SERS based detection of multiple analytes from dye/explosive mixtures using picosecond laser fabricated gold nanoparticles and nanostructures

Selective SERS Detection of TATB Explosives Based on Hydroxy-Terminal Nanodiamond-Multilayer Graphene Substrate

Selective surface-enhanced Raman scattering (SERS) detection of target explosives with good reproducibility is very important for monitoring soldiers' health and ecological environment. Here, the specific charge transfer pathway was constructed between a stable nanodiamond-multilayer graphene (MGD) film substrate and the target explosives. Two-step wet chemical oxidation methods of H2O2 (30%) and HNO3 (65%) solutions were used to regulate the terminal structure of MGD films. The experimental results showed that the hydroxyl (-OH) functional groups are successfully modified on the surface of MGD thin films, and the MGD-OH substrates having good selectivity for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) explosive in mixed solutions of the TATB, 2,2-dinitroethene-1,1-diamine, 2,4,6-trinitrotoluene, and 1,3,5-trinitroperhydro-1,3,5-triazine explosives compared with MGD substrates were demonstrated. Finally, first-principles density functional theory simulations revealed that the SERS enhancement of the MGD-OH substrate is mainly attributed to the transferred electrons between the -NO2 groups of TATB and the -OH groups of the MGD-OH substrate.

Advances in surface-enhanced Raman spectroscopy-based sensors for detection of various biomarkers

Surface enhanced Raman spectroscopy (SERS) allows the ultrasensitive detection of analytes present in traces or even single molecule levels by the generation of electromagnetic fields. It is a powerful vibrational spectroscopic method that is capable to detect traces of chemical and biological analytes. SERS technique is involved in the extremely sophisticated studies of molecules with high specificity and sensitivity. In the vicinity of nanomaterials decorated surfaces, SERS can monitor extremely low concentrations of analytes in a non-destructive manner with narrow line widths. This review article is focused on some recently developed SERS-based sensors for distinct types of analytes like disease-related biomarkers, organic and inorganic molecules, various toxins, dyes, pesticides, bacteria as well as single molecules. This study aims to enlighten the arising sensing approaches based on the SERS technique. Apart from this, some basics of the SERS technique like their mechanism, detection strategy, and involvement of some specific nanomaterials are also highlighted herein. Finally, the study concluded with some discussion of applications of SERS in various fields like food and environmental analysis.

Regulated synthesis of an Au NB-DT@Ag bimetallic core-molecule-shell nanostructure for reliable SERS detection

In recent research, anisotropic plasmonic core-shell nanomaterials have gained a lot of attention in surface-enhanced Raman scattering (SERS) due to their brilliant uniformity and optical properties. Herein, a bimetallic core-molecule-shell (CMS) composite nanorod SERS substrate nanomaterial (Au NB-DT@Ag NRs) was designed and synthesized under precise regulation. The inner core is gold nanobipyramids (Au NBs), which possess superior plasmonic properties. Uniform Au NBs of five different sizes were fabricated via a penta-twinned gold seed mediated growth method. The length varied from 160 to 62 nm and the corresponding diameter varied from 60 to 23 nm while the longitudinal surface plasmonic resonance (SPR) changed from 908 to 715 nm. The SERS activity of five Au NBs were compared and the optimally sized one with a length of 78 nm and width of 28 nm was set as the inner core. After modification with the Raman reporter (DT), different amounts of silver were deposited on the surface of Au NB-DTs to form an Au NB-DT@Ag nanocomposite. The shape of the nanostructure gradually became a rod and lengthened while the longitudinal SPR wavelength varied from 733 nm to 664 nm with an increase in the amount of silver nitrate added. The Au NB-DT@Ag NRs with the best SERS activity (b-3) could realize the quantitative detection of the toxic dyes malachite green (MG) and crystal violet (CV) of concentrations as low as 5 × 10-9 M, showing good reproducibility and stability. This work offers a new design strategy for a SERS substrate for reliable quantitative SERS detection applications.

