Enhanced photoacoustic (PA) intensity from gold nanosphere and nanorod colloidal suspensions in water under tightly-focused femtosecond pulsed laser irradiation was systematically investigated. PA signal amplitudes were measured by ultrasound transducers at frequencies of 5, 10, and 25 MHz. The experimental results revealed a linear-dependence of the relative photoacoustic amplitude on the laser power and the mechanism was attributed to non-radiative relaxation dynamics of surface plasmon oscillations. When gold nanorod with longitudinal absorption/extinction peak at 800 nm coincides with the wavelength of femtosecond laser pulses, the most efficient PA signal is generated. Laser excitation was kept within a thermal stability region of gold nanoparticles, i.e., colloidal suspension can be continuously reused for PA generation.
© 2016 Optical Society of America
Recent advances in photoacoustic (PA) techniques based on non-radiative conversion of absorbed light energy into heat and its accompanying acoustic effects demonstrated a great contribution in various biomedical sensing [1–3], imaging [4,5], and therapeutic applications [6,7]. In most optical imaging techniques, a strong light scattering by biological tissues has been a major problem for a deep penetration imaging. Therefore, new ideas that combine ultrasound and near-infrared (NIR) light to overcome the negative impact of multiple scattering has been investigated in recent years. PA imaging is dependent on the number of incident photons absorbed by the target and converted into heat during thermo-elastic expansion which leads to ultrasound wave generation. It provides higher penetration depth and a better spatial resolution compared to the optical imaging modalities . It combines the resolution of ultrasound imaging and contrast of optical imaging which depends on the optical absorption properties of media. The contrast in PA imaging is based on optical-to-acoustic conversion efficiency and can be greatly improved by the incorporation of various contrast agents .
To enhance the contrast of a photoacoustic image, several contrast agents such as fluorescent dyes [10, 11], metallic , and polymeric  nanoparticles have been developed for better complementary image reconstruction. In particular, plasmonic metal nanoparticles have been introduced as contrast agents in recent years [11–16]. Tailored design of gold nanoparticles and nanostructures is of great importance, primarily due to their biocompatibility , superior optical and catalytic properties such as large absorption cross-section, resistance to high illumination fluences and spectral selectivity based on surface plasmon resonance  for potential applications in photo-electronic, photo-catalytic [18, 19], and solar cells . In metals, the collective oscillation of free electrons causes local temperature increase on the surrounding surface. Gold nanoparticles have tunable absorption peaks in the visible and near infrared ranges, which depends on particle sizes and shapes, allowing them to be very effective energy absorbers at a desired wavelength of light generating a local thermal effect . It has been known that the quantum yield (emitted photon per absorbed photon) from metal nanoparticles is orders of magnitude higher than organic dyes . The large surface area of gold nanoparticles permits them to be applied on targeting strategies for molecular imaging and site-specific therapy . Hence, new techniques on photoacoustic imaging and photothermal therapy have been developed where unique properties of gold nanoparticles are used to achieve enhanced imaging contrast.
Nanoparticle-facilitated absorption of pulsed light leads to the production of ultrasound waves used for PA imaging. Pulsed-laser irradiation to metal nanoparticles or nanoclusters has been extensively investigated for the treatment of cancer cells , bacteria, and viruses in therapeutic and biomedical applications . Photothermal therapy relies on the resonant absorption of light by nanoparticles and the conversion of the electromagnetic energy into heat to destroy malignant tissues. When gold nanoparticles are irradiated by short laser pulses, a rapid increase in temperature resulting in photothermal and other accompanied phenomena leads to highly localized cell damages . In image-guided photothermal therapy, photoacoustic imaging can be used both to confirm the delivery of metal nanoparticles to a desired location in tissue and to visualize temperature maps during photothermal therapy .
