Disaggregation is a Mechanism for Emission Turn-On of ortho-Aminomethylphenylboronic Acid-Based Saccharide Sensors

Intermolecular Proton-Coupled Electron Transfer Reactivity from a Persistent Charge-Transfer State for Reductive Photoelectrocatalysis

Interest in applying proton-coupled electron transfer (PCET) reagents in reductive electro- and photocatalysis requires strategies that mitigate the competing hydrogen evolution reaction. Photoexcitation of a PCET donor to a charge-separated state (CSS) can produce a powerful H-atom donor capable of being electrochemically recycled at a comparatively anodic potential corresponding to its ground state. However, the challenge is designing a mediator with a sufficiently long-lived excited state for bimolecular reactivity. Here, we describe a powerful ferrocene-derived photoelectrochemical PCET mediator exhibiting an unusually long-lived CSS (τ ∼ 0.9 μs). In addition to detailed photophysical studies, proof-of-concept stoichiometric and catalytic proton-coupled reductive transformations are presented, which illustrate the promise of this approach.

Water-soluble fluorescent chemosensor for sorbitol based on a dicationic diboronic receptor. Crystal structure and spectroscopic studies

Selective recognition of saccharides by phenylboronic dyes capable of functioning in aqueous conditions is a central topic of modern supramolecular chemistry that impacts analytical sciences and biological chemistry. Herein, a new dicationic diboronic acid structure 11 was synthesized, structurally described by single-crystal X-ray diffraction, and studied in-depth as fluorescent receptor for six saccharides in pure water at pH = 7.4. This dicationic receptor 11 has been designed particularly to respond to sorbitol and involves two convergent and strongly acidified phenyl boronic acids, with a pKa of 6.6, that operate as binding sites. The addition of sorbitol in the micromolar concentration range to receptor 11 induces strong fluorescence change, but in the presence of fructose, mannitol, glucose, lactose and sucrose, only moderate optical changes are observed. This change in emission is attributed to a static complexation photoinduced electron transfer mechanism as evidenced by lifetime experiments and different spectroscopic tools. The diboronic receptor has a high affinity/selectivity to sorbitol (K = 31 800 M-1) over other saccharides including common interfering species such as mannitol and fructose. The results based on 1H, 11B NMR spectroscopy, high-resolution mass spectrometry and density functional theory calculations, support that sorbitol is efficiently bound to 11 in a 1 : 1 mode involving a chelating diboronate-sorbitol complexation. Since the experimental B⋯B distance (5.3 Å) in 11 is very close to the calculated distance from the DFT-optimized complex with sorbitol, the efficient binding is attributed to strong acidification and preorganization of boronic acids. These results highlight the usefulness of a new diboronic acid receptor with a strong ability for fluorescent recognition of sorbitol in physiological conditions.

Sensitive and specific detection of saccharide species based on fluorescence: update from 2016

Increasing evidence supports the critical role of saccharides in various pathophysiological steps of tumor progression, where they regulate tumor proliferation, invasion, hematogenic metastasis, and angiogenesis. The identification and recognition of these saccharides provide a solid foundation for the development of targeted drug preparations, which are however not fully understood due to their complex and similar structures. In order to achieve fluorescence sensing of saccharides, extensive research has been conducted to design molecular probes and nanoparticles made of different materials. This paper aims to provide in-depth discussion of three main topics that cover the current status of the carbohydrate sensing based on the fluorescence sensing mechanism, including a phenylboronic acid-based sensing platform, non-boronic acid entities, as well as an enzyme-based sensing platform. It also highlights efforts made to understand the recognition mechanisms and improve the sensing properties of these systems. Finally, we present the challenge of achieving high selectivity and sensitivity recognition of saccharides, and suggest possible future avenues for exploration.

