1. Fluorescent nanomaterials
With the advancement of nanotechnology, fluorescent nanomaterials are gradually used for visualization and analysis at the molecular level, significantly advancing the depth of biological and medical research. In order to expand the applications of fluorescent nanomaterials in ion detection, biosensing and signaling, it is crucial to develop and prepare materials with stable quality and excellent performance.
In clinical applications, the homogeneity of nanomaterial particle size is very important. Different particle sizes have different delivery rates and transport mechanisms in organisms, and may be absorbed by multiple sites, thus affecting the imaging effect. Studies have shown that microfluidics can prepare fluorescent nanomaterials with specific morphology, uniform size and superior performance.
2. Classification of microfluidic synthesis systems
Microfluidics builds specific microchannel structures within chips or tubes through fine processing, realizes physical or chemical reactions of fluids on a micro-scale, and integrates synthesis, modification, and separation steps on a tiny platform.
In recent years, advances in new energy technologies such as light, electricity and magnetism have promoted the emergence of advanced microreaction structures such as photoexcited (UV, IR, visible) microreactors, electrochemical microreactors and plasma microreactors. These energy coupling methods and the characteristic structures of the microreactors are crucial for the preparation of fluorescent nanomaterials.
Currently, microfluidic reactors are categorized by structure as chip microreactors and channel microreactors (including tubular and centrifugal microreactors). In chip microreactors, the mass transfer process mainly relies on diffusion, whereas in tubular and centrifugal microreactors, the mass transfer process is influenced by the kinetic energy of the fluid and the flow dissipation rate.
1) Chip microreactor
Based on the principle and technology of common capillary electrophoresis, various microfabricated structures, such as channels and reaction tanks and other functional units, can be constructed on silicon, glass chips, quartz chips or polymer substrates to form microreactors on a chip by micro-nanofabrication technology.
Depending on how the fluid flows,chip microreactors can be categorized into three types: single-phase laminar flow devices, multiphase segmented flow devices, and droplet-based flow devices.
a) Single-phase laminar flow devices
When the two mutually soluble fluids to laminar flow state and parallel flow and reaction, the mass transfer process mainly rely on diffusion, perpendicular to the direction of the flow to form from high to low concentration gradient, and the concentration gradient with the increase of the flow distance and reduce.
Microfluidic devices based on continuous laminar flow are easy to design, operate and scale up, and have been successfully applied to drug screening, membrane-free cell preparation, separation analysis and cell biology research.
b) Multi-phase segmented flow microfluidic device
Multiphase segmented flow refers to the parallel flow of two immiscible fluids in a channel, and a continuous liquid film will be formed between the two phases due to the difference in physical properties. Compared with single-phase laminar flow, the flow state of multiphase fluid is more complex, not only affected by the physical properties of the solution such as density and viscosity, but also related to the interaction force between the fluids.
In a single-phase laminar flow device, the system is relatively static and the mass transfer process is mainly controlled by diffusion; whereas multiphase reactions can be rapidly formed into liquid segments with different conditions by adding a carrier liquid that is immiscible with the reagent to isolate the reagent.
Currently,Multi-phase segmented fluids are mainly classified into liquid-liquid and gas-liquid types.
In liquid-liquid segmentation, discrete droplets are encapsulated by an immiscible inert carrier liquid at nanoliter scales and flow at regular intervals;
The gas-liquid segment separates the liquid by discrete bubbles, limiting reactions in the liquid segment, and internal circulation promotes material transfer from the channel wall to the center of the liquid for efficient convective mixing and thermal mass transfer.
c) Microfluidic devices based on droplet flow
Microchip droplet technology is an important branch of microfluidic technology, which splits the discrete phases of two immiscible fluids into nanoliter scale droplets by adjusting the flow shear or surface tension.
The technology has unique advantages: microdroplets occupy little space and the reactants are confined inside the droplets, which accelerates the mass transfer rate; the reaction time is precisely controllable and the operation is flexible; and each droplet is equivalent to an independent reactor, which significantly reduces the contamination of reagents on the chip channel. According to the way of droplet generation, the technology can be categorized into T-structure method, flow focusing method and coaxial flow confocal method.
2) Tubular Microreactor
Compared with the chip microreactor, the tubular microreactor has a simpler structure, is easy to build, and can be flexibly assembled between modules, which not only improves the processing throughput, but also promotes the development of microfluidic system towards automation, integration and functionalization.
Tubular reactors equipped with micro-mixers or mixing zones have been used to synthesize a wide range of nanomaterials with better mixing performance than chip reactors, stable and less fouling flow patterns, but not as good as chip micro-reactors in terms of reaction precision and controllability.
3) Centrifugal microreactor
Most chip and tube-based microreactors are stationary in operation, with reactions driven by fluid motion or external energy input. Centrifugal force-driven microreactors, on the other hand, rely on the centrifugal force generated by the circular motion of a micromotor as the driving force for the fluid. Compared to other driving methods, this method has the advantages of easy processing, driving multiple fluids, pulsation-free flow, and easy realization of high-throughput analysis.
3. Examples of microfluidic preparation of fluorescent nanomaterials
a) Semiconductor nanoparticles
Semiconductor quantum dots, as fluorescent nanomaterials emitting at wavelengths ranging from the ultraviolet (UV) to the near-infrared (NIR), are widely used in bio-imaging, light-emitting diodes (LEDs), and novel displays. Microfluidics has enabled the continuous and controllable synthesis of a wide range of semiconductor quantum dots.
b) Carbon dots
Carbon dots (CDs) are zero-dimensional nanocarbon materials consisting of carbon cores and shells based on sp2-sp3hybridized structures with similar size and fluorescence properties to semiconductor nanoparticles, but with lower biotoxicity, making them more suitable for use in cellular imaging, disease monitoring and diagnosis.
c) Calcite nanoparticles
Chalcogenide nanoparticles are a new type of photovoltaic material with the structural generalization ABX3.These fluorescent materials have high absorption coefficients, high carrier mobility, and low trap densities, and have attracted much attention in the fields of photovoltaic cells, LEDs, and lasers.
d) Rare earth doped nanoparticles
Rare earth elements, including scandium (Sc), yttrium (Y) and 15 lanthanides, are widely used in luminescence applications due to their rich 4f energy level structure, which provides light emission spectra from the ultraviolet to the near-infrared ranges, and long fluorescence lifetimes.
e) Metal and oxide composite nanomaterials
Metallic nanomaterials are materials with at least one dimension at the nanoscale or consisting of these structural units, combining the unique properties of nanomaterials with the physical and chemical properties of metals, and have a wide range of applications in areas such as electrode surface modification and ion detection.
ZH_CN 
EN