Connecting metallic nanoparticles by optical

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1 Supplementary Information for Connecting metallic nanoparticles by optical printing Julián Gargiulo 1, Santiago Cerrota 1, Emiliano Cortés 1, Ianina L. Violi 1, Fernando D. Stefani* 1,2 1 Centro de Investigaciones en Bionanociencias (CIBION), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Godoy Cruz 2390, C1425FQD Ciudad de Buenos Aires, Argentina 2 Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Güiraldes 2620, C1428EAH Ciudad de Buenos Aires, Argentina Materials Citrate-capped gold nanoparticles (NPs) were purchased from Nanopartz (specified diameter 62±2 nm; average diameter determined by FE-SEM 63±3 nm). Citrate-capped silver NPs (specified diameter 60±8 nm; average diameter determined by FE-SEM 57±5 nm), Polydiallyldimethylammonium chloride (PDDA, MW ), sodium polystyrene sulfonate (PSS, MW = ) and Tween 20 were purchased from Sigma-Aldrich. In order to obtain the appropriate colloidal concentration for printing, the following preparation protocol was performed. Au and Ag NPs were centrifuged at 600 rcf for 5 1

2 minutes in order to remove aggregates, and then the supernatant was diluted in NaCl 1.5 mm and Tween % for Au NPs and in 1mM sodium citrate for Ag NPs. Substrates for optical printing Glass coverslips were modified through layer-by-layer deposition of polyelectrolytes 1 in order to render their surface negatively charged. First, substrates were sonicated in MilliQ water for 20 minutes, then placed in a PDDA solution (1 mg/ml in 0.5M NaCl) for 15 minutes and then placed in a PSS solution (1mg/ml in 0.5M NaCl), with intermediate water-rinsing steps. Automation of printing process and measurement of printing time The optical printing process was automated using a routine programmed in Labview. Scattered light from the printing laser was monitored in confocal configuration at a a maximum frequency of 5 khz. A printing event is detected as an increase in the scattered light signal, due to the presence of a NP in the laser focus. One example time-trace is shown in Figure S1. This particular printing event takes place in less than 1 ms. Figure S1. Time trace of the scattering signal. Printing time in this measurement is 7.8 seconds. 2

3 Printing time We define the printing time as the period between the point in time when printing laser is unblocked and the time when a printing event is detected (Figure 1S). The printing time is dominated by the time that a NP needs to diffuse into the region where optical forces counter-act Brownian motion. Therefore, it becomes shorter for higher laser intensities and higher concentrations of NPs. Since it is defined by diffusion, the printing time is a stochastic variable, and it must be measured for many printing events in order to obtain a representative average value. In the case of Au-Au dimers, we observed a dramatic increase of the average printing time for interparticle separation set points d < 300 nm. Figure S2. Normalized printing time vs. set point d for Au-Au and Ag-Au. Optical fabrication and characterization of Ag-Au heterodimers After printing an extended grid of well separated (e.g NP/m 2 ) Ag NPs, the suspension is removed and replaced with the suspension of Au NPs. Then, the following procedure was implemented: 3

4 1) Two confocal scans of the light scattered by a Ag NP are obtained, one at 405 and the other at 532 nm (Figure S3). The image at 405 nm (Figure S3E) is used to locate the NP center with an accuracy of about 1 nm. The image at 532 nm (Figure S3D) is used in a later step. The sources of elastically scattered light in the sample are the reflection r at the glass surface and the scattering s from the NP. Due to the high coherence of the lasers used, both signals interfere at the detector. The measured signal is then 2 cos where is the phase difference between the reflected and the scattered light, the phase of the NP polarizability e and an additional phase constant. In agreement with the fact that 405 and 532 0, constructive interference is observed at 405 nm (Figure S3E) and destructive interference at 532 nm (Figure S3C). 2) The piezoelectric stage is shifted to a set-point distance d from the Ag NP center. The 532 nm laser is unblocked to focus on the sample with at a power of 1.2 mw (enough to perform printing). The scattering signal is monitored in time at 100 khz. Once the printing of the second NP is detected, the laser is blocked in order to prevent further printing. As a result, a Ag-Au heterodimer is fabricated (Figure S3B). 3) A confocal scan of the Ag-Au dimer is obtained at 532 nm (Figure S3D). Both Ag and Au particles contributes to the scattered signal. Subtracting the contribution from the Ag NP (acquired in step 1, Figure S3C) allows to obtain the image (Figure S3F of the isolated Au NP). The comparison between images F and E provides the measured (experimental) inter-center distance dexp. 4

