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2008, 112, 13320–13323 Published on Web 08/12/2008
Probing the Gold Nanorod-Ligand-Solvent Interface by Plasmonic Absorption and Thermal Decay Aaron J. Schmidt,†,⊥ Joshua D. Alper,†,⊥ Matteo Chiesa,†,‡ Gang Chen,*,† Sarit K. Das,*,†,§ and Kimberly Hamad-Schifferli*,†,| Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts AVenue, Cambridge, Massachusetts 02139, Program of Mechanical Engineering, Masdar Institute of Science and Technology, UAE, Department of Mechanical Engineering, Indian Institute of Technology, Madras, India, and Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Thermal transport between CTAB passivated gold nanorods and solvent is studied by an optical pumpprobe technique. Increasing the free CTAB concentration from 1 mM to 10 mM causes a ∼3× increase in the CTAB layer’s effective thermal interface conductance and a corresponding shift in the longitudinal surface plasmon resonance. The transition occurs near the CTAB critical micelle concentration, revealing the importance of the role of free ligand on thermal transport. The optical properties of Au nanoparticles are attractive for biological applications ranging from novel therapies and imaging to self-assembly. The ability to tune the surface plasmon resonance (SPR) of Au nanorods (NRs) to tissue-transparent wavelengths by controlling their shape1 makes them particularly attractive Au nanoparticles. Ultrafast pulsed laser irradiation can excite the SPR causing the Au NRs to heat. Laser heating of Au NRs has been employed in hyperthermia of tumors,2–4 targeted destruction of microorganisms,5 intracellular imaging of single biomolecules,6 and triggered release of drugs and genes.7,8 Understanding the thermal transport from a nanoparticle in suspension to the surrounding fluid is essential for designing biological applications requiring spatial thermal confinement9,10 and nanofluids with enhanced bulk thermal conductivity.11–13 This thermal transport depends critically on heat transfer across solid-fluid interfaces. The chemical and physical properties of the ligand molecules at the solid-fluid interface heavily influence the thermal transport at the macroscopic and microscopic levels.14–19 The most common Au NR synthesis protocols require the ligand hexadecyltrimethylammonium bromide (CTAB).1 It is hypothesized that the amine headgroups of CTAB coordinate more strongly to certain facets of Au nanoparticles, directing the growth along the [110] crystal direction during NR synthesis.1 This leaves the alkyl chain tails of the CTAB protruding from the surface of the NRs. In the presence of enough free CTAB in aqueous solution, additional CTAB molecules coordinate with these hydrophobic tails to form bilayers. This fluxional bilayer stabilizes the NRs and prevents their aggrega* Corresponding authors. G.C.: phone, (617) 253-0006; fax, (617) 2585802; e-mail:
[email protected]. S.K.D., e-mail:
[email protected]. K.H.D.: phone, (617) 452-2385; fax, (617) 258-0204; e-mail:
[email protected]. † Department of Mechanical Engineering, Massachusetts Institute of Technology. ‡ Masdar Institute of Science and Technology. § Indian Institute of Technology. | Department of Biological Engineering, Massachusetts Institute of Technology. ⊥ These authors contributed equally to this work.
