Self-immolative colorimetric, fluorescent and chemiluminescent chemosensors

Synthetic ratiometric fluorescent probes for detection of ions

Metal cations and anions are essential for versatile physiological processes. Dysregulation of specific ion levels in living organisms is known to have an adverse effect on normal biological events. Owing to the pathophysiological significance of ions, sensitive and selective methods to detect these species in biological systems are in high demand. Because they can be used in methods for precise and quantitative analysis of ions, organic dye-based ratiometric fluorescent probes have been extensively explored in recent years. In this review, recent advances (2015-2019) made in the development and biological applications of synthetic ratiometric fluorescent probes are described. Particular emphasis is given to organic dye-based ratiometric fluorescent probes that are designed to detect biologically important and relevant ions in cells and living organisms. Also, the fundamental principles associated with the design of ratiometric fluorescent probes and perspectives about how to expand their biological applications are discussed.

Thiol-Chromene "Click" Reaction Triggered Self-Immolative for NIR Visualization of Thiol Flux in Physiology and Pathology of Living Cells and Mice

Understanding the pathological process of biological systems can greatly improve the prevention and treatment of diseases. The study of pathological processes has now reached the molecular level, and molecular fluorescent probes have become a powerful tool. Chromene, also known as benzo-pyran molecule, is a structural element of natural products with good biological compatibility and was developed as a fluorescent probe. The thiol-chromene "click" nucleophilic pyran ring-opening reaction allows the quick detection of thiol. In this work, the chromene alcohol can function as an efficient self-immolative spacer, which covalently links NIR fluorophore via a carbonyl ester. Due to its favorable characteristics and superior applicability, the self-immolative amplifier NIR-HMPC achieves the specific, rapid, sensitive, NIR fluorescent detection of thiols. Furthermore, the indoles iodized salt in the system can specifically target thiols in mitochondria. Thus, this probe was used to visualize the fluctuations of thiols during oxidative stress and cell apoptosis, cerebral ischemia reperfusion, demonstrating that it is valuable for elucidating pathophysiology process in living organism. This discovery provides an effective means for studying the pathological process of thiol related diseases.

Reactive Oxygen Correlated Chemiluminescent Imaging of a Semiconducting Polymer Nanoplatform for Monitoring Chemodynamic Therapy

In chemodynamic therapy (CDT), real-time monitoring of reactive oxygen species (ROS) production is critical to reducing the nonspecific damage during CDT and feasibly evaluating the therapeutic response. However, CDT agents that can emit ROS-related signals are rare. Herein, we synthesize a semiconducting polymer nanoplatform (SPN) that can not only produce highly toxic ROS to kill cancer cells but also emit ROS-correlated chemiluminescent signals. Notably, the efficacy of both chemiluminescence and CDT can be significantly enhanced by hemin doping (∼10-fold enhancement for luminescent intensity). Such ROS-dependent chemiluminescence of SPN allows ROS generation within a tumor to be optically monitored during the CDT process. Importantly, SPN establishes an excellent correlation of chemiluminescence intensities with cancer inhibition rates in vitro and in vivo. Thus, our nanoplatform represents the first intelligent strategy that enables chemiluminescence-imaging-monitored CDT, which holds potential in assessing therapeutic responsivity and predicting treatment outcomes in early stages.

An assay for Staphylococcus aureus based on a self-catalytic ampicillin–metal (Fe3+)-organic gels–H2O2 chemiluminescence system with near-zero background noise

A self-catalytic ampicillin–metal (Fe³⁺)-organic gels (AMP–MOGs (Fe))–H₂O₂ CL system, which is not influenced by transition metal ions, was studied.

Proof-of-principle for two-stage photodynamic therapy: hypoxia triggered release of singlet oxygen

Singlet oxygen, which is stored in the form of an endoperoxide, released under hypoxic conditions typically prevalent in most tumors.

