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

Synthetic ratiometric fluorescent probes for detection of ions

Metal cations and anions are essential for versatile physiological processes. Dysregulation of specific ion levels in living organisms is known to have an adverse effect on normal biological events. Owing to the pathophysiological significance of ions, sensitive and selective methods to detect these species in biological systems are in high demand. Because they can be used in methods for precise and quantitative analysis of ions, organic dye-based ratiometric fluorescent probes have been extensively explored in recent years. In this review, recent advances (2015-2019) made in the development and biological applications of synthetic ratiometric fluorescent probes are described. Particular emphasis is given to organic dye-based ratiometric fluorescent probes that are designed to detect biologically important and relevant ions in cells and living organisms. Also, the fundamental principles associated with the design of ratiometric fluorescent probes and perspectives about how to expand their biological applications are discussed.

DFT/MRCI assessment of the excited-state interplay in a coumarin-schiff Mg2+ fluorescent sensor

Fluorescent sensors with selectivity and sensitivity to metal ions are an active field in supramolecular chemistry for biochemical, analytical, and environmental problems. Mg²⁺ is one of the most abundant divalent ions in the cell, and it plays a critical role in many biological processes. Coumarin-based sensors are widely used as desirable fluorophore and binding moieties showing a remarkable sensitivity and fluorometric enhancement for Mg²⁺ . In this work, density functional theory/multireference configuration interaction (DFT/MRCI) calculations were performed in order to understand the sensing behavior of the organic fluorescent sensor 7-hydroxy-4-methyl-8-((2-(pyridin-2-yl)hydrazono)methyl)-2H-chromen-2-one (PyHC) in ethanol to solvated Mg²⁺ ions. The computed optical properties reproduce well-reported experimental data. Our results suggest that after photoexcitation of the free PyHC, a photo-induced electron transfer (PET) mechanism may compete with the fluorescence decay to the ground state. In contrast, this PET channel is no longer available in the complex with Mg²⁺ making the emissive decay more efficient. © 2019 Wiley Periodicals, Inc.

Effect of n-alkyl substitution on Cu(ii)-selective chemosensing of rhodamine B derivatives

Rhodamine B hydrazide-based molecular probes (1-10) were synthesized by derivatization with n-alkyl chains of different lengths at the hydrazide amino end. These probes exhibited selective absorption (A∼557) and fluorescence (I∼580) 'off-on' signal transduction along with a colourless → magenta colour transition in the presence of Cu(ii) ions among all the competitive metal ions investigated. The effective coordination of these probes to Cu(ii) ions under the investigated environment forming [Cu·L]2+ (L = 1-5) and [Cu·L2]2+ (L = 6-10) complexes led to their spiro-ring opening, which in turn was expressed through signatory spectral peaks of ring-opened rhodamine. All these probes exhibited Cu(ii) selectivity in signalling despite structural modifications to the core receptor unit through variation of the nature of the alkyl substituents. However, the sensitivity of the signalling and kinetics of the spiro-ring opening varied and could be correlated with the number of carbon atoms present in the n-alkyl substituents. Structural elucidation with X-ray diffraction and X-ray photoemission spectroscopic analyses provided further insight into the structure-function correlation in their Cu(ii) complexes. These probes with Cu(ii) coordination showed selectivity in signalling, high complexation affinity (log Ka = 4.8-8.8), high sensitivity (LOD = 4.1-80 nM), fast response time (rate = 0.0017-0.0159 s-1) and reversibility with counter anions, which ascertained their potential utility as chemosensors for Cu(ii) ion detection.

Alkali metal complexes of bis-o-xylyl-(17-crown-5): from a dinuclear monomer and a dinuclear polymer to sandwich polymer

The self-assembly reactions of bis-o-xylyl-(17-crown-5) with alkali metal salts afforded binding mode (good fit, perching and sandwich)-dependent supramolecular complexes, including a sandwich-type cesium(i) coordination polymer.

An N-linked disalicylaldehyde together with its caesium ion and dichloromethane sensing performances: ‘dual key & lock’ LMCT-enhanced fluorescence strategy

The disalicylaldehyde-Cs⁺sensing system, a novel approach for quick and reusable detection of Cs⁺together with convenient CH₂Cl₂monitoring.

