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This work shows results of a theoretical survey, based on molecular dynamics simulation, of potential applicability of doxorubicin coadsorption with various dyes molecules on/in carbon nanotubes as a drug delivery system. The central idea is to take advantage of the dyes charge distribution change upon switching the pH of the environment from neutral (physiological 7.4) to acidic one (∼5.5 which is typical for tumor tissues). This work discusses results obtained for four dye molecules revealing more or less interesting behavior. These were bromothymol blue, methyl red, neutral red, and p-phenylenediamine. All of them reveal pKa in the range 5-7 and thus will undergo protonation in that pH range. We considered coadsorption on external walls of carbon nanotubes and sequential filling of the nanotubes inner hollow space by drug and dyes. The latter approach, with the application of neutral red and p-phenylenediamine as blockers of doxorubicin, led to the most promising results. Closer analysis of these systems allowed us to state that neutral red can be particularly useful as a long-term blocker of doxorubicin encapsulated in the inner cavity of (30,0) carbon nanotube at neutral pH. At acidic pH we observed a spontaneous release of neutral red from the nanotube and unblocking of doxorubicin. We also confirmed, by analysis of free energy profiles, that unblocked doxorubicin can spontaneously leave the nanotube interior at the considered conditions. Thus, that system can realize pH controlled doxorubicin release in acidic environment of tumor tissues.

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