Helicene grafting on halloysite nanotubes for drug delivery: layer structure, surface selectivity and pH triggered drug release

Halloysite nanotubes (HNTs) have recently emerged as promising candidates for targeted drug delivery [1]. HNTs are low-toxic and low-cost aluminosilicate clays with nanotubular structure, presenting a positively charged Al(OH)3 inner lumen and a negatively charged SiO2 outer surface, which can support a selective functionalization of the two surfaces. In this work, we investigated the loading and release mechanisms of the tetrathia[7]helicene (7-TH) derivative linked via an imine bond to HNTs. The 7-TH scaffold displays promising intercalation properties for DNA, with a high degree of enantioselective recognition [2]. Moreover, a 7-TH derivative showed potent inhibitory activity against telomerase, demonstrating the great potential of 7-TH as therapeutic cytotoxic molecules [2,3]. We analyzed functionalized HNTs as well as Al2O3 and SiO2 layers, as models of the inner and outer surfaces, by means of surface-sensitive synchrotron-based techniques (XPS, UPS and NEXAFS spectroscopies). The oxide surfaces were analyzed both before and after functionalization with helicene derivatives through a (3-aminopropyl)triethoxysilane (APTES) linker [4]. Furthermore, the effect of a treatment in acidic conditions was investigated to prove the release of the helicene moiety from the oxide carrier at the extracellular pH of tumor cells. The surface state and atomic ratios of key elements within the organic layer determined by XPS proved the successful coupling of the helicene aldehyde to the APTES-functionalized films, clarifying differences in the reactivity of the two oxides. The sulfur peak confirmed the results obtained on the model films, supporting the reliability of the two adopted model surfaces. Moreover, NEXAFS results provided indication of a preferential orientation of helicene moieties at the oxide surface, which is lacking in APTES-functionalized layers. A further confirmation of the complete release of helicene moieties upon treatment in mild acidic conditions was given by NEXAFS spectra, showing a random orientation of the C and N functional groups after the release treatment. Preliminary in vitro toxicity tests on cancer cell lines characterized by different extracellular pH values show data consistent with a pH-triggered release of the 7-TH moiety, as also supported by kinetics data about the release in various physiological conditions. Work is currently under way to achieve a selective functionalization of the inner and outer surfaces by orthogonal functionalization strategies

Halloysite nanotubes as multifarious drug delivery systems : is selective functionalization possible?

Halloysite is an aluminosilicate clay, which naturally comes in the form of nanotubes. One of the most interesting fields of application of these materials is nanomedicine, where their peculiar morphology and low cytotoxicity can be exploited to deliver theragnostic agents to target tissues inside the organism [1]. Halloysite represents one of the rare nanotube systems with a different composition of the inner and outer surfaces, respectively composed by aluminum oxo-hydroxide and silica. This fairly unique structural ambivalence is a potent tool that can be exploited to modify separately the two surfaces, assigning them different tasks. Nonetheless, selective functionalization of halloysite nanotubes is scarcely investigated in the literature [2], as precisely defining the nature and location of molecule adsorption is a complex matter. To help sort this issue out, in this work surface modification of halloysite was carried out alongside the functionalization of purposely prepared model oxides mimicking the inner and outer surfaces, both in the form of powders and thin films. Phosphonic acids were chosen as functionalizing agents as they are known for adsorbing covalently on oxide substrates. Surface modification was followed by several techniques as FTIR, \u3b6-potential, BET and contact angle measurements, particularly relevant due to the presence of hydrophobic chains in the molecule [3]. The role of different parameters was investigated, changing impregnation times and solution pH, while also checking adsorption reversibility. Finally, the selective loading of gold nanoparticles, via a thiolated phosphonic acid linker, inside the inner lumen of halloysite nanotubes will be presented on the grounds of HR-TEM images

Halloysite nanotubes functionalization with phosphonic acids : role of surface charge on molecule localization and reversibility

Halloysite nanotubes (HNT) are aluminosilicates bearing an Al-OH terminated inner lumen and a Si-O-Si exposing outer surface, which hold promise in several research fields due to their intrinsic surface duality. Functionalization with octylphophonic acid (OPA) was here investigated as a means to achieve selective functionalization of the HNT lumen and pH-triggered release. Model oxides were adopted to investigate the role of chemical nature and surface charge on the adsorption mode and reversibility of the OPA bond to the surface. Beside silica and aluminium (oxo)hydroxide used to mimic respectively the outer and inner HNT surfaces, titanium dioxide was also studied due to its intermediate isoelectric point and surface acidity. The effect of the functionalization pH and OPA content, along with the pH-dependent adsorption reversibility, were investigated using both spectroscopic characterization and wetting determinations. Results on both model oxides and HNT support a preferential adsorption of OPA on the Al-OH exposing surface. Functionalized HNT retained their inner lumen porosity and water dispersibility, which are desired properties in terms of application. The specific character of the OPA-HNT interaction is discussed with respect to (oxo)hydroxides, particularly in terms of the pH-dependent adsorption reversibility

