Photothermal DNA Release from Laser-Tweezed Individual Gold

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Photothermal DNA release from laser-tweezed individual gold nanomotors driven by photon angular momentum Hana Šípová, Lei Shao, Nils Odebo Länk, Daniel Andrén, and Mikael Käll ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00034 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Photothermal DNA release from laser-tweezed individual gold nanomotors driven by photon angular momentum Hana Šípová*, Lei Shao, Nils Odebo Länk, Daniel Andrén and Mikael Käll* AUTHOR ADDRESS Department of Physics, Chalmers University of Technology, S-412 96 Göteborg, Sweden KEYWORDS optical tweezers, DNA, nanomotors, photothermal heating, oligonucleotide

ABSTRACT Gold nanoparticles offer a unique possibility for contact-free bioanalysis and actuation with high spatial resolution that increases their potential for bio-applications such as affinity based biosensing, drug delivery and cancer treatment. Here we demonstrate an ultra-sensitive optomechanical method for probing and releasing DNA cargo from individual gold nanoparticles trapped and manipulated by laser tweezers. Single nanorods are operated as rotational nanomotors, driven and controlled by circularly polarized laser light in aqueous solution. By rotational dynamics analysis, we resolve differences in the thickness of adsorbed ultrathin molecular layers, including different DNA conformations, with nanometer resolution. We then utilize photothermal heating to release DNA from single nanomotors while measuring

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the temperature-dependent kinetics and activation energy of the DNA melting process. The method opens new possibilities for optomechanical quantification and application of thermally induced molecular transitions in strongly confined geometries, such as inside microfluidic devices and single cells.


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Gold nanoparticles (GNPs) hold great promise for biophotonics applications1 because they are chemically inert and can function as tiny antennas that dramatically boost the coupling to visible and infrared radiation by virtue of the localized surface plasmon resonance (LSPR) phenomenon 2

. The extreme field confinement and enhanced photothermal properties associated with LSPR is

a powerful asset in fundamental as well as applied biomedical research, such as ultra-sensitive biomolecular detection3 down to the single molecule level 4-6 and novel bioimaging modalities 7. The LSPR can be tuned to the near-infrared (NIR) spectral region where light only weakly interacts with biological matter, allowing for optical addressing of GNPs located deep in tissue 8, while the desired GNP functionality, specificity and cytotoxity 9-10 can be controlled by using a variety of functional coatings based on thiol chemistry 11. This suggests that GNPs can be used as vehicles for delivery and programmed release of biological macromolecules to specific physiological targets, which is a key challenge for molecular therapeutics. Indeed, GNPs have recently received significant attention as drug delivery agents that allow spatial and temporally localized release of cargo by applying light in resonance with the LSPR as an external trigger 1213

. In particular, GNP-DNA conjugates have been delivered into target cells and remotely

triggered to release DNA by resonant illumination

14-17

. In gene therapy the released DNA or

RNA prevents specific mRNA translation into disease-related protein. The type of laser illumination governs the scheme of DNA release: pulsed lasers typically induce a breakage of gold-thiol semi-covalent bond 16, whereas continuous illumination can be used to melt the GNPbound double stranded DNAs (dsDNAs), resulting in release of one of the DNA strands

18

. For

better control of DNA release, it is crucial to fully understand and precisely tune the DNA melting dynamics on the surface of individual nanoparticles in solution.


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Here we demonstrate a new method to probe DNA films and photo-induced DNA release from individual GNPs in aqueous solution with unprecedented resolution using a combination of plasmon enhanced optomechanical and photothermal effects. We use laser tweezers to optically trap individual gold nanorods and spin them at high rotation frequencies by transfer of spinphoton angular momentum 19, creating laser-driven rotational nanomotors. The rotational friction dynamics of such a nanomotor is highly sensitive to its size and shape

20

, and this provides a

means for analyzing the thickness and conformational state of molecular coatings covering the GNP surface with high precision. Moreover, by utilizing plasmon enhanced photothermal heating of the nanomotor, it is possible to rapidly change the temperature of the molecular coating and study thermally induced changes in molecular conformation. We focus on nanomotors coated with different DNA constructs and we utilize the photothermal effect to induce controlled release of DNA cargo and to measure the kinetics and thermodynamics of this process. Moreover, the described methodology can be applied to study the thickness, conformational dynamics and interactions of virtually any kind of molecular coating.

