Microconcentric Ring Electrode/Injector Assembly for Sensitive Voltammetric Analysis in Single Droplets of Ultrasmall Volumes

Abstract

This paper describes the construction of a microring electrode concentric to an inner injection capillary for voltammetric determination of trace analytes in nanoliter- to picoliter-sized droplets. The gold microring is sandwiched between a pulled fused-silica capillary and borosilicate glass tubing. Compared to polymer-coated microring electrodes, the glass-encapsulated electrode is more robust and does not swell in organic solvents. Consequently, the microring electrode is suitable for voltammetric studies of redox-active species and their accompanying ion transfers between two immiscible solvents. Droplets of variable sizes can be conveniently dispensed from front-loaded sample plugs into an immiscible liquid, greatly simplifying the experimental procedure and facilitating analysis of samples of limited availability. The size of the microring and the volume of the droplet deduced from well-defined voltammograms correlate well with those estimated from their geometric dimensions. The thin-layer cell behavior can be attained with well-defined voltammetric peaks and small capacitive current. Exhaustive electrolysis in single droplets can be accomplished in short times (e.g., ∼85 s in a droplet of 1.42 nL at a microring of 11.4 μm in radius). Anodic stripping voltammetry of Ag deposited onto the microring electrode resulted in a detection limit of 0.13 fmol (14 fg) of Ag(+). The microring electrode/injector assembly can be polished repeatedly and is versatile for various applications (e.g., sample plugs can also be back-loaded via a rotary injection valve and an HPLC pump for flow injection analysis).

Continuous Nanoflow-Scanning Electrochemical Microscopy: Voltammetric Characterization and Application for Accurate and Reproducible Imaging of Enzyme-Labeled Protein Microarrays

“…The microconcentric ring electrode/injector assembly contains a thin Au film sandwiched between a pulled fused-silica capillary and a borosilicate glass tubing and is constructed as previously described . The CNF-SECM probe was fabricated by beveling its apex on a microcapillary beveler (model BV-10, Buehler, Lake Bluff, IL).…”

“…These attractive features enabled us to conduct sensitive sample analyses in single droplets of ultrasmall volumes (<100 pL) surrounded by an immiscible liquid. 38 In this work, by beveling such an electrode and connecting its back end to a six-port rotary valve and a precision gas displacement pump, we demonstrate that SECM imaging can be performed in the continuous nanoflow (CNF) mode. The resultant apparatus not only provides controllable and stable flows of fresh solution to the center of the microring electrode (which cannot be achieved with a microinjector in the single injection experiment 33 ) but also facilitates uninterrupted introduction of samples or mediator solutions through the rotary valve.…”

“…35−37 To mitigate these limitations, we recently constructed a robust microring electrode that is concentric to an inner injector capillary. 38 Instead of using electrophoretic paint for insulation, 33,34 the microring electrode is encapsulated by a borosilicate glass tubing. Consequently, the electrode can be polished and sample solutions can be front-or back-loaded.…”

“…The microconcentric ring electrode/injector assembly contains a thin Au film sandwiched between a pulled fused-silica capillary and a borosilicate glass tubing and is constructed as previously described. 38 The CNF-SECM probe was fabricated by beveling its apex on a microcapillary beveler (model BV-10, Buehler, Lake Bluff, IL). During beveling, pressurized air was applied from the back of the capillary to prevent clogging of the injector by polishing materials or broken glass pieces.…”

“…Consequently, the electrode can be polished and sample solutions can be front- or back-loaded. These attractive features enabled us to conduct sensitive sample analyses in single droplets of ultrasmall volumes (<100 pL) surrounded by an immiscible liquid . In this work, by beveling such an electrode and connecting its back end to a six-port rotary valve and a precision gas displacement pump, we demonstrate that SECM imaging can be performed in the continuous nanoflow (CNF) mode.…”