Graphene nanosheets-supported Ag nanoparticles for ultrasensitive detection of TNT by surface-enhanced Raman spectroscopy

Two Stable Pillar-Layered Zn-LMOFs for Highly Fluorescence Sensing of Inorganic Pollutants and Nitro Aromatic Compounds in Water

Luminescent metal-organic frameworks (LMOFs) are a potential class of functional materials for the photoluminescent detection of a wide range of analytes as well as for the detection of pollutants in wastewater. Herein, by using the pillar-layered strategy, two new luminescence Zn-LMOFs (JLU-MOF222 and JLU-MOF223) were successfully solvothermal synthesized. The 2D layers are both consisting of Zn2+ and TPHC [TPHC = (1,1':2',1″-terphenyl)-3,3″,4,4',4″,5'-hexacarboxylic acid] ligands and then pillared by the different N-donor ligands to form the 3D Zn-LMOFs with fsh topology. Benefiting from the uncoordinated carboxylate sites, uncoordinated N atom, or -NH2 group in the pillaring ligands and excellent stability in pH = 2-13 aqueous phase, JLU-MOF222 and JLU-MOF223 not only can sensitively detect trace amounts of inorganic pollutants (Fe3+, Cr2O72-) and nitro aromatic compounds TNP and 2,4-DNP (TNP = 2,4,6- trinitrophenol, 2,4-DNP = 2,4-dinitrophenol) through luminescence quenching but also exhibit high selectivity of other anti-interference competing analytes. The two new Zn-LMOFs can be used as potential luminescent sensors for pollutant detection in water due to their high KSV and low limit of detection (LOD).

Paper and nylon based optical tongues with poly(p-phenyleneethynylene)-fluorophores efficiently discriminate nitroarene-based explosives and pollutants

Discrimination of nitroarenes with hydrophobic dyes in a polar (H2O) environment is difficult but possible via a lab-on-chip, with polymeric dyes immobilized on paper or nylon membranes. Here arrays of 12 hydrophobic poly(p-phenyleneethynylene)s (PPEs), are assembled into a chemical tongue to detect/discriminate nitroarenes in water. The changes in fluorescence image of the PPEs when interacting with solutions of the nitroarenes were recorded and converted into color difference maps, followed by cluster analysis methods. The variable selection method for both paper and nylon devices selects a handful of PPEs at different pH-values that discriminate nitroaromatics reliably. The paper-based chemical tongue could accurately discriminate all studied nitroarenes whereas the nylon-based devices represented distinguishable optical signature for picric acid and 2,4,6-trinitrotoluene (TNT) with high accuracy.

Fouling-Resistant Behavior of Hydrophobic Surfaces Based on Poly(dimethylsiloxane) Modified by Green rGO@ZnO Nanocomposites

In line with global goals to solve marine biofouling challenges, this study proposes an approach to developing a green synthesis inspired by natural resources for fouling-resistant behavior. A hybrid antifouling/foul release (HAF) coating based on poly(dimethylsiloxane) containing a green synthesized nanocomposite was developed as an environmentally friendly strategy. The nanocomposites based on graphene oxide (GO) and using marine sources, leaves, and stems of mangroves (Avicennia marina), brown algae (Polycladia myrica), and zinc oxide were compared. The effectiveness of this strategy was checked first in the laboratory and then in natural seawater. The performance stability of the coatings after immersion in natural seawater was also evaluated. With the lowest antifouling (17.95 ± 0.7%) and the highest defouling (51.2 ± 0.9%), the best fouling-resistant performance was for the coatings containing graphene oxide reduced with A. marina stem/zinc oxide (PrGZS) and graphene oxide reduced with A. marina leaves/zinc oxide with 50% multiwall carbon nanotubes (PrGZHC50), respectively. Therefore, the HAF coatings can be considered as developed and eco-friendly HAF coatings for the maritime industry.

Performance and mechanism of graphene oxide removal from aqueous solutions by calcite: adsorption isotherms, thermodynamics, and kinetics

The flow of graphene oxide (GO) into natural water systems can adversely affect water environments and ecosystems. In this study, the adsorption effect of calcite on GO under different conditions was studied using calcite as adsorbent. Meanwhile, characterized by a combination of microscopic experiments, including SEM, TEM, XRD, FTIR, Raman, XPS, and AFM, additional research on the performance and the mechanism of GO sorption by calcite was conducted. The findings indicated that the highest adsorption efficiency was observed at a temperature of 303 K, pH 3, a mass of 90 mg of calcite, with an initial concentration of 60 mg L-1 GO, resulting in a 95% adsorption rate. The adsorption isotherm conformed to the model of Langmuir and Temkin, and it is a heat absorption process dominated by monolayer adsorption. The thermodynamic analysis showed that the adsorption was spontaneous and heat-absorbing. The adsorption kinetics conformed to the pseudo-second-order kinetic model, and the sorption procedure is chemisorption. In conclusion, calcite has a good sorption capacity for GO, which can provide a reference for the removal of GO in the aqueous environment.

