Current World Environment

Introduction

Organophosphate pesticides (OPs) are the ester forms of phosphoric acid usually considered as secure for agriculture uses due to their relatively fast degradation rates.¹ Although the degradation of OPs is a linear function of microbial composition, pH, temperature, structural arrangement etc. OPs inhibit acetylcholinesterase (AChE) activity not only in insects only, but can also affect the nervous system of other organisms as well as humans.^1-3 Literature data illustrated the OPs persistence in soils years after their application.⁴ But the reason behind this environmental persistence is not very clear. Pesticides has been transferred to humans through the food chain^4,7-9 and number of environmental^4-6 and health^10-14 issues have aroused the public concern during last few years. By kept all these views in mind we are going to carve a review on OPs, divided into following parts.

Structural Properties of OPs

Organophosphate pesticides derived from phosphorus analog PH₃ having phosphorus as a core nuclei involved in oxidation states III and V. Basically organophosphate is the general name for esters of phosphoric acid. Hydrolyzed derivatives of phosphorus formal incorporation of additional oxygen atoms gives phosphinic acid (O=PH₂OH) and phosphonic (phosphorus) acid [O=PH(OH).² Notably, these species may tautomerize between P(V) and P(III), that is, H₂P(O)OH to HP(OH)₂. Also a tetrahedral structure [O=PH(OH)₂] is more established than its isomer phosphonic acid, P(OH)₃. This form can be stabilized by coordination with some metals_.¹⁶

Methods of Detection of OPs

The combination of FTIR, NMR and Mass data is often sufficient to determine completely the structure of an unknown molecule. There has been a longstanding interest, first among inorganic chemists, about how organophosphates bind to metals, and next by analytical chemists, about how an adequate detection device can be engineered. Ligand optimization and reporting media continue to be explored in OPs detection efforts that involve various spectroscopic techniques such as UV-Visible, FTIR, Mass and NMR spectroscopy.

