A Theoretical Approach to Understand the Nonlinear Processes in Enzymatic Electrochemical Biosensors.

The enzymatic biosensors' response can be monitored based on the results of nonlinear differential equations. The nonlinear reaction-diffusion equations proposed for this enzyme-based electrochemical biosensor include a nonlinear term associated with Michaelis-Menten kinetics. Herein, the system of nonlinear reaction-diffusion equations is solved using a modified homotopy perturbation method. For all values of the rate constants, the approximate analytical expressions for the concentration profiles, current, sensitivity, and gradient of biosensor have been determined. Performance factors of an enzymatic electrochemical biosensor, such as response time, sensitivity, accuracy, and resistance, are discussed. The analytical results and numerically simulated outcomes using Matlab software have been compared.

Graphene quantum dots for biosensing and bioimaging.

Graphene Quantum Dots (GQDs) are low dimensional carbon based materials with interesting physical, chemical and biological properties that enable their applications in numerous fields. GQDs possess unique electronic structures that impart special functional attributes such as tunable optical/electrical properties in addition to heteroatom-doping and more importantly a propensity for surface functionalization for applications in biosensing and bioimaging. Herein, we review the recent advancements in the top-down and bottom-up approaches for the synthesis of GQDs. Following this, we present a detailed review of the various surface properties of GQDs and their applications in bioimaging and biosensing. GQDs have been used for fluorescence imaging for visualizing tumours and monitoring the therapeutic responses in addition to magnetic resonance imaging applications. Similarly, the photoluminescence based biosensing applications of GQDs for the detection of hydrogen peroxide, micro RNA, DNA, horse radish peroxidase, heavy metal ions, negatively charged ions, cardiac troponin, etc. are discussed in this review. Finally, we conclude the review with a discussion on future prospects.

Photoelectrochemical detection of superoxide anions released from mitochondria in HepG2 cells based on the synergistic effect of MnO2@Co3O4 core-shell p-n heterojunction.

The detection and comparison of the amount of superoxide anion (O2.-) released by different complexes in mitochondrial electron transport chain (ETC) can locate the main electron leakage sites in mitochondria. In order to realize this, we designed an ultrasensitive photoelectrochemical (PEC) sensor by in situ hydrothermal growth of MnO2 nanosheets on Co3O4 nanowires array modified Ti substrate (NWA|Ti). Due to the formation of a core-shell p-n heterojunction with high specific surface area, tight surface contact and plentiful oxygen vacancies (OVs), MnO2@Co3O4 NWA|Ti possesses a strong visible light absorption, high charges transfer and separation ability. The proposed PEC sensor exhibited a wide linear range of 0.1-50000 nM and a low detection limit of 0.025 nM towards H2O2. Due to the rapid conversion of O2.- to H2O2 inside mitochondria, the PEC sensor can indirectly monitor the electron leakage in the ETC. Specifically, four selected mitochondrial inhibitors specifically inhibited the corresponding complex in mitochondria extracted from living HepG2 cells (hepatocellular carcinoma cells), and the H2O2 levels converted from O2.- was measured by the PEC sensor. It is evident that IQ (ubiquinone binding) site of complex I and Qo (ubiquinol oxidation) site of complex III are the key sites at which electron leakage occurred. This study could provide meaningful information for the diagnosis and treatment of certain disease caused by oxidative stress due to the electron leakage.

Molecular Diagnosis and Cancer Prognosis-A Concise Review.

Cancer is a complicated disease. Globally, it is one of the major causes for morbidity and mortality. A critical challenge associated with it is the difficulty to accurately diagnose it at an early stage. The malignancy due to multistage and heterogeneity that result from genetic and epigenetic modifications poses critical challenge to diagnose and monitor the progress at an early stage. Current diagnostic techniques normally suggest invasive biopsy procedure that can cause further infections and bleeding. Therefore, noninvasive diagnostic methods with high accuracy, safety and earliest detection are the needs of the hour. Herein, we provide a detailed review on the advanced methodologies and protocols developed for the detection of cancer biomarkers based on proteins, nucleic acids and extracellular vesicles. Furthermore, existing challenges and the improvements essential for the rapid, sensitive and noninvasive detection have also been discussed.

Graphene quantum dots: synthesis, properties, and applications to the development of optical and electrochemical sensors for chemical sensing.

