[Show abstract][Hide abstract] ABSTRACT: Laser-induced fluorescence photobleaching anemometer (LIFPA) has been developed in order to measure velocity fluctuations of the unsteady micro electrokinetic turbulent flows in microfluidics. The statistical errors of LIFPA measurement, because of 3-D flows and Taylor’s hypothesis (compared with local Taylor’s hypothesis Pinton and Labbé in J Phys II 4:1461-1468, 1994), are theoretically estimated and compared to hot-wire anemometer (HWA) measurement that is used for conventional turbulence measurement. The correction factor in the direction parallel to the laser beam is estimated, and the influence of directional correction factors of LIFPA is also investigated. It is found that in our investigation, the error due to Taylor’s hypothesis is negligible. The influence of 3-D flows on the first derivative variance of velocity fluctuations in LIFPA is smaller than that in HWA measurement.
Experiments in Fluids 02/2015; 56(2). DOI:10.1007/s00348-014-1850-1 · 1.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A donut-shaped laser that is simultaneously impacted by the coma, astigmatism and spherical aberrations in a high NA focusing system is numerically simulated based on the vectorial diffraction theory. The full-width half-maximum, intensity characters, and dimensions of the dark cores of the donut-shaped laser on the focal plane are quantitatively analyzed. The results are highly consistent with the annular-shaped nano-patterns that are experimentally fabricated by our purpose-built laser direct writing system. The mechanism of asymmetric nano-patterns fabricated through laser writing lithography is also revealed.
Journal of optics 12/2014; 16(12). DOI:10.1088/2040-8978/16/12/125701 · 2.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The surface effect is becoming apparently significant as the miniaturization of fluidic devices. In the micro/nanochannel fluidics, the electrode surface effects have the same important influence on the current signals as the channel surface effects. In this paper, when aqueous solution are driven with non-continuous DC electric field force, the characteristics of current signals of the fluid transferring through microfluidic channel are systematically studied. Six modes of current signal are summarized, and some new significant phenomena are found, e. g. there exists a critical voltage at which the steady current value equals to zero; the absolute value of the steady current decreases at first, however, it increases with the external voltage greater than the critical voltage as the electrode area ratio of cathode and anode is 10 and 20; the critical voltage increases with the enhancing of electrode area ratio of cathode and anode and solution pH, while it decreases with the raising of ion concentration. Finally, the microscopic mechanism of the electrode surface charge effects is discussed preliminarily. The rules will be helpful for detecting and manipulating single biomolecules in the micro/nanofluidic chips and biosensors. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
[Show abstract][Hide abstract] ABSTRACT: We have revisited the synthesis of a series of ICT fluorophores, which were reported to have a core structure of 8-oxo-8H-acenaphtho[1,2-b]pyrrol-9-carbonitrile. However, based on the 2D NMR and X-ray diffraction analysis, their core structure was corrected as 1-oxo-1H-phenalene-2,3-dicarbonitrile (1). Compound 1 shows a highly electron-deficient nature and can easily undergo oxidative SNArH reaction on the naphthyl ring to produce a series of novel ICT fluorophores. The regioselectivity of this substitution reaction was studied by introduction of representative nucleophiles. Moreover, due to the strong rigidity and efficient ICT nature, the obtained fluorescent dyes display very good spectroscopic properties even in an aqueous environment.
[Show abstract][Hide abstract] ABSTRACT: 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., KCl, 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 relationship 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.
[Show abstract][Hide abstract] ABSTRACT: Nanoscale functional structures are indispensable elements in many fields of modern science. In this paper, nanopillar array with a pillar diameter far smaller than Abbe's diffraction limit is realized by a new kind of continuous wave (CW) laser direct lithography technology. With atomic force microscopy technology, the average diameter of nanopillars on thin OIR906 photoresist film is about 65 nm and the smallest diameter is 48 nm, which is about 1/11 of the incident laser wavelength. Also, the influences of coma and astigmatism effects to the shape and size of nanopillar are numerically simulated by utilizing vector integral. As far as we know, it is the first time that nanopillar array is implemented by a donut-shaped 532-nm visible CW laser. The study presents a new, simple, inexpensive, and effective approach for nanopillar/pore array fabrication.
Nanoscale Research Letters 06/2013; 8(1):280. DOI:10.1186/1556-276X-8-280 · 2.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: With unique capability of biomedical diagnosis and chemical analysis, microfluidics has received worldwide applications in various fields. Multiple methods have been proposed to measure velocity profile of flow in microchannels, among which the laser induced fluorescence photobleaching anemometer (LIFPA) provides great performance with not only high spatial resolution but also superior temporal resolution compared with former systems. The measurement sensitivity and temporal resolution of LIFPA system is largely determined by the speed of fluorescence photobleaching effect, meaning that a half-life decay time as short as possible is required to achieve superior temporal resolution. In this paper, several experiments are done to investigate the influence over the half-life decay time by several factors, including incident laser power, dye concentration, buffer concentration and fluidic pH value. Based on the experimental results, a certain dye solution is specially designed, which demonstrates an extremely fast photobleaching effect with a half-life decay time of about 55 μs. Fluorescent dye solution with such a fast photobleaching effect is potentially applicable in the analysis of transient flows, such as the rise time of electroosmotic flow, where high temporal resolution is required.
