Probing Photothermal Effects on Optically Trapped Gold Nanorods by

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Probing Photothermal Effects on Optically Trapped Gold Nanorods by Simultaneous Plasmon Spectroscopy and Brownian Dynamics Analysis Daniel Andrén,*,† Lei Shao,† Nils Odebo Lan̈ k,† Srdjan S. Aćimović,† Peter Johansson,†,‡ and Mikael Kal̈ l*,† †

Department of Physics, Chalmers University of Technology, S-412 96 Göteborg, Sweden School of Science and Technology, Ö rebro University, S-701 82 Ö rebro, Sweden

‡

S Supporting Information *

ABSTRACT: Plasmonic gold nanorods are prime candidates for a variety of biomedical, spectroscopy, data storage, and sensing applications. It was recently shown that gold nanorods optically trapped by a focused circularly polarized laser beam can function as extremely efficient nanoscopic rotary motors. The system holds promise for applications ranging from nanofluidic flow control and nanorobotics to biomolecular actuation and analysis. However, to fully exploit this potential, one needs to be able to control and understand heating effects associated with laser trapping. We investigated photothermal heating of individual rotating gold nanorods by simultaneously probing their localized surface plasmon resonance spectrum and rotational Brownian dynamics over extended periods of time. The data reveal an extremely slow nanoparticle reshaping process, involving migration of the order of a few hundred atoms per minute, for moderate laser powers and a trapping wavelength close to plasmon resonance. The plasmon spectroscopy and Brownian analysis allows for separate temperature estimates based on the refractive index and the viscosity of the water surrounding a trapped nanorod. We show that both measurements yield similar effective temperatures, which correspond to the actual temperature at a distance of the order 10−15 nm from the particle surface. Our results shed light on photothermal processes on the nanoscale and will be useful in evaluating the applicability and performance of nanorod motors and optically heated nanoparticles for a variety of applications. KEYWORDS: photothermal effects, gold nanorod, optical tweezers, nanomotors, thermal reshaping, Brownian dynamics

I

Plasmonic nanostructures, such as gold nanoparticles, interact strongly with optical fields, and this has led to a wealth of studies and applications in areas including sensing,9 medicine,10,11 data storage,12 and solar harvesting.13 In 1994, Svoboda and Block demonstrated that gold colloids could be trapped and manipulated in solution by laser tweezers.14 The combination of optical tweezing and plasmonics has since developed into a new subfield, generating a range of interesting findings.6,15−22 One of the many useful aspects of plasmonic nanoparticles is that they offer several possibilities for spectroscopic analysis of the environment. One example is localized surface plasmon

t is extremely challenging to drive and study nanoscopic processes occurring at and around the surface of a single nanoparticle, and the difficulty increases further when the nanoparticle diffuses in solution. The most commonly used approach in this situation is to use optical tweezers, which offer the possibility to trap a single particle using optical gradient forces induced by a tightly focused laser beam.1 The possibility to exert torque on a trapped particle using circularly polarized laser tweezers provides an additional degree of freedom in optomechanical studies.2,3 Laser tweezers can be used to manipulate and analyze dielectric, metallic, and biological particles, and they have enabled a multitude of intricate studies in, for example, bio- and soft matter physics.4,5 Many of these studies are based on analyzing the positional or orientational fluctuations of the trapped object, which allows for measurements of exceedingly weak forces and small distances.6−8 © 2017 American Chemical Society

Received: June 20, 2017 Accepted: September 5, 2017 Published: September 5, 2017 10053

DOI: 10.1021/acsnano.7b04302 ACS Nano 2017, 11, 10053−10061

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Cite This: ACS Nano 2017, 11, 10053-10061

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Figure 1. Two-dimensional optical trapping and rotation of gold nanorods to probe photothermal effects. (a) Scanning electron microscope image of the two types of gold nanorods analyzed in the experiments. The length and diameter of the “small” (“large”) nanorods are 108 ± 7 and 65 ± 5 nm (155 ± 9 and 88 ± 5 nm), respectively. (b) Ensemble extinction spectra for the two particle types in aqueous solution. The longitudinal localized surface plasmon resonance (LSPR) wavelength is found at 623 (743) nm. The dashed line indicates the laser wavelength (660 nm) used in the experiment. (c) An illustration of the experimental procedure where optical tweezers are combined with instrumentation for dark-field scattering spectroscopic measurements as well as single photon correlation analysis. (d) The scattered white light provides a means of tracking the plasmonic changes the particles experience. (e) Analyzing the scattered laser light yields an autocorrelation signal from which the rotation frequency and the degree of rotational Brownian fluctuation can be obtained.

