Reaction-based Indicator displacement Assay (RIA) for the selective colorimetric and fluorometric detection of peroxynitrite

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

Enzymatic colorimetric method for turn-on determination of l-lactic acid through indicator displacement assay

l-Lactic acid is a natural α-hydroxy carboxylic acid and is commonly used as an addictive. Quantitation of l-lactic acid is indispensable in food and cosmetic industries. An enzymatic colorimetric method was developed for the determination of l-lactic acid by competitive indicator displacement assay. Boric acid inhibited the colorimetric reaction of l-3,4-dihydroxyphenylalanine (l-DOPA) catalyzed by tyrosinase. l-Lactic acid competitively displaced and released l-DOPA bound with boric acid to serve as substrate, and thus restored the tyrosinase activity. Recovery of color reaction could be spectrophotometrically determined at 475 nm and was proportional to the amount of l-lactic acid. A calibration curve between l-lactic acid concentration and recovery of absorbance were built. The concentration range of the l-lactic acid was 0.25-2.25 mM. The limit of detection (LOD) and the limit of quantification (LOQ) for l-lactic acid was estimated to be 0.05 mM and 0.16 mM, respectively. The method achieved turn-on and visual sensing with good precision, accuracy, specificity, and robustness. The assay method exhibited a promising prospect to determine the content of l-lactic acid in foods and cosmetics.

Affinity Studies of Hemicyanine Derived Water Soluble Colorimetric Probes with Reactive Oxygen/Nitrogen/Sulfur Species

Peroxynitrite (ONOO^- ) is an essential endogenous reactive oxygen species (ROS) generated in mitochondria under various pathological and physiological conditions. An increase in its level in mitochondria is related to numerous diseases. Herein, we report a series of hemicyanine-derived water-soluble colorimetric probes (1-4) and the reactivity of which was studied with various reactive oxygen, nitrogen, and sulfur species. Probes 1-4 are formed by conjugating 1,2,3,3-tetramethyl-3H-indolium iodide and 4-hydroxybenzaldehyde or its derivatives through an alkene linkage formed by the Knoevenagel reaction. Oxidative cleavage of the electron-rich double bond of the conjugated hemicyanine dye revealed a discerning affinity of probe 3 towards peroxynitrite among all reactive oxygen species. The rapid change in color of 3 provides a sensitive and selective method for detecting peroxynitrite with a low detection limit of 180 nM. Notably, the water solubility of the probe displays excellent performance for the selective detection of peroxynitrite among ROS and reactive nitrogen (RNS)/sulfur species (RSS). UV-vis, ¹ H NMR, and ¹³ C NMR spectroscopic data and results from theoretical calculations provide further information on the interaction of peroxynitrite with probe 3.

An alkaline phosphatase-induced immunosensor for SARS-CoV-2 N protein and cardiac troponin I based on the in situ fluorogenic self-assembly between N-heterocyclic boronic acids and alizarin red S

Alkaline phosphatase (ALP)-induced in situ fluorescent immunosensor is less investigated and reported. Herein, a high-performance ALP-labeled in situ fluorescent immunoassay platform was constructed. The developed platform was based on a fluorogenic self-assembly reaction between pyridineboronic acid (PyB(OH)₂) and alizarin red S (ARS). We first used density functional theory (DFT) to theoretically calculate the changes of Gibbs free energy of the used chemicals before and after the combination and simulated the electrostatic potential on its' surfaces. The free ARS and PyB(OH)₂ exist alone, neither emits no fluorescence. However, the ARS/PyB(OH)₂ complex emits strong fluorescence, which could be effectively quenched by PPi based on the stronger affinity between PPi and PyB(OH)₂ than that of ARS and PyB(OH)₂. PyB(OH)₂ coordinated with ARS again in the presence of ALP due to the ALP-catalyzed hydrolysis of PPi, and correspondingly, the fluorescence was restored. We chose cTnI and SARS-CoV-2 N protein as the model antigen to construct ALP-induced immunosensor, which exhibited a wide dynamic range of 0-175 ng/mL for cTnI and SARS-CoV-2 N protein with a low limit of detection (LOD) of 0.03 ng/mL and 0.17 ng/mL, respectively. Moreover, the proposed immunosensor was used to evaluate cTnI and SARS-CoV-2 N protein level in serum with satisfactory results. Consequently, the method laid the foundation for developing novel fluorescence-based ALP-labeled ELISA technologies in the early diagnosis of diseases.

