Low-Power Optical Trapping of Nanoparticles and Proteins with

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Low-Power Optical Trapping of Nanoparticles and Proteins with Resonant Coaxial Nanoaperture using 10 nm gap Daehan Yoo, Gurunatha Kargal laxminarayana, Han-Kyu Choi, Daniel A Mohr, Christopher T Ertsgaard, Reuven Gordon, and Sang-Hyun Oh Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00732 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Low-Power Optical Trapping of Nanoparticles and Proteins with Resonant Coaxial Nanoaperture using 10 nm gap

Daehan Yoo,1† Gurunatha Kargal Laxminarayana,2† Han-Kyu Choi,1† Daniel A. Mohr,1 Christopher T. Ertsgaard,1 Reuven Gordon,2,* and Sang-Hyun Oh1,*

1

Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States 2

Department of Electrical and Computer Engineering, University of Victoria, Victoria, British Columbia V8P 5C2, Canada

†

These authors contributed equally to this work.

*Address correspondence to: [email protected] and [email protected]

ACS Paragon Plus Environment

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Abstract We present optical trapping with a 10 nm gap resonant coaxial nanoaperture in a gold film. Large arrays of 600 resonant plasmonic coaxial nanoaperture traps are produced on a single chip via atomic layer lithography with each aperture tuned to match a 785 nm laser source. We show that these single apertures can act as efficient nanotweezers with a sharp potential well, capable of trapping 30 nm polystyrene nanoparticles and streptavidin molecules with laser powers as low as 4.7 mW. Our large arrays of plasmonic tweezers shown here offer a promising platform for the practical and broad application of trapping and manipulating biomolecules and nanoparticles.

Keywords. Optical trapping, optical force, coaxial aperture, gap plasmon, atomic layer lithography, nanogap.

Abstract Graphic


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Optical trapping, when first demonstrated in 1970 using two opposing laser beams focused to a common point acting on dielectric beads,1 provided a new route to trap and manipulate living cells and other biological particles.2-4 Despite these advances, trapping of subwavelength particles using a focused laser beam is restricted by the diffraction limit. As smaller particles require higher and more tightly confined optical fields for stable trapping, such tight focal points can create a dangerously large power density at the trap site and thus damage any trapped particles. Noble metal nanostructures that can confine free-space light into deep subwavelength volumes5-8 provide a promising route to overcome the diffraction limit and boost the efficiency of conventional optical tweezers toward trapping nanometer-sized particles with reduced laser powers. Using this approach, metallic nanostructures interact with incoming light to produce both sharper and deeper trapping potentials than can be created with traditional diffractive optics with the same incident power.9 Early on, the concept of such nano-optical trapping was proposed using the large field enhancement generated by plasmonic nanofocusing with a sharp metallic tip10 by exploiting the confined light transmitted through a metallic nanoaperture11 or the light highly-localized within nanometer-scale gaps.12 Researchers have since experimentally demonstrated subwavelength optical trapping with various nanostructures such as isolated nanoparticles,13,14 nanogap antennas,15-21 and nanostructured films22 to trap particles using lower laser powers than conventional optical tweezers. Works by several groups have also shown great promise of nano-optical trapping using sub-wavelength apertures in metal films. Followed by the demonstration of self-induced back action trapping using circular nanoholes in metal films,23 double nanohole (DNH) metallic apertures have shown highly efficient nano-optical trapping of nanoparticles and biomolecules.24-28


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To further improve the efficiency of aperture-based nano-optical tweezers, it is desirable to aggressively scale down the aperture dimensions and create ultra-narrow gaps (or slits) in metals, which can generate high-gradient optical fields.12,29,30 For example, nanogap antennas formed by two closely spaced gold nanorods can generate much stronger trapping forces than isolated nanorods at the same laser power. To push the limit of nanogap-based optical trapping, coaxial nanoapertures provide a unique geometry to combine the key features of a circular aperture and a nanogap in the same platform. The optical properties of coaxial apertures have been extensively studied in the context of extraordinary optical transmission by many groups.31-37 Among various applications of this versatile nanostructure, the concept of nano-optical trapping of particles with coaxial plasmonic apertures has received attention recently.38-40 While theoretical calculations predict that a coaxial nanoaperture with sub-10 nm gap can trap dielectric particles as small as 2 nm,38 this approach was not demonstrated experimentally likely due to the complexity of fabricating such ultranarrow (sub-10 nm) and high-aspect-ratio (>10) gaps in a metal film. Indeed, fabrication of such coaxial nanoapertures in an optically thick metal film (> 100 nm) is extremely challenging even with the most advanced electron-beam lithography or focused-ion beam (FIB) lithography tools. One

