Tuning of Intermolecular Electron Transfer Reaction by Modulating the Microenvironment Inside Copolymer−Surfactant Supramolecular Assemblies

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.

Supramolecular tuning of thioflavin-T aggregation hosted by polystyrene sulfonate

Tunable and controllable emission is an extremely desirable feature for advanced functional materials that finds usage in optoelectronic utilization, fluorescence probing/sensing, drug-delivery monitoring, etc. In the present contribution, we have employed a macrocyclic host molecule, sulfobutyl ether-β-cyclodextrin (SBE-β-CD), as a tuning agent for an intensely emissive aggregate assembly of a molecular rotor dye, thioflavin-T (ThT), in the presence of an anionic polyelectrolyte, polystyrene sulfonate (PSS). The macrocyclic host breaks the PSS templated ThT aggregates and leads to encapsulation of released ThT molecules, tailoring the emission response of the system in terms of intensity and wavelength. Utilizing the established selectivity of the cyclodextrin-adamantane system, reverse control of this tunable emission has been further achieved. The controllable fluorescence system has been extensively investigated using ground-state absorption, steady-state and time-resolved emission spectroscopy. This kind of supramolecular tailoring of self-assembled aggregate emission has enormous potential in the field of fluorescence sensors and probes, and imaging and tracking in biological systems.

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.

A novel supramolecule-based fluorescence turn-on and ratiometric sensor for highly selective detection of glutathione over cystein and homocystein

A cyclodextrin-based fluorescence light-up and ratiometric sensor is reported for highly selective and sensitive recognition of glutathione over cystein and homocystein. The sensing scheme developed builds up on a supramolecular assembly formed between a molecular rotor dye (ThT) and a polyanionic supramolecular host (sulfated-β-cyclodextrin, SCD). The detection scheme is accomplished as follows: firstly, the bivalent Cu²⁺ quenches the emission from ThT-SCD assembly by causing the dissociation of ThT molecules from SCD surface. Secondly, when GSH is added to the copper-quenched system, owing to specific interaction between Cu²⁺ and GSH, Cu²⁺ is removed from the SCD which again allows the formation of ThT-SCD assembly. Indeed, this scheme of disassembly and reassembly successively caused by Cu²⁺ and GSH in the aqueous solution empowers our sensor framework to work as a good ratiometric sensor for the detection of GSH. The sensor scheme shows a linear response in the range 0-250 μM with a LOD of 2.4 ± 0.2 μM in aqueous solution and 13.6 ± 0.5 μM in diluted human serum sample. The sensor system is excited at 410 nm and the emission signal is plotted as a ratio of intensity at 545 nm (aggregate band) and 490 nm (monomer band). This ratiometric sensor system is highly selective to glutathione over cystein, homocystein, and other amino acids. Additionally, response of the sensor system towards GSH in complex biological media of serum samples demonstrates its potential for practical utility. Graphical abstract.

Kinetics and Energetics of Ultrafast Bimolecular Photoinduced Electron Transfer Reactions in Pluronic-Surfactant Supramolecular Assemblies

