Multiscale electrochemistry of hydrogels embedding conductive nanotubes

Abstract

The local functionalities of biocompatible objects can be characterized under conditions similar to the operating ones, using scanning electrochemical microscopy (SECM). In the case of alginate beads entrapping carbon nanotubes (CNTs), SECM allows evidencing of the local conductivity, organization, and communication between the CNTs. It shows that the CNT network is active enough to allow long range charge evacuation, enabling the use of alginate/CNT beads as soft 3D electrodes. Direct connection or local interrogation by a microelectrode allows visualization of their communication as a network and eventually the study of them individually at the nanoscale.

Fig. 1. (A) Principle of the SECM approach curves for a 2 mm diameter alginate/CNT bead in a 0.1 M KCl + 1 mM CaCl₂ aqueous solution containing 1 mM FcMeOH. (B) Approach curves recorded with a 12.5 μm Pt UME tip toward alginate beads containing CNTs: 0 (), 0.13 (), 0.3 (), 0.55 (), and 1 () wt%. Lines are the simulated curves for irreversible electron transfer kinetics. k _el = 0, 1.82, 3.65, 6.08 and 12.2 ×10^–3 cm^–1 for, respectively, 0, 0.13, 0.3, 0.55 and 1 wt%, using D = 7.8 × 10^–6 cm s^–1 for the diffusion coefficient of FcMeOH in water.

Fig. 2. SECM images of alginate/CNT beads containing (A) 0.3 and (B) 1 wt% CNTs recorded in a 0.1 M KCl + 1 mM CaCl₂ aqueous solution containing 1 mM FcMeOH. Using a 400–500 nm radius Pt nanotip at a 10 μm s^–1 velocity. The nanotip was kept less than 1 μm from the surface. The current scale was normalized using the steady state current recorded in the bulk solution. In (A), the arrows point to the 10 most conductive spots and the inset shows the current profile along the black line of one spot.

Fig. 3. (A and C) Principle of the direct connection of an alginate bead with a 12.5 μm radius UME tip. (B and D) Cyclic voltammograms recorded at a scan rate of v = 100 mV s^–1 for a 3 mm diameter alginate bead containing 1 wt% CNTs: (B) in a 0.1 M KCl + 1 mM FcMeOH aqueous solution (inset: peak current, I _p, as a function of v ^1/2); (D) equilibrated with an aqueous solution of 1 mM hydroquinone (HQ) and immersed into a 1 mM DcFc + 0.1 M NBu₄BF₄ BZN solution.