Introduction

Over the past few decades, artificial enzyme mimics have generated great interest because of their improved properties relative to natural enzymes, such as easy and low costs of preparation, purification and storage, greater resistance to extremes of acidity, temperature and inhibitors1,2,3. Indeed, many enzyme mimics such as hemin, hematin, hemoglobin, cyclodextrin, porphyrin etc. have been developed and applied in the fields of clinical diagnosis, environmental science and biotechnology4,5,6,7,8,9,10,11,12. At present, the synthesis and application of nanomaterials in the field of catalysis has become an active field of scientific research because of their large surface area, high catalytic activity and good selectivity and some nanomaterials have been found to possess enzyme mimetic activity13. For example, Fe3O4 magnetic nanoparticles were first reported to process intrinsic peroxidase-like activity, which could be used to detect H2O2 and thrombin14,15,16. The nanomaterials as artificial enzyme mimics take the advantages of low cost, easy preparation, high activity and good stability. In the past few years, various kinds of nanomaterials have been explored as enzymatic mimics, including metal oxide nanoparticles (NPs) and nanowires (such as Fe3O4 NPs, Co3O4 NPs, CuO NPs, CeO2 NPs, V2O5 nanowires)14,17,18,19,20, metal-based nanomaterials (such as Au NPs, bimetallic alloy NPs, bimetallic hybrid nanorods)21,22,23,24, carbon nanomaterials (such as graphene oxide, carbon nanotubes, carbon nanodots)25,26,27, chalcogenide nanomaterials (such as FeS, FeSe, CuS, CdS NPs)28,29,30,31, multiferroic nanomaterials (such as BiFeO3 NPs, CoFe2O4 NPs)32,33 and even coordination polymer NPs34 or carboxyl functionalized mesoporous polymer35.

During the process of catalytic reactions, the interfaces play an important role, which could result in a better activity and selectivity36. Recently, Yang and co-workers designed multiple metal-metal oxide interfaces to study the catalysis of sequential reactions37. It was found that two different kinds of metal-metal oxide interfaces could catalyze two distinct sequential reactions. Therefore, the fabrication of composite nanomaterials with well-defined composition and interfaces has been extensively studied to enhance or extend the functionality of each component. A variety of graphene or carbon nanotube based nanomaterials have been explored with peroxidase-like catalytic activity38,39,40,41. Typically, the Au NPs/graphene composite nanosheets process an enhanced catalytic activity in comparison to the independent Au-NPs and graphene, which could be due to the synergetic effect at their interfaces41. However, such synergetic effect has few been observed in other systems except the carbon based nanomaterials. In this study, we have shown for the first time that conducting polyaniline (PANI)/Cu9S5 composite nanowires showed a superior catalytic activity as artificial enzyme mimics compared to the independent polyaniline and Cu9S5 nanocomponents due to their synergetic effect.

PANI is one kind of conducting polymers that has been extensively studied in the past few decades. Compared to other intrinsic conducting polymers, the doping level of PANI can be simply controlled by a reversible acid/base doping/dedoping process42. In recent years, PANI nanomaterials have attracted much attention because of their unique physical and chemical properties such as large surface area and high conductivity. Especially, one-dimensional PANI nanostructures such as nanofibers, nanorods and nanowires have become a focus subject in the field of conducting polymer nanotechnology because they process the advantages of both excellent conductivity and low-dimensional systems43. On the other hand, nanocomposites with PANI and inorganic nanomaterials showed improved electrical, thermal and optical properties due to the combined effect of the two components44. For the PANI based nanocomposites, the interactions between PANI and the inorganic nanomaterials are very vital to produce cooperatively enhanced performances. In the past few years, a series of PANI/sulfide nanocomposites have been fabricated for optic, optoelectronic and photoelectrochemcial applications45,46,47. However, it is still a great challenge to prepare low dimensional PANI nanocomposites with strong interactions between PANI and another inorganic nanocomponent to generate synergetic effect, which achieve an enhanced performance of the nanocomposites.

