Minhyuk Yun

Electronic Nose Based on Multipatterns of ZnO Nanorods on a Quartz Resonator with Remote Electrodes

An electrodeless monolithic multichannel quartz crystal microbalance (MQCM) sensor was developed via the direct growth of ZnO nanorod patterns of various sizes onto an electrodeless quartz crystal plate. The patterned ZnO nanorods acted as independent resonators with different frequencies upon exposure to an electric field. The added mass of ZnO nanostructures was found to significantly enhance the quality factor (QF) of the resonator in electrodeless QCM configuration. The QF increased with the length of the ZnO nanorods; ZnO nanorods 5 μm in length yielded a 7-fold higher QF compared to the QF of a quartz plate without ZnO nanorods. In addition, the ZnO nanorods offered enhanced sensitivity due to the enlarged sensing area. The developed sensor was used as an electronic nose for detection of vapor mixtures with impurities.

Evaporation of water droplets from hydrophobic and hydrophilic nanoporous microcantilevers

The evaporation dynamics of water droplets from the surfaces of well-defined nanoporous substrates, anodic aluminum oxide (AAO) cantilevers with various pore sizes, were investigated. The AAO cantilever surfaces were modified to be either hydrophilic or hydrophobic. After placing a water droplet on the cantilevers, variations in the resonance frequency and deflection during evaporation were related to the changes in mass and stress of the cantilever, respectively. The dynamics of water droplet evaporation on a hydrophilic AAO cantilever was found to be significantly different from that measured on a hydrophobic AAO cantilever due to the permeation of water into the hydrophilic nanopores.

CO2-Selective Nanoporous Metal-Organic Framework Microcantilevers.

Nanoporous anodic aluminum oxide (AAO) microcantilevers are fabricated and MIL-53 (Al) metal-organic framework (MOF) layers are directly synthesized on each cantilever surface by using the aluminum oxide as the metal ion source. Exposure of the MIL53-AAO cantilevers to various concentrations of CO2, N2, CO, and Ar induces changes in their deflections and resonance frequencies. The results of the resonance frequency measurements for the different adsorbed gas molecules are almost identical when the frequency changes are normalized by the molecular weights of the gases. In contrast, the deflection measurements show that only CO2 adsorption induces substantial bending of the MIL53-AAO cantilevers. This selective deflection of the cantilevers is attributed to the strong interactions between CO2 and the hydroxyl groups in MIL-53, which induce structural changes in the MIL-53 layers. Simultaneous measurements of the resonance frequency and the deflection are performed to show that the diffusion of CO2 into the nanoporous MIL-53 layers occurs very rapidly, whereas the binding of CO2 to hydroxyl groups occurs relatively slowly, which indicates that the adsorption of CO2 onto the MIL-53 layers and the desorption of CO2 from the MIL-53 layers are reaction limited.

Multi-modal characterization of nanogram amounts of a photosensitive polymer

Here, we demonstrate multi-modal approach of simultaneous characterization of poly(vinyl cinnamate) (PVCN) using a microcantilever sensor. We integrate nanomechanical thermal analysis with photothermal cantilever deflection spectroscopy for discerning ultraviolet (UV) exposure-induced variations in the thermodynamic and thermomechanical properties of the PVCN as a function of temperature and UV irradiation time. UV radiation-induced photo-cross-linking processes in the PVCN are verified with the increase of the Youngs modulus and cantilever deflection as well as the decrease in the hysteresis of deflection and the intensity of C=C peak in the nanomechanical infrared spectrum as a function of UV irradiation time.

Suspended polymer nanobridge on a quartz resonator

A chemical vapor sensor based on a free-standing polystyrene (PS) nanofilm suspended between the tines of a quartz tuning fork (QTF) is demonstrated. Exposure to ethanol vapor decreased the modulus of the PS membrane, which resulted in a decrease in the resonance frequency of the QTF as a function of ethanol concentration. The suspended PS membrane structure on the QTF allowed gas molecules to diffuse into the membrane from both the top and bottom allowing faster response. The QTF response time was found to be 6.5 times faster than the response time of a conventional PS film-coated resonator sensor.

