Formation of Multicomponent Surface Nanodroplets by Solvent Exchange

Multicomponent surface droplets that consist of more than one compound are of great interest for fundamental studies of microwetting, evaporation, and dissolution behaviors, as well as for practical applications in high-throughput screening, microcompartmentalized chemical reactions, and microanalytics. In this work, we study the formation of multicomponent surface nanodroplets from heterogeneous nucleation and growth induced by the process of solvent exchange. In our experiments, as a solution of two oils in their good solvent was displaced by a poor solvent of the oils in the standard solvent exchange, binary droplets of oils were produced on an immersed substrate. The concentration of one oil was constant in the initial solution, whereas the other oil was increased gradually. We characterized the ratio of the two oils inside individual binary droplets by an infrared microspectrometer. Our results show that the ratio of two oils within binary nanodroplets could be varied from 0 to 100% by tuning the composition of the initial solution. However, the ratio of the two oils in the droplets did not simply correspond to that in the solution. Rather, we were able to correlate the ratio of the oils in the droplet to the oversaturation level of each oil based on the ternary phase diagram. We further demonstrate that the principle of the oversaturation level also governs the components in ternary nanodroplets formed by solvent exchange. The quantitative understanding in this work is valuable for the formation of multicomponent surface nanodroplets, which may be applied in nanoextraction, microcompartmentalized reactions, and surface functionalization

Strategy To Enhance the Wettability of Bioacive Paper-Based Sensors

This paper reports a potential method that can restore the wettability of bioactive paper-based sensors while maintaining their bioactivity. This study is driven by the need to increase the wettability of the antibody-loaded blood typing paper devices in order to increase the blood typing assaying speed using such paper devices. Plasma treatment is used to improve the wettability of bioactive paper; the protective effect of bovine serum albumin (BSA) to biomolecules against plasma deactivation is investigated. In the first stage, horseradish peroxidase (HRP) was used as a model biomolecule, because of the convenience of its quantifiable colorimetric reaction with a substrate. By using this protection approach, the inactivation of biomolecules on paper during the plasma treatment is significantly slowed down. This approach enables plasma treatment to be used for fabricating paper-based bioactive sensors to achieve strong wettability for rapid penetration of liquid samples or reagents. Finally, we demonstrate the use of plasma treatment to increase the wettability of antibody treated blood typing paper. After the treatment, the blood typing paper becomes highly wettable; it allows much faster penetration of blood samples into the plasma treated testing paper. Antibodies on the paper are still sufficiently active for blood typing and can report patients’ blood type accurately

Understanding Thread Properties for Red Blood Cell Antigen Assays: Weak ABO Blood Typing

“Thread-based microfluidics” research has so far focused on utilizing and manipulating the wicking properties of threads to form controllable microfluidic channels. In this study we aim to understand the separation properties of threads, which are important to their microfluidic detection applications for blood analysis. Confocal microscopy was utilized to investigate the effect of the microscale surface morphologies of fibers on the thread’s separation efficiency of red blood cells. We demonstrated the remarkably different separation properties of threads made using silk and cotton fibers. Thread separation properties dominate the clarity of blood typing assays of the ABO groups and some of their weak subgroups (A_<i>x</i> and A₃). The microfluidic thread-based analytical devices (μTADs) designed in this work were used to accurately type different blood samples, including 89 normal ABO and 6 weak A subgroups. By selecting thread with the right surface morphology, we were able to build μTADs capable of providing rapid and accurate typing of the weak blood groups with high clarity

Barcode-Like Paper Sensor for Smartphone Diagnostics: An Application of Blood Typing

This study introduced a barcode-like design into a paper-based blood typing device by integrating with smartphone-based technology. The concept of presenting a paper-based blood typing assay in a barcode-like pattern significantly enhanced the adaptability of the assay to the smartphone technology. The fabrication of this device involved the use of a printing technique to define hydrophilic bar channels which were, respectively, treated with Anti-A, -B, and -D antibodies. These channels were then used to perform blood typing assays by introducing a blood sample. Blood type can be visually identified from eluting lengths in bar channels. A smartphone-based analytical application was designed to read the bar channels, analogous to scanning a barcode, interpret this information, and then report results to users. The proposed paper-based blood typing device is rapidly read by smartphones and easy for the user to operate. We envisage that the adaptation of paper-based devices to the widely accepted smartphone technology will increase the capability of paper-based diagnostics with rapid assay result interpretation, data storage, and transmission

Crystallization of Femtoliter Surface Droplet Arrays Revealed by Synchrotron Small-Angle X‑ray Scattering

The crystallization of oil droplets is critical in the processing and storage of lipid-based food and pharmaceutical products. Arrays of femtoliter droplets on a surface offer a unique opportunity to study surfactant-free colloidlike systems. In this work, the crystal growth process in these confined droplets was followed by cooling a model lipid (trimyristin) from a liquid state utilizing synchrotron small-angle X-ray scattering (SAXS). The measurements by SAXS demonstrated a reduced crystallization rate and a greater degree of supercooling required to trigger lipid crystallization in droplets compared to those of bulk lipids. These results suggest that surface droplets crystallize in a stochastic manner. Interestingly, the crystallization rate is slower for larger femtoliter droplets, which may be explained by the onset of crystallization from the three-phase contact line. The larger surface nanodroplets exhibit a smaller ratio of droplet volume to the length of three-phase contact line and hence a slower crystallization rate