Erik B. Berda

Metastable supramolecular polymer nanoparticles via intramolecular collapse of single polymer chains.

We present an elegant approach for fabricating polymeric nanoparticles via the reversible noncovalent intramolecular cross-linking of single polymer chains. By protecting the well-studied 2-ureido-pyrimidinone moiety with an o-nitrobenzyl group, we have introduced a latent quadruple hydrogen bonding motif. Incorporating this group into linear poly(norbornenes) creates a class of supramolecular materials that can be readily dissolved and subsequently cross-linked upon irradiation with UV light, eliminating the need for selective solvent techniques. This is a facile method for producing well-defined particles with narrow polydispersity as confirmed by atomic force microscopy. By virtue of the nature of the intramolecular cross-linking (supramolecular interactions of pendant groups), these metastable nanoparticles are akin to folded biomacromolecules, representing the first step in linking synthetic polymers and self-folded biopolymers.

Single-chain polymer nanoparticles via reversible disulfide bridges

We report the fabrication of single-chain polymer nanoparticles (SCNP) that can reversibly undergo a coil to particle transition via formation and cleavage of intramolecular disulfide cross-links. Characterization of this behaviour via shifts in SEC retention time confirm the changes in solution conformation in response to oxidative or reductive stimuli.

Intra-chain photodimerization of pendant anthracene units as an efficient route to single-chain nanoparticle fabrication.

An efficient route to architecturally defined, sub-20 nm soft nanoparticles fabricated from single polymer chains via intramolecular photodimerization of pendant anthracene units is presented. Photodimerization is confirmed by the disappearance of the characteristic anthracene π-π* absorption peak at ≈ 360 nm measured by UV-vis spectroscopy. Size exclusion chromatography (SEC) with UV, multi-angle light scattering (MALS), and viscometric detection confirms that as photodimers form, the chains fold to form nanoparticles, demonstrated by shifts in the SEC traces to longer retention times as a function of increased irradiation time. These shifts indicate a reduction in hydrodynamic radius, corroborated and quantified by viscometric data. MALS detector traces reveal the presence of a small amount of chain-chain coupling during this process, but confirm that this is primarily a single-chain phenomenon. Electron microscopy provides visual confirmation of nanoparticle formation.

Toward a tunable synthetic [FeFe] hydrogenase mimic: single-chain nanoparticles functionalized with a single diiron cluster

We report two novel “clickable” [(μ-S2C2H4NR)Fe2(CO)6] complexes and their incorporation into single-chain nanoparticles. Variations in the characteristic iron–carbonyl stretching bands caused by the polymer scaffolds confirmed changes in the symmetry and electronics of the complexes. This represents the first SCNP-based model of a metalloenzyme bearing a single, bioinspired active site.

Fabrication of electrochemically responsive surface relief diffraction gratings based on a multifunctional polyamide containing oligoaniline and azo groups

A novel, well-defined multifunctional polyamide bearing pendent aniline tetramer groups has been synthesized via low temperature solution polycondensation. The simultaneous presence of these two functional groups allows the polyamide to display the properties of azo-chromophore and oligoaniline, such as photoinduced birefringence and reversible electroactivity. The electrochromic performance of the polyamide shows the optical change in absorptivity depends on the various redox states of oligoaniline upon electrochemical switching. It is well known that a change in absorptivity is accompanied by a concomitant change in refractive index. By taking advantage of this effect and using the single step fabrication of surface relief gratings based on azo-chromophore, we have fabricated optical diffraction gratings with which the diffraction efficiency can be modulated by an electrochemical signal. Although patterned electroactive films have been made by many techniques, the copolymer approach employing the simple one-step surface relief gratings process displays optimal properties with respect to modulation depth and convenience, which avoids complicated electropolymerization steps or photochemical reactions. The electrochemically-induced modulation in the diffraction efficiency of the polyamide is believed to arise primarily from the effect of the redox state on the films refractive index.

