Chemiluminescence of Conjugated-Polymer Nanoparticles by Direct Oxidation with Hypochlorite

Chemiluminescence (CL) is an advantageous detection tool for in vivo imaging because of the high signal-to-noise ratio of its optical-signal readout, which does not require an external excitation source. Conjugated polymers (CPs) are now used as an energy acceptor in CL nanoparticles to enhance the CL. Here, we demonstrate CL from the direct oxidation of CP backbones in conjugated-polymer nanoparticles (CPNs) by hypochlorite. Such CL CPNs completely avoid the involvement of small-molecule CL donors. The strategy greatly simplifies CL-probes preparation and increases the stability of CL nanoprobes by overcoming the leakage problem of CL donors in nanoparticles. Hypochlorite can oxidize the vinylene bond (C═C) in polyfluorene-vinylene (PFV)/polyphenylenevinylene (PPV) via π²-π² cycloaddition to form a PFV- or PPV-dioxetane intermediate that is unstable and can spontaneously degrade into PFV- or PPV-aldehyde and generate photons. The dioxetane-intermediate formation was confirmed by UV-vis-absorption, fluorescence, nuclear-magnetic-resonance (¹H NMR), and Fourier-transform infrared (FT-IR) spectroscopy. The CL quantum yield (QY) of the brightest CL probe, CPN-poly[(9,9-di(2-ethylhexyl)-9 H-fluorene-2,7-vinylene)- co-(1-methoxy-4-(2-ethylhexyloxy)-2,5-phenylenevinylene)] (90:10 mol ratio, CPN-PFV- co-MEHPV), was 17.79 einsteins/mol (namely, photons per particle). CPN-PFV- co-MEHPV was size-stable, noncytotoxic, selective, and sensitive for hypochlorite detection. The linear range and the LOD of CPN-PFV- co-MEHPV for ClO^- detection are 2-30 and 0.47 μM. Thus, CPN-PFV- co-MEHPV was successfully applied for in vivo imaging of endogenously produced ClO^- in living animals. We expect that the represented strategy could be extended to construct other CL nanoprobes for bioimaging and disease diagnosis by simply optimizing and transforming CP backbones; such CL CPNs will have a profound impact on the field of bioimaging.