Formation of Platinum Nanocrystals on Silicon Nanotubes and In Vitro Anti-Cancer Activity of the Composites
Authors: Le Nguyen T , Akkaraju Giridhar R. , Coffer Jeffery L 
Published: February 2020

The semiconductor Silicon (Si) remains a significant material in the electronic device and photovoltaic industries [1]. Especially, nanostructured forms of Si with a porous morphology (pSi) exhibit interesting properties which can be controlled via modulating pore structure and surface chemistry [1]. Recently, synthesis of a unique one-dimensional form of Si, namely nanotubes, with tunable structure (shell thickness, length, inner diameter and porous morphology) has been demonstrated, thereby suggesting newly emerging applications [2]. For instance, recent works have indicated Si nanotubes (SiNTs) can efficiently serve as a reaction vessel for formation of organometal perovskite nanostructures and a template for superparamagnetic iron oxide (Fe3O4) loading [3], [4]. In an observation of dissolution of SiNTs with a porous morphology (pSiNTs), the material readily resorbed in buffered media at physiological conditions in a similar manner to bioactive nanostructured porous silicon, thereby implying potential therapeutic applications of this material [2].
In chemotherapy, platinum-based cancer drugs, such as cisplatin and carboplatin, are widely used as effective drugs against various types of cancer [5]. Interestingly, while elemental platinum nanoparticles (Pt NPs) have been well investigated in diverse catalytic processes, in recent years, Pt NPs have also been discovered as a potent anti-cancer agent in nanomedicine, implying the use of the nanodrug to counteract chemoresistance in some cancer cell lines [6], [7]. Recent reports have also indicated that enhanced cytotoxicity against selected cancer cell lines is ascribed to ultra-small Pt NPs, especially those with size less than 3 nm [7]. In this report, pSiNTs were investigated as a template for the formation of Pt NPs, and in vitro cytotoxicity of the composites was evaluated against HeLa cancer cells.
Regarding fabrication, pSiNTs with short lengths (~500 nm) and thin walls (~10 nm) were synthesized via a ZnO nanowire sacrificial template method (Fig. 1A). Based on a combination of characterization techniques [High resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray analysis (TEM-EDX)], it is suggested that pSiNTs surface functionalized with 3-aminopropyltriethoxysilane can facilitate formation of Pt nanocrystals (Pt NCs) with size ranging from 1-3 nm utilizing a K2PtCl4 precursor. By varying reaction conditions (concentration of Pt salt and incubation time), the amount of Pt NCs deposited on SiNTs can be sensitively tuned from 20 to 55 wt% (Fig. 1 B, C; Table 1). In terms of cytotoxicity evaluation of the composites against HeLa cells, cellular viability was assessed using CellTiter-Glo assays, which quantified the amount of ATP in metabolically active cells. Our findings suggest that Pt NCs-SiNTs composites were toxic to HeLa cells, and less than 50% cells were still viable after 3 days of treatment with the composites at doses of 35 μg/ml and 50 μg/ml (Fig. 2). Results from caspase 3/7 assays also showed that caspase 3/7 level in cells treated with Pt NCs-SiNTs approximately ranged from 1.5 to 2-fold increase compared to cells without treatment, thereby suggesting apoptosis as the likely mechanism. In vitro cellular uptake studies analyzed by confocal microscopy also confirmed accumulation of the composites within the cytoplasm of the cells after the treatment, consistent with a “Trojan horse” mechanism in which high concentrations of Pt NCs are internalized within cells assisted by pSiNTs and subsequently released via dissolution of the nanotube matrix.
The studies presented herein describe a novel strategy to form and immobilize highly compact clusters of Pt NCs by using pSiNTs as a template. In terms of bio-relevant applications, in vitro studies provide new insights into the anti-cancer properties of the newly discovered composites in inducing apoptosis in HeLa cells, thereby providing significant potential uses of Pt NCs-SiNTs in cancer treatment. Further investigations into gene expression profile(s) may be necessary in order to clarify the impact of the composites on cell survival in terms of molecular mechanisms.

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Figure 1