A controllable honeycomb-like amorphous cobalt sulfide architecture directly grown on the reduced graphene oxide–poly(3,4-ethylenedioxythiophene) composite through electrodeposition for non-enzyme glucose sensing

Alan Meng ab, Liying Sheng b, Kun Zhao a and Zhenjiang Li *b
aState Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, Shandong, P. R. China
bKey Laboratory of Polymer Material Advanced Manufacturing Technology of Shandong Provincial, College of Electromechanical Engineering, College of Sino-German Science and Technology, Qingdao University of Science and Technology, Qingdao 266061, Shandong, P. R. China. E-mail: zhenjiangli@qust.edu.cn; Fax: +86 532 88956228; Tel: +86 532 88956228

Received 15th September 2017 , Accepted 23rd October 2017

First published on 23rd October 2017

A facile, controllable two-step electrodeposition synthesis route was developed, whereby a honeycomb-like amorphous cobalt sulfide architecture was obtained via direct growth on a glassy carbon electrode (GCE) functionalized by a reduced graphene oxide–poly(3,4-ethylenedioxythiophene) (rGO–PEDOT) composite film as an electrode for glucose detection. This electrodeposition method is binder-free, rapid, low-cost and preparation-controlled. The effects of the concentration ratio between CoCl2·6H2O and thiourea, deposition scanning rate and deposition cycles on glucose detection were investigated, and the optimum preparation conditions were determined. The characterization results indicated that the honeycomb-like cobalt sulfide architecture was formed by growing vertically amorphous CoxSy nanosheets with a thickness of about 20–50 nm on the rGO–PEDOT surface, and the morphology of cobalt sulfide could be controlled by regulating the deposition cycles. Under optimal conditions, the sensor exhibited a wide linear range from 0.2 to 1380 μM (R2 = 0.9976), a sensitivity of 113.46 μA mM−1 cm−2, a low detection limit of 0.079 μM and a response time of 3 s. This sensor also displayed good selectivity, reproducibility and repeatability for non-enzyme glucose sensing. More importantly, the sensor was successfully used to determine glucose in human blood serum samples, and the results were consistent with hospital test results.

1. Introduction

Diabetes is a common metabolic endocrine disease that frequently occurs worldwide.1 The disease is characterized by long-term symptoms, such as high blood glucose and results in chronic injury and dysfunction of various tissues, particularly the eyes, kidneys, heart, blood vessels and nerves. The abovementioned problems have attracted significant attention, and the World Health Day 2016 entitled “Beat diabetes” was devoted to this disease. As is well known, the blood sugar concentration or blood glucose level is the amount of glucose present in the blood of a human or animal.2 Therefore, it is of great importance that glucose is detected rapidly, accurately and innocuously. Since Clark and Lyons researched a glucose biosensor by capsulating glucose oxidase,3 several studies regarding enzymatic glucose sensors have been reported due to its high selectivity and good sensitivity.4–8 Another type of sensor, the non-enzymatic glucose sensor, has attracted significant attention in recent years due to its many advantages such as simplicity, reproducibility, and good stability; additionally, it is free from oxygen limitation.9,10 To date, various materials have been applied in non-enzyme glucose sensors, such as carbon materials (carbon nano tubes, rGO11), polymers (PEDOT12), and metal and metallic compound nanomaterials (Au,13 Ni,14 CoOOH,9 CoOx,10 Cu2O,15 Co3O4[thin space (1/6-em)]16). For the past few years, researchers have used cheap and accessible transition metal sulphides, including CuS,17 Cu2S,18 Ni3S2,19etc. to manufacture sensors with outstanding glucose detection performance. Cobalt sulfide, as a type of functional transition metal sulfide, has been applied in the fields of electrocatalysis,20 supercapacitors,21 and dye-sensitized solar cells.22 However, the application of cobalt sulfide in non-enzyme glucose sensors has rarely been reported. Qu et al. has reported that crystalline CoS was decorated on a carbon skeleton via a solvothermal method and used to prepare the non-enzymatic glucose sensor.23 Wu et al. reported that tremella-like crystalline CoS had catalytic activity towards hydrogen peroxide and glucose.24 However, the aforementioned two methods for constructing sensors usually include two steps: crystalline cobalt sulfide is first prepared by a hydrothermal or solvothermal method and then dropped on the electrode in the second step. This might introduce complexity and uncontrollability in the preparation process, morphological variation of the electrode materials and weak adhesion strength between the material and the electrode. Therefore, it is necessary to develop a facile, rapid, binder-free and controllable preparation strategy for synthesizing a new electrode material based on cobalt sulfide with unique architecture, larger surface area, robust adhesion between electrode materials and good performance for glucose detection.

