Excitation-independent carbon dots, from photoluminescence mechanism to single-color application

Abstract

Carbon dots (Cdots) have attracted much interest recently because of their extraordinary and tunable optical properties. However, the poor understanding of their photoluminescence (PL) mechanism limited the universality of Cdots. In this paper, we prepared the raw-Cdots via a facile hydrothermal method; Green- and Blue-Cdots were obtained from the raw-Cdots as they showed excitation-independent green and blue PL emission. The photoluminescence mechanism was therefore studied with the two kinds of single-color Cdots. Two kinds of Cdots have almost the same element contents but different oxygen- and nitrogen-containing groups. The excitation-independent PL mechanism of Cdots was proposed in combination with their optical properties and surface groups. The π-conjugated electron system acts as the center for quantum confinement and photo absorption, surface groups provide different vibration relaxation for their excitation-independent emission and large Stokes shift. This study not only provides the excitation-independent PL mechanism but also shows the insightful guidance for fluorescence applications of Cdots with excitation-independent PL. Given their complementary emission and excitation wavelength properties, the single color fluorescence from Green-Cdots and Blue-Cdots were used for doubly encrypted characters.

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Introduction

Carbon dots (Cdots), as a kind of nascent carbon materials, have received significant breakthrough in last years because of their good photoluminescence (PL) properties.¹ Cdots are expected to act as promising alternative for organic dyes and metal-based quantum dots,² by using their high biocompatibility and photo-stability. In this content, the multi-color and/or controllable color of Cdots becomes critical for their extensive application. However, most of the multi-color of previous Cdots is excitation-dependent color change,^3–6 which limits the application on single excitation.

PL characteristics of Cdots are distinctive and PL mechanism have been studied recently, involving the effects of the quantum confinement, surface state, and molecular state.^7,8 However, it is difficult to characterize the factors influencing their emission precisely. For instance, the emission of Cdots are generally dependent on excitation wavelength,^3–6 whereas excitation-independent emission is seldom observed.^9,10 Cdots show large Stokes shift compared with the other fluorescence probes.^1–3 Surface state of Cdots was considered to govern their emission,¹¹ but the emission was seemingly related to the size distribution of Cdots in terms of quantum confinement.^12,13 Thus, a controversial debate was proposed that how these factors contribute to the optical properties of Cdots.^14,15 The other issue is what factor contributes the large Stokes shift.

Single-particle PL spectrum revealed that Cdots could give the excitation-independent emission at single dot level.^16,17 The excitation-dependent emission was considered to originate from the heterogeneity from different Cdots. So, if homogeneous Cdots can be obtained, they may exhibit the excitation-independent property in their ensemble emission spectroscopy.^15,17 However, a primary question is proposed that how homogeneous particles can be obtained for the real application. Additionally, the origin of excitation-independent emission to homogeneous Cdots is still a subject of scientific debates. The structure–property relationship is pivotal for the development and application of Cdots even we have revealed the mechanism of PL and electrochemiluminescence from Cdots in terms of their carbon core–functional shell structure previously.^6,18

To reveal the PL mechanism of Cdots, here we use the extensively reported Cdots, which are prepared with citric acid and ethylenediamine as the precursors. Although the raw-Cdots exhibited excitation-dependent PL spectra similar to the previous reports,^3–6 the fractions, obtained from size-exclusion chromatography (SEC) with different elution time,^19–21 gave the emission from blue to green. Two fractions of blue and green Cdots were selected because of their excitation-independent blue- and green-emission. Their maximum excitation was also different to each other. The SEC separation mechanism, in combination of the results from transmission electron microscope (TEM), dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), and Fourier Transform Infrared Spectrometer (FTIR), was used to propose the excitation-independent emission mechanism of Cdots. We found that the two kinds of Cdots have almost the same element contents but different oxygen- and nitrogen-containing groups. While the different π-conjugation system and structure of Cdots governed the photon absorption for excitation, different surface functional groups provided different vibration relaxation for their excitation-independent emission. The surface self-traping for vibration relaxation resulted in their large Stokes shift.

This study not only provides the PL mechanism but also shows the insightful guidance for fluorescence application of the Cdots with excitation-independent PL. Different to previous works,^22,23 the different emission from the two kinds of Cdots at the same excition was validated as fluorescence ink for doubly encrypted characters. The calligraphy written with the Cdots was invisible under daylight. While all characters were clearly observed under the excitation of 302 nm, only the parts written with Blue-Cdots were observed under the excitation of 365 nm.

