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# Induction of long-lived room temperature phosphorescence of carbon dots by water in hydrogen-bonded matrices

## Abstract

Phosphorescence shows great potential for application in bioimaging and ion detection because of its long-lived luminescence and high signal-to-noise ratio, but establishing phosphorescence emission in aqueous environments remains a challenge. Herein, we present a general design strategy that effectively promotes phosphorescence by utilising water molecules to construct hydrogen-bonded networks between carbon dots (CDs) and cyanuric acid (CA). Interestingly, water molecules not only cause no phosphorescence quenching but also greatly enhance the phosphorescence emission. This enhancement behaviour can be explained by the fact that the highly ordered bound water on the CA particle surface can construct robust bridge-like hydrogen-bonded networks between the CDs and CA, which not only effectively rigidifies the C=O bonds of the CDs but also greatly enhances the rigidity of the entire system. In addition, the CD-CA suspension exhibits a high phosphorescence lifetime (687 ms) and is successfully applied in ion detection based on its visible phosphorescence.

## Introduction

Room temperature phosphorescence (RTP) has attracted growing attention because of its long-lived luminescence, large stokes shift and high signal-to-noise ratio, allowing its application in the fields of optoelectronic devices1,2,3,4, bioimaging5,6,7, chemical sensors8,9,10 and so forth. Most current RTP materials show phosphorescent features in their anhydrous solid states11,12,13, but phosphorescence quenching usually occurs in the presence of water owing to the presence of dissolved oxygen as well as the occurrence of solvent-assisted relaxation14, impeding the aqueous applications of RTP materials. Though few species with phosphorescence in aqueous media have been reported15,16, most of them exhibit relatively short emission lifetimes. In addition, the present RTP materials are mainly organometallic4,17,18 and crystalline organic compounds19,20,21. However, all of these materials suffer from high costs, cytotoxicity and complicated preparation processes. Therefore, the development of a facile strategy for preparing efficient RTP materials in aqueous environments is desirable.

Recently, carbon dots (CDs) have aroused increasing attention as a new type of photoluminescent (PL) nanomaterial because of their high photostability, low toxicity and small size22,23,24. Generally, CDs show strong fluorescence, but minimal phosphorescence is observed at room temperature25,26,27,28. However, when CDs are fixed in a certain matrix, such as poly(vinyl alcohol)29,30,31,32, KAl(SO4)2·x(H2O)33 or polyurethane34, obvious phosphorescence can be detected under excitation by ultraviolet light. Most recently, in 2016, an ultralong phosphorescence lifetime (up to 1.06 s) was achieved in our laboratory from CDs and composite matrices35. The matrix can effectively protect the CD triplet states from quenching by restricting their intermolecular motions. However, nearly all RTP materials based on CDs emit phosphorescence only in their dry state. Phosphorescence would undergo extensive quenching in the presence of water because water molecules usually break up hydrogen-bonding interactions between CDs and matrices. In addition, the dissolved oxygen present in water can further contribute to phosphorescence quenching. These unfavourable factors severely hinder the aqueous application of RTP materials, particularly in the fields of bioimaging and chemical sensors.

The water that exists in the vicinity of the matrix can generally be divided into three types: bulk water, freezing bound water, and non-freezing bound water. Bound water combines with proteins36,37, phospholipids38 and other solid substances,39 mainly through hydrogen bonds. Compared with bulk water, bound water exhibits unique structural properties, including difficulty freezing, difficult separation, and restricted mobility and solubility, which endow it with unique applications36,40,41. A recent study42 indicated that the bound water confined in a polyelectrolyte brush exhibited a tight arrangement with a high density, which greatly enhanced the hydrogen-bonded network. The strong hydrogen-bonding structures of water in swollen polymer brushes have been reported to suppress protein adsorption, affording biocompatibility43. Based on these conclusions, it is speculated that if we utilise bound water to construct hydrogen-bonded networks between CDs and matrices, effective phosphorescence may be achieved, even in aqueous environments. The achievement of such phosphorescence emission by utilising bound water is considered to be important in biological applications, such as cell imaging and disease diagnosis, as bound water widely exists in cells and biological tissues.