A rhodamine based biocompatible chemosensor for Al3+, Cr3+ and Fe3+ ions: extraordinary fluorescence enhancement and a precursor for future chemosensors

A rhodamine based chemosensor, 3-(((2-(3',6'-bis(ethylamino)-2',7'-dimethyl-3-oxospiro[isoindoline-1,9'-xanthen]-2-yl)ethyl)imino)methyl)-2-hydroxy-5-methylbenzaldehyde (HL-CHO), has been developed for the detection of Al3+, Cr3+ and Fe3+ ions. The absorbance of HL-CHO at 528 nm increases significantly in HEPES buffer in methanol : water (9 : 1, v/v) (pH 7.4) in the presence of Al3+, Cr3+ and Fe3+ ions with the alteration of solution color from colorless to pink. The fluorescence intensity of the probe at 550 nm enhances by 1465, 588 and 800 fold in the presence of Al3+, Cr3+ and Fe3+ ions, respectively. To the best of our knowledge, this huge increase in fluorescence intensity with Al3+ and Cr3+ has not been observed for other rhodamine based chemosensing systems. The weak fluorescence and no coloration of the probe are due to the existence of a spirolactam ring. The trivalent cations induce the opening of the spirolactam ring and consequently change the color and the fluorescence intensity followed by the 1 : 1 complex formation with HL-CHO which are evident from Job's analysis, ESI mass spectral analysis and elemental analysis. The quantum yield and lifetime of HL-CHO have increased considerably in the presence of the trivalent cations. The high sensitivity of the probe towards all the cations is evident from the nM order of LOD values. This has been used in living cell imaging studies with the human neuroblastoma SH-SY5Y cell line. Having appended -CHO groups for Schiff-base condensation with other amines, HL-CHO could be a potential precursor for future chemosensors.

Recent Progress of BODIPY Dyes With Aggregation-Induced Emission

With the development of organic optoelectronic materials and bioimaging technology, to exploit organic luminescent materials with high luminescent efficiency in aggregation-state has become a research hotspot. BODIPYs have become one of the research objects of this kind of material because of their obvious advantages. This review focuses on the design and synthesis of AIE-type BODIPYs, the mechanism of AIE properties and their applications in recent years. Through classification, analysis, and summary, this review aims to explore the structure-activity relationship of AIE-type BODIPYs and to provide ideas for the further design and potential applications of AIE-active fluorescent materials.

Activity-Based Optical Sensing Enabled by Self-Immolative Scaffolds: Monitoring of Release Events by Fluorescence or Chemiluminescence Output

Functional molecular scaffolds comprised of self-immolative adaptors are being used in widespread applications in the fields of enzyme activity analyses, signal amplification, and bioimaging. Optically detected chemical probes are very promising compounds for sensing and diagnosis, since they present several attractive features such as high specificity, low detection limits, fast response times, and technical simplicity. During the last two decades, we have developed several distinct molecular scaffolds that harness the self-immolative disassembly feature of these adaptors to amplify chromogenic output for diagnosis and drug delivery applications. In order to study the molecular behavior of the various amplification systems, an optical output, used to monitor the progress of the disassembly pattern, was required. Therefore, over the course of our research, diverse molecular scaffolds that produce an optical signal in response to a disassembly step, were evaluated. These optically active scaffolds have been incorporated into self-immolative dendrimers and self-immolative polymers to implement unique disassembly properties that result with linear and exponential signal amplification capabilities. In addition, some scaffolds, aimed for linker technology, were used in delivery systems to monitor release of drug molecules. The optical signal used to monitor the release event could be produced by analysis of reporter molecules with chromogenic or fluorogenic properties. Recently, we have also developed molecular scaffolds modified to produce a chemiluminescent signal to monitor the self-immolative disassembly step. The main advantage of these scaffolds over others is the use of chemiluminescence as an output signal. It is well-known that chemiluminescence is considered as one the most sensitive diagnostic methods due to its high signal-to-noise ratio. The unique structures of the self-immolative chemiluminescence scaffolds have been used in the design of three different distinctive concepts: self-immolative chemiluminescence polymers, auto-inductive amplification systems with chemiluminescence signal and monitoring of drug release by a chemiluminescence output. Furthermore, we reported the design and synthesis of the first theranostic prodrug for the monitoring of drug release achieved by a chemiluminescence mode of action. Quinone-methide elimination has proven to serve as a valuable functional tool for composing molecular scaffolds with self-immolative capabilities. Such scaffolds function as molecular adaptors that can almost simultaneously release a target molecule with an accompanied emission of a light signal that is used to monitor the release event. We anticipate that these self-immolative scaffolds will continue to find utility as functional linkers in various chemical and biological research areas such as drug delivery, theranostic applications, and as molecular sensors with signal amplification.

