I have been thinking about how an app might do that and I just don't have any idea how it would work. You said
I was prompted a few weeks ago when I saw a TV ad for an app that works with an iPhone.
In the commercial it looked easy ... you simply put the negative against the iPhone.
but putting a negative, for example, on an iPhone would do what? You would have to put it over one of the camera lenses to take a picture of it, then process it from negative to a positive (not just invert the colors, it's more than that). And any negative or slide would need to be evenly lit from behind or there would be dark areas. And you can't get too close to the camera or it won't focus properly. Even the macro mode needs a few centimeters gap. About all I can think of is maybe a light box, with a stand to hold the iPhone over the slide/negative and then somehow take the picture without disturbing the iPhone, stand, or negative/slide.
Basically, that is what the plustek, and other, scanners do. They hold the image over a scanner that scans at very high resolution with consistent lighting, then the accompanying software does the conversion from negative to positive and cleans up the speckles from dust, light scratches, etc.
Anyway, good luck with it. I tried many things before I went with my dedicated scanner. It paid off in the end in better images and much, much less frustration.