Spatially Controlled Fabrication of Surface-Enhanced Raman Scattering Hot Spots through Photoinduced Dewetting of Silver Thin Films

Mechanically Flexible, Large-Area Fabrication of Three-Dimensional Dendritic Au Films for Reproducible Surface-Enhanced Raman Scattering Detection of Nanoplastics

The escalating crisis of nanoplastic pollution in water and food products demands the development of novel methodologies for detection and recycling. Despite various techniques available, surface-enhanced Raman scattering (SERS) is emerging as a highly efficient technique for the trace detection of micro/nanoplastics. However, the development of highly reproducible and stable, flexible SERS substrates that can be used for sensitive detection in environmental medium remains a challenge. Here, we report a fabrication of large-area, three-dimensional (3D), and highly flexible SERS substrate based on porous dendritic Au films onto a flexible indium tin oxide (ITO) substrate via facile, thermal evaporation of Au over the vacuum-compatible deep eutectic solvent (DES)-coated glass substrate and subsequent direct transfer process. The as-fabricated 3D dendritic Au/ITO flexible substrates can be used for ultrasensitive SERS detection of crystal violet (CV) as probe analyte molecules with the limit of detection (LOD) as low as 6.4 × 10-15 M, with good signal reproducibility (RSD of 11.3%). In addition, the substrate showed excellent sensitivity in detecting nanoplastics such as poly(ethylene terephthalate) (200 nm) and polystyrene (100 nm) with LODs reaching up to 0.051 and 8.2 μg/mL, respectively. This work provides a facile approach for the preparation of highly flexible plasmonic substrates, showing great potential for the SERS detection of a variety of environmental pollutants.

A Scalable Synthesis of Ag Nanoporous Film As an Efficient SERS-Substrates for Sensitive Detection of Nanoplastics

Nanoplastics pollution has led to a severe environmental crisis because of a large accumulation of these smaller nanoplastic particles in the aquatic environment and atmospheric conditions. Detection of these nanoplastics is crucial for food safety monitoring and human health. In this work, we report a simple and eco-friendly method to prepare a SERS-substrate-based nanoporous Ag nanoparticle (NP) film through vacuum thermal evaporation onto a vacuum-compatible deep eutectic solvent (DES) coated growth substrate for quantitative detection of nanoplastics in environmental samples. The nanoporous Ag NP films with controlled pores were achieved by the soft-templating role of DESs over the growth substrate, which enabled the self-assembly of deposited Ag NPs over the surface of DES. The optimized nanoporous Ag substrate provides high sensitivity in the detection of analyte molecules, crystal violet (CV), and rhodamine 6G (R6G) with a limit of detection (LOD) up to 1.5 × 10-13 M, excellent signal reproducibility, and storage stability. Moreover, we analyzed quantitative SERS detection of polyethene terephthalate (PET, size of 200 nm) and polystyrene (PS, size of 100 nm) nanoplastics with an LOD of 0.38 and 0.98 μg/mL, respectively. In addition, the SERS substrate efficiently detects PET and PS nanoplastics in real environmental samples, such as tap water, lake water, and diluted milk. The enhanced SERS sensing ability of the proposed nanoporous Ag NP film substrate holds immense potential for the sensitive detection of various nanoplastic contaminants present in environmental water.

Modes of Nanodroplet Formation and Growth on an Ultrathin Water Film

Using nanobubbles as geometrical confinements, we create a thin water film (∼10 nm) in a graphene liquid cell and investigate the evolution of its instability at the nanoscale under transmission electron microscopy. The breakdown of the water films, resulting in the subsequent formation and growth of nanodroplets, is visualized and generalized into different modes. We identified distinct droplet formation and growth modes by analyzing the dynamic processes involving 61 droplets and 110 liquid bridges within 31 Graphene Liquid Cells (GLCs). Droplet formation is influenced by their positions in GLCs, taking on a semicircular shape at the edge and a circular shape in the middle. Growth modes include liquid mass transfer driven by Plateau-Rayleigh instability and merging processes in and out-of-plane of the graphene interface. Droplet growth can lead to the formation of liquid bridges for which we obtain multiview projections. Data analysis reveals the general dynamics of liquid bridges, including drawing liquids from neighboring residual water films, merging with surrounding droplets, and merging with other liquid bridges. Our experimental observations provide insights into fluid transport at the nanoscale.

