The researchers, led by Surendirakumar Kannaiah and Suguna Paramasivam, collected the fruit pedicel waste, known locally as sundakai kambu, from household kitchens in Pudukkottai, Tamil Nadu. After washing, shade-drying, and grinding the material into a coarse powder, they boiled ten grams of the powder in one hundred milliliters of distilled water at 65 degrees Celsius for one hour. The resulting pale-yellow extract was filtered and then added to a one millimolar solution of silver nitrate, the precursor salt supplying silver ions. Within twenty-four hours of incubation in the dark at room temperature, the mixture shifted in color from pale yellow to orange brown, a visible signature that silver ions had been reduced to metallic silver nanoparticles. The team designated the resulting material ST@AgNPs, reflecting the role of the Solanum torvum extract in their creation.

The chemistry behind this transformation lies in the phytochemical richness of the turkey berry plant. Solanum torvum, a wild relative of the eggplant consumed as a vegetable across Africa and Asia and used extensively in traditional medicine, contains abundant alkaloids, flavonoids, saponins, tannins, and glycosides. In green nanoparticle synthesis, these biomolecules perform two simultaneous jobs: they donate electrons to convert silver ions into neutral silver atoms that nucleate into particles, and they adsorb onto the growing particle surfaces as capping and stabilizing agents that prevent uncontrolled clumping. Fourier-transform infrared spectroscopy of the synthesized particles revealed absorption bands characteristic of O-H and C-H stretching vibrations, amide I bands from proteins, and phenolic and aromatic functional groups, confirming that a cocktail of plant metabolites coats each nanoparticle and governs its formation.

Optical characterization confirmed the success of the synthesis. Ultraviolet-visible spectroscopy showed a distinct surface plasmon resonance peak at 418 nanometers, within the expected 390 to 450 nanometer window for silver nanoparticles and absent from the starting materials. Surface plasmon resonance arises from the collective oscillation of conduction electrons excited by incoming light, and its position and shape report on particle size, distribution, and stability. The researchers also mapped how synthesis conditions affect yield. At acidic pH values of five and six, no meaningful absorption peak appeared because particles agglomerate rather than nucleate, while neutral pH proved optimal. Temperature profiling showed that 35 degrees Celsius encouraged particle formation while preserving the integrity of the heat-sensitive biomolecules, whereas temperatures above 55 degrees Celsius degraded the extract and lowered yields. Time-course measurements revealed that absorbance increased steadily with incubation, reaching a maximum reduction of silver ions around 72 hours.

Structural analysis painted a consistent picture of crystalline, roughly spherical particles. X-ray diffraction produced sharp reflections at 2-theta values of 38.72, 44.16, 63.12, 77.60, and 82.74 degrees, indexed to the (111), (200), (220), (311), and (222) planes of face-centered cubic metallic silver, matching the standard reference pattern for elemental silver. Applying the Debye-Scherrer equation to the diffraction peaks yielded a crystallite size of approximately 22.8 nanometers for the dominant (111) plane. Field-emission scanning electron microscopy showed spherical particles, some with irregular shapes, ranging from roughly 100 to 200 nanometers with mild agglomeration attributable to capillary forces during drying. Energy-dispersive X-ray analysis confirmed elemental silver as the dominant component at 42.46 percent, with an absorption signal near 3 kiloelectronvolts characteristic of metallic silver nanocrystals, alongside contributions from carbon, oxygen, chlorine, and sulfur residues of the plant capping layer.

Transmission electron microscopy resolved individual particles as isotropic spheres with some elliptical and aggregated forms, and the size distribution histogram derived from the images gave an average particle diameter of about 108 nanometers. Selected-area electron diffraction produced concentric rings of elemental silver, confirming high crystallinity. Dynamic light scattering and zeta potential measurements added a critical stability insight: the particles carried a surface charge of negative 16.2 millivolts. This negative charge, attributed to the bioorganic capping layer inherited from the plant extract, generates electrostatic repulsion between particles and keeps the colloid from settling or fusing. The authors note that the hydrodynamic size in this study is larger than the 20 to 27 nanometer particles previously reported from Solanum torvum fruit and root extracts, a difference likely reflecting the distinct phytochemical profile of pedicel tissue compared with other plant organs.

