ABSTRACT
A new colour-based disposable sensor array for a full pH range (0-14) is described. The pH sensing elements are a set of different pH indicators immobilized in plasticized polymeric membranes working by ion-exchange or co-extraction. The colour changes of the 11 elements of the optical array are obtained from a commercial scanner using the hue or H component of the hue, saturation, value (HSV) colour space, which provides a robust and precise parameter, as the analytical parameter. Three different approaches for pH prediction from the hue H of the array of sensing elements previously equilibrated with an unknown solution were studied: Linear model, Sigmoid competition model and Sigmoid surface model providing mean square errors (MSE) of 0.1115, 0.0751 and 0.2663, respectively, in the full-range studied (0-14). The performance of the optical disposable sensor was tested for pH measurement, validating the results against a potentiometric reference procedure. The proposed method is quick, inexpensive, selective and sensitive and produces results similar to other more complex optical approaches for broad pH sensing.
ABSTRACT
This paper presents the characterization of optical sensing membranes for calcium based on ionophore-chromoionophore chemistry. Six different ionophores, 18-membered crown ether derivatives, were studied, coming from 18-crown-6 ether and 4,13-diaza-18-crown-6 ether to a series of double-armed crown ethers with different type of terminal groups. The study of optical membranes containing the same transducer and plasticizer allow drawing some conclusions on the influence of lipophilicity and size of the terminal group of the side chain on calcium selectivity. We have calculated the exchange constant K(ILp)exch for each equilibrium with alkaline and alkaline-earth ions and the selectivity coefficient k(Osel)I,J for each ion against calcium as a way for a full characterization of sensing membranes. In all cases the ion:ionophore stability constants for calcium were the highest and the ionophore V containing an (N-adamantylcarbamoyl) acetyl moiety originated the most selective membrane for calcium. Analytical parameters for calcium determination using prepared membranes were calculated.
