We recently described a technique to monitor heparin anticoagulation therapy in real-time using methylene blue and photoacoustic imaging. The photoacoustic signal of methylene blue was significantly amplified in the presence of heparin, but the exact mechanism underlying this novel photoacoustic behavior remains unclear. Here, we showed that the signal amplification was due to the aggregation of methylene blue. Methylene blue formed different aggregates in water and phosphate buffered saline (PBS). In water, the absorbance maximum of methylene blue with heparin from 0 to 3 U/mL blue shifted from 660 to 570 nm and the corresponding fluorescence intensity decreased 6-fold, which indicated the methylene blue aggregated from monomer to dimer and eventually to high order aggregates. Furthermore, the corresponding 0.04 ppm chemical shift of the proton in the phenothiazine ring of methylene blue from the nuclear magnetic resonance spectrum suggested the electron delocalization and self-aggregation of methylene blue. The coupling of methylene blue molecules results in extra vibrational relaxations within the split exciton states, and this causes enhanced photoacoustic signal. In PBS, we observed the aggregation of methylene blue/heparin complex using transmission electron microscopy (size=150.5 nm), but the absorbance maximum reversed back to 660 nm. This suggested the methylene blue formed monomer bound to heparin—the heparin could not self-aggregate due to electrostatic repulsive forces. The methylene blue bound monomers experienced less degree of freedom than free monomers and therefore caused excess photoacoustic signal.
Junxin Wang, AnanthaKrishnan Soundaram Jeevarathinam, and Jesse V. Jokerst, "Photoacoustic signal amplification of methylene blue via aggregation (Conference Presentation)," Proc. SPIE 10494, Photons Plus Ultrasound: Imaging and Sensing 2018, 1049429 (Presented at SPIE BiOS: January 30, 2018; Published: 15 March 2018); https://doi.org/10.1117/12.2286948.5752215523001.
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