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For use as IDAs, modified CPG samples require removal of weakly bound Cu(II) to so that strong chelate-Cu-dye complexes can form. For all above samples, weakly bound Cu(II) is washed away using HEPES buffer before being acid digested. After digestion, samples are filtered and diluted to a fixed volume Vs with a concentration of Cs. The strong

binding capacity (g Cu per g sample), after buffer wash, is calculated using Equation 3-4.

Strong Cu(II) binding capacity =

€

C_S ×V_S MS

3-4

where ms is the mass of the sample. The calculated Cu(II) binding capacities are in Table 3-4. The Cu(II) binding capacities in Table 3-4 significantly decrease over total binding values in Table 3-3, especially for the two control samples, CPG-CDI and CPG-terpy. One conclusion based on this data is Cu(II) only weakly binds to CPG-CDI surface as it goes to zero after the buffer wash. CPG-terpy capacity decreases ~30% and final value probably represents Cu(II) binding to mostly terpy sites. The Cu(II) binding capacities of CPG-Gx.0 and

much to strong binding of Cu(II), which is consistent with the binding models above pH 7.0 presented in the Introduction section.

Table 3-4. Strong normalized Cu(II) binding capacities determined using acid digestion after buffer-wash treatment at pH 7.0

Terminal group Strong binding capacity (µmol Cu2+ / g sample)

% Cu(II) lost from total binding

CPG-CDI 0.0 100 CPG-terpy 12.0 87.8 CPG-G1.0 18.8 76.2 CPG-G2.0 26.5 69.0 CPG-G3.0 25.4 78.4 CPG-G4.0 24.9 81.8 CPG-G5.0 68.4 63.8 CPG-G3.0-CDI 16.3 83.3 CPG-G4.0-CDI 29.3 75.0 CPG-G5.0-CDI 58.9 59.7 CPG-G3.0-NTA 23.0 72.4 CPG-G4.0-NTA 43.5 64.3 CPG-G5.0-NTA 83.2 39.2 CPG-G3.0-terpy 27.9 41.9 CPG-G4.0-terpy 39.1 39.0 CPG-G5.0-terpy 107.1 -6.9

For most CPG-Gx.0-NTA samples, more than half of the Cu(II) are weakly bound and

washed out by buffer solutions. For example, CPG-G3.0-NTA falls 72.4 %. Meanwhile, the

results show larger binding capacities for ligand (NTA and terpy) terminated surfaces than dendrimer only, such as CPG-G5.0-NTA (83.2 µmol/g) vs. CPG-G5.0 (68.4 µmol/g). This can

be explained by the studies of Diallo11 and Ottaviani15. In their research, only tertiary amine

groups are available for Cu(II) bindings at neutral pH, but not primary amine groups. Therefore, after the primary amine groups are further modified into NTA and terpy groups, there are more binding sites (both tertiary amine groups and terminal NTA or terpy groups) available for Cu(II) ions. Apparently some interior sites are strong for Cu(II) binding. Furthermore, the high kinetics of formation (kf = 2 × 107 M-1s-1) for Cu-terpy complex in aqueous solution could account for the higher loadings for the surfaces with terpy terminal groups, but this seems unlikely as formation rates and formation constants measured at equilibrium are large for most polydentate chelates with Cu(II).20,22 Finally, the percentage

Cu(II) lost from CPG-Gx.0-NTA is higher than that from CPG-Gx.0-terpy. One idea is that the

hydrophobic -(Cu-terpy)2+ sites create some sort of barrier to Cu(II) penetration into or

washing out from the interior Gx.0 sites.

3.4

Conclusion

Cu(II) binds to all kinds of surfaces synthesized for this research work, including primary amines, carboxylic acids, NTA, and terpy terminated surfaces. The results prove that larger dendrimers lead to larger binding capacities, according to both the binding break through curve and the acid digestion method. In addition, the formation of NTA- or terpy- terminated surfaces, considerably increases the strong binding capacities for Cu(II), almost doubling them compared to only PAMAM dendrimer immobilized surfaces. We propose that those are due to the “extra” binding sites created by converting PAMAM dendrimer’s primary amine groups into strong metal chelates for Cu(II), which are available for binding at neutral pH. Finally, this work demonstrates a new utility of the Gx.0 modified surfaces, as

terpy is normally not very soluble in water at pH 7. Thus, the Gx.0 becomes a “solubilizing”

platform for this chelate to operate efficiently to extract heavy metal ions at neutral pH. For future works, since the strong binding capacity of CPG-G5.0-teryincreases (6.9

%) instead of decreases compared to CPG-G3.0-terpy (41.9 %)and CPG-G4.0-terpy (39.0 %),

Cu(II) binding reactions should be redone for explaining Cu(II) binding to terpy-terminated surfaces. The other question that remains unsolved is why CPG-Gx.0 has about the same total binding capacity as CPG-Gx.0-CDI (with carboxylic acid terminal groups).

References

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Biopolymer Template Synthesized Mesoporous Titania Beads of Hundreds of Micrometers Size. Environ. Sci. Technol. 2011,46 (1), 419-425.

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Water Res. 2009,43 (10), 2605-2614.

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Groundwater Contaminated by Heavy- and Transition-Metal Ions by Selective Ion-Exchange Methods. Environ. Sci. Technol. 2002,36 (8), 1851-1855.

8. Xing, Y.; Chen, X.; Wang, D., Electrically Regenerated Ion Exchange for Removal and Recovery of Cr(VI) from Wastewater. Environ. Sci. Technol. 2007,41 (4), 1439-1443.

9. Denizli, A.; Sanli, N.; Garipcan, B.; Patir, S. l.; Alsancak, G. l.,

Methacryloylamidoglutamic Acid Incorporated Porous Poly(methyl methacrylate) Beads for Heavy-Metal Removal. Ind. Eng. Chem. Res. 2004,43 (19), 6095-6101.

