G. Julius Vancso

Interlaboratory round robin on cantilever calibration for AFM force spectroscopy

Single-molecule force spectroscopy studies performed by Atomic Force Microscopes (AFMs) strongly rely on accurately determined cantilever spring constants. Hence, to calibrate cantilevers, a reliable calibration protocol is essential. Although the thermal noise method and the direct Sader method are frequently used for cantilever calibration, there is no consensus on the optimal calibration of soft and V-shaped cantilevers, especially those used in force spectroscopy. Therefore, in this study we aimed at establishing a commonly accepted approach to accurately calibrate compliant and V-shaped cantilevers. In a round robin experiment involving eight different laboratories we compared the thermal noise and the Sader method on ten commercial and custom-built AFMs. We found that spring constants of both rectangular and V-shaped cantilevers can accurately be determined with both methods, although the Sader method proved to be superior. Furthermore, we observed that simultaneous application of both methods on an AFM proved an accurate consistency check of the instrument and thus provides optimal and highly reproducible calibration. To illustrate the importance of optimal calibration, we show that for biological force spectroscopy studies, an erroneously calibrated cantilever can significantly affect the derived (bio)physical parameters. Taken together, our findings demonstrated that with the pre-established protocol described reliable spring constants can be obtained for different types of cantilevers.

Host-Guest Interactions at Self-Assembled Monolayers of Cyclodextrins on Gold

We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of alpha-, beta-, and gamma-cyclodextrins. Monolayers of these cyclodextrin adsorbates were characterized by electrochemistry, wettability studies, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM). The differences in thickness and polarity of the outerface of the monolayers were measured by electro-chemistry and wettability studies. On average about 70% of the sulfide moieties were used for binding to the gold, as measured by XPS. Tof-SIMS measurements showed that the cyclodextrin adsorbates adsorb without any bond breakage. AFM measurements revealed for beta-cyclodextrin monolayers a quasi-hexagonal lattice with a lattice constant of 20.6 A, which matches the geometrical size of the adsorbate. The alpha-cyclodextrin and gamma-cyclodextrin monolayers are less ordered. Interactions of the anionic guests 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) and 2-(p-toluidinyl)naphthalene-6-sulfonic acid (2,6-TNS) and the highly ordered monolayers of heptapodant beta-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a beta-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations (< or = 5 microM). The association constant for the binding of 1,8-ANS (K = 289,000 +/- 13,000M-1) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the beta-cyclodextrin strongly decreases the binding.

Features of the hedritic morphology of β-isotactic polypropylene studied by atomic force microscopy

The lamellar organization of melt-crystallized β-isotactic polypropylene was studied by atomic force microscopy (AFM) after permanganic etching. Hedritic objects grown at a high crystallization temperature (140-143 °C) were investigated. Essential features of the hedritic development were revealed by the characteristic projections exposed at the sample surface. A three-dimensional view of the morphology was obtained by AFM. Hedritic growth proceeded mainly by branching around screw dislocations resulting in new lamellae that further developed. Successive lamellar layers often diverged. Deviation from the planar lamellar habit was observed, varying with the position within the hedrite. Twisting of the lamellae also was observed occasionally in the vicinity of the screw dislocations.

Periodic organic-organometallic microdomain structures in poly(styrene-block-ferrocenyldimethylsilane) copolymers and blends with corresponding homopolymers

A series of poly(styrene-block-ferrocenyldimethylsilane) copolymers (SF) with different relative molar masses of the blocks were prepared by sequential anionic polymerization. The bulk morphology of these polymers, studied by TEM and SAXS, showed well-ordered lamellar and cylindrical domains as well as disordered micellar structures. Temperature-dependent rheological measurements exhibited an order-disorder transition for SF 17/8 (the numbers refer to the relative molar masses in 103 g/mol) between 170 and 180°C, and an order-order transition for SF 9/19 between 190 and 200°C. The morphologies of binary blends of the diblocks with homopolymer were also investigated. In the blends the molar mass of the homopolymer was always less than the molar mass of the matching block. Ordered spheres on a bcc lattice and double-gyroid morphology were observed for the blends. The double-gyroid morphology was found only in F-rich diblock/homopolymer systems.

