Guo Hong-Lian

Effects of Spherical Aberration on Optical Trapping Forces for Rayleigh Particles

The trapping force on Rayleigh particles in an optical tweezers system with an oil immersion objective is calculated by an electromagnetic model. The results indicate that the stability of particles trapped will be affected by spherical aberration, which is caused by refractive difference between objective oil and water solution, when the specimen manipulated is suspended in a water solution. The trapping force and depth of potential well will decrease and the minimum of laser power for ensuring the stability of particles trapped will increase with the enhanced trapping depth.

Mechanical Properties of Breast Cancer Cell Membrane Studied with Optical Tweezers

Membrane tethers are extracted from breast cancer cells using a force generated by an optical trap. It is experimentally obtained that the radius of tether is about 0.1 μm and the static tether force is about 8.5 pN. Calculations based on the experimental measurements give a bending modulus for the tether of 1.35×10−19 N.m and a surface membrane tension of 6.76×10−6 N/m in the breast cancer cell. The treatment with cytochalasin D results in the decreasing bending modulus and decreasing apparent surface tension. When the membrane protein caveolin is over-expressed, similar cases occur in bending modulus and apparent surface tension. In addition, the viscous resistance coefficient of the membrane is calculated to be 1.15 pN.s/μm according to the dynamic tether forces obtained under different pulling velocities.

Effect of spherical aberration introduced by water solution on trapping force

Trapping force of an optical tweezers system with an oil immersion objective is calculated with a ray-optics model. Results indicate that the trapping force will be decreased as a result of the introduction of spherical aberration, which is caused by the refractive mismatch between objective oil and water, when the sample manipulated is suspended in a water solution. The effect of spherical aberration will be serious when the detection depth of the optical tweezers is enhanced.

Measurement of Breaking Force of Fluorescence Labelled Microtubules with Optical Tweezers

Under illumination of excitation light, the force that can make fluorescent dye-labelled microtubules break up is measured by using dual-beam optical tweezers. It is found that this force is about several piconewtons, which is two orders of magnitude smaller than that without fluorescence label. Microtubules can be elongated about 20% and the increase of the tensile force is nonlinear with the microtubule elongation. Some qualitative explanations are given for the mechanisms about the breakup and elongation of microtubules exposed to excitation light.

Membrane Tether Formation on a Cell Surface with Reservoir

We propose a mathematical model to analyse the membrane tether formation process on a cell surface with reservoir. Based on the experimental results, the membrane reservoir density of breast cancer cell was obtained, ? = 8.02. The membrane surface viscosity between membrane and environment ? is 0.021(pN.s/?m3), and the static force F0 = 5.71?pN.

Displacement and force measurements with quadrant photodetector in optical tweezers

A technique of displacement and force measurements with a photodiode quadrant detector in an optical tweezers system is presented. The stiffness of optical trap is calibrated and the leukemia cell membrane tension is measured. The results show that the optical tweezers combined with the quadrant detector is a very useful tool for detecting the displacement and force with a millisecond-order response.

Absorption Range and Energy Shift of Surface Plasmon in Au Monomer and Dimer

The resonance behaviors of local surface plasmon resonance in Au monomer and dimer are characterized systemically by electron energy loss spectroscopy in a scanning transmission electron microscope. The measured absorption range is about 20 nm larger than the physical size of the Au nanoparticles and the resonance peak energy shows a red shift when the electron beam passes of the nanoparticles. The Au dimer displays similar behaviors. Numerical simulation also reproduces those experimental results.

Measurement of Binding Force between Microtubule-Associated Protein and Microtubule

Microtubule-associated proteins (MAPs) are important proteins in cells. They can regulate the organization, dynamics and function of microtubules. We measure the binding force between microtubule and a new plant MAP, i.e. AtMAP65-1, by dual-optical tweezers. The force is obtained to be 14.6±3.5 pN from the data statistics and analysis. This force measurement is helpful to understand the function and mechanism of MAPs from the mechanical point of view and lays the groundwork for future measurements of the mechanical properties of other biological macro-molecules.

Oscillatory disturbance in force calibration of optical tweezers

In the calibration of the optical trap stiffness, it is found that there appears an attenuating oscillation as an oscillatory disturbance added to the trapped bead movement, when the scanner is driven by a triangular wave input. An equivalent oscillator model is put forward to explain the mechanism of the oscillatory disturbance. Both the measurements and calculations show that the attenuating oscillation comes from the oscillation of the scanner and the triangular wave drive causes this additional oscillation of the scanner. Furthermore, the analysis indicates that the oscillatory disturbance will become stronger, when the stiffness of the trap increases or the natural frequency of the scanner decreases. We adopt another driving way, i.e. a sinusoidal wave input is used instead of the triangular wave input. Our experiment has verified that in this case the oscillatory disturbance is eliminated completely.