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

Alternating vs. random amphiphilic polydisulfides: aggregation, enzyme activity inhibition and redox-responsive guest release

Herein, we report the synthesis of an alternating copolymer (ACP) with a bio-reducible amphiphilic polydisulfide backbone and highlight the impact of the alternating monomer connectivity on the self-assembly, morphology, chain-exchange dynamics, drug-release kinetics, and enzyme activity inhibition. Condensation polymerization between hydrophobic 1,10-bis(pyridin-2-yldisulfaneyl)decane and hydrophilic 2,3-mercaptosuccinic acid (1.04 : 1.00 ratio) generated amphiphilic ACP P1 (Mw = 8450 g mol-1, Đ = 1.3), which exhibited self-assembly in water, leading to the formation of an ultra-thin (height <5.0 nm) entangled fibrillar network. In contrast, structurally similar amphiphilic random copolymer P2 exhibited a truncated irregular disc-like morphology under the same conditions. It is postulated that due to the perfect alternating sequence of the hydrophobic and hydrophilic segments in P1, its immiscibility-driven aggregation in water leads to a pleated structure, which further assembles and forms the observed long fibrillar structures, similar to crystallization-driven self-assembly. In fact, wide-angle X-ray diffraction (WXRD) analysis of a lyophilized P1 sample showed sharp peaks, indicating its crystalline nature (approximately 37% crystallinity), and these were completely missing for P2. The effect of such distinct self-assembly on the chain-exchange dynamics was probed by fluorescence resonance energy transfer (FRET) using 3,3'-dioctadecyloxacarbocyanine perchlorate (DiO) and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) as the FRET-donor and -acceptor, respectively. For DiI- and DiO-entrapped solutions of P1, when mixed, no prominent FRET appeared even after 24 h. In sharp contrast, for P2, intense FRET emission occurred, and the FRET ratio (approximately 0.9) reached saturation in approximately 15 h, indicating the greatly enhanced kinetic stability of P1 aggregates. Glutathione-induced release of encapsulated Nile red showed much slower kinetics for P1 compared to that of P2, which was corroborated by the observed slow chain-exchange dynamics of the highly stable alternating copolymer assembly. Furthermore, the well-ordered assembly of P1 exhibited an excellent surface-functional group display (zeta potential of -32 mV compared to -14 mV for P2), which resulted in the effective recognition of the α-chymotrypsin (Cht) protein surface by electrostatic interaction. Consequently, P1 significantly (>70%) suppressed the enzymatic activity of Cht, while in the presence of P2, the enzyme was still active with >70% efficacy.

A Stochastic FRET Study on the Core Dimension of Polystyrene-block-Poly(Polyethylene Glycol Monomethyl Ether Acrylate) Micelles

Polystyrene-b-poly(polyethylene glycol monomethyl ether acrylate) (PSt-b-PPEGA) copolymers featuring pyrene and perylene as the Förster resonance energy transfer (FRET) donor (denoted as D-BCP) and acceptor (denoted as A-BCP), respectively, were synthesized via the reversible addition and fragmentation chain transfer (RAFT) polymerization. These copolymers were then used to form DA-mixed micelles via a dialysis method. The micelles consisted of D-BCP (mole fraction fD = 0.04), A-BCP (fA = 0.04), and label-free PSt-b-PPEGA (fN = 0.92). The decrease in fluorescence intensity of pyrene in the micelles confirmed the occurrence of FRET, with an observed efficiency of 0.32. A Monte Carlo approach was employed to estimate the expected FRET efficiency, assuming the random fractional distribution of D-BCP and A-BCP, along with the random spatial distribution of pyrene and perylene within the micellar core. The observed FRET efficiency suggested a core radius (Rc) of 0.95R0, where R0 was the Förster critical distance. With R0 calculated to be 3.2 nm based on Förster theory, Rc was determined to be approximately 3.0 nm, aligning closely with the dried-out core radius estimated from aggregation number and polystyrene density. This stochastic FRET methodology was further applied to investigate the swelling behavior of the polymer micelles in a mixture of N,N-dimethylformamide (DMF) and water. Dynamic light scattering analysis revealed minimal change in core dimension below 60 vol % DMF content. However, FRET analysis provided a deeper insight, showing an increase in core radius with DMF content up to 20 vol %, followed by saturation up to 50 vol %, offering a more comprehensive understanding of the micelle swelling behavior.

Bioreducible Amphiphilic Hyperbranched Polymer-Drug Conjugate for Intracellular Drug Delivery

This paper reports synthesis of a bioreducible hyperbranched (HB) polymer by A2+B3 approach from commercially available dithiothreitol (DTT) (A2) and an easily accessible trifunctional monomer (B3) containing three reactive pyridyl-disulfide groups. Highly efficient thiol-activated disulfide exchange reaction leads to the formation of the HB polymer (Mw = 21000; Đ = 2.3) with bioreducible disulfide linkages in the backbone and two different functional groups, namely, hydroxyl and pyridyl-disulfide in the core and periphery, respectively, of the HB-polymer. Postpolymerization functionalization of the hydroxyl-groups with camptothecin (CPT), a topoisomerase inhibitor and known anticancer drug, followed by replacing the terminal pyridyl-disulfide groups with oligo-oxyethylene-thiol resulted in easy access to an amphiphilic HB polydisulfide-CPT conjugate (P1) with a very high drug loading content of ∼40%. P1 aggregated in water (above ∼10 μg/mL) producing drug-loaded nanoparticles (Dh ∼ 135 nm), which showed highly efficient glutathione (GSH)-triggered release of the active CPT. Mass spectrometry analysis of the GSH-treated P1 showed the presence of the active CPT drug as well as a cyclic monothiocarbonate product, which underpins the cascade-degradation mechanism involving GSH-triggered cleavage of the labile disulfide linkage, followed by intramolecular nucleophilic attack by the in situ generated thiol to the neighboring carbonate linkage, resulting in release of the active CPT drug. The P1 nanoparticle showed excellent cellular uptake as tested by confocal fluorescence microscopy in HeLa cells by predominantly endocytosis mechanism, resulting in highly efficient cell killing (IC50 ∼ 0.6 μg/mL) as evident from the results of the MTT assay, as well as the apoptosis assay. Comparative studies with an analogous linear polymer-CPT conjugate showed much superior intracellular drug delivery potency of the hyperbranched polymer.

