Highly Efficient Near-Infrared Photothermal Conversion of Single

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Highly Efficient Near-Infrared Photothermal Conversion of a Single Carbon Nanocoil Indicated by Cell Ejection Peng Wang, Lujun Pan,* Chengwei Li, and Jia Zheng School of Physics, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116024, P. R. China

J. Phys. Chem. C Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/27/18. For personal use only.

S Supporting Information *

ABSTRACT: Carbon nanocoil (CNC), which has excellent properties of near-infrared (NIR) absorption and photoelectrical response, is considered as a potential NIR sensing and bioengineering material. In this work, a high-speed living cell ejection is realized in a yeast cell solution based on the photothermal conversion induced by a NIR laser irradiation on the surface of a CNC. The cell ejection reveals a thermal convection of solution induced by the laser irradiated CNC and can be used to evaluate the photothermal conversion ability of CNC. The dynamic behavior of the thermal convection behind cell ejection is studied experimentally and analytically. It is found that the initial solution flow velocity of the dynamic process reaches more than 103 μm/s. Approximately, 60% of the laser energy illuminated on the CNC is converted into thermal energy. The unique helical morphology of CNC enables its high NIR photothermal conversion efficiency. The average convective heat transfer coefficient on the contact area between CNC and surrounding water is deduced to be as high as 7.0 × 105 W/(m2·K). These results indicate that CNC has promising potential applications on microfluidics, laser-operated flow cytometers, bioparticle ejection, and micro-/nano-laser-operated heat generators and exchangers.

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INTRODUCTION Because of great demand for solar energy collection and conversion, thermal imaging, and biological fields, various nanomaterials with high efficient near infrared (NIR) photothermal conversion have been developed.1−3 Among them, carbon-based nanostructures, such as carbon nanotubes (CNTs) and carbon nanodots (CNDs), are developed for photophysical and biological applications because of their low toxicity, intrinsic biocompatibility, and excellent NIR response properties. CNT films and single CNTs show sensitive bolometric NIR photoresponse with short response time and have been fabricated as high-performance NIR sensors and bolometers.4−7 CNDs and CNTs have been used as powerful NIR photothermal agents for biological theranostics and thermal therapy.8−11 However, for above photothermal applications based on CNDs and CNTs, especially for single CND/CNT-based devices, the utilization efficiency of NIR light is low because of their low NIR photothermal conversion efficiency and tiny size. Moreover, the preparation and fabrication process are time-consuming and expensive. As an important branch of carbon nanomaterials, carbon nanocoils (CNCs) are known for their spring-like morphologies. CNCs have been confirmed to have excellent mechanical,12−15 thermal,16 and electrical properties17 and good dispersibility in water. CNCs have many advantages in NIR absorption and utilization. First, because of its unique helical structure and larger line diameter, a single CNC has a longer effective length and surface area for harvesting the NIR © XXXX American Chemical Society

photons, which enhances the utilization efficiency of incident NIR light. Second, a single CNC can be manipulated quite easily under an ordinary optical microscope,18 which benefits the device fabrication process. Finally, CNCs have outstanding NIR photophysical properties. Wen et al. found that the light emission from single CNCs was induced by the synergistic effect of photo- and thermal excitations.19 Ma et al. studied the electrically driven thermal radiation spectra and photoluminescence (PL) spectra of single CNCs.20 They have found that raw CNCs have at least four pairs of energy bands exist around the Fermi energy level, which are suitable for the absorption of wavelengths from 600 to 900 nm. Thus, CNCs may have strong NIR absorption ability. Then, Ma et al. investigated the NIR (λ = 785 nm) response of a single CNC.21 It is found that with absorbing NIR laser energy, the conductivity of the CNC increases significantly in approximately 5 ms because of the bolometric effect. Sun et al. investigated the microbubbles generated by NIR laser induction (λ = 785 nm) on an individual CNC immerged in deionized water because of the conversion from photon energy to thermal energy.22 Therefore, CNCs have great potential to be applied in high performance NIR photothermal conversion devices. However, the NIR photothermal conversion ability of CNC have not been studied quantitatively yet. Received: August 19, 2018 Revised: November 2, 2018 Published: November 7, 2018 A

DOI: 10.1021/acs.jpcc.8b08061 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Here, we report a high-speed cell ejector induced by a NIR laser on a single CNC in a yeast cell solution. The physical mechanism behind in this phenomenon is studied experimentally and analytically. It is evidenced that CNC has excellent NIR photothermal conversion and heat transfer abilities, which may be greatly used in optofluidics, laseroperated flow cytometers, and bioparticle ejection.

