A novel pH/light-triggered surface for DNA adsorption and release.

A simple strategy for the immobilization of Cy3-labeled single strand DNA (Cy3-ssDNA) on a Si(001) surface and its release under control of both light and pH stimuli is presented. In order to prepare a dual pH/light-triggered surface, positively chargeable azobenzene molecules are self-assembled on the Si(001) surface. The surface wettability of this substrate can be changed under influence of both light and pH conditions. The substrates can be positively charged under mildly acidic conditions. The pH-sensitive behavior of the film allows binding of Cy3-ssDNA on the functionalized Si(001) surface through effective electrostatic interactions with the negatively charged polynucleotide backbone. Moreover, irradiation of the film with UVA light induces trans-cis isomerization of the azobenzene units on the surface. As a result, the binding affinity for DNA decreases due to the changing surface hydrophilicity. In order to understand and control the reversible photoswitchable mechanism of this surface, water contact angles are measured after UVA and visible light irradiation. The release of DNA from a dual pH/light-sensitive sample is performed using fluorescence microscopy. The results show that irradiation of the film with UVA light induces trans-cis isomerization of the photoresponsive azobenzene units; this leads to significant changes in the surface hydrophilicity and reduces the binding affinity for DNA.