Applications of the Reversible Boronic Acids/Boronate Switch to Nucleic Acids

Over the last decades, boron and nucleic acids chemistries have gained a lot of attention for biological, medicinal and analytical applications. Our laboratory has a long-standing interest in both chemistries and owing to the ability of boronic acids to react with cis-diol function in aqueous media we developed over the years a variety of applications ranging from molecular recognition and sensing to the development of reversible dynamic systems in which the natural phosphodiester linkage was replaced by a boronate. In this account, we summarize research results from our group from our preliminary studies on molecular recognition of ribonucleosides to the dynamic assembly of functional DNAzymes. In particular, the various parameters influencing the dynamic nature of these reversible covalent bonds able to respond to external stimuli are discussed. Finally, current challenges and opportunities for boron-based nucleic acids are also addressed.

A microfluidic organic transistor for reversible and real-time monitoring of H2O2 at ppb/ppt levels in ultrapure water

A microfluidic organic transistor functionalized with phenylboronic acid firstly succeeded in reversible and real-time monitoring of H₂O₂ at ppb/ppt levels in ultrapure water, which would be used not only as portable chemical sensors but also as monitoring tools to clarify unknown reaction mechanisms of phenylboronic acid with H₂O₂.

A thiocarbonate-caged fluorescent probe for specific visualization of peroxynitrite in living cells and zebrafish

Peroxynitrite (ONOO^-), a highly reactive oxygen species (ROS), is implicated with many physiological and pathological processes including cancer, neurodegenerative diseases and inflammation. In this regard, developing effective tools for highly selective tracking of ONOO^- is urgently needed. Herein, we constructed a concise and specific fluorescent probe NA-ONOO for sensing ONOO^- by conjugating an ONOO^--specific recognition group ((4-methoxyphenylthio)carbonyl, a thiocarbonate derivative) with a naphthalene fluorophore. The probe, NA-ONOO, was in a dark state because the high electrophilicity of (4-methoxyphenylthio)carbonyl disturbs the intramolecular charge transfer (ICT) in the fluorophore. Upon treatment with ONOO^-, the fluorescent emission was sharply boosted (quantum yield Φ: 3% to 56.6%) owing to an ONOO^- triggered release of (4-methoxyphenylthio)carbonyl from NA-ONOO. Optical analyses showed that NA-ONOO presented high selectivity and sensitivity toward ONOO^-. With good cell permeability and biocompatibility, the NA-ONOO probe was successfully applied to imaging and tracing exogenous and endogenous ONOO^- in living cells and zebrafish. The probe NA-ONOO presents a new recognition group and a promising method for further investigating ONOO^- in living systems.

A morpholino hydrazone-based lysosome-targeting fluorescent probe with fast response and high sensitivity for imaging peroxynitrite in living cells

Peroxynitrite (ONOO^-) plays important roles in many pathophysiological processes and its subcellular detection draws increasing attention. In this study, we designed and prepared a novel lysosome-targetable fluorescent probe (E)-2-(benzo[d]thiazol-2- yl)-4-methyl-6-((morpholinoimino)methyl)phenol (BMP) for selective detection of ONOO^- in living systems by incorporating a reactive morpholino hydrazone as new ONOO^- response site into a benzothiazole derivative as fluorophore. After reaction with ONOO^-, an obvious fluorescence increase (83-fold) was observed accompanied with distinct dual colorimetric and fluorescence changes. Probe BMP displayed the merits of fast response (<3 s), ultrasensitivity (LOD = 6 nM) and high selectivity towards ONOO^- over other physiological species including ROS/RNS. Most importantly, the probe was capable of imaging ONOO^- in lysosomes of living cells with good cell permeation and negligible cytotoxicity. Therefore, this research provides an effective tool to study the functions of ONOO^- in lysosomes.

