Thermal Transport across Surfactant Layers on Gold Nanorods in

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Thermal Transport across Surfactant Layers on Gold Nanorods in Aqueous Solution Xuewang Wu, Yuxiang Ni, Jie Zhu, Nathan D. Burrows, Catherine J. Murphy, Traian Dumitrica, and Xiaojia Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12163 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 5, 2016

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ACS Applied Materials & Interfaces

Thermal Transport across Surfactant Layers on Gold Nanorods in Aqueous Solution Xuewang Wu,† Yuxiang Ni,† Jie Zhu,† Nathan D. Burrows,‡ Catherine J. Murphy,‡ Traian Dumitrica,† Xiaojia Wang,*,†

†

Department of Mechanical Engineering, University of Minnesota, Twin Cities, 111 Church St. SE,

Minneapolis, MN 55455, USA ‡

Department of Chemistry, University of Illinois, 600 South Mathews Avenue Urbana, IL 61801, USA

ABSTRACT Ultrafast transient absorption and molecular dynamics simulations are utilized to investigate the thermal transport between aqueous solutions and cetyltrimethylammonium bromide (CTAB)or polyethylene glycol (PEG)-functionalized gold nanorods (GNRs). The transient absorption measurement data are interpreted with a multiscale heat diffusion model, which incorporates the interfacial thermal conductances predicted by molecular dynamics. According to our observations, the effective thermal conductance of the GNR/PEG/water system is higher than that of the GNR/CTAB/water system with the surfactant layer of the same length. We attribute the enhancement of thermal transport to the larger thermal conductance at the GNR/PEG interface as compared with that at the GNR/CTAB interface, in addition to the water penetration into the hydrophilic PEG layer. Our results highlight the role of the GNR/polymer thermal interfaces in designing biological and composite-based heat transfer applications of GNRs, and 1 ACS Paragon Plus Environment


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the importance of multiscale analysis in interpreting transient absorption data in systems consisting of low interfacial thermal conductances.

KEYWORDS: thermal transport, gold nanorods, transient absorption, molecular dynamics simulation, interfacial thermal conductance

1. INTRODUCTION Gold nanorods (GNRs) exhibit surface plasmon resonances (SPR) in the presence of electromagnetic excitation, which leads to significant optical absorption in the visible or near infrared (near IR) regime, and therefore enhances the energy conversion of light to heat.1 The longitudinal near IR plasmonic resonance of GNRs can be tailored to a desirable frequency by engineering the size, shape, surfactant coating materials, aggregation state, and orientation of GNRs, as well as the environment. Additionally, the surfaces of GNRs can be functionalized to improve their hydrophilicity and biocompatibility. The combination of photothermal effects with surface functionalization of the GNRs facilitates potential applications in biomedical areas, such as hyperthermia of tumors, targeted destruction of microorganisms, bio-chemical sensing and imaging, and delivery of targeted drug molecules.2-5 Cetyltrimethylammonium bromide (CTAB) is widely used in GNR synthesis as a structuredirecting agent to control the GNR shape and aspect ratio.6 The bilayer structure of CTAB7 is also essential to keep GNRs from aggregating, as CTAB promotes uniform dispersion in a solution. The toxicity of CTAB is a potential problem if it is to be used in bio-tissues or cells. For GNRs prepared in the presence of CTAB, the CTAB coating of GNRs can be removed or replaced with other surfactant layers through ligand exchange (i.e., surface functionalization) for 2 ACS Paragon Plus Environment

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certain applications. Among the various surfactants, polyethylene glycols (PEG) and PEG derivatives are polymers synthesizable from ethylene oxides, which are abundant from petroleum-based products. The structural flexibility with a wide range of molecular weights, non-toxicity, and hydrophilicity make PEG a promising surfactant material for nanoparticle surface functionalization to improve the water solubility and biocompatibility of nanoparticles. This characteristic is particularly beneficial in bio-medical applications.8-9 Upon heating, both the CTAB bilayer, as the structure-directing agent, and PEG, as the functionalization surfactant, will be thermally destabilized leading to particle aggregation, which is detrimental to the GNR synthesis and its applications. Thus, understanding the fundamental mechanisms of the nanoscale thermal transport of GNRs coated with CTAB or functionalization surfactants is crucial to advance the synthesis technologies and enable the bio-medical applications of GNRs. Owing to the relatively low thermal conductivities of the surfactant layer and the surrounding environment (typically water, ionic solutions, or an in vivo environment for applications in photothermal therapies or drug release), measurement sensitivity of thermal transport across the gold-surfactant-surroundings multilayer is typically low. In addition, the small dimensions of the surfactant molecules (usually on the order of tens of nanometers) make it truly difficult to separate the interfacial thermal resistance from the intrinsic thermal properties of the surfactant layer. There have been a few experimental studies focusing on the thermal conductance of the GNR/surfactant interface by pump-probe transient absorption.10-13 The influence of different types of surfactants14-15 and fluids,16 and the concentration of free CTAB14,12 on the interfacial thermal conductance of GNRs has been discussed. Most of the experimental investigations reported the effective thermal conductance Geff (Geff = 1/Reff, with Reff being the lumped interfacial thermal resistance). This includes contributions from the 3 ACS Paragon Plus Environment


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interface between the GNR and the surfactant layer, the conduction across the surfactant layer, and the interface between the surfactant layer and the surrounding environment. Reported values of Geff for GNRs in liquids span wide ranges. For example, the interfacial thermal conductance between GNRs and organic fluids with small thermal effusivity has been reported to range from 20 to 40 MW m−2 K−1.16 For GNRs in aqueous solutions, Geff has been found to range from 130 to 450 MW m−2 K−1 for GNRs coated with CTAB, and approaches infinity for GNRs coated with thiolated PEG, respectively.12, 15 The effective thermal conductivity (Λeff) can be calculated based on Geff and the surfactant thickness (hs) via the simple relation Λ eff = Geff hs . This would predict an equivalent thermal conductivity of the surfactant layer ranging from 0.5 to 2 W m−1 K−1 for a 4-nm bilayer of CTAB, and an infinitely large thermal conductivity for PEG (regardless of its molecular chain length), where both apparently overestimate the thermal conductivity of the surfactant layer significantly. Therefore, a study on the thermal properties of the PEG surfactant layer is needed. On the theoretical side, many theoretical models have been proposed to predict the heat transfer from the Au nanoparticle to water17-20 but very few21-22 account for the role of the interfaces between the GNR and the surfactant layer. In this work, we conducted transient absorption measurements to investigate the nanoscale thermal transport of GNRs coated with CTAB (~4.7 nm) and PEG layers (1K-PEG ~5 nm and 3K-PEG ~9 nm) in an aqueous environment. For transient absorption measurements, both the pump and probe were set at the near-IR longitudinal plasmonic absorption peak wavelength, ranging from 769 to 785 nm. Utilizing the plasmonic absorption peak wavelength is beneficial for optimizing the absorption signals and avoiding artifacts, thus improving the measurement sensitivity and reliability.14 By taking advantage of thermal confinement with a femtosecond laser, the spatial resolution for probing the thermal transport is on the order of tens of nanometers 4 ACS Paragon Plus Environment

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as determined by the heat diffusion length in the vicinity of the GNRs. Thus, GNRs serve as an ideal platform to probe the nanoscale thermal transport across the GNR/surfactant/environment complex interface assembly. Following the laser excitation, the transient temperature decay in the GNR is analyzed with a multiscale heat diffusion thermal model comprising a GNR/surfactant/water system with explicit thermal interfaces. The interfacial thermal conductance values between Au and the surfactant layers are computed separately by molecular dynamics (MD) simulations and then used as input parameters into the heat diffusion model. The analysis of the transient absorption measurement data is used to quantify thermal transport through the surfactant layer.

