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Facile synthesis of S, N co-doped carbon dots and investigation of their photoluminescence properties† Yue Zhang and Junhui He* A facile one-pot approach to prepare photoluminescent carbon dots (CDs) was developed through hydrothermal treatment of cysteine and citric acid. The obtained CDs show stable and bright blue emission with a quantum yield of 54% and an average lifetime of 11.61 ns. Moreover, the two-photon induced upconversion fluorescence of the CDs was observed and demonstrated. Interestingly, both down and up conversion fluorescence of the CDs show excitation-independent emission, which is quite different from most of the previously reported CDs. Ultrafast spectroscopy was also employed here to study the photoluminescence (PL) properties of the CDs. After characterization using various spectroscopic techniques, a unique PL mechanism for the as-prepared CDs’ fluorescence was proposed accordingly. In addition, the influence of various metal ions on the CD fluorescence was examined and
Received 17th June 2015, Accepted 3rd July 2015
no quenching phenomena were observed. Meanwhile, gold nanoparticles (Au NPs) were found to be good quenchers of CD fluorescence and their quenching behavior was fitted to the Stern–Volmer equation.
DOI: 10.1039/c5cp03498a
This provides new opportunities for fluorescence sensor designs and light energy conversion applications. Finally, the as-prepared CDs were inkjet-printed to form a desirable pattern, which is useful for fluorescent
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patterns, and anti-counterfeiting labeling.
1. Introduction Carbon dots (CDs) have attracted great research interest as promising optical nanomaterials due to their unique characteristics, such as low cost, being environmentally friendly, having high quantum yields, being non-blinking and having high photostability. They have demonstrated prospective applications for bio-imaging,1 sensing,2,3 light-emitting,4 catalysis5,6 and photovoltaics.7 The upconversion fluorescence of CDs has been observed and discussed frequently,8–10 and some possible mechanisms and potential applications have also been proposed.11–13 However, most of the so-called upconversion fluorescence has been characterized by using a spectrofluorometer, which is not convincing. In the debate upon this issue, Wen et al. argued that the reported ‘‘upconversion fluorescence’’ was probably just normal fluorescence excited by the leaking component from the second diffraction in the monochromator of the spectrofluorometer. The same group examined
Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China. E-mail: [email protected]; Fax: +86 10 82543535 † Electronic supplementary information (ESI) available: Detailed method for quantum yield measurement of the CDs and the result about influences of metal ions on CD fluorescence. See DOI: 10.1039/c5cp03498a
20154 | Phys. Chem. Chem. Phys., 2015, 17, 20154--20159
five different synthesized CDs carefully and no upconversion fluorescence was observed, which testified their argument.14 So far, most of the CDs reported by other researchers are sensitive to one or two kinds of metal ions, based on which metal ion sensors have been developed.2,15–20 Other kinds of quenchers for CDs have rarely been reported. A fluorescence resonance energy transfer system containing conjugated organic dye (Ce6) and CDs was designed recently for tumor therapy.21 The energy transfer between the CDs and Ce6 enhanced the singlet oxygen production and thus facilitated in vivo tumor photodynamic therapy treatment. Based on the quenching ability of carbon nanotubes and graphene oxide on CD fluorescence, DNA sensors with high sensitivity and selectivity have been developed as well.22,23 As far as we know, all of the sensing applications based on CDs are on the basis of their fluorescence quenching performance. Thus exploration of various quenchers (electron acceptor/donor) plays a significant role in sensor development based on CDs. The preparation approaches for CDs include top-down and bottom-up methods according to the difference in starting materials. Among various bottom-up methods, hydrothermal treatment of small molecules or polymers provides an efficient and broadly applicable route to CDs.24 It should be noted that the structural features of the CDs derived from hydrothermal approaches are greatly decided by their precursors. Therefore, by delicately selecting the starting materials, CDs with special surface functional groups and/or specified doped elements can
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be obtained. As is known, all of the amino acids contain amino and carboxyl groups, which are of importance for CD formation. Besides, some amino acids possess phenyl, indole, imidazole or thiol groups, which enable the surface functionalization of CDs during synthesis. These advantages thus make amino acids ideal precursors for the bottom-up construction of CDs.25,26 In the current paper, nitrogen (N) and sulfur (S) co-doped CDs with high quantum yield were prepared through hydrothermal treatment of citric acid and cysteine. The photoluminescence (PL) properties of the as-prepared S, N co-doped CDs were intensively investigated. Both steady state and time-resolved fluorescence measurements were conducted in order to interpret the origin of the emission. The two-photon induced upconversion fluorescence was observed and testified. Furthermore, the effect of a variety of metal ions on the CD fluorescence was examined. The energy transfer process between CDs and gold nanoparticles (Au NPs) was studied as well. It is interesting that the CD fluorescence hardly changed with the existence of various metal ions. Meanwhile the Au NPs are good quenchers for CDs, and their quenching behaviors were plotted according to the Stern–Volmer equation. This is a big step toward the construction of nanocatalysts and nanosensors based on CDs and Au NPs that can perform well. Finally, the CDs were used to create fluorescent ink to form desirable patterns, which demonstrated potential for use in anti-counterfeiting.
