The present invention provides a method for coating metal oxide on a PDMS surface. The method includes preparing a mixture that contains a sol-gel precursor, reacting the mixture to form a preconverted sol-gel precursor, where the preconverted sol-gel precursor does not diffuse into PDMS and is not in the form of a gel, forming a reactive PDMS surface, applying the preconverted sol-gel precursor onto the reactive PDMS surface, binding the preconverted sol-gel precursor to the reactive PDMS surface, and converting the bound preconverted sol-gel precursor to a metal oxide to form a metal oxide coating on the PDMS surface. The present invention also provides a PDMS microfiuidic device where one or more channels of the microfluidic device is provided with a metal oxide coating covalently bound only on the surface of the one or more channels.