- Email: [email protected]
CuO nanowire arrays
Electrochimica Acta 334 (2020) 135630
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A highly sensitive non-enzymatic glucose sensor based on CuS nanosheets modified Cu2O/CuO nanowire arrays Chenhuinan Wei a, Xiao Zou b, Qiming Liu a, *, Shuxian Li a, Chenxia Kang a, Wei Xiang c a
School of Physics and Technology, Key Laboratory of Ariticial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan, 430072, China b Hospital of Wuhan University, Wuhan, 430022, China c Wuhan Union Hospital, Tongji Medical College, Wuhan, 430022, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2019 Received in revised form 30 December 2019 Accepted 3 January 2020 Available online xxx
CuS nanosheets modified Cu2O/CuO nanowire arrays (NWAs) were successfully fabricated on Cu foil by in-situ growth and successive ionic layer adsorption and reaction (SILAR) methods without binders. The three-dimensional Cu2O/CuO NWAs not only showed strong binding force with current collector but also provided abundant surface area to support CuS nanosheets. Different morphologies of CuS were obtained by varying the Cu(NO3)2 and Na2S concentrations and the SILAR cycle. When these step-wise prepared products were employed for non-enzymatic glucose sensing, the amperometric response of the optimized CuS/Cu2O/CuO/Cu electrode was approximately twice of that of Cu2O/CuO/Cu electrode. The result is attributed to the modification of CuS nanosheets with suitable amount which increases the active surface area of the electrode. This designed electrode presents an ultrahigh sensitivity of 4262 mA mM1 cm2 in the range from 0.002 to 4.096 mM, as well as the excellent selectivity, reproducibility and stability. Moreover, due to the facile preparation and the low cost of Cu based sensors, the CuS/Cu2O/CuO/Cu electrode will be one kind of promising materials for constructing practical nonenzymatic glucose sensors. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Cu2O/CuO nanowire arrays CuS nanosheets SILAR method Non-enzymatic glucose sensor High sensitivity
1. Introduction Accurate and convenient detection of glucose has attracted global attention because of the ever-growing demands for the living standards of human. The development of low-cost and efficient glucose biosensor is important in many occasions, such as the food industry, diabetes clinical diagnostics and biological research [1,2]. Owing to the good selectivity of enzymatic, glucose oxidase or dehydrogenase based glucose sensors have been applied to construct various amperometric biosensors [3]. However, enzymes are expensive. The catalytic activity of enzyme is highly sensitive to the environmental conditions such as temperature, pH, humidity and operational conditions. Immobilization procedure of enzyme on the conductive electrode is complicated and the amount of enzyme cannot be controlled precisely [4]. Consequently, a great many of efforts have been developed on non-enzymatic glucose sensors. The majority of non-enzymatic electrochemical glucose
* Corresponding author. E-mail address: [email protected] (Q. Liu). https://doi.org/10.1016/j.electacta.2020.135630 0013-4686/© 2020 Elsevier Ltd. All rights reserved.
sensors mainly rely on the electro-catalytic oxidation of glucose directly at the electrode surface without the involvement of enzyme. The absence of enzyme decreases the cost, increases the stability and facilitates the preparation of sensors [5,6]. In recent years, noble metals, such as gold, silver, platinum and their alloys, have been intensively applied in non-enzymatic glucose sensors owing to their special electrocatalytic activity [7,8]. Nevertheless, these metals also have defects like high cost, low selectivity and susceptible to poisoning [9,10]. By comparison, metal oxide/sulfide are low cost and non-toxic. Besides, they have high specific surface area and great electrochemical activity, which enable them to attract increasing attention towards highperformance non-enzymatic glucose sensors [11,12]. Among those metal oxides/sulfides, copper oxides/sulfides have been deeply investigated because of its abundant nature and low cost [13,14]. CuS is a stable semiconductor, and has high catalytic activity and metal like electrical conductivity [15e17]. Microstructured and nanostructured CuS with different morphologies, such as microflowers [18], nanoflowers [19], nanoparticles [20], and nanoflakes [21], have been dropped on glass carbon electrodes and fixed with binders as glucose sensors, and shown good biological
2
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
compatibility and superior catalytic property for fast, sensitive detection of glucose. Copper oxide (Cu2O or CuO) with high isoelectric point and low overpotential for electron-transfer reactions also has been widely applied in glucose sensing [22e25]. Recently, in-situ growth of three-dimensional Cu2O or CuO nanowire arrays on Cu substrates have been largely studied and reported [26e30]. On the one hand, the in-situ growth ensures the great connection between copper oxides and copper substrate, which decreases the inner resistance and promotes the electron transfer between active materials and highly conductive substrate [28]. On the other hands, nanowire arrays possess large active surface area. For instance, Huang and coworkers have confirmed that the CuO nanowire arrays exhibited over 5-fold higher sensitivity compared to CuO nanowire piles [29]. Since the nanowire arrays are binder-free and have abundant surface area, we expect that CuS nanostructure could act as an enhanced sensing material on the Cu2O or CuO NWAs if an optimum amount is deposited. In this work, for the first time we report the CuS nanosheets modified Cu2O/CuO nanowire arrays for electrochemical sensing of glucose. The fabrication process is simple and do not need expensive and complicated equipment. The Cu2O/CuO NWAs were synthesized on Cu foil by fast electrochemical oxidation and facile heat treatment. Next, CuS nanosheets were deposited on Cu2O/CuO NWAs by SILAR method [31e34] to form CuS/Cu2O/CuO/Cu based glucose sensor. The whole process avoided the use of additional binders. After optimizing the concentration of cationic and anionic sources as well as the number of SILAR cycle, amperometric response of CuS/Cu2O/CuO/Cu electrode towards glucose was approximately twice of that of Cu2O/CuO/Cu electrode, confirming the greatly electro-catalytic enhancement of CuS nanosheets. Furthermore, the designed non-enzymatic glucose sensor displayed an ultrahigh sensitivity. 2. Experimental 2.1. Materials High purity Cu foil (>99.9%) were purchased from aladdin Ltd. (China). Chemicals were of analytical grade, obtained from Sinopharm Chemical Reagent and used without further purification. Ultrapure water (18.25 MU cm) was generated from Water Purifier (Pincheng Technology Co., Ltd.) and used as solvent in this experiment. 2.2. Synthesis of Cu2O/CuO NWAs on Cu foil (Cu2O/CuO/Cu) Cu foil (1.0 cm 0.5 cm) was ultrasonically cleaned by acetone, 3 M HCl, distilled water, and ethanol for 10 min then dried under N2 gas. First, to in-situ synthesis Cu(OH)2 NWAs on Cu foil, a cleaned Cu foil, a platinum plate and a Hg|HgO were used as the working, counter and reference electrodes, respectively. The anodization process of Cu foil was conducted at a constant current density (6 mA cm2) in 25 ml NaOH (3 M) at room temperature. After 10 min，a blue film of Cu(OH)2 NWAs was formed on the surface of Cu foil. The product was taken out of NaOH, followed by rinsing with ethanol and drying in air for about 2 h. Second, Cu(OH)2 NWAs precursors were changed to copper oxides NWAs by annealing in air at 400 C for 1 h, the heating rate was 2 C/min, then cooled gradually to room temperature. 2.3. Synthesis of CuS nanosheets on the Cu2O/CuO NWAs (CuS/ Cu2O/CuO/Cu) Successive ionic layer adsorption and reaction (SILAR) method
was used to deposit CuS on Cu2O/CuO NWAs. Cationic and anionic sources were Cu(NO3)2$3H2O aqueous solution and Na2S$9H2O aqueous solution, respectively. Cu2O/CuO/Cu was immersed in a 200 ml breaker which was filled with 60 ml of Cu(NO3)2 (0.005, 0.01, 0.05, 0.1 M) and kept for 60 s, then rinsed with ethanol for 30 s. Subsequently, the Cu2O/CuO/Cu was immersed in 60 ml of Na2S (0.005, 0.01, 0.05, 0.1 M) for 60 s, followed by rinsing in ethanol for 30 s. The above description is a complete cycle. Notably when CuS were synthesized using 0.01 M Cu(NO3)2 and Na2S, the number of SILAR cycle was altered from one to four cycles. The corresponding four kinds of electrodes were denoted as CuS/Cu2O/CuO/Cu-1, CuS/ Cu2O/CuO/Cu-2, CuS/Cu2O/CuO/Cu-3 and CuS/Cu2O/CuO/Cu-4, respectively. 2.4. Characterization FE-SEM images were obtained using an ZEISS-SIGMA operating at 15 kV. A Brucker D8 advance X-ray diffractometer by Cu Ka radiation (l ¼ 1.5406 Å) was used to obtain XRD results. All samples were characterized at a scanning speed of 3 /min in the range (2q) of 10 e80 . Raman patterns were collected by a Raman microscope system from Renishaw Co., Ltd. Ar laser (l ¼ 514.5 nm) was used as the excitation source with a power kept at 4 mW. TEM and HRTEM images were obtained in JEM-2010 FEF transmission electron microscope. XPS results were acquired from a ESCALAB 250Xi X-ray photoelectron spectroscope (Thermo Fisher Scientific, USA) by Al Ka radiation with 1486.6 eV of excitation source. 2.5. Electrochemical measurements All electrochemical measurements were performed by a CHI 660D electrochemical analyzer (Chenhua instrumental Co., Ltd., Shanghai, China) with a three electrode system at ambient temperature. The step-wise prepared products, Cu foil, Cu2O/Cu/Cu and CuS/Cu2O/CuO/Cu, were employed as glucose sensing electrodes directly. The geometry area of the electrode immersed in the NaOH solution was 0.25 cm2. A platinum plate and a saturated Ag|AgCl were used as the counter and the reference electrodes, respectively. Cyclic voltammetry (CV) was investigated at a range of 0.2 Ve0.8 V. For amperometric detection, the solution was stirred at 200 rpm to provide convective transport. A 20 ml NaOH solution was used as electrolyte for glucose detection. Electrochemical impedance spectroscopy (EIS) technique was recorded in 0.1 M NaOH containing 1 mM glucose at open circuit voltage in the frequency range of 0.01 Hze100 kHz with an AC probe amplitude of 5 mV. 3. Results and discussion The three-step procedure for the fabrication of CuS modified Cu2O/CuO NWAs is schematically displayed in Fig. 1a. The Cu foil was first anodized and crystallized into Cu(OH)2 NWAs in the strongly alkaline solution. One dimensional Cu(OH)2 nanowire was formed owing to the shortest inter-planar spacing of (100) in Cu(OH)2 and the relatively stronger hydrogen bonding interaction [29]. Next, heating of Cu(OH)2 NWAs based Cu foil attracted dehydration and transformation into Cu2O/CuO NWAs. The last step is the SILAR process. The Cu2O/CuO/Cu was firstly immersed in the Cu(NO3)2 solution, resulting in the adsorption of Cu2þ on the surface of Cu2O/CuO NWAs. Then, ethanol solution was used to remove redundant adsorbed ions of Cu(NO3)2. When Cu2þ absorbed Cu2O/ CuO/Cu was immersed in Na2S solution, Cu2þ could strongly attract S2. When Cu2þ reacted with S2, CuS was produced. Ethanol solution was used again to remove the unreacted ions from the diffusion layer. The whole process is described by following
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
3
Fig. 1. (a) Schematic illustration of the fabrication process of CuS/Cu2O/CuO/Cu. SEM and TEM images of (b1-b4) Cu(OH)2 NWs, (c1-c4) Cu2O/CuO NWAs and (d1-d4) CuS/Cu2O/CuO/ Cu-2.
equations [35,36].
