Photothermal Microscopy of Nonluminescent Single Particles Enabled

Letter pubs.acs.org/JPCL

Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators Kevin D. Heylman, Kassandra A. Knapper, and Randall H. Goldsmith* Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: A powerful new paradigm for single-particle microscopy on nonluminescent targets is reported using ultrahigh-quality factor optical microresonators as the critical detecting element. The approach is photothermal in nature as the microresonators are used to detect heat dissipated from individual photoexcited nano-objects. The method potentially satisfies an outstanding need for single-particle microscopy on nonluminescent objects of increasingly smaller absorption cross section. Simultaneously, our approach couples the sensitivity of label-free detection using optical microresonators with a means of deriving chemical information on the target species, a significant benefit. As a demonstration, individual nonphotoluminescent multiwalled carbon nanotubes are spatially mapped, and the per-atom absorption cross section is determined. Finite-element simulations are employed to model the relevant thermal processes and elucidate the sensing mechanism. Finally, a direct pathway to the extension of this new technique to molecules is laid out, leading to a potent new method of performing measurements on individual molecules. SECTION: Physical Processes in Nanomaterials and Nanostructures

M

of refraction. Consequently, optical microresonators have emerged as important diagnostic tools15−18 and enabled single-particle detection.19−21 In the typical experimental sensing geometry, the binding of a single analyte molecule or particle results in a fractional spectral shift of the resonance of the microresonator. Detection of single proteins and viruses by microresonators has been achieved via the induced offset in resonance wavelength caused by the presence of a polarizable object interacting with the propagating optical mode.20−24 However, this geometry offers little chemical information about the bound species. Some chemical information may be obtained by selective functionalization of the microresonator surface,16,17 but there is a critical need for more detailed optical characterization of the adsorbed species. Such an advance would significantly amplify the power and utility of microresonator sensing. In this work, we leverage the high intrinsic sensitivity of ultrahigh-Q optical microresonators as the critical detection element for photothermal absorption microscopy. In such an experimental geometry, the goal is not the detection of nonchromophoric analytes as above21,24−26 but the spectral measurement of nonphotoluminescent chromophoric target nano-objects via light absorption. This new approach has the potential to simultaneously provide spectral information for sensing applications, as well as offer a new tool for observing single-molecule dynamics without fluorescence. As a test

easurements on individual particles and molecules are critical to the elucidation of nanoscale behavior. These types of measurements, whether optical, electrical, or scanning probe, are uniquely capable of exposing the tremendous heterogeneity and unsynchronized dynamics that exist at the nanoscale, behavior that directly governs emergent functional macroscopic properties. Optical measurements on individual particles are particularly adept at discerning electronic structure, correlating electronic structure with particle morphology and microenvironment, and doing so in a noncontact experimental geometry. For example, single-particle optical measurements have enabled significant progress in understanding the behavior of nanoparticles,1−7 carbon nanotubes,8,9 and conductive polymers.10−13 The majority of optical single-particle measurements rely on photoluminescence from the target nano-object, an approach that allows high sensitivity and low background.14 However, if the target system is nonluminescent, it is completely opaque to most single-particle optical probes. Consequently, there is an outstanding need to develop new single-particle probes that do not rely on photoluminescence, can operate at room temperature, and can allow singlemolecule sensitivity. This Letter describes a new paradigm for single-particle and ultimately single-molecule microscopy that takes advantage of the tremendous sensitivity of ultrahigh-quality factor (Q) optical microresonators. Ultrahigh-Q optical microresonators are extremely sensitive to small perturbations in their microenvironment. The propagating whispering gallery mode makes many repeated interactions with the microresonator surface, amplifying the effect of small changes in the local index © 2014 American Chemical Society

