发光的异核稀土配合物装饰氧化石墨烯片

李运涛 邱 硕 杜 滕 魏 巍 韩振斌 阮方毅 曹丽慧

（陕西科技大学，陕西省轻化工助剂重点实验室，西安 710021）

0 Introduction

Graphene,a single layer of carbon atoms arranged in honeycomb structures,has attracted extensive attentions since the advent of free-standing graphene sheets in 2004[1].The two-dimensional carbon nonmaterial,such as graphene have been widely explored recently to demonstrate its potential applications in biomedical study and by fabricating graphene based nano-scaled electronic,optoelectronic,photovoltaic and sensing devices[2-4].The optical response of graphene can be described by the surface conductivity which is originated from the contributions of inter-band and intra-band transitions,related to the chemical potential or Fermi level[5-6].Recently a weak and wide photoluminescencepeak of grapheneoxide hasbeen reported[7].In spite of massive investigation has been carried out to address the origin of the fluorescence,there is not a single and strong theory.In addition,it has been established that the sp2units have a vital role in the emission of the light but the π-π*transitions in the sp2unitsof aromatic sectionsarenot the only reason[8-9].

Rare earth （RE）organic complexes not only show high luminous intensity but also high fluorescence quantum yield,they have been widely used in medical testing,drug analysis,environmental monitoring,security fields and other characteristics[10-13].It is well known that fluorescence enhancement could be achieved through ligand sensitization,but using certain second lanthanide ions such as Gd3+and La3+could also increase the fluorescence properties[14-15].Due to the unique properties of GOSs,RE complexes are deposited on the surface of GOSs may obtain high performance luminescent hybrid materials.However,when used in practice,fluorescence performance of GOSs hybrid materials is not as good as expected,the luminescences need to be enhanced,but conventional luminescent tags such as dye molecules have to face a challenge that GOSs have a fluorescence quenching effect[16].Therefore,how to improve the fluorescence properties of hybrid materials becomes a meaningful thing.Recently there are some works of GOSs/RE has been prepared successfully.Cao and co-worker[17]reported GOSs functionalized with RE complexes using a non-covalent approach and GOSs hybrid materials can be easily exfoliated into single-layer sheets.Zhang and co-worker[18]synthesized a novel complex material Eu-modifed reduced GO（graphene oxide）by the simple stirring process through has been proposed.Wang and co-worker[19]reported a simple and effective method synthesized GOSs/Eu-acac-Phen（acac=acetylacetone）via non-covalently functionalized GOSs,the GOSs enhance the fluorescence lifetime of the complexes.Fan and co-worker[20]successfully synthesized luminescent Sm3+complexes [Sm （TTA）3Phen] （TTA=2-thenoyltrifluoroacetone）via covalent functionalization to surface-carboxylated graphene oxide （GO-COOH）.Zhao and co-worker[21]successfully prepared grapheneoxide/Eu（TTA）3Phen complex hybrid materials via an innovation method,which had better stability.Zhang and co-worker[22]reported a novel graphene oxide-rare earth complex hybrid materials[Eu-TTA-PMA/GOSs]（PMA=pyromelliticacids）through non-covalent approach,the material also exhibits stronger luminescence intensity,long decay lifetime and better thermal stability.In this paper,theπelectron of the graphene oxide and the conjugated structure of the ligand are combined by the π-π stacking method of non-covalent bond.The carbon atoms in graphene oxide form a highly delocalizedπ electron by sp2hybridization.Theseπelectrons can be combined with other materials having a largeπ conjugate structure through π-π interaction to achieve good dispersion of graphene oxide,that the noncovalent bond function method of graphene oxide is most commonly used in chemical applications.

In this work,we present a heteronuclear Sm3+and Gd3+RE complex （Sm-Gd）BA3Phen via noncovalently functionalized GOSs based on BA as the main ligand,Phen asthe ancillary ligand.Gd3+ions obviously improved the fluorescence of hybrid material and played a significant role.Moreover,we synthesized the hybrid materials with bright red luminescence,and the characteristic fluorescence of Sm3+is4G5/2-6H5/2,4G5/2-6H7/2,and4G5/2-6H9/2transitioned at 563,596 and 604,and 643 nm excited by UV light.The hybrid materials obtained the best luminescence property when5∶5.

