3D Ultrasensitive Polymers-Plasmonic Hybrid Flexible Platform for In-Situ Detection

Flexible PDMS-Based SERS Substrates Replicated from Beetle Wings for Water Pollutant Detection

The flexible surface-enhanced Raman scattering (SERS) sensor, which has the bionic 3D nanoarray structure of a beetle-wing substrate (BWS), was successfully prepared by replicated technology and thermal evaporation. The bionic structure was replicated with polydimethylsiloxane (PDMS) and then silver (Ag) nanoisland thin films were deposited by thermal evaporation. The deposition times and thicknesses (25-40 nm) of the Ag thin films were manipulated to find the optimal SERS detection capability. The Ag nanoisland arrays on the surface of the bionic replicated PDMS were observed by scanning electron microscope (SEM), X-ray diffraction (XRD), and contact angle, which can generate strong and reproducible three-dimensional hotspots (3D hotspots) to enhance Raman signals. The water pollutant, rhodamine 6G (R6G), was used as a model molecule for SERS detection. The results show that 35 nm Ag deposited on a PDMS-BWS SERS substrate displays the strongest SERS intensity, which is 10 times higher than that of the pristine BWS with 35 nm Ag coating, due to the excellent 3D bionic structure. Our results demonstrate that bionic 3D SERS sensors have the potential to be applied in wearable devices and sensors to detect biomolecules and environmental pollutants, such as industrial wastewater, in the future.

π-Conjugated Small Organic Molecule-Modified 2D MoS2 with a Charge-Localization Effect Enabling Direct and Sensitive SERS Detection

Organic semiconductors have been discovered to exhibit impressive surface-enhanced Raman scattering (SERS) activity recently. However, owing to the underdeveloped candidate materials and relatively low SERS sensitivity, practical application of SERS detection based on organic materials is still a challenge. Herein, we explored ways to further enhance the SERS sensitivity of π-conjugated fluorinated 7,7,8,8-tetracyanoquinodimethane derivatives (F_nTCNQ, n = 2, 4) by utilizing the charge-localization effect induced by two-dimensional (2D) MoS₂ flakes. A strong Raman signal enhancement in SERS has been realized via an organic/2D heterostructure constructed by F_nTCNQ nanostructures grown on a 2D MoS₂ flake. Moreover, F₂TCNQ and F₄TCNQ show different SESR sensitivities due to different numbers of cyano groups leading to different charge transfer (CT) directions. The SERS enhancement factor (EF) of methylene blue (MB) molecules on the optimal F₄TCNQ/MoS₂ nanocomposite substrate can reach as high as 2.531 × 10⁶, and the concentration of the limit of detection (LOD) is as low as 10^-10 M. The SERS results for MB, rhodamine 6G (R6G), and 4-aminothiophenol (4-ATP) molecules demonstrate that high versatility, low cost, good stability, and easy preparation will make the F_nTCNQ/MoS₂ SERS platform promising for the detection of trace molecules. The studies on the complex microscopic interaction of organic/2D composite nanomaterials will provide some novel insights into improved SERS performance and mechanisms.

Detection of Silver Nanoparticles in Seawater Using Surface-Enhanced Raman Scattering

Nanomaterials significantly contribute to the development of new solutions to improve consumer products properties. Silver nanoparticles (AgNPs) are one of the most used, and as human exposure to such NPs increases, there is a growing need for analytical methods to identify and quantify nanoparticles present in the environment. Here we designed a detection strategy for AgNPs in seawater using surface-enhanced Raman Scattering (SERS). Three commercial AgNPs coated with polyvinylpyrrolidone (PVP) were used to determine the relative impact of size (PVP-15nmAgNPs and PVP-100nmAgNPs) and aggregation degree (predefined Ag aggregates, PVP-50-80nmAgNPs) on the SERS-based detection method. The study of colloidal stability and dissolution of selected AgNPs into seawater was carried out by dynamic light scattering and UV-vis spectroscopy. We showed that PVP-15nmAgNPs and PVP-100nmAgNPs remained colloidally stable, while PVP-50-80nmAgNPs formed bigger aggregates. We demonstrated that the SERS-based method developed here have the capacity to detect and quantify single and aggregates of AgNPs in seawater. The size had almost no effect on the detection limit (2.15 ± 1.22 mg/L for PVP-15nmAgNPs vs. 1.51 ± 0.71 mg/L for PVP-100nmAgNPs), while aggregation caused an increase of 2.9-fold (6.08 ± 1.21 mg/L). Our results demonstrate the importance of understanding NPs transformation in seawater since this can influence the detection method performance.

Non-Invasive Approaches for the Evaluation of the Functionalization of Melamine Foams with In-Situ Synthesized Silver Nanoparticles

The use of polymeric nanocomposites has arisen as a promising solution to take advantage of the properties of nanoparticles (NPs) in diverse applications (e.g., water treatment, catalysis), while overcoming the drawbacks of free-standing nanoparticles (e.g., aggregation or accidental release). In most of the cases, the amount and size of the NPs will affect the stability of the composite as well as their performance. Therefore, a detailed characterization of the NPs present on the nanocomposites, including their quantification, is of vital importance for the optimization of these systems. However, the determination of the NPs load is often carried out by destructive techniques such as TGA or ICP-OES, the development of non-invasive approaches to that aim being necessary. In this work, the amount of silver NPs synthesized directly on the surface of melamine (ME) foams is studied using two non-invasive approaches: colorimetry and X-ray radiography. The obtained results show that the amount of silver NPs can be successfully determined from the luminosity and global color changes of the surface of the foams, as well as from the X-ray attenuance.

Melamine Foams Decorated with In-Situ Synthesized Gold and Palladium Nanoparticles

A versatile and straightforward route to produce polymer foams with functional surface through their decoration with gold and palladium nanoparticles is proposed. Melamine foams, used as polymeric porous substrates, are first covered with a uniform coating of polydimethylsiloxane, thin enough to assure the preservation of their original porous structure. The polydimethylsiloxane layer allows the facile in-situ formation of metallic Au and Pd nanoparticles with sizes of tens of nanometers directly on the surface of the struts of the foam by the direct immersion of the foams into gold or palladium precursor solutions. The effect of the gold and palladium precursor concentration, as well as the reaction time with the foams, to the amount and sizes of the nanoparticles synthesized on the foams, was studied and the ideal conditions for an optimized functionalization were defined. Gold and palladium contents of about 1 wt.% were achieved, while the nanoparticles were proven to be stably adhered to the foam, avoiding potential risks related to their accidental release.