Letter pubs.acs.org/acssensors
Sensor Selectivity Enhancement via Electrochemical Shielding in a Recessed Microelectrode Array: The Gatekeeper Geometry David A. Finkelstein* Institut National de la Recherche Scientifique − Énergie, Matériaux, et Télécommunications (INRS-EMT), 1650 Boulevard Lionel-Boulet, Varennes, Quebec, Canada J3X 1S2 S Supporting Information *
ABSTRACT: Selectivity in direct electrochemical sensors is difficult to achieve since Pt, the primary catalyst used for detecting analytes key to workplace safety (NH3, H2S, CO), has cross-sensitivity for a number of interferents. Recent advances in recessed microelectrode arrays readily lend themselves to enhancing selectivity in direct sensors via electrochemical shielding. However, all geometries presented to date are optimized for signal amplification, which requires a reversibly reducible or oxidizable analyte. Indeed, while 50fold improvements in selectivity have been shown, they are a product of 10-fold enhancements via signal amplification and just 5-fold enhancements via shielding. The theoretical study presented herein demonstrates that recessed microelectrode geometries can provide 34to 260-fold improvements in selectivity via shielding alone for measurements lasting 10 to 140 s. These order-of-magnitude improvements in selectivity via electrochemical shielding thus (1) outpace selectivity via signal amplification and (2) greatly broaden the application space of direct electrochemical sensors by improving selectivity for irreverisble analytes, which cannot benefit from signal amplification. This use of electrochemical shielding coupled with the confined dimensions of a recessed microelectrode array is termed the Gatekeeper Geometry. KEYWORDS: electrochemical sensor, irreversible analyte, selectivity, recessed microelectrode array, gatekeeper geometry, comsol, computational fluid dynamics
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irreversible, meaning it cannot benefit from signal amplification to raise its signal above that of interferents. In contrast, improving selectivity via signal amplification for reversibly oxidizable or reducible analytes is readily achieved in interdigitated microelectrode arrays (IDAs). Flat IDAs function by maintaining a short distance in-plane between neighboring working microelectrodes (W1 and W2) such that the two fall within one another’s depletion regions, allowing for signal amplification or shielding of interferents (Figures S2.A−C).8 For a commercial banded IDA, this results in a reported collection efficiency of 70% for ferrocyanide and measured shielding of 48% for O2 after 10 s (Figure S3). Recently, recessed IDAs have been employed to enhance selectivity via signal amplification, shielding, or both in combination, demonstrating selectivity improvements of 5- to 50-fold for reversible analytes.9−14 In a recessed IDA, the sensing electrode, W1, is recessed a fixed distance below the surface, while W2 is extended to the edges of the recess. This creates a cavity that prevents analytes oxidized or reduced at W1 from diffusing away in all directions and instead flux directly toward W2 for signal amplification. Additionally, W2
he development of high performance, direct electrochemical sensors based on the oxidation of NH3 has proved elusory. Most NH3 oxidation electrocatalysts, typically Pt alloys, exhibit rapid poisoining.1,2 Accurate NH3 detection has been demonstrated only at high NH3 concentrations (CNH3) of 3 to 11 mM3 or greater,4 whereas applications in industrial safety and medical diagnosis require detection of CNH3 ≤ 0.1 mM in solution (≤2 ppm in gas).5 At such low CNH3, interference from ambient O2 causes significant signal loss, as O2 is reduced at the same potentials where NH3 is oxidized. These drawbacks are readily apparent even for Pt(100) films, which have greatly enhanced NH3 oxidation current:6 At CNH3 ≤ 0.2 mM, O2 interference causes a loss of 65% of maximum current (Figure S1.A) and even negative signal after just 2.5 s of measurement, when NH3 oxidation has self-poisoned and only negative O2 current remains (Figure S1.B). Interference from O2 can be addressed by employing indirect electrochemical sensors, but cross-sensitivity to reduced gases remains,7 and the sensor lifetime becomes limited by the amount of onboard detecting reagent. Previous efforts have addressed rapid NH3 poisoning in direct electrochemical sensors via an in situ poison removal procedure,1 but detecting NH3 in the presence of interferents like O2 is not easily accomplished. The oxidation of NH3 is © XXXX American Chemical Society
Received: August 11, 2016 Accepted: September 28, 2016
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DOI: 10.1021/acssensors.6b00496 ACS Sens. XXXX, XXX, XXX−XXX
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ACS Sensors
Figure 1. Concentration profiles in the xz plane for O2 (A) and NH3 (B) at a flat IDA, and the concentration profiles in the z-direction for O2 and NH3 at the flat IDA W1c surface (C). Concentration profiles in the xz plane for O2 (D) and NH3 (E) at an IDA with W1 recessed 50 μm from W2, and the concentration profiles in the z-direction for O2 and NH3 at the recessed W1c surface (F).
