Highly efficient non-enzymatic glucose sensor based on CuxS hollow nanospheres

Applied Surface Science 492 (2019) 407–416

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Highly efficient non-enzymatic glucose sensor based on CuxS hollow nanospheres

T

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Miaomiao Caoa,1, Hui Wanga,1, Palanisamy Kannanb, Shan Jia,b, , Xingpu Wanga, Qian Zhaoa, ⁎⁎ Vladimir Linkovc, Rongfang Wanga, a

State Key Laboratory Base for Eco-Chemical Engineering, College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China College of Biological, Chemical Science and Chemical Engineering, Jiaxing University, Jiaxing 314001, China c South African Institute for Advanced Materials Chemistry, University of the Western Cape, Cape Town 7535, South Africa b

A R T I C LE I N FO

A B S T R A C T

Keywords: Copper sulfide Copper oxide Hollow nanostructures Electrocatalysis Glucose sensor

Copper sulfide with copper to sulfur stoichiometric ratios of 2:1, 2.5:1 and 3:1 is rapidly synthesized by varying the amount of sulfur precursor against Cu2O nanospheres. The size, morphology, crystalline nature, and elemental compositions of CuxS hollow nanospheres such as, CuS, Cu2S, Cu7S4, and Cu2O are characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The synthesized CuxS hollow nanospheres display a substantial electrocatalytic activity towards the oxidation of glucose in alkaline medium. Notably, the Cu7S4 hollow-like nanospheres exhibited highest glucose sensitivity of 3728.7 μA mM−1 cm−2 as compared to the other CuxS hollow nanospheres. Further, the Cu7S4 hollow nanospheres show a wide linear concentration responses towards the oxidation of glucose from 1.0 μM to 2.0 mM, with a limit of detection (LOD) of 0.023 μM (S/ N = 3). The obtained results indicate that Cu7S4 hollow nanospheres as a promising candidate for non-enzymatic glucose sensing.

1. Introduction Sensors are vital tool in the fields of clinical, biochemical and biological analysis, environmental screening and food processing industries. [1] Diabetes mellitus is a common chronic disease affecting millions of people over the world due to the abnormal level of glucose concentration in blood (normal range 4.4–6.6 mM) [2]. Monitoring blood glucose concentration is a critical indicator for diabetes as well as endocrine metabolic disorder, thus a rapid and precise detection of glucose is very essential [3,4]. At present, glucose oxidase based strategy is most commonly used for the determination of glucose due to its simplicity, sensitivity, selectivity, and reliability [5]. But the sensitivity of enzyme to temperature, pH, stability and re-usability, considerably obstruct the further development of enzyme-based glucose sensor. Given that characteristic problems of enzyme, there is an urgent need for developing non-enzymatic glucose sensor with rapid response, high sensitivity, selectivity and re-usability. Recently, transition-metal chalcogenides have received significant attention because of their potential applications in photovoltaic,

thermoelectric, confined-space chemical reactors, catalysis, and biomedical diagnosis and therapy [6–8]. Among them, copper sulfide (CuS) is a p-type semiconductor material that showing potential applications in solar cell device, photothermal ablation therapy, and Liion batteries [9–11]. Remarkably, chalcocite (Cu2S) is one of the polymorphs of CuS, and has been explored as potential candidate for electrochemical glucose sensor due to their interesting properties including metal-like electrical conductivity [12–15]. For instance, Lee et al., grown (by in-situ) single crystalline β-Cu2S spherical nanoparticles (NPs) to triangular plates on multiwalled carbon nanotubes (MWCNTs), and incorporated glucose oxidase enzyme to determine the concentration of glucose with the LOD of 10 μM [12]. Zhang et al., synthesized Cu2S nanoplates on the Cu substrate by etching method, and utilized as-prepared Cu2S/Cu substrate for the determination of glucose with the LOD of 0.10 μM [13]. Later, Maji et al., produced Cu2S nanoplates via one step solvothermal decomposition of a single source precursor, and applied for the detection of glucose with a LOD of 1.3 μM [14]. Recently, Lu et al., prepared Cu2S nanorods on 3-D Cu foam as a support for glucose sensor with a LOD of 0.07 μM [15]. Considerably,

