Tile can be laid over a variety of surfaces, but some work better than others. Plastic tiles (e.g., vinyl units) fit well over plywood or even over the original tile, and with a little extra effort, you can also install these types of tiles over concrete or cement floors. The goal is to create a permanent bond between the tile and the flooring in a perfectly smooth, level pattern.
While you can seal particleboard with a normal sealant, you should probably avoid using sealers on particleboard. In fact, you should avoid using particleboard altogether when tiling. Particleboard is literally made up of wood particles that are compressed and bonded together. It tends to expand and contract over time, changing with air, heat and moisture. This provides a very undependable surface on which to place any kind of tile, where any kind of uneven structure or disturbance can ruin the tile pattern.The first step professional tile installers usually take is to rip up particleboard and replace it with plywood, real timber beams that provide the necessary backing for the tile.
If you have no choice but to build over particleboard, then use some kind of cover such as a plastic sheath or laminate that will protect against moisture that might seep up through the particleboard. Applying a sealant straight to the particleboard will not be as effective and may even cause the particle board to degrade faster.
If you can't take out your particleboard, then the next best thing is to put some other type of support over it before tiling. Subfloor timbers work well, provided that you have a secure place to attach them to the particleboard---preferably to joists underneath the particleboard itself. If you do want to use a sealant, make sure that it's a thick, clear version that will harden into a protective surface; be aware that moisture will always create problem with particleboard, even when it is sealed.
When tiling over most types of surfaces, you should consider using a leveling compound before arranging the tile. Even the smoothest of woods can have problems, including nails, knots and slight warps in the grain. The leveling compound will provide a smooth epoxy base on which to bond the tile effectively.
If you can't use a leveling compound, make sure the wood is as clean and smooth as possible. Sand it down well, measuring all the boards with a level and washing away any oil or dirt. Vacuum even the slightest trace of dust, and make sure the wood is completely dry before tiling. Make sure no nails are showing and that there are no cracks from which moisture can escape.