Biocompatible, high refractive index medium for 780 nm?

Question

I am trying to find a medium for optics experiments on living cells. I have been using yeast cells, saccharomyces cerevisiae and saccharomyces boulardii, and need a medium with refractive index (RI) greater than $1.4$ at a wavelength of $780 \,\pu{nm}$ (and temperature $25^\circ \pu{C}$) and in which the cells are still able to live and reproduce. I have been trying to mix their YPD medium with substances such as polyethylene glycol (PEG) and glicerin at different mass fractions, but to no avail: either the cells died or the RI was still not high enough. The “tuning” substance would need to have a RI larger than $1.4$ because it would need to be mixed with YPD (which has basically the same RI as water), and it also would have to be hydrophilic to some degree (preferably shouldn’t be too viscous, but that can be overlooked). Does anyone know about other alternatives? I have heard of iodixanol through microscopy papers, could this work?

Answer 1

First the refractive index of PEG varies by molecular weight and with time, as it gels. You might try using low-MW PEG, and letting it stand mixed with the YPD culture media. It is one of the most innocuous chemicals.

Your thoughts on trying Iodixanol is certainly reasonable, and supported by the literature, but you'd have to test it with your yeast. Actually, that would be a good subject for a paper to help others using high-resolution optical (well, near-IR, possibly barely visible to dark-adapted eye) microscopy, and if you do publish it, make that an answer to replace this, and add citation to it!

You could also try some other liquids to increase the R.I., but likely a high concentration would kill yeast by osmosis, if not outright poisoning. Here are some hydrophilic fluids that might be relatively nontoxic. BTW, the metabolic products of ethylene glycol make it poisonous to mammals, but I don't know about yeasts.

See table for R.I. of glucose/water mixtures.

R.I.  Chemical
1,463 Ethylene Glycol
1.433 Propylene Glycol
1.421 Propylene Carbonate
1.473 Glycerol 
1.400 Glucose 40%, 60% water