Photoacoustic Imaging Using Polysaccharide-Porphyrin Complexes by Photoirradiation at Long Wavelengths.

Photoacoustic (PA) imaging is a novel biological imaging technique with superior depth resolution compared to fluorescence imaging. The efficacy of PA imaging depends on contrast agents that possess considerable absorbance at longer wavelengths, coupled with high permeability in biological tissue and minimal fluorescence, achieved through mitigating aggregation-caused quenching (ACQ) that attenuates PA intensity. Despite the successful transfer of porphyrin 2 featuring amino moieties from polysaccharides to liposomes, most of 2 incorporated within λ-carrageenan (CGN-2 complex) remained in CGN under acidic lysosomal conditions (pH 5.0). Consequently, the CGN-2 complex exhibited a strong PA signal under 680 nm photoirradiation in Colon26 cells owing to the ACQ of 2. Moreover, the PA intensity of the CGN-2 complex was further enhanced under 780 nm photoirradiation owing to the increased absorbance at 780 nm facilitated by the redshift of the Q-band at pH 5.0.