I recommend EEVBlog’s OpAmp tutorial. His explanation is pretty simple to understand. Basically there are two rules (note these rules are ideal, but the exceptions can usually be ignored):
- No current flows into the inverting and non-inverting inputs.
- For negative and positive feedback circuits, the OpAmp wants to keep the inputs the same by changing its output, and will sink power to its positive or negative power rails to achieve this.
I work with a lot of particle detectors. Instead of this, I recommend a photo diode and laser diode. Light dispersal particle counting is relatively easy to pull off by comparison to scraping off coating and all that. You could even use a very small segment of solar panel and a light source or an IR detector and IR LED similar to a smoke detector.