promising approach to overcome the challenge of patterning ultra-small gaps

leverages the precise thickness control afforded by various thin-film deposition techniques.29,41,42 In our previous work, we demonstrated a new lithography scheme utilizing atomic layer deposition (ALD), which can deposit conformal and pinhole-free metal oxide films with Angstrom-scale resolution, to create vertically-oriented single-digit nanometer gaps in metal films with high throughput. After standard lithography (photolithography, electron-beam lithography, or FIB milling) to create base patterns in a metal film (e.g. circular gold pillars for


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the fabrication of coaxial apertures), a conformal Al2O3 film is deposited via ALD, creating ultra-thin insulating layers on the sidewalls of the patterns. We have demonstrated various planarization methods such as blanket ion milling,43,44 peeling with adhesive tape,45,46 and glancing angle ion milling47,48 that can be used afterwards to expose the vertically oriented nanogaps for both optical and electrical applications. Sub-10 nm gaps with various shapes – coaxial apertures,43,45,47,49 rectangular apertures,45 ultra-long slits45,50,51 – have been produced via atomic layer lithography and used for surface-enhanced Raman spectroscopy43,51 and surfaceenhanced infrared absorption spectroscopy on large-area substrates.48,50,52 In this work, we leverage atomic layer lithography to produce an array of resonant coaxial apertures with 10 nm gaps and use them for low-power (sub-5 mW) nano-optical trapping. Since it is possible to precisely control the critical dimension (gap size) of the coaxial trap via ALD and generate ultrastrong optical forces not possible using conventional diffraction-limited optical tweezers, this technology represents an important step toward practical applications of nano-optical tweezers. Coaxial nanoapertures feature multiple Fabry-Pérot (FP) resonances that are easily excited with standard illumination leading to high transmission and, due to the extreme subwavelength nature of the aperture, high field enhancements. Efficient coupling and strong field confinement are both advantageous when considering plasmonic nanostructures for optical trapping of sub-10 nm particles, as optical force generally scales with the third power of the particle radius. Additionally, coaxial apertures have several independent design parameters that can be changed to tune their FP resonances, including gap size, metal film thickness, and diameter. Both the gap size and diameter affect the effective index of the gap plasmon mode traveling though the aperture, while the metal film thickness defines the length of the FP cavity. Using these degrees of freedom, even a modest design space can lead to a large selection of


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resonant wavelengths. For these experiments, we chose to use the first-order FP mode (FP1) of the coaxial nanoaperture due to the ease in tuning the resonance to the near-infrared (NIR), where several common laser wavelengths used for trapping are located (e.g. 785 nm, 850 nm, 1064 nm, 1550 nm). Prior to fabrication, numerical simulations were performed on 10 nm gap coaxial apertures to match their FP1 mode to our 785 nm laser. The optical properties of individual coaxial apertures with 10 nm gaps and diameters ranging from 100-200 nm were then studied using the finite element method (FEM) in COMSOL Multiphysics (Figure 1a). The transmission resonances in Figure 1a arise from a guided TE11 mode traveling along the length of the respective aperture and constructively interfering at the first-order FP resonance condition (cavity length = λ/2), generating high electric field enhancements at the ends of the cavities (Figure 1b and c). At the resonance maximum of the 180 nm diameter aperture, we calculated the optical forces imparted on a 30 nm polystyrene bead (n=1.58) in water (n=1.33) placed 5 nm (measured from the bottom of the bead) above the surface of the aperture by integrating the Maxwell stress tensor across the surface of the bead with the equation53 ⟨𝑭⟩ = ∫,-⟨𝑇⟩ ∙ 𝒏 𝑑𝑎

(1)

where ⟨F⟩ is the time averaged force, ∂V is the surface of the bead, n is the unit vector perpendicular to the surface, and T is the Maxwell stress tensor given as 5

𝑇 = 𝜀0 𝑬𝑬 − 𝜇0 𝑯𝑯 − 6 (𝜀0 𝐸 6 + 𝜇0 𝐻6 ).