Understanding the kinetics and energetics of photoinduced electron transfer (PET) reactions in constrained media has attracted considerable research interest, as constrained media provide a handle to tune the microenvironments and consequently the mechanisms of PET reactions. In this study, PET reactions between excited 7-aminocoumarin acceptors and ground-state N,N-dimethylaniline (DMAN) donor have been investigated in mixed micellar media composed of triblock copolymer, P123, and anionic surfactant, sodium dodecyl sulfate (SDS), with varying SDS-to-P123 molar ratios (n values). The objective is to elucidate the role of the n values in the rates and energetics of PET reactions over the entire time range from the subpicosecond to the subnanosecond domain, especially in regard to the applicability of the two-dimensional ET (2DET) mechanism. It is observed that by changing the n values, there is a significant change in the hydration characteristics of the SDS-P123 mixed micelles, which in turn changes the kinetics to energetic correlations for the PET reactions. Fluorescence from the excited coumarin acceptors undergoes substantial quenching due to PET from DMAN donor in all of the studied micelles as evidenced from steady-state, subnanosecond time-resolved (TR) and ultrafast (subpicosecond/femtosecond) fluorescence up-conversion measurements. The quenching rate constants (k_q), estimated from subnanosecond TR fluorescence studies, and the individual component-wise decay rates (τ_i^-1), estimated from up-conversion measurements, increase gradually with increasing n value, corroborating well with the sequentially increased micropolarity of the mixed micelles. Interestingly, it is observed that the correlations of either k_q (from subnanosecond studies) or τ_i^-1 (from femtosecond studies) with the reaction exergonicity (-ΔG°) show the noteworthy Marcus inversion (MI) behavior in a very consistent and similar manner for the entire time window, from subpicoseconds to subnanoseconds. The onset of MI always appears at an exergonicity (-ΔG°_MI) much lower than solvent reorganization energy (λ_s), suggesting the involvement of 2DET mechanism throughout the subpicosecond to subnanosecond time domains. The present results thus provide a comprehensive picture of the kinetics and energetics of the PET reactions in constrained media for the whole time span and unequivocally establish the applicability of 2DET mechanism for the PET reactions in constrained media, eliminating any apprehensions about the effect of time resolution of the subnanosecond setup on the observed Marcus inversion behavior. This is indeed an important finding, providing valuable insights for PET reactions in constrained media, which has not been explored explicitly in any of the previous studies. Observation of MI behavior and the modulations in the PET reactions by simply changing the composition of SDS in the SDS-P123 mixed micelles are noteworthy findings of the present study and are expected to find suitable applications for better utilization and outcome of the PET reactions.

Effect of Ionic Liquids as Cosurfactants on Photoinduced Electron Transfer in Tetronic Micelles

This study investigates the role of varying alkyl chain lengths of a series of surface-active 1-alkyl-3-methylimidazolium tetrafluoroborate ([C _nMIm][BF₄], n = 4, 6, and 10) ionic liquids (ILs) as cosurfactants in modifying the micellar characteristics of a tetronic star-block copolymer, T1304, and the consequent effects on bimolecular photoinduced electron transfer (PET) reactions carried out in these T1304-IL mixed micellar systems. Using coumarin 153 as the probe dye and following ground-state absorption, steady-state fluorescence, and time-resolved emission measurements, the micropolarity, microviscosity, and solvent relaxation dynamics in the micellar palisade layer have been revealed both in pure T1304 and in T1304-IL systems. With increasing alkyl chain length of the ILs, the palisade layer of the micelles gradually becomes more polar and less viscous, suggesting better incorporation of the longer alkyl chain length ILs as cosurfactants into the T1304 micelles. The bimolecular PET reactions, involving 7-aminocoumarins as acceptors and N, N-dimethylaniline as the donor, are considerably modulated in T1304 micelles by the presence of the ILs, the effect being more prominent for ILs with longer alkyl chain lengths. In all of the micellar systems, correlations of the electron transfer (ET) kinetics with the reaction exergonicity (-Δ G⁰) show clear Marcus inversion (MI) behavior where onsets of MI invariably appear at significantly lower exergonicities, suggesting the involvement of a two-dimensional ET mechanism. Interestingly, the Marcus correlations display significant variations, namely, enhanced reaction rates and gradual shift in the onset of MI toward higher exergonicity, as longer alkyl chain length ILs are sequentially introduced as cosurfactants. From the observed results, it is convincingly realized that 1-alkyl-3-methylimidazolium-based ILs can be used satisfactorily as cosurfactants in tetronic star-block copolymer solutions to modulate PET reactions very significantly for their better utilizations in suitable applied areas.

Self-assembled nanocontainer mediated oxidation of Fe(ii) by Cu(ii)–neocuproine complex: a model system to emulate electron transfer proteins

Catalysis of coordination inspired Fe(ii) oxidation by Cu(ii)–neocuproine complex.

Microstructure of copolymeric micelles modulated by ionic liquids: investigating the role of the anion and cation

The addition of ionic liquids significantly alters the micellization behaviour of, and has a profound effect on, the micelles of T1304.

