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Thermo-Plasmonic Effects in Gain-Assisted Nanoparticle Solutions Giovanna Palermo, Donatello Pagnotto, Loredana Ricciardi, Luigia Pezzi, Massimo La Deda, and Antonio De Luca J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08186 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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The Journal of Physical Chemistry

Thermo-Plasmonic Effects in Gain-Assisted Nanoparticle Solutions Giovanna Palermo,∗,†,‡ Donatello Pagnotto,† Loredana Ricciardi,‡ Luigia Pezzi,† Massimo La Deda,¶,‡ and Antonio De Luca∗,†,‡ Department of Physics, University of Calabria, 87036 Rende (CS),Italy, CNR-Nanotec, 87036, Cosenza, Italy, and Department of Chemistry and Chemical Technologies, University of Calabria, 87036 Rende (CS), Italy E-mail: [email protected]; [email protected]

Abstract We report a detailed characterization of the photo-induced heating observed in gainassisted solutions of gold nanoparticles (AuNPs). AuNPs, with sizes ranging from 14 to 48 nm and concentration of 2.5 · 10−10 M, are exposed to different intensity values of a resonant continuous laser (532 nm), used to excite their Localized Surface Plasmon Resonance (LSPR), responsible of the photo-generation process. In this way the optimal conditions to achieve the maximum temperature variation with the least laser dosage are obtained. By adding an organic dye to the solutions, whose emission band overlaps to the LSPR, we found that the contribution to the photo-thermal efficiency is enhanced if the solutions are excited at 405 nm. This happens in the case of smaller NPs, due to a strong coupling effect between the two sub-units, that causes an increasing of ∗

To whom correspondence should be addressed Department of Physics - University of Calabria ‡ CNR-Nanotec ¶ Department of Chemistry and Chemical Technologies - University of Calabria †

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the extinction cross section of the whole gain-assisted system. On the other hand, for the larger AuNPs, an opposite behavior is found: a loss compensation mechanism, based on a resonant energy transfer process from gain units to plasmonic nanoparticles, limits the increasing of the absorption cross section with a consequent lowering of the photo-thermal efficiency. The presented quantitative analysis of a dispersion of AuNPs results fundamental for bio-medical applications as well as for integrated plasmonic devices based on loss compensation effects, where the impact of undesirable thermal effects can not be ignored.

Introduction The properties of materials at the nanoscale meaningfully differ from those observed at macroscopic range, revealing the opportunity to focus on an intermediate state that resides in between a solid and a single molecule. The possibility to control and modify the chemical and physical properties of nano-structured materials have received great interest both in research fields and industrial areas, especially in the last years. 1,2 Metallic nano-particles (NPs), made by tens to some thousand of atoms, represent a very important class of nanomaterials and their study constitutes one of the greatest topics within the nanotechnology framework. 3–5 The use of these materials ranges from electronics 6–8 to telecommunication, 9 including also chemistry, 10–12 biology and nano-medicine. 13–17 The optical response of a nanoobject represents an important feature in order to understand its complex behavior. For noble metals NPs the most important property is related to the confinement of the electric field on a scale much shorter than the wavelength of an incoming e.m. wave, this leading to collective resonant oscillations of the free electronic charge known as Localized Surface Plasmon Resonance (LSPR). 3 During this process, the oscillating electrons transfer their kinetic energy into the particle lattice through electron-phonon interactions, followed by phononphonon interactions with the surrounding medium. This leads to a significant nano-localized temperature increase around each NP that can be easily estimated in a macroscopic way 2 ACS Paragon Plus Environment