To further improve the electrochemical performance of cobalt sulfide, researchers usually adopt the cobalt sulfide composite. For instance, cobalt sulfide was composited with TiO2,25 N and S co-doped graphene oxide,26 rGO,27 graphene28,29 and so forth. Among the above, graphene, a two-dimensional layer of covalently bonded carbon atoms,30 is a potential modifier in the field of modified electrodes for sensors because of its large specific surface area, excellent electro-conductivity, good biological compatibility and so on. Many materials, such as metal nanoparticles,31–36 metal oxides,37–41 and metal hydroxide, have been integrated with rGO to improve their detection performance.42 Among these materials composited with graphene, conductive polymers, such as polyaniline43 and PEDOT, are commonly applied.43,44 PEDOT, as one conducting polymer material, has low oxidation potential and moderate bandgap with good stability, high electrical conductivity and excellent thermal stability. Due to the superior performances of rGO and PEDOT, it is foreseeable that they can be simultaneously deposited on bare GCE as a composite film to improve sensor performance. Therefore, it is of interest to develop a simple, low cost and controllable method to fabricate a sensor constructed of three materials based on cobalt sulfide, rGO and PEDOT to obtain outstanding electrical conductivity, controllable unique morphology and high glucose sensing performance.

In this study, electrodeposition was first used to obtain a GCE functionalized by a rGO–PEDOT composite film; then, the honeycomb-like amorphous cobalt sulfide architecture was electrodeposited on the rGO–PEDOT surface. The rGO–PEDOT composite film with rough surfaces could provide a large surface area for the growth of honeycomb-like amorphous cobalt sulfide architecture, offering more active sites for the catalytic oxidation of glucose. The honeycomb-like amorphous cobalt sulfide architecture is composed of vertically crossed cobalt sulfide nanosheets, which can be controlled by adjusting the deposition cycles. This preparation method is simple, binder-free, steerable and rapid. The composite sensor has a relatively wide detection range, low detection limit (LOD), good selectivity, reproducibility and repeatability and can potentially be used to detect glucose in human blood serum samples. This study not only obtains a non-enzyme glucose sensor with good detection performance, but also provides a salutary reference for the design of other material systems with controllable morphology and outlines a preparation method for novel design sensors.

2. Experimental

2.1. Materials

Graphene oxide was purchased from Nanjing Xian Feng Nanomaterials Technology Co., Ltd. Moreover, 3,4-ethylenedioxythiophene (EDOT) and D(+)-glucose were purchased from Sigma-Aldrich. Sulfourea (TU), sodium hydroxide (NaOH) and cobalt(II) chloride 6-hydrate were purchased from Sinopharm Chemical Reagent Co., Ltd. Dopamine (DA, >98%), L-ascorbic acid (AA > 99%) and uric acid (UA, >99%) were purchased from Sangon-Biotech Co., Ltd. All other chemicals were of analytical-reagent grade and used without further purification. Ultrapure water was used throughout the experiments. At the same time, human serum samples were provided by Qingdao center medical group and the detection of glucose in serum stored at 4 °C was completed within a week.

2.2. Preparation of CoxSy/rGO–PEDOT modified electrode

GCE (3 mm in diameter) was polished by a 0.3 and 0.05 μm alumina slurry, successively, and then ultrasonically washed in ultrapure water and anhydrous ethanol for 3 min, respectively. At room temperature, the deposition potential was between −1.5 and 1.1 V at a scan rate of 100 mV s−1. The rGO–PEDOT composite film was electrodeposited on the pretreated GCE in a 5.0 mL mixed solution containing 2.0 mg mL−1 GO and 0.02 M EDOT monomer using cyclic voltammetry (CV) for 15 cycles.

The electrochemical deposition of cobalt sulfide was carried out in an aqueous solution with different concentration ratios of CoCl2·6H2O and thiourea by cyclic voltammetry (CV) within a potential range from −1.2 V to 0.2 V (vs. SCE) at a scan rate of 5 mV s−1. Moreover, rGO–PEDOT/GCE and CoxSy/GCE were also prepared under the same condition as those of the contrast experiments. The obtained modified electrode was warily rinsed with deionized water and air dried at room temperature.