particle temperature estimates, which we record as functions of laser power. Finally, the meaning of these two temperatures is discussed in relation to the calculated spatial temperature profile around photothermally heated gold nanorods. We conclude by summarizing the work and by pointing out some possible directions for further research.

resonance (LSPR) sensing, which utilizes the extremely high sensitivity of the surface plasmon resonance to changes in the refractive index (RI) of the medium close to the metal surface. We recently showed that single crystalline gold nanorods, one of the most important classes of plasmonic nanostructures, could be trapped and rotated at record rotation frequencies in water by circularly polarized laser tweezers.23 The possibility to utilize such optically driven nanomotors for molecular analysis was demonstrated as a proof of principle, and several other biomedical and nanofluidic applications were suggested. The high rotation frequencies observed, reaching several tens of kHz, are possible because of the combined effect of a small particle size, which decreases friction, and an amplification of the scattering component of the optical torque for laser wavelengths near the LSPR.23 However, residual ohmic losses in the particles are unavoidable near resonance, and their advantageous properties therefore come at the price of significant heat generation. This can in turn affect molecular adlayers, result in reshaping of the trapped particle, and even induce bubble formation around the particle surface. Photothermal effects can be problematic in some applications, for example, in delicate biomolecular experiments, but they can also be put to an advantage, such as in photothermal imaging,24 plasmonic photothermal therapy,25 and nanosurgery.26 In this work, we examine photothermal processes affecting single gold nanorods that are trapped and actively rotated using optical tweezers. We first briefly describe the sample characteristics and the experimental methodology, which is based on simultaneous plasmon spectroscopy and rotational Brownian motion analysis. Next, we show how these techniques can be used to reveal and quantify miniscule laser-induced changes in nanorod shape as a function of time. We then utilize the RI sensitivity of the LSPR and the viscosity sensitivity of the Brownian fluctuations, respectively, to establish two separate

RESULTS AND DISCUSSION Probing Photothermal Processes via Two Separate Channels. Figure 1 illustrates the sample characteristics and the experimental methods used. We focus on two types of single crystalline gold nanorods, hereafter referred to as “small” and “large”, both prepared via seed-mediated growth.23 The particle dimensions were chosen such that their plasmon spectra overlapped the trapping laser wavelength at 660 nm in order to induce strong photothermal interactions and optical torques. The morphological and spectroscopic differences between the two batches are illustrated by the SEM images and ensemble averaged extinction spectra shown in Figure 1a,b, respectively. The lengths and diameters of the “small” (“large”) nanorods were 108 ± 7 (155 ± 9) nm and 65 ± 5 (88 ± 5) nm, corresponding to aspect ratios (ARs) of ∼1.66 (∼1.76). The spectroscopic and optomechanical properties of a trapped nanorod can be used to access a considerable amount of information about several particle-specific photothermal processes. In order to probe these on the single particle level, an experiment was constructed around two-dimensional (2D) optical tweezers interfaced to a grating spectrometer and a photon correlation system (Figure 1c and Figure S1). The setup and analysis routines allow for continuous and parallel measurements of dark-field (DF) scattering spectra and autocorrelation functions C(τ) versus laser power (see Methods for details). 10054


