In this blog post, we explore in detail how ultra-precise balances utilize the piezoelectric effect to measure the mass of extremely minute substances like gas molecules or DNA.
Balances typically measure mass using the lever principle or by detecting changes in electrical resistance. So how do ultra-precision balances measure the mass of minute substances like gas molecules or DNA? To answer this, an understanding of the piezoelectric effect is necessary. The piezoelectric effect is a phenomenon where mechanical deformation in a solid material induces an electrical response. It is categorized into the primary piezoelectric effect and the secondary piezoelectric effect. The primary piezoelectric effect occurs when mechanical deformation in the material generates a voltage, while the secondary piezoelectric effect occurs when applying a voltage causes mechanical deformation. Materials exhibiting both these piezoelectric effects are called piezoelectric materials, with quartz being the primary material used.
The quartz used as a piezoelectric material is cut and processed in specific directions to form flat, disc-shaped plates. Electrodes are then attached to both sides of the disc. When an alternating voltage with alternating (+) and (-) poles is applied, the quartz vibrates. A quartz crystal oscillator is created by matching the voltage frequency to the quartz’s natural frequency, causing the quartz to vibrate with a large amplitude, making the vibration easier to measure. The natural frequency is the inherent vibration frequency of an object. Even for piezoelectric materials of the same composition, this frequency varies depending on the shape and size of the piezoelectric element.
If any substance adheres to a crystal oscillator, increasing its mass, the frequency at which the oscillator vibrates at its natural frequency decreases. The frequency of a quartz crystal oscillator is highly sensitive to even minute changes in mass, enabling the measurement of the mass of minute substances like gas molecules or DNA. The mass sensitivity of an oscillator is the degree of frequency change divided by the measured mass; the mass sensitivity of a quartz crystal oscillator is extremely high.
Applying the principle of mass measurement with a crystal oscillator enables the detection of the concentration of a specific gas. When a crystal oscillator is treated to allow a specific gas to adhere to it, that gas attaches, causing a mass change that reduces the oscillator’s frequency. At a certain point, the crystal oscillator’s frequency stops decreasing and stabilizes at a constant value. This stabilization occurs because the specific gas does not adhere beyond a certain threshold amount. In a mixed gas, the higher the concentration of a specific gas, the lower the frequency at which it stabilizes. The speed at which a specific gas adheres to the crystal oscillator, causing the frequency to stabilize at a constant value, is called the response time. A shorter response time allows for faster measurement of the specific gas’s concentration.
However, if gases other than the target gas also adhere, it becomes difficult to accurately measure the concentration of the target gas. Furthermore, even if only the target gas attaches, its concentration cannot be directly determined. Therefore, the frequency change of the crystal oscillator must be pre-measured for different concentrations of the target gas. Subsequently, by measuring the frequency change in a mixed gas where the target gas concentration is unknown, the concentration of the target gas can be determined. Dividing the degree of frequency change of the crystal oscillator by the concentration yields the sensitivity to concentration.
Measurements using quartz crystals can be applied not only in laboratories but also across various industrial sectors. For instance, in semiconductor manufacturing processes, ultra-precise scales are used to measure the thickness of materials deposited on substrates, enabling quality control. Furthermore, in the biosensor field, quartz crystals are employed to detect the presence of specific proteins or viruses. These technologies play crucial roles in diverse areas such as medical diagnostics, environmental monitoring, and food safety.
Finally, the potential applications of quartz resonators are virtually limitless. Recent research is actively developing new measurement methods and sensors using quartz resonators. Examples include ultra-high-sensitivity nanoscale balances for measuring nanoparticle masses and high-performance sensors that operate stably in extreme environments. These technological advancements will further enhance the importance of precision measurement using quartz resonators in the future.
