Quartz Crystal Microbalance (QCM) is one of the piezoelectric material common for sensor application in either gas or liquid phase. By immobilizing a thin layer of bio-element on QCM surfaces, it is possible to develop a biosensor suitable for many applications. Interaction between bio-element and its associate materials change the QCM frequency. To improve the quality of the biosensor, a biocompatible material such as Reduced Graphene Oxide (rGO) need to be deposit first. This paper develops a simple technique that immobilize rGO material on the surface of QCM. rGO solution on DMSO with spesific concentration was prepared and then sprayed to the QCM surfaces for several time duration. After drying process, frequency characteristics of the coated QCM was measured. For the measurement, purpose a 24-bit FPGA-based frequency counter with 1 Hz resolution has been designed. Measurement results shows that proposed coating method suitable for biological element immobilization.