Abstract
Introduction
In the present-day scenario, human health, and environmental safety are the foremost concerns worldwide. Hazardous materials are referred to as those which have been determined to be capable of presenting an unreasonable risk to human health, safety, and property. The main characteristics of these materials are ignitability, corrosivity, reactivity, or toxicity. The specific categories among these materials are explosives, flammable liquids, gases, oxidizers, corrosives, flammable solids, radioactive materials, poisonous/infectious substances, and dangerous substances. We start with a short overview of various hazardous materials followed by the introduction of Raman spectroscopy and surface enhanced Rama spectroscopy/scattering (SERS) techniques. This review aims to report on the detection of hazardous materials such as explosives, pesticides, and simulants of chemical warfare agents using flexible SERS substrates.
Hazardous materials
Explosives/high energy materials (HEMs) are those materials that contain nitro groups (which are energetic) and release an enormous amount of energy in the form of light and heat when they are subjected to an external stimulus such as (a) spark (b) shock or even (c) friction. Explosives are commonly categorized as primary and secondary depending on their detonation (velocity, pressure etc.) and sensitivity parameters. Primary explosives are extremely sensitive and release enormous energy even with a small perturbation such as shock/collision. Therefore, the difficulty is generally high while handling the primary explosives. They act as boosters or initiators for detonating secondary explosives. Lead azide and mercury fulminate are a few examples of primary explosives, while 1,3,5,7-Tetranitro-1,3,5,7- tetrazocane (HMX), 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX), trinitrotoluene (TNT), etc. are representative of secondary explosives secondary explosives secondary explosives seconda. Interestingly, there are few home-prepared explosives utilized in the preparation of improvised explosive devices (IEDs). These are now easily synthesized at the laboratory level from simple molecules such as ammonium nitrate (AN), dinitrotoluene (DNT), picric acid (PA), etc.. Pesticides are the chemicals used by farmers/transporters to protect the crops/vegetables/fruits from insects/pests/rodents. The overused pesticides will remain as residues in the food, which may cause risk to human health (cancer/allergies/intoxications) and the ecosystem (surface water/soil)
Therefore, rapid and reliable detection of these hazardous molecules is the primary concern of both governmental agencies and research community to reduce the risk to society. Razdan and co-workers
Raman spectroscopy and variants
Raman spectroscopy is a simple, rapid, and a non-destructive spectroscopic technique based on molecular vibrations as signatures in the spectra. The Raman spectrum of any analyte molecule provides specific information and conveys chemical/structural information. This is important in the case of explosives (in pure form or even in the mixture form) irrespective of solid, liquid, powder, or gas state
In the present times, flexible SERS substrates have received great interest due to them possessing the advantages of (a) easy sampling by swabbing/wrapping directly on any curved/rough surfaces (b) large scalability by printing/roll to roll manufacturing/electrospinning etc. and (c) low overall cost of the sensing system. The development of handy flexible substrates with compact Raman devices/smart-phones can possibly provide portable sensors in real-world sensing/safety applications and serve as a powerful analytical tool for on-field analysis. For example, the possibility of detection of ultralow concentrations [picomolar (10−12 M or pM) to femtomolar (10−15 M or fM)] of two nerve gases, VX and Tabun was reported recently by Hakonen et al
Surface-enhanced Raman scattering (SERS)
Martin Fleischmann and co-workers had reported a fortunate discovery way back in 1974, in which they observed enhanced Raman signals of a pyridine molecule adsorbed on an electrochemically roughened silver surface
Figure 1.
Reviews on different SERS studies
A variety of review reports on SERS have been published over the last decades addressing the issues concerned with fabrication techniques, applications, and their developments. For example, Fan et al
Figure 2.
