3,4-Ethylenedioxythiophene Hydrogels: Relating Structure and Charge Transport in Supramolecular Gels

Ionic charge transport is a ubiquitous language of communication in biological systems. As such, bioengineering is in constant need of innovative, soft, and biocompatible materials that facilitate ionic conduction. Low molecular weight gelators (LMWGs) are complex self-assembled materials that have received increasing attention in recent years. Beyond their biocompatible, self-healing, and stimuli responsive facets, LMWGs can be viewed as a “solid” electrolyte solution. In this work, we investigate 3,4-ethylenedioxythiophene (EDOT) as a capping group for a small peptide library, which we use as a system to understand the relationship between modes of assembly and charge transport in supramolecular gels. Through a combination of techniques including small-angle neutron scattering (SANS), NMR-based Van’t Hoff analysis, atomic force microscopy (AFM), rheology, four-point probe, and electrochemical impedance spectroscopy (EIS), we found that modifications to the peptide sequence result in distinct assembly pathways, thermodynamic parameters, mechanical properties, and ionic conductivities. Four-point probe conductivity measurements and electrochemical impedance spectroscopy suggest that ionic conductivity is approximately doubled by programmable gel assemblies with hollow cylinder morphologies relative to gels containing solid fibers or a control electrolyte. More broadly, it is hoped this work will serve as a platform for those working on charge transport of aqueous soft materials in general.


Table of Contents
In brief, oxalyl chloride (2.55 mL, 30 mmol, 1 eq.) was added in a drop-wise fashion to a solution of EDOT (3.15 mL, 30 mmol, 1 eq.) in dioxane (150 mL).The solution was heated to 100 ⁰ C for 1 hour stirring constantly.Once cooled to room temperature, methanol (90 mL) was added along with triethylamine (5 eq.) following which the mixture was stirred for a further 3 hours.Excess alcohol was then removed in vacuo.The mixture was then diluted with dichloromethane (300 mL), washed with water (100 mL) and brine (100 mL), the separated organic layers dried over MgSO4, filtered and concentrated in vacuo.The resulting residue was then purified by column chromatography using 50% (v/v) ethyl acetate in hexane to yield the desired methyl ester as a yellow solid.This ester was then dissolved in THF (100 mL), then an excess of 1 M aqueous sodium hydroxide (> 5 eq) was added and the reaction mixture stirred for 2 hours at room temperature.Excess tetrahydrofuran was removed in vacuo, then the mixture was acidified with 1 M aqueous hydrochloric acid solution (100 mL) the precipitate was centrifuged and washed with DI water (3 x 100 mL).The solid was dried by lyophilisation yielding the desired oxalyl acid in a 64% yield over 2 steps.Spectroscopic data was consistent with those reported previously.EDOT-peptide synthesis general procedure -(iii) in Figure S1.
Peptide synthesis was achieved either manually or using a Biotage Initiator Alstra+ peptide synthesiser.
Peptides were constructed using conventional Fmoc solid-phase peptide synthesis techniques and capped at the N-terminus with the requisite EDOT oligomer.In brief the resin was swelled in dichloromethane (DCM) (5 mL) inside a capped and fritted syringe.This was expelled and replaced with a solution of the amino acid (3 eq.) and N,N-diisopropylethylamine (DIPEA) (6 eq.) the estimated amount of available chlorotrityl chloride on the resin being designated as 1 eq.This was shaken overnight after which the mixture was expelled, the resin washed 5x with DCM (5 mL), 5x with dimethylformamide (DMF) (5 mL) followed by shaking with 3 mL of a 9:1:0.5 (v/v) DCM:MeOH:DIPEA solution (5 mL) for 20 minutes.If the peptide synthesizer was used this was the point at which they would be loaded after a further 5x DCM, 5x DMF (5 mL each) wash.
Samples would need only be cleaved after automated synthesis (see cleavage protocol below).To cleave Fmoc groups on the N-terminus the mixture was expelled, the resin again washed 5x with DCM and DMF (5 mL) respectively and then shaken with 20% piperidine in DMF (3 mL) for 1 minute.This was expelled and replaced again with 20% piperidine in DMF (3 mL) for 20 minutes.
To couple amino acids to the free amine, the resin was again washed 5x with DCM and DMF (5 mL) respectively following which a 2-3 mL solution in dry DMF of the amino acid (3 eq.), DIPEA (6 eq.) and HATU (0.95 eq.) (5 mL) was taken up and shaken for between 30-60 minutes.The above cleavage and coupling procedures were repeated as many times as necessary to build the required peptide.Between each step a Kaiser test was performed to check for coupling and cleavage success.When coupling the final EDOT capping group the same procedure was used as described above only often with < 3 eq.depending on availability.
To cleave the EDOT-peptides from the resin a solution of 20% trifluoroacetic acid (TFA) in DCM (3 mL) was taken up and shaken for 15-20 minutes.In the case where an aspartic acid was present, a 95:2.5:2.5 (v/v) TFA:water:triisopropylsilane solution (3 mL) was used instead.The solution was transferred into a round-bottom flask (RBF) and concentrated on a rotary evaporator with the assistance of DCM as an azeotrope to remove the TFA if needed.

High-performance liquid chromatography (HPLC) purification
Purification of the low molecular weight gelators (LMWGs) was undertaken using a Shimadzu Prominence LC-20A preparative HPLC with a flow rate of 10 mL/minute.The concentrated crude reaction mixture was dissolved in 40 mL 45-50% (v/v) acetonitrile (MeCN) in water (+ 0.1% (v/v) formic acid).The crude mixture was filtered through a 0.45 μm Teflon filter followed by injection onto the HPLC and eluted with a gradient of either 45-80 B% or 50-80 B% acetonitrile in water (+ 0.1% (v/v) formic acid).Fractions were collected, excess MeCN removed under reduced pressure and the mixture lyophilized to afford the desired products.

EDOT-COOH:
Isolated as a yellow solid in a yield of 79 %.NMR consistent with previous literature.

EDOT-FF:
Isolated as a pale, yellow solid in a yield of 10%.

EDOT-GFF:
Isolated as a pale, yellow solid in a yield of 18%.

Fmoc-GFF:
Isolated as a white solid in a yield of 36%. 1 H NMR consistent with those previously reported.Table S1.All parameters extracted from fitting of small-angle neutron scattering data.The * highlights the large errors found when fitting EDOT-GFF before gelation as mentioned in the main text.All gels at a concentration of 10 mM.

Compound
Note in the case of EDOT-FFF, the gel begins to lose mechanical integrity from 60 ᵒC, as such NMR data was not considered above this temperature when estimating thermodynamic dissociation constants.Of particular interest is the apparent thermal toughening of EDOT-GFFD up to 70 ᵒC.
In Figure S8d is shown the full heating and cooling cycle for EDOT-GFFD.Here can be seen a change of 2 orders of magnitude in storage modulus as mentioned in the main text.Table S4.Parameters generated through modelling of the mid-to-low frequency impedance data.
EDOT-FFF and EDOT-GFF are modelled using the circuit shown in Figure 7b and EDOT-GFFD and the electrolyte control are modelled using the circuit shown in Figure 7a.

Figure S3 .
Figure S3.(a) Frequency and (b) strain sweeps of the three gelators validating the parameters

Figure S4 .
Figure S4.Plots of the residuals generated when fitting the SANS data for both high and low

Figure S6 .
Figure S6.Extinction coefficient of the three gelators in DMSO with dilution.

Table S3 .
GROMACS topology with 22ptimized atom charges and atom types for the EDOT-