Covalent photochemical functionalization of amorphous carbon thin films for integrated real-time biosensing

Recent studies have demonstrated that carbon, in the form of diamond, can be functionalized with molecular and/or biomolecular species to yield interfaces exhibiting extremely high stability and selectivity in binding to target biomolecules in solution. However, diamond and most other crystalline forms of carbon involve high-temperature deposition or processing steps that restrict their ability to be integrated with other materials. Here, we demonstrate that photochemical functionalization of amorphous carbon films followed by covalent immobilization of DNA yields highly stable surfaces with excellent biomolecular recognition properties that can be used for real-time biological detection. Carbon films deposited onto substrates at 300 K were functionalized with organic alkenes bearing protected amine groups and characterized using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The functionalized carbon surfaces were covalently linked to DNA oligonucleotides. Measurements show very high selectivity for binding to the complementary sequence, and a high density of hybridizing DNA molecules. Samples repeatedly hybridized and denatured 25 times showed no significant degradation. The ability to use amorphous carbon films as a basis for real-time biosensing is demonstrated by coating quartz crystal microbalance (QCM) crystals with a thin carbon film and using this for covalent modification with DNA. Measurements of the resonance frequency show the ability to detect DNA hybridization in real time with a detection limit of <3% of a monolayer, with a high degree of reversibility. These results demonstrate that functionalized films of amorphous carbon can be used as a chemically stable platform for integrated biosensing using only room-temperature processing steps.