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Sensors and Actuators B 156 (2011) 546– 552

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

j o ur nal homep a ge: www.elsev ier .com/ locate /snb

highly sensitive and selective colorimetric and off–on fluorescent chemosensoror Cu2+ based on rhodamine B derivative

hihong Xua,∗, Like Zhanga, Rui Guoa,c, Tiancheng Xianga, Changzeng Wua,hi Zhengb, Fengling Yangb,c,∗∗

College of Chemistry and Chemical Engineering, Xuchang University, Bayi Road 88, Xuchang, 461000, PR ChinaInstitute of Surface Micro and Nano Materials, Xuchang University, Xuchang, 461000, PR ChinaDepartment of Chemistry, Zhengzhou University, Zhengzhou, 450052, PR China

r t i c l e i n f o

rticle history:eceived 27 October 2010eceived in revised form 18 January 2011

a b s t r a c t

A new rhodamine B derivative colorimetric and fluorescent sensor (1) was synthesized by condensationreaction of rhodamine B hydrazide and 2,4-dihydroxybenzaldehyde, which showed reversible and highlyselective and sensitive recognition toward Cu2+ over other examined metal ions. Upon addition of Cu2+,

ccepted 29 January 2011vailable online 5 February 2011

eywords:hodamine B schiff baseluorescent and colorimetric chemosensor

sensor 1 exhibits remarkably enhanced absorbance intensity and color change from colorless to pinkin DMSO and MeCN aqueous buffer solution or pure MeCN, and shows significant off–on fluorescenceaccompanied by color changes from colorless to orange in MeCN. The sensor 1 was also successfullyapplied to the determination of Cu2+ in water samples.

© 2011 Elsevier B.V. All rights reserved.

u2+

pirolactam

. Introduction

The design and development of colorimetric or fluorescenthemosensors for detection of environmentally and biologicallymportant metal cations are currently of great importance, sincehey allow nondestructive and prompt detection of metal cationsy a simple absorbance and fluorescence enhancement (turn-on)r quenching (turn-off) response [1].

Among the metal ions, copper, after iron and zinc, is the thirdost abundant essential trace element in the human body, and

erforms an important role in many fundamental physiologicalrocesses in organisms [2]. Alteration in the cellular homeostasisf copper ions is accompanied with some serious diseases, such asenkes and Wilson diseases [3], Alzheimer’s disease [4], familial

myotropic lateral sclerosis [5], and prion diseases [6]. In partic-

lar, exposure to a high level of copper even for a short period ofime can cause gastrointestinal disturbance, and long-term expo-ure causes liver or kidney damage [7], as a result of its ability toisplace other metal ions that act as cofactors in enzyme-catalyzed

∗ Corresponding author. Tel.: +86 374 5979659;ax: +86 374 2968881.∗∗ Corresponding author at: Institute of Surface Micro and Nano Materials,uchang University, Xuchang, 461000, PR China. Tel.: +86 374 5979659; fax: +8674 2968881.

E-mail addresses: Xuzhihong1980@yahoo.com (Z. Xu),angfengling2005@yahoo.com.cn (F. Yang).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.01.066

reactions [8]. Thus, detection of the concentration and subcellulardistribution of copper in physiological processes may greatly con-tribute to understanding its complex physiological functions andnosogenesis.

The rhodamine framework is an ideal mode to construct flu-orescent chemosensors, owing to their excellent photophysicalproperties, such as long absorption and emission wavelengths,large absorption coefficient, and high fluorescence quantum yield[9]. Recently, a few fluorescent chemosensors or chemodosimetersbased on rhodamine B, which are driven by visible light excitationand show turn-on response to the targeted HTM cation [10], such asAg+ [11], Au+ [12], Cu2+ [13], Cr3+ [14], Cr6+ [15], Fe3+ [16], Hg2+ [17]and Pb2+ [18], have been proposed. The cation-sensing mechanismof these probes is based on the change in structure between spiro-cyclic and open-cycle forms [19]. Without cations, these probesexist in a spirocyclic form, which is colorless and nonfluorescent.Addition of metal cation leads to a spirocycle opening resulting inan appearance of pink color and orange fluorescence.

