New methods for the preparation of (bio)sensor surfaces : Molecular gradients and mixed monolayers containing oligo(ethylene glycols)

Alkanethiols form very close-packed and well ordered self-assembled monolayers on gold surfaces. The simple preparation of the organo-metal interface and the possibility to tailor the ƒç-functional group of the thiol individually for each target makes it attractive for a variety of applications. In the recent years many biosensors, for example affinity sensors, DNA chips and array systems, have been developed, which include a thiol sublayer for the covalent binding of receptor molecules. One important problem that must be avoided in biosensor design is non-specific adsorption of (bio)molecules on the sensing surface. Therefore, creating an optimal surface or basis layer is a major goal in biosensor applications. Two main directions for the preparation of thiol basis layers are described in this paper: 1) mixed monolayers and 2) molecular gradients. Oligo(ethylene glycol) terminated thiols (Eg4, Eg6 = HS-(CH2)15-CONH-(CH2)2-O)4,6-H), mercaptooctadecane (MOD), 16-mercaptohexadecan-1-ol (MHD), 16-mercaptohexadecanoic acid (MHA) and 16-mercaptohexadecan-1-amine (MDA) were chosen as model thiols for the investigations. All gold surfaces were cleaned using the TL1 procedure (NH3:H2O2:H2O 1:1:5 at 80¢XC). FT-IR spectroscopy, ellipsometry, impedance and contact angle measurements were used to characterize the monolayers. The combination of optical and electrochemical methods allows detailed statements about quality, structure and stability of the organic layer. The infrared reflection-absorption spectra were recorded at room temperature on a Bruker IFS 66, system equipped with a grazing angle (85o) infrared reflection accessory and a liquid-nitrogen-cooled MCT detector. The measurement chamber was continuously purged with nitrogen gas during the measurements. The acquisition time was around 10 min at 2 cm-1 resolution. A spectrum of a deuterated hexadecanthiolate (HS-(CD2)15-CD3) was used as reference