Letter pubs.acs.org/NanoLett
Tuning DNA Binding Kinetics in an Optical Trap by Plasmonic Nanoparticle Heating Lidiya Osinkina,† S. Carretero-Palacios,*,† Joachim Stehr,‡ Andrey A. Lutich,† Frank Jac̈ kel,*,†,§ and Jochen Feldmann† †
Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Amalienstrasse 54, 80799 Munich, Germany ‡ GNA Biosolutions GmbH, Am Klopferspitz 19, D-82152 Martinsried, Germany S Supporting Information *
ABSTRACT: We report on the tuning of specific binding of DNA attached to gold nanoparticles at the individual particle pair (dimer) level in an optical trap by means of plasmonic heating. DNA hybridization events are detected optically by the change in the plasmon resonance frequency due to plasmonic coupling of the nanoparticles. We find that at larger trapping powers (i.e., larger temperatures and stiffer traps) the hybridization rates decrease by more than an order of magnitude. This result is explained by higher temperatures preventing the formation of dimers with lower binding energies. Our results demonstrate that plasmonic heating can be used to fine tune the kinetics of biomolecular binding events. KEYWORDS: Gold nanoparticles, plasmonic coupling, optical tweezers, optical heating, DNA hybridization
(Bio)molecular binding events are at the heart of any living organism as they are responsible for reproduction, immune defense, growth, metabolism, and many other essential functions.1−4 They also play key roles in biochemical and biomedical analytical applications, bottom-up engineering approaches such as DNA origami, and drug delivery.5−7 These binding processes can be described in terms of diffusion and collision-dependent reaction rates.8,9 Plasmonic nanostructures, on the other hand, are known to provide strongly enhanced and highly localized electromagnetic fields due to their localized surface plasmon resonances.10−13 Particularly large field enhancements can be produced by metal nanoparticle dimers due to plasmonic coupling.14 The strongest fields are located in the gap between the nanoparticles, that is, in the “hot spot”. The field enhancement observed in such dimers has been used to manipulate the optical properties of molecules or nanoparticles placed in the hot spot. Fluorescent signal enhancement,15−18 shaping of fluorescence spectra,19 acceleration of FRET rates,20 and enhancement of Raman signals21−23 have been reported. At the same time, due to absorption plasmonic nanostructures can be employed as nanosized heat sources24,25 that allow for inducing membrane phase transitions, DNA melting, modification of chemical reactions in the vicinity of the nanostructure, and applications in cancer therapy and drug delivery.26−31 Optical trapping of plasmonic nanoparticles can be used to study and employ these properties in free solution with individual or few nanoparticles in a micrometer-scale volume.32,33 This can be employed to quantify the interaction of plasmonic nanoparticles and to create single plasmonic nanoparticle aggregates. Unspecific © 2013 American Chemical Society
dimerization of metal nanoparticles confined in the optical trap has already been investigated.34,35 Here, we show that the kinetics of DNA hybridization between two gold nanoparticles functionalized with complementary DNA strands can be studied and tuned in an optical trap through plasmonic heating. DNA hybridization events are detected via the change of the plasmon resonance frequency due to plasmonic coupling between the gold nanoparticles upon hybridization. We find that with increasing trapping powers (i.e., increasing temperature and trap stiffness) the binding kinetics is slowed down by more than an order of magnitude. This is explained by higher temperatures preventing the formation of loosely bound plasmonic dimers and thus leading to a slower dimerization. Our findings are supported by calculations of temperatures in the trap and of dimer melting temperatures. The results show that plasmonic heating can be employed to fine tune binding processes between biological molecules. Our experimental strategy is illustrated in Figure 1 (full experimental details are given in the Supporting Information). Figure 1a shows the experimental setup. Gold nanoparticles (AuNPs) are trapped in aqueous solution in a tightly focused laser beam with a wavelength of 1064 nm. At the same time, the setup allows for imaging and recording videos of the trapped nanoparticles via their white light Rayleigh scattering in darkReceived: March 26, 2013 Revised: June 14, 2013 Published: June 18, 2013 3140
dx.doi.org/10.1021/nl401101c | Nano Lett. 2013, 13, 3140−3144
Nano Letters
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Figure 1. (a) Experimental setup: combined dark-field and optical trapping microscope. (b) Schematic of a AuNP dimer formed by two complementary A- and T-type nanoparticles fully connected by two hybridized DNA strands. The interparticle distance is ∼8.5 nm. Each particle has ∼1500 single-stranded DNA oligomers attached to its surface. (c) Sketch of the experiment. In dark-field configuration, two optically trapped separate nanoparticles scatter in green. DNA hybridization occurs after the coupling time τ and plasmonic coupling is observed as a color change.
Figure 2. Snapshots from a typical experimental video (Supporting Information video S1): micrographs and corresponding sketches. At t = 0 s, a single DNA-functionalized AuNP in the optical trap scatters green light. At t = 1 s, a second DNA-functionalized AuNP appears nearby the trap and enters the trap at t = 1.16 s leading to an increased scattering intensity. At t = 6.16 s, DNA hybridization occurs leading to plasmonic coupling observed as a color change of the scattered light. The binding time τ (i.e., difference between time of color change and time of entry of the second particle) is 5 s in this case.
field configuration. The concentration of gold nanoparticles is low enough so that virtually only individual particles arrive to the trapping region. Therefore, uncontrolled multiple trapping can be excluded. From the videos, the arrival times of single nanoparticles to the optical trap can be determined with a time resolution of 20 ms. Figure 1b illustrates the two types of DNAfunctionalized AuNPs employed. The 60 nm diameter AuNPs (British Biocell International, size deviation
