Formaldehyde Sensing in Mixed Gas Environments Using Co3O4-Encapsulated ZnO-Based Yolk–Shell Spheres for Breath Analysis

Exhaled breath (EB) contains rich molecular information that can provide insights into an individual’s health. Clinically relevant molecular analytes, such as volatile organic compounds (VOCs), are exhaled in the form of gases, aerosols, or droplets. Lung cancer is a chronic disease characterized by dyspnea and respiratory failure and can be associated with formaldehyde. In this study, formaldehyde was considered as a biomarker of lung cancer. Herein, a chemi-resistive formaldehyde gas sensor based on Co₃O₄ nuclei-encapsulated ZnO-based yolk–shell spheres (ZnO-Co₃O₄ YSSs) was fabricated. This sensor could distinguish the EB of healthy individuals from those of patients, people, and the simulated EB of patients. Physical models based on density functional theory (DFT) demonstrate that the adsorption of formaldehyde molecules is critical for the sensor’s sensitivity. Additionally, the formaldehyde concentration in the mixed gases can be predicted, using the Extreme Learning Machine (ELM) and Multi-Layer Perceptron (MLP), showing potential for detecting formaldehyde in complex expiratory components.