2. EXPERIMENTAL METHODS Figure 1 depicts the structural formula of GNRs coated with CTAB and PEG. The aqueous silver-assisted seed-mediated method was used for GNR synthesis.6, 23 GNRs coated with the CTAB bilayer surfactant were synthesized according to previously published protocols.6, 23-25 The concentrations of GNRs and CTAB were 2.75 nM and 5 mM respectively, for all transient absorption measurements. The samples of GNRs coated with 1K and 3K MW PEG were from Nanopartz. The nominal thickness of the PEG surfactant layer can be estimated using a simple correlation that 1K MW ≈ 5 nm of the PEG molecular chain length, assuming an extended linear conformation. The concentrations of GNRs coated with PEG were 3 nM for all measured samples. Details about the GNR synthesis are provided in Section S1 of the Supplementary Information (SI).

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Figure 1. Structure formula of CTAB and PEG surfactants (not shown to scale). The PEG molecular chains are bonded directly onto the GNR surface through covalent bonding. The CTAB forms a bilayer structure, which is stabilized on the GNR surface through a combination of van der Waals and electrostatic interactions.26

The dimension and aspect ratio of GNRs were characterized by transmission electron microscopy (TEM, FEI Tecnai T12). The geometric parameters of GNRs were tailored so that the longitudinal SPR wavelengths are within the laser operating spectral range,6, 14 as validated by the wavelengths at which pronounced plasmonic absorption peaks were observed from spectroscopic measurements (synergy HTX multi-mode reader). The average hydrodynamic diameters of the GNRs were obtained from dynamic light scattering (DLS, Microtract Nanoflex), and were used to derive the surfactant thickness as input parameters for thermal modeling analysis of the transient absorption measurements. Details on the determination of the surfactant layer dimensions are provided in Section S2 of the SI.


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Figure 2. Optical layout of the ultrafast transient absorption setup in the transmission configuration. EOM, PBS, and NBS are acronyms for electro-optic modulator, polarizing beamsplitter, and nonpolarizing beamsplitter. The GNR samples are loaded in a 100-µm thick capillary tube.

The transient absorption data were collected with an ultrafast pump-probe laser system in the transmission configuration (Figure 2). The optical excitation source is a mode-locked Ti: sapphire laser that produces a train of pulses (~100 fs) at a repetition rate of 80 MHz, which enables the localized heating effect around the GNRs. The laser is divided into a pump beam and a probe beam with a polarizing beamsplitter (PBS). Both pump and probe beams were kept at the longitudinal SPR wavelength specific to each GNR sample. A mechanical delay stage varies the optical path of the pump beam, producing a time delay of up to 4 ns between the pump and probe beams. A 5× objective lens was used to focus both the pump and probe beams on the sample with a beam spot size (radius) of w0 = 15 µm. The GNR absorption has a strong dependence on its orientation with respect to the polarization of the incident electromagnetic waves.16, 27 In order 7 ACS Paragon Plus Environment


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to maximize the excitation of the longitudinal SPR of the GNRs, which were randomly orientated in the aqueous environment, a quarter waveplate was placed before the sample to change the linearly polarized beam into a circularly polarized beam. A second quarter waveplate was positioned after the sample to resolve the circular polarization state of the beam back to the linear polarization. The GNR solution was loaded in a 100-µm thick capillary tube with flat front and back sides to enable focusing of the laser beam. A total laser power of 3 mW was used to optimize the signal-to-noise ratio and sample heating. To ensure that the surfactant layer stays attached on the GNRs upon heating with such a laser power, we took multiple measurements for each sample with a waiting time of ~10 minutes in between to make sure that all the transient absorption signals were repeatable (Section S5 of the SI). Similar to the plasmonic properties of the GNRs, the temperature rise of individual GNRs will vary depending on their size distribution and rod orientation with respect to the electromagnetic fields of the incident beams.14 Considering the extreme case, with the peak power of the Gaussian laser beam and perfect alignment of a GNR (the electric field of the incident beam is along the rod axial direction), the estimated maximum temperature rise is about 60 K with the 3-mW power input and w0 = 15 µm. The average temperature rise of GNRs due to the transient heating within the beam spot of a 30-µm-diameter area is ≈ 6 K, estimated from dividing the energy absorbed by GNRs in per pulse heating by the product of the volumetric heat capacity and total volume of GNRs within the illumination area. For such an average temperature rise, the GNR physical properties can be assumed to be constant.1, 28 The fundamental mechanism enabling transient absorption spectroscopy is the photothermal effect. Upon the longitudinal SPR generation by the laser irradiation of GNRs, most of the light energy is converted to heat through a series of thermophysical processes at different time scales. 8 ACS Paragon Plus Environment

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From short to long time scales, these processes are: (i) thermalization of electrons via electronelectron scattering in hundreds of femtoseconds to reach an electron equilibrium distribution (Fermi distribution); (ii) electron-phonon coupling allowing energy transfer from electrons to the GNR atomic lattice over a few picoseconds;29 (iii) thermal equilibration of the phonons across the GNR, and the excitation of acoustic mode vibrations leading to oscillations in the transient absorption signal within several hundreds of picoseconds;30-31 and (iv) the diffusive regime, in which the GNR lattice dissipates heat into the surroundings and raises the temperature around the particles, occurring over hundreds of picoseconds to a few nanoseconds. With light illumination of GNRs at the longitudinal SPR frequency, a small change in the measured transmitted light can be readily correlated to a change in the magnified absorption of light by GNRs, where light scattering is negligible.32 Thus, the temperature variation of the GNR due to heat generation caused by light absorption and subsequent cooling through heat diffusion into the surroundings can be treated as linearly proportional to the change in transmitted light.3336

More specifically, in the diffusive regime, the transient absorption signal directly represents

the variation of the absorption cross section (σabs) of the sample due to the pump heating. Strong light absorption by GNRs at the plasmonic wavelength creates a temperature change of the sample through heat dissipation, and subsequently alters the refractive index of the sample, which consists of both GNRs and the surrounding medium. Thus the variation of σabs contains contributions from both GNRs and the surroundings, as expressed by Eq. (1) at a certain incident wavelength37

∆σ abs (λ ) = (

∂σ abs dε1 ∂σ abs dε 2 ∂σ dε + )∆Tp + ( abs m )∆Tm , ∂ε1 dTp ∂ε 2 dTp ∂ε m dTm

(1)



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where Tp is the temperature of the GNRs, Tm is the temperature of the surrounding medium (water in this study), λ is the incident wavelength of the laser, εm is the dielectric function of the surrounding medium (assumed to be non-absorbing within the spectral range of interest), and ε1 and ε2 are the real and imaginary parts of the gold dielectric function, respectively. For the small average temperature rise of 6 K, the non-linear temperature variation in the optical properties of GNR and of the surrounding water are negligible,14,