Förster resonance energy transfer within single chain nanoparticles

Single chain nanoparticles (SCNPs) are a highly versatile polymer architecture consisting of single polymer chains that are intramolecularly crosslinked. Currently, SCNPs are discussed as powerful macromolecular architectures for catalysis, delivery and sensors. Herein, we introduce a methodology based on Förster Resonance Energy Transfer (FRET) to evidence the folding of single polymer chains into SCNPs via fluorescence readout. We initially introduce a molecular FRET pair based on a bimane and nitrobenzoxadiazole (NBD) moiety and study its fluorescence properties in different solvents. We subsequently construct a low dispersity polymer chain carrying NBD units, while exploiting the bimane units for intramolecular chain collapse. Upon chain collapse and SCNP formation - thus bringing bimane and NBD units into close proximity - the SCNPs report their folded state by a strong and unambiguous FRET fluorescence signal. The herein introduced reporting of the folding state of SCNPs solely relies on an optical readout, opening avenues to monitoring SCNP folding without recourse to complex analytical methodologies.

Cascade Energy Transfer and White-Light Emission in Chirality-Controlled Crystallization-Driven Two-Dimensional Co-assemblies from Donor and Acceptor Dye-Conjugated Polylactides

Achieving predictable and programmable two-dimensional (2D) structures with specific functions from exclusively organic soft materials remains a scientific challenge. This article unravels stereocomplex crystallization-driven self-assembly as a facile method for producing thermally robust discrete 2D-platelets of diamond shape from biodegradable semicrystalline polylactide (PLA) scaffolds. The method involves co-assembling two PLA stereoisomers, namely, PY-PDLA and NMI-PLLA, which form stereocomplex (SC)-crystals in isopropanol. By conjugating a well-known Förster resonance energy transfer (FRET) donor and acceptor dye, namely, pyrene (PY) and naphthalene monoimide (NMI), respectively, to the chain termini of these two interacting stereoisomers, a thermally robust FRET process can be stimulated from the 2D array of the co-assembled dyes on the thermally resilient SC-PLA crystal surfaces. Uniquely, by decorating the surface of the SC-PLA crystals with an externally immobilized guest dye, Rhodamine-B, similar diamond-shaped structures could be produced that exhibit pure white-light emission through a surface-induced two-step cascade energy transfer process. The FRET response in these systems displays remarkable dependence on the intrinsic crystalline packing, which could be modulated by the chirality of the co-assembling PLA chains. This is supported by comparing the properties of similar 2D platelets generated from two homochiral PLLAs (PY-PLLA and NMI-PLLA) labeled with the same FRET pair.

Fluorescence-readout as a powerful macromolecular characterisation tool

The last few decades have witnessed significant progress in synthetic macromolecular chemistry, which can provide access to diverse macromolecules with varying structural complexities, topology and functionalities, bringing us closer to the aim of controlling soft matter material properties with molecular precision. To reach this goal, the development of advanced analytical techniques, allowing for micro-, molecular level and real-time investigation, is essential. Due to their appealing features, including high sensitivity, large contrast, fast and real-time response, as well as non-invasive characteristics, fluorescence-based techniques have emerged as a powerful tool for macromolecular characterisation to provide detailed information and give new and deep insights beyond those offered by commonly applied analytical methods. Herein, we critically examine how fluorescence phenomena, principles and techniques can be effectively exploited to characterise macromolecules and soft matter materials and to further unravel their constitution, by highlighting representative examples of recent advances across major areas of polymer and materials science, ranging from polymer molecular weight and conversion, architecture, conformation to polymer self-assembly to surfaces, gels and 3D printing. Finally, we discuss the opportunities for fluorescence-readout to further advance the development of macromolecules, leading to the design of polymers and soft matter materials with pre-determined and adaptable properties.

Probing local structure of glass with orientation-dependent luminescence

The local ordering of particles is considered an important process in glass transition. Ordering is usually observed in simulation and micrometer-sized colloid. However, clear information on local ordering at the molecular level is difficult to obtain experimentally. In this study, we prepared an easily glass-forming fluorophore with a color change owing to the intermolecular arrangement in the liquid, glass, and crystal states. The bathochromic shifts of the photoluminescence spectra indicated a change in the intermolecular orientation upon immediate cooling of the melt. The recovery of the spectra by successive heating indicated that rotation contributed to the change in the intermolecular orientation. The orientation in the glass was distinct from that during crystal growth, which was observed as a slow bathochromic shift by maintaining the temperature between the melting points of the blue- and green-luminescent crystals obtained from dichloromethane/ethanol and dichloromethane/hexane, respectively. Our results demonstrate that the anisotropic interaction between glass-forming luminophores is useful for uncovering molecular-level events in the glassy state.

