Paper-based sensing of phytotoxicant gossypol in aqueous media through turn-on visible-light emitting lanthanide-luminescence.

Gossypol, a phytotoxicant in cotton-seed oil, has been found to sensitize Tb(III)-luminescence in a supramolecular hydrogel. Based on this observation, a paper-based sensor has been developed to detect gossypol with a limit of detection (LOD) of 2.9 nM. This is the first report of water-based detection with the highest sensitivity involving turn-on time-gated luminescence. This method was also able to sense gossypol in commercial crude cotton-seed oil.

Augmenting Antimicrobial Resistance Surveillance: Rapid Detection of ß-Lactamase-Expressing Drug-Resistant Bacteria through Sensitized Luminescence on a Paper-Supported Hydrogel.

The emergence of antimicrobial resistance (AMR) in pathogenic bacteria, expedited by the overuse and misuse of antibiotics, necessitates the development of a rapid and pan-territorially accessible diagnostic protocol for resistant bacterial infections, which would not only enable judicious prescription of drugs, leading to infection control but also augment AMR surveillance. In this study, we introduce for the first time a "turn-on" terbium (Tb3+) photoluminescence assay supported on a paper-based platform for rapid point-of-care (POC) detection of ß-lactamase (BL)-producing bacteria. We strategically conjugated biphenyl-4-carboxylic acid (BCA), a potent Tb3+ sensitizer, with cephalosporin to engineer a BL substrate CCS, where the energy transfer to terbium is arrested. However, BL, a major resistance element produced by bacteria resistant to ß-lactam antibiotics, triggers a spontaneous release of BCA, empowering terbium sensitization within a supramolecular scaffold supported on paper. The remarkable optical response facilitates quick assessment with a binary answer, and the time-gated signal acquisition ensues improved sensitivity with a detection limit as low as 0.1 mU/mL. Furthermore, to ensure accessibility, particularly in resource-limited areas, we have developed an in loco imaging device as an affordable alternative to high-end instruments. The integration of the assay with the device readily identified the BL-associated drug-resistant strains in the mimic urinary tract infection samples within 2 h, demonstrating its excellent potential for in-field translation. We believe that this rapid paper-based POC assay, coupled with the in loco device, can be deployed anywhere, especially in developing regions, and will enable extensive surveillance on antibiotic-resistant infections.

A photoluminescence assay with a portable device for rapid, sensitive and selective detection of europium and terbium.

Rare earth elements are essential in many real-life applications, but their steady supply is being affected by multiple challenges. The recycling of lanthanides from electronic and other waste is thus gaining momentum which makes the detection of lanthanides with high sensitivity and selectivity a critical area of research. We now report a paper-based photoluminescent sensor for the rapid detection of terbium and europium with low detection limit (nM), which has the potential to facilitate recycling processes.

Ratiometric rapid distinction of two structurally similar fluoroquinolone antibiotics by a Tb/Eu hydrogel.

Norfloxacin and ofloxacin are two frequently prescribed second-generation fluoroquinolone antibiotics with an identical 4-quinolone chromophore and hence, are difficult to distinguish by conventional methods (UV or fluorescence). We have designed a Tb3+/Eu3+/cholate cocktail that enabled us to differentiate these two drugs and rapidly measure their concentrations when present together. Additionally, a Tb3+-cholate gel-based paper sensor was developed to detect and quantify them in a single drug containing system with a limit of detection (LOD) well below 100 nM.

Secondary Nucleation-Triggered Physical Cross-Links and Tunable Stiffness in Seeded Supramolecular Hydrogels.

Mechanistic understanding and the control of molecular self-assembly at all hierarchical levels remain grand challenges in supramolecular chemistry. Functional realization of dynamic supramolecular materials especially requires programmed assembly at higher levels of molecular organization. Herein, we report an unprecedented molecular control on the fibrous network topology of supramolecular hydrogels and their resulting macroscopic properties by biasing assembly pathways of higher-order structures. The surface-catalyzed secondary nucleation process, a well-known mechanism in amyloid fibrilization and chiral crystallization of small molecules, is introduced as a non-covalent strategy to induce physical cross-links and bundling of supramolecular fibers, which influences the microstructure of gel networks and subsequent mechanical properties of hydrogels. In addition, seed-induced instantaneous gelation is realized in the kinetically controlled self-assembled system under this study, and more importantly, the extent of secondary nucleation events and network topology is manipulated by the concentration of seeds.