GQDs exhibits exceptional electrochemical activity owing to their active edge sites that make them very attractive for biosensing applications. However, their use in the design of new biosensing devices for application to the detection and quantification of toxins, pathogens, and clinical biomarkers has so far not investigated in detail. In this regard, herein we provide a detailed review on various methodologies employed for the synthesis of GQDs, including bottom-up and top-down approaches, with a special focus on their applications in biosensing via fluorescence, photoluminescence, chemiluminescence, electrochemiluminescence, fluorescence resonance energy transfer, and electrochemical techniques. We believe that this review will shed light on the critical issues and widen the applications of GQDs for the design of biosensors with improved analytical response for future applications. HIGHLIGHTS: • Properties of GQDs play a critical role in biosensing applications. • Synthesis of GQDs using top-down and bottom-up approaches is discussed comprehensively. • Overview of advancements in GQD-based sensors over the last decade. • Methods for the design of selective and sensitive GQD-based sensors. • Challenges and opportunities for future GQD-based sensors.

2D metal carbides and nitrides (MXenes) for sensors and biosensors.

MXenes are layered two-dimensional (2D) materials discovered in 2011 (Ti3C2X) and are otherwise called 2D transition metal carbides, carbonitrides, and nitrides. These 2D layered materials have been in the limelight for a decade due to their interesting properties such as large surface area, high ion transport, biocompatibility, and low diffusion barrier. Therefore, MXenes are widely preferred by researchers for applications in electronics, sensing, biosensing, electrocatalysis, super-capacitors and fuel cells. There are a number of methods available for the bulk synthesis of MXene-based nanomaterials. In addition, the possibility of structural modification as required and its outstanding surface chemistry offer a fascinating interface for the development of novel biosensors. In this review, we specifically discuss important MXene synthesis routes. Moreover, critical parameters such as surface functionalization that can dictate the mechanical, electronic, magnetic, and optical properties of MXenes are also discussed. Following this, methods available for the surface functionalization and modification strategies of MXenes are also discussed. Furthermore, the emergence of gas, electrochemical, and optical biosensors based on MXenes since its first report is discussed in detail. Finally, future directions of MXenes biosensors for critical applications are discussed.

Phosphorene quantum dots: synthesis, properties and catalytic applications.

Phosphorene quantum dots (PQDs) belong to a new class of zero-dimensional functional nanostructures with unique physicochemical and surface properties in comparison with few-layer phosphorene and other 2D analogues. Tunable band gap as a function of number of layers, ease of passivation and high carrier mobility of PQDs have attracted considerable attention in catalysis research due to which spectacular progress has been made in PQD research over the last few years. PQDs are now considered as promising catalytic materials for electrocatalytic water splitting and nitrogen reduction, lithium-sulfur batteries, solar light-driven energy devices and biocatalysis, either in pristine form or as an active component for constructing heterostructures with other 2D materials. In the light of these recent advances, it is worthwhile to review and consolidate PQD research in catalytic applications to understand the challenges ahead and suggest possible solutions. In this review, we systematically summarize various synthetic strategies including ultrasonic and electrochemical exfoliation, solvothermal treatment, blender breaking, milling, crushing and pulsed laser irradiation. Furthermore, the physiochemical properties of PQDs are discussed based on both experimental and theoretical perspectives. The potential applications of PQDs in catalysis with special emphasis on photocatalysis (solar light-driven energy devices) and electrocatalysis (oxygen evolution reactions and hydrogen evolution reactions) -are critically discussed along with the present status, challenges and future perspectives.

DNA-Modified Cobalt Tungsten Oxide Hydroxide Hydrate Nanochains as an Effective Electrocatalyst with Amplified CO Tolerance during Methanol Oxidation.

Direct methanol fuel cell technology implementation mainly depends on the development of non-platinum catalysts with good CO tolerance. Among the widely studied transition-metal catalysts, cobalt oxide with distinctively higher catalytic efficiency is highly desirable. Here, we have evolved a simple method of synthesizing cobalt tungsten oxide hydroxide hydrate nanowires with DNA (CTOOH/DNA) and without incorporating DNA (CTOOH) by microwave irradiation and subsequently employed them as electrocatalysts for methanol oxidation. Following this, we examined the influence of incorporating DNA into CTOOH by cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. The enhanced electrochemical surface area of CTOOH offered readily available electroactive sites and resulted in a higher oxidation current at a lower onset potential for methanol oxidation. On the other hand, CTOOH/DNA exhibited improved CO tolerance and it was evident from the chronoamperometric studies. Herein, we noticed only a 2.5 and 1.8% drop at CTOOH- and CTOOH/DNA-modified electrodes, respectively, after 30 min. Overall, from the results, it was evident that the presence of DNA in CTOOH played an important role in the rapid removal of adsorbed intermediates and regenerated active catalyst centers possibly by creating high density surface defects around the nanochains than bare CTOOH.

