Matrix-Free and Highly Efficient Room-Temperature Phosphorescence

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The matrix-free and highly efficient room-temperature phosphorescence of nitrogen-doped carbon dots Yifang Gao, Hui Han, Wenjing Lu, Yuan Jiao, Yang Liu, Xiaojuan Gong, Ming Xian, Shaomin Shuang, and Chuan Dong Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00939 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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Langmuir

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The matrix-free and highly efficient room-

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temperature phosphorescence of nitrogen-doped

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carbon dots

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Yifang Gao a, Hui Han a, Wenjing Lu a, Yuan Jiao a, Yang Liu a, Xiaojuan Gong a,

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Ming Xian a,b , Shaomin Shuang a and Chuan Dong a*

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a

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University, Taiyuan 030006, China

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b

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Keywords: Room-temperature phosphorescence, Matrix-free, Carbon dots, Nitrogen-doped

Institute of Environmental Science, and School of Chemistry and Chemical Engineering, Shanxi

Department of Chemistry, Washington State University, Pullman, WA, 99164, USA

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ACS Paragon Plus Environment

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Abstract

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Efficient room-temperature phosphorescence (RTP) of carbon dots (CDs) usually is seriously

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limited to appropriate solid matrix or introduced heavy atoms responsible for promoting

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intersystem crossing (ISC) and suppress vibrational dissipation between singlet and triplet states.

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So facile preparation efficient RTP of CDs with non-matrix is still a highly difficulty and

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challenging task. Here, we firstly reported a subtle strategy to induce highly-efficient free-matrix

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RTP of nitrogen-doped CDs (NCDs). The NCDs is composed of a core and hydrophilic surface

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of polyaspartic acid-chains arising from high-temperature polymerization by a one-pot heating

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treatment of L-aspartic acid and D-glucose. The obtained NCDs have an ultralong

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phosphorescence lifetime of 747ms and a high phosphorescence quantum yield (PQY) of 35%

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under 320nm excitation in air. To the best of our knowledge, the PQY is current the highest

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values recorded for RTP of CDs. The facile preparation and unique optical features offer these

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NCDs potential application in numerous applications such as anti-counterfeiting and white light-

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emitting diodes.

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Langmuir

Introduction

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Room temperature phosphorescence (RTP) have aroused considerable interest for their

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attracting applications in security aspects1, optoelectronics devices2-4 and biological imaging5-6

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based on their much longer lifetime and higher internal quantum efficiency than fluorescence.

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However, the observation of phosphorescence phenomenon is in principle extremely difficult,

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because it involves spin-forbidden transitions which are responsible for low probability of

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intersystem crossing (ISC) and radiative pathway between singlet and triplet states.7-8 While

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these factors seriously limit the appearance of phosphorescence (Fig. 1A). To date, the effective

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routes to promote intersystem crossing are to incorporate heavy metals, halogen bonding,

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carbonyl bonds, siloxy groups or C-N/C=N bonds etc.9 In another case, it can be realized either

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by embedding a luminescent species in appropriate solid matrices, e.g., filter paper, polymers or

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silica, which can offer dense hydrogen bonding with strong rigidity and good oxygen-barrier

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rigidly to suppress vibrational dissipation.10-11 However, regardless of the methods of introduced

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heavy atoms or embedded appropriate solid matrices, these methods all involved complicated

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preparation process, toxicity, high cost and potential environmental hazards. In addition, most

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materials of RTP relatively short emission lifetimes (i.e. several microseconds to a few

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milliseconds), and such short decay times can’t meet the requirement for the persistent

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luminescence, because tens of milliseconds is generally required for the common human to

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recognize the afterglow emission.12 Therefore, it’s still highly desirable to explore facile

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preparation, environment-friendly RTP materials with a long afterglow emission.

