Optofluidic opportunities in global health, food, water and energy

Highly Accurate Pneumatically Tunable Optofluidic Distributed Feedback Dye Lasers

Optofluidic dye lasers integrated into microfluidic chips are promising miniature coherent light sources for biosensing. However, achieving the accurate and efficient tuning of lasers remains challenging. This study introduces a novel pneumatically tunable optofluidic distributed feedback (DFB) dye laser in a multilayer microfluidic chip. The dye laser device integrates microfluidic channels, grating structures, and vacuum chambers. A second-order DFB grating configuration is utilized to ensure single-mode lasing. The application of vacuum pressure to the chambers stretches the soft grating layer, enabling the sensitive tuning of the lasing wavelength at a high resolution of 0.25 nm within a 7.84 nm range. The precise control of pressure and laser tuning is achieved through an electronic regulator. Additionally, the integrated microfluidic channels and optimized waveguide structure facilitate efficient dye excitation, resulting in a low pump threshold of 164 nJ/pulse. This pneumatically tunable optofluidic DFB laser, with its high-resolution wavelength tuning range, offers new possibilities for the development of integrated portable devices for biosensing and spectroscopy.

Electrically Tunable Lenses for Imaging and Light Manipulation

Optofluidics seamlessly combines optics and microfluidics together to construct novel devices for microsystems, providing flexible reconfigurability and high compatibility. By taking advantage of mature electronic fabrication techniques and flexible regulation of microfluidics, electrically actuated optofluidics has achieved fantastic optical functions. Generally, the optical function is achieved by electrically modulating the interfaces or movements of microdroplets inside a small chamber. The high refractive index difference (~0.5) at the interfaces between liquid/air or liquid/liquid makes unprecedented optical tunability a reality. They are suitable for optical imaging devices, such as microscope and portable electronic. This paper will review the working principle and recent development of electrical optofluidic devices by electrowetting and dielectrophoresis, including optical lens/microscope, beam steering and in-plane light manipulation. Some methods to improve the lens performance are reviewed. In addition, the applications of electrical microfluidics are also discussed. In order to stimulate the development of electrically controlled liquid lens, two novel designs derived from electrowetting and dielectrophoresis are introduced in this paper.

Real-Time Tunable Optofluidic Splitter via Two Laminar Flow Streams in a Microchannel

This paper reports a tunable optofluidic splitter in which the incident light is split via refraction and reflection at the interface between two laminar flows in a microchannel but with different refractive indices. A Y-junction microchannel is used to demonstrate the continuous tuning of the splitting ratio of optical power by smooth adjustment of the ratio of two flow rates. In addition, it has achieved the tuning of split angle from 5° to 19° by the control of the refractive index contrast. The dynamic response gives a fastest switching frequency of 1.67 Hz between the "wave-guiding" and "splitting" status.

Advances and applications of nanophotonic biosensors

Nanophotonic devices, which control light in subwavelength volumes and enhance light-matter interactions, have opened up exciting prospects for biosensing. Numerous nanophotonic biosensors have emerged to address the limitations of the current bioanalytical methods in terms of sensitivity, throughput, ease-of-use and miniaturization. In this Review, we provide an overview of the recent developments of label-free nanophotonic biosensors using evanescent-field-based sensing with plasmon resonances in metals and Mie resonances in dielectrics. We highlight the prospects of achieving an improved sensor performance and added functionalities by leveraging nanostructures and on-chip and optoelectronic integration, as well as microfluidics, biochemistry and data science toolkits. We also discuss open challenges in nanophotonic biosensing, such as reducing the overall cost and handling of complex biological samples, and provide an outlook for future opportunities to improve these technologies and thereby increase their impact in terms of improving health and safety.

Droplet Microfluidics for Food and Nutrition Applications

Droplet microfluidics revolutionizes the way experiments and analyses are conducted in many fields of science, based on decades of basic research. Applied sciences are also impacted, opening new perspectives on how we look at complex matter. In particular, food and nutritional sciences still have many research questions unsolved, and conventional laboratory methods are not always suitable to answer them. In this review, we present how microfluidics have been used in these fields to produce and investigate various droplet-based systems, namely simple and double emulsions, microgels, microparticles, and microcapsules with food-grade compositions. We show that droplet microfluidic devices enable unprecedented control over their production and properties, and can be integrated in lab-on-chip platforms for in situ and time-resolved analyses. This approach is illustrated for on-chip measurements of droplet interfacial properties, droplet-droplet coalescence, phase behavior of biopolymer mixtures, and reaction kinetics related to food digestion and nutrient absorption. As a perspective, we present promising developments in the adjacent fields of biochemistry and microbiology, as well as advanced microfluidics-analytical instrument coupling, all of which could be applied to solve research questions at the interface of food and nutritional sciences.