Supramolecular alternating copolymers with highly efficient fluorescence resonance energy transfer

This article reports alternating supramolecular copolymerization of two naphthalene-diimide (NDI)-derived building blocks (NDI-1 and NDI-2) under thermodynamic control. Both monomers contain a central NDI chromophore, attached to a hydrocarbon-chain and a carboxylic-acid group. The NDI core in NDI-2 is symmetrically substituted with two butane-thiol groups, which makes it distinct from NDI-1. In decane, a 1 : 1 mixture of NDI-1 and NDI-2 shows spontaneous gelation and a typical fibrillar network, unlike the behavior of either of the components individually. The solvent-dependent UV/vis spectrum of the mixed sample in decane shows bathochromically shifted sharp absorption bands and a sharp emission band (holds a mirror-image relationship) with a significantly small Stokes shift compared to those in CHCl3, indicating J-aggregation. In contrast, the aggregated spectra of the individual monomers show broad structureless features, suggesting ill-defined aggregates. Cooling curves derived from the temperature-dependent UV/vis spectroscopy studies revealed early nucleation and a signature of well-defined cooperative polymerization for the mixed sample, unlike either of the individual components. Molecular dynamics simulations predicted the greatest dimer formation tendency for the NDI-1 + NDI-2 (1 : 1), followed by pure NDI-1 and NDI-2. Theoretical studies further revealed a partial positive charge in the NDI ring of NDI-1 when compared to NDI-2, promoting the alternating stacking propensity, which is also favored by the steric factor as NDI-2 is core-substituted with alkyl thiols. Such theoretical predictions fully corroborate with the experimental results showing 1 : 1 stoichiometry (from Job's plot) of the two monomers, indicating alternate stacking sequences in the H-bonded (syn-syn catemer type) supramolecular copolymer. Such alternating supramolecular copolymers showed highly efficient (>93%) fluorescence resonance energy transfer (FRET).

FRET-based analysis on the structural stability of polymeric micelles: Another key attribute beyond PEG coverage and particle size affecting the blood clearance

Various attributes of micelles, such as PEG density and particle size, are considered to be related to blood clearance. The structural stability of micelles is another key attribute that will affect the in vivo fate. This study employed fluorescence resonance energy transfer (FRET) analysis to guide the preparation of polymeric micelles with different structural stability. Micelles prepared using copolymers with longer hydrophobic blocks showed higher structural stability; emulsification was a better method than nanoprecipitation to prepare stable micelles. The fast chain exchange kinetics and the high-water content of micellar cores explained the low structural stability of those micelles. Moreover, this study highlighted the importance of structural stability that affected blood clearance in concert with PEG length and particle size. One-third of the small and stable micelles were detected in the blood 24 h after injection. While unstable micelles would be cleared from the circulation within 4 h. Notably, there would be a threshold of structural stability. Micelles with structural stability below this threshold were quickly cleared even if they possessed a longer PEG length and a smaller size. In contrast, higher structural stability allowed polymeric micelles to maintain higher integrity in vivo and enhance tumor accumulation and anti-tumor efficacy. In conclusion, this study systematically analyzed the importance of the structural stability of micelles on the in vivo fate.

Quantitative Measurement of Drug Release Dynamics within Targeted Organelles Using Förster Resonance Energy Transfer

Measuring the release dynamics of drug molecules after their delivery to the target organelle is critical to improve therapeutic efficacy and reduce side effects. However, it remains challenging to quantitatively monitor subcellular drug release in real time. To address the knowledge gap, a novel gemini fluorescent surfactant capable of forming mitochondria-targeted and redox-responsive nanocarriers is designed. A quantitative Förster resonance energy transfer (FRET) platform is fabricated using this mitochondria-anchored fluorescent nanocarrier as a FRET donor and fluorescent drugs as a FRET acceptor. The FRET platform enables real-time measurement of drug release from organelle-targeted nanocarriers. Moreover, the obtained drug release dynamics can evaluate the duration of drug release at the subcellular level, which established a new quantitative method for organelle-targeted drug release. This quantitative FRET platform can compensate for the absent assessment of the targeted release performances of nanocarriers, offering in-depth understanding of the drug release behaviors at the subcellular targets.

Force-Induced Shuttling of Rotaxanes Controls Fluorescence Resonance Energy Transfer in Polymer Hydrogels

The molecular shuttling function of rotaxanes can be exploited to design mechanoresponsive reporter molecules. Here, we report a new approach to such rotaxane-based mechanophores, in which the fluorescence resonance energy transfer (FRET) between a donor-acceptor pair is mechanically controlled. A cyclic molecule containing a green-light-emitting FRET donor connected to a red-light-emitting FRET acceptor was threaded onto an axle equipped with a quencher at its center and two stoppers in the peripheral positions. In the force-free state, the green emitter is located near the quencher so that charge transfer interactions or photo-induced electron transfer between the two moieties suppress green emission and prevent the FRET from the green to the red emitter. The mechanophore was covalently incorporated into a linear polyurethane-urea (PUU), and stretchable hydrogels were prepared by swelling this polymer with water. Upon deformation of the PUU hydrogels and under an excitation light that selectively excites the donor, the intensity of the red fluorescence increases, as a result of a force-induced separation of the green emitter from the quencher, which enables the FRET. The switching contrast is much more pronounced in the gels than in dry films, which is due to increased molecular mobility and hydrophobic effects in the hydrogel, which both promote the formation of inclusion complexes between the ring containing the green emitter and the quencher.