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Carbon dots (CDs), as an important class of photoluminescent (PL) nanomaterials, have

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attracted significant attention due to their low toxicity, excellent optical properties,

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environmental friendliness and high photostability. There optical applications frequently focus

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on biological imaging13 printing inks4,

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fluorescence properties. However, the rare attention is paid to the phosphorescent phenomenon

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and their related applications. Zhao et al.16 first observed RTP of CDs by introducing a polymer

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poly(vinyl alcohol) as a protective matrix. The generation of CDs RTP was from the hydrogen

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bonding interaction between hydroxyl groups on polymer and carbonyl groups on CDs, which

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effectively limited the intramolecular motions and preventing the nonradiative relaxation of CDs.

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This research released the possibility of CDs phosphorescence creation for the matrix selection

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concern. Numerous polymers, such as polyurethane,17 slica gel18 and polymer composite matrix

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and light emitting devices2-3 from its excellent


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(urea and biuret)1 as matrix materials have been used for RTP of CDs. In addition, inorganic

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matrix (KAl(SO4)2.xH2O19 or NaCl Hybrid Crystals20) were also found as substrate to induce

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RTP of CDs, Which were able to protected their energy from rotational or vibrational loss by

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rigidifying these aromatic carbonyl groups, so as to suppress the nonradiative deactivation

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pathways.21 However, relative deficient dispersion medium and complicated manipulated

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process have to some extent confined further utilization. The RTP CDs-based matrix systems are

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facing a great challenge. Recently, Liu et. al9 induced RTP of CDs based on the aggregation-

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induced effect from PVA conjugated CDs. CDs themselves serve as both solidified host and

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luminescent guest without introducing any heavy atoms and additional supporting matrices.

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The strategy could effectively prevented oxygen from permeating the aggregates and quenching

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the triplet and simplified the required conditions for CDs RTP. This study inspired us to develop

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the facile preparation of CDs with long RTP lifetime for practical application purpose.

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In this work, a highly efficient and matrix-free RTP of novel NCDs was prepared with a one-

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step high-temperature polymerization method22 by conjugating polyaspartic acid-chains onto the

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surface of NCDs. The NCDs themselves serve as both solidified host and luminescent guest

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without any supporting matrices. The polyaspartic acid-chains could play dual roles which both

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efficiently formed the hydrogen bond framework on the surface of NCDs to stabilizing the triplet

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states of NCDs and strongly hindered oxygen quenching to present RTP behavior at air

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atmosphere. What's more, the NCDs possess the intriguing features of ultralong phosphorescence

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lifetime of 747ms and a high PQY of 35%, which are favorable for the purpose of visible

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to naked eye. To the best of our knowledge, the PQY value is current the highest record for RTP

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of NCDs. It will be great potential for anti-counterfeiting and white light-emitting diodes.

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Experimental

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Materials

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D-glucose and L-Aspartic Acid were obtained from Aladdin (USA). Other reagents were

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acquired from Aladdin Ltd (Shanghai, China). All chemicals were of analytical reagent grade

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and used as received without any further purification. Ultrapure water (≥18.25 MΩ cm) from a

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molecular purification system (Shanghai, China) was used in all experiments.

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Apparatus


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The transmission electron microscopic (TEM) images were acquired on a JEOL JEM-2100

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transmission electron microscopy (Tokyo, Japan) with an accelerating voltage of 300 kV. The

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elemental analysis was conducted on an Elementar Analysen systemevario EL cube elemental

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analyser (Hanau, Germany). The X-ray photoelectron spectra (XPS) were acquired on an AXIS

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ULTRA DLD X-ray photoelectron spectrometer (Kratos, Tokyo, Japan) with AlKα radiation

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operating at 1486.6 eV. Spectra were processed by Case XPS v.2.3.12 software using a peak-

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fitting routine with symmetrical Gaussian-Lorentzian functions. The Fourier transform infrared

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spectra (FTIR) were recorded on a Bruker Tensor П FTIR spectrometer (Bremen, German).

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Absorption spectra were recorded using a UV-2450-PC spectrophotometer (Shimadzu, Japan).

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Steady-state PL and phosphorescence spectra were collected on a Hitachi F-4500 fluorescence

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spectrophotometer. Time-resolved luminescence spectra were performed using an Edinburgh

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FLS920 fluorescence spectrophotometer. The absolute quantum efficiency was obtained on an

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Edinburgh FLS920 spectrophotometer equipped with an integrating sphere with an integrating

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sphere from the reconvolution fit analysis.

