Hollow micro and nanostructures for therapeutic and imaging applications

Abstract

Hollow particles have been extensively used in bioanalytical and biomedical applications for almost two decades due to their unique and tunable optoelectronic properties as well as their significantly high loading capacities. These intrinsic properties led them to be used in various bioimaging applications as contrast agents, controlled delivery (i.e. drugs, nucleic acids and other biomolecules) platforms and photon-triggered therapies (e.g. photothermal and photodynamic therapies). Since recent studies showed that imaging-guided targeted therapeutics have higher success rates, multimodal theranostic platforms (combination of one or more therapy and diagnosis modality) have been employed more often and hollow particles (i.e. nanoshells) have been one of the most efficient candidates to be used in multiple-purpose platforms, owing to their intrinsic properties that enable synergistic multimodal performance. In this review, recent advances in the applications of such hollow particles fabricated with various routes (either inorganic or organic based) were summarized to delineate strategies for tuning their properties for more efficient biomedical performance by overcoming common biological barriers. This review will pave the ways for expedited progress in design of next generation of hollow particles for clinical applications.

Graphical Abstract

1. Introduction

Efficient targeted theranostic applications usually require an organic or inorganic based carrier/functional particle and a specific recognition ligand to be tethered to them. These particles usually have unique optoelectronic, magnetic or physical properties (i.e. large surface area as a carrier for drugs), which can facilitate their application as a sensing, imaging or therapeutic agent. Combination of the intrinsic properties of these particles and specificity provided by the recognition ligands (i.e. antibodies, aptamers, peptides and etc.) increases the efficiency of these agents in targeted theranostic applications. Multimodal theranostic systems have been developed and became popular in biomedical research because of synergistic therapeutic and diagnostic functionality of the particles [1,2]. Therefore, new hybrid particles that are capable of demonstrating more than one functionality became the center of interest for such multimodal theranostic applications.

Among different types of particles, hollow micro-/nanostructures have been employed as one of the most favorable types of theranostic agents since they can offer more than one therapeutic or diagnostic modality, based on their tunable optoelectronic (optical-electronic is usually related to light absorption and scattering of the particles) properties and their high capacity to load imaging and therapeutic molecules (i.e. imaging dyes, drugs, genes and etc.) [3]. Their efficacy and optoelectronic properties can be easily adjusted by facile control of their size, density, volume, shell thickness and surface properties. Besides their optoelectronic properties, they can also be used as carriers for various molecules owing to presence of vacancies or voids in their hollow structure. This chemical storage and delivery ability enable them to be used in a broad range of research areas including, but not limited to, delivery of genes and drugs to the cells, imaging with loaded contrast agents, photonics, photovoltaics, supercapacitors, and hydrogen storage. On the other hand, their tunable particle shell thicknesses can adjust their optical properties to the near-infrared (NIR) region, or in other words, optical window of the spectrum where healthy tissues absorb the light minimally, which eases the optical imaging and therapeutic applications. Moreover, the hollow structures are feasible platforms to be coupled with other types of materials and form different hybrid particles for multimodal imaging, sensing or therapeutic applications. Each material that constitutes the composition of those hybrid particles contribute to the multimodal identity of the hybrid hollow structures by demonstrating their own beneficiary therapeutic or diagnostic features.

Particle properties are specified by their size and structures, and their specific applications are determined by their properties. Hollow structures can be fabricated in different shapes such as spherical, cubic, tube-like and etc. by using different templating techniques. On the other hand, based on the outer number of layers on their surface, different groups of hollow structures can be fabricated: a single, double or multi-walled hollow structure. Finally, organic and inorganic structures are categorized according to the composition of the shell structures. Upon tuning their properties using the available fabrication methods, they should be conjugated with specific targeting molecules such as antibodies or peptides like the other nano/micro particles, so that they can perform better mainly due to their higher uptake by the targeted tissue or organs.

In this review, we will summarize the most recent nano/micro hollow structures employed in theranostic applications. Different methods for functionalization of these structures will be discussed based on their specific application and a general map will be provided for design and potential application of new generation of hollow particles in a near future. In contrary to the other reviews about specific types of hollow particles [4–6] in this review, we tried to include various organic and inorganic hollow particles and summarize their therapeutic and imaging applications as well as the synergy provided with their multimodal theranostic applications and early clinical trials.

2. Biomedical applications of hollow micro/nanostructures

2.1. Hollow micro/nanostructures in drug delivery and chemotherapy

Hollow nanostructures are advantageous for drug delivery systems, due to their high surface to volume ratio and high porosity, which can be loaded with high doses of the drugs and biomolecules. Drugs can be encapsulated inside these hollow nanostructures and get released at targeted organs and tissues. On the other hand, functionalization of the external surface with specific targeting ligands enhances the efficacy of the drug delivery using hollow structures and may help to decrease the side effects of the drugs as well, due to their overall lower administration doses. These features make the hollow nanostructures unique and promising candidates for multifunctional drug delivery and chemotherapy. Following two sections is focused on organic and inorganic hollow micro/nanostructures utilized in drug delivery and chemotherapy.

2.1.1. Organic hollow micro/nanostructures

Organic hollow structures have been fabricated based on controlled polymerization or growth of graphite layers (e.g. carbon nanotubes or CNT and carbon nanospheres). Most of the recent researches have been focused on other carbon-based structures such as hollow polymers, carbon nanospheres and capsules as well.

2.1.1.1. Polymeric hollow micro/nanostructures.

Hollow polymeric micro/nanostructures have been widely used for drug delivery, because of the continuous advancements in their synthesis and functionalization, which results in their unique physicochemical properties, especially at the nanoscale [7]. These drug-loaded polymer-based hollow structures have been mainly designed for controlled release of the drugs by external stimuli such as temperature or pH. For example, Ke et al. [8] developed pH-responsive poly (D,L-lactic-co-glycolic acid) hollow particles (PLGA HPs) for delivery of doxorubicin (DOX, a common anti-cancer agent) into multi-drug resistance (MDR) cells (MCF-7/ADR) (Fig. 1B). The shell wall of PLGA HPs was loaded with DiO (a hydrophobic dye), and the aqueous core contained DOX and sodium bicarbonate, which generated carbon dioxide (CO₂) in acidic microenvironment of the tumors. The progressive acidification of the particles internalized into the late endosomes/lysosomes of the cancer cells generated CO₂ bubbles, leading to the disruption of HPs, prompt release of DOX, its accumulation in the nuclei, and finally the death of the MDR cells. Chen et al. [9] prepared hollow poly (acrylic acid) (PAA) nanogels delivery of multiple drugs in vitro on human gastric carcinoma cells (BGC-823). (Fig. 1A). They used bovine serum albumin (BSA), and DOX, as model drugs to investigate the loading efficiency of their nanostructures and found that their nanogels effectively loaded with high amounts of both molecules (e.g. 8.0 mg of BSA was successfully loaded into 1 mg of PAA nanogels). They also demonstrated sustained intracellular release of the drugs from PAA nanogels after their cellular internalization in BCG-823 cancer cells.

Fig. 1.

A) Schematic showing the synthesis of porous hollow PAA nanogels. Adapted with permission from Chen et al. [9]. Copyright © 2010, American Chemical Society. B) In order to overcome the MDR effect, the structure and composition of the pH-responsive PLGA HPs along with the drug loading and releasing mechanism should be accurately adjusted. When internalized by drug resistive MCF-7/ADR cells, the encapsulated NaHCO₃ reacts with the acidic environment in late endosomes/lysosomes to promptly generate CO₂ bubbles which disrupt the particle shell wall and release the drug into the cell. Adapted with permission from Ke et al. [8]. Copyright © 2010, American Chemical Society.

