Chaoxiong Ma

Redox cycling in nanoscale-recessed ring-disk electrode arrays for enhanced electrochemical sensitivity.

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.

Recessed Ring–Disk Nanoelectrode Arrays Integrated in Nanofluidic Structures for Selective Electrochemical Detection

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-).

Redox Cycling on Recessed Ring-Disk Nanoelectrode Arrays in the Absence of Supporting Electrolyte

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.

Ion Accumulation and Migration Effects on Redox Cycling in Nanopore Electrode Arrays at Low Ionic Strength

Ion permselectivity can lead to accumulation in zero-dimensional nanopores, producing a significant increase in ion concentration, an effect which may be combined with unscreened ion migration to improve sensitivity in electrochemical measurements, as demonstrated by the enormous current amplification (∼2000-fold) previously observed in nanopore electrode arrays (NEA) in the absence of supporting electrolyte. Ionic strength is a key experimental factor that governs the magnitude of the additional current amplification (AFad) beyond simple redox cycling through both ion accumulation and ion migration effects. Separate contributions from ion accumulation and ion migration to the overall AFad were identified by studying NEAs with varying geometries, with larger AFad values being achieved in NEAs with smaller pores. In addition, larger AFad values were observed for Ru(NH3)6(3/2+) than for ferrocenium/ferrocene (Fc(+)/Fc) in aqueous solution, indicating that coupling efficiency in redox cycling can significantly affect AFad. While charged species are required to observe migration effects or ion accumulation, poising the top electrode at an oxidizing potential converts neutral species to cations, which can then exhibit current amplification similar to starting with the cation. The electrical double layer effect was also demonstrated for Fc/Fc(+) in acetonitrile and 1,2-dichloroethane, producing AFad up to 100× at low ionic strength. The pronounced AFad effects demonstrate the advantage of coupling redox cycling with ion accumulation and migration effects for ultrasensitive electrochemical measurements.

Electrochemistry at single molecule occupancy in nanopore-confined recessed ring-disk electrode arrays

Electrochemical reactions at nanoscale structures possess unique characteristics, e.g. fast mass transport, high signal-to-noise ratio at low concentration, and insignificant ohmic losses even at low electrolyte concentrations. These properties motivate the fabrication of high density, laterally ordered arrays of nanopores, embedding vertically stacked metal-insulator-metal electrode structures and exhibiting precisely controlled pore size and interpore spacing for use in redox cycling. These nanoscale recessed ring-disk electrode (RRDE) arrays exhibit current amplification factors, AFRC, as large as 55-fold with Ru(NH3)62/3+, indicative of capture efficiencies at the top and bottom electrodes, Φt,b, exceeding 99%. Finite element simulations performed to investigate the concentration distribution of redox species and to assess operating characteristics are in excellent agreement with experiment. AFRC increases as the pore diameter, at constant pore spacing, increases in the range 200-500 nm and as the pore spacing, at constant pore diameter, decreases in the range 1000-460 nm. Optimized nanoscale RRDE arrays exhibit a linear current response with concentration ranging from 0.1 μM to 10 mM and a small capacitive current with scan rate up to 100 V s-1. At the lowest concentrations, the average pore occupancy is 〈n〉 ∼ 0.13 molecule establishing productive electrochemical signals at occupancies at and below the single molecule level in these nanoscale RRDE arrays.

Nanopore-enabled electrode arrays and ensembles

AbstractThis review (with 116 refs.) addresses recent developments in nanoelectrode arrays and ensembles with particular attention to nanopore-enabled arrays and ensembles. Nanoelectrode-based arrays exhibit unique mass transport and ion transfer properties, which can be exploited for electroanalytical measurements with enhanced figures-of-merit with respect to microscale and larger components. Following an introduction into the topic, we cover (a) methods for fabrication of solid-state nanopore electrodes, (b) chemical and biochemical sensors, (c) nanochannel arrays with embedded nanoelectrodes; (d) recessed nanodisk electrode arrays; (e) redox cycling in nanopore electrode arrays, (f) finally discuss novel nanoarrays for electrochemistry, and then give a future outlook. A wide variety of nanoelectrode array-based chemical and biochemical sensors properties are discussed in addition to faradaic, ion transfer and spectroelectrochemical applications. Graphical abstractThis review addresses recent developments in nanoelectrode arrays and ensembles with particular attention to nanopore-enabled arrays and ensembles. A wide variety of nanoelectrode array-based chemical and biochemical sensors properties are discussed in addition to faradaic, ion transfer and spectroelectrochemical applications.

Closed bipolar electrode-enabled dual-cell electrochromic detectors for chemical sensing

Bipolar electrodes (BPE) are electrically floating metallic elements placed in electrified fluids that enable the coupling of anodic and cathodic redox reactions at the opposite ends by electron transfer through the electrode. One particularly compelling application allows electron transfer reactions at one end of a closed BPE to be read out optically by inducing a redox-initiated change in the optical response function of a reporter system at the other end. Here, a BPE-enabled method for electrochemical sensing based on the electrochromic response of a methyl viologen (MV) reporter is developed, characterized, and rendered in a field-deployable format. BPE-enabled devices based on two thin-layer-cells of ITO and Pt were fabricated to couple an analytical reaction in one cell with an MV reporter reaction, producing a color change in the complementary cell. Using Fe(CN)63/4- as a model analyte, the electrochemically induced color change of MV was determined initially by measuring its absorbance via a CCD camera coupled to a microscope. Then, smartphone-based detection and RGB analysis were employed to further simplify the sensing scheme. Both methods produced a linear relationship between the analyte concentration, the quantity of MV generated, and the colorimetric response, yielding a limit of detection of 1.0 μM. Similar responses were observed in the detection of dopamine and acetaminophen. Further evolution of the device replaced the potentiostat with batteries to control potential, demonstrating the simplicity and portability of the device. Finally, the physical separation of the reporter and analytical cells renders the device competent to detect analytes in different (e.g. non-aqueous) phases, as demonstrated by using the electrochromic behavior of aqueous MV to detect ferrocene in acetonitrile in the analytical cell.