Poly(amino acids) at Si-oxide interfaces - bio-colloidal interactions, adhesions and 'conformation'

Biopolymers in a confined solution environment will be subject to long-range electrostatic and short-range van der Waals interactions. The interactions between two model poly(amino acids), poly-l-lysine and poly-l-glutamic acid, in solution with a Si-oxide surface have been investigated. The model amino acids were adsorbed, or covalently coupled, to colloidal probes consisting of a micro-sphere attached to a force-sensing lever. The methodology was based on sensing interaction between the probe and a flat surface through carrying out force versus distance analysis with a scanning force microscope. The framework of the conventional DLVO theory was used to arrive at a qualitative description. The outcomes illustrate both repulsive and attractive long-range interactions that will hinder, or promote, transport of colloidal biospecies into the region of attractive short-range interactions at the physical interface. Large 'snap-on' distances were observed for some systems and were ascribed to compression of the 'soft' functionalised layers. Those observations and measurements of adhesion provided insights into conformation of the adsorbed species and strength of attachment. The results are relevant to design and functionality of Si-based microfluidics technology