Microsphere-Enabled Micropillar Array for Whispering Gallery Mode Virus Detection.

Rapid detection of pathogens and analytes at the point of care offers an opportunity for prompt patient management and public health control. This paper reports an open microfluidic platform coupled with active whispering gallery mode (WGM) microsphere resonators for the rapid detection of influenza viruses. The WGM microsphere resonators, precoated with influenza A polyclonal antibodies, are mechanically trapped in the open micropillar array, where the evaporation-driven flow continuously transports a small volume (∼µL) of sample to the resonators without auxiliaries. Selective chemical modification of the pillar array changes surface wettability and flow pattern, which enhances the detection sensitivity of the WGM resonator-based virus sensor. The optofluidic sensing platform is able to specifically detect influenza A viruses within 15 min using a few microliters of sample and displays a linear response to different virus concentrations.

Ultra-high-resolution greyscale fluorescence images via UV-exposure of thin flexible phosphor films.

Thin films of BaFCl:Sm nanocrystals prepared using a polymer binder were used to create fluorescence images. The phosphor films were exposed to a UV-C mercury lamp light source via chromium-coated quartz greyscale masks to create 4 µm resolution greyscale fluorescence images. The mechanism relies on the highly efficient conversion of Sm3+ to Sm2+ ions upon exposure to UV-C light which displays a large linear dynamic range. The red fluorescence around 688 nm of the Sm2+ is then read-out using blue-violet illumination under a laser scanning confocal microscope. The greyscale images with 16 greyscale levels had a resolution equivalent to ∼125 line pairs per mm or ∼6400 dpi. Improvements in the resolution would be possible using collimated UV-C laser exposure of the film or the use of higher resolution photomasks. Ultra-high resolution binary fluorescence images were also created with resolutions down to 2 µm (∼250 line pairs per mm, ∼12 700 dpi). Downstream applications of the technology could include tailored covert or overt anti-counterfeiting labelling.

Reversible colorimetric sensing of volatile analytes by wicking in close proximity to a photonic film.

Isolation of volatile analytes from environmental or biological fluids is a rate-determining step that can delay the response time for continuous sensing. In this paper, we demonstrate a colorimetric sensing system that enables the rapid detection of gas-phase analytes released from a flowing micro-volume fluid sample. The sensor platform is an analyte-responsive metal-insulator-metal (MIM) thin-film structure integrated with a large area quartz micropillar array. This allows precise planar alignment and microscale separation (310 µm) of the optical and fluidic structures. This configuration offers rapid and homogeneous color changes over large areas that permits detection by low-resolution optics or eye, which is well-suited to portable/wearable devices. For our proof-of-principle demonstration, we utilized a poly(methyl methacrylate) (PMMA) spacer and evaluated the sensor's response (color change) to ethanol vapor. We show that the RGB color value is quantitatively linked to the spacer swelling, which is reversible and repeatable. The optofluidic platform reduces the sensor response time from minutes to seconds compared with experiments using a conventional chamber. The sensor's concentration-dependent response was examined, confirming the potential of the reported sensing platform for continuous, compact, and quantitative colorimetric analysis of volatile analytes in low-volume samples, such as biofluids.

Caged-Sphere Optofluidic Sensors: Whispering Gallery Resonators in Wicking Microfluidics.

The rapid development of optofluidic technologies in recent years has seen the need for sensing platforms with ease-of-use, simple sample manipulation, and high performance and sensitivity. Herein, an integrated optofluidic sensor consisting of a pillar array-based open microfluidic chip and caged dye-doped whispering gallery mode microspheres is demonstrated and shown to have potential for simple real-time monitoring of liquids. The open microfluidic chip allows for the wicking of a thin film of liquid across an open surface with subsequent evaporation-driven flow enabling continuous passive flow for sampling. The active dye-doped whispering gallery mode microspheres placed between pillars, avoid the use of cumbersome fibre tapers to couple light to the resonators as is required for passive microspheres. The performance of this integrated sensor is demonstrated using glucose solutions (0.05-0.3 g/mL) and the sensor response is shown to be dynamic and reversible. The sensor achieves a refractive index sensitivity of ~40 nm/RIU, with Q-factors of ~5 × 103 indicating a detection limit of ~3 × 10-3 RIU (~20 mg/mL glucose). Further enhancement of the detection limit is expected by increasing the microsphere Q-factor using high-index materials for the resonators, or alternatively, inducing lasing. The integrated sensors are expected to have significant potential for a host of downstream applications, particularly relating to point-of-care diagnostics.

