No abstract

To utilize carbon nanotubes (CNTs) in various commercial and scientific applications, the graphene sheets that comprise CNT surfaces are often modified to tailor properties, such as dispersion. In this article, we provide a critical review of the techniques used to explore the chemical and structural characteristics of CNTs modified by covalent surface modification strategies that involve the direct incorporation of specific elements and inorganic or organic functional groups into the graphene sidewalls. Using examples from the literature, we discuss not only the popular techniques such as TEM, XPS, IR, and Raman spectroscopy but also more specialized techniques such as chemical derivatization, Boehm titrations, EELS, NEXAFS, TPD, and TGA. The chemical or structural information provided by each technique discussed, as well as their strengths and limitations. Particular emphasis is placed on XPS and the application of chemical derivatization in conjunction with XPS to quantify functional groups on CNT surfaces in situations where spectral deconvolution of XPS lineshapes is ambiguous.

As with all nanomaterials, a large fraction of the atoms in carbon nanotubes (CNTs) reside at or near the surface. Consequently, surface chemistry will play a crucial role in determining the fate and transport of CNTs in aquatic environments. Frequently, oxygen-containing functional groups (surface oxides) are deliberately grafted into the CNT surface to promote colloidal stability. To study the influence that both the oxygen concentration and the oxygen functional-group distribution have on the colloidal stability of multiwalled carbon nanotubes (MWCNTs), a suite of oxidized MWCNTs (O-MWCNTs) were created using different oxidizing agents and reaction conditions. Stable colloidal suspensions were prepared by low-power sonication of O-MWCNT powders in Milli-Q water. Results from TEM, AFM, DLS, and XPS measurements revealed that, irrespective of the surface chemistry, the colloidal suspensions were composed of individual nanotubes with comparable length distributions. The critical coagulation concentrations (CCC) of O-MWCNTs that exhibited different surface chemistries were measured with time-resolved dynamic light scattering (TR-DLS) using NaCl as the electrolyte. Over a range of environmentally relevant pH values, linear correlations were found to exist between the CCC, total oxygen concentration, and surface charge of O-MWCNTs. In contrast to surface charge, electrophoretic mobility did not prove to be a useful metric of colloidal stability. Information obtained from chemical derivatization studies, carried out in conjunction with XPS, revealed that the distribution of oxygen-containing functional groups also influences the colloidal stability of O-MWCNTs, with carboxylic acid groups playing the most important role. This study highlights the fact that quantitative relationships can be developed to rationalize the influence of surface chemistry on the behavior of nanomaterials in aquatic environments.

Grafting oxygen-containing functional groups onto carbon nanotubes (CNTs) by acid treatment improves their dispersion in aqueous solutions, but there is a lack of quantitative information on the colloidal properties of oxidized CNTs. We have studied the influence that pH and electrolytes have in determining the colloidal stability of oxidized multiwalled carbon nanotubes (O-MWCNTs), prepared by refluxing pristine MWCNTs in nitric acid. The acid-treated MWCNTs contained oxygen predominantly in the form of carboxyl groups. Colloidal suspensions of O-MWCNTs were prepared by low-power sonication and contained negatively charged, individual MWCNTs with an average length of approximately 650 nm. Time-resolved dynamic light scattering revealed that the aggregation rate of O-MWCNTs exhibited both reaction and mass-transport limited regimes in the presence of different electrolytes and as a function of pH. Particle stability profiles constructed from aggregation rate data allowed for the determination of critical coagulation concentrations (CCC), a metric of colloidal stability. The CCC values of O-MWCNTs varied with counterion concentration and valence in a manner consistentwith DLVO theory. Potentiometric measurements of surface charge correlated well with the observed pH-dependent variations in the O-MWCNT's colloidal stability. Electrophoretic mobility was also a diagnostic of particle stability, but only in neutral and acidic conditions.

Lubrication is key for the efficient function of devices and tissues with moving surfaces, such as articulating joints, ocular surfaces and the lungs. Indeed, lubrication dysfunction leads to increased friction and degeneration of these systems. Here, we present a polymer-peptide surface coating platform to non-covalently bind hyaluronic acid (HA), a natural lubricant in the body. Tissue surfaces treated with the HA-binding system exhibited higher lubricity values, and in vivo were able to retain HA in the articular joint and to bind ocular tissue surfaces. Biomaterials-mediated strategies that locally bind and concentrate HA could provide physical and biological benefits when used to treat tissue-lubricating dysfunction and to coat medical devices.