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We investigated the effect of specific surface area on the electrochemical properties of NiCo2O4 (NCO) for glucose detection. NCO nanomaterials with controlled specific surface area have been produced by hydrothermal synthesis with additives, and self-assembling nanostructures with hedgehog, pine needle, tremella and flower like morphology have also been produced. The novelty of this method lies in the systematic control of the chemical reaction path by adding various additives during synthesis, which leads to the spontaneous formation of various morphologies without any differences in the crystal structure and chemical state of the constituent elements. This morphological control of NCO nanomaterials leads to significant changes in the electrochemical performance of glucose detection. In conjunction with material characterization, the relationship between specific surface area and electrochemical performance for glucose detection was discussed. This work may provide scientific insight into the surface area tuning of nanostructures that determines their functionality for potential applications in glucose biosensors.
Blood glucose levels provide important information about the metabolic and physiological state of the body1,2. For example, abnormal levels of glucose in the body can be an important indicator of serious health problems, including diabetes, cardiovascular disease, and obesity3,4,5. Therefore, regular monitoring of blood sugar levels is very important for maintaining good health. Although various types of glucose sensors using physicochemical detection have been reported, low sensitivity and slow response times remain barriers to continuous glucose monitoring systems6,7,8. In addition, currently popular electrochemical glucose sensors based on enzymatic reactions still have some limitations despite their advantages of fast response, high sensitivity and relatively simple fabrication procedures9,10. Therefore, various types of non-enzymatic electrochemical sensors have been extensively studied to prevent enzyme denaturation while maintaining the advantages of electrochemical biosensors9,11,12,13.
Transition metal compounds (TMCs) have a sufficiently high catalytic activity with respect to glucose, which expands the scope of their application in electrochemical glucose sensors13,14,15. So far, various rational designs and simple methods for the synthesis of TMS have been proposed to further improve the sensitivity, selectivity, and electrochemical stability of glucose detection16,17,18. For example, unambiguous transition metal oxides such as copper oxide (CuO)11,19, zinc oxide (ZnO)20, nickel oxide (NiO)21,22, cobalt oxide (Co3O4)23,24 and cerium oxide (CeO2) 25 is electrochemically active with respect to glucose. Recent advances in binary metal oxides such as nickel cobaltate (NiCo2O4) for glucose detection have demonstrated additional synergistic effects in terms of increased electrical activity26,27,28,29,30. In particular, precise composition and morphology control to form TMS with various nanostructures can effectively increase the detection sensitivity due to their large surface area, so it is highly recommended to develop morphology controlled TMS for improved glucose detection20,25,30,31,32,33. 34, 35.
Here we report NiCo2O4 (NCO) nanomaterials with different morphologies for glucose detection. NCO nanomaterials are obtained by a simple hydrothermal method using various additives, chemical additives are one of the key factors in the self-assembly of nanostructures of various morphologies. We systematically investigated the effect of NCOs with different morphologies on their electrochemical performance for glucose detection, including sensitivity, selectivity, low detection limit, and long-term stability.
We synthesized NCO nanomaterials (abbreviated UNCO, PNCO, TNCO and FNCO respectively) with microstructures similar to sea urchins, pine needles, tremella and flowers. Figure 1 shows the different morphologies of UNCO, PNCO, TNCO, and FNCO. SEM images and EDS images showed that Ni, Co, and O were evenly distributed in the NCO nanomaterials, as shown in Figures 1 and 2. S1 and S2, respectively. On fig. 2a,b show representative TEM images of NCO nanomaterials with distinct morphology. UNCO is a self-assembling microsphere (diameter: ~5 µm) composed of nanowires with NCO nanoparticles (average particle size: 20 nm). This unique microstructure is expected to provide a large surface area to facilitate electrolyte diffusion and electron transport. The addition of NH4F and urea during synthesis resulted in a thicker acicular microstructure (PNCO) 3 µm long and 60 nm wide, composed of larger nanoparticles. The addition of HMT instead of NH4F results in a tremello-like morphology (TNCO) with wrinkled nanosheets. The introduction of NH4F and HMT during synthesis leads to aggregation of adjacent wrinkled nanosheets, resulting in a flower-like morphology (FNCO). The HREM image (Fig. 2c) shows distinct grating bands with interplanar spacings of 0.473, 0.278, 0.50, and 0.237 nm, corresponding to the (111), (220), (311), and (222) NiCo2O4 planes, s 27 . Selected area electron diffraction pattern (SAED) of NCO nanomaterials (inset to Fig. 2b) also confirmed the polycrystalline nature of NiCo2O4. The results of high-angle annular dark imaging (HAADF) and EDS mapping show that all elements are evenly distributed in the NCO nanomaterial, as shown in Fig. 2d.
