In all measured NAM-IR spectra of PS/PVP mixtures, spectral elements from both PS and PVP are visible, with those for PVP becoming dominant as higher concentrations of this polymer are added. For instance, the large peak at 1685 cm−1, caused by carbon–oxygen double bonds and deriving from PVP, increases in intensity, while the sharp PS-derived peak at 1484 cm−1, stemming from carbon–hydrogen bonds in the aromatic ring, is fading out.
Based on these initial measurements, the NAM-IR method appears to be well suited to analyze mixed and polluted samples. Peaks generally appear sharp and with little tendency to merge, making individual spectral features relatively clear. However, currently samples with absorption peaks of very different intensities are difficult to measure; for large intensities, the duty cycle of the IR laser has to be quite low, in order for the phase-locked loop to stay locked in, and this makes the smaller absorption peaks difficult to identify.
4. Conclusion
Nanomechanical IR spectroscopy (NAM-IR), with its effective intrinsic ENM sampling, has been presented as an excellent tool for fast chemical analysis of minute sample amounts.
The geometry of the mechanical string resonators used for NAM-IR is important, and in accordance with the appertaining theory, the results presented here show that long and narrow strings are preferable for both efficient sampling and sensitive measurements. However, the longer relaxation time for long and narrow strings will have to be accounted for.
NAM-IR is still in its infancy, and accordingly several issues can be improved, including the bulky and not very user-friendly experimental setup, and the IR laser. Furthermore, the sensitivity when measuring a mixture, or another complex sample, can be enhanced by improving the frequency setup to measure larger intensity differences.
However, even at this early stage, NAM-IR exhibits several extraordinary features; the time necessary for preparing a sample for IR spectroscopy is no more than a few minutes with this method, a drastic improvement from the ∼2 days that is typically needed for freeze-drying of a sample for ATR-FTIR, which is still considered state-of-the-art within IR spectroscopy. With respect to the necessary amount of sample, NAM-IR also outmatches ATR-FTIR, requiring only picograms of deposited sample as opposed to milligrams. NAM-IR spectra closely resemble those of ATR-FTIR and can be obtained at similar resolutions.
Acknowledgements
The authors thank Maksymilian Kurek for the cleanroom support. Financial support from the Villum Foundation's Young Investigators Programme (project no. VKR023125) and the Villum Kann Rasmussen Centre of Excellence “NAMEC” (contract no. 65286) is gratefully acknowledged.
NAM-IR is still in its infancy, and accordingly several issues can be improved, including the bulky and not very user-friendly experimental setup, and the IR laser. Furthermore, the sensitivity when measuring a mixture, or another complex sample, can be enhanced by improving the frequency setup to measure larger intensity differences.