A Nanoreactor for Tuning the Chemical Reactivity of a Solute

Expanding the scope of self-assembled supramolecular biosensors: a highly selective and sensitive enzyme-responsive AIE-based fluorescent biosensor for trypsin detection and inhibitor screening

Trypsin, a pancreatic enzyme associated with diseases like pancreatic cancer and cystic fibrosis, requires effective diagnostic tools. Current detection systems seldom utilize macrocyclic molecules and tetraphenyl ethylene (TPE) derivative-based supramolecular assemblies, known for their biocompatibility and aggregation-induced emission (AIE) properties, for trypsin detection. This study presents an enzyme-responsive, AIE-based fluorescence 'Turn-On' sensing platform for trypsin detection, employing sulfated-β-cyclodextrin (S-βCD), an imidazolium derivative of TPE (TPE-IM), and protamine sulfate (PrS). The anionic S-βCD and cationic TPE-IM formed a strongly fluorescent supramolecular aggregation complex in an aqueous buffer. However, PrS suppresses fluorescence because of its strong binding affinity with S-βCD. The non-fluorescent TPE-IM/S-βCD/PrS supramolecular assembly system exhibits trypsin-responsive properties, as PrS is a known trypsin substrate. Trypsin restores fluorescence in the TPE-IM/S-βCD system through the enzymatic cleavage of PrS, correlating linearly with trypsin catalytic activity in the 0-10 nM concentration range. The limit of detection is 10 pM. This work contributes to the development of self-assembled supramolecular biosensors using charged TPE derivatives and β-cyclodextrin-based host-guest chemistry, offering an innovative fluorescence 'Turn-On' trypsin sensing platform. The sensing system is highly stable under various conditions, selective for trypsin, and demonstrates potential for biological analysis and disease diagnosis in human serum. Additionally, it shows promise for the screening of trypsin inhibitors.

Al3+-Responsive Ratiometric Fluorescent Sensor for Creatinine Detection: Thioflavin-T and Sulfated-β-Cyclodextrin Synergy

Kidney disorders are a rising global health issue, necessitating early diagnosis for effective treatment. Creatinine, a metabolic waste product from muscles, serves as an ideal biomarker for kidney damage. The existing optical methods for creatinine detection often involve labor-intensive synthesis processes and present challenges with the aqueous solubility and sensitivity to experimental variations. In this study, we introduce a straightforward fluorescence "turn-on" ratiometric sensor system for creatinine detection in aqueous media with a limit of detection of 0.5 μM. The sensor is based on sulfated-β-cyclodextrin (SCD)-templated H-aggregate of a commercially available, ultrafast rotor dye thioflavin-T (ThT). The Al3+ ion-induced dissociation of ThT-SCD aggregates, followed by reassociation upon creatinine addition, generates a detectable signal. The modulation of monomer/aggregate equilibrium due to the disassembly/reassembly of the ThT-SCD system under Al3+/creatinine influence serves as the optimal strategy for ratiometric creatinine detection in aqueous media. Our sensor framework offers several advantages: utilization of the readily available dye ThT, which eliminates the need for a laborious synthesis of custom fluorescent probes; ratiometric sensing, which improves quantitative analysis accuracy; and compatibility with complex aqueous media. The sensor's practical utility has been successfully demonstrated in artificial urine samples. In summary, our sensor system represents a significant advancement in the rapid, selective, and sensitive detection of the clinically crucial bioanalyte creatinine, offering potential benefits for the early diagnosis and management of kidney disorders.

