A novel acetylcholinesterase inhibition based colorimetric biosensor for the detection of paraoxon ethyl using CUPRAC reagent as chromogenic oxidant

A novel colorimetric biosensor for the sensitive and selective detection of an organophosphate pesticide, paraoxon ethyl (POE), was developed based on its inhibitory effect on the acetylcholine esterase (AChE) enzyme. The bis-neocuproine copper (II) complex ([Cu(Nc)2]2+) known as the CUPRAC reagent, was used as a chromogenic oxidant in the AChE inhibition-based biosensors for the first time. To initiate the biosensor, an enzymatic reaction takes place between AChE and its substrate acetylthiocholine (ATCh). Then, enzymatically produced thiocholine (TCh) reacts with the light blue [Cu(Nc)2]2+ complex, resulting in the oxidation of TCh to its disulfide form. On the other hand, [Cu(Nc)2]2+ reduces to a yellow-orange cuprous complex ([Cu(Nc)2]+) which gives maximum absorbance at 450 nm. However, the absorbance of [Cu(Nc)2]+ proportionally decreased with the addition of POE because the inhibition of AChE by the organophosphate pesticide reduced the amount of TCh that would give a colorimetric reaction with the CUPRAC reagent. Based on this strategy, the linear response range of a colorimetric biosensor was found to be between 0.15 and 1.25 μM with a detection limit of 0.045 μM. The fabricated biosensor enabled the selective determination of POE in the presence of some other pesticides and metal ions. The recovery results between 92% and 104% were obtained from water and soil samples spiked with POE, indicating that the determination of POE in real water and soil samples can be performed with this simple, accurate, sensitive, and low-cost colorimetric biosensor.

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.

Fabrication of a Novel Optical Glucose Biosensor Using Copper(II) Neocuproine as a Chromogenic Oxidant and Glucose Dehydrogenase-Immobilized Magnetite Nanoparticles

This study describes a novel optical glucose biosensor based on a colorimetric reaction between reduced nicotinamide adenine dinucleotide (NADH) and a copper(II) neocuproine complex ([Cu(Nc)2]2+) as a chromogenic oxidant. An enzymatic reaction takes place between glucose and glucose dehydrogenase (GDH)-chitosan (CS) immobilized on silanized magnetite nanoparticles (CS@SiO2@Fe3O4) in the presence of coenzyme NAD+. The oxidation of glucose to gluconolactone via the immobilized enzyme is coupled with the reduction of NAD+ to NADH at the same time. After the separation of GDH-immobilized SiO2@Fe3O4 with a magnet, the enzymatically produced NADH chemically reduces the chromogenic oxidant cupric neocuproine to the cuprous chelate. Thus, the glucose biosensor is fabricated based on the measurement of the absorbance of the formed yellow-orange complex ([Cu(Nc)2]+) at 450 nm. The obtained results show that the colorimetric biosensor has a wide linear response range for glucose, between 1.0 and 150.0 μM under optimized conditions. The limit of detection and limit of quantification were found to be 0.31 and 1.02 μM, respectively. The selectivity properties of the fabricated biosensor were tested with various interfering species. This biosensor was applied to various samples, and the obtained results suggest that the fabricated optical biosensor can be successfully used for the selective and sensitive determination of glucose in real samples.

Heteroatom-Doped Carbon Quantum Dots and Polymer Composite as Dual-Mode Nanoprobe for Fluorometric and Colorimetric Determination of Picric Acid