Schematic illustration of the process of formation of NiCo2O4 nanostructures with controlled morphology. Schematics and SEM images of various nanostructures are also shown.
Morphological and structural characterization of NCO nanomaterials: (a) TEM image, (b) TEM image along with SAED pattern, (c) grating-resolved HRTEM image and corresponding HADDF images of Ni, Co, and O in (d) NCO nanomaterials. .
X-ray diffraction patterns of NCO nanomaterials of various morphologies are shown in Figs. 3a. The diffraction peaks at 18.9, 31.1, 36.6, 44.6, 59.1 and 64.9° indicate the planes (111), (220), (311), (400), (511) and (440) NiCo2O4, respectively, which have a cubic spinel structure (JCPDS No. 20-0781) 36. The FT-IR spectra of the NCO nanomaterials are shown in Figs. 3b. Two strong vibrational bands in the region between 555 and 669 cm–1 correspond to metallic (Ni and Co) oxygen drawn from the tetrahedral and octahedral positions of the NiCo2O437 spinel, respectively. To better understand the structural properties of NCO nanomaterials, Raman spectra were obtained as shown in Fig. 3c. The four peaks observed at 180, 459, 503, and 642 cm-1 correspond to the Raman modes F2g, E2g, F2g, and A1g of the NiCo2O4 spinel, respectively. XPS measurements were performed to determine the surface chemical state of elements in NCO nanomaterials. On fig. 3d shows the XPS spectrum of UNCO. The spectrum of Ni 2p has two main peaks located at binding energies of 854.8 and 872.3 eV, corresponding to Ni 2p3/2 and Ni 2p1/2, and two vibrational satellites at 860.6 and 879.1 eV, respectively. This indicates the existence of Ni2+ and Ni3+ oxidation states in NCO. Peaks around 855.9 and 873.4 eV are for Ni3+, and peaks around 854.2 and 871.6 eV are for Ni2+. Similarly, the Co2p spectrum of two spin-orbit doublets reveals characteristic peaks for Co2+ and Co3+ at 780.4 (Co 2p3/2) and 795.7 eV (Co 2p1/2). Peaks at 796.0 and 780.3 eV correspond to Co2+, and peaks at 794.4 and 779.3 eV correspond to Co3+. It should be noted that the polyvalent state of metal ions (Ni2+/Ni3+ and Co2+/Co3+) in NiCo2O4 promotes an increase in electrochemical activity37,38. The Ni2p and Co2p spectra for UNCO, PNCO, TNCO, and FNCO showed similar results, as shown in fig. S3. In addition, the O1s spectra of all NCO nanomaterials (Fig. S4) showed two peaks at 592.4 and 531.2 eV, which were associated with typical metal-oxygen and oxygen bonds in the hydroxyl groups of the NCO surface, respectively39. Although the structures of the NCO nanomaterials are similar, the morphological differences in the additives suggest that each additive may participate differently in the chemical reactions to form NCO. This controls the energetically favorable nucleation and grain growth steps, thereby controlling particle size and degree of agglomeration. Thus, the control of various process parameters, including additives, reaction time, and temperature during synthesis, can be used to design the microstructure and improve the electrochemical performance of NCO nanomaterials for glucose detection.
(a) X-ray diffraction patterns, (b) FTIR and (c) Raman spectra of NCO nanomaterials, (d) XPS spectra of Ni 2p and Co 2p from UNCO.
