A mimicking enzyme analogue for chemical sensors

Ultrasensitive detection of carcinogenic chromium(VI) species below the WHO limit using a LaCeO3/carbon black screen printed electrode in batch injection analysis

The widespread industrial use of chromium and its subsequent release into the environment as toxic and carcinogenic hexavalent chromium (Cr(VI)) species pose significant risks to human health and the environment. The World Health Organization (WHO) has established a limit of 50 ppb (960 nM) for Cr(VI) in water samples. Developing simple, selective, and separation-free methods for the direct detection of Cr(VI) species in the environment remains a challenging task. Herein, we present a highly crystalline lanthanum cerate/carbon black chemically modified screen-printed electrode (SPE/CB@LaCeO3) as an effective electrochemical system for the high-performance and selective electrochemical reduction of toxic Cr(VI) species in pH 2 KCl-HCl solution. The CB@LaCeO3 composite is characterized by its high-density electroactive sites and enhanced electrical conductivity, which facilitate the efficient diffusion-controlled reduction of Cr(VI) species at a low reduction potential of 0.55 V vs. Ag/AgCl. The modified electrode demonstrated stability and resistance to surface fouling during continuous voltammetry analysis of high Cr(VI) concentrations. A batch-injection analysis using a three-in-one screen-printed electrode, comprising carbon working, silver-ink reference, and CB@LaCeO3 modified carbon working electrodes, exhibited excellent concentration linearity within the ranges of 2-30 ppb and 10-35 ppm, with a low detection limit of 682 ppt (signal-to-noise ratio, 3). This method was not interfered by dissolved oxygen or other common chemicals present in environmental and water systems. The linear range and detection limit achieved in this study surpass those reported in several previous works involving precious metal and organic molecule-based chemically modified electrodes. The analytical method was validated with t-test analysis. To demonstrate the applicability of this new system, batch injection analysis was performed on a wide range of real samples, including water (tap, ground, well, and reverse osmosis), consumable products (coffee, tea and milk powders), and tannery effluent, using the standard addition method. This approach yielded accurate and sensitive detection of Cr(VI) species in the samples, with recovery values of approximately 100%.

Electrical Wiring of Malarial Parasite Intermediate Hematin on a Tailored N-Doped Carbon Nanomaterial Surface and Its Bioelectrocatalytic Hydrogen Peroxide Reduction and Sensing

Hematin, an iron-containing porphyrin compound, plays a crucial role in various biological processes, including oxygen transport, storage, and functionality of the malarial parasite. Specifically, hematin-Fe interacts with the nitrogen atom of antimalarial drugs, forming an intermediate step crucial for their function. The electron transfer functionality of hematin in biological systems has been scarcely investigated. In this study, we developed a biomimicking electrical wiring of hematin-Fe with a model N-drug system, represented as {hematin-Fe---N-drug}. We achieved this by immobilizing hematin on a multiwalled carbon nanotube (MWCNT)/N-graphene quantum dot (N-GQD) modified electrode (MWCNT/N-GQD@Hemat). N-GQD serves as a model molecular drug system containing nitrogen atoms to mimic the {hematin-Fe---N-drug} interaction. The prepared bioelectrode exhibited a distinct redox peak at a measured potential (E1/2) of -0.410 V vs Ag/AgCl, accompanied by a surface excess value of 3.54 × 10-9 mol cm-2. This observation contrasts significantly with the weak or electroinactive electrochemical responses documented in literature-based hematin systems. We performed a comprehensive set of physicochemical and electrochemical characterizations on the MWCNT/N-GQD@Hemat system, employing techniques including FESEM, TEM, Raman spectroscopy, IR spectroscopy, and AFM. To evaluate the biomimetic electrode's electroactivity, we investigated the selective-mediated reduction of H2O2 as a model system. As an important aspect of our research, we demonstrated the use of scanning electrochemical microscopy to visualize the in situ electron transfer reaction of the biomimicking electrode. In an independent study, we showed enzyme-less electrocatalytic reduction and selective electrocatalytic sensing of H2O2 with a detection limit of 319 nM. We achieved this using a batch injection analysis-coupled disposable screen-printed electrode system in physiological solution.