Naked-Eye Detection of Hydrogen Peroxide on Photoluminescent Paper Discs.

We have developed a turn-on photoluminescence protocol to detect hydrogen peroxide (H2O2) utilizing a supramolecular hydrogel as a sensing platform. Hydrogen peroxide is widely used in formulations, starting from healthcare products to explosives. It is also known to induce deleterious health effects at its irregular physiological concentration and considered as a biomarker in various disease conditions. We designed molecule 2, which releases the Tb3+ sensitizer biphenyl-4-carboxylic acid (1) upon unmasking by hydrogen peroxide. This chemistry led us to develop a sensitive photoluminescence assay for H2O2 through the 1-induced photoluminescence of terbium (Tb3+) in a hydrogel matrix. Paper discs (0.45 cm) were coated with the soft hydrogel to make the sensing process simple and cost-effective. The green luminescence from the paper discs, observed under a UV lamp, allowed naked-eye detection of H2O2 in the micromolar level without any sophisticated instrumentation. Image processing software or a plate reader can be used for the accurate quantification of the analyte in micromolar and nanomolar ranges. Several commercial hand sanitizers containing hydrogen peroxide were tested by this method. The results indicated that this low-cost system could be practically adopted, especially in resource-limited areas, to quantify/detect H2O2 for quality control purposes or other applications.

Energy transfer in FRET pairs in a supramolecular hydrogel template.

Fluorescence resonance energy transfer (FRET) in pairs of chromophores has mostly been achieved using covalently bound chromophores. In this study, we have demonstrated energy transfer in FRET pairs by taking advantage of the self-assembly of the chromophores on metal cholate hydrogel fibers.

An Inexpensive Paper-Based Photoluminescent Sensor for Gallate Derived Green Tea Polyphenols.

This work describes a terbium luminescence-based protocol to selectively detect gallate-derived green tea polyphenols on a supramolecular gel immobilised paper platform for the first time. This user-friendly, inexpensive (€â€…0.0015) approach requires very low sample volumes for the analysis. The developed strategy enables simultaneous detection of gallate polyphenols in multiple tea samples with the potential for practical applications.

An inexpensive and sensitive turn-on luminescence protocol for sensing formaldehyde.

Formaldehyde (FA), the simplest and most widely-used aldehyde, can pose serious health issues when present at elevated concentrations. Here, we report a "turn-on" terbium photoluminescence method for the efficient detection of FA. A pro-sensitizer molecule was designed and synthesised, which releases the sensitizer in the presence of FA inside the terbium cholate hydrogel matrix, resulting in a "turn-on" luminescence response. The introduction of a paper-based sensing approach makes the protocol simpler and cost-effective, and has a detection limit as low as 100 nM.

Supramolecular Gelation of Europium and Calcium Cholates through the Nucleation-Elongation Growth Mechanism.

A detailed understanding of gelation mechanism can enable the properties of gels to be tuned for various applications, and may possibly help in understanding the aggregation of different biomolecules. We report a detailed study of the morphological and physio-chemical changes, dynamics (of a probe), and kinetics during the gelation of europium and calcium cholate hydrogels, leading to the development of a growth model. AFM images showed the transition of aggregated particles (100-150 nm) in the sol phase growing to a fibrous network in the gel through the entanglement of fibres, and not by dendritic growth (height analysis). The dynamic changes during this phase transformation were studied using a fluorescence probe (change in intensity and lifetime). We have been able to delineate the growth mechanism by using a combination of Eu(III) luminescence and a polarity sensitive fluorescence probe. The growth was found to follow the nucleation-elongation model, and these two phases responded in distinctly different fashions in rheological and luminescence measurements.

