Utilizing Nafion antifouling coatings on stencil-printed electrodes for sensitive electrochemical detection of the therapy drug carbamazepine from saliva

Introduction:: Due to the complex composition of human saliva and the associated potential for signal interference, quantification of analytes in human saliva can be challenging. Further, analyte concentration in saliva is often at substantially lower levels than in serum. Thus, signal enhancement methods are critical to robust quantification of salivary analytes and specifically for our application of monitoring the anticonvulsant carbamazepine for guiding therapeutic dosing. In this study, we investigate the use of Nafion™, a perfluorosulfonic acid ion-exchange polymer, as an electrode coating, in combination with sodium dodecyl sulfate (SDS), for the signal enhancement of carbamazepine in saliva. We show that Nafion™ in combination with SDS is critical for carbamazepine quantification at the expected therapeutic levels of carbamazepine in saliva.

Materials and Methods:: The CBZ sensors were composed of stencil-printed electrodes on polyester sheets (Fig. 1). The working and counter electrodes were made of carbon paste (C2180626D6, Sun Chemicals, NJ USA), and created by using a squeegee to spread the paste over the stencils. The same process was then used to pattern the reference electrode using Ag/AgCl ink (CI-4002, EMS, OH, USA). The electrodes were cured in an oven, at 100 °C, for 10 min. A fluidic channel was cut from 10-mil-thick polyester with adhesive and attached to the substrate with the three stencil-printed electrodes. Materials were cut using a CO₂ laser. The channels were filled with a diluted Nafion solution of 1% from a stock solution of 20% Nafion in alcohols and dried in a vacuum oven for 45 minutes at 50 °C. SDS in different concentrations was applied from a 100 mM stock solution. Human saliva collected at Oregon State University (OSU) using a protocol approved by the OSU Institutional Review Board. Saliva was aliquoted, and stored at −20 °C and spiked with 15-μM CBZ from a 10-mM CBZ solution in ethanol stock (resulting in a < 1% dilution) before use. Square-wave voltammetry measurements were performed using a benchtop potentiostat (CH Instruments). SWV parameters included a potential range of 0.5–1.2 V with a step size of 0.004 V, an amplitude of 0.025 V, a frequency of 15 Hz, and an incubation time of 4 minutes. The resulting data of current vs. potential were analyzed using our LDNH method [1].

Results, Conclusions, and Discussions:: The effect of electrode coating with Nafion™ on the carbamazepine electrochemical signature was investigated using saliva samples spiked with 15 μM carbamazepine and with SDS in solution. The raw voltammograms of Fig. 2 indicate that the 1% Nafion™ coating visibly enhances the carbamazepine signal in the voltammograms of two different saliva samples (with different viscosities) over the analogous voltammograms without the Nafion™ coating. Motivated by this, three different approaches of incorporating SDS with the Nafion™ film in the saliva sample were investigated: (i) SDS in the saliva solution above the Nafion™ film, (ii) mixing SDS and Nafion™ and then applying a composite film onto the electrode, and (iii) drying down an SDS film on top of a Nafion™ film. For the latter two methods, two different concentrations of SDS were evaluated. The results in Fig. 3 indicate the highest carbamazepine signal was achieved for SDS in the solution above the Nafion™ electrode film. The second highest carbamazepine signal was achieved by fabricating a SDS film on top of the original Nafion™ electrode film with the lower SDS concentration producing a higher signal. The lowest signals, about 50% that of the SDS in solution case, corresponded to the composite film of SDS and Nafion™ applied to the electrode.

We have demonstrated that a Nafion™ coating in combination with SDS substantially improves the carbamazepine signal in saliva. For a field use device, the method of SDS film on top of the Nafion™ film would be more suitable than requiring SDS in solution. Ongoing work will further fine-tune the optimal protocol for creating the multi-layer SDS and Nafion™ structure for enhanced carbamazepine detection.




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References (Optional): : [1]. Wentland L, Cook J, Minzlaff J et al., Journal of Applied Electrochemistry (2023), 53, 523-534