Imaging-Based Optofluidic Biolaser Array Encapsulated with Dynamic Living Organisms

Optofluidic biolasers have emerged as promising tools for biomedical analysis due to their strong light-matter interactions and miniaturized size. Recent developments in optofluidic lasers have opened a new Frontier in monitoring biological processes. However, most biolasers require precise recording of the lasing spectrum at the single cavity level, which limits its application in high-throughput applications. Herein, a microdroplet laser array encapsulated with living Escherichia coli was printed on highly reflective mirrors, where laser emission images were employed to reflect the dynamic changes in living organisms. The concept of image-based lasing analysis was proposed by quantifying the integrated pixel intensity of the lasing image from whispering-gallery modes. Finally, dynamic interactions between E. coli and antibiotic drugs were compared under fluorescence and laser emission images. The amplification that occurred during laser generation enabled the quantification of tiny biological changes in the gain medium. Laser imaging presented a significant increase in integrated pixel intensity by 2 orders of magnitude. Our findings demonstrate that image-based lasing analysis is more sensitive to dynamic changes than fluorescence analysis, paving the way for high-throughput on-chip laser analysis of living organisms.

Development of a portable lab-on-a-valve device for making primary diagnoses based on gold-nanoparticle aggregation induced by a switchable linker

We have developed a low-cost, portable lab-on-a-valve (LOV) integrated microdevice for the detection of pathogens in primary-diagnosis settings. This system was designed for field-based pathogen detection based on the aggregation of gold nanoparticles induced by a switchable linker. A three-way valve, which has attracted much attention as a functional mesofluidic platform for pressure-driven flow, has been designed as a universal reaction platform that combines the functions of fluid flow and a reaction chamber. In addition, we obtain rapid and enhanced visual signals by the use of a syringe filter to remove gold nano-aggregates (Au NAs). Using this device, Salmonella Typhimurium down to 101 CFU mL-1 can be visually detected within 30 min by performing a simple operation that requires no complex equipment. This prototype device has great potential for use in the semi-quantitative and qualitative identification of pathogens in on-site primary diagnoses.

Piezophototronic gated optofluidic logic computations empowering intrinsic reconfigurable switches

Optofluidic nano/microsystems have advanced the realization of Boolean circuits, with drastic progression to achieve extensive scale integration of desirable optoelectronics to investigate multiple logic switches. In this context, we demonstrate the optofluidic logic operations with interfacial piezophototronic effect to promote multiple operations of electronic analogues. We report an optofluidic Y-channeled logic device with tunable metal-semiconductor-metal interfaces through mechanically induced strain elements. We investigate the configuration of an OR gate in a semiconductor-piezoelectric zinc oxide nanorod-manipulated optofluidic sensor, and its direct reconfiguration to logic AND through compressive strain-induced (−1%) piezoelectric negative polarizations. The exhibited strategy in optofluidic systems implemented with piezophototronic concept enables direct-on chip working of OR and AND logic with switchable photocurrent under identical analyte. Featured smart intrinsic switching between the Boolean optoelectronic gates (OR↔AND) ultimately reduces the need for cascaded logic circuits to operate multiple logic switches on-a-chip.

Designing optofluidic nano/microsystems to realize large-scale Boolean circuits remains a challenge. Here, the authors propose a flexible optofluidic framework to perform binary computations with an integrated piezophototronic mechanism controlling the optofluidic switching of logic gates (PPOF).

An ultraviolet and electric field activated photopolymer-ferroelectric nanoparticle composite for the performance enhancement of triboelectric nanogenerators

For the development of high performance triboelectric generators (TENGs), it is required to have facile methods to adjust the triboelectric properties of the friction surfaces. In this work, we present the surface charge density modulation of the photopolymer-ferroelectric nanoparticle composite surface by applying ultraviolet (UV) and electric field. By using the photopolymer, the triboelectric surface property was modulated by exposure to UV. In addition, lithographic surface patterning can be easily adopted to enlarge the frictional surface area as well. Furthermore, the use of the PP allows a facile integration of ferroelectric nanoparticles (NPs) in the form of a nanocomposite structure, which can effectively increase the surface charge density by spontaneous dipole coupling of NPs embedded in the PP layer. As a result, approximately 4-fold higher output power has been achieved by applying this approach. The developed TENGs have also demonstrated superior mechanical durability, generating consistent outputs during 10⁴ cyclic frictional contacts. The approach proposed here is a simple and reliable way to enhance the output performance of TENGs.

