necessitating precise adjustments in imaging conditions such as lens defocusing, astigmatism, and beam alignment When specimen thickness is less than 10, phase-contrast images can be derived from the projected crystal potential Advanced HRTEM techniques enable the resolution of individual atomic columns or single atoms in crystalline materials, requiring high electron doses of approximately 500–2000 electrons per square Angstrom Consequently, specimens suitable for high-resolution imaging are typically more resistant to electron irradiation effects.
In the case of polymers and certain organic materials, it is very difficult to obtain such images carried out under intense imaging conditions
For over 40 years, High-Resolution Transmission Electron Microscopy (HRTEM) has been instrumental in the morphological analysis of inorganic nanomaterials It focuses on examining the microstructure of crystalline defects, interfaces, and grain boundaries, while also investigating the nanocrystalline characteristics of amorphous films HRTEM is essential for characterizing nanoparticles, their arrangements, nanofibers, fiber diameters, and alignments, particularly in the context of magnetic thin films and multilayers The high-resolution images produced by HRTEM offer more detailed insights into nanoscale materials compared to other microscopic techniques.
The evolution of high-resolution episcopic microscopy (HREM) has been driven by advancements in mechanical, thermal, and electrical strengths over the years Notably, the resolution of electron microscopes surpassed that of optical microscopes in the mid-1930s, and by the early 1970s, the first structural images of large-unit-cell block oxides were captured Significant breakthroughs in resolution were achieved in the early 1980s, surpassing the 2.0 Å barrier.
At this level of resolution, it becomes frank to infer structural models directly from the observation of electron micrographs recorded at the optimum defocus
Selected Area Electron Diffraction Pattern
Electrochemical measurements
2.4.1 Characterization of electrochemical properties of the synthesized materials
Cyclic voltammetry (CV) is used to determine the diffusion coefficient D and to consider the reversible variation (ability to charge of research material)
Cyclic Voltammetry (CV) is an electrochemical technique used to assess the current produced in an electrochemical cell when subjected to voltages that surpass the Nernst equation limits This method involves applying a cyclic potential to a working electrode while measuring the resulting current densities.
This method involves applying a study electrode to scan potential in the anode or cathode direction, allowing for the observation of the corresponding current The polarization curve, a cyclic line, illustrates the relationship between current density (I) and potential (E) The characteristics of the polarization curve are influenced by the selection of solvent, base electrolyte, and electrode materials.
The cyclic polarization curve, representing the relationship of I-E, is a curve with maximum current I p corresponding to potential E p
With the reversible process Ox + ne- Red, Randles-Sevcik proposed a relationship between current density and scan rates: i p = 2.687 10 5 n 3/2 A D 0 1/2 C 0 v 1/2 E.q 2.1 where, n - electrons participate in the reaction; v - scan rates (V/s);
A - area of the electrode (cm 2 )
Potential of anote peak E pa and cathode peak E pc in the reversible process are described by the equation:
In irreversible process Ox + ne- → R, maximum current equation followed by Nicholson – Shain: i p = 2.99 10 5 n ( n a ) 1/2 A C 0 D 0 1/2 v 1/2 E.q 2.3
Where n, A, C 0 , D 0 , v - the same meaning as above
Cyclic voltammetric measurements were performed at room temperature using a NaOH solution, with a potential range of −1.0 to 1.5 V relative to an Ag/AgCl reference electrode, at a scan rate of 10 mV/s on an Autolab PGSTAT302N electrochemical workstation The experimental setup employed a three-electrode configuration, consisting of an Ag/AgCl reference electrode, a platinum wire counter electrode, and the synthesized Ni(OH)2 nanohives/nickel foam as the working electrode.
Electrochemical behaviors of the synthesized electrodes toward glucose were measured by both cyclic voltammetry and chronoamperometry
In cyclic voltammetry measurements for glucose detection, a three-electrode system was utilized, immersed in a solution containing 0.1 M NaOH and glucose concentrations ranging from 0 to 6 mM The applied potential was systematically varied to assess the electrochemical behavior of glucose in this setup.
−1.0 to 1.5 V versus Ag/AgCl electrode at the scan rate of 10 mV/s
For chronoamperometry measurement, the principle is that the potential applied to a working electrode is constant, and the change of current density is recorded by time [64]
The voltage applied to a working electrode is regulated by implementing a potential step that transitions from a state with no Faraday current to a state where the concentration of the electrochemical substance on the electrode's surface reaches zero This principle is illustrated in the accompanying figure.
Figure 2.9: a) Potential step, b) the decrease of concentration of electrochemical substance, c) relationship between current and time [63]
When a potential is applied to the working electrode (WE), its surface undergoes changes due to the formation of double layer and capacitive currents, while also triggering an electrochemical reaction that generates Faraday current The total measured current is the sum of these capacitive and Faraday currents Initially, at potential E1, no electrochemical reaction occurs However, upon applying potential E2, the electrochemical reaction begins, creating a concentration gradient between the electrode surface and the surrounding solution, which drives a liquid flow towards the electrode This flow of substances, along with the resulting current, is directly proportional to the concentration gradient at the surface, leading to observable changes in concentration and current over time.
This study conducted chronoamperometry measurements for glucose detection at a constant stirring rate of 200 rpm and room temperature, utilizing three applied potentials within the lower glucose concentration range of 0-0.5 mM.
Figure 2.10: Autolab electrochemical workstation (PGSTAT302N, Netherlands)
Figure 2.11: Scheme for electrochemical measurement diagram.
