Composite Silica-Based Films as Platforms for Electrochemical Sensors.

Sol-gel-derived silica thin films generated onto electrode surfaces in the form of organic-inorganic hybrid coatings or other composite layers have found tremendous interest for being used as platforms for the development of electrochemical sensors and biosensors. After a brief description of the strategies applied to prepare such materials, and their interest as electrode modifier, this review will summarize the major advances made so far with composite silica-based films in electroanalysis. It will primarily focus on electrochemical sensors involving both non-ordered composite films and vertically oriented mesoporous membranes, the biosensors exploiting the concept of sol-gel bioencapsulation on electrode, the spectroelectrochemical sensors, and some others.

An enzyme-free sensor based on La-doped CoFe-layered double hydroxide decorated on reduced graphene oxide for sensitive electrochemical detection of urea.

The successful synthesis of La-doped CoFe LDH@rGO nanocomposite is reported combining the advantages of LDH and rGO and shows promising performances in electrochemical sensors. The structure of the obtained nanocomposite was investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction pattern (XRD), and field emission scanning electron microscope images (FE-SEM). Then, it was directly utilized to construct a carbon paste electrode (CPE) for urea detection. The electrochemical performance of the sensor was evaluated by various electrochemical methods. The La-CoFe LDH@rGO electrode exhibited excellent electrocatalytic properties, including a wide linear working range of 0.001-23.5 mM, very high sensitivity of 1.07 ± 0.023 µA µM-1 cm-2, a low detection limit of 0.33 ± 0.11 µM, and rapid response time of 5 s towards urea detection at the working potential of 0.4 V. Furthermore, the sensor displayed a high selectivity in different matrices, appropriate reproducibility, and long shelf life without activity loss during 3 months of storage under ambient conditions. Further tests were performed on serum and milk samples to confirm the capability of the proposed sensor for practical applications, demonstrating a reasonable recovery of 94.8 to 102% with an RSD value below 3%. Consequently, the synergistic effect of each component led to the good electrocatalytic activity of the modified electrode towards urea.

Ratiometric electrochemical immunosensor for simultaneous detection of C-myc and Bcl-2 based on multi-role alloy composites.

A ratiometric electrochemical immunosensor is proposed for simultaneous detection of cellular-myelocytomatosis oncoprotein (C-myc) and B-cell lymphoma 2 (Bcl-2) via the potential-resolved strategy. It relied on multi-role co-loaded alloy composites (CLACs) and poly(3,4-ethylenedioxythiophene) (PEDOT)-graphene oxide (GO)-multiwalled carbon nanotubes (MWCNTs) (PGM) modified electrodes. CLACs with good catalytic and enzyme-like properties were synthesized in one step by loading tetramethylbenzidine (TMB) or methylene blue (MB) into Pt-Pd alloy and used as label materials. After immunological reactions, CLACs showed distinguishable dual differential pulse voltammetry signals at - 0.26 V and 0.38 V, corresponding to C-myc and Bcl-2, and the PGM had an electrochemical signal at 1.2 V, which could be used as a reference signal to construct a ratiometric sensor. CLACs had a satisfactory synergistic effect with the PGM, and eventually achieved quadruple signal amplification. Thus, benefiting from multiple magnification and ratiometric self-calibration functions, sensitive detections of C-myc and Bcl-2 were achieved, with detection limits as low as 0.5 and 2.5 pg mL-1, respectively. Additionally, when the designed method was applied to blood samples from lymphoma patients, results consistent with the ELISA kit were obtained. This will open avenues for constructing multiple protein detection sensors.

Voltammetric Sensor Based on the Combination of Tin and Cerium Dioxide Nanoparticles with Surfactants for Quantification of Sunset Yellow FCF.

