By: Dan Wang, Ph.D., Professor, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China.
Luminescent nanomaterials have found many potential applications in health, energy, information and safety area. In 2016, Nature commented that luminescent nanomaterials emission in every color could revolutionize applications from cancer treatment to television displays (Nature, 2016, 531, 26-28). For example, in the field of fluorescence bioimaging, the luminescent nanomaterials have shown much better performance than conventional used fluorescein and fluorescent proteins since 2000. The applications of fluorescent nanomaterials have also demonstrated to be excellent agents for super-resolution optical imaging recently (Nature, 2017, 543, 229-233). Therefore, there is an increasing demand for luminescent nanomaterials as the fluorescence bioimaging moves from microscopy to nanoscopy. However, the key materials still present challenges. The properties of luminescent nanomaterials are extremely sensitive to structures, which allow the control and tuning of characteristics in lab scale, but make them very difficult to reproduce for commercial applications. Therefore, the development of mass-scale nanoparticle synthesis methods have been one of key materials challenges. The issues related to scale-up, cost and compatibility must be considered at an earlier stage of development.
Similar to most chemical processes, the process to synthesize luminescent nanomaterials is a complex combination of a series of unit operations such as mixing of the reactants, critical nuclei, growth of particles, separation and drying (Engineering, 2017, 3, 402-408). Many studies have focused on the reactions in autoclave reactors and the influence of the molar ratio of reactants, solvent types, surfactants, reaction temperature, reaction time and other related factors. However, other unit operations (mixing of the reactants, separation of nanoparticles, drying of the solid powders), which can more or less affect the final products of luminescent nanomaterials, have been rarely focused on. Therefore, the process intensification and process integration of all the types of units during the whole processing offers another angle to solve the problem of scale-up for high quality
luminescent nanomaterials (Powder Technology, 2018, 340, 208-216; AIChE Journal, 2019, 65, e16714).
Due to the extraordinary number of levers that chemical engineering affords, there will be a growing dialogue between chemical engineers and researchers in other fields (such as chemists, physics and materials) who, otherwise, might only have worked with luminescent nanomaterials in laboratory scale. However, broad application of luminescent nanomaterials will not be realized if there is no low-cost and large-scale fabrication capability for them with a well-defined size and shape. Some important issues remain to be addressed (Ind. Eng. Chem. Res. 2018, 57, 1790-1802): (1) the use of green raw materials without expensive or unnecessary dangerous compounds has become an everlasting pursuit. There are concerns about the possible side effects (e.g., environmental toxicity and pollution characteristics) derived from the use of nanomaterials, especially heavy-metal based quantum dots. Discovery of novel luminescent nanomaterials that are completely green in elementary composition may provide an alternative. (2) The development of green processes for the synthesis of luminescent nanomaterials with precise control of all reaction parameters for multi-step procedures is still at an early stage. It is difficult to determine which type of reactor or technology is favored for the synthesis of luminescent nanomaterials. The quality of the product and the cost should be taken into account for practical application. Process intensification based on micro-channels devices and/or high gravity reactors is promising for continuous production of luminescent nanomaterials. Introduction of built-in analytical and feedback mechanisms for real-time tuning and optimization of products is required.
Looking forward to the future, chemical engineers play important roles in accelerating the translation of luminescent nanomaterials in practical applications.
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