Ionic Liquid-Assisted Thermal Evaporation of Bimetallic Ag-Au Nanoparticle Films as a Highly Reproducible SERS Substrate for Sensitive Nanoplastic Detection in Complex Environments

Nanoplastic particles are emerging as an important class of environmental pollutants in the atmosphere that have adverse effects on our ecosystems and human health. While many methods have been developed to quantitatively detect nanoplastics; however, sensitive detection at low concentrations in a complex environment remains elusive. Herein, we demonstrate a greener method to fabricate a surface-enhanced Raman spectroscopy (SERS) substrate consisting of self-assembled plasmonic Ag-Au bimetallic nanoparticle (NP) films for quantitative SERS detection of nanoplastics in complex media. The self-assembly of Ag-Au bimetallic NPs was achieved through thermal evaporation onto a vapor-phase compatible ionic liquid based on deep eutectic solvent over the growth substrate. The finite-difference time-domain simulation revealed that the localized field enhancement is strong in the gaps, which generate uniform SERS "hotspots" in the obtained substrate. Benefiting from highly accessible SERS "hotspots" at the gaps, the SERS substrate exhibits excellent sensitivity for detecting crystal violet with a limit of detection (LOD) as low as 10-14 M and excellent reproducibility (RSD of 5.8%). The SERS substrate is capable of detecting PET nanoplastics with LOD as low as 1 μg/mL and about 100 μg/mL in real samples such as tap water, lake water, diluted milk, and wine. Moreover, we also validated the feasibility of the designed SERS substrate for the practical detection of PET nanoplastics collected from commercial drinking water bottles, and it showed great potential applications for sensitive detection in actual environments.

Sulfur Vacancy-Rich CuS for Improved Surface-Enhanced Raman Spectroscopy and Full-Spectrum Photocatalysis

There are growing interests in the development of bifunctional semiconducting nanostructures for photocatalysis and real-time monitoring of degradation process on catalysts. Defect engineering is a low-cost approach to manipulating the properties of semiconductors. Herein, we prepared CuS nanoplates by a hydrothermal method at increasing amounts of thioacetamide (CS-1, CS-2, and CS-3) and investigated the influence of sulfur vacancy (V_s) on surface-enhanced Raman spectroscopy (SERS) and photocatalysis performance. SERS intensity of 4-nitrobenzenethiol on CS-3 is 346 and 17 times that of CS-1 and CS-2, respectively, and enhancement factor is 1.34 × 10⁴. Moreover, SERS is successfully applied to monitor the photodegradation of methyl orange. In addition, CS-3 also exhibited higher efficiency of Cr(VI) photoreduction than CS-1 and CS-2, and removal rate is 88%, 96%, and 73% under 2 h UV, 4 h visible, and 4 h near-infrared illumination, respectively. A systematic study including electron paramagnetic resonance spectra, photoelectrochemical measurements, and nitrogen adsorption isotherms were conducted to investigate the underlying mechanism. This work may help to understand the impact of vacancy defect on SERS and photocatalysis, and provide an effective and low-cost approach for the design of multifunctional materials.

Confined growth of Ag nanoflakes induced by LSPR-driven carrier transfer in periodic nanopatterned arrays

The growth of metal nanostructures induced by surface plasmons has attracted widespread attention and provides a wide range of applications in the development of plasmonic nanochemistry, biosensors, photoelectrochemical coupling reactions, etc. Herein, a simple method is reported for the fabrication of Ag nanoflakes induced by the surface plasma on two-dimensional periodic nanopatterned arrays with the aid of 4-MBA molecules. The light radiation, molecules, and environmental gases are selected to track the formation mechanism of Ag nanoflakes. The in situ Raman observations and theoretical analyses confirm that small aromatic molecules with carboxyl groups play important roles in Ag nanoflake formation derived by localized surface plasmon resonance (LSPR)-driven carriers, which provide profound insights into the study of LSPR-driven carriers, participating in chemical reactions and the reconstruction of dense hot spots in nanogaps.

A microfluidic chip using Au@SiO2 array-based highly SERS-active substrates for ultrasensitive detection of dual cervical cancer-related biomarkers

In this work, a microfluidic chip using Au@SiO₂ array-based highly active SERS substrates was developed for quantitative detection of squamous cell carcinoma antigen (SCCA) and carcinoembryonic antigen (CEA) associated with cervical cancer. The chip consisted of six functional units with pump-free design, enabling parallel detection of multiple samples in an automatic manner without external pumps and improving the portability. Ag nanocubes (AgNCs) were labeled with Raman reporters and coupled with antibodies (labeling) to prepare SERS tags, while the Au nanoparticle-modified SiO₂ microsphere (Au@SiO₂) array was conjugated with antibodies (coating) to generate the highly SERS-active capturing substrate. In the presence of target biomarkers, they were captured by SERS tags and capturing substrate, resulting in the formation of "sandwich" structures which were trapped in the detection chamber. As the immune reaction proceeded, a large number of "hot spots" were generated by the proximity of the Au@SiO₂ array substrate and AgNCs, greatly amplifying SERS signals. With this chip, the limits of detection of the SCCA and CEA levels in human serum were estimated to be as low as 0.45 pg mL^-1 and 0.36 pg mL^-1, respectively. Furthermore, the good selectivity and reproducibility of this chip were confirmed. Finally, clinical serum samples were analyzed by this chip, and the outcomes were consistent with those of enzyme-linked immunosorbent assay (ELISA). Thus, the proposed microfluidic chip can be potentially applied for the clinical diagnosis of cervical cancer.