Highly specific colorimetric detection based on aggregation of l-cysteine functionalized gold nanoparticles for cypermethrin in water samples.

A fast, simple, and selective colorimetric assay for quantifying cypermethrin in water samples is proposed using l-cysteine functionalized gold nanoparticles (AuNPs@Cyst). Cypermethrin is hydrolyzed by potassium hydroxide to obtain hydrolyzed cypermethrin in the form of 3-phenoxybenzaldehyde by-product (HCy). The detection strategy is based on the aggregation of AuNPs@Cyst caused by hydrogen-bonding recognition between the aldehyde group of HCy and the amine group of l-cysteine on the surface of AuNPs@Cyst. As a result, in the presence of HCy under optimal pH 7, AuNPs@Cyst aggregates within 7 min, exhibiting a distinct color change from red to blue-gray, which can be evaluated with the naked eye and UV-visible spectrophotometry. From FE-TEM image, the stable and spherical AuNPs@Cyst had an average size of 13.8 ± 1.6 nm, and from zeta potential analysis, the charge of AuNPs@Cyst was -25.04 ± 1.66 mV. The surface plasmon resonance band of dispersed AuNPs@Cyst was red shifted from 525 nm to 634 nm when AuNPs@Cyst was aggregated. The absorbance ratio (A634/A525) was linearly related to cypermethrin concentrations from 0.5 to 13.0 mg L-1. The limit of detection was 0.2 mg L-1 and precision, expressed as relative standard deviations (RSDs), ranged from 1.9 to 7.3%. In the presence of interfering pesticides (carbaryl, ethion, profenofos and abamectin), only cypermethrin produced a significantly different response, confirming the selectivity of AuNPs@Cyst. Finally, AuNPs@Cyst was applied to determine cypermethrin in water samples, achieving very satisfied recoveries (>98.6%) and RSDs lower than 6.1%.

A novel double metal-dithizone functionalized polyurethane electrospun nanofiber and film for colorimetric determination of hexavalent chromium.

This work proposes a highly specific method of Cr6+ determination based on the double reactions of two metals, Co2+ with dithizone to form a (DTZ)-Co2+ complex, and the replacement of Co2+ in the formed complex with Cr6+. The fast degradation of DTZ in solution in wet analysis was overcome by preparing dithizone functionalized polyurethane nanofibers that were electrospun into a membrane (DTZ/PU-NF) and a microwell plate film (DTZ/PU-MPF). For comparison, the performance of diphenylcarbazide (DPC), a currently used complexing agent for Cr6+, was also investigated. Colour changes were detected as red-green-blue values. The DTZ/PU-NF was smooth, with an average diameter of 384.09 nm and no bead appeared. A dense network structure was formed. The best formulation of DTZ, PU and Co2+ was also applied as a microwell plate film. In the presence of Cr6+, the colour of DTZ-Co2+ changed from red to magenta. Among the three studied methods, the colorimetric DTZ-Co2+/PU-NF presented the best results. Its linearity range was 0.001-1.0 mg L-1, with a regression equation of Cr6+ = -0.189 + (0.0056 × red) + (0.0086 × green) - (0.0129 × blue), R 2 of 0.990. The limit of detection was 0.001 mg L-1 and the precision was 1.7%. The applicability of DTZ/PU-NF was validated for Cr6+ in vegetable oils with recoveries of 89.5-116.8%. The sensitivity of DTZ/PU-NF was ten times higher than that of DTZ/PU-MPF. The methods based on DTZ-Co2+/PU-NF and DTZ-Co2+/PU-MPF proved to be highly selective, rapid, user-friendly, simple and reliable.

Simultaneous quantification of volatile fatty acids and nonvolatile organic acids in Hevea brasiliensis latex.

The current method used in latex industries to determine the volatile fatty acids contents of Hevea brasiliensis latex is steam distillation. However, the accuracy of the method has been debated for some time. We assessed the accuracy of the method and developed a new, more reliable high-performance liquid chromatographic method of determining acids in latex. The volatile fatty acids (formic, acetic, propionic, butyric, and valeric acids) and nonvolatile organic acids (oxalic, malic, lactic, citric, and succinic acids) in latex are directly determined simultaneously for the first time with high sensitivity and without losses during sample preparation. To avoid errors from derivatization, an acid-resistant Prevail HPLC column and a gradient mobile phase of 25 mM potassium dihydrogen phosphate (pH 2.5) and acetonitrile were employed. Under optimum conditions, the calibrations of both types of acids demonstrated satisfactory correlation coefficients of  ≥0.990, with limits of detection ranging from 0.02 to 395 mM. The developed method demonstrated the profiles of acids in field and concentrated latex of the same batch. Moreover, the evolution of the profiles of all studied acids in both types of latex during a 3-month period was also revealed.

Colorimetric determination of fumonisin B1 based on the aggregation of cysteamine-functionalized gold nanoparticles induced by a product of its hydrolysis.

