Dual functional carbonaceous nanodots exist in a cup of tea

Abstract

Highly photoluminescent (PL) carbonaceous nanodots (CNDs) with a PL quantum yield (PLQY) of 6.8% have been discovered and obtained from tea water. Characterization indicated that the CNDs show amorphous structure, good monodispersity, excellent water-solubility, interesting PL properties, acceptable PL lifetime, and fairly good photostability and biocompatibility, which can enable the obtained CNDs to be applied to Hg²⁺ detecting and cell imaging.

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1. Introduction

Carbon is one of the most common elements in nature, and is abundant in the atmosphere and crust in a variety of forms. For centuries, carbon has been known as a macroscopic, black material. While recently, a variety of carbon materials with remarkable physical and chemical properties were developed; fullerenes,^1,2 carbon nanotubes,³ graphene,⁴ and carbonaceous nanodots (CNDs).^5–7 Among such discoveries, photoluminescent CNDs are attracting more and more interest due to their small particle size, excitation wavelength dependent behavior, non-blinking photoluminescence emission, and favourable biocompatibility, with potential application in heavy metal ions detection,^8–10 biological labeling,^11,12 optical sensors,¹³ optoelectronic devices,¹⁴ and photocatalysis.^15–17 To date, methods that have been developed to prepare photoluminescent CNDs include laser ablation,¹⁸ microwave-assist,^19,20 electrochemical oxidation,²¹ arc-discharge,²² and acid dehydration.²³ However, these approaches usually involve complex reactions and post-treatment processes, and expensive carbon sources. Thus, the practical applications of CNDs have been severely limited.

Lately, several unprecedented scientific researchers have demonstrated new simple synthetic pathways of carbonaceous nanomaterials. Mohapatra’s group and Dong et al. prepared high-quality, photoluminescent, functional CNDs from orange juice and soy milk, respectively.^24,25 Inspired by these works, our group has been working on finding simpler, milder methods to prepare photoluminescent CNDs using renewable biomass as carbon source. On one occasion, we found that a glass of tea water showed blue photoluminescence upon exposure to UV light. Subsequent to this discovery, we investigated the tea water by means of abundant characterization and testing. These investigations demonstrate that water-soluble and highly photoluminescent CNDs exist in the tea water. The applications of tea in materials science have, to the best of our knowledge, not yet been reported, despite being used as a beverage for thousands of years. The unique photoluminescence properties and fairly good biocompatibility can enable the as-prepared CNDs to be applied to Hg²⁺ detecting and cell imaging.

2. Experimental sections

2.1. The preparation of photoluminescent CNDs

The synthesis procedure is shown in Scheme 1. Briefly, 3 g dry tea leaves (Tieguanyin, China) were infused in 200 mL boiling deionized (DI) water (about 100 °C), and 20 minutes later a cup of tea was obtained (Scheme 1). The faint yellow tea water was centrifuged at high speed (22 000 rpm) to separate the precipitate. Subsequently, excess methylbenzene was added into the tea water to extract the organics (e.g. protein, polycyclic aromatic hydrocarbons (PAHs)), the mixture was shaken for 30 minutes, and the upper solution containing CNDs was collected after 1 hour standing. Finally, the CNDs solution was dialyzed against DI water for two days by dialysis membrane (molecular weight cut-off (MWCO): 3500) to remove almost all impurity inorganic ions and molecules, and the clear and transparent CNDs solution without any precipitation was obtained (Scheme 1). Furthermore, the prepared solution was still clear and transparent even though it was exposed to the open air for several months. The CNDs solution shows a light green color under daylight and changes to a blue color on exposure to 365 nm UV light (Scheme 1). These observations imply that the resultant CNDs possess fairly good water solubility and photoluminescence.

Scheme 1 Illustration of the preparation procedure of the CNDs (photograph of the samples excited by daylight and a 365 UV lamp).

2.2. Characterizations

UV-Vis absorbance spectroscopic (Abs) studies were performed with a TU-1901 dual beam UV-Vis spectrophotometer. PL measurements were performed with a FLs-920 steady state/transient state spectro xsort. Mn element was determined by means of inductively coupled plasma mass spectrometry (ICP-MS, TJA, IRIS ER/S). The structure of the CNDs was characterized by X-ray diffraction (XRD, Rigaku D/Max-2400) using Cu-Kα radiation (40 kV, 60 mA). Transmission electron microscopy (TEM), high resolution TEM (HRTEM) images and selected area electron diffraction (SAED) results were acquired using a Tecnai-G² F30 TEM operating at an acceleration voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried on an ESCALAB 250xi photoelectron spectrometer. Fourier transform infrared spectroscopy (FTIR) spectra were measured using a Thermo Nicolet Nexus FTIR model 670 spectrometer. The cellular image was obtained with a laser scanning confocal microscope (LSCM, LSM 510 Meta).

