Probing the charge-transfer of Ag/PEDOT:PSS/4-MBA by surface-enhanced raman scattering

Surface-enhanced Raman spectroscopy for environmental monitoring using gold clusters anchored on reduced graphene oxide

Surface-enhanced Raman spectroscopy is a strong and sensitive analysis tool that can realize single-molecule level detection and provide the fingerprint information of molecules, which has been widely applied in analysing chemistry and biomolecules and monitoring environment. However, it is still a challenge to design and prepare SERS substrates with high enhancement factor, simple synthesis, stability and reproducibility. Here, we synthesized gold clusters anchored on reduced graphene oxide (Au clusters@rGO) using co-reduction method to achieve high SERS enhancement. The substrate of gold clusters anchored on reduced graphene oxide combines the chemical enhancement of reduced graphene oxide and the electromagnetic enhancement of gold clusters, leading to an ultrahigh enhancement factor of 3.5 × 10⁷. The efficient SERS was ascribed to the high localized surface plasmon resonance (LSPR) of aggregations of gold clusters, the synergistic effect of gold clusters and reduced graphene oxide, and the charge transfer between graphene and the molecules. This research will provide an invaluable strategy to design and prepare superior-property SERS substrates.

Biocompatible Ionic Liquids in High-Performing Organic Electrochemical Transistors for Ion Detection and Electrophysiological Monitoring

Organic electrochemical transistors (OECTs) have recently attracted attention due to their high transconductance and low operating voltage, which makes them ideal for a wide range of biosensing applications. Poly-3,4-ethylenedioxythiophene:poly-4-styrenesulfonate (PEDOT:PSS) is a typical material used as the active channel layer in OECTs. Pristine PEDOT:PSS has poor electrical conductivity, and additives are typically introduced to improve its conductivity and OECT performance. However, these additives are mostly either toxic or not proven to be biocompatible. Herein, a biocompatible ionic liquid [MTEOA][MeOSO₃] is demonstrated to be an effective additive to enhance the performance of PEDOT:PSS-based OECTs. The influence of [MTEOA][MeOSO₃] on the conductivity, morphology, and redox process of PEDOT:PSS is investigated. The PEDOT:PSS/[MTEOA][MeOSO₃]-based OECT exhibits high transconductance (22.3 ± 4.5 mS μm^-1), high μC* (the product of mobility μ and volumetric capacitance C*) (283.80 ± 29.66 F cm^-1 V^-1 s^-1), fast response time (∼40.57 μs), and excellent switching cyclical stability. Next, the integration of sodium (Na⁺) and potassium (K⁺) ion-selective membranes with the OECTs is demonstrated, enabling selective ion detection in the physiological range. In addition, flexible OECTs are designed for electrocardiography (ECG) signal acquisition. These OECTs have shown robust performance against physical deformation and successfully recorded high-quality ECG signals.

New Trends in Nanoarchitectured SERS Substrates: Nanospaces, 2D Materials, and Organic Heterostructures

This article reviews recent fabrication methods for surface-enhanced Raman spectroscopy (SERS) substrates with a focus on advanced nanoarchitecture based on noble metals with special nanospaces (round tips, gaps, and porous spaces), nanolayered 2D materials, including hybridization with metallic nanostructures (NSs), and the contemporary repertoire of nanoarchitecturing with organic molecules. The use of SERS for multidisciplinary applications has been extensively investigated because the considerably enhanced signal intensity enables the detection of a very small number of molecules with molecular fingerprints. Nanoarchitecture strategies for the design of new NSs play a vital role in developing SERS substrates. In this review, recent achievements with respect to the special morphology of metallic NSs are discussed, and future directions are outlined for the development of available NSs with reproducible preparation and well-controlled nanoarchitecture. Nanolayered 2D materials are proposed for SERS applications as an alternative to the noble metals. The modern solutions to existing limitations for their applications are described together with the state-of-the-art in bio/environmental SERS sensing using 2D materials-based composites. To complement the existing toolbox of plasmonic inorganic NSs, hybridization with organic molecules is proposed to improve the stability of NSs and selectivity of SERS sensing by hybridizing with small or large organic molecules.