Sensitivity is the biggest virtue of a good SERS substrate is the detection of molecules at very low concentrations [traces meaning parts per billion (ppb) or parts per trillion (ppt) or parts per quadrillion (ppq)]. The sensitivity is generally expressed in terms of the lowest quantity of probe molecule detection possible with a given SERS substrate. The Raman signal disappears when the molecule concentrations reach a limit value. The sensitivity of the SERS substrate varies from molecule to molecule. The sensitivity of the SERS substrate is typically represented by the enhancement factor (limit of detection for a particular vibration mode of the probe molecule). Therefore, one should be judicious with the SERS substrate and select one with a higher enhancement factor or a lower limit of detection (LOD) over a wide range of analytes. Reproducibility is related to the variation of SERS intensity of the probe molecule over the NS surface. The smaller the variation in the signal, the higher the reproducibility and it is generally reported in terms of RSD (relative standard deviation) of the SERS signal. This depends mainly on the distribution of hotspots on the substrate. Low reproducibility of any SERS substrate affects the potential usage in practical applications. It is highly challenging to produce a highly reproducible SERS platform along with a homogeneous distribution of hotspots. The fluctuations of the SERS signals are calculated statistically with RSD of the particular mode intensity in the SERS spectrum. The magnitude of %RSD, indicative of the coefficient of variation, provides uncertainty in the measurement. Lower RSD values indicate a superior substrate in terms of reproducibility. Recyclability is another essential factor to test the usage of the same SERS substrate after detecting one/two molecules followed by proper cleaning procedures
The important results from the literature survey over the last 5−10 years concerning the usage of flexible SERS substrate for various hazardous materials detection is also summarized in this article. A large number of papers have been published in this area. To demonstrate the magnitude of research, a simple search for papers published in the journals and conferences, including the title/keywords/abstract “flexible Surface Enhanced Raman Spectroscopy” or “flexible Surface Enhanced Raman Scattering” or “flexible SERS” as indexed by the Scopus search engine, resulted in typically >100 papers in 2019, >100 papers in 2020 and >40 in the year 2021 alone. The corresponding data obtained is plotted as a bar graph and is shown inFig. 3. The identification of all the developments and practical applications of flexible SERS studies in various fields will be difficult to be presented in this review. Therefore, we have acknowledged the most important recent review articles and those are listed in the Table 1 below. The readers are suggested to select and pursue the review based on their interest(s). This review is limited to the recent studies (typically during the last 3−4 years) on flexible SERS substrates used in the detection of hazardous materials, rather than including broad discussions on solid SERS substrates (nanostructures on solid targets and metal NPs suspension on the solid platform) and their developments, which is a huge field. This review is warranted because of the extremely rapid developments in the area of different nanomaterials synthesized (for SERS studies including plasmonic and non-plasmonic), novel methodologies developed for incorporating various nanoparticles in different flexible platforms, and detection of diverse analyte molecules.
Figure 3.
S. No. | Author | Review topic | Ref. |
1 | Zhang et al. | Flexible SERS substrates and recent advances in food safety analysis | ref. |
2 | Yin et al. | Recent process of 2D materials in SERS | ref. |
3 | Klapec et al. | 2016–2019 published literature on the forensic related molecules and their various detection techniques using SERS | ref. |
4 | Li et al. | Fabrication and applications of flexible, transparent SERS substrates | ref. |
5 | Forbes et al. | Developed and challenges of SERS sensor in the detection of inorganic based explosives | ref. |
6 | Ji Sun et al. | SERS substrate developments and combination with other technologies in on-site analysis using portable Raman spectrometer | ref. |
7 | Jingjing et al. | Different dimensional (0D, 1D, 2D and 3D) SERS substrates for explosive detection | ref. |
8 | Shvalya et al. | Plasmonic NPs and 3D plasmonic NSs sensors with biological, medical, military, and chemical applications | ref. |
9 | To et al. | Explosive trace detection technologies and latest advances | ref. |
10 | Ren et al. | Qualitative and quantitative analysis; strategies of practical application of SERS substrates | ref. |
11 | Huang et al. | Paper SERS substrates in food safety | ref. |
12 | Chen et al. | 2D SERS substrates in chemical and biosensing | ref. |
13 | Dinesh et al. | Flexible sensor fabrication with various printing techniques | ref. |
14 | Xue et al. | Flexible nanofiber-based substrates fabrication and application | ref. |
15 | Ogundare et al. | Cellulose-based SERS substrates: fundamentals and principles | ref. |
16 | Zamora Sequeira et al. | Various methods for the determination of pesticides | ref. |
17 | Piolt et al. | Key aspects of SERS and application in the biomedical field | ref. |
18 | Ogundare et al. | Cellulose substrate fundamental, preparation methods, and applications | ref. |
19 | Lee et al. | Analyte manipulation and hybrid SERS platforms for real-world applications | ref. |
20 | Xu et al. | Latest advances of flexible SERS substrates in point of care diagnostic in tunable, sample swapping and in-situ SERS detection highlights | ref. |
21 | Zhang et al. | Electrospinning NPs based material and their sensing application | ref. |
22 | Restaino et al. | Plasmonic paper SERS substrates-preparation methods and sample collections | ref. |
Table 1. Important review articles on various applications of SERS that have been reported in the last three-years (2019–2021).
