Quantitative Evaluation and Optimization of Photothermal Bubble

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C: Physical Processes in Nanomaterials and Nanostructures

Quantitative Evaluation and Optimization of Photothermal Bubble Generation around Overheated Nanoparticles Excited by Pulsed Lasers Siqi Wang, Lei Fu, Yong Zhang, Jing Wang, and Zhenxi Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07672 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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The Journal of Physical Chemistry

Quantitative Evaluation and Optimization of Photothermal Bubble Generation around Overheated Nanoparticles Excited by Pulsed Lasers Siqi Wanga, Lei Fua, Yong Zhangb, Jing Wanga*, Zhenxi Zhanga* a

Xi’an Jiaotong University, Key Laboratory of Biomedical Information Engineering

of Education Ministry, School of Life Science and Technology, No.28, Xianning West Road, Xi’an, China, 710049 b

Xi’an Jiaotong University, State Key Laboratory of Multiphase Flow in Power Engineering, No. 28 Xianning West Road, Xi’an, China,710049

Abstract Photothermal nanobubble induced by laser-excited plasmonic nanoparticle (NP) presents promising applications in the following fields: optoporation, biosensing, photoacoustics, and theranostics. The diversity of nanoplasmonic systems affects plasmonic response in a highly intricate way and necessitates a systematic design approach based on comprehensive theoretical studies, which currently remains scarce. To palliate this issue, we developed an efficient extended two-temperature model (ETTM). Through a self-built multimodal experimental set-up, the photothermal behavior under nanosecond−laser excitation was studied and the ETTM applicability was further demonstrated.

Furthermore, we investigated the photothermal

performance of gold NPs excited by pulsed-lasers in the screening of large libraries of excitation wavelength (350–800 nm) and pulse duration (100 fs–5 ns). Results showed that two prevailing factors were involved in rationalizing photothermal

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bubble generation in the sub-nanosecond and nanosecond excitation regimes: the thermo-induced NP melting and the bleaching of localized surface plasmon resonance (LSPR). With considering the NP durability for biosafety, the optimization of the photothermal effect for bubble generation was found to be closely associated with Kapitza conductance, excitation wavelength, and pulse duration. Our study provides a general and quantitative insight on the influence of nanoplasmonic system on photothermal effect. The simulation-based design guidelines can be broadly used in engineering systems for biological applications.

1. INTRODUCTION Photo-based diagnoses and clinical treatments have attracted widespread attention because of their minimally invasive and precise modalities in biomedical fields

1-2

. Noble metal nanoparticles (NPs) exhibit significant advantages, such as

tunable optical and photothermal characteristics, and have thus been adopted in photo-based biomedical applications 3-4, particularly in the NP-mediated optoporation, biosensing 5, photoacoustic spectroscopy 6, and in vivo theranostics 7. When the laser energy irradiation on colloid metallic NP solutions is sufficient, the thermo-dynamical stability of the ambient medium adjacent to overheated NPs is broken and results in the formation of photothermal bubbles with nanometer sizes. Laser-excited plasmonic bubbles are associated with the selectivity of bubble generation 8, the amplification of optical scattering and photoacoustic signal

9-10

, and thermal insulation effect 9, all of

which can be realized at the nanoscale. Here, the thermal, optical, and mechanical properties of the bubbles were modulated using NP parameters, including the shape,


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structure, material, clusterization state, and excitation laser

8, 11

. The above physical

effects facilitate the improvement of the sensitivity of photoacoustic imaging efficiency of macromolecular drug delivery

12

, the

13

, the precise localization of

photothermal therapy 12, and biosensing 5. The rationalization of the critical fluence threshold must be prioritized for the investigation of medium vaporization around overheated NPs that are excited by pulsed laser. To date, outstanding experimental studies on bubble generation threshold have been conducted with X-ray scattering (SERS)

15

, multimodal optical detection

14

, surface-enhanced Raman scattering

16

, and transient spectrum technology

17

.

However, a nanoplasmonic system consists of variable NP geometric structures, different types of materials, and multiparameter excitation sources (e.g., the polarization, wavelength, pulse duration, and fluence). These variabilities contribute to the complexity of the system and render the systematical characterization of the bubble generation extremely difficult, expensive, and time or effort consuming. When the potential application of nanoplasmonics is extended to the biomedical field, where biosafety must be considered, a controllable, biosafe, and effective nanoplasmonic system is necessary 18. Thus, a metal NP acting as a direct thermal source in triggering bubble generation must remain intact, so that irreversible structural damage or harm due to potential toxic of photothermal production is prevented in collateral tissues. Thus, the issue comes down to the determination of an optimal NP size that enables the use of extremely low laser fluence for bubble generation and subsequent creation of controllable and localized tissue damage without causing structural damage to the



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NP. Given the high time requirement and limitations of the experimental study, theoretical methods may be promising alternatives for the characterization of photothermal behavior induced by nanoplasmonic system in an efficient way. The non-monotonic dependence of fluence threshold on NP size was observed previously

8, 17, 19

. This observation indicates that obtaining an optimal NP size is

feasible. To date, theoretical methods for predicting optimal NP size remain imperfect. As far as we know, a detailed elucidation regarding an optimal NP size was already presented

17, 20

, but NP thermo-stability was not considered. Meunier et al.

18

were

among the first researchers who explored optimization under an ultrafast laser excitation regime, where the enhancement of near-filed for plasma generation dominated the formation of cavitation bubble rather than the photothermal bubbles discussed in the present study. Here, photothermal bubble generation was induced by short pulsed laser, which was considered applicative to most medical applications 7, 9, 21

. In this work, we aimed to determine the fluence threshold for photothermal

bubble generation, considering practical application and “biosafety” concept. In particular, we developed an extended two-temperature model (ETTM) based on classical thermal diffusion equations. Compared with conventional two-temperature models (TTMs), the proposed ETTM incorporated the effects of NP thermal transition and heat loss, and the environment-temperature-induced damping of the localized surface plasmon resonance (LSPR) band. To validate the feasibility of ETTM, as well as enhance the robustness and applicability of the algorithm, we constructed a


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proof-of-concept experiment set-up, integrating the multimodal method of time-resolved optical response, scattering imaging, and photoacoustic detection, for the probing of photothermal response to the excitation of a nanosecond pulsed laser. Building on the experimental and ETTM-based theoretical studies, we investigated a large variety of spherical gold NPs with diameters ranging from 10 nm to 200 nm. The NPs were excited by a wide range of pulse duration from 100 fs to 5 ns and optimized for a selection of wavelength (λ) window covering the UV-visible and NIR scopes. For the prediction of optimal NP size, the fluence thresholds for the bubble generation and NP structural damage were evaluated. Here, we described the physics process of heat generation and diffusion within the nanosystem containing the NP and surrounding medium. The effect of NP thermal transition, the role of thermal resistance on NP–liquid interface, and the existence of a spinodal temperature of water were intensively discussed.

