Materials and Methods
Hydrogel PreparationAll reagents were purchased from Sigma-Aldrich, unless otherwise noted, and were used as received. Poly(dimethylsiloxane) (PDMS) was commercially purchased locally, under the brand name GE Silicone II Kitchen & Bath. COOH-functionalized multi-walled carbon nanotubes (COOH-MWNTs, diameter: 8-15 nm, length: 10-50 μm, 2.56 % (w/w) functional content) were purchased from cheaptubes.com.
Typical hydrogel preparationSodium acrylate (4.8 g, 51 mmol) and sodium (4-styrene sulfonate) (3.2 g, 14.3 mmol) were combined with 36 mL of deionized water and gently heated (< 40 C) until all solids were dissolved. Poly(ethylene glycol) diacrylate, (PEG-DA, M n = 575, 400 μL, 0.78 mmol) was added and the mixture was purged with N 2 for 5 min. Ammonium persulfate (APS, 72 mg, 0.47 mol%) was added as a radical initiator for copolymerization. Note: adjustments in PEG-DA stoichiometry relative to the other monomers produced hydrogels that were qualitatively stiffer (increased PEG-DA) or softer (decreased PEG-DA).Depending on the dimensions of the sample being prepared, 2 to 8 mL of the reaction mixture was pipetted into a mold. For electrochemical experiments the mold for the sample was created by temporarily affixing, using PDMS adhesive, a square glass cell to a Teflon base bearing a freshly polished glassy carbon electrode (GCE). The mold/sample combination was then heated at 85 °C for 1.5 h. After cooling to RT, the hydrogel was doped by simple submersion in either a solution of 2.0 M FeCl 2 /0.5 M citric acid or 2.0 M FeCl 3 /0.5 M citric acid for a period of 20-48 h (Supplementary Fig. S1). A 1:3 ratio by volume of doping solution to pre-polymer was used.