Permeation into but not across the cornea: Bioimaging of intact nanoemulsions and nanosuspensions using aggregation-caused quenching probes

Combination of PEGylation and Cationization on Phospholipid-Coated Cyclosporine Nanosuspensions for Enhanced Ocular Drug Delivery

Strong precorneal clearance mechanisms including reflex blink, constant tear drainage, and rapid mucus turnover constitute great challenges for eye drops for effective drug delivery to the ocular epithelium. In this study, cyclosporine A (CsA) for the treatment of dry eye disease (DED) was selected as the model drug. Two strategies, PEGylation for mucus penetration and cationization for potent cellular uptake, were combined to construct a novel CsA nanosuspension (NS@lipid-PEG/CKC) by coating nanoscale drug particles with a mixture of lipids, DSPE-PEG2000, and a cationic surfactant, cetalkonium chloride (CKC). NS@lipid-PEG/CKC with the mean size ∼173 nm and positive zeta potential ∼+40 mV showed promoted mucus penetration, good cytocompatibility, more cellular uptake, and prolonged precorneal retention without obvious ocular irritation. More importantly, NS@lipid-PEG/CKC recovered tear production and goblet cell density more efficiently than the commercial cationic nanoemulsion on a dry eye disease rat model. All results indicated that a combination of PEGylation and cationization might provide a promising strategy to coordinate mucus penetration and cellular uptake for enhanced drug delivery to the ocular epithelium for nanomedicine-based eye drops.

OCTN2- and ATB0,+-targeted nanoemulsions for improving ocular drug delivery

Traditional eye drops are administered via topical instillation. However, frequent dosing is needed due to their relatively rapid precorneal removal and low ocular bioavailability. To address these issues, stearoyl L-carnitine-modified nanoemulsions (SC-NEs) were fabricated. The physicochemical properties of SC-NEs in terms of size, morphology, zeta potential, encapsulation efficiency, and in vitro drug release behavior were characterized. The cellular uptake and mechanisms of SC-NEs were comprehensively studied in human corneal epithelial cells and the stearoyl L-carnitine ratio in SC-NEs was optimized. The optimized SC-NEs could target the novel organic cation/carnitine transporter 2 (OCTN2) and amino acid transporter B (0 +) (ATB0,+) on the corneal epithelium, which led to superior corneal permeation, ocular surface retention ability, ocular bioavailability. Furthermore, SC-NEs showed excellent in vivo anti-inflammatory efficacy in a rabbit model of endotoxin-induced uveitis. The ocular safety test indicated that the SC-NEs were biocompatible. In general, the current study demonstrated that OCTN2 and ATB0,+-targeted nanoemulsions were promising ophthalmologic drug delivery systems that can improve ocular drug bioavailability and boost the therapeutic effects of drugs for eye diseases.

Recent advances in nanocrystal-based technologies applied for ocular drug delivery

INTRODUCTION

The intricate physiological barriers of the eye and the limited volume of eye drops impede efficient delivery of poorly water-soluble drugs. In the last decade, nanocrystals have emerged as versatile drug delivery systems in various administration routes from bench to bedside. The unique superiorities of nanocrystals, mainly embodied in high drug-loading capacity, good mucosal adhesion and penetration, and greatly improved drug solubility, reveal a promising prospect for ocular delivery of poorly water-soluble drugs.

AREAS COVERED

This article focuses on the ophthalmic nanocrystal technologies and products that are in the literature, clinical trials, and even on the market. The recent research progress in the preparation, ocular application, and absorption of nanocrystals are highlighted, and the pros and cons of nanocrystals in overcoming the physiological barriers of the eye are also summarized.

EXPERT OPINION

Nanocrystals have demonstrated success as glucocorticoid eye drops in the treatment of anterior segment diseases. However, the thermodynamic stability of nanocrystals remains the major challenge in product development. New technologies for efficiently optimizing stabilizers and sterilization processes are still expected. Strategies to confer more diverse functions via surface modification are also worth exploration to improve the potential of nanocrystals in delivering poorly water-soluble drugs to posterior segment of the eye.