RESULTS AND DISCUSSTION
Morphologies and structural characteristics of the synthesized materials33
3.1.1 FESEM images of the synthesized materials
Figure 3.1: FESEM images of (a) the bare NF and (b-d) the Ni(OH) 2 /NF electrodes
Figure 3.1 illustrates the FESEM images of the bare NF substrate and the synthesized Ni(OH)2 nanohives at various synthesis durations, specifically Ni(OH)2 – 10m, Ni(OH)2 – 30m, and Ni(OH)2 – 50m The NF substrate appears dense and lacks nanoporous structures, while the surface transforms into a porous configuration filled with nanohives after Ni(OH)2 growth Increased synthesis duration leads to a higher density of cavities per unit area, resulting in a homogeneous morphology Although some spherical nanohives are present, likely due to the substrate's uneven surface, the Ni(OH)2 cavity arrays are randomly stacked, creating unique structures that enhance the performance of electrochemical sensors.
The FESEM images reveal that the cavities have a diameter of approximately 300–500 nm, while the nanowalls forming the nanohives measure around 15-20 nm in thickness These ultra-thin nanowalls enhance the surface area available for biochemical adsorption, making them highly beneficial for chemical and biosensor applications.
The hydrothermal method typically yields Ni(OH)2 nanostructures with an average thickness of approximately 20 nm, necessitating high temperatures and extended growth periods However, the in situ growth approach utilized in this study eliminates the need for additional binders, conductors, lengthy hydrothermal reactions, or costly equipment.
Figure 3.2: Higher magnification of FESEM images of the Ni(OH)2/NF electrodes with different reaction time
3.1.2 HRTEM images of the synthesized materials
The morphology and local nanostructure of the synthesized Ni(OH)2 were analyzed using TEM, cross-sectional HRTEM, and SAED images The TEM image reveals porous structures and a homogeneous connection of narrow areas within the polycrystalline material Additionally, the presence of evenly spaced fringes, approximately 0.25 nm apart, aligns closely with the d(100) value in the hexagonal α-Ni(OH)2 phase The SAED pattern confirms the polycrystalline nature of the synthesized Ni(OH)2 Furthermore, the small area of local homogeneous space contributes to the challenges in detecting peaks in the XRD pattern of the produced materials.
Figure 3.3: (a) TEM images, (b) HRTEM image and the inset is SAED pattern of the synthesized Ni(OH) 2
3.1.3 Components of the synthesized materials
Figure 3.4a displays the EDX elemental composition spectrum of the fabricated materials, revealing the presence of nickel (Ni), oxygen (O), and carbon (C) in the sample The detected carbon is attributed to contamination from hydrocarbon compounds in the laboratory environment and/or surface adsorption.
The Ni(OH)2 sample revealed atomic percentages of nickel (Ni) at 21.34% and oxygen (O) at 40.49% This results in an atomic ratio of Ni to O of approximately 0.5, confirming the formation of the Ni(OH)2 phase on the surface of the nanofiber (NF).
Figure 3.4: (a) EDX analysis, and (b) Raman spectra of the synthesized
The formation of Ni(OH) 2 onto the surface of the NF film can be explained by the equations [68]:
The Raman spectra of synthesized Ni(OH)2, displayed in the frequency range of 200–2000 cm−1, reveals the vibrational modes of both α- and β-Ni(OH)2 phases Key vibrational modes for α-Ni(OH)2 are identified at 464, 787, 1007–1059, and 1472 cm−1, with the 464 cm−1 peak corresponding to the lattice mode Additionally, second-order lattice modes are observed around 784 and 1007–1059 cm−1, while the peak at 1472 cm−1 is attributed to the O–H bending mode of the OH lattice The vibrational mode of β-Ni(OH)2 is also present in the spectrum.
510 cm −1 is assigned to the second-order acoustic mode [69].
Cyclic voltammetry measurement of the synthesized materials in alkaline
3.2.1 Influence of reaction time on the electrochemical properties of the synthesized materials
The CV curves displayed in Figure 3.5 illustrate the electrochemical behavior of the bare nickel foam (NF) and the synthesized nickel hydroxide (Ni(OH)2) working electrodes, assessed at varying reaction times The experiments were conducted in a 0.1 M NaOH solution, utilizing a potential range of -1.0 to 1.5 V and a scan rate of 10 mV/s.
The cyclic voltammograms (CV) of the bare NF and Ni(OH) 2 /NF working electrodes were performed in 0.1 M NaOH with a potential window range of
−1.0 to 1.5 V and a scan rate of 10 mV s −1 (figure 3.5), in which Ni(OH) 2 nanohives were synthesized with different duration As shown in figure 3.5, the
The cyclic voltammetry (CV) curve of the synthesized Ni(OH)₂ reveals an anodic peak at 0.6 V and two cathodic peaks at +0.2 V and -0.55 V, indicating the presence of β and α phases in the material.
The β-Ni(OH)₂ exhibits a lower proton count compared to α-Ni(OH)₂, resulting in a higher cathodic peak potential for β-Ni(OH)₂ Furthermore, the cyclic voltammetry (CV) analysis reveals that the anodic and cathodic peak currents for Ni(OH)₂/NF are significantly greater than those observed for the bare NF electrode This increase in current peaks can be attributed to the porous structure of the Ni(OH)₂/NF electrode, which offers a larger surface area, numerous active sites, and enhanced charge transfer capabilities The nanocavity structures within Ni(OH)₂ further boost the contact area with the electrolyte solution, thereby improving the electrode reactions and overall electrochemical performance.
CV plots of the electrodes with different synthesized time
3.2.2 CV measurements toward glucose in alkaline medium
Cyclic voltammetry (CV) at a scan rate of 10 mV s−1 was utilized to detect glucose concentration in a 0.1 M NaOH solution The study focused on the changes in the CV plots of Ni(OH)2/NF working electrodes When 0.5 mM glucose was added to the 0.1 M NaOH solution, the anodic peak current values recorded after 10, 30, and 50 minutes were 0.35, 0.20, and 0.23 mA, respectively These current values showed no significant differences, leading to the selection of the Ni(OH)2-10m electrode for further electrochemical property measurements in subsequent experiments.