Experimental

Reagents

Citric acid (CA, C₆H₈O₇·H₂O) and ethylenediamine (EDA, C₂H₈N₂) were obtained from Tianjin chemical reagent Co., Tianjin, China. Ethanol was purchased from Concord Reagent Co., Tianjin, China. Sodium borohydride (NaBH₄) was purchased from MYM Biological Technology Co., Beijing, China. All of the reagents were analytically pure and used without any purification. Milli-Q grade water was used throughout. Size-exclusion column chromatography was conducted over Sephadex G-25 gel which was obtained from GE Healthcare, Connecticut, USA.

Equipments

Transmission electron microscopy (TEM) images were recorded with Tecnai G2 F20, (FEI Co., U.S.A.) operated at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos Axis Ultra DLD spectrometer fitted with a monochromated Al Kα X-ray source (hν = 1486.6 eV), hybrid (magnetic/electrostatic) optics, a multichannel plate and delay line detector (Kratos Analytical, Manchester, UK). The hydrodynamic sizes of Cdots were tested at 25 °C and recorded with a Zetasizer Nano ZS (Malvern, British) with a 633 nm He–Ne laser. Absorption spectra were recorded on a UV-3600 UV-vis-NIR spectrophotometer (Shimadzu, Tokyo, Japan). The PL measurements were performed on a Hitachi F-4600 spectrofluorometer (Tokyo, Japan). PL quantum yields were obtained in a calibrated integrating sphere in FLS920 spectrometer (Edinburgh, U.K.). Fourier transform infrared (FTIR) spectra (4000–400 cm⁻¹) in KBr were recorded on a Magna-560 spectrometer (Nicolet, Madison, WI, USA).

Preparation of raw-Cdots

Cdots were first synthesized by hydrothermal procedure, similar to previous work and denoted as raw-Cdots. Briefly, 0.920 g of CA and 335 μL of EDA were dissolved in 10 mL of distilled water, and sonicated for 15 min to obtain a homogeneous solution. Then the solution was transferred to a 30 mL Teflon-lined autoclave and heated at constant temperature of 200 °C for 4 h. After the reaction, the autoclaves were cooled to room temperature naturally. The resulted dark brown and transparent solution was then loaded into dialysis membrane with 1000 Da MWCO for dialysis against ultra-pure water for 48 h to obtain the raw-Cdots.

Separation of raw-Cdots into blue fraction and green fraction

Raw-Cdots were separated by column chromatography on a Sephadex G-25 gel filtration column using water–ethanol (1 : 1, v/v) as the eluent. The gel (5 g) was swollen in eluent for 6 h, and the supernatant (including the suspended ultrafine gel) was discarded for the preparation of the column. Slurry media was settled in a ratio of 75% gel to 25% eluent and degassed ultrasonically. A glass column (13 mm inner diameter) was filled with eluent to remove air bubbles, and then closed. Flush the column with eluent, leaving a few milliliters at the bottom, the column was then opened for the continuous addition of the gel suspension. The gel-filled column was washed until no change in height (11 cm) was observed.¹⁹ The raw-Cdots aqueous solution was then added to the gel column and eluted with the eluent. The wavelength of 302 nm was used to monitor the specimens eluting from the column, and the eluted solution was collected for further use. As a result, two fractions of Cdots, named Blue-Cdots and Green-Cdots, were collected according to their PL properties.

Treatment of Cdots with NaBH₄

The Green-Cdots and Blue-Cdots solutions obtained via SEC separation were treated with NaBH₄ (100 mg) under vigorous stirring at room temperature for 1 h, respectively. Then, HNO₃ solution was added dropwise to adjust to pH 7.0. The obtained NaBH₄-treated Cdots were tested to validate the effect of surface groups on the emission.

Fluorescence images of Cdots stained species

The two kinds of Cdots were dissolved in distilled water. Two pieces of dialysis membranes were placed into Blue- and Green-Cdots solutions and vibrated for 2 h to adsorb the Cdots on the membrane. Then the membranes were washed with pure water three times and soaked into water for imaging. Cotton thread was soaked by distilled water, Blue-Cdots, and Green-Cdots, respectively. The coated threads were dried naturally for imaging. The Cdots-imbedded agar gel membrane was obtained by boiling a mixture of agar powder, Cdots and water. Paper sheet used in calligraphy were written with “n”, “k”, and “u” in different Cdot solutions, which were Blue-Cdots, mixed-Cdots and Green-Cdots, respectively. And all the fluorescent images of the calligraphy were captured after dried naturally.

