Surfactant-Controlled Photothermal Assembly of Nanoparticles and

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Surfactant-Controlled Photothermal Assembly of Nanoparticles and Microparticles for Rapid Concentration Measurement of Microbes Yasuyuki Yamamoto,†,‡,§ Shiho Tokonami,*,‡,§ and Takuya Iida*,†,‡ †

Department of Physical Science, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan Research Institute for Light-Induced Acceleration System, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan § Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan ‡

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S Supporting Information *

ABSTRACT: Light-induced heating on a solid−liquid interface can generate a vapor submillimeter bubble and fluid flow, which enables us to densely and rapidly assemble dispersoids into a desired position (photothermal assembly). Here, we revealed that the surface modulation of the light-induced bubble by a surfactant dominates the assembly dynamics of nanoparticles and microparticles as dispersoids, which results in highly efficient photothermal assembly under the surfactant-controlled fluid flow. This mechanism can facilitate the concentration measurement of small objects (microparticles, bacteria, viruses, etc.). Particularly, we found that the surfactant-controlled fluid flow and bubble enable high-density assembly of dispersoids and remarkable enhancement of assembly efficiency, achieving 10−20 times in comparison with the case of no surfactant. This result can extend the limit of measurable concentration by one order. Furthermore, this study revealed the influence of concentration, size, and constituent material of the dispersoids on the assembly efficiency for the improvement of measurement precision. These findings are crucial for laser-induced assembly for the rapid concentration measurement of various microbes without a cultivation process as bioanalysis, for the high-sensitivity detection of harmful particles, and for the colloidal lithography. KEYWORDS: photothermal assembly, laser heating, Marangoni convection, microbubble, microparticle, nanoparticle, surfactant, polystyrene, bacteria

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lithography,25,26,32−35 optoacoustic tweezers31), [B] generation and control of the convection can be performed (e.g., microfluidic pump,27 sorting of microparticles28), and [C] dispersoids can be rapidly and densely assembled in a local area (e.g., detection of proteins29 and molecules,30 measurement of bacterial concentration36,37). In relation to these, fundamental research findings have also been reported on laser-induced bubble growth dynamics, its components,38−40 and convection pattern and speed.33,41 Our research group focused on the third ability [C], developed a quantitative evaluation method for the number of assembled dispersoids (ND) by PTA, and discovered that there is a high correlation between ND and concentration of dispersoids.36,37 Based on this observation, we proposed an application of PTA to highspeed concentration measurement of small objects in a liquid (e.g., nanoparticles, microparticles, and bacteria) and demonstrated the technique for 108 and 105 cells/mL in a previous report.36 While there are many other concentration measurement methods, as listed in Table S1, PTA enables rapid concentration measurement of small objects, such as microparticles, bacteria, and viruses. Since the concentration is

INTRODUCTION Local heat generation induced by laser illumination in lightabsorptive materials (e.g., metal nanoparticles or metal thin films) has attracted the attention of many researchers, and various studies have been reported.1−4 Applied research concerning this technique has been conducted in a wide range of fields such as plasmonic thermotherapy5−7 and nanosurgery,8 chemical vapor deposition,9,10 solvothermal synthesis,11−14 catalytic method,15 thermophoresis in a fluid,16−18 microfluidic valves and pumps of fluid flow,19−21 photothermal imaging,22,23 and nanolithography.24 In particular, studies on laser-induced fluid flow and a vapor bubble (diameter: 1−100 μm) have attracted attention25−42 since they can be used for local assembly of dispersoids in dispersions. The fluid flow generated by laser illumination in light-absorptive materials transports these dispersoids toward the bubble, which traps them in the fluid medium. Then, the dispersoids can be assembled in a ring shape around the bubble by a series of phenomena called photothermal assembly (PTA). In addition, horizontally moving the laser illumination point changes the fluid flow patterns,27,28 and further movement enables us to assemble dispersoids at arbitrary places through the bubble tracking.25,26 There are mainly three kinds of abilities (and applications) of PTA: [A] dispersoids can be trapped in an arbitrary place (e.g., colloidal © XXXX American Chemical Society

