Modulation of Fluorescence through Coassembly of Molecules in Organic Nanostructures

Cascade Förster resonance energy transfer between layered silicate edge-linked chromophores

Förster resonance energy transfer (FRET) serves as a critical mechanism to study intermolecular interactions and the formation of macromolecular assemblies. Cascade FRET is a multi-step FRET process which can overcome limitations associated with traditional single-step FRET. Herein, a novel organic-inorganic hybrid composed of a nile red derivative attached to the edge of the layered silicate clay Laponite (Lap-NR) was used to facilitate cascade FRET between Laponite sheets. Utilizing naphthalene-diimide edge-modified Laponite (Lap-NDI) as the initial donor, Rhodamine 6G on the basal surface of Laponite as the first acceptor, and Lap-NR as the second acceptor, cascade FRET was achieved. The influence of solvent composition in a DMF/water mixture on cascade FRET was investigated, revealing that a higher water content led to an enhancement of the cascade FRET process, which is attributed to increased aggregation-induced emission of Lap-NDI and the enhanced quantum yield of R6G in water. This study provides a unique approach to achieve cascade FRET by taking advantage of the anisotropic surface chemistry of a two-dimensional nanomaterial, providing a colloidally-driven alternative with improved tunability compared to macromolecular routes. The flexibility and simplicity of this approach will advance the state of the art of organic-inorganic hybrids for applications in optoelectronics, sensors, and hybrid photovoltaics.

Co-assembly-Directed Enhancement of the Thermochromic Reversibility and Solvatochromic Selectivity of Supramolecular Polydiacetylene

The construction of functional materials via the co-assembly of multimolecular systems has recently emerged as a fascinating topic. The co-assembled multicomponent could promote the evolution of supramolecular assemblies into a high-order nanoarchitecture with improved functional properties. We report the successful preparation of a dual-functional polydiacetylene (MCPDA-Tz-CA) having thermochromic and solvatochromic properties via facile co-assembly of MCDA-Tz and cyanuric acid (MCDA-Tz-CA) followed by ultraviolet-induced polymerization. Molecular packing patterns from powder X-ray diffraction and density functional theory calculations of molecular self-assembly processes confirm highly ordered co-assembled lamellar structures. MCPDA-Tz-CA showed excellent reversible thermochromism properties when the temperature was increased from 30 to 150 °C with a reversible blue-to-red color transition that could be detected by the naked eye. Also, MCPDA-Tz-CA displayed selective blue-to-red solvatochromism against dimethylformamide and dimethyl sulfoxide. Detailed investigations revealed that the enhanced thermochromic reversibility and solvatochromic selectivity could be attributed to the hydrogen-bonding interactions and the formation of a network structure in the MCDA-Tz/cyanuric acid co-assembly. Our research opens a promising route for improving the performance of functional materials via noncovalent multicomponent arrangements at the molecular level.

Smart fluorescent polysaccharides: Recent developments and applications

Polysaccharides are ubiquitous, generally benign in nature, and compatible with many tissues in biomedical situations, making them appealing candidates for new materials such as therapeutic agents and sensors. Fluorescent labeling can create the ability to sensitively monitor distribution and transport of polysaccharide-based materials, which can for example further illuminate drug-delivery mechanisms and therefore improve design of delivery systems. Herein, we review fluorophore selection and ways of appending polysaccharides, utility of the product fluorescent polysaccharides as new smart materials, and their stimulus-responsive nature, with focus on their biomedical applications as environment-sensitive biosensors, imaging, and as molecular rulers. Further, we discuss the advantages and disadvantages of these methods, and future prospects for creation and use of these self-reporting materials.

Contrasting luminescence in heparin and DNA-templated co-assemblies of dimeric cyanostilbenes: efficient energy transfer in heparin-based co-assemblies

Dimeric cationic cyanostilbenes with peripheral alkyl chains demonstrated aggregation in aqueous media depending on the length of the hydrophobic segment and produced luminescent spherical nano-assemblies in the case of long alkyl chain derivatives. In the presence of heparin, a bio-polyanion that is routinely used as an anticoagulant, the self-assembled structures obtained from the amphiphilic dimers showed the formation of higher-order structures whereas the non-assembling dimers exhibited heparin-induced supramolecular structure formation. In both cases, a significant enhancement in the emission was observed. This led to the detection of heparin in aqueous buffer, serum and plasma with a "turn-on" fluorescence response. Interestingly, these derivatives also exhibited luminescence variation in the presence of ctDNA. However, the response towards DNA was opposite to that observed in the case of heparin i.e., "turn-off'' fluorescence response. Notably, depending on the length of the alkyl segment, divergent DNA binding modes of these derivatives were observed. Due to their enhanced luminescence, the heparin-based co-assemblies were further explored as artificial light-harvesting systems exhibiting an efficient energy transfer process to embedded acceptor dyes with a high antenna effect.

