Trapping and Detection of Nanoparticles and Cells Using a Parallel Photonic Nanojet Array Yuchao Li, Hongbao Xin, Xiaoshuai Liu, Yao Zhang, Hongxiang Lei,* and Baojun Li* State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *
ABSTRACT: In advanced nanoscience, there is a strong desire to trap and detect nanoscale objects with high-throughput, single-nanoparticle resolution and high selectivity. Although emerging optical methods have enabled the selective trapping and detection of multiple micrometer-sized objects, it remains a great challenge to extend this functionality to the nanoscale. Here, we report an approach to trap and detect nanoparticles and subwavelength cells at low optical power using a parallel photonic nanojet array produced by assembling microlenses on an optical fiber probe. Benefiting from the subwavelength confinement of the photonic nanojets, tens to hundreds of nanotraps were formed in three dimensions. Backscattering signals were detected in real time with single-nanoparticle resolution and enhancement factors of 103−104. Selective trapping of nanoparticles and cells from a particle mixture or human blood solution was demonstrated using the nanojet array. The developed nanojet array is potentially a powerful tool for nanoparticle assembly, biosensing, single-cell analysis, and optical sorting. KEYWORDS: optical trapping, optical detection, nanoparticles, subwavelength cells, photonic nanojets
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addition, plasmonic tweezers can be used to trap and detect a single nanoparticle by exploiting the strong field confinement within a plasmonic nanoantenna.18−21 If a periodic array of plasmonic nanoantennas on a metallic substrate is precisely fabricated, plasmonic tweezers can be designed to trap multiple nanoparticles.22−24 However, these techniques rely on the resonant excitation of the photonic crystal resonator or plasmonic nanoantennas associated with resonant light absorption, which may lead to local heating effects such as strong fluid convection and thermophoresis.25−27 Moreover, fabricating precisely designed photonic crystal resonators or plasmonic nanoantennas on a substrate requires accurate nanofabrication processes like thin film deposition, nanolithography, and focused ion beam milling, which are complex. Therefore, a simple and effective method to trap and detect nanoparticles and subwavelength cells is strongly desired. In this work, we present a parallel photonic nanojet array generated by assembling and binding microlenses on an optical fiber probe that can be used to trap and detect nanoparticles and subwavelength cells with high-throughput, single-nanoparticle resolution and high selectivity. Here, the detected volume, defined as the product of the area of the fiber probe and the effective trapping region (∼1 μm) of the photonic
ptical trapping and detection of nanoscale objects, such as nanoparticles, viruses, and subwavelength cells (e.g., pathogenic bacteria), are becoming increasingly important for applications ranging from biomedical diagnostics and biosensing to environmental monitoring and nanostructure assembly.1−6 However, most dielectric nanoparticles and subwavelength cells have weak light−matter interactions because of their small size and low refractive index contrast, which leads to weak trapping strength and low detection sensitivity.7 Therefore, efficient trapping and accurate detection of nanoparticles and subwavelength cells remain challenging. Although optical tweezers have become powerful tools to trap and detect micrometer-sized particles and cells,8−12 they have limits when extended to the nanoscale because of the inherent diffraction limit of far-field laser beams.3 Moreover, relatively bulky optical elements like the high numerical aperture (NA) objective of optical tweezers restrict their miniaturization and integration.13 Several near-field techniques have been developed to trap and detect nanoparticles and subwavelength cells that can overcome the diffraction limit and avoid the use of the bulky structures in conventional optical tweezers.14−24 For example, a photonic crystal resonator based on the optical resonance of a resonant cavity in a one-dimensional silicon photonic crystal has been used to trap and detect a single nanoparticle.14−17 When trapping time was lengthened, multiple nanoparticles were aggregated and trapped in the single resonant cavity of the photonic crystal resonator.16,17 In © 2016 American Chemical Society
Received: December 22, 2015 Accepted: May 10, 2016 Published: May 10, 2016 5800
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Figure 1. Experimental design, schematic illustration, and material characterization. Optical microscope images of arrays of (a) 60 and (b) 130 PS microlenses. (c) Schematic illustration of selective trapping and detection of multiple nanoparticles and subwavelength E. coli cells in human blood solution by a parallel photonic nanojet array. (d) SEM image of fluorescent PS nanoparticles with an average diameter of 190 nm. (e) Optical image of the fluorescent PS nanoparticles excited by a 398 nm laser in the solution. (f) SEM image of E. coli cells with an average diameter of 400 nm and average length of 2.6 μm.