Flexible SERS substrates
A forthright method to achieve the SERS-active substrates is to dry the colloidal NPs (preferably plasmonic) solution on any of the glass/silicon/paper/metal surfaces.
The capabilities of flexible SERS substrates have gained tremendous research interest due to
• Inexpensive fabrication procedures making it possible to prepare large area substrates.
• Easy-to-use nature for on-site detection of a wide range of probe molecules.
• Flexibility in sample collection, i.e., possible to collect the probe molecules/sample directly from any rough surface (e.g., suitcase, bag, table surface, fruit, etc.) with the substrate by simple swabbing/swiping.
The merits of the SERS technique with the portable Raman spectrometer now widely used in national security, food safety, and environmental monitoring.
Recently explosives detection was approached by fabricating various flexible SERS substrates. Liyanage et al
Figure 4.
Paper-based SERS substrates
A detailed literature survey revealed that a variety of papers were used (as a base material) for preparing the SERS substrates such as filter paper
Figure 5.
Kim et al
Figure 6.Filter paper based SERS substrate by aggregated Ag/Au NPs for explosive molecule detection (Left side) (
Lin et al
Figure 7.(
Polymer-based SERS substrates
Nanofiber mats
Electrospinning is a method of translation of polymeric solution/melt (with or without additives) into solid nanofibers by applying the electric field
Figure 8.
The SERS performance of nanofiber depends on the properties of
• nanofibers (polymer nature, fiber diameter, the morphology of the nanofibers, and spinning time, etc.) and
• nanoparticles
• Decoration of NPs on the fiber
• The loading of NPs on the nanofiber mat leads to the NPs assembly with extremely small spacing providing scope for abundant hot spots. These play a crucial factor in SERS response.
Electrospinning polymer fibers can be used as SERS substrates by loading plasmonic NPs; similar to paper substrates, several methods are reported for embedding metal NPs onto the electrospun polymer films like dispersion of metal precursor and pre-mixing of metal NPs into the polymer solution and surface medications after electrospinning. Chamuah et al
Figure 9.(
Recently flexible polymer-based (PDMS
Figure 10.(
Fang et al
Textile based SERS substrates
The textile fabrics have also been investigated as an attractive SERS substrate (akin to paper and electrospun fiber substrate) because the fabric is naturally strong, flexible, soft, and a lightweight material. In textiles, various materials are available such as cotton, wool, silk, etc.. Comparable to other flexible substrates, the loading of NPs can be done in two ways, i.e., in-situ synthesis [soaking in different metal salts] and direct deposition of NPs [anisotropic silver nano-prisms and nano-disks to wool fabric has been reported recently
Figure 11.(
Table 2 summarizes the most important details of recently reported flexible SERS substrates including their preparation methods, materials used in those studies, and the sensitivities achieved. Such data is extremely important since the developments are occurring at a rapid pace and it is imperative to identify the strengths and weakness of each of these methodologies to come up with a viable and practical technique for making robust flexible SERS substrates. These flexible SERS substrates find niche applications in the detection of various hazardous materials in Defence, food, and environmental safety issues. Sensitivity estimations are reported in various parameters such as Molar (M), parts per billion (ppb), nanogram (ng), ng/cm2 and mg/kg. For example, in case of Thiram molecule (molecular weight of 240.44) 10 ppb is ~0.42 nM which is equivalent to ~1 pg in 10 µL; 1 ppb = 1 µg/kg; 1 ppm = 1000 ppb. Table 3 represents a summary of the commercially available SERS substrates (which is not exhaustive) and it is evident that each one of them have varied properties including the sensitivity, stability, and cost. Liu et al
