Monitoring the Transient Thermal Infrared Emission of Gold

Dec 14, 2016 - The transient photothermal process of gold nanoparticles (AuNP) capped with different molecules, namely, citrate, cetyltrimethylammoniu...
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Monitoring the Transient Thermal Infrared Emission of Gold Nanoparticles upon Photoexcitation with a Step-Scan FourierTransform Spectrometer Jia-Ling Liu,† Ya-Ting Yang,‡ Chia-Te Lin,† Yi-Ju Yu,† Jen-Kun Chen,‡ and Li-Kang Chu*,† †

Department of Chemistry, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Rd., Hsinchu 30013, Taiwan Institute of Biomedical Engineering & Nanomedicine, National Health Research Institutes, Miaoli 35053, Taiwan

‡

S Supporting Information *

ABSTRACT: The transient photothermal process of gold nanoparticles (AuNP) capped with different molecules, namely, citrate, cetyltrimethylammonium bromide (CTAB), and methoxyl-polyethylene glycol thiol (mPEG), has been investigated by time-resolved infrared emission spectroscopy monitored with a step-scan Fourier-transform spectrometer. Upon photoexcitation of the surface plasmonic resonance band of AuNPs with a 532 nm nanosecond pulsed laser, the transient infrared emission was observed within about 1 μs, referring to the duration of the laser heating and thermalization of AuNPs. Comparing the infrared emission contours with the blackbody radiation spectra at different temperatures revealed that the temperature reached 400 ± 100 °C in 90−120 ns as the 24 nm mPEG-capped AuNPs were excited by a peak power of 5 × 1010 W m−2 (25 mJ cm−2 from a 5 ns pulsed laser) at 532 nm. The insignificant changes in the morphology and size distribution of mPEG-AuNP suggested that the surface modification via covalent bonding helped retention of the morphology of the nanostructures after laser heating. In addition, photoexcitation of 35 nm CTAB-AuNPs generated a higher transient temperature than that of 89 nm CTAB-AuNP; this is consistent with the prediction by Mie theory that smaller nanoparticles possess a higher contribution of absorption in the extinction coefficient, which leads to higher photothermal efficiency. This is the first time that the transient broadband thermal infrared emission of the photoexcited gold nanoparticles has been recorded within a submicrosecond, which is close to the nascent condition. The duplexity in the temporal capability and broadband spectroscopic window of the time-resolved emission infrared spectroscopy provides a promising noncontact thermometer to illustrate the photothermal process and quantify the transient temperatures of miscellaneous metallic nanostructures upon photoexcitation.

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vibrations,14−17 and thermal relaxation to the surroundings17−19 by monitoring the transient transmittance, absorbance difference, and reflectance signals associated with the temperature dependent dielectric properties of medium and metallic nanoparticles at given probe wavelengths. The two-temperature model (TTM) is commonly used to quantify the energy transfer between the electron and lattice upon pulsed excitation of metallic nanostructures.13,20−22 In addition to the ultrafast excitation, theoretical methods have been employed to illustrate the temperature evolution of the nanoparticles and surroundings upon nanosecond-pulsed excitation.9,23 Sassaroli et al. demonstrated that a 6 ns pulse with a peak power of 5 × 109 W m−2 can cause water to boil at the nanoparticle surface during the pulse.9 A higher excitation power (1.8 × 1010 W m−2) increases the temperature up to ca. 250 °C within the duration of the pulse.9 Chen et al. demonstrated that the temperature of the nanoparticle can reach ca. 800 K in 4 ns upon excitation with a critical pulse fluence of 0.035 J cm−2 (equivalent to 8.75