2.3. In vitro cytotoxicity study and cell imaging

Cytotoxicity study. The cytotoxicity of CNDs was assessed by using the MTT assay. L02 human hepatocyte cells (or S180 sarcoma cells) (1 × 10⁴ cells per well) were grown at 37 °C and under a 5% CO₂ atmosphere in a RPMI-1640 medium in a 96-well plate, supplemented with calf serum (10%) and 1% penicillin streptomycin in a fully humidified incubator. Then, CND solutions with a concentration of 30, 60, 120, 240, 480 and 960 μg mL⁻¹ were added to the cell dishes, and then these cell dishes were put into incubators at 37 °C for 12 h. After incubation for a defined time, the culture medium was removed and 20 μL of MTT reagent (diluted in culture medium, 0.5 mg mL⁻¹) was added, followed by incubating for another 2 h. The MTT/medium was removed carefully and DMSO (150 μL) was added to each well to dissolve the formazan crystals. Absorbance of the colored solution was measured at 570 nm using a microplate reader.

Cell imaging. After confirming the photoluminescence from the CNDs and no distinct auto-photoluminescence from the cell itself under similar conditions, the cellular image was obtained with a laser scanning confocal microscope (LSCM, ZEISS, LSM 510 Meta, Germany). S180 sarcoma cells (6 × 10⁴ cells per well) were seeded on a 6-well plate at 37 °C for 24 h. After that, CND solution with a concentration of 3 mg mL⁻¹ was added to the cell dishes. After further 2 h incubation, these CNDs-loaded cells were washed with PBS three times to remove the free CNDs attached on the outer surface of the cell membrane. Cell luminescence was detected by the LSCM under excitation wavelengths of 364 nm and 488 nm.

3. Results and discussion

Fig. 1a presents a typical powder XRD profile of the obtained CNDs. A broad peak centered at 2θ = 23.4° indicates the poor crystalline nature of the CNDs, which can be attributed to amorphous carbon composed in a considerably random fashion.^26,27 The corresponding interlayer spacing of the CNDs (d = 0.38 nm) is larger than that of graphite (d = 0.33 nm), indicating the CNDs are not graphitic carbon.²⁸ The typical annular SAED (Fig. 1b) also demonstrates the amorphous structure of the obtained CNDs. A TEM image is shown in Fig. 1c; it is found that the CNDs show highly uniform quasi-spherical morphology and fairly good monodispersity. As shown in Fig. 1d, the absence of any discernible lattice structures from the HRTEM image also suggests an amorphous nature of the obtained CNDs, which agrees well with the XRD analysis. The inset of Fig. 1c shows the particle size distribution histogram, which indicates that these quasi-spherical particles are mainly distributed in the range of 3–9 nm (6.8 nm average diameter). The particle size of the obtained CNDs from tea water is similar to that of the CNDs reported previously.^29,30

Fig. 1 (a) XRD pattern, (b) SAED pattern, (c) TEM and (d) HRTEM images of the CNDs (inset: size distribution histogram).

The component and surface states of the CNDs were characterized by XPS and FTIR. The wide scan of XPS is shown in Fig. 2a, which reveals that the obtained CNDs contain mainly carbon and oxygen as well as limited nitrogen and potassium. Calculated from the XPS spectrum, the C : O : N : K atomic ratio in the CNDs is 41.4 : 15.4 : 1.1 : 1. In detail, the C_1s spectrum (Fig. 2b) shows three peaks at 283.0, 284.4 and 286.8 eV, which are attributed to C–C, C–O and C=O/C=N, respectively. The O_1s spectrum exhibits two peaks at 531.0 and 533.7 eV, which are ascribed to C=O and C–OH/C–O–C, respectively.^22–24,31 Moreover, the N_1s and K_2s spectra are displayed in the ESI, Fig. S1.† The FTIR spectrum exhibits characteristic absorption bands of O–H, C–H, C=O, C=C, C–N, and C–O at 3430, 2931, 1698, 1631, 1517 and 1043 cm⁻¹, respectively. These observations clearly indicate that the CNDs are functionalized with carbonyl, carboxyl, hydroxyl, epoxy and amino groups.^32,33 The presence of these functional groups imparts excellent solubility in water without further chemical modification, which suggests that the obtained CNDs are most suitable for biological applications.

Fig. 2 (a) XPS, (b) C_1s and (c) O_1s spectra of the CNDs, (d) FTIR spectrum of the CNDs.