Flexible substrate type | Hazardous material type studied | Method used | SERS active material | Molecules investigated - sensitivity | Ref. |
Paper/Cellulose | Explosives | Inkjet printing | PABT modified-Ag NPs-A4 paper | TNT- pM | ref.132 |
In-situ | Ag NPs in agarose film supported on filter paper | TNT- 10−8 M | ref.78 | ||
Immersion | Ag nano triangles-filter paper | PA- 10−6 M p-ATP- 10−8 M | ref.88 | ||
Soaking | Aggregated Ag/Au NPs-filter paper | PA- 5 µM DNT- 1 µM NTO- 10 µM | ref.94 | ||
Drop casting | Star-shaped Au NPs | PA-5 µM | ref.133 | ||
Reduction | Ag Nanostructures- filter paper Whatman 42 | Urea nitrate- 10−6 M CV- 10−8 M | ref.134 | ||
Drugs | Inkjet printing | Ag- chromatography paper | Organophosphate malathion –413 pg, Heroin –9 ng, Cocaine –15 ng | ref.135 | |
Plasma assisted chemical deposition | Au-Whatman filter paper grade 1 | Cocaine- 1 ng/ml | ref.136 | ||
Dyes | In-situ | Ag NPs-polydopamine -Filter paper | R6g- 10−10 M MG residue on Fish scales- 0.04635 pg/cm2, Crab shells- 0.06952 pg/cm2 and Shrimp skins- 0.09270 pg/cm2 | ref.137 | |
Inkjet-printing | MoO3−x nanosheets on Chromatographic paper, printing paper, filter paper | R6g- 10–7 M CV- 10–6 M and MG- 10–6 M on fish surface | ref.74 | ||
In-situ | Au-filter paper (Advantec #1) | MG-damped fish– 10 ppb | ref.138 | ||
Pesticides | Silver mirror reaction | Ag- filter paper | Thiram- 10−7 M | ref.139 | |
Pen on paper | Au NPs (15–120 nm); Au NRs (50 nm long, 14 nm thick); Ag NPs (50-80 nm) –A4 paper, Filter paper | Thiabendazole < 20 ppb | ref.73 | ||
Airbrush spray method | Ag NPs -glass fibre paper | Enoxacin & Enrofloxacin- 10 −5 M | ref.140 | ||
Printing | Au@Ag 30 nm Au core & 7 nm Ag shell -filter paper | Thiram- 10−9 M | ref.141 | ||
Screen printing | Ag NPs/GO- paper | Thiram 0.26 ng cm−2Thiabendazole 28 ng cm−2Methylparathion 7.4 ng cm−2 | ref.142 | ||
Immersion followed by APTMS | Ag NPs-PDMS sponge | Triazophos 0.79 ng Methyl Parathion 1.58 ng | ref.143 | ||
Vacuum-assisted filtration | AuNPs- cellulose nanofiber | Thiram- 1 pM Tricyclazole- 10 pM | ref.144 | ||
In-situ | Au NPs-pseudo-paper | Thiram- 1.1 ng/cm2 | ref.145 | ||
Laser techniques | Au/Ag film-print paper | Fungicide mancozeb (Dithane DG) and insecticide thiamethoxam (Aktara 25 BG) | ref.146 | ||
Immerson in NaCl solution for 5 min +dip-coating | Ag NPs- filter Paper | Melamine- 1 ppm Thiram- 1 ppm | ref.147 | ||
Immersion | FP-Au NPs | Methyl parathion- 0.011 μg/cm2 | ref.148 | ||
In-situ | Nanocellulose fibers-Ag NPs | Thiram- 0.05 ppm Thiabendazole- 0.09 ppm, MG 0.0014 ppm Enrofloxaci- 0.069 ppm | ref.149 | ||
Silicon rubber mask and a vacuum filtration | Au NRs -cellulose hydrogels | Thiram- 100 fM | ref.92 | ||
Drop casting | Quartz paper/Cellulose nanofiber/ mixture (Ag NPs+Au NSs) | Ferbam on kale leaves (50 µg/kg) | ref.150 | ||
Vacuum filtration | Cellulose nanofibers-Au NPs | Thiram- 10−8M | ref.151 | ||
Drop casting, inkjet printing | Au NPs-Whatman 44 FP | Benzenethiol chemical aerosol Pyridine | ref.152 | ||
Vacuum filtration | Glass-fiber filter paper-Ag NWs coupled with polymerase chain reaction (PCR) | DNA | ref.153 | ||
Electrochemical deposition | Mesoporous Au film@Ag NWs@cellulose nanofiber paper | R6g - 100 fM Thiram - 10 fM 2-naphthalenethiol-1 ppb | ref.154 | ||
Self-assembling | Cellulose nanofibers -Ag@DNA/PDA (polydopamine) | Rhodamine 6G. Thiamethoxamon- 0.003 mg/kg. | ref.155 | ||
Cotton buds | Antibiotics | In situ reduction | Ag NPs-cellulose nanocrystals-Filter paper | Phenylethanolamine A-10−9 M Metronidazole- 10−7 M | ref.93 |
Explosives | Self-assembly & In situ | Ag NPs-cotton swab | 2,4 DNT- 5 ng | ref.156 | |
Pesticides | Soaking, freezing, and drying | Ag NPs-chitosan foam | Triasophols Methidathion Isocrabophos | ref.157 | |