A mitochondria-targeting NIR fluorescent potassium ion sensor: real-time investigation of the mitochondrial K+ regulation of apoptosis in situ

TAC-Rh, as the first mitochondria-targeting NIR K⁺ sensor, was applied to explore mutual regulation between mitochondrial K⁺ and apoptosis.

A fluorescent naphthalimide NADH mimic for continuous and reversible sensing of cellular redox state

A new naphthalimide based NADH mimic that functions as a fully reversible fluorescent “on off” probe for redox state has been synthesised and evaluated.

Carbon dot-based fluorometric optical sensors: an overview

Fluorescent carbon dots (CDs) are a new class of carbon nanomaterials and have demonstrated excellent optical properties, good biocompatibility, great aqueous solubility, low cost, and simple synthesis. Since their discovery, various synthesis methods using different precursors were developed, which were mainly classified as top-down and bottom-up approaches. CDs have presented many applications, and this review article mainly focuses on the development of CD-based fluorescent sensors. The sensing mechanisms, sensor design, and sensing properties to various targets are summarized. Broad ranges of detection, including temperature, pH, DNA, antibiotics, cations, cancer cells, and antibiotics, have been discussed. In addition, the challenges and future directions for CDs as sensing materials are also presented.

Recent Advances in AIEgens for Metal Ion Biosensing and Bioimaging

Metal ions play important roles in biological system. Approaches capable of selective and sensitive detection of metal ions in living biosystems provide in situ information and have attracted remarkable research attentions. Among these, fluorescence probes with aggregation-induced emission (AIE) behavior offer unique properties. A variety of AIE fluorogens (AIEgens) have been developed in the past decades for tracing metal ions. This review highlights recent advances (since 2015) in AIE-based sensors for detecting metal ions in biological systems. Major concerns will be devoted to the design principles, sensing performance, and bioimaging applications.

A near-infrared ratiometric fluorescence probe base on spiropyran derivative for pH and its application in living cells

The intracellular pH has a significant effect on several essential biological processes such as material transfer, enzymatic action, cell apoptosis et al. Thus, it is necessary to monitor pH fluctuation in living cells. Here, we designed a near-infrared ratiometric fluorescence probe for pH detection. The spectroscopic responses of probe to pH variations were investigated in CH₃OH/PBS (v/v, 1:1) mixed solution at different pH values. The experimental results showed that the probe is sensitive to acidity, and the pK_a of probe is calculated to be 4.85. When the pH was decreased from 9.0 to 1.0, the color of probe solution change from purple to yellow was found by naked eye. Moreover, the probe can be a practical tool for assessing cellular pH by cell imaging.

Near-Infrared Fluorescent Probes for Imaging of Intracellular Mg2+ and Application to Multi-Color Imaging of Mg2+, ATP, and Mitochondrial Membrane Potential

The magnesium ion (Mg²⁺) is an essential cation to maintain proper cellular activities. To visualize the dynamics and functions of Mg²⁺, there is a great need for the development of Mg²⁺-selective fluorescent probes. However, conventional Mg²⁺ fluorescent probes are falling behind in low selectivity and poor fluorescence color variation. In this report, to make available a distinct color window for multi-color imaging, we designed and synthesized highly Mg²⁺-selective and near-infrared (NIR) fluorescent probes, the KMG-500 series consisting of a charged β-diketone as a selective binding site for Mg²⁺ and a Si-rhodamine residue as the NIR fluorophore, which showed photoinduced electron transfer (PeT)-type OFF-ON response to the concentration of Mg²⁺. Two types of KMG-500 series probes, tetramethyl substituted Si-rhodamine KMG-501 and tetraethyl substituted Si-rhodamine KMG-502, were synthesized for the evaluation of cell permeability. For intracellular application, the membrane-permeable acetoxymethyl derivative KMG-501 (KMG-501AM) was synthesized and allowed to stably stain cultured rat hippocampal neurons during imaging of intracellular Mg²⁺. On the other hand, KMG-502 was cell membrane permeable without AM modification, preventing the probe from staying inside cells during imaging. KMG-501 distributed mainly in the cytoplasm and partially localized in lysosomes and mitochondria in cultured rat hippocampal neurons. Mg²⁺ increase in response to the FCCP uncoupler inducing depolarization of the mitochondrial inner membrane potential was detected in the KMG-501 stained neurons. For the first time, KMG-501 succeeded in imaging intracellular Mg²⁺ dynamics with NIR fluorescence. Moreover, it allows one to simultaneously visualize changes in Mg²⁺ and ATP concentration and also mitochondrial inner membrane potential and their interactions. This probe is expected to be a strong tool for multi-color imaging of intracellular Mg²⁺.

