Multiphase Electrochemiluminescence of Microdroplets and Radical Salts

Abstract

Over the past decade, experiments involving microdroplets have challenged the framework of chemistry. These droplets constitute a multiphase system, where the dynamic interplay between solid, liquid, and gas influences reactions. Multiphase systems are prevalent in nature and possess unique physicochemical properties. However, in chemistry, phase boundary reactivity is often overlooked because molecules experience “bulk” reactivity. These systems are prevalent in biological processes, such as cell mitosis, biological sensor technology, organic synthesis, and pollution remediation.

Recently, our group has developed different strategies of electrochemiluminescence (also called electrogenerated chemiluminescence, both shortened to ECL) microscopy and imaging to understand the unique properties and dynamics at electrified interfaces. ECL takes advantage of the reactivity between a luminophore that radically annihilates with a strong oxidizing or reducing reagent. If the enthalpy of annihilation is high enough, the luminophore will be left in an excited state and radiatively decay, producing light. Thus, ECL requires no incident light, and ECL microscopy has unique analytical figures of merit due to the light being emitted close to the electrified interface (1–10 μm), providing insight into reactivity within the electrode’s proximity.

This Account will detail our group’s efforts in discovering ECL reactions in environments exhibiting native triphasic (liquid|liquid|electrode) interfaces and reactions where new phases are formed (e.g., bubble nucleation and electroprecipitation). We first began developing the tools necessary to image liquid|liquid interfaces and discovered that, when neighboring droplets fuse together, small pockets (inclusions) of the continuous phase existed inside the merged droplets. Studies of inclusion chemical reactivity have led to the interesting observation that small droplets on electrified interfaces can act as gas micropumps, protecting the electrode from gas buildup during electrocatalytic reactions.

Even though a strength of ECL is that emission is confined directly to the surface, this can also be a significant weakness, considering interesting chemical phenomena can occur far away from the electrode surface. One recent thrust in the community is discovering new ways of using ECL far away from the electrode surface, a phenomenon termed “Through-Space ECL”. Our group has used this technique to measure bubble forces at phase boundaries far from the electrode surface.

By playing on the relative solubilities of ECL reactants and products, we showed that, if a radical ion can be generated and precipitates more quickly than its radical lifetime, radical salts can be formed. These radical salts are a way to fossilize highly energetic molecules. We have used this seemingly new chemical tenet to effectively bottle up ECL, fossilizing the reactants to be used elsewhere in space and time. Given our passion to teach the world the beauties of electrochemistry, we have also used concepts surrounding multiphase ECL to pedagogically innovate new experiments that can be performed by children to visualize competing reactions.

In this Account we will depict the current state of Multiphase ECL spectroscopy and microscopy with a focus on our group’s contributions to this burgeoning field. We will detail where the field has been and discuss its high probability of significant impact across chemical sciences moving forward.