Journal of Nanotechnology Nanomedicine & Nanobiotechnology Category: Medical Type: Research Article
Photothermal Effect of PEG-Functionalized Single-Walled Carbon Nanotubes
- Haishan Zeng1*, Naiyan Huang2, Yue Tian3, Hequn Wang4, Jianhua Zhao5, Harvey Lui6, Mladen Korbelik7
- 1 Imaging Unit Integrative Oncology Department, BC Cancer Agency Research Centre, 675 West 10th Avenue, Vancouver, BC, Canada V5Z 1L3, Canada
- 2 Department Of Laser Medicine, BC Cancer Agency Research Centr, University Of British Columbia & Vancouver Coastal Health Research Institute, 835 West 10th Avenue, Vancouver, BC835 West 10th Avenue, Vancouver, BC, Canada
- 3 Department Of Laser Medicine, Chinese PLA General Hospital, Beijing 100853, China
- 4 Imaging Unit Integrative Oncology Department, BC Cancer Agency Research Centre, University Of British Columbia & Vancouver Coastal Health Research Institute, 835 West 10th Avenue, Vancouver, BC, Canada
- 5 Imaging Unit Integrative Oncology Department, BC Cancer Agency Research Centre, University Of British Columbia & Vancouver Coastal Health Research Institute, 835 West 10th Avenue, Vancouver, Bc, Canada
- 6 Imaging Unit Integrative Oncology Department, Bc Cancer Agency Research Centre, University Of British Columbia & Vancouver Coastal Health Research Institute, 835 West 10th Avenue, Vancouver, Bc, Canada
- 7 Imaging Unit Integrative Oncology Department, BC Cancer Agency Research Centre, 675 West 10th Avenue, Vancouver, BC, Canada
*Corresponding Author:Haishan Zeng
Imaging Unit Integrative Oncology Department, BC Cancer Agency Research Centre, 675 West 10th Avenue, Vancouver, BC, Canada V5Z 1L3, Canada
Tel:+1 604 675 8099,
Received Date: Jan 23, 2017 Accepted Date: Mar 09, 2017 Published Date: Mar 28, 2017
Materials and methods: Eight concentrations of PEG-SWNTs solutions were prepared by 2-fold serial dilution at concentrations of 1.0, 0.5, 0.25, 0.125, 0.0625, 0.0312, 0.0156 and 0.0078 mg/ml. The solvent without PEG-SWNTs was used as control. The temperature changes of the PEG-SWNTs solution during laser irradiation were monitored with an infrared thermometer. Three levels of laser power densities were chosen for the treatment including 100, 200 and 500 mW/cm2.
Results and conclusion: The temperature of PEG-SWNTs solution increases linearly with laser power density, but nonlinearly with irradiation time and concentration. The solution temperature increases much faster within the first 3 minutes of irradiation and then gradually levels off as the irradiation approaching 10 minutes. The maximum temperature change (?Tmax) in each solution increases sharply with increasing PEG-SWNTs concentration up to 0.125 mg/ml and eventually levels off beyond 0.25 mg/ml. Based on this observation, optimized treatment parameters (laser power density and SWNT concentration) could be obtained to raise the temperature by 10-30?C sufficiently for causing cell apoptosis and/or necrosis. For in vitro cellular and in vivo tissue studies, similar experiments could be carried out to find the optimal treatment parameters.
Carbon nanotubes have the capability to efficiently convert Near Infrared (NIR) radiation into heat by photoexcitation of their quantized lattice vibrations (phonons), which is advantageous for the development of next generation photothermal agents for laser cancer treatment [9,13-17]. Exposure to high temperatures for sufficient amount of time could cause permanent physical damage such as protein denaturation or membrane lysis . These effects have been used for direct cancer cell necrosis or apoptosis . Carbon nanotubes have extremely broad electromagnetic absorption spectrum, covering the visible, NIR, radio frequency and microwave bands [20,21] which provides the flexibility in the choice of treatment wavelength. As transmission of NIR and radio frequency through the body is minimally attenuated , photothermal agent based on carbon nanotubes is of particular interest for the treatment of non-superficial cancerous lesions in vivo.
Most of the studies involving SWNTs or other nanoparticles-assisted photothermal treatment were based on high laser power density of 1-40 W/cm2 [23-28]. However, high laser power density could cause non-selective photothermal damage to the surrounding normal tissues. A mild treatment temperature with medium power density is more ideal for targeted phototherapy. It was found that hyperthermic treatments with modest temperature enhanced the efficacy of systemic chemotherapy of cancers . Photothermal heating with SWNTs and doxorubicin killed more cancer cells than photothermal heating with doxorubicin or SWNTs alone . These heating-assisted effects in combination with immunological therapy  or drug release switch  only need modest temperatures, which is easy to achieve with SWNTs.
