Excitation-wavelength-dependent charge transfer resonance of bipyridines on silver nanoparticles: surface-enhanced Raman scattering study

Interfacial charge transfer complex formation between silver nanoparticles and aromatic amino acids

The optical properties of surface-modified silver nanoparticles (Ag NPs) with aromatic amino acids tryptophan (Trp) and histidine (His) were examined using the cluster model for density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Also, the redistribution of electronic charges upon chemisorption of ligand molecules onto silver's surfaces is determined. The obtained theoretical data, on one side, undoubtedly indicate the the formation of an interfacial charge transfer (ICT) complex between silver and this type of ligand, and, on the other side, partial oxidation of surface silver atoms accompanied by an increase of electron density in ligand molecules. The ICT complex formation, based on noble metal nanoparticles, has never been reported previously to the best of our knowledge. The experimental spectroscopic measurements support the theoretical data. A new absorption band in the visible spectral range appears upon surface modification of Ag NPs, and, when exposed to air, oxidation of surface-modified Ag NPs is significantly faster than the oxidation of the unmodified ones.

AgNPs and MIL-101(Fe) self-assembled nanometer materials improved the SERS detection sensitivity and reproducibility

Recently, in the research of Surface-enhanced Raman scattering (SERS) technology, it is found that the preparation of enhanced substrate is particularly important. In this work, the most commonly used methods were used to synthesize AgNPs and MIL-101(Fe), and AgNPs/MIL-101(Fe) nanocomposite was obtained through self-assembly of the two substances. Four different probe molecules were detected with the self-assembled substrate and compared with the results of same probe molecules with AgNPs and MIL-101(Fe) as SERS substrate separately, it was found that AgNPs/ MIL-101 (Fe) nanocomposites had a strong enhancing effect as SERS substrate. The Enhancement Factor (EF) value of 10^-6 mol/L Rhodamine 6G (R6G) was calculated as 2.09 × 10⁹, and the Raman intensities of the peak relative standard deviation (RSD) of R6G Raman attribution was calculated as 7.55%. The time stability of the material was studied and it was found that the reduced Raman signal and poor reproducibility were due to the AgNPs placement time. AgNPs/ MIL-101 (Fe) nanocomposites were used as SERS substrate to detect Paraquat with a minimum concentration of 10^-12 mol/L. The signal values of Paraquat Raman detected at 10^-6 mol/L in different pH environments were relatively stable.

Recent advances in cancer bioimaging using a rationally designed Raman reporter in combination with plasmonic gold

Gold nanomaterials (AuNMs) have been widely implemented for the purpose of bioimaging of cancer and tumor cells in combination with Raman spectral markers.

Arrays of Ag and Au Nanoparticles with Terpyridine- and Thiophene-Based Ligands: Morphology and Optical Responses

The assembly of Ag and Au nanoparticles (NPs) into nanoparticulate arrays mediated by terpyridine (tpy), 4'-(2-thienyl)terpyridine (T-tpy), and short α,ω-bis(tpy)oligothiophene ligands has been accomplished at the interface between the Ag or Au NP hydrosol and a solution of the molecular species in dichloromethane. The relationship between the morphology and the optical responses of the arrays has been investigated by advanced methods of TEM (transmission electron microscopy) image analysis and surface plasmon extinction (SPE) spectra. It has been established that the size of islands of closely spaced NPs rather than the average interparticle distance affects the extent of delocalization of the surface plasmon excitations and thus also the SPE spectra. Furthermore, the structure of surface-adsorbate complexes formed in these arrays has been investigated by SERS spectral measurements carried out as a function of the excitation wavelength. Photoinduced charge transfer (CT) transitions from the neutral Ag_s⁰ and Au_s⁰ adsorption sites on metal NPs to antibonding orbitals of the adsorbates have been identified for Ag/tpy, Ag/T-tpy, Au/tpy, and Au/T-tpy nanoparticulate arrays. Although the surface-adsorbate complexes displaying a photoinduced CT are known for Ag NPs, the Au_s⁰ surface complexes with this CT are newly reported. Bis(tpy)oligothiophenes were found to be attached to both Ag and Au NPs via the tpy group(s). The match between the interparticle distances within the NP islands and the lengths of the oligomers molecules indicates that the molecules act as interparticle linkers. In this case, unequivocal spectral marker band evidence of the Ag_s⁰ as well as Au_s⁰ surface complex formation has not been obtained.

Optimizing Gold Nanoparticle Cluster Configurations (n ≤ 7) for Array Applications

Nanoparticle cluster arrays (NCAs) are novel electromagnetic materials whose properties depend on the size and shape of the constituent nanoparticle clusters. A rational design of NCAs with defined optical properties requires a thorough understanding of the geometry dependent optical response of the building blocks. Herein, we systematically investigate the near- and far-field responses of clusters of closely packed 60 nm gold nanoparticles (n ≤ 7) as a function of size and cluster geometry through a combination of experimental spectroscopy and generalized Mie Theory calculations. From all of the investigated cluster configurations, nanoparticle trimers with D(3h) geometry and heptamers in D(6h) geometry stand out due to their polarization insensitive responses and high electric (E-) field intensity enhancement, making them building blocks of choice in this size range. The near-field intensity maximum of the D(6h) heptamer is red-shifted with regard to the D(3h) trimer by 125 nm, which confirms the possibility of a rational tuning of the near-field response in NCAs through the choice of the constituent nanoparticle clusters. For the nanoparticle trimer we investigate the influence of the cluster geometry on the optical response in detail and map near- and far-field spectra associated with the transition of the cluster configuration from D(3h) into D(∞h).

pH-dependent surface-enhanced resonance Raman scattering of yeast iso-1-cytochrome c adsorbed on silver nanoparticle surfaces under denaturing conditions at pH<3

We measured the pH-induced spectral changes of yeast iso-1-cytochrome c on silver nanoparticle surfaces using surface-enhanced resonance Raman scattering (SERRS) at 457.9 nm. At a pH of 3, the Met80 ligand in yeast iso-1-cytochrome c is assumed to dissociate, leading to a marked conformational change as evidenced by the vibrational spectral shifts. The Soret band at 410 nm in the UV-Vis spectrum shifted to 396 nm at pH 3, indicating a transition from a low spin state to a high spin state from a weak interaction with a water molecule. Thus, SERRS spectroscopy can measure the pH-induced denaturalization of cyt c adsorbed on metal nanoparticle surfaces at a lower concentration with a better sensitivity than ordinary resonance Raman spectroscopy.