Figure 1: Probing DNA adlayers on rotating nanomotors. A single gold nanorod is trapped in 2D by a focused circularly polarized laser beam (λ = 830nm) in water and forced to rotate (top). Adsorption of


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biomolecules increases the effective nanorod friction coefficient, which slows down the rotation and increases the rotational Brownian fluctuation of the nanomotor. Bottom: Measured scattering autocorrelation functions (ACF) of rotating nanomotors coated with single-stranded (a) and doublestranded DNA (b). The data have been fitted with = + 0.5 −/ cos4, where is the rotation frequency and the decay constant. The nanomotors with ssDNA coating experience lower friction than those coated with dsDNA, which implies that τ0 (ssDNA) < τ0 (dsDNA) and f (ssDNA) > f (dsDNA).

RESULTS AND DISCUSSION Rotational dynamics of nanomotors coated with ss-, ds- and hairpin-DNA: Figure 1 illustrates how molecular surface layers influence the rotational dynamics of the Au nanomotors through a comparison of the scattering autocorrelation functions (ACFs) measured from one nanomotor coated with singlestranded (ss-) DNA and one coated with double-stranded (ds-) DNA but rotated under otherwise identical trapping conditions (for details of the experimental setup, see Methods and Section 1 of SI). The ACFs are well described by exponentially decaying periodic oscillations with characteristic decay times and rotation frequencies and it is obvious that both of these parameters differ substantially between the two cases. An analysis of the equation-of-motion for the nanomotor system yields that the average rotation frequency follows = /2π , where is the optical driving torque and is the rotational friction coefficient20. Similarly, the decay time can be found to follow = /4!" #$, where #$ is the rotational Brownian temperature and !" is Boltzmann’s constant. The sensitivity of f and to the presence of molecular adlayers primarily stems from variations of the friction coefficient # = %#&'( , where %# is the temperature dependent dynamical viscosity of the surrounding water, & is a geometrical shape factor depending on the nanorod eccentricity (SI, Section 5) and L is the nanorod


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length. A nanomotor coated with a thick molecular layer, corresponding to an increased effective L, is thus expected to exhibit lower and higher , as seen in Fig. 1. Figure 2a displays results for nanomotors coated with short single-stranded and double-stranded DNAs with the same nominal chain-length (69 nucleotides). The rotation frequency data in Fig. 2a show a remarkable difference of a factor of two or more between ss- and dsDNA, signaling a large increase in layer thickness due to DNA duplex formation. A corresponding difference in the ACF decay times is also obvious. The differences in rotation frequency and are caused by differences in friction coefficients. As a consistency check, we also plot the product ∝ /# , in which the friction coefficients cancel out and the data points for the two DNA variants therefore overlap. To analyze this behavior, we first note that the significant variation in resonance wavelength, +,-./ = ~700-780 nm, reflects the variation in nanorod length due to the size dispersion of the colloid. Longer nanorods exhibit more red-shifted +,-./ and couple more strongly to the laser field21. The positive slope of the +,-./ curves is thus due to the stronger optical torque (Fig. S2) and the higher photothermal heating (Fig. S3), which leads to decreased interfacial viscosity, experienced by nanorods with +,-./ closer to the laser wavelength, +0123 = 830 nm 20.

Figure 2: The rotational dynamics of gold nanomotors varies with DNA layer thickness and DNA conformation. a) Rotational frequency (, top), decay time ( , center) and their product ( , bottom) versus plasmon wavelength measured for individual nanomotors coated with 69nt long ssDNA or dsDNA.


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(b) Rotational dynamics of nanomotors coated with 69nt long ssDNA compared to DNA probes of the same length but containing a 19nt self-complementary sequence that allows hairpin formation. (c) Histograms of layer thicknesses extracted from data for ssDNA (top), hairpin DNA (center) and dsDNA (bottom) including reported lengths of dsDNA of given number of nucleotides 22.

