The study, published in the Spanish Journal of Soil Science by Darina Zheleva and Desislava Angelova of the University of Chemical Technology and Metallurgy together with Ekaterina Krumova, Lyudmila Yovchevska and Yana Gocheva of the Stephan Angeloff Institute of Microbiology at the Bulgarian Academy of Sciences, addresses a gap in leather nanotechnology. Most work on antimicrobial leather has focused on the pre-tanning stage, where nanomaterials are added to the tanning solution itself. Far fewer investigations have explored the very last step of leather processing—the finishing stage—where a thin protective film is laid on top of the material. It is precisely here that the Bulgarian team chose to intervene, synthesizing and depositing zinc oxide nanoparticles in situ rather than simply mixing a pre-made powder into a coating.

The choice of zinc oxide is deliberate. Among metal oxide nanomaterials such as titanium dioxide, copper oxide, silver and copper, ZnO stands out for its strong antimicrobial properties, its biocompatibility, its relatively low toxicity and its cost-effectiveness. Positively charged zinc-containing particles can interact electrostatically with the negatively charged surfaces of bacterial cell walls, and the nanoparticles are known to generate reactive oxygen species—hydrogen peroxide, superoxide anions and hydroxyl radicals—that inflict oxidative stress on microbial cells. Released zinc ions additionally interfere with bacterial metabolism and enzyme function, and the particles’ small size allows them to penetrate membranes and damage intracellular components, including DNA.

To anchor the nanoparticles to the leather, the researchers used a cleverly matched scaffold: a cross-linked gelatin hydrogel, chemically similar in structure to the collagen that constitutes natural leather itself. Gelatin and collagen carry abundant amino, amide and carboxyl groups in their side chains, and these polar groups can bind metal atoms. The amino and carboxyl functional groups act as binding sites for zinc ions, seeding localized nucleation of ZnO crystals on and within the coating. In effect, the organic matrix becomes a template that guides the growth of the inorganic particles.

The team prepared chrome-tanned pig leather samples, each 50 by 50 millimeters and 0.89 millimeters thick, and treated them with three different methods that varied the sequence of steps. In the first method, gelatin was cross-linked with glutaraldehyde before leather impregnation, followed by the addition of zinc nitrate solution and sodium hydroxide. The second method immersed the leather in gelatin first and introduced the crosslinking agent later. The third method, performed in an ultrasonic bath, pre-mixed the zinc nitrate and sodium hydroxide solutions before impregnation and crosslinking. In all cases, sodium hydroxide was added in tenfold excess relative to zinc ions, driving the precipitation of zinc hydroxide and its conversion to zinc oxide during thermal treatment and drying.

Scanning electron microscopy revealed what happened next: the spherical nanoparticles morphed into striking flower-like structures, a sign that nucleation of ZnO crystal architecture had begun on the leather surface. Individual particles measured between 50 and 200 nanometers, but clusters and agglomerations swelled to more than 500 nanometers and beyond one micrometer. The team attributes the flower-like morphology to heterogeneous nucleation on the gelatin matrix: supersaturation after the addition of sodium hydroxide drives crystal growth along specific crystallographic planes of the wurtzite structure, while the surrounding organic network restricts orientation and promotes radial, petal-like assembly. Elemental analysis added a dramatic quantitative contrast. In the first sample type, the coating contained roughly 11.6 percent zinc oxide by weight within the probed cubic-micrometer region; in the second, where the particles sat atop the hydrogel surface, the figure jumped to approximately 64.6 percent—suggesting heavy agglomeration and, critically, higher surface activity exactly where it matters for microbial contact.

Spectroscopy confirmed the chemistry underneath. Fourier-transform infrared spectra showed the expected absorption peaks for amide and amine groups from collagen and gelatin, and demonstrated that positively charged zinc species can bind electrostatically to negatively charged carboxylate residues or to the lone electron pairs on amino acid nitrogen atoms. A characteristic peak at 852 wavenumbers confirmed the formation of tetrahedrally coordinated zinc oxide, most clearly expressed in the ultrasonically treated sample. Ultraviolet-visible spectroscopy sealed the case with a low absorption peak at 363 nanometers—the unambiguous optical fingerprint of nanostructured zinc oxide.

Then came the biological tests, conducted with the Kirby–Bauer disc diffusion method against four bacterial strains: Pseudomonas aeruginosa and Escherichia coli, both Gram-negative, and Bacillus subtilis and Erysipelothrix rhusiopathiae, both Gram-positive. The results revealed a clear selectivity. Only the ultrasonically prepared sample produced a measurable zone of inhibition, and only against Bacillus subtilis, with a modest inhibition zone of about 4 millimeters—small compared with the 18-millimeter zones produced by a reference antibiotic. The Gram-negative bacteria proved entirely resistant to all the modified samples. This pattern matches earlier findings that Gram-positive bacteria are generally more vulnerable to ZnO nanoparticles than their Gram-negative counterparts, possibly owing to differences in cell wall architecture, surface charge, membrane composition and cellular physiology. The researchers also note that the relatively weak antibacterial effect may stem from limited diffusion of active zinc species out of the cross-linked gelatin matrix, which restricts the particles’ availability at the bacterial interface—a trade-off inherent in immobilizing nanoparticles for durability.