Rapid Fabrication of Superhydrophobic Virtual Walls for Microfluidic Gas Extraction and Sensing.

Based on the virtual walls concept, where fluids are guided by wettability, we demonstrate the application of a gas phase extraction microfluidic chip. Unlike in previous work, the chip is prepared using a simple, rapid, and low-cost fabrication method. Channels were cut into double-sided adhesive tape (280 µm thick) and bonded to hydrophilic glass slides. The tape was selectively made superhydrophobic by 'dusting' with hydrophobic silica gel to enhance the wettability contrast at the virtual walls. Finally, the two glass slides were bonded using tape, which acts as a spacer for gas transport from/to the guided liquids. In our example, the virtual walls create a stable liquid-vapor-liquid flow configuration for the extraction of a volatile analyte (ammonia), from one liquid stream to the other through the intermediate vapor phase. The collector stream contained a pH indicator to visualize the mass transport. Quantitative analysis of ammonium hydroxide in the sample stream (<1 mM) was possible using a characteristic onset time, where the first pH change in the collector stream was detected. The effect of gap length, flow rates, and pH of the collector stream on the onset time is demonstrated. Finally, we demonstrate the analysis of ammonium hydroxide in artificial human saliva to show that the virtual walls chip is suitable for extracting volatile analytes from biofluids.

Plasma Deposited Polyoxazoline Films Integration Into Spiral Microfluidics for the Targeted Capture of Size Selected Cells.

Biomolecules readily and irreversibly bind to plasma deposited Polyoxazoline thin films in physiological conditions. The unique reactivity of these thin films toward antibodies is driving the development of immunosensing platforms for applications in cancer diagnostics. However, in order for these coatings to be used as advanced immunosensors, they need to be incorporated into microfluidic devices that are sealed via plasma bonding. In this work, the thickness, chemistry and reactivity of the polyoxazoline films were assessed following plasma activation. Films deposited from methyl and isopropenyl oxazoline precursors were integrated into spiral microfluidic devices and biofunctionalized with prostate cancer specific antibodies. Using microbeads as model particles, the design of the spiral microfluidic was optimised to enable the size-based isolation of cancer cells. The device was tested with a mixed cell suspension of healthy and malignant prostate cells. The results showed that, following size-specific separation in the spiral, selective capture was achieved on the immunofunctionalised PPOx surface. This proof of concept study demonstrates that plasma deposited polyoxazoline can be used for immunosensing in plasma bonded microfluidic devices.

PCR-assisted impedimetric biosensor for colibactin-encoding pks genomic island detection in E. coli samples.

A fast PCR-assisted impedimetric biosensor was developed for the selective detection of the clbN gene from the polyketide synthase (pks) genomic island in real Escherichia coli samples. This genomic island is responsible for the production of colibactin, a harmful genotoxin that has been associated with colorectal cancer. The experimental protocol consisted of immobilizing the designated forward primer onto an Au electrode surface to create the sensing probe, followed by PCR temperature cycling in blank, positive, and negative DNA controls. Target DNA identification was possible by monitoring changes in the system's charge transfer resistance values (Rct) before and after PCR treatment through electrochemical impedance spectroscopy (EIS) analysis. Custom-made, flexible gold electrodes were fabricated using chemical etching optical lithography. A PCR cycle study determined the optimum conditions to be at 6 cycles providing fast results while maintaining a good sensitivity. EIS data for the DNA recognition process demonstrated the successful distinction between target interaction resulting in an increase in resistance to charge transfer (Rct) percentage change of 176% for the positive DNA control vs. 21% and 20% for the negative and non-DNA-containing controls, respectively. Results showed effective fabrication of a fast, PCR-based electrochemical biosensor for the detection of pks genomic island with a calculated limit of detection of 17 ng/µL.

On-chip absorption spectroscopy enabled by graded index fiber tips.

This paper describes the design and characterization of miniaturized optofluidic devices for sensing based on integrating collimating optical fibers with custom microfluidic chips. The use of collimating graded-index fiber (GIF) tips allows for effective fiber-channel-fiber interfaces to be realized when compared with using highly-divergent standard single-mode fiber (SMF). The reduction in both beam divergence and insertion losses for the GIF configuration compared with SMF was characterized for a 10.0 mm channel. Absorption spectroscopy was demonstrated on chip for the measurement of red color dye (Ponceau 4R), and the detection of thiocyanate in water and artificial human saliva. The proposed optofluidic setup allows for absorption spectroscopy measurements to be performed with only 200 µL of solution which is an order of magnitude smaller than for standard cuvettes but provides a comparable sensitivity. The approach could be integrated into a lab-on-a-chip system that is compact and does not require free-space optics to perform absorption spectroscopy.