UV- Visible Spectroscopy

In the field of pesticides UV-Vis. Spectroscopy play the vital role in the detection and interaction of metal ions with organic ligands i.e. pesticides, especially with transition metal ions. P=O can obscure absorbance. Agents themselves are not significant absorbers or emitters in the UV-Vis spectral region¹⁷ unless they are specifically modified with a fluorescent coumarin-type leaving group.¹⁸ With the inclusion of a photoabsorber material, such as a porphyrin, photocatalytic degradation of acephate and monocrotophos in the presence of TiO₂ indicated that the decomposition of acephate begin from the destruction of C-N and P-N bonds.^19-23 The ZnFe₂O₄-TiO₂ composite photocatalyst is prepared by sol-gel method and used to degrade acephate successfully.²⁴ Cu(II)-Glyphosate complex was studied in tea at pH 5 at absorbance 250 nm.²⁵ A UV-Visible based study to detection of monocrotophos at 490 nm included the effect of temperature and different regent viz. 2, 4, dinitrophenylhydrazine and NaOH at different pH  level.²⁶ The photolysis of phorate has been studied as a thin film on a glass surface and in a solution of methanol‐water (60:40) by ultraviolet light (λ > 290 nm). The rate of disappearance of phorate in the solution show first order kinetics with a rate constant of 4.9 × 10^–5 s ^–1.²⁷ Metal complexes of Fe(III), Al(III), Co(II) and Zn(II) studied by using UV-Spectrophotometer, it observed that the  complexes of methylphosphonic acid,  formed the M-(CH₃PO₃)₃·3H₂O, and aminomethylphosphonic acid, formed the M-OH(NH₃CH₂PO₃)₂·H₂O. Also complexes of N-phosphonomethylglycine were prepared and the formula was M-OH(−OOC–CH₂–NH₂⁺–CH₂–PO₃²⁻)·2.25H₂O is proposed. UV-Vis. study indicating that the complexes form by the chelation of P=O and N-H bonds by consuming two ligands and two water molecules.²⁸ FTIR Spectroscopy: IR spectroscopy is indispensible for many systems mentioned herein; direct access to monitoring the phosphoryl stretch is very useful. Electron-poor ions such as Fe(III) and Cu(II) favor stronger coordination and give [P=O] stretching frequencies concomitantly lower by 30-100 cm^-1. Some other functionality, such as perchlorate Cl-O stretches, can obscure P=O absorbance. In OPs from literature following main stretching and bending frequencies observed in different solvents.^29,30 OPs having the thio as well as amino salts, for the thio group ν(S-H) and ν(C-S) observed at 2550 and 730 cm^-1. Ammonium salts are divided into following parts as per their IR frequencies; these are primary, secondary and tertiary ammonium salts. In all these salts ν(C-N) observed between the region 1380 – 1250 cm^-1 and ν(N-H) between the 1550 – 1630 cm^-1. For the ammonium salt a strong band of ν(N-H) at 1430cm^-1 also reported in literature. In the FTIR study of copper complexes, for the yellow complex; [Cu(Ph₃P=O)₂Cl₂] the ν(P=O) observed at 1142 cm^-1. For the dark red complex; [Cu(Ph₃P=O)₂Br₂] the ν(P=O)  observed at 1145,1169 cm^-1. For free Ph₃P=O, ν(P=O) is 1195 cm^-1. These compounds were found to be tetrahedral. The cation moiety Cu(II)-(Me₃PO)₄ was square planar. Arsenic analogs were yellow-brown ν(P=O) at 840 cm^-1 and olive green ν(P=O) at 842 cm^-1. Perchlorate species were also obtained, but the C=O stretching bands in the IR spectrum obscured the P=O stretches. In MgI₂ complex with diphenylphosphinates; it was found that the PO-C bond, not the P-OC bond, was cleaved.¹⁵ In the interaction of Cu(II) with DIMP the IR frequency of ν(P-O) and ν(P-O…H) observed at 1016 and 1206 cm^-1.³¹ Metal complexes of Fe(III), Al(III), Co(II) and Zn(II) studied by using FTIR, it observed that the  complexes of methylphosphonic acid,  formed the M-(CH₃PO₃)₃·3H₂O, and aminomethylphosphonic acid, formed the M-OH(NH₃CH₂PO₃)₂·H₂O. Also complexes of N-phosphonomethylglycine, were prepared and the formula was M-OH(−OOC–CH₂–NH₂⁺–CH₂–PO₃²⁻)·2.25H₂O is proposed. FTIR study indicating that the complexes form by the chelation of P=O and N-H bonds.²⁸  In the FTIR study of interaction of OPs herbicide glyphosate with Fe(III) in aqueous solution at pH 4, suggests that coordination of Fe(III), or more likely Fe(OH)²⁺ species, occurs through the phosphonic group, glyphosate shows no evidence of coordinating the metal through the carboxylate anion or the amino group; however, significant changes are observed in the range for the phosphonic group vibrations as expected for metal-phosphonate coordination.³²