2.1.1.2. Hollow carbon nanospheres and capsules.

Hollow carbon spheres (HCSs), or carbon capsules, are micron or even nanometer sized hollow structures of carbon with thin shells [10] with the advantages of consisting of both microporous organic polymers and non-porous nanocapsules. Chen et al. [11] showed the synthesis of stable hollow carbon nanospheres (HCSs) to deliver DOX and usage of near-infrared laser irradiation to generate cellular reactive oxygen species that triggered cell death through the apoptotic pathway. In this study, laser irradiation stimulated the release of DOX from lysosomal DOX@ HCSs into the cytoplasm, leading to the nucleus penetration. Laser irradiation increased the drug release rate drastically by laser-induced heating which led the dissociation of strong interactions between DOX and the carbon capsules. Consequently, DOX@HCSs overcame the resistance of human breast cancer cells (MCF-7/ADR) to DOX due to the synergy between chemotherapy, photothermal and photodynamic therapies. Sun and Li [12] developed hollow carbonaceous capsules with reactive surface layers that could be coated via hydrothermal methods using sodium dodecyl sulfate (SDS) as an anionic surfactant and glucose as a precursor. Size and shell thickness of the voids were adjusted by altering the hydrothermal reaction parameters such as incubation time, temperature, and glucose and SDS concentrations. Xiao et al. [13] fabricated photo-switchable microcapsules based on “host-guest” interactions between α-cyclodextrin (α-CD) and azobenzene (Azo). Supramolecular structures using “host-guest” interactions were based on weak and reversible noncovalent interactions, such as hydrogen bonding, metal coordination, hydrophobic attractions, van der Waals forces, π–π stacking, as well as electrostatic interactions. α-CD-rhodamine B (α-CD-RhB) was loaded into the microcapsule by host-guest interaction (Fig. 2). Due to photosensitive interactions between α-CD and Azo, capsules could dissociate using UV light irradiation, then α-CD-RhB could be released. Compared with conventional approaches that are mostly based on chemical or physical bonds, this supramolecular approach seems to be more feasible due to its ideal bond strength and light sensitive performance.

Fig. 2.

Schematic showing the synthesis and degradation of the (PAA- C12-Azo)/(CMD-g-α-CD&α-CD modified drug) hollow microcapsules. The photoswitchable Layer-by-Layer (LbL) capsule utilized supramolecular interaction as the driving force for LbL assembling and drug loading. Adapted with permission from Xiao et al. [13]). Copyright © 2010, American Chemical Society.

2.1.1.3. Hollow Carbon Dots.

Carbon dots (CDs) are luminescent nanomaterials displaying unique physical properties that are ideal for bioimaging, biosensing and biomolecule/drug delivery applications [14]. Chenet al. used a solvothermal reaction to prepare hollow CDs (HCDs) from bovine serum albumin.[11]These HCDs had a diameter of 6.8 nm with pore sizes of 2 nm and had a quantum yield of 7%, resulting in bright photoluminescence that could be helpful for cellular imaging. They loaded these HCDs with DOX and demonstrated a pH-controlled drug release followed by the rapid uptake of the particles by A549 cells (adenocarcinomic human alveolar basal epithelial cells).

2.1.2. Inorganic hollow micro/nanostructures

2.1.2.1. Hollow silica materials.

Hollow silica particles have been extensively used for drug delivery applications owing to their advantages such as biocompatibility, chemical stability, and facile bio-conjugation via silane chemistry [15–18]. Wang et al. [19] reported the fabrication of monodispersed hollow mesoporous silica (HMS) nanocages synthesized through a template-coating-etching process (Fig. 3A). In this study, in vitro and in vivo results showed that DOX-loaded HMS nanocages were taken up by liver cancer cell lines (Hep-A-22) and released DOX to the tumors so that tumor growth rate could be reduced compared to the case where free DOX was introduced in vivo. In addition, Shen et al. [20] used hollow mesoporous silica nanospheres (HMSNs) for encapsulation of Bortezomib (BTZ), the first clinically approved proteasome inhibitor for treating multiple human malignancies (Fig. 3B). In vivo tumor-suppression analyses indicated that HMSNs-BTZ showed approximately 1.5 folds stronger anti-tumor activity compared to free BTZ treatment. Overall, these results suggested that the HMSNs-based nanoparticles can potentially be used as promising platforms to deliver therapeutic agents for desired clinical applications in which lowering the administration dosage and frequency of drugs are necessary to reduce potential side effects.

Fig. 3.

A) Synthesis of the hollow mesoporous silica (HMS) nanocages as vehicles drug delivery to liver cancer. CTAB: cetyltrimethylammonium bromide; TEOS: tetraethyl orthosilicate; APTMS: 3-aminopropyltriethoxysilane; PEG: poly-ethylene glycol; DOX: doxorubicin. Adapted with permission from Wang et al. [19]. Copyright © 2011, Royal Society of Chemistry. B) Schematic illustrating the loading, delivery and release of Bortezomib (BTZ) to the cancer cells by HMSNs. Adapted with permission from Shen et al. [20] Copyright © 2014, Royal Society of Chemistry. C) Schematic showing the structure of HMSNs-PDEAEMA and different mechanisms of triggered drug release. Adapted with permission from Zhang et al. [21] Copyright © 2015, Elsevier. D) (i) Fabrication of redox-triggered HMSNs by using a disulfide bond as the intermediate linker. (ii) Schematic illustration of the intracellular redox-triggered HMSNs for targeted in vitro and in vivo tumor therapy. Adapted with permission from Luo et al. [22] Copyright © 2014, Elsevier.

According to the previous studies, using stimuli-responsive structures for dosage-controlled drug delivery can overcome the drug resistance and increase the success rate of the chemotherapy. Thus, as another representative study, Zhang et al. [21] proposed triple-responsive (pH, reduction and light) nanocarriers utilizing hollow mesoporous silica nanoparticles (HMSNPs) modified with poly (2-(diethylamino)-ethyl methacrylate) (PDEAEMA) via radical polymerization (Fig. 3C). When DOX was loaded into the carriers, HeLa cancer cells were incubated with these nanocarriers, and fast drug release was achieved using a reducing agent, acidic environment or UV light irradiation. HeLa cancer cells internalized the nanocarriers and DOX was efficiently released into the cytoplasm under external UV light irradiation. Enhanced cytotoxicity against HeLa cells was also achieved. Also, hollow mesoporous silica nanoreservoirs that could release a cargo triggered by intracellular redox-stimuli were developed by Luo et al. [22] (Fig. 3D). Initially, adamantanamine was grafted onto HMSNs using a redox-cleavable disulfide bond as linker. Then, lactobionic, a synthetic functional molecule, acid-grafted-b-cyclodextrin (b-CD-LA), was immobilized on the surface of HMSNs by complexation with the adamantyl group, where b-CD served as a capping agent to maintain the drug within HMSNs. DOX-loaded nanoreservoirs selectively penetrated HepG2 cells via endocytosis and thereby releasing the drug into cytoplasm and leading to apoptosis. In vivo studies showed that DOX-loaded nanoreservoirs could infiltrate into the tumors and vigorously interact with tumor cells, which stopped the tumor growth. Limited side effects were reported in these studies.