An Open Microfluidic Chip for Continuous Sampling of Solute from a Turbulent Particle Suspension.

High solids content complicates in situ analysis of chemical processing, biological suspensions, and environmental streams. In most cases, analytical methods require at least one pre-treatment step of a small volume of sample before a particle-free fluid can be analyzed. We have developed a continuous in situ sampler that can "sip" particle-free solution from a turbulent high solids content stream (a slurry). An open microfluidic chip with an extended slit opening shields the internal laminar flow from the turbulence outside. Unlike other open chips, our chip does not require close proximity to a solid surface and operates in turbulent environments for hours without maintenance. Two applications are demonstrated: monitoring FeIII in a stirred slurry of mixed ore particles at high solids loading (4 %wt) and paracetamol tablet dissolution profiles for two different formulations.

Evaporation-Driven Flow in Micropillar Arrays: Transport Dynamics and Chemical Analysis under Varied Sample and Ambient Conditions.

Microfluidic flow in lab-on-a-chip devices is typically very sensitive to the variable physical properties of complex samples, e.g., biological fluids. Here, evaporation-driven fluid transport (transpiration) is achieved in a configuration that is insensitive to interfacial tension, salinity, and viscosity over a wide range. Micropillar arrays ("pillar cuvettes") were preloaded by wicking a known volatile fluid (water) and then adding a microliter sample of salt, surfactant, sugar, or saliva solution to the loading zone. As the preloaded fluid evaporates, the sample is reliably drawn from a reservoir through the pillar array at a rate defined by the evaporation of the preloaded fluid (typically nL/s). Including a reagent in the preloaded fluid allows photometric reactions to take place at the boundary between the two fluids. In this configuration, a photometric signal enhancement is observed and chemical analysis is independent of both humidity and temperature. The ability to reliably transport and sense an analyte in microliter volumes without concern over salt, surfactant, viscosity (in part), humidity, and temperature is a remarkable advantage for analytical purposes.

Microfluidic Screening to Study Acid Mine Drainage.

Acid mine drainage (AMD) is the most significant environmental pollution problem associated with the mining industry. Case-specific testing is widely applied and established in the mining and consulting businesses for AMD prediction, and any improvements in its efficiency, while reducing its environmental impact, are of utmost societal importance. In this study, we develop a microfluidic screening method as a useful tool in the prediction and, potentially, prevention and remediation of AMD. The new approach offers key advantages including high throughput screening of reaction conditions, better spatiotemporal control over the process, and ability to conduct field-based measurements, which will account for specific interactions between mineral ores and their environment. Reagent and sample consumptions are greatly reduced to mL and mg levels, compared with those in conventional bulk-scale screening. Parallel (multichip) screening of ferric ion concentration gradients (0-40 mM) and temperature (23-75 °C) is demonstrated here, showing that the dissolution rate of pyrite significantly changes with the pH, temperature, and the ferric ion concentration, consistent with previous bulk-scale studies. To verify the robustness of the method, a mine waste rock was also tested in the microchip with natural waters. This study demonstrates the application of microfluidic screening to the challenging issue of AMD and, more generally, forecasting and optimization of mineral leaching in industry.

Photometric Sensing of Active Chlorine, Total Chlorine, and pH on a Microfluidic Chip for Online Swimming Pool Monitoring.

A microfluidic sensor was studied for the photometric detection of active chlorine, total chlorine, and pH in swimming pool samples. The sensor consisted of a four-layer borosilicate glass chip, containing a microchannel network and a 2.2 mm path length, 1.7 mL optical cell. The chip was optimised to measure the bleaching of methyl orange and spectral changes in phenol red for quantitative chlorine (active and total) and pH measurements that were suited to swimming pool monitoring. Reagent consumption (60 mL per measurement) was minimised to allow for maintenance-free operation over a nominal summer season (3 months) with minimal waste. The chip was tested using samples from 12 domestic, public, and commercial swimming pools (indoor and outdoor), with results that compare favourably with commercial products (test strips and the N,N'-diethyl-p-phenylenediamine (DPD) method), precision pH electrodes, and iodometric titration.

Microvolume Screening of Extraction and Phase Behavior in a Liquid-Liquid Microsystem.

