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Abstract
In this study, an optical manipulation and micro-surface-enhanced Raman scattering (microSERS) setup based on a microcavity was developed for efficient capture of gold nanoparticles using the photothermal effect. In addition, optical manipulation of gold nanoparticles and SERS signal detection were performed using only one laser. The results show that the SERS enhancement effect based on the microcavity was more than 20 times that based on a gold colloid solution. The laser power and velocity of nanoparticles exhibited a good linear relationship, and the velocity of nanoparticles decreased with decreasing radius r, which verifies the detriment of the radial thermophoresis in this study. This method can be used to quickly and efficiently drive metal nanoparticles and provides a promising approach for analysis of substances in the fields of chemistry and biology.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) enables molecules to be identified by their vibrational “fingerprints” [1,2]. The enhancement effect is sensitive to metal nanoparticles, which are usually the molecular carriers [3]. Research shows that aggregates of metal nanoparticles have a higher enhancement effect than single metal nanoparticles; their enhancement factor can reach 14 orders of magnitude even if the resonance Raman effect is excluded [4–6]. Aggregation of nanoparticles results in a large number of particle junctions, which are called “hot spots” [7,8]. The plasma coupling of metal nanoparticles at these “hot spots” produces strong local electromagnetic fields, which can greatly enhance the SERS signal intensities of molecules in the “hot spot” regions [9,10].
Optical manipulation is one of the methods used to produce metal nanoparticle aggregates. Ashkin first captured micron-sized dielectric particles in a stable optical potential well generated by a laser beam in 1970 [11], and subsequently, optical manipulation technology was rapidly developed. However, due to the high reflection and absorption of metal nanoparticles, few reports about the optical trapping of metal nanoparticles have been presented. Until 1994, gold nanoparticles were trapped using a 1064 nm laser, which is far from the plasma resonance wavelength [12]. In 2006, optical tweezers was first used to aggregate silver nanoparticles and detect the Raman signal of thiophenol, which could not be detected with single nanoparticles [13]. The combination of optical manipulation and SERS can achieve a higher enhancement effect [14–16].
Optical manipulation can be realized using a gradient force, a scattering force [10,17,18], and a photothermal effect [19,20]. Scattering force and gradient force are associated with momentum changes in the electromagnetic wave due to scattering by the dipole and the Lorenz force acting on the induced dipole, respectively [21,22]. The photothermal effect mainly includes photophoresis, thermophoresis and convection. Photophoresis is caused by uneven heat distribution on the surface of a particle irradiated by a light beam, and the direction of the force is determined by the thermal conductivity of the particles [23,24]. Thermophoresis is caused by the temperature gradient force of liquid, which repels particles along the temperature gradient from a heated spot [19]. The absorbed laser power produces a temperature difference in the solution, which causes thermal convection. Then, water flows in the solution, and the flowing water has the ability to drag the particles [25]. Until now, metal nanoparticles have been trapped using gradient force and scattering force, and the photothermal effect has primarily been used to trap transparent dielectric particles. However, optical gradient force and scattering force exert only a few femtonewtons to piconewtons [26], and can only affect the particles in a region the size of a few microns. Therefore, the main challenge is that the weak gradient force on nanoparticles is insufficient to overcome the destabilizing effect of the scattering force and Brownian motion. The experimental setup used to trap metal nanoparticles using a gradient force is usually complex, and two laser beams are used to manipulate particles and excite Raman signals. However, trapping and manipulating particles in a larger region is still difficult; the number of particles trapped is limited, and the process takes a long time.
In this study, for the first time, an optical manipulation and microSERS setup based on a microcavity was developed for efficient capture of gold nanoparticles using the photothermal effect. This optical manipulation and microSERS setup can trap gold nanoparticles and excite SERS signals using only one laser, and the morphology and size of gold nanoparticle aggregates can be controlled by reconfiguring the microcavity. Based on this system, the effect of the photothermal effect on the capture of gold nanoparticles and the influence of the microcavity on the gold nanoparticle aggregates were studied. In this study, the influence of the laser power on the flow velocity of gold nanoparticles was calculated and the detriment of radial thermophoresis for optical trapping of gold colloid was verified. The enhancement effect for different aggregation times was studied using a polycyclic aromatic hydrocarbon (pyrene) as the analyte molecule.
2. Experimental
2.1. Chemicals and materials
To prepare gold colloid solution, the following chemicals were purchased from Sinopharm (Beijing, China): chloroauric acid (HAuCl4•4H2O), trisodium citrate and methanol. Pyrene was purchased from Sigma-Aldrich (St. Louis, Mo).
