Supramolecular Construction of Optoelectronic Biomaterials

Supramolecular Construction of Optoelectronic Biomaterials JOHN D. TOVAR* Department of Chemistry, Department of Materials Science and Engineering, and Institute for NanoBioTechnology Johns Hopkins University 3400 North Charles Street (NCB 316), Baltimore, Maryland 21218, United States RECEIVED ON OCTOBER 28, 2012

CONSPECTUS

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eptide self-assembly is a powerful method to create functional nanoscale materials such as optoelectronically relevant organic nanostructures. The enormous potential that may come from bringing π-conjugated electronic function into biological environments is poised to impact cell and tissue engineering, biosensors, and related biomedical applications. However, very little synthetic guidance is available with respect to uniting these two different materials sets in a generally applicable manner. In this Account, I describe my group's work to synthesize and assemble peptidic nanostructures built around organic electronic elements. The Account begins with a very brief background to the area of supramolecular electronics, followed by a description of areas where these nanomaterials could be useful in biology. I then discuss the synthetic approaches that we utilized to embed a variety of π-electron units directly within peptide backbones. A key supramolecular challenge with respect to subsequent self-assembly of these new molecules is balancing electrostatic contributions within the resulting nanomaterials, because the suitable geometries for stabilizing peptide assemblies may not necessarily correspond to those suitable for maximizing intermolecular π-electron interactions. Regardless of the respective magnitudes of these two major influences, the assembly paradigm is fairly robust. Variation of the π-electron units and the peptide sequences that make up the “peptide-π-peptide” triblock molecules consistently leads to fairly uniform tape-like nanostructures that maintain strong electronic coupling among the component π-electron units. We explored a diverse range of π-electron units spanning fluorescent oligo(phenylene vinylene)s, electron-accepting rylene diimides, and hole-transporting oligothiophenes. I then describe the characterization of the nanomaterials that form after molecular self-assembly in order to understand their internal structures, electronic interactions, and morphologies as existing within self-supporting hydrogel matrices. I also describe how a facile shearing process provided globally aligned macroscopic collections of one-dimensional electronic fibrils in hydrogel matrices. These general assembly processes influence intermolecular π-stacking among the embedded chromophores, and the assemblies themselves can facilitate the covalent cross-linking and polymerization (for example, of reactive diyne units). The latter offers an exciting possibility to create peptidic nanostructures comprised of single polymer chains. Finally, I discuss electronic properties as manifested in the interactions of transition dipoles within the nanomaterials and electrical properties resulting from field-effect gating. The ability to tune the observable electrical properties of the nanostructures externally will allow for their transition to in vitro or in vivo platforms as a powerful new approach to regulating biological interactions at the nanoscale.

Introduction

least one size dimension in the 1100 nm range, offers

Supramolecular electronic materials bridge the critical size regime that lies between those available from conventional microscale fabrication and “technology-forcing” prospects of molecular electronics. This intermediate regime, with at

nanoscale dimensionality and more technically feasible ma-

www.pubs.acs.org/accounts 10.1021/ar3002969 & XXXX American Chemical Society

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Supramolecular Construction of Optoelectronic Biomaterials Tovar

FIGURE 1. Supramolecular electronic nanomaterials depicted with varying degrees of chemical abstraction, as adapted with permission from refs 26 (ae, respectively). Copyrights 2001 American Chemical Society, 2004 Benjamin Messmore, 2005 National Academy of Sciences (USA), 2010 Wiley-VCH Verlag GmbH & Co. KGaA, and 2012 Royal Society of Chemistry.

components into nanomaterials with semiconductor properties such as light emission, light harvesting, and charge carrier transport.1 The supramolecular self-assembly process serves as the fabrication step for novel nanoscale electronic materials by guiding or controlling the nature of the intermolecular π-electron interactions and, thus, the nature of the electronic delocalization within the newly formed nanomaterials. Using a carefully engineered balance of hydrogen bonding, quadrupole interactions, and amphiphilic interactions, thermodynamically favorable environments can be constructed within supramolecular aggregates with extended π-electron delocalization arising from specific interactions among molecular components (Figure 1).26 Electrically conductive π-conjugated materials are attracting interest in a variety of biomedical applications7 ranging from neural electrode coatings8 to biosensors,911 cell and tissue engineering,1214 and artificial muscles.15,16 This emerging field is equally exciting for self-assembling bioelectronic materials because the size scales of supramolecular aggregates are on the same order of magnitude as common structural proteins found in the extracellular matrix that provide the critical adhesive foundation for living cells. π-Electron functional materials that in some ways mimic the structures of these native proteins could offer a powerful means to integrate non-natural electronic properties into living organisms. However, the severe insolubilities of typical π-conjugated organic electronic materials in aqueous environments coupled to the strong abilities for water to effectively outcompete or screen the desired intermolecular B ’ ACCOUNTS OF CHEMICAL RESEARCH

