INTRODUCTION

      International Diabetes Federation stated that about 537 million adult people worldwide suffered from diabetes in 2021. Diabetes affects neural systems in humans and leads to many health problems. Daily monitoring and maintenance of glucose levels are essential to prevent as well as avoid life-threatening complications [1–5]. At present, several reliable non-enzymatic glucose monitoring sensing products have been put into application for diabetes diagnosis due to their high sensitivity, wide linear ranges, high anti-interference, and stability [6–8]. In nonenzymatic glucose sensors, noble metal/alloys [9–11] and transition metal/metal oxides [12–15] could work as enzymes for the electrocatalytic oxidation of glucose. Among them, copper-based materials (Cu, Cu2O and CuO) have been considered as one kind of candidate catalytic materials in non-enzymatic glucose sensors owing to their low cost, environmental friendliness, abundant nature, possible accelerated electron transfer at low overpotential, and unique spatial structures for glucose catalytic activity [14–17].

      For glucose detection, low-valence Cu (Cu0 , Cu1+ and Cu2+) is first converted to oxidative Cu3+ in an alkaline solution. Then, glucose is catalyzed to gluconolactone and further oxidized into glucose acid by Cu3+ electron transfer media species [18–21]. Nowadays, the non-enzymatic glucose sensing mechanism focuses specifically on the glucose electrooxidation with sluggish kinetics in the Faraday process. In general, Faraday processes refer mainly to a redox reaction process on the surface of a catalytic material, and subsequently electrons transfer between the active/catalytic material and the carrier/electrode [22,23]. Based on this, an effective strategy to improve the Faraday process is to combine copper species of different valence states with a high glucose adsorption energy, resulting in an improved efficiency of glucose electrocatalysis [15–17,24,25]. Reducing the electron transfer resistance is another critical strategy to improve the sensor properties [22,23]. Plenty of reports have shown that the introduction of electrical conductors (such as metal [6,14], and graphene [13,24,26]) could effectively promote the electron transfer between the catalytic materials and the working electrodes [27,28]. For example, Yuan et al. [29] fabricated porous Cu/Cu2O by the calcination of CuC2O4·2H2O in N2, which exhibited efficient electrocatalytic performance for glucose. Single-atom Pt on Cu@CuO core-shell nanowires also exhibited boosted glucose catalysis and sensing due to their strongly synergistic binding energy of glucose [30]. Therefore, promoting the kinetics of mass-produced multivalent Cu-based catalytic materials for glucose oxidation in the Faraday process remains very attractive.

      In this work, Cu@CuO is fabricated by an in-situ oxidation process at room temperature (RT) and then screen-printed onto silver-carbon working/counter electrodes, forming Cu@CuO modified disposable strip electrodes for nonenzymatic glucose sensors. The sensors exhibit enhanced electrochemical catalytic behaviors for glucose electrooxidation due to the promoted kinetics in the Faraday process. It is worth mentioning that the final sensors could analyze several milliliters of analytes in the natural environment.

EXPERIMENTAL SECTION

Preparation of Cu@CuO

     Commercial Cu power (1.00 g, 30 nm) was dispersed in 100 mL H2O2 solution (20 wt%) by sonication for 5 min, then stirred magnetically for 3 h at RT. Finally, Cu@CuO could be obtained after being filtrated and dried at 50°C.

Fabrication of screen-printed electrodes

      A mixture solution was formed by dissolving 3-(N-morpholino)- propanesulfonic acid sodium salt (1.15 g), hydroxyethyl cellulose (0.15 g), acrylamide (3.50 g), and ammonium persulfate (0.10 g) in 50 mL deionized (DI) water. Meanwhile, 20 mg Cu@CuO was dispersed into 1 mL DI water by ultrasound, forming a Cu@CuO dispersion solution. Then, the Cu@CuO-based screenprinted ink was prepared by mixing the Cu@CuO dispersion solution and the former mixture solution with a volume ratio of 1:10. The silver-carbon working/counter electrode and Cu@CuO-based ink were separately screen-printed onto a polyethylene terephthalate (PET) substrate, obtaining Cu@CuO modified electrodes in non-enzymatic glucose sensors.

