Orientation Sensing with Color Using Plasmonic Gold Nanorods and

Letter pubs.acs.org/JPCL

Orientation Sensing with Color Using Plasmonic Gold Nanorods and Assemblies Sushmita Biswas, Dhriti Nepal, Kyoungweon Park, and Richard A. Vaia* Air Force Research Laboratory, 2941 Hobson Way, Wright Patterson Air Force Base, Ohio 45433, United States S Supporting Information *

ABSTRACT: Colorimetric analysis of broadband illumination scattered from isolated gold nanorods and reduced symmetry Dolmen structures provide a visible measure of the local nanoscale orientation of the nanostructures relative to the laboratory frame of reference. Polarized dark-field scattering microscopy correlated with scanning electron microscopy of low and high aspect ratio gold nanorods demonstrated accuracies of 2.3 degrees, which is a 5-fold improvement over photothermal and defocused imaging methods. By assigning the three color channels of the imaging detector (red, green, and blue) to the plasmon resonance wavelengths of the nanostructure, the quantitative display of orientation improved by 200%. The reduced symmetry of a gold nanorod Dolmen structure further improved the sensitivity of colorimetric orientation by a factor of 2 due to the comparative intensities of the resonances. Thus the simplicity, high accuracy, and sensitivity of visual colorimetric sensing of local nanoscale orientation holds promise for high throughput, inexpensive structure and dynamics studies in biology and material science. SECTION: Plasmonics, Optical Materials, and Hard Matter

R

imaging of a single AuNR by deciphering defocused dark-field images caused by the electric field distribution in the image space.19 A challenge with the defocused imaging method is the accurate determination of the electric field components without degrading the image quality. Sönnichsen et al. demonstrated local orientation sensing with AuNRs by analyzing the polarization of the scattered light.20 Orientation angles of AuNRs were deduced by fitting the angular intensity profile to a cosine squared function. Additional information contained in the spectral distribution of the scattered light was not utilized though. Since the longitudinal and transverse dipoles in a AuNR have different magnitudes and are orthogonal, the corresponding resonances are spectrally distinct and have different response to the relative orientation of the AuNR and the incident polarization. This results in a polarization dependent color change, which is detectable by the collection of intensity within a few spectral bands rather than the incorporation of an expensive complex spectrophotometer in the detection optics. Kim et al. recently reported such results from low aspect ratio (AR=1.5), large diameter (>30 nm) AuNRs in a dark-field microscope.21 Polarization dependent color change and imaging with AuNRs of higher AR, which are important for biomedical and defense applications, was challenged due to their near-infrared plasmon resonances and nonoptimal assignment of the color channels of the imaging detector.

ecently gold nanorods (AuNRs) have been recognized as next generation optical probes for nanoscale orientation and polarization sensing.1−5 The high accuracy of spatial orientation of these optical probes is of tremendous importance to biomedical science as they unveil vital biological mechanisms including conformation and dynamics of single polymer chains,6,7 rotational motion of nano-objects in live cells8 and molecular dynamics of early virus-host coupling mechanisms in infected cells.9 Relative to conventional fluorescent materials used for analysis of structure and dynamics of biomolecules, the AuNRs offer biocompatibility10 and robustness against photobleaching11 and photoblinking.12 The shape anisotropy of AuNR gives rise to longitudinal and transverse surface plasmon resonances. These resonances are sensitive to the relative orientation of a AuNR with respect to the polarization of incident light. Thus, the magnitude of resonant scattering from, or absorption of, the incident linearly polarized light reflects local orientation. A number of methods have been developed so far to determine the orientation of anisotropic nanoparticles, for example, absorption based photothermal methods,13,14 one photon luminescence,15 and defocused imaging with two photon luminescence.16 These methods rely on a laser as an excitation source, which often induces thermal deformation of nanoparticles, thereby reducing the accuracy of measuring orientation.13,14,17,18 The local heat generation also poses limitations for applications in biology and soft matter studies. Alternatively, dark-field scattering with a lower intensity broadband light source facilitates wide-field hyperspectral imaging of hundreds of nanoparticles at the same time. Yeung et al. demonstrated three-dimensional orientational This article not subject to U.S. Copyright. Published 2012 by the American Chemical Society

Received: July 19, 2012 Accepted: August 28, 2012 Published: August 28, 2012 2568