In separate studies, chemical assembly of organic-inorganic hybrid hollow mesoporous organosilica nanoparticles (HMONs) was used for enhanced therapeutic functionality. Zhang et al. reported a multiple-hybridized HMONs with different functional organic groups [21]. These HMONs were modified with physiologically active thioether groups consisting three distinctive disulfide bonds and loaded with DOX so that they became intrinsic reducing-, acidic- and external high intensity focused ultrasound (HIFU)-responsive drug-releasing agents, offering improved in vitro and in vivo histocompatibility, controlled chemotherapy and ultrasonography capability. Fang et al. [23] developed a multi-purpose structure for simultaneous cancer chemotherapy and photothermal therapy. They used Pd nanosheet-covered hollow mesoporous silica nanoparticles. Hollow mesoporous silica core was used to load DOX, when the Pd nanosheets on the surface of particles were used to convert the near-infrared (NIR) light into heat for photothermal therapy. The loading of Pd nanosheets on hollow mesoporous silica nanospheres considerably amplified the rate of cell internalization (11 times higher than silica nanoparticles without Pd nanosheets). Rattle-type Fe₃O₄@SiO₂ hollow mesoporous spheres with various particle sizes, mesoporous shell thicknesses, and levels of Fe₃O₄ content were also prepared by Zhu et al. [24], using carbon spheres as templates. The spheres were able to quickly penetrate into cells without showing any cytotoxicity with increased concentrations of up to 150 μg mL⁻¹. DOX was loaded into the Fe₃O₄@SiO₂ hollow mesoporous spheres exhibiting enhanced cytotoxicity compared to free DOX.

2.1.2.2. Magnetic hollow structures.

Magnetic nanoparticles have been extensively used for drug delivery or as contrast agents for magnetic resonance imaging (MRI) [25,26]. Cheng et al. [27] reported a new approach for loading and controlled release of cisplatin (as an anti-cancer drug) using porous hollow nanoparticles (PHNPs) of Fe₃O₄ (Fig. 4A). The PHNPs were synthesized using controlled oxidation of Fe NPs at 250 °C followed by acid etching. The generated pores (~2–4 nm) enabled the cisplatin penetration into the hollow structure cavity. The porous shell was stable in neutral or basic physiological conditions. Cisplatin leaked from the cavity through the same pores by a slow diffusion-controlled process with a half-life time (t_1/2) of ~16 h. Nevertheless, in acidic (pH < 6) conditions, acidic etching increased the pore gaps and triggered the release of cisplatin with t_1/2 < 4 h. Also, herceptin was attached to the surface to allow the cisplatin-loaded hollow NPs to target SK-BR-3 breast cancer cells. This decreased the concentration to kill half of the cells (IC₅₀) of loaded cisplatin (2.9 μM) compared to free cisplatin (6.8 μM). He et al. [28] reviewed that assembly of Fe₃O₄ or γ-Fe₂O₃ nanosheets with high magnetization properties could be achieved through a novel precursor-templated conversion method. Their synthesis route enabled them to control hierarchy of the fabricated magnetic hollow spheres. They also demonstrated that, high doses of ibuprofen, an anti-inflammatory drug, could be loaded into these assemblies efficiently. Benyettou et al. [29] prepared mesoporous iron oxide nanoparticles (mNPs) following a modified nanocasting approach with mesoporous carbon as a hard template, and used the nanoparticles for combined chemo- and thermo-therapies. (Fig. 4B). The mNPs were loaded with DOX and then coated with the thermosensitive polymer Pluronic F108 to avoid the drug leakage from the mesoporous structure. DOX@F108-mNPs was stable at room temperature and physiological pH and released its DOX cargo slowly under acidic conditions or by magnetically induced heating. DOX@F108-mNPs were harmless to noncancerous cells, due to minimal internalization. However, the drug-loaded particles drastically reduced the viability of various cancer cell lines. Furthermore, using both DOX@F108-mNPs and subsequent alternating magnetic-field-induced hyperthermia was more efficient at killing HeLa cancer cells, compared with either DOX or DOX@F108-mNP treatment alone. Wu et al. [30] developed an in situ method to synthesize hollow magnetic and mesoporous double-shell nanostructures (HMMNSs) by decomposition and reduction of a β-FeOOH nanorods and organosilicate-incorporated silica-shell precursor. The surface of HMMNSs were then modified by rhodamine B isothiocyanate (RBITC) and poly (ethylene glycol) (PEG) chains covalently. It was demonstrated that surface modified particles showed minimal cytotoxicity against HeLa and MCF-7 cells and had outstanding blood compatibility. In addition, these nanocomposites displayed a high capacity for loading hydrophobic anticancer drugs such as docetaxel and camptothecin, due to their large surface area and pore volumes. Additionally, the drug-loaded nanocomposites exhibited superior cytotoxicity compared to free drugs. Xhing et al. [31] used the dopamine-plus-human serum albumin (HSA) process to build hollow iron oxide nanoparticles (HIONPs). DOX was encapsulated within the hollow porous structure. The HIONPs-DOX enabled pH-dependent drug release. The HIONPs-DOX were highly internalized by the multidrug resistant OVCAR8- ADR cells compared to free DOX, therefore were more efficient in killing these cancer cells. The dual mode, drug delivery and MRI imaging using the HIONPs-DOX formulation was demonstrated for both DOX-sensitive and DOX-resistant cancer cells.

Fig. 4.

A) Schematic illustration of simultaneous surfactant exchange and cisplatin loading into a PHNP and functionalization of this PHNP with Herceptin. Adapted with permission from Cheng et al. [27] Copyright © 2009, American Chemical Society. B) Schematic showing loading of DOX into the mesoporous particles and its subsequent heat- and acid-triggered release. Adapted with permission from Benyettou et al. [29] Copyright © 2016, Wiley & Sons.

2.1.2.3. Noble metal hollow structures.

Noble metal hollow structures have been used as versatile nanoplatforms for thermal therapy and drug delivery because of their easily tunable internal structures, shell compositions and surface modification. You et al. [32,33] reported hollow gold nanospheres (HAuNS, ~40-nm diameter) capable of both photothermal ablation of cancer cells and drug release upon near-infrared (NIR) light irradiation. Polyethylene glycol (PEG)-coated HAuNS was loaded with DOX both inside and outside the hollow structure. Photothermal conversion was induced by irradiation with NIR laser, thus triggering fast DOX release from the structure. DOX release was also pH dependent and could be enhanced in intracellular acidic environment. Cell toxicity was increased when NIR light was applied to the MDA-MB-231 cells incubated with DOX-loaded HAuNS because of the synergistic effect of HAuNS-mediated photothermal ablation and chemotherapy triggered by NIR irradiation and DOX release, respectively. Jang et al. [34] developed biocompatible hybrid hollow Au_Ag nanostructures using dextran-coated Ag nanoparticles that could offer two simultaneous therapeutic modalities. Carbonyl groups of the oxidized dextran alcohols were used for surface modification of these particles. These functional groups were also for conjugation of DOX to the particles via Schiff base (imine) mechanism. On the other hand, these nanoparticles could be used for photothermal heat generation in response to near-infrared light, which made these particles as potential candidates for combined chemo-thermotherapy of the tumors.

2.1.2.4. Hydroxyapatite hollow structures.

Hydroxyapatite (HA) nanocrystals are calcium-based materials with porous and hollow structure that have been frequently used for different biomedicine applications, particularly for the repair of hard tissues including bone and tooth [35]. Yang et al. [36] used an opposite ion core/shell strategy to synthesize pH-responsive, biocompatible and biodegradable hydroxyapatite nanoparticles constituted of a hollow core and a mesoporous shell (hmHANPs). The hollow and mesoporous structure showed enhanced drug-loading capability, while the hydroxyapatite shell allowed controlled drug release. DOX was loaded and the DOX release from hmHANPs was controlled by pH variation. The toxicity of DOX-loaded hmHANPs was drastically enhanced compared to free DOX on breast cancer cells (BT-20). Wang et al. [37] used a rapid microwave-assisted hydrothermal route to fabricate flower-like nanostructured hydroxyapatite hollow spheres (NHHS) formed as a result of self-assembly of nanosheets to shape a hierarchical morphology. The NHHS were used as anticancer drug carriers for cellular delivery of mitoxantrone (MIT). The MIT-loaded NHHS showed promising cell internalization and sustained-drug-release during in vitro tests.

