Patterned organic self-assembling monolayers (SAMs) have many applications in both fundamental surface chemistry and various technological fields. Examples include: The study of microscopic mechanisms of friction and wetting, nanoscale surface chemistry, organic light emitting diodes, biochips etc., etc.
We modify SAMs by nanografting1, a process by which the surface structure of a SAM is characterized and modified by atomic force microscopy (AFM). To identify flat areas onto which we can easily graft molecules of a different height, we image the matrix (e.g. initial) SAM at the highest possible resolution. The best resolved AFM images are taken at low imaging forces (~0.05nN for thiols) in a liquid cell containing a solution of the molecules to be nanografted.
Under (low) imaging forces, the AFM tip exerts pressure on the order of 107 Pa on the SAM, causing only local deformations. At higher pressures, the SAM becomes disordered and adsorbates are displaced. Above the imaging force that causes displacement of adsorbates, called the "displacement threshold", the image will show the substrate lattice rather than the SAM. If this is done in the presence of a (larger) molecule solution which has, therefore, a higher concentration than the displaced SAM, the new molecules will immediately graft onto the cleared substrate surface. When the displaced area of the matrix SAM has been replaced by the new adsorbates, the modified surface structure can be characterized by scanning at low imaging forces.
What We Have Done:
Using nanografting we have patterned, imaged and characterized inhomogeneous SAMs of thiols on Au(111) at nanometer scale. We can more accurately compare the molecular properties of different adsorbates in a patterned SAM than in separate experiments because the different molecules of the patterned SAM share the same environment and are being studied by exactly the same AFM tip. Using the lateral force microscopy, we also have studied the friction of the SAMs and correlated the results with the topographical information obtained by height measurements.
What We Plan to Do:
Oligo (ethylene glycol)-terminated SAMs are one of the very few surfaces in nature that do not absorb proteins. These SAMs exhibit different structures on gold and silver.2 As a result, these substratesí resistance to protein adsorption is likely to vary. By nanografting PEG into SAMs of thiols, we should obtain a good reference point for further research on protein adsorption. Since SAM friction is related to SAM density,3 PEG SAMs should have different friction vs load curves on gold and silver substrates.
The electronic properties of de novo protein [Ru(P20)3] in solution have been studied extensively. By nanografting [Ru(P20)3] in a matrix SAM of thiols, we can use conducting-tip AFM to correlate electronic properties to the conformation of the molecules.
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Application of Nanografting to Differential Measurements of
Organic Surface Properties
1. Liu, GY, Xu S and Qian Y. Acc.Chem.Res. 33 (2000).
2. Feldman K, Hahner G, Spencer ND, harder P and Grunze MJ. Am. Chem.Soc. 121 (1999).
3. Barrena E, Ocal C and Salmeron M. J.Chem.Phys. 113 (2000).