SERS-Active Substrates

Fig. 4.56. Schematic diagram of a SERS-active substrate and the measurement arrangement. Alumina nanoparticles are deposited on a glass surface and produce the required roughness. A thin silver layer is evaporated on to the nanoparticles and serves for the enhancement. Organic molecules adsorbed on the silver surface can be detected by irradiation with a laser and collecting the Raman scattered light.

DEVELOPMENT OF SERS-ACTIVE SUBSTRATES 4.1 SERS-Active Metal Electrodes... 

Tarabara V.V., Nabiev I.R., Feofanov A.V., Surface-enhanced Raman scattering (SERS) study of mercaptoethanol monolayer assemblies on silver citrate hydrosol. Preparation and characterization of modified hydrosol as a SERS-active substrate, Langmuir 1998 14 1092-1098. 

For an indirect detection of viral DNA the capture DNA is labeled with either a marker molecule and/or a SERS-active substrate. One approach is the use of a DNA hairpin. Here, the capture DNA is attached at one side to... 

In order to investigate molecules adsorbed at the solid-liquid interface roughened electrode surfaces or metal colloids in solution (sols) are prepared. For investigations of the solid-gas or solid-vapour interface several methods are available to produce metal island films on SERS-active substrates. 

Other SERS-active substrate techniques include mechanical polishing polycristalline silver (Vo-Dinh et al., 1988) or ion bombardment in vacuo (Wood and Zwemer, 1981 Davies et al., 1986). A chemical procedure to prepare silver island films using Tollen s reagent was developed by Ni and Cotton (1986) which turned out to be simple, rapid and highly reproducible. In addition, the surface roughness and hence the enhancement... 

Lucas BD, Kim JS, Chin C, Guo LJ (2008) Nanoimprint lithography based approach for the fabrication of large-area, uniformly oriented plasmonic arrays. Adv Mater 20 1129 Alvarez-Puebla R, Cui B, Bravo-Vasquez J, Veres T, Fenniri H (2007) Nanoimprinted SERS-active substrates with tunable surface plasmon resonances. J Phys Chem C 111 6720... 

The surface-enhanced Raman scattering (SERS)-active substrates were prepared by electrodeposition of Ag nanoparticles in multiwalled carbon nanotubes (MWCNTs)-based nanocomposites for SERS sensor application. 

The novel SERS-active substrates were prepared by electrodeposition of Ag nanoparticles in the MWCNTs-based nanocomposites. The formation of Ag-MWCNTs nanocomposite was characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The application of the Ag-MWCNTs nanocomposite in SERS was investigated by using rhodamine 6G (R6G). The present methodology demonstrates that the Ag-MWCNTs nanocomposite is suitable for SERS sensor. 

Further enhancement of the SERS can be achieved through precise control over the parameters at the metal particle size scale [10] Most SERS-active substrates were made from pure metallic nanostructures such as metal nanoparticles [33-35], metal particle arrays [5], roughened metal surfaces [36], or a combination with metal nanostructures and other nanomaterials [17, 18, 29, 37-39]. Recently, many strategies have shown the adsorbation of molecules on the surface of Ag and Au substrates for SERS applications [40]. SERS-active Ag nanostructures substrates are required to satisfy certain conditions with good reproducibility and stability [39]. For this reason, it is indispensable to develop and optimize the methods to prepare the SERS-active Ag substrates [41]. 

Lithography techniques such as nanosphere lithography and electron-beam lithography are ideal methods to fabricate the reproducible SERS substrates [51]. They have been shown to improve the substrates performance by controlling the size and shape of colloidal nanoparticles and interparticle spacing [52]. Lithography techniques are simple to implement and of low cost. These advantages make them suitable platforms for the fabrication of SERS-active substrate. 

Ag-MWCNTs-ACS-coated ITO and Ag-coated ITO substrates were studied. The Raman spectra of the R6G in aqueous solution at the surfaces of Ag-modified ITO and Ag-MWCNTs-ACS-modified ITO substrates are shown in Fig. 6.6a, b, respectively. The Raman intensity of R6G obtained at the Ag-MWCNTs-ACS-coated ITO is greater than that of Ag-coated ITO. It can be seen that the Ag- MWCNTs-ACS-coated ITO has a considerable effect on the Raman spectra with improvements of more than four times of magnitude as compared with the Ag-coated ITO. This may be attributed to the large surface area of MWCNTs. These results indicate that the Ag-MWCNTs-ACS nanocomposite is a highly SERS-active substrate. Raman spectra of Ag-MWCNTs-ACS nanocomposite to the additions of varying concentrations of R6G in aqueous solution are shown in Fig. 6.7 and the Raman peak intensity of R6G at 1,509 cm obtained at the Ag-MWCNTs-ACS-coated ITO versus different R6G concentration in a logarithmic scale is shown in the inset in Fig. 6.7. The Raman intensities of R6G at 1,509 cm vary linearly with the concentration of R6G in the range of lO -lO M. These results indicate that the Ag-MWCNTs-ACS nanocomposite is suitable for SERS-active substrates. 