Nanofiber-Directed Anisotropic Self-Assembly of CdSe-CdS Quantum Rods for Linearly Polarized Light Emission Evidenced by Quantum Rod Orientation Microscopy.

Hybrid soft materials composed of CdSe-CdS nanorods or "quantum rods" (QRs) and the fluorescent 2,3-didecyloxyanthracene (DDOA) low molecular weight organogelator are obtained through self-assembly. Spectroscopy, microscopy, and rheology studies show that the QRs and DDOA coassemble, thereby stabilizing the organogels. Depending on the QR load and excitation wavelength, single nanofibers (NFs) of the hybrid gel display either sharp polarized red luminescence (under green excitation), or dual perpendicularly polarized blue and red emissions (under UV excitation). Transmission electron microscopy, microspectroscopy, and quantum rod orientation microscopy (QROM) reveal that QRs align along the organogel NFs with order parameters reaching 76% and 87%. This paves the way for obtaining surfaces of QR/NF assemblies yielding sharp red linearly polarized emission. In addition, this work demonstrates that QRs can be used more generally to probe nanostructured soft materials, even nonemissive ones. QROM allows to establish maps of the orientation of single QRs dispersed onto or within a gel network by measuring the polarization of the emission of the individual QRs. As occurs within this work in which QRs and NFs interact, the orientation of each QR reveals information on the underlying nanostructure (such as surface striation, bundle formation, and helicity).

A self-assembled CdSe QD-organogel hybrid: photophysical and thermoresponsive properties.

A luminescent hybrid gel was prepared by incorporating organic ligand capped CdSe quantum dots (QDs) into a steroid-dimer derived organogel. Photophysical measurements and electron microscopy studies allowed us to understand the nature of the hybrid. Detailed analysis of the excited state dynamics of the hybrid was carried out using a kinetic decay model. The luminescence of the QDs in the hybrid was unaltered by taking it through a gel-sol-gel cycle induced by thermal stimuli. We believe that the results obtained herein provide a route to develop a thermoresponsive device for practical applications, because of the spatial assembly between soft organic scaffolds and colloidal QDs.

Eu/Tb luminescence for alkaline phosphatase and ß-galactosidase assay in hydrogels and on paper devices.

Simple technologies for efficient detection of important (bio)molecules are always in great demand. We now report the detection and assay of two biologically important enzymes, alkaline phosphatase and ß-galactosidase, in Eu- or Tb-based cholate hydrogels, respectively, and on filter paper discs coated with such hydrogels. Pro-sensitizers derived from 1-hydroxypyrene and 2,3-dihydroxynaphthalene were incorporated into Eu or Tb cholate hydrogels, respectively. Upon enzyme action, these artificial substrates liberate free sensitizers both in the gel and on gel-coated discs, resulting in turn-on luminescence, red/magenta for Eu, and green for Tb. The detection of enzymes was also demonstrated in natural/biological samples using low-cost systems.

Instant room temperature synthesis of self-assembled emission-tunable gold nanoclusters: million-fold emission enhancement and fluorimetric detection of Zn2+ .

Facile synthesis of luminescent metal nanoclusters (NCs) accompanied by emission color tuning is currently an active area of research. In this work we describe a rapid (1 s) room temperature synthesis of luminescent Au NCs from completely nonluminescent NCs through the incorporation of Zn2+. The nanoclusters are initially stabilized by mercaptopropionate, and the coordination of Zn2+ with the carboxylate groups of the ligands rigidifies the Au(i) thiolates restricting the intramolecular rotation-vibrational motion. This significantly reduces the nonradiative relaxation of the excited state to produce yellow luminescent NCs (λem = 580 nm, QY: 6%, τ = 0.2 ms) with almost a million-fold emission enhancement. The enhanced luminescence is due to the self-assembly mediated aggregation induced emission (AIE) of NCs. These NCs on aging for 24 hours transform to highly ordered green emitting NCs (λem = 500 nm, QY: 20%, τ = 20 ns). The blue shift in emission is due to the dominance of inter Au(i)-Au(i) interaction and inter-NC Zn2+ interaction over the intra modes. TEM images show this distinct transition, a decrease in inter NC distance with increased self-assembly. Excited state relaxation dynamics associated with Au(i) thiolate shell dynamics in yellow and green emitting NCs is explained based on the time resolved fluorescence study. The rapid formation of luminescent NCs from nl-NCs has been used for efficient visual and fluorimetric detection of Zn2+.

