Onset of Nonlinear Electroosmotic Flow under an AC Electric Field.

Nonlinearity of electroosmotic flows (EOFs) is ubiquitous and plays a crucial role in ion transport, specimen mixing, electrochemistry reaction, and electric energy storage and utilization. When and how the transition from a linear regime to a nonlinear one occurs is essential for understanding, prohibiting, or utilizing nonlinear EOF. However, due to the lack of reliable experimental instruments with high spatial and temporal resolutions, the investigation of the onset of nonlinear EOF still remains in theory. Herein, we experimentally studied the velocity fluctuations of EOFs driven by an alternating current (AC) electric field via ultrasensitive fluorescent blinking tricks. The linear and nonlinear AC EOFs are successfully identified from both the time trace and energy spectra of velocity fluctuations. The transitional electric field (EA,C) is determined by both the convection velocity (U) and AC frequency (ff) as EA,C ∼ ff0.48-0.027U. We hope the current investigation could be essential in the development of both theory and applications of nonlinear EOFs.

A Stable NanoPAA-ZnO/ZnCl2 Composite with Variable 3D Structured Morphology and Sustained Superhydrophilicity.

A ZnO/ZnCl2 composite with stable 3D structural morphologies and long lasting superhydrophilicity was synthesized on the top surface of a nano porous anodic alumina (nanoPAA) substrate. The wettability of a nanoPAA-ZnO/ZnCl2 was systematically characterized and the experimental data indicated that the water contact angle (WCA) of 0° could be achieved as well as maintained over 7 days and still remained at 4.36° after 50 days, and its 3D structural morphology had no clearly observable change during this period. The mechanism for the superhydrophilicity of the composites was interpreted in terms of the inherent hydrophilicity of ZnO/ZnCl2 nanofilm, the three-dimensional structures of wrinkled nanoflakes, the nanogaps between neighbor nanoflakes, the difference of structual morphologies (i.e., size, shape, and upright posture of nanoflakes), and the measured True Volume of voids in the nanocomposite. The structural morphologies were mainly determined by the parameters such as the original concentration of precursor ZnCl2 and the pore diameter of nanoPAA substrate. The study proposes a promising superhydrophilic nanomaterial and a cost-effective synthesis method, which will play a practical role in the fields of biomedical molecular sensors and micro/nanofluidic chips.

Rapid AC Electrokinetic Micromixer with Electrically Conductive Sidewalls.

We report a quasi T-channel electrokinetics-based micromixer with electrically conductive sidewalls, where the electric field is in the transverse direction of the flow and parallel to the conductivity gradient at the interface between two fluids to be mixed. Mixing results are first compared with another widely studied micromixer configuration, where electrodes are located at the inlet and outlet of the channel with electric field parallel to bulk flow direction but orthogonal to the conductivity gradient at the interface between the two fluids to be mixed. Faster mixing is achieved in the micromixer with conductive sidewalls. Effects of Re numbers, applied AC voltage and frequency, and conductivity ratio of the two fluids to be mixed on mixing results were investigated. The results reveal that the mixing length becomes shorter with low Re number and mixing with increased voltage and decreased frequency. Higher conductivity ratio leads to stronger mixing result. It was also found that, under low conductivity ratio, compared with the case where electrodes are located at the end of the channel, the conductive sidewalls can generate fast mixing at much lower voltage, higher frequency, and lower conductivity ratio. The study of this micromixer could broaden our understanding of electrokinetic phenomena and provide new tools for sample preparation in applications such as organ-on-a-chip where fast mixing is required.

Large-Scale Flow in Micro Electrokinetic Turbulent Mixer.

In the present work, we studied the three-dimensional (3D) mean flow field in a micro electrokinetic (µEK) turbulence based micromixer by micro particle imaging velocimetry (µPIV) with stereoscopic method. A large-scale solenoid-type 3D mean flow field has been observed. The extraordinarily fast mixing process of the µEK turbulent mixer can be primarily attributed to two steps. First, under the strong velocity fluctuations generated by µEK mechanism, the two fluids with different conductivity are highly mixed near the entrance, primarily at the low electric conductivity sides and bias to the bottom wall. Then, the well-mixed fluid in the local region convects to the rest regions of the micromixer by the large-scale solenoid-type 3D mean flow. The mechanism of the large-scale 3D mean flow could be attributed to the unbalanced electroosmotic flows (EOFs) due to the high and low electric conductivity on both the bottom and top surface.

