Quantum Effects Enter Semiconductor-Based SERS: Multiresonant MoO3·xH2O Quantum Dots Enabling Direct, Sensitive SERS Detection of Small Inorganic Molecules

Cellulose Surface Nanoengineering for Visualizing Food Safety

Food safety is vital to human health, necessitating the development of nondestructive, convenient, and highly sensitive methods for detecting harmful substances. This study integrates cellulose dissolution, aligned regeneration, in situ nanoparticle synthesis, and structural reconstitution to create flexible, transparent, customizable, and nanowrinkled cellulose/Ag nanoparticle membranes (NWCM-Ag). These three-dimensional nanowrinkled structures considerably improve the spatial-electromagnetic-coupling effect of metal nanoparticles on the membrane surface, providing a 2.3 × 108 enhancement factor for the surface-enhanced Raman scattering (SERS) effect for trace detection of pesticides in foods. Notably, the distribution of pesticides in the apple peel and pulp layers is visualized through Raman imaging, confirming that the pesticides penetrate the peel layer into the pulp layer (∼30 μm depth). Thus, the risk of pesticide ingestion from fruits cannot be avoided by simple washing other than peeling. This study provides a new idea for designing nanowrinkled structures and broadening cellulose utilization in food safety.

Noble metal-free SERS: mechanisms and applications

Surface-enhanced Raman scattering (SERS) is a very important tool in vibrational spectroscopy. The coupling of nanomaterials induces local surface plasmon resonance (LSPR), which contributes greatly to SERS. Due to its remarkable sensitivity in trace detection, SERS has gained prominence in the fields of catalysis, biosensors, drug tracking, and optoelectronic devices. SERS activity is believed to be closely related to the LSPR and charge transfer (CT) of the material. Noble metal nanostructures have been commonly used as SERS-active substrates due to their strong local electric fields and relatively mature preparation, application, and enhancement mechanisms. In recent years, SERS research based on semiconductor materials has attracted significant attention because semiconductor materials have advantages such as repeatable preparation, simple pretreatment, stable SERS spectra and superior biocompatibility, stability, and reproducibility. Semiconductor-based SERS has the potential to enrich SERS theory and applications. Thus, the development of semiconductor materials will introduce a new epoch for SERS-based research. In this review, we outline the two main kinds of semiconductor SERS-active substrates: inorganic and organic semiconductor SERS-active substrates. We also provide an overview of the SERS mechanism for different kinds of materials and SERS-based applications.

Enhancing charge transfer in a W18O49/g-C3N4 heterostructure via band structure engineering for effective SERS detection and flexible substrate applications

Chemical mechanism (CM)-related surface-enhanced Raman spectroscopy (SERS) has received tremendous interest due to its exceptional stability and excellent uniformity. Nevertheless, there remains a demand for ingenious methodologies for promoting effective charge transfer (CT) to improve SERS sensitivity further. Herein, a band structure engineered W18O49/g-C3N4 heterostructure (WCN) was first employed as a CM-based SERS substrate with remarkable enhancement and sensitivity. To investigate the Raman enhancement properties of the substrate, malachite green (MG) was employed as the Raman probe with the excitation of a 633 nm laser. The WCN substrate exhibits a Raman enhancement factor (EF) of 2.6 × 107, achieving a limit of detection (LOD) of 1.9 × 10-10 M for MG. The outstanding Raman amplification behavior can be attributed to the heterojunction-induced efficient CT process, energy band matching resonance due to minor doping with g-C3N4 serving as a band gap modifier, and improved photo-induced charge transfer (PICT) efficiency via the oxygen vacancies in the W18O49 units. Additionally, a flexible SERS substrate based on WCN was constructed using a vacuum filtration method and utilized to detect prohibited pharmaceutical residues on fish skin. The integration of this WCN and a nylon membrane not only preserves the Raman activity of the WCN for sensitive detection but also endows the Raman substrate with high flexibility and good mechanical durability, making it a potential candidate for in situ detection in particular environments.