Correction: Bismuthene nanosheets produced by ionic liquid assisted grinding exfoliation and their use for oxygen reduction reaction.

[This corrects the article DOI: 10.1039/D0RA09763B.].

Ultrasensitive photoelectrochemical aptasensor for diclofenac sodium based on surface-modified TiO2-FeVO4 composite.

Herein, a photoelectrochemical (PEC) aptasensing platform was designed by integrating surface oxygen vacancy (OV) defects, Ti3+ self-doping, the heterojunction, and resonance energy transfer (RET) effect into one platform for the detection of diclofenac sodium (DCF). Briefly, OV defects were introduced on TiO2 nanospheres with simultaneous Ti3+ self-doping, followed by a well-separated deposition of FeVO4 nanoparticles on TiO2 to obtain a Ti3+-O-TiO2/FeVO4 heterojunction. The surface modification of OVs, Ti3+ doping, and deposition of FeVO4 were confirmed by SEM, XPS, EPR, DRS, and PEC measurements. The surface OVs and doping of Ti3+ species created a new donor (defect) energy level under the conduction band of TiO2, which minimized the bandgap and thereby improved the visible light absorption of TiO2. Moreover, the capture of photo-excited electrons by surface OVs could hinder the electron-hole recombination. Due to the intimate surface contact and perfect energy matching between TiO2 and FeVO4, the formation of heterojunction decreased the bandgap and facilitated the electron-hole separation of TiO2. All these above events contributed to the enhancement of the PEC signals, which were then quenched by the RET effect between Ti3+-O-TiO2/FeVO4 and Au nanoparticle (AuNP)-labeled cDNA that had been attached to its complementary DCF aptamer on Ti3+-O-TiO2/FeVO4|ITO. The addition of target-DCF detached AuNP-labeled cDNA from the electrode to recover the photocurrent, resulting in a "signal-on" PEC aptasensor that exhibited a 0.1-500-nM linear range and a detection limit of 0.069 nM for DCF, attributed to the excellent amplification of the proposed aptasensing platform.

An integrated energy-efficient electrochromic device for salt water purification.

Herein, we report a multifunctional device with an integrated energy-desalination-smart electrochromic display in one single cell. Specifically, a novel chemically assisted electrochromic desalination set-up was demonstrated using a Prussian blue cathode and an anode comprising Ag or a redox polymer film in salt water without any external electrical energy usage.

Synthesis of a PEDOT-TiO2 heterostructure as a dual biosensing platform operating via photoelectrochemical and electrochemical transduction mode.

A new organic-inorganic heterostructure was prepared by the hydrothermal deposition of poly (3,4-dioxoethylthiophene) (PEDOT) on TiO2 nanowire arrays (TiONWs) to construct a biosensor that can simultaneously function as photoelectrochemical (PEC) and electrochemical (EC) sensor to detect lactate. In both cases, the PEDOT-TiONWs heterostructure not only acted as an immobilization platform for lactate dehydrogenase (LDH) and coenzyme NAD+, but also generated current signals, which were further amplified by the cyclic catalytic mechanism. Specifically, LDH catalytically converted lactate to pyruvate, meanwhile NAD+ was transformed to NADH. For PEC sensing, the photo-generated holes from PEDOT-TiONWs could oxidize NADH back to NAD+, fulfilling a catalytic cycle. Herein, PEDOT significantly promoted the separation of electron-hole pairs and enhanced PEC signals due to its well-matched energy levels with TiONWs, high conductivity and strong visible light absorption. A dynamic range of 0.5-300 µM was observed between the PEC signals and lactate concentration, based on which a sensitivity of 0.1386 ± 0.0053 µA µM-1 and a detection limit of 0.08 ± 0.0032 µM were estimated. For EC sensing, PEDOT-TiONWs could directly oxidize NADH to NAD+ at ~0.54 V to realize the cyclic amplification due to the high conductivity and strong electrocatalytic capability of the heterostructure. The EC biosensor displayed a similar performance upon PEC mode of operation, except the relatively poor selectivity due to the possible oxidation of the interferences at the potentials > 0.54 V.

Microporous Metal-Organic Framework (MOF)-Based Composite Polymer Electrolyte (CPE) Mitigating Lithium Dendrite Formation in All-Solid-State-Lithium Batteries.