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Figure 2. Spectroscopic and optomechanical changes induced by laser tweezing and optical rotation of “small” (“large”) gold nanorods at constant laser power of 16 (20) mW. (a) Intensity autocorrelation functions recorded at the start and end of a 30 min experiment. Note the shift of the first valley of the oscillation toward longer lag times for the “small” nanorod, corresponding to a decrease in rotation frequency. Data for the “large” nanorod are shifted vertically by 0.025. (b) Dark-field scattering spectra recorded at the start and end of the experiment. Note the blueshifts of the long wavelength longitudinal plasmon resonances. The spectral region between 630 and 670 nm is blocked by the notch filter used to introduce the trapping laser beam. The inset illustrates the corresponding change in aspect ratio (not to scale). (c,d) Simultaneously recorded longitudinal plasmon peak position (blue) and rotation frequency (red) of the two particle types versus time. (e,f) Calculated optical torque spectra and absorption cross sections for “small” and “large” nanorods of constant volume but slightly different aspect ratios. The dimensions of the nanorods were selected within the standard deviations of the measured sizes and adjusted to match the experimental scattering spectra recorded at the start of the experiments. The dashed line marks the laser wavelength used in the experiments.

A gold nanorod supports two electric dipolar LSPRs, a longitudinal mode at long wavelengths and a two-fold degenerate transverse mode at shorter wavelengths. The mode splitting monotonically increases as a function of increasing aspect ratio. The resonance wavelengths can be accessed through a biLorentzian curve fit of DF scattering spectra (Figure 1d). The LSPRs provide information on the dimensions of the nanorods and of the (temperature dependent) RI of the surrounding medium. The laser light backscattered from a trapped and rotating nanorod can be used to determine its average rotation frequency favg and rotational decay time τ0 due to Brownian fluctuations (Figure 1e). These parameters can be extracted by polarized photon correlation spectroscopy and curve fitting to a theoretical autocorrelation function (see Methods). The rotation parameters are determined by the optical torque Mopt, the rotational friction coefficient γr, and the temperature T according to

favg (T ) =

τ0(T ) =

Mopt 2πγr(T )

(1)

γr(T ) 4kBT

(2)

For a nanorod with length L, we have γr(T) = πη(T)gL3, where η(T) is the temperature-dependent dynamical viscosity of the surrounding water and g is a geometrical factor depending on the nanorod eccentricity.23,27 Continuous Reshaping at Constant Trapping Power. The nanomotors were first operated at constant power for extended periods of time. Figure 2 illustrates typical behaviors observed for one exemplary nanorod of each type. During 30 min of continuous trapping, the “small” particle exhibits a pronounced decrease in rotation frequency, from ∼16.5 to ∼14.5 kHz, whereas the “large” particle rotated with a constant, 10055


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Figure 3. Time evolutions of the extracted properties, that is, longitudinal LSPR peak position, rotation frequency, and autocorrelation decay time as the laser power is swept in sawtooth ramps with linearly increasing maxima, for the two different particle types. For each nanorod, as the laser power increases, the LSPR peak blue-shifts because of a temperature-induced decrease in the RI of the water environment. The rotation frequency has a positive linear dependence on laser power, whereas the autocorrelation decay time has a negative dependence. (a) “Small” NR case. The dashed blue line is a fit to the low laser power spectral peak positions, illustrating the continuous reshaping behavior. (b) “Large” NR case. Again, the dashed blue line indicates irreversible reshaping. The black marks indicate a change in the slope of the linear relation between the laser power and λLSPR that occurs at Pth, where nucleation of vapor is suggested to occur.

or even slightly increasing, frequency of ∼14.5 kHz (Figure 2a). This indicates that the focused laser field induces structural changes in the “small” but not in the “large” nanorod. However, the recorded plasmon spectra reveal clear changes due to photothermal reshaping for both nanorod types (Figure 2b). Specifically, the long-wavelength longitudinal LSPR peaks continuously blue-shifted by a few nm during the course of the experiment, indicating a slow decrease in aspect ratio toward a more spherical shape, for both the “small” and the “large” rod (Figure 2c,d). The transverse LSPR of the “small” nanorod exhibits a concomitant red-shift as is expected for a decreasing aspect ratio (Figure S2a). We performed electrodynamic simulations of changes in spectral and rotational properties to explain this behavior (see Methods for details). As seen in Figure 2e, the driving optical torque Mopt at the trapping wavelength decreases significantly if the aspect ratio of the “small” nanorod is slightly decreased (by ∼3%, keeping the volume constant). This will result in a decreased favg according to eq 1. In contrast, the torque driving the “large” nanorod is essentially constant for a similar change in particle shape, in spite of an even larger spectral shift toward shorter wavelengths (Figure 2f, see Figure S3 for calculated scattering spectra). Figure 2e,f shows the corresponding changes in absorption spectra caused by the decrease in aspect ratio. The absorption cross-section σabs at 660 nm obviously decreases for the “small” particle due to the reshaping. This will result in a lower surface temperature, which will in turn increase the viscosity and rotational friction coefficient around the particle, thus further reducing the rotation frequency. The “large” particle, in contrast, shows a minor increase in absorption at 660 nm, which will thus tend to increase its rotation frequency over time. The simulations hence qualitatively explain the contrasting experimental behavior observed for the two types of nanorods,

