Infrared-spectroscopic, dynamic near-field microscopy of living cells and nanoparticles in water

Abstract

Infrared fingerprint spectra can reveal the chemical nature of materials down to 20-nm detail, far below the diffraction limit, when probed by scattering-type scanning near-field optical microscopy (s-SNOM). But this was impossible with living cells or aqueous processes as in corrosion, due to water-related absorption and tip contamination. Here, we demonstrate infrared s-SNOM of water-suspended objects by probing them through a 10-nm thick SiN membrane. This separator stretches freely over up to 250 µm, providing an upper, stable surface to the scanning tip, while its lower surface is in contact with the liquid and localises adhering objects. We present its proof-of-principle applicability in biology by observing simply drop-casted, living E. coli in nutrient medium, as well as living A549 cancer cells, as they divide, move and develop rich sub-cellular morphology and adhesion patterns, at 150 nm resolution. Their infrared spectra reveal the local abundances of water, proteins, and lipids within a depth of ca. 100 nm below the SiN membrane, as we verify by analysing well-defined, suspended polymer spheres and through model calculations. SiN-membrane based s-SNOM thus establishes a novel tool of live cell nano-imaging that returns structure, dynamics and chemical composition. This method should benefit the nanoscale analysis of any aqueous system, from physics to medicine.

Nanoscale Probing of the Supramolecular Assembly in a Two‐Component Gel by Near‐Field Infrared Spectroscopy

“…The interferometric recording of the scattered light provides infrared spectra at a spatial resolution approximately equivalent to that of the tip (20 nm) [21–28] . sSNOM has been used for the analysis of polymeric thin film surfaces, [24,29–35] imaging of cells, [36–40] viruses [41,42] and neurons [43] and the structural elucidation of protein fibrils [44–46] …”

Scattering-type Scanning Near-Field Optical Microscopy of Polymer-Coated Gold Nanoparticles

“…We consider these observations to be important in light of s-SNOM’s capabilities for nanoscale spatial resolution, which represent an important advantage when studying Au NP-related aspects, such as their distribution in doped materials or inside cells. Such latter studies are greatly facilitated by recent efforts that have been focused on s-SNOM imaging of living and fixed cells, which have resulted in valuable investigation platforms that can characterize various types of cell structures and processes. We argue that these currently introduced methodologies can be straightforwardly used to assess as well subtle aspects related to the interactions occurring between cells and biomedically functionalized (or hazardous) nanomaterials, to resolve important pending issues such as nanoparticle trafficking by cells .…”

“…We consider our observations to be important, as they may facilitate studies and applications in nanomedicine and nanotechnology where the precise positioning of Au NPs with nanoscale resolution is needed. In this context, we find it worthy to mention that s-SNOM has been demonstrated to date as a very useful tool to image proteins, , viruses, − prokaryotic and eukaryotic cells, − both fixed and living tissues and various types of nanostructured materials, including nanoparticles of different kinds, ,, providing valuable information based on the sample intrinsic contrast. Such capabilities render s-SNOM as a superb tool for supporting studies that focus on the interaction of advanced materials and biological organisms and structures, at nanoscale.…”

Toward augmenting tip-enhanced nanoscopy with optically resolved scanning probe tips

“…The contrast mechanisms of s-SNOM enable mapping the dielectric properties 28 – 31 and chemical composition 14 of samples, which has so far facilitated a wide range of discoveries in condensed phase and two-dimensional materials 32 – 40 Additionally, s-SNOM applications in biology enable precise mapping of specific cellular and subcellular structures of interest, 41 – 44 and even mapping of the exact values of important optical parameters such as the refractive index 45 . Although s-SNOM is generally acknowledged as a surface characterization technique, it nonetheless exhibits valuable capabilities for subsurface and 3D imaging 46 …”