We report on two-photon fluorescence excitation (TPE) action
cross sections for five widely used molecular
fluorophores. Measurements were performed by use of ultrashort
(∼100-fs) Ti:sapphire pulsed excitation over the range 690–960
nm. TPE spectra were obtained by comparison with a fluorescein
calibration standard. Large cross sections were found for the
cyanine reagent Cy 3 (∼140 GM) and for Rhodamine 6G (∼150
GM), both at 700 nm [1 GM = 10-50
(cm4 s)/photon]. Several fluorophores show
interesting and desirable blue shifts with respect to twice the
one-photon absorption wavelength. Fluorophore fluorescence
intensities showed no significant departure (±4%) from quadratic
illumination power dependence, indicating genuine two-photon
processes. Implications of these measurements for two-photon
laser-scanning microscopy are discussed.

Two-photon fluorescence excitation cross sections of biomolecular probes from 690 to 960 nm

Organic fluorescent bioprobes provide robust and powerful analytical techniques for direct detection and monitoring of important biological species and their related biological processes in live samples. In contrast to conventional fluorophores with aggregation-caused quenching effect, aggregation-induced emission (AIE) luminogens (AIEgens) exhibit remarkable photophysical properties such as bright fluorescence in aggregate or solid state, large Stokes shift, excellent photostability, and long retention time in live samples. Benefiting from these advantages, AIEgen-based bioprobes have been widely applied in diverse biomedical applications. In this review, we first introduce the AIE concept and working principles of AIE bioprobes. Then we briefly summarize their applications in biosensing and bioimaging with some representative examples recently reported mainly in our group. The challenge and future development of AIE bioprobes are also discussed and highlighted. Hopefully, this review can inspire more researchers to participate in this fascinating area and expand the versatile biomedical applications.

AIE luminogens as fluorescent bioprobes

https://onlinelibrary.wiley.com/doi/pdf/10.1002/agt2.51

Activity-based smart AIEgens for detection, bioimaging, and therapeutics: Recent progress and outlook

Compounds with a nitrobenzoxadiazole (NBD) skeleton exhibit prominent useful properties including environmental sensitivity, high reactivity toward amines and biothiols (including H2S) accompanied by distinct colorimetric and fluorescent changes, fluorescence-quenching ability, and small size, all of which facilitate biomolecular sensing and self-assembly. Amines are important biological nucleophiles, and the unique activity of NBD ethers with amines has allowed for site-specific protein labelling and for the detection of enzyme activities. Both H2S and biothiols are involved in a wide range of physiological processes in mammals, and misregulation of these small molecules is associated with numerous diseases including cancers. In this review, we focus on NBD-based synthetic probes as advanced chemical tools for biomolecular sensing. Specifically, we discuss the sensing mechanisms and selectivity of the probes, the design strategies for multi-reactable multi-quenching probes, and the associated biological applications of these important constructs. We also highlight self-assembled NBD-based probes and outline future directions for NBD-based chemosensors. We hope that this comprehensive review will facilitate the development of future probes for investigating and understanding different biological processes and aid the development of potential theranostic agents.

NBD-based synthetic probes for sensing small molecules and proteins: design, sensing mechanisms and biological applications

Since attributed to the involvement in important physiological and pathological processes, hydrogen sulfide (H2S) is regarded as another gasotransmitter like nitric oxide (NO) and carbon monoxide (CO). Traditional H2S detection methods are limited by the fast volatility and catabolism of H2S and difficult to real-time detect H2S in biological systems. Hence, numerous small-molecule fluorescent probes have been developed. Besides high sensitivity, the selectivity, response capability and optical performance are improved by combining new response groups and extending the emission wavelength to near infrared (NIR) region. Moreover, probes tend to selectively locate in a specific organelle, which detect H2S more accurately and contribute to understand the physiological roles of H2S in diseases. In this review, recent developments of small-molecule fluorescent probes for H2S imaging were summarized, focusing on researches conducted between January 2018 and June 2020, and their structures and biological applications were highlighted.

Small-molecule fluorescent probes for H2S detection: Advances and perspectives

An ultra-sensitive and ratiometric fluorescent probe for hypochlorous acid (HOCl) detection based on the mechanism of aggregation induced emission (AIE) and through-bond energy transfer (TBET) has been reported herein. By exploiting the advantages of AIE and TBET, which eliminates emission leakage from dark donors, the probe exhibits ultra-high sensitivity towards HOCl by an enhancement of over 7000-fold in the fluorescence ratio (I589 nm/I477 nm), which is one of the highest recorded so far. The reaction mechanism has been discussed in detail, and the effects of interferents and the reaction kinetics have also been investigated. Lastly, the successful result of exogenous/endogenous HOCl imaging detection in different cell lines indicates the potential use of the probe in living systems.

An ultra-sensitive ratiometric fluorescent probe for hypochlorous acid detection by the synergistic effect of AIE and TBET and its application of detecting exogenous/endogenous HOCl in living cells.

https://scholars.cityu.edu.hk/files/71110009/58667491.pdf

A pyrene-based ratiometric fluorescent probe with a large Stokes shift for selective detection of hydrogen peroxide in living cells

Hydrogen sulfide (H2S) has been recognized to influence a wide range of physiological and pathological processes. Its underlying molecular events, however, are still poorly understood. An activatab...

Controllable Cleavage of C–N Bond-Based Fluorescent and Photoacoustic Dual-Modal Probes for the Detection of H2S in Living Mice

We report herein fluorescent probes equipped with dual-quenching groups exhibiting superior sensitivity than probes with mono-quenching groups. Importantly, with this strategy, the probe with dual-quenching groups react with HOCl through two distinct reaction mechanisms, which reduce the plausible side reactions with other competing analytes and enhance the probe’s selectivity. As a proof-of-concept study, we designed and synthesized a probe with dual-quenching groups DQ-HOCl to detect HOCl, which is one of the most important ROS and linked with a number of diseases. In addition, two control probes with mono-quenching groups, MQ-HOCl-1 and MQ-HOCl-2, were also synthesized for comparison purpose. Fluorescent assays demonstrated that DQ-HOCl indeed shows ultra-high sensitivity and selectivity compared with probes with mono-quenching groups. Furthermore, the probe has been successfully utilized to imaging exogenous/endogenous HOCl in living cells. Moreover, DQ-HOCl was applied to visualize HOCl in kidney tissues from rat due to the increased penetration depth and lower tissue autofluorescence from the nature of two-photon probes.

Two quenching groups are better than one: A robust strategy for constructing HOCl fluorescent probe with minimized background fluorescence and ultra-high sensitivity and its application of HOCl imaging in living cells and tissues

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An ultra-sensitive ratiometric fluorescent probe for hypochlorous acid detection by the synergistic effect of AIE and TBET and its application of detecting exogenous/endogenous HOCl in living cells.

A pyrene-based ratiometric fluorescent probe with a large Stokes shift for selective detection of hydrogen peroxide in living cells

Controllable Cleavage of C–N Bond-Based Fluorescent and Photoacoustic Dual-Modal Probes for the Detection of H2S in Living Mice

Two quenching groups are better than one: A robust strategy for constructing HOCl fluorescent probe with minimized background fluorescence and ultra-high sensitivity and its application of HOCl imaging in living cells and tissues