Environment-dependent guest exchange in supramolecular hosts

Structural Determinants of Stimuli-Responsiveness in Amphiphilic Macromolecular Nano-assemblies

Stimuli-responsive nano-assemblies from amphiphilic macromolecules could undergo controlled structural transformations and generate diverse macroscopic phenomenon under stimuli. Due to the controllable responsiveness, they have been applied for broad material and biomedical applications, such as biologics delivery, sensing, imaging, and catalysis. Understanding the mechanisms of the assembly-disassembly processes and structural determinants behind the responsive properties is fundamentally important for designing the next generation of nano-assemblies with programmable responsiveness. In this review, we focus on structural determinants of assemblies from amphiphilic macromolecules and their macromolecular level alterations under stimuli, such as the disruption of hydrophilic-lipophilic balance (HLB), depolymerization, decrosslinking, and changes of molecular packing in assemblies, which eventually lead to a series of macroscopic phenomenon for practical purposes. Applications of stimuli-responsive nano-assemblies in delivery, sensing and imaging were also summarized based on their structural features. We expect this review could provide readers an overview of the structural considerations in the design and applications of nanoassemblies and incentivize more explorations in stimuli-responsive soft matters.

Fluorescence Resonance Energy Transfer Measurements in Polymer Science: A Review

Fluorescence resonance energy transfer (FRET) is a non-invasive characterization method for studying molecular structures and dynamics, providing high spatial resolution at nanometer scale. Over the past decades, FRET-based measurements are developed and widely implemented in synthetic polymer systems for understanding and detecting a variety of nanoscale phenomena, enabling significant advances in polymer science. In this review, the basic principles of fluorescence and FRET are briefly discussed. Several representative research areas are highlighted, where FRET spectroscopy and imaging can be employed to reveal polymer morphology and kinetics. These examples include understanding polymer micelle formation and stability, detecting guest molecule release from polymer host, characterizing supramolecular assembly, imaging composite interfaces, and determining polymer chain conformations and their diffusion kinetics. Finally, a perspective on the opportunities of FRET-based measurements is provided for further allowing their greater contributions in this exciting area.

Fluorescence Resonance Energy Transfer (FRET): A Powerful Tool for Probing Amphiphilic Polymer Aggregates and Supramolecular Polymers

This Review Article highlights the utility of the fluorescence resonance energy transfer (FRET) to probe the dynamics and related issues in amphiphilic polymeric aggregates and supramolecular polymers. Amphiphilic polymers are more attractive compared to their small molecule analogues because they exhibit significantly lower critical aggregation concentration, relatively larger particle size (suitable for the enhanced permeation and retention effect), and a much slower dynamics of exchange between the unimer and the aggregate. Representative examples of exchange dynamics in amphiphilic polymer aggregates and their noncovalent encapsulation stability as a function of the structure of the macromolecule, cross-linking, environmental parameters, and biological conditions, as probed by FRET studies, have been included in this article. Further, related observations on the utility of FRET in studying the exchange dynamics in supramolecular polymers, particularly in aqueous medium, have been discussed at length, revealing a strong impact of chirality, side chain polarity, and other parameters. Overall, this Review Article brings out the strength of this technique to probe dynamics of aggregates and assembled systems, mostly in water medium, which has a paramount importance in designing future biomaterials.

Understanding complex supramolecular landscapes: non-covalent macrocyclization equilibria examined by fluorescence resonance energy transfer