Deep learning approach to overcome signal fluctuations in SERS for efficient On-Site trace explosives detection

Surface-enhanced Raman spectroscopy (SERS) is an improved Raman spectroscopy technique to identify the analyte under study uniquely. At the laboratory scale, SERS has realised a huge potential to detect trace analytes with promising applications across multiple disciplines. However, onsite detection with SERS is still limited, given the unwanted glitches of signal reliability and blinking. SERS has inherent signal fluctuations due to multiple factors such as analyte adsorption, inhomogeneous distribution of hotspots, molecule orientation etc. making it a stochastic process. Given these signal fluctuations, validating a signal as a representation of the analyte often relies on an expert's knowledge. Here we present a neural network-aided SERS model (NNAS) without expert interference to efficiently identify reliable SERS spectra of trace explosives (tetryl and picric acid) and a dye molecule (crystal violet). The model uses the signal-to-noise ratio approach to label the spectra as representative (RS) and non-representative (NRS), eliminating the reliability of the expert. Further, experimental conditions were systematically varied to simulate general variations in SERS instrumentation, and a deep-learning model was trained. The model has been validated with a validation set followed by out-of-sample testing with an accuracy of 98% for all the analytes. We believe this model can efficiently bridge the gap between laboratory and on-site detection using SERS.

A novel portable Raman scattering platform for antibiotic screening in pig urine

Background and Aim

Public health and food safety are gaining attention globally. Consumer health can be protected from chemical residues in meat by early detection or screening for antibiotic residues before selling the meat commercially. However, conventional practices are normally applied after slaughtering, which leads to massive business losses. This study aimed to use portable surface-enhanced Raman spectroscopy (SERS) equipped with multivariate curve resolution-alternation least squares (MCR-ALS) to determine the concentrations of enrofloxacin, oxytetracycline, and neomycin concentrations. This approach can overcome the problems of business loss, costs, and time-consumption, and limit of detection (LOD).

Materials and Methods

Aqueous solutions of three standard antibiotics (enrofloxacin, oxytetracycline, and neomycin) with different concentrations were prepared, and the LOD for each antibiotic solution was determined using SERS. Extracted pig urine was spiked with enrofloxacin at concentrations of 10, 20, 50, 100, and 10,000 ppm. These solutions were investigated using SERS and MCR-ALS analysis. Urine samples from pigs at 1 and 7 days after enrofloxacin administration were collected and investigated using SERS and MCR-ALS to differentiate the urinary enrofloxacin concentrations.

Results

The LOD of enrofloxacin, oxytetracycline, and neomycin in aqueous solutions were 0.5, 2.0, and 100 ppm, respectively. Analysis of enrofloxacin spiking in pig urine samples demonstrated the different concentrations of enrofloxacin at 10, 20, 50, 100, and 10,000 ppm. The LOD of spiking enrofloxacin was 10 ppm, which was 10 times lower than the regulated value. This technique was validated for the first time using urine collected on days 1 and 7 after enrofloxacin administration. The results revealed a higher concentration of enrofloxacin on day 7 than on day 1 due to consecutive administrations. The observed concentration of enrofloxacin was closely correlated with its circulation time and metabolism in pigs.

Conclusion

A combination of SERS sensing platform and MCR-ALS is a promising technique for on-farming screening. This platform can increase the efficiency of antibiotic detection in pig urine at lower costs and time. Expansion and fine adjustments of the Raman dataset may be required for individual farms to achieve higher sensitivity.

Core-satellite-satellite hierarchical nanostructures: assembly, plasmon coupling, and gap-selective surface-enhanced Raman scattering

The plasmonic properties of gold nanoparticles (AuNPs), such as color tunability, electric field generation, hot carrier generation, and localized heating, are significantly enhanced in the nanogaps between AuNPs. Therefore, the creation and control of nanogaps are key to developing advanced plasmonic nanomaterials. Most AuNP nanoassemblies, including dimers, trimers, and core-satellites, have a single type of nanogap within the assembly. In this study, we construct core-satellite-satellite (CSS) hierarchical, fractal-like nanostructures featuring two types of nanogaps, namely first generation nanogaps (Gap1) between the core and first satellite (Sat1) AuNPs and second generation nanogaps (Gap2) between Sat1 and second satellite (Sat2) AuNPs. The sequential and alternating immersion of glass slides in different-sized AuNPs and linkers forms CSS with perfect yield. The UV-vis spectroscopy, combined with charge density distribution calculations, reveals the nature of the plasmon coupling between the AuNPs that constitute CSS nanoassemblies. The plasmon coupling can be tuned by independently varying Gap1 and Gap2. Furthermore, we explore the electric field amplification in Gap1 and Gap2 by comparing the surface-enhanced Raman scattering signal intensity selectively from each nanogap. This new type of nanostructure provides a great flexibility to control and enhance the plasmonic properties of noble metal nanoparticles.