INTRODUCTION The photothermal effect of metallic nanoparticles has been extensively utilized in many aspects, such as controllable drug delivery and release,1,2 thermotherapeutic treatment,3,4 and catalysis.5,6 The thermal energy evolution of gold nanoparticles upon photoexcitation involves the thermalization of the electron clouds within ca. 0.5 ps7,8 and the thermalization of electrons and phonons within 1 ps.7,8 Consequently, the heat exchange between the nanoparticles and the surroundings increases the temperature in the vicinity of the nanoparticle surface.9 The tunability of the excitation wavelengths, ranging from visible to infrared depending on the morphologies of the metallic nanostructures, allows versatile utilization of metallic nanostructures as nanoheaters.4,10,11 The present work illustrates the temperature evolution of gold nanoparticles upon photoexcitation by collecting the thermal infrared emission. Extensive experimental and theoretical works have been carried out to quantify the heat dissipation upon pulsed excitation of gold nanoparticles.12−25 The femtosecond pump− probe method has been widely employed to characterize the dynamics and kinetics of electrons and lattices,12,13 acoustic © XXXX American Chemical Society

Received: October 4, 2016 Revised: December 14, 2016 Published: December 14, 2016 A

DOI: 10.1021/acs.jpcc.6b10044 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C × 1010 W m−2).23 In addition to the theoretical works, the nanosecond excitation is capable of generating nanobubbles around gold nanoparticles,24,25 and the corresponding kinetics is strongly dependent on the nanoparticle size, heat capacity, aggregation status, and pump laser pulse duration.24 In the nanosecond domain, the temperature evolution involved the heat transfer from the gold nanoparticle lattice to the surroundings. However, no direct experimental data have been reported to illustrate the temperature of the AuNPs upon irradiation after the electron−phonon thermalization in the nanosecond domain. No contact methods, such as using thermocouples, are capable of providing sufficient temporal resolution. Previous reports demonstrated that the thermal infrared emission reflected the temperature evolution during the laser heating of a tungsten surface26 and the matrices of MALDI (matrix-assisted laser desorption ionization),27 treating the heating surface as a blackbody radiator. In the present work, we intended to quantify the temperature of the gold nanoparticles upon the nanosecond pulsed photoexcitation using infrared emission spectroscopy, which was compared with the observed infrared emission contours of the blackbody radiation. A stepscan Fourier-transform infrared spectrometer was employed to monitor the thermal infrared emission because of its wavenumber multiplexity and temporal resolution within submicroseconds.28 Previous studies demonstrated that metallic nanostructures having larger absorption portions in the extinction coefficients possess higher photothermal efficiency.29,30 In this work, AuNPs capped with different molecules, namely, citrate, CTAB, and mPEG, and CTABcapped AuNPs of different sizes were employed to serve as the photoinduced nanoheaters. The successful illustration of the photothermal processes of AuNPs solidifies further applications of the photothermal effect of various metallic nanostructures in areas such as chemical catalysis, thermotherapy, and controllable drug release.