A novel electrochemical sensor based on phosphate-stabilized poly-caffeic acid film in combination with graphene nanosheets for sensitive determination of nitro-aromatic energetic materials

This work offers a novel approach and sensor electrode for electrocatalytic reduction of nitro-aromatic explosives (NAEs). This sensor was created by combining electrochemically reduced graphene nanosheets (GNSs) -through cyclic voltammetric reduction of a graphene oxide colloidal solution- with phosphate-stabilized poly-caffeic acid (pCAF) film-modified glassy carbon electrode (GCE). The poly-caffeic acid-modified nonconductive electrode was stabilized with a H2PO4-/HPO42- phosphate buffer at pH 7 and made conductive. The novel electrode, called phosphate stabilized-GC/GNSs/pCAF, was characterized by electrochemical methods and scanning electron microscopy (SEM). The sensor exhibited high performance for trinitrotoluene (TNT) detection with a linear response between 50 and 500 μg L- 1 and a detection limit of 6 μg L-1. In addition to TNT, precise determinations of NAEs such as 2,4-dinitrotoluene (2,4-DNT), tetryl (2,4,6-trinitrophenyl methyl nitramine), trinitro phenol (TNP or picric acid; PA), 2,4-dinitrophenol (2,4-DNP), and 4-amino dinitrotoluene (4A-DNT, an aerobic bacterial degradation product of TNT) were made using the developed sensor electrode and DPV technique. Simultaneous quantification of TNT and DNT was performed with the aid of a computational technique known as multiple linear regression (MLR). The optimized electrode was resistant to interference effects. Satisfactory results on real samples were obtained by applying the modified electrode to the determination of TNT, tetryl, and TNP in contaminated soil. The validation of the proposed method was made against a literature LC-MS/MS method. A statistical comparison of the obtained results was provided using F- and Student's t-tests.

A highly sensitive and selective detection of 2,4,6-trinitrotoluene (TNT) using a peptide-functionalized silicon nanowire array sensor

A highly sensitive and specific detection of 2,4,6-trinitrotoluene (TNT), a typical nitrated aromatic explosive, was demonstrated by a silicon nanowire (SiNW) array sensor. The SiNW array devices were self-assembled and functionalized with the anti-TNT peptide to obtain unique sensitivity toward TNT. Also, the effect of the biointerfacing linker's chemistry and Debye screening with varied ionic strength of phosphate buffer solution (PBS) on TNT binding response signals were investigated. The optimization of the peptide-functionalized SiNW array sensor showed high sensitivity for TNT with a detection limit of 0.2 fM, the highest sensitivity reported to date. These initial promising results may help accelerate the development of portable sensors for femtomolar level TNT detection.

Fabrication of flexible AuNPs@ polyimide heating chips for in situ explosives SERS sensing in nature samples

In this study, highly sensitive flexible AuNPs@ polyimide SERS heating chips (APHC) were fabricated for in situ collecting and detecting TNT. Large-scale AuNPs arrays were synthesized by liquid-liquid interface self-assembly and transferred to polyimide heating film as SERS substrates. 4-ATP and AgNPs functionalized on APHC were used as capture means and signal amplifiers, combining with TNT to form the AuNPs-TNT-AgNPs "sandwich" structure. This flexible APHC chip showed high sensitivity as enhancement factor was 5.5×10⁵, and good repeatability and stability (RSD<10%). It was applied to detect TNT solutions with a low concentration of 10^-9 M, and showed a good linear response in the range from 10^-5 to 10^-9 M (R² = 0.986). In addition, the detection method also had good selectivity and no response to various TNT analogs. More important, combing with the thermal enrichment strategy, TNT dispersed in environmental samples such as soil, fruit and clothing would be enriched as vapor then collected and detected by APHC. This APHC device shows great potential for in situ sensing platforms, due to its sensitivity, high efficiency, and excellent portability.

Hydrogen-Bond-Connected 2D Zn-LMOF with Fluorescent Sensing for Inorganic Pollutants and Nitro Aromatic Explosives in the Aqueous Phase

Herein, a novel luminescent Zn-LMOF, JLU-MOF109 ([Zn(PBBA)(H₂O)]·3DMF·2H₂O, PBBA = 4,4'-(2,6-pyrazinediyl)bis[benzoic acid], DMF = N,N-dimethylformamide), was successfully synthesized under solvothermal conditions. Zinc ions are connected by PBBA ligands to form two-dimensional (2D) layers, and the layers are further propped up through hydrogen-bonding interactions. JLU-MOF109 exhibits good sensitivity to inorganic pollutants, Fe³⁺ and Cr₂O₇^2-, as well as nitro aromatic explosives, 2,4,6-trinitrophenol and 2,4-dinitrophenol. JLU-MOF109 exhibits high K_sv (at 10⁴ M^-1 level) and low limit of detection values (∼10^-6 mol/L) for the abovementioned hazardous pollutants, which is better than a majority of previously reported MOF-based fluorescent sensors. With good stability in the aqueous phase, JLU-MOF109 can serve as a promising chemical sensor for pollutant detection in wastewater.

