Biomaterials by Design: Layer-By-Layer Assembled Ion-Selective and Biocompatible Films of TiO2 Nanoshells for Neurochemical Monitoring

Titania nanoshells with an external diameter of 10–30 nm and a wall thickness of 3–5 nm were prepared by dissolving the silver cores of Ag@TiO2 nanoparticles in a concentrated solution of ammonium hydroxide. The nanoshells were assembled layer-by-layer (LBL), with negatively charged poly(acrylic acid) (PAA) to produce coatings with a network of voids and channels in the interior of the film. The diameter of the channels in the titania shells was comparable to the thickness of the electrical double layer in porous matter (0.3–30 nm). The prepared nanoparticulate films demonstrated strong ion-sieving properties due to the exclusion of some ions from the diffuse region of the electrical double layer. The permeation of ions could be tuned effectively by the pH and ionic strength of a solution between “open” and “closed” states. The ion-separation effect was utilized for the selective determination of one of the most important neurotransmitters, dopamine, on a background of ascorbic acid. Under physiological conditions, the negative charge on the surface of TiO2 facilitated the permeation of positively charged dopamine through the LBL film to the electrode, preventing the access of the negatively charged ascorbic acid. The deposition of the nanoshell/polyelectrolyte film resulted in a significant improvement to the selectivity of dopamine determination. The prepared nanoshell films were also found to be compatible with nervous tissue secreting dopamine. Although the obtained data demonstrated the potential of TiO2 LBL films for implantable biomedical devices for nerve tissue monitoring, the problem of electrode poisoning by the by-products of dopamine reduction has yet to be resolved.