We report the assembly and optical characterization of stimulus-responsive gold nanoparticle dimers using azobenzene-modified hairpin DNA. We demonstrate a reversible, light-triggered actuation of the interparticle distance using reversible trans-to-cis photoisomerization of azobenzene. UV exposure leads to an extension of the dimers and concomitant blue shift of their scattering spectra, whereas blue light reverses the process. We use single-particle dark-field scattering spectroscopy to quantify the interparticle distances of the DNA-hairpin-linked nanoparticle heterodimers in the open and closed hairpin states. Analyzing the plasmon peak shifts for nearly 100 dimers with the plasmon ruler equation yields an average interparticle distance of 14.3 ± 1.7 nm in the closed (transazobenzene) and 17.7 ± 1.5 nm in the open (cis-azobenzene) state, which are in good agreement with finite-difference timedomain (FDTD) electrodynamic simulations of the dimers. We conclude that larger photoreversible plasmon shifts can be achieved by designing photoswitchable linkers that are more rigid in the unzipped state.

We study the photoisomerization quantum yield of azobenzene-modified DNA as a function of temperature for various DNA sequences. We find that even though the photoisomerization quantum yield of free azobenzene is essentially temperature-independent, the trans-to-cis photoisomerization quantum yield depends strongly on temperature when the azobenzene is incorporated into DNA. We show that this temperature dependence is DNA-sequence-dependent and closely linked to the melting temperature of the host DNA. While the trans-to-cis isomerization quantum yield is sequenceand temperature-dependent, in contrast, the thermal cis-totrans isomerization of azobenzene embedded in DNA is sequence-independent and exhibits Arrhenius-like behavior with an activation energy of 88.8 ± 0.693 kJ/mol and a smaller pre-exponential factor than free azobenzene, yielding first-order cis-to-trans kinetics with a rate constant of (2.08 ± 0.00952) × 10 −6 s −1 at 25 °C. These results provide an understanding of the azobenzene isomerization mechanisms in DNA sequences to enable more efficient design of optically reprogrammable nanomaterials and biosensors.