In the research of bio-molecular chips and sensors, extra electric biases are most often employed to control and manipulate the DNA and protein molecules moving through micro/nano-fluidic channels. In order to accurately and flexibly control the bio-molecules as they move within the channels, a clear understanding of how the current changes within the buffer solution caused by an applied bias is fundamental. In this report, the current changed value of different buffer solutions, e.g., KC1, TE, and TBE was systematically studied with real-time monitoring and quantitative analysis in the situation of the buffers moving through a fluidic channel with a 5 μm inner diameter, driven by biases of 50 or 100 mV. The results revealed that the relation- ship between the current changed value and the pause interval of the applied electric field is highly consistent with the Hill Equation, which is helpful for accurately detecting and manipulating single biomolecules in microfluidic sensors and biochips.
As one of the most important realizations of stimulated emission depletion(STED)microscopy,the continuous-wave(CW)STED system,constructed by using CW lasers as the excitation and STED beams,has been investigated and developed for nearly a decade.However,a theoretical model of the suppression factors in CW STED has not been well established.In this investigation,the factors that affect the spatial resolution of a CW STED system are theoretically and numerically studied.The full-width-at-half-maximum(FWHM)of a CW STED with a doughnut-shaped STED beam is also reanalyzed.It is found that the suppression function is dominated by the ratio of the local STED and excitation beam intensities.In addition,the FWHM is highly sensitive to both the fluorescence rate(inverse of fluoresce lifetime)and the quenching rate,but insensitive to the rate of vibrational relaxation.For comparison,the suppression function in picosecond STED is only determined by the distribution of the STED beam intensity scaled with the saturation intensity.Our model is highly consistent with published experimental data for evaluating the spatial resolution.This investigation is important in guiding the development of new CW STED systems.
Haiyun QinWei ZhaoChen ZhangYong LiuGuiren WangKaige Wang
The electrodynamic characteristics of single DNA molecules moving within micro-/nano-fluidic channels are important in the design of biomedical chips and bimolecular sensors. In this study, the dynamic properties of λ-DNA molecules transferring along the microchannels driven by the external electrickinetic force were systemically investigated with the single molecule fluorescence imaging technique. The experimental results indicated that the velocity of DNA molecules was strictly dependent on the value of the applied electric field and the diameter of the channel. The larger the external electric field, the larger the velocity, and the more significant deformation of DNA molecules. More meaningfully, it was found that the moving directions of DNA molecules had two completely different directions：（i） along the direction of the external electric field, when the electric field intensity was smaller than a certain threshold value;（ii） opposite to the direction of the external electric field, when the electric field intensity was greater than the threshold electric field intensity.The reversal movement of DNA molecules was mainly determined by the competition between the electrophoresis force and the influence of electro-osmosis flow. These new findings will theoretically guide the practical application of fluidic channel sensors and lab-on-chips for precisely manipulating single DNA molecules.
