Human Exhalation CO₂ Sensor Based on the PEI-PEG/ZnO/NUNCD/Si Heterojunction Electrode

Abstract

Gas sensors based on semiconductors have outstanding sensitivity compared with the oxide-based devices; however, the high operation temperature greatly hinders its development in practical applications. Chronic obstructive pulmonary disease (COPD) is one of the leading causes of death worldwide, and the patients with severe COPD with or without exacerbation tend to have airflow obstruction, which results in an increase of CO 2 concentration and subsequent hypercapnic respiratory failure. At present, COPD detection relies on professional operation; however, the patients suffer great discomfort during the arterial blood sampling. All these facts reduce patient’s willingness to test their physical health. Thus, noninvasive monitoring of CO 2 levels is crucial for the early diagnosis of high-risk COPD patients. A nitrogen-incorporated ultrananocrystalline diamond (NUNCD) film exhibits excellent properties in biosensing and polyetherimide-polyethylene glycol (PEI-PEG) polymer possesses a great capability of CO 2 capturing. By incorporating NUNCD into PEI-PEG film, this work focuses on ameliorating the sensitivity and selectivity of the present semiconductor CO 2 sensor. From the theoretical regression analyses of the experimental results, it is found that the excellent performance of the PEI-PEG/ZnO/NUNCD/Si electrode is contributed by two main reaction layers, the adsorption layer (PEI-PEG) and the electric transfer layer (ZnO/NUNCD). The selectivity is dominated by the PEI-PEG adsorption layer and the sensitivity is directly related to the changes in the work function of the ZnO/NUNCD interface. The high aspect ratio (>10) of the flower-like ZnO structure, growth from ZnO nanoparticles, can provide a more active adsorption area, as a result, extremely enhancing the sensitivity of the CO 2 sensor.

Noninvasive Salivary Sensor Based on Ferrocene/ZnO/Nitrogen-Incorporated Nanodiamond/Si Heterojunction Nanostructures for Glucose Sensing in Neutral Conditions

“…In the impedance measurements, the semicircle diameter indicates the electron transfer resistance ( R ct ) that controls the electron transfer kinetics of the redox probe at the electrode interface. − The R ct values of NUNCD/Si, b-ZnO/NUNCD/Si, and FC/b-ZnO/NUNCD/Si are 238.1, 134.5, and 140.8 Ω, respectively (Nyquist plot not shown). The R ct was decreased when the b-ZnO was coated, indicating that the efficiency of electron transfer is improved, further verifying the capability of the heterojunction structure . However, the R ct increased slightly due to the poor conductivity nature of FC.…”

“…This phenomenon is attributed to the junction of two semiconductor materials with different energy gaps, resulting in reconciliation at the interface. Thus, the onset energy of secondary electrons and the Fermi energy level were changed, which can improve the current density and sensitivity by reducing the work function . Therefore, the structure of FC/b-ZnO/NUNCD/Si was selected for further investigation.…”

“…Thus, the onset energy of secondary electrons and the Fermi energy level were changed, which can improve the current density and sensitivity by reducing the work function. 26 Therefore, the structure of FC/b-ZnO/NUNCD/Si was selected for further investigation.…”

“…In our glucose sensing system, the FC/b-ZnO/NUNCD/Si electrode with liquid GOx can directly sense glucose in neutral conditions due to high sensitivity and high selectivity of the electrode because of the high specific surface area and direct electron transfer ability. 26 In addition, the FC, acting as a mediator, performs a redox reaction to transfer the electron from the electrolyte to electrode. During the glucose sensing process, two half-reactions from the solution and electrode were proposed in this system.…”

“…A glucose biosensor using nanoscale materials as the primary materials could effectively provide many advantages in glucose sensing. , A nitrogen-incorporated ultra-nanocrystalline diamond (NUNCD) film exhibits excellent biosensing properties, with high surface area and conductivity; it also combines good biocompatibility and chemical and electrochemical stability. , All of these features fulfill the requirement of a superior sensor. Zinc oxide (ZnO) is a promising metal oxide for use in sensors owing to its good electrical properties, low cost, large surface-to-volume ratio, reliable performance, and wide band gap of 3.37 eV. − Ferrocene (FC) is one of the most efficient electron transfer mediators that can be incorporated into sensors to increase the redox cofactor for charge transfer between glucose oxidase (GOx) and the sensing electrode. − GOx is commonly used to improve glucose sensing performance as it catalyzes the conversion of glucose to gluconate. Furthermore, the incorporation of GOx during glucose detection can also improve the selectivity because of the specificity.…”