38

thus those six

coefficients in Eq. (1), including ∂σ abs / ∂ε1 , d ε1 / dTp , ∂σ abs / ∂ε 2 , d ε 2 / dTp , ∂σ abs / ∂ε m , and

dε m / dTm , are nearly constant during the measurement and corresponding model fitting. In all of the transient absorption measurements, both pump and probe were set at the plasmonic peak wavelength. For different GNRs, the plasmonic wavelength ranges from 769 to 785 nm (see Table 1). Under this plasmonic resonance condition, the energy of the laser is mostly absorbed by GNRs rather than the surrounding water, leading to a GNR temperature rise ߂Tp much larger than that of water ߂Tm under transient heating (see Section S3 in the SI). In addition, the coefficient of ߂TP (~ −5.7 nm2 K−1) is two orders larger than that of ߂Tm (~ −0.2 nm2 K−1) in Eq. (1) at the excitation wavelength.38 The combined effects make the direct heating of water by the laser negligible compared with that of the GNRs, resulting in ߂σabs linearly proportional to the temperature change of the GNRs.

3. THEORETICAL MODELING 3.1. Three-Dimensional Thermal Transport Modeling

Interfaces between metals and molecular chains of polymers or liquids are dissimilar with relatively large interfacial thermal resistance.39 To capture this effect, a heat diffusion thermal 10 ACS Paragon Plus Environment

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model with explicit thermal interfaces is applied to extract the thermal properties of the sample system consisting of a GNR, the surfactant, and the surrounding water.12 It should be noted here that there is not significant pulse accumulation in the transient absorption measurements of GNRs due to the small nanoparticle density. Therefore for studying the thermal transport across the surfactant layer, the numerical solutions to the heat diffusion equation for linear radial heat flow give similar results to the analytical solutions to the heat diffusion equation for the radial heat flow in cylindrical symmetry40 or spherical symmetry10-11. In addition, the numerical approach can be easily applied to nanoparticles that are different in shape41 or even do not have simple symmetry. Owing to the geometric symmetry, the thermal modeling can be simplified to a two-dimensional axially symmetric heat transfer problem (with r and z coordinates). Thus only a quarter of one GNR can be taken as the computational region (due to reflection symmetries). As shown in Fig. 3a, the sub-areas in the computational region, from the inside to the outside, are the GNR, the GNR-surfactant interface 1, surfactant, the surfactant-water interface 2, and the surrounding water.



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(a) z

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Water

Interface 1 Surfactant Interface 2 Au r1

r2

TGNR T1 T2 Twater

r Heat flux TGNR

T1

T2

Twater

GNR-Surfactant Surfactant Surfactant-Water Interface 1 Interface 2

(b) Thermal resistance network

Figure 3. (a) Schematic of the thermal modeling for a surface functionalized GNR with axial symmetry. r1 is the radius of the bare GNR and r2 = r1 + hs is the radius of the interface 2 with hs being the thickness of the surfactant layer. (b) Network of thermal resistance in series illustrating the heat flux from GNR to the surrounding water.

The interfacial thermal conductance, G, specifies the finite temperature drop for a given heat flux across an interface. In general, the Kapitza length (LK = Λ/G in Cartesian coordinates) is used to conveniently describe the length of a material with a thermal conductivity of Λ which provides a thermal resistance equivalent to that of the interface. Determination of G requires the dimension of the system of interest to be comparable to or less than LK.42 In our thermal modeling, the thicknesses of interface 1 (GNR/surfactant) and interface 2 (surfactant/water) are chosen as 0.05 nm. This dimension is much smaller than the typical values of Kapitza length at a metal/polymer interface and a polymer/liquid interface (~9 to 15 nm). The choice of interface thickness will then result in negligible influence on G in the thermal model fitting. The


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corresponding thermal conductivity of the interfacial subareas 1 and 2 can be obtained by taking the product of the interfacial thermal conductance and interface thickness (Λ = Gh). Figure 3b depicts the proposed thermal resistance network used to model the heat transfer from the GNR to the surrounding water. The lumped thermal conductance (Geff) is defined by ravg 1 ravg 1 = + + Geff G1r1 Gs G2 r2

.

(2)

In the cylindrical coordinates, 1/Geff consists of three parts: the thermal conductance term of the GNR-surfactant interface (ravg/G1r1), the intrinsic thermal conductance term of the surfactant layer (1/Gs), and the thermal conductance term of the surfactant-water interface (ravg/G2r2). ravg = (r1+r2)/2 is the average radius of the surfactant layer and Gs = Λs /  ln(r2 / r1 ) × ravg  with Λs being the thermal conductivity of the surfactant layer obtained from transient absorption measurements. For the dimension of the sample systems used in this work, the difference in Gs calculated based on the formula in cylindrical coordinates and Cartesian coordinates (Gs = Λs/hs) is within the overall uncertainty (Section S9 of the SI). A better understanding of the mechanisms of thermal transport from GNRs coated with different surfactants to water would be possible if the individual contributions of these three parts in the effective thermal conductance could be obtained simultaneously. Unfortunately, it is extremely challenging, if not impossible, to separate these parts through direct fitting of the transient absorption data to the heat diffusion model. For this reason, we rely on MD data of the interfacial thermal conductance. 3.2. Molecular Dynamics Simulations for Interfacial Thermal Conductance MD simulations are performed in order to understand the interfacial heat transport mechanisms and to compute G1 at interfaces of Au/PEG and Au/CTAB. The simulations are 13 ACS Paragon Plus Environment


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conducted with the LAMMPS software package.43 The embedded-atom-method potential is used for Au-Au interactions.44 The Dreiding model is used to describe the interactions of the polymer atoms and water-PEG interaction.45 The Au-S thiol bond is described by the Morse potential.46 The “Optimized” parameters for the TIP3P model with Ewald methods for long-range electrostatics are used to simulate water molecules.47 The Lennard-Jones potential is used for the non-bonded van der Waals interactions between Au-water and Au-CTAB.21, 48 Note that with these potentials we capture both the covalent bonding of PEG to Au and the CTAB adsorbed on the Au surface.26 Figure 4 illustrates the simulated MD supercell of the Au/PEG/water system. Water molecules are added on the top of PEG chains and allowed to move freely. After energy minimization, water molecules penetrate into the space between the PEG chains. The thermal conductance G1 between these two subsystems with temperature difference ∆T is calculated with the following equation:49

1 1  1 1  +∞ ∆T ( 0 ) ∆T ( t ) = + dt ,   2 G 3kB  N1 N 2  ∫0 ∆T ( 0 )

(3)


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Figure 4. MD relaxed configuration of Au-PEG-water model for molecular dynamic simulations. Inset shows the bonding of a PEG chain covalently bonded onto the Au surface.

where N1 and N2 denote the number of atoms in subsystem 1 (Au) interacting with subsystem 2 (a surfactant layer that could have water penetration) via the interatomic potential. The angular brackets denote a microcanonical ensemble average and kB is the Boltzmann constant. This “∆T” method has been proven to be robust in calculating thermal interface conductance in various systems.22, 50-51 The MD time step used is 0.5 fs. More than 30 independent simulations, each lasting 600 ps, are performed to ensure the convergence of G values averaged from different MD runs. Details on the convergence of ∆T autocorrelation function and the determination of the error bars in theoretical calculations are provided in Section S7 of the SI. Note that when periodic boundary conditions are applied to the supercell shown in Fig. 4, an additional interface is created, where one face of the Au slab is directly exposed to water. This interface is not considered in our calculations of G using Eq. (3). The experimental relevance of the MD predictions depends not only on the fidelity of the 15 ACS Paragon Plus Environment


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interatomic potentials describing the system but also on the packing of the surfactant layer on the metal surface.21 In our MD calculation, the CTAB packing density is 3 molecules per nm2, which is slightly larger than the maximum packing density (2.44 molecules per nm2) that has been calculated on the basis of the cross-sectional area of the head group of CTAB (0.32 nm2).52 This slightly higher packing density is taken in order to allow the formation of a closely packed layer on the gold surface.26 The PEG packing density is 2 molecules per nm2 as specified by Nanopartz.

