Plasmonic enhanced fluorescence spectroscopy using side-polished microstructured optical fiber

We report excitation of surface plasmon in a gold-coated side-polished D-shape microstructure optical fiber (MOF). As the leaky evanescent field from the fiber core becomes highly localized by the plasmon wave, its intensity also gets amplified significantly. Here we demonstrate an efficient use of this intensified field as excitation in fluorescence spectroscopy. The so-called plasmonic enhanced fluorescence emission from Rhodamine B has been investigated experimentally. First, plasmonic effect alone was found to provide an immediate fluorescence enhancement factor of two. Second, experimental results show a good agreement with theoretical modeling. Strong evanescent field generation and surface enhancement with simple metallic coating makes this fiber based device a good candidate for compact fluorescence spectroscopy.     

Numerical investigation of micro- and nanochannel deformation due to discontinuous electroosmotic flow

Large pressures can induce detrimental deformation in micro- and nanofluidic channels. Although this has been extensively studied for systems driven by pressure and/or capillary forces, deflection in electrokinetic systems due to internal pressure gradients caused by non-uniform electric fields has not been widely explored. For example, applying an axial electric field in a channel with a step change in conductivity and/or surface charge can lead to internally generated pressures large enough to cause cavitation, debonding, and/or channel collapse. Finite electric double layers within nanofluidic channels can further complicate the physics involved in the deformation process. In order to design devices and experimental procedures that avoid issues resulting from such deformation, it is imperative to be able to predict deformation for given system parameters. In this work, we numerically investigate pressures resulting from a step change in conductivity and/or surface charge in micro- and nanofluidic channels with both thin and thick double layers. We show an explicit relation of pressure dependence on concentration ratio and electric double layer thickness. Furthermore, we develop a numerical model to predict deformation in such systems and use the model to unearth trends in deformation for various electric double layer thicknesses and both glass and PDMS on glass channels. Our work is particularly impactful for the development and design of micro- and nanofluidic-based devices with gradients in surface charge and/or conductivity, fundamental study of electrokinetic-based cavitation, and other systems that exploit non-uniform electric fields.     

Experimental study of the separation behavior of nanoparticles in micro- and nanochannels

In this article, we investigate the effects of pH, ionic strength, and channel height on the mobility and diffusivity of charged spherical particles within planar microfluidic channels. Specifically, we report results of a broad experimental study on the transport and separation behavior of 50 and 100 nm spherical carboxylated polystyrene nanoparticles, confined in 20 μm, 1 μm, and 250 nm deep fluidic channels. We find that pH, ionic strength, and channel height have coupled impacts on mobility changes. In particular, we show that, depending on pH, the dependence of particle mobility on channel size can have opposing behavior. In addition, we also show that at the nanoscale, at lower ionic strengths, there is a substantial increase in mobility, due to enhanced electric fields within the nanochannels. These effects are important to understand in order to avoid potential downfalls in terms of separation efficiency as well as design for better tuning of separation performance in micro- and nanochannels. Finally, we propose a method to estimate the effective zeta potential of spherical particles from measured electrophoretic mobility data. This could prove useful in characterizing a heterogeneous collection of particles having a distribution over a range of values of the zeta potential.     

Towards a multidimensional view of tourist mobility patterns in cities: A mobile phone data perspective

The last decade has witnessed a wealth of studies on characterizing human mobility patterns using movement datasets. Such efforts have highlighted a few salient dimensions of individual travel behavior relevant to urban planning and policy analysis. Despite the fruitful research outcomes, most of the findings are drawn upon urban residents. The behavioral characteristics of other population groups, such as tourists, remain underexplored. In this study, we introduce an analytical framework to gain insights into tourist mobility patterns. By analyzing mobile phone trajectories of international travelers to three different cities in South Korea, we introduce nine mobility indicators to capture different facets of tourist travel behavior (e.g., duration of stay in a city, spatial extent of activities, location visited and trips conducted, and mobility diversity), and examine their statistical properties across cities. An eigendecomposition approach is then introduced to better understand the interdependency of these mobility indicators and inherent variations among individual travelers. Based on the eigendecomposition results, we further employ a dimension reduction technique to describe the key characteristics of each traveler. Since the mobile phone dataset captures the nationality of tourists, we use such information to quantify the behavioral heterogeneity of travelers across countries and regions. Finally, we select a few traveler groups with distinctive mobility patterns in each city and examine the spatial patterns of their activities. Substantial differences are observed among traveler groups in their spatial preferences. The implications for location recommendation and deployment of tourism services (e.g., transportation) are discussed. We hope the study brings a synergy between classic human mobility analysis and the emerging field of tourism big data. The framework can be applied or extended to compatible datasets to understand travel behavior of tourists, residents, and special population groups in cities.     

