High-Precision Tattoo-Based Multiplexed Sensor for Real-Time Neurotransmitter and Electrolyte Ion Detection in Sweat

Wearable biosensors are rapidly advancing, making health and wellness monitoring accessible without specialized tools or expertise. Non-invasive fluids such as sweat, and saliva offer significant advantages due to their ease of collection, safety, and the presence of key biomarkers similar to those found in blood [1]. This study presents the development of a tattoo-based multiplexed electrochemical sensor designed to quantify neurotransmitters (dopamine, DA), electrolytes (sodium and ammonium), and pH levels in sweat. The platform integrates with a portable and customizable electronic readout system with wireless capability. The sensor is fabricated on tattoo paper with four working electrodes (WEs), a counter electrode (CE), and a reference electrode (RE), patterned in an origami design, which is then transferred to the skin. Conductive graphene ink forms the base electrode layer, while pseudo-reference electrodes are created by electrochemically depositing and chlorinating silver over the graphene. Polyvinyl chloride (PVC) insulation is applied to all electrodes except the WEs to prevent non-specific reactions. For dopamine detection, a nanocomposite of molybdenum disulfide (MoS₂) and nickel metal-organic framework (NiMOF), coated with Nafion, enables selective oxidation and amperometric detection of DA molecules amidst potential interferents. The sodium (Na⁺) ion sensor employs an ionophore-based ion-selective electrode (ISE), and the pH sensor is developed through the electrodeposition of polyaniline (PANI) nanofibers. The ammonium sensor involves a multi-step process: electrodeposition of Prussian blue (PB) on the WE, drop-casting copper hexacyanoferrate (CuHCF) and applying an ISE layer. Changes in the Na⁺, NH₄⁺, and pH levels were measured using open circuit potential (OCV). Sensor data is collected after 15 minutes of exercise, continuously using a custom miniaturized readout circuit for the next 20 minutes. Testing with artificial sweat confirms high selectivity against common interferents, validating effectiveness in real sweat analysis [2,3]. The DA sensor demonstrates a sensitivity of 87.27 μA/log(DA). In contrast, the Na⁺, NH₄⁺, and pH sensors exhibit 55.25 mV/decade, 49.98 mV/decade, and -49.31 mV/pH for Na⁺, NH₄⁺, and pH, respectively, showing near-Nernstian behavior. The detection limits (LOD) for DA, Na⁺, and NH₄⁺ are 0.089 nM, 0.082 mM, and 8.66 μM, respectively. The developed epidermal multiplex tattoo-based biosensing platform offers considerable promise for the non-invasive monitoring of multiple biomarkers in practical settings, inspiring a new era of health monitoring. It can be readily expanded to monitor additional analytes. Future developments will focus on interfacing the sensor with energy harvesting platforms and employing machine learning techniques to analyze data and differentiate various exercise activities.<br/><b>KEYWORDS: </b>Tattoo-Sensor<b>, </b>Dopamine, Nickel metal-organic framework, Graphene, Copper hexacyanoferrate, Ions, Sweat<br/><br/><b>References:</b><br/>[1] Hossain, N.I., Sharma, A., and Sonkusale, S., (2023), IEEE Sensors Letters, 7, 9.<br/>[2] Terse-Thakoor, T., Punjiya, M., Matharu, Z. et al. (2020), npj Flex. Electron<i>.,</i> 4, 18.<br/>[3] Kammarchedu. V., Butler, D., Ebrahimi, A. (2022), Analytica Chimica Acta, 1232, 340447.