2.1.2.5. Y₂O₃:Yb/Er hollow spheres.

In order to combine photothermal therapy (PTT) with chemotherapy for enhanced anticancer efficiency, Lv et al. [38] designed a multifunctional material consisted of Cu_xS nanoparticles attached onto the surface of Y₂O₃:Yb/Er hollow spheres. This was achieved through a combined coprecipitation and subsequent hydrothermal route (Fig. 5). By varying the pH during the synthesis of precursors, fine tuning of the size as well as structural properties of the final composites could be achieved. Conjugation of folic acid (FA) added cell recognition properties to the system in addition to their intrinsic photothermal functionality. Drug release from the carrier was triggered by both pH change and near-infrared (NIR) irradiation. Especially, laser irradiation (at 980 nm) was able to achieve both PTT and chemotherapy simultaneously. The synergistic therapeutic effect led to great cytotoxicity in vitro and highly strong inhibition of animal H22 tumor in vivo.

Fig. 5.

Schematic representation showing the design strategy of Y2O3:Yb/Er-CuxShollow spheres synthesis, where Y(OH)CO3:Yb/Er spheres were synthesized by a coprecipitation process, were used as the sacrificed precursor to prepare Y2O3:Yb/Er hollow spheres, calcined, and then, folic acid (FA) and CuxS nanoparticles were conjugated to recognize the cancer cells and act as PTT agent, respectively (I); and their use as potential candidate to bioimaging, chemotherapy, and PTT carriers simultaneously (II). Adapted with permission from Lv et al. [38]. Copyright © 2015, American Chemical Society.

2.2. Hollow micro/nanostructures in gene delivery

Gene therapy is a technique using nucleic acid (such as plasmid, Minivector DNA or siRNA) as medicine to repair defective genes which are responsible for genetic disorders [39]. The success of gene therapy is largely dependent on the development of an ideal delivery system that can selectively and efficiently deliver genetic materials to target cells without causing any associated pathogenic effects. Therefore, development of highly efficient nonviral gene delivery vectors still remains a great challenge. The biocompatibility and unique structures of hollow nanoparticles make them ideal to act as biomolecule carriers.

2.2.1. Organic hollow micro/nanostructures

Organic hollow nanostructures synthesized via self-assembly method could be used in various in vitro and in vivo applications due to their physiochemical features such as, size tunability, bio-stability and compatibility, high drug loading capacity and stimuli-controlled delivery of the biomolecules.

2.2.1.1. Polypeptide hollow spheres.

Elastin-like polypeptide (ELP) is one of the suitable candidates for gene delivery because of its biodegradable, non-toxic, non-inflammatory properties and efficient pharmacokinetics for the delivery of therapeutics. Dash et al. [40] reported the fabrication of monodispersed ELP hollow spheres with tunable sizes of 100, 300, 500 and 1000 nm by utilizing the self-assembly affinity and net positive charge of ELP molecules. The robustness and stability of the hollow spheres was further increased by the microbial transglutaminase (mTGase) cross-linking. Moreover, the DNA release could be controlled via protease and elastase treatment. Additionally, better cell viability was obtained with polyplex-loaded hollow spheres compared with the polyplex alone, due to efficient protection against endosomal degradation while encapsulated in spheres.

2.2.1.2. Cyclodextrin hollow nanospheres.

Fan et al. [41] studied self-assembly of b-cyclodextrins (b- CDs) and poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (pluronic F127) hollow nanospheres for gene delivery (Fig. 6). These hollow nanospheres enabled loading of PEI10K/DNA, which resulted in high gene delivery capability in the absence or presence of serum, as well as enhanced cytotoxicity.

Fig. 6.

Synthetic procedure to obtain F127NH₂bCD hollow nanospheres by self-assembly F127NH₂ (pluronic F127) and bCD (b-cyclodextrins) and the structure and encapsulation process of PEI10K/DNA complexes. The resulting F127NH₂bCD/(PEI10K/DNA) were demonstrated to enable similar or more efficient gene transfection in comparison with PEK10K/DNA. Adapted with permission from Fan et al. [41] Copyright © 2011, Kirsten Severing.

2.2.2. Inorganic hollow micro/nanostructures

2.2.2.1. Hollow mesoporous silica structures.

Wu et al. [42] developed rattle/hollow mesoporous silica/organosilica nanovehicles (R/HMSVs or R/HMOVs) using salt-assisted acid etching (SAAE) strategy. This technique allowed to overcome the general drawbacks of silica etching methods, namely undesirable by-products, by alkaline etching and strong corrosion of the HF etching process. High cargo-loading capacity and pH-responsive drug releasing behavior were achieved due to the hollow structure and phenylene-bridged framework of HMOVs based on the special cargo-framework interaction. Particularly, the molecularly organic-inorganic hybrid HMOVs have been constructed to concurrently deliver anticancer drugs and P-gp-associated shRNA molecules for increasing the intracellular drug concentrations and overcoming the multi-drug resistance (MDR) of cancer cells. Ma et al. [43] designed a co-delivery system of drug/siRNA utilizing hollow mesoporous silica nanoparticle (HMSNP), which allowed targeted cancer therapy and overcame multi-drug resistance (MDR) in cancer cells. The perpendicular nanochannels connecting to the internal hollow cores were responsible for the drug loading and release from the HMSNPs. In order to add specificity to the HMSNPs, their surfaces were coated with polyethyleneimine-modified folic acid (PEI-FA) through electrostatic interactions between the positively charged amino groups of PEI-FA and negatively charged phosphate groups on the HMSNP surfaces. This also blocked the mesopores and prevented drug leakage. PEI-FA-coated HMSNPs demonstrated enhanced siRNA binding capability and pH-controlled release due to the electrostatic interactions. In order to prove the efficiency of the targeted pH-responsive drug/siRNA co-delivery, the modified HMSNPs were applied to HeLa and MCF-7 cell lines with high and low folic acid receptor expressions, respectively. Effective targeted co-delivery of DOX and siRNA to the HeLa cells was demonstrated and thereby the anti-apoptotic protein Bcl-2 was successfully silenced. Chen et al. [44] developed hollow silica nanocapsules with precisely tunable pore sizes using a reversible Si–O bond breakage and reformation process under mild conditions (e.g., Na₂CO₃ solution). Those pores were excellent for siRNA-loading as well as in vitro intracellular transfection. Furthermore, superparamagnetic nanoparticles could be doped inside those large pores to add MRI imaging properties to these silica nanocapsules. This also demonstrated the importance of hybrid particles that can combine different theranostic modalities. Wu et al. [45] also employed large pore-sized hollow mesoporous organosilica nanoparticles (HMONs) fabricated through their selective bond breakage method. They demonstrated that surface-functionalized HMONs could successfully co-deliver P-gp (responsible for effluxing of drugs and other foreign biomolecules) modulator siRNA and anticancer drug DOX to overcome the multidrug resistance of cancer cells.

