The team, led by Makarand K. Sakate and Balasaheb N. Kokare, collected unripe papaya fruit from cultivated land near Raje Ramrao College in Jath, following national and international guidelines for plant collection. A shallow longitudinal incision in the green fruit releases the milky latex, which the researchers collected in sterile bottles and homogenized with an equal volume of deionized water. That aqueous extract was then blended with a cadmium nitrate solution and stirred for 48 hours at 37 degrees Celsius in darkness, yielding a white precipitate. After centrifugation, washing with water and ethanol, drying at 110 degrees Celsius, and annealing at 600 degrees Celsius, the result was a fine black powder of cadmium oxide nanoparticles functionalized by the papaya latex biomolecules.

The chemistry behind the process hinges on papain, the dominant proteolytic enzyme in papaya latex, along with chymopapain, cysteine proteases, proteins, amino acids, and polyphenolic compounds. According to the proposed mechanism, cadmium ions bind to amino acid residues at papain’s active site, forming a cadmium-papain complex. When heated, the complex loses thermal stability, and subsequent calcination leaves behind latex-functionalized cadmium oxide nanoparticles. The functional groups involved, including hydroxyl, amino, carbonyl, carboxyl, and amide moieties, coordinate with cadmium ions and mediate nucleation, growth, and surface passivation. Fourier-transform infrared spectroscopy supported this picture: broad bands near 3400 wavenumbers correspond to O-H and N-H stretching of phenolics and proteins, while a characteristic absorption below 600 wavenumbers confirms the formation of cadmium-oxygen bonds.

Comprehensive characterization established the structural and optical credentials of the material. X-ray diffraction revealed a polycrystalline, face-centered cubic structure with peaks matching the standard reference pattern for cadmium oxide, and an average crystallite size of 35.67 nanometers calculated via the Scherrer equation. Raman spectroscopy showed second-order scattering peaks at roughly 257 and 400 wavenumbers, attributed to two-phonon overtones, plus a weak band near 960 wavenumbers linked to cadmium-oxygen stretching. Scanning electron microscopy with energy-dispersive spectroscopy showed predominantly quasi-spherical particles in the 40 to 100 nanometer range, with some irregularity and agglomeration typical of green synthesis routes, and elemental peaks confirming cadmium and oxygen.

X-ray photoelectron spectroscopy added a chemical-state analysis, resolving the cadmium 3d spin-orbit doublet at binding energies of 407.12 and 413.99 electron-volts, with a spin-orbit splitting of 6.875 electron-volts. The oxygen 1s signal appeared at 532.4 and 529.1 electron-volts, corresponding to cadmium-oxygen and cadmium hydroxide environments. Notably, no peaks for secondary phases such as cadmium dioxide or cadmium hydroxide were detected, and the cadmium 3d binding energy was shifted about 1.7 electron-volts higher than bulk cadmium oxide, which the authors attribute to nanoscale size effects. Ultraviolet-visible spectroscopy showed a strong absorption peak around 234 nanometers, and Tauc analysis yielded optical band gaps of 4.69 electron-volts for the direct transition and 3.47 electron-volts for the indirect transition.

Colloidal behavior was assessed by dynamic light scattering and zeta potential measurements. The hydrodynamic diameter averaged 324.9 nanometers, considerably larger than the crystallite size, a discrepancy the authors explain by noting that light scattering captures the solvation layer, adsorbed biomolecules, and possible agglomerates, whereas X-ray diffraction and electron microscopy measure the physical core particles. Zeta potential values of 28.9 and minus 17.3 millivolts were reported, indicating electrostatic repulsion strong enough to inhibit agglomeration and support long-term dispersion stability in suspension.

