Gold nanoparticles (Figure 16-29) can be titrated with the oxidizing agent TCNQ in the presence of excess of Br to oxidize Au(0) to AuBr in deaerated toluene. Gold atoms in the interior of the particle are Au(0). Gold atoms bound to C₁₂H₂₅S (dodecanethiol) ligands on the surface of the particle are Au(I) and are not titrated.

Reduced TCNQ has a low-energy electronic absorption peak at 856 nm. The table gives the absorbance at 856 nm as 0.700 mL of 1.00×10^-4 M TCNQ + 0.05 M (C₈H₁₇)4N⁺Br^- in toluene is titrated with gold nanoparticles (1.43 g/L in toluene) from a microsyringe with a Teflon-coated needle. Absorbance in the table has already been corrected for dilution.

(a) Make a graph of absorbance versus volume of titrant and estimate the equivalence point. Calculate the mmol of Au(0) in 1.00 g of nanoparticles.

(b) From other analyses of similarly prepared nanoparticles, it is estimated that 25 wt% of the mass of the particle is dodecanethiol ligand (C₁₂H₂₅S , FM 201.40). Calculate mmol of C₁₂H₂₅S in 1.00 g of nanoparticles.

(c) The Au(I) content of 1.00 g of nanoparticles should be 1.00 mass of Au(0) mass of C₁₂H₂₅S. Calculate the micromoles of Au(I) in 1.00 g of nanoparticles and the mole ratio Au(I):C₁₂H₂₅S. In principle, this ratio should be 1:1. The difference is most likely because C₁₂H₂₅S was not measured for this specific nanoparticle preparation.

Figure 16-29

Transcribed Image Text:

NC CN NC CN TCNQ Tetracyanoquinodimethane Reduced radical anion λ 856 nm Total μL Absorbance at l Total μL nanoparticles 856 nm Absorbance at 4.9 8.0 11.0 13.7 16.2 0.208 0.301 0.405 0.502 0.610 nanoparticles 856 nm 19.1 22.0 25.0 30.0 35.9 0.706 0.770 0.784 0.785 0.784