I am trying to avoid any chemicals in my lysate of transfected HEK cells because I intend to use the supernatant of the lysate for ELISA. Therefore I wonder if I can use water or PBS alone for cells lysis.
To scrape them -maybe- as HEK293T cells are not the most attached cells but you need to lyse them either chemically by detergents or physical by sonication
Cell lysis is easiest after swelling them, for example, add 9 volumes of dd water to one volume of cell suspension in PBS. Keep them on ice for 30 min, then give them a few strokes with a Dounce or Potter homogeniser. Ball bearing homogenisers (e.g., http://isobiotec.com/cell-homogenizer.html) also work very well. With some cells, it is even sufficient to pipette them quickly (check with a microscope).
You can use PBS along with mechanical disruption methods. One ease way is to use a blunted 25G needle and push through 10-15 times, you could even introduce a freeze thaw cycle as well. Spin and check supernatant for protein to ensure lysis. Add protease inhibitors to PBS.