Recently, the interaction of nanosecond laser and metal nanoparticles has been explored in photoacoustic and photothermal studies but much less is known for the control of acoustical signals with ultra-short laser pulses [11, 23]. The photoacoustic signal generation through optical energy deposition induced by femtosecond near-infrared (NIR) pulses has potential in photo-acoustics  since it can deliver energy deposition faster than the electron-phonon relaxation time (picoseconds). By employing NIR light excitation, the imaging depth can be potentially extended to deeper regions in tissue as compared to visible light excitation to metal nanoparticles [23, 25, 26]. Sensitivity as well as tunability of surface plasmon resonance wavelength, which is closely related to the size and shape of metal nanoparticles, can provide control for PA imaging. Tunability of the plasmon resonance wavelength to the near-IR region where human tissue exhibits high sensitivity and transmission is considered to be highly beneficial.
Here, acoustic wave emission from femtosecond laser-irradiated Au nano-colloidal suspensions in water was systematically investigated. Three different ultrasound transducers at 5, 10, and 25 MHz were used to compare the PA signals generated by the interaction between femtosecond near-IR laser pulses and Au nanoparticles. Dependence of the photoacoustic amplitude on the laser power (<100 mW) was investigated in the case of nanoparticle-facilitated absorption of laser pulses. Au nanospheres and nanorods were utilized to demonstrate sensitivity of acoustical response to plasmonic excitation band.
2. Materials and methods
2.1. Sample preparation
All chemicals used in this study were used as received: gold (III) chloride hydrate (HAuCl4(aq), Sigma-Aldrich), trisodium citrate (C6H9Na3O9, Sigma-Aldrich), cetyltrimethylammoniumbromide (CTAB, Sigma), sodium borohydride (NaBH4, Aldrich), silver nitrate (AgNO3, Aldrich) and L-ascorbic acid (C6H8O6, Sigma-Aldrich). Milli-Q water with a resistivity of 18.2 MΩcm at 25°C was used in all experiments.
Synthesis of Au nanospheres
Chemical syntheses were followed by the procedures reported previously . Briefly, a kinetically-controlled seeded growth synthesis of citrate-stabilized Au nanospheres was used. In a 250 mL three-necked round-bottomed flask, a solution of 2.2 mM sodium citrate in Milli-Q water (150 mL) was heated at 115°C for 15 min under vigorous stirring. A reflux condenser and an oil bath were used to prevent the evaporation of the solvent. After it reached the boiling point, 1 mL of HAuCl4(aq) (25 mM) was added. The color of the solution changed from yellow to bluish gray and then to light pink in 10 min. The resulting Au seed particles (∼ 10 nm in diameter) were coated with negatively charged citrate ions and completely dispersed in water.
Immediately after the synthesis of the Au seed solution, the temperature was cooled down to 90°C and seeded growth of Au nanospheres was carried out. Then, 1 mL of HAuCl4(aq) solution (25 mM) was injected on the reaction vessel. The reaction was finished after 30 min and the process was repeated twice. After that, the sample was diluted by extracting 55 mL of the sample and adding 53 mL of Milli-Q water and 2 mL of 60 mM sodium citrate. This solution was then used as seed solution, and the process was repeated again. By changing the volume in each growth step, it is possible to tune the seed particle concentration. Au nanospheres with a diameter of ∼ 20 nm and concentration of ∼ 1.4×10−4 M were used for photoacoustic detection and measurements.
Synthesis of Au nanorods
CTAB-stabilized Au nano-rods were synthesized by seed-mediated growth method [28,29] wherein 5 mL of 0.20 M CTAB solution was first mixed with 5 mL of 0.5 mM HAuCl4(aq). Then, 0.6 mL of iced-cold 0.01 M NaBH4 solution was added to the mixture and vigorously stirred for 2 min under 25°C which resulted to the formation of a brownish yellow seed solution. The growth solution was carried out by adding 0.15–0.2 mL of 4 mM AgNO3 and then 5 mL of 1 mM HAuCl4 to 5 mL of 0.2 M CTAB solution under gentle stirring. Then, 70 μL of 7.9 × 10−2 M L-ascorbic acid was added. For the growth of Au nano-rods, 12 μL of seed solution was added at 27–30°C under gentle stirring for 30 sec. The solution transparency was changed to burgundy red after 10–20 min and then aged for another 12 hours at 27–30°C before twice centrifugation at 6000 rpm for 90 min. The collected Au nanorods with absorption peak at ∼ 800 nm and concentration of ∼ 1.4 × 10−4 M were re-dispersed in Milli-Q water and used in the experiments.
