Nanomechanical Properties of Polyethylene Glycol Brushes on Gold Substrates

A necessary step in advancing the use of polyethylene
glycol (PEG)
surface coatings in critical biotechnological applications such as
cancer treatments is to provide direct and reliable nanoscale property
characterization. Measurements for such characterization are currently
provided by scanning probe methods, which are capable of assessing
heterogeneity of both surface coverage and properties with nanoscale
spatial resolution. In particular, atomic force microscopy (AFM) can
be used to detect and quantify the heterogeneity of surface coverage,
whereas atomic force spectroscopy can be used to determine mechanical
properties, thereby revealing possible heterogeneity of properties
within coatings. In this work, AFM and force spectroscopy were used
to characterize the morphology and mechanical properties of thiol-functionalized
PEG surface coatings on flat gold substrates in aqueous PEG solution.
Thiol-functionalized PEG offers a direct and simple method of attachment
to gold substrates without intermediate anchoring layers and therefore
can be exploited in developing PEG-functionalized gold nanoparticles.
AFM was used to investigate the morphology of the PEG coatings as
a function of molecular weight; the commonly observed coverage was
in the form of sparse, brushlike islands. Similarly, force spectroscopy
was utilized to study the mechanical properties of the PEG coatings
in compression and tension as a function of molecular weight. A constitutive
description of the mechanical properties of PEG brushes was achieved
through a combinatorial analysis of the statistical responses acquired
in both compression and tension tests. Such a statistical characterization
provides a straightforward procedure to assess the nanoscale heterogeneity
in the morphology and properties of PEG coverage.