Results and discussion

Preparation of Blue- and Green-Cdots

To illustrate the generalization of proposed mechanism, a general preparation strategy for Cdots was selected with the hydrothermal treatment of citric acid and ethylenediamine. Fig. 1 illustrated the procedure for the preparation of single-color Cdots. When the clear and transparent citric acid and ethylenediamine solution became dark-brown after hydrothermal treatment (Fig. 1a), strong fluorescence was observed under UV excitation, the same to previous reports and denoted as raw-Cdots.^15,17 Hydrothermal reaction has been extensively used to prepare Cdots. However, most of Cdots were reported with the emission of blue or green fluorescence even if different precursors were used.²⁴ This is the reason why raw-Cdots showed excitation-dependent emission (Fig. 1b). Inadvertently, the complicated PL spectra attracted our attention: what factors influence the emission of Cdots and whether can we obtain the Cdots with single-color emission, that is, excitation-independent emission?

Fig. 1 (a–d) The synthesis and separation of the raw-Cdots. Photographs of the effluent fraction of raw-Cdots arranged in the order of retention time under (e) white light and (f) UV lamp.

We separated the raw-Cdots with SEC column as shown in Fig. 1c. The Cdots with the emission from blue to green were obtained selectively (Fig. 1d). The emission peaks of the two Cdots remain at around 460 and 530 nm, indicating that the emission of raw-Cdots was composited with that of different fractions. Fig. 1e illustrated the different effluent fractions from raw-Cdots with Sephadex G-25 SEC column and water–ethanol (1 : 1, v/v) as eluent. Under UV light excitation, bright emission with different color is strong enough to be distinguished with each other by naked eye. Two main parts with blue-emission Cdots (Blue-Cdots) and green-emission Cdots (Green-Cdots), as well as some mixed-color Cdots, were separated from the raw-Cdots, as shown in Fig. 1f. Moreover, the emission of Cdots was obviously dependent on their retention time. We selected the Cdots with “ture” blue and green emission for the study of the emission mechanism and single-color application. The SEC separation mechanism indicated that the size of Green-Cdots should be smaller than that of Blue-Cdots.

Characterization

To determine whether the effluent order is size-dependent, the dried and hydrodynamic sizes of Blue-Cdots and Green-Cdots were characterized by TEM and DLS. TEM images (Fig. 2a and d) showed that both Blue-Cdots and Green-Cdots possess good monodispersity with the diameter of about 1.8 and 2.6 nm (Fig. 2b and e), which also indicated that the two kinds of Cdots should be monodispersed in solution. The DLS size of Blue-Cdots and Green-Cdots are about 4.2 nm and 3.1 nm (Fig. 2c and f), respectively. The size from DLS was agreed with the SEC principle as the hydrodynamic size of the Cdots decreases with increasing SEC retention time. Moreover, the hydrodynamic size of Cdots should play an important role for the different emission of different effluents and their color because SEC separation achieved in aqueous phase.

Fig. 2 (a) TEM images of Blue-Cdots (inset: high resolution TEM images of Blue-Cdots). The size distributions of Blue-Cdots obtained from (b) TEM and (c) DLS. (d) TEM images of Green-Cdots (inset: high resolution TEM images of Green-Cdots). The size distributions of Green-Cdots obtained from (e) TEM and (f) DLS.

TEM and DLS sizes also reveal the structure of different Cdots. Dried size from TEM images illustrated their graphite core size because the water molecules attached on the surface of Cdots were removed in dried state. Since the structure properties, the Cdots interact with solvent molecules to form solvent shell. The hydrodynamic sizes provide the evidence that Green-Cdots and Blue-Cdots have different solvent shell thickness, which provides their excellent hydrophilicity. The hydrodynamic shell thickness was attributed to the hydration of the oxygen- and nitrogen-containing groups on the Cdots' surface, so the content of oxygen and nitrogen in Blue-Cdots should be higher than that in Green-Cdots. However, the XPS data (Fig. 3) illustrated that the two kinds of Cdots have almost the same C, O and N content with the calculated values of 68.35% C, 24.34% O, and 7.32% N for Green-Cdots, and 68.34% C, 23.29% O, and 8.37% N for Blue-Cdots. The maximum peak in the C1s spectra of XPS measurement at 284.6 eV was assigned to C–C and C=C for graphite structure.

