Seth C. Burkert

Nano-gold corking and enzymatic uncorking of carbon nanotube cups

Because of their unique stacked, cup-shaped, hollow compartments, nitrogen-doped carbon nanotube cups (NCNCs) have promising potential as nanoscale containers. Individual NCNCs are isolated from their stacked structure through acid oxidation and subsequent probe-tip sonication. The NCNCs are then effectively corked with gold nanoparticles (GNPs) by sodium citrate reduction with chloroauric acid, forming graphitic nanocapsules with significant surface-enhanced Raman signature. Mechanistically, the growth of the GNP corks starts from the nucleation and welding of gold seeds on the open rims of NCNCs enriched with nitrogen functionalities, as confirmed by density functional theory calculations. A potent oxidizing enzyme of neutrophils, myeloperoxidase (MPO), can effectively open the corked NCNCs through GNP detachment, with subsequent complete enzymatic degradation of the graphitic shells. This controlled opening and degradation was further carried out in vitro with human neutrophils. Furthermore, the GNP-corked NCNCs were demonstrated to function as novel drug delivery carriers, capable of effective (i) delivery of paclitaxel to tumor-associated myeloid-derived suppressor cells (MDSC), (ii) MPO-regulated release, and (iii) blockade of MDSC immunosuppressive potential.

Oxidative Unzipping of Stacked Nitrogen-Doped Carbon Nanotube Cups

We demonstrate a facile synthesis of different nanostructures by oxidative unzipping of stacked nitrogen-doped carbon nanotube cups (NCNCs). Depending on the initial number of stacked-cup segments, this method can yield graphene nanosheets (GNSs) or hybrid nanostructures comprised of graphene nanoribbons partially unzipped from a central nanotube core. Due to the stacked-cup structure of as-synthesized NCNCs, preventing complete exposure of graphitic planes, the unzipping mechanism is hindered, resulting in incomplete unzipping; however, individual, separated NCNCs are completely unzipped, yielding individual nitrogen-doped GNSs. Graphene-based materials have been employed as electrocatalysts for many important chemical reactions, and it has been proposed that increasing the reactive edges results in more efficient electrocatalysis. In this paper, we apply these graphene conjugates as electrocatalysts for the oxygen reduction reaction (ORR) to determine how the increase in reactive edges affects the electrocatalytic activity. This investigation introduces a new method for the improvement of ORR electrocatalysts by using nitrogen dopants more effectively, allowing for enhanced ORR performance with lower overall nitrogen content. Additionally, the GNSs were functionalized with gold nanoparticles (GNPs), resulting in a GNS/GNP hybrid, which shows efficient surface-enhanced Raman scattering and expands the scope of its application in advanced device fabrication and biosensing.

Corking Nitrogen-Doped Carbon Nanotube Cups with Gold Nanoparticles for Biodegradable Drug Delivery Applications.

Carbon nanomaterials have been proposed as effective drug delivery devices; however their perceived biopersistence and toxicological profile may hinder their applications in medical therapeutics. Nitrogen doping of carbon nanotubes results in a unique “stacked‐cup” structure, with cups held together through van der Waals forces. Disrupting these weak interactions yields individual and short‐stacked nanocups that can subsequently be corked with gold nanoparticles, resulting in sealed containers for delivery of cargo. Peroxidase‐catalyzed reactions can effectively uncork these containers, followed by complete degradation of the graphitic capsule, resulting in effective release of therapeutic cargo while minimizing harmful side effects. The protocols reported herein describe the synthesis of stacked nitrogen‐doped carbon nanotube cups followed by effective separation into individual cups and gold nanoparticle cork formation resulting in loaded and sealed containers.

Uncondensed Graphitic Carbon Nitride on Reduced Graphene Oxide for Oxygen Sensing via a Photoredox Mechanism

Melon, a polymeric, uncondensed graphitic carbon nitride with a two-dimensional structure, has been coupled with reduced graphene oxide (rGO) to create an oxygen chemiresistor sensor that is active under UV photoactivation. Oxygen gas is an important sensor target in a variety of areas including industrial safety, combustion process monitoring, as well as environmental and biomedical fields. Because of the intimate electrical interface formed between melon and rGO, charge transfer of photoexcited electrons occurs between the two materials when under UV (λ = 365 nm) irradiation. A photoredox mechanism wherein oxygen is reduced on the rGO surface provides the basis for sensing oxygen gas in the concentration range 300-100 000 ppm. The sensor response was found to be logarithmically proportional to oxygen gas concentration. DFT calculations of a melon-oxidized graphene composite found that slight protonation of melon leads to charge accumulation on the rGO layer and a corresponding charge depletion on the melon layer. This work provides an example of a metal-free system for solid-gas interface sensing via a photoredox mechanism.