Figure 2a clearly shows that vary weakly with +,-./ , suggesting that it is more suitable for quantitative analysis of biomolecular layer thicknesses than . We thus developed a methodology to extract shell thickness d from experimental data based on the simplifying assumption that the temperature distribution around a rotating nanorod is independent of layer thickness (for details, see Methods section). To approximately fulfill this assumption, we only utilize data from particles with +,-./ far from the laser line (ranges marked by horizontal lines in Fig. 2). The method utilizes the average decay time 〈 5

36 〉

recorded for reference particles with assumed known coating thickness 5

measured at the same experimental setting to generate a calibration curve 5⁄〈 5

36 〉

36

and

that can in

turn be used to extract 5 (see Fig. S5). Figure 2c shows the resulting histograms of d fitted with Gaussian distribution functions. The data indicate that the layer thickness increases from ~3.9±1.9 nm for ssDNA (based on using native reference particles (5 dsDNA (based on ssDNA reference with 5

36 =

36 =

0 nm) within the same +,-./ range) to 21.2±2.8 nm for

3.9 nm). The latter value can be compared to the length of

60nt-long B-form dsDNA, that is 20.4 nm according to x-ray diffraction analysis 22. Though the length of the DNA complex used in the present study might be slightly longer due to the 9-nucleutide spacer incorporated between the duplex and nanorod surface, the result obviously agrees very well with the previous reports and highlights the very different viscoelastic properties of ss- and dsDNA molecules23-24. The persistence length of ssDNA has been reported to be only ~ 0.7 nm (increasing strongly with decreasing salt content) compared to ~45 nm for dsDNA

25-26

. Our results are in line with data obtained

using more established techniques and consistent with a picture in which the dsDNA molecules behaves as stiff rods extending perpendicular out from the nanomotor, whereas the ssDNA is coiled and


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compressed at the metal surface (though it is likely that steric hindrance and repulsion between neighboring strands prevents the ssDNA molecules to contract below the measured ~3.9 nm). The high sensitivity of the nanomotor dynamics to coating morphology opens intriguing possibilities to use the nanomotors to analyze differences in biomacromolecule conformations. As an example, Figure 2b shows the measured and for two DNA probes of the same nominal oligonucleotide length (69-nt) but different secondary structure: the linear ssDNA probe used previously and a probe containing a 19-nt long self-complementary sequence that allows formation of a hairpin, that is, an intramolecular DNA duplex. The data show that the hairpin probe generates slightly higher friction (i.e. lower and higher ) than the linear probe, indicating a stiffer structure similar to the dsDNA. However, the difference between the two conformations essentially disappears for nanorods with λLSPR > ~775 nm. We interpret this as an effect of dsDNA thermal denaturation (melting) due to photothermal heating of the nanomotors with λLSPR approaching the laser wavelength, that is, the surface temperature of these nanomotors is likely to be high enough to “unzip” the hairpin and induce a conformational transition to a structure similar to the ssDNA. In fact, a similar effect is seen for the two nanomotors with dsDNA and highest λLSPR (Fig. 2a), which also exhibit rotational dynamics approaching those coated with ssDNA. We extracted the difference in layer thickness d between the linear and hairpin DNA probes as above using ssDNA data for nanomotors with λLSPR ≤ 755 nm as reference. The resulting histograms (Fig. 2c) indicate that the total thickness of the layer is ~10.2 ± 2.2 nm nm. Thus, the difference in layer thickness is ~6.3 nm compared to the reference linear probes, which is in perfect agreement with expected length of an 19nt-long B-form DNA duplex, which is indeed 6.3 nm22.

Using gold nanomotors to drive and study the kinetics of dsDNA melting: We next investigated the kinetics of dsDNA melting by photothermal control of the nanomotor temperature. Figure 3a shows realtime measurements of the rotation dynamics of a single nanomotor coated with 60nt-long duplexes during a step-by-step increase in laser power from 2.8 mW to 7.4 mW and then back to 2.8 mW. The duration of each step is ~2 min, which is much longer than the thermal equilibration time of the nanoparticle system.


