Chiral Inversion and Recovery of Supramolecular Luminescent Copper Nanocluster Hydrogels Triggered by Polyethyleneimine and Polyoxometalates

Self-assembly of boc-protected diphenylalanine functionalized with boron-dipyrromethane

Peptides functionalized with fluorescent dyes can form aggregated structures that show potential for photonic and/or theranostic applications. In this study, we used the good-bad solvent method to induce the self-assembly of chromopeptides into various products, which were characterized using a multi-technique spectroscopic approach. Two hydrophobic chromopeptides were designed by attaching boron-dipyrromethane dyes to Boc-protected diphenylalanine, resulting in spherical aggregates with tunable dimensions, influenced by the compound's molar concentration and solvent composition. These aggregates exhibit distinct optical properties, including bathochromic shifts in absorption and emission bands, as well as induced chirality, suggesting different chromophore orientations within the microspheres. Moreover, the aggregates show accelerated excited-state relaxation compared to monomeric chromopeptides, primarily through non-radiative processes. This work establishes a foundation for future studies focused on optimizing the desired photophysical properties of similar chromopeptide aggregates, depending on their intended application.

Pathway Complexity of Kinetically Trapped Dipeptide-Based Metastable State: Supramolecular Structural Transformation and Helicity Tuning

Understanding the complexity of nanostructures involved during the supramolecular polymerization process can be achieved by kinetic control rather than thermodynamic stability. Study on supramolecular pathway complexity and associated nanostructures will provide precise control over the materials' properties. This work illustrates the pathway complexity and structural transformation of a naphthalimide-(NMI)-conjugated dipeptide from monomer to thermodynamically stable aggregated state in a binary mixed solvent system (DMSO and water). The self-assembly propensity can be modulated by changing the ratio of water, which offers an effective approach to provide kinetic stability to the on-pathway gel state before reaching its thermodynamically stable crystalline precipitate state. An in-depth spectroscopic and microscopic investigation suggested that the self-assembly process initiated the formation of tiny particles, which further nucleated to form a helical nanofibrilar assembly. At higher water percentages, the supramolecular gel state showed a transient behavior and proceeded toward its thermodynamic stability. However, at lower water percentage, the self-assembly process is kinetically trapped in its gel state. Here, the helicity of nanofibers can be modulated by altering the percentage of water in the mixed solvent. The self-assembled system is completely thermoreversible and can retain its chiral memory even after complete dissolution in the respective solvent composition.

Chiral co-assembly of a polyoxometalate complex with an achiral pyrene derivative enables redox-modulated circularly polarized luminescence

We report the fabrication of helical structures with responsive circularly polarized lumunescence (CPL) via the chiral co-assembly of a cholesterol-modified Lindqvist type polyoxometalate (POM) and an achiral pyrenyl derivative. The chiral surfactant encapsulated POM (CSEP) complex was synthesized by combining (TBA)2[Mo6O19] with cholesterol-containing organic surfactants through ion exchange. It was found that the CSEP complex self-assembled into left-handed helical structures in mixed organic solvents, which could serve as a chiral template that enables achiral pyrenyl fluorophores (Py) to exhibit chiroptical properties. When doping Py at a ratio of 5 wt% into the system, the chiral co-assembly with CSEP in the mixed organic solvent results in the formation of helical nanofibers, which emit blue CPL signals. Furthermore, the chiral helical structures can be dynamically transformed to spherical aggregates upon UV illumination, accompanied by photochromism. The disappearance of CPL signals corresponded to the disruption of the chiral morphology in the co-assembled nanostructures. More importantly, the morphology transformation is reversible. The nanospheres transform into helical nanofibers under the oxidation of H2O2, which could trigger the regeneration of CPL signals. This work contributes to the understanding and development of chiral supramolecular systems featuring stimulus-responsive CPL switches.

