Surface Modification
Keywords: surface modification, organic monolayers, sensors, nanowires
Introduction
Within this research theme we study functionalized Si–C linked monolayers for the development of chemical sensors and biosensors with a label-free, electrical read-out. We envisage two approaches: frequency-dependent capacity-voltage measurements of monolayers and electrical conductance measurements on modified silicon nanowires (SiNWs).
Si – C Linked Organic Monolayers on Silicon Surfaces
Organic molecules having an unsaturated bond can be chemically attached on hydrogen-terminated silicon surfaces via a hydrosilylation reaction, resulting in Si-C linked monolayer (Scheme 1). This type of monolayers has continued to attract attention ever since the first reports of Linford and Chidsey in 1993 and 1995.1,2
Scheme 1. Schematic representation of the etching of a native oxide-covered silicon surface yielding an oxide-free hydrogen-terminated surface, followed by the reaction with 1-alkenes, resulting in the formation of a Si–C linked monolayer.
Si-C linked monolayers are highly stable, also at elevated temperatures and under highly acidic conditions. They also have very good electrical insulating properties; even much better than silicon dioxide insulators of the same thickness.3 Functionalization of the monolayer is also possible.
References
1) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631; Linford, M.R.;Fenter, P.; Eisenberger, P.M.; Chidsey C.E.D. J. Am. Chem. Soc. 1995, 117, 3145
2) Buriak Chem. Rev. 2002, 102, 1272. More recent reviews: Aswal et al. Anal. Chim. Acta 2006, 568, 84 (molecular electronics), Stutzmann et al. Phys. Stat. Sol.(a) 2006, 203, 3424 (biofunctionalization)
3) Faber, E.J.; de Smet, L.C.P.M.; Olthuis, W.; Zuilhof, H.; Sudhölter, E.J.R., Bergveld, P., van den Berg, A. ChemPhysChem 2005, 6, 2153
Capacitive Measurements
A structure consisting of a silicon substrate and an attached organic monolayer can be considered as an electrical capacitor consisting of two parallel capacitor plates. The electrical contacts in the Electrolyte-Insulator-Silicon (EIS) structure for capacitive measurements (Figure 1) are on the back side of the modified silicon substrate (working electrode, WE) and the reference electrode in the electrolyte solution, which is also in contact with the monolayer. Using the hydrosilylation chemistry as mentioned above, we aim to construct hybrid systems for capacitive measurements with a focus on bio-functionalized monolayers that are exposed to aqueous analyte solutions.
Figure 1. Schematic representation of an Electrolyte-Insulator-Silicon (EIS) structure with a back contact (BC), counter electrode (CE), and a reference electrode (RE).
Collaboration
Silicon Nanowires
In a silicon nanowire field effect transistor (SiNW FET) set-up the current through the wire can change upon the binding of a charged analyte (for a fixed voltage). The modification of the surface of the nanowire allows the possibility to facilitate specific binding to design nanowire-based sensors.4 In this project we study new approaches to (bio-)functionalize SiNW surfaces.
Figure 2. (left) Schematic cross section of a silicon nanowire field effect transistor, where D en S stand for Drain and Source, respectively, and (right) an EM picture of a silicon nanowire (courtesy of Prof. Cees van Rijn, Nanosens).
References to recent review articles on nanowire sensing
4) E.T. Carlen, A. van den Berg, Lab Chip 2007, 7, 19; C.M. Lieber, Z.L. Wang, MRS Bull.2007 32, 99; A.K. Wanekaya, W.Chen, N. V. Myung, A. Mulchandani, Electroanalysis 2006,18, 533; F. Patolsky, C. M. Lieber, Materials Today 2005, 8, 20
Collaboration
Prof. H. van der Zant, Molecular Electronics and Devices, Kavli Institute of Nanoscience, Delft University of Technology