Therefore, Au nanospheres were synthesized via citrate reduction method wherein the seeded growth synthesis of size and shape controlled citrate-stabilized Au nanoparticles was described. It is attributed to the inhibition of secondary nucleation during the homogenous growth process, allowing enlargement of Au nanoparticles via surface-catalyzed reduction of Au3+ by sodium citrate. On the other hand, Au nanorods were synthesized via the CTAB-stabilized seeded growth method where the use of cationic surfactant (CTAB) and AgNO3 as ”shape-inducing” agents for size and shape control leads to tunability of optical properties in the visible and NIR spectral regions. Au nano-colloidal suspensions of spheres and rods with Au atomic concentration of ∼ 1.4 × 10−4 M from HAuCl4(aq) dominates over the other chemical components such as AgNO3 with a concentration of ∼ 1.6 × 10−5 M. The major contribution of light absorption was from Au nanoparticles and other chemical components had a negligible contribution.
2.2. Sample characterization
UV-visible absorption spectra were recorded by an absorption spectrometer (Spectrostar Nano, BMG Labtech). A spectral analysis of Au colloidal suspension on quartz cell was conducted in the 300–1000 nm range at room temperature. The absorption spectra of Au nanospheres and nanorods are shown in Fig. 1. The spectrum of Au nanospheres (∼ 20 nm in diameter) shows a characteristic strong absorption band at ∼ 520 nm which is the surface plasmon resonance of Au nanoparticles [30, 31] originating from a strong light absorption/scattering by coherent oscillation of the conduction band electrons . Different particle sizes of Au nanospheres have its representative extinction band (total losses due to absorption and scattering) . In the case of Au nanorods, the absorption spectrum is represented by transverse and longitudinal extinction bands. The shorter wavelength band at ∼ 520 nm is attributed to the transverse surface plasmon resonance which represents spherical nanoparticle impurities. While, the longer wavelength band at ∼ 800 nm is due to the longitudinal surface plasmon resonance defined by the length of the nanorod. The longitudinal band can be controlled by changing the aspect ratio of Au nanorods depending on the desired wavelength of interest . An 800 nm longitudinal absorption band of Au nanorod with an aspect ratio of ∼ 2.92 which coincides with the wavelength of femtosecond laser at 800 nm was chosen for the generation of ultrasound.
The shape and morphology of the synthesized Au nanospheres and nanorods were observed by field emission transmission electron microscopy (FE-TEM, JEOL JEM-2100F). For TEM sample preparation, a drop of Au nanoparticle suspension was casted on copper-Formvar grids and air dried at 25°C. The grids were imaged and the morphology of nanoparticles was analyzed. TEM images of Au nanospheres and nanorods are shown in Fig. 1(a). A uniform dispersion of spherical nanoparticles with a diameter size of 20 ± 2 nm was observed [Fig. 1(a)]. The nanorods [Fig. 1(b)] had length of 35 ± 4 nm and width of 12 ± 2 nm. For detailed comparison of photoacoustic signal generation, the same concentration and volume of nanoparticles was used for photoacoustic experiments. Namely, Vsphere = 4πr3/3 with a diameter of 20 nm has the same volume of ∼ 4.2 × 103 nm3 as nanorods Vrod = πr2h with length of 35 nm and width of 12 nm or ∼ 4.0 × 103 nm3. Au nano-colloidal solutions with Au atomic concentration of ∼ 1.4 × 10−4 mol/L, particle concentration of ∼ 4.21 × 1018 NPs/mL and volume of ∼ 4.0 × 103 nm3 were used in the experiments. Therefore, the only parameter controlled in this study is the shape of Au nanoparticles with different absorption/extinction wavelengths.
