Levitated Plasmonic Nanoantennas in an Aqueous Environment Yazgan Tuna,†,‡,∥ Ji Tae Kim,†,#,∥ Hsuan-Wei Liu,†,‡ and Vahid Sandoghdar*,†,‡ †
Max Planck Institute for the Science of Light, Staudt-Straße 2, 91058 Erlangen, Germany Department of Physics, Friedrich Alexander University of Erlangen-Nürnberg, 91058 Erlangen, Germany
‡
S Supporting Information *
ABSTRACT: We report on the manipulation of a plasmonic nanoantenna in an aqueous solution using an electrostatic trap created between a glass nanopipette and a substrate. By scanning a trapped gold nanosphere in the near field of a single colloidal quantum dot embedded under the substrate surface, we demonstrate about 8-fold fluorescence enhancement over a lateral full width at halfmaximum of about 45 nm. We analyze our results with the predictions of numerical electromagnetic simulations under consideration of the electrostatic free energy in the trap. Our approach could find applications in a number of experiments, where plasmonic effects are employed at liquid−solid interfaces. KEYWORDS: plasmonics, plasmofluidics, electrostatic trapping, fluorescence enhancement, quantum dot
P
20 nm has been achieved using a simple spherical antenna geometry.32 One can imagine different approaches for the manipulation of a plasmonic nanoantenna close to an aqueous interface. Attaching an antenna23 or fabricating it33−35 at the end of a fiber tip is one possibility. Using optical tweezers36,37 can be another option although trapping of very small metallic nanoparticles38,39 within a few tens of nanometers comes at the cost of prohibitive heating. Recently, we reported on trapping of small gold nanoparticles at the junction between a glass nanopipette and a glass substrate in water40 based on a geometry-induced electrostatic potential.41 Here, we show that this technique can yield a considerable plasmonic enhancement of fluorescence from a single semiconductor quantum dot (qdot) in water within a lateral range well below 100 nm.
lasmonic nanoantennas have attracted much attention in the past decade both from the fundamental and technological points of view because of their potential in a number of areas such as the enhancement of the Raman scattering cross section1 or the enhancement of the absorption cross section and the modification of the spontaneous emission rate.2 While the great majority of the existing efforts have concerned dry conditions, plasmonic effects have also begun to be used in the liquid phase, giving rise to the emergence of plasmof luidics.3−7 A particularly promising area of application for plasmofluidics is in biophysics and biosensing,8−11 where biological nanoparticles and macromolecules diffuse or undergo directed transport.12,13 A critical and central feature of plasmonic interactions is their near-field character, which demands subwavelength separations between a nanoantenna and the substance to be affected.2 Various methods such as self-assembly,14 lithographic nanofabrication,15−17 random distribution of emitters,18−20 chemical synthesis,21,22 and use of scanning probe technology for nanopositioning23−26 have been employed to achieve nanometer spacing between emitters and antennas. Among these, nanopositioning with scanning probes has delivered the most quantitative results because it allows one to study the very same emitter at different orientations and separations from the nanoantenna. Scanning a nanoantenna also allows the controlled realization of apertureless near-field microscopy27−29 in a complementary fashion to conventional scanning near-field optical microscopy (SNOM) using subwavelength apertures.30,31 Indeed, spatial resolution down to the order of © 2017 American Chemical Society
RESULTS AND DISCUSSION Figure 1a shows the schematics of our experiment. A gold nanoparticle (GNP) is trapped and manipulated in the near field of individual semiconductor CdSe/CdS core/thick shell qdots,42,43 which were embedded at the upper interface of a thin polymer film44,45 at large lateral separations of several tens of micrometers. The GNPs used in this experiment had a diameter of 80 nm and were trapped out of the aqueous Received: May 12, 2017 Accepted: July 11, 2017 Published: July 11, 2017 7674
DOI: 10.1021/acsnano.7b03310 ACS Nano 2017, 11, 7674−7678
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Figure 1. (a) Schematic illustration of the experimental setup. An 80 nm GNP electrostatically trapped between a scannable nanopipette and a substrate is scanned across a CdSe/CdS giant qdot embedded in a PMMA layer in an aqueous bath. The fluorescence signal of individual qdots is collected by a total internal reflection microscope. (b) Interferometric scattering (iSCAT) detection images showing a guided single GNP by lateral scanning of a nanopipette placed at height h ≈ 140 nm. (c) Axial variations of the GNP in the trap during the scan trajectory depicted in panel b.
