Chemical pesticides have proven to be a boon for the development of nations to exterminate the pest-borne diseases to improve the quality of food products and for the protection of forests, fibres and plantation. But the continuous use of the pesticides for the protection of crops from pests has greatly affected the human life as well as the ecosystem1. Amongst various pesticides, organophosphates (OPs) are one of the highly toxic ones. Commonly used OPs are parathion, malathion, paraoxon and chloropoyrifos.
Presence of organophosphates either in high amount or even in trace amount in agricultural products and water reservoirs possess a great danger to humans, such as abdominal pain, respiratory problems, eye pain, paralysis and can even cause death2. Their toxicity is attributable to their irreversible binding with acetyl cholinesterase (AchE) enzyme that is essential for proper functioning of nervous system3. Hence due to the serious health threats posed by OPs have made them a target for the determination.
Great efforts have been made by researchers for the development of methods for the determination of OPs residues in crops. Various methods like gas/liquid chromatography, mass spectroscopy, high performance liquid chromatography (HPLC) have been carried out to detect OPs, but these are costly, require extensive labour and are time consuming4. This has led to the development of biosensors that have high sensitivity and are useful for the rapid detection of analyte. Amongst the recently developed biosensors, the most commonly used biosensors are the electrochemical biosensors. Earlier enzyme based electrodes were used for the detection of pesticides, but due to the denaturation of enzymes under harsh conditions such as temperature and pH , the working of sensors were highly affected. Moreover, the enzyme also loses selectivity due to the contamination of heavy metals and pesticides5. Therefore researchers have changed their focus to non-enzymatic biosensors for the detection of pesticides as these possess good reproducibility and are highly stable.
In this regard in the last few decades owing to remarkable development in the field of nanotechnology various novel materials like carbon nanotubes, graphene, molecular imprinted polymer, quantum dots etc. have been utilised as matrices for the non-enzymatic detection of pesticides.
Metal nanoparticles and particularly gold nanoparticles have high surface area to volume ratios, high conductivity and better interface dominated properties and owing to these properties they have been widely used for the sensing purpose. Zhang et al.6 have developed an electrochemical sensor for the detection of parathion by depositions of gold NPs on multiwalled carbon nanotubes modified carbon electrodes. A high detection limit of 1×10-7 M with signal to mole ratio of 3 was observed. It was also used for the real sample analysis. Sun et al.7 have prepared Quartz/APES/Au-NP/L-Cys/ID fluorescent assembly on quartz surface using chemisorption for the detection of organophosphates. Yunhe Qu and his co-workers8 have developed a nanocomposite of Au-TiO2 using chitosan as conjuct. This nanocomposite has high sensitivity for parathion with a detection limit of 0.5 ng/m .
Liu et al.9 have utilised zirconia nanoparticles on gold electrode for simple, fast and sensitive detection of nitroaromatic OPs. Zirconia being thermal stable, chemically inert, less toxic, and because of strong affinity for phosphoric acid has shown an enhanced sensitivity for the detection of OPs. Zirconia has also been used as a sorbent in the detection of OPs. Sorbent based electrochemical sensors have been widely used for the determination of OPs. The sensitivity of these sensors can be improved by combining them with nanomaterials. Shuo Wu et al.10 have developed a sorbent from electrochemical reduced β-cyclodextrin dispersed graphene (β-CD-graphene) for sensing of methyl parathion.
CuO because of high surface area and better electrochemical properties has been used as an alternate to electrode material. Soomro et al.11 have developed CuO nanostructures using hydrothermal growth method at low temperature using green amino acids as biocampatible tamplets for the detection of OPs.
Graphene because of its high surface area, thermal conductivity and mechanical properties has been combined with nanoparticles, chitosan and have been investigated as an electrochemical sensor. A nanocomposite of graphene/carbon nanotubes/chitosan for enzymeless detection of OPs using methyl parathion as an analyte was developed by Liu et al.12. Gong et al.13 have developed an enzymeless sensor by modifying glass electrode with nanoassembly of two dimensional graphene decorated with gold NPs. Synergic combination of two have dramatically increased the electrolytic activities for the reduction of parathion. Zhi et al.14 have developed a unique enzymeless sensor based on nanocomposite of copper (II) chelating functionalized graphene. The ligand interaction has resulted in enhanced amperometric response allowing injection flow detection mode with the traits of self- regeneration and reproducibility. This sensor has the advantage of differentiating between sulfarated organophosphate pesticide and generic organophosphate agents. The interference from generic organophosphates and inorganic sulfur containing ions was also negligible. Graphene and chitosan composites have also been utilised for the detection of OPs.
Solid phase extraction (SPE) is the most popular and actively used technique for sample preparation. Carbon nanotubes due to π-conjugative structure and hydrophobic structure has been used as a useful SPE sorbents. Du and his co-workers15 have employed this technique for the extraction of nitroaromatic OPs where multiwalled carbon nanotubes were acting as sorbents where the extraction of large amount of pesticide was made possible in less than 5 minutes. Similarly Gong et al.16 have also developed an electrochemical stripping sensor based on the technique of solid phase extraction using platinum intercalated Ni/Al layered double hydroxides. The sensor has both ideal-stability and good reproducilbility. Yang et al.17 have developed a method for the determination of OPs within 5 minutes by synthesising Au-ZrO2-SiO2 nanocomposite that acts as selective sorbents for the SPE of organophosphorus pesticide.
Solid phase microextraction (SPME) is another technique that uses coated-fused silica fibres for extraction of analyte from aqueous or gaseous phase. Presently this technique has been coupled with analytical methods such as SPME/HPLC, SPME/GC, SPME/MS, SPME/UV etc. Huang et al.18 have used this SPME coupled with SnO2 gas sensor for the detection of organophosphorus pesticide residues such as dichlorofos in cabbage. Polar plots and plot area have been used for quantitative analysis.
Chemiluminscence (CL) method being highly reliable and sensitive has been used by the researchers for the detection of organophosphate pesticide. A molecular self assembly technique for imprinting of polymer membrane of chlorpyrifos at the surface of TiO2 nanoparticles was designed where the catalysis of TiO2 has made possible the chemiluminescence detection of pesticides. Xie et al.19have developed molecular imprinted polymer and chemiluminiscence reaction for detecting the system
Based on the literature survey, we can say that non-enzymatic biosensing techniques have been proven to be beneficial for the fast, simple, green and sensitive detection of organophosphate pesticides.
Plan of work:
The present project focuses on:
- Synthesis of carbon based nanostructures to be utilised as matrix for sensing pesticide.
- Microscopic, optical and electrochemical investigation of the designed matrices.
- Optimization studies in terms of various parameters (e.g. pH, incubation time, etc.) for ideal sensing.
- Determination of the pesticides via electrochemical methods by finding various sensing parameter.
- Interference and recovery studies of the sensor matrix.
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(11). Soomro,A.R.; Hellem,K.R.; Ibupoto,Z.H.; Tahira,A.; Sherazi,S.T.H.; Sirajjuddin; Memon,S.S.; Willander,M. Amino acid assisted growth of CuO nanostructures and their applications in electrochemical sensing of organophosphate pesticide. , Electrochim. Acta, 2016, 190, 972-979
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(17). Yang,Y.; Tu,H.; Zhang,A.; Du,D.; Lin,Y. Preparation and caharacterization of Au-ZrO2-SiO2 nanocomposite spheres and their applications in enrichment and detection of organophosphorus pesticide. J. Mater. Chem., 2012, 22, 4977-4981.
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