Oxygen- and nitrogen-heteroatom-doped, water-dispersible, and bright blue-fluorescent carbon dots (ON-CDs) were prepared for the selective and sensitive determination of 2,4,6-trinitrophenol (picric acid, PA). ON-CDs with 49.7% quantum yield were one-pot manufactured by the reflux method using citric acid, d-glucose, and ethylenediamine precursors. The surface morphology of ON-CDs was determined by scanning transmission electron microscopy, high-resolution transmission electron microscopy, dynamic light scattering, Raman, infrared, and X-ray photoelectron spectroscopy techniques, and their photophysical properties were estimated by fluorescence spectroscopy, UV-vis spectroscopy, fluorescence lifetime measurement, and 3D-fluorescence excitation-emission matrix analysis. ON-CDs at an average particle size of 3.0 nm had excitation/emission wavelengths of 355 and 455 nm, respectively. With the dominant inner-filter effect- and hydrogen-bonding interaction-based static fluorescence quenching phenomena supported by ground-state charge-transfer complexation (CTC), the fluorescence of ON-CDs was selectively quenched with PA in the presence of various types of explosives (i.e., 2,4,6-trinitrotoluene, tetryl, 1,3,5-trinitroperhydro-1,3,5-triazine, 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, pentaerythritol tetranitrate, 3-nitro-1,2,4-triazole-5-one, and TATP-hydrolyzed H2O2). The analytical results showed that the emission intensity varied linearly with a correlation coefficient of 0.9987 over a PA concentration range from 1.0 × 10-9 to 11.0 × 10-9 M. As a result of ground-state interaction (H-bonding and CTC) of ON-CDs with PA, an orange-colored complex was formed different from the characteristic yellow color of PA in an aqueous medium, allowing naked-eye detection of PA. The detection limits for PA with ON-CDs were 12.5 × 10-12 M (12.5 pM) by emission measurement and 9.0 × 10-10 M (0.9 nM) by absorption measurement. In the presence of synthetic explosive mixtures, common soil cations/anions, and camouflage materials, PA was recovered in the range of 95.2 and 102.5%. The developed method was statistically validated against a reference liquid chromatography coupled to tandem mass spectrometry method applied to PA-contaminated soil. In addition, a poly(vinyl alcohol)-based polymer composite film {PF(ON-CDs)} was prepared by incorporating ON-CDs, enabling the smartphone-assisted fluorometric detection of PA.

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.

Direct Determination of Peroxide Explosives on Polycarbazole/Gold Nanoparticle-Modified Glassy Carbon Sensor Electrodes Imprinted for Molecular Recognition of TATP and HMTD

Since peroxide-based explosives (PBEs) lack reactive functional groups, they cannot be determined directly by most detection methods and are often detected indirectly by converting them to H₂O₂. However, H₂O₂ may originate from many sources, causing false positives in PBE detection. Here, we developed a novel electrochemical sensor for the direct sensitive and selective determination of PBEs such as triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD) using electrochemical modification of the glassy carbon (GC) electrode with PBE-memory polycarbazole (PCz) films decorated with gold nanoparticles (AuNPs) by cyclic voltammetry (CV). The prepared electrodes were named TATP-memory-GC/PCz/AuNPs (used for TATP determination) and HMTD-memory-GC/PCz/AuNPs (used for HMTD detection). The calibration lines of TATP and HMTD were found in the concentration range of 0.1-1.0 mg L^-1 using the net current intensities of differential pulse voltammetry (DPV) versus analyte concentrations. The limit of detection (LOD) commonly found was 15 μg L^-1 for TATP and HMTD. The sensor electrodes could separately determine intact TATP and HMTD in the presence of nitro-aromatic, nitramine, and nitrate ester energetic materials. The proposed electrochemical sensing method was not interfered by electroactive substances such as paracetamol, caffeine, acetylsalicylic acid, aspartame, d-glucose, and detergent (containing perborate and percarbonate) used as camouflage materials for PBEs. This is the first molecularly imprinted polymeric electrode for PBEs accomplishing such low LODs, and the DPV method was statistically validated in contaminated clay soil samples against the GC-MS method for TATP and a spectrophotometric method for HMTD using t- and F-tests.

