Selective detection of phosphaphenanthrene-containing luminophors with aggregation-induced emission enhancement to transition metal ions

Abstract

Transition metal ions (Pb²⁺, Zn²⁺, Cd²⁺, Co²⁺, Mn²⁺, Cu²⁺, Ni²⁺, Hg²⁺, Ag⁺, Fe³⁺) in water are used to quench emission of 2-(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)-1,4-phenylene-bis(p-pentyloxylbenzoate)s (MD5) with aggregation-induced emission enhancement (AIEE) in water-acetonitrile (AN) mixture (80:20 by volume). Among all metal ions, Fe³⁺ exhibits the highest quenching efficiency on AIEE of MD5 even when the concentration of Fe³⁺ is lower than 1×10^-6 mol/L. The quenching efficiency of Hg²⁺ is lower than that of Fe³⁺ at the same concentration, though MD5 is used to detect Hg²⁺ efficiently, too. To other metal ions, low quenching efficiency has few relations with a wider concentration range. The UV absorbance spectra show only red shift of absorbance wavelength in the presence of Hg²⁺ and Fe³⁺, which indicates a salt-induced J-aggregation. SEM photos reveal larger aggregation and morphological change of nanoparticles of MD5 in water containing Hg²⁺ and Fe³⁺, which reduce the surface area of MD5 emission for further aggregation. The selective quenching effect of transition metal ions to emission of MD5 has a potential application in chemical sensors of some metal ions.

Keywords

AIEE / phosphaphenanthrene / transition metal ions / quenching effect

Introduction

The development of selective and sensitive imaging tools capable of monitoring heavy- and transition-metal ions Fe³⁺ and Hg²⁺ has attracted considerable attention because Fe³⁺ plays a pivotal role in many biochemical processes in a cellular level, and Hg²⁺ is one of the most hazardous components in the environment [1-6]. Many fluorescent probes for the detection of Fe³⁺ and Hg²⁺ have been proposed so far [7-13].

A number of compounds exhibiting aggregation-induced emission (AIE), aggregation-induced emission enhancement (AIEE) or crystallization-induced emission enhancement (CIEE) have been developed in recent years [14-33]. Because they exhibit stronger emission in the solid state than in solution, these compounds show great potentials for such applications as highly selective and stable fluorescence sensors for proteins [25-27], DNA [28], organic vapors [29,30], explosives [31,32], metal ions [33], and so on.

In our previous study, as one type of AIEE compound family, 2-(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)-1,4-phenylene-bis(p-pentyloxylbenzoate)s (MD5) can be observed with AIEE and CIEE effects [34] due to the progressively fortified restriction of its intramolecular rotation in the different physical phases. The morphological structure of the thin solid film of MD5 between amorphous and crystalline phases can be changed by vapor-fuming process as well as fuming–heating and heating–cooling cycles, leading to emission switching between bright and dark states [34]. The phosphaphenanthrene-containing compound is obviously different from others reported, which has negative P = O bond and refers capable of easy interaction with positive molecules or ions. If positive ions are introduced during forming the aggregation of MD5, the aggregation packing structure and organizational morphology of MD5 would be obviously changed and lead to “bad” emission of MD5 aggregates to some extent. In this paper, we will report that some transition metal ions, such as Fe³⁺ and Hg²⁺, have rapid and sensitive quenching effect on MD5 AIEE, which means that it has a factual application like chemosensor of some transition metal ions.

Experimental

Materials

2-(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)-1,4-phenylene-bis(p-pentyloxylbenzoate)s (MD5) was prepared in our laboratory. Acetonitrile (AN) (of pesticide residue grade) was purchased from American Honeywell B&J. Pb(NO₃)₂, Zn(NO₃)₂, Cd(NO₃)₂, Co(NO₃)₂, Mn(NO₃)₂, CuSO₄, Ni(NO₃)₂, Hg(NO₃)₂, AgNO₃, and FeCl₃, Ce(NO₃)₃ were all analytical reagent and purchased from Beijing Chemical Reagent Co. Ltd.

Instrumentation

UV spectra were recorded on a HITACHI Spectrophotometer U-2800. Photoluminescence (PL) spectra were recorded on a Perkin-Elmer VARIAN 55 spectrofluorometer. SEM images were taken on a HITACHI S-4800 Scanning Electron Microscope with EDS.

Preparation of MD5 nanoaggregates in mixture containing metal ions

The solution of MD5 in AN with concentration of 1 × 10^-4 mol/L was first prepared. Aliquots of the solution were transferred into 10-mL volumetric flasks, into which appropriate volumes of AN and water were added dropwise under vigorous stirring to furnish 1 × 10^-5 mol/L solutions with different water contents (0-90 vol%) in the absence or presence of metal ions with appropriate concentration. UV and PL spectra were immediately performed once the solutions were prepared.

