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Improving the molecular specificity of surface-enhanced Raman spectroscopy

The synthesis of silver colloid particles with controlled size significantly increases the application range of surface-enhanced Raman spectroscopy.
23 September 2007, SPIE Newsroom. DOI: 10.1117/2.1200709.0835

Silver nanoparticles are currently generating significant interest due to their attractive physicochemical properties. In recent years, they have demonstrated their potential in catalysis and electronics applications,1,2 in the fabrication of highly sensitive detectors,3 and as substrates for surface-enhanced Raman spectroscopy (SERS).4

Since their usage depends on particle characteristics such as size, size distribution, and charge, our present research is focused on the fabrication of SERS−active nanoparticles using synthesis approaches that allow particle size control. Recently, our group used sugars as reducing agents and ammonia as a complexing agent to produce silver nanoparticles ranging in size from tens to hundreds of nanometers (see Figures 1 and 2).5

Figure 1. Surface-enhanced Raman spectra of 1-methyl adenine (10−5 mol L−1) in presence of a silver hydrosol prepared by the reduction of [Ag(NH3)2]+ with D(+)−xylose. The average particle size is 55nm. Silver hydrosols were activated by 0.002 (A), 0.01 (B), and 0.1 mol L−1 (C) of NaCl. The Raman spectrum of 1−methyl adenine (10−2 mol L−1) in deionised water (D) is included for comparison purposes .5

Our interest in producing SERS−active silver nanoparticles is motivated by the wide use of SERS, a powerful chemical sensing technique that combines extremely low detection limits with high molecular specificity.6 SERS and surface−enhanced resonant Raman spectroscopy (SERRS) are routinely used for investigating the adsorption of simple organic molecules on silver films and colloids. Recent examples include 1,10−phenanthroline,7 p−nitrothiophenol,8 1,4−dihydrazinophthalazine,9 fatty acids,10 and nucleic bases and their derivatives.11 More recently, oxygen release in hemoglobin, the oxygen carrier in blood, was studied with SERRS using silver colloids.12

Having overcome earlier signal reproducibility problems associated with the preparation and activation of silver particles, SERS has now emerged as a reliable analytical method. It can now be used as a sensitive probe of ultra−low DNA concentrations adsorbed on colloidal silver11, of capillary electrophoresis eluents13, and also for the quantitative determination of picomolar amounts of important industrial and biological compounds.

The high level of signal enhancement in SERS and SERRS also enables the detection of individual molecules adsorbed on silver particles. Several studies have shown that Raman enhancement factors of the order of 1014 to 1015 can be gained using particles of an appropriate size, which depends on the selected laser excitation wavelength. For example, particles in the 70 to 200nm size range require excitation in the 488 to 647nm interval. This is in good agreement with results showing that 80 to 100nm−sized particles can be excited with the 514.5nm line of an Argon laser, commonly used in SERS experiments with silver colloids.14 A high degree of enhancement can be also be obtained with smaller particles (≈ 50nm in diameter). In this case, however, it is necessary to activate the silver particles, which results in a partial aggregation process producing slightly enlarged particles.15 Halide ions (excluding fluorites) are the most commonly used activation agent.16

Our research suggests that the synthesis of size−controlled silver colloid nanoparticles has great potential for the development of novel SERS applications in chemistry and biology.

Figure 2. Atomic force microscopy image of silver nanoparticles prepared by the reduction of [Ag(NH3)2]+ with D(+)−maltose. The average particle size is ≈ 25nm.

Jana Soukupová, Robert Prucek, Libor Kvítek, Aleš Panácek 
Department of Physical Chemistry
Centre for Nanomaterial
Research Palacky University
Olomouc, Czech Republic