Microfluidic-assisted polymer-protein assembly to fabricate homogeneous functionalnanoparticles.

Functional polymer-protein nanoparticles (NPs) have broad applications in biotechnology and nanotechnology. In principle, controllable and vigorous mixing is required to fabricate homogeneous NPs, which remains a challenge via conventional bulk synthetic methods. In this study, an electrokinetics (EK) based microfluidic reactor with fast mixing is explored to assemble functional proteins with polymers in an ethanol/water co-solvent system. The resultant NPs show significantly improved size distribution by comparison with the ones prepared using conventional bulk method, while the NPs size can be tuned by adjusting the mass ratio of polymer to protein. The functionalities of the assembled proteins are sustained upon the EK based microfluidic mixing, indicating the application potential of our method in the controlled assembly of different functional proteins.

Fluorescent Nanoparticles for Stimulated Emission Depletion Microscopy.

While most of the fluorescent nanoparticles used in stimulated emission depletion (STED) microscopy have a long excitation wavelength, many applications need shorter wavelength fluorophores, which are yet to be developed for STED microscopy applications. Here, three kinds of fluorescent nanoparticles, namely silica nanoparticles (NFv465), fluoro-max blue aqueous fluorescent nanoparticles (FBs) and light yellow nanoparticles (LYs) with short excitation wavelength in violet band have been studied to assess whether they are applicable in STED microscopy. The experimental configuration utilizes a 405 nm continuous wave (CW) laser as excitation beam and a 532 nm CW laser as depletion beam. We compare the photostability, photobleaching and depletion efficiency of three kinds of fluorescent nanoparticles in a series of experiments. Light yellow nanoparticles are proved to be a good candidate as fluorophore in STED microscopy.

Publisher Correction: A special three-layer step-index fiber for building compact STED systems.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

A special three-layer step-index fiber for building compact STED systems.

Up to now, most of stimulated-emission-depletion (STED) systems were lens-based bulky systems. Exchanging some spatial light paths with optical fiber components will make the systems more flexible and will benefit various fields. A big problem to achieve this goal is that the STED beam generated by the traditional method of bulky systems cannot be maintained in an optical fiber due to its birefringence. In this article, we will introduce a type of special optical fiber. With the special fiber, a dark hollow beam with doughnut-shaped focal spot and a concentric beam with Gaussian-shaped focal spot can be generated at the same time. Parameters of a sample and a compact STED system based on it are demonstrated.

Artificial Cellulosome Complex from the Self-Assembly of Ni-NTA-Functionalized Polymeric Micelles and Cellulases.

Polymer-protein core-shell nanoparticles have been explored for enzyme immobilization. This work reports on the development of functional polymeric micelles for immobilizing His6 -tagged cellulases with controlled spatial orientation of enzymes, resulting in "artificial cellulosomes" for effective cellulose hydrolysis. Poly(styrene)-b-poly(styrene-alt-maleic anhydride) was prepared through one-pot reversible addition-fragmentation chain-transfer polymerization and modified with nitrilotriacetic acid (NTA) to afford an amphiphilic block copolymer. The self-assembled polymer was mixed with a solution of NiSO4 to form Ni-NTA-functionalized micelles, which could successfully capture His6 -tagged cellulases and form hierarchically structured core-shell nanoparticles with cellulases as the corona. Because the anchored enzymes are site-specifically oriented and in close proximity, synergistic catalysis that results in over twofold activity enhancement has been achieved.

Drug screening assay based on the interaction of intact Keap1 and Nrf2 proteins in cancer cells.

BACKGROUND: The Nrf2-Keap1 interaction is the major regulatory pathway for cytoprotective responses against oxidative and electrophilic stresses. Keap1, a substrate protein of a Cul3-dependent E3 ubiquitin ligase complex, is a negative regulator of Nrf2. The use of chemicals to regulate the interaction between Keap1 and Nrf2 has been proposed as a strategy for the chemoprevention of degenerative diseases and cancers. RESULTS: The interactions between Keap1 and Nrf2 in vitro and in vivo were investigated using fluorescence resonance energy transfer (FRET) and bimolecular fluorescence complementation (BiFC) strategies in our study. Nrf2 with its N-terminal fused to eGFP and Keap1 with its C-terminal fused to mCherry were expressed and purified in vitro. When purified eGFP-Nrf2 and Keap1-mChrry proteins were mixed together, a strong FRET signal could be detected, indicating an efficient energy transfer from eGFP to mCherry. Moreover, the FRET was detected in vivo using confocal microscopy in colon cancer HCT-116 cells that were co-transfected with eGFP-Nrf2 and Keap1-mCherry. Finally, using an eGFP BiFC approach, the Keap1-Nrf2 interaction was also detected in MCF7 cells by transfecting eGFP N-terminal fused to Nrf2 (eN158-Nrf2) and eGFP C-terminal fused to Keap1 (eC159-Keap1). Using the BiFC and FRET systems, we demonstrated that the prototypical Nrf2-activiting compound tBHQ and the antitumor drug F-dUrd might interfere with the intracellular interaction between Keap1 and Nrf2 whereas the 5-Fu have little role in activating the protective response of Nrf2 pathway in cancer cells. CONCLUSIONS: By analyzing the perturbation of the energy transfer between the donor and acceptor fluorophores and the bimolecular fluorescence complementation of eGFP, we can screen potential inhibitors for the interaction between Keap1 and Nrf2.