anodization Cu þ 2OH !Cu OH þ 2e 2
D
Cu þ CuðOHÞ2 !Cu2 O þ H2 O D
(1)
(2)
CuðOHÞ2 !CuO þ H2 O
(3)
Cu2þ þ S2 /CuS
(4)
The morphology of the synthesized products was characterized by FESEM and TEM. From the overview image in Fig. b1, the dense and straight Cu(OH)2 nanowires are regularly ranged on Cu foil. At high magnification (Fig. 1b), the Cu(OH)2 nanowire is a circular truncated cone and the diameter of the top of the nanowire is about 300 nm. It illustrates the “root” of nanowires have the large surface area to connect the Cu foil. The surface of the nanowire is smooth. After annealing (Fig. 1c), the nanowires become a little bend and
the porosity in Cu2O/CuO NWAs are shown up. This morphology is caused by the evaporation of H2O vapor during the dehydration. Compared with smooth Cu(OH)2 nanowires, the porous nanostructures provide increased surface area which may suit to mediate the nucleation and growth of foreign materials. Besides, the Cu2O/CuO product remains macroscopically uniform nanowire structure (Fig. 1c1), which supplied good backbone for supporting CuS. The open space between each nanowire also supplied enough volume for growing CuS and space for the mass transport. Fig. 1d is the SEM and TEM images of CuS/Cu2O/CuO/Cu-2. After two cycles of SILAR, CuS nanosheets are deposited on Cu2O/CuO NWAs without aggregation. The CuS modified NWAs still remains nanowire structure. TEM image in Fig. 1d4 further displays that there are amounts of thin nanosheets on the nanowires. The integration of CuS nanosheets on Cu2O/CuO NWAs presents the branched architecture with high surface area. It is clearly that there is no binder that covers the active materials on Cu foil. The in-situ growth and SILAR method ensure the direct synthesis of materials on Cu substrate. Without the use of binder, the modified electrode can realize the direct electron transfer between the active materials and the
4
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
conducting substrate, which is important to improve the performance of the sensing. Fig. 2a is the XRD patterns of the step-wise prepared products. The Cu foil shows two diffraction peaks at 43.3 and 50.4 , which agree with the (111) and (200) planes of cubic Cu (JCPDS no. 04e0836). For Cu(OH)2 NWAs (black line), all of the diffraction peaks are well fitted to the XRD pattern of the orthorhombic structured Cu(OH)2 (JCPDS no. 72e0140). In red line, diffraction peaks at 29.6 , 36.4 , 42.3 and 61.4 are in good accordance with the planes of Cu2O (JCPDS no. 78e2076), diffraction peaks at 35.5 , 38.7 and 48.7 can be assigned to the planes of CuO (JCPDS no. 45e0937). It illustrates that the nanowires after annealing are consist of Cu2O and CuO. In particular, no characteristic diffraction peak of CuS was obtained when the SILAR cycle was up to four. The reason was that CuS was too little to be detected. In this case, we synthesized CuS modified Cu2O/CuO NWAs by further increasing the SILAR cycle. A small diffraction peak at 47.9 can be identified (blue line) until eight SILAR cycles. The peak is well fitted to the (110) planes of CuS (JCPDS no. 06e0464). Furthermore, the crystal structure of CuS/Cu2O/CuO/Cu-2 was characterized by TEM and HRTEM as shown in Fig. S1. The lattice fringe distances of 0.273 nm and 0.304 nm match well with the lattice spacing (006) and (102) planes of CuS (JCPDS no. 06e0464), respectively. Raman spectra are shown in Fig. 2b. The peaks at 286, 333 and 625 cm1 can be identified to Cu2O and CuO heterostructure. After 2 cycles of SILAR, a new strong peak at 470 cm1 is emerged, which is the result from the SeS bond stretching in the lattice vibrations of CuS [37]. Furthermore, XPS analysis was used to further characterize the products. Compared to the Cu2O/CuO NWAs, the product synthesized after SILAR process has two additional peaks that correspond to S 2s and S 2p (Fig. 2c). In Cu 2p spectrum (Fig. 2d), two peaks located at 932.5 and 952.4 eV are assigned to Cu 2p3/2 and Cu 2p1/2
of Cu2O. Two peaks at 934.2 and 954.3 eV and the remaining two shake-up satellite peaks at 943.9 and 962.8 eV are a well evidence of the presence of Cu(II). For S 2p spectrum (Fig. 2e), the S 2p3/2 and S 2p1/2 peaks are observed at 161.8 and 163.0 eV, further confirming the successful deposition of CuS on Cu2O/CuO NWAs surface [38]. All the results in Fig. 2 illustrate the final product is the hybrid of CuS, Cu2O, CuO and Cu. To investigate the influence of the concentrations of cationic and anionic sources on the morphology of CuS crystal, the concentrations of Cu(NO3)2 and Na2S were ranged from 0.005 mM to 0.1 mM. The corresponding FESEM images are shown in Fig. 3. Initially, when 0.005 mM Cu(NO3)2 and Na2S were used, the number of nucleation sites formed on the Cu2O/CuO nanowires were limited due to the small amount of reduced CuS monomer. 2D CuS nanosheets were grown after thermodynamic growth through the diffusion of Cu2þ in the vicinity of the nucleation sites, see Fig. 3a. When the concentration Cu(NO3)2 and Na2S was increased to 0.01 mM, more nucleation sites are available and consequently higher density of CuS nanosheet were grown on the nanowires (Fig. 3b). In contrast, further increasing the concentration of Cu(NO3)2 and Na2S to 0.05 mM produced a great number of nucleation sites and leaded to the fast isotropic growth of CuS nanoparticles. These nanoparticles aggregated and merged, eventually covering the Cu2O/CuO nanowires completely, as shown in Fig. 3c. When Cu(NO3)2 and Na2S were 0.1 mM, the surplus reduced CuS monomer rendered the nucleation occurred both in solution and on the nanowires, forming clusters that partly broke the structure of nanowire arrays (see Fig. 3d). Collapse of nanowires appeared when the concentrations of Cu(NO3)2 and Na2S were larger than 0.05 mM. The reason may be that too much CuS agglomerates loaded on the bend Cu2O/CuO nanowires. The effect of the SILAR cycle were also investigated. Fig. 4aed are the FESEM
Fig. 2. (a) XRD patterns of Cu(OH)2/Cu (black line), Cu2O/CuO/Cu (red line) and CuS/Cu2O/CuO/Cu (blue line). (b) Raman spectra of Cu2O/CuO/Cu and CuS/Cu2O/CuO/Cu-2. (c) Typical XPS survey spectra of the Cu2O/CuO/Cu and CuS/Cu2O/CuO/Cu-2. XPS spectra of the (d) Cu 2p and (e) S 2p of CuS/Cu2O/CuO/Cu-2. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
5
Fig. 3. SEM images of CuS/Cu2O/CuO/Cu synthesized in (a) 0.005 M, (b) 0.01 M, (c) 0.05 M and (d) 0.1 M Cu(NO3)2 and Na2S with 2 SILAR cycles.
Fig. 4. SEM images of CuS/Cu2O/CuO/Cu synthesized in 0.01 M Cu(NO3)2 and Na2S with (a) 1, (b) 2, (c) 3 and (d) 4 SILAR cycles.
images of CuS/Cu2O/CuO/Cu with the increased SILAR cycle. Repeating the SILAR cycle, new nucleation sites were formed continuously thus the amount of CuS nanosheets were increased. After three SILAR cycles (Fig. 4c), CuS nanosheets based thick shell almost covers the entire Cu2O/CuO nanowires. Intuitively, increasing the SILAR cycles will absorb more Cu2þ and S2, thereby increasing the amount of CuS nanosheets. The electro-catalytic activities of the step-wise prepared electrodes in our study were tested at a scan rate of 50 mV s1 in 0.1 M NaOH (Fig. 5a). For CuS/Cu2O/CuO/Cu-2, the CV curve of electrode in the absence of glucose displays a broad redox behaviour through Cu2þ/Cu3þ redox couple (curve 5). When 2 mM glucose is added, there is an increase of the anodic peak current (curve 6). This indicates the proposed involvement of Cu2þ and Cu3þ surface species in the oxidation of glucose [39]. By comparing the CV curves of step-wise prepared electrodes in the presence of 2 mM glucose (curve 2, 4 and 6), the CuS/Cu2O/CuO/Cu-2 electrode exhibits the best CV response, illustrating that CuS/Cu2O/CuO/Cu-2 electrode had an enhanced electro-catalytic performance when compared with the Cu2O/CuO/Cu electrode and the pure Cu foil. It may because of the improved electron transfer from the synergistic effect of ternary compositions [40,41]. Fig. 5b shows the current responses of step-wise prepared electrodes toward increased
concentration of glucose and Fig. 5c is the corresponding relationship between glucose concentration and response current. The CuS/Cu2O/CuO/Cu-2 electrode displays the maximum oxidation current and the highest sensitivity. It is noted that the current response of CuS/Cu2O/CuO/Cu-2 electrode is approximately twice of that of Cu2O/CuO/Cu electrode, which further demonstrates that CuS nanosheets are effective for enhancing the electrooxidation of Cu2O/CuO/Cu electrode towards glucose. Besides, with the increase of glucose concentration, the response currents increase linearly with satisfactory linearity (Fig. 5c). CV curves of CuS/Cu2O/CuO/Cu2 electrode with increased scan rates in the presence of 2 mM glucose are exhibited in Fig. 5d. The anodic peak currents around 0.6 V increase with the increase of the scan rate. The inset in Fig. 5d exhibits a great linear dependence between the square root of scan rate (v1/2) and oxidation peak current with a correlation coefficient of 0.998, revealing that the electro-catalysis oxidation of glucose on CuS/Cu2O/CuO/Cu-2 electrode is controlled by diffusion [42]. According to the previous researches, the possible mechanism of the glucose oxidation process on Cu based sensors is presented in Fig. S2. The detail reactions are proposed as follows [18,22,27,28]. Cu2O þ 2OH þ H2O / Cu(OH)2 þ 2e
(5)
6
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
Fig. 5. (a) CVs of pure Cu (black curve), Cu2O/CuO/Cu and CuS/Cu2O/CuO/Cu-2 electrodes in 0.1 M NaOH in the absence (dotted curve 1, 3 and 5) and in the presence (solid curve 2, 4 and 6) of 2 mM glucose. (b) Amperometric response of the step-wise prepared electrodes in 0.1 M NaOH with successive addition of 6 mL, 8 mL and 10 mL of 0.5 M glucose in 0.1 M NaOH. (c) The relationship between glucose concentration and current response of step-wise prepared electrodes. (d) CVs of CuS/Cu2O/CuO/Cu-2 electrode in 0.1 M NaOH in the present of 2 mM glucose with scan rates of 30, 50, 70, 90 and 110 mV s1 and the corresponding linear curve of the peak current vs. scan rate. Amperometric responses of CuS/Cu2O/ CuO/Cu electrodes synthesized in (e) different concentration of Cu(NO3)2 and Na2S and (f) different SILAR cycles with successive addition of increased concentration of glucose in 0.1 M NaOH. (h) Current responses of electrodes synthesized with different Cu(NO3)2 and Na2S concentrations and SILAR cycle to 0.6 mM glucose in 0.1 M NaOH (n ¼ 3). (i) Amperometric response of the CuS/Cu2O/CuO/Cu-2 electrode in 0.1 M NaOH at various detection potentials with successive addition of increased concentration of glucose in 0.1 M NaOH.
