An electrochemical sensor based on a Co₃O₄–ERGO nanocomposite modified screen-printed electrode for detection of uric acid in artificial saliva

Abstract

In this study, we report the fabrication of a nanocomposite consisting of Co₃O₄ nanoparticles (Co₃O₄ NPs) and electrochemically reduced graphene oxide (ERGO) on a screen-printed electrode (SPE) and its sensing performance in the electrochemical detection of uric acid (UA). The surface modification of the electrode was confirmed by using a variety of characterization techniques (FE-SEM, XRD, AFM, EDX, WCA, FTIR, and Raman spectroscopy). In addition, the surface modification was electrochemically characterized step by step through CV, EIS and DPV techniques, and the results showed that the Co₃O₄–ERGO nanocomposite exhibited highly sensitive and selective sensing performance towards the oxidation of UA in 0.1 M (pH 7.0) phosphate buffer solution (PBS). The sensor (Co₃O₄–ERGO/SPE) signals were observed to be linear to the UA concentration in the range of 5 μM to 500 μM (R² = 0.9985). After revealing its other performance characteristics, such as repeatability, reproducibility, stability, sensitivity, and selectivity, the sensor was successfully applied to the analysis of UA in artificial saliva samples.

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1. Introduction

Human saliva is basically a fluid secreted from the salivary glands, keeping the oral cavity moist and lining the teeth together with mucosa.¹ Although human saliva is composed of 98% water, the remaining 2% is made up of many important substances such as electrolytes (sodium, potassium, magnesium, calcium, chloride, bicarbonate, phosphate, and iodine), mucus, antibacterial compounds (thiocyanate, hydrogen peroxide, and IgA), and various enzymes (lysozyme, α-amylase, lingual lipase, and lactoferrin).^2,3 In addition, saliva, which correlates strongly with blood components, provides an alternative to direct blood analysis.^4,5 One of these components is uric acid (UA), which is found in a lower concentration in saliva than in blood serum. UA is the end product of the metabolic degradation of purines in the human body. In healthy individuals, the normal concentration of UA in the blood is in the range of 240–520 μM.⁶ The abnormal concentration of UA in a human body may cause several diseases, such as gout, hyperuricemia, and Lesch–Nyhan syndrome,⁷ and poses a risk for obesity,⁸ cardiovascular disease,⁹ and kidney disease.¹⁰ Therefore, it is very important to determine UA, which is a critical biomarker in biological fluids. In addition to various conventional methods,^11–16 the electrochemical techniques^17,18 are also used for the determination of UA. Among these methods, electrochemical sensing systems exhibit an effective approach to target analyte detection due to their advantages such as fast response, high sensitivity, and easy applicability to the sample.^19,20 Also, different sensing platforms applied to saliva samples have been reported for the detection of UA.^6,21 However, the development of electrochemical (bio)sensors applicable to biological fluids for UA detection with high sensitivity and selectivity is still ongoing.

In this work, we aimed to design a new electrochemical sensor to electrochemically detect UA in saliva samples using a screen-printed electrode (SPE) that allows detection of the analyte in a few drops of samples as well as point-of-care testing. The SPE was modified with ERGO and Co₃O₄ NPs to improve its sensing performance towards the electrooxidation of UA. After the structural and the morphological characterization of Co₃O₄–ERGO, its current response was examined in detail utilizing EIS, CV, and DPV techniques. As a form of graphene oxide (GO), reduced graphene oxide (rGO) is processed using chemical,¹⁹ electrochemical, thermal, and other methods to reduce oxygen-containing functional groups in the structure of GO. Recently, electrochemical reduction of GO to produce graphene has shown great promise since it can be performed without any hazardous reagents.^22,23 In addition, the electrochemical reduction method is more environmentally friendly and cost-effective, and more appropriate for making rGO sheets with fewer defects.²³

