Article pubs.acs.org/journal/apchd5
Precise Sorting of Gold Nanoparticles in a Flowing System Wei Wu,† Xiaoqiang Zhu,† Yunfeng Zuo,† Li Liang,† Shunping Zhang,† Xuming Zhang,‡ and Yi Yang*,† †
School of Physics and Technology, Wuhan University, Wuhan 430072, China Department of Applied Physics, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
‡
S Supporting Information *
ABSTRACT: Precise sorting of gold nanoparticles is important, but it still remains a big challenge. Traditional methods such as centrifugation can separate nanoparticles with a high throughput but at the cost of low precision. Optical tweezers enable the precise manipulation of a single nanoparticle in steady liquid environments. However, this method may become problematic when dealing with a considerable amount of nanoparticles in a flowing system due to the difficulties in balancing the additional Stokes forces by the fast velocity of streams and in controlling all dispersed nanoparticles with disorderly positions. Here, we exploit optical and hydrodynamic forces to sort gold nanoparticles in the flowing system, obtaining simultaneously high precision and considerable throughput. This is accomplished by utilizing opposite impinging streams to generate a stagnation point, near which the flow velocity becomes very small to reduce the Stokes force and to prolong the optical acting time. Nanoparticles of different sizes, confined in a narrow region by the hydrodynamic focusing, can then be separated by a laser beam of moderate power. Experimental demonstrations have been presented by sorting gold nanoparticles with diameters of 50 nm from those of 100 nm, and 100 nm from 200 nm. The sorting fidelities is ≥92% for the 50/100 nm combination and ≥86% for the 100/200 nm set, with a sorting throughput of 300 particles/min. Sorting of gold nanoparticles with smaller heterogeneity (50 and 70 nm) has also been realized with a lower throughput of 0.99); the corresponding shift of the stagnation point is less than 1 μm and can be ignored. Simulation of the velocities of the laminar streams and the particles is clearly shown in Figure 2b. Similar to the laminar streams, the particles also have a very slow velocity when they get close to the stagnation point, as shown in Figure 2c. The velocity of particles near the stagnation in the laser working area is less than 80 μm/s, and the velocities of the innermost particle and the outermost particle have a small difference (less than 50 μm/s) at Qsheath/Qcore = 10. More in depth information about the velocity analysis is presented in Figure S1 (see the Supporting Information). Figure 2d plots the velocity variation of one particle in the innermost particle stream, when it travels from the inlet to the outlet of the chip. The particles in the main channel have high velocity, which is difficult for
running flow, providing a powerful tool for high-precision, high-throughput gold nanoparticle sorting.
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RESULTS AND DISCUSSION
The schematics of the working principle is illustrated in Figure 1. Gold nanoparticles have strong scattering of incident light, which makes them easily manipulated in water by the optical force. Larger particles have stronger optical force due to the much stronger scattering and absorption. Figure 1a presents the schematic of the sorting process. Nanoparticles are confined in a narrow central stream by the hydrodynamic focusing in the left side of the central flow, and they are slowed down after entering the impinging zone. The larger ones are sorted when they go through the laser. As shown in Figure 1a, the red dots indicate the trajectory of larger particles, whereas the blue dots represent the trajectory of smaller ones. Figure 1b shows the design of the optofluidic chip, which consists of five stream inlets. The main stream consists of two core inlets (3 with nanoparticles, 4) and two sheath inlets (1 and 2). The sheath inlet streams are used to confine the sample in a narrow region with a width from sub-micrometer to several micrometers. The opposite stream (stream 5) acts as the counter flow. The opposite streams (1−4 and 5) impinging in the