Nanomechanical microcantilever sensors as new analytical systems

Abstract

The advanced nanotechnology and the MEMS (microelectro-mechanical systems) technologies can be utilized in developing ultrasensitive sensor systems for detection of biological and chemical analytes as well as physical properties. These micromechanical sensors can be micro or nanoresonator device enabling to sensitively measure the mass of analytes. The change in the frequency of the resonators can be converted to the change in the mass of the resonators. The microresonators generally show the mass sensitivity of nanogram (10-9) to fecogram (10-15), but the minimum detectable mass of the commercial mass balance is microgram level. Therefore, the microresonators offer a clear path to development of ultrasensitive mass sensor system. Although the microresonators show high mass sensitivity, multiscreening function, and fast thermal equilibrium, most studies about microresonators have focused on the detection of physical properties and gases and biomolecules. Therefore, there are still need to find out new application areas with fundamental studies and model systems. In this works, there are three categories which are fundamental studies of microresonators for mass sensitivity enhancement and mass loading positions, applications to the development of new analysis tool, and fundamental studies for model systems with microresonators. Firstly, for fundamental studies of microresonaors about mass sensitivity of microresonators and mass loading position, we directly synthesized and fabricated various nanostructures such as ZnO nanorods, Si nanowires, and AAO nanopores on microresonators to enhance mass sensitivity for the first time. We also demonstrated that the size and shape of the nanostructures are readily tunable by controlling the reactant conditions. Mass sensitivity of nanostructures-grown microresonators was found to be superior to that of unmodified microresonators. In addition, effects of gold patterning on the bending profile and frequency response of a microcantilever was investigated for fundamental studies of mass loading position on a microresonaor. We reported a reliable and systematic method to study variations in bending profiles and frequency responses of cantilevers that have been treated with gold coatings that vary in position and area. Changes in the frequency modes were determined by the position and area of the gold film because the mass and the flexural rigidity had opposite effects on the resonance frequencies. We also determined the highest and lowest sensitivity of mass and flexural rigidity by using frequency shifts of the increasing gold patterns from the free end or clamping region. This systematic study is also one of methods of mass sensitivity enhancement due to reduction of the mass detection errors. Secondly, a new analysis tool, micro-TGA, was developed for investigating synthesized materials such as analysis of a single microcapsule and CO2 sorbents using silicon microcantilevers. We have demonstrated the first use of a silicon microcantilever for investigating the thermomechanical properties of single microcapsules and CO2 adsorption capacity and kinetics of amine functionalized mesoporous silica. In the study of a single capsule, the rupture temperature of the capsule and thermal degradation temperature of the polyurethane shell were clearly determined. In addition, the permeability of the polyurethane shell with respect to chlorobenzene was measured. In the analysis of CO2 sorbent, we have synthesized various types of mesoporous silica having different pore size and grafting various amines and measured their CO2 adsorption and desorption using silicon microcantilevers. The CO2 adsorption or desorption capacity, kinetics and selectivity of the sorbents at various temperatures can be measured from resonance frequency measurements. Therefore, we have demonstrated the first use of a silicon microcantilever for investigating the properties of synthesized materials.Finally, one of most important properties in the synthesized materials is diffusion kinetics. The diffusion and release kinetics of the synthesized materials was well defined by variation in the mass of microcantilevers. In particular, the mesoporous materials with nanopores have not well ordered structure but randomly oriented nanopore, whereas the AAO nanopores have well ordered geometry and can easily control the pore size and length. Therefore, the AAO cantilever with well ordered nanowell/nanochannel can be good model systems for the diffusion and release kinetics.For example, the amine functionalized AAO cantilever is a great substitute of amine functionalized mesoporous silica. The amine efficiency of amine functionalized mesoporous silica is around 0.3, which is smaller than the theoretical value (0.5). However, in the amine functionalized AAO cantilever, the molecular ratio between CO2 and amine is calculated to be 0.6, which might be attributed to a diffusion problem into micropores. The AAO cantilever is useful to study not only gas adsorption and desorption but also water desorption and release of large molecules. The evaporation rates of water in AAO cantilever with various pores were calculated by the variation in the mass against time, and the diffusion coefficients was also calculated, however, in spite of AAO cantilever with various pores, the diffusion coefficients is similar to each other, which is attributed to large size of AAO cantilever having low mass sensitivity. Although the water evaporation from various sizes of nanopores was not differentiated, larger molecules, PEG 20k showed the different releasing kinetics from various sizes of nanopores. The releasing kinetics from the smaller pores is slower than that from the larger pores. Fast and slow kinetics was linearly fitted in logarithm plot of the relative eluting mass, which might be attributed to interaction PEG molecules with or without surfaces.Therefore, these results as shown above suggested solutions about microresonators’ drawbacks and new application areas with fundamental studies and model systems. These studies can be extended to investigate for not only developments of various sensors and new analysis tools systems but also fundamental studies of the various materials’ properties.