Patent ID: 7135342

Claim:
An electro-optical sensing device for detecting the presence of an analyte in a medium, said sensing device comprising: a. a radiation source that emits radiation; b. a first and a second photosensitive elements configured to receive radiation and output an electrical signal in response thereto; c. a first and a second indicator elements positioned to receive radiation from the radiation source and to transmit radiation to said first and second photosensitive elements, said first and second indicator elements each containing indicator molecules with an optical characteristic responsive to the presence of an analyte; and d. a hermetically-sealed chamber covering said second indicator element so as to render said second indicator element insensitive to the presence of the analyte in a medium external to the device, wherein said first indicator element is in fluid communication with said external medium and said second indicator element is not in fluid communication with said external medium, and wherein said hermetically-sealed chamber contains an analyte-containing fluid in contact with said second indicator element.