Fluorescent organometallic dyads and triads: establishing spatial relationships

FRET pairs involving up to three different Bodipy dyes are utilized to provide information on the assembly/disassembly of organometallic complexes. Azolium salts tagged with chemically robust and photostable blue or green or red fluorescent Bodipy, respectively, were synthesized and the azolium salts used to prepare metal complexes [(NHC_blue)ML], [(NHC_green)ML] and [(NHC_red)ML] (ML = Pd(allyl)Cl, IrCl(cod), RhCl(cod), AuCl, Au(NTf₂), CuBr). The blue and the green Bodipy and the green and the red Bodipy, respectively, were designed to allow the formation of efficient FRET pairs with minimal cross-talk. Organometallic dyads formed from two subunits enable the transfer of excitation energy from the donor dye to the acceptor dye. The blue, green and red emission provide three information channels on the formation of complexes, which is demonstrated for alkyne or sulfur bridged digold species and for ion pairing of a red fluorescent cation and a green fluorescent anion. This approach is extended to probe an assembly of three different subunits. In such a triad, each component is tagged with either a blue, a green or a red Bodipy and the energy transfer blue →green → red proves the formation of the triad. The tagging of molecular components with robust fluorophores can be a general strategy in (organometallic) chemistry to establish connectivities for binuclear catalyst resting states and binuclear catalyst decomposition products in homogeneous catalysis.

Designs and Applications of Multi-stimuli Responsive FRET Processes in AIEgen-Functionalized and Bi-fluorophoric Supramolecular Materials

Materials capable of displaying strong ratiometric fluorescence with Förster resonance energy transfer (FRET) processes have attracted much research interest because of various chemosensor and biomedical applications. This review highlights several popular strategies in designing FRET-OFF/ON mechanisms of ratiometric fluorescence systems. In particular, the developments of organic and polymeric FRET materials featuring aggregation-induced emission-based luminogens (AIEgens), supramolecular assemblies, photochromic molecular switches and surfactant-induced AIE/FRET mechanisms are presented. AIEgens have been frequently employed as FRET donor and/or acceptor fluorophores to obtain enhanced ratiometric fluorescences in solution and solid states. Since AIE effects and FRET processes rely on controllable distances between fluorophores, many interesting fluorescent properties can be designed by regulating aggregation states in polymers and supramolecular systems. Photo-switchable fluorophores, such as spiropyran and diarylethene, provide drastic changes in fluorescence spectra upon photo-induced isomerizations, leading to photo-switching mechanisms to activate/deactivate FRET processes. Supramolecular assemblies offer versatile platforms to regulate responsive FRET processes effectively. In rotaxane structures, the donor-acceptor distance and FRET efficiency can be tuned by acid/base-controlled shuttling of the macrocycle component. The tunable supramolecular interactions are strongly influenced by external factors (such as pH values, temperatures, analytes, surfactants, UV-visible lights, etc.), which induce the assembly and disassembly of host-guest systems and thus their FRET-ON/FRET-OFF behavior. In addition, the changes in donor or acceptor fluorescence profiles upon detections of analytes can also sufficiently alter the FRET behavior and result in different ratiometric fluorescence outputs. The strategies and examples provided in this review offer the insights and toolkits for future FRET-based material developments.

In situ self-assembly of amphiphilic dextran micelles and superparamagnetic iron oxide nanoparticle-loading as magnetic resonance imaging contrast agents

Polymeric micelles have long been considered as promising nanocarrier for hydrophobic drugs and imaging probes, due to their nanoscale particle size, biocompatibility and ability to loading reasonable amount of cargoes. Herein, a facile method for dextran micelles preparation was developed and their performance as carriers of superparamagnetic iron oxide (SPIO) nanocrystals was evaluated. Amphiphilic dextran (Dex-g-OA) was synthesized via the Schiff base reactions between oxidized dextran and oleylamine, and self-assembled in situ into nano-size micelles in the reaction systems. The self-assembling behaviors of the amphiphilic dextran were identified using fluorescence resonance energy transfer technique by detection the energy transfer signal between the fluorophore pairs, Cy5 and Cy5.5. Hydrophobic SPIO nanoparticles (Fe₃O₄ NPs) were successfully loaded into the dextran micelles via the in situ self-assembly process, leading to a series of Fe₃O₄ NPs-loaded micelle nanocomposites (Fe₃O₄@Dex-g-OA) with good biocompatibility, superparamagnetism and strongly enhanced T ₂ relaxivity. At the magnetic field of 0.5 T, the Fe₃O₄@Dex-g-OA nanocomposite with particle size of 116.2 ± 53.7 nm presented a higher T ₂ relaxivity of 327.9 mM Fe - 1 ·s^-1. The prepared magnetic nanocomposites hold the promise to be used as contrast agents in magnetic resonance imaging.

Far-field lithography through saturated resonance energy transfer

A method based on resonance energy transfer in a donor-acceptor pair is proposed for writing three-dimensional features in the far field. The saturation of the energy transfer between the donor-acceptor pair induces a nonlinear intensity response in the radical population. This optical nonlinearity can enable three-dimensional nanofabrication without requiring the use of the ultrafast excitation that is compulsory for non-resonant multiphoton excitation. Addition of a second, spatially shaped beam can enable nanopatterning far below the diffraction limit.

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.

Chemical crosslinking in 'reactive' multicomponent gels

We show that the hydrolysis of EDC can be used to construct a reactive system to trigger permanent covalent crosslinking between the components in multicomponent gels comprising gelators with a carboxylic acid and amine group yielding an amide functionalized gel with enhanced mechanical properties.

