Diaminocyclohexane-Functionalized/Thioglycolic Acid-Modified Gold Nanoparticle-Based Colorimetric Sensing of Trinitrotoluene and Tetryl

MOFs as versatile scaffolds to explore environmental contaminants based on their luminescence bustle

Metal-Organic Frameworks (MOFs) with luminescent properties hold significant promise for environmental remediation. This review critically examines recent research on these materials design, synthesis, and applications, mainly focusing on their role in combating environmental pollutants. Through a comprehensive analysis of metal ions, ligands, and framework compositions, the review discusses the importance of tailored design and synthesis approaches in achieving desired luminescent characteristics. Key findings highlight the effectiveness of luminous MOFs as fluorescent sensors for a wide range of contaminants, including heavy metals, reactive species, antibiotics, and explosives. Considering all this, the review discusses future research needs and opportunities in the field of luminous MOFs. It emphasizes the importance of developing multifunctional materials, refining design methodologies, exploring sensing mechanisms, and ensuring environmental compatibility, scalability, and affordability. By providing insights into the current state of research and outlining future directions, this review is a valuable resource for researchers seeking to address environmental challenges using MOF-based solutions.

Chemical reduction as a facile colorimetric approach for selective TNT detection by spectrophotometry and photothermal lens spectroscopy

In this study, the simple determination of TNT is achieved through the vivid stable red color products generated after chemically reduction by NaBH₄ as a common and accessible reducing/colorimetric reagent. Some other nitroaromatics were impressed under reduction reaction and led to the colorful products. The color of these reduced nitroaromatics were unstable and approximately vanished after some few minutes which ameliorated the selectivity in TNT determination. Utilizing the time-dependent selectivity, the method was applied specifically for discriminating of TNT from other nitroaromatic compounds (NACs). UV-vis spectrophotometry and photothermal lens spectrometry were employed as detection techniques. The former was simpler and more available in various laboratories while the latter provides higher sensitivity. It was revealed that the photothermal lens responses were linear from 2.0 to 55.0 nM with a limit of detection (LOD) of about 0.8 nM. The LOD of the photothermal lens measurement were found to be 241 times lower than that of the UV-vis spectrophotometry in TNT quantification. The evolved method was successfully carried out for TNT vapor determination after trapping into the colorimetric reagent. The recoveries and relative standard deviations (RSD, n = 3) calculated for 3 gas samples were ≥91% and ≤7%, respectively.

Visual colorimetric sensor for nitroguanidine detection based on hydrogen bonding-induced aggregation of uric acid-functionalized gold nanoparticles

A colorimetric assay is proposed for the quantification of nitroguanidine (NQ), based on triggering the aggregation of uric acid-modified gold nanoparticles (AuNPs@UA) by intermolecular hydrogen bonding interaction between uric acid (UA) and NQ. The red-to-purplish blue (lavender) color change of AuNPs@UA with increasing NQ concentrations could be perceived with the naked eye or detected by UV-vis spectrophotometry. The absorbance versus concentration correlation gave a linear calibration curve in the range of 0.6-3.2 mg L^-1 NQ, with a correlation coefficient of 0.9995. The detection limit of the developed method was 0.063 mg L^-1, lower than those of noble metal aggregation methods in the literature. The synthesized and modified AuNPs were characterized using UV-vis spectrophotometry, scanning transmission electron microscopy (STEM), dynamic light scattering (DLS), and Fourier transform infrared spectroscopy (FTIR). Some critical parameters such as modification conditions of AuNPs, UA concentration, solvent environment, pH, and reaction time were optimized for the proposed method. The non-interference of common explosives (i.e., nitroaromatic, nitramine, nitrate ester, insensitive and inorganic explosives), common soil and groundwater ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Cl^-, NO₃^-, SO₄^2-, CO₃^2-, PO₄^3-) and possible interfering compounds (used as camouflage agents for explosives; D-(+)-glucose, sweeteners, acetylsalicylic acid (aspirin), household powder detergents, and paracetamol) on the proposed method was demonstrated, proving that the procedure was fairly selective for NQ, due to special hydrogen bonding interactions between UA-functionalized AuNPs and NQ. Finally, the proposed spectrophotometric method was applied to NQ-contaminated soil, and the obtained results were statistically compared with those of the liquid chromatography-tandem mass spectrometric (LC-MS/MS) method in the literature.

Using a dual-emission Sm(iii)-macrocycle as the perceptive lab-on-a-molecule chemosensor toward selective and discriminative detection of nitroaromatic explosives

A dual-emission Sm(iii)-macrocycle Sm-2l is designed as the perceptive lab-on-a-molecule toward selective and discriminative detection of nitroaromatic explosives by statistical analysis.

