There are few examples of polymers that exhibit upper critical solution temperature (UCST) behavior under physiological conditions of temperature, pH, and ionic strength. In this study, we demonstrated that polymers with ureido groups undergo UCST-type phase transitions under physiologically relevant conditions. Poly(allylurea) copolymers showed UCST behavior at pH 7.5 in 150 mM NaCl even at the low polymer concentration of 0.13 mg/mL. Their phase separation temperatures (T(p)) could be controlled up to 65 °C. Similar thermosensitivity was observed with copolypeptides consisting of L-citrulline having an ureido group. This is the first demonstration of a non-vinyl polymer that shows UCST behavior under physiologically relevant conditions. We suggest that the ureido modification will be useful for production of polymer materials with UCST behavior in aqueous media.

Ureido-derivatized polymers, such as poly(allylurea) (PU) and poly(L-citrulline) derivatives, exhibited upper critical solution temperature (UCST) behavior under physiological buffer conditions as we previously reported. The PU derivatives having amino groups (PU-Am) also showed UCST behavior. In this study, we modified the amino groups of the polymer with succinyl anhydride (PU-Su) or acetyl anhydride (PU-Ac) to determine the effects of these ionic groups on the UCST behavior and to control interactions between the PU derivatives and biocomponents such as proteins and cells. Succinylation of PU-Am resulted in a significant decrease in phase separation temperature (Tp), whereas acetylation of PU-Am resulted in an increase in Tp. As expected, the Tp of PU-Am and PU-Su changed when the pH of the solution was changed. The Tp of PU-Am increased at higher pH, whereas that of PU-Su increased at lower pH, indicating that ionic charge decreases Tp of PU derivatives by increasing osmotic pressure and by increasing hydrophilicity of the polymer chains. Interestingly, these groups did not significantly change UCST when these groups were nonionic. We then examined capture and separation of particular proteins from a protein mixture by cooling-induced phase separation. Selective and rapid capture of particular proteins from protein mixture by PU derivatives was shown, indicating that the ureido-derivatized polymers are potential media for bioseparation under biofriendly conditions.

Two-photon photoemission spectroscopy (2PPE) was employed to investigate the electronic interactions at the interface of size-selected metal oxide clusters (Mo3O9, W3O9, Ti3O6, Mo3O6, W3O6, and Ti5O10) and a Cu(111) surface. The cluster–Cu interactions were probed by work function shifts measured by 2PPE as a function of local cluster coverage. For all the clusters studied, the work functions shifted to higher energies after cluster deposition, indicating negative interfacial dipole moments pointing toward the surface. The magnitudes of the derived interfacial dipoles are found to be in the order Mo3O9 ≈ W3O9 > W3O6 ≈ Mo3O6 > Ti5O10 > Ti3O6. DFT calculations of the electrostatic potentials at the interface and Bader charge analyses were used to assess the relative contributions of electron transfer and the structure-dependent cluster dipole moment to the observed work function shifts (ΔΦ). For the fully oxidized Mo3O9 and W3O9 clusters (+6 cation oxidation states), DFT calculations indicate that electron transfer from the Cu(111) support to the cluster is the dominant contribution. The smaller interfacial dipole moments for the Mo3O6 and W3O6 clusters are qualitatively consistent with the decreased ability of the reduced cations (+4 oxidation state) to accommodate charge from the Cu surface. The DFT calculations also predict small changes in ΔΦ for the titania clusters on Cu(111) but in the opposite direction of that observed experimentally. In the case of the Ti5O10/Cu(111) surface, this result is due to the net balance of cluster dipole and electron transfer contributions that have opposite signs. Overall, the results presented in this study show that a combination of coverage-dependent work function measurements and DFT calculations can be a powerful tool to investigate the electronic interactions, especially electron transfer, at the metal oxide–metal interface.

Size-selected niobium oxide nanoclusters (Nb 3 O 5 , Nb 3 O 7 , Nb 4 O 7 , and Nb 4 O 10 ) were deposited at room temperature onto a Cu(111) surface and a thin film of Cu 2 O on Cu(111), and their interfacial electronic interactions and reactivity toward water dissociation were examined. These clusters were specifically chosen to elucidate the effects of the oxidation state of the metal centers; Nb 3 O 5 and Nb 4 O 7 are the reduced counterparts of Nb 3 O 7 and Nb 4 O 10 , respectively. From twophoton photoemission spectroscopy (2PPE) measurements, we found that the work function increases upon cluster adsorption in all cases, indicating a negative interfacial dipole moment with the positive end pointing into the surface. The amount of increase was greater for the clusters with more metal centers and higher oxidation state. Further analysis with DFT calculations of the clusters on Cu (111) indicated that the reduced clusters donate electrons to the substrate, indicating that the intrinsic cluster dipole moment makes a larger contribution to the overall interfacial dipole moment than charge transfer. X-ray photoelectron spectroscopy (XPS) measurements showed that the Nb atoms of Nb 3 O 7 and Nb 4 O 10 are primarily Nb 5+ on Cu(111), while for the reduced Nb 3 O 5 and Nb 4 O 7 clusters, a mixture of oxidation states was observed on Cu(111). Temperature-programmed desorption (TPD) experiments with D 2 O showed that water dissociation occurred on all systems except for the oxidized Nb 3 O 7 and Nb 4 O 10 clusters on the Cu 2 O film. A comparison of our XPS and TPD results suggests that Nb 5+ cations associated with NbO terminal groups act as Lewis acid sites which are key for water binding and subsequent dissociation. TPD measurements of 2-propanol dehydration also show that the clusters active toward water dissociation are indeed acidic. DFT calculations of water dissociation on Nb 3 O 7 support our TPD results, but the use of bulk Cu 2 O(111) as a model for the Cu 2 O film merits future scrutiny in terms of interfacial charge transfer. The combination of our experimental and theoretical results suggests that both Lewis acidity and metal reducibility are important for water dissociation.

The interfacial electronic structure of various size-selected metal oxide nanoclusters (M 3 O x ; M = Mo, Nb, Ti) on Cu(111) and a thin film of Cu 2 O supports were investigated by a combination of experimental methods and density functional theory (DFT). These systems explore electron transfer at the metal-metal oxide interface which can modify surface structure, metal oxidation states and catalytic activity. Electron transfer was probed by measurements of surface dipoles derived from coverage dependent work function measurements using two-photon photoemission (2PPE) and metal core level binding energy spectra from x-ray photoelectron spectroscopy (XPS). The measured surface dipoles are negative for all clusters on Cu(111) and Cu 2 O/Cu(111), but those on the Cu 2 O surface are much larger in magnitude. In addition, sub-stoichiometric or "reduced" clusters exhibit smaller surface dipoles on both the Cu(111) and Cu 2 O surfaces. Negative surface dipoles for clusters on Cu(111) suggest Cucluster electron transfer, which is generally supported by DFT-calculated Bader charge distributions. For Cu 2 O/Cu(111), calculations of the surface electrostatic potentials show that the charge distributions associated with cluster adsorption structures or distortions at the cluster-Cu 2 O-Cu(111) interface are largely responsible for the observed negative surface dipoles. Changes observed in the XPS spectra for the Mo 3d, Nb 3d and Ti 2p core levels of the clusters on Cu(111) and Cu 2 O/Cu(111) are interpreted with help from the calculated Bader charges and cluster adsorption structures, the latter providing information about the presence of inequivalent cation sites. The results presented in this work illustrate how the combined use of different experimental probes of along with theoretical calculations can result in a more realistic picture of cluster-support interactions and bonding.