Co-assemblies of Silver Nanoclusters and Fullerenols With Enhanced Third-Order Nonlinear Optical Response

Exploring potential third-order nonlinear optical (NLO) materials attracts ever-increasing attention. Given that the atomically precise and rich adjustable structural features of silver nanoclusters (Ag NCs), as well as the unique π-electron conjugated system of carbon-based nanomaterials, a supramolecular co-assembly amplification strategy to enhance the luminescent intensity and NLO performance of the hybrids of the two components, are constructed and the relationship between structures and optical properties are investigated. By combining water soluble Ag NCs [(NH4)6[Ag6(mna)6] (H2mna = 2-mercaptonicotinic acid, abbreviated to Ag6─NCs hereafter) containing uncoordinated carboxyl groups with water-soluble fullerene derivatives modified with multiple hydroxyl groups (fullerenols, C60─OH), the π-electron delocalization is expanded owing to non-covalent hydrogen bonding effect between Ag6─NCs and C60─OH, which provides a feasible basis for realizing the NLO response. Then, the co-assemblies are doped into a PMMA matrix to prepare composite film and its NLO properties are evaluated by Z-scan technique. Remarkably, the effective nonlinear absorption coefficients β is of two orders magnitude higher than those of the Ag6─NCs assemblies at the absence of C60─OH. This work showcases a new approach for amplifying NLO responses which greatly facilitates the development of integrated photonic devices.

Supramolecular Luminescent Copper-Nanocluster-Based Dough with Excellent Electrical Conductivity Sensing Properties

In recent years, the rapid advancement of flexible conductive materials has significantly increased the demand for dough materials that offer high flexibility and conductivity for diverse applications. Here, we developed a flexible, stretchable, and self-healing dough utilizing hydrogen-bonding interactions between glutathione-stabilized copper nanoclusters (GSH-Cu NCs) and poly(acrylic acid) (PAA). The dough materials can be kneaded, readily reshaped, and further processed to create bulk materials of arbitrary form factors. The incorporation of PAA not only preserved the vibrant blue emission of GSH-Cu NCs but also enhanced their electrical conductivity and stretchability. The dough can be stretched up to 25 times its initial length and achieves complete self-healing in a short time. Moreover, the dough can automatically repair physical damage and return to its initial conductivity levels after healing. Surprisingly, the electrical conductivity of the dough can reach as high as 2.97 S/m, which is relatively superior compared to that of conventional conductive materials. This study presents a dough that serves as a highly sensitive strain sensor, capable of effectively monitoring human movement across a broad range of strains.

Room-Temperature Persistent Phosphorescence of Aggregated Gold Nanoclusters under Molecular Crystal Confinements

We report color-tunable and solvent-processable persistent fluorescence to phosphorescence switching at room temperature by doping gold nanoclusters (AuNCs) inside molecular crystals. This provides a significant insight into the tunability of the photoluminescence property of the dopant depending on the crystal environment and compactness of confinement, with the possibility of energy transfer from crystal to aggregated AuNCs. For test cases, we have doped histidine-stabilized AuNCs (HIS-AuNCs) inside histidine (HIS-AuNCs-HIS) and isophthalic acid (HIS-AuNCs-IPA) crystals, respectively, and glutathione-stabilized AuNCs (GSH-AuNCs) inside histidine crystals (GSH-AuNCs-HIS). The maximal phosphorescence decay time recorded for crystal doped aggregated AuNCs was 9.38 ms, and the photoluminescence quantum yield value was measured as 25%. The possible energy states and potential interactions between aggregated NCs and host crystals were accounted for through density functional theory calculations and docking techniques, respectively. This finding opens new possibilities for designing and producing color-tunable persistent AuNC-based luminous crystals for multilayer information encryption, display, and biological applications.