Probing the Role of the Bridging Nitrogen in the Signaling Mechanism of an Anthracene-Boronic Acid Sugar Sensor and a Different Version of the PET-Based Mechanism

The N-quaternized derivative 5 of the James-Shinkai anthracene-boronic acid fluorescence sugar sensor 1 was prepared to probe the role of the bridging nitrogen in the signaling mechanism of 1. Both 5 and 1 contain positively charged bridging groups NMe⁺ or NH⁺, respectively, but 5 lacks the ability to form the intramolecular ammonium-boronate doubly ionic hydrogen bond present in 1. Receptors 1 and 5 display opposite fluorescence vs pH profiles with a small turn-on effect of the sugar binding to the zwitterion of 5 in contrast to a large effect observed with 1. It is concluded that the ammonium-boronate hydrogen bond is essential for the signaling mechanism of 1. Its possible function is enabling the PET quenching effect by shifting the NH⁺ proton toward boronate anion inside the hydrogen bond, the degree of which is modulated by the ester formation with diols affecting the basicity of boronate anion. This mechanism agrees with observed signaling selectivity of 1 toward a series of di- and polyols of variable structures as well as with the behavior of 1 in buffered D₂O and methanol solvents at controlled pH and provides an addition to the established "loose bolt" mechanism signaling mode essential for receptors with nonpolar fluorophores.

Fluorescence Sensing of Monosaccharides by Bis-boronic Acids Derived from Quinolinium Dicarboxamides: Structural and Spectroscopic Studies

Three new diboronic acid-substituted bisquinolinium salts were synthesized, structurally described by single-crystal X-ray diffraction, and studied in-depth as fluorescent receptors for six monosaccharides and two open-chain polyols in water at physiological pH. The dicationic pyridine-2,6-dicarboxamide-based receptors contain two N-quinolinium rings as the fluorescent units covalently linked to three different isomers of phenylboronic acid (ortho, 2; meta, 3; and para, 4) as chelating binding sites for polyols. Additions of glucose/fructose in the micromolar concentration range to receptors 2 and 3 induce significant fluorescence changes, but in the presence of arabinose, galactose, mannose, and xylose, only modest optical changes are observed. This optical change is attributed to a static photoinduced electron transfer mechanism. The meta-diboronic receptor 3 exhibited a high affinity/selectivity toward glucose (K = 3800 M^-1) over other monosaccharides including common interfering species such as fructose and mannitol. Based on multiple spectroscopic tools, electrospray ionization high-resolution mass spectrometry, crystal structures, and density functional theory calculations, the binding mode between 3 and glucose is proposed as a 1:1 complex with the glucofuranose form involving a cooperative chelating diboronate binding. These results demonstrate the usefulness of a new set of cationic fluorescent diboronic acid receptors with a strong ability for optical recognition of glucose in the sub-millimolar concentration range.

Fluorescent Molecular Logic Gates and Pourbaix Sensors in Polyacrylamide Hydrogels

Polyacrylamide hydrogels formed by free radical polymerisation were formed by entrapping anthracene and 4-amino-1,8-naphthalimide fluorescent logic gates based on photoinduced electron transfer (PET) and/or internal charge transfer (ICT). The non-covalent immobilisation of the molecules in the hydrogels resulted in semi-solid YES, NOT, and AND logic gates. Two molecular AND gates, examples of Pourbaix sensors, were tested in acidic aqueous methanol with ammonium persulfate, a strong oxidant, and displayed greater fluorescence quantum yields than previously reported. The logic hydrogels were exposed to aqueous solutions with chemical inputs, and the fluorescence output response was viewed under 365 nm UV light. All of the molecular logic gates diffuse out of the hydrogels to some extent when placed in solution, particularly those with secondary basic amines. The study exemplifies an effort of taking molecular logic gates from homogeneous solutions into the realm of solid-solution environments. We demonstrate the use of Pourbaix sensors as pE-pH indicators for monitoring oxidative and acidic conditions, notably for excess ammonium persulfate, a reagent used in the polymerisation of SDS-polyacrylamide gels.

Esterase-Activated Precipitating Strategy to Achieve Highly Specific Detection and Long-Term Imaging of Calcium Ions by Aggregation-Induced Phosphorescence Probe

Spatial and temporal monitoring of bioactive targets such as calcium ions is vitally significant for their essential roles in physiological and biochemical functions. Herein, we proposed an esterase-activated precipitating strategy to achieve highly specific identification and long-term bioimaging of calcium ions via lighting up the calcium ions by precipitation using a water-soluble aggregation-induced phosphorescence (AIP) probe. The designed probe CaP2 has an AIP behavior and can be efficiently aggregated by calcium ions through the coupling coordination of carboxylic acid and cyanide groups, which enables it to light up Ca²⁺ by precipitating-triggered phosphorescence. Four hydrophilic groups of tetraethylene glycol were introduced to endow the resulting probe CaP3 with extraordinary water solubility as well as excellent cellular penetration. Only when the probe CaP3 penetrates inside the live cells the existing esterase in cells can activate the probe to be transformed active CaP2 probe selectively binding with calcium ion in the surroundings. The probe was used to further evaluate the imaging of intracellular calcium ions in model organisms. The excellent imaging performance of CaP3 in Arabidopsis thaliana seedling roots demonstrates that CaP3 has the excellent capability of monitoring calcium ions in live-cell imaging, and furthermore CaP3 exhibits much better photostability and thereby greater potential in long-term imaging. This work established a general esterase-activated precipitating strategy to achieve specific detection and bioimaging in situ triggered by esterase in live cells, and established a water-soluble aggregation-induced phosphorescence probe with high selectivity to achieve specific sensing and long-term imaging of calcium ions in live cells.