Received: December 28, 2018 Accepted: February 25, 2019

A

DOI: 10.1021/acsabm.8b00838 ACS Appl. Bio Mater. XXXX, XXX, XXX−XXX

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ACS Applied Bio Materials

Figure 1. (a) Optical transmission images at 10 and 180 s after laser illumination and schematic illustrations of the assembly pattern (not to scale) at each concentration of the surfactant. The length of the scale bar is 50 μm. (b) Bubble radius and (c) NPS as a function of laser illumination time at each concentration of the surfactant (n = 5). (d) Convective velocity experimentally obtained from particle tracking (black squares plot, at 100 μm away from the laser illumination point) and surface tension gradient in the direction of the height h as a function of NS concentration (blue circle plot, surface tension gradient where the convective velocity obtained by FEM and the experimental value coincide with each other) (n = 5). Without NS, the convective velocity was more than 100 μm/s and ∂σ/∂h = 3.3 × 106 mN/m2 (see also Figure S7). (e) Simulation of the convection obtained by FEM in the case without NS and (f) that with NS (assumed 300 s after laser illumination). In the case with NS, a structure assuming the assembled PS was placed between the bubble and the substrate. Black arrows and solid lines indicate the directions of convections and streamlines, and the colormap indicates the convective velocity.

estimated from ND in this method, the assembly efficiency of PTA (= ND/ total number of dispersoids in liquid) should be increased in order to apply this method to lower concentrations of dispersoids. In addition to our concentration measurement, improvement of the assembly efficiencies can contribute to improving the detection sensitivities of the specimens because they can be assembled at high densities in the observation region. However, there is little knowledge from the physicochemical viewpoint on the assembly dynamics of dispersoids using PTA and the control of ND, which makes it challenging to improve

the assembly efficiency. Zheng et al. have suggested that the convection pattern changes according to the amount of adsorbed microparticles on the bubble surface by performing numerical calculations.33 Inspired by this report, we have attempted to control the state of the bubble surface using surfactants (especially, a nonionic surfactant (NS) type) rather than microparticles in order to improve the assembly efficiency. Because the state of the bubble surface that is controlled by the surfactants is different from the dispersoids, the laser-induced fluid flow and assembly dynamics can be expected not to change during the assembly process. B

DOI: 10.1021/acsabm.8b00838 ACS Appl. Bio Mater. XXXX, XXX, XXX−XXX

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ACS Applied Bio Materials

Figure 2. Optical transmission images at 300 s after laser illumination in each concentration of dispersoids: (a) 4.55 × 108, (b) 4.55 × 107, (c) 4.55 × 106, (d) 4.55 × 105, (e) 4.55 × 104 particles/mL. The length of the scale bar is 20 μm. (f) NPS as a function of laser illumination time at each concentration of PS (n = 5). (g) The assembly efficiency of NPS and NSA at 300 s after laser illumination as a function of concentration of the dispersoid (n = 5). The regions with different assembly efficiencies are divided into three regions ((i)−(iii)), and the respective concentrations are (i) 4.55 × 104−4.55 × 106, (ii) 4.55 × 106−9.10 × 107, and (iii) 9.10 × 107−9.10 × 108 particles/mL. (h) Schematic diagrams of the assembly dynamics at each concentration of PS and assembly efficiencies in three regions ((i)−(iii)). The blue arrow represents the direction of convection, and the trap efficiency is represented by the number of arrows toward the stagnant area.

dispersoid was verified. Finally, the concentration of bacteria was estimated using PTA, and the results were compared with those of the cultivation method to examine precision and detection limit.

In this study, we improved the assembly efficiency in surfactant-controlled PTA based on quantitatively analyzing dispersoids (nanoparticles and microparticles), where ND was increased by a factor of 10−20 as compared to the case without NS. Furthermore, we investigated the influence of surfactants on PTA dynamics due to their adsorption on the bubble. In particular, by comparing N D for different concentrations of the surfactant, we obtained the appropriate concentration to realize high-density assembly and achieve control of the PTA dynamics. After that, under optimal conditions, we experimentally and numerically investigated the relationship between the assembly efficiency and dispersoid concentration to improve the precision of concentration measurement by PTA. Subsequently, the change of assembly dynamics depending on the size and constituent material of the

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RESULTS AND DISCUSSION Effect of Surfactants on Photothermal Assembly. We illuminated a gold thin film (thickness 10 nm) with a continuous wave (CW) laser of wavelength 1064 nm (laser power: 100 mW, spot diameter: 2.5 μm) and performed PTA with polystyrene microparticles (PS; diameter is 1.0 μm) at different concentrations of NS (Supporting Information Movies 1−4). Figure 1a shows the optical transmission images obtained at 10 and 180 s after laser illumination and the schematic illustrations of the assembly patterns in each C