2. Experimental 2.1
Materials
All reagents were purchased from Beihua Fine Chemicals and used without further purification. Ultrapure water was used in all experiments, and was obtained from a three-state Millipore Milli-Q Plus185 purification system (Academic). Au NPs were prepared by following the procedure in ref. 27. 2.2
Synthesis of S, N co-doped CDs
0.364 g cysteine (3 mmol) and 0.21 g citric acid (1 mmol) were added to 10 mL of water to form a transparent solution, which was then transferred into a 50 mL Teflon lined stainless autoclave. The sealed autoclave was subsequently heated at 200 1C in an electric oven for 5 h. Acetone was added into the obtained product solution, which was then subjected to centrifugation at 15 000 rpm for 5 min. After removal of the supernatant, the yellow brown gel-like product was redispersed into water for later use. 2.3
CD characterization
Fluorescence emission spectra and UV-Vis absorption spectra were recorded on an F-4600 fluorescence spectrophotometer (HITACHI) and a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co.), respectively. Attenuated total reflectionFourier transform infrared (ATR-FTIR) spectra of the CDs were recorded using a Varian Excalibur 3100 spectrometer. For transmission electron microscopy (TEM) observations, samples were cast on carbon-coated copper grids and observed on a JEOL
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JEM-2100F transmission electron microscope at an acceleration voltage of 200 kV. Transient absorption spectra were recorded on a LP920 Laser Flash Photolysis Spectrometer (Edinburgh Instruments Ltd) with an excitation wavelength of 355 nm. Fluorescence lifetimes were measured using an F900 timecorrected single photon counting (TCSPC) system (Edinburgh analytical instruments) with an excitation wavelength of 375 nm and a detection wavelength of 418 nm. 2.4 Measurements of CD fluorescence in the presence of metal ions and Au NPs For assays of the metal ion influence on CD fluorescence, 50 mL of the CD dispersion (0.03 mg mL 1) was mixed with 15 mL of a metal ion aqueous solution (20 mM). Then the mixture was diluted with 2.335 mL of PBS solution (pH = 7.4). Eleven metal ions were examined here including Fe3+, Cu2+, Ba2+, Mg2+, Zn2+, Mn2+, Co2+, Ni2+, Cd2+, Pb2+ and Hg2+. For assays of the Au NP influence on CD fluorescence, varied amounts of the Au NP suspension were added into 30 mL of the CDs dispersion (0.03 mg mL 1), followed by dilution with water to make the mixture’s volume 3 mL.
3. Results and discussion 3.1
Characterization of the CDs
It has been reported that doped N and S render the resulting CDs to contain heteroatoms in the carbon framework, which influence their photoluminescence properties accordingly.5 Cysteine was used for the facile synthesis of S, N co-doped CDs here. Carboxyl and hydroxyl groups have been found to be important in enhancing the quantum yield of CDs.18 Thus citric acid was used in preparing the CDs to provide carboxyl and hydroxyl groups. The CD formation process included the condensation of citric acid and cysteine, and a carbonization process. The UV-Vis absorption, excitation and emission spectra of the CD solution are shown in Fig. 1. The absorption spectrum features a peak focused at 342 nm, which corresponds to the carbonyl or conjugated carbonyl groups. Meanwhile, the excitation spectrum shows two peaks centered at 246 nm and 345 nm, which correspond to the p - p* transition from the carbogenic core and n - p* from the surface/molecule state region, respectively.28 Interestingly, the fluorescence emission of the CDs is around 418 nm showing no significant change upon different excitation wavelengths (see Fig. 1b). Thus it is reasonable to say that the fluorescence emission wavelength of the CDs does not depend on the excitation wavelength. This is quite different from the usually observed excitation-dependent multicolor emissions of CDs prepared by other methods, which usually have broad featureless excitation/absorption spectra at different excitation wavelengths.20,29–32 This phenomenon should be attributed to uniform chemical features on the surface of the as-prepared CDs.25,33,34 The fluorescence quantum yield of the CDs was estimated to be 54% using a quinine sulphate solution in 0.1 M sulphuric acid (quantum yield 54%) as a reference (see Fig. S1, ESI†).
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Fig. 1 (a) UV-Vis absorption (Abs), excitation (Ex) and emission (Em) spectra of the CDs dispersed in water. Inset in (a): photographs of the CD dispersion under daylight (left) and UV light (365 nm, right). (b) Emission spectra at excitation wavelengths ranging from 220 nm to 380 nm. Inset in (b): dependence of PL intensity on excitation wavelength.
Fig. 2 TEM image of the CDs with the inset showing a high-resolution TEM image of an individual CD.