Real-time optofluidic surface-enhanced Raman spectroscopy based on a graphene oxide/gold nanorod nanocomposite

We demonstrate a glass microcapillary fiber as an optofluidic platform for surface enhanced Raman spectroscopy (SERS), the inner walls of which are coated with a graphene oxide (GO)/gold nanorod (AuNR) nanocomposite. A simple thermal method is used for the coating, allowing for the continuous deposition of the nanocomposite without surface functionalization. We show that the AuNRs can be directly and nondestructively identified on the GO inside the capillaries via identification of the Au-Br SERS peak, as Br- ions from the AuNR synthesis remain on their surface. The coated microcapillary platform is, then, used as a stable SERS substrate for the detection of Rhodamine 6G (R6G) and Rhodamine 640 (RH640) at concentrations down to 10^-7 and 10^-9 M, respectively. As the required sample volumes are as low as a few hundred nanoliters, down to ~75 femtograms of analyte can be detected. The fiber also allows for the detection of the molecules at acquisition times as low as 0.05 s, indicating the platform's suitability for real-time sensing.

Optofluidic Tunable Lenses for In-Plane Light Manipulation

Optofluidics incorporates optics and microfluidics together to construct novel devices for microsystems, providing flexible reconfigurability and high compatibility. Among many novel devices, a prominent one is the in-plane optofluidic lens. It manipulates the light in the plane of the substrate, upon which the liquid sample is held. Benefiting from the compatibility, the in-plane optofluidic lenses can be incorporated into a single chip without complicated manual alignment and promises high integration density. In term of the tunability, the in-plane liquid lenses can be either tuned by adjusting the fluidic interface using numerous microfluidic techniques, or by modulating the refractive index of the liquid using temperature, electric field and concentration. In this paper, the in-plane liquid lenses will be reviewed in the aspects of operation mechanisms and recent development. In addition, their applications in lab-on-a-chip systems are also discussed.

Scalable Spatial-Spectral Multiplexing of Single-Virus Detection Using Multimode Interference Waveguides

Simultaneous detection of multiple pathogens and samples (multiplexing) is one of the key requirements for diagnostic tests in order to enable fast, accurate and differentiated diagnoses. Here, we introduce a novel, highly scalable, photonic approach to multiplex analysis with single virus sensitivity. A solid-core multimode interference (MMI) waveguide crosses multiple fluidic waveguide channels on an optofluidic chip to create multi-spot excitation patterns that depend on both the wavelength and location of the channel along the length of the MMI waveguide. In this way, joint spectral and spatial multiplexing is implemented that encodes both spatial and spectral information in the time dependent fluorescence signal. We demonstrate this principle by using two excitation wavelengths and three fluidic channels to implement a 6x multiplex assay with single virus sensitivity. High fidelity detection and identification of six different viruses from a standard influenza panel is reported. This multimodal multiplexing strategy scales favorably to large numbers of targets or large numbers of clinical samples. Further, since single particles are detected unbound in flow, the technique can be broadly applied to direct detection of any fluorescent target, including nucleic acids and proteins.

Rainer Gross Award Lecture 2016: A Laboratory in Your Pocket: Enabling Precision Nutrition

The need for improving methods of nutritional assessment and delivering primary health care globally cannot be overemphasized. While advances in medical technology typically create more disparities because of access being limited to resource-rich settings, a transition of health care to a mobile platform is increasingly leveling the field. Technological advances offer opportunities to scale laboratory procedures down to mobile devices, such as smartphones and tablets. Globalization also provides the required infrastructure and network capacity to support the use of mobile health devices in developing settings where nutritional deficiencies are most prevalent. Here, we discuss some of the applications and advantages provided by expanding markets of biomarker measurement coupled with primary health care and public health systems and how this is enhancing access and delivery of health services with significant global impact.

In-line optofluidic refractive index sensing in a side-channel photonic crystal fiber

An in-line optofluidic refractive index (RI) sensing platform is constructed by splicing a side-channel photonic crystal fiber (SC-PCF) with side-polished single mode fibers. A long-period grating (LPG) combined with an intermodal interference between LP₀₁ and LP₁₁ core modes is used for sensing the RI of the liquid in the side channel. The resonant dip shows a nonlinear wavelength shift with increasing RI over the measured range from 1.3330 to 1.3961. The RI response of this sensing platform for a low RI range of 1.3330-1.3780 is approximately linear, and exhibits a sensitivity of 1145 nm/RIU. Besides, the detection limit of our sensing scheme is improved by around one order of magnitude by introducing the intermodal interference.