Abnormal Rheological Phenomena in Newtonian Fluids in Electroosmotic Flows in a Nanocapillary.

Abnormal rheological phenomena arising in Tris-borate-ethylenediaminetetraacetic acid solutions (believed to be Newtonian fluids) were observed in direct current electroosmotic flows within a nanocapillary with a diameter of 200 nm under a low electric field of tens of volts per meter. In solutions with different concentrations and pH values, the flow behavior indices of the power-law fluids were calculated on the basis of current-voltage relations. When the electric field intensity was below a critical value of 6.7 V/m, the fluids exhibited dilatant (shear thickening) effects. Fluid viscosity changed with electric field intensity because the near-wall shear rate of an electroosmotic flow changes with electric field intensity via a power-law relation. When the electric field intensity surpasses the critical electric field, the fluid again becomes Newtonian and has constant viscosity. The investigation shows that in nanocapillaries, fluids commonly believed to be Newtonian can become non-Newtonian near walls as a result of strong nanoscale interfacial effects. The results can also improve our understanding of electroosmosis-related transport phenomena in nanofluidics and biomedical science.

Study of Oscillating Electroosmotic Flows with High Temporal and Spatial Resolution.

Near-wall velocity of oscillating electroosmotic flow (OEOF) driven by an AC electric field has been investigated using a laser-induced fluorescence photobleaching anemometer (LIFPA). For the first time, an up to 3 kHz velocity response of OEOF has been successfully measured experimentally, even though the oscillating velocity is as low as 600 nm/s. It is found that the oscillating velocity decays with the forcing frequency ff as ff-0.66. In the investigated range of electric field intensity (EA), below 1 kHz, the linear relation between oscillating velocity and EA is also observed. Because the oscillating velocity at high frequency is very small, the contribution of noise to velocity measurement is significant, and it is discussed in this manuscript. The investigation reveals the instantaneous response of OEOF to the temporal change of electric fields, which exists in almost all AC electrokinetic flows. Furthermore, the experimental observations are important for designing OEOF-based micro/nanofluidics systems.

Studying compaction-decompaction of DNA molecules induced by surfactants.

The mechanism and detailed processes of DNA compaction and decompaction are essential for the life activities, as well as for the researches in the molecular biology, genetics and biomedicine. The compaction of two kinds of DNA molecules caused by Cetyltrimethyl Ammonium Bromide (CTAB) and their decompaction induced with sodium dodecyl sulfate (SDS) or excessive amount of CTAB have been investigated with multiple perspectives such as the UV-VIS spectrophotometry, dynamic light scattering, and zeta potential. The compaction phenomenon of DNA can easily be observed when the CTAB combines with the DNA, not just when the molar ratio QCTAB/QDNA is approximately equal to 1 as the conventional recognition, but also when QCTAB/QDNA <1,DNA can be compacted; Molecular state of DNA is only changed in the conformational structure, but not in the chemical structure. Finally, a model is suggested to help catch on the biophysical mechanism of DNA chain conformational change.

Experimental Investigations on Fluorescence Excitation and Depletion of Carbon Dots.

Carbon dots (CDs) can be readily synthesized and utilized as attractive fluorescent probes for a variety of applications. In this study, we have synthesized CDs using a previously published method and characterized their photo-physical properties. The resultant CDs possess prominent photo-stability and short emission wavelength in the violet region. Our study reveals that CDs, with weak photo-bleaching, enable them to be employed to achieve high spatial resolution in stimulated emission depletion (STED) microscopy. The depletion efficiency can reach 60%. More importantly, the shorter excitation wavelength of CDs contributes to further improvement of resolution for STED microscopy. An excellent candidate for fluorophores, these CDs have potential to be used in super-resolution imaging for STED microscopy.