[Show abstract][Hide abstract] ABSTRACT: Cyanamide was introduced into the rhodamine spirolactam framework to produce a colorless and non-fluorescent compound . It shows a reversible ring-opening/ring-closure process in response to the solution pH, which exhibits an "ON/OFF" switching in its fluorescence. Different from other rhodamine-type dyes, the ring-open form of is stable in protic solvents under neutral, near neutral and basic conditions, showing a pink color and very strong fluorescence. We also demonstrated the potential of in live cell imaging.
[Show abstract][Hide abstract] ABSTRACT: A novel fluorescent dye named ATTO 390 is tested and verified to be
applicable in STED microscopy with great performance. The experimental
configuration uses a 405nm continuous wave (CW) laser as excitation beam
and a 532nm CW laser as depletion beam. The two collinear beams are
focused on the dye using an objective with NA=1.4 and the fluorescence
is detected by avalanche photodiode detector. With short wavelength in
violet band, high fluorescence quantum yield, good photostability, and
relatively low Ps demonstrated in a series of experiments,
ATTO 390 is proved to be a potential good choice for fluorescent dye in
[Show abstract][Hide abstract] ABSTRACT: The present work demonstrates the use of a dielectrophoretic lab-on-a-chip device in effectively separating different cancer cells of epithelial origin for application in circulating tumor cell (CTC) identification. This study uses dielectrophoresis (DEP) to distinguish and separate MCF-7 human breast cancer cells from HCT-116 colorectal cancer cells. The DEP responses for each cell type were measured against AC electrical frequency changes in solutions of varying conductivities. Increasing the conductivity of the suspension directly correlated with an increasing frequency value for the first cross-over (no DEP force) point in the DEP spectra. Differences in the cross-over frequency for each cell type were leveraged to determine a frequency at which the two types of cell could be separated through DEP forces. Under a particular medium conductivity, different types of cells could have different DEP behaviors in a very narrow AC frequency band, demonstrating a high specificity of DEP. Using a microfluidic DEP sorter with optically transparent electrodes, MCF-7 and HCT-116 cells were successfully separated from each other under a 3.2 MHz frequency in a 0.1X PBS solution. Further experiments were conducted to characterize the separation efficiency (enrichment factor) by changing experimental parameters (AC frequency, voltage, and flow rate). This work has shown the high specificity of the described DEP cell sorter for distinguishing cells with similar characteristics for potential diagnostic applications through CTC enrichment.
[Show abstract][Hide abstract] ABSTRACT: We report an electrokinetically driven micro-turbulent flow phenomenon in a microchannel, where the Reynolds number (Re) based on bulk flow and channel width is in the order of 10. As we know the critical Re in a channel flow is about 2000 and a pressure driven flow with Re in the order of 10 is definitely laminar, hence, the fluid mixing is very flow. Although there have been electrokinetical micromixers published that can increase mixing, the flows are believed not to be turbulence, but chaotic. Here we design a novel electrokinetically forced pressure driven flow with Re in the order of 10 to achieve turbulence like flow. To confirm that the flow is turbulence, Laser Induced Fluorescence Photobleaching Anemometer (LIFPA) is used to measure the corresponding flow velocity features. LIFPA enables ultrahigh temporal and spatial resolution measurement. We report the measured flow features: high dissipation and diffusion rate, multiscale continuous power spectra and 3-D flow. The results open a new view of turbulence at microscale.
[Show abstract][Hide abstract] ABSTRACT: We present a non-intrusive molecular dye based method, i.e., laser-induced fluorescence photobleaching anemometer (LIFPA),
to significantly increase temporal resolution (TR) for velocity measurement of fast transient electrokinetic flows. To our
knowledge, the TR has been for the first time achieved to 5–10μs, about 100 times better than that published from state-of-the-art
micro particle image velocimetry (μPIV), which is currently the most widely used velocimetry in the microfluidics community.
The new method provides us with new opportunities to study experimentally the fundamental phenomena of unsteady electrokinetics
(EK) and to validate relevant theoretical models. One application of the new method is demonstrated by measuring the rise
time of DC electroosmotic flows (EOFs) in a microcapillary of 10μm in diameter.