2. THEORETICAL AND EXPERIMENTAL METHOD 2.1 LASER ENERGY DEPOSITION The laser energy deposition per unit time and unit volume, S [W/m3], obeys the Gaussian distribution 22 and can be described by Equations (1) and (2),

S=

P(t ) =

Cabs (Tm , DNP ) ⋅ P(t ) VNP

 4 ⋅ ln (2)  2 ⋅ F ⋅ exp  − ⋅ t − ( 1.5 ) τ  p  τ p2 π ⋅τ p  

2 In(2)

(1)

(2)

where P [W/m2] is time-dependent laser power density that is characterized by laser



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pulse duration τp [s] and incident laser fluence F [J/m2], and VNP denotes the NP volume. The evaluation of the NP absorption cross-section, Cabs [m2], is calculated with the dielectric function of bulk gold material, which is experimentally determined by Johnson et al. 23. The calculation was based on the Mie scattering theory, which is known as a rigorous solution of the Maxwell equations

24

. From the femtosecond to

the picosecond irradiation regimes, the time scale for heat dissipation to the surrounding water is usually longer than τp 25, and thus Cabs can be expressed as a function of the absorption properties of the ambient: Cabs = f (T0, DNP), where T0 is the initial ambient temperature and DNP [m] is the NP diameter. If extended to the nanosecond pulse laser irradiation, the temperature evolutions of electrons, lattices, and surrounding water are considered simultaneous

25

because the heating rate of an

electron–lattice system is comparable to its cooling process. The heating-cooling dynamics caused a local temperature gradient in the fluid medium perpendicular to the NP surface. This gradient correspondingly changed the refractive index of the surrounding water layers, as illustrated in Fig. 1. The thermo-modified optical properties of the NP coated by water shells can be modeled with a multi core-shell system

26

. In the model of laser energy deposition, the water layers around the NP

were assumed to have different thermo-optical properties. Each layer can be characterized by a predefined shell thickness (usually a few nanometers) with constant refractive index

27-28

. Basing on the Mie theory24,

29

, we developed a

multilayered spherical scattering model to describe the nonlinear thermo-optical properties of Cabs 27-28, which was expressed as the function of Tm(r, t) and DNP, where


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Tm(r, t) represents the temperature of the fluid medium with spatial-temporal variables, r and t.

Fig. 1 Schematic of the calculation model of nonlinear thermo-optical absorption cross section (Cabs) of the high temperature NP coated by water shells. The interlayer spacing was 2 nm, defined by referring to the thickness of thermal boundary resistance 30.

In fact, a generalized theoretical description of the thermodynamic kinetics after the water ebullition is still challenging for the pulsed laser irradiation with long pulse duration (e.g., nanosecond laser). On the one hand, the energy deposition process cannot isolate itself from the occurrence of water explosive evaporation and subsequent bubble dynamics. On the other hand, the heat flux crossing the interface between an NP and surrounding “hot” and high-pressure vapor remains unpredictable. The uncertainty of the evolution of such vapor bubble during pulse duration poses problems for the quantitative analysis of optical limitation

31

, subsequently reducing

NP optical absorbance. The heating up of the thermally insulated NP to the boiling phase and the subsequent recrystallization possibly occur with a time scale of nanoseconds

30, 32

, thus, it can be neglected in picosecond and femtosecond laser

excitation. In the nanosecond laser excitation, recrystallization aggravates the



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complexity of a modeled bubble evolution. Therefore, in this paper, we defined the upper limit of the time range under discussion as tu, which is a piecewise function of

τp as shown in Equation (3). Where tc = 1 ns and τbubble is the time when the water vaporization occurs and that depends on pulse duration, laser fluence, and NP size 30.

 min ( tc ,τ bubble ( D,τ p , F ) )  tu =  min (τ p ,τ bubble ( D,τ p , F ) )

t < tc

(3)

t ≥ tc

2.2 MODELING THE LASER HEATING NP IN WATER The quasi-free electrons and ions have distinct thermophysical parameters, and thus gold NPs are usually considered a system composed of electrons and lattices 22. Thus, TTM was applied to describe the temperature evolution of the electron and lattice subsystem. The previous TTM had deficiencies because the thermo-physical parameters

of

the

temperature

response

(such

as

temperature-dependent

electron-phonon coupling factor, NP absorption cross-section and Fermi distribution) were coarsely treated as physical constants and the effect of the melting thermodynamics of NP on heat diffusion was neglected. Based on the conventional TTM, we developed an ETTM to evaluate electron and lattice temperatures (denoted by Te and Tl, respectively). Comparing with the classical TTM

33

, the ETTM

incorporates variable (temperature-dependent) thermal physical parameters, the effect of environment heating on NP absorption characteristics, and the probability of NP phase transition. The ETTM can be expressed by:


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C e (Te )

C l (Tl )

Cm (Tm )

dTe

dTl dt

∂Tm

S2 =

= −G (Te ) ⋅ (Te − Tl ) + S1

(4)

= G(Te ) ⋅ (Te − Tl ) − S 2 − S 3

(5)

dt

∂t

=

2 1 ∂ ( km r ( ∂Tm ∂r ) ) + S2 ∂r r2

(6)

3 ⋅ g ⋅ (Tl − Tm ( DNP )) DNP / 2

S3 = Π(Tl /Tmelt )ϕ , ϕ =

(7)

∆Vmelt ⋅ hmelt Vnp ⋅ dt

(8)

The ETTM adopts the lumped parameter method to calculate NP temperature, which assumes a uniform distribution because of the much higher thermal conductivity of the NP compared with that of the surrounding water. Given that the conventionally used free electron gas (FEG) model is unsuitable for the description of electronic conductivity Ce and electron-photon coupling factor G under high laser intensity 22, 34, the proposed ETTM adopts the two parameters (Ce and G) on the basis of the real parabolic electron density of states (EDOS) and the electron-temperature dependent Fermi distribution 34-35. The heat flux across the interface of NP and the surrounding water per unit volume was denoted by S2 [W/m3], which reflected the NP cooling dynamics. Owing to the wide range of excitation laser (fluence), NP temperature tends to exceed the melting threshold

14

, especially under a femtosecond laser excitation regime, where

the pulse duration is far shorter than the collision time of water molecules


36

and


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rapidly and effectively heats the lattice. Thus, NP phase transition is worthy of being considered. In Equation (8), ∆Vmelt represents the melting volume within the time step dt, and hmelt is the melting enthalpy, equaling to 1.24 ×109 J/m3. NP is assumed to be reshaped from a polyhedron to spherical geometry when NP temperature reaches the melting point of bulk gold, that is, Tmelt = 1337 K. At this moment, temperature does not increase until the complete melting of NP. When NP temperature increases to boiling point Tvap, NP size starts to decrease via evaporation 25, 27, as shown in Fig. 2. In the process above, the specific heat of gold Cl [J/m3K] can be described by the Tl-dependent function 22. Meanwhile, Π is a gate function operator, where Π(Tl − Tmelt) equals to 1 when NP temperature is greater than Tmelt; otherwise, it is equal to 0. Owing to the heat losses to the surrounding media, electron temperature hardly exceeds the critical temperature (about 7000–8000 K for gold), where NP fragmentation occurs because of Coulomb explosion

34

. This extreme condition

occurs under some situations, especially when a bubble is generated before the end of laser heating and NP temperature becomes uncontrollable and unpredictable because of the strong suppression of heat release to the surrounding medium by the bubble 25. In the present study, the possibility of Coulomb explosion effect was not considered because the time range we interested in was confined to the trigger time of bubble generation. Thus, the proposed ETTM incorporated the mechanism of photothermal melting and evaporation even at the femtosecond laser excitation condition. The thermal interface (boundary) conductance g is set to 105 MW/(m2·K)27, 37-38

, which is reciprocal to the Kapitza resistance (also named thermal interface


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resistance)

39-41

. Details about the boundary conditions at the interface of NP and the

surrounding water could be found in Ref 27, 34.

