Water-Soluble Small Organic Fluorophores for Oncological Theragnostic Applications: Progress and Development

Cancer is one of the major noncommunicable diseases, responsible for millions of deaths every year worldwide. Though various cancer detection and treatment modalities are available today, many deaths occur owing to its late-stage detection and metastatic nature. Noninvasive detection using luminescence-based imaging tools is considered one of the promising techniques owing to its low cost, high sensitivity, and brightness. Moreover, these tools are unique and valuable as they can detect even the slightest changes in the cellular microenvironment. To achieve this, a fluorescent probe with strong tumor uptake and high spatial and temporal resolution, especially with high water solubility, is highly demanded. Recently, several water-soluble molecules with emission windows in the visible (400-700 nm), first near-infrared (NIR-I, 700-1000 nm), and second near-infrared (NIR-II, 1000-1700 nm) windows have been reported in literature. This review highlights recently reported water-soluble small organic fluorophores/dyes with applications in cancer diagnosis and therapeutics. We systematically highlight and describe the key concepts, structural classes of fluorophores, strategies for imparting water solubility, and applications in cancer therapy and diagnosis, i.e., theragnostics. We discuss examples of water-soluble fluorescent probes based on coumarin, xanthene, boron-dipyrromethene (BODIPY), and cyanine cores. Some other emerging classes of dyes based on carbocyclic and heterocyclic cores are also discussed. Besides, emerging molecular engineering methods to obtain such fluorophores are discussed. Finally, the opportunities and challenges in this research area are also delineated.

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.

New Rhodamine-based sensor for high-sensitivity fluorescence tracking of Cys and simultaneously colorimetric detection of H2S

Sulfhydryl-containing compounds including cysteine (Cys), homocysteine (Hcy), glutathione (GSH) and hydrogen sulfide (H2S) are involved in many physiological processes. The development of single-molecule optical sensor for the distinguish detection of these bio-thiols is a critical and challenging effort. In this work, we designed a one-step synthesis of the Rhodamine-based sensor FR for specific fluorescent response of Cys and simultaneously colorimetric detection of H2S, in which the aldehyde and fluorine groups act as response sites. Sensor FR displays significant fluorescence enhancement at 565 nm toward Cys with high selectivity and low detection limits (49 nM) due to the low background fluorescent signal of the spirocyclic closed-state in Rhodamine structure. Meantime, after treatment of H2S, the color of the sensor changes significantly from colorless to blue-purple, which can be used as a visual colorimetric method to detect H2S. These response mechanisms were systematically characterized by 1H NMR and Mass spectrometry. Finally, sensor FR could be used to monitor exogenous and endogenous of intracellular Cys changes.

A single-wavelength excited NIR fluorescence probe for distinguishing GSH/H2S and Cys/Hcy in living cells and zebrafish through separated dual-channels

Biothiols and hydrogen sulfide, as critical sulfur-containing reactive substances, serve essential functions in various human pathological processes, making it challenging to simultaneously distinguish them due to their similar reactivity and structures (-SH). Here, we rationalized the development of a single-wavelength excitation near-infrared (NIR) fluorescence probe, FC-NBD, for distinguishing GSH/H₂S and Cys/Hcy by separated fluorescence dual channels. In this probe, FC-NBD, composed of coumarin-benzopyrylium derivatives linked with nitro benzoxadiazole (NBD) via ether bonds, could quantitatively and selectively distinguish GSH/H₂S and Cys/Hcy with a low limit of detection (LOD) of 0.199/0.177 μM and 0.106/0.076 μM, respectively. As expected, under single-wavelength excitation (470 nm), FC-NBD demonstrated distinctly separable green and NIR fluorescence emissions towards Cys/Hcy at 550 and 660 nm, but only exhibited a noticeable NIR fluorescence emission towards GSH/H₂S at 660 nm. Moreover, FC-NBD could simultaneously visualize and discriminate GSH/H₂S and Cys/Hcy in living cells as well as zebrafish through green and NIR channels at a single excitation wavelength.

Rational design of a dual-reactive probe for imaging the biogenesis of both H2S and GSH from L-Cys rather than D-Cys in live cells

Biothiols and their interconversion are involved in cellular redox homestasis as well as many physiological processes. Here, a dual-reactive dual-quenching fluorescent probe was rationally developed based on thiolysis reactions of...