nanojet, is about 6.1 × 102 μm3, and the amount of traps is about 60 for a single fiber probe with a diameter of 28 μm. This method requires only an optical fiber probe and microlenses, avoiding the use of elaborate nanostructures and bulky optical systems. Due to the strong trapping strength of the photonic nanojet array, operation optical power intensity is 1−2 orders of magnitude lower than that of conventional optical tweezers (typically 1 × 1011 to 1 × 1012 W/m2), so no heating effect is observed. This array can also be used to selectively trap nanoparticles and cells in a mixture or human blood solution.
focusing microlenses29−31 because of their relatively high refractive index and low optical absorption. Highly focused beams were generated near the shadow-side surface of the microlenses because of the constructive interference of the optical field. These near-field beams, termed “photonic nanojets”, have a subwavelength full width at half-maximum (fwhm) that can break the diffraction limit31,32 and provide a possibility for optical trapping and detection of nanoparticles and subwavelength cells. Figure 1c schematically illustrates multiple nanoparticles and subwavelength Escherichia coli (E. coli) cells selectively trapped and detected in human blood solution by the parallel photonic nanojet array. Both the trapping light and detection signals propagate through the fiber probe, making the device highly autonomous and free of bulky optical elements. The flexibility and small size of this device allow it to operate in narrow spaces such as blood vessels, in vivo, biocapillaries, or lab-on-a-chip devices.33−36 To experimentally verify the trapping and detection ability of the photonic nanojet array, fluorescent PS nanoparticles with an average diameter of 190 nm (Figure 1d), which can be considered as rough approximate models for viruses and small spherical bacteria, were used. When excited by a 398 nm laser, the emission of the fluorescent PS nanoparticles at 518 nm was observed with an optical microscope (Figure 1e). As an example of a biological application of the device, E. coli cells with an average diameter of 400 nm and average length of 2.6 μm (see Figure 1f for an SEM image and Figure S3 for size distributions) were also prepared for subsequent experiments (preparation details are provided in the Methods section).
RESULTS AND DISCUSSION To fabricate a parallel photonic nanojet array, a twodimensional array of microlenses was regularly patterned to the end face of an optical fiber probe using a photophoretic assembly technique, which can efficiently assemble twodimensional crystals on a transparent flat surface.28 Fabrication details are described in the Methods section. Figure 1a shows an optical microscope image of 60 microlenses assembled on a fiber probe with a diameter of 28 μm. Figure 1b depicts a largescale microlens array formed by assembling 130 microlenses on a fiber probe with a diameter of 45 μm. To clearly characterize the morphologies of the microlense arrays, as an example, Figure S1 in the Supporting Information presents scanning electron microscopy (SEM) images of 43 microlenses assembled on a fiber probe with a diameter of 23 μm and 133 microlenses assembled on a fiber probe with a diameter of 48 μm. Each microlens is a polystyrene (PS) microsphere with a diameter of 3 μm. PS microspheres (see Figure S2 for an SEM image) were used because they are known to act as 5801
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Figure 2. Numerical simulations and calculations. (a) Electric field (E) intensity distribution (x−y plane) of an optical fiber probe with a microlens array. The wavelength of the input laser beam was set as 808 nm to match the experiments, and the average optical power released onto a single microlens was ∼1 mW. (b) E intensity at the focal plane (indicated in (a) by the dashed yellow line) of the output light in the x direction. The fwhm of the photonic nanojets is 360 nm. Optical forces (Fi) and potentials (Ui) of a 190 nm PS nanoparticle (NP) as functions of (c) x and (d) y. The insets show the calculation models. (e) Optical potential depth ΔU (as indicated in the inset) of the particles in the photonic nanojets as a function of particle diameter Dp. (f) Normalized relative intensity of the nanoparticles with different diameters as a function of position x of the nanoparticle at y = 0. Insets f1 and f2 show the E distributions when a 190 nm nanoparticle was located in the photonic nanojet (x = 0) and near the photonic nanojet (x = 0.2 μm), respectively.
