In previous studies, AuAg colloidal nanostar formulations
were
developed with the two-fold aim of producing optimized surface-enhanced
Raman spectroscopy (SERS) substrates and investigating the nature
of the capping process itself. Findings demonstrated that the nanoparticle
metals are alloyed and neutral, and capping by stabilizers occurs
via chemisorption. This study utilizes citrate as the model stabilizer
and investigates the mechanistic aspects of its interaction with mono-
(Au20) and bimetallic (Au19Ag) surfaces by density
functional theory (DFT) calculations. Citrate was modeled according
to the colloid’s pH and surrounded by a water and sodium first
solvation shell. A population of stable cluster–citrate structures
was obtained, and energies were refined at the uB3LYP//LANL2TZ(f)/cc-pVTZ
level of theory. Solvation was accounted for both explicitly and implicitly
by the application of the continuum model SMD. Results indicate that
both direct binding and binding by water proxy through the charge-transfer
complex formation are thermodynamically favorable. Water participation
in citrate adsorption is supported by the adsorption behavior observed
experimentally and the comparison between experimental and DFT-simulated
IR spectra. Vibrational mode analysis suggests the possible presence
of water within a crystal in dried nanostar residues. All ΔG
ads(aq) indicate a weak chemisorptive process,
leading to the hypothesis that citrate could be displaced by analytes
during SERS measurements.