Received: April 18, 2014 Accepted: May 16, 2014 Published: May 16, 2014 1917

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Figure 1. Photothermal detection of nonluminescent objects using optical microresonators. (a) SEM of a toroidal microresonator. (Inset) Resonance scan of a toroidal microresonator. The fitted Q factor is 1.3 × 107 (predeposition). (b) Schematic of the optical path of the pump laser (red) and probe laser (blue/black). Abbreviations: ND = neutral density; CW = continuous-wave. (c) Rendering (Blender) of the photothermally pumped nanotube on a microresonator. (d) Coarse photothermal map of an entire resonator, showing three distinct nanotube signals. The photothermal background from the silicon substrate is faintly visible.

probe of local temperature could allow substantial increases to the sensitivity of nonluminescent single-particle and singlemolecule measurements, ultimately enabling high-resolution spectroscopy of such objects and time-resolved fast dynamics. Toroidal microresonators, Figure 1a, are fabricated by lithographically defining silica disks on a silicon substrate41 and undercutting the silica with an isotropic SF6/Ar etch.42 This procedure is followed by a laser-induced reflow step.41 The details of our fabrication process have been reported.43 Toroidal optical microresonators are used due to their unique combination of ultranarrow line widths41 and small mode volumes.44 Probe light is coupled into the microresonator via a tapered optical fiber controlled by a three-axis piezopositioner (Attocube ECS 3030), Figure 1b. Two orthogonal microscopes are used for alignment (Navitar Ultrazoom) and excitation beam delivery (Nikon FN1). A fiber-coupled tunable diode laser (New Focus; velocity, 1560 nm) is used to probe the resonant wavelengths. The fiber-coupled probe power is kept low (∼250 nW) to avoid thermal broadening of the toroid resonance.45 Nanotubes are photoexcited with a pump beam (Blue Sky Research, 640 nm) focused by a microscope objective (60×, 0.95 NA, Nikon) to a near-diffraction-limited spot (780 nm 1/e2 diameter). A gimbal-mounted mirror placed at a plane conjugate to the objective back aperture is used to control the beam position on the resonator,43 Figure 1b. A polarizer and half-wave plate are used to adjust the linear

system, individual multiwalled carbon nanotubes (MWCNTs) are pumped with a focused free-space laser. The local increase in temperature caused by heat dissipation from the absorbing nanotube red shifts the resonant wavelength of the microresonator due to the positive thermo-optic coefficient of silica.27 A resonance shift of almost 40 line widths is observed at modest pump power (Q = 3.3 × 106, Ipump = 9 × 104 W/cm2). Photothermal shifts as a function of excitation beam position and input polarization are used to image the nanotubes and explore their electronic transitions. A number of other approaches have been used to image or spectrally analyze individual absorbing nano-objects without photoemission, including detection of Rayleigh scattering,8 spatial modulation,28 balanced detection,29 coherent antiStokes Raman scattering,30 detection of conductance,31 highfrequency signal modulation from ground-state depletion,32 and photothermal detection.33−36 The first optical detection of individual molecules employed frequency modulation to enable detection via direct absorption but required cryogenic temperatures.37 In a particularly successful implementation of photothermal detection, scattering is used as a readout of the local temperature shift.35,36 This approach enabled singlemolecule detection38 as well as insights into the electronic structure of nanotubes39 and nanoparticles.40 However, scattering is an inherently low-cross-section process for small objects, limiting the ultimate sensitivity. A more responsive 1918

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Figure 2. Gallery of photothermal maps. (a−f) Representative samples of photothermal maps taken on separate nanotubes under identical conditions. The incident power was 440 μW, the resolution was 250 nm/pixel, and each pixel is an average of five scans.