Results and discussion

Figure 1 shows the fluorescence spectrum of MD5 in the absence or presence of three metal ions (Fe³⁺, Hg²⁺ and Cu²⁺) in water-AN mixture (80∶20 by volume). MD5 shows strong emission in absence of metal ions because of its AIEE property. After addition of metal ions, the PL intensity of MD5 at 359nm decreased, which contributed to the quenching effect of the transition metal ions on AIEE of MD5. Outstanding differences in emission intensity were observed with samples mixed with Fe³⁺ and Hg²⁺ whose relative quenching efficiency reached 56% and 28%, as shown in Figs. 1 and 2. Cu²⁺ shows low quenching efficiency (about 10%) which may be deemed having no quenching ability.

The fluorescence spectra of MD5 in the aggregation state were also measured in the presence of other metal ions including Pb²⁺, Zn²⁺, Cd²⁺, Co²⁺, Mn²⁺, Ni²⁺ and Ag⁺ under identical conditions. As shown in Fig. 2, quenching efficiency of the eight metal ions on MD5 aggregates at 359 nm was rather small (less than 10%) after addition of 10.0 equiv of the respective metal ions. It is obvious that the emission of MD5 has different responses to all the 10 transition metal ions, with the highest response ability to Fe³⁺ and next to Hg²⁺. Other competitive metal ions such as Pb²⁺, Mn²⁺, Ni²⁺, Co²⁺ etc., however, induce a rather low interference effect on this fluorescence assay for Fe³⁺ and Hg²⁺.

The PL intensities of MD5 were measured in water-AN mixture (80∶20 by volume) solution containing different concentrations of metal ions from 1 × 10^-3 mol/L to 1 × 10^-6 mol/L, respectively, as shown in Figs. 3 and 4. The experimental results indicate that the detection sensitivity distinctly increases with the increase of the concentration of metal ions. Among all metal ions used in our work, Fe³⁺, however, exhibits the highest quenching efficiency even when the concentration of Fe³⁺ was lower than 1 × 10^-6 mol/L. The quenching efficiency of Hg²⁺ is lower than that of Fe³⁺ at the same concentration. For Cu²⁺, the low quenching efficiency has little relation with its concentration ranged from 1 × 10^-3 mol/L to 1 × 10^-6 mol/L. Thus, the emission of MD5 aggregation is thought to have selective responses to Hg²⁺ and Fe³⁺ and thereby has a potential application such as transition metal ion chemosensors.

To find the reason of different quenching effects of metal ions on MD5 AIEE, UV spectra of MD5 aggregation in water-AN mixture (80∶20 by volume) in the presence of these metal ions were detected. Figure 5 shows that UV spectra of MD5 aggregation state in the presence of Fe³⁺ and Hg²⁺ ions exhibit an obvious difference from those of other ions, which show the outstanding red shift from 268 nm for other metal ions to 276 nm for Fe³⁺ or 281 nm for Hg²⁺. The absorption wavelength red shift and PL emission quench of MD5 in the presence of Fe³⁺ and Hg²⁺ reveal only that the packing style of MD5 is a salt-induced J-aggregate state [34]. The polar P = O structure in MD5 molecule strongly interacts with Hg²⁺ or Fe³⁺ and their hydrolysate because part of Hg²⁺ or Fe³⁺ hydrolyzes to form Hg(OH)⁺ or FeO(OH) in neutral solution, respectively [35-37]. Compared with pure Hg²⁺ and Fe³⁺ ion, their hydrolysate is a kind of sorbents which tend to adhere to the particle of MD5 at initial stages. It is the main reason of higher aggregation degree of MD5 in water containing Hg²⁺ or Fe³⁺. Except for Fe³⁺ and Hg²⁺, all other metal ions havehave UV absorbance curves that show the similar absorption feature of MD5 , which corresponds to their weak quenching efficiency on emission of MD5.

The morphology of MD5 aggregated in water containing different transition metal ions was further detected by SEM in Fig. 6. The results reveal that the average diameter of nanoparticles with irregular shape in the absence of metal ions is about 30nm. With the addition of metal ions, the average diameters of nanoparticles are increased to 100 nm (Cu²⁺), 150 nm (Fe³⁺) and 300 nm (Hg²⁺) respectively, which are 3-10 folds bigger than those with the absence of metal ions. The increase of diameter directly affects the aggregation degree. Therefore, it can be concluded that the increasing volume of MD5 nanoparticles reduces their emission area and weakens the emission [38]. The change of MD5 aggregation reveals that the molecule stacking style in metal ion solution is absolutely different. Moreover, it should be emphasized that the nanoparticles from Hg²⁺ solution have the biggest size in a shape of regular sphere. It is a direct evidence of change of aggregation morphology between aggregation in pure water and in water containing transition metal ions. It has relation to the high quenching efficiency of some ions (Fe³⁺ and Hg²⁺).

