Surface chemical composition of chromatographically fractionated graphite nanofiber-derived carbon dots
Introduction
A variety of emerging carbon-based materials (graphitic and amorphous) having dimensions in the lower-nanometer range, mostly grouped into carbon nanoparticles (CNPs), have continued to gain momentum as nascent nanomaterials for a variety of applications, including chromatographic stationary phases [1], drug delivery vehicles [2] and photothermal therapies [3]. Carbon dots (C-dots) and the closely-related graphene quantum dot [4] are perhaps most promising for their intrinsic luminescent properties for applications such as photocatalysis [5], bioimaging [6], and chemical sensing [7]. However, as the large number of reports on the synthesis of these materials grow, fundamental studies and understanding are lacking, which may hinder future applications and the direction of research in the field. We have demonstrated with various starting materials and synthetic conditions that the products obtained from C-dot syntheses can be very complex [6], [8], [9] with regard to both size and surface of the nanomaterial.
Although vastly underutilized, the application of high-resolution separation techniques such as high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE) can provide valuable and relatively quick information to allow a better understanding of the complexity of a mixture of C-dots while simultaneously gaining valuable insight such as on-line spectral information of individual C-dot components. This information can rapidly guide a synthetic protocol and allow for the collection of fractions of interest for further study or enhanced application [6], [8]. We have found that anion-exchange HPLC (AE-HPLC) allows for the high-resolution separation of many C-dot species with scale-up, fractionation capability and that hidden properties of C-dots can be found by studying the fractionated nanomaterial. While a mixture of C-dots reflects the average of the different properties of the various charge/size species present, many AE-HPLC fractions showed to be mostly single charge/size entities by high-resolution CE analysis. Many simplified fractions displayed unique properties that would have been missed by studying the unseparated mixture alone [6]. Considering the reports indicating that the surface properties of CNPs are directly related to their luminescence properties, both of the as-synthesized material [8] and surface-passivated products [4], it is of high interest to understand and accurately model the surface properties of C-dot fractions of interest. Herein, we report on the surface properties of the previously isolated fractions having relatively high photoluminescence. We used X-ray photoelectron spectroscopy (XPS) and Fourier transform-infrared (FTIR) spectroscopy to learn about the surface properties of the graphite nanofiber-derived C-dots separated by AE-HPLC. Our findings indicate that the fractionated GNF-derived C-dots with the higher quantum yields were those with less oxygen content.
Section snippets
Reagents
Nitric acid (65%) along with ammonium carbonate were obtained from Mallinckrodt Baker (Phillipsburg, NJ). Sulfuric acid (96.5%) was purchased from EMD Millipore (Billerica, MA). Graphite nanofibers (GNFs), batch H700, were purchased from Catalytic Materials, LLC (Pittsboro, NC), which were reported to have > 98% carbon content and 80 nm average diameter. Quinine hemisulfate salt monohydrate was purchased from Fluka Analytical/Sigma-Aldrich (St. Louis, Missouri), and rhodamine 6G (laser grade) was
Results and discussion
After modification of a procedure for the synthesis of graphene oxide nanocolloids from graphite nanofibers (GNFs) [10], we synthesized C-dots with a variety of sizes and properties [6]. The GNF starting material was oxidized by refluxing in acid; the resulting solution was then dialyzed and subjected to AE-HPLC separation using LIP detection. A variety of luminescent C-dot fractions were obtained on the basis of their ion-exchange interactions with the chromatographic column. Twelve fractions
Conclusions
The high-resolution fractionation of C-dots, with a separation mechanism mainly based on surface charge, has been absolutely crucial to fully appreciate the many components present in a C-dot mixture prepared by the oxidation of graphite nanofibers. Without such fractionation, reporting properties of C-dots may reflect the average of the many species contained within the mixture. The fractionated C-dot nanomaterials appear to be of discrete sizes, emission profiles, and with defined surface
Acknowledgments
We acknowledge the financial support for this work by the National Science Foundation, USA [Grants CHE 1058373 and CHE 1048740 (CRIF-MU)]. The authors thank Dr. David Watson for the use of the UV–visible and fluorescence spectrophotometers, Dr. Frank V. Bright for the use of the FTIR spectrometer and Dr. Joseph A. Gardella for discussions of XPS. We also acknowledge Dr. Ivonne M. Ferrer for assistance with the capillary electrophoresis experiments.
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