698 Sen.wrs and Actuators B, 18-19 (1994) 698-703 Significance of mass and viscous loads discrimination for an AT-quartz blood group immunosensor L. Tessier, F. Patat, N. Schmitt and M. Lethiecq GIP Ultrasons, 2 Bis Bd Tomelk!, B.P. 3223, 37032 Tours Ceder (France) Y. Frangin D&artment de Chimie, UFR des Sciences et Techniques de I’lJnivmite’ de Tours, Part Gmmhnont, 37200 Tours (France) D. Guilloteau INSERM lJ316, Lnboratoire de Biophysique Mkdicale et Pharmaceutique, 2 Bd TonneliP,37032 Tours Cede* (France) Abstract An immunosensor based on a thickness shear mode (TSM) acoustic resonator has been fabricated and tested for ABO blood grouping. The resonant frequency f., and the quality factor Q,, of the loaded resonator are deduced from the electrical conductance spectrum measured by an impedance analyzer. An acoustic transmission line model is used to describe the acoustic immunosensor and the two resonance parameters, fn and Q., are shown to be related to the shear acoustic impedance of the loading material. Experimental results show the ability of the TSM immunosensor to discriminate between A and B groups. Furthermore, it is shown that red cells immobilized by immuno complexing induce significant viscous liquid changes with no additional rigid mass deposition. This is interpreted as an additional viscous load owing to the intraerythrocyte liquid. Introduction The extensive use of chemical and biological analyses in research and industrial applications has stimulated the development of piezoelectric acoustic wave sensors [l, 21. These devices are used to monitor mechanical and electrical changes occuring on their surfaces [3-51. In particular, thickness shear mode (TSM) resonators (AT-quartz) have been extensively used as micro bal- ances in vacuum deposition procedures [6], and as dynamic viscometers [7-91. TSM acoustic sensors have also been developed for various biological applications in both gas and liquid phases [lO-151. In previous investigations, the resonant frequency shift owing to molecular interactions at the sensor surface was in- terpreted as an additional rigid mass which was cal- culated using Sauerbrey’s equation [16]. Recently, it has been emphasized in the literature that mass de- position due to molecular binding is often accompanied by significant interfacial viscosity variations which also affect the AT-quartz resonance [17, 181. It is therefore necessary to measure simultaneously the viscous load and the rigid mass deposit. Accordingly, the purpose of this paper is to present a convenient method for monitoring viscous liquid and rigid mass changes with TSM sensors and to enhance the significance of mass and viscous load discrimination in the context of a blood group immuno-detector de- velopment. Experimental results for A and B groups are reported using the resonant frequency and the quality factor and then using the real and imaginary parts of the acoustic impedance of the loading material in order to exhibit the changes in inter-facial viscous load in contrast to rigid mass deposit. Theory The loaded TSM resonator A TSM resonance is characterized by its frequency fn (for the nth overtone) and its quality factor Q,,. Let us first derive a simple way how these values are modified when a semi-infinite medium, of acoustic impedance Zci =Z, +jZIm, is coupled to one side of the TSM resonator. The unloaded TSM resonator is a propagation line whose length is nh/2, where h is the acoustic wave length, and whose acoustic impedance is Z, = c with the quartz elastic modulus &, and the quartz density po. The energy is trapped inside the resonator since the reflection coefficient at each in- terface is one (Fig. 1). When this propagation line is 0925.4005/94/$7.00 0 1994 Elsevier Sequoia. All rights reserved SSDI 0925-4005(93)01134-P