Synthesis of Nitrogen and Sulfur Co-doped Carbon Dots from Garlic for Selective Detection of Fe3+
© Sun et al. 2016
Received: 28 October 2015
Accepted: 22 February 2016
Published: 29 February 2016
Garlic was used as a green source to synthesize carbon dots (CDs) with a systematic study of the optical and structure properties. Ethylenediamine was added into the synthesis to improve the photoluminescence quantum yield (PL QY) of the CDs. Detailed structural and composition studies demonstrated that the content of N and the formation of C–N and C=N were critical to improve the PL QY. The as-synthesized CDs exhibited excellent stability in a wide pH range and high NaCl concentrations, rendering them applicable in complicated and harsh conditions. Quenching the fluorescence of the CDs in the presence of Fe3+ ion made these CDs a luminescent probe for selective detection of Fe3+ ion.
Fluorescent materials have drawn considerable attention owing to their potential applications in a variety of fields, such as bioimaging, optoelectronic devices, and sensing [1–7]. Semiconductor quantum dots (QDs) have become one of the most promising nanomaterials due to their high photostability, large molar extinction coefficients [8, 9], high photoluminescence quantum yields (PL QYs), and size-tunable emission . However, these popular QDs have raised concerns over serious toxicity and environmental hazard . In addition, their superior optical features are usually applied in organic solvents, thus restricting their direct analytical and biological applications to a great degree. Various methods have been applied to make these luminescent QDs water-soluble, such as surface passivation with hydrophilic protective layers [12–14] and water-phase synthesis. However, these protocols are compromised at the expense of reducing the PL QYs and the time-consuming, complicated, expensive processes .
Carbon nanoparticles, or carbon dots (CDs) are free of toxic elements. They have recently emerged as the most attractive candidates to replace QDs with outstanding advantages, such as the excellent water solubility, strong luminescence, high biocompatibility, and good photostability [16–23].
Typically, the strategies for synthesizing CDs can be divided into two major categories: top-down and bottom-up methods. Top-down methods consist of laser ablation , arc discharge , and chemical oxidation , where the CDs are formed by cutting a large carbon structure into small pieces. In bottom-up methods, the CDs are synthesized by carbonization of organic molecular precursors through solvothermal methods [27, 28], microwave treatment , ultrasonic-assisted synthetic methods , and so on. Among them, the top-down methods often require sophisticated and expensive energy-consuming equipment . The ultrasonic-assisted synthetic methods rely on strong acids or bases . Typically, CDs can be synthesized within minutes by microwave irradiation, while they suffer from uncontrollable reaction conditions. The hydrothermal route is mostly preferred because of its simplicity, controlled reaction conditions, and cost-effectiveness .
Despite several advancements in the field of CDs synthesis [27, 28, 32], there are some by-products using conventional chemicals, which do not meet the standard of non-toxicity. Besides, expensive precursors and complicated post-treatment processes for surface passivation also hinder the broad application of CDs. Therefore, many efforts have been made for the preparation of CDs using easily accessible and natural precursors as starting materials. For example, milk , potato , grape juice , lime juice , orange juice , pomelo peel , grass , willow bark , and even waste biomass  have been used as the green sources for CDs. Nevertheless, it still remains a major challenge to achieve biomass-derived CDs with high PL QYs.
Recently, heteroatom-doped CDs have been reported with enhanced optical and electronic properties. It has revealed that heteroatom doping plays a vital role in tuning compositions and structures of CDs [42, 43]. Particularly, there were intensive investigations of nitrogen-doped CDs , while nitrogen and sulfur co-doped CDs were rarely reported. Garlic is a cheap, easily available natural condiment, containing carbohydrate, proteins, and thiamine, and abundant in carbon, nitrogen, and sulfur elements. Consequently, dehydration, polymerization, carbonization, and passivation may involve in the formation of CDs under high temperature and pressure during the hydrothermal treatment [22, 23]. We report herein a green synthetic method for N and S co-doped CDs from garlic by a one-step hydrothermal synthesis. Systematic study of the optical and structure properties of CDs is presented in the work. In comparison to CDs prepared by other natural materials such as grass, potato, pomelo peel, willow bark, and waste biomass which have relatively low content of N and S elements, the QY of our CDs exhibited nearly double increase. The as-synthesized CDs can be applied in sophisticated and harsh conditions because of their excellent stability in a wide range of pH values and high ion strength solutions. Selective fluorescence quenching of CDs qualifies them as a probe to detect Fe3+ ion.