Indicator displacement assays: from concept to recent developments

Overcoming the synthetic burden related to covalently connected receptors with appropriate indicators for sensing various analytes via an indicator spacer receptor (ISR) approach, the indicator displacement assay (IDA) seems to be a very sophisticated and versatile supramolecular sensing paradigm, and it has taken the phenomenon of molecular recognition to the next level in the realm of host-guest chemistry. Due to the unavailability of a comprehensive report on what has been done in the last decade in relation to IDAs, we decided to set down this account illustrating diverse indicator displacement assays (IDAs) in detail from the concept stage to recent developments relating to the detection of cationic, anionic, and neutral analytes. The authors conclude this account with future perspectives and highlight the limitations and challenges relating to IDAs which need to be overcome in order to realize the full potential of this popular sensing phenomenon. While we were finalizing our account for publication, a tutorial review by the research groups of Anslyn, Sessler, and Sun was published, which focuses mainly on diverse aspects of the chemistry related to IDAs. As can be seen, our review, besides discussing various basic IDA concepts, has a vast collection of information published in the past decade and hence, hopefully, will be very informative for the supramolecular community. We believe that this work will offer new insights for the construction of novel sensors operating through the IDA approach.

Principles of odor coding in vertebrates and artificial chemosensory systems

The biological olfactory system is the sensory system responsible for the detection of the chemical composition of the environment. Several attempts to mimic biological olfactory systems have led to various artificial olfactory systems using different technical approaches. Here we provide a parallel description of biological olfactory systems and their technical counterparts. We start with a presentation of the input to the systems, the stimuli, and treat the interface between the external world and the environment where receptor neurons or artificial chemosensors reside. We then delineate the functions of receptor neurons and chemosensors as well as their overall I-O relationships. Up to this point, our account of the systems goes along similar lines. The next processing steps differ considerably: while in biology the processing step following the receptor neurons is the "integration" and "processing" of receptor neuron outputs in the olfactory bulb, this step has various realizations in electronic noses. For a long period of time, the signal processing stages beyond the olfactory bulb, i.e., the higher olfactory centers were little studied. Only recently there has been a marked growth of studies tackling the information processing in these centers. In electronic noses, a third stage of processing has virtually never been considered. In this review, we provide an up-to-date overview of the current knowledge of both fields and, for the first time, attempt to tie them together. We hope it will be a breeding ground for better information, communication, and data exchange between very related but so far little connected fields.

Electrochemical and Electrochemiluminescence Dendrimer-based Nanostructured Immunosensors for Tumor Marker Detection: A Review

The usage of dendrimers or cascade molecules in the biomedical area has recently attracted much attention worldwide. Furthermore, dendrimers are interesting in clinical and pre-clinical applications due to their unique characteristics. Cancer is one of the most widespread challenges and important diseases, which has the highest mortality rate. In this review, the recent advances and developments (from 2009 up to 2019) in the field of electrochemical and electroluminescence immunosensors for detection of the cancer markers are presented. Moreover, this review covers the basic fabrication principles and types of electrochemical and electrochemiluminescence dendrimer-based immunosensors. In this review, we have categorized the current dendrimer based-electrochemical/ electroluminescence immunosensors into five groups: dendrimer/ magnetic particles, dendrimer/ferrocene, dendrimer/metal nanoparticles, thiol-containing dendrimer, and dendrimer/quantum dots based-immunosensors.

A high-performance genetically encoded fluorescent biosensor for imaging physiological peroxynitrite

Peroxynitrite is a reactive nitrogen species (RNS) that plays critical roles in signal transduction, stress response, and numerous human diseases. Advanced molecular tools that permit the selective, sensitive, and noninvasive detection of peroxynitrite are essential for understanding its pathophysiological functions. Here, we present pnGFP-Ultra, a high-performance, reaction-based, genetically encodable biosensor for imaging peroxynitrite in live cells. pnGFP-Ultra features a p-boronophenylalanine-modified chromophore as the sensing moiety and exhibits a remarkable ~110-fold fluorescence turn-on response toward peroxynitrite while displaying virtually no cross-reaction with other reactive oxygen/nitrogen species. To facilitate the expression of pnGFP-Ultra in mammalian cells, we engineered an efficient noncanonical amino acid (ncAA) expression system that is broadly applicable to the mammalian expression of ncAA-containing proteins. pnGFP-Ultra robustly detected peroxynitrite production in activated macrophages and primary glial cells. pnGFP-Ultra fills an important technical gap and represents a valuable addition to the molecular toolbox for probing RNS biology.

