In-situ deposition of silver nanoparticles on silver nanoflowers for ultrasensitive and simultaneous SERS detection of organic pollutants

Ag nanoisland functionalized hollow carbon nanofibers as a non-invasive, label-free SERS salivary biosensor platform for salivary nitrite detection for pre-diagnosis of oral cancer

A highly selective, label-free, surface-enhanced Raman spectroscopy (SERS) based sensor platform employing hollow carbon nanofibers functionalized with silver nanoparticles (Ag@HCNFs) has been developed to monitor anomalous concentrations of potential biomarkers, such as salivary nitrite facilitating pre-diagnosis of oral cancer. Co-axial electrospinning was used for the fabrication of the nanofibrous Ag@HCNFs followed by thermal treatment of PAN/PVP core-shell nanofibers and chemical reduction of silver nanoislands. The developed plasmonic Ag@HCNFs was structurally and morphologically characterized using X-Ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy, which clearly demonstrated the successful anchoring of silver nanoparticles on hollow carbon nanofibers. The properties of Ag@HCNFs showed significant SERS enhancement of the order of 107 with a detection limit of 10-11 M with R6G, demonstrating its efficacy to investigate real-time salivary samples, particularly towards the detection of salivary nitrite within the clinically relevant range (50 μM-300 μM) towards the pre-diagnosis of oral cancer. The proposed SERS-based salivary platform has the potential to be used as a low-cost, non-invasive pre-diagnostic tool for early diagnosis and mass screening of oral cancer.

Silver nanopopcorns decorated on flexible membrane for SERS detection of nitrofurazone

The synthesis of three-dimensional silver nanopopcorns (Ag NPCs) onto a flexible polycarbonate membrane (PCM) for the detection of nitrofurazone (NFZ) on the fish surface by surface-enhanced Raman spectroscopy (SERS) is presented. The proposed flexible Ag-NPCs/PCM SERS substrate exhibits significant Raman signal intensity enhancement with the measured enhancement factor of 2.36 × 106. This is primarily attributed to the hotspots created on Ag NPCs, including numerous nanoscale protrusions and internal crevices distributed across the surface of Ag NPCs. The detection of NFZ by this flexible SERS substrate demonstrates a low limit of detection (LOD) of 3.7 × 10-9 M and uniform and reproducible Raman signal intensities with a relative standard deviation below 8.34%. It also exhibits excellent stability, retaining 70% of its efficacy even after 10 days of storage. Notably, the practical detection of NFZ in tap water, honey water, and fish surfaces achieves LOD values of 1.35 × 10-8 M, 5.76 × 10-7 M, and 3.61 × 10-8 M, respectively,  which highlights its effectiveness across different sample types. The developed Ag-NPCs/PCM SERS substrate presents promising potential for sensitive SERS detection of toxic substances in real-world samples.

Surface-enhanced Raman scattering spectroscopy monitoring and degradation of organic pollutants using a novel nanowire

A multifunctional Ag/AlOOH nanowires (ANW) composite substrate was constructed, which not only accomplishes highly sensitive detection of organic dye molecules, but also has excellent performance in the degradation of pollutants. The ANW in the Ag/ANW substrate possesses a high aspect ratio, which extends the distribution area of Ag and enables a large number of hot spots on the active substrate. Additionally, due to the abundant OH groups on the ANW, there is an increased number of anchor sites for adsorbed metal ions in the Ag/ANW compound, thus contributing to the enhancement and degradation of molecules. Moreover, the constructed multifunctional Ag/ANW nanocomplexes also show great promise for practical applications, providing a reference for the detection and degradation of contaminants.

