Noble metal nanomaterials have been widely used in
sensors mainly due to their enhanced localized surface plasmon
resonance (LSPR), which depends on the size, shape,
composition, and separation distance of nanoparticles, as well
as the dielectric environment of the surrounding medium
(Petryayeva and Krull, 2011). The high conductivity of noble
metal nanoparticles also promotes the electron transfer between
electrode surface and the analyst (Lei and Ju, 2012). The
presence of enhanced LSPR leads to high molar extinction
coefficient and Rayleigh scattering, as well as enhanced local
electromagnetic fields near the nanoparticle surface. Based
on theoretical calculation, nano-Au spheres of w40 nm in
diameter have an absorption cross-section 5 orders of
magnitude higher than conventional dyes, while 80-nm nano-
Au spheres scatter light 5 orders of magnitude more than
fluorescence dyes (Jain et al., 2006). Nobel metals were used
mainly in colorimetric and surface-enhanced Raman spectroscopy
(SERS) sensing. Colorimetric assays are fast and
simple. The signal transduction relies on the color change of
the nanoparticle suspension due to different interparticle
distance or aggregation state (Kelly et al., 2003). It has been
studied for detection of DNA, diagnosis of pathogen infection
and pathogen monitoring in water samples. However, the
aggregation state of nanoparticles is sensitive to the solution
chemistry and difficult to control. The coexisting water/
wastewater constitutes will greatly affect results, reducing
reproducibility. The SERS phenomenon is attributed to both
electromagnetic effect and chemical mechanisms related to
the charge transfer between the noble metals and the target
molecules (Moskovits, 2005). As a result, the efficiency of
Raman scattering can be enhanced more than 1014 fold, which
is even capable of detecting a single molecule (Nie and Emery,
1997).