SFM Characterization of Poly(isocyanodipeptide) Single Polymer Chains in Controlled Environments: Effect of Tip Adhesion and Chain Swelling Wei Zhuang, † Christof Ecker, † Gerald A. Metselaar, ‡ Alan E. Rowan, ‡ Roeland J. M. Nolte, ‡ Paolo Samorı ´,* ,†,§,| and Ju 1 rgen P. Rabe* ,† Department of Physics, Humboldt University Berlin, Newtonstrasse15, 12489 Berlin, Germany, Department of Organic Chemistry, NSR center, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands, Instituto per la Sintesi Organica e la Fotoreattivita ` , C.N.R. Bologna, via Gobetti 101, 40129 Bologna, Italy, and Nanochemistry Laboratory, Institut de Science et d’Inge ` nierie Supramole ` culaires (ISIS), Universite ´ Louis Pasteur of Strasbourg, 8 alle ´ e Gaspard Monge, 67083 Strasbourg, France Received June 19, 2004; Revised Manuscript Received November 5, 2004 ABSTRACT: Isolated slightly hydrophilic chains of poly(isocyanodipeptides) (PICs) adsorbed on mica were studied by intermittent contact mode-scanning force microscopy (IC-SFM) in an ambient atmosphere controlled both with respect to the relative humidity (RH) and the presence of CHCl 3 vapor. SFM revealed that the average chain height increases up to more than an order of magnitude with decreasing RH, leading to the highest value at RH ) 0%. This is due to both a minimization of the capillary forces between the SFM tip and the hydrophilic substrate surface and a collapse of the side chains in the poor solvent. In a saturated CHCl 3 vapor atmosphere, the chain heights increase up to twice this value, which is close to the polymer diameter measured by powder X-ray diffraction. This indicates that the PIC chains are solvated by CHCl 3 molecules, causing the swelling of the single polymers. Achieving a control over the thickness of the polymer chains is fundamental for their optimal observation by SFM. Moreover, the understanding of the conformational properties of single macromolecules adsorbed on surfaces under different environmental conditions is of importance for unraveling their physicochemical properties and their dynamics, including their reactivity. Introduction Intermittent contact mode-scanning force microscopy (IC-SFM), known also as tapping mode SFM, 1 offers direct access to the structural and mechanical properties of single macromolecules 2-4 as well as to supramolecular architectures 5 adsorbed on a surface. It is a valuable method for the investigation of soft organic and biologi- cal layers, since it applies very small lateral forces to the sample surface. 1 However, the measurements of the heights of the adsorbed molecules, which are typically performed at the solid-air interface, are not easily reproducible and the obtained values are far smaller than the diameters of the investigated molecules them- selves. 3,6,7 This can even lead to such small chain’s heights that these are hardly detectable by SFM. To overcome this problem contrasting agents have been added to the polymers, 8 altering in this way their physicochemical properties. Besides, Van Noort et al. introduced a model describing the relationship between the height anomalies in IC-SFM at the solid-air interface and the tip-sample adhesion caused by capil- lary force. 9 This experimental finding prompted us to find new routes to avoid height anomalies in IC-SFM imaging of neat macromolecules adsorbed at surfaces by removing the capillary force. The capillary forces caused by tip-sample adhesion have been systematically studied in contact mode SFM. 7,10 The curvature at the contact between the SFM tip and the sample causes the formation of a “water neck”, namely the condensation of vapor from the ambient atmosphere. Also surfaces exposed to air are typically coated by a layer of water, whose thickness depends on the relative humidity (RH) of the atmo- sphere and on the physicochemical nature of the com- ponents. 11 It can result in a strong attractive capillary force (10 -7 -10 -8 N) that holds the tip in contact with the surface. 10 To avoid capillary forces the ambient relative humidity should be zero, although some previ- ous work on the tip-mica system has demonstrated that below RH ) 10% the capillary forces do not further decay. 12 Thus, two experimental procedures to minimize the effect of capillary forces can be followed: the first is to flood a sealed measurement chamber with a dry inert gas such as N 2 , He or Ar; the second is to perform measurements with both the tip and the sample im- mersed in a liquid medium making use of a fluid cell. 10 Both have been undertaken for visualizing hydrophilic macromolecules, such as double stranded (ds) DNA. 13 It was found that the apparent height of ds-DNA decreases with increasing ambient humidity, which was ascribed to the capillarity forces. 7,14 Differently, at the solid-liquid interface the height of DNA was 0.6 nm higher than that of ds-DNA measured in ambient air. 15 If one wishes to apply the liquid medium approach to macromolecules soluble in organic solvents, an aqueous solution becomes impractical for molecular conformation studies and an organic solvent is required for the SFM experiment. A series of peptide based poly(isocyanides) has re- cently been synthesized and their unique physicochem- ical properties (including the mechanical ones), have been reported. 3,16 † Humboldt University Berlin. ‡ University of Nijmegen. § C.N.R. Bologna. | Universite ´ Louis Pasteur of Strasbourg. * Corresponding authors: (J.P.R) fax, +49-(0)30-20937632; e-mail, rabe@physik.hu-berlin.de; (P.S.) fax, +39-051-6399844; e-mail, samori@isof.cnr.it. 473 Macromolecules 2005, 38, 473-480 10.1021/ma048786z CCC: $30.25 © 2005 American Chemical Society Published on Web 12/30/2004