Silver Nanoparticles: Multifunctional Tool in Environmental Water Remediation

Water pollution is a worldwide environmental and health problem that requires the development of sustainable, efficient, and accessible technologies. Nanotechnology is a very attractive alternative in environmental remediation processes due to the multiple properties that are conferred on a material when it is at the nanometric scale. This present review focuses on the understanding of the structure-physicochemical properties-performance relationships of silver nanoparticles, with the objective of guiding the selection of physicochemical properties that promote greater performance and are key factors in their use as antibacterial agents, surface modifiers, colorimetric sensors, signal amplifiers, and plasmonic photocatalysts. Silver nanoparticles with a size of less than 10 nm, morphology with a high percentage of reactive facets {111}, and positive surface charge improve the interaction of the nanoparticles with bacterial cells and induce a greater antibacterial effect. Adsorbent materials functionalized with an optimal concentration of silver nanoparticles increase their contact area and enhance adsorbent capacity. The use of stabilizing agents in silver nanoparticles promotes selective adsorption of contaminants by modifying the surface charge and type of active sites in an adsorbent material, in addition to inducing selective complexation and providing stability in their use as colorimetric sensors. Silver nanoparticles with complex morphologies allow the formation of hot spots or chemical or electromagnetic bonds between substrate and analyte, promoting a greater amplification factor. Controlled doping with nanoparticles in photocatalytic materials produces improvements in their electronic structural properties, promotes changes in charge transfer and bandgap, and improves and expands their photocatalytic properties. Silver nanoparticles have potential use as a tool in water remediation, where by selecting appropriate physicochemical properties for each application, their performance and efficiency are improved.

Template-Free Synthesized Gold Nanobowls Composed with Graphene Oxide for Ultrasensitive SERS Platforms

Engineering of plasmonic properties of gold nanostructures expands the field of their applications from photocatalysis and photothermal effects to ultrasensitive surface-enhanced Raman spectroscopy (SERS). The known methods of preparation of gold nanobowls involve the deposition of gold layer on polymers or silicon nanotemplates and the removal of the top layer of gold together with the template. Such gold nanobowls are characterized by very broad plasmonic bands due to the plasmon hybridization. The sharp edges on the top of nanobowls are potential sources of the strong electromagnetic field beneficial for SERS. We present a novel template-free synthesis of gold nanobowls (AuNBs). The AuNB layers are deposited on graphene oxide (GO) layers. We compare AuNBs with gold nanospheres (AuNSs) and gold nanourchins (AuNUs) having similar size. The gold nanoparticles are combined with pristine GO or graphene oxide conditioned in ammonia (GONH3) or graphene oxide conditioned in sodium hydroxide (GONaOH). The SERS properties of the hybrid supports were studied using rhodamine 6G (R6G) as the SERS probe. The 633 nm laser line was used, which falls out of the molecular resonance with R6G. The results indicate that AuNBs show largely higher enhancement factors when compared to AuNUs and AuNSs. Furthermore, the GO materials are able to modify the SERS enhancement by 1 order of magnitude. We explain the influence of the GO material by three factors: (1) enabling or disabling the charge transfer between gold and R6G, which is crucial for the chemical part of SERS enhancement; (2) causing the aggregation of gold nanoparticles and formation of hot spots; (3) dipole contribution to the electromagnetic enhancement through the abundance of polar groups on the surface.

Dendrimer-based magneto-plasmonic nanosorbents for water quality monitoring using surface-enhanced Raman spectroscopy

In this work, we report the synthesis of magneto-plasmonic dendrimer-based nanosorbents containing Au nanostars and we demonstrate that they can be used as versatile optical sensors for the detection of pesticides in spiked water samples. The magnetic hybrid nanoparticles were obtained by conjugating silica-functionalized G5-NH₂ PAMAM dendrimers to silica-coated magnetite cores. The resulting magnetic-PAMAM conjugates were then used to reduce and sequester Au seeds for the subsequent in situ growth of Au nanostars. The dendrimer-based magneto-plasmonic substrates containing the Au anisotropic nanophases were then investigated regarding their ability to monitor water quality through surface-enhanced Raman scattering (SERS) spectroscopy. As a proof-of-concept, the ensuing multifunctional materials were investigated as SERS probing systems to detect dithiocarbamate pesticides (ziram and thiram) dissolved in water samples. It was observed that the magneto-plasmonic hybrid materials enhance the Raman signal of these pesticides under variable operational conditions, suggesting the versatility of these systems for water quality monitoring. Moreover, a detailed analysis of the SERS data was accomplished to predict the adsorption profile of the dithiocarbamate pesticides to the Au surface.

