Maybe this discussion could help

https://www.researchgate.net/post/Coating_of_samples_for_SEM-EDX?

Platinum is better (for higher magnifications). Au-Pd alloy also is better than pure gold.

Stefan Baunack, Thank you for sharing the link.

Vladimir Dusevich Thank you. Could you please clarify more on 'high magnification'? specific range?

In term of grain size, platinum-iridium alloy or just iridium is the best one. But even a standard gold target is fine (2-3 nm grains) for most of biological applications. For example, see the Figure 2C here:

https://www.thelancet.com/journals/ebiom/article/PIIS2352-3964(22)00226-2/fulltext

The sample was coated with just gold, using a really old and simple coater (SCD 005, BAL-TEC).

Good luck!

As the per the answers given by other people, it depends on the magnification you are going for. The purpose of coating is to increase conductivity to prevent image charging when observed under vacuum. Do take note that Au have the best conductivity and the conductivity drops as you go down the list. Meaning, it may cause charging and adjustment need to be made (not an issue if you can use low vacuum mode - it have became quite widely available for newer SEM and FE-SEM).

However, there is no hard and fast rule on which one to use. The magnification values given below are just a rough guide (depends on how big the feature you need to observe - microns or nanometer) as it also depends on the resolution of the SEM/FE-SEM you are using.

Types of coating that are usually use are as follow.

Carbon - This is suitable for EDS analysis. Grains might be quite large and can be visible but does not affect the EDS results as much compared to other types of coating. Not suitable for high resolution observation as grain size is very big. But these days, I do compromise by using the same coating and timing so that comparison can be made between two samples. Not necessary for carbon coating unless accuracy of the EDS is very important.

For observation/imaging:

Gold (Au) - Best conductivity. A few hundred to 10-20k magnification

Au-Pd - smaller grain size compared to Au.

Pt-Pd - smaller grain size than AU-pd. Can be visible if your FE-SEM's resolution is high (newer ones or cold FE type gun) visible at 50k magnification or above.

Pt - Even smaller grain compared to Pt-Pd but lower conductivity compared to Gold. Suitable for 50k to 400k magnification (maybe can push for 500k).

Osmium (Os) - Amorphous, therefore no grain size as it has no crystalline structure. Use for Ultra high resolution analysis (400k to 1000k magnification). The only problem is that Os is toxic.

Denis Korneev Thanks you

In general, coating is not a big deal for biological samples. If one has such a microscope, which is available to see the difference between 1.5 nm and 3 nm grain size, he definitely has a proper coating equipment and protocols. Practically, there are plenty of more important things in biological SEM imaging, e.g. fixation, drying, imaging parameters, etc.

The coating material itself is not a big deal too, there are some more important issues:

1. Optimal thickness. The thinner one is not necessarily the best one. Indeed, if the conductivity is not good enough, the charged areas can completely spoil the image;

2. Conductivity of the substrate. It's always better to deposit your cells on a conductive substrate. Then the final coating can be much thinner. The easiest way - gold-coated pieces of glass slides, in addition a fresh gold coating improves adhesion of the cells;

3. Shadowing. It's not important for such small objects as bacteria, but it's getting critical when you working with bigger objects. That's why almost all modern advanced coaters have tilt and rotation of the stage.

With a proper design of sample preparation even the oldest and primitive "manual" coaters with a conventional gold target are fine for biological imaging with ultrastructural resolution. For example, this work:

https://www.frontiersin.org/articles/10.3389/fcell.2021.672592/full

was done using a really old one. On the photo.

Good luck!