A mitochondria-targeted two-photon fluorescent probe for sensing and imaging pH changes in living cells

A multifunctional fluorescent probe for sequential detection of hydrogen sulfide and pH in foodstuffs, living cells and mice

BACKGROUND

Cancer as a leading cause of premature death worldwide has become a major threat to human health due to the high incidence and mortality. Monitoring tumor markers are reliable and significantly important for early detection of cancers. In complex biological systems, it is of great urgency but still remains challenging to conceive a fluorescent probe with multiple tumor markers detection property. Hydrogen sulfide (H2S) and pH are two target biomarkers for diagnosis of early cancer. The preparation of a novel probe with H2S and pH dual detection functions is highly anticipated.

RESULTS

Herein, a novel sequential detection probe HTPQ-HS for H2S and pH has been developed. In this system, HPQ (2-(2 -hydroxyphenyl)-4(3H)-quinazolinone) structure combined with triphenylamine is applied as the fluorophore, and 2, 4-dinitrophenylsulfonyl group is used as the recognition group. In the presence of H2S, HTPQ-HS is transformed into product HTPQ-OH which shows fluorescence enhancement (29-fold) at 525 nm in less than 4 min and further displays repeatable acid-base responsive ability. HTPQ-HS is able to sequentially response to H2S and pH in living cells and does not react directly with pH. Owing to the low cytotoxicity, HTPQ-HS is able to detect exogenous and endogenous H2S in colon cancer cells and mice, monitor H2S in inflammation model and in foodstuffs. As the environment changes from acidic to alkaline, the fluorescence intensity ratio (I470/I530) of product HTPQ-OH changes remarkably, illustrating the ratiometric fluorescent responsiveness to pH.

SIGNIFICANCE AND NOVELTY

A multifunctional fluorescent probe HTPQ-HS for sequential detection of H2S and pH is synthesized. Probe HTPQ-OH realizes the monitoring of dynamic changes in intracellular pH and displays prospective application in security printing. We expect that our work could offer an important guidance on the development of multifunctional fluorescent probes for visualizing H2S and pH in biology and environment.

Ratiometric fluorescent probe based on non-conjugated polymer dots for pH measurements in ordinary Portland cement-based materials

An organic fluorescent probe, citric acid-1,3-Propanediamine-Rhodamine B (CPR) was synthesized to determine pH of ordinary Portland cement requiring only a small sample size (less than 500 μL cement leachate). The SEM, XRD, and FTIR investigations demonstrate that citric acid-1,3-Propanediamine are polymer dots with a fusiform structure. Ratio pH probe is constructed by rhodamine B with polymer dots, which exhibits a linear response in high alkaline range. A 6-fold increase in fluorescence intensity (455 nm) is achieved at pH from 12.00 to 13.25. Combined with measurements of the isothermal calorimeter, mineral composition, and microscopic morphology, variation of pH is used to evaluate the changes of components during hydration. Furthermore, CPR can be applied to measure pH of high-dose pulverized fuel ash blending systems, the non-pure cement with slightly lower alkalinity.

Mitochondrial Targeting and Imaging with Small Organic Conjugated Fluorophores: A Review

The last decade has seen an increasingly large number of studies reporting on the development of novel small organic conjugated systems for mitochondrial imaging exploiting optical signal transduction pathways. Mitochondria are known to play a critical role in a number of key biological processes, including cellular metabolism. Importantly, irregularities on their working function are nowadays understood to be intimately linked to a range of clinical conditions, highlighting the importance of targeting mitochondria for therapeutic benefits. In this work we carry out an in-depth evaluation on the progress to date in the field to pave the way for the realization of superior alternatives to those currently existing. The manuscript is structured by commonly used chemical scaffolds and comprehensively covers key aspects factored in design strategies such as synthetic approaches as well as photophysical and biological characterization, to foster collaborative work among organic and physical chemists as well as cell biologists.

Recent Advancements in Colorimetric and Fluorescent pH Chemosensors: From Design Principles to Applications

Due to the immense biological significance of pH in diverse living systems, the design, synthesis, and development of pH chemosensors for pH monitoring has been a very active research field in recent times. In this review, we summarize the designing strategies, sensing mechanisms, biological and environmental applications of fluorogenic and chromogenic pH chemosensors of the last three years (2018-2020). We categorized these pH probes into seven types based on their applications, including 1) Cancer cell discriminating pH probes; 2) Lysosome targetable pH probes; 3) Mitochondria targetable pH probes; 4) Golgi body targetable pH probes; 5) Endoplasmic reticulum targetable pH probes; 6) pH probes used in nonspecific cell imaging; and 7) pH probes without cell imaging. All these different categories exhibit diverse applications of pH probes in biological and environmental fields.

Recent Advances in Small Molecule-Based Intracellular pH Probes

Intracellular pH plays an important role in many biological and pathological processes. Small-molecule based pH probes are found to be the most effective for pH sensing because of ease of preparation, high sensitivity, and quick response. They have many advantages such as small perturbation to the functions of the target, functional adaptability, cellular component-specific localization, etc. The present review highlights the flurry of recent activity in the development of such probes. The probes are categorized based on the type of fluorophore used like quinoline, coumarin, BODIPY, rhodamine, indolium, naphthalimide, etc., and their analytical performance is discussed.

