Electrochemical Sensors Based on the Electropolymerized Natural Phenolic Antioxidants and Their Analytical Application

Lab on skin: real-time metabolite monitoring with polyphenol film based subdermal wearable patches

The advent of digital technologies has spurred the development of wearable sensing devices marking a significant shift in obtaining real-time physiological information. The principal objective is to transition from blood-centric monitoring to minimally invasive modalities, which will enable movement from specialised settings to more accessible environments such as the practices of general practitioners or even home settings. While subcutaneously implanted continuous monitoring devices have demonstrated this transition, detection of analytes from sample matrices like skin interstitial fluid (ISF), is a frontier that offers attractive minimally invasive routes for detection of biomarkers. This manuscript presents a comprehensive overview of our work in subdermal wearable biosensing patches for the simultaneous monitoring of glucose and lactate from ISF in ambulatory conditions. The performance of the subdermal wearable glucose monitoring patch was evaluated over a duration of three days, which is the longest reported duration reported till date. The subdermal wearable lactate sensing patch was worn for the duration of the exercise. Our findings highlight a critical observation that biofouling effects become apparent after a 24 h period. The data presented in this manuscript extends on the knowledge in the areas of continuous metabolite monitoring by introducing multifunctional polyphenol polymer films that can be used for both glucose and lactate monitoring with appropriate modifications. This study underscores the potential of subdermal wearable patches as versatile tools for real-time metabolite monitoring, positioning them as valuable assets in the evolution of personalised healthcare in diverse settings.

Iodine-Doped Hollow Carbon Nanocages without Templates Strategy for Boosting Zinc-Ion Storage by Nucleophilicity

Zn-ion hybrid supercapacitors (ZHCs) combining merits of battery-type and capacitive electrodes are considered to be a prospective candidate in energy storage systems. Tailor-made carbon cathodes with high zincophilicity and abundant physi/chemisorption sites are critical but it remains a great challenge to achieve both features by a sustainable means. Herein, a hydrogen-bonding interaction-guided self-assembly strategy is presented to prepare iodine-doped carbon nanocages without templates for boosting zinc-ion storage by nucleophilicity. The biomass ellagic acid contains extensional hydroxy and acyloxy groups with electron-donating ability, which interact with melamine and ammonium iodide to form organic supermolecules. The organic supermolecules further self-assemble into a nanocage-like structure with cavities under hydrothermal processes via hydrogen-bonding and π-π stacking. The carbon nanocages as ZHCs cathodes enable the high approachability of zincophilic sites and low ion migration resistance resulting from the interconnected conductive network and nanoscale architecture. The experimental analyses and theoretical simulations reveal the pivotal role of iodine dopants. The I5-/I3- doping anions in carbon cathodes have a nucleophilicity to preferentially adsorb the Zn2+ cation by the formation of C+-I5--Zn2+ and C+-I3--Zn2+. Of these, the C+-I3- shows stronger bonding with Zn2+ than C+-I5-. As a result, the iodine-doped carbon nanocages produced via this template-free strategy deliver a high capacity of 134.2 mAh/g at 1 A/g and a maximum energy and power density of 114.1 Wh/kg and 42.5 kW/kg.

Electrochemical DNA Sensor for Valrubicin Detection Based on Poly(Azure C) Films Deposited from Deep Eutectic Solvent

A novel electrochemical DNA sensor was developed for the detection of the anthracycline drug, valrubicin, on the base of poly(Azure C) electropolymerized from the deep eutectic solvent reline and covered with adsorbed DNA from calf thymus. Biosensor assembling was performed by multiple scanning of the potential in one drop (100 µL) of the dye dissolved in reline and placed on the surface of a screen-printed carbon electrode. Stabilization of the coating was achieved by its polarization in the phosphate buffer. The electrochemical characteristics of the electron transfer were determined and compared with a similar coating obtained from phosphate buffer. The use of deep eutectic solvent made it possible to increase the monomer concentration and avoid using organic solvents on the stage of electrode modification. After the contact of the DNA sensor with valrubicin, two signals related to the intrinsic redox activity of the coating and the drug redox conversion were found on voltammogram. Their synchronous changes with the analyte concentration increased the reliability of the detection. In the square-wave mode, the DNA sensor made it possible to determine from 3 µM to 1 mM (limit of detection, 1 µM) in optimal conditions. The DNA sensor was successfully tested in the voltammetric determination of valrubicin in spiked artificial urine, Ringer-Locke solution mimicking plasma electrolytes and biological samples (urine and saliva) with a recovery of 90-110%. After further testing on clinical samples, it can find application in the pharmacokinetics studies and screening of new drugs' interaction with DNA.