2.3. Apparatus of material characterization and electrochemical measurements

The morphology of the samples was characterized by scanning electron microscopy (SEM) at the accelerating voltage of 20 kV (SEM, JEOL JSM-6700F). Energy dispersive spectroscopy (EDS) was carried out using an SEM system equipped with an energy dispersive X-ray analyzer. Transmission electron microscopy (TEM, JEOL JEM-200EX) measurements were performed with the accelerating voltage of 200 kV. The powder X-ray diffraction (XRD) pattern was recorded with Cu Kα radiation (k = 1.542 Å) over the 2θ range of 10–80° using an Advance D8 X-ray diffractometer. An X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250XI) with an X-ray source of Mg Kα was used to further analyze the chemical compositions in the CoxSy/rGO–PEDOT composite. Fourier transform infrared (FTIR) spectra were characterized using a Nicolet 510P Fourier transform infrared spectrometer.

All electrochemical measurements were carried out on a CHI660E electrochemical analyzer (Shanghai CH Instrument Company, China). Electrochemical impedance spectroscopy (EIS) measurements were recorded in a frequency range of 0.1–105 Hz in a 5.0 mM [Fe(CN)6]3−/4− solution containing 0.1 M KCl. The amplitude of the applied sine wave was 5 mV with the direct current potential set at 0.2 V. The cyclic voltammetry curve (CV) of glucose detection was tested in a 0.1 M NaOH solution with a scanning range from −0.4 to 0.8 V at a scan rate of 100 mV s−1. The current response was obtained by adding glucose with a specific concentration at a constant potential of +0.65 V. A saturated calomel electrode (SCE) as the reference electrode, a modified GCE as the working electrode and a platinum wire as the counter electrode were applied in a three-electrode system.

3. Results and discussion

3.1. Optimization of the conditions for constructing CoxSy/rGO–PEDOT/GCE

To obtain superior sensor performance, the effects of the concentration ratio between CoCl2·6H2O and thiourea and the deposition scanning rate on the detection performance of CoxSy/rGO–PEDOT/GCE towards glucose were first investigated. The CV responses of CoxSy/rGO–PEDOT/GCE fabricated under different concentration ratios and scan rates towards glucose are shown in Fig. S1 and S2 (ESI), respectively. From the above data, the change in response differentials (ΔCurrent) of the electrodes prepared under different CoCl2·6H2O[thin space (1/6-em)]:[thin space (1/6-em)]thiourea concentration ratios and different scan rates was obtained, and the results are shown in Fig. 1. We found that the optimum concentration ratio and deposition scan rate was 1[thin space (1/6-em)]:[thin space (1/6-em)]20 (Fig. 1a) and 5 mV s−1 (Fig. 1b), respectively, which were taken as the optimum parameters in subsequent experiments.
image file: c7tb02482g-f1.tif
Fig. 1 Effects of the concentration ratio between CoCl2·6H2O and thiourea (a) and deposition scanning rate (b) on the oxidation current response differentials of CoxSy/rGO–PEDOT/GCE towards glucose.

It is well known that the morphology and performance of the material prepared by electrodeposition rely mainly on the deposition cycles, which may affect its thickness and compactness, and ultimately its detection performance. Therefore, the deposition cycle of cobalt sulfide is considered a critical factor. Fig. 2a shows the CVs of CoxSy/rGO–PEDOT/GCE prepared under different deposition cycles in 0.1 M NaOH without and with 1.0 mM glucose at a scan rate of 100 mV s−1. Fig. 2b reveals the ΔCurrent of the electrodes prepared under different deposition cycles of cobalt sulfide. Based on the results of Fig. 2, it was observed that the current response increased correspondingly with an increase in electrodeposition cycles from 1 to 3 cycles and then decreased beyond 3 cycles. The optimized number of electrodeposition cycles is 3 cycles.

image file: c7tb02482g-f2.tif
Fig. 2 CVs of different deposition cycles of cobalt sulfide nanosheets modified on rGO–PEDOT/GCE in 0.1 M NaOH without and with 1.0 mM glucose at a scan rate of 100 mV s−1 (a). Effect of the deposition cycles of cobalt sulfide nanosheets on the oxidation current response differentials of CoxSy/rGO–PEDOT/GCE towards glucose (b).