Wafer-scale Silver Nanodendrites with Homogeneous Distribution of Gold Nanoparticles for Biomolecules Detection

We report the fabrication and demonstrate the superior performance of robust, cost-effective, and biocompatible hierarchical Au nanoparticles (AuNPs) decorated Ag nanodendrites (AgNDs) on a Silicon platform for the trace-level detection of antibiotics (penicillin, kanamycin, and ampicillin) and DNA bases (adenine, cytosine). The hot-spot density dependence studies were explored by varying the AuNPs deposition time. These substrates’ potential and versatility were explored further through the detection of crystal violet, ammonium nitrate, and thiram. The calculated limits of detection for CV, adenine, cytosine, penicillin G, kanamycin, ampicillin, AN, and thiram were 348 pM, 2, 28, 2, 56, 4, 5, and 2 nM, respectively. The analytical enhancement factors were estimated to be ∼10⁷ for CV, ∼10⁶ for the biomolecules, ∼10⁶ for the explosive molecule, and ∼10⁶ for thiram. Furthermore, the stability of these substrates at different time intervals is being reported here with surface-enhanced Raman spectroscopy/scattering (SERS) data obtained over 120 days.

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Wafer-scale surface-enhanced Raman spectroscopy/scattering (SERS) substrate of Ag nanodendrites decorated with Au nanoparticles prepared

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Trace level detection of antibiotics achieved

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Versatility of these substrates demonstrated by detecting explosive, dye molecules

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Typical enhancement factors achieved were 10⁵–10⁷

Picosecond Laser-Ablated Nanoparticles Loaded Filter Paper for SERS-Based Trace Detection of Thiram, 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX), and Nile Blue

Recently, filter paper (FP)-based surface-enhanced Raman scattering (SERS) substrates have stimulated significant attention owing to their promising advantages such as being low-cost, easy to handle, and practically suitable for real-field applications in comparison to the solid-based substrates. Herein, a simple and versatile approach of laser-ablation in liquid for the fabrication of silver (Ag)-gold (Au) alloy nanoparticles (NPs). Next, the optimization of flexible base substrate (sandpaper, printing paper, and FP) and the FP the soaking time (5−60 min) was studied. Further, the optimized FP with 30 min-soaked SERS sensors were exploited to detect minuscule concentrations of pesticide (thiram-50 nM), dye (Nile blue-5 nM), and an explosive (RDX-1,3,5-Trinitroperhydro-1,3,5-triazine-100 nM) molecule. Interestingly, a prominent SERS effect was observed from the Au NPs exhibiting satisfactory reproducibility in the SERS signals over ~1 cm2 area for all of the molecules inspected with enhancement factors of ~105 and relative standard deviation values of <15%. Furthermore, traces of pesticide residues on the surface of a banana and RDX on the glass slide were swabbed with the optimized FP substrate and successfully recorded the SERS spectra using a portable Raman spectrometer. This signifies the great potential application of such low-cost, flexible substrates in the future real-life fields.

Hybrid Surface-Enhanced Raman Scattering Substrates for the Trace Detection of Ammonium Nitrate, Thiram, and Nile Blue

We report the fabrication and performance evaluation of hybrid surface-enhanced Raman scattering (SERS) substrates involving laser ablation and chemical routes for the trace-level detection of various analyte molecules. Initially, picosecond laser ablation experiments under ambient conditions were performed on pure silver (Ag) and gold (Au) substrates to achieve distinct nanosized features on the surface. The properties of the generated surface features on laser-processed portions of Ag/Au targets were systematically analyzed using UV-visible reflection and field emission scanning electron microscopy studies. Later, hybrid-SERS substrates were achieved by grafting the chemically synthesized Au nanostars on the plain and laser-processed plasmonic targets. Subsequently, we employed these as SERS platforms for the detection of a pesticide (thiram), a molecule used in explosive compositions [ammonium nitrate (AN)], and a dye molecule [Nile blue (NB)]. A comparative SERS study between the Au nanostar-decorated bare glass, silicon, Ag, Au, and laser-processed Ag and Au targets has been established. Our studies and the obtained data have unambiguously determined that laser-processed Ag structures have demonstrated reasonably good enhancements in the Raman signal intensities for distinct analytes among other substrates. Importantly, the fabricated hybrid SERS substrate of "Au nanostar-decorated laser-processed Ag" exhibited up to eight times enhancement in the SERS intensity compared to laser-processed Ag (without nanostars), as well as up to three times enhancement than the Au nanostar-loaded plain Ag substrates. Additionally, the achieved detection limits from the Au nanostar-decorated laser-processed Ag SERS substrate were ∼50 pM, ∼5 nM, and ∼5 μM for NB, thiram, and AN, respectively. The estimated enhancement factors accomplished from the Au nanostar-decorated laser-processed Ag substrate were ∼10⁶, ∼10⁶, and ∼10⁴ for NB, thiram, and AN, respectively.

