Lasers are the pillars of modern optics and sensing. Microlasers based on whispering-gallery modes (WGMs) are miniature in size and have excellent lasing characteristics suitable for biosensing. WGM lasers have been used for label-free detection of single virus particles, detection of molecular electrostatic changes at biointerfaces, and barcode-type live-cell tagging and tracking. The most recent advances in biosensing with WGM microlasers are described in this review. We cover the basic concepts of WGM resonators, the integration of gain media into various active WGM sensors and devices, and the cutting-edge advances in photonic devices for micro- and nanoprobing of biological samples that can be integrated with WGM lasers.

UV-A radiation (320–400 nm) induces damage to the DNA molecule and its components through photosensitized reactions. Biopterin (Bip) and its photoproducts 6-formylpterin (Fop) and 6-carboxypterin (Cap) accumulate in the skin of human beings suffering from vitiligo, a depigmentation disorder where the protection against UV radiation fails because of the lack of melanin. This study was aimed to evaluate the photosensitizing properties of oxidized pterins present in the skin and to elucidate the mechanisms involved in the photosensitized oxidation of purine nucleotides by pterins in vitro. For this purpose, steady-state and time-resolved experiments in acidic (pH 5.0–5.8) aqueous solution were performed using Bip, Fop and Cap as photosensitizers and the nucleotide 2′-deoxyguanosine 5′-monophosphate (dGMP) as an oxidizable target. The three pterin derivatives are able to photosensitize dGMP, being Fop the most efficient sensitizer. The reactions proceed through two competing pathways: (1) electron transfer from dGMP to triplet excited-state of pterins (type I mechanism) and (2) reaction of dGMP with ¹O₂ produced by pterins (type II mechanism). Kinetic analysis revealed that the electron transfer pathway is the main mechanism and the interaction of dGMP with the triplet excited-state of pterins and the formation of the corresponding dGMP radicals were demonstrated by laser flash photolysis experiments. The biological implications of the results obtained are also discussed.

Pterins belong to a class of heterocyclic compounds present in a wide range of living systems and accumulate in the skin of patients affected by vitiligo, a depigmentation disorder. The study of the emission of 7,8-dihydropterins is difficult because these compounds are more or less unstable in the presence of O₂ and their solutions are contaminated with oxidized pterins which have much higher fluorescence quantum yields (Φ_F). In this work, the emission properties of six compounds of the dihydropterin family (6-formyl-7,8-dihydropterin (H₂Fop), sepiapterin (Sep), 7,8-dihydrobiopterin (H₂Bip), 7,8-dihydroneopterin (H₂Nep), 6-hydroxymethyl-7,8-dihydropterin (H₂Hmp), and 6-methyl-7,8-dihydropterin (H₂Mep)) have been studied in aqueous solution. The fluorescence characteristics (spectra, Φ_F, lifetimes (τ_F )) of the neutral form of these compounds have been investigated using the single-photon-counting technique. Φ_F and τ_F values obtained lie in the ranges 3–9 × 10⁻³ and 0.18–0.34 ns, respectively. The results are compared to those previously reported for oxidized pterins.