Fluorescent metal nanoclusters: From luminescence mechanism to applications in enzyme activity assays

Metal nanoclusters (MNCs) have outstanding fluorescence property and biocompatibility, which show widespread applications in biological analysis. Particularly, evaluation of enzyme activity with the fluorescent MNCs has been developed rapidly within the past several years. In this review, we first introduced the fluorescent mechanism of mono- and bi-metallic nanoclusters, respectively, whose interesting luminescence properties are mainly resulted from electron transfer between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Meanwhile, the charge migration within the structure occurs through ligand-metal charge transfer (LMCT) or ligand-metal-metal charge transfer (LMMCT). On such foundation, diverse enzyme activities were rigorously evaluated, including three transferases and nine hydrolases, in turn harvesting rapid research progresses within past 5 years. Finally, we summarized the design strategies for evaluating enzyme activity with the MNCs, presented the major issues and challenges remained in the relevant research, coupled by showing some improvement measures. This review will attract researchers dedicated to the studies of the MNCs and provide some constructive insights for their further applications in enzyme analysis.

Tyrosine Mutation in the Characteristic Motif of the Amorphous Region of Spidroin for Self-Assembly Capability Enhancement

Spidroin, with robust mechanical performance and good biocompatibility, could fulfill broad applications in material science and biomedical fields. Development of miniature spidroin has made abundant fiber production economically feasible, but the mechanical properties of artificial silk still fall short of natural silk. The mechanism behind mechanical properties of spidroin usually focuses on β-microcrystalline regions; the effect of amorphous regions was barely studied. In this study, residue tyrosines (Y) were designed to replace asparagine (N)/glutamic acid (Q) in the characteristic motifs (GGX)n in amorphous regions for performance enhancement of spidroin; the mutants presented lower free energy and significantly exhibited stronger van der Waals and electrostatic interactions, which might result from π-π stacking interactions between the phenyl rings in the side chain of tyrosine. Additionally, the soluble expressions of wild-type spidroin and mutant spidroin were achieved when heterologously expressed in E. coli, with yields of 560 mg/L (2REP), 590 mg/L (2REPM), 240 mg/L (4REP), and 280 mg/L (4REPM). Significantly, secondary structure analysis confirmed that the mutant spidroin more avidly forms more β-sheets than the wild-type spidroin, and aggregation morphology suggested that mutant spidroin displayed better self-assembly capacity and was easier to form artificial spider silk fibers; in particular, self-assembled 4REPM nanofibrils had an average modulus of 11.2 ± 0.35 GPa, about 2 times higher than self-assembled B. mori silk nanofibrils and almost the same as that of native spider dragline silk fibers (10-15 GPa). Thus, we first demonstrated a new influence mechanism of the amorphous region's characteristic motif on the self-assembly and material properties of spidroin. Our study provides a reference for the design of high-performance material proteins and their heterologous preparation.

Divergent self-assembly propensity of enantiomeric phenylalanine amphiphiles that undergo pH-induced nanofiber-to-nanoglobule conversion

This study presents the pathway diversity in the self-assembly of enantiomeric single phenylalanine derived amphiphiles (single F-PDAs), viz.L-NapF-EDA and D-NapF-EDA, that form supramolecular hydrogels at varied concentrations (≥1 mg mL-1 and ≥3 mg mL-1, respectively). By fitting the variable temperature circular dichroism (VT-CD) data to the isodesmic model, various thermodynamic parameters associated with their self-assembly, such as association constant (K), changes in enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG), were extracted. The self-assembly of these single F-PDAs was found to be enthalpy-driven but entropically-disfavored. Although self-assembly of the D-isomer was slow, it also exhibited greater free energy of association than the L-isomer. Consequently, thermally and mechanically more robust self-assemblies were formed by the D-isomer than the L-isomer. We term these results as the "butterfly effect in self-assembly" wherein the difference in the stereochemical orientation of the residues at a single chiral center present in these molecules resulted in strong differences in the self-assembly propensity as well as in their thermal and mechanical stability. These single F-PDAs form helical nanofibers of opposite chirality upon self-assembly at basic pH (≥8) that produce intense CD signals. However, upon decreasing the pH, a gradual nanofiber-to-nanoglobular transformation was noticed due to protonation-induced structural changes in the PDAs.