In this study, pyrene was detected as an analyte molecule. In the process of preparing the pyrene sample, solid pyrene was dissolved in methanol and then diluted with distilled water. A 500 nM pyrene solution was prepared.
2.2. Gold colloid preparation
The gold colloid was obtained according to the method of Frens [27]. Based on the results of our previous study [28] and the excitation wavelength used in this study, the molar density of the gold colloid solution was 0.78 nM and the average diameter of the colloidal gold nanoparticles prepared here was 60 ± 10 nm.
2.3. Microcavity preparation
Linear microcavities of different sizes were etched on the surface of support substrates using a femtosecond laser by controlling the power of the pulsed laser, the size of the focal spot, the scanning speed and the number of scans. Quartz wafers were selected as support substrates (1.5 cm long, 0.5 cm wide and 0.2 cm thick) for transmission imaging. Linear microcavities 30 µm deep and 10 µm wide, 30 µm deep and 30 µm wide, and 30 µm deep and 60 µm wide were etched on the surface of the quartz wafers.
2.4. Optical manipulation and microSERS setup
We built an optical manipulation and microSERS setup that has the simultaneous functions of microimaging, optical manipulation and microSERS spectrum detection. An array light source (Thorlabs, LIUCWHA) was used for white light illumination. One 785 nm laser (CrystaLaser, DL-785-500-N) was focused on the sample through an objective lens (Nikon S Plan Fluor 40X, NA=0.65, WD=3.6∼2.8 mm) to manipulate gold nanoparticles and excite the Raman signal. A Raman spectrometer (QE65000, Ocean Optics) equipped with a grating of 1200 g/mm allowed a 6 cm−1 spectral resolution. The gold nanoparticle aggregation process in the microcavity was recorded by a CCD camera (Infinity 2-1R).
3. Results and discussion
The light path diagram of the optical manipulation and microSERS setup is shown in Fig. 1. Optical manipulation of gold nanoparticles and SERS detection shared the same laser, which simplified the experimental setup. A quartz support substrate with a microcavity was placed at the bottom of the vessel, which was filled with a mixture of gold colloid and pyrene solution (V:V=1:3). Optical manipulation of gold nanoparticles and SERS spectrum detection were carried out simultaneously. The laser power at the sample ranged from 60 mW to 120 mW, the diameter of the focal spot was approximately 16 µm, the integration time was 1 s, and the SERS signals were collected every 1 min until the signal reached stability, which indicated that the number of gold nanoparticles in the microcavity reached saturation.
Our calculation based on the dipole approximation [29] is shown in Fig. 2, and indicates that when the field radius r exceeds 20 µm, the horizontal gradient force is very small and can be ignored, thus, the gold nanoparticles cannot be trapped in a large region by the gradient force. Therefore, in this study, the large-scale capture of gold nanoparticles was achieved by increasing the laser power to generate a photothermal effect. The high energy laser beam was focused on the microcavity in the gold colloid solution through a microscopic objective, and thus, the photothermal effect on the gold colloid caused by the laser drove the gold nanoparticles towards the spot center, thereby inducing aggregation of the gold nanoparticles. Moreover, the morphology and size of the gold nanoparticle aggregates could be reconfigured by controlling the shape and size of the microcavity; thus, the best electromagnetic enhancement effect of gold nanoparticle aggregates could be obtained.
Fig. 2. Transverse component of the radiation force for a gold nanoparticle located in the plane of the beam-waist (z=0 µm) and off the beam-waist (z=0 µm, z=50 µm, z=100 µm, z=200 µm, z=300 µm, z=400 µm, z=500 µm, and z=600 µm) as a function of the transverse position r.
The scanning electron microscopy (SEM) image of gold nanoparticles presented in Fig. 3(a) shows that the gold nanoparticles were uniform. The extinction spectrum of the gold colloid solution is presented in Fig. 3(b), which shows that the plasma resonance of a single gold nanoparticle occurs at 533 nm. Figures 3(c) and Fig. 3(d) show a comparison of gold nanoparticles aggregated on a microcavity before and after optical manipulation. By adjusting the position of the microcavity, the aggregated gold nanoparticles in the microcavity could be clearly imaged at the white light focal plane. The width and depth of the microcavity in Fig. 3 are 60 µm and 30 µm, respectively.
Fig. 3. Scanning electron microscopy (SEM) image, extinction spectrum of gold colloid solution and contrast micrographs of gold nanoparticles aggregated on a microcavity before and after optical manipulation. (A) SEM image of gold nanoparticles. (B) Extinction spectrum of gold colloid solution. (C), (D) Micrographs of a microcavity (60 µm×30 µm) in gold colloid solution before optical manipulation and after laser illumination for 8 min.
