Photodynamic inactivation, or PDI, rests on the interplay of three components: a photosensitizer compound, light of an appropriate wavelength, and molecular oxygen. When a photosensitizer in its ground singlet state absorbs a photon, it is promoted to an excited singlet state and, through intersystem crossing, reaches a longer-lived triplet excited state. From there, two photochemical pathways unfold. In the type I pathway, the excited photosensitizer engages in charge transfer with surrounding biomolecules or oxygen, generating superoxide anion, hydrogen peroxide, and hydroxyl radicals. In the type II pathway, energy is transferred directly to triplet oxygen, producing singlet oxygen, a potent oxidant with a nanosecond-scale half-life and a diffusion range of only 10 to 20 nanometers. Either way, the resulting oxidative burst overwhelms microbial antioxidant defenses, disrupting membranes, inactivating enzymes, and killing the cell. Because this damage is multi-target rather than pathway-specific, the likelihood of resistance development is reduced, and no stable classical resistance to PDI has been demonstrated to date.

Yet molecular photosensitizers have stubborn limitations. Many, including porphyrins, phthalocyanines, phenothiazines, and curcumin, suffer from low water solubility, a tendency to aggregate, limited selectivity, and suboptimal photophysical stability. These flaws reduce reactive oxygen species generation and therapeutic performance, particularly inside biofilms, where penetration and accumulation are already difficult. The Brazilian review, which surveyed studies published between 2022 and 2025, argues that inorganic nanoparticles offer a versatile toolkit to overcome these barriers. By tuning size, morphology, and surface chemistry, researchers can engineer nanomaterials that stabilize photosensitizers, prevent aggregation, boost light-induced ROS production, and promote accumulation at infection sites—including hard-to-reach microenvironments that free drugs struggle to reach.

Some inorganic nanoparticles go further and act as photosensitizers themselves. Metal oxide nanoparticles such as zinc oxide and titanium dioxide, along with semiconductor quantum dots, generate reactive oxygen species upon light irradiation through photocatalytic bandgap excitation. When photons with energy above the bandgap are absorbed, conduction band electrons and valence band holes form and migrate to the particle surface, where electrons reduce oxygen to superoxide and holes oxidize water or hydroxide ions into hydroxyl radicals. These nanostructures also offer improved photostability, controllable intrinsic antimicrobial activity in the dark, surface functionalization for targeted delivery, and a broader excitation range. Titanium dioxide, however, absorbs mainly in the ultraviolet, so researchers have doped it with metal ions such as iron, copper, and silver to shift its bandgap into the visible range, enabling activation with blue light around 450 nanometers.

Metallic nanoparticles, particularly those made of silver and gold, operate through a different and mechanistically richer phenomenon: localized surface plasmon resonance. When incident light overlaps with a nanoparticle’s extinction band, its free conduction electrons oscillate collectively, creating a dipole that amplifies the local electromagnetic field. If a photosensitizer’s absorption band overlaps the plasmon band and the two are within roughly 10 nanometers of each other, this field amplification can enhance photosensitizer excitation and increase ROS generation. Plasmon-to-photosensitizer energy transfer and nanoparticle-facilitated electron transfer may also contribute. There is a catch: at very short distances, generally below 5 nanometers, nonradiative energy transfer from the photosensitizer to the nanoparticle can quench the excited state and reduce singlet oxygen output. The plasmon band itself can be tuned across roughly 390 to 700 nanometers by adjusting nanoparticle size, morphology, and composition, and gold nanostructures can additionally convert light into heat, opening the door to combined photothermal and photodynamic therapy.

The review’s bacterial case studies illustrate these principles in action. Luo and colleagues built a platform of silica-coated gold nanorods conjugated to Rose Bengal, combining photothermal heating—reaching 70 degrees Celsius under 808-nanometer laser irradiation—with photodynamic action. The system achieved 100 percent inactivation of Escherichia coli and Staphylococcus aureus under xenon lamp irradiation, outperforming either component alone, with negligible cytotoxicity toward human embryonic kidney cells and minimal hemolytic activity. Wang and colleagues developed a titanium dioxide–hydroxyapatite nanocomposite activated by a dental curing LED, which reduced Streptococcus mutans by roughly 83 percent and simultaneously promoted enamel remineralization in extracted human teeth, suggesting a dual-purpose tool against dental caries. Zhan and colleagues loaded silver nanoparticles into a titanium-based metal–organic framework, where the silver contributed intrinsic light-independent killing; the minimum bactericidal concentration against E. coli dropped about 64-fold under white light, with activity against MRSA as well.