Facile preparation of mesoporous silica coated nitrogen doped carbon dots for sensitive detection of picric acid

In this work, a nanocomposite suitable for long-term storage was constructed for efficient and highly selective detection of picric acid (PA). For this purpose, nitrogen-doped carbon dots (N-CDs) were synthesized by a simple hydrothermal reaction one-step method, and the synthesized nitrogen-doped carbon dots were loaded into amine-modified mesoporous silica nanoparticles (MSN-NH₂) to form N-CDs@MSN-NH₂ nanocomposites. The as-synthesized N-CDs@MSN-NH₂ was detected by X-ray photoelectron spectroscopy (XPS) and the Fourier transform infrared (FT-IR) analysis methods. After being coated with MSNs, the as-synthesized N-CDs@MSN-NH₂ exhibits excellent photo-stability in storage for 60 days at room temperature. Furthermore, PA can significantly quench the fluorescence signal of N-CDs@MSN-NH₂ through the fluorescence resonance energy transfer (FRET) effect, while other metal ions and nitro compounds only cause little change. The a-synthesized composites were used to detect PA with a detection limit of 50 nM in an aqueous solution. These results indicate that the synthesized composites have promise for application in PA detection in aqueous solution.

The Theory of Surface-Enhanced Raman Spectroscopy on Organic Semiconductors: Graphene

Drawing on a theoretical expression previously derived for general semiconductor substrates, we examine the surface-enhancement of the Raman signal (SERS) when the substrate is chosen to be monolayer graphene. The underlying theory involves vibronic coupling, originally proposed by Herzberg and Teller. Vibronic coupling of the allowed molecular transitions with the charge-transfer transitions between the molecule and the substrate has been shown to be responsible for the SERS enhancement in semiconductor substrates. We then examine such an expression for the Raman enhancement in monolayer graphene, which is dependent on the square of the derivative of the density of states of the graphene. On integration, we find that the discontinuity of the density-of-states function leads to a singularity in the SERS intensity. Knowledge of the location of this resonance allows us to maximize the Raman intensity by careful alignment of the doping level of the graphene substrate with the charge-transfer transition.

The novel sandwich composite structure: a new detection strategy for the ultra-sensitive detection of cyclotrimethylenetrinitramine (RDX)

This study presents a novel sandwich composite structure that was designed for the ultra-sensitive detection of cyclotrimethylenetrinitramine (RDX). Au nanorod arrays (Au NRAs) were prepared and bound to 10^-7M 6-MNA as adsorption sites for RDX, while Au nanorods (Au NRs) were modified using 10^-5M 6-MNA as SERS probes. During detection, RDX molecules connect the SERS probe to the surface of the Au NRAs, forming a novel type of Au NRAs-RDX-Au NRs 'sandwich' composite structure. The electromagnetic coupling effect between Au NRs and Au NRAs is enhanced due to the molecular level of the connection spacing, resulting in new 'hot spots'. Meanwhile, Au NRAs and Au NRs have an auto-enhancement effect on 6-MNA. In addition, the presence of charge transfer in the formed 6-MNA-RDX complex induced chemical enhancement. The limits of detection of RDX evaluated by Raman spectroscopy using 6-MNA were as low as 10^-12mg ml^-1(4.5 × 10^-15M) with good linear correlation between 10^-12and 10^-8mg ml^-1(correlation coefficientR² = 0.9985). This novel sandwich composite structure accurately detected RDX contamination in drinking water and on plant surfaces in an environment with detection limits as low as 10^-12mg ml^-1and 10^-8mg ml^-1.

Plasmonic Metal Nanoparticles Hybridized with 2D Nanomaterials for SERS Detection: A Review

In SERS analysis, the specificity of molecular fingerprints is combined with potential single-molecule sensitivity so that is an attractive tool to detect molecules in trace amounts. Although several substrates have been widely used from early on, there are still some problems such as the difficulties to bind some molecules to the substrate. With the development of nanotechnology, an increasing interest has been focused on plasmonic metal nanoparticles hybridized with (2D) nanomaterials due to their unique properties. More frequently, the excellent properties of the hybrids compounds have been used to improve the drawbacks of the SERS platforms in order to create a system with outstanding properties. In this review, the physics and working principles of SERS will be provided along with the properties of differently shaped metal nanoparticles. After that, an overview on how the hybrid compounds can be engineered to obtain the SERS platform with unique properties will be given.