The biological performance of the nanoparticles was tested against four human pathogenic bacteria: the Gram-positive Staphylococcus aureus and Staphylococcus haemolyticus, and the Gram-negative Klebsiella pneumoniae and Pseudomonas aeruginosa. In agar well diffusion assays, activity rose with dose. At the highest concentration of 100 micrograms per milliliter, the nanoparticles produced inhibition zones of 18 millimeters against Staphylococcus aureus, 16 millimeters against Klebsiella pneumoniae, 15 millimeters against Pseudomonas aeruginosa, and 8 millimeters against Staphylococcus haemolyticus, with a clearly dose-dependent effect when compared against the standard antibiotic ampicillin. Broth dilution assays quantified the minimum inhibitory concentrations, yielding IC50 values of 68.54 micrograms per milliliter for Staphylococcus aureus, 53.12 for Klebsiella pneumoniae, 72.78 for Pseudomonas aeruginosa, and 81.40 for Staphylococcus haemolyticus, along with marked reduction of bacterial colony formation on agar plates even at the lowest tested doses.

The mechanism of this antibacterial action operates on several fronts. Silver nanoparticles bind to the negatively charged bacterial cell surface, disrupting membrane integrity and interfering with permeability, osmoregulation, electron transport, and respiration. Particles that penetrate the cell interact with DNA, proteins, and other phosphorus- and sulfur-containing biomolecules, triggering cellular dysfunction and death. In addition, the particles release silver ions that amplify the bactericidal effect in a size- and concentration-dependent manner. Because antibiotic resistance continues to erode the effectiveness of conventional drugs, metal nanoparticles are increasingly viewed as a promising platform for new antimicrobial coatings, wound dressings, food preservation systems, and antiseptic formulations, and the low minimum inhibitory concentrations observed here support that trajectory.

Perhaps the most striking result came from the cancer experiments. Using the MTT viability assay on A549 human lung adenocarcinoma cells, the team found that ST@AgNPs suppressed cell proliferation in a dose-dependent manner with an IC50 value of 38.2 plus or minus 0.8 micrograms per milliliter. For comparison, the clinical chemotherapy drug doxorubicin, used as the positive control, showed an IC50 of 24.3 plus or minus 1.05 micrograms per milliliter. While the plant-derived nanoparticles did not outperform the commercial drug, the authors emphasize that achieving potent cytotoxicity with a material synthesized from discarded kitchen waste, without toxic chemical reagents, represents a meaningful step toward affordable bioactive therapeutics. The team cautions that further clinical studies are needed before any therapeutic application, but the convergence of waste valorization, green chemistry, and demonstrated antibacterial and anticancer activity makes ST@AgNPs a compelling example of how nanotechnology can convert environmental liabilities into biomedical assets and advance a more circular bioeconomy.

Subject of Research: Green synthesis of antibacterial and anticancer silver nanoparticles from Solanum torvum fruit pedicel waste

Article Title: Antibacterial and cytotoxic effect of green synthesized silver nanoparticles using Solanum torvum fruit pedicel waste extract

Article References: Kannaiah, S., Paramasivam, S., Sanasam, B., Sekar, N., Nongthombam, K. S., Palanivel, K., & Pandiyan, J. (2026). Antibacterial and cytotoxic effect of green synthesized silver nanoparticles using Solanum torvum fruit pedicel waste extract. Discover Biotechnology, 3(1), Article 2. https://doi.org/10.1007/s44340-026-00049-y

Image Credits: AI Generated

DOI: 10.1007/s44340-026-00049-y

Keywords: silver nanoparticles, green synthesis, Solanum torvum, food waste valorization, antibacterial activity, A549 lung cancer cells, MTT assay, phytochemicals, zeta potential, nanobiotechnology, Turkey berry, sustainable nanomaterials