Herein, a facile strategy for preparing a long-afterglow RTP material by utilising water molecules to construct hydrogen-bonded networks between CDs and cyanuric acid (CA) particles is reported. Our results show that water molecules not only cause no phosphorescence quenching but also greatly enhance the phosphorescence emission. This abnormal enhancement behaviour may be attributed to the water-induced formation of hydrogen-bonded networks between CDs and CA particles. In the presence of water, the surfaces of the CA particles can strongly absorb a layer of highly ordered water molecules (non-freezing bound water). These bound water molecules can construct robust bridge-like hydrogen-bonded networks between the CDs and CA particles, which not only effectively rigidifies the C=O bonds of CDs but also greatly enhances the rigidity of the entire system, resulting in the enhancement of phosphorescence. In addition, the CD-CA suspension exhibits a phosphorescence lifetime of 687 ms under 373 nm excitation and is successfully applied in ion detection based on its visible phosphorescence. As an example, the detection limit of Fe3+ ions is as low as 32 µM. The CD-CA suspension can also be successfully applied to the determination of Fe3+ in real lake water and complicated protein solutions.

## Results

### Characterisation of the CDs and CD-CA powder

The preparation of CDs and the CD-CA powder is described in Fig. 1. The CD aqueous solution and CD-CA powder emitted blue light under illumination by UV (365 nm) light. Interestingly, when the ultraviolet source was removed from the CD-CA powder, a green afterglow was observed with the naked eye at room temperature. The morphologies and optical properties of the CDs in solution and the CD-CA powder were studied. As shown in Fig. 2a, a transmission electron microscopy (TEM) image of the CDs demonstrates the formation of well-dispersed nanoparticles with a mean particle diameter of 4.8 nm (Supplementary Fig. 1a). An atomic force microscopy image of the CDs reveals that their topographic heights are in the range of 0.5–2.5 nm, suggesting that the prepared CDs are anisotropic (Supplementary Fig. 1b). The related high-resolution TEM (HRTEM) image and X-ray diffraction (XRD) results reveal the presence of both crystalline graphite and amorphous phases (Supplementary Fig. 1c, d). In the presence of CA, the CDs evidently cover the surface of CA in their assembled state, and the morphology of the CDs mediated by CA exhibits no significant changes relative to that of the original CDs (Fig. 2b). The Fourier transform infrared (FTIR) spectrum of the CDs (Fig. 2c) displays characteristic peaks at 1705, 1565 and 1400 cm−1 that are attributed to C=O, N–H, and C–N stretching vibrations, respectively. The results of X-ray photoelectron spectroscopy (XPS) experiments indicate that the CDs mainly consist of carbon, nitrogen, and oxygen (Supplementary Fig. 1e). The C 1 s XPS spectrum (Fig. 2d) shows three peaks at 284.6, 285.6 and 288.1 eV for C–C, C–N and C=O, respectively44. Fig 2e shows the corresponding phosphorescence spectra of the CD-CA powder under different excitation wavelengths, where an optimal excitation at 373 nm was obtained. To investigate the origin of the observed phosphorescence, the phosphorescence excitation and absorption spectra of the CD-CA powder were investigated (Fig. 2f). In the absorption spectrum, two bands centred at 255 and 360 nm were observed, which are owing to the π–π* transition of the aromatic sp2 domains and the n–π* transition of the C=O bonds, respectively45. The phosphorescence emission of the CD-CA powder at 480 nm could be excited within the range of 300–400 nm, which overlaps with the absorption bands of C=O, suggesting that the observed phosphorescence may come from the surface C=O bonds of the CDs.

### The role of water in phosphorescence

To obtain better insight into the bonding interactions between the CDs and CA molecules in the completely dissolved state of the CD-CA powder, their thermodynamic parameters and binding constants were measured by isothermal titration calorimetry (ITC) and microscale thermophoresis (MST) (Supplementary Fig. 3a–d and Supplementary Note 1). The obtained endothermic signal and positive entropy values suggest that the binding of CDs to CA molecules is partly driven by hydrophobic interactions, which may be attributed to the hydrophobic carbon skeleton of the CDs. A low binding constant indicates the presence of weak binding interactions between the CDs and CA, explaining the absence of phosphorescence in the completely dissolved state of the CD-CA powder.

