In this work spontaneous oxidation of Prussian white (PW), which is completely reduced form of Prussian blue (PB), has been investigated at ambient conditions and stable form of PW-PB as an intermediate oxidation state based on the presence of both redox forms (PW and PB) was identified and investigated. Two types of optical slides were manufactured bymodifying optically transparent indium tin oxide (ITO) coated glass electrode (glass/ITO) (i) with PW layer (glass/ITO/PW optical slide) and (ii) with PW-PB layer (glass/ITO/PW-PB optical slide). Further modification of the developed optical slides with glucose oxidase (GOx) showed that glass/ITO/PW-PB/GOx optical slide was insensitive toward glucose, but glass/ITO/PW/GOx optical slide acted as glucose biosensor and showed linear sensitivity toward glucose in the concentration range from 20 mu M to 200 mu M (R-2 = 0.99). (C) 2019 The Electrochemical Society.