Microfabricated self-referencing pulstrodes

Abstract

Screen printing and inkjet printing are attractive processes to produce low-cost and mass producible electroanalytical sensors. Despite important advances in the field, obtaining a printed electrochemical reference element that satisfies analytical requirements has not yet been realized satisfactorily. This paper investigates the use of screen printing and inkjet printing to produce a self-contained, all-solid state reference element that can be integrated with a wide range of electroanalytical sensing principles. The principle relies on a silver/silver iodide element that self-generates its potential by the application of a so-called pulstrode protocol. Specifically, a defined quantity of iodide is released by a short cathodic current pulse, and the reference potential defined by the released iodide is subsequently recorded at zero current. Both screen and inkjet-printed reference electrodes are fabricated and characterized, and the methodology optimized and assessed. As an application example, a single-point calibration method is used to quantify ions in undiluted filtered urine samples by potentiometry. The screen-printing approach was less successful owing to the low purity of the silver ink used. The inkjet printing approach allowed one to quantify chloride and sodium in urine. Using a conventional silver/silver chloride reference electrode as standard, relative errors of respectively 7.7 and 14.1% for chloride and sodium were obtained. While the approach would benefit from further optimization for long term applications, especially the use of high purity silver inks, it is a promising strategy for the realization of fully integrated all-solid-state microfabricated sensing systems.

Fig. 1. Illustration of the principle (a) and potential and current changes (b).

Fig. 2. Pictures and schematic representations of the three different type of electrodes used in the present work. Commercially available macroelectrode (a). For printed electrodes, the silver electrode used as reference element is shown in grey (bottom right electrode for (b) and middle electrode for (c)). Its diameter is 2 mm.

Fig. 3. Experimental traces for the pulstrode protocol steps with a macro-electrode: (a) open-circuit measurement for 3 s (black trace), galvanostatic pulse at 5 μA for 5 s (red trace) and EMF measurement for 0.25 s (blue trace) and (b) potentiostatic (OCP + 50 mV) regeneration pulse for 30 s. Background electrolyte: 0.15 M NaCl.

Fig. 4. Potentiometric traces for the deposition of the silver iodide layer, corresponding to a conversion of 30% of the silver layer (a). Response to changes in iodide activity (b) and reference values produced by the pulstrode protocol (c) of electrodes with 30% conversion.

Fig. 5. Potentiometric traces under application of galvanostatic currents of 15.7 (blue) and −15.7 μA (red) in 0.1 M NaI solution for one inkjet-printed electrode.

Fig. 6. Cyclic voltammetry experiment of bare silver macro-electrode (a) and inkjet-printed electrode (c) in 0.15 M potassium nitrate solution. 25 cycles of pulstrode protocol in 0.15 M potassium nitrate after electrochemical cleaning on (b) macro-electrode and (d) inkjet-printed electrode (3 cycles, 50 mV s⁻¹, from −0.6 to 0.3 V).

Fig. 7. Potentiometric quantification of urine ions using inkjet-printed electrodes (N = 5) (a) potential differences between synthetic (calibrant) and biological urine and (b) concentration measured in biological urine.