Highly Fluorescent Water Soluble Spirobifluorene-based Dye as Sensitive Probe for Protein and Live Cell Imaging

Fluorescent markers suitable for cellular organelles or biomolecules staining require a series of features, like large Stokes shift, high molar extinction coefficient (e) and high quantum yield (QY) to give high brightness values in aqueous solution,[1] as well as low cytotoxicity. Despite the large variety of dyes reported so far, the combination of the whole set of requirements in the same molecule is still challenging. Among polyaromatic compounds, spirobifluorene-based emitters attract growing interest due to chemical stability and versatility allowing their use in several technological applications.[2] Nonetheless, the lack of water solubility precluded the application of spirobifluorene compounds in biological field to date. Here, we disclose the synthesis and photophysical properties of the first water soluble spirobifluorene-based fluorescent dye and its interaction with proteins as model system for biological purposes.[3] The dye is characterized by high blue-greenish photoluminescence QY (50-70%) and very large Stokes shift (> 6000 cm-1) in both organic and aqueous solution. Noteworthy, in presence of bovine serum albumin (BSA) the emission peak shows a hypsochromic shift with increased QY in water from 50% to 95%. By using this dye as non covalent probe for BSA, we were able to reach the outstanding detection limit of 7.3 nM in a wide range with small concentration of dye. The effect of the BSA:dye interactions and the local microenvironment on the emission will be also presented. Investigations on cellular uptake and cytotoxicity revealed that this chromophore is biocompatible and taken up by living cells, indicating the potential application for live cell imaging. To conclude, we show the earliest example of a new class of spirobifluorene-based emitters with outstanding fluorescent properties in water, opening novel scenario for bio applications.