Biosensors and Bioelectronics 24 (2009) 2643–2648 Contents lists available at ScienceDirect Biosensors and Bioelectronics journal homepage: www.elsevier.com/locate/bios User-friendly, miniature biosensor flow cell for fragile high fundamental frequency quartz crystal resonators Brigitte P. Sagmeister a , Ingrid M. Graz a,1 , Reinhard Schwödiauer a,∗ , Hermann Gruber b , Siegfried Bauer a a Institute of Experimental Physics, Soft Matter Physics Division, Johannes Kepler University, Altenbergerstrasse 69, 4040 Linz, Austria b Institute of Biophysics, Johannes Kepler University, Linz, Austria article info Article history: Received 30 October 2008 Received in revised form 23 December 2008 Accepted 19 January 2009 Available online 29 January 2009 Keywords: Quartz crystal microbalance High frequency fundamental Flow cell abstract For the application of high fundamental frequency (HFF) quartz crystal resonators as ultra sensitive acous- tic biosensors, a tailor-made quartz crystal microbalance (QCM) flow cell has been fabricated and tested. The cell permits an equally fast and easy installation and replacement of small and fragile HFF sen- sors. Usability and simple fabrication are two central features of the HFF-QCM flow cell. Mechanical, thermal, electrical and chemical requirements are considered. The design of the cell combines these, partially contradictory, requirements within a simple device. Central design concepts are discussed and a brief description of the fabrication, with a special focus on the preparation of crucial parts, is provided. For test measurements, the cell was equipped with a standard 50MHz HFF resonator which had been surface-functionalised with a self-assembled monolayer of 1-octadecanethiol. The reliable performance is demonstrated with two types of experiments: the real time monitoring of phospholipid monolayer formation and its removal with detergent, as well as step-wise growth of a protein multilayer system by an alternating immobilisation of streptavidin and biotinylated immunoglobulin G. © 2009 Elsevier B.V. All rights reserved. 1. Introduction During the last decade acoustic biosensors which detect mass- and viscosity-alterations of surface adsorbed biofilms via a corre- sponding shift in resonance frequency (Steinem and Janshoff, 2007; Bizet et al., 1999; Ballantine et al., 1997), have found widespread acceptance as versatile tools for the analysis of biomolecular interactions and related phenomena. Especially the quartz crystal microbalance (QCM) with a thickness-shear mode (TSM) resonator in combination with a flow cell is gaining increasing relevance for biological and biochemical research (Steinem and Janshoff, 2007). This trend is well reflected by the steady growth of related publi- cations as recently reported by Cooper and Singleton (2007), who documented, for the period 2001–2005, more than 1400 articles referencing “quartz crystal microbalance” or “QCM” in the Web of Science database. Various properties of QCM systems may account for their exten- sive usage. Beside the scientific merits of QCM biosensors (Marx, 2007), the principle of operation is simple and the basic elements of such a system are easily available at relatively low costs. Hence, for many research groups home made systems with a reasonable per- ∗ Corresponding author. Fax: +43 732 2468 9273. E-mail address: reinhard.schwoediauer@jku.at (R. Schwödiauer). 1 Present address: University of Cambridge, Nanoscience Centre, 11 JJ Thomson Ave, Cambridge, Cambridgeshire CB3 0FF, United Kingdom. formance are easy to build and an alternative to more sophisticated, but also far more expensive commercial machines. A crucial factor for the performance of QCM systems – home made or commercial – is the fundamental resonance frequency, f 0 , of the oscillating piezoelectric sensor. A mass alteration per unit area, m, of a thin biofilm on the sensor surface is, to a first order approximation, the main detectable effect of biochemical binding interactions under study. It can be measured via a corresponding frequency shift, f , according to the Sauerbrey relation (Sauerbrey, 1959)f =-2f 2 0 / √  · m, with the density  = 2648 g/cm 3 , and the shear modulus  = 29.47GPa (AT-cut) of the quartz crystal. Since the theoretical sensitivity increases with f 2 0 , the use of a sensor with a high fundamental resonance frequency is clearly desirable. Following the resonance condition f 0 =  //2h, the fabrication of TSM resonators with higher values for f 0 is basically achieved by reducing the plate thickness h of the sensor. For practical purposes however, the minimum thickness is con- strained by mechanical stability issues. Most applications utilise either 5 MHz or 10 MHz resonators, with a corresponding plate thickness of 0.33mm or 0.17mm. Such thicknesses, in combina- tion with a diameter of approximately 14mm, provide a sufficient mechanical stability for an easy installation in a flow cell. Occa- sionally sensors with resonance frequencies between 20 MHz and 30 MHz are used (Okahata et al., 2000, 2007; Sota et al., 2002; Michalzik et al., 2005; Sota et al., 2002). These sensors are smaller in diameter (approximately 8 mm) and already rather thin and frag- ile. The corresponding plate thicknesses between 83 m and 56 m 0956-5663/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2009.01.023