Resonant-Cantilever-Detected Kinetic/Thermodynamic Parameters for Aptamer-Ligand Binding on a Liquid-Solid Interface.

Nucleic acid aptamers have been widely used as recognition elements on various biosensing interfaces, but quantitative kinetic/thermodynamic analysis for revealing the aptamer-ligand binding mechanism, which occurs on a liquid-solid interface, has not been realized due to a lack of usable biophysical tools. Herein we apply a resonant microcantilever sensor to continuously record the frequency shift according to the binding-induced mass change on the liquid-solid interface. The frequency-shift curve is used for tracing the reaction process and is fitted with classic equations to calculate a set of kinetic/thermodynamic parameters, such as rate constants (ka = 902.95 M-1 s-1, kd = 0.000141 s-1), equilibrium constants (KD = 1.55 µM), the Gibbs free energy (ΔG° = -32.57 kJ/mol), and the activation energy (Ea = 38.03 kJ/mol) for the immobilized aptamer and free ATP. This quantitative analysis method is label-free, calibration-free, and highly sensitive. The kinetic/thermodynamic parameter detection method provides new resolution to the in-depth understanding of the ligand-aptamer interaction on the liquid-solid interface for biosensing or lab-on-a-chip applications.

In situ construction of metal-organic framework (MOF) UiO-66 film on Parylene-patterned resonant microcantilever for trace organophosphorus molecules detection.

The detection of organophosphorus (OP) compounds, which are extremely toxic, is a requirement in many application fields such as food security. Mass-type chemical sensors based on ultra-sensitive resonant microcantilevers exhibit high comparative advantages in OP compound detection. However, it is still a big challenge to construct a sensing film in situ from corrosive precursors on resonant cantilevers for batch fabrication. In this work, Parylene-C is patterned and a sample reservoir is formed on the free-end of resonant microcantilever for constructing the sensing material directly. Not only utilized for corrosive precursor loading and in situ sensing material construction, the Parylene-C film can also be used to effectively protect the integrated elements from damage by corrosive substances. For extremely toxic OP molecule detection, a typical metal-organic framework (MOF) of UiO-66 film has been regioselectively constructed in situ on the Parylene-C patterned microcantilevers. A limit of detection (LOD) of 5 ppb for the OP simulant dimethyl methylphosphonate (DMMP) is achieved. The in situ MOF construction method manifests satisfactory consistency for sensor batch fabrication. The DMMP sensing mechanism is identified as the specific host-guest interaction between the UiO-66 and OP molecule.

Design, Fabrication, and Testing of a Monolithically Integrated Tri-Axis High-Shock Accelerometer in Single (111)-Silicon Wafer.

In this paper, a monolithic tri-axis piezoresistive high-shock accelerometer has been proposed that has been single-sided fabricated in a single (111)-silicon wafer. A single-cantilever structure and two dual-cantilever structures are designed and micromachined in one (111)-silicon chip to detect Z-axis and X-/Y-axis high-shock accelerations, respectively. Unlike the previous tri-axis sensors where the X-/Y-axis structure was different from the Z-axis one, the herein used similar cantilever sensing structures for tri-axis sensing facilitates design of uniform performance among the three elements for different sensing axes and simplifies micro-fabrication for the multi-axis sensing structure. Attributed to the tri-axis sensors formed by using the single-wafer single-sided fabrication process, the sensor is mechanically robust enough to endure the harsh high-g shocking environment and can be compatibly batch-fabricated in standard semiconductor foundries. After the single-sided process to form the sensor, the untouched chip backside facilitates simple and reliable die-bond packaging. The high-shock testing results of the fabricated sensor show linear sensing outputs along X-/Y-axis and Z-axis, with the sensitivities (under DC 5 V supply) as about 0.80⁻0.88 µV/g and 1.36 µV/g, respectively. Being advantageous in single-chip compact integration of the tri-axis accelerometers, the proposed monolithic tri-axis sensors are promising to be embedded into detection micro-systems for high-shock measurement applications.

Detection of volatile-organic-compounds (VOCs) in solution using cantilever-based gas sensors.

Micromechanical resonant sensor offers many advantages for chemical detection, but it fails to maintain high quality factor (Q-factor) when working directly in liquid because of the viscous damping. To solve the problem, a gas/liquid separated sensing method is introduced to detect volatile organic compounds (VOCs) in solution with a resonant cantilever gas sensor. With the help of a waterproof and breathable expanded polytetrafluoroethylene (ePTFE) film, the resonant sensor can be physically isolated from the analyte solution. Thus, the sensor can resonate in gas phase environment with a high Q-factor, meanwhile the interference from the solvent emission can be significantly suppressed. Loaded with the sensing-group functionalized mesoporous-silica nanoparticles (MSNs), the resonant cantilever can detect the target VOC molecules that permeate from the flowing solution sample at the other side of the film. Two typical kind of resonant microcantilever VOC sensors are tested to verify the proposed method, which are loaded with carboxyl (-COOH) and amino (-NH2) sensing groups functionalized MSNs, respectively. The sensors exhibit highly sensitive (mg/L level resolution) and reproducible detection ability to aniline and acetic-acid solution, respectively. This gas/liquid separated sensing technique is promising in various on-site chemical detection applications.