Large-Area Nanosphere Self-Assembly Monolayers for Periodic Surface Nanostructures with Ultrasensitive and Spatially Uniform SERS Sensing

Colloidal lithography provides a rapid and low-cost approach to construct 2D periodic surface nanostructures. However, an impressive demonstration to prepare large-area colloidal template is still missing. Here, a high-efficient and flexible technique is proposed to fabricate self-assembly monolayers consisting of orderly-packed polystyrene spheres at air/water interface via ultrasonic spray. This "non-contact" technique exhibits great advantages in terms of scalability and adaptability due to its renitent interface dynamic balance. More importantly, this technique is not only competent for self-assembly of single-sized polystyrene spheres, but also for binary polystyrene spheres, completely reversing the current hard situation of preparing large-area self-assembly monolayers. As a representative application, hexagonal-packed silver-coated silicon nanorods array (Si-NRs@Ag) is developed as an ultrasensitive surface-enhanced Raman scattering (SERS) substrate with very low limit-of-detection for selective detection of explosive 2,4,6-trinitrotoluene down to femtomolar (10^-14 m) range. The periodicity and orderliness of the array allow hot spots to be designed and constructed in a homogeneous fashion, resulting in an incomparable uniformity and reproducibility of Raman signals. All these excellent properties come from the Si-NRs@Ag substrate based on the ordered structure, open surface, and wide-range electric field, providing a robust, consistent, and tunable platform for molecule trapping and SERS sensing for a wide range of organic molecules.

Electrochemical determination of nitroaromatic explosives using glassy carbon/multi walled carbon nanotube/polyethyleneimine electrode coated with gold nanoparticles

The on site/in field detection of explosives has become a rising priority for homeland security and counter-terrorism measures. This work presents the sensitive detection of nitroaromatic explosives using glassy carbon/multi-walled carbon nanotubes/polyethyleneimine (GC/MWCNTs/PEI) electrode coated with gold nanoparticles (AuNPs). MWCNTs and PEI could be well dispersed in ethanol/water solution, giving rise to a thin and homogeneous film on GCE. The GC/MWCNTs/PEI electrode was electrochemically modified with AuNPs and used for the differential pulse voltammetric (DPV) detection of nitroaromatics. The enhanced detection sensitivities were achieved through π-π and charge-transfer (CT) interactions between the electron-deficient nitroaromatic explosives and donor amine groups in PEI to which gold nanoparticles were linked, providing increased analyte affinity toward the modified GCE. Calibration curves of current intensity versus concentration were linear in the range of 0.05-8 mg L^-1 for TNT, 0.2-4 mg L^-1 for 2,4-dinitrotoluene (DNT), 1-20 mg L^-1 for 2,4-dinitrophenol (2,4-DNP), 0.25-10 mg L^-1 for picric acid (PA), and 0.05-4 mg L^-1 for 2,4,6-trinitrophenyl-N-methylnitramine (tetryl) with detection limits (LOD) of 15 μg L^-1, 45 μg L^-1, 135 μg L^-1, 30 μg L^-1, and 12 μg L^-1, respectively. The proposed method was successfully applied to the analysis of nitroaromatics in synthetic explosive mixtures and military composite explosives (comp B and octol). The electrochemical method was not affected by possible interferents of electroactive camouflage materials and common soil ions. Method validation was performed against the reference LC-MS method on TNT and PA-contaminated clay soil samples separately.

A 'sandwich' structure for highly sensitive detection of TNT based on surface-enhanced Raman scattering

Ultra-sensitive detection of 2,4,6-trinitrotoluene (TNT) plays an important role in society security and human health. The Raman probe molecule p-aminothiophenol (PATP) can interact with TNT in three ways to form a TNT-PATP complex. In this paper, a 'sandwich' structure was developed to detect TNT with high sensitivity. Au nano-pillar arrays (AuNPAs) substrates modified by low-concentration PATP through Au-S bonds were acted as capture probe for TNT. Meanwhile, Ag nano-particles (AgNPs) modified by PATP at higher concentration were employed as tags for surface-enhanced Raman scattering (SERS). The formation of the TNT-PATP complex is not only the means by which AuNPAs substrates recognize and capture TNT, but also links the SERS tags to TNT, forming an AuNPAs-TNT-AgNPs 'sandwich' structure. The Raman signal of PATP was greatly enhanced mainly because novel 'hot spots' formed between the AuNPAs and AgNPs of the 'sandwich' structure. The Raman signal of PATP was further amplified by the chemical enhancement effect induced by the TNT-PATP complex formation. Based on this mechanism, the limit of detection (LOD) of TNT was determined from the Raman signal of PATP. The LOD reached 10^-9 mg/mL (4.4 × 10^-12 M), much lower than that suggested by the US Environmental Protection Agency (88 nM). Moreover, TNT was selectively detected over several TNT analogues 2,4-dinitrotoluene (DNT), p-nitrotoluene (NT) and hexogen (RDX). Finally, the 'sandwich' structure was successfully applied to TNT detection in environmental water and sand.