Manipulating electrokinetic conductance of nanofluidic channel by varying inlet pH of solution

The electrokinetic conductivity of micro-/nanofluidic systems, which strongly depends on the local solution properties (e.g., pH and ionic strength), has wide applications in nanosystems to control the system performance and ion rectification. Accurate and active manipulation of this parameter is proven to be very challenging since, in nanoscale, the ion transport is particularly dominated by the acquired surface charge on the solid–liquid interfaces. In this study, we propose an approach to manipulate the nanochannel electrokinetic conductivity by changing the pH value of the solution at the inlet in order to impose asymmetrical conditions inside nanochannel. The variable surface charge of walls is determined by considering the chemical adsorption on the solid–liquid interface and the electrical double layer interaction. The presented numerical model, which couples Poisson–Nernst–Planck and Navier–Stokes equations, can fully consider the electro-chemo-mechanical transport phenomena and predict the electrokinetic conductivity of nanofluidic channels with good accuracy. Modeling results show that the electrokinetic conductivity of the nanofluidic systems can be regulated by varying the solution pH at the inlet. It is revealed that the stronger electric double layers interaction can enhance the sensitivity of the nanochannel electrokinetic conductance to the inlet pH. This unique behavior of the nanochannel electrokinetic conductivity could broaden potential applications in biomedical, energy, and environmental systems using nanofluidic devices.     

Discovering and modeling meta-structures in human behavior from city-scale cellular data

For a long time, researchers explore spatio-temporal properties in mobility to understand human behavior. They have discovered many statistical laws about human dynamics. Unfortunately, we still have limited knowledge about the spatio-temporal structure of individuals’ movement at a large scale. In this paper, we studied the unified spatio-temporal structures (i.e., meta-structures) in human mobility. We hereby propose a meta-structure discovery algorithm by coupling both topology and spatio-temporal attributes of mobility graphs. With the construction of individual profiles from meta-structure analyses, we provided a novel mobility model from a process-driven perspective, which reduced the dependence of many existing models on the consistency between local and global mobility statistics. We gained some insights on the dominating meta-structures in human mobility by leveraging mobile data in a large city. The statistical distribution of meta-structures is found to be determined by the intrinsic heterogeneity of spatio-temporal properties in human behavior. Our model evaluation showed that a process with basic rules could demonstrate the key statistical properties in mobility meta-structures. We believe that these approaches and observations would be a good reference for management of human mobility in mobile networks and transportation systems.     

Modulation of electroosmotic flow by electric field-responsive polyelectrolyte brushes: a molecular dynamics study

Molecular dynamics simulations were done to study the electroosmotic flow (EOF) transport in a nanochannel grafted with polyelectrolytes under the control of an electric field normal to the channel wall. This study first addresses some problems on the interplay between complex EOF and non-equilibrium conformational behavior of polyelectrolyte brushes at a molecular level. We demonstrated that changing the normal electric field has a significant impact on the conformational transition of polyelectrolytes and ion distributions, further leading to some new flow phenomena. The coupling mechanisms of polyelectrolyte chain dynamics and electrohydrodynamics were discussed. A remarkable result obtained is that fluid flux depends nonmonotonically on the normal electric field. Our work provides fundamental understanding of the EOF modulation using polyelectrolyte brushes and guidance for the design of smart nanofluidic channels.     