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Synthesis of NCDs

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The synthesis of NCDs was carried out as shown in Fig. 1B. A typical synthetic route is

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described as follow. Glucose (0.11g, 0.62 mmol) and L- Aspartic Acid (0.33g, 2.5 mmol) were

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sufficiently mixed via sonication with 3 ml of aqueous sodium hydroxide solution, and then the

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mixture was heated to 150 ºC by an oil bath (Fig. 1B). The reaction was completed when the

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mixture became a pale-yellow solid. The products were completely dissolved into 5 mL water

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and freeze-dried to obtain solid. It only takes a few minutes. The product was cooled to room

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temperature and was completely dissolved into 20 mL water, and then placed in a dialysis bag

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(cutoff Mn: 500-1000 Da) and dialyzed against water for 1 d to remove small molecules. The

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water was replaced every 4 h, and finally, NCDs in the dialysis bag was freeze-dried to obtain

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light yellow solid.


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Fig. 1. (A) Energy-level diagram of the relevant photophysical processes (S0 = ground state, S1 =

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singlet excited state, T1 = triplet excited state; ISC = intersystem crossing; Abs. = absorption;

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Non Rad. = non radiative). (B) Synthesis of NCDs.

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Results and discussion

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Characterization of NCDs

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As is shown in Fig.2A, SEM images of NCDs powder displays like microsheets, which can

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re-dissolve in water without aggregation. The TEM image of the nanoparticles is displayed in

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Fig.2B, which indicates that the nanoparticles are uniformly dispersed without any aggregation.

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The statistical particle-size distribution as shown in Fig. 2D is calculated using 100

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nanoparticles. The size-distribution ranges between 5.5 nm and 8.5 nm, with an average particle

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size of about 6.7 nm. The HRTEM image shows that most of the NCDs are amorphous in

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structure without any lattices.

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To confirm the particle organization, we took NCDs powder/ethanol suspension (the NCDs

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don’t dissolve in ethanol so that their aggregated morphologies may mostly retain) for HRTEM

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image of a typical NCDs and expectedly observed that the NCDs particle is composed of a core

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and shell (Fig. 2C). NCDs and polyaspartic acid (without adding D-glucose) powder are similar

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in infrared absorbance to polyaspartic acid-chains (Fig. S1), which is a clue to existence of


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polyaspartic acid-chains even though after hydrothermal treatment of 150℃. The above indicate

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that the incompletely carbonizing lead to a more disorder surface. These TEM of the NCDs and

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CDs in the absence of L-Aspartic Acid are somewhat different, that is, CDs remain high-

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crystallinity (Fig. S3) but NCDs has hardly any high-crystallinity. Therefore, we predict that

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NCDs particle may be depicted as a graphitizing core in amorphous surface. This similar

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structure is also suggested by previous study9 which revealed that numerous polyaspartic acid-

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chains radially wrap the carbon cores. Thus, everyone nanoparticles may tightly crosslink with

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each other via hydrogen bonds after removing the water, just like the formation solid film.23

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Surface analysis was carried out using X-ray photoelectron spectroscopy (Fig. 2E). The C 1s, N

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1s, and O 1s peaks are observed at 284.6, 399.4, and 530.9 eV respectively. The high-resolution

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spectrum of C1s (Fig. S4.A) shows three peaks at 284.8, 286.5, and 288.6 eV for C−C/C=C,

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C−N/C−O, and COOH, respectively. The N 1s spectrum (Fig. S4.B) shows the characteristic

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peaks of a pyrrole-like N and N−H groups at about 399.9, and 401.7 eV, respectively. The O 1s

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XPS spectrum of CDs (Fig. S4.C) can be deconvoluted into two peaks at 531.7 eV and 533.1 eV,

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indicating the existence of C=O and C−OH, respectively.24 These XPS results indicate that

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NCDs are rich in oxygen and nitrogen atoms.

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Fig. 2. (A) SEM images of NCD powder. (B, C) TEM and HRTEM images of NCDs and (D)

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size distribution of NCDs powder in ethanol. (E) XPS spectra of NCDs. (F) FTIR spectra of

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NCDs.