Eu-Doped MOF-based high-efficiency fluorescent sensor for detecting 2,4-dinitrophenol and 2,4,6-trinitrophenol simultaneously

Nitroaromatic explosives pose a great threat to the environment and human safety. It is very important to design simple, highly efficient and multifunctional sensors for detecting nitroaromatic explosives. However, a few sensors can determine multicomponent nitroaromatic explosives simultaneously. Eu functionalized MOF-253 (Eu@MOF-253) hybrid material was synthesized using the post-synthetic modification method. The introduction of Eu³⁺ in MOF-253 caused the fluorescence peak of the ligand to show a distinct red-shift due to its polarization enhancement effect in the presence of 2,4-DNP. The emission and excitation spectra of the Eu@MOF-253 sensor showed overlap with the ultraviolet-visible (UV-vis) absorption spectra of the representative nitroaromatic explosives 2,4-dinitrophenol (2,4-DNP) and 2,4,6-trinitrophenol (TNP). Therefore, it is feasible to discriminate and quantify TNP and 2,4-DNP simultaneously. As proposed, the Eu@MOF-253 luminescent sensor was highly sensitive and selective towards TNP and 2,4-DNP. The other coexisting nitroaromatic explosives did not interfere with the determination. Upon addition of TNP, the fluorescence of the Eu@MOF-253 sensor decreased dramatically and showed an excellent quenching constant (K_sv) of 1.58 × 10⁶. The fluorescence intensities of the Eu@MOF-253 sensor presented good linear relationships with concentrations of TNP and 2,4-DNP ranging from 0.01-100 μM and 0.01-25 μM, respectively. Low limits of detection (LOD) for both 2,4-DNP and TNP were approximately 10 nM. The determination mechanism is mainly ascribed to the internal filtration effect (IFE) and electron transfer. This work provides a practical method for the highly efficient determination of nitroaromatic explosives.

A Simple Determination of Trinitrotoluene (TNT) Based on Fluorescence Quenching of Rhodamine 110 with FRET Mechanism

Sensitive and selective detection of nitroaromatic explosives is an important issue in regard to human health, environment, public security and military issues. In this study, a simple and sensitive fluorescence quenching - based assay utilizing Rhodamine 110 as fluorophore probe was developed for the determination of trinitrotoluene (TNT). This sensitive fluorometric method could measure the decrease in fluorescence of Rhodamine 110 (λex = 490 nm, λem = 521 nm) owing to the primary amine groups of Rhodamine 110 (different from other rhodamines) capable of donor-acceptor interaction with TNT. The resulting TNT-amine complex can strongly quench the fluorescence emission of Rhodamine 110 by fluorescence resonance energy transfer (FRET) which occurs as the excited Rhodamine 110 fluorophore (donor) transfers its energy to TNT (acceptor) by non-radiative dipole-dipole interaction. Fluorescence quenching varied linearly with TNT concentration, with LOD and the LOQ of 0.71 and 2.38 mg L- 1 TNT, respectively. Similar explosives, common soil ions, and possible camouflage materials were found not to interfere with the proposed method, offering significant advantages with its easy methodology, low-cost, sensitivity, and rapidity of analysis. FRET mechanism based on dye donor-TNT acceptor interaction.

Cost Effective Silver Nanowire-Decorated Graphene Paper for Drop-On SERS Biodetection

The use of SERS for real-world bioanalytical applications represents a concrete opportunity, which, however, is being largely delayed by the inadequacy of existing substrates used to collect SERS spectra. In particular, the main bottleneck is their poor usability, as in the case of unsupported noble metal colloidal nanoparticles or because of the need for complex or highly specialized fabrication procedures, especially in view of a large-scale commercial diffusion. In this work, we introduce a graphene paper-supported plasmonic substrate for biodetection as obtained by a simple and rapid aerosol deposition patterning of silver nanowires. This substrate is compatible with the analysis of small (2 μL) analyte drops, providing stable SERS signals at sub-millimolar concentration and a detection limit down to the nanogram level in the case of hemoglobin. The presence of a graphene underlayer assures an even surface distribution of SERS hotspots with improved stability of the SERS signal, the collection of well-resolved and intense SERS spectra, and an ultra-flat and photostable SERS background in comparison with other popular disposable supports.