This quantum yield value is comparable or higher than those of previously reported S, N co-doped CDs.5,20,26 Fig. 2 shows a TEM image of the CDs illustrating their morphology and structure. Some aggregation was observed, which may be attributed to strong intermolecular hydrogen bonding between the CDs. The particle diameters were estimated to be 2–6 nm. A discernible lattice structure of the CDs was observed displaying a lattice spacing distance of 0.28 nm (inset in Fig. 2). The IR spectrum (Fig. 3) shows a strong and wide absorption band at around 2974 cm 1 which is the stretching vibration of O–H from intermolecular association. The bands at 3179 cm 1 and 3418 cm 1 correspond to the stretching vibrations of O–H
Fig. 3
IR spectrum of the CDs.
20156 | Phys. Chem. Chem. Phys., 2015, 17, 20154--20159
and N–H, respectively. The sharp bands at 1555 cm 1 and 775 cm 1 are related to the stretching vibration and bending vibration of –COO , respectively. These observations indicate that the CD surface is rich in amino, hydroxyl and carboxyl groups, which results in the CDs having good hydrophilic properties and readily forming intermolecular hydrogen bonds. The strong sharp band at 1389 cm 1 is related to the bending vibration of –C–N. Moreover, the small sharp bands at 1300 cm 1 and 845 cm 1 are assigned to the stretching vibrations of C–O and NH–CHz, respectively. In addition, the small band at 1211 cm 1 corresponds to the stretching vibration of C–S. The relatively weak bands at 1697 cm 1 and 1450 cm 1 are assigned as the stretching vibrations of CQO from amide and CQN, respectively. It should be noted that no significant absorption band of –SH (at 2596 cm 1) was observed but the stretching vibration band of SO–O appears at 1011 cm 1. This implies that –SH has been oxidized to –SO2– in the hydrothermal process, which is consistent with the previous reported results.20 Fig. 4a depicts a fluorescence decay profile of the CDs. The decay curve of the CDs can be fitted by a double-exponential function with lifetimes (t) of 10.08 ns (96.09%) and 49.28 ns (3.91%) at 375 nm excitation yielding an intensity-weighted average lifetime of 11.61 ns. It is clear that the CDs with a shorter lifetime form the majority and dominate the luminescence property. This is consistent with the property of excitationindependent emission, thus suggesting relatively uniform fluorescence radiative processes in this kind of CD.25,33,34 To gain more insight into the PL properties explaining the high quantum yield and long lifetime of CDs, nanosecond broadband (380–700 nm) transient absorption spectroscopy was performed (see Fig. 4b). After the pump light excitation, the photo-excited electrons may further absorb probe light and transfer to higher levels. This process is called excited-state absorption (ESA) and shows positive signals. Besides, the electrons in the excited state could return to the ground state via radiation. This process is called stimulated emission (SE) and shows negative signals. Despite most of the transient absorptions of CDs having been investigated using ultra-fast spectroscopy with femtosecond to picosecond resolution,35–37 the results in this study using nanosecond laser excitation also provided much valuable information. At around 410–420 nm, there are strong negative signals, which
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Fig. 4 (a) Fluorescence decay profile of the CDs. (b) Nanosecond transient absorption spectra for CDs.
are consistent with their blue fluorescence emissions. Thus this state is assigned to SE. The large SE signals illustrate the high fluorescence quantum yield of CDs. Interestingly, the negative signals (SE) could be observed until 500 ns after excitation. This result further illustrates the long lifetime of the as-prepared CDs. In addition, both the positive signal peaks around 400 nm and the positive signal valleys around 420 nm were observed after 500 nanoseconds as the negative signals diminished (see inset in Fig. 4b). The positive signal peaks are assigned to ESA and the signal valleys are attributed to the weak fluorescence emission disturbance. Basically, all of the curves show wide positive absorption peaks at around 550 nm, which are designated to ESA.38
dominant process. To further elucidate this phenomenon, the dependence of fluorescence intensity on excitation power was investigated (see Fig. 5b). The inset in Fig. 5b shows the relationship between the logarithm of excitation power and the logarithm of luminescence intensity plotted by linear fit. The slope of the plot is ca. 2, thus it is confirmed that the upconversion fluorescence is induced by two-photon absorption. The upconversion fluorescence of the CDs opens up new opportunities for cell imaging with two-photon luminescence microscopy, as well as highly efficient catalyst design.
3.2
By summarizing the spectroscopic observations, a PL mechanism was proposed for the as-prepared CDs, and is depicted in Fig. 6. An intrinsic band gap state (246 nm) from the carbogenic core accompanied with a surface/molecule state (345 nm) accounts for the strong blue fluorescence centered at 418 nm. Since the fluorescence emissions are irrespective of the excitation wavelengths, it is reasonable to conclude that non-radiative transitions (thermal relaxation/electron transfer) between the carbogenic core and surface state are very likely to occur. Moreover, in the upconversion PL profile of the CDs, electrons jump to the excited state by absorbing two photons simultaneously and return to the ground state through emitting blue fluorescence as well.