The optical spectra of CD-CA powders with various CD contents were recorded (Supplementary Fig. 4a and Table 2 and Supplementary Note 1). At low CD concentrations, the phosphorescence lifetime and fluorescence quantum yield remained nearly unchanged, whereas both properties showed obvious quenching at high concentrations, which is owing to an increase in the level of self-absorption by the CDs. In addition, we investigated the effects of the aqueous environment pH on phosphorescence . The emission intensity of the CD-CA system showed a negligible change over the pH range from 3.1 to 4.8 and a sudden reduction at pH 7.4. To explain this phenomenon, the zeta potentials of the CDs and CA particles were measured. The sum of the absolute values of the CD and CA particle zeta potentials exhibited an opposite trend as that of the emission intensities of the CD-CA system at different pH values (Fig. 3f and Supplementary Table 1), suggesting that electrostatic interactions have an important role in phosphorescence. In addition, an obvious reduction in the amount of bound water in the CD-CA system at pH 7.4 compared with that at pH 4.8, which causes a further reduction in the phosphorescence of the CD-CA system at pH 7.4, was detected by DSC tests (Supplementary Fig. 4b). Dissolved oxygen is known to often cause phosphorescence quenching in water. Thus, the phosphorescence spectra of CD-CA suspensions with and without nitrogen bubbling were measured, and almost identical optical curves were obtained (Fig. 3g), which may be attributed to the restricted solubility of oxygen in bound water. In a further set of experiments, we investigated the effects of the temperature on the photoluminescence of the CD-CA system containing 40% water in the range of 78–301 K. (Supplementary Fig. 4c, d, Table 3 and Supplementary Note 1). Usually, the phosphorescence intensity increases as the temperature decreases30,46; however, our results were different. As shown in Supplementary Fig. 3c, the phosphorescence of the CD-CA system decreased with a decrease in the temperature, which might be attributed to the freezing water in the CD-CA system. Small phosphorescent materials are very important in biology and related fields. The phosphorescence image, confocal laser scanning microscopy (CLSM) results and dynamic light scattering (DLS) data (Fig. 3h) illustrated that the CD-CA suspension still exhibited visible phosphorescence under illumination by UV light at the nanometre scale, which is crucial for real applications.

### Application in ion detection

In further experiments, we applied the CD-CA probe to Fe3+ determination in complicated peptone solutions containing numerous amino acids and proteins. As anticipated, the phosphorescent sensor still showed a good linear correlation in the concentration range from 0.1−0.5 mM with a detection limit of 28 µM, but the fluorescent sensor based on CDs did not work well because of interference by scattering light and fluorescent organics (Fig. 4f and Supplementary Fig. 5d, e). In addition, the CD-CA suspension was also successfully applied to the determination of Fe3+ in real lake water (Supplementary Fig. 5f).

### Universality confirmation

To confirm the universality of our technology, we designed a series of phosphorescent materials containing a variety of host-guest materials. The materials a-CD31, b-CD35 and c-CD32 have been reported in previous work, and a full description of their syntheses is described in the Supplementary Information. As anticipated, the prepared materials all exhibited long phosphorescence lifetimes in solution (Fig. 5a, b, Supplementary Table 4 and Supplementary Note 1), indicating that our method can be generally applied to the establishment of RTP in metal-free organic materials in aqueous environments.

## Discussion

In summary, we have developed an efficient metal-free RTP material system in aqueous media by introducing multiple rationally designed hydrogen-bonding interactions between the CDs, CA and water molecules. The CA particle surfaces contain a large number of polar groups; thus, CA particles adsorb a layer of highly ordered water molecules (non-freezing bound water) through strong hydrogen-bonding interactions. In the presence of CDs, the particles can form robust hydrogen-bonded networks, which not only effectively rigidifies the C=O bonds of the CDs but also greatly enhances the rigidity of the entire system, leading to an enhancement of the phosphorescence. The CD-CA suspension exhibited a high phosphorescence lifetime of 687 ms under 373 nm excitation and was successfully applied in ion detection based on its visible phosphorescence. Phosphorescence detection more effectively eliminates the fluorescence background and scattered light than the fluorescence method. Therefore, our CD-CA system shows great potential for the detection of Fe3+ in real lake water and complicated protein solutions. We believe that the demonstrated strategy based on bound water for enabling effective RTP in aqueous media can be utilised for the development of novel metal-free organic phosphorescence materials for chemical sensors as well as for bioimaging applications in real water environments.