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interactions has made the exploration of supramolecular materials in the aqueous and high ionic strength environments of biology very difficult. One class of synthons useful for the aqueous supramolecular assembly of π-conjugated nanostructures is the oligo(ethylene glycol) (OEG) motif. Lee and co-workers reported several self-assembling structures based on OEG-modified oligophenylenes leading to micelles, vesicles, and nanotubes.17,18 Rybtchinski used OEG-modified perylene diimides to create complex aqueous polymeric nanomaterials.19,20 However, these motifs can be envisioned to resist protein adhesion or other biological interaction, and to address this issue, Lee and co-workers further modified their materials with carbohydrates and other biological motifs to overcome these resistances.21 Peptide sequences have also been exploited to render π-conjugated systems soluble in aqueous environments. The most common methods to add π-electron functionality to peptide systems would be through amide bond formation or Michael-type addition at an ionizable peptide residue (lysine amines, cysteine thiols, etc.) thus leading to proteins with site-specific spin labels or environmentally sensitive dyes. More recently, these synthetic techniques have been used to construct peptides with unusual electronic function. R-Helical oligopeptides have been labeled with fluorescent dyes at controlled positions along the helical backbone leading to unusual intramolecular chromophore interactions,22 and helical polypeptides appended with semiconductive oligothiophenes have been used to fabricate organic field-effect transistors (OFETs).23 Oligomeric π-conjugated units have also


FIGURE 3. Generic solid-phase synthetic routes to prepare π-conjugated peptides.

FIGURE 2. Representative examples π-electron units incorporated into designer oligopeptides (truncated for clarity). The portions of the molecules that were fabricated independently of peptide coupling reactions are shown in red.

been placed at the peptide termini rather than onto side chains.24 These “diblock” architectures usually lead to a 2-D lattice-like network of interdigitated materials whereby the π-electron and the peptide components self-segregate in the solid state. Although there have been several examples of amide-based self-association phenomena being used to encourage the intermolecular delocalization of π-conjugated oligomers,25 relatively few examples have been executed in completely aqueous environments. Research over the past decade has established that π-electron subunits can be embedded directly into the backbones of oligopeptides to allow for subsequent manipulation in aqueous environments (Figure 2).2630 Merging π-electron systems with biomimetic structures holds great promise for the development of the next generation of bioelectronic materials.31 This Account will describe our work toward this goal by describing molecular synthesis, spectroscopic and morphological characterization, and emerging prospects in electrical device fabrication and biological application.

Design and Synthetic Development We hypothesized that, in line with established peptide assembly paradigms, peptide aggregation would drive embedded π-electron units into close-packed electronically perturbed channels. As noted above, functional π-electron units have been previously incorporated quite successfully

FIGURE 4. Representative π-conjugated Fmoc-protected amino acids (top) and peptides subsequently prepared from them (middle and bottom).

into peptide scaffolds through reactive side-chain functionalities of specific amino acid residues. However, general synthetic routes to systematically install π-electron materials directly within peptide backbones were lacking. We developed three distinct approaches to prepare peptide-conjugated electronic materials that depend on structurally distinct chromophore subunits: π-conjugated “amino acids”,32 bis-electrophiles,33 and most recently, a Pd-catalyzed route that eliminates the need for the up-front construction of preassembled π-electron systems (Figure 3).34 They were intentionally restricted to be compatible with standard Fmoc-based solidphase peptide synthesis (SPPS) in order to alleviate the insolubilities often associated with π-conjugated oligomers when doing solution chemistry. Via π-Conjugated “Amino Acids”. The first-generation strategy used Rω amino acids where a π-conjugated unit was appended on one end of the linear conjugation pathway with an Fmoc-protected amine group and the other end with a carboxylic acid group.32 We anticipated that such building blocks could be used just as any commercially available Fmoc-protected amino acids used for SPPS. This idea was validated by Chmielewski and by Nowick to prepare photoswitchable diazo-containing peptides and rod-like Vol. XXX, No. XX

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FIGURE 5. Development of a general on-resin dimerization synthesis approach. The anticipated reactivity and isolated product are shown on the left, and the experimental results are shown on the right.

peptide macromolecules, respectively.26,27 We first targeted a bithiophene amino acid due to the high interest in oligothiophene electronic materials, followed by ter- and quaterthiophene amino acids35 and oligo(phenylene vinylene) amino acids (Figure 4).36 All of these building blocks were subsequently carried into peptide backbones except for the quaterthiophene, which was not soluble enough in typical SPPS solvents. The synthetic complexities involved with amino acid construction limited availabilities to proof-ofprinciple quantities, and the multiple equivalents often

FIGURE 6. Representative diacid units employed in the on-resin dimerization (top) and peptides subsequently prepared from them (middle and bottom).

necessary to drive amidations to completion represent a substantial waste of precious and synthesis-intensive build-

drive this to reasonable yields of isolated peptideπpeptide

ing blocks. Via π-Conjugated Dianhydrides and Diacids: “On-Resin Dimerization”. A major synthetic advance came by accident. We intended to use naphthalene and perylene dianhydrides as precursors to electron-deficient amino acids but encountered some problematic but well-precedented cyclization complications.37 To avoid this, we treated a resinbound oligopeptide (presenting deprotected N-terminal amines) with the dianhydrides directly expecting to monoimidate the amines and present the other anhydride moiety

molecules (ca. 4050%).33 We subsequently extended this finding to π-conjugated Rω diacid electrophiles with comparably modest yields (Figure 6). Relative to the π-conjugated amino acids, the synthesis of π-conjugated diacids was operationally much simpler. For example, quaterthiophene R,R0 -diacids could be

at the peptide termini ready for imidation with, for example, ethylene diamine or phenylene diamine (Figure 5, left), thus leaving a free amine attached to the diimide core ready for continued SPPS. However, mass spectral analysis of the resulting products was inconsistent with those anticipated. After some discussion, we reasoned that the observed products were

obtained in one step from commercially available quaterthiophene. Although these streamlined syntheses allowed for a greater diversity of electronic materials in shorter times, longer oligomeric structures still exhibited problematic solubilities that frustrated larger scale chemical synthesis. Via Resin-Bound Pd-Catalyzed Cross-Couplings. The third and most recent route used Pd-catalyzed chemistry to piece together the electronic units directly on the solid phase

line with comparable reactivity shown previously for aliphatic

rather that requiring the prior synthesis of amino acids or diacids (Figure 7, left).34 Smaller carboxylic acid subunits with aryl bromide functionality could be reacted with the terminal resin-bound amines for initial amide bond formation. Treat-

cross-linking agents.38 Optimized reaction conditions could

ment of the now-terminal aryl bromides presented on the resin

resulting from the “cross-linking” of two resin-bound terminal amines with the dianhydride biselectrophile (Figure 5, right), in

