Electrochemical Immunosensors Based on Zinc Oxide Nanorods for Detection of Antibodies Against SARS-CoV-2 Spike Protein in Convalescent and Vaccinated Individuals

Even after over 2 years of the COVID-19 pandemic, research on rapid, inexpensive, and accurate tests remains essential for controlling and avoiding the global spread of SARS-CoV-2 across the planet during a potential reappearance in future global waves or regional outbreaks. Assessment of serological responses for COVID-19 can be beneficial for population-level surveillance purposes, supporting the development of novel vaccines and evaluating the efficacy of different immunization programs. This can be especially relevant for broadly used inactivated whole virus vaccines, such as CoronaVac, which produced lower titers of neutralizing antibodies. and showed lower efficacy for specific groups such as the elderly and immunocompromised. We developed an impedimetric biosensor based on the immobilization of SARS-CoV-2 recombinant trimeric spike protein (S protein) on zinc oxide nanorod (ZnONR)-modified fluorine-doped tin oxide substrates for COVID-19 serology testing. Due to electrostatic interactions, the negatively charged S protein was immobilized via physical adsorption. The electrochemical response of the immunosensor was measured at each modification step and characterized by scanning electron microscopy and electrochemical techniques. We successfully evaluated the applicability of the modified ZnONR electrodes using serum samples from COVID-19 convalescent individuals, CoronaVac-vaccinated with or without positive results for SARS-CoV-2 infection, and pre-pandemic samples from healthy volunteers as controls. ELISA for IgG anti-SARS-CoV-2 spike protein was performed for comparison, and ELISA for IgG anti-RBDs of seasonal coronavirus (HCoVs) was used to test the specificity of immunosensor detection. No cross-reactivity with HCoVs was detected using the ZnONR immunosensor, and more interestingly, the sensor presented higher sensitivity when compared to negative ELISA results. The results demonstrate that the ZnONRs/spike-modified electrode displayed sensitive results for convalescents and vaccinated samples and shows excellent potential as a tool for the population's assessment and monitoring of seroconversion and seroprevalence.

Metal-Oxide FET Biosensor for Point-of-Care Testing: Overview and Perspective

Metal-oxide semiconducting materials are promising for building high-performance field-effect transistor (FET) based biochemical sensors. The existence of well-established top-down scalable manufacturing processes enables the reliable production of cost-effective yet high-performance sensors, two key considerations toward the translation of such devices in real-life applications. Metal-oxide semiconductor FET biochemical sensors are especially well-suited to the development of Point-of-Care testing (PoCT) devices, as illustrated by the rapidly growing body of reports in the field. Yet, metal-oxide semiconductor FET sensors remain confined to date, mainly in academia. Toward accelerating the real-life translation of this exciting technology, we review the current literature and discuss the critical features underpinning the successful development of metal-oxide semiconductor FET-based PoCT devices that meet the stringent performance, manufacturing, and regulatory requirements of PoCT.

Preparation of uniform gold nanoparticles of different quantity deposited on zinc oxide nanorods for photoelectrochemical water splitting

For the photocatalytic test, gold nanoparticles (AuNPs) were prepared using trisodium citrate dehydrate (TCD), following which they were combined on the surface of zinc oxide (ZnO) to prepare ZnO decorated with uniform AuNPs (ZnO/AuNP) photocatalysts. The photocatalytic performance with the ZnO/AuNP was estimated through the rhodamine B (RB) dye degradation under solar irradiation. ZnO/AuNP-30 showed the greatest photocatalytic performance, achieving dye degradation efficiency up to 78.65%. Photoelectrochemical (PEC) measurements were performed using the ZnO/AuNP photoanodes. With AuNP doping amounts of 10, 20, and 30 mL on the ZnO surface, photocurrent densities of 47.46, 63.74, and 68.64 mA cm^-2, respectively, were achieved at an applied voltage of 1.5 V. These values indicated that the doping of AuNPs on the ZnO surface is advantageous for enhancing its PEC water-splitting activity. The highest solar-to-hydrogen (STH) efficiency is 22% with the ZnO/AuNP-30 photoanode at an applied voltage of 0.88 V. The interfacial charge-transfer resistances at the interface were 40 and 2.2 kΩ cm² for the ZnO and ZnO/AuNP-30 photoanodes, respectively.

