Simplified diagram illustrating energy levels of a quantum dot compared to its semiconductor material in a bulk crystal. Electrons and holes in the quantum dot and the semiconductor crystal obey Pauli’s exclusion principle when filling the energy states and their positions are shown schematically only. Adapted from Ref. [11].

Enhancement of fluorescence of quantum dots (8 nM QD655-carboxyl terminated) endocytosed in Du145 cells (5×10⁵ cells/ml). A) Fluorescence microscopy photograph (objective 40×) taken within 0–1 min of observation after around 24 h incubation. Nuclei are stained with Hoechst 33342. B) The same spot on the culture dish photographed under the same conditions after 5 min irradiation with the microscope excitation light (395–440 nm). Red fluorescence appears while the Hoechst dye is photobleached during irradiation. C) Fluorescence photograph taken at lower magnification (10×) highlights the red fluorescent spot caused by the irradiation using the 40× objective. The scale bars are 100 μm.

Quenching of ADPA photoluminescence (PL) with X-ray irradiation in water (a), ADPA in MTCP solution (b), ADPA in LaF₃:Tb³⁺-MTCP conjugate (c) and ADPA in LaF₃:Tb³⁺-MTCP-folic acid conjugate (d). Reprinted with permission from Ref. [162] ^© 2008 American Institute of Physics.

Schematic presentation of the most likely events occurring when a photon hits a quantum dot. Situation for a low-energy photon (ultraviolet, visible light, near infrared radiation): I) Charge transfer, where QD is a donor and A is an acceptor molecule, ΔE_g is the energy gap of the quantum dot, e⁻ and h⁺ is an electron-hole pair generated by the absorption of the photon, II) Photosensitization, where the quantum dot is promoted from its ground state (a) to a higher energy excited state (b) and then to a “trap” state (c), from which by TET triplet ground-state oxygen (³O₂) is promoted into a highly reactive singlet oxygen (¹O₂). Vibrational levels are depicted for illustrative purpose. Dashed arrows show non-radiative transitions. Situation for a high-energy photon (X-rays and gamma rays): III) Ejection of a high speed electron from an atom constituting the quantum dot due to the photon energy transfer to the electron (photoelectric ionization effect) or Compton scattering, where the incident photon is scattered continuing its voyage with lower energy until next event, IV) Photon annihilation on a nucleus of an atom and generation of an electron-positron pair. The positron will annihilate with a free electron releasing two 0.51 MeV photons, which will further lose their energy through photoelectric effect or Compton scattering. Electrons generated in the events III and IV will induce secondary high speed electrons as well as Auger electrons. Such electrons that succeed to escape into environment will be captured by an acceptor (water, biomolecule, oxygen, nitrogen oxides) in the vicinity of the quantum dot and induce biomolecular radicals, superoxide, hydroxyl radical, peroxynitrite anion or nitric oxide radical.

Fluorescence microscopy detection of PPEI-EI-functionalized carbon dots in MCF-7 cells with one-photon (458 nm) and two-photon (800 nm) excitations. Shown in the insets are signals likely associated with single carbon dots.

Schematic presentation of the nanoparticle-based X-ray-induced PDT. Under ionizing radiation a nanoparticle starts to scintillate transferring its energy into a conjugated porphyrin molecule, which then generates singlet oxygen necessary to produce photosensitizing effect. This methodology will help to treat nodular and deeper tumours due to higher penetrating capacity of X-rays and gamma rays compared to that of visible light commonly used in PDT.

Cytotoxicity (left bars for each sample) and phototoxicity (right bars for each sample) of 3×10⁻⁵ M ZnO, 3×10⁻⁵ M MTCP and 3×10⁻⁵ M ZnO-MTCP on NIH-OVCAR-3 cells. Reprinted with permission from Ref. [164] ^© 2008 American Institute of Physics.