Organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH for biosensing, bioimaging and biotherapeutics applications

In recent years, organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH (DFR-MPs-pH) have been attracting much interest in fundamental application research fields. More and more scientific publications have reported the exploration of various DFR-MPs-pH systems that have unique dual-fluorescence ratiometry as the signal output, in-built and signal self-calibration functions to improve precise detection of targets. DFR-MPs-pH systems possess high-performance applications in biosensing, bioimaging and biomedicine fields. This review has comprehensively summarized recent advances of DFR-MPs-pH for the first time. First of all, the compositions and types of DFR-MPs-pH are introduced by summarizing different organic fluorophores-based molecule systems. Then, construction strategies are analyzed based on specific components, structures, properties and functions of DFR-MPs-pH. Afterward, biosensing and bioimaging applications are discussed in detail, primarily referring to pH sensing and imaging detection at the levels of living cells and small animals. Finally, biomedicine applications are fully summarized, majorly involving bio-toxicity evaluation, bio-distribution, biomedical diagnosis and therapeutics. Meanwhile, the current status, challenges and perspectives are rationally commented after detailed discussions of representative and state-of-the-art studies. Overall, this present review is comprehensive, in-time and in-depth, and can facilitate the following further exploration of new and versatile DFR-MPs-pH systems toward rational design, facile preparation, superior properties, adjustable functions and highly efficient applications in promising fields.

A flavonol-labelled cellulose fluorescent probe combined with composite fluorescent film imaging and smartphone technology for the detection of Fe3

Fe3+ is one of the most widely distributed and abundant elements on earth. Realizing efficient and real-time monitoring of Fe3+ is of great significance for the natural environment and the health of living organisms. In this paper, a flavonol-labelled cellulose-based fluorescent probe (ACHM) was synthesized by using dialdehyde cellulose (DAC) as the backbone and combining with flavonol derivatives (AHM - 1). The mechanism of recognizing Fe3+ was verified by characterizing the structure of ACHM by NMR, HRMS (High Resolution Mass Spectrometry), FTIR (Fourier Transform Infrared Spectroscopy), XRD (X-ray Diffraction), TG (Thermogravimetry) and SEM (Scanning Electron Microscopy). The H2O solution of the probe ACHM showed good fluorescence properties. It has quenching fluorescence properties for Fe3+, with a low limit of detection (LOD) of 0.10 μM and a fast response time of only 20 s. In addition, in order to expand the application range of the probe, ACHM was prepared as a fluorescent composite film with an average tensile strength of 32.9 MPa and an average elongation at break of 3.39 %. It shows its superiority in mechanical properties. The probe also demonstrated its practical application value for detecting Fe3+ in smartphone imaging applications.

Detection of breast cancer cells by a near-infrared fluorescent probe targeting mitochondrial viscosity

Monitoring abnormal viscosity in biological systems is important for basic research and clinical applications. Fluorescence imaging technology is adaptable for the visualization of tumor tissues due to its comprehensive features. However, fluorescence detection of intracellular viscosity in clinical samples remains challenging. We developed a promising near-infrared fluorescent probe, M556, for viscosity measurement. M556, which targets mitochondria, was successfully applied to monitor the mitochondrial viscosity in living cells. Furthermore, M556 was demonstrated to effectively discriminate tumors from normal tissues in a mouse tumor model and in clinical specimens from breast cancer patients, thus indicating the potential perioperative use of this probe by clinicians to assist with biopsy procedures.

Screening of aggregation-induced emission and multi-response acrylonitrile-bridging fluorescent molecules tailored for rapid turn-on detection of HClO as well as ratiometric visualizing of extreme basicity

Realizing the rapid and sensitive tracing of multiple analysis indicators using single molecular probe through structural designing is urgently desired for exploring novel multi-response chemosensors. Herein, a series of acrylonitrile-bridging organic small molecules have been rationally designed. Among these donor-π-acceptor (D-π-A) compounds with efficient aggregation-induced emission (AIE) characteristics, a unique derivative, 2-(1H-benzo[d]imidazole-2-yl)-3-(4-(methylthio)phenyl) acrylonitrile, named MZS, has been screened out for multifunctional utilizing. First, probe MZS can respond to hypochlorous acid (HClO) through specific oxidation reaction, showing a marked fluorescence turn-on signal (I₄₉₅). This special sensing reaction is ultra-fast with a rather low detection limit (LOD = 13.6 nM). Next, versatile MZS is also sensitive to the extreme pH fluctuation, displaying an intriguing ratiometric signal variation (I₅₄₀/I₄₅₀), facilitating the real-time and naked-eye visualizing, which is even stable and reversible. Furthermore, probe MZS has been used for the monitoring of HClO in real water and commercially available disinfectant spray samples with satisfactory results. We envision that probe MZS would be a flexible and powerful tool for monitoring of environmental toxicity and industrial operations under realistic scenarios.

