Decoration of Ag nanoparticle on MXene sheets for SERS studies.

MXene sheets with the unique electrical and optical properties show the excellent potential for surface-enhanced Raman spectroscopy (SERS) applications. In this study, we chose Ti3C2Tx, a type of MXene, to decorate silver nanoparticles (Ag NPs) on the ultrathin two-dimensional (2D) MXene sheets. The designed Ag-MXene substrates with SERS activity showed high sensitivity, high stability, and reproducibility. The SERS signal was enhanced by the synergistic contribution of both charge-transfer (CT) and surface plasmon resonance (SPR) involving the Ag NPs and the MXene sheets. Due to the strong interaction between the probe molecules and Ag NPs which provided the nanoscale gap, the substrate exhibited remarkable SERS performance. A novel experimental strategy was developed to facilitate the controlled synthesis of noble metal NPs and MXene sheets and provide insights for further improving the practical applications of these materials in SERS detection.

SERS monitoring of methylene blue degradation by Au-Ag@Cu2O-rGO nanocomposite.

A combination of multiple materials effectively improves and enhances the performance of the materials. Thus, a gold-silver@cuprous oxide (Au-Ag@Cu2O)-reduced graphene oxide (rGO) structure was designed and fabricated. We decorated the Au nanoparticles (NPs) on the Ag@Cu2O-rGO composite surface by a redox reaction to form a Au-Ag@Cu2O-rGO structure with two noble metals attached to a Cu2O semiconductor. A comparable Au-Ag@Cu2O structure was also fabricated. After decorating Au NPs into the Ag@Cu2O-rGO composite, the Au-Ag@Cu2O composite structure was loosened, and the surface and interior of the Cu2O shell were decorated with Au and Ag NPs. Moreover, the addition of Au NPs resulted in a proper surface plasmon resonance effect and a significant broadening of the absorption range. The loose structure increased the adsorption of the probe molecules, which increased the surface-enhanced Raman scattering (SERS) intensity. In addition, the fabricated Au-Ag@Cu2O-rGO exhibited excellent catalytic reduction of methylene blue (MB) with sodium borohydride (NaBH4). Therefore, the SERS-based monitoring of the MB degradation was obviously improved.

Effect of a Ag-rGO structure on the SERS activity of PEDOT:PSS films.

π-Conjugated organic semiconductors with tunable electronic structures are new prospective active substrate materials for surface-enhanced Raman scattering (SERS). However, observing higher SERS activity when using organic semiconductors as substrates could be difficult because there is no plasmonic effect of hot electrons. Here, we designed a Ag-reduced graphene oxide (rGO) structure, introduced it into a poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) solution, and spin-coated the solution to obtain a Ag-rGO/PEDOT:PSS (ARPP) film. Our analyses demonstrate that the introduction of this Ag-rGO structure can not only enhance the electromagnetic field effect based on plasmon resonance but also improve the interaction between the target molecule and the substrate in the ARPP film. This innovative approach not only improves the SERS activity of π-conjugated organic polymers but also provides novel ideas for the preparation of other organic semiconductor-based SERS substrates.

Ag decoration on Na2Ti3O7 nanowires for improved SERS and PHE performance.

Na2Ti3O7 (NTO) is recognized as an authenticated promising photocatalyst and surface-enhanced Raman scattering (SERS) active material, although its performance is limited by its high carrier recombination rate, wide band gap and inadequate utilization of visible light. In this study, to solve these issues, sea urchin-shaped NTO nanowires directly grown on a substrate were fabricated, and then Ag nanoparticles were decorated on NTO nanowires using sputtering equipment. The as-prepared Ag-NTO substrates exhibited different morphologies and high SERS activity, which was confirmed by finite-difference time-domain (FDTD) simulations, showing that appropriate Ag decoration can bring more nanogaps and thus enhance the electromagnetic field (EM) contribution. We visualized the charge transfer (CT) mechanism in SERS and further investigated the catalytic hydrogen production process similarly induced by photogenerated CT. The optimal SERS substrate (Ag-NTO-3) was adopted to verify the photocatalytic hydrogen evolution (PHE) activity, and the hydrogen evolution rate of Ag-NTO-3 was 106.7 µmol h-1 (twice that of pristine NTO). Photoelectrochemical measurements and photoluminescence (PL) analysis were used to elucidate the potential enhancement mechanisms for the photocatalytic performance and CT process. This study can provide a valuable reference for performance and mechanism studies of SERS substrates and photocatalysts.