Cation Triggered Self-Assembly of α-Lactalbumin Nanotubes

Metal ions play a dual role in biological systems. Although they actively participate in vital life processes, they may contribute to protein aggregation and misfolding and thus contribute to development of diseases and other pathologies. In nanofabrication, metal ions mediate the formation of nanostructures with diverse properties. Here, we investigated the self-assembly of α-lactalbumin into nanotubes induced by coordination with metal ions, screened among the series Mn2+, Co2+, Ni2+, Zn2+, Cd2+, and Au3+. Our results revealed that the affinity of metal ions toward hydrolyzed α-lactalbumin peptides not only impacts the kinetics of nanotube formation but also influences their length and rigidity. These findings expand our understanding of supramolecular assembly processes in protein-based materials and pave the way for designing novel materials such as metallogels in biochip and biosensor applications.

High Internal Phase Emulsion for Constructing Chiral Helical Polymer-Based Circularly Polarized Luminescent Porous Materials

Polymerized high internal phase emulsions (polyHIPEs) with circularly polarized luminescence (CPL), as an interesting class of porous materials, are of great significance for the development of CPL porous materials but have not been reported so far. Herein, we report the construction of polyHIPE-based CPL porous materials, taking advantage of an adsorption strategy. The pristine polyHIPEs constructed by chiral helical polymers, which acted as a chiral microenvironment, were fabricated by coordination polymerization of chiral acetylene monomers (R/S-SA) using HIPEs as templates. Achiral fluorescent small molecules were dispersed in the pores of the 3D porous organic chiral polymer matrix provided by polyHIPEs through the adsorption strategy, and CPL-active porous materials with blue, cyan, and green emissions were constructed using a fluorescence-selective absorption mechanism that does not rely on chirality transfer at the molecular level. The maximum luminescence dissymmetry factor (glum) value was -2.6 × 10-2. This work establishes a new and simple way for developing CPL porous materials.

Chiral Aggregation-Induced Emission Carbon Dot-Based Multicolor and Near-Infrared Circularly Polarized Delayed Fluorescence via a Light-Harvesting System

Circularly polarized luminescence (CPL) materials are the research frontier of chiral luminescence. As a kind of luminescent carbon material, carbon dots (CDs) are expected to become excellent candidates for the construction of CPL materials. However, the construction of CD-based circularly polarized afterglow emission, especially multicolor and near-infrared emission, remains a great challenge due to aggregation-caused quenching and the instability of triplet excitons. In this work, we synthesized chiral CDs with aggregation-induced emission using dithiosalicylic acid and l/d-arginine as precursors through a one-step solvothermal method. Notably, the CDs exhibit green delayed fluorescence (DF) in poly(vinyl alcohol) films. Furthermore, multicolor and near-infrared circularly polarized delayed fluorescence is successfully realized via engineering a chiral light-harvesting system in which the CDs with green DF emission act as energy donors and fluorescent dyes with emission colors ranging from yellow to the near infrared serve as energy acceptors.

Ratiometric fluorescence detection of Hg2+ based on gold nanocluster/carbon quantum dots nanohybrids

Ratiometric fluorescence sensing methods are widely used in analysis and detection due to their high sensitivity and stability. In this work, a ratiometric fluorescence method for sensitive detection of Hg2+ was established using a gold nanoclusters/carbon quantum dots (AuNCs/CQDs) nanohybrid probe. The AuNCs/CQDs nanohybrids probe were simply constructed by mixing blue-light-emitting gold nanoclusters (AuNCs) with an orange-emissive carbon quantum dots (CQDs). The probe had two fluorescence emission peaks at 434 nm and 561 nm when the excitation wavelength was 375 nm. With the addition of Hg2+, the fluorescence at 434 nm decreased and the fluorescence at 561 nm remained unchanged; the fluorescence intensity ratio Δ(F434/F561) and Hg2+ concentration have a good linear relationship in the range of 8.32 × 10-7 to 7.69 × 10-5 mol L-1, and the limit of detection (LOD) is 3.58 × 10-7 mol L-1. The method was applied in the detection of Hg2+ in cosmetics and wastewater, and has potential applications for detecting Hg2+ in other samples.