Thermal properties of the suspended nanoparticles

Chapter 3 Experiment

3.3. Thermal properties of the suspended nanoparticles

The thermal property of materials which we used is shown in Table 3.1 and Table 3.2.

Table 3.1

Chapter 4 Two-dimensional Assembly Formation of Various Nanoparticles Suspended in Aqueous Solution

4.1. Introduction

Based on the numerical prediction in Chapter 2, we know that the temperature of gold nanoparticle is elevated by laser irradiation due to large adsorption cross section, as given by the calculation using Mie theory. Such the temperature elevation of a single gold nanoparticle (we called this single gold nanoparticle as “mother gold nanoparticle”) attracts the nanoparticles suspended in the solution to the mother gold nanoparticle via convection flow, which is induced by the temperature gradient. We found that the gathered nanoparticles form a two-dimensional assembly. In this study, we employed 1064-nm CW laser tightly focused on the mother gold nanoparticle (d: 200 nm) in the presence of a various kind of nanoparticles suspended in aqueous solution. Dark-field microscopy was utilized to observe the scattering light around the mother gold nanoparticle.

4.2. Two-dimensional assembly formation of fluorescent polystyrene beads

Fluorescent polystyrene beads (with excitation wavelength at 488 nm) are used as suspended nanoparticles. We observed that the scattering light intensity increased when the mother gold nanoparticle is irradiated, as shown by a series of images in Fig. 4.1. Importantly, the scattering light intensity after irradiation is higher than that before irradiation. To confirm such scattering light intensity change, we evaluate the fluorescence image around the mother gold nanoparticle by using confocal microscopic imaging and SEM, as shown in Fig. 4.2 and Fig. 4.3, respectively. As the result, they clearly show that there is a doughnut-shaped two-dimensional assembly formation of the fluorescent polystyrene beads around the mother gold nanoparticle. The assembly formation is also supported by the line profile of the fluorescence image, where the fluorescence intensity around the center is extremely higher than surrounding area (Fig. 4.2 (b)). The local minimum of fluorescence intensity at the center indicates that there are no polystyrene beads on the top of the mother gold nanoparticle. Based on this finding, we interpret that during laser irradiation there are a large number of polystyrene beads gathered around the mother gold nanoparticle by either gradient force due to optical trapping phenomenon or convection flow related to laser heating. The temporary gathered polystyrene beads are not entirely formed into an assembly, and thus they partially are dispersed into the surrounding area when the trapping laser is switched off. Therefore, we

only observed two-dimensional assembly formation attached on the glass substrate.

Fig. 4.1 A series of dark-field scattering images of the mother gold nanoparticle (d: 200 nm) upon a focused laser beam irradiation in fluorescent polystyrene beads (d: 50 nm) solution.

The scattering intensity of the mother gold nanoparticle (a) before laser irradiation, (b) during laser irradiation, and (c) after switching off laser by a programmed mechanical shutter after 1 second irradiation. The suspension polystyrene beads are too small to be observed from dark-field image. The laser power, irradiation time, and particle density of solution are 0.5 W, 1 second, and 3.9×10¹¹ particles/mL, respectively.

Fig. 4.2 (a) Fluorescence image shows a doughnut shape of an assembly formation of fluorescent polystyrene beads (d: 50 nm) around the mother gold nanoparticle (d: 200 nm).

The hollow center is the position of the mother gold nanoparticle and brighter green signal is the fluorescence intensity of polystyrene. (b) Fluorescence intensity profile gives large intensity around the center.

(a) (b)

Mother gold nanoparticle (200 nm)

Polystyrene beads (50 nm)

(a) (b) (c)

Fig. 4.3 SEM image of two-dimensional polystyrene beads (d: 50 nm) assembly upon a focused laser beam irradiation on the mother gold nanoparticle (d: 200 nm). The laser power, irradiation time, and particle density of solution are 0.5 W, 1 second, and 3.9×10¹¹ particles/mL, respectively.

4.2.1. Probability of two-dimensional assembly formation 4.2.1.1. Power dependence

Experimentally, we examined 30 mother gold nanoparticles as the samples under various laser powers with a fixed irradiation time of 1 second. We found that the two-dimensional assembly formation of the polystyrene beads depends on the laser power. The probability of the assembly, defined as the number of assembly divided by the total number of samples done under the same condition, as a function of laser power is shown in Fig. 4.4 (a). In contrary to the assembly formation, we also plot the probability of disappearance, which means that the mother gold nanoparticle evaporated or melted and then detached from substrate. We also repeated the same experiment for the mother gold nanoparticle in pure water solution in the absence of polystyrene beads as reference and the result is shown in Fig. 4.4 (b).