selected and probed, allowing for faster throughput, though more dense surface coverage will ultimately compromise the resonator quality factor. Other techniques used to enhance sensitivity in nanoparticle−resonator binding studies, such as Pound−Drever−Hall locking,46−48 backscattered light detection,49 and all-fiber interferometry,25 could be used to improve the sensitivity. A representative family of photothermal images is shown in Figure 2. MWCNTs are observed to be up to several micrometers in length but appear with a diffraction-limited thickness, consistent with their high aspect ratio. Very high contrast is observed against the background. For example, the nanotube in Figure 2f exhibits signal-to-noise = 425 and signalto-background = 36. A case study of four MWCNTs is presented in Figure 3, with photothermal maps (a), SEM images (b), and polarization dependence (c,d) presented side by side. The photothermal image matches the SEM image, including photothermal hotspots corresponding to the carbonrich globules surrounding residual catalyst evident in nanotubes in Figure 3b−d. To calibrate our photothermal technique, the absorption cross section was determined on a per-atom basis to account for the variation in size between nanotubes, Table 1. The absorption cross section of nanotubes is a critical parameter for understanding their optical and device parameters and has been the subject of several recent single-particle studies. All cross section measurements were performed on regions of MWCNTs that were distant from any residual catalyst particles to avoid their influence on the calculation. Calculation of the absorption cross section requires knowledge of the amount of light absorbed by the target, which can be determined from the magnitude of heat released. The heat dissipated can be calculated from the measured resonance wavelength shift of the microresonator. However, the relationship between the heat dissipated and the photothermal shift is a function of the position of the nanotube along the microresonator, which was determined from the SEM images. Then, finite element

polarization angle at the target nano-object. Additional experimental details are available in the Supporting Information. Photothermal images at variable resolution are taken by scanning the pump beam across the top surface of the resonator, Figure 1c. Wide-area maps (50 μm × 50 μm) at low resolution (2 μm/pixel) are used to locate individual absorbers, Figure 1d. High-resolution maps (3 μm × 3 μm at 250 nm/ pixel) are taken to define the shape of the nanotube, Figure 2. These maps highlight the utility of using a whispering gallery mode-coupled probe beam while performing excitation with a scanning free-space pump beam. At every pixel, the resonance wavelength is measured with the pump beam off and on, allowing for background subtraction, and five successive scans are averaged at each pixel. Scanning electron micrographs (SEMs) taken before and after photothermal experiments confirm that the nanotubes were not significantly altered by the measurement and allowed correlation between the observed photothermal map and the physical dimensions, Figure 3. MWCNTs (Sigma, >90% carbon basis) were dissolved in Nmethyl pyrrolidone (Sigma) with ultrasonication at a concentration of 60 μg/mL without further purification and deposited onto resonator wafer chips by spin-coating. Microresonators were plasma-activated immediately prior to spincoating (O2 plasma, 250 W, 5 min) to enhance adhesion. High Q factors were largely maintained (Q > 2 × 106) after nanotube deposition (Q ≈ 107 predeposition). Nanotubes typically stick to the inside of the silica rim during deposition (Figure 3b) and are able to substantially thermally influence the propagating mode after photoexcitation (Figure 4). On the other hand, the nanotubes do not strongly influence the mode via direct absorption, as confirmed by the small reduction in Q and shown in Figure 4c. Both coarse and high-resolution photothermal maps are acquired in 7 min, although the thermal equilibration time of the microresonator (∼250 μs)43 could allow for much faster acquisition. Multiple molecules or particles on a single resonator can be individually spatially 1919

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Figure 3. Photothermal characterization of four different nanotubes. Nanotubes are labeled I−IV from top to bottom. (a) Photothermal map of MWCNTs. The pump beam is scanned at 250 nm/pixel with a 440 μW incident power at 640 nm. (b) SEMs. (c) Polarization dependence of the photothermally induced resonance shift. (d) Polarization dependence plotted in polar coordinates for clarity.

