Utilization of Carica papaya latex on coating of SPIONs for dye removal and drug delivery

Latex, a milky substance found in a variety of plants which is a natural source of biologically active compounds. In this study, Latex was collected from raw Carica papaya and was characterized using UV–Vis, FTIR and GC–MS analyses. Super Paramagnetic Iron Oxide Nanoparticles (SPIONs) were synthesized, coated with C. papaya latex (PL-Sp) and characterized using UV–Vis, FT-IR, SEM–EDX, XRD, VSM and Zeta potential analyses. SPIONs and latex coated SPIONs (PL-Sp) were used in batch adsorption study for effective removal of Methylene blue (MB) dye, where (PL-Sp) removed MB dye effectively. Further the PL-Sp was used to produce a nanoconjugate loaded with curcumin and it was characterized using UV–Vis spectrophotometer, FT-IR, SEM–EDX, XRD, VSM and Zeta potential. It showed a sustained drug release pattern and also found to have good antibacterial and anticancer activity.


Characterization of SPIONs and Latex coated SPIONs.
In UV-Vis Spectroscopy the adsorption maxima of the synthesized iron oxide nanoparticle were found to be at 260 to 340 nm (Fig. 1a) and for Latex coated SPIONs from 270 to 280 nm (Fig. 1b). This can be due to the presence of alkyl group present in the latex 20 . Samrot et al. 11 synthesized SPIONs of which had absorption maxima around 260 nm.
FT-IR spectra of SPIONs was found in the range of 569 cm −1 representing the stretching vibrations of Fe-O bond (Fig. 2a). Similar results were obtained by Samrot et al. 18 . The bands near 2921 cm −1 , 1640 cm −1 , 1458 cm −1 of papaya latex coated SPIONs indicated the presence of -CH, N-H, C-N-vibration of amino group. The band at 2925 cm −1 indicated the vibration of C-H stretching (Fig. 2b) 18,26 where these functional groups were rendered by aqueous extract of papaya latex.  www.nature.com/scientificreports/ SEM image showed that the synthesized SPIONs were in the size ranging between 30 and 60 nm (Fig. 3a). The SEM images showed that the papaya latex coated SPIONs were in the size ranging between 45 and 62 nm, the size increase might be due to coating with aqueous extract of C. papaya latex (Fig. 3b). Both the SPIONs and Latex coated SPIONs were spherical in shape. Samrot et al. 27 produced SPIONs in the size of 25 nm whereas, Aghazadeh and Karimzadeh 28 , synthesized SPIONs in the size of 10 nm with no aggregation.
Zeta potential of the SPIONs and latex coated SPIONs was around 40.2 mV and − 15.2 mV respectively ( Fig. 5a,b). Khatami et al. 32 produced SPIONs which had zeta potential of 40.1 mV. The shift in the zetapotential was due to the coating with the aqueous extract of C. papaya and the coating made the SPIONs more stable. VSM of the synthesized SPIONs and Latex coated SPIONs were found to be superparamagnetic in nature as the magnetization increased around origin and no hysteresis loop was recorded (Fig. 6a,b). Mahmoudi et al. 33 produced SPIONs with negligible remanence, coercivity in the hysteresis loop.
Adsorption studies in batch systems. Optimization of adsorbate concentration. Here, fixed concentration of both the adsorbents SPIONs and Latex coated SPIONs was taken as 0.01 g and the concentration of methylene blue dye was varied from 1 to 10 ppm. The adsorbent and the adsorbate were allowed to interact for 1 h. 0.01 g of latex coated SPIONs was able to remove 10 ppm of MB dye with maximum removal efficiency of 80.01% ( Supplementary Fig. 7). 40 mg of Magnetic nanoparticles (Fe 3 O 4 ) were able to remove methylene blue dye of 70 ppm with removal efficiency of 89% 34 . SPIONs showed less removal efficiency compared with latex coated SPIONs (results not shown). Thus, latex coated SPIONs were subjected for further studies.   www.nature.com/scientificreports/ Optimization of adsorbent concentration. Here, the concentration of adsorbent which showed maximum removal from the aforementioned studies was varied from 0.01 to 0.1 g and it was allowed to interact for 1 h. In the above studies latex coated SPIONs showed maximum removal at 10 ppm hence, 10 ppm of dye was kept as constant and latex coated SPIONs was varied from 0.01 to 0.1 g. Here, 0.08 g of latex coated SPIONs was able to remove 10 ppm of dye with maximum removal efficiency of 98.75% ( Supplementary Fig. 8). 8 g/l of SPIONs has been reported to remove 10 ppm of basic crystal violet dye with maximum removal of 94.7% 10 .