4. RESULTS AND DISCUSSION 4.1. Optical and Structural Characteristics

The dimensions of GNRs were statistically averaged over more than 150 GNRs in TEM images (Fig. 5a). The aspect ratio distributions of GNRs determined from TEM were fitted with Gauss functions (Fig. 5b). The hydrodynamic diameters (dH) of both the CTAB and PEG surfactants were obtained from DLS measurements, which were used to derive the thickness of the surfactants (hs) according to literature correlations (Section S2 of the SI). Figure 5c shows the optical absorption spectra of GNRs with CTAB and PEG surfactants characterized with a spectrophotometer. The wavelengths of the longitudinal plasmonic absorption peak are 771 nm for GNRs with CTAB, 785 nm for GNRs with 1K-PEG, and 769 nm for GNRs with 3K-PEG. The plasmonic properties and geometric parameters of GNRs and their associated uncertainties are summarized in Table 1. The values in Table 1 are used as input parameters for thermal modeling.


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200

(a)

GNR-PEG Gauss fit

(b)

Number

150 100 50

50 nm

1.0

0 2.0

2.4

2.8 3.2 3.6 Aspect Ratio

GNR-CTAB 771 nm GNR-3K PEG 769 nm

4.0

GNR-1K PEG 785 nm

Plasmon wavelength

0.8 Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4 0.2

(c)

0.0 400 500 600 700 800 900 1000 Wavelength (nm) Figure 5. (a) Transmission electron micrograph (TEM) of the GNRs with the CTAB surfactant. The scale bar is 50 nm. (b) Distribution of the aspect ratio of GNRs coated with 1K-PEG analyzed from TEM micrographs. (c) UV-VIS absorption spectrum of GNRs in aqueous solution.

Table 1. Geometric parameters of the GNR samples with CTABa and PEGb coatings Sample (#) A. GNRs with CTAB B. GNRs with 1K-PEG C. GNRs with 3K-PEG a b

λSPR

LGNR

dGNR

(nm) 771 785 769

(nm) 55 ± 4.5 29 ± 4 30.8 ± 4

(nm) 14.5 ± 1.5 9.1 ± 1 10.2 ± 0.8

Aspect Ratio

dH

hs

3.8 3.1 3.0

(nm) 43 ± 2 31.5 ± 2 42 ± 2

(nm) 4.7 ± 0.9 5.4 ± 0.9 9.0 ± 1.0

CTAB refers to cetyltrimethylammonium bromide bilayer structure PEG refers to polyethylene glycol with varying chain lengths. The nominal thickness is 1K ≈ 5 nm.

4.2. Molecular Dynamics Simulation Results 17 ACS Paragon Plus Environment


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The thermal conductances predicted by MD are summarized in Table 2. The MD simulations account for the water penetration into the space between the PEG chains. The direct contact between the water molecules and Au provide an additional thermal path. Thus, G1 for the GNR-PEG system comprises GNR-PEG (GAu-PEG) and GNR-water (GAu-water) components. The value of GAu-PEG = 146 ± 22 MW m−2 K−1 is comparable to the Au-alkanedithiol interfacial thermal conductance reported previously (113 ± 9 and 200 ± 60 MW m−2 K−1)53-54, which are calculated from non-equilibrium MD simulations. The good agreement with the available experimental data justifies the choice of the inter-atomic potentials employed in MD. The value of GAu-CTAB = 83 ± 15 MW m−2 K−1 is much smaller than GAu-PEG, indicating that the covalent Au-PEG bonding plays a significant role in enhancing the Au-surfactant interface thermal conductance. Indeed, covalent bonding has often been proven to be efficient in enhancing interfacial thermal transport.50, 55-56 Our simulations also suggest that the penetration of water in the PEG layer adds a significant conduction path. The value of GAu-water = 84 ± 15 MW m−2 K−1 is comparable with our result for GAu-CTAB. Overall, the very small values obtained for both surfactants indicate that the heat flow from GNR to the water environment is significantly obstructed. The heat flow mediated by PEG is more efficient as the value for G1 at GNR/PEG interface is nearly three times larger than that found at the GNR/CTAB interface. Our MD calculations are based on force fields in which hydrogen atoms are explicitly represented. It is expected that the inclusion of the high-frequency hydrogen vibrations overestimates the thermal properties of the surfactant and water. However, our present investigation is restricted to the interface behavior. According to the earlier MD investigations of Ref. 22, the phonon transmission between gold and polymer has a frequency cut-off at around


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10 THz. Since the vibrational frequency of hydrogen in the polymer and water is around 90 THz, the hydrogen vibrations should have a negligible contribution to G1. Regarding the thermal conductance of the second interface, in the case of PEG, it comprises two components: polymer-water and penetrated water-outside water. Only the first component is present in the case of CTAB. We note that recent MD simulations revealed efficient heat flow from the polymer to water.22 The G2 values are expected to be larger than G1 for both the PEG and CTAB cases. This is due to the better match in the phonon density of states between the surfactant and water than that between the surfactant and Au.22 The reported G2 value of 11.9 GW m−2 K−1 for polymer-water is already two orders of magnitude larger than the G1 value listed in Table 2. G2 would be even higher for the PEG case than for the CTAB case due to the additional thermal channel formed by the penetrated water and outside water components (we anticipate some difficulties in computing this last component with our MD method). We do not focus our MD modeling on this second interface since the subsequent thermal modeling analysis (Section 4.4) indicates that the explicit accounting of the interfacial conductance is not necessary when G values are sufficiently high. Table 2. Interface thermal conductance [MW m−2 K−1] in the Au-surfactant system. Surfactant

GAu-CTAB/GAu-PEG

GAu-water

G1

CTAB PEG

83 ± 15 146 ± 22

NAa 84 ± 15

83 230

a

Due to the hydrophobic nature of CTAB, the additional thermal path between Au and water does not exist at the Au/surfactant interface for GNRs coated with CTAB. Therefore, GAu-water is denoted as not available for this case.

