Synthesis of tunable near infrared absorbance of gold nanorods (AuNRs) and their gold oxidation states during nanorod formation following by synchrotron based X-ray absorption spectroscopy (XANES)

Abstract

A study of growth mechanism on AuNRs synthesized using seed-mediated method has been conducted via time resolved synchrotron based XAS. The synthesized AuNRs were characterized by TEM and UV–vis spectroscopy. It was found that the volume change of AgNO3 solution influenced their surface plasmon resonances (SPR) of AuNRs. An increase of silver ions concentration resulted in an increase of the aspect ratio (AR). For the growth mechanism investigation, time-resolved XANES in fluorescent mode and UV–vis measurement were employed. The potential growth mechanism was proposed according to the XANES spectrums. The reduction of Au(3+) oxidation state to Au(0) occurs after the reducing agent were added. The change in oxidation state of the AuNRs corresponded to the XANES spectrum of AuCl illustrating the oxidation states were reduced from Au(3+) to Au(+) then Au(0), respectively. Moreover, the rate of change in oxidation state depended on the AR. The formation of AuNRs was completed for the shorter rods (AR∼2.0) while the longer rods (AR∼4.2) still exhibit higher feature of oxidation state compared at the same reaction time. In addition, the change in absorbance from UV–vis spectrums revealed that AuNRs with AR∼2.0 took 50 min to complete rod formation which is faster than that of the AuNRs with AR∼4.2 (60 min). These UV–vis results were in a good agreement with the XANES spectrums. This finding leaded to conclude that the rod formation of AuNRs with lower AR could occurred faster than the formation of AuNRs with higher AR.