Continuous minimally-invasive alcohol monitoring using microneedle sensor arrays

Abstract

The present work describes an attractive skin-worn microneedle sensing device for the minimally invasive electrochemical monitoring of subcutaneous alcohol. The device consists of an assembly of pyramidal microneedle structures integrated with Pt and Ag wires, each with a microcavity opening. The microneedle aperture was modified by electropolymerizing o-phenylene diamine onto the Pt wire microtransducer, followed by the immobilization of alcohol oxidase (AOx) in an intermediate chitosan layer, along with an outer Nafion layer. The resulting microneedle-based enzyme electrode displays an interference-free ethanol detection in artificial interstitial fluid without compromising its sensitivity, stability and response time. The skin penetration ability and the efficaciousness of the biosensor performance towards subcutaneous alcohol monitoring was substantiated by the ex vivo mice skin model analysis. Our results reveal that the new microneedle sensor holds considerable promise for continuous non-invasive alcohol monitoring in real-life situations.

Keywords: Alcohol; Biosensor; Microneedle; Minimally-invasive monitoring; Wearable sensor.

Figure 1

Schematic representation of a (A) hollow microneedle, (B) press fitting of Pt and Ag wires to the aperture of hollow microneedle and (C) Pt and Ag wires integrated microneedle array. (D) An image showing the microneedle array mounted on the fingertip. (E) Optical micrograph of the microneedle array integrated with Pt and Ag wires. (F) Schematics demonstrating the construction of alcohol biosensor with its multi-layer reagent coating along with the biocatalytic reaction of the immobilized AOx.

Figure 2

(A) Effect of applied potential (−0.2 V to 0.6 V) on the chronoamperometric responses of 250 μM H₂O₂ measured at Pt wire integrated microneedle electrode. Also shown (inset) the corresponding cyclic voltammogram for 500 μM H₂O₂. (B) Linear chronoamperometric responses of H₂O₂ (50 μM to 400 μM) measured at Pt wire integrated microneedle electrode (inset shows the oxidation of H₂O₂ on the Pt wire integrated microneedle electrode).

Figure 3

Chronoamperometric responses of the alcohol biosensor recorded in (A) phosphate buffer of pH 7 and (B) in artificial ISF from 0 mM to 80 mM alcohol in 5 mM increments (from a–q) at 0.6 V vs. Ag/AgCl electrode. (Inset shows the corresponding calibration plots. Current Sampling interval: 0.1 s over 35 s. Other conditions, as in Figure 2.

Figure 4

Chronoamperometric response of the alcohol biosensor for 20 mM alcohol followed by the addition of electroactive interferents such as AC, UA, L-Cysteine and AA, respectively (0.6 V vs. Ag/AgCl electrode). Inset shows the percentage deviation of relative current responses based on (a) 20 mM alcohol signal for 100 μM each of (b) AC, (c) UA and (d) L-Cysteine and 10 μM AA, respectively. Also shown is the blank response (black current). Conditions, as in Figure 3.

Figure 5

Operational stability of the microneedle alcohol biosensor. Response to 30 mM alcohol in (A) phosphate buffer and (B) artificial ISF containing 20.1 mg/mL BSA over 100 min periods at 10 min intervals. Conditions, as in Figure 3.

Figure 6

(A) Schematic showing the detection of alcohol in artificial ISF using the microneedle penetrated through the mice skin. (B) Real-time alcohol detection from the artificial ISF using the microneedle penetrated through the mice skin. A close up image showing the (C) microneedle biosensor array, (D) mice skin sample and (E) the penetrated microneedle through the mice skin. Other conditions, as in Figure 3.