2.2.2.2. Calcium phosphate nanoshells.

Calcium phosphates can naturally be found in skeletons and teeth of vertebrate animals, and therefore are considered to be biocompatible and biodegradable [46]. Therefore, calcium phosphate [Cax (PO4)yOHz] particles have extensively been explored for their potential biomedical applications. Roy et al. [47] synthesized calcium phosphate nanospheres to encapsulate DNAs for targeted gene delivery. The nanoshell form of calcium phosphate was reported by Schmidt and Ostafin [48], who used liposomes as templating agents to obtain shell thickness of 2–10 nm around a liposome/water core of 45–100 nm in diameter. After encapsulation of pSVbGal plasmid DNAs inside calcium phosphate nanoparticles, they were delivered to different body tissues and in vivo expression of beta-galactosidase could be detected. This demonstrated the efficacy of calcium phosphate nanoshells as drug delivery vehicles.

2.2.2.3. Hollow manganese oxide nanoparticles.

Hollow Manganese oxide (MnO) nanoparticles have recently been explored as a new T₁ MRI contrast agent, due to their high surface area-to-volume ratio and water accessibility through the pores [49]. Bae et al. [49] employed hollow manganese oxide nanoparticles (HMON) for both cancer diagnosis via MRI imaging and therapy through targeted siRNA delivery. HMONs were functionalized with Herceptin, a therapeutic monoclonal antibody with the aid of polyethylenimine-DOPA (3,4-dihydroxy-l-phenylalanine) for specific cancer detection and therapy using T₁-weighted MRI and intracellular delivery of siRNA, respectively.

2.3. Hollow micro/nanostructures in thermal therapy

2.3.1. Optical tunability of nanoshells

Nanoshell particles can be potentially used in various biomedical applications such as bio-imaging and photothermal therapy, due to their photoelectric characteristics, in which the electron motions can be controlled via the exposure of the photons. This is mainly because of higher absorption rate of these nanoshells at the selected optical wavelengths, especially in therapeutically desired optical windows (i.e. near-infrared or NIR range of the electromagnetic spectrum, 650–1350 nm). Adjustability to absorb NIR-radiation is advantageous in many non-invasive clinical applications, due to the high penetration depth (up to ~10 cm) and minimal scattering of the NIR photons in tissues [50–53].

Nanoshells or nanohollow particles with absorption bands (Surface Plasmon Resonance (SPR) peaks) in NIR region were first synthesized by coating silica particles with gold shells in different thicknesses [54]. Gold nanoshells were synthesized by deposition of small gold particles (1–2 nm in size) on silica bead previously prepared with Stöber method. These silica beads were first modified with amine groups (-NH₂) using 3-aminopropyltriethoxysilane to help ultrasmall gold particles getting tethered to their surfaces by electrostatic interactions.

The thickness of the gold shell can be altered by adjusting the chemical reduction of gold on silica bead surfaces [55,56] and was feasibly used to tune the SPR bands of these nanostructures to the desired wavelengths. Generally, the SPR bands demonstrated red shift by decreasing the shell thickness. These absorption bands were highly sensitive to even small changes in the shell thickness. Even one nm decrease in the shell thickness can result in about hundred nm of red shift in SPR bands as it can be seen in Fig. 7 (i.e. between t = 2 and t = 3) [57].

Fig. 7.

The extinction spectra calculated for different thicknesses of gold nanoshells on silica cores using DDA method. The diameter of the silica core is fixed at 60 nm while the shell thickness varies from 2 to 15 nm. Adapted with permission from Hu et al. [57]. Copyright © 2006 Royal Society of Chemistry.

2.3.2. Thermal therapy using nanoshells

Generally, when plasmonic nanostructures such as nanoshells are irradiated with NIR lasers, they can convert the absorbed light into heat (i.e. photothermal effects) due to electron-phonon collisions. This photothermal phenomenon results in apoptosis of the malignant cells such as cancers [58] since cells are highly susceptible to temperature variations, and temperatures above 42 °C can be lethal for them [59]. Nanoshells can be delivered to tumor cells either by “passive targeting” utilizing enhanced permeability and retention (EPR) effect or via “active targeting” using biomolecules such as antibodies, peptides and aptamers conjugated or loaded to the nanoshell surface [35,60].

Nanoshells were first used for photothermal ablation of the tumors in 2003 by Hirsch et al., where human breast epithelial carcinoma cells (SKBr3) were incubated with nanoshells in vitro and then irradiated with NIR light (cells were exposed to NIR light (coherent, 820 nm, 35 W/cm²) for 7 min) [61]. In this passive targeting experiment, cells that weren’t incubated with nanoshells were also irradiated with NIR laser as a control. After the incubations all the cells were stained with Calcein AM (causes viable cells to fluoresce green), and a circle of cell death was observed for the nanoshell-treated cells matching with the spot size of the laser and there was no cell death for the cells treated with the laser only (Fig. 8). In addition, fluorescein dextran dye that was impermeable to healthy cells, penetrated the cells treated with nanoshells, suggested the ruptures in the membranes of the nanoshell/laser treated cells as the main cell death mechanism (Fig. 8).

Fig. 8.

Calcein AM staining to show cell death (a, b), and fluorescein dextran dye cellular uptake to show cell membrane destruction (c, d) after photothermal therapy using nanoshells. Adapted with permission from Hirsch et al. [61]. Copyright © 2003, The National Academy of Sciences.

Passive targeting has limited capability to deliver sufficient levels of nanoparticles or drugs to the tumor sites due to lack of specificity, which decreases the efficiency of therapy dramatically [62]. Therefore, in order to increase the specific delivery of nanoshells to cancers, their surfaces should be modified with targeting ligands such as antibodies. In order to maintain the specificity, Loo et al. employed anti-HER2 antibody-conjugated nanoshells to target human epidermal growth factor receptor 2 (HER2), which is over-expressed on human breast carcinoma cells [63]. On the other hand, nanoshells were also conjugated to a nonspecific control antibody (anti-IgG) as a negative control. Breast carcinoma cells were then incubated with these antibody-functionalized nanoshells, following by their irradiation with NIR laser. Irradiated cells were stained with calcein AM and circular area of cell death corresponding to the spot size of the laser beam was observed only for the cells incubated with the positive control nanoshells (anti-HER2 antibody-conjugated nanoshells) (Fig. 9). There was no cell death for the cells incubated with nonspecific anti-IgG-conjugated nanoshells or for the cells only irradiated with NIR without any other incubation. Note that these nanoshells were designed to be used as dual functional imaging and therapeutic agents. Efficient scattering and absorption of the NIR irradiation enabled these nanoshells to be used for both dark field imaging and photothermal therapy of cancer cells, respectively.

Fig. 9.

Combined imaging and therapy of SKBr3 breast cancer cells using anti-HER2-conjugated nanoshells. Dark field imaging of the negative (left and middle) and positive controls (right, top row). Cell viability test via calcein AM staining (middle row), and silver stain imaging of nanoshell binding (bottom row). Adapted with permission from Loo et al. [63]. Copyright © 2005, American Chemical Society.

