The study on the application of solid-state method for synthesizing the polyaniline/noble metal (Au or Pt) hybrid materials
© Jamal et al.; licensee Springer. 2013
Received: 7 January 2013
Accepted: 14 February 2013
Published: 2 March 2013
The solid-state method was applied for synthesizing polyaniline (PANI)/noble metal hybrid materials with the presence of HAuCl4·4H2O or H2PtCl6·6H2O in the reaction medium. The structure, morphology, and electrochemical activity of the composites were characterized by Fourier transform infrared (FTIR) spectra, UV-visible (vis) absorption spectra, energy dispersive spectrum (EDS), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and cyclic voltammetry. The results from FTIR and UV-vis spectra showed that the oxidation degree and doping level of the PANI in composites can be influenced by HAuCl4·4H2O and H2PtCl6·6H2O. The EDS data demonstrated that the composites contain a certain amount of Au (or Pt) element. XRD analysis indicated the presence of crystalline-state Au particles in PANI matrix prepared from the presence of HAuCl4·4H2O and revealed that the H2PtCl6·6H2O cannot be converted into metal Pt. The TEM and SEM images implied that the Au particles did exist in the polymer matrix with the size of about 20 nm. The enzymeless H2O2 sensor constructed with PANI/Au composite from the presence of HAuCl4·4H2O showed a short response time (within 5 s) and displayed an excellent performance in wide linear range.
KeywordsSolid-state synthesis Polyaniline Noble metal Hybrid materials Sensor
Noble metal nanoparticles such as Au and Pt nanoparticles have high catalytic activity, nontoxicity, and biocompatibility . Conducting polymers are usually used as matrix to noble metal nanoparticles and then applied in biosensors [2, 3], electrocatalysts , and supercapacitors , due to the synergy effect between polymer matrix and inorganic nanoparticles. Among various conducting polymers, polyaniline (PANI) has a potential use in a broad field because of its high environmental stability, low cost, relatively facile preparation, and reversible control of conductivity by charge-transfer doping and protonation . The composite of PANI and Au (or Pt) nanoparticles, which have been intensively investigated, are also attractive materials as they combine the properties of large surface area, high conductivity, and excellent biocompatibility [7, 8]. Up to now, PANI/Au (or Pt) hybrid material can be synthesized chemically or electrochemically. These methods have the advantages of easily controlling operating conditions. However, they have significant disadvantages such as the formation of toxic waste products and are not suitable for mass production. Solid-state synthesis is a mechanochemical reaction that occurs between powders in the solid state . It is a new synthetic method to develop green chemistry with obvious advantages: reduced pollution, low costs, and simplicity in process and handling. Also, these factors are especially important in the industry.
H2O2 as a metabolic intermediate involved in many biological reactions plays an important role in the fields of chemistry, biology, clinical control, and environmental protection; therefore, its detection is of great importance . To date, various techniques including spectrometry, titrimetry, chemiluminescence, and electrochemistry have been employed for determination [1, 11, 12]. Among the developed techniques, electrochemical methods have become one of the predominant analytical techniques due to their high sensitivity, low cost, and low power requirement . Moreover, among the electrochemical methods, amperometric sensors have shown great potential for developing versatile analytical techniques for H2O2 determination . The conducting polymer/metal composite amperometric enzyme electrodes as sensors have been paid particular attention due to their advantages of high sensitivity and specificity [14, 15]. However, an efficient electron transfer between the active site of the enzyme and the electrode surface is not quite stable and depends on the enzyme type, temperature, and pH as a function of time . Therefore, an alternative sensor called ‘enzymeless sensor’, which try to mimic natural enzymes with the same effectiveness and selectivity, has been widely studied [16, 17].
Herein, we report the exploration of synthesizing the polyaniline/noble metal hybrid materials by solid-state synthesis method at room temperature. The structure, morphology, and components of composites were characterized by Fourier transform infrared (FTIR), UV-visible (vis), X-ray powder diffraction (XRD), energy dispersed spectrum (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) methods. Furthermore, the composite from the existence of HAuCl4·4H2O in the reaction medium was selected for designing an enzymeless sensor on a glassy carbon electrode (GCE) for H2O2 detection.