Fig. 2 Schematics of the morphological evolution and phase transition process of Au NP induced by laser thermodynamics.

2.3 MATERIALS AND EXPERIMENTAL SET-UP The experiments were conducted using solid spherical gold NPs with nominal diameters of 10, 15, 20, 40, 50, 60, 80, 100 and 150 nm (BBI Solution Inc.). The concentrations of the NP suspensions with optical densities of 0.5 were adjusted for the prevention of the overlapping effects of neighboring NPs. The measured bubble threshold with a relatively high concentration in the NP suspensions was used for the approximation of the fluence threshold for the onset of bubble formation around a single NP. The optical density was obtained by the UV-visible spectrophotometric measurements (V-550 UV/VIS, Jasco, Japan). The nanoscale bubble generation and detection were performed by a multimodal experimental set-up (shown in Fig. 3), which integrated the time-resolved optical response, side-scattering imaging, and the acoustic detection. A pump laser with a wavelength of 532 nm and pulse duration of 5 ns was provided by a Q-switched, frequency doubled Nd: YAG (Q-smart 450, Quantel) laser device. The device was used for the irradiation of sample chamber containing colloid Au NP suspension. The



excitation energy was controlled by motorized half-wave (λ/2) plate and recorded by a calibrated pyroelectric energy meter (PE-DIF-C, Ophir Optronics). A CW He-NE probe laser (HNL 020RB, Thorlabs) was adjusted to be confocally aligned with the pump laser. The two beams directly went through a long-distance objective lens and then irradiated into the customized sample chamber with high transmittance. The transmitted probe light through the suspensions was collected by a 20× objective lens and projected onto an AC-coupled avalanche photo-diode (APD) (FEMTO, 25 KHz-200 MHz bandwidth). A set of dichroscopes in front of the photo-diode were used for the extraction of excitation irradiation from transmitted light. A detailed description of the experiment principle was summarized below. First, bubble occurrence after vaporization was detected through time-resolved scattering technique. Specifically, changes in probe beam intensity due to the bubble-specific shape of the time-response transmitted laser intensity were monitored. Bubble expansion can improve the NP-based imaging by amplifying the scattering light intensity with orders of magnitude, compared to that induced by a single NP 9. Thus, monitoring bubble generation can also be realized through the side-scattering imaging method. Second, an imaging signal was acquired through a cooled charged coupled device with electronic multiplication (EMCCD) (DU 897, Andor), which was set perpendicularly to the direction of excitation beam propagation. Third, the identification of the generated bubbles was based on the capture of transient pressure waves induced by bubble dynamics. Thus, an ultrasound transducer with a central bandwidth of 25 MHz (V324-N-SU, Olympus) was immersed into the chamber for


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the detection of the bubble-mediated photoacoustic signal in the suspension. The multimodal signals were recorded by using a four-channel digital oscilloscope (PTE1204, Rohde & Schwarz) with sampling frequency of 5 GHz. A digital delay generator (DG645, Stanford Research Systems Inc.) was adopted for the synchronized control of multichannel signal triggering. Each of these detection techniques has its respective strengths and weaknesses (details can be found in Ref 16), but a reliable assessment of the bubble generation threshold can be achieved by combining these techniques.

Fig. 3 The proof-of-concept multimodal platform for tracking the bubble generation excited by a nanosecond laser with λ = 532 nm and τp = 5 ns. The inset figures showed three typical signals of the bubble generation acquired by acoustic detection, time-resolved optical transmitted response and side-scattering imaging, respectively.

3. RESULTS AND DISCUSSIONS Section 3.1 discussed the heating efficiency of NP excited by low excitation fluence where the NP melting, LSPR bleaching, and bubble nucleation were prone not to take place. Thereinto, the heating efficiency of NP was closely associated with the



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bubble generation. Section 3.2 explored the criterion of photothermal bubble generation. Section 3.3-3.6 respectively discussed the influence of the NP thermal phase transition, interfacial wettability-dependent Kapitza conductance, excitation wavelength, and laser pulse duration on the photothermal bubble generation. 3.1 SIZE-DEPENDENT HEATING EFFICIENCY The heat diffusion by phonon–phonon relaxation from the NP lattice to the solvent molecules increased the surrounding temperature, and then the NP itself was cooled. Small NPs were expected to relax rapidly because of a large surface-to-volume ratio of NP 42. In order to quantitatively analyze the influence of size-dependent heat dissipation on NP thermal behavior, we defined the efficiency of thermal exchange as δsw, which was characterized by Esw/Elaser, where Esw and Elaser respectively denoted the energy that diffused to the surroundings and total absorbed laser energy by the NP.

(

tu

)

Esw = ∫ (π ⋅ Dnp2 ) ⋅ g ⋅ Tl (t ) − Tm ( Dnp , t ) ⋅ dt 0

tu

Elaser = ∫ Cabs (Tm , Dnp ) ⋅ P (t ) dt

(9)

0

Fig. 4 discussed the temperature evolution of the Au NPs (with diameters 20−100 nm) irradiated with some given levels of absorbed laser energy far below the fluence threshold of bubble generation or NP melting, with considering the effect of size-dependent δsw. The 355 nm emission laser with relative short pulse duration (τp = 15 ps) was utilized to avoid the effect of LSPR. Fig. 4a exhibited the spatial-temporal temperature evolution of the surrounding medium. With the increase of NP size, the heat-affected region in the medium fluid gradually decreased, although the level of


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absorbed energy increased. This indicated that the lager sized NPs with slow thermal exchange rates had increased potential in thermal field localization. The time evolution of δsw can be described by monotonically increasing curves (see Fig. 4b), where a sharp increase of δsw could be observed in NPs with small diameter. In Fig. 4c, the 20 nm diameter NP heated by the minimal level of absorbed laser energy exhibited the maximum NP temperature rise despite the rapidest thermal exchange. The perplexing thermal behavior of NP can be attributed to decreased heat capacity at decreased NP size. This could help understanding the observed experimental phenomenon 14 wherein small NPs have increased tendency to achieve a molten state under the bubble generation threshold. It can be concluded that although a small NP was expected to have a highly efficient thermal exchange, the existence of low heat capacity will aggravate the possibility of NP thermal phase transition despite a small amount of laser energy stranded in the NP.