Machine learning-assisted ammonium detection using zinc oxide/multi-walled carbon nanotube composite based impedance sensors

We report a machine learning approach to accurately correlate the impedance variations in zinc oxide/multi walled carbon nanotube nanocomposite (F-MWCNT/ZnO-NFs) to NH4+ ions concentrations. Impedance response of F-MWCNT/ZnO-NFs nanocomposites with varying ZnO:MWCNT compositions were evaluated for its sensitivity and selectivity to NH4+ ions in the presence of structurally similar analytes. A decision-making model was built, trained and tested using important features of the impedance response of F-MWCNT/ZnO-NF to varying NH4+ concentrations. Different algorithms such as kNN, random forest, neural network, Naïve Bayes and logistic regression are compared and discussed. ML analysis have led to identify the most prominent features of an impedance spectrum that can be used as the ML predictors to estimate the real concentration of NH4+ ion levels. The proposed NH4+ sensor along with the decision-making model can identify and operate at specific operating frequencies to continuously collect the most relevant information from a system.

Nanowire-based sensor electronics for chemical and biological applications

Detection and recognition of chemical and biological species via sensor electronics are important not only for various sensing applications but also for fundamental scientific understanding. In the past two decades, sensor devices using one-dimensional (1D) nanowires have emerged as promising and powerful platforms for electrical detection of chemical species and biologically relevant molecules due to their superior sensing performance, long-term stability, and ultra-low power consumption. This paper presents a comprehensive overview of the recent progress and achievements in 1D nanowire synthesis, working principles of nanowire-based sensors, and the applications of nanowire-based sensor electronics in chemical and biological analytes detection and recognition. In addition, some critical issues that hinder the practical applications of 1D nanowire-based sensor electronics, including device reproducibility and selectivity, stability, and power consumption, will be highlighted. Finally, challenges, perspectives, and opportunities for developing advanced and innovative nanowire-based sensor electronics in chemical and biological applications are featured.

Mesoporous TiO2 Implanted ZnO QDs for the Photodegradation of Tetracycline: Material Design, Structural Characterization and Photodegradation Mechanism

A sol-gel method was used to prepare a mesoporous TiO2 implanted with a ZnO quantum dot photocatalyst (TZQ) for the photodegradation of tetracycline (TC) under fluorescent light irradiation. Scanning electron microscopy (SEM) shows the presence of cavities on the photocatalyst surface due to the use of starch as a synthetic template, where the nitrogen sorption results indicate that TZQ contains mesopores with reduced size (ca. 4.3 nm) versus the pore size of the parent meso-TiO2 (ca. 7.5 nm). The addition of ZnO quantum dots (QDs) resulted in spherically-shaped binary composite particles in layers onto the surface of TiO2. The coexistence of the ZnO QDs and TiO2 phase was observed using high resolution-transmission electron microscopy (HR-TEM). The photodegradation of TC was carried out in a homemade reactor equipped with two fluorescent lights (24 W each) and within 90 min of irradiation, 94.6% of TC (40 mg L−1) was photodegraded using 250 mg L−1 of TZQ at pH 9. The major reactive oxygen species identified from the scavenging tests were O2●− followed by HO●. The deconvolution of the photoluminescence spectrum of TZQ indicates the presence of a strong quantum confinement effect (QCE) of the ZnO QDs, a defect related to Ti-species and oxygen. The analysis of the intermediates detected by LC-time-of-flight/mass spectrometry (LC/TOF-MS) suggest two photodegradation pathways. The pathways were validated using the Fukui function approach and the Wheland localisation approach. This simple and efficient photocatalytic technology is anticipated to benefit small-scale animal husbandries and aquaculture operators that have limited access to sustainable water treatment technology.

CuO Nanoparticles Decorated ZnO Nanorods Based Extended-Gate Field-Effect-Transistor (EGFET) for Enzyme-Free Glucose Sensing Application

In this work, CuO nanoparticles (NPs) decorated zinc oxide nanorods (ZnO NRs) on fluorine-doped tin oxide (FTO) substrate has been used as a working electrode. This working electrode has been used as an extended gate for field-effect transistor. The main objective is to use the EGFET (extended gate field effect transistor) as a glucose sensor. The proposed glucose sensor has good sensitivity of 6.643 mV/mM with a wide range of linearity (1mM-8mM) which covers the glucose level of human blood ranging from 4.4 mM to 6.6 mM. This novel concept of the glucose sensing using CuO NPs decorated ZnO nanorods based EGFET may be explored for sensing other saccharides such as mannose, fructose, and sucrose. This vertically grown ZnO nanorods decorated with CuO NPs based electrode gives reliable selectivity, good repeatability, and more stability. The performance of proposed sensor is also compared with commercially available glucose sensors. The sensitivity performance of the glucose sensor also confirms the capability to detect the glucose level from human blood and serum.

