Nanoscale electric polarizability of ultrathin biolayers on insulating substrates by electrostatic force microscopy

Abstract

We measured and quantified the local electric polarization properties of ultrathin (∼5 nm) biolayers on mm-thick mica substrates. We achieved it by scanning a sharp conductive tip (<10 nm radius) of an electrostatic force microscope over the biolayers and quantifying sub-picoNewton electric polarization forces with a sharp-tip model implemented using finite-element numerical calculations. We obtained relative dielectric constants εr = 3.3, 2.4 and 1.9 for bacteriorhodopsin, dioleoylphosphatidylcholine (DOPC) and cholesterol layers, chosen as representative of the main cell membrane components, with an error below 10% and a spatial resolution down to ∼50 nm. The ability of using insulating substrates common in biophysics research, such as mica or glass, instead of metallic substrates, offers both a general platform to determine the dielectric properties of biolayers and a wider compatibility with other characterization techniques, such as optical microscopy. This opens up new possibilities for biolayer research at the nanoscale, including nanoscale label-free composition mapping.

Nanoscale dielectric microscopy of non-planar samples by lift-mode electrostatic force microscopy

“…The use of lift mode imaging ensures the maximum sensitivity in all positions of the sample, and our analysis ensures the results are free from topographic crosstalk artefacts. We would like to highlight, however, that for planar samples or low dimensional non-planar samples (like nanoparticles, nanotubes, etc,) the use of constant height imaging mode can be preferred since the accuracy required (very often in the sub-1zF/nm) [26,27,29] cannot be offered by the reconstruction procedure presented here.…”

“…The reason being that for metallic substrates the cantilever contribution is included in both the lift EFM image and the reconstructed crosstalk image, and hence it is automatically subtracted and does not contribute to the intrinsic capacitance gradient image. Note, however, that in the case of thick insulator substrates, the microscopic parts of the probe, such as the cantilever, induce some indirect effects in addition to the direct stray effect mentioned above, and some contribution from them need to be included in the model [29,35].…”

“…Among them, we can cite nanoscale capacitance microscopy [1][2][3], electrostatic force microscopy (EFM) [4][5][6][7][8][9][10], nanoscale impedance microscopy [11,12], scanning polarization force microscopy [13][14][15][16], scanning microwave microscopy (SMM) [17,18] and nanoscale non-linear dielectric microscopy [19]. These techniques have allowed measuring the electric permittivity with nanoscale spatial resolution on planar samples, such as thin oxides, polymer films and supported biomembranes [2][3][4]8,10], and on non-planar ones, such as, single carbon nanotubes, nanowires, nanoparticles, viruses and bacterial cells [20][21][22][23][24][25][26][27][28][29][30].…”

Internal Hydration Properties of Single Bacterial Endospores Probed by Electrostatic Force Microscopy

“…Note that EFM measurements are more sensitive to the hygroscopic properties than the AFM images themselves, as it can be seen by comparing the relative variations produced by a RH change on the electric signals ( Figure 1l) and on the topographic profiles ( Figure 1d). The reason being that the electric permittivity of water (ε r,water~8 0) is much larger than that of the dry biochemical components of the endospore (ε r,proteins ~3-5 for proteins, 48,49 ε r,lipids~2 for lipids 48 and ε r,DNA~8…”

“…EFM is a scanning probe microscopy technique sensitive to the local dielectric properties of the samples. 33,34 Examples showing this ability include numerous applications to samples of non-biological origin (thin and thick oxides, 35 polymer films, [36][37][38] nanowires, 39 nanotubes 40,41 or nanoparticles [42][43][44][45] ), and of biological origin (single bacterial cells, 46,47 single virus particles, 45 solid supported biomembranes, 48 protein complexes 49 or DNA molecules). 50 EFM has two important properties relevant for the present application, namely, (i)…”