fluid dynamics (CFD) simulations, it is shown, perhaps counterintuitively, that maximizing the recess depth of W1 relative to to the shielding electrode, W2, results in the greatest differential flux of analyte to interferent arriving at W1. Thus, the recess depths employed herein are an order of magnitude deeper than those used previously.12 It is further shown that the differential flux arises from the near-total elimination of an interferent within a deep recess, which causes a collapse of its concentration profile and results in a profoundly minimized interferent flux. Since these geometries function by strictly limiting access of interferent to a confined space where interferent concentration has been minimized, they are herein termed Gatekeeper Geometries. The operative principle behind the Gatekeeper Geometry is straightforward. Fick’s first Law, shown in 2D form in the xz plane in eq 1, states that the flux (J) of a given species is proportional to the gradient of its concentration (C) by the species’ diffusion coefficient (D). It follows that a species with a small change in concentration across a great distance would have minimal flux. The Gatekeeper Geometry functions by
can be used to shield interferents from entering the cavity and arriving at W1.12 Other variations involve recessing microdisks instead of microbands, using a recessed microring for W2, and additional solution confinement above the main plane of the sensor.14 Such geometries improve collection efficiency to 90− 99% for Ru(NH3)63+ and shielding up to 80% for ascorbic acid.12,14 Unfortunately, the selectivity for an irreversible analyte, such as NH3, can only be improved in a recessed IDA via shielding and not signal amplification. Since the cited geometries exhibit just a 2- to 5-fold decrease in interferent signal from shielding, owing the remainder of reported selectivity improvements to signal amplification,12,14 they are highly unsuitable for detecting irreversible analytes. These recessed IDA geometries are primarily designed for keeping the analyte inside the microcavity, rather than keeping the interferent out. The geometries presented herein are optimized for excluding interferents (e.g., O2) from microcavities to decrease the flux of interferents to the sensing electrode, W1, enabling enhanced selectivity of irreversibly oxidized analytes (e.g., NH3). Using computational B
DOI: 10.1021/acssensors.6b00496 ACS Sens. XXXX, XXX, XXX−XXX
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ACS Sensors
Figure 2. Flux profiles in the xz plane for O2 (A) and NH3 (B) at a flat IDA, and flux across time for O2 and NH3 at the flat IDA W1c surface (C). Flux profiles in the xz plane for O2 (D) and NH3 (E) at an IDA with W1 recessed 50 μm from W2, and flux across time for O2 and NH3 at the recessed W1c surface (F).
the unseen direction along the bands, and z is the direction away from the electrode surface toward bulk solution. The CFD simulations used a 2D diffusion model in the xz plane to determine species flux and concentration using Fick’s first and second Laws, respectively (eqs 1 and 2; see Supporting Information and Figure S5 for methods). The expected current (i), or signal, at W1 or W2 for a given species is proportional to the species’ total flux to the W1 or W2 surface (eq 3).
providing a deep, confined recess where a given interferent can be nearly completely depleted by both W1 and W2, flattening the interferent’s concentration profile throughout the recess. Thus, even if the interferent is consumed by W1 at the bottom of the recess, the interferent’s downward flux will be minimized.
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⎛ ∂C ∂C ⎞⎟ J = − D⎜ + ⎝ ∂x ∂z ⎠
(1)
MODEL EMPLOYED CFD simulations were employed to explore several variations of the Gatekeeper Geometry. To best illustrate its impact on selectivity, the overall geometry was based on and compared to a commercial flat banded IDA geometry (Micrux) with 10-μmwide W1 and W2 bands separated by 10-μm-wide gaps. The Gatekeeping IDA had 10-μm-wide W1 bands recessed 50 μm from the array surface. The W2 bands were widened to 30 μm to reach the edges of the recesses The xz cross sections of these geometries are shown in Figure 1, where x is the direction along the electrode surface cutting across electrode bands, y is
⎛ ∂ 2C dC ∂ 2C ⎞ = D⎜ 2 + ⎟ dt ⎝ ∂x ∂z 2 ⎠
(2)
i = nFAJ
(3)
where t is time, n is the number of e− involved in the reaction of the species at the electrode, F is Faraday’s constant, and A is electrode area. The true current is dependent on a multitude of other factors, such as NH3 poisoning, so only raw flux data is provided to exhibit the best possible performance of the Gatekeeper Geometry. C
DOI: 10.1021/acssensors.6b00496 ACS Sens. XXXX, XXX, XXX−XXX
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ACS Sensors The model was verified by simulating the shielding of JO2 to the centermost W1 band (W1c) of the flat IDA and comparing the result to shielding at a commercial IDA of identical electrode spacing. After 10 s, the simulation showed a 47% decrease in JO2 (Figure S4), which was highly similar to the 48% decrease in JO2 observed experimentally (Figure S3). In the simulations of the Gatekeeper Geometry, W2 was set to a potential such that O2 was consumed at its mass transport limit throughout the experiment. To simplify the model, both NH3 and O2 were assumed to be consumed at W1 at their respective mass transport limits. Additionally, the 3 e− oxidation of NH3 (eq S1) and 4 e− reduction of H2O (eq S2) were assumed, reducing complexities arising from the production of side products, such as NO and H2O2. Concentration and flux profiles that arose after 10 s of transport are shown in Figures 1 and 2. The establishment of the concentration profiles during 1 s and of the flux profiles during 1.5 s are depicted in a set of corresponding animations (Figures S6 to S13).
Figure 3. Ratio of NH3 flux to O2 flux at W1c across time at recess depths of 0−100 μm (A) and 200−400 μm (B).
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This surprising result is due to a simple reason: decreasing the distance between W1 and W2 benefits signal amplification at the cost of shielding, making the two approaches difficult to use in combination. However, it is likely that microcavityconfined signal amplification between closely spaced W1 and W2, used in combination with shielding from a distant W3, would obtain the greatest possible improvement in analyte selectivity. This general geometry has already been fabricated,15 but W3 was used only as a combined counter and reference electrode rather than as a shielding electrode, and was placed too close to W1 (8 μm) to observe the effects reported herein. Notably, signal amplification improves selectivity against the widest possible range of interferents, whereas multiple shielding electrodes are needed for disparate interferents. The modeling results in Figure 3 do not show a clear optimum for the Gatekeeper Geometry. Rather, they show that sensor rise time increases exponentially with higher selectivities up to 100 μm and increases linearly thereafter. For many applications, the 20-fold selectivity achieved in
