Franck H. Lei

Shear force detection by using bimorph cantilever with the enhanced Q factor

An improved nonoptical shear force detection system based on a rectangular bimorph cantilever incorporating the force feedback technique has been developed for tip–sample distance regulation in shear force microscopy. The force feedback amplifier consisting of a phase shifter and a linear amplifier is adjusted in such a way that the motion of the cantilever is mechanically amplified, resulting in a great enhancement of quality factor Q. Driving a fiber attached bimorph cantilever at its first harmonic resonance, with a phase shift φ=π/2 and an appropriate amplifier gain, allows one to obtain a Q factor greater than 103 in air, which corresponds to a Q enhancement of more than 1 order of magnitude. The effect of Q enhancement leads to an increase in the signal to noise ratio and thus the force detection sensitivity of the system. Typically, the minimum interaction force that can be sensed by the system is about 2 pN/√Hz. Topographic images of a human aorta tissue section in its natural state, taken with th...

Nanospectrofluorometry inside single living cell by scanning near-field optical microscopy

Near-field fluorescence spectra with subdiffraction limit spatial resolution have been taken in the proximity of mitochondrial membrane inside breast adenocarcinoma cells (MCF7) treated with the fluorescent dye (JC-1) by using a scanning near-field optical microscope coupled with a confocal laser microspectrofluorometer. The probe–sample distance control is based on a piezoelectric bimorph shear force sensor having a static spring constant k=5 μN/nm and a quality factor Q=40 in a physiological medium of viscosity η=1.0 cp. The sensitivity of the force sensor has been tested by imaging a MCF7 cell surface.

Shear force near-field optical microscope based on Q-controlled bimorph sensor for biological imaging in liquid.

Shear force near‐field microscopy on biological samples in their physiological environment loses considerable sensitivity and resolution as a result of liquid viscous damping. Using a bimorph‐based cantilever sensor incorporating force feedback, as recently developed by us, gives an alternative force detection scheme for biological imaging in liquid. The dynamics and sensitivity of this sensor were theoretically and experimentally discussed. Driving the bimorph cantilever close to its resonance frequency with appropriate force feedback allows us to obtain a quality factor (Q‐factor) of up to 103 in water, without changing its intrinsic resonance frequency and spring constant. Thus, the force detection sensitivity is improved. Shear force imaging on mouse brain sections and human skin tissues in liquid with an enhanced Q‐factor of 410 have shown a high sensitivity and stability. A resolution of about 50 nm has been obtained. The experimental results suggest that the system is reliable and particularly suitable for biological cell imaging in a liquid environment.

Non-optical bimorph-based tapping-mode force sensing method for scanning near-field optical microscopy

A non‐optical bimorph‐based tapping‐mode force sensing method for tip–sample distance control in scanning near‐field optical microscopy is developed. Tapping‐mode force sensing is accomplished by use of a suitable piezoelectric bimorph cantilever, attaching an optical fibre tip to the extremity of the cantilever free end and fixing the guiding portion of the fibre to a stationary part near the tip to decouple it from the cantilever. This method is mainly characterized by the use of a bimorph, which carries out simultaneous excitation and detection of mechanical vibration at its resonance frequency owing to piezoelectric and anti‐piezoelectric effects, resulting in simplicity, compactness, ease of implementation and lack of parasitic optical background. In conjugation with a commercially available SPM controller, tapping‐mode images of various samples, such as gratings, human breast adenocarcinoma cells, red blood cells and a close‐packed layer of 220‐nm polystyrene spheres, have been obtained. Furthermore, topographic and near‐field optical images of a layer of polystyrene spheres have also been taken simultaneously. The results suggest that the tapping‐mode set‐up described here is reliable and sensitive, and shows promise for biological applications.