Guided transport of nanoparticles by plasmonic nanowires

Herein, we report the optical trapping and directional transport of nanoparticles in an aqueous solution by plasmonic nanowires. A laser illuminated one end of a silver nanowire and excited the localized and propagating surface plasmons. Optical forces were induced by the surface plasmons, which could trap the nanoparticles in an aqueous solution. Interestingly, the trapped nanoparticles moved along the silver nanowires from the trapping site to the excitation spot of the laser. Such movements of nanoparticles were also observed on curved nanowires, in which the trajectories of the particles were explicitly determined by the shape of the nanowires. More importantly, for a V-shaped silver nanowire, the direction of the movement could be modulated by the polarization of the incident laser. The direction of the movement was opposite to the prediction by the scattering force due to the propagation of surface plasmons, and the driving force could involve the thermal convection of local fluid due to a heating effect. Our findings indicate a novel approach to transport nanoparticles by plasmonic waveguides in aqueous solution.

Dual-color plasmonic enzyme-linked immunosorbent assay based on enzyme-mediated etching of Au nanoparticles

Colorimetric enzyme-linked immunosorbent assay utilizing 3′-3-5′-5-tetramethylbenzidine(TMB) as the chromogenic substrate has been widely used in the hospital for the detection of all kinds of disease biomarkers. Herein, we demonstrate a strategy to change this single-color display into dual-color responses to improve the accuracy of visual inspection. Our investigation firstly reveals that oxidation state of 3′-3-5′-5-tetramethylbenzidine (TMB²⁺) can quantitatively etch gold nanoparticles. Therefore, the incorporation of gold nanoparticles into a commercial TMB-based ELISA kit could generate dual-color responses: the solution color varied gradually from wine red (absorption peak located at ~530 nm) to colorless, and then from colorless to yellow (absorption peak located at ~450 nm) with the increase amount of targets. These dual-color responses effectively improved the sensitivity as well as the accuracy of visual inspection. For example, the proposed dual-color plasmonic ELISA is demonstrated for the detection of prostate-specific antigen (PSA) in human serum with a visual limit of detection (LOD) as low as 0.0093 ng/mL.

Recent Developments in Optofluidic Lens Technology

Optofluidics is a rapidly growing versatile branch of adaptive optics including a wide variety of applications such as tunable beam shaping tools, mirrors, apertures, and lenses. In this review, we focus on recent developments in optofluidic lenses, which arguably forms the most important part of optofluidics devices. We report first on a number of general characteristics and characterization methods for optofluidics lenses and their optical performance, including aberrations and their description in terms of Zernike polynomials. Subsequently, we discuss examples of actuation methods separately for spherical optofluidic lenses and for more recent tunable aspherical lenses. Advantages and disadvantages of various actuation schemes are presented, focusing in particular on electrowetting-driven lenses and pressure-driven liquid lenses that are covered by elastomeric sheets. We discuss in particular the opportunities for detailed aberration control by using either finely controlled electric fields or specifically designed elastomeric lenses.

Solar-thermal complex sample processing for nucleic acid based diagnostics in limited resource settings

The use of point-of-care (POC) devices in limited resource settings where access to commonly used infrastructure, such as water and electricity, can be restricted represents simultaneously one of the best application fits for POC systems as well as one of the most challenging places to deploy them. Of the many challenges involved in these systems, the preparation and processing of complex samples like stool, vomit, and biopsies are particularly difficult due to the high number and varied nature of mechanical and chemical interferents present in the sample. Previously we have demonstrated the ability to use solar-thermal energy to perform PCR based nucleic acid amplifications. In this work demonstrate how the technique, using similar infrastructure, can also be used to perform solar-thermal based sample processing system for extracting and isolating Vibrio Cholerae nucleic acids from fecal samples. The use of opto-thermal energy enables the use of sunlight to drive thermal lysing reactions in large volumes without the need for external electrical power. Using the system demonstrate the ability to reach a 95°C threshold in less than 5 minutes and maintain a stable sample temperature of +/- 2°C following the ramp up. The system is demonstrated to provide linear results between 10(4) and 10(8) CFU/mL when the released nucleic acids were quantified via traditional means. Additionally, we couple the sample processing unit with our previously demonstrated solar-thermal PCR and tablet based detection system to demonstrate very low power sample-in-answer-out detection.

Reconfigurable liquid-core/liquid-cladding optical waveguides with dielectrophoresis-driven virtual microchannels on an electromicrofluidic platform

