Optical and tomography studies of water-soluble gold nanoparticles on bacterial exopolysaccharides

Abstract

Gold nanoparticles of different shapes (spherical, rods, and prisms) aggregate when deposited onto Lactobacillus fermentum's exopolysaccharide (EPS), a set of polysaccharides excreted by the bacteria. Transmission electron microscopy studies revealed that gold nanoparticles have high affinity for EPS. UV-vis spectra of aggregated gold nanoparticles showed additional absorbance peaks at lower energies in comparison with isolated nanoparticles. In the case of gold nanoprisms, the aggregation leads to a new absorption at a very low energy centered at 1100 nm. Moreover, the EPS of L. fermentum itself produces gold aggregates from a Au(III) solution. Surface-enhanced Raman spectroscopy performances for the detection of rhodamine B of gold aggregates were drastically different. A tomography study on all samples revealed clear differences in the extension of the EPS coating on the gold nanoparticles. Only the gold aggregate in which gold interparticle surfaces were exposed to RhB showed a drastic increase (two orders of magnitude) of intensity in the Raman spectrum of RhB.