turned transparent. Then 100 μL of 0.029 M HAuCl4 was added to the solution, and the mixture was stirred at 35 °C. Afterward, 1 mL of seed solution was added to the vial, and 50 μL of 0.1 M ascorbic acid (J. T. Baker, > 99.5%) solution was added immediately to the premixed solution to prepare 35 nm CTAB-AuNP. Changing the amounts of the seed solution to 0.25 and 0.05 mL allowed tuning of the sizes of the CTABAuNPs to 55 and 89 nm, respectively. After being stirred for 10 min, the solution turned wine red. Finally, the solution was centrifuged at 3,500, 3,000 and 2,530g for 35, 55, and 89 nm CTAB-AuNP, respectively, for 30 min and the supernatant was discarded. The precipitate was redistributed in 10 mL water and then centrifuged again for 30 min to concentrate the sample to 1 mL. Preparation of mPEG-Capped AuNP. The preparation of mPEG-AuNP was based on previous work with appropriate modifications.32 In brief, auric acid (HAuCl4, Sigma-Aldrich) aqueous solution (5.0 mM, 25 mL) was mixed with 300 μL of 1 M NaOH and adjusted to 500 mL with double deionized water. The mixture was heated to boiling with vigorous stirring, and sodium citrate solution (5%, 5.2 mL) was subsequently added to reduce the reaction. The solution was cooled to room temperature at 2 h after reaction. The methoxyl-poly(ethylene glycol)-thiol (mPEG, NOF Co., Tokyo, Japan) molecules with 5 kDa of average molecular weight were employed to exchange citrate ions on AuNPs. The mPEG solution (2.0 mM, 248 μL) was added into 40 mL of AuNP solution to maintain an mPEG/AuNP molar ratio of 40000:1 overnight (17 h) with gentle mixing (99 rpm). After PEGylation, the solution was subjected to purification using ultrafiltration with 20 kDa molecular weight cutoff membrane at 2,000g for 10 min. The concentrated particulate fraction on the ultrafiltration membrane was collected and pooled for the subsequent experiments. Preparation of Dried AuNP on a CaF2 Window. The concentrations of the above-mentioned samples were adjusted to 1 O.D. around the SPR band by adding deionized water. The samples were centrifuged at ca. 18,000 × g for 5 min and the supernatants were discarded. The concentrated samples were sonicated for 1 min for better dispersivity. 3−5 μL concentrates were dropped onto a CaF2 window (diameter: 1 in.; thickness: 2 mm) and dried for 1 h in ambient conditions for timeresolved experiments. Steady-State Spectroscopy. The ultraviolet−visible (UV−vis) absorption spectra were recorded with a spectrometer from Ocean Optics (USB4000-UV−vis). A Fouriertransform infrared spectrometer (Vertex 80, Bruker) operated in continuous scan mode was employed to collect the absorption spectra of the dried samples on a CaF2 window with spectral resolutions of 4 and 32 cm−1; each spectrum was averaged from 60 scans. Electron Microscope. The morphologies of the gold nanoparticles samples before and after laser irradiation were characterized using a thermal-type field emission scanning electron microscope (JSM-7000F, JEOL). Time-Resolved Emission Spectroscopy Recorded with a Step-Scan Fourier-Transform Spectrometer. The dried sample on the CaF2 window was mounted at the focal point of the parabolic mirror for collecting the emission at the side of the interferometer, as shown in Figure S2. A set of sleeves was connected to the sample compartment, and a small amount of NaOH was added to adsorb the H2O and CO2 to prevent as much of the spectral interference in the infrared optical path as possible. A frequency-doubled Nd:YAG laser (INDI-40-10,

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MATERIALS AND METHODS Preparation of Citrate-Capped AuNP. The citratecapped gold nanoparticles were synthesized by reducing the gold salt with sodium citrate. 0.5 mL of 0.029 M chloroauric acid (HAuCl4·3H2O, Alfa Aeser, 99.99%) and 0.1 g of sodium citrate (Na3C6H5O7, J. T. Baker, > 99%) were added into 50 mL of deionized water simultaneously. The solution was vigorously stirred at 80 °C for 30 min and cooled to room temperature for further use. The average diameter of the asprepared gold nanoparticles was 13 ± 2 nm. In order to remove excess citrate, the gold nanoparticle solution was centrifuged at 8,000g for 30 min and the supernatant was discarded. Then deionized water was added to redistribute the precipitate, followed by centrifugation in the same conditions once again. Preparation of CTAB-Capped AuNP. The preparation of CTAB-capped AuNP followed previous works with modifications.30,31 To prepare a gold seed solution, 500 μL of 0.029 M chloroauric acid (HAuCl4·3H2O, Alfa Aeser, 99.99%) was added to 49.5 mL of deionized water in a vial. Then 0.1 g of sodium citrate (Na3C6H5O7, J. T. Baker, > 99%) was added to the solution, which was stirred for 30 min at 80 °C. The solution turned bright red, indicating that the gold ions were completely reduced to neutral gold atoms. Afterward, 8.85 mL of the aqueous solution containing 4.65 × 10 −2 M cetyltrimethylammonium bromide (CTAB, Sigma-Aldrich, > 99%) was prepared in a vial and sonicated until the solution B