Figure 2. (a−c) A series of CCD camera images of a CdSe/CdS qdot with different lateral positions of a trapped 80 nm GNP a, −400 nm, (b) 0 nm, and (c) 400 nm at a constant pipet-substrate gap, h ≈ 95 nm. The inset shows the unsaturated image of qdot confirming a donut shape. Scale bar is 500 nm. (d) Dependency of the fluorescence signal on the lateral position of a GNP. A fluorescence enhancement of about 8 and a fwhm = 45 nm are obtained.
domain (FDTD) simulations, where we took into account the antenna-induced modifications of the radiative and nonradiative decay rates as well as changes in the excitation rate and collection efficiency (see Supporting Information). Figure 3b shows the outcome for the overall enhancement of fluorescence for emitters of different initial quantum efficiencies. We see that the behavior observed in Figure 2d is well reproduced for an emitter with an internal quantum efficiency of 40% placed at 15 nm under the surface with a tilt angle of 25° with respect to the z-axis. It is important to note, however, that this agreement is only semiquantitative because qdots are known to possess two emission dipole moments55,56 and that we do not have a precise knowledge of the depth of the qdot under the GNP although the finite size of the qdot (about 15−20 nm in diameter42,43) puts a lower bound on this quantity.
solution at the end of a glass nanopipette with surface charge density ∼1016 em−2 [ref 46] and an opening of about 100 nm. A three-dimensional piezo-electric stage allowed us to move and position the trapped particle at will.40 To visualize the GNPs, we performed wide-field interferometric scattering microscopy (iSCAT)47,48 at a frame rate of 1.9 kHz using a CMOS camera at a wavelength λ = 532 nm. Figure 1b displays two iSCAT images of a GNP before and after it was laterally displaced at a speed of 250 nm s−1 by 1.4 μm in a trap with stiffness 10−100 fN/nm (calculated with equipartition theorem).40 As presented by Figure 1c, the interferometric nature of the iSCAT contrast also allows one to monitor the axial height variations of a GNP41,49 without the need for an external shear-force control.50 We note that the electrostatic repulsion in the trap protects the GNP from coming in contact with the PMMA substrate since it carries a surface charge density of about 6.6 × 1015 em−2 in deionized water.51 To excite the qdots, we changed the laser beam path to achieve total internal reflection through the microscope objective. The fluorescence from the qdots was collected with the same objective and imaged on a sensitive CCD camera. More details of the experimental procedure can be found in the Methods section. Figure 2a shows the fluorescence image of an unperturbed qdot, whereas in Figure 2b we present the image of the same qdot when a trapped GNP was positioned above it at a pipet-substrate gap of h ≈ 95 nm. Figure 2c displays the qdot fluorescence after the particle is scanned away. The intensity minimum in the middle of the image in Figure 2b (see inset) indicates that the emission dipole associated with the antenna-qdot composite lies dominantly along the normal to the sample plane.52,53 In Figure 2d, we plot the integrated signal under the Gaussian fits of the fluorescence camera images from a single qdot as a GNP was laterally scanned across it. We find that the fluorescence signal was increased by about 8 times over a profile with a full width at half-maximum (fwhm) of 45 nm, confirming a near-field interaction. We attribute the asymmetric shape of the observed peak in Figure 2d to the variation of the relative orientation of the qdot’s dipole moment with respect to the GNPs radius at the point of closest contact during the scan as illustrated in Figure 3a.24,54 To gain some quantitative insight into the observed effects, we performed finite-difference time-
Figure 3. (a) Schematic illustration of the asymmetry in the relative orientation of the emitter dipole moment with respect to the radial normal to the antenna surface. (b) Total enhancement of the fluorescence as a function of the lateral displacement of a GNP with 5 nm steps. (c) The calculated enhancement factor versus the axial displacement of the GNP at the peak position (−15 nm). (d) fwhm of the lateral scan profile (see panel b) for different antennasubstrate gap values g (see panel a). 7675
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Figure 4. Studies at different nanopipette heights. (a−f) Fluorescence signal of a qdot as a function of the lateral position of a trapped GNP at different antenna-substrate gaps g. Scanning speed was 0.25 μm s−1, and the length of each scan was 1.25 μm. The fluorescence signal was monitored at a frequency of 10 Hz. The red symbols in panel f indicate the region after the qdot was bleached. (g) Enhancement factor of a qdot fluorescence as a function of g; (h) fwhm of the Gaussian fitted to each scan.
The results of the simulations in Figure 3c show the expected drop of the fluorescence signal as a function of the axial separation of the GNP. To verify this behavior experimentally, we adjusted the trap height h (see the Methods section for experimental details), and scanned the antenna at different antenna-substrate gaps g which is calculated from h according to the Helmholtz free energy as shown in Figure 5. In Figure 4a−f, we present the fluorescence signal from a single qdot for h varied between 115 and 90 nm at 5 nm steps. We note in passing that contrary to the conventional shear-force controlled SNOM tip, our method does not rely on a hard contact of the tip with the sample surface. We also remark that the qdot bleached in the latter part of the measurement presented in Figure 4f. The signal recorded after photobleaching (see red symbols) provides a convenient measure for the small fluorescence background, which was subtracted from all data for estimating the enhancement factors. To examine the relationship between h and g, we calculated the Helmholtz free energy (F) in the system by summing the electrostatic field energies and entropies over all charges in three dimensions.57 Here, we considered deionized water with 0.04 mM ionic strength, a 80 nm GNP (σgnp ≈ 7 × 1015 em−2), a glass nanopipette opening of 100 nm in diameter (σglass ≈ 1016 em−2), and a PMMA substrate (σPMMA ≈ 6.6 × 1015 em−2). Figure 5a displays the profiles of the calculated free energy difference (ΔF) as a function of g for traps with different h as indicated by the color codes in each curve. As shown in Figure 5b, g at the minimum free energy (ΔFmin) varies nearly linearly with h. Using the outcome of Figure 5, we can now analyze the enhancement factor and the fwhm of the scan profiles obtained in Figure 4a−f in Figure 4g,h in terms of g. We find that the general trend agrees well with the simulated data in Figure 3c. However, here we observe a steeper dependence on the emitter−antenna gap. We attribute this to the weaker trap stiffness at larger h and, thus, higher fluctuations as indicated by the simulated trap potentials in Figure 5a.