Fabrication and electroosmotic flow measurements in micro- and nanofluidic channels

An easy method for fabricating micro- and nanofluidic channels, entirely made of a thermally grown silicon dioxide is presented. The nanochannels are up to 1-mm long and have widths and heights down to 200 nm, whereas the microfluidic channels are 20-μm wide and 4.8-μm high. The nanochannels are created at the interface of two silicon wafers. Their fabrication is based on the expansion of growing silicon dioxide and the corresponding reduction in channel cross-section. The embedded silicon dioxide channels were released and are partially freestanding. The transparent and hydrophilic silicon dioxide channel system could be spontaneously filled with aqueous, fluorescent solution. The electrical resistances of the micro- and nanofluidic channel segments were calculated and the found values were confirmed by current measurements. Electrical field strengths up to 600 V/cm were reached within the nanochannels when applying a potential of only 10 V. Electroosmotic flow (EOF) measurements through micro- and nanofluidic channel systems resulted in electroosmotic mobilities in the same order of those encountered in regular, fused silica capillaries.     

Application of continuum mechanics to fluid flow in nanochannels

A fundamental understanding of the transport phenomena in nanofluidic channels is critical for systematic design and precise control of such miniaturized devices towards the integration and automation of Lab-on-a-chip devices. The goal of this study is to develop a theoretical model of electroosmotic flow in nano channels to gain a better understanding of transport phenomena in nanofluidic channels. Instead of using the Boltzmann distribution, the conservation condition of ion number and the Nernst equation are used in this new model to find the ionic concentration field of an electrolyte solution in nano channels. Correct boundary conditions for the potential field at the center of the nanochannel and the concentration field at the wall of the channel are developed and applied to this model. It is found that the traditional plug-like velocity profile is distorted in the center of the channel due to the presence of net charges in this region opposite to that in the electrical double layer region. The developed model predicted a trend similar to that observed in experiments reported in the literature for the area-average velocity versus the ratio of Debye length to the channel height.     

光纤倏逝波生物传感器的结构研究

光纤倏逝波生物传感器的性能主要体现在探测能力和简便性两个方面,为了提高其探测极限和野外适应性,针对光纤探头的倏逝场激发能量和系统结构的整体性进行了较为系统的研究,分析了三种不同发展阶段的系统结构,设计和搭建了一种目前发展的基于光纤束的荧光光纤倏逝波生物传感器系统,以此系统为基础进行了实验检测并取得了较为满意的检测效果.最后给出了下一阶段的系统结构发展方向并分析了其优越性和可行性.     

Clustering effects in solution‐based nanoparticle/template hybrid coatings

Abstract— Templating processes with monodisperse polystyrene particles are being explored for a number of application areas, especially in microelectronics and optics. Classically, these depend on rather slow‐drying steps that allow for the polystyrene particles to gradually situate themselves into ordered arrays and patterns. On the other hand, many manufacturing processes are designed to operate much more rapidly (spin‐coating, roll‐coating, spray‐coating, etc.). The present work investigates the behavior of nanoparticle(titania)‐microparticle (polystyrene) hybrid solutions when processed with these rapid techniques. Very thin coatings were examined where the larger‐template particle‐particle interactions can be quantifiably observed and measured. This gives us insight into the conditions when structural order might be achievable in materials systems using these rapid, industrially important, coating techniques. The specific system that was investigated involves nanoscale titanium dioxide particles combined with monodisperse micron‐scale polystyrene template particles aimed at building flexible dye‐sensitized solar cells. For this application, the formation of strong physical networks of titanium dioxide nanoparticles with percolating interparticle channels for the electrolyte phase to reach all surfaces where dye molecules will reside is required.     

Distributed decision fusion under nonideal communication channels with adaptive topology

Multi-sensor decision fusion has attracted some attention in information fusion field, meanwhile, the distributed target detection has been a well-studied topic in the multi-sensor detection theory. This paper investigates the increase in detection reliability that an adaptive network (with adaptive topologies and nonideal channels and decision fusion rules) can provide, compared with a fixed topology network. We consider a network, consisting of K-local uncertainty sensors and a Fusion Center (FC) tasked with detecting the presence or absence of a target in the Region of Interest (ROI). Sensors transmit binary modulated local decisions over nonideal channels modeled as Gaussian noise or fading channels. Assuming that the signal intensity emitted by a target follows the isotropic attenuation power model, we consider three classes of network topology architectures: (1) serial topology; (2) tree topology, and (3) parallel topology. Under the Neyman–Pearson (NP) criterion, we derive the optimal threshold fusion rule with adaptive topology to minimize the error probability. Extensive simulations are conducted to validate the correctness and effectiveness of the proposed algorithms.     