A turn-on fluorescence sensor for detection of heparinase with heparin templated aggregation of tetracationic porphyrin derivative

Heparinase is the only mammalian endoglycosidase that breaks down the commonly used blood-anticoagulant heparin into therapeutically relevant low-molecular-weight-heparin. Importantly, heparinase has been considered a malignant disease diagnostic marker. Thus, it is essential to develop detection scheme for heparinase. However, optical methods for heparinase determination are limited. In the present work, we report a turn-on fluorescence sensor for detection of heparinase that utilizes heparin-templated aggregation of a tetra-cationic porphyrin derivative, TMPyP4+, as a sensing framework. Heparinase cleaves the glycosidic linkage between hexosamine and uronic acid in the structure of heparin to destroy its polyelectrolytic nature that originally causes the aggregation of TMPyP4+. Thus, heparinase leads to dissociation of TMPyP4+ aggregates and generates an optical signal. This system leads to a sensitive and selective response towards heparinase with a Limit of Detection (LOD) of 0.3 pmol/L. Further, the same system is demonstrated to sense a trace amount of Oversulfated Chondrootin Sulphate (OSCS) in heparin, which is a heparin adulterant, by utilizing the fact that OSCS serves as an inhibitor for heparinase activity, which leads to reverse modulation in the photo-physical features of the monomer/aggregate equilibrium of the TMPyP4+-heparin-heparinase system. The sensing mechanism has been thoroughly demonstrated by ground-state absorption, steady-state emission, and time-resolved emission measurements. The selectivity of the sensor was tested using lysozyme, α-amylase, pepsin, trypsin, lipase, and glucose oxidase in the heparinase selectivity study and the method is also validated using another method reported in the literature. The study provides a new approach for the development of optical methods for the detection of heparinase and oversulfated chondroitin sulfate, which is currently limited.

A highly sensitive hemicyanine-based near-infrared fluorescence sensor for detecting toxic amyloid aggregates in human serum

The development of an accurate and sensitive sensor for detecting amyloid plaques, which are responsible for many protein disorders like Alzheimer's disease, is crucial for early diagnosis. Recently, there has been a notable increase in the development of fluorescence probes that exhibit emission in the red region (>600 nm), aiming to effectively tackle the challenges encountered when working with complex biological matrices. In the current investigation, a hemicyanine-based probe, called LDS730, has been used for the sensing of amyloid fibrils, which belong to the Near-Infrared Fluorescence (NIRF) family of dyes. NIRF probes provide higher precision in detection, prevent photo-damage, and minimize the autofluorescence of biological specimens. The LDS730 sensor emits in the near-infrared region and shows a 110-fold increase in fluorescence turn-on emission when bound to insulin fibrils, making it a highly sensitive sensor. The sensor has an emission maximum of ~710 nm in a fibril-bound state, which shows a significant red shift along with a Stokes' shift of ~50 nm. The LDS730 sensor also displays excellent performance in the complicated human serum matrix, with a limit of detection (LOD) of 103 nM. Molecular docking calculations suggest that the most likely binding location of LDS730 in the fibrillar structure is the inner channels of amyloid fibrils along its long axis, and the sensor engages in several types of hydrophobic interactions with neighboring amino acid residues of the fibrillar structure. Overall, this new amyloid sensor has great potential for the early detection of amyloid plaques and for improving diagnostic accuracy.

A cationic AIEgen and hexametaphosphate based simple and convenient fluorometric assay for alkaline phosphatase and its inhibitor

Alkaline phosphatase (ALP) is an important biomarker to diagnose a number of diseases, such as anaemia, hepatobiliary diseases, chronic nephritis, and hypothyroidism. Therefore, the development of simple and convenient assays to monitor levels of ALP is highly desirable. In the present study, an aggregation-induced emission based simple, real-time, and direct fluorescence detection platform has been developed, by using a tetracationic pyridinium derivative of tetraphenylethylene (TPy-TPE) and anionic sodium hexametaphosphate (HMP) as component units. The sensing system, based on the TPy-TPE-HMP assembly, is highly responsive to the ALP dependent disintegration of the TPy-TPE-HMP aggregation complex, owing to HMP digestion by ALP. The sensing platform has an ALP detection limit of 16 mU mL^-1 and linear range of 0-742 mU mL^-1, respectively. The enzyme kinetic parameters, K_m and V_max, have been evaluated. In addition, the potential applicability of the TPy-TPE-HMP sensing system has also been shown with diluted human serum samples. Moreover, the TPy-TPE-HMP probe system is also useful for screening inhibitors of ALP.

