Modified indium tin oxide electrodes: Electrochemical applications in pharmaceutical, biological, environmental and food analysis

Sulfhydryl-functionalized 3D MXene-AuNPs enabled electrochemical sensors for the selective determination of Pb2+, Cu2+ and Hg2+ in grain

Herein, we made 3D MXene-AuNPs by in situ growth of gold nanoparticles (AuNPs) on the surface of MXene by chemical reduction method, and then introduced three sulfhydryl (-SH) compounds as functionalized modifiers attached to the AuNPs to form a highly selective composite material for the detection of Pb2+, Cu2+, and Hg2+, respectively. The doping of AuNPs changes the microstructure of 2D MXene and generates more active sites. On a sensing platform based on ITO array electrodes, the detection system was optimised with sensitivities up to 1.157, 0.846 and 0.799 μA·μg-1Lcm-2 (Pb2+, Cu2+, and Hg2+). The selectivity of MXene@AuNPs was effectively improved by sulfhydryl group modification. In the range of 1-1300 μg L-1, the detection limits of three ions were 0.07, 0.13 and 0.21 μg L-1. In addition, this method can efficiently and accurately detect heavy metal ions in four cereal samples with consistent results with inductively coupled plasma mass spectrometry.

Hydroxylation of the indium tin oxide electrode promoted by surface bubbles

Adherent bubbles at electrodes are generally treated as reaction penalties. Herein, in situ hydroxylation of indium tin oxide surfaces can be easily achieved by applying a constant potential of +1.0 V in the presence of bubbles. Its successful hydroxylation is further demonstrated by preparing a ferrocene-terminated film, which is confirmed by cyclic voltammetry and X-ray photoelectron spectroscopy.

Monitoring the Electrochemical Failure of Indium Tin Oxide Electrodes via Operando Ellipsometry Complemented by Electron Microscopy and Spectroscopy

Transparent conductive oxides such as indium tin oxide (ITO) are standards for thin film electrodes, providing a synergy of high optical transparency and electrical conductivity. In an electrolytic environment, the determination of an inert electrochemical potential window is crucial to maintain a stable material performance during device operation. We introduce operando ellipsometry, combining cyclic voltammetry (CV) with spectroscopic ellipsometry, as a versatile tool to monitor the evolution of both complete optical (i.e., complex refractive index) and electrical properties under wet electrochemical operational conditions. In particular, we trace the degradation of ITO electrodes caused by electrochemical reduction in a pH-neutral, water-based electrolyte environment during electrochemical cycling. With the onset of hydrogen evolution at negative bias voltages, indium and tin are irreversibly reduced to the metallic state, causing an advancing darkening, i.e., a gradual loss of transparency, with every CV cycle, while the conductivity is mostly conserved over multiple CV cycles. Post-operando analysis reveals the reductive (loss of oxygen) formation of metallic nanodroplets on the surface. The reductive disruption of the ITO electrode happens at the solid-liquid interface and proceeds gradually from the surface to the bottom of the layer, which is evidenced by cross-sectional transmission electron microscopy imaging and complemented by energy-dispersive X-ray spectroscopy mapping. As long as a continuous part of the ITO layer remains at the bottom, the conductivity is largely retained, allowing repeated CV cycling. We consider operando ellipsometry a sensitive and nondestructive tool to monitor early stage material and property changes, either by tracing failure points, controlling intentional processes, or for sensing purposes, making it suitable for various research fields involving solid-liquid interfaces and electrochemical activity.