## Methods

### Chemicals

Unless otherwise noted, all chemicals were commercially available and used as received without additional purification. Anhydrous citric acid, urea, CA, folic acid, ethylenediamine triacetic acid (EDTA)-2Na, biuret and the oxidised form of β-nicotinamide-adenine dinucleotide were purchased from J&K Scientific Ltd (Beijing) or Aladdin Chemicals Ltd (Shanghai). Deionised (DI) water (18.2 MΩ.cm at 25 °C) prepared by a Milli-Q water system was used throughout all experiments. Lake water samples were obtained from the Weiminghu Lake of Peking University, China. Before evaluation, the lake water samples were filtered through a 0.20 µm membrane and then centrifuged at 10,000 rpm for 10 min. The resultant water samples were spiked with Fe3+ at different concentrations and then analysed with the proposed method.

### Material characterisation

TEM images were taken by an H-7650 instrument (Hitachi, Japan). HRTEM images were obtained on a JEM2010 high-resolution field-emission transmission electron microscope at 200 KV (JEOL, Japan). XPS was performed using an Escalab 250 XI system (Thermo Electron Corporation, USA). FTIR spectra were recorded on a Bruker Tensor 27 spectrophotometer (Germany). The absorbance spectra were recorded with a Shimadzu UV-3600 spectrometer. Steady-state PL and phosphorescence spectra were collected on a Hitachi F-7000 fluorescence spectrophotometer. Time-resolved luminescence spectra were obtained using an Edinburgh FLS920 fluorescence spectrophotometer. Lifetime decay profiles were measured using an Edinburgh FLS920 fluorescence spectrophotometer equipped with a xenon arc lamp (Xe900), a nanosecond hydrogen flash-lamp (nF920) and a microsecond flash-lamp (µF900). For the temperature-dependent experiment, the sample was placed in an Optistat DN-V liquid nitrogen cryostat with a temperature control range between 80 and 293 K. The lifetimes (τ) of the luminescence were obtained by fitting the decay curve with the following multi-exponential decay function:

$$I\left( t \right) = \mathop {\sum }\limits_i A_i\mathrm{e}^{ - \frac{t}{{\tau _i}}}$$
(1)

where A i and τ i represent the amplitude and lifetime, respectively, of the individual components of the multi-exponential decay profiles. DSC was performed on a DSC Q-2000 from TA Instruments using cooling and heating rates of 5 K min−1. DLS was performed on a Malvern Instruments Zeta Sizer Nano-ZS apparatus. ITC was performed on the Micro Cal ITC-200. Fluorescence images were taken by a CLSM equipped with a diode laser at 405 nm. The binding affinity study was investigated with an MST instrument. Raman spectra were measured on a Horiba-HR-800 Raman spectroscopy system. This system was equipped with a 514 nm diode laser source. NMR spectra were collected with a Bruker Avance Ш HD 500 MHz instrument. XRD patterns were obtained by a Rigaku S2 diffractometer.

### Synthesis of CDs

Anhydrous citric acid (1 g) and urea (0.5 g) were dissolved in DI water (25 mL) and stirred to form a clear solution. Then, the solution was transferred to a poly (tetrafluoroethylene) (Teflon)-lined autoclave (50 mL) and heated at 250 °C for 10 h. The obtained dark brown solution was centrifuged at high speed (10,000 rad min−1) for 20 min to remove large or agglomerated particles. The final products were obtained via freeze-drying. A stock solution (4 mg ml−1) of the obtained CDs was then prepared for further use.

