The only passive 2.45 GHz tag on the market of which I am aware is Hitachi‘s µ-chip (pronounced “mu-chip”), which is very tiny and has an antenna etched onto it (see Hitachi Unveils Smallest RFID Chip, Hitachi Unveils Integrated RFID Tag and RFID News Roundup: Hitachi Shrinks Smallest RFID Chip). The µ-chip is designed for a very short read range (just a few millimeters), and could not support a temperature sensor.
A company called SCS used to manufacture 2.45 GHz passive tags and readers. I know someone who has about 45,000 tags and four reader-writers with antennas available. These tags do not have sensors, but if you are interested, e-mail your request to
firstname.lastname@example.org and I will put you in touch with him.
—Mark Roberti, Founder and Editor, RFID Journal