A theoretical and experimental study on l-tyrosine and citrate mediated sustainable production of near infrared absorbing twisted gold nanorods.

Anisotropic gold nanoparticles displaying plasmon band in the near infrared region can play a crucial role in cancer therapy particularly with techniques such as photothermal therapy (PTT) and photodynamic therapy (PDT). Herein, we report an efficient, sustainable, one pot protocol for the fabrication of an unusual gold anisotropic shape, which we have named as twisted gold nanorods. These particles, though having dimensions in the nanoscale regime comparable to those of gold nanorods, display a continuous flat plasmon band like that of 2-D gold nanowire networks, extended up to the NIR-III (SWIR) range. The proposed strategy is simple and does not require any seed mediation, heating or potential toxic templates or organic solvents. Our process is based on the slow reduction of gold salt in presence of two mild reducing agents viz. l-tyrosine (an amino acid) and trisodium citrate. We observed that when both molecules are present together in particular concentrations, they direct the growth in form of twisted gold nanorods. The mechanism of growth has been described by a Diffusion Limited Aggregation numerical scheme, where it was assumed that both l-tyrosine and the gold ions in solution undergo a stochastic Brownian motion. The predictions of the model matched with the experiments with a good accuracy, indicating that the initial hypothesis is correct. The final structure has been thoroughly characterized in terms of morphology, while SERS and cytotoxic activity have also been demonstrated.