Electrochemical Ultrasensitive Sensing of Uric Acid on Non-Enzymatic Porous Cobalt Oxide Nanosheets-Based Sensor

Abstract

Transition metal oxide (TMO)-based nanomaterials are effectively utilized to fabricate clinically useful ultra-sensitive sensors. Different nanostructured nanomaterials of TMO have attracted a lot of interest from researchers for diverse applications. Herein, we utilized a hydrothermal method to develop porous nanosheets of cobalt oxide. This synthesis method is simple and low temperature-based. The morphology of the porous nanosheets like cobalt oxide was investigated in detail using FESEM and TEM. The morphological investigation confirmed the successful formation of the porous nanosheet-like nanostructure. The crystal characteristic of porous cobalt oxide nanosheets was evaluated by XRD analysis, which confirmed the crystallinity of as-synthesized cobalt oxide nanosheets. The uric acid sensor fabrication involves the fixing of porous cobalt oxide nanosheets onto the GCE (glassy carbon electrode). The non-enzymatic electrochemical sensing was measured using CV and DPV analysis. The application of DPV technique during electrochemical testing for uric acid resulted in ultra-high sensitivity (3566.5 µAmM^-1cm^-2), which is ~7.58 times better than CV-based sensitivity (470.4 µAmM^-1cm^-2). Additionally, uric acid sensors were tested for their selectivity and storage ability. The applicability of the uric acid sensors was tested in the serum sample through standard addition and recovery of known uric acid concentration. This ultrasensitive nature of porous cobalt oxide nanosheets could be utilized to realize the sensing of other biomolecules.

Keywords: cobalt oxide; electrochemical; nanosheets; non-enzymatic; porous; sensor; ultra-sensitive; uric acid.

Scheme 1

Schematic showing the synthesis process of porous cobalt oxide nanosheets, sensor fabrication process, and measurement techniques used.

Figure 1

FESEM images at low- (a,b) and high- (c) magnifications and XRD analysis (d) of porous cobalt oxide nanostructure.

Figure 2

TEM images at low- (a), high- (b) magnifications, SAED pattern (c), and BET Nitrogen adsorption-desorption isotherms of porous cobalt oxide sheet-like nanostructure. Inset (d) shows the pore size distribution plot of cobalt oxide sheet-like nanostructure.

Figure 3

EIS spectra of bare GCE and cobalt oxide/GCE electrodes recorded in the redox probe solution of [Fe(CN)₆]^3−/4− with KCl (0.1 M) “as supporting electrolyte”. (a) Nyquist plots of bare GCE (black line) and cobalt oxide/GCE (red line) electrodes and their respective Bode plots (b,c). Inset a shows the Randle circuit.

Figure 4

(a) CV response curves of bare GCE and cobalt oxide/GCE electrodes recorded in the redox probe solution of [Fe(CN)₆]^3−/4− with KCl (0.1 M) “as supporting electrolyte”. The CV curves were measured at the fixed scan rate (50 mV/s). (b) CV analysis of cobalt oxide/GCE electrode at varying scan rates and respective calibration plot (c). In figure (c), the arrow shows the scan rate increasing direction from 10 mV/s to 250 mV/s.

Figure 5

(a) CV response curves recorded for cobalt oxide/GCE electrode in PBS without and with uric acid (10 µM) at 50 mV/s, (b) CV responses of cobalt oxide/GCE electrode with increasing uric acid concentrations (0−2500 µM), and (c) plot of current response vs. uric acid concentration showing linear and non−linear regions. Inset b shows the magnified CV response curves for low-concentration uric acid. Inset c shows the calibration plot of the linear region (i.e., response vs. uric acid concentration).

Scheme 2

Scheme showing uric acid oxidation over the porous cobalt oxide nanosheets/GCE sensor.

Figure 6

(a) DPV response curves recorded for cobalt oxide/GCE sensor in PBS without and with uric acid (10 µM), (b) DPV responses of cobalt oxide/GCE electrode with increasing uric acid concentrations (0–2500 µM), and (c) plot of current response vs. uric acid concentration showing linear and non-linear regions. Inset b shows the magnified DPV response curves for low-concentration uric acid. Inset c shows the calibration plot of the linear region (i.e., response vs. uric acid concentration).

Figure 7

(a) CV response curves recorded for cobalt oxide/GCE sensor in PBS with 25 µM uric acid only (black curve) and 25 µM uric acid and 100 µM of each interfering species (i.e., lactic acid, L−cysteine, glucose, urea, fructose, sodium chloride, and potassium chloride) (red curve). (b) CV response curves of cobalt oxide/GCE sensor for 25 µM uric acid showing stability of sensor after 30 and 45 days. (c) DPV response curves for 25 µM uric acid only (black curve) and 25 µM uric acid and 100 µM of each interfering species (i.e., ascorbic acid, dopamine, and urea).