Modulation of Excited State Proton Transfer Dynamics of a Lactim-Lactam Tautomeric System in Different Block Copolymer-Surfactant Aggregates

The proton transfer (PT) process in 1-(2-hydroxy-5-chloro-phenyl)-3,5-dioxo-1H-imidazo-[3,4-b]isoindole (ADCL) has been studied in three different copolymer-surfactant supramolecular assemblies prepared in aqueous 1% P123 triblock copolymer micellar solution with varying concentrations of surfactants (sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and triton-X-100 (TX 100)). The aim of the present study is to monitor the modulation of the PT process by changing the degree of micellar hydration inside the P123 micelle with the addition of the three different surfactants (two ionic and one non ionic), that is, in P123-surfactant aggregates. Besides, a comparative study has been done with these results with those in water, pure P123 micellar medium and three different surfactants medium. The micropolarity measurement and time-resolved fluorescence anisotropic measurements have been performed to evaluate the binding location of the probe (ADCL) in the three different copolymer-surfactant supramolecular assemblies. It is found that the micropolarity at the binding site of the molecule in the various environments largely influences the PT rate of ADCL. The PT rate is found to be the slowest in the P123 medium and in P123-surfactant aggregates the rate becomes faster as the micropolarity around the binding locations of the molecule in these aggregates is higher in comparison to that in P123 micelle.

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.

Unusual H-type aggregation of coumarin-481 dye in polar organic solvents

Coumarin-481 (C481) dye shows intriguing time-resolved (TR) fluorescence behavior in polar organic solvents of both protic and aprotic nature, namely, ethanol (EtOH) and acetonitrile (ACN), demonstrating the presence of multiple emitting species in the solution. Following concentration-dependent and wavelength-dependent TR fluorescence measurements and the time-resolved emission spectra (TRES) and time-resolved area-normalized emission spectra (TRANES) subsequently constructed using wavelength dependent decay parameters, we convincingly conclude that in the studied solvents a part of the dissolved dye undergoes H-type of aggregation, even at a very low dye concentration. This is quite an unusual finding because the dye C481 apparently shows reasonably good solubility in these organic solvents. As the TR measurements indicate, major contribution in the fluorescence decays is due to monomeric dye, having reasonably short lifetime (∼0.64-0.68 ns), which is in accordance with the conversion of fluorescent intramolecular charge transfer (ICT) state to nonfluorescent twisted intramolecular charge transfer (TICT) state suggested for the dye in high polarity solvents, causing an efficient nonradiative deexcitation. The minor contributions arising from the aggregated dyes show its clear presence in the decays at the blue edge of the emission spectra and have relatively longer lifetimes (∼1.2-5.2 ns) because the steric hindrance caused by the stacked dyes resists the ICT to TICT conversion. Aggregation of C481 dye as observed in the present study in polar organic solvents is an intriguing finding, as the dye is a widely used fluorescent probe for various photochemical studies, where overlooking such aggregation can mislead the observed results.

Determination of Reaction and Reorganization Free Energies of Electron Transfer Reactions under Restricted Geometry Conditions

In this paper, different methods of obtaining the two parameters controlling the rate of electron transfer processes (reaction and reorganization free energies, Λ and Δ G0’, respectively) under restricted geometry conditions are considered. The main difficulty of accomplishing this comes from lack of knowledge of the properties in the interfacial region, where the reaction occurs. A general method has been presented and illustrated with the study of intermolecular processes in micelles. This method is optimized when the free energies for (at least) the three reactions required are quite different. For excited state electron transfer, the general approach is based on the appearance of the so-called Marcus inverted region: at the starting point of this region the value of Δ G0’ gives the value of Λ directly. These reaction free energies also present some uncertainties because in their calculation it is necessary to know the value of the local dielectric constant. Finally, it should be mentioned that some authors have suggested that the treatments for electron transfer reactions could not be applicable under restricted conditions. However, experiments do seem to show the applicability of the Marcus-Hush treatment under these conditions.