We also detected the elementary disperse of MD5 aggregates by EDS to explain aggregation of MD5 in water containing Hg²⁺. Figure 7 shows that the elements of C, O, P, Cu, Na are detected (Cu and Na elements are from copper net). The Hg element can not, however, be found at the surface of nanoparticles. It means that the nanoparticles are made up of MD5, and the Hg²⁺ did not combine with MD5 evenly in particles. So, we deduce that the Hg²⁺ as a center first forms a complex with MD5 before the process of nanoparticle forming. More detailed mechanism analysis is currently underway in our laboratory.

Conclusion

In summary, transition metal ions (Pb²⁺, Zn²⁺, Cd²⁺, Co²⁺, Mn²⁺,Cu²⁺, Ni²⁺, Hg²⁺, Ag⁺, Fe³⁺) in water are used to quench emission of 2-(6-oxido-6H-dibenz<c,e><1,2>oxaphosphorin-6-yl)-1,4-phenylene-bis(p-pentyloxylbenzoate)s (MD5) with AIEE in water-AN mixture (80∶20 by volume). Among all metal ions, Fe³⁺ exhibits the highest quenching efficiency even when the concentration of Fe³⁺ is lower than 1 × 10^-6 mol/L. The quenching efficiency of Hg²⁺ ion to AIEE of MD5 is lower than that of Fe³⁺ at the same concentration, though MD5 is used to detect Hg²⁺ efficiently, too. To other ions, the low quenching efficiency has, however, little relation with wider concentration range. The UV absorbance spectra show only red shift of absorbance wavelength in the presence of Hg²⁺ and Fe³⁺, which indicates salt-induced J-aggregation. Therefore, the quenching results from the fact that energy of exiting state of MD5 is transferred into thermal energy of metal ions. At the same time, SEM photos reveal larger aggregation and morphological change of nanoparticles of MD5 in water containing Hg²⁺ and Fe³⁺. The larger aggregation reduces the surface area of MD5 emission for further aggregation. However, other transition ions also have slightly quenching effect on emission of MD5 aggregate state. The selective quenching emission of MD5 to transition metal ions has a potential application in chemical sensors of some metal ions.

References

[1] de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E., Chem. Rev.1997, 97, 1515–1566

[2] Liu, J.; Lu, Y., J. Am. Chem. Soc.2003, 125, 6642–6643

[3] Peng, X.; Du, J.; Fan, J.; Wang, J.; Wu, Y.; Zhao, J.; Sun, S.; Xu, T., J. Am. Chem. Soc.2007, 129, 1500–1501

[4] Royzen, M.; Dai, Z.; Canary, J. W., J. Am. Chem. Soc.2005, 127, 1612–1613

[5] Mandal, A. K.; Suresh, M.; Suresh, E.; Mishra, S. K.; Mishra, S.; Das, A., Sens. Actuators B Chem.2010, 145, 32–38

[6] Kim, I. B.; Erdogan, B.; Wilson, J. N.; Bunz, U. H. F., Chemistry2004, 10, 6247–6254

[7] Ono, A.; Togashi, H., Angew. Chem. Int. Ed.2004, 43, 4300–4302

[8] Moon, S. Y.; Youn, N. J.; Park, S. M.; Chang, S. K., J. Org. Chem.2005, 70, 2394–2397

[9] Descalzo, A.; Martò′nez-Manez, R.; Radeglia, R.; Rurack, K.; Soto, J., J. Am. Chem. Soc.2003, 125, 3418–3419

[10] Guo, X. F.; Qian, X. H.; Jia, L. H., J. Am. Chem. Soc.2004, 126, 2272–2273

[11] Zhang, H.; Han, L. F.; Zachariasse, K. A.; Jiang, Y. B., Org. Lett.2005, 7, 4217–4220

[12] Yoon, S.; Miller, E. W.; He, Q.; Do, P. H.; Chang, C. J., Angew. Chem. Int. Ed.2007, 46, 6658–6661

[13] Bricks, J. L.; Kovalchuk, A.; Trieflinger, C.; Nofz, M.; Büschel, M.; Tolmachev, A. I.; Daub, J.; Rurack, K., J. Am. Chem. Soc.2005, 127, 13522–13529

[14] Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z., Chem. Commun.2001, 1740–1741