Reduced graphene oxide: Biofabrication and environmental applications

Green synthesis of high-quality reduced graphene oxide (rGO) from agro-industrial waste resources remains attractive owing to its outstanding environmental benefits. The remarkable properties of rGO include excellent morphology, uniform particle size, good optical properties, high conductivity, nontoxicity, and extraordinary chemical stability. Traditional methods for the synthesis of rGO nanomaterials involve several chemical reactions including oxidation, carbonization, toxic solvent, and pyrolysis which produce harmful byproducts. Green preparation of rGO is an emerging area of research in graphene technology which is cost-effective and sustainable in the procedure. Owing to the uniform particle rGO particle size, these smart nanomaterials have wide applicability, including in metal ions and pollutant sensing and adsorption, photocatalysis, optoelectrical devices, medical diagnosis, and drug delivery. Here we review the physicochemical properties of rGO, the biowaste sources and green methods of rGO synthesis, and the diverse applications of rGO, including in water purification and the biomedical fields. With this review, covering more than 200 research articles published on rGO in the last eight years ending in 2022, we aim to provide a quick guide for researchers seeking up-to-date information on the properties, production, and applicability of rGO, with special attention to rGO applications in water purification and the biomedical fields.

Recent advancements in Janus nanoparticle-based biosensing platforms

Nanoparticles have aided in the development of nano-based sensors for diagnostic applications. However, use of nanoparticles in the development of sensing devices for multiple analyte detection is constrained due to their inability to detect several analytes with a single type of nanoparticle. The term "Janus particle" refers to micro or nanoscale particles that have been divided into sections or compartments, each of which has a distinct set of chemical or physical properties, producing multifunctional particles endowed with distinctive qualities. Furthermore, Janus particles have the ability to perform multiple functions within a single particle at the same time, with no interference from adjacent sections. This review focuses on the use of Janus particles in the fabrication of biosensors as well as in the investigation of various properties endowed by these Janus particles for their use as biosensors. It also discusses the various types of Janus particle-based biosensors that are currently available. Finally, the limitations of Janus particles in sensor technologies and their future scope have been discussed.

Graphical abstract

Protamine gold nanoclusters - based fluorescence turn-on sensor for rapid determination of Trinitrotoluene (TNT)

Determination of trace residues of 2,4,6-trinitrotoluene (TNT) is an analytical challenge as it is widely used in military, mining industry, civilian and counter-terrorism purposes. In this study, a gold nanocluster - based turn-on fluorescence sensor was developed for TNT determination. A one-pot approach was used to synthesize the fluorescent protamine - stabilized gold nanoclusters (PRT-AuNC). The proposed turn-on fluorometric sensor relies on the aggregation-induced emission enhancement mechanism. As a result of the donor-acceptor interaction between the non-fluorescent Meisenheimer anion formed from TNT and the amino groups of weakly fluorescent protamine, the PRT-AuNCs aggregate and an accompanying enhancement in fluorescence intensity is observed with a large Stokes shift (λ_ex = 300 nm, λ_em = 600 nm). The fluorescence enhancement increased linearly with TNT with an LOD of 12.44 µg/L. Similar energetic materials, common soil ions and explosive camouflage materials did not affect the proposed fluorometric sensing method. TNT in artificially contaminated soil was determined, and the results were comparable to those obtained by the HPLC-DAD system. The proposed turn-on sensor is an important tool for simple, fast, rapid and sensitive TNT determination, and has a potential to be converted to a kit format.

A luminescent Zn-MOF for the detection of explosives and development of fingerprints

A luminescent 3D metal-organic framework [Zn(NDA)(AMP)] = PUC1 (where, NDA = naphthalene-2,6-dicarboxylic acid and AMP = 4-aminomethyl pyridine) was synthesized under solvothermal conditions. The synthesized 3D framework was fully characterized with the help of different analytical techniques such as SCXRD, FTIR, TGA, PXRD, SEM, BET, etc. PUC1 exhibited a strong emission peak at 371 nm when excited at 290 nm and the resulting emission was efficiently quenched in the presence of various organic explosive substances like pentaerythritol tetranitrate (PETN), 2,4,6-trinitrophenyl-N-methylnitramine (Tetryl), trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX). PUC1 revealed highly sensitive and selective detection of PETN and Tetryl with high quenching constant values of 0.1 × 10⁶ and 0.12 × 10⁵ M^-1 and low detection limits of 0.315 and 0.404 μM respectively. The strong luminescent properties of PUC1 lead to its successful application in the development of latent fingermarks on different non-porous surfaces using the powder dusting method. The accuracy and applicability of the synthesized material were determined by developing fingerprints by using secretions from eccrine and apocrine glands on a glass slide and various other surfaces, followed by dusting the surfaces. The results so obtained were found to be very accurate and promising.