Efficient and very selective dual mode chemosensor for Cd(II) ions in live cells

A sensitive dual mode chemosensor NS-1 comprising Nitrobenzoxadiazole and salicylhydrazide conjugate has been synthesized via single step reaction. The probe NS-1 is characterized by analytical techniques such as multi nuclear NMR techniques and Mass spectrometry. The probe is showing a strong change in color from yellow to red on treatment of Cd(II) ions, interestingly its shows bright "Switch-ON" fluorescence state upon binding of Cd²⁺ ions in buffer solution whereas other cations did not showed any color change as well as fluorescent change. Interestingly the probe NS-1 did not results in the any color and fluorescence change with the adding together of Zn(II) ions, hence the probe is able to differentiate between Cd(II) ions from Zn(II). Further the color change and turn-on fluorescence can be rationalized by the interruption of internal charge transfer upon binding of Cd(II) ions with NS-1. The Internal charge transfer disturbance led to fluorescence change of the receptor NS-1 upon addition of Cd²⁺ has been further supported by TD-DFT calculations. The limit of detection was found to be 6.31 nano molar. The association constant was found to be 7.97*10⁴ M^-1 using Benesi-Hildebrand plot method. MTT assay suggesting that the probe NS-1 is least toxic to cells and it will be widely applicable to image Cd(II) ions in living cells via fluorescence imaging.

Tuning the Molecular and Cationic Affinity in a Series of Multifunctional Metal-Organic Frameworks Based on Dodecanuclear Zn(II) Carboxylate Wheels

A series of new zinc(II)-thiophene-2,5-dicarboxylate (tdc) MOFs based on novel dodecanuclear wheel-shaped building blocks has been synthesized in almost quantative yields. Single-crystal X-ray diffraction analyses reveal 3D porous frameworks with a complex composition [Zn₁₂(tdc)₆(glycolate)₆(dabco)₃] where glycolate is a deprotonated polyatomic alcohol (ethylene glycol, EgO₂, 1; 1,2-propanediol, PrO₂, 2; 1,2-butanediol, BuO₂, 3; 1,2-pentanediol, PeO₂, 4; glycerol, GlO₂, 5) and dabco is 1,4-diazo[2.2.2.]bicyclooctane. All compounds 1-5 are isostructural except for pendant groups of the diols decorating the surface of channels. The adsorption of small gases (N₂, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆) and larger hydrocarbons (benzene, cyclohexane) both in liquid and vapor phases was thoroughly investigated for all compounds. The zero-coverage adsorption enthalpies, Henry constants, and selectivity factors by various models are calculated and discussed. The versatile adsorption functionality of the title series results from the variable nature of the diol and could be tailored for a specific adsorbate system. For example, 1 shows excellent selectivity of benzene over cyclohexane (20:1 for vapors, 92:1 for liquid phase), while 4 demonstrates unprecedented adsorption preference of cyclohexane over benzene (selectivity up to 5:1). The compound 5 demonstrates great adsorption selectivity for CO₂/N₂ (up to 75.1), CO₂/CH₄ (up to 7.7), C₂H₂/CH₄ (up to 14.2), and C₂H₄/CH₄ (up to 9.4). Also, due to polar nature of the pores, 5 features size-selective sorption of alkaline metal cations in order Li⁺ > Na⁺ > K⁺ > Cs⁺ as well as a notable luminescent response for cesium(I) ions and urea.

Construction of homochiral alkaline-lanthanide heteronuclear helicates with Na+-selective bonding in the self-assembly process

A supramolecular approach to distinguish Na+ from other biologically important metal ions was demonstrated. By designing ligands reasonably, Na+-selective bonding was achieved in the construction of homochiral alkaline-lanthanide heteronuclear helicates, which was further confirmed by mixed-metal self-assembly experiments and 1H-NMR spectra.

