Label-Free Assessment of Neuron-Specific Enolase via Polydopamine over a Carbon-Nanotube-Based Flexible Immunosensor.

A label-free electrochemical immunosensor was developed for the rapid and sensitive detection of neuron-specific enolase (NSE). The electropolymerization of dopamine in conjunction with highly conductive carbon nanotubes offers a simple and quick platform for the direct anchoring of antibodies without the assistance of any coupling agent as well as a blocking agent. The developed immunosensor exhibited a wider detection range from 120 pM (9 ng mL-1) to 3 nM (200 ng mL-1) for NSE with a high sensitivity of 3.9 µA pM-1 cm-2 in 0.1 M phosphate-buffered saline (PBS) at physiological pH (7.4). Moreover, the short recognition time (15 min) for the antigen enabled the detection to be fast and less invasive. Additionally, the evaluation of a rate constant at various concentrations of NSE via feedback mode of scanning electrochemical microscopy (SECM) explained the profound effect of antigen concentration on the rate of flow of electrons. Therefore, the proposed immunosensor can be a promising tool for the early detection of small cell lung cancer in a very short period of time with consistent accuracy.

Realizing the label-free sensitive detection of carcinoembryogenic antigen (CEA) in blood serum via a MNC-decorated flexible immunosensor.

A label-free electrochemical immunosensor utilising nitrogen-rich mesoporous carbon (MNC) as the substrate material was developed for the sensitive quantification of carcinoembryonic antigen (CEA). The synergic interactions between MNC and AbCEA also eliminated the need for coupling agents such as EDC/NHS. The novel immunosensor demonstrated a wide detection range from 500 fM (9.04 pg mL-1) to 50 nM (1 µg mL-1) and a low detection limit (LOD) of 500 fM. Moreover, the immunosensor showed sensitivities of 12.27 mA nM-1 cm-2 and 0.066 mA nM-1 cm-2 for detecting CEA in the linear ranges 10 pM to 1 nM and 2 nM to 50 nM, respectively, while maintaining long-term storage stability of 6 weeks. Analysis of real serum sample analysis yielded highly accurate results with recovery rates ranging from 99.3% to 103.7%. Furthermore, the developed paper-based screen-printed electrode exhibited a similar detection range, suggesting its potential for use in point-of-care detection devices in future applications.

Advances and Challenges in Nanomaterial-Based Electrochemical Immunosensors for Small Cell Lung Cancer Biomarker Neuron-Specific Enolase.

Early and rapid detection of neuron-specific enolase (NSE) is highly significant, as it is putative biomarker for small-cell lung cancer as well as COVID-19. Electrochemical techniques have attracted substantial attention for the early detection of cancer biomarkers due to the important properties of simplicity, high sensitivity, specificity, low cost, and point-of-care detection. This work reviews the clinically relevant labeled and label-free electrochemical immunosensors developed so far for the analysis of NSE. The prevailing role of nanostructured materials as electrode matrices is thoroughly discussed. Subsequently, the key performances of various immunoassays are critically evaluated in terms of limit of detection, linear ranges, and incubation time for clinical translation. Electrochemical techniques coupled with screen-printed electrodes developing market level commercialization of NSE sensors is also discussed. Finally, the review concludes with the current challenges associated with available methods and provides a future outlook toward commercialization opportunities for easy detection of NSE.

Tailoring cellulose paper via electroless CuSnB deposition for selective electrochemical detection of dopamine.

A novel, biodegradable substrate based, and cost-effective flexible electrochemical sensor was developed for the highly selective and sensitive detection of one of the major neurotransmitters, dopamine, which can be utilised as a disposable electrode for point-of-care diagnostic applications. The active material CuSnB decorated over cellulose paper exhibits good sensitivities of 3.92 µA µM-1 cm-2 with a limit of detection of 0.5 nM. Moreover, the flexible sensor demonstrated superior selectivity towards co-existing metabolites such as ascorbic acid, glucose, and uric acid, in addition to stability at various mechanical deformations.