Upconversion fluorescence
Interestingly, the upconversion fluorescence of the CD dispersion was observed as revealed by Fig. 5. It is noteworthy that the upconversion fluorescence also shows a maximum emission at around 418 nm. Moreover, these emissions hardly shift upon excitation at various wavelengths in the range of 720–800 nm. What has been observed here is consistent with that of the down conversion fluorescence. Therefore, it is deduced that both up and down conversion fluorescence emissions occur from the lowest singlet state irrespective of the mode of excitation. Specifically, for a multiple photon induced fluorescence process, the fluorescence intensity will exhibit an excitation intensity (Iex) dependent fluorescence (Iex)n for an n photon
3.3
PL mechanism
Fig. 5 Upconversion fluorescence spectra of CDs at (a) excitation wavelengths from 720 nm to 800 nm and (b) excitation powers from 40 mW to 320 mW. Inset in (a): dependence of upconversion PL intensity of CDs on excitation wavelength (up) and photographs of CDs dispersion under daylight and excited by 730 nm laser (down). Inset in (b): plot of logarithm of excitation power and logarithm of luminescence intensity.
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Fig. 8 Two-dimension code pattern printed by using a CD ink on filter paper under daylight (left) and illuminated by a portable UV lamp (right).
Fig. 6
3.4
The proposed PL mechanism for as-prepared CDs.
Influence of metal ions and Au NPs on CD fluorescence
Quite a few of the CDs previously reported are sensitive to certain metal ions and their fluorescence is readily quenched by interaction with the metal ion. To find out whether it is the same case for our prepared CDs, the effect of various metal ions (100 mM) on the PL behavior of the CDs was studied. It should be noted that PBS buffer solution (pH = 7.4) was used in all of the samples here to avoid pH interference. Interestingly, none of the tested metal ions (100 mM) can quench the CD fluorescence effectively as revealed by Fig. S2 (ESI†), which means that there is no energy transfer between the CDs and the tested metal ions. These results imply that there are few thiol groups on the CDs’ surface, which show strong affinity with some heavy metal ions (Pb2+ and Hg2+). This inference agrees well with the IR results, which do not show any absorption band for the thiol group. Au NPs are known to be a kind of promising quencher for a wide range of fluorophores. In addition, to the best of our knowledge, the energy transfer between Au NPs and CD fluorescence has never been examined. In this work, the quenching effect of Au NPs (B5.5 nm) on CDs was investigated as well. It can be seen from Fig. 7 that Au NPs are effective quenchers of CD fluorescence. As the concentration of Au NPs increases, the fluorescence of the CDs decreases gradually. The PL quenching was analyzed using the Stern–Volmer equation:39 F0/F
1 = K [Q]
Fig. 7 Fluorescence spectra of CDs in the presence of varied amounts of Au NPs. Inset: the relationship of the Au NP concentration and F0/F 1.
20158 | Phys. Chem. Chem. Phys., 2015, 17, 20154--20159
where F0 and F are the fluorescence intensities in the absence and presence of Au NPs respectively; K is the Stern–Volmer quenching constant (fitted to be 0.82); [Q] is the concentration of the Au NPs. Fluorescence quenching is attributed to nonradiative electron transfer from the excited state of the CDs to Au NPs. It is implied that this energy transfer system containing CDs and Au NPs has the potential for sensing applications. Taking advantage of the high quantum yield and high stability, the CDs were utilized to create a fluorescent ink for printing patterns. A piece of filter paper (the paper showed no background UV fluorescence) was chosen as the printing substrate. A colorless aqueous solution of CDs was injected into a vacant cartridge from a commercial inkjet printer. As illustrated in Fig. 8, the invisible image could only be observed under UV illumination. This result demonstrates that the as-prepared CDs are very promising for anti-counterfeiting applications.18,40
4. Conclusions In conclusion, CDs were prepared through hydrothermal treatment of cysteine in the presence of citric acid. The as-prepared CDs demonstrated a high quantum yield (54%) and long lifetime (11.61 ns), which were further illustrated using nanosecond transient spectroscopy. Interestingly, both down and up conversion fluorescence of the CDs were observed and found to be independent of the excitation wavelength, which is quite different from most of the previously reported CDs. Furthermore, a unique PL mechanism for the fascinating CDs was proposed according to the obtained spectroscopic results. Undoubtedly, understanding the PL mechanisms of CDs is of benefit for CD design and application. It was demonstrated that a variety of metal ions did not have an influence on the fluorescence of the CDs, which implied the passivated surface chemistry and stability of the CDs. Meanwhile, the fluorescence quenching behavior of CDs by Au NPs was achieved for the first time and plotted using the Stern–Volmer equation. These interesting energy transfer properties of CDs as electron donors may offer new opportunities for sensor design, light energy conversion, as well as catalysis and related applications. Finally, the CDs were used to create fluorescent inks for pattern printing, which demonstrates that the CDs are promising for anticounterfeiting applications.
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Acknowledgements This research was financially supported by the Innovative Talents Cultivation Project of Technical Institute of Physics and Chemistry, the Chinese Academy of Sciences (2014-Z) and the Laboratory Open Fund of Beijing Institute of Microchemistry (TY2014B01).