Against fungi, the story was considerably more impressive. The untreated leather showed no antifungal activity whatsoever, but all three ZnO-modified variants strongly protected against Aspergillus niger and Candida albicans. Growth of Candida albicans was inhibited for the first 24 hours before the yeast resumed growth, while Aspergillus niger growth was suppressed for 24 hours, followed by long-lasting inhibition of sporulation that persisted through 72 hours of observation. The mechanism likely combines physical effects—disruption of ion transport, membrane integrity and protein function by the particles—with chemical attack through reactive oxygen species generation, zinc ion release and direct membrane interaction. When treated with the nanoparticles, fungal hyphae developed compact vacuole-like structures, lost cytoplasmic volume and became coated with electron-dense particle layers, consistent with nanoparticle adsorption and subsequent cellular damage.

The authors conclude that these ZnO-containing finish films can serve as effective protective antibacterial and antifungal coatings for shoe leather, helping shield the human foot from microbial exposure. Because zinc oxide holds Generally Recognized As Safe status for certain uses and has shown limited dermal penetration and low acute toxicity in most studies, the approach looks promising relative to the hazardous biocides traditionally used to prevent leather biodegradation. Still, the researchers urge caution: the long-term effects of chronic skin contact with zinc ion release remain insufficiently understood, and nanomaterials fall under the EU’s REACH regulatory framework, demanding careful toxicological evaluation. There is also an unsolved sustainability problem at the end of the product’s life. No current technology can extract or regenerate nanoparticles embedded in footwear materials, which limits both the reuse of the embedded metal and the technology’s overall environmental credentials. For now, the work stands as a vivid demonstration that the final finishing step—the one the industry has long overlooked—may be the best place to build infection-fighting power directly into the shoes on our feet.

Subject of Research: Antimicrobial zinc oxide nanoparticle coatings for shoe lining leather

Article Title: Investigation of ZnO Antimicrobial Surface Treatment of Shoe Lining Materials

Article References: Zheleva, D., Angelova, D. S., Krumova, E., Yovchevska, L., & Gocheva, Y. (2026). Investigation of ZnO Antimicrobial Surface Treatment of Shoe Lining Materials. Spanish Journal of Soil Science, 15, Article 14992. https://doi.org/10.3389/sjss.2025.14992

Image Credits: AI Generated

DOI: 10.3389/sjss.2025.14992

Keywords: zinc oxide nanoparticles, antimicrobial coating, shoe lining leather, gelatin hydrogel, footwear hygiene, reactive oxygen species, Aspergillus niger, Candida albicans, nanotechnology, leather finishing, collagen, antifungal activity

The case for moving away from alkaline electrolytes is rooted in the peculiar chemistry of the zinc anode. In concentrated alkaline solutions, zinc metal is thermodynamically restless. It corrodes, converting itself into soluble zincate ions and releasing hydrogen gas as a byproduct. That hydrogen evolution dries out cells, builds pressure, and wastes capacity. At the same time, uneven dissolution and redeposition of zinc during repeated charge and discharge cycles encourages the growth of dendrites, needle-like metallic structures that can pierce separators and short-circuit the cell. These parasitic processes, well documented across the zinc-battery literature, are the principal reason alkaline zinc-air prototypes often fade after relatively few cycles. Neutral electrolytes, operating near pH 7, dramatically slow all three failure modes: corrosion, hydrogen evolution, and dendrite propagation. The catch is that neutral electrolytes are less conductive and less kinetically forgiving, so the rest of the cell, especially the air cathode, has to work harder to compensate.

The Sichuan group attacked both halves of the problem simultaneously. On the electrolyte side, they formulated a hydrogel built from gelatin, the same protein-derived polymer that gives dessert jelly its bounce. Gelatin carries abundant functional groups along its polypeptide chains, including carboxyl and amine groups, which can bind water and interact with dissolved ions, helping the gel retain electrolyte and conduct charge. Because the hydrogel is a soft, conformable solid rather than a free-flowing liquid, it eliminates the leakage and infiltration problems that plague liquid-electrolyte flexible cells, and it maintains intimate contact with both electrodes even when the battery is folded around a wrist or a fingertip. The researchers prepared this gelatin hydrogel under neutral conditions, creating a mechanically resilient ion-conducting bridge between the zinc anode and the air-breathing cathode.

The cathode, however, is where the study makes its most distinctive contribution. Air cathodes are the workhorses of zinc-air chemistry: during discharge they must catalyze the oxygen reduction reaction, in which oxygen molecules from the air accept electrons to form hydroxide or related species, and during recharge they must run the reverse oxygen evolution reaction. Both reactions are notoriously sluggish, and the catalysts that drive them well, typically platinum-group metals or complex transition-metal oxides, are expensive or difficult to synthesize. Titanium dioxide, an abundant, cheap, and chemically robust semiconductor, has attracted attention as an oxygen reduction catalyst when its surfaces are properly engineered, but its poor electrical conductivity has always limited its usefulness in a battery electrode where electrons must arrive at every catalytic site.

