Synthesis, topology, and photoexcited dynamics of amino acid-derived gold nanoparticles
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Abstract
The use of biomolecules as capping and reducing agents in the synthesis of metallic nanoparticles constitutes a promising framework to achieve desired functional properties with minimal toxicity. The system’s complexity and the large number of variables involved represent a challenge for theoretical and experimental investigations to devise precise synthesis protocols. In the center of this dissertation, we use L- asparagine (Asn), an amino acid building block of large biomolecular systems, to synthesize gold nanoparticles (AuNPs) in an aqueous solution at controlled pH. The use of Asn offers a primary system that allows us to understand the role of biomolecules in synthesizing metallic nanoparticles. Our results indicate that AuNPs synthesized in acidic (pH 6) and basic (pH 9) environments present somewhat different morphologies. We examine these AuNPs via Raman scattering experiments and classical molecular dynamics simulations of zwitterionic and anionic Asn states adsorbing on (111)-, (100)-, (110)-, and (311)-oriented gold surfaces. A combined analysis infers that the underlying mechanism controlling AuNPs geometry correlates with amine’s preferential adsorption over ammonium groups, enhanced upon increasing pH. Water molecules strongly interact with the gold face-centered-cubic lattice and create traps that prevent the Asn from diffusing on the more open surfaces. Our simulations expose that Asn (both zwitterionic and anionic) adsorption on gold (111) is essentially different from adsorption on more open surfaces. These results indicate that pH is a relevant parameter in green-synthesis protocols that can control the nanoparticle’s geometry and pave the way to computational studies exploring the effect of water monolayers on the adsorption of small molecules on wet gold surfaces. Additionally, The use of amino acids as capping and reducing agents in the synthesis of metallic nanoparticles constitutes a promising framework to achieve desired functiona properties with minimal toxicity. In the second part of this dissertation, we use L- Asparagine (Asn) to synthesize gold nanoparticles (AuNPs) in an aqueous solution at controlled pH. Our results indicate that size, shape, and localized surface plasmon resonance (LSPR) characteristics strongly depend on the Asn pH values of pH 6 and 8. The particle size obtained for pH 6 is 18±9 nm, while the particle size for pH 8 is 86±25 nm. The electron cooling dynamics of the LSPR were examined by transient absorption spectroscopy (fs-TA), and the results show that the pH 8 sample exhibits slightly slower electron cooling. We discuss these results in the context of our earlier findings regarding the pH-dependent amino acid-binding at the Au surface. The LSPR stability and the high reproducibility of the AuNPs-Asn complex suggest that AuNPs synthesized can be a potential candidate for biocompatible applications with plasmonic structures, such as bioimaging.