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We present a method to calibrate the light to heat conversion in an aqueous fluid containing nanoparticles. Accurate control of light and heat is of dramatic importance in many fields of science and metal nanoparticles have acquired an increased importance as means to address heat in very small areas when irradiated with an intense light. The proposed method enables to measure the temperature in the environment surrounding nanoparticles, as a function of the exposure time to laser radiation, exploiting the properties of thermochromic cholesteric liquid crystals. This method overcomes the problems of miscibility of nanoparticles in liquid crystals, provides temperature reading at the microscale, since the cholesteric liquid crystal is confined in microdroplets, and it is sensitive to a temperature variation, 28°C-49°C, suitable for biological applications.
© 2014 Optical Society of America
1. Introduction
The visualization and control of optical to thermal energy conversion following light absorption in nanostructures is a key challenge in many fields of science with applications to areas as microfluidics [1], nanofluidics [2], nanocatalysis [3], photothermal cancer therapy [4,5], drug delivery [6,7], imaging and spectroscopy [8–10], information storage [11] and processing [12], nanoscale patterning [13] and solar energy harvesting [14]. In the last years, gold and silver nanoparticles (NPs) are drawing attention due to their plasmonic resonances in the visible region of the electromagnetic spectrum [15]. In fact, localized surface plasmons are responsible for both enhanced light scattering and enhanced light absorption. This enhanced absorption and the subsequent NP temperature increase turn metal NPs into nano-sources of heat remotely controllable using light [16,17]. A key issue in this frame of research, necessary for practical applications, is being able to measure the temperature of the medium surrounding NPs. Several techniques for small scale thermometry have been reported, such as nanolithographic, nanomaterial-based, fluorescent materials based, and nanoscale superstructure thermometry methods [18–20]. Among them, liquid crystal thermography, has received great attention due to its capability to quickly reconstruct temperature fields in surfaces and volumes. Liquid crystals (LCs) are organic substances which behave, from a mechanical point of view, as liquids while they simultaneously exhibit optical properties similar to crystalline materials. Among different types of liquid crystals, cholesteric liquid crystals (CLCs) exhibit optical properties sensitive to temperature variations [21]. CLCs are composed of different parallel layers, the average molecular orientation is different in each layer and this results in a characteristic twisted arrangement. The presence of a supramolecular chiral structure leads to a periodical modulation of the refractive index inside the CLC and it behaves as a one-dimensional photonic crystal. CLCs exhibit selective reflection of light, with the presence of the typical photonic band gap in their reflection spectra, that is the effect on which many of their applications are based [22–24]. Since the supramolecular helical structure is temperature sensitive, liquid crystal substances are known to shift the band gap position when subjected to temperature changes [25–28].
In this work, we propose the idea to use thermotropic cholesteric liquid crystal microdroplets for the non-intrusive visualization and measurement of the temperature distribution at the microscale. In particular, we focus on the visualization of the optical energy conversion to heat in metal NPs. Gold or silver NPs can efficiently release heat under optical excitation. When excited with a laser beam, the laser electric field strongly drives charge mobile carriers inside the NPs, and the energy gained by carriers turns into heat. Heat generation becomes especially strong in the regime of plasmon resonance [16]. The NPs temperature may rise significantly and the heat can propagate to the surrounding medium. In our method, the temperature surrounding nanoparticles can be estimated monitoring the spectral shift of the selective reflection peak of a CLC. For this purpose, an emulsion of water, cholesteric liquid crystal and nanoparticles is investigated and the temperature in the environment surrounding nanoparticles, as a function of the exposure time to laser radiation, is monitored.
The proposed technique presents important advantages. It overcomes the problem to suitably cap NPs with molecules necessary to improve the miscibility of NPs in liquid crystals [29,30]. In fact, the CLC is confined in micron size droplets while NPs are suspended in the aqueous matrix. Micron size droplets act as microthermometers and can provide a local visualization of the temperature. The selected CLC is sensitive to a temperature range between 28°C and 49°C, this and the fact that NPs are contained in an aqueous medium makes this technique suitable for biological applications.
2. Experiments and discussion
In the following, we describe the preparation of the cholesteric mixture and the study of its optical properties, followed by the description of the preparation of the thermosensitive emulsion containing NPs. Finally, we investigate the behaviour of the nanocomposite emulsion after laser irradiation.
