Characterization of citrates on gold and silver nanoparticles

• Coordinations of citrates on Au and Ag NPs are confirmed by FTIR and MO calculations.
• Citrate forms an ionic bond with Ag NPs while it forms a unidentate bond with Au NPs.
• LSPR excited on nanoparticles enhances IR vibration mode perpendicular to the surface.

In this paper, we report different coordinations of citrates on gold (AuNP) and silver (AgNP) nanoparticles, as determined using Fourier transform infrared spectroscopy (FTIR) and molecular orbital (MO) calculations. AuNPs and AgNPs are found to have completely different interactions with the carboxylate anchoring groups, as indicated by their unique asymmetric stretching vibrations in the FTIR spectra. The νas (COO−) of citrate exhibits a high-frequency shift resulting from the formation of a unidentate coordination on AuNPs, whereas this vibration exhibits a low-frequency shift as a result of ionic bond formation on AgNPs, as predicted from the MO calculations of the corresponding metal complex salts. The enhancement in the IR signals when their vibration direction was perpendicular to the nanoparticle surface revealed the influence of localized surface plasmons excited on the metal nanoparticles.

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