Nanogap-tailored Au nanoparticles fabricated by pulsed laser ablation for surface-enhanced Raman scattering

Herein, gold nanoparticles (Au NPs) were synthesized by pulsed laser ablation (PLA) in a mixed-phase solvent of acetonitrile and water. The size of Au NPs and the number of graphitic carbon (GC) layers were controlled by varying the ratio of the solvent mixture. The surface-enhanced Raman scattering (SERS) of the Au NPs was investigated using 10^-3 M 4-aminobenzenethiol and 10^-4 M 4-nitrobenzenethiol as probe molecules. The SERS activity strongly depended on the nanogaps between particles owing to the formation of hot spots. In the present work, the nanogaps were controlled by changing the amount of GC layers. No GC layers were produced in water, resulting low SERS intensity. In contrast, Au NPs prepared in 30 vol% of acetonitrile showed significant SERS enhancement, which was attributed to the optimal size of the GC-coated NPs and a reasonable gap between them. The obtained results revealed that Au NPs produced by PLA in liquid could be applied in SERS-based microsensors.

Trace level detection of explosives and pesticides using robust, low-cost, free-standing silver nanoparticles decorated porous silicon

We report results from our extensive studies on the fabrication of ultra-thin, flexible, and cost-effective Ag nanoparticle (NP) coated free-standing porous silicon (FS-pSi) for superior molecular sensing. The FS-pSi has been prepared by adopting a simple wet-etching method. The deposition time of AgNO₃ has been increased to improve the number of hot-spot regions, thereby the sensing abilities are improved efficiently. FESEM images illustrated the morphology of uniformly distributed AgNPs on the pSi surface. Initially, a dye molecule [methylene blue (MB)] was used as a probe to evaluate the sensing capabilities of the substrate using the surface-enhanced Raman scattering (SERS) technique. The detection was later extended towards the sensing of two important explosive molecules [ammonium nitrate (AN), picric acid (PA)], and a pesticide molecule (thiram) clearly demonstrating the versatility of the investigated substrates. The sensitivity was confirmed by estimating the analytical enhancement factor (AEF), which was ∼10⁷ for MB and ∼10⁴ for explosives and pesticides. We have also evaluated the limit of detection (LOD) values in each case, which were found to be 50 nM, 1 µM, 2 µM, and 1 µM, respectively, for MB, PA, AN, and thiram. Undeniably, our detailed SERS results established excellent reproducibility with a low RSD (relative standard deviation). Furthermore, we also demonstrate the reasonable stability of AgNPs decorated pSi by inspecting and studying their SERS performance over a period of 90 days. The overall cost of these substrates is attractive for practical applications on account of the above-mentioned superior qualities.

Broadband SERS Enhancement by DNA Origami Assembled Bimetallic Nanoantennas with Label-Free Single Protein Sensing

Engineering hotspots in surface-enhanced Raman spectroscopy (SERS) through precisely controlled assembly of plasmonic nanostructures capable of expanding intense field enhancement are highly desirable to enhance the potentiality of SERS as a label-free optical tool for single molecule detection. Inspired by DNA origami technique, we constructed plasmonic dimer nanoantennas with a tunable gap decorated with Ag-coated Au nanostars on origami. Herein, we demonstrate the single-molecule SERS enhancements of three dyes with emission in different spectral regions after incorporation of single dye molecules in between two nanostars. The enhancement factors (EFs) achieved in the range of 10⁹-10¹⁰ for all the single dye molecules, under both resonant and nonresonant excitation conditions, would enable enhanced photostability during time-series measurement. We further successfully explored the potential of our designed nanoantennas to accommodate and detect a single thrombin protein molecule after selective placement in the wide nanogap of 10 nm. Our results suggest that such nanoantennas can serve as a broadband SERS enhancer and enable specific detection of target biological molecules with single-molecule sensitivity.