To numerically investigate the trapping and detection ability of the photonic nanojets, a theoretical model was constructed with the finite element method using COMSOL Multiphysics (see the Methods section for details). Figure 2a shows the simulated electric field (E) intensity distribution near the fiber probe in the x−y plane. Because of the presence of the microlens array, the output light from the fiber probe was highly focused and thus formed a parallel photonic nanojet array. The photonic nanojets were generated at a distance of ∼1 μm from the surface of the microlenses. The fwhm values of the nanojets are 360 nm (∼λ/2.2) in the x direction (Figure 2b) and 2.2 μm (∼3λ) in the propagation direction (see Figure S4), which can overcome the λ/2 diffraction limit and correspond to the definition of a photonic nanojet.32 Because the photonic nanojets are highly focused, they can markedly decrease the trapping volume and interact strongly with nanoparticles. The interactions can lead to two types of optical forces. The optical gradient force resulting from the temporary polarization of the nanoparticle in a nonuniform E attracts the nanoparticle toward the higher-intensity region. The optical scattering force generated by the momentum transfer associated with the scattering of the photons drives the nanoparticle along the direction of light propagation. The total optical force F exerted on the nanoparticle can be expressed as37
F=
∮S (⟨TM⟩·n)dS
(1)
where the integration is performed over a closed surface S surrounding the nanoparticle, n is the unit vector outward normal to S, and ⟨TM⟩ is the time-averaged Maxwell stress tensor, which is given by ⟨TM⟩ =
⎤ 1 ⎡ 1 Re⎢ε EE* + μHH* − (ε |E|2 + μ |H|2 )I ⎥ ⎣ ⎦ 2 2 (2)
where EE* and HH* denote the outer product of the optical fields, I is the unit dyadic, and ε and μ are the electric permittivity and magnetic permeability of the surroundings, respectively. As a result, the component Fi (i = x, y) of F in the x and y directions acting on a 190 nm PS nanoparticle was obtained, as illustrated in Figure 2c,d, with the focus position of the output light defined as the origin (x, y) = (0, 0) of the coordinate. The insets of Figure 2c,d show schematic diagrams of the calculated models. In the models, a laser beam with an optical power of 1 mW was released onto a single microlens to form the photonic nanojet used to trap the nanoparticle. According to the optical force profiles, the trap stiffness κtrap values were estimated to be 0.78 and 0.31 pN/nm/W in the x and y directions, respectively. Compared with reported techniques, κtrap of our method was similar to that of slot 5802
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Figure 3. Experimental setup. The red, green, and purple arrows indicate the propagation of the trapping light (808 nm), detection signal (backscattering: 808 nm), and excitation light (398 nm), respectively. Fiber 1 was used to trap and detect nano-objects, while fiber 2 was used to excite fluorescent nanoparticles. SAM, six-axis microstage; CCD, charge-coupled device; PD, photodetector.