Figure 3.6: The change of current value in CV curve when adding 0.5 mM glucose into 0.1 mM NaOH of the fabricated Ni(OH) 2 electrode with different reaction time
The CV curves of the Ni(OH)2/NF electrode were analyzed at a scan rate of 10 mV/s in a 0.1 M NaOH solution, showcasing varying glucose concentrations from 0 to 6 mM Additionally, a plot illustrating the relationship between reduction peak currents and glucose concentrations was generated, highlighting the electrode's responsiveness to glucose levels.
Figure 3.7a shows the CV curves of the sensor measured in different glucose concentrations (0–6 mM) The CV curves exhibit a slight decrease in
The analysis reveals that the cathodic peak current decreases as glucose concentration increases from 0 mM to 6 mM, as illustrated in Figure 3.7a The relationship between cathodic peak currents and glucose concentration is depicted in Figure 3.7b, demonstrating a linear decrease that adheres to the following calibration equation.
E.q 3.4 where C G is the glucose concentration (in mM), and the squared correlation coefficient (R 2 ) is 0.99
The sensing mechanism of the electrochemical sensor for glucose detection relies on the electrocatalyst based on Ni(OH) 2 towards glucose oxidation as established by the following reactions [11][72]:
Nickel oxyhydroxide (NiOOH) serves as a catalyst for the non-enzymatic oxidation of glucose, facilitating its conversion to gluconolactone while being reduced to Ni(OH)2 In the absence of glucose in the electrolyte solution, NiOOH is entirely reduced to Ni(OH)2 However, when glucose is introduced, a portion of NiOOH acts as a catalyst for its oxidation, while the remainder participates in an electrochemical reaction to form Ni(OH)2 This process results in a decrease in the cathodic peak current, indicating the ongoing consumption of NiOOH for glucose oxidation Consequently, the findings suggest that Ni(OH)2 is an effective mediator for enhancing catalytic activity in glucose oxidation.
We conducted cyclic voltammetry (CV) measurements of the sensor at varying scan rates in a 0.1 M NaOH and 1 mM glucose solution to investigate the kinetics of glucose oxidation catalyzed by Ni(OH)₂, as illustrated in Figure 3.8a The results indicate that the cathodic peak currents increase with higher scan rates, as shown by the arrow in the figure This relationship suggests that a faster scan rate results in a shorter time frame and a larger current value necessary to maintain the same charge Furthermore, the cathodic peak currents exhibit a linear correlation with the square root of the scan rate, as depicted in Figure 3.8b, and can be expressed mathematically by the equation relating cathodic peak currents to the square root of the scan rate (x ≈ υ¹/²).
This result indicates that the electrochemical reactions on the Ni(OH) 2 /NF electrode reside in diffusion-controlled regime [46] The gradual displacement of
40 cathodic peak potential with scan rate shows that glucose oxidation is a quasi- reversible mass transfer-controlled process [73]
Figure 3.8 illustrates the cyclic voltammetry (CV) curves of the Ni(OH)2/NF electrode in a 0.1 M NaOH and 1 mM glucose solution, showcasing various scan rates ranging from 10 mV/s to 100 mV/s Additionally, the graph presents a relationship between the reduction peak current and the square root of the scan rates, highlighting the electrochemical behavior of the electrode in different conditions.
Figure 3.9: CV curves of (a) five different Ni(OH) 2 /NF working electrodes synthesized with the same condition, (b) the Ni(OH) 2 /NF working electrode after three months stored in room temperature
The stability and reproducibility of the Ni(OH) 2 /NF electrodes were assessed in a 0.1 M NaOH solution using cyclic voltammetry (CV) measurements Five identical Ni(OH) 2 /NF electrodes were tested to evaluate their reproducibility, with results indicating negligible differences in the CV curves, as shown in figure 3.9a Additionally, amperometric measurements for detecting 0.5 mM glucose demonstrated a relative standard deviation of only 5.9% after five tests These findings confirm that the Ni(OH) 2 /NF electrodes are stable and suitable for repeated glucose measurements.
The long-term stability of the Ni(OH) 2 /NF electrode was also tested by
The CV measurements conducted before and after three months of storage at room temperature indicate that the Ni(OH)₂/NF electrode exhibits excellent stability, as evidenced by the similarity in CV curves (Figure 3.9b) Additionally, amperometric testing at 0.6 V for 0.5 mM glucose in 0.1 M NaOH revealed only a 7% decrease in current response, further confirming the electrode's stability Overall, these findings demonstrate that the Ni(OH)₂/NF electrode possesses good reproducibility and stability, making it a suitable candidate for glucose monitoring applications.
Chronoamperometry measurement of the synthesized materials in alkaline
3.3.1 Optimization of the statical potential applied
Chrono amperometry technique (CA) is widely applied in the determination of sample concentration due to its easy processing, high sensitivity, and good application in realistic
To optimize glucose concentration determination, it is essential to adjust the applied potential, comparing the sensitivity of Ni(OH)2/NF at various potentials (0.5, 0.6, and 0.7 V) with glucose concentrations ranging from 0 to 0.5 mM The relationship between current densities and glucose concentration is illustrated in the CA curves and plots shown in Figure 3.10.
As illustrated in Figure 3.10a, an increase in applied potential from 0.5 to 0.7 V leads to a rise in current Analysis of the calibration curve in Figure 3.10b indicates that a potential of 0.6 V yields the optimal signal.
V is high but the signal is not stable Therefore, 0.6 V was chosen as the optimized potential to construct a calibration curve for the determination of glucose concentration
The I-t curves of the Ni(OH)2/NF electrode were analyzed at voltages of 0.5, 0.6, and 0.7 V versus Ag/AgCl in a 0.1 M NaOH solution, with glucose concentrations ranging from 0 mM to 0.5 mM Additionally, calibration plots were created to illustrate the relationship between response current and glucose concentration.