A wide variety of nanomaterials are used as modifiers in the design of an electrochemical sensor. For example, metal nanoparticles,^24,25 various metal oxide nanoparticles,²⁶ carbonaceous nanomaterials,^27,28 and nanocomposites^29,30 provide many advantages such as high surface-to-volume ratio and strong catalytic properties to the electrochemical sensor. Co₃O₄ as a convenient material has also received significant attention in sensors, batteries, water splitting, catalysis and energy storage³¹ due to its remarkable oxidation ability, nontoxicity, chemical stability, and low cost.^32,33 The composites based on Co₃O₄ can also be formed with various supports such as graphene,^31,34 TiO₂,³⁵ g-C₃N₄,³⁶ and MoS₂ (ref. 37) to improve the activity and stability of Co₃O₄. For example, Ezhil Vilian and coworkers³⁴ reported MOF derived RGO–Co₃O₄ to construct a non-enzymatic glucose biosensor. Herein, Co₃O₄–ERGO was utilized as a modification material for the electrochemical determination of UA. Co₃O₄ NPs can easily catalyze the oxidation reaction of UA, thereby amplifying the electrochemical signal of the sensor for UA detection. UA molecules are catalytically oxidized by CoOOH as a catalyst to dehydrourate (1H-purine-2,6,8(3H)-trione) molecules.³⁸

Co₃O₄ + OH⁻ + H₂O → 3CoOOH + e⁻

By using reduced graphene oxide, an efficient electrical network can be provided through Co₃O₄ anchored directly on the surface of reduced graphene oxide. This can increase the electron transfer rate and accordingly improve the sensitivity of the sensor.³⁹ In the light of these literature reports, the Co₃O₄–ERGO modified SPE (Co₃O₄–ERGO/SPE) shows a linearity in the concentration range of 5 μM to 500 μM with a detection limit of 1.5 μM. Furthermore, the electrochemical results reveal that Co₃O₄–ERGO/SPE can be practically applied for the determination of UA in artificial saliva samples. As far as we can ascertain, no study has been found on the use of Co₃O₄–ERGO for the electrochemical determination of UA. Compared to similar sensing studies, this study presents some new approaches for UA detection in terms of both the modification step and the use of a new generation combined electrode that allows the use of a small amount of sample.

2. Experimental

2.1. Chemicals and apparatus

Chemicals and apparatus are detailed in the ESI.†

2.2. Synthesis of Co₃O₄ NPs

Co₃O₄ NPs were prepared according to the procedure of Shao et al.³⁶ 2.8 g of cobalt nitrate hexahydrate was dissolved in 30 mL water under stirring at 200 rpm. Then, 60 mL of NaOH (0.33 M) and 8 mL of ammonia solution were added. The solution was placed in a stainless steel autoclave and heated at 120 °C for 12 h. Subsequently, the mixture was centrifuged, washed, and dried at 70 °C. The dried Co₃O₄ NPs were annealed at 450 °C for 3 h.

2.3. Preparation of Co₃O₄–ERGO/SPE

Graphene oxide (GO) was prepared by oxidizing graphite according to the synthesis steps reported by Marcano et al.⁴⁰ Then, GO was dispersed in 0.1 M PBS with the aid of ultrasonic treatment to prepare a final concentration of 1 mg mL⁻¹. After a 150 μL volume of the GO dispersion was dropped onto the SPE surface, reduction was performed using the CV technique with a scanning rate of 50 mV s⁻¹ in the potential range of −1.5 V to 0 V.⁴¹ After the electrochemical reduction step, the GO drop on the surface of the SPE was intently removed, and the ERGO modified SPE was quickly cleaned with distilled water. To obtain Co₃O₄–ERGO/SPE, a dispersed mixture of Co₃O₄ (1 mg mL⁻¹) and GO (1 mg mL⁻¹) was used following the same route. Scheme 1 shows a schematic illustration of the modified SPE used in the electrochemical studies and the oxidation mechanism of UA⁴² on the electrode surface.

Scheme 1 Schematic illustration of Co₃O₄–ERGO/SPE and the electro-oxidation mechanism of UA.

3. Results and discussion

3.1. Characterization

Fig. 1a shows the XRD patterns of the synthesized Co₃O₄ nanoparticles, and the characteristic diffraction lines of the cubic spinel Co₃O₄ phase (JCPDS card no. 01-077-0241) were observed. The sharp diffraction peaks indicated that the Co₃O₄ nanoparticles possessed good crystallinity. The strong reflections at 2θ values of 19.2, 31.1, 36.7, 38.6, 44.6, 59.2, and 65.1 degrees were attributed to the typical diffraction peaks of spinel Co₃O₄.⁴³ Raman spectra were also recorded to confirm the structure of the Co₃O₄ nanoparticles and to gain more information about the oxide lattice distortion. As seen in Fig. 1b, five bands were found corresponding to the A_1g, 3 F_2g, and E_g Raman-active modes of the Co₃O₄ spinel structure.⁴⁴ Of these, approximately 190 and 680 cm⁻¹ can be attributed to the tetrahedral and octahedral Co regions, respectively.⁴⁵

Fig. 1 XRD pattern (a) and Raman spectrum (b) of the prepared Co₃O₄.

