Mass-sensitive detection of cells, viruses and enzymes with artificial receptors Oliver Hayden, Roland Bindeus, Claudia Haderspo ¨ck, Karl-Ju ¨rgen Mann, Barbara Wirl, Franz L. Dickert * Institute of Analytical Chemistry, Vienna University, Waehringerstrasse 38, A-1090 Vienna, Austria Abstract Synthetic polymer receptors for the online monitoring of bioanalytes are formed directly onto quartz crystal microbalances using surface imprinting techniques. The molded materials are capable of enriching whole cells, viruses and enzymes on the sensor layer surface. Enzyme imprinted polymer layers are also effective as nucleation site for the induction of protein crystallization. Differential measurements are done with a single piezocrystal having two screen-printed gold electrodes for a sensitive and a reference channel. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Quartz crystal microbalance; Molecular imprinting; Cell; Virus; Enzyme; Crystallization 1. Introduction Self-organized materials with specific properties are a matter of intense research in various research fields. One major application area of those so-called smart or advanced materials is chemical sensing. Templating techniques, such as molecular imprinting (MIP; molecularly imprinted poly- mers), are a promising approach to form receptor sites in the bulk [1,2] for e.g. small organic molecules and on the surface [3]. We have shown in recent publications that yeast imprinted polyurethane sensor layers in combination with microbalances can exclusively detect yeast cells without cross-selectivities to bacteria [4]. The concept of cell imprinting is based on a stamping technique, where tem- plating cells are pressed with little force into a polymerizing sensor layer. A honeycomb patterned surface remains after the polymerization, which recognizes cells depending on geometrical fit and chemical interaction [5]. The detection of viruses and bacteria is of great interest, since both groups of analytes are a matter of big concern regarding the contamination of water or their hazardous impact as biological weapons. However, virus and bacteria detection aregenerally time-consuming tasks [6–8].Furthermore,weare interested in expanding the concept of bioimprinting to enzymes and to mammalian cells, which is crucial due to their lower mechanical stability compared to microorganisms. For chemical sensors the enzyme is still too large for bulk imprint- ing and surface imprinting has to be done again. Here, we present theversatile concept of surface imprinted sensor layers with stamping techniques, which extends the chemical sensing applications of MIPs in aqueous phases to these nano- and micrometer sized groups of bio-analytes. 2. Experimental 2.1. Dual QCMs and oscillator measurements Gold electrodes were screen printed onto AT-cut 10 MHz quartz blanks and subsequently burned in at 400 8C. Elec- trodes facing the aqueous phase have 50% larger diameters than the electrodes oriented to the gas phase [9]. QCMs were mounted into thermostated fluid cells. Electrodes were connected to self-made oscillator circuits by soldering silver wires to the QCM rims of coppered gold electrode contacts. Electrodes in contact with the liquid are grounded as well as the metallic liquid cell, which forms an faradaic cage. The frequencies are recorded with a HP 53131A and data acquisition is done via a HP-IB bus to a computer. 2.2. Monomers and analytes Methacrylic acid was distilled to remove the stabilizer. Radical inhibitors in styrene and divinylbenzene were extracted with 2 M KOH and the monomers dried over NaSO 4 . Other reagents were used as received. Blood sam- Sensors and Actuators B 91 (2003) 316–319 * Corresponding author. Tel.: þ43-1-4277-52317; fax: þ43-1-4277-9523. E-mail address: franz.dickert@univie.ac.at (F.L. Dickert). 0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00093-5