The biological results were striking. In agar well diffusion assays, the nanoparticles produced zones of inhibition measuring 19 millimeters against Bacillus cereus, 18 against Staphylococcus aureus, 8 against Escherichia coli, 18 against Aspergillus niger, and 18 against Candida albicans, benchmarked against streptomycin and fluconazole as positive controls. The authors attribute this broad-spectrum activity to several synergistic mechanisms: electrostatic attraction between the particles and the negatively charged microbial cell envelope, disruption of membrane integrity and leakage of intracellular contents, generation of reactive oxygen species such as hydroxyl and superoxide radicals that damage lipids, proteins, and DNA, and gradual release of cadmium ions that interfere with thiol-containing enzymes. The papaya-derived biomolecular coating may further enhance activity by improving dispersion and contact with microbial cells, although the authors caution that these mechanisms were not directly investigated and are inferred from prior literature on metal oxide nanoparticles.

Anticancer testing against the KB-3-1 human oral cancer cell line produced an IC50 value of 2.69 micrograms per milliliter over 48 hours, a comparatively potent figure that the authors contrast with, for example, Melia dubia-derived silver nanoparticles that required 31.2 micrograms per milliliter against breast cancer cells. Phase-contrast microscopy revealed cytomorphological changes in the treated cancer cells, including cell bursting, growth retardation, clumping, and loss of membrane stability. The researchers emphasize, however, that the molecular mechanisms of the nanoparticles in cancer and normal cells, as well as any effects on the human immune system, remain to be investigated, and toxicity toward healthy cells was not assessed in this study.

On the environmental side, the nanoparticles degraded 98 percent of methylene blue dye within 300 minutes of visible-light irradiation from a 250-watt mercury lamp fitted with an ultraviolet cutoff filter, using 25 milligrams of catalyst in 50 milliliters of a 25 parts-per-million dye solution. The authors themselves note that this photocatalytic efficiency is relatively modest and not strongly convincing, and suggest that doping with active species could improve it. They also acknowledge broader limitations: long-term colloidal stability, catalyst recyclability, identification of degradation intermediates, ecotoxicity toward aquatic organisms, and a detailed photocatalytic reaction mechanism were all left unexamined.

Even with those caveats, the study illustrates why biogenic synthesis has become one of nanoscience’s most active frontiers. Plant extracts supply a rich, self-assembling toolkit of reducing, capping, and stabilizing agents, cutting costs and avoiding hazardous reagents while often imparting synergistic biological activity through the biomolecular coating left on particle surfaces. Prior work has used papaya extracts to make copper oxide, palladium, iron oxide, and gold nanoparticles with antimicrobial, antioxidant, and anticancer properties, and the present report extends that catalog to cadmium oxide. The authors argue that the latex’s unusual abundance of proteolytic enzymes and proteins offers multiple functional groups that promote efficient reduction, surface passivation, and controlled growth, and that systematically linking phytochemical composition to nanoparticle characteristics distinguishes their approach from earlier, more empirical reports. Future work, they say, will target morphology control and composite engineering to boost photocatalysis, mechanistic studies of the biological effects, reusability testing, biocompatibility assessment, and ultimately deployment in real wastewater treatment and advanced biomedical systems. Whether papaya latex-functionalized cadmium oxide can clear the safety and scalability hurdles that stand between laboratory promise and clinical or environmental practice remains an open question, but the fruit’s sap has now demonstrated a remarkable range of talents in a single, sustainably made material.

Subject of Research: Green synthesis of Carica papaya latex-functionalized cadmium oxide nanoparticles with antibacterial, anticancer, and photocatalytic applications

Article Title: Carica papaya latex-functionalized biogenic synthesis of cadmium oxide nanoparticlesto improve antibacterial, anticancer and photocatalytic applications

Article References: Sakate, M. K., Wategaonkar, S. B., Prasad, N. R., Shendage, S. S., Kuldeep, A. R., Sargar, B. M., & Kokare, B. N. (2026). Carica papaya latex-functionalized biogenic synthesis of cadmium oxide nanoparticlesto improve antibacterial, anticancer and photocatalytic applications. Discover Chemistry, 3(1), Article 437. https://doi.org/10.1007/s44371-026-00895-z

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00895-z

Keywords: Carica papaya latex, cadmium oxide nanoparticles, green synthesis, antibacterial activity, anticancer activity, photocatalysis, methylene blue degradation, papain, reactive oxygen species, nanomaterials, biogenic synthesis, KB-3-1 cancer cells