1. Introduction to Ion Concentration Polarization (ICP)
Ion Concentration Polarization (ICP) has been widely used in microfluidic chips as an effective means of separation and enrichment. Microfluidic chips with integrated ICP function have the advantages of high efficiency, rapidity, low sample consumption and wide applicability.
ICP is an electrokinetic transport phenomenon that occurs at the micro- and nanointerfaces under the action of an applied electric field. Taking negatively charged sample particles as an example, when a micrometer channel is connected to a nano-channel, the nano-channel exhibits cation selectivity due to the overlapping double electric layer of the nano-channel and the negatively charged surface.
Driven by an applied electric field, cations can pass through the nano-channels while anions are repelled due to strong electrostatic interactions, leading to the formation of an ion depletion zone and an ion enrichment zone on both sides of the ion-selective membrane. The electroosmotic flow carries the sample to the edge of the depletion zone, where the sample is enriched.
In recent years, the nanostructures required for ICP and the micrometer structures required for microfluidics have been organically combined by means of micro-nanofabrication technology to construct microfluidic analytical chips capable of realizing the integration of highly efficient enrichment by ICP and microfluidic transport, thus significantly improving the detection sensitivity and selectivity of microfluidic chips.
2. Microfluidic ICP chip microfluidic channel design
Microfluidic ICP chips organically combine the nanostructures required for ICP with the microstructures of microfluidics by means of micro-nanofabrication technology, forming a biochemical analytical tool that integrates highly efficient enrichment and microfluidic channel transportation. These chips usually consist of an upper layer of micrometer channels and a lower layer of substrate containing nanostructures.
One of the current research focuses is to design and develop ICP chips with efficient enrichment functions by utilizing the easy integration of microfluidic chips. The shape and size of the micrometer channel play a crucial role in the enrichment efficiency of the chip.
Although the one-dimensional I-shaped single-channel is commonly used for principle analysis due to its simple structure, it has significant limitations in terms of target enrichment range and enrichment efficiency. In contrast, ICP chips based on H-shaped microfluidic channels have been widely used for static enrichment of charged components such as proteins, DNA, fluorescein, etc. Y-shaped microfluidic channels are suitable for the mobile enrichment or separation of targets in complex samples.
Currently, H- and Y-shaped microfluidic channel designs have become the mainstay of ICP chips for static and dynamic enrichment of samples, making them the two main cell structure designs.
3. Preparation and design of nanostructures in microfluidic ICP chips
Nanostructure preparation methods in ICP chips include mask processing, casting, self-assembly, electrical breakdown and embedding.
The mask processing method is the classical preparation method, in which micrometer channels and nano-channels are fabricated on a substrate by means of a high-precision graphically structured mask plate combined with an etching technique. Commonly used materials have evolved from silicon wafers to high hardness materials such as glass and quartz.
However, the anodic bonding method or high-temperature bonding method used in the chip bonding process can easily lead to deformation or blockage of the channel, and thus requires a high level of processing and bonding process.
The casting method is commonly used to prepare micro- and nanostructures on polydimethylsiloxane (PDMS), while the electric breakdown method is commonly used for PDMS and polyethylene terephthalate (PET) materials. These two methods have deficiencies in processing accuracy, and the harsh processing conditions and high energy consumption of the electroporation method have led to a gradual decrease in their use.
Currently, the embedding method and self-assembly method become the most widely used methods for the preparation of nanostructures. The embedding method realizes the organic combination of micron and nanostructures by placing the nanopore membrane between PDMS chips containing micron channels.
This method is inexpensive in material price and simple in process, and is widely used in the preparation of porous membrane ICP chips. The self-assembly method forms nanostructures through the spontaneous ordered arrangement of materials, which is suitable for nanomaterials such as gold nanoparticles, silicon bead nanoparticles, and poly-2-acrylamido-2-methyl-1-propanesulfonic acid.
The self-assembly method is one of the most promising ICP chip preparation methods with various ways, rich material types and mild reaction conditions.
In the preparation of micro- and nanostructures for ICP chips, the embedding and self-assembly methods are dominant, especially in the modification of embedded membrane materials and the in-situ preparation of nano-channels, which have attracted much attention in the research.
4. Optimization of enrichment performance of microfluidic ICP chips
ICP chips based on the electric field effect have made remarkable progress in the enrichment of charged particles, but during the enrichment process, the effect of electroosmotic flow often leads to the enrichment area being difficult to be controlled, which brings difficulties in the subsequent operations such as separation of target objects and fixed-point detection.
To solve this problem, some studies have proposed multi-field coupling strategies to achieve controllability of the enrichment region by enriching the target in a specified region. The most commonly used method is to utilize hydrodynamic counterpressure in a microfluidic chip to counteract the thrust of electroosmotic flow, thus precisely regulating the enrichment region.
In addition, regulating the enrichment effect by designing and integrating valves is also an effective means to achieve the localization of the ICP enrichment region and improve the enrichment efficiency. Although the valve design is highly controllable and can effectively separate, enrich and extract mixed targets, it also makes the structure and peripheral equipment of the ICP chip more complicated and the operation process more cumbersome.
5. Application of microfluidic ICP chips in biochemical analysis
Over the past decade, the ICP effect has been widely used in the development of fast and efficient biosensors due to its advantages of efficient and rapid enrichment. Microfluidic ICP chips constructed on this basis can be applied to the detection of a wide range of target analytes, including cells, proteins, nucleic acids, small molecules, and inorganic ions.
1) Small molecules and inorganic ions
Due to the small scale of small molecules, there is a difference between their real environments in conventional large system environments, and the micro-nanometer scale of microfluidic chip fits the application requirements of small molecules. Combined with the advantages of high efficiency enrichment of ICP, it can meet the application requirements of enrichment, separation and detection. Some scholars have combined ICP with a droplet generator to develop a chip capable of concentrating 1 μmol/L sodium fluorescein up to 100-fold within one hour.
2) Nucleic acids
DNA and RNA have specific sequences and specific causal links in clinical medicine, so their efficient detection is crucial for pathological research and clinical diagnosis. The researchers designed a microfluidic ICP chip to detect miRNAs by Raman technology, which reduces the purification time from nearly 1 hour to less than 10 minutes compared to conventional magnetic detection chips, with a linear range of 1-100 pmol/L. The chip is designed to detect miRNAs by Raman technology.
3) Protein
Qualitative and quantitative analysis of protein molecules is important for assessing the extent of early low-level lesions or damage, and provides an adjunct to early clinical prevention and diagnosis. Currently, the research of ICP chip in protein analysis focuses on signal enhancement and specificity detection. By increasing the concentration of the target material through microfluidic ICP microarray, thus enhancing the detection efficiency, the challenge of detection sensitivity enhancement in protein analysis has been effectively solved.
4) Cells
The microfluidic ICP chip constructed by combining the ICP separation and enrichment mode with microfluidic analysis technology has attracted wide attention in the field of cell analysis. It has been shown that the ICP chip, with its advantages of high enrichment multiplicity and easy integration, has demonstrated significant research value and application prospects in cell concentration, enrichment of cell lysates or secretions.
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