An electrically reconfigurable liquid-core/liquid-cladding (L(2)) optical waveguide with core liquid γ-butyrolactone (GBL, ncore = 1.4341, εcore = 39) and silicone oil (ncladding = 1.401, εcladding = 2.5) as cladding liquid is accomplished using dielectrophoresis (DEP) that attracts and deforms the core liquid with the greater permittivity to occupy the region of strong electric field provided by Teflon-coated ITO electrodes between parallel glass plates. Instead of continuously flowing core and cladding liquids along a physical microchannel, the DEP-formed L(2) optical waveguide guides light in a stationary virtual microchannel that requires liquids of limited volume without constant supply and creates stable liquid/liquid interfaces for efficient light guidance in a simply fabricated microfluidic device. We designed and examined (1) stationary and (2) moving L(2) optical waveguides on the parallel-plate electromicrofluidic platform. In the stationary L-shaped waveguide, light was guided in a GBL virtual microchannel core for a total of 27.85 mm via a 90° bend (radius 5 mm) before exiting from the light outlet of cross-sectional area 100 μm × 100 μm. For the stationary spiral waveguide, light was guided in a GBL core containing Rhodamine 6G (R6G, 1 mM) and through a series of 90° bends with decreasing radii from 5 mm to 2.5 mm. With the stationary straight waveguide, the propagation loss was measured to be 2.09 dB cm(-1) in GBL with R6G (0.01 mM). The moving L-shaped waveguide was implemented on a versatile electromicrofluidic platform on which electrowetting and DEP were employed to generate a precise GBL droplet and form a waveguide core. On sequentially applying appropriate voltage to one of three parallel L-shaped driving electrodes, the GBL waveguide core was shifted; the guided light was switched at a speed of up to 0.929 mm s(-1) (switching period 70 ms, switching rate 14.3 Hz) when an adequate electric signal (173.1 VRMS, 100 kHz) was applied.

Optofluidic tunable lenses using laser-induced thermal gradient

This paper reports a new design of optofluidic tunable lens using a laser-induced thermal gradient. It makes use of two straight chromium strips at the bottom of the microfluidic chamber to absorb the continuous pump laser to heat up the moving benzyl alcohol solution, creating a 2D refractive index gradient in the entrance part between the two hot strips. This design can be regarded as a cascade of a series of refractive lenses, and is distinctively different from the reported liquid lenses that mimic the refractive lens design and the 1D gradient index lens design. CFD simulation shows that a stable thermal lens can be built up within 200 ms. Experiments were conducted to demonstrate the continuous tuning of focal length from initially infinite to the minimum 1.3 mm, as well as the off-axis focusing by offsetting the pump laser spot. Data analyses show the empirical dependences of the focal length on the pump laser intensity and the flow velocity. Compared with previous studies, this tunable lens design enjoys many merits, such as fast tuning speed, aberration-free focusing, remote control, and enabling the use of homogeneous fluids for easy integration with other optofluidic systems.

Disposable (bio)chemical integrated optical waveguide sensors implemented on roll-to-roll produced platforms

Demonstration of disposable multi-analyte polymeric integrated Young interferometers for analyte specific chemical- and biochemical sensing using biological and biomimetic recognition.

Label-free water sensors using hybrid polymer-dielectric mid-infrared optical waveguides

A chip-scale mid-IR water sensor was developed using silicon nitride (SiN) waveguides coated with poly(glycidyl methacrylate) (PGMA). The label-free detection was conducted at λ=2.6-2.7 μm because this spectral region overlaps with the characteristic O-H stretch absorption while being transparent to PGMA and SiN. Through the design of a hybrid waveguide structure, we were able to tailor the mid-IR evanescent wave into the PGMA layer and the surrounding water and, consequently, to enhance the light-analyte interaction. A 7.6 times enhancement of sensitivity is experimentally demonstrated and explained by material integration engineering as well as waveguide mode analysis. Our sensor platform made by polymer-dielectric hybrids can be applied to other regions of the mid-IR spectrum to probe other analytes and can ultimately achieve a multispectral sensor on-a-chip.

Rapid imaging, detection and quantification of Giardia lamblia cysts using mobile-phone based fluorescent microscopy and machine learning

Rapid and sensitive detection of waterborne pathogens in drinkable and recreational water sources is crucial for treating and preventing the spread of water related diseases, especially in resource-limited settings. Here we present a field-portable and cost-effective platform for detection and quantification of Giardia lamblia cysts, one of the most common waterborne parasites, which has a thick cell wall that makes it resistant to most water disinfection techniques including chlorination. The platform consists of a smartphone coupled with an opto-mechanical attachment weighing ~205 g, which utilizes a hand-held fluorescence microscope design aligned with the camera unit of the smartphone to image custom-designed disposable water sample cassettes. Each sample cassette is composed of absorbent pads and mechanical filter membranes; a membrane with 8 μm pore size is used as a porous spacing layer to prevent the backflow of particles to the upper membrane, while the top membrane with 5 μm pore size is used to capture the individual Giardia cysts that are fluorescently labeled. A fluorescence image of the filter surface (field-of-view: ~0.8 cm(2)) is captured and wirelessly transmitted via the mobile-phone to our servers for rapid processing using a machine learning algorithm that is trained on statistical features of Giardia cysts to automatically detect and count the cysts captured on the membrane. The results are then transmitted back to the mobile-phone in less than 2 minutes and are displayed through a smart application running on the phone. This mobile platform, along with our custom-developed sample preparation protocol, enables analysis of large volumes of water (e.g., 10-20 mL) for automated detection and enumeration of Giardia cysts in ~1 hour, including all the steps of sample preparation and analysis. We evaluated the performance of this approach using flow-cytometer-enumerated Giardia-contaminated water samples, demonstrating an average cyst capture efficiency of ~79% on our filter membrane along with a machine learning based cyst counting sensitivity of ~84%, yielding a limit-of-detection of ~12 cysts per 10 mL. Providing rapid detection and quantification of microorganisms, this field-portable imaging and sensing platform running on a mobile-phone could be useful for water quality monitoring in field and resource-limited settings.