The researchers’ solution was to decorate titanium dioxide particles with a small, controlled loading of silver nanoparticles, prepared by chemical reduction. Silver is one of the best electrical conductors known, and even a modest fraction of it, dispersed as nanoscale islands across the oxide surface, creates conductive pathways that let electrons flow efficiently through the composite cathode material. The team designated their best formulation TA-0.05, indicating the optimized silver loading level. Beyond simply wiring the oxide particles together, the silver-titania interface can also influence the catalytic chemistry itself, since metal-semiconductor junctions alter how charge is distributed at active sites. The result is a cathode that conducts better, catalyzes oxygen reduction more effectively, and holds up under the repeated potential swings of cycling, all while using only a low loading of a relatively affordable noble metal rather than a heavy dose of platinum.

When the TA-0.05 cathode was coupled with the gelatin hydrogel electrolyte and a zinc anode to form a complete flexible cell, the performance figures were striking for a neutral-electrolyte device. The battery sustained stable charge-discharge cycling for more than fifty hours at a current density of one milliampere per square centimeter, a benchmark that matters because flexible wearables typically draw currents in precisely this low-to-moderate range. Fifty hours of continuous cycling implies that the cell resisted the corrosion, gas evolution, and dendrite problems that would have degraded an alkaline equivalent, while the conductive cathode kept the voltage gaps between charge and discharge acceptably narrow, preserving round-trip energy efficiency. In the world of neutral zinc-air batteries, where sluggish kinetics often inflate those voltage gaps and erode efficiency, this combination of stability and efficiency is a meaningful advance.

The demonstration that will likely capture the public imagination, though, is the simplest one. The researchers connected two of their flexible batteries in series and used the pair to light a light-emitting diode, not on a laboratory bench but while the assembly was being bent. The LED stayed illuminated under deformation, showing that the gelatin hydrogel maintained ionic contact and the cathode retained its structural integrity through mechanical stress. For a wearable device, that is the test that counts: a battery that only works when it is flat is not a wearable battery at all. The image of a soft, protein-based power cell glowing steadily while flexed is exactly the kind of proof-of-concept that moves a technology from an electrochemistry paper toward a product roadmap.

The broader significance of the work lies in its systems-level design philosophy. Rather than optimizing a single component in isolation, the team matched the electrolyte and the cathode to each other’s constraints. The neutral hydrogel tames the zinc anode’s destructive side reactions, but it demands a cathode with excellent conductivity and catalytic activity to overcome the kinetic penalties of near-neutral operation. The low-loading silver-titania composite answers that demand with inexpensive, scalable materials: titanium dioxide is a commodity chemical, silver is far cheaper than platinum, and gelatin is a biopolymer produced industrially on enormous scales. None of the ingredients is exotic, which matters enormously for any technology hoping to be manufactured by the kilometer for consumer wearables. The chemical reduction synthesis used to deposit the silver nanoparticles is likewise a routine, low-temperature procedure compatible with large-area flexible substrates.

Challenges remain before such cells can power a smartwatch on a store shelf. Neutral zinc-air batteries generally deliver lower power densities than their alkaline cousins, and the reported cycling was conducted at a modest current density, so performance under the heavier loads of, say, a wireless radio burst remains to be demonstrated. Long-term questions about hydrogel dehydration, gelatin biostability over months of use, and the durability of the silver-titania interface across thousands of cycles will need answers from extended testing. The authors themselves frame the work as a promising strategy for next-generation wearable electronics rather than a finished product. Still, by showing that a food-safe polymer gel and a sprinkle of silver on cheap titanium dioxide can keep a flexible battery glowing through fifty hours of cycling and through repeated bending, the Sichuan team has sketched a credible, low-cost path to power sources that flex as naturally as the skin they sit on.

Subject of Research: Development of a silver nanoparticle-decorated TiO2 air cathode with a gelatin hydrogel electrolyte for flexible neutral zinc-air batteries

Article Title: Low-loading Ag nanoparticle-decorated TiO2 air cathode coupled with a gelatin hydrogel electrolyte for flexible neutral zinc–air batteries

Article References: Kong, X., Qin, Q., Wang, Z., Li, J., & Zeng, M. (2026). Low-loading Ag nanoparticle-decorated TiO2 air cathode coupled with a gelatin hydrogel electrolyte for flexible neutral zinc–air batteries. Ionics. https://doi.org/10.1007/s11581-026-07495-y

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07495-y

Keywords: zinc-air battery, flexible battery, gelatin hydrogel, titanium dioxide, silver nanoparticles, air cathode, oxygen reduction reaction, neutral electrolyte, wearable electronics, electrocatalysis, energy storage, nanomaterials