Scaling of velocity and scalar structure functions in ac electrokinetic turbulence.

Electrokinetic (EK) turbulence or electrohydrodynamic (EHD) turbulence has been recently achieved in different fluids under both ac [G. Wang et al., Lab Chip 14, 1452 (2014)10.1039/C3LC51403J; Phys. Rev. E 93, 013106 (2016)10.1103/PhysRevE.93.013106] and dc electric fields [A. Varshney et al., Soft Matter 12, 1759 (2016)10.1039/C5SM02316E]. Here, through dimensional analysis, scaling laws of both velocity and electric conductivity structure functions in the forced cascade region of ac EK turbulence can be predicated (similar to Bolgiano-Obukhov scaling law in turbulent Rayleigh-Bénard convection), in either macroscale or microscale flows. In the forced cascade region, EK force, which relies on the direct cascade of conductivity structures, injects energy directly into a wide spectral region to sustain the flow disturbance. The scaling exponents of the second-order velocity and conductivity structures are 2/5 and 4/5, respectively. In addition to the scaling regions, two characteristic small length scales are derived for both weak and strong electric body forces, respectively. This theoretical investigation can significantly enhance our understanding of EK or EHD turbulence while forced by an ac electric field. It can further broaden our understanding of the forced cascade region of forced turbulence and make the manipulation of the turbulent cascade process more flexible and controllable.

Influence of concentration on distribution properties of stretched-DNA in the MEC studied with fluorescence imaging and drop shape analyzing.

Stretching and manipulating DNA efficiently is significant for exploring the properties and applications of single DNA molecules. Here, the influence of concentrations of buffer and DNA on properties of stretched DNA molecules in the molecular evaporation combing (MEC) is investigated systematically with the single molecule fluorescence imaging microscopy and the high-precision drop shape analyzing technology. The stretched degree and uniformity of combed DNA molecules decrease as the buffer concentration are increased from 7 to 20mM. When the buffer concentration changes from 12 to 15mM, the stretched DNA molecules are apt to form a ringlike pattern. During the MEC process, there exist two kinds of evaporation modes, i.e., the constant contact angle mode and the constant contact radius mode. The former only takes effect in the lower concentration of buffer and DNA, enabling the uniform stretching. While the latter plays the leading role in the higher concentration, promoting the formation of the ringlike pattern of DNA molecules.

Microelectrokinetic turbulence in microfluidics at low Reynolds number.

There is commonly no turbulence in microfluidics, and the flows are believed to be either laminar or chaotic, since Reynolds number (Re) in microflows is usually on the order of unity or lower. However, we recently demonstrated that it is possible to achieve turbulence with low Re (based on the measured flow velocity and the width of the channel entrance) when a pressure-driven flow is electrokinetically forced in a quasi T-microchannel. To be able to measure high frequency velocity fluctuations in microchannels, a velocimeter with submicrometer spatial resolution and microsecond temporal resolution, called a laser-induced fluorescence photobleaching anemometer, is developed. Here we characterize the microelectrokinetic turbulence and observe some typical and important features of high Re flows, such as Kolmogorov -5/3 spectrum of velocity fluctuation, which usually can be realized only at very high Re in macroturbulent flows.

On micro-electrokinetic scalar turbulence in microfluidics at a low Reynolds number.

We recently demonstrated the direct observation of micro-electrokinetic turbulence in a microchannel at a low Reynolds number (Re) when a pressure-driven flow was forced electrokinetically. Here, we characterize the corresponding scalar turbulence and surprisingly find that the corresponding turbulent mixing has some typical and important features of scalar turbulence, such as the Obukhov-Corrsin (O-C) -5/3 spectrum of concentration fluctuation, which can commonly be realized only at high Re in macroflows. This discovery could provide a new perspective of scalar turbulence and an avenue for control of transport phenomena in lab-on-a-chip platforms. This will deepen our fundamental understanding of transport phenomena in microfluidics.

Nanopillar array with a λ/11 diameter fabricated by a kind of visible CW laser direct lithography system.

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.

An acid catalyzed reversible ring-opening/ring-closure reaction involving a cyano-rhodamine spirolactam.

Cyanamide was introduced into the rhodamine spirolactam framework to produce a colorless and non-fluorescent compound RBCN. 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 RBCN 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 RBCN in live cell imaging.