Nondestructive imaging of grain boundaries in polycrystalline materials using evanescent microwave probes

Nondestructively imaging electrical properties of materials with very high spatial resolution is of great importance in analyzing electronic, semiconductor, superconductor, biological, and other materials. In this work, we introduce and discuss a new family of probes that use evanescent microwave fields to image microwave properties of materials. We discuss and explain new equipment and programming methods that have been incorporated into our setup to improve its resolution and imaging capabilities. In addition, new scanning techniques are discussed that allow for the affluence region of microwave probe to be characterized. Finally, images that were obtained from ceramics, diamond, and other types of materials are discussed to demonstrate the improved imaging capability of the evanescent microwave probe in detecting grain boundaries and film uniformities among other parameters.     

Efficient prototyping of large-scale pdms and silicon nanofluidic devices using pdms-based phase-shift lithography

In this study, we explore the potential of poly-dimethylsiloxane (PDMS)-based phase-shift lithography (PPSL) for the fabrication of nanofluidic devices. We establish that this technology, which was already shown to allow for the generation of 100?nm linear or punctual features over squared centimeter surfaces with conventional photolithography systems, is readily adequate to produce some of the most popular nanofluidic systems, namely nanochannels and nanoposts arrays. We also demonstrate that PPSL technology enables to generate PDMS and silicon nanofluidic systems. This technological achievement allows us to perform single DNA molecule manipulation experiments in PDMS and silicon nanochannels, and we observe an unexpectedly slow migration of DNA in PDMS devices, which is independent on salt or pH conditions. Our data in fact hint to the existence of an anomalous response of DNA in PDMS nanofluidic devices, which is likely associated to transient nonspecific interactions of DNA with PDMS walls. Overall, our work demonstrates the efficiency and the performances of PPSL for prototyping nanofluidic systems.     

Zeta-potential Analyses using Micro Electrical Field Flow Fractionation with Fluorescent Nanoparticles

Increasingly growing application of nanoparticles in biotechnology requires fast and accessible tools for their manipulation and for characterization of their colloidal properties. In this work we determine the zeta-potentials for polystyrene nanoparticles using micro electrical field flow fractionation (mu-EFFF) which is an efficient method for sorting of particles by size. The data obtained by mu-EFFF were compared to zeta potentials determined by standard capillary electrophoresis. For proof of concept, we used polystyrene nanoparticles of two different sizes, impregnated with two different fluorescent dyes. Fluorescent emission spectra were used to evaluate the particle separation in both systems. Using the theory of electrophoresis, we estimated the zeta-potentials as a function of size, dielectric permittivity, viscosity and electrophoretic mobility. The results obtained by the mu-EFFF technique were confirmed by the conventional capillary electrophoresis measurements. These results demonstrate the applicability of the mu-EFFF method not only for particle size separation but also as a simple and inexpensive tool for measurements of nanoparticles zeta potentials.     

Laser-induced forward transfer of silver nanoparticle ink: time-resolved imaging of the jetting dynamics and correlation with the printing quality