(2)

This calculation was performed for different particle positions to extract the trapping potential 𝒓

well, defined as10 𝑈(𝒓0 ) = − ∫?> 𝑭(𝒓)𝑑𝒓, as shown in Figures 1d-f. Our results were normalized


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to 100 mW of Gaussian light (N.A.=1.25) illuminating the aperture and demonstrate that a stable optical trap2 (U ≥10kT) can be created with common, commercially available laser powers. One main advantage of using coaxial apertures or other aperture-based trapping devices is the rapid heat dissipation upon illumination. Compared to nanoparticle-based traps, apertures in planar metal films are surrounded by large heat sinks which can dissipate thermal energy quickly throughout the metal film,13,36 while nanoparticle-based devices must often dissipate heat through non-metallic routes. In Figure 2, we simulated the Joule heating of a bowtie antenna (Figure 2a) and a coaxial aperture (Figure 2b) upon resonant illumination, and note a 25-fold reduction in the maximum device temperature when normalized to input power (shown), and 10 fold reduction when normalized to potential well depth (Supporting Figure 1). Furthermore, we expect the Al2O3-filled gap to increase the thermal stability of our structure compared to an open gap or a free-standing device without any dielectric supporting material. Our nano-optical trapping devices were fabricated on 4-in. glass wafers with large, 10 × 10 arrays of isolated coaxial nanoapertures (Figures 3a-c) having diameters ranging from 100200 nm in 20 nm increments (600 coaxial nanoaperture traps per chip). Interspersed with the coaxial apertures are alignment marks that only aid in finding the individual apertures during experiments. Fabrication was performed using electron-beam lithography to first pattern an array of gold pillars (i.e. center conductors) using a liftoff process. Next, alumina (Al2O3) is deposited using ALD to precisely define the gap size (10 nm) along the vertical sidewalls of the pillars followed by a second, conformal gold deposition to create the outer conductor of the apertures. Finally, glancing-angle ion milling47 removes the excess metal from the conformal coating to reveal the final device (Figures 3c). As the ion milling time increases, the resonator cavity becomes shorter, leading to a blueshift of the resonance and allowing us to tune the FP1 mode to


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a range of desired wavelengths (Figure 3d and e). For thicker metallic films, we were able to measure the transmission spectra from single coaxial nanoapertures with various diameters (see Supporting Information). As we increased the ion milling time to tune the FP1 resonance to our 785 nm laser wavelength, however, it became more difficult to measure the transmission resonance from a single coaxial nanoapertures due to the background signal from the thinner metallic film. For this reason, the transmission spectra in Figure 3e and f were measured from the arrays of coaxial nanogap arrays with a period of 1 µm. Although the array is periodic, we still anticipate the resonance position to be representative of the single aperture case as any periodic effect should occur at 1 µm and above. Figure 3f shows the FP1 resonances measured from arrays of coaxial nanoaperutres with various diameters. As the diameter of the aperture increases from 140 nm to 200 nm, the resonance redshifts due to the dispersion of the gap plasmon mode. For our chosen ion milling time, a 180 nm diameter aperture exhibits a resonance very close to our 785 nm laser wavelength, and was therefore selected for all our optical trapping experiments. To demonstrate optical trapping with coaxial apertures, 20 µL of a 30 nm polystyrene (PS) bead (Sigma Aldrich) solution (140 pM) or streptavidin (Sigma Aldrich) in 1× phosphatebuffered saline (PBS) (pH 7) solution (190 nM) were pipetted into a micro chamber formed by an imaging spacer (Secure Seal imaging spacer, Grace Bio-Labs) with a hole diameter of 9 mm and thickness of 90-100 µm on 80-130 µm thick cover glass (GoldSeal, Ted Pella, Inc.). The Au coated glass chip including multiple arrays of individual coaxial apertures was placed on the imaging spacer to seal the well and the whole sample assembly is then mounted on the optical trapping setup. Figure 4 shows the schematic of the experimental setup used for optical trapping in coaxial nanoapertures of different diameters. A home-built optical tweezer setup with a silicon-based avalanche photodiode (APD; SPCM-AQRH-14-FC) was used to measure the