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The FTIR spectrum of NCDs gave the further structure information (Fig.2F). The broad and

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pronounced absorption peaks at 3409 and 3161 cm-1 indicate the existence of large amounts of

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O−H and N−H groups. Peaks at 1397 cm-1 could be ascribed to the bending vibration of C−H,

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while those at 2976, 2934, and 2875 cm-1 could be assigned to the stretching vibration of C−H.

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The strong stretching vibration at 1625 cm-1 corresponded to the C=O of aromatic carbonyl. The

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appearance of characteristic peaks at 1339, 1306, and 1074 cm-1 originated from the C−N,

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C−O−C, and C−O−H stretching vibrations, respectively. For further exploration of the source of

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surface functional groups on the surface of the NCDs, FTIR spectra of L-Aspartic acid,

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polyaspartic acid, glucose and polyglucose have been revealed in Fig. S1. By contrast, C=O, O-

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H, and COOH on the surface of the NCDs can be derived from polyglucose and polyaspartic

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acid.

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The TGA analyses curves of NCDs, polyaspartic acid and polyglucose have been conducted

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to investigated the surface functional groups of NCDs (Fig. 2S). A comparative TGA curves of

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NCDs and others materials (polyaspartic acid and polyglucose) related to weight loss against

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temperature was analyzed and displayed in Fig. 2S. All the profiles showed a little weight losses

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below 150 ºC due to loss of absorbed water.25 The main step of the copolymer degradation starts

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at 303 ºC with the mass loss of 31%, which probably corresponds to the decomposition of amide

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and ester bonds.26 The decomposition temperature of NCDs, polyaspartic acid and polyglucose

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were at 303 ºC, 315 ºC and 272 ºC, respectively. The difference in thermal degradation behaviors

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maybe is attributed to attached carboxylate groups.27 The polyaspartic acid, polyglucose and

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NCDs decomposed with the weight loss up to 66.66%, 71.83% and 79.85% of their actual

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weight at 900 ºC, respectively. TGA data clearly demonstrate more weight loss in the case of

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NCDs over the polyaspartic acid and polyglucose which could be related to the weight

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contribution originated from hydrogen bonds.28 The present results indicate that there exist

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strong hydrogen bonds among NCDs. The TGA identify surface functional groups consistent

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with XPS and FTIR. The XPS, FTIR and TGA results confirmed that the NCDs contain

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abundant functional groups (C=O, C−N, NH, OH, etc.) which impart excellent water solubility

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without further chemical modification.

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Photoluminescence properties of NCDs

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The photoluminescence properties of NCDs are the basis of their applications. We had

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studied its spectroscopic properties by UV-visible absorption and photoluminescence


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(fluorescence and phosphorescence) emission spectra. The UV-visible absorption spectra of

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NCDs (Fig. 3B) exhibit absorption peaks at 297 nm, which is ascribed to the n–π* transition of

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the C=O bond.29-30 Surprisingly, we found that the NCDs not only have intense fluorescence

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under UV light (365 nm) excitation, but also an obvious afterglow when the UV light is turned

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off. The digital photographs of the after-glow are demonstrated at time 0 s, 1 s, and 2 s in air (Fig.

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3A) and the corresponding video is included in supplementary information, which is visible to

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the naked eye for seconds (a corresponding video is provided in the ESI). Fig. 3C shows the

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corresponding fluorescence spectrum (black line) and phosphorescence spectrum (red line) of

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NCDs at an excitation wavelength 320 nm. There is an approximately stokes shift 42 nm. This

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implies that the difference between singlet-triplet energy level is about 0.21eV. According to

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previous reports,16-17, 19, 31-32 the emergence of phosphorescence is mainly due to the aromatic

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carbonyl groups formed at the surfaces of CDs. In the phosphorescence excitation spectrum with

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a broader band emission at 515 nm, only a broad band at 240–340 nm appears and overlaps the

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absorption band of C=O bonds, suggesting that the phosphorescence may come from the C=O

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bonds on NCDs(Fig. S5).

