Scientists led by Kazuaki Sawada at the Toyohashi University of Technology has developed a pH imaging sensor. The sensor enables simultaneous visualisation of the pH and optical imaging of chemical activity of solutions and cell activity. Sawada and his group are looking for industrial partners for the development of other applications of the pH image sensor.
The CMOS device consists of an array of CCDs covered with a functionalised membrane. Changes in the concentration and two dimensional distribution of hydrogen ions are detected by charge accumulation. In addition to monitoring the pH distribution, the device also yields optical images of the test sample.
The sensitivity of the pH imaging sensor is 100 times greater than ISFET devices and enables the determination of pH differences of 0.0001 pH. ‘High sensitivity is possible because we accumulate charge over well-defined periods of time,’ said Sawada. ‘The charge transfer is repeated many times, which gives huge improvements in signal to noise ratio.’
The current pH image sensors consist of 128 x 128 pixels, each with a sensing area of 10 x 25µm. Sawada and his group are developing pH image sensors with one million pixels, with each pixel being 10 x10µm. Sawada has also launched a pH imaging consortium to address issues related to ion image sensing. He said: ‘In the future, plans include pH imaging devices for visualising the movement and distribution of other ions including calcium and sodium.’
Sawada's group has recently reported on the use of the sensor for real-time imaging of acetylcholine (ACh) enzyme reactions. ‘We imaged changes in the distribution of Ach when nerve cells are stimulated with KCL,’ said Sawada. ‘Insights in the variation of the concentration of ACh may lead to new methods for the treatment of Alzheimer’s disease.’
