Stimuli-responsive multimodality imaging agents have broad potential in medical diagnostics. Herein, we report the development of a new class of branched-bottlebrush polymer dual-modality organic radical contrast agents—ORCAFluors—for combined magnetic resonance and near-infrared fluorescence imaging in vivo. These nitroxide radical-based nanostructures have longitudinal and transverse relaxation times that are on par with commonly used heavy-metal-based magnetic resonance imaging (MRI) contrast agents. Furthermore, these materials display a unique compensatory redox response: fluorescence is partially quenched by surrounding nitroxides in the native state; exposure to ascorbate or ascorbate/glutathione leads to nitroxide reduction and a concomitant 2- to 3.5-fold increase in fluorescence emission. This behaviour enables correlation of MRI contrast, fluorescence intensity and spin concentration with tissues known to possess high concentrations of ascorbate in mice. Our in vitro and in vivo results, along with our modular synthetic approach, make ORCAFluors a promising new platform for multimodality molecular imaging.

Cu(II) was used as a probe to investigate the structure and ion binding ability of full-generation poly(amidoamine) starburst dendrimers (nSBDs). Computer-aided analysis of the EPR spectra provided information on the formation of copper complexes in various internal or external locations of the dendrimers, as well as the nSBD structure as a function of the size (generation) of the dendrimers, pH, temperature, Cu(II) concentration, and aging of the samples. At low pH, Cu 2+ competes with protons for binding with external amino groups, which become available for complexation at pH > 3.5. The Cu(H 2 O) 6 2+ complexes are localized in proximity to the SBD surface, where structural modifications of the water solution prevented the occurrence of a freezing transition. A portion (about 20%) of Cu(H 2 O) 6 2+ resides in the water pools in the open structure of the earlier generation dendrimers (G < 4) and are capable of undergoing a freezing transition. Progressive penetration of Cu 2+ ions into the SBD structure occurs with an increase in pH and gives rise to the formation of complexes with various amino groups of the SBD structure. Between pH 4 and 5, the EPR spectra clearly show the superposition of three components corresponding to (a) a Cu(H 2 O) 6 2+ complex, (b) a complex with two surface NH 2 groups, and (c) a complex with two surface NH 2 groups and two internal NR 3 groups (Cu-N 4 ). A fraction of the third component increases with an increase in generation. At pH g 6 the Cu-N 4 complex is the only species present in the Cu(II)-nSBD solution. The low mobility of this complex supports the hypothesis that this complex is located in the external layers of the SBDs, which become densely packed in the later generations. At higher pH, Cu 2+ migrates to the internal SBD structure and the complexes show higher mobilities. The dendrimers decomposed upon aging and decomposition was almost complete for the earlier generation dendrimers at pH > 5.5 and at 60 days after preparation.

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Metal-free magnetic resonance imaging (MRI) agents could overcome the established toxicity associated with metal-based agents in some patient populations and enable new modes of functional MRI in vivo. Herein, we report nitroxide-functionalized brush-arm star polymer organic radical contrast agents (BASP-ORCAs) that overcome the low contrast and poor in vivo stability associated with nitroxide-based MRI contrast agents. As a consequence of their unique nanoarchitectures, BASP-ORCAs possess per-nitroxide transverse relaxivities up to ∼44-fold greater than common nitroxides, exceptional stability in highly reducing environments, and low toxicity. These features combine to provide for accumulation of a sufficient concentration of BASP-ORCA in murine subcutaneous tumors up to 20 h following systemic administration such that MRI contrast on par with metal-based agents is observed. BASP-ORCAs are, to our knowledge, the first nitroxide MRI contrast agents capable of tumor imaging over long time periods using clinical high-field 1H MRI techniques.

A spin label was attached to poly(N4sopropylacrylamide) (PNIPAM) to allow EPR spectroscopy to be used as a probe of the interactions between the solvent and this polymer in water, methanol, and watermethanol mixtures. Labeled polymers were prepared by reaction of a copolymer of N-isopropylacrylamide and N-(acry1oxy)succinimide with 4-amino-2,2,6,6-tetramethylpiperidine 1-oxide. The label contents of the polymers (M, 2.9 X lo6) were 1 X 10" and 1.6 X lo4 mol g*. EPR spectra of solutions of the labeled polymers (3 g L-9 were recorded (a) at constant temperature as a function of solvent composition and (b) for several water-methanol mixtures as a function of temperature (-10 to +35 O C ) . The spectra were analyzed in terms of the isotropic hyperfine coupling constant and the correlation time for the reorientation motion. The temperature and composition dependence of the parameters was determined by fitting the line shapes of the experimental spectra to simulated spectra. The results support a model involving preferential adsorption of methanol to the polymer chains in mixed methanol-water solutions, as the main contributor to the cononsolvency phenomenon.

We describe the parallel, one-pot synthesis of core-photocleavable, poly(norbornene)-co-poly(ethylene glycol) (PEG) brush-arm star polymers (BASPs) via a route that combines the "graft-through" and "arm-first" methodologies for brush polymer and star polymer synthesis, respectively. In this method, ring-opening metathesis polymerization of a norbornene−PEG macromonomer generates small living brush initiators. Transfer of various amounts of this brush initiator to vials containing a photocleavable bis-norbornene cross-linker yielded a series of water-soluble BASPs with low polydispersities and molecular weights that increased geometrically as a function of the amount of bis-norbornene added. The BASP cores were cleaved upon exposure to UV light; the extent of photo-disassembly depended on the amount of cross-linker. EPR spectroscopy of nitroxide-labeled BASPs was used to probe differences between the BASP core and surface environments. We expect that BASPs will find applications as easy-to-synthesize, stimuli-responsive core−shell nanostructures.

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Mn(II) has been used as a probe to investigate the interacting abilities of the various ligand sites at the surface of starburst dendrimers (SBDs) at different protonation conditions, and as a function of generation (G), both for half- and full-generation SBDs (n.5-SBDs and n-SBDs, respectively). The computer-aided analysis of the electron paramagnetic resonance (EPR) spectra of Mn(II) provided information on the location of Mn(II) in the hydration layers of the internal and external SBD surface and on the physical status of water inside the dendrimer structure and at the surface/solution interface. Mn(II) did not show any interaction with the full-generation dendrimers and only interacted, at the second−third solvation shells, with the surface carboxylate groups (SBD-COO-) of n.5-SBDs. Two main components were identified in the EPR spectra:  (1) a minor component due to Mn(II) that is localized in the solvation layers of the internal and external SBD surface and is in fast exchange with the bulk solution. This Mn(II) fraction underwent a freezing transition at about 255 K; (2) a major component arising from complexation of Mn(II) with SBD-COO- groups at the second solvation shell of the ions. The portion of solution that contained the Mn(II) complexes underwent a glass transition with a decrease in temperature. The complexation of Mn(II) was favored for later generation dendrimers (G > 4.5), at high pH and low Mn(II) concentrations.