Cannabidiol nanoemulsion for eye treatment - Anti-inflammatory, wound healing activity and its bioavailability using in vitro human corneal substitute

Cannabidiol (CBD) is the non-psychoactive component of the plant Cannabis sativa (L.) that has great anti-inflammatory benefits and wound healing effects. However, its high lipophilicity, chemical instability, and extensive metabolism impair its bioavailability and clinical use. Here, we report on the preparation of a human cornea substitute in vitro and validate this substitute for the evaluation of drug penetration. CBD nanoemulsion was developed and evaluated for stability and biological activity. The physicochemical properties of CBD nanoemulsion were maintained during storage for 90 days under room conditions. In the scratch assay, nanoformulation showed significantly ameliorated wound closure rates compared to the control and pure CBD. Due to the lower cytotoxicity of nanoformulated CBD, a higher anti-inflammatory activity was demonstrated. Neither nanoemulsion nor pure CBD can penetrate the cornea after the four-hour apical treatment. For nanoemulsion, 94 % of the initial amount of CBD remained in the apical compartment while only 54 % of the original amount of pure CBD was detected in the apical medium, and 7 % in the cornea, the rest was most likely metabolized. In summary, the nanoemulsion developed in this study enhanced the stability and biological activity of CBD.

Selection of an aggregation-caused quenching-based fluorescent tracer for imaging studies in nano drug delivery systems

In order to develop and optimize nano drug delivery systems (NDDSs), it is crucial to understand their in vivo fate. We previously found that P2 (Aza-BODIPY) and P4 (BODIPY) as aggregation-caused quenching (ACQ) probes could be used to unravel the biofate of various nanoparticles owing to their water-sensitive emission. However, previous studies also found that quenched ACQ probe aggregates showed repartition into hydrophobic physiologically relevant constituents, resulting in fluorescence re-illumination. In this paper, we screened various types of fluorophores for ACQ and their re-illumination performance and focused on Aza-BODIPY dyes. BODIPY and Aza-BODIPY dyes were identified to be advantageous over other fluorophores. Some BODIPY and Aza-BODIPY dyes were selected as potential probes with improved performance against re-illumination. The best performing probes were Aza-C7 and Aza-C8. Aza-C7-loaded PMs were found to have decreased fluorescence re-illumination properties over P2 and DiR.

Multi-functional chitosan copolymer modified nanocrystals as oral andrographolide delivery systems for enhanced bioavailability and anti-inflammatory efficacy

Modifying nanocrystals with functional materials have been common strategy to enlarge the enhancing ability on oral absorption via nanocrystals; however, whether the functional materials have played their full enhancing ability in oral absorption is still unknown. In this study, we synthetized a novel chitosan-based copolymer (the copolymer of sodium dodecyl sulfate (SDS), chitosan (CS) and D-α-Tocopherol polyethylene glycol 1000 succinate, SDS-CS-TPGS), and modified nanocrystals with this copolymer, aiming to enhance the oral absorption of polymer andrographolide (ADR). In real-time distribution study, we found the distribution of ADR, SDS, CS and TPGS varies in gastrointestinal tract, while the distribution of ADR and SDS-CS-TPGS was similar, revealing the SDS-CS-TPGS could able to participate in the absorption process of andrographolide timely. To explore the oral absorption enhancing ability of SDS-CS-TPGS, we prepared a series of nanocrystals modified with different materials and explored their pharmacokinetic performances on SD rats. The results showed the nanocrystals modified with SDS-CS-TPGS (S-C-TANs) exhibited the highest bioavailability, which could enhance the AUC_0-∞ of ADR from 1.291 mg/L*h to 5.275 mg/L*h (enhanced for about 4.09-folds). The enhanced anti- inflammatory efficacy was also found on ICR mice by employing ear swelling rate, TNF-α, IL-1β and IL-6 and pharmacodynamic index. These results indicated that modified with synthesized copolymer containing different functional stabilizers is an efficient strategy to enlarge the enhancing ability on oral absorption of nanocrystals.