Molecular Probes, Chemosensors, and Nanosensors for Optical Detection of Biorelevant Molecules and Ions in Aqueous Media and Biofluids

Synthetic molecular probes, chemosensors, and nanosensors used in combination with innovative assay protocols hold great potential for the development of robust, low-cost, and fast-responding sensors that are applicable in biofluids (urine, blood, and saliva). Particularly, the development of sensors for metabolites, neurotransmitters, drugs, and inorganic ions is highly desirable due to a lack of suitable biosensors. In addition, the monitoring and analysis of metabolic and signaling networks in cells and organisms by optical probes and chemosensors is becoming increasingly important in molecular biology and medicine. Thus, new perspectives for personalized diagnostics, theranostics, and biochemical/medical research will be unlocked when standing limitations of artificial binders and receptors are overcome. In this review, we survey synthetic sensing systems that have promising (future) application potential for the detection of small molecules, cations, and anions in aqueous media and biofluids. Special attention was given to sensing systems that provide a readily measurable optical signal through dynamic covalent chemistry, supramolecular host-guest interactions, or nanoparticles featuring plasmonic effects. This review shall also enable the reader to evaluate the current performance of molecular probes, chemosensors, and nanosensors in terms of sensitivity and selectivity with respect to practical requirement, and thereby inspiring new ideas for the development of further advanced systems.

Enantioselective sensing of insect pheromones in water

An arrayed combination of water-soluble deep cavitands and cationic dyes has been shown to optically sense insect pheromones at micromolar concentration in water. Machine learning approaches were used to optimize the most effective array components, which allows differentiation between small structural differences in targets, including between different diastereomers, even though the pheromones have no innate chromophore. When combined with chiral additives, enantiodiscrimination is possible, dependent on the size and shape of the pheromone.

Synthesis of Diboronic Acid-Based Fluorescent Probes for the Sensitive Detection of Glucose in Aqueous Media and Biological Matrices

Reliable and accurate glucose detection in biological samples is of great importance in clinical diagnosis and medical research. Chemical probes are advantageous in simple operation and flexible design, especially for the development of fluorescent probes. Anthracene-based diboronic acid (P-DBA) has shown potential in glucose probing because of its high sensitivity. However, poor solubility limits its applications in aqueous media. In this work, we systemically modify P-DBA by introducing fluoro (F-), chloro (Cl-), methoxyl (MeO-), or cyano (CN-) substituents. Among these probes, the cyano-substituted probe (CN-DBA) displays the highest glucose-binding constant (6489.5 M^-1, 33% MeOH). More importantly, it shows good water solubility in the aqueous solution (0.5% MeOH), with ultrasensitive recognition with glucose (LOD = 1.51 μM) and robust sensing from pH 6.0 to 9.0. Based on these features, the CN-DBA is finally applied to detect glucose in cell lysates and plasma, with satisfactory recovery and precision. These results demonstrate that CN-DBA could serve as an accurate, sensitive fluorescent probe for glucose assays in biological samples.

Molecular Boronic Acid-Based Saccharide Sensors

Boronic acids can reversibly bind diols, a molecular feature that is ubiquitous within saccharides, leading to their use in the design and implementation of sensors for numerous saccharide species. There is a growing understanding of the importance of saccharides in many biological processes and systems; while saccharide or carbohydrate sensing in medicine is most often associated with detection of glucose in diabetes patients, saccharides have proven to be relevant in a range of disease states. Herein the relevance of carbohydrate sensing for biomedical applications is explored, and this review seeks to outline how the complexity of saccharides presents a challenge for the development of selective sensors and describes efforts that have been made to understand the underpinning fluorescence and binding mechanisms of these systems, before outlining examples of how researchers have used this knowledge to develop ever more selective receptors.