The team, led by Khouloud Benmarzoug of the University of Gabes, collected sand from the Albian continental formation in the Douiret region of Tataouine in southern Tunisia. Rather than relying on energy-intensive furnace calcination at around 700 °C for hours, the researchers used ordinary sodium hydroxide and a domestic-style microwave oven to convert the sand’s crystalline quartz into soluble sodium silicate. A 5-gram sample of washed, dried sand was ground with 1.77 grams of sodium hydroxide and irradiated at 750 watts for only five minutes, triggering an alkaline fusion reaction that transforms silicon dioxide into sodium silicate and water. The extraction cycle was repeated three times with hot distilled water to maximize yield, after which citric acid was added dropwise to bring the solution to a near-neutral pH of about 5.8, converting the silicate into silicic acid in a deliberately greener alternative to hydrochloric acid.

Subsequent microwave steps, first a gentle 160-watt warming for ten minutes to promote gelation and then a 350-watt calcination for ten minutes, condensed the silicic acid into a fine white powder of silica nanoparticles. The entire sequence replaces what would conventionally be a harsh, corrosive, and lengthy industrial workflow with roughly twenty-five minutes of total microwave processing. The chemical logic is elegant: high microwave power delivers rapid, uniform volumetric heating that breaks down the rigid quartz lattice, while the milder stages encourage silanol groups to polycondense into a continuous network, preserving the mesoporosity that makes the final material so reactive.

Characterization confirmed a dramatic transformation. X-ray diffraction revealed that the raw sand’s sharp quartz and cristobalite peaks evolved into a hybrid structure combining amorphous silica, marked by a broad diffraction hump, with residual crystalline phases, while impurity peaks from magnetite and calcite vanished entirely. Infrared spectroscopy showed the carbonate bands of the original sand disappearing, replaced by the characteristic silicon-oxygen-silicon stretching and bending signatures of a silica framework. Electron microscopy captured the morphological shift from dense, angular quartz grains to loosely packed, cauliflower-like clusters of interconnected sub-micron aggregates, the hallmark of freshly precipitated amorphous silica.

The purification achieved is arguably the study’s most striking technical result. Energy-dispersive X-ray analysis showed the silicon dioxide content jumping from 77.52 percent in the raw sand to 94.34 percent in the nanoparticles, with iron, potassium, titanium, magnesium, and calcium all falling below detection limits. Conventionally, such purification demands treatment with hydrochloric or even hydrofluoric acid, generating corrosive effluents and risking the co-dissolution of aluminum. Here, a simple distilled-water wash accomplished the same goal, leaving only a residual 3.82 percent aluminum oxide locked within a stable, water-insoluble aluminosilicate framework. The method also carries a compelling economic profile: because the silicon source is essentially free sand and the reagents are limited to sodium hydroxide and citric acid, the researchers estimate the combined precursor and reagent cost is roughly two orders of magnitude lower than a conventional synthesis based on tetraethyl orthosilicate, the expensive organosilicon compound used in most laboratory photocatalysis studies.

Textural analysis sealed the case for photocatalytic use. Nitrogen adsorption measurements showed the surface area expanding nearly sevenfold, from 9.59 to 65.48 square meters per gram, while pore size grew from essentially negligible to 4.25 nanometers and pore volume to 0.0696 cubic centimeters per gram, textbook signatures of a well-defined mesoporous framework. The optical properties proved equally intriguing. Pure, defect-free silica is a wide-band-gap insulator with almost no intrinsic photoactivity, yet Tauc analysis of the ultraviolet-visible absorption data yielded a direct band gap of 3.55 electron volts, notably lower than the 3.8 to 4.4 electron volts typical of pure amorphous silica. The reduction stems from structural defects, lattice disorder inherited from the natural precursor, and a dense population of surface silanol groups, all of which introduce sub-bandgap states that allow the material to harvest light at the near-visible edge.