Herein, we have developed a novel method to fabricate PANI/Cu9S5 composite nanowires using the dopant of mercaptoacetic acid (MAA) in the PANI matrix as the sulfur source under a hydrothermal reaction. During the process of the reaction, the Cu9S5 nanocrystals could be in-situ formed on the surface of PNAI nanowires with strong interactions because the sulfur source of MAA was interacted with PANI nanowires through ionic bonds. Furthermore, it was found that the as-prepared PANI/Cu9S5 composite nanowires exhibited an enhanced catalytic activity toward the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H2O2 over the independent PANI and Cu9S5 nanocomponents due to the synergetic effect between them.

Results

In this study, PANI/Cu9S5 composite nanowires were prepared by mixing MAA doped PANI nanowires and CuCl2·2H2O under a hydrothermal reaction. The synthetic strategy mainly involves three steps: (1) PANI nanowires doped with HCl was synthesized through a rapid-mixing method. (2) After treated by NH3·H2O, the dedoped PANI nanowires were again doped with MAA to form MAA doped PANI nanowires. (3) Mixing MAA doped PANI nanowires with copper ions under a hydrothermal condition to form PANI/Cu9S5 composite nanowires. During the reaction, the Cu9S5 nanocrystals were in-situ formed on the surface of PANI nanowires (Figure 1a). Figure 1b shows the TEM image of HCl doped PANI nanowires. It can be seen that the diameter of the PANI nanowires is in the range of 50–100 nm with a relatively smooth surface. After dedoping with NH3·H2O and again doping with MAA, the size of nanowires did not change much, but showed a little aggregation (Figure S1 and 1c). After hydrothermal reaction, it can be clearly seen that Cu9S5 nanocrystals with a size in the range of 5–20 nm were uniformly formed on the surface of PANI nanowires (Figure 1d). A high resolution TEM (HRTEM) image exhibits that the as-prepared Cu9S5 nanocrystals showed a good crystallinity, with the lattice spacing of 0.20 nm which could be corresponding to the {0120} facet of Cu9S548 (Figure 1e). A control experiment was also performed to synthesize pristine Cu9S5 nanocrystals, which exhibit a particle or rod shape with a size of about 20–80 nm (Figure S2). This result indicates that the presence of PANI nanowires make for the Cu9S5 nanocrystals with a smaller size, which should be due to the strong interactions between the dopant of MAA and PANI nanowires during the hydrothermal reaction.

Figure 1
figure 1

Fabrication and morphology of the PANI/Cu9S5 composite nanowires.

(a) Schematic illustration of the fabrication process to synthesize PANI/Cu9S5 composite nanowires. TEM images of the as-prepared (b) HCl doped PANI nanowires, (c) MAA doped PANI nanowires and (d) PANI/Cu9S5 composite nanowires. (e) HRTEM image of the as-prepared PANI/Cu9S5 composite nanowires.

Full size image

The formation of Cu9S5 nanocrystals on the surface of PANI nanowires were characterized by XRD measurement. Figure 2a shows the XRD patterns of the as-prepared PANI/Cu9S5 composite nanowires and Cu9S5 nanocrystals alone. From Figure 2a (red line), the diffraction peaks at 2θ = 27.8°, 29.5°, 32.2°, 37.6°, 42.0°, 46.3°, 48.8°, 54.7°are indexed to the (0015), (107), (1010), (0114), (0117), (0120), (119), (1115) planes of Cu9S5, which is in good agreement with the Rhomb-centered structure (JCPDS 47-1748). Moreover, most of the strong diffraction peaks including (0015), (1010), (0120), (1115) have also been observed in the pattern of the as-prepared PANI/Cu9S5 composite nanowires, indicating the formation of Cu9S5 nanocrystals on the surface of PANI nanowires. We also estimated the size of Cu9S5 nanocrystals in accordance with the Scherrer's equation and the average particle size of the Cu9S5 nanocrystals was calculated to be about 7.5 nm, which is consistent with the results originated from the TEM images.

Figure 2
figure 2

Surface composition of the PANI/Cu9S5 composite nanowires.

(a) XRD patterns of pristine Cu9S5 nanocrystals (red) and the as-prepared PANI/Cu9S5 composite nanowires (black). (b) FTIR spectra of HCl doped PANI nanowires (black), dedoped PANI nanowires with NH3·H2O (red), MAA doped PANI nanowires (green) and PANI/Cu9S5 composite nanowires (blue). XPS spectra of the as-synthesized PANI/Cu9S5 composite nanowires: (c) Cu2p, (d) S2p, (e) C1s and (f) N1s.