Sustainable Electropolymerization of Zingerone and Its C2 Symmetric Dimer for Amperometric Biosensor Films

Polymeric permselective films are frequently used for amperometric biosensors to prevent electroactive interference present in the target matrix. Phenylenediamines are the most commonly used for the deposition of shielding polymeric films against interfering species; however, even phenolic monomers have been utilized in the creation of these films for microsensors and biosensors. The purpose of this paper is to evaluate the performances of electrosynthesized polymers, layered by means of constant potential amperometry (CPA), of naturally occurring compound zingerone (ZING) and its dimer dehydrozingerone (ZING DIM), which was obtained by straight oxidative coupling reaction. The polymers showed interesting shielding characteristics against the main interfering species, such as ascorbic acid (AA): actually, polyZING exhibited an AA shielding aptitude comprised between 77.6 and 99.6%, comparable to that obtained with PPD. Moreover, a marked capability of increased monitoring of hydrogen peroxide (HP), when data were compared with bare metal results, was observed. In particular, polyZING showed increases ranging between 55.6 and 85.6%. In the present work, the molecular structures of the obtained polymers have been theorized and docking analyses were performed to understand their peculiar characteristics better. The structures were docked using the Lamarckian genetic algorithm (LGA). Glutamate biosensors based on those polymers were built, and their performances were compared with biosensors based on PPD, which is the most widespread polymer for the construction of amperometric biosensors.

Recent Advances in Electrochemical Sensors for Sulfur-Containing Antioxidants

Sulfur-containing antioxidants are an important part of the antioxidant defense systems in living organisms under the frame of a thiol-disulfide equilibrium. Among them, l-cysteine, l-homocysteine, l-methionine, glutathione, and α-lipoic acid are the most typical representatives. Their actions in living systems are briefly discussed. Being electroactive, sulfur-containing antioxidants are interesting analytes to be determined using various types of electrochemical sensors. Attention is paid to the chemically modified electrodes with various nanostructured coverages. The analytical capabilities of electrochemical sensors for sulfur-containing antioxidant quantification are summarized and discussed. The data are summarized and presented on the basis of the electrode surface modifier applied, i.e., carbon nanomaterials, metal and metal oxide nanoparticles (NPs) and nanostructures, organic mediators, polymeric coverage, and mixed modifiers. The combination of various types of nanomaterials provides a wider linear dynamic range, lower limits of detection, and higher selectivity in comparison to bare electrodes and sensors based on the one type of surface modifier. The perspective of the combination of chromatography with electrochemical detection providing the possibility for simultaneous determination of sulfur-containing antioxidants in a complex matrix has also been discussed.

Layer-by-Layer Combination of MWCNTs and Poly(ferulic acid) as Electrochemical Platform for Hesperidin Quantification

The electrochemical polymerization of suitable monomers is a powerful way to create voltammetric sensors with improved responses to a target analyte. Nonconductive polymers based on phenolic acids were successfully combined with carbon nanomaterials to obtain sufficient conductivity and high surface area of the electrode. Glassy carbon electrodes (GCE) modified with multi-walled carbon nanotubes (MWCNTs) and electropolymerized ferulic acid (FA) were developed for the sensitive quantification of hesperidin. The optimized conditions of FA electropolymerization in basic medium (15 cycles from -0.2 to 1.0 V at 100 mV s^-1 in 250 µmol L^-1 monomer solution in 0.1 mol L^-1 NaOH) were found using the voltammetric response of hesperidin. The polymer-modified electrode exhibited a high electroactive surface area (1.14 ± 0.05 cm² vs. 0.75 ± 0.03 and 0.089 ± 0.003 cm² for MWCNTs/GCE and bare GCE, respectively) and decreased in the charge transfer resistance (21.4 ± 0.9 kΩ vs. 72 ± 3 kΩ for bare GCE). Under optimized conditions, hesperidin linear dynamic ranges of 0.025-1.0 and 1.0-10 µmol L^-1 with a detection limit of 7.0 nmol L^-1 were achieved, which were the best ones among those reported to date. The developed electrode was tested on orange juice and compared with chromatography.

