Environmental-sentinels

More about the science

The enabling core technology behind our next-generation environmental sensor is the novel tuning fork sensor developed in our lab. There are three main components to our sensor:

1.    Sensing platform - Tuning fork

We take a quartz crystal tuning fork as a mechanical resonator and use it as the sensor platform. The resonator, made of quartz, has piezoelectric properties and is shaped in the form of a tuning fork. As a result, an alternating voltage can be applied across the two electrodes to set it into oscillation. It is designed to resonate in a balanced mode; thus canceling noise that is common to both prongs of the tuning fork. The tuning fork typically has a quality factor (Q) of ~10,000; allowing high sensitivity as well as low noise, power consumption, and temperature dependence at room temperature. The exposed tuning fork can then be modified with a sensing material either by coating the exposed surface with the sensing material or by stretching a tight wire across the tips of the prongs with the material. As targeted chemical analytes interact with the sensing material, the overall mass (in the case of coating) or spring constant (in the case of the wire) of the tuning fork sensor will change, resulting in a change in its natural resonant frequency according to the following equation:

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where f is the resonant frequency, k is the effective spring constant, and m is the effective mass.

2.    Sensing materials – molecularly imprinted polymers

We have designed and synthesized sensing materials to help tune our sensor’s selectivity and sensitivity. The typical technique employed in the synthesis of these materials is molecular imprinting. Basically, we use the target analyte molecules as imprinting templates with cross-linked monomers as the matrix. The imprinting templates will be removed after the matrix is formed. As a result, the synthesized material shows higher affinity towards molecules structurally similar to the template molecules. Due to the large surface area to volume ratio of the synthesized materials, the sensitivity towards the targeted analytes increases while the response time decreases.

3.    Electronics – Frequency detection circuits

To take full advantage of the high sensitivity of the tuning fork sensor, state-of-the-art electronics are used to produce low-cost and yet high performance driving and detection circuitries. Our frequency detection circuit is essentially a digital frequency counter capable of detecting small resonant frequency change down. This combines with the high sensitivity of the tuning fork that can give our sensor sensitivity in the low parts-per-billion range. This is the best mass sensitivity ever reached for tuning fork resonators.