3.3.2 Construct the calibration curve for the determination of glucose concentration
The I-t curves of the Ni(OH)2/NF electrode were analyzed at a potential of 0.6 V versus Ag/AgCl in a 0.1 M NaOH solution, with glucose concentrations varying from 0 mM to 0.5 mM Additionally, a calibration plot was created to illustrate the relationship between response current and glucose concentration.
Amperometric measurement was utilized for the detection of glucose concentrations ranging from 0 to 0.5 mM in a 0.1 M NaOH solution at a potential of 0.6 V versus Ag/AgCl The sensor's current response, depicted in figure 3.11, demonstrates an increase in current with rising glucose levels, indicating a strong sensor response Current plots as a function of glucose concentration, measured at the applied voltage of 0.6 V, reveal that the base current remains stable at approximately the same value.
1 mA thanks to the high conductivity of the NF substrate Additionally, there is a linear increase in the current towards the increase in glucose concentration from
The sensor demonstrated a sensitivity of 12.55 mA mM −1 cm −2 within the concentration range of 0 mM to 0.5 mM, calculated by dividing the slope by the working electrode area of 0.65 cm² The excellent conductivity of the nanofiber (NF) significantly enhanced the sensor's signal, leading to a limit of detection of approximately 57 aM, determined based on a signal-to-noise ratio of 3.
Table 3.1: Comparison of the performance of the synthesized Ni(OH) 2 /NF and other nickel-based materials for non-enzymatic glucose sensors
Our findings were compared with recent studies on glucose sensors, as illustrated in Table 3.1 The developed electrode features ultrathin walls, approximately 15 nm thick, composed of Ni(OH)2 nanohives, built on a highly conductive nickel foam substrate.
Ni(OH) 2 nanohives/NF 12.55 57 0.05–6 This study
Recent studies have revealed that nickel-based non-enzymatic glucose sensors exhibit the highest sensitivity, with a notable enhancement attributed to the increased surface area provided by the porosity of Ni(OH)2 nanohives This advancement allows for a significantly large linear detection range of 0.05–6 mM, surpassing many previous findings The results indicate that the Ni(OH)2/NF electrode demonstrates promising potential for glucose detection in alkaline environments.
The effect of interferences of Ni(OH) 2 /NF
Figure 3.12: Amperometric response for successive addition of 0.5 mM glucose,
0.05 mM AA, 0.05 mM CA, 0.05 mM DA, 15 mM NaCl, and 0.1 mM glucose to the Ni(OH) 2 /NF electrode in 0.1 M NaOH at the potential of 0.6 V
Application of the synthesized electrode for glucose measurement in real
We evaluated the selectivity of the Ni(OH)2/NF electrode for glucose detection by testing its response to 0.5 mM glucose alongside potential interferents: 0.05 mM AA, 0.05 mM CA, 0.05 mM DA, 15 mM NaCl, and 1 mM glucose, which reflect standard levels found in human blood The results, illustrated in Figure 3.12, indicate that the current response to 0.5 mM and 0.1 mM glucose is significantly higher than that of the interfering substances, with interference percentages of only 2.53% for AA, 2.95% for CA, 2.11% for DA, and 5.59% for NaCl These findings demonstrate that the Ni(OH)2/NF electrode exhibits remarkable selectivity for glucose detection in an alkaline environment.
3.5 Application of the synthesized electrode for glucose measurement in real samples
To assess the performance of the fabricated sensor in real-world applications, measurements were conducted using human serum samples from a local hospital A 350 µL sample of serum was combined with 20 mL of 0.1 NaOH electrolyte solution, and the amperometric response was recorded at 0.6 V, with results detailed in Table 3.2 The findings indicated a strong correlation between the hospital's measurements and those obtained with the synthesized Ni(OH)2/NF electrode The glucose recovery values, achieved by adding pure glucose to the serum samples, ranged from 97.83% to 116.41% These encouraging results confirm the sensor's capability for rapid and accurate glucose detection in human blood serum.
Table 3.2: Measurement of glucose concentration of real human blood serum samples.
NiO nanohives based on Ni(OH) 2 -precursors for glucose measurement in
3.6.1 Formation of NiO from Ni(OH) 2 -precursors
Besides Ni(OH) 2 nanomaterials, NiO is also a potential candidate for non- enzymatic electrochemical glucose sensor In this thesis, NiO nanohives are
The synthesis of NiO involves using Ni(OH)2 as a precursor, which is formed on a substrate The Ni(OH)2-modified electrode is then heated in an oven at 300°C for two hours to facilitate the transition to the NiO phase After this process, the electrode is allowed to cool to room temperature and is stored in a vacuum chamber until it is needed for further use.
The EDS pattern of the synthesized materials, depicted in Figure 3.13, reveals the presence of nickel and oxygen with atomic percentages of 56.72% and 43.28%, respectively This approximately 1:1 atomic ratio indicates the formation of the NiO phase on the surface of the nickel foam electrode.
Note that in our study, the sample was treated at temperature of 300 o C, which is much lower than the initial oxidation of Ni at temperature of about
The EDX signal depicted in figure 3.13 is primarily attributed to the NiO nanohives rather than the Ni foam, with the composition of the nanohives closely resembling stoichiometric NiO This suggests that the sample exhibits high quality.
Int ens ity (co un ts)
Figure 3.13: EDS pattern of the synthesized NiO.
Figure 3.14 displays FE-SEM images of synthesized NiO, revealing uniformly structured nanohives at lower magnification (figure 3.14a) As magnification increases (figure 3.14b), the walls of the NiO nanohives are observed to be 20-30 nm thick Importantly, there is no discernible difference in the morphologies between the Ni(OH)2 precursors and the resulting NiO.