Indicator displacement assays (IDAs): the past, present and future

Indicator displacement assays (IDAs) offer a unique and innovative approach to molecular sensing. This Tutorial review discusses the basic concepts of each IDA strategy and illustrates their use in sensing applications.

Alizarin-based molecular probes for the detection of hydrogen peroxide and peroxynitrite

Phenol fluorophores are a large family of fluorophores, which have attracted more and more attention in the design of probes.

A highly selective ratiometric fluorescent probe based on naphthalimide for detection and imaging of CYP1A1 in living cells and zebrafish

Real-time monitoring of the cytochrome P450 1A1 (CYP1A1) activity in complex biological systems via a practical tool is highly sought after because of its significant role in the metabolism and bioactivation of various xenobiotics. Herein, according to slight differences in the 3D structure and substrate preference between CYP1A1 and its homologous CYP1A2, a series of novel ratiometric fluorescent probes were designed and synthesized using 1,8-naphthalimide because of its trait of naked-eye visualization and ratiometric fluorescence to achieve the detection of CYP1A1 in biological samples. Among these probes, NEiPN showed good water solubility, highly isoform selectivity and great sensitivity (LOD = 0.04874 nM) for CYP1A1 under simulated physiological conditions, which makes it favorable for monitoring CYP1A1 in vivo. Remarkably, NEiPN exhibited excellent reproducibility when it was used to detect the CYP1A1 content in human liver microsomes, which indicated that it has a great potential for quantifying the CYP1A1 content in real biological samples. Furthermore, NEiPN showed relatively low cytotoxicity and has been successfully applied in biological imaging in living cells and zebrafish. These findings indicate that NEiPN is capable of real-time monitoring of the activity of endogenous CYP1A1, which could provide support for CYP1A1-associated pathological processes.

Product-boosted fluorescence signal: a new approach for designing small-molecule probes for detection of peroxynitrite

In situ self-assembled boronate ester comprising commercially available 2-formylphenylboronic acid and 2-(2′,3′-bihydroxyphenyl)benzothiazole (BHBT) is explored for the detection of ONOO⁻ with product-boosted fluorescence.

A new long-wavelength fluorescent probe for tracking peroxynitrite in live cells and inflammatory sites of zebrafish

Design of a long-wavelength fluorescent probe for tracking peroxynitrite in live cells and inflammatory sites of zebrafish.

Reaction-based indicator displacement assay (RIA) for the development of a triggered release system capable of biofilm inhibition

Here, a reaction-based indicator displacement hydrogel assay (RIA) was developed for the detection of hydrogen peroxide (H2O2) via the oxidative release of the optical reporter Alizarin Red S (ARS). In the presence of H2O2, the RIA system displayed potent biofilm inhibition for Methicillin-resistant Staphylococcus aureus (MRSA), as shown through an in vitro assay quantifying antimicrobial efficacy. This work demonstrated the potential of H2O2-responsive hydrogels containing a covalently bound diol-based drug for controlled drug release.