Capacitive Bionic Magnetic Sensors Based on One-Step Biointerface Preparation

Magnetic biomolecule-based bionic magnetic field sensors are anticipated to open up novel pathways for magnetic field detection. The detection range and accuracy of current bionic magnetic field sensors are limited, and little work is based on the capacitive response principle. We successfully developed a biochemical interface with an extralarge target-receptor size ratio, which can be manufactured in a single step for weak magnetic field detection across a wide frequency range, and we used electrochemical capacitance as a magnetic field change conduction strategy. The thickness-controllable nanoscale bovine serum albumin/graphene layer on an indium tin oxide working electrode combines with the one-step preparation method to immobilize the MagR/Cry4 complex. This capacitive bionic magnetic sensor can achieve the detection range of 0-120 mT. This biointerface design strategy obtains the further improvement of the performance of this bionic magnetic field sensor. Furthermore, the biointerface construction and optimization methodology in this proposal has potential applications in the design of other medical biosensors.

Detection of depression marker ASS1 in urine by gold nanoparticles based dual epitope-peptides imprinted sensor

Trace detection of argininosuccinate synthetase 1 (ASS1), a depression marker, in urine samples is difficult to achieve. In this work, a dual-epitope-peptides imprinted sensor for ASS1 detection in urine was constructed based on the high selectivity and sensitivity of the "epitope imprinting approach". First, two cysteine-modified epitope-peptides were immobilized onto gold nanoparticles (AuNPs) deposited on a flexible electrode (ITO-PET) by gold-sulfur bonds (Au-S), then a controlled electropolymerization of dopamine was carried out to imprint the epitope peptides. After removing epitope-peptides, the dual-epitope-peptides imprinted sensor (MIP/AuNPs/ITO-PET) which with multiple binding sites for ASS1 was obtained. Compared with single epitope-peptide, dual-epitope-peptides imprinted sensor had higher sensitivity, which presented a linear range from 0.15 to 6000 pg ml^-1 with a low limit of detection (LOD = 0.106 pg mL^-1, S/N = 3). It had good reproducibility (RSD = 1.74%), repeatability (RSD = 3.60%), stability (RSD = 2.98%), and good selectivity, and the sensor had good recovery (92.4%-99.0%) in urine samples. This is the first highly sensitive and selective electrochemical assay for the depression marker ASS1 in urine, which is expected to provide help for the non-invasive and objective diagnosis of depression.

Cobalt oxide modified sulfur and phosphorus Co-doped g-C3N4 for screening of urinary human albumin

The fabrication of a heteroatom-doped nanocomposite based on cobalt oxide modified sulfur, phosphorus co-doped carbon nitride (Co3O4/SP-CN) with increased active sites is reported. The synthesized nanocomposite offers surprisingly high electrocatalytic oxidation efficacy toward human albumin (HA) despite its agglomeration. This improved efficacy of Co3O4/SP-CN nanocomposite could be attributed to its increased adsorption sites and surface defects, fast charge transportation capability, and conductivity. Additionally, morphological and compositional analysis of the fabricated Co3O4/SP-CN material has been performed  through scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photon spectroscopy (XPS), and Raman spectroscopy. The fabricated electrode shows remarkable amperometric response against the HA with a limit of detection of 8.39 nM and linear range of 20-4000 nM at applied potential of 0.25 V versus Ag/AgCl in 0.1 M PBS (pH 8.2). The designed Co3O4/SP-CN electrode has been successfully applied to monitor HA in  urine samples of diabetic patient with recovery percentage from 94.1 and 92.1% and with relative standard deviation (RSD) values of 5.8 and 7.8%. According to the best of our knowledge, this is the first report to use a Co3O4/SP-CN-based graphitic pencil (GP) electrode for monitoring of HA for early diagnosis of diabetic nephropathy.

Recent Advances in Metal-Organic Frameworks (MOFs) and Their Composites for Non-Enzymatic Electrochemical Glucose Sensors

In recent years, with pressing needs such as diabetes management, the detection of glucose in various substrates has attracted unprecedented interest from researchers in academia and industry. As a relatively new glucose sensor, non-enzymatic target detection has the characteristics of high sensitivity, good stability and simple manufacturing process. However, it is urgent to explore novel materials with low cost, high stability and excellent performance to modify electrodes. Metal-organic frameworks (MOFs) and their composites have the advantages of large surface area, high porosity and high catalytic efficiency, which can be utilized as excellent materials for electrode modification of non-enzymatic electrochemical glucose sensors. However, MOFs and their composites still face various challenges and difficulties that limit their further commercialization. This review introduces the applications and the challenges of MOFs and their composites in non-enzymatic electrochemical glucose sensors. Finally, an outlook on the development of MOFs and their composites is also presented.