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Facile synthesis of S, N co-doped carbon dots and investigation of their photoluminescence properties† Yue Zhang and Junhui He* A facile one-pot approach to prepare photoluminescent carbon dots (CDs) was developed through hydrothermal treatment of cysteine and citric acid. The obtained CDs show stable and bright blue emission with a quantum yield of 54% and an average lifetime of 11.61 ns. Moreover, the two-photon induced upconversion fluorescence of the CDs was observed and demonstrated. Interestingly, both down and up conversion fluorescence of the CDs show excitation-independent emission, which is quite different from most of the previously reported CDs. Ultrafast spectroscopy was also employed here to study the photoluminescence (PL) properties of the CDs. After characterization using various spectroscopic techniques, a unique PL mechanism for the as-prepared CDs’ fluorescence was proposed accordingly. In addition, the influence of various metal ions on the CD fluorescence was examined and
Received 17th June 2015, Accepted 3rd July 2015
no quenching phenomena were observed. Meanwhile, gold nanoparticles (Au NPs) were found to be good quenchers of CD fluorescence and their quenching behavior was fitted to the Stern–Volmer equation.
DOI: 10.1039/c5cp03498a
This provides new opportunities for fluorescence sensor designs and light energy conversion applications. Finally, the as-prepared CDs were inkjet-printed to form a desirable pattern, which is useful for fluorescent
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patterns, and anti-counterfeiting labeling.
1. Introduction Carbon dots (CDs) have attracted great research interest as promising optical nanomaterials due to their unique characteristics, such as low cost, being environmentally friendly, having high quantum yields, being non-blinking and having high photostability. They have demonstrated prospective applications for bio-imaging,1 sensing,2,3 light-emitting,4 catalysis5,6 and photovoltaics.7 The upconversion fluorescence of CDs has been observed and discussed frequently,8–10 and some possible mechanisms and potential applications have also been proposed.11–13 However, most of the so-called upconversion fluorescence has been characterized by using a spectrofluorometer, which is not convincing. In the debate upon this issue, Wen et al. argued that the reported ‘‘upconversion fluorescence’’ was probably just normal fluorescence excited by the leaking component from the second diffraction in the monochromator of the spectrofluorometer. The same group examined
Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China. E-mail: [email protected]; Fax: +86 10 82543535 † Electronic supplementary information (ESI) available: Detailed method for quantum yield measurement of the CDs and the result about influences of metal ions on CD fluorescence. See DOI: 10.1039/c5cp03498a
20154 | Phys. Chem. Chem. Phys., 2015, 17, 20154--20159
five different synthesized CDs carefully and no upconversion fluorescence was observed, which testified their argument.14 So far, most of the CDs reported by other researchers are sensitive to one or two kinds of metal ions, based on which metal ion sensors have been developed.2,15–20 Other kinds of quenchers for CDs have rarely been reported. A fluorescence resonance energy transfer system containing conjugated organic dye (Ce6) and CDs was designed recently for tumor therapy.21 The energy transfer between the CDs and Ce6 enhanced the singlet oxygen production and thus facilitated in vivo tumor photodynamic therapy treatment. Based on the quenching ability of carbon nanotubes and graphene oxide on CD fluorescence, DNA sensors with high sensitivity and selectivity have been developed as well.22,23 As far as we know, all of the sensing applications based on CDs are on the basis of their fluorescence quenching performance. Thus exploration of various quenchers (electron acceptor/donor) plays a significant role in sensor development based on CDs. The preparation approaches for CDs include top-down and bottom-up methods according to the difference in starting materials. Among various bottom-up methods, hydrothermal treatment of small molecules or polymers provides an efficient and broadly applicable route to CDs.24 It should be noted that the structural features of the CDs derived from hydrothermal approaches are greatly decided by their precursors. Therefore, by delicately selecting the starting materials, CDs with special surface functional groups and/or specified doped elements can
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be obtained. As is known, all of the amino acids contain amino and carboxyl groups, which are of importance for CD formation. Besides, some amino acids possess phenyl, indole, imidazole or thiol groups, which enable the surface functionalization of CDs during synthesis. These advantages thus make amino acids ideal precursors for the bottom-up construction of CDs.25,26 In the current paper, nitrogen (N) and sulfur (S) co-doped CDs with high quantum yield were prepared through hydrothermal treatment of citric acid and cysteine. The photoluminescence (PL) properties of the as-prepared S, N co-doped CDs were intensively investigated. Both steady state and time-resolved fluorescence measurements were conducted in order to interpret the origin of the emission. The two-photon induced upconversion fluorescence was observed and testified. Furthermore, the effect of a variety of metal ions on the CD fluorescence was examined. The energy transfer process between CDs and gold nanoparticles (Au NPs) was studied as well. It is interesting that the CD fluorescence hardly changed with the existence of various metal ions. Meanwhile the Au NPs are good quenchers for CDs, and their quenching behaviors were plotted according to the Stern–Volmer equation. This is a big step toward the construction of nanocatalysts and nanosensors based on CDs and Au NPs that can perform well. Finally, the CDs were used to create fluorescent ink to form desirable patterns, which demonstrated potential for use in anti-counterfeiting.
