An array of nanoscale-recessed ring-disk electrodes was fabricated using layer-by-layer deposition, nanosphere lithography, and a multistep reactive ion etching process. The resulting device was operated in generator-collector mode by holding the ring electrodes at a constant potential and performing cyclic voltammetry by sweeping the disk potential in Fe(CN)6(3-/4-) solutions. Steady-state response and enhanced (~10×) limiting current were achieved by cycling the redox couple between ring and disk electrodes with high transfer/collection efficiency. The collector (ring) electrode, which is held at a constant potential, exhibits a much smaller charging current than the generator (disk), and it is relatively insensitive to scan rate. A characteristic feature of the nanoscale ring-disk geometry is that the electrochemical reaction occurring at the disk electrodes can be tuned by modulating the potential at the ring electrodes. Measured shifts in Fe(CN)6(3-/4-) concentration profiles were found to be in excellent agreement with finite element method simulations. The main performance metric, the amplification factor, was optimized for arrays containing small diameter pores (r < 250 nm) with minimum electrode spacing and high pore density. Finally, integration of the fabricated array within a nanochannel produced up to 50-fold current amplification as well as enhanced selectivity, demonstrating the compatibility of the device with lab-on-a-chip architectures.

In canonical electrochemical experiments, a high-concentration background electrolyte is used, carrying the vast majority of current between macroscopic electrodes, thus minimizing the contribution of electromigration transport of the redox-active species being studied. In contrast, here large current enhancements are achieved in the absence of supporting electrolyte during cyclic voltammetry at a recessed ring-disk nanoelectrode array (RRDE) by taking advantage of the redox cycling effect in combination with ion enrichment and an unshielded ion migration contribution to mass transport. Three distinct transport regimes are observed for the limiting current as a function of the concentration of redox species, Ru(NH3)6(2+/3+), revealed through the strong dependence of ion transport on ionic strength. Behavior at low analyte concentrations is especially interesting. In the absence of supporting electrolyte, ions accumulate in the nanopores, resulting in significantly increased current amplification compared to redox cycling in the presence of supporting electrolyte. Current enhancements as large as 100-fold arising from ion enrichment and ion migration effects add to the ~20-fold enhancement due to redox cycling, producing a total current amplification as large as 2000-fold compared to a single microelectrode of the same total area, making these RRDE arrays interesting for electrochemical processing and analysis.

Mechanical forces are critical but poorly understood inputs for organogenesis and wound healing. Calcium ions (Ca) are critical second messengers in cells for integrating environmental and mechanical cues, but the regulation of Ca signaling is poorly understood in developing epithelial tissues. Here we report a chip-based regulated environment for microorgans that enables systematic investigations of the crosstalk between an organ's mechanical stress environment and biochemical signaling under genetic and chemical perturbations. This method enabled us to define the essential conditions for generating organ-scale intercellular Ca waves in Drosophila wing discs that are also observed in vivo during organ development. We discovered that mechanically induced intercellular Ca waves require fly extract growth serum as a chemical stimulus. Using the chip-based regulated environment for microorgans, we demonstrate that not the initial application but instead the release of mechanical loading is sufficient, but not necessary, to initiate intercellular Ca waves. The Ca response depends on the prestress intercellular Ca activity and not on the magnitude or duration of the mechanical stimulation applied. Mechanically induced intercellular Ca waves rely on IPR-mediated Ca-induced Ca release and propagation through gap junctions. Thus, intercellular Ca waves in developing epithelia may be a consequence of stress dissipation during organ growth.

Electroosmotic flow (EOF) is used to enhance the delivery of Fe(CN)(6)(4-)/Fe(CN)(6)(3-) to an annular nanoband electrode embedded in a nanocapillary array membrane, as a route to high efficiency electrochemical conversions. Multilayer Au/polymer/Au/polymer membranes are perforated with 10(2)-10(3) cylindrical nanochannels by focused ion beam (FIB) milling and subsequently sandwiched between two axially separated microchannels, producing a structure in which transport and electron transfer reactions are tightly coupled. The middle Au layer, which contacts the fluid only at the center of each nanochannel, serves as a working electrode to form an array of embedded annular nanoband electrodes (EANEs), at which sufficient overpotential drives highly efficient electrochemical processes. Simultaneously, the electric field established between the EANE and the QRE (>10(3) V cm(-1)) drives electro-osmotic flow (EOF) in the nanochannels, improving reagent delivery rate. EOF is found to enhance the steady-state current by >10× over a comparable structure without convective transport. Similarly, the conversion efficiency is improved by approximately 10-fold compared to a comparable microfluidic structure. Experimental data agree with finite element simulations, further illustrating the unique electrochemical and transport behavior of these nanoscale embedded electrode arrays. Optimizing the present structure may be useful for combinatorial processing of on-chip sample delivery with electrochemical conversion; a proof of concept experiment, involving the generation of dissolved hydrogen in situ via electrolysis, is described.

Arrays of recessed ring-disk (RRD) electrodes with nanoscale spacing fabricated by multilayer deposition, nanosphere lithography, and multistep reactive ion etching were incorporated into nanofluidic channels. These arrays, which characteristically exhibit redox cycling leading to current amplification during cyclic voltammetry, can selectively analyze electroactive species based on differences in redox reversibility, redox potential, or both. Using Ru(NH3)6(3+) and ascorbic acid (AA) as model reversible and irreversible redox species, the selectivity for electrochemical measurement of Ru(NH3)6(3+) against a background of AA improves from ∼10, for an array operated in a fluidically unconstrained geometry, to ∼70 for an array integrated within nanofluidic channels. RRD arrays were also used for the detection of dopamine in the presence of AA by cyclic voltammetry. A linear response ranging from 100 nM to 1 mM with a detection limit of 20 nM was obtained for dopamine alone without nanofluidic confinement. In nanochannel-confined arrays, AA was depleted by holding the ring electrodes at +0.5 V versus Ag/AgCl, allowing interference-free determination of dopamine at the disk electrodes in the presence of a 100-fold excess of AA. For selective detection of electrochemically reversible interfering species on an RRD array without nanochannel confinement, a ring potential can be chosen such that one species exhibits exclusively cathodic (anodic) current, allowing the other species to be determined from its anodic (cathodic) current. This approach for selective detection is demonstrated in a mixture of Ru(NH3)6(3+) and Fe(CN)6(3-), which have resolved redox potentials. The same principle was successfully applied to differentiate species with overlapping redox potentials, such as dopamine/Fe(CN)6(3-) and ferrocenemethanol/Fe(CN)6(4-).