(a)



(b)

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(c)

Fig. 4 Size dependent thermal exchange for Au NPs with diameters of 20, 60, 80 and 100 nm excited by pulsed laser (τp = 15 ps and λ = 355 nm). The excitation energies were respectively 0.0075, 0.0179, 0.0303 and 0.043 pJ for the 20, 60, 80 and 100 nm NPs, respectively. (a) The temperature evolution of the surrounding water as a function of radial position and time. (b) Time-resolved thermal exchange efficiency of different NPs. (c) Temporal profiles of the NP temperature.

In fact, the optical properties of the NPs were also sensitive to NP size. According to the Mie theory

24, 29

, a NP with low surface-to-volume ratio

(corresponding to a large size) usually has a high absorption cross-section regardless of wavelength, as shown in Fig. 5.

Fig. 5 Relation between the absorption cross section and the NP diameter


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The NP temperature increase was in fact a reflection of the energy conversion from light to heat. An ideal situation is that laser energy is totally deposited onto an NP and heat diffusion between the NP and surroundings is nonexistent. In this case, NP temperature can be depicted by 20:

Tl ( DNP )=

Cabs ⋅ F VNP ⋅ Cl

+ T0

(10)

Equation (10) can roughly estimate the maximum NP temperature. The temperature increase was proportional to Cabs/VNP, where Cabs was absorption cross section and VNP was NP volume. Thus, the temperature increase was more determined by Cabs/VNP than by Cabs. By applying the ETTM, where the size-dependent thermal exchange (heat diffusion) was considered, we were able to quantitatively estimate the maximum interfacial temperature of a water layer, which was strongly and negatively associated with medium vaporization

17

. The ETTM evaluated the interfacial temperature at a

given delay time after the triggering of the excitation pulse for Au NPs that have diameters of 20, 60, and 100 nm irradiated by the pulsed laser (τp = 15 ps and λ = 355 nm) with incident fluence (F) of 20 J/m2, which was below the bubble generation threshold. As shown in Fig. 6a, at the delay time of 50 ps, the highest interfacial temperature was predicted in the 60 nm NP instead of the 100 nm NP, although the 100 nm NP had the largest absorption cross-section (see Fig. 5). Meanwhile, the lowest interfacial temperature was observed in the 100 nm NP instead of the 20 nm NP, although the 20 nm NP possessed of the fastest thermal exchange rate with surroundings. The interfacial temperatures of the 60 and 100 nm NP declined in turn



because of the damping of the absorption cross-section with respect to the NP volume. By contrast, the slightly low interfacial temperature of the 20 nm NP can be ascribed to the rapid heat release and to the considerable amount of energy that was stored in the invalid heating region of the fluid (out of the interfacial water layer). Meanwhile, the interfacial temperature is indeed an indicator of heating efficiency, which is not only influenced by NP size but also associated with excitation wavelength. To incorporate the influence of the excitation wavelength on the laser heating efficiency, we calculated the interfacial temperature, as shown in Fig. 6b, where the NPs with diameters ranging from 10 nm to 200 nm were irradiated by a 15 ps pulsed laser with frequently-used wavelengths of 355, 400, 532, and 800 nm. The non-monotonic profiles of the interfacial temperature along with NP size were observed. The extreme points on the temperature curve representing the optimal heating efficiency moved from 50–60 nm in the UV-visible (350-700 nm) region to approximately 150 nm in the NIR (800 nm) region. Compared with the ETTM, the rough estimation of the NP temperature by Equation (10) obviously excessively evaluated the temperature evolution. However, Equation (10) can qualitatively explain the NP thermal behavior in Fig. 6b. Thus, in the case with low excitation fluence (where bubble nucleation, NP melting and LSPR bleaching were prone not to occur), the existence of the optimal NP size (the extreme points) was a consequence of the balance between heat conversion capacity related to Cabs/VNP and thermal exchange capacity damping with NP size.


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(a)

(b)

Fig. 6. Effects of size-dependent optical properties on temperature evolution under different laser excitations: (a) Radical temperature distribution for different NPs at the delay time of 50 ps with F = 20 J/m2, τp = 15 ps and λ = 355 nm; (b) Size-dependent interfacial temperature profiles with respect to different NP sizes and laser wavelengths.

3.2 CRITERION OF PHOTOTHERMAL BUBBLE GENERATION The criterion of plasmonic bubble generation has been remained to be discussed up to now both experimentally and theoretically. In 2006, Kotaidis et al. 14 conducted a femtosecond experiment and used a 100 fs pulsed laser to discuss the size-dependent bubble generation threshold by the pump/X-ray probe method. Moreover, Lukianova-Helb et al.

8

investigated the bubble generation threshold of

NPs with diameters of 10, 30, 60, 80 and 250 nm under 0.5 ns laser excitation (532 nm) through the optical scattering method. In 2013, Fang et al.

15

studied the bubble

generation around a 100 nm NP that was irradiated by a CW laser via SERS. Years later, Katayama et al.

17

discussed the formation and evolution of incipient

nanobubble after the excitation of 20–150 nm NPs by pulsed laser (355 nm and 15 ps), exploiting the transient (picosecond) pump-probe spectrum measurement. Meunier et



al.

43

observed the bubble dynamics of a single 100 nm NP illuminated by a 40

fs-pulsed laser at the off-resonance wavelength of 800 nm by coupling the ultrafast shadow graphic imaging with in situ dark field imaging. In our experiment, we applied a self-built proof-of-concept multifunctional system to study the thermal response in colloid gold NP suspensions containing NPs that had diameters of 10–150 nm and were excited by a pulsed laser with pulse duration (τp) of 5 ns and wavelength (λ) of 532 nm. Nanobubble generation at the level of fluence threshold can be identified according to optical scattering phenomena, namely the bubble-based scattering imaging and time-resolved optical response mediated by bubble dynamics. Moreover, bubble dynamics-induced sound pressure measured by a photoacoustic signal detector can facilitate detection sensitivity. Given the inherent particle-to-particle variations, the ensemble studies of fluence threshold for bubble generation using a colloid solution was adopted according to statistical results: The threshold fluence indicates a 50% bubble generation probability, as shown in Fig. 7. The detailed detection method of individual or combined modality could be found in Ref 8, 16, 21.

Fig. 7. Experimental measurements of bubble generation for 20, 60 and 100 nm diameter Au


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NPs versus the laser fluence with 5 ns pulse duration and 532 nm wavelength. A sigmoidal function (the red lines) was used to approximate the experimental results. The dashed line denoted the 50 % bubble generation probability.