Potentiometric Performance of a Highly Flexible-Shaped Trifunctional Sensor Based on ZnO/V2O5 Microrods

A trifunctional flexible sensor was fabricated on a polyethylene terephthalate (PET) fiber surface. Synthesized ZnO and ZnO/V2O5 composite were coated on ZnO seed layer sputtered PET fiber. X-ray diffraction (XRD) and photoelectron spectroscopy (XPS) techniques confirmed the exact formation of ZnO and ZnO/V2O5. The fabricated ZnO/V2O5 on ZnO seeds base temperature sensor recorded better electrical properties and reversibility with a maximum temperature coefficient resistance (TCR) of 0.0111 °C-1. A calibration curve (R = 0.9941) within glucose concentration of (10 µM-10 mM) was obtained at +0.8 V vs. Ag/AgCl from current-voltage curves which assisted in calculating glucose sensitivity, limit of detection (LOD), limit of quantification (LOQ). The electrode achieved an outstanding performance of sensitivity (72.06 µAmM-1cm-2), LOD (174 µM), and LOQ (582 µM) at optimum deposition time. Interference from oxidation of interfering biomolecules such as ascorbic acid, dopamine, and uric acid were negligible compared to glucose. Finally, the fabricated electrode was employed as a pH sensor and displayed a pH sensitivity of 42.26 mV/pH (R = 0.9922). This fabricated ZnO/V2O5 electrode exhibited high sensitivity and a stable combined temperature, glucose, and pH sensor which is promising for development of multifunctional sensors in next generation wearables.

Highly sensitive non-enzymatic electrochemical glucose sensor based on dumbbell-shaped double-shelled hollow nanoporous CuO/ZnO microstructures

A high-performance non-enzymatic glucose sensor based on hybrid metal-oxides is proposed. Dumbbell-shaped double-shelled hollow nanoporous CuO/ZnO microstructures (CuO/ZnO-DSDSHNM) were prepared via the hydrothermal method using pluronic F-127 as a surfactant. This structure is studied by various physicochemical characterizations such as scanning electron microscopy, X-ray diffraction spectroscopy, inductively coupled plasma atomic emission spectroscopy, elemental mapping techniques, X-ray photoelectron spectroscopy, and transmission electron microscopy. This unique CuO/ZnO-DSDSHNM provides both a large surface area and an easy penetrable structure facilitating improved electrochemical reactivity toward glucose oxidation. The prepared CuO/ZnO-DSDSHNM was used over the glassy carbon electrode (GCE) as the active material for glucose detection and then coated by Nafion to provide the proposed Nafion/CuO/ZnO-DSDSHNM/GCE. The fabricated glucose sensor exhibits an extremely wide dynamic range from 500 nM to 100 mM, a sensitivity of 1536.80 µA mM^-1 cm^-2, a low limit of detection of 357.5 nM, and a short response time of 1.60 s. The proposed sensor also showed long-term stability, good reproducibility, favorable repeatability, excellent selectivity, and satisfactory applicability for glucose detection in human serum samples. The achieved high-performance glucose sensing based on Nafion/CuO/ZnO-DSDSHNM/GCE shows that both the material synthesis and the sensor fabrication methods have been promising and they can be used in future researches.

Real-time in situ auto-correction of K+ interference for continuous and long-term NH4+ monitoring in wastewater using solid-state ion selective membrane (S-ISM) sensor assembly

Potassium ions (K⁺) present in wastewater has caused severe interference for NH₄⁺ monitoring, over-estimation of NH₄⁺ concentration and ultimately leads to extra energy consumption. Past effort for enhancing the selectivity of NH₄⁺ over K⁺ were oftentimes complex, costly, or compromised the selectivity and accuracy of the NH₄⁺ ion selective membrane (ISM) sensors. This study targeted this imminent challenge by developing an integrated NH₄⁺/K⁺ auto-correction solid-state ISM (S-ISM) sensor assembly combined with a data-driven model to monitor [NH₄⁺] under different [NH₄⁺] and [K⁺] concentrations. The results showed that the interference of K⁺ was substantially alleviated for NH₄⁺ measurement. The accuracy was enhanced by over 70% when examined using real wastewater and energy consumption was expected to reduce by 26% for a wastewater treatment plant, especially for wastewater with high [K⁺]. Furthermore, the uniquely structured S-ISMs were made by embedding the ionophores in a robust polyvinyl chloride (PVC) matrix containing plasticizers and a layer of carbon nanotubes (CNT) as ion-to-electron transducer, which maintained the selectivity and accuracy of the S-ISM sensor for 4 weeks in wastewater. NH₄⁺/K⁺ sensor assembly integrated with data-driven correction models poses great potential in high-efficiency and energy-saving wastewater treatment and water reuse processes.