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Correspondence to: S. Ji, College of Biological, Chemical Science and Chemical Engineering, Jiaxing University, Jiaxing, 314001, China. Corresponding author. E-mail addresses: [email protected] (S. Ji), [email protected] (R. Wang). 1 Miaomiao Cao and Hui Wang contributed equally. ⁎⁎

https://doi.org/10.1016/j.apsusc.2019.06.248 Received 20 February 2019; Received in revised form 13 June 2019; Accepted 25 June 2019 Available online 26 June 2019 0169-4332/ © 2019 Published by Elsevier B.V.

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Quantachrome Autosorb-1 volumetric analyzer, using nitrogen adsorption-desorption and the Brunauer-Emmett-Teller (BET) method.

the above Cu2S based nanostructures were successfully applied for glucose sensor, though the following disadvantages limits their potential use in clinical analysis, including (i) MWCNTs or Cu foam used as supports or backbone, high temperature, enzyme based glucose detection, less sensitivity etc., Hollow structured nanomaterials with void space available inside the distinct shell can effectively enhance the electrocatalytic properties due to their high specific surface area, volume buffer and shell permeability, and the hollow structured materials usually exhibit many unique features. In this work, we explore the synthesis of CuxS hollow nanospheres as efficient nanocatalysts for non-enzymatic glucose sensor for the first time. The CuxS hollow nanospheres are obtained by mild solution method without using solid supports, micelles, and structure directing agents. The as-prepared CuxS hollow nanospheres are evaluated as nanocatalyst for non-enzymatic detection of glucose in alkaline medium. Notably, the Cu7S4 hollow nanospheres exhibits significantly higher electrocatalytic activity towards the detection of glucose as compared to the other CuxS hollow nanospheres such as Cu2O, Cu2S, and CuS. Further, the Cu7S4 hollow nanospheres are highly selective for the detection of glucose in presence of important electroactive interfering compounds and displayed excellent stability. Thus, the Cu7S4 hollow nanospheres showing great potential in developing new sensor design and other biological applications.

2.4. Electrochemical measurements All electrochemical experiments were carried out by using a conventional three-electrode electrochemical cell system from CHI 650D Electrochemical Work Station, Texas, USA. In the three-electrode electrochemical cell, a platinum wire, 5 mm diameter glassy carbon disc and Ag/AgCl (saturated KCl solution) were used as working, counter, and reference electrodes, respectively. The CuxS hollow nanospheres modified working electrode was prepared as follows: First, 2 mg of the catalyst was ultrasonically dispersed in 0.4 mL of Nafion - ethanol solution (Nafion: 25 wt%); next, 8 μL of the above solution was dropped onto the glassy carbon surface and dried under nitrogen atmosphere for all the electrochemical experiments. The amperometric i-t curves were obtained at an applied potential of 0.5 V in 0.1 M NaOH solution under constant stirring. 3. Results and discussion 3.1. Morphological characterization of CuxS hollow nanospheres In this work, we used Cu2O nanospheres as seed/nuclei for synthesizing CuxS hollow nanospheres by mild solution method. The size and morphology of as-synthesized CuxS hollow nanospheres were characterized by SEM and TEM techniques, as shown in Fig. 1. The SEM image of as-prepared Cu2O sample clearly displayed spherical nanosphere morphology with an average particle size of 300 ± 20 nm and have a narrow size distribution (Fig. 1a). The corresponding TEM image further confirms the nanospheres morphology of Cu2O sample with a compact structure, which is composed by many tiny nanoparticles (Fig. 1b). The SEM image of Cu2S sample showed hallow-like nanospheres with similar particles size like Cu2O nanospheres (Fig. 1c), and corresponding TEM image shown in Fig. 1d; the contrast difference between the light center and dark boundaries further confirms the spheres have a hollow-like nanostructures. In the case of both CuS (Fig. 1e, and f) and Cu7S4 (Fig. 1g, and h) samples, hollow-like nanostructures were highly retained (see respective SEM and TEM images) during the synthesis with an average size of 250 ± 20 nm. Notably, the size of CuS and Cu7S4 hollow nanostructures was slightly smaller than the Cu2O and Cu2S nanostructures due to change in ratios of the reaction precursor Na2S from 2 to 3. The obtained results demonstrate that successful preparation of CuS polymorphs namely Cu2S, Cu7S4 and CuS, with identical hollow nanospheres, by tuning the quantity of Na2S in the reaction mixture. During the first phase of reaction, spherical nanospheres were formed by hydrolysis of copper(II) acetate. In the second phase of the reaction, the growth process was extended, thus these spherical nanospheres were subject to inside-out ripening. This emptying process could initiate at regions either near the surface (at the bottom inside) or around the nanosphere midpoint depends on the ripening characteristics of chemical species (for instance, polarity of the solvent) [16,17] which leads to hollow-like nanospheres.

