IR spectroscopy has been a valuable research tool for decades in various fields, such as pharmacy, forensic science and industrial process control [3], [4], [5] and [6]. However, conventional IR spectroscopy techniques such as attenuated total reflectance-Fourier transform IR (ATR-FTIR) spectroscopy still require relatively large sample amounts and extensive sample preparation. A sample mass of 1–6 mg is typically used for ATR-FTIR [7],[8] and [9], although samples down to 0.25 mg have been analyzed by this method [10]. Aqueous dispersions of e.g. drugs or colloidal systems are typically dried before measurement, since water is known to disturb IR spectroscopic measurements. Depending on sample concentration, freeze-drying enough material for one measurement can take several days.
In this paper, nanomechanical IR spectroscopy (NAM-IR) is demonstrated to be highly suitable for analysis of ENMs, yielding results similar to those obtained with ATR-FTIR, but in a few minutes, sample preparation included. 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.