Optimization investigation for high-power 1034 nm all-fiber narrowband Yb-doped superfluorescent source

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In-situ green synthesis of nitrogen-doped carbon dots-based room temperature phosphorescence materials for visual iron ion detection Xueyun Wu, Chunhui Ma, Jiancong Liu, Yushan Liu, Sha Luo, Mingcong Xu, Peng Wu, Wei Li, and Shou-Xin Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b03281 • Publication Date (Web): 04 Oct 2019 Downloaded from pubs.acs.org on October 6, 2019

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In-situ green synthesis of nitrogen-doped carbon dots-based room temperature phosphorescent materials for visual iron ion detection Xueyun Wu,† Chunhui Ma,† Jiancong Liu,‡ Yushan Liu,† Sha Luo,† Mingcong Xu,† Peng Wu,† Wei Li,†,* Shouxin Liu†,* † Key laboratory of Bio-based Material Science and Technology of Ministry of Education, Northeast Forestry University, Harbin 150040, Heilongjiang, Peoples R China. ‡Key Laboratory of Functional Inorganic Material Chemistry Ministry of Education, Heilongjiang University, Harbin 150080, Heilongjiang, Peoples R China.

AUTHOR INFORMATION: *Corresponding Authors Wei Li, E-mail address: [email protected] and phone: 13796673316 Shouxin Liu, E-mail address: [email protected] and phone: 13351003378

ORCID Wei Li: 0000-0002-3008-9865 Shouxin Liu: 0000-0002-0491-8885

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ABSTRACT: Room temperature phosphorescent (RTP) shows great potential for ion detection due to the long-lifetime and a high signal-to-noise ratio. Most RTP materials are limited to organometallic or organic compounds that are costly, toxic and complex. Here, an insitu green synthesis of carbon dot (CDs)-based RTP materials using Schisandra chinensis polysaccharide (SCP) as the only carbon source, which has a natural nitrogencontaining structure for endogenous nitrogen doping, is prepared. RTP measurements show that the CDs-based RTP materials had lifetimes up to 271.2 ms under 350 nm excitation and with smaller energy gap (0.32 eV). In addition, they exhibit adequate quenching in the presence of iron ions (Fe3+). The visible RTP intensity is inversely proportional to the Fe3+ concentration over the range of 0.1–2 mM, with a 0.57 μM detection limit. Further, the prepared CDs-based RTP materials have highly stable optical and physical properties, which open a new perspective as luminescent sensor for Fe3+ detection with inexpensive and green raw materials. KEYWORDS: Carbon Dots, Schisandra chinensis polysaccharide, Room Temperature Phosphorescent, Carbon Dots/Polyvinyl alcohol film, Visual sensor

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INTRODUCTION Phosphorescent detection has advantages over fluorometry as a luminescence sensor because of the elimination of interference from fluorescent backgrounds and scattered light.1-6 However, phosphorescent detection has not received the same degree of development as fluorescence detection because of the difficulty of producing phosphorescent materials that function at room temperature. The obtaintion of room temperature phosphorescent (RTP) materials are focused on toxic inorganic materials with rare earth-containing or organic compounds.7-10 Cai et al. developed enhanced ultralong lifetime organic phosphorescence, but needed introducing π-type halogen bonding to obtaining RTP materials.11 Li et al. developed efficient RTP materials for detecting iron ions, but it has limited the applications only in liquid state.12 Rare earth elements are usually expensive and toxic,13 while organic compounds usually require incorporation of heavy atoms or aromatic carbonyls for phosphorescence in ambient conditions.14-16 Fortunately, carbon dots (CDs) can be utilized as ideal agents for CDs-based RTP materials in chemical sensors fields, because they have good optical stability, low toxicity, excellent water solubility, a cost-effective synthesis.17-19 However, CDs-based RTP materials are still challenging because triplet excitons concern the nature of spin-forbidden transitions causuing inefficient intersystem crossing (ISC), and they can be easily deactivated by nonradiative decay processes or by oxygen quenching in ambient conditions.20-22 In general, the following conditions are required to prepare CDs-based RTP materials. One is the introduction of elements like N or O that enhance n-π* transitions can promote the process of ISC and further produce triplet excitons.23, 24 The other is the incorporation of external particular functional groups such as heavy atoms, halogen atoms, and nitrogen heterocycles that can form hydrogen bonds that inhibit non-radiative transitions and consequently stabilize the triplet states.25-28 Long et al. introduced F and N to achieve CDs-based self-protective RTP by suppressing triplet exciton quenching and improving hydrogen-bonding interactions.29 Jiang et al. developed

