Kinetic and Mechanistic Insight into the Surfactant-Induced Aggregation of Gold Nanoparticles and Their Catalytic Efficacy: Importance of Surface Restructuring

Abstract

Understanding the fundamental interactions between plasmonic metal nanoparticles (MNPs) and small molecules is of utmost importance in various applications such as catalysis, sensing, drug delivery, optoelectronics, and surface-enhanced Raman spectroscopy. Herein, we have investigated the early stage of the aggregation pathway of citrate-stabilized Au NPs with surfactants and explored their catalytic efficacy. Our findings reveal that (17 ± 2)-nm-sized citratestabilized Au NPs undergo concentration and time-dependent aggregation with positively charged cetyltrimethylammonium bromide (CTAB). Kinetic analyses revealed the presence of two distinct kinds of aggregates, namely, smaller clusters and a larger branched network of Au nanochains. At longer times and in the presence of higher concentrations of CTAB, these branched networks of Au nanochains transform into dense compact globular aggregates. The catalytic efficacy of Au NPs, branched Au nanochains, and dense compact aggregates has been explored with respect to the reductive hydrogenation of 4-nitophenol in the presence of excess NaBH 4 . Our study revealed that the catalytic rate decreases in the order of Au NPs > branched Au nanochains > compact aggregates. Interestingly, pre-equilibrating different Au NP samples with excess NaBH 4 prior to the onset of the reaction results in similar catalytic activity irrespective of the aggregation state of Au NPs. This observation has been explained by considering efficient surface restructuring via ligand exchange with H − ions and the subsequent disruption of CTAB-induced aggregates of Au NPs. Moreover, the aggregated Au NPs can be recycled over several consecutive cycles for the reductive hydrogenation of 4-NP upon ligand exchange with H − ions. Taken together, our present study highlights the early-stage aggregation kinetics of Au NPs with CTAB surfactants and demonstrates the importance of the surface restructuring of Au NPs on their catalytic efficacy.

Microscopic Insights into the Mechanism of White Light Generation by Disruptive Interaction between Human Serum Albumin Amyloid Fibrils and Surfactant-AIEgen Nanorods

“…The generation of white light has been an area of active research, due to its potential applications in organic electronics and biophysical chemistry. − Its inherent mechanism is usually believed to involve a cascade energy transfer pathway in complex systems containing multiple emitting species; still, it is also possible that the observed white light is simply a sum of emissions in different spectral regions, from the components of the system. Of the various strategies for assembling multichromophoric systems for this purpose, surfactant-induced aggregation is convenient and reliable. − In recent times, this modality has been exploited to obtain aggregation-induced enhancement of emission (AIEE). − The study presented here explores the potential of one such aggregate in the design of novel white light-emitting platforms, in conjunction with protein amyloid fibrils. − The implications of amyloid fibrils in neurological diseases − prompted a frenzy of activity in the field of inhibition of aggregation and/or fibrillation. − More recently, however, more attention has been paid to the prospect of amyloid fibrils as functional biomaterials, , with potential applications in drug delivery, analyte sensing, and bioengineering and as templates for self-assembly . Hence, the domain has expanded from conventional disease-related fibril-forming proteins like amyloid protein and prion protein to globular proteins like myoglobin and albumins, ,, in which fibrillation can be induced under experimental conditions.…”

Water-soluble ionic carbon nitride as unconventional stabilizer for highly catalytically active ultrafine gold nanoparticles

“…50 Therefore, the "naked" (i.e., ligand-free) Au NPs stabilized by excess NaBH 4 represent an ideal benchmark for comparative investigations of the effect of the proposed stabilizers on the size, long-term stability, and catalytic activity of Au NPs. This choice is further substantiated by the fact that though also other weakly binding ligands (e.g., citrate) do allow for good accessibility of active sites on the Au nanoparticle surface, they are typically not able to stabilize Au NPs with sizes below 10 nm when using NaBH 4 as a reducing agent, [51][52][53] prohibiting thus comparative studies in the ultrafine size regime. A 10-fold excess of NaBH 4 with respect to HAuCl 4 was selected as the optimum molar ratio of NaBH 4 : HAuCl 4 to produce the benchmark ligand-free Au NPs (denoted as Au10) without agglomeration (for details see ESI 2.1 †).…”

“…This choice is further substantiated by the fact that though also other weakly binding ligands ( e.g. , citrate) do allow for good accessibility of active sites on the Au nanoparticle surface, they are typically not able to stabilize Au NPs with sizes below 10 nm when using NaBH 4 as a reducing agent, 51–53 prohibiting thus comparative studies in the ultrafine size regime.…”

Plasmon resonance dynamics and enhancement effects in tris(2,2′-bipyridine)ruthenium(ii) gold nanosphere oligomers

“…[15][16][17][18][19] Elucidating the optimal nanoscale conditions for surface-molecule interactions is especially important because uncontrolled aggregation or instability of assumed active sites can severely limit applications for catalysis, spectroscopy and light-harvesting processes. 15,16,20,21 Early observations of adsorbate-molecule induced aggregation of metal nanoparticles were reported by Moskovits and Vlčková where the aggregation rate constant depends nontrivially on adsorbate concentration. 22 For gold and other metallic nanoparticles in general, there exists a range of competing interactions such as van der Waals, electrical double layers, hydration forces, hydrophobic forces, steric electronic interactions, and electrostatic forces that drive aggregation reactions.…”