An efficient TP-FRET-based lysosome-targetable fluorescent probe for imaging peroxynitrite with two well-resolved emission channels in living cells, tissues and zebrafish

Peroxynitrite (ONOO^-), an important role of reactive oxygen species (ROS) in vivo, and studies showed abnormal of ROS can induce lysosomal membrane permeabilization (LMP) and lead to the death of cells. Thus, it is of great significance for designing an effective method for investigating relationship between physiology and pathology between ONOO^- and lysosome. Herein, for the first time, we adopted a Förster resonance energy transfer (FRET) strategy to construct a lysosome-targetable small molecular ratiometric two-photon (TP) fluorescent probe (NpRh-ONOO) for detecting ONOO^- in living cells, tissues and zebrafish. Specifically, a TP fluorophore and a rhodamine B fluorophore are directly connected by a flexible piperidine linker to form the TP-FRET-scaffold, a hydrazide as ONOO^- reactive set, and a dimethylamino as lysosome targeting-group, which shows a target-modulated ratiometric TP fluorescence response, two well-resolved emission peaks separated by 73 nm, rapid response (<10 s), high selectivity and sensitivity with the detection limit is as low as 3.3 nM for ONOO^-. These prominent features of probe were then applied for ratiometric bioimaging both exogenous and endogenous peroxynitrite in living cells, tissues and zebrafish, demonstrating it can be used as a powerful tool for biological research of lysosomal peroxynitrite in biological systems.

A specific AIE and ESIPT fluorescent probe for peroxynitrite detection and imaging in living cells

Developing specific and sensitive method for endogenous peroxynitrite (ONOO^-) in living systems is valuable to understand its various pathological events. In this work, a simple and novel fluorescent probe (HPP) based on aggregation induced emission (AIE) as well as excited state intramolecular proton transfer (ESIPT) processes for endogenous ONOO^- detection was constructed containing a diphenylphosphinate as the reactive site and a salicylaldehyde azine as the fluorophore. The probe showed specific fluorescence turn-on response toward ONOO^- owing to the diphenylphosphinate group of HPP reacted with ONOO^-, releasing the salicylaldehyde azine with both AIE and ESIPT characteristics. In addition, the probe exhibited a large Stokes shift, an excellent light-up ratio, high selectivity and excellent sensitivity with a low detection limit of 8 × 10^-8 M for endogenous ONOO^-. Moreover, the probe could be applied to bioimaging of endogenous ONOO^- in live cell, which demonstrated the probe can be used as effective tool for investigation of ONOO^- in biological systems.

Reaction-Based Fluorescent Probes for the Detection and Imaging of Reactive Oxygen, Nitrogen, and Sulfur Species

This Account describes a range of strategies for the development of fluorescent probes for detecting reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive (redox-active) sulfur species (RSS). Many ROS/RNS have been implicated in pathological processes such as Alzheimer's disease, cancer, diabetes mellitus, cardiovascular disease, and aging, while many RSS play important roles in maintaining redox homeostasis, serving as antioxidants and acting as free radical scavengers. Fluorescence-based systems have emerged as one of the best ways to monitor the concentrations and locations of these often very short lived species. Because of the high levels of sensitivity and in particular their ability to be used for temporal and spatial sampling for in vivo imaging applications. As a direct result, there has been a huge surge in the development of fluorescent probes for sensitive and selective detection of ROS, RNS, and RSS within cellular environments. However, cellular environments are extremely complex, often with more than one species involved in a given biochemical process. As a result, there has been a rise in the development of dual-responsive fluorescent probes (AND-logic probes) that can monitor the presence of more than one species in a biological environment. Our aim with this Account is to introduce the fluorescent probes that we have developed for in vitro and in vivo measurement of ROS, RNS, and RSS. Fluorescence-based sensing mechanisms used in the construction of the probes include photoinduced electron transfer, intramolecular charge transfer, excited-state intramolecular proton transfer (ESIPT), and fluorescence resonance energy transfer. In particular, probes for hydrogen peroxide, hypochlorous acid, superoxide, peroxynitrite, glutathione, cysteine, homocysteine, and hydrogen sulfide are discussed. In addition, we describe the development of AND-logic-based systems capable of detecting two species, such as peroxynitrite and glutathione. One of the most interesting advances contained in this Account is our extension of indicator displacement assays (IDAs) to reaction-based indicator displacement assays (RIAs). In an IDA system, an indicator is allowed to bind reversibly to a receptor. Then a competitive analyte is introduced into the system, resulting in displacement of the indicator from the host, which in turn modulates the optical signal. With an RIA-based system, the indicator is cleaved from a preformed receptor-indicator complex rather than being displaced by the analyte. Nevertheless, without a doubt the most significant result contained in this Account is the use of an ESIPT-based probe for the simultaneous sensing of fibrous proteins/peptides AND environmental ROS/RNS.

