Advances in polydopamine surface modification for capillary electrochromatography

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Capillary electrophoresis (CE) has a wide range of applications in analytical fields due to its advantages of low sample consumption, short separation time, and high separation efficiency. The cathodic electroosmotic flow (EOF) and single electrophoretic separation mechanism are not optimal for many CE applications. Hence, the use of an unmodified fused-silica capillary leads to insufficient separation performance that cannot meet the requirements for various complex sample systems, especially neutral and chiral compounds. Therefore, it is necessary to introduce various capillary modification strategies in CE so that its potential for practical application can be expanded. Mussel-inspired polydopamine (PDA) and PDA-derived coating materials have fascinating advantages such as simple surface coating procedures, strong surface adhesiveness, good chemical stability, latent reactivity with many functionalized molecules, and good biocompatibility. Thus, they have been widely utilized in different research fields, including catalysis, sensing, water treatment, sample pretreatment, biomedicine, chromatographic separation, and CE. The preparation of PDA coatings is simple as it involves physical adsorption, and the obtained surface adhesive coatings possess good stability similar to covalently bonded coatings. Therefore, PDA and PDA-derived coatings are well suited for the modification of fused-silica capillaries. More importantly, the PDA coating can be utilized as an intermediate reaction platform for diverse subsequent surface modification because of its strong surface adhesive property and strong latent reactivity with many functionalized molecules (such as polymers, proteins, and nanomaterials). Consequently, various chromatographic retention mechanisms can be introduced on the inner wall of the capillary, thereby contributing to the fabrication of multi-functional PDA-based stationary phases for CEC. Owing to these outstanding advantages, researchers are paying increasing attention to the great application potential of PDA and PDA-derived coatings in CEC. In this paper, recent advances in the methods for preparing PDA coatings, especially the recently developed fabrication strategies and various applications of PDA-modified silica capillary in open tubular-capillary electrochromatography (OT-CEC) and capillary electrochromatography monolithic columns, are summarized and discussed. Furthermore, the application prospects of PDA-based coating materials in CEC are prospected in this review. Although PDA and PDA-derived coating materials are seeing widespread utilization in field of CEC, researchers have still not reached a definite conclusion regarding the PDA formation and coating mechanisms, and further investigation is needed in this direction. The PDA coatings formed using existing methods are generally thin. In the early stage, many studies adopted the strategy of repeated coating to improve the coating effect of PDA in capillaries, but this method was found to be time-consuming and less efficient. In order to improve the preparation efficiency of PDA-modified CEC columns, many researchers have focused on fast deposition induced by a strong oxidant to obtain PDA-coated columns. However, the controllability of the PDA coating obtained by this method is poor. Thus, it is necessary to further explore new preparation strategies for PDA-coated CEC capillaries with better reproducibility and stronger operability. On the other hand, although a strategy for directly synthesizing functional PDA-coated CEC columns in the organic phase has been proposed, its application potential in CEC remains to be further explored. In addition, the PDA coating itself has poor porosity and a small specific surface area, which may be significantly improved by modifying the coating on the porous monolithic column surface. However, there has been limited research on the use of PDA coatings in monolithic columns, and their application potential remains to be expanded. With in-depth research into the formation mechanism and preparation methodologies of PDA coatings, PDA, which is a highly malleable biomimetic material, will play a more important role in advances in the fields of CE and CEC.