Biological and Intracellular Fates of Drug Nanocrystals through Different Delivery Routes: Recent Development Enabled by Bioimaging and PK Modeling

Nanocrystals have contributed to exciting improvements in the delivery of poorly water-soluble drugs. The biological and intracellular fates of nanocrystals are currently under debate. Due to the remarkable commercial success in enhancing oral bioavailability, nanocrystals have originally been regarded as a simple formulation approach to enhance dissolution. However, the latest findings from novel bioimaging tools lead to an expanded view. Intact nanocrystals may offer long-term durability in the body and offer drug delivery capabilities like those of other nano-carriers. This review renews the understanding of the biological fates of nanocrystals administered via oral, intravenous, and parenteral (e.g., dermal, ocular, and pulmonary) routes. The intracellular pathways and dissolution kinetics of nanocrystals are explored. Additionally, the future trends for in vitro and in vivo quantification of nanocrystals, as well as factors impacting the biological and intracellular fates of nanocrystals are discussed. In conclusion, nanocrystals present a promising and underexplored therapeutic opportunity with immense potential.

Drug-peptide supramolecular hydrogel boosting transcorneal permeability and pharmacological activity via ligand-receptor interaction

Boosting transcorneal permeability and pharmacological activity of drug poses a great challenge in the field of ocular drug delivery. In the present study, we propose a drug-peptide supramolecular hydrogel based on anti-inflammatory drug, dexamethasone (Dex), and Arg-Gly-Asp (RGD) motif for boosting transcorneal permeability and pharmacological activity via the ligand-receptor interaction. The drug-peptide (Dex-SA-RGD/RGE) supramolecular hydrogel comprised of uniform nanotube architecture formed spontaneously in phosphate buffered saline (PBS, pH = 7.4) without external stimuli. Upon storage at 4 °C, 25 °C, and 37 °C for 70 days, Dex-SA-RGD in hydrogel did not undergo significant hydrolysis, suggesting great long-term stability. In comparison to Dex-SA-RGE, Dex-SA-RGD exhibited a more potent in vitro anti-inflammatory efficacy in lipopolysaccharide (LPS)-activated RAW 264.7 macrophages via the inhibition of nuclear factor кB (NF-κB) signal pathway. More importantly, using drug-peptide supramolecular hydrogel labeled with 7-nitro-2,1,3-benzoxadiazole (NBD), the Dex-SA-K(NBD)RGD showed increased performance in terms of integrin targeting and cellular uptake compared to Dex-SA-K(NBD)RGE, as revealed by cellular uptake assay. On topical instillation in rabbit's eye, the proposed Dex-SA-K(NBD)RGD could effectively enhance the transcorneal distribution and permeability with respect to the Dex-SA-K(NBD)RGE. Overall, our findings demonstrate the performance of the ligand-receptor interaction for boosting transcorneal permeability and pharmacological activity of drug.

Nanocrystals based mucosal delivery system: Research Advances

Nanocrystal technology is a new way to increase the solubility and bioavailability of poorly soluble drugs. As an intermediate preparation technology, nanocrystals are widely used in drug delivery for oral, venous, percutneous and inhalation administration, which exhibits a broad application prospect. By referring to the domestic anforeign literatures, this paper mainly reviews the preparation methods of nanocrystals for poorly soluble natural products and its application in the mucosal delivery for skin, eye, oral cavity and nasal cavity. This can provide the reference for the research and development of nanocrystal technology in natural product preparations.

Gastrointestinal lipolysis and trans-epithelial transport of SMEDDS via oral route

Self-microemulsifying drug delivery systems (SMEDDSs) have recently returned to the limelight of academia and industry due to their enormous potential in oral delivery of biomacromolecules. However, information on gastrointestinal lipolysis and trans-epithelial transport of SMEDDS is rare. Aggregation-caused quenching (ACQ) fluorescent probes are utilized to visualize the in vivo behaviors of SMEDDSs, because the released probes during lipolysis are quenched upon contacting water. Two SMEDDSs composed of medium chain triglyceride and different ratios of Tween-80 and PEG-400 are set as models, meanwhile Neoral® was used as a control. The SMEDDS droplets reside in the digestive tract for as long as 24 h and obey first order kinetic law of lipolysis. The increased chain length of the triglyceride decreases the lipolysis of the SMEDDSs. Ex vivo imaging of main tissues and histological examination confirm the trans-epithelial transportation of the SMEDDS droplets. Approximately 2%-4% of the given SMEDDSs are transported via the lymph route following epithelial uptake, while liver is the main termination. Caco-2 cell lines confirm the cellular uptake and trans-epithelial transport. In conclusion, a fraction of SMEDDSs can survive the lipolysis in the gastrointestinal tract, permeate across the epithelia, translocate via the lymph, and accumulate mainly in the liver.