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FIGURE 7. A generic depiction of the palladium-catalyzed on-resin dimerization procedure (left) and representative peptides prepared with this procedure (right).

with standard difunctional transmetalation agents under Pd catalysis then led to the formal cross-link between the peptide chains. A much broader scope of electronic structures could be achieved through this chemistry in vastly shorter time including arylene ethynylenes, mixed thienylphenyl oligomers and oligothiophenes up to sexithiophene (Figure 7, right). These longer oligomers as discrete amino acids or diacids would have severe molecular insolubilities thus rendering them unfit for proper purification and solid-phase chemistry. We could employ commercially available building blocks (aryl halides and transmetalation agents) or could use synthetically tractable and scaleable units for more exotic precursors. Regardless of the synthetic method employed, all peptides were cleaved from their resin synthesis supports using standard trifluoroacetic acid treatment without any obvious decomposition of the π-electron segment. The peptide residues were chosen based on literature precedent for peptide assembly along with our own empirical findings. For example, we have found the “DFAG” tetrapeptide sequence to be a reliable motif to encourage self-assembly among these types of π-conjugated peptides. Remarkably, the overall material assembly process as described below is tolerant of substantial variations both in the specific peptide sequences attached to the conjugated core and the nature of this core itself. There is one subtle but important structural aspect that is dependent on the choice of synthesis method: the orientation of the N-to-C directionality (or “polarity”) of the oligopeptide chain with respect to the central core. The use of π-conjugated amino acids allows for the directionality of the peptide sequence to be maintained through the conjugated linkage: there is one overall N-to-C directionality and only one unique N- and C-terminus as expected in natural proteins. In contrast, the N-terminal amines on the resin are joined to a common conjugated core when using

the on-resin dimerization approaches, and the final peptide cleavage unveils the C-termini of these fragments. Thus, the N-to-C polarity emanates in two different directions from cores prepared through the on-resin dimerization routes.

Molecular Assembly into Peptidic Nanostructures The self-assembly of small oligopeptides has been well studied in the context of understanding the formation of insoluble protein deposits associated with prions and neurological diseases such as Alzheimer's.3942 These assemblies come in a variety of morphologies including coiled ribbons, twisted tapes, and higher-order stacked structures. The widths of these materials range from the length of the molecular components up to dozens of nanometers, and their lengths can exceed micrometers. These dimensions have thus rendered peptide aggregates to be attractive targets for materials design,4345 and they provide a logical platform to bring organic electronic function into robust 1-D nanomaterials provided that the internal π-electron unit does not interfere with these assembly processes. A careful balancing of molecular electrostatics is required when considering any self-assembly scheme. Our initial peptides included several carboxylic acid groups that should be deprotonated at slightly basic or even neutral pH. The resulting localized centers of anionic charge thus lead to Coulombic repulsion that minimizes any extent of intermolecular association. At acidic pH, carboxylate protonation formally screened these charges and reduced the intermolecular electrostatic repulsions. This screening then allowed for other favorable intermolecular interactions to be established such as peptide hydrogen bonding. Although we find this fairly extreme change in pH to be convenient in practice to quickly assess self-assembly, it is certainly not required. We Vol. XXX, No. XX

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FIGURE 8. Energy-minimized depictions of possible aggregate structures showing the tape-like nature (a) and the inherent twist when looking down the long axis of the nanostructure (b). The π-conjugated segment (here, sexithiophene) is colored yellow.

have shown in many cases that much more benign charge screening conditions can be employed to initiate self-assembly (e.g., physiologically relevant pH conditions, buffered cell culture media). These are fairly common approaches employed to trigger self-assembly within nonconjugated peptidic materials.4648 Our molecular design was predicated on the premise that the establishment of the enthalpically stabilizing β-sheet hydrogen bonding arrays between peptide molecules would subsequently force the π-units into cofacial intermolecular interactions. Completely cofacial ππ interactions are not entirely favorable from an electrostatic point of view (although dispersive contributions might counter this),49 so aromatics tend to adopt other habits in organized assemblies consisting of slipped stacks or edge-to-face orientations.50 Natural β-sheets have slight twists (not being entirely flat tapes) so it would be expected that this twist might serve to alleviate the completely cofacial quadrupolar electrostatic repulsion (see Figure 8). If the amyloid model were applicable to our modified π-conjugated peptides, the long axis of the intermolecular electronic delocalization arising from the π-stacking would run fairly coincident with the long axes of the individual peptide nanostructures. Molecular modeling of generic peptide sequences with stilbene as a representative electronic core is shown in Figure 9. As described above, the chemical details of the synthetic technique have a direct impact on the nature of the β-sheet hydrogen-bonding networks that must be established in order to facilitate the intermolecular electronic F ’ ACCOUNTS OF CHEMICAL RESEARCH

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FIGURE 9. Energy-minimized structures showing the subtle differences of aggregate internal structure due to differing hydrogen-bonding networks among the nonsymmetric (a, forming the antiparallel β-sheets) and the symmetric (b, forming the parallel β-sheets) stilbene-based peptides.

delocalization. Peptides prepared from the π-conjugated amino acids necessarily must form antiparallel β-sheets in order to achieve the electronically delocalized π-stacked architecture, while peptides prepared from the dimerization routes require the formation of parallel β-sheets. In principle, these bonding arrangements can be ascertained through detailed interpretation of infrared (IR) and circular dichroism (CD) spectra, but in practice for these types of materials, such analyses are ambiguous at best. The intermolecular π-stacking dictated by these hydrogen-bonding networks was at less than optimal 5 Å distances.