Synthesis of Unzipping Polyester and a Study of its Photochemistry

Preparation of an unzipping polyester is reported. The monomer was prepared from benzoic acid in a four-step sequence. Step growth polymerization of the monomer provides the target polymer. Efficient depolymerization upon irradiation at 254 nm was confirmed with a quantum yield of >0.8. The photolysis mechanism was investigated, and the results of radical trapping experiments are consistent with an initial Norrish type I like homolysis followed by a radical mediated depropagation reaction driven by aromatization.

Design Principles, Sensing Mechanisms, and Applications of Highly Specific Fluorescent Probes for HOCl/OCl

Hypochlorous acid/hypochlorite (HOCl/OCl^-), one of the most important reactive oxygen species (ROS), plays vital roles in various physiological and pathological processes. At normal concentrations, OCl^- acts as part of an immune defense system by destroying invasive bacteria and pathogens. However, nonproperly located or excessive amounts of OCl^- are related to many diseases, including cancers. Thus, detection of OCl^- has great importance. Owing to their high sensitivities, selectivities, fast response times, technical simplicities, and high temporal and spatial resolution, fluorescent probes are powerful tools for in vitro and in vivo sensing of target substances. This Account focuses on the development of new chemosensors for detection of OCl^-, which operate by undergoing a chemical reaction with this ROS in conjunction with a change in emission properties. As part of the presentation, we first introduce several important factors that need to be considered in the design of fluorescent chemosensors for OCl^-, including fluorophores, reaction groups, cosolvents, and buffers. Discussion here revolves around the need to select fluorophores that resist oxidation by OCl^-. As well, attention is given to the sensitivities and selectivities of groups in the sensors that react with OCl^- to trigger a fluorescence response. Moreover, well-known reaction groups, which react with highly reactive ROS (hROS), have been redesigned to be specific for OCl^-. In addition, it is pointed out that several cosolvents and buffers such as DMSO and HEPES are not suitable for use in systems for the detection of OCl^- because they are readily oxidized by this ROS. We further discuss recent investigations carried out by us and others aimed at the development of fluorescent probes for in vitro and in vivo detection of OCl^-. These efforts led to the new "dual lock" strategy for designing OCl^- chemosensors as well as several new specific reaction groups such as imidazoline-2-thiones and imidazoline-2-boranes. Probes created using this strategy and the new reacting groups have been successfully applied to imaging exogenous and endogenous OCl^- in live cells and/or tissues. The design concepts and strategies emanating from our studies of fluorescent OCl^- probes have provided insight into the general field of fluorescent probes. Despite the progress made thus far, challenges still remain in developing and applying fluorescent OCl^- probes. For example, more highly specific and sensitive fluorescent OCl^- probes are still in great demand for studies of the biological roles played by OCl^-. Thus, interdisciplinary collaborations of chemists, biologists, and medical practitioners are needed to drive future developments of OCl^- probes for disease diagnosis and drug screening.

