Applied infrared spectroscopy by nanomechanical resonators

Abstract

Modern analytical demands for expedient characterization and identification of nanomaterials, ranging from environmental contaminants to developing nanopharmaceutical drugs, call for innovative techniques. While state-of-the-art methods like mass spectrometry provide high sensitivity in the picogram range, they require intensive sample preparation, concentration, and separation of low-abundance analytes. These drawbacks make the analyte susceptible to cross-contamination and sample loss.A commonly used alternative for characterizing chemical composition based on functional groups is infrared (IR) spectroscopy. However, the low energy of IR photons limits the sensitivity of conventional transmission spectroscopy to the lower milligram range. Recent advancements, such as photothermal induced resonance (PTIR), which combines atomic force microscopy and photothermal expansion, have enabled the analysis of single nanoparticles down to the classification of 103 molecules. Nevertheless, these sophisticated methods are time-consuming and lack high throughput capabilities. A promising technique for low abundant sample analysis introduced over the past decade is nanomechanical IR spectroscopy (NAM-IR), which exploits thermal-inducedfrequency detuning of nanomechanical resonators in response to absorbed infrared light. Due to photothermal enhancement over conventional transmission spectroscopy, this technique has demonstrated exceptional sensitivity comparable to the level of mass spectrometry. Despite the potential of NAM-IR spectroscopy to analyze various compounds such as femtograms of explosives, nanoparticles, pharmaceuticals, and thin films down to a few nanometers, its application to complex samples remains challenging. Moreover, its practical implementation as a widely applicable analytical tool has remained unrealized.This thesis presents the scientific journey and development of a setup for applied NAMIR spectroscopy from a proof-of-concept stage to an advanced state-of-the-art system. The novel system facilitates fast spectral acquisition featuring thermal desorption capabilities. For the first time, the technique of NAM-IR spectroscopy was combined with thermogravimetric analysis, demonstrating sensitivities as low as 5 pg (∼30 fmol for caffeine) for single compounds and mixtures. The thermal control of the resonator and analyte further enables the acquisition of desorption kinetics and in situ separation of mixed compounds. This suggests the potential of the developed techniques for applications across various scientific domains.The sensitivity of NAM-IR spectroscopy was evaluated across various resonator geometries, analytes, and sampling methods, including impaction-based sampling of generated and environmental aerosols, drop-casted proteins, and spin-coated thin films. In addition to operation with a quantum cascade laser, the NAM-IR system was successfully integrated with Fourier-transform spectroscopy. This has significantly enlarged the accessible spectral range from the so-called fingerprint region to the entire MIR range.