The absorption spectrum of the CND solution is shown in Fig. 3a, a distinct absorption peak is seen at 273 nm, which can be attributed to the π–π* transition of nanocarbon.³⁴ The CND solution presents a 455 nm PL emission peak when exciting at a 365 nm wavelength, which corresponds to the blue photoluminescent image (inset of Fig. 3a). To further investigate the photoluminescent properties of the as-prepared CNDs, we carried out a detailed PL study with different excitation wavelengths. As shown in Fig. 3b, when the excitation wavelengths range from 320 to 500 nm, the PL emission spectra show unusual emission peaks, which are very different from the PL emission of other research reports.^35,36 When the excitation wavelength ranges from 320 to 400 nm, the PL emission spectra show two peaks. The first emission peak shifts from 447 to 464 nm, presenting familiar excitation-dependent PL behavior of the CNDs, which can probably be attributed to the presence of different particle sizes and the distribution of the different surface emission traps of the CNDs. Interestingly, the intensity of the second PL emission peak, which is located at a position of about 516 nm, gradually increased with the red-shift of the excitation wavelength. When the excitation wavelength ranges from 420 to 500 nm, the emission spectra show only one peak around 516 nm, which is characteristic of the Mn(II) internal ⁴T₁–⁶A₁ transition. Comparing to the emission peak reported previously at 585 nm,^37–39 this blue-shift of about 69 nm appears to be caused by the smaller size of the particles.⁴⁰ It is common knowledge that tea is rich in Mn element, and the CNDs are indeed demonstrated through plasma ICP-MS element analysis to have a Mn content of 0.046%. It should be noted that the Mn element does not appear in the XPS spectrum just because of the minute amounts of Mn present. What is not expected is that the PLQY is as high as 6.8%, which is comparable with that of the CNDs previously reported.^41,42 Moreover, the Raman spectrum was not collected just because of the strong PL interference when exciting over a wide wavelength range.²⁵

Fig. 3 (a) UV-Vis absorption and PL emission (λ_ex = 360 nm) spectrum, (b) the photoluminescent spectra of the CNDs at different excitation wavelengths as indicated.

The photostability of the CNDs has also been investigated. The PL intensity has almost no change when the CNDs were dissolved in DI water, upon exposure to daylight and a xenon lamp with high intensity UV light for 6 hours (as shown in Fig. S2a†). The PL intensity hardly changed, demonstrating that the CND solution reveals fairly good photostability. Likewise, when the CNDs were dissolved in 0.1 mol L⁻¹ NaCl solution, the CNDs also showed fairly good photostability (Fig. S2b†). Additionally, a negative charge (zeta potential = −33.1 mV, Fig. S3†) was observed on the surfaces of the CNDs, which confirms the electrostatic stabilization of the CND aqueous solution.⁴³ The PL decay profile obtained from 455 nm emission at a 360 nm excitation wavelength (Fig. S4†) shows treble exponential decay kinetics. The parameters generated from iterative reconvolution of the decay with the instrument response function (IRF) are listed in the inset of Fig. S4.† The average lifetime was calculated to be 4.36 ns, and the lifetime with a magnitude of nanoseconds suggests that the synthesized CNDs are most suitable for optoelectronic and biological applications.^44,45 Furthermore, the PL decay profile obtained from 516 nm emission at a 360 nm excitation wavelength was shown in Fig. S5;† the decay lifetime is about 0.78 ms, which is corresponding to the Mn(II) decay lifetime.^46,47

The feasibility of using this photoluminescent CND solution for detecting Hg²⁺ was investigated in Phosphate Buffer Solution (PBS, pH = 7.0). It is seen that the CND solution in the absence of Hg²⁺ exhibits a strong fluorescence (Fig. 4a, curve A). In contrast, the presence of Hg²⁺ leads to an obvious PL intensity decrease (Fig. 4a, curve B), indicating that Hg²⁺ can effectively quench the photoluminescence of the CNDs. This observation can be attributed to the Hg²⁺ being able to quench the photoluminescence of CNDs via electron or energy transfer.^48–50 Furthermore, the PLQY of the CNDs is dropped to 3.5% after addition of 100 μM Hg²⁺. For sensitivity study, different concentrations of Hg²⁺ ranging from 1 nM to 100 μM were investigated. Fig. 4b shows the PL spectra of CND dispersion in the presence of different Hg²⁺ concentrations, revealing that the CND solution is sensitive to Hg²⁺ concentration and the PL intensity decreases with the increase of Hg²⁺ concentration. The detection limit is estimated to be 1 nM, which exhibits superior sensitivity than previously reported sensing systems (as shown in Table S1†).^50–52 The detection limits also have been investigated when the PBS detection system was changed to a real water system (lake water, or tap-water). In this real water system, the interference factor is significant, such as ions, bacteria, etc. The lake water (the Yellow River, China) samples were filtered and then centrifuged at 15 000 rpm for 30 min. The resultant water samples were spiked with Hg²⁺ at different concentration levels and then analyzed with the proposed method. The analysis results indicate that the detection limits can reach up to 700 nM and 200 nM for the lake water system and tap-water system, respectively (as shown in Fig. S6†). That is, in spite of the interference from numerous minerals and organics existing in the real water, the CND-based “sensor” can still distinguish between fresh real water and that spiked with Hg²⁺.