1 Experimental

1.1 Materials and instrumentation

Natural flake graphite was purchased from Sinopharm Chemical Reagent Co.,Ltd.Samarium oxide （Sm2O3,99.99%）were purchased from Sinopharm Chemical Reagent Co.,Ltd.Benzoic acid were purchased from Tianjin Bai Shi Chemical Co.,Ltd.1,10-phenanthroline （Phen）were purchased from Tianjin Branch Miou Chemical Reagent Co.,Ltd.All the reagents for the analysis are of pure and used without further purification.

Fourier Transform infrared spectroscopy （FT-IR）the United States Perkin Elmer 2000 Fourier transform infrared spectrometer （KBr tablet）.Fluore-scence spectrometry （FS）was obtained by with Germany HORTBA Scientific Fluormax-4P Spectrofluorometer.Thethermo-gravimetric analyzer（TAG）was performed Germany Netzsch PLT-400 type integrated thermal analyzer.X-ray diffraction （XRD）was obtained by the Shimadzu XRD-7000S/Ltypeautomatic X-ray diffractometer with a voltage of 40 kV and a current of 30 mA at a scanning rate of 8.0°·min-1.The diffraction angle was conducted in a range of 4°～50°with Cu Kα1radiation （λ=0.154 06 nm）.Scanning electron microscopy （SEM）was obtained by Japanese Hitachi Model S-4800.The secondary electron resolution was 1.0 nm（15 kV）,themagnification was 8 000 and the acceleration voltage was 3.0 kV.

1.2 Preparation of pure(Sm-Gd)BA 3Phen complex

Graphene Oxide was prepared by modified Hummers methods[23-24].Firstly,1 mmol of samarium oxide （Sm2O3）was completely dissolved in 15 mL of concentrated hydrochloric acid,and slowly steamed in the water bath to remove excess acid.The 20 mL of 99%（V/V）absolute ethanol was added to obtain 0.05 mol·L-1SmCl3ethanol solution.Then 1 mmol of gadolinium chloride （GdCl3）was dissolved in 20 mL ethanol solution to prepare 0.05 mol·L-1GdCl3ethanol solution.A certain amount of BA and Phen was weighted according to nSmCl3∶nBA∶nPhen=1∶3∶2,mixed in a three-necked flask and kept stirring.The SmCl3and GdCl3ethanol solution was added dropwise to the system andwas 5∶5.Then,the pH value was adjusted to 6 ～7 with 0.1 mol·L-1NaOH aqueous solution.The mixture was heated and stirred for 5 h at 70℃.The white precipitate was obtained and placed for 24 h.Finally the mixture was filtered and washed with ethanol several times.The precipitate was dried at 60℃.

1.3 Synthesis of GOSs/(Sm-Gd)BA3Phen complex hybrids

Firstly,10 mg as-prepared GOSs were dispersed in 20 mL ethanol and vibrierened for 2 h.Secondly the obtained 0.05 mol·L-1SmCl3ethanol solution and 0.05 mol·L-1GdCl3ethanol solution were added to the solution of GOSs in ethanol to vibrieren for 0.5 h.After these treatments,0.73 g （6 mmol）of BA and 0.72 g （4 mmol）of Phen were dissolved in 20 mL ethanol and dropped into the above suspension under the strong agitation.The pH value of solution was adjusted to 6～7 by 0.1 mol·L-1NaOH aqueous solution.The mixture was heated and stirred for 5 h at 70℃.Finally,after being placed for 24 h,the product were washed with ethanol several times and dried at 60℃.Schematic illustration of the preparation of GOSs/（Sm-Gd）BA3Phen complex hybrid is shown in Fig.1.

Fig.1 Schematic illustration of preparation of GOSs/（Sm-Gd）BA3Phen complex hybrid （RE=Sm,Gd）

