LDHs films with excellent luminescent

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Fabrication of CuNCs/LDHs films with excellent luminescent properties and exploration of thermosensitivity Liyang Fu, Huimin Liu, Liang Yan, Yanyan Fu, Yu Zhu, Lan Jin, and Ruizheng Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Industrial & Engineering Chemistry Research

The fabricated CuNCs/LDHs films indicate the strong orange-red emission with QY and prolonged fluorescence lifetime. Moreover, it exhibited excellent luminescence stability, temperature-responsive in photoluminescence (PL) and electrogenerated chemiluminescence (ECL). 75x38mm (150 x 150 DPI)

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Fabrication of CuNCs/LDHs films with excellent luminescent properties and exploration of thermosensitivity Liyang Fu†, Huimin Liu†, Liang Yan, Yanyan Fu, Yu Zhu, Lan Jin*, Ruizheng Liang* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

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ABSTRACT Fluorescent copper nanoclusters (CuNCs) have been drawing great research interest because of their fascinating physicochemical properties. However, the low quantum yield (QY) and poor stability of CuNCs have limited their applications. In this work, CuNCs were anchored onto the layered double hydroxides (LDHs) by layer-by-layer (LBL) assembly method, and the obtained CuNCs/LDHs ultrathin films displayed an ordered periodic long-range structure and uniform morphology. The CuNCs/LDHs films indicate the strong orange-red emission with higher QY (16.66%) and prolonged fluorescence lifetime (9.05 μs), which is obviously better than that of individual CuNCs. The experimental studies and theoretical calculations both reveal that the location and confinement effects derived from LDHs nanosheets enhance the QY of CuNCs. In addition, the CuNCs/LDHs films show temperature responsive in photoluminescence

(PL)

and

electrogenerated

chemiluminescence

(ECL),

respectively. Consequently, it provides a new and facile approach to fabricate CuNCs-based films accompanied with excellent luminescent properties, which indicate the great potential for temperature sensing applications.

2



1. INTRODUCTION As a kind of promising fluorescent material, metal nanoclusters have received extensive attention because of their fascinating physicochemical properties such as discrete energy levels, molecule-like electronic transitions.1-5 In comparison with the organic dyes and inorganic quantum dots, these fluorescence nanoclusters possess large Stokes shift, low toxicity and good biocompatibility.2-6 In the past decades, fluorescent AuNCs and AgNCs have been intensively investigated because of their unique features such as ultrasmall hydrodynamic diameters, tailorable surface properties and chemical stability.7-10 However, studies of CuNCs are still deficient because of their weak fluorescence properties and surface oxidation upon exposure to air.11 Therefore, researchers have been striving for the improvement of the quantum yield (QY), and the fluorescence efficiency of CuNCs can be greatly enhanced through aggregation induced emission (AIE) in the organic solvent.12,13 However, the AIE process is difficult to control and the resulted CuNCs aggregation is usually water insoluble, which suppresses the promised application in optical devices and biosensing. Moreover, extensive efforts have been made to synthesize stable CuNCs using a variety of protecting ligands, such as DNA,14 sulfhydryl compounds15,16 and protein.17 Although greatly improved, it is still far from the satisfactory. Therefore, how to fabricate CuNCs with the high QY and excellent stability by an effective approach is highly needed but still a big challenge. The layered double hydroxides (LDHs) are a typical kind of two-dimension inorganic nanomaterials with the property of tunable chemical composition.18 Positively-charged monolayer LDHs nanosheets as building block or substrate can be prepared through a common exfoliation strategy.19,20 Based on the host-guest interactions, LDHs functional materials have received growing attention and widely used in the fields of photofunctional materials,21,22 biology,23,24 catalysis,25-28 and electrochemistry.29 It is well known that the fluorescence QY is defined as the ratio of the emitted photons to the absorbed photons.30 If the excited electrons are confined by a layered matrix which has a broad band gap, a promoted recombination of electrons and holes and enhanced QY will be obtained. This inspires us that if CuNCs are 3