The average crystal size of Co₃O₄ NPs was calculated using the Scherrer equation:^46,47

where λ is the wavelength of the X-ray source in nm, β is the peak width at half maximum of the diffraction peak, θ is the Bragg diffraction angle of the diffraction peak, and D is the average crystal size in nm. From the Scherrer equation, the average crystal size of Co₃O₄ NPs was calculated to be 58.5 nm.

FE-SEM analysis was performed to study the morphology of the prepared surfaces. The FE-SEM image of ERGO (Fig. 2a) shows the sheet like structure. On the other hand, Co₃O₄ nanoparticles are seen well deposited and distributed between the ERGO sheets (Fig. 2b). The distribution of elements present on the Co₃O₄–ERGO/SPE surface was examined by energy-dispersive X-ray spectroscopy (EDX) elemental mapping analysis. C (red color), O (green color), and Co (blue color) elements were observed as shown in the EDX mapping profile of the Co₃O₄–ERGO/SPE surface (Fig. 2c). The EDX spectrum of the Co₃O₄–ERGO/SPE surface (Fig. 2d) shows elemental C, O, and Co signatures, which confirms the material on the electrode surface. In addition, the independent elemental C, O, and Co distributions are shown in Fig. 2e, f and g.⁴⁸

Fig. 2 FE-SEM images of ERGO/SPE (a) and Co₃O₄–ERGO/SPE (b) surfaces. EDX mapping (c) and the EDX spectrum (d) of Co₃O₄–ERGO/SPE. Elemental maps of Co₃O₄–ERGO/SPE from FESEM-EDX for carbon (e), oxygen (f), and cobalt (g).

The surface morphologies of the prepared ERGO/SPE and Co₃O₄–ERGO/SPE were also examined via AFM. The 2D and 3D AFM images of the ERGO/SPE (Fig. 3a and b) exhibit a typical sheet-like morphology of the reduced graphene and the average roughness (R_a) and root mean square (rms) values are 144.66 nm and 189.168 nm, respectively. After deposition of Co₃O₄, the AFM images of the Co₃O₄–ERGO/SPE (Fig. 3c and d) show the decrease in roughness (R_a = 64.85 nm, rms = 80.54 nm) over the ERGO sheets due to the presence of smooth and nanostructured Co₃O₄ nanoparticles. It can be clearly seen that the intermediate spaces are covered by the Co₃O₄ nanoparticles compared with ERGO/SPE.

Fig. 3 Two- and three-dimensional AFM images of ERGO/SPE (a and b) and Co₃O₄–ERGO/SPE (c and d) surfaces.

The investigations on the presence of functional groups in ERGO/SPE and Co₃O₄–ERGO/SPE were done by FTIR analysis and are shown in Fig. S1.† In the FTIR spectrum of ERGO/SPE, the peak at around 3300 cm⁻¹ belonging to the hydroxyl functionality has completely disappeared. The peaks at around 1030, 1380, 1450, and 1730 cm⁻¹ correspond to the alkoxy (–C–O), alkenyl (–C=C), carboxyl (–COOH), and ketone (–C=O) groups, respectively. In the case of ERGO, either the intensity of these peaks is lower or these peaks disappear.⁴⁹ This shows that the electrochemical reduction process has been carried out successfully. Furthermore, the Co₃O₄–ERGO/SPE surface exhibits two absorption peaks at 662 cm⁻¹ and 582 cm⁻¹ associated with the Co–O stretching vibrations.⁵⁰

The Raman spectrum of the Co₃O₄–ERGO/SPE surface (Fig. S2†) displays two prominent peaks, a G-peak at 1578 cm⁻¹ along with a D-peak at 1311 cm⁻¹ that belongs to ERGO. The ratio of I_D/I_G is found to be 1.106. The peaks corresponding to Co₃O₄ (192, 472, 516, 613, and 685 cm⁻¹) are also present in the prepared sensor, which is evidence for existence of Co₃O₄ in ERGO.^51,52

To prove the formation of films on the SPE, the change of wettability characteristics on the surface can be observed by measuring the water contact angle (WCA).⁵³ The wetting characteristics of bare SPE, GO/SPE, ERGO/SPE, and Co₃O₄–ERGO/SPE surfaces are illustrated in Fig. S3.† The water contact angles were measured to be 127°, 37°, 125°, and 122° for bare SPE, GO/SPE, ERGO/SPE, and Co₃O₄–ERGO/SPE, respectively.

