Multicomponent ligand interactions are involved in essentially all practical nanoparticle (NP) applications. Presented herein is the finding that multicomponent ligand binding to gold nanoparticles (AuNPs) can be highly dependent on the sequence of ligand mixing with AuNPs. Quantitative study revealed that the competitive adenine and glutathione (GSH) adsorption onto both as-synthesized and pegylated AuNPs is predominantly kinetically controlled, and adenine that binds only nonspecifically to AuNP adsorbs faster than GSH. This raises concerns about the validity of the popular practice in current NP research of using the Langmuir isotherm or its variants to model multicomponent ligand adsorption on NPs. Mechanistically, this sequence dependency is due to the fact that there is no spontaneous ligand desorption even for the model protein and small molecules that can bind only nonspecifically to AuNPs. The insights and experimental methods provided in this work should be important for molecular-level understanding of nanoparticle interfacial interactions.
Gold nanoparticles (AuNPs) have been used extensively as surface-enhanced Raman spectroscopic (SERS) substrates for their large SERS enhancements and widely believed chemical stability. Presented is the finding that iodide can rapidly reduce the SERS intensity of the ligands, including organothiols adsorbed on plasmonic AuNPs through both iodide-induced ligand desorption and AuNP fusion. The organothiols trapped inside the fused AuNPs have negligible SERS activities. Multiple photochemical processes were involved when organothiol-containing AuNP aggregates were treated with KI under photoillumination. The photocatalytically produced I 3 − reacts with both organothiol and AuNPs. Chloride and bromide also induce partial organothiol displacement and the fusion of the as-synthesized AuNPs, but neither of the two halides has detectable effects on the morphology and Raman signals of the organothiol-containing AuNP aggregates. The insight provided in this work should be important for the understanding of interfacial interactions of plasmonic AuNPs and their SERS applications. ■ INTRODUCTIONSurface-enhanced Raman spectroscopy (SERS) has been used extensively to study molecular adsorption, desorption, displacement, and reaction on plasmonic gold and silver nanoparticle (AuNP and AgNP) surfaces. 1−4 Plasmonic nanoparticles (NPs) can modify Raman signal of molecular adsorbates through three competing processes that include electromagnetic enhancement, 5−7 chemical enhancement, 8−10 and nanoparticle (NP) inner filter attenuation. 11,12 While the electromagnetic enhancement strengthens the Raman signal of the molecules adsorbed onto the NP surfaces, the NP inner filter effect reduces the analyte Raman and SERS signal by absorbing and scattering the incident and Raman photons. The nature and contribution of chemical enhancement are less understood. While it bears the term of "enhancement" in its name by convention, the chemical interactions between the molecular adsorbates and SERS substrate should be able to enhance or reduce the Raman signal of the surface adsorbates depending on its structural perturbations imposed by SERS substrates. Indeed, recent reports showed that the resonance enhancement factor of Rhodamine 6G on nanoparticle is far less than that in water. 13 While significant progresses have been made on the mechanistic understanding of SERS signal phenomena and on the SERS substrate fabrications, limited information is available on the postsynthesis morphological change of the SERS substrate and its potential effect on the Raman activity of the molecules on NPs. For example, it has been demonstrated that iodide can eliminate SERS signal of adventitious molecules adsorbed onto AuNPs by ligand displacement. 14 However, the applicability of this observation to general SERS-active molecules is unclear. In addition, the potential impact of iodide-induced AuNP structural modification on the ligand SERS activity has, to our knowledge, not been explored. Such a study is especially relevant for the ligand-containing AuNPs ...
Ligand displacement from gold is important for a series of gold nanoparticle (AuNP) applications. Complete nondestructive removal of organothiols from aggregated AuNPs is challenging due to the strong Au–S binding, the steric hindrance imposed by ligand overlayer on AuNPs, and the narrow junctions between the neighboring AuNPs. Presented herein is finding that monohydrogen sulfide (HS–), an anionic thiol, induces complete and nondestructive removal of ligands from aggregated AuNPs. The model ligands include aliphatic (ethanethiol(ET)) and aromatic monothiols, methylbenzenethiol (MBT), organodithiol (benzenedithiol (BDT)), thioamides (mercaptobenzimidazole (MBI) and thioguanine (TG)), and nonspecific ligand adenine. The threshold HS– concentration to induce complete ligand displacement varies from 105 μM for MBI and TG to 60 mM for BDT. Unlike using HS–, complete ligand displacement does not occur when mercaptoethanol, the smallest water-soluble organothiol, is used as the incoming ligand. Mechanistically, HS– binding leads to the formation of sulfur monolayer on AuNPs that is characterized with S–S bonds and S–Au bonds, but with no detectable S–H spectral features. The empirical HS– saturation packing density and Langmuir binding constant on AuNPs are 960 ± 60 pmol/cm2 and (5.5 ± 0.8) × 106 M–1, respectively. The successful identification of an effective ligand capable of inducing complete and nondestructive removal of ligands from AuNPs should pave the way for using AuNP for capture-and-release enrichment of biomolecules that have high affinity to AuNP surfaces.
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