Sunset Yellow FCF (SY FCF) is one of the widely used synthetic azo dyes in the food industry whose content has to be controlled for safety reasons. Electrochemical sensors are a promising tool for this type of task. A voltammetric sensor based on a combination of tin and cerium dioxide nanoparticles (SnO2-CeO2 NPs) with surfactants has been developed for SY FCF determination. The synergetic effect of both types of NPs has been confirmed. Surfactants of various natures (sodium lauryl sulfate (SLS), Brij® 35, and hexadecylpyridinium bromide (HDPB)) have been tested as dispersive media. The best effects, i.e., the highest oxidation currents of SY FCF, have been observed in the case of HDPB. The sensor demonstrates a 4.5-fold-higher electroactive surface area and a 38-fold-higher electron transfer rate compared to the bare glassy carbon electrode (GCE). The electrooxidation of SY FCF is an irreversible, two-electron, diffusion-driven process involving proton transfer. In differential pulse mode in Britton-Robinson buffer (BRB) pH 2.0, the sensor gives a linear response to SY FCF from 0.010 to 1.0 µM and from 1.0 to 100 µM with an 8.0 nM detection limit. The absence of an interferent effect from other typical food components and colorants has been shown. The sensor has been tested on soft drinks and validated with the standard chromatographic method.

Modification of Glassy Carbon Electrodes with Complexes of Manganese(II) with Some Phenanthroline Derivatives Immobilized in Nafion Layer.

Manganese(II) complexes with phenanthroline derivatives modified with different substituents were synthesized and incorporated into Nafion layers covering the surfaces of glassy carbon electrodes and were studied electrochemically. Formal potentials and apparent diffusion coefficients were calculated and discussed. The suitability for electrocatalytic oxidation of ascorbic acid and glycolic acid was examined. The surfaces of modified electrodes were characterized using atomic force microscopy.

Harnessing Graphene-Modified Electrode Sensitivity for Enhanced Ciprofloxacin Detection.

Increased evidence has documented a direct association between Ciprofloxacin (CFX) intake and significant disruption to the normal functions of connective tissues, leading to severe health conditions (such as tendonitis, tendon rupture and retinal detachment). Additionally, CFX is recognized as a potential emerging pollutant, as it seems to impact both animal and human food chains, resulting in severe health implications. Consequently, there is a compelling need for the precise, swift and selective detection of this fluoroquinolone-class antibiotic. Herein, we present a novel graphene-based electrochemical sensor designed for Ciprofloxacin (CFX) detection and discuss its practical utility. The graphene material was synthesized using a relatively straightforward and cost-effective approach involving the electrochemical exfoliation of graphite, through a pulsing current, in 0.05 M sodium sulphate (Na2SO4), 0.05 M boric acid (H3BO3) and 0.05 M sodium chloride (NaCl) solution. The resulting material underwent systematic characterization using scanning electron microscopy/energy dispersive X-ray analysis, X-ray powder diffraction and Raman spectroscopy. Subsequently, it was employed in the fabrication of modified glassy carbon surfaces (EGr/GC). Linear Sweep Voltammetry studies revealed that CFX experiences an irreversible oxidation process on the sensor surface at approximately 1.05 V. Under optimal conditions, the limit of quantification was found to be 0.33 × 10-8 M, with a corresponding limit of detection of 0.1 × 10-8 M. Additionally, the developed sensor's practical suitability was assessed using commercially available pharmaceutical products.

Laser-induced graphene electrodes on polyimide membranes modified with gold nanoparticles for the simultaneous detection of dopamine and uric acid in human serum.