Chemical Enhancement Effect of Icotinib-Au Complex Studied by Combined Density Functional Theory and Surface-Enhanced Raman Scattering

Icotinib is an epidermal growth factor receptor tyrosine kinase inhibitor used in the treatment of non-small cell lung cancer. The charge transfer effect between gold nanoparticles (AuNPs) and icotinib molecules can be used as a model to study the adsorption mechanism between molecules and metal. The adsorption of icotinib on the AuNP surface was confirmed by UV-vis and transmission electron microscopy (TEM) experiments. To explain the nature of chemisorption between icotinib and AuNPs from a theoretical perspective, the molecular correlation properties of the complex model of icotinib-Au₆ were studied by the density functional theory method. By studying the molecular electrostatic potential of an icotinib molecule, four potential binding sites of the icotinib molecule were predicted. The calculation results of binding energy showed that the complex formed by chemisorption of icotinib through acetylene group and Au₆ was the most stable one. The molecular frontier orbitals of icotinib and icotinib-Au₆ confirmed that the charge transfer effect occurred on the acetylene group, benzene ring, and quinazoline ring of the icotinib molecule. The Herzberg-Teller surface selection rule was used to explain selective enhancement in the theoretically calculated Raman spectra. By comparing the spectra of theory and experiment, the cause of spectral peak shift and broadening that appeared in the surface-enhanced Raman scattering spectrum compared with the normal Raman spectrum was explained as well. This work would contribute to the development and application of the icotinib-Au drug carrier system.

The characteristics of Raman spectroscopy of fenbendazole-gold nanoparticles based on the chemical adsorption effect

Fenbendazole, a benzimidazole derivative with anti-tubulin polymerization properties, has been widely used in the treatment of parasitic infections. Because of its anticancer activity similar to that of many anticancer drugs, low cost and few side effects, fenbendazole has attracted wide research attention. The chemical adsorption of fenbendazole and gold nanoparticles are studied by the UV-Vis spectrophotometry, density functional method, Raman spectroscopy and surface-enhanced Raman spectroscopy. By comparing and analyzing the theoretical and experimental Raman spectra, this paper explains the reasons for the difference between the theoretical and experimental Raman spectra. Meanwhile, it is also found that the frequencies at 851 cm^-1, 1222 cm^-1, 1425 cm^-1 and 1566 cm^-1 are greatly enhanced. It is found that imidazole is adsorbed vertically to the surface of the substrate. It is concluded that Fenbendazole is vertically adsorbed on the surface of AuNPs through imidazole.

Effective Electrochemical Modulation of SERS Intensity Assisted by Core-Shell Nanoparticles

An effective and reversible tuning of the intensity of surface-enhanced Raman scattering (SERS) of nonelectroactive molecules at nonresonance conditions by electrochemical means has been developed on plasmonic molecular nanojunctions formed between Au@Ag core-shell nanoparticles (NPs) and a gold nanoelectrode (AuNE) modified with a self-assembled monolayer. The Au@Ag nanoparticle on nanoelectrode (NPoNE) structures are formed in situ by the electrochemical deposition of Ag on AuNPs adsorbed on the AuNE and can be monitored by both the electrochemical current and SERS signals. Instead of introducing molecular changes by the applied electrode potential, the highly effective SERS intensity tuning was achieved by the chemical composition transformation of the ultrathin Ag shell from metallic Ag to insulating AgCl. The electrode potential-induced electromagnetic enhancement (EME) tuning in the Au@Ag NPoNE structure has been confirmed by finite-difference time-domain simulations. Moreover, the specific Raman band associated with Ag-molecule interaction can also be tuned by the electrode potential. Therefore, we demonstrated that the electrode potential could effectively and reversibly modulate both EME and chemical enhancement in Au@Ag NPoNE structures.