Electrochemical Quantification of Accelerated Rates in Individual Nanodroplets For centuries, scientists have assumed that chemical reactivity in bulk, continuous water is conserved at all levels in nature, from reactions in the ocean to enzymatic digestion in a lysosome. However, there is growing evidence using mass spectrometry that chemical reaction rates are enhanced by orders of magnitude in micro- and nanodroplets. A limitation of these electrospray experiments is the generation of droplets of various sizes, prohibiting robust quantification of reaction rates as a function of droplet size. Here, we demonstrate an electrochemical method for quantification of reactivity within single attoliter-nanoliter droplets where we rigorously account for the effect of nanodroplet size on rate acceleration. When single nanodroplets irreversibly adsorb onto an ultramicroelectrode surface, reactivity is apparent in the amperometric current-time trace if the ultramicroelectrode generates one of the homogeneous reactants, and the homogeneous reaction recycles this electrochemical product in a rate-limiting step. Nanodroplet volume is easily accessible by integrating the current-time response and using Faraday’s Law. The single nanodroplet technique allows us to plot reactivity as a function of nanodroplet size. Finite element simulations are used to confirm our experimental results, assign rate constants, and offer insights into parameters influencing single nanodroplet reactivity. As a model system, we first presented the nano-confined catalysis of a glucose dehydrogenase enzyme, where we reported a rate enhancement of up to two orders of magnitude and demonstrated a clear inverse relationship between nanodroplet size (radii = 250 nm – 2.5 µm) and reaction acceleration (turnover rates = 265 s-1 - 12 s-1). Here we present the measurement of important bimolecular rates, exploring the nanoconfined-enhancement effect on common EC’ reactions, like the dimerization of cysteine by ferricyanide, and the reduction of dopamine quinone (the oxidized form of dopamine) by ascorbate. These results provide a framework to profoundly influence understanding of chemical reactivity at the nanoscale.

Electrochemical quantification of accelerated rates in individual nanodroplets

ACS Spring 2022

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