There are about as many protocols to prepare coverslips as there are ways to make tuna casserole. You can spend from 5 seconds to 2 days, depending on what your lab prefers. But in the end, what’s really needed? I’ve tried many protocols over the years and I’ve questioned some steps. Here I share my opinions on some coverslip preparation techniques – from the seemingly absurd to the down-right reasonable to help you to separate the helpful from unjustified traditions in your experiments.
Acid washing is often recommended by the manufacturers of coverslips, such as Corning, because it is not only reported to enhance the cleanliness of the coverslip but the acid also etches the glass making it a better substrate for cellular attachment. And while I have done it in the past, many people skip acid washing their coverslips. For the most part acid washing is a step that can be reserved if quicker methods of cleaning and sterilizing aren’t working for you, or if you are having attachment problems.
I was surprised to find this step in protocols, but with the advent of sonicating toothbrushes I see how people use the logic that sonication scrubs. But I find it hard to believe that sonicating is going to remove anything significant from an otherwise clean and polished coverslip (comment below if you have evidence otherwise). So I’d skip sonicating unless you are desperate to get rid of some sort of observed particulates not removable by other means.
Phosphate Buffered Saline versus water.
Some people recommend rinsing your coverslips in PBS instead of water. And while the coverslip does appear to coat better when there is salt in the water, I do not use PBS. Instead I use pure water because I do not want the possibility of salts drying onto the coverslips, since salt could carry over and cause problems in future steps.
Many labs consider autoclaving a necessary step for complete sterilization. In this method, typically after various washes, coverslips are placed in a glass petri dish and sent through the dry cycle of the autoclave. The upside of autoclaving is that you are sure your coverslips are tissue culture sterile, which is necessary if you plan on growing cells on these. The downside is that autoclaving takes time. Also, don’t autoclave fancy commercially-purchased precoated coverslips.
70% ethanol is frequently used for sterilizing coverslips. However, for sterilization ethanol is inferior to autoclaving, therefore it is often followed up by exposure to UV light or flaming. Doing both may still be faster than autoclaving and the subsequent cooling. Remember though, if you choose to use this method, some microorganisms can persist in 70% ethanol, so do not reuse your alcohol.
While ethanol sterilization is typically performed at a concentration of 70%, flaming is often done with a higher concentration of ethanol, such as 90% (although 70% will still catch on fire). Flaming is nice because it is super-fast and better at eliminating spores and other hard-to-disinfect microorganisms than ethanol alone. However, because one of the most dangerous hazards in the laboratory is the Bunsen burner in the tissue culture hood, some universities have outright banned the practice. Not only is it dangerous due to the open flam aspect but also because some people (*cough cough* the summer undergrad) accidentally turn the gas UP till the flame goes out, thinking they have turned it off when in reality they are creating A BIG BOMB. Don’t let this happen to you, and as fun as it may be, I recommend that you just skip the flame when in the tissue culture hood.
This is my favorite, it’s quick, it’s easy and can kill just about any microorganism in its path: Yes, UV light is the way to go for fast and efficient sterilization. After cleaning your coverslips and giving them a preliminary sterilization in 70% ethanol, pop them under the germicidal UV light of your tissue culture hood for 15 minutes and voilà! you have very sterile coverslips. Incubations under the UV light vary from 15 minutes, up to 1 or 4 hours, or even overnight. The only negative to this method is that your lab mates are going flock to that blue light like a moth and will be annoyed that the hood isn’t being used for tissue culture work. They may even move your coverslips to the side and begin working, compromising all your sterility and cleanliness. So take care to coordinate and explain what you are doing to your lab mates ahead of time.
This is a tried and true method. Coating with poly-L-lysine provides a positive charge for cell attachment and is used for a wide variety of cell types. If you are working with a cell type that does not attach to glass easily, like HEK293 cells, coating your coverslips with poly-L-lysine can make the difference between having one cell left after treatment, or a million. Some prefer to use poly-D-lysine since it is resistant to breakdown by cell-released proteases. Regardless, it IS important to spend the time rinsing off excess poly-lysine to prevent its accumulation into the media which induces cellular toxicity.
As a major constituent of extracellular matrixes, collagen I, II and IV can be used to coat coverslips for a wide variety of cell types including primary cells, myocytes, chondrocytes and endothelial cells. Do be sure that you are using the recommended collagen as there are many different types, and take care to sterilize your coverslips properly.
Fibronectin is another major component of the extracellular matrix. Fibronectin can serve as a substrate for a wide variety of cell types including neurons.
As an important component of basement membranes, laminins are also a good substrate for many different cell types. They are especially well known as a substrate for neurons and in assays for chemotaxis.
Why do people bust out the nail polish and paint it around the edges of their coverslips? Some say 1) to prevent the sample from drying out 2) to prevent oxidation 3) to prevent the mounting medium from leeching onto the objective or 4) to keep the coverslip in place. It’s a mixture of all those reasons, but the reason varies depending on the type of mounting medium and the time after mounting. If you are using a mounting medium that does not harden, it might not make any difference when you seal it. But if you are using one that hardens, such as Prolong gold, and you seal it before it hardens, it may never cure properly. So make yourself familiar with the recommendations for your mounting medium, and also to check if there are any reports of the nail polish quenching your fluorophore of choice
This homemade solution isn’t toxic like nail polish and is recommended in many protocols especially those that involve live imaging. The anacronym is derived from its ingredients which are Vaseline, Lanolin and Paraffin wax. I’m not sure why anyone wouldn’t use VALAP over nail polish, unless it’s a time issue (or you just secretly like to get high off the smell of nail polish).
In theory you could use all sorts of glue to hold down your coverslip. But when using an unknown/untested epoxy you run the risk of it quenching your fluorophores or interfering with the anti-quench agents in your mounting medium. Not a risk worth taking.
That’s right, good old paraffin wax. Why not? It may harden quickly but at least it’s non-toxic and a simpler than VALAP (just melt). Now if only you had some melted on-hand, like all the time.
Do you have your own insights into coverslip preparation? If so, please share below. But let’s focus on facts and experience – not unjustified traditions.