Synthesis of gold nanorods with longitudinal surface plasmon resonance wavelength up to 1245 nm using gallic acid as a reductant in the presence of a binary surfactant system

Gold nanorods with the longitudinal surface plasmon resonance (LSPR) wavelength up to 1245 nm are obtained by using an improved seed-mediated wet chemistry growth method where gallic acid is used as a reductant in the presence of a binary surfactant system. The effects of the gallic acid and the seed on the synthesised gold nanorods are investigated by ultraviolet-visible-near infrared spectroscopy and transmission electron microscopy (TEM). It is found that by increasing the gallic acid amount from 200 to 600 L, the LSPR wavelength shifts from 975 to 1170 nm; at the same time, the absorption intensity of both the transverse surface plasmon resonance and LSPR increases, indicating a more complete reduction of chloroauric acid. To achieve a longer wavelength of the gold nanorods' LSPR peak, the seed amount was changed to a condition of 600 L gallic acid. A redshift of the LSPR wavelength from 1135 to 1245 nm can be obtained as the seed amount increases from 30 to 70 L. Furthermore, it was found that the redshift of the LSPR peak with increasing seed amount is in accordance with the increase trend of the aspect ratio obtained by analysing the dimensions of the gold nanorods through the TEM images. The method offers a new strategy to obtain gold nanorods with tunable LSPR wavelengths, which have wide application in many areas, such as biosensing and ultrafast optical information.