I am looking for a material which is fully transparent to X-rays.
In addition, it should be solid enough to serve as a substrate on which I will deposit some biomolecules.
X-rays do not cast image of low density materials. For example, when X-ray photo is aimed to see whether there is any bone fracture in the case of an accident, soft tissue all around bone becomes transparent and unclear. The high density bone is seen clearly. Low density renal stones (stones in kidneys) are transparent to X-rays so not visible in X-ray picture.