Abstract
Abstract. Quartz-crystal microbalances (QCMs) are commercially available mass sensors which mainly consist of a quartz resonator that oscillates at a characteristic frequency, which shifts when mass changes due to surface binding of molecules. In addition to mass changes, the viscosity of gases or liquids in contact with the sensor also shifts the resonance but also influences the quality factor (Q-factor). Typical biosensor applications demand operation in liquid environments leading to viscous damping strongly lowering Q-factors. For obtaining reliable measurements in liquid environments, excellent resonator control and signal processing are essential but standard resonator circuits like the Pierce and Colpitts oscillator fail to establish stable resonances. Here we present a low-cost, compact and robust oscillator circuit comprising of state-of-the-art commercially available surface-mount technology components which stimulates the QCMs oscillation, while it also establishes a control loop regulating the applied voltage. Thereby an increased energy dissipation by strong viscous damping in liquid solutions can be compensated and oscillations are stabilized. The presented circuit is suitable to be used in compact biosensor systems using custom-made miniaturized QCMs in microfluidic environments. As a proof of concept we used this circuit in combination with a customized microfabricated QCM in a microfluidic environment to measure the concentration of C-reactive protein (CRP) in buffer (PBS) down to concentrations as low as 5 µg mL−1.
Highlights
Quartz-crystal microbalances (QCMs) are commercially available mass sensors which mainly consist of a quartz resonator that oscillates at a characteristic frequency, which shifts when mass changes due to surface binding of molecules
As a proof of concept we used this circuit in combination with a customized microfabricated QCM in a microfluidic environment to measure the concentration of C-reactive protein (CRP) in buffer (PBS) down to concentrations as low as 5 μg mL−1
Quartz-crystal microbalances (QCMs) are mass sensors that are used for tasks like the thickness control during thinfilm deposition and nowadays a lot of biosensor applications are already reported in literature
Summary
Quartz-crystal microbalances (QCMs) are mass sensors that are used for tasks like the thickness control during thinfilm deposition and nowadays a lot of biosensor applications are already reported in literature. Good surveys are offered by the reviews of Becker, Cooper and Speight (Becker and Cooper, 2011; Cooper and Singleton, 2007; Speight and Cooper, 2015) Such biosensor applications include detection and quantification of bacteria, protein–protein interactions, as well as protein adsorption, lipid-film formation, and cell adhesion to just name a few. S. Beißner et al.: Low-cost, in-liquid measuring system using a novel compact oscillation circuit and QCM ble before. Beißner et al.: Low-cost, in-liquid measuring system using a novel compact oscillation circuit and QCM ble before An overview of these improvements is given in Skládal (2016). After a short introduction into the problem of QCMs under high damping, we will introduce a new circuit built with surface mounted devices (SMDs) based on the Borngräber circuit utilizing commercially available SMDs leading to a very compact design resulting in smaller stray capacitances and inductances and to a very stable behaviour. Measurements of artificial samples with Creactive protein were carried out with the new circuitry and microfabricated QCM devices
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