Contactless conductivity sensor as detector for microfluidic paper-based analytical device with application to unique rapid method for quantifying sulfite preservative

Abstract

The work presents the latest exploitation of capacitively coupled contactless conductivity detector (C4D) produced from printed-circuit board as durable detector for in-situ headspace microfluidic paper-based analytical devices (μPADs). This is the first reagent-free operating paper-based method for direct determination of sulfite, with storage of acid reagent during μPAD fabrication. In the procedure, the detection zone is first moistened with deionized water, and the μPAD turned over and placed on the C4D sensing area with the moist-zone faced down. Sample is loaded at the top layer of μPAD where it flows into the adjacent covered reaction zone, containing the absorbed acid, forming SO2(g). The gas diffuses through the in-situ headspace in the middle layer and is absorbed into the moist detection zone at the base layer giving a prompt change in the C4D signal due to the change in conductivity from the generated conductive ionic species, viz. H+(aq) and HSO3−(aq). The working range of the C4D-μPAD is 8–100 mg L-1 sulfite, with the limit of detection of 6.61 mg L-1. Compared with other paper-based methods that detect SO2(g), this is the only in-situ method and most rapid method (3.3 min per sample) for fruit juices, white and red wines.