Sensitive and selective fluorescent probe for hypochlorite in 100% aqueous solution and its application for lysosome-targetable cell imaging

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

A unique phenothiazine-based fluorescent probe using benzothiazolium as a reactivity regulator for the specific detection of hypochlorite in drinking water and living organisms

As a common disinfectant and an essential reactive oxygen species (ROS), hypochlorite (ClO-) plays vital roles in both water treatment and cell metabolism, but its abnormal level can cause serious harm to human health. Therefore, quantifying ClO- level in drinking water and living organisms is extremely significant. Herein, we decorated different cationic heterocycles on phenothiazine core to construct three fluorescent probes for ClO-. According to the results, only benzothiazolium moiety reasonably adjusted the electron cloud density at sulfur atom of phenothiazine core for the specific oxidation with ClO-, thus endowing the prepared probe PT-BT with a perfect selectivity for ClO-. Meanwhile, PT-BT exhibited a low detection limit (38 nM) and a fast response (within 20 s) toward ClO-. Furthermore, this probe was utilized to fabricate a ready-to-use test strip, which could quantitatively measure ClO- level in real water samples by a portable smartphone sensing platform. Notably, PT-BT targeted mitochondria efficiently, and successfully visualized endogenous ClO- in living cells and zebrafish larvae. Especially, PT-BT was able to monitor the dynamic change of ClO- level in inflammatory mice. These results strongly manifested that probe PT-BT was a promising tool for detecting ClO- in drinking water and living organisms.

Advances in the Development of Water-Soluble Fluorogenic Probes for Bioimaging of Hypochlorite/Hypochlorous Acid in Cells and Organisms

This mini-review summarizes the development of intracellular fluorogenic probes for biological investigations of hypochlorous acid/hypochlorite (HOCl/OCl^-) in living cells and tissues. Monitoring the formation or effects of reactive oxygen species (ROS) inside living systems is critical in determining their roles in human physiology. HOCl/OCl^- is considered as an important member of the nonradical ROS family for its decisive microbicidal action in the innate immune system. Even though HOCl/OCl^- plays a defensive role in human health, abnormal or overexpression may have detrimental effects on the host physiology leading to many diseases, including neurodegeneration and cancer. In recent years, progress in the development of fluorescent imaging probes for observing HOCl/OCl^- levels in live cells and tissues has been made. Despite considerable advancement, challenges still exist in areas like working solvent/media, pH, response time, buffer selection, emission region, and others. In addition, this account aims to discuss the design strategies and sensing mechanisms of the representative fluorogenic probes for bioimaging of HOCl/OCl^-, endogenously and exogenously. Herein, we also have tried to provide the future direction to develop HOCl/OCl^- specific probes for disease diagnosis with particular attention to the requirement of the recognition group, solvent, and buffer media, which will be beneficial for those working in the domain of biomedical research.

A novel mitochondria-targetable NIR fluorescent probe for monitoring intracellular hypobromous acid levels

The development of ultrasensitive in situ detection techniques for monitoring hypobromous acid (HBrO) levels in the biological systems is of great significance to reveal its complex pathological and physiological effects. A simple mitochondria-targetable hydrazine-based near-infrared (NIR) fluorescent probe (Mito-NIR) for detecting HBrO in the mitochondria of live cells is presented in this paper. Probe Mito-NIR displays the ultrafast (< 5 s) response for HBrO. It can detect HBrO with high sensitivity. Additionally, it shows high selectivity towards HBrO over other biologically important substances. Finally, it can monitor the changes of endogenous/exogenous HBrO levels in the mitochondria of live cells. A simple mitochondria-targetable NIR fluorescent probe with picomolar sensitivity for HBrO was developed to specifically track mitochondrial HBrO.

A deep-red lysosome-targetable fluorescent probe for detection of hypochlorous acid in pure water and its imaging application in living cells and zebrafish

Hypochlorite plays a significant role in physiological processes, particularly regulation of lysosomal functions, and is involved in various diseases. Thus, it is crucial to develop highly sensitive and selective molecule tools to detect HClO in lysosomes. Herein, a novel 2H-benzo[h]chromene-pyridine derivative (1) was synthesized through condensation reaction, which exhibited a notable deep-red emission at 640 nm in pure water. This deep-red emission was specifically quenched by adding ClO^-. The response of probe 1 toward ClO^- was rapid (within 10 s), sensitive (detection limit of 0.012 μM), and effective over a wide range of pH (1.0-12.0). Due to the existence of morpholine as the lysosome-targeting unit, the probe was successfully utilized to monitor lysosomal ClO^-. Moreover, the probe 1 was also applied to detecting ClO^- in zebrafish.