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The absolute PQY (excitation at 320 nm) of the NCDs is about 35%, which is a relatively

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high PQY for NCDs (Table S2). Compared with other the PQY based on carbon dots，the

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NCDs possess the highest value recorded for RTP of CDs (Table S3). The high PQY is mainly

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due to the existence of O and N atoms. These heteroatoms in the NCDs lattice can disrupt the sp2

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hybridization of carbon atoms, altering the electronic structures and increasing the PL intensity

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of CDs.33-34

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The time-resolved phosphorescence spectra with an excitation wavelength of 320 nm and the

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emission wavelengths of 340 and 650 nm are shown in Fig. 3D. The results of fitting the two

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spectra with bi-exponential functions are presented with an average lifetime of 747 ms in Table

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S2. The two phosphorescence lifetimes of 250.50 ms (47.56%) and 1198.14ms (52.44%) imply

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that the NCDs surface has diverse energy levels.35 The multiple phosphorescence lifetimes imply

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the present of various electronic transition processes, which originate from the various chemical

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environments of the carbonyls on the surface of CDs.36-37


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Fig. 3. (A) Photographs of NCDs before and after turning off the UV excitation (365nm). (B)

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UV-vis absorption spectrum. (C) The fluorescence (black lines) and phosphorescence (red lines)

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emission spectra of NCDs with excitation at 320 nm. (D) Time-resolved phosphorescence

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spectra of NCDs.

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In a further set of experiment, three kinds of NCDs-1, NCDs-2 and NCDs-3 (molar ratio D-

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glucose: L-Aspartic Acid (1:4, 1:2 and1:1) were synthesized, via the same method. The

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elemental analysis indicates (Table S1) that the NCDs contain carbon, hydrogen, nitrogen and

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oxygen elements. The element analysis from XPS and TOC (total organic carbon) be analyzed in

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Table S1, and results consistent with elemental analyser. In addition, we found that

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phosphorescence intensity (Fig. 4A), the absolute PQY and the lifetimes (Table S2) are

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enhanced with the increase of nitrogen content. The digital photos of NCDs-1, NCDs-2 and

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NCDs-3 of the after-glow are demonstrated at time 0 s, 1 s, 2 s and 3s in air (Fig. S6). These

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results confirm that the intensity of phosphorescence and PQY is enhanced with the increase of

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nitrogen content. We infer that more nitrogen content favours n-π transition and hence facilitate

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the spin-forbidden transfer of singlet-to-triplet excited states through intersystem crossing to

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populate triplet excitons.38-39 In addition, the intensity of phosphorescence decreases as the


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temperature increases with the range of 78−301 K (Fig. 4B), confirming major contribution from

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RTP.

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Fig. 4. (A) Phosphorescence spectra of NCDs-1, NCDs-2 and NCDs-3 excited at 320 nm. (B)

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Phosphorescence spectra of NCDs at temperatures from 78 to 301 K. (C) The effect of exposure

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time under UV light on the intensity of phosphorescence. (D) The effect of exposure time under

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in air on the intensity of phosphorescence.

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The PL stability of NCDs was also investigated under UV exposure times and kept in air

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conditions. At different UV exposure times (Fig. 4C), the PL intensities increases and eventually

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reaches a steady-state and peak characteristics show no prominent changes, which indicates that

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NCDs are beneficial for practical applications in optoelectronics and data security. In addition,

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no significant changes was detected when the NCDs were exposed under in air for a month (Fig.

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4D) and further proved the as-prepared NCDs itself provided unique rigid shape based on the

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hydrogen bond framework from polyaspartic acid-chains on the surface of NCDs and can

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obstacle the penetration of moisture and oxygen.

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Application of NCDs

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Taking advantages of the above-mentioned properties (The solution of NCDs has no

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phosphorescence phenomenon, and the solid has phosphorescence phenomenon.), the NCDs

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could also be developed for promising applications in data encryption.23 The letters Chinese

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character “山西大学” have been written on non-fluorescent paper using NCDs mixed solution as

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the ink, which could not be clearly seen in either visible. After drying, the letters are observable

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when the UV light is turned off. When the paper was wetted via water-spraying, the letters

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gradually became dark when the UV light is turned off (Fig. 5A). These observations

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demonstrate that the NCDs aqueous dispersion could potentially be employed as an advanced

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anti-counterfeiting ink for advanced authentication or hiding important information from

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unanticipated people.