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by irradiating at the same time a huge number of NPs. 18,19 The possibility to externally trigger the temperature variation related to nano-objects leads to the development of new, non invasive, cancer therapy approaches in which biological tissues can be exposed to very high temperatures to promote the selective destruction of big cells. 20–22 The state of art shows how it is important to minimize the effect of photo-generation on healthy tissues, by opting for the use of nano-objects, whose LSPR falls in the water window (740 − 1000 nm), where the absorption of skin cells is minimized. 13 However, the visible radiation can be used by strongly monitoring the pump beam intensity values or it can be considered a two-photon absorption process for the excitation. In the past, several groups have proposed experimental and theoretical studies about the photo-induced heat of a single NP with different morphologies, 23,24 or ensemble of NPs arranged in an ordered way but even randomly distributed. 25 Richardson et al. performed a study of heat generation from single to few AuNPs in a ice matrix, using Raman and photo-luminescence spectroscopy in order to determine the amount of photo-generated heat by observing the melting effect. 26 The same group performed an experimental and theoretical study on the photo-thermal effect in a water droplet containing AuNPs. 27 Moreover, Hogan et al. demonstrated that the particlebased photo-thermal processes can concentrate energy absorption into very small volumes, useful for light-induced steam production. 28 The first attempt to estimate the light to heat conversion efficiency of AuNP solutions with different sizes has been proposed by Jiang et al. 29 However, an analysis of the thermal response of AuNPs solutions as a function of the impinging intensity (power density) in a wide range (from 0.5 to 2.2 · 103 W/cm2 ) and the possibility to take advantage of a gain medium added to the solution, have not been yet experimentally investigated. In the past, it has been demonstrated that bringing gain in proximity to metal sub-units can reduce the strong radiation damping, in terms of reduction of the imaginary part of the metal dielectric permittivity. 30–32 The loss mitigation, in this case, may have a negative impact on the thermal efficiency of the AuNPs, but the proper control of the gain medium properties could enable novel photo-thermal aspects.



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In this work, we investigate the photo-thermal mechanisms of gain-assisted AuNPs colloidal solutions, by monitoring the effect of some physical properties on the light to heat conversion efficiency. Mainly we show how the NPs size, as well as the contribution of the pump beam power density and the presence of fluorescent guest molecules in the solutions, may affect the photo-thermal process. The choice of particular dye molecules (Rhodamine 6G-R6G) is not random, but it is linked to the request of a proper overlapping between the dye emission band and the absorption band of the used AuNPs. A CW green laser (λ = 532 nm) was employed to photo-excite the LSPR of a 5 mL solution of AuNPs, in absence of R6G. By monitoring with a thermographic camera the hot-spot related to the maximum increase of temperature at the liquid/air interface of a 10 mm quartz cuvette, we are able to determine the size and laser dosage responsible of the maximal heating effect with minimal power density. The effect of the gain medium on the temperature variations was estimated by exciting the gain-assisted solutions with a CW blue laser (λ = 405 nm), corresponding to a wavelength falling into the R6G absorption band but, at the same time, far from the LSPR maximum.

Methods Synthesis Gold(III) chloride trihydrate (HAuCl4 · 3H2 O) and trisodium citrate were purchased from Sigma-Aldrich and used as received. All solutions were prepared using distilled water. The citrate-stabilized AuNPs with an average diameter respectively of 14, 26 and 48 nm were synthesized following the well-developed kinetically controlled seeded growth method, via the reduction of HAuCl4 by sodium citrate. Briefly, the gold seeds (∼ 10 nm) were prepared by adding 1 mL of HAuCl4 (25 mM) into a 150 mL of reducing solution containing sodium citrate (2.2 mM) at 100◦ C under reflux and vigorous stirring. Within 10 min the solution turned from pale yellow to blue-gray, pink and then red-wine indicating the formation of 4 ACS Paragon Plus Environment

