Concentration Enrichment in a Dissolving Microdroplet: Accessing Sub-nanomolar Electroanalysis

Abstract

Droplet evaporation has previously been used as a concentration enrichment strategy; however, the measurement technique of choice requires quantification in rather large volumes. Electrochemistry has recently emerged as a method to robustly probe volumes even down to the attoliter (10 −18 L) level. We present a concentration enrichment strategy based on the dissolution of a microdroplet placed on the surface of a Au ultramicroelectrode (radius ∼ 6.25 μm). By precisely positioning a 1,2-dichloroethane microdroplet onto the ultramicroelectrode with a microinjector, we are able to track the droplet's behavior optically and electrochemically. Because the droplet spontaneously dissolves over time, given the relative solubility of 1,2-dichloroethane in the water continuous phase, the change in volume with time enriches the concentration of the redox probe (Cp 2 *(Fe) II ) in the droplet. We demonstrate robust electrochemical detection down to sub-nM (800 pM) concentrations of Cp 2 *(Fe) II . For this droplet, 800 pM constitutes only about 10 6 molecules. We extend the strategy in a single-blind study to determine unknown concentrations, emphasizing the promise of the new methodology. These results take voltammetric quantification easily to the sub-μM regime.

Amplifying the electrochemical footprint of <1000 molecules in a dissolving microdroplet

“…The apparent standard potential for the redox couple (Cp*) 2 Fe III /(Cp*) 2 Fe II is observed at −0.1 V vs. Ag/AgCl. 39 The ClO 4 − species in the aqueous phase allow for maintaining electroneutrality inside the droplet. For instance, applying a potential more positive than −0.1 V allows for the oxidation of (Cp*) 2 Fe II to couple (Cp*) 2 Fe III , which involves partitioning of the ClO 4 − ion into the DCE phase.…”

“…As the droplet shrinks over time, the magnitude of observed current in the voltammograms increases due to the spontaneous enrichment in the concentration of the redox analytes confined in the droplet. 39 The voltammograms transition from an initial sigmoid (navy curve) to a duck-shape (green curve) and finally transition into a Gaussian pair of peaks indicating thin-layer conditions (orange curve). Note that the voltammogram recorded subsequent to the orange curve (CV 81 and optical micrograph 6 in panel (b) of Fig.…”

Single Entity Electrocatalysis

“…Stochastic electrochemical methods are commonly employed for single entity measurements, including single doped partially oxidized polyaniline for nitrite reduction, Nafion-based particles for permanganate reduction, and pyrroloquinoline quinone (PQQ) modified multiwalled carbon nanotube for hydrazine oxidation and poly(vinylferrocene)-modified graphene nanoplatelets for cysteine oxidation . One area of great interest moving forward is the development of new measurement modalities to study curious chemistry in electrochemically unreachable environments. ,− One potentially exciting new avenue of inquiry for such studies is single entity electrocatalysis in a dissolving microdroplet. , …”