Charge Transfer at the TiO₂/N3/Ag Interface Monitored by Surface-Enhanced Raman Spectroscopy

The interface of semiconductor–dye–metal system is a crucial issue for investigating dye-sensitized solar cells (DSSCs), where the electron transfer takes place. In this work, a series of assemblies of TiO₂/N3 (<i>cis</i>-bis­(isothiocyanato)­bis­(2,2′-bipyridyl-4,4′-dicarboxylato)­ruthenium­(II)) and TiO₂/N3/Ag have been fabricated, which were employed for the investigation of the adsorption configuration and conformational change of N3 molecules. We plot degree of charge transfer (CT) (ρ_CT) as a function of excitation wavelength of TiO₂/N3 and TiO₂/N3/Ag assemblies, which contributes to the understanding of the CT process in the series of N3 assemblies. According to the variation tendency of ρ_CT, when laser energy exceeds the CT energy threshold 2.071 eV, ρ_CT shows an obvious increasing trend with the increasing laser energy. In the case of TiO₂/N3/Ag assembly, when the laser energy exceeds the CT energy threshold 1.877 eV, ρ_CT becomes lager with the increase in the laser energy, until asymptotic behavior appears under higher laser energy. To explain the variation tendency of ρ_CT and the shift of CT energy threshold, we have proposed two models about the energy level scheme of TiO₂/N3 and TiO₂/N3/Ag assemblies. Furthermore, we investigated the influence of crystal structure of TiO₂ NPs on the CT process by the fabrication TiO₂/N3/Ag assemblies based on anatase and rutile TiO₂ NPs. It is noted that the TiO₂/N3/Ag assembly based on TiO₂ NPs calcinated at 450 °C with highest ρ_CT and lowest CT energy threshold is most in favor of CT process. Besides the specific chemical binding mode in the TiO₂/N3/Ag system, this study also found the relationship between the ρ_CT and the CT process, which is of considerable importance and relevance to solar energy conversion

Magnetic Silver Hybrid Nanoparticles for Surface-Enhanced Resonance Raman Spectroscopic Detection and Decontamination of Small Toxic Molecules

Magnetic hybrid assemblies of Ag and Fe₃O₄ nanoparticles with biocompatibly immobilized myoglobin (Mb) were designed to detect and capture toxic targets (NO₂^–, CN^–, and H₂O₂). Mb was covalently attached to chitosan-coated magnetic silver hybrid nanoparticles (M-Ag-C) <i>via</i> glutaraldehyde that serves as a linker for the amine groups of Mb and chitosan. As verified by surface-enhanced resonance Raman (SERR) spectroscopy, this immobilization strategy preserves the native structure of the bound Mb as well as the binding affinity for small molecules. On the basis of characteristic spectral markers, binding of NO₂^–, CN^–, and H₂O₂ could be monitored and quantified, demonstrating the high sensitivity of this approach with detection limits of 1 nM for nitrite, 0.2 μM for cyanide, and 10 nM for H₂O₂. Owing to the magnetic properties, these particles were collected by an external magnet to achieve an efficient decontamination of the solutions as demonstrated by SERR spectroscopy. Thus, the present approach combines the highly sensitive analytical potential of SERR spectroscopy with an easy approach for decontamination of aqueous solutions with potential applications in food and in environmental and medical safety control

Multiplex Immunochips for High-Accuracy Detection of AFP-L3% Based on Surface-Enhanced Raman Scattering: Implications for Early Liver Cancer Diagnosis

α-Fetoprotein (AFP) is an important tumor biomarker. In particular, the overexpression of AFP-L3 is associated with hepatocellular carcinoma (HCC). Accordingly, several hospitals have begun to employ the ratio of AFP-L3 to the total AFP level (AFP-L3%) as new diagnostic evidence for HCC owing to its high diagnostic accuracy. However, current methods of detection for AFP and AFP-L3 are time-consuming, require multiple samples, and lack in sensitivity and specificity. Herein, we present a novel concept for the early diagnosis of HCC based on the combination of Raman frequency shift and intensity change, and developed surface-enhanced Raman scattering (SERS)-based immunochips via AFP-L3%. In the first step of the study, the frequency shift of 4-mercapto­benzoic acid (MBA) was applied for the quantitative determination of total AFP based on the AFP and anti-AFP interaction on MBA-modified silver chips. 5,5-Dithiobis­(succinimidyl-2-nitro­benzoate) (DSNB)-modified immunogold was then incorporated with AFP-L3 antibodies for sandwich immunoreaction on the chips. As a result, we found that a typical Raman band intensity of DSNB presented an exponential linear relationship with the concentration of AFP-L3. Thus, the AFP-L3% can be calculated according to the concentrations of AFP-L3 and total AFP. The most important advantage of the proposed method is the combination of AFP-L3% and frequency shifts of SERS, which exhibits excellent reproducibility and high accuracy, and significantly simplifies the conventional detection procedure of AFP-L3%. Application of the proposed method with the serum of patients with HCC demonstrated its great potential in early liver cancer diagnosis