2. Experimental section 2.1. Synthesis of Cu2O nanospheres All the chemicals were of analytical grade and used as received without further purification. Firstly, Cu2O nanospheres were prepared by according to the following procedure. 0.3993 g of (CH3COO)2Cu∙H2O was dissolved in 25 ml of DMF and 3 ml of deionized water, then the mixture was treated ultrasonically for 3 min. Subsequently, the solution was constantly stirred at 85 °C for 10 min. Finally, the solution was cooled down to room temperature and allowed for 2 h. Then, the obtained product was separated by centrifugation process, washed thoroughly with 95% ethanol and dried in a vacuum oven. 2.2. Preparation of hollow-like structured CuxS nanospheres The hollow-like CuxS nanospheres were prepared by following procedure. 25 mg of as-synthesized Cu2O nanospheres were added into 15 ml of deionized water, and ultrasonicated for 5 min to obtain homogeneous solution. Next, various amounts of Na2S∙9H2O (molar ratios of Na2S∙9H2O/Cu2O 2:1, 2.5:1 and 3:1 for CuS, Cu2S and Cu7S4 nanostructures, respectively) were added into the above reaction vessel, and then ultrasonication process continued for another 5 min in an ice bath. Finally, black color product was obtained at the end of the ultrasonication process. The obtained product was separated by centrifugation process, washed with 95% ethanol, and then dried in a vacuum oven at 40 °C for 12 h. 2.3. Characterization of CuxS nanomaterials

3.2. X-ray diffraction analysis of CuxS hollow nanospheres X-ray diffraction (XRD) analysis was carried out on a Shimadzu XD3A goniometer with Ni filtered monochromatic CuKα (λ = 1.54056 Å) as radiation source and operated at 40 kV, to study the crystalline structure of the as-prepared CuxS samples. The size and morphologies of the as-prepared CuxS nanostructures were examined by using scanning electron microscope (SEM: Carl Zeiss Ultra Plus field emission) and transmission electron microscopy (TEM: JEOL JEM-2010 Electron Microscope) techniques. The X-ray photoelectron spectroscopy (XPS) analysis was carried out using PHI-5702 spectrometer and C1s peak at 285.0 eV was used as a reference to calibrate the all binding energies. The specific surface area of all CuxS nanostructures were measured on a

The crystalline nature of as-prepared CuxS hollow nanospheres was examined by XRD technique. For comparison, the diffraction pattern of Cu2O nanospheres is also shown in Fig. 2. The intense diffraction patterns were located at 35.4, 41.5, 60.5, 72.7 and 76.6°, which corresponding to the (110), (111), (200), (220), and (311) crystalline planes of cubic Cu2O respectively (black line) (JCPDS 05-0667) [18]. The absence of other diffraction peaks indicated that Cu2O nanospheres were composed by a single phase crystalline nature. For Na2S/Cu2O, molar ratio of 2:1, the diffraction peaks at 2θ ≈ 27.51, 32.23, 46.10, and 54.32° were matched with (111), (200), (220) and (311) plane 408