Spectrophotometric Fluoride Determination Using St. John's Wort Extract as a Green Chromogenic Complexant for Al(III)

In this study, we applied an innovative approach of green analytical chemistry to develop a novel and eco-friendly chromogenic agent for fluoride determination by making use of the nontoxic Al(III)-flavonoid complex in a natural extract from St. John's wort plant. The initial intensely yellow-colored Al(III)-flavonoid complex formed in the plant extract was converted to a colorless AlF₆ ^3- complex with increasing amounts of fluoride, and color bleaching of the Al-flavonoid chromophore (measured as absorbance decrement) was proportional to fluoride concentration. The developed method gave a linear response within the F^- concentration range of 0.11-1.32 mM with the LOD and LOQ values of 0.026 mM (0.5 mg L^-1) and 0.079 mM (1.5 mg L^-1), respectively. The LOD value for fluoride was below the WHO-permissible limit (1.5 mg L^-1) and the US-EPA-enforceable limit (4 mg L^-1) in water. The possible interference effects of common anions (Cl^-, Br^-, I^-, NO₃ ^-, HCO₃ ^-, SO₄ ^2-, and PO₄ ^3-) and cations (K⁺, NH₄ ⁺, Ag⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, and Fe³⁺) were investigated; the observed interferences from Fe²⁺, Fe³⁺, and PO₄ ^3- were easily eliminated by masking iron with the necessary amount of Na₂EDTA without affecting the blank absorbance of the Al(III)-flavonoid complex, precipitating phosphate with Ag(I) salt, and partly neutralizing alkaline water samples to pH 4 with acetic acid. The developed method was applied to real water samples and also validated against a reference spectroscopic method at the 95% confidence level.

Ionic Liquid-Modified Gold Nanoparticle-Based Colorimetric Sensor for Perchlorate Detection via Anion-π Interaction

A rapid and convenient nanoparticle(NP)-based colorimetric sensor was developed for determining the propellant oxidant, ammonium perchlorate (AP). The sensing element was manufactured by modifying gold nanoparticles (AuNPs) with [(1-methyl-1H-imidazol-2-yl)sulfanyl]acetic acid, which is an imidazolium-based ionic liquid (IL), to produce the IL@AuNP nanosensor stabilized by polyvinylpyrrolidone. The used IL is an exceptional IL which can attach to AuNPs through the sulfanyl-S atom. The sensing principle was based on observing the red shift in the surface plasmon resonance band of AuNPs leading to NP aggregation as a result of anion-π interaction of perchlorate anion with the zwitterionic form of IL@AuNPs so as to bring opposite charges face-to-face, thereby reducing the overall surface charge of NPs. The surface plasmon resonance band of AuNPs at 540 nm shifted to 700 nm as a result of aggregation. The ratiometric sensing was performed by dividing the absorbance at 700 nm to the absorbance at 540 nm and correlating this ratio to the AP concentration. The limit of detection and limit of quantification of the sensor for AP were 1.50 and 4.95 μM, respectively. Possible interferences of other energetic substances and common soil ions in synthetic mixtures were also investigated to achieve acceptable recoveries of analyte. This work may pioneer similar sensing systems where the overall anionic charges of IL-functionalized AuNPs are exceptionally reduced by an analyte anion (perchlorate), thereby forcing NPs to aggregate.

A novel flow injection amperometric method for sensitive determination of total antioxidant capacity at cupric-neocuproine complex modified MWCNT glassy carbon electrode