The level control of biological active molecules in human body fluids is important for the surveillance of several human diseases. Dopamine (DA) and uric acid (UA) are two important biomarkers of neurological and bone diseases, respectively. Design of sensitive and cost-effective sensors for their detection is an effervescent research field. We report on the straightforward design of laser-induced graphene electrodes (LIGEs) from the laser ablation of a polyimide substrate and their modification by electrochemical deposition of gold nanoparticles (AuNPs/LIGE) and their uses as chemosensors. Electrochemical investigations showed that the presence of gold nanoclusters onto the electrode surface improved the electrochemical surface area (ECSA) and the heterogenous electron transfer (HET) rate. Furthermore, the AuNPs/LIGEs can be used to detect simultaneously low concentrations of DA and UA in presence of ascorbic acid (AA) as an potentially interfering substance at redox potentials of 300 mV, 230 mV and 450 mV and 91 mV, respectively, compared with the Ag/AgCl (3 M KCl) reference electrode in cyclic voltametric. The method displayed linear ranges varying from 2 to 20 µM and 5 to 50 µM, led to limits of detection of 0.37 µM and 0.71 µM for DA and UA, respectively. The AuNPs/LIGE was applied to simultaneously detect both analytes in scarcely diluted human serum with good recoveries. The data show that the recovery percentages ranged from 94% ± 2.1 to 102 % ± 0.5 and from 94% ±0.3 to 112% ± 1.4 for dopamine and uric acid, respectively. Thus, the AuNPs/LIGEs are promising candidates for the detection of other biologically active molecules such as drugs, pesticides, and metabolites.

Electrochemical sensing platform based on screen-printed carbon electrode modified with plasma polymerized acrylonitrile nanofilms for determination of bupropion.

A original electrochemical sensing platform, based on screen-printed electrodes modification with plasma polymerized acrylonitrile (pp-AN) nanofilms is proposed. For that purpose, plasma-enhanced chemical vapor deposition (PECVD) process was conducted in a parallel plate (13.56 MHz) plasma reactor for 2 min with discharge power of 10 W. The surface topography and electrochemical properties of prepared sensors were investigated by X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersion spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry. The electrochemical characteristics of pp-AN/SPCE and pp-AN/SPAuE sensors was investigated for model redox pair [Fe(CN)6]4-/3-. Conducted research confirmed the excellent chemical stability, durability, wide potential window, high signal-to-noise (S/N) ratio, and, most importantly, the ability to standardize the sensors. The pp-AN/SPCE sensor was applied to the determination of bupropion, an antidepressant drug whose intake has increased dramatically during the COVID-19 pandemic. The voltammetric response of pp-AN/SPCE for BUP was linear in two concentration ranges of 0.63-10.0 and 10.0-50.0 µmol L-1, with a detection limit of 0.21 µmol L-1. Satisfactory recoveries (96.2-102%) and good precision (RSD below 4.1%) obtained for environmental and biological samples confirmed the usefulness of the sensor for the analysis of various kinds of samples.

Exploring the potential of molecularly imprinted polymers and metal/metal oxide nanoparticles in sensors: recent advancements and prospects.

Metal/metal oxide nanoparticles have gained increasing attention in recent years due to their outstanding features, including optical and catalytic properties, as well as their excellent conductivity. The implementation of metal/metal oxide nanoparticles, combined with molecularly imprinted polymers (MIPs) has paved the way for a new generation of building blocks to engineer and enhance the fascinating features of advanced sensors. This review critically evaluates the impact of combining metal/metal oxide nanoparticles with MIPs in sensors. It covers synthesis strategies, advantages of coupling these materials with MIPs, and addresses questions about the selectivity of these hybrid materials. In the end, the current challenges and future perspectives of this field are discussed, with a particular focus on the potential applications of these hybrid composites in the sensor field. This review highlights the exciting opportunities of using metal/metal oxide nanoparticles along with MIPs for the development of next-generation sensors.

Selective Voltammetric Sensor for the Simultaneous Quantification of Tartrazine and Brilliant Blue FCF.

Tartrazine and brilliant blue FCF are synthetic dyes used in the food, cosmetic and pharmaceutical industries. The individual and/or simultaneous control of their concentrations is required due to dose-dependent negative health effects. Therefore, the paper presents experimental results related to the development of a sensing platform for the electrochemical detection of tartrazine and brilliant blue FCF based on a glassy carbon electrode (GCE) modified with MnO2 nanorods, using anodic differential pulse voltammetry. Homogeneous and stable suspensions of MnO2 nanorods have been obtained involving cetylpyridinium bromide solution as a cationic surfactant. The MnO2 nanorods-modified electrode showed a 7.9-fold increase in the electroactive surface area and a 72-fold decrease in the electron transfer resistance. The developed sensor allowed the simultaneous quantification of dyes for two linear domains: in the ranges of 0.10-2.5 and 2.5-15 µM for tartrazine and 0.25-2.5 and 2.5-15 µM for brilliant blue FCF with detection limits of 43 and 41 nM, respectively. High selectivity of the sensor response in the presence of typical interference agents (inorganic ions, saccharides, ascorbic and sorbic acids), other food dyes (riboflavin, indigo carmine, and sunset yellow), and vanillin has been achieved. The sensor has been tested by analyzing soft and isotonic sports drinks and the determined concentrations were close to those obtained involving the chromatography technique.