Herein, we synthesized a new rhodamine B derivative 1 whichwas utilized as selective colorimetric and fluorescent sensor forCu2+. Among the various metal ions, sensor 1 exhibits remarkablyenhanced absorbance intensity and color change from colorless to

pink in DMSO and MeCN aqueous buffer solution or pure MeCN,and shows significant off–on fluorescence accompanied with colorchanges from colorless to orange in MeCN. The sensor 1 wasalso successfully applied to the determination of Cu2+ in watersamples.

Z. Xu et al. / Sensors and Actuators B 156 (2011) 546– 552 547

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tallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif.The crystal structure of 1 clearly reveals the unique spirolactamring formation, with the lactam and xanthene moiety form verticalplanes (Fig. 2), which breaks the conjugation of the whole system,

Fig. 1. Synthes

. Experimental

.1. Apparatus

Absorption spectra were measured on a Shimadzu UV-2550pectrophometer (Tokyo, Japan). Fluorescence spectra measure-ent was performed on a Sanco CRT-970 spectrofluorimeter

Shanghai, China). The pH was measured with a Model pHs-3Ceter (Shanghai, China). The melting points were determined

n an X-4 microscopic melting point apparatus with a digitalhermometer (Shanghai, China). Carbon, hydrogen, and nitro-en were analyzed on an Elemental Vario EL Analyzer. 1H NMRpectra were measured on a Varian VR 300-MHz spectrome-er. Electrospray ionization (ESI) mass spectra was conductedy ABI-057-TY4675 instrument. The crystal data were collectedn a BRUKER SMART 1000 CCD diffractometer equipped with araphite crystal monochromatized Mo Ka radiation (� = 0.71073 A)t 298(2) K. The structure was solved by direct methods withHELXS-97 program, and refined anisotropically by the full-matrixeast-squares methods for all non-H atoms. All H atoms were addedccording to theoretical calculations and refined isotropically.

.2. Reagents

All the reagents were purchased from commercial suppliersSinopharm Chimical Reagent Co. Ltd., China; Alfa Aesar Chemi-al Co. Ltd.). The solutions of cations and anions were preparedrom their chloride and tetrabutylammonium salts. All chemicalssed in this work were of analytical grade and used without fur-her purification. Double distilled water was used throughout thexperiment.

Stock solution (1.0 × 10−3 mol/L) of 1 was prepared by dissolv-ng the requisite amount of it in different solvent. Stock solutionsf various ions were prepared by dissolving their chlorides andetrabutylammonium salts in double distilled water.

.3. Calculation method

All the calculations were carried out by using the Gaussian 03rogram package [20]. The geometries of the products were opti-ized using DFT calculations at the B3LYP/6-31G (d,p) level.

.4. Synthesis of 1

As shown in Fig. 1, the compound 1 was prepared by reactinghodamine B hydrazide with 2,4-dihydroxybenzaldehyde. Rho-amine B hydrazide was synthesized according to literature [13j].hodamine B hydrazide (0.46 g, 1 mmol) was dissolved in 20 mlthanol, then excess dihydroxybenzaldehyde (0.16 g, 1.2 mmol)nd 3 drops HAc was added, then the mixture was refluxed for

h under N2 atmosphere. The resulting solution was cooled and

oncentrated to 10 ml and allowed to stand at ambient tempera-ure. Pink crystals were obtained after several days. 1 was filterednd washed with ethanol and then dried under reduced pressure.ield: 0.48 g (83%). m.p. 296 ◦C. ESI mass spectrometry: m/z 577.6M+H]+, elemental analysis, calcd. for 1 (C35H36N4O4), C 72.90, H

te of sensor 1.

6.29, N 9.72%; found, C 72.98, H 6.32, N 9.68%. 1H NMR (300 MHz,DMSO-d6) ı (ppm): 1.07–1.11 (t, 12H, NCH2CH3), 3.30–3.47 (q, 8H,NCH2CH3), 4.36 (s, 1H, Ben-H), 6.17 (d, 1H, Ben-H), 6.25–6.45 (m,6H, Xanthene–H), 7.07–7.12 (d, 2H, Ar–H), 7.59 (d, 2H, Ar–H), 7.88-7.91 (d, 1H, Ben–H), 9.01 (s, 1H, N C–H), 9.99 (s, 1H, OH), 10.63 (s,1H, OH).