Polyanionic Cyclodextrin-Induced Supramolecular Assembly of a Cationic Tetraphenylethylene Derivative with Aggregation-Induced Emission

The combination of supramolecular chemistry and aggregation-induced emission-based luminogens (AIEgens) has recently attracted tremendous attention because of its ability to offer large emission enhancement even in substantially dilute solutions. In this work, a new aggregation-induced emission (AIE)-based supramolecular assembly has been reported, which consists of a polyanionic cyclodextrin derivative and a tetracationic tetraphenylethylene (TPE) derivative. Ionic cyclodextrins have attracted significant attention in host-guest supramolecular chemistry and pharmaceutical industry. However, ionic derivatives of β-cyclodextrins have not been explored to establish noncovalent interactions-based aggregation assembly of the most popular class of AIEgens, i.e., tetraphenylethylene derivatives. The current report demonstrates AIE of a tetracationic methyl pyridinium derivative of tetraphenylethylene (TPy-TPE) induced by a polyanionic sulfated β-cyclodextrin (S-βCD). The AIE-based supramolecular assembly has been thoroughly investigated using steady-state fluorescence, ground-state absorbance, and time-resolved fluorescence measurements. Further, the response of the supramolecular assembly towards external stimuli, such as, ionic strength, pH, and temperature, has been investigated. In addition, the complexation behavior of the TPE derivative has also been compared with the native neutral β-cyclodextrin derivative, which delineates the important role of the negatively charged portal of S-βCD in inducing aggregation of the TPy-TPE. The stoichiometry of the complex has been found to be 3:1 for TPy-TPE:S-βCD, using Job's plot analysis. Finally, to get insights into the underlying interactions between the supramolecular assembly components, molecular docking calculations have been performed.

Anionic Polyelectrolyte-Induced Aggregation of Basic Orange 21: A Clue toward Metachromasia

The change in the color of chromophore upon being embedded in a biological tissue is known as metachromasia. Basic Orange 21 (BO21) is a cationic polymethine dye that has been implied as a supravital dye, which produces metachromasia in leukocytes. An improved differential counting of leukocytes has been achieved in the clinical setup based on characteristic metachromatic expressions of BO21 for different types of leukocytes. Although BO21 has been utilized as a chromatic indicator for leukocyte counting, there are limited number of investigations that focus on the factors that may be responsible for the spectral shift in absorption and emission spectra of BO21, which leads to its metachromatic behavior. In this work, we have investigated the effect of a synthetic anionic polyelectrolyte, polystyrene sulfonate (PSS), on the photophysical properties of BO21, using steady-state emission, ground-state absorption, and time-resolved emission measurements, to get an understanding of the factors that may be responsible for the spectral shift of BO21 in the cellular environment. PSS induces aggregation of BO21 molecules with large changes in its photophysical properties; this appears to be most likely the mechanism of spectral shift for BO21 reported in the cellular environment. The employment of external stimulus reveals BO21 aggregates to be significantly responsive toward external stimuli, for example, temperature and presence of salt in the medium, which further strengthens the proposal of aggregate formation. Further, we have also employed fluorescence upconversion spectroscopy with subpicosecond time resolution to estimate the excited-state lifetime of BO21.

Supramolecular Control on the Optical Properties of a Dye-Polyelectrolyte Assembly

Control of fluorescent molecular assemblies is an exciting area of research with large potential for various important applications, such as, fluorescence sensing/probing, cell imaging and monitoring drug-delivery. In the present contribution, we have demonstrated control on the extent of aggregation of a dye-polyelectrolyte assembly using a macrocyclic host molecule, sulfobutylether-β-cyclodextrin (SBE-β-CD). Initially, a cationic molecular rotor based organic dye, Auramine-O (AuO), undergoes aggregation in the presence of an anionic polyelectrolyte, polystyrene sulfonate (PSS), and displays a broad intense new emission band along with large variation in its absorption features and excited-state lifetime. A manipulation of the monomer-aggregate equilibrium of the dye-polyelectrolyte assembly has been achieved by introducing a cyclodextrin based supramolecular host, SBE-β-CD, which leads to relocation of AuO molecules from polyelectrolyte (PSS) to supramolecular host cavity, owing to the formation of a host-guest complex between AuO and SBE-β-CD. A reversible control on this manipulation of monomer-aggregate equilibrium is further achieved by introducing a competitive guest for the host cavity i. e., 1-Adamantanol. Thus, we have demonstrated an interesting control on the dye-polyelectrolyte aggregate assembly using a supramolecular host molecule which open up exciting possibilities to construct responsive materials using a repertoire of various host-specific guest molecules.

