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.

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-Aunano) 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-Aunano 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-Aunano) 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.

The two members of peroxide-based explosives, triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD), can be manufactured from readily accessible reagents, and are difficult to detect by conventional analytical methods. TATP and HMTD were securely synthesized, taken up with acetone, hydrolyzed with 4 M HCl to hydrogen peroxide, the acidic solution containing H(2)O(2) was neutralized, and assayed by the copper(II)-neocuproine spectrophotometric method. The chromophore of the reaction was the Cu(I)-neocuproine chelate responsible for light absorption at 454 nm. The molar absorptivity (epsilon) of the method for TATP and HMTD was 3.45 x 10(4) and 4.68 x 10(4) L mol(-1) cm(-1), respectively. The TATP recovery from a synthetically contaminated loamy clay soil was 91-99%. The colorimetric method was also applied to a Cu(ii)-neocuproine-impregnated polymeric Nafion membrane sensor developed for the first time in this work for peroxide explosive assay. The absorbance-concentration response was perfectly linear, and the limit of detection (LOD) of the procedure for both TATP and HMTD was approximately 0.2 mg L(-1). Neither common soil ions (Ca(2+), K(+), Cl(-), SO(4)(2-), Mg(2+) and NO(3)(-)) at 100-fold amounts nor military-purpose nitro-explosives of TNT, RDX, and PETN at 10-fold amounts interfered with the proposed assay. Active oxygen constituents of laundry detergents (perborates and percarbonates), which normally interfered with the assay, could easily be separated from the analytes by solubility differences. The method was statistically validated against standard reference methods of TiOSO(4) colorimetry and GC-MS.

The heterocyclic nitramine compounds, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), are two most important military-purpose high explosives. Differentiation of RDX and HMX with colorimetric methods of determination has not yet been made because of their similar chemical structures. In this study, a sensitive colorimetric method for the determination of RDX and HMX was proposed on the basis of differential kinetics in the hydrolysis of the two compounds (yielding nitrite as a product) followed by their colorimetric determination using 4-aminothiophenol (4-ATP) modified gold nanoparticles (AuNPs) and naphthylethylene diamine (NED) as coupling agent for azo-dye formation, abbreviated as "4-ATP-AuNP+NED" colorimetric method. After alkaline hydrolysis in a 1 M Na2CO3 + 0.04 M NaOH mixture solution at room temperature, only RDX (but not HMX) was hydrolyzed to give a sufficient colorimetric response in neutralized solution, the molar absorptivity (ε) at 565 nm and the limit of detection (LOD) for RDX being (17.6 ± 1.3) × 10(3) L mol(-1) cm(-1) and 0.55 μg mL(-1), respectively. On the other hand, hot water bath (at 60 °C) hydrolysis enabled both nitramines, RDX and HMX, to give substantial colorimetric responses; i.e., ε and LOD for RDX were (32.8 ± 0.5) × 10(3) L mol(-1)cm(-1) and 0.20 μg mL(-1) and for HMX were (37.1 ± 2.8) × 10(3) L mol(-1)cm(-1) and 0.24 μg mL(-1), respectively. Unlike other AuNP-based nitrite sensors in the literature showing absorbance quenching within a relatively narrow concentration range, the developed sensor operated with an absorbance increase over a wide range of nitrite. Synthetic mixtures of (RDX + HMX) gave additive responses, and the proposed method was statistically validated against HPLC using nitramine mixtures.

The explosive triacetone triperoxide (TATP) can be easily manufactured from readily accessible reagents and is extremely difficult to detect, owing to the lack of UV absorbance, fluorescence, or facile ionization. The developed method is based on the acidic hydrolysis of TATP into H2O2, pH adjustment to 3.6, and the addition of magnetite nanoparticles (Fe3O4 MNPs) to the medium to produce hydroxyl radicals from H2O2, owing to the peroxidase-like activity of MNPs. The formed radicals converted the N,N-dimethyl-p-phenylenediamine (DMPD) probe to the colored DMPD(+) radical cation, the optical absorbance of which was measured at a wavelength of 554 nm. The molar absorptivity (ε) of the method for TATP was 21.06 × 10(3) L mol(-1) cm(-1). The colored DMPD(+) product in solution could be completely retained on a cation-exchanger Nafion membrane, constituting a colorimetric sensor for TATP and increasing the analytical sensitivity. The proposed method did not respond to a number of hand luggage items like detergent, sweetener, sugar, acetylsalicylic acid (aspirin), and paracetamol-caffeine-based analgesic drugs. On the other hand, TATP could be almost quantitatively recovered from a household detergent and sweetener that can be used as camouflage for the analyte. Neither common soil and groundwater ions (e.g., Ca(2+), Mg(2+), K(+), Cl(-), SO4(2-), and NO3(-)) at 100-fold ratios nor nitro-explosives of trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and pentaerythritol tetranitrate (PETN) at 10-fold amounts interfered with the proposed assay. The method was statistically validated against the standard GC/MS reference method.