Surface Plasmon Resonance, Formation Mechanism, and Surface Enhanced Raman Spectroscopy of Ag+-Stained Gold Nanoparticles

A series of recent works have demonstrated the spontaneous Ag+ adsorption onto gold surfaces. However, a mechanistic understanding of the Ag+ interactions with gold has been controversial. Reported herein is a systematic study of the Ag+ binding to AuNPs using several in-situ and ex-situ measurement techniques. The time-resolved UV-vis measurements of the AuNP surface plasmonic resonance revealed that the silver adsorption proceeds through two parallel pseudo-first order processes with a time constant of 16(±2) and 1,000(±35) s, respectively. About 95% of the Ag+ adsorption proceeds through the fast adsorption process. The in-situ zeta potential data indicated that this fast Ag+ adsorption is driven primarily by the long-range electrostatic forces that lead to AuNP charge neutralization, while the time-dependent pH data shows that the slow Ag+ binding process involves proton-releasing reactions that must be driven by near-range interactions. These experimental data, together with the ex-situ XPS measurement indicates that adsorbed silver remains cationic, but not as a charged-neutral silver atom proposed by the anti-galvanic reaction mechanism. The surface-enhanced Raman activities of the Ag+-stained AuNPs are slightly higher than that for AuNPs, but significantly lower than that for the silver nanoparticles (AgNPs). The SERS feature of the ligands on the Ag+-stained AuNPs can differ from that on both AuNPs and AgNPs. Besides the new insights to formation mechanism, properties, and applications of the Ag+-stained AuNPs, the experimental methodology presented in this work can also be important for studying nanoparticle interfacial interactions

Electrolyte interactions with ligand functionalized gold nanoparticles

Electrolyte interactions with ligand functionalized gold nanoparticles (AuNPs) have broad implication to a wide range of applications in nanoparticle research field. Among a wide range of electrolytes, halides, nitrates, borohydrides, and sulfides are used to study the AuNP interfacial interactions. Although there are many studies on AuNP interactions with anionic species (halides, nitrates, borohydrides, and sulphides), there is limited information on AuNP interactions with metallic cations. Therefore, studying the nanoparticle interfacial interactions with both anionic and metallic cation species is highly important. The research reported here is focused on deepening the understanding of electrolyte interactions with ligand functionalized AuNPs in aqueous solutions. The stability of citrate-residues on AuNPs against ligand displacement has been controversial. In the first study, we demonstrated the direct experimental evidence for the simultaneous adsorption of both citrate-residues and solution impurities onto citrate-reduced AuNPs by using AuNPs synthesized with deuterated citrate in combination with the surface-enhanced Raman spectroscopic (SERS) analysis. The citrate-residues can be readily displaced from AuNPs by a wide range of specific and non-specific ligands including organosulfur and electrolytes. In the second study, we investigated the charge state and the mechanism of silver ion binding onto organothiol functionalized AuNPs. Mechanistic study reveals that silver binding onto AuNPs proceeds predominantly through reactive pathways with proton generations providing the first direct experimental evidence that Ag+ can disrupt the Au-S binding and enhance the mobility of the organothiols on AuNPs. Ligand displacement from AuNPs is important in a wide range of applications. Complete and non-destructive removal of ligands from AuNPs is important and challenging due to the strong Au-S binding and the steric hindrance imposed by ligand overlayer on AuNPs. In the final study, we investigated hydrogen sulphide (HS-), an anionic thiol as an effective ligand to induce complete and non-destructive removal of ligands from aggregated AuNPs. The new insights and methodologies presented in this dissertation are important for studying the electrolyte interfacial interactions with ligand functionalized AuNPs which have a broad impact on nanoparticle surface chemistry

Determining The Liquid Light Scattering Cross Section And Depolarization Spectra Using Polarized Resonance Synchronous Spectroscopy

Rayleigh scattering is a universal material property because all materials have nonzero polarizability. Reliable quantification of the material light scattering cross section in the liquid phase and its depolarization spectra is, however, challenging due to a host of sample and instrument issues. Using the recently developed polarized resonance synchronous spectroscopic method, we reported the light scattering cross section and depolarization spectra measured for a total of 29 liquids including water, methanol, ethanol, 1-propanol, 1-butanol, dimethylformamide, carbon disulfide, dimethyl sulfoxide, hexane and two hexane isomers (3-methylpentane and 2,3-dimethylbutane), tetrahydrofuran, cyclohexane, acetonitrile, pyridine, chloromethanes including di-, tri, tetrachloromethane, acetone, benzene and eight benzene derivatives (toluene, fluorobenzene, 1,2-, 1,3-, and 1,4-difluorobenzene, chlorobenzene, 1,2- and 1,3-dichlorobenzene, and nitrobenzene). The solvent light scattering depolarization is wavelength-independent for the model solvents, and it varies from 0.023 ± 0.011 for CCl4 to 0.619 ± 0.022 for nitrobenzene. The light scattering cross-section spectra can be approximated with the function of σ(λ) = αλ-4 with the α value varying from 7.2 ± 0.2 × 10-45 cm6 for water to a maximum of 8.5 ± 0.6 × 10-43 cm6 for nitrobenzene. Structural isomerization has no significant effect on either the depolarization or the scattering cross sections for both hexanes and difluorobenzene isomers. This work represents the most comprehensive experimental study on liquid light scattering features. The insight from this work should be important for understanding the correlation between the material structure and optical properties. The described method can be readily implemented by researchers with access to conventional spectrofluorometers equipped with excitation and detection polarizers