Hierarchical Composite Self-Sorted Supramolecular Gel Noodles

Multicomponent supramolecular systems can be used to achieve different properties and new behaviors compared to their corresponding single component systems. Here, a two-component system is used, showing that a non-gelling component modifies the assembly of the gelling component, allowing access to co-assembled structures that cannot be formed from the gelling component alone. The systems are characterized across multiple length scales, from the molecular level by NMR and CD spectroscopy to the microstructure level by SANS and finally to the material level using nanoindentation and rheology. By exploiting the enhanced mechanical properties achieved through addition of the second component, multicomponent noodles are formed with superior mechanical properties to those formed by the single-component system. Furthermore, the non-gelling component can be triggered to crystallize within the multicomponent noodles, allowing the preparation of new types of hierarchical composite noodles.

Peptide-Based Low Molecular Weight Photosensitive Supramolecular Gelators

Over the last couple of decades, stimuli-responsive supramolecular gels comprising synthetic short peptides as building blocks have been explored for various biological and material applications. Though a wide range of stimuli has been tested depending on the structure of the peptides, light as a stimulus has attracted extensive attention due to its non-invasive, non-contaminant, and remotely controllable nature, precise spatial and temporal resolution, and wavelength tunability. The integration of molecular photo-switch and low-molecular-weight synthetic peptides may thus provide access to supramolecular self-assembled systems, notably supramolecular gels, which may be used to create dynamic, light-responsive "smart" materials with a variety of structures and functions. This short review summarizes the recent advancement in the area of light-sensitive peptide gelation. At first, a glimpse of commonly used molecular photo-switches is given, followed by a detailed description of their incorporation into peptide sequences to design light-responsive peptide gels and the mechanism of their action. Finally, the challenges and future perspectives for developing next-generation photo-responsive gels and materials are outlined.

Highly sensitive and ratiometric luminescence sensing of heparin through templated cyanostilbene assemblies

The assembly of organic dyes on bio-molecular templates is an attractive strategy for the creation of bio-materials with intriguing optical properties. This principle is exploited here for the detection of polyanion heparin, a known anticoagulant, by employing di-cationic cyanostilbene derivatives with inherent aggregation induced emission (AIE) features. The cyanostilbene derivatives exhibited weak cyan-blue monomeric emissions in solutions but upon electrostatic co-assembly with heparin, formed highly luminescent clusters on the polyanion surface. The cyanostilbene chromophores in the clusters exhibited greenish-yellow excimer emissions with remarkably longer life-times (up to 70-fold) and higher quantum yields (up to 85-fold) compared to their aqueous solutions. This led to heparin detection in aqueous buffer in low nanomolar concentrations. Additionally, and more importantly, a ratiometric detection of heparin was achieved in highly competitive media such as 50% human serum and 60% human plasma in medically relevant concentrations.

A Remarkable Fluorescence Quenching Based Amplification in ATP Detection through Signal Transduction in Self-Assembled Multivalent Aggregates

Signal transduction is essential for the survival of living organisms, because it allows them to respond to the changes in external environments. In artificial systems, signal transduction has been exploited for the highly sensitive detection of analytes. Herein, a remarkable signal transduction, upon ATP binding, in the multivalent fibrillar nanoaggregates of anthracene conjugated imidazolium receptors is reported. The aggregates of one particular amphiphilic receptor sensed ATP in high pm concentrations with one ATP molecule essentially quenching the emission of thousands of receptors. A cooperative merging of the multivalent binding and signal transduction led to this superquenching and translated to an outstanding enhancement of more than a millionfold in the sensitivity of ATP detection by the nanoaggregates; in comparison to the "molecular" imidazolium receptors. Furthermore, an exceptional selectivity to ATP over other nucleotides was demonstrated.

High affinity heparin detection by multivalent supramolecular polymers through aggregation induced emission

Supramolecular polymers based on aggregation induced emission active cationic cyanostilbenes provide a highly sensitive “light-up” platform for heparin detection.