waveguides (1.4 pN/nm/W)38 and higher than those of nanoplasmonic devices (0.01 pN/nm/W)22 and conventional optical tweezers (0.008 pN/nm/W).39 In this comparison, we scaled all κtrap to the 190 nm nanoparticle used in our experiment because the optical force is proportional to the third power of the radius of the nanoparticle.20 Therefore, the photonic nanojets can provide sufficient strength to stably trap nanoparticles. To further investigate the stability of the optical traps, the trapping potential Ui in the x and y directions was evaluated by integrating Fi over the distance from the origin of the coordinate D according to
Ui = −
∫ FidD
will be a particle diameter for which the scattering force will exceed the gradient force. This particle diameter is defined as the critical diameter Dc. Only particles with sizes smaller than Dc can be stably trapped in the photonic nanojets. Dc was determined by the diameter Dm and refractive index nlens of the microlens, as shown in Figure S5. Therefore, by using a microsphere with a desired size and refractive index as the microlens, particles of specific sizes in a mixture can be selectively trapped. Moreover, by adjusting the optical power of the trapping laser, nanoparticles with a specific refractive index or E. coli cells with a specific length/diameter ratio can also be selectively trapped by the photonic nanojets. This was verified by additional simulations and calculations (see Figure S6). Because of the highly focused beam irradiated on the nanoparticle and high collection efficiency of the microlens, the backscattering signal of the trapped nanoparticles can be markedly enhanced, which is beneficial to detect nanoparticles. The backscattering intensity of nanoparticles was calculated by the finite element method (see detailed simulation and calculation in Figure S7a−d). Figure 2f shows the backscattering intensity relative to the intensity of an isolated nanoparticle without a microlens of nanoparticles with different diameters ranging from 100 to 220 nm with a 30 nm interval as a function of position x of the nanoparticle when y = 0. The profiles reveal that the relative backscattering intensity is dramatically enhanced when the nanoparticle is positioned in the photonic nanojet at (x, y) = (0, 0). The enhancement factors were calculated by comparing the backscattering intensities of nanoparticles positioned inside and outside the photonic nanojets (see detailed simulation and calculation in Figure S7e,f). Calculated enhancement factors were up to 103− 104 for nanoparticles with different diameters, comparable with those of 102−105 reported for nanoplasmonic devices designed to detect nanoparticles.41−43 Specifically, the enhancement factor of the backscattering signal is 1134 for the 190 nm PS nanoparticle used in the experiments. To experimentally demonstrate the trapping and detection of multiple nanoparticles and cells by the parallel photonic nanojet array, the experimental setup shown in Figure 3 was constructed; details are described in the Methods section. In the experiment, an 808 nm laser beam, which is weakly
(3)
The calculated Ui values in the x and y directions are presented in Figure 2c,d, respectively. The minimum Ui occurred at (x, y) = (0, 0), which indicates that the nanoparticle will be stably trapped on the focus of the output light. The potential depths of Ui in the x and y directions ΔU are 4.9 and 2.3 × 106 kBT/W, respectively. Because the optical field in the simulation has rotational symmetry, and ΔU in the z direction is equal to that in the x direction. Therefore, the photonic nanojets can form three-dimensional potential wells to allow stable trapping of nanoparticles. Trapping selectivity was verified by comparing ΔU of particles of different sizes. Ashkin et al.40 stated that stable trapping requires a ΔU of about 10 kBT to compensate for stochastic kicks in the particle’s Brownian motion. Thus, in this work, ΔU = 10 kBT was considered as the threshold for stable optical trapping. Figure 2e shows ΔU (as indicated in the inset of Figure 2e) of trapped particles of different diameter Dp. In the simulation and calculation, the optical power of the trapping laser directed to a single microlens was set as 1 mW, which was approximately equal to the average laser power released onto a single microlens in the following selective trapping experiments. The results show that ΔU exceeds 10 kBT for particles with 50 < Dp < 1000 nm (trapping region). Thus, particles with sizes in this region can be stably trapped by the optical gradient force, while particles with Dp > 1000 nm (pushing region) will be pushed away by the optical scattering force. These findings indicate that, for a given microlens, there 5803
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Figure 4. Trapping and detection of multiple nanoparticles and E. coli cells. (a) Optical microscope image showing trapped 190 nm fluorescent nanoparticles observed at a specific focal plane of the optical microscope. The optical power of the trapping light directed into the fiber with 60 microlenses was 60.2 mW. (b) Optical trapping of multiple E. coli cells by the photonic nanojet array. (c) Real-time detection of the backscattering signal Bs in the trapping process of multiple nanoparticles. Each of the steps (I−VI) of Bs indicates that a singlenanoparticle trapping event has occurred. (d) Histogram of Bs intensity (steps I−VI) with Gaussian fits.