Mother gold nanoparticle (200 nm)

Polystyrene beads (50 nm)

(a) (b)

Fig. 4.4 (a) Probability of polystyrene beads (d: 50 nm) assembly and the mother gold nanoparticle (d: 200 nm) disappearance in fluorescent polystyrene beads solution (particle density is 3.9×10¹¹ particles/mL). (b) Probability of the mother gold nanoparticle disappearance in pure water solution. The irradiation time was fixed at 1 second.

Fig. 4.4 shows that the assembly probability increases with laser power, in contrast to disappearance that decreases with laser power. We can observe that the disappearance probability of gold nanoparticle is 100% above 200 mW laser power in pure water solution, but suppression of laser-induced evaporation of the mother gold nanoparticle is observed in the solution containing fluorescent polystyrene beads.

4.2.1.2. Irradiation time

Since temperature of the mother gold nanoparticle can reach evaporation point due to temperature elevation under long time irradiation, we focus on irradiation time below 1 second as shown in Fig. 4.5 and Fig. 4.6. The irradiation time can be operated at the lowest

time of 0.1 millisecond by using a programmed mechanical shutter. We examined 10 mother gold nanoparticles as the samples under various irradiation times. We observe that the probability of assembly fluctuates around the value which corresponds to those obtained in Fig. 4.4 (a). Such behavior was found under different laser power respectively, and we know that the time of assembly was quite fast just below than 2 millisecond. The results indicated that irradiation time dependence of the assembly formation is out of our instrument resolution.

Fig. 4.5 Irradiation time dependence on assembly probability in suspension fluorescent polystyrene beads solution (particle density is 3.9×10¹¹ particles/mL) with different laser power (a) 300 mW, (b) 500 mW, (c) 700 mW, and (d) 900 mW from 0.1 to 1000 millisecond irradiation time scale.

(a) (b)

Fig. 4.6 Irradiation time dependence on assembly probability in suspension of fluorescent polystyrene beads solution (particle density is 3.9×10¹¹ particles/mL) with different laser power (a) 300 mW, (b) 500 mW, (c) 700 mW, and (d) 900 mW. Here we only focused the time scale from 0.1 to 2 millisecond.

(a) (b)

4.2.1.3. Particle density dependence

Fig. 4.7 The assembly probability with various particle density of polystyrene beads solution (a) pure water, (b) 3.9×10⁹, (c) 3.9×10¹⁰, (d) 3.9×10¹¹, (e) 3.9×10¹² and (f) 3.9×10¹³ particles/mL. Green dashed line means laser power where take place at equivalent point of assembly and disappearance probability. The irradiation time was fixed at 1 second.

(a) (b)

(e) (f)

Fig. 4.7 shows that the assembly probability depends on particle density of polystyrene beads in the solution. Higher particle density means that higher numbers of nanoparticles are gathered around the mother gold nanoparticle. The threshold between assembly and probability shifted to lower laser power when particle density was increased. The shifting down of the threshold as a function of particle density indicates that the mother gold nanoparticle needs more nanoparticles assembled to suppress temperature elevation and to avoid evaporation of the mother gold nanoparticle.

4.2.2. Size of two-dimensional assembly

Based on assembly probability results, we know that the assembly formation is induced under various laser power. Therefore, in this section, we focused on the size of two-dimensional formation with various laser powers and did experiments under the same condition as it is in the previous section.

4.2.2.1. Power dependence

By SEM observation, we can quantify the size of assembly formation precisely. There are five to ten samples were recorded for each point and the diameter was the average diameter of formed samples assembly for each power, respectively. Fig. 4.9 shows that the

assembly size was increased with incident laser power, indicating that higher laser power can enlarge heated area and can induce the convection flow more efficiently.

Fig. 4.8 The size of polystyrene beads assembly formation increased with laser power. The irradiation time and particle density of solution are 1 second and 3.9×10¹¹ particles/mL, respectively.

Fig. 4.9 SEM images of two-dimensional assembly formed at different laser powers (a) 0.3W, (b) 0.5 W, (c) 0.7 W and (d) 0.9 W. The irradiation time and particle density of solution are 1 second and 3.9×10¹¹ particles/mL, respectively.