Fabrication of electroactive oligoaniline functionalized poly(amic acid) nanofibers for application as an ammonia sensor

A novel, well-defined, multifunctional electroactive poly(amic acid) (EPAA) containing oligoaniline pendants was synthesized by a one-step synthetic route. The structure was confirmed spectroscopically via nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectra, and the thermal stability was probed via thermogravimetric analysis (TGA). The oligoaniline pendants imparted excellent electroactivity, photoresponsiveness to chemical redox and electrochemical modulation, and expectable electrochromic performance to the obtained EPAA. Leucoemeraldine base EPAA (LEPAA), emeraldine base EPAA (EEPAA) and HCl-doped emeraldine base EPAA (HCl-doped EEPAA) nanofibers were fabricated by electrospinning and were characterized by scanning electron microscopy (SEM) to test their morphology, and a contact angle goniometer was used to test their hydrophilicity. The as-prepared HCl-doped EEPAA nanofibers were then evaluated for their ability to sense ammonia.

Tuning the fluorescent response of a novel electroactive polymer with multiple stimuli.

A novel polymer featuring oligoaniline pendants that exhibits reversible electroactivity and good electrochromic properties with high contrast value, acceptable switching times, and excellent coloration efficiency is presented. This polymer can undergo reversible changes in fluorescence in response to reductive and oxidative chemical stimulus, pH, and electrical potential. The fluorescence switching operation shows reasonable reversibility and reproducibility when subjected to multiple stimuli. In this elegant fluorescence switching system, the oligoaniline pendants are used as fluorophore and regulatory units simultaneously.

Exploring structural effects in single-chain “folding” mediated by intramolecular thermal Diels–Alder chemistry

We describe a method to fold single polymer chains into nanoparticles using simple thermal Diels–Alder (DA) chemistry. Two different folding strategies are explored, one employing “chain-internal folding” and the other using external, multi-functional cross-linkers. In the first strategy, random terpolymers were designed with varying incorporations of methyl methacrylate (MMA), furfuryl methacrylate (FMA) and a maleimide functionalized methacrylate (MIMA) to achieve internal folding through a thermal DA reaction between pendent furan and maleimide groups. In the second method, the synthesis of random copolymers of MMA and FMA forms nanoparticles after effecting a thermal DA reaction between pendent furan groups and external bi- or tri-maleimide functionalized cross-linkers. This multifaceted approach compares different synthetic designs of linear polymers as well as multiple cross-linker species as a means to explore the effect these synthetic differences have on the resulting SCNP. The two polymer series designed in this study allow for a direct comparison between chain internal cross-linking of multiple internal pendent groups and external cross-linker mediated collapse.

An efficient fluorescent sensor for redox active species based on novel poly(aryl ether) containing electroactive pendant

We describe the synthesis of a novel poly(aryl ether), containing pendant oligoaniline and anthracene moieties (PAE-p-OA), and both side chains present in equal amounts. Structures were confirmed spectroscopically via nuclear magnetic resonance (NMR), morphological data ascertained viaX-ray diffraction (XRD), and thermal stability probed viathermogravimetric analysis (TGA). Electrochemical and photophysical properties were also investigated using cyclic voltammetry and UV-Vis and fluorescence spectroscopy. This material exhibits an interesting fluorescent response to redox active species. When PAE-p-OA is in the reduced state, oxidative materials interact with the oligoaniline side chains resulting in a progression from leucoemeraldine base (LEB) to the emeraldine base (EB). The resulting change in molecular conformation of the oliganiline unit increases its propensity for interaction with the anthracene side chain (corroborated by molecular modeling), leading to decreased fluorescence via quenching and thus “turn off” fluorescence sensing. When PAE-p-OA is in the oxidized state, reduction of the oligoaniline from the EB to the LEB via interaction with a reductive species delivers “turn on” fluorescence sensing via decreased interaction between the oligoaniline and anthracene side chains, again due to a change in the molecular conformation of oligoaniline.

Characterization of single-chain polymer folding using size exclusion chromatography with multiple modes of detection

We highlight here recent work from our laboratory on the subject of fabricating nanostructures from single polymer chains. These so-called single-chain nanoparticles are synthesized by inducing intra-molecular cross-linking on discrete macromolecules in dilute solution. Among the biggest challenges in this rapidly expanding area of research is reliable and accurate means to characterize this process. In this paper, we review our preferred method of characterization: size exclusion chromatography featuring multiple modes of detection. Multi-angle light scattering in conjunction with a concentration detector can provide absolute molecular weight data; viscometric detection can provide information about solution size and conformation. Correlation of these data provides a simple and robust way to quantify the process by which we fold single polymer coils into architecturally defined unimolecular nanostructures.