Spectroscopic ellipsometry meets AFM nanolithography: about hydration of bio-inert oligo(ethylene glycol)-terminated self assembled monolayers on gold

Abstract

For the first time, to our knowledge, spectroscopic ellipsometry (SE) has been combined with state-of-the-art AFM differential height measurements conducted after shaving nano-lithography of ultrathin, soft-matter films for thickness determination. We investigated self-assembled monolayers of SH-(CH2)11-EGn-OH molecules on gold, where EG is ethylene glycol units and n = 3 and 6, a prototypical non-fouling system. We performed SE measurements (245-1200 nm) focusing on the changes induced by the formation of the film (difference spectra). SE measurements, analysed by simple models, confirm the formation of the S-Au interface, transparency of the SAMs and provide a sharp picture of the ability of the EG functionality to protect the surface from unspecific adsorption of proteins. A quantitative assessment of the film thickness by SE was carried out ex situ, thanks to the optical contrast between the film and the ambient, and by AFM in liquid. The cross-check between SE and AFM height measurements combined with the comparison between in-liquid and ex situ SE measurements allowed obtaining non-perturbative information about the vertical density profile of the SAM. The in-liquid SE measurements indicate a refractive index matching between the aqueous medium and the outer part of the SAM, consistent with a disordered configuration of OEG and/or the penetration of water amid the OEG strands. A critical discussion provides a detailed insight into the subtle issues and pitfalls related to the thickness determination of soft-matter films to the monolayer limit.

Selective Adsorption on Fluorinated Plastic Enables the Optical Detection of Molecular Pollutants in Water

“…We show that, in this index-matching condition and for a thickness h of the adsorbed layer much smaller the wavelength of light , the reflected or scattered intensity I is related to the amount of adsorbed molecules by simple optical models. More generally, other well-established techniques enable the characterization of the structure of a thin molecular layer on a planar surface by detailed modeling of the spectrum and polarization of the reflected light [31] [32]. In this work we exploit a different approach: we simply measure the increase of reflected or scattered intensity of monochromatic light upon formation of the layer.…”

Repelling and ordering: the influence of poly(ethylene glycol) on protein adsorption

“…Therefore, in the development of drug nanocarriers, biosensors or anti-biofouling coatings, protein resistant surfaces play an important role to prevent nonspecific protein adsorption. [1][2][3][4][5] A common approach to obtain protein repellent, or so-called anti-fouling, coatings relies on the use of hydrophilic or zwitterionic materials. [6][7][8] Among these materials poly(ethylene glycol) (PEG) is a widely used and thoroughly studied hydrophilic protein repelling polymer.…”

Investigating organic multilayers by spectroscopic ellipsometry: specific and non-specific interactions of polyhistidine with NTA self-assembled monolayers

“…The suffixes 1 and 0 of δΔ 1,0 and δΨ 1,0 correspond to the SAM–Au and to the Au system, respectively. The results from NTA SAMs are compared with those obtained recently for oligo(ethylene glycol)-terminated alkanethiols, HS-C11-EG3 (T-OEG3) and HS-C11-EG6 (T-OEG6), for which the thicknesses were accurately determined by SE and AFM nanolithography [ 49 ].…”

“…In a very recent work we successfully combined our approach with thickness determination based on AFM measurements exploiting smart nanolithography methods, such as the so-called nanoshaving [ 49 ], that considerably helped to reduce the uncertainty related to the correlation between thickness and refractive index that is typical of ultrathin films.…”