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The Journal of Physical Chemistry C Spectra-Physics) provided 5 ns pulses at 532 nm for the excitation of the surface plasmonic resonance band of the AuNPs and operated at a 10 Hz repetition rate by exciting the backside of the sample. A photodiode (DET25K, Thorlabs) was used to collect the scattering of the laser to serve as a trigger. Time-resolved infrared emission spectra were collected using a Fourier-transform infrared spectrometer (Vertex 80, Bruker) operated in the step-scan mode. The raw interferograms were collected using the ac-coupled method. A mercury cadmium telluride (MCT) detector (KMPV8−0.5-J1/DC, Kolmar Technologies) was employed to collect the infrared emission and directly sent to an analog-to-digital convertor (20 MHz, 14 bits, Bruker), without further amplification and electronic filtering. No optical filter was positioned in the midst of the optical path. The spectral range was mainly defined by the IR detector and other light-collection optical components at 900−4000 cm−1, and the undersampling factor of 4 was applied to reduce the acquisition period. The spectral resolution was set at 32 cm−1 and the acquisition of the interferogram required 444 steps of the moving mirrors. The period of a single experiment was 30 min, with 40 laser shots on average at a 10Hz repetition rate at each mirror stop. After the completion of the data acquisition in all the optical retardations, the timeevolved emission spectra were obtained via reverse Fourier transformation of the interferograms. Infrared Emission of Blackbody Radiation. In order to determine the corresponding temperature of the infrared emission of the photoexcited AuNPs, the blackbody radiation contours at different temperatures were collected for comparison. A blackbody radiation simulator (IR-564, Graseby Infrared) provided the standard profiles of the thermal irradiation and was directed to the spectrometer at the same position of the dried nanoparticle sample. A mechanical chopper was mounted in front of the blackbody radiator to modulate the emitting radiance, which was collected using the ac-coupled method with the above-mentioned step-scan spectrometer. The experimental setup is provided in Figure S3. The evolutions of the blackbody emissions at different temperatures are shown in Figure S4. The emission spectra were integrated within the opening slot of the chopper, and the normalized blackbody radiations at different temperatures are plotted in Figure S5. It should be noted that the abovementioned blackbody radiation patterns (Figure S5) are not identical to the theoretical curves because they appeared in the present optical configuration, which included the spectral responses of the optics and MCT detector for collecting infrared emission.

Figure 1. (a) Normalized steady-state extinction spectra of AuNP samples. Electron microscopic images of the AuNPs (b) before and (c) after the 5 ns pulsed 532 nm excitation at 25 mJ cm−2 at 10 Hz for 30 min. The histograms of the size distribution, shown in the inset, and the average sizes are determined on counting 90 particles for each sample.

plasmonic resonance (SPR) band of AuNPs. The size distributions before and after laser excitation were determined by the electron microscopes in Figure 1b,c, respectively. The infrared absorption spectra of three dried AuNP samples (Figure S1) were surveyed, and the vibrational features were attributed solely to their corresponding capping materials. After laser irradiation, their sizes and corresponding distributions (Figure 1c) were broadened relative to those before excitation, suggesting that the excitation fluence (25 mJ cm−2 with 5 ns pulse width) was capable of reshaping or slightly melting the gold nanoparticles. It is noteworthy that the mPEG-AuNPs seemed more resistant to the pulsed excitation, for the shape and size deviations mostly retained the unphotolyzed conditions. Time-Resolved Infrared Emission Spectra. Infrared emission has previously been employed to characterize the temperature evolution during the laser exposure of different systems.26,27 In a previous study, the emissivity of the tungsten surface upon nanosecond pulse excitation was assumed to be constant between the integrated frequencies (equivalent wavelength range of 1−2 μm),26 lacking the multiplexity of wavenumbers in the emission patterns. Moreover, the detector for monitoring the MALDI process is not sensitive when the temperature is lower than 550 K,27 hampering the detection of moderate increases in temperature. Alternatively, step-scan FTIR, having duplexity in temporal and spectral capability,28 is

