Convenient and Efficient FRET Platform Featuring a Rigid Biphenyl Spacer between Rhodamine and BODIPY: Transformation of 'Turn-On' Sensors into Ratiometric Ones with Dual Emission

We have connected a boron-dipyrromethene (BODIPY) donor to the 5' position of a tetramethylrhodamine (TMR) acceptor to form a high efficiency (over 99%) intramolecular fluorescence resonance energy transfer (FRET) cassette, BODIPY-rhodamine platform (BRP). While the good spectral overlap between the emission of BODIPY and the absorption of TMR was one favorable factor, another feature of this FRET system was the rigid and short biphenyl spacer that favored efficient through-bond energy transfer. More importantly, in this system, the 2'-carboxyl group of the rhodamine unit was preserved for the further modifications, which was as convenient as those carbonyl groups on the original rhodamines without connection to donors. For this reason, BRP is clearly differentiated from the previous ratiometric sensors based on donor rhodamine systems. To illustrate its value as a versatile platform, we introduced typical Hg2+ receptors into BRP, through convenient one-pot reactions on the 2'-carboxyl group, and successfully developed two ratiometric sensors, BRP-1 and BRP-2, with different spirocyclic receptors that recognized Hg2+ on different reaction mechanisms. Upon excitation at a single wavelength (488 nm), at which only BODIPY absorbed, both of the FRET sensors exhibited clear Hg2+-induced changes in the intensity ratio of the two strong emission bands of BODIPY and rhodamine. It should be noted that these ratiometric Hg2+ sensors exhibited excellent sensitivity and selectivity Hg2+, as well as pH insensitivity, which was similar to the corresponding (turn-on) rhodamine sensors. While both ratiometric probes were applicable for Hg2+ imaging in living cells, BRP-1 exhibited higher sensitivity and faster responses than BRP-2. Our investigation indicated that on a versatile platform, such as BRP, a large number of highly efficient ratiometric sensors for transition-metal ions could be conveniently developed.