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For each increase in laser power, the rotation frequency goes up instantly due to the increased optical torque. However, there is also a gradual change occurring over longer timescales. In particular, when the power is increased from 4.8 to 6.0 mW, the rotation frequency almost doubles, indicating that the dsDNA melts and dissociates into two single strands, where the one without a thiol-linker diffuses out into the surrounding solution. There is no indication of rebinding during the subsequent step-wise decrease in power, which is expected considering the low concentration of counter-ions and complementary strands in the medium.

Figure 3: Measuring the kinetics of dsDNA melting using photothermally heated gold nanomotors (a) Real-time measurements of rotation frequency of a nanomotor functionalized with dsDNA during a step-by-step change in applied laser power. A clear acceleration of the rotation due to dsDNA melting can be seen at laser power 6 mW (region indicated by dashed circle). (b) Real-time measurements of dsDNA melting on six different nanorods with λLSPR in the range 729-739 nm. An instant change in rotation frequency and autocorrelation decay time can be seen after the increase in laser power, followed by gradual change due to dsDNA melting and dissociation of the nonthiolated DNA strand. (c) Change in effective DNA layer thickness versus time at different nanomotor temperatures


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calculated from changes in the autocorrelation decay time. The data have been fitted with single exponential decays to extract the temperature dependent dissociation rate !9 #. (d) Arrhenius plot of !9 # with each point representing a measurement on a single nanomotor. The dashed line shows a least-square fit to the Arrhenius equation based on # = # . Error bars were calculated from fitting errors of kd and τ0.

To further analyze the melting process, we operated several nanomotors at low power and then increased the laser power in a single step to induce DNA melting. Figure 3b shows that the rotation frequency instantly increases and then gradually stabilizes when the laser power at the sample is raised from 2.8 mW to 4.2 mW, 5.8 mW or 6.8 mW, indicating that the DNA dissociation on the individual nanorods proceeds at different rates depending on laser power. When the laser power is decreased back to 2.8 mW, the rotation is up to twice as fast as at the beginning of the experiment. The time-traces of the decay times for the same particles show a corresponding instant drop when the laser power is raised, caused by increased temperature of the particle, followed by a gradual and power dependent decrease as the thickness of the DNA layer shrinks due melting and unbinding. When the laser power is finally decreased back to 2.8 mW, the nanomotors exhibit a reduction in by a factor ~2 compared to the beginning of experiment, that is, of similar magnitude as seen in the comparison between ss- and dsDNA in Figure 2a. To confirm that the change in is not caused by thermal desorption of thiolated DNAs, we measured of native particles and particles with ssDNA versus increasing laser power (Fig. S7). These data do not exhibit any abrupt changes in that would suggest ssDNA desorption To calculate the rates of dsDNA melting, we first determine the relative change in biolayer thickness d during the melting process from the relative change in using the same methodology as above (see Methods for details). We use the decay time at the end of measurement period as 5 with 5

36

36

= 3.9 nm for ssDNA under the assumption that all dsDNAs had melted. We also calculate the

nanorod rotational temperature # , which we assume to be constant during the melting process, from 5

36 .

Figure 3c shows the calculated changes in d at various temperatures # . The dissociation

process follow first-order kinetics and can be well fitted by single exponential decay ∆5; =


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∆5→= expA−!9 ;B − 1 with dissociation rate constant !9 and ∆5→= ≈ 15 nm being the difference in layer thickness between the ds- and ssDNA. A plot of the dissociation rate constant versus # shows an exponential increase with temperature (Fig 3d). We can then determine the activation energy EF of DNA melting from the Arrhenius equation !9 # ∝ exp A− EF ⁄G# B, where R is the universal gas constant. This yields EF = 91.1±4.0 kJ/mol, which can be compared to reported activation energies of 50-100 kJ/mol for dsDNA dissociation of 20nt duplexes in 0.9-1M NaCl buffer obtained through fluorescence assays

27-28

. The surprising similarity in activation energies for 60nt long dsDNA obtained here and the

reported values for shorter 20nt DNA in solution27-28 can be reconciled by considering that layers of surface-bound DNA duplexes are significantly destabilized by their crowded environment, in particular at low concentrations of counter-ions29, resulting in a decreased EF . We note here, that the dsDNA molecules in reality experience a temperature gradient, with the highest temperature Tmax affecting the part of the molecules that is closest to the nanorod surface (d ≈ 1 nm) and the lowest Tmin at the dangling end of the DNA (d ≈ 21 nm) 30. Fits to Arrhenius plots based on these temperatures give lower and upper limits of the activation energy as EFHIJK = 74±3 kJ/mol and EFHI1L = 94±3 kJ/mol. A slightly more advanced analysis of the data in Figure 3d based on transition state theory allows us to estimate the activation enthalpy (∆M9N) and activation entropy (∆O9N) of duplex dissociation based on Eyring´s theorem31: !9 =