Electrode Based on the MWCNTs and Electropolymerized Thymolphthalein for the Voltammetric Determination of Total Isopropylmethylphenols in Spices

Isopropylmethylphenols, namely thymol and carvacrol, are natural phenolic monoterpenoids with a wide spectrum of bioactivity making them applicable in the cosmetic, pharmaceutical, and food industry. The dose-dependent antioxidant properties of isopropylmethylphenols require their quantification in real samples. Glassy carbon electrode (GCE) modified with multi-walled carbon nanotubes (MWCNTs) and electropolymerized thymolphthalein has been developed for the sensitive quantification of isopropylmethylphenols. Conditions of thymolphthalein electropolymerization (monomer concentration, number of cycles, and electrolysis parameters) providing the best response to thymol have been found. Scanning electron microscopy and electrochemical methods confirm the effectivity of the electrode developed. The linear dynamic ranges of 0.050-25 and 25-100 µM for thymol and 0.10-10 and 10-100 µM for carvacrol with detection limits of 0.037 and 0.063 µM, respectively, have been achieved in differential pulse mode in Britton-Robinson buffer pH 2.0. The selectivity of the isopropylmethylphenols response in the presence of typical interferences (inorganic ions, saccharides, ascorbic acid) and other phenolics (caffeic, chlorogenic, gallic and rosmarinic acids, and quercetin) is a significant advantage over other electrochemical methods. The electrode has been used in the analysis of oregano and thyme spices. Total isopropylmethylphenols contents have been evaluated after a single sonication-assisted extraction with methanol.

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.

Detection of Residual 2-Phenylphenol on Lemon Rind by Electrochemically Deposited Poly(hydroxybenzaldehyde) and Poly(hydroxybenzoic acid) Polymeric Stackings as Electrode Modifiers

This study explores the characteristics of electrodeposition of the three hydroxybenzaldehyde isomers and selected hydroxybenzoic acids (4-hydroxybenzoic acid, salicylic acid, 3,5-dihydroxybenzoic acid) from mesityl oxide solvent. Similar to recent advances of this solvent, used by electrochemical studies, the carbon-carbon double bond had significant influence on the formation of polymers from the outlined molecules. In case of most substrates the peak currents increased to a steady-state but electropolymerization of some substrates caused significant deactivation. Scanning electron microscopic and complementary voltammetric studies facilitated that the electrochemically formed polymers are present on the electrode surface in stackings. In viewpoint of analysis of 2-phenylphenol, the modifying deposit formed from 4-hydroxybenzaldehyde was the best with 5 µM detection limit obtained with differential pulse voltammetry. Furthermore, a new procedure was chosen for the involvement of a cavitand derivative into the organic layers with the purpose to improve the layer selectivity (subsequent electrochemical polymerization in an other solution). Further studies showed that in this way the sensitivities of as-modified electrodes were a little worse than without this step, thus indicating that application of this step is disadvantageous. Recovery studies of 2-phenylphenol were carried out on lemon rind without any treatment, and it was compared with the case when the outer yellow layer was removed by rasping. The inner tissues showed very high adsorption affinity towards 2-phenylphenol.

Electrochemical Detection of Olivetol Based on Poly(L-Serine) Film Layered Copper Oxide Modified Carbon Paste Electrode (p-L-Serine/CuO/CPE)

Olivetol is an important polyphenol compound and intermediate in the synthesis of cannabinoids possessing many types of biological activities. A facile electrochemical sensor for olivetol was fabricated based on p-L-serine, and copper oxide (CuO) nanoparticles modified carbon paste electrode (p-L-serine/CuO/CPE). The proposed p-L-serine/CuO/CPE was applied to the electrochemical detection of olivetol by cyclic voltammetry (CV) and differential pulse voltammetric (DPV). Through the characterizations of materials and modified electrodes, the p-L-serine/CuO/CPE exhibited enhanced electrochemical signals for olivetol compared to bare CPE and CuO/CPE in both CV and DPV methods. Under the optimized conditions, the proposed p-L-serine/CuO/CPE showed a good quantitative analysis ability and a wide analysis range from 20 to 100 μmol L^-1 of olivetol with a limit of detection of 1.04 μmol L^-1. Based on the reproducibility, repeatability, and stability exhibited by this fabricated sensor and the cheap and accessible raw materials, the p-L-serine/CuO/CPE became a novel determination choice for olivetol in the electrochemical method with the advantages of being cost-effective and convenient.

