  Aggregation of the nanoparticles is a natural phenomena due to various
biological and physical parameters but these aggregates are highly unstable. If
aggregates would have been stable then it might be useful for some biomedical
applications. To study the optical properties and photothermal heat generation
of stable aggregates of the metal nanopartilces, we present a forced synthesis
of aggregates of small gold nanorods in a mixture of Dulbecco's Modified
Eagle's medium (DMEM) and Bovine Serum Albumin (BSA). We synthesised
hexadecyltrimethylammonium bromide (CTAB) coated gold nanorods and then
prepared the stable aggregates to study the optical characteristics of these
aggregates. Absorption spectra of aggregates show the redshift compared to the
monodispersive gold nanorods and confirm that the shape and size of aggregate
depend on the amount of BSA in DMEM as well as on the concentration of CTAB in
the stock solution of the gold nanorods suspension. The higher concentration of
the BSA in DMEM and lower concentration of CTAB of monodispersive gold nanorod
suspension causes greater redshift. We tuned the well defined plasmonic
absorption resonance peaks of the aggregates of gold nanorods up to the second
biological therapeutic window. We studied the stability of the synthesised
aggregates of gold nanorods for up to one week and found that the aggregates
were stable for atleast one week. A photothermal study of these aggregates was
also carried out using high power broadband near-infrared light source.
Aggregates were also photothermally stable which warrants their repeated use
for such applications. The photothermal conversion efficiency of the these
stable gold nanorod aggregates were higher than its monodispersive form of the
nanorods.
