Aggregation-induced emission-based fluorescent probes for cellular microenvironment detection

The cellular microenvironment exerts a pivotal regulatory influence on cell survival, function, and behavior. Dynamic analysis and detection of the cellular microenvironment can promptly elucidate changes in cellular microenvironmental information, uncover the pathogenesis of diseases associated with aberrant microenvironments, and aid in predicting disease risk and monitoring disease progression. Aggregation-induced emission (AIE) fluorescent molecules possess unique AIE characteristics and offer significant advantages in imaging and sensing cellular microenvironments. In this review, we present a profile of the remarkable progress achieved in utilizing AIE fluorescent molecules for detecting cellular microenvironments in recent years. We particularly focus on AIE fluorescent probes applied in imaging key parameters of the cellular microenvironment, including pH, viscosity, polarity, and temperature, as well as in analyzing critical biological components of the microenvironment, such as gas signal molecules, metal ions, redox state, and proteins. We underscore the design principles, detection mechanisms, sensing performance, and biological applications of these fluorescent probes. Furthermore, we address the current challenges confronting this field and provide prospects for the future development of AIE probes used for microenvironment detection. We trust that this review will inspire researchers to develop more precise and sensitive AIE fluorescent probes for the detection of cellular microenvironments.

Aggregation-Induced Emission Luminogen: Role in Biopsy for Precision Medicine

Biopsy, including tissue and liquid biopsy, offers comprehensive and real-time physiological and pathological information for disease detection, diagnosis, and monitoring. Fluorescent probes are frequently selected to obtain adequate information on pathological processes in a rapid and minimally invasive manner based on their advantages for biopsy. However, conventional fluorescent probes have been found to show aggregation-caused quenching (ACQ) properties, impeding greater progresses in this area. Since the discovery of aggregation-induced emission luminogen (AIEgen) have promoted rapid advancements in molecular bionanomaterials owing to their unique properties, including high quantum yield (QY) and signal-to-noise ratio (SNR), etc. This review seeks to present the latest advances in AIEgen-based biofluorescent probes for biopsy in real or artificial samples, and also the key properties of these AIE probes. This review is divided into: (i) tissue biopsy based on smart AIEgens, (ii) blood sample biopsy based on smart AIEgens, (iii) urine sample biopsy based on smart AIEgens, (iv) saliva sample biopsy based on smart AIEgens, (v) biopsy of other liquid samples based on smart AIEgens, and (vi) perspectives and conclusion. This review could provide additional guidance to motivate interest and bolster more innovative ideas for further exploring the applications of various smart AIEgens in precision medicine.

Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

Recent progress in the development of fluorescent probes for imaging pathological oxidative stress

Oxidative stress is closely related to the physiopathology of numerous diseases. Reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS) are direct participants and important biomarkers of oxidative stress. A comprehensive understanding of their changes can help us evaluate disease pathogenesis and progression and facilitate early diagnosis and drug development. In recent years, fluorescent probes have been developed for real-time monitoring of ROS, RNS and RSS levels in vitro and in vivo. In this review, conventional design strategies of fluorescent probes for ROS, RNS, and RSS detection are discussed from three aspects: fluorophores, linkers, and recognition groups. We introduce representative fluorescent probes for ROS, RNS, and RSS detection in cells, physiological/pathological processes (e.g., Inflammation, Drug Induced Organ Injury and Ischemia/Reperfusion Injury etc.), and specific diseases (e.g., neurodegenerative diseases, epilepsy, depression, diabetes and cancer, etc.). We then highlight the achievements, current challenges, and prospects for fluorescent probes in the pathophysiology of oxidative stress-related diseases.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

Real-Time Visualization and Monitoring of Physiological Dynamics by Aggregation-Induced Emission Luminogens (AIEgens)

Physiological dynamics in living cells and tissues are crucial for maintenance and regulation of their normal activities and functionalities. Tiny fluctuations in physiological microenvironments can leverage significant influences on cell growth, metabolism, differentiation, and apoptosis as well as disease evolution. Fluorescence imaging based on aggregation-induced emission luminogens (AIEgens) exhibits superior advantages in real-time sensing and monitoring of the physiological dynamics in living systems, including its unique properties such as high sensitivity and rapid response, flexible molecular design, and versatile nano- to mesostructural fabrication. The introduction of canonic AIEgens with long-wavelength, near-infrared, or microwave emission, persistent luminescence, and diversified excitation source (e.g., chemo- or bioluminescence) offers researchers a tool to evaluate the resulting molecules with excellent performance in response to subtle fluctuations in bioactivities with broader dimensionalities and deeper hierarchies.

A mitochondria-targeted fluorescent probe for the detection of endogenous SO2 derivatives in living cells

A unique fluorescent probe (ZACA) for the monitoring of SO2 derivatives was developed from coumarin and benzoindoles based on FRET and ICT. ZACA exhibited an active emission signal, large Stokes shift, wide emission window distance, and high photostability. It also possessed many advantages in the ratiometric detection of HSO3-/SO32- including low detection limit and high selectivity and sensitivity. Importantly, ZACA was successfully applied in the ratiometric detection of endogenous HSO3-/SO32- in living cells with excellent cellular imaging capability (1 μM) and mitochondria-targeting ability (co-localization coefficient: 0.91).