Molecular-Orbital Delocalization Enhances Charge Transfer in π-Conjugated Organic Semiconductors.

π-Conjugated organic semiconductors are promising materials for surface-enhanced Raman scattering (SERS)-active substrates based on the tunability of electronic structures and molecular orbitals. Herein, we investigate the effect of the temperature-mediated resonance-structure transitions of poly(3,4-ethylenedioxythiophene) (PEDOT) in poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT : PSS) films on the interactions between substrate and probe molecules, thereby affecting the SERS activity. Absorption spectroscopy and density functional theory calculations show that this effect occurs mainly due to delocalization of the electron distribution in molecular orbitals, effectively promoting the charge transfer between the semiconductor and probe molecules. In this work, we investigate for the first time the effect of electron delocalization in molecular orbitals on SERS activity, which will provide new design ideas for the development of highly sensitive SERS substrates.

Highly selective and quantitative in situ monitoring of cell surface proteins by SERS immunoassay system.

Due to their pivotal roles in many biological functions, cell surface proteins (CSPs) are often used for cancer prognosis, as evidenced by a number of studies that have reported significant changes in the expression levels of specific surface proteins depending on the stage of tumorigenesis and selection/variety of reprogrammed cells during cell fate conversion. Current CSP detection strategies suffer from poor selectivity and lack the ability for in situ analysis but maintain the spatial information between cells. Here, we have fabricated nanoprobes for surface-enhanced Raman scattering (SERS) immunoassays by conjugating a specific antibody onto silica-coated gold nanoparticles incorporating an individual Raman reporter (Au-tag@SiO2-Ab NPs) for highly sensitive and selective in situ detection in different types of cells. When multiple HEK293 cell lines stably expressing different levels of the CSP, ACE2, were investigated by the SERS immunoassay, we demonstrated that the level of ACE2 expression in each cell line could be statistically distinguished from that in the other cell lines, indicating the quantitative feature of this biosensing system. When detecting living cells without cell lysis or fixation, as well as fixed cells, the levels of the epithelial CSPs, EpCAM (epithelial cell adhesion molecule) and E-cadherin, were successfully determined using our Au-tag@SiO2-Ab NPs and SERS immunoassay system in a highly selective and quantitative manner without significant cytotoxicity. Hence, our work provides technical insight into the development of a biosensing platform for a variety of biomedical applications, such as cancer metastasis prognosis and the in situ monitoring of stem cell reprogramming and differentiation.

Interface Design of 3D Flower-like Ag@ZnSe Composites: SERS and Photocatalytic Performance.

In this work, we cosputtered Ag and ZnSe on a polystyrene template to form a three-dimensional (3D) Ag@ZnSe (x) structure. The 3D surface morphology and material composition that provided abundant "hot spots" were controlled by adjusting the sputtering power of the ZnSe, which was confirmed by finite-difference time-domain (FDTD) simulation. The introduction of ZnSe into the noble metal Ag also introduced a charge-transfer (CT) effect into the system, and the CT path was proven with the two-dimensional correlation spectroscopy (2D-COS)-surface-enhanced Raman scattering (SERS) technique. In addition, the substrate exhibited excellent catalytic activity due to the CT effect. The catalyzed degradation of malachite green (MG) was due to the CT effect in the system, and the catalytic process was successfully monitored by in situ SERS. Most importantly, the catalytic degradation by Ag@ZnSe (x) with different parameters was proportional to the degree of CT (ρCT). The SERS and catalytic mechanisms were analyzed in depth with the 2D-COS-SERS technique, which was also useful in verifying the CT process. The catalytic sites for MG were successfully monitored with the 2D-COS-SERS technique. This study provides a reference for studies of the synergistic effects of the electromagnetic mechanism and CT, as well as a new perspective on photocatalysis with dye molecules and monitoring of the catalytic processes.

Controllable hot electron transfer in the Ag/MoO3 layer by layer system: Thickness-dependent MoO3 layer.

The Ag and MoO3 layer-by-layer sputtering method was employed to fabricate Ag/MoO3 coated on a polystyrene (PS) array (Ag/MoO3@PS), which exhibited excellent surface-enhanced Raman scattering (SERS) activity. The thickness of the MoO3 layer was controlled by changing the sputtering power. The SERS intensity of 4-aminothiophenol (PATP) on Ag/MoO3@PS with a 2 nm thickness of the MoO3 layer was comparable to that on pure Ag coated on the PS array (Ag@PS). This is possible because hot electrons were injected from Ag into the MoO3 layer to enhance the photocatalyst reaction; thus, the SERS spectra of coupled PATP were obtained. The transport of hot electrons rapidly decayed and was blocked with increasing thickness of the MoO3 layer from 2 nm to 9 nm. Thus, the SERS intensity decreased, and interestingly, the b2 mode of PATP decreased and nearly disappeared. This study provides new insight into the control of hot electron reduction for catalytic reduction process monitoring.

