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«Kinetic Investigations of Thiolate Protected Gold Nanoparticles: Protein Interactions, Electron Transfer, and Precursor Formation By Brian N. Turner ...»

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Table 13: Elemental analysis-calculation compared to two different TGA-TEM calculations of molecular formula. Found represents the found percentages from elemental analysis. Expected are the percentages anticipated from the TGA-TEM calculations. Errors are given for each individual element and for the totals percentages expected for tiopronin and inorganic material (sodium was factored in to investigate whether the final solids are heavily sodiated at the carboxylate groups despite the acidification pre-dialysis).

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The formula Au216(C5H9NO3S)146 was obtained from direct calculation of elemental analysis results and TEM core size calculations. Au 216(C5H9NO3S)129 was obtained from a combination of TGA and TEM assuming that all loss was attributed to tiopronin, and Au216(C5H9NO3S)68 was calculated in the same way, except that it was assumed that the loss was due to a loss of AuTiopronin2 staple complexes from the surface. If it is assumed that the elemental analysis gives a true total tiopronin loss, then the conclusion is that the all tiopronin calculation from TGA underestimates the tiopronin thiolate content by 8.12% while the staple calculation underestimates the content by 73.1%.

Therefore, given the assumption, the all tiopronin loss method is more accurate than the staple calculation. Consequently, it is not expected that Au atoms are lost from the sample during thermal analysis. The underestimation could originate from ashing of the ligand on the pan, resulting in residual elemental carbon that was not factored into the total organic mass. Ashing was expected from visual inspection of burned material.

When the TGA was only run up to 550 °C, the residual material in the pan was black.

Running the TGA up to 800 °C or higher resulted in what appeared to be purely solid gold flakes, consistent with a complete loss of organic matter. In order to confirm the identity of the burn off during this final step, TPD-MS was used.

TPD-MS of Tiopronin Gold Nanoparticles Therefore, TPD-MS was used to study the identity of the burn off material. A TGA from the TPD-MS instrument is provided for comparison in Figure 54.

Figure 54: Thermal gravimetric spectrum generated on the TG-MS instrument at Oakridge, under Argon purge.

The magnitude of each loss is identical between the two analyses, but the shape of the final loss is different. The difference in thermal gravimetric plot shape was likely due to an argon purge in the Oak Ridge instrument as supposed to a nitrogen purge with the VINSE instrument. M/z of the burn off compound tracked with temperature is displayed in Figure 57, preceded by burn off for AuHexanethiol in Figure 55 and 56 for comparison.

Figure 55: Selective ion scan of elemental and small molecule fragments thermally lost from AuHexanethiol nanoparticles. The y-axis is total ion current at the selected m/z.

Small room-temperature gas molecules are the easiest to observe in this technique.

Notice how molecules or atoms associated with carbon and sulfur from hexanethiol spike at the beginning of the mass loss (Figure 53) and slowly taper off. Total quantitative measurements of these ions are hampered by their natural presence in the chamber due to 100% purging efficiency.

Figure 56: Larger mass peaks thermally lost from AuHexanethiol nanoparticles. The yaxis is total ion current at the selected m/z, listed at right, in Da, with some m/z identified.

Larger mass fragments were identifiable for this sample, including the intact ligand and some fragments thereof, noted in the key (charges on the ions not shown). Some larger mass fragments could be identified for the AuTiopronin sample, but identification of the ions was not consistent with expectations (see Figure 55). Therefore, only the small mass ions under oxidative (air) conditions are presented.

Figure 57: Small mass fragments thermally lost from AuTiopronin nanoparticles. The yaxis is total ion current at the selected m/z. This particular experiment was run in air instead of argon.

For tiopronin, only small molecule masses were identifiable. The loss that spans 600 to 1000 °C tracks directly with carbon, supporting the idea that tiopronin ashes during the burning process. This attributed to the carboxylate group, as molecules with carboxylate groups tend to have additional high temperature losses (as was observed with tiopronin and glutathione ligands without gold). The hypothesized chemistry of this process, consistent with the observations, might be RCOOH  RH + CO2, CO2  O2 + C(s), C(s) + O2.

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The aim was to prove that carboxylate terminated G1 dendrimers could be used to synthesize gold nanoparticle cored dendrimers (NCDs) by Brust-like methodology. Here, to the best of our knowledge at the time of the study, the first example of a water soluble NCD is presented. The following reaction was studied, in Scheme B-1.

Scheme B-1: Synthesis of gold nanoparticle cored dendrimers (AuNCD)

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AuxG1SHy was synthesized using a modified Brust reaction.1 A solution of HAuCl4 · 3H2O in 5 mL of 6:1 methanol:acetic acid was cooled to -70° C in a dry ice/acetone bath.

To this cold solution was added 60 mg of AuG1SH in 3.5 mL 6:1 methanol:acetic acid.

