Facile Fabrication of Large-Scale Porous and Flexible Three-Dimensional Plasmonic Networks

Detection of thiram on fruit surfaces and in juices with minimum sample pretreatment via a bendable and reusable substrate for surface-enhanced Raman scattering

BACKGROUND

Surface-enhanced Raman scattering (SERS) substrates based on metallic nanoparticles locked in some flexible materials have great potential for rapid detection of pesticide residues in foods, but these substrates are generally not reusable.

RESULTS

A bendable and reusable sponge based on polydimethylsiloxane (PDMS) and Au nanospheres was synthesized and employed as SERS substrate to analyze thiram on the surfaces of apples and grapes (20-1000 ng cm^-2 ) and in their juices (0.5-5.0 mg L^-1 ) with minimum sample pretreatments. The lowest detectible concentrations for thiram in fruit juices and on fruit skins were 0.5 mg L^-1 and 20 ng cm^-2 , respectively. The Au-PDMS substrate had acceptable intra-reproducibility for SERS analysis of thiram in fruit juices and on fruit skins, resulting in 3.6-16.9% relative standard deviation (RSD) for the SERS signal of the primary peak of thiram. Moreover, the Au-PDMS substrate exhibited distinguished reusability and stability, which could provide a reproducible SERS signal of thiram in apple juice even after the substrate being reused ten times (RSDs for the three major characteristic peaks of thiram were 2.7-10.5% during the ten reused cycles).

CONCLUSION

This flexible and reusable Au-PDMS SERS substrate for thiram detection could be readily extended to the analysis of other trace chemicals in a broad range of foods, providing a new possibility for SERS application. © 2022 Society of Chemical Industry.

Noninvasive Diagnosis of Gastric Cancer Based on Breath Analysis with a Tubular Surface-Enhanced Raman Scattering Sensor

SERS-based breath analysis as an emerging technique has attracted increasing attention in cancer screening. Here, eight aldehydes and ketones in the human breath are reported as the VOC biomarkers identified by gas chromatography-mass spectrometry (GC-MS) and applied further for the noninvasive diagnosis of gastric cancer (GC) with a tubular SERS sensor. The tubular SERS sensor is prepared with a glass capillary loaded with ZIF-67-coated silver particles (Ag@ZIF-67), which offers Raman enhancement from the plasmonic nanoparticles and gas enrichment from the metal-organic framework (MOF) shells. The composite materials are modified with 4-aminothiophenol (4-ATP) to capture different aldehyde and ketone compounds. The tubular sensor is served simultaneously as a gas flow channel and a detection chamber, bringing a higher gas capture efficiency than the planar SERS sensor. As a proof-of-concept, the tubular SERS sensor is successfully employed to screen gastric cancer patients with an accuracy of 89.83%, based on the noninvasive, rapid, and easily operated breath analysis. The results demonstrate that the established breath analysis method provides an excellent alternative for the screening of GC and other diseases.

Three-dimensional SERS sensor based on the sandwiched G@AgNPs@G/PDMS film

Three-dimensional (3D) SERS substrate with the denser "hotspots" is synthesized by the constriction of PDMS film decorated with sandwiched graphene@AgNPs@graphene (G@AgNPs@G) nanostructure. Graphene layers above and below the AgNPs are used to absorb molecules onto the "hotspots", and prevent the oxidation of AgNPs in our design. PDMS films can be easily shrunk for 3D structures, causing advantages in enhancement ability and light-matter interaction. Benefiting from the above advantages, a detection limit of 10^-14 M (CV) and enhancement factor (EF) of 3.9 × 10⁹ were obtained in our experiment. Theoretical analyses (FDTD) were also used to study the enhancement mechanism. For practical purposes, in-situ detection of MG molecules on the fish surface and the label-free detection of DNA base of adenine (A) and cytosine (C) were also studied. The high enhancement factor, great sensitivity, reliability, and stability of substrate reasonably proved that it can be used as an excellent SERS substrate for biomolecular detection.

Photocatalytic Degradation of Rhodamine B Dye by TiO2 and Gold Nanoparticles Supported on a Floating Porous Polydimethylsiloxane Sponge under Ultraviolet and Visible Light Irradiation

A combination of plasmonic nanoparticles (NPs) with semiconductor photocatalysts, called plasmonic photocatalysts, can be a good candidate for highly efficient photocatalysts using broadband solar light because it can greatly enhance overall photocatalytic efficiency by extending the working wavelength range of light from ultraviolet (UV) to visible. In particular, fixation of plasmonic photocatalysts on a floating porous substrate can have additional advantages for their recycling after water treatment. Here, we report on a floating porous plasmonic photocatalyst based on a polydimethylsiloxane (PDMS)-TiO₂-gold (Au) composite sponge, in which TiO₂ and Au NPs are simultaneously immobilized on the surface of interconnected pores in the PDMS sponge. This can be easily fabricated by a simple sugar-template method with TiO₂ NPs and in situ reduction of Au NPs by the PDMS without extra chemicals. Its ability to decompose the organic pollutant rhodamine B in water was tested under UV and visible light, respectively. The results showed highly enhanced photocatalytic activity under both UV and visible light compared to the PDMS-TiO₂ sponge and the PDMS-Au sponge. Furthermore, its recyclability was also demonstrated for multiple cycles. The simplicity of fabrication and high photocatalytic performance of our PDMS-TiO₂-Au sponge can be promising in environmental applications to treat water pollution.

Wrapping AgCl Nanostructures with Trimetallic Nanomeshes for Plasmon-Enhanced Catalysis and in Situ SERS Monitoring of Chemical Reactions

Selective chemical control of multiple reactions is incredibly important for the fabrication of sophisticated nanostructures for functional applications. A representative example is the synthesis of plasmonic nanomaterial-silver chloride (AgCl) conjugates, where metal ions should be selectively reduced into metallic nanostructures for plasmon-enhanced catalytic activity, while the reducible AgCl nanomaterials remain intact despite the presence of a chemical reductant. In addition to the selectively controlled reduction, the plasmonic nanostructures should be appropriately designed for the high stability and photoefficiency of catalysts. In this study, we demonstrate how AgCl nanocubes and nanospheres could be comprehensively wrapped by plasmonic three-dimensional nanomesh structures consisting of gold, silver, and palladium by the selective reduction of their ionic precursors while the AgCl nanostructures remain intact. Complete trimetallic wrapping provided the absorption of visible light, while the porosity of the nanomesh structures exposed the photocatalytic AgCl surface to catalyze desired reactions. Platinum in place of palladium was examined to demonstrate the versatility of the wrapping scheme, resulting in an extraordinary catalytic activity. Importantly, the detailed chemical mechanism behind the trimetallic wrapping of the AgCl nanostructures was systematically investigated to understand the roles of each reaction component in controlling the chemical selectivity. The synthesized AgCl-trimetal nanoconjugates excellently exhibit both metal-based and plasmon-enhanced catalytic properties for the removal of environmentally harmful Cr⁶⁺. Moreover, their applications as surface-enhanced Raman-scattering (SERS) probes for the in situ monitoring of catalytic reduction in real-time and as single-nanoparticle SERS probes for molecular detection are thoroughly demonstrated.