Rapid and scalable route to CuS biosensors: a microwave-assisted Cu-complex transformation into CuS nanotubes for ultrasensitive nonenzymatic glucose sensor

Abstract

We report the rapid, scalable and economical strategy—microwave-assisted Cu-complex transformation route to fabricate CuS nanotubes with high quality based on a preferential chemical transformation and crystallization process. With the Cu-complex Cu(TU)Cl·0.5H₂O (TU = thiourea) nanowires formed in ambient conditions as self-sacrificed template, uniform CuS nanotubes with rectangular cross-sections were synthesized in high yield. The diameter of these CuS nanotubes with rectangular cross-sections can be effectively tuned through changing chemical reaction parameters, such as reaction time, temperature, and solvent. Furthermore, uniform cone-like CuS nanotubes were also successfully prepared by adopting Cu-EBT (EBT = eriochrome black T) complex nanorods as precursors. These nanoporous CuS nanotubes with rectangular cross-sections have been confirmed to be very effective as nonenzymatic glucose sensors. The CuS nanotube-modified electrode shows excellent electrocatalytic activity towards the oxidation of glucose and good linear dependence and high sensitively to glucose concentration change.

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Introduction

Glucose detection is extremely important in clinical diagnosis, food analysis, and bioreactor monitoring.¹ Diabetes is a common chronic disease, and its clinical detection and therapy is one of the major health care problems affecting millions of people worldwide.² Some effective noninvasive technologies for glucose detection have been developed.^3,4 Electrochemical glucose sensors, especially amperometric biosensors, hold a leading position among various biosensors. The majority of known amperometric biosensors for glucose monitoring involve the use of the enzyme glucose oxidase, which catalyzes the oxidation of glucose to gluconolactone.^3,4 Although enzymatic detection usually exhibits high performance, the enzyme is easily denatured during the immobilization procedure.⁵ The most common and serious problem with enzymatic glucose biosensors lies in their lack of long-term stability as the enzyme can be easily affected by temperature, pH value, humidity and toxic chemicals.⁵ In order to overcome the drawbacks of enzymatic glucose sensors, nonenzymatic glucose sensors have also been designed and fabricated.^6–8

As a p-type semiconductor metal chalcogenide, CuS is one of the most intensively studied materials owing to its technologically important applications in the field of photothermal conversion, solar cell devices, super ionic materials, optical filters, gas sensors, and Li-ion batteries.^9–12 Recently, CuS has been found to show interesting properties including metal-like electrical conductivity, which may have potential application as an electrochemical sensor.^13–15 To date, various nanostructured CuS, such as zero-dimensional (OD) quantum dots, one-dimensional (1D) nanorods, nanowires, and nanotubes, two-dimensional (2D) nanodisks, and three-dimensional (3D) hierarchical doughnuts have been prepared.^16–27 Hollow structured nanomaterials (e.g.nanotubes and nanocapsules) have been attracting increasing attention because of their widespread applications in catalysis, drug delivery, energy-storage devices and optoelectronic sensors.^28–34 There are even several methods reported for the synthesis of hollow structured CuS tubes,^14,19,23,25 however, these syntheses are commonly tedious and time-consuming, and only sub-gram quantities of products can be produced. Furthermore, the conventionally prepared tubular CuS nanostructures have fewer electron transfer passages resulting in low sensitivity and so are limited in their application as biosensors. An economical mass-production method needs to be developed for the practical application of these functional nanotubes. Herein, we reported a general, rapid, scalable and economical strategy (microwave-assisted metal-complex transformation route) to fabricate CuS nanotubes with high quality based on a preferential chemical transformation and crystallization process with the assistance of diethanolamine. Nanoporous tubular CuS obtained by the rapid microwave-assisted transformation route, which possess porous exteriors (shells) and hollow interiors, was used for the fabrication of nonenzymatic glucose biosensors and exhibited high performance.

Experimental section

Materials synthesis

Cu(TU)Cl·0.5H₂O complex nanowires were prepared by a simple solution process at room temperature (TU = thiourea). In a typical experimental procedure for fabricating ultralong nanowires, 0.2 mol L⁻¹ aqueous solution of TU was mixed with 0.1 mol L⁻¹ aqueous solution of CuCl₂ at room temperature without stirring. The mixed solution immediately turned to blue and gradually to green and finally to colourless. White complex nuclei were immediately formed after ∼5 s. The reaction took about 40 s under ambient conditions. After filtration and washing with absolute ethanol and distilled water several times. The white Cu(TU)Cl·0.5H₂O complex nanowires were transferred to a solution with appropriate amount of diethanolamine. The suspension was microwave-heated (model XH-100B from Beijing Xianghu Science and Technology Development Co., LTD) to 90 °C (taking several seconds) at a power setting of 600 W. The solution was maintained at 90 °C for 5 min, then the microwave heating was terminated and the solution was allowed to cool to room temperature. The green–black products were separated by filtration, washed with absolute ethanol and distilled water several times, and dried at 60 °C. For taper-like CuS nanotubes, eriochrome black T (EBT) was adopted as complex agent for the synthesis of Cu–EBT complex taper-like nanorods.