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Fig. 1. SEM and corresponding TEM images of as-prepared Cu2O (a, b), Cu2S (c, d), CuS (e, f), and Cu7S4 (g, h) hollow nanospheres.

also resulted in the diffraction peaks at 47.40, and 52.54° that corresponding to the (110), and (103) characteristic planes of monoclinic CuS phase (magenta line) (JCPDS 06-0464) [22].

characteristics of Cu2S pure cubic phase (JCPDS 84-1770) [19]. No characteristic peaks of Cu2O were detected during the analysis confirms the pure phase of Cu2S hollow nanospheres (red line). Notably, three intense peaks were observed at 2θ ≈ 47.20, 49.01, and 54.31° which has good agreement with the monoclinic Cu4S7 phase structure (dark blue line) (JCPDF 23–0958) [20,21]. The intensity of (0160) peak is particularly intense, which indicates the preferential growth orientation of hollow-like nanospheres. The CuS sample was

3.3. Structural characterization of CuxS hollow nanospheres X-ray photoelectron spectroscopy (XPS) is a potential tool to determine surface chemical states and elemental composition of as409

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The S 2p highly resolution XPS of Cu2S, Cu7S4 and CuS hollow nanospheres were shown in Fig. 4b and their corresponding electronic orbits along with binding energies were summarized in Table 2. The peak fit reveals the existence of a distinct doublet related to the S 2p3/2 binding energy located at 161.4 eV and S 2p1/2 binding energy located at 162.7 eV. The primary S 2p3/2 element obviously indicates the presence of a Cu-S-Cu bonding conformation and the value reported for sulfur component in chalcocite CuxS samples. [25–27] Besides that, S 2p1/2 high-energy components were present in all samples further evidenced the involvement of Cu–S-type bonding, which perhaps due to the dangling bonds at the hollow sphere surface [28]. Notably, the S 2p3/2 and S 2p1/2 binding energies of Cu7S4 hollow nanospheres exhibited obvious shift in lower binding energy side reveals that the higher electron configuration, resulting enhanced electronic conductivity of the hollow nanospheres, which is highly favorable for accessing large amount of analytes during electrochemical reaction. 3.4. BET surface area of CuxS hollow nanospheres Fig. 2. XRD patterns of as-prepared Cu2O (a), Cu2S (b), Cu7S4 (c) and CuS (d) hollow nanospheres.

Nitrogen (N2) sorption analysis was adopted to investigate the specific surface area of as-synthesized CuxS hollow nanospheres (Fig. 5). N2 sorption plot (P/P0 0 to 1.0) of Cu2O exhibits IUPAC type III isotherms, without significant hysteresis loop, indicating the existence of mesopores. Based on the N2 sorption results, the calculated BET surface area of Cu2O, Cu2S, Cu7S4, and CuS samples were 15.0, 18.0, 18.5 and 17.0 m2 g−1, respectively. The above results indicate that Cu7S4 has slightly larger BET surface area than other CuxS samples.