A simplified thermal dynamic model considering the Kapitza resistance (i.e., the thermal resistance) developed by Merabia group

30

described the kinetics of bubble

formation under sub-nanosecond excitation regimes. In this model, the fluid temperature was assumed to cross the spinodal curve (critical temperature) at a predefined 2 nm water layer from NP surface. Here, the NP melting effect was found to be important to predict the bubble generation. Then, the group 44 further determined whether the threshold temperature at the spatial spinodal position (i.e., the phase transition interface) should arrive at the spinodal temperature (550 K) before it can promote water boiling, according to the hydrodynamics model. The above experimental and theoretical studies implied that the definition of the bubble generation should be associated with pulse duration, interfacial threshold temperature, and the interface thickness. On the basis of the previous study by Meunier et al.

45

, the mechanism for bubble generation was transformed from the

photothermo-mediated effect (which was discussed in this paper) to the plasma-mediated effect with decreasing pulse duration. For instance, in the 40 fs ultrashort laser excitation, the researchers

43

found that thermal diffusion cannot

interpret bubble generation with the excitation condition they set. By contrast, the threshold was in fact the critical laser fluence which could motivate the nanoplasma density to reach 1021/cm3 around the NP poles. Note that the thermal diffusion model



had been demonstrated by Siems 31 for describing the thermodynamic evolution of the water medium around the overheated Au NPs exposed to 100 fs laser. Therefore, pulse duration shorter than 100 fs was not included in the present study (of the photothermal bubble). The upper bound of the time scale in our study was determined by the available experimental results under the pulse duration of 5 ns. Above discussion defined the pulse duration range under which the bubble generation was dominated by photothermal mechanism. The below (of section 3.2) will further explore the criterion of bubble generation under such an excitation condition. As well known, the threshold temperature was the critical factor for the photothermal bubble generation modeling by heat diffusion equations. The threshold temperature at the spinodal curve inducing the water vaporization in fact scaled with the pressure

14, 46

, which meant any transient pressure will change the threshold

temperature for bubble generation and further affect the evaluation of threshold fluence. For instance, a cavitation effect considerably reduced the threshold 14, 47. The predicted threshold temperature for femtosecond-laser-induced nano-cavitation can be reduced to 441 K 47. In the present study, where the pulse duration ranged from 100 fs to 5 ns, the time scale of the maximum interfacial temperature occurrence was about 100 ps–5 ns (as shown in Fig. 4a), which was also the most possible moment for triggering the water phase transition. This time scale was far behind the propagation time of thermal acoustic transients in the heat-affected region, which was about 7 ps for a 10 nm averaged heated region in Fig. 4a. That is, the pressure waves had been released already before the interfacial temperature increased to the threshold level.


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Therefore, the pressure confinement condition cannot be satisfied, and the effect of thermoacoustic pressure on the threshold temperature can be neglected reasonably in the present issue. Thus, considering the above factors, we assumed that water started to boil when the interfacial temperature within a predefined region with the thickness of 2 nm reached the 550 K spinodal temperature (precisely corresponding to 85% of the critical temperature of water), and the threshold temperature was similar to that in the previous studies mentioned above. In the following text, the theoretical framework using the ETTM will be verified based on the available experimental results. Then it was used to evaluate the size-dependent bubble generation threshold and explore the major influencing factors on the bubble generation under different excitation regimes. 3.3 EFFECT OF NP MELTING ON BUBBLE GENERATION In the sub-nanosecond excitation regime, the relaxation time of photon–photon interaction lagged behind the time profile of the pulsed laser. Thus, a heat-unaffected region was present in the surrounding water after the laser pulse ending. Then we can set the thermophysical parameters of the surrounding liquid as constants. Under this regime, the heating rate of the lattice was much faster than the cooling rate. Therefore, the thermal stability of the NP might be destroyed by energy accumulation before the phase transition of the surrounding liquid. The melting of the NPs was verified experimentally and theoretically. For instance, Kotaidis group

14

adopted the

time-resolved X-ray scattering method to detect the nanoscale structural change and revealed the possibility of the occurrence of melting thermodynamics before the bubble generation under the femtosecond excitation. In view of the biosafety issue



where low dose irradiation was preferentially selected and meanwhile the NP was maintained structure intact, it was necessary to find the optimal NP size which can minimize the fluence required for bubble generation and at the same time, avoid the photo-induced thermal instability of the NP from the collateral tissue damage. In this section, the first item under investigation was the influence of NP melting on bubble generation by applying the ETTM described in section 2.2. Then we compared the numerical predication of bubble generation with the available experimental data obtained by Siems et al.

31

for femtosecond laser excitation (τp =

100 fs, λ = 400 nm) and those by Katayama et al. 17 for picosecond laser excitation (τp = 15 ps, λ = 355 nm), as shown in Fig. 8. To explore the thermal behavior of NPs with diameters of 10−200 nm, we calculated the threshold fluence for bubble generation, with consideration of the heat losses caused by the NP thermal transition (dotted line) and without incorporating the contribution of melting latent heat (solid line). The NP melting (or damaging) threshold (dashed line), corresponding to the minimal fluence to initiate the NP melting, was also presented. It was found that the bubble threshold can be used for enhancing the accuracy of the experimental data when NP melting was considered. Thus, considering NP melting in the estimation of the bubble threshold in the sub-nanosecond laser excitation regime will be more reasonable.


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(a)

(b)

Fig. 8. Comparison between the fluence calculated by the ETTM (lines) and the experimental results (scatters) from Siems et al.

31

and Katayama et al.

17

. The red dotted

line and black solid line respectively denoted the case with and without the heat losses due to NP melting. The vertical arrows indicate the location of the pits on the threshold curves for bubble generation in both cases. The NP melting threshold is represented by the green dashed line. (a) Femtosecond case (100 fs and 400 nm laser) and (b) Picosecond case (15 ps and 355 nm laser).

From Fig. 8, it was further found that there was a non-monotonic relation between the fluence threshold (of both the NP melting and bubble generation) and the NP size. The non-monotonic behavior of the melting threshold curves was also experimentally demonstrated in the photo-modification of the geometrical morphology of Au NPs by Cavicchi et al. 19. They applied a series of characterizing approaches, such as dynamic light scattering and electrospray−differential mobility analysis, and found that NPs with diameter of 60 nm diameter yielded extremely low photo-instability (or photo-damaging) thresholds by exposing Au NPs with diameters of 10–100 nm to a single 7 ns laser pulse. Meanwhile, Metwally et al.


20

used a


Runge-Kutta-4 numerical algorithm to theoretically elucidate the non-monotonic profile of bubble generation fluence and found that NP with 60 nm in diameter yielded a minimum fluence threshold rather than NPs with larger diameter. However, few studies had been conducted on the physics of the threshold curve considering the melting thermodynamics which in fact aggravated the complexity of bubble nucleation in plasmonics, and was discussed in the following text. A large excitation fluence over the melting line (green dash lines in Fig. 8) will result in melting. The energy loss used for the NP thermal transition yielded a high fluence for water boiling, thus, the bubble threshold fluence was higher when NP melting was considered. When the bubble fluence was lower than the melting fluence (with large NP size), no NP melting occurred before the bubble generation and thus coincident bubble threshold lines appeared with the increase of NP size. Generally, small NPs experienced thermal transition before bubble generation because small particles had small volumes to be heated, where the lattice heating rate was expected to increase at the same level of deposited energy (see Fig. 4c). Therefore, the high fluence threshold for small NP can be ascribed to the following two aspects: On the one hand, the rapid thermal exchange between the NP and surroundings will result in the diffusion of a large amount of absorbed energy to the medium fluid and alleviation of energy storage in the 2 nm interfacial water layer (showed in Fig. 4a). In one of the most inspiring previous studies, Metwally et al.

