Any standard buffer is allowed (i.e. PBS,Tris, HEPES, etc.). If specific additives are known to be necessary for sample stability, these should be present in the minimum amount possible. The dilution buffer composition can be your formulation buffer, but preferably without sucrose, glycerol, trehalose (or other cryo-protectants), or high salt. These components negatively affect the particle contrast In the images. If you cannot get around adding any of these, NIS would suggest keeping the cryo-protectant below 2% and the salt concentration below 500mM.