KR2‐Hydrogel Opto‐Iontronic Platform for Na ⁺ Sensing Over 0.001–1 M

ABSTRACT

Sodium ions (Na ⁺ ) play pivotal roles in body‐fluid homeostasis, neuronal excitability, and osmotic regulation, motivating sensing approaches that operate across physiologically relevant and high‐ionic‐strength conditions. Here, we present an opto‐iontronic sensor based on the light‐driven Na ⁺ pump Krokinobacter rhodopsin 2 (KR2). Incorporating KR2 into a hydrogel microenvironment markedly increases the measured photoelectrical response relative to neat KR2. The KR2–hydrogel sensor shows a concentration‐dependent Na ⁺ response from 0.001 to 1 M and much smaller responses to K ⁺ and Ca ^{2 +} in separate single‐salt measurements. The upper limit of 1 M is treated as a high‐ionic‐strength proof‐of‐concept condition rather than a typical physiological concentration. Microscopy, impedance, and photocurrent measurements establish morphological and electrical differences after hydrogel incorporation, but do not by themselves identify a unique microscopic transport mechanism or a net KR2 orientation. These results demonstrate a proof‐of‐concept coupling strategy between a protein ion pump and a soft ionic matrix for low‐bias Na ⁺ sensing; mixed‐ion and real‐sample validation remain necessary for biological or continuous‐monitoring applications.