Niepelt et al. Nanoscale Research Letters 2011, 6:511 http://www.nanoscalereslett.com/content/6/1/511 Page 4 of 7 Figure 2 Dark field (a and c) and fluorescence (b and d) images of biofunctionalized ZnO nanowires. Nanowires in upper row have been modified with Cy3-marked DNA and nanowires in bottom row with FITC-marked DNA. Note that the nanowires marked with the red and yellow circles in (b) show different fluorescence intensities. The nanowire marked blue in (d) only shows fluorescence at one side; the nanowire marked green shows higher fluorescence intensity at the side that is appearing darker in the corresponding dark field image. biofunctionalized nanowires can be explained by the interplay of nanowire thickness variations and non- homogeneous surface coverage. Recourse to either can be solved using a more advanced nanowire growth setup than the one utilized here. As ZnO nanostructures are known to show a green defect luminescence that can be dramatically enhanced when the nanowires are coated [21], one has to check if the wires themselves might cause a significant lumines- cence signal that could be interpreted falsely as fluores- cence. Therefore, all samples have been examined with different band pass fluorescence filters. All samples only showed fluorescence at wavelengths referable to the spe- cific dye used for the biofunctionalization; a broader defect luminescence was not detected. Additionally, necessary reference experiments have been performed with untreated ZnO wires as well as nanowires, which were silanized but not biofunctiona- lized (with DNA). Figure 4 confirms the absence of autofluorescence or luminescence of the nanowires. It is also known that ZnO nanostructures act fluores- cence enhancing themselves through affecting the self- quenching properties of fluorophores [22]. This could lead to fluorescent wires on top of a dye-covered sub- strate. As mentioned already above, the wires in our study were first functionalized and then subsequently transferred to a clean and dye-free substrate. Thus, fluorescence enhancement of substrate-bonded fluores- cence markers can also be excluded as the origin of the observed fluorescence. Upon having clarified that the observed fluorescence is caused by DNA attached to the ZnO nanowires, the question regarding the character and stability of this attachment still remains. To investigate these issues, several control samples have been prepared. In Figure 5, a typical fluorescence image of a silanized and biofunc- tionalized nanowire (a) is compared to an image of a nanowire that was treated the same way, but leaving out the silanization step and using DNA without a 5’-amino linker (b). One can see that the observed luminescence