Spectroscopic investigation of water nanocluster structures formed by AOT reverse micelles

Abstract

AOT reverse micelles can solubilize relatively large amounts of water in the micelle center and provide a unique opportunity to study different aggregation of water molecules. In this work, various sizes of water nanocluster were prepared from AOT-alkane-water solution and their microstructures were studied using steady-state, time-resolved luminescence spectros- copy. As luminescence probes, 2-naphthol derivatives and lanthan- ide(III) ion were used because the excited lifetime of naphthol in aqueous core depends on local environment due to the proton trans- fer reaction and the luminescence lifetime of lanthanide(III) ion in aqueous solution also depends on the microstructure of water. The luminescence spectra of 2-naphthol derivatives reflect the competition between emission from the excited acidic form NSOH^* and basic form NSO^-*: addition of water cause the NSOH^* emission band to decrease with a concomitant increase in the NSO^-* emssion band. The emission behaviors for two 2-naphthol derivatives of different charge, with varying w ([H_2O]/[AOT]), indicated that 2-naphthol-6,8-disulfonate is localized around the center of the water pool whereas 2-naphthol-6-sulfonate resides in the bound water. Luminescence properties of lanthanide ion(III) in AOT reverse micelle were also observed: The relatively large luminescence intensity of the hypersensitive transition of lanthanide(III) ion significantly decreases as the size of reverse micelle is increased. Lifetimes of the europium nitrate solution and the terbium nitrate solution ranged from 143μs to 355μs, 500μs to 1141μs, with the longer lifetime corresponding to solution with a smaller AOT reverse micelle size. All lifetimes are significantly longer than those observed in bulk solution. This results from the increase of inner-sphere interaction between nitrate and lanthanide(III) ion. It is concluded that the luminescence properties of these probes in the water pool formed by AOT reverse micelle were very sensi- tive to the nanocluster size implying that the excited-state dynamics of these probes are controlled by the local structure of the water cluster. ; AOT reverse micelle은 내부에 많은 양의 물을 포함할 수 있고 bulk 상태와는 다른 물분자 배치를 보인다. 본 연구에서는 AOT- alkane-water solution을 사용하여 여러 크기의 water nanocluster를 만들고 steady-state, time-resolved spectroscopy 방법을 사용하여 이들의 미세구조를 관찰하였다. Luminescence probe로 2-naphthol 유도체와 lanthanide(III) ion을 사용하였는데 naphthol은 excited-state에서 proton transfer 반응을 일으키므로 local environment에 따라 다른 lifetime을 나타내고 lanthanide(III) ion의 luminescence lifetime도 물의 미세구조에 따라 변한다. 2-naphthol 유도체의 형광 스펙트럼은 excited-state acidic form에 대한 emission band와 excited-state deprotonation에 의한 basic form의 emission band로 되어 있는데 물이 첨가되어 reverse micelle의 크기가 증가하면 acidic form의 emission band 세기는 감소하고 basic form의 emission band 세기는 증가한다. 2-naphthol derivatives로 서로 다른 charge를 갖는 2-naphthol-6,8-disulfonate(2N68DS)와 2-naphthol -6-sulfonate (2N6S)를 사용하여 형광 스펙트럼의 변화를 관찰하고 lifetime을 측정하였으며 이로부터 heterogeneous한 AOT reverse micelle의 water phase내에서 2N68DS는 중앙부분에, 2N6S는 bound water 부분에 위치함을 알 수 있었다. 또다른 probe인 lanthanide(III) ion의 형광 스펙트럼은 작은 reverse micelle에서 상대적으로 큰 hypersensitive transition의 형광 세기를 나타내었으며 그 세기는 reverse micelle의 크기가 증가함에 따라 감소하였다. 이는 lanthanide(III)와 nitrate ion 사이의 inner-sphere interaction에 기인한 것으로 lifetime에도 상당한 변화가 나타는데 AOT reverse micelle의 크기가 증가함에따라 europium nitrate solution과 terbium nitrate solution의 lifetime이 355에서 143μs, 1141에서 500μs의 범위로 감소하였다. 그러나 모두 bulk solution에서의 lifetime보다 상당히 큰 값을 나타내었다. 위의 결과로부터 AOT reverse micelle에 의해 형성된 water pool에서 사용한 probe의 luminescence properties는 nanocluster size에 의존하며 따라서 이들 probe의 excited-state dynamics는 water cluster의 local structure에 의해 조절될 수 있음을 알 수 있었다.