### Preparation of CD-CA powders

Different masses of the CDs (0.2 mg, 0.4 mg, 0.8 mg, 2 mg, 4 mg, and 16 mg) were first diluted with 4 mL of DI water and mixed with CA (0.75 g) under constant stirring for 12 h. The reaction mixtures were then centrifuged (8000 rpm, 10 min) to remove the unabsorbed CDs, and the solid powders remaining on the bottoms of the centrifuge tubes were fully dried under vacuum. Finally, the CD-CA powders were obtained and then redissolved in water for further use. Unless stated otherwise, the CD-CA powder in this article was prepared by mixing CDs (2 mg) and CA (0.75 g) in 4 mL of DI water.

### Synthesis of a-CDs

The a-CDs were prepared according to previous work. EDTA-2Na (1.6 g) was placed in a quartz boat and heated in a tube furnace under a N2 atmosphere at a rate of 10 °C min−1 to 400 °C. After annealing for 4 h at 400 °C, the reaction was allowed to cool to room temperature, and the black product was dispersed in DI water (80 mL) under ultrasonication for 30 min. Then, the solution was centrifuged at 9500 rpm for 16 min to remove larger and insoluble particles. Solid-state luminescent CDs were obtained via freeze-drying.

### Synthesis of b-CDs

The b-CDs were prepared according to previous work. In brief, folic acid (FA) (1 g) was dissolved in DI water (100 mL). Then, the solution was transferred to a Teflon-lined autoclave (200 mL) and heated at 260 °C for 2 h. After the reaction, the reactor was cooled to room temperature naturally. The final product was collected following centrifugation at 10,000 rpm for 20 min.

### Synthesis of c-CDs

The c-CDs were prepared according to previous work. Isophorone diisocyanate (8 g, 36 mmol) was added to a Teflon vessel that was sealed with an explosion-proof enclosure. Then, raw N-doped carbon quantum dots (CQDs) were prepared under 700 W microwave irradiation at 250 °C for 10 min. The oil-soluble CQDs were collected after centrifugation and dialysed. Then, the oil-soluble CQDs were ultrasonically dispersed in DI water (50 mL) and heated at 180 °C for 3 h. After the reaction, the mixture was cooled to room temperature, and the obtained yellow solution was centrifuged (10,000 rpm, 20 min) and filtered through a 0.22 mm filter membrane to remove large and insoluble particles. Solid-state luminescent a-CDs were obtained via freeze-drying.

### Fluorescent detection of Fe3+ ions

The detection of Fe3+ was performed in an ultrapure water solution. An aqueous solution of CDs (0.01 mg mL−1) was used as the blank solution. Different concentrations of Fe3+ were added to the CD blank solution, and the mixed solutions were equilibrated for 10 min. In addition, the PL emission spectra were collected at an excitation wavelength of 373 nm. The selectivity for Fe3+ was confirmed by adding other metal ion stock solutions instead of Fe3+ in a similar way. All experiments were performed at room temperature.

### Phosphorescent detection of Fe3+ ions

The detection of Fe3+ was performed in an ultrapure water solution. Different concentrations of Fe3+ were added to 3 mL of the CD solution (0.01 mg mL−1), and the mixed solutions were equilibrated for 10 min. Then, the same amount of CA powder (0.3 g) was added to each mixed solution, which was stirred for 1 h. Then, the PL emission spectra were collected at an excitation wavelength of 373 nm. The selectivity for Fe3+ was confirmed by adding other metal ion stock solutions instead of Fe3+ in a similar way. All experiments were performed at room temperature. The detection of Fe3+ in lake water and complicated protein solutions was carried out using lake water and complicated protein solutions, respectively, instead of ultrapure water. The complicated protein solution was prepared by adding peptone (0.5 g) to ultrapure water (50 mL).

### Data availability

The data that support the findings of this study are available from the corresponding author.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

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## References

1. 1.

Baldo, M. A. et al. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 395, 151–154 (1998).

2. 2.

Wei, J. et al. Induction of strong long-lived room-temperature phosphorescence of N-phenyl-2-naphthylamine molecules by confinement in a crystalline Dibromobiphenyl matrix. Angew. Chem. Int. Ed. Engl. 55, 15589–15593 (2016).