PQR H ∆-UV exp T S /

−

∆WUV X, /H

where kB is Boltzmann´s constant, h is Planck

constant, and κ is a transmission coefficient, which was assumed to have a value of 1. This yields ∆M9N = 17.5±0.7 kcal mol-1 and ∆O9N = −18±1 cal mol-1 K-1. The activation enthalpy is in reasonable agreement with literature values 32, though a direct comparison is difficult since the activation enthalpy depends on various parameters, such as DNA length and sequence, the presence of a gold surface and reaction conditions, such as ionic strength

32

. In particular, there is lack of experimental data on

measurements in aqueous solutions without addition of positive ions. The negative value of ∆O9N indicates an entropic penalty for formation of the activated complex. This might be a consequence of unscreened negative charge on the DNA sugar-phosphate backbone since the displacement of one of the DNA strands


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can be expected to require significant reordering of water and neighboring DNA molecules in the absence of screening ions 29.

Probing ultrathin self-assembled monolayers: The results above demonstrate that the optomechanical properties of the gold nanomotors are highly sensitive to changes in coating thickness of the order 5-10 nm. We investigated the thickness limit of the sensitivity using nanomotors coated with ultrathin carboxyl-terminated alkanethiols of varying chain-lengths33 (see Methods for details). Alkanethiols chemisorbed on gold surfaces can form compact self-assembled monolayers (SAMs) to an extent determined by alkyl chain-length, temperature etc.

34,35

. We studied nanomotors functionalized with

alkanethiols containing either 6, 11, 16 or 32 atoms in the main chain (6-MHA, 11-MUA, 16-MHDA and 11-MU-EG6-AA, respectively, see Fig. 4a) using a fixed laser power (P ≈ 6 mW). A clear trend appeared when the rotation frequency was plotted against the corresponding +,-./ for each individual nanomotor (Fig. 4b). As in the DNA-case above, the systematic decrease in f for nanorods with the two longest alkanethiols (16-MHDA and 11-MU-EG6-AA), compared to native nanorods with similar +,-./ , reflects the size change induced by the molecular layers. The data is in good agreement with theoretical +,-./ calculated for rods covered with dielectric layers of varying thickness 5 (Fig. 4c), which take changes in nanorod dimensions, LSPR wavelength and plasmonic heating into account (see Fig. S2 and associated discussion for details), although the experiments show a slightly higher sensitivity of +,-./ to changes in d than the theoretical prediction. Layer thicknesses were determined from analysis of changes in as before (see SI, Section 7 and Fig. S6 for details), which yielded 5 = 2.2 ± 1.2 and 3.5 ± 2.1 nm for the particles coated with 16-MHDA and 11MU-EG6-AA, respectively. This is in good agreement with experimental data for gold films 36 and further substantiate the analysis methodology (see Fig S6 and associated discussion for details). One may note that there is no significant difference in rotation dynamics between the native particles and particles coated with the two shortest alkyl chains (6-MHA and 11-MHA). This indicates that these alkanethiols do


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not form compact monolayers on the nanomotors, which is consistent with previous studies on a flat gold 36 37

. However, it could also be that the native nanomotors contain residual CTAB with similar layer

thickness as the shortest alkanethiols, resulting in similar friction properties. We note that the absolute spread in d values is of the order ~ ±2 nm for all particle sets, irrespective of thickness and type of molecular coating (DNA or thiols). We primarily attribute this variability to variations in nanorod shape and size that are not accounted for in the analysis, although the higher standard deviation in d values for dsDNA might also indicate a variation in the number of bound molecules per particle. Nevertheless, it is clear from the results that accuracy increases with the number of analyzed nanorods, which indicates that sub-nanometer differences in layer thickness could in principle be distinguished given large enough data-sets.