Voltammetric Sensor Based on the Poly(p-aminobenzoic Acid) for the Simultaneous Quantification of Aromatic Aldehydes as Markers of Cognac and Brandy Quality.

Cognac and brandy quality control is an actual topic in food analysis. Aromatic aldehydes, particularly syringaldehyde and vanillin, are one of the markers used for these purposes. Therefore, simple and express methods for their simultaneous determination are required. The voltammetric sensor based on the layer-by-layer combination of multi-walled carbon nanotubes (MWCNTs) and electropolymerized p-aminobenzoic acid (p-ABA) provides full resolution of the syringaldehyde and vanillin oxidation peaks. Optimized conditions of p-ABA electropolymerization (100 µM monomer in Britton-Robinson buffer pH 2.0, twenty cycles in the polarization window of -0.5 to 2.0 V with a potential scan rate of 100 mV·s-1) were found. The poly(p-ABA)-based electrode was characterized by scanning electron microscopy (SEM), cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). Electrooxidation of syringaldehyde and vanillin is an irreversible two-electron diffusion-controlled process. In the differential pulse mode, the sensor allows quantification of aromatic aldehydes in the ranges of 0.075-7.5 and 7.5-100 µM for syringaldehyde and 0.50-7.5 and 7.5-100 µM for vanillin with the detection limits of 0.018 and 0.19 µM, respectively. The sensor was applied to cognac and brandy samples and compared to chromatography.

Electrochemical Chemically Based Sensors and Emerging Enzymatic Biosensors for Antidepressant Drug Detection: A Review.

Major depressive disorder is a widespread condition with antidepressants as the main pharmacological treatment. However, some patients experience concerning adverse reactions or have an inadequate response to treatment. Analytical chromatographic techniques, among other techniques, are valuable tools for investigating medication complications, including those associated with antidepressants. Nevertheless, there is a growing need to address the limitations associated with these techniques. In recent years, electrochemical (bio)sensors have garnered significant attention due to their lower cost, portability, and precision. Electrochemical (bio)sensors can be used for various applications related to depression, such as monitoring the levels of antidepressants in biological and in environmental samples. They can provide accurate and rapid results, which could facilitate personalized treatment and improve patient outcomes. This state-of-the-art literature review aims to explore the latest advancements in the electrochemical detection of antidepressants. The review focuses on two types of electrochemical sensors: Chemically modified sensors and enzyme-based biosensors. The referred papers are carefully categorized according to their respective sensor type. The review examines the differences between the two sensing methods, highlights their unique features and limitations, and provides an in-depth analysis of each sensor.

Graphene-Based Electrochemical Sensing Platform for Rapid and Selective Ferulic Acid Quantification.

Due to the multitude of physiological functions, ferulic acid (FA) has a wide range of applications in the food, cosmetic, and pharmaceutical industries. Thus, the development of rapid, sensitive, and selective detection tools for its assay is of great interest. This study reports a new electroanalytical approach for the quantification of ferulic acid in commercial pharmaceutical samples using a sulphur-doped graphene-based electrochemical sensing platform. The few-layer graphene material (exf-SGR) was prepared by the electrochemical oxidation of graphite, at a low applied bias (5 V), in an inorganic salt mixture of Na2S2O3/(NH4)2SO4 (0.3 M each). According to the morpho-structural characterization of the material, it appears to have a high heteroatom doping degree, as proved by the presence of sulphur lines in the XRD pattern, and the C/S ratio was determined by XPS investigations to be 11.57. The electrochemical performances of a glassy carbon electrode modified with the exf-SGR toward FA detection were tested by cyclic voltammetry in both standard laboratory solutions and real sample analysis. The developed modified electrode showed a low limit of detection (30.3 nM) and excellent stability and reproducibility, proving its potential applicability as a viable solution in FA qualitative and quantitative analysis.