Dipping & drying | Ag NPs-cotton swab with NaCl | Thiabendazole (TBZ), thiram, TBZ + thiram | ref.158 | ||
3D- sponge | Explosives | In situ | Ag NPs -polyurethane sponge | Perchlorates- 0.13 ng CChlorates- 0.13 ng Nitrates- 0. 11 ng | ref.159 |
Nanofiber mat | Pesticides | Electrospinning | Au coated PVA nanofiber | Deltamethrin- 0.33 mg/kg Quinalphos- 0.28 mg/kg Thiacloprid- 0.26 mg/kg | ref.104 |
CWA simulants | Electrospinning | Au NPs –PVA nanofiber | Methyl salicylate | ref.160 | |
Dyes | Electrospinning | Ag NPs-PVA nanofiber | R6G-10−5 M | ref.161 | |
Electrospinning and in-situ | Ag NPs-Polyimide (PI) nanofabric | p-Aminothiophenol (p-ATP)- 10−14 mol/L), | ref.162 | ||
Fabric | Pesticides | Self-assembly/in-situ | Ag NPs- non woven fabric | Isocarbophos Sumicidin Phosgene | ref.163 |
Dip coating | Triangular Ag nanoplates-Cotton fabric | Carbaryl- 10−5M | ref.164 | ||
In situ | Polydopamine mediated Ag-Au NPs – cotton fabric | Carbaryl- 10−6M | ref.165 | ||
Magneton sputtering | Ag NPs-cotton fabric | Thiram - 1 ppm | ref.127 | ||
Magnetron sputtering | Ag-polyester fabric | R6G on cucumber, MG and Thiram | ref.166 | ||
Photochemical deposition (254 nm) | Ag NPs on TiO2 coated polyester fiber membranes | Sodium saccharin in soft drinks- 0.3 mg/L, (cola and sprite) | ref.167 | ||
In-situ growth | Ag NPs-Cotton fabrics | PATP-10−8 M | ref.168 | ||
Vacuum evaporation | Ag coated (10 nm) nylon fabrics | PATP-10−9 M Thiram on cucumber surface-10−7 M | ref.169 | ||
Dyes | Vacuum thermal evaporation and high-temperature annealing | Ag NPs-carbon fiber cloth | R6g- 10−14 mol·L−1 | ref.170 | |
Polymers | Explosives | Oriented stacking and in-situ | Ag and Au–Ag nanoplates- PET | TNT- 10 nM RDX- 10 nM | ref.171 |
Self-assembling | Au triangular nanoprisms on adhesive film (Scotch magic-tape) | TNT- 900 ppq RDX- 50 ppq and PETN- 50 ppq | ref.58 | ||
Incubated overnight followed by thorough rinsing drying | Au NPs,Au NRs and Au NCs on elastomeric film (PDMS) | TNT vapor | ref.172 | ||
Gravure printing | Ag NPs-PET | DNT vapor | ref.173 | ||
Sol–gel method and magnetron sputtering | Ag NPs-Porous silica aerogels | NTO- 7.94×10−10 M | ref.174 | ||
UV lithography and Au deposition | Ag NPs-Au coated -nanowrinkled zigzag micropattern on PDMS layer | TNT- 10−13 mol·L−1TNT residue(10−9 mol·L−1) on cloth bag | ref.175 | ||
Dyes | Electron-beam evaporation-uniaxial stretching | Stretched Ag coated poly(ε-caprolactone) film | MG-green mussel surface- 0.1×10−6 M | ref.176 | |
Pyramid Si template | MoS2/AgNPs/inverted pyramidal PMMA | R6G+MG | ref.177 | ||
Pyramid Si template | GO/Ag NPs/ pyramidal PMMA | MG on shrimp | ref.178 | ||
Ar plasma etching and Au evaporation | Worm-like Au NSs – PET film | R6G-10−9 M | ref.179 | ||
Self-assembly and in situ chemical reduction | Raspberry-like polyamide@Ag hybrid nanoarray film | R6g-10−14 M Adenosine- 10−9 M | ref.180 | ||
Pesticides | Drop-dry method | Au NPs (25 nm) - adhesive tape | Parathion-methyl- 2.60 ng/cm2Thiram 0.24 ng/cm2Chlorpyrifos 3.51 ng/cm2on apples, oranges, cucumbers, and green vegetables surfaces | ref.181 | |
Spin coating and manual peeling | AgNP@AgNW network-PDMS | Thiram (0.1µM) on a leaf surface and MG (0.1µM) on a living fish scale | ref.182 | ||
Paste and peeling of self-assembled NPs from Si | Adhesive acrylic polymer tape and polyethene terephthalate (PET) film (T/Au@Ag/PET) | Thiram on apple, tomato, and cucumber peels (5 ng/cm2) | ref.113 | ||
Seed mediated | Gold nanobush+PDMS | Thiabendazole (TBZ) on cherry – 0.64 ng/ml Carbaryl TBZ+Carbaryl | ref.183 | ||
Femtosecond laser induced plasma assisted ablation | Ag NPs and Au NPs FEP (fluorinated ethylene propylene) | Thiram on apple- 7.96 ng/cm2 | ref.184 | ||
Drop casting | Ag NS with spikes-adhesive tape | Phosmet & carbaryl on apple-surface 10 −7M | ref.185 |
Table 2. Summary of the recent flexible SERS substrates, their preparation methods, materials used, and the sensitivities achieved (2014-2021).
