Real-time and accurate biomarker detection is crucial for point-of-care diagnosis, food freshness monitoring, and hazardous leakage warning. Current technologies, however, face challenges in achieving this objective. Ammonia (NH3) detection is particularly important due to its relevance in various fields. Abnormal NH3 concentrations in exhaled breath indicate kidney diseases, offering a non-invasive diagnostic tool. NH3 released from food spoilage serves as an indicator of food freshness, enabling better food safety and waste reduction. In industrial settings, detecting NH3 leaks from pipelines is crucial for safety and environmental protection. Traditional methods like gas chromatography-mass spectrometry are accurate but expensive, bulky, and require specialized personnel. Fluorescent devices offer portability but lack quantitative accuracy. Chemiresistive sensors often require wired equipment, hindering on-site application, and are often susceptible to humidity interference. Existing humidity compensation methods add complexity and bulk to the system. Therefore, a real-time, accurate, portable, and humidity-independent NH3 sensor is needed. Wireless passive devices, particularly LC sensors, offer a promising solution. However, challenges remain in maintaining a stable electric field for charge transfer, dealing with high background noise, and balancing material properties for sensitivity, conductivity, and semiconductor type. Existing sensitization strategies struggle to achieve ultra-low detection limits and wide detection ranges. Humidity interference is a major concern in NH3 sensing, and current solutions introduce complexity and compromise accuracy. This research aims to address these challenges by developing a wireless LC sensor with self-humidity compensation capabilities.

The literature extensively covers portable biomarker detection across various applications, including disease diagnosis and health monitoring. Existing portable devices offer advantages in extending analyte analysis from restricted settings to wider environments. NH3 detection holds significant importance in healthcare (detecting kidney disease), food safety (evaluating freshness), and industrial safety (detecting leaks). While methods like gas chromatography-mass spectrometry offer high accuracy, their size, cost, and operational requirements limit their practical application in these scenarios. Fluorescent sensors offer portability but lack quantitative analysis, and chemiresistive sensors require wired connections and are susceptible to humidity. Existing humidity compensation methods introduce complexity. Wireless passive sensors, specifically LC devices, have emerged as a promising alternative but face challenges in sensitivity, detection range, and humidity interference. Prior LC-based NH3 sensors have not attained the ultra-low detection limits or wide ranges of wired devices. Existing strategies such as heterojunction and Schottky junction designs struggle to optimize both sensitivity and detection range, and effective humidity compensation remains a significant hurdle. This work builds on this existing literature by addressing these challenges through innovative material selection and sensor design.

This study presents a wireless LC sensor based on platinum-doped partially deprotonated polypyrrole (Pt-PPy and PPy) for NH3 detection. The sensor design consists of a square inductor (L), an interdigital electrode-based capacitor (C), and a Pt-PPy and PPy chemiresistor (R) on a flexible polyimide substrate. The working principle relies on the electromagnetic coupling between an interrogating antenna and the sensor's inductance (Ls) for energy supply and signal transmission. The NH3 sensing mechanism involves conductivity changes in Pt-PPy and PPy upon exposure to NH3, altering Rs and shifting the sensor's S11. The NH3 concentration is then quantified by wirelessly reading the S11-f curve. To enhance S11, the conductivity of Pt-PPy and PPy is chemically tuned by adjusting PPy's protonation doping, confirmed by density functional theory (DFT) calculations. To improve NH3 sensing sensitivity, transition metal doping (Cu, Pd, Pt) is used to enhance the adsorption energy and charge transfer of partially deprotonated PPy (PPy and PPy0). DFT calculations show Pt-PPy and PPy exhibits the highest adsorption energy and charge transfer, resulting in a wide detection range. The sensor’s frequency (f) is used as a humidity-transducing parameter for self-compensation. The Pt-PPy and PPy materials were synthesized using chemical oxidative polymerization with Fe3+, followed by reduction with BH4- to control the oxidation state and introduce Pt. Various characterization techniques (TEM, XPS, FTIR, Raman, conductivity measurements) were employed to analyze the material properties. The sensor's wireless transmission capability was evaluated by measuring S11 and f at different bending radii and working distances. Gas sensing performance was assessed using a custom gas sensing system, measuring the sensor's response to varying NH3 concentrations. The sensor's self-humidity compensation capability was investigated by measuring its response to NH3 at different relative humidity levels. The sensor's selectivity was evaluated by testing its response to various interfering gases. The sensor's robustness and repeatability were tested by subjecting it to repeated bending and exposure cycles. Finally, the sensor was integrated into portable systems for real-world demonstrations in food freshness monitoring (smart package), point-of-care diagnosis (POC tester), and hazardous leakage detection (leak guard).

The developed wireless LC sensor based on Pt-PPy and PPy achieved a significant 180% increase in S11air compared to a PPy-only sensor. The sensor demonstrated an unprecedented NH3 detection range of 125 ppb to 2000 ppm. The 10 Pt-PPy and PPy sensor showed the highest response to 100 ppm NH3. The frequency (f) response was insensitive to NH3, indicating that the sensing mechanism is primarily based on changes in resistance due to adsorption-charge transfer dynamics. The Pt NPs enhanced the sensitivity by 16.2 times compared to PPy-only sensor. The sensor exhibited p-type semiconductor behavior. The sensor achieved a fast response time (tres) of 19 s at 2000 ppm NH3. Self-humidity compensation was achieved by using the frequency response to humidity for calibration, minimizing the impact of humidity changes on NH3 detection accuracy. The sensor showed good selectivity towards NH3 compared to other gases, including biogenic amines. The sensor exhibited high robustness and repeatability under bending and repeated gas exposure. The sensor demonstrated successful application in real-time food freshness monitoring, POC diagnosis, and hazardous leakage detection in experimental settings.

The findings successfully addressed the research question by demonstrating a wireless LC sensor with high accuracy, wide detection range, and self-humidity compensation for NH3 detection. The wide detection range and fast response time make it suitable for various applications, including food quality assessment, medical diagnostics, and industrial safety monitoring. The self-humidity compensation mechanism eliminates the need for additional humidity sensors, simplifying the device and improving its portability. The sensor's high selectivity minimizes the interference from other gases. The results significantly advance the state-of-the-art in wireless NH3 sensing, offering a practical solution for real-world applications. This approach offers a compelling alternative to existing methods, combining accuracy and portability.

This study successfully developed a highly sensitive and selective wireless LC chemical sensor for NH3 detection, featuring a wide detection range (125 ppb to 2000 ppm), self-humidity compensation, and high robustness. This sensor shows great promise for various real-world applications. Future research could explore further miniaturization, integration with more advanced signal processing techniques, and large-scale deployment in diverse settings.

The study used a limited number of samples and a controlled testing environment. More extensive testing across a wider range of conditions and with a larger sample size is necessary to fully validate the sensor's performance and generalizability in diverse real-world scenarios. Further investigation into the long-term stability and reliability of the sensor under continuous operation is also needed. The selectivity studies included a limited set of interfering gases, and more comprehensive evaluation is recommended to assess the sensor's performance in complex gas mixtures.