2 Results and discussion

2.1 FT-IR characterization

In order to provide a more detailed composition of the chemical bonds and the functional groups,GOSs,（Sm-Gd）BA3Phen, GOSs/（Sm-Gd）BA3Phen complex hybrid were characterized by FT-IR as shown in Fig.2 and the spectral region was 4 000～400 cm-1.The characteristic absorption peak of GOSs at 1 728 cm-1was assigned to C=O stretching vibration,the peak of C=C was at 1 620 cm-1,the peak of-OH deformation vibration was at 1 403 cm-1and the peak of symmetric stretching vibration of epoxy bond was at 1 050 cm-1indicating successful synthesis of GOSs[25].The characteristic peak of BA are assigned to C=O （1 660 cm-1）and C-O （1 281 cm-1）,and disappeared after （Sm-Gd）BA3Phen complex was formed.The peak of-COO stretching vibration wasat 1 410 cm-1,which illustrated that the acid radical was involved in the coordination.The characteristic absorption peaks of Phen at 1 639 and 1 578 cm-1correspond to C=Cand C=N stretching vibrations[26],respectively,which were shifted to 1 613 and 1 573 cm-1,indicating that N atoms had coordinated with RE ions.The peak of C-H bending vibration at 744 and 850 cm-1were shifted to 731 and 846 cm-1,which also indicated that N atoms had coordinated with RE ions.In addition,around 426 and 660 cm-1correspond to Sm-O stret-ching vibration[27].Fig.2 shows the peak at 1 519 cm-1of GOSs/（Sm-Gd）BA3Phen complex hybrid compared with （Sm-Gd）BA3Phen complex was broaden.It may provide strong evidence for deprotonated-COOH coordinated to Sm3+as-COO-.This shows that RE ion is easy to carboxyl and carbonyl coordination.

Fig.2 FT-IR spectra of GOSs,（Sm-Gd）BA3Phen and GOSs/（Sm-Gd）BA3Phen

2.2 XRD

X-ray diffraction was used to study the crystal structure of GOSs, （Sm-Gd）BA3Phen complex and GOSs/（Sm-Gd）BA3Phen complex hybrid （Fig.3）.There is a sharp peak at 10.32°in Fig.3（a）,which is the characteristic peak of GOSs[28].The interlayer spacing of GOSs was 0.759 nm which is much larger than that of pristine graphite （about 0.337 nm）due to the presence of the oxygen-containing functional groups on the surfaces of the graphite sheet[29].The XRD of（Sm-Gd）BA3Phen complex in Fig.3（b） shows two diffraction peaks at 2θ=23.77°and 27.8°,which belong to the reflection of Phen and SmCl3[30],respe-ctively.This means that Phen and SmCl3did not react completely during the complex formation.As shown in Fig.3（c）the reflection of GOSs disappeared after GOSs combined with Sm complex.This suggests that the interlayer structure of GOSs have been destroyed by（Sm-Gd）BA3Phen complex[31-32].The diffraction peaks of GOSs/（Sm-Gd）BA3Phen complex hybrid at 2θ=12.57°and 8.6°appeared.Hence,the results illustrate the crystal structureof（Sm-Gd）BA3Phen complexand GOSs disappeared and GOSs/（Sm-Gd）BA3Phen complex hybrid formed a new crystal structure,which is different from both GOSs and （Sm-Gd）BA3Phen complex[21].

Fig.3 XRD patterns of GOSs （a）,（Sm-Gd）BA3Phen （b）and GOSs/（Sm-Gd）BA3Phen （c）

2.3 SEM characterization

In order to study the specific morphology of the materials,the SEM images of GOSs,（Sm-Gd）BA3Phen complex and GOSs/（Sm-Gd）BA3Phen complex hybrid were obtained.There were many overlapping sheet structures textures in the layer structure in Fig.4 （a）,which were graphene oxide sheet structure,because they are self-fold and rolling up making GOSs keep a steady state[33].As can be seen in Fig.4（b）, （Sm-Gd）BA3Phen complex was irregular sheet structure and closely packed together like a flower-like structure.Many seaweed shape structures were adsorbed on the surface of the GOSs in Fig.4（c）.After the combination with the graphene oxide,some changes were observed in the morphology of RE complexes.（Sm-Gd）BA3Phen complex dispersed in the lamellar structure of GOSs,which showed that the （Sm-Gd）BA3Phen complex coated in the graphene oxide layer structure.The boundaries,curls and wrinkles of GOSs were clearly observed.However,thedistribution of（Sm-Gd）BA3Phen was not uniform because oxygen-containing groups of GOSs were scattered and irregular[22].In addition,the structure of the seaweed shape could illustrate the reduction of π-π stacking and oxygen-containing groups of GOSs.

