Pillar cuvettes: capillary-filled, microliter quartz cuvettes with microscale path lengths for optical spectroscopy

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.

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.

High-Performance Passive Plasma Separation on OSTE Pillar Forest

Plasma separation is of high interest for lateral flow tests using whole blood as sample liquids. Here, we built a passive microfluidic device for plasma separation with high performance. This device was made by blood filtration membrane and off-stoichiometry thiol-ene (OSTE) pillar forest. OSTE pillar forest was fabricated by double replica moldings of a laser-cut polymethylmethacrylate (PMMA) mold, which has a uniform microstructure. This device utilized a filtration membrane to separate plasma from whole blood samples and used hydrophilic OSTE pillar forest as the capillary pump to propel the plasma. The device can be used to separate blood plasma with high purity for later use in lateral flow tests. The device can process 45 μL of whole blood in 72 s and achieves a plasma separation yield as high as 60.0%. The protein recovery rate of separated plasma is 85.5%, which is on par with state-of-the-art technologies. This device can be further developed into lateral flow tests for biomarker detection in whole blood.

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.

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.

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.

Directed Growth of Orthorhombic Crystals in a Micropillar Array

We report directed growth of orthorhombic crystals of potassium permanganate in spatial confinement of a micropillar array. The solution is introduced by spontaneous wicking to give a well-defined film (thickness 10-15 μm; volume ∼600 nL) and is connected to a reservoir (several microliters) that continuously "feeds" the evaporating film. When the film is supersaturated, crystals nucleate and preferentially grow in specific directions guided by one of several possible linear paths through the pillar lattice. Crystals that do not initially conform are stopped at an obstructing pillar, branch into another permitted direction, or spontaneously rotate to align with a path and continue to grow. Microspectroscopy is able to track the concentration of solute in a small region of interest (70 × 100 μm²) near to growing crystals, revealing that the solute concentration initially increases linearly beyond the solubility limit. Crystal growth near the region of interest resulted in a sharp decrease in the local solute concentration (which rapidly returns the concentration to the solubility limit), consistent with estimated diffusion time scales (<1 s for a 50 μm length scale). The ability to simultaneously track solute concentration and control crystal orientation in nanoliter samples will provide new insight into microscale dynamics of microscale crystallization.

Influence of Sample Volume and Solvent Evaporation on Absorbance Spectroscopy in a Microfluidic "Pillar-Cuvette"

Spectroscopic analysis of solutions containing samples at high concentrations or molar absorptivity can present practical challenges. In absorbance spectroscopy, short optical path lengths or multiple dilution is required to bring the measured absorbance into the range of the Beer's Law calibration. We have previously reported an open "pillar-cuvette" with a micropillar array that is spontaneously filled with a precise (nL or μL) volume to create the well-defined optical path of, for example, 10 to 20 μm. Evaporation should not be ignored for open cuvettes and, herein, the volume of loaded sample and the rate of evaporation from the cuvette are studied. It was found that the volume of loaded sample (between 1 and 10 μL) had no effect on the Beer's Law calibration for methyl orange solutions (molar absorptivity of (2.42 ± 0.02)× 10(4) L mol(-1) cm(-1)) for cuvettes with a 14.2 ± 0.2 μm path length. Evaporation rates of water from methyl orange solutions were between 2 and 5 nL s(-1) (30 - 40% relative humidity; 23°C), depending on the sample concentration and ambient conditions. Evaporation could be reduced by placing a cover slip several millimeters above the cuvette. Importantly, the results show that a "drop-and-measure" method (measurement within ∼3 s of cuvette loading) eliminates the need for extrapolation of the absorbance-time data for accurate analysis of samples.