6_SERS

1 Electrochemical SERS Objectives 1. To prepare SERS-active gold and silver electrodes for the electrochemical study 2. To perform electrochemical surface enhanced Raman measurements 3. To interpret the observed experimental data 2 GENERAL BACKGROUND Raman spectroscopy can sensitively provide structural information of molecules by observation of the vibrational spectrum of the target species. By exciting a sample with a monochromatic laser light, a Raman spectrum can be easily obtained to the red (Stokes) and to the blue (anti-Stokes) of the excitation wavelength, corresponding to the vibrational frequencies of the allowed vibrational modes. In common practice, only the Stokes Raman frequencies are measured and shown in the Raman spectrum, owing to their stronger signal intensity. Raman spectra can be easily obtained over a wide frequency range of molecular vibrations for gas, solid, and liquid samples. However, the Raman process is of very low quantum yield, leading to very weak signals especially for species at the solid/liquid and solid/gas interfaces. The Surface-Enhanced Raman Scattering (SERS) effect, discovered in the mid of 1970s, can greatly enhance the signal (typically by 106 fold) for species adsorbed on metal nanoparticles or substrates consisting of nanostructures of Ag, Cu, Ag, other transition metals, and some semiconductors. It can even provide sensitivity down to the single-molecule level under optimal conditions. In the present two sets of experiments, we will show how to perform SERS experiment on assembled metal colloids and electrochemically roughened SERS substrates. EXPERIMENTAL PROCEDURES Instrument Scheme 1: Schematic configuration of a confocal microprobe Raman instrument. Raman instruments: Confocal microprobe Raman instruments are the dominant instruments in the market. The simplified optical path of a Raman instrument is displayed above, which contains the following parts: laser source, incident and collection objective, sample chamber, monochromator, and detector. Two types of Raman instruments will be used: Renishaw R- 1000 and the JY Labram I. Electrochemical Instrument: A CHI instruments system will be used for recording the cyclic voltammogram and recording and controlling the potential during electrochemical Raman study. Other instruments: ultrasonic bath, centrifuge Other necessary materials and devices: pipetts, electrochemical Raman cells with embedded counter electrode, Au electrode, Ag electrode, SCE reference, 0.01 M pyridine 3 +0.1 M KCl solution, 1mM adenine + 0.1 M NaClO4solution, 0.5 M pyridine solution in a sealed glass tube, saturated adenine solution, adenine powder, 0.1 M KCl solution, 55 nm Au colloids Au and Ag Electrode pretreatment: Au and Ag electrode should be polished successively with 6 μm emery paper and then 3 and 1μm alumina polishing power on a polishing cloth (Buehler, micropolishing cloth). After each polishing step, the electrode should be ultrasonically cleaned 3 times to ensure cleanliness and a mirror-finish surface. After polishing, the Au electrode should be further electrochemically cleaned in 0.5 M H2SO4 in the potential range of -0.3 V to 1.45 V vs. SCE for several cycles until a reproducible voltammetric feature is obtained. Raman measurement: One may consider choosing the proper objective, extending the acquisition time (e.g. 10 s) and/or increasing the accumulation numbers to get high quality spectra,. For the sake of comparison, the spectra should cover a spectral range of 400-1800 cm-1 in the case of adenine, and 150 to 3500 cm-1 in the case of pyridine. One may use a ×50 objective with a long working distance for acquiring the SERS spectrum and the normal Raman spectrum of the solid sample. A ×10 objective is used to collect the solution spectrum. Experiment 1: SERS study of adenine adsorption on an assembled Au colloid surface Electrode preparation: 1. Cleaning of colloidal solution: the colloids should be cleaned before use by centrifugating for 3 times to remove the major amount of reductant remained from the synthesis process for stabilizing the colloids. After each round of cleaning, the colloids should be redistributed in the equal amount of ultrapure water by sonication. 2. Dispersing of Au colloids on Au electrode: place a droplet of colloid solution on the Au electrode, and repeat for 3 times to obtain a SERS-active Au substrate. Raman measurement In EC-SERS study, it is necessary to acquire the solution spectrum and/or the spectrum of solid sample to compare with the spectral feature of the adsorbed species. 1. Acquisition of solution spectrum: Use the full laser power to obtain the solution spectrum. For the transparent sample, it is recommended to use an objective of lower NA, such as ca. 0.2, to ensure a longer collection path therefore a stronger signal. 