Real-time screening of NixBy bifunctional electrocatalysts for overall NH3 synthesis via SG-TC SECM.

Electrochemical ammonia synthesis, which couples oxygen evolution at the anode with nitrogen reduction at the cathode, holds great significance for future food and energy needs. Both of these half-cell reactions determine the overall cell potential and efficiency of the process. However, the employment of different catalysts on either side, due to discrete mechanisms, increases the complexity and material processing costs of the system, where the designing of a bifunctional catalyst active towards both the NRR and OER is of huge significance. Unfortunately, the initial screening of the designed catalysts via physical characterizations, optical methods and other techniques, does not provide details about the electrochemical activity. The scanning electrochemical microscopy (SECM) technique can be useful to screen multi-catalysts at the same time for their electrochemical activities. Herein, we employed the sample generation-tip collection (SG-TC) mode of SECM to screen the designed NixBy catalysts before half-cell investigations, which suggested that the catalyst synthesized via sonochemical reduction (SR), i.e. NixBy (SR), was a better catalyst. This inference was in accordance with the half-cell NRR and OER measurements (FE: 49% for NH3 production, OER overpotential: 300 mV). By virtue of this remarkable bifunctional activity, the NRR-OER coupled full cell was assembled, which initiated the NH3 production at just 1.7 V and produced NH3 (1.08 mg h-1 mgcat-1) at the cathode and O2 (0.81 mg h-1 mgcat-1) at the anode after 2 h of electrolysis at 1.9 V.

Flexible electrochemical sensor for highly sensitive and selective non-enzymatic detection of creatinine via electrodeposited copper over polymelamine formaldehyde.

A non-enzymatic electrochemical biosensor was developed for highly sensitive detection of creatinine using copper nanoparticles supported over polymelamine formaldehyde. The synergy between the electrodeposited copper nanoparticles over the highly porous polymer (eCu-PMF) provided a greener platform to boost up the electron transport at the electrode electrolyte interface by eliminating the role of redox species as well as interference of major interferents like glucose, dopamine, and ascorbic acid in physiological media 0.1 M PBS (pH 7.4). The proposed sensor exhibited a wide detection range of 100 fM-60 mM with high sensitivities of 0.320 mA nM-1 cm-2 and 3.8 mA nM-1 cm-2. Moreover, the sensor was applied to real samples of serum creatinine and recoveries of 97 to 114% were found. Additionally, a paper-based flexible screen-printed electrode was fabricated which displayed an excellent activity with the same detection range of 100 fM-60 mM and long-term storage stability of 15 days.

Electrochemical Production of Hydrogen from Hydrogen Sulfide Using Cobalt Cadmium Sulfide.

The electrocatalytic decomposition of H2S is a promising technology for H2 production as well as for targeting environmental pollution. But due to the lack of low-cost and efficient electrocatalysts, this technology for H2 production is not being explored much. Moreover, the highly toxic and copious waste H2S released from industries is rarely encountered in the scientific domain. Herein, we have designed a highly efficient electrocatalyst, i.e., CoCd(x:y)Sn, as an anode catalyst for sulfide oxidation reaction (SOR). This optimized catalyst could drive the anode reaction at an onset potential of 0.25 V vs reversible hydrogen electrode (RHE), which was 1.27 V lower than that required for the water oxidation reaction. Moreover, we have achieved 98% H2 Faradaic efficiency with remarkable stability of 120 h. Thus, this method paves a path to high-value utilization of hazardous waste H2S and demonstrates its great potential for hydrogen production and sulfur toward sustainable energy applications.

Sustainable ammonia synthesis through electrochemical dinitrogen activation using an Ag2VO2PO4 catalyst.

