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Mitochondrial pH (pHmito) is known to be alkaline (near 8.0) and has emerged as a potential factor for mitochondrial function and disorder. We have developed a ratiometric two-photon probe (CMP1) for quantitative analysis of pHmito in live cells and tissues. This probe is designed to function by controlling the intramolecular charge transfer from 2-naphthol, having an ideal pKa value (7.86 ± 0.05) in the cells to monitor pHmito. This transition results in a marked yellow to red emission color change in response to pH alterations from 6.0 to 9.0. CMP1 exhibits easy loading, selective and robust staining ability of mitochondria, low cytotoxicity, and bright two-photon excited fluorescence in situ, thereby allowing quantitative imaging of the pHmito in live cells and tissues. The ratiometric TPM imaging clearly reveals that subcellular distribution of the pHmito values is heterogeneous, with the pHmito values in the perinuclear region being higher than those at the periphery of the cells. The changes of pHmito values on carbonyl cyanide m-chlorophenyl hydrazone (CCCP) treatment and autophagic processes were also investigated along with their morphological alterations at specific subcellular positions. We also used CMP1 to visualize the pHmito values of Parkinson's disease model astrocytes as well as living hippocampal tissues. Our results demonstrate that CMP1 will be useful as a quantitative imaging probe to study pHmito in biomedical research.

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The formation of beta amyloid (Aß) plaques in specific brain regions is one of the early pathological hallmarks of Alzheimer's disease (AD). To enable the early detection of AD and related applications, a method for real-time, clear 3D visualization of Aß plaques in vivo is highly desirable. Two-photon microscopy (TPM) which utilizes two near-infrared photons is an attractive tool for such applications. However, this technique needs a sensitive and photostable two-photon (TP) probe possessing bright TP exited fluorescence to impart high signal-to-noise (S/N) visualization of Aß plaques. Herein, we report a quadrupolar TP fluorescent probe (QAD1) having large TP action cross section (Φδmax = 420 GM) and its application for in vivo TPM imaging of Aß plaques. This probe, designed with a centrosymmetric D-A-D motif with a cyclic conjugating bridge and solubilizing unit, displays bright TP excited fluorescence, appreciable water solubility, robust photostability, and high sensitivity and selectivity for Aß plaques. Using the real-time TPM imaging of transgenic 5XFAD mice after intravenous injection of QAD1, we show that this probe readily enters the blood brain barrier and provides high S/N ratio images of individual Aß plaques in vivo. We also used QAD1 in dual-color TPM imaging for 3D visualization of Aß plaques along with blood vessels and cerebral amyloid angiopathy (CAA) inside living mouse brains. These findings demonstrate that this probe will be useful in biomedical applications including early diagnosis and treatments of AD.

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We report a two-photon fluorescent probe which shows a strong two photon excited fluorescence enhancement in response to Zn(2+), easy loading into the cells, Golgi-localizing ability, low cytotoxicity, and high photostability. Two-photon microscopy imaging revealed that this probe allows for real-time monitoring of the changes in Golgi Zn(2+) as well as their 3D distributions in live cells and tissues.

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Two-photon microscopy (TPM) is a new imaging tool that can detect biological targets deep inside a live tissue. To faciltate the use of TPM in biomedical research, a variety of two-photon (TP) probes for specific applications are needed. In this Forum Article, we describe the design strategy, photophysical properties, and biological imaging applications of a selection of our recent studies in the development of TP probes for metal ions. Small-molecule TP turn-on probes, organelle-targeted probes, and multicolor emissive probes for dual-color imaging are briefly reviewed.

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We report a small-molecule two-photon fluorescent probe (ANa2) for Na(+) that shows a strong TPEF enhancement in response to Na(+) and can be easily loaded into live cells and can real time monitor the fluctuation of [Na]i in live cells and living tissue at more than 100 µm depth.

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A simple pyridinium-based tripodal chemosensor, 1, effectively recognizes AMP over ATP and ADP through indicator displacement assay (IDA) technique in water at pH 6.4. The good recognition of 1 is due to the better accommodation of AMP at the core of 1 as well as functional interaction involving hydrogen bonding and charge-charge interaction. The sensor 1 also recognizes intracellular AMP.

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The pyridinium-based fluororeceptor 1 and the sensor bead 2 recognize hydrogen pyrophosphate effectively through the 'Indicator Displacement Assay' (IDA) technique over a series of other anions in aq CH(3)CN (CH(3)CN/H(2)O = 4:1, v/v, pH = 6.5). The sensor bead 2 is also capable of sensing the same anion selectively in pure water.

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