prepared CuxS hollow nanospheres. By the analysis of binding energy (BE) values, we have identified the nature of Cu metal in CuxS hollow nanospheres. The survey XPS spectra (Fig. 3a) of CuS, Cu2S, Cu4S7 hollow nanospheres and Cu2O nanospheres indicate the presence of elements such as Cu, S, C, and O components in the samples. The elemental composition reveals that the atomic ratio of S/Cu was about 0.50 for Cu2S, 0.57 for Cu4S7, and 1 for CuS (Fig. 3b). As expected, the S content in the hollow nanospheres increased while increasing the quantity of Na2S precursors in respective CuxS samples. The chemical state of Cu was determined from core-level high resolution XPS spectra of Cu 2p as shown in Fig. 4a and corresponding specific electronic orbits along with binding energies were summarized in Table 1. The two main peaks were observed at ca. 932.3 and 952.4 eV that corresponding to Cu 2p3/2 and Cu 2p1/2 respectively due to the presence of Cu(I) in all the samples [23]. The obtained binding energy values were well agreed with the literature based on monovalent state of copper Cu(I) in chalcocite Cu2S. Moreover, a small peak was observed at a binding energy of 938.8 eV which indicated the presence of Cu(II) resulting from the partial oxidation of the surface due to its exposure to the ambient atmosphere. Besides that, a very weak peak was observed at 942.4 eV corresponding to the satellite peak of Cu+ in Cu7S4 and CuS hollow nanospheres [24], particularly the later was slightly dominant due to the influence of the precursors mole ratio during the synthesis. As presented in Table 1, Cu 2p peaks of Cu7S4 visibly shifted to higher binding energies compared to those of Cu2S and CuS, suggested the formation of Cu7S4 hollow nanostructures.

3.5. Ultraviolet absorption spectrum of CuxS hollow nanospheres Since the optical energy band gap of semi-conductor was dependent on their microstructures, UV–vis diffusive reflectance spectra were carried out to investigate the microstructure for as-prepared samples. The obtained UV–vis spectra and their corresponding plots of (αhv)2 vs. hv were presented in Fig. 6. The estimate value (the dashed lines to the x-axis) of hv at α = 0 shows an absorption edge energy corresponding to Egap = 1.31 eV for Cu2S,1.50 Ev for Cu7S4 and 1.29 eV for CuS. In comparison with the data of values of 1.2 eV [49] and 1.34 eV [50] for their bulk counterparts, it is reasonable to conclude that a considerable blue shift has occurred. It presumably resulted from the quantum size effects considering the polycrystalline feature of the hollow nanospheres [51]. 3.6. Electrochemical oxidation of glucose at CuxS hollow nanospheres We have examined electrocatalytic activity of as-prepared CuxS hollow nanospheres modified GCE towards electrochemical oxidation of glucose. The cyclic voltammetry (CVs) measurements were carried

Fig. 3. (a) XPS survey spectra of Cu2O, Cu2S, Cu7S4 and CuS; (b) Bar plot showing atomic ratio of S/Cu in products vs. molar ratio of Na2S/Cu2O precursors. 410

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Fig. 4. (a) Cu 2p XPS of Cu2O, Cu2S, Cu7S4 and CuS. (b) S 2p XPS of Cu2S, Cu7S4 and CuS hollow nanospheres.

nanospheres, Cu2S, Cu7S4 and CuS hollow nanospheres modified GCE [29–31]. Remarkably, Fig. 5b shows Cu7S4 hollow nanospheres modified GCE displayed obviously higher catalytic current response as compared to Cu2O nanospheres, Cu2S, and CuS hollow nanospheres modified GCE in 0.1 M NaOH solution containing 3 mM glucose (Fig. 5b). The typical oxidation of glucose to gluconolactone at Cu7S4 hollow nanospheres modified GCE was electrocatalyzed by Cu(I)/Cu(II) redox couple, according to the following reactions: [30].

Table 1 Showing 2p binding energies of Cu(I) and Cu(II) in Cu2S, Cu7S4 and CuS hollow nanospheres. Sample

Binding energy/eV Cu (I)

Cu2O Cu2S Cu7S4 CuS

Cu (II)

2p3/2

2p1/2

2p3/2

2p1/2

932.1 931.9 932.3 932.1

952.1 952.0 952.3 952.2

933.3 933.2 933.8 933.3

953.4 953.4 954.1 953.4

Binding energy/eV 2p3/2

Cu2S Cu7S4 CuS

2p1/2

161.4 160.9 161.5

(1)

Cu(II) + glucose → Cu(I) + gluconolactone

(2)