4.3. Transient Absorption Measurements of Surface Functionalized GNRs 19 ACS Paragon Plus Environment


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Transient absorption measurements performed for all three samples are listed in Table 1. Representative transient absorption signal data sets from all three samples of GNRs coated with CTAB (black open circles) and PEG (blue and gold open circles) are shown in Fig. 6a. As described in Eq. (1), the transient absorption signal has a linear relationship with the temperature change of GNRs. Note that the process of fitting transient absorption data to the transient heat diffusion model involves only the temperature decay rate rather than the absolute amplitude of temperature variation. To facilitate the fitting process, the transient absorption signals are normalized at 10 ps. From Fig. 6a, the transient absorption signal of GNRs with PEG decays faster than that of GNRs with CTAB, indicating that the PEG coated GNRs have an overall larger thermal conductance than GNRs coated with CTAB. In addition to the thermal decay, the transient signals feature acoustic fringes at times as high as hundreds of picoseconds. These correspond to the extensional coherent phonon vibrations along the GNR axial direction resulting from the laser excitation, as illustrated in Fig. 6b. The breathing mode associated with the transverse phonon vibrations is not reflected in the transient absorption signals, as the heating of GNRs is dominated by the longitudinal surface plasmonic absorption excited along the axial direction, the same direction as the extensional mode of the coherent phonon vibration. The elastic modulus E of GNRs can be correlated to the vibrational fringe periodicity (Text), the average GNR length, and the density of gold (ρ), via the simple correlation Text = 2 LGNR / E / ρ . The measured coherent phonon vibrational fringes have periods of 82 ps, 54 ps, and 47 ps for GNRs coated with CTAB, 1K-PEG, and 3K-PEG, respectively. The elastic modulus E is derived to be 34 ± 6 GPa, 22 ± 4.5 GPa, and 33 ± 8 GPa for CTAB, 1K-PEG, and 3K-PEG, respectively. The dominant uncertainties quoted are


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propagated from the variation in the GNR dimensions. All of these values are in good agreement with the literature values for GNRs of similar dimensions.30 2

(a)

1

(b) Extensional mode Breathing mode

(c) 15 10

|∆abs|-|∆abs|base

Normalized |∆abs|

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

82 ps x 2

0

10

0.01 10

GNR-CTAB GNR-1K PEG GNR-3K PEG 100

GNR/CTAB

54 ps x 2

GNR/1K PEG

47 ps x 2

GNR/3K PEG

0

10 0

1000

Time delay (ps)

-10 0

50

100

150

200

250

300

Time delay (ps)

Figure 6. (a) Fitting of the transient absorption signal to the heat diffusion model. Open circles are the normalized transient absorption signals of GNRs with CTAB (black), 1K-PEG (blue), and 3K-PEG (dark gold), and solid red lines represent the best fit of thermal modeling to the transient absorption measurement data with G1 calculated from MD (model 3). (b) Schematics of the extensional (deformation of GNR lattice along the axial direction) and breathing (deformation of GNR lattice along the radial direction) modes of the coherent phonon vibration. Color contrast (green) is added to indicate the shape vibrational deformation of the GNRs. (c) Picosecond acoustic fringes indicating the extensional mode (axial) of coherent phonon vibration. The thermal decay background has been removed in (c) for better illustration.

4.4. Transient Absorption Data Analysis with the Multiscale Thermal Model As discussed above, it is challenging, if not impossible, to separate those three parts of the thermal resistance network depicted in Figure 3b through direct fitting of the transient absorption 21 ACS Paragon Plus Environment


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signal to the thermal diffusion model. To examine the impact of the interfacial thermal conductance to the intrinsic thermal conductivity of the surfactant layer and Geff, the thermal analysis is performed for three models. In model 1, the thermal model considers Λs only by treating interfaces 1 and 2 as though they have zero thicknesses to exclude G1 and G2 in model analysis. In model 2, the intrinsic thermal conductivity of the surfactant layer, Λs, is extracted by fitting the measurement data with the thermal model, in which G1 and G2 are assigned 1 GW m−2 K−1. This model serves only as a reference, to examine how large interfacial thermal conductance values impact Geff. In model 3, MD predictions of G1 are used to determine Λs and Cs. G2 is set as infinite (> 104 MW−1 m−2 K−1),22 of which the value will not affect the fitting results. In this case, the effect of interface 2 is lumped into the thermal transport of the surfactant layer (see Section S11 of the SI).

Table 3. Summary of the thermal properties of the GNR/surfactant/water structure. Model 3 refers to the multiscale thermal model, which incorporates G1 from MD simulation. Sample A

B

C

Model (#) & System 1. Au/CTAB/water 2. Au/G1/CTAB/G2/water 3. Au/G1(MD)/CTAB/G2/water 1. Au/1K-PEG /water 2. Au/G1/1K-PEG/G2/water 3. Au/G1(MD) /1K-PEG/G2/water 1. Au/3K-PEG /water 2. Au/G1/3K-PEG/G2/water 3. Au/G1(MD) /3K-PEG/G2/water

G1

G2

Geff −2

1000 83 1000 230 1000 230

−1

(MW m K ) 45 ± 10 1000 44 ± 10 4 >10 34 ± 8 62 ± 12 1000 59 ± 12 >104 42 ± 8 43 ± 8 1000 42 ± 7 >104 32 ± 6

Λs −1

Cs −1

(W m K ) 0.22 ± 0.02 0.22 ± 0.03 0.36 ± 0.06 0.36 ± 0.03 0.34 ± 0.04 0.34 ± 0.05 0.41 ± 0.03 0.41 ± 0.04 0.41 ± 0.05

(J cm−3 K−1) 1.6 ± 0.5 1.6 ± 0.6 2.4 ± 0.8 3.0 ± 0.7 3.1 ± 0.9 4.1 ± 1.4 3.1 ± 0.7 3.2 ± 0.9 3.8 ± 1.1

The results of thermal model analysis for all models are summarized in Table 3. The overall uncertainties of Λs, Cs and Geff are estimated as a combination of measurement uncertainty from 22 ACS Paragon Plus Environment

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the transient absorption characterization (Section S8 of the SI) and calculation uncertainty for MD simulation (Section S7 of the SI). Despite the relatively large uncertainties, comparison of the fitting results with all three models indicate that the PEG surfactant (Sample B), with a larger Geff, performs better in thermal transport than the CTAB surfactant (Sample A) that has a similar thickness. Similar observations of the enhancement in Geff for GNRs coated with other hydrophilic surfactant have also been found in previous works.57-58 Our further examination of the differences among models and samples in Table 3 provided the following new insights: (i)

The standard data analysis based on lumped thermal conductivity (model 1)

overestimates Geff12, 14-15 when small interfacial thermal conductances are involved. Indeed, with models 1 and 2, our fitting results show no observable difference in the thermal properties of each sample. Thus, in general the lumped thermal conductivity obtained from the standard treatment as for model 1 appears to be a good approximation for Λs only when G1 and G2 are sufficiently large. Nevertheless, in model 3 we reveal that a small G1 value leads to a notable increase in Λs and Cs. Furthermore, Geff calculated with Eq. (2) becomes smaller than the values given by previous models. Specifically, for the three samples investigated in this work, Geff is overestimated in model 1 by ~30% for GNRs coated with CTAB and 3K-PEG, and ~40% for GNRs coated with 1K-PEG. (ii) For all three models, the heat capacity of 1K-PEG and 3K-PEG is nearly a factor of two higher than that of CTAB. This observation suggests substantial water penetration into the PEG layer. The water penetration effect agrees with previous literature study on the hydrated polyelectrolyte multilayers,14 although the PEG-only surfactant in this work does not show the