The morphology of the adapted NCO nanomaterials is closely related to the formation of the initial phases obtained from various additives depicted in Figure S5. In addition, X-ray and Raman spectra of freshly prepared samples (Figures S6 and S7a) showed that the involvement of different chemical additives resulted in crystallographic differences: Ni and Co carbonate hydroxides were mainly observed in sea urchins and pine needle structure, while as structures in the form of tremella and flower indicate the presence of nickel and cobalt hydroxides. The FT-IR and XPS spectra of the prepared samples are shown in Figures 1 and 2. S7b-S9 also provide clear evidence of the aforementioned crystallographic differences. From the material properties of the prepared samples, it becomes clear that additives are involved in hydrothermal reactions and provide different reaction pathways to obtain initial phases with different morphologies40,41,42. The self-assembly of different morphologies, consisting of one-dimensional (1D) nanowires and two-dimensional (2D) nanosheets, is explained by the different chemical state of the initial phases (Ni and Co ions, as well as functional groups), followed by crystal growth42, 43, 44, 45, 46, 47. During post-thermal processing, the various initial phases are converted into NCO spinel while maintaining their unique morphology, as shown in Figures 1 and 2. 2 and 3a.
Morphological differences in NCO nanomaterials can influence the electrochemically active surface area for glucose detection, thereby determining the overall electrochemical characteristics of the glucose sensor. The N2 BET adsorption-desorption isotherm was used to estimate the pore size and specific surface area of ​​the NCO nanomaterials. On fig. 4 shows BET isotherms of various NCO nanomaterials. The BET specific surface area for UNCO, PNCO, TNCO and FNCO were estimated at 45.303, 43.304, 38.861 and 27.260 m2/g, respectively. UNCO has the highest BET surface area (45.303 m2 g-1) and the largest pore volume (0.2849 cm3 g-1), and the pore size distribution is narrow. The BET results for the NCO nanomaterials are shown in Table 1. The N2 adsorption-desorption curves were very similar to type IV isothermal hysteresis loops, indicating that all samples had a mesoporous structure48. Mesoporous UNCOs with the highest surface area and highest pore volume are expected to provide numerous active sites for redox reactions, leading to improved electrochemical performance.
BET results for (a) UNCO, (b) PNCO, (c) TNCO, and (d) FNCO. The inset shows the corresponding pore size distribution.
The electrochemical redox reactions of NCO nanomaterials with various morphologies for glucose detection were evaluated using CV measurements. On fig. 5 shows CV curves of NCO nanomaterials in 0.1 M NaOH alkaline electrolyte with and without 5 mM glucose at a scan rate of 50 mVs-1. In the absence of glucose, redox peaks were observed at 0.50 and 0.35 V, corresponding to oxidation associated with M–O (M: Ni2+, Co2+) and M*-O-OH (M*: Ni3+, Co3+). using the OH anion. After the addition of 5 mM glucose, the redox reaction on the surface of the NCO nanomaterials significantly increased, which may be due to the oxidation of glucose to gluconolactone. Figure S10 shows the peak redox currents at scan rates of 5–100 mV s-1 in 0.1 M NaOH solution. It is clear that the peak redox current increases with increasing scan rate, indicating that NCO nanomaterials have similar diffusion controlled electrochemical behavior50,51. As shown in Figure S11, the electrochemical surface area (ECSA) of UNCO, PNCO, TNCO, and FNCO is estimated to be 2.15, 1.47, 1.2, and 1.03 cm2, respectively. This suggests that UNCO is useful for the electrocatalytic process, facilitating the detection of glucose.
CV curves of (a) UNCO, (b) PNCO, (c) TNCO, and (d) FNCO electrodes without glucose and supplemented with 5 mM glucose at a scan rate of 50 mVs-1.