Probe-Free Identification of RNA Virus Variants with Point Mutations by Surface-Enhanced Raman Spectroscopy.

As observed in the COVID-19 pandemic, RNA viruses continue to rapidly evolve through mutations. In the absence of effective therapeutics, early detection of new severely pathogenic viruses and quarantine of infected people are critical for reducing the spread of the viral infections. However, conventional detection methods require a substantial amount of time to develop probes specific to new viruses, thereby impeding immediate response to the emergence of viral pathogens. In this study, we identified multiple types of viruses by obtaining the spectral fingerprint of their surface proteins with probe-free surface-enhanced Raman scattering (SERS). In addition, the SERS-based method can remarkably distinguish influenza virus variants with several surface protein point mutations from their parental strain. Principal component analysis (PCA) of the SERS spectra systematically captured the key Raman bands to distinguish the variants. Our results show that the combination of SERS and PCA can be a promising tool for rapid detection of newly emerging mutant viruses without a virus-specific probe.

Recent Advances in Monitoring Stem Cell Status and Differentiation Using Nano-Biosensing Technologies.

In regenerative medicine, cell therapies using various stem cells have received attention as an alternative to overcome the limitations of existing therapeutic methods. Clinical applications of stem cells require the identification of characteristics at the single-cell level and continuous monitoring during expansion and differentiation. In this review, we recapitulate the application of various stem cells used in regenerative medicine and the latest technological advances in monitoring the differentiation process of stem cells. Single-cell RNA sequencing capable of profiling the expression of many genes at the single-cell level provides a new opportunity to analyze stem cell heterogeneity and to specify molecular markers related to the branching of differentiation lineages. However, this method is destructive and distorted. In addition, the differentiation process of a particular cell cannot be continuously tracked. Therefore, several spectroscopic methods have been developed to overcome these limitations. In particular, the application of Raman spectroscopy to measure the intrinsic vibration spectrum of molecules has been proposed as a powerful method that enables continuous monitoring of biochemical changes in the process of the differentiation of stem cells. This review provides a comprehensive overview of current analytical methods employed for stem cell engineering and future perspectives of nano-biosensing technologies as a platform for the in situ monitoring of stem cell status and differentiation.

Evaluation of SERS activity for cosputtered Ag-ZnX@PS (X = O, S, Se) composites: Carrier density dependence.

Ag-ZnX (X = O, S, Se) composites coated on polystyrene (PS) arrays (Ag-ZnO@PS, Ag-ZnS@PS, Ag-ZnSe@PS) were successfully fabricated by using cosputtering technology. We found that ZnX doping decreased the carrier densities of these composites compared to that of pure Ag@PS, which was due to redistribution of electrons between Ag and ZnX. Thus, the carrier density of Ag was decreased, and the surface plasmon resonance (SPR) of Ag was redshifted in the Ag-ZnX composites. As the redshift of the SPR of Ag induced a high SPR contribution to the surface-enhanced Raman scattering (SERS), the SPR and charge transfer (CT) contributions were simultaneously increased with increasing carrier density in the Ag-ZnX composites. This study opens a new path to designing metal-semiconductor composites with controllable carrier density. Regulation of the carrier density will be of great help in understanding SPR and CT contributions.

Novel insight into charge transfer regulation based on carrier density-dependent Ag/ITO composite films.

Cosputtering technology was utilized to prepare a Ag and indium tin oxide (ITO) composite on a flat polystyrene (PS) microsphere array. The carrier density estimated by Hall effect testing of different Sn concentrations in the cosputtered films can be tuned from 1018 to 1020 cm-3. The bandgap calculated based on ultraviolet photoelectron spectroscopy can be adjusted within the range of 3.95-4.02 eV. We explored the possible mechanism of charge transfer (CT) by varying the bandgap and explained the causes of the surface-enhanced Raman scattering (SERS). Surprisingly, a synchronous change in the CT process with the carrier density was discovered. This observation suggests that the CT process can be precisely regulated by changes in the composition of the metal-semiconductor nanostructures. Our study provides a reference for the application of Ag/ITO films as alternative near-infrared plasmonic materials.