Multi-functional nanoshells are highly beneficial due to their ability for simultaneous diagnosis and therapy. In addition, besides combination of imaging and therapy, nanoshells can be designed to embody more than one type of imaging or therapeutic functionalities within a single platform owing to the unique optoelectronic properties of the plasmonic particles. Liang et al. conjugated gold nanoshells with an anticancer drug (doxorubicin, DOX) using a pH-dependent biodegradable copolymer thiol-poly (ethylene glycol) and used them for synergistic targeted chemotherapy and photothermal therapy [64]. They used A54 peptides for targeting hepatocarcinoma cell lines, which also enhances the cellular uptake. The nanoshells were designed to release DOX in acidic environment of the intercellular organelles (endosomes or lysosomes). In addition, under NIR laser irradiation, the targeted nanoshells generated heat due to their photothermal effects. This double-armed strategy enhanced the efficiency of the nanoshells to kill targeted cancer cells. Coughlin et al. developed gadolinium-conjugated gold nanoshells as multi-functional cancer imaging and photothermal therapy agents [65]. Their gadolinium-conjugated gold nanoshells could enhance contrast for magnetic resonance imaging (T₁-weighted), X-Ray, optical coherence tomography, reflectance confocal microscopy, and two-photon luminescence. Moreover, the photothermal response of their nanoshells ablated the B16–F10 melanoma cancer cells under NIR irradiation which was further confirmed by calcein AM staining (Fig. 10). Overall, their results suggested the possibility of image-guided photothermal ablation of cancer cells using effective multimodal imaging contrast agents based on nanoshell design.

Fig. 10.

(A) Gadolinium-nanoshells (Gd-NS) effectively ablated B16–F10 melanoma cells after particle incubation under NIR irradiation (808 nm, 35 W/cm², 3 min). Fluorescent viability staining was performed with calcein AM and ethidium homodimer-1, which show live and dead cells in green and red, respectively. The red area shows dead cells only limited to the irradiation zone. (B) Cells irradiated under the same conditions with no prior particle incubation remained viable. Non-irradiated cells incubated (C) with and (D) without particles also remained viable. Scale bar = 300 μm. Adapted with permission from Coughlin et al. [65]. Copyright © 2014, Small.

Gold nanoshells were also conjugated with cholesteryl succinyl silane (CSS) nanomicelles loaded with doxorubicin and Fe₃O₄ magnetic nanoparticles in a separate study. This CSS-DOX-Fe₃O₄–Au-shell complex was used as a multifunctional nanoplatforms for magnetic resonance imaging (T₂-weighted MRI) and magnetic-targeted drug delivery due to presence of magnetic nanoparticles, light-triggered drug release, and photothermal therapy based on surface Plasmon of the gold nanoshells [66]. As shown in Fig. 11, the highest therapeutic efficacy of these nanoplatforms (i.e. the lowest survival rate of the cells after treatments) happened when both NIR irradiation and a magnetic field were applied to the system, leading to simultaneous magnetic-targeted drug delivery and photothermal therapy. Therefore, such multifunctionality seems to help improving the performance of these nanoshells for cancer diagnosis and therapy, owing to the benefits of each therapeutic or diagnostic modality and the targeting strategies used to enhance the cellular uptake.

Fig. 11.

Cell viabilities after incubation of HeLa cells with different dosages (0.05 μM, 0.5 μM, 5 μM) of DOX, CDF (CSS-DOX-Fe₃O₄) nanomicelles, and CDF-Au-shell nanomicelles, with or without a magnetic field and laser irradiation (808 nm, 4 W/cm², 10 min). The CF-Au and CDF-Au samples had the same iron concentrations. M and L represent the magnetic field and laser, respectively. The data are shown as mean ± standard deviation (SD), n = 3. Adapted with permission from Ma et al. [66]. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gold nanoshells have also been used as photothermal therapy agents in early clinical trials. Only one FDA approved gold nanoshell particle platform that provides photothermal therapy could progress to clinical trials. PEGylated gold nanoshell particles (AuroLase(TM) and AuroShell, Nanospectra Biosciences Inc.) were injected to patients intravenously for the photothermal ablation of head and neck cancers using FDA approved NIR lasers [67]. According to the clinical trials that were updated lastly in February 2017, the selected participants (11 patients) were divided into three groups, in which they were treated with three different laser intensities. These groups were named as AuroShell-3.5, 4.5 and 5.0, where the participants were treated with the laser powers of 3.5, 4.5 and 5.0 W and AuroShell particles in the dosages of 4.5, 7.5 and 7.5 mL/kg, respectively [67].

As shown in Table 1, the study was unfortunately limited due to early termination of the treatment by the patients, which led to the analysis of small numbers of participants and therefore the data is unreliable or uninterpretable. Even it is hard to attribute the possible adverse device effects that the participants experienced to only the AuroShell particle administration (Table 2).

Table 1.

Participant flow for the overall study of the AuroShell particle treatment during the time frame of 6 months [67].

	AuroShell-3.5	AuroShell-4.5	AuroShell-5.0
STARTED	5	5	1
Day 1	5	5	1
Day 2	5	5	1
Day 3	5	5	1
Day 8	5	5	1
Day 15	4	5	1
Month 1	4	5	1
Month 2	1	5	1
Month 3	1	4	1
Month 4	1	4	1
Month 5	1	3	1
Month 6	1	3	1
COMPLETED	1	3	1
NOT COMPLETED	4	2	0
Death	1	2	0
Other Therapy	3	0	0

Table 2.

The number of participants with any adverse device effects that can be considered to be attributable to AuroShell particle administration [67].

Measured Values	AuroShell-3.5	AuroShell-4.5	AuroShell-5.0
Participants Analyzed [Units: Participants]	5	5	1
Number of Participants with any adverse device effects considered attributable to AuroShell Particle administration [Units: Participants]	2	2	0

According to these results, more clinical trials with a higher number of participants are required to interpret the fate and reliability of the nano shells in therapeutic applications. Thus, it is quite early to have statistically supported conclusions.

Hollow or semi-hollow magnetic nanoparticles have also shown applications in magneto-hyperthermia when placed in a in a in a magnetic field where high heat generation is induced by magnetic fields. via mag. As demonstrated in section 2.1.2.2, using both DOX@F108-mNPs and subsequent alternating magnetic-field-induced hyperthermia was more efficient at killing HeLa cancer cells, compared with either DOX or DOX@F108-mNP treatment alone [29]. Thus, thermal therapy (either photon or magnetic induced) can still be combined with other therapies quite efficiecntly. Urries et al. [68] synthesized SPIONs-SiO₂ (superparamagnetic iron oxide nanoparticles coated with silica) with gold nano shells. These magneto-plasmonic particles possess the capability of being used as an T2 MRI agent, due to the presence of SPIONs in their core. Also, the gold shell provides the region required for conjugating drugs to the inner core and can be used for drug delivery, chemotherapeutic purposes as well as photothermal therapy. These structures have the optical properties of gold nano shells in NIR as well as magnetic properties of SPIONs when placed in presence of an external magnetic field. In a separate study, Rhodamine was absorbed onto the etched silica shell and in the presence of laser light irradiation a fast release of drug was observed from the irradiated samples. Goswami et al. [69] synthesized hollow nanospheres using a new mechanism, and found that hollow nanospheres performed better than solid nanoparticle counter-parts. They have a good absorbent quality, as well as the hollow region can be used for loading materials such as drugs. Cytotoxicity tests were conducted using the synthesized particles in the presence of AC magnetic field. A549 cancer cell line was used for this experiments and up to 50% cell death was observed in cancer cells, which was more than control PBMC cells. These results showed efficacy of these particles for hyperthermia therapy applications.

2.4. Hollow micro/nanostructures for diagnosis and bioimaging

Below, different types of magnetic and optical properties of the hollow micro/nanostructures will be discussed to show how they can be used for the imaging and diagnosis of cancer cells. Hollow nanoparticles have emerged as important diagnostic tools during the last two decades as they provide controlled pore volume and shell thickness that can be used for different biomedical purposes [16,70–72]. These nanostructures were beneficial as they had unique intrinsic optoelectronic properties and offered high surface area-to-volume ratios that could be used to entrap a variety of drugs, dyes (optical labels), nanoparticles and biomolecules for controlled release or imaging [71]. In this section, we reviewed the recent advances of hollow nanocarriers in biomedical applications with emphasis on diagnostics [17,73–76].