Full size image

Figure 2b shows the FTIR spectra of HCl doped PANI nanowires, dedoped PANI nanowires, MAA doped PANI nanowires and PANI/Cu9S5 composite nanowires, respectively. It was found that the characteristic peaks at 1581, 1495, 1304, 1244, 1142, 821 cm−1 which are corresponding to C = C stretching of the quinonoid and benzenoid rings, C-N and C = N stretching vibrations, N = Q = N (where Q denotes the quinonoid ring) bending vibration and the in-plane bending of C-H, respectively, were clearly observed for HCl doped PANI nanowires45. After dedoping by aqueous ammonia, the characteristic peaks of C = C stretching of the quinonoid and benzenoid rings shift to 1591 and 1500 cm−1. And these two peaks shift to 1581 and 1498 cm−1 after doping with MAA. And a new peak at 1664 cm−1 appears for the MAA doped PANI nanowires, which is corresponding to the C = O stretching vibrations of MAA dopant. For PANI/Cu9S5 composite nanowires, the characteristic peaks were observed at 1591, 1500, 1300, 1130 and 822 cm−1, indicating that the strong interactions between PANI nanowires and Cu9S5 nanocrystals.

Insight information of the surface chemical composition of the as-prepared PANI/Cu9S5 composite nanowires has been characterized by XPS analysis. As shown in Figure 2c, two strong peaks at 932.4 and 952.3 eV for Cu 2p3/2 and Cu 2p1/2, respectively, were clearly observed. The binding energy for Cu 2p3/2 in the Cu(2p) core level spectrum is close to the previous report of Cu+49. In addition, a weak broad peak at about 945.0 eV are also observed, which could be attributed to the Cu2+50. These results suggested that both Cu+ and Cu2+ ions existed in the PANI/Cu9S5 composite nanowires. Figure 2d shows the S 2p spectrum of PANI/Cu9S5 composite nanowires. The binding energy of S 2p appears at about 162.9 eV, which is generally attributed to the formation of Cu-S bond. And another weak peak centered at about 168.7 eV could be ascribed to the oxidized sulfur species such as sulfonate51. On the other hand, the C 1s of PANI in PANI/Cu9S5 composite nanowires has also been detected and could be deconvoluted into two components (Figure 2e). The peak at about 284.6 eV is attributed to C-C bond, while the other peak at about 285.2 eV is related to C-N bond. Similarly, the N1s spectrum could also be deconvoluted into two components, with binding energy at 400.1 and 399.1 eV, respectively (Figure 2f). The former peak is attributed to neutral nitrogen moieties in PANI52. The later signal at 399.1 eV is usually ascribed to uncharged deprotonated imine ( = N-) atoms52. This result indicated that the doping degree was low in the PANI/Cu9S5 composite nanowires.

The formation of the MAA doped PANI nanowires and PANI/Cu9S5 composite nanowires has been proved by UV-vis absorption spectra. As shown in Figure 3a, HCl doped PANI nanowires exhibited characteristic color in green. After dedoping by ammonia water, the color turned to blue. Then it became back to light green after again doping with MAA. This result indicated that MAA had been doped in the PANI chains. The UV-vis absorption spectrum of the HCl doped PANI nanowires showed three characteristic absorption bands at around 360, 425 and > 800 nm (Figure 3b). The first absorption peak could be ascribed to the π–π* electron transition within benzenoid segments. The other two absorption peaks at 425 and > 800 nm are attributing to the polaron to π* electron transition and the π to the localized polaron band, respectively53. For the dedoped PANI nanowires, the obvious peak centered at about 670 nm is corresponding to the benzenoid to quinonoid excitonic transition, which is the characteristic band of the dedoped state of the PANI nanowires53. As for the MAA doped PANI nanowires, the characteristic peaks of the π–π* electron transition, polaron-π* electron transition and π-localized polaron band were observed at 358, 450 and > 800 nm, indicating that PANI nanowires are in doped state. The UV-vis absorption spectrum of PANI/Cu9S5 composite nanowires showed the characteristic peaks at about 333 and 630 nm, indicating the low doping level of the PANI in the composite nanowires.