Spontaneous formation of a third immiscible phase during liquid-liquid solvent extraction presents an enormous technical challenge for industry. Insight from current empirical investigations is greatly limited by the lack of methodologies that simultaneously report the progress of the extraction, third-phase onset time, and chemical and physical nature. The microfluidic strategy presented here answers this challenge by supporting an optically transparent submicroliter organic-phase film in a micropillar array surrounded by the aqueous phase. To demonstrate, we used 1 M Cyanex 572 in Shellsol D70 (organic phase) to extract Yb3+ and Dy3+ from a pH 2 aqueous phase. Real-time optical tracking confirmed that the visual onset of third-phase formation is consistent with the cessation of extraction (at the loading limit). Spectroscopic analysis of the solid-like third phase was carried out successfully. The new analytical approach offers a step change in speed and efficiency for reagent development, process control, and fundamental studies of complex phase behavior in reactive multiphase systems.

A Multiplexed Microfluidic Platform toward Interrogating Endocrine Function: Simultaneous Sensing of Extracellular Ca2+ and Hormone.

Extracellular Ca2+ ([Ca2+]ex) is an important regulator of various physiological and pathological functions, including intercellular communication for synchronized cellular activities (e.g., coordinated hormone secretion from endocrine tissues). Yet it is rarely possible to concurrently quantify the dynamic changes of [Ca2+]ex and related bioactive molecules with high accuracy and temporal resolution. This work aims to develop a multiplexed microfluidic platform to enable monitoring oscillatory [Ca2+]ex and hormone(s) in a biomimetic environment. To this end, a low-affinity fluorescent indicator, Rhod-5N, is identified as a suitable sensor for a range of [Ca2+]ex based on its demonstrated high sensitivity and selectivity to Ca2+ in biomedical samples, including human serum and cell culture medium. A microfluidic chip is devised to allow for the immobilization of microscale subjects (analogous to biological tissues), precise control of the perfusion gradient at sites of interest, and integration of modalities for fluorescence measurement and enzyme-linked immunosorbent assay. As this analytical system is demonstrated to be viable to quantify the dynamic changes of Ca2+ (0.2-2 mM) and insulin (15-150 mU L-1) concurrently, with high temporal resolution, it has the potential to provide key insights into the essential roles of [Ca2+]ex in the secretory function of endocrine tissues and to identify novel therapeutic targets for human diseases.

The Timing of Application and Inclusion of a Surfactant Are Important for Absorption and Translocation of Foliar Phosphoric Acid by Wheat Leaves.

Introduction: Foliar applied phosphorus (P) has the potential to provide a more tactical approach to P fertilization that could enhance P use efficiency. The aims of this study were to investigate the influence of adjuvant choice and application timing of foliar applied phosphoric acid on leaf wettability, foliar uptake, translocation, and grain yield of wheat plants. Materials and Methods: We measured the contact angles of water and fertilizers on wheat leaves, and the uptake, translocation and wheat yield response to isotopically-labelled phosphoric acid in combination with five different adjuvants when foliar-applied to wheat at either early tillering or flag leaf emergence. Results: There was high foliar uptake of phosphoric acid in combination with all adjuvants that contained a surfactant, but only one treatment resulted in a 12% increase in grain yield and two treatments resulted in a decrease in grain yield. Despite the wettability of all foliar fertilizers being markedly different, foliar uptake was similar for all treatments that contained a surfactant. The translocation of phosphorus from foliar sources was higher when applied at a later growth stage than when applied at tillering despite the leaf surface properties that affect wettability being similar across all leaves at both growth stages. Discussion: Both the timing of foliar application and the inclusion of a surfactant in the formulation are important for absorption and translocation of phosphoric acid by wheat leaves, however high foliar uptake and translocation will not always translate to a yield increase.

Intracellular delivery of mRNA to human primary T cells with microfluidic vortex shedding.

Intracellular delivery of functional macromolecules, such as DNA and RNA, across the cell membrane and into the cytosol, is a critical process in both biology and medicine. Herein, we develop and use microfluidic chips containing post arrays to induce microfluidic vortex shedding, or µVS, for cell membrane poration that permits delivery of mRNA into primary human T lymphocytes. We demonstrate transfection with µVS by delivery of a 996-nucleotide mRNA construct encoding enhanced green fluorescent protein (EGFP) and assessed transfection efficiencies by quantifying levels of EGFP protein expression. We achieved high transfection efficiency (63.6 ± 3.44% EGFP + viable cells) with high cell viability (77.3 ± 0.58%) and recovery (88.7 ± 3.21%) in CD3 + T cells 19 hrs after µVS processing. Importantly, we show that processing cells via µVS does not negatively affect cell growth rates or alter cell states. We also demonstrate processing speeds of greater than 2.0 × 106 cells s-1 at volumes ranging from 0.1 to 1.5 milliliters. Altogether, these results highlight the use of µVS as a rapid and gentle delivery method with promising potential to engineer primary human cells for research and clinical applications.