Fig. 3 (a) FTIR spectra and (b) XPS survey spectra of Blue-Cdots and Green-Cdots. High-resolution C1s XPS spectra of (c) Blue-Cdots and (d) Green-Cdots. (e) C1s XPS normalized spectra of Blue-Cdots and Green-Cdots.

Different to the simple use of element contents of oxygen and nitrogen, we try to explore the precise surface groups on Cdots in combination with FTIR and XPS results. Sharp band at 1697 cm⁻¹ (assigned to ν_C=O) and medium strength band at 1570 cm⁻¹ (assigned to ν_C–N, δ_N–H) in FTIR spectrum (Fig. 3a) of Blue-Cdots illustrated the existence of secondary amide group. Sharp band at 1642 cm⁻¹ is assigned to ν_C=N, which indicates the imine groups in Green-Cdots. Amide group is easily hydrated compared with imine group for the big DLS size of Blue-Cdots. This was further validated by the high resolution XPS spectra of C1s in the two kinds of Cdots (Fig. 3c and d). The deconvoluted C1s XPS spectrum of Blue-Cdots around 287.7 eV was pointed to the C=O of amide group. While C=O component at 288.4 eV was pointed to C=O of carboxyl group in C1s XPS spectrum of Blue-Cdots and Green-Cdots. Lower C=O of carboxyl group content was found from Green-Cdots than from Blue-Cdots, which provide the intuitive evidence (Fig. 3e). So, Cdots contains sp²-graphene structure and amorphous oxygen- and nitrogen-containing groups. The results were validated by the high resolution images in the inset of Fig. 2a and d. So, their different sizes of π-conjugated structure and surface groups attribute to the different emission of Blue- and Green-Cdots.

Optical properties of Blue-Cdots and Green-Cdots

The optical properties of Blue-Cdots and Green-Cdots were demonstrated in Fig. 4. The emission peaks of the raw-Cdots exhibit excitation-dependent characteristic and remain at around 460 and 530 nm (Fig. 4a). The profile of excitation vs. emission wavelength clearly illustrated the excitation-dependent characteristics of the raw-Cdots (inset of Fig. 4a). However, both Blue-Cdots and Green-Cdots have an excitation-independent emission as shown in Fig. 4b and c. Blue-Cdots have strong UV-vis absorption at approximately 340 nm, which is attributable to n–π* transition of C=O bonds and is the typical absorption of the Cdots synthesized from citric acid and ethylenediamine.^4,25 Green-Cdots have an absorption peak around 300 nm, which is attributable to n–π* transition in the π-conjugated structure. The broad absorption around 420–460 nm attributed to n–π* transition of C=O bonds (Fig. 4c). So, the different absorption states were resulted from the different π-conjugated electron system. It should be noted that their absorption characteristic is consistent with their respective PL excitation spectrum, as the optimal excitation was same as the maximum absorption.²⁶ The result confirmed that the excitation was governed by the π-conjugated electron structure of Cdots, so the broad absorption peak of Green-Cdots in visible region is not attributed to Mie scattering but from the π-conjugated structure.²³

Fig. 4 (a) Fluorescence spectra of raw-Cdots. Inset: the profile of maximum emission wavelength vs. excitation wavelength. The UV-vis and fluorescence spectra of (b) Blue-Cdots and (c) Green-Cdots. Inset: photographs of Blue-Cdots (left) and Green-Cdots (right) solutions under the excitation at (b) 365 nm and (c) 302 nm, which are the optimal excitation wavelengths of Blue-Cdots and Green-Cdots. (d) Comparison of fluorescence spectra of Blue-Cdots and Green-Cdots solutions at different excitation. Almost zero emission was observed from Green-Cdots at the maximum emission of Blue-Cdots at 460 nm.