Upon allowing the solution to slowly warm to -30° C, the solution turned from yellow to a light brown-orange, indicating the formation of a Au(III)-G1SH complex. To this solution was rapidly added a 25x molar excess (50 mg) of NaBH 4 in 1 mL of deionized water. CAUTION! A violent exothermic reaction results from the reduction of Au(III) to Au0. Upon reduction a black precipitate formed and was allowed to stir for 15 hrs.

The solvent was removed by rotary evaporation, and the resulting black solid was resuspended in 0.5M NaOH. Purification was accomplished by dialysis in 10,000 MWCO tubing with MeOH:0.5M NaOH/H2O. Dialysis proceeded for 5 days, with changes 4x daily. NMR: Consistent with the ligand, desensitization of protons that would be closest to the gold core, consistent with other ligands on gold (Figure 58).

UV/Vis: No resulting SPR band. TEM: average diameter 1.3 ± 0.6 nm (Figure 59). TGA analysis was obscured by the presence of salt, but was measured as 60.2% organic.

Average formula Au186G1S50.

Place Exchange G1SH was stirred with AuTiopronin nanoparticles in 50:50 MeOH:H 2O (20 mg nanoparticle in 4 mL solvent, 30:1 G1SH:tiopronin feed ratio). 1H NMR is displayed in Figure 60. Quantification was not possible due to solvent peaks.

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Figure 58: 1H NMR of G1 dendrimer protecting a gold nanoparticle Figure 59: TEM micrograph and size distribution of AuG1S NCDs.

Figure 60: 1H NMR data G1 place exchanged onto AuTiopronin. Green spectrum (top) is gold tiopronin nanoparticles, blue spectrum (middle) is G1SH, and red spectrum (bottom) is the place exchanged product.

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Standard Route to p-thioacetylphenylethynylbenzene (PEPSAc)157 This route was not practical in the current research, as the starting material (4iodophenylthioacetate) was not commercially available. Therefore, the following syntheses were pursued before turning to the commercially available PEPEPSAc.

Other routes to PEPSAc Synthesis of PEPSAc Through a Solvent Free, Copper Free Sonogashira Coupling185

0.5g BrPhSAc (2.16 mmol), 0.045g PdCl2(PPh3)2 (3 mol %), and 2.05 g TBAF (6.5 mmol) were weighed into a 3 neck flask and purged with Ar. 290 µL of phenylacetylene (2.6 mmol) was added via syringe. The reaction mixture was heated to 80 °C in an oil bath and moderately stirred for 18 hours. The product was extracted in ether, washed 3x with water, dried over MgSO4, filtered, and evaporated. The product was further purified on a silica gel column with DCM/hexanes. The reaction yielded 0.49 g of a yellowish solid. 1H NMR: Product appeared to be deprotected disulfide, but coupled to phenylacetylene. The next reaction was chosen in order to work at a lower temperature so that disulfide formation might be avoided.

Synthesis of PEPSAc Through a Copper Free, Amine Free Sonogashira Coupling186

0.5g BrPhSAc (2.16 mmol), 0.045g Pd2(dba)3 (0.5 mol %), and 0.82 g TBAF (2.6 mmol), and 11.3 g of PPh3 (2 mol%) were weighed into a 3 neck flask and purged with Ar. 6 mL of anhydrous THF was added via syringe. 290 µL of phenylacetylene (2.6 mmol) was added via syringe. The reaction mixture was moderately stirred for 24 hours at room temperature. The product was extracted in ether, washed 3x with water, dried over MgSO4, filtered, and evaporated. The product was further purified on a silica gel column with DCM/hexanes. The reaction yielded 0.54 g of a yellow oil. The correct product did not seem to be formed. The next reaction was chosen to work at an intermediate temperature, and to follow more closely the original Sonogashira reaction.

Synthesis of PEPSAc Through a Standard Sonogashira Coupling187

0.5g BrPhSAc (2.16 mmol), 0.027g PdCl2(PPh3)2 (0.039 mmol), and 0.023 g CuI (0.119 mmol) were weighed into a 3 neck flask and purged with Ar. 3 mL of anhydrous THF and 3 mL of anhydrous Hunig’s base was added via syringe. 290 µL of phenylacetylene (2.6 mmol) was added via syringe. The reaction mixture was heated to 50 °C in an oil bath and moderately stirred for 24 hours. The product was extracted in ether, wahed 3x with water, dried over MgSO4,filtered, and evaporated. The product was further purified on a silica gel column with DCM/hexanes. An orange solid was obtained. 1H NMR: 7.55ppm, 7.36ppm, and 7.29 ppm, m, 9H (aromatic); 2.42 ppm, s, 3H (thioacetate methyl group). Yield: 42%, questionable purity.

Iodine Exchange with p-bromothioanisole (BrPhSCH3)188 An amount of 1.0g of BrPhSCH3 (4.92 mmol) was weighed into a 3 neck flask and purged with Ar. The solid was dissolved in 30 mL anhydrous THF, added via syringe.