3. 3.

Kabe, R., Notsuka, N., Yoshida, K. & Adachi, C. Afterglow organic light-emitting diode. Adv. Mater. 28, 655–660 (2016).

4. 4.

Xu, H. et al. Recent progress in metal-organic complexes for optoelectronic applications. Chem. Soc. Rev. 43, 3259–3302 (2014).

5. 5.

Zhang, G., Palmer, G. M., Dewhirst, M. W. & Fraser, C. L. A dual-emissive-materials design concept enables tumour hypoxia imaging. Nat. Mater. 8, 747–751 (2009).

6. 6.

Xiong, X. et al. Thermally activated delayed fluorescence of fluorescein derivative for time-resolved and confocal fluorescence imaging. J. Am. Chem. Soc. 136, 9590–9597 (2014).

7. 7.

Palner, M., Pu, K., Shao, S. & Rao, J. Semiconducting polymer nanoparticles with persistent near-infrared luminescence for in vivo optical imaging. Angew. Chem. Int. Ed. 127, 11639–11642 (2015).

8. 8.

Zhang, G. et al. Multi-emissive difluoroboron dibenzoylmethane polylactide exhibiting intense fluorescence and oxygen-sensitive room-temperature phosphorescence. J. Am. Chem. Soc. 129, 8942–8943 (2007).

9. 9.

Shu, X., Chen, Y., Yuan, H., Gao, S. & Xiao, D. H2O2 sensor based on the room-temperature phosphorescence of Nano TiO2/SiO2 composite. Anal. Chem. 79, 3695–3702 (2007).

10. 10.

Yang, X. & Yan, D. Long-afterglow metal–organic frameworks: reversible guest-induced phosphorescence tunability. Chem. Sci. 7, 4519–4526 (2016).

11. 11.

Lee, D. et al. Room temperature phosphorescence of metal-free organic materials in amorphous polymer matrices. J. Am. Chem. Soc. 135, 6325–6329 (2013).

12. 12.

Kwon, M. S. et al. Suppressing molecular motions for enhanced room-temperature phosphorescence of metal-free organic materials. Nat. Commun. 6, 8947 (2015).

13. 13.

Kwon, M. S., Lee, D., Seo, S., Jung, J. & Kim, J. Tailoring intermolecular interactions for efficient room-temperature phosphorescence from purely organic materials in amorphous polymer matrices. Angew. Chem. Int. Ed. Engl. 53, 11177–11181 (2014).

14. 14.

Mukherjee, S. & Thilagar, P. Recent advances in purely organic phosphorescent materials. Chem. Commun. 51, 10988–11003 (2015).

15. 15.

Pfister, A., Zhang, G., Zareno, J., Horwitz, A. F. & Fraser, C. L. Boron polylactide nanoparticles exhibiting fluorescence and phosphorescence in aqueous medium. ACS Nano 2, 1252–1258 (2008).

16. 16.

Gui, R. et al. Room-temperature phosphorescence logic gates developed from nucleic acid functionalized carbon dots and graphene oxide. Nanoscale 7, 8289–8293 (2015).

17. 17.

Li, Z. et al. Direct aqueous-phase synthesis of sub-10 nm “luminous pearls” with enhanced in vivo renewable near-infrared persistent luminescence. J. Am. Chem. Soc. 137, 5304–5307 (2015).

18. 18.

Abdukayum, A., Chen, J. T., Zhao, Q. & Yan, X. P. Functional near infrared-emitting Cr3+/Pr3+ co-doped zinc gallogermanate persistent luminescent nanoparticles with superlong afterglow for in vivo targeted bioimaging. J. Am. Chem. Soc. 135, 14125–14133 (2013).

19. 19.

An, Z. et al. Stabilizing triplet excited states for ultralong organic phosphorescence. Nat. Mater. 14, 685–690 (2015).

20. 20.

Bolton, O., Lee, K., Kim, H. J., Lin, K. Y. & Kim, J. Activating efficient phosphorescence from purely organic materials by crystal design. Nat. Chem. 3, 205–210 (2011).

21. 21.