2.5. General UV–vis and fluorescence spectra measurements

Since the chemosensor was not fully soluble in 100% aqueousmedia, DMSO and MeCN were used as a cosolvent. Stock solutionsof 1 were prepared in DMSO or MeCN. The UV–vis spectra wereobtained in mixed DMSO/Tris–HCl and MeCN/Tris–HCl aqueousbuffer solutions. The fluorescence spectra were measured in MeCN.Fluorescence measurements were carried out with excitation andemission slit width of 10 and 5 nm (�ex = 561 nm).

3. Results and discussion

3.1. Synthesis and characterization of 1

Probe 1 was easily synthesized by condensation of rhodamine Bhydrazide and 2,4-dihydroxyaldehyde. The structure of 1 was con-firmed by 1H NMR spectroscopic, MS data, elemental analysis andsingle-crystal X-ray diffraction methods. The crystal suitable forX-ray analysis was obtained by slow evaporation of the methanolsolution. The molecule crystallizes in monoclinic system withthe space group P21/c. The cell parameters are a = 9.4461 (4) A,b = 26.6905 (12) A, c = 12.2543 (5) A and Z = 4 [21]. CCDC 781454contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crys-

Fig. 2. Crystal structure of 1, all hydrogen atoms were omitted for clarity.

548 Z. Xu et al. / Sensors and Actuators B 156 (2011) 546– 552

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due to its five-membered spirolactam structure. When 40 equiv.metal ions of Ag+, Ca2+, Cd2+, Co2+, Cr3+, Fe3+, Hg2+, K+, Mn2+,Na+, Ni2+, Zn2+ were added and measured immediately, no obvi-

ig. 3. Changes in absorption spectra of 1 (10 �M) in DMSO/Tris–HCl buffer (1:9,/v, pH 7.0) solutions with various amounts of Cu2+ ions (0–30 equiv.). Inset: colorhange of 1 in the visible region.(For interpretation of the references to color in thisgure legend, the reader is referred to the web version of the article.)

hus leading to the non-fluorescence of the molecule. In addition,eak intermolecular O2–H2. . .O1 stabilize the crystal structure to

orm one-dimensional infinite chains (Fig. S1).

.2. UV–vis spectral responses of 1

An optimized DMSO/Tris–HCl buffer (1:9, v/v, pH 7.0) solutionas selected for the spectroscopic investigation. The UV/visible

pectrum of 1 (10 �M) shows only a very weak band above 500 nm,hich was ascribed to the trace ring-opened form of molecules of

in solution. Upon addition of Cu2+ to a solution of 1, the solu-ion turned from colorless to pink (Fig. 3 inset), and the absorbanceas significantly enhanced with a new peak appearing at around

61 nm (Fig. 3), clearly suggesting the formation of the ring-openedmide form of 1 as a result of Cu2+ binding [10]. Other cationsr anions had negligible response (Figs. S2 and S3). Accordingo the linear Benesi–Hildebrand expression [22], the measuredbsorbance [1/(A − A0)] at 561 nm varied as a function of 1/[Cu2+]n a linear relationship (R = 0.99306), while the Cu2+ concentration

aried from 1 × 10−5 to 3 × 10−4 mol L−1, indicating the 1:1 stoi-hiometry between the Cu2+ ion and 1 (Fig. 4). On the basis of 1:1toichiometry and UV–vis titration data in Fig. 3, the associationonstant of 1 with Cu2+ ion in DMSO/Tris–HCl buffer (1:9, v/v, pH

ig. 4. Benesi–Hilderbrand plot of 1 with Cu2+ in DMSO/Tris–HCl buffer (1:9, v/v,H 7.0) solutions (UV/vis spectra).

Fig. 5. Normalized response of absorbance calibration value as a function of Cu2+

concentration in DMSO/Tris–HCl buffer (1:9, v/v, pH 7.0) solutions.

