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Sensing & Measurement

Sensor monitors exposure to airborne nanoparticles

Airborne nanoparticles are detected through a frequency shift in the resonance of a silicon sensor triggered by their mass.
11 July 2011, SPIE Newsroom. DOI: 10.1117/2.1201105.003685

Nanoparticles (NPs) are increasingly present in industrial and consumer products such as food packaging, cosmetics, pharmaceuticals, medical devices, odor-resistant textiles, and household appliances. Given their ubiquity, they could constitute a potential source of toxicity for human health and the environment. In particular, occupational health professionals have raised concerns that repeated exposure to these particles could lead to adverse health effects such as respiratory disorders.1 Thus, there is now a demand for sensors to detect airbone NPs, particularly for air-quality monitoring in the workplace.

The most common detection methods rely on optical fibers and laser beams.2–4 However, they require heavy equipment and complex alignment, and they are time-consuming. Microelectromechanical system (MEMS) sensors have been widely used in chemistry and biology due to their high precision and effectiveness with very small samples. These sensors work by detecting a shift in the resonance frequency of microscale resonators that occurs as airborne NPs are deposited on them. The value of the change in frequency can be used to determine the mass of the particles and then their concentration in the air sample. Resonant mass sensors that are thermally actuated—i.e., brought to their standard resonance mode thanks to a temperature change—have thus been used for detecting particles of ∼115 picograms (pg).5 However, this method is only applicable to particles of ∼1μm in diameter deposited on the resonator under partial vacuum conditions.

Figure 1. (a) View of the cantilever sensor and (b) of its integrated full Wheatstone bridge, consisting of p-type resistors on a 200μm-wide suspension, designed to measure a change in resistance.

Among other MEMS, micromachined cantilever sensors have recently become popular due to their simplicity, high sensitivity, portability, and low-cost batch fabrication. For example, such sensors made of silicon-on-insulator (SOI) wafers have measured glycerine beads as small as 1μm and exhibited a mass sensitivity of 3.33Hz/femtogram.6 However, this method has limitations due to the high production costs of SOI wafers and the inability to change their thickness to modulate the sensor's performance.

To overcome these issues, we have developed a novel silicon cantilever sensor. Low-cost silicon was favored over SOI because of its high mechanical-quality factor, stability, and the degree of freedom it brings for cantilever design. This type of sensor reaches its standard resonance mode by means of a piezoactuator located at the base of the beam on a supporting frame, which works by expanding and bending the cantilever following application of a low voltage. The sensor detects NPs by determining changes in its resonance frequency that can be deduced from a change in resistance. To do so, it relies on a device known as a full Wheatstone bridge, used as a piezoresistive strain gauge. This helps measure changes in electrical resistance following the application of mechanical stress induced by the deposition of NPs. Our method offers several advantages, including simplicity, large output signal of the order of the millivolts (obviating the need for a low-noise amplifier), low-power consumption, and high sensitivity. As a result, it can be used in applications requiring a high mass-sensitivity and high quality factor to probe microstructures such as the deep and narrow spray holes of direct-injection fuel nozzles in cars (see Figure 1).7

Figure 2. Principle of the dielectrophoresis mechanism for trapping nanoparticles (NPs).

Figure 3. Magnified view of the free end of a cantilever sensor in the (a) unloaded state and (b) loaded state where airborne NPs are trapped on the edges.

We tested our sensor using a process that releases standardized NPs under controlled conditions, known as airborne NP sampling. This was performed under normal workplace conditions in a sealed glass chamber containing a stable carbon aerosol, with particles of ∼20nm in diameter, monitored by a fast mobility particle sizer (FMPS). A nanometer aerosol sampler, consisting of a grounded cylindrical sampling case with an electrode at the bottom, was used to enhance the sampling efficiency of the cantilevers. Finally, we relied on dielectrophoresis (DEP)8 to detect NPs (see Figure 2). There, carbon NPs were subject to an electric force in a nonuniform electric field. A number of carbon NPs were thus trapped on the cantilever surface in a nonhomogeneous manner (see Figure 3). Most of them were deposited on the edges of the free end of the cantilever and agglomerated where the highest electrical field strengths and gradients occurred.

Evaluation of sensor performance showed a mass sensitivity of 8.33Hz/nanogram with a frequency resolution of 0.04Hz, a mass resolution of 4.8pg, and a quality factor of 1230 in air. The mass sensitivity of these sensors can be improved by miniaturizing their geometry, i.e., by decreasing the effective mass—that of the cantilever without its supporting frame—and by increasing the frequency of the mechanical resonator. Further improvement of mass-sensing resolution will require increasing the quality factor of the sensor, expected for higher resonant modes. This can be done by vibrating the cantilever in in-plane—as opposed to out-of-plane—bending modes and by integrating a closed-loop feedback electronic circuit.

In summary, our work has shown the principle of a novel micromachined silicon cantilever beam operating as a resonant sensor to detect airborne NPs based on the measurement of their mass. In future work, we intend to develop mobile sensors by optimizing their geometry, detection method, and the NP collection system to yield a higher quality factor, mass sensitivity, sensing resolution, and sampling efficiency.

This work has been performed as part of the NanoExpo collaborative project funded by the German Federal Ministry of Education and Research within the NanoCare cluster (contract 03X0098A).

Hutomo Suryo Wasisto, Andreas Waag, Erwin Peiner
Institute of Semiconductor Technology (IHT)
Technical University Braunschweig
Braunschweig, Germany

Hutomo Suryo Wasisto is a PhD candidate working in the field of semiconductor sensors and MEMS-based technology for airborne NP-sensing applications.

Andreas Waag received his PhD in physics from the University of Würzburg, Germany (1990). In 1996, he was the recipient of the Gaede Award of the German Vacuum Society for the development of novel II–VI materials for blue-green laser diodes. Since 2003, he has been a full professor and head of IHT. His research activities range from applications of oxides and nitrides in optoelectronics to sensor technologies.

Erwin Peiner received his PhD from the University of Bonn, Germany (1988). In 2000 he became a lecturer in semiconductor technology at the Faculty of Mechanical and Electrical Engineering of the Technical University Braunschweig. Currently, he heads the Semiconductor Sensors and Metrology group at IHT. He is also coordinator of the NanoExpo collaborative project funded by the German Federal Ministry of Education and Research.

Erik Uhde
Material Analysis and Indoor Chemistry
Fraunhofer Institute for Wood Research (WKI)
Braunschweig, Germany

Erik Uhde received his PhD in chemistry (1998) from the Technical University Braunschweig, Germany. In 1995, he started as a project manager at the Fraunhofer WKI and in 2001 became the deputy head of the Material Analysis and Indoor Chemistry Department, which specializes in the characterization of gaseous and solid air pollutants indoors.

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