Visualizing energy transfer between redox-active colloids

Abstract

Redox-active colloids (RACs) represent a novel class of energy carriers that exchange electrical energy upon contact. Understanding contact-mediated electron transfer dynamics in RACs offers insights into physical contact events in colloidal suspensions and enables quantification of electrical energy transport in nonconjugated polymers. Redox-based electron transport was directly observed in monolayers of micron-sized RACs containing ethyl-viologen side groups via fluorescence microscopy through an unexpected nonlinear electrofluorochromism that is quantitatively coupled to the redox state of the colloid. Via imaging studies, using this electrofluorochromism, the apparent charge transfer diffusion coefficient D_CT of the RAC was easily determined. The visualization of energy transport within suspensions of redox-active colloids was also demonstrated. Our work elucidates fundamental mechanisms of energy transport in colloidal systems, informs the development of next-generation redox flow batteries, and may inspire new designs of smart active soft matter including conductive polymers for applications ranging from electrochemical sensors and organic electronics to colloidal robotics.

Fig. 1.. RAC electrofluorochromism.

(A) Three-electrode characterization cell schematic. (B) Fluorescence imaging of RAC monolayer (scale bars, 2 μm) in oxidized (i and iv) and reduced (ii and iii) states. (C) Reversible reduction and oxidation of the pendant EV group. The dication EV²⁺ (oxidized) form is fluorescent, and the radical cation EV^+• (reduced) form is dark. (D) CV on the RAC monolayer over the viologen first electron-transfer voltage range at a sweep rate of 1 mV/s. Fluorescence images presented in (B) are taken at the four indicated points (i, ii, iii, and iv). (E) Top: Fluorescence intensity versus time of a field of 166 colloids (fig. S4) at a sweep rate of 5 mV/s. Bottom: Voltage versus time (cycling between −0.05 and −0.5 V versus Ag/AgCl). (F) Tangent line drawing (indicatory) for extraction of Faradaic charges going into/out of RACs from the first round of 5 mV/s CV data in fig. S5. Enclosed area (cross-hatched region of zoomed-in image between red tangent line and blue redox peak signatures) is the Faradaic charge. (G) First-cycle fluorescence intensity (normalized) versus percent reduced EV groups at sweep rates of 5 and 10 mV/s.

Fig. 2.. RAC quenching photophysics.

(A) Stern-Volmer plot of reduced monomer (EV^+•) quenching oxidized RAC dispersion (10 mM total viologen groups). (B) Spectral superimposition of reduced RAC (0.1 mM) absorbance and normalized PL emission for oxidized RAC (under 488-nm excitation). (C) Time-resolved PL measurement and single-exponential fit of fluorescence lifetime of oxidized RAC dispersion (0.1 M viologen groups). IRF, instrument response function.

Fig. 3.. Intercolloid electron diffusion.

(A) Schematic illustrating interparticle lateral charge transport (yellow: oxidized RAC; gray: reduced RAC; blue: metallic electrode; pink: glass slide). (B) Fluorescence images with fluorescence front tracking, red line, extracted from movie S2, during reduction and oxidation. Scale bars, 4 μm. Colloids above the platinum electrode appear brighter than those above the glass substrate because of reflection from the electrode and possibly metal-enhanced fluorescence. (C) Chronoamperometry (CA) performed on a RAC monolayer, with reduction potential held at −0.6 V versus Ag/AgCl and oxidation potential held at +0.2 V versus Ag/AgCl. (D) Tracking result from movie S2: lateral distance squared versus time (plotted from fig. S17), with least-squared linear fits for two sections. (E) Lateral quenched limit (L) tracked for four cycles on the same sample. (F) Lateral charge-transport diffusion coefficients (D₁ and D₂) determined as shown in (D); note the different y-axis scales.

Fig. 4.. Collision-induced energy exchange.

(A) Schematic of net electron exchange between two colliding RACs at different oxidation states (pendant groups not drawn to scale) and optical microscopy image of oxidized (bright) and reduced (dark) RAC when both transmitted light and fluorescence excitation are turned on for imaging. (B) Thirty-one discrete oxidized RAC (color-boxed) pinned to the substrate. (C) Fluorescence intensity (normalized) change (red) over 300 s of the 31 oxidized particles in (B) and six oxidized particles (separate experiment) that underwent photobleaching only (black). Scale bars, 3 μm (A) and 50 μm (B).