In situ formation of luminescent CdSe QDs in a metallohydrogel: a strategy towards synthesis, isolation, storage and re-dispersion of the QDs.

A one step, in situ, room temperature synthesis of yellow luminescent CdSe QDs was achieved in a metallohydrogel derived from a facially amphiphilic bile salt, resulting in a QD-gel hybrid. An ordered self-assembly and homogeneous distribution of the CdSe QDs in the hydrogel network was observed from optical and electron micrographs. The different excited state behavior of the CdSe QDs in the hybrid was revealed for the first time using time resolved spectroscopy. We also describe the successful isolation of the photoluminescent CdSe QDs from the gel followed by their re-dispersion in an organic solvent using suitable capping ligands.

Metallogels of indium(iii) with bile salts: soft materials for nanostructured In2S3 synthesis.

This work describes sonication mediated facile metallo-hydrogel formation from biodetergents sodium cholate and deoxycholate in the presence of indium(iii). The resulting thermo-irreversible gels behaved as viscoelastic soft solids as observed by rheological measurements. Scanning electron and atomic force microscopy analysis revealed the transition from spherical aggregates before gelation to typical entangled three-dimensional fibrous networks in gels. The In-Ch hydrogel was further used to prepare nanostructured In2S3 in which the cholate units acted as surfactants to limit the growth of the nanoflakes.

Luminescence Resonance Energy Transfer in a Multiple-Component, Self-Assembled Supramolecular Hydrogel.

Cascade energy transfer from a sensitizer to TbIII then to fluorescent dyes was studied for the first time in a supramolecular hydrogel. Efficient energy transfer from TbIII to the dyes was observed, as established by time-delayed emission and excitation spectral analysis, lifetime data, and microscopic studies.

Hierarchical self-assembly of photoluminescent CdS nanoparticles into a bile acid derived organogel: morphological and photophysical properties.

We have described a strategy towards integrating photoluminescent semiconductor nanoparticles into a bio-surfactant derived organogel. A facially amphiphilic bile thiol was used for capping CdS nanoparticles (NPs) which were embedded in a bile acid derived new organogelator in order to furnish a soft hybrid material. The presence of CdS NPs in a well-ordered 1D array on the organogel network was confirmed using microscopic techniques. Photophysical studies of the gel-NP hybrid revealed resolved excitation and emission characteristics. Time resolved spectroscopic studies showed that the average lifetime value of the CdS NPs increased in the gel state compared to the sol phase. A kinetic model was utilized to obtain quantitative information about the different decay pathways of the photoexcited NPs in the sol and gel states.

A Stimuli-Responsive Metallohydrogel Exhibiting Cyclohexane-Like Hydrophobicity.

Silver(I) forms a hydrogel in the presence of cholate with unusual properties, which are not observed with other cations. Polarity-sensitive probes have shown that the spherical aggregates observed in the gel have 'pockets' with hydrophobicity comparable to that of degassed cyclohexane. The gel exhibited thermo- and mechanoresponsive properties. Color tunability from blue to cyan and green was observed with prodan. The two sol phases of the gel formed by applying stress and temperature showed very different properties.

A novel strategy towards designing a CdSe quantum dot-metallohydrogel composite material.

We have described here an efficient method to disperse hydrophobic CdSe quantum dots (QDs) in an aqueous phase using cetyltrimethylammonium bromide (CTAB) micelles without any surface ligand exchange. The water soluble QDs were then embedded in 3D self assembled fibrillar networks (SAFINs) of a hydrogel showing homogeneous dispersibility as evidenced from optical and electron microscopic techniques. The photophysical studies of the hydrogel-QD composite are reported for the first time. These composite materials may have potential applications in biology, optoelectronics, sensors, non-linear optics and materials science.