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RESULTS AND DISCUSSION The nanoparticles were characterized using steady-state UV− vis and infrared absorption spectroscopy to confirm the resonant plasmonic band of AuNP and the attachment of the capping materials on the nanoparticle surfaces, respectively. Electron microscope images were collected to confirm the morphologies of the nanoparticles before and after the laser excitation. Then the time-resolved thermal infrared emission contours were collected to illustrate the temperature evolution and to estimate the near-nascent temperatures after laser heating. Characterization of the AuNPs. The absorption spectra of AuNPs capped with different materials (Figure 1a) exhibited similar spectral contours with a maximum at ca. 530 nm, implying that a 532 nm laser is suitable to excite the surface C

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Hopkins et al. demonstrated that the electron and phonon nonequilibrium only persists within the time scale on the order of picoseconds in metals.36 As a result, the observed infrared emission in submicroseconds results from the thermal infrared emission as the lattices of the gold nanoparticles reach thermalization upon nanosecond pulsed excitation. Since the infrared emission is associated with thermal relaxation, the temperature will gradually decrease as the time evolves, resulting in the gradual redshift of the emission on the assumption of blackbody radiation. The temporal profiles of the integrated intensity at different wavenumber regions, avoiding the vibrational features of the capping materials, possess the character of a rise and a decay, as shown in Figure 3. Regardless

advantageous for detecting the transient infrared emission, which could reflect the temperature evolution. Upon excitation of the SPR band, three dried AuNP samples emitted infrared photons within ca. 1 μs, as shown in Figure 2.

Figure 2. Contours and representative stack plots of the time-resolved infrared emission spectra of the dried samples upon photoexcitation at 532 nm with 25 mJ cm−2 with pulse width of 5 ns: (a) citrate-AuNP, (b) CTAB-AuNP, and (c) mPEG-AuNP. The infrared absorption spectra of sodium citrate, CTAB, and mPEG are also shown by white solid lines in each contour. The spectral resolution is 32 cm−1.

The emission contours extended to higher wavenumbers in the early period and gradually shifted toward lower wavenumbers in the prolonged periods. For citrate-AuNP (Figure 2a), a significant dip at 1579 cm−1 and a medium dip at 1394 cm−1 coincided with the asymmetric stretch and symmetric stretch of the vibration of COO− in citrate, respectively.33 As for CTABAuNP (Figure 2b), an emission contour extended to 2000 cm−1 with a dip at 1470−1480 cm−1, in association with the CH2 scissoring and C−H symmetric stretch of N+−CH3 of CTAB.34 For mPEG-AuNP (Figure 2c), a continuous emission contour extended from 2300 to 900 cm−1 with multiple dips, which were mainly attributed to stretching of the C−O ether linkage (1280−1080 cm−1) and C−H bending (1480 cm−1) of the mPEG.35 It is noteworthy that the vibrational absorption features of the capping materials were inversely correlated with the infrared emission contours. Thus, we demonstrated that the infrared emission was not attributed to the radiative relaxation of the capping materials from the higher vibrational states, which were populated due to the heating by photoexcitation of gold nanoparticles. The broadband emission can most likely be attributed to the thermal infrared emission, and those dips are attributed to the optical absorption of the capping materials.

Figure 3. Normalized traces of evolutions at different spectral regions for (a) citrate-AuNP, (b) CTAB-AuNP, and (c) mPEG-AuNP, respectively, upon photoexcitation at 532 nm with 25 mJ cm−2 with pulse width of 5 ns.

of the capping materials, the normalized traces of integrated intensity at higher wavenumbers decay faster than those at lower wavenumbers, referring to the thermalization process. The spectral radiance of an ideal blackbody, I(ν,T), at a given wavenumber (ν) and temperature (T in K) is expressed as follows: I (ν , T ) =

8π hν 3 1 c 3 (ehν / kT − 1)

(1)

Because the temperature changes with time, the temporal profiles of the infrared emission at different wavenumbers reflect the temperature evolution, T(t). Choosing the detection wavenumbers at ca. 1000 and 2000 cm−1 to represent the high and low wavenumbers, respectively, their corresponding timeresolved radiances can be expressed as follows: D

