Functionalization of Carbon Nanotubes with 5-Phenylvaleryl, Pentadecanoyl, and Adipoyl Chloride: Synthesis and Spectroscopic Surface Analysis Toward Biomarker Sensing

Carbon nanotubes (CNTs) were covalently modified with aromatic (5-phenylvaleryl, 5PC), long-chain aliphatic (pentadecanoyl, PDC), and oxygen-rich/carboxylated (adipoyl, ACC) chemistries to create a tunable surface interface for detecting volatile and semivolatile chemical biomarkers. Following oxidative activation, 3-aminopropyltriethoxysilane (APTES) amination, and acyl-chloride coupling, successful attachment and differentiation of linker classes were evaluated using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and C KLL Auger analysis. Distinct spectroscopic fingerprints corresponding to aromatic, aliphatic, and oxygenated functionalities were resolved despite the intrinsically carbon-rich substrate. These orthogonal signatures provide a compact quality-control strategy for CNT functionalization and establish a framework for correlating surface chemistry with predicted analyte interactions, including aromatic interactions, nonpolar adsorption, and acid–base or hydrogen-bond pairing. This surface chemistry palette is intended to enable flexible CNT-based sensor arrays for noninvasive monitoring of clinically relevant biomarkers, including ketones associated with diabetes, nitrogen-containing species linked to chronic kidney disease, and volatile compounds implicated in preictal seizure states. Sensor-level performance was not evaluated in the present study and is the subject of ongoing device work.