Cu(OH)2 þ OH / CuOOH þ H2O þ e
(6)
CuO þ OH / CuOOH þ e
(7)
Cu3þ þ glucose / Cu2þ þ gluconic acid
(8)
In order to optimize the electrochemical sensing of sensor, the concentration of Cu(NO3)2 and Na2S and the SILAR cycle were investigated by assessing the amperometric response. To ensure the stability and the nanowire structure of the electrode, we only compared the current responses of CuS modified electrodes synthesized in 0.005 M and 0.01 M reaction solutions. Obviously, the electrode synthesized in 0.01 M reaction solution has the higher current response (Fig. 5e). It illustrates that increasing CuS nanosheets increased the surface active sites of sensor (Fig. 3a and b). Next, Fig. 5f shows the current responses of electrodes synthesized with different SILAR cycle. The CuS/Cu2O/CuO/Cu-2 electrode presents the highest current response among these four sensors. The variation of the morphology of the CuS nanosheets has been shown in Fig. 4aed. The amount of CuS nanosheets is enhanced by increasing the SILAR cycle, thus the current response increases from CuS/Cu2O/CuO/Cu-1 electrode to CuS/Cu2O/CuO/Cu-2 electrode. However, the current response decreases when the CuS nanosheets were synthesized through three SILAR cycles. It is due
to the thicker covered layer which decreased the interaction between the electrolyte and inner active materials, and slowed down the glucose diffusion to the inner active materials, so the electrocatalytic contribution from Cu2O/CuO was restricted. EIS measurements were carried out to investigate the charge transfer activity at the electrode/electrolyte interface. All Nyquist plots consisted of a semicircle in the high frequency region and a straight line in the low frequency region. The diameter of the semicircle associates with the charge transfer resistance (Rct), indicative of the conducting capability. From the result in Fig. 5g, CuS/Cu2O/CuO/Cu2 electrode owns the smallest semicircle diameter, indicating the better electron transfer behaviour of the CuS/Cu2O/CuO/Cu-2 electrode, which is beneficial for the glucose electro-oxidation. The optimized results are collected in Fig. 5h, the product synthesized in 0.01 M Cu(NO3)2 and Na2S with two SILAR cycles displays the highest current response. The influence of detection potential on the current response of CuS/Cu2O/CuO/Cu-2 electrode was also investigated. As shown in Fig. 5i, applied potential of 0.60 V causes a notable increase of amperometric response upon each addition when compared with 0.50 V and 0.55 V. Further increasing the applied potential to 0.65 V and 0.70 V, the higher background currents appears [43]. In this case, 0.60 V was chosen as the suitable detection potential for glucose sensing in the subsequent study. The enzymatic glucose sensors based on the bio-redox
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
interaction between the enzyme and glucose can be used in the physiological condition to realize the detection of glucose. Nevertheless, alkaline electrolyte is necessary for the electro-oxidation of carbohydrates on Cu based sensing electrodes attributing to their electro-catalytic effect on Cu2þ/Cu3þ couples. Thus, the concentration of NaOH was varied from 0.05 M to 0.5 M. Fig. S3a is the amperometric response of CuS/Cu2O/CuO/Cu electrodes in different NaOH concentrations. It can be seen that all of the electrodes have responses in different NaOH concentrations. The background current increases with the increase of NaOH concentration, which is adverse to the detection limit of sensor. The current density increments for 0.6 mM glucose in different NaOH concentration were also calculated (Fig. S3b). When NaOH concentration increases from 0.05 M to 0.1 M, the current density increment of CuS/Cu2O/ CuO/Cu electrode increases because glucose is easily oxidized at high pH. However, the current density increment decreases since the NaOH concentration is up to 0.15 M. The reason may be that too much OH blocks the electro-adsorption of glucose anion on the electrode [44,45]. The result in Fig. S3b illustrates that CuS/Cu2O/ CuO/Cu electrode tested in 0.1 M NaOH is most sensitive for glucose sensing. Thus, 0.1 M NaOH is the optimal alkaline electrolyte. Fig. 6a displays the amperometric response of the CuS/Cu2O/ CuO/Cu-2 electrode at 0.60 V with successive injection of glucose into 0.1 M NaOH solution. Inset is the enlarged response from 350 to 800 s. The amperometric response increases with the glucose level and the corresponding calibration curve based on current density versus glucose concentration is displayed in Fig. 6b. The CuS/Cu2O/CuO/Cu-2 electrode provided a high sensitivity of 4262 mA mM1 cm2 in the range of 0.002e4.096 mM (R2 ¼ 0.996) and a low detection limit (LOD) of 0.89 mM. LOD was calculated by 3 s/s, where s is the standard deviation of the background current and s represents the slope of the calibration curve. It is worth nothing that the designed electrode possesses prominent
7
sensitivity when compared with those of previously reported sensing electrodes (Table 1). Four factors are considered to have contributed to this high sensitivity: (1) In-situ growth of Cu2O/CuO NWAs on Cu foil ensures the strong binding force between current collector (Cu foil) and active material (Cu2O/CuO NWAs). Directly deposition of CuS without binders shortens the path length for electrons transportation between the electrolyte and electrode. (2) The nanowire arrays can provide more internal spaces for fast electrolyte penetration. Besides, Cu2O/CuO NWAs employed as an effective medium for electron transfer as well as a large-area support for CuS nanosheets. (3) These ultra-thin CuS nanosheets on the Cu2O/CuO NWAs further increase the accessible reaction surface area and the electro-catalytic active sites for glucose sensing. (4) The modification of CuS contributes to the enhancement of the mixed-valence Cu2þ/Cuþ and thus facilitates the redox reactions between glucose and CuS/Cu2O/CuO/Cu electrode. The sensitivity of our product to glucose also greatly outperforms the sensors with enzyme [54e56]. It has been reported that thick enzyme layer is prone to cause the surface blocking effect [57]. Enzyme immobilization creates an extra barrier for the electron transfer process between the electrode and the electrolyte interface. By contrast, non-enzymatic glucose sensors could avoid this disadvantage of enzyme. The absence of enzyme ensures the direct contact of glucose and electrode surface and the fast electron transfer between electrolyte and conductive electrode, which is beneficial to promote the sensitivity of glucose detection. The selectivity is an important factor for non-enzymatic glucose sensors. It has been reported that Cl can poison some metal based sensors. Ascorbic acid (AA), uric acid (UA), acetaminophen (AP) and reducing sugars may cause unwanted interference. In human blood, the normal concentration of glucose is at least 30 times higher than that of some interfering species [29,39]. To ensuring the feasibility of our electrode, the concentration of the added glucose was
Fig. 6. (a) Amperometric response of the CuS/Cu2O/CuO/Cu-2 electrode with successive addition of glucose into 0.1 M NaOH at 0.60 V. (b) The corresponding calibration curve of the CuS/Cu2O/CuO/Cu-2 electrode. (c) Amperometric response of the CuS/Cu2O/CuO/Cu-2 electrode to successive addition of 1 mM glucose and 0.1 mM interferences. (d) Long stability of the CuS/Cu2O/CuO/Cu-2 electrode measured for three weeks.
8
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
Table 1 Comparison of sensing performance of our proposed CuS/Cu2O/CuO/Cu electrode with other non-enzymatic glucose sensors. Electrode
Sensitivity (mA mM1 cm2)
Linear range (mM)
Ref.
CuS nanosheets/Cu2O/CuO NWAs/Cu foil Cu2O biscuit/SPCE CHT/Cu@Cu2O NPs/FTO Nafion/Au@Cu2O NPs/GCE Cu NPs/ITO Nafion/Cu2O nanosheets/Cu NWs/GCE Nafion/HKUST-1 derived CuO nanorod/GCE Cu(OH)2 nanotube arrays/Cu foil Cu2O/CuO nanoporous layer/Cu foil Cu2O/CuO nanosheets/Cu foil CuO NWAs/Cu foil/PET CuO NWAs/Cu foil Nafion/CuS microflower/GCE Nafion/CuS NPs/GCE
4262 309 618 715 1005 1420 1524 418 1066 1541 1085 1886 1007 1085
0.002e4.1 0.0005e4.03 0.01e2.0 0.05e2.0 0.0033e3.9 0.0007e2.0 Up to 1.25 Up to 3 0.1e2.04 Up to 4.0 Up to 2.55 0.002e3.56 0.01e5.4 0.02e2.5
This work [46] [47] [48] [49] [50] [51] [52] [25] [53] [29] [26] [18] [38]
Note: NW—Nanowire, NWAs—Nanowire arrays, NPs—Nanoparticles, CHT—Chitosan, GCE—Glass Carbon electrode, SPCE—Screen-Printed Carbon Electrode, ITO—Indium Tin Oxide, PET—Polyethylene terephthalate, HKUST-1—Cu3(BTC)2 (BTC: benzene tricarboxylate).
decreased to 10 times of the interfering species. The selectivity result of CuS/Cu2O/CuO/Cu-2 electrode is displayed in Fig. 6c. When 1 mM glucose was added into NaOH solution, the steady current gains a significant increase immediately. And there are negligible responses for 0.1 mM AP, 0.1 mM UA, 0.1 mM AA, 0.1 mM KCl, 0.1 mM sucrose and 0.1 mM lactose. The result ascertains that the prepared sensing electrode has great selectivity toward glucose. Thus, it can be speculated that the influence of the interfering species in human serum is slight for our sensor in glucose detection. A possible reason for the good selectivity of our prepared electrode against the major interfering specie of AA can be ascribed to the electrostatic repelling effect [58,59]. The isoelectric point (IEP) of the copper oxides is ~9.5, which means that the surface of our sensor would be negatively charged in 0.1 M NaOH (pH ¼ 13). The AA is negatively charged in 0.1 M NaOH solution because of the loss of proton. Therefore, the negative surface of sensor could strongly repel the negatively charged molecules. The reproducibility of sensor also plays significant role for practical application. Fig. S4a is the current responses of five electrodes with 0.2 mM glucose and Fig. S4b is the current response of one electrode with five successive glucose additions. The relative standard deviation (RSD) of current responses of five electrodes is calculated to be 2.85%, and the RSD of current increments of one electrode for five consecutive measurements with 0.2 mM glucose is 3.29%. These data confirm the good reproducibility of CuS/Cu2O/CuO/Cu-2 electrode. The stability of sensing electrode stored in air at room temperature (about 20 C/50% RH) was also tested. The current responses to 0.2 mM glucose were checked every 3 days within a period of three weeks (Fig. 6d). The sensing electrode exhibits the retention current about 92% from its original response after three weeks. This results suggests that CuS/Cu2O/CuO/Cu-2 electrode has the acceptable stability towards the glucose detection. The reason may be attributed to the fact that the coverage of stable semiconductors decreased the expose area of metal copper. To confirm the sensor’s feasibility for practical analysis, the designed CuS/Cu2O/CuO/Cu-2 electrodes were used for detecting the glucose level in real human serum. Two serum samples were obtained from local hospital without further pre-treatment before detection. 50 mL of serum samples and three successive additions of 10 mL glucose (0.1 M) were injected in 20 ml of 0.1 M NaOH at 0.60 V under stirring. According to the obtained calibration curve, the glucose concentrations of serum samples were calculated in Table S1. As expected, the results are well fitted with the given values from hospital, demonstrating that our sensors are suitable for monitoring the glucose in human blood.
4. Conclusion In summary, CuS nanosheets modified Cu2O/CuO nanowire arrays was fabricated on Cu foil as a novel binder-free non-enzymatic glucose sensor. The amount of CuS nanosheets on the Cu2O/CuO NWAs was increased with the increase of SILAR cycles and concentrations of reaction solution. However, further increasing the concentrations of reaction solution, the CuS aggregated, large cluster appeared and the structure of nanowire arrays was damaged. After optimizing, the CuS/Cu2O/CuO/Cu synthesized in 0.01 M Cu(NO3)2 and Na2S with 2 SILAR cycles owned the highest current response towards glucose. The response of CuS/Cu2O/CuO/ Cu-2 electrode was approximately twice of that of Cu2O/CuO/Cu electrode. Moreover, the CuS/Cu2O/CuO/Cu-2 electrode exhibits a high sensitivity of 4262 mA mM1 cm2 in the range of 0.002e4.096 mM, great reproducibility, acceptable selectivity and stability, together with the feasibility of glucose detection in human serum. Thus, the CuS nanosheets modified Cu2O/CuO nanowire arrays has promising potential to be applied as a non-enzymatic glucose sensor. Declaration of competing interest The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Chenhuinan Wei: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Xiao Zou: Methodology, Formal analysis. Qiming Liu: Conceptualization, Methodology, Supervision. Shuxian Li: Writing - review & editing. Chenxia Kang: Writing - review & editing. Wei Xiang: Resources. Acknowledgments This research work was financially supported by the National Natural Science Foundation of China (51572202) and the Natural Science Foundation of Jiangsu Province (BK20171234). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135630.
