Tunable hierarchical surfaces of CuO derived from metal–organic frameworks for non-enzymatic glucose sensing

Yumei Luo ab, Qingyong Wang c, Jinghua Li a, Fen Xu *a, Lixian Sun *ab, Yiting Bu a, Yongjin Zou a, Heinz-Bernhard Kraatz d and Federico Rosei e
aGuangxi Collaborative Innovation Center of Structure and Property for New Energy, Guangxi Key Laboratory of Information Materials, Guilin 541004, P.R. China. E-mail: sunlx@guet.edu.cn; xufen@guet.edu.cn
bSchool of Electronic Engineering and Automation, Guilin University of Electronic Technology, Guilin 541000, P.R. China
cKey Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
dDepartment Physics & Environment Science, University of Toronto Scarborough, Toronto, ON M1C 1A4, Canada
eInstitut National de la Recherche Scientifique—Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel-Boulet, J3X 1S2 Varennes, QC, Canada

Received 26th January 2020 , Accepted 23rd February 2020

First published on 24th February 2020


Non-enzyme glucose sensors constructed using transition metal oxides present several advantages such as their low cost, high stability, and high sensitivity. Herein, a kind of porous nanosphere-stacking CuO structure has been synthesized by optimizing the thermal decomposition atmosphere, which was derived from Cu-metal–organic framework (Cu-MOF) microrods. Such hierarchical CuO structures, with controllable porosity and adjustable surface area, are efficient catalytic materials for glucose sensing. Benefiting from structural advantages, the obtained porous hierarchical CuO nanospheres exhibit enhanced sensing performance compared to hierarchical CuO clusters. Based on CuO, the effects of surface and morphology on the sensing performance of glucose are also discussed. The sensitivity of CuO porous hierarchically nanospheres for glucose is found to be 1806.1 μA cm−2 mM−1 in the wide linear range of 0–6.535 mM with a low detection limit (S/N = 3) of 0.15 μM. Glucose detection in artificial saliva is then performed, which shows excellent capability in the low concentration range (5 μM–1.165 mM) for non-invasive sensing performance. The sensor also demonstrates a good recovery in real saliva. The novel MOF-templated CuO hierarchical nanospheres are expected to be effective sensing materials for developing non-enzyme and non-invasive glucose sensors.


1. Introduction

According to the International Diabetes Association (IDF), the number of people with diabetes will increase to 642 million in 2040, accounting for 10.4% of the world population.1,2 Since the first glucose sensor was invented, glucose monitoring tools have become critical medical devices for diabetes management.3–7 Except for collecting blood samples by finger pricking or venipuncture, non-invasive detection methods have focused on continuous monitoring of blood glucose.8 Diabetes can be also diagnosed by glucose levels in other body fluids such as saliva, sweat, tears and urine, which are simpler and painless in the collection process.9,10 A high correlation between saliva and blood glucose levels in diabetic patients has been previously reported,11–15 also indicating that the detection of glucose in saliva is a promising alternative for diabetes monitoring. However, the reliable, accurate, sensitive, fast and low-cost detection of non-invasive glucose is still in its infancy. Despite the high selectivity and sensitivity, enzyme-based technologies have some inherent disadvantages, such as high cost, difficult enzyme immobilization, and poor stability.16 Thus, non-enzymatic electrochemical glucose sensors with high sensitivity, good stability, and fast response are considered to be one of the most efficient methods for detecting glucose.17

Metal–organic frameworks (MOFs), wherein metal ions or clusters and organic ligands self-assemble, are widely studied due to their crystal structure, large surface area, controllable pore size, and good adsorption affinity.18–21 Depending on the organic and inorganic units, MOFs with different morphologies can be designed and realized with highly precise structure, large surface area, and controlled porosity.22–26 For example, the MOF-template synthesis method involves a controllable porous structure, which can promote the electron transfer and ionic diffusion to achieve high electrochemical activity.23,27,28 MOF-derived materials can be considered as one of the most desirable choices for designing suitable structures with a large number of active species, making MOFs a template of choice for higher activated species with diversified treatments.29 However, these activating processes usually involve with the structural collapse of the MOF, which largely defeats the purpose of using MOFs as templates. Therefore, developing a proper activating process is crucial for achieving superior structure with enhanced electrochemical performance.