When tested against Rhodamine B, a cationic xanthene dye widely used as a model water pollutant, the sand-derived catalyst delivered complete degradation in thirty minutes under a solar simulator calibrated to Tunisian noon sunlight, at a modest catalyst loading of 0.4 grams per liter and an acidic pH of 3. Control experiments confirmed that neither photolysis nor adsorption alone could account for the removal. The surface chemistry explains why acidity matters: the catalyst’s point of zero charge sits at pH 5.85, and under strongly acidic conditions protonated silanol groups create a high density of active adsorption sites where hydrogen bonding and van der Waals forces outweigh electrostatic repulsion between the positively charged surface and the cationic dye. At neutral pH, by contrast, the surface turns negative, the dye adopts a zwitterionic form, and degradation stalls.

Mechanistically, the picture that emerges is one of surface-mediated oxidation rather than textbook band-to-band excitation. Radical scavenger experiments showed degradation efficiency plummeting from 100 percent to 28.59 percent when photogenerated holes were intercepted, to 38.35 percent when superoxide radicals were quenched, and to 61.13 percent when hydroxyl radicals were scavenged, establishing a reactivity hierarchy of holes, then superoxide, then hydroxyl radicals. Mulliken electronegativity calculations placed the conduction band edge at approximately +0.20 volts and the valence band edge at +3.75 volts versus the normal hydrogen electrode. The deeply positive valence band gives photogenerated holes a powerful thermodynamic driving force to oxidize both adsorbed dye and water, while the conduction band position suggests superoxide generation proceeds through defect-mediated trap states rather than direct one-electron oxygen reduction at the nominal band edge.

The degradation is not merely cosmetic decolorization. The distinctive hypsochromic shift in the absorption spectrum, from 554 nanometers toward shorter wavelengths, tracks the stepwise removal of ethyl groups from the dye molecule, and chemical oxygen demand measurements confirmed genuine mineralization, with COD falling by 60.76 percent in thirty minutes and 73.12 percent in forty-five minutes as the conjugated backbone is cleaved into progressively smaller fragments. Non-linear kinetic modeling showed the reaction follows pseudo-first-order kinetics with a rate constant of 0.059 per minute, consistent with a quasi-steady-state concentration of reactive species at the catalyst surface.

Robustness testing revealed a nuanced picture of real-world applicability. Chloride ions actually accelerated the process, cutting complete removal time from thirty minutes to fifteen at 5 millimolar sodium chloride and down to nine minutes at 50 millimolar, likely because chloride scavenges holes to form reactive chlorine species that contribute additional oxidation. Sulfate ions suppressed removal to a plateau near 50 percent by consuming hydroxyl radicals and holes to form the less reactive sulfate radical, while carbonate ions proved the most detrimental, limiting removal to under 12 percent through radical scavenging and calcium-driven passivation of surface silanol sites. The catalyst also demonstrated versatility beyond a single dye, degrading more than 90 percent of methylene blue within thirty minutes and achieving 84 to 86 percent removal of the anionic Acid Green 25, although a ternary dye mixture slowed Rhodamine B removal to about sixty minutes through competition for active sites, and the pharmaceutical acetaminophen required six hours to reach roughly 69 percent removal, reflecting the greater resistance of drug molecules to photocatalytic oxidation.

Perhaps most importantly for practical deployment, the nanoparticles retained high activity across five consecutive degradation cycles with only marginal efficiency loss, while X-ray diffraction and infrared spectroscopy confirmed that the bulk silica framework remained structurally intact after repeated use. Compared against the published literature, the performance is exceptional: titanium dioxide-silica composites typically require 40 to 210 minutes and higher catalyst loadings under artificial light to achieve comparable removal rates. The authors are candid about limitations, noting that X-ray photoelectron spectroscopy, electron paramagnetic resonance radical detection, and zeta-potential measurements remain to be performed, and that full life-cycle cost assessment awaits future work. Even so, the study stands as a compelling proof of concept that desert sand, a material so abundant it is essentially free, can be upgraded in minutes inside a microwave into a robust, reusable solar photocatalyst, opening a path toward affordable water treatment technologies for regions where clean water and expensive reagents are equally scarce.

