Graphitic carbon nitride/metal-organic framework microextraction fiber for enriching volatile organic compounds in the exhaled breath of patients with lung cancer

Volatile organic compounds (VOCs) in human exhaled breath can act as biochemical indicators for the objective determination and evaluation of physiological or pathological conditions. In humans, the pathophysiology associated with malignant tumors causes metabolic changes that alter the types and concentrations of VOCs in exhaled breath. However, as exhaled breath has a complex composition and low VOC content, meeting the direct detection requirements of analytical instruments is difficult. Therefore, the direct detection of VOCs in exhaled breath requires the development of a simple, fast, and efficient enrichment technique. In this study, graphitic carbon nitride (g-C3N4) and the metal-organic framework UiO-66 were combined to prepare a three-dimensional nanoflower-like g-C3N4/UiO-66 solid-phase microextraction (SPME) coating. The g-C3N4/UiO-66 composite was characterized using Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy. UiO-66, which exhibited a regular ortho-octahedral structure with sharp corners, was embedded among g-C3N4 nanoflowers, and the surface was encapsulated in g-C3N4 to form a hydrophobic barrier. Five biomarkers (benzene, toluene, ethylbenzene, o-xylene, and decanal) in the exhaled breath of patients with lung cancer were enriched using the developed SPME fiber and then detected using gas chromatography-mass spectrometry. The optimum extraction time and temperature of the nanoflower-like SPME fiber were 10 min and 60 ℃, respectively, whereas the optimum desorption time and temperature were 3 min and 260 ℃, respectively. The adsorption efficiencies of the g-C3N4/UiO-66 composite coating, a single UiO-66 coating, and a 30 μm polydimethylsiloxane commercial coating were compared under optimal conditions. The five VOCs showed good linearity in the investigated concentration ranges, with correlation coefficients of 0.9919-0.9977, limits of detection of 0.16-4.50 μg/L, and limits of quantification of 0.54-9.18 μg/L. The intra- and inter-day precisions for six parallel determinations with the SPME fiber were 3.3%-7.3% and 4.0%-6.6%, respectively. Owing to the presence of nanoflower-like interlayer channels and g-C3N4 micropores in g-C3N4/UiO-66, the composite coating exhibited a high enrichment efficiency and good repeatability. The spiked recoveries of the five VOCs were in the range of 84.6%-113.2%. Finally, the method was applied to analyze exhaled breath for the detection of lung cancer. The benzene contents in the exhaled breath of both healthy individuals and patients with lung cancer were generally low. However, the toluene contents differed significantly, with patients with lung cancer having much higher levels than healthy individuals. The established method is expected to provide a reference for the noninvasive early diagnosis of lung cancer.