Real-time fluorometric assay for acetylcholinesterase activity and inhibitor screening through the pyrene probe monomer-excimer transition

Two Fluorescent Probes for Recognition of Acetylcholinesterase: Design, Synthesis, and Comparative Evaluation

In this study, two "on-off" probes (BF2-cur-Ben and BF2-cur-But) recognizing acetylcholinesterase (AChE) were designed and synthesized. The obtained probes can achieve recognition of AChE with good selectivity and pH-independence with a linear range of 0.5~7 U/mL and 0.5~25 U/mL respectively. BF2-cur-Ben has a lower limit of detection (LOD) (0.031 U/mL), higher enzyme affinity (Km = 16 ± 1.6 μM), and higher inhibitor sensitivity. A responsive mechanism of the probes for AChE was proposed based on HPLC and mass spectra (MS) experiments, as well as calculations. In molecular simulation, BF2-cur-Ben forms more hydrogen bonds (seven, while BF2-cur-But has only four) and thus has a more stable enzyme affinity, which is mirrored by the results of the comparison of Km values. These two probes could enable recognition of intracellular AChE and probe BF2-cur-Ben has superior cell membrane penetration due to its higher log p value. These probes can monitor the overexpression of AChE during apoptosis of lung cancer cells. The ability of BF2-cur-Ben to monitor AChE in vivo was confirmed by a zebrafish experiment.

A S-substituted Nile Blue-derived bifunctional near-infrared fluorescent probe for in vivo carboxylesterase imaging-guided photodynamic therapy of hepatocellular carcinoma

The development of theranostic probes that integrate both diagnostic and therapeutic functions still remains an intractable challenge in precise cancer treatment. Herein, a novel bifunctional near-infrared (NIR) fluorescent probe (CEP1) for carboxylesterase (CE) imaging and photodynamic therapy (PDT) of hepatocellular carcinoma (HCC) has been firstly developed and successfully applied in vitro and in vivo. The probe was constructed by introducing carbamate as both the recognition unit and the fluorescence quenching unit into the fluorophore S-substituted Nile Blue (ENBS) via a self-eliminating spacer with substituted chloride. It can be activated by CE and hydrolyzed into fluorescent ENBS, which recover fluorescence at about 700 nm, and can generate superoxide radical anions under NIR irradiation. Additionally, the probe could effectively distinguish tumor cells from normal cells by CE imaging of live cells. Furthermore, it could achieve CE imaging in vivo and significantly inhibits tumor growth by imaging-guided PDT. Therefore, this study offers a promising and attractive platform for activatable imaging-guided PDT of HCC.

Construction of a Label-Free Ratiometric Biosensor Based on Target Recycling Amplification and Hg-ZnSe QDs for Assay of BChE and OPs

Herein, we constructed a label-free ratiometric fluorescence biosensing strategy for the determination of butyrylcholinesterase (BChE) activity and organophosphorus (OPs) concentration. BChE promoted the hydrolysis of iodized s-butyrylthiocholine (BTCh) into a reducing substance thiocholine, which can decompose CoOOH nanosheets (CoOOH NSs) to Co2+. Subsequently, the single-stranded DNA (ssDNA) on the surface of CoOOH NSs was released. Then, ssDNA hybridized with hairpin DNA (h-DNA) and triggered the target recycling amplification process, producing large amounts of G-quadruplex. After adding thioflavin T (ThT), the target BChE was converted into activatable G-quadruplex/ThT with an amplified yellow fluorescence signal. The addition of OPs could significantly inhibit the hydrolysis of BTCh by BChE and thus unable to produce the yellow fluorescence G-quadruplex/ThT complex. Throughout the entire process, the fluorescence intensity of Hg-ZnSe QDs as a reference signal remained unchanged at 630 nm. Furthermore, this work provided an effective approach for detecting the BChE activity in serum samples and OPs in fruits and vegetables.