Process monitoring of photocatalytic degradation of 2,4-dinitrotoluene by Au-decorated Fe3O4@TiO2 nanoparticles: surface-enhanced Raman scattering method.

Recently, the degradation and detection of 2,4-dinitrotoluene (2,4-DNT) capable of producing 2,4,6-trinitrotoluene (TNT) for environmental and human health risks have been developed. We prepared photoresponsive Au-decorated Fe3O4@TiO2 nanoparticles (Fe3O4@TiO2-Au NPs) under sunlight simulated Xe lamp irradiation. The photodegradation process of 2,4-DNT by Fe3O4@TiO2-Au NPs was successfully monitored by surface-enhanced Raman scattering (SERS). Since SERS monitoring shows intrinsic information about the molecular structure, it was possible to predict the photodegradation of 2,4-DNT. The 2,4-DNT photodegradation mechanism based on two-dimensional correlation spectroscopy (2D-COS), which provides very beneficial information for a deeper understanding of systems, has been identified. We confirmed that Fe3O4@TiO2-Au NPs can be widely used in organic pollutant degradation under sunlight. Furthermore, the combination of SERS based process monitoring and 2D-COS can be a convincing analytical technique for photodegradation studies of organic pollutants.

Ag-decorated polymer chip for the determination of the respective concentrations of TTD and Hg2+ by surface-enhanced raman scattering.

A "hotspot"-rich Ag-nanoparticle-decorated three-dimensional polymer substrate was fabricated, exhibiting an excellent surface-enhanced Raman scattering (SERS) activity. 4-Mercaptobenzoic acid (MBA) was selected as a probe molecule for comparing the SERS activity on selected substrates. The proposed detection chip with the adsorption of tetramethylthiuram disulfide (TTD) shows an excellent sensitivity for the quantitative determination of TTD and mercury ions (Hg2+). This chip exhibited a high sensitivity for the trace detection of the targets. Interestingly, we found that the adsorbed TTD is selectively sensitive to Hg2+. The SERS band had a significant frequency shift of 11 cm-1 as the concentration of Hg2+ increased from 10-10 to 10-3 mol L-1. More importantly, the frequency shift of the SERS band exhibited an excellent linear relationship with the concentration of Hg2+, and the determination limit for Hg2+ was 10-10 mol L-1. Furthermore, the proposed detection chip shows great application potential for the determination of pesticides and Hg2+.

Trapping analytes into dynamic hot spots using Tyramine-medicated crosslinking chemistry for designing versatile sensor.

HYPOTHESIS: Due to the intrinsic nature of the surface-enhanced Raman scattering (SERS), the detection of molecules with weak binding affinities toward metal substrates is critical for development of a universal SERS sensing platform. We hypothesized the physical trapping of small pesticide molecules for active hot spot generation using tyramine-mediated crosslinking chemistry and silver nanoparticles (Ag NPs) enhances SERS detection sensitivity. EXPERIMENTS: Tyramine-mediated crosslinking chemistry for sensor application was validated by ultraviolet-visible absorption spectroscopy, scanning electron microscopy, dynamic light scattering, and Raman spectroscopy. SERS sensing platform using tyramine-mediated crosslinking reaction was systematically studied for detection of 1,4-dyethylnylbenzene as a model analyte. This sensor system was applied to detect two other pesticides, thiabendazole and 1,2,3,5-tetrachlorobenzene, which have different binding affinities toward metal surfaces. FINDINGS: The SERS signal of 1,4-dyethylnylbenzene obtained using this sensor system was 3.6 times stronger than that obtained using the Ag colloidal due to the nanogap of approximately 1.3 nm within the generated hot spots. This sensor system based on tyramine-mediated crosslinked Ag NPs was evaluated as a promising tool to achieve a solution based sensitive detection of various pesticide molecules that cannot be adsorbed on the surfaces of typical SERS substrates such as metal nanoparticles.

Tumor immune microenvironment lncRNAs.