Explosive vapour/particles detection using SERS substrates and a hand-held Raman detector

We developed and optimized surface-enhanced Raman spectrometry (SERS) methods for trace analysis of explosive vapour and particles using a hand-held Raman spectrometer in the field. At first, limits of detection (LODs) using SERS methods based on a colloidal suspension of gold nanoparticles were measured under alkaline conditions and are as follows: pentaerythritol tetranitrate (PETN) (1.5 × 10⁻⁶ M, 6.9 ng), 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX), 8.1 × 10⁻⁶ M, 35 ng; urea nitrate (UN), 9.2 × 10⁻⁴ M, 165 ng; 2,4,6-trinitrotoluene (TNT), 1.1 × 10⁻⁷ M, 0.35 ng. We developed SERS substrates that demonstrate the wide applicability of this technique for use in the field for explosive vapour and particles adsorbed on a surface based on Au nanoparticles that were optimal for the detection of the target materials in solution. Au nanoparticles were modified onto quartz fibres or a polyurethane sponge for vapour/particles detection. SERS detection of vapours of 2,4-dinitrotoluene (2,4-DNT) and 1,3-dinitrobenzene (1,3-DNB) was shown by sampling vapours onto Au-modified quartz fibres followed by hand-held Raman analysis with estimated minimum detection levels of 3.6 ng and 54 ng, respectively. The detection of 2,4-DNT using sponge-based SERS decorated with Au nanoparticles was also demonstrated; however, the sensitivity was lower than that observed using quartz fibres. The detection of TNT on a surface was performed by utilizing quartz-fibres precoated with alumina and modified with Au nanoparticles, and the detection of 10 μg (0.53 μg cm⁻²) of TNT was demonstrated.

We developed and optimized surface-enhanced Raman spectrometry (SERS) methods for trace analysis of explosive vapour and particles using a hand-held Raman spectrometer in the field.

Ultra-sensitive reusable SERS sensor for multiple hazardous materials detection on single platform

We demonstrate the detection of dipicolinic acid, (DPA), a biomarker of bacterial spores for Bacillus anthracis, 2,4-Dinitrotoluene (DNT) and picric acid (PA) nitroaromatic hazardous chemicals on ultra-sensitive, reusable femtosecond laser textured Au nanostructures decorated with hierarchical AuNPs as a SERS substrate. The AuNPs were achieved by ablating an Au sheet using two different laser scan speeds (1 and 0.1 mm/s) in linear and crossed patterns. The morphological studies revealed dense hierarchical nanostructures decorated with spherical AuNPs possessing 30-40 nm in size in 0.1 mm/s laser scan. The limits of detection (LOD) of the sensor were determined from the detailed SERS measurements and were estimated to be 0.83 pg/L, 3.6 pg/L and 2.3 pg/L for DPA, DNT, and PA, respectively. To the best of our knowledge, the achieved sensitivity is nearly 2 orders improved for DPA when compared with the currently reported LODs using other techniques and 1 order in the case of SERS. Moreover, for DNT and PA the LODs were found to be either superior or comparable with recent reports. We have also demonstrated the competence of our SERS substrates by testing a few real samples (water spiked with these analytes) and again obtained very good sensitivity.

Silver nanoribbons achieved by picosecond ablation using cylindrical focusing and SERS-based trace detection of TNT

We report the fabrication of silver nanoribbons by picosecond laser ablation of bulk silver (Ag) targets submerged in double distilled water (DDW) using a cylindrical focusing geometry. The laser ablation was performed by ∼2 picosecond laser pulses and the corresponding light sheet engendered by a cylindrical lens of focal length ∼4.5 cm. The input pulse energies employed at a wavelength ∼800 nm in the experiments were ∼1000 μJ, ∼1200 μJ, and ∼1400 μJ. In contrast to the case of ablation with spherical lenses, cylindrical lens ablation produced nanoparticles (NPs) and nanostructures (NSs) in 20% less time. The data obtained from the optical characterizations exemplify that localized surface plasmon resonance (LSPR) was observed at 406 nm, 408 nm, and 410 nm for the input energies of ∼1000 μJ, ∼1200 μJ, and ∼1400 μJ, respectively. Interestingly, it was observed that the ablation performed at an input energy of ∼1200 μJ demonstrated the fabrication of Ag nanoribbons rather than the formation of Ag NPs. Selected area electron diffraction (SAED) data of the nanoribbons recorded revealed their crystalline phase and linear morphology. Ag nanomaterials (NPs and ribbons) synthesized in these experiments were employed to detect the explosive molecules of 2,4,6-trinitrotoluene (TNT) at a concentration 25 nM using the technique of surface enhanced Raman scattering. The enhancement factor in the case of Ag nanoribbons (width of ∼20-30 nm, length of ∼0.6-2 μm), obtained using the cylindrical focussing geometry at input pulse energies of ∼1200 μJ, was estimated to be ∼10⁷ for the 1362 cm^-1 mode, corresponding to the symmetric NO₂ stretch of TNT.