Induced Self-Assembly and Disassembly of Alkynylplatinum(II) 2,6-Bis(benzimidazol-2'-yl)pyridine Complexes with Charge Reversal Properties: "Proof-of-Principle" Demonstration of Ratiometric Förster Resonance Energy Transfer Sensing of pH

A series of pH-responsive alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) complexes with charge-reversal properties was synthesized, and the supramolecular assemblies between conjugated polyelectrolyte, PFP-OSO₃^-, and [Pt{bzimpy(TEG)₂}{C≡C-C₆H₃-(COOH)₂-3,5}]Cl (1) have been studied using UV-vis absorption, emission, and resonance light scattering (RLS) spectroscopy. An efficient Förster resonance energy transfer (FRET) from PFP-OSO₃^- donor to the aggregated 1 as acceptor with the aid of Pt(II)···Pt(II) interactions has been presented, which leads to a growth of triplet metal-metal-to-ligand charge transfer (³MMLCT) emission in the low-energy red region. The two-component PFP-OSO₃^--1 ensemble was then exploited as a "proof-of-principle" concept strategy for pH sensing by tracking the ratiometric emission changes. With the aid of judicious molecular design on the pH-driven charge-reversal property, the polyelectrolyte-induced self-assembly and the FRET from PFP-OSO₃^- to the platinum(II) aggregates have been modulated. Together with its excellent reversibility and photostability, the extra stability provided by the Pt(II)···Pt(II) and π-π stacking interactions on top of the electrostatic and hydrophobic interactions existing in polyelectrolye-complex assemblies has led to a selective and sensitive pH sensing assay.

Fluorescent probes for the detection of chemical warfare agents

Chemical warfare agents (CWAs) are toxic chemicals that have been intentionally developed for targeted and deadly use on humans. Although intended for military targets, the use of CWAs more often than not results in mass civilian casualties. To prevent further atrocities from occurring during conflicts, a global ban was implemented through the chemical weapons convention, with the aim of eliminating the development, stockpiling, and use of CWAs. Unfortunately, because of their relatively low cost, ease of manufacture and effectiveness on mass populations, CWAs still exist in today's world. CWAs have been used in several recent terrorist-related incidents and conflicts (e.g., Syria). Therefore, they continue to remain serious threats to public health and safety and to global peace and stability. Analytical methods that can accurately detect CWAs are essential to global security measures and for forensic analysis. Small molecule fluorescent probes have emerged as attractive chemical tools for CWA detection, due to their simplicity, ease of use, excellent selectivity and high sensitivity, as well as their ability to be translated into handheld devices. This includes the ability to non-invasively image CWA distribution within living systems (in vitro and in vivo) to permit in-depth evaluation of their biological interactions and allow potential identification of therapeutic countermeasures. In this review, we provide an overview of the various reported fluorescent probes that have been designed for the detection of CWAs. The mechanism for CWA detection, change in optical output and application for each fluorescent probe are described in detail. The limitations and challenges of currently developed fluorescent probes are discussed providing insight into the future development of this research area. We hope the information provided in this review will give readers a clear understanding of how to design a fluorescent probe for the detection of a specific CWA. We anticipate that this will advance our security systems and provide new tools for environmental and toxicology monitoring.

Recent Advances in Excimer-Based Fluorescence Probes for Biological Applications

The fluorescent probe is a powerful tool for biological sensing and optical imaging, which can directly display analytes at the molecular level. It provides not only direct visualization of biological structures and processes, but also the capability of drug delivery systems regarding the target therapy. Conventional fluorescent probes are mainly based on monomer emission which has two distinguishing shortcomings in practice: small Stokes shifts and short lifetimes. Compared with monomer-based emission, excimer-based fluorescent probes have large Stokes shifts and long lifetimes which benefit biological applications. Recent progress in excimer-based fluorescent sensors (organic small molecules only) for biological applications are highlighted in this review, including materials and mechanisms as well as their representative applications. The progress suggests that excimer-based fluorescent probes have advantages and potential for bioanalytical applications.

Rapid detection of rocuronium based on host/guest complex between a pyrene derivative and sugammadex

Rocuronium is widely used in surgery as a neuromuscular relaxant, but it has been difficult to accurately control its specific dosage in clinical operation. Therefore, the development of fast and instant rocuronium detection methods has important application value for reducing risks and safeguarding health. In this study, N, N, N-trimethyl-4-(pyrene-1-butyl)-ammonium bromide (PyBTA) was designed as a probe to detect rocuronium rapidly. The method relied on replacing PyBTA in sugammadex with rocuronium to induce changes in fluorescence intensity of PyBTA, thereby realizing quantitative detection. Its sensing performance and detection mechanism were explored systematically by spectroscopy. The linear range of this method was 0.5-10 μM and the detection limit of it was 0.3 μM. In addition, we confirmed that the host-guest interaction among PyBTA, sugammadex, and rocuronium was mainly driven by electrostatic and hydrophobic interactions.