Calcination strategies can not only shorten the reaction time without any post-treatment and simultaneously maintain the morphology of the initial MOF with large surface area.30–32 With this treatment, Cu-MOF derived copper oxide (CuO) stands out due to the intrinsic p-type semiconductor property, nontoxicity, and structural advantage of high stability, capability for electron transfer and regular periodicity.33 Thermal treatment has been chosen as an effective way to produce CuO samples, which can expose more Cu atoms active-sites for better electrochemical performance. However, such heat treatment is often accompanied by changes in crystallinity, structure and morphology. The properties of the final samples can be tuned by controlling the heat treatment conditions. Hence, choosing an appropriate MOF precursor and facile, economical, and effective pyrolysis process are promising ways to achieve higher sensitivity and wider linear range for glucose sensing.

Herein, we report a facial calcination strategy with the capability of adjusting different surfaces of electroactive materials by converting MOFs into metal oxide nanostructures (Scheme 1). First, a rod-like Cu-MOF was chosen as the precursor, then, the morphologies of CuO derived from the Cu-MOF were designed by tuning the temperature and thermal atmosphere without any post-treatment, with obtaining stacked porous CuO nanospheres. When applied as electrode materials to construct non-enzymatic glucose electrochemical sensors, the as-prepared CuO thermal treated under nitrogen gas first and in air followed by showed higher sensitivity, wider linear range, lower detection limit, and excellent anti-interference ability, compared with the CuO only thermal treated in air. Thus nano-sphere CuO is promising for applications in non-invasive glucose sensing.


image file: d0qi00104j-s1.tif
Scheme 1 Synthetic strategy of CuO samples with different hierarchical surfaces derived from Cu-MOF microrods.

2. Experiment

2.1 Synthesis of the CuO

Synthesis of Cu-MOF. The synthesis of Cu-MOF was modified according to previous work.34 Typically, copper chloride (CuCl2, 2 mmol) and 3,5-pyridinedicarboxylic acid (3,5-pdc, 1 mmol) were dissolved in 10 mL distilled water, respectively. After stirring for 10 min, the two solutions were mixed. Then the solution became homogeneous through continuous stirring for another 30 min and transferred into a 40 mL Teflon-lined autoclave. The reaction was driven at 140 °C for 12 h. After cooling down to room temperature, lightblue precipitates were obtained by filtration, then alternately washed with ethanol and deionized water several times, and finally vacuum dried at 80 °C for 12 h.
Synthesis of CuO-350-NA. There were two steps in the calcination process. First, the Cu-MOF (100 mg) was poured into a ceramic boat and then placed in a horizontal tube furnace with N2 atmosphere by heating to 350 °C for 3 h with the ramp rate of 2 °C min−1. After cooling to room temperature, the obtained sample was placed in the oven with air atmosphere by heating to 350 °C for 3 h with the ramp rate of 2 °C min−1.
Synthesis of CuO-350-AIR. There was one step of calcination process. The sample of Cu-MOF was heated with the ramp rate of 2 °C min−1 to 350 °C for 6 h in the air.

2.2 Characterization and electrochemical measurements

Material characterization. The crystal structures of all samples were analyzed using an X-ray diffractometer (XRD, Bruker D8 Advance) with a CuKα radiation at 40 kV and 40 mA. A Nicolet 6700 infrared spectrophotometer was measured to record Fourier transform infrared (FTIR) spectra of the samples in the wavenumber range of 4000–400 cm−1 in transmission mode. Scanning electron microscopy (SEM; Quanta 200, FEI) was used to characterize microstructures and morphologies of the samples. Structural analysis was conducted by transmission electron microscopy (TEM; FEI Titan G2 80-200). Energy dispersive spectroscopy (EDS) was performed during the scanning transmission electron microscopy (STEM) tests. The specific surface areas of the samples were measured based on N2 adsorption at 77.3 K using the Brunauer–Emmett–Teller (BET) method (Quantachrome Autosorb iQ2). The pore diameter was obtained by using the desorption isotherm of the Barrett–Joyner–Halenda (BJH) method. The samples were degassed at 160 °C until the vacuum pressure was lower than 10−4 Pa. The average value of three measurements was determined for each sample. The surface element chemical states of samples were analyzed by X-ray photoelectron spectroscopy (XPS; Thermo Electron ESCALAB 250).
Electrochemical measurements. First, a glassy carbon electrode (GCE) was carefully polished with 1.0, 0.3 and 0.05 mm Al2O3 powder in turn, then rinsed thoroughly with ultra-pure water and ethanol and dried with nitrogen gas. Simultaneously, 5.0 mg the hierarchical CuO was dispersed into 1.0 mL N,N-dimethylformamide (DMF) and 25 μL Nafion (0.25%) to obtain homogeneous suspensions by ultrasound. Then, 4 μL suspensions were dropped on the polished GCE surface and dried in air. The diameter of the glassy carbon electrode that we used was 3 mm, so that the loading of catalyst was calculated to be 0.28 mg cm−2. The as-prepared electrodes are marked as CuO-350-NA/GCE and CuO-350-AIR/GCE, respectively.