The optimal excitation wavelength of Blue-Cdots is 340 nm (3.65 eV) with the quantum yield of 40.69%. Green-Cdots show the maximum excitation wavelength at 300 nm (4.14 eV) and their quantum yield was 69.30% under excited at 300 nm. Fig. 4b and c show the dramatic decrease of the PL intensity but no redshift was observed with the increased excitation wavelength. Moreover, the overlaps between absorptions and emissions of both Blue-Cdots and Green-Cdots are negligible. While the Stokes shift of 120 nm was observed from Blue-Cdots, the ultralarge Stokes shift was 230 nm for Green-Cdots. The large Stokes shift decreases the PL interference from excitation light.

The photographs of two Cdots in the insets of Fig. 4b and c validated their excitation-independent property; blue and green emission is clearly observed from Blue-Cdots and Green-Cdots even with different excitations. Fig. 4d further demonstrated the excitation-independent property by their narrow peaks with full width at half maxima (FWHM) of 99 and 92 nm, respectively, whereas broad emission peak (FWHM = 112 nm) was observed for the raw-Cdots (Fig. 4a).^27,28 The fluorescence intensity of Green-Cdots is almost zero at the maximum emission of 460 nm for Blue-Cdots (Fig. 4d). So the combination of the two Cdots can be used for doubly encrypted characters.

Photoluminescence mechanism of Cdots

3D-PL plots also revealed the excitation-independent emission and narrow PL peak of Blue-Cdots and Green-Cdots (Fig. 5). Based on the 3D plots, the energy levels and electron transition were proposed and schematically illustrated in Fig. 5. The excitation wavelength is the same to absorption wavelength, therefore indicating that the π-conjugated electron systems in Cdots correspond to their excitation energy levels. As for the emission, the absorbed photons produce electron–hole pair (exciton) after electron transition procedure (1, 2, and 3 in Fig. 5). The exciton emits blue (2.70 eV/460 nm) or green (2.34 eV/530 nm) light produced through radiative recombination after vibration relaxation (4 in Fig. 5) for Blue-Cdots and Green-Cdots, respectively. So, the excitation procedure and energy is related to the π-conjugated electron structure, and the different surface state and groups governs the emission.^10,12,29,30

Fig. 5 3D-PL plots of (a) Blue-Cdots and (b) Green-Cdots (intensity rises from blue to red) and the proposed energy levels and electron transition diagrams. Diagram shows the representative excitation procedures (solid-line arrows of 1, 2, and 3), vibration relaxation procedure (4) and emission procedure (5 and 6). Dashed-line arrows of 1, 2, and 3 refer to the excitonic radiative emission. 1 (3.65 eV) denotes the optimal excitation energy of Blue-Cdots for the maximum excitation of 340 nm. 2 (4.14 eV) and 3 (2.89 eV) denote the peak excitation energy of Green-Cdots for the maximum excitation of 300 nm and 430 nm. Different excitation wavelength is related to different π-conjugated electron system of Cdots. Blue (5) and green (6) lines indicate the single emission procedure of Blue-Cdots and Green-Cdots, corresponding to their 3D-PL plots in (a) and (b).

The excitonic radiative emission (dashed-line arrows of 1, 2, and 3) is almost the same as the absorption wavelength in principle, but decayed by surface absorption.^16,17 The π-conjugated electron system of Blue-Cdots results in a single maximum excitation wavelength, 340 nm or 3.65 eV. For Green-Cdots, the excitation peaks, 300 nm (4.14 eV) and 430 nm (2.89 eV), result from not only the sp² π-conjugated electron systems, but also the imine and carboxyl groups connecting to the π-conjugated electron carbon structure. While the energy decays, caused by the surface functional groups, lead to a longer wavelength emission,^6,31 which is 460 nm and 530 nm for Blue-Cdots and Green-Cdots, respectively.

Redox-tuning of the surface state of Cdots was used to further validate our proposed mechanism. If the fluorescence properties of Cdots can be modulated, the structure and surface state should be changed and have significant effect on the emission after redox.³² To this end, Green-Cdots were first treated with NaBH₄. Fig. 6a illustrated that the emission from NaBH₄-treated Green-Cdots was closely resemble the excitation-dependent emission of Cdots.^3–6 Moreover, the Green-Cdots lose the second excitation peaks at 430 nm. The imine groups on Green-Cdots' surface are easily reduced to become amine groups. So, the reduction process changes the original uniform structure and surface groups and the independent emission becomes excitation-dependent. As a comparison, Blue-Cdots were also treated with NaBH₄. The excitation-independent emission was still kept as shown in Fig. 6b. Since the surface of Blue-Cdots contains amide and carboxyl groups, which react with NaBH₄ difficultly, so the emission from Blue-Cdots was not changed. The result of redox process further demonstrates our elaboration to the relationship between the structure and PL mechanism of Cdots.