Optimization of pH. After optimizing adsorbate and adsorbent concentration, pH was optimized. pH was varied from 5 to 9 and maximum removal after interaction of 1 h was found to be 98.87% at pH 9 ( Supplementary  Fig. 9). pH is one of the most important factors to be optimized as ionic charge of adsorbent mostly depend on pH of the adsorbate solution. The functional groups present in the latex coated SPIONs are responsible for the binding of positively charged methylene blue dye at alkaline pH. Alizadeh et al. 35  Optimization of contact time. After the adsorbent-adsorbate and pH was optimized, contact time was varied in order to check how much interaction time was required to achieve maximum removal efficiency. 98.92% was removed at 3 h of interaction between the adsorbent and adsorbate at alkaline pH ( Supplementary Fig. 10). Due to the increased surface area of the nanoparticle, it provided an active site for binding of MB dye till 3 h and further the removal percentage was found to decrease. Singh et al. 36 used SPIONs-chitosan nanocomposite to remove oil from water and reported that the maximum removal efficiency was found to be till 3 h later the removal was found to be decreasing.
Adsorption isotherm. R 2 value is the correlation factor which helps us to determine the best fit isotherm model for the particular adsorption reaction. Here, adsorption isotherm was calculated by keeping the adsorbent (0.01 g SPIONs) as constant and adsorbate concentration (1-10 ppm) was varied. In this study the R 2 value was 0.906, 0.901 and 0.84 for Langmuir, Freundlich and Tempkin isotherms respectively. Hence, the removal of MB dye by latex coated SPIONs followed Langmuir isotherm as the R 2 value of the isotherm model was high  www.nature.com/scientificreports/ compared to other isotherms (Fig. 7). Hosseinzadeh and Mohammadi 37 , also reported that anionic dyes were removed efficiently by magnetic iron oxide nanoparticles and followed Langmuir isotherm.
Characterization of drug loaded nanoconjugate. UV-Vis of the synthesized drug loaded nanoconjugate was found to be around 260 nm and 290 nm, characteristics of SPIONs, curcumin and latex and further the absorbance found to be decreasing ( Supplementary Fig. 11). Samrot et al. 38 produced a drug loaded nanocarrier using A. heterophylla which showed maximum absorbance of 210 nm which indicated the presence of xylose content in polysaccharide. FTIR was performed to identify the functional group present in the synthesized drug loaded nanoconjugate. The band near 1300 cm −1 -1450 cm −1 indicated the presence of CH 3 and CH 2 bending 39 . The band near 1645 cm −1 indicated the presence of enolic CO group of the drug curcumin 40 . The band at 3450 cm 1 confirmed the presence of hydroxyl group of curcumin in drug loaded nanocarrier ( Supplementary Fig. 12) 41 .
SEM analysis revealed that the surface of the drug loaded nanoconjugate was rough and was aggregated. The synthesized nanoconjugate was size around 111 nm in size (Fig. 8). The increase in size was due to the addition of drug curcumin. When SLS, CTAB, and SPAN 20 were used as surfactants, Pradeepkumar et al. 42 was able to synthesize doxorubicin-loaded C. gigantea nanocarriers with a size around 100 nm.
