resonant photothermal sensitizers

Transcription

1 Supporting Information White and brightly coloured 3D printing based on resonant photothermal sensitizers Alexander W. Powell, Alexandros Stavrinadis, Ignacio de Miguel, Gerasimos Konstantatos*, Romain Quidant* This document contains additional information about the synthesis and characterization of GNRs, GNR-PA12 nanocomposite powders, including: Description of experimental methods Absorption spectra for CB and GNRs in solution. SEM images of GNRs with coatings of PEG and silica. The extended plot and spectral plots showing the peak shifts for GNRs coated in PEG and silica after storage at high temperature. A description of the relationship between GNR concentration, heating and colouration in GNR-PA12 powder mixes. A description of the method to calculate the photothermal conversion efficiency. A diagram of the custom-built laser-scanning setup used to take optical heating measurements and to sinter 2D shapes for colour tests. A description of the CIE L*a*b colour space. Experimental Methods Gold nanorods were synthesized in-house via the seed-mediated method of Nikoobackht & El-Sayed 1, and either coated in PEG following the procedure described by Liao & Hafner 2 or in silica following Gorelikov and Matsuura 3. Carbon black powder was purchased from PlasmaChem.

2 The GNRs and CB were dispersed in ethanol (aqueous solutions were not found to mix well with the polymer powders), then mixed with pristine PA12 powder from Advanc3Dmaterials at differing wt % concentrations (see Fig S5 for studies on the effect of concentration), by first pouring the solution over the powder, then mixing mechanically for several minutes until nearly dry to ensure even coverage of the powder particles. Powders were then dried in an oven at 60 C overnight. For coloured 3D objects, the NP s were mixed in solution with commercially available fabric dyes and then added to the powder as described above. After drying powders were sieved prior to printing. For sintering the 3D objects, the composite powders were the deposited into the chamber of a Sintratec Kit printer. The printer has been modified by switching the 2W 445 nm laser for a 1W 808 nm laser from Lasertack, and replacing the scanning mirrors, lenses and optical windows with components tuned to maximise efficiency in the NIR. After printing, all samples were cleaned using brushes and a small sandblaster. The 2-photon luminescence measurements were carried out in a STED CW microscope in a Leica SP5, modified in-house to implement 2PL. In these images, green represents the presence of gold in the sample, with brighter green signifying a higher gold concentration. To perform the comparative tests between Carbon black and GNRs, the powder, small samples of powder in a shallow aluminium holder were placed into a custom-made test bed (See Figure S7), where illumination was supplied by either a 18W diode laser from lumics for the heating tests, using a 1 cm 2 beam cross-section operating at a power density of 1 W/cm 2, or, for the colouration tests, a 1W 808 nm diode laser from Thorlabs was used. Pre-heating was supplied from a hotplate below and IR lamps from above in order to mimic the conditions in a SLS printer. The bulk of the powder was heated to 150C using the hotplate and then the top layer heated to C using the lamps. The temperature of all samples was measured using a FLIR ax5 thermal camera, and the colour tests on printed samples was performed using a PCE instruments PCE-CSM 4 colorimeter. Photothermal efficiency tests were performed by illumination 2 ml of GNR or CB solution at a concentration of 0.01 g/l in a cuvette via a 808 nm laser at an intensity of 1 W/cm 2 and measuring the heating and cooling curves via a thermal camera before calculating the efficiencies.

3 Figure S1 : Absorption spectra of 0.01 g/l solutions of CB and GNRs coated in Silica, suspended in ethanol. Figure S2 : SEM images of GNRs coated in (a) PEG and (b) Silica. The scale bar shows 500 nm.

4 Figure S3 : (a) The resonance wavelength of GNRs coated in PEG and Silica in PA12 powders after prolonged heating in an oven at 150 C for the entire duration measured. Normalised absorption spectra of (b) PEG and (c) silica coated GNRs mixed with PA12 powder in an oven at 150 C. The blueshift of the PEG particle resonance over time can be clearly seen.