Advances in colorimetric biosensors of exosomes: novel approaches based on natural enzymes and nanozymes

Exosomes are 30-150 nm vesicles derived from diverse cell types, serving as one of the most important biomarkers for early diagnosis and prognosis of diseases. However, the conventional detection method for exosomes faces significant challenges, such as unsatisfactory sensitivity, complicated operation, and the requirement of complicated devices. In recent years, colorimetric exosome biosensors with a visual readout underwent rapid development due to the advances in natural enzyme-based assays and the integration of various types of nanozymes. These synthetic nanomaterials show unique physiochemical properties and catalytic abilities, enabling the construction of exosome colorimetric biosensors with novel principles. This review will illustrate the reaction mechanisms and properties of natural enzymes and nanozymes, followed by a detailed introduction of the recent advances in both types of enzyme-based colorimetric biosensors. A comparison between natural enzymes and nanozymes is made to provide insights into the research that improves the sensitivity and convenience of assays. Finally, the advantages, challenges, and future directions of enzymes as well as exosome colorimetric biosensors are highlighted, aiming at improving the overall performance from different approaches.

Plastic Chip Electrode: An Emerging Multipurpose Electrode Platform

The properties of electrodes play a crucial role in the processes occurring on them. Therefore, a variety of materials have been tried as electrodes. Carbon composite materials are among the most admired ones. Use of composites as electrode material dates back to the mid of the last century when polymer-carbon composites were tried as general-purpose electrode platforms and epoxy impregnated graphite paste/ solid electrodes were tried in polarography. Later the composite electrodes have seen several phases of development. Plastic Chip Electrode (PCE) is a class of polymer composite electrode developed by our group. This monographic review gives a bird's eye account of polymer composite electrodes and appurtenant work, followed by elaborating on various aspects and state-of-the-art plastic chip electrodes.

Edge and Basal Plane Anisotropy of a Preanodized Pencil Graphite Electrode Surface Revealed Using Scanning Electrochemical Microscopy and Electrocatalytic Dopamine Oxidation as a Molecular Probe

Pencil graphite (PGE), an ultralow-cost and ready-to-use disposable-type electrode, has been used for various electrochemical and electroanalytical applications after its surface anodization (PGE*, * means preanodized surface). Indeed, systematic studies on mechanistic and surface features of PGE* have not yet been explored. Herein, we report anodized pencil graphite as a model system to study molecular level insights into the surface using a scanning electrochemical microscopy (SECM) technique and dopamine (DA) electrocatalytic oxidation reaction as a molecular probe. The as-prepared PGE* showed an appreciable electronic conductivity similar to the edge-plane graphitic sites (EPPG) of the highly pyrolytic graphitic electrode (HOPG) but without any surface deterioration that occurs with HOPG due to the instability of the EPPG. Physicochemical characterizations by FESEM, FTIR, Raman, and XPS techniques revealed a flake-like exfoliated PGE* surface with higher contents of carbon-oxygen especially phenolic/alcoholic functional groups than the PGE surface. Based on the chronocoulometric experiment, the number of functional groups formed on the PGE* was calculated as 10.9 × 10-10 mol cm-2. An independent SECM technique using ferricyanide as a redox probe showed the existence of a heterogeneous surface and exhibited an improved electron transfer activity due to the flake-like graphitic island on the PGE* surface. Investigated DA electrochemical oxidation on PGE* yielded about three times enhancement in the peak current signal and about 200 mV reduction in the oxidation potential over the PGE without any serious surface fouling feature that is related to the intermediate polydopamine formation on the basal-plane graphitic surface of the underlying electrode. As an independent electroanalytical study, a prototype electrochemical sensor using PGE* as a working electrode for instant detection of DA-containing pharmaceutical samples in a 1 mL Eppendorf vial has been demonstrated.

