Dmitry S. Koktysh

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.

Fast energy transfer in layer-by-layer assembled CdTe nanocrystal bilayers

We report on efficient resonant energy transfer in bilayers of water-soluble CdTe quantum dots. The bilayers of CdTe nanocrystals of two different sizes capped by short-chain thiols were formed by layer-by-layer assembly. Temporally and spectrally resolved fluorescence spectroscopy reveals spectral diffusion of the fluorescence signal for quantum dots within one layer as well as rapid (254 ps) energy transfer from layers of small dots to layers of larger dots, which is fast for nanocrystal pairs. Subspecies within the inhomogeneous distribution of donor nanocrystals even show energy transfer rates of (134u2009ps)−1 due to a large spectral overlap with acceptor nanocrystals.

Layer-By-Layer Assembly of Collagen Thin Films: Controlled Thickness and Biocompatibility

The extracellular matrix molecular collagen is the one of the most widely utilized scaffolding materials in tissue engineering. However, obtaining uniform bioactive collagen films in the nanoscale range and precisely controlling the physical and chemical properties of these biological films is still a challenge for biomedical engineering. Layer-by-layer assembly, i.e., sequential adsorption of oppositely charged macromolecular species, is a powerful new film preparation technique that can be applied to the design of versatile biomaterials, with well-controlled interfacial, mechanical and biological functions. To demonstrate the feasibility of biomaterial design by means of layer-by-layer assembly, type-I collagen thin films were prepared by using this technique with poly(styrene) sulfonate as a partner polyelectrolyte. The gradual build-up of the collagen films was confirmed by UV-vis spectroscopy and ellipsometry, while their surface morphology was assessed by atomic force microscopy. The thickness of the collagen layers can be changed by increasing the number of bilayers adsorbed with an increment of 13 nm. It was found that the layer-by-layer assembled collagen scaffolds can support the attachment and growth of C2C12 myoblast cells and PC12 pheochromocytoma cells. Accurate thickness control and compatibility with nerve cell precursors indicate the utility of layer-by-layer assembled films in neuroprosthesis.

II-VI semiconductor nanocrystals in thin films and colloidal crystals

Semiconductor nanocrystals whose optical properties are largely determined by the quantum confinement effect are currently being extensively studied in both physics and chemistry. Highly luminescent thiol-capped CdTe and HgTe nanocrystals with desirable sizes ranging from less than two to approximately 8 nm have been recently synthesized in aqueous solutions by a wet chemical route. They were used for the preparation of composite multilayer thin films by the layer-by-layer (LBL) deposition technique. Films containing luminescent nanocrystals were made both on planar substrates and on submicron-sized monodisperse polystyrene spheres. Alternatively, nanocrystals have been incorporated as cores into silica spheres of desirable sizes. Composite nanocrystal/silica and core-shell latex/nanocrystal spheres have been used as building blocks for 3-D colloidal photonic crystals.