You've mostly got the setup, but there are a few issues. Note, we (Ansys staff) can only give guidance so you've got some reading to do.
- The mesh is not suitable for CFD. You want at least 10 cells across the channel, ideally more and to resolve the surface in more detail. With the above it'll be hard to tell what's a mesh artifact and what's biological.
- The UDF looks OK, but check how it performs. I've looked at it, not checked the code.
- Inlet diffusion shouldn't make much difference here, but check the manual for more information.
- You may want to review diffusivity coefficients. For liquid systems you're a little off what I'd expect.