Fig. 4 (a) PL emission spectra of the CND dispersion in the absence (curve A) and presence (curve B) of Hg²⁺ ions, (b) PL spectra of the CND dispersion in the presence of different Hg²⁺ concentrations (from top to bottom: 0, 0.0001, 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 20, 30, 40, 50 and 100 μM), (c) the dependence of F⁰/F on the concentrations of Hg²⁺ ions within the range of 0–100 μM, (d) the dependence of F/F⁰ with a blank and solutions containing different metal ions (excitation at 360 nm, [ions] = 100 μM).

The photoluminescence quenching was best described by the Stern–Volmer equation,⁵³

where F and F⁰ are the PL intensity of the CND solution in the presence and absence of Hg²⁺, respectively. [Q] is the concentration of the quencher (i.e., Hg²⁺), and K_SV is the Stern–Volmer constant. The Stern–Volmer plot shown in Fig. 4c does not fit a conventional linear Stern–Volmer equation, indicating both dynamic and static quenching processes occur in this sensor system.^54,55

To evaluate the selectivity of this sensing system, we examined the PL intensity changes in the presence of representative metal ions under the same conditions (Fig. 4d). It is seen that a much lower PL intensity was observed for the CNDs upon addition of Hg²⁺. Moreover, the addition of Co²⁺, (Mn²⁺, Ca²⁺, Al³⁺, [ions] = 100 μM) into the CND–Hg²⁺ mixture gives a slight effect on the detection of Hg²⁺ (Fig. S7†). All these observations indicate that the sensing platform exhibits high selectivity for Hg²⁺ detection. The outstanding selectivity and specificity can be probably attributed to the chemical properties of Hg²⁺ which has a stronger affinity for the carboxylic group on the CND surface than other metal ions. These above observations imply that the CND solution can be used as Hg²⁺ sensor and, upon further development, has potential for practical Hg²⁺ detection.

The possible application of obtained CNDs as cell-imaging agents was explored. The biocompatibility of CNDs was evaluated using human sarcoma cell (S180) lines and human hepatocyte (L02) lines through MTT assay. As shown in Fig. 5a, the obtained CNDs do not impose any significant toxicity to cells and are tolerable even at high dose (960 mg mL⁻¹), hence it is safe for in vitro and in vivo applications. Having established the biocompatibility, we performed in vitro cellular uptake experiments in S180 cells. As shown in Fig. 5b, the S180 cells incubated with CNDs in the medium presented bright, while showing blue and green colours upon excitation at 364 and 488 nm, respectively, by using a laser scanning confocal microscopy (Fig. 5c and d), which is corresponding to the above PL properties. Obviously, bright green luminescence of CNDs in the cell membrane and cell cytoplasm regions was observed, while the image is dark in the cell nucleus, which demonstrates that the CNDs are able to label the cell membrane and cell cytoplasm. Such observations indicate that the CNDs from green tea serve as a potential, ideal probe for cell imaging.

Fig. 5 (a) Cell viability by MTT assay, (b) S180 cells under bright field, by excitation at (c) 364 nm and (d) 488 nm.

4. Conclusion

In summary, water-soluble, manganiferous, photoluminescent carbonaceous nanomaterials have been discovered in a cup of tea. Such CNDs exhibit small particles, amorphous structure and novel photoluminescent properties. The component and surface state investigated by FTIR and XPS demonstrated the excellent water solubility of the CNDs. Moreover, these CNDs can serve as an efficient photoluminescent sensing probe for sensitive, selective detection of Hg²⁺ and cell-imaging. These observations are interesting and important because they provide a very simple, immediate and low cost route toward environmentally friendly production of photoluminescent carbon nanomaterials for sensing, bioimaging, optical imaging and a wide range of other applications.

Acknowledgements

This study is supported by Chongqing Natural Science Foundation (cstc2013jcyjA20023), Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1401111).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11152d

‡ These authors contributed equally to this work.

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