Charge-transfer-driven ultrasensitive SERS sensing in a two-dimensional titanium carbonitride MXene.

Two-dimensional (2D) MXenes stand out as promising platforms for surface-enhanced Raman scattering (SERS) sensing owing to their metallic feature, various compositions, high surface area, compatibility with functionalization, and ease of fabrication. In this work, we report a high-performance 2D titanium carbonitride (Ti3CN) MXene SERS substrate. We reveal that the abundant electronic density of states near the Fermi level of Ti3CN MXene boosts the efficiency of photo-induced charge transfer at the interface of Ti3CN/molecule, resulting in significant Raman enhancement. The SERS sensitivity of Ti3CN MXene is further promoted through a 2D morphology regulation and molecular enrichment strategies. Moreover, prohibited drugs are detectable on this substrate, presenting the potential of trace-amount analysis on Ti3CN MXene. This work provides a deep insight of the SERS mechanisms of Ti3CN MXene and broadens the practical application of transition metal carbonitride MXene SERS substrates.

VSe2-x O x @Pd Sensor for Operando Self-Monitoring of Palladium-Catalyzed Reactions.

Operando monitoring of catalytic reaction kinetics plays a key role in investigating the reaction pathways and revealing the reaction mechanisms. Surface-enhanced Raman scattering (SERS) has been demonstrated as an innovative tool in tracking molecular dynamics in heterogeneous reactions. However, the SERS performance of most catalytic metals is inadequate. In this work, we propose hybridized VSe2-x O x @Pd sensors to track the molecular dynamics in Pd-catalyzed reactions. Benefiting from metal-support interactions (MSI), the VSe2-x O x @Pd realizes strong charge transfer and enriched density of states near the Fermi level, thereby strongly intensifying the photoinduced charge transfer (PICT) to the adsorbed molecules and consequently enhancing the SERS signals. The excellent SERS performance of the VSe2-x O x @Pd offers the possibility for self-monitoring the Pd-catalyzed reaction. Taking the Suzuki-Miyaura coupling reaction as an example, operando investigations of Pd-catalyzed reactions were demonstrated on the VSe2-x O x @Pd, and the contributions from PICT resonance were illustrated by wavelength-dependent studies. Our work demonstrates the feasibility of improved SERS performance of catalytic metals by modulating the MSI and offers a valid means to investigate the mechanisms of Pd-catalyzed reactions based on VSe2-x O x @Pd sensors.

Flexible plasmonic nanocavities: a universal platform for the identification of molecular orientations.

The molecular orientation provides fundamental images to understand molecular behaviors in chemistry. Herein, we propose and demonstrate sandwich plasmonic nanocavities as a surface-selection ruler to illustrate the molecular orientations by surface-enhanced Raman spectroscopy (SERS). The field vector in the plasmonic nanocavity presents a transverse spinning feature under specific excitations, allowing the facile modulation of the field polarizations to selectively amplify the Raman modes of the target molecules. It does not require the knowledge of the Raman spectrum of a bare molecule as a standard and thus can be extended as a universal ruler for the identification of molecular orientations. We investigated the most widely used Raman probe, Rhodamine 6G (R6G) on the Au surface and tried to clarify the arguments about its orientations from our perspectives. The experimental results suggest concentration-dependent adsorption configurations of R6G: it adsorbs on Au primarily via an ethylamine group with the xanthene ring lying flatly on the metal surface at low concentrations, and the molecular orientation gradually changes from "flat" to "upright" with the increase of molecular concentrations.

Investigation of the Plasmon-Activated C-C Coupling Reactions by Liquid-State SERS Measurement.

The implementation of plasmonic materials in heterogeneous catalysis was limited due to the lack of experimental access in managing the plasmonic hot carriers. Herein, we propose a liquid-state surface-enhanced Raman scattering (SERS) technique to manipulate and visualize heterogeneous photocatalysis with transparent plasmonic chips. The liquid-state measurement conquers the difficulties that arise from the plasmon-induced thermal effects, and thus the plasmon based strategies can be extended to investigate a wider range of catalytic reactions. We demonstrated the selection of reaction products by modulating the plasmonic hot carriers and explored the mechanisms in several typical C-C coupling reactions with 4-bromothiophenol (4-BTP) as reactants. The real-time experimental results suggest brand new mechanisms of the formation of C-C bonds on plasmonic metal nanoparticles (NPs): the residue of 4-BTP, but not thiophenol (TP), is responsible for the C-C coupling. Furthermore, this technique was extended to study the evolution of the Suzuki-Miyaura reaction on nonplasmonic palladium metals by establishing the charge transfer channels between palladium and Au NPs. The cleavage and formation of chemical bonds in each individual reaction step were discerned, and the corresponding working mechanisms were clarified.

Electromagnetic Mechanisms or Chemical Mechanisms? Role of Interfacial Charge Transfer in the Plasmonic Metal/Semiconductor Heterojunction.

The plasmonic metal/semiconductor heterojunction provides a unique paradigm for manipulating light to improve the efficiency of plasmonic materials. Previous studies suggest that the improvement originates from the enhanced carrier exchanges between the plasmonic component of the heterojunction and molecules. This viewpoint, known as the chemical mechanism, is reasonable but insufficient, because the construction of the heterojunction will lead to a charge redistribution in the plasmonic component and cause changes in its physical characteristics. Herein, we will try to clarify that these changes are decisive factors in specific applications by investigating the surface-enhanced Raman scattering (SERS) behavior of a typical Ag/TiO2 heterojunction. We observed significant changes in SERS spectra by modulating the band alignment of the heterojunction in a loop. Identical trends in SERS spectra were observed despite the fact that the charge transfer from the heterojunction to molecules was blocked, suggesting that the major SERS enhancement originates from electromagnetic mechanisms rather than chemical ones.

