Embodiments herein relate to chemical sensors for detecting a physiological analyte. In an embodiment, an implantable medical device including a chemical sensor for detecting an ion concentration in a bodily fluid is provided. The chemical sensor can include a sensing element having an outer barrier layer forming a top, a bottom, and opposed sides, where the top of the outer barrier layer can be created from a polymeric matrix permeable to sodium ions, potassium ions, and hydronium ions. An active agent can be disposed within the top of the outer barrier layer, the active agent having antiinflammatory effects. The chemical sensor can include an optical excitation assembly configured to illuminate the sensing element. The chemical sensor can also include an optical detection assembly configured to receive light from the sensing element. Other embodiments are also included herein.