Substitution-rearrangement-cyclization strategy to construct fluorescent probe for multicolor discriminative analysis biothiols in cells and zebrafish

Discriminative detection of biothiols (Cysteine, homocysteine and glutathione) is of great significance to clarificate their complex physiological processes, the occurrence and development of related diseases. However, similar structure and reactivity among such species pose huge challenges in developing fluorescent probes to distinguish among of them. In this work, a dual-site probe CTT reacted with the analytes to regulate molecular conjugation through substitution-rearrangement-cyclization strategy, utilizing a multi-channel signal combination mode to realize the distinguishing detection of the three biothiols. Cell and zebrafish imaging experiments sufficiently demonstrated that CTT could semiquantify biothiols, which will provide valuable chemical tool for elucidating the complex biological functions of biothiols.

Sulphide activity-dependent multicolor emission dye and its applications in in vivo imaging

Reactive sulfur species (RSS) play pivotal roles in various pathological and physiological processes. There exists an intricate relevance in generation and metabolism among these substances. Although they are nucleophilic, there are still some differences in their reactivity. There are many methods to detect them by using reactive fluorescent probes, but the systematic study of their reactivity is still lacking. In our study, we designed a multiple reaction site fluorescent probe based on benzene conjugated benzopyrylium and NBD. The study revealed that besides both biothiols and hydrogen sulfide, sulfur dioxide (SO₂) can cleave the ether bond. There are two reaction forms for GSH with low reactivity: cutting the ether bond and adding the conjugated double bond of benzopyrylium. Nevertheless, Cys/Hcy with higher activity can further rearrange with NBD after cutting the ether bond. In addition, SO₂ can not only cleave the ether bond, but also continue to add the conjugated double bond of benzopyrylium. The above processes lead to multicolor emission of the probe, thus realizing the characteristic analysis of different sulfides. Thus the probe can be used for the detection of sulfide in mitochondria, and further for the imaging of sulfide in cells and zebrafish. This effective analysis method will provide a broad application prospect for practical applications.

NBD-based synthetic probes for sensing small molecules and proteins: design, sensing mechanisms and biological applications

Compounds with a nitrobenzoxadiazole (NBD) skeleton exhibit prominent useful properties including environmental sensitivity, high reactivity toward amines and biothiols (including H2S) accompanied by distinct colorimetric and fluorescent changes, fluorescence-quenching ability, and small size, all of which facilitate biomolecular sensing and self-assembly. Amines are important biological nucleophiles, and the unique activity of NBD ethers with amines has allowed for site-specific protein labelling and for the detection of enzyme activities. Both H2S and biothiols are involved in a wide range of physiological processes in mammals, and misregulation of these small molecules is associated with numerous diseases including cancers. In this review, we focus on NBD-based synthetic probes as advanced chemical tools for biomolecular sensing. Specifically, we discuss the sensing mechanisms and selectivity of the probes, the design strategies for multi-reactable multi-quenching probes, and the associated biological applications of these important constructs. We also highlight self-assembled NBD-based probes and outline future directions for NBD-based chemosensors. We hope that this comprehensive review will facilitate the development of future probes for investigating and understanding different biological processes and aid the development of potential theranostic agents.

Development of a piperazinyl-NBD-based fluorescent probe and its dual-channel detection for hydrogen sulfide

To selectively detect H₂S based on the thiolysis reaction of 7-nitro-1,2,3-benzoxadiazole (NBD), amines attracted increasing attention since NBD amine is regarded as a new H₂S reaction site. Herein, a novel fluorescent probe, triphenylamine piperazine NBD (TPA-Pz-NBD), was developed. The results showed that it exhibited high selectivity towards H₂S via fluorescence spectroscopy and solution color. Furthermore, TPA-Pz-NBD not only detected H₂S by a dual-channel, turn-on fluorescence signal at 500 nm and turn-off fluorescence signal at 545 nm, respectively, but also displayed a wide detection range of 0-125 μM. In addition, living cell imaging results indicated that TPA-Pz-NBD holds potential for the detection of intracellular H₂S.

