Control of Submillimeter Phase Transition by Collective Photothermal

Article pubs.acs.org/JPCC

Control of Submillimeter Phase Transition by Collective Photothermal Effect Yushi Nishimura,†,‡ Keisuke Nishida,†,‡ Yojiro Yamamoto,§ Syoji Ito,∥ Shiho Tokonami,*,†,# and Takuya Iida*,†,⊥,# †

Nanoscience and Nanotechnology Research Center, Osaka Prefecture University, 1-2 Gakuencho, Nakaku, Sakai, Osaka 599-8570, Japan ‡ Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuencho, Nakaku, Sakai, Osaka 599-8531, Japan § GreenChem Inc., 930-1-202 Fukuda, Nakaku, Sakai, Osaka 599-8241, Japan ∥ Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan ⊥ Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuencho, Nakaku, Sakai, Osaka 599-8531, Japan S Supporting Information *

ABSTRACT: Local molecular states and biological materials in small spaces ranging from the microscale to nanoscale can be modulated for medical and biological applications using the photothermal effect (PTE). However, there have been only a few reports on exploiting the collective phenomena of localized surface plasmons (LSPs) to increase the amount of light-induced heat for the control of material states and the generation of macroscopic assembled structures. Here, we clarify that microbeads covered with a vast number of Ag nanoparticles can induce a large PTE and generate a submillimeter bubble within several tens of seconds under the synergetic effect of the light-induced force (LIF) and photothermal convection enhanced by collective phenomena of LSPs. Control of the phase transition induced by such a “collective photothermal effect” enables rapid assembling of macroscopic structures consisting of nanomaterials, which would be used for detection of a small amount of proteins based on light-induced heat coagulation.

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INTRODUCTION There have been a number of attempts to control the material state in a microscopic space by utilizing the photothermal effect (PTE),1 and various biological applications of PTE include the destruction of pathogenic cells using metallic nanoparticles (NPs),2−5 cell and tissue imaging through a change in the optical response of an organic fluorescent dye, semiconductor quantum dots (QDs), or nanodiamond under laser irradiation,6,7 and high-density optical recording with a nanoscale thermal phase transition.8,9 There have also been reports on microfabrication using the PTE for improving lithium ion batteries by photothermal reduction of graphene oxide,10 threedimensional etching of agarose gel with an optically heated microneedle,11 and assembly of QDs, glycine molecules, and soft oxometalates on substrates by using optically generated microbubbles.12−14 Moreover, for optical control of the kinetic state of a material, many researchers in a variety of fields have been involved in fundamental and applied studies of the lightinduced force (LIF), which is utilized in optical tweezers for trapping small objects.15,16 The LIF enables trapping and control of the mechanical motion of nanoscale objects such as metallic NPs of several tens of nanometers in diameter.17,18 © 2014 American Chemical Society

Since localized surface plasmons (LSPs) in metallic NPs show sensitive changes in the optical response under the adhesion of biological materials, metallic NPs can be used as biosensors.19,20 In terms of chemical applications, it has been reported that the crystallization of glycine molecules can be accelerated by causing clustering with laser irradiation, and a 50 μm crystal was fabricated in a supersaturated solution within 30 s purely through the LIF without any light-induced heat.21 In these past studies, the PTE and LIF were used separately to assemble nanomaterials and molecules. We have noted with interest that the degrees of freedom of optical control can be dramatically increased if we choose photoresponsive materials exhibiting both a large PTE and strong LIF. In particular, theoretical predictions have shown that the collective phenomena of LSPs in metallic NPs produce an enhanced LIF for incident infrared (IR) light because of the large redshift and spectral broadening that occurs under optical trapping conditions;22 this was also demonstrated experimenReceived: June 27, 2014 Revised: July 15, 2014 Published: July 16, 2014 18799

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tally in coupled Ag nanorods.23 Other theoretical predictions revealed that the PTE under IR light irradiation can be enhanced in a small number of densely assembled metallic NPs of several nanometers in diameter.4 If we consider an assembled structure of a vast number of metallic NPs under dense concentration conditions24,25 then we can expect a further simultaneous enhancement of the PTE and LIF. Recent experimental reports show that chemically self-assembled metallic NPs (Au and Ag) on plastic beads exhibit a prominently red-shifted and broadband optical spectrum in the ultraviolet (UV) to near-IR region.26−28 These interesting properties of metallic NP-fixed beads are promising for generating enhanced PTE and LIF. In this letter, we report on an investigation into the IR-laserinduced dynamics of Ag NP-fixed beads (AgNP-FBs) and the possibility for controlling the biological material state based on the synergetic effect of both the PTE and LIF arising from the collective phenomena of LSPs. We attempted to control the macroscopic phase transition of the surrounding medium with this synergetic effect and investigated the potential application of the obtained principle to heat-denatured proteins.