Supramolecular Fluorescent Sensors: An Historical Overview and Update

Since as early as 1867, molecular sensors have been recognized as being intelligent "devices" capable of addressing a variety of issues related to our environment and health (e.g., the detection of toxic pollutants or disease-related biomarkers). In this review, we focus on fluorescence-based sensors that incorporate supramolecular chemistry to achieve a desired sensing outcome. The goal is to provide an illustrative overview, rather than a comprehensive listing of all that has been done in the field. We will thus summarize early work devoted to the development of supramolecular fluorescent sensors and provide an update on recent advances in the area (mostly from 2018 onward). A particular emphasis will be placed on design strategies that may be exploited for analyte sensing and corresponding molecular platforms. Supramolecular approaches considered include, inter alia, binding-based sensing (BBS) and indicator displacement assays (IDAs). Because it has traditionally received less treatment, many of the illustrative examples chosen will involve anion sensing. Finally, this review will also include our perspectives on the future directions of the field.

Anion recognition by anthracene appended ortho-aminomethylphenylboronic acid: a new PET-based sensing mechanism

Carboxylate, phosphate and sulphate anions form hydrogen bonded complexes with the B(OH)2 group of the receptor 1 producing a turn-off fluorescence response, while the malonate monoanion induces the opposite turn-on effect.

Glycoform Differentiation by a Targeted, Self-Assembled, Pattern-Generating Protein Surface Sensor

A method for generating targeted, pattern-generating, protein surface sensors via the self-assembly of modified oligodeoxynucleotides (ODNs) is described. The simplicity by which these systems can be created enabled the development of a sensor that can straightforwardly discriminate between distinct glycoform populations. By using this sensor to identify glycosylation states of a therapeutic protein, we demonstrate the diagnostic potential of this approach as well as the feasibility of integrating a wealth of supramolecular receptors and sensors into higher-order molecular analytical devices with advanced properties. For example, the facile device integration was used to attach the well-known anthracene-boronic acid (An-BA) probe to a biomimetic DNA scaffold and consequently, to use the unique photophysical properties of An-BA to improve glycoform differentiation. In addition, the noncovalent assembly enabled us to modify the sensor with a trinitrilotriacetic acid (tri-NTA)-Ni²⁺ complex, which endows it with selectivity toward a hexa-histidine tag (His-tag). The selective responses of the system to diverse His-tag-labeled proteins further demonstrate the potential applicability of such sensors and validate the mechanism underlying their function.

Boronic acid sensors with double recognition sites: a review

Boronic acids reversibly and covalently bind to Lewis bases and polyols, which facilitated the development of a large number of chemical sensors to recognize carbohydrates, catecholamines, ions, hydrogen peroxide, and so on. However, as the binding mechanism of boronic acids and analytes is not very clear, it is still a challenge to discover sensors with high affinity and selectivity. In this review, boronic acid sensors with two recognition sites, including diboronic acid sensors, and monoboronic acid sensors having another group or binding moiety, are summarized. Owing to double recognition sites working synergistically, the binding affinity and selectivity of sensors can be improved significantly. This review may help researchers to sort out the binding rules and develop ideal boronic acid-based sensors.

Enhanced ion binding by the benzocrown receptor and a carbonyl of the aminonaphthalimide fluorophore in water-soluble logic gates

Fluorescent logic gates with benzocrown ethers attached at the imide naphthalimide exhibit synergistic binding of Na⁺ and K⁺ in aqueous methanol and water.

Hypochlorite fluorescence sensing by phenylboronic acid-alizarin adduct based carbon dots

The selective fluorescence sensing of hypochlorite (ClO^-) was achieved at pH 7.4 by a simple analytical procedure through the fluorescence quenching of autoclave synthesized carbon dots (CDs), which used as precursor an adduct formed between 3-aminophenylboronic acid (APBA) and alizarin red S (ARS). The use of this adduct allowed the preparation of CDs with a red shifted emission (560 nm) and excitation in the visible range (490 nm). Quantification of hypochlorite was achieved at physiological pH (pH 7.4) in aqueous solutions by fluorescence quenching with a linearity range of 0-200 μM (limit of detection of 4.47 μM, and limit of quantification of 13.41 μM). The selectivity of hypochlorite sensing was confirmed by comparison with other potential analytes, such as glucose, fructose and hydrogen peroxide. Finally, the validity of the proposed assay was further demonstrated by performing recovery assays in different matrices. Thus, this CDs allows the fluorescent sensing of ClO^- with spectral properties more suitable for in vitro/in vivo applications.

