BACKGROUND AND OBJECTIVE:Nanotechnology offers the possibility of creating multi-functional structures that can provide solutions for biomedical problems. The nanoprobes herein described are an example of such structures, where nano-scaled particles have been designed to provide high specificity and contrast potential for optical detection of cancer. Specifically, enzymatically activated fluorescent nanoprobes (EANPs) were synthesized as cancer-specific contrast agents for optical imaging. STUDY DESIGN/MATERIALS AND METHODS:EANPs were prepared by nanoprecipitation of blends of poly(lactic acid)-b-poly(ethylene glycol) and poly(lactic-co-glycolic acid)-b-poly(l-lysine). The lysine moieties were then covalently decorated with the near infrared (NIR) fluorescent molecule AlexaFluor-750 (AF750). Close proximity of the fluorescent molecules to each other resulted in fluorescence quenching, which was reversed by enzymatically mediated cleavage of poly(l-lysine) chains. EANPs were characterized by dynamic light scattering and electron microscopy. Enzymatic development of fluorescence was studied in vitro by fluorescence spectroscopy. Biocompatibility and contrast potential of EANPs were studied in cancerous and noncancerous cells. The potential of the nanoprobes as contrast agents for NIR fluorescence imaging was studied in tissue phantoms. RESULTS:Spherical EANPs of ∼100 nm were synthesized via nanoprecipitation of polymer blends. Fluorescence activation of EANPs by treatment with a model protease was demonstrated with up to 15-fold optical signal enhancement within 120 minutes. Studies with MDA-MB-231 breast cancer cells demonstrated the cytocompatibility of EANPs, as well as enhanced fluorescence associated with enzymatic activation. Imaging studies in tissue phantoms confirmed the ability of a simple imaging system based on a laser source and CCD camera to image dilute suspensions of the nanoprobe at depths of up to 4 mm, as well as up to a 13-fold signal-to-background ratio for enzymatically activated EANPs compared to un-activated EANPs at the same concentration. CONCLUSION:Nanoprecipitation of copolymer blends containing poly(l-lysine) was utilized as a method for preparation of highly functional nanoprobes with high potential as contrast agents for fluorescence based imaging of cancer. Lasers Surg. Med. 47:579-594, 2015. © 2015 Wiley Periodicals, Inc.