2.4.1. Hollow micro/nanostructures for diagnosis using photo acoustic tomography (PAT) and optical coherence tomography (OCT)

Hollow gold (Au) nanocages were developed as imaging and therapeutic agents for tumors since AuNPs (gold nanoparticles) have several advantages for biomedical applications. AuNPs offer several advantages such as chemical stability, biocompatibility, and facile labeling via Au-thiolate chemistry [70,77]. Optical properties of Au make it a suitable candidate for optical coherence tomography (OCT) because metallic nanoparticles such as those of gold, silver, copper are capable of being OCT contrast agents, where the image reconstruction mainly comes from scattering and absorption of light by tissues. OCT can act as a high-resolution imaging technology for medical sample examination. Au nanocages were used as an efficient optical contrast agent as it absorbed the light strongly in the near infrared region (~5 times higher magnitude than indocyanine green, ICG), which has been the most widely used contrast agent [70,78,79]. As another example, polypyrrole coated nanoparticles have a high absorption in NIR regions which makes them potential candidates as polymer-based OCT contrast agents. Mono-dispersed nanoparticles of size near 100 nm were synthesized for cancer cell imaging based on this approach and showed efficacy of such polymer-based contrast agents for OCT as a result of their strong contrast generation around 1300 nm [80].

PAT utilizes both optical and imaging techniques as it measures changes in ultrasonic waves caused by thermal expansion of biological specimen due to absorption of light and provides high spatial resolution [81]. Au nanocages provided large cross-sectional absorption area for photons that further aided the photothermal effect so that the efficient heat conversion enhanced the contrast between the targeted object and its surrounding environment. PAT images were taken after three consecutive injections of PEGylated Au nanocages that showed gradual increment in the optical contrast up to 81% in the cerebral cortex of the rats, justifying efficiency of Au nanocages as contrast agents. Also, in vitro studies were conducted in breast cancer cells where Au nanocages labeled with antibodies were used as photothermal contrast agents [82]. Other biomedical applications based on diagnosis and therapy involved the use of hollow micro or nano-swimmers that could be activated by applying low strength rotating magnetic field. These Spirulina like nanostructures were superparamagnetic in nature, possessed ultrahigh surface area and permitted diffusion-based drug loading/release. These nano-swimmers adopted to form individual nanoparticles by using ultrasound waves that made them suitable tools for in vitro and in vivo targeted drug delivery.

In a separate study, silica nanospheres were fabricated through shell by shell assembly to be used as an imaging and delivery vehicle. The in vitro assessment of delivery of silica nanospheres loaded with doxorubicin, an anti-cancer drug, could be done in cell nucleus by fluorescence [19]. A Bifunctional hollow nanostructure could be synthesized using the shell of BaMoO₄:Pr³⁺ without a template in the diameter of 17.5 nm through solution chemistry. These porous nanostructures were fluorescent in nature that could demonstrate red emission at 643 nm, when excited in the range of 430–500 nm, so that they could be used in imaging and diagnostics [83].

2.4.2. Hollow micro/nanostructures for magnetic resonance imaging (MRI)

In recent years, theranostic agents attracted enormous interest since they could combine both imaging and therapeutic functions within a single entity [44,84,85]. MRI has always been a major focus in the area of contrast imaging and diagnostics [26,86]. Due to magnetic properties of iron oxide nanoparticles (IONPs), they have been used in various applications such as waste water treatment, lithium ion batteries [74, 87], drug delivery [74] and MRI [88,89]. Iron oxide nanoparticles (IONPs) were the first particles used as T₂ MRI contrast agents, due to their superparamagnetic properties and high magnetic moments compared to other contrast agents such as magnetic metal complexes [86,90]. Multifunctional IONPs were also developed that could transport drugs and provide contrast for MRI [91]. On the other hand, labeling IONPs with fluorescent molecules further helped to investigate the internalization of nanoparticles inside the cancer cells. For this purpose, nanocapsules were functionalized with polymers such as polyethylenimine (PEI) and polyacrylic acid (PAA) to control the drug release trends. Some groups also reported PVP-modified silica/Fe₃O₄ core/shell nanoparticles based on their stimuli responsive behavior for drug release [61]. At the time of delivery to the target site, very high frequency magnetic field was generated and the biomolecules were released due to expansion of shell due to alternating magnetic field. Recently, hollow nanoparticles being used for various applications such as FePt- CoS₂ yolk—shell nanoparticles were synthesized by Kirkendall effect using FePt/Co core/shell nanoparticles and showed seven fold higher cytotoxicity as compared to cisplatin than Pt [88,92]. The nanoshells entered through the endocytic pathway where oxidation reaction led to release of Pt⁺². This Pt⁺² entered inside the nucleus and mitochondria lead to cell death after binding with DNA. In this line, bifunctional FePt–Fe₂O₃ yolk-shell nanocarriers were prepared to increase the cytotoxicity to enhance the MRI contrast image [79]. This provides a promising area where surface functionalization with specific biomolecules, can help to target tumor cells or tissues for the detection or transformation of cancer stages by noninvasive procedure by MRI [93].

Nanocomposites consist of hollow gold nanospheres and IONPs have great potential to be used in theranostic applications since these multifunctional nanomaterials could offer outstanding optical and magnetic properties for bioimaging purposes [94,95]. For example, a remarkable higher contrast for T₂-weighted MRI and enhancement of signals of photoacoustic imaging were shown in in vitro studies for those composite particles. These studies also indicated that the combination of MRI and photoacoustic imaging with photothermal therapy could be used as an effective tool in multimodal theranostic applications [79,96–98]. Different shapes of these nanomaterials were also available (i.e. rod, cage, shell and star) [96,99–101] which allowed them to have tunable near-infrared (NIR) absorption that offered minimal invasiveness for the tissues and cancer cells [95,102]. Over the years much efforts have been made to develop multifunctional nanocomposites consisting of gold or iron core-shell materials [95,103]. To the best of our knowledge there is no report on hollow gold nanosphere and magnetic hybrid nanomaterials related to its application in photothermal therapy (PTT) and MRI [104]. Thus, besides bioimaging, hollow hybrid nanostructures were useful as carriers that could be loaded with biomolecules and drugs so that combinatorial effect could be seen for PTT and chemotherapy [105–108].

2.4.3. Hollow micro/nanostructures for near-infrared (NIR) imaging

Hollow conjugated gold (Au) micro/nanostructures have been used for NIR imaging mostly based on their surface enhanced Raman spectroscopy (SERS) signal. Most SERS substrates operate in a typical range of molecular Raman scattering throughout the near-infrared and visible regions (~400–1000 nm). Two important parameters should be considered when using these nanostructures for SERS imaging: molecular species to be detected (the probe) and the metallic structure on which the molecules are absorbed (the SERS substrates). The most used structures for SERS and plasmonic surfaces are generally gold (Au) and silver (Ag) nanoparticles, because they show suitable optical quality. Plasmon resonances on metal film surfaces is usually based on electromagnetic interaction of light with metals. The substrates (molecules) must be attached on metal surface or be very close (around 10 nm) to the surface to receive enough plasmon resonances throughout the visible/second IR range that is almost 400–3000 nm. The structures with sizes in sub-wavelength range and sub-100 nm are more suitable for this approach.