2. Experimental 2.1
Materials
All reagents were purchased from Beihua Fine Chemicals and used without further purification. Ultrapure water was used in all experiments, and was obtained from a three-state Millipore Milli-Q Plus185 purification system (Academic). Au NPs were prepared by following the procedure in ref. 27. 2.2
Synthesis of S, N co-doped CDs
0.364 g cysteine (3 mmol) and 0.21 g citric acid (1 mmol) were added to 10 mL of water to form a transparent solution, which was then transferred into a 50 mL Teflon lined stainless autoclave. The sealed autoclave was subsequently heated at 200 1C in an electric oven for 5 h. Acetone was added into the obtained product solution, which was then subjected to centrifugation at 15 000 rpm for 5 min. After removal of the supernatant, the yellow brown gel-like product was redispersed into water for later use. 2.3
CD characterization
Fluorescence emission spectra and UV-Vis absorption spectra were recorded on an F-4600 fluorescence spectrophotometer (HITACHI) and a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co.), respectively. Attenuated total reflectionFourier transform infrared (ATR-FTIR) spectra of the CDs were recorded using a Varian Excalibur 3100 spectrometer. For transmission electron microscopy (TEM) observations, samples were cast on carbon-coated copper grids and observed on a JEOL
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JEM-2100F transmission electron microscope at an acceleration voltage of 200 kV. Transient absorption spectra were recorded on a LP920 Laser Flash Photolysis Spectrometer (Edinburgh Instruments Ltd) with an excitation wavelength of 355 nm. Fluorescence lifetimes were measured using an F900 timecorrected single photon counting (TCSPC) system (Edinburgh analytical instruments) with an excitation wavelength of 375 nm and a detection wavelength of 418 nm. 2.4 Measurements of CD fluorescence in the presence of metal ions and Au NPs For assays of the metal ion influence on CD fluorescence, 50 mL of the CD dispersion (0.03 mg mL 1) was mixed with 15 mL of a metal ion aqueous solution (20 mM). Then the mixture was diluted with 2.335 mL of PBS solution (pH = 7.4). Eleven metal ions were examined here including Fe3+, Cu2+, Ba2+, Mg2+, Zn2+, Mn2+, Co2+, Ni2+, Cd2+, Pb2+ and Hg2+. For assays of the Au NP influence on CD fluorescence, varied amounts of the Au NP suspension were added into 30 mL of the CDs dispersion (0.03 mg mL 1), followed by dilution with water to make the mixture’s volume 3 mL.
3. Results and discussion 3.1
Characterization of the CDs
It has been reported that doped N and S render the resulting CDs to contain heteroatoms in the carbon framework, which influence their photoluminescence properties accordingly.5 Cysteine was used for the facile synthesis of S, N co-doped CDs here. Carboxyl and hydroxyl groups have been found to be important in enhancing the quantum yield of CDs.18 Thus citric acid was used in preparing the CDs to provide carboxyl and hydroxyl groups. The CD formation process included the condensation of citric acid and cysteine, and a carbonization process. The UV-Vis absorption, excitation and emission spectra of the CD solution are shown in Fig. 1. The absorption spectrum features a peak focused at 342 nm, which corresponds to the carbonyl or conjugated carbonyl groups. Meanwhile, the excitation spectrum shows two peaks centered at 246 nm and 345 nm, which correspond to the p - p* transition from the carbogenic core and n - p* from the surface/molecule state region, respectively.28 Interestingly, the fluorescence emission of the CDs is around 418 nm showing no significant change upon different excitation wavelengths (see Fig. 1b). Thus it is reasonable to say that the fluorescence emission wavelength of the CDs does not depend on the excitation wavelength. This is quite different from the usually observed excitation-dependent multicolor emissions of CDs prepared by other methods, which usually have broad featureless excitation/absorption spectra at different excitation wavelengths.20,29–32 This phenomenon should be attributed to uniform chemical features on the surface of the as-prepared CDs.25,33,34 The fluorescence quantum yield of the CDs was estimated to be 54% using a quinine sulphate solution in 0.1 M sulphuric acid (quantum yield 54%) as a reference (see Fig. S1, ESI†).
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Fig. 1 (a) UV-Vis absorption (Abs), excitation (Ex) and emission (Em) spectra of the CDs dispersed in water. Inset in (a): photographs of the CD dispersion under daylight (left) and UV light (365 nm, right). (b) Emission spectra at excitation wavelengths ranging from 220 nm to 380 nm. Inset in (b): dependence of PL intensity on excitation wavelength.
Fig. 2 TEM image of the CDs with the inset showing a high-resolution TEM image of an individual CD.
