Detection Of Nitroaromatic Compounds Research Articles

A Novel Composite of Zinc-based Metal Organic Framework Embedded with SnO2 Nanoparticle as a Photocatalyst for Methylene Blue Dye Degradation as well as Fluorometric Probe for Nitroaromatic Compounds Detection.

A facile bottom up synthesis technique is opted for the preparation of novel composite SnO2@Zn-BTC. This synthesized composite is fully characterized by Fourier Transform Infrared (FTIR) Spectroscopy, Powder X-Ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and Elemental mapping techniques. Optical analysis was performed using UV-Visible absorption spectroscopy and fluorescence studies. Further this composite was utilized for the first time as a photocatalyst for methylene blue (MB) dye degradation under sunlight irradiation. This photocatalyst shows degradation efficiency of 89% within 100min of exposure of sunlight. In addition to that, the synthesized composite can be utilized as a fluorescence probe for detection of NACs via 'turn-off" quenching response. This composite is extremely sensitive towards 3-NA in aqueous medium with quenching efficiency of 75.42%, which is highest quenching rate till reported. There occurs no interference for detecting 3-NA in the presence of other NACs. The linear fitting of the Stern-Volmer plot for 3-NA shows large quenching constant (KSV) of 0.0115ppb-1 with correlation coefficient R2 = 0.9943 proves higher sensitivity of composite in sensing process. The outstanding sensitivity of composite for 3-NA is certified by the low detection limit (LOD) of 25ppb (0.18µM). Photoinduced Electron Transfer (PET)and Fluorescence Resonance Energy Transfer (FRET) are the mechanisms used for clarification of quenching response of PL intensity by NACs via density functional theory (DFT) calculations and extent spectral overlap, respectively. Hence, synthesized composite is verified as multi-component system to act as excellent photocatalyst as well as fluorescent sensor.

Fluorescent porous organic polymers for detection and adsorption of nitroaromatic compounds

A fluorescent porous organic polymer (FPOP) with strong fluorescence and tunable emission colors, was synthesized through a simple cost-effective method via Scholl coupling reaction. Experiments proved the stability and excellent detection and adsorption ability, and microporous nature of the material. Luminescence of FPOP was quenched when addition of nitroaromatic compounds. The properties along with large-scale and low-cost preparation make these FPOP potential candidates for fluorescence detection of nitroaromatic compounds. Additionally, FPOP shows higher adsorption capacity and rate than other reported adsorbents, and has the possibility of being an effective adsorbent for industrial usage. Moreover, a fluorescent test paper was further developed and is found to be sensitive to 10–8 M level, complete with a rapid response time and visual detection. This newly developed strategy may open up an avenue for exploring porous polymers, particularly those with a strong fluorescence, for the large-scale fabrication of FPOP for various advanced applications.

Extremely Stable Thorium‐MOF Assembly of Tetraphenylethylene Derivative With Tunable AIE Property and Highly Selective Detection of Nitro Aromatic Compounds

AbstractThorium‐based metal–organic frameworks (MOFs) are unique, which are substantially less reported compared to the MOFs based on transition metal and rare earth elements. Here, a thorium‐MOF (Th‐TCBPE) based on 4',4'',4''',4''''‐(ethene‐1,1,2,2‐tetrayl)tetrakis([1,1'‐biphenyl]‐4‐carboxylic acid) (H4TCBPE) and thorium nitrate is synthesized first. Th‐TCBPE is an interpenetrated 2D network, where Th (IV) is ten‐coordinated with a bicapped dodecahedron coordination geometry. Further studies about the structure by time‐resolved emission spectra (TRES) analysis are also discussed. What's more, Th‐TCBPE has excellent thermal stability and good tunable AIE property on concentration in THF. The photophysical properties of Th‐TCBPE in different solvents, different concentrations in THF, and different ratios of THF/H2O are studied. The ability of Th‐TCBPE to detect nitro aromatic compounds is also investigated. It is also believed that this work can provide theoretical guidance and inspiration for the development of Th‐MOF and MOF with AIEgens in the future.