A novel amperometric method is presented for the determination of total antioxidant capacity in flow injection analysis (FIA) system using copper(II)-neocuproine complex modified on Nafion-functionalized multi-walled carbon nanotube-glassy carbon electrode ([Cu(Ncp)₂²⁺]/Nf@f-MWCNT/GCE). Cyclic voltammetric studies showed that the modified electrode exhibits a very well-formed reversible redox couple for Cu(II)-/Cu(I)-complex. In addition, the [Cu(Ncp)₂²⁺]/[Cu(Ncp)₂⁺] redox pair shows very good electrocatalytic activity towards the oxidation of polyphenolic compounds (PPhCs) such as trolox, catechin, and quercetin due to the enhancement of the anodic peak current of the redox couple in the presence of these analytes. This electrocatalytic oxidation current at the [Cu(Ncp)₂²⁺]/Nf@f-MWCNT/GCE was used for flow injection (FI) amperometric determination of PPhCs. FI amperometric-time curves recorded under optimized conditions (applied potential: + 0.6 V vs. Ag/AgCl/KCl_(0.10 M), flow rate: 2 mL/min) showed that the proposed electrode had a wide linear range (LR) with a very low detection limit (LOD) for PPhCs. LR and LOD were 0.5-800 and 0.2 µM for trolox, respectively and 0.50-250 and 0.14 µM, respectively, for both quercetin and catechin. This sensitive method was successfully applied to the amperometric measurement of total antioxidant capacity (TAC) of some herbal teas, giving compatible results with the spectrophotometric CUPRAC method. The proposed method gave higher rank to fast-reacting antioxidants; it was equally precise but had a wider linear range and lower LOD than the spectrophotometric CUPRAC assay (e.g., LOD for ascorbic acid and gallic acid were 0.07 and 0.08 μM, respectively), and similar electroanalytical methods using the CUPRAC reagent.

Detection of nitric oxide radical and determination of its scavenging activity by antioxidants using spectrophotometric and spectrofluorometric methods

Although reactive nitrogen species (RNS) may attack biomacromolecules and cause tissue damage when unbalanced by natural antioxidant defenses of the organism, they can also take part in cell signaling under different physiological states and defend against certain pathogens. Since there is a scarcity of analytical methods to detect radicalic NO and its scavengers, a functionalized gold nanoparticle-based spectrophotometric method and a spectrofluorometric method have been separately developed to test antioxidant activity toward scavenging of NO produced from sodium nitroprusside (SNP). The spectrophotometric method involves conversion of NO to nitrite, followed by the formation of an azo dye with 4-aminothiophenol (4-ATP)-modified gold nanoparticles (AuNPs) and N-(1-naphthyl)-ethylene diamine dichloride (NED) and its absorbance measurement at 565 nm. Calibration equations were established by taking the absorbance difference in the presence and absence of antioxidants. In the spectrofluorometric method, the excess of NO radicals, after being scavenged by thiol type antioxidants, caused a decrease in resorcinol fluorescence. The developed spectrophotometric method was applied to orange juice and its trolox equivalent (TE) antioxidant activity was found. By further applying the developed methods to real samples such as bovine serum albumin (BSA), fetal bovine serum (FBS), saliva and certain biomolecules, it is envisaged that these novel methods improving the selectivity of previous methods can be useful in human health and disease research associated with nitric oxide. The developed methods were compared and validated against the conventional Griess assay with Student t-test and F tests.

A novel electrochemical sensor for nitroguanidine determination using a glassy carbon electrode modified with multi-walled carbon nanotubes and polyvinylpyrrolidone

The GC/PVP/MWCNTs electrode is the first electrode for the electrochemical determination of insensitive explosive nitroguanidine using intermolecular hydrogen bonding interactions.

The effect of alexithymic characteristics on perceived stress and health anxiety during the COVID-19 pandemic

OBJECTIVE

The anxiety of life that comes with the pandemic process increases the health anxiety and the level of perceived stress. However, there are uncertainties about which individuals are more sensitive. This study aims to investigate the effects of alexithymic characteristics on health anxiety and perceived stress.

MATERIALS AND METHODS

The authors invited the participants to study via social media and e-mail. The data of 793 individuals, aged 18-65, collected over the internet (Google Forms) between November and December 2020 were statistically evaluated. Evaluations were made with the sociodemographic data form, the General Health Questionnaire-12 (GHQ-12), the Health Anxiety Scale (HAS), the Toronto Alexithymia Scale (TAS-20), and the Perceived Stress Scale (PSS).

