A highly sensitive carbon nanotube-based sensor capable of detecting multidrug-resistant myeloid leukaemia cells even when present at very low concentration of 10 cells per ml has been fabricated by Delhi-based researchers.
Vapour deposition
Using chemical vapour deposition technique, the team led by Prof. S.S. Islam from the Centre for Nanoscience and Nanotechnology at the Jamia Millia Islamia produced vertically aligned multi-walled carbon nanotubes. The carbon nanotubes were grown on a silicon wafer substrate. In order to miniaturise the sensor and make it flexible, moisture-resistant and stable even at 180 degree C, the researchers transferred the carbon nanotubes to a flexible substrate made of polyethylene terephthalate (PET).
As a first step to converting the carbon nanotubes into a sensor, the researchers functionalised the nanotubes. Nanotubes that are functionalised have better ability to bind to a leukaemia antibody, which in turn binds to the leukaemia cells present in the sample.
“We used vertically aligned carbon nanotubes to increase the surface area available for the antibodies to bind to them,” says Payal Gulati from Jamia Millia Islamia and first author of a paper published in the journal Sensors and Actuators B: Chemical. “As more antibodies get bound to carbon nanotubes, the sensitivity improves.”
The portions of the functionalised nanotubes that do not contain the antibody are blocked with bovine serum albumin to prevent the leukaemia cells from binding there.
Drug-resistant cells
Compared with drug-sensitive leukemic cells, multidrug-resistant leukaemia cells have better ability to bind to the antibody found on the nanotubes. “The expression of P-glycoprotein transporter present on leukaemia cell surface gets enhanced when the cell becomes multidrug-resistant,” says Gulati. So to make the cells multidrug-resistant, the researchers treated them with chemotherapy drug doxorubicin
A solution containing six different leukaemia cell populations ranging from 150 cells per ml to 15 million cells per ml were tested to assess the sensitivity. The carbon nanotubes are allowed to stand in a solution containing leukaemia cells for three hours at room temperature to allow sufficient time for the cells to bind to the antibody found on the nanotube surface.
Superior sensitivity
“We found the sensitivity of the sensor to be superior. It was able to detect multidrug-resistant leukaemia cells even when just 10 cells per ml were present,” says Prof. Islam.
The presence of leukaemia cells bound to the antibody is determined by measuring a drop in the conductivity of the carbon nanotubes. To measure changes in conductivity, three electrodes — carbon nanotubes as a working electrode, a counter electrode made of platinum and a reference electrode — are immersed in a electrolyte and a constant voltage is applied and a change in current is measured.
Leukaemia cells are insulating in nature. So the conductivity of carbon nanotubes reduces as the cells bind to the antibody. The more the concentration of cells bound to the antibody the greater is the reduction in the current flow.
“We were able to measure a change in current when just 10 leukaemia cells bind to the antibody present on the carbon nanotubes,” says Gulati.
The researchers found that there was negligible variation in current even after three weeks when the carbon nanotubes containing leukaemia cells are stored at 4 degree C in a buffer solution. “The response of the sensor remains almost unchanged for three weeks,” she says.