Intracellular uptake of nanocrystals: Probing with aggregation-induced emission of fluorescence and kinetic modeling

Nanocrystal formulations have been explored to deliver poorly water-soluble drug molecules. Despite various studies of nanocrystal formulation and delivery, much more understanding needs to be gained into absorption mechanisms and kinetics of drug nanocrystals at various levels, ranging from cells to tissues and to the whole body. In this study, nanocrystals of tetrakis (4-hydroxyphenyl) ethylene (THPE) with an aggregation-induced emission (AIE) property was used as a model to explore intracellular absorption mechanism and dissolution kinetics of nanocrystals. Cellular uptake studies were conducted with KB cells and characterized by confocal microscopy, flow cytometry, and quantitative analyses. The results suggested that THPE nanocrystals could be taken up by KB cells directly, as well as in the form of dissolved molecules. The cellular uptake was found to be concentration- and time-dependent. In addition, the intracellular THPE also could be exocytosed from cells in forms of dissolved molecules and nanocrystals. Kinetic modeling was conducted to further understand the cellular mechanism of THPE nanocrystals based on first-order ordinary differential equations (ODEs). By fitting the kinetic model against experimental measurements, it was found that the initial nanocrystal concentration had a great influence on the dynamic process of dissolution, cellular uptake, and exocytosis of THPE nanocrystals. As the nanocrystal concentration increased in the culture media, dissolution of endocytosed nanocrystals became enhanced, subsequently driving the efflux of THPE molecules from cells.

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Nanocrystals of Tetrakis(4-hydroxyphenyl) ethylene (THPE), an aggregation-induced emission (AIE) probe was used as a model.

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THPE nanocrystals could be taken up in forms of dissolved molecules and nanocrystals.

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The dynamic process of dissolution, cellular uptake, and exocytosis of THPE nanocrystals was concentration-dependent.

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Exocytosis of intracellular THPE-NCs bore different kinetics and/or mechanisms compared with endocytosis.

Nanostructure of DiR-Loaded Solid Lipid Nanoparticles with Potential Bioimaging Functions

The fluorescence dye-loaded nanoparticles are widely used as bioimaging agents in the field of nanotheranostics. However, the nanoparticles for nanotheranostics usually consist of synthetic materials, such as metal, silica, and organic polymers, which are often biologically incompatible and may arouse toxicity issues. Herein, the potential of near-infrared probe DiR-containing solid lipid nanoparticle suspensions (DiR-SLNS) as the bioimaging agent, which was prepared by lipids and surfactants with excellent biocompatibility, was investigated in this study. The nanostructure of DIR-SLNS system and the distribution of DiR were studied by dissipative particle dynamics (DPD) simulations. The stability of physicochemical properties and fluorescence spectra of DIR-SLNS system were investigated using dynamic laser scattering (DLS), nanoparticle tracking analysis (NTA), and fluorescence spectra. The fluorescence intensity-concentration correlation of DIR-SLNS was also evaluated. As a result, DiR-SLNS demonstrated a "core-shell"-like nanostructure and DiR was mainly distributed in the cetyl palmitate (CP) core rather than the surface of SLNS, which was beneficial to its potential applications in bioimaging. DiR-SLNS exhibited remarkable physicochemical stability as the nanoparticles maintained ~ 90% fluorescence intensity during the 10-day storage time. The correlation between fluorescence intensity and concentration was established and validated using a linear regression model. This study proposed a type of promising candidates in nano-scale with higher safety and fluorescence stability for bioimaging.

Utility of Pickering emulsions in improved oral drug delivery

Pickering emulsions are surfactant-free emulsions stabilized by solid particles. Their unique structure endows them with good stability, excellent biocompatibility, and environmental friendliness. Pickering emulsions have displayed great potential in oral drug delivery. Several-fold increases in the oral bioavailability or bioaccessibility of poorly soluble drugs, such as curcumin, silybin, puerarin, and rutin, were achieved by using Pickering emulsions, whereas controlled release was found for indomethacin and caffeine. The shell of the interfacial particle stabilizers provides enhanced gastrointestinal stability to the cargos in the oil core. Here, we also discuss general considerations concerning particle stabilizers and design strategies to control lipid digestion.

