Various squaraine dyes have been developed for biological imaging. Nevertheless, squaraine dyes with emission in the second window (NIR-II, 1000–1700 nm) have few reports largely due to the short of a simple and universal design strategy. In this contribution, molecular engineering strategy is explored to develop squaraine dyes with NIR-II emission. First, NIR-I squaraine dye SQ2 is constructed by the ethyl-grafted 1,8-naphtholactam as donor units and square acid as acceptor unit in a donor–acceptor–donor (D–A–D) structure. To red-shift the fluorescence emission into NIR-II window, malonitrile, as a forceful electron-withdrawing group, is introduced to strengthen square acid acceptor. As a result, the fluorescence spectrum of acceptor-engineered squaraine dye SQ1 exhibits a significant red-shift into NIR-II window. To translate NIR-II fluorophores SQ1 into effective theranostic agents, fibronectin-targeting SQ1 nanoprobe was constructed and showed excellent NIR-II imaging performance in angiography and tumor imaging, including lung metastatic foci in deep tissue. Furthermore, SQ1 nanoprobe can be used for photoacoustic imaging and photothermal ablation of tumors. This research demonstrates that the donor–acceptor engineering strategy is feasible and effective to develop NIR-II squaraine dyes.

No abstract

Elevated hydrogen peroxide (H2O2) in biological tissues is generally recognized to be relevant to the carcinogenesis process that regulates the proliferative activity of cancer cells and the transformation of malignant features. Inspired by this observation, it can be hypothesized that imaging H2O2 in the tumor microenvironment (TME) could help diagnose tumor types and malignancy, and even guide precise therapy. Thus, in this study, a noninvasive photomedicine strategy is demonstrated that leverages the different levels of H2O2 in the TME, and two representative skin cancers, malignant melanoma (MM, clinically higher incidence of metastasis and recurrence) and cutaneous squamous cell carcinoma (cSCC, relatively less dangerous), are differentially diagnosed. The working probe used here is one we previously developed, namely, intelligent H2O2 responsive ABTS-loaded HRP@Gd nanoprobes (iHRANPs). In this study, iHRANPs have advantages over ratiometric imaging due to their bimodal imaging elements, in which the inherent magnetic resonance imaging (MR) mode can be used as the internal imaging reference and the H2O2 responsive photoacoustic (PA) imaging modality can be used for differential diagnosis. Results showed that after intravenous injection of iHRANPs, the tumor signals on both MM and cSCC are obviously enhanced without significant difference under the MR modality. However, under the PA modality, MM and cSCC can be easily distinguished with obvious variations in signal enhancement. Particularly, guided by PA imaging, photothermal therapy (PTT) can be precisely applied on MM, and a strong antitumor effect was achieved owing to the excessive H2O2 in the TME of MM. Furthermore, exogenous H2O2 was injected into cSCC to remedy H2O2 deficiency in the TME of cSCC, and an evident therapeutic efficacy on cSCC can also be realized. This study demonstrated that MM can be differentially diagnosed from cSCC by noninvasive imaging of H2O2 in the TME with iHRANPs; meanwhile, it further enabled imaging-guided precision PTT ablation, even for those unsatisfactory tumor types (cSCC) through exogenously delivering H2O2.