Quantitative Surface Enhanced Raman Spectroscopy

Abstract: Surface enhanced Raman spectroscopy (SERS) is a very attractive technique for detection of various organic and inorganic molecules due to its sensitivity and selectivity. The surface enhancement is achieved when the molecules of interest are on or in the vicinity of a metal surface, usually gold and silver. This proximity can increase the Raman signal with up to 14 orders of magnitude.

The aim of this thesis was to investigate SERS as a quantitative technique for determination of organic compounds in complex matrices such as different biological fluids and single living cells. Metal colloids were used for surface enhancement due to their high enhancing capacity, ease of production and high flexibility.

Determination of the anti-cancer drug doxorubicin, in blood plasma, was performed using silver particles as enhancers of Raman scattering and quenchers of the fluorescent background. The spectra obtained were evaluated with multivariate statistics, principal components analysis (PCA) and partial least squares (PLS). Even if the surface enhancement was large enough to enable determinations at low concentrations, the reproducibility, due to uncontrollable variation in the enhancement was poor.

To increase the reproducibility, a novel technique to normalise the intensities of the spectral bands is presented. The internal standard was organised in a self-assembled monolayer (SAM) onto the gold or silver surfaces. The collective behaviour among the enhancing colloids, such as aggregation, could thereby be corrected for, and an increase of the precision with one order of magnitude was reached. Different SAM layers were evaluated for spectral stability and signal strength. Selection criteria for such an internal standard should be spectral and chemical stability, few and selective bands and weak to moderate signal intensities.

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