Figure 5. (a) Dependence of the trapping free energy ΔF on particle−substrate gaps g for different pipet-substrate gaps h ranging from 80 to 130 nm. (b) The extracted value of g at the minima of the trapping potentials, ΔFmin, as a function of the pipetsubstrate gap, h.
enhancement of fluorescence in aqueous environments. While we demonstrated this for stationary semiconductor qdots embedded at an interface, our experimental platform can also be used for diffusing fluorophores, for example, when they act as a label for proteins or lipids in biomembranes. Adaptation of the trap geometry to support asymmetric antennas such as nanorods58 will also allow rotational diffusion studies.
METHODS Sample Preparation. Glass nanopipettes with about ∼100 nm aperture diameter and optimized taper shapes were fabricated by a multistep pulling process using a laser pipet puller (P-2000, Sutter Instrument). The nanopipettes were cleaned by standard rinsing and an O2 plasma process. Gold nanospheres with a diameter of 80 nm (British Biocell International) were centrifuged and resuspended in deionized water (18 MΩ cm−1) twice to remove traces of salt or other contaminants. These nanoparticles carry a charge59 of the order of ∼100 e. Topography-free samples of giant CdSe/CdS core/thick shell qdots42,43 embedded in PMMA film were fabricated as follows: The CdSe/CdS quantum dots were deposited on a cleaned cover glass by spin coating at 2000 rpm for 60 s. In addition, the quantum dots were covered with a PMMA film of 800 nm thickness by spin coating at 2000 rpm for 60 s. To place the qdots at the PMMA−air interface, the resulting film was floated in water, subsequently flipped, and placed on a glass substrate.45 A customized liquid chamber on a scanning stage and an inverted microscope was used for trapping and nanoantenna experiment.
CONCLUSION In this work, we demonstrated the application of an electrostatically confined gold nanoantenna for plasmonic 7676
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ACS Nano Scanning the Nanopipette. The position of the glass nanopipette was controlled with 3-axis piezo scanner (P-620.2CD, P620.ZCD, Physik Instrumente). The pipet−substrate gaps were precisely controlled via iSCAT detection. The contact point between the nanopipette and the substrate was detected by approaching the nanopipette to the surface at 10 nm steps until the interference pattern created by the light scattered of the nanopipette and the light reflected from the substrate no longer changes. Then, we pulled back the pipet and set the initial gap of 120 nm. The stepwise approach of the pipet during fluorescence imaging was 5 nm. The speed and range of lateral scanning were typically 250 nm s−1 and 1.25 μm. Interferometric Scattering Detection and Fluorescence Imaging. The detection of gold nanoparticles (80 nm) and CdSe/ CdS qdots was performed using an inverted optical microscope designed for interferometric scattering detection (iSCAT) and total internal reflection fluorescence imaging. For iSCAT, a 10 mW CW laser at λ = 532 nm illuminated the sample using an oil-immersion microscope objective (100×, NA 1.4, Olympus). The light scattered from the GNP and reflected from the substrate were collected by the objective and imaged on a CMOS camera (MV-D1024-160-CL-12, PhotonFocus). The qdots were excited under total internal reflection illumination and their fluorescence was collected using the same objective, a 600 nm long pass filter, and a CCD camera (Andor Luca) with 10 frames per second. Data Analysis. CCD images were cut to create a 615 nm × 615 nm box around localized qdot in the absence of the GNP. Then the fluorescence image was fitted to a 2D Gaussian for each image separately and the fit function was integrated over a range of 5 μm × 5 μm. The resulting total intensity values were plotted as in Figure 4a−f. Average background values of these fits were taken as a reference and the enhancement factor was calculated by dividing the peak values to the reference.
Energy (DOE) Office of Science by Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000).
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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.7b03310. Calculations of the electrostatic potential landscape via Helmholtz free energy and Comsol Multiphysics 5.2a simulation details; discussion about the calculation of enhancement factor; finite-difference time-domain (FDTD) simulation results with parametric scan (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Yazgan Tuna: 0000-0003-4499-4396 Present Address
# J.T.K.: Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China.
Author Contributions ∥
Y.T. and J.T.K. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was financed by the Max Planck Society and an Alexander von Humboldt professorship. We thank S. Götzinger, K. Matsuzaki, X.-L. Chu, and T. Utikal for experimental support. Thick-shell qdots were synthesized at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of 7677
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