nanoIR Journal Club

As part of our work in nanoscale infrared (nanoIR) spectroscopy, we regularly come across great research articles. Members of our nanoIR Journal Club receive brief reviews of select papers, collected below. Sign up to automatically receive the monthly Journal Club via email: 

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PROBE-SAMPLE INTERACTION-INDEPENDENT ATOMIC FORCE MICROSCOPY-INFRARED SPECTROSCOPY: TOWARD ROBUST NANOSCALE COMPOSITIONAL MAPPING
Seth Kenkel, Anirudh Mittal, Shachi Mittal, and Rohit Bhargava
ANAL. CHEM, 2018, 90, 8845-8855
DOI: 10.1021/acs.analchem.8b00823
(This article is paywalled, but if you have access to it, you can find it at: https://pubs.acs.org/doi/pdf/10.1021/acs.analchem.8b00823)

Nanoscale topological imaging using atomic force microscopy (AFM) combined with infrared (IR) spectroscopy (AFM-IR) is a rapidly emerging modality to record correlated structural and chemical images. Although the expectation is that the spectral data faithfully represents the underlying chemical composition, the sample mechanical properties affect the recorded data (known as the probe–sample-interaction effect). Although experts in the field are aware of this effect, the contribution is not fully understood. Further, when the sample properties are not well-known or when AFM-IR experiments are conducted by nonexperts, there is a chance that these nonmolecular properties may affect analytical measurements in an uncertain manner. Techniques such as resonance-enhanced imaging and normalization of the IR signal using ratios might improve fidelity of recorded data, but they are not universally effective. Here, we provide a fully analytical model that relates cantilever response to the local sample expansion which opens several avenues. We demonstrate a new method for removing probe–sample-interaction effects in AFM-IR images by measuring the cantilever responsivity using a mechanically induced, out-of-plane sample vibration. This method is then applied to model polymers and mammary epithelial cells to show improvements in sensitivity, accuracy, and repeatability for measuring soft matter when compared to the current state of the art (resonance-enhanced operation). Understanding of the sample-dependent cantilever responsivity is an essential addition to AFM-IR imaging if the identification of chemical features at nanoscale resolutions is to be realized for arbitrary samples.


BOUNDARY-INDUCED AUXILIARY FEATURES IN SCATTERING-TYPE NEAR-FIELD FOURIER TRANSFORM INFRARED SPECTROSCOPY
Jiong Yang, Mohannad Mayyas, Jianbo Tang, Mohammad B. Ghasemian, Honghua Yang, Kenji Watanabe, Takashi Taniguchi, Qingdong Ou, Lu Hua Li, Qiaoliang Bao, Kourosh Kalantar-Zadeh
ACS Nano, 2020, XXXX, XXX, XXX-XXX
DOI: 10.1021/acsnano.9b08895
(view article: https://pubs.acs.org/doi/10.1021/acsnano.9b08895#)

Phonon-polaritons (PhPs) in layered crystals, including hexagonal boron nitride (hBN), have been investigated by combined scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared (FTIR) spectroscopy. Nevertheless, many of such s-SNOM-based FTIR spectra features remain unexplored, especially those originated from the impact of boundaries. Here we observe real-space PhP propagations in thin-layer hBN sheets either supported or suspended by s-SNOM imaging. Then with a high-power broadband IR laser source, we identify two major peaks and multiple auxiliary peaks in the near-field amplitude spectra, obtained using scattering-type near-field FTIR spectroscopy, from both supported and suspended hBN. The major PhP propagation interference peak moves toward the major in-plane phonon peak when the IR illumination moves away from the hBN edge. Specific differences between the auxiliary peaks in the near-field amplitude spectra from supported and suspended hBN sheets are investigated regarding different boundary conditions, associated with edges and substrate interfaces. The outcomes may be explored in heterostructures for advanced nanophotonic applications.


DETERMINATION OF POLYPEPTIDE CONFORMATION WITH NANOSCALE RESOLUTION IN WATER
Georg Ramer, Francesco Simone Ruggeri, Aviad Levin, Tuomas P. J. Knowles, Andrea Centrone
ACS Nano, 2018, 12, 7, 6612-6619
DOI: 10.1021/acsnano.8b01425
(view article: https://pubs.acs.org/doi/10.1021/acsnano.8b01425)

The folding and acquisition of proteins native structure is central to all biological processes of life. By contrast, protein misfolding can lead to toxic amyloid aggregates formation, linked to the onset of neurodegenerative disorders. To shed light on the molecular basis of protein function and malfunction, it is crucial to access structural information on single protein assemblies and aggregates under native conditions. Yet, current conformation-sensitive spectroscopic methods lack the spatial resolution and sensitivity necessary for characterizing heterogeneous protein aggregates in solution. To overcome this limitation, here we use photothermal-induced resonance to demonstrate that it is possible to acquire nanoscale infrared spectra in water with high signal-to-noise ratio (SNR). Using this approach, we probe supramolecular aggregates of diphenylalanine, the core recognition module of the Alzheimer’s β-amyloid peptide, and its derivative Boc-diphenylalanine. We achieve nanoscale resolved IR spectra and maps in air and water with comparable SNR and lateral resolution, thus enabling accurate identification of the chemical and structural state of morphologically similar networks at the single aggregate (i.e., fibril) level.