I couldn't find such narrow tubing, but you might be able to use this device:

https://www.thermofisher.com/us/en/home/life-science/protein-biology/protein-purification-isolation/protein-dialysis-desalting-concentration/dialysis-products/slide-a-lyzer-mini-dialysis-devices.html

Here are alternatives to Adam's suggestion but also no

The Slide-A-Lyzers suggested above are excellent for dialysis and retaining sample. Additionally, I've been very happy with Pur-A-Lyzers from Sigma, which come in several different sizes and are simple to use.

http://www.sigmaaldrich.com/labware/labware-products.html?TablePage=108679809

Thank you for all your suggestions! Unfortunately, none of these products will fit into the vessel I will be using, which is a circular well about 1 cm in diameter and less than 1 cm deep. I will have to keep looking for small-diameter classical tubing, or make one from a larger dialysis tube.

Have you tried capillary flow dialyzers?. They are full of small cellulose capillaries that can be the right size for your applications.

Good luck

Jaime Llodra

Thank you Jaime, this is exactly what I was looking for!