In conclusion, it has been shown that the SERS techniques offer a means of sensitive detection of probe molecules. An efficient and simple SERS-active substrate prepared by electrodeposition of Ag on MWCNTs has been developed. The prepared Ag-MWCNT nanocomposites exhibited good SERS performance and also featured a simple application process. The technique may have a potential use for in situ determination of analytes. Therefore, such a work will lead to a very promising future for applications in SERS chemical sensors. 

Cejkova J, Prokopec V, Brazdova S, Kokaislova A, Matejka P, Stepanek F (2009) Characterization of copper SERS-active substrates prepared by electrochemical deposition. Appl Surf Scl 255 7864-7870... 

Hossain MK, Shibamoto K, Ishioka K, Kitajima M, Mitani T, Nakashima S (2007) 2D nanostructure of gold nanoparticles an approach to SERS-active substrate. J Lumin 122-123 792-795... 

The highly localized electromagnetic fields of SERS active substrates have been studied for a number of years by various groups utilizing near-field visualization techniques. The groups of Martin Moskovits and Vladimir Shalaev investigated the electromagnetic field distribution of laser-excited optical modes of fractal clusters... 

SERS-active substrate SERS-active substrate... 

Figure 11.9 illustrates a representative system of SERS-based immunoassays. Rohr et al. first proposed an SERS-based immunoassay using dye-labeled antibodies and silver-island films coated with a capture antibody [60]. In their system, a silver-island film is used for the base substrate linked to the capture antibody as well as the SERS-active substrate that enhances Raman signals of reporter molecules when the reporter dye-linked antibody conjugate is bound to the captured... 

Optical Trapping of Raman Active Objects Approached to a SERS-Active Substrate... 

Instead of creating SERS-active sites in an optical trap, Raman enhancement can also be obtained by moving Raman active probes to the vicinity of a SERS-active substrate using an optical tweezers [50]. As schemed in Eig. 18.4a, gold nanoparticles were pre-immobUized on the surface of a glass slide facing the solution where biological spores were trapped. When the spore is far from the... 

Surface-enhanced Raman scattering (SERS) has attracted considerable attention as a sensitive technique for the detection of chemical, environmental and biological agents in extremely low concentrations [1], The fabrication of reproducible SERS-active substrates with well-defined nanoscale geometries is an important challenge of current research in order to SERS spectroscopy would become a powerful analytical tool of practical purposes. 

Porous anodic aluminum oxide (AAO) which characterized by a closely packed regular array of columnar cells is well-established and widely-used material for formation of nanostructures for SERS [2,3]. Particularly, promising SERS-active substrates were prepared by vacuum deposition of silver onto commercially available alumina filters with open pores of 200-300 nm diameters [4], Nanowires and nanorods have been fabricated by filling the AAO pores with transition- or noble-metals. However, due to multistage procedure these nanoarrays being sensitive are rather complicated in fabrication. 

Figure 2. Absorption spectra of SERS-active substrates on the base of AAO with varying thickness of silver film. Bottom trace presents intrinsic absorption of the AAO plate.

The dependence of SERS intensity of CuTMPyP4 on the mass thickness of the silver evaporated on the surface of AAO has been studied to establish the optimum thickness of Ag films. The results obtained will be useful for fabrication of efficient AAO-based SERS-active substrates. 

The observation and understanding of SERS are clearly very important developments in the study of surface chemistry and surface physics. The combination of molecular information and extraordinary sensitivity provides a valuable probe of surface structure and behavior. Out of the broad study of SERS by both chemists and physicists have emerged several approaches to using SERS for chemical analysis. A common analytical situation involves preparation of a SERS active substrate by one of several methods, then exposure of the substrate to a liquid or gaseous sample. Subsequent Raman spectroscopy of the adsorbed layer provides the analytical signal, enhanced by whatever chemical or field enhancement is provided by the adsorbate-substrate interaction. The current and next section are not intended to address SERS substrates comprehensively, but several of analytical interest are described. 

An interesting possibility is inducing SERS activity, in a non-SERS-active substrate, by depositing submonolayer quantities of silver on its surface. Van Duyne and Haushalter used this method to measure Raman scattering from a GaAs semiconductor interface. There was also an experiment to use a silver underlayer to induce SERS in a layer covering it. ... 

Nitzan and Brus" " have proposed that photochemical reactions on SERS-active substrates may be also enhanced. Goncher and Harris" reported photofragmentation of several molecules adsorbed on a silver surface, with an incident wavelength of 363.8 nm( ). They attribute the reaction to enhanced singlet-triplet transitions or to multiphoton processes. Chen and Osgood" reported enhanced photodeposition on several metals, but not on gold, at 257 nm. 

To prepare highly SERS-active substrates, proper surface roughening procedures are necessary. 

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