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.

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.

Rapid and Selective Discrimination of Gram-Positive and Gram-Negative Bacteria by Boronic Acid-Modified Poly(amidoamine) Dendrimer

There is an urgent need to develop a rapid and selective method for the detection of bacteria because delayed diagnosis and the overuse of antibiotics have triggered drug resistance in bacteria. To this end, we prepared boronic acid-modified poly(amidoamine) generation 4 (B-PAMAM(G4)) dendrimer as cross-linking molecules that form aggregates with bacteria. Within 5 min of adding B-PAMAM(G4) dendrimer solution to a bacterial suspension, large aggregates were observed. Interestingly, the aggregate formation with various bacteria was pH-dependent. In basic pH, both Gram-positive and Gram-negative bacteria formed aggregates, but in neutral pH, only Gram-positive bacteria formed aggregates. We revealed that this bacteria-selective aggregation involved the bacterial surface recognition of the phenylboronic acid moiety of B-PAMAM(G4) dendrimer. In addition, we demonstrated that the spherical structure of B-PAMAM(G4) was one of the important factors for the formation of large aggregates. The aggregation was also observed in the presence of ≤10 mM fructose. B-PAMAM(G4) dendrimer is expected to be a powerful tool for the rapid and selective discrimination between Gram-positive and Gram-negative bacteria.

Insights into the role of nanostructure in the sensing properties of carbon nanodots for improved sensitivity to reactive oxygen species in living cells

The surface states of carbon nanodots (CDs) were engineered by controlling the chemical structure on the surface of the CDs, which play an important role in the chemiluminescence sensing properties of CDs towards peroxynitrite. Their application in monitoring exogenous and endogenous release of peroxynitrite in living cells is demonstrated.

Dye Displacement Assay for Saccharides using Benzoxaborole Hydrogels

Dye displacement assays are a simple but effective method to determine the concentration of target analytes. Previously, we have shown that phenylboronic acid pinacol ester hydrogels (borogels) can be used to develop a boronic acid–Alizarin red S dye displacement assay for the determination of fructose (orange to red). In this work, benzoxaborole hydrogels (BOBgels) were developed, and these BOBgels demonstrated an enhanced apparent binding affinity towards monosaccharides, in particular towards glucose.

Are two better than one? Comparing intermolecular and intramolecular indicator displacement assays in pyrophosphate sensors

Peptide receptors with Zn(ii)-DPA units and a covalently bound fluorescent coumarin indicator on an oxazole-containing scaffold are shown to function as more selective pyrophosphate sensors than the analogous chemosensing ensembles in indicator displacement assays.

Colorimetric and fluorimetric detection of Hg2+ and Cr3+ by boronic acid conjugated rhodamine derivatives: Mechanistic aspects and their bio-imaging application in bacterial cells

Colorimetric and fluorimetric detection of toxic metal ions such as Hg²⁺ and Cr³⁺ has gained tremendous popularity over the conventional methods due to their operational simplicity, high selectivity, and speediness. Although numerous colorimetric and fluorescent receptors for Hg²⁺ or Cr³⁺ were reported in the literature, boronic acid-based receptors for these metal ions are rather scarce in the literature. Hence, in the present study dual function boronic acid conjugated rhodamine derivatives were developed, and their toxic metal ion detection abilities were studied by absorption, emission and visual detection methods. Absorption and emission spectral studies revealed that these derivatives displayed selectivity towards Hg²⁺, Cr³⁺ and Fe³⁺ among the other metal ions studied by forming new absorption band. Both the derivatives exhibited colorimetric response towards Hg²⁺ and Cr³⁺ by the change in color of the solution to pink and reddish pink with Fe³⁺. The detailed mechanism involved in the detection of Hg²⁺ was deduced by ¹H NMR and ESI-MS studies. Further, these derivatives were used for fluorescence imaging of Hg²⁺ and Cr³⁺ in S. aureus bacterial cells. Thus the present manuscript demonstrated the use of boronic acid conjugated rhodamine derivatives as a dual function (colorimetric and fluorescent) probes and as imaging agents for Hg²⁺ and Cr³⁺, which are known for their toxic influence on bacterial cells.