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immobilized into the interlayer of LDHs nanosheets, an intrinsic localization and confinement effects would boost the fluorescence QY and stability of CuNCs. Herein, the immobilized CuNCs/LDHs films with high QY and stability were conducted through the LBL assembly of CuNCs and LDHs nanosheets (Scheme 1). The assembled CuNCs/LDHs films have a periodic long-range ordered structure and uniform morphology, confirmed by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and atomic force microscope (AFM). Compared with pristine CuNCs aqueous solution, the fabricated CuNCs/LDHs films show much higher QY (16.66% vs.2.56%), prolonged fluorescence lifetime (9.05 µs vs. 1.17 µs) and better storage stability. The enhancement of these fluorescence properties can be ascribed to confinement effects of LDHs layered structure, which was confirmed by periodic

density

functional

theoretical

(DFT)

calculation.

Moreover,

the

CuNCs/LDHs films exhibit temperature-responsive in the photoluminescence (PL) and electrochemluminescence (ECL) measurements, which can be potentially applied in the fields of optical sensing and devices in the future.

Scheme 1. Schematic diagram for (A) preparation of GSH-capped CuNCs in aqueous solution; (B) the LBL fabrication of fluorescence (CuNCs/LDHs)n films and (C) the structure of CuNCs localized into interlayer of LDHs. 2. EXPERIMENTAL SECTION 2.1. Materials

4



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Analytical pure CuSO4·5H2O, NaOH, Al(NO3)3·9H2O, Mg(NO3)2·6H2O, as well as urea and other reagents were purchased from Beijing Chemical Co. Ltd. L-Glutathione, formamide were purchased from Aladdin Chemical. Co. Ltd. Deionized water was used in the experimental processes. 2.2. Preparation of CuNCs The glutathione-stabilized Cu nanoclusters (CuNCs) were synthesized referring to the previous work with a little modification.31 In a typical experiment, 5.0 mL 50 mgmL−1 of GSH solution was added into 5.0 mL 10 mM of CuSO4 solution under stirring. The colour of the solution changed from transparent to white hydrogel. Subsequently, NaOH (1 M) was dropwise added into the solution until pH value increased to 5.5. Finally, the resulting solution was incubated at 37°C for 1 h. The as-prepared CuNCs solution were subjected to 500 MWCO of dialysis membrane for further purification, and then stored at 4 °C. 2.3. Fabrication of CuNCs/LDHs Films The construction of MgAl-LDHs nanosheets was referred to our previous experiments.28 Typically,

0.002

mol

of

Mg(NO3)2·6H2O,

0.001

mol

of

Al(NO3)3·9H2O and 0.012 mol of urea were dissolved in 70 mL of deionized water. The mixture was transferred into a 90 mL Teflon-lined stainless steel autoclave and put it in an oven at 110 ° C for 24 h. The obtained sample was washed with water and dried at 60 °C. After dissolving 0.3 g of this sample in an aqueous solution (300 mL, containing 0.50 mol NaNO3), 0.08 mL of concentrated HNO3 was added slowly and then the solution was stirred at room temperature under N2 atmosphere for 24 h. The obtained LDHs was washed with deionized water, and dried at 60 °C under vacuum. Afterwards, 0.1 g LDHs sample was added in 100 mL formamide solution under the protection of N2, and the mixture was stirred for 48 hours to obtain a colloidal suspension of the exfoliated Mg-Al-LDH layer. Before LBL assembly, the quartz glass substrate was firstly steeped into the mixed solution of H2SO4-30% H2O2 (7:3, v/v), followed in ethanol to clear away the organics on the surface, and finally washed thoroughly and dipping in deionized water. In order to fabricate the CuNCs/LDHs films, the substrate was immersed into LDHs nanosheets solution (1.0 gL1) for ten 5