KeywordsLaser-induced fluorescence photobleaching anemometer–LIFPA–DC electroosmotic flow–Rise time–Transient electrokinetic flow
Microfluidics and Nanofluidics 09/2011; 11(3):353-358. DOI:10.1007/s10404-011-0800-y · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have developed two new microfluidic cell sorters based on conventional negative dielectrophoresis (DEP) for continuous flow operations. The first is a cascade configuration sorter designed to increase purity of isolated target cell. The second has two staggered side channels in opposite side walls to increase sample throughput without compromising enrichment factor. Particles (carboxylate microspheres) of different sizes were first used to demonstrate the feasibility of the present DEP sorters for cell isolation. Then biological cells, i.e. human prostate cancer cell line LNCaP and human colorectal cancer cell line HCT116 were used to test the performance of the DEP sorters. In the present work, applied voltage was in the range of 0-20 V(p-p) , and frequency was from 0 to 10 MHz. Comparing to a single side channel DEP cell sorter, the isolation purity was improved from 80 to 96% by a single cascade sorter and the sample throughput was increased from 0.2 to 0.65 μL/min by a single staggered side channel sorter. In this article, we report the cell sorter designs, cell separation and enrichment factors.
[Show abstract][Hide abstract] ABSTRACT: In this Letter, the nanoporous anodic alumina membranes (nanoPAAM) were first fabricated by the two-step anodic oxidisation method, and then based on these nanoPAAM master molds, polymer polymethylmethacrylate nanopillars with controllable heights and diameters were achieved by the direct casting method. Subsequently, combined with the ultraviolet lithography or electron beam lithography, a kind of useful platform of microfluidic channel that was embedded with nanopillar arrays was achieved. Finally, the transferring processes of DNA molecules around the entrance of the hybrid channel were monitored with an epi-fluorescence microscopy and electron multiplying charge coupled device camera in real time. The hybrid structure of microchannel with nanopillar arrays provides critical advantages when using the fluorescent microscopy to detect the dynamics of single DNA molecules.
[Show abstract][Hide abstract] ABSTRACT: In order to understand transport phenomena in nanofluidics, we study the flow velocity profile in a nanocapillary. Laser Induced Fluorescence Photobleaching Anemometer (LIFPA) and Stimulated Emission Depletion (STED) are combined to establish a far-field nanoscopic velocimeter for flow velocity measurement in a nanochannel. LIFPA uses molecular dye as tracer to avoid issues involved in particles as tracer in PIV, when one dimension of the channels is nearly in the same order of particle diameter. STED is applied to overcome conventional diffraction limit in physics to increase spatial resolution. To apply LIFPA, calibration is first required to establish the relationship between the fluorescence intensity signal and flow velocity. Current monitoring, time-fly of fluorescence and metering from syringe pump are used for the calibration respectively. Then the velocity profile is measured with a spatial resolution of about 70 nm in a nanocapillary with inner diameter of 360 nm. The conductivity influence on the velocity profile is investigated.
[Show abstract][Hide abstract] ABSTRACT: A dynamic micropumping phenomenon has been observed, which is based on acoustic streaming generated by a piezotransducer actuated
31 mode without check valve. The use of d
31 mode of a piezoelectric microtransducer can significantly enhance the pumping performance for application in miniaturization.
Experimental study of the maximum local velocity has been conducted to investigate the effect of actuator tip configuration
on the micropumping performance. In addition, this study also shows the quantitative measurement of the pumping performance
such as the pressure head generated as a function of different relevant parameters like applied electrical field, AC frequency,
and length of the actuator.
KeywordsDynamic micropumping-Acoustic streaming-Piezoelectric microtransducer-
31 mode-Miniaturization-Actuator tip configuration
Microfluidics and Nanofluidics 08/2010; 9(2):385-396. DOI:10.1007/s10404-009-0556-9 · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Stimulated emission depletion (STED) microscopy is one of the breakthrough technologies that belong to far-field optical microscopy and can achieve nanoscale spatial resolution. We demonstrate a far-field optical nanoscopy based on continuous wave lasers with different wavelengths, i.e., violet and green lasers for excitation and STED, respectively. Fluorescent dyes Coumarin 102 and Atto 390 are used for validating the depletion efficiency. Fluorescent nanoparticles are selected for characterizing the spatial resolution of the STED system. Linear scanning of the laser beams of the STED system along one line of a microscope slide, which is coated with the nanoparticles, indicates that a spatial resolution of about 70 nm has so far been achieved. A two-dimensional image of the particle pattern of the STED system is constructed and compared with scanning confocal microscope. The present work has further extended the application of the STED microscopy into the blue regime.
The Review of scientific instruments 05/2010; 81(5):053709. DOI:10.1063/1.3432001 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: For the first time we have been able to measure the flow velocity profile for nanofluidics with a spatial resolution better than 70 nm. Due to the diffraction resolution barrier, traditional optical methods have so far failed in measuring the velocity profile in a nanocapillary or a closed nanochannel without an opened sidewall. A novel optical point measurement method is presented which applies stimulated emission depletion (STED) microscopy to laser induced fluorescence photobleaching anemometer (LIFPA) techniques to measure flow velocity. Herein we demonstrate this far-field nanoscopic velocimetry method by measuring the velocity profile in a nanocapillary with an inner diameter of 360 nm. The closest measuring point to the wall is about 35 nm. This method opens up a new class of functional measuring techniques for nanofluidics and for nanoscale flows from the wall.
Lab on a Chip 01/2010; 10(2):240-5. DOI:10.1039/b917584a · 6.12 Impact Factor