DOI: 10.1021/acsabm.8b00838 ACS Appl. Bio Mater. XXXX, XXX, XXX−XXX

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case without NS. We believe that this result indicates the difference in the trap efficiency of PS to a stagnant area between the bubble and substrate. Without NS, the growth rate of the bubble was large, and the dispersoids were assembled in a single layer; however, with NS, the bubble did not grow much, and the dispersoids were assembled in multiple layers. The same phenomenon was confirmed with surfactants other than NS, for example, amphoteric and anionic surfactants (Figures S3 and S4). In consideration of these factors, the convection around the bubble was calculated using FEM in order to investigate the difference in NPS (Figures 1e, 1f, and S5). Without NS, the space between the bubble and substrate was narrow, and as a result, the flowing dispersoids were affected by Marangoni convection near the bubble surface (Figure 1e). With NS, we believe that the stagnant area expanded owing to the presence of multiple layers of PS, and the trap efficiency was therefore improved (Figure 1f). Furthermore, we verified that a concentration of 9.0 × 10−5 M of NS (almost the same as the CMC) was suitable for highdensity assembly of the dispersoids. In contrast, while micellar surfactant inhibited the assembly at a higher concentration than the CMC, the dispersoids are adsorbed on the bubble surface and difficult to be assembled in the stagnant area at a concentration lower than CMC by one order. Furthermore, when the NS concentration becomes lower or absent, the dispersoids are hardly trapped around the bubble. Correlation between Assembly Efficiency and Dispersoids’ Concentration, Size, and Material. We investigated the relationship between NPS and concentration of PS (4.55 × 104 to 9.10 × 108 particles/mL) under an NS concentration of 9.0 × 10−5 M. While Figures 2a−2e show the transmission images at 300 s after laser illumination, Figure 2f shows NPS as a function of laser illumination time (30−300 s) at each concentration of PS. In order to analyze this result, the assembly efficiency (= NPS/NTotal PS) at 300 s at each concentration was plotted in Figure 2g. It is seen that the assembly efficiency changes depending on the concentration of the dispersoid (Figure 2g (i)−(iii)). While the assembly efficiency changed slightly up to a concentration of 4.55 × 106 particles/mL (Figure 2g (i)), it increased sharply when this concentration was exceeded (Figure 2g (ii)). In order to understand the increase in the assembly efficiency at 9.10 × 106 to 9.10 × 107 particles/mL, we investigated the stagnant area and convective velocity using FEM simulations (Figure S8). According to the simulation results, the stagnant area increased with NPS (Figure S8a), whereas the convective velocity slightly reduced owing to the reduction of the bubble− liquid interface because of the increase in NPS (Figure S8b). This result suggests that the increase in the stagnant area has a large influence on trap efficiency, as described in the previous section. As the concentration of PS increased, the assembly efficiency decreased (Figure 2g (iii)), which is believed to be because the stagnant area was filled with PS (the total number of PS increases beyond the limit of space between the bubble and substrate). As a summary, Figure 2h shows the schematic diagrams of the assembly dynamics at each concentration of PS. Furthermore, in order to confirm the size dependence of the stagnant area with respect to the assembly efficiency, similar experiments were carried out on PS of different sizes (diameter = 50−1000 nm), and the assembly efficiency was investigated (Figure 3). Similar to this result, the assembly efficiency decreased as the particle size decreased, which means