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
References [1] K. Fisher, L. Griffith, A. Gruneir, D. Panjwani, S. Gandhi, L.L. Sheng, A. Gafni, P. Chris, M. Markle-Reid, J. Ploeg, Comorbidity and its relationship with health service use and cost in community-living older adults with diabetes: a population-based study in Ontario, Canada, Diabetes Res. Clin. Pract. 122 (2016) 113e123. [2] K. Dhara, D.R. Mahapatra, Electrochemical nonenzymatic sensing of glucose using advanced nanomaterials, Mikrochim. Acta 185 (2017) 49. [3] J. Wang, Electrochemical glucose biosensors, Chem. Rev. 108 (2008) 814e825. [4] S.Y. Tee, C.P. Teng, E. Ye, Metal nanostructures for non-enzymatic glucose sensing, Mater Sci Eng C Mater Biol Appl 70 (2017) 1018e1030. [5] P. Si, Y. Huang, T. Wang, J. Ma, Nanomaterials for electrochemical nonenzymatic glucose biosensors, RSC Adv. 3 (2013) 3487e3502. [6] D.W. Hwang, S. Lee, M. Seo, T.D. Chung, Recent advances in electrochemical non-enzymatic glucose sensors - a review, Anal. Chim. Acta 1033 (2018) 1e34. [7] Z. Li, F. Gao, Z. Gu, Vertically aligned Pt nanowire array/Au nanoparticle hybrid structure as highly sensitive amperometric biosensors, Sens. Actuators B Chem. 243 (2017) 1092e1101. [8] M.A. Poletti Papi, F.R. Caetano, M.F. Bergamini, L.H. Marcolino-Junior, Facile synthesis of a silver nanoparticles/polypyrrole nanocomposite for nonenzymatic glucose determination, Mater Sci Eng C Mater Biol Appl 75 (2017) 88e94. [9] A. Esmaeeli, A. Ghaffarinejad, A. Zahedi, O. Vahidi, Copper oxide-polyaniline nanofiber modified fluorine doped tin oxide (FTO) electrode as nonenzymatic glucose sensor, Sens. Actuators B Chem. 266 (2018) 294e301. [10] K.J. Babu, T. Raj Kumar, D.J. Yoo, S.-M. Phang, G. Gnana kumar, Electrodeposited nickel cobalt sulfide flowerlike architectures on disposable cellulose filter paper for enzyme-free glucose sensor applications, ACS Sustain. Chem. Eng. 6 (2018) 16982e16989. [11] X. Niu, X. Li, J. Pan, Y. He, F. Qiu, Y. Yan, Recent advances in non-enzymatic electrochemical glucose sensors based on non-precious transition metal materials: opportunities and challenges, RSC Adv. 6 (2016) 84893e84905. [12] H. Zhu, L. Li, W. Zhou, Z. Shao, X. Chen, Advances in non-enzymatic glucose sensors based on metal oxides, J. Mater. Chem. B 4 (2016) 7333e7349. [13] P.V. Dat, N.X. Viet, Facile synthesis of novel areca flower like Cu2O nanowire on copper foil for a highly sensitive enzyme-free glucose sensor, Mater Sci Eng C Mater Biol Appl 103 (2019) 109758. [14] W.B. Kim, S.H. Lee, M. Cho, Y. Lee, Facile and cost-effective CuS dendrite electrode for non-enzymatic glucose sensor, Sens. Actuators B Chem. 249 (2017) 161e167. [15] X. Zhang, G. Wang, A. Gu, Y. Wei, B. Fang, CuS nanotubes for ultrasensitive nonenzymatic glucose sensors, Chem. Commun. (2008) 5945e5947. [16] W. Ke, G. Fang, H. Lei, P. Qin, H. Tao, W. Zeng, J. Wang, X. Zhao, An efficient and transparent copper sulfide nanosheet film counter electrode for bifacial quantum dot-sensitized solar cells, J. Power Sources 248 (2014) 809e815. [17] L. Qian, J. Mao, X. Tian, H. Yuan, D. Xiao, In situ synthesis of CuS nanotubes on Cu electrode for sensitive nonenzymatic glucose sensor, Sens. Actuators B Chem. 176 (2013) 952e959. [18] S. Radhakrishnan, H.-Y. Kim, B.-S. Kim, A novel CuS microflower superstructure based sensitive and selective nonenzymatic glucose detection, Sens. Actuators B Chem. 233 (2016) 93e99. [19] Y.J. Yang, J. Zi, W. Li, Enzyme-free sensing of hydrogen peroxide and glucose at a CuS nanoflowers modified glassy carbon electrode, Electrochim. Acta 115 (2014) 126e130. [20] N. Karikalan, R. Karthik, S.M. Chen, C. Karuppiah, A. Elangovan, Sonochemical synthesis of sulfur doped reduced graphene oxide supported CuS nanoparticles for the non-enzymatic glucose sensor applications, Sci. Rep. 7 (2017) 2494. [21] X. Yan, Y. Gu, C. Li, B. Zheng, Y. Li, T. Zhang, Z. Zhang, M. Yang, A non-enzymatic glucose sensor based on the CuS nanoflakesereduced graphene oxide nanocomposite, Analytical Methods 10 (2018) 381e388. [22] X. Niu, J. Pan, F. Qiu, X. Li, Y. Yan, L. Shi, H. Zhao, M. Lan, Anneal-shrinked Cu2O dendrites grown on porous Cu foam as a robust interface for highperformance nonenzymatic glucose sensing, Talanta 161 (2016) 615e622. [23] P. Yang, X. Wang, C.-y. Ge, X. Fu, X.Y. Liu, H. Chai, X. Guo, H.-C. Yao, Y.X. Zhang, K. Chen, Fabrication of CuO nanosheets-built microtubes via Kirkendall effect for non-enzymatic glucose sensor, Appl. Surf. Sci. 494 (2019) 484e491. [24] S. Cheng, X. Gao, S. DelaCruz, C. Chen, Z. Tang, T. Shi, C. Carraro, R. Maboudian, In situ formation of metal-organic framework derived CuO polyhedrons on carbon cloth for highly sensitive non-enzymatic glucose sensing, J. Mater. Chem. B 7 (2019) 4990e4996. [25] C. Li, M. Kurniawan, D. Sun, H. Tabata, J.J. Delaunay, Nanoporous CuO layer modified Cu electrode for high performance enzymatic and non-enzymatic glucose sensing, Nanotechnology 26 (2015), 015503. [26] P. Ni, Y. Sun, Y. Shi, H. Dai, J. Hu, Y. Wang, Z. Li, Facile fabrication of CuO nanowire modified Cu electrode for non-enzymatic glucose detection with enhanced sensitivity, RSC Adv. 4 (2014) 28842e28847. [27] R. Li, X. Liu, H. Wang, Y. Wu, K.C. Chan, Z. Lu, Sandwich nanoporous framework decorated with vertical CuO nanowire arrays for electrochemical glucose sensing, Electrochim. Acta 299 (2019) 470e478. [28] C. Yu, J. Cui, Y. Wang, H. Zheng, J. Zhang, X. Shu, J. Liu, Y. Zhang, Y. Wu, Porous HKUST-1 derived CuO/Cu2O shell wrapped Cu(OH)2 derived CuO/Cu2O core
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43] [44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
9
nanowire arrays for electrochemical nonenzymatic glucose sensors with ultrahigh sensitivity, Appl. Surf. Sci. 439 (2018) 11e17. J. Huang, Y. Zhu, X. Yang, W. Chen, Y. Zhou, C. Li, Flexible 3D porous CuO nanowire arrays for enzymeless glucose sensing: in situ engineered versus ex situ piled, Nanoscale 7 (2015) 559e569. X. Liu, W. Yang, L. Chen, J. Jia, Three-dimensional copper foam supported CuO nanowire arrays: an efficient non-enzymatic glucose sensor, Electrochim. Acta 235 (2017) 519e526. Y. Nicolau, J. Menard, Solution growth of ZnS, CdS and Zn1-xCdxS thin films by the successive ionic-layer adsorption and reaction process; growth mechanism, J. Cryst. Growth 92 (1988) 128e142. Y. Nicolau, Solution deposition of thin solid compound films by a successive ionic-layer adsorption and reaction process, Appl. Surf. Sci. 22 (1985) 1061e1074. H. Pathan, C. Lokhande, Deposition of metal chalcogenide thin films by successive ionic layer adsorption and reaction (SILAR) method, Bull. Mater. Sci. 27 (2004) 85e111. Y. Nicolau, M. Dupuy, M. Brunel, ZnS, CdS, and Zn1xCdxS thin films deposited by the successive ionic layer adsorption and reaction process, J. Electrochem. Soc. 137 (1990) 2915e2924. J. Luo, L. Steier, M.K. Son, M. Schreier, M.T. Mayer, M. Gratzel, Cu2O nanowire photocathodes for efficient and durable solar water splitting, Nano Lett. 16 (2016) 1848e1857. X. Chen, R. Chen, X. Zhu, Q. Liao, Y. Zhang, D. Ye, B. Zhang, Y. Yu, J. Li, A solar responsive cubic nanosized CuS/Cu2O/Cu photocathode with enhanced photoelectrochemical activity, J. Catal. 372 (2019) 182e192. X. Zhang, W. Guo, G. Gao, M. Que, C. Pan, Z.L. Wang, CuS nanotrough-networks for highly stable transparent conducting electrodes, J. Mater. Chem. C 4 (2016) 4733e4739. A. Venkadesh, S. Radhakrishnan, J. Mathiyarasu, Eco-friendly synthesis and morphology-dependent superior electrocatalytic properties of CuS nanostructures, Electrochim. Acta 246 (2017) 544e552. S.Y. Tee, E. Ye, P.H. Pan, C.J. Lee, H.K. Hui, S.Y. Zhang, L.D. Koh, Z. Dong, M.Y. Han, Fabrication of bimetallic Cu/Au nanotubes and their sensitive, selective, reproducible and reusable electrochemical sensing of glucose, Nanoscale 7 (2015) 11190e11198. R. Ponnusamy, A. Gangan, B. Chakraborty, C. Sekhar Rout, Tuning the pure monoclinic phase of WO3 and WO3-Ag nanostructures for non-enzymatic glucose sensing application with theoretical insight from electronic structure simulations, J. Appl. Phys. 123 (2018). R. Ponnusamy, A. Gangan, B. Chakraborty, D.J. Late, C.S. Rout, Improved nonenzymatic glucose sensing properties of Pd/MnO2 nanosheets: synthesis by facile microwave-assisted route and theoretical insight from quantum simulations, J. Phys. Chem. B 122 (2018) 7636e7646. M.-m. Guo, Y. Xia, W. Huang, Z. Li, Electrochemical fabrication of stalactite-like copper micropillar arrays via surface rebuilding for ultrasensitive nonenzymatic sensing of glucose, Electrochim. Acta 151 (2015) 340e346. X. Liu, W. Yang, L. Chen, J. Jia, Synthesis of copper nanorods for non-enzymatic amperometric sensing of glucose, Microchimica Acta 183 (2016) 2369e2375. Y.Z. Wang, H. Zhong, X.M. Li, F.F. Jia, Y.X. Shi, W.G. Zhang, Z.P. Cheng, L.L. Zhang, J.K. Wang, Perovskite LaTiO3-Ag0.2 nanomaterials for nonenzymatic glucose sensor with high performance, Biosens. Bioelectron. 48 (2013) 56e60. B. Zheng, G. Liu, A. Yao, Y. Xiao, J. Du, Y. Guo, D. Xiao, Q. Hu, M.M.F. Choi, A sensitive AgNPs/CuO nanofibers non-enzymatic glucose sensor based on electrospinning technology, Sens. Actuators B Chem. 195 (2014) 431e438. M. Velmurugan, N. Karikalan, S.-M. Chen, Synthesis and characterizations of biscuit-like copper oxide for the non-enzymatic glucose sensor applications, J. Colloid Interface Sci. 493 (2017) 349e355. G. Wang, H. Sun, L. Ding, G. Zhou, Z.S. Wang, Growth of Cu particles on a Cu2O truncated octahedron: tuning of the Cu content for efficient glucose sensing, Phys. Chem. Chem. Phys. 17 (2015) 24361e24369. Y. Su, H. Guo, Z. Wang, Y. Long, W. Li, Y. Tu, Au@Cu2O core-shell structure for high sensitive non-enzymatic glucose sensor, Sens. Actuators B Chem. 255 (2018) 2510e2519. C. Wei, Y. Liu, Q. Liu, W. Xiang, Uniform and dense copper nanoparticles directly modified indium tin oxide electrode for non-enzymatic glucose sensing, J. Electroanal. Chem. 835 (2019) 273e280. Y. Zhao, L. Fan, Y. Zhang, H. Zhao, X. Li, Y. Li, L. Wen, Z. Yan, Z. Huo, Hyperbranched Cu@Cu2O coaxial nanowires mesh electrode for ultra-sensitive glucose detection, ACS Appl. Mater. Interfaces 7 (2015) 16802e16812. K. Kim, S. Kim, H.N. Lee, Y.M. Park, Y.-S. Bae, H.-J. Kim, Electrochemically derived CuO nanorod from copper-based metal-organic framework for nonenzymatic detection of glucose, Appl. Surf. Sci. 479 (2019) 720e726. S. Zhou, X. Feng, H. Shi, J. Chen, F. Zhang, W. Song, Direct growth of vertically aligned arrays of Cu(OH)2 nanotubes for the electrochemical sensing of glucose, Sens. Actuators B Chem. 177 (2013) 445e452. J. Lv, C. Kong, Y. Xu, Z. Yang, X. Zhang, S. Yang, G. Meng, J. Bi, J. Li, S. Yang, Facile synthesis of novel CuO/Cu2O nanosheets on copper foil for high sensitive nonenzymatic glucose biosensor, Sens. Actuators B Chem. 248 (2017) 630e638. K.-J. Huang, L. Wang, J. Li, T. Gan, Y.-M. Liu, Glassy carbon electrode modified with glucose oxidaseegrapheneenano-copper composite film for glucose sensing, Measurement 46 (2013) 378e383. M.E. Ghica, R.C. Carvalho, A. Amine, C.M.A. Brett, Glucose oxidase enzyme
10
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
inhibition sensors for heavy metals at carbon film electrodes modified with cobalt or copper hexacyanoferrate, Sens. Actuators B Chem. 178 (2013) 270e278. [56] C.X. Guo, C.M. Li, Direct electron transfer of glucose oxidase and biosensing of glucose on hollow sphere-nanostructured conducting polymer/metal oxide composite, Phys. Chem. Chem. Phys. 12 (2010) 12153e12159. [57] J. Mohapatra, B. Ananthoju, V. Nair, A. Mitra, D. Bahadur, N.V. Medhekar,
M. Aslam, Enzymatic and non-enzymatic electrochemical glucose sensor based on carbon nano-onions, Appl. Surf. Sci. 442 (2018) 332e341. [58] Y. Ding, Y. Liu, L. Zhang, Y. Wang, M. Bellagamba, J. Parisi, C.M. Li, Y. Lei, Sensitive and selective nonenzymatic glucose detection using functional NiOePt hybrid nanofibers, Electrochim. Acta 58 (2011) 209e214. [59] H. Li, C.Y. Guo, C.L. Xu, A highly sensitive non-enzymatic glucose sensor based on bimetallic Cu-Ag superstructures, Biosens. Bioelectron. 63 (2015) 339e346.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A highly sensitive non-enzymatic glucose sensor based on CuS nanosheets modified Cu2O/CuO nanowire arrays Chenhuinan Wei a, Xiao Zou b, Qiming Liu a, *, Shuxian Li a, Chenxia Kang a, Wei Xiang c a
School of Physics and Technology, Key Laboratory of Ariticial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan, 430072, China b Hospital of Wuhan University, Wuhan, 430022, China c Wuhan Union Hospital, Tongji Medical College, Wuhan, 430022, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2019 Received in revised form 30 December 2019 Accepted 3 January 2020 Available online xxx
CuS nanosheets modified Cu2O/CuO nanowire arrays (NWAs) were successfully fabricated on Cu foil by in-situ growth and successive ionic layer adsorption and reaction (SILAR) methods without binders. The three-dimensional Cu2O/CuO NWAs not only showed strong binding force with current collector but also provided abundant surface area to support CuS nanosheets. Different morphologies of CuS were obtained by varying the Cu(NO3)2 and Na2S concentrations and the SILAR cycle. When these step-wise prepared products were employed for non-enzymatic glucose sensing, the amperometric response of the optimized CuS/Cu2O/CuO/Cu electrode was approximately twice of that of Cu2O/CuO/Cu electrode. The result is attributed to the modification of CuS nanosheets with suitable amount which increases the active surface area of the electrode. This designed electrode presents an ultrahigh sensitivity of 4262 mA mM1 cm2 in the range from 0.002 to 4.096 mM, as well as the excellent selectivity, reproducibility and stability. Moreover, due to the facile preparation and the low cost of Cu based sensors, the CuS/Cu2O/CuO/Cu electrode will be one kind of promising materials for constructing practical nonenzymatic glucose sensors. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Cu2O/CuO nanowire arrays CuS nanosheets SILAR method Non-enzymatic glucose sensor High sensitivity
1. Introduction Accurate and convenient detection of glucose has attracted global attention because of the ever-growing demands for the living standards of human. The development of low-cost and efficient glucose biosensor is important in many occasions, such as the food industry, diabetes clinical diagnostics and biological research [1,2]. Owing to the good selectivity of enzymatic, glucose oxidase or dehydrogenase based glucose sensors have been applied to construct various amperometric biosensors [3]. However, enzymes are expensive. The catalytic activity of enzyme is highly sensitive to the environmental conditions such as temperature, pH, humidity and operational conditions. Immobilization procedure of enzyme on the conductive electrode is complicated and the amount of enzyme cannot be controlled precisely [4]. Consequently, a great many of efforts have been developed on non-enzymatic glucose sensors. The majority of non-enzymatic electrochemical glucose
* Corresponding author. E-mail address: [email protected] (Q. Liu). https://doi.org/10.1016/j.electacta.2020.135630 0013-4686/© 2020 Elsevier Ltd. All rights reserved.