The process of gold nanoparticle aggregation towards the center of the microcavity is shown in Video 1 (see Visualization 1). Figures 4(a) and 4(b) captured from Video 1 (see Visualization 1) present instantaneous diagrams of nanoparticle motion, with a time interval of 1 s. Some nanoparticles deviate from the white light focal plane within 1 s because of the uneven longitudinal temperature distribution of the beam on the particle. Due to the limitation of CCD resolution, single nanoparticles cannot be clearly viewed through the video. However, a few micron-sized particles can be clearly observed to aggregate towards the center of the laser spot because the addition of the analyte and the collision of particles during the aggregation process lead to agglomeration of the gold nanoparticles, the average size of the gold nanoparticle aggregates is approximately 3 µm. Therefore, to more clearly show the movement of gold nanoparticles, the ten gold nanoparticle aggregates circled in Fig. 4 were selected for observation, and all the gold nanoparticles clearly moved towards the center of the microcavity. The SERS spectra of 5×10−7 mol/L pyrene in gold colloid solution (with the focal spot far from the quartz) and based on the microcavity are presented in Fig. 4(c). The width and depth of the microcavity used in this figure were 60 µm and 30 µm, the integration time was 1 s, and the laser power was 90 mW. The SERS peaks of pyrene were significantly enhanced compared with those for the gold colloid solution [Fig. 4(a)]. The Raman peak at 586 cm−1 was more than 20 times larger than that for the gold colloid solution. This is mainly due to the formation of three-dimensional gold nanoparticles aggregates in the microcavity. The electromagnetic field will be enhanced in the gaps between metal nanoparticles, and more “hot spots” will be generated; thus, the Raman signals of samples in the gaps are greatly enhanced. Although the laser power used in this study was high, bubbles were not found in the solution because a large volume of gold colloid solution (20 mm×10 mm×2.5 mm) was present in the vessel and the temperature gradient generated by the Gaussian beam was not sufficient to form bubbles.
Fig. 4. Instantaneous diagrams of nanoparticle motion captured from Video 1 (see Visualization 1), and SERS spectrum of 5×10−7 mol/L pyrene based on different SERS substrates. (A) and (B) Instantaneous diagrams of nanoparticle motion with a time interval of 1 s. (C) Comparison of the SERS spectrum of 5×10−7 mol/L pyrene based on a gold colloid solution (a) and a microcavity (b).
The SERS signal increased with increasing aggregation time of gold nanoparticles in the microcavity. As shown in Fig. 5, the SERS signal linearly increased within 8 min, and the SERS signal tended to be stable after this time. The reason is that when the gold nanoparticles aggregate in the microcavity, the electromagnetic field becomes increasingly stronger with the increase in the number of aggregated nanoparticles and the decrease in the distance between the gold nanoparticles. However, the number of gold nanoparticles aggregated in the microcavity eventually reaches saturation, and the space between nanoparticles is stable because the limitation of the microcavity is reached. Thus, the enhancement effect finally tends to stabilize.
Fig. 5. SERS spectrum of pyrene at different times under optical manipulation. (A) SERS spectrum of 5×10−7 mol/L pyrene at different aggregation times of gold nanoparticles in the microcavity. (B) The SERS peak intensities at 586 cm−1 (a) and 1233 cm−1 (b) increase with the aggregation time of particles.
In this study, the relationship between the velocity of gold nanoparticles and laser power was studied by changing the laser power. The motion states of gold nanoparticles under different laser powers, 60 mW, 70 mW, 80 mW, 90 mW, 100 mW, 110 mW and 120 mW, were recorded with a CCD. Five gold nanoparticles randomly selected in the same annular region (r=70 ∼ 100µm) [ Fig. 6(a)] were tracked, and their velocities were calculated. Figure 6(b) shows a fitting curve between the laser power and the instantaneous velocity of the five gold nanoparticle aggregates, and the determination coefficient is 0.959. The results show that the velocity and laser power exhibit a good linear relationship.
Fig. 6. Dependence of the velocity of gold nanoparticles on the laser power. (A) Annular region of tracked nanoparticles. (B) Fitting curve between the laser power and the instantaneous velocity of nanoparticles in the same annular region.