Enhanced transdermal delivery of curcumin nanosuspensions: A mechanistic study based on co-localization of particle and drug signals

Nanosuspensions have received much attention in enhanced transdermal delivery. However, the corresponding mechanisms have not been clarified. In particular, whether nanosuspensions can directly penetrate across the stratum corneum (SC) and what is the transdermal route for the enhanced penetration. Therefore, curcumin (CUR) was adopted in this study as a model drug, while an aggregation-caused quenching (ACQ) probe was physically embedded in CUR nanosuspensions, i.e., the CUR hybrid nanosuspensions (CUR-HNSs), for bioimaging. The ACQ properties enable identification of intact CUR-HNSs. The co-localization of particle and CUR signals was exploited to outline the translocation profiles of intact nanosuspensions as well as the cargoes. Three sizes of CUR-HNSs are prepared, which are spherical and amorphous. CUR is poor in transdermal transport even in propylene glycol solution, which was enhanced by nanosuspensions. Although 400 nm CUR-HNSs present higher steady state flux than 140 nm and 730 nm ones, the cumulative amount of permeated CUR is yet less than 2% of the applied dose at 12 h. Co-localization of CUR and ACQ probe signals indicates that CUR-HNSs can infiltrate into the SC layer and accumulate in the hair follicles. The intact CUR-HNSs cannot enter into the skin. On the contrary, CUR molecules diffuse into the whole skin tissues following dissolution of CUR-HNSs in the SC and the hair follicles. In conclusion, nanosuspensions are advantageous for transdermal delivery of poorly permeable drugs by filtrate into the SC and accumulate in hair follicles.

The prominence of the dosage form design to treat ocular diseases

The eye is susceptible to various diseases commonly difficult to treat. To overcome the barriers imposed by this organ for required drugs penetration, technological strategies have been implemented to ocular formulations. Among them are the use of temperature or electric stimuli and the development of nanoparticles. The objective of this review is to present the main barriers to ocular drug delivery and to discuss strategies used in the development of ocular dosage forms, primarily for topical delivery, to increase the local bioavailability of drugs, target their delivery and increase patient compliance. Results obtained in the last years related to the topical administration of liposomes, dendrimers, iontophoresis, among other nanoparticulate systems focused on ophthalmic delivery, will be addressed. Finally, some clinical trials and marketed formulations that use nanotechnology to topically treat eye diseases will be presented.

Enhanced Oral Bioavailability of Felodipine from Solid Lipid Nanoparticles Prepared Through Effervescent Dispersion Technique

Felodipine (FLD), a dihydropyridine calcium channel blocker with excellent antihypertensive effect, is poorly soluble and undergoes extensive hepatic metabolism, which lead to poor oral bioavailability (about 15%) and limit its clinic application. The goal of this study was to develop solid lipid nanoparticles (SLNs) loading FLD to improve the oral bioavailability. The FLD loaded solid lipid nanoparticles (FLD-SLNs) were prepared by the effervescent dispersion technique developed by our laboratory, which might have some advantages over traditional methods. The FLD-SLNs showed desired particle characteristics with particle size (198.15 ± 1.82 nm), poly dispersity index (0.26 ± 0.02), zeta-potential (- 25.53 ± 0.60 mV), entrapment efficiency (95.65 ± 0.70%), drug loading (2.33 ± 0.10%), and a spherical appearance. Pharmacokinetic results showed that the FLD-SLNs presented 3.17-fold increase in area under the curve (AUC_(0-t)) compared with free FLD after oral administration in beagle dogs, which indicated that SLNs prepared using the effervescent dispersion technique can improve the bioavailability of lipophilic drugs like felodipine by enhancement of absorption and reduction first-pass metabolism.

What is the future for nanocrystal-based drug-delivery systems?

Nanocrystals are used as a drug-delivery platform for poorly water-soluble drugs and have had commercial success in oral drug delivery. We assert that the future of this technique is with cancer treatment and in the development of parenteral preparations. Advances in techniques for uniform and high-quality nanocrystals as well as deciphering the in vivo fate of nanocrystals are critical. The bottom-up technique allows for better control of particle properties, while the hybrid nanocrystal technique provides a novel approach to explore the in vivo fate of nanocrystals. Breakthroughs in these two techniques to further the development of nanocrystals are also discussed.