All electrochemical measurements were performed on a Zahner electrochemical workstation (Germany) at room temperature. A conventional three-electrode system was employed, including the modified GCE as the working electrode, a platinum wire as auxiliary electrode, and an Ag/AgCl electrode as reference electrode. The cyclic voltammetric (CV) experiments were performed without stirring. The amperometric and selectivity experiments were carried out under continuous stirring with 0.1 M NaOH electrolyte. Electrochemical impedance spectroscopy (EIS) measurements were performed in 0.5 M KCl solutions within the frequency range of 0.01–100 kHz using 0.5 mM K3Fe(CN)6.

Saliva glucose detection. Analyses of glucose in artificial saliva samples were carried out with Fusayama's artificial saliva, which consisted of 0.078 g NaH2PO4·2H2O, 0.043 g KCl, 0.081 g of CaCl2·2H2O, 0.04 g NaCl, 0.1 g urea, 1 mL of 0.05% of Na2S·9H2O in 100 mL of distilled water.35 15 mL of artificial saliva was homogeneously mixed with 5 mL of 0.1 M NaOH using a magnetic stirrer.

Human raw saliva samples were collected from healthy volunteers after rinsing the oral cavity with water. The obtained saliva samples were heated to 100 °C for 60 min, and then were centrifuged at 10[thin space (1/6-em)]000 rpm for 20 min. The supernatant was then collected and analyzed for the recovery test.36,37

3. Results and discussion

The initial Cu-MOF was prepared in a simple hydrothermal reaction (Fig. 1a and b). Microrods with widths and lengths of about 2–4 μm and 30–50 μm, respectively, were distributed uniformly with smooth surfaces, and the cross-sections of Cu-MOF exhibited as tri-prism or tetrahedron structures. Obtained from single-crystal analysis, the unit structure of the obtained MOF (Fig. 1c) consisted of trigonal bipyramidal copper centers coordinated by one pyridyl group and two carboxylate groups of 3,5-pdc ligands at the equatorial positions, and two water molecules coordinated to the metal center at the axis.34 The powder XRD patterns of the as-prepared Cu-MOF (Fig. 1d) further identified a good crystalline state that was consistent with the simulated patterns, indicating the successful synthesis of Cu-MOF. Meanwhile, sharp and intense diffraction peaks can also be observed, confirming high purity and good crystal quality of the Cu-MOF. These aspects are beneficial for electrochemical sensors and provide the possibility for producing uniform metal species.
image file: d0qi00104j-f1.tif
Fig. 1 SEM images (a and b) of Cu-MOF; (c) the coordination environment of Cu-MOF; (d) XRD patterns of the simulated and as-prepared Cu-MOF; (e) FT-IR spectrums of the organic ligand 3,5-pdc and the as-prepared Cu-MOF; (f) TG curves of as-prepared Cu-MOF under N2 and in the air; (g) the schematic diagram of the decomposition mechanism of Cu-MOF.

FT-IR spectra of 3,5-pdc and Cu-MOF are displayed in Fig. 1e. The FT-IR spectrum of 3,5-pdc showed a band at 1704 cm−1 corresponding to a C[double bond, length as m-dash]O stretch of the COOH group, which disappeared in the spectrum of Cu-MOF, suggesting the existence of the coordinate bond of Cu–O. The absorption bonds at 520 and 421 cm−1 correspond to Cu–O and Cu–N vibrations, respectively, which confirmed the formation of the coordinate bonds of Cu-MOF.38,39 Both of the above results confirmed that the rod-like Cu-MOF was successfully synthesized. TG analysis was conducted to simulate the heat treatment mechanism of Cu-MOF under nitrogen gas and in air (Fig. 1f). Two obvious weight loss steps could be observed whether in air or under nitrogen. In the 25–274 °C temperature range, the weight losses of Cu-MOF were almost the same under the two atmospheres. The first mass loss, ranging from 80 °C to 142 °C, was about 13.4%, which was related to the removal of water molecules. As the temperature increased, the framework of Cu-MOF began to collapse at about 274 °C. In air, Cu-MOF showed a second weight loss (71.1%) in the range of 274–331 °C. After that, no further weight loss was observed during temperature increase, which indicated that the Cu-MOF completely decomposed and totally converted to the final product CuO directly. The totally broken of Cu–N bonds along with the simultaneous formation of Cu–O bonds may lead to aggregation and the formation of CuO products.40 The decomposition mechanism of the Cu-MOF is outlined in Fig. 1g. Under nitrogen, Cu-MOF showed a second weight loss (51.1%) in the range of 274–320 °C. The follow-up long descent platform suggested that Cu-MOF did not completely collapse at 320 °C, which may slow down the aggregation of CuO particles; this is the basis for the optimized thermal activation condition for Cu-MOF. Hence, the sample of CuO-350-NA (under nitrogen gas first and in air followed by) was produced and CuO-350-air (only air) was also synthesized for comparison.