Simultaneous Sensing of Cd(II), Pb(II), and Cu(II) Using Gold Nanoparticle-Modified APTES-Functionalized Indium Tin Oxide Electrode: Effect of APTES Concentration

In this work, indium tin oxide (ITO) electrodes were functionalized with varying 3-aminopropyltriethoxysilane (APTES) concentration percentages (0.5, 0.75, 1.0, and 2.0 wt %) to obtain the optimum conditions for the assembly of the as-synthesized gold nanoparticles (AuNPs). The AuNP coverage, wettability, and electrochemical performance of the modified electrodes were evaluated. The AuNP/0.75% APTES-ITO-modified electrode exhibited uniform coverage of AuNPs and high electrochemical performance for the simultaneous detection of Cd(II), Pb(II), and Cu(II) ions. Under the optimum conditions, the AuNP/0.75% APTES-ITO-modified electrode showed a linear detection range of 5-120 ppb and limit of detection of 0.73, 0.90, and 0.49 ppb for the simultaneous detection of Cd(II), Pb(II), and Cu(II) ions, respectively, via square wave anodic stripping voltammetry. The modified electrode demonstrated good anti-interference toward other heavy metal ions, good reproducibility, and suitability for application in environmental sample analysis.

A new Schiff base derived from 5-(thiophene-2-yl)oxazole as "off-on-off" fluorescence sensor for monitoring indium and ferric ions sequentially and its application

A new Schiff base sensor (E)-N'-((8-hydroxy-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)methylene)-5-(thiophen-2-yl)oxazole-4-carbohydrazide (TOQ) was synthesized and found to emit yellowish green fluorescence upon introduction of In³⁺. Furthermore, the resulting complex TOQ-In³⁺ was quenched selectively by Fe³⁺. The detection limits of TOQ for In³⁺ and Fe³⁺ were 1.75 × 10^-10 M and 8.45 × 10^-9 M, respectively. The complex stoichiometry of TOQ with target ions was determined to be 1:2 via Job's plot analysis, which further was verified by ESI-MS titration and theoretical calculations. Moreover, TOQ can be used for the determination of target ions in environmental water samples. A portable paper sensor of TOQ was successfully developed for detecting In³⁺ to assess its applicability.

Structural, Electronic Properties, and Relative Stability Studies of Low-Energy Indium Oxide Polytypes Using First-Principles Calculations

Materials made of indium oxide (In₂O₃) are now being used as a potential component of the next generation of computers and communication devices. Density functional theory is used to analyze the physical, electrical, and thermodynamical features of 12 low-energy bulk In₂O₃ polytypes. The cubic structure In₂O₃ is majorly used for many of the In₂O₃-based transparent conducting oxides. The objective of this study is to explore other new stable In₂O₃ polytypes that may exist. The structural properties and stability studies are performed using the Vienna ab initio simulation package code. All the In₂O₃ polytypes have semiconductive properties, according to electronic band structure investigations. The full elastic tensors and elastic moduli of all polytypes at 0 K are computed. Poisson's and Pugh's ratio confirms that all stable polytypes are ductile. The phonon and thermal properties including heat capacity are obtained for mechanically stable polytypes. For the first time, we report the Raman and infrared active modes of stable polytypes.