Mercury ion-engineering Au plasmonics on MoS2 layers for absorption-shifted optical sensors

Semiconducting MoS₂ layers offer the electrons, reducing conjugated Au(I) to Au atoms, and sebsequently serve as desirable substrates for supporting the interfacial growths of gold nanostructures. Au-covering MoS₂ heterostructures perform morphology-varied optical characteristics, and the surface engineering of MoS₂ involved by Hg²⁺ ions results in the differential growths of nanostructures and morphological diversities. Naked-eye colorimetric responses to mercury ions, with a low limit of detection of 1.27 nM, are achieved based on the in situ grown heterostructures.

Colorimetric optical nanosensors for trace explosive detection using metal nanoparticles: advances, pitfalls, and future perspective

Warfare threats and acts of terror are challenging situations encountered by defense agencies across the globe and are of growing concern to the general public, and security-minded policy makers. Detecting ultra-low quantities of explosive compounds in remote locations or under harsh conditions for anti-terror purposes as well as the environmental monitoring of residual or discarded explosives in soil, remains a major challenge. The use of metal nanoparticles (NPs) for trace explosive detection has drawn considerable interest in recent years. For nano-based explosive sensor devices to meet real-life operational demands, analytical parameters such as, long-shelf life, stability under harsh conditions, ease-of-use, high sensitivity, excellent selectivity, and rapid signal response must be met. Generally, the analytical performance of colorimetric-based nanosensor systems is strongly dependent on the surface properties of the nanomaterial used in the colorimetric assay. The size and shape properties of metal NPs, surface functionalisation efficiency, and assay fabrication methods, are factors that influence the efficacy of colorimetric explosive nanosensor systems. This review reports on the design and analytical performances of colorimetric explosive sensor systems using metal NPs as optical signal transducers. The challenges of trace explosive detection, advances in metal NP colorimetric explosive design, limitations of each methods, and possible strategies to mitigate the problems are discussed.

Ethylenediamine-bound magnetite nanoparticles as dual function colorimetric sensor having charge transfer and nanozyme activity for TNT and tetryl detection

A reusable, low-cost, and convenient ethylenediamine (EDA)-bound magnetite nanoparticles (MNPs)-based colorimetric sensor has been developed for dual function colorimetric determination of nitroaromatic explosives such as TNT and tetryl. Colorimetric detection of analytes may occur through two independent routes: (1) nano-Fe₃O₄- EDA- NH₂ as σ-donor may interact with the σ- and π-acceptor aromatic-poly(NO₂) groups to produce a colored charge-transfer (CT) complex; (2) nano-Fe₃O₄-EDA-NH₂ as a Fenton-type nanozyme may generate reactive species that comprise hydroxyl radicals (^•OH) with H₂O₂ to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to a blue-colored diimine (oxTMB-TMB) CT complex, where this color is bleached with TNT/tetryl because of donor-acceptor interactions between the explosive -NO₂ groups and the -NH₂ group of Fe₃O₄-EDA nanoparticles of restricted nanozyme activity. Both methods can quantify TNT well below the EPA recommended TNT residential screening level in soil, LOD being in the micromolar range. As EDA was covalently bound to MNPs, the same sensor can be separately reused six times for TNT and eight times for tetryl determination, using method (1). Common metal ions, anions, energetic materials, several camouflage materials, and soil components such as humates did not interfere with the nanosensor performance for TNT and tetryl. The combination of charge-transfer and nanozyme ability of Fe₃O₄- EDA-NH₂ nanoparticles may bring a new approach to dual function colorimetric sensor design. To the best of our knowledge, this is the first dual function colorimetric sensor for TNT and tetryl using the same nanoparticles as sensing elements in two different detection systems involving either formation or bleaching of colored species. The proposed colorimetric sensor can determine nitroaromatic explosives in two different ways: method-1 for TNT and tetryl sensing with EDA-MNPs relies on the donor-acceptor interaction between the electron-deficient nitroaromatics and electron-rich amine groups covalently functionalized on MNPs to produce an absorbance at 512 nm. In method-2, EDA-MNPs having nanozyme activity react with H₂O₂ to form reactive species that can oxidize TMB to its blue-colored charge-transfer (CT) complex, where TNT and tetryl addition may partially inhibit the nanozyme activity of EDA-MNPs and cause color bleaching (decrement of 650 nm absorbance) by disrupting the CT complex formed from TMB. This is the first dual function colorimetric sensor for nitro explosives uniquely combining charge-transfer and nanozyme ability of EDA-Fe₃O₄ nanoparticles in the same nano-sensor.