Enzyme-mediated reversible deactivation radical polymerization for functional materials: principles, synthesis, and applications

Enzymes provide a potential and highly efficient way to mediate the formation of various functional polymer materials with wide applications.

Electrostatic-driven self-sorting and nanostructure speciation in self-assembling tetrapeptides

Significant efforts in the field of supramolecular materials have strived to co-assemble small molecules in order to realize individual nanostructures with multiple, tunable activities. The design of self-assembling motifs bearing opposite charges is one commonly used method, with favorable electrostatic interactions used to promote mixing in a resulting co-assembly. This approach, at the same time, contrasts with a typical thermodynamic preference for self-sorting. Moreover, rigorous experimental techniques which can clearly elucidate co-assembly from self-sorting are limited. Here we describe the self-assembly of two oppositely charged tetrapeptides yielding highly disparate nanostructures of fibrillar and spherical assemblies. Upon mixing at different ratios, the disparate nanostructure of the parent peptides remain. Interestingly, while the assemblies appear self-sorted, surface-mediated interactions between spherical and fibrous assemblies translate to increased mechanical properties through enhanced fiber bundling. Moreover, the observed self-sorting is a thermodynamic product and not a result of kinetically trapped pre-existing structures. Taken together, and with the benefit of disparate nanostructures in the parent peptides, we have shown in our system experimental evidence for electrostatic-driven self-sorting in oligopeptide self-assembly.

Minimalistic peptide supramolecular co-assembly: expanding the conformational space for nanotechnology

Molecular self-assembly is a ubiquitous process in nature and central to bottom-up nanotechnology. In particular, the organization of peptide building blocks into ordered supramolecular structures has gained much interest due to the unique properties of the products, including biocompatibility, chemical and structural diversity, robustness and ease of large-scale synthesis. In addition, peptides, as short as dipeptides, contain all the molecular information needed to spontaneously form well-ordered structures at both the nano- and the micro-scale. Therefore, peptide supramolecular assembly has been effectively utilized to produce novel materials with tailored properties for various applications in the fields of material science, engineering, medicine, and biology. To further expand the conformational space of peptide assemblies in terms of structural and functional complexity, multicomponent (two or more) peptide supramolecular co-assembly has recently evolved as a promising extended approach, similar to the structural diversity of natural sequence-defined biopolymers (proteins) as well as of synthetic covalent co-polymers. The use of this methodology was recently demonstrated in various applications, such as nanostructure physical dimension control, the creation of non-canonical complex topologies, mechanical strength modulation, the design of light harvesting soft materials, fabrication of electrically conducting devices, induced fluorescence, enzymatic catalysis and tissue engineering. In light of these significant advancements in the field of peptide supramolecular co-assembly in the last few years, in this tutorial review, we provide an updated overview and future prospects of this emerging subject.

Multicomponent peptide assemblies

Self-assembled peptide nanostructures have been increasingly exploited as functional materials for applications in biomedicine and energy. The emergent properties of these nanomaterials determine the applications for which they can be exploited. It has recently been appreciated that nanomaterials composed of multicomponent coassembled peptides often display unique emergent properties that have the potential to dramatically expand the functional utility of peptide-based materials. This review presents recent efforts in the development of multicomponent peptide assemblies. The discussion includes multicomponent assemblies derived from short low molecular weight peptides, peptide amphiphiles, coiled coil peptides, collagen, and β-sheet peptides. The design, structure, emergent properties, and applications for these multicomponent assemblies are presented in order to illustrate the potential of these formulations as sophisticated next-generation bio-inspired materials.

Nanofiber electrospinning in samarium complex-doped PMMA

Herein, an electrospinning process of samarium complex-doped PMMAs was carried out to fabricate ultrafine fibers with a uniform diameter of about 230 nm.

Hydrogen-Bonded Multifunctional Supramolecular Copolymers in Water

We have investigated the self-assembly in water of molecules having a single hydrophobic bis-urea domain linked to different hydrophilic functional side chains, i.e., bioactive peptidic residues and fluorescent cyanine dyes. By using a combination of spectroscopy, scattering, and microscopy techniques, we show that each one of these molecules can individually produce well-defined nanostructures such as twisted ribbons, two-dimensional plates, or branched fibers. Interestingly, when these monomers of different functionalities are mixed in an equimolar ratio, supramolecular copolymers are preferred to narcissistic segregation. Radiation scattering and imaging techniques demonstrate that one of the molecular units dictates the formation of a preferential nanostructure, and optical spectroscopies reveal the alternated nature of the copolymerization process. This work illustrates how social self-sorting in H-bond supramolecular polymers can give straightforward access to multifunctional supramolecular copolymers.