Thermoresponsive Self-Assembly of Twofold Fluorescently Labeled Block Copolymers in Aqueous Solution and Microemulsions

A nonionic double hydrophilic block copolymer with a long permanently hydrophilic and a small thermoresponsive block is synthesized by reversible addition-fragmentation chain-transfer polymerization (RAFT). By employing a specifically designed chain-transfer agent, the polymer is functionalized with complementary end groups which are suited for Förster resonance energy transfer (FRET). The end group attached to the permanently hydrophilic block of poly(N,N-dimethylacrylamide) pDMAm is designed as a permanently hydrophobic segment ("sticker") comprising a long alkyl chain and the 4-aminonaphthalimide fluorophore. The other end attached to the thermoresponsive block of poly(N-isopropylacrylamide) pNiPAm incorporates a coumarin fluorophore. The temperature-dependent self-assembly of the twofold fluorescently labeled copolymer is studied in pure aqueous solution as well as in an o/w microemulsion by several techniques including turbidimetry, dynamic light scattering (DLS), and fluorescence spectroscopy. It is compared to the behaviors of the analogous twofold-labeled pDMAm and pNiPAm homopolymer references. The findings indicate that the block copolymer behaves as a polymeric surfactant at low temperatures, with one relatively small hydrophobic end block and an extended hydrophilic chain forming "hairy micelles". At elevated temperatures above the LCST phase transition of the pNiPAm block, however, the copolymer behaves as an associative telechelic polymer with two nonsymmetrical hydrophobic end blocks, which do not mix. Thus, instead of a network of bridged "flower micelles", large dynamic aggregates are formed. These are connected alternatingly by the original micellar cores as well as by clusters of the collapsed pNiPAm blocks. This type of structure is even more favored in the o/w microemulsion than in pure aqueous solution, as the microemulsion droplets constitute an attractive anchoring point for the hydrophobic dodecyl sticker but not for the collapsed pNiPAm chains.

Single molecule detection; from microscopy to sensors

Single molecule detection is necessary to find out physical, chemical properties and their mechanism involved in the normal functioning of body cells. In this way, they can provide a new direction to the healthcare system. Various techniques have been developed and employed for their successful detection. Herein, we have emphasized various traditional methods as well as biosensing technology which offer single molecule sensitivity. The various methods including plasmonic resonance, nanopores, whispering gallery mode, Simoa assay and recognition tunneling are discussed in the initial part which has been followed by a discussion about biosensor-based detection. Plasmonic, SERS, CRISPR/Cas, and other types of biosensors are focused in this review and found to be highly sensitive for single molecule detection. This review provides an overview of progression in different techniques employed for single molecule detection.

Reversible, controllable white-light emission of dye systems by dynamic covalent furan moiety exchange

New exo-Diels-Alder adduct dyes were synthesized. Pure exo and endo dye isomers were isolated and prepared. Two high-quality, blue-light emitting crystals were obtained. X-ray single crystal diffraction reveals the precise structure of the exo isomer, and longer Diels-Alder bonds (1.59 Å) than common C-C bonds (1.54 Å). The adduct dye displays high intramolecular energy transfer efficiency from the pyrene donor to the dansyl acceptor, where blue-violet donor emission is prohibited. By dynamic furan moiety exchange, the donor fluorescence restores, which combines with complementary green-yellow acceptor fluorescence to give white-light emission of the dye systems. White and green-light emitting dye systems could be reversibly, controllably, and mutually transformed.

Ultrabright Pdots with a Large Absorbance Cross Section and High Quantum Yield

Semiconducting polymer dots (Pdots) are increasingly used in biomedical applications due to their extreme single-particle brightness, which results from their large absorption cross section (σ). However, the quantum yield (Φ) of Pdots is typically below 40% due to aggregation-induced self-quenching. One approach to reducing self-quenching is to use FRET between the donor (D) and acceptor (A) groups within a Pdot; however, Φ values of FRET-based Pdots remain low. Here, we demonstrate an approach to achieve ultrabright FRET-based Pdots with simultaneously high σ and Φ. The importance of self-quenching was revealed in a non-FRET Pdot: adding 30 mol % of a nonabsorbing polyphenyl to a poly(9,9-dioctylfluorene) (PFO) Pdot increased Φ from 13.4 to 71.2%, yielding an ultrabright blue-emitting Pdot. We optimized the brightness of FRET-based Pdots by exploring different D/A combinations and ratios with PFO and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-phenylene)] (PFP) as donor polymers and poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(1,4-phenylene)] (PFPV) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1',3}-thiadiazole)] (PFBT) as acceptor polymers, with a fixed concentration of poly(styrene-co-maleic anhydride) as surfactant polymer. Ultrabright blue-emitting Pdots possessing high Φ (73.1%) and σ (σ_R = σ_abs/σ_all, 97.5%) were achieved using PFP/PFPV Pdots at a low acceptor content (A/[D + A], 2.5 mol %). PFP/PFPV Pdots were 1.8 times as bright as PFO/PFPV Pdots due to greater coverage of acceptor absorbance by donor emission─a factor often overlooked in D/A pair selection. Ultrabright green-emitting PFO Pdots (Φ = 76.0%, σ_R = 92.5%) were obtained by selecting an acceptor (PFBT) with greater spectral overlap with PFO. Ultrabright red-emitting Pdots (Φ = 64.2%, σ_R = 91.0%) were achieved by blending PFO, PFBT, and PFTBT to create a cascade FRET Pdot at a D:A₁:A₂ molar ratio of 61:5:1. These blue, green, and red Pdots are among the brightest Pdots reported. This approach of using a small, optimized amount of FRET acceptor polymer with a large donor-acceptor spectral overlap can be generalized to produce ultrabright Pdots with emissions that span the visible spectrum.

Improved Core Viscosity Achieved by PDLLA10kCo-Incorporation Promoted Drug Loading and Stability of mPEG2k-b-PDLLA2.4k Micelles

PURPOSE

This study aims to investigate the effect of poly(D, L-lactic acid)10K (PDLLA10K) incorporation on the drug loading and stability of poly(ethylene glycol)2K-block-poly(D, L-lactide)2.4K (mPEG2k-b-PDLLA2.4k) micelles. In addition, a suitable lyophilization protector was screened for this micelle to obtain favorable lyophilized products.