Initially, a thermotropic CLC mixture is prepared doping the nematic liquid crystal BL006 with a left-handed chiral agent ZLI-811, both available from Merck, in the following percentage in weight: 71% BL-006 + 29% ZLI-811. The mixture is stirred in its isotropic phase at about 110°C for 30 min. To investigate its thermo-optical properties, the liquid crystalline mixture is confined in a cell, prepared with two glass plates. The spacing between the plates is set to 12 µm using mylar films. Thin layers of polyvinyl alcohol (PVA, Sigma-Aldrich) are deposited on glasses and are rubbed to obtain planar alignment of the liquid crystal material. The mixture is infiltrated by capillarity in the cell. A hot stage (CaLCTec, mod. FB-250), with 0.1°C accuracy, is used to control the cell temperature. The optical properties of the mixture, absorption and reflection spectra, as a function of temperature are investigated with a fiber coupled spectrophotometer (Avantes, mod. AvaSpec-2048), having 1 nm resolution.
Red, green and blue curves in Fig. 1(a) show the shift of the photonic band gap of the CLC mixture at 28°C, 38°C and 49°C respectively. The curve in Fig. 1(b) reports the shift of the position of the middle-point of the photonic band gap as a function of temperature.
Fig. 1 (a) CLC photonic band gap at different temperatures: 28°C (red line), 38°C (green line) and 49°C (blue line), (b) position of the middle-point of the photonic band gap as a function of temperature.
An emulsion of the cholesteric mixture in water and glycerol is prepared with the following percentage in weight: 95% (90% water + 10% glycerol) + 5% CLC. Glycerol is added in order to reduce the evaporation rate of the emulsion. A cuvette with 1mm gap is filled with the emulsion and stirred at 600 rpm, at room temperature for 25min. As a result, a homogeneous distribution of CLC microdroplets suspended in an aqueous glycerol matrix is obtained. The diameter range of CLC microdroplets is 10-15μm. Depending on the CLC concentration and stirring speed, CLC microspheres with different diameter sizes and packing density may be obtained.
The cholesteric liquid crystal in the microdroplets exhibits the same optical properties shown in Fig. 1 for the cholesteric film. To visualize the temperature distribution inside the cuvette when the emulsion is heated up from room temperature, the cuvette is placed on a heating stage. Figure 2(a) shows the local temperature at different height in the cuvette, also showing the vertical temperature gradient. Blue color at the bottom of the cuvette corresponds to the highest temperature, and it turns to green, yellow, red and brown as we move to the top of the cuvette, indicating a decrease in temperature. Figure 2 shows the same microscopic area of a liquid crystal cell at 25°C (b) and 42°C (c).
Fig. 2 (a) Temperature distribution in a cuvette containing CLC microdroplets in a water and glycerol mixture. The temperature scale is built using data from Fig. 1. Optical microscope images of CLC microdroplets in a liquid crystals cell. Red color (b) is visible at room temperature, 25 °C, while blue (c) is visible at 42°C.
Silver nanoparticles are, then, added to the prepared emulsion as described in the following procedure. Silver NPs (Sigma-Aldrich), 40 nm in size, dispersed in an aqueous buffer (0.02 mg/mL) containing sodium citrate as stabilizer, are used. To probe their optical properties, a cuvette with 1mm gap is filled with the suspension of water, glycerol (10% in weight) and silver NPs and it is stirred at room temperature for 20 min. The resonance absorption of this mixture shows a peak at 450nm, as expected when NPs are not aggregated.
The final nanocomposite emulsion (NCE) is obtained mixing cholesteric liquid crystal, glycerol and the NP aqueous solution in the following concentrations in weight: 95% (90% NP suspension + 10% glycerol) + 5% CLC. As in the previous case, glycerol is used to reduce the evaporation rate of water during experiments. Finally, the NCE is stirred at 600 rpm, at room temperature to produce homogeneously distributed CLC microdroplets.
To visualize the effect of energy harvesting from silver nanoparticles, the experimental setup sketched in Fig. 3, is used.
Fig. 3 Experimental setup: cuvette (1), impinging laser beam @ 457nm (2), NCE (3), optical shutter (4), edge filter (5), QTH light source (6), fiber-coupled spectrometer (7).The laser beam travels inside the cuvette, parallel to its larger faces. The system to observe the reflected light from the suspension is formed by elements 4, 5, 6, 7, which illuminates and collects light perpendicularly to the laser beam.
As excitation source a 457 nm diode pumped solid state laser (Shangai Dream Lasers Technology Co. Ltd.), with a maximum power of 500 mW, is employed. The power at the sample is set to 300mW using attenuators, the probe beam spot is free from aberrations. To measure the variation in color of the NCE, a quartz tungsten halogen (QTH) lamp is used. The reflected light is collected with a fiber-coupled spectrometer. The Mie scattering of laser irradiation is cut by an edge filter, at 460nm, placed in front of the spectrometer. To avoid heating caused by the QTH lamp, an optical shutter is used. Optical images are recorded using a digital camera.