Fig.4 SEM images of GOSs （a）,（Sm-Gd）BA3Phen （b）,GOSs/（Sm-Gd）BA3Phen （c）

Fig.5 TGA curves （a）and DTGcurves （b）of GOSs,（Sm-Gd）BA3Phen and GOSs/（Sm-Gd）BA3Phen

2.4 TGA

Fig.5 Shows the thermogravimetric analysis measurements of GOSs,（Sm-Gd）BA3Phen complex and GOSs/（Sm-Gd）BA3Phen complex hybrid.TGA was carried out under nitrogen atmosphere with a heating rate of 10℃·min-1,and the temperature interval was 20～800 ℃ in the TGA curves.The first stage of GOSs had a weight loss process under 137℃,which is due to evaporation of water,corresponding to about 17%in Fig.5（a）.The second stage of weight loss was in the temperature interval of 137～289 ℃,corresponding to a weight loss of about 37%,which could be attributed to decomposition of oxygen functional groups[34].The curve of （Sm-Gd）BA3Phen complex shows decomposition stages in 300～600 ℃,which could be attributed to the decomposition of （Sm-Gd）BA3Phen complex.And the process of weight loss under 200℃was due to the evaporation of adsorbed water molecules from the surface of the complex.The weight loss below 268℃ of GOSs/（Sm-Gd）BA3Phen complex hybrid was due to the loss of water and decomposition of oxygen functional groups,correspon-ding to a weight loss of about 2.2%.The second stage of weight loss was between 260 and 487℃,corresponding to a weight loss of about 22.2%,which could be attributed to the decomposition of （Sm-Gd）BA3Phen complex.The third stage of weight loss occured from 487 to 600℃,corresponding to a weight loss of about 31.4%,which is due to the loss of GOSs[35].The DTG curves（derivative thermogravimetry） of （Sm-Gd）BA3Phen complex in Fig.5（b）had main mass loss peaks.The peak appeared at 372℃,which was different from the GOSs/（Sm-Gd）BA3Phen at 516 ℃.The results illustrate that the protective effect of oxygen groups in GOSs could improve the decomposition temperature of GOSs/（Sm-Gd）BA3Phen.We could conclude that the GOSs/（Sm-Gd）BA3Phen complex hybrid has good thermal stability.

Fig.6 Digital images:（Sm-Gd）BA3Phen under daylight（a）and under 365 nm UV light（c）;GOSs/（Sm-Gd）BA3Phen under daylight（b）and under 365 nm UV light（d）

Fig.7 Excitation （a）and emission （b）spectra of（Sm-Gd）BA3Phen,GOSs/（Sm-Gd）BA3Phen

2.5 Fluorescence characterization

Fig.6 shows the digital images of（Sm-Gd）BA3Phen complex and GOSs/（Sm-Gd）BA3Phen complex hybrid under daylight and 365 nm UV light.As we can see from Fig.6（b）,under daylight,the color of hybrid materials was gray after reacting with GOSs.In addition,the （Sm-Gd）BA3Phen complex and GOSs/（Sm-Gd）BA3Phen complex hybrid could present bright red emissions under UV light irradiation under 365 nm UV light.The fluorescence of complex hybrid was stronger than （Sm-Gd）BA3Phen complex.The results illustrate that GOSs and RE complex are bound together by non-covalent bonds and improve the fluorescence properties of the complex[22].

Luminescence emission and excitation spectra of（Sm-Gd）BA3Phen complex and GOSs/（Sm-Gd）BA3Phen complex hybrid have been showed in Fig.7.In the excitation spectra of （Sm-Gd）BA3Phen complex and GOSs/（Sm-Gd）BA3Phen complex hybrid,the bands from 310 to 400 nm was related to theπ→π transition based on the conjugated double bands of two ligands.The maximum absorption peak was 352 nm.The two spectra are similar,and the fluorescence intensity of GOSs/（Sm-Gd）BA3Phen complex hybrid was stronger than that of （Sm-Gd）BA3Phen complex.The result illustrates the presence of GOSs obviously improved the excitation intensity of the complex hybrid.

