Surface-Enhanced Infrared Absorption Spectroscopic Studies In Gold Nanoparticles Aqueous Solution

The effect of the infrared absorption on molecules on metal nano film (island film) is10to20times stronger than the absorption on bright substrate has come to be known as surface-enhanced infrared absorption (SEIRA). Surface-enhanced infrared spectroscopy (SEIRAS) with highly sensitivity, which is the important means of studying solid-gas interface, solid-liquid interface and solids-vacuum surface and interface. However, conventional electrochemical methods are difficulty to detect the real-time information of interface reaction directly, substance concentration and the changes in morphology of electrode and electrolyte. Moreover, it is difficult to conduct more in-depth research on various electrochemical phenomena and reaction mechanisms. But the Electrochemical Surface-enhanced Infrared Spectroscopy (EC-SEIRAS) can be able to provide specific information on the molecular structure, which is convenient to make a research on material status and reaction mechanism. Although there are deficiencies that low wave number cannot be measured and the differential p lanogram needed to be processing in EC-SEIRAS. But there are also some benefits on EC-SEIRAS, the absorption enhancement effect is little influenced by the type of metal; the reversibility of spectrum signal potential is good; the simple surface selection rules and the highly sensitive to small polar molecules. Years of practice shows that EC-SEIRAS has been successfully applied to the study of electrodes adsorbed molecular configuration, small organic molecules catalyzed and electrode interface coordination reaction for its unique advantages. In recent years, lots of research groups devoted many efforts on the phenomenon of surface-enhanced infrared. As we known, most the methods of studying surface-enhanced infrared are based on the attenuated total reflection set-ups ATR, whereas, literature have also been reported surface-enhanced infrared external reflection spectroscopy, but all of these use roughened metal substrate deposited on the surface as working electrode [10,11],and the operation of electrodepositing metals film (electroless deposition or vacuum evaporated) is complicated and low reproducibility. In this report, we make a thorough inquiry into the phenomenon of surface-enhanced infrared in gold nanoparticles for the first time. The infrared absorption enhancement phenomenon can be observed in different electrical properties gold nanoparticles solution. The biggest advantage of this method is that there is no need to prepare the special ATR reflecting device and the working electrode. Only while adding gold nanoparticles in solution simultaneously can be achieved indicate infrared enhanced. The main contents are summarized as follows:1. In view of gold nanoparticle has the good adsorption properties, the normal characteristics of nanoparticle:size effect, surface effect, small size effect and so on. What is more, gold nanoparticle also has good biocompatibility, which can keep the biological activity of enzyme after the enzyme adsorbed on gold nanoparticle. Different particle size, concentration and opposite electric charge gold nanoparticles aqueous solution has been prepared. Then the gold nanoparticles aqueous solution has also been special processed and characterized, which would be the reaction environment of the electron transfer of dopamine, ascorbic acid and the cobalt-phenanthroline complex.2. The surface-enhanced infrared phenomenon could be seen in negatively charged gold nanoparticles aqueous solution with DA and cobalt-phenanthroline complex by time-resolved rapid scan infrared spectroscopy. The results show that in the gold nanoparticles aqueous solution the cyclic voltammetry behaviour of DA and cobalt-phenanthroline complex observed in negatively charged GNP aqueous solution is no different with that in aqueous GNP absence. However the infrared absorption enhancement phenomenon can be clearly observed in the infrared spectroscopy. And the absorbance of IR was significantly larger with the decrease of the particle size of GNP. The smaller particle size of GNP was, the more enhanced infrared absorbance of peak could be observed. The result implies that the gold nanoparticle does not affect the redox mechanism of DA on platinum electrode. But SEIRA cannot be discovered on ascorbic acid and benzoquinone. The results suggest that molecular with positive charge can adsorbed on GNP thorough electrostatic force and SEIRA spectra can be observed. 3. The negatively charged ascorbic acid was also tested in positively charged gold nanoparticles aqueous solution by time-resolved rapid scan infrared spectroscopy. The phenomenon of surface-enhanced infrared could be observed in this system. And the experimental phenomena implied that the gold nanoparticle does not affect the redox mechanism of AA on platinum electrode. At the same time, phenomenon of surface-enhanced infrared cannot be observed on positively charged DA and electrically neutral benzoquinone. Based on the infrared spectra3D scene and derivative cyclic voltabsorptometry, we guess that the surfece-enhanced infrared here is caused by the adsorption of the tested substances on GNP. There are two kinds of interaction force between the tested substances and the GNP. First, as the atoms which located on the surface of the nanoparticles have surface energy, the surface energy of GNP continues to increase as atoms assembling on the surface, so the tested substances can be adsorbed on the surface of GNP. Secondly, silver and gold nano aqueous solution usually takes charge and the tested substances is also chargeability, electrostatic forces can contribute to the adsorption. So the charged gold nanoparticles can adsorb the opposite charged tested substance.