Biodetection Based on Resonance Light Scattering of Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 174

8.3.2 Ag NPs as RLS Probes

Like Au NPs, Ag NPs are useful nanomaterials for many applications,
mainly because of their interesting optical, electronic, and catalytic
properties. 61-63 Ag NPs interact with photons more eficiently than
any other type of NP of the same dimension, making them useful in a
wide variety of optical applications. 61-63 The LSPR bands of spherical
Ag NPs occur at around 410 nm, which are shorter that those of
spherical Au NPs, which are around 520 nm. Based on the fact that
RLS intensity is proportional to the fourth power of frequency, Ag
NPs are more useful RLS probes than Au NPs. However, Ag NPs are
not as stable as Au NPs. In addition, it is more dificult to prepare
high-purity Ag NPs that have a narrow size distribution. In order to
overcome this disadvantage, seeding methods have been suggested
for the preparation of Au-Ag NPs. 64-66

8.3.2.1 Small analytes

By taking advantage of the high reduction potential of Ag ions,
analytes can be detected by RLS through the induced formation of
Ag NPs. To determine the analytes that are capable of the reduction
of Ag ions, Ag ions and the analytes are mixed to form Ag NPs. The
RLS intensity of the formed Ag NPs is then measured. This method is
useful for the determination of Sudan dyes in food products. 67 Sudan
dyes including I, II, III, and IV posses nitrogen-nitrogen double bonds
and phenol groups in their molecular structures, which are able to
reduce Ag ions to form Ag NPs. The formation of Ag NPs induced
by Sudan IV can be easily observed as light scattering when using a
laser pointer and a light emitting diode to irradiate solutions (Fig.
8.12). Upon increasing Sudan IV concentration, the RLS intensity
increases. Although the wavelength of the laser pointer (653 nm) is
located at the far end of the RLS spectra, it is still a useful light source
for generating these RLS signals because its output power (2.0 mW)
is much higher than that of the light emitting diode (0.5 mW, 458
nm). Although color change from the red of Sudan to the brown
of Ag NPs can be observed by the naked eye, its sensitivity is too
poor (1.0 μM) for colorimetric detection to be practical. In order to
improve the sensitivity, a commercial spectroluorometer is effective
to detect the enhanced RLS signals characterized at 452 nm. The RLS
intensities are proportional to the dye concentrations over the range
of 0.2-2.4 μM Sudan I, 0.1-2.4 μM Sudan II, 0.1-2.4 μM Sudan III, and