Detection of dithiocarbamate, chloronicotinyl and organophosphate pesticides by electrochemical activation of SERS features of screen-printed electrodes

Enhancement of Raman intensity due to the electrochemical surface-enhanced Raman scattering (EC-SERS) effect is an interesting alternative to overcome the lack of sensitivity traditionally associated with Raman spectroscopy. Furthermore, activation of metallic screen-printed electrodes (SPEs) by electrochemical route leads to the reproducible generation of nanostructures with excellent SERS properties. EC-SERS procedure proposed in this work for the detection of several pesticides (thiram, imidacloprid and chlorpyrifos) with different nature, uses gold SPEs as SERS substrates, but also includes a preconcentration step as the initial and essential stage. Taking into account the small volume of solution employed, only 60 µL, the preconcentration cannot be performed for more than 15 min in order to ensure the proper contact of the solution with WE, RE and CE. Furthermore, selected temperature, 34 °C, is not very high to allow the exhaustive control of the drop volume. Optimization of preconcentration parameters (time and temperature) displays a crucial step, particularly in the detection of low concentrations of pesticides, because it will provide higher Raman intensity in EC-SERS experiments. After the initial step, gold SPEs were electrochemically activated by cyclic voltammetry, allowing the detection of very low concentration (µg·L^-1) of pesticides due to the generation of fresh nanostructures with SERS effect.

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.

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.

Nitrogen and Phosphorus Co-Doped Carbon Dots for Selective Detection of Nitro Explosives

In this work, a highly selective and sensitive method has been developed for the detection of trinitrophenol (TNP), which is a dangerous explosive. For this purpose, N and P co-doped carbon dots (NP-Cdots) have been used. Synthesis of N and P co-doped carbon dots has been carried out by a simple and quick method. X-ray photoelectron spectroscopy analysis was carried out to detect the doping of N and P. These carbon dots are insoluble in water (inNP-Cdots). These carbon dots were functionalized by treating them with conc. HNO3 so that they become water-soluble (wsNP-Cdots). These dots were characterized by different analytical techniques such as IR, UV-vis, and fluorescence spectroscopy. The as-prepared wsNP-Cdots have good fluorescence properties. The average diameter of wsNP-Cdots is found to be 5.7 nm with an interlayer spacing (d-spacing) of 0.16 nm. The as-prepared wsNP-Cdots are highly sensitive and selective toward TNP, as observed using a fluorescence quenching technique. The quenching constant for TNP is found to be very high (8.06 × 104 M-1), which indicates its high quenching ability. The limit of detection is found to be 23 μM.

Controllable fabrication of Ag-NP-decorated porous ZnO nanosheet arrays with superhydrophobic properties for high performance SERS detection of explosives

An efficient hydrophobic condensation surface was developed and used as an ultrasensitive and stable SERS sensor based on ZnO–Ag hybrid mesoporous nanosheet (MNS) arrays for natural explosive sample detection.

Ultrasensitive SERS detection of rhodamine 6G and p-nitrophenol based on electrochemically roughened nano-Au film

Quantitative analysis of organic pollutants in environmental water is an important issue for ecological environment and human health. In this paper, the quantitative analysis of rhodamine 6G (R6G) and p-nitrophenol (PNP) is performed by the surface enhanced Raman scattering (SERS) technology. The enhancement of Raman signals is achieved on the surface of an electrochemically roughened nano-Au film. The SERS performance depends on the microstructure of roughened nano-Au films, which is affected by the thickness of Au films and electrochemical roughening parameters. The structure-dependence of SERS performance is validated by finite element simulation of local electromagnetic field distribution. An obvious SERS effect of R6G with an enhancement factor of 10⁸ is obtained on the roughened nano-Au film. A sensitive SERS detection of R6G with a linear range of 10^-9-10^-5 M and a detection limit of 10^-11 M is realized. Moreover, a wide linear range of 10^-9-10^-3 M is obtained for the detection of PNP. The roughened nano-Au film is an effective substrate for the SERS detection of organic pollutants with high reproducibility and good stability. Therefore, the electrochemical technology in this study is expected to be a very promising method for the fabrication of high-performance SERS substrate.

Chemical and Bio Sensing Using Graphene-Enhanced Raman Spectroscopy

Graphene is a two-dimensional (2D) material consisting of a single sheet of sp² hybridized carbon atoms laced in a hexagonal lattice, with potentially wide usage as a Raman enhancement substrate, also termed graphene-enhanced Raman scattering (GERS), making it ideal for sensing applications. GERS improves upon traditional surface-enhanced Raman scattering (SERS), combining its single-molecule sensitivity and spectral fingerprinting of molecules, and graphene's simple processing and superior uniformity. This enables fast and highly sensitive detection of a wide variety of analytes. Accordingly, GERS has been investigated for a wide variety of sensing applications, including chemical- and bio-sensing. As a derivative of GERS, the use of two-dimensional materials other than graphene for Raman enhancement has emerged, which possess remarkably interesting properties and potential wider applications in combination with GERS. In this review, we first introduce various types of 2D materials, including graphene, MoS₂, doped graphene, their properties, and synthesis. Then, we describe the principles of GERS and comprehensively explain how the GERS enhancement factors are influenced by molecular and 2D material properties. In the last section, we discuss the application of GERS in chemical- and bio-sensing, and the prospects of such a novel sensing method.