Au nanoparticles modified CuO nanowireelectrode based non-enzymatic glucose detection with improved linearity

This paper explores gold nanoparticle (GNP) modified copper oxide nanowires(CuO NWs)based electrode grown on copper foil for non-enzymatic glucose detection in a wide linear ranging up to 31.06 mM, and 44.36 mM at 0.5 M NaOH and 1 M NaOH concentrations. The proposed electrode can be used to detect a very low glucose concentration of 0.3 µM with a high linearity range of 44.36mM and sensitivity of 1591.44 µA mM-1 cm-2. The electrode is fabricated by first synthesizing Cu (OH)2 NWs on a copper foil by chemical etching method and then heat treatment is performed to convert Cu (OH)2 NWs into CuO NWs. The GNPs are deposited on CuO NWs to enhance the effective surface-to-volume ratio of the electrode with improved catalytic activity. The surface morphology has been investigated by XRD, XPS, FE-SEM and HR-TEM analysis. The proposed sensor is expected to detect low-level of glucose in urine, and saliva. At the same time, it can also be used to measure extremely high sugar levels (i.e. hyperglycemia) of ~ 806.5454 mg/dl. The proposed sensor is also capable of detecting glucose after multiple bending of the GNP modified CuO NWs electrode. The proposed device is also used to detect the blood sugar level in human being and it is found that this sensor's result is highly accurate and reliable.

Nozzle-Jet-Printed Silver/Graphene Composite-Based Field-Effect Transistor Sensor for Phosphate Ion Detection

High concentration of dissolved phosphate ions is the main responsible factor for eutrophication of natural water bodies. Therefore, detection of phosphate ions is essential for evaluating water eutrophication. There is a need at large-scale production of real-time monitoring technology to detect phosphorus accurately. In this study, facile enzymeless phosphate ion detection is reported using a nozzle-jet-printed silver/reduced graphene oxide (Ag/rGO) composite-based field-effect transistor sensor on flexible and disposable polymer substrates. The sensor exhibits promising results in low concentration as well as real-time phosphate ion detection. The sensor shows excellent performance with a wide linear range of 0.005-6.00 mM, high sensitivity of 62.2 μA/cm²/mM, and low detection limit of 0.2 μM. This facile combined technology readily facilitates the phosphate ion detection with high performance, long-term stability, excellent reproducibility, and good selectivity in the presence of other interfering anions. The sensor fabrication method and phosphate detection technique yield low-cost, user-friendly sensing devices with less analyte consumption, which are easy to fabricate on polymer substrates on a large scale. Besides, the sensor has the capability to sense phosphate ions in real water samples, which makes it applicable in environmental monitoring.

Reusable nano-BG-FET for point-of-care estimation of ammonia and urea in human urine

A back-gate-field-effect-transistor (BG-FET) has been developed to selectively detect ammonia and urea. The BG-FET was prepared on a p-type Si substrate with an n-type channel of CdS-TiO₂ nanocomposite and poly-methyl methacrylate film as dielectric layer. The reusability of the sensor was ensured by putting it as a cover to a chamber where samples were detected. The BG-FET showed an increase in drain current with the increase in ammonia release from chamber because higher numbers of charge carriers were created when ammonia adsorped on CdS-TiO₂ nanostructures. Control experiments suggested that the variation in current-to-voltage response of BG-FET could also be calibrated to measure the activity of a host of other hazardous gases. The lowest concentration of ammonia detected was ∼0.85 ppm with a response time of 30 s at a gate voltage of 0.5 V or less, which were superior than available field effect transistors ammonia sensors. Addition of urease in urine liberated ammonia equivalent to urea content in urine, which could be detected by the proposed BGFET. The urea-urease enzyme catalysis reaction made the sensor specific in detecting the biomarker. The accuracy, sensitivity, and reusability of the device was found to be suitable to develop a point-of-care testing device for ammonia and urea detection.