dispersion. Four main changes were confirmed depending on the presence or absence of NS: (i) assembly pattern of PS, (ii) size of the generated bubble, (iii) convective velocity, and (iv) number of assembled PS (NPS). Regarding the assembly pattern (Figure 1a), the dispersoids were observed to assemble in a single layer between the bubble and substrate in the absence of NS. However, with the NS (9.0 × 10−6 M), the dispersoids assembled in multiple layers between the bubble and substrate, and they were also adsorbed on the bubble surface. At a particular concentration of NS (9.0 × 10−5 M), no dispersoids were adsorbed on the bubble surface, and a majority of them assembled between the bubble and substrate. When the concentration of NS was further increased (9.0 × 10−4 M), a binding behavior was observed between the microparticles. Although the dispersoids assembled by PTA were generally immobilized between the bubble and substrate during laser illumination (Supporting Information Movies 1−3), we observed that a part of the assembled dispersoids was not immobilized and moved under the increased NS condition (9.0 × 10−4 M) (Supporting Information Movie 4). Since this concentration of the surfactant exceeds the critical micelle concentration (CMC), micelle formation of the surfactant in the high-temperature region near the laser illumination point would cause such a behavior. Without NS, the radius of the bubble was approximately 100 μm at 60 s after laser illumination, whereas with NS (9.0 × 10−6 to 9.0 × 10−4 M), the radius was approximately 35 μm for the same time interval (Figure 1b). It is believed that the bubble became smaller owing to a decrease in surface tension on the bubble surface (eq 2 in Supporting Information). The velocity of the dispersoids near a substrate (height of ∼0.5 μm) was found to be more than 100 μm/s without NS, whereas it was ∼1.5 μm/s with NS ranging from 9.0 × 10−6 M to 9.0 × 10−4 M (Figure 1d), as determined by the trajectory analysis of PS. The fluid flow transporting the dispersoids was mainly of the Marangoni convection41,43 type, which occurs at the upper surface of the liquid sample in the glass-bottom dish (Figure S1a) and at the surface of the light-induced bubble (Figure 1). In both cases, the Marangoni convection was suppressed by NS, and the convective velocity was reduced. Upon examining the temperature of the bubble surface (Figure S6), since adsorption of NS molecules is an exothermic reaction, they are observed to be densely adsorbed on the upper part of the bubble away from the laser illumination point. Furthermore, the surface tension depends on the temperature and concentration of the NS (see also Figure S7). Therefore, the change in surface tension σ was canceled owing to the temperature and concentration gradients of NS on the bubble surface. Figure 1d shows the surface tension gradient (in the direction of height h from the substrate) when the convective velocity obtained by the finite element method (FEM) with COMSOL Multiphysics (Figures S5 and S7) corresponds to that from the experimental result. When NS was not considered, ∂σ/∂h = 3.3 × 106 mN/m2 (calculated from Figure S7d), whereas it was ∂σ/∂h = 76.7−95.4 mN/m2 when NS was present. This result suggests that the surface tension gradient was relaxed owing to the cancellation by the temperature and concentration gradients, as mentioned earlier. Figure 1c shows NPS as a function of laser illumination time for each concentration of NS. Although the bubble size and velocity of the dispersoids decreased with the surfactant, NPS (at 300 s) increased by a factor of 10−20 as compared to the D

DOI: 10.1021/acsabm.8b00838 ACS Appl. Bio Mater. XXXX, XXX, XXX−XXX

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ACS Applied Bio Materials that the volume of the stagnant area affects the assembly efficiency.

Figure 4. Comparison of the concentrations of bacteria estimated by PTA (5 min laser illumination) and the cultivation method (approximately 24 h) for five samples with different concentrations (numbers in this graph denote relative errors) (n = 5).

Figure 3. Assembly efficiency as a function of particle diameter (n = 5).

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Since the relationship between NPS and PS concentration became clear, we can measure the PS concentration within 4.55 × 104 to 9.10 × 108 particles/mL by PTA. In order to apply this concentration measurement to bacteria, it was verified whether the assembly dynamics of the bacteria was different from that of PS. We examined the relationship between the number of assembled Staphylococcus aureus (SA) (NSA) and the concentration of SA (6.42 × 104 to 6.42 × 108 cells/mL), whose size and shape were almost the same as those of PS (the size of the dispersoid is required for this concentration measurement). As with the PS analysis, Figure 2g shows the assembly efficiency of NSA at 300 s at each concentration. Since the analysis result showed almost the same behavior as PS, there was no difference in the assembly dynamics between the two (a slight decrease in the assembly efficiency is thought to be due to the difference in size between PS and SA). Therefore, this result indicates that even if the constituent material of PS and SA is different it does not affect the assembly efficiency, and there is a possibility that the concentration measurement by PTA is applicable to dispersoids of various constituent materials. Finally, we carried out concentration measurement by PTA for SA whose concentration was 6.42 × 104 to 6.42 × 108 cells/ mL to investigate the measurement precision. As it was confirmed that the bacteria were also assembled in a manner similar to PS, the concentration of each bacterial dispersion was estimated using relational expressions between NPS and CPS (the PS concentration) (high-density assembly: NPS = 1.4 × 10−3 × CPS0.87, low-density assembly: NPS = 2.5 × 10−5 × CPS1.1). These equations were obtained from the fitting of the power function of NPS vs CPS in the region (i) and (ii) of Figure 2g, respectively (the function with the smallest R value was the power function). From the microscopic image after the assembly of dispersoids as in Figures 2a−e, it can be decided which of these equations to apply. Then, we estimated the concentration of the same dispersion by the cultivation method and compared it with the result obtained by PTA (Figure 4). From this result, it was clarified that the PTA method could perform concentration measurements for a few minutes with an average relative error of 11% with the cultivation method (24 h) and the limit concentration as 6.42 × 104 cells/mL (the lowest concentration in these demonstration experiments).