To further reveal the influence of the deposition cycles on CoxSy/rGO–PEDOT/GCE towards glucose, the effect of deposition cycles on the morphology of cobalt sulfide grown on GCE functionalized by rGO–PEDOT was investigated, which is illustrated in Fig. 3. As is shown, only a small quantity of cobalt sulfide was deposited on the surface of rGO–PEDOT with 1 deposition cycle (Fig. 3a); some wrinkled structures of rGO–PEDOT could still be observed. When the deposition cycle was increased to 2 (Fig. 3b), the structure of the honeycomb-like architecture was found to be preliminarily formed, but the quantity of nanosheets was insufficient, which should cause a lower catalytic activity of CoxSy/rGO–PEDOT/GCE towards glucose. As the deposition cycle of cobalt sulfide increased to 3 cycles (Fig. 3c), we could clearly observe the honeycomb-like amorphous cobalt sulfide architecture, which might provide larger effective contact sites towards glucose, and thus improve the catalytic oxidation of glucose. When the deposition cycle was increased to 4 cycles (Fig. 3d), it is probable that a large amount of cobalt sulfide accumulated on the surface of the honeycomb-like amorphous cobalt sulfide architecture, which would make the honeycomb-like holes disappear and reduce the specific surface. It would reduce the effective contact sites of cobalt sulfide towards glucose, leading to a negative effect on the glucose detection performance. The electrocatalytic performance of CoxSy/rGO–PEDOT/GCE towards glucose was determined by the morphology and quantity of CoxSy. When the deposition cycle was 3 cycles, CoxSy possessed optimal morphology and suitable quantity. Therefore, CoxSy/rGO–PEDOT/GCE exhibited better electrocatalytic performance towards glucose.

image file: c7tb02482g-f3.tif
Fig. 3 Effect of deposition cycles (a: 1 cycle, b: 2 cycle, c: 3 cycle, d: 4 cycle) on the morphology of cobalt sulfide architecture grown on GCE functionalized by rGO–PEDOT.

Above all, a deposition cycle of 3, a concentration ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]20 and a deposition scanning rate of 5 mV s−1 were the optimal preparation conditions.

A construction process of CoxSy, directly grown on GCE functionalized by rGO–PEDOT, is illustrated in Scheme 1. As seen in Scheme 1, CoxSy/rGO–PEDOT/GCE was fabricated through a two-step electrodeposition process. First, rGO–PEDOT composite film was directly grown on GCE by cyclic voltammetry. In the second step, the as-prepared rGO–PEDOT/GCE was immersed into an aqueous solution containing CoCl2 and TU; then, Co2+ reacted with TU to form CoxSy, which was mutually interconnected and uniformly grown on the surfaces of rGO–PEDOT/GCE to produce a well-defined honeycomb-like amorphous architecture. The reaction equation can be expressed as follows:

Co2+ + 2TU → Co(TU)22+(1)
Co(TU)22+ + 2e → CoS(2)

image file: c7tb02482g-s1.tif
Scheme 1 Schematic of the construction process of CoxSy/rGO–PEDOT/GCE.

Based on the aforementioned analysis, the novel CoxSy/rGO–PEDOT nanostructure could be acquired.

3.2. Characterization of CoxSy/rGO–PEDOT/GCE

A typical SEM image of the rGO–PEDOT composite film is shown in Fig. 4a. The rGO–PEDOT composite film displayed a rough surface with many wrinkles, which would be of greater benefit to the deposition of cobalt sulfide. Fig. 4b shows the FTIR spectra of GO and the rGO–PEDOT composite film. A broad peak located at 3386 cm−1 and some other characteristic peaks located at 1056, 1224, 1400 and 1728 cm−1 were attributed to the −OH stretching mode, C–O (alkoxy) stretching, C–O–H (epoxy) stretching, O–H bending and C[double bond, length as m-dash]O stretching from GO, respectively.45 After electrodeposition, rGO–PEDOT showed that the intensity of all absorption peaks of oxygen functional groups (C–O (alkoxy) stretching, C–O–H (epoxy) stretching, O–H bending and C[double bond, length as m-dash]O stretching) decreased, which proved that rGO was formed.44,45 The stretching of C–S in the thiophene rings was observed at 687 cm−1, 842 cm−1 and 984 cm−1, while the stretching of C–C and C[double bond, length as m-dash]C was observed at 1315 cm−1 and 1518 cm−1, respectively, proving the existence of PEDOT in the composite film.45 The aforementioned results indicated that the rGO–PEDOT composite film was obtained in the electrodeposition process. Under optimum conditions, the morphology of the as-prepared cobalt sulfide grown on the rGO–PEDOT surface is shown in Fig. 4c. As can be seen, the structure of cobalt sulfide has a honeycomb-like architecture. The high magnification SEM image is displayed in Fig. 4d. It was found that the thickness of cobalt sulfide nanosheets was about 20–50 nm. Because of the unique structure of the honeycomb-like amorphous cobalt sulfide, it possessed a larger surface area, which could provide more active sites for glucose oxidization, improving the detection performance of this sensor. Furthermore, Fig. 4e and f present energy dispersive spectroscopy (EDS) and elemental mapping images, respectively. They were applied to verify the components of CoxSy/rGO–PEDOT. Co, S, C and O, were found to be present, which confirmed the presence of CoxSy grown on the surface of the rGO–PEDOT composite film.
image file: c7tb02482g-f4.tif
Fig. 4 SEM image of rGO–PEDOT (a), FTIR spectra of GO and rGO–PEDOT (b), SEM image of CoxSy nanosheets grown on the rGO–PEDOT surface (c), high magnification SEM images of CoxSy nanosheets (d), EDS spectra of CoxSy/rGO–PEDOT (e) and CoxSy/rGO–PEDOT nanocomposites elemental mapping images of C, O, Co and S (f).