A multifunctional fluorescent probe for visualizing H2S in wastewater with portable smartphone via fluorescent paper strip and sensing GSH in vivo

In this paper, a bifunctional tri-site fluorescent probe was designed for the first time not only for visualization and quantitative analysis of sensing H₂S in wastewater by coupling paper strip and smartphone (Color recognizer, Xiyi Technology) but also for sensitively monitoring GSH in living cells, which relied on different emission channels and the pH of solutions. The recognition properties of GH towards H₂S/GSH were satisfactorily demonstrated through fluorescence, UV-vis, ¹H NMR and DFT calculations. More importantly, integrated with the paper strip, portable smartphone-sensing platform with a color recognizer app would accomplish cost-effective and rapid assays for colorimetric water quality testing, which displayed huge application potential in fields of environmental monitoring.

Recent advances in probe design to detect reactive sulfur species and in the chemical reactions employed for fluorescence switching

Reactive sulfur species, including hydrogen sulfide, hydropersulfide, and polysulfide, have many roles in biological systems. For example, hydrogen sulfide is involved in the relaxation of vascular smooth muscles and mediation of neurotransmission, while sulfane sulfur, which exists in cysteine persulfide/polysulfide, and glutathione persulfide/polysulfide, is involved in physiological antioxidation and cytoprotection mechanisms. Fluorescence imaging is well suited for real-time monitoring of reactive sulfur species in living cells, and many fluorescent probes for reactive sulfur species have been reported. In such probes, the choice of detection chemistry is extremely important, not only to achieve effective fluorescence switching and high selectivity, but also because the reactions may be applicable to develop other chemical tools, such as reactive sulfur species donors/scavengers. Here, we present an overview of both widely used and recently developed fluorescent probes for reactive sulfur species, focusing especially on the chemical reactions employed in them for fluorescence switching. We also briefly introduce some applications of fluorescent probes for hydrogen sulfide and sulfane sulfur.

Establishment of a Customizable Fluorescent Probe Platform for the Organelle-Targeted Bioactive Species Detection

A customizable fluorescent probe platform that can be used to detect various bioactive analytes offers significant potential for engineering a wide range of bioprobes with diverse sensing and imaging functions. Here, we show a facile and innovative strategy for introducing cis-amino-proline as a carrier scaffold, which is appended with three specific functional groups: a target group, a water-soluble group, and fluorophores with triggers. The potency of the designed strategy could be customized to generate variable multifunctional fluorescent probes for detecting bioactive species of interest, including reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive sulfur species (RSS), ROS/RSS, and even enzymes. We designed and synthesized five representative water-soluble and organelle-targeted compounds, PMB, PMN, PMD, PRB, and PME, with emission wavelengths of these fluorescent probes varying from blue to red (465, 480, 535, 550, 565, and 640 nm). This strategy could be exemplified by its application to develop a mitochondria-/lysosome-targeting multifunctional fluorescent probe capable of imaging bioactive species of interest in live cells and nude mice.

Construction of ratiometric hydrogen sulfide probe with two reaction sites and its applications in solution and in live cells

Hydrogen sulfide (H₂S), as the third multifunctional signaling biomolecule, it acts as a neuromodulator in the human brain and is recognized as an important gas transmitter in human physiology. The abnormal concentrations of H₂S in human cells can result in several common diseases. Therefore, accurate, fast, and reliable methodologies are required for measuring the in vitro and in vivo concentrations of H₂S to further investigate its function. In this study, a novel DR-SO₂N₃ fluorescent probe containing the fluorophore Disperse Red 277 and a sulfonyl azide group was developed and exploited based on the structural characteristic of Disperse Red 277 that contains the active site easily can be attacked by HS^-. Therefore, this probe featured two reaction sites that involved the reduction and Michael addition of H₂S and exhibited rapid ratiometric fluorescence changes and high selectivity towards H₂S with a 619-fold enhancement factor. Further, the density functional theory (DFT)/time-dependent density functional theory (TDDFT) studies are conducted to understand the photophysical properties of DR-SO₂N₃ and the final product DRHS-SO₂NH₂, which makes the proposed mechanism more reasonable. Furthermore, the probe was successfully applied for the ratiometric fluorescence imaging of exogenous H₂S in living cells.

Discrimination of cysteamine from mercapto amino acids through isoelectric point-mediated surface ligand exchange of β-cyclodextrin-modified gold nanoparticles

A pI-mediated R6G-β-CD@AuNPs system was designed for the first time for the discrimination of CA from GSH/Cys/Hcy in human serum samples.

