Whispering - Gallery - Mode type microresonators provide great opportunities for label - free biosensing, allowing detections down to single - molecule levels. Microtoroids as optical resonators are quite sensitive and preferable biosensors due to their high quality factors. However, their surface design should be carefully considered for a selective biodetection. For this purpose, studies on WGM type biosensing for DNA, RNA and protein detections using various surface modifications are herein summarized. Over and above, this thesis mainly focuses on the microfabrication and surface modification of the microtoroids for various selective biosensing purposes such as antigen detection in complex media or detecting single base pair DNA alterations in buffer. With this regard, the developed microtoroid surface modification for dual characteristics (anti - fouling property and bioconjugability) is described in detail. The dual surface approach, used for selective Interleukin - 2 and Exotoxin - A detections in complex media, is applied to the microtoroids. Biosensing in complex media is a challeging task to perform unless the calibration approach is used. To overcome this challenge the suggested surface modification approach for nano - molar level detections is explained in detail. Besides, a novel surface modification approach for a selective single - stranded DNA detection based on discriminating single base pairs is portrayed as well.