After thermal treatment under nitrogen gas and in air sequentially, CuO-350-NA with a rod-shape morphology was obtained (Fig. 2a). Different from CuO-350-AIR sample and Cu-MOF sample, the surface of CuO-350-NA was not smooth, which was caused by the homogeneous nanospheres self-assembly into the microrods (Fig. 2b), indicating a change occurred from the interior to the surface, which may be caused by the organic ligands cross-linking together and forming a partial network as supported materials under nitrogen treatment. Subsequently the organic fraction was removed in the air calcination process. The two-step calcination processes prevented the fast formation of new Cu–O bonds and facilitated the construction of hierarchical nanosphere structures. These aspects are expected to enhance the electrochemical performance of CuO nanospheres.41 Moreover, pores were visible in between the interconnected nanospheres (Fig. 2c and d) with an average grain size of around 70 nm, which may provide multi metal active-sites. Compared with the pristine Cu-MOF, the rod shape of CuO-350-AIR sample was almost destroyed, exhibiting as clusters (Fig. 2e). Higher magnifications (Fig. 2g and h) of CuO clusters could see close packing without apparent space between the clusters, which would restrict the electrolyte permeation and lead to low electrochemical activity. It can be shown that tuning the thermal condition allows to achieve a superior structure for Cu-MOF derived CuO for superior electrochemical performance.


image file: d0qi00104j-f2.tif
Fig. 2 SEM images of the as-prepared CuO-350-NA (a–d) and CuO-350-AIR (e–h).

TEM images (Fig. 3) were acquired to further investigate the detailed morphology of the as-prepared CuO samples. The obtained CuO-350-AIR rods could be seen as consisting of dense copper oxide clusters (Fig. 3a) exhibiting in a polycrystalline state, which was reflected by the selected area electron diffraction (SAED) pattern displayed in the red rectangle (Fig. 3b). Meanwhile, well-defined lattice fringes (Fig. 3c) with d-spacings of 0.232 nm were also measured with a fast Fourier transform (FFT) in the white rectangle. These are attributed to the (111) reflections of monoclinic CuO.42 Uniform CuO-350-NA rods could be observed, composed of homogeneous copper oxide nanospheres with obvious pores (Fig. 3d and e), which was in accordance with SEM images. The corresponding SAED pattern (Fig. 3e) in the red rectangle shows multiple diffraction rings, which also indicated a polycrystalline structure. Moreover, multiple well-defined lattice fringes are observed in the high-resolution transmission electron microscopy (HRTEM) image in Fig. 3f, and the d-spacing of the lattice fringes was measured to be 0.232 nm, which was the same as CuO-350-AIR. Meanwhile, STEM and EDS-mapping analysis (Fig. 3g–i) confirmed that Cu and O species were contained and distributed homogenously across the entire architecture of CuO-350-NA. The two-step pyrolysis processes were not only beneficial for the growth of uniform nanospheres, but also promoted the formation of pores. The rod-like structure yields high surface area, enables large electrolyte–electrode contact area, and reduces redox reaction time. More importantly, a hierarchical structure with nanospheres provides more redox reaction active sites, which is crucial for the proper performance of electrodes as electrochemical sensors. In addition, this porous structure facilitates electrolyte penetration, also shortening ionic diffusion and electron transfer length, thereby increasing the sensitivity performance of CuO-350-NA.


image file: d0qi00104j-f3.tif
Fig. 3 (HR)TEM images (a–c) of CuO-350-AIR, the inset is the corresponding SAED; (HR)TEM images (d–f) of CuO-350-NA, the inset is the corresponding SAED; STEM and the corresponding EDS-mapping of CuO-350-NA (g–i).

The crystal structures of the as-prepared samples were explored by XRD (Fig. 4a). All the diffraction peaks can be indexed to the monoclinic-phase of CuO (JCPDS no. 48-1548).43 No impurity peaks from Cu(OH)2 or Cu2O were observed, indicating the pure phase of the as-prepared CuO samples. The strong peaks suggested good crystallinity of the CuO samples. The average crystalline sizes of CuO-350-NA and CuO-350-AIR samples were calculated using Sherrer's formula as 15.96 and 24.49 nm, respectively.44 This implied a smaller crystallite size and larger extended surface area with structural defects for CuO-350-NA, which indicated a potentially good electrochemical activity.


image file: d0qi00104j-f4.tif
Fig. 4 XRD patterns (a) of CuO-350-NA and CuO-350-AIR; XPS spectrums of Co 2p (b), and O 1s for CuO-350-AIR (c), CuO-350-NA (d); nitrogen sorption isotherms (e) and (f) pore-size distributions of CuO-350-NA and CuO-350-AIR.

