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FIGURE 9.13 Plots of moles transported versus time for Lys (upper) and BSA (lower) across a 40 nm Au i.d. nanotube membrane.

Gold nanotube membranes with PEG-thiol monolayers are placed in a U-tube permeation cell and solution from the feed side of the membrane is forced through the cell by applying 20 psi pressure. The concentration of the protein is monitored by periodically sampling the permeate side of the U-tube using UV-vis spectroscopy. The results of permeation experiments with single protein solutions of lysozyme (Lys, MW = 14 kDa) and bovine serum albumin (BSA, MW = 67 kDa) through a 40 nm i.d. nanotube membrane are shown in Figure 9.13.

The Stokes radii for BSA and Lys are 3.6 and 2 nm, respectively. Using the Stokes-Einstein equation, the calculated diffusion coefficient for Lys should be 1.8 times higher than BSA. From the data in Figure 9.13, a much higher flux for Lys relative to BSA is observed. The higher flux is a consequence of the smaller size of Lys relative to BSA, as BSA is physically hindered from translocating the membrane.

Experiments with two proteins in the feed solution were also performed. In this case, the experiments are carried out in a similar fashion, except the feed side of the membrane contained an equimolar ratio of Lys and BSA, and the results were monitored using HPLC. The results for the two protein Lys and BSA permeation experiments as a function of nanotube i.d. are shown in Figure 9.14. In Figure 9.14A, the HPLC data of the initial feed solution is shown. Figure 9.14B shows HPLC data of the permeate solution after transport through a 45 nm i.d. nanotube membrane. In analogy to the single-protein permeation experiments shown in Figure 9.13, the permeation of BSA is hindered relative to Lys, as observed by the diminished BSA peak. Figure 9.14C shows the HPLC of the permeate solution of a 30 nm i.d.

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