Cu2O nanoparticles for the degradation of methyl parathion

Methyl parathion (MP) is one of the most neurotoxic pesticides. An inexpensive and reliable one-step degradation method of MP was achieved through an aqueous suspension of copper(I) oxide nanoparticles (NPs). Three different NPs sizes (16, 29 and 45 nm), determined with X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), were synthesized using a modified Benedict’s reagent. 1H nuclear magnetic resonance (NMR) results show that the hydrolytic degradation of MP leads to the formation of 4-nitrophenol (4-NPh) as the main product. While the P=S bond of MP becomes P=O, confirmed by 31P NMR. Although Cu2O is a widely known photocatalyst, the degradation of methyl parathion was associated to the surface basicity of Cu2O NPs. Indirect evidence for the basicity of Cu2O NPs was achieved through UV–vis absorption of 4-NPh. Likewise, it was shown that the surface basicity increases with decreasing nanoparticle size. The presence of CuCO3 on the surface of Cu2O, identified using X-ray photoelectron spectroscopy (XPS), passivates its surface and consequently diminishes the degradation of MP.


Introduction
Organophosphorus pesticides (OPPs) are one of many kinds of pesticides that have attracted some attention mainly due to their neurotoxic effect [1][2][3]. The primary mechanism of action of OPPs is that they are effective inhibitors of acetylcholinesterase through the interaction with serine inside the nucleophilic active site of the enzyme to form a phosphorylated enzyme derivative, which is more resistant to subsequent hydrolysis than the normal acetylated derivative. Therefore, the inhibition is essentially irreversible [2]. Inhibition of acetylcholinesterase leads to an accumulation of the neurotransmitter. This, in turn, causes seizures and respiratory failure, which are the main causes of death [3]. O,O-Dimethyl O-(4-nitrophenyl) phosphorothioate, most commonly known as methyl parathion (MP), is among the most acutely toxic pesticides used in agriculture [4][5][6]. MP includes other risks for human health, such as the induction of changes in tertiary villi of the placenta of women exposed to this OPP [7].
It has been estimated that in the year 2020 about 153,000 metric tons of OPPs will be used worldwide [8]. In Mexico, roughly 5,732 metric tons of MP are mainly used annually for the production of beans, cabbage, soy, wheat, lettuce, and tomatoes [5,6], despite the fact that MP is a forbidden pesticide by the United Nations Rotterdam Convention. Due to the large volumes of MP used in agriculture, thousands of metric tons, and because MP is highly neurotoxic, there has been extensive studies about the degradation of MP using different materials [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. All studies about the degradation of MP can more or less be classified as biotic, photocatalytic, or chemical degradation [27]. In aqueous media, chemical degradation of MP can result in either oxidation, isomerization or hydrolysis as some authors have suggested [11][12][13][14]28]. Oxidation of MP leads to the formation of methyl paraoxon, which is much more toxic than MP. Isomerization also leads to the formation of other phosphorothioates that are acetylcholinesterase inhibitors. Therefore, hydrolysis is the desired route of degradation of MP. Strictly speaking all degradations of MP are chemical. What is meant by biotic degradation is that bacteria are used for the degradation of MP, whereas photocatalytic degradation needs photons in the form of UV light and chemical degradation utilizes chemical species, such as copper(I) oxide (Cu 2 O) NPs in this work. Cu 2 O is widely known for its photocatalytic activity [29][30][31][32][33]. However, there are scarce studies of its applications regarding its basicity. In this work, copper(I) oxide NPs of different sizes were synthesized using a modified Benedict's reaction. They were used for the first time in the chemical degradation of MP. The advantages of using Cu 2 O are that it is an inexpensive, abundant, moderately stable, and reliable source of material for the degradation of MP. It is well known that nanoparticles have the advantage of a relatively high surface area. We have used this advantage to increase the basicity of Cu 2 O in the form of surface hydroxy groups (OH). This also decreased the degradation time while increasing the degradation efficacy. Our results suggest that the surface basicity of Cu 2 O NPs leads to degradation of MP without the need of other chemical substances or the use of photocatalysts that generate free radicals. The presence of free radicals is undesired since there is a rising consensus on the damage that these reactive species, formed during the photocatalytic reactions, cause to cell membranes by peroxidation of the polyunsaturated phospholipids [34]. This leads to the subsequent loss of activity that relies on an intact membrane, and ultimately to the death of organisms. While this may be an advantage for water disinfection, it is a great disadvantage for the removal of OPPs in natural waters. Thus, Cu 2 O NPs are an excellent substance for the degradation of MP.

