Nano Today
ReviewFunctionalization of hollow nanoparticles for nanoreactor applications
Graphical abstract
Introduction
Hollow inorganic nanoparticles, bearing a nanosized cavity space encapsulated in a porous inorganic nanoshell, have been attracting great interest from both scientific and application perspectives [1], [2], [3]. Research over the last decade has proved that their distinct properties, which come from the presence of an interior void space, are advantageous for a range of applications, including nanoreactors [4], [5], [6], [7], drug-delivery vehicles [8], [9], [10], contrast agents for molecular imaging [11], [12], [13], and energy- and gas-storage materials [14], [15], [16]. For instance, their high surface area affords the ability to carry a high payload of functional species such as catalysts and molecular imaging agents, which allows highly reactive catalysis and sensitive diagnosis. The interior void space, which is protected by an inert shell, can be employed to load, store, and deliver functional molecules such as drug molecules to the target location. Furthermore, by utilizing their permeable porous nanoshell, we can selectively control the internalization and release of guest molecules such as substrates, products, and drug molecules in catalysis and drug-delivery applications. In addition, their tolerance against volume change and their low density has been proved desirable for high-capacity energy materials such as anode materials for Li batteries.
In particular, when catalytic species such as noble metal nanocrystals are incorporated in the interior cavity of the porous shell, the hollow nanoparticles are considered as attractive candidates for nanoreactor systems; this makes up for the weak point in conventional nanoparticle-based catalysts, aiding the realization of industrially applicable nanocatalyst systems. While conventional nanoparticles are often subject to deactivation owing to the sintering process or aggregation during the reaction, these nanoreactors, based on functionalized hollow nanoparticles, are expected to maintain their activities even under harsh reaction conditions and during the recycling process by providing catalysts with an environment that is well isolated and protected from the neighboring catalysts and molecules in the surrounding solution. In addition, because catalytic nanocrystals are usually entrapped in a surfactant-free state inside the cavity, they can exert a high-activity performance without interference from surface-capping ligands. Moreover, we can also expect that such a hollow reactor can selectively catalyze the transformation of a specific molecule that can access the cavity easily through the selectively permeable pores of the nanoshell. The confinement of the transition state of the transformation inside the nanosized cavity can help to enhance the regio- and enantioselectivity of the catalytic reaction. Besides their use as catalysts, very recently, the utility of the hollow nanoreactor was further extended to the template synthesis of metal nanocrystals within the confines of the cavity, which enables high-concentration production of morphology-controlled metal nanocrystals in a surfactant-free state.
In the performance of the hollow nanoreactor, the selectively functionalized interior cavity is the most vital component, which allows chemical reactions to occur confined within the protective void space. In this context, the selective and differential functionalization of the interior cavity space is an important and challenging subject that must be addressed for full exploitation of the potential of hollow nanoparticles in nanoreactor applications. In this review paper, we provide an overview of the efforts made so far to functionalize hollow nanoparticles with potential for nanoreactor applications. Since many excellent review papers covering broader discussions on hollow nanoparticles [17], [18], [19], [20], [21], [22], [23], [24] or their applications [8], [25], [26], [27], [28] have already been published, in this study, we focus on the synthetic strategies for functionalizing the interior cavities of hollow nanoparticles and their employment as nanoreactor systems to catalyze chemical reactions and template the growth of nanocrystals.
Section snippets
Fabrication of hollow-interior nanoparticles
Owing to their interesting and applicable properties, including a high surface to volume ratio, low density, and a protected and specific environmental interior void space, hollow-interior nanoparticles have received increasing attention, and various synthetic methods have been devised by many researchers over the last decades [29], [30], [31]. The synthetic methods developed so far for the fabrication of hollow nanoparticles can be classified into two categories according to the involvement of
Functionalization of hollow interior space
Hollow nanoparticles have been proved to have remarkable potential in biomedical [8], [9], [10], [11], [12], [13], catalytic [4], [5], [6], [7], and energy applications [14], [15], [16] owing to their distinct and advantageous characteristics, which mostly come from their interior void space and porous shell with selective permeability [1], [2], [3]. In particular, when catalytic species such as noble metal nanocrystals are incorporated inside a cavity enclosed by a shell of chemically inert
Nanoreactor applications for catalytic reactions
Hollow nanoparticles with functionalized interior spaces represent a promising framework for the development of catalytic hollow nanoreactor systems with high activity, selectivity, and recyclability, which can tackle the issues of conventional nanoparticle-based catalysts [3], [23], [28], [29], [31]. Hollow nanoreactors, described in this review papers, mostly belong to a yolk@shell like structure, which bears a single or multiple numbers of catalytic core-nanoparticles or catalytic interior
Nanoreactor applications for confined syntheses of metal nanocrystals
Until recently, most studies with hollow nanoreactors have focused on the possibility of realizing efficient and sustainable nanocatalyst systems that catalyze reactions with high activity and selectivity and can be recycled without any decline in performance [3], [23], [28], [29], [31]. Very recently, however, a new line of research has appeared, with the aim of extending the utility of hollow nanoreactors to synthesize nanocrystalline materials within the confines of the cavity [6], [7], [241]
Conclusion and outlook
In this study, we surveyed the development of functionalized hollow nanoparticles from the perspective of their use in nanoreactor applications. Recently, the synthetic technique for functionalizing the interior surface of hollow nanoparticles has undergone rapid advancement, with an increasing number of reports being published in this field. To develop hollow nanoparticle-based nanoreactor systems, several effective strategies have been devised, which involve introducing catalytic nanocrystals
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (2011-0017377).
Jihwan Lee received his BS in 2012 from the Department of Chemistry at Pohang University of Science and Technology (POSTECH). He is currently pursuing his PhD at the Department of Chemistry of POSTECH under the supervision of Professor In Su Lee. He is working on the development of hollow inorganic nanoparticles for the biomedical and nanoreactor applications.
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Jihwan Lee received his BS in 2012 from the Department of Chemistry at Pohang University of Science and Technology (POSTECH). He is currently pursuing his PhD at the Department of Chemistry of POSTECH under the supervision of Professor In Su Lee. He is working on the development of hollow inorganic nanoparticles for the biomedical and nanoreactor applications.
Soo Min Kim received her BS degree from the Department of Chemistry, Sookmyung Women's University (2005), and PhD (2010) from Seoul National University under the direction of Professor Young Keun Chung. After the postdoctoral work with Dr. Kwang-Deog Jung at Korea Institute of Science and Technology (2010–2011), she is now working as a postdoctoral researcher with Professor In Su Lee at POTECH. Her research interest is in the development of new and facile routes for synthesizing functional nanostructures and their applications.
In Su Lee received his PhD (2000) from Seoul National University under the supervision of Professor Young Keun Chung. After the work as a senior researcher at LG chemical (2000–2003), the post-doctoral research with Jeffrey Long at UC Berkeley (2003–2005), and the research with Taeghawn Hyeon for Creative Research Initiative Center for Oxide Nanocrystalline Materials at Seoul National University (2005–2006), he worked at Kyung Hee University as an assistant and associate professor (2006–2011). After this, he moved to POSTECH and joined the Department of Chemistry as an associate professor. His current research is focused on the synthesis and modification of hollow metal and metal oxide nanoparticles with interior cavity and finding of their applications in catalysis and biomedical field.