Electrolytes play crucial roles in biological functions including but not limited to activating nerve impulses and regulating muscle, heart, and kidney functions. The imbalance of electrolytes in the body can lead to cardiovascular, neurological, ocular, and muscular dysfunctions. Thus, monitoring of electrolytes levels is important for disease diagnosis and monitoring therapies. Photoluminescent single-walled carbon nanotubes (SWCNTs) exhibit outstanding potential in developing molecular probes for optical detection systems. Surface coatings on carbon nanotubes could facilitate effective molecular interactions enabling targeted nanoscale probes. Here, we report development of carbon nanotube-based optical sensors to detect target electrolytes in biological systems. Photoluminescent SWCNTs were encapsulated within synthetic polymers that present electrolyte recognition motifs along the polymer chains. The resulting polymer-SWCNT complexes allowed optical detection of electrolytes at nanomolar range.