The XRD patterns showed that the produced curcumin drug loaded nanoconjugates were slightly amorphous in nature. 2θ peaks at 56.9°, 62.5° were in correspondence with the plane (511), (440) which referred to magnetite nanoparticle 43 . Sharp intense peaks observed at 2θ = 29.7°, 35.3°, 53.3° where these shifts indicated the drug loading onto the nanoconjugate ( Supplementary Fig. 13). Justin et al. 44 was able to maintain the crystalline structure of the curcumin loaded SPIONs nanocarrier even after functionalization of SPIONs and final coating with chitosan.
Zeta potential analysis was performed and the charge of the drug loaded nanoconjugate was found to be − 23.6 mV ( Supplementary Fig. 14). The values obtained by Samrot et al. 39 for the drug loaded nanocarrier synthesized using carboxymethylated polysaccharides of Terminalia catappa and chelated using tri sodium tri meta phosphate was found to be − 38.06 mV. Vibrating sample magnetometer revealed that synthesized drug loaded nanoconjugate was superparamagnetic in nature. The magnetization power was retained around origin (Supplementary Fig. 15), thus the drug loading onto the nanoconjugate did not alter the magnetic property of SPIONs. www.nature.com/scientificreports/ Drug encapsulation efficiency. The drug loaded nanoconjugate was analysed for its drug encapsulation efficiency and it was observed that the maximum encapsulation of the hydrophobic drug curcumin into the nanoconjugate was around 72.75% (Fig. 9). The percentage of encapsulation was found to be increasing as the time increased. At first, the curcumin got entrapped with the readily available sites in the latex coated SPIONs but as the time increased the curcumin entrapped in all the free spaces available. This might be the reason for increase in the encapsulation efficiency as the time increases. Shobana et al. (2019) reported a better encapsulation efficiency of 90% when curcumin was loaded into the STMP chelated chitosan nanoparticles.
Drug release efficiency. Percentage drug release was calculated at every 30 min till 180th minute and the maximum drug release of 69.19% of the total encapsulated drug was observed at 180th minute. (Fig. 10). As the nanoconjugate was coated with aqueous extract of latex, which was getting dissolved when the solvent system Bioactivity studies. Antibacterial activity. SPIONs, C. papaya latex coated SPIONs and drug loaded nanoconjugate were tested for antibacterial activity against a Gram-positive bacterium (Bacillus subtilis) and a Gram-negative bacterium (Pseudomonas aeruginosa). SPIONs did not show any antibacterial activity (results not shown). Only curcumin loaded nanoconjugate showed inhibition against the Gram-negative bacterium at all the concentrations used (Fig. 11, Table. 1). This could be due to the release of curcumin. The growth of P. gingivalis, Fusobacterium nucleatum, Prevotella intermedia, and Treponema denticola was inhibited by curcumin in a dose-dependent manner 46 .
Anticancer activity. MTT assay. The SPIONs, latex coated SPIONs and drug loaded nanoconjugate did not show much activity on normal fibroblast L929 cell lines (Fig. 12a) where SPIONs, latex coated SPIONs and the curcumin loaded nanoconjugate had showed activity against the breast cancer cell lines where drug loaded nanoconjugate showed IC 50 (inhibition concentration 50) at least concentration around 12 µg concentration (Fig. 12b). The enhanced activity was due to the release of curcumin by the nanoconjugate. Interestingly, cytotoxicity of drug loaded nanoconjugate was found against cancer cell line alone and did not show much inhibition on normal cells, thus, so it can be used in cancer therapy. Curcumin loaded nanoparticles have been reported be active against various cancer cell lines including PC3, MCF-7 etc 47 .   (Fig. 13ag), this also due to the action of curcumin release by the nanoconjugate. Apoptotic and necrotic breast cancer cells were observed as red fluorescence due to their loss of membrane integrity when tamoxifen was used as drug in chitosan nanoparticles 48 .
Collection of papaya latex. Latex of Carica papaya was collected from unripe raw papaya (Supplementary Fig. 1a,b,c). Latex was collected in a sterilized glass container ( Supplementary Fig. 1d,e) and stored in refrigerator.