Experimental Reagents
The chemical reagents used for the synthesis of Cu 2

Preparation of Cu 2 O NPs
For the preparation of Cu 2 O nanoparticles Benedict's reagent was used [35], with the variation of a water/dimethyl sulfoxide (DMSO) solvent mixture in order to obtain different NPs sizes. The modified Benedict's reagent was prepared as follows: In 50 mL of distilled water, 1.257 g (5 × 10 −3 mol) of CuSO 4 ·5H 2 O and 2.941 g (10 × 10 −3 mol) of Na 3 C 6 H 5 O 7 ·2H 2 O were dissolved. Then, 1.06 g (10 × 10 −3 mol) of Na 2 CO 3 were added and, lastly, 0.901 g (5 × 10 −3 mol) of C 6 It is important to note that the reaction in Equation 1 is not balanced in order to point out that a 1:1 mole ratio between the copper complex and glucose is needed. Glucose loses an electron while copper gains an electron. Also, the DMSO/H 2 O mixture is only used for the synthesis of Cu 2 O NPs with different sizes, it is not used in the degradation of MP.

Methyl parathion degradation
The degradation of MP was achieved in deionized water by reacting MP with Cu 2 O NPs in a 1:5 molar ratio. This was carried out using 250 mL of a 1.5 × 10 −4 M aqueous solution of MP (3.75 × 10 −5 mol) containing 26.8 mg of Cu 2 O NPs (1.87 × 10 −4 mol). First the MP solution is prepared by dissolving 9.4 mg of MP in 250 mL water. Then, 26.8 mg of Cu 2 O NPs is added to the solution. Since the Cu 2 O NPs do not dissolve in the parathion solution, the NPs were dispersed by sonicating for 90 s. Also, a constant stirring was maintained throughout the degradation of MP with Cu 2 O NPs. Table 1 summarizes the dispersion conditions for the Cu 2 O NPs of different size. The concentration of Cu 2 O was calculated by diving the amount of substance of Cu 2 O by the volume of the dispersion and does not represent the concentration of NPs because they are made up of different amounts of Cu 2 O. When the reactions were over, the NPs were separated by centrifugation. This was done also before each UV-vis absorption and NMR measurement. Degradation of MP with bulk Cu 2 O was also tested, giving similar results. Likewise, degradation experiments in the darkness were also performed giving identical results to those under daylight thus photocatalytic degradation was ruled out.

Results and Discussion
Characterization of Cu 2 O NPs with powder XRD and HRTEM The structural and morphological characterization of Cu 2 O NPs was carried out using powder X-ray diffraction and high-resolution transmission electron microscopy. Copper(I) oxide is practically insoluble in water (K sp = 2 × 10 −15 @ 25 °C). Since the NPs remain in the powder form throughout the entire degradation, XRD is a very useful technique for the characterization of Cu 2 O. Figure 1 shows the powder XRD of the Cu 2 O NPs before and after the degradation of MP. As far as the sensitivity of this technique, the NPs are essentially inert since they do not oxidize in the MP solution. In both cases the XRD results are consistent with the powder diffraction file: PDF #74-1230, which corresponds to cubic crystals of Cu 2 O. XRD diffractograms show a broadening of the peaks with decreasing nanoparticle size, this is best explained by the small crystallite size of the NPs. An approximate size of the NPs can be calculated through measurements of this broadening [36].
Using the Scherrer equation, the three NPs sizes obtained were 16 ± 3 nm (yellow powder), 29 ± 3 nm (orange powder), and 45 ± 9 nm (bright red powder). The colored powders can be seen in Figure 2, as well as their color in aqueous dispersion. It is important to mention that there is no evidence in XRD for the presence of CuO or CuCO 3 , although these compouds are observed in XPS.   Figure 3b shows the FFT from the area marked with a yellow square in Figure 3a. The processed image in Figure 3c was obtained from the same area. Interplanar distances corresponded to the (211) and (110)  Degradation study of MP using NMR 31 P NMR is used as a characterization technique for the degradation of MP [11,21,[37][38][39][40]. Figure 4 and Figure 5 show the 31 P NMR spectra of the products obtained after 14, 44 and 144 h of degradation time using Cu 2 O NPs. In all cases, the spectrum for 0 h corresponds to pure MP with a chemical shift of 65.6 ppm as reported elsewhere in the literature [38][39][40][41]. The 31 P NMR spectra of Figure 4 show the results obtained when Cu 2 O NPs with an approximate size of 29 nm were used for the degradation. In this case, the final product formed is dimethyl hydrogen phosphate with a chemical shift of −4.9 ppm in deuterated chloroform (CDCl 3 ). Similarly, the chemical shift of 42.2 ppm is that of dimethyl phosphorothioate (P=S) [40], which then hydrolyzes after 44 h to form dimethyl hydrogen phosphate (P=O) and the NMR peak at −4.9 ppm developes [42]. The intensity of the chemical shift is relatively low due to the low solubility in CDCl 3 , but when D 2 O is used the intensity increases under the same experimental conditions and there are two chemical shifts: one at −4.3 ppm, which corresponds to protonated form (acid), and the one at 1.6 ppm belonging to the deprotonated form (anion), both of which are in equilibrium [42] (see below Scheme 1 for their corresponding molecular structural formulas).   Figure 4 and Figure 5 is the absence of the chemical shift for MP (65.6 ppm) after 14 and 44 h of degradation time, this does not mean that all the MP was degraded within that reaction time but instead it is attributed to the technique used for dehydration (lyophilization) before the NMR spectra were obtained. In other words, during the lyophilization process when water is removed by lowering the temperature and pressure followed by an increase in temperature so that water is removed by sublimation and consequently methyl parathion is also removed and therefore absent in the NMR spectra.   [14], and not at aliphatic or aromatic carbon atoms (S N 2 @C) [14,39].
Furthermore, Cu 2 O NPs play an important role in the degradation of MP since hydroxy groups are found on its surface (see XPS results below). These surface hydroxy groups can either be directly involved in the S N 2 @P mechanism or they can polarize the oxygen-hydrogen bonds of the water molecules and thus facilitate the hydrolysis of MP. Further research regarding the exact mechanism for the degradation of MP on the surface of Cu 2 O NPs is in progress. Scheme 1 shows the observed degradation pathway considering all the NMR results obtained.