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the colloidal nanoparticles. To further growing steps, the temperature of the solution was decreased to 90◦ C in order to prevent any secondary nucleation of AuNPs. For the growth of the seeds up to a diameter of 14 nm, 1 mL of sodium citrate (60 mM) and 1 mL of HAuCl4 (25 mM) were sequentially injected and then the reaction was run for 30 min. Hence, by subsequently repeating this process (addition of 1 mL of 60 mM sodium citrate and 1 mL of 25 mM HAuCl4 ) three times, AuNPs were grown up to ∼ 26 nm. Finally, AuNPs of 48 nm in diameter were synthesized by means of further growth steps. The sample (AuNPs of ∼ 26 nm) was diluted by extracting 55 mL of sample and adding 53 mL of water and 2 mL of 60 mM sodium citrate. 1 mL of 25 mM HAuCl4 was injected after 1 min and after 30 min. This process, dilution plus two injections, was repeated three times. During the whole synthetic procedure, 1 ml of the solution was taken after each growing step and characterized by UVVIS spectroscopy and Transmission Electron Microscopy (TEM). The as-synthesized AuNPs were purified by ultrafiltration method (Vivaspin20 equipped with 100 kDa membrane).

Characterization The morphology of the synthesized AuNPs was observed by means of a JEOL 2010F transmission electron microscope (TEM). The samples were prepared by depositing a drop of a diluted colloidal solution on 200 mesh carbon-coated copper grids. After evaporation of the solvent, the particles were observed at an operating voltage of 120 kV. The extinction spectra of AuNP solutions were acquired by a UV-VIS spectrometer (Ocean Optics, USB2000+). The CW green laser used in the photo-thermal excitations is a Verdi-V5, by Coherent. The temperature variations were monitored by means of a thermographic camera (E40 by FLIR), characterized by a sensitivity of 0.07 ◦ C and a spatial resolution of 2.72 mrad. Control parameters are appropriately set to take into account both environment and material properties: reflected temperature was set to T0 = 20◦ C, emissivity of water to ε = 0.98 and the working distance between camera and object to 0 m, meaning that it was less then 10 cm. The camera provides images of 19200 pixels (160 x 120 px), where each pixel represents a temperature 5 ACS Paragon Plus Environment


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value. A lens has been used to focus the laser at the air/solution interface (focal length of 3 cm). Steady-state emission spectra were recorded by a HORIBA Jobin-Yvon Fluorolog-3 FL3-211 spectrometer equipped with a 470W xenon arc lamp, double-grating excitation and single-grating emission monochromators (2.1 nm/mm dispersion; 1200 grooves/mm), and a Hamamatsu R928 photomultiplier tube. Emission and excitation spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. To prevent that second order diffraction light from the source could reach detector, cut/off filters were used. Time-resolved measurements were performed using the time-correlated single-photon counting (TCSPC) option of the Fluorolog 3. A NanoLED at 379 nm, fwhm t ; VN P is the volume of each nanoparticle.



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Moreover, the number nw is deeply related to the value of beam diameter w, and can be approximately estimated by considering the beam propagation through the AuNP solutions as a cone whose base size matches the impinging beam diameter. The height of the cone has been estimated from the side views of the propagating beam in each solution (Figures 2d-f). It is well evident how, in case of 14 and 26 nm AuNPs, the pump beam is able to reach the bottom of the cuvette, while in the case of 48 nm nanoparticles, this height is reduced to a fraction of the cuvette height. By following this approach, and by considering the exact NPs concentration, we find a difference of about 3 orders of magnitude in nw for the 48 nm AuNPs, passing from the non focused beam (w0 ) to the most focused one (w2 ). The estimation of nw for 14 nm and 48 nm samples is reported in Table 1. Table 1: Estimation of nw as a function of beam diameter AuNPs diameter (nm)

w0

nw w1

14 48

8.6 · 109 2.0 · 109

1.87 · 109 4.6 · 108

w2 3.0 · 107 7.5 · 106

These experimental results show the existence of an inverse relation between the intensity of the incoming radiation and the number of AuNPs contemporary excited in the photogeneration processes. In fact, once fixed the incident power, by acting on the beam diameter causes a decrease of the number nw but an increase of the intensity value. Even if the exponent values in these numbers are different, the final value of ∆T is ensured by the presence of the square of diameter in the denominator of the intensity expression. Then, the thermal efficiency of our system can be modified by acting on the focus of the pump beam in order to involve a greater or less number of AuNPs in the heat generation process. This allows to have a control of the photo-thermal conversion efficiency by exploiting multiple parameters such as size, nw , P , w and also VN P (the nanoparticle volume) related to the


