First discovered in 1975, carbon nanotubes are graphene sheets rolled up into cylinders. The diameter is typically in the nanometer range, and the length can be up to many micrometers. Carbon nanotubes offer promise in many applications, including electronics, novel materials, and medicine.  Now, researchers from Rice University have developed a spectral triangulation system, intended to pinpoint cancer tumours tagged with antibody-linked carbon nanotubes.  The team state that their spectral analysis of light coming through the skin can reveal the depth of tissue through which that light has passed; this allows the three-dimensional coordinates of the nanotube beacon to be deduced from a small set of noninvasive optical measurements.  The study is published in the journal Nanoscale.

Previous studies show that nanomaterials with luminescence in the short-wave infrared (SWIR) region are of special interest for biological research and medical diagnostics because of favorable tissue transparency and low autofluorescence backgrounds in that region. Single-walled carbon nanotubes show well-known sharp SWIR spectral signatures and therefore have potential for noninvasive detection and imaging of cancer tumours, when linked to selective targeting agents such as antibodies. However, such applications face the challenge of sensitively detecting and localizing the source of SWIR emission from inside tissues.  The current study shows that single-walled carbon nanotubes naturally fluoresce at SWIR wavelengths when excited by visible light, whilst a highly sensitive detector called an InGaAs (indium gallium arsenide), a highly sensitive detector reads faint signals from nanotubes up to 20 millimeters deep in simulated tissue.

The current study utilises a small optical probe mounted following a computer-programmed pattern to take readings from the embedded flourescent nanotubes at points spaced a few millimeters apart.  Results show that SWIR emission is gauged at points on the surface by a scanning fibre optic probe leading to an InGaAs spectrometer or a spectrally filtered InGaAs avalanche photodiode detector. Data findings show that because of water absorption, attenuation of the nanotube fluorescence in tissues is strongly wavelength-dependent.

Results show that the nanotube-probe distance is gauged by analysing differential changes in the measured nanotube emission spectra. Data findings show that nanotube fluorescence can be clearly detected through at least 20 mm of tissue phantom, and the 3D locations of embedded nanotube test samples are found with sub-millimeter accuracy at depths up to 10 mm. The team state that the method can also distinguish and locate two embedded nanotube sources at distinct positions

The team surmise they have used an unusually sensitive detector that hasn’t been applied to this sort of work before and they have detected and localized emission from very small concentrations of nanotubes inside biological tissues. For the future, the researchers state that this has potential applications in medical diagnosis.

Source: Rice University