Indicator displacement-based colorimetric assay for dibutyl phthalate in pharmaceutical products with titanium(IV)-pyridylazo resorcinol (PAR).

Taking advantage of the competitive binding affinity towards Ti(IV) between 4-(2-pyridylazo) resorcinol (PAR) and phthalate, a simple indicator displacement (ID)-based colorimetric assay was designed for indirect determination of a well-known phthalic acid ester, dibutyl phthalate (DBP). The indicator PAR and Ti(IV) formed a purplish-red-colored Ti(IV)-PAR complex (λmax = 540 nm) at a 1:1 ratio. In the presence of pre-hydrolyzed DBP, colorless complex formation of phthalate ion (emerging from alkaline hydrolysis of DBP) with Ti(IV) resulted in a hypsochromic shift in absorbance maximum, accompanying a color change from purplish-red to yellowish-orange (λmax = 390 nm) by the release of PAR from Ti(IV)-PAR system. Based on this mechanism, the linear response range of the system for DBP was found to lie between 0.16 and 0.37 mmol L-1 with an experimental detection limit of 11.6 µmol L-1. The recommended Ti(IV)-PAR system was successfully applied to DBP-containing pharmaceutical products (as real sample) after a simple clean-up process for removing possible water-soluble interferents. The analytical results obtained from the recommended method (by applying the standard addition approach) and the reference liquid chromatography-tandem mass spectrometric (LC-MS/MS) method were statistically compared using DBP-extract of the drug samples. Consequently, a simple and selective colorimetric ID strategy was proposed for the analysis of DBP in pharmaceuticals for the first time.

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.

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.

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.

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-, SO42-, 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.

Colorimetric Determination of (Aminoalkyl)indole-containing Synthetic Cannabimimetics.

Cannabis is an important industrial plant, in addition to its illicit drug use. Compound Δ9-THC (Δ9-tetrahydrocannabinol) is mainly responsible for the hallucinogenic effect on humans. The aminoalkylindole group cannabimimetics targets at the same physiological receptors to mimic the analgesic effects of Δ9-THC. Since there is no reliable colorimetric test to detect these synthetic cannabimimetics on site, a simple colorimetric assay for (aminoalkyl)indole group-containing drugs was developed, based on the silica/sulfuric acid-catalyzed Ehrlich reaction of (aminoalkyl)indoles with p-dimethylaminobenzaldehyde. The electrophilic substitution reaction of indoles with carbonyl compounds resulting in the formation of bis(indolyl)alkanes in an acid-catalyzed reaction has been used for the first time for their spectrophotometric determination by color change from yellow to purple/blue. The method was statistically validated against liquid chromatography tandem mass spectrometry, and applied to certain (aminoalkyl)indole derivatives, with 0.5 - 2.5 µg mL-1 detection limits for AM-2201, JWH-081, MAM-2201, JWH-018, JWH-210, JWH-122, 5F-PB-22 and XLR-11.