Metal nanoparticles have many interesting optical and electrical properties. For many of these, such as sensors and catalysts, it would be advantageous to immobilize the nanoparticles and make them re-usable. In today’s experiment, we will examine methods to grow nanoparticles onto insoluble supports for potential catalysis applications.
- Clay exchanges
- TiO2 supports
Clay is abundant, inexpensive, and non-toxic. Many clays have exchangeable cations, which we will use to immobilize the reducing / stabilizing agents used in AuNP synthesis. We will make the exchanges today and form the AuNP next week.
- Place 1.0 g of montmorillonite (NaMT) and 155 mg of Histidine in a 50 mL Falcon tube. Add 50 mL H2O. Shake overnight.
- Place 1.0 g of NaMT and 174 mg of Arginine in a 50 mL Falcon tube. Add 50 mL H2O. Shake overnight.
- Place 1.0 g of NaMT and 204 mg of Tryptophan in a 50 mL Falcon tube. Add 50 mL H2O. Shake overnight.
The cation exchange capacity (CEC) of NaMT is 92 mequiv / g. To exchange all Na+ with another cation a 10% excess of CEC amount of cation is typically added. The driving force for monovalent cation exchange is that the more hydrophobic cation will replace the hydrophilic cation in the clay galleries to minimize undesirable interactions with water. When the cations have similar hydrophobicity, a greater excess of the replacing cation is needed to drive the exchange. We will assume the amino acids are hydrophobic enough to replace most Na+ under equal molar conditions. (For polyvalent cations, the strong electrostatic attraction towards the clay surface increases its affinity relative for clay to monovalent cations.)
For 1 g of clay, we require 1 mequiv of cation. The amino acids all have 1 mmol of salt per mequiv of cation. As a result, 1 mmol of amino acid is added for a 10% excess of CEC. For a protein, there are typically more than one mequiv of cation per mmol of protein. We want a protein that has a basic pI, since these will have an excess of amines, which will be protonated at neutral pH. BSA has a pI around 5, but lysozyme has a pI around 11. Previous studies (C. T. Johnston et al., Langmuir, 28(1), 2011, 611-619) have shown that there are about 9 mequiv of charge per mmol in pH unadjusted clay – amino acid systems, which would result in a protein concentration of 2 mM, or about 10 times what we typically use. Upon further consideration of the system, one could realize that the large size of the protein will make it sterically difficult to replace every Na+ with protein. Since Na+ should not interfere with the formation of AuNP, using partially exchanged clay should still work for immobilizing the AuNP. We will use a 10% CEC target (200 uM protein) for this study. Lysozyme has 129 AA residues and a molar mass of 14,300 g/mol.
- Place 1.0 g of NaMT and 159 mg of lysozyme in a 50 mL Falcon tube. Add 50 mL H2O. Shake overnight.
Questions to consider: Would this work if the exchanged cation was only a reducing agent? Would it work if it were only the stabilizing agent? Will the AuNP be permanent? If not, what could displace the AuNP and is there a way to make it permanent? What characteristics do the cations have to have in order to be effective?
TiO2 supported AuNP
This procedure is based on the methods published by R. Zanella et al., J. Phys. Chem. B 106, 2002, 7634-7642. We will duplicate the methods using anion adsorption.
- In a small beaker, add 0.25 g of TiO2 to 25 mL of 4 mM HAuCl4.
- Heat the solution to 80 C for 15 min
- Filter the particles, rinsed with water and place in a clean porcelain crucible.
- Heat the particles to 300 C in a muffle furnace for 2 h