Photophysical Outcomes of the Assembly Process Unlike the case for natural amyloids and related protein nanostructures, the π-conjugated units present unique and direct possibilities to ascertain internal structure due to exciton coupling among the transition dipoles that make up the π-conjugated moieties within the nanostructures. These couplings have well-established spectral properties,


FIGURE 10. Chromic variations among different peptide solutions and hydrogels revealed under UVvis irradiation as exemplified by peptides embedded with perylene diimide (a), terthiophene (b), and distyryl benzene (c). The left images in each panel represent basic pH where the peptides are molecularly dissolved, and the right images are at acidic pH that drives the peptide assembly process. These differences can also be assessed spectrally as shown for a distyryl benzene peptide using UVvis and photoluminescence (d) and circular dichroism (e). The dashed lines represent spectra recorded in basic pH and the solid lines represent spectra recorded in acidic pH. Data taken from ref 33.

with the so-called “H-aggregates” showing pronounced absorption blue shifts and diminished photoluminescence intensities shifted to the red.51 H-aggregates are most commonly associated with high degrees of cofacial π-electron overlap, and their presence as deduced via electronic spectroscopy can bode well for semiconductor behavior. Two exciting aspects of these materials are that the peptides can enforce intermolecular π-electron orientations that may not be energetically favorable among the unsubstituted variants and that the π-electron units are able to undergo observable exciton coupling in the first place (vide supra). The assembly processes lead to a variety of visually distinct color changes that vary with the embedded chromophore, as observed in aqueous media both under ambient light and under UVvis irradiation (Figure 10). The photophysics of these processes as observed in spectroscopically dilute solutions (ca. 105106 M) are consistent with the H-like aggregation picture revealing classic blueshifted absorption and quenched red-shifted steady-state photoluminescence profiles after assembly is triggered. Under conditions that promote molecular dissolution, the CD

spectra are featureless in the low-energy regions corresponding to the π-electron units. After assembly, pronounced excitonic couplings are found indicating that the chromophore units are held in locally chiral environments. The sign changes of these split Cotton effects also give information about the collective orientations of transition dipoles within the aggregates (e.g., helical twist sense within the β-sheets) according to the exciton chirality method52 and indicate here that the natural left-handed twisting found in β-sheet peptides is preserved in the π-conjugated derivatives. The geometric origins of exciton coupling can be traced to specific translations, rotations, and attenuated π-stacking distances among the individual transition dipoles that make up an aggregate.53,54 These factors are difficult to control from an empirical standpoint but extensive theoretical treatments have revealed the anticipated photophysical outcomes associated with these geometric perturbations. We are currently exploring how different peptide residues, with varying degrees of steric bulk and hydrophobicity, can “fine-tune” these interactions in the vicinity of the embedded chromophore to directly influence semiconductor behavior.

Morphological Outcomes of the Assembly Process At suitable peptide concentrations (ca. 0.11 wt % in water), the assembly process is accompanied macroscopically by the formation of self-supporting hydrogels (Figure 10ac). These compositions are of interest for biomedicine because they can be thought of as synthetic surrogates of the extracellular matrix and have been used as such for tissue engineering.48,55 We visualized the internal structure of these hydrogels in their dried states using atomic force (AFM), transmission electron (TEM), and scanning electron (SEM) microscopies (Figure 11). Although the nature and extent of the information content is different for the three, all reveal the formation of 1-D structures with widths or diameters well under 10 nm and lengths of micrometers. For reference, the molecular lengths of the peptides used are between 4 and 6 nm. Height profiles of structures deposited on mica or silicon oxide surfaces suggest a variety of morphologies from flat tapes a couple nanometers in height to large hierarchically ordered bundles up to 40 nm in height. TEM corroborates the unbundled nanostructure dimensions as tracking with the molecular peptide length (ca. 68 nm). SEM gives a better feel for the internal structure of the hydrogel showing a densely entangled array of filaments with feature widths on the order of 3040 nm. Unlike the AFM bundles, quantification of the SEM dimensions Vol. XXX, No. XX

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FIGURE 11. Representative microscopy images obtained for the self-assembling nanostructures: atomic force microscopy of nonsymmetric bithiophene peptide nanostructures (a), transmission electron microscopy of symmetric terphenyl peptide nanostructures (b), and scanning electron microscopy of symmetric distyrylbenzene peptide nanostructures (c). Images and acquisition details can be found in refs 32, 34, and 58, respectively.

FIGURE 12. Aligned arrays of peptide nanostructures within hydrogel “noodles” shown at different magnifications from naked eye (a) to micrometer scale (b, c) and a representative image taken through crossed polarizers indicating the homogeneous extent of alignment over the macroscale material. Images taken from ref 58. Copyright 2011 Wiley.

is difficult due to aggressive sample preparation conditions. The π-conjugated peptides usually do not form one specific nanomaterial morphology but rather a diverse array of hierarchically complex bundles (much like amyloid nanomaterials). Nevertheless, the ability to fashion π-electron materials in aqueous media into sub-10-nm nanomaterials is especially noteworthy!

