Efficient bioconjugation of protein capture agents to biosensor surfaces using aniline-catalyzed hydrazone ligation.

Aniline-catalyzed hydrazone ligation between surface-immobilized hydrazines and aldehyde-modified antibodies is shown to be an efficient method for attaching protein capture agents to model oxide-coated biosensor substrates. Silicon photonic microring resonators are used to directly evaluate the efficiency of this surface bioconjugate reaction at various pHs and in the presence or absence of aniline as a nucleophilic catalyst. It is found that aniline significantly increases the net antibody loading for surfaces functionalized over a pH range from 4.5 to 7.4, allowing derivatization of substrates with reduced incubation time and sample consumption. This increase in antibody loading directly results in more sensitive antigen detection when functionalized microrings are employed in a label-free immunoassay. Furthermore, these experiments also reveal an interesting pH-dependent noncovalent binding trend that plays an important role in dictating the amount of antibody attached onto the substrate, highlighting the competing contributions of the bioconjugate reaction rate and the dynamic interactions that control opportunities for a solution-phase biomolecule to react with a substrate-bound reagent.

Solvent-dependent friction force response of poly(ethylenimine)-graft-poly(ethylene glycol) brushes investigated by atomic force microscopy.

Lateral and normal forces between a surface-bound, brushlike copolymer, poly(ethylenimine)-graft-poly(ethylene glycol) (PEI-g-PEG), and a silica colloidal probe were investigated with atomic force microscopy (AFM) and related to the relative mass of the solvent absorbed within the polymer as measured with the quartz crystal microbalance. PEI-g-PEG was adsorbed onto an oxide-passivated silicon wafer through its exposure to physiologically buffered solutions of the polymer. Frictional forces were measured between the colloidal probe and the substrate by AFM as the polarity of the solvent was systematically varied. Reduced friction forces and greater film thicknesses were encountered under solvents of higher polarity, which are attributed to the extended conformation of the brushlike copolymer under these conditions. Lateral and normal forces detected between the colloidal probe and this surface-bound PEI-g-PEG were found to be similar under certain solvent conditions to those measured for poly(L-lysine)-graft-poly(ethylene glycol), a brushlike copolymer with a different molecular architecture. To this end, friction force studies of both symmetric and asymmetric PEI-g-PEG-coated interfaces served to identify the contributions of conformational and bridging effects in the observed tribological behavior.

Tribological properties of poly(L-lysine)-graft-poly(ethylene glycol) films: influence of polymer architecture and adsorbed conformation.

The tribological properties of poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG)-coated oxide interfaces have been investigated with atomic force microscopy (AFM) as a function of the molecular structure. Polymer-bearing surfaces were obtained via spontaneous adsorption of the polymer onto the oxide substrate from a buffered solution of physiological pH. Interfacial friction of these PLL-g-PEG-coated surfaces was found to be highly dependent on the duration of deposition and the architecture of PLL-g-PEG. In terms of the architecture, the PEG chain length and the grafting ratio (i.e., the molar ratio of L-lysine monomer to PEG side chain) of adsorbed PLL-g-PEG significantly influence the interfacial friction; specifically, friction is reduced as the PEG chain length increases and as the molar ratio of L-lysine monomer to PEG side chain decreases. The characteristics of the polymer deposition time and the influence of the lysine/PEG grafting ratio are rationalized in terms of spatial packing density considerations.

Solvent dependent friction force response of polystyrene brushes prepared by surface initiated polymerization.

Polystyrene (PS) brushes were prepared on oxide passivated silicon by the surface initiated polymerization (SIP) technique. From an AIBN-type free radical initiator, which was silanized and immobilized on silicon wafers, styrene brushes were directly polymerized and grafted from the surface. The formation of the initiator monolayer and, subsequently, the polymer brush on the surface were monitored by X-ray photoelectron spectroscopy (XPS) and ellipsometry. Friction force measurements were performed by atomic force microscopy (AFM), using a 5 microm SiO2 colloidal sphere tip and under systematically varied solvent environments (nonpolar to polar), to demonstrate the dependence of brush lubricity on solvation. The relative uptake of solvents in the PS brush was determined by quartz crystal microbalance (QCM), and it correlates well with friction data. It is surmised that, in poor solvent environments, the polymer brush exists in a collapsed conformation, giving rise to the higher observed friction response.