Preparation of CuS nanotube-modified electrodes

CuS nanotubes (10 mg) were dispersed into a mixture of 0.1 mL of Nafion perfluorosulfonated ion-exchange resin (5 wt%) and 0.9 mL of distilled water. Approximately 30 min of ultrasonication was necessary to obtain uniformly dispersed CuS nanotubes. The glassy carbon electrodes (GCE, 3.0 mm in diameter) were firstly polished with 0.3 and 0.05 μm alumina slurries followed by thoroughly rinsing with distilled water. After dropping 10 μL of CuS nanotube solution onto the electrode surface, the electrodes were allowed to dry under ambient conditions to obtain nanostructured CuS nanotube-modified electrodes.

Materials characterization

The collected products were characterized by X-ray diffraction (XRD) on a Rigaku-DMax 2400 diffractometer equipped with the graphite monochromatized Cu-Kα radiation flux at a scanning rate of 0.02° s⁻¹. Scanning electron microscopy (SEM) analysis was carried using a JEOL-5600LV scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDS) microanalysis of these samples was performed during SEM measurements. The structure of these nanotubes was investigated by means of transmission electron microscopy (TEM, Philips, TecnaiG2 20). UV-Vis adsorption spectrum was recorded on UV-Vis-NIR spectrophotometer (JASCO, V-570). The photoluminescence (PL) spectrum was measured at room temperature using a Xe lamp with a wavelength of 325 nm as the excitation source.

Electrochemical measurements

Cyclic voltammetric (CV) was performed on CHI 660D (CHI, USA). A three-electrode system comprising a platinum wire as the auxiliary, a saturated calomel electrode as the reference and the CuS nanotubes-modified electrode as the working electrodes was used for all the electrochemical experiments. The electrochemical experiments were carried out in a cell containing 0.2 mol L⁻¹phosphate buffer solution (PBS, pH 7.2).

Results and discussion

Our design strategy for the preparation of CuS nanotubes with rectangular cross-sections is based on a preferential chemical transformation and crystallization process with the assistance of diethanolamine, and is schematically displayed in Scheme 1. First, Cu(TU)Cl·0.5H₂O complex nanowires were successfully prepared by simply mixing an aqueous solution of CuCl₂ and TU (TU = thiourea) at room temperature (Scheme 1a). The addition of TU into CuCl₂ causes the reduction of Cu²⁺ to Cu⁺, resulting in the formation of Cu(TU)Cl·0.5H₂O nanowires (see Fig. S1 in the ESI†). The formation of Cu–TU complex nanowires is extremely rapid (only taking ∼40 s, see Fig. S2 in the ESI†), low-cost (reaction performed under ambient conditions) and easily scalable (only needing two vessels with large capacity). These metal complex nanowires were employed as the precursors for preparing metal sulfide nanotubes. To realize the chemical transformation from complex nanowires into sulfide nanotubes, the surface of the Cu–TU complex nanowires was first treated with diethanolamine solution at room temperature. Subsequently, these surface-activated nanowires were treated with microwave irradiation for about several minutes (e.g. 5 min, Scheme 1b). During these processes, the Cu(TU)Cl 0.5H₂O nanowires acted as self-sacrificial templates and provided both the copper and sulfur sources to produce CuS nanotubes. As the surface of complex nanowires was activated with diethanolamine, the dissolution or etching of Cu(TU)Cl·0.5H₂O by TU decomposition preferentially took place at the surface of solid nanowires and continued towards their interior (Scheme 1c). In the subsequent chemical transformation, the Cu(TU)Cl·0.5H₂O complexes continued to be consumed, and CuS homogeneously nucleated and grew on the newly-formed surface of these precursors in the presence of coordinated diethanolamine. Thus, the smooth solid complex nanowires gradually converted into nanoporous metal sulfide nanotubes with rectangular cross-section as shown in Scheme 1d.

