FACS-sorted particles reduce the data variance in optical tweezers-assisted dynamic force spectroscopy measurements.

By combining optical tweezers-assisted dynamic force spectroscopy experiments with fluorescence activated cell sorting (FACS), we demonstrate a new approach to reducing the data variance in measuring receptor-ligand interactions on a single molecule level by ensuring similar coating densities. Therefore, the carboxyfluorescein-labelled monophosphorylated peptide tau226-240[pThr231] is anchored on melamine resin beads and these beads are sorted by FACS to achieve a homogeneous surface coverage. To quantify the impact of the fluorescence dye on the bond parameters between the phosphorylated peptide and the corresponding phosphorylation specific anti-human tau monoclonal antibody HPT-104, we perform dynamic force spectroscopy and compare the results to data using unsorted beads covered with the non-fluorescence peptide analogue. Finally, we demonstrate that the data variance of the relative binding frequency is significantly decreased by a factor of 3.4 using pre-sorted colloids with a homogeneous ligand coating compared to using unsorted colloids.

A precise optical determination of nanoscale diameters of semiconductor nanowires.

Electrical and optical properties of semiconducting nanowires (NWs) strongly depend on their diameters. Therefore, a precise knowledge of their diameters is essential for any kind of device integration. Here, we present an optical method based on dark field optical microscopy to easily determine the diameters of individual NWs with an accuracy of a few nanometers and thus a relative error of less than 10%. The underlying physical principle of this method is that strong Mie resonances dominate the optical scattering spectra of most semiconducting NWs and can thus be exploited. The feasibility of this method is demonstrated using GaAs NWs but it should be applicable to most types of semiconducting NWs as well. Dark field optical microscopy shows that even slight tapering of the NWs, i.e. diameter variations of a few nanometers, can be detected by a visible color change. Abrupt diameter changes of a few nanometers, as they occur for example when growth conditions vary, can be determined as well. In addition a profound analysis of the elastic scattering properties of individual GaAs NWs is presented theoretically using Mie calculations as well as experimentally by dark field microscopy. This method has the advantage that no vacuum technique is needed, a fast and reliable analysis is possible based on cheap standard hardware.

Micro-rheology on (polymer-grafted) colloids using optical tweezers.

Optical tweezers are experimental tools with extraordinary resolution in positioning (± 1 nm) a micron-sized colloid and in the measurement of forces (± 50 fN) acting on it-without any mechanical contact. This enables one to carry out a multitude of novel experiments in nano- and microfluidics, of which the following will be presented in this review: (i) forces within single pairs of colloids in media of varying concentration and valency of the surrounding ionic solution, (ii) measurements of the electrophoretic mobility of single colloids in different solvents (concentration, valency of the ionic solution and pH), (iii) similar experiments as in (i) with DNA-grafted colloids, (iv) the nonlinear response of single DNA-grafted colloids in shear flow and (v) the drag force on single colloids pulled through a polymer solution. The experiments will be described in detail and their analysis discussed.

Optical properties of heavily doped GaAs nanowires and electroluminescent nanowire structures.

We present GaAs electroluminescent nanowire structures fabricated by metal organic vapor phase epitaxy. Electroluminescent structures were realized in both axial pn-junctions in single GaAs nanowires and free-standing nanowire arrays with a pn-junction formed between nanowires and substrate, respectively. The electroluminescence emission peak from single nanowire pn-junctions at 10 K was registered at an energy of around 1.32 eV and shifted to 1.4 eV with an increasing current. The line is attributed to the recombination in the compensated region present in the nanowire due to the memory effect of the vapor-liquid-solid growth mechanism. Arrayed nanowire electroluminescent structures with a pn-junction formed between nanowires and substrate demonstrated at 5 K a strong electroluminescence peak at 1.488 eV and two shoulder peaks at 1.455 and 1.519 eV. The main emission line was attributed to the recombination in the p-doped GaAs. The other two lines correspond to the tunneling-assisted photon emission and band-edge recombination in the abrupt junction, respectively. Electroluminescence spectra are compared with the micro-photoluminescence spectra taken along the single p-, n- and single nanowire pn-junctions to find the origin of the electroluminescence peaks, the distribution of doping species and the sharpness of the junctions.

Material and doping transitions in single GaAs-based nanowires probed by Kelvin probe force microscopy.

We demonstrate the potential of Kelvin probe force microscopy for simultaneously probing the topography and the work function of individual nanowires. Our technique allows us to visualize both the material and the doping contrast in single GaAs-based nanowires without the need to electrically contact the nanowires. In a GaAs/GaP heterostructure nanowire, a core-shell structure is found. This is attributed to a thermally activated radial overgrowth of GaAs, while in the GaP region the vertical nanowire growth dominates. In partially p-doped GaAs nanowires the doping transitions can be localized and the width of the depletion layer is estimated.