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EXPERIMENTAL DETAILS CNCs used in this study were synthesized by a thermal chemical vapor deposition (CVD) method using Fe−Sn catalysts.23 A single CNC was fixed onto a tungsten probe with silver paste and was drawn from as-grown CNC grass. Then, the as-prepared CNC was adhered horizontally to a transparent sample chamber, as shown in Figure 2. A diluted yeast cell solution (106/mL) was dripped into the chamber at room temperature. A NIR laser (λ = 785 nm) with a tunable output power (0−80 mW) was focused into the chamber. The focus radius is approximately 2 μm. The dynamic process is monitored by a charge-coupled device camera at 100 frames/s. In the experiments, a toroidal illumination is used to reduce image artifacts and provide high sample contrast.

Figure 2. Schematic diagram for the experimental system under a laser illumination (not in scale). ω0 and r are radius of the laser focus and the trapped yeast cell, respectively.

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RESULTS AND DISCUSSION Design and Fabrication. A single CNC was fixed onto a tungsten probe with silver paste and was drawn from CVDgrown CNC grass. Figure 1 shows the optical micrograph and

Figure 3. Snapshots of “cell ejection” under the laser power of 76 mW. (a−c) Yeast cell is trapped by a focused laser and is moved toward a single CNC. (d−i) Cell is ejected out fleetly and keeps moving for 50 ms.

shown in Figure 3d−i. Figure 3i shows the final position of the cell. The whole dynamic process is shown in Movie S1. The velocity of the cell versus time (v−t curve), under the laser power (P0) of 76 mW, is plotted in Figure 4a. It is found that the maximum ejection velocity (at 10 ms) can reach as high as 340 μm/s. Because of their extremely tiny mass of several nanograms and density similar to water, yeast cells are suitable for revealing the water flow. The cell ejection indicates that there is a high-speed water flow occurs when CNC is illuminated by laser. The relationship between the velocities of the cell at 10 ms and the laser power (P0) is shown in Figure 4b. It is found that higher laser power induces higher ejected speed of the cell. In particularly, there is no such a phenomenon occurs on metallic nanomaterials, such as a gold nanowire, a copper nanowire, and even a CNC covered with 300 nm thick gold film. This is a unique phenomenon, which comes from CNCs’ excellent NIR photothermal properties, rather than optical force effect. The CNC is irradiated by the laser beam when it contact with the trapped cell. With laser irradiating, the photo-energy is absorbed by CNC and is converted to thermal energy promptly. Then, the thermal energy accumulates and releases to the surrounding solution rapidly and continually. The CNCs unique helical morphology with large specific surface areas benefits this process. The temperature difference between CNC and

Figure 1. Optical micrograph of a single CNC. The inset is the SEM image.

the SEM image of the single CNC, which indicates that CNC has unique spring-like morphology. Then, the as-prepared CNC was immersed into a diluted yeast cell solution under an optical microscopy. Figure 2 shows the schematic diagram for the experimental system. The as-prepared CNC was adhered horizontally to a transparent sample chamber. Then, a diluted yeast cell solution was dripped into the chamber at room temperature and submerges the CNC. A NIR laser (λ = 785 nm) focused into the chamber, which can trap single yeast cell optically.24 Observation of Yeast Cell Motion. When a living yeast cell trapped by a focused laser steadily, the centers of the two are coincident. The CNC fixed on the chamber is moved slowly to the trapped cell slowly by adjusting the sample chamber, as shown in Figure 3a−c. With CNC approaching, the trapped yeast cell was ejected immediately and kept moving for dozens of milliseconds. The dynamic process is B

DOI: 10.1021/acs.jpcc.8b08061 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. (a) Dependence of the cell motion velocity on the time under different laser illuminating powers. (b) Velocity of the cell at 10 ms vs the power of laser (P0).