An optopneumatic piston for microfluidics

We demonstrate an optopneumatic piston based on glass capillaries, a mixture of PDMS-carbon nanopowder, silicone and mineral oil. The fabrication method is based on wire coating techniques and surface tension-driven instabilities, and allows for the assembly of several pistons from a single batch production. By coupling the photothermal response of the PDMS-carbon mixture with optical excitation via an optical fiber, we demonstrate that the piston can work either as a valve or as a reciprocal actuator. The death volume of the pistons was between 0.02 and 1.56 μL and the maximum working frequency was around 1 Hz. Analysis of the motion during the expansion/contraction of the piston shows that this machine can be described by a phenomenological equation analogous to the Kelvin-Voight model used in viscoelasticity, having elastic and viscous components.

Electrospinning graphene quantum dots into a nanofibrous membrane for dual-purpose fluorescent and electrochemical biosensors

Graphene quantum dots (GQDs) have become increasingly important for applications in energy materials, optical devices and biosensors. Here we report a facile technique to fabricate a nanofibrous membrane of GQDs by electrospinning water-soluble GQDs with polyvinyl alcohol (PVA) directly. The structure and fluorescence properties of the fabricated PVA/GQD nanofibrous membrane were investigated using scanning and transmission electron microscopy, and fluorescence microscopy. It was found that the electrospun PVA/GQD nanofibrous membrane has a three-dimensional structure with a high surface area to volume ratio, which is beneficial for the adsorption of electrolytes and the diffusion of reactants. For the first time, the created PVA/GQD nanofibrous membrane was utilized to fabricate dual-purpose fluorescent and electrochemical biosensors for highly sensitive determination of hydrogen peroxide (H₂O₂) and glucose. The experimental results indicated that the fluorescence intensity of the nanofibrous membrane decreased linearly with increasing H₂O₂ concentration, because the addition of H₂O₂ leads to fluorescence quenching of the GQDs, which endows the fabricated nanofibrous membrane with fluorescence activity. Besides, after binding glucose oxidase onto the created nanofibrous membrane, the fabricated nanofibrous membrane showed high sensitivity and selectivity for glucose detection. In addition, the PVA/GQD nanofibrous membrane can also be directly electrospun onto an electrode for electrochemical detection of H₂O₂. This novel nanofibrous membrane exhibits excellent catalytic performance and fluorescence activity, and therefore has potential applications for the highly stable, sensitive, and selective detection of H₂O₂ and glucose.

Multiphase optofluidics on an electro-microfluidic platform powered by electrowetting and dielectrophoresis

For diverse material phases used on an electro-microfluidic (EMF) platform, exploiting the electro-optical properties of matter in varied phases is essential to reap the benefits of the optofluidic capabilities of that platform. Materials in the four fundamental phases--solid-phase dielectric layer, liquid-phase droplet, gas-phase bubble, and plasma-phase bubble microplasma--have been investigated to offer electrically tunable optical characteristics for the manipulation of fluids on an EMF platform. Here we present an overview of the basic driving mechanisms for electrowetting and dielectrophoresis on the EMF platform. Three optofluidic examples occurring in multiple phases are described: solid optofluidics--liquid and light regulation by electrowetting on a solid polymer dispersed liquid crystal (PDLC) dielectric layer; liquid optofluidics--transmittance and reflectance modulation with formation of particle chains in a liquid droplet; and gas and plasma optofluidics--ignition and manipulation of a bubble microplasma by liquid dielectrophoresis. By combining the various materials possessing diverse electro-optical characteristics in separate phases, the EMF platform becomes an ideal platform for integrated optofluidics.

A micropillar array for sample concentration via in-plane evaporation

We present a method to perform sample concentration within a lab-on-a-chip using a microfluidic structure which controls the liquid-gas interface through a micropillar array fabricated in polydimethylsiloxane between microfluidic channels. The microstructure confines the liquid flow and a thermal gradient is used to drive evaporation at the liquid-gas-interface. The evaporation occurs in-plane to the microfluidic device, allowing for precise control of the ambient environment. This method is demonstrated with a sample containing 1 μm, 100 nm fluorescent beads and SYTO-9 labelled Escherichia coli bacteria. Over 100 s, the fluorescent beads and bacteria are concentrated by a factor of 10.