A Heparin based dual ratiometric sensor for Thrombin

Thrombin is an important enzyme that plays a pivotal role in the blood clotting pathways. An imbalance in the activity of this enzyme is clinically known to be associated with various diseases, such as thrombosis, inflammation, atherosclerosis, and haemophilia, suggesting the need to devise sensors for Thrombin detection. However, the majority of the fluorescence-based Thrombin assays rely on fluorescence labelling assays or Thrombin specific recognition biomolecules, such as, aptamers or antibody which requires sophisticated techniques and makes it very expensive. Herein, we report a simple, selective, sensitive and label-free fluorescence detection scheme for Thrombin which is based on the interaction between Thrombin and a fluorescent complex of Heparin with a molecular rotor dye, Thioflavin-T. The detection scheme exploits selective interaction between cationic Thrombin and anionic Heparin to modulate the monomer-aggregate equilibrium of the Thioflavin-T-Heparin system. Importantly, the present system offers a ratiometric response that has the ability for robust quantification of Thrombin concentration even in complex medium. The involvement of all commercially available components is a crucial advantage of this detection scheme. Further, the detection scheme also shows reasonable response in diluted serum matrix.

An ATP responsive fluorescent supramolecular assembly based on a polyelectrolyte and an AIE active tetraphenylethylene derivative

Aggregation induced emission (AIE) active probes have attracted enormous attention due to their wide-spread and ever increasing number of applications in the sensing of chemically and biologically important molecules. AIE probes undergo drastic modulation in their photophysical features from a monomeric to aggregated state. In the current work, we report the aggregation of tetra-anionic Su-TPE (AIE active probe) in the presence of a cationic polyelectrolyte, poly(allylaminehydrochloride) (PAH). A supramolecular assembly is formed by the electrostatic interaction between cationic PAH and anionic Su-TPE molecules, which leads to drastic modulations in the spectral features of anionic Su-TPE upon addition of cationic PAH. The Su-TPE-PAH aggregate assembly has been investigated using various photophysical techniques, such as, ground-state absorption, steady-state and time-resolved emission spectroscopic techniques along with 1H NMR measurements. The Su-TPE-PAH aggregate assembly is found to be responsive towards the ionic strength of the medium and temperature which results in drastic modulations of the spectral features of the emissive supramolecular aggregate assembly. Finally, the specific recognition of an important bioanalyte, ATP, has been achieved using the formed Su-TPE-PAH supramolecular aggregate assembly as a sensing platform which displays good selectivity and high sensitivity towards ATP. Importantly, the developed sensor platform could also function in the human serum matrix, hence, demonstrating the potential of the established sensor platform for real-life applications in near future.

An anionic polyelectrolyte induced aggregate assembly of Thioflavin-T: A prospective platform for Protamine sensing

Protamine, a polycation, is biologically and medically relevant protein. Protamine exhibits a wide array of functions in biological processes like gene transfer, tissue and organogenesis, cell reproduction, etc. Medically, Protamine is the only clinically approved antidote for Heparin and is routinely used in various surgical interventions, and hence controlling Protamine dosing in patients is very crucial. Taking into account the medical significance of Protamine, designing simple, reliable and sensitive fluorescence sensors is highly desirable. In this work, we propose one such sensitive and reliable fluorescent sensor which is based on a template of dye-polyelectrolyte assembly constituting a molecular rotor dye, Thioflavin-T and an anionic synthetic polyelectrolyte, polystyrene sulfonate. The addition of Protamine, prompts drastic modulations in spectral features of dye-polyelectrolyte assembly which enables sensitive detection of Protamine in aqueous solution. Apart from sensitive detection, our sensing platform aids in highly selective sensing of Protamine compared to other proteins. Moreover, our sensor system is constructed on label-free, inexpensive, commercially available molecules posing as an advantage over other sensor systems which involve laborious synthesis protocols. Most importantly, our sensor template is able to sense Protamine in diluted serum sample, indicating the potential practical utility of our sensor system.

