Multiple electrokinetic actuators for feedback control of colloidal crystal size

We report a feedback control method to precisely target the number of colloidal particles in quasi-2D ensembles and their subsequent assembly into crystals in a quadrupole electrode. Our approach relies on tracking the number of particles within a quadrupole electrode, which is used in a real-time feedback control algorithm to dynamically actuate competing electrokinetic transport mechanisms. Particles are removed from the quadrupole using DC-field mediated electrophoretic-electroosmotic transport, while high-frequency AC-field mediated dielectrophoretic transport is used to concentrate and assemble colloidal crystals. Our results show successful control of the size of crystals containing 20 to 250 colloidal particles with less than 10% error. Assembled crystals are characterized by their radius of gyration, crystallinity, and number of edge particles, and demonstrate the expected size-dependent properties. Our findings demonstrate successful ensemble feedback control of the assembly of different sized colloidal crystals using multiple actuators, which has broad implications for control over nano- and micro- scale assembly processes involving colloidal components.     

Magnetic properties of nanoparticles of Prussian blue-based molecular magnets M 3[Cr(CN)6]2⋅zH2O (M=Fe, Co, and Ni)

The nanoparticles of Prussian blue-based molecular magnets, M ₃[Cr(CN)₆]₂?zH₂O (where M=Fe, Co, and Ni), prepared by a slow addition (drop by drop) of chemicals using the co-precipitation method, are investigated by means of X-ray diffraction, infra red spectroscopy and dc magnetization measurement techniques. The formation of nanoparticles has been confirmed by scanning electron microscopy, whereas the characteristic peak, observed in the range of 1900–2300 cm^?1 in the infrared spectra, corresponds to the CN stretching frequency of $\mbox{Cr}^{\mathrm{+III}}$ –CN– $M^{\mathrm{+II}}$ , and confirms the formation of Prussian blue compounds. The results, derived from the Rietveld refinement of X-ray diffraction patterns, reveal that all samples are nanocrystalline in nature with a face-centered cubic crystal structure of space group Fm3m. The particle size and the lattice constants decrease with an increasing atomic number of the transition metals (M=Fe, Co and Ni). The magnetization data show a magnetically ordered state of all nanoparticle samples with a low coercivity (except for the Fe₃[Cr(CN)₆]₂?zH₂O) as well as the remanent magnetization. In addition, by varying M with Fe, Co and Ni, the magnetic ordering temperature increases from ～12 to ～28 K, whereas the maximum magnetization and the coercive field decrease from ～14 to ～4.5 μ_B/f.u. and ～554 to ～22 Oe, respectively. The observed magnetization behavior has been discussed in terms of the structural changes due to the decreasing particle size as well as the varying nature of the metal ions.     

Visual chiral recognition of mandelic acid and α-amino acid derivatives by enantioselective gel formation and precipitation

Novel chiral receptors based on l-phenylalanine and l-valine have been synthesized and their chiral recognition properties toward mandelic acid and N-tosyl α-amino acids are studied. The phenylalanine-based receptor undergoes enantioselective gel formation with R-mandelic acid and N-tosyl-d-valine, whereas the valine-linked receptor in their presence results in the formation of precipitates.     

On the Transfer of Energy between Widely Spaced Modes in Structures

An experimental and theoretical study of the response of aflexible cantilever beam to an external harmonic excitation nearthe beam's third natural frequency is presented. For a certain range ofthe excitation frequency, we observed experimentally that the responseincludes a large contribution due to the first mode of the beamaccompanied by a slow modulation of the amplitude and phase of the thirdmode. In addition, we noted that the energy transfer between the thirdand first modes is very much dependent upon the closeness of themodulation (or Hopf bifurcation) frequency to the first-mode naturalfrequency. In earlier studies by Nayfeh and coworkers, the modulationfrequency was close to the first-mode natural frequency, and thereforelarge first-mode swaying was observed. But for higher forcingamplitudes, the present experiments show that the modulation frequencytends to shift away from the first-mode natural frequency, andsubsequently very little swaying is observed. We also developed areduced-order analytical model by discretizing the integralpartial-differential equation of motion, derived by Crespo daSilva and Glenn, using the Galerkin procedure with a four-modeapproximation. The reduced-order model demonstrates the energy transferfrom the third mode to the first mode.     

Fixed-width confidence intervals for contrasts in the means

The sequential procedure developed by Bhargava and Srivastava (1973, J. Roy. Statist. Soc. Ser. B, 35, 147–152) to construct fixed-width confidence intervals for contrasts in the means is further analyzed. Second-order approximations for the first two moments of the stopping time and the coverage probability associated with the sequential procedure, are obtained. A lower bound for the number of additional observations after stopping is derived, which ensures the mxact probability of coverage. Moreover, two-stage, three-stage and modified sequential procedures are proposed for the same estimation problem. Relative advantages and disadvantages of these sampling schemes are discussed and their properties are studied.     

A dipyrromethane based receptor as a dual colorimetric sensor for F− and Cu2+ ions

A novel dipyrromethane based receptor has been synthesized. It works as a highly selective colorimetric sensor for F^? and Cu²⁺ ions, which can be used for their necked eye detection with evident colour change of its solution.     

Chelation-controlled diastereoselective reduction of alpha-fluoroketones

The effect of Ti-based Lewis acids on the reduction of alpha-fluoropropiophenone was examined to determine whether chelation control could be used to direct the diastereoselectivity of conversion to an alpha-fluoro alcohol. Pretreatment of alpha-fluoropropiophenone with TiCl4 followed by reduction with LiBH4 in diethyl ether or methylene chloride provided the syn diastereomer predominantly, while use of Ti(OiPr)4 under identical conditions provided the anti diastereomer as the major product. The products are consistent with a chelation-controlled mechanistic pathway in the former reduction and a nonchelation pathway in the latter case. Detailed 1H, 13C, and 19F NMR studies were consistent with chelation between TiCl4 and alpha-fluoropropiophenone under the reaction conditions utilized in this study. Reduction of other alpha-fluoroketones in the presence of TiCl4 also provided a high degree of diastereoselectivity in the conversion to alpha-fluoro alcohols, showing the generality of this approach.     

Cyclopentane construction by Rh-catalyzed intramolecular C-h insertion: relative reactivity of a range of catalysts

The preparation and Rh-mediated cyclization of the alpha-diazoester 1 are outlined, and its utility in determining the elements that contribute to the reactivity of the intermediate Rh-carbenoid is presented. The rate of disappearance of diazo ester 1 catalyzed by several representative Rh(II) complexes was determined. The observed relative rate constants for the reaction of the Rh(II) complexes with 1 varied over a range of >10(7). The reactivity of the Rh-carbenoid intermediate was explored using the ratio of the sum of (3 + 4 + 5) to 2 (cyclization vs elimination), the ratio of 3 to the sum of (4 + 5) (chemoselectivity), and the ratio of 4 to 5 (diastereoselectivity). It is striking that these four measures of reactivity were found to be independent of each other.     

Rapid and Sensitive HPTLC Method for Determination of Epalrestat in Human Plasma

JPC – Journal of Planar Chromatography – Modern TLC - Epalrestat is a new drug for diabetic neuropathy. A new, simple, rapid very sensitive, and accurate high performance thin-layer...