A molecular design strategy toward enzyme-activated probes with near-infrared I and II fluorescence for targeted cancer imaging

The advance of cancer imaging requires innovations to establish novel fluorescent scaffolds that are excitable and emit in the near-infrared region with favorable Stokes shifts. Nevertheless, the lack of probes with these optimized optical properties presents a major bottleneck in targeted cancer imaging. By coupling of boron dipyrromethene platforms to enzymic substrates via a self-immolative benzyl thioether linker, we here report a strategy toward enzyme-activated fluorescent probes to satisfy these requirements. This strategy is applicable to generate various BODIPY-based probes across the NIR spectrum via introducing diverse electron-withdrawing substituents at the 3-position of the BODIPY core through a vinylene unit. As expected, such designed probes show advantages of two-channel ratiometric fluorescence and light-up NIR (I and II) emission with large Stokes shifts upon enzyme activation, enabling targeted cancer cell imaging and accurate tumor location by real-time monitoring of enzyme activities. This strategy is promising in engineering activatable molecular probes suitable for precision medicine.

Recognition Moieties of Small Molecular Fluorescent Probes for Bioimaging of Enzymes

Enzymes are a class of important substances for life, and their abnormal levels are associated with many diseases. Thus, great progress has been made in the past decade in detecting and imaging enzymes in living biosystems, and in this respect fluorescent probes combined with confocal microscopy have attracted much attention because of their high sensitivity and unrivaled spatiotemporal resolution. Fluorescent probes are usually composed of three moieties: a signal or fluorophore moiety, a recognition or labeling moiety, and an appropriate linker to connect the two aforementioned moieties. At present, however, research and reviews on enzymatic probes mostly focus on fluorophores and/or linkers, whereas those on the recognition moiety are relatively few. Moreover, current enzymatic probes with some recognition moieties have drawbacks such as poor selectivity, high background fluorescence, or/and low sensitivity and are unsatisfactory for practical applications. Thus, developing new recognition moieties with higher specificity or/and sensitivity to the enzyme of interest is very desirable but still challenging. In this Account, we introduce the recognition moieties of fluorescent probes for several enzymes, including tyrosinase, monoamine oxidase A (MAO-A), nitroreductase (NTR), and aminopeptidases. Highlights are given on how new specific recognition moieties of tyrosinase and MAO-A were designed to eliminate the interference by reactive oxygen species (ROS) and MAO-B, respectively. Here we present four recent examples in which designed fluorescent probes are employed to image enzymes in living biosystems. The first example shows that 3-hydroxyphenyl can serve as a new and more specific recognition moiety than the traditional 4-hydroxyphenyl group for tyrosinase, enabling the development of a highly selective fluorescent probe for imaging of tyrosinase without interference by ROS. The second presents a general design strategy for fluorescent probes specific for an enzyme, which involves combining the characteristic structure of an inhibitor of the target enzyme along with its traditional reactive group as a new recognition moiety, and successfully demonstrates it by selective detection of MAO-A in the presence of its isomeric MAO-B. The third mainly illustrates that 5-nitrothiophen-2-yl alcohol with a stronger electron-donating S atom is a better fluorescence quenching and recognition moiety than 5-nitrofuran-2-yl alcohol for NTR, leading to the development of a highly sensitive method for NTR assay. Lastly, on the basis of known observations, we show that besides the specific interaction with the target, another function of some recognition moieties may be responsible for tuning the fluorescence signal, which is exemplified by the linking of several aminopeptidases' recognition moieties to the free hydroxyl or amino group of different fluorophores. It is our wish that this Account will promote the appearance of more specific recognition moieties and fluorescent probes with excellent properties and that new biofunctions of the enzymes will be uncovered.

