Carbon Dots Turn Light Into Heat and Cell-Killing Chemistry, Review Finds

Carbon dots, the tiny fluorescent nanoparticles first discovered in 2004 during the purification of single-walled carbon nanotubes, have quietly become one of the most versatile light-responsive materials in modern chemistry. A comprehensive new review published in Discover Chemistry by William Wilson Anku of the University of Environment and Sustainable Development in Ghana brings together, for the first time in a single mechanistic framework, how these quasi-spherical particles smaller than ten nanometres convert absorbed light into heat and reactive oxygen species, and why that dual capability matters for cancer therapy, water disinfection, agriculture, and beyond.

The central insight of the review is that everything hinges on excited-state dynamics. When a carbon dot absorbs a photon, an electron is promoted from the ground singlet state to an excited singlet level, then rapidly relaxes to the lowest excited singlet state. From there, three competing pathways determine the fate of the energy: radiative fluorescence, non-radiative relaxation that releases heat, or intersystem crossing into the triplet manifold, which enables the production of reactive oxygen species. The relative rates of these processes, described by the rate constants for fluorescence, non-radiative decay, and intersystem crossing, dictate whether a given carbon dot behaves as an imaging agent, a photothermal heater, or a photosensitizer.

Photothermal therapy relies on the second pathway. Carbon dots that channel absorbed energy, particularly in the near-infrared window between 700 and 1100 nanometres where tissue penetration is deepest, dissipate it as localized heat through vibrational relaxation and phonon generation. Reported photothermal conversion efficiencies for carbon dots typically range from 20 to 50 percent, below the values above 60 percent achievable with gold nanorods, but the review argues that carbon dots compensate with biodegradability, lower cytotoxicity, scalable synthesis, and superior water dispersibility. Hybrid materials such as carbon dot reduced-graphene-oxide composites have pushed efficiencies beyond 45 percent through plasmonic coupling and improved charge transfer.

Photodynamic activity follows a different logic. Here the triplet state is the key intermediate, because its long lifetime provides the temporal window for energy or electron transfer to molecular oxygen. In the Type II pathway, energy transfers directly from the excited triplet carbon dot to ground-state oxygen, generating singlet oxygen, the cytotoxic species central to conventional photodynamic therapy. In the Type I pathway, electrons or hydrogen atoms are transferred instead, producing superoxide and hydroxyl radicals. The review stresses that Type I chemistry is more tolerant of the hypoxic conditions found in solid tumours, although it is not fully oxygen-independent, since superoxide formation still requires molecular oxygen as the terminal electron acceptor.

The engineering levers that tip these competing pathways are remarkably accessible. Nitrogen doping introduces electron-rich active sites that enhance fluorescence and catalytic activity, while sulfur and phosphorus doping alters surface charge and extends light absorption. Heavier atoms such as iodine, selenium, and manganese strengthen spin-orbit coupling through the heavy-atom effect, accelerating intersystem crossing and boosting singlet oxygen yields that pristine carbon dots cannot achieve alone. Donor-acceptor structures built through surface functionalization narrow the singlet-triplet energy gap, and polymer passivation with polyethylene glycol or polyethylenimine improves water solubility, biocompatibility, and drug-conjugation chemistry. PEI-functionalized dots, rich in amine groups, have proven particularly effective in antimicrobial photodynamic therapy.

Crucially, the review emphasizes that photothermal and photodynamic outputs compete for the same excited-state population. Excessively rapid non-radiative decay increases heat release but shortens the excited-state lifetime and suppresses reactive oxygen generation, while highly emissive systems lose energy as fluorescence before either heat or triplet formation becomes efficient. Rational design therefore demands a careful kinetic balance, especially for multifunctional dots expected to deliver both heat and oxidative damage under a single excitation wavelength. Nitrogen and sulfur co-doped dots that absorb in the near-infrared while simultaneously forming reactive oxygen species exemplify this dual-modal strategy.

The application landscape is strikingly broad. In oncology, nitrogen-doped carbon dots prepared from folic acid precursors have targeted folate receptor-overexpressing cancer cells and driven complete tumour regression under 808-nanometre laser irradiation in animal models. Heat from photothermal therapy can trigger immunogenic cell death, releasing tumour antigens that, combined with immune checkpoint inhibitors, amplify systemic anti-tumour immunity in the emerging field of photoimmunotherapy. Against antibiotic-resistant bacteria, nitrogen-doped dots under 808-nanometre irradiation have reduced viability of E. coli and Staphylococcus aureus by more than 99.9 percent within minutes, with heat helping reactive oxygen species penetrate biofilms.

Environmental and agricultural uses extend the same photochemistry in different directions. Carbon dots generate singlet oxygen, hydroxyl radicals, and superoxide under visible or near-infrared light to degrade persistent pollutants such as synthetic dyes, antibiotics, and endocrine disruptors; sulfur- and nitrogen-loaded dots completely broke down tetracycline and chloramphenicol within thirty minutes under simulated sunlight. Paper-based sensors built from carbon dots detect mercury at levels as low as 0.1 micrograms through visible fluorescence quenching, and smartphone-assisted fluorescence readers now enable low-cost field measurements. In crop protection, sunlight-activated dots produce reactive oxygen species that disrupt the membranes of phytopathogens such as Xanthomonas, Pseudomonas, and Botrytis cinerea, offering a pesticide-free alternative derived from agricultural waste like citrus peels and rice husks.

The review is candid about the barriers standing between laboratory promise and real-world impact. Batch-to-batch variability arising from precursor composition, reaction temperature, purification methods, and the inherent chemical inconsistency of biomass feedstocks produces apparently similar materials with markedly different properties. Inconsistent performance metrics, unreliable reactive oxygen species assays prone to false positives from probe adsorption and inner-filter effects, shallow optical penetration, incomplete mechanistic understanding, and limited long-term safety data all hinder translation. No carbon-dot phototherapy platform has yet received regulatory approval, and ecotoxicity studies show that zebrafish embryos exposed to high concentrations can suffer hatching retardation and developmental anomalies.

The path forward, the review argues, runs through standardized synthesis and reporting protocols, intrinsically near-infrared-II-active carbon dots in the 1000 to 1700 nanometre window, scalable good-manufacturing-practice production, comprehensive chronic toxicology, and field-scale validation of agricultural applications. Artificial intelligence and machine learning are expected to accelerate the search for optimal precursor and doping combinations by predicting structure-property relationships from large datasets. If those pieces come together, carbon dots could anchor a genuinely integrated platform spanning diagnosis, therapy, and environmental stewardship, a prospect few single material classes can credibly claim.

Subject of Research: Structure-property relationships and excited-state photophysics of carbon dots for photothermal and photodynamic applications

Article Title: Structure property relationships and excited state photophysics of carbon dots for photothermal and photodynamic applications

Article References: Anku, W. W. (2026). Structure property relationships and excited state photophysics of carbon dots for photothermal and photodynamic applications. Discover Chemistry, 3(1), Article 476. https://doi.org/10.1007/s44371-026-00928-7

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00928-7

Keywords: carbon dots, photothermal therapy, photodynamic therapy, reactive oxygen species, intersystem crossing, nanomaterials, heteroatom doping, singlet oxygen, photocatalysis, biomedicine, environmental remediation, agriculture