Fabrication of nano/microstructures for SERS substrates using an electrochemical method

Based on an electrochemical method, three-dimensional arrayed nanopore structures are machined onto a Mg surface. The structured Mg surface is coated with a thin gold (Au) film, which is used as a surface-enhanced Raman scattering (SERS) substrate. A rhodamine 6G (R6G) probe molecule is used as the detection agent for the SERS measurement. Different sizes of arrayed micro/nanostructures are fabricated by different treatment time using the electrochemical process. The topographies of these micro/nanostructures and the thickness of the Au film have an influence on the Raman intensity of the Mg substrate. Furthermore, when the thickness of Au film coating is held constant, the Raman intensity on the structured Mg substrates is about five times higher after a treatment time of 1 min when compared with other treatment times. The SERS enhancement factor ranges from 10⁶ to 1.75 × 10⁷ under these experimental conditions. Additionally, a 10^-6 mol·L^-1 solution of lysozyme was successfully detected using the Mg-Au nanopore substrates. Our low-cost method is reproducible, homogeneous, and suitable for the fabrication of SERS substrates.

Recent Developments in the Field of Explosive Trace Detection

Explosive trace detection (ETD) technologies play a vital role in maintaining national security. ETD remains an active research area with many analytical techniques in operational use. This review details the latest advances in animal olfactory, ion mobility spectrometry (IMS), and Raman and colorimetric detection methods. Developments in optical, biological, electrochemical, mass, and thermal sensors are also covered in addition to the use of nanomaterials technology. Commercially available systems are presented as examples of current detection capabilities and as benchmarks for improvement. Attention is also drawn to recent collaborative projects involving government, academia, and industry to highlight the emergence of multimodal screening approaches and applications. The objective of the review is to provide a comprehensive overview of ETD by highlighting challenges in ETD and providing an understanding of the principles, advantages, and limitations of each technology and relating this to current systems.

Flexible porous aerogels decorated with Ag nanoparticles as an effective SERS substrate for label-free trace explosives detection

Sensitive and reliable methods for explosives detection are of significance in homeland security due to the serious threats of explosives in terrorist attack events. However, such suitable sensors are still rare. Herein, porous silica aerogels decorated with silver nanoparticles (SiO2-Ag hybrids) were prepared and applied as a flexible SERS substrate for ultrasensitive explosives detection. It is worth noting that the silica aerogel we prepared had good flexibility compared with traditional silica aerogels, which effectively avoided structural damage during sample collection. Also, because of excellent adsorption performance provided by the silica aerogel, trace explosive 3-nitro-1,2,4-triazol-5-one (NTO) could be enriched and realized by label-free detection. Combined with the plasma enhancement provided by Ag NPs decorated around these porous aerogels, the limit of detection for explosive NTO was as low as 7.94 × 10-10 M. As far as we know, this SiO2-Ag hybrid SERS substrate was firstly used for the detection of explosives. It presented good sensitivity and reproducibility for analyte sensing. Most importantly, this is a label-free method for trace explosives detection and has a good application prospect in homeland security.

Silver mirror films deposited on well plates for SERS detection of multi-analytes: Aiming at 96-well technology

96-Well technology is associated with automated sample preparation and simultaneous analysis based on the low-cost well plate format. To explore the potential applications of 96-well technology in SERS detection, we examined the surface-bound electroless deposition procedure for the preparation of uniform and stable Ag mirror films on polydopamine (PDA)-coated well plates as active-SERS substrates. In the presented procedure, small Ag seeds assembled on PDA coating were employed as the surface-bound catalyst and provided the active sites for electroless Ag deposition. The high-quality Ag mirror films showed high performance in terms of sensitivity, uniformity, reproducibility and stability using rhodamine 6G (R6G) as the probe molecule. A remarkable enhancement factor of 3.41 × 10⁸ was obtained. The relative standard deviations against well-by-well and batch-by-batch reproducibility were less than 5%. The SERS films on well plates were successfully used to quantify the amounts of organic dyes (R6G and malachite green) in environmental water samples and small biological molecules (adenosine triphosphate and adenine) in urine matrix, displaying satisfactory sensitivity, selectivity and recovery. Their limit of detection values were at nanomolar, even picomolar concentration.