10.1021/jp8051888 CCC: $40.75
tion through steric and electrostatic interactions. A sufficiently sensitive and systematic study of the role of the CTAB ligand molecules is necessary to characterize thermal transport processes in Au NR systems. The optical pump-probe technique is a well-established tool to study the transient absorption of nanoparticles. It has been used to investigate many physical phenomena including the excitation of the electron cloud,20 the thermalization of the electron cloud with the nanoparticle lattice phonons,21,22 the acoustic vibrations in the lattice,17,23 and the thermal decay due to heat diffusion to the solvent.14–16,18,19,24 The thermal decay studies concluded that the structural and chemical properties of the ligand layer and its interaction with the surrounding solvent can strongly influence the thermal interface16 especially if the particle is less than 40 nm in diameter.14 It is important to understand the magnitude of the effect that subtle differences in the ligand layer can make. In particular, there is a need to analyze quantitatively how the concentration of free CTAB, the ubiquitous surfactant ligand for NRs, affects the thermal transport between Au NRs and solvent. We use the pump-probe technique to obtain the transient absorption of Au NRs as a function of free CTAB in solution. After excitation by the pump laser pulse, the Au NR’s longitudinal SPR shifts due to thermal expansion.25 For small temperature rises, the absorption of the Au NR solution is linearly proportional to the temperature of the rods.14 We fit the normalized change in absorption to a continuum finite element model of the heat diffusion from the Au NRs to the surrounding fluid, which allows us to characterize the heat transfer properties of the CTAB ligand layer. We synthesized Au NRs using a modified nonseeding synthesis protocol.26 All chemicals used were purchased from Sigma Aldrich Co. and used without further purification. Briefly, HAuCl4 · 3H2O and AgNO3 were added to a CTAB containing 2.25 mM NaCl solution, which turned yellow upon inversion. L-Ascorbic acid was added, and the solution turned clear. NaBH4 was then added, and the solution was mixed again by inversion. The solution went from clear to a deep purple brown over several hours. After at least 3 h of incubation at room 2008 American Chemical Society
Letters
Vol. 112, No. 35, 2008 13321
Figure 1. Au NR characterization. (a) TEM image of NRs. (b) Distribution of NR aspect ratios. Mean ) 3.6. Standard deviation ) 0.5. Yield ) 93%. Note that aspect ratios above 1.75 are defined to be a NR, and below this it is considered a sphere. (c) Optical absorption spectrum of NRs in 10 mM CTAB with longitudinal SPR peak at 768 nm.
Figure 2. Experimental pump-probe system. Each pulse is divided into pump and probe pulses with a polarizing beam-splitter (B.S.). After the beam splitter, the probe beam passes through a 4X beam expander to minimize divergence over the maximum possible 7 ns of delay, after which it is compressed before being focused through the sample at normal incidence. The pump beam passes through an electro-optic modulator (EOM) and then a bismuth triborate (BIBO) crystal where it is frequency-doubled to 400 nm. The doubled light is directed through the same lens coaxial with the probe beam. Suspensions of Au NRs are held in a 1 mm thick cuvette. A color filter prevents any pump light from reaching the detector.
temperature, excess reactants were removed and the NRs were concentrated by centrifugation. The NRs in 10 mM CTAB have their longitudinal SPR peak at 770 nm (Figure 1c). Transmission electron microscope (TEM) imaging shows (Figure 1a) that the NRs are 9.9 ( 2.2 nm in diameter and are 35.8 ( 6.9 nm long, with an average aspect ratio of 3.6:1 (Figure 1b). Our pump-probe system (Figure 2) is built around a Tisapphire laser that emits a train of 150 fs pulses at a repetition rate of 80 MHz and a power per pulse of ∼15 nJ. The typical laser fluence at the sample is ∼2 J/m2. Compared to more commonly used amplified pump-probe systems, unamplified systems have the advantage that high frequency modulation and lock-in detection can be used to give a very high signal-tonoise ratio,14 making the measurement sensitive to changes in absorption on the order of one part in 107. This allows resolution of fine details in the cooling process following pulsed laser heating. In addition, unlike previously reported unamplified systems,14–16,19 we use a frequency-doubled pump beam to excite our sample. This allows us to prevent scattered pump light from reaching the detector by using color filters with greater efficiency than polarization-based techniques, and without the additional complexity of double-modulation schemes. The spatial resolution of our delay stage translates into better than 0.5 ps temporal
resolution of the probe pulse. The center probe wavelength is 800 nm, which is close to, but not at, the longitudinal SPR peak. More details on the experimental system can be found elsewhere.27 We took transient absorption data on 1 nM NR solutions with the free CTAB concentration varying from