Fig. 5a shows a typical TEM image of the isolated cobalt sulfide nanosheet grown on the rGO–PEDOT film surface. As can be seen, there are some steps on the sides of the cobalt sulfide nanosheet, indicating that the cobalt sulfide nanosheets were composed of many thinner pieces. Furthermore, the upper left corner of Fig. 5a depicts the selected area electron diffraction (SAED) pattern. It was revealed that the product was amorphous. Fig. 5b presents the XRD pattern of the CoxSy/rGO–PEDOT composite. Clearly, no indicative peaks of the crystalline cobalt sulfide could be observed. A broad diffraction peak at around 25° was observed, which maybe attributed to the amorphous cobalt sulfide and the C(002) from PEDOT and rGO.46,47 In addition, the characteristic peak of GO around 2θ = 10° was not observed, indicating GO was reduced rGO.11 It further demonstrates that GO was reduced to rGO, which is consistent with the characterization results of the FTIR spectra. In summary, the above characterization results showed that GO was reduced to rGO during the deposition of the composite films, and cobalt sulfide was acquired in an amorphous form. Hence, cobalt sulfides prepared by this method are named CoxSy due to their amorphous characteristic and various potential chemical constituents.21,48,49

image file: c7tb02482g-f5.tif
Fig. 5 TEM image of CoxSy grown on the rGO–PEDOT film surface (a) (inset: SAED pattern for CoxSy), XRD pattern of the CoxSy/rGO–PEDOT composite (b).

To further confirm the more detailed elemental composition of the CoxSy/rGO–PEDOT composite, the product was investigated using an X-ray photoelectron spectrometer (XPS). As can be seen from Fig. 6a, Co, O, C and S could be detected from the CoxSy/rGO–PEDOT composite. The high resolution Co 2p spectrum showed two major peaks centered at 780.7 eV and 797.2 eV in Fig. 6b, which was attributed to Co2p3/2 and Co2p1/2, respectively. A minor peak at 786.0 eV can be assigned to the Co2p3/2 characteristic peaks of cobalt sulfide. In addition, the difference in binding energy between Co2p3/2 and Co2p1/2 is more than 15 eV, which demonstrates the uncertain chemical constituents of Co and S.46 The results further confirmed that cobalt sulfide prepared in this study was amorphous. Fig. 6c shows the high-resolution S 2p spectrum. Two major peaks located at the binding energies of 163.8 and 165.0 eV are attributed to the spin–orbit couple S 2p3/2 and S 2p1/2, respectively. The characteristic peak at 163.8 eV refers to the S–Co bond, which originates from cobalt sulfide. The high-resolution C1s spectrum of the CoxSy/rGO–PEDOT composite is displayed in Fig. 6d. The peaks around 284.7 eV, 285.7 eV and 286.4 eV are ascribed to the sp2 C–C bond, C–S bond and the C–O bond, respectively, which originated from the thiophene ring in PEDOT. Furthermore, two major peaks were observed at 288.4 eV and 289.3 eV, corresponding to the weak C[double bond, length as m-dash]O bond and O–C[double bond, length as m-dash]O bond in rGO, respectively. The existence of a C–O bond (533.0 eV), a weak C[double bond, length as m-dash]O (531.6 eV) bond and a O–C[double bond, length as m-dash]O bond (530.6 eV) could be discerned from the spectrum of O1s (Fig. 6e). These results further proved that GO was reduced to rGO in the process of electrochemical deposition.

image file: c7tb02482g-f6.tif
Fig. 6 XPS full survey (a) and high resolution XPS spectra of Co 2p (b), S 2p (c), C 1s (d), and O 1s (e) for the CoxSy/rGO–PEDOT composite, (f) Raman spectrum of rGO–PEDOT.