To further investigate the surface element chemical states of the CuO samples, we acquired XPS spectra from the CuO-350-NA and CuO-350-AIR samples. The peaks of C, O, and Cu are observed in both CuO samples (Fig. S1). Meanwhile, the peak for nitrogen could not be observed, accounting for the total oxidation and high purity of the as-prepared CuO samples which was also identified in the XRD results. The electron binding energies (BE) of Cu 2p3/2 and Cu 2p1/2 of CuO-350-NA sample are 933.48 eV and 953.48 eV, respectively (Fig. 4b). The BE of Cu 2p3/2 and Cu 2p1/2 of CuO-350-AIR sample were 933.78 eV and 953.78 eV, respectively. The gaps between Cu 2p3/2 and Cu 2p1/2 peaks of the as-prepared CuO samples were both 20 eV, which indicates the Cu2+ oxidation state.45 The high-resolution spectra of O 1s of the CuO-350-AIR and CuO-350-NA samples are presented in Fig. 4c and d. The two O 1s surface peaks can be fitted by two bands. The band with lower BE was ascribed to the lattice oxygen (OL) of CuO crystal lattice, which corresponds to 529.54 eV and 529.74 eV of CuO-350-AIR and CuO-350-NA samples, respectively. A shoulder band with higher BE was ascribed to the adsorbed oxygen or oxygen in hydroxyl-like groups on the surface of CuO (denoted as OC).46,47 The band was related to the bands at 531.23 and 531.54 eV of CuO-350-AIR and CuO-350-NA samples, respectively. As shown in Table S1, the OC component (56.40%) in CuO-350-NA sample was higher than the OL (43.60%) content, while the CuO-350-AIR sample showed the opposite. The greater the OC content, the higher the density of the surface defects and the surface adsorption sites, as well as the higher the catalytic activity. Hence, the CuO-350-NA sample is expected to show good electrocatalytic ability towards glucose oxidation.

The porosity of the as-prepared CuO samples was also studied by N2 sorption isotherm curves (Fig. 4e) and the pore size distributions were measured (Fig. 4f). Both CuO samples exhibit typical IV isotherms accompanied by H3-type hysteresis loops, which suggests the presence of a mesoporous structure.48 The BET surface areas of CuO-350-NA and CuO-350-AIR were calculated as 28.74 and 6.334 m2 g−1, respectively. The larger specific surface area of CuO-350-NA could expose more activated sites, as also be observed in the SEM images. Also, the pore size distribution showed that CuO-350-NA possessed partial micropores which were missing in the CuO-350-AIR sample. Such features would facilitate ionic adsorption and promote electrochemical activity for improved glucose sensing.

To investigate the electrochemical behavior and glucose sensing application of the as-prepared CuO samples, non-enzyme electrodes were designed by using GCEs. CV curves of CuO-350-NA/GCE and CuO-350-AIR/GCE samples at a scan rate of 50 mV s−1 in 0.1 M NaOH aqueous solution with and without 0.5 mM glucose are displayed in Fig. 5. In the absence of glucose (Fig. 5a), both CuO-350-NA/GCE and CuO-350-AIR/GCE showed a significant reduction peak at about +0.55 V, which was caused by the Cu(III)/Cu(II) redox, compared with the bare GCE. However, the corresponding oxidative peaks from +0.4 V to +0.8 V were not observed. This may due to the water splitting reactions at a large current density that covered the oxidation peaks formed by the Cu(II)/Cu(III) redox.49,50 Moreover, CuO-350-NA/GCE showed higher redox current than CuO-350-AIR/GCE, suggesting a higher electrocatalytic activity. With the addition of 0.5 mM glucose (Fig. 5b), except for the reduction peak, an oxidation peak occurred from +0.25 V to +0.38 V of CuO-350-NA/GCE. More importantly, the oxidation current in case of CuO-350-NA/GCE has been enhanced about 10 times compared to CuO-350-AIR/GCE; this is tentatively ascribed to the porous nanosphere-stacking structure of CuO-350-NA. The overall reaction mechanism can be represented as follows:45,51

 
CuO + OH → CuOOH + e(1)
 
image file: d0qi00104j-t1.tif(2)


image file: d0qi00104j-f5.tif
Fig. 5 Comparation of CV curves of CuO-350-NA, CuO-350-AIR/GCE and bare GCE in 0.1 M NaOH without glucose (a) and with 0.5 mM glucose (b) at the scan rate of 100 mV s−1; CV curves of CuO-350-NA in 0.1 M NaOH without glucose (c) and with 0.5 mM glucose (d) at various scan rates, inset graphs are the corresponding fitting curves.