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The Journal of Physical Chemistry C I(1000cm−1, T (t )) ∝ I(2000cm−1, T (t )) ∝

1 1439/ T (t )

(e

− 1)

(2a)

− 1)

(2b)

1 (e

2879/ T (t )

At the given wavenumbers, the ratio of the intensity at t2 to t1 during the thermal relaxation (t2 > t1), at which the corresponding temperatures are T2 and T1, can be simplified and expressed in the following terms when T is not too high: I(1000cm−1, t 2) I(1000cm−1, t1) I(2000cm−1, t 2) I(2000cm−1, t1)

= e−1439(1/ T2 − 1/ T1) (3a)

= e−2879(1/ T2 − 1/ T1) (3b)

As the time evolves, T2 becomes smaller than T1, and eqs 3a and 3b reveal that the percentage decrease of the infrared intensity at the higher wavenumber is larger than that at the lower wavenumber during the thermalization. Comparing the normalized temporal profiles at different infrared regions, the higher wavenumber (blue dots in Figure 3) exhibits a faster intensity drop than the other two features at lower wavenumbers, which is qualitatively consistent with the relative irradiance of a blackbody during the thermalization, on the basis of eqs 3. Chen et al. modeled the temperature evolution of a 200 nm spherical gold nanoabsorber deposited on BK7 glass, which served as a heat sink.23 After excitation at the Gaussian beam center of the heating laser of 59 mJ cm−2, the temperature dropped to 330 K after 1 μs and reached room temperature after roughly 40 μs.23 Since the thermal conductivity coefficient of CaF2 (9.71 W m−1 K−1, ref 37) is higher than that of BK7 glass (1.115 W m−1 K−1 at 375 K, ref 38), it is expected that the thermalization of AuNPs on a CaF2 window could be slightly faster than that found by Chen et al.23 Because the transient infrared emission was collected from an ac-coupled signal, the diminishing of the infrared intensity at the prolonged period refers to the thermalization of the sample and the surroundings. Surveying the two-dimensional contours of the infrared evolution (Figure 2), the thermalization was completed roughly within 1 μs, which is quantitatively consistent with the prediction on the basis of the results by Chen et al.23 and the values of thermal conductivity of the substrate materials BK7 and CaF2. We thus concluded that the infrared emission was raised by the thermal emission upon the photoexcitation of the gold nanoparticles. Estimation of Transient Temperature. Since the infrared emission could be attributed to thermal radiation, the temperature after the photoexcitation of the AuNPs could be estimated. Upon pulsed photoexcitation, the temperature of a 50 nm AuNP lattice reaches a steady state within 0.1 ns,9 according to the formulization by Carslaw and Jaeger.39 Without using the theoretically predicted emissive contour of the blackbody radiation, apparent blackbody spectra at different temperatures, which include the contribution of the optical components in the spectrometer and light collection optics, were recorded (Figure S5 in the Supporting Information) for comparison with the observed contours. Since the temporal resolution was about 50 ns, as defined by the analog-to-digital convertor (ADC), the normalized infrared emission contours averaged in 90−120 ns (Figure 4) reflected

Figure 4. Comparison of the blackbody emission (colored traces) with the observed emission contours averaged within 90−120 ns (black traces) of (a) citrate-AuNP, (b) CTAB-AuNP, and (c) mPEG-AuNP, respectively. The blackbody radiation contours at different temperatures are provided in Figure S5 in the Supporting Information.