Forces between single pairs of charged colloids in aqueous salt solutions.

Forces between single pairs of negatively charged micrometer-sized colloids in aqueous solutions of monovalent, divalent, or trivalent counter-ions at varying concentrations have been measured by employing optical tweezers. The experimental data have been analyzed by using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and a numerical solution of the Poisson-Boltzmann (PB) equation. With monovalent counterions, the data are well described by the DLVO and PB theories, suggesting that the DLVO theory is adequate to describe the colloidal forces at these conditions. At higher counter-ion valence, the approximations within the two theories become evident.

Binding of TmHU to single dsDNA as observed by optical tweezers.

Optical tweezers are employed to study the action of the histone-like protein from Thermotoga maritima (TmHU) on DNA at a single molecule level. Binding and disruption of TmHU to and from DNA are found to take place in discrete steps of 4-5 nm length and a net binding enthalpy of about 16kBT. This is in reasonable agreement with a microscopic model that estimates the extension of the binding sites of the protein and evaluates the energetics mainly for bending of the DNA in the course of interaction.

The elastic properties of single double-stranded DNA chains of different lengths as measured with optical tweezers.

Optical tweezers are microscopic tools with extraordinary precision in the determination of the position (±2 nm) of a colloid (diameter: ∼2.0 µm) in 3D-space and in the measurement of small forces in the range between 0.1 and 100 pN (pN=10-12 N). Experiments are reported in which single double-stranded (ds)-DNA chains of different length [2,000 base pairs (bp), 3,000, 4,000, and 6,000 bp] are spanned between two colloidal particles by use of appropriate molecular linkers. For the forces applied (≤40 pN) a fully reversible and well reproducible force-extension dependence is found. The data can be well described by both the worm-like chain model or by an approach developed by R. G. Winkler. For the resulting persistence length, a pronounced dependence on the ionic concentration in the surrounding medium is found.

Synthesis, structures, and conformational characteristics of calixarene monoanions and dianions.

The synthesis, complete characterization, and solid state structural and solution conformation determination of calix[n]arenes (n = 4, 6, 8) is reported. A complete series of X-ray structures of the alkali metal salts of calix[4]arene (HC4) illustrate the great influence of the alkali metal ion on the solid state structure of calixanions (e.g., the Li salt of monoanionic HC4 is a monomer; the Na salt of monoanionic HC4 forms a dimer; and the K, Rb, and Cs salts exist in polymeric forms). Solution NMR spectra of alkali metal salts of monoanionic calix[4]arenes indicate that they have the cone conformation in solution. Variable-temperature NMR spectra of salts HC4.M (M = Li, Na, K, Rb, Cs) show that they possess similar coalescence temperatures, all higher than that of HC4. Due to steric hindrance from tert-butyl groups in the para position of p-tert-butylcalix[4]arene (Bu(t)C4), the alkali metal salts of monoanionic Bu(t)C4 exist in monomeric or dimeric form in the solid state. Calix[6]arene (HC6) and p-tert-butylcalix[6]arene (Bu(t)C6) were treated with a 2:1 molar ratio of M(2)CO(3) (M = K, Rb, Cs) or a 1:1 molar ratio of MOC(CH(3))(3) (M = Li, Na) to give calix[6]arene monoanions, but calix[6]arenes react in a 1:1 molar ratio with M(2)CO(3) (M = K, Rb, Cs) to afford calix[6]arene dianions. Calix[8]arene (HC8) and p-tert-butylcalix[8]arene (Bu(t)()C8) have similar reactivity. The alkali metal salts of monoanionic calix[6]arenes are more conformationally flexible than the alkali metal salts of dianionic calix[6]arenes, which has been shown by their solution NMR spectra. X-ray crystal structures of HC6.Li and HC6.Cs indicate that the size of the alkali metal has some influence on the conformation of calixanions; for example, HC6.Li has a cone-like conformation, and HC6.Cs has a 1,2,3-alternate conformation. The calix[6]arene dianions show roughly the same structural architecture, and the salts tend to form polymeric chains. For most calixarene salts cation-pi arene interactions were observed.

Tricalixarenes and pentacalixarenes: synthesis and complexation studies.