Long non-coding ribonucleic acids (RNAs) (lncRNAs) are key players in tumorigenesis and immune responses. The nature of their cell type-specific gene expression and other functional evidence support the idea that lncRNAs have distinct cellular functions in the tumor immune microenvironment (TIME). To date, the majority of lncRNA studies have heavily relied on bulk RNA-sequencing data in which various cell types contribute to an averaged signal, limiting the discovery of cell type-specific lncRNA functions. Single-cell RNA-sequencing (scRNA-seq) is a potential solution for tackling this limitation despite the lack of annotations for low abundance yet cell type-specific lncRNAs. Hence, updated annotations and further understanding of the cellular expression of lncRNAs will be necessary for characterizing cell type-specific functions of lncRNA genes in the TIME. In this review, we discuss lncRNAs that are specifically expressed in tumor and immune cells, summarize the regulatory functions of the lncRNAs at the cell type level and highlight how a scRNA-seq approach can help to study the cell type-specific functions of TIME lncRNAs.

Charge-Transfer Induced by the Oxygen Vacancy Defects in the Ag/MoO3 Composite System.

In this paper, an Ag/MoO3 composite system was cosputtered by Ar plasma bombardment on a polystyrene (PS) colloidal microsphere array. The MoO3 formed by this method contained abundant oxygen vacancy defects, which provided a channel for charge transfer in the system and compensated for the wide band gap of MoO3. Various characterization methods strongly demonstrated the existence of oxygen vacancy defects and detected the properties of oxygen vacancies. 4-Aminothiophenol (p-aminothiophenol, PATP) was used as a candidate surface-enhanced Raman scattering (SERS) probe molecule to evaluate the contribution of the oxygen vacancy defects in the Ag/MoO3 composite system. Interestingly, oxygen vacancy defects are a kind of charge channel, and their powerful effect is fully reflected in their SERS spectra. Increasing the number of charge channels and increasing the utilization rate of the channels caused the frequency of SERS characteristic peaks to shift. This interesting phenomenon opens up a new horizon for the study of SERS in oxygen-containing semiconductors and provides a powerful reference for the study of PATP.

Characterization of the phase transition mechanism of P(NiPAAm-co-AAc) copolymer hydrogel using 2D correlation IR spectroscopy.

A thermo-responsive polymer, poly(N-isopropylacrylamide) (PNiPAAm), was copolymerized with acrylic acid (AAc) in this study. Its phase transitions during the heating and cooling processes were investigated using IR spectroscopy, principal component analysis (PCA), and two-dimensional correlation spectroscopy (2D-COS). During the heating process, the hydrogen bonding between side chain in P(NiPAAm-co-AAc) copolymer hydrogel and H2O was broken first, and then the formation of the intramolecular interaction in P(NiPAAm-co-AAc) copolymer hydrogel occurred. However, unlike the heating process, intensities of bands in the CH stretching region were changed before those in the CO stretching including the NH bending region during the cooling process. The results indicate that the phase transition of P(NiPAAm-co-AAc) copolymer hydrogel is an irreversible process at the molecular levels.

Photo-Induced Charge Transfer Enhancement for SERS in a SiO2-Ag-Reduced Graphene Oxide System.

Understanding and controlling the disorder in materials, especially the disorder caused by structural composition and doping effects, are important keys to studying the optical characteristics of materials. In this study, a SiO2-Ag-reduced graphene oxide (rGO) composite structure was prepared by a simple wet chemical method, in which Ag nanoparticles (NPs) and SiO2 were decorated onto the surface of rGO. The introduction of Si atoms can control not only the plasmon effect of Ag NPs but also, more importantly, the defect concentration of rGO. The formation of defects causes the rGO structure to enter a metastable state, which facilitates charge separation and transfer in the system. It is worth noting that changes in defect concentration can affect the energy band position of rGO; therefore, controlling the defect concentration can be used to achieve charge transfer resonance coupling. This study not only revealed the ultrahigh surface-enhanced Raman scattering activity of the substrate structure but also elucidated in detail the effect of the crystallinity of this rGO-based composite system on its optical properties.

Surface plasmon resonance induced charge transfer effect on the Ag-ZnSe-PATP system.

This work demonstrated the effect of charge transfer (CT) induced by metal surface plasmon resonance (SPR) on surface-enhanced Raman scattering (SERS). We designed an Ag-ZnSe nanostructure and introduced p-aminothiophenol (PATP) molecules to form an Ag-ZnSe-PATP system. The proposed method compensates for the CT difficulty in wide-band-gap semiconductors, which was initiated by the SPR of Ag. The Raman intensity is enhanced differently depending on the action of excitation light of different wavelengths. The concept of the CT degree was introduced to analyze this intriguing phenomenon. The system constructed in this work combines the electromagnetic enhancement mechanism and the chemical enhancement mechanism, which helps further understand the SERS mechanism and provides important references for SERS research on wide-band-gap semiconductors.