Trimethoprim is a widely prescribed antibiotic, usually administered in combination with sulfamethoxazole for urinary tract and respiratory infections, and it is a textbook example of an “emerging contaminant”: a compound that is not effectively removed by conventional wastewater treatment and therefore accumulates in rivers, lakes, and even drinking water sources. Because it is designed to suppress bacterial growth, its continuous presence in aquatic ecosystems exerts selective pressure on microbial communities, accelerating the evolution and dissemination of antibiotic-resistance genes—one of the most pressing public health threats of the century. Environmental surveys documented in the literature report trimethoprim in hospital effluents, municipal wastewater treatment plant discharges, and receiving surface waters worldwide, often at concentrations high enough to exert biological effects. Conventional biological treatment only partially transforms the molecule, sometimes generating transformation products of uncertain toxicity, which has driven intense interest in advanced oxidation processes that can mineralize the antibiotic completely rather than merely relocating it.

The core technical challenge in photocatalytic water purification lies in harnessing visible light efficiently. The archetypal photocatalyst, titanium dioxide, is chemically robust and inexpensive but possesses a wide band gap of roughly 3.2 electronvolts, meaning it absorbs only ultraviolet radiation—a small fraction of the solar spectrum. The Indian team’s Mn₂O₃–Co₃O₄ composite sidesteps this limitation. Optical measurements revealed a narrowed band gap of 2.1 electronvolts, allowing the material to absorb a substantial portion of visible light, including the abundant photons available in ordinary sunlight. This narrowing arises from the electronic structure of the two oxides: both manganese(III) oxide and cobalt(II,III) oxide are semiconducting transition-metal oxides with partially filled d-orbitals that create intermediate electronic states, and when coupled in a heterostructure, their band alignments promote efficient absorption and charge transfer. The researchers attribute the material’s outstanding performance specifically to this synergistic interaction between the two oxide phases, which facilitates rapid separation of photogenerated electron–hole pairs and enhances electron mobility across the interface.

The synthesis itself is deliberately unglamorous, which is part of its appeal. The team used a facile sol–gel method—a wet-chemical technique in which metal precursors are dissolved, gelled, and calcined to form the mixed oxide. Sol–gel processing offers fine control over composition and particle size at low cost, without the high temperatures, pressures, or exotic reagents demanded by hydrothermal or vapor-phase methods. Structural characterization by X-ray diffraction confirmed the coexistence of crystalline Mn₂O₃ and Co₃O₄ phases, while scanning electron microscopy and energy-dispersive X-ray analysis revealed a porous, heterostructured morphology with the expected elemental composition. Brunauer–Emmett–Teller surface area analysis documented the enhanced surface properties of the composite—critical, because photocatalysis and electrochemical sensing are both interfacial processes whose rates scale with accessible active surface area. Porosity also aids adsorption of trimethoprim molecules onto the catalyst surface, bringing them into intimate contact with reactive sites before degradation begins.

Photocatalytic performance was evaluated under natural sunlight irradiation, and the results were striking. Under optimized conditions—a solution pH of 5, a temperature of 35 degrees Celsius, and an initial trimethoprim concentration of 20 parts per million—the nanocomposite destroyed 95 percent of the antibiotic within 50 minutes. Kinetic analysis of the concentration-versus-time data indicated that the degradation follows pseudo-first-order reaction kinetics, a hallmark of heterogeneous photocatalysis in which the reaction rate is proportional to pollutant concentration while the catalyst surface is saturated with light-generated reactive species. To probe the mechanism, the researchers conducted radical scavenging experiments using isopropyl alcohol, benzoquinone, and ammonium oxalate—selective quenchers of hydroxyl radicals (•OH), superoxide radicals (•O₂⁻), and photogenerated holes, respectively. The mechanistic picture that emerges is familiar to photocatalysis researchers: sunlight excites electrons from the valence band to the conduction band of the composite, leaving holes behind. Dissolved oxygen captures conduction-band electrons to form superoxide radicals, while water or hydroxide ions react with holes to generate hydroxyl radicals. These reactive oxygen species then attack the trimethoprim molecule, progressively cleaving its aromatic rings and heteroatom-containing moieties until mineralization products are formed.