Silica-based carriers demonstrated another key advantage: taming dark toxicity. Prieto-Montero and colleagues functionalized silica nanoparticles of different sizes with Rose Bengal and gluconamide, a ligand that targets the lipopolysaccharide of Gram-negative bacteria. Free Rose Bengal reduced bacterial survival to 43 percent even without light, but when covalently anchored to the nanoparticles, dark survival climbed to 83 to 99 percent, while phototoxicity under green LED irradiation was preserved and even enhanced by the targeting ligand. Smaller 20-nanometer particles interacted more effectively with bacterial membranes than 250-nanometer ones. In a separate approach, Chen and colleagues created cellulose discs bearing indium phosphide quantum dots functionalized with 9-anthracene carboxylic acid, which generated singlet oxygen under visible light and reduced MRSA by 99 percent and vancomycin-resistant Enterococcus faecium by 93 percent in the presence of potassium iodide—pointing toward self-sanitizing surfaces for hospitals. Li and colleagues combined mesoporous iron oxide magnetic nanoparticles loaded with the dye IR-820, using magnetic hyperthermia under an alternating magnetic field plus photodynamic irradiation to cut MRSA biofilm biomass by roughly 75 percent and accelerate wound healing in rats.

Fungal results were equally striking. Raposo and colleagues paired zinc(II) porphyrins with roughly 14-nanometer silver nanoparticles whose plasmon peak at 410 nanometers overlapped the porphyrin Soret band. Free porphyrin reduced Candida albicans by only about 2 to 3 log10, but the nanoparticle conjugates achieved complete fungal eradication at 4- to 8-fold lower photosensitizer concentrations, with ROS levels rising about 50 percent and confocal microscopy showing greater cellular internalization. Rodrigues and colleagues conjugated methylene blue to silver nanoprisms, which suppressed the dye’s dimerization—a known quenching pathway—and enabled eradication of a resistant C. albicans strain with just 2 minutes of red light. Wang and colleagues engineered a biomimetic upconversion nanoparticle coated with cell membrane and armed with a DNA aptamer, converting near-infrared excitation into visible emission that activated methylene blue; the system detected and killed C. albicans in infected mouse wounds, reducing fungal burden by 99.8 percent. Liu and colleagues went further, adding ultrasound to an upconversion nanosystem, which loosened dense fungal biofilms and allowed deeper nanoparticle penetration, achieving 99.5 percent biofilm reduction and near-complete wound healing in mice by day 11.

The review’s authors are candid about the field’s challenges. Nearly all fungal studies focused on C. albicans, leaving clinically urgent pathogens such as Candida auris and Aspergillus species underexplored. Experimental parameters—light doses, incubation times, nanoparticle characterization—are inconsistently reported, making direct comparison difficult, and most studies remain in vitro with limited preclinical evidence. Some systems, such as silver-doped titanium dioxide, killed about 30 percent of fungi even without irradiation, a bimodal behavior that can be advantageous in dental materials but complicates interpretation. Mechanistic questions, especially how the plasmonic effect enhances photodynamic action, remain incompletely resolved, and biocompatibility, long-term stability, scalability, and regulatory pathways all demand attention. Still, inorganic nanoparticles already have a human safety track record in sunscreens and silver wound dressings, and the reviewed studies collectively show that these multifunctional agents can act as carriers, photocatalysts, plasmonic amplifiers, photothermal agents, and imaging probes at once. As the authors conclude, careful platform selection matched to the microbial target and clinical scenario could finally translate these luminous nanotools into real therapies against the resistant microbes that conventional drugs can no longer touch.

Subject of Research: Use of inorganic nanoparticles to enhance antimicrobial photodynamic inactivation of resistant bacteria and fungi

Article Title: Inorganic nanoparticles: multifunctional agents for antimicrobial photodynamic inactivation

Article References: Lima, G. S., Souza, T. H. S., Henrique, R. B. L., Bueno-Janice, J. C., Fernandes, M. G. S., Santana, G. S., Santana, O. A., Cabral Filho, P. E., & Fontes, A. (2026). Inorganic nanoparticles: multifunctional agents for antimicrobial photodynamic inactivation. Journal of Nanoparticle Research, 28(10), Article 262. https://doi.org/10.1007/s11051-026-06785-w

Image Credits: AI Generated

DOI: 10.1007/s11051-026-06785-w

Keywords: antimicrobial resistance, photodynamic inactivation, inorganic nanoparticles, photosensitizers, reactive oxygen species, silver nanoparticles, gold nanorods, titanium dioxide, quantum dots, upconversion nanoparticles, biofilms, Candida albicans