Electrospun doping of carbon nanotubes and platinum nanoparticles into the β-phase polyvinylidene difluoride nanofibrous membrane for biosensor and catalysis applications

A novel β-phase polyvinylidene difluoride (PVDF) nanofibrous membrane decorated with multiwalled carbon nanotubes (MWCNTs) and platinum nanoparticles (PtNPs) was fabricated by an improved electrospinning technique. The morphology of the fabricated PVDF-MWCNT-PtNP nanofibrous membrane was observed by scanning electron microscopy, and the formation of high β-phase in the hybrid nanofibrous membrane was investigated by Fourier transform infrared spectroscopy and differential scanning calorimetry. The uniform dispersion of MWCNTs and PtNPs in the PVDF hybrid nanofibrous membrane and their interaction were explored by transmission electron microscopy and X-ray diffraction. For the first time, we utilized this created PVDF-MWCNT-PtNP nanofibrous membrane for biosensor and catalysis applications. The nonenzymatic amperometric biosensor with highly stable and sensitive, and selective detection of both H2O2 and glucose was successfully fabricated based on the electrospun PVDF-MWCNT-PtNP nanofibrous membrane. In addition, the catalysis of the hybrid nanofibrous membrane for oxygen reduction reaction was tested, and a good catalysis performance was found. We anticipate that the strategies utilized in this work will not only guide the further design of functional nanofiber-based biomaterials and biodevices but also extend the potential applications in energy storage, cytology, and tissue engineering.

Thermocapillary flow in glass tubes coated with photoresponsive layers

Thermocapillary flow has proven to be a good alternative to induce and control the motion of drops and bubbles in microchannels. Temperature gradients are usually established by implanting metallic heaters adjacent to the channel or by including a layer of photosensitive material capable of absorbing radiative energy. In this work we show that single drops can be pumped through capillaries coated with a photoresponsive composite (PDMS + carbon nanopowder) and irradiated with a light source via an optical fiber. Maximum droplet speeds achieved with this approach were found to be ~300 μm/s, and maximum displacements, around 120% of the droplet length. The heat generation capacity of the coatings was proven having either a complete coating over the capillary surface or a periodic array of pearls of the photoresponsive material along the capillary produced by the so-called Rayleigh-Plateau instability. The effect of the photoresponsive layer thickness and contact angle hysteresis of the solid-liquid interface were found to be important parameters in the photoinduced thermocapillary effect. Furthermore, a linear relationship between the optical intensity I(o) and droplet velocity v was found for a wide range of the former, allowing us to analyze the results and estimate response times for heat transfer using heat conduction theory.

Microfluidic reactors for photocatalytic water purification

Photocatalytic water purification utilizes light to degrade the contaminants in water and may enjoy many merits of microfluidics technology such as fine flow control, large surface-area-to-volume ratio and self-refreshing of reaction surface. Although a number of microfluidic reactors have been reported for photocatalysis, there is still a lack of a comprehensive review. This article aims to identify the physical mechanisms that underpin the synergy of microfluidics and photocatalysis, and, based on which, to review the reported microfluidic photocatalytic reactors. These microreactors help overcome different problems in bulk reactors such as photon transfer limitation, mass transfer limitation, oxygen deficiency, and lack of reaction pathway control. They may be scaled up for large-throughput industrial applications of water processing and may also find niche applications in rapid screening and standardized tests of photocatalysts.

Solar thermal polymerase chain reaction for smartphone-assisted molecular diagnostics

Nucleic acid-based diagnostic techniques such as polymerase chain reaction (PCR) are used extensively in medical diagnostics due to their high sensitivity, specificity and quantification capability. In settings with limited infrastructure and unreliable electricity, however, access to such devices is often limited due to the highly specialized and energy-intensive nature of the thermal cycling process required for nucleic acid amplification. Here we integrate solar heating with microfluidics to eliminate thermal cycling power requirements as well as create a simple device infrastructure for PCR. Tests are completed in less than 30 min, and power consumption is reduced to 80 mW, enabling a standard 5.5 Wh iPhone battery to provide 70 h of power to this system. Additionally, we demonstrate a complete sample-to-answer diagnostic strategy by analyzing human skin biopsies infected with Kaposi's Sarcoma herpesvirus (KSHV/HHV-8) through the combination of solar thermal PCR, HotSHOT DNA extraction and smartphone-based fluorescence detection. We believe that exploiting the ubiquity of solar thermal energy as demonstrated here could facilitate broad availability of nucleic acid-based diagnostics in resource-limited areas.