(b)

(d) (a)

Gold nanoparticle (200 nm)

Polystyrene beads (50 nm)

(c)

4.2.2.2. Particle density

In this section, we employed two different particle densities of suspended polystyrene

beads solutions of 3.9×10¹² and 3.9×10¹¹ particles/mL. Fig. 4.10 shows that the assembly size was not depend on particle density obviously, indicating that higher particle density can not affect the assembly size so much in contrast to power effect.

Fig. 4.10 SEM images of two-dimensional assembly formed under different particle densities, (a) 3.9×10¹² particles/mL and (b) 3.9×10¹¹ particles/mL, respectively. The laser power and irradiation time are 0.7 W and 1 second, respectively.

4.2.2.3. Irradiation time dependence

Since the temperature elevation induced by laser irradiation is time dependent, we focus here on different irradiation time such as 200 milliseconds, 1 second and 2 seconds, respectively. Fig. 4.11 shows that assembly size is increased with irradiation time from 200 milliseconds to 1 second, but the assembly size does not grow up further when the irradiation

1.9 m

time more than 1 second. We can observe that the assembly size is time dependent when irradiation time below 1 second but it is time independent when irradiation time above 1 second.

Fig. 4.11 SEM images of two-dimensional assembly formation at different irradiation times, which are (a) 200 milliseconds, (b) 1 second, and (c) 2 seconds, respectively. The laser power and particle density of solution are 0.5 W and 3.9×10¹¹ particles/mL, respectively.

0.4 m 0.4 m

(a)

1.3 m

(b)

1.4 m

(c)

4.3. Assembly formation of quantum dots, gold nanoparticles, and PNIPAM molecules

Here we applied this method to different materials such as semiconductor (quantum dots) and metallic nanoparticles (gold nanoparticles). We also observe two-dimensional assembly formation upon a focused laser beam on the mother gold nanoparticle in a quantum dots (d:

20 nm) solution similarly to the case of polystyrene beads. The assembly probability, scattering intensity image, and SEM image is shown in Fig. 4.12, Fig. 4.13 and Fig. 4.14, respectively.

Fig. 4.12 Probability of quantum dots (d: 20 nm) assembly and the mother gold nanoparticle (d: 200 nm) disappearance in quantum dots solution. The irradiation time and concentration of solution are 1 second and 8 nM, respectively.

Fig. 4.13 A series of dark-field scattering images of the mother gold nanoparticle (d: 200 nm) upon a focused laser beam irradiation in quantum dots solution. The scattering intensity of the mother gold nanoparticle (a) before laser irradiation, (b) during laser irradiation, and (c) after switching off laser by a programmed mechanical shutter after 1 second irradiation. The suspension quantum dots (d: 20 nm) are too small to be observed from dark-field image. The laser power, irradiation time and concentration of solution are 0.5 W, 1 second and 8 nM, respectively.

Fig. 4.14 SEM image of two-dimensional quantum dots (d: 20 nm) assembly upon focused laser beam irradiation on single gold nanoparticle (d: 200 nm). The laser power, irradiation time and concentration of solution are 0.5 W, 1 second and 8 nM, respectively.

Gold nanoparticle (200 nm)

Quantum dots (20 nm)

(a) (b) (c)

We used smaller gold nanoparticles (d: 100 nm) as assembled nanoparticles in solution, and was similarly irradiated the mother gold nanoparticle. The assembly probability is shown in Fig. 4.15. The assembly formation in the gold nanoparticles case shows different phenomena compare with polystyrene beads and quantum dots. Microbubble was formed by focused laser beam heating, as shown in Fig. 4.16. Due to the microbubble formation, we obtained another shape of two-dimensional assembly formation and this rose-like-shaped concentric multiple-rings was shown in Fig. 4.17. The mechanism of these two different types of assembly will be discussed in the next section.

Fig. 4.15 Probability of gold nanoparticles (d: 100 nm) assembly and the mother gold nanoparticle (d: 200 nm) disappearance in gold nanoparticles solution. The irradiation time and particle density of solution are 1 second and 5.6×10⁶ particles/mL, respectively.

