They are expensive, and it doesn't have a built in sensor, but I have an IQAir healthpro compact and have confirmed with my own particle detector that it filters out > 99% of PM0.1 particles, and 100% of PM2.5 particles.

If you don't care what it looks like, you can easily build your own air purifier for much cheaper. All it is is a blower fan which forces air through 2 or 3 filters, with the last being a HEPA filter. You can get all the requisite pieces for about $100.