Figure 3.14: (a) Low and (b) high magnification FE-SEM images of the synthesized NiO/NF electrode
3.6.2 Electrochemical behaviors and glucose measurements of the synthesized NiO
The electrochemical behaviors of materials were assessed in 0.1 M NaOH at room temperature, as depicted in Figure 3.15, which shows the cyclic voltammograms (CVs) of bare and NiO-modified nickel foam electrodes at a scan rate of 50 mV s -1 The NiO/NF electrodes exhibited higher peak current signals, indicating improved electrochemical properties following the deposition of NiO nanohives Furthermore, the CVs of three different NiO/NF electrodes synthesized under identical conditions displayed no significant shifts, demonstrating good reproducibility of the synthesized electrodes.
Figure 3.15: CVs plots of the NF and three NiO/NF electrodes synthesized under the same conditions
Figure 3.16: CVs plots of NiO/NF electrode in 20 mL of 0.1 M NaOH and 0, 3,
The synthesized electrodes demonstrated effective glucose detection, as illustrated in Fig 4, which shows the CV plots of the NiO/NF electrode in 20 mL of 0.1 M NaOH with varying glucose concentrations (0, 3, 4, 5, 6, and 7 mM) Notably, the cathodic peak currents decreased with increasing glucose concentration, indicating a clear relationship between glucose levels and electrode response The electrochemical oxidation of glucose is facilitated by the NiO electrocatalyst, which is first oxidized to NiO(OH) and subsequently oxidizes glucose to gluconic acid while regenerating NiO This process is defined by specific electrocatalytic oxidation mechanisms.
When glucose is introduced to a NaOH solution, NiO(OH) partially oxidizes it to gluconic acid, leading to a decrease in the amount of NiO(OH) participating in the reaction and consequently reducing the cathodic peak current This indicates that synthesized NiO/Ni exhibits effective catalytic activity for glucose oxidation in electrochemical sensors The electrochemical properties of NiO are significantly influenced by its morphology and the substrates used For instance, research by Salazar et al highlights these dependencies.
51 thin film NiO on screen printed electrodes showed a redox reaction at high potential (~0.55 V) with Ag pseudo-reference electrode
Figure 3.17: (a) Amperometric response of NiO/NF toward glucose with a concentration range between 0 and 0.5 mM, and (b) The plot of dependence between response current and glucose concentration at different potentials
He et al., [71], the hollow porous NiO modified glassy carbon was used as working electrode, where the anodic and cathodic peaks were found at about 0.48
The study observed anodic and cathodic peaks at approximately 0.58 V and 0.28 V (vs Hg/HgO) using a porous honeycomb-like NiO modified NF as a working electrode for glucose sensing The addition of glucose resulted in an increased anodic peak current and a slight shift to higher potential, while the cathodic peak current decreased and shifted slightly to a more positive potential This indicates that the anodic and cathodic peaks of Ni²⁺/Ni³⁺ can vary based on the substrate materials and the morphology of synthesized NiO In our sensor, the anodic peak was recorded at +0.6 V and the cathodic peak at +0.1 V versus an Ag/AgCl reference electrode in 0.1 M glucose solution.
M NaOH The difference in CVs of the NiO prepared in our study can be ascribed due to the difference in the morphology of the synthesised materials and the substrate
Table 3.3: To compare the glucose electrochemical sensing of the fabricated sensors with other nickel-based sensors
To investigate the electrocatalytic properties of NiO/NF for glucose oxidation, chronoamperometry was utilized to assess glucose levels at three different voltage settings: 0.5, 0.6, and 0.7 V The results indicate that as glucose concentration increases from 0 to 0.5 mM, the response current at 0.7 V rises linearly In contrast, at 0.5 V and 0.6 V, the response currents exhibit a nonlinear increase with rising glucose concentrations The relationship between the response current at 0.7 V and glucose concentration can be described by a specific function.
Where C G is glucose concentration (mM) The sensitivity of the sensor calculated by dividing the slope to the working electrode area of 0.65 mm 2 is 6.8 mA mM -
The fabricated sensor demonstrated a detection limit (LOD) of 5.7 àM, determined based on a signal-to-noise ratio of 3 This performance is compared with previously reported nickel-based sensors, highlighting the effectiveness of the developed sensor in detecting low concentrations.
1) All results indicated that the NiO/NF electrode is a capable candidate for clinical glucose detection
CONCLUSIONS
After the implementation of the topic and based on the analysis results presented above, we draw some conclusions as follows:
Ni(OH) 2 nanohives/NF electrode was synthesized successfully by using a facile, low-cost, and instant chemical method
Nanohives with an average cavity diameter of 300–500 nm and a nanowall thickness of approximately 15 nm were homogenously formed on the surface of NF
The Ni(OH)2/NF electrode exhibits excellent electrocatalytic activity for glucose oxidation in a NaOH solution, demonstrating a broad linear detection range of 0 to 6 mM (R² = 0.999) It also shows high sensitivity at 12.55 mA mM⁻¹ cm⁻² and a detection limit of 57 µM.
In addition, the sensor presented good reproducibility, high stability, and good selectivity
The fabricated electrode is a potential candidate for practical application with the values of glucose recovery from 97.83% to 116.41% in real samples measured
Successfully synthesis of NiO nanohives based on Ni(OH) 2 -precursors, the synthesized NiO nanohives also exhibit the good electrocatalytic activity toward glucose oxidation in NaOH solution
The fabricated sensor demonstrates low cost, high sensitivity, excellent stability, and strong selectivity, highlighting its significant potential for use in non-enzymatic electrochemical biosensors designed for blood glucose detection.