Figure 5. (a) Velocity distribution of the solution domain. The dots represent real-time positions and velocities of the yeast cell at every 10 ms, from 10 to 50 ms under the PQ of 0.5 mW. (b) Temperature distribution of the solution domain at 50 ms under the PQ of 0.5 mW. The inset is temperature distribution of CNC. (c) Time-dependent velocities with different PQ values. (d) PQ values vs the laser power P0.

square error (marked as S) of the analytical and the experimental v−t curve, according to eq S15 (see more details in Supporting Information). Then, the photothermal conversion efficiency of CNC can be deduced according to PQ = η· PCNC, where PCNC is the laser power illuminated on CNC. Figure 5a shows the simulated velocity distribution of solution at 50 ms under the PQ of 0.5 mW, where PQ is the thermal power generated on CNC. The initial ejection velocity reaches as high as 1100 μm/s, which shows CNCs promising potential application in microfluidic system. A particle tracing node is added to analyze the cell motion and position versus time. As shown in Figure 5a, five dots represent the yeast cell’s positions at every 10 ms from 10 to 50 ms, which are in accordance with Figure 3e−i. The upper two dots overlap with each other because the flow velocity decreases to nearly zero. The simulation results of time-dependent velocities with different trial PQ values are shown in Figure 5c. According to Eq S15, the optimal PQ value is 0.5 mW, corresponding to S = 6.4 at the P0 of 76 mW. Through repeating above operations, the trial PQ values and the root-mean-square errors versus the

surrounding cell solution induces a high-speed thermal microconvection, which may be the real reason for the cell motion. Dynamics Simulation and Analysis. We go further to confirm our assumption analytically. On the basis of the fluid dynamics and the heat transfer theory, we have further used the finite element analysis method by Comsol Multiphysics software to confirm our assumption. Briefly, a 3D transientstate study based on Stokes flow module and liquid−solid conjugate heat transfer module is solved (more details are described in Supporting Information). Obviously, the whole velocity field and temperature field are determined by the thermal power generated on CNC, PQ. However, the photothermal conversion efficiency η, which is the most important intrinsic factor for evaluating the thermal power generated on CNC, is unknown and have not been studied before. Here, the fluidic field distribution and variation with time are determined through giving five possible trial values to PQ in order to find out the most fitting and veritable case. The optimal PQ value is determined by calculating the root-meanC

DOI: 10.1021/acs.jpcc.8b08061 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Table 1. Root-Mean-Square Errors S (Calculated According to Eq S15) Versus the Trial Values of the PQ and the Incident Laser Power P0 PQ (mW) P0 (mW)

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

41 52 61 68 76

48.1 51.6

9.2 35.4 40.4

27.4 8.9 20.7 49.2

39.8 30.1 10.4 21.3

57.5 63.1 35.2 9.5 52.7

59.3 26.4 28.0

57.5 6.4

28.4

52.6

Figure 6. (a) Absorption spectra of CNCs. (b) PL spectra of CNCs at excitations of 532 nm.

Figure 7. (a) Distribution of the surface heat transfer coefficient on the surface of CNC under PQ = 0.5 mW. (b) Relationships between the surface heat transfer coefficient (h) and Nusselt number (Nu) vs incident laser power P0.

incident laser power P0 is shown in Table 1. The relationship of optimal PQ values with P0 is shown in Figure 5d. It is found that PQ has a nearly linear relationship with P0. The photothermal conversion efficiency η of single CNC is calculated to be 60%, which is equivalent to or even higher than the other nanostructures, such as CdX (X = S, Se, and Te) nanocrystals (∼22%) and CNTs/CNDs (∼50%).1,11,25,26 The absorption spectra and the PL spectra of CNCs are investigated and shown in Figure 6. CNCs exhibit two strong absorption bands in the visible and NIR region and significant PL emissions in the NIR region. The excellent optical properties may help CNCs for widely using in photothermal therapy and biological imaging in vivo. The temperature distribution is shown in Figure 5b, the maximum temperature of CNC reaches 353 K (approximately 80 °C) under the PQ of 0.5 mW. It has been proved theoretically and experimentally that the heating rate on surrounding water at the laser trap position is approximately 0 °C/W, when the laser wavelength is lower than 800 nm. There is no notable temperature increment of the surrounding water even though the incident power is up to 200 mW.27 In our condition, the maximum laser power is 76 mW; thus, the local temperature increment and thermal convection of surround-

ings are attributed to the laser-excited CNC heating. The curves of the maximum temperature of CNC (TCNC‑max) versus time under different incident laser powers are shown in Figure S2a. TCNC reaches maximum in less than 10 ms and keeps constant. Obviously, TCNC shows a positive relationship with P0. When higher power laser is used to illuminate on CNC, the temperature of surrounding solution near CNC becomes higher and may become even higher than 373.15 K (100 °C), which may cause the generation of micro water vapor bubble on CNC.22 In order to explore how the CNC influences such an interesting phenomenon, contrastive analytical studies on a single “gold nanocoil” and a single “copper nanocoil” are performed. The velocity field and temperature field distributions of metal nanocoils are shown in Figure S3. Compared with the results of single CNC shown in Figure 5a,b, it is found that because of their high specific heat capacity and high thermal conductivity, single metal nanocoil induces evidently lower thermal flow velocity, lower temperature increment, and larger heating area to surroundings, under the same condition. Thus, the low specific heat capacity and low thermal diffusivity of CNC benefit the local temperature increment in a D