Characterizations and theoretical calculations

      The crystal structure, specific surface area, and surface chemical bonds of Cu@CuO were characterized by X-ray diffraction (XRD, 1.5406 Å), N2 adsorption-desorption isotherms, and Xray photoelectron spectroscopy (XPS), respectively. The information of morphologies and structures was obtained by fieldemission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The electrocatalytic behaviors were studied on an electrochemical workstation (CHI 560) with ~10 μL analytical solution for each electrode.

      Electron structure and adsorption calculations were performed based on density functional theory (DFT), respectively. For the electron structure calculations, the sizes of Cu and CuO bulks were set as 0.72 nm × 0.72 nm × 0.72 nm and 0.93 nm × 0.68 nm × 0.51 nm, respectively. The models were first optimized with the generalized gradient approximation (GGA) method combined with the Perdew–Burke–Ernzerhof (PBE) functional. Then, the band structures and density of states (DOS) were calculated with the energy cutoffs of 353.7 eV for Cu and 489.8 eV for CuO. The on-the-fly generation (OTFG) ultrasoft pseudopotential was used to deal with core-electron interactions. It could provide consistent results of all electron calculations and describe the ground-state structures accurately. For the adsorption calculations, Cu (200) and CuO (111) models were constructed to adsorb glucose according to the TEM results. The DFT-D2 Tkatchenko-Scheffler method was used for dispersion corrections. Adsorption energy (Eads) can be calculated as follows:

Eads = Etot − Esub − Eet, where Esub is the energy of a clean Cu or CuO surface, Eet is the energy of an isolated glucose molecule, and Etot is the total energy of the Cu or CuO surface adsorbed with the glucose molecule.

RESULTS AND DISCUSSION

      Fig. 1 presents the fabrication processes of Cu@CuO and Cu@CuO-modified electrodes for glucose sensor. First, commercial Cu powders are partially oxidized by H2O2 at RT, obtaining Cu@CuO. Then, the silver-carbon electrode material and the obtained Cu@CuO catalytic material are separately screen-printed onto a PET substrate, forming Cu@CuO-modified electrodes.

      XRD pattern in Fig. 2a shows typical diffraction peaks of Cu and CuO (JCPDS card No. 04-0836 and 48-1548). In detail for Cu, the peaks locate at 2θ = 43.3° and 50.4°, respectively, corresponding to the (111) and (200) planes. Meanwhile, all the other peaks at 32.5°, 35.4°/35.5°, 38.7°/38.9°, 48.7°, 53.5°, 58.3°, 61.5°, 66.2°/66.4°, 68.1°, 72.9° and 75.0° belong to crystalline CuO [31]. It indicates that Cu and CuO coexist, forming a Cu@CuO compound. According to the N2 adsorption-desorption isotherms (Fig. 2b), the calculated value of the specific surface area is 4.35 m2 g−1 . Cu@CuO exhibits porous structures with porous sizes mainly distributed around 3.89 nm (Fig. 2c), which could provide more electrocatalytic sites as well as electron transfer channels for glucose electrooxidation [32–34]. XPS was further characterized to gain insight into Cu@CuO composition and surface bonding information. The XPS curve in Fig. 2d shows the Cu 3p, Cu 3s, C 1s (from the atmosphere [31,35]), O 1s, and Cu 2p core regions. High-resolution Cu 2p spectrum in Fig. 2e include 2p3/2 and 2p1/2 peaks. The Cu 2p3/2 peak at 933.8 eV belongs to Cu2+ 2p3/2. The Cu 2p1/2 peak can be divided into two peaks. A peak at 952.2 eV is indexed as Cu(0) 2p1/2, confirming the existence of Cu [31,35]. The other at 954.1 eV is caused by Cu2+ on the (B)-sites (Cu2+ 2p1/2). The XPS results on the Cu 2p peak indicate that Cu exhibits valence states of 0 and +2. In the case of the O 1s spectrum in Fig. 2f, the peak also shows two divided peaks. One at 530.8 eV indicates the binding energy of O2− in the CuO lattice, and the other at 529.4 eV could be indexed as a satellite peak (adsorbed oxygen) [25,31,35,36]. Based on the above analysis, XPS results further verify the synthesis of Cu@CuO.