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These challenges can be overcome without restriction to the visible region using a tunable spectral representation of the data, three color channels, and a charge-coupled device (CCD) camera. Integration of spectral intensity with a single data channel and gray scale presentation limits visual sensitivity since a human eye (or a detector) generally detects fewer shades in gray scale (256) than it can for color.22 According to Grassmann’s law and the CIE RGB chromaticity diagram, the color perception is additive (i.e., created by mixing) in terms of the primary colors red (R), green (G), and blue (B).22−25 The true colors R, G, and B are the basic colors detected by the three kinds of cone cells responsible for human color vision. CCDs replicate the response of the human eye through RGB tricolor imaging using color filters for R, G, and B with sensitivities in accordance with the Bayer model.22−29 The standard reference wavelengths for RGB true color imaging are 640, 540, and 450 nm, respectively.26,27,29 By redefining the color sensing channels to correspond to the spectral regions of maximum intensity change but preserving the true color projection channels, false color images provide a facile route to convert the broad spectral data to a simpler three-data-point spatial array that preserves the location and orientation information with spectrally unique characteristics of the nanoparticle. Thus, orientation sensing using the scattered colors from a particle with known spectral profile provides a cost efficient alternative, where the detection optics can be greatly simplified by replacing the spectrophotometer with bandpass filters. In this paper, we demonstrate using software-implemented bandpass filters an inexpensive, highly efficient, and reliable method of nanoscale orientation sensing with a unique visual approach using the scattered colors from single AuNRs and unique Dolmen structures. Using false color representations of the color channels, this method offers great advantages, including high throughput sensing, higher sensitivity, and robustness against photobleaching and photoblinking. The salient features of colorimetric orientation sensing provide an outstanding platform for optical imaging and sensing in biomedical applications and material science. The ultraviolet−visible (UV−vis) spectra of an aqueous solution of AuNRs (details of synthesis in Supporting Information S.I. I−III) with aspect ratios (AR) of 2.5 and 3.5 are shown in Figure 1a.30,31 The corresponding transmission electron microscope (TEM) images are shown in Figure 1b,c. The diameter and average AR of the AuNRs are 20 ± 3 nm, 2.5 and 19 ± 3 nm, 3.5, respectively. The longitudinal and the transverse peaks of the plasmon resonances are at 666 and 520 nm for AR 2.5 and at 786 and 511 nm for AR 3.5, respectively. The strength of the extinction is determined by the corresponding polarizabilities of the resonances.32 These two bands have different sensitivities to the polarization of incident light with the longitudinal resonance being more sensitive in part due to its larger dipole moment.32−37 The amount of light coupling to the two resonances is determined by the relative orientation of the electric field polarization to the long axis of a AuNR. This coupling can be tuned by rotating the direction of polarization of the incident broadband light. The dark-field scattering image and spectra of AR 2.5 and 3.5 AuNRs on a glass substrate are shown in Figure 2 (experimental details in Supporting Information S.II). The true color, 24-bit RGB image with conventional channel assignments (i.e., software bandpass filter) of 640, 540, and 450 nm,26,27 respectively, are displayed in Figure 2a,d. The

Figure 1. (a) UV−vis absorption spectra of AuNRs of AR 2.5 (blue) and 3.5 (red); the vertical red, green, and blue lines represent the conventional R→640, G→540, and B→450 channel assignments. TEM images of (b) AuNR of AR 2.5 (c) AuNR of AR 3.5.

bandwidth of each of the three channels in the RGB image is ∼1.2 nm (S.I. II). The false color image, with R, G and B channels shifted (through hyperspectral imaging software ENVI-IDL, details in S.I. II) corresponding to the ensemble average resonances of the AR 2.5 AuNR (668 nm (longitudinal peak), 525 nm (transverse peak), and 450 nm, respectively) on glass substrate, is shown in Figure 2b. The peak shifts experienced (from water to glass and air) are due to the change in the local dielectric environment13,21 and the size distribution.13,21 The false color image increases the number of visible particles by 20% due to the appropriate assignment of the RGB sensing channels. In the absence of a third resonance, the B channel is used to detect off-resonance scattering in the short wavelength region, and functions as a polarization independent reference for the location of a AuNR. For comparison, the true and false color images and spectra from AR 3.5 AuNRs are displayed in Figure 2d,e,f, respectively. The true R color channel (640 nm) is substantially off the longitudinal resonance at 787 nm. As a result, most of the particles are not visible. However, when R is tuned to 787 nm and G is shifted slightly to 527 nm, 2.5 times more particles are visible. This optimal channel assignment for any AuNR and environment can be decided a priori since the resonance peaks of AuNR are known from their AR, and the dielectric constant of the surrounding medium and is well characterized in the literature both theoretically and experimentally.30−38 Ideally, determination of the polydispersity of a sample’s aspect ratio should be possible by careful analysis and modeling of particle density at different imaging channels. Thus, the proper assignment of the 2569

