| Single-walled carbon nanotubes (CNTs) have unique properties and promising applications in a variety of fields including medical diagnostics and therapeutics. Recent doubts about the safety and efficacy of modern Gadolinium-based contrast agents (GBCAs) in magnetic resonance imaging (MRI) have led to a number of reports on novel alternatives to marketed GBCAs, including CNTs (Gd-doped CNTs). However, debate exists over the location and electron state of Gd, a matter of significance given the threat of Gd as a transmetallating agent. The present study is an investigation and discussion of the properties and application of these particles. Gd-doped CNTs were made by incubating acid/sonication-processed CNTs with Gadolinium Chloride hexa-hydrate (GdC1 3·6H2O) and washing to remove unreacted GdCl 3. Dodecylbenzene sulfonic acid sodium salt (NaDBS) was added to disperse the CNTs and to test its effects on both CNTs and Gd-doped CNTs. CNTs in water (ddH2O), CNTs in NaDBS, Gd-doped CNTs in ddH2O, and Gd-doped CNTs in NaDBS were imaged by Ultraviolet-Visual Absorption Spectrophotometry (UV/Vis) and Atomic Force Microscopy (AFM). Gd-doped CNTs in NaDBS had the least overall absorbance except in the UV range while CNTs in NaDBS had the highest, indicating that NaDBS dispersed the CNTs, as intended, but had quite the opposite effect on Gd-doped CNTs. Likewise, Gd-doped CNTs in ddH 2O were on the order of 3x larger than CNTs in ddH2O (12.02 nm and 3.95 nm, respectively) and Gd-doped CNTs in NaDBS were around 7x larger than CNTs in NaDBS (18.49 nm and 2.79 nm, respectively). Length was not a significant effect of the treatments. Electrochemical characterization was performed by Cyclic Voltammetry (CV) using Potassium Chloride (KC1) as a reference. Addition of NaDBS to Gd-doped CNTs caused a loss of ionic activity apparent in Gd-doped CNTs in water. The data suggests that, in water, Gd attaches to the CNT surface where it develops ionic bridges between CNTs. However, there is a strong indication that, in the presence of NaDBS, an entirely different effect takes place whereby the CNT is enveloped in a metallic Gd nanoparticle. By modeling the particle as a prolate spheroid, the crystalline Gd concentration per particle was found to be around 0.5 fmol for Gd and 0.2 fmol for Gd 2O3, although a mixture of the two is the most likely form. A great molar excess of GdCl3 (100 mM) was added to the CNTs (3.7 muM), which may explain the formation of such large particles. Both ionic and metallic Gd-doped CNTs may still be tenable GBCAs provided measures are taken to encapsulate the particles in biocompatible agents. Furthermore, these particles may also find use as X-Ray contrast agents where electron state is less important or as a neutron capture agent in cancer therapy due to the high absorptive radius of Gd atoms. Clearly, more research should be devoted to optimizing particle production for specific tasks. |
