Two-Photon Fluorescent Probes for Long-Term Imaging of Calcium Waves in Live Tissue

A Nano-Aggregatable Acedan Derivative for Clathrin-Mediated Cellular Uptake and Two-Photon Imaging of Diabetes-Associated Lipid Droplets

Lipid droplets (LDs), the essential cytosolic fat storage organelles, have emerged as pivotal regulators of cellular metabolism and are implicated in various diseases. The noninvasive monitoring of LDs necessitates fluorescent probes with precise organelle selectivity and biocompatibility. Addressing this need, we have engineered a probe by strategically modifying the structure of a conventional two-photon-absorbing dipolar dye, acedan. This innovative approach induces nanoaggregate formation in aqueous environments, leading to aggregation-induced fluorescence quenching. Upon cellular uptake via clathrin-mediated endocytosis, the probe selectively illuminates within LDs through a disassembly process, effectively distinguishing LDs from the cytosol with exceptional specificity. This breakthrough enables the high-fidelity imaging of LDs in both cellular and tissue environments. In a pioneering investigation, we probed LDs in a diabetes model induced by streptozotocin, unveiling significantly heightened LD accumulation in cardiac tissues compared to other organs, as evidenced by TP imaging. Furthermore, our exploration of a lipopolysaccharide-mediated cardiomyopathy model revealed an LD accumulation during heart injury. Thus, our developed probe holds immense potential for elucidating LD-associated diseases and advancing related research endeavors.

Novel chemical sensor for detection Ca(II) ions based on ferutinin

A new optical chemical sensor based on medicinal compound, jaeschkeanadiol p-hydroxyben-zoate (Ferutinin), has been designed and utilized for Ca(II) ions detections. This natural optical sensor exhibits immense selectivity including fluorescence and absorption ratiometric for Ca(II) ions within precious physiological pH range. Further, the chelation process of the Ca(II) ions with the medicinal optical sensor ferutinin yields a 1:1 (metal: ligand) complex which is accompanied by fluorescence enhancement of the main emission band centered at 355 nm of the medicinal probe. The fluorescence "turn-on" effect is a clear evidence for the chelation process between ferutinin medicinal probe and Ca(II) ions and this could be simply detected. The new sensor was proposed depending on significant fluorescence mechanism, (MLCT, metal-ligand charge transfer). However, the interaction of the medicinal optical sensor with Ca(II) in presence of other cations was examined without any significant interference. Also, the new developed optical sensor consecutively exhibits low limit of detection (LOD) 1.5 nM for Ca(II) which is detected in very significant physiological pH range (pH = 7.4). Also, the probe provides high binding affinity towards Ca(II) with large binding constant K_b 5.97 × 10⁴ M^-1. As a result, this optical sensor may apply for detection Ca(II) in cell or biological samples.

Two-photon probes for in vivo multicolor microscopy of the structure and signals of brain cells

Imaging the brain of living laboratory animals at a microscopic scale can be achieved by two-photon microscopy thanks to the high penetrability and low phototoxicity of the excitation wavelengths used. However, knowledge of the two-photon spectral properties of the myriad fluorescent probes is generally scarce and, for many, non-existent. In addition, the use of different measurement units in published reports further hinders the design of a comprehensive imaging experiment. In this review, we compile and homogenize the two-photon spectral properties of 280 fluorescent probes. We provide practical data, including the wavelengths for optimal two-photon excitation, the peak values of two-photon action cross section or molecular brightness, and the emission ranges. Beyond the spectroscopic description of these fluorophores, we discuss their binding to biological targets. This specificity allows in vivo imaging of cells, their processes, and even organelles and other subcellular structures in the brain. In addition to probes that monitor endogenous cell metabolism, studies of healthy and diseased brain benefit from the specific binding of certain probes to pathology-specific features, ranging from amyloid-β plaques to the autofluorescence of certain antibiotics. A special focus is placed on functional in vivo imaging using two-photon probes that sense specific ions or membrane potential, and that may be combined with optogenetic actuators. Being closely linked to their use, we examine the different routes of intravital delivery of these fluorescent probes according to the target. Finally, we discuss different approaches, strategies, and prerequisites for two-photon multicolor experiments in the brains of living laboratory animals.

A naphthalene-based fluorescent probe with a large Stokes shift for mitochondrial pH imaging

A naphthalene-based fluorescent pH probe with a pK_a of 8.8 for imaging mitochondrial pH changes in live cells.