Figure 3
figure 3

Absorption spectra property and peroxidase-like catalytic activity of the PANI/Cu9S5 composite nanowires.

(a) Photographs (from left to right) and (b) UV-vis absorption spectra of aqueous dispersions of HCl doped PANI nanowires (black), dedoped PANI nanowires (red), MAA doped PANI nanowires (green) and PANI/Cu9S5 composite nanowires (blue). (c) Photographs (from left to right) and d) UV-vis spectra of TMB solution (black line), TMB-H2O2 (blue line), PANI/Cu9S5 solution (purple line), PANI/Cu9S5 + TMB (green line) and PANI/Cu9S5 + TMB + H2O2 (red line) in pH 4.0 acetate buffer at room temperature ([TMB]: 0.1 mM; [H2O2]: 65 mM; [PANI/Cu9S5]: 20 μg/mL).

Full size image

It is well known that some transition metal compound nanoparticles such as Fe3O4, FeS and CuS process a peroxidase-like activity14,28,30. In this study, we evaluated the catalytic activity of the PANI/Cu9S5 composite nanowires by using a peroxidase-like catalytic reaction involving the oxidation of a peroxidase substrate TMB in the presence of H2O2. Similar to HRP, PANI/Cu9S5 composite nanowires could quickly catalyze the oxidation of TMB in the presence of H2O2 to produce a typical blue color in only several minutes, showing their good peroxidase-like activity. Control experiments were also performed to demonstrate that the peroxidase-like activity was attributed to the nanocatalysts of PANI/Cu9S5 composite nanowires. As shown in Figure 3c, the TMB + H2O2 system without PANI/Cu9S5 composite nanowires was colorless and also no blue color was generated for the PANI/Cu9S5 composite nanowires + TMB system under experimental conditions. This result suggested that neither H2O2 nor PANI/Cu9S5 composite nanowires alone could catalyze the oxidation of TMB. UV-vis spectra have also been used to evaluate the peroxidase-like activity of PANI/Cu9S5 composite nanowires (Figure 3d). It was seen that no obvious adsorption peaks ranging from 330 to 750 nm were observed for TMB solution, PANI/Cu9S5 composite nanowires dispersions, TMB + H2O2 system and PANI/Cu9S5 composite nanowires + TMB system. However, three strong adsorption peaks centered at 369, 453 and 652 nm appeared for the PANI/Cu9S5 composite nanowires + TMB + H2O2 system, which originates from the oxidation of TMB. This result revealed that PANI/Cu9S5 composite nanowires could catalyze the oxidation of TMB in the presence of H2O2. It is well known that Cu2+ and Fe3+ are common Fenton regents, which could also efficiently catalyze the oxidation of TMB in the presence of H2O2. To rule out the possibility that the Cu2+ leaching from PANI/Cu9S5 composite nanowires act as the catalysts for the oxidation of TMB, we also performed a comparison experiment to test the peroxidase-like activity of the leaching solution. It was found that no obvious change of absorbance was observed when the leaching solution was used as the catalysts under the same reaction conditions (Figure S3). These results suggested that the peroxidase-like activity was due to the PANI/Cu9S5 composite nanowires.