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The phosphorescence mechanism of NCDs

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To further understand the phosphorescence mechanism of NCDs. We investigate the RTP

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emission of an aqueous solution of NCDs, but no RTP can be detected when NCDs is dissolved

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in the water (WNCDs). We compared the FT-IR spectrum of NCDs and WNCDs (Fig. S7). In

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the FT-IR analysis, the appearance of characteristic peaks at 2732, 2664, 2515, 1330, 1305,1248

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and 925 cm-1 originate from the carboxylic acid dimer vibrations.40 It has been confirmed that the

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NCDs are immobilized on the surface of nCOOH through the formation of C＝O┄H-O bonds,

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but no the formation of C＝O┄H-O bonds when the CDs is dissolved in the water. The reason

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may be that intra-NCDs hydrogen bonds are disrupted by the solvent effect of water molecules

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(Fig. 5B). So we think that the formation of intra-NCDs hydrogen bonds between carboxylic

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acids is also a key factor for RTP. Moreover, the rigidity of hydrogen bonding play a key role in

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rigidifying the C=O bonds on the surface of NCDs, minimizing the nonradiation transitions of

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triplet excitons. In addition, the long chain of the NCDs surface forms a carboxylic acid dimer

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has a good oxygen barrier performance, which can hinder the direct collisions between C=O and

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oxygen molecules and promote the phosphorescence.


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Fig. 5. (A) Security protection application. (B) Phosphorescence mechanism of NCDs.

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Conclusion

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In summary, we provided a kind of highly efficient RTP of NCDs by one-step with non-

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matrices. The obtained NCDs have an ultralong phosphorescence lifetime of 747ms and a high

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PQY of 35% under 320 nm excitation. It is found that polyaspartic acid-chains on the surface of

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NCDs gave much advantage to the moisture-resistance that makes NCDs powder stand rigidly

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under ambient condition. And this polyaspartic acid-chains structure also acts as tight barrier that

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effectively prevents oxygen from permeating the aggregates and quenching the triplet. Its

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potential applications include in security aspects and optoelectronics devices. This discovery of a

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new class of CDs-based RTP materials with ultralong lifetimes and high efficiency is developed

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as a proof of concept, which we believe will promote both fundamental understanding and future

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practical applications of RTP materials.

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ASSOCIATED CONTENT

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Supporting Information

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FTIR spectra of NCDs, polyaspartic acid and L-Aspartic Acid, TEM and HRTEM (inset)

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images of CDs, High-resolution C1s XPS, High-resolution N1s XPS, High-resolution O1s XPS


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Page 14 of 19

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spectra of the NCDs, UV-vis absorption and excitation spectra of the as-prepared NCDs, The

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digital photos of NCDs-1, NCDs-2 and NCDs-3 of the after-glow are demonstrated at time 0 s, 1

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s, 2 s and 3s in air, FTIR spectra of NCDs and WNCDs, Elemental analysis of the NCDs,

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Lifetime data obtained from the time-resolved decay curves of NCDs at λex/λem of 320/515 nm

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and the absolute PQY, The NCDs of fluorescence and phosphorescence.

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AUTHOR INFORMATION

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Corresponding Author

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* Fax: +86-351-7018613

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E-mail: [email protected] (C. Dong)

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ACKNOWLEDGMENT

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The authors gratefully acknowledge financial support from the National Natural Science

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Foundation of China (No. 21705101, 21475080 and 21575084) and Shanxi Province Hundred

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Talents Project.

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Langmuir

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A subtle strategy to construct efficient room-temperature phosphorescence of carbon dots

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without any supporting matrix was reported. The obtained carbon dots have an ultralong

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phosphorescence lifetime of 747ms and a high phosphorescence quantum yield (PQY) of 35% in

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air.

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Matrix-Free and Highly Efficient Room-Temperature Phosphorescence

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