Photo-pH dual stimuli-responsive multicolor fluorescent polymeric nanoparticles for multicolor security ink, optical data encryption and zebrafish imaging

Stimuli-responsive fluorescent polymeric nanoparticles are widely used in bioimaging, anti-counterfeiting and data encryption due to their advantages of simple preparation, good water-dispersibility, high brightness and photostability. In this paper, novel photo-pH dual stimuli-responsive multicolor fluorescent polymeric nanoparticles (SMFPNs) were fabricated via combination of one-pot miniemulsion polymerization and surface modification. Based on fluorescence resonance energy transfer (FRET) principle, 4-ethoxy-9-allyl-1,8-naphthalimide (EANI), fluorescein isothiocyanate (FITC) and spiropyran-linked methacrylate (SPMA) can be used to constitute three pairs of FRET systems. In the nanoparticles, the blue fluorescence of EANI (energy donor) decreases and the green fluorescence of FITC (energy receptor) increases via increasing pH. While pH decreases, it can return to blue fluorescence. Meanwhile, after UV light irradiation, SPMA converts from non-fluorescent spiropyrans (SP) state to red fluorescent merocyanine (MC) state, the transformation from blue or green emission to red emission can be realized. When irradiated with visible light, it reverts to its original blue or green fluorescence. Therefore, the system can achieve the multi-color transformation of blue, green and red under changing pH and illumination conditions. In addition, the as-prepared nanoparticles also exhibited high fluorescence contrast, rapid photo-responsiveness and excellent photoreversibility. Therefore, SMFPNs displayed great potential in the fields of multicolor security ink, advanced anti-counterfeiting, multilevel data encryption and in vivo bioimaging.