We have investigated the effect of 3-mercaptopropyltrimethoxysilane (MPS) concentration, water content, and pH on the thickness and insulation quality of a MPS coating layer on a gold electrode obtained in an MPS solution in ethanol. We showed that controlling the MPS concentration, water content, and pH were all important for the insulation coating. With 1% MPS and 0% water, the coating rate remained well below 1 nm/h at all pH and at 2 and 8 nm/h in 0.1% MPS with 0.5% water at pH 9 and in 1% MPS with 1% water at pH 9, respectively. Furthermore, the surface of the MPS coating obtained with 0.1% MPS and 0.5% water at pH 9 was smooth for optimal binding surface stress induced detection resonance shift in a piezoelectric plate sensor. The width-mode resonance frequency of a piezoelectric plate sensor insulated with a MPS coating obtained in 0.1% MPS with 0.5% water at pH 9 for 36 h was shown to be stable for more than 24 h in a phosphate buffered saline solution.