In this study, we investigate the performance of two piezoresistive micro-electro-mechanical system (MEMS)-based silicon cantilever sensors for measuring target analytes (i.e., ultrafine particulate matters). We use two different types of cantilevers with geometric dimensions of 1000 × 170 × 19.5 µm3 and 300 × 100 × 4 µm3, which refer to the 1st and 2nd types of cantilevers, respectively. For the first case, the cantilever is configured to detect the fundamental in-plane bending mode and is actuated using a resistive heater. Similarly, the second type of cantilever sensor is actuated using a meandering resistive heater (bimorph) and is designed for out-of-plane operation. We have successfully employed these two cantilevers to measure and monitor the changes of mass concentration of carbon nanoparticles in air, provided by atomizing suspensions of these nanoparticles into a sealed chamber, ranging from 0 to several tens of µg/m3 and oversize distributions from ~10 nm to ~350 nm. Here, we deploy both types of cantilever sensors and operate them simultaneously with a standard laboratory system (Fast Mobility Particle Sizer, FMPS, TSI 3091) as a reference.

Organic−inorganic hybrids are ideal for gas detection, considering their selectivity and sensitivity to single gas species under moderate working conditions. However, the poor surface-to-volume ratio and low electron density of organic materials hinder their application in high-performance resistive gas sensors. Instead herein, a gravimetric sensor is realized on the basis of an in-plane self-actuating and self-reading piezoresistive microcantilever-chip (PMC), which is patterned with an (inorganic) 3D framework of ZnO nanorods on a Si-nanopillar array (3D ZnO-NRs@Si-NPLs) and functionalized by a thin (organic) self-assembled monolayer (SAM, (3-aminopropyl)trimethoxysilane (APTES)) for interacting with NO 2 . For stable adsorption/desorption rates of NO 2 , this SAM-on-3D ZnO-NRs@Si-NPL PMC (S3-PMC) was exposed to constant light illumination by an LED (wavelength: 530 nm, intensity: 10 mW/cm 2 ), realizing a limit of detection (LOD) of about 2 parts per billion by volume (ppbv) for NO 2 at room temperature, together with fast response and complete recovery within times of 42.1 ± 6.6 s and 112 ± 17.4 s, respectively, to NO 2 concentrations ranging up to 1000 ppbv. Moreover, the sensor shows reliable stability under both short-and long-time (31 days) exposure to NO 2 , where resonance frequency-shift deviations of merely at most ±5% and ±9%, respectively, are observed. These unprecedented results indicate an enormous potential of the S3-PMC for portable gas sensor arrays in high-resolution real-time-monitoring applications.

In this paper, fabrication and testing of a miniaturized microcantilever-based particulate matter detector with integrated electrostatic on-chip ultrafine particle (UFP) separation and collection are presented. Mass added to the sensor causes a resonance frequency shift. To attract naturally charged particles, the cantilever is equipped with a collection electrode. In addition, a µ-channel is integrated, to improve the particle collection efficiency and to enable a size/mass-related particle separation. For electrical read-out, piezo-resistive struts are attached to the cantilever sidewalls near its clamping. This design offers high miniaturization potential, since no integration of transducing electronics on the cantilever beam is needed. The sensors are fabricated using Si bulk material and standard micromachining technology; the cantilevers have a thickness of 3  ±  0.5 µm, a width of 3.1  ±  0.3 µm, 5.9  ±  0.4 µm or 10.5  ±  0.4 µm and a length of 118.7  ±  0.8 µm, 168.8  ±  0.8 µm or 171.2  ±  1 µm, respectively. To this end, a front-side release process using cryogenic inductive-coupled plasma reactive ion etching was developed, which does not require additional sidewall passivation steps. Testing of the resonator function by operating the sensor inside a scanning electron microscope and reference measurements inside a temperature-controlled test chamber using synthetic carbon UFPs (~160 nm average mass concentration distribution) and a fast mobility particle sizer as a reference instrument were carried out. Here, the ability to detect low UFP mass concentrations in the range  <10 µg m−3 could be shown with a limit of detection of ~1 µg m−3 and a collection time of ~10 min. In addition, a voltage dependence of the collection efficiency was found at constant UFP-concentration conditions, which is an indication of size-selective UFP collection.

The asymmetric resonance response in electro-thermal piezoresistive cantilever resonators causes a need of an optimization treatment for taking parasitic actuation-sensing effects into account. An electronic reference circuit for signal subtraction, integrated with the cantilever resonator has the capability to reduce the effect of parasitic coupling. Measurement results demonstrated that a symmetric amplitude shape (Lorentzian) and an optimized phase characteristic (i.e., monotonically decreasing) were successfully extracted from an asymmetric resonance response. With the monotonic phase response, real-time frequency tracking can be easier to implement using a phase-locked loop (PLL) system. In this work, an electro-thermal piezoresistive cantilever resonator functionalized with self-assembled monolayers of chitosan-covered ZnO nanorod arrays as sensitive layers has been investigated under different relative humidity (rH) levels. Enhancement of resonance phase response has been demonstrated by implementing the reference signal subtraction. Subsequently, a lock-in amplifier integrated with PLL system (MFLI, Zurich Instruments, Zurich, Switzerland) was then employed for continuously tracking the resonant frequency. As a result, we find a good correlation of frequency shift (∆f0) with change in rH monitored using a commercial reference sensor.