Cellulose-Based Fluorescent Material for Extreme pH Sensing and Smart Printing Applications

Environment-responsive fluorescence materials are being widely investigated for instrument-free determination of various environmental factors. However, developing an eco-friendly cellulose-based fluorescent pH sensor for sensing extreme acidity and alkalinity is still challenging. Herein, a highly fluorescent and multifunctional material is developed from biopolymer-based cellulose acetate. A biopolymer-based structure containing responsive functional groups such as -C═O and -NH is constructed by chemically bonding 5-amino-2,3-dihydrophthalazine-1,4-dione (luminol) onto cellulose acetate using 4,4'-diphenylmethane diisocyanate (MDI) as a cross-linking agent. The prepared material (Lum-MDI-CA) is characterized by UV-vis, Fourier transform infrared, ¹H NMR, ¹³C NMR spectroscopies, and fluorescence techniques. The material exhibits excellent aqua blue fluorescence and demonstrates extreme pH sensing applications. Interesting results are further revealed after adding a pH-unresponsive dye such as MTPP as the reference to develop the ratiometric method. The ratiometric system clearly differentiates the extreme acidic pH 1 from pH 2 and extreme alkaline pH 12, 13, and 14 by visual and fluorescence color change response under a narrow pH range. In addition, the material is fabricated into transparent flexible fluorescent films which demonstrate an outstanding UV shielding, security printing, and haze properties for smart food packaging and printing applications.

A novel aggregation-induced emission fluorescent probe with large Stokes shift for sensitive detection of pH changes in live cells

The detection of intracellular pH is crucial for elucidating the pathological process of cancers, as well as for medical diagnostic applications. Here, we developed an aggregation-induced emission active pH-responsive fluorescent probe (TPE-DCP) for sensitively detecting cell pH changes. The probe shows obvious pH-sensing properties at ~615 nm, with a pKa value of 6.82 and a good linear pH response ranging from 8.5 to 4.5. TPE-DCP holds advantages such as excellent anti-interference performance, good photostability, and low cytotoxicity, and has been successfully used to image intracellular pH changes in cells.

A new near-infrared fluorescent probe for sensing extreme acidity and bioimaging in lysosome

Since the intracellular pH plays an important role in the physiological and pathological processes, however, the probes that can be used for monitoring pH fluctuation under extreme acidic conditions are currently rare, so it is necessary to construct fluorescent probes for sensing pH less than 4. In this work, we developed a new near-infrared (NIR) fluorescent probe Cy-SNN for sensing pH fluctuation under extremely acidic conditions. For the preparation of this probe, benzothiozolium moiety was chosen as lysosomal targeting unit and NIR fluorophore, and barbituric acid moiety was fused in the polymethine chain of probe to introduce protonation center. Surprisingly, on the basis of the balance of quaternary ammonium salts and free amines, the pKa value of Cy-SNN was calculated as low as 2.96, implying that Cy-SNN can be used in acidic conditions with pH < 4. Moreover, Cy-SNN exhibited highly selective response to H+ over diverse analytes in real-time with dependable reversibility. Importantly, Cy-SNN can be used to specifically target lysosome, providing potential tools for monitoring the function of lysosome in autophagy process.

Recent progress of organic small molecule-based fluorescent probes for intracellular pH sensing

Fluorescent probes along with fluorescence microscopy are essential tools for biomedical research. Various cellular ubiquitous chemical factors such as pH, H₂O₂, and Ca²⁺ are labeled and traced using specific fluorescent probes, therefore helping us to explore their physiological function and pathological change. Among them, intracellular pH value is an important factor that governs biological processes, generally ∼7.2. Furthermore, specific organelles within cells possess unique acid-base homeostasis, involving the acidic lysosomes, alkalescent mitochondria, and neutral endoplasmic reticulum and Golgi apparatus, which undergo various physiological processes such as intracellular digestion, ATP production, and protein folding and processing. In this review, recently reported fluorescent probes targeted toward the lysosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, and cytoplasm for sensing pH change are discussed, which involves molecular structures, fluorescence behavior, and biological applications.

An ESIPT based versatile fluorescent probe for bioimaging live-cells and E. coli under strongly acidic conditions

A BTNN probe undergoes a 146 times increase in fluorescence intensity at 530 nm on lowering the pH from 7.0 to 2.0 and has been deployed for the bioimaging of MG-63 live cells and E. coli bacteria at different pH levels.