Gong, Y. et al. Achieving persistent room temperature phosphorescence and remarkable Mechanochromism from pure organic Luminogens. Adv. Mater. 27, 6195–6201 (2015).

22. 22.

Yu, H. et al. Smart utilization of carbon dots in semiconductor photocatalysis. Adv. Mater. 28, 9454–9477 (2016).

23. 23.

Lim, S. Y., Shen, W. & Gao, Z. Carbon quantum dots and their applications. Chem. Soc. Rev. 44, 362–381 (2015).

24. 24.

Baker, S. N. & Baker, G. A. Luminescent carbon nanodots: emergent nanolights. Angew. Chem. Int. Ed. Engl. 49, 6726–6744 (2010).

25. 25.

Xu, S., Chen, R., Zheng, C. & Huang, W. Excited state modulation for organic afterglow: materials and applications. Adv. Mater. 28, 9920–9940 (2016).

26. 26.

Xie, Z., Wang, F. & Liu, C. Y. Organic-inorganic hybrid functional carbon dot gel glasses. Adv. Mater. 24, 1716–1721 (2012).

27. 27.

Kwon, W. et al. Freestanding luminescent films of nitrogen-rich carbon nanodots toward large-scale phosphor-based white-light-emitting devices. Chem. Mater. 25, 1893–1899 (2013).

28. 28.

Li, X. et al. Intercrossed carbon nanorings with pure surface states as low-cost and environment-friendly phosphors for white-light-emitting diodes. Angew. Chem. Int. Ed. Engl. 54, 1759–1764 (2015).

29. 29.

Jiang, K. et al. Triple-mode emission of carbon dots: applications for advanced anti-counterfeiting. Angew. Chem. Int. Ed. Engl. 55, 7231–7235 (2016).

30. 30.

Al-Attar, H. A. & Monkman, A. P. Room-temperature phosphorescence from films of isolated water-soluble conjugated polymers in hydrogen-bonded matrices. Adv. Funct. Mater. 22, 3824–3832 (2012).

31. 31.

Deng, Y. et al. Long lifetime pure organic phosphorescence based on water soluble carbon dots. Chem. Commun. 49, 5751–5753 (2013).

32. 32.

Tan, J., Zhang, J., Li, W., Zhang, L. & Yue, D. Synthesis of amphiphilic carbon quantum dots with phosphorescence properties and their multifunctional applications. J. Mater. Chem. C. 4, 10146–10153 (2016).

33. 33.

Dong, X. et al. Efficient long lifetime room temperature phosphorescence of carbon dots in a potash alum matrix. J. Mater. Chem. C. 3, 2798–2801 (2015).

34. 34.

Tan, J. et al. Large-scale synthesis of N-doped carbon quantum dots and their phosphorescence properties in a polyurethane matrix. Nanoscale 8, 4742–4747 (2016).

35. 35.

Li, Q. et al. Efficient room-temperature phosphorescence from nitrogen-doped carbon dots in composite matrices. Chem. Mater. 28, 8221–8227 (2016).

36. 36.

Garczarek, F. & Gerwert, K. Functional waters in intraprotein proton transfer monitored by FTIR difference spectroscopy. Nature 439, 109–112 (2006).

37. 37.

Fisette, O. et al. Hydration dynamics of a peripheral membrane protein. J. Am. Chem. Soc. 138, 11526–11535 (2016).

38. 38.

Weng, L., Ziaei, S. & Elliott, G. D. Effects of water on structure and dynamics of trehalose glasses at low water contents and its relationship to preservation outcomes. Sci. Rep. 6, 28795 (2016).

39. 39.

Gezici-Koç, Ö., Erich, S. J. F., Huinink, H. P., van der Ven, L. G. J. & Adan, O. C. G. Bound and free water distribution in wood during water uptake and drying as measured by 1D magnetic resonance imaging. Cellulose 24, 535–553 (2017).

40. 40.

Wang, Y. et al. Water-mediated structuring of bone apatite. Nat. Mater. 12, 1144–1153 (2013).

41. 41.

Ball, P. Water is an active matrix of life for cell and molecular biology. Proc. Natl. Acad. Sci. USA 114, 13327–13335 (2017).