Endogenous Cys-Assisted GSH@AgNCs-rGO Nanoprobe for Real-Time Monitoring of Dynamic Change in GSH Levels Regulated by Natural Drug

Glutathione (GSH) levels are closely related to the homeostasis of redox state which directly affects human disease occurrence by regulating cell apoptosis. Hence, real-time monitoring of dynamic changes in intracellular GSH levels is urgently needed for disease early diagnosis and evaluation of therapy efficiency. In this study, an endogenous cysteine (Cys)-assisted detection system based on GSH@AgNCs and reduced graphene oxide (rGO) with high sensitivity and specificity was developed for GSH detection. Compared with GSH, GSH@AgNCs with weaker affinity and bonding force was quite easier to extrude from the rGO surface when competing against GSH, leading to the obvious change in fluorescence signal. This phenomenon was termed as "a crowding out effect". Furthermore, the presence of Cys can improve GSH assay sensitivity by enhancing the quenching efficiency of rGO on the GSH@AgNCs. In vitro assay indicated that the efficiency of fluorescence recovery was positively related with GSH concentration in the range from 0 to 10 mM. In addition, the method was employed for real-time monitoring of the dynamic changes in GSH levels regulated by natural drugs. The imaging results showed that the natural compound 3 (C3) can downregulate GSH levels in HepG2 cells, which was accompanied by reactive oxygen species (ROS) release and apoptosis induction. Finally, the method was used to monitor the change of GSH levels in serum samples with chronic hepatitis B (CHB) infection. The results demonstrated that the occurrence and development of CHB may be positively correlated with GSH levels to some extent. Overall, the above results demonstrate the potential application of this new nanosystem in anticancer natural drug screening and clinical assay regarding GSH levels.

A ratiometric fluorescent probe for simultaneous detection of Cys/Hcy and GSH

GSH, Cys and Hcy are the main intracellular thiols to play crucial roles in human pathologies. It is a great challenge to differentiate these three biothiols using single molecular fluorescent probes due to their close similarities in chemical structure and reactivity. In this work, based on the fluorescence resonance energy transfer (FRET) mechanism, a fluorescent probe CPR was constructed to simultaneously distinguish GSH and Cys/Hcy by means of ratiometric fluorescence changes: from red (584 nm) to green (542 nm) for GSH and from red (584 nm) to blue (472 nm) for Cys/Hcy. This probe showed high sensitivity and selectivity with low limits of detection (LOD = 12 nm, 13 nm and 30 nm for Cys, Hcy and GSH, respectively) and was capable of imaging GSH and Cys/Hcy in cells and zebrafish in a ratiometric manner with low toxicity.

Dual-quenching NBD-based fluorescent probes for separate detection of H2S and Cys/Hcy in living cells

Dual-quenching fluorescent probes based on thiolysis of NBD thioether/ether/amine for fast and separate detection of H₂S and Cys/Hcy in living cells were rationally constructed.

A lysosome-targetable fluorescent probe for the simultaneous sensing of Cys/Hcy and GSH from different emission channels

A lysosome-specific fluorescent probe, Lyso-AC, for biothiols was developed by incorporation of a 4-nitrophenol moiety into a coumarin dye. The Cys/Hcy-triggered substitution-rearrangement cascade, and GSH-induced substitution reaction lead to the corresponding blue emissive amino-coumarin and yellow emissive thiol-coumarin, thereby enabling Cys/Hcy and GSH detection from distinct emissions. Moreover, this probe displayed an excellent lysosome targeting property with a 0.92 Pearson's colocalization coefficient by using Neutral Red as a reference. Significantly, biological experiments indicated Lyso-AC has the potential to monitor lysosome Cys/Hcy and GSH simultaneously in living HeLa cells from distinct emissions.

A novel lysosome targetable fluorescent probe, Lyso-AC, that can selectively sense lysosome Cys/Hcy and GSH from different emission channels was developed.

Functional synthetic probes for selective targeting and multi-analyte detection and imaging

In contrast to the classical design of a probe with one binding site to target one specific analyte, probes with multiple interaction sites or, alternatively, with single sites promoting tandem reactions to target one or multiple analytes, have been developed. They have been used in addressing the inherent challenges of selective targeting in the presence of structurally similar compounds and in complex matrices, as well as the visualization of the in vivo interaction or crosstalk between the analytes. Examples of analytes include reactive sulfur species, reactive oxygen species, nucleotides and enzymes. This review focuses on recent innovations in probe design, detection mechanisms and the investigation of biological processes. The vision is to promote the ongoing development of fluorescent probes to enable deeper insight into the physiology of bioactive analytes.