A colorimetric method was developed for the determination of the mold toxin fumonisin B1 (FB1). It is based on the aggregation of cysteamine-capped gold nanoparticles (Cys-AuNPs). The assay involves alkaline hydrolysis of FB1 to obtain hydrolyzed fumonisin B1 (HFB1). The latter induces the aggregation of Cys-AuNPs which results in a color change from wine-red to blue-gray, best at a pH value of 9.0. A plot of absorbance ratio at 645/520 nm versus FB1 concentration is linear in the 2-8 µg kg-1 FB1 concentration range, and the detection limit is 0.90 µg kg-1. Inter-day and intra-day precisions are <6.2%, and recoveries from spiked samples ranged from 93 to 99%. The assay was successfully applied to the determination of FB1 in corn samples. It has a high selectivity over other competitive mycotoxins including aflatoxin, zearalenone, citrinin and patulin. The method is more selective than the detection of FB1 directly which may lead to false-positive errors. Graphical abstract Schematic representation of colorimetric assay of fumonisin B1 (FB1). FB1 was alkali-hydrolyzed and its product (hydrolyzed fumonisin B1) induces cysteamine-capped gold nanoparticles (Cys-AuNPs) via hydrogen bondings. The aggregation of Cys-AuNPs causes changes in color from wine-red to blue-gray.

Selective magnetic nanographene oxide solid-phase extraction with high-performance liquid chromatography and fluorescence detection for the determination of zearalenone in corn samples.

A magnetic nanographene oxide sorbent as a selective sorbent for the magnetic solid-phase extraction combined with high-performance liquid chromatography and fluorescence detection was developed and proved to be a robust method for zearalenone determination in corn samples. Optimum extraction of zearalenone (20 mg magnetic nanographene oxide sorbent, extraction for 15 min, desorption time of 15 min using 1 mL of 0.5% formic acid in methanol) resulted in low limits of detection (05 mg/L) and quantitation (0.13 mg/L) and good linearity range of 0.13-1.25 mg/L with the correlation coefficient of 0.9957. Acceptable recoveries (79.3-80.6%) with relative standard deviations below 4% and satisfactory intra- and interday precisions (2-7.4%) were achieved. Additionally, the proposed method has been proved to be good in several aspects: easily prepared sorbent with high affinity to zearalenone, convenient and fast procedure, and high extraction efficiency.

Colorimetric determination of sialic acid based on boronic acid-mediated aggregation of gold nanoparticles.

The authors describe a rapid, sensitive and selective colorimetric assay for sialic acid (SA) based on the use of gold nanoparticles (AuNPs) modified with 3-aminophenylboronic acid (3-APBA) which acts as the recognition probe for SA. 3-APBA contains amino groups and boronic acid groups through which it can assemble on citrate stabilized AuNPs. It reacts with the cis-diol groups of SA by reversible formation of a cyclic boronate ester in slightly acidic buffer. Detection involves the sequential addition of AuNPs, phosphate buffer, 3-APBA and SA in a tube, vortex mixing, acquisition of photographic images or absorption spectra, and calculation of the result. The method is simple, rapid, and does not require cumbersome steps such as the preparation of stable boronic acid functionalized AuNPs as used in colorimetric sensing of saccharides. Under the optimum conditions, the ratio of absorbances at 700 and 520 nm increases linearly in the 0.15-1.00 mM SA concentrations range, and the detection limit is 60 µM. This is comparable to the detection limit obtained in other colorimetric assays reported. Acceptable intra- and inter-day precisions of three SA concentrations (0.50, 1.00 and 2.00 mM) ranged from 1.9-4.2% and 4.2-6.4%, respectively. The efficacy of the method was demonstrated by analyzing simulated human saliva which gave recoveries ranging from 98.7-106.0%. Graphical abstract Schematic of a colorimetric method for detection of sialic acid (SA) in simulated saliva. It is based on aggregation of gold nanoparticles with 3-aminophenyl boronic acid (3-APBA) which assembles on AuNPs while the boronic acid group binds to cis-diols of SA to form a boronate ester.

Robust colorimetric detection based on the anti-aggregation of gold nanoparticles for bromide in rice samples.

Inorganic bromide (Br-) is an important contaminant ion as it can originate from the overuse of illegal methyl bromide as a fumigant in stored rice samples. Herein, we developed a simple and highly sensitive colorimetric sensor for bromide ion detection in rice samples. The sensor is based on the anti-aggregation of gold nanoparticles (AuNPs) by Br- in the presence of Cr3+, which made the method more selective than other typical aggregations of nanoparticles. The AuNPs underwent an aggregation process as a result of the coordination of Cr3+ and the carboxylate group of a citrate ion stabilized the AuNPs, resulting in a red-to-blue color change. When Br- was pre-mixed with the AuNPs and Cr3+ was added, the solution color changed from blue to red with an increase in the Br- concentration. The anti-aggregation process can be detected with the naked eye and monitored using UV-vis spectrophotometry. The linear calibration curve ranged between 0.31 and 3.75 µM Br- with a low LOD and LOQ of 0.04 and 0.13 µM. The recovery was excellent, ranging from 79.9-92.2% with an RSD of less than 4.0%. The good inter-day and intra-day precisions were 2.9-6.4% and 3.1-7.1%, respectively. The developed sensor has proved to provide a robust method for Br- detection in rice samples.