Scheme 1 Schematic illustration of CuS nanotubes formation through microwave-assisted metal-complex transformation strategy: (a) Cu(TU)Cl·0.5H₂O complex nanowires were prepared by simply mixing an aqueous solution of CuCl₂ and TU at room temperature; (b) activation of the surface of the complex nanowires with diethanolamine solution; (c) dissolution or etching of Cu(TU)Cl·0.5H₂O by TU decomposition preferentially took place at the surface of the solid nanowires and continues towards the interior; (d) complex continued to be consumed, CuS homogeneous nucleated and grew on the newly-formed surface of precursors in the presence of coordinated diethanolamine, and eventual formation of the CuS nanotubes.

Fig. 1a shows a representative scanning electron microscopy (SEM) image of the as-prepared Cu-complex nanowires. These nanowires have been observed to be straight and several tens of micrometres in length. The as-obtained precursor nanowires have smooth surfaces and are clearly uniform through their entire length. Fig. 1b shows the corresponding high-magnification SEM image of the complex nanowires. Further characterization (inset of Fig. 1b) reveals that these complex nanowires have width of ∼250 nm and thickness of ∼200 nm. An X-ray diffraction (XRD) pattern of the as-prepared complex nanowires is shown in Fig. 1c. All the diffraction peaks have been indexed to the monoclinic polymorph of Cu(TU)Cl·0.5H₂O (JCPDS no. 53-0121). A chemical transformation from metal complex nanowires into metal sulfide nanotubes can be clearly observed, as shown in the XRD pattern of Fig. 1d, in which all diffraction peaks can be indexed as the pure hexagonal CuS with lattice constants a = 3.792 and c = 16.344 Å, which agree well with the reported data (JCPDS no. 06-0464). The greater full-width half maximum of these peaks compared to that of bulk of CuS indicates thin shell of nanotubes. The EDS result (Fig. S3d, ESI†) clearly confirms the presence of Cu and S elements in these nanotube products (their atomic ratio is about 1 : 1, which is in agreement with the stoichiometric ratio of CuS), and Cl was not detected, which indicates that all those metal complex precursors have completely converted into sulfide products. Low-magnification SEM images of metal sulfide CuS nanotube products are shown in Fig. 2a,b, which serve to confirm that the overall 1D structural and morphological homogeneity of the complex precursors are preserved in the products. Fig. 2c and d show high-magnification SEM images of these sulfide nanotubes, and the inset of Fig. 2d shows the hollow rectangular cross-section of the nanotubes. The average outer diameter of the nanotubes is ∼250 nm with a shell thickness of 25–35 nm. Furthermore, the structural characteristics of these nanotubes are clearly visible in the transmission electron microscopy (TEM) images in Fig. 3a,b. Fig. 3c is a high-magnification TEM image of a segment of such an individual nanotube. The external surface of this tube is rough, which can not be discerned in the SEM images (Fig. 2). The thickness and diameter of this nanotube are ∼30 nm and ∼250 nm, respectively, which is consist with the SEM characterizations. A high-resolution TEM (HRTEM) image taken of this nanotube shows that the shell consists of tiny nanocrystals with a lattice spacing of 0.305 nm, corresponding to the lattice spacing of (102) in the hexagonal CuS (Fig. 3d).

Fig. 1 Microstructure and composition characterizations of Cu(TU)Cl·0.5H₂O precursor nanowires and CuS nanotube products. (a,b) SEM images show that these nanowires have a uniform size. The inset of Fig. 1b reveals the rectangular cross-section. (c) XRD pattern of Cu(TU)Cl·0.5H₂O nanowires. All the diffraction peaks can be indexed to the monoclinic phase of Cu(TU)Cl·0.5H₂O (JCPDS no. 53-0121). (d) XRD pattern of CuS nanotubes with rectangular cross-sections. All the diffraction peaks can be indexed to the hexagonal phase of CuS (JCPDS no. 06-0464).

Fig. 2 SEM images of CuS nanotubes with rectangular cross-sections. (a,b) Low-magnification SEM images, (c,d) high-magnification SEM images. All these SEM images clearly show that these uniform CuS nanotubes have a diameter of ∼250 nm, and length of several micrometres. The inset of Fig. 2d shows the hollow rectangular cross-section of the nanotubes.

Fig. 3 TEM characterizations of CuS nanotubes with rectangular cross-sections synthesised via a microwave-assisted metal-complex transformation route. (a) Low-magnification TEM image, (b,c) high-magnification TEM images, (d) HRTEM image clearly shows the lattice plane of CuS and nanoporous shell of nanotubes. The inset of Fig. 3d shows the SAED pattern of a single nanotube, which indicates that these uniform nanoporous nanotubes are polycrystalline.