Ammonia (NH3) is the second most produced chemical commodity with a worldwide production of 235 million tonnes in 2019, by virtue of its importance in fertilizer production, energy storage and transportation, and in the production of industrial chemicals. The most frequent method of NH3 production in large plants (1000 to 1500 t day-1) is the Haber-Bosch process, which has drawbacks of high greenhouse gas emissions (2.16 tonnes CO2 per tonne of NH3) and high energy consumption (over 30 GJ per tonne of NH3) due to high pressure and temperature conditions. For sustainable NH3 production, we require alternative green routes, wherein the electrochemical route holds huge potential due to reduced energy consumption and plant costs, higher selectivity, lower temperatures and pressures, and small to medium scale utilization of NH3. However, there are a number of challenges faced during the same viz. low production rates due to difficult N2 activation and reduced faradaic efficiency due to competing side reactions in aqueous electrolytes. Therefore, the most crucial aspect of electrochemical ammonia production technology is the design of an electrocatalyst which could activate the strong NN triple bond and effectively suppress the competing hydrogen evolution reaction (HER). In addition, the true NH3 yield estimation is of major concern due to the presence of possible N-contaminants, which may possibly lead to false estimation or overestimation of NH3. In this context, we have synthesized an Ag2VO2PO4 electrocatalyst with rice-grain like morphology via an energy efficient and less time consuming sonochemical method to carry out low temperature NH3 synthesis in an alkaline electrolyte. The choice of Ag metal and an alkaline environment effectually suppresses the HER and the bimetallic phosphate materials (Ag and V metals) induce high activity during nitrogen reduction, while rigorous analysis for tracing/elimination of N-labile and reducible species is considered for true NH3 production and assessment.

Dinitrogen Reduction Coupled with Methanol Oxidation for Low Overpotential Electrochemical NH3 Synthesis Over Cobalt Pyrophosphate as Bifunctional Catalyst.

Electrochemical dinitrogen (N2 ) reduction to ammonia (NH3 ) coupled with methanol electro-oxidation is presented in the current work. Here, methanol oxidation reaction (MOR) is proposed as an alternative anode reaction to oxygen evolution reaction (OER) to accomplish electrons-induced reduction of N2 to NH3 at cathode and oxidation of methanol at anode in alkaline media thereby reducing the overall cell voltage for ammonia production. Cobalt pyrophosphate micro-flowers assembled by nanosheets are synthesized via a surfactant-assisted sonochemical approach. By virtue of structural and morphological advantages, the maximum Faradaic efficiency of 43.37% and NH3 yield rate of 159.6 µg h-1 mgca -1 is achieved at a potential of -0.2 V versus RHE. The proposed catalyst is shown to also exhibit a very high activity (100 mA mg-1 at 1.48 V), durability (2 h) and production of value-added formic acid at anode (2.78 µmol h-1 mgcat -1 and F.E. of 59.2%). The overall NH3 synthesis is achieved at a reduced cell voltage of 1.6 V (200 mV less than NRR-OER coupled NH3 synthesis) when OER at anode is replaced with MOR and a high NH3 yield rate of 95.2 µg h-1 mgcat -1 and HCOOH formation rate of 2.53 µmol h-1 mg-1 are witnessed under full-cell conditions.

Selective Electrochemical Conversion of N2 to NH3 in Neutral Media Using B, N-Containing Carbon with a Nanotubular Morphology.

Electrochemical dinitrogen reduction (NRR) has riveted substantial attention as a greener method to synthesize ammonia (NH3) under ambient conditions. Here, B, N-containing carbon catalysts with a discrete morphology were synthesized from the metal-organic framework-ionic liquid (MOF-IL) composite for NRR in a neutral electrolyte medium (pH = 7). Morphology-dependent activity is witnessed, wherein C-BN@600 with a nanotubular morphology is able to achieve a high NH3 yield rate of 204 µg h-1 mgcat-1 and an F.E. of 16.7% with a TOF value of 0.2 h-1 at -0.2 V vs RHE. Further, a rigorous protocol is put forward for true NH3 estimation by tracing/eliminating any source of contamination in catalysts, electrolytes, or gas supply via ultraviolet-visible (UV-vis) spectroscopy, gas-purification methods, and isotope labeling experiments. Density functional theory predicts BN to be the favorable active site for N2 adsorption with a reduced energy barrier in the first reduction step and sequential stabilization of the B-N bond by an adjacent carbon atom.