Therapeutic nanoparticles, which were designed for photodynamic diagnosis, have been known as innovative breakthrough through personalized nanomedicine [109–112]. These nanoparticles were employed as both imaging and therapeutic agents [90,109,111,113]. When used as therapeutic agents, these materials enable photothermal effect based on their surface plasmon resonance/nonradiative transitions [90,114,115]. Their photo conversion abilities make a significant contribution to fluorescence or photoacoustic (PA) imaging. These capabilities are also used for photothermal therapy (PTT) or photodynamic therapy (PDT), which provide important superiority of simple composition, single wavelength excitation, and excellent in vivo theranostic performance [116,117]. One of the therapeutic nanoparticles having this conversion capability is semiconductor metal sulfide nanoparticles involving CuS and Ag₂S nanocrystals. Nowadays, these crystals have been broadly examined for PTT and fluorescence imaging based on their enhanced near-infrared (NIR) absorbance, better photobleaching protection and effective photoconversion efficiency [118–120]. Ag₂S is more favorable compared to other semiconductor materials (Fig. 12), which demonstrate narrowband-gap (~1.5 eV), and poor photoconversion into a thermal effect or reactive oxygen species [118–120]. These hollow nanodots have been prepared with distinct nanostructures for fluorescence applications in the NIR and second NIR (NIR-II) regions [121–124]. However, large scale fabrication of these materials for applications such as cancer theranostics is still a major challenge, as shown in schematic below.

Fig. 12.

a) Schematic synthesis illustration of theranostic Ag₂S nanodots, b) in vivo applications of NIR-II fluorescence/photoacoustic imaging and photothermal therapy, and c) Fluorescence mapping of Ag2S-NDs in the NIR-II region at various concentrations. Adapted with permission from Yang et al. [125]. Copyright © 2017 American Chemical Society.

NIR-SERS spectra can be detected in different living cells and tissues, for example, in pig tissues, as shown in Fig. 13(a and b), at different injection depths up to 8 mm. In separate studies, three types of NIR SERS dots encoded with 4-BBT, 4-CBT and 4-FBT Raman reporters were injected into a mouse subcutaneously and SERS spectra were collected at the injection sites to evaluate multiplex detection in a live animal model. Measurements showed that NIR SERS dots produced strong individual SERS signals without any interference from deep tissues. These NIR SERS dots can therefore have great potential for multiple detection of specific target molecules in vivo.

Fig. 13.

a) Schematic illustration for depth profiling by using a 785-nm laser beam. b) Normalized SERS intensity and SERS spectra of NIR SERS dots encoded with 4-CBT as a function of injection depth. c) Setup used for micro-Raman measurements in a mouse (top-photo) and in vivo multiplex detection (bottom-photo) of subcutaneously injected NIR-SERS dots. d) SERS spectra obtained after injecting NIR SERS dots into the mouse. Adapted with permission from Kang et al. [126]. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Also, NIR-SERS dots have been proven to exhibit broad adsorption band from plasmonic surfaces prepared using Ag nanoshell (400–1400 nm) [127]. Indeed, the enhancement of the E - field of this rough Ag nano structure proved to be four times stronger than smooth Ag NPs with SERS enhancement factor 6.4 × 10⁵ [128]. Raman label markers such as 4-CBT and 4-BBT were then fixed on these Ag–Au SS-NPs to provide single particle level SERS activity. Such a strong SERS activity in particles with different structural and physical properties can provide opportunities for multimodal diagnosis.

In addition to SPR (surface plasmon resonance), LSPR (localized surface plasmon resonance) is another terminology especially for the small metallic nanoparticles in which the size becomes comparable or smaller than the irradiation wavelength. The LSPR is obviously dependent on the metal, environment through its dielectric constant, and the size of the sphere. The LSPR of typically Au/Ag metal nanostructures originates from regular variation in magnitude or position around an incident light in which the collective oscillations occurred by conduction electrons in response to the incident light. The shape and size of nanostructures and the medium surrounding them are significant factors determining the LSPR peak position. According to basic calculations, it is indicated by Birnboim et al. that LSPR signals having tunable wavelengths on a broad range of the spectrum have been obtained by hollow metal nanostructures involving nanoshells, and nanocages. Au nanoshells based on depositing Au onto nanosilica beads were fabricated first by Halas and co-workers with tunable LSPR peaks in the near infrared region. They demonstrated that the LSPR peaks of Au nanoshells can be manipulated from 700 to 1050 nm by reducing the shell thickness from 20 to 5 nm. Additional to those developments, Skrabalak, and co-workers synthesized Au nanocages, which consist of hollow porous walls based on redox reaction between silver and HAuCl₄. The alterations of the LSPR peak positions were proportional to the volume of the HAuCl₄ solution used for synthesis. It is indicated by theoretical calculations that the resonance peak position is highly dependent on shell thickness; however, both the scattering and the absorption cross-sections are very sensitive to the pores of the nanocages [129].

Hollow gold nanostructures which show extremely large scattering and absorption cross sections can be implemented for photo acoustic imaging and photo thermal therapy in biomedical technologies. The optical properties of hollow nanostructures made of oxides have also been examined by Zeng and co-workers. It was also proven by Zhang and co-workers that the optical properties of Cu₂O nanoshells are simply adapted in the visible range with tunable inner and outer dimensions of the nanoshells [130]. The color change was based upon the geometry-dependent light absorption and scattering. Also, the optical properties of Cu₂O nanoshells at various thicknesses, which verify the experimental extinction spectra, were calculated by the Mie scattering theory. Optical signals based upon resonant Mie scattering of hollow nanostructures attract much interest. One fabrication method for polystyrene silica hybrid particles through a polymerization technique was proposed by Fielding and co-workers [131]. By calcination process of PS core, hollow silica nanostructures were also prepared. A blue staining has been detected resulting from reaction product if placed on a dark medium. The observed color is dependent on the diameter of silica that is approximately one-half the wavelength of visible light. It was reported by Ye et al. [132] that magnetite (Fe₃O₄) hollow spheres of 650 nm diameter and with 40 nm shell thickness show a bright green color, when compared to black magnetite solids (as seen Fig. 14a–d). The color could be altered with dimension and shape of the nanoparticles. It has been shown that the color is able to be explained by resonant Mie scattering rather than Bragg diffraction, verified by the dark ground microscopy and diffuse reflectance spectroscopy. The convenient thicknesses of hollow silica spheres could positively enhance the transport mean free path of light and also these structures prevent the resonantly scattered light from multiple scattering [133]. The fused particulate silica shells also exhibit visible Mie scattering phenomenon [131]. Same results have also been demonstrated for hollow titania spheres studied by Li et al. [134].

Fig. 14.

TEM images of the magnetite (a) hollow and (b) dense spheres. (c) hollow and (d) dense spheres are shown by photographs of powder samples. Adapted with permission from Ref. [132]. Copyright © 2009 AIP Publishing LLC …

3. Conclusions and remarks

We summarized various techniques for fabrication and functionalization of hollow nano/micro structures. Also, major applications of these materials as potential candidates for diagnostic and therapeutic applications were discussed. Major studies are limited to in vitro applications, with some promising in vivo and clinical translations. Targeting and delivery of these particles seem to be major areas that should be further studied in future works. Also, toxicity, degradability and body clearance of these materials are still unknown which require additional considerations. Addressing these challenges, will enable researchers to further take advantage of the unique physical and chemical properties these materials offer for sensitive diagnostic and effective therapeutic applications.

Highlights.

• Hollow particles have unique and tunable optoelectronic properties with high loading capacities for multimodal therapeutics.

• Their tunable physical structures and so their optoelectronic properties are suitable for different bioimaging modalities.

• Imaging-guided targeted therapeutics demonstrated higher success rates so multimodal theranostic platforms are desired are amore.

• The theranostic applications and surface modifications of hollow particles fabricated with various routes were summarized.