Simple spectrophotocolorimetric method for quantitative determination of gold in nanoparticles

A simple spectrophotocolorimetric method devoted to the measurement of gold content in nanoparticles (NPs) was developed. It includes two steps: (i) metal gold NPs (Au NPs) are oxidized into the AuCl₄^? anion using a 5 × 10^?2 M HCl-1.5 × 10^?2 M NaCl-7 × 10^?4 M Br₂ solution, next (ii) AuCl₄^? concentration is measured using a spectrophotometric assay based on the reaction of AuCl₄^? with the cationic form of Rhodamine B to give a violet ion pair complex. This latter is extracted with diisopropyl ether and the absorbance of the organic complex is measured at 565 nm. The method is linear in the range 6-29 μM of AuCl₄^? with a limit of detection of 4.5 μM.The analytical method was optimized with respect of bromine excess to obtain complete Au NPs oxidation. The method was applied to two types of Au NPs currently under investigation: citrate-stabilized Au NPs and Au NPs capped with dihydrolipoic acid (Au@DHLA). Both the gold content of Au NPs and the concentration of NPs (using NP diameter measured by transmission electron microscopy) have been calculated.