absorbed by biological matter,44 with an optical power of 60.2 mW was directed into fiber 1 with a diameter of 28 μm and 60 microlenses for trapping and detection. That is, an average optical power of ∼1.0 mW was released onto a single microlens and then focused into a photonic nanojet with fwhm of 360 nm, so the estimated local intensity within the trap is 1.0 mW/ (3602 × π/4) nm2, that is, 1.0 × 1010 W/m2. This intensity is 1−2 orders of magnitude lower than that of conventional optical tweezers (typically 1 × 1011 to 1 × 1012 W/m2) and compatible with biological objects.19 Meanwhile, a 398 nm laser beam was directed through fiber 2 to excite the fluorescent nanoparticles. Once irradiated by the 808 nm laser beam, the fluorescent nanoparticles near the fiber probe will be trapped in the photonic nanojet array by the strong optical gradient force exerted on the nanoparticles. By controllably moving fiber 1 in the solution, more fluorescent nanoparticles can be efficiently trapped by the nanojets. Figure 4a shows an optical microscope image of a row of trapped fluorescent nanoparticles observed at a specific focal plane of the optical microscope. A single nanoparticle was trapped by each photonic nanojet, which agrees with the above simulation result. During the experiment, no heating effect such as fluid convection or thermophoresis was observed, so the trapped nanoparticles were stably held until the 808 nm laser was switched off. In addition to nanoparticles, the photonic nanojets can also trap biological species with subwavelength sizes such as E. coli cells, as illustrated in Figure 4b. Interestingly, all the trapped E. coli cells aligned their long axis along the optical axis. An additional simulation and calculation was performed to explain this orientation phenomenon (Figure S8), which revealed that the aligned configuration is the most stable state (optical torque of zero) for the rod-like E. coli cells. In addition, careful experiments demonstrated that the trapped nanoparticles and E. coli cells
can be released when the trapping laser is switched off (Figures S9 and S10). This also confirmed that the nano-objects were actually trapped by the optical forces rather than the physical interactions between the microlenses and trapped nano-objects. Once the nanoparticles or cells were trapped in the parallel photonic nanojet array, their backscattering signal Bs could be detected in real time with single-nanoparticle resolution. As an example, we present the results of the detection of the fluorescent nanoparticles (Figure 4c). There are several obvious abrupt increases in Bs caused by the presence of the nanoparticles. These step changes (steps I−VI) correspond to single-nanoparticle trapping events, which were also confirmed by charge-coupled device (CCD) observations with microscopic images (Figure S11). The photonic nanojets achieved highly sensitive detection with single-nanoparticle resolution. Figure 4d shows histograms of Bs intensity (steps I−VI), which are Gaussian distributions. The center intensity and fwhm become larger when a nanoparticle was added in the photonic nanojet, which results from the stronger light backscattering and Brownian motion of the nanoparticle. To gain insight into Bs of an individual nanoparticle, steps I−VI were analyzed independently and each trapping signal was extracted from background noise (see Figure S12). The analytical results show that each trapping signal has a Gaussian distribution, indicating the nanoparticles were trapped in harmonic optical potential wells, which agrees with the simulation results. Because Bs is sensitive and specific to trapped nano-objects, it can be used to distinguish between different nano-objects. For example, the detection of Bs can be used to distinguish platelet-derived microparticles (0.1−1.0 μm) from E. coli cells in human blood solution because of their different shapes (see Figure S13). When used to distinguish nano-objects with similar shapes, the current method is limited because of the uneven optical field distribution on the end face of the fiber probe. However, this 5804
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Figure 5. Selective trapping of nanoparticles and subwavelength cells in mixtures. (a,b) Selective trapping of 700 nm PS nanoparticles from a mixture of 700 nm and 2 μm PS particles. The optical power of the trapping light direct into the fiber with 60 microlenses was 60.2 mW. (a) At t1 = 0 s, the fiber probe was controllably moved to approach particles A and B after the trapping light was turned on for 12.1 s. Particles A and B immediately started to move in opposite directions because of the optical gradient force and scattering force, respectively. Time zero was defined as the starting point for recording the movement of particles A and B. (b) At t1 = 0.5 s, particle A was trapped in the photonic nanojet, while multiple 2 μm PS particles (including particle B) were pushed further away from the fiber probe. (c) Measured distance d between particles A and B along the x direction as a function of t1. The separation distance Δd between particles A and B is 12.3 μm at t1 = 0.5 s. (d−f) Selective trapping of E. coli cells in human blood solution. The optical powers of the trapping light and killing light directed into the fiber with 26 microlenses were 26.5 mW and 200 μW, respectively. (d) At t2 = 0 s, the fiber probe was controllably moved to approach a red blood cell (indicated by a red dashed circle) after the trapping light was turned on for 14.8 s. The red blood cell started to move forward immediately under the action of an optical scattering force. Time zero was defined as the starting point for recording the movement of the red blood cell. At (e) t2 = 0.9 s and (f) t2 = 2.1 s, multiple red blood cells were pushed further away from the fiber probe. (g) Measured time to kill all the trapped E. coli cells tk as a function of the ultraviolet laser power P. The red line is linearly fitted to the measured data, and the error bars represent the standard deviations of five repeated experiments.