METHODS

The incorporation ratios of PDLLA10k were screened based on the particle size and drug loading. The dynamic stability, core viscosity, drug release, stability in albumin, and in vivo pharmacokinetic characteristics of PDLLA10k incorporated micelles were compared with the original micelles. In addition, the particle size variation was used as an indicator to screen the most suitable lyophilization protectant for the micelles. DSC, FTIR, XRD were used to illustrate the mechanism of the lyophilized protectants.

RESULTS

After the incorporation of 5 wt% PDLLA10K, the maximum loading of mPEG2k-b-PDLLA2.4k micelles for TM-2 was increased from 26 wt% to 32 wt%, and the in vivo half-life was increased by 2.25-fold. Various stability of micelles was improved. Also, the micelles with hydroxypropyl-β-cyclodextrin (HP-β-CD) as lyophilization protectants had minimal variation in particle size.

CONCLUSIONS

PDLLA10k incorporation can be employed as a strategy to increase the stability of mPEG2k-b-PDLLA2.4k micelles, which can be attributed to the viscosity building effect. HP-β-CD can be used as an effective lyophilization protectant since mPEG and HP-β-CD form the pseudopolyrotaxanesque inclusion complexes.

Separated Micelles Formation of pH-Responsive Random and Block Copolymers Containing Phosphorylcholine Groups

The self-assembly of pH-responsive random and block copolymers composed of 2-(N,N-diisopropylamino)ethyl methacrylate and 2-methacryloyloxyethyl phosphorylcholine was investigated in aqueous media. Their pH-responsive behaviors were investigated in aqueous media by dynamic light scattering (DLS) and fluorescence measurements using a pyrene hydrophobic fluorescence probe. In an acidic environment, these copolymers existed as single polymer chains that did not interact with each other. In contrast, upon increasing the pH of the solution above the critical value of ~8, separated micelles were formed in the mixture, which was indicated by bimodal distribution in DLS results with radius of 4.5 and 10.4 nm, corresponding to the random and block copolymer micelles, respectively. Fluorescence resonance energy transfer efficiencies were near to zero in the mixture of the donor labeled block and acceptor labeled random copolymers under both acidic and basic pH. These results demonstrated the coexistence of two distinct micelles.

Hyperbranched vs. linear poly(disulfide) for intracellular drug delivery

This communication reports comparative studies between amphiphilic hyperbranched and linear poly(disulfide) with regard to their aggregation and glutathione-responsive intracellular drug delivery.

Deep-blue-emitting nanoaggregates from carbazole-based dyes in water

New amphiphilic carbazole-based dyes assemble in water into deep-blue-emitting, highly fluorescent helical aggregates as observed by transmission electron microscopy and atomic force microscopy. Single crystal X-ray diffraction and NMR spectroscopy reveal that self-complementary, antiparallel H-bonding and π-π stacking interactions are the driving forces for the formation of these dye aggregates.

Upper Critical Solution Temperature-Type Responsive Cyclodextrins with Characteristic Inclusion Abilities

Water-soluble and thermoresponsive macrocycles with stable inclusion toward guests are highly valuable to construct stimuli-responsive supramolecular materials for versatile applications. Here, we develop such macrocycles - ureido-substituted cyclodextrins (CDs) which exhibit unprecedented upper critical solution temperature (UCST) behavior in aqueous media. These novel CD derivatives showed good solubility in water at elevated temperature, but collapsed from water to form large coacervates upon cooling to low temperature. Their cloud points are greatly dependent on concentration and can be mediated through oxidation and chelation with silver ions. Significantly, the amphiphilicity of these CD derivatives is supportive to host-guest binding, which affords them inclusion abilities to guest dyes. The inclusion complexation remained nearly intact during thermally induced phase transitions, which is in contrast to the switchable inclusion behavior of lower critical solution temperature (LCST)-type CDs. Moreover, ureido-substituted CDs were exploited to co-encapsulate a pair of guest dyes whose fluorescence resonance energy transfer process can be switched by the UCST phase transition. We therefore believe these novel thermoresponsive CDs may form a new strategy for developing smart macrocycles and allow for exploring smart supramolecular materials.

Dynamics in supramolecular nanomaterials

Self-assembly of amphiphilic small molecules in water leads to nanostructures with customizable structure-property relationships arising from their tunable chemistries. Characterization of these assemblies is generally limited to their static structures -e.g. their geometries and dimensions - but the implementation of tools that provide a deeper understanding of molecular motions has recently emerged. Here, we summarize recent reports showcasing dynamics characterization tools and their application to small molecule assemblies, and we go on to highlight supramolecular systems whose properties are substantially affected by their conformational, exchange, and water dynamics. This review illustrates the importance of considering dynamics in rational amphiphile design.