Self-standing Fe3O4 decorated paper electrode as a binder-free trifunctional electrode for electrochemical ammonia synthesis and Zn-O2 batteries.

The conversion of the abundant biodegradable material into electroactive electrode material can be a good resource for sustainable energy conversion and storage applications. Herein, we present a simple, cost-effective and green approach for the fabrication of a flexible cellulose paper electrode using an electroless-electrodeposition method. The one-step electroless deposition route is followed to induce conductivity into a non-conductive cellulose paper substrate without using any expensive activators or sensitisers. The Fe3O4 is then electro-deposited as an active catalyst over the conductive paper substrate for use in electrochemical activities. The as-fabricated paper electrode shows promising activity and stability during the dinitrogen reduction reaction (NRR) as well as oxygen bifunctional electrocatalysis. A faradaic efficiency of 4.32% with a yield rate of 245 µg h-1 mgcat-1 at -0.1 V is achieved for NRR whereas a very small overpotential of 180 mV is required to reach 10 mA cm-2 during OER, and the ORR reaction starts at the onset potential of 0.86 V. The practical applicability of the paper electrode is validated by assembling a Zn-O2 battery showing a peak power density of 81 mW cm-2 and a stability up to 35 h during charge-discharge cycles, which can power the NRR to produce NH3 under full cell conditions.

Copper nanoparticles embedded in polyaniline derived nitrogen-doped carbon as electrocatalyst for bio-energy generation in microbial fuel cells.

Microbial fuel cells (SC-MFCs) have emerged as green energy devices to resolve the growing energy and environmental crisis. However, the technology's application depends on the sluggish oxygen reduction reaction (ORR) kinetics. Among the electrocatalysts explored, transition metal-nitrogen-carbon composites exhibit satisfactory ORR activity. Herein, we investigate the performance of copper-nitrogen-carbon (Cu/NC) electrocatalysts for ORR, highlighting the effect of temperature, role of nitrogen functionalities, and Cu-Nx sites in catalyst performance. Cu/NC-700 demonstrated satisfactory ORR activity with an onset potential of 0.7 V (vs. RHE) and a limiting current density of 3.4 mA cm-2. Cu/NC-700 modified MFC exhibited a maximum power density of 489.2 mW m-2, higher than NC-700 (107.3 mW m-2). These observations could result from synergistic interaction between copper and nitrogen atoms, high density of Cu-Nx sites, and high pyridinic-N content. Moreover, the catalyst exhibited superior stability, implying its use in long-term operations. The electrocatalytic performance of the catalyst suggests that copper-doped carbon catalysts could be potential metal-nitrogen-carbon material for scaled-up MFC applications.

Fabrication of NiFeB flexible electrode via electroless deposition towards selective and sensitive detection of dopamine.

A novel cost-effective and eco-friendly flexible electrochemical sensor was designed for highly selective and sensitive detection of dopamine to deal with the problems related to serious neurological disorders. The novel flexible paper electrode with NiFeB synthesised by simple dip-coating exhibits a high sensitivity of 35.35 µA µM-1 cm-2, 2.36 µA µM-1 cm-2 and 0.215 µA µM-1 cm-2 in the linear ranges from 10 nM to 1 µM, 5 µM to 50 µM and 100 µM to 400 µM, respectively, with an ultra-low detection limit of 2.1 nM. Besides, the free-standing flexible electrode for the detection of DA, the flexible paper sensor displayed superior selectivity towards various interferents, such as ascorbic acid, glucose and uric acid, as well as stability under various deformations.

A novel NiVP/Pi-based flexible sensor for direct electrochemical ultrasensitive detection of cholesterol.

