This probably is not a project you could do for your school science project, unless you are really, really keen. Chemists at Queen’s University Belfast have developed a molecular system that acts like an ion-selective electrode across a pH range of 9. Prasanna de Silva and his colleagues, who featured in a special issue of Intute Spotlight recently, explain that their device has the advantage of operating on the nanoscale and so could be incorporated into microelectromechanical systems, or so-called lab-on-a-chip devices. The team used a set of four sensor molecules in relay to act as the quantitative version of universal indicator paper. One sensor does the hard work of sensitively responding to pH across approximately 2 pH units and then hands on the baton to the next, which in turn works hard over the next 2 pH units and so on covering 9.5 units on the pH scale. de Silva says that the molecular devices show an almost linear intensity-pH profile that is reminiscent of the response of a conventional glass pH electrode but a thousand times smaller than even micrometer-scale intracellular pH electrodes.
“Our research shows a simple and predictive way of greatly extending the sensing range of molecular sensors,” de Silva told Sciencebase, “We do it by using a set of four sensors to act somewhat like a 4×100 m relay team. One sensor does the hard work of sensitively responding to pH across approximately 2 pH units and then hands over the task to the next one, which in turn works hard over the next 2 pH units (approximately) and then hands over the task to the next one etc. In this way we get a range of 9.5 pH units whereas the normal range of a molecular sensors is only 2 (as you learn in high school science).”
Scientists have thought about this problem before (resulting in the famous universal indicator), there was no quantitative version of a molecular universal indicator until now. The nearly linear intensity-pH profile is reminiscent of the glass pH electrode’s response, even though the electrode is obviously a lot bigger in size (the smallest ones are micrometer-sized and used for intracellular pH measurements), de Silva told me.
You can get the full experimental details in their JACS paper on this: Analog Parallel Processing of Molecular Sensory Information