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