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characteristics of the resonances, the zero direction is chosen as the direction where the longitudinal peak is strongest in intensity. The spectra reveal the decrease in the intensity of the longitudinal peak and concomitant increase in the intensity of transverse peak with the rotation of the polarization direction. The transverse peaks, normally difficult to observe in unpolarized light due to the overwhelming intensities of the longitudinal peak, are revealed using polarized incident light. The longitudinal peak intensities have greater sensitivity to the change in polarization compared to those of the transverse peak, as has been observed before by various groups.13,20,21 This has been attributed to the depolarization from a high numerical aperture (N.A.) objective lens whereby out of sample-plane components couple to the transverse resonance13 and also the interband transitions of gold overlapping the transverse resonance,39 thereby reducing directional sensitivity of this resonance. The AuNR of AR 2.5 and 3.5 changes color from bright red, reddish brown, and orange, to green with the rotation of the relative orientation of the AuNR axis with the polarization direction in concert with the spectral changes. The change in color is prominent when the transverse component appears in the spectrum and has comparative intensities with the R component. As a result, there is an abrupt change of color to green when the transverse peak is stronger than the longitudinal peak. This happens within an angle of approximately ±3° in the transverse orientation when the polarization direction is perpendicular to the AuNR long axis. The low bandwidth of the color channels, (G channel in particular) ensures the accuracy of this method. Note that, although a lower bandwidth increases the accuracy of the largest population of AuNRs, it also compromises the use of AuNRs that populate the tails of the distribution in particle size or local environment. Bandwidth reflects a tradeoff between sensitivity and parallel particle imaging. Scanning electron microscopy (SEM)-correlated polarized dark-field scattering of AuNRs provide calibration to the AuNR orientation determined by the color change referenced to the maximum intensity in the R channel. SEM and corresponding dark-field images of three AuNRs of AR 2.5 denoted by A, B, and C, marked with circles, are depicted in Figure 4a. Markers have been made on indium tin oxide (ITO)-coated glass substrate with focused ion beam milling (FIB). The vertical axis

Figure 2. Hyperspectral image of AuNR AR 2.5 with (a) default assignment of wavelengths to RGB color channels with R→640 nm, G→540 nm, and B→450 nm, (b) optimal assignment with R→668 nm, G→525 nm, and B→450 nm, (c) dark-field scattering spectra, and AuNR of AR 3.5 with (d) default color channels, (e) optimal assignment with R→787 nm, G→527 nm, and B→450 nm and (f) dark-field scattering spectra.

resonance wavelengths to the RGB color channels can be optimized for visualization of the AuNRs. Figure 3 summarizes polarized dark-field scattering results and associated optimal false color images using the above RGB assignments of a single AR 2.5 and 3.5 AuNR at different polarization directions (0°−360°) of the incident linearly polarized broadband light. On the basis of known optical

Figure 3. Polarized single particle dark-field scattering spectra and image with optimal RGB choice of AuNR AR 2.5 (top row) and AR 3.5 (bottom row) at different polarization angles with respect to the long axis of AuNR (a) 0°, (b) 30° top (60° bottom), (c) 60° top (70° bottom), (d) 80° and (e) 90°. The colors in (d) and (e) have been intensified for better illustration. 2570

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the polarization direction is perpendicular to the long axis of the AuNRs. This limits the abruptness of the observed color change. A possible reason for these differences arises from the depolarization of the high N.A. dark-field condenser lens. The dark-field condenser in general does not preserve the linear polarization of incident polarized light due to its complex optical train. Depolarization as high as 10% in the focal plane has been reported in high N.A. dark-field condensers with annular illumination.40,41 There is also a significant amount of components out of the focal plane. As a result, elliptical polarization is generated from incident linear polarization. A detailed investigation of the impact of depolarization on AuNR and ways to counteract is ongoing. As an alternative to measuring the depolarization factor and rectifying the false color image, the angular resolution and accuracy can be further improved by invoking the angular dependence of intensities of the three independent color channels of the CCD. The angular change of R, G, and B intensities for an AR 2.5 AuNR is summarized in Figure 5. As expected, the R and G data display a cosine squared functional dependence on (θ − ϕ) and can be fitted by I = I0M cos2(θ − φ) + C

(1)

Figure 4. (a) SEM image of three NRs A, B, and C of AR 2.5 (marked with circles) with a nearby FIB marker and their corresponding darkfield scattering image. (b) A plot of ϕvis with ϕsem for AuNRs of AR 2.5 (red squares) and AR 3.5 (green triangles) with a linear fit.

