Excitation Wavelength-Dependent Study to Investigate the Mechanism of Light-Induced Spectral Redshift of a Single Gold Nanorod

The paper presents the study of light-induced spectral redshift observed for a single gold nanorod immersed in hydroquinone solution. Two lasers emitting at 532 nm and 633 nm were used to excite the nanorods to perform a wavelength-dependent study and thereby investigate the mechanism behind the spectral redshift. Gold nanorods with longitudinal surface plasmon resonance (LSPR) at ~650 nm was chemically synthesized by the seed mediated growth method [1]. The nanorods were then deposited on glass substrate for the single particle study. A home-built confocal microscopy setup that utilized the photoluminescence property of the gold nanorods was used to obtain single-particle images and spectra. A gradual redshift (~60 nm on average) of the LSPR of the single gold nanorods was observed on continuous illumination by a 532 nm laser in the presence of hydroquinone solution. This was surprising because hydroquinone does not react with the gold nanorods in a solution and is therefore usually used as a mild reducing agent during the synthesis of gold nanorods. Also, single gold nanorods immersed in hydroquinone solution and kept in the dark did not show any shift of the LSPR. This observation proved that the red shift of the LSPR is driven by light. The mechanism behind the spectral shift can be either due to reshaping of the nanorods causing a change in its aspect ratio, or due to the change in the local refractive index. In a follow up experiment, the LSPR of the gold nanorods was restored to its initial position on washing with cetrimonium bromide (CTAB) solution without laser excitation. This observation rules out the possibility of reshaping of the nanorods as the cause of the redshift. The redshift is likely due to the change in local refractive index by virtue of some reversible rection taking place at the surface of the gold nanorod. As reported in the literature, excitation with light causes the generation of hot carriers at the surface of gold nanorods [2]. Moreover, hydroquinone, being a reducing agent, is expected to actively participate in making its free electrons available for recombination with the photo-induced hot holes and adsorbing on the gold nanorod surface. Incubation in CTAB helps in removing this adsorbed layer of hydroquinone on the nanorod surface. The experiment was repeated at an excitation wavelength of 633 nm. A similar trend of redshift was observed in this case but at a slower rate. This indicates that the phenomenon is dependent on the excitation wavelength. Excitation at 633 nm, which is closer to the LSPR wavelength, leads to more heating at the nanorod's surface. This in turn rules out the possibility of heat playing a role in the spectral shift, and we conclude the phenomenon to be driven primarily by hot carriers. This work serves not only as another way of tuning the LSPR spectrum of gold nanorods but should also help in understanding photo-induced activities at the surface of the gold nanorods at single particle level [3]. The wavelength-dependence on the rate of the shift can be of major interest for controlled photocatalytic activities.