The mechanisms of boronate ester formation and fluorescent turn-on in ortho-aminomethylphenylboronic acids

ortho-Aminomethylphenylboronic acids are used in receptors for carbohydrates and various other compounds containing vicinal diols. The presence of the o-aminomethyl group enhances the affinity towards diols at neutral pH, and the manner in which this group plays this role has been a topic of debate. Further, the aminomethyl group is believed to be involved in the turn-on of the emission properties of appended fluorophores upon diol binding. In this treatise, a uniform picture emerges for the role of this group: it primarily acts as an electron-withdrawing group that lowers the pKa of the neighbouring boronic acid thereby facilitating diol binding at neutral pH. The amine appears to play no role in the modulation of the fluorescence of appended fluorophores in the protic-solvent-inserted form of the boronic acid/boronate ester. Instead, fluorescence turn-on can be consistently tied to vibrational-coupled excited-state relaxation (a loose-bolt effect). Overall, this Review unifies and discusses the existing data as of 2019 whilst also highlighting why o-aminomethyl groups are so widely used, and the role they play in carbohydrate sensing using phenylboronic acids.

Facile construction of boranil complexes with aggregation-induced emission characteristics and their specific lipid droplet imaging applications

A rational strategy was reported to construct boranil complexes (DPFB derivatives) with unique aggregation-induced emission effects by installing phenyl rings in the anil ligand as the intramolecular rotors. In view of the good biocompatibility and suitable lipophilicity, DPFB derivatives can serve as excellent fluorescent probes for specific imaging of lipid droplets in living cells and yolk lipids in zebrafish.

Modeling Boronic Acid Based Fluorescent Saccharide Sensors: Computational Investigation of d-Fructose Binding to Dimethylaminomethylphenylboronic Acid

Designing organic saccharide sensors for use in aqueous solution is a nontrivial endeavor. Incorporation of hydrogen bonding groups on a sensor's receptor unit to target saccharides is an obvious strategy but not one that is likely to ensure analyte-receptor interactions over analyte-solvent or receptor-solvent interactions. Phenylboronic acids are known to reversibly and covalently bind saccharides (diols in general) with highly selective affinity in aqueous solution. Therefore, recent work has sought to design such sensors and understand their mechanism for allowing fluorescence with bound saccharides. In past work, binding orientations of several saccharides were determined to dimethylaminomethylphenylboronic acid (DMPBA) receptors with an anthracene fluorophore; however, the binding orientation of d-fructose to such a sensor could not be determined. In this work, we investigate the potential binding modes by generating 20 possible bidentate and six possible tridentate modes between fructose and DMPBA, a simplified receptor model. Gas phase and implicit solvent geometry optimizations, with a myriad functional/basis set pairs, were carried out to identify the lowest energy bidentate and tridentate binding modes of d-fructose to DMPBA. An interesting hydrogen transfer was observed during selected bidentate gas phase optimizations; this transfer suggests a strong sharing of the hydrogen atom between the boronate hydroxyl and amine nitrogen.

A new approach to developing diagnostics and therapeutics: Aggregation-induced emission-based fluorescence turn-on

Fluorescence imaging is a promising visualization tool and possesses the advantages of in situ response and facile operation; thus, it is widely exploited for bioassays. However, traditional fluorophores suffer from concentration limits because they are always quenched when they aggregate, which impedes applications, especially for trace analysis and real-time monitoring. Recently, novel molecules with aggregation-induced emission (AIE) characteristics were developed to solve the problems encountered when using traditional organic dyes, because these new molecules exhibit weak or even no fluorescence when they are in free movement states but emit intensely upon the restriction of intramolecular motions. Inspired by the excellent performances of AIE molecules, a substantial number of AIE-based probes have been designed, synthesized, and applied to various fields to fulfill diverse detection tasks. According to numerous experiments, AIE probes are more practical than traditional fluorescent probes, especially when used in bioassays. To bridge bioimaging and materials engineering, this review provides a comprehensive understanding of the development of AIE bioprobes. It begins with a summary of mechanisms of the AIE phenomenon. Then, the strategies to realize accurate detection using AIE probes are discussed. In addition, typical examples of AIE-active materials applied in diagnosis, treatment, and nanocarrier tracking are presented. In addition, some challenges are put forward to inspire more ideas in the promising field of AIE-active materials.