This solution was cooled in a dry ice/acetone bath. 5.8 mL of 1.7 M tert-butyllithium in heptanes was added. CAUTION: tert-butyllithium should be handled with extreme caution as it readily acts with air and water in an extremely exothermic reaction capable of producing large flames. This solution was stirred for 5 minutes. 1.37 g iodine (5.42 mmol) was dissolved in 15 mL anhydrous THF. The iodine solutions was added to the organolithium reagent via cannula. The reaction was brought to room temperature and stirred for about 30 minutes. The reaction was quenched with 20 mL of 1M HCl, and 2 moderate scoops of sodium sulfite. The solution was evaporated, and 20 mL of water was added. The organic product was extracted into ether and washed 3x with water.

Recrystallization from hot iso-octane/toluene yielded a blue-green crystalline product.

1H NMR matches literature values.

Synthesis of p-thiomethylphenylethynylbenzene (PEPSCH3) via Songoashira Coupling of IPhSCH3157

0.3 g of IPhSCH3 from the previous reaction (1.27 mmol) and 0.010 g CuI (1.8 mol%) were weighted into a 3 neck flask and purged with Ar. 5.5 mL THF, 5.5 mL Hunig’s base, and 522 µL of phenylacetylene were added via syringe. Then, 48.6 mg (0.6 mol %) PdCl2(PPh3)2 was quickly added, and the solution purged. The solution was heated to 50 °C in an oil bath and stirred overnight. The product was extracted in ether, wahed 3x with water, dried over MgSO4,filtered, and evaporated. Recrystallization from hot hexanes yielded an orange solid. 1H NMR: 7.5 ppm, m, 2H; 7.42 ppm, d, 2H, J = 8.52 Hz; 7.32 ppm, m, 3H; 7.19 ppm, d, J = 8.52 Hz (aromatic); 2.48 ppm, s, 3H (thiomethyl).

GC/MS M+• = 224.3 Da observed, pure product. Deprotection of this product proved impractical by the methods of Young, Gauthier, and Coombs, and also by NaH reaction.

Route from p-iodophenylethynylbenzene (PEPI) to p-triisopropylsilylphenylethynylbenzene (PEPSSiTIP) Synthesis of PEPI via Sonogashira Coupling of Diiodobenzene189 Triethylamine was distilled over CaH2 under Ar. 1 g diiodobenzene (3.0 mmol), 28.3 mg PPh3 (0.1 mmol), and 8 mg CuI (10 mol%) were added to 180 mL of freshly distilled triethylamine. Then, 12.4 mg Pd2dba3 (5 mol%) was quickly added. 100 µL of phenyacetylene was added (1.0 mmol). The solution was stirred under Ar for 1.5 hours, the filtered, and evaporated. The yellowish solid was the dissolved in dichloromethane and poured over an excess of silica gel. The dichloromethane was then removed in vacuo, and the dry silica gel with the product absorbed onto it is loaded on top of a silica gel column. Chromatography yields high purity crystals of diiodobenzene in hexanes, and high purity PEPI. Yield 78% white flaky solid. 1H NMR: 7.69 ppm, d, 2H, J = 8.52 Hz; 7.53 ppm, 2H, m; 7.36 ppm, m, 3H; 7.26 ppm, d, 2H, J = 8.52 Hz, values match literature value.

Synthesis of PEPSSiTIP from PEPI190

0.9 g of PEPI from the previous reaction (2.96 mmol), 200 mg Pd(PPh3)4 (5.8 mol %), 212 mg CsCO3 (3.69 mmol) were dissolved in 30 mL of toluene, previous dried over molecular sieves, and purged with Ar. 850 µL of TIPSiSH (3.83 mmol) was added via syringe. The mixture was brought to 100 °C in an oil bath and stirred for 18 hours.

Afterwards, 30 mL saturated NH4Cl was added, and the aqueous solution was extracted twice with diethyl ether. The organic fractions were washed 3 times with water, and evaporated to yield a red oil with red/orange precipitate. Residual solvent was removed with heating in vacuo. The product was extracted into hexanes. 1H NMR: 7.54 ppm, m and 7.50 ppm, d, total 4H; 7.40 ppm, d, and 7.35 ppm, m, total 5H (aromatic); 1.12 ppm, m, 21H (triisopropyl silane protons). Crude product could not be further purified on a column as the product decomposed. Recrystallization strategies were not very successful.

Synthesis of p-iodotetramethylthiurylbenzene (IPhSTMT)191 An amount of 1.0 g diiodobenzene (3.0 mmol) was weighed into a 3 neck flask and purged with Ar. 6 mL of freshly distilled THF was added. The solution was chilled in an ice bath. 3.54 mL of 0.9M (as determined by iodine titration) iPrMgCl-LiCl solution was added. CAUTION: The Grignard reagent will react exothermically with air and water.

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