and they illustrate the rather complex relation between particle shape, the optical torque spectrum, and the photothermal properties of gold nanorods.23 Quantification of Photothermal Reshaping. Photothermal reshaping of gold nanorods can be understood as a diffusion process where gold atoms primarily migrate from high to low curvature areas, driving the particle toward a thermodynamically stable spherical shape.28 Previous studies of this process have mainly concerned immobilized particles subject to ultrafast laser pulses,12,28−33 and there are few studies based on CW laser illumination.31,34,35 Thermal reshaping of optically trapped gold nanorods have only been mentioned in passing (e.g., in SI of ref 6). It is therefore interesting to quantify the reshaping in terms of actual number of atoms involved. To do this, we first need to connect the measured LSPR shifts to actual changes in aspect ratio. We used anisotropic etching36 to shrink the length of ensembles of particles while monitoring the longitudinal LSPR and aspect ratio changes through extinction spectroscopy and SEM analysis, respectively. The LSPR shifts observed in this experiment were similar to those seen in the single nanoparticle trapping experiments. We obtained values for dλLSPR/dAR of 148 and 221 nm for the “small” and the “large” particle types, respectively, in excellent agreement with finite difference time domain (FDTD) simulations of particles with constant volume and decreasing aspect ratio (Figure S4). By approximating the nanorod shape with a hemispherically capped cylinder (capsule), we then estimated the absolute change in nanorod dimensions during the time course of an experiment (Supporting Information Discussion S1, Figure S5). This analysis shows that the nanorods shrink in length by only a few Å during the 30 min experiments shown in Figure 2. The corresponding migration rate amounts to a few hundred atoms per minute (Supporting Information Table S1). We find it quite 10056


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estimate the reversible LSPR shifts ΔλLSPR induced by the RI change, by fitting and subtracting the linear trend from the spectral shifts in Figure 3 measured at the lowest laser power (Figure S6a). This procedure hence effectively removes the contribution from reshaping (Figure S6b). We here assume that no adsorption or desorption of stabilizing ligand molecules occur during the experiment, which is reasonable considering the highly diluted colloid solution used in the experiments. To dλ dλ dn = dLSPR proceed, we then utilize the chain rule ( dLSPR ) and T n dT the RI temperature dependence of water, which is well described by n(T) = 1.335 − AT − BT2, where A and B are constants (Figure S7).41 The RI sensitivity of the longitudinal LSPRs can be obtained from extinction spectra of nanorods dispersed in mixtures of water/glycerol of varying concentration, yielding dλLSPR/dn = 230 (331) nm for the “small” (“large”) nanorods (Figure S8). Integrating from an initial peak position λLSPR(T0) to a position at temperature T = TRI, which thus defines ΔλLSPR, then yields