Fluorescent Marangoni Flows under Quasi-Steady Conditions

Marangoni flow is among the most intriguing effects in complex fluids and interfacial science. We report here on a fluorescent surfactant that enables to monitor Marangoni flows under quasi-steady conditions, without the need of invasive tracers. The Marangoni zone is clearly visible, and its dynamics can be quantitatively probed both at the air-water interface and within the bulk. In particular, we show that the Marangoni zone exhibits unexpected dependencies with the container size and water depth with the pyrene-tailed surfactant. Additionally, recirculation flows are evidenced by fluorescence near the bottom of the container. This fluorescent probe may find other useful applications in deciphering the complexity of the ubiquitous Marangoni effect.

Fluorine-free synthesis of Ti3C2 MQDs for smartphone-based fluorescent and colorimetric determination of acetylcholinesterase and organophosphorus pesticides

Ti₃C₂ MQDs were synthesized using an effective fluorine-free method with excitation/emission maxima at 390/490 nm and a fluorescence quantum yield of 11.78%. In contrast to the traditional, hazardous, and time-consuming process of HF pretreatment, our fluorine-free method is safe and simple. Acetylcholinesterase (AChE) could catalyze the hydrolysis of acetylthiocholine (ATCh) to produce thiocholine which was further reacted with Ehrman's reagent and decomposed to form a yellow product 2-nitro-5-thiobenate anion (TNB). Due to the obvious overlap between the excitation spectrum of Ti₃C₂ MQDs and the absorption spectrum of TNB, AChE catalyzed the hydrolysis of substrate DTNB/ATCh to form TNB, which can effectively quench the fluorescence of Ti₃C₂ MQDs through the inner filter effect (IFE). However, the presence of organophosphorus (OPs) inhibited the activity of AChE, leading to a less expressed IFE and increasing recovery of fluorescence. This was used for the quantification of OPs with a detection limit of 0.20 μg·L^-1. Moreover, with the constant increase of AChE activity, the color of the reaction system changed visibly from colorless to yellow, and then from yellow to colorless with further continuous addition of OPs. A colorimetric detection with a paper-based sensor of AChE activity and OP concentration was also fabricated by analyzing changes in RGB value using a smartphone APP. In this work, we proposed an effective fluorescence/colorimetric two-mode detection method, which opened a new horizon to detect other targets.

A molecular approach to rationally constructing specific fluorogenic substrates for the detection of acetylcholinesterase activity in live cells, mice brains and tissues

Acetylcholinesterase (AChE) is an extremely critical hydrolase tightly associated with neurological diseases. Currently, developing specific substrates for imaging AChE activity still remains a great challenge due to the interference from butyrylcholinesterase (BChE) and carboxylesterase (CE). Herein, we propose an approach to designing specific substrates for AChE detection by combining dimethylcarbamate choline with a self-immolative scaffold. The representative P10 can effectively eliminate the interference from CE and BChE. The high specificity of P10 has been proved via imaging AChE activity in cells. Moreover, P10 can also be used to successfully map AChE activity in different regions of a normal mouse brain, which may provide important data for AChE evaluation in clinical studies. Such a rational and effective approach can also provide a solid basis for designing probes with different properties to study AChE in biosystems and another way to design specific substrates for other enzymes.

Near-Infrared Fluorescence Probe for Evaluating Acetylcholinesterase Activity in PC12 Cells and In Situ Tracing AChE Distribution in Zebrafish

Acetylcholinesterase (AChE) plays crucial roles in numerous physiological processes such as cell differentiation, cell apoptosis, and nerve tissue developments. Hence, it is highly necessary to design a fluorescent probe for monitoring AChE activity in complex living organisms. In this work, a near-infrared (NIR) off-on probe (CyN) was developed for AChE detection. CyN was exactly synthesized by introducing an N,N-dimethyl carbamyl moiety to hemicyanine (CyOH). AChE can "light up" strong NIR fluorescence through a cleavage special ester bond and transform CyN into CyOH. Moreover, CyN was qualified for imaging the dynamic change of AChE activity in PC12 cells with retinoic acid or hypoxia stimulation. In particular, the probe has been successfully applied for in situ tracing the intact distribution of AChE in living zebrafish. The observations indicate that major occurrence sites of endogenic AChE on zebrafish are the yolk sac and neuromasts. Overall, CyN shows great potential for use in AChE-related physiological studies.

