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Keywords:

  • bioelectronics;
  • cytochrome c;
  • plasmon coupling;
  • silica coatings;
  • surface enhanced Raman spectroscopy

Abstract

A novel Ag–silica–Au hybrid device is developed that displays a long-range plasmon transfer of Ag to Au leading to enhanced Raman scattering of molecules largely separated from the optically excited Ag surface. A nanoscopically rough Ag surface is coated by a silica spacer of variable thickness from ∼1 to 21 nm and a thin Au film of ∼25 nm thickness. The outer Au surface is further functionalized by a self-assembled monolayer (SAM) for electrostatic binding of the heme protein cytochrome c (Cyt c) that serves as a Raman probe and model enzyme. High-quality surface-enhanced resonance Raman (SERR) spectra are obtained with 413 nm excitation, demonstrating that the enhancement results exclusively from excitation of Ag surface plasmons. The enhancement factor is estimated to be 2 × 104–8 × 103 for a separation of Cyt c from the Ag surface by 28–47 nm, corresponding to an attenuation of the enhancement by a factor of only 2–6 compared to Cyt c adsorbed directly on a SAM-coated Ag electrode. Upon immobilization of Cyt c on the functionalized Ag–silica–Au device, the native structure and redox properties are preserved as demonstrated by time- and potential-dependent SERR spectroscopy.