Protein Binding by Functionalized Multiwalled Carbon Nanotubes Is Governed by the Surface Chemistry of Both Parties and the Nanotube Diameter

The protein binding propensity of nanoparticles determines their in vivo toxicity and their fate to be opsonized and cleared by human defense systems. In this work, protein-binding mechanisms of pristine and functionalized multiwalled carbon nanotubes (f-MWNTs) were investigated by varying f-MWNTs' diameters, nanotube surface chemistry, and proteins using steady-state and time-resolved fluorescence, and circular dichroism (CD) spectroscopies. The f-MWNTs with a larger diameter (∼40 nm) generally exhibited stronger protein binding compared to those with a smaller diameter (∼10 nm), demonstrating that the curvature of nanoparticles plays a key role in determining the protein binding affinity. Negative charges or steric properties on f-MWNTs enhanced binding for some proteins but not others, indicating that the electrostatic and stereochemical nature of both nanotubes and proteins govern nanotube/protein binding. Protein fluorescence lifetime was not altered by the binding while the intensity was quenched indicating a static quenching through complex formation. The binding-induced conformational changes were further confirmed by CD studies.