NMR Spectroscopy

Nuclear magnetic resonance (NMR) is a spectroscopic method that is even more important to the structure elucidation than other spectroscopic techniques. Many nuclei may be studied by NMR techniques viz. ¹³C, ¹H, ³¹P, ¹⁵N, and ¹⁹F. Any atomic nucleus that possesses either odd mass, odd atomic number split in the radiofrequency region. NMR gives information about the number of magnetically distinct atoms of the type being studied.²⁸ NMR spectroscopy supports many areas of chemistry and science. NMR spectra have found great utility in monitoring reactions and characterizing new compounds and also in detection of OPs and their fragments.^33,34 There are numerous nuclei that can be brought to bear in OPs studies, especially the ³¹P nucleus. Analytes that contain characteristic ¹³C, ¹H, and ³¹P, as well as ¹⁵N and ¹⁹F, NMR signals can be probed.³⁵ Other NMR active nuclei and related experiments also are found in the literature. The types of experiments have involved HSQC NMR, as well as magic angle spinning.¹⁵ Next, shift reagents can change the phosphorus δ value.¹⁵ One early study involved mixtures of (Me₂N)₃P=O, DMMP, or (MeO)₃P=O, in the presence of Be²⁺ and Al³⁺ ionic centers and a series of phosphates was treated with Co²⁺ and Fe³⁺.³⁶ The species(MeO)₃P=O, (EtO)₃P=O, and (MeO)₂P=O(Me) were all found to give shifts upon binding; also P-H decoupling was observed. With this technique, changes in P-S bonding can be monitored.³⁷ Variable-temperature ³¹P NMR spectroscopy was used in studying resin-based systems with two DMMP adsorption sites, the macro-reticular region and the quaternary ammonium hydroxide ion-exchange sites.³⁸ It was found that DMMP may migrate from one site to another. There is also a report of lanthanide-induced shifts from authors who have also been active in the organophosphonate sensor area.³⁹ An NMR spectroscopic assay was also developed to conveniently determine the purity of live agents of OPs derivatives.³¹ P NMR spectroscopy can also be used in studies that involve enzymes that degrade agents. Studies that successfully determine discrete cleavage events have used ³¹P.⁴⁰ Nuclei other than ¹³C, ¹H, and ³¹P NMR also occasionally hold prominence. The ²⁷Al, ¹¹³Cd, and ¹⁹⁹Hg nuclei have been utilized in terms of monitoring the adducts and mineralization.¹⁵ A ¹H-NMR study of phorate deals to photodegredation of phorate, indicated the P-S bond destruction.