De Novo Iron Oxide Hydroxide, Ferrihydrite Produced by Comamonas testosteroni Exhibiting Intrinsic Peroxidase-Like Activity and Their Analytical Applications

Natural enzyme mimics have attracted considerable attention due to leakage of enzymes and their easy denaturation during their storage and immobilization procedure. Here in this study, for the first time, a new iron oxide hydroxide, ferrihydrite - Fe_1.44O_0.32 (OH) _3.68 magnetic nanoparticles were synthesized by bacterial strain named Comamonas testosteroni. The characterization of the produced magnetic nanoparticles was confirmed by transmission electron microscopy (TEM), Fourier-transform spectroscopy (FTIR), X-ray diffraction (XRD), and magnetization hysteresis loops. Further, these extracted nanoparticles were proven to have biogenic magnetic behavior and to exhibit enhanced peroxidase-like activity. It is capable of catalyzing the oxidation of 3, 3', 5, 5'-Tetramethylbenzidine (TMB) by H₂O₂ to produce blue color (typical color reactions). Catalysis was examined to follow Michaelis-Menton kinetics and the good affinity to both H₂O₂ and TMB. The K _m value of the Fe_1.44O_0.32 (OH) _3.68 with H₂O₂ and TMB as the substrate was 0.0775 and 0.0155 mM, respectively, which were lower than that of the natural enzyme (HRP). Experiments of electron spin resonance (ESR) spectroscopy proved that the BMNPs could catalyze H₂O₂ to produce hydroxyl radicals. As a new peroxidase mimetic, the BMNPs were exhibited to offer a simple, sensitive, and selective colorimetric method for determination of H₂O₂ and glucose and efficiently catalyze the detection of glucose in real blood samples.

Size- and shape-dependent peroxidase-like catalytic activity of MnFe2O4 Nanoparticles and their applications in highly efficient colorimetric detection of target cancer cells

FA- and FITC-labeled MnFe₂O₄ nanohybrid exhibits highly efficient colorimetric detection of target cancer cells.

Room temperature aerobic oxidation of amines by a nanocrystalline ruthenium oxide pyrochlore nafion composite catalyst

The aerobic oxidation of primary amines to their respective nitriles has been carried out at room temperature using a highly reusable nanocrystalline ruthenium oxide pyrochlore Nafion composite catalyst (see figure).

Polynuclear Nickel Hexacyanoferrate/Graphitized Mesoporous Carbon Hybrid Chemically Modified Electrode for Selective Hydrazine Detection

A hybrid polynuclear nickel hexacyanoferrate (NiHCFe)/graphitized mesoporous carbon- (GMC-) modified glassy carbon electrode (GCE/NiHCFe@GMC) has been prepared by a sequential method using electrodeposited Ni on a GMC-modified glassy carbon electrode (GCE/Ni@GMC) as a template and [Fe(CN)6]3−as anin-situchemical precipitant, without any additional interlinking agent. Physicochemical and electrochemical characterizations reveal the presence of NiHCFe units within the porous sites of the GMC. The GCE/NiHCFe@GMC electrode showed highly stable and well-defined redox behaviors with surface-confined electron-transfer mechanism in a pH 7 phosphate buffer solution. The GCE/NiHCFe@GMC showed about 20 times enhancement in hydrazine oxidation peak current along with 500 mV reduction in overpotential over the corresponding unmodified GCE/GMC. Hydrazine calibration plots by CV and amperometrici-tmethods were linear up to 1 mM and 220 μM with current sensitivity values of 15.86 μA/mM and 7.37 nA/μM, respectively. Calculated detection limit by the amperometrici-tmethod was 23.2 nM. The hybrid GCE/NiHCFe@GMC exhibits remarkable tolerance to important industrial and biological interferents. Finally determination of hydrazine in cigarette smoke sample was successfully demonstrated.