Imprinted Polymers AND Carbon Nanotubes? Is That Even Allowed?

I ran across a paper a few days ago that caught my eye for two reasons.  The first reason is that the paper deals with applications of molecularly imprinted polymers, and applies molecular imprinting to a sensing scheme that I hadn't previously considered.  The second reason for my keen interest in this paper is that the analyte that the authors are trying to detect with their sensor is a cardiac protein called Troponin T (TnT), which is what much of my Masters research focused on - developing a fluorescence-based immunosensor for TnT.  The paper, titled Artificial antibodies for troponin T by its imprinting on the surface of multiwalled carbon nanotubes: Its use as sensory surfaces (subscription is likely required) is published in the October 2011 edition of Biosensors and Bioelectronics, which is one of my favorite journals.

I should probably briefly explain why TnT is important.  Cardiac TnT is part of a greater, multi-protein Troponin complex, and is found exclusively in cardiac muscle tissue.  When necrosis of cardiac tissue occurs, a hallmark of heart attack, the Troponin complex fragments into its three individual subunits (TnI, TnC, and TnT).  These subunits then become bloodborne and can be detected in the blood using standard laboratory testing.  Under normal, healthy conditions, the subunits are absent from the blood.  The current laboratory testing techniques for detection of TnT to diagnose heart attack are unfortunately somewhat lengthy, and there is a great deal of interest in developing a much more rapid diagnostic test that is both accurate and sensitive.  This was the goal of a large part of my Masters research.  And obviously, this is the goal of the authors of the paper described herein.

I'm not going to go into depth on the technical details of the sensing system that the authors developed, but it's important to have a general understanding of how the thing works.  The researchers started with multi-wall carbon nanotubes (MWCNTs) and chemically modified them, allowing them to bind covalently to TnT when it was introduced.  The MWCNTs with bound TnT were then coated in a functional polymer capable of forming a number of weak noncovalent bonds with the TnT.  Then, when the TnT was removed using a chemical extraction procedure, a molecularly imprinted binding site for TnT was left behind.  I've described this process previously, and it is diagrammed in the figure below, which is from the paper.

The MWCNTs coated in molecularly imprinted polymer were then suspended in PVC and coated onto a wire electrode.  The electrode was then exposed to a solution of TnT and voltage measurements were used to determine TnT binding.

Although I found this paper quite fascinating and an interesting use of molecularly imprinted materials, I have to say that the methods the authors used are somewhat flawed.  The most glaring problem that I noticed was the extraction procedure to remove the TnT template.  The authors claim that the TnT was removed by introducing the MWCNTs with the imprinted polymer to a solution of oxalic acid.  Now, I'm not all that familiar with oxalic acid, but I'm not entirely sure that this extraction procedure is robust enough to disrupt the covalent amide bond that was used to link the TnT to the surface of the MWCNT.  This would explain why the authors observed large peaks that are normally associated with proteins when x-ray spectroscopy measurements were taken.  Another fairly major problem is that the modified MWCNTs were embedded in PVC for the potential measurements.  Although TnT is not an abnormally large protein, it is a protein nonetheless, and is therefore a substantially sized molecule in the grand scheme of things.  Due to its size, it is unlikely that TnT would be able to infiltrate the PVC matrix.  That would mean only those MWCNTs that are exposed at the surface of the PVC layer would be able to effectively bind to TnT, rendering the rest of the MWCNTs - probably a majority of them - inaccessible to the analyte.


All in all, the idea behind this project is pretty cool.  It was poorly executed, though, and I have difficulty believing that they actually got the results that they claim to have gotten.  I can think of a number of ways to go about developing a carbon nanotube-based electrochemical sensor that uses molecular imprinting to impart selectivity, and I can honestly say that I wouldn't have decided on the method described here.