β-Naphthothiazolium-based ratiometric fluorescent probe with ideal pKa for pH imaging in mitochondria of living cells

The weakly alkaline microenvironment (pH ~8.0) in mitochondria plays a vital role in maintaining its morphology and function. Thus monitoring mitochondrial pH (pH_mito) is of great significance. Herein, a ratiometric fluorescent probe (ENBT) for pHmito imaging in mitochondria of living cells is reported. pH variation closely correlates to intramolecular charge transfer (ICT) from naphthol to β-naphthothiazolium. ENBT exhibits a remarkable decrease on ratiometric fluorescence at λ_em1/λ_em2 = F₅₉₅/F₇₀₀ in response to pH variation within 6.30-9.29. In addition, ENBT has an ideal pKa value of 7.94 ± 0.08, which is advantageous in accurate sensing of pH_mito. Moreover, ENBT has a Stokes shift of >150 nm, which effectively eliminates the potential interference from the excitation irradiation. ENBT shows excellent capability for specific staining of mitochondria with low cytotoxicity, which is most suitable for pH_mito imaging in live cells. The probe was applied for monitoring pH_mito variation in mitochondria of live cells caused by H₂O₂, NAC (N-Acetyl-l-cysteine), NH₄Cl, carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and lactate/pyruvate. The morphological alterations of mitochondria in living cells after treatment by CCCP were further evaluated.

Imidazolyl-benzocoumarins as ratiometric fluorescence probes for biologically extreme acidity

A rational approach to develop a fluorescent probe for sensing biologically "extreme" acidity (pH <3) is disclosed. The probe, a push-full type 3-(imidazolyl)benzocoumarin dye, has the lowest pK_a = 1.3 among ratiometric probes known so far, which is ascribed due to a unique sensing mechanism. The probe has high quantum yields, high chemical stability and good aqueous solubility. The probe was successfully applied to ratiometric fluorescence imaging of intrabacterial acidity from pH 4.0-1.0, offering a practical means for studying biological systems under the extreme pH conditions.

The development of a highly selective fluorescent probe for the rapid detection of HClO in living cells and zebrafish

A new turn-on fluorescent probe, BM-HA, for the rapid detection of HClO in living cells and zebrafish is proposed, and DFT/TDDFT calculations provide insights into the optical properties of the BM-HA probe.

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.

A benzothiazolium-based fluorescent probe with ideal pKa for mitochondrial pH imaging and cancer cell differentiation

A mitochondrial pH sensing fluorescent probe namely 2-(2-(6-hydroxynaphthalen-2-yl)vinyl)-3-(6-(triphenyl-phosphonio)hexyl)benzothiazol-3-ium bromide (HTBT2) was designed and facilely synthesized via the Knoevenagel condensation reaction. HTBT2 displayed a linear fluorescence enhancement at 612 nm in response to pH changes between 8.70 and 7.20. The pK_a value was determined to be 8.04 ± 0.02, which might be ideal for mitochondrial pH (pH_mito∼8.0) detection. HTBT2 also exhibited a remarkable large Stokes shift of 176 nm, which could diminish the interference of excitation light. The results of live cell imaging studies suggested that HTBT2 showed excellent targeting ability for mitochondria. Importantly, it was successfully applied to visualize mitochondrial pH changes in live cells and differentiate the pH_mito difference between cancer cell lines and normal cell lines. Our results consistently supported that HTBT2 held practical promise for the investigation of physiological processes related to pH_mito changes and clinical potential for cancer cell differentiation.

Indolenine-Based Derivatives as Customizable Two-Photon Fluorescent Probes for pH Bioimaging in Living Cells

Novel pH probes based on 2-(6-methoxynaphthalen-2-yl)-3,3-dimethyl-3H-indole have been synthesized and characterized. These compounds display excellent "off-on" fluorescence responses to acidic pH especially under two-photon (TP) excitation conditions as well as strong selectivity and sensitivity toward H⁺. These features are supported by fluorescence quantum yields over 35%, TP cross sections ∼60 GM, and good resistance to photodegradation under acidic conditions. The synthetic versatility of this model allows subcellular targets to be tuned through minor scaffold modifications without affecting its optical characteristics. The effectiveness of the probes' innate photophysical properties and the structural modifications for different pH-related applications are demonstrated in mouse embryonic fibroblast cells.

Design and applications of a novel fluorescent probe for detecting glutathione in biological samples

This study aimed to develop a novel and practical fluorescent method for GSH detection in complex biological samples. To this end, a series of coumarin-based fluorescent probes was designed and synthesized using various aliphatic halogens as the sensing group. By using a new evaluation method of GSH/Cys/Hcy coexisting conditions, the probe with chloropropionate (CBF3) showed a high selectivity, excellent sensitivity, good stability for GSH detection. The reaction mechanism is proposed as nucleophilic substitution/cyclization and intramolecular charge transfer (ICT), which was confirmed by LC-MS and NMR analysis, as well as density functional theory calculations. In addition, CBF3 was demonstrated to be competent not only for the quantitative detection of GSH in real serum samples, but also for sensing GSH changes in different oxidative stress models in living cells and nematodes. This study showed a practical strategy for constructing GSH-specific fluorescent probes, and provided a sensitive tool for real-time sensing of GSH in real biological samples. The findings would greatly facilitate further investigations on GSH-associated clinical diagnosis and biomedical studies.