The second, equally consequential application is electrochemical detection. The same nanocomposite was immobilized on an electrode and tested for its ability to oxidize trimethoprim in phosphate buffer solution. Cyclic voltammetry established that the modified electrode exhibits excellent electrocatalytic activity toward the antibiotic, with a well-defined oxidation signal whose current increases systematically with trimethoprim concentration. Quantitative calibration using differential pulse voltammetry—a pulsed technique that suppresses background charging current and therefore improves sensitivity—demonstrated a wide linear detection range spanning 0.05 to 25 micromolar, a limit of detection of 0.5 micromolar, and a high sensitivity of 10 microamperes per micromolar per square centimeter. These figures of merit compare favorably with previously reported trimethoprim sensors, including those based on noble-metal nanoparticles, carbon fiber paper, and graphene oxide–zinc oxide quantum dot composites, yet the underlying material is composed of two earth-abundant, inexpensive oxides prepared in a single synthesis. The improved electrochemical response again reflects the synergy between the two oxide phases: efficient charge separation within the composite translates into faster heterogeneous electron transfer between the trimethoprim molecule and the electrode, amplifying the analytical signal.

What makes this work resonate beyond the laboratory is the elegance of its dual-purpose design. Environmental monitoring of pharmaceuticals currently depends on labor-intensive analytical techniques such as liquid chromatography coupled to mass spectrometry, which require expensive instrumentation, trained operators, and centralized facilities. Electrochemical sensors, by contrast, are compact, fast, inexpensive, and amenable to field deployment—and a sensor built from the same material that degrades the pollutant offers a compelling vision of integrated remediation systems in which treatment and verification happen side by side. A treatment plant or a decentralized rural water-treatment unit could, in principle, load sunlight-active composite onto a photo-reactor while equipping an electrode downstream with the same composite to continuously verify that antibiotic levels have fallen below safe thresholds. The low fabrication cost and reliance on freely available sunlight make the approach particularly attractive for low-resource settings where antibiotic contamination and monitoring gaps are most severe.

The findings also add to a growing body of evidence that carefully engineered heterojunctions between cheap metal oxides can rival more exotic and costly photocatalysts. Prior studies have explored Z-scheme and p–n heterojunction systems—such as Co₃O₄/BiOI for ibuprofen and trimethoprim degradation, g-C₃N₄/AgMoO₄ composites for antibiotic destruction, and Mn/Fe oxide-functionalized ceramic membranes for catalytic ozonation—but relatively few materials have been validated for both photocatalytic degradation and electrochemical sensing of the same target molecule. The Mn₂O₃–Co₃O₄ system demonstrates that the same interfacial charge-transfer physics that drives photocatalysis can be exploited for amperometric detection, unifying two branches of applied materials chemistry under one synthesis. The mechanistic understanding that superoxide and hydroxyl radicals are the dominant degrading species, confirmed through selective scavenger tests, provides a blueprint that other groups can use to rationalize and optimize related composite systems.

Important work remains before the technology can leave the bench. Real wastewater contains competing organic matter, suspended solids, and mixed pharmaceutical cocktails that can foul catalysts and interfere with electrochemical signals, and the study’s optimized conditions—moderately acidic pH and relatively warm temperatures—will need to be tested against the variable chemistry of actual effluents. Long-term catalyst stability, recyclability across repeated sunlight cycles, and the identity and toxicity of degradation intermediates are all questions that scale-up studies must answer. The authors, who also include Dhanyashree Savithree Vishwakumar of Davangere University, Jagadish Krishnegowda of Sarada Vilas College, University of Mysore, and Sucheta Mallikarjunaiah of Bangalore University, report no external funding for the work and state that all supporting data are contained within the article. Nonetheless, the combination of a 2.1-electronvolt band gap, 95 percent degradation in under an hour of sunlight, pseudo-first-order kinetics, and a sub-micromolar electrochemical detection limit establishes the Mn₂O₃–Co₃O₄ nanocomposite as one of the more versatile entries yet in the quest to tame antibiotic pollution—and a reminder that sometimes the most impactful materials science begins with the humblest of ingredients.