The enhanced catalytic current response of Cu7S4 hollow nanospheres towards the electrochemical oxidation of glucose was occurred due to the following key reasons. (i) the Cu7S4 hollow nanostructures offers a large specific (active) surface area; (ii) good electronic conductivity of Cu7S4 hollow nanospheres could provide reliable interaction towards the glucose molecules; and (iii) the presence of inside out or void-like open structure configuration in Cu7S4 hollow nanospheres access large amount of glucose molecules to the inner and outer surface for efficient oxidation. Fig. 7c–f displays the CVs obtained for Cu2O nanospheres (c), Cu2S (d), Cu7S4 (e) and CuS (f) hollow nanospheres modified electrodes in 0.1 M NaOH containing different concentrations of glucose (0.05, 0.1, 0.5, 1, 2, 3, 4 and 5 mM) at a scan rate of 50 mVs−1. Upon successive scanning, a significant enhancement in the catalytic current along with a small potential shift during each addition of glucose was prominently observed; this indicated that as-prepared Cu2O nanospheres, Cu2S, Cu7S4 and CuS hollow nanospheres have shown good sensitivity. From the above analysis, Cu7S4 hollow nanospheres exhibited highest catalytic current responses indicating that Cu7S4 as an excellent candidate for this electrochemical glucose sensor.

Table 2 S 2p binding energies of Cu2S, Cu7S4 and CuS hollow nanospheres. Sample

Cu(I) → Cu(II) + e−

162.6 162.0 162.7

3.7. Amperometric determination of glucose at CuxS hollow nanospheres The amperometry method provides an effective way to establish convective mass transport on the electrode surface resulting in rapid detection of glucose. Indeed, for practical application of any sensor, it is necessary to use continuous potential amperometry, for evaluating the performance of transducer. The electrochemical oxidation of glucose was significantly enhanced at the ipa of +0.50 V in the CV measurements, thus we choose +0.50 V as an applied potential for amperometric detection of glucose using CuxS hollow nano-spheres modified GCE electrodes (Fig. 8a-Cu2O, c-Cu2S, e-Cu7S4, and g-CuS) under constantly stirred 0.1 M NaOH solution. The typical amperometric i–t curves obtained on successive injection of glucose (1 to 8000 μM), and the i-t curves were reached steady-state current within 2 s, which is identical to the previously reported non-enzymatic glucose sensors [32–34]. The concentration range from 1 to 8000 μM of glucose was tested for all CuxS modified electrodes, and corresponding amperometric current response was increased in the given order, Cu2O nanospheres < Cu2S < CuS < Cu7S4 hollow nanospheres. The respective

Fig. 5. N2 sorption isotherms of CuxS hollow nanospheres.

out by using CuS, Cu2S, Cu7S4 hollow nanospheres and Cu2O nanospheres modified GCE in 0.1 M NaOH solution without (Fig. 5a) and with 0.1 mM glucose (Fig. 5b). In addition, CV response of bare GCE towards the oxidation of glucose was also presented in both figures for comparison. The unmodified GCE shows no noticeable CV redox response in the absence as well as in the presence of 0.1 M glucose indicating that bare GCE was electrochemically inactive to catalyze the oxidation of glucose (black lines). On the other hand, a distinguished redox peaks were observed between 0.2 and 0.8 V in the CV response, which is associated to the oxidation of Cu(I) to Cu(II) in the forward scan, and reduction of Cu(II) to Cu(I) during the reverse scan on Cu2O 411

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Fig. 6. (a) UV–vis spectra for CuxS, and (b, c and d) (αhv)2 vs. hv plots of Cu2S, Cu7S4 and CuS hollow nanospheres, respectively.