Simultaneous Voltammetric Determination of Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) Using a Modified Carbon Paste Electrode and Chemometrics

This work presents the simultaneous quantification of four non-steroidal anti-inflammatory drugs (NSAIDs), paracetamol, diclofenac, naproxen, and aspirin, in mixture solutions, by a laboratory-made working electrode based on carbon paste modified with multi-wall carbon nanotubes (MWCNT-CPE) and Differential Pulse Voltammetry (DPV). Preliminary electrochemical analysis was performed using cyclic voltammetry, and the sensor morphology was studied by scanning electronic microscopy and electrochemical impedance spectroscopy. The sample set ranging from 0.5 to 80 µmol L^-1 was prepared using a complete factorial design (3⁴) and considering some interferent species such as ascorbic acid, glucose, and sodium dodecyl sulfate to build the response model and an external randomly subset of samples within the experimental domain. A data compression strategy based on discrete wavelet transform was applied to handle voltammograms' complexity and high dimensionality. Afterward, Partial Least Square Regression (PLS) and Artificial Neural Networks (ANN) predicted the drug concentrations in the mixtures. PLS-adjusted models (n = 12) successfully predicted the concentration of paracetamol and diclofenac, achieving correlation values of R ≥ 0.9 (testing set). Meanwhile, the ANN model (four layers) obtained good prediction results, exhibiting R ≥ 0.968 for the four analyzed drugs (testing stage). Thus, an MWCNT-CPE electrode can be successfully used as a potential sensor for voltammetric determination and NSAID analysis.

Progress of Enzymatic and Non-Enzymatic Electrochemical Glucose Biosensor Based on Nanomaterial-Modified Electrode

This review covers the progress of nanomaterial-modified electrodes for enzymatic and non-enzymatic glucose biosensors. Fundamental insights into glucose biosensor components and the crucial factors controlling the electrochemical performance of glucose biosensors are discussed in detail. The metal, metal oxide, and hybrid/composite nanomaterial fabrication strategies for the modification of electrodes, mechanism of detection, and significance of the nanomaterials toward the electrochemical performance of enzymatic and non-enzymatic glucose biosensors are compared and comprehensively reviewed. This review aims to provide readers with an overview and underlying concept of producing a reliable, stable, cost-effective, and excellent electrochemical performance of a glucose biosensor.

Amanitins in Wild Mushrooms: The Development of HPLC-UV-EC and HPLC-DAD-MS Methods for Food Safety Purposes

Mushroom poisoning remains a serious food safety and health concern in some parts of the world due to its morbidity and mortality. Identification of mushroom toxins at an early stage of suspected intoxication is crucial for a rapid therapeutic decision. In this study, a new extraction method was developed to determine α- and β-amanitin in mushroom samples collected from central Portugal. High-performance liquid chromatography with in-line ultraviolet and electrochemical detection was implemented to improve the specificity of the method. The method was fully validated for linearity (0.5-20.0 µg·mL^-1), sensitivity, recovery, and precision based on a matrix-matched calibration method. The limit of detection was 55 µg mL^-1 (UV) and 62 µg mL^-1 (EC) for α-amanitin and 64 µg mL^-1 (UV) and 24 µg mL^-1 (EC) for β-amanitin. Intra- and inter-day precision differences were less than 13%, and the recovery ratios ranged from 89% to 117%. The developed method was successfully applied to fourteen Amanita species (A. sp.) and compared with five edible mushroom samples after extraction with Oasis^® PRIME HLB cartridges without the conditioning and equilibration step. The results revealed that the A. phalloides mushrooms present the highest content of α- and β-amanitin, which is in line with the HPLC-DAD-MS. In sum, the developed analytical method could benefit food safety assessment and contribute to food-health security, as it is rapid, simple, sensitive, accurate, and selectively detects α- and β-amanitin in any mushroom samples.

