Quartz formation within a geode - While there is yet to be a conclusion as to how geodes form, it's widely accepted that they form by means of solidification of gas bubbles (vesicles) within basaltic lava during igneous rock formation and in round cavities within sedimentary rock. In some instances, a surplus of these minerals will result in a solid vein of quartz where individual crystals can no longer be identified by the naked eye. The crystals will continue to grow as long as they're provided with the environment necessary for formation. Long crystals, also known as spears, can form in these cavities when the solution drips more from one location of the rock than the other, explaining why vugs often form a wide variety of crystal sizes in close proximity. Quartz formation within a vug/cavity - Silicon-rich solutions will deposit themselves along the walls of the rock cavities. These cavities most often occur by means of tectonic activity, hollow tubes formed by lava, dissolution (breakdown of rock resulting in cavities) or even as solidified bubbles of gas within the earth (geodes).
In this environment, the silicon atoms will bond to oxygen and begin building layers that over time result in cavities lined with thousands of crystals. This combination typically occurs when water that's high in silicon content (often gained through the breakdown of the surrounding rock) seeps through cracks in rocks and ends up exposed to oxygen within cavities. 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 is primarily made up of two elemental components, silicon (Si) and Oxygen (O), both of which are in high abundance on our planet. The presented circuit is suitable to be used in compact biosensor systems using custom-made miniaturized QCMs in microfluidic environments. Thereby an increased energy dissipation by strong viscous damping in liquid solutions can be compensated and oscillations are stabilized. 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. 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.
Typical biosensor applications demand operation in liquid environments leading to viscous damping strongly lowering Q-factors. 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). 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.
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