Molecular recognition using nanomechanical photothermal spectroscopy

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Despite advantages such as small size, high sensitivity, array-based detection, and low power consumption, miniature sensors such as microcantilevers still remain as a laboratory tool. This is mainly due to the lack of selectivity and reproducibility in detection, especially when detecting small molecules. Selectivity challenge has been a recurring problem with indirect sensing platforms such as those based on variations in mass, electrical, optical, and mechanical properties induced by molecular adsorption. Reproducibility issues arise from the lack of uniformity in chemical/biological interfaces that serves as the basis for selective adsorption of target molecules. These problems can be overcome by combining the physical sensitivity of a cantilever with an orthogonal, physics-based signal generation, for example, mid-infrared spectroscopy. Resonant excitation of vibrational modes of molecules adsorbed on a bimaterial cantilever creates small temperature changes that can be detected as bending of the cantilever. This technique is label-free as well as receptor-free and depends only on the physical properties of the molecules. The technique, therefore, combines the extremely high sensitivity of the cantilever with very high selectivity of molecular vibrations for molecular recognition. The reproducibility challenge is overcome by monitoring the adsorbed mass simultaneously from resonance frequency variation. The technique has the potential of investigating a single molecule using a nanocantilever. In addition to serving as a platform for chemical and biological sensing, the concept can be used for investigating the thermal characteristics of molecules.