Direct Measurements of Heating by Electromagnetically Trapped Gold Nanoparticles on Supported Lipid Bilayers Poul M. Bendix, S. Nader S. Reihani, † and Lene B. Oddershede* Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark. † Permanent address: IASBS, Zanjan, Iran. O wing to the plasmonic properties of gold nanoparticles, the absorp- tion and scattering cross sections vary dramatically across the optical spectrum. Their high optical extinction makes gold nanoparticles useful as nonfading optical la- bels which has been demonstrated by de- tection of single gold nanoparticles as small as 2.5 nm in biological environments. 1-3 Gold nanoparticles are extensively used for nanoengineering purposes to assist the self- assembly of DNA into three-dimensional ar- chitectures 4 or to allow remote release from liposomes. 5 While gold nanoparticles absorb predominantly in the visible spectral range a biological transparency window ex- ists around near-infrared wavelengths 6 which has led to development of infrared resonant nanoparticles. 7 Gold nanoparticles efficiently convert electromagnetic radia- tion into heat 8 and hence, they have poten- tial in photothermal applications. 7,9-11 In particular, nanostructures with enhanced IR absorption, like gold nanorods, have proven promising for photothermal cancer therapy and gene delivery. 12 With the extensive use of irradiated gold nanoparticles to investigate or ma- nipulate biological specimens it is crucial to know the exact temperature profile around the particle. Different approaches have been applied to measure the optical heating of nanoparticles; the vaporization of surrounding liquid, 13 the melting of glass surfaces, 8 or the melting around gold nano- particles embedded in ice 14 have been used as measures, however, these types of experiments are typically performed either with high energy pulsed lasers yielding ex- treme transient heating or in a matrix and at a temperature which is far from biologi- cally relevant. However, recently heating around metallic nanoparticles irradiated by 532 nm laser light has been shown to in- duce reversible phase transitions in biologi- cally relevant lipid bilayer systems. 15 In that and several other studies, calculations incorporating estimates of physically rel- evant parameters constituted an essential part of finding the temperature profile sur- rounding the irradiated particle. 14,15,16 The strong interaction of metallic nanoparticles with electromagnetic ra- diation also leads to a strong polariza- tion of the particle, thus enabling effi- cient optical trapping of individual metallic nanoparticles. 16-21 Here, we present direct quantitative measurements of the heating associated with infrared opti- cal trapping of spherical gold nanoparticles *Address correspondence to oddershede@nbi.dk. Received for review December 2, 2009 and accepted March 24, 2010. Published online April 6, 2010. 10.1021/nn901751w © 2010 American Chemical Society ABSTRACT Absorption of electromagnetic irradiation results in significant heating of metallic nanoparticles, an effect which can be advantageously used in biomedical contexts. Also, metallic nanoparticles are presently finding widespread use as handles, contacts, or markers in nanometer scale systems, and for these purposes it is essential that the temperature increase associated with electromagnetic irradiation is not harmful to the environment. Regardless of whether the heating of metallic nanoparticles is desired or not, it is crucial for nanobio assays to know the exact temperature increase associated with electromagnetic irradiation of metallic nanoparticles. We performed direct measurements of the temperature surrounding single gold nanoparticles optically trapped on a lipid bilayer, a biologically relevant matrix. The lipid bilayer had incorporated fluorescent molecules which have a preference for either fluid or gel phases. The heating associated with electromagnetic radiation was measured by visualizing the melted footprint around the irradiated particle. The effect was measured for individual gold nanoparticles of a variety of sizes and for a variety of laser powers. The temperatures were highly dependent on particle size and laser power, with surface temperature increments ranging from a few to hundreds of degrees Celsius. Our results show that by a careful choice of gold nanoparticle size and strength of irradiating electromagnetic field, one can control the exact particle temperature. The method is easily applicable to any type of nanoparticle for which the photothermal effect is sought to be quantified. KEYWORDS: Gold nanoparticles · heating · optical trapping · molecular partitioning · photothermal effect · lipid bilayer ARTICLE VOL. 4 ▪ NO. 4 ▪ BENDIX ET AL. www.acsnano.org 2256