Adsorption of DNA Bases onto a Semiconductor Surface:  Evidence for Surface-Mediated Promotion and Detection of Complementary Base Pair Formation

The adsorption of DNA basesadenine (A), thymine (T), guanine (G), and cytosine (C)and base pairs onto single-crystal n-CdSe substrates has been studied in several solvents, using the band gap photoluminescence (PL) of the semiconductor as a probe. In methanol solution, all four bases cause similar, reversible PL quenching, consistent with their acting as Lewis acids toward the surface. The PL changes can be well fit by a dead-layer model, indicating that adsorption increases the depletion width of the semiconductor by ∼200−300 Å. In DMSO solution, there is no PL response to individual bases. However, the complementary A−T and G−C base pairs yield a PL response, providing evidence that the surface can promote base pair formation. The A−T and C−G responses in DMSO correspond to depletion width increases of ∼100 and 200 Å and persist to ∼45 and 75 °C, respectively. Competition experiments reveal a preference for C−G binding at elevated temperatures. In chloroform solution, the PL response of C−G base pairs can be distinguished from those of C and G individually, whereas A, T, and A−T are experimentally indistinguishable. Electronic and hydrogen-bonding effects that may contribute to the PL responses are discussed.