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Summary The rational design of two-photon absorption (2PA) organic dyes with 2PA cross-section (δ) values tunable by mild stimuli under physiological conditions remains a challenge. Using a simulation-assisted structure screening method, we have designed an acetal terminated distyrylbenzene derivative (Ace-DSB), which can be readily converted into aldehyde terminated molecules (Ald-DSB) to exhibit a significant increase in δ values at 760 nm. The conversion reaction, which dramatically turns on the 2PA absorption capability of the molecules, can occur in an intracellular environment. Furthermore, Ald-DSB exhibits 40 times higher efficiency of two-photon generation of singlet oxygen than Ace-DSB. These turn-on features allow enhanced two-photon laser confocal scanning microscopic imaging and two-photon excited photodynamic therapy (2PE-PDT) for MCF-7 cancer cells and melanoma tumors. This work opens a new avenue toward the design of turn-on-type two-photon absorption molecules for both in vitro and in vivo tumor imaging and 2PA-PDT.

We report the in vivo distribution, toxicity and metabolism of micro-sized fluorescent organic particles and their applications in cerebral blood flow tracing. The fluorescent microparticles exhibit bright fluorescence, good photo-stability and low toxicity; therefore, they are ideal for long-term non-invasive in vivo tracing. In contrast to conventional fluorescent labeling agents, which stain the entire blood vessel, the tracer microparticles can be easily tracked individually and provide vital information about blood flow behavior. Furthermore, we observed stimulated emission from these microparticles in living animals. These microparticles can provide unprecedented contrast for simultaneous observation of the distribution of blood vessels and the dynamics of microcirculation. Pathological examination revealed that the injected microparticles eventually collected in the spleen and liver. We found no observable toxicity of the microparticles to cells or mouse organs. We demonstrate that these fluorescent microparticles are suitable for applications in the field of non-intrusive blood flow tracing and could play a complementary role to traditional imaging agents.