3.2. Electrochemical characterization

Each modification step in the preparation of the proposed sensor was characterized by CV, DPV, and EIS techniques, respectively. Fig. 4a shows the CVs of SPE, ERGO/SPE, and Co₃O₄–ERGO/SPE in 5.0 mM K₃Fe(CN)₆/K₂Fe(CN)₆ (1 : 1) solution with 0.1 M KCl as a supporting electrolyte. The two well-defined redox peaks with ΔE_p of 251 mV, ΔE_p of 210 mV, and ΔE_p of 184 mV were obtained for SPE, ERGO/SPE, and Co₃O₄–ERGO/SPE, respectively. Co₃O₄–ERGO/SPE showed a smaller peak-to-peak separation than both SPE and ERGO/SPE. Additionally, Co₃O₄–ERGO/SPE exhibited a maximum enhanced peak current compared to ERGO/SPE and Co₃O₄–ERGO/SPE, which can be attributed to an easier electron transfer process between the electrode and Fe(CN)₆^3−/4−.⁵⁴ The electrochemical surface areas of SPE, ERGO/SPE, and Co₃O₄–ERGO/SPE were calculated by the use of the Randles–Sevcik formula.⁵⁵ The effective surface areas of SPE, ERGO/SPE, and Co₃O₄–ERGO/SPE were found to be 0.080, 0.129 and 0.145 cm², respectively. According to the results, the effective surface area of Co₃O₄–ERGO/SPE is maximum as compared with that of other electrodes, which indicates that Co₃O₄–ERGO/SPE provides an effective sensor performance towards the electrooxidation of UA. In order to investigate the electron transfer ability of the modified SPEs, the Nyquist plots (Fig. 4b) of bare SPE, ERGO modified SPE, and Co₃O₄–ERGO modified SPE were obtained in a 0.1 M KCl solution in the presence of 5.0 mM K₃Fe(CN)₆/K₂Fe(CN)₆ (1 : 1). The electron transfer resistance (R_ct) values which are correlated with the diameter of the semicircles can be found to be 500 Ω for Co₃O₄–ERGO/SPE, 700 Ω for ERGO/SPE and 1250 Ω for the SPE, respectively. The smaller R_ct means that the Co₃O₄–ERGO nanocomposite exhibits a lower electron transfer resistance and a higher electron transfer ability.⁵⁶Fig. 4c shows the DPV peaks of bare SPE, ERGO/SPE, and Co₃O₄–ERGO/SPE in 0.1 M PBS (pH 7.0) in the presence of 90 μM UA. The peak potential obtained at the Co₃O₄–ERGO modified SPE is 0.108 V, which is a more negative potential than those of other electrodes (0.112 V for the ERGO/SPE and 0.180 V for the SPE). Also, Co₃O₄–ERGO/SPE has a higher peak response than both bare SPE and ERGO modified SPE, which is due to the enhanced electron transfer rate.

Fig. 4 (a) CVs and (b) Nyquist plots of different electrodes (SPE, ERGO/SPE and Co₃O₄–ERGO/SPE) in 0.1 M KCl in the presence of 5 mM K₃Fe(CN)₆/K₂Fe(CN)₆ (1 : 1). (c) DPVs of 90 μM UA in 0.1 M PBS at different electrodes (SPE, ERGO/SPE and Co₃O₄–ERGO/SPE).

