Droplet microfluidics offers advantages in high-throughput analysis due to reduced sample consumption and automation. While droplet trapping techniques are well-established, selective droplet retrieval often requires complex device fabrication or sophisticated control. Current droplet release methods include breaking pressure balance, regulating pneumatic valves, and light-induced bubbles. Methods involving pressure manipulation or pneumatic valves are complex, while light-induced bubble methods often require complicated chip fabrication incorporating photothermal materials or suffer from slow release times. This research addresses these limitations by utilizing a photoresponsive fluorosurfactant composed of fluorinated plasmonic nanoparticles (f-Au@SiO2). These nanoparticles enable rapid, light-driven droplet release due to their intense photothermal response, leading to oil vaporization and bubble formation upon laser illumination. The aim is to develop a simpler, faster, and more scalable method for selective droplet release.

The existing literature extensively covers droplet microfluidics applications in drug screening, directed evolution, droplet digital PCR, and single-cell analysis. Various droplet trapping methods exist, including passive traps (hydrodynamic and floating traps) and active traps using electric fields. Selective droplet retrieval is crucial for downstream analysis, and current strategies involve manipulating pressure balance, pneumatic valves, or light-induced bubbles. While light-induced bubble methods offer non-contact control, they typically require complex chip designs incorporating photothermal materials or show slow release speeds. Existing fluorosurfactants primarily act as stabilizers, lacking additional functionalities. The authors' previous work demonstrated a photoresponsive fluorosurfactant based on f-PNPs, enabling light-driven droplet movement. This study builds upon this prior work to develop a selective droplet release method.

The study involved synthesizing fluorinated gold-silica core-shell nanoparticles (f-Au@SiO2) via a three-step wet chemistry procedure: gold nanoparticle synthesis, silica shell coating, and fluorination. A fluorescence-activated droplet release (FADR) system was constructed using a dual-laser system. A 532 nm laser triggered droplet release via photothermal effects of f-Au@SiO2, while a 488 nm laser excited laser-induced fluorescence (LIF) for droplet identification. A motorized stage facilitated automated positioning. Hydrodynamic and floating traps were designed and fabricated using soft lithography with PDMS. Hydrodynamic traps utilized a main flow channel and parallel trapping channels with optimized hydraulic resistance. Floating traps were double-layered, exploiting the density difference between water droplets and fluorocarbon oil. The microfluidic device operated by injecting droplets, trapping them, exchanging the continuous phase with pure HFE-7500, and then releasing them using the 532 nm laser. The release process was characterized and optimized by adjusting laser parameters and flow rates.

The f-Au@SiO2-stabilized water-in-oil droplets were successfully trapped in both hydrodynamic and floating traps. Laser illumination generated a vapor bubble at the droplet interface, enabling on-demand droplet release. In the hydrodynamic trap, the bubble propelled the droplet back into the main flow channel, while in the floating trap, it pushed the droplet downwards. Droplet release was achieved with high efficiency (over 95% for floating traps under optimized conditions). The release time was significantly faster compared to previously reported light-induced bubble methods (50 ms for hydrodynamic traps and 5 ms for floating traps). The laser power required for bubble generation was relatively lower. The system demonstrated the successful integration of droplet trapping, fluorescence detection, and on-demand release in a fully automated platform.

The developed method offers significant advantages over existing droplet release techniques. The use of a photoresponsive fluorosurfactant simplifies chip fabrication by eliminating the need for integrated photothermal materials. The rapid release speeds enable high-throughput screening applications. The automated FADR system demonstrates the scalability of this approach. The success in both hydrodynamic and floating traps demonstrates the versatility of the method. Further optimization of laser parameters and trap design could improve release efficiency and speed further.

This study successfully demonstrates a novel, facile, and rapid method for on-demand light-driven release of droplets in microfluidic systems. The use of a photoresponsive fluorosurfactant simplifies chip fabrication and significantly accelerates the release process compared to existing methods. The fully automated FADR platform highlights the potential of this technology for large-scale screening applications. Future work could explore the application of this technology to other droplet-based assays and investigate the use of other photoresponsive materials for enhanced performance.

While the study demonstrates high release efficiency under optimized conditions, further investigation is needed to evaluate the robustness of the method under varied experimental conditions. The current system relies on fluorescence detection for selective release, limiting its applicability to fluorescently labeled samples. The long-term stability of the photoresponsive fluorosurfactant and its potential biocompatibility also warrant further investigation.