Silica nanoconstruct cellular toleration threshold in vitro
Graphical Abstract
Treatment with three different silica nanogeometries, presented with limited geometric impact. Following uptake, a toleration threshold appeared. Native cell mechanistic processes may be utilized as defense mechanisms to compartmentalize the constructs.
Introduction
Silica nanoparticles are an appealing biomedical platform for nanomedicine since ease in physicochemical modification, economic affordability and potential for reasonably simplistic scale up provide the feasibility of rapid translation. This has led to an increased academic and industrial interest in the creation of new silica nanomaterials for therapeutic, diagnostic, prognostic and combinatory applications.
The chemistry and science of nanomaterials has drastically improved over the last several decades, facilitating the development of silica nanoparticles with significantly different physicochemical characteristics such as surface functionalization and alterations in geometry [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Silica nanoparticles can be engineered to facilitate controlled release rates, targetability, biocompatibility, and protective stability with which one can encapsulate therapeutics or contrast agents [1], [2], [8], [9], [10], [11], [12], [13]. New mesoporous silica chemistries have allowed for the development of nanoconstructs with variations in nanopore geometry [1], [2]. Such variations provide different mechanisms and path lengths for small molecule encapsulation, which is shown to significantly alter their diffusion patterns [1], [2]. Additionally, mesoporous silica can have stimuli sensitive capabilities, where encapsulated drug content can remain protected in the nanoparticle until it reaches the desired delivery site where release can take place via changes in the local environment such as pH, reactive species or magnetic fields [3], [4], [5], [6], [7]. Investigators have utilized capping agents such as gold nanoparticles, polymeric supports, dendrimers, cadmium sulfide and magnetic nanoparticles to facilitate encapsulation [3], [4], [5], [6], [7]. Additionally, the sol–gel chemistries can facilitate the enclosure of a variety of non-releasing molecular agents, such as organic dyes, iron oxide, quantum dots and gold [8], [9], [10], [11], [12], [13]. These doped materials can be utilized in optical, magnetic resonance or photonic imaging and evidence suggests an increase in biocompatibility and signal yields [14], [15], [16].
With the development of new nanoscale silica-based materials however, comes the necessity to fully understand how they interact with the biological environment to ensure safety. Evidence suggests that both the material's physicochemical properties and the cell type which the experiments are performed on alter the mode and mechanisms of induced cellular toxicity. For example, epithelial cells treated with silica nanoparticles show very little to no cytotoxic effects [17], while cells with longer population doubling times (i.e., fibroblasts), or phagocytic capabilities (i.e., macrophages and endothelial cell types) have a substantial increase in toxicity [18], [19]. Nanoparticle size, geometry and surface modification have also proven to alter uptake and toxicity patterns. For example, smaller nanoparticles are widely thought of as having an increased uptake and thus toxicity, mostly contributed to their increase in surface area and exposure to cell surfaces [20], [21]. Likewise, tubular silica nanostructures showed a decrease in uptake when compared to their spherical counter parts [22]. Mesenchymal stem cells have been shown to endocytose positively charged silica nanoparticles to a greater extent than their unmodified systems [23].
To provide validity to these existing and future investigations it will be imperative to correlate physicochemical properties with specified induced biological mechanisms in a systematic fashion. This can lead to the development of safer nanoconstructs, and further lead to the potential for effective manipulation of these properties to facilitate better bioengineered materials for use in nanomedicine. In this work, we set out to investigate the safety and biocompatibility of three silica nanoconstructs, namely spherical, worm-like and cylindrical nanoparticles. The induced biological mechanisms following in vitro treatment of these constructs on model epithelial and phagocytic cell lines is further elucidated.
Section snippets
Silica nanoparticle synthesis and characterization
Spherical silica nanoparticles, nanoworms, and nanocylinders were prepared utilizing previously reported modified Stober methods [24], [25]. Changes in geometry were facilitated by altering the ratios during synthesis of cetyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), sodium hydroxide (NaOH), aminopropyltrimethoxysilane (APTMS) and water respectively. The following ratios were used respectively to synthesize the worm-like (1.0:8.16:3.85:2.55:4857), cylinder-like
Particle synthesis and characterization
Three silica nanoparticle types with various geometries were synthesized; worms, cylinders, and spheres. Each construct had a common dimension of approximately 200 nm, with an average equivalent positive charge density and an approximate 20% variation in the size of the nanoparticle (Table 1, Fig. 1).
Variations in nanoparticle geometries were due to alterations in incorporated APTMS and CTAB ratios. CTAB is a surfactant and self associates at a critical concentration, facilitating the formation
Conclusion
This work demonstrates cell type dependent toxicity of cationic silica nanoparticles and illustrates limited geometric dependence in biological results across the two cell lines, and in the size range studied. Evidence collected suggests the existence of a cellular toleration concentration threshold and the involvement of autophagic processes in cellular coping mechanisms for silica nanoparticles.
Acknowledgments
This research was supported by the National Institutes of Health (R01-DE19050), National Science Foundation (NSF-NIRT: 0835342) and the Utah Science Technology and Research (USTAR) Initiative. The authors would also like to thank Nancy Chandler from the Health Sciences Center Research Microscopy Facility at the University of Utah for her help with transmission electron microscopy.
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