Supramolecular Complex of Photochromic Diarylethene and Cucurbit[7]uril: Fluorescent Photoswitching System for Biolabeling and Imaging

Abstract

Photoswitchable fluorophores—proteins and synthetic dyes—whose emission is reversibly switched on and off upon illumination, are powerful probes for bioimaging, protein tracking, and super-resolution microscopy. Compared to proteins, synthetic dyes are smaller and brighter, but their photostability and the number of achievable switching cycles in aqueous solutions are lower. Inspired by the robust photoswitching system of natural proteins, we designed a supramolecular system based on a fluorescent diarylethene ( DAE ) and cucurbit[7]uril (CB7) (denoted as DAE @CB7). In this assembly, the photoswitchable DAE molecule is encapsulated by CB7 according to the host–guest principle, so that DAE is protected from the environment and its fluorescence brightness and fatigue resistance in pure water improved. The fluorescence quantum yield (Φ fl ) increased from 0.40 to 0.63 upon CB7 complexation. The photoswitching of the DAE @CB7 complex, upon alternating UV and visible light irradiations, can be repeated 2560 times in aqueous solution before half-bleaching occurs (comparable to fatigue resistance of the reversibly photoswitchable proteins), while free DAE can be switched on and off only 80 times. By incorporation of reactive groups [maleimide and N -hydroxysuccinimidyl (NHS) ester], we prepared bioconjugates of DAE @CB7 with antibodies and demonstrated both specific labeling of intracellular proteins in cells and the reversible on/off switching of the probes in cellular environments under irradiations with 355 nm/485 nm light. The bright emission and robust photoswitching of DAE-Male3 @CB7 and DAE-NHS @CB7 complexes (without exclusion of air oxygen and addition of any stabilizing/antifading reagents) enabled confocal and super-resolution RESOLFT (reversible saturable optical fluorescence transitions) imaging with apparent 70–90 nm optical resolution.

Small-molecule photoswitches for fluorescence bioimaging: engineering and applications

“…4a and b). 63 Upon interaction with CB7, the DAE showed an increase in fluorescence quantum yield (1.6-fold) and photostability (430-fold). Conjugated to antibodies, the probe enabled RESOLFT imaging of microtubules on fixed cells, achieving 70-90 nm spatial resolution.…”

Current Trends in Nanotheranostics: A Concise Review on Bioimaging and Smart Wearable Technology

“…Quantum dots, which are also semiconductor nanocrystals, have particularly high fluorescence efficiency and broad absorption spectra, making them useful for multiplexing and deep tissue imaging 68 , 69 . Gold nanoparticles have strong light scattering properties and can be used to enhance the signal of fluorescent probes 70 , 71 , while fluorescent dyes can be used to label specific biomolecules or structures 72 . The use of nanomaterials in fluorescence bioimaging is expected to have a significant impact on the early diagnosis and treatment of diseases, as well as on our understanding of biological processes at the molecular level.…”

Persistent Fluorescence Switching of a Probe Using a Photochromic Quencher with High Photostability Assisted by Protein-Surface Modification

“…More recently, diarylethene included by cucurbit [7]uril was reported to improve photostability compared to free diarylethene in an aqueous solution. 18 However, those supramolecular complexes have a sterically crowded structure, and they require longer linker lengths to conjugate with other biomolecules. Here, to address this limitation, we established a protein-surface-assisted strategy that achieves persistent reversible fluorescence photoswitching of a PSFM in an aqueous solution.…”