Molecular systems that can be remotely controlled by light are gaining increasing importance in cell biology, physiology, and neurosciences because of the spatial and temporal precision that is achievable with laser microscopy. Two-photon excitation has significant advantages deep in biological tissues, but raises problems in the design of "smart" probes compatible with cell physiology. This Review discusses the chemical challenges in generating suitable two-photon probes.

Numerous research efforts are investigating the possibility of using light interactions with metallic nanoparticles to improve the fluorescence properties of nearby molecules. Few investigations have considered the encapsulation of molecules in metallic nanocavities. In this paper, we present the optical properties of new hybrid nanoparticles consisting of gold nanoshells and fluorescent organic dyes in their liquid cores. Microspectroscopy on single nanoparticle demonstrates that the extinction spectra are in good agreement with Mie's theory. Finite difference time domain (FDTD) calculations reveal that excitation and emission radiations are efficiently transmitted through the thin gold nanoshells. Thus, they can be considered as transparent plasmonic nanocontainers for photoactive cores. In agreement with FDTD calculations, measurements show that fluorophores encapsulated in gold nanoshells keep their brightness, but they show fluorescence lifetimes 1 order of magnitude shorter. As a salient consequence, the photoresistance of encapsulated organic dyes is also improved by an order of magnitude. This unusual ultraviolet photoresistance results from the reduced probability of triplet-singlet conversion that eventually exposes dyes to singlet oxygen photodegradation.

High two-photon photolysis cross sections and water solubility of probes are important to avoid toxicity in biomedical applications of photolysis. Systematic variation of the position of a carboxyl electron-withdrawing group (EWG) on photolysis of 8-dimethylaminoquinoline protecting groups identified the C5-substituted isomer as a privileged dipole. The 5-benzoyl-8-DMAQ substitution yields a caging group with an enhanced two-photon uncaging cross section (δu = 2.0 GM) and good water solubility (c ≤ 50 mM, pH 7.4).

No abstract

The synthesis and photophysical properties of two lipophilic quadrupolar chromophores featuring anthracenyl (1) or dibromobenzene (2) were described. These two chromophores combined significant two-photon absorption cross-sections with high fluorescence quantum yield for 1 and improved singlet oxygen generation efficiency for 2, in organic solvents. The use of Pluronic nanoparticles allowed a simple and straightforward introduction of these lipophilic chromophores into biological cell media. Their internal distribution in various cell lines was studied using fluorescence microscopy and flow-cytometry following a successful staining that was achieved upon 2 h of incubation. Finally, multiphoton excitation microscopy and photodynamic therapy capability of the chromophores were demonstrated by cell exposure to a 820 nm fs laser and cell death upon one photon resonant irradiation at 436 ± 10 nm, respectively.

Over the last 20 years, the number of pathogenic multi-resistant microorganisms has grown steadily, which has stimulated the search for new strategies to combat antimicrobial resistance. Antimicrobial photodynamic therapy (aPDT), also called photodynamic inactivation, is emerging as a promising alternative to treatments based on conventional antibiotics. We have explored the effectiveness of methylene blue-loaded targeted mesoporous silica nanoparticles (MSNP) in the photodynamic inactivation of two Gram negative bacteria, namely Escherichia coli and Pseudomonas aeruginosa. For E. coli, nanoparticle association clearly reduced the dark toxicity of MB while preserving its photoinactivation activity. For P. aeruginosa, a remarkable difference was observed between amino-and mannose-decorated nanoparticles. The details of singlet oxygen production in the nanoparticles have been characterized, revealing the presence of two populations of this cytotoxic species. Strong quenching of singlet oxygen within the nanoparticles is observed. OPEN ACCESS

The systematic SAR study of a "caging" group showed a strong influence of the position of the donor dimethylamino group on the efficiency of photolysis of the DMAQ (2-hydroxymethylene-(N,N-dimethylamino)quinoline) caged acetate under one-photon near-UV or two-photon near-IR excitation. Photorelease of l-glutamate by the most efficient 8-DMAQ derivative strongly and efficiently activated glutamate receptors, generating large, fast rising responses similar to those elicited by glutamate photoreleased from the widely used MNI-caged glutamate.

A chromophore featuring a diyne bridge that connects two dibromobenzene moieties shows a much improved two-photon singlet oxygen generation ability in the biological transparency window compared to a related and relevant literature benchmark, as a result of a distorted ground state.