Fabrication of COOH-terminated self-assembled monolayers for DNA sensors

Surface-functionalization of the self-assembled monolayer (SAM) can be achieved by introducing a functional group in a molecule at the terminal. To immobilize biomolecules such as nucleotides and proteins on a gold substrate, COOH-terminated thiol SAMs are preferably employed. Here, we fabricated and characterized several kinds of SAMs of the COOH-terminated dithiol compounds by surface plasmon resonance (SPR), X-ray photoemission spectroscopy (XPS). Amino-terminated oligonucleotide was chemically bound on the SAM via the amide coupling using a condensing agent. Moreover, hybridization the DNA-immobilized SAM with target DNA was in situ monitored by SPR. S spectra of the SAMs showed that the S atoms are chemically bound on the Au surface, suggesting the formation of the SAM on Au (1 1 1) substrate. The efficiency of DNA immobilization on the SAMs was evaluated by Nls XPS. The efficient hybridization by the SAM was observed by SPR compared with a COOH-terminated normal alkane thiol SAM system.