3.3. Optimization study

Several important parameters such as pH of the buffer solution, Co₃O₄ : ERGO ratio, and Co₃O₄–ERGO composite thickness were optimized to obtain more selective and sensitive detection of UA. First, the effect of the Co₃O₄ : ERGO ratio on the current signal of 60 μM UA at the proposed sensor was examined using various electrodes fabricated with various ratios (0.5 : 1, 1 : 1, 2 : 1, and 3 : 1) of Co₃O₄ : ERGO. As shown in Fig. 5a, Co₃O₄–ERGO/SPE fabricated at a ratio of 1 : 1 possessess the highest DPV signal. In the following ratios, the peak currents decreased, which indicates that the decrease in the amount of ERGO in the composite composition decreases the sensitivity of Co₃O₄–ERGO/SPE. As another parameter, the effect of the Co₃O₄–ERGO composite thickness on the current response of the proposed sensor was examined with various electrodes prepared by changing the number of scan cycles (between 5 and 35) in the potential range of −1.5 to 0 (Fig. 5b). The peak current of Co₃O₄–ERGO/SPE began to decrease after 20 cycles. The findings can be attributed to the slower electron transfer process between UA and Co₃O₄–ERGO/SPE as a consequence of more cycles due to the formation of a thicker film on the surface of the SPE.⁵⁷ In subsequent experiments, the modification of the SPE with the Co₃O₄–ERGO composite was carried out between −1.5 V and 0 V for 20 cycles at a scan rate of 50 mV s⁻¹. To decide on the optimal pH value of the buffer solution, the DPV signals of 60 μM UA at the Co₃O₄–ERGO/SPE was monitored in 0.1 M PBS at different pH values in the range of 5.0–9.0 (Fig. 5c). The DPV current signal obtained at pH 6.5 is slightly larger than that obtained at pH 7. However, the pH of the electrolyte was chosen as 7.0 for all subsequent measurements. Because the applicability of Co₃O₄–ERGO/SPE to human saliva for UA detection has been aimed at in this study, the pH value of the electrolyte has been chosen in the pH region (6.8–7.8) suitable for healthy human saliva.⁵⁸

Fig. 5 Effect of the (a) Co₃O₄ : ERGO ratio, (b) Co₃O₄–ERGO composite thickness, and (c) pH of the buffer solution on the current response of 60 μM UA at Co₃O₄–ERGO/SPE.

3.4. Electrochemical detection of UA

In order to find out the sensitivity of the present sensor towards the electrooxidation reaction of UA under the optimal conditions, the electrochemical signals of Co₃O₄–ERGO/SPE were monitored by the DPV technique in the presence of increasing concentrations of UA (Fig. 6a). Fig. 6b shows that the current signals are linear with UA concentrations ranging from 5 μM to 500 μM. The linear regression of the calibration curve yielded the equation I_UA (μA) = 0.0465 + 0.0517C_UA (μM) with a linear correlation coefficient of 0.9985. The detection limit was calculated to be 1.5 μM, as estimated using the equation of 3SD/m.⁵⁹ Based on the slope of the calibration curve, the analytical sensitivity of Co₃O₄–ERGO/SPE was obtained to be 0.0465 μA μM⁻¹. The analytical characteristics of Co₃O₄–ERGO/SPE were also compared with those of several sensors for the detection of salivary UA in the literature, and these characteristics are summarized in Table 1. By comparison, accordingly, Co₃O₄–ERGO/SPE shows either better or comparable analytical performance. This sensing performance of Co₃O₄–ERGO/SPE may be due to the fact that ERGO increases the electron flow rate in the oxidation reaction of UA⁶⁰ and Co₃O₄ NPs exhibit high catalytic activity as well as its large surface area and highly conductive nature.⁶¹

Fig. 6 (a) DPVs of Co₃O₄–ERGO/SPE for increasing concentrations of UA (5–500 μM). Supporting electrolyte: 0.1 M PBS (pH 7.0). (b) Calibration curve of the changed peak current of DPVs vs. the UA concentration.

Table 1 Sensing performances of several sensors for electrochemical determination of UA

Sensor	Linear range (μM)	LOD (μM)	Sample	Reference
Uricase/MWCNTs/SPE	5–1000	0.33	Artificial saliva	21
α-Ni0.75Zn0.25(OH)2-modified FTO	100–1400	0.023	Saliva and artificial saliva	62
SPE-AuNPs	20–200	14.64	Artificial saliva	6
PEDOT–GO/ITO	2–1000	0.75	Artificial saliva	63
UOx–GO/PANI/Nf–Gr/Pt	3–300	3	Saliva	64
β-CD/CPE	10–170	4.6	Saliva	65
Co3O4–ERGO/SPE	5–500	1.5	Artificial saliva	Present work

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3.5. Repeatability, reproducibility, stability and interference