the transient temperature of the acceptable ratio of signal-tonoise. Ignoring the absorption dip due to the absorption of the capping molecules, the observed emission profiles met the blackbody emission profiles at 200 ± 100, 200 ± 100, and 400 ± 100 °C for citrate-AuNP, CTAB-AuNP, and mPEG-AuNP, respectively, by comparing the emission contour in 1500−2200 cm−1 since the emission in the higher wavenumber could be less distinguishable due to the weak intensity. Although the temporal capability was slightly belated and prevents the precise determination of the nascent condition due to the response of the ADC, the observed temperatures in 90−120 ns were already close to the modeling works, which demonstrate temperature increases to 527 °C upon 35 mJ cm−2 pulsed excitation with a pulse width of 4 ns.23 Moreover, the resistance of the shape and size distribution of mPEG-AuNP (Figure 1b,c) at even higher transient temperature suggested that the surface modification through covalent bonding helped to retain the morphology of the nanostructures. Previous studies have reported that the melting point of metallic nanoparticles decreases as the diameter of the nanoparticles is reduced40 and depends on the shapes.41 However, when the diameter of the AuNP is larger than 10 nm, the melting point, which is not strongly dependent on the size, is higher than 1,200 K, close to the melting point of bulk gold.40 Without complete melting, the simulation revealed that the surface premelting of metallic nanoparticles occurred even when the temperature was lower than its normal melting point. The ratio of the number of liquid-like surface atoms to the total number of surface atoms increases with temperature.42 In addition, without extra chemical modification of the gold nanoparticle surface, the particles sinter together and lose their spherical shape completely at 290 °C.43 Our observed relationships of the morphology (Figure 1b,c) and transient temperatures (Figure 4) are consistent with these reports, except for those of the mPEG-AuNP. E

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Figure 5. (a) Contours of the time-resolved infrared emission spectra of the dried CTAB-AuNP of different diameters upon photoexcitation at 532 nm with 15 mJ cm−2; (b) before and (c) after the photoexcitation. The histograms of the size distribution, shown in the inset, and the average sizes are determined on counting 90 particles for each sample.

relaxation could be only carried out by the molecular chains of mPEG. Since the thermal conductivity of polyethylene glycol (ca. 0.25 W m−1 K−1, ref 46) is much smaller than gold (314 W m−1 K−1), the transient temperature of mPEG-AuNP could be higher than the other two. Temperature Evolution upon Excitation of Different Sizes of CTAB-AuNP. The infrared emission contours of 35, 55, and 89 nm CTAB-AuNPs were collected and are shown in Figure 5a. The normalized integrated intensities at 1000−1250 cm−1 are shown in Figure 6, indicating that the thermal relaxation of the smaller AuNPs was slower than that of the larger ones. This observation suggested that the initial temperature of the 35 nm AuNPs was higher than that of the 89 nm ones. In addition, the morphologies of the AuNPs were

According to the report by Tsuji et al., the agglomeration of the NPs prior to the laser-induced melting can generate spherical particles on the submicron scale.44 The laser irradiation causes the decomposition and removal of citrate molecules on the surface of the source NPs, dynamically causing the agglomeration of the source NPs. Since the citrate and CTAB are bound to the NP surface through electrostatic forces, the rising temperature can cause drifting of these capping materials, followed by the aggregation and fusion via the surface premelting, leading to the increase in size. Furthermore, Tsuji et al. also demonstrated that dispersed NPs will not fuse upon laser irradiation.44 For mPEG-AuNPs, gold atoms were covalently bound with sulfur via the thio group of mPEG, resulting in higher dissociation energy to separate the mPEG and AuNP. Moreover, the end-to-end length of 5 kDa mPEG is about 8 nm, according to a Gaussian random coil model,45 supporting the better dispersivity of mPEG-AuNP exhibited in the TEM image (Figure 1b). Accordingly, even though the instantaneous temperature of mPEG-AuNP was higher than those of the other two samples and premelting probably occurred, no significant changes in the morphology and dispersivity were observed. It is essential for photothermal applications to prevent the degradation of nanostructures during photoexcitation. In addition, since CTAB and citrate are not strongly and covalently bound to the gold nanoparticle surface, the heat generation upon photoexcitation of the gold nanoparticles probably leads to desorption of CTAB and citrate. Subsequently, the potential surface melting leads to the aggregation and enhances the thermal relaxation via gold lattices, resulting in the lower transient temperature. However, the mPEG on the gold nanoparticles are not going to desorb due to the covalent bonding of S−Au. Therefore, the heat

