Hair fiber as a precursor for synthesizing of sulfur- and nitrogen-co-doped carbon dots with tunable luminescence properties
Introduction
Fluorescent carbon-based materials have attracted tremendous research interest and have been widely applied in various fields, owing to their appealing advantages such as high aqueous solubility, high chemical inertness, facile functionalization, high resistance to photobleaching, low toxicity and good biocompatibility [1], [2], [3]. These superior properties make these materials promising alternatives to common toxic metal-based quantum dots (QDs) for numerous exciting applications such as bioimaging, biosensing, drug delivery, and optoelectronic devices [4], [5], [6]. While researchers have reported a number of fluorescent carbon-based materials, including fullerenes, carbon nanotubes, nanodiamonds and carbon nanoparticles, carbon dots (C-dots) have generated especially high amounts of excitement because of their strong fluorescence with adjustable parameters, as well as facile synthetic routes with abundant and cheap raw materials [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].
Until now, tremendous effort has been spent on developing synthesis methods for various types of C-dots. These approaches can be classified into two main groups: top-down [2], [7], [17] and bottom-up methods [18], [19], [20]. Recently, carbon nanomaterials containing heteroatoms have been actively pursued, as they are considered the most promising candidate to complement carbon in materials applications because of their tunable intrinsic properties, such as their electronic properties as well as their surface and local chemical reactivities [21], [22]. Particularly, considerable research efforts have been focused on preparing nitrogen-doped C-dots. To incorporate nitrogen into the carbon framework, several strategies have been developed, such as electrochemical synthesis [2], ultrasonic synthesis [23], hydrothermal treatment, and more [14], [24], [25]. C-dots doped with heteroatoms other than N, whether alone or with other dopants, have not been widely reported in the literature. Accordingly, developing simple methods suitable for synthesizing heteroatom-doped or co-doped C-dots is still a great challenge.
On the other hand, nature provides nearly boundless resource, which is widely distributed in nature and easily available to replace conventional chemical reagent [26], [27], [28], [29]. Several reports have explored the preparation of nanomaterials using natural biomass, which has many potential applications [11], [12], [13], [14], [30], [31], [32], [33]. For example, C-dots [11] and graphene [33] were produced from natural precursors with plasma treatments. As we all know, hair fiber mainly consists of carbon, nitrogen, oxygen, sulfur and hydrogen elements owing to the proteins it is made from. Thus, we expect this natural material to be a promising precursor for synthesizing advanced carbon nanomaterials. To the best of our knowledge, no reports exist on synthesizing S–N–C-dots from hair fiber.
In this work, a facile and low-cost strategy is developed for the synthesis of S–N–C-dots with sulfuric acid carbonization and etching of hair fiber. Fig. 1 shows the synthesis procedure (see experimental section details). It is found that both sulfur element and nitrogen element are doped in carbon dots and higher reaction temperature is in favor of the formation of S–N–C-dots with smaller sizes, higher S contents, and longer emission wavelengths from photoluminescence (PL). The prepared S–N–C-dots also have good luminescence stability, low toxicity, good biocompatibility, and high solubility. This approach may provide an efficient strategy to synthesize heteroatom-co-doped C-dots.
Section snippets
Materials and apparatus
Human hair fiber was obtained from a barbershop. H2SO4 (98%) was purchased from Nanjing Chemical Reagents Factory (Nanjing, China). 3-(4,5-Dime-thylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was purchased from Sigma (St. Louis, MO, USA). Doubly deionized water (18.2 MΩ*cm at 25 °C) prepared by a Milli-Q (MQ) water system was used throughout all experiments. All other reagents were of analytical grade and used as received without further treatment.
Ultraviolet–visible (UV–Vis) absorption
Characterization
XPS was used to analyze the elemental composition. Fig. 2a shows the XPS spectra of S–N–C-dots, which reveals sulfur (around 164 eV), carbon (around 285 eV), nitrogen (around 398.5 eV), and oxygen (around 531 eV) atoms. The content of each element is shown in Table 1. In the high-resolution C1s XPS spectrum of the S–N–C-dots (Fig. 2b), five peaks are observed with the binding energies of about 284.5, 285.3, 286, 286.5 and 288.2 eV, which are attributed to C–C/CC, C–S, C–N, C–O (epoxy and alkoxy) and
Conclusion
In conclusion, we developed a simple method to the synthesis of S–N–C-dots on a large scale using carbonization and etching of hair fiber by sulfuric acid. It was verified that the S content of the as-prepared S–N–C-dots increased with reaction temperature, while the N content remained almost unchanged; this control over atomic composition allowed us to change the down-conversion and up-conversion PL properties of the S–N–C-dots. The obtained S–N–C-dots also exhibit good luminescence stability,
Acknowledgments
We greatly appreciate the National Natural Science Foundation (21175065, and 21121091). This work was also supported by National Basic Research Program of China (2011CB933502).
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