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minutes and washed with water thoroughly, then immersed into the CuNCs aqueous solution for another ten minutes. Followed by washing and drying, this construction process was repeated for n cycles. 2.4. Characterizations The powder x-ray diffraction (XRD) patterns of the film were recorded by a Rigaku 2500 VB2 + PC diffractometer, using Cu Ka radiation (λ= 0.154 nm) of 40 kV, 50 mA, with scanned in step of 0.04 (2θ) in the range from 1° to 10°. Transmission electron images (TEM) images were collected on a JEOLJEM-2100 with an accelerating voltage of 200 kV. XPS measurements were carried out by an ESCALAB 250 equipment with Al Ka radiation. The UV-vis absorption spectra were recorded using a Shimadzu U-3000 spectrophotometer with the slit width of 1.0 nm in the range from 250 to 700 nm. The fluorescence spectra were recorded via a F-7000 fluorescence spectrophotometer (Hitachi Ltd., Japan) in the range from 500 to 700 nm with emission slit of 5 nm. The Fourier transform infrared (FT-IR) spectra were performed via a Nicolet 605 XB FT-IR spectrometer in the range from 4000 to 400 cm−1. The loading density of Cu was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Shimadzu ICPS-7500. The morphology of the films was analysed by a scanning electron microscope (SEM, HitachiS-3500) with an accelerating voltage of 20 kV. The surface roughness data of films were collected through a NanoScopeIIIa AFM. Photoluminescence (PL) QY measurements were carried out on the reconvolution fit analysis (Edinburgh F980 analysis software) with an integrating sphere equipment. Steady-state luminescence lifetime was measured on an Edinburgh FLS 980 lifetime spectrometer. The experiments of PL responses to temperature

were

conducted

by

a

CRAIC22/30PV

Spectra

Vision.

The

electrogenerated chemiluminescence (ECL) experiments were conducted on a three-electrode system, involving platinum foil and saturated Ag/AgCl electrode as the auxiliary and reference electrode respectively. The ECL signals were obtained through a MPI-B chemiluminescent analytical system (Remax Electronic Co. Ltd., China) with the voltage of the photomultiplier tube on 800 V. 3. RESULTS AND DISCUSSION 6



3.1. The Assembling and Monitoring of (CuNCs/LDHs)n Films The water-soluble functional CuNCs with orange-red emission were prepared according to the reported method, in which GSH was used both as reducing reagent to reduce Cu(II) and as ligand to protect the prepared CuNCs.31 The morphology of CuNCs was characterized by transmission electron microscope (TEM), which indicated the spherical morphology and good monodispersity of CuNCs (Figure S1A). The mean diameter of CuNCs can be estimated to be 2.6 nm. In the HRTEM image (inset of Figure S1A), the interplanar spacing of 2.11Å indicates the (111) diffraction plane of face-centered Cu (JCPDS 89-2838). XPS study was performed to determine the oxidation state of copper. And the absence of satellite peak in 942.58 eV from the Cu 2p3/2 suggests the complete reduction of Cu (II) (Figure S1B).15,32,33 We further utilized the UV-vis absorption spectroscopy and fluorescence spectra to demonstrate the formation of CuNCs. The characteristic absorbance peak around 280 nm is ascribed to the ligand-to-metal charge transfer transition (LMCT) and the lack of the absorption band at 560 nm indicates that the size of obtained CuNCs is too small to support plasmon resonance,13,34,35 both of which illuminate the successful fabrication of CuNCs (Figure S1C). Moreover, the CuNCs aqueous solution displays weak orange-red emission under the UV irradiation (inset of Figure S1C) and the typical emission peak of CuNCs is located at 599 nm (Figure S1C). Furthermore, FT-IR measurements were applied to get some insight into the structural properties of CuNCs. As shown in Figure S1D, the completely vanishment of SH stretching vibration at 2524 cm‒1 in the FT-IR spectra of CuNCs sample suggests that GSH and CuNCs are connected through the formation of CuS bond.33,36 In addition, the zeta potential of CuNCs is found to be electronegative (−7.7 mV), which can be assembled with the positively-charged LDHs nanosheets (43 mV) through electrostatic interaction (Figure S2).

7


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Figure 1. (A) UV-vis absorption spectra of (CuNCs/LDHs)n (n= 630) films; (B) linear relation between absorbance intensity and cycle number n at 280 nm.

Figure 2. (A) The top-view SEM images (n= 30); (B) the AFM topographical images (n= 30); (C) the side-view SEM images of (CuNCs/LDHs)30 film and (D) XRD patterns of (CuNCs/LDHs)n (n= 12, 30).