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ultralong-lived RTP CDs by using ethanolamine and phosphoric acid for N and P doping that enhanced n-π* transitions, facilitating ISC for effectively populating triplet excitons.30 Jiang et al. reported efficient RTP through forming H-bonds between hydroxyl groups and C-N/C=N groups on polyvinyl alcohol (PVA) and on CDs surfaces, respectively. C-N/C=N groups can increase the number of triplet excited states and facilitated originates of RTP.31 Deng et al. obtained CDs-based RTP under extremely harsh synthesis conditions via pyrolysis the organic salt. The aromatic carbonyl groups, which have high spin-orbit coupling degree, contributed to the production of RTP in view of both the small energy gap and favoring ISC process.32 The introduction of external compounds or the use of organic complexes as carbon sources for RTP make the synthesis complex, difficult, and costly. Thus, it’s urgent to prepare efficient RTP materials, which features simple and ecofriendly preparations, are cost effective, have low toxicity, and have no need to introduce external compounds. Possible structures of CDs for RTP should have mostly N and O for favoring n-π* transitions to facilitate ISC, and functional groups for forming hydrogen bonds that stabilize the excited triplet states.30, 31-35 Natural products can be suitable carbon sources because they contain heteroatoms and have natural polymer-like structures without the need for additional N-containing compounds or for the incorporation of particular groups to facilitate ISC and to stabilize the triplet states.36, 37 Water-soluble Schisandra chinensis polysaccharide (SCP), can be isolated from Schisandra chinensis fruit by simple and ecofriendly ethanol precipitation.38, 39 The SCP can be an ideal carbon source for CDs because it not only converts natural products to functional nanomaterials, but it also can be prepared without toxic reagents and external compounds. Thus, they are ideal agents for preparing CDs-based RTP materials. Here, RTP materials with lifetimes up to 271.2 ms are obtained by embedding CDs into PVA matrix. The only carbon source is SCP, without incorporating external compounds. Based on characterization results, CDs have abundant C=N/C=O groups on its surface,

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which increased the probability of ISC and inhibited non-radiative transitions by forming H-bonds with PVA matrix. CDs/PVA film can then be used as visible RTP sensors for detecting Fe3+ ions.

EXPERIMENTAL SECTION Materials. Absolute ethyl alcohol is purchased from Tianjin. Schisandra chinensis is bought via Beijing. PVA is bought from Shanghai. The chemicals, which is used in the experiment, are the analytical grade and used as specified.

Synthesis of Schisandra chinensis polysaccharide (SCP). As noted above, the SCP with water-soluble, is extracted from Schisandra chinensis fruit via ethanol precipitation. The extraction process of dried Schisandra chinensis is that 500 g fruit with 1 L of distilled water for 4–6 h at 100 ℃, and further filtered when cooling in room temperature. By adding four volumes of 95% EtOH into filtrate, the precipitation can be obtained via standing overnight in air conditions. And after centrifugating at 6000 rpm for 20 min, SCP precipitate is obtained and further vacuum-dried at 40 ℃.

Synthesis of carbon dots (CDs). CDs are prepared from SCP through eco-friendly hydrothermal carbonization. 0.5 g SCP is mixed with 15 mL distilled water, the obtained uniform solution is placed in a 30 mL Teflon-lined autoclave for heating at 200 ℃ with 8 h. Then, the product is centrifuged for 10 min at 12000 r/min. The precipitate is filtered via a membrane with 0.22 μm pores and purified in 1000 Da dialysis bags for 12 h, and then dry to a powder.