A fluorescent turn-on probe based on isophorone for the rapid detection of alkaline phosphatase and its application in bioimaging

Biological species analyses on account of fluorescence detection technology are receiving increasing attention, because they combine the advantages both powerful detection capability and excellent imaging technology. By effectively integrating isophorone and phosphate group via p-hydroxybenzaldehyde, the alkaline phosphatase (ALP) detection probe was obtained. Based on the enzyme-catalyzed dephosphorization course, phosphate group was separated from the probe by ALP and released yellow fluorescence signal. Upon addition with ALP, the probe exhibited high selectivity, short response time (6 min) and longer emission peak shift (570 nm). Furthermore, bioimaging experiment results indicated that the probe could detect endogenous ALP effectively.

A photostable Si-rhodamine-based near-infrared fluorescent probe for monitoring lysosomal pH during heat stroke

Heat stroke is a symptom of hyperthermia with a temperature of more than 40 °C, which usually leads to all kinds of physical discomfort and even death. It is necessary to study the mechanism of action of heat stroke on cells or organelles (such as cytotoxicity of heat) and the processes of cells or organelles during heat stroke. Recent studies have shown that there is a certain correlation between heat stroke and lysosome acidity. In order to clarify their relationship, Lyso-NIR-pH, a photostable Si-rhodamine-based near-infrared fluorescent probe, was developed for sensing pH changes in lysosomes during heat stroke in this paper. For Lyso-NIR-pH, a morpholine group is employed as the lysosome-targeting unit and a H⁺-triggered openable deoxylactam is employed as the response unit to pH. Lyso-NIR-pH can detect pH with a high selectivity and a sensitivity, and its pK_a is 4.63. Lyso-NIR-pH also has outstanding imaging performances, such as excellent lysosome-targeting ability, low autofluorescence and photostable fluorescence signal, which are in favor of long-term imaging of pH with accurate fluorescence signals. Moreover, we successfully applied Lyso-NIR-pH to monitor lysosomal pH increases induced by chloroquine and apoptosis in live cells. Finally, we successfully applied Lyso-NIR-pH for monitoring changes of lysosomal pH during heat stroke. These results confirmed that Lyso-NIR-pH is a powerful tool to monitor pH change in lysosomes and study its possible effects.

Fluorescent probes with high pKa values based on traditional, near-infrared rhodamine, and hemicyanine fluorophores for sensitive detection of lysosomal pH variations

Sterically hindered fluorescent probes (A-C) have been developed by introducing 2-aminophenylboronic acid pinacol ester to a traditional, A, a near-infrared rhodamine dye, B, and a near-infrared hemicyanine dye, C, forming closed spirolactam ring structures. Probe A was non-fluorescent under basic pH conditions whereas probes B and C were moderately fluorescent with fluorescence quantum yields of 9% and 5% in pH 7.4 PBS buffer containing 1% ethanol, respectively. With all probes increasing acidity leads to significant increases in fluorescence at 580 nm, 644 and 744 nm for probes A, B and C with fluorescence quantum yields of 26%, 21% and 10% in pH 4.5 PBS buffer containing 1% ethanol, respectively. Probes A, B and C were calculated to have pKa values of 5.81, 5.45 and 6.97. The difference in fluorescence under basic conditions is ascribed to easier opening of the closed spirolactam ring configurations due to significant steric hindrance between the 2-aminophenylboronic acid pinacol ester residue and an adjacent H atom in the xanthene derivative moiety in probe B or C. The probes show fast, reversible, selective and sensitive fluorescence responses to pH changes, and are capable of sensing lysosomal pH variations in living cells.

Ratiometric Fluorescent Strategy for Localizing Alkaline Phosphatase Activity in Mitochondria Based on the ESIPT Process

Fluorescent probes are powerful tools for detecting and mapping the species of interest in vitro and in vivo. Although the probes always show high selectivity and sensitivity, they are usually affected by some factors, such as detecting conditions and the probe concentrations. Ratiometric fluorescent strategies, possessing advantage of low background noise, would solve the problem effectively and lead to a higher sensing performance. Thus, an ESIPT-based ratiometric probe (HBTP-mito) was developed on the basis of a phosphorylated 2-(2'-hydroxyphenyl)-benzothiazole derivative for the determination of ALP activity. HBTP-mito is water soluble and emits green fluorescence in TBS buffer due to the blockage of ESIPT. Upon the introduction of ALP, the phosphate ester of HBTP-mito was hydrolyzed and the ESIPT process was restored. Accordingly, the fluorescence at 514 nm decreases, while emission at 650 nm shows a "turn-on" response. The ratio of intensity (I_514nm/I_650nm) decreases linearly with ALP activity increasing from 0 to 60 mU/mL, obtained an LOD of 0.072 mU/mL. The favorable performance of the probe enables its application not only in the detection of ALP activity in biological samples, but also in the localization of the ALP levels in living cells and in vivo.

