RESUMEN
The distance dependence and kinetics of the heterogeneous electron transfer (ET) reaction for the redox protein azurin adsorbed to an electrode modified with a gold nanoparticle film are investigated using cyclic voltammetry. The nanoparticle films are comprised of nonaqueous nanoparticles, known as monolayer-protected clusters (MPCs), which are covalently networked with dithiol linkers. The MPC film assembly serves as an alternative adsorption platform to the traditional alkanethiolate self-assembled monolayer (SAM) modified electrodes that are commonly employed to study the ET kinetics of immobilized redox proteins, a strategy known as protein monolayer electrochemistry. Voltammetric analysis of the ET kinetics for azurin adsorbed to SAMs of increasing chain length results in quasi-reversible voltammetry with significant peak splitting. We observed rate constants (k degrees (ET)) of 12-20 s(-1) for the protein at SAMs of shorter alkanethiolates that decays exponentially (beta = 0.9/CH(2) or 0.8/A) at SAMs of longer alkanethiolates (9-11 methylene units) or an estimated distance of 1.23 nm and is representative of classical electronic tunneling behavior over increasing distance. Azurin adsorbed to the MPC film platforms of increasing thickness results in reversible voltammetry with very little voltammetric peaks splitting and nearly negligible decay of the ET rate over significant distances up to 20 nm. The apparent lack of distance dependence for heterogeneous ET reactions at MPC film assemblies is attributed to a two-step mechanism involving extremely fast electronic hopping through the MPC film architecture. These results suggest that MPC platforms may be used in protein monolayer electrochemistry to create adsorption platforms of higher architecture that can accommodate greater than monolayer protein coverage and increase the Faradaic signal, a finding with significant implications for amperometric biosensor design and development.
Asunto(s)
Azurina/química , Nanopartículas del Metal/química , Adsorción , Azurina/metabolismo , Proteínas Bacterianas/química , Proteínas Bacterianas/metabolismo , Ácidos Carboxílicos/química , Electroquímica , Transporte de Electrón , Proteínas Inmovilizadas/química , Proteínas Inmovilizadas/metabolismo , Cinética , Modelos Moleculares , Conformación Proteica , Pseudomonas aeruginosa , Electricidad EstáticaRESUMEN
We have used a combinatorial gradient technique to map precisely how the terrace structure and microdomain lattice alignment in a thin film of a sphere-forming diblock copolymer are affected by both the thickness of the copolymer film and the height of a series of parallel step edges fabricated on the substrate. We find that for film thicknesses slightly incommensurate with integer numbers of sphere layers, the step edges act as nucleation sites for regions with one more or one fewer layers of spheres. We also find that for our system, the hexagonal lattice formed by a single layer of spheres on the low side of a step edge is aligned along the direction of the step edge only where the film on the high side is sufficiently thin to support only a wetting layer of copolymer material. This work will guide the tuning of film thickness and step height in future studies and applications of graphoepitaxy in block copolymer films.