Site-specific adsorption of metallic and biological nanoparticles on nanostructured silicon surfaces

Abstract

The (photo)electrochemical preparation of nanostructures on single crystalline Si surfaces is described and surfaces are characterized by tapping mode atomic force microscopy (TM-AFM), Kelvin probe microscopy (KPM), high-resolution electron energy loss spectroscopy (HREELS), and synchrotron radiation photoelectron spectroscopy (SRPES). The H-terminated Si(111) surface and the step-bunched Si surface that exhibits multiatomic bilayer (BL) steps are prepared. HREELS demonstrates the low contamination levels achieved with the used in-system combined electrochemistry/surface analysis apparatus. With a miscut in the direction, the surfaces prepared show a zigzag structure of the atomic terraces. These unique features provide the substrate for the (electro)deposition of platinum and for the immobilization of enzymes. Pt deposition, of relevance for the development of photovoltaic or fuel-generating solar cells, occurs predominantly at the edges of steps. On step-bunched surfaces, where KPM shows negative charging along the step edges and SRPES evidences the presence of an accumulation layer, the Pt nanoislands are considerably smaller than on H-terminated surfaces that are in flat band situation before deposition. SRPES analysis of the chemical and electronic condition after electrodeposition shows silicon oxide formation on both surfaces, suboxidic species, and the typical asymmetric Pt 4f core level lines. The results are discussed based on the details of the Pt deposition process at Si surfaces. As biomolecule, the enzyme reverse transcriptase (RT) of the avian myeloblastosis virus (AMV) was used. Immobilization is observed in TM-AFM experiments at the negatively charged step edges of the step-bunched surface. It is attributed to a superposition of attractive van der Waals and electrostatic interactions. The latter is related to the rather large Debye length of the carrier solution (∼4 nm) and the overall positive charge of the RT where the pH of the carrier solution (pH 7) is smaller than that for the isoelectric point (IP) of 8.3.