Characterization of Carica papaya latex. Latex was dried and 10 g dissolved in 100 mL distilled water and was centrifuged at 5000 rpm, supernatant collected was subjected for characterization. UV-Vis spectrum range between 200 and 800 nm was recorded using UV-Visible spectroscopy (Shimadzu UV-1800, Japan). FTIR analysis for was performed (Shimadzu, Japan) to identify the functional group present in the latex. GC-MS (Shimadzu, QP2010 Plus) analysis was performed to identify the bioactive compounds present in the latex. Thin layer Chromatography was performed for the latex having ethanol: water (2:1) as mobile phase. Rf value was determined.
TLC-bioautography for antioxidant activity. Thin layer Chromatography was performed for the latex as above mentioned. DPPH solution (0.004% (w/v) in 90% of methanol) was prepared and sprayed on TLC plates which was run with latex sample with aforementioned solvent system and checked for presence of yellow spot 25 .

Synthesis of latex coated SPIONs.
Latex coated SPIONs was prepared following the modified method of Zhang et al. 50 . 10 g of latex was dissolved in 100 ml of nitrogen purged Milli Q water (10%) and centrifuged for 15 min at 3000 rpm and the supernatant collected was used for further coating. 100 mg of SPIONs was added into the extracted latex solution and was sonicated for 30 min. They were separated by applying external magnetic field. The obtained coated SPIONS were washed thrice with nitrogen purged Milli Q water and lyophilized.

Adsorption studies in batch experimental systems. Preparation of methylene blue (MB) dye solu-
tion. For this study, methylene blue (MB) was prepared at the concentration of 10 ppm. Different ppm of the dye solution was obtained by making further dilutions from the prepared 10 ppm of stock solution. The dye solution was taken in an aliquot and measured spectroscopically at 663 nm (Results not shown). A graph was plotted with concentration in X-axis and optical density (OD) in Y-axis. www.nature.com/scientificreports/ Optimization of adsorbate concentration-best adsorbent identification. 0.01 g of SPIONs and Latex coated SPI-ONs was taken as adsorbent materials and was kept as constant and the ppm concentration of MB was varied from 1 to 10 ppm. Adsorbate and the adsorbent were allowed to interact for 1 hour. Later the mixture was centrifuged at 5000 rpm for 15 min and the supernatant was collected and it was measured spectroscopically at 663 nm. Removal percentage was calculated from the standard curve. The % removal was calculated from the following formula A graph was plotted by having concentration in terms of ppm in X-axis and % removal in Y-axis. The adsorbent material which showed highest percentage of removal was taken for the further optimization studies.
Optimization of adsorbent concentration. Best adsorbent was identified from the previous study and now keeping the adsorbate concentration in which, it showed maximal removal as constant and the concentration of the best adsorbent was varied from 0.01 to 0.1 g and it was allowed to interact with the adsorbate for 1 h. Later, the mixture was centrifuged at 5000 rpm for 15 min and the supernatant was collected and it was measured spectroscopically at 663 nm. Removal was calculated from the standard curve. A graph was plotted by having adsorbent concentration in X-axis and % removal in Y-axis.
Optimization of pH. After the optimization of adsorbent and adsorbate concentration, pH was optimized in order to enhance the removal efficiency. pH was varied from 5 to 9 and it was altered using 0.1 N NaOH and 1 N HCl. It was allowed to interact and centrifuged at 5000 rpm for 15 min. Supernatant was collected and measured spectroscopically at 663 nm. Removal percentage was calculated from the standard curve. A graph was plotted by having pH in X-axis and % removal in Y-axis.
Optimization of contact time. Contact time was further optimized after optimizing adsorbent-adsorbate concentration and pH. Adsorbent and adsorbate (optimized) were allowed to interact in an optimized pH in varied contact hours (1-5 h). After the end of each hour the solution was centrifuged at 5000 rpm for 15 min. Supernatant was collected and measured spectroscopically at 663 nm. Removal percentage was calculated from the standard curve. A graph was plotted by having Contact time in terms of hours in X-axis and % removal in Y-axis.