To better understand the electrochemical behavior of the CuO/GCE, we also measured CV curves (Fig. 5c) CuO-350-NA/GCE without glucose in 0.1 M NaOH at different scan rates (25, 50, 75, 100, 125, 150, 175 and 200 mV s−1). As seen from the inset of Fig. 5c, the anodic peak current showed good linearity with the scan rate, indicating that the glucose sensing of CuO-350-NA/GCE was controlled by diffusion in the catalytic oxidation of glucose.52 After adding 0.5 mM glucose, the fitting curve between scan rate and anodic peak current also showed a good linear relationship, suggesting that the oxidation of glucose for CuO-350-NA/GCE is a typical surface-controlled chemical reaction. The same electrochemical behavior of CuO-350-AIR/GCE is also observed in Fig. S2, which also confirms the surface-controlled chemical mechanism of CuO sensing towards glucose. Analyzing CV curves of CuO-350-NA/GCE (a) and CuO-350-AIR/GCE (b) in response to increased concentration of glucose (Fig. S3), the oxidation peak current increased with an increase in the glucose concentration, which also indicates the glucose sensing ability of two CuO samples.53 Moreover, the increase in the oxidation peak current of CuO-350-NA/GCE was more obvious with adding higher concentrations of glucose, suggesting the higher amperometric response and higher catalytic activity towards the glucose. Such higher electrochemical activity is attributed to the more active sites which was exposed by higher surface area of CuO-350-NA.

To achieve a sensitive signal, the optimal applied potential for glucose sensing was investigated. Fig. S4a shows the amperometric curve of CuO-350-NA in 0.1 M NaOH with the continuous addition of 0.2 mM glucose at different potentials. Stable and rapid changes in the current response were observed with the increase in the applied potential, and the biggest response was obtained when the applied potential was set at +0.55 V. Therefore, +0.55 V was selected as the optimum potential in the following experiments. As shown in Fig. S4b, after adding 0.1 mM glucose, the current response of CuO-350-NA immediately increased and achieved 90% of the steady-state current (t90) within 3 s, suggesting a fast response to glucose oxidation.54

To further evaluate the sensitivity, linear range, and detection limit of the as-prepared CuO/GCEs for glucose detection, amperometric tests (Fig. 6a and c) and the corresponding calibration curves (Fig. 6b and d) for CuO-350-NA/GCE and CuO-350-AIR/GCE samples were obtained, respectively. When 5 μM glucose was added into the 0.1 M NaOH at the applied potential of +0.55 V, the response current of CuO-350-NA/GCE increased substantially (inset of Fig. 6a). According to the linear fitting equation (Fig. 6b), the sensor sensitivity of the CuO-350-NA/GCE sensor can be estimated at 1806.1 μA cm−2 mM−1 in the linear range of 0–6.535 mM with a detection limit (S/N = 3) of 0.15 μM. On the other hand, the sensitivity of CuO-350-AIR/GCE was 1614.4 μA cm−2 mM−1 in the linear range of 0–4.035 mM with a detection limit (S/N = 3) of 0.63 μM, which were obtained from Fig. 6c and d. The sensitivity of CuO-350-NA/GCE sensor was nearly 1.2 times higher than that of CuO-350-AIR/GCE. After adding glucose with the same concentration, the response time of the CuO-350-NA/GCE sensor (about 3 s) was apparently shorter than that of CuO-350-AIR/GCE (about 8 s) (Fig. 7a), confirming the higher sensitivity of CuO-350-NA/GCE which was mainly attributed to more surface-active sites due to the in situ growth of nanospheres. More importantly, the lower detection limit and a wider linear range of CuO-350-NA/GCE were very attractive for glucose detection. Compared to other enzyme-free glucose sensors fabricated using pure CuO, the as-prepared CuO-350-NA/GCE displayed a much wider linear range,55–61 as well as a low detection limit and high sensitivity, as shown in Table S2. The large specific area of the ordered nanospheres provided more active sites, and its porosity were good for electron and mass transfer. On this basis, we concluded that the superior porous structure composed by nanosphere-stacking of CuO-350-NA presented unique advantages towards glucose sensing.