Post-assembly functionalization of supramolecular nanostructures with bioactive peptides and fluorescent proteins by native chemical ligation

Post-assembly functionalization of supramolecular nanostructures has the potential to expand the range of their applications. We report here the use of the chemoselective native chemical ligation (NCL) reaction to functionalize self-assembled peptide amphiphile (PA) nanofibers. This strategy can be used to incorporate specific bioactivity on the nanofibers, and as a model, we demonstrate functionalization with the RGDS peptide following self-assembly. Incorporation of bioactivity is verified by the observation of characteristic changes in fibroblast morphology following NCL-mediated attachment of the signal to PA nanofibers. The NCL reaction does not alter the PA nanofiber morphology, and biotinylated RGDS peptide was found to be accessible on the nanofiber surface after ligation for binding with streptavidin-conjugated gold nanoparticles. In order to show that this strategy is not limited to short peptides, we utilized NCL to conjugate yellow fluorescent protein and/or cyan fluorescent protein to self-assembled PA nanofibers. Förster resonance energy transfer and fluorescence anisotropy measurements are consistent with the immobilization of the protein on the PA nanofibers. The change in electrophoretic mobility of the protein upon conjugation with PA molecules confirmed the formation of a covalent linkage. NCL-mediated attachment of bioactive peptides and proteins to self-assembled PA nanofibers allows the independent control of self-assembly and bioactivity while retaining the biodegradable peptide structure of the PA molecule and thus can be useful in tailoring design of biomaterials.

Insights into the coassembly of hydrogelators and surfactants based on aromatic peptide amphiphiles

The coassembly of small molecules is a useful means of increasing the complexity and functionality of their resultant supramolecular constructs in a modular fashion. In this study, we explore the assembly and coassembly of serine surfactants and tyrosine-leucine hydrogelators, capped at the N-termini with either fluorenyl-9-methoxycarbonyl (Fmoc) or pyrene. These systems all exhibit self-assembly behavior, which is influenced by aromatic stacking interactions, while the hydrogelators also exhibit β-sheet-type arrangements, which reinforce their supramolecular structures. We provide evidence for three distinct supramolecular coassembly models; cooperative, disruptive, and orthogonal. The coassembly mode adopted depends on whether the individual constituents (I) are sufficiently different, such that effective segregation and orthogonal assembly occurs; (II) adhere to a communal mode of self-assembly; or (III) act to compromise the assembly of one another via incorporation and disruption. We find that a greater scope for controllable coassembly exists within orthogonal systems; which show minimal relative changes in the native gelator's supramolecular structure by Fourier transform infrared spectroscopy (FTIR), circular dichroism (CD), and fluorescence spectroscopy. This is indicative of the segregation of orthogonal coassembly constituents into distinct domains, where surfactant chemical functionality is presented at the surface of the gelator's supramolecular fibers. Overall, this work provides new insights into the design of modular coassembly systems, which have the potential to augment the chemical and physical properties of existing gelator systems.

Tunable light emission from co-assembled structures of benzothiadiazole molecules

Tunable light emission (∼510–690 nm) achieved from co-localized, co-assembled structures of two different 4,7-substituted benzothiadiazole molecules.

Oligo(phenylenevinylene) hybrids and self-assemblies: versatile materials for excitation energy transfer

The engineering of the nanostructure of OPV based self-assemblies allows control of photoinduced energy transfer processes, leading to materials exhibiting tunable luminescence colours, including white.

Supramolecular Nanofibers of Peptide Amphiphiles for Medicine

Peptide nanostructures are an exciting class of supramolecular systems that can be designed for novel therapies with great potential in advanced medicine. This paper reviews progress on nanostructures based on peptide amphiphiles capable of forming one-dimensional assemblies that emulate in structure the nanofibers present in extracellular matrices. These systems are highly tunable using supramolecular chemistry, and can be designed to signal cells directly with bioactive peptides. Peptide amphiphile nanofibers can also be used to multiplex functions through co-assembly and designed to deliver proteins, nucleic acids, drugs, or cells. We illustrate here the functionality of these systems describing their use in regenerative medicine of bone, cartilage, the nervous system, the cardiovascular system, and other tissues. In addition, we highlight recent work on the use of peptide amphiphile assemblies to create hierarchical biomimetic structures with order beyond the nanoscale, and also discuss the future prospects of these supramolecular systems.

