In Vivo Assembly and Disassembly of Probes to Improve Near-Infrared Optical Bioimaging.

The near-infrared range (NIR, 700-1700 nm) has been used as a superior optical window for non-invasive bioimaging. Increasing signal-to-noise ratio (SNR) is the most fundamental method to improve NIR bioimaging. However, the low delivery efficiency of fluorescent contrast agents leads to weak signal at lesions. Moreover, non-specific accumulation and "always on" signals will cause "false positive" signals and high background noise, all of which result in low SNR and potential long-term biotoxicity. Thus, to reach precise detection of lesions, strong bioimaging signals and low background interference are the two important pre-requisites. This review provides an overview of in vivo assembly and disassembly strategies to improve tumor-specific accumulation, "turn-on" the silent signals, and reduce the background noise in NIR bioimaging windows. In vivo assembly and disassembly occurring spontaneously, responding to disease micro-environment or external stimuli, including pH, enzymes, reactive oxygen species, redox, light, and specific recognition is summarized, which may provide ideas and approaches to further enhance bioimaging and reduce long-term biotoxicity concerns.