Pressurized Gold Separations – Part 2

ByThomas Gaborski

I continued work with a couple of new wafers and settled on one that lowered the cut-off to between 5 and 10 nm gold particles. For completeness, I will also include some of the old data that has been further analyzed.

All experiments were performed with BBI stock gold solutions (0.01% w/w?) diluted 1:1 with DI water, except for separation experiments where the solution is equal parts of non-diluted stock solutions.

The following table shows the concentration of the gold in the filtrate solution as a % of the stock/feed solution. All experiments were run at 10 PSI until approximately half the starting solution had passed through the membrane. Concentration of the feed and filtrate solutions was determined by measuring the absorbance peak between 510-530 nm and subtracting from a DI water control blank. (NS – No signal above background)

wafer# gold size filtrate conc %
SC225 5 nm 87%
SC225 10 nm NS
SC225 15 nm NS
SC199 5 nm 90%
SC199 10 nm 54%
SC199 15 nm NS
SC199 20 nm NS

The numbers suggest that the small species is passing nearly as fast as water molecules and experiencing only minor resistance from the membrane. It also demonstrates the nearly absolute cut-offs.

Below is a new picture showing the results with SC225 (Left-filtrate, Center-Stock, Right-Retentate).

img_3898

For some reason I had great difficulty using the Malvern to measure sizes of 5 nm particles. In multiple instances the 5nm beads in the filtrate as well as stock solutions measured larger than the 10 nm particles – it appears that some larger contaminants in my solution were skewing the measurements since 5 nm clearly made it through our membranes based on absorbance. For this reason I do not have a similar plot to the 10/15 cut-off from SC199 (see below for a re-post)

gold_separation_labeled1

For SC225, water permeability was measured at 4 cc/cm2-min-bar (DI water Flow rate is 11.25 uL/min at 10 PSI in our two-slot SepCon device).
The flow rate reduces approximately 1/3 when introducing 5 nm particles that pass through the filter.
The flow rate reduces approximately 1/2 when introducing 10 or 15 nm particles which are completely retained.
The flow rate was approximately 1/2 when in the presence of both 5 and 10 nm particles.

In the four experiments for SC225 (5, 10, 15 and 5/10 nm gold), I collected all of the filtrate and retentate. I measured absorbances and calculated mass and compared to the starting solution. On average I recovered 98% (SD=6) of the starting mass.

The mass transport for both SC199 and SC225 is between 2-5 mg/hour for the small species in our 2-slot SepCon device. If you recall the Martin news piece in Nature Materials (2004) on the pore within pore nanomembranes, he commented that those membranes would only transport 1g of material over 1 year with 100 square cm of area. Our transport rates suggest we would transport nearly 1g per MINUTE (per 100 cm2 active area).

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