Multi-ion detection by one-shot optical sensors using a colour digital photographic camera

Abstract

The feasibility and performance of a procedure to evaluate previously developed one-shot optical sensors as single and selective analyte sensors for potassium, magnesium and hardness are presented. The procedure uses a conventional colour digital photographic camera as the detection system for simultaneous multianalyte detection. A 6.0 megapixel camera was used, and the procedure describes how it is possible to quantify potassium, magnesium and hardness simultaneously from the images captured, using multianalyte one-shot sensors based on ionophore–chromoionophore chemistry, employing the colour information computed from a defined region of interest on the sensing membrane. One of the colour channels in the red, green, blue (RGB) colour space is used to build the analytical parameter, the effective degree of protonation (1 − α_eff), in good agreement with the theoretical model. The linearization of the sigmoidal response function increases the limit of detection (LOD) and analytical range in all cases studied. The increases were from 5.4 × 10⁻⁶ to 2.7 × 10⁻⁷ M for potassium, from 1.4 × 10⁻⁴ to 2.0 × 10⁻⁶ M for magnesium and from 1.7 to 2.0 × 10⁻² mg L⁻¹ of CaCO₃ for hardness. The method's precision was determined in terms of the relative standard deviation (RSD%) which was from 2.4 to 7.6 for potassium, from 6.8 to 7.8 for magnesium and from 4.3 to 7.8 for hardness. The procedure was applied to the simultaneous determination of potassium, magnesium and hardness using multianalyte one-shot sensors in different types of waters and beverages in order to cover the entire application range, statistically validating the results against atomic absorption spectrometry as the reference procedure. Accordingly, this paper is an attempt to demonstrate the possibility of using a conventional digital camera as an analytical device to measure this type of one-shot sensor based on ionophore–chromoionophore chemistry instead of using conventional lab instrumentation.