Effect of environment pH on the photophysics of fisetin in solid lipid nanoparticles

•Photophysics of fisetin has been studied in solid lipid nanoparticle (SLN).•pH of the solutions was varied from 5 to 9.•The neutral and anionic forms of fisetin coexist in ground state.•In the excited state and at low pH, the anionic form of fisetin predominates.•Limited percolation of H+ and OH− ions occurs in the SLNs.

Photophysical modulation of fisetin has been extensively studied in bulk aqueous as well as solid lipid nanoparticles (SLN) by varying the pH of the medium. The solution pH was varied from 5 to 9 to mimic biological environments. Neutral and anionic forms of fisetin coexist in ground state in both acidic and alkaline conditions. However, in the excited state and at low pH, the anionic form of fisetin predominates over the proton transferred form, whereas in SLNs, the proton transferred form is the major emitting species. Higher pH showed enhancement in anionic emission to different extent in the two types of environments. Limited percolation of H+ and OH− ions inside the SLNs that host fisetin molecules controls their photophysics. The experimental results encourage usage of fisetin as a drug depending on the ratio of the neutral and anionic as well as the proton transferred forms under various pH conditions.

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