The electrochemical performance of NCO nanomaterials for glucose detection was investigated and the results are shown in Fig. 6. Glucose sensitivity was determined by the CA method by stepwise addition of various concentrations of glucose (0.01–6 mM) in 0.1 M NaOH solution at 0.5 V with an interval of 60 s. As shown in fig. 6a–d, NCO nanomaterials show different sensitivities ranging from 84.72 to 116.33 µA mM-1 cm-2 with high correlation coefficients (R2) from 0.99 to 0.993. The calibration curve between glucose concentration and the current reaction of NCO nanomaterials is shown in fig. S12. The calculated limits of detection (LOD) of NCO nanomaterials were in the range of 0.0623–0.0783 µM. According to the results of the CA test, UNCO showed the highest sensitivity (116.33 μA mM-1 cm-2) in a wide detection range. This can be explained by its unique sea urchin-like morphology, consisting of a mesoporous structure with a large specific surface area providing more numerous active sites for glucose species. The electrochemical performance of the NCO nanomaterials presented in Table S1 confirms the excellent electrochemical glucose detection performance of the NCO nanomaterials prepared in this study.
CA responses of UNCO (a), PNCO (b), TNCO (c), and FNCO (d) electrodes with glucose added to 0.1 M NaOH solution at 0.50 V. The insets show calibration curves of current responses of NCO nanomaterials: (e) KA responses of UNCO, (f) PNCO, (g) TNCO, and (h) FNCO with stepwise addition of 1 mM glucose and 0.1 mM interfering substances (LA, DA, AA, and UA).
The anti-interference ability of glucose detection is another important factor in the selective and sensitive detection of glucose by interfering compounds. On fig. 6e–h show the anti-interference ability of NCO nanomaterials in 0.1 M NaOH solution. Common interfering molecules such as LA, DA, AA and UA are selected and added to the electrolyte. The current response of NCO nanomaterials to glucose is evident. However, the current response to UA, DA, AA and LA did not change, which means that the NCO nanomaterials showed excellent selectivity for glucose detection regardless of their morphological differences. Figure S13 shows the stability of NCO nanomaterials examined by the CA response in 0.1 M NaOH, where 1 mM glucose was added to the electrolyte for a long time (80,000 s). The current responses of UNCO, PNCO, TNCO, and FNCO were 98.6%, 97.5%, 98.4%, and 96.8%, respectively, of the initial current with the addition of an additional 1 mM glucose after 80,000 s. All NCO nanomaterials exhibit stable redox reactions with glucose species over a long period of time. In particular, the UNCO current signal not only retained 97.1% of its initial current, but also retained its morphology and chemical bond properties after a 7-day environmental long-term stability test (Figures S14 and S15a). In addition, the reproducibility and reproducibility of UNCO were tested as shown in Fig. S15b, c. The calculated Relative Standard Deviation (RSD) of reproducibility and repeatability was 2.42% and 2.14%, respectively, indicating potential applications as an industrial grade glucose sensor. This indicates the excellent structural and chemical stability of UNCO under oxidizing conditions for glucose detection.
It is clear that the electrochemical performance of NCO nanomaterials for glucose detection is mainly related to the structural advantages of the initial phase prepared by the hydrothermal method with additives (Fig. S16). The high surface area UNCO has more electroactive sites than other nanostructures, which helps improve the redox reaction between the active materials and the glucose particles. The mesoporous structure of UNCO can easily expose more Ni and Co sites to the electrolyte to detect glucose, resulting in a fast electrochemical response. One-dimensional nanowires in UNCO can further increase the diffusion rate by providing shorter transport paths for ions and electrons. Because of the unique structural features mentioned above, the electrochemical performance of UNCO for glucose detection is superior to that of PNCO, TNCO, and FNCO. This indicates that the unique UNCO morphology with the highest surface area and pore size can provide excellent electrochemical performance for glucose detection.
The effect of specific surface area on the electrochemical characteristics of NCO nanomaterials was studied. NCO nanomaterials with different specific surface area were obtained by a simple hydrothermal method and various additives. Different additives during synthesis enter into different chemical reactions and form different initial phases. This has led to the self-assembly of various nanostructures with morphologies similar to the hedgehog, pine needle, tremella, and flower. Subsequent post-heating leads to a similar chemical state of the crystalline NCO nanomaterials with a spinel structure while maintaining their unique morphology. Depending on the surface area of ​​different morphology, the electrochemical performance of NCO nanomaterials for glucose detection has been greatly improved. In particular, the glucose sensitivity of NCO nanomaterials with sea urchin morphology increased to 116.33 µA mM-1 cm-2 with a high correlation coefficient (R2) of 0.99 in the linear range of 0.01-6 mM. This work may provide a scientific basis for morphological engineering to adjust specific surface area and further improve the electrochemical performance of non-enzymatic biosensor applications.
