Core shell particles

The formal study of core-shell particles may be traced back to the late sixties in the development of a novel sorbent of HPLC comprising porous spherical soft shells supported by fluid-impermeable glass microspheres [1]. Such ‘pellicular’ structures were found to be more stable at increasing pressures, changing column temperatures, and eluent compositions than conventional monolithic soft sorbents. Since then, many core-shell-type particles have been designed for various purposes because such composite particles exhibit the combined properties of the core and shell materials.

The study of core-shell particles has expanded exponentially in the past few decades, given the variety of structures possible and the vast gamut of applications they can support (Figure 1). The composites that have been studied have been of various constituents, gross sizes, relative sizes, and morphologies (Figure 2). While the shell is almost always solid, the cores contain solids, liquids (capsules), or gases (hollow particles). These composite nanoparticle cores and shells have been in various combinations of inorganic, organic, polymeric, and crystalline materials. The entire ensembles have been nanometric or micrometric in size. The core has comprised single particles or multiple ones, and shells that have been studied have been continuous, multi-layered, or discontinuous domains. These particles themselves have been of various shapes as well.

Figure 1. Number of publications on core-shell particles. Data derived from Scopus using keywords ‘core’ and ‘shell’.

Figure 2. Types of core-shell particulates.

Morphology-based classification of core-shell particles

The particle sizes of core-shell structures have varied from nanometric to micrometric dimensions. The size factor is particularly important for biomedical applications such as drug delivery in which the drug molecules are encapsulated by a biocompatible/biodegradable shell. The advantages of micronized core-shell particles as drug carriers are the higher encapsulation efficiency and protection possible with the thicker shells than with the relatively finer shells in nanosized particles. However, nanosized core-shell particles can carry a higher payload and release the drug faster with a more direct ‘burst’ release, which makes core-shell nanocapsules relevant in fast-release drug applications. Thus, while micrometer-sized particles are ideal as a long-term reservoir of drugs upon local administration, nanometric particles are better suited for the rapid and targeted delivery of drugs. Nanometric capsules are also effective carriers of biomolecules such as DNA and RNA fragments that must be introduced into cells that cannot be penetrated by larger micrometer-sized particles.

On the other hand, micrometer-sized core-shell particles are suited for the design of ‘artificial cells’ because of the possibility of having multiple compartments encapsulated by a common shell, similar to living cells (Figure 3).

Figure 3. An example of a core-shell structured artificial cell. Image adapted from Ref. [2].

The core-shell particles are of numerous shapes beyond spherical; cubical [3], prismatic [4], hexagonal [5], octahedral [6], disc-like [7], tubular [8], and wire-shaped [9], core-shell nano and microparticles have been studied (Figure 4). The anisotropy of their morphology has led to unique catalytic [10], optical [10], magnetic [11], and electrical [12] properties.

Figure 4. Variously shaped core-shell particles reported in literature.

Composition-based classification of core-shell particles

Apart from the physical state (solid/liquid/gas), core-shell particles may be inorganic/inorganic, inorganic/organic [13], organic/inorganic, and organic/organic materials. The choice of the core and shell materials depends upon the end application.

Inorganic–organic core/shell nanoparticles are typically studied for their unique electrical and magnetic properties, wherein the core inorganic particle has the desired property and the shell of an organic compound allows it to be incorporated in organic-based matrices. Such composite particles are used in displays, batteries, optical sensors, magnetic imaging applications, etc. When the organic shell is an electrical conductor, like PANI, core-shell composites find use in transparent electronics, light emitters, light absorbers (Figure 5), piezoelectric devices, microwave absorbers, chemical sensors, and spin electronics.

Figure 5. An example of inorganic core-organic shell particles for use in light emitting and light absorption devices. Image reproduced without modification from [14].

Bimetallic core-shell nanoparticles have shown improved magnetic [15], catalytic [16], and optics [17] properties for a variety of applications. The segregation phenomena have dictated the structure and properties of multi-metallic core-shell structures (Figure 6). Simulation studies have shown that the surface segregation in the bimetallic combinations depends on the cohesive energy and Wigner–Seitz radius of the two metals in the core-shell structure [18].

Figure 6. Four types of structures possible as seen from molecular dynamics (MD) and Monte Carlo (MC) simulations for 50:50 bimetallic composition. Image adapted from Ref. [18].

Hollow nanostructures, which are essentially core-shell particles in which the shell encapsulates an empty or partially filled space, are also of significant academic interest because the presence of the cavities results in the reduction of the density compared to the dense solid counterparts for equal volumes of materials'. Hollow nanoparticles with empty or partially filled cores (Figure 7) have been studied for use as catalytic hollow nanoreactor systems with high activity, selectivity, and recyclability. The shells are usually permeable, and allow only specific substrate molecules to access the interior cavity that houses the catalytic site, thereby enabling substrate-selective catalysis. Such nanoreactor cores are usually made of noble metals and their alloys, or magnetic/transition metal oxides, and the shells are made of ceramic oxide materials or carbon-based materials, such as amorphous carbon and reduced graphene oxide, and organic polymers [19].

Figure 7. Types of hollow nanostructures.

Synthesis of core-shell particles

Core-shell particles have been synthesized by bottom-up approaches by which the core particles are formed followed by envelope formation. Chemical synthesis, chemical vapour deposition, laser-induced assembly, self-assembly, colloidal aggregation, film deposition, and growth, are some of the methods commonly reported in the literature. A comprehensive review of various synthesis methods for various kinds of core-shell particles was published in 2012 [20]. Since then, the field of core-shell particulate materials has grown so vast that no single review can provide a complete picture of the synthesis methods. A recent review [21] summarizes some of the recent trends in the area of core-shell systems. Almost all synthesis methods reported fall under the categories represented in Figure 8.

Figure 8. General synthesis methods for core-shell systems.

The application areas of core-shell particles are numerous. The main areas of study of core-shell structures have been

• Biomedical (molecular imaging, drug delivery, and therapy) [22];

• Energy (supercapacitors, li-ions batteries, hydrogen storage systems, quantum dot solar cells, dye-sensitized solar cells, silicon/organic solar cells, and fuel cells) [23];

• Catalysis (photocatalysis, electrocatalysis, bimetallic catalysis) [2425] and

• Photonics [26].

In many applications that require nanostructured particles to impart specialized properties such as superhydrophobicity [27], lubrication [28], corrosion resistance [29], etc., core-shell particles are used to stabilize the nanoparticles and prevent aggregation. Thus, while the core-shell structure in itself does not contribute to the property sought, it plays a supporting role. In some cases, the core shell approach provides for easier sintering.

An interesting approach to self-healing structures has been to encapsulate healing agents inside micron-sized or nanosized capsules that break to release the healing agents. Scopus lists 71 papers in 2022 alone, on the use of core-shell particles for self-healing coatings and other applications.

Biomedical and energy applications can be expected to drive future developments in core-shell-type particles. The ability to converge different properties into a single system offers tremendous scope for new materials development required for futuristic applications.