In addition, to further demonstrate that GO was reduced to rGO in the electrodeposition process, the Raman spectrum of rGO–PEDOT was acquired and presented in Fig. 6f. As is clearly observed from Fig. 6f, there are peaks located at 1367 and 1582 cm−1, which were attributed to the D band and G band, respectively. Moreover, the intensity of D band was stronger than that of G band, further indicating that GO was reduced to rGO. Additionally, the peaks around 1432 and 1513 cm−1 were attributed to symmetric Cα[double bond, length as m-dash]Cβ (–O) stretching and asymmetric C[double bond, length as m-dash]C stretching in PEDOT, respectively. The peak observed at 990 cm−1 was ascribed to the oxyethylene ring deformation in PEDOT. The peak located at 1270 cm−1 was due to Cα–Cα inter-ring stretching. The abovementioned results are highly consistent with the results of FTIR.

Fig. 7a depicts Nyquist plots of bare GCE, CoxSy/GCE, rGO–PEDOT/GCE and CoxSy/rGO–PEDOT/GCE. The electron transfer resistance (Ret) value of bare GCE was 238 Ω. The Ret value was decreased to 95 Ω, when CoxSy was modified onto bare GCE. Compared with that of the above two electrodes, the Ret value of rGO–PEDOT/GCE was the smallest. This may be attributed to the large specific surface area and excellent conductivity of the rGO–PEDOT composite film, which promotes electronic transmission. Furthermore, the Ret value of CoxSy/rGO–PEDOT/GCE was 75 Ω, which was smaller than that of CoxSy/GCE. The possible reason was as follows: on one hand, it could be confirmed that the rGO–PEDOT composite film played an important role in electron transmission. On the other hand, the honeycomb-like amorphous CoxSy was grown directly on the surface of the rGO–PEDOT composite film by electrodeposition, which improved the adhesion strength between CoxSy and the rGO–PEDOT composite film, promoting its conductivity and the glucose detection performance of the modified electrode.

image file: c7tb02482g-f7.tif
Fig. 7 Nyquist plots of bare GCE, rGO–PEDOT/GCE, CoxSy/GCE and CoxSy/rGO–PEDOT/GCE in a 5 mM [Fe(CN)6]3−/4−, 0.1 M KCl solution with the frequency range from 0.1 to 105 Hz (a). CVs of the GCE electrode modified by different materials in NaOH (b): a, b-bare GCE, c, d-rGO–PEDOT/GCE, e, f-CoxSy/GCE, g, h-CoxSy/rGO–PEDOT/GCE, a, c, e, g-without 1 mM glucose, b, d, f, h-with 1 mM glucose (potential: −0.4–0.8 V, v: 100 mV s−1).

CVs of CoxSy/rGO–PEDOT/GCE in 0.1 M NaOH containing various concentrations of glucose (0, 0.5, 1.0, 2.0 and 3.0 mM) are shown in Fig. S3 (ESI), displaying the electrocatalytic behavior of CoxSy/rGO–PEDOT/GCE towards glucose with different concentrations. With the increase of glucose concentration, the current response gradually increased, which indicated that the as-prepared CoxSy/rGO–PEDOT/GCE could be used for non-enzymatic electrochemical detection of glucose.

Fig. 7b shows CVs of bare GCE (a, b), rGO–PEDOT/GCE (c, d), CoxSy/GCE (e, f) and CoxSy/rGO–PEDOT/GCE (g, h) in 0.1 M NaOH solution without and with 1 mM glucose. It is evident that the current responses of bare GCE and rGO–PEDOT/GCE towards glucose were very low. When the electrode was modified by the honeycomb-like amorphous CoxSy, the CV revealed a high current response towards glucose in NaOH solution. When CoxSy/rGO–PEDOT was modified into bare GCE, the current response towards glucose was most obvious in NaOH solution, with potentials positioned at around 0.5–0.7 V; this indicates that CoxSy/rGO–PEDOT possessed excellent electro-catalytic activity for glucose detection. The reason for this result is as follows: on one hand, the rGO–PEDOT composite film was deposited on GCE, which promoted the strong combination between the rGO–PEDOT composite film and GCE. The rGO–PEDOT composite film with its rough surface not only possessed high conductivity, promoting the electronic transmission, but also could provide numerous nucleation sites for CoxSy growth. On the other hand, a honeycomb-like amorphous cobalt sulfide architecture was grown directly on the surface of the rGO–PEDOT composite film by electrodeposition, promoting strong adhesion strength between CoxSy and rGO–PEDOT, which further improved the electrical conductivity. In addition, honeycomb-like amorphous CoxSy can offer more active sites for the catalytic oxidation of glucose, promoting the detection of glucose. The synergistic effect of these various factors improves the performance of this non-enzyme glucose sensor.