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EXPERIMENTAL PROCEDURES Optical Trapping Setup. The IR continuous wave laser light (1064 nm) was introduced into the inverted microscope (Eclipse Ti−U; Nikon, Japan) using a back port adapter (LMSAD-NI-BP; Sigma-Koki, Japan) for the optical trapping (Figure 1a). The trapping laser was focused to a ∼1 μm diameter spot by a 100× oil immersion objective lens (numerical aperture = 1.30). The input laser power (0−2.0 W at the light source) was reduced to approximately 1/4 (0−500 mW) after the objective, which was measured by a power detector (UP17P-6S−H5 with TUNER; Gentec Electro-Optics, Canada). A sample-dispersed liquid (5 μL) was dropped onto the coverslip (thickness ∼0.17 mm) and set on the sample stage. The laser spot was set to be incident on the center of the sample droplet in the xy plane and at z = 5 μm, where the upper surface of the coverslip is at z = 0 μm. The optical trapping and assembling process was recorded (transmission image) during the 1 min laser irradiation under bright field conditions using a charge-coupled device (CCD) camera with a frame rate of 15 frames/s. Preparation of AgNP-FBs. Ag NPs (mean diameter: 3.7 nm; 0.0738 g/L) were densely self-assembled on each acrylate resin bead (mean diameter: ∼400 nm) through binder molecules.25 Plastic beads (50 mg) and 3 mL of the binder molecules (p-aminothiophenol; 0.01 M) were added to 300 mL of the Ag NP dispersed solution, and the mixture was stirred at room temperature for 3 h. After the beads were filtered, they were washed with an ample amount of water and dried in a vacuum. Each AgNP-FB consists of 64001 Ag NPs on the bead surface (Figure 1b). The concentration of obtained AgNP-FBs was 2.49 × 1011 beads/mL. For the optical measurement and optical trapping experiments, the AgNP-FB suspension was diluted to 1/50 (4.98 × 109 beads/mL) by ultrasonic dispersion. In addition, for the albumin solvent experiment, AgNP-FB suspension of 1/25 (9.96 × 109 beads/mL) and 1/10 (2.49 × 1010 beads/mL) were also prepared. Observation of Extinction Spectra and Raman Scattering Spectra. Measurement of the extinction spectrum in the Ag NP suspension was carried out using a UV−vis spectrophotometer (UV-2400-PC; Shimadzu, Japan). Raman scattering spectra were observed with a laser Raman microscope (RAMAN-DM; Nanophoton, Japan) at a wavelength of

Figure 1. (a) Experimental setup for photothermal assembly. (b) Schematic AgNP-FB (left) and scanning electron microscope (SEM) image (right). (c) Extinction spectra of AgNP-FBs and single Ag NPs. Spectra were normalized to the maximum values in the wavelength range shown.

532 nm. The laser beam was expanded into a line (400 points) and scanned over the sample, which was illuminated through the 100× objective. The exposure time for each line was 10 s. Albumin Dispersion. A total of 5 mg of powdered albumin from egg (Wako Pure Chemical Industries, Japan) was dissolved in 4.995 mL of ultrapure water on ice to a final concentration of ∼0.1%.

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RESULTS AND DISCUSSION Figure 1a shows the experimental setup for controlling the dynamics of AgNP-FBs (Figure 1b) dispersed in water. The extinction spectrum of the aqueous AgNP-FB suspension shows a significantly large red-shift and broadening in comparison with that of single Ag NPs before fixation on a bead (Figure 1c). Also, as theoretically discussed in ref 28, since the nonradiative absorption in used AgNP-FB is much higher than that of the scattering process in the IR region, the strong heat can be expected under the IR laser irradiation. Optical transmission images (Figure 2, panels a−d) taken during 0.2 and 0.6 W laser irradiation show that a bubble of several tens of micrometers in diameter was generated around the focused laser spot and that within several tens of seconds AgNP-FBs assembled on the surface of the bubble. Since no bubbles could be generated with higher laser powers in the case of suspensions of single Ag NPs or bare polymer beads (Figure 3), this phenomenon can be considered as being specific to 18800

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of the trapped AgNP-FBs accelerates the convection flow and leads to the generation of a bubble. The bubble size gradually increased, but convection continued because of the enhancement of the PTE as a result of the assembled AgNP-FBs. This continuous flow increased the number of AgNP-FBs adsorbed onto the surface of the bubble. The bubble size is to some extent limited and depends on the input laser power, which means that an equilibrium state exists at a given energy. To understand this tendency and the bubble generation, we theoretically evaluated the dependence of the bubble diameter DB on the laser power by assuming the model shown in Figure 4a. The difference between the transmitted

