Real-time monitoring of out-of-equilibrium Janus droplets for liquid sensing applications

Liquid sensing platforms are increasingly vital for environmental protection, product quality assurance, and scientific innovation. Conventional methods for detecting aqueous pollutants and chemical targets, such as High-Performance Liquid Chromatography (HPLC) and Gas Chromatography–Mass Spectrometry (GC–MS), remain laborious, costly, and largely confined to laboratory settings. To overcome these limitations, there is growing interest in alternative approaches capable of delivering real-time, on-site analysis. Liquid–liquid chemo- and biosensors, including Janus emulsion droplets, represent a promising alternative because they can be engineered to operate in controlled microenvironments that facilitate specific chemical interactions, enabling sensing behaviors analogous to those found in biological systems and offering complementary capabilities to traditional techniques. Janus emulsion droplets, consisting of immiscible hydrocarbon (HC) and fluorocarbon (FC) oils stabilized by surfactants, exhibit gravity-aligned morphologies and significant refractive index (RI) contrasts between phases. These droplets operate in dynamic, thermodynamically out-of-equilibrium states, with continual molecular exchange between the droplet interior and the surrounding aqueous medium. Their hydrophobic–hydrophilic interfaces facilitate highly selective interactions between surfactants and analytes, allowing Janus droplets to function as versatile “messenger colloids.” This enables the real-time transduction of chemical or biological stimuli into measurable optical or morphological changes, achieving exceptional detection sensitivities, sometimes at femtomolar levels. Their simple preparation, tunable interfaces, and adaptability across diverse conditions make them highly suitable for rapid and flexible sensing applications, including environmental monitoring, biomedical diagnostics, and industrial process control. A high-throughput analytical platform for rapid screening and analyte differentiation using Janus droplets stabilized by both inert and stimuli-responsive surfactants was developed. This platform integrates sideview microscopy with a real-time monitoring system to track droplet morphology change upon analyte addition. Critical geometric parameters, including contact angles, snowman angles, surface area ratios, and volume ratios, were quantitatively extracted via custom MATLAB algorithms. These measurements enabled the construction of calibration curves and the accurate prediction of analyte identities and concentrations. Additionally, this system demonstrated the capability to determine the critical micelle concentration (CMC) of active surfactants in complex media, broadening its utility for surfactant characterization. A key optical feature of Janus droplets is their refractive optical properties. In this work, emissive dyes were selectively positioned within the higher refractive index HC phase to exploit guided luminescence through total internal reflection (TIR) along droplet boundaries. A custom-built rotating fluorescence microscope was constructed to capture angularly resolved fluorescence profiles around individual droplets and investigate directional emission behavior. Experimental measurements of ratiometric luminescence, supported by ray-tracing simulations, revealed distinct angle-dependent signal patterns with excellent agreement between theory and experiment, exhibiting maximum intensity variations of up to 90%. Furthermore, localizing fluorescent dyes at the droplet’s inner interface enhanced emission intensity by a factor of 1.3 to 1.5 compared to dyes dispersed randomly within the HC phase. To enable a stable, portable, and scalable platform for rapid sensing applications, a hydrogel matrix was engineered to encapsulate individual Janus droplets and facilitate analyte detection. Due to the partitioning behavior of surfactants, the HC volume within droplets gradually decreases as surfactant molecules migrate and extract HC components. This results in the formation of localized water pockets surrounding each droplet, creating discrete microenvironments that allow droplets to remain spatially separated and freely mobile within the hydrogel. Upon exposure to target analytes, droplets exhibit rapid morphological transitions that can be readily observed, simplifying and accelerating the sensing process. This approach advances the feasibility of integrating Janus droplets with hydrogel matrices for industrial-scale production of sensing patches and diagnostic devices. Altogether, this thesis advances the development of Janus emulsion droplets as versatile, reconfigurable modular transducers and lays the foundation for automated sensing devices characterized by low cost, rapid response times, and high analytical precision. These innovations hold significant promise for diverse applications, including environmental monitoring, public health diagnostics, industrial process control, and sustainable resource management.