Scientists at the National Institute for Materials Science, Tsukuba, Japan, have designed and tested a novel miniaturized pH sensor based on multi-walled BN nanotubes (BNNTs). The nanotubes have an outer diameter in the region of 100 nm. The submicrometre-sized sensors consist of biotin-fluorescein-functonalized BN nanotubes with anchored silver nanoparticles (see figure above).

Intrinsic pH-dependent photoluminescence and Raman signals from attached fluorescein molecules enhanced by silver nanoparticles (around 20 nm in size) allow these novel nanohybrids to perform as practical three-dimensional pH mapping probes. Thanks to its compact design, the device is able to work in a submicrometre-sized space. For example, the sensor may determine the environmental pH of subunits in living cells where a traditional optical fiber sensor fails because of spatial limitations. The idea is based on the fact that a laser spot for Raman measurements on a BNNT can be restricted to less than 1 µm in size. Thus, spatially resolved pH mapping in living cells could be achieved through simple tracing of one identical spot on a controllable BNNT probe.

Reference curve

The Raman signal ratios of different bands in the silver surface-enhanced Raman spectra reflected well the pH values in a submicrometer-sized environment. The spatial resolution of the pH measurements is determined by the laser spot size and the diameter of the BNNT.

If one can fix a Raman laser spot on a specific site on a hybrid BN nanotube and assemble the tubes into a robotic control system, the probe should be able to detect the pH values in any position of interest without the need to provide other labels.

The researchers presented their work in Nanotechnology.