Figure 2. Snapshots (6 s intervals) of bubble generation with (a) 0.2 W laser and (b) 0.6 W laser. Recording started at the same time as laser irradiation. (c) and (d) Enlarged views of the trapped AgNP-FBs in white boxes in (a) and (b) before bubble generation with the 0.2 and 0.6 W lasers, respectively. (e) Schematic of the generation process of light-induced bubble in the AgNP-FB suspension. Figure 4. (a) Analysis model. (b) Bubble diameter DB as a function of the input laser power. Fits were obtained by changing the parameter α in eq 1. The bubble diameter was measured 1 min after the start of bubble generation for each input laser power, and the average of three time measurements was taken. Vertical bars indicate the standard deviation of DB. (c) Left: Raman spectra of assembled AgNP-FBs after IR-laser irradiation (input laser power: 0.2, 0.6, and 1.0 W) and selfassembled AgNP-FBs after natural drying. Right: Optical reflection images. Figure 3. Snapshots (6 s intervals) of dynamics of bare polymer beads (dispersed in 10 μL water with the concentration of 1.25 × 1010 beads/mL) with 1.0 W laser. Recording started at the same time as the laser irradiation.

laser power through an equivalent amount of water (5 μL) and the power through the suspension of the AgNP-FBs was determined by the power detector. Such a difference (ΔP) is considered as the absorption rate of light energy by AgNP-FBs. The total amount of absorbed energy is given by ΔPt, and it is assumed to be completely converted into heat (Figure 4b), where t is the laser irradiation time. DB is given by the thermodynamic equation as follows

AgNP-FBs. As shown in Figure 1c, the AgNP-FBs absorb 1064 nm light more efficiently than single Ag NPs and emit the absorbed energy as heat, causing the surrounding water to evaporate and the formation of a bubble. Careful observations of the bubble generation process (see the movies of the Supporting Information) led to an understanding of the assembly process (Figure 2e), in which a gradual convection flow, caused by the laser-induced heat just after irradiation, acts on the AgNP-FBs and results in some AgNP-FBs being trapped by the gradient force near the focal point of the laser. The PTE

3

DB = 2

H 2O ⎞ ⎛ 3 ⎜ m V gas ⎟ H 2O ⎟ 4π ⎜⎝ ρ V liq ⎠

(1)

where m = ΔPtα/[c(Tb − T0) + q] is the mass of H2O as vapor in the bubble, ρ = 1.0 g/cm3 is the density of liquid H2O, Tb = 18801

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373 K (100 °C) is the boiling point of water, T0 = 298 K (25 °C) is taken as room temperature, c = 4.18 J/gK is the specific heat of H2O around T0, q = 2259 J/g is the vaporization heat of H2O, VHgas2O = 24.8 L is the volume of 1 mol H2O vapor, and HO Vliq2 = 18 mL is the volume of 1 mol liquid H2O at standard ambient temperature and pressure. The irradiation time was set to t = 60 s in Figure 4b. The laser intensity is significantly strong near the laser spot. Therefore, it was assumed that part of the light energy ΔPtα absorbed by the AgNP-FBs in a volume of the cone-shaped region Vspot (shaded region of Figure 4a) mainly contributes to the generation of a bubble in the total laser-irradiated volume Vlaser rather than ΔPt, where the parameter α is a multiple of α0 = Vspot/Vlaser. The parameter α was varied to investigate the dependence of DB on the input laser power as shown in Figure 4b; the experimental data were well-fitted by assuming α = 10α0. Since DB is proportional to α1/3, this result implies that the size of the generated bubble depends on the heat arising from the AgNP-FBs around the region with a length dimension 2 to 3 times larger than the cone-shaped region (101/3 ≈ 2.15). Laser Raman spectroscopy was employed to investigate the optically assembled structure around the bubble (Figure 4c). For the Raman scattering measurement, assembled AgNP-FB samples were dried after soft destruction of the bubble with an ultrasonic wave because simultaneous observation of optical trapping and Raman scattering is difficult with our experimental system. The assembled AgNP-FBs formed a ringlike structure, independent of the input laser power for the bubble generation (right upper panel in Figure 4c), and the signal originating from the region corresponding to the bubble surface was much stronger than that originating from the focal point. The AgNPFBs are considered to be deposited at the stagnation region between the substrate and bubble as a template.13 For comparison, we also observed Raman scattering from the region, including the highest density of AgNP-FBs after natural drying in the absence of laser irradiation (right lower panel in Figure 4c). The Raman spectrum in this case can be attributed to p-aminothiophenol as the binder fixing the Ag NPs to the polymer bead.27 The maximum Raman scattering signals from optically assembled AgNP-FBs for laser powers of 0.2, 0.6, and 1.0 W were approximately twice as high as the average value of the signal from a rectangular region (400 × 50 pt) of the naturally dried structure. This would indicate that the AgNPFBs were densely assembled by the IR laser, and that the number of binder molecules contributing to the Raman scattering increased in the observed region. We also confirmed that the spectral shape was modified when a high-intensity (0.6 or 1.0 W) laser irradiated the sample for a longer time of several tens of minutes. This was thought to be due to heat degeneration of the beads by the PTE. These results indicate that there is an optimum irradiation time and laser power for the AgNP-FBs that maintain the original properties. Since water vaporization occurs during bubble generation, we expected the local temperature of the AgNP-FB suspension to be very high around the laser focal point. We thus added 5 μL of aqueous albumin (0.10%) solution to 5 μL AgNP-FB suspensions with different concentrations to investigate the temperature change (Figure 5, panels a−c, and the movies of the Supporting Information). Remarkably, we found that the mixed solvent solidified in a region several tens of micrometers wide (much larger than the laser focused area), even with lowdensity AgNP-FBs (4.98 × 109 beads/mL; Figure 5a). The local temperature around the laser spot was thus at least 78 °C,