remarkable that such modest structural changes can be resolved for single optically trapped nanoparticles. Reversible Photothermal Heating Effects. We next tried to separate the slow irreversible reshaping process from reversible heating effects caused by an increase in temperature of the water surrounding the trapped particle. We applied a sawtooth laser power pattern with linearly increasing amplitude while tracking the DF spectra and rotational dynamics of the trapped particles. Figure 3 summarizes data for one exemplary particle of each kind, “small” and “large”. It is obvious that the sawtooth laser pattern is essentially reproduced in the three extracted parameters, the longitudinal LSPR peak position (λLSPR), the rotation frequency (f), and the autocorrelation decay time (τ0), for both particle types (see shaded area in each panel). Because of the high thermal conductivity of Au, the particle temperature equilibrates within nanoseconds of changing the laser power.37 An increase in laser intensity will thus, almost instantaneously, increase the temperature of the water surrounding the particle surface and decrease its RI (n). Since dλLSPR/dn > 0, this qualitatively explains the observed negative correlation between λLSPR and laser power seen in Figure 3 (see Figure S2b for data on the “small” nanorod’s transverse LSPR mode). Further, according to eqs 1 and 2, an increased laser power will increase the optical driving torque, hence the positive correlation between f and P, and the concomitant temperature rise will decrease the viscosity of the interfacial water, resulting in the observed decrease in decay time τ0. Nevertheless, as in the constant power case, there is a long-term trend caused by a continuous reshaping of the nanorods. Reshaping clearly occurs with a more or less constant rate throughout the experiment in the “small” particle case (Figure 3a) and up until the last power ramp for the “large” particle (Figure 3b). Estimates of the corresponding shape changes and atomic migration rates can be found in the Supporting Information (Discussion S1 and Table S1). However, the final power ramp for the “large” nanorod induces an irreversible blue-shift that is even more pronounced (∼4 nm). It is also possible to discern a rather abrupt change in slope of the λLSPR(t) trace at a particular threshold power Pth ≈ 22 mW. The same effect was observed for several “large” nanorods at similar laser powers. We tentatively interpret this point as the onset of vapor formation around the trapped particle, since this would abruptly decrease the RI sensed by the particle, leading to a more pronounced LSPR blue-shift. Vapor formation would at the same time reduce the heat conduction away from the nanorod, which could explain the accelerated reshaping and the considerable irreversible peak shift. We return to the question of possible vapor formation at the end of the article. Two Separate Temperature Estimations. We now try to estimate the absolute temperature near the nanorod’s surface based on the Brownian dynamics and spectroscopic measurements in Figure 3. As described in refs 23 and 38, the effective rotational Brownian temperature Tr is most easily obtained from the decay time τ0 because this parameter, in contrast to favg, does not explicitly depend on the optical torque. We thus first set the nanorods lengths L and shape parameters g equal to average values obtained from SEM analysis of “small” and “large” particles. Tr can then be obtained from eq 2 by inserting an analytical expression for the viscosity of water η(T).38,39 A more in-depth analysis of temperature estimates from rotational dynamics, including stochastic simulations and a discussion of the role of translational diffusion can be found in ref 40. The spectroscopic data can be translated to a corresponding temperature, which we call TRI, in a similar fashion. We first

TRI(Δλr ) = −

A + 2B

ΔλLSPR A2 + AT0 + BT0 2 − 2 dλ 4B

( dn )

(3)

where the initial temperature T0 is the only unknown.

Figure 4. Temperature determination based on rotational Brownian dynamics (blue data points) and LSPR spectroscopy (red data points) obtained from the data shown in Figure 3 for the (a) “small” and the (b) “large” particle case.

Figure 4 shows the estimated Tr and TRI extracted from the data in Figure 3. In anticipation of the theoretical analysis below, we have set T0 = Tr (t = 0). This results in an excellent match between the two temperature estimates in the case of the “small” nanorod. For the “large” nanorod, there is a deviation between Tr and TRI for the higher laser powers. It is possible that this originates from our use of the experimentally obtained ensemble LSPR RI sensitivity in eq 3 rather than the sensitivity for the particular nanoparticle at hand. For example, setting dλLSPR/dn to 270 nm instead of to 331 nm, which is not unrealistic considering the particle polydispersity, results in an almost complete overlap of the two curves (not shown). Theoretical Treatment of the Heated Particle. In order to further interpret the experimentally obtained temperatures, we 10057