Investigation of Charge Transfer in Ag/N719/TiO₂ Interface by Surface-Enhanced Raman Spectroscopy

The interfaces of metal–dye molecule–semiconductor sandwich structure are very important in the investigation of dye-sensitized solar cells (DSSCs) where metals are used to enhance absorption. In this work, we first designed and synthesized Ag/N719 and Ag/N719/TiO₂ sandwich systems to investigate the chemical binding type at the interfaces of Ag/N719/TiO₂. The results of the Raman spectra under the laser excitations of 532, 633, and 785 nm clarified that the SCN groups adsorbed on the Ag surface via the S terminal and the TiO₂ layer possibly bound to Ag/N719 via the ester linkage (OCO) of the COOH group in N719. Then, we optimized the Ag substrate as an SERS detection platform and selected the Ag sol film as the substrate. Last, the relationship between the “degree of CT (ρ_CT)” in the SERS spectra and the charge transfer (CT) process was investigated by tuning the contribution from the chemical effect. We found that, owing to the introduction of TiO₂, the intensity and ρ_CT first increased (<i>n</i> = 0–2) and then decreased (<i>n</i> = 2–3) with the increase of the number of TiO₂ layers under 532 and 633 nm laser excitations. However, the intensity decreased (<i>n</i> = 0–3) with the increase of the number of TiO₂ layers under the laser excitation of 785 nm, and there was no obvious change about ρ_CT. Meanwhile ρ_CT became higher with the increase of the laser excitation energy at the interfaces with the same TiO₂ layer number. In order to explain these variations about ρ_CT, we utilize ultraviolet photoelectron spectroscopy (UPS) and UV–vis spectra to calculate energy levels for better understanding the charge transfer (CT) process, and the calculation result is in accordance with the variation tendency of ρ_CT

Reduced Charge-Transfer Threshold in Dye-Sensitized Solar Cells with an Au@Ag/N3/<i>n</i>‑TiO₂ Structure As Revealed by Surface-Enhanced Raman Scattering

Interfacial information, such as the binding type and charge transfer (CT) processes, is critical for the investigation of dye-sensitized solar cells (DSSCs). Herein, Au@Ag core–shell nanoparticles (NPs) were employed for the fabrication of Au@Ag/N3/<i>n</i>-TiO₂ systems. With the help of the CT degree (ρ_CT), surface-enhanced Raman scattering (SERS) spectra are utilized to evaluate the CT processes upon layer-by-layer addition of TiO₂ to metal/N3/<i>n</i>-TiO₂ systems based on Au@Ag, Ag, and Au. The layer-dependent SERS intensity and ρ_CT for Au@Ag/N3/<i>n</i>-TiO₂ revealed a CT enhancement involving TiO₂ at excitation wavelengths of 488, 514.5, 647, and 785 nm, whereas Ag/N3/<i>n</i>-TiO₂ presented such enhancement only at excitation wavelengths of 488 and 514.5 nm. The mechanisms of CT process are proposed to explain such reduced CT energy threshold: In Au@Ag/N3/<i>n</i>-TiO₂, an equivalent CT process involving TiO₂ is first proposed, in which the electrons are directly transferred from the HOMO level of N3 to the much lower CB₃ level of the Au@Ag/TiO₂ complex due to energy level equilibration, which reduces the CT threshold involving TiO₂ below1.58 eV, extends the CT response region involving TiO₂ from 488 to 514.5 nm (Ag/N3/<i>n</i>-TiO₂) to 488–785 nm and enhances the CT efficiency in the high-energy region. Finally, Au@Ag1–5 with different Ag/Au ratios were prepared for the fabrication of bimetal/N3/<i>n</i>-TiO₂ systems. In the Au@Ag1/N3/<i>n</i>-TiO₂ and Au@Ag2/N3/<i>n</i>-TiO₂ systems, the decreased CT threshold is induced by energy level equilibration; in the Au@Ag4/N3/<i>n</i>-TiO₂ system, it is due to the dual effects of energy level equilibration and activation of Ag at the core–shell surface induced by the inner Au