Poly(ethylene oxide) (PEO)-based composite polymer electrolytes (CPEs) containing the amine-functionalized, zirconium-based metal-organic framework @silica (UiO-66-NH2@SiO2) and lithium, LiN(CF3SO2)2 salt (LiTFSI) are prepared using a simple hot press method. The electrochemical properties such as compatibility of the electrolyte with the Li metal anode, Li transference number, and ionic conductivity are investigated for the different systems containing different relative concentrations of the additives. The incorporation of UiO-66-NH2@SiO2 in the PEO-LiTFSI matrix not only enhanced ionic conductivity by one order of magnitude but also offered better compatibility and suppressed the formation of lithium dendrites appreciably. X-ray photoelectron spectroscopy studies on post-cycled materials revealed the formation of lithium alkoxide (RO-Li) on the cathode and Li2O on the anode. The coin cell (2032-type) consisting of LiFePO4/CPE/Li with UiO-66-NH2@SiO2 as filler provided a discharge capacity of 151 mA h g-1 at 0.1 C-rate at 60 °C, measurably higher than control experiments utilizing SiO2 and UiO-66-NH2. The notable enhancement of electrochemical properties when incorporating the UiO-66-NH2@SiO2 at the CPE was attributed to formation of more uniform ion conduction pockets and channels within the PEO matrix, facilitated by the presence of the microporous UiO-66-NH2@SiO2. The enhanced distribution of microporous channels, where Li ions are assumed to percolate through within the matrix, is assumed to desirably reduce formation of Li dendrites by increasing diffusion channels and therefore reducing crystallization and growth of dendrites at the electrode surface.

Bismuthene nanosheets produced by ionic liquid assisted grinding exfoliation and their use for oxygen reduction reaction.

We report the simple synthesis of bismuthene nanosheets (BiNS) by ionic liquid assisted grinding exfoliation, followed by size selection sequential centrifugation steps for the first time. The exfoliation process results in the formation of self-assembled spherule-like superstructures with abundant edge sites, which are able to catalyze the oxygen reduction reaction (ORR) via a two-electron pathway, with a higher efficiency than the bulk Bismuth. We rationalize the enhanced ORR activity of the BiNS to: (i) the presence of 1 dimensional topological edge states, which provide strong conduction channels for electron hopping between the bismuth layers and (ii) the more active role of edge sites in facilitating O2 adsorption and dissociation of O-O bonds compared to the basal plane. The present study provides a pathway for employing 2D topological insulators as a new class of electrocatalysts for clean energy applications.

A photoelectrochemical aptasensor for aflatoxin B1 detection based on an energy transfer strategy between Ce-TiO2@MoSe2 and Au nanoparticles.

In this work, a novel photoelectrochemical (PEC) aptasensor was developed for the sensitive detection of aflatoxin B1 (AFB1) based on a resonance energy transfer strategy between the Ce-TiO2@MoSe2 heterostructure and Au nanoparticles (AuNPs). The Ce-TiO2@MoSe2 composite was obtained by growing MoSe2 nanosheets on a TiO2 nanocube doped by the Ce element with a facile hydrothermal method. The composite effectively extended the absorption of TiO2 to the visible region and avoided the self-aggregation of MoSe2 nanosheets, leading to the excellent photocurrent response under visible light excitation. The PEC aptasensor was then fabricated by immobilizing the Ce-TiO2@MoSe2 composite on an ITO electrode, followed by the modification of the aminated AFB1 aptamer. An AuNP-labeled DNA sequence was subsequently hybridized with the aptamer to fabricate a sandwich structure, which was destroyed after the introduction of AFB1, decreasing the amount of the energy acceptor (AuNPs) at the electrode surface. Accordingly, the photocurrent was increased with the increase of AFB1 concentration. Under the optimal conditions, the PEC aptasensor showed a wide linear range of 0.03-200 ng mL-1 and a low detection limit of 0.01 ng mL-1 for AFB1 determination.

Role of Structural Distortion in Stabilizing Electrosynthesized Blue-Emitting Phosphorene Quantum Dots.

Luminescent phosphorene quantum dots (PQDs) have emerged as fascinating nanomaterials for potential applications in optoelectronics, catalysis, and sensing. Herein, we investigate the structural distortion of black phosphorus (BP) under an applied electric field to yield blue luminescent PQDs [average diameter 8 ± 1.5 nm ( N = 60)]. The electrosynthesized PQDs exhibit photoluminescence emission independent of excitation wavelength with 84% quantum efficiency. Structural distortion that occurred during the transformation of BP to PQDs is confirmed by results obtained during transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Further, using first-principles-based density functional theory, calculations on oxygenated and nonoxygenated PQDs augment the experimental observations that an optimum oxygen content maintains the structural integrity of PQDs, above which the structural robustness of PQDs is drastically diminished.