As molecular self-assembled systems increase in complexity, due to a large number of participating entities and/or the establishment of multiple competing equilibria, their full understanding becomes likewise more complicated, and the use of diverse analytical techniques that can afford complementary information is required. We demonstrate in this work that resonance excitation energy transfer phenomena, measured by fluorescence spectroscopy in combination with other optical spectroscopies, can be a valuable tool to obtain supplementary thermodynamic data about complex supramolecular landscapes that other methods fail to provide. In particular, noncovalent macrocyclization processes of lipophilic dinucleosides are studied here by setting up a competition between intra- and intermolecular association processes of Watson-Crick H-bonding pairs. Multiwavelength analysis of the monomer emission changes allowed us to determine cyclotetramerization constants and to quantify chelate cooperativity, which was confirmed to be substantially larger for the G-C than for the A-U pair. Furthermore, when bithiophene-BODIPY donor-acceptor energy transfer probes are employed in these competition experiments, fluorescence and circular dichroism spectroscopy measurements in different regions of the visible spectrum additionally reveal intermolecular interactions occurring simultaneously at both sides of the macrocyclization reaction: the cyclic product, acting as a host for the competitor, and the monomer reactant, ultimately leading to macrocycle denaturation.

Importance of Encapsulation Stability of Nanocarriers with High Drug Loading Capacity for Increasing in Vivo Therapeutic Efficacy

Current drug delivery systems are hampered by poor delivery to tumors, in part reflecting poor encapsulation stability of nanocarriers. Although nanocarriers such as polymeric micelles have high colloidal stability and do not aggregate or precipitate in bulk solution, nanocarriers with low encapsulation stability can lose their cargo during circulation in blood due to interactions with blood cells, cellular membranes, serum proteins, and other biomacromolecules. The resulting premature drug release from carriers limits the therapeutic efficacy at target sites. Herein, we report a simple and robust technique to improve encapsulation stability of drug delivery systems. Specifically, we show that installation of disulfide cross-linked noncovalent polymer gatekeepers onto mesoporous silica nanoparticles with a high loading capacity for hydrophobic drugs enhances in vivo therapeutic efficacy by preventing premature release of cargo. Subsequent release of drug cargos was triggered by cleavage of disulfide cross-linking by glutathione, leading to improved antitumor activity of doxoroubicin in mice. These findings provide novel insights into the development of nanocarriers with high encapsulation stability and improved in vivo therapeutic efficacy.

Chlorophyll-Inspired Red-Region Fluorophores: Building Block Synthesis and Studies in Aqueous Media

Fluorophores that absorb and emit in the red spectral region (600-700 nm) are of great interest in photochemistry and photomedicine. Eight new target chlorins (and 19 new chlorins altogether)-analogues of chlorophyll-of different polarities have been designed and synthesized for various applications; seven of the chlorins are equipped with a bioconjugatable tether. Hydrophobic or amphiphilic chlorins in a non-polar organic solvent (toluene), polar organic solvent (DMF), and aqueous or aqueous micellar media show a sharp emission band in the red region and modest fluorescence quantum yield (Φ_f = 0.2-0.3). A Poisson analysis implies most micelles are empty and few contain >1 chlorin. Water-soluble chlorins each bearing three PEG (oligoethyleneglycol) groups exhibit narrow emission bands (full-width-at-half maximum <25 nm). The lifetime of the lowest singlet excited state and the corresponding yields and rate constants for depopulation pathways (fluorescence, intersystem crossing, internal conversion) are generally little affected by the PEG groups or dissolution in aqueous or organic media. A set of chlorin-avidin conjugates revealed a 2-fold increase in Φ_f with increased average chlorin/avidin ratio (2.3-12). In summary, the chlorins of various polarities described herein are well suited as red-emitting fluorophores for applications in aqueous or organic media.

