Charge Transfer between Metal Nanoparticles Interconnected with a Functionalized Molecule Probed by Surface-Enhanced Raman Spectroscopy

Orientation-independent reaction activity monitoring with single particle and data analytics

Single-particle analysis is the most powerful method to obtain accurate local information for understanding and monitoring chemical reactions. However, investigations about obtaining comprehensive information at the single-particle level to overcome individual errors and sampling randomness have not been reported to date. Plasmonic nanorods, which have excellent anisotropic optical and chemical properties, make us in situ acquisition of conformation and dynamics of the biological information. On the basis of their anisotropic optical properties of the plasmonic nanorods such as Au nanorods (AuNRs) and data analytics, herein we developed a high-throughput resonance scattering imaging method of AuNRs under dark-field microscopy (DFM) to monitor orientation-independent reaction activity of AuNRs. Data analytics are introduced to determine a large number of AuNRs orientation obtained from a series of polarized DFM images, allowing us to real-time monitor reaction activity of AuNRs at all orientations, and also makes it possible to study the global and local reaction processes of AuNRs at single-particle level. Our method is expected to provide a new strategy for analytical study and single-particle sensing in chemistry.

Ligand-Supported Hot Electron Harvesting: Revisiting the pH-Responsive Surface-Enhanced Raman Scattering Spectrum of p-Aminothiophenol

The discussion of the surface-enhanced Raman scattering (SERS) spectra of p-aminothiophenol (PATP) and of its photocatalytic reaction product 4,4'-dimercaptoazobenzene (DMAB) is important for understanding plasmon-supported spectroscopy and catalysis. Here, SERS spectra indicate that DMAB forms also in a nonphotocatalytic reaction on silver nanoparticles. Spectra measured at low pH, in the presence of the acids HCl, H₂SO₄, HNO₃, and H₃PO₄, show that DMAB is reduced to PATP when both protons and chloride ions are present. Moreover, the successful reduction of DMAB in the presence of other, halide and nonhalide, ligands suggests a central role of these species in the reduction. As discussed, the ligands increase the efficiency of hot-electron harvesting. The pH-associated reversibility of the SERS spectrum of PATP is established as an observation of the DMAB dimer at high pH and of PATP as a product of its hot-electron reduction at low pH, in the presence of the appropriate ligand.

Self-Assembled Ag-Cu₂O Nanocomposite Films at Air-Liquid Interfaces for Surface-Enhanced Raman Scattering and Electrochemical Detection of H₂O₂

We employ a facile and novel route to synthesize multifunctional Ag-Cu₂O nanocomposite films through the self-assembly of nanoparticles at an air-liquid interface. In the ethanol-water phase, AgNO₃ and Cu(NO₃)₂ were reduced to Ag-Cu₂O nanoparticles by NaBH₄ in the presence of cinnamic acid. The Ag-Cu₂O nanoparticles were immediately trapped at the air-liquid interface to form two-dimensional nanocomposite films after the reduction reaction was finished. The morphology of the nanocomposite films could be controlled by the systematic regulation of experimental parameters. It was found that the prepared nanocomposite films serving as the substrates exhibited strong surface-enhanced Raman scattering (SERS) activity. 4-aminothiophenol (4-ATP) molecules were used as the test probes to examine the SERS sensitivity of the nanocomposite films. Moreover, the nanocomposite films synthesized by our method showed enhanced electrocatalytic activity towards hydrogen peroxide (H₂O₂) and therefore could be utilized to fabricate a non-enzymatic electrochemical H₂O₂ sensor.