Application of SWNTs-assisted photothermal treatment requires knowledge of the temperature increases associated with SWNTs absorption . It has been shown that the threshold time of cell necrosis by thermal therapy changes with the temperature . Longer treatment time is needed for lower temperature, or vice versa. For example, cell necrosis requires 135 min at 43ºC, but only 5 sec at 57ºC [34,35]. Different temperature is needed depending on the treatment protocol or treatment objective. It is important to measure the temperature in SWNTs-assisted photothermal treatment in order to design a proper treatment plan.
In this paper, we will focus on the photothermal properties of polyethylene glycol functionalized SWNTs (PEG-SWNTs) solution irradiated by 785 nm laser and investigate the dependence of photothermal effect on concentration of PEG-SWNTs, laser power density and laser treatment time.
Materials and Methods
Nine concentrations of PEG-SWNTs solution were prepared with each solution in triplicate. Eight concentrations of PEG-SWNTs solution were prepared by two-fold serial dilution from 1 mg/ml to 0.0078 mg/ml (i.e., 1, 0.5, 0.25, 0.125, 0.0625, 0.0313, 0.0156, 0.0078 mg/ml). One solution without PEG-SWNTs was used as control (i.e., pure double distilled water). The solutions were placed in a 96-well plate. The diameter and depth of each well were 0.65 ± 0.01 cm and 1.09 ± 0.01 cm, respectively. Each well could hold 350 ?l of the PEG-SWNTs solution. In this experiment, 200 ?l of each PEG-SWNTs solution was used to avoid spillover. This diameter and volume of PEG-SWNTs solution was chosen to simulate the size of the tumor in the experimental animals.
Laser and laser power densities
Each sample was irradiated with the specified laser power densities for 10 minutes. The temperature of the PEG-SWNTs solutions was measured at a 1-minute interval after irradiation for 10 minutes with each reading being taken about 5 seconds. Each sample was irradiated once, and every concentration of PEG-SWNTs solution was repeated 3 times. The average of the three measurements of each concentration was used for data analysis. The change of the temperature (Temperature change ?T), which is the difference of the temperature at any time point and the room temperature before laser irradiation, was used as a measure of the treatment effect.
The dependence of the maximum temperature change ?Tmax on laser power density for various concentrations of PEG-SWNTs is shown in (figure 2A). It is found that ?Tmax increases linearly with laser power density for any concentration of PEG-SWNTs. The slope of the lines represents the rate of ?Tmaxincreasing as a function of laser power density at a specific concentration of PEG-SWNTs, which is shown in (figure 2B). It shows that the rate of ?Tmax increasing is a fast increasing function at low concentrations (< 0.3 mg/ml) and levels off at high concentrations (>0.3 mg/ml), indicating that the rate of ?Tmax increasing cannot be further improved by solely increasing the concentration of PEG-SWNTs.
Figure 2B: The rate of ?Tmax increasing with laser power density versus the concentration of PEG-SWNTs. The rate of ?Tmax increasing is defined as the slope of the lines in (a). Higher rate of ?Tmaxincreasing indicates that the temperature is more sensitive to laser power density. ?Tmax was the temperature change at 10 minutes after laser irradiation.
The temperature of PEG-SWNTs solution increases with the length of laser irradiation for any given concentration of PEG-SWNTs and laser power densities (Figures 1A-C). Therefore, increasing treatment time is an effective way to raise the temperature of the PEG-SWNTs solutions. However, the temperature increases rapidly for the first few minutes and then levels off after about 10 minutes. This is because the heat dissipation increases with the raising temperature. The temperature leveling off reflects that a balance has been reached between the heat generation and the heat dissipation. The maximum temperature that a solution can achieve depends on the concentration of PEG-SWNTs and laser power density. Therefore if certain temperature is needed to generate a clinical treatment effect, it is insufficient by solely increasing the treatment time. Optimal concentration of PEG-SWNTs and laser power density are needed.