limitation can be overcome by using a special optical fiber with a relatively uniform distribution of output light or by trapping the target nano-objects in positions that are symmetrical to the optical axis of the fiber. In addition, a contrast experiment was also performed using the same optical fiber probe without the microlenses. The results show that Bs of the nanoparticles cannot be detected by the bare optical fiber probe. This experiment confirms that the real-time detection with singlenanoparticle resolution arose from the signal enhancement ability of the photonic nanojets. To experimentally demonstrate selective trapping, we first carried out an experiment using a mixture containing PS particles of two sizes (700 nm and 2 μm). Figure 5a,b shows that 700 nm PS nanoparticles were trapped by the photonic nanojets, while 2 μm PS particles were pushed away from the fiber probe. As illustrated in Figure 5a, the fiber tips were controllably moved to approach a 700 nm particle (labeled A) and 2 μm particle (labeled B) at t1 = 0 s by adjusting the six-axis
microstage and translation stage. Particles A and B immediately started to move in opposite directions because of the optical gradient force and scattering force, respectively, exerted on them. At the next time point (t1 = 0.5 s, Figure 5b), particle A was attracted into the photonic nanojet, while particle B was driven further away. This result agrees with the simulation in which Dc is 1000 nm for the microlens used in the experiments. Figure 5c shows the measured distance d along the x direction of particles A and B as a function of t1. Here, the position of particle A at t1 = 0 was defined as the origin (x = 0, y = 0) of the coordinate. The results reveal that particle A moved in the −x direction and particle B moved in the +x direction with average velocities of vA = −5.3 μm/s and vB = 14.8 μm/s, respectively. The separation distance Δd between particles A and B is 12.3 μm when t1 = 0.5 s. In the experiment, the final Δd is 64.5 μm when t1 = 6.5 s. This experiment confirmed that the photonic nanojets can selectively trap a specific size of nanoparticles in a mixture. 5805
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particles and cells, the presented method is also expected to find applications in nanoparticle assembly, single-cell analysis, optical sorting, and biosensing of pathogenic bacteria.
As an example of a biological application, selective trapping of E. coli cells in human blood solution was also performed (Figure 5d−f). The mixture of human blood and E. coli cells was prepared as described in the Methods section. Figure 5d illustrates how at t2 = 0, the fiber probe was controllably moved to approach a red blood cell. The red blood cell started to move forward immediately under the action of optical scattering force. At the next time point (t2 = 0.9 s), the red blood cells had moved further away (Figure 5e) and then reached a distance of about 20 μm in the x direction at t = 2.1 s (Figure 5f). It should be emphasized that E. coli cells can cause serious diseases (e.g., septicemia) when they invade human blood. To kill the E. coli cells in human blood without damaging the red blood cells is an important goal in the relevant disease treatment. In this work, the trapped E. coli cells were killed by directing an ultraviolet laser beam with a wavelength of 253 nm into the fiber. The viability of the E. coli cells was observed in real time by injecting a trypan blue solution (Solarbio, Beijing, China) with a concentration of 0.4% into the cell solution with a volume ratio of ∼1:9. The results show that the trapped E. coli cells were stained blue, which indicates that the trapped cells were all dead, while the dispersed cells without UV irradiation were not stained (see Figure S14a,b). Because the output ultraviolet light was confined in the photonic nanojets, there was no damage to the red blood cells, which were pushed away from the nanojets. The viability of the red blood cells after ultraviolet irradiation of the mixture was determined by observing their shapes and surfaces (see Figure S14c−f). Figure 5g shows the measured time to kill all the trapped E. coli cells tk as a function of the optical power P of the ultraviolet laser. The results show that tk decreases linearly with increasing P. Therefore, this method can be used to quickly trap and kill pathogenic bacteria in human blood. It should be pointed out that there are some platelets and platelet-derived microparticles with similar sizes to E. coli cells in human blood solution. However, their Bs can be clearly distinguished from that of E. coli cells, and the selective trapping of E. coli cells was not affected by the platelets and plateletderived microparticles, as shown in Figures S13 and S15. In addition, there are many exosomes secreted by cells in blood. The exosomes could not be trapped and detected in our experiment because of their relatively small size (diameter: 30− 100 nm) and low refractive index (n = ∼1.39).45−47 However, it can be solved by using an array of microlenses with smaller Dm or higher n.