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Figure 5. Theoretical analysis of temperature estimates. (a) Calculated temperature decay along the longitudinal and transverse directions from a “small” gold nanorod, modeled as a hemispherically capped cylinder with dimensions 108 × 65 nm, in water compared to the analytical temperature decay for a sphere with identical volume (radius r0 ∼ 40 nm), surface temperature (Ts = 136 °C), and ambient temperature (T0 = 20 °C). The absorbed optical power in the nanorod was obtained from an FDTD simulation, yielding σabs = 4203 nm2 at 660 nm for circularly polarized light, and an incident intensity of I = 9.7 mW/μm2 similar to experiment. The inset displays the corresponding temperature map around the nanorod. (b) Gradients in RI (blue) and viscosity (red) as functions of distance from the surface of the equivalent spherical particle in (a). The inset indicates the radial distances that correspond to the effective RI (neff) and viscosity (η(Tr)) probed by the particle through its plasmon resonance position and rotational Brownian motion, respectively. (c) Theoretical and experimental Tr and TRI, together with calculated surface temperature Ts plotted against applied laser power.

need to analyze how the spectroscopic and Brownian measurements sample the temperature distribution around a heated nanorod. Figure 5a shows a calculation of the temperature distribution around a “small” nanorod obtained from a finite element simulation based on an absorption cross section estimated from an FDTD simulation and an incident laser intensity similar to experiments (see Methods). The simulation is compared to the analytical temperature distribution around a spherical particle with identical volume, T(r) = ΔTr0/r + Tamb, where Tamb is the ambient temperature, r0 is the radius of the equivalent sphere and Ts = T(r0) = Tamb + ΔT is the surface temperature. It can be seen that the temperature drops slightly slower (faster) along the long (short) axis of the rod compared to the sphere, approaching the same 1/r decay at long distances. However, the discrepancy is modest, and to simplify the analysis, we therefore use the spherical approximation in the following. Figure 5b shows the variations in viscosity and RI that result from the temperature gradient shown in Figure 5a based on the same analytical temperature dependences as before. Due to the rapid spatial decay of the optical near-field associated with LSPR excitation, the particle samples the RI very close to the surface. To analyze this effect, it is convenient to treat the nanoparticle as if it was immersed in a uniform medium characterized by an effective RI defined by42

6

∞

neff =

r ∫r 4πr 2( r0 ) n(r )dr 0

∞

6

r ∫r 4πr 2( r0 ) dr 0

(4)

where r0 is the particle radius. This expression is based on the Rayleigh approximation (r0 ≪ λ), for which the induced nearfield intensity decays as (r0/r)6 outward from the surface. We can thus insert n(r) = n(T(r)) from the calculated RI variation (Figure 5b) and obtain and equivalent effective RI. This quantity in turn corresponds to a point in the radial RI distribution at a certain distance reff from the particle surface (inset in Figure 5b). reff is set by the size of the particle and essentially independent of surface temperature. From here, an effective spectroscopic temperature TRI can be defined as the temperature value at the distance reff from the particle. In essence, this is the theoretical representation of the experimentally obtained spectroscopic temperature. A similar analysis can be applied to the spatially varying viscosity η(T(r)) sampled by the rotational fluctuations of the particle. Following ref 43, we thus calculate the effective temperature characterizing hot rotational Brownian motion 43 4 ∞ 4 according to Tr = (∫ ∞ r0 T(r)/(η(r)r )dr)/(∫ r0 1/(η(r)r )dr). Again, this temperature will correspond to a certain distance from the particle surface, determined by matching Tr to that 10058


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incident laser powers. Acceleration of the reshaping process at the highest laser powers indicates vapor formation above a certain particle-specific power threshold, though further studies are required to characterize this phenomenon in detail. Moreover, we show that the two separate measurement channels, LSPR sensing and Brownian rotation dynamics, can be used to probe the temperature profile around a trapped particle via the temperature-dependent RI and viscosity of water, respectively. A theoretical analysis shows that the effective temperatures are very similar and correspond to the actual temperature value at a distance of 10−15 nm from the metal surface for the particle sizes studied. We hope that the presented experimental procedure and methodology will facilitate quantitative applications of laser tweezed plasmonic particles and other resonant nanostructures susceptible to photothermal heating. We have previously demonstrated that Brownian analysis of rotating nanorods can be used to detect binding of organic molecules.23 One interesting possibility for further research is to take advantage of the photothermal effect to drive and study thermally induced conformational transitions of molecules near the particle surface. Optically tweezed and heated plasmonic particles are also perfectly suited for fundamental studies of nonequilibrium thermodynamics and heat transfer at the nanoscale. Simultaneous LSPR spectroscopy will clearly facilitate such studies, as would other spectroscopic information channels, like Raman or fluorescence analysis.