CONCLUSION In summary, we systematically clarified the effect of some kinds of surfactants (nonionic, amphoteric, and anionic) on PTA and improved the performance of concentration measurement of the dispersoids (PS and SA). The main part of the discussion is focused on the PTA with the nonionic surfactant, which provides less damage to bacteria, and the potential applications with keeping biological functions would be extended. The effects of the surfactant on PTA are summarized as follows: (i) changes in the assembly pattern of dispersoids, (ii) reduction in the size of the laser-induced bubble, (iii) deceleration of the convective velocity, (iv) increase in NPS. In particular, (i) is sensitive to the concentration of the surfactant, and the dispersoids can be densely assembled at a specific concentration (in the case of NS, 9.0 × 10−5 M ≈ CMC). In the relationship between ND and the concentration of dispersoids, the assembly efficiency changed depending on the concentration of the dispersoid because of the difference in the trap efficiency due to the change of stagnant area. Similar results were obtained with regard to the size of the dispersoid. Furthermore, if the size and shape of the dispersoid were almost the same, there was no change in the assembly dynamics even when the constituent material was different. Then, estimating the concentration of the bacteria by PTA without the amplification process (cultivation), we succeeded in improving the measurement precision (the average relative error with the cultivation method was 11%), and the measurement limit concentration (6.42 × 104 to 6.42 × 108 cells/mL) by one order compared to a previous report.36 Although only concentration measurement of spherical bacteria was performed in this study, it is necessary to investigate the influence of morphology on the assembly pattern and ND. The clarified mechanisms on PTA will improve the versatility of the dispersoids and the precision in concentration measurement of various microbes (bacteria, viruses, etc.), the detection of a small amount of harmful particles, and for the colloidal lithography for functional assembled structures. The obtained results and discussions will be an important foundation for the “Photothermal Fluidics” based on the physicochemical process of light-induced fluidic effect under the molecularly controlled surface energy. E

DOI: 10.1021/acsabm.8b00838 ACS Appl. Bio Mater. XXXX, XXX, XXX−XXX

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calculated by multiplying these volumes by the filling rate 0.74 (assuming close-packed condition) and dividing by the volume of the dispersoid. When the dispersoids were assembled into a single layer, ND was calculated by dividing the projection area by the area of the dispersoid. When ND was small, the dispersoids captured in the transmission images were counted. The detailed information in calculation of the convection performed in this study is described in Supporting Information.

METHODS

Materials Used in Experiments. Polystyrene (PS) microparticles with diameters of 1.0 μm (Polysciences, USA), 0.50 μm, 0.20 μm, 0.10 μm, and 0.050 μm (Micromod Partikeltechnologie GmbH, Germany) with COOH modification and Staphylococcus aureus (SA) bacteria with a diameter of approximately 0.75 μm were used as dispersoids. In the experiments, the concentrations of the PS microparticles were 4.55 × 104−9.10 × 108 particles/mL, and the concentration of SA was 6.42 × 104−6.42 × 108 cells/mL, estimated by a cultivation method. In this cultivation method, the concentration measurement was performed three times for 24 h using commercially available Petrifilm (Petrifilm; 3 M Health Care, USA), and the average value was used. Ultrapure water was used as the solvent of the dispersion (PS and SA). Polyoxyethylene sorbitan monolaurate (T20; nonionic surfactant; Wako Pure Chemical Industries, Japan), 3-(3-cholamidepropyl) dimethyammonio-1-propanesulfonate (CHAPS; amphoteric surfactant; GE Healthcare Life Sciences, USA), and sodium dodecylsulfate (SDS; anionic surfactant; GE Healthcare Life Sciences, USA) were used as the surfactant (each CMC is 6.0 × 10−5 M (T20), 8.0 × 10−3 M (CHAPS), 8.0 × 10−3 M (SDS)). Each surfactant was added to the dispersion described above so that the concentration became 9.0 × 10−l M (l = 4−6; T20), 1.6 × 10−m M (m = 3−5; CHAPS), and 3.4 × 10−n M (n = 3−5; SDS). Experimental Setup for Photothermal Assembly. Figure S1a shows the optical system used to induce the fluid flow and a bubble by laser illumination. A gold thin film (thickness: 10 nm) formed by ion sputtering (E-1010; Hitachi, Japan) on a glass bottom dish (IWAKI, Japan) was used as the light-absorptive substrate. The film thickness of the gold thin film was measured with a profilometer (Dectak150; Takaoshin, Japan). This substrate was placed on the stage of the microscope (Eclipse Ti−U; Nikon, Japan) using a back port adapter (MMS-2L-800/1064; Sigma Koki, Japan), and 100 μL of the sample dispersion was dropped. Thereafter, the infrared continuous-wave laser (wavelength: 1064 nm) (FLS-1064-2000F, Sigma Koki, Japan) was focused by an objective lens (Olympus 40 ×, 0.6 NA) and illuminated onto the gold thin film from the bottom. The focusing position of the laser was aligned with the dispersion/gold thin-film interface, and the diameter of the laser spot was 2.5 μm. The laser power after passing through the objective lens was set to be 100 mW by utilizing a power meter (UP17P-6S−H5 and TUNER; Gentec Electro-Optics, Canada) because this laser power and NPS are closely related (Figure S2). During laser illumination (300 s), the PTA process was monitored by a cooled charge-coupled device camera (DS-Filc-L3, Nikon, Japan) and saved in time-lapse images under Köhler illumination. All the experiments in this study were performed five times, and all experimental results are shown with error bars representing standard deviation. Calculation Method for the Number of Assembled Dispersoids. ND by PTA was calculated from the projected area between the bubble and the substrate in a transmission image. The projection areas at each laser illumination time were measured by using an image analysis software (NIS-Elements Analysis; Nikon, Japan). Under the assumption that the assembled dispersoids fill the space between the bubble and the substrate such as in regions (i) and (ii) in Figure S1b, the relational expressions between the projection areas and the volume of the assembled region are, respectively, as follows