Random versus Aligned Nanomaterials The nanostructure images shown in Figure 11 show a clear lack of alignment, and the ability to align these nanoscale “wires” could open up additional opportunities for electronic or biological investigation. We were therefore pleased to learn about a technique developed by Samuel Stupp's group at Northwestern that provided highly aligned arrays of nanostructures within hydrogel “noodles”.56 Shuming Zhang, the lead author of the Nature Materials paper, had just started his postdoctoral period with Hai-Quan Mao (JHU Materials Science), and he inspired us to extend the procedure to our materials. Mao found previously that aligned matrices of polymer microfibers can influence neural cell physiology,57 and comparable alignment of sub-10-nm nanostructures with electronic function could be an exciting addition to his work. H ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 000–000

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The technique in brief involves dispensing a peptide solution manually through a pipet into another solution that initiates assembly while applying a gentle drag to the pipet tip (Figure 12a). The local shearing of the peptide solution at the point of mixing presumably promotes a pseudoepitaxial growth of aligned peptide nanostructures. The versatility of the Stupp technique is evident in the minimal experimental tweaking that was necessary to achieve alignment among our π-conjugated peptide nanostructures. The alignment was observed locally via SEM analysis of critical-point dried samples and globally via polarized optical microscopy (Figure 12bd).58 This simple technique is thus in one procedural step able to produce macroscopically aligned collections of electronically delocalized nanostructures starting from isotropic molecular solutions, with no requirements for the application of external magnetic or electric fields.

Supramolecular Chemistry within the Nanostructures: Topochemical Polymerization We focused on peptide-driven assembly to organize π-conjugated units within supramolecular polymer nanostructures. It is also conceivable to preorganize reactive diacetylene units in order to promote topochemical polymerizations into conjugated


FIGURE 13. A truncated illustration of diphenyl butadiyne aggregates before and after topochemical polymerization (top) and the resulting nanomaterials as visualized by AFM and by modeling software (bottom).

polydiacetylenes within the assembled nanostructures. These polymerizations have tight geometric specifications necessary for successful reactivity,59 and it turns out that these nanomaterials maintain sufficient internal order to promote them.60 Diphenyl butadiyne chromophores were embedded in the peptide backbones and triggered to self-assemble (Figure 13, top). Exposing the nanomaterials to UV irradiation led to the formation of blue material with spectral properties consistent with conjugated polydiacetylenes. Diacetylene polymerizations within self-assembling architectures are known to provide responsive chromic materials,61,62 and the single polymer nanostructures now available from this approach will allow us to explore biosensory responses associated with isolated (and perhaps even isolable) polydiacetylene chains (e.g., those emerging from the bundle in Figure 13, bottom left). The chromic properties of these polymer assemblies could be reversibly altered through changes in pH, presumably due to changes in side-chain residue protonation and accompanying alteration of the peptide hydrogen-bonding network leading to conformational changes along the conjugated polymer backbone.

Interrogation of Nanostructure Electrical Properties Finally, the electrical transport properties of the nanomaterials were measured in collaborative efforts via two distinct methods to assess the presence and mobility of mobile

FIGURE 14. Force microscopy images showing the standard height profile (a) and the phase-shift images recorded in the absence of (b) and during the application of a 5 V (c) and þ5 V (d) bias to the conductive tip. Reprinted from ref 60 with permission. Copyright 2012 American Chemical Society.

charge carriers within the nanostructures. Both measurements were conducted on dried samples and lay a critical foundation for our future research. Electrostatic Force Microscopy. Using a variant of conductive AFM, Nikolaus Hartman in Nina Markovic's group (JHU Physics) showed the ambipolar transport properties within polydiacetylene nanostructures.60 This scanning probe technique can distinguish features with topographical differences in heights from features with variations in conductance.63 Figure 14 shows the height profile of polydiacetylene nanostructures (panel a) along with the images recorded in the absence of an applied potential that would induce mobile charge carriers (panel b). As expected, no induced bias leads to no additional tipsurface interactions. However, when a “gate bias” is provided (panel c and d), a signal is now evident that corresponds to the nanomaterials in panel a, a signature that mobile carriers have been created. An attractive aspect of this measurement is that it allows for a qualitative visualization of electrical properties directly without the need to deposit electrodes on the peptidic materials. Field-Effect Transistors. Thomas Dawidcyzk in Howard Katz's group (JHU Materials Science) fabricated transistors using the nanomaterials as the active semiconductor layer in order to measure gate-induced field effects.58 Clear anisotropies in hole transport (ca. 20-fold) through the aligned noodles containing a “p-type” quaterthiophene core were Vol. XXX, No. XX

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evident when the source and drain electrodes were probed parallel vs perpendicular to the alignment axis. The mobilities measured in aligned samples are on the order of 0.03 cm2/(V s) while those measured on randomly dispersed nanostructures are in the 103105 cm2/(V s) range. It is worth noting that these materials are roughly composed of 70% peptidic matter, which should contribute minimally to electrical transport if not provide a major source of contact resistance at the electrode surfaces, so these data do not necessarily reflect the performance inherent to the embedded π-conjugated cores. For comparison, “high performance” crystalline organic semiconductors can now routinely achieve mobilities in the 0.11.0 cm2/(V s) range. Clearly, the selfassembled semiconductors are nowhere near this metric, but the present values are sufficiently viable to be relevant for future bioelectronics study.