RESULTS

In mediation analyzes between TAS subscales and HAS, Difficulty in Identifying Feelings (DIF) most strongly predicted HAS (B=0.469, p<0.001) and indirectly affected HAS only through GHQ (CS: 0.08, B=0.108, SE:0.021, CI: 0.070, 0.153). However, both PSS (CS: 0.0128, B=0.084, SE:0.027, CI: 0.032, 0.139) and GHQ (CS: 0.02, B=0.139, SE:0.028, CI: 0.090, 0.198) played a mediating role between Difficulty Describing Feelings (DDF) and HAS.

CONCLUSIONS

The present study suggests that individuals with alexithymic features are more sensitive to stress during periods of health-related increased stress, such as pandemics, and that individuals with alexithymic features should be given priority in psychotherapeutic interventions.

Direct Electrochemical Determination of Peroxide-Type Explosives Using Well-Dispersed Multi-Walled Carbon Nanotubes/Polyethyleneimine-Modified Glassy Carbon Electrodes

The sensitive and selective determination of peroxide-based explosives (PBEs) in the field/on site is an important analytical challenge. Most methods claiming to detect PBEs are indirect, actually detecting their decomposition product, H₂O₂. Here, we present an electrochemical sensor for direct detection of organic peroxide explosives, that is, triacetone triperoxide (TATP) and hexamethylenetriperoxide diamine (HMTD), using well-dispersed multiwalled carbon nanotubes/polyethyleneimine (MWCNTs/PEI)-modified glassy carbon (GC) electrode, namely, GC/MWCNTs/PEI electrode. This is the first use of the conductive polyelectrolyte PEI as an electrode modifier for pristine PBE sensing. The potential range, scan rate, solvent selection, and supporting electrolyte concentration were optimized for PBEs. As a distinct advantage over other similar methods, our sensor electrode responded to intact TATP solutions in neutral medium, meaning that TATP did not interact with acids/bases that would transform it into H₂O₂. Calibration curves were linear in the range of 10-200 mg L^-1 for TATP and 25-200 mg L^-1 for HMTD. Using differential pulse voltammetry, detection limits of 1.5 mg L^-1 TATP and 3.0 mg L^-1 HMTD were obtained from direct electrochemical reduction in 80/20% (v/v) H₂O-acetone solvent medium. Electroactive camouflage materials such as passenger belongings (e.g., sweetener, detergent, sugar, and paracetamol-caffeine-based analgesic drugs), common ions, and other explosives were shown not to interfere with the proposed method. The nonresponsive behavior of our electrode to H₂O₂ prevents "false positives" from other peroxide materials of everyday use. This electrochemical sensor could also detect other nitro-explosives at different potentials and was statistically validated against standard GC-MS and spectrophotometric methods.

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.

Development of a gold nanoparticles-based colorimetric sensor for the indirect determination of ammonium dinitramide and tetryl

Ammonium dinitramide (ADN) is a strong, environmentally friendly oxidizer used in composite solid rocket propellants. As there is no reliable colorimetric sensor for ADN assay applicable to in-field screening, we developed a sensitive and practical sensing method to determine it in the presence of other explosives and possible interferents, based on the detection of nitrite formed from ADN degradation under UV light in a slightly alkaline (i.e. of lower alkalinity than needed to hydrolyze nitramines) solution by a nanoparticle-based colorimetric sensor. The ADN-derived nitrite formed a colored product via a Griess reaction using gold nanoparticles modified with 4-aminothiophenol (4-ATP-AuNPs) along with a coupling reagent N-(1-naphthyl)ethylene diamine (NED) for forming an azo dye. The method used for ADN detection could also be applied to tetryl samples at a different wavelength. The limit of detection (LOD) was 0.012 mg L^-1 for ADN and 0.615 mg L^-1 for tetryl. Interference effects of energetic materials like trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and pentaerythritol tetranitrate (PETN) to ADN determination could be overcome. In addition, common soil ions did not adversely affect the nanosensor performance. The developed method was statistically validated against reference voltammetric, UV, and HPLC methods using t- and F- tests.

