Preparation of a Highly Conductive Seed Layer for Calcium Sensor Fabrication with Enhanced Sensing Performance.

The seed layer plays a crucial role in achieving high electrical conductivity and ensuring higher performance of devices. In this study, we report fabrication of a solution-gated field-effect transistor (FET) sensor based on zinc oxide nanorods (ZnO NRs) modified iron oxide nanoparticles (α-Fe2O3 NPs) grown on a highly conductive sandwich-like seed layer (ZnO seed layer/Ag nanowires/ZnO seed layer). The sandwich-like seed layer and ZnO NRs modification with α-Fe2O3 NPs provide excellent conductivity and prevent possible ZnO NRs surface damage from low pH enzyme immobilization, respectively. The highly conductive solution-gated FET sensor employed the calmodulin (CaM) immobilization on the surface of α-Fe2O3-ZnO NRs for selective detection of calcium ions (Ca2+). The solution-gated FET sensor exhibited a substantial change in conductance upon introduction of different concentrations of Ca2+ and showed high sensitivity (416.8 µA cm-2 mM-1) and wide linear range (0.01-3.0 mM). In addition, the total Ca2+ concentration in water and serum samples was also measured. Compared to the analytically obtained data, our sensor was found to measure Ca2+ in the water and serum samples accurately, suggesting a potential alternative for Ca2+ determination in water and serum samples, specifically used for drinking/irrigation and clinical analysis.

Fabrication of a solution-gated transistor based on valinomycin modified iron oxide nanoparticles decorated zinc oxide nanorods for potassium detection.

There are considerable interests to detect and monitor the abnormal level of minerals in water for avoiding/preventing any toxic effects after consumption. Herein, we report the fabrication of solution-gated field-effect-transistor (FET) based potassium sensor using iron oxide nanoparticles (Fe2O3 NPs) modified directly grown zinc oxide nanorods (ZnO NRs). The Fe2O3 NPs modification of ZnO NRs provided stability to nanorods surface and improved surface area for valinomycin immobilization. As-fabricated potassium sensor (valinomycin-Fe2O3 NPs-ZnO NRs/SiO2/Si) provided enhanced current response with increasing potassium concentration. During sensing measurements, FET sensor showed high sensitivity (4.65 µA/µM/cm2) in the linear range of 0.1 µM to 125 µM, low limit of detection (∼0.04 µM), good stability, excellent reproducibility, and favorable selectivity. Thus, good sensing performance of the FET based potassium sensor presents it as simple, low-cost, and convenient device for selective detection of potassium in solution.

Synthesis of manganese oxide nanorods and its application for potassium ion sensing in water.

Potassium is an important body mineral that control the cellular and electrical functions in the body. The potassium ion concentration change in human serum causes the risk of acute cardiac arrhythmia. Hence, it is important to monitor the potassium level in drinking water/food to control the intake and prevent its effect. This paper reports synthesis of manganese oxide (MnO2) nanorods using low-temperature sol-gel method for the fabrication of non-enzymatic potassium ion sensor. The detailed investigation of the as-synthesized MnO2 nanorods were carried out using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The morphological and structural observations revealed that this method yield small nanorods with average length and diameters of about 210 ±â€¯10 nm and 20 ±â€¯3 nm, respectively. Further, as-synthesized α-MnO2 nanorods were used to fabricate non-enzymatic potassium ion sensor following the deposition of α-MnO2 nanorods on glassy carbon electrode (GCE) with the help of conductive binder. The electrochemical characterizations of fabricated non-enzymatic potassium sensor showed good sensing performance (i.e. sensitivity, selectivity, long term stability, and reproducibility). Moreover, applicability of the sensor to detect potassium ion in water samples were also demonstrated.

Fabrication of sensitive non-enzymatic nitrite sensor using silver-reduced graphene oxide nanocomposite.

There are increasing demands of more sensitive sensors for monitoring potential hazards in real water that may cause serious problems to human health. Herein, we report the development of a non-enzymatic nitrite sensor using nanocomposite of reduced graphene oxide decorated with silver nanoparticle (Ag-rGO). First, Ag-rGO nanocomposite was synthesized using a facile and cost-effective microwave-assisted approach. Then, as-synthesized Ag-rGO nanocomposite was used to modify glassy carbon electrode (GCE) and applied for the sensitive and selective detection of nitrite in the aqueous medium with increasing concentration of nitrite. Under optimized conditions, sensor achieved high sensitive response (18.4 µA/µM·cm2) in a wide linear range (0.1-120 µM), low limit of detection (∼0.012 µM), and good selectivity using differential pulse voltammograms (DPV). The applicability of fabricated non-enzymatic nitrite sensor was checked in real sample with satisfactory results.