Ratiometric surface-enhanced Raman scattering strategy using gold nanoparticles confined on an ultrathin polydimethylsiloxane grafted gold mirror film substrate for ferbam screening in fruit juice

BACKGROUND

In surface-enhanced Raman scattering (SERS) detection methods, the intricacies in the synthesis and recognition processes, along with non-uniform substrate morphologies, induce spectral irreproducibility. Metal (gold) nanoparticles (AuNPs) on gold (Au) mirror film configuration along with a ratiometric approach, constitute a potential system to resolve this issue.

RESULTS

To acquire a reproducible and stable SERS response, an ultrathin polydimethylsiloxane (PDMS) spacer layer was grafted onto the Au mirror film via a contact heating step. The AuNPs-supported ultrathin PDMS grafted Au mirror film system was extended for ratiometric sensing of ferbam residue in real fruit juice samples. The hydrophobic PDMS localizes the AuNPs, 4-nitrophenol probe, and ferbam to a smaller region on the PDMS-grafted Au mirror film and prevents their spreading and diffusion. The ratiometric SERS response for ferbam target and probe ratio at I1376/I1326 cm-1 was monitored on the AuNPs@PDMS grafted Au mirror film substrate with good linear fitting. A detection limit of 1.09 × 10-8 mol L-1 and a relative standard deviation of 11.90% were obtained. In addition, ferbam residues in grape and orange juice samples were successfully recovered (96.86%-99.76%).

SIGNIFICANCE

The AuNPs@PDMS grafted Au mirror film substrate, coupled with ratiometric analysis, showed excellent SERS activity with high sensitivity and reproducibility. The proposed platform can be adequately extended to detect other pesticide types in complex food settings.

Nanosphere Lithography-Enabled Hybrid Ag-Cu Surface-Enhanced Raman Spectroscopy Substrates with Enhanced Absorption of Excitation Light

We demonstrated a low-cost, highly sensitive hybrid Ag-Cu substrate with enhanced absorption for the excitation laser beam via the nanosphere lithography technique. The hybrid Ag-Cu surface-enhanced Raman spectroscopy (SERS) substrate consists of a Cu nanoarray covered with Ag nanoparticles. The geometry of the deposited Cu nanoarray is precisely determined through a self-assembly nanosphere etching process, resulting in optimized absorption for the excitation laser beam. Further Raman enhancement is achieved by incorporating plasmonic hotspots formed by dense Ag nanoparticles, grown by immersing the prepared Cu nanoarray in a silver nitrate solution. The structural design enables analytical enhancement factor of hybrid Ag-Cu SERS substrates of 1.13 × 105. The Ag-Cu SERS substrates exhibit a highly sensitive and reproducible SERS activity, with a low detection limit of 10-13 M for Rhodamine 6G detection and 10-9 M for 4,4'-Bipyridine. Our strategy could pave an effective and promising approach for SERS-based rapid detection in biosensors, environmental monitoring and food safety.

Orthogonal Chemical Reporter Strategy Enables Sensitive and Specific SERS Detection of Hydrazine Derivatives

Hydrazine and its derivatives are well-known environmental hazards and biological carcinogens; therefore, there is a great need for a powerful workflow solution for protecting the public from unexpected exposure to toxic contaminants. Recently, functional surface-enhanced Raman scattering (SERS) exhibits enormous benefits in sensing trace biochemical substances due to its fingerprint-like identification of individual molecules, making it an ideal method for detecting and quantifying hydrazine. Herein, for the first time, we integrated the orthogonal chemical reporter strategy with SERS to build an intelligent hydrazine detection platform (orthogonal chemical SERS, ocSERS), in which 4-mercaptobenzaldehyde was incorporated on a nanoimprinted gold nanopillar array, which acted as an orthogonal coupling partner of hydrazine to form Raman active benzaldehyde hydrazone, allowing for sensitively detecting hydrazine with a detection limit of 10^-13 M in complex circumstances. Particularly, ocSERS could effectively identify the carcinogen N-nitrosodimethylamine (NDMA) after its reduction to dimethylhydrazine (UDMH), enabling ultrasensitive detection of UDMH (10^-13 M). Importantly, ocSERS could not only monitor elevated levels of NDMA in ranitidine due to improper storage but also quantify NDMA in urine and blood after oral administration of NDMA-containing drugs, thereby preventing NDMA overexposure. Therefore, ocSERS represents the first click SERS sensor and may open up a new analytical field.

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.