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periodic “odd-even-effect” as widely seen for alternative anionic and cationic polyelectrolyte films. (iii) Our measurements likely capture the effects of the polymer-chain alignment in the surfactant layer. For all three models of samples B and C, Cs values extracted from transient absorption measurements are similar for 1K- and 3K-PEG, while Λs is consistently higher for 3K-PEG. If Cs is treated as a volumetric average of the heat capacities of water (≈ 4.2 J cm−3 K−1) and PEG (≈ 2 J cm−3 K−1), then the rate of water penetration (ranging from ~50% to 85% depending on the value of G1) is nearly the same for 1K- and 3K-PEG. The higher Λs for 3KPEG likely implies a better alignment for the 3K-PEG sample having longer molecular chain lengths. It has been reported that well-aligned polyethylene can possess a thermal conductivity nearly 20 times higher than water.59 (iv) In general, the difference in thermal behavior of the two surfactants (captured by Geff) could be attributed to the differences in GNR/surfactant interfacial thermal transport and/or to the potential effects of the hydrophilicity or hydrophobicity on Λs, considering that the thermal conductivity of water (~0.6 W m−1 K−1) is typically higher than that of amorphous polymer materials (~0.2 W m−1 K−1). Assuming another extreme case of no water penetration into PEG, we refitted the transient experimental data using G1 = 146 ± 22 MW m−2 K−1 and obtained Geff = 36 ± 8 MW m−2 K−1, which is lower than the Geff for 1K PEG solvated wholly by water molecules (42 ± 8 MW m−2 K−1). Thus Geff of 1K PEG with partial water penetration should locate within the range of 36 to 42 MW m−2 K−1, which is larger than Geff for CTAB (34 ± 8 MW m−2 K−1). Our analysis with model 3 supports the former explanation, as Λs of PEG turns out to be comparable to that of CTAB. For model 1, the values of Λs for PEG are higher than that


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of CTAB. However, this standard analysis lumps the effect of the interface into Λs and thus it does not allow for distinguishing between the two possible explanations.

5. CONCLUSIONS We have performed ultrafast transient absorption experiment to investigate the heat transfer from CTAB- and PEG-functionalized GNRs to the aqueous surroundings. MD simulations reveal that the GNR/surfactant interface severely obstructs the heat flow from the GNR, in addition to the intrinsic thermal resistance from the surfactant layer. For this reason, a multiscale heat diffusion thermal modeling with explicit interfaces (model 3) is used to extract the effective interfacial conductance from the experimental absorption measurement data. We obtain that PEG performs noticeably better in thermal transport than the CTAB surfactant and attribute the difference in thermal performance to the difference in interfacial conductance between the GNR/PEG and GNR/CTAB. At the microscopic level, this is due to heat conduction paths set by the covalent bonding between Au and PEG and the van der Waals interactions occurring between Au and the penetrated water, which are not present at the GNR/CTAB interface due to the hydrophobic nature of CTAB. Overall, this multiscale heat diffusion thermal modeling enables the revealing of possible explanations of the enhanced thermal transport in GNR/PEG/water, including better interfacial thermal conductance and molecular chain alignment, in addition to water penetration as reported in literature. The fundamental understanding of the thermal transport within tens of nanometers of GNRs uncovered in this study could offer promising strategies to resolve the thermal stability issue in GNR synthesis and to develop applications of functionalized GNRs in biological and heat transfer areas.



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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publication website. Gold Nanorod (GNR) Synthesis, Determination of the Surfactant Thickness, Wavelength Selection for Pump and Probe Beams, Control Measurements on the Water Solvent, Repeatability of Transient Absorption Measurements on Three Samples, Thermal Transport Model, Uncertainty of Interfacial Conductance Calculations from MD Simulations, Sensitivity Analysis of Transient Absorption and Uncertainty Estimation, Comparison of Geff in the Cylindrical and Cartesian Coordinates, Discussion of Parameters that Influence the Transient Temperature Decay Process, Effect of G2 on the Fitting Results.

ACKNOWLEDGMENTS This work was partially supported by the National Science Foundation (NSF) through the University of Minnesota MRSEC under Award Number DMR-1420013 (XWW and XJW), NSF CMMI-1000415 (YXN and TD), and NSF CHE-1306596 (NDB and CJM). JZ would like to thank the support from the National Natural Science Foundation of China (Grant No. 51206167 and

No. 51373184).

Computations

were

performed

at

the

Minnesota

Supercomputing Institute.


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REFERENCES (1) Huang, X.; Neretina, S.; El-Sayed, M. A. Gold Nanorods: From Synthesis and Properties to Biological and Biomedical Applications. Adv. Mater. 2009, 21 (48), 4880-4910. (2) Alivisatos, A. P. The Use of Nanocrystals in Biological Detection. Nat. Biotechnol. 2004, 22 (1), 47-52. (3) Day, E. S.; Morton, J. G.; West, J. L. Nanoparticles for Thermal Cancer Therapy. J. Biomech. Eng. 2009, 131 (7), 074001. (4) Huang, H. C.; Barua, S.; Sharma, G.; Dey, S. K.; Rege, K. Inorganic Nanoparticles for Cancer Imaging and Therapy. J. Control. Release 2011, 155 (3), 344-357. (5) Jain, P. K.; El-Sayed, I. H.; El-Sayed, M. A. Au Nanoparticles Target Cancer. Nano Today 2007, 2 (1), 18-29. (6) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. Anisotropic Metal Nanoparticles: Synthesis, Assembly, and Optical Applications. J. Phys. Chem. B 2005, 109 (29), 13857-13870. (7) Gomez-Grana, S.; Hubert, F.; Testard, F.; Guerrero-Martínez, A.; Grillo, I.; Liz-Marzán, L. M.; Spalla, O. Surfactant (Bi) Layers on Gold Nanorods. Langmuir 2011, 28 (2), 1453-1459. (8) Niidome, T.; Akiyama, Y.; Yamagata, M.; Kawano, T.; Mori, T.; Niidome, Y.; Katayama, Y. Poly(Ethylene Glycol)-Modified Gold Nanorods as a Photothermal Nanodevice for Hyperthermia. J. Biomat. Sci.-Polym. E. 2009, 20 (9), 1203-1215. (9) Niidome, T.; Ohga, A.; Akiyama, Y.; Watanabe, K.; Niidome, Y.; Mori, T.; Katayama, Y. Controlled Release of Peg Chain from Gold Nanorods: Targeted Delivery to Tumor. Bioorgan. Med. Chem. 2010, 18 (12), 4453-4458. (10) Wilson, O. M.; Hu, X.; Cahill, D. G.; Braun, P. V. Colloidal Metal Particles as Probes of Nanoscale Thermal Transport in Fluids. Phys. Rev. B 2002, 66 (22), 224301. (11) Ge, Z.; Kang, Y.; Taton, T. A.; Braun, P. V.; Cahill, D. G. Thermal Transport in AuCore Polymer-Shell Nanoparticles. Nano Lett. 2005, 5 (3), 531-535. (12) Schmidt, A. J.; Alper, J. D.; Chiesa, M.; Chen, G.; Das, S. K.; Hamad-Schifferli, K. Probing the Gold Nanorod− Ligand− Solvent Interface by Plasmonic Absorption and Thermal Decay. J. Phys. Chem. Lett. 2008, 112, 13320-13323. (13) Hu, M.; Wang, X.; Hartland, G. V.; Salgueiriño-Maceira, V.; Liz-Marzán, L. M. Heat Dissipation in Gold–Silica Core-Shell Nanoparticles. Chem. Phys. Lett. 2003, 372 (5), 767-772. (14) Huang, J.; Park, J.; Wang, W.; Murphy, C. J.; Cahill, D. G. Ultrafast Thermal Analysis of Surface Functionalized Gold Nanorods in Aqueous Solution. ACS Nano 2012, 7 (1), 589-597. (15) Alper, J. D.; Hamad-Schifferli, K. Effect of Ligand on Thermal Dissipation from Gold Nanorods. Langmuir 2010, 26 (6), 3786-3789. (16) Park, J.; Huang, J. Y.; Wang, W.; Murphy, C. J.; Cahill, D. G. Heat Transport between Au Nanorods, Surrounding Liquids, and Solid Supports. J. Phys. Chem. C 2012, 116 (50), 26335-26341. (17) Baffou, G.; Rigneault, H. Femtosecond-Pulsed Optical Heating of Gold Nanoparticles. Phys. Rev. B 2011, 84, 035415. (18) Chen, X. L.; Munjiza, A.; Zhang, K.; Wen, D. S. Molecular Dynamics Simulation of Heat Transfer from a Gold Nanoparticle to a Water Pool. J. Phys. Chem. C 2014, 118, 12851293. 27 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