In Situ Fluorescent and Photoacoustic Imaging of Golgi pH to Elucidate the Function of Transmembrane Protein 165

Golgi pH homeostasis affects many different biological processes, including glycosylation. Recent studies have demonstrated that transmembrane protein 165 (TMEM165) deficiency leads to Golgi glycosylation abnormalities by disturbing Golgi pH homeostasis. However, due to the lack of specific tools to measure Golgi pH in situ, evidence for TMEM165 involvement in H⁺ transport in the Golgi apparatus is still absent. Herein, the photoacoustic and fluorescent dual-mode probe CPH was developed for ratiometric detection of Golgi pH. CPH was proved to accumulate in the Golgi apparatus and reversibly image Golgi pH in real-time with high sensitivity in cells. Furthermore, we found that the absence of TMEM165 influenced H⁺ equilibrium and caused Golgi apparatus acidification. Our work provides strong evidence that TMEM165 regulates Golgi pH homeostasis. Moreover, we believe that CPH has the potential to be a practical tool to monitor Golgi pH in various biological processes.

Visual and Precise Detection of pH Values under Extreme Acidic and Strong Basic Environments by Cellulose-Based Superior Sensor

Precise determination of pH values with an instrument-free mode is challenging and practical in industry processes, life science, and environmental monitoring. Here, taking advantage of the intramolecular charge transfer process, we construct a cellulose-based superior pH sensor, Phen-MDI-CA, which contains a highly sensitive, pH-responsive, and extended conjugation structure by combining phenanthroline moiety as a chromophore with urea group as a bridge. The resultant Phen-MDI-CA can precisely and visually determine pH values under extreme acidic and strong basic environments, depending on the different colors. In visible light mode, Phen-MDI-CA can readily discriminate the pH values of 14.0, 13.0, 12.0, and 11.0 and distinguish pH 2.0 from 1.0 by the naked eye. In fluorescent mode, Phen-MDI-CA can recognize more accurate pH values of pH 11.6-13.2 at a 0.2-0.4 pH interval by the high-contrasting color change. After addition of pH-irresponsive dyes as the reference, the corresponding ratiometric systems exhibit different colors at a 0.2-0.4 pH interval during the pH values of 11.0-14.0 and 1.0-2.0. Benefiting from the excellent formability, the Phen-MDI-CA was readily fabricated into pH test strips by coating method. To our knowledge, this is the first study to monitor extreme acidity and strong basicity in such precise pH values with an instrument-free mode.

A novel water-soluble naked-eye probe with a large Stokes shift for selective optical sensing of Hg2+ and its application in water samples and living cells

A water-soluble and colorimetric fluorescent probe with a large Stokes shift (139 nm) for rapidly detecting Hg²⁺, namely Hcy-mP, was synthesized by using an indole derivative and 2,4-dihydroxybenzaldehyde as starting materials. This probe demonstrates good selectivity for Hg²⁺ over other metal ions including Ag⁺, Pb²⁺, Cd²⁺, Cr³⁺, Zn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, K⁺, Na⁺, Mg²⁺, and Ca²⁺ in aqueous solution. With the increase in concentration of Hg²⁺, the color of the solution changed from pale yellow to pink and the fluorescence intensity decreased slightly. When 5-equivalents of EDTA were added to the solution with Hg²⁺, the fluorescence intensity of this probe was restored. The probe has been applied to the detection of Hg²⁺ in real water samples. Moreover, this probe was confirmed to have low cytotoxicity and excellent cell membrane permeability. The effect of Hcy-mP–Hg²⁺ towards living cells by confocal fluorescence was also investigated.

A water-soluble and colorimetric fluorescent probe with a large Stokes shift (139 nm) for rapidly detecting Hg²⁺, namely Hcy-mP, was synthesized by using an indole derivative and 2,4-dihydroxybenzaldehyde as starting materials.

From aggregation-induced to solution emission: a new strategy for designing ratiometric fluorescent probes and its application for in vivo HClO detection

The fluorescence of aggregation-induced emission is converted into that from ICT-induced emission in the presence of HClO.

Imaging of lysosomal pH changes with a novel quinoline/benzothiazole probe

A fluorescent pH probe BTVQ for imaging lysosomal pH fluctuations in live cells and extreme acidity in E. coli cells.

Inhibition of the prototropic tautomerism in chrysazine by p-sulfonatocalixarene hosts

This study reveals the unusual inhibition of excited-state prototropic tautomerism of Chrysazine by p-sulfonatocalix[4,6]arene hosts.