42. 42.

Yamazoe, K. et al. Enhancement of the hydrogen-bonding network of water confined in a polyelectrolyte brush. Langmuir 33, 3954–3959 (2017).

43. 43.

Nagasawa, D., Azuma, T., Noguchi, H., Uosaki, K. & Takai, M. Role of interfacial water in protein adsorption onto polymer brushes as studied by SFG spectroscopy and QCM. J. Phys. Chem. C. 119, 17193–17201 (2015).

44. 44.

Song, Z. et al. Invisible security ink based on water-soluble graphitic carbon nitride quantum dots. Angew. Chem. Int. Ed. Engl. 55, 2773–2777 (2016).

45. 45.

Li, X., Zhang, S., Kulinich, S. A., Liu, Y. & Zeng, H. Engineering surface states of carbon dots to achieve controllable luminescence for solid-luminescent composites and sensitive Be2+ detection. Sci. Rep. 4, 4976–4983 (2015).

46. 46.

Hirata, S. et al. Efficient persistent room temperature phosphorescence in organic amorphous materials under ambient conditions. Adv. Funct. Mater. 23, 3386–3397 (2013).

47. 47.

Zhang, J. & Yu, S.-H. Carbon dots: large-scale synthesis, sensing and bioimaging. Mater. Today 19, 382–393 (2016).

48. 48.

Zhu, S. et al. Highly photoluminescent carbon dots for multicolor patterning, sensors, and bioimaging. Angew. Chem. Int. Ed. Engl. 52, 3953–3957 (2013).

49. 49.

Ding, H., Wei, J. S. & Xiong, H. M. Nitrogen and sulfur co-doped carbon dots with strong blue luminescence. Nanoscale 6, 13817–13823 (2014).

50. 50.

Ramon-Marquez, T., Medina-Castillo, A. L., Fernandez-Gutierrez, A. & Fernandez-Sanchez, J. F. A novel optical biosensor for direct and selective determination of serotonin in serum by solid surface-room temperature phosphorescence. Biosens. Bioelectron. 82, 217–223 (2016).

## Acknowledgements

This research was supported by the National Basic Research Program of China (973 Program, Grant No. 2011CB013004) of the Ministry of Science and Technology of China and the Major Project of Chinese State Key Laboratory of Tribology (Grant No. SKLT2014A01).

## Author information

Q.L. and M.Z. conceived the experiments. Q.L. synthesised all materials, made all photophysical measurements and wrote the paper. M.Z., M.Y., Q.Y., Z.Z. and J.S. gave suggestions to optimise the system. All authors discussed the results and commented on the manuscript at all stages.

### Competing interests

The authors declare no competing financial interests.

Correspondence to Ming Zhou.

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•  & Wei Huang

Nature Photonics (2019)

• ### Controlling the fluorescence and room-temperature phosphorescence behaviour of carbon nanodots with inorganic crystalline nanocomposites

• David C. Green
• , Mark A. Holden
• , Mark A. Levenstein
• , Shuheng Zhang
• , Benjamin R. G. Johnson
• , Julia Gala de Pablo
• , Andrew Ward
• , Stanley W. Botchway
•  & Fiona C. Meldrum

Nature Communications (2019)

• ### Enhancing the performance of pure organic room-temperature phosphorescent luminophores

• Kenry
• , Chengjian Chen
•  & Bin Liu

Nature Communications (2019)

• ### Recent Advances in Carbon Nanodots: Properties and Applications in Cancer Diagnosis and Treatment

• Xiaofeng Li
• , Lihong Shi
• , Lin Li
• , Chuan Dong
• , Chen-zhong Li
•  & Shaomin Shuang

Journal of Analysis and Testing (2019)

• ### A dual emission nanocomposite prepared from copper nanoclusters and carbon dots as a ratiometric fluorescent probe for sulfide and gaseous H2S

• Zhuoqi Wen
• , Shanliang Song
• , Tantan Hu
• , Chuanxi Wang
• , Fengdong Qu
• , Pei Wang
•  & Minghui Yang

Microchimica Acta (2019)