Besides the good results that the thesis has achieved, there are still some issues that need to be studied further Thus, the next research orientation should be:
Continue to study the influence of other synthesized conditions on the formation of different structural morphologies of Ni(OH) 2 , to enhance the sensitivity toward glucose
In addition, the working electrodes will be decorated with metals (such as
Au, Pt, Cu,…) or combined with other metal oxides (such as CuO, NiO,…) to improve the electrochemical behaviors for glucose measurements
1 Vu Thi Oanh, Chu Thi Xuan, Le Manh Tu, and Nguyen Duc Hoa A
A straightforward chemical method for the direct synthesis of nickel oxide (NiO) on nickel foam electrodes has been developed, enabling its application in non-enzymatic glucose electrochemical measurements This innovative approach enhances the performance of glucose sensors, offering a promising solution for efficient and accurate monitoring of glucose levels The findings are documented in the Lecture Notes in Networks and Systems, Volume 366, pages 100–106, published by Springer Science and Business Media Deutschland GmbH.
2 Vu Thi Oanh, Chu Thi Xuan, and Nguyen Duc Hoa A highly sensitive non-enzymatic glucose electrochemical sensor based on NiO nanohives
Advances in Natural Sciences: Nanoscience and Nanotechnology 12,
Vu Thi Oanh, Chu Thi Xuan, and Nguyen Duc Hoa have developed a straightforward method for the in situ growth of Ni(OH)2 nanohives on nickel foam, aimed at enhancing non-enzymatic electrochemical glucose sensors This innovative approach is detailed in their complete manuscript prepared for submission to a Q1 ISI journal.
[1] Ngoc NB, Lin ZL, Ahmed W Diabetes: What challenges lie ahead for Vietnam Annals of Global Health 2020;86:1–9 https://doi.org/10.5334/aogh.2526.
A study by Jarosz et al (2017) presents an amperometric glucose sensor utilizing a Ni(OH)2/Al(OH)4− electrode derived from a thin Ni3Al foil The research, published in Applied Surface Science, highlights the innovative approach to glucose detection, emphasizing the electrode's efficiency and potential applications in biosensing technologies The findings contribute to advancements in sensor design and performance For more details, refer to the article [here](https://doi.org/10.1016/j.apsusc.2017.02.188).
[3] Pal N, Banerjee S, Bhaumik A A facile route for the syntheses of Ni(OH)2 and NiO nanostructures as potential candidates for non-enzymatic glucose sensor Journal of Colloid and Interface Science 2018;516:121–7 https://doi.org/10.1016/j.jcis.2018.01.027
A groundbreaking study by Yue et al (2019) introduces innovative non-enzymatic dopamine sensors utilizing a hybrid of nickel oxide and reduced graphene oxide nanosheets Published in the Journal of Materials Science: Materials in Electronics, this research highlights the potential of these advanced materials in enhancing the sensitivity and efficiency of dopamine detection.
In a 2019 study published in RSC Advances, Mishra et al explored the development of a gold nanoparticles modified CuO nanowires electrode aimed at enhancing non-enzymatic glucose detection The research demonstrates the potential of this innovative electrode design in improving the sensitivity and efficiency of glucose sensors, contributing to advancements in biomedical applications For more details, refer to the publication [here](https://doi.org/10.1039/C8RA07516F).
[6] Chen C, Xie Q, Yang D, Xiao H, Fu Y, Tan Y, et al Recent advances in electrochemical glucose biosensors: A review RSC Advances 2013;3:4473–91 https://doi.org/10.1039/c2ra22351a
Recent advancements in non-enzymatic electrochemical glucose sensors utilizing non-precious transition metal materials present significant opportunities and challenges These developments aim to enhance the efficiency and affordability of glucose monitoring technologies The study by Niu et al (2016) highlights the potential of these materials in improving sensor performance, while also addressing the obstacles that must be overcome for practical applications For more detailed insights, refer to the full article in RSC Advances.
[8] Wilson R, Elizabeth Q Glucose oxidase : An ideal enzyme Review article Glucose oxidase Biosensors t Biwlecrronia 2016;5663:165–85
[9] Park S, Boo H, Chung TD Electrochemical non-enzymatic glucose sensors
Analytica Chimica Acta 2006;556:46–57 https://doi.org/10.1016/j.aca.2005.05.080
[10] Safavi A, Maleki N, Farjami E Fabrication of a glucose sensor based on a novel nanocomposite electrode Biosensors and Bioelectronics 2009;24:1655–60 https://doi.org/10.1016/j.bios.2008.08.040
[11] Singer N, Pillai RG, Johnson AID, Harris KD, Jemere AB Nanostructured nickel oxide electrodes for non-enzymatic electrochemical glucose sensing Microchimica Acta 2020;187:15–20 https://doi.org/10.1007/s00604-020-4171-5
A recent study published in Materials Chemistry and Physics details the development of a solid-state glucose sensor utilizing copper (Cu) and iron (Fe)-doped carbon nitride This innovative sensor demonstrates significant potential for improving glucose monitoring technology, offering enhanced sensitivity and reliability The research, conducted by Dante RC and colleagues, emphasizes the effectiveness of the doped carbon nitride material in sensor applications, paving the way for advancements in biomedical devices For more information, refer to the article at https://doi.org/10.1016/j.matchemphys.2020.124023.
[13] Chawla M, Pramanick B, Randhawa JK, Siril PF Effect of composition and calcination on the enzymeless glucose detection of Cu-Ag bimetallic nanocomposites Materials Today Communications 2021;26:101815 https://doi.org/10.1016/j.mtcomm.2020.101815
In the study by Zhao et al (2017), a novel CuO/rGO/Cu2O nanocomposite was successfully deposited on copper foil using a hydrothermal method This innovative approach enables the sensitive nonenzymatic voltammetric detection of glucose and hydrogen peroxide, showcasing significant advancements in electrochemical sensing technologies The findings are published in Microchimica Acta, highlighting the potential applications of this nanocomposite in biomedical and environmental monitoring For further details, refer to the article at https://doi.org/10.1007/s00604-017-2229-9.
