Zhao Xin-Sheng

Optical properties of Langmuir-Blodgett film of hemicyanine containing the rare earth complex anion Dy(BPMPHD)-2

Abstract ( E )- N -hexadecyl-4-(2-(4-dimethylaminophenyl)ethenyl)pyridinium bis(1,6-bis(1′-phenyl-3′-methyl-5′-pyrazolone-4′)hexanedionato- [1,5]) dysprosium (III) was synthesized. The monolayers formed on a pure water subphase (pH 5.6, 18 °C) were transferred onto hydrophilic quartz, calcium fluoride, and glass substrates successively with a transfer ratio of around unity. From second-harmonic generation (SHG) experiments, the second-order molecular hyperpolarizability β was evaluated to be about 4.8 × 10 -48 C m 3 V -2 . The results of UV-visible, IR and X-ray photoelectron spectroscopy of the Langmuir-Blodgett films are also reported.

A Microfluidic Mixer for Single-Molecule Kinetics Experiments

We designed and built a microfluidic mixer based on the principle of hydrodynamic focusing governed by Navier-Stokes equation for single-molecule kinetics experiments. The mixer is a cast of poly(dimethylsiloxane) (PDMS) sealed with transparent fused-silica coverglass, which results in low fluorescence background and broad biological compatibility and this enables single-molecule fluorescence detection under nonequilibrium conditions. The pressure regulated sample delivery system is convenient for loading a sample and allows for precise and stable flow velocity control. The combination of microfluidic mixer and single-molecule fluorescence resonance energy transfer (smFRET) allows us to measure the time course of the distribution of the smFRET efficiency in protein folding. We used the fact that denatured protein collapses much faster than the mixing process to characterize the mixing time using donor and acceptor dyes labeled staphylococcal nuclease (SNase) as an smFRET efficiency indicator. By monitoring the smFRET efficiency of denatured SNase during the course of mixing, we determined that the mixing time was 150 ms under conditions suitable for single-molecule detection.

Maximum Entropy Method for Analyses of Fluorescence Correlation Spectra of Oligonucleotide Intra-Chain Collision

It has been a challenge to accurately extract dynamic information from experimental fluorescence correlation spectroscopy (FCS) data. In this paper, we compare three major fitting methods: the model-dependent multiple exponential function (MultiExp), the empirical stretched exponential function (StreExp), and the exponential function based on the model-free maximum entropy method (MemExp). MultiExp has straight forward physical significance but it is difficult to implement and interpret in a complex system. StreExp has simple form and is easy to use but its physical picture is obscure. MultiExp is model free but its results are sensitive to experimental noise. A good choice seems to be a combination of MemExp and MultiExp. In our example, we have unraveled that two independent processes exist in the intra-chain collision of a single-stranded DNA when base pair formation is possible, which has not been observed by previous investigators. MemExp is recommended for the FCS data analysis, although caution should be exercised in the practice.

Spectra of Single Cyanobacterial Cells

With confocal microscopy and grating spectrograph, the absorption and fluorescence emission spectra of single Anabaena strain PCC 7120 cells have been studied in intact filaments in situ. The different spectra between vegetative cells and heterocysts are observed. The evident heterogeneities in heterocysts that have not been reported previously are attributed to the different stages of the evolution of phycobilisomes and photosystems in heterocysts.

The Role of Sulfite in the Wetting Etching of Silicon and Silica

The effects of sulfite on the etching processes of silicon and silica in 40% (ω) NH4F aqueous solution have been studied by a method based on photolithography and atomic force microscopy (AFM). It is demonstrated that the silicon and silica etching rates depend on the sulfite concentrations. The spectra of the F 1 s core level in the high resolution X-ray photoelectron spectroscopy (XPS) for surfaces treated in solutions with/without sulfite suggest the chemistry of the surfaces is different in the two cases. The experimental results indicate that sulfite not only acts as an oxygen scavenger, but also influences the surface reactions in the wetting etching of Si and SiO2.