2. Raman measurement of the solid adenine sample: Uniformly disperse a small amount of adenine powder over a clean glass slide. Focus the laser on the powder with an objective of high NA. The laser power should be adjusted to a value (normally <1 mW) that will not lead to the decomposition of the spectra. Increasing the accumulation time will be helpful in obtaining a better signal to noise ratio. To avoid the interference of the fluorescence of the commercially available adenine, it is better to prepare a saturated adenine solution, which is then dropped over a glass slide. In the coffee ring formed during the drying process of the solution, some tiny crystals with a higher purity and less fluorescence will form to ensure a better Raman signal. 4 3. Perform the cyclic voltammetric measurement in 0.1 M NaClO4 + 1 mM adenine solution to check the electrochemical potential window. 4. Perform the SERS measurement with the colloids assembled Au electrodes : Mount the SERS active Au electrode in the electrochemical Raman cell. Then 0.1 M NaClO4 + 1 mM adenine solution was added into the cell. Cover the cell with the quartz window and connect the three electrode to the potentiostat. After the electrode has been stabilized for about 5-10 min., the SERS measurement can be performed. One should properly focus the laser onto the electrode surface rather on the window. SERS signal may be acquired from the open circuit potential to -1.0 V and then back to positive potential until the appearance of the Au oxide bands at about 500 cm-1. The potential step can be 100 mV. After each change of the potential, the system should be stabilized for about 30 s before the spectrum is acquired. 5. Data Analysis. By comparing the change in the relative signal intensity and frequency to propose the interaction of adenine on the surface. Experiment 2 : Electrochemical SERS study of adsorption of pyridine on Ag electrode surface Preparation of the SERS active silver electrode 1. Polish a silver electrode according to the procedure described in the section of experimental procedure until a mirror-finish surface is obtained. 2. Roughen the silver electrode in 0.1 mol/L KCl solution by scanning from -1.3 V to 0.2 V at a rate of 0.1 V s-1, and then back to 0.05 V at 0.01 V s-1, and then back to -0.3 V at 0.001 V s-1for a complete reduction. The roughened surface should now have a milky finish. Raman measurement 1. Solution spectrum of 0.5 M pyridine: due to the strong odor of pyridine solution, the pyridine solution should be sealed into a glass tube prior to the experiment. A x10 objective is used to collect the solution Raman spectrum by directly focusing the laser into the tube. Normally 900 s is recommended to ensure a good signal to noise ratio and the room light should be turned off during the acquisition of the solution spectrum. 2. Mount the SERS active Ag electrode in the electrochemical Raman cell. Then add 0.1 M KCl +0.01 M pyridine solution to the cell. Cover the cell with the quartz window and connect the three electrodes to the potentiostat. After the electrode has been stabilized for about 5-10 min., the SERS measurement can be performed. One should properly focus the laser spot onto the electrode surface rather on the window. 3. Suggested experimental conditions: acquisition time: 1 s; accumulation time: 5; spectral range: 200 -3400 cm-1. Laser power: less than 1 mW. The grating can be 1800 g/mm. The experiment can be done with a potential step of 100 mV from the open circuit potential to -1.0 V. After each change of the potential, the system should be stabilized for about 30 s before the spectrum is acquired. 5 Data Analysis Compare the solution spectrum to that of SERS spectra and analyze both the dependence of band position and intensity on the applied potential. References: 1. M. Fleischmann, P. J. Hendra, A. J. McQuillan, Raman spectra of pyridine adsorbed at a silver electrode, Chem. Phys. Lett., 26(1974)163～166. 2. D. L. Jeanmaire , R. P. Van Duyne, Surface Raman spectroscopy. 1. Heterocyclic , amromatic , and aliphatic amines adsorbed on anodized silver electrode, J . Electroanal. Chem., 84(1977)1～20. 3. B. Ren, X. F. Lin, Y. X. Jiang, P. G. Cao, Y. Xie, Q. J. Huang, Z. Q. Tian, Optimizing detection sensitivity on SERS of transition metal electrodes with confocal microprobe Raman spectroscopy, Appl. Spectrosc., 57(2003)419～427. 4. D. Y. Wu, J. F. Li, B. Ren, Z. Q. Tian, Electrochemical Surface-enhanced Raman Spectroscopy of Nanostructures, Chem. Soc. Rev., 37(2008) 1025～1041. 5. Z. Q. Tian, B. Ren, Raman spectroscopy of electrode surfaces, in: Encyclopedia of Electrochemistry (Ed. A. J. Bard), Wiley, Vol. 3(2003)572～659. 6. B. Ren, D. Y. Wu, Z. Q. Tian, In situ Raman Spectroscopic Studies of Pyridine Adsorption on Different Transition Metal Surfaces, in: In-Situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis (Eds. S. G. Sun, P. A. Christensen and A. Wieckowski), Elsevier, (2008)299～337.