Discussion

As comparisons, we have evaluated the peroxidise activity of PANI/Cu9S5 composite nanowires, Cu9S5 nanocrystals, PANI nanowires and even the physical mixture of Cu9S5 nanocrystals and PANI nanowires, respectively. As shown in Figure 4a and b, free PANI nanowires almost showed no catalytic activity and Cu9S5 nanocrystals alone had a low catalytic activity. The physical mixture of Cu9S5 nanocrystals and PANI nanowires showed a similar catalytic activity with that of pristine Cu9S5 nanocrystals. However, the PANI/Cu9S5 composite nanowires exhibited a surprising high activity. The catalytic activity of the PANI/Cu9S5 composite nanowires processed a factor of about 30 fold higher than that of Cu9S5 nanocrystals and more than 1000 fold excess higher than that of PANI nanowires. The high catalytic activity might be due to the small size of the Cu9S5 nanocrystals on the surface of PANI nanowires. And the synergetic coupling effects between PANI nanowires and Cu9S5 nanocrystals in the PANI/Cu9S5 composite nanowires might also play an important role on mimicking peroxidases. It has been reported that the interfaces is important to achieve a better activity and selectivity during the catalytic reactions36,37. Therefore, the synergetic catalytic effect in the composites should be due to the strong contact between PANI nanowires and Cu9S5 nanocrystals. An early report has shown that Au/graphene hybrid with strong covalent interfacing exhibited a synergetic catalytic effect in peroxidase-like catalysis41. In this paper, we have synthesized Cu9S5 nanocrystals on the surface of PANI nanowires by using the dopant of PANI nanowires as the sulfur source, producing a strong contact between the two nanocomponents, which enhances the catalytic activity for the oxidation of TMB. In order to verify the synergetic catalytic effect between PANI nanowires and Cu9S5 nanocrystals, we have also synthesized multi-walled carbon nanotubes (MWCNT)/Cu9S5 composite nanotubes using a hydrothermal reaction and studied their catalytic activity toward the oxidation of TMB for mimicking peroxidises. TEM image showed that Cu9S5 nanocrystals were well dispersed on the surface of MWCNT nanotubes with a very small size, most of which was in the range of 5–30 nm (Figure S4). However, the as-synthesized MWCNT/Cu9S5 composite nanotubes showed a low catalytic activity toward the oxidation of TMB compared with that of PANI/Cu9S5 composite nanowires (Figure S5). This result further proved that the synergetic catalytic effect between PANI nanowires and Cu9S5 nanocrystals contributed the high catalytic activity of PANI/Cu9S5 composite nanowires for the oxidation of TMB.

Figure 4
figure 4

Synergetic catalytic effect and the mechanism of the PANI/Cu9S5 composite nanowires toward the oxidation of TMB.

(a) The time-dependent absorbance changes at 652 nm in the presence of pristine Cu9S5 nanocrystals, PANI nanowires, the physical mixture of PANI nanowires and Cu9S5 nanocrystals and the as-prepared PANI/Cu9S5 composite nanowires in pH 4.0 acetate buffer (0.1 M NaAc-HAc) at room temperature; (b) Comparison of the catalytic activities of PANI nanowires, pristine Cu9S5 nanocrystals, the physical mixture of PANI nanowires and Cu9S5 nanocrystals and PANI/Cu9S5 composite nanowires under the same conditions. The difference of the absorbance between 10 and 0 min stands for the activity and the maximum point was set as 100%. (c) A possible mechanism of the synergetic catalytic effect of the PANI/Cu9S5 composite nanowires toward the oxidation of TMB.

Full size image

A possible mechanism for the enhanced catalytic activity of the PANI/Cu9S5 composite nanowires toward the oxidation of TMB has been demonstrated. Based on the previous reports, the blue color product should be due to the oxidation of TMB by OH·54,55. Cu(II) on the surface of Cu9S5 nanocrystals might produce OH· by the Fenton reaction between Cu(II) and H2O230. However, this pathway to produce OH· could not explain the enhanced catalytic activity of the PANI/Cu9S5 composite nanowires compared to that of Cu9S5 nanocrystals. We think that the adsorbed H2O2 could also be catalyzed by the PANI/Cu9S5 heterostructures. As shown in Figure 4c, when PANI was coupled with a narrow band gap semiconductor such as Cu9S5 under visible light irradiation, the photogenerated electrons excited by Cu9S5 will transfer from Cu9S5 to PANI, which effectively prevent the recombination of the electron-hole pairs. And H2O2 is a good electron scavenger to form OH· on the surface of composite nanowires, which largely enhanced the oxidation ability of TMB55. As a result, the PANI/Cu9S5 composite nanowires exhibited an enhanced catalytic activity compared to the individual Cu9S5 nanocrystals and PANI nanowires alone.

