1. Droplet microfluidics and surface-enhanced Raman spectroscopy (SERS) techniques
Droplet microfluidics utilizes microfluidic control technology to precisely generate, manipulate and merge tiny droplets in microchannels, providing a platform for efficient, flexible and controllable microscale fluid manipulation.
SERS technology significantly enhances the Raman detection signal by adsorbing the analyte onto the surface of metal nanostructures, which has the advantages of specificity, rapidity, and sensitivity, and is an irreplaceable means for the structural analysis and quantitative detection of chemical substances.
SERS detection based on droplet microfluidics has the following advantages:
1. Avoid liquid evaporation and memory effect: The droplet microfluidic system prevents liquid evaporation, avoids the sample from staying on the channel wall for a long time in continuous microfluidics, and reduces the influence of memory effect.
2. Improve analyte diffusion and concentration efficiency: Through methods such as electrodynamic concentration, magnetic capture, hydrodynamic focusing, or dielectric electrophoresis, droplet microfluidic systems can improve the diffusion efficiency of analytes into the enhanced substrate and increase analytical sensitivity.
3. High-throughput and high-efficiency analysis: The droplet microfluidic system generates and manipulates a large number of discrete droplets, enabling high-throughput analysis and high-efficiency reaction processes, improving analytical speed and efficiency.
4. Improved Repeatability and Stability: By precisely controlling the droplet size and separation, the droplet microfluidic system is able to control the experimental conditions with a high degree of stability, thus improving the repeatability and stability of Raman analysis.
2. Application of droplet microfluidics-SERS technology
1) In situ characterization of chemical reactions
In situ characterization is crucial in understanding and optimizing chemical reactions, revealing the structural changes, kinetic behavior and catalytic mechanisms of substances during the reaction.
Microfluidic droplets are attractive as stand-alone reaction vessels for on-line monitoring of chemical reactions due to their ability to precisely control reaction reagents and rapidly optimize reaction conditions independent of the external environment.
For example, the high interfacial area of droplets facilitates the study of catalytic processes in Pickering emulsions. Conventional batch Pickering emulsions prepared by vigorous stirring show wide droplet size distributions, making it difficult to measure kinetics and low interfacial areas between the two phases.
The opacity and tight phase mixing of Pickering emulsions make on-line studies difficult, requiring sampling and breaking of the emulsion for further analysis.
An on-line in situ study of Pickering emulsion catalysis was realized using droplet microfluidics-SERS, which resulted in a 9-fold increase in acid-catalyzed deacetalization reaction yield due to the high interfacial area of the droplets.
Droplet microfluidics-SERS has the uniqueness and potential in monitoring special reactions, especially in reducing the interference of external environment, excluding the memory effect, enlarging the interfacial effect, and continuous dynamic monitoring, which brings important significance and broad application prospect for in situ chemical reaction research.
2) Analysis of single cells
Single-cell analysis plays an important role in revealing individual cellular heterogeneity and providing insights into cellular function, development, and disease mechanisms, as well as providing a foundation for personalized medicine.
The classification of cells and droplets in microfluidic systems is of great significance to researchers from the laboratory to clinical applications.
In contrast to common fluorescence-activated cell sorting and magnetically-activated cell sorting techniques, Raman-activated cell sorting is capable of label-free, non-invasive identification and isolation of individual cells of a target type, state, or environment from homogeneous populations or complex cell communities.
However, flux remains one of the main factors limiting its wider application.
The droplet microfluidic-SERS technique has also been used for molecular analysis of single cells.
The researchers detected the expression of cell surface molecules by encapsulating individual cells and wheat germ agglutinin-functionalized SERS probes in microdroplets, enabling the study of oncology targets and the observation of intercellular heterogeneity.
Microdroplets provide a living space for single cells to accumulate and analyze cell-secreted metabolites. The clever reaction design and immuno-sandwiching system enables highly sensitive detection of single-cell metabolites and simultaneous analysis of multiple metabolites, revealing the functional state and metabolic characteristics of cells.
Droplet microfluidics-SERS technology is also important in single-cell sequencing.
By precisely encapsulating target cells in microdroplets and accurately exporting them in subsequent sequencing, it is possible to obtain complete genome sequence analysis from a single cell.
This method can be used to isolate and characterize clinically significant cells, such as drug-resistant bacteria, with the advantage of high genome coverage and high resolution.
Therefore, droplet microfluidics-SERS technology provides a high-throughput, high-sensitivity and non-invasive monitoring means for single-cell analysis, which is of great significance in the study of cellular heterogeneity and the exploration of cellular functional and metabolic characteristics, and demonstrates a broad application prospect.
3) Controlled synthesis of active particles for the detection of small molecules
Structurally controllable SERS-active particles are particularly important for the reproducibility of Raman detection, yet the preparation of SERS-active particles with good homogeneity is challenging.
The SERS active particles prepared by the droplet microfluidic system have controllable and uniform size and morphology, which mainly include polymer matrix particles and photonic crystal particles.
Combining the advantages of top-down (microdroplet generation and metal film deposition) and bottom-up (nanoparticle self-assembly) processes, the researchers propose a fast and reliable method for generating plasma microsensors.
High-quality SERS active substrates with >107 average enhancement factor were successfully prepared with good reproducibility by high-frequency and continuous droplet generation by microfluidics, self-assembly of spatially confined silica nanoparticles for controlling the microsphere volume fraction and filler density, and physical thin-film deposition on the surface of the microspheres for the precise formation of plasma nanostructures.
The particles prepared based on droplet microfluidics are also size-selective, pH-sensitive, and temperature-responsive, demonstrating good SERS detection of biosmall molecules.
Therefore, the SERS active particles prepared based on the droplet microfluidic system can effectively improve the sensitivity and reproducibility of Raman detection and be applied to complex sample systems through flexible structural design, which is expected to be widely used in the future in the fields of health monitoring, disease diagnosis and drug residue detection.
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