LA ethanol solution (0.3 mL) of 0.1 M was added to 10.0 mL of obtained citrate stabilized AuNPs with a pre-adjusted pH 11, and then the LA plus the AuNPs solution was stirred overnight (12–18 h, Lin et al., 2004). To purify the LA capped AuNPs, the mixture were centrifuged for 20 min at 14,000 rpm and decantated by supernatants, then the resulting AuNPs were resuspended in ultrapure water. Surface functionality of lipoic acid was characterized by FT-IR (Thermo Scientific, Nicolet 6700). For doing that, LA capped AuNPs were purified twice by centrifugation, lyophilized and prepared in a KBr pallet. The particle size and shape of LA capped AuNPs were characterized using TEM. The absorbance spectra of LA capped AuNPs was scanned in UV–vis spectra.