Molecular Recognition Driven Bioinspired Directional Supramolecular Assembly of Amphiphilic (Macro)molecules and Proteins

Bioinspired self-assembly has been explored with diverse synthetic scaffolds, among which amphiphiles are perhaps the most extensively studied systems. Classical surfactants or amphiphilic block copolymers, depending on the hydrophobic-hydrophilic balance, produce distinct nanostructures, which hold promise for applications ranging from biology to materials sciences. Nevertheless, their immiscibility-driven aggregation does not provide the opportunity to precisely regulate the internal order, morphology, or functional group display, which is highly desirable, especially in the context of biological applications.A new class of amphiphiles have emerged in the recent past in which the hydrophilic segment(s) is appended with a hydrophobic supramolecular-structure-directing-unit (SSDU), consisting of a π-conjugated chromophore and a H-bonding group. Self-recognition of the SSDU by attractive directional interactions governs the supramolecular assembly, which is fundamentally different than the repulsive solvent-immiscibility driven aggregation of traditional amphiphiles. Such SSDU-appended hydrophilic polymers exhibit entropy-driven highly stable self-assembly producing distinct nanostructures depending on the H-bonding functional group. For example, polymers with the hydrazide-functionalized SSDU attached form a polymersome, while in a sharp contrast, the same polymers when connected to an amide containing SSDU produce a cylindrical micelle via a spherical-micelle intermediate. This relationship holds true for a series of SSDU-attached hydrophilic polymers irrespective of the hydrophobic/hydrophilic balance or chemical structure, indicating that the supramolecular-assembly is primarily controlled by the specific molecular-recognition motif of the SSDU, instead of the packing parameter-based norms. Beyond synthetic polymers, SSDU-attached proteins also exhibit similar molecular-recognition driven self-assembly as well as coassembly with SSDU-attached polymers or hydrophilic wedges, producing multi-stimuli-responsive nanostructures in which the protein gains remarkable protection from thermal denaturation or enzymatic hydrolysis and exhibits redox-responsive enzymatic activity.Furthermore, SSDU-derived bola-shape π-amphiphiles have been recognized as a useful scaffold for the synthesis of unsymmetric polymersomes, rarely reported in the literature. The building block consists of a hydrophobic naphthalene-diimide (NDI) π-system attached to a hydrophilic functional group (ionic or nonionic) and a nonionic wedge on its two opposite arms. Extended H-bonding among the hydrazide groups, placed only on one side of the central chromophore by design, ensures stacking of the NDIs with parallel orientation and induces a preferred direction of curvature so that the H-bonded chain and consequently the functional groups attached to the same side remain at the inner-wall of the supramolecular polymersome. Automatically, the functional groups, located on the other side, are displayed at the outer surface. This design works for different amphiphiles, which by virtue of efficient and predictable functional group display, strongly influences the multivalent binding with different biological targets resulting in efficient enzyme inhibition, glycocluster effect, or antibacterial activity, depending on the nature of the functional group. By taking advantage of the electron accepting nature of the NDI, electron rich pyrene-containing amphiphiles can be costacked in alternating sequence, producing temperature and redox-responsive supramolecular polymers with NDI/pyrene stoichiometry-dependent morphology, lower critical solution temperature (LCST), functional group display, and antibacterial activity.

Tetraphenylethene-based fluorescent probe with aggregation-induced emission behavior for Hg2+ detection and its application

A tetraphenylethene (TPE) derivative was designed and synthesized upon conjugation with bis(thiophen-2-ylmethyl) amine (BTA) containing a mercury-binding moiety and further characterized by using Nuclear magnetic resonance (NMR), LC-MS, UV-Vis, and fluorescence spectroscopic methods. The resulting TPE-BTA exhibited comprehensive aggregation-induced emission while expressing a high quantum yield and emission intensity at 70% water fraction. The probe exhibited a good photochromic effect with a Stokes shift of 178 nm, and the emission intensity at 550 nm increased considerably with the color turning from dark green to bright green under a UV lamp upon the addition of 5 μM Hg²⁺. The lowest-energy conformation of the probe showed that two thiophene rings were perpendicular to the phenyl ring, while two BTA molecules were situated in a staggered form to each other. The sulfur and nitrogen atoms present in TPE-BTA were coordinated to the Hg²⁺ ion, and these binding sites were confirmed by the NMR parameters, X-ray photoelectron spectroscopy signals, and structural calculations. The binding of Hg²⁺ to TPE-BTA was believed to restrict the intramolecular motion of TPE-BTA, thus inducing it to shine brighter according to the unique aggregation-induced emission effect. The concentration of Hg²⁺ was determined based on the enhancement of the emission intensity, and the present probe showed an extremely high sensitivity with a limit of detection of 10.5 nM. Furthermore, TPE-BTA enabled selective detection of Hg²⁺ even in the presence of a 1000-fold excess of other interfering metal ions. The proposed method was successfully employed to determine Hg²⁺ in living HeLa cells and real water samples.

Energy transduction through FRET in self-assembled soft nanostructures based on surfactants/polymers: current scenario and prospects

The self-assembled systems of surfactants/polymers, which are capable of supporting energy funneling between fluorophores, have recently gained significant attraction. Surfactant and polymeric micelles form nanoscale structures spanning a radius of 2-10 nm are generally suitable for the transduction of energy among fluorophores. These systems have shown great potential in Förster resonance energy transfer (FRET) due to their unique characteristics of being aqueous based, tendency to remain self-assembled, spontaneous formation, tunable nature, and responsiveness to different external stimuli. This review presents current developments in the field of energy transfer, particularly the multi-step FRET processes in the self-assembled nanostructures of surfactants/polymers. The part one of this review presents a background and brief overview of soft systems and discusses certain aspects of the self-assemblies of surfactants/polymers and their co-solubilization property to bring fluorophores to close proximity to transduce energy. The second part of this review deals with single-step and multi-step FRET in the self-assemblies of surfactants/polymers and links FRET systems with advanced smart technologies including multicolor formation, data encryption, and artificial antenna systems. This review also discusses the diverse examples in the literature to present the emerging applications of FRET. Finally, the prospects regarding further improvement of FRET in self-assembled soft systems are outlined.

Controlling fluorescence resonance energy transfer of donor–acceptor dyes by Diels–Alder dynamic covalent bonds

Two new dyes, consisting of an aromatic amine donor and dansyl acceptor connected by Diels–Alder bonds, display a switchable energy transfer. Dynamic covalent properties enable the mutual conversion of the two dyes by maleimide exchanges.