Fig. 6 Fluorescence spectra of Green-Cdots (a) and Blue-Cdots (b) after being treated by NaBH₄.

Fluorescence ink application of Cdots for doubly encrypted characters

Their high quantum yields and excitation-independent emission make Blue- and Green-Cdots be valuable ink, but solid support is essential for the fluorescence ink application.^33,34 Fig. 4 illustrated their excitation-independent properties in aqueous state. We first simulate aqueous environment with dialysis membrane adsorbed Blue- and Green-Cdots. As shown in Fig. 7a, when the dialysis membranes were soaked in distilled water, blue and green emission was clearly observed under UV lamp. Moreover, not gradation was observed, indicating the strong adhesion of the Cdots on the substrate for real application. Cotton threads, which have the basic structural unit of α-glucose, contain dense polar groups to interact to Cdots.^33,34 As expected, the cotton threads coated with Cdots showed strong blue and green fluorescence staining under UV excitation, whereas nearly no fluorescence was observed in the uncoated cotton threads under the same conditions (Fig. 7b and c). We further examined their solid-state fluorescence ability by imbedding them into the non-luminescent polar agar gel. Blue and green emission of such hybrids was also clearly observed under UV lamp (Fig. 7d). The dipole–dipole interaction or hydrogen bonding between surface groups of the Cdots and polar groups of the substrate tunes the electronic structure of Cdots and enhances their PL efficiency.³⁴

Fig. 7 (a) Fluorescent images captured under UV lamp (302 nm) of dialysis membranes after adsorption of Blue-Cdots and Green-Cdots observed from different angles. Images under daylight (b) and UV lamp (302 nm) (c) of cotton threads without Cdots coating (top), with Blue-Cdots coating (middle) and Green-Cdots (bottom) coating, respectively. (d) Images under daylight (top) and UV lamp (302 nm) (bottom) of Blue-Cdots-imbedded (left) and Green-Cdots-imbedded (right) agar gel membrane, respectively. Calligraphy with Cdots-based fluorescent inks on paper. “n”, “k”, and “u” were written with Blue-Cdots, mixed-Cdots and Green-Cdots, respectively. Photos were captured under daylight (e), UV lamp excited at 302 nm (f) and 365 nm (g).

The excitation-independent PL properties and complementary excitation wavelength endow the two Cdots-based fluorescent inks for doubly encrypted characters. Calligraphy of letters (“nku”) was then achieved with the two Cdots-based fluorescent inks (Fig. 7e–g). This calligraphy was invisible under daylight, but was clearly observed under UV lamp. Under the excitation of 302 nm, all fluorescent characters “nku” are clearly observed. The part written in Green-Cdots ink are invisible, while the blue fluorescent characters “n” from Blue-Cdots ink and the part of mixed-Cdots ink appear under the excitation of 365 nm. This phenomenon indicates that the Cdots-based fluorescent ink can be used in anti-counterfeit labels and message encryption.

Conclusions

Two kinds of Cdots with different maximum emission wavelength and excitation-independent emission were obtained from raw-Cdots for the study of their emission mechanism. The results indicated that different π-conjugated electron structure from sp² graphite structure and the heteroatom-containing π-electron groups result in the different UV-vis absorption and maximum excitation wavelength. Surface functional groups provide the radiative recombination after vibration relaxation for different emission wavelength and large Stokes shift. The results may provide the answer for the question why almost of all previous Cdots give blue and green emission even they have different TEM size and are prepared with different methods. As a conclusion for the emission of Cdots, their nano-size provides quantum confinement for the energy level, which can be tuned by the heteroatom-containing π-electrons, such as those from imine, amide, and carboxyl groups. The π-conjugated electron level governs the excitation wavelength of Cdots, while the emission wavelength is affected by the vibration relaxation from surface functional groups. The single-color emission from the two kinds of Cdots was validated by their fluorescence ink applications for doubly encrypted characters.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grants 21375064 and 21435001), 973 projects (2015CB932001) and Tianjin Natural Science Foundation (15ZCZDSF00060).

Notes and references

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