5 The effect of GNR concentration in GNR-PA12 nanocomposites. The absorption of silica coated GNRs and CB in an ethanol solution are shown in Fig. S1: it can be seen that near resonance, the GNRs are significantly better absorbers than the CB, leading to the superior heating properties shown in Fig. S4 where the GNRs are shown to heat significantly more than CB, and in fact show an heating equal to 3 times the concentration in CB, as highlighted in Fig. S4(a)&(b). It can be seen that for every doubling of GNR concentration, the increase in heating is less, which is in agreement with the Beer-Lambert law and signifies that there are diminishing returns in terms of heating for higher concentrations of GNRs. Figure S4 : (a) Showing the maximum temperature of GNR-PA12 (silica coated) and CB-PA12 composites as a function of concentration, on illumination with a 808 nm laser at a power of 1W/cm 2 and an illumination area of 1cm 2. (b) Heating vs time curves of GNR-PA12 and CB-PA12 composites for a concentration of CB three times that of the GNRs by weight. For any plasmonic absorber, there will always be some amount of absorption in the visible, in this case owing to the interband absorption of gold and the secondary mode of the GNRs, which arises from their geometry. It is therefore important to achieve a balance between heating ability and colouration in the GNR-PA12 nanocomposite powders. It was already shown that there is a diminishing return in terms of heating ability with concentration in Fig. S4, and in Fig S5 the colouration is discussed. With the increase in GNR concentration, the powders can be seen visually, and in the CIELAB plots to grow pinker and darker. Looking at the relationship between coloration and concentration in Fig. S5(d) & (e), the largest changes can be seen for small concentrations of GNRs, with both coloration (d) and lightness (e) showing significant shifts for small changes in GNR concentration.

6 Figure S5 : Showing the effect of changing Au concentration on changing color properties. (a) Photo of GNR-PA12 mixes with GNR concentration of wt %. (b),(c) CIE L*a*b plots of colouration and lightness respectively. The direction of the red arrows point to increasing GNR concentration. (d) Plot of the coloration of powders vs GNR concentration. (e) Plot of the lightness of powders vs concentration. As we require materials to heat as much as possible, but remain as white as possible, a balance must be found. For the purposes of this study, we found a concentration of 0.01 wt % GNRs to heat effectively whilst still creating visually white powders and printed objects.

7 Calculating photothermal efficiency Following the method described by Feng et al 4, the photothermal conversion efficiencies nanoparticles in solution can be determined from the heating and cooling curves of the solution under illumination. Starting from the steady-state equilibrium of the system, the photothermal conversion efficiency, η of the PTS can be found via the following: = h 1 10 Where h is the heat transfer coefficient, S is the surface area of the container, T Max & T Sur are the maximum equilibrium temperature under illumination, and the ambient temperature respectively, Q s is the energy input due to absorption in the ethanol solvent and the cuvette, I is the incident radiation intensity and A 808 is the absorbance at the laser wavelength, hs can be solved as: h =, Where m i, c i ae the mass and heat capacitance of the solvent and the cuvette and τ s is a time constant for the system assosciated with the cooling. When the laser is switched off, and the sample is in the cooldown phase, τ s can be written: Where: = ln = Therefore to find the photothermal conversion efficiency of the system, one needs to measure the absorbance of the system at the laser wavelength, heating curves to extract the ambient and maximum temperatures, cooling curves to extract the time constant and thus the hs values, and also perform an identical test on a cuvette containing only ethanol to obtain Qs. The data used for these calculations is shown below in Figure S6.

8 Fig S6 : (a) Heating and cooling curves for GNRs in ethanol under 1 W/cm 2 illumination at 808 nm. (b) Data extracted from the cooling curves to obtain the time constant, τ s. Experimental test-bed for 2D sintering and coloration tests Figure S7 : The custom-built laser scanning setup used for measuring heating and 2D writing. The laser can be interchanged between a high power Osram laser for optical heating tests and a low-power Thorlabs laser for 2D writing.

9 The CIELAB colour space The colouration of objects or powders can be defined quantatively using the CIELAB (or CIEL*a*b*) colour space. The CIELAB space is designed to approximate human vision and is therefore a good choice for comparing real-world objects where aesthetics are important. This scale divides visual colour into three dimensions, of L* for the lightness, and a* and b* define colour opposites green-red and blue-yellow. This scale can alternately be described using cylindrical co-ordinates L*c*h* where c* describes how strongly coloured something is (how close it is to the edge of the circle in Fig. S7) and h* defines that actual hue. Figure S8 : A depiction of the CIELAB (and CIELCH) colour space showing the pure colors achieved when all other values are equal to zero (square boxes).

10 References (1) Nikoobakht, B. N.; El-Sayed, M. A., Chemisre Mater. 2003, 15, (2) Liao, H.; Hafner, J. H., Chem. Mater. 2005, 17, (3) Gorelikov, I.; Matsuura, N., Nano Lett. 2007, 8, (4) Feng, W.; Chen, L.; Qin, M.; Zhou, X.; Zhang, Q.; Miao, Y.; Qiu, K.; Zhang, Y.; He, C., Sci. Rep. 2015, 5,