Phosphonic acids as selective functionalizing agents: a study on halloysite surface modification based on model oxides

Phosphonic acids are hetero-organic compounds bearing a C-PO(OH)2 group, which are known for adsorbing covalently on oxide substrates and can be used to create self-assembled monolayers. Their selectivity towards certain oxides [1] can be exploited for the selective functionalization of inherently dual systems, such as halloysite nanotubes. Halloysite is a polymorph of kaolinite which naturally wraps itself to form nanotubes. Its numerous fields of application range from polymeric nanocomposites with superior mechanical and thermal properties, to catalysis and drug delivery [2]. Halloysite is one of the few nanotubular systems presenting an inner lumen and an outer surface characterized by different surface charge and structural composition: the inner lumen exposes aluminum hydroxyl groups, while their outermost layer is silica. This characteristic structural ambivalence holds potential for the separate modification of the two surfaces, which can thus be assigned different tasks. However, precisely defining the nature and location of molecule adsorption is a complex matter and the selective functionalization of halloysite inner and outer surfaces has been scarcely investigated in the literature [3]. In this work, the surface modification of halloysite with octylphosphonic acid (OPA) was investigated together with the functionalization of purposely prepared model oxides mimicking the inner and outer nanotube surfaces. Evidence of the preferential location of the OPA molecules in the halloysite inner lumen was gathered by both comparative studies on the model oxides as well as direct measurements on the functionalized nanotubes. Furthermore, the effect of the surface charge of the oxide on the functionalization efficiency and reversibility was investigated in detail. The isoelectric point of the oxide plays a major role in determining a stable OPA adsorption, as proved by functionalization isotherms on the model oxides measured at different pH values. An amphiphilic oxide (TiO2) was also investigated as a reference [4]. The pH-triggered selective release of the adsorbed OPA molecules could be obtained and release conditions were determined for both the halloysite nanotubes and the model oxides. The combination of the inner location of the functionalizing agents and the pH-dependent reversibility of their adsorption make of the phosphonic acid-functionalized halloysite nanotubes promising candidates in numerous fields

Halloysite nanotubes as innovative carriers for (bio)organic molecules

The recent growth in nanoscale technology has led to advanced investigations of various types of nanocarriers. Among those, Halloysite nanotubes (HNTs) arrive as the best inorganic material for potential applications in a wide range of areas, including anticancer therapy, sustained delivery for certain agents, as a template or nanoreactor for biocatalyst. [1] HNTs are inexpensive, biocompatible and naturally occurring vehicles that exhibit high specific surface area and large aspect ratio. They are composed of rolled bilayer of aluminol (Al-OH) inner-layer and siloxane (Si-O-Si) outer-layer. The two mentioned layers exhibit different properties including the surface charge and potential reactivity. Specifically, the outer lumen surface is negatively charged while the inner lumen surface is positively charged and on average more reactive. Moreover, there is a significant content of hydroxyl groups present on defects and edges of the HNTs tubes. Taking into account mentioned properties HNTs exhibit prominent potential in various modifications and loading possibilities. The inner-lumen hence promoting the loading of negatively charged molecules while the outer surface opens the possibility for the adsorption of positively charged molecules and its relative functionalization. In our study we have explored the complete HNTs analysis, including physical characterization, investigation on the HNTs functionalization with several different bi-functional organic molecules and their loading capacities for certain pharmacological agents in the context of bladder cancer treatment. We characterized HNTs with respect to the length and inner-/outer-lumen diameter size, the specific surface area, the main pore size and its decomposition with respect to the temperature increase. Mainly we focused our attention on HNTs functionalization capacities taking into account the influence of naturally adsorbed water molecules on its outer surface. A series of functionalized HNTs have therefore been obtained and completely characterized by means of FT-IR, zeta potential, BET, TEM. We performed mimics of HNTs inner-/outer-lumen and compared their reactivity. In this communication we will show the results of the study of HNTs loading procedure, loading efficiency and drug release