Biodetection Based on Resonance Light Scattering of Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 162

8.3.1 Au NPs as RLS Probes

Au NPs have been studied extensively in recent years due to their
interesting optical properties and bio-compatibility. 25 Au NPs display
intense colors ranging from rose red to purple. This is due to intense
surface plasmon absorption and scattering at different wavelengths,
which is highly dependent upon the NP size and shape. 26 The
localized surface plasmon resonance (LSPR) band is due to electron
oscillations in the metallic particles, induced by the incident light
ield. Upon increasing the size of Au NPs, LSPR bands undergo red
shifts. Because of the strong oscillations from incident light in the
visible region, RLS signals of analytes on the surface of Au NPs can
be enhanced with magnitudes greater than 100-fold. 27 As a result,
Au NPs have been shown to be effective matrices for the quantitative
detection of analytes by RLS.

8.3.1.1 Small analytes

Because of strong interactions of thiol compounds with Au NPs
through Au-S bonding, Au NPs have shown great selectivity for thiol-
containing compounds such as cysteine and glutathione (GSH). 28,29
The determination of thiol-containing pharmaceutical compounds
like thiamazole by RLS has been demonstrated using 13 nm
diameter Au NPs. 27 At pH 5.2, the RLS spectrum of Au NPs shows
a maximum peak at 555 nm and the RLS intensity is enhanced by
trace amounts of thiamazole. The binding of Au NPs to thiamazole
results in ligand-induced aggregation of Au NPs and signiicantly
enhances the RLS intensity. Figure 8.1 demonstrates the relationship
between relative RLS intensity and the ratio of thiamazole/Au
NPs at a constant thiamazole concentration. An Au NP solution at
higher particle concentration is less prone to aggregation, which is
possibly due to the lack of enough thiamazole molecules per particle
to induce aggregation. In term of sensitivity, it is suggested to use
low concentrations of Au NPs when the analyte concentration is
low. The RLS approach provides a linear range from 2.36 nM to
1.05 μM for thiamazole, with a limit of detection (LOD) at signal-to-
noise ratio 3 ( S / N = 3) of 2.01 nM ( n = 11, RSD = 1.86%). Similarly,
a RLS spectroscopy has been successfully applied to investigation
of the aggregation process of Au NPs (10 nm diameter) induced by
captopril. 30 In an aqueous medium of pH 2.09, Au NPs and captopril
form core-shell assemble of [(Au) 31000 ]@[(Captopril) 2200 ]. As a