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Preparation and Characterization of Strong and Uniform Surface-Enhanced Raman Spectroscopy Substrate

Author: GaoMinXia
Tutor: RenBin
School: Xiamen University
Course: Physical and chemical
Keywords: Surface-enhanced Raman Spectroscopy Electrodeposition in-situ UV Absorption Spectroscopy Highly Sensitive Detection Au
CLC: TB383.1
Type: Master's thesis
Year: 2007
Downloads: 248
Quote: 1
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Abstract

Surface-enhanced Raman spectroscopy（SERS） is becoming a potential and widely used analytical technique,because it can provide extremely high detection sensitivity up to single molecule level and vibrational information spreading the whole molecular vibration spectrum.However,whether SERS will be developed into a technique with practical importance depends very much on the stability of the substrate and reproducibility of the data obtained.The key issue is how to obtain uniform and highly SERS-active substrates.The present thesis employed the reported method,which is to chemically assemble Au nanoparticles on ITO surfaces,to obtain uniform SERS substrates in a large area.Thereafter,we proposed the electrochemical deposition method to enlarge the nanoparticles to induce the effective coupling of the electromagnetic field of neighboring nanoparticles to significantly enhance the SERS activity of the substrates.We systematically explored the condition for preparing SERS substrates,characterized the SERS activity and detection sensitivity of the substrates,and use SERS mapping to check the substrate uniformity The main results and conclusions are summarized as follows:1.The condition for assembling the nanoparticle with different diameters on APTMS modified ITO substrate was systematically explored.For Au nanoparticles with diameter about or smaller than 30 nm,very uniform substrates could be obtained by assembling for 12 h.For 50 nm nanoparticles,a uniform surface could only be obtained by concentrating the sol solution for about 3 times and assembling for ca. 5-7 h.For nanoparticles with even larger size,direct assembly in original sol solution for 24 h can produce a relative uniform surface.2.The influence of electrodeposition potential and time on the SERS activity and growing process of assembled Au nanoparticles of different size was systematically investigated.It was found that for all the investigated nanoparticles, the nanoparticles could remain the spherical shape during the electrochemical growing process and the deposition would not occur on the ITO surface by controlling the deposition potential at -20 mV.In situ electrochemical UV-vis absorption spectroscopy was constructed to monitor the electrochemical deposition potential and time on the SERS activity with pyridine as a probe molecule to monitor the SERS activity of the substrate.It was found that different deposition time was required for nanoparticles of different size.The optimal deposition times for nanoparticles with a size of 12,30,50 and 70 nm were 1.5,5, 7,and 2 min,respectively.It was further found that the substrate prepared from 50 and 70 nm nanoparticles showed the highest SERS enhancement.3.It was found that the combined chemical assembly and electrochemical deposition method can effectively exclude the interference of the impurities on the substrates to systems with the low concentration or weak adsorption.The main reason is that it is not necessary to use organic molecules or capping reagents and the negative potential applied on the nanoparticle surface will lead to the desorption of capping reagent during electrodeposition process.On such kind of clean substrate,a non-resonant molecule,4,4’-bipyridine was used as a probe molecule to demonstrate the detection sensitivity.On both substrate prepared from 50 nm and 70 nm nanoparticles,a detection limit of as low as 10^-13 M was achieved.4.The uniformity of the SERS substrates was characterized by using SERS mapping. It was found that with the increase of the particle size,the uniformity of the substrate was decreasing.The signal deviation of the strongest signal to the weakest signal was about 7%,19%,and 16%for nanoparticles with a size of 12 nm,30 nm,and 50 nm,respectively,which are all less than the acceptable value of 20%for a commercialized SERS substrate.However,for nanoparticles of 70 nm,the signal deviation was larger than 20%.

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