Tuning of electron transfer reactions in pluronic-surfactant supramolecular assemblies

Photoinduced electron transfer (ET) reaction between an anionic acceptor, coumarin-343 (C343), and a neutral donor, N,N-dimethylaniline (DMAN), has been investigated in composite supramolecular assemblies (mixed micelles) comprised of a pluronic copolymer (P123: EO20-PO70-EO20 or F88: EO103-PO39-EO103 where EO: ethylene oxide and PO: propylene oxide) and a cationic surfactant (CTAC: cetyltrimethylammonium chloride), following fluorescence quenching studies. Systematic increase in the quenching rates for the studied donor-acceptor system with the increasing CTAC to pluronic molar ratio in the mixed micelles demonstrates a large modulation in the ET rates. The mixed micellar systems in the present cases are formed through the incorporation of the hydrocarbon chains of CTAC into the poly-PO core of the pluronic micelles whereby the cationic head groups of CTAC are placed at the periphery of the micellar core, protruded into the hydrated poly-EO corona region, leading to the formation of a positively charged layer deep inside these mixed micelles. Thus, the anionic C343 dye, initially dissolved at the micelle-water interface, experiences a gradually increasing electrostatic attraction and is therefore systematically dragged deeper inside the micellar corona, as the CTAC composition is increased in the mixed micellar systems. Consequently, the ET rate of the C343-DMAN pair undergoes a large enhancement in the studied mixed micellar systems with the increasing CTAC to pluronic molar ratio. The present strategy of modulating ET reactions using such composite supramolecular assemblies can find applications in areas where bimolecular ET is an integral reaction step.

Does the optimum hydrophilic lipophilic balance condition affect the physical properties of mixed reverse micelles? A spectroscopic investigation

Synergism in several physical properties as realized in many mixed surfactant reverse micellar (RM) systems often manifests optimum hydrophilic-lipophilic balance (HLB), interdroplet interaction, or both. Such synergism is often desired for specific applications of RM systems; however, a proper rationale on the effect of such phenomenon imparted on the structure, dynamics, and activity of water molecules in RM waterpool is strongly demanded. In the present contribution we have investigated how the optimum HLB condition of mixed RM composed two nonionic surfactants (Igepal 210 and Igepal 630) affects the physical properties of entrapped water molecules in the RM waterpool. The studied mixed RM exhibits synergistic water solubilization behavior as a function of the mixing ratio with a maximum in solubilization capacity being reached at X(Ig630) = 0.3. Dynamic light scattering (DLS) studies show a bimodal distribution of droplet size in this region, whereas it is monomodal in terminal compositions. Fourier transform infrared spectroscopy (FTIR) study in the 3000-3800 cm(-1) region identifies a linear trend in which the content of "bound" water increases at the expense of the "network" water as the content of the hydrophilic surfactant Igepal 630 is increased in the mixture. Subnanosecond relaxation dynamics of the entrapped water as revealed by the fluoroprobe coumarin 500 corroborates a similar linear trend as observed in the FTIR measurements as the rotational diffusion gets retarded with the increase of ethylene oxide chain length of Igepal. Reaction kinetics of solvolysis of benzoyl chloride reaction, however, does not offer any linear trend as it gets slower in the optimum HLB region, the nonlinearity being a consequence of the distribution of the substrate in the different phases.

Nanoreactors for pH controlled sequential activity switching in multistep enzymatic processes

This theoretical model predicts that the activity of multiple enzymes may be controlled simultaneously with superior efficiency in nanosized reactors by adjusting pH. Multistep enzymatic processes employed for various purposes including organic biotransformation may require application of multiple reactions and isolation of intermediates. Sequential activity switching would offer substantial advantages. Nanoreactors would provide better option to fully appreciate the pH switching approach.