20

expressed in fact the similar

viewpoint although the interfacial thickness was neglected (i.e., different criterions of bubble nucleation were set). On the other hand, the possibility of molten behavior can


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interpret the high fluence threshold for bubble generation in the case of small sized NP. When stretched to large NPs, the low heating rate due to increased heat capacity, reduced the NP melting possibility, and the low thermal exchange rate enabled the storage of a considerable amount of energy in the NP rather than the effective energy deposition in the interfacial water layer within the time scale of τbubble. Thus, it was the NP size-dependent thermal behavior that dominated the non-monotonic profile of fluence threshold along the NP diameter. With regards to the effect of NP melting on the optimal size, the pit (the lowest points, indicating the most efficient heating of the surroundings) shift denoted the effect of thermal transition on the bubble threshold curves. More specifically, seen in Fig. 8a, a predicted pit location (60 nm in diameter) was extremely close to that (56 nm in diameter) of the above optimal NP size for the highest heating efficiency under the excitation wavelength of 400 nm (see the prediction in Fig. 6b in section 3.1) when NP melting was not considered. If it was considered, the 60 nm NP melted before bubble generation. For NPs with diameter larger than 70 nm, the bubble generation occurred before the NPs melted (where the bubble threshold line was below the melting line). As a result, a relatively low threshold fluence for bubble generation was observed. Thus, as shown in Fig. 8a, the optimal size of Au NP was about 70 nm in diameter (located in the curve pit) under the irradiation of a 100−fs pulse with the wavelength of 400 nm. As a summary, in the femtosecond case of Fig. 8a, there was a deviation between the optimal NP size (70 and 60 nm) for heating efficiency with and without consideration of the melting possibility. By contrast, at



the picosecond case, the deviation cannot be observed, as shown in Fig. 8b, and the size range of the NP that was affected by the melting thermodynamics decreased to 54 nm. This can be attributed to the decrease in NP heating rate (which became closer to the NP cooling rate) because of the increase in pulse width. 3.4 EFFECT OF KAPITZA RESISTANCE ON BUBBLE THRESHOLD The Kapitza resistance, also the thermal boundary resistance, provided a resistance against the heat diffusion. The delayed heat diffusion (corresponding to slow cooling kinetics) resulted in the accumulation of absorbed energy in the NP, which was prone to yield an irreversible damage to the NP via thermal transition (i.e., the melting behavior). Given the important role of NP melting in the determination of the bubble threshold fluence, the first issue we need to quantitatively discuss was the effect of the Kapitza resistance on the cooling kinetics. As shown in Fig. 9, the 20 nm NP was exposed to pulsed lasers with the wavelength of 355 nm and the pulse duration of 15 ps. The NP cooling kinetics under the excitation of three different threshold fluences were investigated under different assumptions. The first one (Fig. 9a) modeled the case where the interface temperature gradient of the NP and surrounding water was negligible (i.e., the boundary resistance was set approximating to zero). The second case (Fig. 9b) ignored the melting latent heat resulting from NP thermal transition, notwithstanding it was possible that the NP was heated up to the melting temperature. Comparing the two cases above, a longer trigger time of bubble generation was observed in the latter case with a slow cooling kinetics (high temperature in NP), because the existence of the finite Kapitza


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resistance delayed the energy release to the surrounding, thus prevented the rapid medium heating and bubble generation 30. This also induced the NP melting to occur before bubble generation. When incorporating the boundary resistance and the relevant NP melting, the cooling rate was further weakened, because the NP melting suppressed the heat flux across the interface, as shown in case 3 (Fig. 9c). The prolonged trigger time was not an accidental phenomenon only occurring at the limit of bubble nucleation, but a direct result of the NP melting here which had been demonstrated in Supporting Information.

(a)

(b)

(c) Fig. 9. Influencing factors of NP cooling kinetics excited by a 15 ps laser at a wavelength of 355 nm. (a) Temperature evolution considering an infinity thermal interface conductance g∞,



where g∞ >> g. The solid line showed the NP temperature. The dashed lines represented the interfacial temperature of water layer at different positions, where the interface 1 and interface 2 were located at r =10 nm and r =12 nm. The interfacial space of 2 nm was related to the spinodal crossing 30 (b) and (c) respectively denoted the temperature evolution without and with considering the energy loss due to NP thermal transition where g = 105 MW/(m2⋅K) .

Based on the analysis above, we further investigated the effect of Kapitza resistance on the bubble threshold curve. Ge and Shenogina 41, 48 had revealed that the interfacial wettability and adhesion properties could influence the (Au) NP/SW contact angle, as well as the angle-dependent Kapitza resistance. A typical value of the thermal interface conductance (indicated by g) was 105 MW/(m2⋅K). g = 140 MW/(m2⋅K) was corresponding to a 50 degree of the contact angle 30. This was close to the highest reported value of g = 150 MW/(m2⋅K) of g was 50 MW/ (m2⋅K)

20, 39

. The lowest reported value

20, 41

. As shown in Fig. 10, we calculated three cases of g,

including the lowest and the typical value of g, as well as a 25 times larger value than the typical one. With g = 25 × 105 MW/(m2⋅K), the interface temperature gradient can be expected to be very small. Seen in Fig. 10, the thermal boundary conductance g (reciprocal of the interfacial thermal resistance) had a strong effect on the NP melting, especially for the case of g = 50 MW/(m2⋅K). With the increase of g, the range of NP diameter affected by the melting dynamics reduced gradually, which indicated that the delay effect of the energy release to the surrounding fluid was weakened. A small g always signified that


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large amount of absorbed energy stranded in the NP and used for NP melting before any phase change of the fluid occurred. This will lead to a higher bubble threshold fluence due to the delayed energy release for a large resistance. This can also explain why there was a jump point existed on the fluence threshold curve of g = 50 MW/(m2⋅K). In turn, we can further infer that the optimal NP size will become larger (from 54 nm to 90 nm in diameter) with the increase of the Kapitza resistance (i.e., the decrease of g), as shown in the inset figure of Fig. 10.

Fig. 10. Effect of the thermal boundary conductance (the reciprocal of the Kapitza resistance) on the NP Melting. Gold NPs with diameter ranging from 10-200 nm were exposed to a 15 ps-pulsed laser, with the wavelength of 355 nm. The predicated NP size regions affected by the NP melting was shaded by different labeled color areas. The inset figure showed the optimal NP size as a function of g.