Colorimetric assay of acetylcholinesterase inhibitor tacrine based on MoO2 nanoparticles as peroxidase mimetics

Molybdenum dichalcogenides MoX₂ (X=S, Se) have been found to possess intrinsic peroxidase-like activity. However, molybdenum oxides (MoO₂) as peroxidase mimetics have not been exploited yet. Herein, MoO₂ nanoparticles were synthesized by a simple hydrothermal method and found to possess the peroxidase-like activity for the first time. MoO₂ nanoparticles could catalyze the oxidation of 3,3',5,5'-tetrametylbenzidine (TMB) by H₂O₂ to produce a blue-color product (oxTMB). The catalytic property and mechanism were investigated by stead-state kinetics experiment and free radicals scavenging experiment, respectively. Acetylcholinesterase (AChE) could catalyze the hydrolysis of acetylthiocholine chloride (ATCh) into thiocholine (TCh), which could reduce oxTMB to decrease the absorbance in solution. In the presence of AChE inhibitor tacrine, the generation of TCh was inhibited and the absorbance was preserved. Based on these properties, a colorimetric assay method was developed for AChE inhibitor tacrine. This work not only broadens the application of the peroxidase mimetics, but also overcome the disadvantages of traditional methods such as expensive, complex and vulnerable to background interference for colorimetric assay of AChE inhibitor.

Copper(II) complex as a turn on fluorescent sensing platform for acetylcholinesterase activity with high sensitivity

Acetylcholinesterase (AChE) is an important enzyme associated with many nervous diseases, demonstrating the great need for smarter sensing platform with improved sensitivity, selectivity and simplified operation. A "turn on" fluorometric assay is described herein for AChE activity detection, according to the specific enzyme catalyzed reaction of acetylcholine (ATCh) by AChE, which generates thiocholine (TCh) as the product. The well-designed fluorescent probe HBTP possesses ESIPT (Excited State Intramolecular Proton Transfer) nature, leading to a larger Stokes shift, which could be quenched upon coordination with Cu²⁺. The fluorescence-silent HBTP-Cu²⁺ complex could be broken by TCh generated from reaction of ATCh with AChE, giving rise to HBTP release which originates from competitive coordination of TCh with Cu²⁺. This complex probe HBTP-Cu²⁺ offers a limit detection as low as 0.02 mU mL^-1, which is lower than most reported literatures. Furthermore, both HBTP-Cu²⁺ and HBTP show little toxicity to live cells and is available in visualizing cellular AChE activity.

Metal coordination polymer induced perylene probe excimer fluorescence and its application in acetylcholinesterase sensing and alpha-fetoprotein immunoassay

Perylene probe excimer fluorescence based acetylcholinesterase sensing and alpha-fetoprotein immunoassay.

Multiresponsive Supramolecular Theranostic Nanoplatform Based on Pillar[5]arene and Diphenylboronic Acid Derivatives for Integrated Glucose Sensing and Insulin Delivery

A closed-loop "smart" insulin delivery system with the capability to mimic pancreatic cells will be highly desirable for diabetes treatment. This study reports a multiple stimuli-responsive insulin delivery platform based on an explicit supramolecular strategy. Self-assembled from a well-designed amphiphilic host-guest complex formed by pillar[5]arene and a diphenylboronic acid derivative and loaded with insulin and glucose oxidase, the obtained insulin-GOx-loaded supramolecular vesicles can selectively recognize glucose, accompanied by the structure disruption and efficient release of the entrapped insulin triggered by the high glucose concentration as well as the in situ generated H₂ O₂ and acid microenvironment during the GOx-promoted specific oxidation of glucose into gluconic acid. Moreover, such a "smart" supramolecular theranostic nanoplatform is able to function as both a glucose sensor and a controlled insulin delivery actuator. In vivo experiments further demonstrate that this smart supramolecular nanocarrier shows fast response to hyperglycemic circumstances and can effectively regulate the glucose levels in a mouse model of type I diabetes.

