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Today, carbon nanotubes manufacturers as well as users such as molecular electronics, nanomedicine, nano-biotechnology and similar industries are facing a major challenge: lack of length uniformity of carbon nanotubes in mass production. An effective solution to this major issue is the use of a length-based ladder. We are, for the first time, presenting such a valuable tool to determine the length purity. Our length-based carbon nanotubes ladder, containing a series of carbon nanotubes markers with different lengths, is made based on three combined techniques - bio-conjugation, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and silver staining. Creating an indicator using conjugation of a biomolecule with carbon nanotubes to make a carbon nanotubes ladder is a novel idea and a significant step forward for length-based carbon nanotubes separation. The very sensitive silver staining technique allows a precise visualization and quantification of the gel. This ladder with a pending patent by Northeastern University is an effective quality control tool when bulk quantities of nanotubes with a desirable length are manufactured.

Parametric separation of carbon nanotubes, especially based on their length is a challenge for a number of nano-tech researchers. We demonstrate a method to combine bio-conjugation, SDS-PAGE, and silver staining in order to separate carbon nanotubes on the basis of length. Egg-white lysozyme, conjugated covalently onto the single-walled carbon nanotubes surfaces using carbodiimide method. The proposed conjugation of a biomolecule onto the carbon nanotubes surfaces is a novel idea and a significant step forward for creating an indicator for length-based carbon nanotubes separation. The conjugation step was followed by SDS-PAGE and the nanotube fragments were precisely visualized using silver staining. This high precision, inexpensive, rapid and simple separation method obviates the need for centrifugation, additional chemical analyses, and expensive spectroscopic techniques such as Raman spectroscopy to visualize carbon nanotube bands. In this method, we measured the length of nanotubes using different image analysis techniques which is based on a simplified hydrodynamic model. The method has high precision and resolution and is effective in separating the nanotubes by length which would be a valuable quality control tool for the manufacture of carbon nanotubes of specific lengths in bulk quantities. To this end, we were also able to measure the carbon nanotubes of different length, produced from different sonication time intervals.