Detection of COVID-19 with the naked eye using C₆₀-based sensors for isobutyric acid in exhaled air: A DFT/TDDFT study

The global COVID-19 pandemic has underscored the critical need for rapid, non-invasive, and reliable diagnostic methods. In this study, we propose a novel approach using C60-based sensors functionalized with metal dopants (Al, Zn, and Ni) to detect isobutyric acid, a volatile organic compound identified as a biomarker in exhaled breath of COVID-19 patients. Through Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations, we systematically investigated the electronic, optical, and adsorption properties of pristine and doped C₆₀ structures and their complexes with isobutyric acid. The results reveal that Zn-doped C60 (ZnC₅₉) outperforms other candidates due to its smallest energy gap (1.42 eV), highest chemical softness (0.70 eV⁻¹), and most pronounced redshift in UV–Vis absorption after complexation, shifting from 420 nm to 460 nm. The ZnC₅₉@ISO-But complex exhibits exceptional dipole moment (6.42 D), polarizability (790.83 a.u.), and hyperpolarizability (1520.43 a.u.), highlighting its superior sensitivity and responsiveness. Adsorption energy and recovery time analyses indicate that while pristine C60 exhibits the strongest adsorption (−75.30 kcal/mol), ZnC₅₉@ISO-But demonstrates the fastest recovery (2.2 × 10⁻¹⁰ s), making it ideal for real-time sensing applications. These findings establish ZnC₅₉@ISO-But as a promising candidate for designing efficient, portable, and non-invasive sensors for COVID-19 detection. By leveraging the synergistic effects of metal doping and complexation, this work provides valuable insights into the development of advanced nanostructure-based diagnostic tools, paving the way for broader applications in biosensing technologies.