pH detection range is widest yet for a precise chemical test

Chemists from Queen’s University, Belfast, UK, have taken inspiration from relay runners to create a molecular version of a conventional glass pH electrode. Their molecular sensor can measure pH over a wide range (0-9.5) and should allow scientists to begin measuring pH levels in microscopic volumes, such as within individual cells.

To create their pH sensor, the team - led by A. Prasanna de Silva - turned to a process known as photoinduced electron transfer (PET), which they had previously used to develop various molecular logic gates. 

In PET, a fluorescent compound is connected (using a spacer molecule) to a receptor compound that can bind to particular chemical groups. Normally, the receptor prevents the fluorescent compound from emitting light when excited, because the energy is instead used to oxidise the receptor. But this process is interrupted when the receptor binds with its target molecule, allowing the fluorescent compound to shine.

There are a number of receptors that bind with hydrogen ions, the chemical species that determine pH. Different receptors work best at different ion concentrations of hydrogen ion, and therefore different pH values. But the stumbling block was that each receptor can only work over a narrow range of around two pH units. So de Silva and his team wondered what would happen if they utilised a number of PET sensors, containing different receptors, in parallel.

’We use a set of four sensors to act somewhat like a relay team,’ explained de Silva. ’One sensor does the hard work of sensitively responding to pH across approximately two pH units and then hands over the task to the next one, which in turn works hard over the next two pH units, and so on,’ he told Chemistry World. De Silva and his team found that the total fluorescence emitted by an equal mixture of these four sensors fell directly in line with increasing pH, showing the same linear response as a glass pH electrode.

The team hope that the relay approach could now be applied to extend the range of other chemical sensors.

Jon Evans