METHODS Fabrication of the Optical Fiber Probe. The optical fiber probe was fabricated from a commercial multimode optical fiber (MMJ-62.5/ 125-3.0-5m-FP/FP, Corning Inc.). The buffer and polymer jacket of the fiber were stripped off with a fiber stripper to obtain a 2 cm length of bare fiber. Before being heated, the fiber was sheathed with a glass capillary (inner diameter = 0.9 mm, wall thickness = 0.1 mm, length = 120 mm) to prevent the fiber from bending and breaking. The bare fiber outside the capillary was heated for about 40 s until it reached its melting point. Then, the fiber was drawn through a heating zone of approximately 3 mm at a speed of about 2 mm/s, which then gradually tapered off, causing its diameter to decrease from 125 to 45 μm over a length of approximately 1.6 mm. The optical fiber was then removed from the flame and cooled to room temperature for 2 min. Finally, the tapered region of the optical fiber was cut by a fiber cleaver to obtain a smooth end surface. The final diameter of the fiber probe can be modified by controlling the heating time and drawing speed. Preparation of the PS Microsphere Suspension. Commercially available PS microspheres were modified with aliphatic amine, causing them to become positively charged. The PS microsphere powder was suspended in deionized water and then diluted to a concentration of about 5.1 × 104 particles/μL. Ultrasonic treatment was performed for 5 min to obtain a monodisperse suspension. Assembly of the Microlens Array on the Fiber Probe. First, the PS microsphere suspension was added dropwise on the end face of the fiber probe, which was fixed vertically with a fiber positioner. Second, upon directing a laser beam with a wavelength of 1550 nm and optical power of 100 mW through the fiber, the PS microspheres were irradiated by the light and began to move toward the center of the surface of the fiber probe because of the photophoretic effect. After 2 min, the PS microspheres formed a two-dimensional array and bound to the end face of the fiber probe through the electrostatic attraction in the aqueous environment. The microlens array was stabilized by naturally evaporating the water at room temperature and then used in subsequent experiments under different conditions. The stability of the fabricated device was tested by exposing the device in an aqueous solution to an environmental vibration or fluid flow. The results revealed that the device remained stable, and no PS microspheres were released into the aqueous sample (Supplementary Video S1). Preparation of Nanoparticle and E. coli Cell Suspensions. The commercially available fluorescent PS nanoparticles (Shanghai Huge Biotechnology Co., Ltd.) had a refractive index of 1.58 and emission maximum at 518 nm when excited by a 398 nm laser. The nanoparticles were diluted to a concentration of about 6.1 × 104 particles/μL with deionized water. The nanoparticle suspension was then added dropwise into the microfluidic chamber using a micropipette for subsequent experiments. E. coli cells were grown at room temperature in lysogeny broth and washed and diluted with phosphate-buffered saline to obtain a suitable concentration of about 8.1 × 104 E. coli cells per microliter. Preparation of Particle or Cell Mixtures. The particle mixture was obtained by mixing equal amounts (1 mL) of 700 nm and 2 μm PS particles with comparable concentrations (about 1.1 × 104 particles/μL) in deionized water. Ultrasonic treatment was performed for 3 min to obtain a monodisperse particle suspension. The cell mixture was obtained by mixing an E. coli cell suspension (1 mL) with human blood solution (3 mL). The human blood solution was composed of 10 μL of blood from the fingertip of a healthy adult and diluted with 4 mL of phosphate-buffered saline (pH 7.44), which contained 121.5 mM NaCl, 25.2 mM Na2HPO4, and 4.8 mM KH2PO4. Simulations and Calculations. Simulations were performed by the finite element method using COMSOL Multiphysics 4.4 with the radio frequency module (electromagnetic wave, frequency domain)
