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The atomic force microscope (AFM) and related nanomechanical instruments have become fundamental tools for studying molecular, colloidal, and surface forces in physiological fluid environments down to the pN range.1-5 A key component of the AFM is the soft microfabricated cantilever force transducer and fine probe tip (with a typical end radius of RTIP < 50 nm) that deflects when interacting with a sample surface. Standard Si3N4 probe tips have been used for studying the nanomechanical properties of macromolecules on surfaces,1 and probe tips functionalized with proteins, ligands and receptors, cells, and nanotubes have enabled studies of biologically relevant intermolecular interactions.2-5 Probe tip functionalization has been achieved by covalent immobilization, nonspecific physisorption, and conventional adhesives for larger structures (g1 μm).6-10 However, attachment of macromolecules to the apex of a probe tip with a prespecified orientation, conformation, and density is difficult due to the small surface area involved and, for polyelectrolytes, the presence of fixed charge groups. Functionalization and subsequent characterization of parameters such as the chain grafting density, Γ (chains/nm2), in the vicinity of the tip apex are also difficult and critical to the interpretation of nanomechanical data relating interaction force to tip-sample separation distance. Our recent studies of the biological polyelectrolyte molecules in cartilage,11 chondroitin sulfate glycosaminoglycans (GAGs), called for a new method of attachment and assessment on nanoscale probe tips in order to carry out nanomechanical experiments between two opposing, end-grafted GAG brushes which more closely mimic the physiological conformations found in native tissue. GAGs were chemically end-grafted to an Aucoated Si3N4 probe tip by means of an electric field applied between the probe tip and a nearby Pt electrode. Previously, an electric field was used to attract charged DNA oligonucleotides to a monolayer of single-stranded DNA that had been immobilized on an Au-coated sensor surface through a Au-thiol attachment and a 300 mV potential between the sensor and a Pt electrode was found to enhance hybridization of the DNA oligonucleotides to the DNA monolayer.13 We extended this methodology to use an electric field to drive charged GAG molecules to a nanosized probe tip, thus increasing the local polyelectrolyte concentration in the vicinity of the probe tip and chemisorption via an end-terminal functionality.

Preparation of End-Grafted Polyelectrolyte Brushes on Nanoscale Probe Tips Using an Electric Field