In this work, we used time-resolved imaging to study the dynamics of the laser-induced forward transfer (LIFT) process of a silver nanoparticle (NP) ink (NP size: 30–50 nm). LIFT is a versatile direct write technique in which a variety of functional materials can be transferred from a donor substrate to a receiving substrate with high spatial resolution. Two different LIFT configurations were employed: (a) dynamic release layer (DRL)-assisted LIFT, in which direct expose of the silver NP ink to laser irradiation is prevented, and (b) DRL-free LIFT, in which the silver NP ink is exposed to laser irradiation. Jetting dynamic behavior, initiated by a cavitation bubble generation and expansion, was observed in both LIFT configurations. However, jetting dynamics were significantly milder in the case of DRL-assisted LIFT, resulting in a wide laser fluence processing window (100–230 mJ/cm²) for high uniformity and reproducibility printing of silver NP ink droplets. On the contrary, DRL-free LIFT resulted in smooth jetting dynamics only for a narrow laser fluence window (30–40 mJ/cm²). The explanation of the different dynamics is based on the different mechanisms that govern the conversion of the laser pulse energy to a dynamic cavitation bubble for each LIFT configuration, i.e., (a) heat diffusion, mediated by the DRL layer, in DRL-assisted LIFT and (b) microcavitation around the silver NPs due to near field enhancement when no DRL is used. In addition, the mechanism of formation of undesirable satellite droplets around the main deposited droplets was studied by using a flexible polymeric receiving substrate. The importance of the smooth jetting behavior, achieved by DRL-assisted LIFT, was highlighted for high-resolution printing of silver NP ink as well as for ensuring enhanced LIFT processing stability.     

A novel bonding method for fabrication of PMMA nanofluidic chip with low deformation of the nano-trenches

All PMMA-based nanofluidic chips are becoming increasingly important for biological and medical applications. To fabricate PMMA nanofluidic chips, the open nano-trenches should be sealed by thermal bonding method. However, the present thermal bonding method suffers from high deformation of nano-trenches due to PMMA softening near glass transition temperature. In this work, a novel bonding technique, based on acetone and ethanol (v:v, 8:2) treatment, is developed to adjust the Young’s modulus of PMMA in its surface layer. By optimizing nanoimprinting and bonding process, PMMA nanofluidic chip was fabricated without undesired nano-trench deformation. The integrity of the enclosed PMMA nanofluidic system was verified by a fluorescence filling experiment.     

A Monolithic Dual-Color Total-Internal-Reflection-Based Chip for Highly Sensitive and High-Resolution Dual-Fluorescence Imaging

We report a dual-color total-internal-reflection (TIR)-based chip that can generate two overlapping evanescent fields with different wavelengths for simultaneous imaging of two types of fluorophores. We derived a general relationship among the dimensions of the components of the chip to guarantee the overlap of two evanescent fields. Optical simulation results also confirm the generation and overlap of two evanescent fields. Using Si bulk micromachining and poly(dimethylsiloxane) (PDMS) casting, our fabrication method integrates all miniaturized optical components into one monolithic PDMS chip. Thus, assembly is unnecessary, and misalignment is avoided. Our PDMS chip can be employed with various sample delivery platforms, such as glass slide, flow cell, microchannel, etc. We first demonstrated the capability of the chip by imaging TIR fluorescent spots of a mixture of two fluorophores, namely, fluorescein and tetramethylrhodamine. We then employed the chip to observe the Brownian motion of a mixture of nile-red and dragon-green 500-nm microbeads. Our chip could potentially be integrated into a micro-total analysis system for highly sensitive and high-resolution dual-fluorescence imaging applications.$hfill$[2009-0188]      

Capillarity-driven dynamics of water–alcohol mixtures in nanofluidic channels

We investigated the spontaneous capillarity-driven filling of nanofluidic channels with a thickness of 6 and 16 nm using mixtures of ethanol and water of variable composition. To improve the visibility of the fluid, we embedded metal mirrors into the top and bottom walls of the channels that act as a Fabry–Pérot interferometer. The motion of propagating liquid–air menisci was monitored for various concentrations in transmission with an optical microscope. In spite of the visible effects of surface roughness and different affinity of water and ethanol to the channel walls, the dynamics followed the classical t ^1/2—dependence according to Lucas and Washburn. While the prefactor of this algebraic relation falls short of the expectations based on bulk properties by 10–30%, the relative variation between mixtures of different composition follows the expectations based on the bulk surface tension and viscosity, implying that—despite the small width of the channels and the large surface-to-volume ratio—specific adsorption or chemical selectivity effects are not relevant. We briefly discuss the impact of surface roughness on our experimental results.