2.3. Photoacoustic system experimental set-up
Figure 2 shows the experimental setup of the photoacoustic detection and measurements. NIR femtosecond laser pulses (duration 35 fs, wavelength 800 nm, repetition rate 1 kHz, horizontal polarization, Legend Elite HE USP, Coherent, Inc.) were focused in water onto the sample using an objective lens (10×, numerical aperture NA = 0.28, Mitutoyo). A custom-made ultra-sound detection system was used for the measurement of photoacoustic signal generated by Au nano-colloidal suspensions. A 5-mm-diameter glass tube controlled by a 3D mechanical stage was immersed in an acrylic water tank with a fused silica optical window (the water enclose emulates, as a first approximation, a tissue response). The glass tube contains an inlet and an outlet for the injection of sample solution and was allowed to circulate using pump. A 50 mL Au nanosphere or nanorod colloidal suspension of 1.4 × 10−4 mol/L concentration was injected into the tube.
Three single element unfocused ultrasound transducers: (V310-SU, Olympus) 5 MHz, (V327-SU, Olympus) 10 MHz, and (V324-SM, Olympus) 25 MHz with −6 dB fractional bandwidth of ∼ 55% were mounted on a 3D positioning stage. The V310 has fractional bandwidth of 3.635–6.375 MHz, V327 7.25–12.75 MHz, and V324 18–31.875 MHz. The distance of the ultrasound transducer and glass tube filled sample was kept constant at 15 mm during the entire experiment. Femtosecond laser pulses were introduced through the optical window into the water tank and focused on the center part of the glass tube filled sample (Fig. 2). Photoacoustic signals were detected and amplified using an ultrasound preamplifier (Olympus, 5678). The acquired amplified PA signals were recorded and analyzed using oscilloscope (Agilent Tech., DSO-X 3034A).
3. Results and discussion
3.1. Photoacoustic measurements
The change in plasmonic resonance wavelength at the maximum extinction (absorption or scattering) is sensitive to the change of physical and chemical environment on the surface of the nanoparticle. Thus, a high sensitivity of the spectral response of plasmonic resonance band to the changes in the refractive index of the surrounding is achieved . Since the PA amplitude is proportional to the optical absorption, a strong increase in the PA signal is expected at resonance band of Au nanoparticles depending on size, shape, and environment.
Indeed, Chen et al. reported the PA signals generated by the gold silica core-shell nanorods with controllable silica thickness and its stability in aqueous solution for nanosecond laser pulses . Fukusawa et al. have demonstrated that the amplitude of PA signals generated from the aqueous suspensions of spherical Au nanoparticles at an ambient temperature decreases with increasing particle size . The shape and size dependence of the surface plasmon resonance of gold nanoparticles was studied by El-Brossy et al. for PA measurement which is in agreement with the optical absorption wavelength . These results revealed that the PA signal generated from particle suspension is mainly affected by the optical absorption of the nanoparticles, the physicochemical properties of the surrounding medium and the particle-medium interface. Acoustic wave emission by femtosecond laser-irradiated Au nano-colloidal suspensions was acquired by PA detection [Fig. 2(a)]. The representative PA signal in time domain is shown in Fig. 2(c). The first peak represents the fundamental ultrasound signal (a pressure wave) and the second peak corresponds to the internal reflection of ultrasound waves from the inner surface of the glass tube with a diameter of d = 5mm. The time interval between the two peaks is t = 4.56 μs, which can be ascribed to the difference in travel distances Δd of the two signals in water solutions where the speed of sound is (vH2O = 1500 m/s). The distance, Δd, can be estimated to be 6.84 mm. Therefore, the laser focus, which is the ultrasound source, is about 1 mm above the central position of the glass tube. The first peak signal which corresponds to the fundamental ultrasound signal was used as a measure of photoacoustic response from femtosecond laser-irradiated Au nano-colloidal suspensions.