Selective colorimetric sensing of deferoxamine with 4-mercaptophenol- and mercaptoacetic acid-functionalized gold nanoparticles via Fe(iii) chelation

The multidentate deferoxamine ligand can selectively aggregate the Fe(iii)-attached AuNPs@(4MP–MAA) colorimetric nanoprobe, whereas other bidentate iron chelators cannot bridge the nanoparticles.

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.

Titanium dioxide nanoparticles-based colorimetric sensors for determination of hydrogen peroxide and triacetone triperoxide (TATP)

Due to its relatively simple preparation and readily available precursors, determination of triacetone triperoxide (TATP) by portable devices has become important. In this work, two different titanium dioxide nanoparticles (TiO₂NPs)-based colorimetric sensors based on complex formation on the solid surface were developed for determination of H₂O₂ and TATP. The first sensor, (3-aminopropyl)triethoxysilane (APTES) modified-TiO₂NPs-based paper sensor (APTES@TiO₂NPs), exploits peroxo-titanate binary complex formation between APTES@TiO₂NPs and H₂O₂ on chromatographic paper. The second sensor, 4-(2-pyridylazo)-resorcinol-modified-TiO₂NPs-based solid sensor (PAR@TiO₂NPs), relies on the formation of a ternary complex between Ti(IV), PAR and H₂O₂. The developed sensors were also applied to TATP determination after acidic hydrolysis of samples to H₂O₂. The limits of detection (LODs) of APTES@TiO₂NPs-based paper sensor were 3.14 × 10^-4 and 5.13 × 10^-4 mol L^-1 for H₂O₂ and TATP, respectively, whereas the LODs of PAR@TiO₂NPs solid sensor were 6.06 × 10^-7 and 3.54 × 10^-7 mol L^-1 for H₂O₂ and TATP, respectively. Possible interferences of common soil ions, passenger belongings used as camouflage materials during public transport (e.g., detergent, sweetener, acetylsalicylic acid and paracetamol-caffeine based analgesic drugs) and of other explosives were examined. The developed methods were statistically validated using t- and F- tests against the titanyl sulfate (TiOSO₄) colorimetric literature method.

Silver Nanoparticle Formation-Based Colorimetric Determination of Reducing Sugars in Food Extracts via Tollens' Reagent

A simple, sensitive, and nonenzymatic nanospectrophotometric method was developed for the determination of reducing sugars. The silver mirror reaction-assisted method is based on the in situ formation of silver nanoparticles in the presence of reducing sugars. All simple reducing sugars (glucose, galactose, fructose, mannose, maltose, and lactose) examined had perfectly linear regression equations. The detection limit for glucose was 40 nM. The proposed method could be selectively applied to various synthetic mixtures of reducing sugars with polyphenolic compounds, and to honey, milk, and commercial fruit juice as real samples using solid phase extraction as a clean-up process. The developed method was also statistically validated against conventional alkaline CUPRAC (cupric-neocuproine, Cu(II)-Nc) spectrophotometric method using Student's t- and F-tests.