Determining the Liquid Light Scattering Cross Section and Depolarization Spectra Using Polarized Resonance Synchronous Spectroscopy

Rayleigh scattering is a universal material property because all materials have nonzero polarizability. Reliable quantification of the material light scattering cross section in the liquid phase and its depolarization spectra is, however, challenging due to a host of sample and instrument issues. Using the recently developed polarized resonance synchronous spectroscopic method, we reported the light scattering cross section and depolarization spectra measured for a total of 29 liquids including water, methanol, ethanol, 1-propanol, 1-butanol, dimethylformamide, carbon disulfide, dimethyl sulfoxide, hexane and two hexane isomers (3-methylpentane and 2,3-dimethylbutane), tetrahydrofuran, cyclohexane, acetonitrile, pyridine, chloromethanes including di-, tri, tetrachloromethane, acetone, benzene and eight benzene derivatives (toluene, fluorobenzene, 1,2-, 1,3-, and 1,4-difluorobenzene, chlorobenzene, 1,2- and 1,3-dichlorobenzene, and nitrobenzene). The solvent light scattering depolarization is wavelength-independent for the model solvents, and it varies from 0.023 ± 0.011 for CCl₄ to 0.619 ± 0.022 for nitrobenzene. The light scattering cross-section spectra can be approximated with the function of σ­(λ) = αλ^–4 with the α value varying from 7.2 ± 0.2 × 10^–45 cm⁶ for water to a maximum of 8.5 ± 0.6 × 10^–43 cm⁶ for nitrobenzene. Structural isomerization has no significant effect on either the depolarization or the scattering cross sections for both hexanes and difluorobenzene isomers. This work represents the most comprehensive experimental study on liquid light scattering features. The insight from this work should be important for understanding the correlation between the material structure and optical properties. The described method can be readily implemented by researchers with access to conventional spectrofluorometers equipped with excitation and detection polarizers

Reactive Ag⁺ Adsorption onto Gold

Proposed mechanisms of monolayer silver formation on gold nanoparticle (AuNP) include AuNP-facilitated under-potential reduction and antigalvanic reduction in which the gold reduces Ag⁺ into metallic atoms Ag(0). Reported herein is the spontaneous reactive Ag⁺ adsorption onto gold substrates that include both as-obtained and butanethiol-functionalized citrate- and NaBH₄-reduced gold nanoparticles (AuNPs), commercial high-purity gold foil, and gold film sputter-coated onto silicon. The silver adsorption invariably leads to proton releasing to the solution. The nominal saturation packing density of silver on AuNPs varies from 2.8 ± 0.3 nmol/cm² for the AuNPs preaggregated with KNO₃ to 4.3 ± 0.2 nmol/cm² for the AuNPs prefunctionalized with butanethiol (BuT). The apparent Langmuir binding constant of the Ag⁺ with the preaggregated AuNPs and BuT-functionalized AuNPs are 4.0 × 10³ M^–1 and 2.1 × 10⁵ M^–1, respectively. The silver adsorption has drastic effects on the structure, conformation, and stability of the organothiols on the AuNPs. It converts disordered BuT on AuNPs into highly ordered <i>trans</i> conformers, but induces near complete desorption of sodium 2-mercaptoethanesulfonate and sodium 3-mercapto-1-propyl sulfonate from AuNPs. Mechanically, the Ag⁺ adsorption on AuNPs most likely proceeds by reacting with molecules preadsorbed on the AuNP surfaces or chemical species in the solutions, and the silver remains as silver ion in these reaction products. This insight and methodology presented in this work are important for studying interfacial interactions of metallic species with gold and for postpreparation modulation of the organothiol structure and conformation on AuNP surfaces

Teaching Old Polymers New Tricks: Improved Synthesis and Anomalous Crystallinity for a Lost Semi-Fluorinated Polyaryl Ether via Interfacial Polymerization of Hexafluoroacetone Hydrate and Diphenyl Ether

A practical and direct electrophilic polymerization of hexafluoroacetone hydrate with diphenyl ether toward the preparation of semi-fluorinated polyaryl ethers (PAE) is reported. Electrophilic aromatic substitution (EAS) polymerization under interfacial conditions with phase transfer catalyst (Aliquat 336) proceeds in trifluoromethanesulfonic anhydride by generation of trifluoromethanesulfonic acid and the protonated hexafluoroacetone (HFA) in situ affording 1,1,1,3,3,3-hexafluoroisopropylidene (6F) PAE with high regioselectivity (4,4’-DPE) and high molecular weight (≈60 kDa). Although first reported in a 1966 US Patent by DuPont using harsh conditions, improved synthetic methods or modern characterization has not been disclosed until now. Despite the presence of the 6F group, known to impart disordered morphology, this simple semi-fluorinated PAE exhibits anomalous crystallinity with polymorphic melting points (Tm) ranging from 230–309 °C, high solubility in common organic solvents, a glass transition (Tg) of 163 °C, and thermo-oxidative stability above 500 °C. Tough optically clear films prepared from solution give transmittance higher than 90% throughout the visible region. Synthesis, mechanistic aspects, and characterization including surface and dielectric properties are discussed

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