Aggregation Induced Emission (AIE)‐Active Poly(acrylates) for Electrofluorochromic Detection of Nitroaromatic Compounds

AbstractTwo aggregation induced emission (AIE)‐active poly(acrylates) bearing triphenylvinyl‐functionalized phenylcarbazole and triphenylamine pendants, respectively, were synthesized. Their fluorescence response to nitroaromatics and electrofluorochromic (EFC) properties were studied, whereby both the presence of nitroaromatics and applied voltages led to fluorescence quenching. In a very first attempt to perform EFC detection of nitroaromatics, it was surprisingly found that after initial fluorescence quenching, the presence of nitroaromatics unexpectedly turned on fluorescence at high applied voltages (>+1.0 V), demonstrating a promising approach for the EFC detection of nitroaromatic explosives.

Investigation of Chemosensing and Color Properties of Schiff Base Compounds Containing a 1,2,3-triazole Group.

A series of Schiff base compounds (ER1-ER5) containing a 1,2,3-triazole and carboxylic acid groups were synthesized and their chemosensory properties towards anions (I-, CO32-, SO42-, NO2-, NO3-, CH3COO-, ClO3-, CNO-, N3-) and cations (Al3+, Ag+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, Mn2+, Ni2+, Zn2+, Cd2+, Pb2+). The compounds were also used as fluorescence probs for the detection of nitroaromatic compounds. The structural characterization of the synthesized compounds was elucidated using methods such as FT-IR, UV, FL, LC-MS, MALDI-TOF MS, 1H(13C) NMR. The effect of substitute groups (-CH3, -OCH3, -OH, -Cl and -Br) on the synthesized Schiff bases (ER1-ER5) on the chemosensory properties were compared. As the groups changed, the sensor and quenching effects of the molecule against anions and cations changed. Compound ER3 having methoxy (OCH3) group exhibited selective sensor properties against Fe3+ ion while compound ER5 with a chloride substitute (Cl) group showed selectivity for Cr3+ ion under 254 nm UV-lamp. The substitute effect was also observed for the sensing of anions. Under 254 nm UV-lamp, ER2 having the -OH group has a selective sensing property for CNO- and ER4 with the bromide (Br) group exhibited selectivity for N3- ion. The synthesized Schiff base compounds were also tested as fluorescence probs for the sensing of some nitroaromatic explosives.

A Complementary Silicon Quantum Dot-Enzyme Platform for the Selective Detection of Nitroaromatic Compounds: Explosives versus Nerve Agents

A Complementary Silicon Quantum Dot-Enzyme Platform for the Selective Detection of Nitroaromatic Compounds: Explosives versus Nerve Agents

Oxacalix[4]arene based dual-signalling fluorimetric and electrochemical chemosensor for the selective detection of nitroaromatic compounds

A new Bi-naphthylated oxacalix[4]arene viz. BNOC has been designed and synthesized as a potential fluorescence probe for selective recognition of 4-NT and 2,3-DNT compared to the other nitroaromatic compounds due to the geometry and hydrophobicity of the modified cavity. The interaction of BNOC with 4-NT and 2,3-DNT is confirmed by ESI-MS study. A highly sensitive dual-signaling electrochemical detection based on host–guest interaction between BNOC as signal probe and 4-NT, 2,3-DNT as target molecules was developed. The LOD for 4-NT and 2,3-DNT was determined to be 34.1 nM and 36.7 nM respectively. The concentration of 2,3-DNT and 4-NT in the water sample was determined using a standard addition procedure. Computational calculation further indicates that strong π-π interaction between host BNOC and guest NACs executes selective sensing of 4-NT and 2,3DNT.

2H–MoS2/Co3O4 nanohybrid with type I nitroreductase-mimicking activity for the electrochemical assays of nitroaromatic compounds

