A facile microwave-hydrothermal approach towards highly photoluminescent carbon dots from goose feathers

Abstract

Biomass such as hair, silk and feathers is regarded as an appealing candidate for the fabrication of heteroatom-doped carbon nanomaterials. In this work, we report a facile and efficient approach to synthesise photoluminescent carbon dots (CDs) from goose feathers by microwave-hydrothermal treatment. These goose feather generated CDs possess a uniform two-dimensional morphology with a diameter of ∼21.5 nm and a height of ∼4.5 nm. Inheriting the heteroatom-rich nature of goose feathers, the resulting CDs contain a large amount of oxygen, nitrogen and sulfur atoms and have a high photoluminescence efficiency of ∼17.1%. Used as label-free photoluminescence probes, the goose feather derived CDs exhibit highly sensitive and selective detection behavior of Fe³⁺ ions with a low detection limit of 196 nM.

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1. Introduction

Photoluminescent carbon dots (CDs) have attracted intensive attention due to their diverse and fascinating virtues including alluring photo-physical behavior, good biocompatibility as well as low toxicity,^1–8 which render them appealing photoluminescent materials for a wide variety of applications, such as bioimaging, drug delivery and optoelectronic devices.^9–16 In the last few years, heteroatom-doping of CDs has become one of the most popular and effective approaches to fabricate unprecedented CDs since the heteroatoms such as nitrogen (N), oxygen (O) and sulfur (S) in the carbon frameworks provide the opportunities to tune both the optic/electronic properties and the chemical reactivities of the resultant CDs.^17–22 Generally, heteroatom-doped CDs are derived from the thermal or solvothermal treatment of heteroatom-containing organic molecules, while the high cost of these precursors retards the practical application of such methods.^23–27 In fact, nature offers numerous raw materials for various heteroatom-containing carbon nanomaterials with diversified components, morphology and properties. For instance, N-doped CDs can be obtained by the hydrothermal treatment of grass,²⁸ soy milk,²⁹ and cocoon silk.³⁰ On the other hand, S- and N-doped CDs were prepared by sulfuric acid carbonication and etching of hair fiber.³¹ Very recently, another type of N and S-doped CDs were generated by microwave-assisted pyrolysis of rice as carbon source and N-acetyl-L-cysteine as dopant.³² It should be noted that the compositions and PL behaviors of the CDs derived from above methods greatly depend on their precursors.^23–32 Therefore, the pursuing of novel biomass as the raw material for CDs is still intriguing and attractive.

As the major waste from the poultry industry, feather is rich in carbon, nitrogen, sulfur and oxygen elements due to the β-keratin it is composed of. Inspired by pervious results, we envision that the manufacturing of heteroatom-doped carbon nanomaterials with feather as the precursors will be experimentally feasible, which is unfortunately still absent so far. In this work, a simple and economical microwave-hydrothermal approach was developed to convert goose feathers to highly photoluminescent CDs. Enriched with N, O and S containing functional groups, these feather generated CDs possess a uniform two-dimensional (2D) morphology with a diameter of ∼21.5 nm and a height of ∼4.5 nm. Compared with most of the reported heteroatom-doped CDs from biomass,^{24,28,29,31–33} the as-prepared CDs exhibit a superior photoluminescence efficiency (QY) of about 17.1%, resulting from the synergistic effect of the doped heteroatoms. Moreover, it is found that the obtained CDs can serve as the label-free photoluminescence sensors for the highly sensitive and selective detection of Fe³⁺ ions with low detection limit of 196 nM.

2. Experimental section

2.1 Materials

All the chemicals used in this work were purchased from Sigma-Aldrich Corp. and Shanghai Chemical Corp. All the reagents were of analytical grade and used as received without further purification. Goose feather was purchased from Rose Feather & down Sells Co., Ltd., Anhui, China. Milli-Q water was used in all experiments. Environmental water sample was obtained from the river in Shanghai University.

