The authors thank the DFG (Hu 997/2-2) for financial support. FS acknowledges the Hanns-Seidel-Stiftung (HSS). SKr was supported by the Elitenetzwerk Bayern in the framework of the doctorate program Material Science of Complex Interfaces. SKi thanks the SFB 863 for financial support.

NOTE: See Figure 2 for an overview of the process flow comprising the preparation, the data acquisition and data analysis steps.
1. Reagent Setup
NOTE: All chemicals must be handled with care, and thus a lab coat, gloves and eye protection should be used. All operations must be performed in a laboratory hood. In particular, special gloves should be worn in case of chloroform use.
<ol>
	<li>Use chemicals with low water content such as dry chlorofor...

<ol>
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L., Bechhoefer, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Calibration+of+atomic-force+microscope+tips.">Calibration of atomic-force microscope tips.</a> Review of Scientific Instruments. 64 (7), 1868 (1993).</li><li>Krysiak, S., Liese, S., Netz, R. R., Hugel, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Peptide+desorption+kinetics+from+single+molecule+force+spectroscopy+studies.">Peptide desorption kinetics from single molecule force spectroscopy studies.</a> Journal of the American Chemical Society. 136 (2), 688-697 (2014).</li><li>Li, I. T. S., Walker, G. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interfacial+free+energy+governs+single+polystyrene+chain+collapse+in+water+and+aqueous+solutions.">Interfacial free energy governs single polystyrene chain collapse in water and aqueous solutions.</a> Journal of the American Chemical Society. 132 (18), 6530-6540 (2010).</li><li>Dudko, O. K., Hummer, G., Szabo, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Theory,+analysis,+and+interpretation+of+single-molecule+force+spectroscopy+experiments.">Theory, analysis, and interpretation of single-molecule force spectroscopy experiments.</a> Proceedings of the National Academy of Sciences of the United States of America. 105 (41), 15755-15760 (2008).</li><li>Stetter, F. W. S., Cwiklik, L., Jungwirth, P., Hugel, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single+Lipid+Extraction:+The+Anchoring+Strength+of+Cholesterol+in+Liquid-Ordered+and+Liquid-Disordered+Phases.">Single Lipid Extraction: The Anchoring Strength of Cholesterol in Liquid-Ordered and Liquid-Disordered Phases.</a> Biophysical Journal. 107 (5), 1167-1175 (2014).</li><li>Horinek, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Peptide+adsorption+on+a+hydrophobic+surface+results+from+an+interplay+of+solvation,+surface,+and+intrapeptide+forces.">Peptide adsorption on a hydrophobic surface results from an interplay of solvation, surface, and intrapeptide forces.</a> Proc Natl Acad Sci U S A. 105 (8), 2842-2847 (2008).</li><li>Ebner, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Functionalization+of+probe+tips+and+supports+for+single-molecule+recognition+force+microscopy.">Functionalization of probe tips and supports for single-molecule recognition force microscopy.</a> Topics in current chemistry. 285 (April), 29-76 (2008).</li><li>Geisler, M., Balzer, B. 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The authors declare that they have no competing financial interests.

During the last decades, single molecule experiments have provided unprecedented insights into molecular mechanisms and turned out to be an invaluable approach in life science and beyond. To achieve good and meaningful statistics from SMFS experiments, ideally one and the same molecule is used over the whole course of the experiment. In contrast to experiments with ensembles of molecules, SMFS experiments are able to detect rare events and hidden molecular states. Another advantage of single molecule experiments is that ...