Vregion(i) =

Vregion(ii) =

i2 y π 3 H + π(R2 − D2)jjj R2 − D2 − H zzz 3 k3 { πH ijjj 2 2 2 jD + H − 3R + 3 jjk

yz zz zz H2 + D2 z{

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsabm.8b00838. Experimental data and simulation data (PDF) Movie of illuminated gold thin film (thickness 10 nm) with a continuous wave (CW) laser of wavelength 1064 nm (laser power: 100 mW, spot diameter: 2.5 μm) at different concentrations of NS (MPG) Movie of illuminated gold thin film (thickness 10 nm) with a continuous wave (CW) laser of wavelength 1064 nm (laser power: 100 mW, spot diameter: 2.5 μm) at different concentrations of NS (MPG) Movie of illuminated gold thin film (thickness 10 nm) with a continuous wave (CW) laser of wavelength 1064 nm (laser power: 100 mW, spot diameter: 2.5 μm) at different concentrations of NS (MPG) Movie of illuminated gold thin film (thickness 10 nm) with a continuous wave (CW) laser of wavelength 1064 nm (laser power: 100 mW, spot diameter: 2.5 μm) at different concentrations of NS where part of the assembled dispersoids was not immobilized (MPG)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T. I.) *E-mail: [email protected] (S. T.). ORCID

Takuya Iida: 0000-0003-1313-7025 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Dr. Y. Nishimura and E. Shimizu for their advice on the sample preparation. In addition, we thank Dr. M. Tamura for his advice and support from a theoretical viewpoint. A major part of this work was supported by a Grantin-Aid for Scientific Research (A) (No. 17H00856); Grant-inAid for Scientific Research (B) (No. 15H03010, No. 18H03522); Grant-in-Aid for JSPS Fellows (No. 18J13307); Grant-in-Aid for Scientific Research on Innovative Areas (No. 16H06507) from JSPS; JST-Mirai Program (No. JPMJMI18GA); the Canon Foundation; and the Key Project Grant Program of Osaka Prefecture University, and the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”.

(1)

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2R3

(2)

REFERENCES

(1) Govorov, A. O.; Richardson, H. H. Generating Heat with Metal Nanoparticles. Nano Today 2007, 2, 30−38. (2) Qin, Z.; Bischof, J. C. Thermophysical and Biological Responses of Gold Nanoparticle Laser Heating. Chem. Soc. Rev. 2012, 41, 1191− 1217.

where V is the total volume of the dispersoids assembled around the bubble; H is the height of the center of a bubble; R is the radius of a bubble; and D is the radius of the region where the dispersoids assembled. The average number of assembled dispersoids was F

DOI: 10.1021/acsabm.8b00838 ACS Appl. Bio Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsabm.8b00838 ACS Appl. Bio Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsabm.8b00838 ACS Appl. Bio Mater. XXXX, XXX, XXX−XXX