7.0) solution was found to be 2.83 × 104 M−1. As shown in Fig. 5,according to the result of titrating experiment, the absorbance cal-ibration values were normalized between the minimum intensityand the maximum intensity. A linear regression curve was thenfitted to these normalized data, and the point at which this linecrossed the ordinate axis was considered as the detection limit(3.42 × 10−6 M) [23]. In addition, the sensor 1 exhibits response toCu2+ in DMSO/Tris–HCl buffer (4:6, v/v, pH 7.0) and pure MeCNsolution (Figs. S4 and S5), the association constant and detectionlimit calculated by using the same method (Figs. S6–S9) in thesedifferent media were schemed in Table 1.

3.3. Fluorescence properties

The high selectivity of 1 for Cu2+ was further observed in the flu-orescent spectra. As shown in Fig. 6, 1 is essentially non-fluorescent

ous changes of fluorescence intensity besides the interfere peak at

Fig. 6. Fluorescence emission changes of 1 (10 �M) in MeCN upon addition of vari-ous ions (the concentration of Cu2+ was 200 �M, the concentration of each of all theother ions was 400 �M). The fluorescence spectra were recorded with �ex = 561 nm(the other metal ions refer to Ag+, Ca2+, Cd2+, Co2+, Cr3+, Fe3+, Hg2+, K+, Mn2+, Na+,Ni2+, Zn2+).

Z. Xu et al. / Sensors and Actuators B 156 (2011) 546– 552 549

Table 1The association constant and detection limit in different solvents by using UV-spectral titration experiment.

Solvent

DMSO/Tris–HCl buffer (1:9, v/v, pH 7.0)

Association constant 2.83 × 104 M−1

Detection limit 3.42 × 10−6 M

Fig. 7. Fluorescent intensity (� = 561 nm) changes of 1 (10 �M) in MeCN upon theau

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3.5. Tolerance of 1 to Cu2+ over other metal ions

The possible interferences by other metal ions were assessedthrough competitive experiments. The absorbance changes of 1

ex

ddition of Cu2+ ions (0–40 equiv., interval: 20 �M). Inset: fluorescent change of 1nder 365 nm lamp excitation.

xcitation wavelength 561 nm could be observed. However, underhe identical conditions, upon addition of 20 equiv. Cu2+, the fluo-escence intensity of 1 enhanced significantly.

To further investigate the interaction of Cu2+ and 1, a fluores-ence titration experiment was carried out. A linear increasingf fluorescence intensity of 1 could be observed with increasingu2+ concentration accompanied by color changes from colorlesso orange (Fig. 7). Binding analysis using the Benesi–Hildebrandxpression established that the stoichiometry of 1-Cu2+ also was:1, and the association constant(Fig. S10) and detection limitFig. S11) were summarized in Table 2.

.4. Optimization of experimental conditions

.4.1. Effect of reaction timeThe kinetic characteristics of the reaction system in

MSO/Tris–HCl buffer (1:9, v/v, pH 7.0) solution were inves-igated (Fig. S12). No obvious absorbance variation of 1 (10 �M)t 561 nm was observed even over a period of 3 h, indicating theve-membered spirolactam structure of sensor 1 is stable. Uponddition of 20 equiv. Cu2+, the interaction of 1 with Cu2+ was

ompleted in less than 30 s. Thus, this system might be used forhe real-time monitoring of Cu2+.

able 2he association constant and detection limit in MeCN by using fluorescence titrationxperiment.

SolventMeCN

Association constant 1.33 × 103 M−1

Detection limit 3.85 × 10−6 M

DMSO/Tris–HCl buffer (4:6, v/v, pH 7.0) MeCN

6.99 × 104 M−1 3.17 × 104 M−1

1.05 × 10−6 M 3.55 × 10−6 M

3.4.2. Effect of pHTo investigate the influence of the different acid concentration

on the spectra of sensor 1 and find a suitable pH span in whichsensor 1 can selectively detect Cu2+ efficiently, the acid titrationexperiments were performed. As shown in Fig. 8, the absorptiontitration curve of free sensor did not show obvious characteristiccolor of rhodamine between pH 1.0 and 12.0, suggesting that spiro-lactam tautomer of sensor 1 was insensitive to the pH changes inthis range. However, the addition of Cu2+ led to the absorbanceenhancement over a comparatively wide pH range (4.0–7.0), whichis attributed to opening of the rhodamine ring. Consequently, sen-sor 1 may be used to detect Cu2+ in approximate physiologicalconditions. Therefore, further UV/vis studies were carried out inDMSO/Tris–HCl mixed buffer solution (pH 7.0).