sensors mainly rely on the electro-catalytic oxidation of glucose directly at the electrode surface without the involvement of enzyme. The absence of enzyme decreases the cost, increases the stability and facilitates the preparation of sensors [5,6]. In recent years, noble metals, such as gold, silver, platinum and their alloys, have been intensively applied in non-enzymatic glucose sensors owing to their special electrocatalytic activity [7,8]. Nevertheless, these metals also have defects like high cost, low selectivity and susceptible to poisoning [9,10]. By comparison, metal oxide/sulfide are low cost and non-toxic. Besides, they have high specific surface area and great electrochemical activity, which enable them to attract increasing attention towards highperformance non-enzymatic glucose sensors [11,12]. Among those metal oxides/sulfides, copper oxides/sulfides have been deeply investigated because of its abundant nature and low cost [13,14]. CuS is a stable semiconductor, and has high catalytic activity and metal like electrical conductivity [15e17]. Microstructured and nanostructured CuS with different morphologies, such as microflowers [18], nanoflowers [19], nanoparticles [20], and nanoflakes [21], have been dropped on glass carbon electrodes and fixed with binders as glucose sensors, and shown good biological
2
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
compatibility and superior catalytic property for fast, sensitive detection of glucose. Copper oxide (Cu2O or CuO) with high isoelectric point and low overpotential for electron-transfer reactions also has been widely applied in glucose sensing [22e25]. Recently, in-situ growth of three-dimensional Cu2O or CuO nanowire arrays on Cu substrates have been largely studied and reported [26e30]. On the one hand, the in-situ growth ensures the great connection between copper oxides and copper substrate, which decreases the inner resistance and promotes the electron transfer between active materials and highly conductive substrate [28]. On the other hands, nanowire arrays possess large active surface area. For instance, Huang and coworkers have confirmed that the CuO nanowire arrays exhibited over 5-fold higher sensitivity compared to CuO nanowire piles [29]. Since the nanowire arrays are binder-free and have abundant surface area, we expect that CuS nanostructure could act as an enhanced sensing material on the Cu2O or CuO NWAs if an optimum amount is deposited. In this work, for the first time we report the CuS nanosheets modified Cu2O/CuO nanowire arrays for electrochemical sensing of glucose. The fabrication process is simple and do not need expensive and complicated equipment. The Cu2O/CuO NWAs were synthesized on Cu foil by fast electrochemical oxidation and facile heat treatment. Next, CuS nanosheets were deposited on Cu2O/CuO NWAs by SILAR method [31e34] to form CuS/Cu2O/CuO/Cu based glucose sensor. The whole process avoided the use of additional binders. After optimizing the concentration of cationic and anionic sources as well as the number of SILAR cycle, amperometric response of CuS/Cu2O/CuO/Cu electrode towards glucose was approximately twice of that of Cu2O/CuO/Cu electrode, confirming the greatly electro-catalytic enhancement of CuS nanosheets. Furthermore, the designed non-enzymatic glucose sensor displayed an ultrahigh sensitivity. 2. Experimental 2.1. Materials High purity Cu foil (>99.9%) were purchased from aladdin Ltd. (China). Chemicals were of analytical grade, obtained from Sinopharm Chemical Reagent and used without further purification. Ultrapure water (18.25 MU cm) was generated from Water Purifier (Pincheng Technology Co., Ltd.) and used as solvent in this experiment. 2.2. Synthesis of Cu2O/CuO NWAs on Cu foil (Cu2O/CuO/Cu) Cu foil (1.0 cm 0.5 cm) was ultrasonically cleaned by acetone, 3 M HCl, distilled water, and ethanol for 10 min then dried under N2 gas. First, to in-situ synthesis Cu(OH)2 NWAs on Cu foil, a cleaned Cu foil, a platinum plate and a Hg|HgO were used as the working, counter and reference electrodes, respectively. The anodization process of Cu foil was conducted at a constant current density (6 mA cm2) in 25 ml NaOH (3 M) at room temperature. After 10 min，a blue film of Cu(OH)2 NWAs was formed on the surface of Cu foil. The product was taken out of NaOH, followed by rinsing with ethanol and drying in air for about 2 h. Second, Cu(OH)2 NWAs precursors were changed to copper oxides NWAs by annealing in air at 400 C for 1 h, the heating rate was 2 C/min, then cooled gradually to room temperature. 2.3. Synthesis of CuS nanosheets on the Cu2O/CuO NWAs (CuS/ Cu2O/CuO/Cu) Successive ionic layer adsorption and reaction (SILAR) method
was used to deposit CuS on Cu2O/CuO NWAs. Cationic and anionic sources were Cu(NO3)2$3H2O aqueous solution and Na2S$9H2O aqueous solution, respectively. Cu2O/CuO/Cu was immersed in a 200 ml breaker which was filled with 60 ml of Cu(NO3)2 (0.005, 0.01, 0.05, 0.1 M) and kept for 60 s, then rinsed with ethanol for 30 s. Subsequently, the Cu2O/CuO/Cu was immersed in 60 ml of Na2S (0.005, 0.01, 0.05, 0.1 M) for 60 s, followed by rinsing in ethanol for 30 s. The above description is a complete cycle. Notably when CuS were synthesized using 0.01 M Cu(NO3)2 and Na2S, the number of SILAR cycle was altered from one to four cycles. The corresponding four kinds of electrodes were denoted as CuS/Cu2O/CuO/Cu-1, CuS/ Cu2O/CuO/Cu-2, CuS/Cu2O/CuO/Cu-3 and CuS/Cu2O/CuO/Cu-4, respectively. 2.4. Characterization FE-SEM images were obtained using an ZEISS-SIGMA operating at 15 kV. A Brucker D8 advance X-ray diffractometer by Cu Ka radiation (l ¼ 1.5406 Å) was used to obtain XRD results. All samples were characterized at a scanning speed of 3 /min in the range (2q) of 10 e80 . Raman patterns were collected by a Raman microscope system from Renishaw Co., Ltd. Ar laser (l ¼ 514.5 nm) was used as the excitation source with a power kept at 4 mW. TEM and HRTEM images were obtained in JEM-2010 FEF transmission electron microscope. XPS results were acquired from a ESCALAB 250Xi X-ray photoelectron spectroscope (Thermo Fisher Scientific, USA) by Al Ka radiation with 1486.6 eV of excitation source. 2.5. Electrochemical measurements All electrochemical measurements were performed by a CHI 660D electrochemical analyzer (Chenhua instrumental Co., Ltd., Shanghai, China) with a three electrode system at ambient temperature. The step-wise prepared products, Cu foil, Cu2O/Cu/Cu and CuS/Cu2O/CuO/Cu, were employed as glucose sensing electrodes directly. The geometry area of the electrode immersed in the NaOH solution was 0.25 cm2. A platinum plate and a saturated Ag|AgCl were used as the counter and the reference electrodes, respectively. Cyclic voltammetry (CV) was investigated at a range of 0.2 Ve0.8 V. For amperometric detection, the solution was stirred at 200 rpm to provide convective transport. A 20 ml NaOH solution was used as electrolyte for glucose detection. Electrochemical impedance spectroscopy (EIS) technique was recorded in 0.1 M NaOH containing 1 mM glucose at open circuit voltage in the frequency range of 0.01 Hze100 kHz with an AC probe amplitude of 5 mV. 3. Results and discussion The three-step procedure for the fabrication of CuS modified Cu2O/CuO NWAs is schematically displayed in Fig. 1a. The Cu foil was first anodized and crystallized into Cu(OH)2 NWAs in the strongly alkaline solution. One dimensional Cu(OH)2 nanowire was formed owing to the shortest inter-planar spacing of (100) in Cu(OH)2 and the relatively stronger hydrogen bonding interaction [29]. Next, heating of Cu(OH)2 NWAs based Cu foil attracted dehydration and transformation into Cu2O/CuO NWAs. The last step is the SILAR process. The Cu2O/CuO/Cu was firstly immersed in the Cu(NO3)2 solution, resulting in the adsorption of Cu2þ on the surface of Cu2O/CuO NWAs. Then, ethanol solution was used to remove redundant adsorbed ions of Cu(NO3)2. When Cu2þ absorbed Cu2O/ CuO/Cu was immersed in Na2S solution, Cu2þ could strongly attract S2. When Cu2þ reacted with S2, CuS was produced. Ethanol solution was used again to remove the unreacted ions from the diffusion layer. The whole process is described by following
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
3
Fig. 1. (a) Schematic illustration of the fabrication process of CuS/Cu2O/CuO/Cu. SEM and TEM images of (b1-b4) Cu(OH)2 NWs, (c1-c4) Cu2O/CuO NWAs and (d1-d4) CuS/Cu2O/CuO/ Cu-2.
equations [35,36].