simulations were performed using the COMSOL Multiphysics package to calculate the temperature elevation at every point in the mode volume as a result of a nanoscopic heat source, Figure 4a,b. The same approach was shown to correctly model the resonance shift of the microresonator in the absence of absorbing nano-objects as a result of the absorption from the silicon substrate.43 The temperature along the rim of the microresonator is measured in a series of 2D slices at regular azimuthal spacing (5°). Using the literature value for the thermo-optic coefficient,27 the shifted refractive index was calculated at each point. The shift in the refractive index was then weighted by the relative intensity (|E|2) of the propagating mode,50 Figure 4c. The total resonance shift of the toroid was calculated by averaging the resonance shift contribution from each slice. The dissipated thermal power was then determined by matching the observed resonance shift. The pump intensity can be easily determined from the optical pump power and the spot size. Finally, the known atom density and physical dimensions

of the tube can be used to extract the absorption cross section per carbon atom, as summarized in eq 1 σabs =

⎡ ⎛ w ⎞2 ⎤ Pthermal 1 × × ⎢π ⎜ 0 ⎟ ⎥ (1 − φlumin) ⎣ ⎝ 2 ⎠ ⎦ Poptical M × ×β d 2 π 2 LρNa

()

(1)

where σabs is the absorption cross section per carbon atom, Pthermal the heat dissipated, Poptical the pump beam power (440 μW), φlumin the quantum yield for luminescence (0 for MWCNT), w0 the 1/e2 beam diameter (780 nm), d and L the nanotube diameter and length, respectively, ρ the density (1.75 g/cm3),51 Na Avogadro’s number, M the molar mass (12.01 g/mol carbon), and β the fraction of atoms in the nanotube excited by the pump beam. β is calculated by taking an overlap integral between the measured spot size of the pump laser and the physical dimensions of the nanotube as measured 1920

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by SEM (see the Supporting Information). This correction is necessary because the spot size is the same order of magnitude as the diameter (100−170 nm) and length (1000−4000 nm) of the nanotubes. This procedure ultimately yields a value of 2.3 ± 0.5 × 10−18 cm2/C. MWCNTs of this size (>100 nm diameter) should have a per-atom optical absorption cross section approaching that of bulk graphite due to the MWCNTs’ relatively gentle curvature. Reported per-atom cross sections of graphite range over 2.5− 2.8 × 10−18 cm2/C,52−55 in excellent agreement with our results. The absorption cross section of MWCNTs has been roughly estimated56 by dividing the bulk absorption of a MWCNT sample by the carbon atom density at 3.4 × 10−18 cm2/C. To our knowledge, our work reports the first per-atom absorption cross section of MWCNTs made on individual nanotubes. Single-walled carbon nanotubes are less directly analogous but serve as a useful comparison. Literature reports for the per-carbon cross section of single-walled carbon nanotubes vary over (1−3) × 10−17 cm2/C.28,34,57,58 The polarization dependence of the photothermal signal was investigated, Figure 3c,d. Angular dependence clearly follows a sin2 θ dependence, with the maximum photothermal shift observed with the pump beam polarized parallel to the long axis of the nanotube, as expected.9,28,31 The contrast ratio between parallel and perpendicular polarization was approximately 2:1. The lack of complete extinction is consistent with previous measurements of single-walled nanotubes on silicon and on silicon dioxide,28,31,59 where a 2:1 contrast ratio was also observed, and largely resulted from the symmetry-breaking influence of the substrate,59 though the weaker curvature of a MWCNT can also play a role. Looking forward, the tremendous sensitivity of ultrahigh-Q optical microresonators can also enable single-molecule photothermal microscopy and spectroscopy. Extrapolation to the absorption cross section of a single chromophore (∼1 × 10−16 cm2)14 from a nanotube suggests a molecular shift of 2.1 fm, a value significantly greater than the smallest detectable resonance shifts in toroidal microresonators achieved thus far.19,25,49 The most significant technical barrier to realizing single-molecule resolution is pump absorption by silicon, a factor that allows convenient tuning of resonance spectral positions43 but ultimately contributes a prohibitively high background photothermal shift. Future efforts are focused on minimizing the effect of this background absorption via chemical modification and signal processing. To summarize, we have presented a new method for measuring optical absorption at the single-particle level by combining the sensitivity of ultrahigh-Q optical microresonators with the spectral specificity of photothermal absorption. We successfully measured the polarization-dependent per-atom absorption cross section of individual MWCNTs by mapping out the spatially resolved optical absorption and heat dissipation. The ultimate limit of sensitivity was calculated to be several orders of magnitude smaller, allowing for detection of absorption by nonluminescent single molecules under ambient conditions. This technique places few requirements on the target system’s properties and does not require multiple objectives or a transparent substrate. Photothermal approaches are also immune to contributions from scattered light and, when combined with direct absorption approaches, provide complementary information in regard to characterizing the flow of energy in nano-objects. Further, toroidal microresonators are compatible with water60 and could be used to