Recent advances in nanowires-based field-effect transistors for biological sensor applications.

Nanowires (NWs)-based field-effect transistors (FETs) have attracted considerable interest to develop innovative biosensors using NWs of different materials (i.e. semiconductors, polymers, etc.). NWs-based FETs provide significant advantages over the other bulk or non-NWs nanomaterials-based FETs. As the building blocks for FET-based biosensors, one-dimensional NWs offer excellent surface-to-volume ratio and are more suitable and sensitive for sensing applications. During the past decade, FET-based biosensors are smartly designed and used due to their great specificity, sensitivity, and high selectivity. Additionally, they have the advantage of low weight, low cost of mass production, small size and compatible with commercial planar processes for large-scale circuitry. In this respect, we summarize the recent advances of NWs-based FET biosensors for different biomolecule detection i.e. glucose, cholesterol, uric acid, urea, hormone, proteins, nucleotide, biomarkers, etc. A comparative sensing performance, present challenges, and future prospects of NWs-based FET biosensors are discussed in detail.

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.

Solution Process Synthesis of High Aspect Ratio ZnO Nanorods on Electrode Surface for Sensitive Electrochemical Detection of Uric Acid.

This study demonstrates a highly stable, selective and sensitive uric acid (UA) biosensor based on high aspect ratio zinc oxide nanorods (ZNRs) vertical grown on electrode surface via a simple one-step low temperature solution route. Uricase enzyme was immobilized on the ZNRs followed by Nafion covering to fabricate UA sensing electrodes (Nafion/Uricase-ZNRs/Ag). The fabricated electrodes showed enhanced performance with attractive analytical response, such as a high sensitivity of 239.67 µA cm-2 mM-1 in wide-linear range (0.01-4.56 mM), rapid response time (~3 s), low detection limit (5 nM), and low value of apparent Michaelis-Menten constant (Kmapp, 0.025 mM). In addition, selectivity, reproducibility and long-term storage stability of biosensor was also demonstrated. These results can be attributed to the high aspect ratio of vertically grown ZNRs which provides high surface area leading to enhanced enzyme immobilization, high electrocatalytic activity, and direct electron transfer during electrochemical detection of UA. We expect that this biosensor platform will be advantageous to fabricate ultrasensitive, robust, low-cost sensing device for numerous analyte detection.

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.

Highly stable hydrazine chemical sensor based on vertically-aligned ZnO nanorods grown on electrode.

Herein, we report a binder-free, stable, and high-performance hydrazine chemical sensor based on vertically aligned zinc oxide nanorods (ZnO NRs), grown on silver (Ag) electrode via low-temperature solution route. The morphological characterizations showed that the NRs were grown vertically in high density and possess good crystallinity. The as-fabricated hydrazine chemical sensors showed an excellent sensitivity of 105.5 µAµM-1cm-2, a linear range up to 98.6µM, and low detection limit of 0.005µM. It also showed better long-term stability, good reproducibility and selectivity. Furthermore, the fabricated electrodes were evaluated for hydrazine detection in water samples. We found the approach of directly growing nanostructures as a key factor for enhanced sensing performance of our electrodes, which effectively transfers electron from ZnO NRs to conductive Ag electrode. Thus it holds future prospective applications as binder-free, cost-effective, and stable sensing devices fabrication.

Development of highly-stable binder-free chemical sensor electrodes for p-nitroaniline detection.

Herein, pre-seeded fluorine doped tin oxide (FTO) glass substrates were used as an electrode for zinc oxide nanorods (ZnO NRs) growth by a low-temperature solution route in order to fabricate binder-free high-sensitive chemical sensor. The vertically-grown ZnO NRs exhibited a more favorable active morphology and improved sensing properties for p-nitroaniline (pNA) detection. On investigation with different concentrations of pNA, the ZnO NRs/FTO electrode showed an excellent sensitivity (10.18µAµM-1cm-2) and low detection limit (0.5µM) with good selectivity, outstanding long-term stability, and high reproducibility. Collectively, the present work emphasizes the potency of ZnO NRs/FTO electrodes for fabrication of an efficient and reliable chemical sensing device with improved performances.