Mid-infrared spectrometer using opto-nanofluidic slot-waveguide for label-free on-chip chemical sensing

A mid-infrared (mid-IR) spectrometer for label-free on-chip chemical sensing was developed using an engineered nanofluidic channel consisting of a Si-liquid-Si slot-structure. Utilizing the large refractive index contrast (Δn ∼ 2) between the liquid core of the waveguide and the Si cladding, a broadband mid-IR lightwave can be efficiently guided and confined within a nanofluidic capillary (≤100 nm wide). The optical-field enhancement, together with the direct interaction between the probe light and the analyte, increased the sensitivity for chemical detection by 50 times when compared to evanescent-wave sensing. This spectrometer distinguished several common organic liquids (e.g., n-bromohexane, toluene, isopropanol) accurately and could determine the ratio of chemical species (e.g., acetonitrile and ethanol) at low concentration (<5 μL/mL) in a mixture through spectral scanning over their characteristic absorption peaks in the mid-IR regime. The combination of CMOS-compatible planar mid-IR microphotonics, and a high-throughput nanofluidic sensor system, provides a unique platform for chemical detection.

Industrial lab-on-a-chip: design, applications and scale-up for drug discovery and delivery

Microfluidics is an emerging and promising interdisciplinary technology which offers powerful platforms for precise production of novel functional materials (e.g., emulsion droplets, microcapsules, and nanoparticles as drug delivery vehicles- and drug molecules) as well as high-throughput analyses (e.g., bioassays, detection, and diagnostics). In particular, multiphase microfluidics is a rapidly growing technology and has beneficial applications in various fields including biomedicals, chemicals, and foods. In this review, we first describe the fundamentals and latest developments in multiphase microfluidics for producing biocompatible materials that are precisely controlled in size, shape, internal morphology and composition. We next describe some microfluidic applications that synthesize drug molecules, handle biological substances and biological units, and imitate biological organs. We also highlight and discuss design, applications and scale up of droplet- and flow-based microfluidic devices used for drug discovery and delivery.

Microfluidic opportunities in the field of nutrition

Nutrition has always been closely related to human health, which is a constant motivational force driving research in a variety of disciplines. Over the years, the rapidly emerging field of microfluidics has been pushing forward the healthcare industry with the development of microfluidic-based, point-of-care (POC) diagnostic devices. Though a great deal of work has been done in developing microfluidic platforms for disease diagnoses, potential microfluidic applications in the field of nutrition remain largely unexplored. In this Focus article, we would like to investigate the potential chances for microfluidics in the field of nutrition. We will first highlight some of the recent advances in microfluidic blood analysis systems that have the capacity to detect biomarkers of nutrition. Then we will examine existing examples of microfluidic devices for the detection of specific biomarkers of nutrition or nutrient content in food. Finally, we will discuss the challenges in this field and provide some insight into the future of applied microfluidics in nutrition.

Ratiometric highly sensitive luminescent nanothermometers working in the room temperature range. Applications to heat propagation in nanofluids

There is an increasing demand for accurate, non-invasive and self-reference temperature measurements as technology progresses into the nanoscale. This is particularly so in micro- and nanofluidics where the comprehension of heat transfer and thermal conductivity mechanisms can play a crucial role in areas as diverse as energy transfer and cell physiology. Here we present two luminescent ratiometric nanothermometers based on a magnetic core coated with an organosilica shell co-doped with Eu(3+) and Tb(3+) chelates. The design of the hybrid host and chelate ligands permits the working of the nanothermometers in a nanofluid at 293-320 K with an emission quantum yield of 0.38 ± 0.04, a maximum relative sensitivity of 1.5% K(-1) at 293 K and a spatio-temporal resolution (constrained by the experimental setup) of 64 × 10(-6) m/150 × 10(-3) s (to move out of 0.4 K--the temperature uncertainty). The heat propagation velocity in the nanofluid, (2.2 ± 0.1) × 10(-3) m s(-1), was determined at 294 K using the nanothermometers' Eu(3+)/Tb(3+) steady-state spectra. There is no precedent of such an experimental measurement in a thermographic nanofluid, where the propagation velocity is measured from the same nanoparticles used to measure the temperature.

Electrically tunable optofluidic light switch for reconfigurable solar lighting

We describe a reconfigurable lighting system for indoor solar illumination which provides a new way for solar energy conservation. We have experimentally demonstrated an electrically tunable optofluidic light switch which takes the key role for the reconfigurability. The working principle of the switch is based on applying a dielectrophoretic force on a thin oil film hence inducing a surface deformation and consequent leakage of the guided light propagating along the waveguide. A maximum modulation frequency of 2 Hz was achieved. The switch has the advantages of simple fabrication, compact size and low power consumption. The potential applications of such an optofluidic switch are also discussed.