The emission spectrum of （Sm-Gd）BA3Phen complex was obtained under excitation at 352 nm.In the emission spectrum,there were three characteristic fluorescence emission peaks,related to 4f orbital electronic transitions of4G5/2→6H5/2（563 nm）,4G5/2→6H7/2（596,604 nm）,4G5/2→6H9/2（643 nm）,which are thecharacteristic peaks of Sm3+[36].It iswell known that Gd3+ions did not have characteristic emission peaks owing to its stable structure （4f7）[14].Moreover,hybrid materials had bright red luminescence.The strongest fluorescence emission peak corresponds to the4G5/2→6H7/2electron transition at 596 nm.According to the description in the introduction,GOSs had a strong quenching effect.However,the luminescence intensity of GOSs/（Sm-Gd）BA3Phen complex hybrid was higher than （Sm-Gd）BA3Phen complex.It could intimate that GOSs do not quench the luminescence of （Sm-Gd）BA3Phen complex that attached on their surface.It may be due to the unique properties of RE ions that trivalent lanthanide ions have photoluminescence properties.Since the 4f valence shells of the trivalent lanthanides were well shielded by the peripheral 5s and 5p ones,they had preventive effect on external potential quenchers[37-38].

Fig.8 Emission spectra （a）and curves （b）of GOSs/（Sm-Gd）BA3Phen complexes with different Gd3+molar fractions

The emission spectra of GOSs/（Sm-Gd）BA3Phen complex hybrid with different Gd3+or Sm3+molar ratios under excitation 352 nm has been showed in Fig.8.Table 1 shows the result of the excitation and emission intensity,where Ixis the measured fluorescence excitation intensity and Imis the measured fluorescence emission intensity of complex hybrid.In order to further study the relationship between fluorescent intensity and the concentration of Gd3+,more analyses were carried out.As we can see in Fig.8,all the emission spectra were similar and had the characteristic peaks of Sm3+.Fig.8（b）shows the different Gd3+molar fractions under the wavelength of 596 nm.It could also be seen from the Table 1 that when the amount of Gd3+or Sm3+reached the maximum,the fluorescence intensity of the hybrid was the weakest.The most suitable conditions was obtained through different molar ratios.The intensity reached maximum whenwas 5∶5.It may be due to the energy transfer from Gd3+to Sm3+enhancing the fluorescence performance of the complex hybrids,but excess Gd3+or Sm3+will quench the fluorescence performance.

In order to study the influence of the addition of inert RE element Gd3+on RE complexes,the emission spectrum of（Sm）BA3Phen and （Sm-Gd）BA3Phen complexes withbeing 5∶5 under 352 nm excitation have been obtained （Fig.9）.The two emission spectra were similar and had the maximum emission peak at 596 nm.Comparing the emission intensity of the two compounds,the fluorescence intensity of REcomplexes added with Gd3+was significantly higher.The result illustrates that Gd3+ions play a significant role in improving the luminescence intensity.

2.6 Decay lifetime

In order to further understand the properties of material,the study of decay lifetime is also very meaningful.The luminescent lifetime decay curves of（Sm-Gd）BA3Phen and GOSs/（Sm-Gd）BA3Phen are shown in Fig.10.In Fig.10,there are two luminescent lifetime decay,the lifetime of （Sm-Gd）BA3Phen were 0.80 and 63.52 μs,and the lifetime of GOSs/（Sm-Gd）BA3Phen were 2.19 and 77.64μs.The luminescent lifetime of GOSs/（Sm-Gd）BA3Phen was longer than（Sm-Gd）BA3Phen,which could demonstrate that the complex was deposited on the surface of GOSs.Due to the honeycomb crystal structure of GOSs which limited the rotation and vibration of the RE complex to some extent,the non-radiative transition is reduced to prolong the fluorescence lifetime of the material[19].

Fig.10 Luminescence decay curves of（Sm-Gd）BA3Phen and GOSs/（Sm-Gd）BA3Phen

3 Conclusions

In conclusion,we successfully synthesized a novel heteronuclear RE complex GOSs/（Sm-Gd）BA3Phen hybrid through non-covalent approach.All products show the characteristic emission peaks of Sm3+.The hybrid materials exhibited strong luminescence intensity,long lifetime and good thermal stability.Whenwas 5∶5,the fluorescence intensity was strongest,due to the intramolecular energy transfer.The result proved the existence of GOSs does not quench the luminescence of the Sm3+complex.The great property of the hybrid could be used in many fields and provides an important basis for potential applications such as drug carriers and biomarkers.

Acknowledgments:Thanks to the National Natural Science Foundation Youth Project（Grant No.21701106）and the experimental conditions provided by Shaanxi University of Science and Technology.