In this blog post, we will take a closer look at how ultra-precision balances use the piezoelectric effect to measure the mass of microscopic substances such as gas molecules and DNA.
Scales often measure mass by using the principle of the lever or by measuring changes in electrical resistance. So how do ultra-precise scales measure the mass of microscopic substances such as gas molecules or DNA? To answer this question, we need to understand the piezoelectric effect. The piezoelectric effect is a phenomenon in which mechanical deformation in a solid material causes an electrical response, and is divided into the primary and secondary piezoelectric effects. The primary piezoelectric effect is when voltage is generated when mechanical deformation occurs in the material, and the secondary piezoelectric effect is when mechanical deformation occurs when voltage is applied. Materials that exhibit both of these piezoelectric effects are called piezoelectric materials, and quartz is mainly used.
The quartz crystal used as an electromechanical element is cut and processed in a specific direction to make it into a flat disc shape. After creating electrodes on both sides of the disc, a voltage is applied to the electrodes in alternating polarity (+) and (-) so that the quartz crystal vibrates. The quartz crystal vibrator makes it easy to measure the vibration by matching the frequency of the voltage to the natural frequency of the quartz crystal so that the quartz crystal vibrates in a wide range. The natural frequency is the unique vibration frequency of an object, which varies depending on the shape and size of the piezoelectric material, even if it is made of the same material.
If a substance adheres to a quartz crystal and its mass increases, the frequency of the quartz crystal, which was vibrating at its natural frequency, decreases. The frequency of a quartz crystal is sensitive to very small changes in mass, which makes it possible to measure the mass of microscopic substances such as gas molecules and DNA. The mass sensitivity of an oscillator is the change in frequency divided by the measured mass, and the mass sensitivity of a quartz crystal is very high.
The principle of measuring mass with a quartz crystal oscillator can be applied to detect the concentration of a specific gas. If a quartz crystal oscillator is treated to attract a specific gas, the specific gas will stick to it, causing a change in mass and decreasing the frequency of the quartz crystal oscillator. At a certain point, the frequency of the quartz crystal oscillator will not decrease further and will remain at a constant value. The reason for maintaining this constant value is that the specific gas will not stick to it in excess of a certain amount. In a mixture of gases, the greater the concentration of a specific gas, the more stable the frequency at a smaller frequency. The measure of how quickly a specific gas attaches to a crystal oscillator and stabilizes the frequency is called the response time. The shorter the response time, the faster the concentration of the specific gas can be measured.
However, if a gas that is not the target of measurement attaches to it, it is difficult to accurately measure the concentration of the target gas. In addition, even if only the target gas is attached, it is not possible to know the concentration of that gas. For this reason, the frequency change of the quartz crystal should be measured in advance according to the concentration of the target gas. Then, if the frequency change is measured in a mixed gas whose concentration is unknown, the concentration of the target gas can be determined. The sensitivity to concentration can be obtained by dividing the frequency change of the quartz crystal by the concentration.
Measurements using quartz crystals can be used not only in laboratories but also in various industrial fields. For example, in the semiconductor manufacturing process, a high-precision scale is used to measure the thickness of the material deposited on the substrate, and quality control is performed using this. In addition, in the field of biosensors, quartz crystals are used to detect the presence of specific proteins or viruses. This technology plays an important role in various fields such as medical diagnosis, environmental monitoring, and food safety.
Finally, the potential applications of quartz crystals are endless. Recent research is actively developing new measurement methods and sensors using quartz crystals. For example, ultra-high-sensitivity nano-scales that measure the mass of nanoparticles and high-performance sensors that operate stably in extreme environments are being developed. These technological advancements will further increase the importance of precision measurements using quartz crystals.