A model for the controlled assembly of semiconductor peptides

The self-assembly of small molecules provides a potentially powerful method to create functional nanomaterials for many applications ranging from optoelectronics to oncology. However, the design of well-defined nanostructures via molecular assembly is a highly empirical process, which severely hampers efforts to create functional nanostructures using this method. In this review, we describe a simple strategy to control the assembly of functionalized peptides by balancing attractive hydrophobic effects that drive assembly with opposing electrostatic repulsions. Extended π-π contacts are created in the nanostructures when assembly is driven by π-stacking interactions among chromophores that are appended to the peptide. The formation of insoluble β-sheet aggregates are mitigated by incorporating charged side-chains capable of attenuating the assembly process. Although the application of this approach to the assembly of organic semiconductors is described, we expect this strategy to be effective for many other functional organic materials.

Self-assembling glucagon-like peptide 1-mimetic peptide amphiphiles for enhanced activity and proliferation of insulin-secreting cells

Current treatment for type 1 diabetes mellitus requires daily insulin injections that fail to produce physiological glycemic control. Islet cell transplantation has been proposed as a permanent cure but is limited by loss of β-cell viability and function. These limitations could potentially be overcome by relying on the activity of glucagon-like peptide 1 (GLP-1), which acts on β-cells to promote insulin release, proliferation and survival. We have developed a peptide amphiphile (PA) molecule incorporating a peptide mimetic for GLP-1. This GLP-1-mimetic PA self-assembles into one-dimensional nanofibers that stabilize the active secondary structure of GLP-1 and can be cross-linked by calcium ions to form a macroscopic gel capable of cell encapsulation and three-dimensional culture. The GLP-1-mimetic PA nanofibers were found to stimulate insulin secretion from rat insulinoma (RINm5f) cells to a significantly greater extent than the mimetic peptide alone and to a level equivalent to that of the clinically used agonist exendin-4. The activity of the GLP-1-mimetic PA is glucose-dependent, lipid-raft dependent and partially PKA-dependent consistent with native GLP-1. The GLP-1-mimetic PA also completely abrogates inflammatory cytokine-induced cell death to the level of untreated controls. When used as a PA gel to encapsulate RINm5f cells, the GLP-1-mimetic PA stimulates insulin secretion and proliferation in a cytokine-resistant manner that is significantly greater than a non-bioactive PA gel containing exendin-4. Due to its self-assembling property and bioactivity, the GLP-1-mimetic PA can be incorporated into previously developed islet cell transplantation protocols with the potential for significant enhancement of β-cell viability and function.

Nanostructure-templated control of drug release from peptide amphiphile nanofiber gels

High aspect ratio peptide nanofibers have potential as biodegradable vehicles for drug delivery. We report here the synthesis of four self-assembling peptide amphiphiles (PAs) containing a lysine ε-amine-derivatized hydrazide that was systematically placed at different positions along the backbone of the peptide sequence C(16)V(2)A(2)E(2) (where C(16) = palmitic acid). Hydrazones were formed from each hydrazide by condensation with the solvatochromic dye 6-propionyl-2-dimethylaminonaphthalene (Prodan), which is typically used to probe cell membranes. All four compounds were found to self-assemble into nanofibers, and Prodan release was measured from filamentous gels prepared by screening PA charges with divalent cations. Near zero-order release kinetics were observed for all nanofibers, but release half-lives differed depending on the position of the fluorophore in the PA sequence. Dye release kinetics were rationalized through the use of cryogenic transmission electron microscopy, small-angle X-ray scattering, fluorescence spectroscopy, fluorescence anisotropy, circular dichroism, and partition coefficient calculations. Relative release rates were found to correlate directly with fluorophore mobility, which varied inversely with packing density, degree of order in the hydrophobic PA core, and the β-sheet character of the peptide.