The catalytic oxidation mechanism of CoxSy as an active substance can be shown by the following reactions:23

Co(II) + OH → Co(III)–(OH) + e(3)
Co(III) + OH → Co(IV)–(OH) + e(4)
Co(IV)–(OH) + glucose → Co(III) + OH + gluconolactone(5)

CVs of CoxSy/rGO–PEDOT/GCE at various scan rates in 0.1 M NaOH with 1 mM glucose were investigated, and the result is shown in Fig. S4a (ESI). It was found that the catalytic oxidation of glucose was a diffusion-controlled process (correlation coefficient, R2 = 0.9997 and 0.9976, respectively). In addition, the applied potential was optimized because it strongly affected the current response of the sensor, and the result is shown in Fig. S4b (ESI). From Fig. S4b (ESI), the current response reached the maximum value when the applied potential was 0.65 V. Therefore, 0.65 V was adopted as the optimum applied potential for the detection of glucose in the following experiments.

3.3. Amperometric determination of glucose

Fig. 8a shows the response of CoxSy/rGO–PEDOT/GCE to different concentrations of glucose added successively into NaOH solution under stirring. 95% of the steady-state current response time of this sensor was obtained within three seconds (right inset of Fig. 8a), proving a faster response to glucose. It was observed that the current responses of catalyzing glucose are linear in the glucose concentration range from 0.2 μM to 1380 μM (R2 = 0.9976), with a sensitivity of 113.46 μA mM−1 cm−2 and a detection limit (LOD) of 0.079 μM at a signal-to-noise of 3. The relatively low LOD may be due to the synergistic effect of CoxSy and the rGO–PEDOT composite film. The rGO–PEDOT composite film possessed high conductivity and CoxSy with a honeycomb-like architecture provided a high catalytic performance towards glucose. The current responses reached stabilization when the concentration of glucose exceeded 1380 μM. The comparison of CoxSy/rGO–PEDOT/GCE with other non-enzyme glucose sensors is shown in Table 1. CoxSy/rGO–PEDOT/GCE has good comprehensive performance compared with other reported glucose sensors, such as a relatively wide detection range, a low detection limit and so on.
image file: c7tb02482g-f8.tif
Fig. 8 Amperometric current response (a) of CoxSy/rGO–PEDOT/GCE to glucose with the concentrations of 0.2, 0.5, 1, 2, 5, 20, 40, 80, 140, 210, 290, 380, 480, 680, 980, 1380, 1980, 2780, 3780, 4980, 6380, 7980 μM added successively into a stirred 0.1 M NaOH solution (inset, left corner: magnified portion of the amperometric response curve of the glucose sensor; right corner: magnified portion of typical response time), respectively, at the applied potential of 0.65 V. The calibration curve for current vs. concentration of glucose (b).
Table 1 Comparison of non-enzyme glucose sensing performances based on various nanomaterials
Materials Response time (s) Linear range (mM) LOD (μM) Sensitivity (μA mM−1 cm−2) Ref.
CoOOH nanosheets/Co substrate <4 up to 0.5 10.9 967 9
CoOxNPs/ERGO <5 0.01 to 0.55 2 79.3 10
Cu2O/GNs <9 0.3 to 3.3 3.3 15
CoS/3D porous carbon skeleton 0.01 to 0.9 2 679 23
Co3O4 nanofibers <7 up to 2.04 0.97 36.25 39
Ni(OH)2–RGO <7 0.002 to 3.1 600 11.43 42
PVdF–HFP/Ni/Co <10 0.001 to 7.0 0.26 7.56 50
CoxSy/rGO–PEDOT/GCE <3 0.0002 to 1.38 0.079 113.46 This work

