The structure and binding mode of citrate in the stabilization of gold nanoparticles

Abstract

Elucidating the binding mode of carboxylate-containing ligands to gold nanoparticles (AuNPs) is crucial to understand their stabilizing role. A detailed picture of the three-dimensional structure and coordination modes of citrate, acetate, succinate and glutarate to AuNPs is obtained by C andNa solid-state NMR in combination with computational modelling and electron microscopy. The binding between the carboxylates and the AuNP surface is found to occur in three different modes. These three modes are simultaneously present at low citrate to gold ratios, while a monocarboxylate monodentate (1κO) mode is favoured at high citrate:gold ratios. The surface AuNP atoms are found to be predominantly in the zero oxidation state after citrate coordination, although trace amounts of Au are observed. Na NMR experiments show that Na ions are present near the gold surface, indicating that carboxylate binding occurs as a 2e L-type interaction for each oxygen atom involved. This approach has broad potential to probe the binding of a variety of ligands to metal nanoparticles.

Experimental probes of silver metal nanoparticle formation kinetics: Comparing indirect versus more direct methods

“…These results are consistent with the role of citrate as a reducing agent. The apparent plateauing of k 2 values could indicate that the surface of the growing nanoparticle is getting saturated at higher citrate concentrations, thus blocking the way for incoming metal ions to be reduced on the surface …”

On the nature of citrate-derived surface species on Ag nanoparticles: Insights from X-ray photoelectron spectroscopy

“…The increased quantity of K + before the washing operation, which completely removes K and N, is due to potassium nitrate, with the ratio of carboxylate to K + being 1.2 after subtraction of the KNO3 content. The results obtained for the fractions (i) and (ii) suggest that alkali cations are weakly (electrostatically) attached to AgNPs; the cations are probably localized near both unbound COO -groups and those bound to Ag surface, similar to Na + in gold NPs -citrate system [20], albeit some minor citrate salts from the dried solution may still contribute to the spectra.…”

“…Citrate ions are widely utilized in the wet chemical synthesis of silver [3][4][5][6][7][8][9][10][11][12][13][14], gold [15][16][17][18][19][20][21], platinum [22] and other nanoparticles, acting as reducing (direct reduction of aqueous Ag + ions by citrate occurs under boiling or hydrothermal conditions [9][10][11][12][13]), complexing, and stabilizing agent, although the precise mechanisms are far from being fully understood. Сitrate is a key reagent for the preparation of silver nanoplates, cubes, disks, etc., as the selective adsorption of citrate on Ag (111) facets impedes their growth and promotes the yield of anisotropic particles [12,[23][24][25].…”

Negative‐Imaging of Citrate Layers on Gold Nanoparticles by Ligand‐Templated Metal Deposition: Revealing Surface Heterogeneity

“…Center‐to‐center distance between individual Pt clusters measured at different pH values is shown to be dependent on citrate intermolecular spacing. Considering adatom‐mediated Au−OH and Au−(H)OOC surface complexations, we propose the binding mode of citrate molecules (H 3 Cit, H 2 Cit ‒ , HCit 2‒ , and Cit 3‒ ), which is more diverse than the previously determined . The heterogeneity of metal surfaces or molecular layers on nanoparticles is crucial in the local surface chemical/physical properties and thus in the accurate description of the interfacial phenomena of nanoparticles.…”

“…A variety of spectroscopy and microscopy applying photon, electron, or ion excitation have been used to obtain structural, electronic, and compositional information on the metallic side of nanoparticle interface . On the contrary, the structural study of molecular adlayers on metal nanoparticles, in particular for citrate‐stabilized metal nanoparticles that are widely used in nanoparticle research, has been done mostly with several limited spectroscopic methods such as nuclear magnetic resonance (NMR) and vibrational spectroscopy including IR spectroscopy and surface‐enhanced Raman scattering (SERS) . Those spectroscopic methods generally provide global spectral information on the local bonding states of molecules and partially allow for determination of the structure of individual bound molecules.…”