Fluorescence interference-contrast microscopy on oxidized silicon using a monomolecular dye layer

A silicon chip is covered by a monomolecular film of a fluorescence dye with silicon dioxide used as a spacer. The fluorescence depends on the distance of the dye from the silicon. The modulation of the intensity is described quantitatively by an optical theory which accounts for interference of the exciting light and of the emitted light. The effect is used to obtain a microscopic picture of the surface profile with a precision of a few Angströms. The perspectives for an application in wet systems such as neuron-silicon junctions and lipid membranes on silicon are pointed out. PACS: 42.80; 68.55; 78.65 A living cell in close contact to a surface forms a twodimensional electrical cable with the cell membrane as a capacitive coat and the narrow cleft between membrane and surface as a resistive core [1, 2]. The electrical properties of that cable can be studied by field-effect transistors if the cell is attached to a silicon chip [3, 4, 5]. In the future such hybrid systems of cells and oxidized silicon may allow the assembly of integrated circuits of nerve cells and silicon microstructures and the development of technical devices such as drug sensors. A critical point in the investigation of cell-silicon junctions is the unknown distance of the membrane from the surface which may be in the range of tens of nanometers [5]. Here we propose a method which is suitable to solve this problem. We take advantage of the modulation of fluorescence of dye molecules in front of the reflecting surface of silicon. Distance dependent excitation and emission is known since the classical experiments of Wiener [6], Drude and Nernst [7], Selenyi [8] and Kossel [9]. It was studied in particular by Kuhn and Drexhage in their elegant experiments with fluorescent dyes on metal mirrors spaced by Langmuir-Blodgett films [10, 11, 12]. The effect was observed also in soap lamellas [13]. A change of fluorescence *correspondence author was noted on oxidized silicon [14, 15] and assigned to optical interference [15]. In the present paper we investigate the fluorescence of a cyanine dye embedded in a Langmuir-Blodgett film which is spaced from silicon by a layer of silicon dioxide (Fig. 1). The observations are made in a microscope. We describe the modulation of fluorescence intensity by classical optics. On that basis we show that changes of fluorescence intensity on a microscopic scale can be used to determine small changes of the surface profile with a precision of a few Angströms. At the end the perspectives for an application in cell-silicon junctions are pointed out.