The LBL assembly course of (CuNCs/LDHs)n (n= 630) films were evaluated. As shown in Figure 1A, the absorption curves of (CuNCs/LDHs)n films don’t exhibit any shift or broadening, and the absorption intensities of different cycles at 280 nm heighten along with the growth of layer number n. Figure 1B shows a favourable linear relation between absorbance intensity and cycle number n, suggesting a uniformly and regularly prepared procedure. The loading contents of Cu for (CuNCs/LDHs)n (n= 630) films were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). It can be seen that the loading density of 8



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Cu increases linearly from 1.86 to 9.65 µg·cm‒2 with cycle number n changing from 6 to 30 (Table S1 and Figure S3). Subsequently,

the

morphological

and

structural

characteristics

of

(CuNCs/LDHs)n films were investigated. Figure 2A and Figure S4 are the corresponding top-view SEM images, which exhibit the uniform surface morphology of (CuNCs/LDHs)n films. Figure 2B and Figure S5 are the AFM topographical images, showing a homogeneous surface morphology in these films. Side-view SEM and small-angle XRD were further used to determine the thickness of films. As shown in Figure 2C and Figure S6, the thickness of each layer is calculated to be around 3.20 ± 0.04 nm and an orderly increase of the film thickness along with the layer number n is observed (Table S2). The small-angle XRD pattern (Figure 2D) shows a reflection at 2θ=2.77°, suggesting that the repeated thickness of films is 3.19 nm. Those results are approximately according with the summation of the monolayer of CuNCs (2.6 nm) and LDHs nanosheets (0.48 nm). Thus, we can conjecture a model of the arrangement of CuNCs among the LDHs nanosheets (shown in Scheme 1C). 3.2. The Fluorescence Properties of (CuNCs/LDHs)n Films

Figure 3. (A) Fluorescence emission spectra of (CuNCs/LDHs)n (n=630) films; (B) linear relations between emission intensity and cycle number n (Inset: the photographs of the films under UV irradiation); (C) the fluorescence decay curve of 9


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CuNCs aqueous solution and (D) the fluorescence decay curve of (CuNCs/LDHs)30 film.

Comprehensive experiments were performed to explore the fluorescence properties of the assembled (CuNCs/LDHs)n films. Figure 3A displays the fluorescence emission spectrum of the assembled (CuNCs/LDHs)n films with an emission peak at 586 nm. The emission intensities were enhanced with the increase of assembled layers, which can be intuitively confirmed by the photographs of the films under UV irradiation (Figure 3B inset). A linear relation between emission intensity and cycle number n is further observed in Figure 3B. Compared with that of pristine CuNCs aqueous solution, the fluorescence emission peak of the (CuNCs/LDHs)n films shifted from 599 to 586 nm, which may be resulted from the confinement effect derived from LDHs laminate.29,30 We further conducted the steady-state fluorescence lifetime and QY measurements to get a better understanding of the properties in excited state. The fluorescence lifetimes for original CuNCs and the assembled (CuNCs/LDHs)30 film are described with a tri-exponential fitting. The average fluorescence lifetime for original CuNCs solution is 1.17 µs [0.14 µs (17.72%), 0.79 µs (48.65%) and 2.25 µs (33.63%)], while that of the (CuNCs/LDHs)30 film prolonged to 9.05 µs [0.54 µs (28.17%), 7.15 µs (42.96%) and 20.19 µs (28.88%)] (Table S3). There is an almost 8-fold increased fluorescence lifetime, which is visually shown at the typical fluorescence decay curves (Figure 3C and 3D). Moreover, the QY for the pristine CuNCs aqueous solution and (CuNCs/LDHs)30 film are 2.56% and 16.66%, respectively (Table 1). The QY of assembled (CuNCs/LDHs)30 film also shows almost 7-fold improvement compared with the pristine CuNCs aqueous solution. In addition, the stability of assembled (CuNCs/LDHs)30 film is also obviously improved. As shown in Figure S7, the fluorescence intensity of (CuNCs/LDHs)30 film decreased only ~6% after five days storage, while the fluorescence intensity of CuNCs solution almost quenched completely after three days. All these excellent fluorescent performances of the CuNCs/LDHs film may guarantee its further applications. 10



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Table 1.The fluorescence lifetime and quantum yield of CuNCs aqueous solution and the (CuNCs/LDHs)30 film. Samples

QY

τ

CuNCs aqueous solution

1.17 µs

2.56 %

(CuNCs/LDHs)30 film

9.05 µs

16.66 %

According to the potential principle of fluorescence lifetime and QY, if the photo-induced electrons and holes of the fluorescent agents are confined by some matrixes with the broad band gap, the excited electrons and hole can’t transfer to the conduction band and valence band, respectively. Thus the recombination process of electrons and holes are facilitated, followed with the improved fluorescence properties.