A type I nitroreductase-mimicking nanocatalyst based on 2H–MoS2/Co3O4 nanohybrids for trace nitroaromatic compounds detection is reported in this work. For the preparation of nanocatalyst, ultrathin Co3O4 nanoflakes array was in-situ grown onto 2H–MoS2 nanosheets forming three-dimensional (3D) nanohybrid with large specific surface area as well as abundant active sites. The as-prepared nanocatalyst shows a specific affinity as well as high catalytic activity towards nitroaromatic compounds. Given the favorable nitroreductase-mimicking catalytic activity of 2H–MoS2/Co3O4 nanohybrid, a sensitive and efficient electrochemical microsensor has been constructed for the detection of 2, 4, 6-trinitrotoluene (TNT). Under optimized conditions, the microsensor displayed sensitive response from μM to pM levels with a limit of detection (LOD) of 1 pM. We further employed photoelectron spectroscopy (XPS) analysis and high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) method to identify the nitroreductase-mimicking mechanism of 2H–MoS2/Co3O4 nanohybrids towards 2, 4, 6- TNT. It was found that the abundant oxygen vacancies in ultrathin Co3O4 nanoflakes played an essential role in determining its catalytic performance. Moreover, the developed MoS2/Co3O4 nanozyme has a lower Michaelis-Menten constant (km) than that of nature nitroreductase demonstrating a good enzymatic affinity towards its substrates, and further generating a high catalytic activity. This research not only proposed a new type of nanozyme, but also developed a portable electrochemical microsensor for the detection of 2, 4, 6-TNT.

3,6‐Diaminocarbazole doped fluorescent electrospun nanofibers for highly sensitive detection of nitroaromatics

AbstractElectrospinning is an efficient technique to fabricate nanofibers of controlled diameter and uniform morphology. Herein, we report a low‐cost and high‐yield route to fabricate PAN/DAC composite nanofibers and its use for vapor sensing of 2,4‐Dinitrotoluene (DNT) and 2,4,6‐Trinitrotoluene (TNT) with a detection level in the range of parts‐per‐billion. Furthermore, the sensing ability and photo‐induced electron transfer mechanism of DAC towards DNT and TNT in the solution phase were also investigated in detail via absorption, steady‐state, and time‐resolved fluorescence spectroscopy and further supported by density function theory (DFT). The calculated Stern‐Volmer quenching constants, 810 M−1 (DNT) and 1170 M−1 (TNT), revealed that DAC is not much sensitive in solution‐phase because of the self‐condensation phenomenon of DAC molecules. This concern was addressed by the development of a fluorescent nanofiber probe constituting π‐electron‐rich carbazole (CZ) derivatives, namely 3,6‐Diaminocarbazole (DAC) as a fluorescent material and polyacrylonitrile (PAN) as a support polymatrix. This fluorophore‐doped nanofiber matrix was fabricated at a relatively lower wt% ratio (PAN:DAC::10:6), obtaining an average diameter of 857 nm and exhibited promising features in the vapor detection of DNT and TNT such as fast response time, excellent sensitivity and selectivity. Reduction of self‐condensation caused DAC molecules' fluorescence self‐quenching by simply employing the electrospinning technique to cast PAN/DAC nanofibrous film. This work promises a new aspect of sensitivity enhancement of carbazole molecular unit by amine modification and further incorporation into the polymer matrix. This nanofiber‐based sensor can lead to the design and development of highly efficient and field‐deployable vapor sensors for the detection of nitroaromatic compounds, with application in both explosive sensing and environment pollution.

One step synthesis of crosslinked fluorescent microspheres for the effective and selective sensing of explosives in aqueous media

Herein, new pyrene-based polymeric microbeads were simply synthesized in one-step as fluorescence chemosensor for high precision detection of nitroaromatic compounds (NACs) such as 2,4,6-trinitro toluene (TNT), 2,4-dinitro toluene (DNT) and 2,4,6-trinitrophenol (picric acid, PA). Firstly, nonhydrolyzable 4-(1-pyrenyl)-styrene (PySt) monomer, in which the pyrene units are directly attached to styrene group, was synthesized by Suzuki Coupling reaction and polymerized in the presence of ethylene glycol dimethacrylate (EGDMA) as crosslinker by distillation precipitation polymerization. Highly crosslinked almost monodisperse poly(4-(1-pyrenyl)-styrene-co-ethyleneglycoldimethacrylate), P(PySt-EGDMA) microspheres with 850–1000 nm particle size distribution were successfully obtained by an effortless and very quick -one hour- polymerization reaction. One of the most important advantages of the microbeads with this particle size range was that they give homogeneous and stable dispersions in both aqueous and organic media, which facilitates detection studies. Detection studies and quenching activities of these microbeads against NACs were investigated under different solvent conditions such as water, acetonitrile, methanol and tetrahydrofuran. During the fluorescence measurements, each dispersion of P(PySt-EGDMA) microbeads showed very strong fluorescence activity in their bare form, even at very low concentrations (6.67 µg/ml solvent). Moreover, among all suspension, the responses in aqueous media against NACs demonstrated the best fluorescence quenching activities. When the responses of the aqueous dispersion to NACs are compared over the Ksv values, the highest quenching ability of the microbeads was found against DNT (Ksv: 1.4 × 106M−1). At the same time, DNT could be detected at the nanomolar level with the lowest calculated limit of detection (LOD) value, 1.39 ppb. All these facts demonstrate that the aqueous dispersion of P(PySt-EGDMA) microbeads is highly selective and sensitive to DNT.