5 Figure S3. Dark field images of a) 60 nm Ag NP; b) Ag-Au dimer at d=200 nm with Ag NP on top. Confocal scattering images of c) Ag NP scanned with 532 nm laser; d) heterodimer scanned with 532 nm laser; e) Ag NP scanned with 405 nm laser; f) image obtained after subtracting the signal received by a single Ag NP (image c) to the dimer confocal scattering at 532 nm (image d). Intensity of image C is proportional to 2 cos Intensity of image D is proportional to 2 cos 2 cos 2 cos Subtracting D-C 2 cos 2 cos 5

6 The first two terms have a maximum at the Au NP position. The last term can be considered as an estimation of the error for the optical characterization. Numerical simulations considering the experimental levels of signal, background and noise, show that the error is always lower than 20 nm. It is important to remark that light polarized perpendicularly to the dimer axis is used, in order to avoid a scattering signal from electromagnetically coupled NPs. This process was repeated for at least 20 dimers, and for each inter-center set point d, obtaining a mean value in each case. As an example, Figure S4 shows the measured intercenter distance dexp for 17 dimers, fabricated with a set point of d=400 nm. The obtained mean value was (401±4) nm. Figure S4. Experimental interparticle distance dexp for each fabricated Ag-Au dimer. The red line indicates the mean value. 6

7 Gaussian beam description Both for the calculations of temperature and optical forces, we considered our focused beam as Gaussian, propagating along the z direction:, 2 1 tan 1 where is the wave-vector in the propagation medium of refractive index n. is the maximum field amplitude at (0,0), and /. The parameter is the beam waist at the focal plane (z = 0), also called the Gaussian beam radius, and represents the radius at which the field has decreased to 1/e, and the intensity (see below) has decreased to 1/e 2. w is the beam waist along the z direction. The time-averaged intensity (irradiance [W/m 2 ]) Re, is given by: where, is the intensity at (0,0), and / is the wave impedance in the non-magnetic propagation medium. Or in terms of the total power of the beam P [W]:, 2 / The value of was determined experimentally by detecting the light dispersed as the laser focus (532 nm) was scanned over a Au NP. This measurement is proportional to,. By fitting a Gaussian function to detected signal we obtained a value of = (266 ± 3) nm. 7

8 Temperature calculations This calculation considers a sphere generating heat in an infinite surrounding medium. At first, the NP is in thermal equilibrium with the surrounding medium. Upon (light absorption and) heat generation, there is a temperature increase of the NP and its immediate surrounding with respect to the initial temperature. The temperature increase Δ at the surface of the printed NP is calculated according to Baffou et al 2 : Δ with being the absorbed power by the NP, is the NP radius, and the heat conductivity of the surrounding medium. For a focused Gaussian beam at a distance d from the NP, Q takes the following form:, 0 2 Printed particle temperature increase [K] Au Ag d [nm] Figure S5. Surface temperature increase of a 60 nm NP (Au or Ag) when it is illuminated with a focused Gaussian beam (w0 = 266 nm, = 532 nm, and P = 1.2 mw) centered at a distance d from the center of the NP. 8

9 Optical force calculations Gradient and scattering forces were calculated following the work by Agayan et al 3., 1, 2, 2, 2 The polarizability was calculated using the Mie theory. The optical forces shown in figure S6 were calculated considering = 532 nm, = 1,2 mw and = 266 nm. z = 0 corresponds to the glass substrate. Figure S6. Optical forces acting on a spherical gold 60 nm NP at different positions relative to a focused Gaussian beam ( = 532 nm, w0 = 266 nm, P = 1.2 mw) centered at (0,0) and propagating in the z direction. The arrows denote the direction of the force and its magnitude is color coded. 9

10 Corresponding Author * fernando.stefani@cibion.conicet.gov.ar References (1) Urban, A. S.; Lutich, A. A.; Stefani, F. D.; Feldmann, J. Nano Lett. 2010, 10, (2) Baffou, G.; Quidant, R. Laser Photon. Rev. 2013, 7, (3) Agayan, R. R.; Gittes, F.; Kopelman, R.; Schmidt, C. F. Appl. Opt. 2002, 41,