Small molecule-based ratiometric fluorescence probes for cations, anions, and biomolecules

Abstract

Quantitative determination of specific analytes is essential for a variety of applications ranging from life sciences to environmental monitoring. Optical sensing allows non-invasive measurements within biological milieus, parallel monitoring of multiple samples, and less invasive imaging. Among the optical sensing methods currently being explored, ratiometric fluorescence sensing has received particular attention as a technique with the potential to provide precise and quantitative analyses. Among its advantages are high sensitivity and inherent reliability, which reflect the self-calibration provided by monitoring two (or more) emissions. A wide variety of ratiometric sensing probes using small fluorescent molecules have been developed for sensing, imaging, and biomedical applications. In this research highlight, we provide an overview of the design principles underlying small fluorescent probes that have been applied to the ratiometric detection of various analytes, including cations, anions, and biomolecules in solution and in biological samples. This highlight is designed to be illustrative, not comprehensive.

Fig. 1

(a) Schematic representation of the spectral shifts expected for ICT-based sensors as the result of cation binding to the constituent electron donor and electron acceptor groups. (b) A naphthalimide-based ICT molecule that functions as a theragnostic agent. (c) Structures of the ICT-based molecules and corresponding ratiometric fluorescence responses, Probes 1-7. The illustration in Figure (b) is reproduced with permission of the American Chemical Society from ref. 28. Copyright © 2012.

Fig. 2

(a) An illustration of ESIPT showing the photo- and thermal driven changes that occur upon photoexcitation of 2-(20-hydroxyphenyl)-benzoxazole (HBO). (b) Chemical structures of ESIPT-based molecules, Probes 8-10. (c) Structure of BTTPB, a probe fluoride ions and spectral traces show the analyte-induced absorption and emission changes observed upon exposure to F^– in micellar suspensions in water. CIE 1931 (x,y) chromaticity diagram of test papers for the detection of NaF that are based on 3-BTHPB as derived from fluorescence spectra recorded at different analyte concentrations. The absorption and emission spectra and the CIE chromaticity diagram are reproduced with permission of the Wiley from ref. 44. Copyright © 2010.

Fig. 3

(a) Through-space and (b) through-bond energy transfer cassettes. (c) Structures of the FRET/TBET-based molecules, Probes 11-17. (d) Schematic illustration of FRET-based probes used for the dual emission sensing of ATP and its ratiometric analysis in living cells. The fluorescence images of obtained in solution and in cells are reproduced from ref. 54 with permission of the American Chemical Society. Copyright © 2010.

Fig. 4

(a) Molecular structure of probe 18. (b) Schematic illustration of probe 18 and its display of (right) pyrene monomer emission and (left) excimer formation and emission as applied to ratiometric DNA sensing. The fluorescence image of the solution is reproduced from ref. 60 with permission from the American Chemical Society. Copyright © 2012.