Colorimetric paper sensor for sensitive detection of explosive nitroaromatics based on Au@Ag nanoparticles

Rapid, reliable, onsite approaches for detection trace level of trinitrotoluene (TNT) is a pressing necessity for both homeland security and environmental protection. To this end, hydrophilic amine(-NH₂) protected Au@Ag nanoparticles (NPs) were developed and fabricated as colorimetric paper sensor for delicate detection of TNT. The as-developed nanoprobe selectively reacts with TNT through classic Meisenheimer complex formation by means of charge transfer process from an electron-rich NH₂ group of β-cysteamine to an electron-deficient nitro group on TNT. Due to the absence of this particular interaction of other nitroaromatics, the proposed probe is highly selective for TNT detection with a better linear range (0-20 μg/mL) and limit of detection (LOD) of 0.35 μg/mL. The present work provides a novel and facile strategy to fabricate colorimetric paper sensors with rapid and selective recognition ability for label free analysis of TNT.

Electrocatalytic reduction of trace nitrobenzene using a graphene-oxide@polymerized-manganese-porphyrin composite

A more stable composite of graphene oxide encapsulated by the polymerized porphyrins realize the electrocatalytic reduction of nitrobenzene to nitrosobenzene.

Graphene-based nanoplatforms for surface-enhanced Raman scattering sensing

Surface-enhanced Raman scattering (SERS) is one of the important techniques for sensing applications in biological analysis, disease diagnosis, environmental science, and food safety. Graphene provides an excellent nanoplatform for SERS sensing due to its two-dimensional flat structure, uniform electronic and photonic properties, excellent mechanical stability, atomic uniformity, and high biocompatibility. In this review, we summarize recent advances in the fabrication of various graphene-based nanoplatforms for SERS sensing. We present the strategies, such as self-assembly, in situ synthesis, one-pot synthesis, liquid phase reduction, and biomimetic synthesis, for the fabrication of graphene-based hybrid metallic and alloy nanoplatforms, and then demonstrate the potential applications of graphene-based nanoplatforms for the SERS sensing of ions, organic dyes, pesticides, bacteria, DNA, proteins, cells, and other chemicals in great detail. In addition, we also discuss the future development of this interesting research field and provide several perspectives. This work will be helpful for readers to understand the fabrication and sensing mechanisms of graphene-based SERS sensing nanoplatforms; meanwhile, it will promote the development of new materials and novel methods for high performance sensing and biosensing applications.

Advancement of Ag-Graphene Based Nanocomposites: An Overview of Synthesis and Its Applications

Graphene has been employed as an excellent support for metal nanomaterials because of its unique structural and physicochemical properties. Silver nanoparticles (AgNPs) with exceptional properties have received considerable attention in various fields; however, particle aggregation limits its application. Therefore, the combination of AgNPs and graphene based nanocomposites (Ag-graphene based nanocomposites) has been widely explored to improve their properties and applications. Excitingly, enhanced antimicrobial, catalytic, and surface enhanced Raman scattering properties are obtained after their combination. In order to have a comprehensive knowledge of these nanocomposites, this Review highlights the chemical and biological synthesis of Ag-graphene nanocomposites. In particular, their applications as antimicrobial agents, catalysts, and sensors in biomedicine, agricultural protection, and environmental remediation and detection are covered. Meanwhile, the factors that influence the synthesis and applications are also briefly discussed. Furthermore, several important issues on the challenges and new directions are also provided for further development of these nanocomposites.

Electrospun flexible poly(bisphenol A carbonate) nanofibers decorated with Ag nanoparticles as effective 3D SERS substrates for trace TNT detection

A flexible 3D hybrid PC/Ag surface-enhanced Raman scattering (SERS) substrate was fabricated through the combination of electrospinning and in situ chemical reduction. Due to the rough surface morphology and the intricate 3D structure, a high density of Raman "hotspots" was formed at the junctions of cross-linked nanofibers, resulting in excellent sensitivity to a probe molecule (4-aminothiophenol). The nanofibers were modified with l-cysteine to capture TNT molecules by the formation of a Meisenheimer complex, after which positively charged 4-ATP-labelled AgNPs were introduced to the system, which both generated more hotspots and led to a linear relationship between the TNT concentration and the SERS intensity of the labelled molecules. As a result, a good linear response ranging from 10^-8 to 10^-12 M was achieved, and the detection limit for TNT was as low as 2.05 × 10^-13 M. This strategy demonstrates an ultra-sensitive approach for the detection of trace amounts of TNT, and a promising method for the detection of many other analogous explosives.