of glucose sensor is an important parameter in clinical point-view. Fig. 9 shows the amperometric i-t curve responses obtained for Cu2O (black line), Cu2S (red line), Cu7S4 (blue line), and CuS (magenta line) hollow nanospheres modified GCE under constantly stirred 0.1 M NaOH solution at an applied potential of +0.50 V. Upon injection of 1.0 mM glucose, a substantial increase in the amperometric current response was observed for all the electrodes (step a), indicating catalytic activity towards the detection of glucose. Next, no change in the amperometric current responses was observed while injecting 0.1 mM of AA (step b), UA (step c), and NaCl (step d) into the same solution. Finally, the addition of 1 mM glucose (step e) into the same solution results in an identical amperometric current response obtained for all electrodes like step “a”, indicating that the present modified electrodes are highly selective towards the detection of glucose in the presence of major interferents. Based on the value of isoelectric points for CuO and CuS, the surfaces of these materials were negatively charged in an alkaline environment. Especially, the interfering species were also negatively charged due to the deprotonating effect in the alkaline medium [44–47], allowing the surface of CuS to repel negatively charged interfering species and which resulted in highly selective detection of glucose. It should be noted that the amperometric response of Cu7S4 was higher than the other modified electrodes, which demonstrates Cu7S4 hollow nanospheres as promising candidate for non-enzymatic glucose sensor.

linear calibration plots (Fig. 6b-Cu2O, d-Cu2S, f-Cu2S4, and h-CuS) were obtained by plotting amperometric current versus concentration of glucose with a correlation coefficient of 0.993, 0.995, 0.999,and 0.995 for Cu2O nanospheres, Cu2S, Cu7S4 and CuS hollow nanospheres modified electrodes, respectively. Based on the calibration curves, sensitivity values of signal to noise ratio of 3 (S/N = 3) and linear concentration ranges were obtained and listed in Table 3. The limit of detection (LOD) of the present sensor was estimated using the following equation [35]:

LOD =

3Sb m

where sb is standard deviation obtained from 10 individual measurements of the blank signal and m is the slope value of the calibration plot. We found that Cu7S4 hallow nanospheres modified electrode shows enhanced sensitivity, wide linear range and lowest LOD (0.023 M) among the other CuxS hollow nanospheres modified electrodes. The sensitivity and detection limits of Cu2S, Cu7S4 and CuS hollow nanospheres were clearly surpassing recently reported literature as shown in Table 3. Further, it is important to mention that morphology of nanomaterial, porosity of the nanostructure, large specific surface area, and composition plays a critical role in catalytic performance of the nanomaterials [38–42]. In addition to the hollow nanospheres with identical particle size (see above), the electronic effect between Cu and S also played a supporting role in enhanced catalytic performance of this glucose sensor. Particularly, the positive shifts of Cu 2p and negative shifts of S 2p3/2 lead to increase the electric dipole in Cu7S4 hollow nanospheres, which facilitate electron transfer from Cu to S. This electronic effect could reduce the energy barriers in the electrocatalytic reaction and facilitates accessing large amount of glucose for efficient oxidation [43].

3.9. The stability and reproducibility of Cu7S4 hollow nanospheres To examine the stability of all Cu7S4 hollow nanospheres modified GCE, the CVs were obtained in 0.1 M NaOH solution containing 0.1 mM glucose for every 5 min interval. It was found that the oxidation peak current remains unchanged with a relative standard deviation of 2.4% for 10 repetitive measurements, representing that the electrode has a good reproducibility and does not undergo surface fouling. After CV measurements, the Cu7S4 hollow nanospheres modified GCE was kept in 0.1 M NaOH solution at 4 °C. The current response decreased about 3.1% in one week and 6.8% in about two weeks. To ascertain the reproducibility of the results, three individual GCE were modified with the Cu7S4 hollow nanospheres modified GCE and their electrochemical

3.8. Electrochemical interference analysis at CuxS hollow nanospheres Ascorbic acid (AA), uric acid (UA), and some carbohydrate compounds usually co-exists with glucose in biological samples and consequently interferes towards the detection of glucose. Thus, selectivity 412

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Fig. 7. CVs obtained for Cu2O nanospheres, Cu2S, Cu7S4 and CuS hollow nanospheres on GCE in 0.1 M NaOH (a) and 0.1 M NaOH +3 mM glucose (b) at a scan rate of 50 mV s−1. CVs obtained for Cu2O nanospheres (c), Cu2S (d), Cu7S4 (e) and CuS (f) hollow nanospheres on GCE in 0.1 M NaOH +0.05, 0.1, 0.5, 1, 2, 3, 4 and 5 mM glucose at a scan rate of 50 mV s−1.