Fresh garlic were purchased from local supermarket (Changchun, Jilin Province). Ethylenediamine (EA) was attained from Aladdin. Na2S·9H2O were purchased from Xi Long Chemical Reagent Co., Ltd. Na2SO4, NaCl, KCl, AlCl3, ZnCl2, Ba(NO3)2, FeCl3, MgCl2, Ni(NO3)2·6H2O, MnCl2·4H2O, CuCl2·2H2O,PbCl2, and HgCl2 were obtained from Sinopharm Chemical Reagent Co., Ltd. and used without further purification.
Preparation of CDs
The garlic were peeled and washed and cut into small pieces and set in the oven at 50 °C for 24 h. After dehydration, the garlic pieces were ground into fine powder in a mortar; 0.5 g of garlic powder and 10 mL of deionized water were added in an autoclave with a polytetrafluoroethylene (PTFE) inner chamber and heated at 200 °C for 6 h (for N- and S-rich CDs, additional EA, Na2S·9H2O and Na2SO4 were added into the reaction). After the reaction, the autoclave was cooled to room temperature naturally. The reaction product was centrifuged at 5000 rpm for 10 min to remove the black precipitates, and then, the resulted supernatant was purified by filtering out large-sized carbon nanoparticles using a syringe filter with pores of 0.22 μm. Finally, the product was subjected to dialysis (MWCO = 1000 Da) in order to obtain the pure CDs.
Fluorescence emission spectral measurements were carried out using a Shimadzu RF-5301 fluorescence spectrophotometer. Absorbance spectra were acquired by using a Shimadzu UV-3600 spectrophotometer. The surface morphology of the as-prepared CDs was investigated using a transmission electron microscope (TEM) (JEOL). X-ray diffraction (XRD) patterns of CDs were obtained using a Bruker D8 Advance X, Pert diffractometer (Cu Kα: λ = 1.5406 Å), in the range of 10°–70° at a scan rate of 4°min−1. Atomic force microscopy (AFM) images were recorded with a Veeco DI-3100 instrument. Fourier transform infrared spectroscopy (FTIR) was performed on an FTIR spectrophotometer (IFS-66 V/S). X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB250 spectrometer. Binding energy of C 1 s at 284.7 eV was set as the calibration. The absolute PL QYs of the solution sample were measured by a fluorescence spectrometer (FLS920P, Edinburgh Instruments) equipped with an integrating sphere with its inner face coated with BENFLEC.
Metal Ion Detection of the CDs
Many different kinds of metal cations have been applied for the detection, such as Na+, K+, Al3+, Zn2+, Ba2+, Fe3+, Mg2+, Ni2+, Mn2+, Cu2+, Pb2+, and Hg2+. Briefly, a metal salt aqueous solution (250 μM, 2 mL) was mixed with a CD solution (0.05 mg/mL, 2 mL). After the mixture was vibrated for 5 min, the fluorescence spectra of the mixture were recorded.
pH Stability of CDs
HCl (2 M) or NaOH (2 M) was used to adjust the pH of the resultant CD solution (0.05 mg/mL, 2 mL), and the mixed solution was vibrated for 5 min. Then, the fluorescence spectra of the mixture were carried out.
Ion Strength Stability of CDs
NaCl aqueous solution (2 mL) with certain concentrations was mixed with a solution of CDs (0.05 mg/mL, 2 mL). Then, the fluorescence spectra of the mixture were recorded after the mixed solution was equilibrated for 5 min with vibrating.
Results and Discussion
The structure and components of CDs were identified by FTIR and XPS. As illustrated in Fig. 1b, the broad band at 3399~3283 cm−1 was attributed to O-H and N-H bonds . A small band at 2931 cm−1 was ascribed to the C-H bonds . The peak at 1769 cm−1 could be ascribed to C=O stretching vibration, while the peak at 1608 cm−1 was attributed to the C=C [27, 42]. Moreover, two bands at 1704 and 1454 cm−1 were assigned to the C=N and C–N stretching vibrations, respectively [42, 47]. In addition, the peak at 1121 cm−1 could be identified as the C–O, C–N, and C–S bonds  and the peak at 1048 cm−1 could be ascribed to –SO3−, C–O–C, and C–O bonds [44, 49].
