Self-Assembled Peptide Nanotube Films with High Proton Conductivity

Abstract

Design flexibility and modularity have emerged as powerful tools in the development of functional self-assembled peptide nanostructures. In particular, the tendency of peptides to form fibrils and nanotubes has motivated the investigation of electron and, more recently, proton transport in their fibrous films. In this study, we present a detailed characterization by impedance spectroscopy of films of self-assembled cyclic octa-d,l-α-peptide self-assembled nanotubes with amine side chains that promote proton transport. We show that the conductivity of the peptide nanotube film, which is in the range of 0.3 mS cm–1, is within the same order of magnitude as that of ultrathin films of Nafion, a benchmark proton conducting polymer. In addition, we show that while slow diffusion processes at the interface are present for both films, additional interface effects occur in the peptide nanotube films at the same rate as their bulk proton transport effects, further limiting charge transport at the interface. Overall, our studies demonstrate the great potential of using peptides as building blocks for the preparation of bioinspired supramolecular proton conducting polymers with improved conductivity with respect to that of natural systems.

Proton‐Conductive Melanin‐Like Fibers through Enzymatic Oxidation of a Self‐Assembling Peptide

“…Kinetic isotope effect (KIE) experiments were used to further confirm the dominance of protons to charge transport processes for the entire relative humidity range. [40,43,44] As expected for proton transport, lower currents were observed when D 2 O was used instead of H 2 O to control the relative humidity conditions ( Figure 4A). The measured KIE values provided, in addition, insight to the proton transport mechanisms.…”

“…This represents an upper limit value since it was calculated considering the vertical contact cross section. While previous studies from our group indicate that this calculation closely represents the actual conductivity value, [ 40 ] we note that a more conservative estimation, taking into account the larger film cross section, yields a conductivity value of ≈0.35 mS cm −1 . In any case, both values represent a high in‐plane conductivity, comparable to that of ultrathin films of Nafion, [ 40 ] a polymer electrolyte which is commonly used as the membrane in fuel cells.…”

“…While previous studies from our group indicate that this calculation closely represents the actual conductivity value, [ 40 ] we note that a more conservative estimation, taking into account the larger film cross section, yields a conductivity value of ≈0.35 mS cm −1 . In any case, both values represent a high in‐plane conductivity, comparable to that of ultrathin films of Nafion, [ 40 ] a polymer electrolyte which is commonly used as the membrane in fuel cells. [ 41 ] This high in‐plane conductivity is also similar to the out‐of‐plane conductivity measured for ultrathin films of synthetic melanin, [ 12 ] suggesting that the highly anisotropic fibrillar morphology, and possibly the degree of order, play a key role in maintaining high in‐plane conductivity over mesoscopic distances.…”

The role of the protein–water interface in dictating proton conduction across protein-based biopolymers

“…Inspired by nature, several recent works have utilized proteins for the formation of proton conducting materials. [5][6][7][8][9] Such bioinspired polymeric conductive materials are gaining momentum toward their integration in biomedical applications. 10,11 In addition to their applicative route, much effort was targeted toward deciphering how protons are being transported along such protein-based materials.…”

“…12,13 Based on this knowledge, they recently showed a highly proton conducting peptide fibril network, which has merely 5-7 times lower conductivity than the Nafion s films. 6 The role of charged amino acids has also been proven to be essential in supporting very high proton conductivity across protein-based polymers. 5,9 Another example is the bovine serum albumin (BSA) protein that has been manipulated (by denaturing the protein) through the electrospinning process to receive a free-standing material with a fibrillar structure.…”