Honglian Guo

Angular dependent magnetoresistance with twofold and fourfold symmetries in A-type antiferromagnetic Nd0.45Sr0.55MnO3 thin film

The angular dependent magnetoresistance (AMR) of Nd0.45Sr0.55MnO3 thin film epitaxially grown on SrTiO3 (001) is examined at different temperatures and magnetic fields. Twofold and fourfold symmetric AMR and a transition between them are observed under two different measurement modes and are found to be dependent on temperature and/or strength of a magnetic field. In comparison with AMR occurring in other systems, we believe that the twofold/fourfold symmetric AMR observed here corresponds to different spin-canted states induced by the magnetic field at certain temperatures below the Neel temperature.

Transmission modulation in the passband of polystyrene photonic crystals

The periodic disturbance in transmittance of polystyrene photonic crystal is detected in our experiment. The peak-to-peak intervals of the periodic disturbance vary with the wavelength. It is attributed to the propagation of the whispering gallery mode in polystyrene spheres. The results show that the periodically arrayed particles may affect light transfer outside the gap band, which implies the possibility in applications of photonic crystal films.

Kinetics of microtubule-AtMAP65-1 bond studied with dual-optical tweezers

The unbinding force of microtubule-AtMAP65-1 bond is measured with dual-optical tweezers. Force histograms reveal quantized force distributions under physiological condition and with 100 mM NaCl treatment. Based on Bell-Evans-model of multiple bonds, histograms are fitted and kinetic parameters are obtained. Under the force loading rate of 9pN/s, the most probable unbinding forces for the single bond are 16.8 +/- 0.8 and 14.7 +/- 0.8 pN under the physiological and treatment conditions, respectively. Position of transition state of the bond is given to be Delta x = 2.1 +/- 0.1 nm both with and without treatment of NaCl. Intrinsic dissociation rate constants k(0) are 0.0010 +/- 0.0002 and 0.0020 +/- 0.0005 s(-1) corresponding to without and with NaCl treatment, which means that 100 mM NaCl shortens the natural lifetime of the bond by 50%. In addition, the activation barrier also changes by 3.4% after treatment with NaCl, which indicates that NaCl makes the dissociation of the bond easier.

Method used to measure interaction of proteins with dual-beam optical tweezers

In the force measurement of protein-protein interaction, proteins are usually attached to microbeads, so the coated beads serve as both handles and force transducers. Due to the short interaction distance between proteins, the beads are usually close enough to each other. When dual-beam optical tweezers and quadrant photodiode detector are used to investigate the interaction of proteins, it is found that the signal of detected beads is greatly affected by adjacent beads. Analysis reveals that the contribution of two beads to the quadrant detector signal is independent. A method for extracting the real interaction signal from a disturbed one is presented. Based on this method, interaction between microtubules and AtMAP65-1 is measured. The results show that this method is useful for measuring short-distance interaction with the precision of piconewton and nanometer scales.

Measurements of leucocyte membrane elasticity based on the optical tweezers

A 1-μm in diameter polystyrene bead coated with lec tin is trapped by optical tweezers which is formed by a focused laser beam (740 nm). A leucocyte adhered to the bottom of sample cell is chosen to close to the trapped bead. When their stable combination is confirmed, the leucocyte is displaced by moving the sample stage. A “tether” is then formed between the trapped bead and the membrane. The force acting on the tether is measured by a detector equipped on the optical tweezers system. The deformation of the membrane is diverse in different contact conditions. When the contact area is small, the tether is very thin and the force on it is 7.4 pN (10−12 N), while the tether is much thicker under a larger contact area and its corresponding force is about 14 pN. It is presumed that the latter is related to the deformation of cytoskeleton.

Unbinding strength between C-terminal segment of AtMAP65-1 and microtubule studied with dual-optical tweezers

It has been reported that AtMAP65-1 dimer induces the formation of large microtubule bundles by forming cross-bridges between microtubules. More important, it has been reported that the C terminus of AtMAP65-1 contributes to microtubule binding and the N terminus to AtMAP65-1 dimerization. The interaction between microtubule and AtMAP65-1 is crucial since it can directly affect mechanical properties of microtubules and further the organizations and some functions of microtubules. In order to quantitatively know the interaction between microtubule and the C-terminal segment of microtubule associated protein (AtMAP65-1), we measure the unbinding force between them with dual-optical tweezers. Force histograms reveal quantized force distributions. Based on Bell-Evans-model of multiple bonds, histograms are fitted and kinetic parameters are obtained. The most probable unbinding force for the single bond is 20.0plusmn1.1 pN. Position of transition state of the bond is given to be Deltax=2.1 plusmn 0.1 nm. Intrinsic dissociation rate constant k0 is 0.0002plusmn0.00005 s-1.

INFLUENCE OF ILLUMINATION CONDITIONS ON THE MEASUREMENT OF OPTICAL TWEEZERS SYSTEM

In an optical tweezers system, the output signal of a photodiode quadrant detector and the temperature in a sample cell are two key factors for the quantitative measurements of mechanical properties of living biological objects such as cells, organelles and macro-molecules. In order to enhance the output of a quadrant detector and effectively control the temperature in a sample cell, the dependence of the temperature in the sample cell and the output of the quadrant detector for different illumination conditions are studied. The results show that appropriate illumination conditions can ensure both nearly constant temperatures in the cell and the desired output signal, which provides for the possibility of high precision and damage-free analysis of living biological objects.

Superposed oscillation in force calibrations driven by triangular-wave in optical tweezers system

In force calibration, when triangular-wave input is exerted on the piezoelectric-driven substage, there is a damped oscillation superposed on the square-wave movement of the trapped bead in our experiments. This will bring about instability for well trapping bead and become the noise source of the bead signal. Consequently, the superposed vibration will influence the precision and the range of force that can be calibrated. In order to analyze derivation of the oscillation and solve it, a model of equivalent oscillator is put forward. Based on this model, we calculate the initial amplitude of oscillation and frequency spectra of the movement of trapped bead, which is well in agreement with the experimental one. We apply sinusoidal-wave input substituted for triangular one to the substage, as a result, oscillation disturbance is avoided effectively, so the trapping stability of optical trap increases and the range of calibrated force has been extended greatly.