Encapsulation of Copper Nanoparticles in Electrospun Nanofibers for Sustainable Removal of Pesticides

The excellent catalytic properties of copper nanoparticles (CuNPs) for the degradation of the highly toxic and recalcitrant chlorpyrifos pesticide are widely known. However, CuNPs generally present low stability caused by their high sensitivity to oxidation, which leads to a change of the catalytic response over time. In the current work, the immobilization of CuNPs into a polycaprolactone (PCL) matrix via electrospinning was demonstrated to be a very effective method to retard air and solvent oxidation and to ensure constant catalytic activity in the long term. CuNPs were successfully anchored into PCL electrospun fibers in the form of Cu2O at different concentrations (from 1.25 wt % to 5 wt % with respect to the PCL), with no signs of loss by leaching out. The PCL mats loaded with 2.5 wt % Cu (PCL-2.5Cu) almost halved the initial concentration of pesticide (40 mg/L) after 96 h. This process was performed in two unprompted and continuous steps that consisted of adsorption, followed by degradation. Interestingly, the degradation process was independent of the light conditions (i.e., not photocatalytic), expanding the application environments (e.g., groundwaters). Moreover, the PCL-2.5Cu composite presents high reusability, retaining the high elimination capability for at least five cycles and eliminating a total of 100 mg/L of chlorpyrifos, without exhibiting any sign of morphological damages.


S1. Additional SEM images of electrospun fibers.
The following micrographs at higher magnifications provided a more complete vision of the morphology of the electrospun fibers. Herein, the formation of small cracks resulting from the environmental conditions of the electrospinning process is clearly observed. Figure S1. SEM images at high magnifications of (a) neat PCL and (b) PCL-5Cu acquired using a secondary electron detector and a backscattered electrons detector, respectively.

S2. Study of the copper oxidation through XRD and infrared spectroscopy
XRD patterns of the samples are displayed in Figure S2(a). Main peaks at 21.4º, 22.0º, and 23.7º, can be ascribed to (110), (111) and (200) planes, respectively, of PCL in an orthorhombic crystal form 1 . Peaks of Cu species are masked by the PCL ones.

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The study of the presence of metallic species with infrared spectroscopy is highly challenging. The infrared-assigned peaks of neat PCL and PCL-2.5Cu, and the rest of the PCL-Cu mats are presented in Figure   S2(b,c). The infrared spectra of all samples showed identical infrared bands typical of PCL polymer 2 . The lack of band-shifting indicated that a strong interaction between the matrix and the metallic nanoparticles was not established 3 .

S3. Raman spectra of copper powder
The inspection of the Raman spectra confirmed the lack of oxides in the copper powder stored.
4 Figure S3. Raman spectra of copper powder stored. The absence of peaks corresponding to oxidizing species of copper indicates the purity of the reagent.

S4. XPS Auger signals of copper
The inspection of this section of the XPS spectra confirmed the presence of Cu(II) in PCL-5Cu x O sample, and of Cu(I) in PCL-1.25Cu. S5. The degradation mechanism of CP for PCL-Cu fibers and CuNPs studied through UV-Vis spectroscopy 5 The same absorbance peaks in the UV-Vis spectra of PCL-Cu fibers and free CuNPs indicated that the same chemical reaction took place. Besides, the differences in intensity over the time indicated that the content of CuNPs in PCL mats positively affect to the degradation reaction kinetics. Figure S5. The UV-Vis of (a) PCL-1.25Cu, (b) PCL-5Cu, and (c) CuNPs after 0h, 24h, and 96h.

S6. Mass-spectrometry of the reaction solution
Mass-spectrometry spectra was used to identify the degradation products obtained in the catalytic reaction.  S8. Study of the catalytic activity of copper for the degradation of CP By plotting ratio between the absorbance peak associated to CP (at 289 nm) and the reaction product (at 321 nm), is possible to demonstrate that neat PCL mats did not remove the pesticide via chemical degradation. The kinetic parameter of CuNPs was obtained to evaluate the degradation efficacy PCL-Cu mats.

S9. Study of the degradation reaction under light and dark conditions.
The UV-Vis spectra of the Chlorpyrifos solution in contact with the free copper nanoparticles and PCL-2.5Cu sample under light and dark conditions. The great similarity between the spectra at all measuring times and band overlapping indicates that the reaction was performed through the same mechanism.

S10. Study of the particle's distribution
The micrographs at higher magnifications of the samples PCL-1.25Cu, PCL-2.5Cu, and PCL-5Cu showed that the dispersion of the nanoparticles worsened when the concentration of Cu overcame the 2.5 wt.%.

S12. Identification of CuNPs in the UV-Vis spectra
The lack of signals related to a plasmon response of copper nanoparticles indicated that CuNPs were not leached out from the PCL mats. Figure S12. The UV-Vis spectra of the reaction solution of the sample PCL-2.5Cu after being immersed for 72 hours the fifth cycle. Only the peaks related to the pesticide were observed.

S13. Fabrication parameters of samples
In the preparation of the samples, the concentration of PCL in the chloroform solution was constant (1.6 g/mL), while the concentration of CuNPs added varied from 0 to 5 wt.% with respect to the polymer content. The compositional variability of samples resulted into different fabrication parameters. Table S2. Composition and fabrication parameters of the mats studied in this work.