Near-infrared hydrophobic probes as molecular light switch for CMC determination of Triton X-100 solution

The fluorescence behavior of two near-infrared (NIR) chromophores with linear alkyl chains of different lengths, 2-[4′chloro-7′(3″ethyl-2″benzothiaolinylidene)-3′，5′-(1″′3′″-propanediyl)-1′3′，5′-heptantriene-1′-yL]-3-ethylbenzothiazolium iodide (Probe Ⅰ) and 2-[4′chloro-7′(3″hexadecyl-2″benzothiazolinylidene)-3′，5′-(1′″，3′″-propanediyl)-1′,3′,5′-heptantriene-1′-yl]-3-ethylbenzothiazolium iodide (Probe Ⅱ), in aqueous solution containing different concentrations of surfactants was studied. The fluorescence of the probe with a short chain (probe Ⅰ) was completely quenched in water and aqueous solution containing a low concentration (below the critical micelle concentration,CMC) of surfactant Triton X-100. However, the fluorescence reappeared and reached maximum rapidly once the concentration of the surfactant approached the CMC. The probe with a long chain (probe II) displayed a similar fluorescence behavior but more dramatically fluorescent recovery in Triton X-100 system, which gave a direct in- dication for the micelle forming process and provided a simple method for the determination of the critical micelle concentration of the surfactant. The CMC values determined by this method were in good agreement with those obtained by other techniques. The fluorescence behavior of the two probes in other surfactant systems was also investigated.