Figure 6. Normalized traces of evolutions at 1000−1250 cm−1 for different sizes of CTAB-AuNPs upon photoexcitation at 532 nm with 15 mJ cm−2 with pulse width of 5 ns. F

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The Journal of Physical Chemistry C significantly altered after pulsed photoexcitation, as shown in Figure 5c. According to the results in the previous section, we found that the surface modification of AuNPs without covalent bonding was not capable of retaining its original morphology. Most of the 35 nm CTAB-AuNPs aggregated and melted, whereas the 85 nm AuNPs partially retained their original morphology; this is consistent with the temperature evolution in Figure 6 in that the initial temperature of the smaller CTABAuNP is higher than that of the larger CTAB-AuNP. Mie theory can be used to differentiate the contribution of scattering and absorption in the extinction coefficient of metallic nanoparticles. The larger gold nanoparticle possesses higher content of scattering. Our observations were qualitatively consistent with the Mie theory prediction of the extinction coefficient, which contains a lower contribution of absorption as the diameter of the AuNP increases.29 The contribution of absorption in light extinction can result in the photothermal process29,30 and the consequent increase in the AuNP lattice temperature.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 886-3-5715131 ext. 33396. Fax: 886-3-5711082. Email: [email protected]. ORCID

Li-Kang Chu: 0000-0001-6080-9598 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the grants from the Ministry of Science and Technology of Taiwan (MOST 103-2113-M-007010-MY2 and MOST 105-2113-M-007-014- for L.K.C. and MOST 105-2113-M-400-005- for J.K.C.) and National Health Research Institutes (NHRI-BN-105-PP-27 for J.K.C.).

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CONCLUSIONS The photothermal phenomenon of the AuNPs capped with different molecules, namely, citrate, CTAB, and mPEG, has been investigated by time-resolved infrared emission spectroscopy. A step-scan Fourier-transform interferometer provides duplexity on the temporal resolution and broadband spectroscopic window to illustrate the thermal evolution of the AuNPs in a noncontact fashion. Upon 5 ns pulsed photoexcitation at 532 nm, the observed transient thermal infrared emission lasted about 1 μs, which is consistent with the predicted thermal conduction. Comparing the transient infrared contours in 90− 120 ns with the blackbody radiation revealed that the temperatures reached 400 ± 100 °C as the 24 nm mPEGcapped AuNP was excited by a fluence of 25 mJ cm−2. Moreover, the resistance of the morphology and size distribution of mPEG-AuNP indicated that the surface modification through covalent bonding helped to retain the morphology of the nanostructures. In addition to the different capping materials, the photoexcitation of 35 nm CTAB-AuNPs led to a higher transient temperature than that of 89 nm ones, indicating that the smaller AuNPs had higher photothermal efficiency because the smaller nanoparticles possessed a higher contribution of absorption in the extinction coefficient. Our observations are of great significance and importance because the transient temperature is detectable in the near-nascent condition, which is crucial for the applications of photothermal effects of metallic nanostructures in areas such as catalysis, thermotherapy, and controllable drug release. For further application, the combination of a wavelength-tunable laser system and time-resolved emission spectroscopy could serve as a promising noncontact instantaneous spectroscopic thermometer for characterizing the photothermal processes of various nanostructures.

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time-resolved step-scan Fourier-transform spectrometer operated in emission mode (PDF)

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ABBREVIATIONS CTAB = cetyltrimethylammonium bromide mPEG = methoxyl-poly(ethylene glycol)-thiol AuNP = gold nanoparticle REFERENCES

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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.6b10044. Infrared absorption spectra of citrate-AuNP, CTABAuNP, and mPEG-AuNP, blackbody emission spectra at different temperatures, and the experimental setup of the G

DOI: 10.1021/acs.jpcc.6b10044 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.jpcc.6b10044 J. Phys. Chem. C XXXX, XXX, XXX−XXX