In situ hydrodynamic lateral force calibration of AFM colloidal probes

Lateral force microscopy (LFM) is an application of atomic force microscopy (AFM) to sense lateral forces applied to the AFM probe tip. Recent advances in tissue engineering and functional biomaterials have shown a need for the surface characterization of their material and biochemical properties under the application of lateral forces. LFM equipped with colloidal probes of well-defined tip geometries has been a natural fit to address these needs but has remained limited to provide primarily qualitative results. For quantitative measurements, LFM requires the successful determination of the lateral force or torque conversion factor of the probe. Usually, force calibration results obtained in air are used for force measurements in liquids, but refractive index differences between air and liquids induce changes in the conversion factor. Furthermore, in the case of biochemically functionalized tips, damage can occur during calibration because tip-surface contact is inevitable in most calibration methods. Therefore, a nondestructive in situ lateral force calibration is desirable for LFM applications in liquids. Here we present an in situ hydrodynamic lateral force calibration method for AFM colloidal probes. In this method, the laterally scanned substrate surface generated a creeping Couette flow, which deformed the probe under torsion. The spherical geometry of the tip enabled the calculation of tip drag forces, and the lateral torque conversion factor was calibrated from the lateral voltage change and estimated torque. Comparisons with lateral force calibrations performed in air show that the hydrodynamic lateral force calibration method enables quantitative lateral force measurements in liquid using colloidal probes.     

Kinetics of hypericin association with low-density lipoproteins

Steady‐state and time‐resolved fluorescence spectroscopy have been used for the study of the incorporation kinetics of hypericin (Hyp) into low‐density lipoproteins (LDL). Biphasic kinetics of Hyp association with LDL was observed when solutions of Hyp and LDL were mixed at various concentration ratios. The rapid phase of Hyp incorporation is completed within seconds, while the slow phase lasts several minutes. The relative contributions of the individual phases show that a higher amount of Hyp molecules (65%) are incorporated into LDL in the second phase. The kinetics of the incorporation of Hyp into LDL particles preloaded with Hyp (Hyp/LDL = 25:1) was also investigated. The decreased intensity of Hyp fluorescence is a sign of the formation of Hyp aggregates after penetration of additional Hyp molecules into Hyp/LDL = 25:1 complex. The time dependence of Hyp fluorescence was measured after mixing the complex Hyp/LDL = 200:1 with appropriate amounts of free LDL molecules. For each final Hyp/LDL ratio, an increase in the intensity and lifetime of Hyp fluorescence was observed, suggesting a monomerization of Hyp aggregates. The half‐time of Hyp transfer from Hyp/LDL complex to LDL particles is similar to the half‐time of the slow phase of Hyp incorporation into free LDL particles.     

Design of cross-linked semicrystalline poly(ε-caprolactone)-based networks with one-way and two-way shape-memory properties through Diels-Alder reactions

Cross‐linked poly(ε‐caprolactone) (PCL)‐based polyesterurethane (PUR) systems have been synthesized through Diels–Alder reactions by reactive extrusion. The Diels–Alder and retro‐Diels–Alder reactions proved to be useful for enhancing the molecular motion of PCL‐based systems, and therefore their crystallization ability, in the design of cross‐linked semicrystalline polymers with one‐way and two‐way shape‐memory properties. Successive reactions between α,ω‐diol PCL (PCL₂), furfuryl alcohol, and methylene diphenyl 4,4′‐diisocyanate straightforwardly afforded the α,ω‐furfuryl PCL‐based PUR systems, and subsequent Diels–Alder reactions with N,N‐phenylenedimaleimide afforded the thermoreversible cycloadducts. The cross‐linking density could be modulated by partially replacing PCL‐diol with PCL‐tetraol. Interestingly, the resulting PUR systems proved to be semicrystalline cross‐linked polymers, the melting temperature of which (close to 45 °C) represented the switching temperature for their shape‐memory properties. Qualitative and quantitative measurements demonstrated that these PUR systems exhibited one‐way and two‐way shape‐memory properties depending on their cross‐linking density.     