Heteroatom-doped CDs can modulate compositions and structures of CDs to enhance the optical and electronic properties [42, 43]. In order to elucidate the effect of N element and further increase the QY of the obtained CDs, an additional N-containing chemical was added into the hydrothermal synthesis. EA is a commonly used N precursor to synthesize N doped CDs, which was chosen in this work. In detail, different volumes of EA were added to the garlic powder and heated at 200 °C for 6 h. As shown in Fig. 3c, the QY firstly increased with the EA added and then dropped gradually with more EA. The highest QY could be tuned to 20.5 % with 1 mL of EA. When the ratio of EA was high, the CDs would be carbonized hardly  which led to lower QYs.
The absorption of two typical samples (1 mL EA denoted as CD-1 and 4 mL EA denoted as CD-4) was studied as shown in Additional file 1: Figure S2a. A new absorption band at 358 nm appeared for the EA addition, which is related to surface states [16, 27, 51]. Absorption and QY of CD-1 with different reaction times were shown in Additional file 1: Figures S2b and S3a. When the reaction time was 2 h, the absorption peak at 287 nm dominated and the QY was very low. However, when the reaction time was prolonged to 3 h, the absorption peak at 358 nm appeared and the QY was still not high. With the reaction time further prolonged, the absorption peaks remained unchanged. As shown in Additional file 1: Figure S3a, the QY firstly increased with the increase of reaction time up to 6 h; then, no apparent change of QY was observed with the reaction time to 8 h. Additional file 1: Figure S3b shows that the particle size of CD-1 was the same as that of the CDs without EA addition. The PL property of CD-1 was shown in Fig. 3d. The emission had a redshift of 10 nm compared to the one without EA.
The XPS and FTIR experiments were also carried out to identify the structure and components of these EA-assisted CDs. The band at 1769 cm−1 corresponding to C=O almost disappeared, and the bands at 1704 and 1454 cm−1corresponding to C=N and C–N became more obvious in the FTIR spectra of CD-1 and CD-4 (Additional file 1: Figure S4), suggesting that N atom gradually substituted O atom during the dehydrolysis process, which was consistent with the elemental compositions result (Additional file 1: Table S3). The content of pyrrole N of CD-1 was higher than that of CD-4 which could be seen from Additional file 1: Figures S5 and S6. No other states of N and distinct differences were observed in the XPS, which proved that N atoms entered CD framework through the dehydrolysis and the content of N was helpful to acquire high QY.
Based on the above results, the role of EA in the synthesis of CDs could be explained as below. Firstly, the –COOH or the C=O, C–O of the garlic may react with EA, facilitating dehydrolysis and carbonization process. Then N atoms substitute part of O atoms to form amides, which leads to a newly formed surface state induced by N atoms. In this case, surface passivation may facilitate a high yield of the radiative recombination and depress the non-radiative recombination . As a consequence, the PL QY of the resulting CDs increases as the N content of CDs increases.
Compared to the role of N, S shows relatively weak influence. Firstly, Na2S·9H2O was used as the S precursor to synthesize S-doped CDs. However, as shown in Additional file 1: Figure S7a, the QY of the CDs was not enhanced; conversely, the QY of the CDs declined with the increasing mass of S precursor. This is mainly because Na2S is a strong base which has an effect on the PL intensity of the CDs. Then Na2S was substituted by Na2SO4 whose pH was neutral. As shown in Additional file 1: Figure S7b, the QY of the CDs increased slightly with the increasing mass of S precursor, which is not as effective as the N precursor. The absorption and PL spectra of two S precursors were studied as shown in Additional file 1: Figure S8. For the Na2S case, the absorption was the same to the one without S precursor when the amount of Na2S was small. However, with the increasing of Na2S amount, the absorption peak became less clear and changed as the absorption of CDs in pH value of 13. This is mainly because Na2S is a base which could change the pH value of the reaction solution and led to the decline of PL intensity. The absorption did not change with the addition of the Na2SO4, indicating no energy level changed. The PL spectra of these two S precursors were the same, and a little difference was achieved from the CDs without S. Therefore, compared to the N precursor, the S precursors have inconspicuous influence on the dehydrolysis and carbonization process.
A direct, simple, and green synthetic approach for CDs using garlic as carbon source was presented. The content of N and the formation of C–N, C=N were critical to improve the PL QY. The as-prepared CDs showed no obvious photobleaching toward UV light. Fluorescence quench effect of the CDs in the presence of Fe3+ ion provided a platform for detecting Fe3+ ion from 0.2 to 500 μM.
This work was financially supported by the National Natural Science Foundation of China (51272084, 61306078, 61225018, 61475062), the Jilin Province Key Fund (20140204079GX), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2013KF17), the Ministry of Education, Youth and Sports of the Czech Republic (LO1305), and the funding from Palacky University institutional support.
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