37,38

To verify the confinement effect for CuNCs/LDHs film, we studied

the band edge structures of MgAl-LDHs and CuNCs by density functional theory calculations (DFT). The conduction band minimum energy (ECBM) of periodic LDHs is calculated to be −0.67 eV and valence band maximum energy (EVBM) is −5.25 eV (Figure 4A, the Mg-2p, O-2p, H-1s orbitals played a major contribution in LDHs). The HOMO and LUMO energy of the non-periodic pristine CuNCs is calculated to be −3.78 eV and −1.03 eV, respectively (Figure 4B). Compared with CuNCs, LDHs possess higher LUMO energy but lower HOMO energy, and a mechanism for the enhanced fluorescence properties of the (CuNCs/LDHs)n film is proposed in Figure 4B. The excited electrons and hole in CuNCs can’t transfer to the conduction band and to the valence band maximum of MgAl-LDHs, which indicates that the photo-induced electrons and holes of CuNCs are trapped in the LDHs confined microenvironment, thus the recombination process of electrons and holes are facilitated and improved fluorescence properties (prolonged lifetime and enhanced quantum yield) are obtained. Moreover, the molecular dynamics (MD) simulation was further employed to study the changes of CuNCs geometric structure after immobilized on LDHs nanosheets. As shown in Figure S8A, the CuNCs consist of 11


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metal-core and outstretched ligands (GSH), forming a spherical structure. When the CuNCs are located on the LDHs layers, the ligands of CuNCs are tightly localized and gathered toward adjacent LDHs nanosheets caused by the electrostatic interactions and hydrogen bonds between the CuNCs and LDHs nanosheets (Figure S8B). Therefore, we conclude that the prolonged lifetime and enhanced quantum yield of (CuNCs/LDHs)30 film are attributed to the confinement effect of LDHs layers and the attendant host-guest interaction.

Figure 4. (A) Density of states for LDHs (inset: the optimized structure of MgAl-NO3-LDHs) and (B) the band edge placement of LDHs and CuNCs. 3.3. The Luminescence Responsive Toward Temperature of (CuNCs/LDHs)30 Film To

explore

its

temperature-dependent

potential

as

a

photoluminescence

luminescent (PL)

and

thermometer,

the

electrogenerated

chemiluminescence (ECL) properties of the (CuNCs/LDHs)n films were measured. As shown in Figure 5A, the fluorescence emission intensity decreases gradually with the temperature elevating from 10 °C to 60 °C with no obvious shift in emission peak. Visual changes of emission intensity are observed by the microcosmic fluorescence pictures under UV light (Figure 5B). As shown in Figure 5C, the fluorescent intensity of (CuNCs/LDHs)30 film shows a decrease trend with a good double-exponential form. Moreover, the measurement of temperature-response was repeated for 10 cycles, and the fluorescent intensity is almost recovered when the temperature changes from 60 °C to 10 °C, which indicates a good cycling stability (Figure 5D). However, the pristine CuNCs aqueous solution is unsuitable for temperature response since it is unstable even at room temperature. Based on the experimental results and previous

reports,15,39

the

probable

temperature-response 12


mechanism

for


(CuNCs/LDHs)30 film is as following: thermal collision frequency and non-radiative transition would increase with the temperature rising, and then radiative transition and fluorescence emission intensity decrease correspondingly.

Figure 5. Temperature-responses photoluminescence for (CuNCs/LDHs)30 film in the range from 10 °C to 60 °C: (A) fluorescent emission spectra; (B) microcosmic pictures taken under UV irradiation and (C) the decrease of fluorescence intensity at 586 nm and (D) reversibility test.