Switching Transport through Nanopores with pH-Responsive Polymer Brushes for Controlled Ion Permeability

Several nanoporous platforms were functionalized with pH-responsive poly(methacrylic acid) (PMAA) brushes using surface-initiated atom transfer radical polymerization (SI-ATRP). The growth of the PMAA brush and its pH-responsive behavior from the nanoporous platforms were confirmed by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM). The swelling behavior of the pH-responsive PMAA brushes grafted only from the nanopore walls was investigated by AFM in aqueous liquid environment with pH values of 4 and 8. AFM images displayed open nanopores at pH 4 and closed ones at pH 8, which rationalizes their use as gating platforms. Ion conductivity across the nanopores was investigated with current-voltage measurements at various pH values. Enhanced higher resistance across the nanopores was observed in a neutral polymer brush state (lower pH values) and lower resistance when the brush was charged (higher pH values). By adding a fluorescent dye in an environment of pH 4 or pH 8 at one side of the PMAA-brush functionalized nanopore array chips, diffusion across the nanopores was followed. These experiments displayed faster diffusion rates of the fluorescent molecules at pH 4 (PMAA neutral state, open pores) and slower diffusion at pH 8 (PMAA charged state, closed pores) showing the potential of this technology toward nanoscale valve applications.

Towards a nanomechanical basis for temporary adhesion in barnacle cyprids (Semibalanus balanoides)

Cypris larvae of barnacles are able to use a rapidly reversible temporary adhesion mechanism for exploring immersed surfaces. Despite decades of research interest, the means by which cyprids maintain attachment with surfaces prior to permanent settlement remain poorly understood. Here, we present novel observations on the morphology of ‘footprints’ of a putative adhesive secretion deposited by cyprids during surface exploration. Atomic force microscopy (AFM) was used to image footprints at high resolution and to acquire measurements of interaction forces. R–CH3- and R–NH2-terminated glass surfaces were used for comparison of footprint morphology, and it was noted that on R–NH2 each footprint comprised three times the volume of material deposited for footprints on R–CH3. Direct scaling of adhesion forces derived from AFM measurements did not adequately predict the real attachment tenacity of cyprids, and it is suggested that a mixture of ‘wet’ and ‘dry’ adhesive mechanisms may be at work in cyprid adhesion. High-resolution images of cyprid footprints are presented that correlate well with the known morphology of the attachment structures.

Reversible ph-controlled switching of poly(methacrylic acid) grafts for functional biointerfaces

Responsive polymeric brushes of poly(methacrylic acid) (PMAA) were grafted from silicon surfaces using controlled surface-initiated atom-transfer radical polymerization (SI-ATRP). The growth kinetics of PMAA was investigated with respect to the composition of the ATRP medium by grafting the polymer in mixtures of water and methanol with different ratios. The dissociation behavior of the polymer layers was characterized by FTIR titration after incubating the polymer-grafted substrates in PBS buffer solutions with different pH values. PMAA layers show a strong pH-dependent behavior with an effective pK(a) of the bulk polymer brush of 6.5 ± 0.2, which is independent of the polymer brush thickness and methanol content of the ATRP grafting medium. The pH-induced swelling and collapse of the grafted polymer layers were quantified in real time by in situ ellipsometry in liquid environment. Switching between polymer conformations at pH values of 4 and 8 is rapid and reversible, and it is characterized by swelling factors (maximum thickness/minimum thickness) that increase with decreasing the methanol content of the SI-ATRP medium.

Free-Standing 3 D Supramolecular Hybrid Particle Structures†

Make a stand: The formation of stable and ordered free-standing particle bridges and hollow capsule structures with controllable sizes and geometries is demonstrated by combining the directed assembly of submicrometer particles, transfer printing, and supramolecular layer-by-layer assembly.

Surface‐Confined Metallodendrimers: Isolated Nanosize Molecules

The incorporation of dialkyl sulfide side chains in metallodendrimers is a simple method for their insertion into a monolayer of decanethiol formed by self-assembly on a gold surface. The dendrimer binds through the sulfide group to a defect in the monolayer on the gold surface (see picture). The surface concentration of the isolated dendrimer adsorbate can be regulated by the adsorption time (for example, 55 adsorbates on a surface of 200x200 nm(2) after 20 h).

Surface Morphology of Polymer Films Imaged by Atomic Force Microscopy

Abstract The surface morphology of commercial polymer films has been studied by contact mode and tapping mode atomic force microscopy. Flame-treated isotropic HDPE obtained by cast-film extrusion exhibited randomly oriented, 20–50-nm thick lamellar features of 200–400 nm in length, while uniaxially oriented films showed a shish-kebab-like morphology. The surface features of extrusion-blown films of blends of low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) appeared as stacks of lamellae arranged in a cauliflower-like pattern. Images of isotactic polypropylene (PP) films obtained by cast-film extrusion consisted of branched fibrillar features. Corona treatment of the PP films resulted in the formation of 400–500 nm large ‚droplet-like‘ features at the surface. During metallization, 20–40-nm diameter A1 particles were deposited. The metal layer formed followed the topography of the surface. The surface of ethylene-vinyl alcohol (EVOH) copolymer skins on PP consisted of radiating l...