A novel fluorescent probe for sensitive detection and imaging of hydrazine in living cells

A turn-on fluorescent probe (Naphsulf-O) for hydrazine was developed by protecting the hydroxy group of the fluorophore 6-acetyl-2-hydroxynaphthalene via O-4-nitrobenzenesulfonylation, where 4-nitrobenzene was used as a fluorescence quenching moiety as well as an electrophile. Upon nucleophilic aromatic substitution (NAS) reaction of hydrazine toward the probe, the protecting group was removed and fluorophore was released. The probe exhibits a large Stokes shift, excellent selectivity and high sensitivity for hydrazine detection in aqueous solution with a detection limit of 0.716 ppb (22nM), which is of great importance in both environmental and biological system. Furthermore, it was successfully applied to imaging of hydrazine in living cells.

A Highly Selective Fluorescence Turn-On Sensor for Extracellular Calcium Ion Detection

A highly water-soluble, fluorescence turn-on sensor for Ca(2+) is reported. The sensor affords high selectivity in sensing Ca(2+) over other biologically important metal cations. The dissociation constant of the sensor in binding Ca(2+) is 0.92 mm. Fluorescence microscopy experiments demonstrate that the sensor is cell-impermeable and capable of detecting extracellular Ca(2+) .

Fluorescent probes and bioimaging: alkali metals, alkaline earth metals and pH

All living species and life forms have an absolute requirement for bio-functional metals and acid-base equilibrium chemistry owing to the critical roles they play in biological processes. Hence, a great need exists for efficient methods to detect and monitor biometals and acids. In the last few years, great attention has been paid to the development of organic molecule based fluorescent chemosensors. The availability of new synthetic fluorescent probes has made fluorescence microscopy an indispensable tool for tracing biologically important molecules and in the area of clinical diagnostics. This review highlights the recent advances that have been made in the design and bioimaging applications of fluorescent probes for alkali metals and alkaline earth metal cations, including lithium, sodium and potassium, magnesium and calcium, and for pH determination within biological systems.

Excited-state localization and energy transfer in pyrene core dendrimers with fluorene/carbazole as the dendrons and acetylene as the linkages

Multi-scale theoretical model and spectra simulation for dendrimers combining TD-DFT/DFT and semi-empirical methods.

Near-IR Two-Photon Fluorescent Sensor for K(+) Imaging in Live Cells

A new two-photon excited fluorescent K(+) sensor is reported. The sensor comprises three moieties, a highly selective K(+) chelator as the K(+) recognition unit, a boron-dipyrromethene (BODIPY) derivative modified with phenylethynyl groups as the fluorophore, and two polyethylene glycol chains to afford water solubility. The sensor displays very high selectivity (>52-fold) in detecting K(+) over other physiological metal cations. Upon binding K(+), the sensor switches from nonfluorescent to highly fluorescent, emitting red to near-IR (NIR) fluorescence. The sensor exhibited a good two-photon absorption cross section, 500 GM at 940 nm. Moreover, it is not sensitive to pH in the physiological pH range. Time-dependent cell imaging studies via both one- and two-photon fluorescence microscopy demonstrate that the sensor is suitable for dynamic K(+) sensing in living cells.

A membrane permeable fluorescent Ca2+ probe based on bis-BODIPY with branched PEG

The cellular uptake of MPFCP-2 is improved by the PEG encapsulation method, and then MPFCP-2 could pass through the cell membrane by itself, and monitor the changes of the intracellular Ca²⁺ signal.

N-heterocyclic carbene-assisted, bis(phosphine)nickel-catalyzed cross-couplings of diarylborinic acids with aryl chlorides, tosylates, and sulfamates

Efficient bis(phosphine)nickel-catalyzed cross-couplings of diarylborinic acids with aryl chlorides, tosylates, and sulfamates have been effected with an assistance of N-heterocyclic carbene (NHC) generated in situ from N,N'-dialkylimidazoliums, e.g., N-butyl-N'-methylimidazolium bromide ([Bmim]Br), in toluene using K3PO4·3H2O as base. In contrast to bis(NHC)nickel-catalyzed conventional Suzuki coupling of arylboronic acids, mono(NHC)bis(phosphine)nickel species generated in situ from Ni(PPh3)2Cl2/[Bmim]Br displayed high catalytic activities in the cross-couplings of diarylborinic acids. The structural influences from diarylborinic acids were found to be rather small, while electronic factors from aryl chlorides, tosylates, and sulfamates affected the couplings remarkably. The couplings of electronically activated aryl chlorides, tosylates, and sulfamates could be efficiently effected with 1.5 mol % NiCl2(PPh3)2/[Bmim]Br as catalyst precursor to give the biaryl products in excellent yields, while 3-5 mol % loadings had to be used for the couplings of non- and deactivated ones. A small ortho-substitutent on the aromatic ring of aryl chlorides, tosylates, and sulfamates was tolerable. Applicability of the nickel-catalyzed cross-couplings in practical synthesis of fine chemicals has been demonstrated in process development for a third-generation topical retinoid, Adapalene.

