I want to switch PhD programme I started a PhD programme 6 months ago.  During the selection process you were asked which areas would u like to work in. My studies specialization is in microbiology and molecular biology, I clearly stated that i wanted to carry on my research in food microbiology because I carry out my master thesis in this area. What is more, after the interview I was contacted by a professor working in this area because my profile was totally fitting the PhD they were looking for.

However, a big professor was also interested in me (the rector of the university) and directly contacted me to work with him in biosensors applied to food. I tried to go back to my interest but I found the door closed (some matter of power around...). Despite of this, I though biosensors sounded very interesting and I would enjoy it.

After the PhD kick-off I found out that we did not share the same definition of a biosensor.. they were meaning just sensors made with organic polymers. I was told to do a PhD proposal in a short time of a topic I had no knowledge in.

Nowadays, my supervisor does not follow me and they group is so new that there is only a postdoc researcher who is an engineer, we have nobody from biotech,food science or a chemist who works with sensors in the "bio" part. I am totally alone in my research in a  field I do not know. I told my supervisor and he told me to look for somebody who help me in the literature to help me. what the .....?? I really fear i do not manage to finish this PhD successfully.

I started looking for another PhD opportunities and I found one which I totally felt in love with. I would like to write the professor in advance but I am not sure whether I should mention that I am already a PhD student. 

What would you do? In the applications I am doing I am commenting I am a PhD student that does not have interest nor the background in the area I was assigned..

Thank you,
Yes you can change if you want. Your advisor will probably be unhappy about this, but you have to do what's best for yourself. 

And yes you can tell other professors that your position is not what you thought it would be, so you are looking to change. Yes it won't be the best story for them to hear, but it isn't universally poison either. I presume you don't exactly want them to ask for a reference from your current employer (on that note, probably should be quite careful your current advisor doesn't find out until your plan is set). 

But finally consider that many (if not most) people probably feel overwhelmed, like an imposter, in over their head, etc. when they're six months into grad school. And it's probably never exactly what they expected to be doing. So just be sure this particular position is really the whole problem.