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temperature variation ∆T by a relation of the type:

∆T ∝ q · VN P ∝

nw V N P P (w/2)2

(2)

From this relation, it is evident how, by focusing the beam at a diameter w2 , a decreasing of nw of 3 orders of magnitude is well balanced by 2 orders of magnitude in the intensity value (the square of diameter at the denominator ensures it). The presence of the nanoparticle volume in the expression of ∆T permits, then, to obtain a final value that is not one order of magnitude less (passing from w0 to w2 ) but only 3.5◦ C (see Fig. 2c). Another interesting point in the study of photo-induced heat generation in AuNPs solutions is the use of gain media to induce photo-thermal effect by exciting the solutions far from the LSPR. In order to study the effect of a gain medium on the photo-thermal mechanism of AuNPs, a low concentration (CR6G = 2.92 · 10−6 M) of Rhodamine 6G (R6G) in H2 O has been added to the solutions characterized by a diameter of 14 and 48 nm. The choice of R6G is related to the possibility of exploiting the strong coupling between the gain medium and the AuNPs plasmonic resonance in terms of an enhancement of the photo-induced heating process or in its mitigation. To determine the contribution of R6G in the heat production, the considered solutions were thermally re-characterized after the addition of 0.5 mL of R6G dispersed in water. The same amount of distilled H2 O was added to the previous two solutions in absence of R6G, in order to permit the comparison between iso-concentrated samples of AuNPs. Then, we used as continuous pump source a blue laser (405 nm), corresponding to a wavelength that falls into the absorption band of R6G but far from the AuNPs LSPR. By exciting the pure R6G solution with a low intensity blue laser (I = 7 W/cm2 ), we were not able to observe any temperature variation compared to room temperature, while for the pure AuNPs samples a ∆T of 2.5 ± 1 ◦ C and ∼ 10 ± 1 ◦ C has been observed in the case of 14 nm and 48 nm samples, respectively, by using the same intensity of the blue laser. These temperature variations are related to the interband transitions that for the noble metals



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occur in the UV-Vis spectrum region. 34 This photo-generation process is less efficient than the one related to intraband transitions. 34 No significant results have been obtained in the samples with a NP concentration of 2.5 · 10−10 M, demonstrating no substantial differences between the solutions in presence and absence of R6G (see Supporting Information). Significant results have been, instead, achieved by increasing the AuNP concentration to 2 · 10−9 M, almost an order of magnitude higher. In this case, we observed a temperature variation of about 6.0±1◦ C for the 14 nm NPs in presence of R6G, while for larger NPs we find almost the same temperature variation (10.0 ± 1◦ C, Figure 3a).

(a)

.

.

(b)

Dye molecules

w2

Au NP

(d)

(c)

(

)

w0

Figure 3: (a) Temperature variations for the AuNPs samples and for the AuNPs gain-assisted samples. (b) Time-resolved fluorescence intensity decays for the R6G and 14 nm AuNPsR6G and 48 nm AuNPs-R6G samples.(c) Excitation (dashed line) and emission spectra (solid line) of R6G (black), (14 nm + R6G) system (red) and (48 nm + R6G) system. (d) Absorption spectra of R6G dye in H2 O and plasmon bands of 14 nm gain-assisted AuNPs.


