Miniaturization of Photothermal Cantilever Deflection Spectroscopy with an Electrical Readout

Abstract

Detection, differentiation, and quantification of trace amounts of chemical analytes with a portable device are of great interest in many applications such as energy, environment, law-enforcement, national security, and health. With the recent progress of micro/nanofabrication and system integration technologies, micro/nanoelectromechanical systems are regarded as very strong candidates to satisfy the current requirements of portable sensors such as high sensitivity, miniature size, multiple detection, low-cost, and low-power consumption. The enhancement in quality of life and economic growth are anticipated by realizing and exploiting high-performance portable chemical sensing systems in the near future. However, achieving this goal using the current approach based on the development of chemoselective interfaces is proving to be a challenging task, especially for the realization of reversible sensors at room temperature. Responses from a sensor with immobilized chemoselective interfaces often show interference due to the presence of other similar molecules and give false signals. This chemical selectivity challenge can be overcome by photothermal cantilever deflection spectroscopy (PCDS), which exploits the high thermomechanical sensitivity of a bi-material microcantilever. A bi-material cantilever responds to heat generated by non-radiative decay process when the adsorbed molecules are resonantly excited with infrared (IR) light. Monitoring the variation in cantilever deflection amplitude as a function of illuminating IR wavelength corresponds to the conventional IR absorption spectrum of the adsorbed molecules. In addition, the mass of the adsorbed molecules can be determined by measuring the resonance frequency shift of the cantilever for the quantitative analysis. This photothermal spectroscopy technique offers unprecedented opportunities for highly selective as well as sensitive chemical sensing without relying on chemoselective interfaces. However, conventional optical signal readout schemes employed for microcantilever sensors limit miniaturization and system level integration. Here, we demonstrate PCDS using a piezoresistive cantilever which can be easily miniaturized and integrated into a portable sensor platform.