The maximum temperature change ?Tmax is found to increase linearly with laser power density for any given concentration of PEG-SWNTs solution (Figure 2A). This linear dependence indicates that the absorption of PEG-SWNTs is a linear effect. Therefore, increasing laser power density is an effective way to raise the temperature for any given PEG-SWNTs solutions. The rate of ?Tmax increasing with laser power density depends on the concentration of PEG-SWNTs (Figure 2B). Higher rate of ?Tmax increasing indicates that the temperature change is more sensitive to laser power density. The optimal concentration for photothermal therapy can be determined from the rate of ?Tmax increasing plot, which is around 0.25 mg/ml in this study, as the ?Tmax increasing cannot be further increased by solely increasing the concentration beyond this value. In this regime, the maximum temperature change ?Tmax can only be increased by laser power density (assuming laser irradiation time is fixed). The maximum rate of temperature change is ~0.057?C/mW/cm2 in this experiment. Therefore, if the laser power density is increased by 100 mW/cm2, the temperature will be increased by about 5.7?C.
The maximum temperature change ?Tmax of the PEG-SWNTs solutions increases with the concentration of PEG-SWNTs for a given laser power density and saturates at about 0.125 – 0.25 mg/ml (Figure 3). This phenomenon was observed in other carbon nanotube studies as well but with no explanation of the underlying mechanisms. For example, Ghosh et al.,  studied aqueous solution of DNA-encased Multi-Walled carbon Nanotubes (MWNTs) irradiated with a 1064 nm laser at 2W, 3W and 4W power. They found that the solution temperature was increased for 1, 5, 10, 25 and 50 μg/ml, and saturated at 75 and 100 μg/ml. Whitney et al.,  studied single-walled carbon nanohorns heated with a c.w. laser at wavelength 1064 nm and irradiance of 40 W/cm2 for duration of 0–6 minutes. They found that the temperature was saturated at 50ºC for concentrations between 0.085 and 0.333 mg/ml. The saturation of temperature at certain concentration of PEG-SWNTs may be related to the depletion of light . The underlying mechanism could be that when the concentration of PEG-SWNTs is high enough the light is completely absorbed by the solution and therefore, even if the concentration of PEG-SWNTs is further increased, there are no more photons that could be absorbed to generate more heat. The level off of the rate of ?Tmax increases versus PEG-SWNTs concentration curve (Figure 2B) at and above 0.25 mg/ml is due to the same mechanism of depletion of light at high concentrations.
At the laser power densities of 100 and 200 mW/cm2, we observed a maximum at 0.125 mg/ml on the maximum temperature change ?Tmax versus PEG-SWNTs concentration curves (Figure 3). This could be explained by considering the high thermal conductivity of PEG-SWNTs, which is approximately 6,600 W/m?K at room temperature , about four orders higher than the thermal conductivity of the solvent (double distilled water, 0.6 W/m?K). Because the laser beam is depleted, there is no more energy being absorbed by the solution even if the concentration of PEG-SWNTs is further increased. However, the thermal conductivity of the solution is increased in higher concentrations, which dissipates more heat and thus makes the temperature drop slightly for higher concentrations. This maximum was not observed for our high laser power experiment (500 mW/cm2) and in other studies that used higher laser power densities of 2 – 4 W/cm2 and 40 W/cm2 [24,26]. This might be because the heat dissipation effect of the thermal conductivity increasing occurred before the laser depletion, thus no maximum appeared. For higher laser powers, the light depletion occurs at high PEG-SWNT concentrations.
The observed temperature saturation effect when the PEG-SWNT concentration is higher than certain values could have significant implementations for in vitro cellular and in vivo tissue experiments. When the concentration is increased beyond the threshold value, it will not help with increasing the temperature anymore. Staying at lower concentration will help with minimizing any side effects associated with SWNTs. For similar reasons, our discovery that the temperature maximizes at an optimized concentration (Figure 3) could lead to optimized treatment parameters: lower laser power density, lower SWNT concentration, and yet maximum temperature increase.
This study was performed with PEG-SWNTs solution with water as solvent, generating a homogeneous SWNTs suspension. While in an in vitro cell culture or an in vivo tissue system, SWNTs could form aggregates with different absorption capacity, the above detailed studies need to be repeated to find out the system specific optimal parameters (SWNTs concentrations and laser power densities) for desired photothermal therapies. The endpoint will include not only the temperature change, but also cell/tissue viability.
Conflict of Interest:
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Citation:Huang N, Tian Y, Wang H, Zhao J, Liu H, et al. (2017) Photothermal Effect of PEG-Functionalized Single-Walled Carbon Nanotubes. J Nanotechnol Nanomed Nanobiotechnol 4: 012.
Copyright: © 2017 Haishan Zeng, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.