Over the past 30 years, atomic force microscopy (AFM) has turned out to be a valuable imaging technique to study biological 1,2 and synthetic 3 materials and surfaces since it provides molecular spatial resolution in all three dimensions and can be operated in various solvent environments. In addition, AFM-single molecule force spectroscopy (SMFS) enables to measure forces ranging from the pN to &#181;N regime and has given unprecedented insight for example into protein folding 4,5, polymer physics 6&#8211;8 , and single molecule-surface interaction9&#8211;12.The rationale behind studying single molecules rather than an ensemble of molecules is to avoid averaging effects which often mask rare events or hidden molecular states. Furthermore, a multitude of molecular parameters such as the contour length, the Kuhn length, the adhesion free energy, etc. can be obtained. This is detailed in the examples below. In a typical AFM-SMFS experiment, the probe molecule is coupled to a very sharp tip via a linker molecule. The tip itself is located at the end of a bendable cantilever. If the tip is brought into contact with the surface the probe molecule will interact with this surface. By observing the deflection of the cantilever upon retraction of the tip, the force, and hence the free energy, to detach the molecule from the surface can be determined. To obtain meaningful statistics, a large number of so called force-distance curves have to be acquired. Furthermore, to have true single molecule experiments (i.e., using one and the same probe molecule over the duration of the whole experiment) the probe molecule should be coupled covalently to the AFM tip. Here, an experimental protocol for cantilever functionalization with a single molecule via a covalent bond is presented. The single molecule can either be coupled via an amino or a thiol group to the AFM tip. The conjugation process can be performed in a broad variety of solvents (organic and aqueous) to account for the solvation properties of the polymers used.
In the first part, a general protocol to covalently attach a single molecule (&#8220;probe molecule&#8221;) via a linker molecule to an AFM tip is described. To this end, organic NHS- or maleimide-chemistry is used13. Along with the protocol for three example molecules, the data acquisition and data analysis processes are described and references for further reading are provided. The example molecules are: the (linear) polymer tyrosine, the graft polymer PI-g-PS and the lipid POPE. This includes slight variations of the protocol, for example to covalently attach cysteines. In addition, a section is dedicated to the preparation of different surfaces such as a diamond surface, a CH3-self-assembled monolayer and lipid bilayers. These interfaces have proven to be good references and examples.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>Materials</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Hellmanex III alkaline liquid concentrate (detergent solution)</td>
			<td>Hellma</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>RCA (ultrapure water, hydrogen peroxide (35 %), ammonia (32%); 5:1:1(v/v/v))</td>
			<td>Sigma</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Vectabond reagent / APTES (3-Aminopropyl)triethoxysilane</td>
			<td>Vectorlabs</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Dry acetone (&#60; 50 ppm H2O)</td>
			<td>Sigma</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Dry chloroform (&#62; 99.9 %)</td>
			<td>Sigma</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Triethylamine</td>
			<td>Sigma</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrapure water</td>
			<td>Biochrom, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Di-sodium tetraborate (&#62; 99.5 %)</td>
			<td>Biochrom, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Boric Acid</td>
			<td>Biochrom, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Monofunctional &#945;-methoxy-&#969;-NHS PEG, 5kDa, &#8220;methyl-PEG-NHS&#8221;</td>
			<td>Rapp, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Heterobifunctional &#945;,&#969;-bis-NHS PEG, 6 kDa, &#8220;NHS-PEG-NHS&#8221;</td>
			<td>Rapp, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Heterobifunctional &#945;-maleimidohexanoic- &#969;-NHS PEG, 5 kDa, &#8220;Mal-PEG-NHS&#8221;</td>
			<td>Rapp, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Probe molecule (polymer, lipid, etc.)</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Sufficient amount of glass crystallising dishes with spout (10 ml), glass petri dishes (500 &#181;l) and glass lids</td>
			<td>VWR International GmbH, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Laboratory oven model UF30</td>
			<td>Memmert, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Temperature controlled sonicator</td>
			<td>VWR International GmbH, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Plasma system &#34;Femto&#34;, 100 W</td>
			<td>Diener, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>One separate glass syringe for each organic solvent</td>
			<td>VWR International GmbH, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Vortex mixer</td>
			<td>VWR International GmbH, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Microcentrifuge tubes (0.5 ml or 1.5 ml)</td>
			<td>Eppendorf</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Pipettes: 10-100 &#181;l, 50-200 &#181;l and 100-1000 &#181;l</td>
			<td>Eppendorf</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>AFM with temperature controlled fluid cell (e.g. MFP-3D with BioHeater)</td>
			<td>Asylulm Research, Santa Barbara</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Soft SiN cantilevers cantilever, typically made from silicon nitride (SiN) (spring constant less than 100 pN/nm, e.g. MLCT)</td>
			<td>Bruker AXS, Santa Barbara</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

investigating single molecule adhesion by atomic force spectroscopy

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. At the same time the AFM tip is passivated to prevent unspecific interactions between the tip and the substrate, which is a prerequisite to study single molecules attached to the AFM tip. Analyses to determine the adhesion force, the adhesion length, and the free energy of these molecules on solid surfaces and bio-interfaces are shortly presented and external references for further reading are provided. Example molecules are the poly(amino acid) polytyrosine, the graft polymer PI-g-PS and the phospholipid POPE (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine). These molecules are desorbed from different surfaces like CH3-SAMs, hydrogen terminated diamond and supported lipid bilayers under various solvent conditions. Finally, the advantages of force spectroscopic single molecule experiments are discussed including means to decide if truly a single molecule has been studied in the experiment.

In the following, the results for the above described example molecules, namely the polymers poly(amino acid) polytyrosine, the graft polymer PI-g-PS and the phospholipid POPE, are presented. First for each example, experiment specific details for the data acquisition and data preparation are provided. Then, the exemplary results for experiments where these molecules were desorbed from different surfaces (CH3-SAMs, hydrogen terminated diamond and lipid bilayers) are shown. Determination of the adhesio...

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Investigating Single Molecule Adhesion by Atomic Force Spectroscopy

A protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. Procedures and examples to determine the adhesion force and free energy of these molecules on solid supports and bio-interfaces are provided.

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single ...

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10.2K Görüntüleme.  Technische Universität München. A protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. Procedures and examples to determine the adhesion force and free energy of these molecules on solid supports and bio-interfaces are provided.