Adsorption isotherms. The mechanism taking place between adsorbent and the adsorbate was studied using adsorption isotherm. Here, adsorption isotherm was calculated by keeping the adsorbent as constant and adsorbate concentration was varied. Adsorption capacity at equilibrium qe= (Ci-Ce)V/m, Ci and Ce are the initial and final concentration; V and mM are the volume of the adsorbate and mass of the adsorbent respectively. Langmuir (1/qe vs. 1/Ce); Freundlich (ln qe vs. ln Ce) and Tempkin (qe vs. ln Ce) isotherm models were calculated.
Production of drug loaded nanoconjugate using latex coated SPIONs. Nanoconjugate was produced by slight modification of Akbarian et al. 51 . 0.2 g of latex coated SPIONs was taken and was added to 100 ml of nitrogen purged Milli Q water and sonicated for 10 min. 0.01 g of drug (curcumin) was added to 10 ml of non-polar solvent methanol and mixed. Methanol containing curcumin was added to the latex coated SPIONs solution and was sonicated for 30 min. Later, the solution was centrifuged at 5000 rpm for 20 min and the pellet was collected. The mixture was washed thrice and lyophilized.

Drug encapsulation efficiency.
After loading the drug curcumin, the nanoconjugates were centrifuged at 7000 rpm for 15 min, 1 ml of supernatant was collected and subjected to UV-Vis spectroscopy at 427 nm. Drug encapsulation efficiency was performed as reported 39,52 . Graph was plotted by taking time in the X-axis and % drug encapsulation in Y axis.
Drug release efficiency. 10 mg of synthesized drug loaded nanoconjugate were dissolved in 1 mL of nitrogen purged Milli Q water. Phosphate buffer solution (PBS) of pH 7.4 (Sodium chloride-8 g, Potassium chloride-200 mg, Disodium hydrogen phosphate-1.44 g, Potassium dihydrogen phosphate-245 mg) was prepared. The nanoconjugates were put in dialysis bag and dialyzed against phosphate buffer with slight modifications in the composition of PBS and its pH 41 . Absorbance was measured by taking the OD values at 30 min interval. % Drug release was calculated at every 30th minute using the formula 53 Graph was plotted by taking time in the X-axis and % drug release in Y axis. www.nature.com/scientificreports/ Bioactivity studies. SPIONs, Latex coated SPIONs, Drug loaded nanoconjugate were checked for its antibacterial activity-agar well diffusion method 54 and anticancer activity against normal fibroblast L929 and breast cancer MDA-MB-231 cell lines (procured from National Centre for Cell Sciences (NCCS), Pune, India) was performed using MTT assay 55 and AO/ EB staining 56 .

Statistical analysis.
Triplicates were performed for all the experiments and all the value are given as mean ± standard deviation value here.
Graphical abstract. The complete work is given in terms of graphical abstract (Supplementary Fig. 16).

Conclusion
C. papaya L latex was collected and characterized. The aqueous extract was utilized to coat Super paramagnetic iron oxide nanoparticles (SPIONs), then characterized and utilized for various application including dye removal and drug delivery. In batch adsorption studies, latex coating onto SPIONs was enhancing the removal efficiency of methylene blue dye, 10 ppm of MB dye was removed by 0.08 g of latex coated SPIONs with maximum removal of 98.92% at alkaline pH (pH 9) after 3 h of interaction time and it obeyed Langmuir isotherm with maximum R 2 value of 0.906. Curcumin loaded nanoconjugate was produced and it showed a sustain drug release pattern. The drug loaded nanoconjugate showed better antibacterial activity and it also had excellent anticancer activity against breast cancer cell line. From this study, it is clear that C. papaya L latex can be used for SPIONs coating can be used for dye removal and drug delivery.

Data availability
The datasets used and/or analysed during the present study are available from the corresponding author on reasonable request.