3.5

WAVELENGTH-DEPENDENT

NP

MELTING

AND

BUBBLE

THRESHOLD To characterize the effect of the excitation wavelength on the NP melting and further on the bubble fluence threshold, we first applied the ETTM to estimate the bubble generation/melting threshold with the NP diameters ranging from 20-150 nm



Page 32 of 79

and the excitation wavelengths ranging from 355-800 nm. The pulse duration was 15 ps. Results were shown in Fig. 11.

(a)

(b)

(c)

(d)

Fig. 11. Wavelength-dependent NP melting thermodynamics in NPs that had variable sizes and were exposed to a 15 ps pulsed laser. The solid line indicated the fluence threshold for bubble generation under different wavelengths. The white dashed line represented the threshold curve corresponding to initial NP melting. The shadow area showed that the NP underwent the total melting and reduced the NP size via evaporation. The red region (0.8−1) indicated that total melting might have occurred, but no NP evaporation occurred. Vmelt represented the possible molten volume. Vmelt/VNP was evaluated on the basis of the ratio between the energy loss that was induced by NP thermal transition and the energy needed


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for the total melting. Size reduction occurred when Vmelt/VNP > 1. The NP diameters were (a) 20 nm, (b) 60 nm, (c) 100 nm and (d) 150 nm.

It was found that the irradiation wavelength had a strong influence on the thermal behavior of the NPs and surrounding water. In the UV-visible region (350– 700 nm), the NPs reached a molten state even at low fluence regardless of the NP size. A possible reason was that the strength of the absorption band in the visible spectrum region for the noble metallic particles increased because of the surface plasmon oscillation modes of the conduction electrons coupling with the external electromagnetic field. When considering the cases in the NIR region (especially in the frequently-used wavelength of 800 nm), most of the NPs were unaffected by the excitation laser regardless of NP size, even when the fluence was promoted to 500 J/m2. Such a high threshold can be ascribed to the reduced plasmonic resonance efficiency at the excitation wavelength, corresponding to a weak absorption property, as shown in Fig. 5. Laser fluence, which initiated the bubble generation or melting thermodynamics, was highly dependent on the NP size. The intactness of the small NPs was difficult to maintain even below the threshold fluence of bubble generation regardless of wavelength, as shown in Fig. 11a. The fluence threshold for the 60 nm NP was close to the melting threshold curve but just right below the melting threshold curve limit and thus prevented structural deformation, as shown in Fig. 11b. In the large NPs, bubble generation was hardly affected by the melting behavior at any wavelength, as shown in Fig. 11c and 11d. The relations between the optimal NP diameters (50–150 nm) and excitation



Page 34 of 79

laser wavelength (350–800 nm) emitted by 100 fs and 15 ps pulsed laser were summarized in Fig. 12. The optimal NP size considering the effect of NP melting was denoted by red spherical symbols and the case without considering NP melting was represented by black triangle symbols. It was found that in the UV-VIS range, the curves of the optimal NP size were quite different from each other, especially for the shorter pulsed laser excitation with τp=150 fs. The difference gradually dribbled away with the increase of the wavelength, which was ascribed to that the lower plasmonic resonance efficiency under longer wavelength could suppress the NP heating rate and further reduce the possibility of NP melting. Additionally, a shorter pulse width usually indicated an efficient NP heating but a relatively insufficient NP cooling. This will also aggravate the probability of NP thermal transition. The corresponding fluence thresholds of the optimal NP size to provoke the bubble generation was also presented with considering the NP thermal phase transition, as shown in the inset figure of Fig. 12. The bubble threshold fluence increased with wavelength after 550 nm. Due to a lower heating efficiency, a higher fluence was required at a longer wavelength.

(a)

(b)


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Fig. 12. The optimal NP size as a function of wavelength in the cases with and without considering the NP melting effect. The pump laser was emitted by (a) 150 fs pulsed laser and (b) 15 ps pulsed laser. The inset figures indicated the corresponding fluences to trigger the bubble generation for NPs with optimal size where NP melting effect was involved in the predication.

3.6 EFFECT OF PULSE DURATION ON NP MELTING AND BUBBLE THRESHOLD At increased pulse duration, the heat loss due to phonon–phonon relaxation from the NP to the surrounding water molecules was comparable with photon absorption. This relaxation behavior reduced the possibility that the NP underwent the melting process before the fluid phase change. To characterize the influence of pulse duration on the bubble generation, we used the ETTM to evaluate the energy deposition in 20 nm NPs that were irradiated with the bubble threshold fluence with wavelength of 532 nm and pulse duration ranging from 7 ps to 1 ns. As shown in Fig. 13, with the increase of pulse width, the ratio of energy loss Emelt (due to the NP melting) to the absorbed laser energy Elaser decreased, implying that the effect of NP melting dribbled away with enlarged pulse duration. Under the near ns-pulsed excitation (0.4-1 ns), the ratio of energy stored in the surrounding water Efluid to Elaser was close to 80%. This implied that the heat diffusion to the surrounding water molecules took the major role in medium evaporation at large pulse width, where high fluence was expected to provoke the bubble generation because a significant amount of energy was released to the surrounding water rather than stored in the interfacial water layer. Thus, the heat



dissipation loss should not be neglected in the nanosecond pulsed laser excitation.

Fig. 13. Influence of pulse duration on energy allocation during NP melting. ENP and Efluid respectively represented the energy stored in the NP and fluid, Emelt was the energy loss due to NP melting, Elaser was the total absorbed laser energy.

As specified in section 2.1, the ETTM considered the effect of environment heating on the NP absorption characteristics, i.e., the absorption cross-section Cabs during the nanosecond pulsed laser excitation. Essentially, environment heating was affected by NP temperature evolution under different conditions. In turn, the medium-temperature-dependent Cabs affected the NP heating process. The above issues were detailed in this section. Strasser group

27

demonstrated that medium

heating in the nanosecond regime resulted in the bleaching of LSPR band under pressurized conditions, where the bubble formation was prevented. In fact, any factor that reduced Cabs can be considered capable of bleaching the LSPR band. In this study, we focused on the influence of medium heating on the process of LSPR band bleaching without additional environmental pressurized controls. It was demonstrated that medium heating will change the laser deposition and further influence the optimal heating efficiency. Detailed results were shown below. The NP temperature and the


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absorption characteristics for different NP sizes excited by the 5 ns pulsed laser were shown in Fig. 14 and 15. As discussed in section 3.1, the optimal NP size of the highest heating efficiency was 56 nm in diameter (exposing to the 532 nm excitation pulse laser). Owing to the influence of environmental temperature on the absorption cross-section (shown in Fig.14a and 14b), the optimal NP size of heating efficiency was shifted to about 70 nm.

(a)

(b)

(c)

(d)

Fig. 14. Lattice temperature Tl of differently sized NPs under different laser fluences at delay times (relative to the initial time of pump laser) of (a) 5 ns, (b) 10 ns, (c) 20 ns and (d) 30 ns. A 5 ns pulsed laser with a wavelength of 532 nm was adopted. The NP diameter increased from 10 nm to 200 nm.



