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The interaction of methylene blue (MB) and sodium dodecyl sulfate (SDS) leads to a reversible spectral shift during SDS micellization, but the underlying mechanism has remained unclear. Here, we measured photoacoustic (PA) intensity, micelle concentration, and spectral shift of MB-SDS complex to elucidate this interaction mechanism. We observed a switchable PA effect of MB, which is sensitive to critical micelle concentration (CMC) of SDS (i.e. 8 mM). The addition of 3.47 mM SDS increased the PA intensity of 0.05 mM MB 492-fold because of fluorescence quenching. Then, the PA intensity decreased by 54-fold when the SDS concentration was increased above the CMC at 8.67 mM due to decrease of MB aggregation. Meanwhile, we observed increased number of non-micellar MB-SDS clusters, ranging from 80 to 400 nm, as the SDS concentration approaching to CMC and then the number decreased once the SDS concentration was above CMC. The correlation between PA intensity and nanoparticle number indicated that the formation of MB-SDS cluster was responsible for the PA enhancement. Further controlled studies using MB/hexadecyltrimethylammonium bromide, MB/sodium octyl sulfate, and MB/sodium chloride showed that the binding between MB and SDS occurred at the sulfate moiety of SDS. They also found that MB-SDS clusters disassociated to micelles MB-SDS monomers at the SDS micellar concentrations. These findings further elucidate the binding mechanism of MB and SDS and presented the potential for developing an activatable MB PA contrast agent.
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