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Photothermal Spectroscopy of Confined Liquids Using Microfluidic Cantilevers
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Due to their ability to measure extremely small displacements and forces, nanomechanical cantilevers have attracted considerable attention from numerous scientific communities, each exploring a variety of applications. Though typically operated in either vacuum or air, operation of the device in liquid media remains highly challenging, primarily due to strong viscous damping. In order to overcome this limitation a variation of the microfluidic cantilever sensor capable of confining 1-300 picoliter volumes of liquid sample was fabricated. The capabilities of this platform were then investigated in both the static and dynamic mode of operation.
While calorimetry-based bi-material cantilever spectrometer has been shown to be a very promising platform when operated in air, this technique loses its sensitivity when operated in the liquid phase due to significantly reduced extinction lengths and increased thermal losses. It is thus not suitable for infrared spectroscopic measurements therein and an alternate approach is required.
Confining a liquid inside the microcantilever affords a means to overcome the limitations inherent to standard bi-material microcantilevers by decreasing thermal loss and viscous damping, and allowing for the study of calorimetry-based spectroscopy of liquids. Simultaneously, the low resolution and signal-to-noise ratio of mid infrared (MIR) spectroscopy in aqueous media can be addressed by employing a quantum cascade laser as the light source. The effects of solvent-solute interactions on the absorption peaks related to in ethanol have been investigated using this micromechanical calorimetric spectroscopy platform to collect infrared (IR) spectra of ethanol-water mixtures. The results revealed a power law dependence of the IR absorption peak positions to the induced dipole moments of ethanol in the ethanol-water mixtures. Using such a microfluidic based spectroscopy method can provide high-resolution liquid spectroscopy measurements to further investigate intermolecular interactions.
The thermal sensitivity of a bi-material microcantilever plays a critical role in calorimetric spectroscopy when the device is used to collect photothermal spectra of liquid samples. However, further improvements in their applications as a spectroscopic platform depends on an enhanced understanding of the device’s response to heat. In this work, a new model applicable to the bi-material microfluidic cantilever is presented when the device undergoes uniform heating. The presented analysis indicates that an increase in the thermal sensitivity, resulting from reducing the channel height, can improve the photothermal response of this platform and allow for improving the sensitivity, resolution, and selectivity of photothermal deflection spectroscopic measurements.
Piezoelectric crystals in feedback loops have been mainly employed as driving mechanisms for microfluidic cantilever resonators when the device was used to analyze liquid samples. However, there has not yet been any report on actuating a microfluidic cantilever by applying AC voltage on a confined electrolyte solution. In this work, nanograms of NaHSO4 and NaCl solutions in combination with an AC potential difference were used to actuate microfluidic cantilevers. The results indicate that the resonance amplitude increases as a function of applied voltage. However, an increase in the concentration of the electrolyte does not necessarily result in an increase in the amplitude. This concept can possibly be employed to design a new online actuation method in the future. -
- Date created
- 2018-04-01
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- Type of Item
- Research Material