We next sought to stabilise the origami-TFO-PEG complexes through directed photocrosslinking using the psoralen functionality on the 5′ end of the TFO. It was expected that irradiation at 365 nm would induce formation of either mono-or bis-adducts between the pso-TFO and the hairpin staples. To test this, we folded the 258HP-pso-TFO-PEG5K origami in one-pot reaction and irradiated the complex using a home-built light-emitting diode To target these hairpins, we used a pso-TFO labelled with a biotin group at the 3′ end of the oligonucleotide (pso-TFO-bio), which enabled the recruitment of streptavidin tetramers (STV) to the bound oligonucleotides. The 27HP-pso-TFO-bio origami complex was folded, purified, irradiated, and incubated with STV for 1 hr. AGE analysis at pH 8.0 revealed that the complexes were progressively shifted by STV, with apparent saturation achieved at a ratio of shown). TEM imaging of 27HP-pso-TFO-bio:STV complexes did not provide sufficient contrast to reveal locations of STV-loaded hairpins (data not shown). In contrast, AFM imaging of the STV-saturated 27HP-pso-TFO-bio origami revealed, as expected, triangles featuring "blobs" in three clusters at the expected locations of all three sectors (Fig ). AFM images of 0HP structures incubated with the saturating concentration of STV, and AFM images of 27HP-pso-TFO-bio complexes obtained without STV, revealed no such clusters (Fig ). We conclude that TFOs can accurately target specific sites in HP-containing DNA origami, and that the number and positioning of introduced groups can be varied through HP incorporation.