TOPOGRAPHY AND RECOGNITION IMAGING OF CELLS - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 144

ligands onto the AFM tip via a long, lexible tether, such as polyethylene glycol
(PEG) chains. 7 The immobilization of the sensor molecule via the lexible
linker gives the ligand the freedom to adopt the correct orientation, and this
indeed increases the chances of receptor detection on the surface.
The attachment of ligands onto AFM tips via PEG chains is usually
performed in three steps as illustrated in Fig. 7.1 . Firstly, amino (-NH 2 )
groups are produced on the tip either by the esteriication of the supericial
silicon oxide layer with ethanolamine hydrochloride in dimethylsulfoxide 8
( Fig. 7.1 , step Ia) or gas phase silanization with 3-aminopropyltriethoxysilane
similar to the procedure described by Lyubchenko

and co-workers 9
( Fig. 7.1 , step Ib). It has proved critical to use methods that do not signiicantly
increase roughness and/or stickiness of the tip surface. In the second step,
heterobifunctional PEG chains are attached by one end to the amino group

(a) (b)

Figure 7.1. AFM tip functionalization with proteins via PEG linkers. (I) Amino-
functionalization of silicon nitride tips either via (a) esteriication with ethanolamine
or (b) silanization with 3-aminopropyltriethoxysilane (APTES) from the gas phase.
(II) Use of heterobifunctional NHS-PEG-aldehyde linker for lexible attachment of
underivatized protein onto the AFM tip. (III) The C=N double bond is usually ixed by
a reaction with sodium cyanoborohydride (NaCNBH 3 ).