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transmitted signal through single coaxial nanoapertures during trapping. A NIR laser (OBIS 785 nm LX from Coherent) was then focused onto a single coaxial nanoaperture using a 100× oil immersion objective (N.A.=1.25). The trapping experiments were performed with the incident laser powers as low as 4.7 mW, measured before the objective lens. Transmitted light through the aperture is collected using a 10× microscope objective (N.A.=0.25) as a condenser and measured by an APD (0.5 msec acquisition time). First, we carried out optical trapping of 30 nm PS beads using our coaxial nanoapertures. In Figure 5, we plot over time the laser transmission through the coaxial nanoaperture in a diluted solution with and without 30 nm PS beads. Figure 5a shows time traces of transmitted optical intensity in a PS beads solution (140 pM). Single step-like jumps in the transmitted intensity are observed, typically within 3 minutes, which are characteristic of an aperture-based nano-optical trapping event. To ensure that the step-like changes in the intensity can be attributed to PS beads being captured by the coaxial nanoaperture, optical trapping in PBS solution without beads was performed as a control experiment. Over the course of 5 minutes for our control experiment, we observed no signal changes as plotted in Figure 5b. Additionally, the cycle of trapping and then releasing PS beads in an individual coaxial nanoaperture was repeated by blocking and un-blocking the laser beam. As shown in Figure 5c, once an intensity change was recorded, the laser was blocked to release the trapped particle from the coaxial nanoaperture. After 30 seconds, we re-illuminated the coaxial nanoaperture, leading to another step-like signal showing nearly identical trapped and released intensity levels as the previous trapping event. This reversible trapping/releasing cycle was repeated several times. Based on these observations, we conclude that our coaxial nanoapertures work as nano-optical nanotweezers for reversible trapping of nanoparticles.


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Next, we carried out experiments of optically trapping proteins using our coaxial nanoapertures. Figure 6 shows time-resolved transmission intensity data recorded from a single 180 nm-diameter coaxial nanoaperture in a 190 nM solution of streptavidin molecules (molecule size ~4.5 nm × 4.5 nm × 5.8 nm).54 All the time traces shown in Figure 6a exhibit a single steplike intensity change, which we attribute to successful optical trapping events. For this set of experiments, all optical trapping events occurred within 3 minutes. To prove the discontinuous signals observed during our trapping experiments result from streptavidin molecules captured by the sharp potential landscape induced by a coaxial nanoaperture rather than from passive adsorption, we again performed trapping and releasing cycles as shown in Figure 6b. If this transmission increment was induced by protein adsorption on the metal surface, it should increase with time. However, all trapping signals were reversible with respect to the laser state, and thus indicate active optical trapping events. With our coax-based optical trapping platform, it is now possible to perform a large number of trapping experiments in a reproducible manner. Extensive statistical analysis of single-molecule trapping events using molecules of different sizes at varying concentrations is currently under way and will be reported in the future. In summary, we have experimentally demonstrated nano-optical trapping of protein molecules and nanoparticles using individual coaxial apertures with 10 nm gaps produced via atomic layer lithography. Our structures are fabricated at the chip scale (600 coaxial traps per chip) using electron-beam lithography and ALD, and the process can be scaled up using highthroughput fabrication methods such as nanoimprint, as the critical dimension, i.e. the 10 nm gap size, is defined by ALD. In addition to trapping demonstrations, we numerically study our devices to determine the trapping potential. Coaxial apertures exhibit high transmission efficiencies with large field enhancements, increasing the efficiency of optical trapping and


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reducing incident laser power trapping thresholds. As a result, we are able to trap 30 nm polystyrene beads and streptavidin molecules with incident light powers as low as 4.7 mW. As nanogap structures are also promising for surface-enhanced Raman scattering (SERS)28-30,55 and surface-enhanced infrared absorption,42,48,56 our devices can act as a multifunctional platform to integrate single-molecule manipulation and spectroscopic analysis.57 Also, the unique ring-shape geometry of the coaxial nanoaperture trap can be combined with circularly polarized light to explore optical rotation.58-61

Supporting Information Computer simulations comparing the optical trapping efficiency of bowtie antennas and coaxial apertures and additional details on thermal modeling. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Contributions R.G. and S.-H.O. conceived the experiments. D.Y. performed sample design, fabrication, and optical characterization. G.K.L. and H.-K.C. performed optical trapping experiments. D.A.M. performed numerical simulations. H.-K.C. and C.T.E. constructed an optical trapping apparatus. All authors analyzed the data and wrote the paper together.

