Ratiometric fluorescent probes for pH mapping in cellular organelles

The intracellular pH (pHi) in organelles, including mitochondria, endoplasmic reticulum, lysosomes, and nuclei, differs from the cytoplasmic pH, and thus maintaining the pH of these organelles is crucial for cellular homeostasis. Alterations in the intracellular pH (ΔpHi) in organelles lead to the disruption of cell proliferation, ion transportation, cellular homeostasis, and even cell death. Hence, accurately mapping the pH of organelles is crucial. Accordingly, the development of fluorescence imaging probes for targeting specific organelles and monitoring their dynamics at the molecular level has become the forefront of research in the last three decades. Among them, ratiometric fluorescent probes minimize the interference from the excitation wavelength of light, auto-fluorescence from probe concentration, environmental fluctuations, and instrument sensitivity through self-correction compared to monochromatic fluorescent probes, which are known as turn-on/off fluorescent probes. Small-molecular ratiometric fluorescent probes for detecting ΔpHi are challenging yet demanding. To date, sixty-two ratiometric pH probes have been reported for monitoring internal pH alterations in cellular organelles. However, a critical review on organelle-specific ratiometric probes for pH mapping is still lacking. Thus, in the present review, we report the most recent advances in ratiometric pH probes and the previous data on the role of mapping the ΔpHi of cellular organelles. The development strategy, including ratiometric fluorescence with one reference signal (RFRS) and ratiometric fluorescence with two reversible signals (RFRvS), is systematically illustrated. Finally, we emphasize the major challenges in developing ratiometric probes that merit further research in the future.

Hypochlorite Detection by Fluorescent Sensors Bearing Long Alkyl Chains: The Role of Chain Length in Sensing Properties

Three pyridinium derivatives bearing alkyl chains of different lengths (C1, C8, and C18) that show aggregation-enhanced emission were synthesized. These compounds can be used to detect ClO- ion as the reaction releases the fluorescent core with an increase in emission intensity and change in absorption wavelength. The lowest detection limit of TPA-Pyr-18C was 6.04 μM. The length of the alkyl chain and resulting lipophilicity allowed the targeting of different subcellular structures. TPA-Pyr-18C could be used for staining yolk lipids in zebrafish.

Discrimination of Cell Death Types with an Activatable Fluorescent Probe through Visualizing the Lysosome Morphology

Cell death plays a vital role in body development, maintenance of tissue function, and homeostasis. Accurate evaluation of cell death types is of great importance for pharmacological and pathological research. However, there is a lack of efficient fluorescent probes to discriminate various cell states. Here, we design and synthesize a novel activatable fluorescent probe PNE-Lyso to detect intracellular pH and hexosaminidases with two kinds of fluorescence signals. PNE-Lyso could distinguish dead cells from healthy cells based on a dual-color mode by targeting the lysosome and evaluating lysosomal hexosaminidase activity. Significantly, PNE-Lyso could also discriminate apoptotic and necrotic cells through visualizing lysosome morphology that is adjusted by the integrity of the lysosome membrane. Moreover, probe PNE-Lyso was successfully applied to investigate the drug-induced cell death process. To the best of our knowledge, this work is the first time cell death types have been distinguished based on a single fluorescent probe.

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.

A benzaldehyde-indole fused chromophore-based fluorescent probe for double-response to cyanide and hypochlorite in living cells

With the rapid development of various industries, cyanide (CN-) and hypochlorite (ClO-) have a tremendously adverse effect on the health of humans and animals. In this study, a fluorescent probe HHTB based on a benzaldehyde-indole fused chromophore was designed to detect cyanide and hypochlorite simultaneously. The synthesized probe was found to have strong anti-interference ability. In addition, the designed probe could respond rapidly to ClO- in just 80 s, while the color changed visibly from red to colorless. Moreover, the response time to CN- was longer (about 160 s), with the apparent color change from red to light red. The ratiometric and colorimetric absorbance variation of HHTB was due to the nucleophilic attack of CN- on the indole C[double bond, length as m-dash]N functional group and the strong oxidization of ClO- which destroyed the C[double bond, length as m-dash]C bonds and the conjugation systems. Furthermore, the probe HHTB responding to ClO- and CN- presented high sensitivity, as the calculated detection limits were 1.18 nM and 1.40 nM, respectively. The probe was also found to have low biological toxicity and was used in living cells successfully. Therefore, it has good application prospect in the field of cell imaging and biomedicine. The binding mechanism of HHTB-CN and the reaction mechanism of HHTB and ClO- were further elucidated by a series of experiments.

Visual monitoring of the lysosomal pH changes during autophagy with a red-emission fluorescent probe

Autophagy plays crucial roles in maintaining normal intracellular homeostasis. Molecular probes capable of monitoring lysosomal pH changes during autophagy are still highly required yet challenging to develop. Here, a lysosome-targeting fluorescent pH probe, RML, is presented by introducing a methylcarbitol unit as the lysosome-targeting group to rhodamine B, which is highly sensitive to pH changes. RML exhibits remarkable pH-dependent behavior at 583 nm with a fluorescent enhancement of more than 148-fold. The pK_a value is determined as 4.96, and the linear response with pH changes from 4.50-5.70, which is favorable for lysosomal pH imaging. We also confirm that RML diffuses selectively into lysosomes using confocal fluorescence microscopy. Using RML, we have successfully visualized autophagy by monitoring the lysosomal pH changes.

Dual-Functional NIR AIEgens for High-Fidelity Imaging of Lysosomes in Cells and Photodynamic Therapy

Design and synthesis of water-soluble near-infrared (NIR) emissive fluorescent molecules with aggregation-induced emission (AIE) characteristics, perfect signal-to-noise ratio for imaging of organelle, and photodynamic therapy (PDT) functions has received enormous attention. However, the dual-functional NIR AIEgens of high-fidelity tracking lysosome and ablation cancer cells was rarely reported. Herein, a series of AIE luminogens (AIEgens) with a typical AIE effect, good biocompatibility, superior photostability, high brightness, and excellent reactive oxygen species (ROS) generation ability were developed, which had different electronic push-pull strength and conjugate system size in the molecular structure. These AIEgens could specifically "light up" and dynamically long-term track the lysosomes in living cells and zebrafish with ultrahigh colocalization imaging Pearson's correlation coefficients (Rr: 0.9687) and overlap coefficient (R: 0.9967). Additionally, the MPAT of NIR luminescence as a photosensitizer was used for photodynamic ablation of cancer cells, owing to prompt generation of the ROS under green light irradiation (495-530 nm, 10 mW cm^-2). Hence, this research not only expands the application range of NIR AIEgens but also provides useful insights into design of split-new method for the treatment of cancer.

Simultaneous imaging of lysosomal and mitochondrial viscosity during mitophagy using molecular rotors with dual-color emission

A schematic illustration of rotors to detect mitochondrial and lysosomal viscosity during mitophagy.

A novel water-soluble fluorescent probe with ultra-sensitivity over a wider pH range and its application for differentiating cancer cells from normal cells

A water-soluble fluorescent probe with ultra-sensitivity over a wider pH range was developed to differentiate cancer cells from normal cells.