Near-infrared fluorescein dyes containing a tricoordinate boron atom

Tricoordinate boron imparts near-infrared absorption/emission and unusual multi-stage changes in the photophysical properties to fluorescein dyes.

Bora-fluoresceins (BFs), fluorescein analogues containing a tricoordinate boron atom instead of an oxygen atom at the 10-position of the fluorescein skeleton, were synthesized as a new family of fluorescein analogues. The deprotonated BFs exhibited absorption and fluorescence in the near-infrared region, which were significantly red-shifted relative to those of hitherto-known heteroatom-substituted fluorescein analogues on account of the orbital interaction between the tricoordinate boron atom and the fluorescein skeleton. BFs also showed multi-stage changes resulting from a Lewis acid–base equilibrium at the boron center in combination with a Brønsted acid–base equilibrium at the phenolic hydroxy group.

Near-infrared fluorescent probes with BODIPY donors and rhodamine and merocyanine acceptors for ratiometric determination of lysosomal pH variance

Three fluorescent probes have been developed by conjugating three different BODIPY donors to rhodamine and merocyanine acceptors for ratiometric determination of lysosomal pH variations. Probe A consists of a 1,3,5,7-tetramethyl-BODIPY donor and a near-infrared rhodamine acceptor bearing a lysosome-targeting morpholine residue. Probe B is composed of a 3,5-dimethyl-BODIPY donor and a near-infrared rhodamine acceptor modified with an o-phenylenediamine residue. Probe C contains a 3-styrene-functionalized BODIPY donor with longer wavelength emission and a near-infrared merocyanine acceptor containing a morpholine residue. Under neutral or basic pH conditions, the probes only show fluorescence from the BODIPY donors under BODIPY excitation because the rhodamine and merocyanine acceptors maintain closed spirolactam configurations. However, excitation at BODIPY absorption wavelengths concomitant with gradual pH decrease results in fluorescence decreases with the BODIPY donors and fluorescence increases from the rhodamine and merocyanine acceptors due to through-bond energy transfer from the donors to the acceptors. This is because the spirolactam ring opens under more acidic conditions and fluorescence of the acceptors results from significantly improved π-conjugation. These experimental results are substantiated with theoretical calculations on models of the different probes. The probes have all been used to determine lysosome pH variations in HeLa cells. Probe B was further utilized to successfully detect pH fluctuations in HeLa cells under oxidative stress and with treatment of NH4Cl and chloroquine.

A 7-Hydroxybenzoxazinone-Containing Fluorescence Turn-On Probe for Biothiols and Its Bioimaging Applications

In this work, a novel 7-hydroxybenzoxazinone-based fluorescent probe (PBD) for the selective sensing of biothiols is reported. Upon treatment with biothiols, PBD shows a strong fluorescence enhancement (up to 70-fold) and a large Stokes shift (155 nm). Meanwhile, this probe exhibits high resistance to interference from other amino acids and competing species. PBD features good linearity ranges with a low detection limit of 14.5 nM for glutathione (GSH), 17.5 nM for cysteine (Cys), and 80.0 nM for homocysteine (Hcy), respectively. Finally, the potential utility of this probe for biothiol sensing in living HeLa cells is demonstrated.

The ratiometric fluorescent probe with high quantum yield for quantitative imaging of intracellular pH

Intracellular pH, especially cytoplasmic pH (~7.2) plays a crucial role in cell functions and metabolism. A ratiometric fluorescent probe namely, 6-(2-(benzothiazol-2-yl)vinyl)naphthalen-2-ol (BTNO) was facilely synthesized by the condensation of 6-hydroxy-2-naphthaldehyde and 2-methylbenzothiazole. BTNO exhibited a remarkable ratiometric emission (F₄₅₆/F₅₂₆) enhancement in response to a pH change with a linear range of pH = 9.50-7.00 and a pKa value of 7.91 ± 0.03, which is desirable for measuring and monitoring the cytoplasmic pH fluctuations. In addition, because of the high fluorescence quantum yield of BTNO (Φ = 0.88 in DMSO and 0.61 in water relative to quinine sulfate solution in 0.1 M H₂SO₄), the interferences of the probe on the physiological functions could be greatly reduced. This could also provide enhanced measurement sensitivity. The successful demonstration of BTNO in detecting and monitoring the intracellular pH changes in live HeLa cells via a ratiometric approach confirmed that BTNO held a practical potential in biomedical research.