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Time-resolved fluorescence spectroscopy, along with absorption spectra, have been carried out on the gain-assisted systems to perform a comparative analysis of the related thermoplasmonic effects. Figure 3b reports the time-correlated single-photon counting (TCSPC) data acquired at 560 nm for the R6G-AuNPs systems with respect to the pure R6G dye solution when irradiated with a 379 nm NanoLed pulsed diode laser. The time resolved fluorescence intensity decay of the H2 O solution of pure R6G molecules is fitted with a single exponential function (see magenta squares and cyan line fit in Figure 3b), giving a decay time constant of τR6G = 3.9 ± 0.2 ns (χ2 = 0.979). The gain-assisted system, characterized by AuNPs of 14 nm (red squares and cyan line fit), shows the same TCSPC response with respect to pure R6G τR6G ≈ 3.9 ns (χ2 = 0.979); on the contrary, for the 48 nm NPs (green squares and black line fit) two time components can be identified in the decay dynamics: a fast decay (τ1 = 300 ± 20 ps; χ2 = 0.978, Fig. 3b) is accompanied by a long-living emission (τ2 = 3.6 ± 0.2 ns; χ2 = 0.978) in which the decay kinetics resembles the fluorescence decay of the pure R6G dye molecules. The first decay time is attributed to the fraction of dye molecules that experience a resonant energy transfer process due to the coupling with AuNPs; the long-living emission, similar to the value recorded from pure R6G solution, is related to the fraction of dye molecules (the largest fraction) present in solution but that are not coupled to the plasmonic NPs. In the case of small NPs, we did not observe any modification of the fluorescent lifetime, but the static emission spectra show that part of the emitted radiation is quenched (see Figure 3c). This allows us to establish that, although there is no resonant energy transfer from dye molecules to metal NPs, we are anyway in presence of an exciton-plasmon coupling between the two sub-units that is able to modify the behavior of the entire system. This coupling can be investigated by monitoring the total extinction cross-section (σext ) of the individual system components and compare it to the response of the gain-assisted sample. As reported in Fig. 3d (red curve), the total σext results substantially higher for all the wavelengths in the visible range, and a remarkable increase is obtained in correspondence to the maximum of the plasmonic band;



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this increase results also higher than the mathematical sum of the NPs and R6G curves, reported in the same figure as the dashed-dot black curve. The obtained data suggest that the exciton-plasmon coupling for the considered parameters (NPs sizes, concentrations of NPs and dye ratio, spectral overlapping of the NP absorption band with the R6G emission band) does not results in the mitigation of the absorptive losses of metallic NPs 30,35 but demonstrates an opposite mechanism, by showing the possibility of creating a super-absorber system. The broadening and red-shift of about 5 nm of the plasmonic band of the gainassisted sample (red curve) with respect to the main sample (green curve) can be, in part, related to a major polidispersivity of the new nano-objects (AuNPs+R6G) formed in the solution and on a change of the surrounding environment; indeed, the R6G molecules, as a result of their positive charge, electrostatically interact with the surface of AuNPs that are negative charged, due to the outer citrate capping. But the main effect responsible of the increased photo-induced heat generation remains the close proximity between R6G molecules and AuNPs, able to promote the exciton-plasmon coupling process. No aggregates have been observed in the two solutions, as evidenced by the dynamical light scattering (DLS) measurements performed on the two samples, evidencing that it is not present any substantial change in the average hydro dynamical diameter of the gain-assisted sample with respect to the AuNPs one (see Figure S4 in Supporting Information). As it can be seen from Fig. 3b-c, the largest NPs (48 nm), instead, show a non-radiative resonant energy transfer process that move in the direction of a loss compensation effect. Indeed, both the observed static quenching and the birth of a fast decay time in the TCSPC measurement are the symptom of a faster energy dissipation channel, typical of these types of processes. 30–32 This is confirmed by the extinction cross section of the gain-assisted sample that shows a σext lower than in the case of pure AuNPs sample (see Supporting Information). This means that, by choosing opportunely the sub-units parameters of the gain-assisted system, we can use dye molecules not only to mitigate ohmic losses as seen in previous works, but also to enhance the thermo-plasmonic response of metallic NPs by obtaining a


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macroscopic enhanced photo-thermal effect through a strong exciton-plasmon coupling.