Figure 4: Rotation frequency of a nanomotor varies with alkanethiol monolayer thickness (a) Chemical structure of the alkanethiols used for functionalization of the gold nanomotors, including reported thickness for 6-MHA, 11-MUA and 16-MHDA 36, 38. The structure of a CTAB molecule is also indicated (b) Measured rotation frequency versus plasmon resonance wavelength for nanomotors functionalized with various alkanethiols. Each point represents one nanomotor and dashed lines are fits to the data using Lorentzian peaks centered at +0123 = 830 nm. All particles were driven by the same fixed laser power (6 mW). (c) Calculated rotation frequencies versus plasmon resonance wavelength for nanorods coated with dielectric shells of thicknesses 0-10nm.


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CONCLUSIONS We presented a sensitive method to probe the thicknesses and conformational dynamics of DNA monolayers on single nanoparticles in solution. The method is based on observing the motion of optically trapped and photothermally heated gold nanorods operated as rotational nanomotors. The thicknesses of adsorbed DNA layers and self-assembled monolayers of alkanethiols could be determined with nanometer precision and were found to be in excellent agreement with previously reported values. Real-time measurements of dsDNA melting kinetics illustrate the possibility to perform and study controlled DNA release from single nanoparticles, which opens exciting possibilities for precise remote control of biological activity through gene transfection in individual cells. However, the methodology is generic and can be easily adapted to other types of nano- and microparticles and to other types of molecular interactions and transitions with the only prerequisite being that suitable particle functionalization strategies are available and that the light-particle interaction is strong enough to allow for optical tweezing, photothermal heating and observation of rotational dynamics.

METHODS Nanorod synthesis and functionalization: The nanorods were prepared by a seed-mediated growth method using cethydriltrimethylammonium bromide (CTAB) as a stabilizing agent

20, 39

. The as-

synthesized nanorods were washed twice by first centrifuging the solution and then removing the supernatant, resulting in a CTAB concentration of approximately 1 mM. The particles were analyzed by standard spectrophotometry as well as by scanning electron microscopy (SEM) in order to determine optical properties and size variation. The nanorods average length L and width W were L = 139.5±10.4 nm and W = 69.0±4.4 nm (n=65), respectively.


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DNA constructs were ordered with HPLC purification (Integrated DNA Technologies, IDT). The particles were functionalized with thiolated ssDNA probes and self-assambled monolayers of alkanethils (see Supplementary information for details). Optical rotation experiments: We use a circularly polarized laser tweezers setup built around an inverted microscope to trap and drive individual colloidal gold nanorods to rotate at kHz frequencies (Fig. 1 and S1a) as described in 20, 40. The nanorods are trapped in 2D against a cover glass because the optical gradient force induced by the focused laser field that confines the nanorods laterally is insufficient to counter the radiation pressure. The nanorods are therefore pushed towards the glass surface by radiation pressure until the optical force is balanced by Coulomb repulsion. The photon spin angular momentum, provided by the circularly polarized laser light, is transfered to the nanorods primarily through resonant light scattering and forces them to rotate around their short axis20, 40. The rotation frequency can be varied by changing the laser power, or it can be kept stable for periods up to an hour or more20. The laser wavelength used, +0123 = 830 nm, is positioned at the low energy flank of the ensemble averaged longitudinal plasmon peak of the nanorods +̅,-./ ≈ 750 nm (Supplementary Fig. S1)

20

and optimal for

biophotonics applications because of the low absorption in water and tissue. A dark-field spectroscopy system integrated into the setup provides means for determining +,-./ of each individual trapped nanomotor while their rotational dynamics can be quantified by recording the autocorrelation function (ACF) of the back-scattered laser light from a trapped particle. The average rotation frequency f and the autocorrelation decay time τ0 can be obtained from the ACF by fitting the function by = + 0.5 −/ cos4, where I0 is the average intensity and I1 is the amplitude of the intensity fluctuation

20, 40

. The optical trapping experiments were performed using droplets of diluted colloidal

nanorod suspensions placed between two glass slides separated by a 100 µm spacer. Estimating the thickness of a molecular layer from the ACF decay constants: We use ACF decay time ( ) to estimate changes in layer thickness d as follows: We first calculate the average decay constant 〈 5 = 0〉 for native particles measured at a specific experiment condition (e.g. a certain laser


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power P). We assume that this value represents nanorods with the same length and width as the average structure parameters established through SEM analysis, that is ' = 139 nm and W0 = 69 nm and the average shape factor & as detailed in Section 4 of SI. We can then calculate the average rotational temperature # 5 = 0 from = %# & '( /4!] # , in which we use the temperature dependent viscosity of water according to %# = % exp ^E ⁄!] # + #′`, where % =2.4152×10-5 Pa·s, E = 4.7428 kJ/mol and #′ = -139.86 K are constants.41 When the nanorod acquires a coating of thickness d, the above parameters changes to: ' → ' + 25; & → & + a5 and # 5 = 0 → # 5. We now assume that # 5 = 0 equals the surface temperature of the native nanorods, #- , and obtain the temperature at the surface of the coating layer from # 5 = #1Ib + G#c − #1Ib /G + 5 as outlined in Section 3 of SI. The temperature decay also determines the viscosity at a distance d from the surface, %d# 5e. Inserting the parameters then yields a calibration curve according to: fg 9 fg

H

i9

9 (

H lHm

= Hh9 1 + j T1 + , X exp AHh9lHmB. h

k

g

h

Figure S5 shows calculated curves 5/ 0 for various # 0 values that allows us to estimate d from changes in . Analysis of dsDNA melting kinetics: The rotation dynamics of nanorods capped with dsDNA was monitored continuously. During the measurement, the power of the trapping laser was increased from P0 to P1 in a single step and then decreased back to P0 after the rotational dynamics had stabilized, which implies that the DNA melting process is completed. For each P1, we extract τ0 at this point of stabilization and then calculate the corresponding rotational Brownian temperature Tr(P1) from = %# &'( / 4!] # , where g and L now correspond to a native particle with a 3.9 nm coating layer representing ssDNA. We also extrapolate Tr(P) to P = 0 to obtain a measure of Tamb, which turned out to be ~34 oC (the difference from room temperature is due to heating from the dark-field condenser illumination). Similar to the previous paragraph, we can calculate a calibration curve 5/ 5 = 3.9 nm which


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allows us to obtain the change in d as a function of time during the melting process (Fig. 3c in the main text) for different Tr(P1).

ACKNOWLEDGEMENTS This work was supported by the Knut and Alice Wallenberg Foundation and the Swedish Research Council.

ASSOCIATED CONTENT The Supporting information is available free of charge on the ACS publication website. 1. Details of experimental setup; 2. Nanorod functionalization 3. Electrodynamics simulations; 4. Temperature distribution around a photothermally heated nanorod; 5. Friction shape factor; 6. Estimating the thickness of a molecular layer from the ACF decay constants; 7. Estimating the thickness of alkanethiol SAMs; 8. Thermal stability of nanomotors functionalized with ssDNA.

AUTHOR INFORMATION Corresponding Author * Hana Šípová, [email protected]; *Mikael Käll, [email protected] Author Contributions


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HS performed all optical experiments, nanorod functionalization, and data analysis and wrote a draft of the manuscript. LS synthesized Au nanorods and assisted in setting up the experiment. NOL performed electrodynamics simulations. MK supervised the project. All authors contributed to the final version of the manuscript.

Funding Sources This work was supported by Knut and Alice Wallenberg Foundation, the Swedish Research Council, and the Swedish Foundation for Strategic Research.

ABBREVIATIONS SAM, self-assembled monolayer; DNA, deoxyribonucleic acid; ssDNA, single-stranded deoxyribonucleic acid; dsDNA, double-stranded deoxyribonucleic acid; GNP, gold nanoparticle; DLS, dynamic light scattering; LSPR, localized surface plasmon resonance; ACF, autocorrelation function; SEM, scanning electron microscopy.

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Title: Photothermal DNA release from laser-tweezed individual gold nanomotors driven by photon angular momentum Authors: Hana Šípová*, Lei Shao, Nils Odebo Länk, Daniel Andrén and Mikael Käll*


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Photothermal DNA Release from Laser-Tweezed Individual Gold

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