3.2. Photoacoustics from Au nanospheres and nanorods
The detailed results of PA signals generated by water, Au nanospheres, and nanorods irradiated at 10–100 mW laser power are shown in Fig. 3. Three ultrasound transducers at different central frequencies of 5, 10, and 25 MHz were used for comparison. A hydrophone (HNA-0400, ONDA) with a frequency range from 1–20 MHz was used for the calibration of the three transducers. A close to linear dependence of the photoacoustic signal amplitude on laser power was observed with amplitude proportional to laser power. The PA signal amplitude of Au nanorods was enhanced more than 2 times as compared to that of water while Au nanospheres showed slight enhancement of 1.5 times than the case of water at the frequency of 5 MHz when the laser power is at 100 mW (Fig. 3). The enhancement is due to the surface plasmon resonance of Au nanorods at 800 nm matching excitation wavelength. Off-resonance excitation of Au nanospheres leads to lower PA signal [30, 35]. Though the ultrasound intensity is enhanced with Au nanorods, indeed, due to resonant surface plasmon contribution [Fig. 1(b)], the local field enhancement factor is yet another factor contributing to absortion and heating. In solution phase, Au nanoparticles are homogeneously distributed and randomly oriented. Therefore, effective light absorption when the light is linearly polarized is less pronounced as compared with calculations for perfect alignment. The local field enhancement at the solution–nanoparticle interface is indeed comparable for the resonant and non-resonant cases as shown in the simulations (Fig. 5). In addition, when electrons oscillate along with the polarization, they enhance heat transfer along polarization . This can smear the difference between rods and spheres. The PA signal is produced mainly by the non-radiative decay of the absorbed energy therefore the reduction of this non-radiative decay leads to the reduction in the PA signal . Figure 3(a), (b), and (c) show the PA signal amplitude generated by water, Au nanospheres, and nanorods respectively. PA signal amplitude decreases while the frequency increases, i.e., the highest PA signal intensity was detected by 5 MHz ultrasound transducer and lowest at 25 MHz. This phenomenon is a drawback since a high frequency ultrasound is applied for high-spatial resolution ultrasound imaging. An increase in PA generation efficiency is a targeted solution. Nanoparticle-facilitated absorption of pulsed light mechanism basically relies on the non-radiative relaxation dynamics of the surface plasmon oscillations of Au nanoparticles . The ratio of radiative to non-radiative losses depends on particle size and wavelength , and can be engineered to maximize one versus the other; for nanospheres of radius r, the extinction cross section σext = σabs + σscat ∝ r3/λ + r6/λ4 . Smaller particles absorb stronger than scatters and vice versa.
3.3. Frequency difference
Figure 3(d) shows the frequency dependence on PA signal amplitude using water, Au nanospheres and nanorods. Based on the results, the PA amplitude at 5 MHz has the highest signal dectection sensitivity compared to 10 and 25 MHz ultrasound transducers. The selection of laser pulse duration correlates with the optimization of the PA emission for detection. With a 75 ns pulsed laser, previously Eghtedari  showed that the PA signal can be detected to up to 5 MHz level. Wang  revealed that the PA emission can still be measured with ultra-sound transducer of 25 MHz using 6.5 ns laser pulses. Similarly, Agarwal  showed that the PA signal with a frequency of up to 50 MHz can be detected when a 5 ns pulsed laser was used. Here, we showed for the first time that with the laser pulses of tens-of-fs can be used to generate sufficiently high PA signal. This is potentially valuable in high resolution diagnosis and imaging, since the PA imaging resolution highly depends on the PA emission frequency. A femtosecond laser pulse can induce PA emission up to GHz region  which could be used to perform nano-scale mechanical actuation on a single-cell surface.
3.4. Thermal stability of Au nanospheres and nanorods
During PA detection and measurements, Au nanoparticles were exposed to femtosecond laser pulses. The nanoparticles absorbed a portion of light and generate substantial heat that can lead to nanoparticle reshaping and reduction via atom surface diffusion even at temperatures considerably lower that the bulk melting temperature , moreover, the effect is stronger for high-aspect ratio nanorods. Nanoparticle melting has been reported for nanospheres and nanorods to occur at significantly lower temperature than bulk material because surface reorganization process dominates [45–47]. Hence, thermal stability of nanoparticles has to be addressed for practical applications and choice of irradiance conditions as discussed next.