Not always what closes best opens better: mesoporous nanoparticles capped with organic gates

Four types of calcined MCM-41 silica nanoparticles, loaded with dyes and capped with different gating ensembles are prepared and characterized. N1 and N2 nanoparticles are loaded with rhodamine 6G and capped with bulky poly(ethylene glycol) derivatives bearing ester groups (1 and 2). N3-N4 nanoparticles are loaded with sulforhodamine B and capped with self-immolative derivatives bearing ester moieties. In the absence of esterase enzyme negligible cargo release from N1, N3 and N4 nanoparticles is observed whereas a remarkable release for N2 is obtained most likely due to the formation of an irregular coating on the outer surface of the nanoparticles. In contrast, a marked delivery is found in N1, N3, and N4 in the presence of esterase enzyme. The delivery rate is related to the hydrophilic/hydrophobic character of the coating shell. The use of hydrophilic poly(ethylene glycol) derivatives as gating ensembles on N1 and N2 enables an easy access of esterase to the ester moieties with subsequent fast cargo release. On the other hand, the presence of a hydrophobic monolayer on N3 and N4 partially hinders esterase enzyme access to the ester groups and the rate of cargo release was decreased.

Target Enzyme-Activated Two-Photon Fluorescent Probes: A Case Study of CYP3A4 Using a Two-Dimensional Design Strategy

The rapid development of fluorescent probes for monitoring target enzymes is still a great challenge owing to the lack of efficient ways to optimize a specific fluorophore. Herein, a practical two-dimensional strategy was designed for the development of an isoform-specific probe for CYP3A4, a key cytochrome P450 isoform responsible for the oxidation of most clinical drugs. In first dimension of the design strategy, a potential two-photon fluorescent substrate (NN) for CYP3A4 was effectively selected using ensemble-based virtual screening. In the second dimension, various substituent groups were introduced into NN to optimize the isoform-selectivity and reactivity. Finally, with ideal selectivity and sensitivity, NEN was successfully applied to the real-time detection of CYP3A4 in living cells and zebrafish. These findings suggested that our strategy is practical for developing an isoform-specific probe for a target enzyme.

Near-Infrared Aggregation-Induced Emission-Active Probe Enables in situ and Long-Term Tracking of Endogenous β-Galactosidase Activity

High-fidelity tracking of specific enzyme activities is critical for the early diagnosis of diseases such as cancers. However, most of the available fluorescent probes are difficult to obtain in situ information because of tending to facile diffusion or inevitably suffering from aggregation-caused quenching (ACQ) effect. In this work, we developed an elaborated near-infrared (NIR) aggregation-induced emission (AIE)-active fluorescent probe, which is composed of a hydrophobic 2-(2-hydroxyphenyl) benzothiazole (HBT) moiety for extending into the NIR wavelength, and a hydrophilic β-galactosidase (β-gal) triggered unit for improving miscibility and guaranteeing its non-emission in aqueous media. This probe is virtually activated by β-gal, and then specific enzymatic turnover would liberate hydrophobic AIE luminogen (AIEgen) QM-HBT-OH. Simultaneously, brightness NIR fluorescent nanoaggregates are in situ generated as a result of the AIE-active process, making on-site the detection of endogenous β-gal activity in living cells. By virtue of the NIR AIE-active performance of enzyme-catalyzed nanoaggregates, QM-HBT-βgal is capable of affording a localizable fluorescence signal and long-term tracking of endogenous β-gal activity. All results demonstrate that the probe QM-HBT-βgal has potential to be a powerful molecular tool to evaluate the biological activity of β-gal, attaining high-fidelity information in preclinical applications.

Tracking CO release in cells via the luminescence of donor molecules and/or their by-products

Carbon monoxide (CO) is a bioactive signalling molecule that is produced endogenously via the breakdown of heme. Beneficial health effects associated with the delivery of CO gas have spurred the development of CO-releasing molecules (CORMs) that can be used to provide specific amounts of the gas. In addition to their potential use as therapeutics, CORMs are needed to provide insight into the biological targets of CO. In this regard, light-activated CO-releasing molecules (photoCORMs), are valuable for examining the effects of localized CO release. Herein we examine luminescent CORMs and photoCORMs that have been reported for tracking CO delivery in cells. A variety of motifs are available that exhibit differing luminescence properties and cover a wide range of wavelengths. Trackable CO donors have been successfully applied to targeting CO delivery to mitochondria, thus demonstrating the feasibility of using such molecules in detailed investigations of the biological roles of CO.