A dual-response fluoran-phenothiazine hybrid fluorescent probe for selective sensing of Fe3+ and ClO- and cell imaging application

Bifunctional fluorescent probes with dual-emission response attract extensive attention. A novel fluorescent probe FP, a hybrid of fluoran and phenothiazine, has been designed and synthesized for selective sensing of Fe³⁺ and ClO^- with dual-emission changes, which involes mechanisms of Fe³⁺-promoted spirolactone ring opening and ClO^--induced oxidation of phenothiazine moiety, respectively. In addition, the detection limits for Fe³⁺ and ClO^- were estimated to be 49.1 and 35.9 nM, respectively. Significantly, FP can be employed as an tracer for the detection of Fe³⁺ ions within living HeLa cells by fluorescence imaging.

A Unique Multifunctional Luminescent Probe for Self-Monitoring Photodynamic Therapy by Detecting H2S in Cancer Cells

With the increasing interest in photodynamic therapy (PDT), the assessment of the level of reactive oxygen species produced during PDT has also become increasingly important. However, most of the fluorescent probes for reactive oxygen species (ROS) evaluation were separated from photosensitizers in the PDT process, resulting in ex situ and asynchronous treatment feedback. Additionally, the consumption of ROS by these fluorescent probes themselves will inevitably affect the therapeutic effect. Herein, inspired by the redox balance in the cell, we developed a multifunctional hydrogen sulfide (H₂S) probe Ru-NBD for reporting the therapeutic effect during the PDT process by detecting hydrogen sulfide. The probe Ru-NBD could not only serve as an effective PDT reagent both before and after H₂S activation but could also be used for real-time and in situ monitoring of the therapeutic effect via restored luminescence during the PDT process. As the phototherapy process progresses, the luminescent signal of Ru-NBD changes accordingly. The experimental results show that there is a certain correlation between the luminescence intensity and the cell inhibition rate; thus, we can monitor the phototherapy process by detecting the changes in the probe's luminescent signal. This study provides an idea for the design and adjustment of PDT.

Preparation of Yellow Fluorescent N,O-CDs and its Application in Detection of ClO. J Fluoresc 2021;31:659-666. [PMID: 33534115 DOI: 10.1007/s10895-021-02686-4]

Accurate and efficient detection of ClO^- was extremely important due to the harm of ROS in the environment and organism. In this paper, yellow fluorescent N,O-CDs were successfully prepared by the solvothermal method. The microscopic size of the N,O-CDs was approximately spherical with an average particle size of 4.8 ± 0.8 nm. The fluorescence quantum yield in ethanol solution was calculated as 10.5 % using fluorescein as the standard reference. The as-fabricated N,O-CDs had high sensitivity and low detection limit (7.5 µM) for quantitatively detecting ClO^- with a linear range from 0.07 mM to 0.16 mM. The probe not only shows good selectivity and anti-interference to metal ions, anions and amino acids but also has excellent light stability and thermal stability. Also, a wide selection range for pH was demonstrated.

One-step construction of a novel AIE probe based on diaminomaleonitrile and its application in double-detection of hypochlorites and formaldehyde gas

A novel and efficient probe with AIE property was designed and synthesized for application in double-detection of hypochlorites and formaldehyde gas.

Synthesis of Dihydroquinolines as Scaffolds for Fluorescence Sensing of Hydroxyl Radical

A mild synthetic method to prepare dihydroquinolines has been presented. These dihydroquinolines, for the first time, showed great potential for fluorescence detection of the important biorelevant hydroxyl radicals (^•OH). Sensitive and selective ^•OH detection and intracellular organelle-targeted fluorescence imaging of ^•OH have been demonstrated by using one of the synthetic dihydroquinolines. Moreover, dihydroquinoline has also exhibited promising potential to construct advanced fluorescence probes for ^•OH with tunable photophysical properties.