Fuel-Free Light-Powered TiO2/Pt Janus Micromotors for Enhanced Nitroaromatic Explosives Degradation

Nitroaromatic explosives such as 2,4,6-trinitrotoluene (2,4,6-TNT) and 2,4-dinitrotoluene (2,4-DNT) are two common nitroaromatic compounds in ammunition. Their leakage leads to serious environmental pollution and threatens human health. It is important to remove or decompose them rapidly and efficiently. In this work, we present that light-powered TiO₂/Pt Janus micromotors have high efficiency for the "on-the-fly" photocatalytic degradation of 2,4-DNT and 2,4,6-TNT in pure water under UV irradiation. The redox reactions, induced by photogenerated holes and electrons on the TiO₂/Pt Janus micromotor surfaces, produce a local electric field that propels the micromotors as well as oxidative species that are able to photodegrade 2,4-DNT and 2,4,6-TNT. Furthermore, the moving TiO₂/Pt Janus micromotors show an efficient degradation of nitroaromatic compounds as compared to the stationary ones thanks to the enhanced mixing and mass transfer in the solution by movement of these micromotors. Such fuel-free light-powered micromotors for explosive degradation are expected to find a way to environmental remediation and security applications.

Bicomponent β-sheet assembly of dipeptide fluorophores of opposite polarity and sensitive detection of nitro-explosives

Fluorescent hydrogels formed by the bicomponent β-sheet co-assembly of dipeptide–pyrene amphiphiles of opposite polarity provide a 3D microenvironment to detect toxic nitro-explosives.

Recent progress on graphene-based substrates for surface-enhanced Raman scattering applications

Graphene-based SERS substrates are classified and introduced, and their applications in biosensing-related fields are reviewed.

Sensitive and specific detection of explosives in solution and vapour by surface-enhanced Raman spectroscopy on silver nanocubes

Surface-enhanced Raman spectroscopy (SERS) has been widely utilised as a sensitive analytical technique for the detection of trace levels of organic molecules. The detection of organic compounds in the gas phase is particularly challenging due to the low concentration of adsorbed molecules on the surface of the SERS substrate. This is particularly the case for explosive materials, which typically have very low vapour pressures, limiting the use of SERS for their identification. In this work, silver nanocubes (AgNCs) were developed as a highly sensitive SERS substrate with very low limit-of-detection (LOD) for explosive materials down to the femtomolar (10^-15 M) range. Unlike typical gold-based nanostructures, the AgNCs were found suitable for the detection of both aromatic and aliphatic explosives, enabling detection with high specificity at low concentration. SERS studies were first carried out using a model analyte, Rhodamine-6G (Rh-6G), as a probe molecule. The SERS enhancement factor was estimated as 8.71 × 10¹⁰ in this case. Further studies involved femtomolar concentrations of 2,4-dinitrotoluene (DNT) and nanomolar concentrations of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), as well as vapour phase detection of DNT.

Sensitive chemiluminescence determination method for 2,4,6-trinitrotoluene based on the catalytic activity of amine-capped gold nanoparticles

TNT can efficiently quench the high intensity CL emission of a rhodamine B–KMnO₄–EDA capped AuNP CL system.

Graphene based nanosensor for aqueous phase detection of nitroaromatics

Nitroaromatics sensor composed of monolayer graphene and molecularly imprinted chitosan thin film was fabricated and responded selectively against imprinted nitrotriazolone.

Highly selective detection of nitrotoluene based on novel lanthanide-containing ionic liquids

Two novel rare-earth ionic liquids demonstrate high selectivity toward nitrotoluene in the presence of other aromatic compounds.

Silver Nanoparticle-Embedded Thin Silica-Coated Graphene Oxide as an SERS Substrate

A hybrid of Ag nanoparticle (NP)-embedded thin silica-coated graphene oxide (GO@SiO₂@Ag NPs) was prepared as a surface-enhanced Raman scattering (SERS) substrate. A 6 nm layer of silica was successfully coated on the surface of GO by the physical adsorption of sodium silicate, followed by the hydrolysis of 3-mercaptopropyl trimethoxysilane. Ag NPs were introduced onto the thin silica-coated graphene oxide by the reduction of Ag⁺ to prepare GO@SiO₂@Ag NPs. The GO@SiO₂@Ag NPs exhibited a 1.8-fold enhanced Raman signal compared to GO without a silica coating. The GO@SiO₂@Ag NPs showed a detection limit of 4-mercaptobenzoic acid (4-MBA) at 0.74 μM.