Space-confined indicator displacement assay inside a metal-organic framework for fluorescence turn-on sensing

The indicator displacement assay (IDA) is for the first time performed within a metal-organic framework (MOF) to achieve ultrasensitive fluorescence turn-on sensing. A Zr(iv) ion MOF (UiO-67-DQ-PsO) furnished with electron-deficient diquat units (DQ2+, as the receptor) on the wall and electron-rich 1-pyrenesulfonate anions (PsO-, as the fluorescent indicator) in the pores was prepared by postsynthetic anion exchange. The MOF is capable of sensing alkylamines owing to the competing PsO--DQ2+ and alkylamine-DQ2+ charge-transfer interactions, the former interaction causing a fluorescence OFF state and the latter displacing PsO- to trigger its emission. Significant advantages have been demonstrated for the IDA inside the MOF. The turn-on assay exhibits much higher sensitivity and anti-interference than the turn-off sensing using the MOF without indicators (the sensitivity is enhanced by as much as six orders of magnitude to the subnanomolar level). The integration of both the receptor and indicator in the porous solid enables facile regeneration and recyclability of the IDA ensemble. Furthermore, we show that the confined space provided by the MOF significantly enhances the supramolecular interactions to make possible the IDA impossible in solution. This work not only demonstrates a novel conceptual approach to fabricate superior fluorescence turn-on sensors using porous materials but also has important implications for supramolecular chemistry in porous materials.

3-Hydroxyphenylboronic Acid-Based Carbon Dot Sensors for Fructose Sensing

The selective fluorescence sensing of fructose was achieved by fluorescence quenching of the emission of hydrothermal-synthesized carbon quantum dots prepared by 3-hydroxyphenylboronic acid. Quantification of fructose was possible in aqueous solutions with pH of 9 (Limit of Detection L_OD and Limit of Quantification L_OQ of 2.04 and 6.12 mM), by quenching of the emission at 376 nm and excitation ~380 nm with a linearity range of 0-150 mM. A Stern-Volmer constant (K_SV) of 2.11 × 10^-2 mM^-1 was obtained, while a fluorescent quantum yield of 31% was calculated. The sensitivity of this assay towards fructose was confirmed by comparison with other sugars (such as glucose, sucrose and lactose). Finally, the validity of the proposed assays was further demonstrated by performing recovery assays in different matrixes. Graphical Abstract.

An aggregation-induced phosphorescence probe for calcium ion-specific detection and live-cell imaging in Arabidopsis thaliana

A molecular probe with aggregation-induced phosphorescence (AIP) properties for calcium ion-specific detection and imaging in vivo was designed.

Synthesis of water-soluble anthracene-appended benzoxaboroles and evaluation of their cis-1,2-diol recognition properties

Three series of water-soluble anthracene-appended benzoxaboroles 1a-c were developed; their binding affinity toward cis-1,2-diols was explored by conventional fluorescence titrations to demonstrate the role of benzoxaborole as a general recognition motif of cis-1,2-diols for fluorescent probes. The complex structures of the tetra-coordinated boronate adducts between 1 and the cis-1,2-diols were revealed.

Light-Switchable Self-Assembly of Non-Photoresponsive Gold Nanoparticles

Herein, an interesting light-induced self-assembly behavior from non-photoresponsive gold nanoparticles (Au NPs) was reported. Specifically, a pH-responsive amphiphile SPBwas developed that contained a particular phenylboronic acid moiety and showed excellent surface activity at the neutral and basic conditions, thereby stabilizing Au NPs well. Accordingly, the SPB-functionalized Au NPs showed strong pH dependence that there presented the pH-induced reversible self-assembly behavior. Furthermore, the introduction of a small amount of commercially available photoacid generator named diphenyliodonium nitrate (DIN) into the system could endow it with apparent light-switchable self-assembly behavior. The pH- and light-induced self-assembly behaviors of SPB-functionalized Au NPs in the absence and presence of DIN, respectively, were systematically studied by various techniques including UV-vis spectrum, transmission electron microscope, nuclear magnetic resonance, and Fourier transform infrared spectroscopy, which evidently confirmed that the stimuli-responsive self-assembly was controlled by the hydrogen-bonding interactions between phenylboronic acid moieties. Attributing to the light-induced obvious color change from bright-red to deep purple, the system was applied in particle imprinting successfully.