Recent development of two-photon fluorescent probes for bioimaging

Fluorescent probes are essential tools for studying biological systems.

A parallel G-quadruplex-selective luminescent probe for the detection of nanomolar calcium(II) ion

A parallel G-quadruplex-selective iridium(III) complex has been synthesized and employed as a luminescent probe in a label-free G-quadruplex-based detection assay for Ca(2+) ions in aqueous solution. In this assay, a guanine-rich oligonucleotide (G4, 5'-G4T4G4-3') initially exists in an antiparallel G-quadruplex conformation, resulting in a low luminescence signal. Upon incubation with Ca(2+) ions, the antiparallel G-quadruplex is induced into a parallel G-quadruplex conformation, which greatly enhances the luminescence emission of the iridium(III) probe. This method was highly sensitive for Ca(2+) ions with a limit of detection in the nanomolar range, and was selective for Ca(2+) over other metal ions.

A colorimetric selective sensing probe for calcium ions with tunable dynamic ranges using cytidine triphosphate stabilized gold nanoparticles

Cytidine triphosphate (CTP) stabilized-Au nanoparticles (CTP-AuNPs) allow a quantitative assay of Ca(2+) down to a concentration of 10(-4) M with high selectivity towards Ca(2+) ions over various metal ions including Mg(2+) and the detection range of this probe also can be easily tuned by changing the concentration of CTP.

Enantiospecific synthesis of genetically encodable fluorescent unnatural amino acid L-3-(6-acetylnaphthalen-2-ylamino)-2-aminopropanoic acid

Fluorescent unnatural amino acids (UAAs), when genetically incorporated into proteins, can provide unique advantages for imaging biological processes in vivo. Synthesis of optically pure L-enantiomer of fluorescent UAAs is crucial for their effective application in live cells. An efficient six-step synthesis of L-3-(6-acetylnaphthalen-2-ylamino)-2-aminopropanoic acid (L-Anap), a genetically encodable and polarity-sensitive fluorescent UAA, has been developed. The synthesis takes advantage of a high-yield and enantiospecific Fukuyama-Mitsunobu reaction as the key transformation.

Two-photon fluorescent probes for metal ions

Two-photon microscopy (TPM) has become an indispensible tool in biology and medicine owing to the capability of imaging the intact tissue for a long period of time. To make it a versatile tool in biology, a variety of two-photon probes for specific applications are needed. In this context, many research groups are developing two-photon probes for various applications. In this Focus Review, we summarize recent results on model studies and selected examples of two-photon probes that can detect intracellular free metal ions in live cells and tissues to provide a guideline for the design of useful two-photon probes for various in vivo imaging applications.

Two-photon probes for intracellular free metal ions, acidic vesicles, and lipid rafts in live tissues

Optical imaging with fluorescence microscopy is a vital tool in the study of living systems. The most common method for cell imaging, one-photon microscopy (OPM), uses a single photon of higher energy to excite the fluorophore. However, two-photon microscopy (TPM), which uses two photons of lower energy as the excitation source, is growing in popularity among biologists because of several distinct advantages. Using TPM, researchers can image intact tissue for a long period of time with minimum interference from tissue preparation artifacts, self-absorption, autofluorescence, photobleaching, and photodamage. However, to make TPM a more versatile tool in biology, researchers need a wider variety of two-photon probes for specific applications. In this Account, we describe a series of two-photon probes that we developed that can visualize the distribution of intracellular metal ions, acidic vesicles, and lipid rafts in living cells and tissues. The development of these probes requires a significant two-photon cross section for the bright image and receptors (sensing moieties) that triggers the emission of the two-photon excited fluorescence upon binding with the ions or membrane in the living system. These probes also must be sensitive to the polarity of the environment to allow selective detection of cytosolic and membrane-bound probes. In addition, they need to be cell-permeable, water-soluble for the staining of cells and tissues, and highly photostable for long-term imaging. The resulting probes-AMg1 (Mg(2+)), ACa1-ACa3 (Ca(2+)), AZn1 and AZn2 (Zn(2+)), AH1, AH2, and AL1 (acidic vesicles), and CL2 (membrane)-use 2-acetyl-6-aminonaphthalene as the fluorophore and receptors for the target ions or membrane. All of these two-photon turn-on probes can detect the intracellular free metal ions, acidic vesicles, and lipid rafts at 100-300 microm depth in live tissues. Moreover, with ACa1-AM, we could simultaneously visualize the spontaneous Ca(2+) waves in the somas of neurons and astrocytes at approximately 120 microm depth in fresh hypothalamic slices for more than 1000 s without appreciable decay. Furthermore, AL1 could visualize the transport of the acidic vesicles between cell body and axon terminal along the axon in fresh rat hippocampal slices at approximately 120 microm depth.