DOI: 10.1021/acs.jpcc.8b08061 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C synergistic fashion28 and increase the thermal convection velocity eventually. The unique helical morphology of CNC also plays an important role during this photothermal process. A simulation of “carbon nanowire”, with straight-line morphology, is also performed, as shown in Figure S4. The maximum velocity and maximum temperature induced by nanowire are both much lower than those induced by CNC under the same condition, which further indicates that the helical structure of CNC helps absorbing photo-energy and releasing thermal energy more efficiently. Overall, the excellent NIR absorption, particular thermal properties, and unique helical morphology benefit CNCs excellent optical absorption and conversion ability. The convective heat transfer coefficient, h, is calculated by Comsol Multiphysics software automatically (see more details in Supporting Information). The distribution of the convective heat transfer coefficient under the PQ of 0.5 mW (corresponding to the P0 of 76 mW) is shown in Figure 7a. It is found that the convective heat transfer coefficient has a similar distribution with the temperature and velocity fields. Obviously, high flow speed of the surrounding solution and high temperature difference between CNC and surrounding solution are beneficial for the heat transfer improvement. As shown in Figure 6b, because of the higher flow velocity and temperature difference between CNC and surrounding solution under higher power laser, the average convective heat transfer coefficient h shows a positive increase with P0. h reaches as high as 7.0 × 105 W/(m2·K) under the P0 of 76 mW, which is higher than metal microstructures.29,30 Such a high h value mainly comes from the microscale size, the large effective area (due to its unique helical morphology) for the laser energy absorption and heat transfer, the high photothermal conversion ability of CNC, which helps providing a high heat flux, and high flow speed as well. The dependences of the average h values on the time under different incident laser powers are shown in Figure S2b. It is found that the convective heat transfer coefficient turns to be stable in less than 10 ms. The Nusselt numbers under different incident laser powers are also calculated according to eq S10 and are shown in Figure 7b. It is observed that Nu shows a slight increase with the rise of laser power. Such a small Nu value indicates that heat conduction dominates the heat transfer process for microscale systems.31

unique helical morphology, the tiny size, and the large effective area of CNC. Therefore, as a kind of low density, tiny size, high specific surface area, and photothermal responsive carbon nanomaterial, CNCs may have great potential applications in near-infrared photothermal sensing, laser-operated micro/nano heat generators and exchangers.

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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/acs.jpcc.8b08061.

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Further details on numerical simulations and analyzations; 3D stereogram of FEA model; variation of the maximum temperature of the CNC and the convective heat transfer coefficient versus time; velocity distribution and temperature distribution of solution domain based on “gold nanocoil”; and velocity distribution and temperature distribution of solution domain based on “carbon nanowire” (PDF) Photothermal conversion dynamic process (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lujun Pan: 0000-0003-3666-408X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (nos. 51661145025, 11274055, and 61520106013).

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CONCLUSIONS In this work, we have demonstrated the laser-activated living cell ejection and near-infrared photothermal conversion property of CNCs. A high-speed living cell ejection assisted by a single NIR laser-irradiated CNC is developed. With absorbing the incident NIR laser energy, the thermal energy generated on CNC releases to the surrounding water rapidly, which results in a high-velocity heat convection and induces the cell ejection. A further analytical study is also performed for a clear illustration of the dynamics process. It is found that the maximum velocity of solution flow, also the cell ejection velocity, reaches higher than 103 μm/s with the increase of laser power, which indicates the promising applications in opto-microfluidic systems and laser-operated flow cytometers. The photothermal conversion efficiency of CNC is deduced to be 60%, indicating excellent NIR absorption and photothermal conversion properties. The average convective heat transfer coefficient on the surface of CNC reaches as high as 7.0 × 105 W/(m2·K). Such outstanding properties mainly come from the E

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