image file: d0qi00104j-f6.tif
Fig. 6 Amperometric responses of CuO-350-NA/GCE (a) and CuO-350-AIR/GCE (c) in 0.1 M NaOH upon consecutive addition of glucose at 0.55 V (vs. Ag/AgCl); the corresponding calibration curves of the CuO-350-NA/GCE (b) and CuO-350-AIR/GCE (d) for the glucose detection.

image file: d0qi00104j-f7.tif
Fig. 7 (a) Amperometric responses of CuO-350-NA/GCE and CuO-350-AIR/GCE in 0.1 M NaOH upon consecutive addition of 0.5 mM glucose at 0.55 V (vs. Ag/AgCl); (b and c) influence of interfering substances (AA, UA and L-cysteine) on the amperometric response to glucose by CuO-350-NA/GCE sensor; (d) the long-term stability of the CuO-350-NA/GCE electrode was measured for every two days during 9 days.

Selectivity is another important factor for characterizing the performance of a glucose sensor. We thus studied the impact of interferences on the determination of glucose of the as-prepared CuO-350-NA/GCE (Fig. 7b and c). The response current to 0.1 mM glucose at a potential of +0.55 V was compared to the addition of 0.01 mM of sucrose, fructose, NaCl, ascorbic acid (AA), uric acid (UA), dopamine hydrochloride (DA), and L-cysteine. After adding the interference substances, the response current did not change significantly. After the subsequent addition of glucose, the response current showed a significant step-like jump, which indicated good anti-interference properties of CuO-350-NA/GCE in glucose detection. In contrast with the test for glucose (100%), current signals from interferences were insignificant, and a clear response from glucose was obtained again in the presence of interferences. However, CuO-350-AIR/GCE displayed poorer selectivity when adding interferents (Fig. S5). Moreover, the stability of CuO-350-NA/GCE was characterized every 2 days during a period of 9 days by adding 0.5 mM glucose into 0.1 M NaOH solution, as shown in Fig. 7d. The sensor showed about 17% loss in current response, which demonstrated that the non-enzymatic glucose sensor constructed by CuO-350-NA has good stability.

Considering the good electrochemical sensing performance of CuO-350-NA, it is possible to measure the glucose concentration quantitatively. Amperometric tests of the CuO-350-NA/GCE in artificial saliva with 0.1 M NaOH was also conducted (Fig. 8a). As seen from the corresponding calibration curves (Fig. 8b), the concentration of glucose was also linear with the response current within the concentration range of 5 μM–1.165 mM, which was large than the normal range of saliva glucose concentration as well as larger than the previous work (30–80 μM).62–64 The strong linear relationship between glucose concentration and the response current facilitated the quantitative detection of glucose in saliva. Moreover, the sensitivity of glucose of the CuO-350-NA/GCE in artificial saliva was 963.28 μA cm−2 mM−1, which was higher than the previous report.65 Furthermore, we also added the known concentration of glucose to raw saliva sample and examined the level of glucose by the standard addition method by amperometric test. Table S3 showed the results from three samples, and the CuO-350-NA/GCE sensor had a good recovery rate in saliva.


image file: d0qi00104j-f8.tif
Fig. 8 (a) Amperometric responses of CuO-350-NA/GCE in artificial saliva with 0.1 M NaOH upon consecutive addition of glucose at 0.55 V (vs. Ag/AgCl); (b) the corresponding calibration curves of the CuO-350-NA/GCE for the glucose detection.

To explore the reasons behind the good sensing properties of CuO-350-NA/GCE, EIS spectra of CuO-350-NA/GCE and CuO-350-AIR/GCE were measured, and the corresponding Nyquist plots are presented in Fig. 9a. The Nyquist semicircle of CuO-350-NA/GCE decreased dramatically compared with CuO-350-AIR/GCE. As expected, the electron transfer resistance of CuO-350-NA was remarkably reduced, which confirmed the better sensing performance. To further understand the internal relations between the surface morphologies and the sensing properties, CV curves of two CuO/GCE at 100 mV s−1 in 0.5 mM K3Fe(CN)6/0.1 M KCl electrolyte were conducted (Fig. 9b). CuO-350-NA showed a higher anodic peak current and larger curve integral area, indicating a better electrochemical performance.


image file: d0qi00104j-f9.tif
Fig. 9 (a) Electrochemical impedance spectroscopy of CuO-350-NA/GCE and CuO-350-AIR/GCE; (b) CV curves of CuO-350-NA/GCE and CuO-350-AIR/GCE in 0.5 mM K3Fe(CN)6/0.1 M KCl electrolyte at the scan rate of 100 mV s−1; CV curves of CuO-350-NA/GCE (c) and CuO-350-AIR/GCE (e) in 0.5 mM K3Fe(CN)6/0.1 M KCl electrolyte at different scan rate, the corresponding fitting curves of CuO-350-NA/GCE (d) and CuO-350-AIR/GCE (f).