Naphthalimide‐Hydroxyquinoline Conjugates for Discriminative Detection of Nitro Aromatic Compounds in Aqueous Medium and Soil Sample

AbstractNaphthalimide‐hydroxyquinoline based conjugates (NQ1 and NQ2) were synthesized for the detection of nitroaromatics in HEPES buffer/DMSO (9/1, v/v, pH=7.4) and Fe3+/Pd2+ in CH3CN solvent. The NQ1 and NQ2 were characterized using NMR (1H, 13C, COSY) and IR spectroscopies. Theoretical studies revealed that 8‐HQ moiety in NQ1 and NQ2 placed at orthogonal position with respect to naphthalimide core to minimize steric crowding. The energy gap between HOMO and LUMO for NQ1 and NQ2 were found to be −3.1 eV. The emission spectrum of NQ1 and NQ2 showed decrease in the emission intensity at 450 nm upon addition of picric acid (PA) and trinitrotoluene (TNT) whereas addition of 2‐nitroaniline caused quenching with concomitant red‐shift of the emission band in 90 % HEPES buffer‐DMSO. The detection limit of NQ1 for PA, TNT and 2‐NA detection was calculated as 8.7×10−6 M, 2.2×10−5 M and 1.8×10−5 M. The complexation of NQ1 with PA, TNT and 2‐NA was further supported by Job's plot, dynamic light scattering studies and NMR titrations. We successfully utilized NQ1 and NQ2 for the detection of PA, TNT and 2‐NA in real soil samples with excellent recovery. NQ1 also showed quenching of the emission intensity at 450 nm upon addition of Fe3+ and Pd2+ ions in CH3CN. Thus, NQ1 showed solvent‐dependent dual chemosensor type behavior. The functioning of NOR gate has been achieved using Fe3+/NACs with NQ1.

Comparison of aggregation-induced emission enhancement effect between π-σ* and σ-σ* conjugation on silole and application in explosives detection

Herein we have synthesized and characterized the novel π-σ* conjugated 1,4-bis(1-methyl-2,3,4,5-tetraphenyl-1-silacyclopentadienyl)benzene nanoaggregates based on aggregation-induced emission enhancement (AIEE) property for the detection of nitroaromatic compounds by fluorescence quenching. The new photoluminescent silole nanoaggregates exhibited unique AIEE property by increasing the emission intensity 180 times in THF/water mixtures. We investigated that the π-σ* hyper conjugation of Si–C phenyl ring is more effective on AIEE due to the less non-radiative decay than the σ-σ* hyper conjugation of Si–Si bond of 1,4-bis(1-methyl-2,3,4,5-tetraphenyl-1-silacyclopentadienyl). The π-σ* conjugated nanoaggregates showed excellent sensitivity for sensing explosives. High Stern-Volmer constant was observed during quenching of photoluminescence (PL) intensity upon adding explosives. The absolute quantum yield and average particle size were measured for the nanoaggregates and tuned by controlling at different fraction of water. These results demonstrate that AIEE property of π-σ* conjugated silole nanoaggregates will provide guidance to develop fluorescent materials for detecting explosives.