X-ray Diffraction

X-ray diffraction is among the unswerving techniques to studies and elucidates the intricacies of metal ligand structure and probable binding possibilities that help open up a casement for future sensing possibilities. In study of [R₃P=O---Mⁿ⁺] moieties (R = alkyl, aryl) and (M = any metal), the mean P=O bond length in [ P=O---Mn⁺] interactions is 1.48 Å and the mean M---O bond length is 2.33 Å.¹⁵ The importance of these structures is that they resemble a deprotonated acid fragment bound through three atoms to one metal center. (CH₂)₂ could be thought of as holding the place of [-P=OMe(OR)-]. This motif is pragmatic with respect to a degraded sample in which the (-OR) has been hydrolyzed off. Metal complexes of Fe(III), Al(III), Co(II) and Zn(II) studied by using XRD-Spectrophotometer, it observed that the  complexes of methylphosphonic acid formed the M-(CH₃PO₃)₃·3H₂O, and aminomethylphosphonic acid, formed the M-OH(NH₃CH₂PO₃)₂·H₂O. Also complexes of N-phosphonomethylglycine were prepared and the formula was M-OH(−OOC–CH₂–NH₂⁺–CH₂–PO₃²⁻)·2.25H₂O is proposed. X - ray study representing that metal ions concerned in octahedral structure.²⁸ Electrochemical: Almost all current instrumental techniques that explicitly determine the presence of pesticides are generally exclusive and non-portable, such as FTIR, mass chromatography and NMR spectrophotometer etc. But electrochemical methods are simple, versatile, in terms of controlling and altering the behavior of redox materials. Transition metal ions here have vulnerable properties. Such equipment is not too bulky and has a portable device, principally in regards to recent lab-on-a-chip research efforts. Reviews in this area mostly deal with biosensing. Organophosphonate electrochemistry with sarin, (EtO)₂(EtSCH₂-CH₂S)P=O, parathion, and malathion. There were reports of a mercury surface with which polaragram data was recorded. Chemical groups point outward and waters are replaced stepwise in a Cu²⁺-complex by the surface-active compounds R₃P=O. In the late 1990s, there are outstanding electrochemistry papers concerning with organophosphorus hydrolase.^41-43 Also, some studies involve the use of phthalocyanines. Thus, this section involves structurally positioned redox active metal ions that do not directly bind with OPs donor atoms. Recently, electronic tongue array, consisting of an eight working electrodes (Au, Pt, Ir, Rh, Cu, Co, Ni, and Ag) was used to detect nerve agent stimulants DCP and DECP in aqueous environments.^44,15 Mass Spectrometric: The fundamental principles of mass spectroscopy (MS) to determined the mass-to-charge ratio of the molecule by ionizing it by using different procedures. The number of ions with a particular mass-to-charge ratio is plotted as a function of that ratio. The types of MS techniques include MALDI-TOF, GC-MS, ESI, SPAMS, and desorption electrospray. MALDI-TOF and MS-TOF was presented as an effective way of determining widespread emergency events.⁴⁰ A detection by MS of species that include (RO)₂P=O(R₁), (RO)P=O(R₁)F, (RO)P=O(R₁)(SR₂), and (R₂N)P=O(OR₁)(CN).⁴⁴ MS also provides support for some surface-based studies. A study include the development and inter-laboratory verification of LC–MS libraries for organic chemicals of environmental concern, includes the 129 pesticides in which the monocrotophos and phorate having the m/z at 193, 98 and 75. In the study it was observed that more than 90% data was accepted in both modes.¹⁵ The determination of OPs in human blood and water using solid-phase microextraction and gas chromatography with mass (GC-MS) spectrometric detection performed.^45,46 A Multiresidue detection of pesticide in fishery products was conducted by using the tandem solid-phase extraction technique. Study includes more than 50 OPs. Zero recovery was obtained from samples fortified with acephate and monocrotophos and 118.2 and 125.4 % in sample amino-propyl, the maximum label spiked (mg/L) for both is 5.⁴⁷ Method validation and comparison of acetonitrile and acetone extraction for the analysis of 169 pesticides in soya grain by liquid chromatography–tandem mass spectrometry.^45-47 The interesting disulfide derivative [bis(diisopropylaminoethyl)disulfide] was determined from a soil sample (detected at 1 μg per 1.0 g of soil).¹⁵ Secondary ionization (IM-TOF-MS) was used in detecting pinacolyl methylphosphonate, diethyl phosphoramidate, and 2-(butylamino)ethanethiol.¹⁵ Some reports involve the mention of pesticides: malathion was studied with GC-FID.⁴⁷ ES-MS was used to support microsynthesis of various O,Odialkyl- N,N-dialkylphosphoramidates to generate a library of mass spectra.⁴⁸ Additionally the methyl esters of N,N-dialkylaminoethane- 2-sulfonic acids, R₂NCH₂CH₂S(O)₂OCH₃, were analyzed by GC-EI mass spectroscopy.⁴⁹ Rapid determination of pesticide residues in Chinese materia medica using QuEChERS sample preparation followed by gas chromatography–mass spectrometry.⁵⁰ In a minipig model, plasma was used in studying (iPrO(P=O)Me-(OH)) and cyclohexyl-O(P=O)Me(OH). In another study, albumin was studied; peptide fragments of human serum albumin were analyzed in response to chlorpyrifos oxon, dichlorvos, diisopropylfluorophosphate, and sarin.^50-53 Complexes of Ni, Co, and Fe, were evaluated for the OPs sensing, but Cu²⁺ gave the best response indicating that the ligand had to accommodate enough vacancy. Thus, at this early point it was concluded that “copper complexes may adsorb and desorb phosphorus esters in air.”¹⁵

Conclusion

Among the all methods of detections mass and electrochemical based methods are flourished in recent years but there is slow development in FTIR and UV based sensing and detection of OPs which is highlighting a gap in study. Even OPs have the short life time of decomposition but they persist and leach out in soil and environment which is the matter of huge concern, so there is need to develop the new moieties which minimize all these risks and hazards. For the future safeguard, there is need to development of antidotes for intoxication with neurotoxic is  one of the most important task, not only because their potential use as chemical warfare defense agents, but also for the treatment of intoxication with organophosphorus pesticides, which are very intensively used in agriculture. There is the need to develop quicker, cheapest, portable methods for agent and pesticides sensing..

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