An unprecedented oxidised julolidine-BODIPY conjugate and its application in real-time ratiometric fluorescence sensing of sulfite

Reaction of a julolidine-based BODIPY compound with silver(i) ions in the presence of white light produced the oxidised julolidine version (OXJUL) containing a quaternary nitrogen. The oxidation of one ring at the julolidine site is highly unusual and there is no other reported literature example. The fluorescence maximum of OXJUL is altered from 648 nm to 608 nm by the addition of an aqueous solution of Na2SO3 over several minutes. In the presence of a large excess of sulfite a further slower reaction is observed which further shifts the emission maximum to 544 nm. The alterations form the basis of a real-time ratiometric sensor for sulfite and its detection in a white wine.

Recent Advances in Aggregation-Induced Emission Chemosensors for Anion Sensing

The discovery of the aggregation-induced emission (AIE) phenomenon in the early 2000s not only has overcome persistent challenges caused by traditional aggregation-caused quenching (ACQ), but also has brought about new opportunities for the development of useful functional molecules. Through the years, AIE luminogens (AIEgens) have been widely studied for applications in the areas of biomedical and biological sensing, chemosensing, optoelectronics, and stimuli responsive materials. Particularly in the application of chemosensing, a myriad of novel AIE-based sensors has been developed to detect different neutral molecular, cationic and anionic species, with a rapid detection time, high sensitivity and high selectivity by monitoring fluorescence changes. This review thus summarises the recent development of AIE-based chemosensors for the detection of anionic species, including halides and halide-containing anions, cyanides, and sulphur-, phosphorus- and nitrogen- containing anions, as well as a few other anionic species, such as citrate, lactate and anionic surfactants.

Tuning the pKa of Carboxyfluorescein with Arginine-Rich Cell-Penetrating Peptides for Intracellular pH Imaging

5-Carboxylfluorescein (FAM) is a conventional pH-responsive fluorophore widely used in fluorescence labeling and imaging. Because of its nonfluorescent structure under acidic conditions, FAM has long been limited to pH determination in a neutral-basic environment. Here, we modified the optical properties of FAM with cationic arginine-rich cell-penetrating peptides (CPPs), tuning the pK_a value of FAM to adapt well to pH measurement under diverse pH conditions. With increasing length of polyarginine, the pK_a value of FAM was tuned from 6.20 ± 0.06 to 5.17 ± 0.05. The key mechanism for pK_a variations was attributed to intramolecular electrostatic attraction and the positive charge of cationic CPPs tend to stabilize the fluorescent dianionic form of FAM. Apart from tunable pK_a, arginine-rich CPPs also improved the water solubility, membrane permeability, and organelle-specific localization of FAM. Two conjugated probes FAM-R₁₂ and FAM-(F_xr)₃ were selected to monitor intracellular pH fluctuations. Compared to FAM-(F_xr)₃, highly positively charged FAM-R₁₂ was more effective in lower pH condition and realized targeted visualization of lysosomal pH changes. The arginine-rich CPP-based strategy offers a promising approach to obtain optimized fluorescent pH probes with adjustable pK_a values for organelle-specific pH measurement.

Role of hydrophobicity in tuning the intracellular uptake of dendron-based fluorophores for in vitro metal ion sensing

Fluorophores are used for sensing biologically relevant ions, toxic metals or pathogenic markers. However, the mode of entry of such fluorophores into the cell greatly depends on their size, shape, surface charge, functional groups, and hydrophobicity. In particular, the influence of hydrophobicity on the intracellular uptake of fluorophores is poorly investigated. Self-assembly is a recent strategy to tune the intracellular uptake of fluorophores, facilitating increased intracellular sensing and fluorescence. Herein, self-assembly of three novel poly(aryl ether) dendron derivatives that contain rhodamine units was used to investigate the effect of hydrophobicity on the intracellular uptake of self-assembled fluorophores. The results suggest that monomer hydrophobicity plays an important role in the uptake. The dendron-based fluorophores, which upon self-assembly, formed stable spherical aggregates ranging from 300 to 500 nm. The rhodamine-based dendrons could selectively sense Hg²⁺ ions in the presence of other competing metal cations. Intracellular imaging of the dendron-based fluorophores displayed bright red fluorescence in human embryonic kidney cells. The rate of intracellular uptake of the three dendron-based fluorophores was analyzed by flow cytometry. The results establish the importance of the hydrophilic-lipophilic balance of the self-assembled amphiphiles for tuning the intracellular uptake.