Initially the laser beam, with a 1mm cross section, is directed towards the cuvette. With the passing of time, the cholesteric microdroplets, due to local heating induced by the laser beam, change their color, and the shift of their photonic band gap can be precisely measured.
Figure 4(a) shows the laser beam propagating through the cuvette after 10s of irradiation and Fig. 4(b) after 30s.The “heat trace” penetrates inside the NCE highlighting the profile of the temperature distribution. In Fig. 5 the color distribution inside the cuvette (a) before and (b) immediately after the switching off of the laser beam is shown. The red arrow indicates the area exhibiting a larger temperature (green) than the surroundings (yellow-red). The highest temperature is obviously reached along the previous laser beam propagation path. An high-pass filter can be added in front of the camera to cut the impinging laser light, in order to follow the dynamics of the heat propagating inside the sample.
Fig. 5 (a) Image of the laser beam propagating inside the NCE and (b) red arrow indicating the highest local temperature in correspondence of the previous laser beam propagation path immediately after switching off the laser beam.
To observe the dynamics of the temperature evolution in the region of the cuvette illuminated by the pumping laser beam, the reflection spectra from the NCE is acquired as a function of the exposure time. A blue shift of the photonic band gap of about 200nm after 90 s of exposure is observed. Figure 6(a) shows the reflection spectra at three different exposure times: 6 s (red line), 35 s (green line), 90s (blue line), always in presence of the pumping laser beam, which wavelength also appears at 457nm.
Fig. 6 (a) NCE selective reflection at 6 s (red line), 35 s (green line) and 90 s (blue line), under pumping laser irradiation. (b) Selective reflection of the emulsion not containing NPs, as a function of the exposure time: 6 s (red line), 35s (green line) and 90 s (blue line).
The shift of the photonic band gap practically stops after 90s of exposure. This is mainly due to the fact that the photonic band gap is less sensitive to temperature variations in the blue range of the CLC spectrum, as represented by the plateau at 50° shown in Fig. 1(b). Another reason could be the balance between the energy gained from laser irradiation and the energy released to the environment by the NPs. Note that the CLC has been chosen with a photonic band gap shift in the shown temperature range because it is the most convenient for biological applications. Other cholesteric materials could satisfy other technical requests.
As counter-proof, to demonstrate that silver NPs play the crucial role in light to heat conversion, an emulsion without silver NPs is prepared and the same experiment is carried out. In this case, we observe that the photonic band gap of the new emulsion shifts, but significantly less than in the case of the NCE (Fig. 6(b)), about six times less than in presence of NPs. This shift could be explained by the presence of impurities in the emulsion, that can act as absorptive centers for the laser light, and by the water heat absorption.
To have a better mapping of the temperature distribution in the cuvette in the case of the NCE suspension, when irradiated by the laser beam, we used “Origin Lab” software to visualize the local temperature along and around the laser beam propagation path. This program converted an image in a XYZ worksheet, then transformed it in a 3D Surface plot, using a customized color map. The temperature-wavelength dependence has been calibrated by using data from Fig. 1(b) and the temperature profile is shown in pseudo colors in Fig. 7. The upward tail of the temperature distribution indicates that phenomena of heat convection can also be monitored.
3. Conclusion
We describe a new method that enables to evaluate the temperature of the medium surrounding silver NPs as a function of the exposure time to light radiation. This method relies on the optical properties of cholesteric liquid crystals, confined into microdroplets dispersed in the medium, and combines the advantages of high spatial resolution and good temperature accuracy with fast readout. This method can be used for any NPs, disregarding their size or shape, for instance silver or gold based, and it is useful to calibrate the local temperature increase due to laser irradiated NPs in a variety of materials including particles in an inorganic or organic matrix. It is worth noting that the CLC microdroplets can be obtained in a water based environment and with a size comparable to the one of living cells. Hence, the micron size droplets act as microthermometers, providing a local visualization of the temperature reached by the medium surrounding NPs. Moreover, the proposed technique overcomes the problem to suitably cap NPs with molecules necessary to improve their miscibility in liquid crystals. The NCE used for the experiments described in this work contains a cholesteric liquid crystal with a working temperature range around 28°C-49°C, which is well suitable for applications in the biological field. Nevertheless, the cholesteric liquid crystal optical properties can be tailored to change the temperature interval to suit other applications.
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