2.2 Synthesis of photoluminescent CDs

In a typical procedure, 4 g goose feather was first soaked in 300 mL water. The mixture was transferred to 500 mL Teflon-line autoclave and heated for 40 min at 180 °C in a microwave autoclave (WB-GSH, manufactured by XINGJIAN CHEMICAL MACHINERY CO., LTD., maximum heating power: 2 kW). Subsequently, the suspension of CDs was obtained by further dialysis against Milli-Q water with a cellulose ester membrane bag (M_w = 3500). Finally, a clear yellow suspension containing CDs was collected for further characterization and use by simply filtration.

2.3 Characterizations

Transmission electron microscope (TEM) measurements were performed on JEM-2010F at operating voltage of 200 kV. The sample was diluted in water and then dropped on carbon-coated copper grid. For atomic force microscopy (AFM) characterization, 1 μL aqueous dispersion of the resulting CDs was spotted onto freshly cleaved mica surface and dried in air. The samples were measured by tapping mode on Nanonavi E-Sweep (SII). The elemental analysis was measured on PE 2400 II made by Perkin Elmer Company in America. UV-Vis spectra were recorded at room temperature on a Hitachi J-4100 spectrophotometer. Photoluminescence spectra were recorded on a Horiba Fluoromax-4 spectrometer. X-ray diffraction (XRD) patterns were taken with Bruker AXS D8 Advance X-ray diffraction instrument using Cu Kα irradiation. X-ray photoelectron spectra (XPS) were determined by an X-ray photoelectron spectrometer (AXIS UltraDLD, Kratos, Japan). Fourier transform infrared spectroscopy (FTIR) spectra were recorded using Spectrum 100 (Perkin Elmer, Inc., USA) spectrometer. Dynamic light scattering (DLS) analysis was carried out by Malvern NANO ZS90 with He–Ne Laser (633 nm) in a sample cell.

2.4 Calculation of quantum yields (QYs)

QY of CDs was calculated according to the following eqn (1) using quininesulfate (QY = 54% in water) as the standard sample:where Q is the quantum yield, I is the measured integrated emission intensity, n is the refractive index, and E is the extinction. The subscript “R” refers to the standard with known QY.

2.5 Detection of Fe³⁺ ions

The detection of Fe³⁺ ions was performed at room temperature in aqueous solution. In a typical run, 1 mL CDs dispersion was added to 20 mL of water. Fe³⁺ aqueous solutions of different concentrations together with other metal ion solutions were freshly prepared before use. To evaluate the sensitivity towards Fe³⁺, different concentrations of Fe³⁺ were added into the aqueous suspension containing the same amount of CDs and the mixed dispersions were equilibrated for 5 min before spectral measurements. The photoluminescent spectra were recorded by operating the fluorescence spectrophotometer with an excitation wavelength of 340 nm.

3. Results and discussion

The morphology of the CDs from goose feather was first explored by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Their TEM images (Fig. 1A and B) clearly reveal the uniform morphology and narrow size distribution of the CDs. The statistical analysis of one hundred CDs further demonstrates the majority diameters of the CDs are fell within the range from 19.0 to 23.0 nm (Fig. 1C). The following dynamic light scattering (DLS, Fig. S1†) results indicate the average hydrodynamic diameter of CDs in water is mainly ranging from 12 to 25 nm, which is very close to the observation from their TEM images. Their AFM image also suggest that the CDs are monodispersed nanoparticles with a diameter of 21 ± 5 nm (Fig. S2†). It is interesting to note that the height of these CDs from the AFM analysis is around 4.5 nm (Fig. S2B†), suggesting that they have a two-dimensional (2D) disc-like morphology. The unique microstructures of the goose feather generated CDs should be attributed to their precursors, β-keratins, which are consist of four repeating units of two β-sheets.³⁴ The high resolution TEM image clearly reveals that the diffraction contrast of the CDs is very low and without any obvious lattice fringes (Fig. 1B), implying that they are the amorphous nature. In the XRD results, only a broad peak with the 2θ ranging from 10–30° can be observed, which confirms the poorly crystallized carbon framework of the CDs (Fig. S3†).