10. Kadirvelu, K.; Faur-Brasquet, C.; Cloirec, P. L., Removal of Cu(II), Pb(II), and Ni(II) by Adsorption onto Activated Carbon Cloths. Langmuir 2000,16 (22), 8404-8409. 11. Diallo, M. S.; Christie, S.; Swaminathan, P.; Balogh, L.; Shi, X.; Um, W.; Papelis, C.; Goddard, W. A.; Johnson, J. H., Dendritic Chelating Agents. 1. Cu(II) Binding to Ethylene Diamine Core Poly(amidoamine) Dendrimers in Aqueous Solutions. Langmuir

2004,20 (7), 2640-2651.

12. Diallo, M. S.; Balogh, L.; Shafagati, A.; Johnson, J. H.; Goddard, W. A.; Tomalia, D. A., Poly(amidoamine) Dendrimers: A New Class of High Capacity Chelating Agents for Cu(II) Ions. Environ. Sci. Technol. 1999,33 (5), 820-824.

13. Diallo, M. S.; Christie, S.; Swaminathan, P.; Johnson, J. H.; Goddard, W. A., Dendrimer Enhanced Ultrafiltration. 1. Recovery of Cu(II) from Aqueous Solutions Using PAMAM Dendrimers with Ethylene Diamine Core and Terminal NH2 Groups. Environ. Sci. Technol. 2005,39 (5), 1366-1377.

14. Ottaviani, M. F.; Montalti, F.; Turro, N. J.; Tomalia, D. A., Characterization of Starburst Dendrimers by the EPR Technique. Copper(II) Ions Binding Full-Generation Dendrimers. J. Phys. Chem. B 1997,101 (2), 158-166.

15. Ottaviani, M. F.; Bossmann, S.; Turro, N. J.; Tomalia, D. A., Characterization of starburst dendrimers by the EPR technique. 1. Copper complexes in water solution. J. Am. Chem. Soc. 1994,116 (2), 661-671.

16. Block, B. P.; McIntyre, G. H., The Calculation of Formation Constants for Systems Involving Polydentate Ligands1. J. Am. Chem. Soc. 1953,75 (22), 5667-5669.

17. Jonassen, H. B.; Reeves, R. E.; Segal, L., Inorganic Complex Compounds Containing Polydentate Groups. XI. Effect of Hydroxide Ion on the Bis-ethylenediaminecopper(II) Ion.

J. Am. Chem. Soc. 1955,77 (10), 2748-2749.

18. Whitlow, S. H., Structure of sodium nitrilotriacetatocopper(II) monohydrate. Inorg. Chem. 1973,12 (10), 2286-2289.

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22. Wood, R. D.; Nakon, R.; Angelici, R. J., Copper(II) complex catalysis of amino acid ester hydrolysis. A correlation of rates with complex stability. Inorg. Chem. 1978,17 (4), 1088-1090.

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Applications of Indicator Displacement Assays on Metal Loaded Surfaces and

Formation of Ligand-Metal-Indicator Ternary Complexes In Aqueous Solution

Copper(II) binding capacities of generation 3.0-5.0 PAMAM dendrimers immobilized CPG-Gx.0-NTA and CPG-Gx.0-terpy samples were determined via the acid digestion method in Chapter 3. This chapter describes work in which, the resulting Cu(II) loaded materials were applied to indicator displacement assays for recognition of small molecules using a fluorescent dye, 6,7-dihydroxy-4-methylcoumarin (DHMC). In addition, formation constants of ternary complexes (ligand-metal-indicator) of Cu(II), Ni(II), and Zn(II) with nitrilotriacetic acid (NTA), 2,2':6',2''-terpyridine (terpy) as ligands, and bromopyrogallol red (BPR) and pyrocatechol violet (PV) as indicators, in aqueous solutions, are reported as a reference to solid support based studies.

4.1

Introduction

Previous studies suggested the formation of a tetragonally distorted octahedral structure for Cu(II)-PAMAM dendrimer complexes at neutral pH.1,2 _{A Cu(II) central metal}

ion is coordinated to four dendrimer tertiary amine groups and two axial water molecules inside of the dendrimer.3 _{The bound water molecules can be displaced by an incoming dye}

molecule or a substrate, forming a five-membered ring structure with the central Cu(II) metal ion. In this work, CDI-activated CPG-Gx.0 are modified with two metal chelators, lysine-NTA

(NTA) and aminofunctionalized terpyridine (terpy) (Figure 4-1), which convert dendrimer exterior primary amine groups into NTA and terpy groups, respectively. The resulting CPG surfaces are able to bind Cu(II) by not only their interior tertiary amine groups but also the newly generated surfaces groups. Due to steric effects (bad accessibility) and electrostatic

effects, the dendrimer interior tertiary amine groups are not responsible for most of the binding of Cu(II), especially after metal chelators are attached to the dendrimer exterior primary amine groups. These terminal groups further block the incoming molecules from reacting with the dendrimer interior. Herein, metal chelator (NTA or terpy) sites on the exterior of dendrimers bind most of the Cu(II), since they are exposed to the solution environment. In the case of IDAs, these metal chelator-Cu(II) sites are ready for incoming dyes and substrates.

Figure 4-1. A Cu(II) metal ion binds to three terpy amine groups and two hydroxyl groups on DHMC. The bound DHMC can be displaced by an incoming substrate.

The formation constants of ternary complexes of Cu(II), Ni(II), and Zn(II) metal ions with NTA or terpy and indicator dye in aqueous solutions are also reported to help guide selection of an appropriate indicator and to help confirm its reactivity with the surfaces.