3.4.3. Effect of mediaReaction media were studied firstly to obtain the suitable reac-

tion system. Sensor 1 exhibits sensitive UV–vis spectral responseto Cu2+ in DMSO–water mixed solution, MeCN and MeCN–watermixed solution (Fig. S13) and insensitive response in DMF, acetoneand pure DMSO solution, and show analogous response to Cu2+ andFe3+ in methanol and ethanol solution. Therefore, DMSO–water andMeCN–water solution is favorable for the UV–vis spectral measure-ment. But only pure MeCN solution was selected for fluorescentassay cause the fluorescence can be quenched by water moleculeunder the identical condition.

The effect of the water content on the UV–vis spectral measure-ment of Cu2+ was investigated, as shown in Fig. 9, it can be observedthat the sensor 1 exhibits sensitive response to Cu2+ at 1:9–5:5DMSO aqueous and 2:8–10:0 MeCN aqueous solution. Hence, thesemedia can be selected for the UV–vis spectral method.

Fig. 8. Variation of absorbance (561 nm) of free sensor 1 (10 �M) and in the presenceof 20 equiv. Cu2+ in DMSO/Tris–HCl buffer (1:9, v/v, pH 7.0) solutions with differentpH conditions.

550 Z. Xu et al. / Sensors and Actuators B 156 (2011) 546– 552

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ig. 9. Effect of the water content on the absorbance intensity of 1 (10 �M) in theresence and absence of Cu2+ (20 equiv.) at pH 7.0.

ere measured by the treatment of 1 equiv. Cu2+ ions in the pres-nce of 10 equiv. other interfering metal ions including Ag+, Ca2+,d2+, Co2+, Cr3+, Fe3+, Hg2+, K+, Mn2+, Na+, Ni2+, Zn2+. The resultsemonstrated that the absorbance of 1 enhanced effectively by Cu2+

ons with these metal ions as background (Fig. 10). This observationonfirms that the selective Cu2+ signaling of 1 was not influencedy the presence of common metal ions including major competingetal ions of Fe3+.

.6. The reversibility of the interaction of 1 and Cu2+

The reversibility of the chemosensor is a very important aspectn practical application. The interaction between 1 and Cu2+ waseversible, which can be verified by the introduction of EDTA intohe system containing 1 (10 �M) and Cu2+ (200 �M). The experi-

ent showed that the introduction of EDTA (1 equiv. to Cu2+) could

mmediately restore the absorbance of 1. When Cu2+ was added tohe system again, the absorbance of 1 was enhanced. This processould be repeated at least three times (Fig. S14). This regenerationndicated that the sensor 1 could be reused with proper treatment.

ig. 10. The UV–vis absorbance response of 1 (10 �M) to various cations inMSO/Tris–HCl buffer (1:9, v/v, pH 7.0) solution. The red bars represent thebsorbance of 1 in the presence of cations of interest. The green bars representhe changes that occur upon subsequent addition of Cu2+. (For interpretation of theeferences to color in this figure legend, the reader is referred to the web version ofhe article.)

Fig. 11. Calculated energy-minimized structure of 1 with Cu2+.

3.7. Reaction mechanism

Similar to many reported rhodamine spirolactam-based fluores-cent chemosensors, the absorbance and fluorescence enhancementresponse of chemosensor 1 toward Cu2+ is most likely the resultof the spiro ring-opening mechanism (Fig. S15) rather than anion-catalyzed hydrolysis reaction. The above mentioned EDTAexperiment could serve as experimental evidence to support thisreversible spiro ring-opening mechanism.