This quantum yield value is comparable or higher than those of previously reported S, N co-doped CDs.5,20,26 Fig. 2 shows a TEM image of the CDs illustrating their morphology and structure. Some aggregation was observed, which may be attributed to strong intermolecular hydrogen bonding between the CDs. The particle diameters were estimated to be 2–6 nm. A discernible lattice structure of the CDs was observed displaying a lattice spacing distance of 0.28 nm (inset in Fig. 2). The IR spectrum (Fig. 3) shows a strong and wide absorption band at around 2974 cm 1 which is the stretching vibration of O–H from intermolecular association. The bands at 3179 cm 1 and 3418 cm 1 correspond to the stretching vibrations of O–H
Fig. 3
IR spectrum of the CDs.
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and N–H, respectively. The sharp bands at 1555 cm 1 and 775 cm 1 are related to the stretching vibration and bending vibration of –COO , respectively. These observations indicate that the CD surface is rich in amino, hydroxyl and carboxyl groups, which results in the CDs having good hydrophilic properties and readily forming intermolecular hydrogen bonds. The strong sharp band at 1389 cm 1 is related to the bending vibration of –C–N. Moreover, the small sharp bands at 1300 cm 1 and 845 cm 1 are assigned to the stretching vibrations of C–O and NH–CHz, respectively. In addition, the small band at 1211 cm 1 corresponds to the stretching vibration of C–S. The relatively weak bands at 1697 cm 1 and 1450 cm 1 are assigned as the stretching vibrations of CQO from amide and CQN, respectively. It should be noted that no significant absorption band of –SH (at 2596 cm 1) was observed but the stretching vibration band of SO–O appears at 1011 cm 1. This implies that –SH has been oxidized to –SO2– in the hydrothermal process, which is consistent with the previous reported results.20 Fig. 4a depicts a fluorescence decay profile of the CDs. The decay curve of the CDs can be fitted by a double-exponential function with lifetimes (t) of 10.08 ns (96.09%) and 49.28 ns (3.91%) at 375 nm excitation yielding an intensity-weighted average lifetime of 11.61 ns. It is clear that the CDs with a shorter lifetime form the majority and dominate the luminescence property. This is consistent with the property of excitationindependent emission, thus suggesting relatively uniform fluorescence radiative processes in this kind of CD.25,33,34 To gain more insight into the PL properties explaining the high quantum yield and long lifetime of CDs, nanosecond broadband (380–700 nm) transient absorption spectroscopy was performed (see Fig. 4b). After the pump light excitation, the photo-excited electrons may further absorb probe light and transfer to higher levels. This process is called excited-state absorption (ESA) and shows positive signals. Besides, the electrons in the excited state could return to the ground state via radiation. This process is called stimulated emission (SE) and shows negative signals. Despite most of the transient absorptions of CDs having been investigated using ultra-fast spectroscopy with femtosecond to picosecond resolution,35–37 the results in this study using nanosecond laser excitation also provided much valuable information. At around 410–420 nm, there are strong negative signals, which
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Fig. 4 (a) Fluorescence decay profile of the CDs. (b) Nanosecond transient absorption spectra for CDs.
are consistent with their blue fluorescence emissions. Thus this state is assigned to SE. The large SE signals illustrate the high fluorescence quantum yield of CDs. Interestingly, the negative signals (SE) could be observed until 500 ns after excitation. This result further illustrates the long lifetime of the as-prepared CDs. In addition, both the positive signal peaks around 400 nm and the positive signal valleys around 420 nm were observed after 500 nanoseconds as the negative signals diminished (see inset in Fig. 4b). The positive signal peaks are assigned to ESA and the signal valleys are attributed to the weak fluorescence emission disturbance. Basically, all of the curves show wide positive absorption peaks at around 550 nm, which are designated to ESA.38
dominant process. To further elucidate this phenomenon, the dependence of fluorescence intensity on excitation power was investigated (see Fig. 5b). The inset in Fig. 5b shows the relationship between the logarithm of excitation power and the logarithm of luminescence intensity plotted by linear fit. The slope of the plot is ca. 2, thus it is confirmed that the upconversion fluorescence is induced by two-photon absorption. The upconversion fluorescence of the CDs opens up new opportunities for cell imaging with two-photon luminescence microscopy, as well as highly efficient catalyst design.
3.2
By summarizing the spectroscopic observations, a PL mechanism was proposed for the as-prepared CDs, and is depicted in Fig. 6. An intrinsic band gap state (246 nm) from the carbogenic core accompanied with a surface/molecule state (345 nm) accounts for the strong blue fluorescence centered at 418 nm. Since the fluorescence emissions are irrespective of the excitation wavelengths, it is reasonable to conclude that non-radiative transitions (thermal relaxation/electron transfer) between the carbogenic core and surface state are very likely to occur. Moreover, in the upconversion PL profile of the CDs, electrons jump to the excited state by absorbing two photons simultaneously and return to the ground state through emitting blue fluorescence as well.
