Over the past few years, researchers have been keen to develop microresonators, especially for gravimetric sensing to measure changes in mass in a system. Microscale sensors that are built from high-frequency bulk-acoustic-wave (BAW) resonators consist of a piezoelectric layer sandwiched between two electrodes, to which a variable-frequency signal is applied. The resonator vibrates at a given frequency, and the properties of the resulting acoustic wave allow researchers to determine what is occurring in the environment – a change in the acoustic wave being measured denotes a change of mass that occurs when an object of interest is absorbed into the resonant surface.

Sense and sensitivity

But the major hurdle in using BAWs for these commercial applications is that they are very sensitive to temperature. A change in temperature causes a change in the acoustic wave and it is impossible to work out if the change is a response to the mass actually being measured or a temperature variation. Because of this, the sensors are currently only used in the laboratory, where all conditions, including temperature, can be strictly monitored and controlled.

In the past, many research groups have tried to eliminate the effect that a change in temperature has on the sensor, but have failed. The changes are nonlinear and so can only be minimized and never completely eliminated, explains Luis Garcia-Gancedo of Cambridge University in the UK. Instead, what Garcia-Gancedo and colleagues have done is to live with the changes in temperature and account for them. "We decided to measure both the mass and the temperature each time we measure the mass, so that we can then eliminate the effects of the temperature if we wish," explained Garcia-Gancedo.

Resonances and responses

The team designed a new type of thin-film bulk-acoustic-wave resonator that allows simultaneous measurement of temperature and mass-loading measurements in a single device. The new sensor is a multilayered device that has two fundamental frequencies of resonance, which react differently to mass and temperature changes. An extra layer of passive material is added underneath the piezoelectric material and this passive layer generates the second resonance.

Garcia-Gancedo explained that if only a change in mass occurs, then both resonances remain passive and only the mass is measured. But, if a change in temperature occurs as well as a change in mass, one resonance shows a positive frequency shift with a temperature rise, while the second shows a negative frequency shift for the same temperature variation. Simply put, one resonance increases while the other decreases. So, by simultaneously measuring both resonances, any change in frequency can be expressed as a combination of a mass-load component and a temperature-change component.

"This has two consequences," said Garcia-Gancedo. "First, we are able to eliminate the effects of temperature completely, regardless of its nonlinearity. Second, we are able to measure mass and temperature with extremely high sensitivity at exactly the same location, which we have not been able to do before." He explained that this proved to be useful, as many biological interactions are temperature dependent and having the extra information about the temperature measurement is therefore an added bonus.

Detecting viruses

Garcia-Gancedo also pointed out that the new device is not bulky and has the same electronics as most existing sensors. As a result, it can easily be integrated with existing technologies. While the researchers have not done this yet, they have successfully used the resonator as a biosensor to detect proteins in a sample at a very low concentration, thereby proving its sensitivity.

In the future, the team hopes to use its device specifically for biological systems and physical sensing. Because of their sensitivity and size, the resonators could play a crucial part in the healthcare industry or in environmental monitoring. The microresonator can be easily embedded into small medical devices and can detect masses as small as 10–15 grams – the approximate mass of a virus. It could also detect contaminated water or measure air quality. The researchers, along with commercialization specialists Cambridge Enterprise, are currently seeking commercial partners to develop these technologies.

The research is published in Biosensors and Bioelectronics.