Quantification of Resonance Raman Enhancement Factors for Rhodamine 6G (R6G) in Water and on Gold and Silver Nanoparticles: Implications for Single-Molecule R6G SERS

The resonance Raman (RR) enhancement factors of Rhodamine 6G (R6G) in water and on gold and silver nanoparticles (AuNPs and AgNPs) were determined using a double ratiometric method where adenine is used as the internal reference. The RR enhancement factor for R6G on AgNPs upon laser excitation at 532 nm is 537.6 ± 214.8. This is ∼5 times lower than the experimental (2.7 ± 0.3) × 10³ RR enhancement factor for R6G in water. These experimental RR enhancement factors for R6G in water and on AgNPs are 10⁴ smaller than the 10⁷ RR enhancement proposed in literature for R6G in water and on SERS substrates. In addition, a simple back-of-the-envelope calculation showed that even with this damped RR for R6G on AgNPs in comparison to R6G in water, a SERS enhancement factor of 10⁶ is sufficient to explain the single-molecule resonance SERS activities reported for R6G located in nanoparticle junctions. This conclusion is deduced from fact that normal Raman spectrum could be readily obtained with 24 fmol of adenine at laser focal volume of ∼150 fL at 532 nm excitation. This work provides the first direct experimental evidence for the recent theoretical predication that plasmonic nanoparticles quench the resonance Raman signal. In addition, the double ratiometric method reported in this work represents a significant technique development in Raman and SERS, which should pave the way for quantitative investigations of the RR for dye molecules dissolved in solution or adsorbed on plasmonic nanoparticles

Anisotropic Nanoparticles Contributing to Shear-Thickening Behavior of Fumed Silica Suspensions

Rheological characteristics of a concentrated suspension can be tuned using anisotropic particles having various shapes and sizes. Here, the role of anisotropic nanoparticles, such as surface-functionalized multiwall carbon nanotubes (MWNTs) and graphene oxide nanoplatelets (GONPs), on the rheological behavior of fumed silica suspensions in poly­(ethylene glycol) (PEG) is investigated. In these mixed-particle suspensions, the concentrations of MWNTs and GONPs are much lower than the fumed silica concentration. The suspensions are stable, and hydrogen-bonded PEG solvation layers around the particles inhibit their flocculation. Fumed silica suspensions over the concentration range considered here display shear-thickening behavior. However, for a larger concentration of MWNTs and with increasing aspect ratios, the shear-thickening behavior diminishes. In contrast, a distinct shear-thickening response has been observed for the GONP-containing suspensions for similar mass fractions (MFs) of MWNTs. For these suspensions, shear thickening is achieved at a lower solid MFs compared to the suspensions consisting of only fumed silica. A significant weight reduction of shear-thickening fluids that can be achieved by this approach is beneficial for many applications. Our results provide guiding principles for controlling the rheological behavior of mixed-particle systems relevant in many fields

NaHS Induces Complete Nondestructive Ligand Displacement from Aggregated Gold Nanoparticles

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

Removal of Molecular Adsorbates on Gold Nanoparticles Using Sodium Borohydride in Water

The mechanism of sodium borohydride removal of organothiols from gold nanoparticles (AuNPs) was studied using an experimental investigation and computational modeling. Organothiols and other AuNP surface adsorbates such as thiophene, adenine, rhodamine, small anions (Br^– and I^–), and a polymer (PVP, poly­(<i>N</i>-vinylpyrrolidone)) can all be rapidly and completely removed from the AuNP surfaces. A computational study showed that hydride derived from sodium borohydride has a higher binding affinity to AuNPs than organothiols. Thus, it can displace organothiols and all the other adsorbates tested from AuNPs. Sodium borohydride may be used as a hazard-free, general-purpose detergent that should find utility in a variety of AuNP applications including catalysis, biosensing, surface enhanced Raman spectroscopy, and AuNP recycle and reuse

Making Flavone Thioethers Using Halides and Powdered Sulfur or Na₂S₂O₃

The method for constructing C–S bonds is very important in organic synthesis. Here a new sulfenylation method to generate flavone thioether derivatives was developed by employing aromatic or alkyl halides, S powder and Na₂S₂O₃ as reactants. Good yields of regioselective C_alkyl–S and C_aryl–S-substituted flavones were generated under relatively environmentally friendly and simple conditions. This method might be potentially applicable to large scale production, and it enriches current sulfenylation methods

Desulfurization of Mercaptobenzimidazole and Thioguanine on Gold Nanoparticles Using Sodium Borohydride in Water at Room Temperature

Organosulfur compounds are known to poison metallic nanoparticle catalysts. Herein NaBH₄ is shown to desorb and desulfurize 2-mercaptobenzimidazole (2-MBI) and 6-thioguanine (6-TG) adsorbed on 10, 15, and 50 nm diameter gold nanoparticles (AuNPs). The desulfurization rates decrease significantly with increasing AuNP sizes. Isotope labeling experiments, conducted with NaBD₄ in H₂O, indicate that this desulfurization reaction proceeds through a pathway requiring hydrogen uptake onto AuNP surfaces prior to the 2-MBI or 6-TG desulfurization reaction, rather than direct hydride attack from BH₄^– on the sulfur-bearing carbon in 2-MBI or 6-TG, or H₂ reaction with 2-MBI or 6-TG . In addition to serving as the hub for electron charge transfer between hydride and proton, AuNPs capture the cleaved sulfide, facilitating sulfur separation from the desulfurized products

