You need a CAPACITIVE sensor. What have you got?
See page 12 here
https://cdn.sick.com/media/docs/4/44/944/Product_overview_PROXIMITY_SENSORS_PRODUCTS_AT_A_GLANCE_en_IM0067944.PDF
Or
https://www.ifm.com/us/en/category/010/010_020/010_020_010#!/S/BD/DM/1/D/0/F/0/T/50