Detection of o‐nitro aniline by bovine serum albumin based self‐fluorescent hydrogel via FRET process

AbstractSelective and sensitive detection of nitroaromatic compounds is recent concern for both security purpose and pollution control. Nitroaromatics are either explosive in nature or used in preparation of explosives and they inherently carry the toxic property. Herein, a selective and rapid detection of ortho nitro aniline (o‐NA) by glutaraldehyde cross‐linked bovine serum albumin (BSA) protein functionalized auto fluorescent hydrogel (BG) in aqueous medium is reported. The hydrogel is characterized by Scanning electron microscope, rheology, Fourier transform infrared spectroscopy and dynamic light scattering. The change in conformation due to gelation of BSA is supported by FTIR with alternation of peak intensity and position, DLS with increased average particle size, UV–vis spectra having different kind of peaks after gelation and SEM image with porous morphology. A good linear relationship of fluorescence detection of o‐NA is observed in range of 0–3 × 10−4 (M) with the detection limit of 35 μM. The sensing procedure is performed by involving dried BG gel in aqueous medium. The possible quenching process is established via FRET mechanism between BG gel and o‐NA showing high selectivity and sensitivity towards the detection of o‐NA over the other nitroaromatics and common ions present in water.

Multi-responsive luminescent coordination polymer nanosheets for selective detection of nitroaromatics.

Sensitive sensing of nitroaromatic compounds (NACs) is realized by using luminescent lanthanum-tricarboxytriphenylamine (La-TCA) nanosheets fabricated by a top-down sonication assisted strategy. The accessible Lewis base sites and electron-rich fluorophores on the surface of the La-TCA nanosheets enable them to interact with electron-deficient NACs, delivering multi-responsive behaviours (emission intensity quenching, wavelength red-shift and valley) for hydroxyl group NACs.

Efficient Electropolymerization of Β-Cd on Pretreated Carboxylated Multiwalled Carbon Nanotubes/Gce to Improve Electrochemical Simultaneous Detection of Nitroaromatic Compounds

Efficient Electropolymerization of Β-Cd on Pretreated Carboxylated Multiwalled Carbon Nanotubes/Gce to Improve Electrochemical Simultaneous Detection of Nitroaromatic Compounds

Coordination Polymers as a Functional Material for the Selective Molecular Recognition of Nitroaromatics and ipso‐Hydroxylation of Arylboronic Acids

We report the synthesis and structural characterization of two coordination polymers (CPs), namely; [{Zn(L)(DMF)4 } ⋅ 2BF4 ]α (1) and [{Cd(L)2 (Cl)2 } ⋅ 2H2 O]α (2) (where L=N2 ,N6 -di(pyridin-4-yl)naphthalene-2,6-dicarboxamide). Crystal packing of 1 reveals the existence of channels running along the b- and c-axis filled by the ligated DMF and lattice anions, respectively. Whereas, crystal packing of 2 reveals that the metallacycles of each 1D chain are intercalating into the groove of adjacent metallacycles resulting in the stacking of 1D loop-chains to form a sheet-like architecture. In addition, both 1 and 2 were exploited as multifunctional materials for the detection of nitroaromatic compounds (NACs) as well as a catalyst in the ipso-hydroxylation of aryl/heteroarylboronic acids. Remarkably, 1 and 2 showed high fluorescence stability in an aqueous medium and displayed a maximum 88% and 97% quenching efficiency for 4-NPH, respectively among all the investigated NACs. The mechanistic investigation of NACs recognition suggested that the fluorescence quenching occurred via electron as well as energy transfer process. Furthermore, the ipso-hydroxylation of aryl/heteroarylboronic acids in presence of 1 and 2 gave up to 99% desired product yield within 15 min in our established protocol. In both cases, 1 and 2 are recyclable upto five cycles without any significant loss in their efficiency.

Rational Design and Facile Synthesis of Dual-State Emission Fluorophores: Expanding Functionality for the Sensitive Detection of Nitroaromatic Compounds.

Six novel benzimidazole-based D-π-A compounds 4 a-4 f were concisely synthesized by attaching different donor/acceptor units to the skeleton of 1,3-bis(1H-benzimidazol-2-yl)benzene on its 5-position through an ethynyl link. Due to the twisted conformation and effective conjugation structure, these dual-state emission (DSE) molecules show intense and multifarious photoluminescence, and their fluorescence quantum yields in solution and solid state can be up to 96.16 and 69.82 %, respectively. Especially, for excellent photostability, obvious solvatofluorochromic and extraordinary wide range of solvent compatibility, DSE molecule 4 a is a multifunctional fluorescent probe for the visual detection of nitroaromatic compounds (NACs) with the limit of detection as low as 10-7 M. The quenching mechanism has been proved as the results of photoinduced electron transfer and fluorescence resonance energy transfer processes. Importantly, probe 4 a can sensitively detect NACs not only in real water samples, but also on 4 a-coated strips and 4 a@PBAT thin films.

