Noble metal nanoparticles show high light absorption at the frequency of the particle-plasmon resonance. When such nanoparticles are heated by absorption of laser light, they transfer their thermal energy to their environment. This heat transfer leads to a temperature increase in the vicinity of the surface of the metal nanoparticle and allows a spatially confined control of nearby (e.g. attached) temperature-sensitive objects. This concept of a nanoparticle-assisted photothermal heating has been investigated in several studies in the context of destructive processes, such as the hyperthermia of malignant cells. In contrast, the photothermal control of non-destructive, i.e. reversible, biomolecular reaction processes have rarely been studied.
In this study we investigated the reversible photothermal control of the dehybridization process of DNA. For this purpose, gold nanoparticle networks were synthesized using complementary single-stranded DNA as linker molecules. The well-known color change of the suspension that occurs upon disassembly or assembly of the nanoparticle networks is applied to continuously monitor the state of the networks using optical spectroscopy. A focused laser beam (c.w., 532 nm wavelength, 5 kW/cm² intensity, 100 µm focus size) is used to irradiate a DNA-nanoparticle network suspension. The spatially confined temperature increase that is induced by this photothermal treatment gives rise to the disassembly of the networks, indicating the dehybridization ('melting') of the DNA double strands. After the end of the laser irradiation, the networks reassemble, thus indicating the reversibility of the dehybridization process. It is shown that the DNA melting occurs predominantly within the 100 µm wide laser focus, where the intensity is highest, thus enabling a highly local control of the reaction.

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