S. No. | Company | SERS substrate | Sensitivity | Stability | Cost | Ref |
1 | Cellulose with Au NPs | ~106 | 3 months | $199 (pack of 30) | ref. | |
2 | Glass coated with Au nanorods processed by dynamic oblique vacuum evaporation | − | − | − | ref. | |
3 | Electrodeposition of silver and gold nanoparticles on an ITO glass surface | ~105–106 | 4 months | 5 pcs Ag- €115
| ref. | |
4 | Si/Glass passivated with a thin transparent dielectic layer. | ~106 | Stable when unpacked | − | ref. | |
5 | Nanostructured Si deposited with Gold (Au), Silver (Ag) | − | − | 5 units €350 | ref. | |
6 | Au NS on polypropylene | − | 3 months
| − | ref. | |
7 | Ag/Au coating on silicate glass. | − | 2 months | Ag- €15
| ref. | |
8 | Si | − | − | 100 USD for single 2 mm × 2 mm sample. | ref. | |
9 | Au NSs on Si (5 mm × 5 mm) | ppb to ppm | 6 months (package)
| 2 units | ref. | |
10 | Ag, Au based Filter paper | − | − | − | ref. |
Table 3. A summary of the commercially available SERS substrates, their costs, sensitivities and their stability (non-exhaustive).
Conclusions and outlook
In recent years the development and applications of flexible SERS substrate has received incredible attention towards the detection of hazardous materials. In this review, we summarized the most recent research (focusing particularly on the last 3−4 years of research) on flexible based SERS substrates, including paper/cellulose, polymer nanofibers, 3D sponges, fabrics, etc., and their potential on-site detection of explosives, pesticides, chemical warfare agents, drugs for homeland security, food safety, and medical fields. There is a tremendous scope for the flexible SERS substrates in the above-mentioned fields and many others not listed here. Particularly in the field of explosive trace detection, these substrates will be highly beneficial. For example, explosives trace swiping/swabbing from luggage surfaces, clothing, vehicle surfaces, post-blast sites will be easier with such flexible substrates. These explosive molecules are sticky and leave behind small traces while handling and transporting them (on various surfaces). Such traces can be easily detected using efficient SERS substrates. Combined with a portable or handheld Raman spectrometers enriched with database/libraries of all explosive molecules, it presents a very attractive methodology for identification and prevention of terrorist activities. Similarly, testing food materials with these substrates enables prevention of easy adulteration (e.g., drinking water, milk, edible oils). Although there are several issues (e.g., further improvements in the sensitivity, long-term stability, reducing the costs) that need to be addressed for each of these methods. But there is also a huge scope for research in these areas, and we firmly believe the developments in these research areas will lead to practical devices.
Additionally, the recent developments in the understanding of SERS substrates (both plasmonic and non-plasmonic) and their potential have increased by leaps and bounds, the proof of which is evident from the number of review articles published in this area
Different real-world applications that can be envisaged with these SERS substrates include
(a) Biomedical applications, bioimaging and biosensing
(b) Inspection in food quality and safety
(c) Biochemical and medical analysis
(d) Virus detection (including COVID-19)
(e) Plant disease diagnostics
(f) Forensics
Since there are numerous methods by which SERS substrates can be fabricated
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