Arrayed nanopore silver thin films for surface-enhanced Raman scattering

Active substrates are crucial for surface-enhanced Raman scattering (SERS). Among these substrates, large uniform area arrayed nanoporous silver thin films have been developed as active substrates. Arrayed nanoporous silver thin films with unique anisotropic morphologies and nanoporous structures can be fabricated onto the nanoporous anodic aluminum oxide (AAO) of controlled pore size and interspacing by precisely tuning the sputtering parameters. These thin films preserve locally enhanced electromagnetic fields by exciting the surface plasmon resonance, which is beneficial for SERS. In this study, nanoporous silver thin films were transferred into polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS) substrates using our recently invented template-assisted sol-gel phase inverse-imprinting process to form two different nanopore thin films. The as-formed Ag nanoporous thin films on PMMA and PDMS exhibited intensively enhanced SERS signals using Rhodamine 6G (R6G) as the model molecule. The two nanopore thin films exhibited opposite pore size-dependent SERS tendencies, which were elucidated by the different enhancement tendencies of the electric field around pores of different diameters. In particular, the Ag nanoporous thin film on PMMA exhibited an R6G detection limit of as low as 10-6 mol L-1, and the SERS enhancement factor (EF) was more than 106. The low detection limit and large EF demonstrated the high sensitivity of the as-prepared SERS substrates for label-free detection of biomolecules. Compared with conventional smooth films, this nanopore structure can facilitate future application in biomolecular sensors, which allows the detection of single molecules via an electronic readout without requirement for amplification or labels.

Surface enhanced Raman scattering substrate for the detection of explosives: Construction strategy and dimensional effect

Surface-enhanced Raman spectroscopy (SERS) technology has been reported to be able to quickly and non-destructively identify target analytes. SERS substrate with high sensitivity and selectivity gave SERS technology a broad application prospect. This contribution aims to provide a detailed and systematic review of the current state of research on SERS-based explosive sensors, with particular attention to current research advances. This review mainly focuses on the strategies for improving SERS performance and the SERS substrates with different dimensions including zero-dimensional (0D) nanocolloids, one-dimensional (1D) nanowires and nanorods, two-dimensional (2D) arrays, and three-dimensional (3D) networks. The effects of elemental composition, the shape and size of metal nanoparticles, hot-spot structure and surface modification on the performance of explosive detection are also reviewed. In addition, the future development tendency and application of SERS-based explosive sensors are prospected.

Instantaneous trace detection of nitro-explosives and mixtures with nanotextured silicon decorated with Ag-Au alloy nanoparticles using the SERS technique

The development of recyclable surface enhanced Raman scattering (SERS) based sensors has been in huge demand for trace level explosives detection. A simple, hybrid Silicon (Si) nanotextured target-based SERS platform is fabricated through patterning micro square arrays (MSA) on Si using femtosecond (fs) laser ablation technique at different fluences. Using the hybrid target Si MSA substrate loaded/decorated with Ag-Au alloy NPs (obtained using femtosecond ablation in liquids) we demonstrate the trace level detection of organic nitro-explosives [picric acid (PA), 2,4-dinitrotoluene (DNT), and 1, 3, 5-trinitroperhydro-1, 3, 5-triazine (RDX)] and their mixtures. The microstructures/nanostructures of MSA fabricated at an input fluence of 9.55 J/cm², and decorated with Ag-Au alloy NPs, exhibited exceptional SERS enhancement factors (EFs) up to ∼10¹⁰ for MB, ∼10⁶ for PA, and ∼10⁴ for RDX with the detection limits obtained being ∼5 pM, ∼36 nM, and ∼400 nM for MB, PA and RDX respectively. Furthermore, we demonstrate these SERS substrates possess good reproducibility (RSD values < 15%) and a superior performance compared to a commercial Ag substrate (SERSitive, Poland). Three binary mixtures, i.e. MB-PA, MB-DNT, PA-DNT at different concentrations, were also investigated using the same SERS substrate to test the efficacy. Further, the SERS spectra of dyes, explosives, and complex mixtures were utilized for discrimination/classification using principal component analysis.

