Synthesis and Functionalization of Dextran-Based Single-Chain Nanoparticles in Aqueous Media IK4-CIDETEC, Pº Miramón 196, 20014, Donostia-San Sebastián, Spain. b. Radiochemistry and Nuclear Imaging Group, CIC biomaGUNE, Pº Miramón 182, 20014, Donostia-San Sebastián, Spain. Water-dispersible dextran-based single-chain polymer nanoparticles (SCPNs) were prepared in aqueous media and mild conditions. Radiolabeling of the resulting biocompatible materials allowed the study of lung deposition of aqueous aerosols after intratracheal nebulization by means of single-photon emission computed tomography (SPECT), demostrating their potential use as imaging contrast agents.Advances in engineering polymer chains at the molecular level 1 have pushed the development of synthetic strategies for the controlled compaction of single polymer coils into unimolecular soft nano-objects, named single-chain polymer nanoparticles (SCPNs).2-4 SCPNs based on synthetic polymers benefit from the possibility of a controlled construction of the precursors in order to prepare tuned SCPNs with desired size and functionality.3 Additionally, a wide variety of biocompatible, non-toxic and ready-to-use natural polymers are available. Consequently, SCPNs have gained interest as potential mimetics of biomacromolecules such as proteins 5,6 and for application in different fields including nanomedicine.7-9 Among natural polymers, polysaccharides can be envisaged as natural analogues of polyethylene glycol (PEG). For example, dextrans have been used in several biomedical applications due to aqueous solubility, biocompatibility, biodegradability, wide availability, ease of modification and non-fouling properties.10,11 However, in vivo application of SCPNs can be limited due to their small size. Rapid blood clearance is expected after intravenous administration of particles below 5 nm size. 12 In the last years, non-invasive lung administration route has been broadly studied, 13 and SCPNs could be beneficial due to their small size.14 Synthetic routes to SCPNs are mainly based on intrachain homo-and hetero-coupling or cross-linking-induced collapseof pre-functionalized polymers through covalent, dynamic covalent, and non-covalent bonding. 15,16 Most of the covalent strategies are performed in organic solvents and require highly diluted polymer solutions (usually <1 mg/mL), high temperatures and/or the presence of metal catalysts. The preparation of SCPNs through "continuous addition" avoids ultra-dilution conditions, which is beneficial for multigram scale preparations. 17 Recently, it has been described that the presence of oligo(ethylene glycol) brushes as side-chains allows to achieve SCPNs at high polymer concentrations (100 mg/mL). 18 However, the development of general procedures to obtain functionalizable SCPNs in aqueous media, mild conditions and scalable conditions is still a challenging issue.Our group reported a strategy to generate structurally defined and water-dispersible poly(methacrylic acid)-based SCPNs.19 In pursuit of novel types of...