Borate-modified carbon dots as a probe for quercetin in plants

Schematic presentation of the PBA-CDs enhancing the fluorescence of quercetin in contrast to N-CDs.

Emerging analytical tools for the detection of the third gasotransmitter H2S, a comprehensive review

BACKGROUND

Hydrogen sulfide (H2S) is currently considered among the endogenously produced gaseous molecules that exert various signaling effects in mammalian species. It is the third physiological gasotransmitter discovered so far after NO and CO. H2S was originally ranked among the toxic gases at elevated levels to humans. Currently, it is well-known that, in the cardiovascular system, H2S exerts several cardioprotective effects including vasodilation, antioxidant regulation, inhibition of inflammation, and activation of anti-apoptosis. With an increasing interest in monitoring H2S, the development of analysis methods should now follow.

AIM OF REVIEW

This review stages special emphasis on the several analytical technologies used for its determination including spectroscopic, chromatographic, and electrochemical methods. Advantages and limitations with regards to the application of each technique are highlighted with special emphasis on its employment for H2S in vivo measurement i.e., biofluids, tissues.

KEY SCIENTIFIC CONCEPTS AND IMPORTANT FINDINGS OF REVIEW

Fluorescence methods applied for H2S measurement offer an attractive non-invasive and promising approach in addition to its selectivity, however they cannot be considered as H2S-specific probes. On the other hand, colorimetric assays are among the most common methods used for in vitro H2S detection, albeit their employment in vivo H2S measurement has not yet been possible . Separation techniques such as gas or liquid chromatography offer higher selectivity compared to direct spectrophotometric or fluorescence methods especially for suitable for endpoint H2S measurements i.e. plasma or tissue samples. Despite all the developed analytical procedures used for H2S determination, the need for highly selective, much work should be devoted to resolve all the pitfalls of the current methods.

Mitochondria-Targeted Sensor Array with Aggregation-Induced Emission Luminogens for Identification of Various Cells

Accurate discrimination of cancerous cells is a good solution for early diagnosis of tumors. The mitochondrion plays an important role in cells. Herein, the five aggregation-induced emission luminogens (AIEgens) with various double positive charges are synthesized to image mitochondria. Tetraphenylethylene (TPE) molecules are modified by methoxy groups, conjugated donor-acceptor, and different positive charges to achieve multicolor emission. The five AIEgens form the PTx-Sa (positive mitochondria-target molecular sensor array) to perform cross-fluorescence response based on the mitochondria-targeted imaging to achieve the discrimination of various cells. Principal component analysis of the cross-response fluorescence data of PTx-Sa shows that 100% accurate identification of various cells, including cancer cells and normal cells, digestive tract cancer cells, gastric cancer cells, and mixed gastric cancer cells. By support vector machine to show the predictive ability of PTx-Sa to unknown cells by using blind samples. This is the first time to apply mitochondria-targeted sensor array to identification of various cells.

Combined tetraphenylethylene fluorogens with positive charge for imaging capsule-covered pathogens

Capsule-covered pathogens can cause serious infectious diseases, and are highly pathogenic to humans. Herein, we developed four positively charged tetraphenylethylene derivatives (PC-TPEgens) that in certain combinations were applied to identify capsule-bearing pathogens using fluorescence imaging. The dual-charged probes were used to visualize the entire process of phagocytosis of pathogens into macrophages.

Fluorescent fibrous mats assembled with self-propagating probes for visual sensing of hydrogen peroxide and choline

Challenges remain in the facile, rapid and sensitive detection of substances at ultralow levels. In the current study, visual sensors of hydrogen peroxide (H2O2) and choline are developed via the integration of an ultrafine fibrous substrate and self-propagating and aggregation-induced emission (AIE) probes. Self-immolative probes (SIPs) composed of phenylboronic acid triggers and choline units are grafted on electrospun polyethylene terephthalate (PET) fibers, followed by electrostatic adsorption of tetraphenylethene derivatives (TPE-SO3) to obtain fluorescent PET-Ch/TPE fibers. Choline oxidase (ChOX) is immobilized on polystyrene-co-maleic anhydride (PSMA) fibers to obtain PSMA-ChOX, followed by assembly into PET-Ch/TPE@PSMA-ChOX composite mats. The presence of H2O2 initiates the cleavage of phenylboronic acid triggers in SIPs to release choline and choline/TPE complexes from PET-Ch/TPE fibers. The released choline is oxidized by PSMA-ChOX fibers to generate H2O2 that then activates a cascade of self-propagating reactions until the release of all choline/TPE complexes, leading to the alleviation of AIE effect and gradual fluorescence fading of fibrous mats. Thus, the hydrogen peroxide and choline concentrations can be read out from the fluorescence fading time of fibrous mats with a detection limit of 0.5 μM H2O2 within 30 min, providing potential self-test devices for a real-time, naked-eye and sensitive detection of bioactive substances.