specific location in the temperature distribution (rr in inset of Figure 5c). Hence, we can theoretically assess the temperature sampled by rotational Brownian motion and the LSPR shift for given particle size, absorption cross-section, and illumination condition. Based on the above analysis, we calculated theoretical TRI and Tr for the same range of laser powers as used in the experiments. Figure 5c shows the results together with the experimental data (from Figure 4) and the calculated surface temperature plotted against applied laser power for the “small” and the “large” particle types. Generally, the theoretical and experimental temperatures are seen to match very well, which clearly indicates that the assumption to set the initial temperature T0 in eq 3 equal to Tr is reasonable. Further, the analysis shows that the rotational Brownian motion and the RI induced shift of the LSPR both measure temperatures very close to the particle surface. For the model spherical particle in Figure 5a,b, which has the same volume as the “small” nanorod, the distances are of the order 10− 15 nm, or ∼30% of the equivalent particle radius r0 = 40 nm. An even smaller particle would naturally probe the temperature even closer to its surface. Importantly, this high “surface sensitivity” indicates that the particle rotational dynamics and the LSPR are essentially insensitive to the presence of cover glass that is used to keep the rotating nanorods in the optical trap. Still, the calculations indicate that there is a substantial difference between the actual surface temperature and the temperatures measured through Brownian analysis and LSPR spectroscopy. For the “small” nanorod, the data in Figure 5c indicate a temperature difference approaching 100 K at the highest laser powers. Vapor Formation. The establishment of a temperature scale TS(P) in Figure 5c allows us to briefly revisit the question of possible vapor formation discussed in conjunction with Figure 3b. By taking into account the Laplace pressure due to surface tension and the vapor pressure using the Clausius−Clapeyron relation, one can estimate the critical temperature Tc required to initiate a bubble around a nucleation site of radius R from C exp(−h/(kBTc)) = p0 + 2γ(Tc)/R, where C = 95 GPa, h = 6.7501 × 10−20 J is the enthalpy of evaporation for a water molecule, p0 = 1 atm is the external pressure, and γ(T) is the temperature-dependent surface tension of water.44,45 Inserting Tc = Ts = 200 °C from Figure 5c, corresponding to the estimated surface temperature of the nanorod at the threshold laser power Pth in Figure 3b, yields a radius R ≈ 25 nm, which is smaller than but not far from the radius of curvature of the “large” nanorod (R ≈ 44 nm, assuming a hemispherically capped cylindrical shape). One could speculate whether this might indicate that vapor only forms around certain parts of the rod, for example, at the rods ends or at specific facet edges. Previous studies of vapor formation around optically heated plasmonic nanoparticles have mainly utilized pulsed laser sources.46−48 However, it was recently shown that also CW laser illumination at powers similar to those used here could generate nanobubbles around metallic nanostructures.44,45

METHODS Experimentals and Data Analysis. The optical tweezers system is based on an inverted microscope (Nikon Eclipse Ti) equipped with a 40× objective with NA = 0.95 (Nikon CFI Plan Apo Lambda 40X) and white-light dark-field illumination for imaging and spectroscopy. The trapping laser (Cobolt Flamenco 05−01 Series) emits at 660 nm, and the output power can be remotely controlled via a computer interface. The measured power stability was found to be within the specified range of ∼2% over a period of 2 h. We used the backscattered laser light to probe the rotational dynamics of the trapped nanorods, as described in ref 23. The dark-field scattering from the trapped particles is filtered (Semrock 658 nm Stopline, Thorlabs 658 nm Notch filter) to remove the laser line and then spectrally analyzed by a grating spectrometer (Princeton Instruments IsoPlane SCT320) equipped with a liquid nitrogen cooled detector (Princeton Instruments PyLoN). A schematic of the complete system is shown in Figure S1. An experiment is initiated by placing a 4 μL droplet of the colloid, which is highly diluted in deionized water to avoid simultaneous trapping of multiple particles, in a 100 μm thin sample cell formed between a microscope slide and a cover glass. A particle is then selected through DF observation and trapped by the laser beam against the upper cover glass in the cell. The useful minimum and maximum trapping power is dictated by the necessity to form a stable 2D trap and at the same time avoid that the particle gets stuck on the upper cover glass due to the radiation pressure, respectively. The applicable power range varies with particle type but can be selected based on previous experience. Vertical sample drift is compensated by using a commercial laser-based continuous focus correction system (Nikon PFS). The measured autocorrelation signal can be well fitted to C(τ) = I02 + 2 I1 /2 × exp[−τ/τ0] cos(2Nπfavgτ), where τ is the autocorrelation lag time, favg is the average rotation frequency, I0 is the average intensity, and I1 is the amplitude of the intensity fluctuation. Dark-field scattering spectra were normalized to the incident white light spectrum and fitted by two Lorentzian functions, representing the transverse and the longitudinal LSPRs, respectively, and a linear baseline to account for interband transitions in gold. Simulations. The optical properties of nanorods were simulated using a commercially available FDTD software (Lumerical Solutions,