Sensors for Detection of the Synthetic Dye Rhodamine in Environmental Monitoring Based on SERS

This article presents a review of many types of SERS sensors for food safety and environmental pollution monitoring based on detecting rhodamine. It introduces the basic concepts of substrates, enhancement factors, and mechanisms, devices' sensors integrated with the microstructure. Here, we review the state-of-the-art research in the field of rhodamine monitoring and highlight the applications of SERS sensors. The trends in the development of substrates for different applications have been mentioned with the aim of providing an overview of the development of different SERS substrates. Thus, an efficient approach for rhodamine detection has a good perspective for application in environmental monitoring.

Three-Dimensional Dendritic Au-Ag Substrate for On-Site SERS Detection of Trace Molecules in Liquid Phase

The development of a facile surface-enhanced Raman scattering (SERS) sensor for the on-site detection of trace molecules in liquid phase is a compelling need. In this paper, a three-dimensional (3D) dendritic Au–Ag nanostructure was constructed by a two-step electro displacement reaction in a capillary tube for the on-site liquid phase detection of trace molecules. The multiplasmon resonance mechanism of the dendritic Au–Ag structure was simulated using the finite-difference time domain (FDTD) method. It was confirmed that the highly branched 3D structure promoted the formation of high-density “hot spots” and interacted with the gold nanoparticles at the dendrite tip, gap, and surface to maximize the spatial electric field, which allowed for high signal intensification to be observed. More importantly, the unique structure of the capillary made it possible to achieve the on-site detection of trace molecules in liquids. Using Rhodamine 6G (R6G) solution as a model molecule, the 3D dendritic Au–Ag substrate exhibited a high detection sensitivity (10⁻¹³ mol/L). Furthermore, the developed sensor was applied to the detection of antibacterial agents, ciprofloxacin (CIP), with clear Raman characteristic peaks observed even at concentrations as low as 10⁻⁹ mol/L. The results demonstrated that the 3D dendritic Au–Ag sensor could successfully realize the rapid on-site SERS detection of trace molecules in liquids, providing a promising platform for ultrasensitive and on-site liquid sample analysis.

In situ detection of fluid media based on a three-dimensional dendritic silver surface-enhanced Raman scattering substrate

A simple substitution reaction was used to grow 3D dendritic silver structures in microfluidic channels, and a highly active SERS detection platform was formed. The system can realize in situ detection of 10−10 mol L−1 R6G solution.

A photo-responsive p-Si/TiO2/Ag heterostructure with charge transfer for recyclable surface-enhanced Raman scattering substrates

A Si/TiO2/Ag heterostructure is prepared as a recyclable SERS substrate with EF of 1.23 × 1012 and excellent repeatability, which can boost performance effectively by the synergistic contribution of the EM and CT enhancement effects.

A photochemical approach to anchor Au NPs on MXene as a prominent SERS substrate for ultrasensitive detection of chlorpromazine

As a novel two-dimensional (2D) material, metal carbide (MXene) has been identified as a hotspot research topic in the field of surface-enhanced Raman spectroscopy (SERS). Herein, we report the increment of SERS activity of titanium carbide (TiC) by incorporation of gold nanoparticles (Au NPs) by a facile photoreduction process for the detection of antipsychotic drug. TiC anchored with Au NPs produce a remarkable SERS enhancement by the synergistic action of chemical and electromagnetic mechanisms. The hotspots are formed in the nanometer-scale gaps between Au NPs on the TiC surface for the effective interaction with probe molecules. The proposed TiC/Au-NPs SERS substrate was employed for the detection of chlorpromazine (CPZ) with the wide linear range of 10^-1-10^-10 M and the ultra-low limit of detection of 3.92 × 10^-11 M. Besides, the SERS effect of the optimized TiC/Au-NPs for the 532 nm excitation exhibits the enhancement factor in the order of 10⁹ with the relative standard deviation of < 13% for the uniformity and < 8.80% for the reproducibility. To ensure the practical feasibility of the proposed TiC/Au-NPs SERS substrate, the spike and recovery method was used for the detection of CPZ in human biological fluids like urine and saliva. This work can open up a new approach to improve the SERS activity of MXene-based SERS substrate for practical applications, especially the determination of antipsychotic drugs in environmental pollution management.