Electropolymerized 4-Aminobenzoic Acid Based Voltammetric Sensor for the Simultaneous Determination of Food Azo Dyes

Electrochemical sensors with polymeric films as a sensitive layer are of high interest in current electroanalysis. A voltammetric sensor based on multi-walled carbon nanotubes (MWCNTs) and electropolymerized 4-aminobenzoic acid (4-ABA) has been developed for the simultaneous determination of synthetic food azo dyes (sunset yellow FCF and tartrazine). Based on the voltammetric response of the dyes' mixture, the optimal conditions of electropolymerization have been found to be 30-fold potential scanning between -0.3 and 1.5 V, at 100 mV s^-1 in the 100 µmol L^-1 monomer solution in phosphate buffer pH 7.0. The poly (4-ABA)-based electrode shows a 10.5-fold increase in its effective surface area and a 17.2-fold lower electron transfer resistance compared to the glassy carbon electrode (GCE). The sensor gives a sensitive and selective response to sunset yellow FCF and tartrazine, with the peak potential separation of 232 mV in phosphate buffer pH 4.8. The electrooxidation parameters of dyes have been calculated. Simultaneous quantification is possible in the dynamic ranges of 0.010-0.75 and 0.75-5.0 µmol L^-1 for both dyes, with detection limits of 2.3 and 3.0 nmol L^-1 for sunset yellow FCF and tartrazine, respectively. The sensor has been tested on orange-flavored drinks and validated with chromatography.

Electrochemical Sensors Based on Au Nanoparticles Decorated Pyrene-Reduced Graphene Oxide for Hydrazine, 4-Nitrophenol and Hg2+ Detection in Water

Monitoring hazardous chemical compounds such as hydrazine (N2H4), 4-nitrophenol (4-NP) and Hg2+ in natural water resources is a crucial issue due to their toxic effects on human health and catastrophic impact on the environment. Electrochemical nanostructured platforms integrating hybrid nanocomposites based on graphene derivatives and inorganic nanoparticles (NPs) are of great interest for such a purpose. In this work, disposable screen-printed carbon electrodes (SPCEs) have been modified with a hybrid nanocomposite formed by reduced graphene oxide (RGO), functionalized by 1-pyrene carboxylic acid (PCA), and decorated by colloidal Au NPs. These hybrid platforms have been tested for the electrocatalytic detection of N2H4 and 4-NP by differential pulse voltammetry and have been modified with an electropolymerized film of Hg2+ ions imprinted polycurcumin for the electroanalytical detection of Hg2+ by DPV. LODs, lower and in line with the lowest ones reported for state-of-the-art electrochemical sensors, integrating similar Au-graphene < nanocomposites, have been estimated. Additionally, good repeatability, reproducibility, and storage stability have been assessed, as well as a high selectivity in the presence of a 100-fold higher concentration of interfering species. The applicability of the proposed platforms for the detection of the compounds in real complex matrices, such as tap and river water samples, has been effectively demonstrated.

Highly selective electrochemical sensing based on electropolymerized ion imprinted polyaniline (IIPANI) on a bismuth modified carbon paste electrode (CPE-Bi) for monitoring Nickel(ii) in river water

Electrochemical sensors based on ion-imprinting polymers have emerged as an effective analytical tool for heavy metal tracking. This study describes a simple and facile technique for manufacturing a highly selective and sensitive electrode using an ion imprinting polymer on a bismuth-modified carbon paste electrode. The developed sensor applied aniline as a functional monomer and was used for tracking Ni(ii) ions. The proposed sensor was thoroughly characterized by scanning electron microscopy, cyclic voltammetry, and differential pulse striping anodic voltammetry. The analytical evaluation showed that the proposed sensor has a linear dynamic range (R² = 0.999) for the Ni(ii) concentration range of 0.01 to 1 μM and a limit of detection value of 0.00482 μM. The proposed sensor showed excellent performance when tested for tracking Ni(ii) ions in the presence of interfering ions (Cd(ii), Co(ii), Cu(ii), and Zn(ii) ions) at a 1000-fold higher concentration. When the proposed sensor was tested for tracking Ni(ii) concentration in an actual river sample, our modified sensor showed similar results compared to the atomic absorption spectroscopy evaluation (p > 0.05, n = 3). In summary, our proposed sensor is promising for monitoring Ni(ii) ions in the aquatic environment.