Aniline and ammonium peroxydisulfate were obtained from Xi’an Chemical Reagent Company (Xi’an, China). Chloroauric acid hydrated (HAuCl4·4H2O), chloroplatinic acid hydrated (H2PtCl6·6H2O), and p-toluenesulfonic acid (p-TSA) were purchased from Shanghai Aladdin Reagent Company (Shanghai, China). H2O2 (30 wt.%) was obtained from Tianjin Chemical Reagent Company (Tianjin, China). Nafion, a 5-wt.% solution in a mixture of lower aliphatic alcohols and 20% water, was obtained from Sigma-Aldrich (St. Louis, MO, USA). Before use, it was diluted with 0.5 wt.% isopropanol. All the reagents were of analytical grade, aniline was purified by distillation under reduced pressure and stored in a refrigerator, and all other chemicals and solvents were used as received without further purification. Phosphate buffer saline (PBS; 0.1 M) was prepared by mixing stock solutions of NaH2PO4 and Na2HPO4.
The FTIR spectra of the composites were obtained using a Bruker Equinox-55 Fourier transform infrared spectrometer (Bruker, Billerica, MA, USA) (frequency range 4,000 to 500 cm−1). The UV-vis spectra of the samples were recorded on a UV-vis spectrophotometer (UV4802, Unico, Dayton, NJ, USA). XRD patterns have been obtained using a Bruker AXS D8 diffractometer with monochromatic Cu Kα radiation source (λ = 0.15418 nm), the scan range (2θ) was 5° to 70°. SEM measurements were performed on a Leo 1430VP microscope (Zeiss, Oberkochen, Germany) with Oxford Instruments (Abingdon, Oxfordshire, UK). EDS experiments were carried out with a pellet which was pressed at 200 MPa and then adhered to copper platens.
A three-electrode system was employed to study the electrochemical performances of composites. Pt electrode was used as a counter electrode and saturated calomel electrode as a reference electrode. PANI(HAuCl4·4H2O)-modified GCE (diameter = 3 mm) was used as a working electrode. The working electrode was fabricated by placing a 5-μL dispersion (30 mg/L) on a bare GCE surface and air-dried for 10 min. All the experiments were carried out at ambient temperature and air atmosphere.
Results and discussion
Generally, the Q/B (I~1,580 cm−1/I~1493 cm−1) value indicates the oxidation degree of PANI . A comparison indicates that the composites exhibit a higher intensity ratio of Q to B ring modes than pure PANI, suggesting that there are more quinoid units in the composites than pure PANI. This result can be attributed to the adding of HAuCl4 and H2PtCl6, which can serve not only as the resource of metal particles, but also as strong oxidants, which can enhance the oxidation degree of the PANI in composites [22, 23].
In this paper, the synthesis of the polyaniline/noble metal hybrid materials by solid-state method in the presence of HAuCl4·4H2O or H2PtCl6·6H2O in the reaction system was investigated. These composites were characterized by FTIR, UV-vis, X-ray, TEM, SEM, and EDS as well as by the electrochemical measurements. The results showed that the strong oxidation ability of HAuCl4·4H2O and H2PtCl6·6H2O was a main factor in increasing the oxidation degree and doping level of the PANI in composites. Furthermore, the results also indicated that the HAuCl4·4H2O can be converted into Au nanoparticles, while that of the H2PtCl6·6H2O cannot be converted into metal Pt, suggesting the formation of [PtCl6]2−, [PtCl5(H2O)]−, and [PtCl4(H2O)2] in the polymer matrix. Compared with the existing methods, the method demonstrated here was facile but effective and could be readily used for a large-scale preparation of the PANI/Au. However, the PANI/Pt was not successfully synthesized by this solid-sate method which may be a result of the fully suppressed deprotonation reaction of aqua ligands of H2PtCl6 by the high concentration of protons in the reaction system. These interesting results indicated the potential application of the solid-state method for polymer complex such as PANI-type conducting polymer Pt(IV) complexes. Furthermore, the electrochemical measurements indicated that the obtained PANI/Au displayed a fast response to H2O2 and excellent performance in wide linear range. The sensor could catalyze the oxidation and reduction of H2O2 at the same time, and it exhibited a fast amperometric response (about 5 s) to the reduction of H2O2 in a wide linear range.
We gratefully acknowledge the financial support from the National Natural Science Foundation of China (nos. 20964004 and 21064007) and Xinjiang University institution cooperation project (XJDX1108-2012-03).
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