Prospects for the Future The versatility of this peptide platform, in terms of controlling peptide composition, optoelectronic properties, and macroscale alignment, will enable us to address a variety of scientific and technological problems. We are exploring how peptide sequence variation can exert subtle but important control of chromophore orientations, nanostructural morphologies, and interfacial chemistries. Collaborations with William Wilson (UIUC) explore how “noodle quality” (as reflected in the extent of global ordering within a hydrogel noodle) impacts photophysics in an effort that involves new fabrication techniques and new spectroscopic setups. Finally, we are including bioactive signals to use the nanomaterial surfaces to influence cell biology. Collaborations with Mao and Katz are exploring how these optoelectronic hydrogels can be exploited for tissue engineering applications. All of these ongoing efforts rely on the now simple ability to “dial-in” electronic function at the molecular level and have this function be included within nano- and macrostructured constructs. We look forward to sharing our findings from these inquiries in the near future. I am extremely grateful to all of my current and former students and collaborators for making this research come to life. Our work has been generously supported by JHU, the Institute for NanoBioTechnology (INBT), and the Department of Energy, Office of Basic Energy Sciences (Grant DE-SC0004857). BIOGRAPHICAL INFORMATION J. D. Tovar was born in Waterloo, Iowa, in 1975. His educational path includes a B.S. from UCLA, a Ph.D. from MIT, and postdoctoral J ’ ACCOUNTS OF CHEMICAL RESEARCH

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studies at Northwestern. He started his independent career at Johns Hopkins University where he is currently an Associate Professor. His group's research interests involve several aspects of πconjugated materials ranging from non-benzenoid aromaticity to bioelectronics. FOOTNOTES *Fax: þ1-410-516-7044. E-mail: [email protected]. The authors declare no competing financial interest. REFERENCES 1 Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. About supramolecular assemblies of π-conjugated systems. Chem. Rev. 2005, 105, 1491–1546. 2 Schenning, A.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. Hierarchical order in supramolecular assemblies of hydrogen-bonded oligo(p-phenylene vinylene)s. J. Am. Chem. Soc. 2001, 123, 409–416. 3 Shao, H.; Seifert, J.; Romano, N. C.; Gao, M.; Helmus, J. J.; Jaroniec, C. P.; Modarelli, D. A.; Parquette, J. R. Amphiphilic self-assembly of an n-type nanotube. Angew. Chem., Int. Ed. 2010, 49, 7688–7691. 4 Messmore, B. W.; Hulvat, J. F.; Sone, E. D.; Stupp, S. I. Synthesis, self-assembly, and characterization of supramolecular polymers from electroactive dendron rodcoil molecules. J. Am. Chem. Soc. 2004, 126, 14452–14458. 5 Tian, L.; Szilluweit, R.; Marty, R.; Bertschi, L.; Zerson, M.; Spitzner, E.-C.; Magerle, R.; Frauenrath, H. Development of a robust supramolecular method to prepare well-defined nanofibrils from conjugated molecules. Chem. Sci 2012, 3, 1512–1521. 6 Jin, W.; Fukushima, T.; Niki, M.; Kosaka, A.; Ishii, N.; Aida, T. Self-assembled graphitic nanotubes with one-handed helical arrays of a chiral amphiphilic molecular graphene. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10801–10806. 7 Owens, R. M.; Malliaras, G. G. Organic electronics at the interface with biology. MRS Bull. 2010, 35, 449–456. 8 Abidian, M. R.; Corey, J. M.; Kipke, D. R.; Martin, D. C. Conducting-polymer nanotubes improve electrical properties, mechanical adhesion, neural attachment, and neurite outgrowth of neural electrodes. Small 2011, 6, 421–429. 9 Wang, Y.; Corbitt, T. S.; Jett, S. D.; Tang, Y.; Schanze, K. S.; Chi, E. Y.; Whitten, D. G. Direct visualization of bactericidal action of cationic conjugated polyelectrolytes and oligomers. Langmuir 2012, 28, 65–70. 10 Traina, C. A.; Bakus, R. C., II; Bazan, G. C. Design and synthesis of monofunctionalized, water-soluble conjugated polymers for biosensing and imaging applications. J. Am. Chem. Soc. 2011, 133, 12600–12607. 11 Wigenius, J.; Persson, G.; Widengren, J.; Inganas, O. Interactions between a luminescent conjugated oligoelectrolyte and insulin during early phases of amyloid formation. Macromol. Biosci. 2011, 11, 1120–1127. 12 Wong, J. Y.; Langer, R.; Ingber, D. E. Electrically conducting polymers can noninvasively control the shape and growth of mammalian cells. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 3201–3204. 13 Zhao, H.; Zhu, B.; Sekine, J.; Luo, S.-C.; Yu, H.-h. Oligoethylene-glycol-functionalized polyoxythiophenes for cell engineering: Syntheses, characterizations, and cell compatibilities. ACS Appl. Mater. Interfaces 2012, 4, 680–686. 14 Gumus, A.; Califano, J. P.; Wan, A. M. D.; Huynh, J.; Reinhart-King, C. A.; Malliaras, G. G. Control of cell migration using a conducting polymer device. Soft Matter 2011, 6, 5138– 5142. 15 Otero, T. F.; Sansinena, J. M. Soft and wet conducting polymers for artificial muscles. Adv. Mater. 1998, 10, 491–494. 16 Foroughi, J.; Spinks, G. M.; Wallace, G. G.; Oh, J.; Kozlov, M. E.; Fang, S.; Mirfakhrai, T.; Madden, J. D. W.; Shin, M. K.; Kim, S. J.; Baughman, R. H. Torsional carbon nanotube artificial muscles. Science 2011, 334, 494–497. 17 Ryu, J.-H.; Hong, D.-J.; Lee, M. Aqueous self-assembly of aromatic rod building blocks. Chem. Commun. 2008, 1043–1054. 18 Huang, Z.; Kang, S.-K.; Banno, M.; Yamaguchi, T.; Lee, D.; Seok, C.; Yashima, E.; Lee, M. Pulsating tubules from noncovalent macrocycles. Science 2012, 337, 1521–1526. 19 Krieg, E.; Rybtchinski, B. Noncovalent water-based materials: Robust yet adaptive. Chem.;Eur. J. 2010, 17, 9016–9026. 20 Ustinov, A.; Weissman, H.; Shirman, E.; Pinkas, I.; Zuo, X.; Rybtchinski, B. Supramolecular polymers in aqueous medium: Rational design based on directional hydrophobic interactions. J. Am. Chem. Soc. 2011, 133, 16201–16211. 21 Ryu, J. H.; Lee, E.; Lim, Y. B.; Lee, M. Carbohydrate-coated supramolecular structures: Transformation of nanofibers into spherical micelles triggered by guest encapsulation. J. Am. Chem. Soc. 2007, 129, 4808–4814.


