ANALYST FULL PAPER THE www.rsc.org/analyst Synthetic receptors as sensor coatings for molecules and living cells Franz L. Dickert,* Oliver Hayden and Konstantinos P. Halikias Institute of Analytical Chemistry, Vienna University, Waehringerstrasse 38, A-1090, Vienna, Austria. E-mail: Franz.Dickert@univie.ac.at Received 11th December 2000, Accepted 2nd May 2001 First published as an Advance Article on the web 18th May 2001 Non-covalent molecularly imprinted polymers are applied as sensitive coatings to planar waveguides and mass-sensitive devices for the selective detection of various groups of analytes in the gaseous and aqueous phases. Cavity imprinting in the bulk of the sensor material as well as surface imprinting techniques are used to enrich analytes ranging from sub-nanometres to micrometres in analyte size. The coated devices provide sensitivity to e.g. polycyclic aromatic hydrocarbons, xanthine derivatives, complex coffee samples and whole microorganisms. Introduction The introduction of molecularly imprinted polymers (MIPs) for sensor applications attracts increasing attention. 1,2 Molecular imprinting for chemical sensors 3,4 and solid phase extraction 5,6 represent the most promising fields for industrial implementa- tion. Our main interest in non-covalent molecular imprinting 7 for chemosensors is directed towards the capabilities of a low cost and highly versatile procedure for demanding areas concerning long-lasting serviceability of the sensor device and resistance against chemical and physical stress. The imprinting technology is favourable in matters of availability/diversity of monomers, coating procedures and nearly unlimited adaptabil- ity to the type and site of application. In the meantime, we have been able to apply our MIP coated chemosensors to a vast range of analytes, such as vapours, 8,9 complex mixtures 10 and most recently microorganisms. Our aim is to develop highly sensitive chemical sensors with selective enrichment capabilities, especially in the aqueous phase, 11 with good reversibility and suitable response times to allow effortless on-line monitoring in gaseous and liquid phases. Here, we mainly review our latest results and present preferably unpublished data of our research work. The influence of templates for sensitivity and selectivity tuning of sensor layers is demonstrated in the case of PAH (polycyclic aromatic hydrocarbons) imprinted polyurethane. Compared to imprinting with a single template, the combination of two PAHs in a binary template mixture can drastically improve the selectivity for a distinct PAH analyte. The imprinting with complex template mixtures enables us to characterize multi-component analytes as a whole, such as automotive oils. 10 According to this strategy, “fingerprints” of these analytes are formed with the polymer matrix. This technique allows quality control measurements not only with a sensor array but even with a single MIP coated sensor device. A further innovative area of synthetic receptors is the usage of micrometre structures, such as bacteria 12 or yeasts. We have attained regular patterns of imprinted sites on the surface of polymers with a stamping procedure in a single operation. Both the imprinting of bulk material with template mixtures and the surface imprinting process for chemical sensors are illustrated in Fig. 1. The coating process is performed preferentially as on- chip polymerization. In Fig. 1 a 428 MHz shear transverse wave resonator (STW) with two channels is shown, which is briefly described in the following section. Experimental The chemicals of analytical grade were used as received from Fluka and Merck. The radical inhibitor from divinylbenzene was exctracted by a 1 M KOH solution and dried over Na 2 SO 4 prior to usage. Active dry yeasts and lyophilized bacteria were purchased from Uvaferm and Sigma. Cell concentrations were determined using a Neubauer improved erythrometer. The freshly prepared cell suspensions for the measurements showed no significant agglomeration of microorganisms (controls performed with light microscopy). Polyurethane layers for PAH MIPs Bisphenol A (2,2-bis(4-hydroxyphenyl)propane), 30% of cross- linker phloroglucinol and p,pA-diisocyanatodiphenyl-methane (isomer mixture with 30% triisocyanate) were dissolved in THF to form a stoichiometric solution. Template PAHs were added Fig. 1 Dual 428 MHz shear transverse wave resonators with on-chip polymerized MIP coatings. Small analytes are detected through enrichment in the polymer bulk. Typical layer thicknesses are in the range of 100 nm to 2 μm. Large biopolymers or whole cells can only be detected by selective surface adhesion due to a hindered diffusion into the bulk. Regular patterns of imprinted sites are preferred for a high number of packed imprinted sites. For a maximum in flexibility the imprinting process is preferentially performed non-covalently. Spin coating procedures ensure excellent homogenous layers. This journal is © The Royal Society of Chemistry 2001 766 Analyst, 2001, 126, 766–771 DOI: 10.1039/b009893k