(498d) Uncorking and Oxidative Decomposition Dynamics of Gold Nanoparticle Corked Carbon Nanotube Cups for Drug Delivery Studied Via in Situ Transmission Electron Microscopy

Nitrogen-doped carbon nanotube cups (NCNCs) are an intriguing material for drug-delivery applications due to their cup-shaped morphology and their propensity for chemical modification. NCNCs can readily uptake small molecules, which can be effectively sealed within the inner cavity through the formation of nanoparticles (NP) on the open end of the cup. The resulting nanocapsules can then undergo oxidative biodegradation through biologically available peroxidase enzymes, [1] effectively releasing their loaded cargo. This mechanism has been applied for Au NP-corked NCNCs loaded with chemotherapeutic molecules for immunotherapy in melanoma models.[2] However, a better understanding of the mechanisms of cargo release – through uncorking and/or nanotube degradation – is crucial for improving their effectiveness as drug delivery platforms. The environmental transmission electron microscope (ETEM) is a powerful tool to directly investigate these questions, combining sub-nanoscale spatial resolution with the ability to image Au NP-NCNCs under similar gas, thermal, and liquid environments as their intended applications.

In this work, we present our in situ ETEM study of the dynamic oxidation-dependent uncorking and degradation of Au NP-NCNCs. NCNCs were synthesized using a liquid (a mixture of xylenes, acetonitrile, and ferrocene) injection chemical vapor deposition method. The resulting stacked NCNCs were separated into short stacked segments [3,4] and corked with Au NPs through a sodium citrate reduction of hydrogen tetrachloroauric acid. TEM specimens were prepared from aqueous solutions of the NCNCs. We used a Hitachi H9500 ETEM equipped a homebuilt multi-species gas injection system and specialized sample heating holders to study the structure of the Au NP-capped NCNCs and the dynamics of their uncorking and subsequent degradation in situ.

Structural examination of the NP-capped NCNCs revealed a multilayered tube structure, typically with multiple internal compartments. The Au NP morphology consisted of either a cap (interacting only with the uppermost lip of the NCNC) or plug (extending deeper into the tube). Under vacuum, the corked NCNCs exhibited remarkable tolerance to temperatures of up to 800 °C, though the internal structures of the carbon tubes exhibited restructuring at around 500 °C. The segmented cavities transformed from truncated cones in shape to spherical cavities, decreasing in apparent volume by 50-60%. This may be a result of increasing pressure on the walls from the encapsulated cargo as it is heated or due to thermally induced stresses. The Au NP caps were slowly "pulled" into the tubes at high temperatures (700-750 °C) while the Au plugs showed no meaningful change.

Upon exposure to ~10^-2 Pa of O₂, however, the ingrown Au plugs were ejected from the tubes at a temperature-dependent rate that increased 2-3 orders of magnitude from 400 °C to 800 °C. This uncorking was of a stepwise, punctuated nature, rather than continuous. Importantly, the onset of uncorking occurred prior to any observed oxidative degradation of the NCNCs, significant decomposition of which required temperatures of ~500 °C. The exterior walls and any interior walls of cavities that were exposed (e.g., from a breach) exhibited the same manner of attack. The oxidation primarily initiated at the rims (exterior wall) and cup bottoms (interior wall) of each of the NCNCs, progressing along the cup walls. Oxidation of the exterior walls typically proceeded at a slower rate and after a period of delay, attributed to the presence of an amorphous carbon layer on the outside of the tube, which could act as a sacrificial barrier hindering oxidation. In all cases, once oxidation began, it occurred along the entire length of the tube, with no preference for the ends. In contrast, interior cavities without an obvious breach exhibited an isotropic thinning of their walls, indicating a different mechanism was involved. This behavior is currently under investigation. It is important to note that the oxidative degradation of the NCNCs in aqueous solution may not necessarily proceed by the same mechanisms as these solid-gas reactions. For this reason, ETEM experiments are underway using an in situ liquid cell holder to study the uncorking and degradation reactions in the enzymatic solution environments. Surface chemistry changes are being explored by in situ X-ray photoelectron spectroscopy experiments to correlate with the gas-based ETEM structural changes. The knowledge gained from these studies will enhance the tailoring of the Au NP-corked NCNCs properties for a more effective therapeutic delivery.

[1] I Vlasova, et al., Toxicology Applied Pharmacology 299 (2016), p. 58.

[2] Y Zhao, et al., Journal of the American Chemical Society 137 (2015), p. 675.

[3] Y Tang, et al., Journal of Physical Chemistry C 117 (2013), p. 25213.

[4] Y Zhao, et al., ACS Nano 6 (2012), p. 6912.