Colorimetric sensors and nanoprobes for characterizing antioxidant and energetic substances

The development of analytical techniques for antioxidant compounds is important, because antioxidants that can inactivate reactive species and radicals are health-beneficial compounds, also used in the preservation of food and protection of almost every kind of organic substance from oxidation. Energetic substances include explosives, pyrotechnics, propellants and fuels, and their determination at bulk/trace levels is important for the safety and well-being of modern societies exposed to various security threats. Most of the time, in field/on site detection of these important analytes necessitates the use of colorimetric sensors and probes enabling naked-eye detection, or low-cost and easy-to-use fluorometric sensors. The use of nanosensors brings important advantages to this field of analytical chemistry due to their various physico-chemical advantages of increased surface area, surface plasmon resonance absorption of noble metal nanoparticles, and superior enzyme-mimic catalytic properties. Thus, this critical review focuses on the design strategies for colorimetric sensors and nanoprobes in characterizing antioxidant and energetic substances. In this regard, the main themes and properties in optical sensor design are defined and classified. Nanomaterial-based optical sensors/probes are discussed with respect to their mechanisms of operation, namely formation and growth of noble metal nanoparticles, their aggregation and disaggregation, displacement of active constituents by complexation or electrostatic interaction, miscellaneous mechanisms, and the choice of metallic oxide nanoparticles taking part in such formulations.

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.

Visual determination of azodicarbonamide in flour by label-free silver nanoparticle colorimetry

A very practical and competitive sensing strategy for the detection of azodicarbonamide in flour samples was developed by using label-free Ag NPs as a colorimetric probe. Well-dispersed Ag NPs in suspension can form aggregates upon reacting with glutathione (GSH) via Ag-SH covalent bonds and electrostatic attraction, with the color changing from bright yellow to red. However, azodicarbonamide can oxidize the -SH of GSH, preventing the aggregation of Ag NPs. Under the optimum conditions, the A₅₅₀/A₃₉₈ of Ag NPs is linearly related to the concentration of azodicarbonamide in the range of 0.33 μM to 1.7 μM. The proposed method can be used for the detection of azodicarbonamide in flour, with a detection limit of 0.09 μM and recovery between 95% and 97.4% (RSD < 6%). When the azodicarbonamide concentration reaches 0.33 μM, the color change can be detected by the naked eye.

Carbon Dots for Forensic Applications: A Critical Review

Owing to their superior fluorescence performance, inexpensive synthesis and nontoxic nature, carbon dots (C-dots) are systematically explored in a variety of applications; in this review, we outline and critically discuss recent trends with respect to their potential exploitation in criminal investigation, forensic toxicology and anti-counterfeit interventions. Capitalising on their colour-tuneable behaviour (in the sense that they adopt different colours with respect to the incident radiation), C-dot-based compositions are ideal for the visual enhancement of latent fingerprints, affording improved contrast against multicoloured and patterned backgrounds. As highly sensitive and highly selective optical nanoprobes, C-dots show excellent analytical performance in detecting biological compounds, drugs, explosives, heavy metals and poisonous reactants. In addition, benefiting from their versatile structural and chemical composition, C-dots can be incorporated into ink and polymeric formulations capable of functioning as a new generation of cost-effective barcodes and security nanotags for object authentication and anti-counterfeit applications. Translating these encouraging research outcomes into real-life innovations with significant social and economic impact requires an open, multidisciplinary approach and a close synergy between materials scientists, biologists, forensic investigators and digital engineers.

Interpol review of detection and characterization of explosives and explosives residues 2016-2019

This review paper covers the forensic-relevant literature for the analysis and detection of explosives and explosives residues from 2016-2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/Resources/Documents#Publications.

Urea-Functionalized Poly(ionic liquid) Photonic Spheres for Visual Identification of Explosives with a Smartphone

Current effort merging rational design of colorimetric sensor array with portable and easy-to-use hand-held readers delivers an effective and convenient method for on-site detection and discrimination of explosives. However, on the one hand, there are rare relevant reports; on the other hand, some limitations regarding direct sensing, color retention, and array extendibility still remain. Herein, urea-functionalized poly(ionic liquid) photonic spheres were employed to construct a brand-new colorimetric sensor array for directly identifying five nitroaromatic explosives with a smartphone. It is found that the strong hydrogen bonding between the urea motifs and the nitro groups offers the spheres high affinity for binding the targets, whereas the existence of other abundant intermolecular interactions in poly(ionic liquid) units renders one single sphere eligible for prominent cross-responses to a broad range of analytes. Besides, in our case, opal-like photonic crystal structures other than chemical dyes are used to fabricate a new style of colorimetric array. Such structural colors can be vivid and unchanged over a long period even in hazard environments. Importantly, through simply altering the preparation conditions of our PIL spheres, a pool of sensing elements could be added to the developed array for discrimination of extended target systems such as more explosives and even their mixtures in real-world context.

Mechanistic and thermodynamic aspects of a pyrene-based fluorescent probe to detect picric acid

Thermodynamic investigation of picric acid sensing using a pyrene-based fluorescent probe.