Functionalized nanostructures with application in regenerative medicine

In the last decade, both regenerative medicine and nanotechnology have been broadly developed leading important advances in biomedical research as well as in clinical practice. The manipulation on the molecular level and the use of several functionalized nanoscaled materials has application in various fields of regenerative medicine including tissue engineering, cell therapy, diagnosis and drug and gene delivery. The themes covered in this review include nanoparticle systems for tracking transplanted stem cells, self-assembling peptides, nanoparticles for gene delivery into stem cells and biomimetic scaffolds useful for 2D and 3D tissue cell cultures, transplantation and clinical application.

Fluorogenic Enzyme-Responsive Micellar Nanoparticles

In this paper we describe enzyme-responsive fluorogenic micellar nanoparticles with detectable spectrophotometric properties unique to the particles and their aggregated state. These micelles are assembled from peptide-polymer amphiphiles (PPAs) labeled with either fluorescein or rhodamine. This is achieved by labelling otherwise similar block copolymer amphiphiles with each of the dyes. When mixed together, signals from the FRET-pairs can be utilized to detect particle assembly and hence enzymatic activity. Furthermore, we show FRET signals within the shell of the assembled micelles can be used to estimate particle stability (critical aggregation concentration) and enable a determination of intraparticle distances between amphiphiles in the micellar aggregates leading to elucidation of the packing arrangement of amphiphilic copolymers within the micelles.

Complementary π-π interactions induce multicomponent coassembly into functional fibrils

Noncovalent self-assembled materials inspired by amyloid architectures are useful for biomedical applications ranging from regenerative medicine to drug delivery. The selective coassembly of complementary monomeric units to provide ordered multicomponent fibrils is a possible strategy for enhancing the sophistication of these noncovalent materials. Herein we report that complementary π-π interactions can be exploited to promote the coassembly of phenylalanine (Phe) derivatives that possess complementary aromatic side-chain functionality. Specifically, equimolar mixtures of Fmoc-Phe and Fmoc-F(5)-Phe, which possess side-chain groups with complementary quadrupole electronics, readily coassemble to form two-component fibrils and hydrogels under conditions where Fmoc-Phe alone fails to self-assemble. In addition, it was found that equimolar mixtures of Fmoc-Phe with monohalogenated (F, Cl, and Br) Fmoc-Phe derivatives also coassembled into two-component fibrils. These results collectively indicate that face-to-face quadrupole stacking between benzyl side-chain groups does not account for the molecular recognition between Phe and halogenated Phe derivatives that promote cofibrillization but that coassembly is mediated by more subtle π-π effects arising from the halogenation of the benzyl side chain. The use of complementary π-π interactions to promote the coassembly of two distinct monomeric units into ordered two-component fibrils dramatically expands the repertoire of noncovalent interactions that can be used in the development of sophisticated noncovalent materials.

Dendronized triangular oligo(phenylene ethynylene) amphiphiles: nanofibrillar self-assembly and dye encapsulation

Triangular-shaped oligo(phenylene ethynylene) amphiphiles 1a and 1b decorated in their periphery with two- and four-branched hydrophilic triethyleneglycol dendron wedges, have been synthesized and their self-assembling properties in solution and onto surfaces investigated. The steric demand produced by the dendritic substituents induces a face-to-face rotated pi stacking of the aromatic moieties. Studies on the concentration and temperature dependence confirm this mechanism and provide binding constants of 1.2 x 10(5) and 1.7 x 10(5) M(-1) in acetonitrile for 1a and 1b, respectively. Dynamic and static light scattering measurements complement the study of the self-assembly in solution and demonstrate the formation of rod-like supramolecular structures in aqueous solution. The nanofibers formed in solution can be efficiently transferred onto surfaces. Thus, TEM images reveal the presence of strands of various thickness, with the most common being several micrometers long and with diameters of around 70 nm. Some of these nanofibers present folded edges that are indicative of their ribbon-like nature. Interestingly, compound 1b can also form thick filaments with a rope-like appearance, which points to a chiral arrangement of the fibers. AFM images under highly diluted conditions also reveal long fibers with height profiles that fit well with the molecular dimensions calculated for both amphiphiles. Finally, we have demonstrated the intercalation of the hydrophobic dye Disperse Orange 3 within the filaments and its subsequent release upon increasing the temperature.