A novel non-enzymatic electrochemical sensor was constructed to achieve a highly selective and ultrasensitive detection of cholesterol to address the problems related to serious coronary heart disease. The NiVP/Pi-based sensor exhibited ultra-high sensitivity of 5510.18 µA µM-1 cm-2 and 36.8 µA µM-1 cm-2 for 1 nM to 10 µM and 100 µM to 10 mM of cholesterol, respectively, with an ultra-low detection limit of 1 aM, along with superior selectivity for cholesterol when exposed to various interferents such as ascorbic acid, glucose and uric acid. Also, a novel NiVP/Pi-based flexible sensor coated onto Whatman filter paper was developed and displayed superior sensitivity, even with a human blood serum sample at physiological pH.

Ultrasensitive electrochemical biosensors for dopamine and cholesterol: recent advances, challenges and strategies.

The rapid and accurate determination of the dopamine (neurotransmitter) and cholesterol level in bio-fluids is significant because they are crucial bioanalytes for several lethal diseases, which require early diagnosis. The level of DA in the brain is modulated by the dopamine active transporter (DAT), and is influenced by cholesterol levels in the lipid membrane environment. Accordingly, electrochemical biosensors offer rapid and accurate detection and exhibit unique features such as low detection limits even with reduced volumes of analyte, affordability, simple handling, portability and versatility, making them appropriate to deal with augmented challenges in current clinical and point-of-care diagnostics for the determination of dopamine (DA) and cholesterol. This feature article focuses on the development of ultrasensitive electrochemical biosensors for the detection of cholesterol and DA for real-time and onsite applications that can detect targeted analytes with reduced volumes and sub-picomolar concentrations with quick response times. Furthermore, the development of ultrasensitive biosensors via cost-effective, simple fabrication procedures, displaying high sensitivity, selectivity, reliability and good stability is significant in the impending era of electrochemical biosensing. Herein, we emphasize on recent advanced nanomaterials used for the ultrasensitive detection of DA and cholesterol and discuss in depth their electrochemical activities towards ultrasensitive responses. Key points describing future perspectives and the challenges during detection with their probable solutions are discussed, and the current market is also surveyed. Further, a comprehensive review of the literature indicates that there is room for improvement in the miniaturization of cholesterol and dopamine biosensors for lab-on-chip devices and overcoming the current technical limitations to facilitate full utilization by patients at home.

Nickel Iron Phosphide/Phosphate as an Oxygen Bifunctional Electrocatalyst for High-Power-Density Rechargeable Zn-Air Batteries.

The evolution of an effective oxygen electrocatalyst is of great importance for the widespread application of Zn-air batteries but remains an immense challenge. Thus, an efficient catalyst toward the oxygen evolution reaction and oxygen reduction reaction (OER and ORR) is highly essential for high-performance Zn-air batteries. Here, we have reported bifunctional nickel iron phosphide/phosphate (NiFeP/Pi) catalysts with various Ni/Fe ratios toward oxygen electrocatalysis in alkaline media. These catalysts are highly active toward OER and ORR, wherein NiFe(1:2)P/Pi exhibits a low OER overpotential of 0.21 V at 10 mA cm-2 and a high ORR onset potential (0.98 V vs RHE) with the lowest potential difference (ΔE = E10 - E1/2) of 0.62 V, which surpasses that of the benchmark Pt/C and RuO2 catalyst as well as those of most previously reported bifunctional catalysts. Furthermore, the NiFe(1:2)P/Pi-based Zn-air battery demonstrates a very high power density of 395 mW cm-2 and outstanding discharge capacity of 900 mAh g-1@10 mA cm-2 along with steady cyclability, maintaining 98% of the round trip efficiency over 300 cycles. These results are helpful for a good understanding of the composition-activity relation with a certain band gap toward high-performance Zn-air batteries.

Highly sensitive non-enzymatic electrochemical glucose sensor surpassing water oxidation interference.