Figure 5. Solidification of albumin by photothermal effect of AgNPFBs in the case of (a) low density, (b) medium density, and (c) high density. Input laser power was set to 1.0 W.

which is the solidification point of albumin. Also, from Figure 4b, the input laser power dependence of bubble diameter indicates that the vapor of H2O with boiling point 100 °C was the main component of the generated bubble, which is consistent with this result. In examining the PTE of AgNPFBs on the heat coagulation of albumin, we found that suspensions with a higher AgNP-FB concentration (9.96 × 109 and 2.49 × 1010 beads/mL; Figure 5, panels b and c) led to clustering of the AgNP-FBs due to the high viscosity of the albumin solution. As a result, higher concentration AgNP-FB suspensions provide a higher probability of rapid heat coagulation to make the area around the laser spot transparent and assemble more AgNP-FBs around the solidified region. This indicates that clusters of AgNP-FBs lead to highly efficient heat generation. If the initial albumin concentration (1 mg/ mL) is maintained, the total mass of solidified albumin in the mixed suspension of AgNP-FBs can be estimated as 0.5 mg/mL × (4π/3) × (8 μm)3 = 1.0 pg (the diameter of the transparent region is about 16 μm from Figure 5c). While this is only a rough estimation, there is a possibility of observing a few picograms of albumin in the area around the laser spot by collective photothermal effect of AgNP-FBs. This would enable us to detect a significantly small amount of heat-denatured protein within a very short time, ranging from several seconds to several minutes. Moreover, paying attention to conventional biosensors such as ELISA29,30 that can detect a few picograms of analytes, the above estimated value is comparable to this. In addition, a commercial protein detection kit requires several hours to detect about 100 pg−5 ng of protein, whereas the method here would pave the way for rapid and highly sensitive detection of heat-denatured biomaterials with different solid18802

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ification points. In fact, we have confirmed that the aqueous solution of egg white including albumin as the major component31 can be solidified under the same condition in Figure 5. The tendency of the solidification process near the laser spot and the assembling of AgNP-FBs is similar to the result for albumin. Furthermore, the similar experiment was performed in the solution of egg yolk, including the lipoprotein (lipovitellin) as the major component,32 where the solidification and the assembling of AgNP-FBs started within a few seconds after the laser irradiation, faster than those of egg white. Similarly to the boiled egg, such a difference would be attributed to the components in the egg white and the egg yolk, respectively.

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CONCLUSION In conclusion, we have studied the process of a submillimeter phase transition under the synergetic effect of the LIF and PTE using a suspension of AgNP-FBs irradiated by an IR laser. The experimental conditions for fabricating macroscopic structure of assembled AgNP-FBs that maintain their original properties were determined, and remarkably, the phase transition of several picograms of heat-denatured proteins could be induced through photothermal assembling processes of AgNP-FBs. These results will open up a promising avenue for highly efficient bottom-up fabrication methods of small objects based on self-induced PTE leading to “photothermal chemistry” and form the basis for the microfabrication, the medical application, and for the rapid and highly sensitive biosensors.

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ASSOCIATED CONTENT

S Supporting Information *

Movies of assembling processes of AgNP-FBs and solidification of proteins. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions #

S.T. and T.I. contributed equally.

Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS The authors would like to thank Mr. S. Hidaka, Mr. M. Tamura, Prof. C. Kojima, Prof. H. Shiigi, Prof. T. Nagaoka, Prof. H. Ishihara, and Prof. H. Miyasaka for their useful advice and kind support. A major part of this work was supported by Special Coordination Funds for Promoting Science and Technology from MEXT [Improvement of Research Environment for Young Researchers (FY 2008-2012)], Grants-in-Aid for Exploratory Research (Grants 23655072, 24654091, and 26610089), Grants-in-Aid for Young Researcher (A) (Grant 24685013), and a Grant-in-Aid for Scientific Research (B) (Grants 23310079 and 26286029) from JSPS.

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