anodization Cu þ 2OH !Cu OH þ 2e 2
D
Cu þ CuðOHÞ2 !Cu2 O þ H2 O D
(1)
(2)
CuðOHÞ2 !CuO þ H2 O
(3)
Cu2þ þ S2 /CuS
(4)
The morphology of the synthesized products was characterized by FESEM and TEM. From the overview image in Fig. b1, the dense and straight Cu(OH)2 nanowires are regularly ranged on Cu foil. At high magnification (Fig. 1b), the Cu(OH)2 nanowire is a circular truncated cone and the diameter of the top of the nanowire is about 300 nm. It illustrates the “root” of nanowires have the large surface area to connect the Cu foil. The surface of the nanowire is smooth. After annealing (Fig. 1c), the nanowires become a little bend and
the porosity in Cu2O/CuO NWAs are shown up. This morphology is caused by the evaporation of H2O vapor during the dehydration. Compared with smooth Cu(OH)2 nanowires, the porous nanostructures provide increased surface area which may suit to mediate the nucleation and growth of foreign materials. Besides, the Cu2O/CuO product remains macroscopically uniform nanowire structure (Fig. 1c1), which supplied good backbone for supporting CuS. The open space between each nanowire also supplied enough volume for growing CuS and space for the mass transport. Fig. 1d is the SEM and TEM images of CuS/Cu2O/CuO/Cu-2. After two cycles of SILAR, CuS nanosheets are deposited on Cu2O/CuO NWAs without aggregation. The CuS modified NWAs still remains nanowire structure. TEM image in Fig. 1d4 further displays that there are amounts of thin nanosheets on the nanowires. The integration of CuS nanosheets on Cu2O/CuO NWAs presents the branched architecture with high surface area. It is clearly that there is no binder that covers the active materials on Cu foil. The in-situ growth and SILAR method ensure the direct synthesis of materials on Cu substrate. Without the use of binder, the modified electrode can realize the direct electron transfer between the active materials and the
4
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
conducting substrate, which is important to improve the performance of the sensing. Fig. 2a is the XRD patterns of the step-wise prepared products. The Cu foil shows two diffraction peaks at 43.3 and 50.4 , which agree with the (111) and (200) planes of cubic Cu (JCPDS no. 04e0836). For Cu(OH)2 NWAs (black line), all of the diffraction peaks are well fitted to the XRD pattern of the orthorhombic structured Cu(OH)2 (JCPDS no. 72e0140). In red line, diffraction peaks at 29.6 , 36.4 , 42.3 and 61.4 are in good accordance with the planes of Cu2O (JCPDS no. 78e2076), diffraction peaks at 35.5 , 38.7 and 48.7 can be assigned to the planes of CuO (JCPDS no. 45e0937). It illustrates that the nanowires after annealing are consist of Cu2O and CuO. In particular, no characteristic diffraction peak of CuS was obtained when the SILAR cycle was up to four. The reason was that CuS was too little to be detected. In this case, we synthesized CuS modified Cu2O/CuO NWAs by further increasing the SILAR cycle. A small diffraction peak at 47.9 can be identified (blue line) until eight SILAR cycles. The peak is well fitted to the (110) planes of CuS (JCPDS no. 06e0464). Furthermore, the crystal structure of CuS/Cu2O/CuO/Cu-2 was characterized by TEM and HRTEM as shown in Fig. S1. The lattice fringe distances of 0.273 nm and 0.304 nm match well with the lattice spacing (006) and (102) planes of CuS (JCPDS no. 06e0464), respectively. Raman spectra are shown in Fig. 2b. The peaks at 286, 333 and 625 cm1 can be identified to Cu2O and CuO heterostructure. After 2 cycles of SILAR, a new strong peak at 470 cm1 is emerged, which is the result from the SeS bond stretching in the lattice vibrations of CuS [37]. Furthermore, XPS analysis was used to further characterize the products. Compared to the Cu2O/CuO NWAs, the product synthesized after SILAR process has two additional peaks that correspond to S 2s and S 2p (Fig. 2c). In Cu 2p spectrum (Fig. 2d), two peaks located at 932.5 and 952.4 eV are assigned to Cu 2p3/2 and Cu 2p1/2
of Cu2O. Two peaks at 934.2 and 954.3 eV and the remaining two shake-up satellite peaks at 943.9 and 962.8 eV are a well evidence of the presence of Cu(II). For S 2p spectrum (Fig. 2e), the S 2p3/2 and S 2p1/2 peaks are observed at 161.8 and 163.0 eV, further confirming the successful deposition of CuS on Cu2O/CuO NWAs surface [38]. All the results in Fig. 2 illustrate the final product is the hybrid of CuS, Cu2O, CuO and Cu. To investigate the influence of the concentrations of cationic and anionic sources on the morphology of CuS crystal, the concentrations of Cu(NO3)2 and Na2S were ranged from 0.005 mM to 0.1 mM. The corresponding FESEM images are shown in Fig. 3. Initially, when 0.005 mM Cu(NO3)2 and Na2S were used, the number of nucleation sites formed on the Cu2O/CuO nanowires were limited due to the small amount of reduced CuS monomer. 2D CuS nanosheets were grown after thermodynamic growth through the diffusion of Cu2þ in the vicinity of the nucleation sites, see Fig. 3a. When the concentration Cu(NO3)2 and Na2S was increased to 0.01 mM, more nucleation sites are available and consequently higher density of CuS nanosheet were grown on the nanowires (Fig. 3b). In contrast, further increasing the concentration of Cu(NO3)2 and Na2S to 0.05 mM produced a great number of nucleation sites and leaded to the fast isotropic growth of CuS nanoparticles. These nanoparticles aggregated and merged, eventually covering the Cu2O/CuO nanowires completely, as shown in Fig. 3c. When Cu(NO3)2 and Na2S were 0.1 mM, the surplus reduced CuS monomer rendered the nucleation occurred both in solution and on the nanowires, forming clusters that partly broke the structure of nanowire arrays (see Fig. 3d). Collapse of nanowires appeared when the concentrations of Cu(NO3)2 and Na2S were larger than 0.05 mM. The reason may be that too much CuS agglomerates loaded on the bend Cu2O/CuO nanowires. The effect of the SILAR cycle were also investigated. Fig. 4aed are the FESEM
Fig. 2. (a) XRD patterns of Cu(OH)2/Cu (black line), Cu2O/CuO/Cu (red line) and CuS/Cu2O/CuO/Cu (blue line). (b) Raman spectra of Cu2O/CuO/Cu and CuS/Cu2O/CuO/Cu-2. (c) Typical XPS survey spectra of the Cu2O/CuO/Cu and CuS/Cu2O/CuO/Cu-2. XPS spectra of the (d) Cu 2p and (e) S 2p of CuS/Cu2O/CuO/Cu-2. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
5
Fig. 3. SEM images of CuS/Cu2O/CuO/Cu synthesized in (a) 0.005 M, (b) 0.01 M, (c) 0.05 M and (d) 0.1 M Cu(NO3)2 and Na2S with 2 SILAR cycles.
Fig. 4. SEM images of CuS/Cu2O/CuO/Cu synthesized in 0.01 M Cu(NO3)2 and Na2S with (a) 1, (b) 2, (c) 3 and (d) 4 SILAR cycles.
images of CuS/Cu2O/CuO/Cu with the increased SILAR cycle. Repeating the SILAR cycle, new nucleation sites were formed continuously thus the amount of CuS nanosheets were increased. After three SILAR cycles (Fig. 4c), CuS nanosheets based thick shell almost covers the entire Cu2O/CuO nanowires. Intuitively, increasing the SILAR cycles will absorb more Cu2þ and S2, thereby increasing the amount of CuS nanosheets. The electro-catalytic activities of the step-wise prepared electrodes in our study were tested at a scan rate of 50 mV s1 in 0.1 M NaOH (Fig. 5a). For CuS/Cu2O/CuO/Cu-2, the CV curve of electrode in the absence of glucose displays a broad redox behaviour through Cu2þ/Cu3þ redox couple (curve 5). When 2 mM glucose is added, there is an increase of the anodic peak current (curve 6). This indicates the proposed involvement of Cu2þ and Cu3þ surface species in the oxidation of glucose [39]. By comparing the CV curves of step-wise prepared electrodes in the presence of 2 mM glucose (curve 2, 4 and 6), the CuS/Cu2O/CuO/Cu-2 electrode exhibits the best CV response, illustrating that CuS/Cu2O/CuO/Cu-2 electrode had an enhanced electro-catalytic performance when compared with the Cu2O/CuO/Cu electrode and the pure Cu foil. It may because of the improved electron transfer from the synergistic effect of ternary compositions [40,41]. Fig. 5b shows the current responses of step-wise prepared electrodes toward increased
concentration of glucose and Fig. 5c is the corresponding relationship between glucose concentration and response current. The CuS/Cu2O/CuO/Cu-2 electrode displays the maximum oxidation current and the highest sensitivity. It is noted that the current response of CuS/Cu2O/CuO/Cu-2 electrode is approximately twice of that of Cu2O/CuO/Cu electrode, which further demonstrates that CuS nanosheets are effective for enhancing the electrooxidation of Cu2O/CuO/Cu electrode towards glucose. Besides, with the increase of glucose concentration, the response currents increase linearly with satisfactory linearity (Fig. 5c). CV curves of CuS/Cu2O/CuO/Cu2 electrode with increased scan rates in the presence of 2 mM glucose are exhibited in Fig. 5d. The anodic peak currents around 0.6 V increase with the increase of the scan rate. The inset in Fig. 5d exhibits a great linear dependence between the square root of scan rate (v1/2) and oxidation peak current with a correlation coefficient of 0.998, revealing that the electro-catalysis oxidation of glucose on CuS/Cu2O/CuO/Cu-2 electrode is controlled by diffusion [42]. According to the previous researches, the possible mechanism of the glucose oxidation process on Cu based sensors is presented in Fig. S2. The detail reactions are proposed as follows [18,22,27,28]. Cu2O þ 2OH þ H2O / Cu(OH)2 þ 2e
(5)
6
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
Fig. 5. (a) CVs of pure Cu (black curve), Cu2O/CuO/Cu and CuS/Cu2O/CuO/Cu-2 electrodes in 0.1 M NaOH in the absence (dotted curve 1, 3 and 5) and in the presence (solid curve 2, 4 and 6) of 2 mM glucose. (b) Amperometric response of the step-wise prepared electrodes in 0.1 M NaOH with successive addition of 6 mL, 8 mL and 10 mL of 0.5 M glucose in 0.1 M NaOH. (c) The relationship between glucose concentration and current response of step-wise prepared electrodes. (d) CVs of CuS/Cu2O/CuO/Cu-2 electrode in 0.1 M NaOH in the present of 2 mM glucose with scan rates of 30, 50, 70, 90 and 110 mV s1 and the corresponding linear curve of the peak current vs. scan rate. Amperometric responses of CuS/Cu2O/ CuO/Cu electrodes synthesized in (e) different concentration of Cu(NO3)2 and Na2S and (f) different SILAR cycles with successive addition of increased concentration of glucose in 0.1 M NaOH. (h) Current responses of electrodes synthesized with different Cu(NO3)2 and Na2S concentrations and SILAR cycle to 0.6 mM glucose in 0.1 M NaOH (n ¼ 3). (i) Amperometric response of the CuS/Cu2O/CuO/Cu-2 electrode in 0.1 M NaOH at various detection potentials with successive addition of increased concentration of glucose in 0.1 M NaOH.