CONCLUSION In summary, we have studied individual gold nanorods operated as rotational nanomotors driven by circularly polarized light in water using two simultaneous but independent measurement schemes: rotational Brownian dynamics analysis and localized surface plasmon resonance (LSPR) spectroscopy. The LSPR data show that photothermal heating of the optically trapped nanorods causes slow but measurable particle reshaping due to rearrangement of a few tens of Au atoms per second at moderate 10059


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ACS Nano Inc., Canada). An isolated gold nanoparticle was modeled as an elongated tetrahexahedron with crystal facets. This was accomplished by covering each face of a rectangular block with triangular prisms with base angle of 24°, in accordance with previous reports.49 The triangular facets of the prisms were created at the same angle, 24°. The length and width of the nanorod were defined as the distance between the respective tips and were varied between L = 93 to 102 nm and L = 144 to 156 nm as well as D = 70 to 65 nm and D = 93 to 88 nm for the “small” and “large” particles, respectively. The dielectric function of gold used in the simulations is based on experimental data.49 The nanorods were excited by a circularly polarized plane wave propagating in the z-direction, perpendicular to the rod axis. The scattering and absorption cross sections were calculated by normalizing the scattered and absorbed powers by the source intensity. The total torque was obtained from ⃗ through placing a closed surface, S, around the particle M⃗ tot = r ⃗ × Ftot and integrating the Maxwell stress tensor.23 Calculations of thermal profiles around optically heated nanorods were performed using finite element simulations (COMSOL Multiphysics 4.3). We calculate the temperature profile around a 108 × 65 nm gold nanorod with an absorption cross section estimated from FDTD to 4203 nm2 at 660 nm for circularly polarized light. The nanorod is illuminated with a laser power of 9.7 mW/μm2 which is comparable to that used in experiments.

We thank F. Hajizadeh and S. Jones for valuable comments and assistance.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04302. Detailed experimental setup; extracted transverse LSPR peak position further displaying reshaping; FDTD simulations of scattering cross sections for the two nanorod types; determination of sensitivity to changes in aspect ratio; contours of reshaping particle to reach atomic migration rates; spectroscopic data separated into reshaping and heating contributions for triangular ramps; quadratic model for temperature dependent refractive index; determination of sensitivity to changes in RI; procedure for analyzing the reshaping particle and reaching atomic migration rate; table over extracted information regarding the reshaping particles (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Daniel Andrén: 0000-0003-0682-5129 Nils Odebo Länk: 0000-0003-1769-1082 Srdjan S. Aćimović: 0000-0001-8379-0316 Author Contributions

D.A. performed all optical experiments and data analysis and wrote a draft of the paper. L.S. made the nanoparticles and assisted in the data analysis. N.O.L. performed the electrodynamics simulations. D.A. and S.S.A. constructed the experimental setup. P.J. assisted in simulation and data interpretation. M.K. supervised the project. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Knut and Alice Wallenberg Foundation, the Swedish Foundation for Strategic Research, and Chalmers Area of Advance Nanoscience and Nanotechnology. 10060


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Probing Photothermal Effects on Optically Trapped Gold Nanorods by

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