Preparation of CDs-based RTP materials (CDs/PVA films). A series of CDs/PVA films are fabricated via drop-coating from PVA aqueous solution (wt=10%) containing different CDs concentrations (0.01, 0.1, 0.2, 0.4, 0.6, 0.8, 1, and 2

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mg/L). The homogeneous CDs/PVA solution is obtained by mixing 1 mL CDs solution with 2 mL PVA solution. Then, the homogeneous solution is drop-casted on polytetrafluoroethylene petri dish substrates, further dried at 45 °C in an oven.

Fluorescence detection of Fe3+ ions. Different Fe3+ concentrations are added to the blank solution and equilibrated for 5 min. The experiment has used 0.01 mg/mL CDs solution as the blank solution. When the excitation wavelength at 350 nm, it will have an optimized fluorescence emission spectra. CDs toward Fe3+ is compared with other metal ions like Cd2+, Hg2+, Zn2+, Pb2+, Mn2+, Ca2+, K+, Cu2+, Na+, Ag+, Ni2+, Cr3+ and Co2+ under identical conditions to confirm its selectivity.

Phosphorescence detection of Fe3+ ions. RTP detection of Fe3+ is carried out with CDs/PVA films. The films are cut into 1 cm2 cm pieces. Then, different Fe3+ concentrations (0–10 mM) are added (10 μL) into the films and equilibrate, further dried about 30 min at ambient environment. The selectivity of CDs/PVA films toward Fe3+ is compared with other metal ions like Pb2+, Hg2+, Ag+, Ni2+, Mn2+, Ca2+, K+, Cu2+, Zn2+, Cd2+, Na+, Cr3+ and Co2+ under identical conditions. All experiments are performed at room temperature.

Characterizations. TEM and high-resolution TEM (HRTEM) is detected in a JEM-2100F electron microscope (JEOL, Ltd., Japan). The Nicolet iS10 FT-IR spectrometer (Thermo Scientific, USA) are used to record the FT-IR spectra. XPS spectra are recorded on a Thermo ESCALAB 250 Xi photoelectron spectrometer (Thermo Fisher Scientific, USA). The TU-1950 UV–vis spectrometer (Persse, China) is recorded to obtain UV–vis absorption spectra. A D/max-r B X-ray diffractometer (Rigaku Corp., Tokyo, Japan) is performed to obtain XRD patterns. An Agilent Cary Eclipse Fluorescence Spectrophotometer (Agilent Technologies, U.S.A) is recorded to obtain fluorescence emission spectra. A Zetasizer Nano ZS90 (Malvern

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Instruments, Manchester, U.K.) is performed to obtain dynamic light scattering and zeta potential. FLS980 fluorescence spectrophotometer (Edinburgh, UK) is used to recorded the phosphorescent emission and excitation spectra, and decay curves of the CDs/PVA films. The luminescence lifetimes (𝜏) are obtained by fitting the decay curves: < 𝜏 >= ∑𝛼𝑖𝜏2𝑖 /∑𝛼𝑖𝜏𝑖 ,

(1)

where 𝛼𝑖 and 𝜏𝑖 are the individual components of the multi-exponential decay profiles, which represent the amplitudes and lifetimes, respectively.

RESULTS AND DISCUSSION Characterization and fluorescence properties of CDs. Figure 1a depicts the CDs synthesis from SCP with a natural N-containing structure. TEM image is showed in Figure 1b, revealing that CDs are spherical and well-dispersed with a 2.31 nm average diameter. The HR-TEM image (the insert of Figure 1b) indicates that CDs have well-resolved lattice fringes. The 0.215 nm interplanar spacing is consistent with the (100) facet of sp2 graphitic carbon.40,41 Figure 1c shows the particle size distribution and a size distribution spanning 0.8–5.2 nm. There have two peaks in the XRD pattern, showing an intense peak at 2θ=21.5° and a weak peak at 2θ=45.5° of the CDs (Figure S1), respectively. The weak peak of character result of XRD at 2θ=45.5° is assigned to the (100) facet of graphitic carbon, which is corresponded to lattice fringes with 0.251 nm in HRTEM images.29,42 The inset in Figure S2 shows bright blue CDs fluorescence excited by an UV lamp. Two UV-vis absorption peaks of CDs solution are at 257 nm and a shoulder at 350 nm, which can attribute to aromatic sp2 domain of C=C groups from π–π* transitions and conjugated C=N/C=O groups from the n–π* transition, respectively (Figure S2).43 The CDs excitation-emission map in Figure 1d exhibits a single narrow emission center. When excitation wavelength varied from 310 to 350 nm, CDs solution have only a change in the fluorescence intensity but no shift in the peak position, which might contribute to the