Figure 4. Simulations of photothermal absorption and propagating modes. (a) Three-dimensional temperature distribution resulting from an absorbing nano-object on the surface of a microresonator. The physical parameters are equal to those used to model nanotube I. log(T − 293.15) is plotted, where T is the temperature in Kelvin. Room temperature is defined as 293.15 K. (b) Two-dimensional slice of (a) at an azimuthal plane crossing the nanotube. (c) Propagating mode in the toroidal microresonator (log[E × E*]). The physical dimensions of the resonator are marked with dashed lines. The log scales are shifted by a constant for clarity. 1921

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Table 1. Parameters for Photothermal Measurement of MWCNTsa resonance shift (pm)

diameter (μm)

length (μm)

dissipated thermal power (μW)

carbon fraction absorbing, β (%)

per-atom absorption cross section (1 × 10−18 cm2/ atom)

polarization extinction ratio

13.5 14.1 24.0 16.6

0.104 0.111 0.157 0.154

1.41 1.71 4.04 2.99

88 92 163 111

33.0 27.2 11.5 15.4

2.7 2.5 2.3 1.6

2.0:1 2.3:1 1.8:1 1.6:1

I II III IV a

This table lists the relevant parameters for the four MWCNTs presented in Figure 3. The measured resonance shift, physical dimensions, and polarization extinction ratio are directly measured via photothermal absorption or SEM. The dissipated thermal power is calculated using finiteelement simulations. The carbon fraction absorbing (β) and the per-atom absorption cross section are calculated by analyzing the combined results of measurements and simulation. (3) Empedocles, S. A.; Neuhauser, R.; Shimizu, K.; Bawendi, M. G. Photoluminescence from Single Semiconductor Nanostructures. Adv. Mater. 1999, 11, 1243−1256. (4) Slaughter, L.; Chang, W. S.; Link, S. Characterizing Plasmons in Nanoparticles and Their Assemblies with Single Particle Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 2015−2023. (5) Fernee, M. J.; Tamarat, P.; Lounis, B. Spectroscopy of Single Nanocrystals. Chem. Soc. Rev. 2014, 43, 1311−1337. (6) Staleva, H.; Hartland, G. V. Transient Absorption Studies of Single Silver Nanocubes. J. Phys. Chem. C 2008, 112, 7535−7539. (7) Link, S.; El-Sayed, M. A. Shape and Size Dependence of Radiative, Non-Radiative and Photothermal Properties of Gold Nanocrystals. Int. Rev. Phys. Chem. 2000, 19, 409−453. (8) Sfeir, M. Y.; Wang, F.; Huang, L. M.; Chuang, C. C.; Hone, J.; O’Brien, S. P.; Heinz, T. F.; Brus, L. E. Probing Electronic Transitions in Individual Carbon Nanotubes by Rayleigh Scattering. Science 2004, 306, 1540−1543. (9) Freitag, M.; Martin, Y.; Misewich, J. A.; Martel, R.; Avouris, P. H. Photoconductivity of Single Carbon Nanotubes. Nano Lett. 2003, 3, 1067−1071. (10) Gliboff, M.; Sulas, D.; Nordlund, D.; deQuilettes, D. W.; Nguyen, P. D.; Seidler, G. T.; Li, X.; Ginger, D. S. Direct Measurement of Acceptor Group Localization on Donor−Acceptor Polymers Using Resonant Auger Spectroscopy. J. Phys. Chem. C 2014, 118, 5570− 5578. (11) Barbara, P. F.; Gesquiere, A. J.; Park, S. J.; Lee, Y. J. SingleMolecule Spectroscopy of Conjugated Polymers. Acc. Chem. Res. 