22 Kas, O. Y.; Charati, M. B.; Rothberg, L. J.; Galvin, M. E.; Kiick, K. L. Regulation of electronic behavior via confinement of PPV-based oligomers on peptide scaffolds. J. Mater. Chem. 2008, 18, 3847–3854. 23 Kumar, R. J.; MacDonald, J. M.; Singh, T. B.; Waddington, L. J.; Holmes, A. B. Hierarchical self-assembly of semiconductor functionalized peptide R-helices and optoelectronic properties. J. Am. Chem. Soc. 2011, 133, 8564–8573. 24 Klok, H. A.; R€osler, A.; G€otz, G.; Mena-Osteritz, E.; B€auerle, P. Synthesis of a silk-inspired peptide oligothiophene conjugate. Org. Biomol. Chem. 2004, 2, 3541–3544. 25 Gonzalez-Rodríguez, D.; Schenning, A. P. H. J. Hydrogen-bonded supramolecular π-functional materials. Chem. Mater. 2011, 23, 310–325. 26 Ulysse, L.; Cubillos, J.; Chmielewski, J. Photoregulation of cyclic peptide conformation. J. Am. Chem. Soc. 1995, 117, 8466–8467. 27 Gothard, C. M.; Rao, N. A.; Nowick, J. S. Nanometer-sized amino acids for the synthesis of nanometer-scale water-soluble molecular rods of precise length. J. Am. Chem. Soc. 2007, 129, 7272–7273. 28 Schillinger, E. K.; Mena-Osteritz, E.; Hentschel, J.; B€orner, H. G.; B€auerle, P. Oligothiophene versus β-sheet peptide: Synthesis and self-assembly of an organic semiconductor-peptide hybrid. Adv. Mater. 2009, 21, 1562–1567. 29 Stone, D. A.; Hsu, L.; Stupp, S. I. Self-assembling quinquethiophene-oligopeptide hydrogelators. Soft Matter 2009, 5, 1990–1993. 30 Mba, M.; Moretto, A.; Armelao, L.; Crisma, M.; Toniolo, C.; Maggini, M. Synthesis and selfassembly of oligo(p-phenylenevinylene) peptide conjugates in water. Chem.;Eur. J. 2011, 17, 2044–2047. 31 Jatsch, A.; Schillinger, E. K.; Schmid, S.; B€auerle, P. Biomolecule assisted self-assembly of π-conjugated oligomers. J. Mater. Chem. 2010, 20, 3563–3578. 32 Diegelmann, S. R.; Gorham, J. M.; Tovar, J. D. One-dimensional optoelectronic nanostructures derived from the aqueous self-assembly of π-conjugated oligopeptides. J. Am. Chem. Soc. 2008, 130, 13840–13841. 33 Vadehra, G. S.; Wall, B. D.; Diegelmann, S. R.; Tovar, J. D. On resin dimerization incorporates a diverse array of π-conjugated functionality within aqueous self-assembling peptide backbones. Chem. Commun. 2010, 46, 3947–3949. 34 Sanders, A. M.; Dawidczyk, T. J.; Katz, H. E.; Tovar, J. D. Peptide-based supramolecular semiconductor nanomaterials via facile Pd-catalyzed solid-phase “dimerizations”. ACS Macro Lett. 2012, 1, 1326–1329. 35 Diegelmann, S. R. Chemistry; Johns Hopkins University: Baltimore, MD, 2011. 36 Wall, B. D. Unpublished results. 37 Langhals, H.; Jaschke, H. Naphthalene amidine imide dyes by transamination of naphthalene bisimides. Chem.;Eur. J. 2006, 12, 2815–2824. 38 Farrall, M. J.; Frechet, J. M. J. Reaction of cross-linked chloromethyl polystyrene with 1,4butanedithiol: sitesite interactions and their control. J. Am. Chem. Soc. 1978, 100, 7998–7999. 39 Nowick, J. S. Chemical models of protein β-sheets. Acc. Chem. Res. 1999, 32, 287–296. 40 Dobson, C. M. Protein folding and misfolding. Nature 2003, 426, 884–890. 41 Lashuel, H. A.; LaBrenz, S. R.; Woo, L.; Serpell, L. C.; Kelly, J. W. Protofilaments, filaments, ribbons, and fibrils from peptidomimetic self-assembly: Implications for amyloid fibril formation and materials science. J. Am. Chem. Soc. 2000, 122, 5262–5277. 42 Aggeli, A.; Nyrkova, I. A.; Bell, M.; Harding, R.; Carrick, L.; McLeish, T. C. B.; Semenov, A. N.; Boden, N. Hierarchical self-assembly of chiral rod-like molecules as a model for peptide beta-sheet tapes, ribbons, fibrils, and fibers. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11857–11862. 43 Channon, K.; MacPhee, C. E. Possibilities for `smart' materials exploiting the self-assembly of polypeptides into fibrils. Soft Matter 2008, 4, 647–652.

