Characterization of polyethylene glycol self-assembled monolayers by means of sum-frequency generation spectroscopy for biosensor applications

Abstract

Protein biochips are miniaturized biological sensors intended to analyze and characterize biomolecule interactions with high throughput. An important issue when developing such biochips is substrate passivation. The support is rendered inert in order to avoid non-specific adsorption. Strategic control of the non-specific protein adsorption can be achieved by creating a resistant self-assembled monolayer (SAM) based on polyethylene glycol (PEG). The degree of resistance depends on the PEG surface density, i.e. the number of PEG units the molecule contains. Infrared-visible sum-frequency generation (SFG) spectroscopy (Lambert et al., Appl Spectrosc Rev 40:103–145, 2005) is used to in-vestigate the vibrational fingerprint of a PEG self-assembled monolayer adsorbed on a flat platinum surface, in the 2,750–3,050 cm-1 frequency range. The objec-tive is to characterize the SFG baseline of the biosensor that will be further developed by mixing the PEG antifouling layer with bioactive host molecules. Nanostructures will then be implemented on the substrate in order to enhance the SFG signal through localized surface plasmon resonances (Lis et al., Adv Opt Mater 1:244–255, 2013). The ultimate goal will be to detect the SFG signature of the antigen/antibody recognition process at the interface of the above biosensing layer. The general chemical formula of the molecule investigated is C3H5S2 -(CH2)4 –C= O-NH-(CH2 -CH2 -O)n -CH3. The molecule studied here holds 7 polyethylene glycol units (n= 7). Former studies carried out by quartz crystal microbalance (QCM) and electrochemistry showed that molecules containing 7 PEG chains have the best antifouling properties. The SFG spectra present several vibrational modes (Sartenaer et al., Biosens Bioelectron 22:2179–2183, 2007; Cimatu et al., J Phys Chem C 112:14529–14537, 2008; Even et al., Macromolecules 39:9396–9401, 2006). The most significant contributions come from the CH2, O-CH2 and O-CH3 modes.