Cu(OH)2 þ OH / CuOOH þ H2O þ e
(6)
CuO þ OH / CuOOH þ e
(7)
Cu3þ þ glucose / Cu2þ þ gluconic acid
(8)
In order to optimize the electrochemical sensing of sensor, the concentration of Cu(NO3)2 and Na2S and the SILAR cycle were investigated by assessing the amperometric response. To ensure the stability and the nanowire structure of the electrode, we only compared the current responses of CuS modified electrodes synthesized in 0.005 M and 0.01 M reaction solutions. Obviously, the electrode synthesized in 0.01 M reaction solution has the higher current response (Fig. 5e). It illustrates that increasing CuS nanosheets increased the surface active sites of sensor (Fig. 3a and b). Next, Fig. 5f shows the current responses of electrodes synthesized with different SILAR cycle. The CuS/Cu2O/CuO/Cu-2 electrode presents the highest current response among these four sensors. The variation of the morphology of the CuS nanosheets has been shown in Fig. 4aed. The amount of CuS nanosheets is enhanced by increasing the SILAR cycle, thus the current response increases from CuS/Cu2O/CuO/Cu-1 electrode to CuS/Cu2O/CuO/Cu-2 electrode. However, the current response decreases when the CuS nanosheets were synthesized through three SILAR cycles. It is due
to the thicker covered layer which decreased the interaction between the electrolyte and inner active materials, and slowed down the glucose diffusion to the inner active materials, so the electrocatalytic contribution from Cu2O/CuO was restricted. EIS measurements were carried out to investigate the charge transfer activity at the electrode/electrolyte interface. All Nyquist plots consisted of a semicircle in the high frequency region and a straight line in the low frequency region. The diameter of the semicircle associates with the charge transfer resistance (Rct), indicative of the conducting capability. From the result in Fig. 5g, CuS/Cu2O/CuO/Cu2 electrode owns the smallest semicircle diameter, indicating the better electron transfer behaviour of the CuS/Cu2O/CuO/Cu-2 electrode, which is beneficial for the glucose electro-oxidation. The optimized results are collected in Fig. 5h, the product synthesized in 0.01 M Cu(NO3)2 and Na2S with two SILAR cycles displays the highest current response. The influence of detection potential on the current response of CuS/Cu2O/CuO/Cu-2 electrode was also investigated. As shown in Fig. 5i, applied potential of 0.60 V causes a notable increase of amperometric response upon each addition when compared with 0.50 V and 0.55 V. Further increasing the applied potential to 0.65 V and 0.70 V, the higher background currents appears [43]. In this case, 0.60 V was chosen as the suitable detection potential for glucose sensing in the subsequent study. The enzymatic glucose sensors based on the bio-redox
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
interaction between the enzyme and glucose can be used in the physiological condition to realize the detection of glucose. Nevertheless, alkaline electrolyte is necessary for the electro-oxidation of carbohydrates on Cu based sensing electrodes attributing to their electro-catalytic effect on Cu2þ/Cu3þ couples. Thus, the concentration of NaOH was varied from 0.05 M to 0.5 M. Fig. S3a is the amperometric response of CuS/Cu2O/CuO/Cu electrodes in different NaOH concentrations. It can be seen that all of the electrodes have responses in different NaOH concentrations. The background current increases with the increase of NaOH concentration, which is adverse to the detection limit of sensor. The current density increments for 0.6 mM glucose in different NaOH concentration were also calculated (Fig. S3b). When NaOH concentration increases from 0.05 M to 0.1 M, the current density increment of CuS/Cu2O/ CuO/Cu electrode increases because glucose is easily oxidized at high pH. However, the current density increment decreases since the NaOH concentration is up to 0.15 M. The reason may be that too much OH blocks the electro-adsorption of glucose anion on the electrode [44,45]. The result in Fig. S3b illustrates that CuS/Cu2O/ CuO/Cu electrode tested in 0.1 M NaOH is most sensitive for glucose sensing. Thus, 0.1 M NaOH is the optimal alkaline electrolyte. Fig. 6a displays the amperometric response of the CuS/Cu2O/ CuO/Cu-2 electrode at 0.60 V with successive injection of glucose into 0.1 M NaOH solution. Inset is the enlarged response from 350 to 800 s. The amperometric response increases with the glucose level and the corresponding calibration curve based on current density versus glucose concentration is displayed in Fig. 6b. The CuS/Cu2O/CuO/Cu-2 electrode provided a high sensitivity of 4262 mA mM1 cm2 in the range of 0.002e4.096 mM (R2 ¼ 0.996) and a low detection limit (LOD) of 0.89 mM. LOD was calculated by 3 s/s, where s is the standard deviation of the background current and s represents the slope of the calibration curve. It is worth nothing that the designed electrode possesses prominent
7
sensitivity when compared with those of previously reported sensing electrodes (Table 1). Four factors are considered to have contributed to this high sensitivity: (1) In-situ growth of Cu2O/CuO NWAs on Cu foil ensures the strong binding force between current collector (Cu foil) and active material (Cu2O/CuO NWAs). Directly deposition of CuS without binders shortens the path length for electrons transportation between the electrolyte and electrode. (2) The nanowire arrays can provide more internal spaces for fast electrolyte penetration. Besides, Cu2O/CuO NWAs employed as an effective medium for electron transfer as well as a large-area support for CuS nanosheets. (3) These ultra-thin CuS nanosheets on the Cu2O/CuO NWAs further increase the accessible reaction surface area and the electro-catalytic active sites for glucose sensing. (4) The modification of CuS contributes to the enhancement of the mixed-valence Cu2þ/Cuþ and thus facilitates the redox reactions between glucose and CuS/Cu2O/CuO/Cu electrode. The sensitivity of our product to glucose also greatly outperforms the sensors with enzyme [54e56]. It has been reported that thick enzyme layer is prone to cause the surface blocking effect [57]. Enzyme immobilization creates an extra barrier for the electron transfer process between the electrode and the electrolyte interface. By contrast, non-enzymatic glucose sensors could avoid this disadvantage of enzyme. The absence of enzyme ensures the direct contact of glucose and electrode surface and the fast electron transfer between electrolyte and conductive electrode, which is beneficial to promote the sensitivity of glucose detection. The selectivity is an important factor for non-enzymatic glucose sensors. It has been reported that Cl can poison some metal based sensors. Ascorbic acid (AA), uric acid (UA), acetaminophen (AP) and reducing sugars may cause unwanted interference. In human blood, the normal concentration of glucose is at least 30 times higher than that of some interfering species [29,39]. To ensuring the feasibility of our electrode, the concentration of the added glucose was
Fig. 6. (a) Amperometric response of the CuS/Cu2O/CuO/Cu-2 electrode with successive addition of glucose into 0.1 M NaOH at 0.60 V. (b) The corresponding calibration curve of the CuS/Cu2O/CuO/Cu-2 electrode. (c) Amperometric response of the CuS/Cu2O/CuO/Cu-2 electrode to successive addition of 1 mM glucose and 0.1 mM interferences. (d) Long stability of the CuS/Cu2O/CuO/Cu-2 electrode measured for three weeks.
8
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
Table 1 Comparison of sensing performance of our proposed CuS/Cu2O/CuO/Cu electrode with other non-enzymatic glucose sensors. Electrode
Sensitivity (mA mM1 cm2)
Linear range (mM)
Ref.
CuS nanosheets/Cu2O/CuO NWAs/Cu foil Cu2O biscuit/SPCE CHT/Cu@Cu2O NPs/FTO Nafion/Au@Cu2O NPs/GCE Cu NPs/ITO Nafion/Cu2O nanosheets/Cu NWs/GCE Nafion/HKUST-1 derived CuO nanorod/GCE Cu(OH)2 nanotube arrays/Cu foil Cu2O/CuO nanoporous layer/Cu foil Cu2O/CuO nanosheets/Cu foil CuO NWAs/Cu foil/PET CuO NWAs/Cu foil Nafion/CuS microflower/GCE Nafion/CuS NPs/GCE
4262 309 618 715 1005 1420 1524 418 1066 1541 1085 1886 1007 1085
0.002e4.1 0.0005e4.03 0.01e2.0 0.05e2.0 0.0033e3.9 0.0007e2.0 Up to 1.25 Up to 3 0.1e2.04 Up to 4.0 Up to 2.55 0.002e3.56 0.01e5.4 0.02e2.5
This work [46] [47] [48] [49] [50] [51] [52] [25] [53] [29] [26] [18] [38]
Note: NW—Nanowire, NWAs—Nanowire arrays, NPs—Nanoparticles, CHT—Chitosan, GCE—Glass Carbon electrode, SPCE—Screen-Printed Carbon Electrode, ITO—Indium Tin Oxide, PET—Polyethylene terephthalate, HKUST-1—Cu3(BTC)2 (BTC: benzene tricarboxylate).
decreased to 10 times of the interfering species. The selectivity result of CuS/Cu2O/CuO/Cu-2 electrode is displayed in Fig. 6c. When 1 mM glucose was added into NaOH solution, the steady current gains a significant increase immediately. And there are negligible responses for 0.1 mM AP, 0.1 mM UA, 0.1 mM AA, 0.1 mM KCl, 0.1 mM sucrose and 0.1 mM lactose. The result ascertains that the prepared sensing electrode has great selectivity toward glucose. Thus, it can be speculated that the influence of the interfering species in human serum is slight for our sensor in glucose detection. A possible reason for the good selectivity of our prepared electrode against the major interfering specie of AA can be ascribed to the electrostatic repelling effect [58,59]. The isoelectric point (IEP) of the copper oxides is ~9.5, which means that the surface of our sensor would be negatively charged in 0.1 M NaOH (pH ¼ 13). The AA is negatively charged in 0.1 M NaOH solution because of the loss of proton. Therefore, the negative surface of sensor could strongly repel the negatively charged molecules. The reproducibility of sensor also plays significant role for practical application. Fig. S4a is the current responses of five electrodes with 0.2 mM glucose and Fig. S4b is the current response of one electrode with five successive glucose additions. The relative standard deviation (RSD) of current responses of five electrodes is calculated to be 2.85%, and the RSD of current increments of one electrode for five consecutive measurements with 0.2 mM glucose is 3.29%. These data confirm the good reproducibility of CuS/Cu2O/CuO/Cu-2 electrode. The stability of sensing electrode stored in air at room temperature (about 20 C/50% RH) was also tested. The current responses to 0.2 mM glucose were checked every 3 days within a period of three weeks (Fig. 6d). The sensing electrode exhibits the retention current about 92% from its original response after three weeks. This results suggests that CuS/Cu2O/CuO/Cu-2 electrode has the acceptable stability towards the glucose detection. The reason may be attributed to the fact that the coverage of stable semiconductors decreased the expose area of metal copper. To confirm the sensor’s feasibility for practical analysis, the designed CuS/Cu2O/CuO/Cu-2 electrodes were used for detecting the glucose level in real human serum. Two serum samples were obtained from local hospital without further pre-treatment before detection. 50 mL of serum samples and three successive additions of 10 mL glucose (0.1 M) were injected in 20 ml of 0.1 M NaOH at 0.60 V under stirring. According to the obtained calibration curve, the glucose concentrations of serum samples were calculated in Table S1. As expected, the results are well fitted with the given values from hospital, demonstrating that our sensors are suitable for monitoring the glucose in human blood.
4. Conclusion In summary, CuS nanosheets modified Cu2O/CuO nanowire arrays was fabricated on Cu foil as a novel binder-free non-enzymatic glucose sensor. The amount of CuS nanosheets on the Cu2O/CuO NWAs was increased with the increase of SILAR cycles and concentrations of reaction solution. However, further increasing the concentrations of reaction solution, the CuS aggregated, large cluster appeared and the structure of nanowire arrays was damaged. After optimizing, the CuS/Cu2O/CuO/Cu synthesized in 0.01 M Cu(NO3)2 and Na2S with 2 SILAR cycles owned the highest current response towards glucose. The response of CuS/Cu2O/CuO/ Cu-2 electrode was approximately twice of that of Cu2O/CuO/Cu electrode. Moreover, the CuS/Cu2O/CuO/Cu-2 electrode exhibits a high sensitivity of 4262 mA mM1 cm2 in the range of 0.002e4.096 mM, great reproducibility, acceptable selectivity and stability, together with the feasibility of glucose detection in human serum. Thus, the CuS nanosheets modified Cu2O/CuO nanowire arrays has promising potential to be applied as a non-enzymatic glucose sensor. Declaration of competing interest The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Chenhuinan Wei: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Xiao Zou: Methodology, Formal analysis. Qiming Liu: Conceptualization, Methodology, Supervision. Shuxian Li: Writing - review & editing. Chenxia Kang: Writing - review & editing. Wei Xiang: Resources. Acknowledgments This research work was financially supported by the National Natural Science Foundation of China (51572202) and the Natural Science Foundation of Jiangsu Province (BK20171234). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.135630.
