Citation of a book chapter that was put online
I would cite this as a book section, including the DOI (because that's where you found it/how others will be able to find it) but not including &quot;date accessed&quot;, because that is intended for more volatile resources like web pages. (If a reference has a DOI you can usually treat it as non-volatile.)
Sticking as precisely as you can to a reference format is important, but the most important thing is to make sure that the information is present that allows readers to (1) attribute credit appropriately and (2) re-locate the original source.

(1) Carver, A.; Gallicchio, V. S. Heavy Metals and Cancer. In Cancer Causing Substances; Atroshi, F., Ed.; 2018. https://doi.org/10.5772/intechopen.68247.