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carbon core states. However, the fluorescence emission peak correspondingly shifts from 440 nm to 510 nm with the excitation wavelength changing from 350 to 450 nm, which might contribute to the abundant surface states. The reasons of the CDs fluorescence spectrum in Figure S3 may due to the carbon core statesand the surface states.44, 45 Figure S4a shows the FTIR spectrum of the CDs. The nitrogen peaks are showed with C=N (1613 cm-1) and aromatic C-N heterocycles ranging over 1240–1500 cm-1 (including 1395 and 1290 cm-1).37, 46, 47 The stretching vibrations at 3310, 3292, 2927, 1705, 1641 and 1102 cm-1 revealed hydroxyl, amidogen, methyl, methylene groups, C=O in carboxyl, and C-O-C groups,12, 48 respectively. 1510 cm-1, which is a intense peak, can attribute to the C=C groups absorption, consistenting with the result for the HR-TEM lattice spacing of carbon core structure.49 These results indicate that there are rich in O- and N-containing groups on CDs surfaces, producing excellent water solubility.30, 50 Figure 1e mainly shows C, N and O elements in XPS spectra. Figure S4b reveals the high-resolution spectrum of C1s, which has four peaks at 284.7, 285.7, 286.6 and 288.4 eV for C=C/C-C, C-N, C-O, and C=N/C=O, respectively. Figure S4c reveals the high-resolution of N1s spectrum, there are the presence of C-N=C (399.5 eV), N-(C)3 (400.08 eV) and amide/amino N-H groups (401.2 eV).51, 52 High-resolution O1s spectrum (Figure S4d) is divided into two peaks at 531.8 and 532.8 eV, which can attribute to C=O and C-O-C/O-H,53 respectively. These FTIR and XPS results confirm that CDs have C=N/C=O groups without the need to incorporate external N-containing compounds.

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Figure 1 (a) Schematic depiction of the one-step synthesis CDs. (b) TEM image of CDs. Insert: HRTEM lattice fringe image of CDs. (c) Size distribution of CDs (average diameter=2.31 nm). (d) 3D fluorescence plots of CDs. (e) Fluorescence spectra (350 nm excitation) of CDs with different concentration of Fe3+: 0, 0.0001, 0.001, 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.4, 0.6, 0.8, and 1 mM. The inset plots the relative fluorescence intensity (I/I0) of CDs recorded at 350 nm versus the concentration of Fe3+. (f) Normalized fluorescence intensity of CDs with various metal ions. (g) XPS survey spectrum of CDs. (h) Fluorescence decay curve of CDs in the absence and presence of Fe3+. CDs stability is examined in Figure S5a and Figure S5b with increasing NaCl concentrations over the range 0–200 mM. There are no significant changes in the fluorescence intensities. Moreover, there have no decrease in fluorescence intensity when CDs are dispersed in various pH values over the range 3–12 (Figure S5c and Figure S5d). The CDs exhibit strong stability after exposure to 365 nm light for one hour (Figure S5e