To understand the selectivity and the configuration of 1-Cu2+,we carried out density functional theory (DFT) calculations withB3LYP exchange functionals using the Gaussian 03 package. Theoptimized configuration is shown in Fig. 11, which shows that Cu2+

ions well occupy the acylhydrazone coordination centers of 1 andthe molecule forms a planar structure. The Cu–N bond length is1.865 A, and the Cu–O bond lengths are 1.786 A and 1.850 A, respec-tively.

3.8. Application of 1 in Cu2+ analysis in water samples

2+

Synthesized water (by adding Cu and other metal ionsto drinking water) and Kangshifu Drinking Mineralized Waterobtained from the local supermarket were analyzed by the pro-posed absorption method under optimized conditions (Table 3).From the above results, it can be seen that 1 can measure the con-

Table 3Determination of Cu2+ in water samples.

Sample Cu2+ added(�M)

Cu2+ found(�M)

Recovery (%) R.S.D. a(%)

Drinking water 0.00 0.0012.50 12.65 101 4.3

Synthesized waterb 0.00 10.20 102 2.312.50 22.76 101 2.1

a n = 3.b Synthesized by: drinking water, 10.00 �M Cu2+, 10.00 �M Pb2+, Cd2+, Zn2+ and

50.00 �M Na+, K+, Mg2+ and Ca2+. Conditions: 10 �M 1 in DMSO/Tris–HCl buffer (1:9,v/v, pH 7.0) solutions.

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entration of Cu2+ in water samples with good recovery and R.S.D.esults, suggesting the potential application of real sample analysisy 1.

. Conclusion

A new rhodamine-based derivative 1 was synthesized and char-cterized. Sensor 1 exhibits reversible and highly selective andensitive recognition toward Cu2+ over other metal ions. Upon addi-ion of Cu2+, sensor 1 exhibits remarkably enhanced absorbancentensity and color change from colorless to pink in DMSO and

eCN aqueous buffer solution or pure MeCN, and shows significantff–on fluorescence accompanied by color changes from colorlesso orange in MeCN. Under the optimized condition of 10 �M 1 inMSO/Tris–HCl buffer (1:9, v/v, pH 7.0) solutions, the quantifi-ation of Cu2+ by 1 was satisfactory in a linear working range of0–300 �M, with a detection limit of 3.42 × 10−6 M. The detectionf Cu2+ in water samples by 1 was also successful.

cknowledgments

The authors thank the Program for New Century Excellent Tal-nts in University (NCET-08-0665), the Program for Science andechnology Innovation Talents in Universities of Henan Province2008HASTIT016), Henan Province Science and Technology Keyroject (082102230036), Natural Science Foundation of Henanrovince (2010B150029) and Xuchang University for financial sup-ort.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.snb.2011.01.066.

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Biographies

Zhihong Xu received his PhD degree in 2008 from Lanzhou University, China. He isa vice professor in College of Chemistry and Chemical Engineering at Xuchang Uni-versity. His current research interests includes development of complex synthesis,chemosensors and nanosensors for biological and environmental relevance.

Like Zhang received his master degree in 2005 form Henan Normal University,China. He has been a faculty member in College of Chemistry and Chemical Engi-neering at Xuchang University. His current research interests focus mainly onenvironmental analytical chemistry.

Rui Guo is studying for master in the Department of Chemistry at Zhengzhou Uni-versity. He is engaged in the synthesis, characterization and studies of Chemosensorfor the detection of biological and environmental relevance.

Tiancheng Xiang received his PhD degree in 2008 from College of Chemistry andChemical Engineering, Graduate University of Chinese Academy of Sciences, China.He is a vice professor in College of Chemistry and Chemical Engineering at XuchangUniversity. His current research interests focus mainly on Quantum CalculationChemistry.

Changzeng Wu is studying for PhD degree at Henan Normal University. He isengaged in the synthesis, characterization and studies of Chemosensor for the detec-tion of biological and environmental relevance.

Zhi Zheng is a professor in Institute of Surface Micro and Nano Materials, XuchangUniversity major in Inorganic Chemistry and Surface Micro and Nano Materials.

Fengling Yang is a professor in Institute of Surface Micro and Nano Materials,Xuchang University major in Organic Chemistry and Surface Micro and Nano Mate-rials.