Photothermal cantilever deflection spectroscopy of a photosensitive polymer

The mechanical and chemical information of a poly(methyl methacrylate) (PMMA) film on a microcantilever were simultaneously acquired by photothermal cantilever deflection spectroscopy as a function of ultraviolet (UV) irradiation time. Nanomechanical infrared (IR) spectra from the PMMA-coated microcantilever agreed well with the Fourier transform infrared spectroscopy (FTIR) spectra of PMMA on gold-coated silicon wafer. The decreasing intensities of nanomechanical IR peaks represent chemical as well as mechanical information of UV radiation-induced photodegradation processes in the PMMA which cannot be obtained by a conventional FTIR technique. The observed decrease in the resonance frequency of the microcantilever is related to the change in the Young’s modulus of the PMMA under UV exposure.

Quartz Resonator for Simultaneously Measuring Changes in the Mass and Electrical Resistance of a Polyaniline Film

A novel quartz resonator was developed to measure, simultaneously, changes in the mass and electrical resistance of a polyaniline film during the absorption of water vapor. Interdigitated gold electrodes were vacuum-deposited on the sensing surfaces of the quartz crystals, and polyaniline films were drop-cast on the electrodes used to measure the changes in the electrical resistance. Two symmetric semicircular gold electrodes were deposited on the bottom surface of the quartz crystal. These electrodes were used to measure the changes in the mass of absorbed water based on the changes in the resonance frequency. The simultaneous measurements of mass and electrical resistance shed light on the interactions between the water vapor and the polyaniline film. The resonator was exposed to various organic gases, including ethanol, acetone, or chloroform, and each gas was found to produce characteristic changes in the normalized electrical resistance.

Modulus-tunable magnetorheological elastomer microcantilevers

Modulus-tunable microcantilevers are fabricated from magnetorheological elastomers (MREs) consisting of polydimethylsiloxane and carbonyl iron particles by using a simple sandwich molding method. Depending on the presence or absence of an external magnetic field during curing, isotropic or anisotropic MRE cantilevers are obtained. Randomly distributed particles are present in the polymer matrix of the isotropic microcantilevers, whereas the particles in the anisotropic microcantilevers are aligned in the direction of the magnetic field. The fractional changes in the resonance frequencies of the MRE cantilevers are measured as functions of the magnetic field intensity and the quantity of particles in the matrix. The anisotropic microcantilevers undergo greater changes in frequency than the isotropic microcantilevers when exposed to external magnetic fields, which indicates that larger changes in modulus are induced in the anisotropic microcantilevers. In addition, the dissipation and damping ratios of the MRE microcantilevers are determined by fitting the exponential decays of their deflection amplitudes with time.

Facile Phase Transition Measurements for Nanogram Level Liquid Samples Using Suspended Microchannel Resonators

We investigated phase transitions of a PEO-PPO-PEO triblock copolymer and n-heptadecane using a suspended microchannel resonator (SMR). After filling the microchannel of the SMR with each sample in liquid state, changes in the resonance frequency of the SMR were measured as a function of temperature, and then converted into changes in the density of each sample. As temperature increases, PEO-PPO-PEO unimers aggregate and form micelles (unimer-micelle transition), so the density of the polymer decreases and the resonance frequency of the SMR increases. As temperature decreases, n-heptadecane undergoes liquid to rotator phase (liquid-rotator transition), which increases the sample density and decreases the resonance frequency of the SMR. In addition, the liquid-rotator transition of n-heptadecane exhibits a sudden change in the quality factor of the SMR.

A tunable microresonator sensor based on a photocrosslinking polymer microwire

A polyvinylcinnamate (PVCN) microwire was attached between the two tines of a quartz tuning fork (QTF) to form a polymer bridge. Exposure of a PVCN wire-connected QTF to ethanol vapor decreased the modulus of the wire, resulting in a decrease in the resonance frequency. The resonance frequency and Q factor of the resonator were measured as a function of the ethanol vapor concentration. The photocrosslinking of the PVCN wire enhanced the sensitivity of the QTF sensor and offered a facile route to developing a sensor with a tunable resonance frequency.