      The structure determines performance. Therefore, the information of morphology, composition, and crystallography for Cu@CuO was analyzed by SEM and TEM (Fig. 3), respectively. In a low-magnification SEM image (Fig. 3a), Cu@CuO exhibits hierarchical dimensions with plenty of irregular structures. In a larger-magnification view shown in Fig. 3b, the irregular particles appear to be composed of nanoparticles. The TEM image in Fig. 3c shows that the particles have diameters of about 30 nm. Elemental mappings of Cu@CuO (Fig. 3d) exhibit the uniform distributions of Cu and O elements. The high-resolution TEM (HR-TEM) image in Fig. 3e displays two lattice spacings. One is 0.18 nm indexed to the (200) planes of metal Cu, and the other is 0.23 nm corresponding to the (111) planes of CuO [31,35]. The selected area electron diffraction (SAED) pattern in Fig. 3f exhibits multiple concentric circles and a set of well-defined spots, confirming the coexistence of metal Cu and polycrystalline CuO. According to the results of inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 5110), the calculated mass percentages of Cu and CuO are 14.5% and 85.5%, respectively.

       To gain the voltametric signals of the obtained Cu@CuOmodified electrodes, differential pulse voltammetry (DPV) responses were performed in 0.1 mol L−1 NaOH. Fig. 4a shows the average DPV curves of five electrodes at each glucose concentration (0, 0.5, 1, 1.5, 2, and 2.5 mmol L−1 ) between 0 and 0.8 V. Broad peaks at around 0.5–0.6 V are observed in all Cu@CuO-modified electrodes due to the Cu2+/Cu3+ redox pairs [37–40]. Furthermore, as the glucose concentrations increase, the signal intensity of the oxidation peak current becomes stronger. Fig. 4b shows the corresponding calibration plot of glucose concentrations (C, mmol L−1 ) and the current density at 0.55 V (I, μA) with a linear regression equation of I = −1.31 − 0.08C, as well as a correlation coefficient of 0.986 (R2 ). This indicates the surface-controlled electrochemical reactions on the Cu@CuO-modified electrodes [41]. To further demonstrate the glucose catalytic properties of Cu@CuO in non-enzymatic sen sors, current–time (i–t) responses were performed at 0.55 V using an amperometric technique in alkaline buffer. Fig. 4c displays the average i–t curves of five electrodes. All the electrodes exhibit typical amperometric responses to the changed glucose concentrations. The fitting linear regression equation in Fig. 4d is I (μA) = −0.14 − 0.03C (mmol L−1 ) (R2 = 0.991). The limit of detection (LOD) of the Cu@CuO-modified electrodes is estimated to be 1.3 μmol L−1 by LOD = 3SB/b (Signal/Noise = 3). Compared with the previous reports on non-enzymatic glucose sensors in Table 1, Cu@CuO-modified electrodes show advantages in simple fabricating method, higher sensitivity, lower LOD and more suitable linear range. Therefore, Cu@CuO has potential applications in glucose sensors.

      The reproducibility of Cu@CuO-modified sensor electrode was carried out 20 times in a mixture solution containing 1 mmol L−1 glucose and 0.1 mol L−1 NaOH, and the relative standard deviation (RSD) of the response current is 4.28% (Fig. 5a). This indicates that the nonenzymatic electrochemical sensor has good reproducibility. To discriminate concomitant electroactive species with jamming effects on glucose detection in serums, interference testing is required. Fig. 5b shows the average amperometric responses of five electrodes with some exogenous interfering maltose (5 mmol L−1 ), galactose (GAL, 0.83 mmol L−1 ), icodextrin (INN, 1.647 M), and creatinine (CR, 44 mmol L−1 ) in the mixed solution of 0.1 mol L−1 NaOH and 1 mmol L−1 glucose at 0.55 V. It clearly shows that the effect of maltose, GAL, and CR additives on glucose detection was small with relative variation values of 3.9%, 5.3%, and 6.8%, whereas the INN has a relatively highly influential but acceptable value of 17.8%. The results indicate that the Cu@CuO-modified electrodes could be used for glucose determination even in the presence of interference. Long-term storage stability in air at RT is another important evaluation parameter for the fabricated electrodes. Amperometric responses are checked over three weeks. Fig. 5c shows the average peak current of five electrodes on the 1st, 5th, 9th, 13th, 16th, 19th, and 22nd day with a low RSD of 2.82%, indicating the reliability of the Cu@CuO-modified electrodes for glucose detection.