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

Detection of explosive residues in soil and postblast debris is an important issue in sensor design for environmental and criminological purposes. An easy-to-use and low-cost gold nanoparticle (AuNP)-based colorimetric sensor was developed for the determination of nitroaromatic explosives, i.e., trinitrotoluene (TNT) and tetryl, capable of analyte detection at picomolar (pM) levels. The sensor nanoparticles were synthesized by functionalizing the negatively charged thioglycolic acid (TGA)-modified AuNPs with positively charged (±)- trans-1,2-diaminocyclohexane (DACH) at a carefully calculated pH. The working principle of the sensor is charge-transfer (CT) interaction between the electron-rich free amino (-NH₂) group of DACH and the electron-deficient -NO₂ groups of TNT/tetryl, added to possible nanoparticle agglomerization via electrostatic interaction of TNT-Meisenheimer anions with more than one cationic DACH-modified AuNP. The limit of detection (LOD) and limit of quantification (LOQ) of the sensor were 1.76 pM and 5.87 pM for TNT and 1.74 pM and 5.80 pM for tetryl, respectively. TNT, tetryl, and tetrytol, extracted from a nitroaromatic explosive-contaminated soil sample, were determined with the proposed sensor, yielding good recoveries. The sensor could be selectively applied to various mixtures of TNT with common energetic materials such as RDX, HMX, and PETN. Additionally, common soil ions (Cl^-, NO₃^-, SO₄^2-, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Fe²⁺, Fe³⁺, and Al³⁺) as well as detergents, sugar, sweeteners, acetylsalicylic acid (aspirin), caffeine, and paracetamol-based painkiller drugs, which may be used as camouflage materials for explosives, either had no adverse effects or removable interferences on the detection method. The developed method was statistically validated against a GC-MS literature method.

A novel cerium oxide nanoparticles-based colorimetric sensor using tetramethyl benzidine reagent for antioxidant activity assay

Antioxidant activity (AOA) assays using nanotechnology are recently developed utilizing nanoparticles of transition metal oxides, especially nanoceria that can switch between trivalent and tetravalent oxidation states of cerium. Cerium oxide nanoparticles (CeO-NPs) may act as both an oxidant and an antioxidant, depending on the preparation method and particle size. A novel colorimetric sensor for AOA assay is proposed with the use of poly(acrylic acid) sodium salt (PAANa)-coated CeO-NPs. PAANa-coated CeO-NPs oxidized tetramethyl benzidine (TMB), a peroxidase substrate, in a slightly acidic solution at pH 4.0 to a blue charge-transfer complex. Antioxidants decreased the color intensity of the nanoceria suspension, and were indirectly determined by absorbance difference. Detection limits, linearity, additivity and precision were calculated, e.g., quercetin quantification with the proposed assay showed a detection limit of 8.25 × 10^-9 mol L^-1. The trolox equivalent antioxidant capacities of hydrophilic and lipophilic antioxidants were compatible with those of conventional antioxidant assays. Potential interferents such as glucose, citric acid, mannitol, sorbitol and benzoic acid did not adversely affect AOA determination. The developed sensor is more sensitive and selective than similar colorimetric sensors relying on the intrinsic color change of nanoceria. The measurement wavelength is sufficiently red-shifted, preventing possible interferences from plant pigments.

Novel Spectroscopic and Electrochemical Sensors and Nanoprobes for the Characterization of Food and Biological Antioxidants

Since an unbalanced excess of reactive oxygen/nitrogen species (ROS/RNS) causes various diseases, determination of antioxidants that can counter oxidative stress is important in food and biological analyses. Optical/electrochemical nanosensors have attracted attention in antioxidant activity (AOA) assessment because of their increased sensitivity and selectivity. Optical sensors offer advantages such as low cost, flexibility, remote control, speed, miniaturization and on-site/in situ analysis. Electrochemical sensors using noble metal nanoparticles on modified electrodes better catalyze bioelectrochemical reactions. We summarize the design principles of colorimetric sensors and nanoprobes for food antioxidants (including electron-transfer based and ROS/RNS scavenging assays) and important milestones contributed by our laboratory. We present novel sensors and nanoprobes together with their mechanisms and analytical performances. Our colorimetric sensors for AOA measurement made use of cupric-neocuproine and ferric-phenanthroline complexes immobilized on a Nafion membrane. We recently designed an optical oxidant/antioxidant sensor using N,N-dimethyl-p-phenylene diamine (DMPD) as probe, from which ROS produced colored DMPD-quinone cationic radicals electrostatically retained on a Nafion membrane. The attenuation of initial color by antioxidants enabled indirect AOA estimation. The surface plasmon resonance absorption of silver nanoparticles as a result of enlargement of citrate-reduced seed particles by antioxidant addition enabled a linear response of AOA. We determined biothiols with Ellman reagent-derivatized gold nanoparticles.