The Cu7S4 hollow nanostructures offers (i) a large specific (active) surface area, (ii) good electronic conductivity which could provide reliable interaction towards the glucose molecules; and (iii) the presence of inside out or void-like open structure configuration in Cu7S4 hollow nanostructures access large amount of glucose molecules to the inner and outer surface for higher electrocatalytic activity towards the detection of glucose. Moreover, enhanced electric dipole movement facilitates electron transfer from Cu to S in Cu7S4 hollow nanospheres, which reduces the energy barriers in the electrocatalytic reaction and accessing large amount of glucose for efficient oxidation. Finally, Cu7S4 hollow nanospheres demonstrated higher selectivity towards the detection of glucose in the presence of major interfering species such as ascorbic acid (AA), uric acid (UA), and NaCl. In summary, the Cu7S4 hollow nanospheres showed significant potential for the development of a non-enzymatic glucose sensor.

response towards the oxidation of 0.10 mM glucose was tested by 5 repeated measurements, and the catalytic responses were identical for all the electrodes. The above results showed that the Cu7S4 hollow nanospheres modified GCE was highly stable and reproducible towards detection of glucose. 4. Conclusions In conclusion, we have synthesized CuxS hollow nanospheres with different compositions using Cu2O nanoparticles as seed source by mild solution method without using any solid supports, micelles, and structure directing agents. The as-synthesized CuxS hollow nanospheres were successfully used for electrochemical non-enzymatic detection of glucose. Among the other CuxS, Cu7S4 hollow nanospheres exhibited excellent electrocatalytic activity towards the oxidation of glucose with a high sensitivity of 3876 μA mM−1 cm−2. The Cu7S4 hollow nanospheres showed a wide-linear responses towards various concentrations of glucose from 1.0 μM to 2.0 mM, with a LOD of 0.023 μM (S/N = 3). 413

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Fig. 8. Amperometric i-t curve responses and the corresponding calibration plots obtained for Cu2O nanospheres (a–b), Cu2S (c–d), Cu7S4 (e–f), and CuS (g–h) hollow nanospheres modified GCE electrodes with increasing glucose concentrations (1 to 8000 μM) in 0.1 M NaOH at an applied potential of +0.50 V vs. Ag/AgCl. All the insets show magnified view of amperometric i-t curves response at micro molar level glucose concentrations (1 to 20 μM).

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Table 3 Comparison of other chemically modified electrodes for the determination of glucose with CuxS hollow nanospheres modified electrode. Electrode Cu2O Cu2S Cu7S4 CuS Cu2S@Cu Sphere-like CuS CuS NPs Cu2S nanoparticles Cu-Cu2S Cu2S/MWCNT

Detection potential/V vs. Ag/AgCl

Sensitivity/μA cm−2 mM−1

Linear range/mM

LOD/μM

Reference

0.5 0.5 0.5 0.5 0.45 0.05 0.5 −0.35 −0.1 0.55

2504.5 3610.1 3728.7 3594.9 11,750.8 117.3 1085 61.67 361.58 0.075

0.001–0.8 0.001–1.0 0.001–2.0 0.001–1.0 0.0002–0.63 0.001–12 0.02–2.5 0.01–3.1 0.002–8.1 0.010–1

0.061 0.055 0.023 0.053 0.07 0.015 2.0 1.3 0.1 1.0

This work This work This work This work [15] [36] [37] [14] [13] [12]

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Fig. 9. Amperometric i-t curve responses obtained for Cu2O nanospheres (black line), Cu2S (red line), Cu7S4 (blue line), and CuS (magenta line) hollow nanospheres on GCE in 0.1 M NaOH and in presence of glucose (1.0 mM) as well as interfering compounds such as AA (0.1 mM), UA (0.1 mM), and NaCl (0.1 mM) at an applied potential +0.50 V vs. Ag/AgCl. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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