Rational Design of Near-Infrared Aggregation-Induced-Emission-Active Probes: In Situ Mapping of Amyloid-β Plaques with Ultrasensitivity and High-Fidelity

High-fidelity mapping of amyloid-β (Aβ) plaques is critical for the early detection of Alzheimer's disease. However, in vivo probing of Aβ plaques by commercially available thioflavin derivatives (ThT or ThS) has proven to be extremely limited, as evident by the restriction of enrichment quenching effect, low signal-to-noise ( S/ N) ratio, and poor blood-brain barrier (BBB) penetrability. Herein, we demonstrate a rational design strategy of near-infrared (NIR) aggregation-induced emission (AIE)-active probes for Aβ plaques, through introducing a lipophilic π-conjugated thiophene-bridge for extension to NIR wavelength range with enhancement of BBB penetrability, and tuning the substituted position of the sulfonate group for guaranteeing specific hydrophilicity to maintain the fluorescence- off state before binding to Aβ deposition. Probe QM-FN-SO₃ has settled well the AIE dilemma between the lipophilic requirement for longer emission and aggregation behavior from water to protein fibrillogenesis, thus making a breakthrough in high-fidelity feedback on in vivo detection of Aβ plaques with remarkable binding affinity, and serving as an efficient alternative to the commercial probe ThT or ThS.

Molecular Design Strategy to Construct the Near-Infrared Fluorescent Probe for Selectively Sensing Human Cytochrome P450 2J2

Cytochrome P450 2J2 (CYP2J2), a key enzyme responsible for oxidative metabolism of various xenobiotics and endogenous compounds, participates in a diverse array of physiological and pathological processes in humans. Its biological role in tumorigenesis and cancer diagnosis remains poorly understood, owing to the lack of molecular tools suitable for real-time monitoring CYP2J2 in complex biological systems. Using molecular design principles, we were able to modify the distance between the catalytic unit and metabolic recognition moiety, allowing us to develop a CYP2J2 selective fluorescent probe using a near-infrared fluorophore ( E)-2-(2-(6-hydroxy-2, 3-dihydro-1 H-xanthen-4-yl)vinyl)-3,3-dimethyl-1-propyl-3 H-indol-1-ium iodide (HXPI). To improve the reactivity and isoform specificity, a self-immolative linker was introduced to the HXPI derivatives in order to better fit the narrow substrate channel of CYP2J2, the modification effectively shortened the spatial distance between the metabolic moiety ( O-alkyl group) and catalytic center of CYP2J2. After screening a panel of O-alkylated HXPI derivatives, BnXPI displayed the best combination of specificity, sensitivity and applicability for detecting CYP2J2 in vitro and in vivo. Upon O-demethylation by CYP2J2, a self-immolative reaction occurred spontaneously via 1,6-elimination of p-hydroxybenzyl resulting in the release of HXPI. Allowing BnXPI to be successfully used to monitor CYP2J2 activity in real-time for various living systems including cells, tumor tissues, and tumor-bearing animals. In summary, our practical strategy could help the development of a highly specific and broadly applicable tool for monitoring CYP2J2, which offers great promise for exploring the biological functions of CYP2J2 in tumorigenesis.

A FRET probe for the detection of alkylating agents

A fluorogenic FRET probe allows efficient detection of toxic alkylating agents through a self-immolative reaction.

A paper-based chemosensor for highly specific, ultrasensitive, and instantaneous visual detection of toxic phosgene

A chemosensor containing an o-hydroxyaniline unit as the reaction site was developed for colorimetric and fluorimetric detection of phosgene, which showed fast response (15 s), high specificity, and an extremely low detection limit.