Surface-enhanced Raman spectroscopy combined with gold nanorods for the simultaneous quantification of nitramine energetic materials

A nanosensing method based on surface-enhanced Raman spectroscopy was proposed for simultaneous quantification of nitramine compounds, HMX and RDX.

Electrochemical Determination of Food Preservative Nitrite with Gold Nanoparticles/p-Aminothiophenol-Modified Gold Electrode

Due to the negative impact of nitrate and nitrite on human health, their presence exceeding acceptable levels is not desired in foodstuffs. Thus, nitrite determination at low concentrations is a major challenge in electroanalytical chemistry, which can be achieved by fast, cheap, and safe electrochemical sensors. In this work, the working electrode (Au) was functionalized with p-aminothiophenol (p-ATP) and modified with gold nanoparticles (Au-NPs) to manufacture the final (Au/p-ATP-Au_nano) electrode in a two-step procedure. In the first step, p-ATP was electropolymerized on the electrode surface to obtain a polyaminothiophenol (PATP) coating. In the second step, Au/p-ATP-Au_nano working electrode was prepared by coating the surface with the use of HAuCl₄ solution and cyclic voltammetry. Determination of aqueous nitrite samples was performed with the proposed electrode (Au/p-ATP-Au_nano) using square wave voltammetry (SWV) in pH 4 buffer medium. Characteristic peak potential of nitrite samples was 0.76 V, and linear calibration curves of current intensity versus concentration was linear in the range of 0.5–50 mg·L⁻¹ nitrite with a limit of detection (LOD) of 0.12 mg·L⁻¹. Alternatively, nitrite in sausage samples could be colorimetrically determined with high sensitivity by means of p-ATP‒modified gold nanoparticles (AuNPs) and naphthylethylene diamine as coupling agents for azo-dye formation due to enhanced charge-transfer interactions with the AuNPs surface. The slopes of the calibration lines in pure NO₂⁻ solution and in sausage sample solution, to which different concentrations of NO₂⁻ standards were added, were not significantly different from each other, confirming the robustness and interference tolerance of the method. The proposed voltammetric sensing method was validated against the colorimetric nanosensing method in sausage samples.

Determination of total antioxidant capacity of humic acids using CUPRAC, Folin-Ciocalteu, noble metal nanoparticle- and solid-liquid extraction-based methods

Total antioxidant capacity (TAC) of humic acid (HA) samples was determined using CUPRAC (CUPric Reducing Antioxidant Capacity), FC (Folin-Ciocalteu), QUENCHER-CUPRAC, QUENCHER-FC, Ag-NP (Silver nanoparticle)‒ and Au-NP (Gold nanoparticle)‒based methods. Conventional FC and modified FC (MFC) methods were applied to solid samples. Because of decreased solubility of Folin-Ciocalteu's phenol reagent in organic solvents, solvent effect on TAC measurement was investigated using QUENCHER-CUPRAC assay by using ethanol:distilled water and dimethyl sulfoxide:distilled water with varying ratios. To see the combined effect of solubilization (leaching) and TAC measurement of humic acids simultaneously, QUENCHER experiments were performed at 25°C and 50°C; QUENCHER-CUPRAC and QUENCHER-FC methods agreed well and had similar precision in F-statistics. Although the Gibbs free energy change (ΔG°) of the oxidation of HA dihydroxy phenols with the test reagents were negative, the ΔG° was positive only for the reaction of CUPRAC reagent with isolated monohydric phenols, showing CUPRAC selectivity toward polyphenolic antioxidants. This is the first work on the antioxidant capacity measurement of HA having a sparingly soluble matrix where enhanced solubilization of bound phenolics is achieved with coupled oxidation by TAC reagents.