Ni(NO3)2 6H2O, Co(NO3)2 6H2O, urea, hexamethylenetetramine (HMT), ammonium fluoride (NH4F), sodium hydroxide (NaOH), d-(+)-glucose, lactic acid (LA), dopamine hydrochloride (DA), L-ascorbic acid (AA) and uric acid (UA) were purchased from Sigma-Aldrich. All reagents used were of analytical grade and were used without further purification.
NiCo2O4 was synthesized by a simple hydrothermal method followed by heat treatment. Briefly: 1 mmol of nickel nitrate (Ni(NO3)2∙6H2O) and 2 mmol of cobalt nitrate (Co(NO3)2∙6H2O) were dissolved in 30 ml of distilled water. In order to control the morphology of NiCo2O4, additives such as urea, ammonium fluoride and hexamethylenetetramine (HMT) were selectively added to the above solution. The whole mixture was then transferred to a 50 ml Teflon-lined autoclave and subjected to a hydrothermal reaction in a convection oven at 120° C. for 6 hours. After natural cooling to room temperature, the resulting precipitate was centrifuged and washed several times with distilled water and ethanol, and then dried overnight at 60°C. After that, freshly prepared samples were calcined at 400°C for 4 h in ambient atmosphere. The details of the experiments are listed in the Supplementary Information Table S2.
X-ray diffraction analysis (XRD, X’Pert-Pro MPD; PANalytical) was performed using Cu-Kα radiation (λ = 0.15418 nm) at 40 kV and 30 mA to study the structural properties of all NCO nanomaterials. Diffraction patterns were recorded in the range of angles 2θ 10–80° with a step of 0.05°. Surface morphology and microstructure were examined using field emission scanning electron microscopy (FESEM; Nova SEM 200, FEI) and scanning transmission electron microscopy (STEM; TALOS F200X, FEI) with energy dispersive X-ray spectroscopy (EDS). The valence states of the surface were analyzed by X-ray photoelectron spectroscopy (XPS; PHI 5000 Versa Probe II, ULVAC PHI) using Al Kα radiation (hν = 1486.6 eV). The binding energies were calibrated using the C 1 s peak at 284.6 eV as a reference. After preparing the samples on KBr particles, Fourier transform infrared (FT-IR) spectra were recorded in the wavenumber range 1500–400 cm–1 on a Jasco-FTIR-6300 spectrometer. Raman spectra were also obtained using a Raman spectrometer (Horiba Co., Japan) with a He-Ne laser (632.8 nm) as the excitation source. Brunauer-Emmett-Teller (BET; BELSORP mini II, MicrotracBEL, Corp.) used the BELSORP mini II analyzer (MicrotracBEL Corp.) to measure low temperature N2 adsorption-desorption isotherms to estimate specific surface area and pore size distribution.
All electrochemical measurements, such as cyclic voltammetry (CV) and chronoamperometry (CA), were performed on a PGSTAT302N potentiostat (Metrohm-Autolab) at room temperature using a three-electrode system in 0.1 M NaOH aqueous solution. A working electrode based on a glassy carbon electrode (GC), an Ag/AgCl electrode, and a platinum plate were used as the working electrode, reference electrode, and counter electrode, respectively. CVs were recorded between 0 and 0.6 V at various scan rates of 5-100 mV s-1. To measure ECSA, CV was performed in the range of 0.1-0.2 V at various scan rates (5-100 mV s-1). Acquire the sample’s CA reaction for glucose at 0.5 V with stirring. To measure sensitivity and selectivity, use 0.01–6 mM glucose, 0.1 mM LA, DA, AA, and UA in 0.1 M NaOH. The reproducibility of UNCO was tested using three different electrodes supplemented with 5 mM glucose under optimal conditions. The repeatability was also checked by making three measurements with one UNCO electrode within 6 hours.
All data generated or analyzed in this study is included in this published article (and its supplementary information file).
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