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Silver Nanoparticles for the Simultaneous Raman Detection of Anions, Cations and Neutral Molecules

N.M.S. Sirimuthu and S.E.J. Bell
Innovative Molecular Materials Group,Queen’s University, UK

Raman, analysis, SERS

Citrate-reduced silver colloids are used extensively for SERS (surface-enhanced Raman spectroscopy) detection of low concentrations of cations but are typically ineffective for detection of anions, since the metal surface bears a negatively charged citrate layer. This problem can be reduced by using surface modifying agents which create a positive metal surface or, in our approach, by using materials which aggregate the colloids but also allow negatively-charged molecules to access the surface directly.
Here we extend this approach to the simultaneous detection of even oppositely-charged analytes. The main problem to overcome is that the experimental conditions required for detection of positively and negatively charged analytes are very different. However, we have developed a method of preparing arrays of polymer modified nanoparticle clusters, in which each of several <100 m diameter clusters is modified so that it is sensitive to a different class of analyte (e.g. anions or cations). When a complex mixture of compounds must be analysed, a small (<l) drop is applied to the array and the Raman spectra from each of the different clusters recorded simultaneously. Since the individual signals from each cluster are spatially resolved, the spectra from each class of analyte can be detected separately.

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