device generate a stagnation point whose nearby zone has a flow velocity near zero. The flow rates of the inlet flows are named as Q1, Q2, Q3, Q4, and Q5, respectively. Qsheath is the sum rate of the sheath flow rates (Qsheath = Q1 + Q2), Qcore is the sum of the core flow rates (Qcore = Q3 + Q4), and Q5 = Qsheath + Qcore. The left core inlet (inlet 3) feeds the mixed particles, and the right one (inlet 4) injects deionized (DI) water for a buffer region. To confine the sample on the left, the buffering flow rate is slightly larger than the sample flow. When the laser is turned on, the smaller particles remain in the sample stream 3 and go to the outlet 1 on the left side, whereas the larger particles are pushed to the buffer stream 4 and are finally conveyed to the outlet 2 on the right side, as shown by the trajectories in Figure 1b. The laser is supplied by a single-mode fiber, and the laser from the optical fiber is collimated by a microfabricated cylindrical lens. In the working region for particle sorting, the laser beam is 25 μm wide in the horizontal direction (i.e., the xy plane) and slightly 2499
DOI: 10.1021/acsphotonics.6b00737 ACS Photonics 2016, 3, 2497−2504
ACS Photonics
Article
conventional optical manipulation. Then they are slowed to the stagnation point for sorting. After they leave the stagnation point for the outlet, they can be accelerated again to high velocity. The summed input flow rate of the sample and the buffer flow is 2 nL/s (the flow rates of the sample and the buffer are 0.8 and 1.2 nL/s, respectively), the two sheath flow rates are both 10 nL/s, and the opposite flow rate is the summation of them. As a result, the velocity of the gold nanoparticles is decreased from 450 μm/s to 30 μm/s for optical operation, while the nanoparticles remain in the narrow steam without a decrease in flow rate. Figure 2e and f plot the relationship between the particle diameter and the particle velocity at the x-axis when the laser power is 0.36 and 0.12 mW/μm2, respectively. Red and black lines show the velocity variations of the inside and outside particles in the focused stream in Figure 2c. The inside particles are easier to sort because of the relatively lower velocity. For the nanoparticles smaller than 100 nm, this enables the sorting of nanoparticles with a diameter difference of about 20 nm. Additionally, the sorting of a given size depends on the flow rate and the optical power, and an operation map is presented in Supporting Information Figure S2. The hydrodynamic focusing can effectively focus the particles in the center of the channel from the microscale to nanoscale size.33−35 Figure 3 shows the experimental results near the stagnation point without the optical force. In Figure 3a−c, the particle focusing states at different flow rate ratios are shown by the micrographs. The ratio of each figure is Qsheath/Qcore = 20 for (a), Qsheath/Qcore = 10 for (b), and Qsheath/Qcore = 2.5 for (c). In the experiments, the summed flow rate of each main stream is fixed (i.e., Q5 = Qsheath + Qcore = 22 nL/s), but the ratio (i.e., Qsheath/Qcore) of each flow stream (Q1, Q2, Q3, Q4) is changed according to the requirement. The green line indicates the trajectory of the outer particles, whereas the other lines show the trajectories of inner particles. The orange dashed line indicates the central line of the chip, and the transparent green rectangle marks the region of the laser spot. The white circle indicates the region of the ream and the buffer stream. Figure 3d plots the relationship between the focused width and the flow rate ratio. The width narrows down with the increase of the flow rate ratio, and the experiments agree well with the theory. To get the appropriate width and velocity in the experiment, the flow rate ratio is chosen to be 10. All the particles are confined near the stagnation point, and they are 3 times higher than the smaller one. This indicates that it is practicable to identify the particle size by the scattering intensity. The
Figure 4. (a) Optical images of the gold nanoparticles illuminated by the 532 nm laser. (b) Integrated scattering intensity showing nanoparticles of different sizes.