The key for successful synthesis of high-quality CuS nanotubes with very high aspect ratio are the selection of long metal-complex nanowires as sacrificial templates and the use of diethanolamine to control the transformation reaction kinetics. Compared with other templates, the uniform complex nanowires have several advantages. First, the facile prepared metal complex nanowires can be well dispersed in water, ethanol, or glycol, facilitating the formation of homogeneous shells on the templates. Secondly, these complex nanowires with different diameters can be completely consumed in the chemical transformation (decomposition) process in a short reaction time, avoiding the time-consuming step for the removal of core templates. Finally, these uniform precursor nanowires can be facilely synthesized with different diameters, which determines the diameter of the sulfide final product nanotubes. Through tuning the reaction time, temperature and solvent or adding some organic compounds with a longer chain length, the diameter of the complex nanowires can be changed accordingly. For example, when organic ethanol solvent was used instead of water, nanowires with smaller diameter were achieved (Fig. 4a,b), which might due to the lower polarity of ethanol. While introducing some organic compounds into the interlayer of the metal complex, the diameter of these precursor nanowires can be expanded (Fig. 4c,d). The diameter of these complex nanowires can also be easily increased through prolonging the reaction time and increasing the reaction temperature. Additionally, diethanolamine plays a key role in the microwave-assisted transformation reaction. Due to its moderate coordination ability and relatively low alkalinity, diethanolamine can effectively reduce the formation rate of sulfide to facilitate the formation of a smooth and continuous shell on the templates, while we can only obtain CuS nanoparticles in the presence of NaOH (or other organic amines with high alkalinity such as ethylenediamine) or retain the precursor nanowires in the absence of diethanolamine. To understand the formation process of these sulfide nanotubes in detail, structural characterization of the intermediates was carried out. White and green–black particles coexist in the products obtained after reaction for 2 min, which means that only part of the metal complex nanowires have been converted into CuS. From Fig. S4 (in the ESI†), we can see that these intermediates consist of core/shell nanotubes. The cores of these nanotubes are not completely hollow, but are partially filled with precursor segments, which is similar to that reported previously in the case of the formation of metal sulfides.²⁵

Fig. 4 SEM images of CuS nanotubes with tunable diameters through controlling reaction parameters. Adopting ethanol instead of water as the solvent leads to nanowires with smaller diameter (∼150 nm), while insertion of CTAB into the interlayer of the complex expands nanowires (∼500 nm in diameter). The inset of Fig. 4b shows the diameter of nanotubes.

To confirm the generality of our microwave-assisted metal-complex transformation strategy, other organic ligands were processed. For example, uniform Cu–EBT complex taper-like nanorods (Fig. 5) can be easily fabricated when cupric acetate, EBT, and CTAB were used as the copper source, organic ligand and surfactant, respectively (EBT = eriochrome black T, CTAB = hexadecyl trimethyl ammonium bromide). EBT is a kind of azo dye, and its chemical structure is shown in Fig. 5d. As expected, cone-like CuS nanotubes were successfully prepared by simply adopting this strategy (Fig. 6). From Fig. 6a and 6b, it can see that cone-like CuS nanotubes closely resemble the shape and size of the Cu–EBT complex nanorod precursors. The detailed microstructure information is supported by the high-magnification image shown in Fig. 6c, which shows some typical broken CuS nanotubes with wall thickness of ∼150 nm. The XRD pattern of products shown in Fig. 6d reveals a pure phase, and all the diffraction peaks are very consist with the reported XRD profile of the hexagonal CuS (JCPDS no. 75-2234).

Fig. 5 SEM images of Cu–EBT complex cone-like nanorods (a–c) and the chemical structure of EBT (d). All SEM images show that these uniform Cu–EBT complex nanorods have a diameter of 900–950 nm and length of 8–9 μm.

Fig. 6 SEM images (a–c) and XRD pattern (d) of cone-like CuS nanotubes. The inset of Fig. 6c shows the cross-section of a single nanotube. All the diffraction peaks in Fig. 6d can be indexed to the hexagonal phase of CuS (JCPDS no. 75-2234).