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and Figure S5f). Thus, the CDs from SCP exhibit stability with respect to ions, pH, and UV light for practical applications. From above characterization results, there are number of functional groups on CDs surfaces, and especially, carboxyl and hydroxyl groups are known to be good for chelation of metals and further have potential applications in sensing.54-56 Using the same concentrations and testing conditions, the results indicate that the fluorescence of CDs is selectively quenched by Fe3+, and not by other metal ions (Figure 1f and Figure S6a). In Figure 1h, the fluorescence intensity decayed as the Fe3+ concentration increased, with good linearity over the range 0–1 mM and a correlation coefficient (R2) of 0.993 (Figure S6b). The limit of detection for the fluorescence is estimated to be 0.125 μM at a signalto-noise ratio of three times. To further explore the CDs quenching mechanism, FTIR, fluorescence decay assay, zeta potential measurement, the dynamic light scattering (DLS) and Stern-Volmer equation are performed to characterize the change of CDs during the response of Fe3+. The most critical parameter to estimate static or dynamic quenching is the average timeresolved fluorescence lifetime (𝜏). Figure 1h and Table S1 show that the calculated 𝜏 of the CDs after coordination of Fe3+ ions is 3.61 ns, which is nearly the same as that before adding Fe3+ ions (3.76 ns). Thus, the quenching mechanism of CDs belongs to static quenching process. In addition, the binding sites between CDs and Fe3+ are confirmed by FTIR spectrum (Figure S4a). When adding Fe3+ into CDs solution, the relative intensity shows a sharp decrease at 1641 cm-1 and the peak at 3310 cm-1 diminishes gradually, indicating that fluorescence quenching may because of the interaction between Fe3+ and carboxyl and hydroxyl groups on CDs surfaces. Zeta potential of pure CDs is exhibited in Figure S7 by the form of histogram. The value of pure CDs is -21.0 mV, which confirms the presence of negatively charged groups existing on CDs surfaces. However, with adding Fe3+ ions, the absolute value of zeta potential has been decreased, which can attribute to electrostatic

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interaction between CDs and Fe3+, further comsuming surface negatively charged groups on CDs. The DLS is performed to measure the particle size of CDs with Fe3+ (in Figure S8), which shows 222.6 nm, confirming that the complex have sharply aggregated comparing with pure CDs. The result confirms that the fluorescence quenching may attribute to the formation of weak-binding non-fluorescent complex between Fe3+ ions and CDs.57 According to the Stern-Volmer Eq. (2), the quenching equation can be calculated to explore the fluorescence quenching mechanism of the complex: I0 I = Ksv[Q] +1

(2)

where Ksv, Q represent the Stern-Volmer quenching constant and Fe3+ concentration. I0 and I are the fluorescence intensities of CDs under the excitation of 350 with Fe3+and no Fe3+, respectively. Figure S9 shows a good linear relationship between I0/I and Fe3+ concentration, and the correlation coefficient (R2) is 0.99624 in the range of 0-100 μM. The calculated Ksv value (Ksv = 0.00187) is much less than 10-2 mol/L, indicating that Fe3+ is not an effective dynamic quencher for CDs.58 Therefore, the fluorescence quenching mechanism of the complex should be attributed to the static quenching process.

RTP of CDs/PVA films. The CDs/PVA films obtained by embedding CDs into the PVA matrix via drop-coating (Figure 2a) have strong blue fluorescence under UV excitation (Figure 2b left), and intense long-lived green RTP when turning off the UV lamp (Figure 2b right). CDs in water have no emission when the excitation is turned off, which may have been due to deactivated triplet states via the non-radiative decay processes. The PVA matrix has hydroxyl groups that easily formed inter-/intra-hydrogen bonds. It also forms intermolecular hydrogen bonds with functional groups on the CDs that suppress the non-radiative decay processes (Figure 2c). The RTP spectra of CDs/PVA films with different concentrations of CDs (Figure S10) indicate that the optimum CDs concentration in the PVA matrix is 0.8 mg/mL,

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which is thus used in experiments below.

Figure 2 (a) Fabrication of CDs/PVA films. (b) CDs/PVA film excited with a UV lamp. (c) Schematic of the RTP mechanism. Fluorescence excitation and emission spectra of CDs/PVA films are acquired with 350 nm excitation (Fig. S11). The excitation spectrum (em=450 nm) have a peak at 350 nm, which can attribute to n–π* transitions of the C=N/C=O groups on CDs. CDs/PVA films exhibit a broad RTP emission center in three-dimensional (3D) plot, and the optimal broad emission peak is located at 480–540 nm under 350 nm excitation (Figure 3a). The excitation-dependent RTP emission spectra have a maximum at 510 nm under 350 nm excitation, which is consistent with CDs fluorescence (Figure S12). The fluorescence and RTP emission spectra (Figure 3b) of the CDs/PVA films reveal 60 nm Stokes shifts