Gold nanoparticles (100 nm)

Fig. 4.16 A series of dark-field scattering images of the mother gold nanoparticle (d: 200 nm) upon a focused laser beam irradiation in gold nanoparticles (d: 100 nm) solution. The scattering intensity of the mother gold nanoparticle (a) before laser irradiation, (b) and (c) during laser irradiation, and (d) after switching off laser by a programmed mechanical shutter after 900 second irradiation. Images of (c) and (d) were taken under reduced intensity of illumination light. The laser power, irradiation time and particle density of solution are 0.5 W, 900 second and 5.6×10⁶ particles/mL, respectively.

Fig. 4.17 SEM image of two-dimensional gold nanoparticles (d: 100 nm) assembly upon a focused laser beam irradiation on the mother gold nanoparticle (d: 200 nm). A rose-like multiple-rings structure is formed around the mother gold nanoparticle while the mother gold nanoparticle disappears. The laser power, irradiation time, and particle density of solution are 0.5 W, 900 second and 5.6×10⁶ particles/mL, respectively.

(a) (b)

We used poly(N-isopropylacrylamide) (PNIPAM) as assembled nanoparticles in solution, and employed the same laser irradiation for the mother gold nanoparticle. Based on light scattering spectrum and CCD image which shown in Fig. 4.18, we know that PNIPAM assembly around gold nanoparticle after laser irradiation and the heated area increases with laser power. The dark-field observation revealed that PNIPAM molecules exhibited phase transition when they were transported to the vicinity of heated gold nanoparticle. We find the scattering spectrum peak shift to longer wavelength during and after laser irradiation (Fig.

4.18(d)), which indicates that the PNIPAM is gathered on gold nanoparticle surface.

Fig. 4.18 A series of dark-field scattering images of the mother gold nanoparticle (d: 200 nm) upon a focused laser beam irradiation in PNIPAM solution (500 mg/mL). The scattering intensity of the mother gold nanoparticle (a) before laser irradiation, (b) during laser irradiation, and (c) after switching off laser by a programmed mechanical shutter after 60 second irradiation. The heated suspended PNIPAM near the focal spot can be observed from dark-field image. The laser power, irradiation time and concentration of solution are 0.7 W, 43 second and 200 mg/mL, respectively. (d) The scattering spectra show the intensity changed before, during and after laser irradiation.

(d)

(a) (b)

(c)

According to thermo phase transition of PNIPAM, laser-induced temperature elevation is clearly observed. Then we confirm that the heat transfer from the mother gold nanoparticle to surrounding medium and the heated area are enlarged with laser power, as shown in Fig. 4.19 and Fig. 4.20. The mechanism of PNIPAM assembly also will be discussed in the next section.

Fig. 4.19 Dark-field scattering image and scattering light intensity profile with various laser powers (a) 0.3 W, (b) 0.5 W and (c) 0.7 W. The irradiation time and concentration of solution are 40 second and 500 mg/mL, respectively.

Fig. 4.20 The size of PNIPAM molecule assembly formation increased with laser power. The irradiation time and concentration of solution are 40 second and 500 mg/mL, respectively.

(a) (b) (c)

4.4. Discussion

4.4.1. Probability of two-dimensional assembly formation

The result presented in previous section reveals that the assembly probability depends on laser power and particle density. A higher laser power enhanced heated area from focused position to surrounding medium and induced convection flow not only broader but also more efficiently, while higher particle density meant much more nanoparticles at near the focused spot. Larger heated area and higher particle density contributes to accumulate nanoparticle efficiently and suppresses the evaporation of heated mother gold nanoparticle due to temperature elevation. Based on the result, we know that temperature reaches an equilibrium within less than 0.1 millisecond, and then convection flow and assembly formation are induced simultaneously. Therefore, the assembly of nanoparticles is formed efficiently within a short irradiation time, and laser power and particle density are the main factors governing the probability of the assembly formation.

4.4.2. Size of two-dimensional assembly

According to our results, the assembly size depends on laser power obviously and not depends on particle density. From the above section, we know that the laser power affect not only assembly probability but also assembly size, due to higher laser power enlarged heated area and induced convection flow more efficiently. From Fig. 4.10, we know that particle

density does not affect assembly size markedly but assembly probability obviously. It strongly suggests that suppression of temperature elevation does not depend on numbers of assembly nanoparticles dominantly but depends on numbers of suspended nanoparticles in the vicinity around the mother gold nanoparticle. Therefore, the results show particle density dependence of assembly probability but not of assembly size. The assembly size increases with irradiation time till 1 second but stops to grow over 1 second. Therefore, laser power and irradiation time are the main factors of nanoparticles assembly size.

