Microfluidics is a field of science that manipulates and controls small volumes of fluid as well as suspended micro- and nanoparticles within microscale channels of a chip-sized device. In the intricate world of microfluidics, controlling the flow of fluids at a microscopic level is not just a necessity; it’s an art form that intersects with the realms of biology, chemistry, and medicine. This field, essential for the development of flow cell assays and lab-on-chip devices, is witnessing a transformative phase with the advent of optofluidics, a sub-discipline that harnesses light to manipulate fluid flow. However, the journey toward precise fluid control at the microscale is fraught with challenges such as lack of flexibility, prompting researchers to delve into the realm of optical nanostructures.

Traditional techniques used metal nanostructures interacting with light to manipulate fluids, as when illuminated, they can confine energy at subwavelength scales. This energy concentration was exploited to create nanosources of heat - a study known as thermoplasmonics. It has numerous applications in nanotechnology, such as photothermal cancer therapy, targeted drug delivery, and solar-powered steam generation. Yet, the high temperatures attained by these structures pose a risk to delicate biological samples coming into contact with them, driving scientists to explore alternative materials.

To address the limitations of metal nanostructures in microfluidics, researchers have pivoted towards dielectric nanostructures. These structures, composed of non-metallic, insulating materials, avoid the strong Ohmic losses typical of their metallic counterparts. Previous research works typically capitalize on their low light absorption to minimize heat generation, thus preventing unwanted fluid motion or destabilizing thermal effects that could adversely affect particle manipulation. This characteristic offers a refined approach to controlling suspended nanoparticles. However, the low absorption also limits their effectiveness in microfluidic control, presenting a new set of challenges for researchers.

At Vanderbilt University, the innovative work of Justus Ndukaife, an assistant professor of electrical engineering, and his former Ph.D. student Sen Yang, is advancing the field of all-dielectric thermonanophotonics. This emerging field aims to control optical heating on a subwavelength scale by fine-tuning optical losses in dielectrics. Their research, published in Light: Science & Applications, leverages an all-dielectric metasurface supporting quasi-bound states in the continuum (quasi-BIC) resonances. These resonances, originating from quantum mechanics, enable the creation of optical resonances with high quality factors (a measure of a resonator’s bandwidth relative to its center wavelength, abbreviated as Q), and strong electromagnetic field enhancements matching or surpassing those in plasmonic structures.

Water absorption is used as heat sources in this quasi-BIC system. More specifically, when approaching the resonance, the total heat generation comprises of the global absorption by the bulk water when laser passing through the microfluidic chamber as well as the heat dissipation from the water layer close to the resonators which serves as local heat sources. By harnessing water absorption and manipulating the resonances, the team has demonstrated precise control over the heat dissipation and fluid flow within microfluidic chambers. They have achieved a level of control over temperature field distribution and flow dynamics by adjusting laser wavelengths within several nanometers, a feat unattainable with plasmonic systems due to their low-Q properties. This precision facilitates the rapid transport of micro- and nanoparticles suspended in a microfluidic chamber and their aggregation on the metasurface. The research also illustrates varied particle aggregation distributions upon the introduction of a cationic surfactant to the nanoparticle colloid, indicating how the temperature field distribution is influenced by the quasi-BIC resonance. The researchers have demonstrated these effects using fluorophore-labeled tracer polystyrene beads, predicting the same effects could be achieved for particles below 100 nm. 

The implications of this research are profound, particularly in the transport of analytes to biosensing surfaces. The high-Q dielectric metasurface can act as a sensor, detecting aggregated particles and molecules by refractometry or enhancing Raman or fluorescence signals from the molecules, thus improving the detection sensitivity. This method could significantly impact the detection of cancer-associated vesicles, aiding in patient treatment monitoring and early disease detection. 

This pioneering work represents the first experimental demonstration of using quasi-BICs for fluid and particle manipulation, marking a pivotal advancement in controlling thermally induced microfluidic dynamics with dielectric nanostructures. It heralds new opportunities in biology and medicine, such as enhancing the concentration and detection of extracellular vesicles and viruses, and boosting the sensitivity of biosensors to detect minute particles. The ability of this system to concentrate particles in a liquid could revolutionize biosensing technology, offering more sensitive, efficient, and early-stage disease detection tools. 

In summary, the integration of dielectric nanostructures into microfluidics signifies a substantial advancement in our capability to manipulate fluids at the microscopic level. As researchers continue to explore these structures’ potential, we are entering a new era in medical diagnostics, where disease detection and monitoring are more precise, efficient, and accessible. This journey in microfluidics, fueled by relentless innovation, not only expands our scientific understanding but also brings us closer to a future where diseases can be detected and treated with unparalleled precision and efficiency.