(19) Tribelsky, M. I.; Miroshnichenko, A. E.; Kivshar, Y. S.; Luk'yanchuk, B. S.; Khokhlov, A. R. Laser Pulse Heating of Spherical Metal Particles. Phys. Rev.X 2011, 1, 021024. (20) Volkov, A. N.; Sevilla, C.; Zhigilei, L. V. Numerical Modeling of Short Pulse Laser Interaction with Au Nanoparticle Surrounded by Water. Appl. Surf. Sci. 2007, 253, 6394–6399. (21) Merabia, S.; Shenogin, S.; Joly, L.; Keblinski, P.; Barrat, J.-L. Heat Transfer from Nanoparticles: A Corresponding State Analysis. Proc. Natl. Acad. Sci. USA. 2009, 106 (36), 15113-15118. (22) Soussi, J.; Volz, S.; Palpant, B.; Chalopin, Y. A Detailed Microscopic Study of the Heat Transfer at a Water Gold Interface Coated with a Polymer. Appl. Phys. Lett. 2015, 106 (9), 093113. (23) Sau, T. K.; Murphy, C. J. Seeded High Yield Synthesis of Short Au Nanorods in Aqueous Solution. Langmuir 2004, 20 (15), 6414-6420. (24) Murphy, C. J.; Thompson, L. B.; Chernak, D. J.; Yang, J. A.; Sivapalan, S. T.; Boulos, S. P.; Huang, J.; Alkilany, A. M.; Sisco, P. N. Gold Nanorod Crystal Growth: From Seed-Mediated Synthesis to Nanoscale Sculpting. Curr. Opin. Colloid. IN. 2011, 16 (2), 128-134. (25) Lohse, S. E.; Eller, J. R.; Sivapalan, S. T.; Plews, M. R.; Murphy, C. J. A Simple Millifluidic Benchtop Reactor System for the High-Throughput Synthesis and Functionalization of Gold Nanoparticles with Different Sizes and Shapes. ACS Nano 2013, 7 (5), 4135-4150. (26) Meena, S. K.; Sulpizi, M. Understanding the Microscopic Origin of Gold Nanoparticle Anisotropic Growth from Molecular Dynamics Simulations. Langmuir 2013, 29 (48), 1495414961. (27) Pérez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzán, L. M.; Mulvaney, P. Gold Nanorods: Synthesis, Characterization and Applications. Coord. Chem. Rev. 2005, 249 (17), 1870-1901. (28) Petrova, H.; Juste, J. P.; Pastoriza-Santos, I.; Hartland, G. V.; Liz-Marzán, L. M.; Mulvaney, P. On the Temperature Stability of Gold Nanorods: Comparison between Thermal and Ultrafast Laser-Induced Heating. Phys. Chem. Chem. Phys. 2006, 8 (7), 814-821. (29) Wang, W.; Cahill, D. G. Limits to Thermal Transport in Nanoscale Metal Bilayers Due to Weak Electron-Phonon Coupling in Au and Cu. Phys. Rev. Lett. 2012, 109 (17), 175503. (30) Hartland, G. V. Measurements of the Material Properties of Metal Nanoparticles by Time-Resolved Spectroscopy. Phys. Chem. Chem. Phys. 2004, 6 (23), 5263-5274. (31) Hu, M.; Wang, X.; Hartland, G. V.; Mulvaney, P.; Juste, J. P.; Sader, J. E. Vibrational Response of Nanorods to Ultrafast Laser Induced Heating: Theoretical and Experimental Analysis. J. Am. Chem. Soc. 2003, 125 (48), 14925-14933. (32) Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine. J. Phys. Chem. B 2006, 110 (14), 72387248. (33) Baral, S.; Green, A. J.; Livshits, M. Y.; Govorov, A. O.; Richardson, H. H. Comparison of Vapor Formation of Water at the Solid/Water Interface to Colloidal Solutions Using Optically Excited Gold Nanostructures. ACS Nano 2014, 8 (2), 1439-1448. (34) Govorov, A. O.; Richardson, H. H. Generating Heat with Metal Nanoparticles. Nano Today 2007, 2 (1), 30-38. (35) Green, A. J.; Alaulamie, A. A.; Baral, S.; Richardson, H. H. Ultrasensitive Molecular Detection Using Thermal Conductance of a Hydrophobic Gold-Water Interface. Nano Lett. 2013, 13 (9), 4142-4147. 28 ACS Paragon Plus Environment

