IntroductionDopamine (DA) serves as a neurotransmitter in the central and sympathetic nervous systems, or as a hormone in vesicles of the adrenal medulla for regulating the heart beat rate and blood pressure. 1 A loss of DA-containing neurons may result in some serious disease, such as Parkinson's disease. Therefore, detailed information regarding its reaction at the molecular level and a determination of the concentration of DA are important.The fact that DA and other catecholamines are easily oxidizable compounds makes their detection possible by electrochemical methods based on anodic oxidation. Several studies of electrochemical redox processes of DA have been carried out. 2,3 However, the redox process of DA is very complicated due to the formation of a series of intermediates, and the mechanism is still uncertain.In situ Fourier transform infrared (FTIR) spectroelectrochemistry is one of the useful methods to characterize the intermediates in redox processes. To our knowledge, a study of the redox pathway of DA using the in situ FTIR spectroelectrochemical method has not been reported previously. In this work the electrochemical redox processes of DA were studied by thinlayer cyclic voltammetry and an in situ FTIR technique. It is our goal to identify the absorbance characteristics of any intermediates in the electrochemical oxidation of DA and to clearly understand the reaction pathway of DA in detail.
Experimental
ReagentsDopamine hydrochloride was obtained from Sigma Chemical Company (USA). KCl, K2HPO4 and KH2PO4 were purchased from Shanghai Chemical Company (Shanghai), Hongxing Chemical Company (Beijing), and Xilong Chemical Company (Guangdong), respectively. All chemicals were of analytical reagent grade. All solutions were prepared in D2O and oxygen was removed by purging high-purity nitrogen.
Instruments and in-situ FTIR spectroelectrochemical measurementsThe in-situ FTIR experiments were carried out on a NicoletNexus 870 spectrometer equipped with a variable-angle specular reflectance accessory (VeeMax II) and an MCT-A detector cooled with liquid nitrogen. The electrochemical equipment used was an EG&G PAR Model 283 potentiostat/galvanostat. The in situ FTIR and voltammetric measurements were performed in a spectroelectrochemical cell. The working, counter and reference electrodes were a glassy carbon electrode (EG&G), a Pt wire and an Ag/AgCl electrode, respectively. The IR transparent window was a disk of calcium fluoride, and the angle of incidence on the CaF2 window was 50˚. A total of 50 interferometric scans with a resolution of 4 cm -1 were accumulated for the spectrum, and the current was recorded simultaneously. The spectra were successively taken while the electrode potential was scanned at a rate of 2 mV s -1 . All of the IR spectra obtained are represented as ∆R/R in the normalized form according to the formulawhere R(ES) and R(Eb) are the reflected intensities measured at the sample and base potentials, respectively.
Results and Discussion
Thin-layer cyclic voltammogramThe thin-layer c...