The theoretical calculation shows that the scattering force has no transverse component and that the transverse gradient force is limited to 20 µm. When the laser spot is near the bottom of the cuvette, a large number of nanoparticles move towards the microcavity on the quartz plate beyond the photopressure area in the field of view (200 µm), which indicates that the nanoparticles are affected by forces other than the radiation force. The velocities of nanoparticles in the region from rn+1 to rn (Fig. 7) were measured when the laser energy was 70 mW. The r values were r1=40 µm, r2=70 µm, r3=94 µm, r4=120 µm, r5=134 µm, r6=155 µm and r6=178 µm. Three nanoparticles were randomly selected in each annular region to calculate the mean and variance of the velocity of nanoparticles in the annular region. Figure 7 shows that as r decreases, the nanoparticle velocity gradually decreases. When the nanoparticles converge at the center of the laser spot, the velocity does not decrease to zero. Therefore, collisions occur between gold nanoparticles, which destroy the electrostatic repulsion between the nanoparticles, and the irreversible aggregation of nanoparticles results in the formation of gold nanoparticle aggregates. When the laser spot is far from the bottom of the cuvette, circulation of the gold nanoparticles is observed. These phenomena are inferred to be caused by convection ① and thermophoresis ② (Fig. 8) since photophoresis can be ignored because the thermal conductivity of the gold nanoparticles (∼318 W/mK) is much higher than that of water (∼0.6 W/mK): (1) The gold nanoparticles are repelled from the heated center by radial thermophoresis. Convection makes the gold nanoparticles circulate in the solution, but the high temperature gradient towards the cooling quartz destroys the circulation of gold nanoparticles at the liquid and quartz interface, which results in deposition of the gold nanoparticles into the microcavity. When the laser spot is far from the bottom of cuvette, there is no cooling quartz to produce high temperature gradient to break the circulation. (2) The thermophoretic force is proportional to the temperature gradient [30], and the convection force exerted on nanoparticles is proportional to the temperature difference [31]. In the radial direction, if two areas with the same temperature difference are selected, the temperature gradient near the heated center is larger than that far from the heated center. Therefore, the closer the gold nanoparticles are to the laser spot, the greater the thrust of the thermophoresis on the gold nanoparticles; and therefore, the nanoparticle velocities gradually decrease with decreasing r.
Fig. 7. Different annular regions of tracked nanoparticles and fitting curve of the relationship between the velocities of gold nanoparticles and the field radius r. (A) Different annular regions rn∼rn+1, r1=40 µm, r2=70 µm, r3=94 µm, r4=120 µm, r5=134 µm, r6=155 µm, and r7=178 µm. (B) Fitting curve of the relationship between the mean velocity of five nanoparticles and the field radius r.
Fig. 8. Schematic diagram of the photothermal effect of a laser on the gold colloid solution. (① convection, ② thermophoresis).
The morphology and size of the microcavity on the surface of the quartz support substrate will affect the morphology and size of gold nanoparticle aggregates. In this study, SERS spectra of pyrene (5×10−7 mol/L) on microcavities of different sizes [width × depth: 10 µm × 30 µm (Fig. 9(a)), 30 µm × 30 µm (Fig. 9(b)), and 60 µm × 30 µm (Fig. 9(c))] were detected. The SERS enhancement effect for pyrene using a microcavity of 60 µm × 30 µm was better than that using the other sizes. Considering that the laser spot was constant and smaller than all the gold nanoparticle aggregations, it can be inferred that the size of the microcavity will influence the size of the gold nanoparticle aggregates, and thus, influence the electromagnetic enhancement effect.
Fig. 9. SERS spectrum of 5×10−7 mol/L pyrene based on microcavities of different sizes. (width × depth: 10 µm × 30 µm (a), 30 µm × 30 µm (b), 60 µm × 30 µm (c)).
Conclusion
In this study, a system that combined optical manipulation with microSERS spectroscopy was built. Optical manipulation of gold nanoparticles and SERS signal detection were realized using only one laser. The optical manipulation and efficient capture of a large number of gold nanoparticles in a large region using the photothermal effect were realized for the first time. Circulation of the gold nanoparticles was observed when the laser spot was far from the bottom of the cuvette, and the gold nanoparticles were deposited into the microcavity when the laser spot was near the surface of quartz substrate. The SERS enhancement effect based on the microcavity was more than 20 times that based on the gold colloid solution, and the SERS enhancement effect was stable within 7-8 min. The laser power and velocity of nanoparticles exhibited a good linear relationship, and the velocity of the nanoparticles decreased with decreasing radius r, which verifies the detriment of the radial temperature gradient force of liquid for optical trapping of gold colloid. The results also show that the morphology and size of metal aggregates can be controlled by changing the shape and size of the microcavity, which is helpful for studying the electromagnetic enhancement mechanism of metal nanoparticle aggregates. This method can be used to quickly and efficiently form gold nanoparticle aggregates and provides a promising approach for detection and analysis of substances in the fields of chemistry and biology.
Funding
National Natural Science Foundation of China (40906051, 41476081); Key Technology Research and Development Program of Shandong (2016GSF115020, 2019GHY112027).
Disclosures
The authors declare no conflicts of interest.
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