Highly Stable Core-Shell Structured Semiconducting Polymer Nanoparticles for FRET-Based Intracellular pH Imaging

Highly stable semiconducting polymer nanoparticles (NPs) (poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO)/ poly(fluorene-2,7-ylenevinylene-co-phenylene) (PFV)-dopamine (DA) NPs) with previously unreported core-shell structure are developed for ratiometric sensing of intracellular pH values. PFO/PFV-DA NPs comprise central polyfluorene (PFO) as donor and PFV as acceptor, in which the donor and acceptor are spatially separated into the central core and nanoparticle shell. Specifically, thick PFV shells can not only significantly minimize the quenching interference of dopamine on the emission of standard reference (PFO), but are also able to maximize its accessibility to pH-sensitive dopamine and lead to sensitive response to pH changes. The resulting core-shell PFO/PFV NPs are structurally and optically stable, which can avoid the photobleaching and leakage of materials issues compared to traditional semiconducting polymer nanoparticles (SPNs)-based fluorescence resonance energy transfer (FRET) systems containing small molecules. Additionally, the designed compact PFO/PFV-DA NPs show quantitative response to the pH values in aqueous media and are capable of mapping intracellular pH fluctuations by ratiometric imaging. This work may open up opportunities for the generalizability of the consistent ratiometric emission intensity strategy based on core-shell structured SPNs nanoprobes for highly sensitive biological sensing.

Red-Emitting Fluorescence Sensors for Metal Cations: The Role of Counteranions and Sensing of SCN- in Biological Materials

The spatiotemporal sensing of specific cationic and anionic species is crucial for understanding the processes occurring in living systems. Herein, we developed new fluorescence sensors derived from tetrapyrazinoporphyrazines (TPyzPzs) with a recognition moiety that consists of an aza-crown and supporting substituents. Their sensitivity and selectivity were compared by fluorescence titration experiments with the properties of known TPyzPzs (with either one aza-crown moiety or two of these moieties in a tweezer arrangement). Method of standard addition was employed for analyte quantification in saliva. For K⁺ recognition, the new derivatives had comparable or larger association constants with larger fluorescence enhancement factors compared to that with one aza-crown. Their fluorescence quantum yields in the ON state were 18× higher than that of TPyzPzs with a tweezer arrangement. Importantly, the sensitivity toward cations was strongly dependent on counteranions and increased as follows: NO₃^- < Br^- < CF₃SO₃^- < ClO₄^- ≪ SCN^-. This trend resembles the chaotropic ability expressed by the Hofmeister series. The high selectivity toward KSCN was explained by synergic association of both K⁺ and SCN^- with TPyzPz sensors. The sensing of SCN^- was further exploited in a proof of concept study to quantify SCN^- levels in the saliva of a smoker and to demonstrate the sensing ability of TPyzPzs under in vitro conditions.

In Vivo Multicolor Imaging with Fluorescent Probes Revealed the Dynamics and Function of Osteoclast Proton Pumps

In vivo two-photon fluorescence imaging is a powerful modality to monitor cell dynamics in biomedical studies. To detect protein functions in living animals in real-time, fluorescent probes must show a quick response to the target function in specific tissues. Here, we developed a rhodamine-based small-molecule fluorescent probe called Red-pHocas (red pH-activatable fluorescent probe for osteoclast activity sensing) to reversibly detect the acidic environments for the spatiotemporal analysis of the function of osteoclast proton pumps. The introduction of electron-withdrawing N-alkyl substituents in the rhodamine spirolactam fluorophore remarkably increased the kinetics of the fluorescence response to acidic pHs, which allowed the rapid and reversible monitoring of acidic compartments and the analysis of the dynamics of osteoclast proton pumps during osteoclastic bone resorption. In vivo multicolor two-photon imaging using Red-pHocas in fluorescent reporter mice revealed that bone acidification occurred synchronously with the accumulation of proton pumps onto the bone surfaces. To our knowledge, this is the first study to demonstrate the direct involvement of osteoclast proton pumps in bone acidification under intravital conditions by means of an imaging probe.