100th Anniversary of Macromolecular Science Viewpoint: Enabling Advances in Fluorescence Microscopy Techniques

In the past few decades there has been a revolution in the field of optical microscopy with emerging capabilities such as super-resolution and single-molecule fluorescence techniques. Combined with the classical advantages of fluorescence imaging, such as chemical labeling specificity, and noninvasive sample preparation and imaging, these methods have enabled significant advances in our polymer community. This Viewpoint discusses several of these capabilities and how they can uniquely offer information where other characterization techniques are limited. Several examples are highlighted that demonstrate the ability of fluorescence microscopy to understand key questions in polymer science such as single-molecule diffusion and orientation, 3D nanostructural morphology, and interfacial and multicomponent dynamics. Finally, we briefly discuss opportunities for further advances in techniques that may allow them to make an even greater contribution in polymer science.

pH-responsive high stability polymeric nanoparticles for targeted delivery of anticancer therapeutics

The practical application of nanoparticles (NPs) as chemotherapeutic drug delivery systems is often hampered by issues such as poor circulation stability and targeting inefficiency. Here, we have utilized a simple approach to prepare biocompatible and biodegradable pH-responsive hybrid NPs that overcome these issues. The NPs consist of a drug-loaded polylactic-co-glycolic acid (PLGA) core covalently 'wrapped' with a crosslinked bovine serum albumin (BSA) shell designed to minimize interactions with serum proteins and macrophages that inhibit target recognition. The shell is functionalized with the acidity-triggered rational membrane (ATRAM) peptide to facilitate internalization specifically into cancer cells within the acidic tumor microenvironment. Following uptake, the unique intracellular conditions of cancer cells degrade the NPs, thereby releasing the chemotherapeutic cargo. The drug-loaded NPs showed potent anticancer activity in vitro and in vivo while exhibiting no toxicity to healthy tissue. Our results demonstrate that the ATRAM-BSA-PLGA NPs are a promising targeted cancer drug delivery platform.

Stochastic or Not? Method To Predict and Quantify the Stochastic Effects on the Association Reaction Equilibria in Nanoscopic Systems

The stochastic nature of chemical reaction and impact of the stochasticity on their evolution is soundly documented. Both theoretical predictions and emerging experimental evidence indicate the influence of stochastic effects on the equilibrium state of association reaction. In this work simple mathematical formulas are introduced to estimate these effects. First, the dependence of the ratio of observed reactants (apparent association constant, equivalent of macroscopic association constant in stochastic analysis) on the volume and the number of molecules of reagents is discussed and the limiting factors of this effect are shown. Next, the apparent association constant is approximated for nanoscale systems by closed-form formulas derived for this purpose. Finally, an estimation for the macroscopic constant value from the apparent one is provided and validated on the published experimental data. This work was inspired by chemical reactions occurring in biological compartments, but the results can be used for all systems belonging to the stochastic regime of chemical reactions.

Disulfide chemistry in responsive aggregation of amphiphilic systems

The dynamic nature of the disulfide bond has enhanced the potential for disulfide based amphiphiles in the emerging biomedical field. Disulfide containing amphiphiles have extensively been used for constructing wide ranging soft nanostructures as potential candidates for delivery of drugs, proteins and genes owing to their degradable nature in the presence of intracellular glutathione (present in a many fold excess compared to the extracellular milieu). This degradable nature of amphiphiles is not only useful to deliver therapeutics but it also eliminates the toxicity issues associated with the carrier after delivery of such therapeutics. Therefore, these bioreducible and biocompatible nano-aggregates inspired researchers to use them as vehicles for therapeutic delivery and as a result the literature of disulfide containing amphiphiles has been intensified. This review article highlights the structural diversity in disulfide containing amphiphilic small molecule and polymeric systems, structural effects on their aqueous aggregation, redox-responsive disassembly and biological applications. Furthermore, the use of disulfide chemistry towards the design of cell penetrating polymers has also been discussed. Finally a brief perspective on some future opportunities of these systems is provided.

Measuring nanoparticle-induced resonance energy transfer effect by electrogenerated chemiluminescent reactions

Electrogenerated chemiluminescence (ECL) efficiencies, redox potentials, photoluminescent (PL) (quenching and coupling) effects, and AFM images for the [Ru(bpy)₃]²⁺/Au@tiopronin system were determined in aqueous solutions of the gold nanoparticles (NPs) at pH 7.0. The most remarkable finding was that ECL measurements can display the nanoparticle-induced resonance energy transfer (NP-RET) effect. Its effectiveness was quantified through a coefficient, K_(NP-RET)ECL, which measures how much an ECL reaction has been enhanced. Moreover, the NP-RET effect was also checked using PL measurements, in such a way that a coefficient, K_(NP-RET)PL, was determined; both constants, K_(NP-RET)ECL and K_(NP-RET)PL being in close agreement. It is important to highlight the fact that the NP-RET effect is only displayed in diluted solutions in which there is no NPs self-aggregation. The existence of the NPs self-aggregation behavior is revealed through AFM measurements.

Electrogenerated chemiluminescence efficiencies, redox potentials, photoluminescent (quenching and coupling) effects, and AFM images for the [Ru(bpy)₃]²⁺/Au@tiopronin system were determined in aqueous solutions of the gold nanoparticles at pH 7.0.

Hyperbranched polydisulfides

Synthesis, aqueous aggregation, hydrophobic guest encapsulation, non-covalent encapsulation stability and glutathione responsive degradation of amphiphilic hyperbranched polydisulfides have been reported.