ECL is another crucial method to produce fluorescence from high-energy electron-transfer electrochemical reactions in solutions.8,36,40 The ECL performances of the (CuNCs/LDHs)30 film were tested in a buffer solution (PBS, 0.1 M), which contain KCl as electrolyte and N2H4·H2O as co-reactants (both 0.1 M concentration, respectively). As shown in Figure 6A and 6B, bare ITO showed no distinct CV signal and ECL emission (black line) while (CuNCs/LDHs)30 film shows obvious CV and ECL signals (red line) after CuNCs and LDHs assembled on ITO. The ECL intensity of (CuNCs/LDHs)30/ITO was quite stable during five cycles (Figure 6C), indicating an excellent stability. For control experiments, the ECL performances of pristine CuNCs and drop-casted (CuNCs/LDHs)30/ITO film were explored. CuNCs aqueous 13


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solution displays no obvious CV and ECL signal (Figure S9). The ECL signal of drop-casted CuNCs/ITO film can’t keep stable and decreases continuously (Figure S10). The stability of (CuNCs/LDHs)30/ITO film in ECL further verifies the importance of the confinement effect of the LDHs matrix. According to previous reports, the possible ECL mechanism could be conjectured as follows:36

Figure 6.(A) CV curves; (B) ECL-potential curves; (C) ECL signals of the ITO glass and the (CuNCs/LDHs)30/ITO film and (D) The ECL response to temperature for the (CuNCs/LDHs)30/ITO film.

CuNCs firstly coordinated with N2H4·H2O through the nitrogen atom. When N2H4·H2O was oxidized, CuNCs would get the energy to generate excited state CuNCs* (eq. 1). As a result, the excited state CuNCs* produced the strong ECL emission when CuNCs* further returned to CuNCs (eq. 2). H2N-NH2 (CuNCs/LDHs) → (CuNCs*/LDHs)

(1)

(CuNCs*/LDHs) → (CuNCs/LDHs) + hv

(2)

Moreover, temperature-dependent ECL performance of (CuNCs/LDHs)30/ITO film was measured at different temperatures (Figure 6D). The ECL intensity of 14



(CuNCs/LDHs)30/ITO film is stable at each temperature (10 °C, 30 °C and 50 °C) and decreases gradually along with increase of temperature. To the best of our knowledge, there are few reports about temperature responsive ECL properties about the CuNCs systems. Thus, the assembled CuNCs/LDHs film material reported here may possess the potential application as an original temperature responsive ECL device. 4. CONCLUSIONS In summary, we fabricated CuNCs/LDHs hybrid ultrathin films by assembling CuNCs with the exfoliated LDHs nanosheets via the LBL technique, which possessed of high QY (16.66%), long fluorescence lifetime (9.05 μs) and superior emission stability. CuNCs were stabled on LDHs layers tightly by electrostatic and hydrogen bonding interactions. The confinement effect and the host-guest interactions between CuNCs and LDHs nanosheets restrains the transfer of excited electrons, promotes the electrons/holes recombination and then results in dramatically improved fluorescence properties of CuNCs/LDHs films. In addition, the obtained CuNCs/LDHs films possess PL and ECL thermo-responsive properties. Taking advantage of the confinement-induced-PL enhancement, we have successfully fabricated the ultrathin films of CuNCs/LDHs, which possess the potential applications in optical devices and biosensing fields. ASSOCIATED CONTENT Supporting Information The Supporting Information available: Detailed description of the theoretical calculations including the model construction, computational method; Characteristic of CuNCs aqueous and CuNCs/LDHs films; The optimized geometry of pristine CuNCs and CuNCs anchored on the LDHs layers; CV curves and ECL signals of CuNCs aqueous solution; ECL signal of drop-casted CuNCs/ITO film.

AUTHOR INFORMATION Corresponding Author *Tel.: +86-10-64412131; Fax: +86-10-64425385. E-mail: [email protected] (Lan Jin), [email protected] (R. Liang). 15


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ORCID: Lan Jin: 0000-0003-1673-2890 Author contributions †L.

Y. F. and H. M. L. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (NSFC), the National Key Research and Development Program (Grant No. 2017YFA0206804), the 973 Program (2014CB932103), the Fundamental Research Funds for the Central Universities (XK1802-6) and the Beijing Natural Science Foundation (2174082).

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LDHs films with excellent luminescent

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