Surfactant modulated aggregation induced enhancement of emission (AIEE)-a simple demonstration to maximize sensor activity

A new type of easily synthesized rhodamine-based chemosensor L-3, with potential NO2 donor atoms, selectively and rapidly recognizes Hg2+ ions in the presence of all biologically relevant metal ions and toxic heavy metals. A very low detection limit (78 nM) along with cytoplasmic cell imaging applications with no or negligible cytotoxicity indicate good potential for in vitro/in vivo cell imaging studies. SEM and TEM studies reveal strongly agglomerated aggregations in the presence of 5 mM SDS which turn into isolated core shell microstructures in the presence of 9 mM SDS. The presence of SDS causes an enhanced quantum yield (phi) and stability constant (K-f) compared to those in the absence of SDS. Again, the FI of the [L-3-Hg](2+) complex in an aqueous SDS (9 mM) medium is unprecedentedly enhanced (similar to 143 fold) compared to that in the absence of SDS. All of these observations clearly manifest in the enhanced rigidity of the [L-3-Hg](2+) species in the micro-heterogeneous environment significantly restricting its dynamic movements. This phenomenon may be ascribed as an aggregation induced emission enhancement (AIEE). The fluorescence anisotropy assumes a maximum at 5 mM SDS due to strong trapping (sandwiching) of the doubly positively charged [L-3-Hg](2+) complex between two co-facial laminar microstructures of SDS under pre-miceller conditions where there is a strong electrostatic interaction that causes an improved inhibition to dynamic movement of the probe-mercury complex. On increasing the SDS concentration there is a phase transition in the SDS microstructures and micellization starts to prevail at SDS >= 7.0 mM. The doubly positively charged [L-3-Hg](2+) complex is trapped inside the hydrophobic inner core of the micelle which is apparent from the failure to quench the fluorescence of the complex on adding 10 equivalents of H(2)EDTA(2)-solution but in the absence of SDS it is quenched effectively.