The authors demonstrate a novel method for the stable and selective surface functionalization of germanium (Ge) embedded in silicon dioxide. The Ge(001) surface is functionalized using α-lipoic acid (ALA), which can potentially be used for the immobilization of a wide range of biomols. The authors present a detailed pH-dependence study to establish the effect of the incubation pH value on the adsorption layer of the ALA mols. A threshold pH value for functionalization is identified, dividing the examined pH range into two regions. Below a pH value of 7, the formation of a disordered ALA multilayer is observed, whereas a stable well-ordered ALA mono- to bi-layer on Ge(001) is achieved at higher pH values. Furthermore, the authors analyze the stability of the ALA layer under ambient conditions, revealing the most stable functionalized Ge(001) surface to effectively resist oxidation for up to one week. The authors' established functionalization method paves the way towards the successful immobilization of biomols. in future Ge-based biosensors. (c) 2018 American Institute of Physics.