Quantification of protein interaction kinetics in a micro droplet

Photothermal-Driven Droplet Manipulation: A Perspective

Optofluidics, which utilizes the interactions between light and fluids to realize various functions, has garnered increasing attention owing to the advantages of operational simplicity, exceptional flexibility, rapid response, etc. As one of the typical light-fluid interactions, the localized photothermal effect serving as a stimulus has been widely used for fluid manipulation. Particularly, significant progress on photothermal-driven droplet manipulation has been made. In this perspective, recent advancements in localized photothermal effect driven droplet manipulation are summarized. First, the photothermal manipulation of droplets on open surfaces is outlined. An attractive droplet manipulation of light droplet levitation above the gas-liquid interface via localized photothermal effect is then discussed. Besides, the photothermal-driven manipulation of droplets in an immiscible liquid phase is also discussed. Although promising, further development of photothermal-driven droplet manipulation is still needed. The challenges and perspectives of this light droplet manipulation strategy for broad implementation are summarized, which will help future studies and applications.

Label-Free Immunoassay Using Droplet-Based Brewster's Angle Straddle Interferometry

We report the detection of antigen capture by immobilized antibodies using a simple, label-free version of monochromatic reflective interferometry. The technique is implemented on silicon with its native oxide and relies on choosing an incident angle between the Brewster angles for the air/oxide and oxide/silicon interfaces. We demonstrate sensitivity to anti-human and anti-rabbit immunoglobulin (anti-IgG) concentrations less than 100 nM using only 10 nL droplets of the analyte. We have introduced a protocol using a model sugar to reduce nonspecific binding and have been able to detect anti-IgG even in the presence of 100-fold larger concentrations of bovine serum albumin. The limit of detection is not yet associated with the optical method but is imposed by nonspecific binding. Evaluated in terms of pg/mm², our sensors are comparable in sensitivity to surface plasmon resonance (SPR) but are advantaged with respect to SPR in the tolerance of the optical components and alignment, the low material usage, and the ability to exploit multiplex detection without modification. The simplicity and convenience of the method are promising for eventual application to portable diagnostic applications.

Label-Free Sensing in Microdroplet-Based Microfluidic Systems

Droplet microfluidic systems have evolved as fluidic platforms that use much less sample volume and provide high throughput for biochemical analysis compared to conventional microfluidic devices. The variety of droplet fluidic applications triggered several detection techniques to be applied for analysis of droplets. In this review, we focus on label-free droplet detection techniques that were adapted to various droplet microfluidic platforms. We provide a classification of most commonly used droplet platform technologies. Then we discuss the examples of various label-free droplet detection schemes implemented for these platforms. While providing the research landscape for label-free droplet detection methods, we aim to highlight the strengths and shortcomings of each droplet platform so that a more targeted approach can be taken by researchers when selecting a droplet platform and a detection scheme for any given application.

Kinetics of PTEN-mediated PI(3,4,5)P3 hydrolysis on solid supported membranes

Phosphatidylinositides play important roles in cellular signaling and migration. Phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P3) is an important phosphatidylinositide because it acts as a secondary messenger to trigger cell movement and proliferation. A high level of PI(3,4,5)P3 at the plasma membrane is known to contribute to tumorigenesis. One key enzyme that regulates PI(3,4,5)P3 levels at the plasma membrane is phosphatase and tensin homologue deleted on chromosome 10 (PTEN), which dephosphorylates PI(3,4,5)P3 through hydrolysis to form phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2). It has been reported that PI(4,5)P2 is involved in positive feedback in the PI(3,4,5)P3 hydrolysis by PTEN. However, how PI(3,4,5)P3 dephosphorylation by PTEN is regulated, is still under debate. How other PI(3,4,5)P3-binding proteins affect the dephosphorylation kinetics catalyzed by PTEN also remains unclear. Here, we develop a fluorescent-protein biosensor approach to study how PI(3,4,5)P3 dephosphorylation is regulated by PTEN as well as its membrane-mediated feedback mechanisms. Our observation of sigmoidal kinetics of the PI(3,4,5)P3 hydrolysis reaction supports the notion of autocatalysis in PTEN function. We developed a kinetic model to describe the observed reaction kinetics, which allowed us to i) distinguish between membrane-recruitment and allosteric activation of PTEN by PI(4,5)P2, ii) account for the influence of the biosensor on the observed reaction kinetics, and iii) demonstrate that all of these mechanisms contribute to the kinetics of PTEN-mediated catalysis.

Recent advances in hybrid measurement methods based on atomic force microscopy and surface sensitive measurement techniques

This review summaries the recent progress of the combination of optical and non-optical surface sensitive techniques with the atomic force microscopy.

Microfluidic Surface Plasmon Resonance Sensors: From Principles to Point-of-Care Applications

Surface plasmon resonance (SPR) is a label-free, highly-sensitive, and real-time sensing technique. Conventional SPR sensors, which involve a planar thin gold film, have been widely exploited in biosensing; various miniaturized formats have been devised for portability purposes. Another type of SPR sensor which utilizes localized SPR (LSPR), is based on metal nanostructures with surface plasmon modes at the structural interface. The resonance condition is sensitive to the refractive index change of the local medium. The principles of these two types of SPR sensors are reviewed and their integration with microfluidic platforms is described. Further applications of microfluidic SPR sensors to point-of-care (POC) diagnostics are discussed.

Quantification of protein interaction kinetics in a micro droplet

Characterization of protein interactions is essential to the discovery of disease biomarkers, the development of diagnostic assays, and the screening for therapeutic drugs. Conventional flow-through kinetic measurements need relative large amount of sample that is not feasible for precious protein samples. We report a novel method to measure protein interaction kinetics in a single droplet with sub microliter or less volume. A droplet in a humidity-controlled environmental chamber is replacing the microfluidic channels as the reactor for the protein interaction. The binding process is monitored by a surface plasmon resonance imaging (SPRi) system. Association curves are obtained from the average SPR image intensity in the center area of the droplet. The washing step required by conventional flow-through SPR method is eliminated in the droplet method. The association and dissociation rate constants and binding affinity of an antigen-antibody interaction are obtained by global fitting of association curves at different concentrations. The result obtained by this method is accurate as validated by conventional flow-through SPR system. This droplet-based method not only allows kinetic studies for proteins with limited supply but also opens the door for high-throughput protein interaction study in a droplet-based microarray format that enables measurement of many to many interactions on a single chip.