Biosensors and Bioelectronics 22 (2007) 912–919 Lipoate-based imprinted self-assembled molecular thin films for biosensor applications Kirsi Tappura ∗ , Inger Vikholm-Lundin, Willem M. Albers VTT Technical Research Centre of Finland, P.O.Box 1300, FIN-33101 Tampere, Finland Received 7 November 2005; received in revised form 26 January 2006; accepted 15 March 2006 Available online 25 April 2006 Abstract Lipoate derivatives were used for the formation of imprinted self-assembled molecular thin films for the recognition of morphine. A large collection of lipoate derivatives was screened by molecular dynamics simulations in various solvents. A set of ligands showing favourable interactions with morphine in aqueous environment was selected for synthesis. Morphine-imprinted layers were then produced on gold substrates in mixed monolayers with morphine added as the template. The binding of ligands and morphine to gold, as well as the association/dissociation of morphine to the formed layers were studied with Surface Plasmon Resonance. Imprinted factors were highly variable and were dependent on ligand/morphine mixing ratio, lipoate derivative and monolayer preparation method. The imprinted factors were as high as 100 and 600 for one of the ligands. The results show that the simulations are able to provide correct information of the relative strengths of the molecular interactions between the ligand and morphine molecules in different solutions. The liquid phase simulations are, however, not able to predict the imprinted factors (i.e. distinguish between specific and non-specific binding), because the specificity is not formed before self-assembly on the surface. © 2006 Elsevier B.V. All rights reserved. Keywords: Self-assembly; Molecular imprinting; Morphine; Surface plasmon resonance; Lipoates; Molecular dynamics simulations 1. Introduction Self-assembled monolayers have been widely used for the construction of biosensors (Chaki and Vijayamohanan, 2002; Ulman, 1996). However, due to the relatively high instability of biomolecules incorporated in the biosensor devices, molecularly imprinted polymers (MIPs) have been intensively investigated as more stable, selective sensor materials (Wulff, 1995; Haupt and Mosbach, 2000). MIPs have already been used in some prototype gas sensor devices (Jakoby et al., 1999; Dickert et al., 1999) and liquid-phase sensors (Blanco-L´ opez et al., 2003; Kriz and Mosbach, 2005; Zhang et al., 2005; Sergeyeva et al., 1999). Computational methods are nowadays used successfully for the design and optimisation of MIPs (Chianella et al., 2002). Imprinted self-assembled monolayers (i-SAMs) would be con- ceptually more straightforward to fabricate, because polymer- ization steps can be omitted. The i-SAM approach can also be considered as a more suitable method for applying thinner recep- ∗ Corresponding author. Tel.: +358 20 722 3317; fax: +358 20 722 3319. E-mail address: kirsi.tappura@vtt.fi (K. Tappura). tor layers and, thus, for producing a more sensitive response with surface sensing methods, such as surface plasmon resonance, SPR and surface acoustic wave devices (Gabl et al., 2004). However, only a few reports have appeared so far on i-SAMs. Jacob Sagiv (1979) first reported the production of imprinted self-assembled silane layers, and more recently Piletsky et al. (1999) and Mirsky et al. (1999) described imprinted self- assembled monolayers for the detection of cholesterol and barbi- turic acid, respectively. The imprinting of cholesterol was based on hydrophobic interactions with hexadecylmercaptan (Piletsky et al., 1999). Cholesterol was incorporated in the self-assembled monolayer in a more loosely bound fashion, and could be washed out with ethanol, leaving a two-dimensional print in the layer. Barbituric acid and thiobarbituric acid were used in a mixed monolayer with dodecylmercaptan, where thiobarbituric acid assumed an underlying mold for the recognition of barbituric acid (Mirsky et al., 1999). The main idea of the present work is to introduce more ver- satile, bifunctional self-assembling ligands (building blocks), which in the presence of the template are capable of forming imprinted self-assembled layers. Instead of using monofunc- tional thiol compounds, bifunctional self-assembling ligands 0956-5663/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2006.03.014