Electrostatic Assemblies of Well-Dispersed AgNPs on the Surface of Electrospun Nanofibers as Highly Active SERS Substrates for Wide-Range pH Sensing

Surface-enhanced Raman scattering (SERS) has shown high promise in analysis and bioanalysis, wherein noble metal nanoparticles (NMNPs) such as silver nanoparticles were employed as substrates because of their strong localized surface plasmon resonance (LSPR) properties. However, SERS-based pH sensing was restricted because of the aggregation of NMNPs in acidic medium or biosamples with high ionic strength. Herein, by using the electrostatic interaction as a driving force, AgNPs are assembled on the surface of ethylene imine polymer (PEI)/poly(vinyl alcohol) (PVA) electrospun nanofibers, which are then applied as highly sensitive and reproducible SERS substrate with an enhancement factor (EF) of 10(7)-10(8). When p-aminothiophenol (p-ATP) is used as an indicator with its b2 mode, a good and wide linear response to pH ranging from 2.56 to 11.20 could be available, and the as-prepared nanocomposite fibers then could be fabricated as excellent pH sensors in complicated biological samples such as urine, considering that the pH of urine could reflect the acid-base status of a person. This work not only emerges a cost-effective, direct, and convenient approach to homogeneously decorate AgNPs on the surface of polymer nanofibers but also supplies a route for preparing other noble metal nanofibrous sensing membranes.

In situ monitoring of catalytic process variations in a single nanowire by dark-field-assisted surface-enhanced Raman spectroscopy

In this communication, we provide a new method for characterizing the kinetics of a catalytic process on multiple sites of a single nanowire by dark-field-assisted surface-enhanced Raman spectroscopy (DFSERS).

Different behaviors in the transformation of PATP adsorbed on Ag or Au nanoparticles investigated by surface-enhanced Raman spectroscopy - a study of the effects from laser energy and annealing

In order to explore the key role of surface plasmon resonance (SPR) and active (3)O2 for the chemical transformation to 4,4-dimercaptoazobenzene (DMAB) from p-aminothiophenol (PATP) adsorbed on Ag or Au NPs, we systematically investigated the laser wavelength and temperature dependent surface-enhanced Raman spectra of PATP capped Ag and Au NPs. DMAB can be easily observed at the 514.5nm laser for Ag NPs but at the 632.8nm laser for Au NPs, indicating that a suitable energy level is necessary for the formation of DMAB. The tendency is consistent with the wavelength dependent SPR properties of Ag or Au NPs accordingly. With the energy provided by annealing, the transformation of PATP to DMAB is much easier on Ag NPs at a lower temperature, and more DMAB can be observed at the same temperature, compared to the case of Au NPs under the same condition. It is mainly due to the active (3)O2 on Ag surfaces could be more easily formed than that on Au surfaces.

Enhanced Raman scattering as a probe for 4-mercaptopyridine surface-modified copper oxide nanocrystals

We report on the application of Raman scattering as a probe to study 4-mercaptopyridine (4-Mpy) surface-modified CuO nanocrystals. Enhanced Raman scattering from 4-Mpy adsorbed on CuO nanocrystals was observed. The Raman signals displayed 10(2) enhancement compared with those of 4-Mpy in solution. We compared the Raman spectra of 4-Mpy molecules adsorbed on CuO nanocrystals and Ag, Cu substrate. A particular feature of the semiconductor substrate that was different from the metal substrate was revealed. The excitation wavelength-dependent behavior was clearly observed.

Contribution of Charge-Transfer Mechanisms to Surface-Enhanced Raman Scattering with Near-IR Excitation

Surface-enhanced Raman scattering under near-IR excitation is investigated for p-aminothiophenol (PATP) molecules that are either adsorbed on an electrochemically roughened silver electrode or embedded in an Au/PATP/Ag molecular junction assembled on an indium-doped tin oxide electrode. The contribution from chemical enhancement can be amplified relative to the contribution from electromagnetic enhancement, because the energy of the near-IR excitation is far from the surface plasmon resonance of the nanosized metal particles. The energy required for the charge-transfer process for the Au/PATP/Ag molecular junction is much lower than that of the PATP molecules adsorbed on the electrochemically roughened silver electrode. Coadsorption of chloride ions on the metal nanoparticles may result in an alteration of the local Fermi level of the metal nanoparticles, thus leading to better energy matching between the energy level of the interconnecting PATP molecules and the Fermi level of the metal nanoparticles.