CONCLUSIONS Based on a theoretical prediction, we experimentally demonstrated a near-field approach to trap and detect nanoparticles and subwavelength cells with high-throughput, single-nanoparticle resolution and high selectivity. The technique makes use of the parallel photonic nanojet array generated by assembling a two-dimensional microlens array on a compact fiber probe, which is simple to fabricate and integrate. The subwavelength confinement of the photonic nanojet array provides strong trapping strength, allowing the stable trapping of nanoparticles and cells at relatively low P. Bs of the trapped nanoparticles and subwavelength cells was detected in real time with single-nanoparticle resolution and enhancement factors of 103−104 by the photonic nanojet arrays. Moreover, the arrays were used to selectively trap one type of nanoparticle or cell in a mixture or human blood solution. With the features of selective trapping and signal-enhanced detection of nano5806
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ACS Nano and perfectly matched layer boundary conditions. The trapping light directed into the optical fiber probe was set as a Gaussian beam with a wavelength of 808 nm. The mesh sizes of the regions of the fiber probe, microlens, water, nanoparticle, and E. coli were set as 200, 80, 150, 5, and 10 nm, respectively. The refractive indices of the fiber probe, microlens, PS nanoparticle, E. coli cells, and water were set as 1.44, 1.60, 1.58, 1.39, and 1.33, respectively. Experimental Setup. Two optical fibers sheathed with glass capillaries were mounted on tunable six-axis microstages (Kohzu Precision Co., Ltd.), which could be controllably moved in three dimensions with 50 nm resolution. The tips of the optical fibers were introduced into a microfluidic chamber, which contained the solution of nanoparticles or cells and was positioned on a two-dimensional translation stage (resolution = 50 nm). An 808 nm laser beam was sent to fiber 1 through the 10% arm of a Y-branch coupler (1:9) to trap nanoparticles and cells. The 90% arm of the coupler was connected to an oscilloscope through an InGaAs-biased photodetector (Thorlabs DET01CFC) with a band-pass filter of 790−1200 nm to detect the 808 nm backscattering signal in real time. A 398 nm laser beam was directed into fiber 2 to excite the fluorescent nanoparticles. An optical microscope (Union, HISOMET II-DH II) with a 100× objective (NA = 0.73) was used to observe the trapping process. A CCD camera (Sony iCY-SHOT, DXC-S500) connected to a personal computer was used to capture images and record videos. The total magnification in the field of view was 1000×.
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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/acsnano.5b08081. Details of the SEM images, counting method, simulations and calculations of fwhm, trapping selectivity, backscattering signal and torque, experiments of trapping of nanoparticles and E. coli cells, selective trapping in human blood solution and viability testing, and analyses of backscattering signals (PDF) Video S1: Details of the experiments of stability testing (MPG)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
B.L. supervised the project; Y.L. and H.L. designed the study; Y.L., H.X., and X.L. experiments; Y.L., H.X., and Y.Z. analyzed H.L., and B.L. discussed the results and wrote
conceived and performed the the data; Y.L., the paper.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors thank Jiyi Wu, Jiahao Yan, and Chang Cheng from the School of Physics and Engineering, Sun Yat-Sen University, for experimental assistance. This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (IRT13042) and the National Natural Science Foundation of China (No. 61205165). REFERENCES (1) Vollmer, F.; Arnold, S. Whispering-Gallery-Mode Biosensing: Label Free Detection Down to Single Molecules. Nat. Methods 2008, 5, 591−596. 5807
DOI: 10.1021/acsnano.5b08081 ACS Nano 2016, 10, 5800−5808
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ACS Nano
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DOI: 10.1021/acsnano.5b08081 ACS Nano 2016, 10, 5800−5808