Preparation of high-efficiency near-infrared aggregation-induced emission nanoparticles based on FRET and their use in bio-imaging

Recently, the development of fluorescent probes has contributed to significant advances in cell biology and medical diagnostic imaging. In this work, we use biocompatible bovine hemoglobin (BHb) molecules to co-coat aggregation-induced emission (AIE) molecules amino tetraphenylethylene (TPE-NH₂) and near-infrared emission molecules 2-(4-aminophenyl)-3-(4-(4-(diphenylamino)styryl)phenyl) fumaronitrile (TPAADFN), to get TPE-NH₂/TPAADFN@BHb nanoparticles. Due to the fluorescence resonance energy transfer (FRET) between the two fluorescent molecules, the prepared fluorescent nanoparticles have high fluorescence quantum efficiency. The prepared TPE-NH₂/TPAADFN@BHb nanoparticles also have large Stokes shift, which helps to avoid the cross-talk between the absorption and emission of the particles themselves. This is beneficial to avoid the self-absorption of biological tissues and obtain very high detection sensitivity. Furthermore, due to the good biocompatibility of BHb, TPE-NH₂/TPAADFN@BHb nanoparticles have good mono-dispersity, low toxicity and high brightness, which is very propitious in the application of bio-imaging.

Fabrication of ultra-small nanocrystals by formation of hydrogen bonds: In vitro and in vivo evaluation

Poor water solubility and low bioavailability hinder the clinical application of about 70% of newly synthesized compounds. Nanocrystal technology has become a preferred way to improve bioavailability by improving solubility. However, it remains challenging to produce nanocrystals with ultra-small particle sizes to further enhance the extent of bioavailability. Herein, we constructed ultra-small puerarin nanocrystals (Pue-NCs) (20-40 nm) via formation of hydrogen bond during HPH. We confirmed the formation of hydrogen bonds by ¹H NMR and FTIR, and observed the distribution of polymer chains by SEM and TEM. The absorption mechanisms were studied in Caco-2 cell monolayers, and the results showed that the major transport mechanism for puerarin was passive diffusion, meanwhile, for Pue-NCs, the passive transport and micropinocytosis-mediated endocytosis coexisted. The absolute bioavailability of Pue-NCs was 35.28%, which was 11.54 folds compared to that of puerarin. Therapeutic equivalence was demonstrated between Pue-NCs and puerarin injection at 50 mg/kg and 15 mg/kg, respectively, in isoproterenol-induced myocardial ischemia model. This study provides a novel strategy for preparing ultra-small nanocrystals by HPH to increase bioavailability of poorly soluble drugs.

Hybrid drug nanocrystals

Nanocrystals show promise to deliver poorly water-soluble drugs to yield systemic exposure. However, our knowledge regarding the in vivo fate of nanocrystals is in its infancy, as nanocrystallization is simply viewed as an approach to enhance the dissolution of drug crystals. The dying crystal phenomenon inspired the development of hybrid nanocrystals by physically embedding fluorophores into the crystal lattice. This approach achieved concurrent therapy and bioimaging and is well-established to study pharmacokinetics and nanocrystal dissolution in vivo. Nanocrystals also offer the advantage of long-term durability in the body for interacting with biological tissues and cells. This review introduces the hybrid nanocrystal technique, including the theoretical concepts, preparation, and applications. We also discuss the latest development in self-discriminative hybrid nanocrystals utilizing environment-responsive probes. This review will stimulate further development and application of nanocrystal-based drug delivery systems for theranostic strategies.

Drug nanocrystals: Fabrication methods and promising therapeutic applications

The drug nanocrystals (NCs) with unique physicochemical properties are now considered as a promising drug delivery system for poorly water-soluble drugs. So far >20 formulations of NCs have been approved in the market. In this review, we summarized recent advances of NCs with emphasis on their therapeutic applications based on administration route and disease states. At the end, we present a brief description of the future perspectives of NCs and their potential role as a promising drug delivery system. As a strategy for solubilization and bioavailability enhancement, the NCs have gained significant success. Besides this, the function of NCs is still far from developed. The emerging NC-based drug delivery approach would widen the applications of NCs in drug delivery and bio-medical field. Their in vitro and in vivo fate is extremely unclear; and the development of hybrid NCs with environment-sensitive fluorophores may assist to extend the scope of bio-imaging and provide better insight to their intracellular uptake kinetics, in vitro and in vivo.