Electrostatic Modification for Promotion of Flavin-Mediated Oxidation of a Probe for Flavin Detection

Electrostatic effects on the redox photochemistry of synthetic probes (1, 2, and 1-Zn) are examined by adjusting the thermodynamic driving force of their oxidation reactions. The redox photochemistry was simply controlled by introducing a zinc binding site (2,2'-dipicolylamine (DPA)) on the coumarin moiety of probe 2. Zinc complexation produced a positively charged environment on the coumarin (1-Zn), which lowered the electron density of a nearby 9 H-xanthene ring, attenuating the auto-oxidation of 1-Zn by 45 % compared with that of probe 1 at 298 K. The positive net charge of 1-Zn also provided an attractive Coulombic force toward the phosphate of flavin mononucleotide and flavin adenine dinucleotide, which lowered the reduction potential of the electron acceptor (isoalloxazine) and improved intermolecular electron transfer from the 9 H-xanthene ring to isoalloxazine. The flavin-mediated oxidation rate of 1-Zn was increased to 1.5 times that of probe 2. Probe 1-Zn showed highly selective sensing behaviour toward flavins, producing an intense brightness (ϵΦ_F =2.80×10³  m^-1  cm^-1 ) in the long-wavelength regions (λ_max =588 nm) upon flavin-mediated oxidation. Furthermore, probes 1-Zn and 2 were successfully applied to eosinophil imaging and the differential diagnosis of eosinophilia; this demonstrates their use as diagnostic tools.

The Strong Light-Emission Materials in the Aggregated State: What Happens from a Single Molecule to the Collective Group

The strong light emission of organic luminogens in the aggregated state is essential to their applications as optoelectronic materials with good performance. In this review, with respect to the aggregation-induced emission and room-temperature phosphorescence luminogens, the important role of molecular packing modes is highlighted. As demonstrated in the selected examples, the molecular packing status in the aggregate state is affected by many factors, including the molecular configurations, the inherent electronic properties, the special functional groups, and so on. With the consideration of all these parameters, the strong fluorescence and phosphorescence in the aggregated state could be achieved in the rationally designed organic luminogens, providing some guidance for the further development.

Live cell imaging of bacterial cells: Pyrenoylpyrrole-based fluorescence labeling

A novel substituted pyrenoylpyrroles was synthesized by the reaction of pyrenoyl chalcone, TosMIC and methyl iodide under mild condition. All the synthesized compounds were screened for their bioactivity, and the MIC was determined, among which few compounds showed moderate antibacterial activity toward Gram-positive as well as Gram-negative bacteria. Further, cytotoxicity assay ascertained that these compounds were non-toxic to mammalian cells as well. The pyrene chromophore in the synthesized compounds (3a-e) and (5a-e) is responsible for the good photophysical properties which have an absorbance at λ 340 nm and emission at λ 410 nm. Hence, two of the selected novel synthesized compounds with non-cytotoxic nature prospected for bio-imaging of bacterial cells using high-content screening analysis show that the molecule is suitable for microbial imaging in pathological diagnostic studies.

Discovery of a butyrylcholinesterase-specific probe via a structure-based design strategy

We report herein the structure-based design and application of a fluorogenic butyrylcholinesterase probe that could discriminate butyrylcholinesterase from acetylcholinesterase.

Approach Based on Polyelectrolyte-Induced Nanoassemblies for Enhancing Sensitivity of Pyrenyl Probes

We have developed a unique approach for enhancing the sensitivity of pyrenyl probes based on polyelectrolyte-induced nanoassemblies and explored its sensing application toward 2,4,6-trinitrophenol (TNP). The key issue of the method is the formation of the nanoassemblies which possess high-density charges, specific surface area, and inner hydrophobic regions. These properties would help increase the loading of analytes and promote probe-analyte interactions, thereby leading to the prominent enhancement of the sensitivity. In the course of TNP detection, pyrene nanoassemblies can bind TNP efficiently through cooperative noncovalent interactions including electrostatic, π-π stacking, and charge-transfer interactions, resulting in the distinct fluorescent responses of pyrene moieties. This system has excellent selectivity and sensitivity for TNP detection. The detection limit is as low as 5 nM. It may be used for monitoring the TNP concentrations in real-world samples.

A simple fluorescent probe based on a pyrene derivative for rapid detection of protamine and monitoring of trypsin activity

We report the synthesis of a simple pyrene derivative and its application in protamine detection and monitoring of trypsin activity. This assay can be conducted in aqueous solution and features rapid response, visual detection, high sensitivity and selectivity. The limit of detection of protamine was 0.5 μg mL(-1). The IC50 value of a soybean trypsin inhibitor was estimated to be 0.51 U mL(-1).