Improving the Sensitivity of Solid-Contact Ion-Selective Electrodes by Using Coulometric Signal Transduction.

Tingting Han,Johan Bobacka,Ulriika Mattinen

doi:10.1021/acssensors.8b01649

Tingting Han, Johan Bobacka + Show 1 more

Open Access

https://doi.org/10.1021/acssensors.8b01649

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Journal: ACS Sensors	Publication Date: Mar 7, 2019
Citations: 69	License type: cc-by

Affiliation: Åbo Akademi University

Abstract

A fundamental limitation of potentiometric ion sensors is their relatively low sensitivity due to the logarithmic dependence between potential and activity. Here we address this issue by exploring a recently developed coulometric transduction method for solid-contact ion-selective electrodes (SCISEs). Spin-coated thin-layer ion-selective membranes are used to lower the membrane resistance and shorten the response time of the SCISEs. When using coulometric transduction, an optimized design of the K+-SCISE is able to detect a concentration change of 5 μM at a concentration level of 5 mM, corresponding to a 0.1% change in K+ activity. This indicates that SCISEs can provide extremely high sensitivity when employing coulometric transduction. Impedance measurements show that the coulometric transduction process for the K+-SCISE is limited by diffusion even for very thin ion-selective membranes. On the other hand, the H+-SCISE shows a low impedance and a fast coulometric response that is related to the high mobility of H+ in the H+-selective polymeric membrane as well as in the solid contact layer. The coulometric transduction method was used to detect small changes of pH in seawater and found to improve the sensitivity compared to classical potentiometry. The coulometric method was briefly tested also for determining activity changes of K+ in a serum sample.

Highlights

A fundamental limitation of potentiometric ion sensors is their relatively low sensitivity due to the logarithmic dependence between potential and activity
Thin-layer K+-selective membranes were spin-coated on top of the Glassy carbon (GC)/PEDOT(PSS) solid contact in order to minimize the membrane resistance, thereby facilitating the ion transfer through the thin membrane and shortening the response time for K+-SC-Ion-selective electrodes (ISEs) when applying the coulometric signal read-out method.[12,13]
Typical chronoamperometric and coulometric signal responses of K+-solidcontact ISEs (SCISEs) with spin-coated thin-layer membranes are presented in Figure. 1

Summary

■ RESULTS AND DISCUSSION

Thin-layer K+-selective membranes were spin-coated on top of the GC/PEDOT(PSS) solid contact in order to minimize the membrane resistance, thereby facilitating the ion transfer through the thin membrane and shortening the response time for K+-SC-ISEs when applying the coulometric signal read-out method.[12,13] Typical chronoamperometric and coulometric signal responses of K+-SCISEs with spin-coated thin-layer membranes are presented in Figure. 1. 1. Figure 1a shows an enlargement of the first and second dilution steps (Δlog aK+ = 0.18 per dilution step) of the chrononamperograms and the cumulated charge vs time for K+-SCISEs with 4 mC PEDOT(PSS) solid contact and different thickness of the spin-coated thin-layer ISMs (1, 2, and 3 drops). The K+-SCISEs with a spin-coated (thin) ISM showed a comparatively larger amount of charge than K+-SC-ISEs with a drop-cast (thick) membrane These results indicate that the transport of H+ through the bulk of the H+-selective ISM and the PEDOT(PSS) solid contact is faster compared to the corresponding transport of K+ in the K+SCISE. (a) Chronoamperograms and (b) the cumulated charge vs time for K+-SCISEs and H+-SCISEs with 10 mC PEDOT(PSS) solid contact and spin-coated (3 drops) ISM during sequential dilution steps In order to fully utilize the advantages of signal amplification offered by the coulometric method it will be important to eliminate any potential drift of the SCISE (and reference electrode)

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES