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.

Development of a carbon quantum dot-based sensor for the detection of acetylcholinesterase and the organophosphate pesticide

In this study, a proper and reliable fluorometric method is introduced for screening acetylcholinesterase (AChE) and its inhibitors, using carbon quantum dots (CQDs) as the signal reporter. Pure, S-doped, and P-doped CQDs, were synthesized and their recoverable fluorescence quenching properties were observed, when exposed to Hg2+, Cu2+, and Fe3+ quenching ions, respectively. The study on the recovery of their emission showed that after the introduction of another guest substance with a stronger affinity to the quenching ions, their fluorescence is restored. The Design Expert software was employed to compare the performance of the three CQDs, as fluorescent probes, based on their quenching efficiency and the percentage of their emission recovery in the presence of AChE and acetylthiocholine (ATCh). Based on the statistical analysis, among the studied CQDs, S-doped CQD was the most suitable candidate for sensor designing. The detection mechanism for the proposed S-doped CQD-based sensor is as follows: The strong binding of Cu2+ ions to carboxyl groups of S-doped CQD quenches the fluorescence signal. Then, hydrolysis of ATCh into thiocholine (TCh) in the presence of AChE causes fluorescence recovery, due to the stronger affinity of Cu2+ to the TCh, rather than the CQD. Finally, in the presence of malathion and chlorpyrifos inhibitors, AChE loses its ability to hydrolyze ATCh to TCh, so the fluorescence emission remains quenched. Based on the proposed detection technique, the designed sensor showed detection limits of 1.70 ppb and 1.50 ppb for malathion and chlorpyrifos, respectively.

Determination of methomyl in grain using deep eutectic solvent-based extraction combined with fluorescence-based enzyme inhibition assays

A deep eutectic solvent (DES)-based extraction method is established to facilitate the determination of methomyl in grain via enzyme inhibition fluorescence. The environmentally-friendly DES was synthesized from proline and ethylene glycol and used as a green replacement for traditional extraction solvents that are generally toxic. The DES was added to grain samples and vortex extraction of methomyl, the supernatant was then collected for fluorescence detection. Biomass carbon quantum dots (CQDs) synthesized from millet were used as fluorescent probes. Acetylcholinesterase catalyzes the hydrolysis of acetylthiocholine iodide to thiocholine. The positively-charged thiocholine interacts electrostatically with the negatively-charged quantum dots resulting in the quenching of their fluorescent emission. The pesticide extract solution blocks the enzyme activity and thus recovers the fluorescent from the quantum dots. The fluorescence response was correlated with the amount of methomyl residue in the grain over the range 0.01 to 5 mg kg^-1. The limit of detection was found to be 0.003 mg kg^-1, and the limit of quantification 0.01 mg kg^-1. Recoveries of 86.5% to 107.8% were obtained using real samples, including millet, rice, wheat, and barley, with a relative standard deviation of less than 3.8%. The method is efficient and convenient and has good application prospects for extracting and detecting pesticides in grain samples.

Recent advances in synthesis and modification of carbon dots for optical sensing of pesticides

Serious threat from pesticide residues to the ecosystem and human health has become a global concern. Developing reliable methods for monitoring pesticides is a world-wide research hotspot. Carbon dots (CDs) with excellent photostability, low toxicity, and good biocompatibility have been regarded as the potential substitutes in fabricating various optical sensors for pesticide detection. Based on the relevant high-quality publications, this paper first summarizes the current state-of-the-art of the synthetic and modification approaches of CDs. Then, a comprehensive overview is given on the recent advances of CDs-based optical sensors for pesticides over the past five years, with a particular focus on photoluminescent, electrochemiluminescent and colorimetric sensors regarding the sensing mechanisms and design principles by integrating with various recognition elements including antibodies, aptamers, enzymes, molecularly imprinted polymers, and some nanoparticles. Novel functions and extended applications of CDs as signal indicators, catalyst, co-reactants, and electrode surface modifiers, in constructing optical sensors are specially highlighted. Beyond an assessment of the performances of the real-world application of these proposed optical sensors, the existing inadequacies and current challenges, as well as future perspectives for pesticide monitoring are discussed in detail. It is hoped to provide powerful insights for the development of novel CDs-based sensing strategies with their wide application in different fields for pesticide supervision.

Recent advances and perspectives of enzyme-based optical biosensing for organophosphorus pesticides detection

The overuse or abuse of organophosphorus pesticides (OPs) can bring about severe contamination problems in foodstuff and the environment, which will seriously threaten human health and the ecosystem's cycle. Hence, it is in high demand to establish sensitive, portable, specific, and cost-effective methods for monitoring OPs to control food safety, protect the ecosystem, and prevent disease. The optical biosensor with enzyme as bio-recognition elements has been an effective alternative for OPs detection. Herein, we firstly introduce various enzymes, sensing mechanisms, advantages and disadvantages used as bio-recognition elements in optical sensing for OPs detection. Then, we review various optical biosensing strategies based on enzymes as recognition elements that were ingeniously designed and successfully utilized for OPs detection, with a particular emphasis on photoluminescence (PL), chemiluminescence (CL), electrochemiluminescence (ECL), and colorimetric (CM) biosensing strategies. We not only highlight the state-of-art developments and the construction strategies of the enzyme-based optical biosensing method but also summarize the existing deficiencies, current challenges, and the future perspectives of OPs detection.

Spectroscopic detection of Hg2+ in water samples using fluorescent carbon quantum dots as sensing probe

Mercury (Hg2+) is remarked as toxic and hazardous element to global environment. Here, carbon quantum dots (CQDs) were synthesized by simple microwave assisted technique for Hg2+ detection in water samples via. fluorescence quenching and FT-IR spectroscopic approach. The morphology and chemical structure of synthesized CQDs was investigated by TEM, FT-IR, 13C-NMR, fluorescence and UV-vis spectroscopic technique. The resultant CQDs bears spherical morphology with an average size of 2–4 nm. The binding parameters, as Stern-Volmer quenching constant (Ksv) and binding constant for CQDs-Hg system was investigated by fluorescence method, whereas UV-vis techniques was employed for determination of thermodynamic parameters, as Gibb’s free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) at three different temperature (295, 298 and 305 K). Moreover, selectivity assay for Hg2+ detection has been studied in presence of other metal ions by FT-IR as well as fluorescence spectroscopy. Analytical assay was also successfully applied for Hg2+ in spiked water samples collected from different areas of Chhattisgarh, with 98–99 recovery %. The detection of Hg2+ has been demonstrated in the range of 0 to 5.0μM with 3.25 nM detection limit. The present method is found to be simple, highly sensitive and selective for sensing of Hg2+ in aquatic environmental samples using CQDs as sensing probe.