Fig. 1 (A) Low- and (B) high-resolution TEM images of the CDs from goose feather; (C) the size distribution histogram of the CDs.

X-ray photoelectron spectroscopy (XPS) was applied to analyze the chemical states of the elements in the CDs. The XPS survey spectra of CDs show four typical peaks of C1s, N1s, O1s and inconspicuous S2p (Fig. 2A). The corresponding contents of C, N, S and O are calculated to be around 48.4, 16.3, 1.9, and 33.3 wt%, respectively. The deconvolutions of the C1s spectra (Fig. 2B) are fitted by four peaks, which are assigned to C–C (284.5 eV), C–N (285.1 eV), C–O (286.1 eV) and C=N/C=O (287.8 eV), respectively.³⁰ The N1s spectra (Fig. 2C) reveal three nitrogen species of C–N–C (399.4 eV), N–C₃ (400.0 eV), and N–H (400.9 eV).²⁸ Moreover, the S2p spectra (Fig. 2D) are mainly consist of two peaks centered at 163.7 and 168.2 eV, suggesting that sulfur atoms exist in two forms as thiophene-S and sulfate/sulfonate.³¹

Fig. 2 XPS spectra of the goose feather produced CDs: (A) survey spectra; (B) C1s, (C) N1s and (D) S2p spectra.

The high contents of nitrogen and oxygen in the goose feather produced CDs suggest that they are enriched with a large number of hydroxyl, carbonyl, carboxylic or amide groups on the surface, which are further confirmed by their Fourier transform infrared spectroscopy (FTIR) results. As indicated in the FTIR spectra (Fig. S3†), the absorption band located at around 3287 cm⁻¹ is characteristic of O–H stretching for carboxyl groups. And the bands at 2963 and 2880 cm⁻¹ can be assigned to the stretching vibrations of C–H.²⁴ On the other hand, the absorption bands at 1657, 1545 and 1452 cm⁻¹ correspond to the C=C stretching mode of polycyclic aromatic hydrocarbons.³⁰ Furthermore, the elemental analysis reveals the compositions of CDs are C 43.5 wt%, N 14.4 wt%, S 1.83 wt%, H 5.57 wt% and O (calculated) 34.7 wt%, which are in good agreement with the XPS results. All the observations confirm that the CDs contain plenty of heteroatoms including N, O and S on their surface.

In order to explore the optical properties of the goose feather generated CDs, their UV-Vis absorption and photoluminescence emission spectra were recorded accordingly. The UV-Vis spectra of the CDs show a broad absorption band with a shoulder peak at 270 nm (Fig. 3A), which can be ascribed to the π–π* transition within the carbon framework.³⁵ Under the excitation at 365 nm, the light yellow CDs suspension exhibits strong blue luminescence (Fig. 3B). As shown in Fig. 3A, the emission spectra of CDs are broad, ranging from 410 to 560 nm, with a dependence on the excitation wavelengths. Generally, the surface passivation of CDs is indispensable for their photoluminescent behaviors since it is believed that their emission presumably arise from the radiative recombination of the excitons trapped by the surface defects generated during the passivation process.^3,4,36,37 In this case, the abundant functional groups on the surface of CDs such as carboxyl acids and amines can introduce plenty of defects to act as the excitation energy traps and lead to the photoluminescent behavior.²⁴ Using quinine sulfate as a reference and 340 nm as the excitation wavelength, the QY of CDs is calculated as 17.1%, much higher than most of the biomass-generated CDs previously reported.^{24,28,29,31–33} Moreover, the CDs stored for more than one year still manifest almost identical emission spectra as the freshly prepared samples (Fig. S5†), indicating the very high photostability of the CDs derived from goose feather.