[15] Qian J, Wang Y, Pan J, Chen Z, Wang C, Chen J, et al Non-enzymatic glucose sensor based on ZnO–CeO2 whiskers Materials Chemistry and Physics 2020;239:122051 https://doi.org/10.1016/j.matchemphys.2019.122051
[16] Niu X, Lan M, Zhao H, Chen C Highly Sensitive and Selective Nonenzymatic Detection of Glucose Using Three-Dimensional Porous Nickel Nanostructures Analytical Chemistry 2013;85:3561–9 https://doi.org/10.1021/ac3030976
Palladium-Nickel nanoparticles have been successfully decorated on functionalized multi-walled carbon nanotubes (MWCNT) to enhance non-enzymatic glucose sensing This innovative approach, detailed in the study by Karimi-Maleh et al., published in *Materials Chemistry and Physics*, showcases significant advancements in precision for glucose detection The findings contribute to the development of more efficient sensors in the field of biosensing technology.
The research article discusses the straightforward synthesis of a nickel hydroxide (Ni(OH)2) modified disposable pencil graphite electrode This innovative electrode is designed for use in a highly sensitive non-enzymatic glucose sensor, showcasing its potential application in electroanalysis The findings contribute to advancements in glucose sensing technology, emphasizing the electrode's efficacy and practicality in real-world scenarios.
[19] Faure C, Delmas C, Fouassier M Characterization of a turbostratic α-nickel hydroxide quantitatively obtained from an NiSO4 solution Journal of Power Sources 1991;35:279–90 https://doi.org/10.1016/0378-7753(91)80112-B
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The study by Ma et al (2014) presents a straightforward phase transformation technique for synthesizing a three-dimensional, flower-like β-Ni(OH)₂/GO/CNTs composite This innovative composite demonstrates remarkable performance as a supercapacitor, highlighting its potential applications in energy storage technologies.
The study by Li et al (2020) explores the in situ growth of β-Ni(OH)₂ nanosheet arrays on nickel foam, presenting an innovative integrated electrode for supercapacitors This approach demonstrates significant potential for achieving high energy density in energy storage applications The findings are documented in Dalton Transactions, volume 49, pages 4956-4966 However, the DOI link provided (https://doi.org/10.1039/d0dt00687d) could not be located, indicating possible issues with the DOI registration or its accuracy.
spectroscopy (Hitachi S-4800)
M NaOH with glucose concentration from 0 to 6 mM, (b) plot of
3.3.2 Construct the calibration curve for the determination of glucose concentration
The I-t curves of the Ni(OH)2/NF electrode were analyzed at a voltage of 0.6 V vs Ag/AgCl in a 0.1 M NaOH solution, with glucose concentrations varying from 0 mM to 0.5 mM Additionally, a calibration plot was created to illustrate the relationship between the response current and glucose concentration.
Amperometric measurement was utilized for detecting glucose concentrations ranging from 0 to 0.5 mM in a 0.1 M NaOH solution at a potential of 0.6 V versus Ag/AgCl The current versus time plots demonstrate a clear increase in current with rising glucose levels, indicating the sensor's effective response Current plots, measured at the applied voltage of 0.6 V, reveal that the base current remains stable, further supporting the reliability of the sensor's performance.
1 mA thanks to the high conductivity of the NF substrate Additionally, there is a linear increase in the current towards the increase in glucose concentration from
The sensitivity of the sensor was measured at 12.55 mA mM −1 cm −2, calculated from the slope divided by the working electrode area of 0.65 cm², across a concentration range of 0 mM to 0.5 mM The excellent conductivity of the nanofiber (NF) significantly enhanced the sensor's signal, resulting in a calculated limit of detection of approximately 57 àM, based on a signal-to-noise ratio of 3.
Table 3.1: Comparison of the performance of the synthesized Ni(OH) 2 /NF and other nickel-based materials for non-enzymatic glucose sensors
Recent publications on glucose sensors were compared with our findings, as detailed in Table 3.1 Our study features a fabricated electrode with ultrathin walls, approximately 15 nm thick, composed of Ni(OH)2 nanohives, supported by a highly conductive nickel foam substrate.
Ni(OH) 2 nanohives/NF 12.55 57 0.05–6 This study
The study reports the highest sensitivity among nickel-based non-enzymatic glucose sensors, attributed to the increased surface area from the porosity of Ni(OH)2 nanohives Additionally, the linear detection range of 0.05–6 mM is significantly broader than in many prior studies These findings indicate that the Ni(OH)2/NF electrode holds promise for effective glucose detection in alkaline environments.
3.4 The effect of interferences of Ni(OH) 2 /NF
Figure 3.12: Amperometric response for successive addition of 0.5 mM glucose,
0.05 mM AA, 0.05 mM CA, 0.05 mM DA, 15 mM NaCl, and 0.1 mM glucose to the Ni(OH) 2 /NF electrode in 0.1 M NaOH at the potential of 0.6 V
Selectivity is a crucial characteristic of chemical sensors, particularly due to the presence of interfering species like ascorbic acid (AA), citric acid (CA), dopamine (DA), and sodium chloride (NaCl) in real samples To evaluate the sensor's selectivity, we tested its response to 0.5 mM glucose alongside low concentrations of AA, CA, DA, and NaCl, which reflect typical levels found in human blood The results, illustrated in Figure 3.12, indicate that the current response to glucose is significantly higher than that of the interfering substances Specifically, the interference from 0.05 mM AA, 0.05 mM CA, 0.05 mM DA, and 15 mM NaCl was measured at only 2.53%, 2.95%, 2.11%, and 5.59%, respectively, when compared to the 0.5 mM glucose signal These findings demonstrate that the Ni(OH)2/NF electrode exhibits excellent selectivity for glucose detection in an alkaline medium.
