Simple Method for Tuning the Optical Properties of Thermoresponsive Plasmonic Nanogels

We report a straightforward
way for forming and tuning the optical
properties of thermally responsive plasmonic nanogels. Upon functionalization,
a small red shift (2–3 nm) of the pNIPAM@AuNPs was observed
due to changes in the refractive index surrounding the AuNP. By adding
thermoresponsive poly-N-isopropylacrylamide (pNIPAM)
into the pNIPAM@AuNP, its optical response was significantly increased.
Heating the nanogel such that the pNIPAM collapsed and acted as a
cross-link resulted in the aggregation of the AuNPs. The plasmonic
response with red shifts of up to 20 nm was observed. The enlarged
red shift was due to the increase in the dielectric constant around
the particles and the interparticle interaction of the AuNPs. The
interparticle interaction also leads to the broadening of the spectra.
Experimental data and finite-difference time-domain (FDTD) calculation
are in agreement with this observation. The temperature-dependent
optical properties were reversible through multiple cycles of heating
and cooling.