Oct 29, 2009
Boron nitride nanotubes measure pH locally
Monitoring the pH value of a microscale environment is important because the biochemical reactivity and biophysical function of cells and body liquids are highly sensitive to the concentration of protons [H+]. To provide chemical information on a local scale, researchers in Japan have developed a miniaturized pH sensor that allows tracing of intracellular physiological or pathogenic processes at high spatial resolution.
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.
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.
About the author
Dmitri Golberg, PhD, is a principal investigator and a nanotube group leader at the International Center for Materials Nanoarchitectonics (MANA), which is part of the National Institute for Materials Science in Tsukuba, Japan. He is also an associate professor at the University of Tsukuba. He has been engaged in boron nitride and other inorganic nanotube research over the last two decades. This has yielded numerous published data related to novel electrical, mechanical, magnetic and chemical properties of these interesting nanotubes that so far have been underestimated and sit in the shadow of widely studied carbon nanotubes. The regarded work was performed under a NIMS post-doctoral project of Dr Qing Huang, who is now based at the University of California at Davis, US. Other authors include Prof. Y Bando and Drs L Zhao and C Y Zhi, all from NIMS. Based on this and other research, the authors are confident that in many respects BN nanotubes should rival or even surpass the practical application prospects of conventional carbon nanotubes.