Assess transmissibility of different influenza subtypes: Based on a SEIABR model.

OBJECTIVE: Influenza is a worldwide public health problem which causes a serious economic and health burden. In order to provide a scientific basis for improving the prevention and control level of influenza, using dynamic model to evaluate the infection rates of influenza different subtypes from 2010 to 2019 in China. METHODS: This article established SEIABR model based on influenza cases reported by China National Influenza Center from 2010 to 2019. And calculated the transmission rate and Re by combined the natural birth rate, natural death rate, infectious rate, proportion of asymptomatic patients, proportion of untreated patients, recovery rate and fatality rate. RESULTS: The average infection rate of influenza was (2.38 ± 0.59) × 10-10, and influenza A was (2.24 ± 0.51) × 10-10, influenza B was (2.21 ± 0.68) × 10-10. And average Re were 1.60, 1.51, 1.49. In addition, the infection rates of A /H1N1, A/H3N2, B/Yamagata and B/Victoria were (2.47 ± 0.51) × 10-10, (2.25 ± 0.48) × 10-10, (2.15 ± 0.61) × 10-10, and (2.30 ± 0.66) × 10-10 and average Re were 1.67, 1.52, 1.44, 1.56. CONCLUSION: Between each year, flu transmission capacity had fluctuation. Influenza A was more transmissible than influenza B, and during the major subtypes, influenza A/H1N1 was the most transmissible.

Verification and Analysis of Single-Molecule SERS Events via Polarization-Selective Raman Measurement.

We propose polarization-selective Raman measurement as a decent method for single-molecule surface-enhanced Raman scattering (SMSERS) verification. This approach features rapid acquisition of SMSERS events and appeals liberal requirements for analyte concentration. It is demonstrated as an efficient tool in sorting out dozens of SMSERS events from a large-scale plasmonic dimer array. In addition, it allows identification of a mixed SMSERS event containing two different individual molecules. In this article, the RPM method is employed to explore the underlying mechanisms of signal blinking, spectral wandering, and other unique characteristics in SMSERS. We observed synchronized blinking of different modes from one Rhodamine 6G (R6G) molecule, but a disagreement is found in a mixed SMSERS event containing one R6G molecule and one crystal violet molecule. Our approach offers a reliable means to interpret SMSERS events in statistical terms and facilitate the fundamental understanding of SMSERS.

Manipulating Hot-Electron Injection in Metal Oxide Heterojunction Array for Ultrasensitive Surface-Enhanced Raman Scattering.

Efficient photoinduced charge transfer (PICT) resonance is crucial to the surface-enhanced Raman scattering (SERS) performance of metal oxide substrates. Herein, we venture into the hot-electron injection strategy to achieve unprecedented enhanced PICT efficiency between substrates and molecules. A heterojunction array composed of plasmonic MoO2 and semiconducting WO3-x is designed to prove the concept. The plasmonic MoO2 generates intense localized surface plasmon resonance under illumination, which can generate near-field Raman enhancement as well as accompanied plasmon-induced hot-electrons. The hot-electron injection in direct interfacial charge transfer and plasmon-induced charge transfer process can effectively promote the PICT efficiency between substrates and molecules, achieving a record Raman enhancement factor among metal oxide substrates (2.12 × 108) and the ultrasensitive detection of target molecule down to 10-11 M. This work demonstrates the possibility of hot-electron manipulation to realize unprecedented Raman enhancement in metal oxides, offering a cutting-edge strategy to design high-performance SERS substrates.

Enlarging spin-dependent transverse displacement of surface plasmon polaritons focus.

By introducing additional spin angular momentum (SAM) induced spiral phase, the spin-dependent transverse displacement of surface plasmon polaritons (SPPs) focus is effectively enlarged. The separation between the SPPs focuses generated by left circularly polarized (LCP) light and right circularly polarized (RCP) light reaches 1500 nm, which is six times larger than the previously reported values with semicircular plasmonic lens. The relationship between the displacement of the SPPs focus and the total spiral phase that consisted of the intrinsic and the additional spiral phase is theoretically established. Furthermore, the flexibility and versatility of the proposed mechanism is demonstrated by reversing or continuously controlling the SPPs focus. These findings hold great promise for spin-based plasmonic devices and the related applications, such as on-chip communication.

Spin-Independent Plasmonic Lens.

For the semicircular plasmonic lens, the spiral phase is the origin of the spin-dependent surface plasmon polariton (SPP) focusing. By counterbalancing the spin-dependent spiral phase with another spiral phase or Pancharatnam-Berry phase, we realized the SPP focusing independent from the spin states of the excitation light. Analyses based on both Huygens-Fresnel principle for SPPs and numerical simulations prove that the position, intensity, and profile of the SPP focuses are exactly the same for different spin states. Moreover, the spin-independent SPP focusing is immune from the change of the radius, the central angle, and the shape of the semicircular slit. This study not only further reveals the mechanism of spin-dependent SPP devices but also provides effective approaches to overcome the influence of spin states on the SPPs field.