3.5 Application of the synthesized electrode for glucose measurement in real samples
To assess the effectiveness of the developed sensor, real human serum samples from a local hospital were tested A volume of 350 µL of serum was mixed with 20 mL of 0.1 NaOH electrolyte solution, and the amperometric response was recorded at 0.6 V, with results presented in Table 3.2 The findings showed a close correlation between the hospital measurements and those obtained using the synthesized Ni(OH)2/NF electrode The glucose recovery rates, determined by adding pure glucose to the serum samples, ranged from 97.83% to 116.41% These encouraging results indicate that the sensor can detect glucose in human blood serum both rapidly and accurately.
Table 3.2: Measurement of glucose concentration of real human blood serum samples.
3.6 NiO nanohives based on Ni(OH) 2 -precursors for glucose measurement in alkaline
3.6.1 Formation of NiO from Ni(OH) 2 -precursors
Besides Ni(OH) 2 nanomaterials, NiO is also a potential candidate for non- enzymatic electrochemical glucose sensor In this thesis, NiO nanohives are
The synthesis of NiO involves using Ni(OH)2 as a precursor, which is first formed on a substrate Following this, the Ni(OH)2-modified electrode is heated in an oven at 300°C for two hours to facilitate the transformation into the NiO phase After the heating process, the electrode is allowed to cool to room temperature and is then stored in a vacuum chamber until needed for future applications.
Figure 3.13 illustrates the EDS pattern of the synthesized materials, revealing the presence of nickel and oxygen with atomic percentages of 56.72% and 43.28%, respectively The nearly 1:1 atomic ratio indicates the formation of the NiO phase on the surface of the nickel foam electrode.
Note that in our study, the sample was treated at temperature of 300 o C, which is much lower than the initial oxidation of Ni at temperature of about
The EDX signal observed in figure 3.13 is primarily attributed to the NiO nanohives rather than the Ni foam The composition of these nanohives closely resembles stoichiometric NiO, highlighting the high quality of the sample.
Int ens ity (co un ts)
Figure 3.13: EDS pattern of the synthesized NiO.
FE-SEM images of the synthesized NiO, displayed in figure 3.14, reveal uniformly structured nanohives at lower magnification (figure 3.14a) As magnification increases (figure 3.14b), the walls of the NiO nanohives are observed to be 20-30 nm thick Importantly, there is no observable difference in the morphologies between the Ni(OH)2 precursors and the resulting NiO.
Figure 3.14: (a) Low and (b) high magnification FE-SEM images of the synthesized NiO/NF electrode
3.6.2 Electrochemical behaviors and glucose measurements of the synthesized NiO
The electrochemical behaviors of materials were evaluated in a 0.1 M NaOH solution at room temperature, as depicted in Figure 3.15, which shows the cyclic voltammograms (CVs) of both bare and NiO-modified nickel foam electrodes at a scan rate of 50 mV s⁻¹ The NiO/NF electrodes exhibited significantly higher peak current signals, indicating an enhancement in their electrochemical properties following the deposition of NiO nanohives Furthermore, the CVs of the three different NiO/NF electrodes, synthesized under identical conditions, demonstrated good reproducibility with no notable shifts in their plots.
Figure 3.15: CVs plots of the NF and three NiO/NF electrodes synthesized under the same conditions
Figure 3.16: CVs plots of NiO/NF electrode in 20 mL of 0.1 M NaOH and 0, 3,
The synthesized electrodes were utilized for glucose detection, as demonstrated by the CV plots of the NiO/NF electrode in a 20 mL solution of 0.1 M NaOH with varying glucose concentrations (0, 3, 4, 5, 6, and 7 mM) The results indicate a decrease in cathodic peak currents with increasing glucose concentration, highlighting the electrochemical oxidation mechanism facilitated by the NiO electrocatalyst Specifically, NiO is oxidized to NiO(OH), which subsequently oxidizes glucose to gluconic acid while being reduced back to NiO This process underscores the electrocatalytic oxidation mechanism involved in glucose detection.
When glucose is introduced to a NaOH solution, NiO(OH) partially oxidizes it to gluconic acid, leading to a reduction in the amount of NiO(OH) involved in the reaction and consequently a decrease in the cathodic peak current This indicates that the synthesized NiO/Ni exhibits catalytic activity for glucose oxidation in electrochemical sensors The electrochemical properties of NiO are significantly influenced by its morphology and the substrates used For instance, a study by Salazar et al highlights these dependencies in their findings.
51 thin film NiO on screen printed electrodes showed a redox reaction at high potential (~0.55 V) with Ag pseudo-reference electrode
Figure 3.17: (a) Amperometric response of NiO/NF toward glucose with a concentration range between 0 and 0.5 mM, and (b) The plot of dependence between response current and glucose concentration at different potentials
He et al., [71], the hollow porous NiO modified glassy carbon was used as working electrode, where the anodic and cathodic peaks were found at about 0.48
The study observed anodic and cathodic peaks at approximately 0.58 V and 0.28 V (vs Hg/HgO) when using porous honeycomb-like NiO modified NF as a working electrode for glucose sensing The addition of glucose resulted in an increased anodic peak current and a slight shift to higher potential, while the cathodic peak current decreased and shifted positively This indicates that the anodic/cathodic peaks of Ni²⁺/Ni³⁺ can vary based on substrate materials and the morphology of synthesized NiO In our sensor, the anodic peak was recorded at +0.6 V and the cathodic peak at +0.1 V versus an Ag/AgCl reference electrode in a 0.1 M glucose solution.
M NaOH The difference in CVs of the NiO prepared in our study can be ascribed due to the difference in the morphology of the synthesised materials and the substrate
Table 3.3: To compare the glucose electrochemical sensing of the fabricated sensors with other nickel-based sensors