Ni(II)-Salophen-Comprehensive Analysis on Electrochemical and Spectral Characterization and Biological Studies.

New aspects of the Ni(II)-salophen complex and salophen ligand precursor were found during deep electrochemical and optical characterization, as well as biological studies for new pharmacological applications. Physicochemical and spectroscopic methods (1H- and 13C-NMR, FT-IR and UV-Vis, electrospray ionization mass spectroscopy, thermogravimetric analysis, and molar conductance measurements) were also used to prove that the salophen ligand acts as a tetradentate and coordinates to the central metal through nitrogen and oxygen atoms. The electrochemical behavior of the free Schiff salophen ligand (H2L) and its Ni(II) complex (Ni(II)L) was deeply studied in tetrabutylammonium perchlorate solutions in acetonitrile via CV, DPV, and RDE. Blue films on the surfaces of the electrodes as a result of the electropolymerization processes were put in evidence and characterized via CV and DPV. (H2L) and Ni(II)L complexes were tested for their antimicrobial, antifungal, and antioxidant activity, showing good antimicrobial and antifungal activity against several bacteria and fungi.

Electrochemical Characterization of the Antioxidant Properties of Medicinal Plants and Products: A Review.

Medicinal plants are an important source of bioactive compounds with a wide spectrum of practically useful properties. Various types of antioxidants synthesized in plants are the reasons for their application in medicine, phytotherapy, and aromatherapy. Therefore, reliable, simple, cost-effective, eco-friendly, and rapid methods for the evaluation of antioxidant properties of medicinal plants and products on their basis are required. Electrochemical methods based on electron transfer reactions are promising tools to solve this problem. Total antioxidant parameters and individual antioxidant quantification can be achieved using suitable electrochemical techniques. The analytical capabilities of constant-current coulometry, potentiometry, various types of voltammetry, and chrono methods in the evaluation of total antioxidant parameters of medicinal plants and plant-derived products are presented. The advantages and limitations of methods in comparison to each other and traditional spectroscopic methods are discussed. The possibility to use electrochemical detection of the antioxidants via reactions with oxidants or radicals (N- and O-centered) in solution, with stable radicals immobilized on the electrode surface, via oxidation of antioxidants on a suitable electrode, allows the study of various mechanisms of antioxidant actions occurring in living systems. Attention is also paid to the individual or simultaneous electrochemical determination of antioxidants in medicinal plants using chemically modified electrodes.

Organic Additive-derived Films on Cu Electrodes Promote Electrochemical CO2 Reduction to C2+ Products Under Strongly Acidic Conditions.

Electrochemical CO2 reduction (CO2 R) at low pH is desired for high CO2 utilization; the competing hydrogen evolution reaction (HER) remains a challenge. High alkali cation concentration at a high operating current density has recently been used to promote electrochemical CO2 R at low pH. Herein we report an alternative approach to selective CO2 R (>70 % Faradaic efficiency for C2+ products, FEC2+ ) at low pH (pH 2; H3 PO4 /KH2 PO4 ) and low potassium concentration ([K+ ]=0.1 M) using organic film-modified polycrystalline copper (Modified-Cu). Such an electrode effectively mitigates HER due to attenuated proton transport. Modified-Cu still achieves high FEC2+ (45 % with Cu foil /55 % with Cu GDE) under 1.0 M H3 PO4 (pH≈1) at low [K+ ] (0.1 M), even at low operating current, conditions where HER can otherwise dominate.

On the Mediated Electron Transfer of Immobilized Galactose Oxidase for Biotechnological Applications.

