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

Quantification of amino groups on halloysite surfaces using the Fmoc-method

The functionalization of halloysite nanotube (HNT) surfaces with aminosilanes is an important strategy for their further decoration with organic molecules to obtain hybrid inorganic-organic nanoarchitectures to be used in catalysis and drug delivery. The exact quantification of amino groups on the surface is an important aspect in view of the obtainment of systems with a known number of loaded molecules. In the present study, we describe a simple and reliable method for the correct quantification of groups present on HNT surfaces after their reaction with aminopropyltriethoxysilane (APTES). This method, applied for the first time to HNT chemistry, was based on the use of Fmoc groups as probes covalently bound to APTES and quantified by UV-Vis after release from the HNT-APTES-Fmoc system. Interestingly, this method showed great accordance with the already employed quantitative thermogravimetric analysis (TGA), with some benefits such as simple and non-destructive procedure, besides the possibility to monitor the deprotection reaction

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

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