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.

Transparent, Flexible Plasmonic Ag NP/PMMA Substrates Using Chemically Patterned Ferroelectric Crystals for Detecting Pesticides on Curved Surfaces

Transparent and flexible surface-enhanced Raman scattering (SERS) substrates have attracted much interest for the detection of probe molecules on the curved surfaces of real samples, but a facile route to fabricate such substrates is still lacking. Herein, we present a rationally designed, high-performance flexible SERS substrate fabricated using a simple drop and peel-off technique for the ultrasensitive detection of pesticides. The proposed SERS substrate consists of a polymethyl methacrylate (PMMA) film anchored with plasmonic silver nanoparticles (Ag NPs), which are photoreduced using chemically patterned ferroelectric templates. The photoreduced Ag NPs extracted onto the PMMA film offer strong electromagnetic enhancement and produce intensive hotspots for the effective enhancement of the Raman signal. They provide superior SERS performance for the detection of parathion (PT) and fenitrothion (FNT) at trace-level concentrations of 10^-9 M and 10^-10 M with excellent enhancement factors in the order of 10⁸ and 10⁹, respectively. Moreover, the Ag NP/PMMA SERS substrate has good spot-to-spot uniformity and batch-to-batch reproducibility with the reservation of high detection sensitivity even after the mechanical deformation of bending and torsion up to 50 cycles. The multiplex detection ability is also investigated for the simultaneous detection of PT and FNT. To ensure the practical feasibility, the in-situ, real-time detection of PT and FNT on the curved surfaces of tomato and lemon using a fiber-coupled Raman probe is performed with limits of detection of 4.24 × 10^-8 M and 2.74 × 10^-9 M. The proposed Ag NP/PMMA flexible SERS substrate possesses unique features, such as easy fabrication through a simple, economical, rapid process, and facilitates straightforward implementation of in-situ SERS detection on curved fruit/vegetable surfaces.

Photochemical decoration of silver nanoparticles on silver vanadate nanorods as an efficient SERS probe for ultrasensitive detection of chloramphenicol residue in real samples

Aquaculture and farming industries have been seriously threatened by the illegal use of antibiotics as feed-additives to benefit the animal growth. Although various conventional chemical sensing approaches have been widely explored for the trace-level detection of antibiotics, the effective and accurate monitoring techniques are still highly demanded. Herein, we propose a novel surface-enhanced Raman scattering (SERS) substrate with the heterogeneous integration of silver nanoparticles (Ag NPs) on silver vanadate nanorods (β-AgVO₃ NRs) for the ultrasensitive detection of popular antibiotic, chloramphenicol (CAP). The photochemical decoration of Ag NPs on the surface of β-AgVO₃ NRs remarkably enhances the Raman signal intensity of CAP molecules by the synergistic action of the mechanisms of electromagnetic and chemical enhancement. The structural features of Ag-NPs@β-AgVO₃-NRs favor the formation of hotspots at the interface between NPs and NRs by enhanced surface area and numerous active sites for the interaction with CAP molecules. The SERS measurement of CAP molecules on the Ag-NPs@β-AgVO₃-NRs shows a trace-level limit of detection (10^-10 M), high uniformity (5.29%), good reproducibility (3.89%), and high analytical enhancement factor (2.05 × 10⁸). The proposed SERS substrate possesses excellent detection ability in monitoring real samples like tap water, milk and eye drops.