Gold-Nanoparticle-Coated Magnetic Beads for ALP-Enzyme-Based Electrochemical Immunosensing in Human Plasma

A simple and sensitive AuNP-coated magnetic beads (AMB)-based electrochemical biosensor platform was fabricated for bioassay. In this study, AuNP-conjugated magnetic particles were successfully prepared using biotin-streptavidin conjugation. The morphology and structure of the nanocomplex were characterized by scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDX) and UV-visible spectroscopy. Moreover, cyclic voltammetry (CV) was used to investigate the effect of AuNP-MB on alkaline phosphatase (ALP) for electrochemical signal enhancement. An ALP-based electrochemical (EC) immunoassay was performed on the developed AuNP-MB complex with indium tin oxide (ITO) electrodes. Subsequently, the concentration of capture antibodies was well-optimized on the AMB complex via biotin-avidin conjugation. Lastly, the developed AuNP-MB immunoassay platform was verified with extracellular vesicle (EV) detection via immune response by showing the existence of EGFR proteins on glioblastoma multiforme (GBM)-derived EVs (10⁸ particle/mL) spiked in human plasma. Therefore, the signal-enhanced ALP-based EC biosensor on AuNP-MB was favorably utilized as an immunoassay platform, revealing the potential application of biosensors in immunoassays in biological environments.

Cyclic and Linear Sweep Voltammetric Studies of a Modified Carbon Paste Electrode with Nickel Oxide Nanoparticles toward Tamoxifen: Effects of Surface Modification on Electrode Response Kinetics

Due to the serious adverse futures of some anticancer drugs, the determination of trace amounts of these drugs by simple analytical techniques is of great interest. In this regard, knowing about the mechanism of the analyte with the sensing material plays an important role. Nickel oxide nanoparticles (NiO NPs) modified by a carbon paste electrode (NiO-CPE) showed an irreversible cyclic voltammetric (CV) behavior in the NaOH (pH 13) supporting electrolyte based on the peak separation of 311 mV. Its peak current was decreased by adding tamoxifen (TAM), confirming that TAM molecules can consume NiO before participating in the electrode reaction. For this goal, TAM can be oxidized or reduced, and the corresponding mechanisms are schematically illustrated in the text. This study focused on the kinetic aspects of the process. Based on the CV results, a surface coverage (Γ) value of 2.72 × 10^-5 mol NiO per cm² was obtained with charge transfer coefficients α_a and α_c of 0.317 and 0.563, respectively. α_a and α_c values were changed to 0.08 and 0.72 in the presence of TAM. Further, the rate constant (k _s) value was 0.021 ± 0.01 s^-1 in the presence of TAM. In linear sweep voltammetry (LSV), an α value of about 0.636 ± 0.023 and an exchange rate constant (k _o) value of about 0.097 ± 0.031 s^-1 were obtained in the absence of TAM, which changed to 0.62 ± 0.081 and 0.089 ± 0.021 s^-1 in the presence of TAM, respectively. Despite more published papers, when the TAM analyte was added to the NaOH supporting electrolyte, both anodic and cathodic peak currents of the modified NiO-CPE decreased. We suggested some reasons for this decreased peak current, and four mechanisms were illustrated for the electrode response in the presence of TAM.

Investigation on Transparent, Conductive ZnO:Al Films Deposited by Atomic Layer Deposition Process

Transparent electrodes are a core component for transparent electron devices, photoelectric devices, and advanced displays. In this work, we fabricate fully-transparent, highly-conductive Al-doped ZnO (AZO) films using an atomic layer deposition (ALD) system method of repeatedly stacking ZnO and Al₂O₃ layers. The influences of Al cycle ratio (0, 2, 3, and 4%) on optical property, conductivity, crystallinity, surface morphology, and material components of the AZO films are examined, and current conduction mechanisms of the AZO films are analyzed. We found that Al doping increases electron concentration and optical bandgap width, allowing the AZO films to excellently combine low resistivity with high transmittance. Besides, Al doping induces preferred-growth-orientation transition from (002) to (100), which improves surface property and enhances current conduction across the AZO films. Interestingly, the AZO films with an Al cycle ratio of 3% show preferable film properties. Transparent ZnO thin film transistors (TFTs) with AZO electrodes are fabricated, and the ZnO TFTs exhibit superior transparency and high performance. This work accelerates the practical application of the ALD process in fabricating transparent electrodes.