Samples are collected on nanomechanical string resonators and exposed to monochromatic IR light from a quantum cascade laser (QCL). IR light absorbed by the sample is transferred into a measurable frequency detuning of the string, due to photothermal heating. The resonance frequency shift is directly proportional to the absorbed energy. A NAM-IR spectrum is thus readily obtained by recording this photothermal frequency detuning of the resonator.
NAM-IR has the particular benefit of ultra-fast sample preparation, owing to the non-diffusion limited sampling method used to collect material on the strings. Measurement including sample preparation takes only a few minutes, a significant reduction from that required for conventional methods. Sampling on strings is done by nebulizing ENMs directly from dispersion using an electrospray or jet nebulizer setup. The aerosol containing the material to be analyzed flows through an orifice in the sensor chip, over which the resonator is spanned. In this way, the nanomechanical resonator functions as a single filter-fiber (Fig. 1) [11] and [12].