The thermal stability of Au nanospheres and nanorods in aqueous solutions was investigated by UV-vis absorption spectroscopy. Figure 4 shows the absorption spectra of Au nanospheres (a) and nanorods (b) before and after the laser irradiation experiments shown in Fig. 3. In the case of Au nanospheres, high thermal stability was observed due to its off-resonance wavelength. Meanwhile, in the case of Au nanorods, it was found that the longitudinal surface plasmon resonance band was slightly shifted to shorter wavelength. In the case of excitation at the on-resonance wavelength of Au nanorod, a rapid temperature increase leads to a possible shape deformation. It has been reported that shape deformation is inhibited by reducing the concentration of CTAB . Zou et al. showed that ∼ 10−3–10−2 M CTAB concentration can enhance the thermal stability of Au nanorods even at 95°C . In our experiments, the Au nanorod colloidal suspension with CTAB concentration at 10−2 M after centrifugation was used. Au nano-colloidal suspensions were irradiated by the laser pulses at 10–100 mW varying by 10 mW steps. The irradiation time for each laser power was 1 min, in total of 10 mins, with the laser repetition rate of 1 kHz. Therefore, Fig. 4(b) shows the spectrum of the Au nano-colloidal suspension after the irradiation of 6 × 105 laser pulses. The total volume of the suspension used in the experiment was 5 × 10−5 m3 while the effective laser focus volume was estimated to be 1.09 × 10−11 m3 based on an imaging (not shown). Therefore, the total solution volume which is laser-irradiated can be estimated to be 6.54 × 10−6 m3 which is comparatively small; about 10% of the total volume of the sample suspension.
Since surface diffusion driven reshaping of Au nanorods occurs at low temperature it is informative to simulate light intensity distribution around the nanoparticles in water. Different E-field components are plotted for planar cross section of Au nanorod and nanosphere in Fig. 5. In the case of Au nanorods, a response to non-polarized light illumination was simulated to reveal the polarization effect at two wavelengths which correspond to the T- and L-modes [Fig. 4(b)] of the nanorods, respectively (top rows in Fig. 5). For Au nanosphere, it was enough to use a linearly polarized light source for illumination due to symmetry arguments; pattern of the light intensity distribution is similar and only differs by an absolute value according to the scattering cross section σscat ∝ r6/λ4. It is noteworthy that considerable light concentration occurs at the interface between nanoparticle and solution for the E-fields perpendicular to the interface. Also, a considerable portion of light intensity is in the longitudinal Ez (along light propagation) due to depolarization. The normal to the surface and Ez components are expected to deliver the most of absorption and heating. Laser ablation of surfaces in the light field component perpendicular to the interface has been demonstrated . An enhanced heat diffusion occurs along the E-field orientation as was shown by polymerization using ultra-short laser pulses . This augmented heat transport along the light field at the interface is a factor which needs further investigations and could be harnessed for generation of stronger PA signals. It is well known that finite difference time domain (FDTD) calculations predict well light field enhancement as was demonstrated in polymerization experiments . Since nanoparticle size in the regime is comparable with skin depth, a substantial portion to light intensity is absorbed by nanoparticle and drives an acoustic response.
It is important to select laser irradiation conditions such that thermal reshaping of nanoparticles would not take place. However, due to a linear scaling of PA signal on irradiance, one could consider a flow system where fresh nanorod solution should be fed into the generation chamber where reshaping would yield spherical nanoparticles. At high laser fluence, such method is used to make nanospheres with narrow size distribution .
Nanoparticle-facilitated absorption of pulsed light leads to the generation of ultrasound waves which can be used for PA imaging and photothermal therapy. A higher PA signal intensity was observed for the plasmonic longitudinal resonance (L-mode) band of Au nanorods compared to Au nanospheres. The mechanism of interaction between femtosecond laser and Au nanoparticles is attributed to a non-radiative relaxation dynamics of surface plasmon oscillations. No significant shape transformation of nanorods into spheres after femtosecond laser irradiation was observed under low laser power of 10–100 mW. Enhanced acoustic wave emission from gold nano-colloidal suspensions is a promising technique for PA imaging since it provides wide band frequency detection with high PA signal intensity sensitivity even at high frequencies. The high frequency PA signal is necessary for high resolution photoacoustic microscopy.
SJ is grateful for partial support via the Australian Research Council DP130101205 Discovery project. TY is grateful for the partial support from Murata Foundation.
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