2005, 38, 602−610. (12) Kaake, L. G.; Barbara, P. F.; Zhu, X. Y. Intrinsic Charge Trapping in Organic and Polymeric Semiconductors: A Physical Chemistry Perspective. J. Phys. Chem. Lett. 2010, 1, 628−635. (13) Palacios, R. E.; Fan, F. R. F.; Bard, A. J.; Barbara, P. F. SingleMolecule Spectroelectrochemistry (SMS-EC). J. Am. Chem. Soc. 2006, 128, 9028−9029. (14) Moerner, W. E.; Fromm, D. P. Methods of Single-Molecule Fluorescence Spectroscopy and Microscopy. Rev. Sci. Instrum. 2003, 74, 3597−3619. (15) Vollmer, F.; Arnold, S. Whispering-Gallery-Mode Biosensing: Label-Free Detection down to Single Molecules. Nat. Methods 2008, 5, 591−596. (16) Washburn, A. L.; Luchansky, M. S.; Bowman, A. L.; Bailey, R. C. Quantitative, Label-Free Detection of Five Protein Biomarkers Using Multiplexed Arrays of Silicon Photonic Microring Resonators. Anal. Chem. 2010, 82, 69−72. (17) Soteropulos, C. E.; Zurick, K. M.; Bernards, M. T.; Hunt, H. K. Tailoring the Protein Adsorption Properties of Whispering Gallery Mode Optical Biosensors. Langmuir 2012, 28, 15743−15750. (18) Farca, G.; Shopova, S. I.; Rosenberger, A. T. Cavity-Enhanced Laser Absorption Spectroscopy Using Microresonator WhisperingGallery Modes. Opt. Express 2007, 15, 17443−17448. (19) He, L. N.; Ozdemir, K.; Zhu, J. G.; Kim, W.; Yang, L. Detecting Single Viruses and Nanoparticles Using Whispering Gallery Microlasers. Nat. Nanotechnol. 2011, 6, 428−432. (20) Shao, L.; Jiang, X.-F.; Yu, X.-C.; Li, B.-B.; Clements, W. R.; Vollmer, F.; Wang, W.; Xiao, Y.-F.; Gong, Q. Detection of Single

study chemical or biological reactions in real time. Looking forward, the simple fixed-wavelength pump laser used in this experiment could be replaced with a tunable source to perform broad-band visible absorption spectroscopy using toroidal microresonators. Extending spectroscopy of nonluminescent targets toward single molecules and smaller nano-objects would provide a powerful new tool for investigating structure and dynamics at the nanoscale while also offering needed additional chemical information for label-free sensing.

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ASSOCIATED CONTENT

S Supporting Information *

Further details of experimental methods, additional SEMs of nanotubes I−IV, discussion of the background and noise level, explanation of the pump beam and nanotube overlap integral, time dependence testing of the COMSOL simulations, measured pump power dependence of the resonance shift, and testing of the effects of various simulation parameters on the calculated resonance shift. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript and Supporting Information. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Q. Leonard, H. Gilles, and K. Kupcho for technical assistance with microfabrication. The authors would also like to thank E. Vasiukevicius, K. Schneider, and B. Goebel for assistance with instrument construction. This work was supported by the Defense Advanced Research Projects Agency (N66001-12-1-4215) and the University of WisconsinMadison.

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