Fig. 3 (A) UV-Vis absorption and photoluminescence emission spectra (recorded for progressively longer wavelengths with 20 nm increments from 300 nm to 500 nm) of a dilute aqueous suspension of the CDs; (B) photographs of the suspension of the CDs taken under daylight (left) and excitation at 365 nm (right).

The rich content of heteroatoms and high QY render these goose feather produced CDs promising candidates for label-free photoluminescence probes to sense metal ions. To study the sensitivity of the CDs to Fe³⁺, the photoluminescent spectra of the CDs with the addition of Fe³⁺ at different concentrations were measured and compared. It is obvious that the photoluminescent spectra of the CDs are very sensitive to Fe³⁺ ions and the photoluminescence intensities of the CDs decrease drastically with the incremental of Fe³⁺ concentration (Fig. 4A). The dependence of F/F₀ on the concentrations of Fe³⁺ ions is illustrated in Fig. 4B, where F₀ and F represent the photoluminescence intensities of CDs at 425 nm in the absence and presence of Fe³⁺, respectively. The quenching efficiency can be fitted to the Stern–Volmer eqn (2):^38,39

Fig. 4 (A) Photoluminescent spectra of the CDs in the presence of different concentrations of Fe³⁺ (0–14 μM); (B) the relationship between the photoluminescence of the CDs and Fe³⁺ from 0–10 μM; (C) a linear region of 2–7 μM (F₀ and F are the highest photoluminescence intensities of the CDs excited at 340 nm in the absence and presence of Fe³⁺, respectively); (D) the difference in the relative photoluminescence intensity of the CDs dispersions containing different metal ions (excitation at 340 nm; [Mn⁺] = 10 μM).

The obtained Stern–Volmer plot shown in Fig. 4C fits a linear equation over the concentration range of 2–7 μM, while the plot does not fit a linear equation over the whole concentration range of 1–14 μM, indicating both dynamic and static quenching processes occur in this sensing system. Such a result suggests that the quenching of Fe³⁺ on the photoluminescence of the CDs occurs by affecting their surface states.^38,40 The sensing application of the CDs in environmental water samples was also explored in this work (Fig. S6†). The sensing behaviors of the CDs in environmental water samples are very similar to those in Milli-Q water. The remarkable photoluminescence sensitivity of the goose feather derived CDs towards Fe³⁺ can be ascribed to the heteroatom-doping induced modulation of the chemical and electronic characteristics and the easy formation of complexes between the CDs and Fe³⁺. The detection limit is estimated to be 196 nM at a signal-to-noise ratio of 3.

To evaluate the selectivity of this sensing system, the photoluminescence intensity change in the presence of representative metal ions under the same conditions including K⁺, Mg²⁺, Ca²⁺, Co²⁺, Cu²⁺, Fe²⁺, Cr³⁺ and Al³⁺ was examined. As shown in Fig. 4D, high photoluminescent quenching is observed upon the addition of Fe³⁺ and the other cations have no distinct influence on the detection results. The high selectivity of these CDs for Fe³⁺ might be due to the faster chelating process of Fe³⁺ ions with CDs through N, O and S in comparison with other metal ions.⁴⁰

4. Conclusion

We developed a microwave hydrothermal protocol to fabricate highly photoluminescent CDs using the low-cost goose feathers as the starting materials. With excellent photoluminescent properties, the obtained CDs exhibit the intriguing potential as the label-free photoluminescence probe for the detection of Fe³⁺ ions. More importantly, the one-step synthesis process is simple but effective, requiring neither a strong acidic solvent nor surface modification reagent, which makes it very suitable for large-scale production of photoluminescent carbon nanomaterials.

Acknowledgements

This work was financially supported 973 Program of China (2013CB328804 and 2014CB239701), National Natural Science Foundation of China (21343002, 21102091), Program for Professor of Special Appointment (Eastern Scholar) and Program for Innovative Research Team in University (no. IRT13078). The authors also thank Lab for Microstructure, Instrumental Analysis and Research Center, Shanghai University, for materials characterizations.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12077a

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