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between fluorescence and phosphorescent under the excitation of 350 nm, which is consistent with the energy gap of 0.32 eV. The small energy gap makes the spin-orbit coupling efficiency and further facilitate the generation of RTP. To examine the origin of the RTP, a comparison of CDs the UV-vis absorption spectrum and CDs/PVA RTP excitation spectrum is shown in Figure 3c. The absorption spectrum have a shoulder at 350 nm, which is attributed to the n–π* transitions of the C=N/C=O groups. In RTP excitation spectrum for emission at 510 nm, there have a broad band at 330–350 nm, which overlapped with the absorption band of the C=N/C=O groups, suggesting that RTP derives from those groups on the CDs surfaces. The C=O groups are reported as the origin of RTP due to the highly spin-orbit coupling degree and the smaller energy gap between singlet and triplet levels.59 C=N groups can increase the population of triplet excitons by facilitating ISC.60 However, there have no RTP observed in CDs solution because the triplet excitons are deactivated by collisions with oxygen in solution or by intermolecular vibrations. In contrast, the CDs/PVA films exhibit long-lived RTP because of the PVA matrix. A possible mechanism for the formation of CDs-based RTP materials is depicted in Figure 3d. The C=N/C=O groups on CDs surfaces induced hydrogen bonds with the PVA matrix, which further facilitate the stability of the triplet excited states. The PVA has lower oxygen permeability that reduced C=N and C=O collisions with ambient oxygen, enabling efficient RTP.61 The 0.32 eV energy gap between the singlet and triplet levels also allow efficient spin-orbit coupling and ISC, resulting in RTP.62

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Figure 3 (a) 3D RTP of CDs/PVA films. (b) Spectra of fluorescence emission (green line) and RTP emission (blue line) of CDs/PVA films. (c) RTP excitation spectrum (red line) of CDs/PVA films and the absorption spectrum (black line) of CDs dispersed in water. (d) Illustration of possible RTP mechanism in CDs/PVA films.

Phosphorescent detection of Fe3+ ions in CDs/PVA films. Phosphorescent detection can exclude scattered light and fluorescence background, which exhibits good sensitivity and signal-to-noise ratio relative to fluorescence sensing.5 The phosphorescent of CDs/PVA films exhibit good selectivity for Fe3+ ions (Figure 4a), and no significant quenching with other metal ions. Moreover, RTP quenching can be visually observed with a UV lamp. Therefore, it has potential applications as a luminescent sensor for the sensing of Fe3+ ions. The RTP intensity is measured for different Fe3+ concentrations for evaluating the sensitivity of Fe3+ ions. With Fe3+ concentrations increasing, the RTP intensity of the

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CDs/PVA films are decreased (figure 4b). Figure S13 plots a reasonable linear correlation (R2=0.98814) between quenching efficiency and Fe3+ concentration over the range 0.1–2 mM. The limit of detection is estimated to be 0.57 μM for three times the standard deviation. In addition, Fe3+ has the strongest ability to quench the CDs fluorescence (Figures 1f, g). The fluorescence and RTP are simultaneously quenched by Fe3+, because of static quenching from the non-fluorescent complexes between the CDs and Fe3+.48 These complexes change the CDs energy level structure that is created by C=O and hydroxyl groups, which may be attributed to the electrons in the triplet states partially transferring to the Fe3+ orbital via nonradiative electron-transfer.53 C=N/C=O groups play an irreplaceable role in obtaining RTP. However, the energy level structure created by the C=O groups is damaged and led to RTP quenching (Figure 4c). The lifetimes, which are very important parameters to assess the quenching mechanism, are examined in Figure 1f and Figure 4d.48 As noted earlier, the CDs fluorescence lifetimes calculated with Eq. (1) are almost the same when the Fe3+ ions absent and present (3.76 ns and 3.61 ns, respectively), as did the RTP lifetimes of the CDs/PVA films (271.2 ms and 267.4 ms, respectively) in Table S1. The above results prove that RTP quenching of the CDs/PVA films by Fe3+ occurs via static quenching process.