(a)

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(b)

(c)

(d)

Fig. 15. Change in Cabs values at differently sized NPs under different laser fluences at delay times (relative to the initial time of pump laser) of (a) 5 ns, (b) 10 ns, (c) 20 ns and (d) 30 ns. The change of Cabs was defined as the ratio of the temperature-dependent Cabs(Tm) value to the constant Cabs(T0). A 5 ns pulsed laser with a wavelength of 532 nm was adopted, and the NP diameter increased from 10 to 200 nm.

Considering the important role of medium heating on Cabs, we further applied the ETTM incorporating the temperature dependent Cabs to estimate the bubble threshold fluence and compared with the experimental results obtained in colloid Au suspension using the proof-of-concept multimodal platform in section 2.3. As shown in Fig. 16, the numerical predicated fluence thresholds (with or without considering the effect of


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medium heating on Cabs) were generally lower than our experimental results (blue triangles) excited by 5 ns pulsed laser. But considering the medium heating effect in the ETTM made the predication closer to the experimental measurement. The deviation (i.e., the predicated results were lower than the measured ones) can be ascribed to that the bubble threshold fluence in the suspension was expected to be higher than that of a single NP. In the suspension, the relative NP position to the laser focus was uncertain because of the nonuniform laser intensity distribution (e.g., Gaussian distribution in our study) and uncontrollable NP fluctuation. The bubble generation was very likely to occur with the location deviated from the expected one, which was corresponding to the highest laser intensity. Although denser optical density (e.g., associated with high NP concentration and small NP-to-NP space) used in our experiment can enhance the frequency of the statistical events (i.e. the bubble occurrence), and make it closer to that with a single NP in an ideal location (with the highest laser intensity), the NP suspension was expected to inevitably underestimate the bubble generation, i.e., yield a higher fluence threshold. Besides, our predicated threshold can well agree with the experimental data for the single NP measurement (green triangle symbol)

21

when the effect of medium heating was considered under

the same condition (Fig. 16).



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Fig. 16. Comparison between the predicated threshold fluences by ETTM and the experimentally measured threshold fluences for bubble generation as a function of NP size excited by a 5 ns laser pulse with a wavelength of 532 nm. The green star symbol represents the experimental threshold fluence around a single 90 nm NP

21

. An inverted microscope

system coupled with a dark-field epi-illumination scheme was used. The blue triangle symbols represent the experimental data in a colloid gold NP suspension with optical densities of 0.5 in this work.

Section 3.1 discussed the optimal NP size with the highest heating efficiency at the excitation wavelength of 532 nm under an irradiation fluence that was far below the melting and bubble threshold fluence. Hereinto, the optimal size was about 60 nm for 15 ps pulsed excitation. When the ultrashort pulsed laser was adopted, the heating efficiency of a small NP was easily weakened by the NP melting thermodynamics. The optimal NP size shifted to the large size direction. When stretched to nanosecond excitation, the optimal heating efficiency of an NP was influenced by the bleaching of LSPR, which also shifted to the large size direction. Therefore, we summarized the influence of the pulse duration on the determination of the optimal NP size in Table 1, where a nonmonotonic tendency with the pulse duration ranging from fs to ns existed.


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Table 1 The optimal NP size under different pulse durations Pulse duration 100 fs 15 ps 5 ns

Optimal NP size 72 nm 60 nm 70 nm

Fluence threshold 2

27 J/m 24.5 J/m2 560 J/m2

Wavelength 532 nm 532 nm 532 nm

4. CONCLUSIONS In this paper, we presented a model based theoretical analysis of the photothermal nanobubble generation around NPs excited by pulsed lasers, screening large libraries of nanoparticle sizes and excitation conditions on the basis of the ETTM. To validate the feasibility of ETTM, as well as enhance the robustness and applicability of the algorithm, we constructed a proof-of-concept experimental set-up, integrating the multimodal method of time-resolved optical response, scattering imaging and photoacoustic detection, for the investigation of photothermal response to the excitation of a nanosecond pulsed laser. Then, key influencing factors for the rational evaluation of the bubble threshold fluence under different excitation regimes were systematically studied. When “biosafety” was considered in a biomedical application, an optimal NP size was required, with which the excitation fluence that promoted bubble generation was minimized and the intactness of NPs was retained for the prevention of potential collateral damage to surrounding tissues. The major findings of this work were summarized as follows: 1) The non-monotonic dependence of the fluence threshold (of NP melting and bubble generation) with regards to NP size was extracted. It can be attributed to two aspects. On the one hand, small NPs with high laser fluences were attributed to the



rapid thermal exchange between the NPs and the surrounding medium. This exchange suppressed the efficient interfacial energy deposition for medium boiling. And besides, the feature that small NPs were prone to be affected by NP melting thermodynamics will aggravate the difficulty of bubble generation. On the other hand, large NPs increased the fluence threshold because of the increased heat capacity, which retarded the heating of the surrounding medium. 2) In the sub-nanosecond excitation regime, the possibility of NP melting was found important in the rationalization of the bubble generation threshold, because a high NP heating efficiency but a low NP cooling dynamics efficiency will both present under this condition. When the irradiation laser was stretched to the nanosecond excitation regime, the bleaching of LSPR band due to the inevitable medium heating within the time scale of the excitation pulse outperformed the influence of NP melting, during the evaluation of bubble generation. 3) For the design of an efficient and durable NP-based biomedical regimen, the optimization concept with the purpose of biosafety was introduced. Optimization was found to be closely related with the characteristic of interfacial heat transfer, laser excitation wavelength, and pulse duration. More specifically, the optimal NP size decreased with increasing Kapitza resistance (i.e., the interface thermal conductance) and decreasing excitation wavelength and had a non-monotonic dependence on laser pulse duration. This work was expected to clarify the mechanisms underlying bubble nucleation in plasmonics under pulsed laser illuminations. Theoretical predictions can be


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confirmed or validated through previous experimental reports and our own experimental studies. This work may help establish a new effective routine for the design of efficient and biosafe photothermal bubble nanosources. ASSOCIATED CONCENT SUPPORTING INFORMATION

The effect of NP melting thermodynamics on the trigger time of bubble nucleation based on the extended two-temperature model (ETTM)was discussed in this material. AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] ORCID Jing Wang: 0000-0001-5875-4049 NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was jointly supported by the National Natural Science Foundation of Grant (61335012, 61727823, 61575156, 61705177, 61775178, 61505159). REFERENCES (1). Ackroyd, R.; Kelty, C.; Brown, N.; Reed, M. The History of Photodetection and Photodynamic Therapy. Photochem. Photobiol. 2010, 74, 656−669. (2). Rai, P.; Mallidi, S.; Zheng, X.; Rahmanzadeh, R.; Mir, Y.; Elrington, S.; Khurshid, A.; Hasan,



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TABLE OF CONTENTS GRAPHIC


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Quantitative Evaluation and Optimization of Photothermal Bubble

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