Multiparameter toxicity screening on a chip: Effects of UV radiation and titanium dioxide nanoparticles on HaCaT cells.

Microfluidic screening is gaining attention as an efficient method for evaluating nanomaterial toxicity. Here, we consider a multiparameter treatment where nanomaterials interact with cells in the presence of a secondary exposure (UV radiation). The microfluidic device contains channels that permit immobilization of HaCaT cells (human skin cell line), delivery of titanium dioxide nanoparticles (TNPs), and exposure to a known dose of UV radiation. The effect of single-parameter exposures (UV or TNP) was first studied as a benchmark, and then multiparameter toxicity (UV and TNP) at different concentrations was explored. The results demonstrate a concentration-dependent protective effect of TNP when exposed to UV irradiation.

Investigation of Chalcopyrite Leaching Using an Ore-on-a-Chip.

This paper reports on an ore-on-a-chip that enables efficient investigations of mineral leaching using real ore samples. Here, chalcopyrite (CuFeS2) ore samples are cut, polished flat, and sealed against a polydimethylsiloxane microchannel. The leach solution is collected for analysis, and the ore sample is then recovered for surface analysis. Compared to conventional bulk-scale leach tests, the ore-on-a-chip allows for faster, more efficient screening of leach parameters using real ore samples obtained from mine sites. Insight and optimization of leach conditions is demonstrated here for chalcopyrite, which has been extensively studied, yet leach performance is still strongly dependent on the origin of the ore. Two grades of chalcopyrite were chosen for this study (moderate and high purity), and the effect of ferric ion concentration and pH was studied in moderate and high purity chalcopyrite ores, respectively. The leach rate of Cu was faster in the presence of ore impurities (moderate grade) compared to the higher purity ore under the same conditions. The results also suggest that Fe is preferentially leached in the early stages to form an iron-deficient sulfide, according to X-ray photoelectron spectroscopy. Longer leach studies (48 h) reported no measurable surface passivation for the conditions studied. The ore-on-a-chip offers a new approach of case specific leach studies, which will enable rapid and tailored optimization of leach strategies for mineral processing.

Microfluidic Platform for High-Throughput Screening of Leach Chemistry.

We demonstrate a microfluidic screening platform for studying thiosulfate leaching of Au in a transparent microchannel. The approach permits in situ (optical) monitoring of Au thickness, reduced reagent use, rapid optimization of reagent chemistry, screening of temperature, and determination of the activation energy. The results demonstrate the critical importance of the (1) preparation and storage of the leach solution, (2) deposition and annealing of the Au film, and (3) lixiviant chemistry. The density of sputter-deposited Au films decreased with depth resulting in accelerating leach rates during experiments. Atomic leach rates were determined and were constant throughout each experiment. Annealing above 270 °C was found to prevent leaching, which can be attributed to diffusion of the chromium adhesion layer into the Au film. The microfluidic analysis revealed leach rates that are sensitive to the stoichiometric ratio of thiosulfate, ammonia, and copper in the leach solution and optimized for 10 mM CuSO4, 1 M Na2S2O3, and 1 M NH4OH. The temperature dependence of the leach rate gave an apparent activation energy of ∼40 kJ mol-1, based on Arrhenius's relationship.

Microfluidic Cell Microarray Platform for High Throughput Analysis of Particle-Cell Interactions.

With advances in nanotechnology, particles with various size, shape, surface chemistry, and composition can be easily produced. Nano- and microparticles have been extensively explored in many industrial and clinical applications. Ensuring that the particles themselves are not possessing toxic effects to the biological system is of paramount importance. This paper describes a proof of concept method, in which a microfluidic system is used in conjunction with a cell microarray technique aiming to streamline the analysis of particle-cell interaction in a high throughput manner. Polymeric microparticles, with different particle surface functionalities, were first used to investigate the efficiency of particle-cell adhesion under dynamic flow. Silver nanoparticles (AgNPs, 10 nm in diameter) perfused at different concentrations (0 to 20 µg/mL) in parallel streams over the cell microarray exhibited a higher toxicity compared to the static culture in the 96-well-plate format. This developed microfluidic system can be easily scaled up to accommodate a larger number of microchannels for high throughput analysis of the potential toxicity of a wide range of particles in a single experiment.