Investigating the usefulness of chitosan based proton exchange membranes tailored with exfoliated molybdenum disulfide nanosheets for clean energy applications.

Chitosan based proton exchange membranes (PEMs) has been synthesized by a facile solution casting strategy using two-dimensional exfoliated molybdenum disulfide (E-MoS2) nanosheets. The prepared PEMs are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Field-emission scanning electron microscopy (FESEM) with Energy dispersive X-ray spectroscopy (EDX), water uptake, Thermogravimetric analysis (TGA), AC impedance spectroscopy and cyclic voltammetry. In comparison with pure chitosan membrane, E-MoS2 nanosheets incorporated membranes exhibit excellent water absorbing capacity, ion-exchange capacity and proton conductivity. Moreover, the changes in roughness of nanocomposite membranes is investigated by atomic force microscopy (AFM) and the results confirm that the E-MoS2 nanosheets content enhances the surface roughness as well as provide good mechanical and thermal resistivity to the chitosan/E-MoS2 membranes. Chitosan membranes with 0.75% E-MoS2 nanosheets demonstrated higher proton conductivity of 2.92 × 10-3 Scm-1 and membrane selectivity of 8.9 × 104 Scm-3 s with reduced methanol permeability of 3.28 × 10-8 cm2 s-1. Overall, results evidenced that the chitosan/E-MoS2 nanocomposite membranes will be an alternate to Nafion in direct methanol fuel cells (DMFCs).

A single-step, electrochemical synthesis of nitrogen doped blue luminescent phosphorene quantum dots.

Herein, we report a one-step strategy for the electrochemical synthesis of nitrogen doped blue luminescent phosphorene quantum dots (NPQDs) from black phosphorus at room temperature. Nitrogen percentage in NPQDs can be varied by the appropriate choice of solvent and supporting electrolyte. NPQDs [average size 6 ± 1.5 nm (N = 50)] obtained in this work exhibit ca. 88.7% quantum efficiency.

In Situ Synthesis of Silver Nanospheres, Nanocubes, and Nanowires over Boron-Doped Graphene Sheets for Surface-Enhanced Raman Scattering Application and Enzyme-Free Detection of Hydrogen Peroxide.

An effective in situ synthesis strategy is demonstrated for the preparation of silver nanostructures (nanospheres (NSs), nanocubes (NCs), and nanowires (NWs)) on the surface of boron-doped graphene (BG). Further, these functional nanomaterials are employed for the surface-enhanced Raman scattering (SERS) and non-enzymatic electrochemical detection of H2O2. The results confirm the superior performance of BG-Ag nanostructures as SERS platform. Among various geometries of silver nanoparticles studied in this work, we find that the AgNCs over BG (BG-AgNC) present outstanding SERS performance for detecting 4-mercaptobenzoic acid, with a limit of detection of 1.0 × 10-13 M. Furthermore, BG-AgNC exhibits excellent capability to detect melamine as low as 1.0 × 10-9 M. Electrochemical results confirm that the BG-AgNW-based platform exhibits a superior biosensing performance toward H2O2 detection. The enhanced performance is due to the presence of graphene, which improves the conductivity and provides more active sites. The synthesis of doped graphene with metallic nanoparticles described in this work is expected to be a key strategy for the development of an efficient SERS and electrochemical sensor that offers simplicity, cost-effectiveness, long-term stability, and better reproducibility.

Co3Fe7/nitrogen-doped graphene nanoribbons as bi-functional electrocatalyst for oxygen reduction and oxygen evolution.

In this work, we report a one-pot solvothermal strategy to synthesize Co3Fe7 incorporated graphene nanoribbons (Co3Fe7). An improved bi-functional electrocatalytic activity over the traditional electrocatalysts is exhibited by the Co3Fe7/nitrogen-doped graphene nanoribbon (NGNR) composite. For instance, this composite Co3Fe7/NGNRs depicted a lower overpotential of 350 mV than NGNRs (380 mV) and IrO2 (450 mV) to sustain 10 mA cm-2 for an oxygen evolution reaction in 1.0 M KOH. Furthermore, during an oxygen reduction reaction, the catalyst exhibited a four-electron pathway and it is interesting to note that its electrocatalytic behavior is on a par with commercial Pt/C. The enhancement in the electrochemical performance can be attributed to the synergistic effect that stems from the electrocatalytically active nitrogen atoms and metal alloy nanoparticles distributed uniformly over the graphene matrix. This unique composition of electrocatalyst is extremely beneficial for practical applications in fuel cells and metal-air batteries due to its high stability and sustained electrochemical activity.