      The band structure and DOS of Cu and CuO were calculated based on DFT, respectively. The results show that near the Femilevel (0 eV), Cu has lower electron density at p- and d-orbitals than CuO (Fig. 6a). Electrons could transfer from CuO to Cu at the interface of Cu@CuO, resulting in higher electrical conductivity than pure CuO [6,15]. In order to describe the adsorption of glucose on Cu and CuO surfaces qualitatively, the adsorption energies to oxygen atoms on the surface of CuO (111) and Cu (200) were calculated (Fig. 6b). Calculations show that oxygen atoms of glucose may form coordination with several Cu atoms, resulting in high adsorption energy between glucose and Cu [42]. In the case of CuO, the adsorption energy is smaller due to the single point adsorption between oxygen atoms of glucose and Cu atoms. Based on the above calculations, Cu plays a key role in increasing the electrical conductivity [35,39] and adsorption energy of oxygen atoms [42], leading to enhanced electrochemical catalytic behaviors for glucose electrooxidation. As a result, the Cu@CuO-modified disposable sensors have great potential for daily glucose detection.

CONCLUSIONS

      In summary, gram-scale Cu@CuO has been synthesized via insitu oxidation of commercial Cu at RT. After being screenprinted, Cu@CuO-modified disposable strip electrodes are formed for nonenzymatic glucose sensors. Benefiting from the high adsorption energy and electron conductivity of the Cu component, the sensors exhibit some excellent features, including high sensitivity, low detection limit, wide linear range, good selectivity, repeatability, and stability over a three-week storage period. It should be noted that the sensors could operate in a natural environment with ~10 μL of analytes. The test in neutral solution and the application in serum samples of Cu@CuO-modified electrodes would be analyzed in the future.

      These facts suggest a considerable and sustainable improvement in mass production of glucose sensors for nonenzymatic sensing applications.

Received 27 June 2023; accepted 21 August 2023;

published online 10 October 2023

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Acknowledgements

This work was financially supported by the Scientific Research Start-up Funds of Hexi University (KYQD2022004) and the 13th Innovation Program Science and Technology for college students of Hexi University (138). The authors would like to thank Prof. Zhenlai Liu of Hexi University for the initial support of laboratory construction.

Author contributions

Li S designed, engineered, characterized, and analyzed the samples with the support from Liu Y, Cao C, Li S, Wang X, Tian N, Peng S and Luo J; Xia H, Quan C, and Liu L performed the electrochemical experiments; Lu P built the models and performed the calculations. Li S wrote the paper with the support from Xia H and Peng S. All authors contributed to the general discussion.

Conflict of interest

The authors declare that they have no conflict of interest. 

Suyuan Li received his PhD degree from Lanzhou University in 2015. He works as an associate professor at Hexi University. His current research interests are focused on biosensors and lithium ion battery.

Changyun Quan is pursuing his PhD degree at the School of Chemistry and Chemical Engineering, Central South University. Since 2020, he has been working at Cofoe Medical Technology Co., Ltd. engaged in the development of biosensors. His current research interest is focused on developing a series of portable chronic disease detection and monitoring systems.

Shanglong Peng is a professor of Lanzhou University. From 2010 to 2016, he worked at the University of Washington, Seoul National University and the Hong Kong University of Science and Technology. Currently, he is mainly engaged in the design of nanomaterials, interface regulation and their applications in energy conversion and storage, including supercapacitors, solar cells and flexible wearable integrated energy conversion and storage integrated devices.

This article is excerpted from the Science China Press 2023 by Wound World.