Metal sulfide nanocrystals, being representative of metal chalcogenide nanomaterials, have been the most studied due to their great number of applications in different technological areas including biological labeling and diagnostics, photovoltaic devices, sensors, and electroluminescent devices.^35,36Fig. 7 shows the typical UV-Vis and PL spectra of the CuS nanotubes, which clearly exhibits that these CuS nanotubes have a strong UV emission peak at 363 nm and weak blue emission peak at 462 nm (Fig. 7b). The absorbance of CuS nanotubes has a broad shoulder around 325 nm and reaches a minimum around 540 nm, but not zero intensity (Fig. 7a). The absorbance never reaches zero intensity but rises for longer wavelengths again, which is thought to come from the free-carrier intra-band absorbance.³⁷Fig. 8a shows cyclic voltammetric (CV) curves at a bare glassy carbon (GC) electrode and the as-prepared CuS nanotube-modified GC electrode. It can be seen that glucose shows no redox peak on the bare GC electrode even in a glucose solution with the concentration as high as 5 μmol L⁻¹ (curve 0). In contrast, the as-prepared CuS nanotube-modified GC electrode, shows obvious redox peaks (curves 1–14), indicating that nanoporous CuS nanotubes exhibit strong electrocatalytic activity in response toglucose oxidation and improve the electron transfer between glucose and the GC electrode. With the linear increase of glucose concentration, the CV peak becomes stronger, indicating that the CuS glucose sensor is sensitive to the concentration of glucose. Fig. 8b shows that the as-prepared CuS nanotube-modified electrode exhibits a linear dependence (correlation coefficient, R = 0.9998) in the glucose concentration range of 0.5–7.5 μmol L⁻¹ with a detection limit of 0.25 μmol L⁻¹ and a sensitivity of 9.9005 μA μmol L⁻¹. Exhilaratingly, the nanoporous CuS nanotubes-modified electrode also exhibits high stability and reproducibility in the detection of glucose. All of the above results demonstrate that these obtained CuS nanotubes would have great potential application as nonenzymatic glucose sensors with high sensitivity. For comparison, CuS spherical particles and Nafion composite film on a GCE was also fabricated. From the CV curves (Fig. S5 in the ESI†), we can see that the conventionally CuS solid particles also exhibit catalytic response towards glucose oxidation. However, with the same amount of glucose added, the peak current increase of these CuS spherical particles is much smaller than that of nanoporous CuS nanotubes.

Fig. 7 UV-Vis (a) and PL (b) spectra of CuS nanotubes with rectangular cross-sections. Room-temperature PL spectrum was obtained with an excitation wavelength of 325 nm.

Fig. 8 (a) CV performance of nanoporous CuS nanotube-modified GC electrodes in the presence of different amounts of glucose in 0.2 mol L⁻¹PBS (pH = 7.2) at a scan rate of 50 mV s⁻¹. (scans 1–15 correspond to 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5 μmol L⁻¹glucose in 0.2 mol L⁻¹PBS, respectively; scan 0 corresponds to 5 μmol L⁻¹glucose in 0.2 mol L⁻¹PBS on the bare GC electrode). (b) The linear response for glucose concentrations between 0.5 μmol L⁻¹ and 7.5 μmol L⁻¹. The as-prepared CuS nanotube modified electrode give a linear dependence (R = 0.9998) in the glucose concentration range of 0.5–7.5 μmol L⁻¹ with a sensitivity of 9.9005 μA μmol L⁻¹.

Conclusions

In summary, we have developed a rapid microwave-assisted metal-complex transformation strategy, based on a preferential chemical transformation and crystallization process with the assistance of diethanolamine, for the preparation of metal sulfide nanotubes. Uniform CuS nanotubes with rectangular cross-sections and cone-like CuS nanotubes have been synthesized by chemical transformation of the corresponding Cu(TU)Cl·0.5H₂O nanowires and Cu–EBT complex taper-like nanorods, respectively. The obtained nanoporous CuS nanotubes have been confirmed to be very effective as nonenzymatic glucose sensors. The CuS nanotube-modified electrode shows excellent electrocatalytic activity towards the oxidation of glucose and good linear dependence and high sensitively to glucose concentration change.

Acknowledgements

We thank Wanyan Xiao for fruitful discussion. This work was supported by the National Natural Science Foundation of China (Grant Nos. 50872016, 20973033).

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Footnote

† Electronic supplementary information (ESI) available: Reaction routes for Cu(TU)Cl·0.5H₂O complex; photographic images showing the rapid formation process of Cu(TU)Cl·0.5H₂O complex nanowires; EDS patterns of Cu(TU)Cl·0.5H₂O complex nanowire precursors and CuS nanotubes; SEM images of intermediate products; CV performance of CuS spherical particles. See DOI: 10.1039/c0jm01714k

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