Self-assembly of peptide amphiphiles: from molecules to nanostructures to biomaterials

Peptide amphiphiles are a class of molecules that combine the structural features of amphiphilic surfactants with the functions of bioactive peptides and are known to assemble into a variety of nanostructures. A specific type of peptide amphiphiles are known to self-assemble into one-dimensional nanostructures under physiological conditions, predominantly nanofibers with a cylindrical geometry. The resultant nanostructures could be highly bioactive and are of great interest in many biomedical applications, including tissue engineering, regenerative medicine, and drug delivery. In this context, we highlight our strategies for using molecular self-assembly as a toolbox to produce peptide amphiphile nanostructures and materials and efforts to translate this technology into applications as therapeutics. We also review our recent progress in using these materials for treating spinal cord injury, inducing angiogenesis, and for hard tissue regeneration and replacement.

Fibrillar peptide gels in biotechnology and biomedicine

Peptides, peptidomimetics, and peptide derivatives that self-assemble into fibrillar gels have received increasing interest as synthetic extracellular matrices for applications in 3D cell culture and regenerative medicine. Recently, several of these fibrillizing molecules have been functionalized with bioactive components and chemical features such as cell-binding ligands, degradable sequences, drug eluting compounds, and cross-linkable groups, thereby producing gels that can reliably display multiple factors simultaneously. This capacity for incorporating precise levels of many different biological and chemical factors is advantageous given the natural complexity of cell-matrix interactions that many current biomaterial strategies seek to mimic. In this review, recent efforts in the area of fibril-forming peptide materials are described, and advantages of biomaterials containing multiple modular elements are outlined. In addition, a few hurdles and open questions surrounding fibrillar peptide gels are discussed, including issues of the materials' structural heterogeneity, challenges in fully characterizing the diversity of their self-assembled structures, and incomplete knowledge of how the materials are processed in vivo.

Emerging peptide nanomedicine to regenerate tissues and organs

Peptide nanostructures containing bioactive signals offer exciting novel therapies of broad potential impact in regenerative medicine. These nanostructures can be designed through self-assembly strategies and supramolecular chemistry, and have the potential to combine bioactivity for multiple targets with biocompatibility. It is also possible to multiplex their functions by using them to deliver proteins, nucleic acids, drugs and cells. In this review, we illustrate progress made in this new field by our group and others using peptide-based nanotechnology. Specifically, we highlight the use of self-assembling peptide amphiphiles towards applications in the regeneration of the central nervous system, vasculature and hard tissue along with the transplant of islets and the controlled release of nitric oxide to prevent neointimal hyperplasia. Also, we discuss other self-assembling oligopeptide technology and the progress made with these materials towards the development of potential therapies.

Tunable gel formation by both sonication and thermal processing in a cholesterol-based self-assembly system

A family of asymmetric cholesterol-based fluorescent organogelators containing naphthalimide, connected by two acylamines and different alkyl-chain spacers, have been designed and prepared. These compounds can gelate a variety of organic solvents with both ultrasound stimuli and general sol-gel processes. The self-assembly and gelling properties of the compounds depend on the length of the alkyl chains and can be controlled by ultrasound stimuli and renewed by a thermodynamic process. The morphologies and surface wetabilities of the xerogels prepared from these gelators are strongly affected by environmental stimuli. The mechanism of the process was investigated by confocal laser scanning microscopy, transmission or scanning electron microscopy, wide-angle X-ray scattering analysis, and rheological experiments. The studies reveal that the cooperation and relative competition of multiple intermolecular interactions, influenced by the sonication or thermal stimulus, are the main contributors for the aggregation or nucleation processes; this results in the macrodifferences in morphology and surface properties. These results provide a deeper understanding of the intermediate transition state of the gel during use of an ultrasound stimulus.

Supramolecular domains in mixed peptide self-assembled monolayers on gold nanoparticles

Self-organization in mixed self-assembled monolayers of small molecules provides a route towards nanoparticles with complex molecular structures. Inspired by structural biology, a strategy based on chemical cross-linking is introduced to probe proximity between functional peptides embedded in a mixed self-assembled monolayer at the surface of a nanoparticle. The physical basis of the proximity measurement is a transition from intramolecular to intermolecular cross-linking as the functional peptides get closer. Experimental investigations of a binary peptide self-assembled monolayer show that this transition happens at an extremely low molar ratio of the functional versus matrix peptide. Molecular dynamics simulations of the peptide self-assembled monolayer are used to calculate the volume explored by the reactive groups. Comparison of the experimental results with a probabilistic model demonstrates that the peptides are not randomly distributed at the surface of the nanoparticle, but rather self-organize into supramolecular domains.