Robust and cost-effective silver dendritic nanostructures for SERS-based trace detection of RDX and ammonium nitrate

We report the fabrication and performance evaluation of cost-effective, reproducible silver nanodendrite (AgND) substrates, possessing high-density trunks and branches, achieved by a simple electroless etching process and subsequently utilized them for the trace detection of 1,3,5-trinitroperhydro-1,3,5-triazine (Research Development Explosive, RDX) and Ammonium Nitrate (AN). The intricate structural features in AgNDs offer high-density hotspots for effective molecular detection based on the surface enhanced Raman scattering (SERS) technique. The active SERS-substrate was initially tested with standard Rhodamine 6G (R6G) molecules at 1 nM concentration, which established an effective enhancement factor (EF) of ∼10⁸. The AgNDs were subsequently utilized in the detection of the explosives RDX and AN, down to concentrations of 1 μM. The typical EF achieved in the case of RDX and AN was ∼10⁴. The sensitivity of 1 μM R6G was further enhanced by two-fold through the deposition of Au nanoparticles on the AgNDs. The reproducibility of the low-cost substrate was also demonstrated, with a ∼9% RSD value in the measurements.

We report the fabrication and performance evaluation of cost-effective, reproducible silver nanodendrite (AgND) substrates, possessing high-density trunks and branches, achieved by a simple electroless etching and used for the trace detection of RDX and Ammonium Nitrate.

Hierarchical Laser-Patterned Silver/Graphene Oxide Hybrid SERS Sensor for Explosive Detection

We demonstrate an ultrafast laser-ablated hierarchically patterned silver nanoparticle/graphene oxide (AgNP/GO) hybrid surface-enhanced Raman scattering (SERS) substrate for highly sensitive and reproducible detection of an explosive marker 2,4-dinitrotoluene (2,4-DNT). A hierarchical laser-patterned silver sheet (Ag-S) is achieved by ultrafast laser ablation in air with pulse energies of 25, 50, and 100 μJ. Multiple laser pulses at a wavelength of 800 nm and a pulse repetition rate of 50 fs at 1 kHz are directly focused on Ag-S to produce and deposit AgNPs onto Ag-S. The surface morphology of ablated Ag-S was evaluated using atomic force microscopy, optical profilometry, and field emission scanning electron microscopy (FESEM). A rapid increase in the ablation rate with increasing laser energy was observed. Selected area Raman mapping is performed to understand the intensity and size distribution of AgNPs on Ag-S. Further, GO was spin-coated onto the AgNPs produced by ultrafast ablation on Ag-S. The hierarchical laser-patterned AgNP/GO hybrid structure was characterized using FESEM, high-resolution transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy. Further, hierarchical laser-patterned AgNP/GO hybrid structures have been utilized as SERS-active substrates for the selective detection of 2,4-DNT, an explosive marker. The developed SERS-active sensor shows good stability and high sensitivity up to picomolar (pM) concentration range with a Raman intensity enhancement of ∼10¹⁰ for 2,4-DNT. The realized enhancement of SERS intensity is due to the cumulative effect of GO coated on Ag-S as a proactive layer and AgNPs produced by ultrafast ablation.

Magnetic tuning of SERS hot spots in polymer-coated magnetic-plasmonic iron-silver nanoparticles

Plasmonic nanostructures are intensively studied for their ability to create electromagnetic hot spots, where a great variety of optical and spectroscopic processes can be amplified. Understanding how to control the formation of hot spots in a dynamic and reversible way is crucial to further expand the panorama of plasmon enhanced phenomena. In this work, we investigate the ability to modulate the hot spots in magnetic-plasmonic iron-doped silver nanoparticles dispersed in aqueous solution, by applying an external magnetic field. Evidence of magnetic field induction of hot spots was achieved by measuring the amplification of surface enhanced Raman scattering (SERS) from analytes dispersed in the solution containing Ag-Fe NPs. A polymeric shell was introduced around Ag-Fe NPs to confer colloidal stability, and it was found that the length and density of the polymer chains have a significant influence on SERS performance, and therefore on the formation of electromagnetic hot spots, under the action of the external magnetic field. These findings are expected to provide an important contribution to understanding the growing field of tuneable electromagnetic enhancement by external stimuli, such as magnetic fields applied to magnetic-plasmonic nanoparticles.