3.4. Interference, repeatability and reproducibility test

In practice, dopamine (DA), uric acid (UA) and ascorbic acid (AA) generally co-exist with glucose in human serum. Therefore, it is quite significant whether CoxSy/rGO–PEDOT/GCE can resist the interference of DA, UA and AA. Fig. 9 shows that the current response increases significantly when 0.5 mM glucose is added into the substrate solution. No obvious current responses were observed when UA, DA, AA were added into the solution, successively. Compared to 0.5 mM glucose, the RSD of the current responses from interference species were 2.65% (DA), 1.35% (UA) and 1.11% (AA), respectively. In addition, the interferences of lactose, fructose and maltose to glucose were also investigated and the results are shown in Fig. S5 (ESI). The results indicate that this sensor has good anti-interference property.
image file: c7tb02482g-f9.tif
Fig. 9 Current results from an it test of CoxSy/rGO–PEDOT/GCE with various interfering agents.

Repeatability is an important criterion for assessing the performance of a sensor. Therefore, the repeatability was examined using the new CoxSy/rGO–PEDOT/GCE to detect 1 mM glucose six consecutive times, which is shown in Fig. S6a (ESI). An RSD of 2.75% (n = 6) was acquired, revealing excellent repeatability for the detection of glucose. Moreover, 5 new non-enzyme sensors modified with the same CoxSy/rGO–PEDOT nanocomposites were used to detect 1 mM glucose. The results are shown in Fig. S6b (ESI). The RSD was 8.17% (n = 5), confirming the excellent reproducibility for the detection of glucose.

3.5. Detection of glucose in human blood serum samples

To investigate the application of this sensor, CoxSy/rGO–PEDOT/GCE was used to detect glucose in human blood serum samples. The human blood serum samples, with glucose concentrations of 3.15 mM, 6.3 mM, 9.5 mM and 12.6 mM, were provided by the Qingdao center medical group. Furthermore, 40 μL blood samples with different concentrations were injected into 5 mL NaOH solutions under stirring. The amperometric responses of the sensor to glucose in different serum samples were recorded at an applied potential of 0.65 V.

The results are shown in Table 2, the RSDs are less than 4.60% for each serum sample, and the relative deviation for each sample is less than 5.7%. All the above results indicate that this non-enzyme sensor has promising potential in estimating the glucose concentration in human serum.

Table 2 Comparison between the data obtained in hospital and those obtained by using our sensor for the determination of glucose in serum samples
Samples C G determined in the hospital (mM) C G determined by our sensor (mM) RSDb (%) Relative deviation (%)
a Glucose concentration in serum samples. b RSD calculated from three parallel tests using our sensor.
1 3.15 3.33 4.60 5.7
2 6.3 6.26 3.41 −0.63
3 9.45 9.44 1.72 −0.11
4 12.6 12.47 3.87 −1.03

4. Conclusions

In summary, an rGO–PEDOT composite film was first electrodeposited on GCE, and then a honeycomb-like amorphous cobalt sulfide architecture was directly grown on rGO–PEDOT/GCE. By controlling the deposition cycles, the morphology of cobalt sulfide nanosheets was controlled. Under optimum preparation conditions, the honeycomb-like amorphous cobalt sulfide architecture consisted of 20–50 nm amorphous cobalt sulfide nanosheets. The reasonable design of the electrode material systems and the electrodeposition method also resulted in strong adhesion strength in CoxSy, rGO–PEDOT and electrode, which led to an excellent detection performance towards detecting glucose. It exhibits a low detection limit (0.079 μM), a relatively wide detection range (0.2 μM to 1380 μM), a rapid response (<3 s), and high sensitivity (113.46 μA mM−1 cm−2). Moreover, it also had good reproducibility, repeatability and good selectivity. Furthermore, the sensor was successfully applied to detect glucose in human serum samples. All these results suggest that the controllable honeycomb-like amorphous cobalt sulfide architecture grown on the rGO–PEDOT composite film is a promising electrode material for non-enzymatic glucose sensors with high performance and practical value.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the National Natural Science Foundation of China under Grant No. 51672144, 51572137, 51502149 and 51702181, the Natural Science Foundation of Shandong Province under Grant No. ZR2016EMB25, ZR2017PEM006 and ZR201702210482, the Higher Educational Science and Technology Program of Shandong Province under Grant No. J16LA10 and J17KA014, the Application Foundation Research Program of Qingdao under Grant No. 15-9-1-28-jch, the Taishan Scholars Program of Shandong Province under No. ts201511034 and the Qingdao center medical group. We express our gratitude to them for their financial support.


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Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tb02482g

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