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
References [1] K. Fisher, L. Griffith, A. Gruneir, D. Panjwani, S. Gandhi, L.L. Sheng, A. Gafni, P. Chris, M. Markle-Reid, J. Ploeg, Comorbidity and its relationship with health service use and cost in community-living older adults with diabetes: a population-based study in Ontario, Canada, Diabetes Res. Clin. Pract. 122 (2016) 113e123. [2] K. Dhara, D.R. Mahapatra, Electrochemical nonenzymatic sensing of glucose using advanced nanomaterials, Mikrochim. Acta 185 (2017) 49. [3] J. Wang, Electrochemical glucose biosensors, Chem. Rev. 108 (2008) 814e825. [4] S.Y. Tee, C.P. Teng, E. Ye, Metal nanostructures for non-enzymatic glucose sensing, Mater Sci Eng C Mater Biol Appl 70 (2017) 1018e1030. [5] P. Si, Y. Huang, T. Wang, J. Ma, Nanomaterials for electrochemical nonenzymatic glucose biosensors, RSC Adv. 3 (2013) 3487e3502. [6] D.W. Hwang, S. Lee, M. Seo, T.D. Chung, Recent advances in electrochemical non-enzymatic glucose sensors - a review, Anal. Chim. Acta 1033 (2018) 1e34. [7] Z. Li, F. Gao, Z. Gu, Vertically aligned Pt nanowire array/Au nanoparticle hybrid structure as highly sensitive amperometric biosensors, Sens. Actuators B Chem. 243 (2017) 1092e1101. [8] M.A. Poletti Papi, F.R. Caetano, M.F. Bergamini, L.H. Marcolino-Junior, Facile synthesis of a silver nanoparticles/polypyrrole nanocomposite for nonenzymatic glucose determination, Mater Sci Eng C Mater Biol Appl 75 (2017) 88e94. [9] A. Esmaeeli, A. Ghaffarinejad, A. Zahedi, O. Vahidi, Copper oxide-polyaniline nanofiber modified fluorine doped tin oxide (FTO) electrode as nonenzymatic glucose sensor, Sens. Actuators B Chem. 266 (2018) 294e301. [10] K.J. Babu, T. Raj Kumar, D.J. Yoo, S.-M. Phang, G. Gnana kumar, Electrodeposited nickel cobalt sulfide flowerlike architectures on disposable cellulose filter paper for enzyme-free glucose sensor applications, ACS Sustain. Chem. Eng. 6 (2018) 16982e16989. [11] X. Niu, X. Li, J. Pan, Y. He, F. Qiu, Y. Yan, Recent advances in non-enzymatic electrochemical glucose sensors based on non-precious transition metal materials: opportunities and challenges, RSC Adv. 6 (2016) 84893e84905. [12] H. Zhu, L. Li, W. Zhou, Z. Shao, X. Chen, Advances in non-enzymatic glucose sensors based on metal oxides, J. Mater. Chem. B 4 (2016) 7333e7349. [13] P.V. Dat, N.X. Viet, Facile synthesis of novel areca flower like Cu2O nanowire on copper foil for a highly sensitive enzyme-free glucose sensor, Mater Sci Eng C Mater Biol Appl 103 (2019) 109758. [14] W.B. Kim, S.H. Lee, M. Cho, Y. Lee, Facile and cost-effective CuS dendrite electrode for non-enzymatic glucose sensor, Sens. Actuators B Chem. 249 (2017) 161e167. [15] X. Zhang, G. Wang, A. Gu, Y. Wei, B. Fang, CuS nanotubes for ultrasensitive nonenzymatic glucose sensors, Chem. Commun. (2008) 5945e5947. [16] W. Ke, G. Fang, H. Lei, P. Qin, H. Tao, W. Zeng, J. Wang, X. Zhao, An efficient and transparent copper sulfide nanosheet film counter electrode for bifacial quantum dot-sensitized solar cells, J. Power Sources 248 (2014) 809e815. [17] L. Qian, J. Mao, X. Tian, H. Yuan, D. Xiao, In situ synthesis of CuS nanotubes on Cu electrode for sensitive nonenzymatic glucose sensor, Sens. Actuators B Chem. 176 (2013) 952e959. [18] S. Radhakrishnan, H.-Y. Kim, B.-S. Kim, A novel CuS microflower superstructure based sensitive and selective nonenzymatic glucose detection, Sens. Actuators B Chem. 233 (2016) 93e99. [19] Y.J. Yang, J. Zi, W. Li, Enzyme-free sensing of hydrogen peroxide and glucose at a CuS nanoflowers modified glassy carbon electrode, Electrochim. Acta 115 (2014) 126e130. [20] N. Karikalan, R. Karthik, S.M. Chen, C. Karuppiah, A. Elangovan, Sonochemical synthesis of sulfur doped reduced graphene oxide supported CuS nanoparticles for the non-enzymatic glucose sensor applications, Sci. Rep. 7 (2017) 2494. [21] X. Yan, Y. Gu, C. Li, B. Zheng, Y. Li, T. Zhang, Z. Zhang, M. Yang, A non-enzymatic glucose sensor based on the CuS nanoflakesereduced graphene oxide nanocomposite, Analytical Methods 10 (2018) 381e388. [22] X. Niu, J. Pan, F. Qiu, X. Li, Y. Yan, L. Shi, H. Zhao, M. Lan, Anneal-shrinked Cu2O dendrites grown on porous Cu foam as a robust interface for highperformance nonenzymatic glucose sensing, Talanta 161 (2016) 615e622. [23] P. Yang, X. Wang, C.-y. Ge, X. Fu, X.Y. Liu, H. Chai, X. Guo, H.-C. Yao, Y.X. Zhang, K. Chen, Fabrication of CuO nanosheets-built microtubes via Kirkendall effect for non-enzymatic glucose sensor, Appl. Surf. Sci. 494 (2019) 484e491. [24] S. Cheng, X. Gao, S. DelaCruz, C. Chen, Z. Tang, T. Shi, C. Carraro, R. Maboudian, In situ formation of metal-organic framework derived CuO polyhedrons on carbon cloth for highly sensitive non-enzymatic glucose sensing, J. Mater. Chem. B 7 (2019) 4990e4996. [25] C. Li, M. Kurniawan, D. Sun, H. Tabata, J.J. Delaunay, Nanoporous CuO layer modified Cu electrode for high performance enzymatic and non-enzymatic glucose sensing, Nanotechnology 26 (2015), 015503. [26] P. Ni, Y. Sun, Y. Shi, H. Dai, J. Hu, Y. Wang, Z. Li, Facile fabrication of CuO nanowire modified Cu electrode for non-enzymatic glucose detection with enhanced sensitivity, RSC Adv. 4 (2014) 28842e28847. [27] R. Li, X. Liu, H. Wang, Y. Wu, K.C. Chan, Z. Lu, Sandwich nanoporous framework decorated with vertical CuO nanowire arrays for electrochemical glucose sensing, Electrochim. Acta 299 (2019) 470e478. [28] C. Yu, J. Cui, Y. Wang, H. Zheng, J. Zhang, X. Shu, J. Liu, Y. Zhang, Y. Wu, Porous HKUST-1 derived CuO/Cu2O shell wrapped Cu(OH)2 derived CuO/Cu2O core
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43] [44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
9
nanowire arrays for electrochemical nonenzymatic glucose sensors with ultrahigh sensitivity, Appl. Surf. Sci. 439 (2018) 11e17. J. Huang, Y. Zhu, X. Yang, W. Chen, Y. Zhou, C. Li, Flexible 3D porous CuO nanowire arrays for enzymeless glucose sensing: in situ engineered versus ex situ piled, Nanoscale 7 (2015) 559e569. X. Liu, W. Yang, L. Chen, J. Jia, Three-dimensional copper foam supported CuO nanowire arrays: an efficient non-enzymatic glucose sensor, Electrochim. Acta 235 (2017) 519e526. Y. Nicolau, J. Menard, Solution growth of ZnS, CdS and Zn1-xCdxS thin films by the successive ionic-layer adsorption and reaction process; growth mechanism, J. Cryst. Growth 92 (1988) 128e142. Y. Nicolau, Solution deposition of thin solid compound films by a successive ionic-layer adsorption and reaction process, Appl. Surf. Sci. 22 (1985) 1061e1074. H. Pathan, C. Lokhande, Deposition of metal chalcogenide thin films by successive ionic layer adsorption and reaction (SILAR) method, Bull. Mater. Sci. 27 (2004) 85e111. Y. Nicolau, M. Dupuy, M. Brunel, ZnS, CdS, and Zn1xCdxS thin films deposited by the successive ionic layer adsorption and reaction process, J. Electrochem. Soc. 137 (1990) 2915e2924. J. Luo, L. Steier, M.K. Son, M. Schreier, M.T. Mayer, M. Gratzel, Cu2O nanowire photocathodes for efficient and durable solar water splitting, Nano Lett. 16 (2016) 1848e1857. X. Chen, R. Chen, X. Zhu, Q. Liao, Y. Zhang, D. Ye, B. Zhang, Y. Yu, J. Li, A solar responsive cubic nanosized CuS/Cu2O/Cu photocathode with enhanced photoelectrochemical activity, J. Catal. 372 (2019) 182e192. X. Zhang, W. Guo, G. Gao, M. Que, C. Pan, Z.L. Wang, CuS nanotrough-networks for highly stable transparent conducting electrodes, J. Mater. Chem. C 4 (2016) 4733e4739. A. Venkadesh, S. Radhakrishnan, J. Mathiyarasu, Eco-friendly synthesis and morphology-dependent superior electrocatalytic properties of CuS nanostructures, Electrochim. Acta 246 (2017) 544e552. S.Y. Tee, E. Ye, P.H. Pan, C.J. Lee, H.K. Hui, S.Y. Zhang, L.D. Koh, Z. Dong, M.Y. Han, Fabrication of bimetallic Cu/Au nanotubes and their sensitive, selective, reproducible and reusable electrochemical sensing of glucose, Nanoscale 7 (2015) 11190e11198. R. Ponnusamy, A. Gangan, B. Chakraborty, C. Sekhar Rout, Tuning the pure monoclinic phase of WO3 and WO3-Ag nanostructures for non-enzymatic glucose sensing application with theoretical insight from electronic structure simulations, J. Appl. Phys. 123 (2018). R. Ponnusamy, A. Gangan, B. Chakraborty, D.J. Late, C.S. Rout, Improved nonenzymatic glucose sensing properties of Pd/MnO2 nanosheets: synthesis by facile microwave-assisted route and theoretical insight from quantum simulations, J. Phys. Chem. B 122 (2018) 7636e7646. M.-m. Guo, Y. Xia, W. Huang, Z. Li, Electrochemical fabrication of stalactite-like copper micropillar arrays via surface rebuilding for ultrasensitive nonenzymatic sensing of glucose, Electrochim. Acta 151 (2015) 340e346. X. Liu, W. Yang, L. Chen, J. Jia, Synthesis of copper nanorods for non-enzymatic amperometric sensing of glucose, Microchimica Acta 183 (2016) 2369e2375. Y.Z. Wang, H. Zhong, X.M. Li, F.F. Jia, Y.X. Shi, W.G. Zhang, Z.P. Cheng, L.L. Zhang, J.K. Wang, Perovskite LaTiO3-Ag0.2 nanomaterials for nonenzymatic glucose sensor with high performance, Biosens. Bioelectron. 48 (2013) 56e60. B. Zheng, G. Liu, A. Yao, Y. Xiao, J. Du, Y. Guo, D. Xiao, Q. Hu, M.M.F. Choi, A sensitive AgNPs/CuO nanofibers non-enzymatic glucose sensor based on electrospinning technology, Sens. Actuators B Chem. 195 (2014) 431e438. M. Velmurugan, N. Karikalan, S.-M. Chen, Synthesis and characterizations of biscuit-like copper oxide for the non-enzymatic glucose sensor applications, J. Colloid Interface Sci. 493 (2017) 349e355. G. Wang, H. Sun, L. Ding, G. Zhou, Z.S. Wang, Growth of Cu particles on a Cu2O truncated octahedron: tuning of the Cu content for efficient glucose sensing, Phys. Chem. Chem. Phys. 17 (2015) 24361e24369. Y. Su, H. Guo, Z. Wang, Y. Long, W. Li, Y. Tu, Au@Cu2O core-shell structure for high sensitive non-enzymatic glucose sensor, Sens. Actuators B Chem. 255 (2018) 2510e2519. C. Wei, Y. Liu, Q. Liu, W. Xiang, Uniform and dense copper nanoparticles directly modified indium tin oxide electrode for non-enzymatic glucose sensing, J. Electroanal. Chem. 835 (2019) 273e280. Y. Zhao, L. Fan, Y. Zhang, H. Zhao, X. Li, Y. Li, L. Wen, Z. Yan, Z. Huo, Hyperbranched Cu@Cu2O coaxial nanowires mesh electrode for ultra-sensitive glucose detection, ACS Appl. Mater. Interfaces 7 (2015) 16802e16812. K. Kim, S. Kim, H.N. Lee, Y.M. Park, Y.-S. Bae, H.-J. Kim, Electrochemically derived CuO nanorod from copper-based metal-organic framework for nonenzymatic detection of glucose, Appl. Surf. Sci. 479 (2019) 720e726. S. Zhou, X. Feng, H. Shi, J. Chen, F. Zhang, W. Song, Direct growth of vertically aligned arrays of Cu(OH)2 nanotubes for the electrochemical sensing of glucose, Sens. Actuators B Chem. 177 (2013) 445e452. J. Lv, C. Kong, Y. Xu, Z. Yang, X. Zhang, S. Yang, G. Meng, J. Bi, J. Li, S. Yang, Facile synthesis of novel CuO/Cu2O nanosheets on copper foil for high sensitive nonenzymatic glucose biosensor, Sens. Actuators B Chem. 248 (2017) 630e638. K.-J. Huang, L. Wang, J. Li, T. Gan, Y.-M. Liu, Glassy carbon electrode modified with glucose oxidaseegrapheneenano-copper composite film for glucose sensing, Measurement 46 (2013) 378e383. M.E. Ghica, R.C. Carvalho, A. Amine, C.M.A. Brett, Glucose oxidase enzyme
10
C. Wei et al. / Electrochimica Acta 334 (2020) 135630
inhibition sensors for heavy metals at carbon film electrodes modified with cobalt or copper hexacyanoferrate, Sens. Actuators B Chem. 178 (2013) 270e278. [56] C.X. Guo, C.M. Li, Direct electron transfer of glucose oxidase and biosensing of glucose on hollow sphere-nanostructured conducting polymer/metal oxide composite, Phys. Chem. Chem. Phys. 12 (2010) 12153e12159. [57] J. Mohapatra, B. Ananthoju, V. Nair, A. Mitra, D. Bahadur, N.V. Medhekar,
M. Aslam, Enzymatic and non-enzymatic electrochemical glucose sensor based on carbon nano-onions, Appl. Surf. Sci. 442 (2018) 332e341. [58] Y. Ding, Y. Liu, L. Zhang, Y. Wang, M. Bellagamba, J. Parisi, C.M. Li, Y. Lei, Sensitive and selective nonenzymatic glucose detection using functional NiOePt hybrid nanofibers, Electrochim. Acta 58 (2011) 209e214. [59] H. Li, C.Y. Guo, C.L. Xu, A highly sensitive non-enzymatic glucose sensor based on bimetallic Cu-Ag superstructures, Biosens. Bioelectron. 63 (2015) 339e346.