Biodetection Based on Resonance Light Scattering of Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 164

free solution. The last two images clearly exhibit the localization of
Nile Red-absorbed 32 nm Au NPs, showing that this dual detection
system allows clear observation of GSH located in the central part in
individual cells.
Figure 8.2 Comparison of the (A) phase contrast, (B) scattering, and (C)
luorescence images of erythrocytes incubated with 1 × Nile
Red-absorbed 32 nm Au NPs for 12 h. The exposure times were
(A) 0.25 s, (B) 0.5 s, and (D) 4 s, respectively. Light was collected
using a 40 objective for the phase contrast and scattering
measurements and a 100 objective for the luorescence
measurements. Reprinted from Ref. 31 with permission. See
also Color Insert.
RLS spectroscopy can also be used to study the interactions
between cysteine and Au NPs. 32 Through covalent interaction with
the -SH group and the electrostatic binding with the -N H 3 + group of
cysteine, the self-assembling property of the Au NPs forms a network
structure, which results in greatly enhanced RLS intensity. This RLS
approach provides a linear range of cysteine concentrations, from
0.01 to 0.25 μg/mL, with an LOD of 2.0 ng/mL (16.5 nM). To minimize
the interference of GSH, detection of cysteine by this RLS approach
at pH 5.0 is proper. The RLS signals of GSH on Au reach a maximum
at pH 3.2 and there is a sharp decrease in the RLS signal over the
pH range 3.2-4.3. However, homocysteine interference cannot be
prevented; homocysteine also induces an increased RLS signal at
pH 5.0.
The RLS technique is also practical for the determination of
glucose, based on the glucose-induced dissociation of aggregates of
dextran (MW. 170 K) coated Au NPs with concanavalin A (Fig. 8.3). 33
Tetravalent protein concanavalin A is readily cross-linked with
dextran-coated Au NPs (20 nm diameter), leading to a broadening
and red shift in the LSPR band of Au NPs. 34 Addition of glucose
that competitively binds to concanavalin A leads to aggregation of
dextran-Au NPs with concanavalin A to a lesser extent. As a result,