Rational Design of a Near‐infrared Fluorescence Probe for Ca²⁺ Based on Phosphorus‐substituted Rhodamines Utilizing Photoinduced Electron Transfer

Abstract

Fluorescence imaging in the near‐infrared (NIR) region (650–900 nm) is useful for bioimaging because background autofluorescence is low and tissue penetration is high in this range. In addition, NIR fluorescence is useful as a complementary color window to green and red for multicolor imaging. Here, we compared the photoinduced electron transfer (PeT)‐mediated fluorescence quenching of silicon‐ and phosphorus‐substituted rhodamines (SiRs and PRs) in order to guide the development of improved far‐red to NIR fluorescent dyes. The results of density functional theory calculations and photophysical evaluation of a series of newly synthesized PRs confirmed that the fluorescence of PRs was more susceptible than that of SiRs to quenching via PeT. Based on this, we designed and synthesized a NIR fluorescence probe for Ca2+, CaPR‐1, and its membrane‐permeable acetoxymethyl derivative, CaPR‐1 AM, which is distributed to the cytosol, in marked contrast to our previously reported Ca2+ far‐red to NIR fluorescence probe based on the SiR scaffold, CaSiR‐1 AM, which is mainly localized in lysosomes as well as cytosol in living cells. CaPR‐1 showed longer‐wavelength absorption and emission (up to 712 nm) than CaSiR‐1. The new probe was able to image Ca2+ at dendrites and spines in brain slices, and should be a useful tool in neuroscience research.

Deep-Red/Near-Infrared Turn-On Fluorescence Probes for Aldehyde Dehydrogenase 1A1 in Cancer Stem Cells

“…37−39 Chan and co-workers developed an ALDH1A1-responsive turn-on green fluorescence probe whose fluorescence off−on nature is controlled by photoinduced electron transfer (PeT). 37 However, because the fluorescence off−on nature in the DR/NIR region is theoretically difficult to control through PeT, 15,40 there are no examples of turn-on fluorescence probes working in the DR/NIR region. A new method for controlling the nature of off−on fluorescence based on ALDH1A1-mediated transformation is required to develop DR/NIR turn-on fluorescence probes.…”

“…In contrast, “turn-on” fluorescence probes, which reveal their fluorescence signal in response to a specific analyte, avoid the background signal of fluorescence probes from nontarget areas, leading to high-contrast imaging in vitro and in vivo. − Turn-on probes can be developed by utilizing ALDH1A1-mediated transformation of an aldehyde into a carboxylate. − Chan and co-workers developed an ALDH1A1-responsive turn-on green fluorescence probe whose fluorescence off–on nature is controlled by photoinduced electron transfer (PeT) . However, because the fluorescence off–on nature in the DR/NIR region is theoretically difficult to control through PeT, , there are no examples of turn-on fluorescence probes working in the DR/NIR region. A new method for controlling the nature of off–on fluorescence based on ALDH1A1-mediated transformation is required to develop DR/NIR turn-on fluorescence probes.…”

Advancement of fluorescent aminopeptidase probes for rapid cancer detection–current uses and neurosurgical applications

“…Recent advancements in aminopeptidase probes, particularly those based on HMRG and 2MeSiR, are showing significant promise in improving cancer detection and monitoring as disease biomarkers ( 90 , 91 ). These probes offer distinctive advantages, such as rapid activation and reduced background signals.…”

Emerging Trends in Fluorescence Bioimaging of Divalent Metal Cations Using Small‐Molecule Indicators

“…In 2020, Urano and coworkers proposed that reducing the difference of energy level between the LUMO of the fluorophore and the HOMO of the chelator can effectively improve the efficiency of PeT. [24] They performed density functional theory (DFT) calculations to predict calculated the HOMO and LUMO energy levels of SiR, phosphorus-substituted rhodamine (PR), and phenylphosphorus-rhodamine (PPhR) and found that PR has the lowest LUMO among the three. In subsequent experiments, CaPR-1 (compound 12 in Figure 1c) presented the highest turn-on ratio as 5.8 folds.…”