Electrochemical sensors based on ion-imprinting polymers have emerged as an effective analytical tool for heavy metal tracking.

Nanomaterials-based electrochemical sensors for the detection of natural antioxidants in food and biological samples: research progress

Antioxidants are healthy substances that are beneficial to the human body and exist mainly in natural and synthetic forms. Among many kinds of antioxidants, the natural antioxidants have great applications in many fields such as food chemistry, medical care, and clinical application. In recent years, many efforts have been made for the determination of natural antioxidants. Nano-electrochemical sensors combining electrochemistry and nanotechnology have been widely used in the determination of natural antioxidants due to their unique advantages. Therefore, a large number of nanomaterials such as metal oxide, carbon materials, and conducting polymer have attracted much attention in the field of electrochemical sensors due to their good catalytic effect and stable performance. This review mainly introduces the construction of electrochemical sensors based on different nanomaterials, such as metallic nanomaterials, metal oxide nanomaterials, carbon nanomaterials, metal-organic frameworks, polymer nanomaterials, and other nanocomposites, and their application to the detection of natural antioxidants, including ascorbic acid, phenolic acids, flavonoid, tryptophan, citric acid, and other natural antioxidants. In the end, the limitations of the existing nano-sensing technology, the latest development trend, and the application prospect for various natural antioxidant substances are summarized and analyzed. We expect that this review will be helpful to researchers engaged in electrochemical sensors.

A Review on Electrochemical Sensors and Biosensors Used in Assessing Antioxidant Activity

Currently, there is growing interest in screening and quantifying antioxidants from biological samples in the quest for natural and effective antioxidants to combat free radical-related pathological complications. Antioxidants play an important role in human health and provide a defense against many diseases. Due to the valuable dietary role of these compounds, the analysis and determination of their amount in food is of particular importance. In recent years, many attempts have been made to provide simple, fast, and economical analytical approaches for the on-site detection and determination of antioxidant activity in food antioxidants. In this regard, electrochemical sensors and biosensors are considered promising tools for antioxidant research due to their high sensitivity, fast response time, and ease of miniaturization; thus, they are used in a variety of fields, including food analysis, drug screening, and toxicity research. Herein, we review the recent advances in sensors and biosensors for the detection of antioxidants, underlying principles, and emphasizing advantages, along with limitations regarding the ability to discriminate between the specific antioxidant or quantifying total antioxidant content. In this work, both direct and indirect methods for antioxidants detecting with electrochemical sensors and biosensors are analyzed in detail. This review aims to prove how electrochemical sensors and biosensors represent reliable alternatives to conventional methods for antioxidant analysis.

Simultaneous Determination of Ferulic Acid and Vanillin in Vanilla Extracts Using Voltammetric Sensor Based on Electropolymerized Bromocresol Purple

Natural phenolic antioxidants are one of the widely studied compounds in life sciences due to their important role in oxidative stress prevention and repair. The structural similarity of these antioxidants and their simultaneous presence in the plant samples stipulate the development of methods for their quantification. The current work deals with the simultaneous determination of vanillin and its bioprecursor ferulic acid using a voltammetric sensor for the first time. A sensor based on the layer-by-layer deposition of the polyaminobenzene sulfonic acid functionalized single-walled carbon nanotubes (f-SWCNTs) and electropolymerized bromocresol purple has been developed for this purpose. The best response of co-existing target analytes was registered for the polymer obtained from the 25 µM dye by 10-fold potential cycling from 0.0 to 1.2 V with the scan rate of 100 mV s⁻¹ in 0.1 M phosphate buffer (PB), pH 7.0. Scanning electron microscopy (SEM), cyclic voltammetry and electrochemical impedance spectroscopy (EIS) confirmed the effectivity of the sensor developed. The linear dynamic ranges of 0.10–5.0 µM and 5.0–25 µM for both analytes with the detection limits of 72 nM and 64 nM for ferulic acid and vanillin, respectively, were achieved in differential pulse mode. The sensor was applied for the analysis of vanilla extracts.