4.4.3. Two-dimensional assembly formation mechanism

For polystyrene beads, quantum dots and PNIPAM molecules, we observed two-dimensional assembly around the mother gold nanoparticle as typically shown in Fig. 4.3, Fig. 4.14 and Fig. 4.19. Since the assembly size is much larger than that of the laser spot, convection flow generated near the gold nanoparticles should contribute to the nanoparticles accumulation. On the other hand, in the case of gold nanoparticles assembly, they form concentric multiple-rings structure around the irradiated mother nanoparticle while the mother nanoparticle disappeared as exhibited in Fig. 4.17. It is noteworthy that thermal bubbling is observed at the laser focus during the accumulation only in this case. This strongly suggests that photoabsorption and energy dissipation modes depend on the coupling of the mother gold nanoparticle and gathered nanoparticles, leading to different accumulation and structure

formation.

Photothermal heating of gold nanoparticle and heat transfer to medium take place upon focused laser beam irradiation on the mother gold nanoparticle. Then, temperature gradient induced by heat transfer from the mother gold nanoparticle with local heating to surrounding medium leads convection flow. Heat-induced convection flow results in mass transfer toward the heated position. Suspended nanoparticles in solution were drawn toward to the mother gold nanoparticle via convection flow and formed assembly around the mother gold nanoparticle. The nanoparticles are fixed on substrate through electrostatic force between nanoparticles and substrate. The nanoparticles were adhered strongly on the substrate after removing solution, the result indicate that the binding of the nanoparticles to substrate is quite stable. The assembly formation of these two different mechanisms was shown in Fig. 4.21, Fig. 4.22 and Fig. 4.23, respectively.

Fig. 4.21 The schematic illustration of the assembly formation mechanism in the cases of polystyrene beads and quantum dots. (a) Photo-thermal heating of the mother gold

(a)

(d) (e)

(b) (c)

nanoparticle by focused laser beam and heat transfer to surrounding medium. (b) Convection flow was induced due to temperature gradient by laser heating and drew nanoparticles toward to the mother gold nanoparticle. (c) Nanoparticles adsorbed on the mother gold nanoparticle, giving two-dimensional assembly. (d) Heat transfer from the mother gold nanoparticle to surrounding medium suppressed the former temperature elevation leading to no evaporation.

(e) After switching off laser, nanoparticles assembly formation was finished.

Fig. 4.22 The schematic illustration of the assembly formation mechanism in the case of gold nanoparticles. (a) Photo-thermal heating of the mother gold nanoparticle by focused laser beam and heat transfer to surrounding medium. (b) Convection flow was induced due to temperature gradient by laser heating and drew nanoparticle toward to the mother gold nanoparticle. (c) Gold nanoparticles were adsorbed on the mother gold nanoparticle, giving two-dimensional assembly. (d) Heat transfer from the mother gold nanoparticle to surrounding medium suppressed the former temperature elevation leading to no evaporation.

(e) Fusion of gold nanoparticles enhanced temperature elevation efficiently and formed a microbubble on the mother gold nanoparticle surface. (f) Temperature elevated efficiently near focused spot led to evaporation of the mother gold nanoparticle, inducing wider heated area and forming the microbubble. Due to the microbubble formation, we obtained concentric shape of the two-dimensional assembly. (g) After switching off laser, nanoparticles assembly formation was finished, while the mother gold was not observed anymore.

(a) (b) (c) (d)

(e) (f) (g)

Fig. 4.23 The schematic illustration of the assembly formation mechanism in the case of PNIPAM molecules. (a) (b) Photo-thermal heating of the mother gold nanoparticle by focused laser beam and heat transfer to surrounding medium. PNIPAM has phase transition to globule at heated area. (c) Convection flow was induced due to temperature gradient by laser heating, and PNIPAM comes to heated area around the mother gold nanoparticle via convection flow and goes phase transition giving globule conformation. (d) PNIPAM molecules were adsorbed on the mother gold nanoparticle, and forming two-dimensional assembly. (e) Heat transfer from the mother gold nanoparticle to surrounding medium suppressed the former temperature

在文檔中雷射聚焦單一金奈米造成溫度上升引發二維組裝的結構之研究 (頁 47-0)