To prove the precision of Co₃O₄–ERGO/SPE, the repeatability and the reproducibility of the present sensor were examined by recording the DPV responses in 0.1 M PBS (pH 7.0) containing 10 μM UA. The repeatability was analyzed by monitoring five successive current responses using the same sensor for UA detection, and the current responses illustrated in Fig. 7a provided an RSD value of 3.62%. The reproducibility was evaluated by preparing five identical Co₃O₄–ERGO/SPEs designed using the similar protocol for UA detection, and the RSD value of the current responses was found to be 3.38%, as presented in Fig. 7b. The long-term stability of Co₃O₄–ERGO/SPE was examined by comparing the DPV responses obtained for 20 μM UA over 26 days. When the sensor was not used, it was stored in a desiccator under N₂ at RT. The proposed sensor retained 95% of its initial response after 8 days. At the end of 26 days of storage, the peak current was approximately 90% of its initial response, indicating that the proposed sensor has good stability (Fig. 7c). To determine the selectivity of the prepared sensor towards UA, the effect of various possible interfering species (AA, LA, Glc, H₂O₂, Li⁺, Na⁺, Mg²⁺, K⁺, and Ca²⁺) was studied by recording the peak responses of 30 μM of UA and each compound in 0.1 M PBS (pH 7.0). Fig. 7d shows a comparison of the current signals of the interfering species in the peak potential of UA. The RSD values of the current responses for the interfering species were found in the range of 2.71–3.35%. According to the DPV signals of all species recorded on Co₃O₄–ERGO/SPE, it is clear that the proposed sensor shows an acceptable selectivity for the detection of UA.

Fig. 7 (a) Repeatability, (b) reproducibility, (c) stability, and (d) selectivity analysis of Co₃O₄–ERGO/SPE. (e) DPVs of Co₃O₄–ERGO/SPE in the artificial saliva samples (I: 100 μL of 0.1 M PBS + 100 μL of artificial saliva containing 200 μM UA, II: 100 μL of 0.1 M PBS + 100 μL of artificial saliva containing 300 μM UA, and III: 100 μL of 0.1 M PBS + 100 μL of artificial saliva without UA).

3.6. Sample analysis

To assess the applicability of the proposed sensor in practical analysis, the electrochemical response of Co₃O₄–ERGO/SPE was examined in artificial saliva which has a similar electrolyte concentration to human saliva.⁴ The artificial saliva was prepared by dissolving 5 mM NaCl, 1 mM CaCl₂, 15 mM KCl, 1.1 mM KSCN, 4 mM NH₄Cl, and 1 mM citric acid in ultrapure water, and the pH of the solution was adjusted to 6.7.²¹ Prior to the electrochemical detection of UA, the saliva samples containing UA were prepared in an artificial saliva solution. Then, the saliva samples and 0.1 M PBS were mixed in the same volume ratio. The concentrations of UA in the final mixture were chosen in keeping with the level of UA (100–250 μM) in healthy human saliva.⁶² UA concentrations corresponding to the peak signals (peak I and II) recorded for the artificial saliva samples were found by means of the calibration plot of the proposed sensor (Fig. 7e). Peak III corresponds to the DPV peak of 0.1 M PBS + artificial saliva without UA in the same volume ratio. This peak also proves that none of the species in the artificial saliva samples showed an interference effect for the detection of UA at the proposed sensor. The recoveries summarized in Table 2 are in the range of 98.20–98.78%, which confirms the reliability of Co₃O₄–ERGO/SPE for the determination of salivary UA.

Table 2 Detection of UA in artificial saliva samples using Co₃O₄–ERGO/SPE

Sample no.	UA in the sample (μM)	Found (μM)	Recovery (%)
I	100	98.20	98.20
II	150	148.17	98.78

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4. Conclusions

We presented in this study a new electrochemical sensor prepared using Co₃O₄–ERGO as a sensing material. After detailed optimization and characterization studies, it has been proven that the fabricated sensor can be utilized for the detection of UA. The sensor demonstrated better or comparable analytical performance (linear range: 5–500 μM, detection limit: 1.5 μM, and sensitivity: 0.0465 μA μM⁻¹) as compared with the sensing characteristics of the electrodes given in Table 1. Moreover, the Co₃O₄–ERGO film was found to have a good selectivity for UA against various interfering molecules, such as ascorbic acid, lactic acid, glucose, hydrogen peroxide and some ions. With the aim of examining the applicability of Co₃O₄–ERGO/SPE in practical analysis, the current signals were monitored for UA detection in artificial saliva samples, and satisfactory recovery results were obtained. Eventually, the selectivity and sensitivity of Co₃O₄–ERGO towards the electrooxidation of UA could provide a new platform for electrochemical sensing applications.

Conflicts of interest

All authors declare that they have no conflict of interest.

Acknowledgements

The authors greatly thank the Research Foundation of Selcuk University (BAP Project No. 20201062) for the financial support.

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Footnote

† Electronic supplementary information (ESI) available: Chemicals and apparatus, FTIR analysis, Raman analysis, and contact angle measurements. See DOI: 10.1039/d1ay01744f

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