The fluorescent probe Singlet Oxygen Sensor Green is able to produce singlet oxygen under exposure to UV or visible radiation.

Two recent advances in optoelectronics, namely novel near-IR sensitive photomultipliers and inexpensive yet powerful diode-pumped solid-state lasers working at kHz repetition rate, enable the time-resolved detection of singlet oxygen (O2(a1Deltag)) phosphorescence in photon counting mode, thereby boosting the time-resolution, sensitivity, and dynamic range of this well-established detection technique. Principles underlying this novel approach and selected examples of applications are provided in this perspective, which illustrate the advantages over the conventional analog detection mode.

Antimicrobial photodynamic therapy (aPDT) is an emerging treatment for bacterial infections that is becoming increasingly more attractive because of its effectiveness against multi-antibiotic-resistant strains and unlikelihood of inducing bacterial resistance. Among the strategies to enhance the efficacy of PDT against Gram-negative bacteria, the binding to a cationic antimicrobial peptide offers the attractive prospect for improving both the water solubilty and the localization of the photoactive drug in bacteria. In this work we have compared a number of free and apidaecin-conjugated photosensitizers (PSs) differing in structure and charge. Our results indicate that the conjugation of per se ineffective highly hydrophobic PSs to a cationic peptide produces a photosensitizing agent effective against Gram-negative bacteria. Apidaecin cannot improve the phototoxic activity of cationic PSs, which mainly depends on a very high yield of singlet oxygen production in the surroundings of the bacterial outer membrane. Apidaecin-PS conjugates appear most promising for treatment protocols requiring repeated washing after sensitizer delivery.