Synthesis and Tetraphenylethylene-Based Aggregation-Induced Emission Probe for Rapid Detection of Nitroaromatic Compounds in Aqueous Media.

Tetraphenylethylene (TPE) can be used to construct fluorescent probes with typical aggregation-induced emission (AIE) behavior for next-generation sensing applications. McMurry coupling and Suzuki cross coupling strategies provided the desired sensor thiophene-substituted tetraphenylethylene (THTPE). The synthesized TPE analogues were characterized by NMR spectroscopy and mass spectrometry. Maximum AIE of THTPE was observed in 90% water (H2O/THF) content due to extensive formation of aggregates. The AIE properties of THTPE have been utilized for facile detection of nitroaromatic compounds (NACs) (1.0 nM) through a fluorescence quenching mechanism. A paper strip adsorbed with the AIE-based THTPE fluorophore is developed for rapid and convenient detection of NAC-based analytes. Further, interaction of THTPE with analytes is also studied via Gaussian software at the DFT/B3LYP/6-31G(d) level of theory. Interaction energy, frontier molecular orbitals (FMOs), and non-covalent interaction (NCI) analyses are studied by using the same method. Computational results revealed that nitrobenzene (NB) has the strongest interaction while 1,3-dinitrobenzene (DNB) exhibits the least interaction with the sensor molecule. These computational results clearly demonstrate good agreement with experimental data.

Inter- and Intra-Hydrogen Bonding Strategy to Control the Fluorescence of Acylhydrazone-Based Conjugated Microporous Polymers and Their Application to Nitroaromatics Detection

Acylhydrazone-based fluorescent conjugated microporous polymers (CMPs) with inter-and intra-hydrogen bonding-controlled emissive properties were prepared. The synthesized CMPs (BH-CMP and ABH-CMP) were characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, solid-state 13C cross polarization/magic angle spinning nuclear magnetic resonance spectroscopy, and photoluminescence spectroscopy. Interestingly, BH-CMP exhibited emission enhancement via adsorption of water molecules, whereas the emission of ABH-CMP, which possesses free amine groups, decreased upon the addition of water molecules. The differences in the emission trends of BH-CMP and ABH-CMP in the presence of water molecules originate from the formation of different hydrogen-bonding networks in each CMP. The acylhydrazone-based CMPs were applied to the detection of nitroaromatic compounds. As a result, ABH-CMP in DMF exhibited high selectivity for 1,3,5-trinitrotoluene (TNT) over other nitroaromatic compounds nitrobenzene, 1-chloro-4-nitrobenzene, 2,3-dichloronitrobenzene, and 2,4-dinitrotoluene.

Olefin-linked conjugated fluorescent oligomer: Design, synthesis, photophysical studies and detection of nitroaromatic compounds (NACs) in aqueous media

The development of novel fluorescent polymers has attracted significant attention owing to their potential applications in energy and environmental sciences. This study reports novel oligomer (TRZBT) synthesis via Wittig-Horner reaction using bithiophene aldehyde and triazine triethyl phosphite. The resulting oligomer possesses large porous morphology, which was confirmed using SEM analysis. Further, the chemical structure of TRZBT was confirmed by FT-IR, NMR, and GPC studies. The triazine acceptor (A) unit linked with bithiophene donor (D) unit through olefin π-conjugation resulted in a Donor-π-Acceptor (D-π-A) type oligomer. Detailed studies of the solvent-dependent photophysical properties of TRZBT were performed. Due to the alternate d-A structure of oligomer, enhanced intramolecular charge transfer (ICT) properties between D and A moiety was observed. Further, TRZBT in THF and THF: H2O (10:90) system effectively detected nitro-explosives in aqueous media, which exhibited high sensitivity, selectivity, and quick response via fluorescence quenching. TRZBT showed a higher quenching constant (Ksv) value of 4.91 × 105 M−1 towards picric acid as compared with other nitroaromatics. Significantly, the detection limit of 1.58 nM in a 100 % aqueous medium confirms that the synthesized material can detect even trace levels of the analyte.