A new perylene-based fluorescent pH chemosensor for strongly acidic condition

A highly selective and sensitive pH chemosensor N,N-bis[(2-thiophene)-ethyl]-3,4,9,10-perylene tetracarboxylic diimide (TEPTD) was designed and synthesized through Schiff-base condensation reaction. It exhibited large Stokes shifts, good water solubility, excellent selectivity and outstanding photo-stability. The pK_a of the probe was 3.0, which indicated that it could be used in highly acid conditions. With the addition of H⁺, the fluorescence intensity increased gradually. The sensing mechanisms involved photo-induced electron transfer, protonation and deprotonation, which were confirmed by ¹H NMR titration experiment with trifluoroacetic acid. The probe can be used as a convenient probe to distinguish acidic from neutral or alkaline solutions by "naked-eye".

A Schiff Base Fluorescence Enhancement Probe for Fe(III) and Its Sensing Applications in Cancer Cells

We report a new Schiff base fluorescent probe which senses ferric ion, Fe(III), with a significant fluorescence enhancement response. The probe showed high sensitivity (0.8 ppb), and fast response time (<10 s) of Fe(III) in aqueous media. In addition, the probe showed the ability to sense Fe(III) in a HeLa cancer cell line, with very low cytotoxicity. As a new bio-imaging probe for Fe(III), it gave bright fluorescent images in confocal laser scanning microscopy (CLSM).

Highly fluorescent triazolopyridine-thiophene D-A-D oligomers for efficient pH sensing both in solution and in the solid state

Conjugated fluorophores have been extensively used for fluorescence sensing of various substances in the field of life processes and environmental science, due to their noninvasiveness, sensitivity, simplicity and rapidity. Most existing conjugated fluorophores exhibit excellent light-emitting performance in dilute solutions, but their properties substantially decrease or even completely vanish due to severe aggregation quenching in the solid state. Herein, we synthesize a series of triazolopyridine-thiophene donor-acceptor-donor (D-A-D) type conjugated molecules with high absolute fluorescence quantum yields (ΦF) ranging from 80% to 89% in solution. These molecules also show unusual light-emitting properties in the solid state with ΦF of up to 26%. We find that owing to the protonation-deprotonation process of the pyridine ring, these compounds display obvious changes in both fluorescence wavelength and intensity upon addition of acids, and these changes can be readily recovered by the successive introduction of bases. By harnessing this phenomenon, we further show that these fluorophores can be employed for efficient and reversible fluorescence sensing of hydrogen ions in a broad pH range (0.0-7.0). With the fabrication of pH testing papers and ink-printed complex patterns including butterflies and letters on substrates, we demonstrate the application of such sensors to fluorescence indication or solid state pH detection for real samples such as volatile acidic/basic gas and water-quality analysis.

Hybrid drug nanocrystals

Nanocrystals show promise to deliver poorly water-soluble drugs to yield systemic exposure. However, our knowledge regarding the in vivo fate of nanocrystals is in its infancy, as nanocrystallization is simply viewed as an approach to enhance the dissolution of drug crystals. The dying crystal phenomenon inspired the development of hybrid nanocrystals by physically embedding fluorophores into the crystal lattice. This approach achieved concurrent therapy and bioimaging and is well-established to study pharmacokinetics and nanocrystal dissolution in vivo. Nanocrystals also offer the advantage of long-term durability in the body for interacting with biological tissues and cells. This review introduces the hybrid nanocrystal technique, including the theoretical concepts, preparation, and applications. We also discuss the latest development in self-discriminative hybrid nanocrystals utilizing environment-responsive probes. This review will stimulate further development and application of nanocrystal-based drug delivery systems for theranostic strategies.

Frontiers in Probing Alzheimer's Disease Biomarkers with Fluorescent Small Molecules

Alzheimer's disease (AD) is the most common form of dementia. The pathogenesis of the disease is associated with aggregated amyloid-β, hyperphosphorylated tau, a high level of metal ions, abnormal enzyme activities, and reactive astrocytes. This outlook gives an overview of fluorescent small molecules targeting AD biomarkers for ex vivo and in vivo imaging. These chemical imaging probes are categorized based on the potential biomarkers, and their pros and cons are discussed. Guidelines for designing new sensing strategies as well as the desirable properties to be pursued for AD fluorescence imaging are also provided.