Two Orders of Magnitude Fluorescence Enhancement of Aluminum Phthalocyanines by Gold Nanocubes: A Remarkable Improvement for Cancer Cell Imaging and Detection

The metal-enhanced fluorescence (MEF) by metal nanoparticles is a useful technique for fluorescence detections in biological systems. The MEF effects with gold nanorods (AuNRs) and nanocubes (AuNCs) for fluorescence enhancements of sulfonated aluminum phthalocyanine (AlPcS), a commonly used and clinical approved photosensitizer for photodynamic therapy of cancers, were studied in this work. For the AuNRs which have the low aspect ratios with the corresponding longitudinal surface plasma resonance (LSPR) band in the region of 600-750 nm, the fluorescence quenching of conjugated AlPcS was found. Whereas for the AuNRs that have the LSPR bands of 800-900 nm, the MEF of AlPcS was obtained with the enhancing factor of 2-6 times, respectively. Using AuNCs, a great enhancement of AlPcS fluorescence was achieved with an enhancing factor of 150 times. Using two cancer cell lines as in vitro models, an outstanding fluorescence enhancement of AlPcS-AuNCs conjugates in cells, relative to AlPcS alone, was obtained under one-photon excitation (OPE) of 405 nm. Moreover, the bright fluorescence image of AlPcS-AuNCs in cells was also achieved under the two-photon excitation (TPE) of an 800 nm femtosecond laser. The high-quality cell imaging with either OPE or TPE demonstrated the potential of AlPcS-AuNCs in cancer cell detections.