Acknowledgments This research was supported by the U.S. National Science Foundation (ECCS 1610333 to D.Y., H.-K.C., S.-H.O.) and NSERC CRD (Grant No. 469469-2014 to G.K.L. and R.G.). D.A.M. acknowledges the National Institutes of Health Biotechnology Training Grant (NIH T32


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GM008347). C.T.E. was supported by the NSF Graduate Research Fellowship Program. Device fabrication was performed at the Minnesota Nanofabrication Center at the University of Minnesota, which receives partial support from NSF through the National Nanotechnology Coordinated Infrastructure (NNCI). Electron microscopy measurements were performed at the Characterization Facility, which has received capital equipment from NSF MRSEC.

Notes. The authors declare no competing financial interest.


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Figure 1. (a) Transmission spectra (substrate illumination) calculated from single coaxial apertures with 10 nm gaps and varying diameters. (b, c) The horizontal (taken 1 nm below the metallic surface) and vertical electric field distributions for a 180 nm diameter aperture, respectively. (d) Transverse trapping potential created by the optical force (Fx and Fy) imparted on a 30 nm polystyrene bead placed 5 nm away from the surface of the 180 nm coaxial aperture and normalized to 100 mW of incident light (NA=1.25, water illumination). (e, f) Cross-sections of the transverse optical trapping potential (Uxy) along y = 0 nm and x = 90 nm, respectively.


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Figure 2. Horizontal (top panels) and vertical (lower panels) thermal profiles upon Gaussian illumination for a (a) bowtie antenna (30 nm-thick film, 105 nm side length) and a (b) coaxial aperture (80 nm-thick film, 180 nm diameter). Both structures were illuminated on resonance from above (water side) in the NIR. Convective fluid flow was not simulated for simplicity. All scale bars are 100 nm.


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Figure 3. (a) Bright-field, optical microscopy image (transmission mode) of a 10×10 array of individual coaxial nanoapertures interspersed with alignment-marks. (b) Zoomed-in bright-field image of a single unit cell consisting of a single coaxial nanoaperture and four alignment-marks. (c) SEM image of coaxial nanoapertures with a 160 nm diameter. (d) Illustration of controlling the cavity length using glancing-angle ion milling. (e) Spectra measured from the coaxial nanogap array with a 180 nm diameter and 1 µm period while reducing the cavity length. (f) Spectra measured from 1 µm-periodic coaxial nanoapertures with varying diameters.


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Figure 4.

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Schematic of the optical trapping apparatus. A 785 nm diode laser is used to

illuminate the coaxial aperture through the sample solution and its optical power is regulated with an optical density filter (ODF). A beam expander magnifies the beam to fill the back-focal plane of a 100× oil immersion objective (N.A.=1.25). The transmission intensity from the coax is then collected with 10× microscope objective (N.A.=0.25) and focused onto an avalanche photodiode (APD). The circular inset shows a schematic of a coaxial aperture (inverted in our setup).


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Figure 5. (a) Time trace of the transmitted intensity through a coaxial nanoaperture (180 nm inner diameter and 10 nm gap) in 1×PBS solution containing suspended 30 nm polystyrene (PS) beads (140 nM). (b) No signal change is observed through a coaxial nanoaperture in a PBS solution with no PS beads. (c) Time trace of the transmitted intensity through a coaxial nanoaperture with suspended PS beads in 1×PBS as a function of the laser state. Gray regions indicate when the laser radiation is blocked triggering a release of the PS bead.


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Figure 6. (a) Four separate time traces of the optical power transmitted through a single coaxial nanoaperture in 1×PBS solution containing streptavidin (190 nM). The incident optical power applied was 4.7 mW. Each experiment demonstrates the same intensity increment fixed in time indicating repeatable active trapping. (b) Time trace of the transmitted intensity through a coaxial nanoaperture in the same solution as a function of the laser state. Gray regions indicate when the laser radiation is blocked triggering a release of the streptavidin molecule.


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