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Figure 4 (a) Normalized phosphorescent intensities of CDs/PVA films with other metal ions. (b) RTP emission (350 nm excitation) spectra of CDs/PVA films with different Fe3+ concentrations: 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, 2 mM. (c) Schematic of RTP quenching in CDs/PVA films by Fe3+. (d) RTP decay curve of CDs/PVA films in the absence and presence of Fe3+. CDs/PVA films may open new perspectives for ion detection based on the long-lived RTP and high stability. The fluorescence and RTP intensities slightly decreased after 75 min of continuous excitation with a UV lamp, indicating that the CDs/PVA films had excellent photo-stability (Figure 5a). Moreover, after about half a year in the air, there have also long-lived RTP of the CDs/PVA films. The phosphorescent emission bands and lifetimes are nearly constant, indicating high stability (Figure 5b, c). CDs/PVA films can

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be used as solid-state luminescence sensors to detect Fe3+, which has priority over solution sensors due to the ability to carry and store. Figure 5d shows photographs of the change in CDs/PVA films before and after exposure to drops of Fe3+ ions. The intense blue fluorescence and green phosphorescence is visible in the absence Fe3+. Whereas, there is no visible fluorescence or phosphorescence in the presence of Fe3+. Because of the easily preparation in an inexpensive and eco-friendly process, featuring excellent stability under ambient conditions, the CDs/PVA films will be very suitable as luminescent sensor for Fe3+ detection.

Figure 5 (a) Fluorescence and RTP intensities of CDs/PVA films during continuous excitation with a UV lamp up to 75 min. (b) The RTP spectrum of CDs/PVA film before and after half a year in the air. (c) RTP decay curve of CDs/PVA film before and after half a year in the air. (d) Photographs of CDs/PVA films in sunlight, with UV excitation, after

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the UV was turned off, and with the addition of Fe3+, respectively.

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CONCLUSIONS In conclusion, a facile and green synthesis strategy is used to obtain CDs-based RTP materials by embedding CDs into PVA matrix. The RTP CDs are prepared by using SCP as the only carbon source, which provided natural nitrogen-containing structure for endogenous nitrogen doping. The C=N/C=O groups on CDs surfaces can promote n-π* transitions that increased the probability of ISC for populating triplet excitons responsible for RTP. The PVA matrix plays a dual role in stabilizing the triplet excitons and protecting RTP from quenching by ambient oxygen through forming hydrogen bonds between C=N/C=O groups on CDs surfaces and hydroxy groups on PVA matrix. The CDs/PVA films have RTP lifetimes up to 271.2 ms under 350 nm excitation in air. Moreover, the CDs/PVA films are good luminescent sensors, which shows a good response to Fe3+ quenching both the RTP and fluorescence. The RTP intensity is thus inversely proportional to the Fe3+ concentration over the range 0.1–2 mM, with a detection limit of 0.57 μM. CDs/PVA films are prepared via a synthesis that was green, facile, and cost-effective, and thus expanded the way to the design of RTP materials using natural products. Overall, it will open a new perspective as luminescent sensors for ion detection based on the longlived visible phosphorescence, with high optical and physical stability.

ASSOCOATED CONTENT Supporting Information XRD; UV-vis absorption spectrum and fluorescence spectra; FTIR spectrum and highresolution XPS spectra; Fluorescence emission and intensities under different ionic strengths of NaCl, under different pH values from 3 to 12 and under a UV lamp at various times; Fluorescence spectra indicating selectivity to Fe3+ ions; Linear plot; zeta potential measurements; DLS measurement; Stern-Volmer plot; Phosphorescent spectra of

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CDs/PVA films with different CDs concentrations; Fluorescence excitation and emission spectra; Phosphorescent emission spectra under different excitation wavelengths; Linear plot; and the table of Fluorescence and phosphorescent lifetime data of CDs, CDs+Fe3+, CDs/PVA film and CDs/PVA film+Fe3+

CONFLICT OF INTEREST The authors declare that they have no conflicts of interest.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 31890773, 31570567), the Fundamental Research Funds for the Central Universities (2572017ET02).

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Synopsis: CDs-based RTP material is fabricated with in-situ nitrogen dopping strategy for visual iron ion detection

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