Drift-Minimized, Highly Sensitive Carbon Nanotube and Amine-Based CO ₂ Sensors with Humidity Compensation for Indoor/Outdoor and Breath Monitoring

Reliable, real-time CO₂ detection under varying environmental conditions remains a major challenge in developing low-power, stable, scalable, and flexible gas sensors. Among solution-processed flexible sensors, signal drift, humidity sensitivity, and limited long-term stability hinder practical deployment. Signal drift is often overlooked when reporting highly sensitive flexible CO₂ sensor materials, yet it is a critical factor for real-world sensor deployment. In this work, we report a drift-minimized, humidity-compensated CO₂ sensor based on COOH-functionalized single-walled carbon nanotubes (COOH-f-SWCNTs) functionalized with a synergistic Polyethyleneimine-Diaminonaphthalene (PEI/DAN) composite. The introduction of DAN reduced baseline drift, enhanced film homogeneity, and stabilized the amine environment, improving CO₂ detection. To ensure functionality under high humidity (>60% RH), a Nafion top layer enabled dual-mode sensing via resistance and capacitance changes. XPS, FTIR, Raman, and SEM confirmed the successful functionalization and structural integrity of the fabricated sensor. The devices exhibited stable responses across 500-2500 ppm CO₂, with a sensitivity of 2.67 × 10^-3 ppm^-1, a response (Δ<i>R</i>%) of 7.5% at 2500 ppm under 30% RH, along with excellent linearity, repeatability, and batch-to-batch reproducibility. Time-dynamic analysis further revealed that while resistive recovery slowed under low and high RH, the Nafion-based capacitive channel maintained rapid and consistent response/recovery (1-3 min), underscoring the benefit of dual-mode sensing. Real-world validations, including indoor-outdoor monitoring and breath sensing, demonstrated the robustness and versatility of the platform. This work uniquely addresses drift, introduces DAN as a stabilizing agent, and highlights the role of Nafion in enabling reliable dual-mode operation, advancing flexible CO₂ sensors toward practical deployment for health and environmental monitoring.