PSi and pnc-Si Degradation

Anant and I found a protocol in Biomaterials a few weeks ago that allows colorimetric determination of silicic acid.  It was used to track the degradation of PSi samples in Tris-HCl buffer.

I optimized this protocol and got standard linear curves, as reported in the protocols section of the blog here.  Here, I report the results from my first experiment.

I measured the mass of various samples, placed them in wells of a 24-well and then added 1.5mL PBS to the samples.  I left the samples at room temperature, but to speed things up, I moved the samples to the 37C oven after 7 days.  Silicic acid concentrations were calculated once a day by taking 40uL duplicate samples per well.  The umol of Si was calculated and then normalized to the sample mass.

Samples:

1. Porous Silicon (PSi) – from Lisa DeLouise
2. Silicon wafer (from VA Semi)
3. Silicon wafer with 50nm AJA oxide, 25W bias, RT deposition
4. w606 pnc-Si
5. SC100 pnc-Si
6. Plastic retention ring (negative control)
7. PBS (blank)

This assay clearly detected PSi degradation.  I think the PSi signal decreases because it was completely degraded after 3-4 days and then I was removing signal from the wells for the next 2 samples.  Here is the same graph but zoomed to show low y-values.

The retention ring, made of plastic, floated ~0 umol Si for the entire experiment.  The pnc-Si samples (w606 and SC100) didn’t seem to degrade until I placed them in the oven.  On day 6, they were not discolored.  On day 8, after 1 day in the oven, they were completely discolored and it looks like the signal increased significantly from day 6-8.  Ditto for the Si wafer and the Si+oxide wafer.

So, this assay certainly works for PSi. I’m not sure why the signal for pnc-Si is so much lower than pnc-Si since these values are normalized to sample mass.  It could be that pnc-Si is not degrading into silicic acid.  Alternatively, the the 15nm pnc-Si film may degrade but not release lots of silicic acid before the oxide and bulk silicon degrade – I’m repeating this but I’ll take pictures to track discoloration and take more post-discoloration samples.