Effect of the Metal–Ligand Interface on the Chiroptical Activity of Cysteine‐Protected Nanoparticles

Abstract

Gold, silver, and copper small nanoparticles (NPs), with average size ≈2 nm, are synthesized and afterward protected with l‐ and d‐cysteine, demonstrating emergence of chiroptical activity in the wavelength range of 250–400 nm for all three metals with respect to the bare nanoparticles and ligands alone. Silver‐cysteine (Ag‐Cys) NPs display the higher anisotropy factor, whereas gold‐cysteine (Au‐Cys) NPs show optical and chiroptical signatures slightly more displaced to the visible range. A larger number of circular dichroism (CD) bands with smaller intensity, as compared to gold and silver, is observed for the first time for copper‐cysteine (Cu‐Cys) NPs. The manifestation of optical and chiroptical responses upon cysteine adsorption and the differences between the spectra corresponding to each metal are mainly dictated by the metal–ligand interface, as supported by a comparison with calculations of the oscillatory and rotatory strengths based on time‐dependent density functional theory, using a metal–ligand interface motif model, which closely resembles the experimental absorption and CD spectra. These results are useful to demonstrate the relevance of the interface between chiral ligands and the metal surfaces of Au, Ag, and Cu NPs, and provide evidence and further insights into the origin of the transfer mechanisms and induction of extrinsic chirality.

Interaction Mechanisms and Interface Configuration of Cysteine Adsorbed on Gold, Silver, and Copper Nanoparticles

“…Compared to models with one cysteine, the main differences when the second molecule is introduced are two: first, the formation of a previously proposed interface motif with arrangement S−metal−S 26 for the three metals that resembles the staple motif known for gold NPs. This interface motif is anchored to the metal surface mainly by the sulfur from the thiol group, increasing the coordination number of sulphur which in turn decreases the strength of the C β S bond of the molecule, and by one of the oxygens of the carboxyl group.…”

“…In terms of adsorption sites, the interaction of cysteine on Ag and Cu NPs is also mediated mainly by the S−metal interaction, in accordance with the previous results due to the nucleophilic character of the groups. 26,71 However, a Bader analysis reveals that the COO − group binds to the metal surface for Au, Ag, and Cu NPs via a free pair electron and brings with that a small charge transfer toward the NP. This interaction also generates a back donor charge transfer from the NP toward the molecule that is reflected in the increase in one of the CO bond lengths (Table S1), observed as a redshift of the corresponding Raman bands of the three metals (Figure 8a−c), in agreement with the experimental results (Figure 4a− c).…”

“…The strong covalent bonds between the sulfur atom belonging to cysteine’s thiol group, and these three metals result in the formation of the Au–S, Ag–S, and Cu–S bonds, respectively, − which anchors the zwitterionic cysteine ligand to the surface of such metal NPs, often also adsorbed via its carboxylate group, as reported for the case of gold and silver. ,, Adsorption configuration and binding sites of cysteine on gold NPs have been previously ascertained, ,− paving the way for a better understanding of how the intermolecular and the molecular–NP interactions determine the electronic and geometric configuration of the system and therefore its chiral activity . Copper and silver belong to the same group as gold in the periodic table; thus, it is expected to form similar core/shell geometries and ligand environments .…”

“…34 Other studies suggest that the amino and carboxyl groups of cysteine are involved in mutual intermolecular interaction 35 or in interaction with the surface of Ag NPs in the case of nonzwitterionic cysteine. 9,10 In the case of cysteine-capped copper NPs, a composition of Cu(0) and Cu(I), 6,36,37 similar to Au NPs, 5,26 has been reported: core atoms forming Cu−Cu bonds, expected to have oxidation state zero and atoms in the metal−organic framework with Cu−ligand bonds to have oxidation state +1, 27 determining the whole atomic structure of the ligandprotected NPs and therefore the adsorption configuration of cysteine.…”

“…11,20,21 Adsorption configuration and binding sites of cysteine on gold NPs have been previously ascertained, 20,22−25 paving the way for a better understanding of how the intermolecular and the molecular−NP interactions determine the electronic and geometric configuration of the system and therefore its chiral activity. 26 Copper and silver belong to the same group as gold in the periodic table; thus, it is expected to form similar core/shell geometries and ligand environments. 27 However, there is theoretical and experimental evidence, which demonstrates that silver and copper NPs have preferential binding motifs and core geometries that differ from gold NPs due to the different bonding nature and electronegativity existing between the three metals.…”