44 Cherny, I.; Gazit, E. Amyloids: Not only pathological agents but also ordered nanomaterials. Angew. Chem., Int. Ed. 2008, 47, 4062–4069. 45 Ulijn, R. V.; Woolfson, D. N. Peptide and protein based materials in 2010: From design and structure to function and application. Chem. Soc. Rev. 2010, 39, 3349–3350. 46 Schneider, J. P.; Pochan, D. J.; Ozbas, B.; Rajagopal, K.; Pakstis, L.; Kretsinger, J. Responsive hydrogels from the intramolecular folding and self-assembly of a designed peptide. J. Am. Chem. Soc. 2002, 124, 15030–15037. 47 Aggeli, A.; Bell, M.; Carrick, L. M.; Fishwick, C. W. G.; Harding, R.; Mawer, P. J.; Radford, S. E.; Strong, A. E.; Boden, N. pH as a trigger of peptide β-sheet self-assembly and reversible switching between nematic and isotropic phases. J. Am. Chem. Soc. 2003, 125, 9619–9628. 48 Silva, G. A.; Czeisler, C.; Niece, K. L.; Beniash, E.; Harrington, D. A.; Kessler, J. A.; Stupp, S. I. Selective differentiation of neural progenitor cells by high-epitope density nanofibers. Science 2004, 303, 1352–1355. 49 Ringer, A. L.; Sherrill, C. D. Substituent effects in sandwich configurations of multiply substituted benzene dimers are not solely governed by electrostatic control. J. Am. Chem. Soc. 2009, 131, 4574–4575. 50 Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry; University Science Books, Sausalito, CA, 2006. 51 Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. The exciton model in molecular spectroscopy. Pure Appl. Chem. 1965, 11, 371–392. 52 Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry; University Science Books: Mill Valley, CA, 1983. 53 Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Bredas, J. L. Influence of interchain interactions on the absorption and luminescence of conjugated oligomers and polymers: A quantum-chemical characterization. J. Am. Chem. Soc. 1998, 120, 1289–1299. 54 Siddiqui, S.; Spano, F. C. H- and J-aggregates of conjugated polymers and oligomers: A theoretical investigation. Chem. Phys. Lett. 1999, 308, 99–105. 55 Slaughter, B. V.; Khurshid, S. S.; Fisher, O. Z.; Khademhosseini, A.; Peppas, N. A. Hydrogels in regenerative medicine. Adv. Mater. 2009, 21, 3307–3329. 56 Zhang, S. M.; Greenfield, M. A.; Mata, A.; Palmer, L. C.; Bitton, R.; Mantei, J. R.; Aparicio, C.; de la Cruz, M. O.; Stupp, S. I. A self-assembly pathway to aligned monodomain gels. Nat. Mater. 2010, 9, 594–601. 57 Lim, S. H.; Liu, X. Y.; Song, H.; Yarema, K. J.; Mao, H.-Q. The effect of nanofiber-guided cell alignment on the preferential differentiation of neural stem cells. Biomaterials 2010, 31, 9031–9039. 58 Wall, B. D.; Diegelmann, S. R.; Zhang, S.; Dawidczyk, T. J.; Wilson, W. L.; Katz, H. E.; Mao, H. Q.; Tovar, J. D. Aligned macroscopic domains of optoelectronic nanostructures prepared via the shear flow assembly of peptide hydrogels. Adv. Mater. 2011, 23, 5009–5014. 59 Wegner, G. Topochemical reactions of monomers with conjugated triple bonds. I. Polymerization of 2.4-hexadiyn-1.6-diols derivatives in crystalline state. Z. Naturforsch. B 1969, B24, 824. 60 Diegelmann, S. R.; Hartman, N.; Markovic, N.; Tovar, J. D. Synthesis and alignment of discrete polydiacetylene-peptide nanostructures. J. Am. Chem. Soc. 2012, 134, 2028– 2031. 61 Charych, D. H.; Nagy, J. O.; Spevak, W.; Bednarski, M. D. Direct colorimetric detection of a receptor-ligand interaction by a polymerized bilayer assembly. Science 1993, 261, 585– 588. 62 Biesalski, M.; Tu, R.; Tirrell, M. V. Polymerized vesicles containing molecular recognition sites. Langmuir 2005, 21, 5663–5666. 63 Bockrath, M.; Markovic, N.; Shepard, A.; Tinkham, M.; Gurevich, L.; Kouwenhoven, L. P.; Wu, M. S. W.; Sohn, L. L. Scanned conductance microscopy of carbon nanotubes and λ-DNA. Nano Lett. 2002, 2, 187–190.

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