Structures and Conformations of Alkanedithiols on Gold and Silver Nanoparticles in Water

Organodithiols with two distal thiols have been used extensively in gold and silver nanoparticle (AuNP and AgNP) applications. However, understanding the structures and conformations of organodithiols on these nanoparticles is challenging. Reported in this work is a combined surface enhanced Raman spectroscopy (SERS), transmission electron microscope (TEM), inductively coupled plasma mass-spectrometry (ICP-MS), and localized surface plasmonic resonance (LSPR) study of alkyldithiol (ADT, (HS-(CH₂)_<i>n</i>-SH, <i>n</i> = 2, 4, and 6) interactions with AuNPs and AgNPs in water. These complementary techniques revealed a series of new insights that would not be possible using individual methods. A large-fraction of ADTs lies flat on AuNP surfaces. The upright ADTs are dimerized horizontally through disulfide-bond, or remain as monothiolates on the AuNP surfaces. The possibility of a significant amount of vertically disulfide-linked organodithiol on the surface is excluded on the basis of ICP-MS and AuNP LSPR experiments. ADTs induced significant AgNP disintegrations in which ADTs are predominantly in dithiolate forms. This work highlights the extraordinary complexity of organodithiol interactions with plasmonic nanoparticles. The insights provided in this work will be important for enhancing fundamental understanding of the structure and properties of organothiol-functionalized AgNPs and AuNPs

Ligand Desorption and Desulfurization on Silver Nanoparticles Using Sodium Borohydride in Water

We recently reported that a wide range of ligands, including organothiols (OTs), can be completely desorbed from gold nanoparticles (AuNPs) by NaBH₄ in water. In addition, NaBH₄ induces desulfurization of 2-mercaptobenzimidazole (2-MBI) and 6-thioguanine (6-TG) on AuNPs. Reported herein is a systematic investigation of treating ligands adsorbed onto silver nanoparticles (AgNPs) with NaBH₄. These results are compared and contrasted to those previously reported for the same set of ligands adsorbed onto AuNPs. Complete desorptions from AgNPs by NaBH₄ in water were observed for nonspecifically adsorbed ligands that include Rhodamine 6G, adenine, thiophene, and halides (Cl^–, Br^–, and I^–). These cleaned AgNPs can be reused for surface-enhanced Raman spectroscopy acquisition. However, OT ligands could not be completely desorbed from AgNPs regardless of the amount of NaBH₄ used in this work. NaBH₄ can induce complete 6-TG desulfurization adsorbed on AgNPs, but the desulfurization rate is significantly slower than that on AuNPs. Transmission electron microscope analysis revealed that NaBH₄ induced more extensive nanoparticle fusion for AgNPs than for AuNPs. A mechanistic study indicates that AgNPs serve as electron transfer hubs for hydrides in BH₄^– to protons in water. In addition to providing new insights for AgNP recycle, reuse, and catalytic applications, this work also highlights the significant differences in the structure and properties of OTs adsorbed on AuNPs and AgNPs

Simultaneous and Sequential Protein and Organothiol Interactions with Gold Nanoparticles

Proteins and organothiols (OTs) are known to have high affinity for gold nanoparticles (AuNPs). Systematic investigation of protein and OT coadsorption onto AuNPs is, however, mostly an unexplored area. Presented here is a comparison of simultaneous and sequential protein and OT interactions with AuNPs in which a protein and an OT are either simultaneously or sequentially added to colloidal AuNPs. Using bovine serum albumin (BSA) as the model protein and eight model organothiols, both the protein and the OT were coadsorbed onto AuNPs in samples formed by sequential or simultaneous addition. AuNP stability against OT-adsorption-induced AuNP aggregation differed significantly among the AuNP/OT and AuNP/BSA/OT mixtures. The stability of AuNPs in the AuNP/BSA/OT mixtures with the same compositions increased from (AuNP/OT)/BSA to AuNP/(BSA/OT) and finally (AuNP/BSA)/OT (where the two components inside the parentheses are mixed first followed by the addition of the third component). Aging the (AuNP/BSA) mixtures before OT addition also increased the AuNP stability in (AuNP/BSA)/OT samples. This sequence and aging dependence of AuNP stability indicates that protein and OT coadsorption onto AuNPs is kinetically controlled. It also offers a plausible explanation to the large discrepancy in the binding constants reported for the BSA interaction with AuNPs (from 10⁵ to 10¹¹ M^–1). The work is important for AuNP biological/biomedical applications because AuNPs encounter a mixture of proteins and OTs in addition to other molecular species in biofluids