An electrochemical non-enzymatic sensor based on a NiVP/Pi material was developed for the selective and sensitive determination of glucose. The novel sensor showed a high sensitivity of 6.04 mA µM-1 cm-2 with a lowest detection limit of 3.7 nM in a wide detection range of 100 nM-10 mM. The proposed sensor exhibited a superior selectivity without any interference from the oxygen evolution reaction during glucose sensing. We also found that this glucose sensor showed negligible interference from various interferents, such as ascorbic acid, uric acid, dopamine and sodium chloride. Additionally, a novel flexible sensor was developed by coating the NiVP/Pi over Whatman filter paper, which exhibited two linear ranges of 100 nM to 1 µM and 100 µM to 10 mM with an ultra-sensitivity of 1.130 mA µM-1 cm-2 and 0.746 mA µM-1 cm-2, respectively, in 0.1 M NaOH. The proposed sensor was tested with human blood serum samples demonstrating its practical application. Our findings provide a new route by fine tuning the composition of nickel and vanadium that sheds new light on better understanding the processes. This NiVP/Pi-based sensor offers a new approach towards the electrochemical detection of glucose, enabling glucose monitoring in a convenient way.

Ultrasensitive and highly selective detection of dopamine by a NiFeP based flexible electrochemical sensor.

A ultrasensitive NiFeP based electrochemical sensor was developed for the selective electrochemical detection of dopamine to address the issue associated with neurological disorders including Parkinson's and Alzheimer's diseases. The novel sensor shows superior selectivity and sensitivity with a lowest detection limit of 0.3 nM and a wide detection range of 10 nM-500 µM and is immune to the interference of ascorbic acid at physiological pH (7.4). A novel flexible sensor was developed with NiFeP coated over Whatman filter paper, which exhibits two liner ranges of 10 nM-1µM and 10 µM-500 µM with an ultra high sensitivity of 756 µA µM-1 cm-2 and 4.6 µA µM-1 cm-2 respectively with a wide detection range of 10 nM to 500 µM.

Investigation of New B-Site-Disordered Perovskite Oxide CaLaScRuO6+δ: An Efficient Oxygen Bifunctional Electrocatalyst in a Highly Alkaline Medium.

Energy storage and conversion driven by electro- or photocatalyst is a highly exciting field of research, and generations of effective and durable oxide catalysts have received much attention in this field. Here, we report A-site lanthanum-doped oxygen-rich quinary oxide CaLaScRuO6+δ synthesized by adopting the solid-state reaction method and characterized by various techniques such as powder X-ray diffraction, neutron diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma-atomic emission spectrometry, Raman spectroscopy, and temperature-programmed reduction in the presence of a hydrogen atmosphere (H2-TPR). X-ray absorption study confirms the existence of mixed valent Ru ions in the structure, which enhances the oxygen stoichiometry for the partial balance of an extra cationic charge. Neutron powder diffraction and reduction of the material in a hydrogen atmosphere (H2-TPR) can confirm the oxygen overstoichiometry of the catalyst. The present material works as an efficient and robust oxygen bifunctional electrocatalyst for ORR/OER (oxygen evolution reaction/oxygen reduction reaction) followed by four-electron transfer pathway in a strong (1 M KOH) alkaline medium. The catalytic nature of the designed structural and chemical flexible perovskite is a novel example of an electrocatalyst for the oxygen bifunctional activity.

Non-Noble Cobalt Tungstate Catalyst for Effective Electrocatalytic Oxidation of Borohydride.

Morphologically tuned cobalt tungstate (CoWO4), a new entrant toward borohydride oxidation reaction (BOR), was explored as it exhibited negligible H2 evolution while enabling rapid BOR. A simple synthetic strategy was employed and fine-tuned to obtain different morphologies of CoWO4 whose urchin-shaped variant gave exciting activity toward BOR. An early and quite negative onset potential of -1.14 V was observed giving a maximum obtainable specific current density of 105.3 mA mg-1. The synthesized variants were investigated in depth by various electrochemical measurements and assessed in light of previous reports toward BOR activity. Hydrodynamic studies were also performed to ascertain the nature of these static electrochemical measurements. Quantitative assessment of the evolved H2, a prominent competitive reaction to BOR, was performed suggesting minimal interference. The probable origin of such morphology-dependent activity was subsequently studied in detail by high-resolution transmission electron microscopy (HR-TEM) analysis, revealing nanometric structures in the urchin-like variant, which enhance the obtainable BOR activity.