An ESIPT based naphthalimide chemosensor for visualizing endogenous ONOO− in living cells

An ESIPT based naphthalimide chemosensor with high sensitivity and selectivity for visualizing endogenous ONOO⁻ in living cells was developed.

A selective colorimetric sensing strategy for cysteine based on an indicator-displacement mechanism

Rapid determination of cysteine in aqueous solution is important for the diagnosis and treatment of some diseases.

A synthetic approach toward a pH and sugar-responsive diblock copolymer via post-polymerization modification

A novel pH- and sugar-responsive diblock copolymer containing phenylboronic acid was synthesized by RAFT and a post-polymerization modification strategy.

CO2-Acidolysis of iminoboronate ester based polymersomes

The iminoboronate-terminalized star-like prodrug N₃-(OEG-IBCAPE)₄ was prepared to investigate the CO₂-acidolysis of polymersomes with a tunable release feature of CAPE molecules.

A ratiometric fluorescent probe for peroxynitrite prepared by de novo synthesis and its application in assessing the mitochondrial oxidative stress status in cells and in vivo

A ratiometric fluorescent probe is prepared by de novo synthesis for assessing the mitochondrial oxidative stress status in cells and in vivo.

A Self-Assembled Ratiometric Polymeric Nanoprobe for Highly Selective Fluorescence Detection of Hydrogen Peroxide

In this study, a dual-emission fluorescence resonance energy transfer (FRET) polymeric nanoprobe by single-wavelength excitation was developed for sensitive and selective hydrogen peroxide (H₂O₂) detection. Polymeric nanoprobe was prepared by simple self-assembly of functional lipopolymers, which were 4-carboxy-3-fluorophenylboronic acid (FPBA)-modified DSPE-PEG (DSPE-PEG-FPBA) and 7-hydroxycoumarin (HC)-conjugated DSPE-PEG (DSPE-PEG-HC). Subsequent binding of alizarin red S (ARS) to FPBA endowed the nanoprobe with a new fluorescence emission peak at around 600 nm. Because of the perfect match of the fluorescence emission spectra of HC with the absorbance spectra of ARS-FPBA, FRET was achieved between them. The sensing strategy for H₂O₂ was based on H₂O₂-induced deboronation reaction and boronic acid-mediated ARS fluorescence. Interaction between phenylboronic acid and ARS was revisited herein and it was found that electron-donating or -withdrawing group on phenylboronic acid (PBA) has significant influence on the fluorescence property of ARS, which enabled sensitive and selective H₂O₂ sensing. The nanoprobe displayed two well-separated emission bands (150 nm), providing high specificity and sensitivity for ratiometric detection of H₂O₂. Further application was exploited for the determination of glucose and the results demonstrated that the proposed strategy showed ratiometric response capability for glucose detection. The current method does not involve complicated organic synthesis and opens a new avenue for the construction of multifunctional polymeric fluorescent nanoprobe.

Fluorogenic boronate-based probe-lactulose complex for full-aqueous analysis of peroxynitrite

A selective fluorogenic boronate-based probe-lactulose complex was evaluated for the rapid analysis of peroxynitrite (ONOO^-) based on a reaction-based indicator displacement assay (RIA). The probe was synthesised by a simple nucleophilic substitution reaction between a boronic acid moiety and a well known laser dye, DCM. Fluorescence analyses showed that the probe had an off-on response to lactulose, forming a fluorogenic probe-lactulose complex. The subsequent addition of ONOO^- selectively quenched the fluorescence of the complex over other Reactive Oxygen/ Nitrogen Species (ROS/RNS) tested. The complex can be applied for the rapid determination of ONOO^- in full aqueous solution with good linear range, and has also proven suitable for monitoring ONOO^- in living cells and real water samples.