scattering intensity analysis of 100 and 200 nm is presented in Figure S3 (see the Supporting Information). Figure 5 shows the experimental sorting result of the particles with a laser power of 900 mW. Because the laser beam is 25 μm in width and about 100 μm in height, the intensity of the laser is 0.36 mW/μm2. Figure 5a shows a time sequence of sorting processes in a mixture of two types of gold nanoparticles (50 and 100 nm in diameter) using the realtime particle identification method. Both processes are extracted from Video S in the Supporting Information, recorded in the same region and under identical laser illumination. The orange dashed line represents the central line of the chip, and the triangular points indicate the positions of the nanoparticles. As expected, the “low-intensity” scattering nanoparticle (50 nm) moves in one direction, while the “highintensity” scattering particle (100 nm) moves in the opposite direction. Particle sizes are distinguished from the scattering light intensity of the laser, as the picture shows. For gold nanoparticles, it is difficult to accurately measure the size of nanoparticles by the scattering intensity (see Figure 4 and Figure 5a) when the diameter of the nanoparticle is below the wavelength. We measured the real-time spectra of nanoparticles to confirm the particle size in outlet 1 and outlet 2, respectively. The supercontinuum white laser and the CCD with Andor’s line of Shamrock imaging spectrograph are used to detect the selected regions. The results show that the peaks of spectra in each outlet channel have nearly the same wavelength, which also indicates that the nanoparticles have good monodispersion. Figure 5b,c show the spectra of nanoparticles when they go through outlet 1 and outlet 2. Five of those peaks are located at a wavelength of 550 nm (collected from outlet 1), and the remaining five are located at 580 nm (collected from outlet 2). The results of the spectra confirm that the sorting processes are effective. More information about the sorting process and spectra of 100 and 200 nm nanoparticles are presented in Figure S4 (see the Supporting Information). Figure 6a shows snapshots of the sorting events of one large (100 nm) and one small (50 nm) nanoparticle in the experiments. Clearly, the small particle is slightly affected but still remains on the left side, while the large one is sorted out due to the much stronger optical force. To study the details of the sorting process, the large and small particles in Figure 6a are 2500
DOI: 10.1021/acsphotonics.6b00737 ACS Photonics 2016, 3, 2497−2504
ACS Photonics
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Figure 5. (a) Sequence of the frames of four selected particle events. The left sequence corresponds to the smaller nanoparticles (50 nm), and the right sequence shows the larger one (100 nm). (b, c) Spectra of the particles excited by the white light laser in outlet 1 and outlet 2, respectively.
agrees well with the theoretical predictions. Examples of the nanoparticle trajectories during the sorting process are shown in Figure 6c. The green arrows show the trajectories of the smaller particles, and the red ones indicate those of the larger particles; the sorting trajectory also shows the laser beam is 25 μm in width. For further demonstration, another experiment has been conducted to sort the gold nanoparticles with diameters of 100 and 200 nm using a laser power of 300 mW (0.12 mW/μm2), and the results are presented in Figure 7. Similarly, Figure 7a−c
Figure 6. Sorting experiments of the gold nanoparticles with diameters of 50 and 100 nm. (a) Sorting with the laser power of 900 mW. (b) Average horizontal velocities of the 100 and 50 nm nanoparticles at the horizontal axis in (a). (c) Trajectories of the nanoparticles in the sorting process.
calculated to find the horizontal average velocities. The red line in Figure 6b plots the average horizontal velocity of the large particles. It becomes positive and is then slowed down from the highest velocity (120 μm/s). The large particle is accelerated by the optical force and is then slowed down because they are exiting the laser and suffering from drag force. When it moves out of the laser, it is accelerated to outlet 2. In contrast, the green line in Figure 6b shows the average horizontal velocity of the smaller nanoparticles. It is relatively slow in the laser irradiation and is dragged into outlet 1. The experimental result
Figure 7. Sorting experiment of the gold nanoparticles with diameters of 100 and 200 nm. (a) Sorting with a laser power of 300 mW. (b) Average horizontal velocities of 200 and 100 nm nanoparticles. (c) Trajectories of the nanoparticles in the sorting process. 2501
DOI: 10.1021/acsphotonics.6b00737 ACS Photonics 2016, 3, 2497−2504
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Figure 8. Statistical results of (a) sorting accuracies of the 50 and 100 nm nanoparticles and (b) sorting accuracies of the 100 and 200 nm nanoparticles.
nm nanoparticles are found in outlet 2. As a result, the sorting accuracies of the 100 and 50 nm particles in test 1 are 97.0% and 92.3%, respectively. In test 2, the accuracies are 96.6% and 96.3%, respectively, showing also high accuracy and good repeatability. The same procedure is applied to sort the 100 and 200 nm nanoparticles. The results are presented in Figure 8b. Sorting accuracies of the 200 nm nanoparticles reach 92.1% in test 3 and 86.7% in test 4, whereas those of the 100 nm nanoparticles measure 94.9% in test 3 and 85.9% in test 4. According to the statistical data, the sorting throughput is 300 particles/min. This technology can also separate gold nanoparticles with even smaller heterogeneity, such as 50 and 70 nm nanoparticles. This separation can be realized using a tighter hydrodynamic focusing (Qsheath/Qcore = 20, Figure 3a) and a higher laser intensity (0.94 mW/μm2) with a lower throughput of