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(36) Richardson, H. H.; Carlson, M. T.; Tandler, P. J.; Hernandez, P.; Govorov, A. O. Experimental and Theoretical Studies of Light-to-Heat Conversion and Collective Heating Effects in Metal Nanoparticle Solutions. Nano Lett. 2009, 9 (3), 1139-1146. (37) Juvé, V.; Scardamaglia, M.; Maioli, P.; Crut, A.; Merabia, S.; Joly, L.; Del Fatti, N.; Vallée, F. Cooling Dynamics and Thermal Interface Resistance of Glass-Embedded Metal Nanoparticles. Phys. Rev. B 2009, 80 (19), 195406. (38) Winsemius, P.; Van Kampen, F.; Lengkeek, H.; Van Went, C. Temperature Dependence of the Optical Properties of Au, Ag and Cu. J. Phys. Met. Phys. 1976, 6 (8), 1583-1606. (39) Lyeo, H.-K.; Cahill, D. G. Thermal Conductance of Interfaces between Highly Dissimilar Materials. Phys. Rev. B 2006, 73 (14), 144301. (40) Kang, S. D.; Lim, S. C.; Lee, E.-S.; Cho, Y. W.; Kim, Y.-H.; Lyeo, H.-K.; Lee, Y. H. Interfacial Thermal Conductance Observed to Be Higher in Semiconducting Than Metallic Carbon Nanotubes. ACS Nano 2012, 6 (5), 3853-3860. (41) Park, J.; Cahill, D. G. Plasmonic Sensing of Heat Transport at Solid-Liquid Interfaces. J. Phys. Chem. C 2016, 120 (5), 2814–2821. (42) Losego, M. D.; Moh, L.; Arpin, K. A.; Cahill, D. G.; Braun, P. V. Interfacial Thermal Conductance in Spun-Cast Polymer Films and Polymer Brushes. Appl. Phys. Lett. 2010, 97 (1), 011908. (43) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comp. Phys. 1995, 117 (1), 1-19. (44) Foiles, S. M.; Baskes, M. I.; Daw, M. S. Embedded-Atom-Method Functions for the Fcc Metals Cu, Ag, Au, Ni, Pd, Pt, and Their Alloys. Phys. Rev. B 1986, 33 (12), 7983-7991. (45) Mayo, S. L.; Olafson, B. D.; Goddard, W. A. Dreiding - a Generic Force-Field for Molecular Simulations. J. Phys. Chem.-US 1990, 94 (26), 8897-8909. (46) Liang, Z.; Evans, W.; Desai, T.; Keblinski, P. Improvement of Heat Transfer Efficiency at Solid-Gas Interfaces by Self-Assembled Monolayers. Appl. Phys. Lett. 2013, 102 (6), 061907. (47) Price, D. J.; Brooks, C. L. A Modified Tip3p Water Potential for Simulation with Ewald Summation. J. Chem. Phys. 2004, 121 (20), 10096-10103. (48) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. Uff, a Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc. 1992, 114 (25), 10024-10035. (49) Rajabpour, A.; Volz, S. Thermal Boundary Resistance from Mode Energy Relaxation Times: Case Study of Argon-Like Crystals by Molecular Dynamics. J. Appl. Phys. 2010, 108 (9), 094324. (50) Ni, Y.; Han, H.; Volz, S.; Dumitrica, T. Nanoscale Azide Polymer Functionalization: A Robust Solution for Suppressing the Carbon Nanotube-Polymer Matrix Thermal Interface Resistance. J. Phys. Chem. C 2015, 119, 12193–12198. (51) Ni, Y.; Jiang, J.; Meletis, E.; Dumitricǎ, T. Thermal Transport across Few-Layer Boron Nitride Encased by Silica. Appl. Phys. Lett. 2015, 107 (3), 031603. (52) Nakahara, H.; Shibata, O.; Moroi, Y. Examination of Surface Adsorption of Cetyltrimethylammonium Bromide and Sodium Dodecyl Sulfate. J. Phys. Chem. B 2011, 115 (29), 9077-9086. (53) Majumdar, S.; Sierra-Suarez, J. A.; Schiffres, S. N.; Ong, W.-L.; Higgs, C. F.; McGaughey, A. J. H.; Malen, J. A. Vibrational Mismatch of Metal Leads Controls Thermal Conductance of Self-Assembled Monolayer Junctions. Nano Lett. 2015, 15 (5), 2985-2991. 29 ACS Paragon Plus Environment


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(54) Luo, T.; Lloyd, J. R. Non-Equilibrium Molecular Dynamics Study of Thermal Energy Transport in Au–Sam–Au Junctions. Int. J. Heat Mass Transfer 2010, 53 (1), 1-11. (55) Kaur, S.; Raravikar, N.; Helms, B. A.; Prasher, R.; Ogletree, D. F. Enhanced Thermal Transport at Covalently Functionalized Carbon Nanotube Array Interfaces. Nat. Commun. 2014, 5, 3082. (56) Losego, M. D.; Grady, M. E.; Sottos, N. R.; Cahill, D. G.; Braun, P. V. Effects of Chemical Bonding on Heat Transport across Interfaces. Nat. Mater. 2012, 11 (6), 502-506. (57) Ge, Z.; Cahill, D. G.; Braun, P. V. Thermal Conductance of Hydrophilic and Hydrophobic Interfaces. Phys. Rev. Lett. 2006, 96 (18), 186101. (58) Sun, F.; Zhang, T.; Jobbins, M. M.; Guo, Z.; Zhang, X.; Zheng, Z.; Tang, D.; Ptasinska, S.; Luo, T. Molecular Bridge Enables Anomalous Enhancement in Thermal Transport across Hard‐Soft Material Interfaces. Adv. Mater. 2014, 26 (35), 6093-6099. (59) Wang, X.; Ho, V.; Segalman, R. A.; Cahill, D. G. Thermal Conductivity of HighModulus Polymer Fibers. Macromolecules 2013, 46 (12), 4937-4943.


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Tables of Contents (TOC)

1.5 1

∆abs

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Figure 1. Structure formula of CTAB and PEG surfactants (not shown to scale). The PEG molecular chains are bonded directly onto the GNR surface through covalent bonding. The CTAB forms a bilayer structure, which is stabilized on the GNR surface through a combination of van der Waals and electrostatic interactions.26 230x162mm (150 x 150 DPI)

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Figure 2. Optical layout of the ultrafast transient absorption setup in the transmission configuration. EOM, PBS, and NBS are acronyms for electro-optic modulator, polarizing beamsplitter, and non-polarizing beamsplitter. The GNR samples are loaded in a 100-µm thick capillary tube. 227x117mm (150 x 150 DPI)



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Figure 3. (a) Schematic of the thermal modeling for a surface functionalized GNR with axial symmetry. r1 is the radius of the bare GNR and r2 = r1+ hs is the radius of the interface 2 with hs being the thickness of the surfactant layer. (b) Network of thermal resistance in series illustrating the heat flux from GNR to the surrounding water. 159x201mm (150 x 150 DPI)


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Figure 4. MD relaxed configuration of Au-PEG-water model for molecular dynamic simulations. Inset shows the bonding of a PEG chain covalently bonded onto the Au surface. 213x190mm (150 x 150 DPI)



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Figure 5. (a) Transmission electron micrograph (TEM) of the GNRs with the CTAB surfactant. The scale bar is 50 nm. (b) Distribution of the aspect ratio of GNRs coated with 1K-PEG analyzed from TEM micrographs. (c) UV-VIS absorption spectrum of GNRs in aqueous solution. 171x183mm (150 x 150 DPI)


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Figure 6. (a) Fitting of the transient absorption signal to the heat diffusion model. Open circles are the normalized transient absorption signals of GNRs with CTAB (black), 1K-PEG (blue), and 3K-PEG (dark gold), and solid red lines represent the best fit of thermal modeling to the transient absorption measurement data with G1 calculated from MD (model 3). (b) Schematics of the extensional (deformation of GNR lattice along the axial direction) and breathing (deformation of GNR lattice along the radial direction) modes of the coherent phonon vibration. Color contrast (green) is added to indicate the shape vibrational deformation of the GNRs. (c) Picosecond acoustic fringes indicating the extensional mode (axial) of coherent phonon vibration. The thermal decay background has been removed in (c) for better illustration. 343x193mm (150 x 150 DPI)


Thermal Transport across Surfactant Layers on Gold Nanorods in

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