Tricalix[4]arene 4, tricalix[5]arene 14, and pentacalix[4]arene 10 have been synthesized from O-alkylcalixarene mono- and dialdehydes by a two-step conversion to the corresponding monoethynyl ketones or diethynyl ketones followed by aryne trimerization in refluxing DMF containing a dialkylamine. The tricalixarenes 4 and 14 were converted, in turn, to calixarenes 6 and 16, which carry OH groups on the lower rim and methylenes as the bridging moieties to the benzene ring. Complexation studies with the tricalix[5]arene 16 show that it forms (a) a 1:3 complex with N,N-dimethylethylenediamine in which each calixarene unit contains a molecule of the amine, (b) a 1:1 complex with tris(aminomethyl)amine in which each calixarene unit contains one of the three arms of the guest, and (c) a 1:1 complex with C60 in which the guest presumably resides in the cavity provided by the three calixarene units acting cooperatively.

Synthesis and Reactions of Calix[4]arene Bisanhydrides(1).

The tetrabenzyl ether, tetra-p-bromobenzenesulfonate, and tetra-p-methylbenzenesulfonate of p-carboxymethylcalix[4]arene in the 1,3-alternate conformation have been converted to the corresponding bisanhydrides. Reactions of the bisanhydrides with alcohols or amines afford calix[4]arenes carrying two carboxymethyl and two carboalkoxymethyl or two amidomethyl groups on the upper rim positions. The substitution pattern that is established confers molecular chirality on the calixarenes.

Calixarene monolayers as quartz crystal microbalance sensing elements in aqueous solution.

We have examined p-tert-butylcalix[4]arenetetrathiolate (BCAT) monolayers for their potential use as molecular recognition elements for in situ aqueous chemical sensors. Spectroscopic and wetting studies of BCAT monolayers on Au{111} reveal that the calixarene molecules exist in monolayers, preferentially oriented with their phenyl rings parallel to the surface normal axis. Using quartz crystal microbalance (QCM) sensors with gold-coated electrodes, the chemical specificity of monolayers and thin films to a variety of aromatic and aliphatic analytes in aqueous solution was examined. The response of BCAT sensors was compared to the responses of p-tert-butylcalix[4]arene (BCA)- and decanethiolate (DT)-coated QCM electrodes. BCAT is very selective for alkylbenzenes, much more so than either its spray-coated thin-film analogue, BCA, or the highly ordered DT monolayer. From these measurements, the factors behind molecular differentiation in each film are explored. Drawing upon these findings, the roles of cavitation and film order in molecular recognition for calixarene films are discussed.

Instrumentation of furcation with modified sonic scaler inserts: study on manikins, part I.

The purpose of this study was to determine the suitability of different scaling instruments for the debridement of furcations. 12 upper and 12 lower replicated molars with through-and-through furcations were instrumented 3x with 5 different types of instruments: (i) hand instruments; (ii) hand instruments in conjunction with diamond burrs; (iii) a conventional ultrasonic scaler insert; (iv) a conventional sonic scaler insert; (v) a set of 2 modified diamond coated sonic scaler inserts with different angulated shafts. The plastic replicas were fixed in a manikin head without replicated soft tissues. Following debridement, weight loss and % of instrumented furcation area were assessed. In the case of lower molars, it made little difference whether they were treated with hand instruments, hand instruments combined with diamond burrs, or diamond-coated sonic scaler inserts. On upper molars, however, significantly more area was instrumented with the diamond-coated inserts than with the other instruments. Substance removal was greater with diamond-coated inserts than with the other devices. In conclusion, an effective debridement of the furcation seems possible only with an odontoplasty, in which a furcation is fitted to the instrument by means of intensive instrumentation, thus leading to substance loss. To improve results with these instruments, further research is necessary.

Capillary Electrokinetic Chromatography Employing p-(Carboxyethyl)calix[n]arenes as Running Buffer Additives.

Calixarenes, a class of macrocyclic phenolic compounds with a basket-like shape, are used as capillary electrophoresis reagents for separations of native and substituted polycyclic aromatic hydrocarbons. The p-(carboxyethyl)calix[n]arenes reported herein are a series of charged, moderately water soluble macrocyclic molecules that can form complexes with neutral molecules. Electrokinetic chromatographic separations are based on the differential distribution of molecules between a running buffer phase, which is transported by electroosmotic flow, and an electrophoretically mediated calixarene. The size of the calixarene influences separation performance, illustrating the importance of cavity size and geometry in the complexation process. p-(Carboxyethyl)calix[7]arene provides the best efficiency (>10(5) plates/m) and selectivity in these studies. The influences of pH, organic solvent, and field strength on elution range, capacity factors, efficiency, and selectivity are also reported. In general, capacity factors are rather low, but the high charge-to-mass ratios of certain calixarenes produce relatively wide elution ranges. Molecular modeling data and solubility data are used to interpret the observed selectivity.