In summary, PANI/Cu9S5 composite nanowires have been fabricated under a hydrothermal reaction between MAA doped PANI nanowires and copper chloride. This method affords an effective route to decorate Cu9S5 nanocrystals on the surface of PANI nanowires with a strong contact. The obtained Cu9S5 nanocrystals were well dispersed on the surface of PANI nanowires. The as-synthesized PANI/Cu9S5 composite nanowires show a high catalytic activity towards the oxidation of TMB for mimicking peroxidases. The catalytic activity of the PANI/Cu9S5 composite nanowires is much higher than that of Cu9S5 nanocrystals and PANI nanowires alone. The synergetic catalytic effect should be attributed to the strong interactions between PANI nanowires and Cu9S5 nanocrystals. It was expected that the PANI/Cu9S5 composite nanowires could have great potential practical applications in catalysis and bioassays.

Methods

Chemicals and materials

Aniline was purchased from Xilong Chemical Co. Ltd and was distilled under reduced pressure before use. MAA was purchased from Aladdin. Copper chloride (CuCl2·2H2O) and TMB were purchased from Sinopharm group chemical reagent Co. Ltd. Other reagents such as ammonium peroxydisulfate (APS), hydrochloric acid (HCl), ammonia solution (NH3·H2O), hydrogen peroxide (H2O2) and ethanol were of analytical grade and were used as received without purification. Distilled water was used in all experiments.

Preparation of MAA doped PANI nanowires

Firstly, HCl doped PANI nanowires was prepared using a modified rapid-mixing reaction method56,57. In a typical procedure, an aqueous solution of 0.3 mL aniline monomer in 10 mL of 1.0 M HCl and another aqueous solution of 0.18 g of ammonium peroxydisulfate in 10 mL of 1.0 M HCl were prepared and mixed rapidly. After shaking for about 30 s, the mixture was left for 2 h. The as-synthesized green precipitate of PANI nanowires doped with HCl was centrifuged and washed with distilled water for at least three times.

For the preparation of dedoped PANI nanowires, the as-synthesized PANI nanowires was dispersed in 1.0 M NH3·H2O and stirred for about 8 h. The green PANI nanowires doped with HCl immediately changed to blue after the addition of NH3·H2O. The dedoped PANI nanowires was centrifuged and washed with distilled water for at least two times. For the preparation of PANI nanowires doped with MAA, the obtained dedoped PANI nanowires was then dispersed into a MAA solution for about 12 h. Then the MAA doped PANI nanowires was centrifuged and washed with distilled water. Finally, the MAA doped PANI nanowires were dispersed in water for use.

Preparation of PANI/Cu9S5 composite nanowires

PANI/Cu9S5 composite nanowires were prepared by mixing MAA doped PANI nanowires and CuCl2·2H2O under a hydrothermal reaction. In a typical procedure, 1 mL of aqueous solution containing 51 mg CuCl2·2H2O was added into 30 mL of water which containing 15 mg of MAA doped PANI nanowires. After that, the mixed solution was transferred into a 45 mL Teflon-lined stainless steel autoclave at 140°C for 12 h. Finally, the solid product was collected by centrifuge, washing with water and ethanol and then dried in vacuum at 30°C for one night.

Investigation of the peroxidase-like activity of the PANI/Cu9S5 composite nanowires

To evaluate the peroxidase-like activity of the obtained PANI/Cu9S5 composite nanowires, the catalytic oxidation of TMB in the presence of H2O2 was tested. In a typical experiment, 20 μL of 30% H2O2 and 20 μL of 15 mM TMB solution in DMSO solvent (prepared freshly) were added to 0.1 M acetate buffer (pH 4.0). Then, 20 μL of PANI/Cu9S5 composite nanowires suspension in water (3.0 mg/mL) was added into the above mixture. The blue color that developed during the process of the reaction was monitored in time by recording the absorption spectra in a scanning kinetics mode.

Characterization

TEM experiments were performed on JEM-1200 EX (JEOL) electron microscopes with an acceleration voltage of 100 kV. HRTEM imaging and energy dispersive X-ray (EDX) analysis were performed on a FEI Tecnai G2 F20 high-resolution transmission electron microscope operating at 200 kV. XRD patterns are obtained with a PANalytical B.V. Empyrean diffractometer using CuKα radiation. FTIR spectra of KBr powder-pressed pellets were recorded on a BRUKER VECTOR 22 Spectrometer. UV-vis spectra were recorded on a Shimadzu UV-3101 and 2501 PC spectrometer. Analysis of the X-ray photoelectron spectra (XPS) was performed on a Thermo Scientific ESCALAB250 measurement.