To further investigate the influence of surface morphology on electrochemical properties, we calculated the effective electrochemical active area (ECSA) of these two CuO/GCE using the Randles–Sevcik equation.66 CV curves of CuO-350-NA/GCE at different scan rates and the corresponding fitting curve between the anodic peak (Ipa) and the scan rate quadratic (v1/2) were obtained (Fig. 9c and d). The linear relationship was as follows: Ipa (A) = 10−5 × V1/2 (R2 = 100%), and the ECSA can be calculated as 0.28 cm2. The linear relationship of CuO-350-AIR/GCE was as follows: Ipa (A) = 7.35 × 10−5 × V1/2 − 4.32 × 10−5 (R2 = 98.5%), with an ECSA of 0.012 cm2 (Fig. 9e and f). The much higher ECSA of CuO-350-NA, due to the hierarchical nanospheres, could be the main reason for the improved sensing performance over CuO-350-AIR.

According to the mechanism of glucose sensing,67,68 the first step of glucose electro-oxidation was adsorbing glucose molecules on the surface of metal-containing electrocatalysts, as well as forming the bonds between adsorbates and transition metal substrates. The hydrogen removal process was considered as the rate-determining step, which occurred simultaneously with the chemisorption of analytes. Thus, well-spaced adsorption sites on the surface of electrocatalysts with suitable geometry could contribute to the kinetic enhancement of the glucose oxidation process. Based on this theory, the excellent sensing performance of as-synthesized porous nanosphere-stacking CuO derived from Cu-MOF was attributed to two factors. First, the sensing performance largely depends on the structure, morphology, and surface state of the sensing materials. As known from the analysis of the porosity of two CuO samples, the specific surface area of CuO-350-NA was much larger than that of CuO-350-AIR. The larger surface allowed for the adsorption of more glucose molecules, leading to a higher response at the same addition of 5 μM glucose. Lacking a big cavity hindered the diffusion of the particles, so that CuO-350-AIR showed a decreased response rate. The interior structure of CuO-350-AIR cluster was extremely compact compared to the porous nanospheres CuO-350-NA according to the TEM analysis, which also led to longer and fewer paths for electron diffusion. Second, due to the hierarchical nanospheres of CuO-350-NA, more electrochemical active sites were provided and exposed, which was beneficial to higher sensitivity. The hierarchical nanospheres also provided more electrochemical active area for glucose oxidation, which contributed to absorbing more glucose molecules.

4. Conclusions and perspectives

Hierarchical MOF-derived CuO samples were synthesized under different heat treatment atmosphere. The CuO-350-NA sample showed porous nanosphere-stacking structure, which provided more exposed surfaces and active locations. It was concluded that the hierarchical rod shape material with a porous interior structure could be derived from MOF and exhibited superior sensing performance. CuO-350-NA presented excellent sensing performance to glucose with a sensitivity of 1806 μA cm−2 mM−1 in the wide linear range of 0–6.535 mM, as well as a detection limit of 0.15 μM. While the sensitivity of CuO-350-AIR/GCE was 1614.4 μA cm−2 mM−1 with a detection limit of 0.63 μM in the linear range of 0–4.035 mM. Hence, the CuO-350-NA/GCE sensor displayed better glucose sensing performance than CuO-350-AIR/GCE. The above work clearly emphasized architecture-dependent performance in glucose sensing, providing a facile step to synthesize high-performance CuO-based sensors. Moreover, the CuO-350-NA/GCE sensor also displayed excellent capability in the low concentration range (5 μM–1.165 mM) in artificial saliva, which was much larger than the normal range of saliva glucose concentration. The strong linear relationship between glucose concentration and the response current facilitated the quantitative detection of glucose in saliva. The sensor fabricated by CuO porous hierarchically nanospheres also had a good recovery in real saliva sample, which provided a new method way to alternatively diabetes monitoring.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Key R&D Program of China [2018YFB1501203, 2018YFB1501205]; National Natural Science Foundation of China [51971068, 51871065, 51671062, 51863005, 51462006, 51801041 and U1501242]; Guangxi Natural Science Foundation [2017AD23029, 2017JJB150085, 2014GXNSFAA118319, and 2014GXNAFDA118005]; Guangxi Bagui Scholar Foundation, Guangxi Talent Xiaogaodi Project and the study abroad program for the graduate student of Guilin University of Electronic Technology [YXYJ2900].

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Electronic supplementary information (ESI) available. See DOI: 10.1039/d0qi00104j

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