ReviewMicrofluidic fabrication of multifunctional particles and their analytical applications
Graphical abstract
Introduction
Multifunctional particles (MPs) have drawn considerable attention of researchers in recent years. Owing to the unique feature of bearing various functions in an integral whole, MPs have more extensive applications than monofunctional particles. As proved by a growing body of researches, MPs perform excellently in catalysis, sensing, biomedicine and display, etc. [1], [2], [3], [4]. Particles possessing multiple components and properties are what is studied within the scope of MPs, mainly including anisotropic particles among which Janus particles are the most widely investigated, core–shell particles in which the core and the shell act distinct roles, as well as homogeneous particles incorporated or conjugated with other functional species. Notably, structures and functions of MPs can be flexibly adjusted to fulfill the requirements of particular applications, thus MPs are expected to open exciting opportunities in the applied fields where cooperative effect is highly desired.
Bulk production processes of MPs often come across such problems as polydispersity, poor reproducibility and lack of precision in constructing sophisticated structures [4]. The emergence of microfluidic techniques has paved a new way for fabrication of high-quality MPs. High mass and heat transfer rates, accurate control of reaction conditions, fast mixing and low reagent consumption can be achieved in microfluidic platforms [4], [5], and these features make microfluidics a powerful tool in preparation of functional particles. Synthesis of MPs in a microfluidic system is greatly beneficial to controllable size and structure, high homogeneity and reproducibility, as well as simplified fabricating processes, etc. To the best of our knowledge, there are two major strategies in microfluidic fabrication of MPs: droplet-based method and flow-lithography-based method. The former is competent for producing spherical or sphere-like particles, while the latter is an ideal alternative for designing non-spherical particles, such as rods and flakes, which are more desirable in some particular applications. In short, microfluidics can provide a rather flexible platform for fabrication of MPs.
MPs can be designed to possess optical, electrical, magnetic or other properties that can be utilized for sensing and detecting. In fact, MPs have already shown promise in many analytical approaches. They can act as carriers in high-throughput screening, sensors for sensitive and selective detection, valves to control the fluids in lab-on-a-chip systems, as well as reporters for living cell imaging. The MPs always lead to better outcomes in comparison with monofunctional particles in these applications. The above mentioned are quite far from the entire potential applications of MPs, which could be extended to nearly every branches of analytical chemistry as the MPs can be versatilely designed.
An increasing number of researches are devoted to the preparation and applications of MPs, and also there are some reviews that have summarized these studies from different points of view [1], [3], [4], [6], [7]. In this review, we will focus on the microfluidic methods for preparation and applications of MPs both in microscale and nanoscale. Fig. 1 illustrates the general organization of this article. The microfluidic fabrication methods are classified and described in detail, and representative and outstanding work is included while fresh achievements are highlighted. The present review aims to unfold the superiorities of microfluidic methods for MPs preparation and the huge potential that MPs have exhibited in analytical chemistry, which may help the readers to get acquainted with these advanced materials. Meanwhile, possible challenges are also proposed.
Section snippets
Droplet-based devices
Microfluidic droplets are excellent templates in creating MPs, and several polymerization approaches such as UV exposition, thermal treatment, and ion-induced cross-linking are usually employed to solidify the droplets to obtain MPs. In microfluidic systems, droplets are usually formed by shear force between two immiscible fluids, which are described as dispersed phase (droplets) and continuous phase. So far, the most efficient droplet-based devices for fabricating MPs are chip-based flow
Multifunctional encoded microcarriers in multiplexed detections
Multiplex assays are highly significant in the fields of genetic and proteomic analysis, drug screening, clinical diagnosis and so on. Particle-based suspension arrays provide multiplexing strategies with high flexibility, low cost and high throughput [49]. Encoded particles with functions of coding and probing have contributed to high-throughput detection of various proteins [32], [41], [42], [43], [50], [51], [52] and nucleic acids [31], [53], [54], [55], [56], [57], [58], [59]. In a typical
Challenges and outlook
In summary, MPs prepared in microfluidic systems have a host of advantages, such as precisely controlled morphology and size, fine structures, homogeneity, as well as their multiple functions in analytical applications. Nevertheless, the fabrication of MPs based on microfluidic systems is still in its infancy. Preparation of particles in nanoscale and with versatility remains highly challenging, which seriously blocks the particles from being employed in nanoscale applications. Despite the fact
Acknowledgments
Financial support from the Natural Science Foundation of China (21375012 and 21305010), and the Fundamental Research Funds for the Central Universities (N110805001 and N110705002) is gratefully acknowledged.
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