Upconversion fluorescence
Interestingly, the upconversion fluorescence of the CD dispersion was observed as revealed by Fig. 5. It is noteworthy that the upconversion fluorescence also shows a maximum emission at around 418 nm. Moreover, these emissions hardly shift upon excitation at various wavelengths in the range of 720–800 nm. What has been observed here is consistent with that of the down conversion fluorescence. Therefore, it is deduced that both up and down conversion fluorescence emissions occur from the lowest singlet state irrespective of the mode of excitation. Specifically, for a multiple photon induced fluorescence process, the fluorescence intensity will exhibit an excitation intensity (Iex) dependent fluorescence (Iex)n for an n photon
3.3
PL mechanism
Fig. 5 Upconversion fluorescence spectra of CDs at (a) excitation wavelengths from 720 nm to 800 nm and (b) excitation powers from 40 mW to 320 mW. Inset in (a): dependence of upconversion PL intensity of CDs on excitation wavelength (up) and photographs of CDs dispersion under daylight and excited by 730 nm laser (down). Inset in (b): plot of logarithm of excitation power and logarithm of luminescence intensity.
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Fig. 8 Two-dimension code pattern printed by using a CD ink on filter paper under daylight (left) and illuminated by a portable UV lamp (right).
Fig. 6
3.4
The proposed PL mechanism for as-prepared CDs.
Influence of metal ions and Au NPs on CD fluorescence
Quite a few of the CDs previously reported are sensitive to certain metal ions and their fluorescence is readily quenched by interaction with the metal ion. To find out whether it is the same case for our prepared CDs, the effect of various metal ions (100 mM) on the PL behavior of the CDs was studied. It should be noted that PBS buffer solution (pH = 7.4) was used in all of the samples here to avoid pH interference. Interestingly, none of the tested metal ions (100 mM) can quench the CD fluorescence effectively as revealed by Fig. S2 (ESI†), which means that there is no energy transfer between the CDs and the tested metal ions. These results imply that there are few thiol groups on the CDs’ surface, which show strong affinity with some heavy metal ions (Pb2+ and Hg2+). This inference agrees well with the IR results, which do not show any absorption band for the thiol group. Au NPs are known to be a kind of promising quencher for a wide range of fluorophores. In addition, to the best of our knowledge, the energy transfer between Au NPs and CD fluorescence has never been examined. In this work, the quenching effect of Au NPs (B5.5 nm) on CDs was investigated as well. It can be seen from Fig. 7 that Au NPs are effective quenchers of CD fluorescence. As the concentration of Au NPs increases, the fluorescence of the CDs decreases gradually. The PL quenching was analyzed using the Stern–Volmer equation:39 F0/F
1 = K [Q]
Fig. 7 Fluorescence spectra of CDs in the presence of varied amounts of Au NPs. Inset: the relationship of the Au NP concentration and F0/F 1.
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where F0 and F are the fluorescence intensities in the absence and presence of Au NPs respectively; K is the Stern–Volmer quenching constant (fitted to be 0.82); [Q] is the concentration of the Au NPs. Fluorescence quenching is attributed to nonradiative electron transfer from the excited state of the CDs to Au NPs. It is implied that this energy transfer system containing CDs and Au NPs has the potential for sensing applications. Taking advantage of the high quantum yield and high stability, the CDs were utilized to create a fluorescent ink for printing patterns. A piece of filter paper (the paper showed no background UV fluorescence) was chosen as the printing substrate. A colorless aqueous solution of CDs was injected into a vacant cartridge from a commercial inkjet printer. As illustrated in Fig. 8, the invisible image could only be observed under UV illumination. This result demonstrates that the as-prepared CDs are very promising for anti-counterfeiting applications.18,40
4. Conclusions In conclusion, CDs were prepared through hydrothermal treatment of cysteine in the presence of citric acid. The as-prepared CDs demonstrated a high quantum yield (54%) and long lifetime (11.61 ns), which were further illustrated using nanosecond transient spectroscopy. Interestingly, both down and up conversion fluorescence of the CDs were observed and found to be independent of the excitation wavelength, which is quite different from most of the previously reported CDs. Furthermore, a unique PL mechanism for the fascinating CDs was proposed according to the obtained spectroscopic results. Undoubtedly, understanding the PL mechanisms of CDs is of benefit for CD design and application. It was demonstrated that a variety of metal ions did not have an influence on the fluorescence of the CDs, which implied the passivated surface chemistry and stability of the CDs. Meanwhile, the fluorescence quenching behavior of CDs by Au NPs was achieved for the first time and plotted using the Stern–Volmer equation. These interesting energy transfer properties of CDs as electron donors may offer new opportunities for sensor design, light energy conversion, as well as catalysis and related applications. Finally, the CDs were used to create fluorescent inks for pattern printing, which demonstrates that the CDs are promising for anticounterfeiting applications.
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Acknowledgements This research was financially supported by the Innovative Talents Cultivation Project of Technical Institute of Physics and Chemistry, the Chinese Academy of Sciences (2014-Z) and the Laboratory Open Fund of Beijing Institute of Microchemistry (TY2014B01).
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