The use of enzymes as catalysts in chemical synthesis offers advantages in terms of clean and highly selective transformations. Galactose oxidase (GalOx) is a remarkable enzyme with several applications in industrial conversions as it catalyzes the oxidation of primary alcohols. We have investigated the wiring of GalOx with a redox polymer; this enables mediated electron transfer with the electrode surface for its potential application in biotechnological conversions. As a result of electrochemical regeneration of the catalytic center, the formation of harmful H2 O2 is minimized during enzymatic catalysis. The introduced bioelectrode was applied to the conversion of bio-renewable platform materials, with glycerol as model substrate. The biocatalytic transformations of glycerol and 5-hydroxymethylfurfural (HMF) were investigated in a circular flow-through setup to assess the possibility of substrate over-oxidation, which is observed for glycerol oxidation but not during HMF conversion.

Electrochemically Deposited Molecularly Imprinted Polymer-Based Sensors.

This review is dedicated to the development of molecularly imprinted polymers (MIPs) and the application of MIPs in sensor design. MIP-based biological recognition parts can replace receptors or antibodies, which are rather expensive. Conducting polymers show unique properties that are applicable in sensor design. Therefore, MIP-based conducting polymers, including polypyrrole, polythiophene, poly(3,4-ethylenedioxythiophene), polyaniline and ortho-phenylenediamine are frequently applied in sensor design. Some other materials that can be molecularly imprinted are also overviewed in this review. Among many imprintable materials conducting polymer, polypyrrole is one of the most suitable for molecular imprinting of various targets ranging from small organics up to rather large proteins. Some attention in this review is dedicated to overview methods applied to design MIP-based sensing structures. Some attention is dedicated to the physicochemical methods applied for the transduction of analytical signals. Expected new trends and horizons in the application of MIP-based structures are also discussed.

Enhanced Acetaminophen Electrochemical Sensing Based on Nitrogen-Doped Graphene.

Because of the widespread acetaminophen usage and the danger of harmful overdosing effects, developing appropriate procedures for its quantitative and qualitative assay has always been an intriguing and fascinating problem. A quick, inexpensive, and environmentally friendly approach based on direct voltage anodic graphite rod exfoliation in the presence of inorganic salt aqueous solution ((NH4)2SO4-0.3 M) has been established for the preparation of nitrogen-doped graphene (exf-NGr). The XRD analysis shows that the working material appears as a mixture of few (76.43%) and multi-layers (23.57%) of N-doped graphenes. From XPS, the C/O ratio was calculated to be 0.39, indicating a significant number of structural defects and the existence of multiple oxygen-containing groups at the surface of graphene sheets caused by heteroatom doping. Furthermore, the electrochemical performances of glassy carbon electrodes (GCEs) modified with exf-NGr for acetaminophen (AMP) detection and quantification have been assessed. The exf-NGr/GCE-modified electrode shows excellent reproducibility, stability, and anti-interfering characteristics with improved electrocatalytic activity over a wide detection range (0.1-100 µM), with a low limit for AMP detection (LOD = 3.03 nM). In addition, the developed sensor has been successfully applied in real sample analysis for the AMP quantification from different commercially available pharmaceutical formulations.

Electrochemical Impedance Spectroscopy in the Characterisation and Application of Modified Electrodes for Electrochemical Sensors and Biosensors.

Electrochemical impedance spectroscopy is finding increasing use in electrochemical sensors and biosensors, both in their characterisation, including during successive phases of sensor construction, and in application as a quantitative determination technique. Much of the published work continues to make little use of all the information that can be furnished by full physical modelling and analysis of the impedance spectra, and thus does not throw more than a superficial light on the processes occurring. Analysis is often restricted to estimating values of charge transfer resistances without interpretation and ignoring other electrical equivalent circuit components. In this article, the important basics of electrochemical impedance for electrochemical sensors and biosensors are presented, focussing on the necessary electrical circuit elements. This is followed by examples of its use in characterisation and in electroanalytical applications, at the same time demonstrating how fuller use can be made of the information obtained from complete modelling and analysis of the data in the spectra, the values of the circuit components and their physical meaning. The future outlook for electrochemical impedance in the sensing field is discussed.