Using chemical hard modeling for designing optimum pH sensor arrays

pH measurement is widely demanded in biomedical, environmental, and industrial fields [1]. pH is an important physiological signal that plays a critical role in maintaining cellular and tissue homeostasis. Sensing and monitoring minor pH changes, especially at pHs of 5.4–7.4, are thus of great importance. Optical sensors have attracted the attention of many researchers with respect to cost, freedom from electrical interference, safety and the possibility of remote sensing [2]. Optical pH sensors based on the use of pH indicators have two major limitations, i.e. narrow dynamic range and non-linear response for pH measurement. A linear response is desirable because it leads to a simple calibration of the pH sensors, and more importantly, a constant sensitivity and precision in pH measurement over the entire linear range [3]. The useful pH sensing range is centered on the dye’s dissociation constant [4]. Thus, there is a need to fine-tune the probe pKa in a predictable way [5]. There is a body of literature showing how micelle composition can be used to control dye pKa, and chemical model of distribution of solutes between aqueous phase and micellar pseudo-phase in aqueous micellar solutions had been studied [6]. In this study, we have used chemical hard modeling to predict indicator pKa shift, in different micellar pseudo-phase or in different concentration of a micelle. Hard modeling as a systematic approach has been used to design optimum pH sensor array, which has linear response over a broad pH range. To ensure the largest sensing dynamic range, it is helpful to fine-tune the sensor pKa to match the pH of the sample under analysis. In order to increase sensitivity in this pH range, we have designed a sensor array, include 10 sensor which difference indicator pKa in each sensor was 0.2 unit. Using sensor array instead of individual sensor increases sensitivity and linear pH range. Two pH indicators, Alizarin yellow and cresol red in micellar solution of CTAB (Cetyltrimethylammonium bromide), allow the determination of pHs in the range 1 to 12.