Mechanisms for lasing with cold atoms as the gain medium

We realize a laser with a cloud of cold rubidium atoms as gain medium, placed in a low-finesse cavity. Three different regimes of laser emission are observed corresponding, respectively, to Mollow, Raman, and four-wave mixing mechanisms. We measure an output power of up to 300 microW and present the main properties of these different lasers in each regime.     

A q-Factor-Based Digital Image Correlation Algorithm (qDIC) for Resolving Finite Deformations with Degenerate Speckle Patterns

Digital image correlation (DIC) has become a widely utilized non-contact, full-field displacement measurement technique for obtaining accurate material kinematics. Despite the significant advances made to date, high resolution reconstruction of finite deformations for images with intrinsically low quality speckle patterns or poor signal-to-noise content has not been fully addressed. In particular, large image distortions imposed by materials undergoing finite deformations create significant challenges for most classical DIC approaches. To address this issue, this paper describes a new open source DIC algorithm (qDIC) that incorporates cross-correlation quality factors (q-factors), which are specifically designed to assess the quality of the reconstructed displacement estimate during the motion reconstruction process. A q-factor provides a robust assessment of the uniqueness and sharpness of the cross-correlation peak, and thus a quantitative estimate of the subset-based displacement measure per given image subset and level of applied deformation. We show that the incorporation of energy- and entropy-based q-factor metrics leads to substantially improved displacement predictions, lower noise floor, and reduced decorrelation even at significant levels of image distortion or poor speckle quality. Furthermore, we show that q-factors can be utilized as a quantitative metric for constructing a hybrid incremental-cumulative displacement correlation scheme for accurately resolving very large homogeneous and inhomogeneous deformations, even in the presence of significant image data loss.     

A new synthesis of alkane and polyfluoroalkanesulfonyl chlorides

This study describes a new and advantageous procedure for the synthesis of alkanesulfonyl chlorides ( 2 ) by the reaction of alkyl thiocyanates ( 1 ) with sulfuryl chloride in a mixture of acetic acid and water. The alkanesulfonyl chlorides were obtained in good yields. © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:355–361, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20559     

Solid-State and Solution Self-assembly Properties of Mono- and Bis-acetylide RuII Complexes Bearing Hydrogen-Bonding Amide Functions

In this work, we report our latest results regarding the self-assembly properties of two Ru-acetylides complexes and their corresponding free organic ligands, in solution and in the solid state. The four compounds show mesogenic properties, which we have rationalized thanks to extensive DSC, SAXS, POM, FTIR and molecular dynamic investigations. We clearly establish that hydrogen bonds stabilize the supramolecular structures, and that introducing the metal center and its coordination sphere induces large change in the liquid crystal properties. In addition, for the monoacetylide compound Ru₂ , solution-state studies helped us rationalize its supramolecular abilities. In particular, we demonstrate that Ru₂ follows an isodesmic growth supramolecular polymerization in aromatic solvent.     

Si-Tethered Bis- and Tris-Malonates for the Regioselective Preparation of Fullerene Multi-Adducts

Bis- and tris-malonates constructed around a silicon atom have been prepared by reaction of malonate derivatives bearing an alcohol function with di-tert-butylsilyl bis(trifluoromethanesulfonate) and tert-butyl(trichloro)silane, respectively. These compounds have been used for the regioselective bis- and tris-functionalization of C₆₀ under Bingel conditions. By changing the nature of the linker between the central Si atom and the reactive malonate groups, the malonate precursors have been optimized to produce specific bis- and tris-adducts with excellent regioselectivity. A complete understanding of the electronic and stereochemical factors governing the regioselectivity has been obtained by combining computational studies with a complete analysis of the by-products formed during the reactions of the Si-tethered tris-malonates with C₆₀. Finally, desilylation reactions of the resulting fullerene bis- and tris-adducts have been carried out to generate the corresponding acyclic fullerene bis- and tris-adducts bearing alcohol functions.