Conformationally Induced Off-On Cell Membrane Chemosensor Targeting Receptor Protein-Tyrosine Kinases for in Vivo and in Vitro Fluorescence Imaging of Cancers

Molecules capable of monitoring receptor protein-tyrosine kinase expression could potentially serve as useful tools for cancer diagnosis due to the overexpression of tyrosine kinases during tumor growth and metastasis. In this work, a conformationally induced "off-on" tyrosine kinase cell membrane fluorescent sensor (SP1) was designed and evaluated for the detection and imaging of receptor protein-tyrosine kinases in vivo and in vitro. SP1 consists of sunitinib and pyrene linked via hexamethylenediamine and displays quenched fluorescence as a dimer. The fluorescence of SP1 is restored in the presence of receptor protein-tyrosine kinases upon strong interaction with SP1 at the target terminal. The unique signal response mechanism enables SP1 use for fluorescence microscopy imaging of receptor protein-tyrosine kinases in the cell membranes of living cells, allowing for the rapid differentiation of cancer cells from normal cells. SP1 can be used to visualize the chick embryo chorioallantoic membrane and mouse model tumors, suggesting its possible application for early cancer diagnosis.

Tetraphenylethene– and diphenyldibenzofulvene–anthracene-based fluorescence sensors possessing photo-induced electron transfer and aggregation-induced emission enhancement characteristics for detection of water

RS-1 and RS-2 have been developed as PET/AIEE hybrid fluorescence sensors for detection of water in the low and high water content regions in solvents.

Lighting-up protein–ligand interactions with fluorescent PET (photoinduced electron transfer) sensor designs

Extending the versatile fluorescent PET sensing/switching system causes ‘off–on’ signalling when a ligand binds to its appropriate protein.

Bulk Aggregation Based Fluorescence Turn-On Sensors for Selective Detection of Progesterone in Aqueous Solution

Steroids are polycyclic compounds that share tetracyclic ring as core scaffold, and selective detection of a steroid is challenging owing to their structural similarities. The discovery of chemosensors that recognize progesterone by alteration of self-aggregation state is described, and these show significant fluorescence turn-on. A self-aggregated 48-membered dansyl library was screened against a series of metabolites in aqueous buffer and discovered two compounds (PG-1, PG-2) exhibited exceptional selectivity for progesterone. Following studies of aggregation properties of probes using dynamic light scattering and transmission electron microscopy supports progesterone recognition lead to the generation of bulk aggregates that induce fluorescence enhancement. Though many fluorescence sensing mechanisms have been proposed, a sensing mode based on the bulk aggregate formation of fluorophore has never been reported, and this may open a new avenue of chemosensor design.

Two Are Better than One: Halloysite Nanotubes-Supported Surface Imprinted Nanoparticles Using Synergy of Metal Chelating and Low pKa Boronic Acid Monomers for Highly Specific Luteolin Binding under Neutral Condition

Surface-imprinted nanoparticles with double recognition (DM-MIPs) are fabricated onto halloysite nanotubes (HNTs) for highly specific separation of natural flavone luteolin (LTL) under neutral condition. Specifically, a two-step strategy via consecutive surface-initiated atom transfer radical polymerization (SI-ATRP) is employed to introduce inherent recognition of molecular imprinting and reversible covalent affinity of boronic acid ligands and immobilized Zn²⁺ into DM-MIPs. First, Zn²⁺-immobilized poly(vinyl imidazole) (PVLD) shell based on the HNTs via the first SI-ATRP is prepared to capture LTL by metal chelating. Then HNTs-supported surface imprinted nanoparticles are prepared using low pK_a boronic acid monomer 4-(2-acrylamidoethylcarbamoyl)-3-fluorophenylboronic acid (AMC-FPBA) via the second SI-ATRP. Taking advantage of low apparent pK_a of AMC-FPBA and large high-affinity binding site density, DM-MIPs possess a promising binding with cis-diol-containing LTL under neutral condition. In static adsorption, DM-MIPs show large LTL loading amount (83.42 mg g^-1), fast capture kinetics, remarkable selectivity, and excellent recyclability at pH = 7.0. More importantly, by reducing the pH to 4.0, the loaded TLL can be simply released. As a proof of this concept, a commercially available LTL with 85% purity can be easily enriched and further purified, and the product exhibits the similar antibacterial performance with standard substance.