Real-Time in Situ Monitoring of Nitrogen Dynamics in Wastewater Treatment Processes using Wireless, Solid-State, and Ion-Selective Membrane Sensors

Real-time, in situ accurate monitoring of nitrogen contaminants in wastewater over a long-term period is critical for swift feedback control, enhanced nitrogen removal efficiency, and reduced energy consumption of wastewater treatment processes. Existing nitrogen sensors suffer from high cost, low stability, and short life times, posing hurdles for their mass deployment to capture a complete picture within heterogeneous systems. Tackling this challenge, this study presents solid-state ion-selective membrane (S-ISM) nitrogen sensors for ammonium (NH₄⁺) and nitrate (NO₃^-) in wastewater that were coupled to a wireless data transmission gateway for real-time remote data access. Lab-scale test and continuous-flow field tests using real municipal wastewater indicated that the S-ISM nitrogen sensors possessed excellent accuracy and precision, high selectivity, and multiday stability. Importantly, autocorrections of the sensor readings on the cloud minimized temperature influences and assured accurate nitrogen concentration readings in remote-sensing applications. It was estimated that real-time, in situ monitoring using wireless S-ISM nitrogen sensors could save 25% of electric energy under normal operational conditions and reduce 22% of nitrogen discharge under shock conditions.

Field-Effect Biosensors for On-Site Detection: Recent Advances and Promising Targets

There is an explosive interest in the immediate and cost-effective analysis of field-collected biological samples, as many advanced biodetection tools are highly sensitive, yet immobile. On-site biosensors are portable and convenient sensors that provide detection results at the point of care. They are designed to secure precision in highly ionic and heterogeneous solutions with minimal hardware. Among various methods that are capable of such analysis, field-effect biosensors are promising candidates due to their unique sensitivity, manufacturing scalability, and integrability with computational circuitry. Recent developments in nanotechnological surface modification show promising results in sensing from blood, serum, and urine. This report gives a particular emphasis on the on-site efficacy of recently published field-effect biosensors, specifically, detection limits in physiological solutions, response times, and scalability. The survey of the properties and existing detection methods of four promising biotargets, exosomes, bacteria, viruses, and metabolites, aims at providing a roadmap for future field-effect and other on-site biosensors.

Highly Efficient Non-Enzymatic Glucose Sensor Based on CuO Modified Vertically-Grown ZnO Nanorods on Electrode

There is a major challenge to attach nanostructures on to the electrode surface while retaining their engineered morphology, high surface area, physiochemical features for promising sensing applications. In this study, we have grown vertically-aligned ZnO nanorods (NRs) on fluorine doped tin oxide (FTO) electrodes and decorated with CuO to achieve high-performance non-enzymatic glucose sensor. This unique CuO-ZnO NRs hybrid provides large surface area and an easy substrate penetrable structure facilitating enhanced electrochemical features towards glucose oxidation. As a result, fabricated electrodes exhibit high sensitivity (2961.7 μA mM^-1 cm^-2), linear range up to 8.45 mM, low limit of detection (0.40 μM), and short response time (<2 s), along with excellent reproducibility, repeatability, stability, selectivity, and applicability for glucose detection in human serum samples. Circumventing, the outstanding performance originating from CuO modified ZnO NRs acts as an efficient electrocatalyst for glucose detection and as well, provides new prospects to biomolecules detecting device fabrication.

ZnO nanorods array based field-effect transistor biosensor for phosphate detection

A promising field-effect transistor (FET) biosensor has been fabricated based on pyruvate oxidase (PyO) functionalized ZnO nanorods (ZnO NRs) array grown on seeded SiO₂/Si substrate. The direct and vertically grown ZnO NRs on the seeded SiO₂/Si substrate offers high surface area for enhanced PyO immobilization, which further helps to detect phosphate with higher specificity. Under optimum conditions, the fabricated FET biosensor provided a convenient method for phosphate detection with high sensitivity (80.57μAmM^-1cm^-2) in a wide-linear range (0.1µM-7.0mM). Additionally, it also showed very low effect of electroactive species, stability and good reproducibility. Encouraging results suggest that this approach presents a promising method to be used for field measurements to detect phosphate.

Indium-doped ZnO horizontal nanorods for high on-current field effect transistors

High on-current field effect transistors (FETs) are highly desirable for driving information displays such as active matrix organic light-emitting diode displays.