Fluorescent pH sensors that are so sensitive they can detect changes within almost a pH unit, have been developed by Japanese researchers. The digital sensors could be used in molecular computers and to monitor pH changes in biological cells, say the scientists.  

Seiichi Uchiyama and Yumi Makino at the University of Tokyo made the sensors by incorporating water-sensitive fluorophores into pH-responsive polymers. The polymers' structures vary with pH, from a hydrated open form at low pH to a dehydrated globular form at high pH. The sharp fluorescence signals given at each stage within a pH unit make them effective digital sensors. Different polymer structures can cover different pH ranges.

Dongwhan Lee, an expert in molecular sensors from Indiana University, Bloomington, US, says that it is the polymers' innovative design that allows them to give such a sharp fluorescent response to pH change.

'Our sensors can be used to detect a subtle change in intracellular pH,' says Uchiyama. Conventional sensors are sensitive to an 80-fold change in pH whereas Uchiyama's sensors require only a five-fold change. As well as changing the operating pH range, modifying the proton-recognising groups on the polymers can also tune the switching direction (on-off and off-on actions). 

Uchiyama says that in the future, he hopes to incorporate different ion sensors into the structures to detect biological ions other than protons. 

He adds: 'Molecular computing is the most exciting and challenging application for fluorescent digital ion sensors. There are many scientific papers about molecular logic and computing that use conventional fluorescent sensors. Our highly efficient devices could easily be combined with this concept to further develop molecular computing.'

Harriet Brewerton

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