Conclusions In this work we have presented the photo-thermal effects observed mainly in gain-assisted gold nanoparticles (AuNPs) solutions. Different sizes AuNPs have been exposed to three intensity values of a resonant laser beam in order to excite the LSPR, responsible of the photo-generation process. Best results have been obtained with the diameter of 48 nm. By adding in the solutions an organic dye (R6G), whose emission band overlaps to the LSPR, we found that, in the case of smaller NPs, the contribution to the photo-thermal efficiency is enhanced due to a strong coupling effect between the gain medium and the metal nano objects. This is evidenced by an overall increasing of the extinction cross-section of the gain-assisted system with respect to the bare AuNPs, acting as a super-absorber material. On the other hand, for the AuNPs characterized by a larger diameter, a loss compensation mechanism, based on a non-radiative resonant energy transfer (RET) process from gain units to plasmonic nanoparticles has been found. These observations constitute a useful and additional key element in the control of the photo-induced heat generation in a bulk medium, by proving a new way to control the photo-thermal effects related to metallic nano-objects even away from the LSPR.

Supporting Information Available Temperature increase ∆T of the AuNPs solutions as a function of the laser intensity; comparison of the photo-thermal behavior between the gain-assisted solutions (14 and 48 nm) at a concentration of AuNPs of 2.5 · 10−10 M and added R6G at 2.92 · 10−6 M ; extinction spectrum of 48 nm AuNPs compared to the corresponding gain-assisted system at a concentration of 2.5 · 10−10 M. This material is available free of charge via the Internet at http://pubs.acs.org/.



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Acknowledgement The research leading to these results has received support and funding from the Italian Project NanoLase - PRIN 2012, Protocol No. 2012JHFYMC.

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(29) Jiang, K.; Smith, D. A.; Pinchuk, A. Size-dependent photothermal conversion efficiencies of plasmonically heated gold nanoparticles. J. Phys. Chem. C 2013, 117, 27073– 27080. (30) De Luca, A.; Grzelczak, M. P.; Pastoriza-Santos, I.; Liz-Marzán, L. M.; La Deda, M.; Striccoli, M.; Strangi, G. Dispersed and encapsulated gain medium in plasmonic nanoparticles: a multipronged approach to mitigate optical losses. ACS Nano 2011, 5, 5823–5829. (31) Strangi, G.; De Luca, A.; Ravaine, S.; Ferrie, M.; Bartolino, R. Gain induced optical transparency in metamaterials. Appl. Phys. Lett. 2011, 98, 251912. (32) De Luca, A.; Bartolino, R.; Correa-Duarte, M. A.; Curri, M. L.; Steinmetz, N. F.; Strangi, G. Gain-assisted plasmonic metamaterials: mimicking nature to go across scales. Rend. Accad. Naz. Lincei 2015, 26, 161–174. (33) Njoki, P. N.; Lim, I.-I. S.; Mott, D.; Park, H.-Y.; Khan, B.; Mishra, S.; Sujakumar, R.; Luo, J.; Zhong, C.-J. Size correlation of optical and spectroscopic properties for gold nanoparticles. J. Phys. Chem. C 2007, 111, 14664–14669. (34) Palpant, B. Photothermal properties of gold nanoparticles; Imperial College Press: London, 2012. (35) Peng, B.; Zhang, Q.; Liu, X.; Ji, Y.; Demir, H. V.; Huan, C. H. A.; Sum, T. C.; Xiong, Q. Fluorophore-doped core–multishell spherical plasmonic nanocavities: resonant energy transfer toward a loss compensation. ACS Nano 2012, 6, 6250–6259.



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Thermoplasmonic Effects in Gain-Assisted ... - ACS Publications

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