Degradation study of MP using UV-vis spectroscopy
NMR results indicate that one of the degradation products obtained is 4-nitrophenol (4-NPh). The presence of 4-NPh makes quantification of the degradation much easier because 4-NPh absorbs light in the UV-vis range. Hence absorption spectroscopy was used along with the Beer-Lambert law [44]. The molar absorptivity coefficients were determined to be 10080 M −1 ·cm −1 (λ = 320 nm) for 4-nitrophenol and 17632 M −1 ·cm −1 (λ = 400 nm) for 4-nitrophenolate (4-NPh − ), these results are similar to those reported in the literature [39,44]. Figure 7 and Figure 8 are the UV-visible spectra for the degradation of MP with different NPs sizes. Degradation times are indicated with different colors. The band around 280 nm corresponds to MP, the band around 320 nm to 4-NPh, and the band around 400 nm to 4-NPh − .
The absorption band at 400 nm gives a bright yellow color, which can be used to visually determine that the degradation is taking place. The yellow color intensifies as the degradation time increases. It can be seen that the intensity of the band at 280 nm starts to decrease with increasing degradation time while the intensity of the bands at 320 and 400 nm increases. These results are expected but the relative intensities between the 4-nitrophenol (320 nm) and 4-nitrophenolate (400 nm) are different depending on the NP size ( Figure 7 and Figure 8). A  This last result is best explained with Pearson's concept of basicity [45,46], low oxidation number metal oxides are alkaline in aqueous medium. Thus, Cu 2 O is a basic metal oxide. Similarly, as the NP size decreases the surface-to-volume ratio increases. A higher surface area implies a higher amount of hydroxy groups [47,48] and, hence, a higher basicity. MP degradation can be further extended to different metal oxides as others have already reported on the literature [15,16,[22][23][24][25][26]49]. One major difference in this work is the absence of free radicals since the degradation is not photocatalytic. This absence of free radicals makes Cu 2 O NPs a reliable source for the degradation of MP in natural waters. Figure 9 shows the degradation of MP at different reaction times for all three nanoparticle sizes.
Since water is a reactant as well as the solvent, it is prudent to assume a reaction of pseudo first-order kinetics because water is in excess with respect to MP. The degradation of methyl parathion in water is accomplished to about 87% after 44 h of reaction time using 16 nm Cu 2 O NPs, to about 84% with 29 nm

Surface study of Cu 2 O NPs using XPS
It is worth noting that there is a small difference in degradation percentage between Cu 2 O NPs of 16 nm (87%) and those of 29 nm (84%), but a larger difference between the Cu 2 O NPs of 29 nm (84%) and those of 45 nm (75%). The degradation percentage should increase with a reduction in NP size. However, the almost inexistent difference (3%) between the 16 nm and 29 nm NPs suggest the influence of other factors. In order to further study this small difference in degradation percentage between 16 nm and 29 nm NPs, X-ray photoelectron spectroscopy (XPS) analyses were carried out. Figure 10 shows the Cu 2p ( Figure 10a) and O 1s (Figure 10b) XPS spectra obtained for Cu 2 O NPs of 16 nm and 29 nm after the degradation. In Figure 10a, the peak at 932.4 eV was fixed for all samples so that it matches with the Cu 2p 3/2 of Cu 2 O reported in the literature [50,51]. The peak at 952.3 eV is the corresponding spin-orbit splitting (2p 1/2 ) of Cu 2 O. Also, in Figure 10a there is a small peak at 933.6 eV that is assigned to Cu 2p 3/2 of CuO. This last peak was placed in the fitted spectra because there are two peaks at 943.6 eV and 946.4 eV that have been widely accepted as shake-up satellites of Cu 2p and thus implicate the presence of CuO. In Figure 10b, the presence of CuO is more noticeable in the O 1s XPS spectra with a peak at 529.3 eV [50][51][52].  Figure 10b, the 16 nm NPs have a higher relative intensity than the 29 nm NPs. Nonetheless, the amount of surface OH groups seen in XPS is not representative of the reaction conditions because more of these groups should form on the surface of the Cu 2 O NPs when they are placed in water [47,48]. One important difference in the O 1s XPS spectra between 16 nm and 29 nm NPs is the peak at 533.4 eV, which corresponds to CuCO 3 [52]. This carbonate species is also observed in the FTIR spectra.