Effect of Hydrophobic Chain Length on the Stability and Guest Exchange Behavior of Shell-Sheddable Micelles Formed by Disulfide-Linked Diblock Copolymers

Reduction-responsive micelles hold enormous promise for application as drug carriers due to the fast drug release triggered by reducing conditions and high anticancer activity. However, the effect of hydrophobic chain length on the stability and guest exchange of reduction-responsive micelles, especially for the micelles formed by diblock copolymers containing single disulfide group, is not fully understood. Here, shell-sheddable micelles formed by a series of disulfide-linked copolymer poly(ethylene glycol)-b-poly(ε-caprolactone) (PEG-SS-PCL) containing the same chain length of PEG but different chain lengths of hydrophobic block PCL were prepared and well characterized. The influence of the chain length of hydrophobic PCL block on the stability and guest exchange of PEG-SS-PCL micelles was studied by the use of both dynamic laser light scattering (DLS) and fluorescence resonance energy transfer (FRET). The results show that longer PCL chains lead to a slower aggregation rate and guest exchange of micelles in the aqueous solutions containing 10 mM dithiothreitol (DTT). The cell uptake of the shell-sheddable PEG-SS-PCL micelles in vitro shows that the amount of internalization of dyes loaded in PEG-SS-PCL micelles increases with the chain length of hydrophobic PCL block investigated by flow cytometric analysis and confocal fluorescence microscopy.

The power of the ring: a pH-responsive hydrophobic epoxide monomer for superior micelle stability

We developed micelles with superior stability by integrating a novel hydrophobic, pH-responsive epoxide monomer, tetrahydropyranyl glycidyl ether.

Dual-pH-sensitivity and tumour targeting core–shell particles for intracellular drug delivery

The principal problem in the area of drug delivery is achieving better selectivity and controllability.

Fluorophore exchange kinetics in block copolymer micelles with varying solvent-fluorophore and solvent-polymer interactions

Fluorescence spectroscopy was employed to characterize the kinetics of guest exchange in diblock copolymer micelles composed of poly(ethylene oxide-b-ε-caprolactone) (PEO-PCL) diblock copolymers in water/tetrahydrofuran (THF) mixtures which encapsulated fluorophores. The solvent composition (THF content) of the micelle solution was varied as a means of modulating the strength of interactions between the fluorophore and solvent as well as between the micelle core and solvent. A donor-acceptor fluorophore pair was employed consisting of 3,3'-dioctadecyloxacarbocyanine perchlorate (DiO, the donor) and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI, the acceptor). Through the process of Förster resonance energy transfer (FRET), energy was transferred from the donor to acceptor when the fluorophores were in close proximity. A micelle solution containing DiO was mixed with a micelle solution containing DiI at t = 0, and the emission spectra of the mixed solution were monitored over time (at an excitation wavelength optimized for the donor). In micelle solutions containing 5 and 10 vol% THF in the bulk solvent, an increase in the acceptor peak intensity maximum occurred over time in the post-mixed solution, accompanied by a decrease in the donor peak intensity maximum, indicating the presence of energy transfer from the donor to the acceptor. At long times, the FRET ratios (acceptor peak intensity divided by the sum of the acceptor and donor peak intensities) were indistinguishable from that determined from pre-mixed micelle solutions of the same THF content (in pre-mixed solutions, DiO and DiI were encapsulated within the same micelle cores). In the micelle solution containing 20 vol% THF, the fluorophore exchange process occurred too quickly to be observed (the FRET ratios measured from the solutions mixed at t = 0 were commensurate to that measured from the pre-mixed solution). A time constant describing the guest exchange process was extracted from the time-dependence of the FRET ratio through fit of an exponential decay. An increase in the THF content in the micelle solution resulted in a decrease in the time constant, and the time constant varied over five orders of magnitude as the THF content was varied from 5-20 vol%.

Responsive single-chain polymer nanoparticles with host–guest features

We report a facile approach to form ultra-fine single-chain polymer nanoparticles (SCPNs)viadisulfide-based intrachain crosslinking of single polymer chains of a random copolymer poly(HEMA-co-PDSEMA).

Paley's watchmaker analogy and prebiotic synthetic chemistry in surfactant assemblies. Formaldehyde scavenging by pyrroles leading to porphyrins as a case study

Facile exchange of micromolar dialkylpyrrolic constituents among a Poisson distribution of aqueous micelles overcomes immense statistical odds against reaction.