[15] Bhongale, C. J.; Hsu, C. S., Angew. Chem. Int. Ed.2006, 45, 1404–1408

[16] Chung, J. W.; An, B. K.; Park, S. Y., Chem. Mater.2008, 20, 6750–6755

[17] An, B. K.; Lee, D. S.; Lee, J. S.; Park, Y. S.; Song, H. S.; Park, S. Y., J. Am. Chem. Soc.2004, 126, 10232–10233

[18] Qin, A.; Jim, C. K. W.; Tang, Y.; Lam, J. W. Y.; Liu, J.; Mahtab, F.; Gao, P.; Tang, B. Z., J. Phys. Chem. B2008, 112, 9281–9288

[19] Palayangoda, S. S.; Cai, X.; Adhikari, R. M.; Neckers, D. C., Org. Lett.2008, 10, 281–284

[20] Hong, Y. N.; Lam, J. W. Y.; Tang, B. Z., Chem. Commun.2009, 4332–4353

[21] Liu, Y.; Tao, X.; Wang, F.; Dang, X.; Zou, D.; Ren, Y.; Jiang, M., J. Phys. Chem. C2008, 112, 3975–3981

[22] Wu, Y. T.; Kuo, M. Y.; Chang, Y. T.; Shin, C. C.; Wu, T. C.; Tai, C. C.; Cheng, T. H.; Liu, W. S., Angew. Chem. Int. Ed.2008, 47, 1–5

[23] Zeng, Q.; Li, Z.; Dong, Y.; Di, C.; Qin, A.; Hong, Y.; Ji, L.; Zhu, Z.; Jim, C. K.; Yu, G.; Li, Q.; Li, Z.; Liu, Y.; Qin, J.; Tang, B. Z., Chem. Commun.2007, 70–72

[24] Dong, Y.; Lam, J. W. Y.; Qin, A.; Sun, J.; Liu, J.; Li, Z.; Sun, J.; Sung, H. H. Y.; Williams, I. D.; Kwok, H. S.; Tang, B. Z., Chem. Commun.2007, 3255–3257

[25] Tong, H.; Hong, Y.; Dong, Y.; Häussler, M.; Li, Z.; Lam, J. W. Y.; Dong, Y.; Sung, H. H. Y.; Williams, I. D.; Tang, B. Z., J. Phys. Chem. B2007, 111, 11817–11823

[26] Tong, H.; Hong, Y.; Dong, Y.; Häussler, M.; Lam, J. W. Y.; Li, Z.; Guo, Z.; Guo, Z.; Tang, B. Z., Chem. Commun.2006, 3705–3707

[27] Qian, L. J.; Tong, B.; Zhi, J. G.; Yang, F.; Shen, J. B.; Shi, J. B.; Dong, Y. P., Acta Chimi. Sin.2008, 66, 1134–1138

[28] Hong, Y.; Häussler, M.; Lam, J. W. Y.; Li, Z.; Sin, K. K.; Dong, Y.; Tong, H.; Liu, J.; Qin, A.; Renneberg, R.; Tang, B. Z., Chemistry2008, 14, 6428–6437

[29] An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y., J. Am. Chem. Soc.2002, 124, 14410–14415

[30] Itami, K.; Yoshida, J., Chemistry2006, 12, 3966–3974

[31] Yang, J.; Aschemeyer, S.; Martinez, H. P.; Trogler, W. C., Chem. Commun.2010, 46, 6804–6806

[32] Qin, A. J.; Lam, J. W. Y.; Tang, L.; Jim, C. K. W.; Zhao, H.; Sun, J. Z.; Tang, B. Z., Macromolecules2009, 42, 1421–1424

[33] Liu, L.; Zhang, G. X.; Xiang, J. F.; Zhang, D. Q.; Zhu, D. B., Org. Lett.2008, 10, 4581–4584

[34] Qian, L. J.; Tong, B.; Shen, J. B.; Shi, J. B.; Zhi, J. G.; Dong, Y. Q.; Yang, F.; Dong, Y. P.; Lam, J. W. Y.; Liu, Y.; Tang, B. Z., J. Phys. Chem. B2009, 113, 9098–9103

[35] Lu, L. D.; Helgeson, R.; Jones, R. M.; McBranch, D.; Whitten, D., J. Am. Chem. Soc.2002, 124, 483–488

[36] Saito, K., J. Phys. Chem. B2001, 105, 4235–4238

[37] Shankar, S. S.; Patil, U. S.; Prasad, B. L. V.; Sastry, M., Langmuir2004, 20, 8853–8857

[38] Henglein, A.; Brancewica, C., Chem. Mater.1997, 9, 2164–2167

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg