Light Driven Design of Dynamical Thermosensitive Plasmonic

Oct 16, 2018 - Light Driven Design of Dynamical Thermosensitive Plasmonic Superstructures: A Bottom-Up Approach Using Silver Supercrystals...
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Light Driven Design of Dynamical Thermosensitive Plasmonic Superstructures: a Bottom-up Approach using Silver Supercrystals Vitor Brasiliense, Pascal Berto, Pierre Aubertin, Emmanuel Maisonhaute, Catherine Combellas, Gilles Tessier, Alexa Courty, and Frédéric Kanoufi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03140 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Light Driven Design of Dynamical Thermosensitive Plasmonic Superstructures: a Bottom-up Approach using Silver Supercrystals Vitor Brasiliense,a Pascal Berto,b Pierre Aubertin,c Emmanuel Maisonhaute,c Catherine Combellas,a Gilles Tessier,b,e Alexa Courty,*d and Frédéric Kanoufi*a a

Sorbonne Paris Cité, Université Paris Diderot, Interfaces, Traitements, Organisation et Dynamique des Systèmes, CNRSUMR 7086, 15 rue J. A. Baif, F-75013 Paris, France ; b Sorbonne Paris Cité, Université Paris Descartes, Neurophotonics Laboratory, CNRS-UMR 8250, 45 rue des Saints-Pères, F75006 Paris, France ; c Sorbonne Université, Laboratoire Interfaces et Systèmes Electrochimiques CNRS-UMR 8235, 4 place Jussieu, F-75005 Paris France ; d Sorbonne Université Laboratoire MONARIS, CNRS-UMR 8233, 4 place Jussieu, F-75005 Paris France ; e Sorbonne Université, CNRS, Institut de la Vision, 11 Rue Moreau, F-75011 Paris France. KEYWORDS. supercrystals, thermophoresis, self-organization, dynamical plasmonic structures, single particle tracking, holography

ABSTRACT: When narrowly distributed silver nanoparticles (NPs) are functionalized by dodecanethiol, they acquire the ability to self-organize in organic solvents into 3D supercrystals (SCs). The NP surface chemistry is shown to introduce a light-driven thermo-migration effect, thermophoresis. Using a laser beam to heat the NPs and generate steep thermal gradients, the migration effect is triggered dynamically, leading to tailored structures with high density of plasmonic hot spots. This work describes how to manipulate the hot spots, and monitor the effect by holography, thus providing a complete characterization of the migration process on a single object basis. Extensive single object tracking strategies are employed to measure the SCs trajectories, evaluate their size, drift velocity magnitude and direction, allowing the identification of the physical chemical origins of the migration. The phenomenon is shown to happen as a result of the combination of thermophoresis (at short length scales) and convection (long range), and does not require a metallic substrate. This constitutes a fully optical method to dynamically generate plasmonic platforms in situ and on demand, without requiring substrate nanostructuration and with minimal interference on the chemistry of the system. The importance of the proof-of-concept herein described stems from the numerous potential applications, spanning over a variety of fields such as microfluidics and biosensing.

Since the discovery of Surface Enhanced Raman Scattering (SERS) almost forty years ago,1,2 plasmonics has been fueled with the promise of in operando monitoring of nano- and molecular scale events. Although these goals have certainly been achieved in specific situations,3-7 the field still lacks general applicability and widespread commercialization of SERS technology still depends on finding solutions to a few critical bottlenecks. Currently, the most promising SERS platforms rely on nanostructuration of surfaces,910 which introduces manufacturing complications, or in many-step synthetic procedures, which are hardly up-scalable.11 Moreover, localized electromagnetic field enhancement near plasmonic nanostructures (so-called hot spots) strongly depends on the nanostrucutures detailed morphology. Since heterogeneity is the norm at these scales, the enhancement effect often varies from one hot spot to another one and with time.8,12,13 This is a drawback in most industrial applications, where even a more modest but more robust enhancement factor would be desirable.13 For this rea-

son, a considerable effort is currently devoted to render plasmonic technologies more reliable and easy-to-use. One promising strategy is to use plasmonic entities, which gather dynamically, forming SERS active structures either spontaneously or in response to an external excitation.14 In this context, organized ensembles of nanoparticles (NPs) building blocks, such as supercrystals (SCs), emerge as very interesting candidates for dynamical formation of plasmonic platforms. Due to their intrinsic organization, SCs form an array of hot spots, thus multiplying the electromagnetic enhancement factor and reducing variability by averaging many sites. In sensors applications, for example, this may be the key to increase the sensitivity in a robust and reliable way. The formation of SCs and the extent of their organization is the result of a balance between attractive and repulsive particle-particle and solvent-particle interactions. Balance of these forces through a careful choice of the surface functional groups and solvent therefore gives control over the SC average sizes and organization pattern, offering a mechanism through which SCs can be tailored for a given sensor application.15,1617

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Figure 1. Formation, optical tracking and laser-induced aggregation of supercrystals (SCs). A) Principle: C12AgNPs spontaneously organize from favorable solvent-ligand interactions into SCs of characteristic size ∼ 200nm, which migrate towards the hot region created by a green laser spot. B) Two-laser setup used to independently trigger and observe the SCs motion. The observation beam (𝜆𝑜𝑏𝑠 = 785nm) provides a holographic monitoring setup, while the excitation beam (𝜆𝑡𝑟𝑎𝑝 = 532nm) is focused on the sample thus creating a thermal gradient without interfering with the observation beam. The width of the heating beam (𝜔0 ≈ 10µ𝑚) is controlled by a telescope. C1) side-view of the SCs trajectories under a radial temperature gradient. The movement is mainly situated on the sample plane. The monitored volume is 250x250x2000µm 3. Tracking of the SCs position allowed mapping the drift velocity (C3), and measuring its magnitude (C2).

The dynamics of SCs spontaneous growth and aggregation, however, is relatively difficult to control, as it is a problem largely complicated by the number of variables involved, such as molecular interactions, solvent characteristics, particle distribution, to cite a few.18-22 This is a serious issue, since in sensing applications the involved timescales control the overall sensitivity throughput.23 This problem can however be avoided by designing particles that form SCs in response to an external stimulus, such as a thermal gradient generation, therefore allowing active control of the aggregation process (and thus of the hot spot spatial density distribution). Although thermal gradients, through light induction, have been previously proposed to generate the migration and trapping of plasmonic NPs, the mechanisms have so far relied on the establishment of interactions with a plasmonic substrate,24 which can be structurated, 25-27 or on the addition of chemicals such as surfactants (to generate a thermally induced electric field that propels the NPs),28,29 which can interfere with the analyte. These strategies, so far operated in aqueous solutions, make use of thermophoresis, which is a surface phenomenon dependent on a fine balance of interactions between the surface chemistry of the particles and the solution.30 In this respect, organic solvents offer a wider control and tuning of such interfacial NP/solvent interactions, through the ability to

synthetize and dissolve NPs with wider surface chemistry. It should then be possible to promote both controlled aggregation and migration in response to temperature gradients in wider chemical systems. Herein, a proof of concept of this idea is provided. Narrowly distributed 5.9 +/-0.4nm silver NPs that are capped with dodecanethiol (C12AgNP) spontaneously organize in organic solvents into nanometric SCs.37 The interaction of the C12 chains with the solvent also generate migration towards the laser beam, driving the SCs together under the laser spot, forming a micrometric aggregate, as indicated in Figure 1A. The C12AgNP SCs constitute a real functional material, where the Ag core is responsible for creating the thermal gradient and the plasmonic properties, while the C12 organic shell enhances the stability and allows the SC to respond to thermal gradients. We propose to demonstrate the physical chemical origin of such light-driven dynamical trapping of SCs through direct observation and tracking of single SCs motion during the aggregation process using a holographic microscopy monitoring (Figure 1B). Holography is a highly sensitive method, capable of monitoring the movement of small individual objects over a large cell volume.31-33 By tracking single SC objects and comparing

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experimental trajectories to numerical modeling of the thermally-driven transport process, we are able to discriminate between the convective and thermophoretic origins of the migration mechanism. Owing to the SCs SERS properties16 the results herein presented allow far field dynamic control over the formation of plasmonic substrates, aiming at the integration of SERS functionalities to numerous devices, spanning from microfluidics to biophysics. Due to their ephemeral nature, the aggregates can subsequently be dispersed back to SCs, therefore allowing minimal interference on the physics and chemistry of the probed system. This way of creating plasmonic nanostructures constitutes an interesting alternative to much more complicated and permanent photolithographic methods34. RESULTS/DISCUSSION Optical Trapping of Supercrystals: Chemical Conditions. As mentioned in the introduction and depicted in Figure 1A, the illumination of colloidal solution of C12-thiolfunctionnalized Ag NPs (experimental details in METHODS and Sections S1 and S2 in Supporting information, SI) with a green laser light close to the plasmonic resonance of the Ag NPs leads to the formation of a micrometric region of trapping and concentration of SCs (the aggregate). A movie (Movie 1), corresponding to the experiments shown in Figure 2, illustrates the phenomenon, showing as will be described later, the movement and trapping of the SCs to the aggregate when the green laser is turned on, but also the manipulation of the aggregate, or the SCs dynamics, going back to Brownian motion after the laser is switched off. The trapping effect is not observed if the illumination wavelength is significantly different from the NPs Localized Surface Plasmon Resonance (LSPR). This wavelength dependency allowed us to uncouple observation and the onset of the trapping phenomenon using a twolasers setup. Indeed, UV-Vis spectrum of a colloidal solution of the Ag NPs in hexane shows a plasmon band at 420nm (Figure S1 in Supporting information), as expected for well dispersed thiol-modified AgNPs of 5.9nm diameter.18-20 Two wavelengths are then selected. A low power laser with an offresonance wavelength (NIR, λobs = 785nm, NIR, λobs = 785nm, 20mW, weakly focused with NA=0.04 to produce ~3.10 -4 mW/µm² irradiance over a broad region) is used for the observation, associated to a near resonance laser (in the visible range, λtrap = 532nm, capable of delivering up to 2W, but typically 40mW was used) used to trigger the trapping. The NIR laser beam is divided in two branches (object and reference beams) by a beam splitter (Figure 1B). The fluidic cell containing the NP solution (the sample) is illuminated by the object beam through a prism in total internal reflection mode. This configuration allows the rejection of incident light, therefore collecting only the light scattered by the particles. This light is sent to interfere with the reference beam, yielding an interference pattern that is recorded by a CCD camera triggered at a frame rate 𝐹𝐶𝐶𝐷 = 2𝐻𝑧. The holograms are numerically reconstructed, yielding a 3D image, as described in detail elsewhere.35 The second beam is used to induce SCs movement and trapping. It is oriented perpendicularly to the sample (normal incidence), and is focused in the middle of the sample by the same objective used for observation. The waist of the trapping

laser beam is controlled by a telescope, leading to a final beam waist of 𝜔0 ≈ 10µm. This allows control over the size of the focal region, and therefore over the power density, avoiding damage to the particles. Dichroïc mirrors and notch filtering were used to remove the trapping light at trap while observing motion and aggregation at obs. This setup grants the independent control of observation and triggering. Since previous studies have shown a lower observable size limit of ~20nm, individual (5.9nm) Ag NPs cannot be detected. The larger SCs, however, can be localized and tracked individually. The light-driven trapping process has a chemical origin related to the NPs (or generally the SCs) surface functionalization and to their interaction with the solvent employed. Indeed, under similar laser light illumination of tens pM aqueous colloidal solution of 40 to 100nm citrate-capped Ag NPs, such trapping effect was not observed.31,36 It is not observed either by addition of electrolytes, which can induce NPs aggregation through the decrease of the Debye length and of the inter-NP electrostatic repulsion. In such electrolytic situation, a thermoelectric trapping of Au NPs can be operated but requires both the presence of surfactant and the illumination of a nanostructured plasmonic substrate.28,29 Our experimental configuration is completely different. First a glass substrate is used, and the trapping is performed in a mixture of apolar organic solvents while the NP surface is functionalized by a C12-thiol. In a good solvent of the functionalized NPs, such as hexane (solvent-chain ≫ chain-chain interaction), the C12 chains are completely solvated, making a stable colloidal solution, which hardly leads to SCs formation. In this solvent it was not possible to observe the trapping effect under the same conditions of laser power and observation timescale. On the one hand, if a very poor solvent, such as toluene, is used, one can hardly disperse the particles in solution, as they easily form large SCs and precipitate, even before laser light illumination. On the other hand, in mixture of poor and good solvents a compromise between limiting NP aggregation (toluene, poor solvent) and solubilization (hexane, good vent)36,37 can be reached, resulting in the formation of SCs (consisting of arrangement of NPs). Prior Environmental TEM studies37 have shown that in 60% toluene, SCs of radius of ~100-300nm spontaneously form in solution. Since in this size range the SCs can still be kept in suspension by Brownian forces, this composition was chosen for the experiments. Unless otherwise specified, the results shown here were obtained for 60% toluene, but the same trapping phenomenon takes place in 40% toluene, and (to a much lower extent) in pure toluene too. Large Scale Dynamics: Characterizing Trapping Forces. In order to understand the physical and chemical origins of the trapping effect, a multiscale characterization of the motion is required. At first, a low magnification (x20, NA=0.45) is used to provide a large scale characterization. Initially the trapping beam is off, and the system is passively observed using the off-resonance NIR laser. At this point, we only detect a few spontaneously formed SCs moving randomly in Brownian motion. Analysis of their mean square displacement (MSD, Section S3 in SI) distribution reveals a scattered distribution of hydrodynamic radius (𝑟𝐻 ) of ~300-500nm, as previously reported for this solvent composition37. When the trapping laser is turned on, a radical change of behavior occurs: the SCs

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movement switches to directional, towards the laser spot position, initially devoid of visible scattering nanoobjects. These SCs then accumulate into an aggregate at the laser position. It is proposed to discuss this trapping mechanism based on the analysis of the movement of individual SCs toward the laserheated region. The dynamic analysis of the trapping mechanism is intricate as it relies on the analysis of trajectories at its first instant after the heating laser is turned on, and then relies on too few SCs to be statistically sound. Instead, we propose to gather mechanistic insights from steady-state analysis: when the aggregate is bigger than the laser spot, it absorbs a constant power, controlled by the laser spot dimension, regardless of the aggregate size and the system is in quasi-steady thermal equilibrium. Then mechanistic information can be captured, in quasi-steady-state, from the trajectories of individual SCs towards large aggregates which allows exploring different spatial regions (equivalently different time domains) of the characteristic SCs transport. For this purpose, the movement of several SCs towards a large aggregate is tracked using ImarisTrack software (Figure 1C), revealing quasi-linear trajectories for the 66 SCs tracked. The low numerical aperture precludes accurate localization in the z direction. Nevertheless, the large depth of field of holography allows monitoring the full cell depth. The tracking analysis is limited to a region of interest, which excludes the area directly heated by the laser, where individual SCs cannot be distinguished from the forming aggregate. The transition between these motion regimes is better appreciated in Movie 1 analyzed in Figure 2. The SCs instant velocity was measured by taking the time derivative of the radial distance from the center of the trapping laser spot. The change in SCs velocity as the laser is turned on is shown in the histogram included as inset in Figure 2D. While the laser is off, the particles behave as Brownian walkers, with no preferred direction (〈𝑣⃗〉 = 0). Once the trapping laser is turned on, the particles start to move towards the laser beam, with an average velocity of –10µm/s. The SCs aggregate around the laser beam, forming an aggregate, which can be approximated by a disk of ca. 50µm diameter (Figure S4a in SI). If the optical scattering intensity in the aggregation region is monitored over time, one can notice a steady increase from t=50s (time at which the trapping laser is turned on) to t=120s, which agrees with a constant flux of arrival of SCs at constant average velocity. From mean flux/mass balance calculations (Section S4 in SI), for the herein used pM SCs concentration and average velocity, the aggregate covers less than 10% of the laser beam surface area even after few hundreds seconds of illumination, therefore suggesting that the SCs are mostly trapped along a horizontal plane. Force Magnitude and Direction. Simultaneous measurement of the diffusion coefficient and drift velocity allow the determination of the order of magnitude of the force acting on the SCs when submitted to laser illumination. The characteristic time scales for diffusive (𝜏𝐷 ∼ 𝐿2 /𝐷) and ballistic (directional, 𝜏𝑣 ∼ 𝐿/𝑣) movements are compared through the dimensionless Peclet number (𝑃𝑒 = 𝐿𝑣/𝐷), which characterizes the diffusional to ballistic transition. As the trapping laser is turned on, the directive component of the motion largely dominates over diffusive contributions, (𝑃𝑒 = 200 , for a charac-

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teristic distance of 10µm, corresponding to the distance covered by the particle over 2 time frames). In these high 𝑃𝑒 conditions, we can safely neglect diffusion and thus use Stokes’ law to evaluate the order of magnitude of the force that would be necessary to generate the observed velocities (𝜂𝑓 is the fluid dynamic viscosity, rH the SC hydrodynamic radius, taken as 100-300nm): 𝐹𝑡 = 6𝜋𝜂𝑓 𝑟𝐻 𝑣 ≈ 70 ± 10𝑓𝑁

E1

E2

E3

E4

E5

E6

Figure 2. Transition from SCs Brownian motion to directional movement (A-D) and laser-induced manipulation of SCs motion (E). Before the trapping laser is switched on (A, blue), the SCs move by diffusion along a Brownian motion (typical cases in B1 and 2). After, with the trapping beam on (A and C, orange), their movement becomes directional. The transition can be appreciated from the drift velocity, v, histogram ( = 10µm/s), the laser spot center is located at (x,y) = (165, 172) µm (D). The trajectory of individual SCs is modified by moving the laser to new positions. Starting from a formed aggregate, the trapping laser position is changed at times 𝑇1 and 𝑇2 (= 𝑇1 + 50𝑠). The snapshots show the evolution of the aggregate, with a picture every 12.5s. Each blue-green line represents the tracking of a SC position over the past 10s and the red trace circles indicate the stable positions of the laser. Trajectories are quickly affected by the change of laser position (E5, yellow arrow). Each image is 215 × 215µ𝑚2 .

Besides, the velocity vectors are clearly oriented towards the laser spot position, generating the radial migration shown in

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Figures 1 and 2. A change in the position of the laser spot immediately modifies the direction of the SCs migration, thus evidencing the long-range characteristics of the migration phenomena, as seen in Figure 2E, which also shows that it is possible to manipulate the SCs aggregate by modifying the focal point position of the laser beam. This long range migration allows going further on the origin of the trapping mechanism. Optical Contribution to the Mechanism. At first, one wonders about the role played by forces of purely optical origin. When an incident photon interacts with a NP, it can be scattered or absorbed, which transmits momentum to the NP, thus generating forces. These optical forces are only active in the high optical gradient regions near the focus, and therefore cannot account for long-range migration as observed here. Moreover, considering the low power, loose focusing, and the poor trapping efficiency of plasmonic particles, which are efficient scatterers, optical forces are orders of magnitude below what would be necessary for optical tweezer-type trapping. However, two types of optically-induced forces may operate here, particularly during the initiation of the mechanism, due to i) optical absorption and ii) plasmonic interaction, which can respectively contribute to (long range) thermal migration and (short range) SC cohesion. Initially the solution not only contains SCs but also, with a much larger concentration, 5 nm Ag NPs (the building blocks of the SCs). These individual Ag NPs are well below the sensitivity threshold of our system and are not visible. However their plasmonic resonance (420nm) is not far from the wavelength of the heating green laser. Therefore, they have a significant absorption cross-section and can efficiently contribute to a local heating of the solution. In addition, short range ( < /2  100nm) plasmonic interactions can contribute to the formation and cohesion of small NP aggregates, and larger SCs, at the heated region. These larger objects have red-shifted resonance, and can absorb and convert light into heat even more efficiently. Both the individual NPs, their small aggregates or the SCs can therefore generate a temperature gradient in the solution and a long-range migration of SCs. This is definitely observed in Movie 1 (in SI) or in Figure 2E, in the first instant after the laser is turned on or moved to a new location. For example, in Figure 2E5, taken 5s after the laser was moved to its new location, a SC at 50µm from the laser position is quasi-instantaneously changing its direction to the next laser position (𝑇2 + 5𝑠, yellow arrow). If L∼ 50µ𝑚 stands for the characteristic length of the heat source, the temperature gradient difference extends to distances ∼ 5𝐿 ∼ 250µ𝑚, and the time required for heat to diffuse and establish 𝑓 a temperature gradient over ∼ 250µ𝑚 is ~𝐿2 /𝛼 𝑇 (= 0.6𝑠), 𝑓 where 𝛼 𝑇 is the thermal diffusivity of the liquid which is quasi-instantaneous for the imaging frequencies used (𝑓𝑠 = 2 − 10 𝐻𝑧). As discussed above, plasmonic interactions can contribute to the aggregate cohesion: in Figure 2E5, the micrometer-sized aggregate (not illuminated by the green laser) has disassembled into smaller aggregates or SCs as the plasmonic forces contributing to its cohesion are released. These disassembled aggregates or SCs have also slowly moved towards and later (in the image of Figure 2E6) reassembled at the new laser

position. If these optical forces have probably a determinant role in keeping the SCs aggregate cohesion, with strong implication to its internal dynamics and structure, they cannot explain the large scale motion of SCs observed here. Origins of Thermal Mechanism. All evidence gathered so far therefore points towards a mechanism of thermal origin induced by the local illumination of the solution containing both small (undetected) NPs and SCs. Thermal gradients can generate the observed motion through two routes: direct movement of the fluid, generating advection, or of the particles themselves, through thermophoresis. In the first case, the motion is natural convection (from now on, referred to simply as convection), and results from the appearance of buoyancy forces created by a thermal gradient. Heat transfer from a hot source changes the density of the fluid (Δ𝜌) and thus induces fluid motion due to a force ∼ Δ𝜌𝑔 𝑉, where g is the gravity acceleration and V is the characteristic volume of the heated zone, herein evaluated from the aggregate size (𝐿𝐴 ) as 𝐿𝐴 3. The magnitude of the flux can then be estimated from the fluid density (𝜌𝑓 ), thermal expansion coefficient (𝛽) and dynamic viscosity (𝜂𝑓 ), and from the gravity acceleration g and trapping laser beam waist (𝜔0 ). 44 Its magnitude at a given point is expected to scale as: 𝑣0 = 𝜔02 𝜌𝑓 𝑔 |𝛽|Δ𝑇/𝜂𝑓

(1)

In the second mechanism called thermophoresis (also sometimes referred to as the Ludwig-Soret effect, or thermal diffusion), the presence of a temperature gradient along the surface of a nanoobject generates an interfacial fluid flow, which in turns moves the nanoobject in solution. Although relatively well understood for gaseous systems, thermophoresis in liquid media is still debated.38-42 The particles drift velocity is described as proportional to the temperature gradient: ⃗⃗𝑇 𝑣𝑇 = −𝐷𝑇 ∇ ⃗⃗⃗⃗⃗ (2) 𝐷𝑇 is the thermal diffusion coefficient, and has units homogeneous to µ𝑚2 /𝑠𝐾. The Soret number (𝑆𝑇 =𝐷𝑇 /𝐷) is also often used to characterize thermophoretic systems. Thermophoresis has been demonstrated for a variety of chemical systems made composed of solvent (pure, mixture, or electrolyte) and solutes (molecules, colloids, polymers, etc).39,40 In electrolytes and aqueous solutions, it is complicated by double layer charging of the interfaces.28,29 Even in the apparently simpler case of Van der Waals interactions between an uncharged particle and an organic solvent, closer to the present situation, the mechanism is currently poorly understood. Especially in solvent mixture, where a large number of interactions coexist (solventparticle, solvent-cosolvent, ligand-solvent, interparticle…) which are also difficult to appreciate even by molecular dynamics approaches.43 Depending on the exact chemical interactions, the analytes may display a thermophobic (𝐷𝑇 > 0) or thermophilic (𝐷𝑇 < 0) behavior. Moreover, as it relies on the existence of a temperature gradient at the nanoobject surface, both the thermal conductivity of the nanoobject (particularly for metallic nanoobjects) or the variations of the solubility of its capping agent could affect the particle thermal diffusivity. Herein, a proper evaluation of the SC thermal conductivity is complicated as the SCs are organic-metallic composite materials made of the assembly of a metallic core surrounded by an organic shell. The evaluation

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ACS Nano of the thermal conductivity of such a composite nanomaterial is delicate. However, its effect on the local distortion of the temperature gradient can be neglected: under similar conditions of thermal trapping, >100nm Au NPs were shown not to distort by more than 15% the temperature gradient,28 or by 2𝐿𝐴 ), the temperature profile is almost homogeneous, therefore the temperature gradient is small, and thermophoresis can be neglected. The SCs velocity is then solely controlled by convection and estimated from eq.(1). Due to the linear relationship between 𝑢0 and Δ𝑇, the temperature difference between the aggregate and the bulk solution (and thus the gradient profile) can be estimated if the average velocities are measured at a given point, provided by higher spatial resolution trajectories. A higher magnification objective (x100, NA=0.92) was used to restrain the analyzed region to 40x40µm2, located ca. 400µm away from the laser focal position. Information at lower scale can then be assessed, highlighting diffusive effects, which allows us to derive information on the particles movement from the MSD curve. The SCs motion is decomposed in stochastic (Brownian, 𝐵) and deterministic (advective, 𝑣̅ ) components. Here, 𝑣̅ is the average advection velocity, considered constant during the trajectory. After a time lag Δ𝑡, the particle displacement is described as 𝑋(Δ𝑡) = 𝐵(Δ𝑡) + 𝑣̅ Δ𝑡, leading to a MSD such as:

𝑀𝑆𝐷(Δ𝑡) = 2𝑑𝐷Δ𝑡 + 𝑣̅ 2 Δ𝑡 2 (3) with d the number of degrees of freedom of the motion (d=2 for 2D and d=3 for 3D motion). A linear regression on the MSD curves therefore leads to the simultaneous determination of individual SCs size, 𝑟𝐻 (from D, using Einstein-Stokes relation), and advection velocity, 𝑣̅ . Analysis of N=23 trajectories in Figure 3 shows that no correlation between 𝑣̅ and 𝑟𝐻 is observed experimentally, as expected for convective flows. Moreover, the instantaneous velocity, v, can be analyzed. From the distribution of their z position (Figure S5A in SI), even though less well resolved than their x,y position, the 3Dtracked SCs are confined to a 12 µm thick region of liquid adjacent to the substrate surface where the aggregate is formed. At large radial distance from the aggregate, the local instantaneous radial velocities of the SCs, 𝑣𝑟 , were also estimated. 𝑣𝑟 scales as the SC z position (see examples compared to simulation in Figure S5B in SI), a trend in agreement with the FEM simulations, yielding an estimate of Δ𝑇 = 30𝐾. A

B

𝑣̅ (µm/s)

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Figure 3. Small scale behavior images with x100 magnification. (A) Trajectories of small SCs subjected to laser-triggered directed movement. At this scale, diffusive effects are detectable during the particle trajectory, allowing, from MSD, single SCs measurements of (B) the average drift velocity 𝑣̅ as a function of SCs hydrodynamical radius 𝑟𝐻 (no correlation).

Trapping through Competition between Thermophoresis and Convection. Convection in such a diluted solution is however insufficient to explain the trapping effect, as fluid recirculation is expected near the aggregate: the convective flow would be directed upwards (z direction), and have tendency to drive the particles up and then away from the laser spot position, which is only very rarely observed in 3D SC tracking. A second mechanism operating at shorter distance must be invoked, which acts with opposite force (downwards). Since thermal gradients become important at short distance, thermophoretic migration can no longer be neglected. This can be seen clearly in the FEM simulations showing the velocity field in the r-z section of the cell in Figure 4. The total velocity field due to a combination of convection (blue arrows in Figure 4B) and thermophoresis (red arrows in Figure 4B) is evaluated, along with some selected trajectories (in Figure 4A), equivalent to those that would follow model probe SCs that would be placed in the cell. A value of Δ𝑇 = 30𝐾 was assumed in the calculations, but since the magnitude of both effects depends linearly on the temperature difference, the shape of the field remains unaltered by an increase of Δ𝑇.

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Without thermophoresis (𝐷𝑇 = 0 ), model probe SCs are propelled upwards and away from the aggregate. As the thermophoretic term is increased, more model probe SCs are captured, and consequently the escaping rates are reduced. This allows us to define a capture distance (illustrated by the red region in Figure 4A), mostly in the z-direction (𝐿𝑐𝑎𝑝 ), as the value of L for which thermophoresis is comparable to convection (𝑣0 /𝑣𝑇 = 1).

C

Figure 4. Finite Element Method (FEM) simulations, showing the influence of the thermal diffusion coefficient on the vector field around the aggregate (grey region). (A) Total velocity field (taking convection and thermophoresis into account), with selected particle trajectories obtained by integration of the velocity field. As the thermal diffusion coefficient is changed from 0 to −100µ𝑚2 𝑠 −1 𝐾 −1, the capture zone z-distance around the aggregate increases from 0 to 35µm (red). (B) Contributions of convection (blue) and thermophoresis (orange) to the velocity field for 𝐷𝑇 = −100 µ𝑚2 𝑠 −1 𝐾 −1 . (C) Comparison of simulated (lines) and experimental (■) variations of instant radial velocities, vr, variations with the radial distance, r, from the heated zone center. Simulation were performed for T = 25 K in the absence (dashed grey) or presence (black line) of thermophoresis for 𝐷𝑇 = −10 µ𝑚2 𝑠 −1 𝐾 −1 at 𝑧̅ = 7.5µm from the cell bottom. Experimental velocities are averaged over 66 trajectories in r = 5µm radial distance steps. The error bars correspond to the 1st and 3rd quartiles for each point.

The relative importance of the two physical effects can be roughly estimated from equations (1) and (2), and approximating the gradient by (𝐿/Δ𝑇). This leads to:

𝐿𝑐𝑎𝑝 =

𝜂𝑓 𝑆𝑇 𝐷 𝜌𝑓 𝑔 |𝛽|𝜔0

This expression yields a capture distance of 𝐿𝑐𝑎𝑝 ∼ 5µ𝑚 for 𝐷𝑇 = −10µ𝑚2 𝑠 −1 𝐾 −1 and 𝐿𝑐𝑎𝑝 ∼ 50µ𝑚 for 𝐷𝑇 = −100µ𝑚2 𝑠 −1 𝐾 −1 , in fair agreement with FEM calculations (Figure 4A). Interestingly, the occurrence of SCs moving away from the aggregate has seldom been detected, which means that most arriving SCs are being trapped on the aggregate. This suggests a relatively high value of 𝐿𝑐𝑎𝑝 and that the value of the thermal diffusion coefficient 𝐷𝑇 must lay between the two limiting cases shown, and thus 𝐷𝑇 ∼ −10µ𝑚2 𝑠 −1 𝐾 −1 . This estimation of the order of magnitude of the thermal diffusion coefficient is relatively high, but comparable to literature values. Indeed, the diffusivities of polymer NPs or beads in solvents showed thermophobicity with diffusivities in the 𝐷𝑇 ∼ 5/20µ𝑚2 𝑠 −1 𝐾 −1 range.39,45,46 Negative Soret coefficients have also been observed during the thermal diffusion of polymers in mixtures of good and poor solvents or that of octadecyl coated silica beads in toluene. For such situations chemically similar to ours, the measured diffusivities were also in the 𝐷𝑇 ∼ −0.5 /−10µ𝑚2 𝑠 −1 𝐾 −1 range.39,47-49 From the FEM simulation it is then possible to appreciate the contribution of both convection and thermophoresis from their different characteristic length scales, which are explored during the SCs trajectories in the 25 to 150µm distance range from the aggregate center. These regions were investigated, in 2D at low magnification (Figure 1). The instantaneous radial velocity was also estimated at each SC position along a trajectory. Figure 4C presents the variations of vr, the average radial velocity, at various radial distances (with steps of 5µm) over 66 trajectories. The experimental curve is then compared to the simulated velocity profiles calculated for an altitude taken as the average altitude, 𝑧̅, of the SCs. This parameter, 𝑧̅ = 7.5 µm from the substrate surface, was determined using the average value (of the experimental distribution of SCs altitude (Figure S5A in SI) obtained from 3D holography trajectory tracking at x100 magnification (Figure 3), assuming that the z distribution of SCs observed at longer distances (Figure S5A in SI) holds at shorter distances, For r>80µm vr is governed by the convection and the experimental trend follows the simulated trend considering T=25K. When approaching the center of the heated zone, vr decreases. However, this inflection is detected at much shorter distance from the heated zone than expected from a mechanism implying solely convection (dashed grey line in Figure 4C). The agreement with experimental data is better for a simulation taking into account the mixed convection and thermophoresis mechanism (black line), with 𝐷𝑇 = −10µ𝑚2 𝑠 −1 𝐾 −1 . Putting Forth the Transition via Symmetry Break. Finally, the transition between thermophoresis and convection was put forth by analyzing the velocity flow direction change when the radial symmetry of the problem is broken. This was achieved by changing the orientation of the trapping beam from orthogonal to oblique (with an angle 𝜙 = 45𝑜 with respect to the normal direction), as sketched in Figure 5A. The change in orientation modifies the final aggregate form, which becomes elliptical. This is a purely geometric effect, as the intersection of the laser beam with the horizontal glass slide

(4)

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plane defines the hot spot (and therefore the temperature) distribution, thus controlling the aggregate shape. A

B

combination of optical monitoring, single object tracking methods and Finite Elements Method simulations was employed to understand the physical and chemical origins of the trapping. We demonstrate that the trapping is a thermal phenomenon, and suggest that a combination of convection and thermophoresis (with negative Soret number, likely due to the presence of poor and good solvent of the NP coating) is responsible for drawing the SCs together while optical forces together with thermophoresis can contribute to the cohesion of the formed aggregate. These two mechanisms play complementary roles in the aggregate formation. While convection acts at long range, dragging the SCs towards the laser spot, it is thermophoresis that, at short range, draws together the SCs to the warmest regions to form the aggregate.

50µm

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D

Figure 5. Breaking the radial symmetry of the problem by changing the trapping laser orientation,𝜙 = 45𝑜  (A). The resulting SC trajectories are tracked using Imaris tracking software (B). In (C) relative importance of the velocity fields in the xy-plane due to convection (blue) and thermophoresis (orange). The arrows are at the same scale and were obtained via FEM by describing the aggregate as an ellipse held at a fixed temperature and by assuming 𝐷𝑇 = −10 µ𝑚2 𝑠 −1 𝐾 −1 . (D) Integration of the velocities, revealing the expected bent trajectories.

Since convective flows stem from a vertical buoyancy force, the fluid velocity is not expected to rigorously follow the temperature gradient. As a SC initially far from the aggregate approaches it, the thermophoretic component of the velocity field starts to dominate the particle movement, leading to a change in the direction of the motion, as seen in the simulated velocity fields and its expected trajectories (Figure 5C and D). In other words, the trajectories are expected to bend (going from cylindrically-centered to normal to the aggregate tension line) as the flow regime changes from convective to thermophoretic. This effect is visible in the experimental trajectories (Figure 5B). The direction of the SCs is clearly modified by the presence of the aggregate, leading to nonlinear trajectories. This clearly reveals the role of thermophoresis at short scales, as a purely convective field would be linear. It is worth noticing that the particles are attracted not only by the aggregate, but also by regions of increased SCs density, such as the prolongation of the big axis of the ellipse (horizontal in Figure 5B). This is because in this configuration, travelling SCs are also illuminated by 𝜆𝑡𝑟𝑎𝑝 and thus become heat sources. The temperature gradient distribution is consequently distorted, leading to the attractive effect. CONCLUSIONS Supercrystals (SCs) of functionalized silver NPs are a relevant candidate for forming dynamical plasmonic platforms. By adjusting the nature of the organic solvent it is possible to tune its interactions with NP ligands, allowing extension of the SCs functionalities beyond organized aggregation. Taking advantage of the plasmonic properties of the NPs (then SCs) to create a steep temperature gradient upon laser light illumination, tailoring of the interactions leads to interesting transport properties, which introduce migration phenomena, creating a mechanism through which the aggregation of SCs can be controlled dynamically from the far field, using lasers. A

Since surface enhanced methods (such as SERS) engage a very similar geometry as the one used herein in the experiments, we anticipate that the present system can have developments in sensing applications by allowing on-demand assembly or release of the plasmonic sensing platform. Active control of the hot spot density distribution is also likely to enhance the reliability of the plasmonic platform, potentially helping to bridge the gap from fundamental to technological applications of surface-enhanced methods. Although for practical reasons we have chosen to adapt the (organic) solvent to a given surface group, understanding of the chemical origins of the trapping allows the converse strategy: designing functional groups for a given application (and solvent). Although we acknowledge this is no easy task, a wide range of surface modification methods exist and can be employed to reach this goal. The results presented here are therefore expected to be general and widely applicable. METHODS/EXPERIMENTAL C12-AgNP Synthesis The synthesis of narrowly distributed Ag NPs (d=5.9nm, with polydispersivity of 8%, see Section S1b and c in SI) capped with dodecane (C12) thiols is based on a modified Stucky’s procedure, and described in detail elsewhere50. In short, the silver precursor (NO3Ag(PPh3)) is dissolved in 1,2-dichlorobenzene in an inert atmosphere (N2) and heated to 160oC. An excess of dodecanethiol is then added under stirring, prior to the addition of tert-butylamine borane (TBAB), which acts as a reducing agent. After 60min the reaction is stopped by cooling down the solution and the NPs are precipitated by addition of ethanol. A selective precipitation process leads to a population of NPs with a narrow size distribution (polydispersivity < 8%), which are dispersed in apolar solvents such as hexane or toluene. The resulting NPs are characterized by TEM and UV-Vis spectroscopy (Section S1 in SI). Microfluidic Chamber Preparation. A microfluidic chamber (Section S2 in SI) is prepared by superposing a glass coverslip and a glass slide, with a teflon layer spacer (thickness=500 or 100µm) shaping the microfluidic chamber. Plastic capillaries (Eppendorf® Ep T.I.P.S.) are added as inlet and outlet, and an epoxy-based resin (TorrSeal) is used to seal the cell (Section S2 in SI).

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C12AgNPs are dispersed in a mixture of toluene-hexane (60:40, unless specified otherwise), diluting them down to 0.44nM (in NPs, corresponding to a dilution factor by more than 2 orders of magnitude for the SCs, see Section S4 in SI). The NP solution, which is sensitive to O2 is handled and injected, from a glovebox, in the microfluidic chamber. The chamber is kept under argon then closed before being removed from the glovebox and placed in the two-laser holographic setup.

ABBREVIATIONS C12- AgNP, dodecanethiol functionalized silver nanoparticles SCs – supercrystals

REFERENCES

ASSOCIATED CONTENT Supporting Information. Description of Movie 1. S1 AgNPC12characterization. S2 Microfluidic chamber preparation. S3 MSD analysis. S4 Mass balances during aggregate formation. S5 3D Holography. S6 Convection COMSOL models (pdf). Movie 1: video showing the experiment portrayed in Figure 2 (avi). This material is available free of charge via the Internet at http://pubs.acs.org.

TABLE OF CONTENTS

AUTHOR INFORMATION Corresponding Author * e-mails: [email protected], [email protected]

Authors Contributions The authors contributed equally to this work. V.B., P.B., G.T., A.C. and F.K. designed the experiments. V.B., P.B. and P.A. performed the experiments and collected the data. P.A. and L.C. synthesized the AgSCs V.B., P.B., E.M., C.C., G.T., A.C. and F.K. analyzed the data. V.B. and F.K. wrote the manuscript. All authors commented and gave approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here.

Funding Sources This work was financially supported by Labex MiChem, part of the French state funds managed by the Agence Nationale pour la Recherche, ANR, within the Investissement d’Avenir program under reference ANR-11-IDEX-004-02, and through the NEOCASTIP ANR-15-CE09-0015-02 project, CNRS, Paris Diderot University, Paris Descartes University.

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