of the marker has been chosen as a reference for determining the orientation of the AuNRs in the SEM image. The orientation angle of the long axis of a AuNR with the vertical axis of the marker is defined as ϕ (Figure 4a). The angle between the polarization of incident light with the vertical is defined as θ. The incident polarization direction is rotated in a clockwise direction until the AuNR turns green at a specific angle. This angle with an addition or subtraction of 90° gives the orientation of the AuNR. The orientation angles ϕvis determined for AuNRs A, B, C (Figure 4a) with this method are 41°, 76°, and 85°. The corresponding ϕsem are 39.2°, 78.2° and 84°. A summary of ϕsem versus ϕvis for 20 AuNRs is shown in Figure 4b. The root-mean-squared error (RMSE) for AuNR of AR 2.5 (squares) is 2.6° and for AuNR of AR 3.5 (triangles) is 2.3°. Note that prior studies using the photothermal13 and defocused imaging16 technique were less accurate with errors of ∼10° and ∼14°, respectively, due to the overlaying method of SEM and photothermal images as well as depolarization from a high N.A. objective (former), the difference between optical circuits for single and two photon excitation, and the process of fitting defocused images (later). In the course of rotating the direction of polarization, multiple AuNRs turn green at different orientations. In this way, the spatial orientation of multiple AuNRs can be determined simultaneously in a single set of polarization measurements, enabling high throughput and high accuracy sensing. A limitation of this method is that some AuNR do not show a stronger transverse peak compared to the longitudinal when

Figure 5. (a) Longitudinal (R) and transverse (G) peak intensities of a AuNR of AR 2.5 with long axis oriented vertically, plotted for different polarization angles, and intensities of B channel shown by blue line. (b) plot of ϕvis determined from fitting with ϕsem for AuNR of AR 2.5 (red squares) and AR 3.5 (green triangles) with a linear fit (blue dashed line). Color channels defined in Figure 2. 2571

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where I0 and C are constants related to the maximum and minimum intensities, and M is the modulation depth defined as the ratio between the difference of peak and trough intensities divided by their sum intensities.13 (θ − ϕ) equal to zero is the direction of maximum intensity of the longitudinal peak. Maximum value of I is 255 in a 24-bit RGB image when (θ − ϕ) is zero. At this angle, the image is bright red with a maximum value of R and close to the minimum value of G. The B channel, positioned at 450 nm, does not show any angular dependence, verifying that the AuNR remains in focus as the polarization is rotated (B value drops at locations off the AuNR). Now the AuNR is oriented at an arbitrary angle ϕ. The polarization axis is fixed to the vertical direction (followed by a second measurement with polarization at 45° to remove the degeneracy of ϕ within 0−180°) defined earlier as the axis of the marker in Figure 4a. R, G, and B values picked from the image of the AuNR are 165, 16, and 5, respectively. The derived ϕ from the fit is 42°, which is close to the SEM value of 39.2° for AuNR A, circled in Figure 4a. Similarly for AuNRs B and C, the angles derived from fit are 74° and 82°, while ϕsem are 78.2° and 84°, respectively. ϕsem and ϕfit have been plotted in Figure 5b for different AuNRs of AR 2.5 and 3.5. The RMSE determined from this method is 2.3° and 2.1° for AuNRs of AR 2.5 (squares) and 3.5 (triangles), respectively. From this plot one can determine the orientation angle for a set of R, G and B values, which determine the color of the AuNR at that arbitrary orientation. Quantitative orientation sensing directly using the scattered colors can be performed using color matching tools, e.g., color checkers,42 and softwares, e.g., SpectraShop. For comparison, we also determined the orientation with nonoptimal off-resonance wavelength assignment to the three color channels (R→640 nm, G→540 nm, B→450 nm). The average error of orientation measurement was 200%, emphasizing the importance of the optimal channel assignment for quantitative nanoscale orientation information. Finally it is instructive to discuss what determines the ultimate sensitivity of colorimetric orientation sensing of AuNR. The sensitivity of orientation sensing γ using color, in the present context, is defined as the change in the number of different colors shades43 that a sensor can distinguish per unit angle of polarization rotation (dNRGB/dθ, where N and θ vary continuously) and has the units of no. of shades/radian. If R, G, and B represents the intensities of the three channels (where intensity varies continuously from 0 to 255), the total number of discernible colors is given by the volume of the three axes color cube (2553 to the first approximation assuming the noise level to be zero, Figure S2 in S.I. III). However, due to finite noise and different modulation depths of the channels in the case of AuNR (whose two resonances are known have a phase difference of π/2), only a certain number of these colors are available. When B has a fixed value, the number of available colors will lie on a RG plane. On this plane, if R varies and G is constant or vice versa, it is equivalent to a monochromatic case. If R and G both vary, it is a chromatic case, which represents the present context of a AuNR. In this case of a AuNR, the relationship of the R and G intensities with each other are given by (details in S. I. III)

intensities of the two channels respectively. The total number of detected shades is given by the length of this line segment in the RG plane (Figure S2 in S.I. III). Hence the sensitivity γ is γNR = =

dNRGB dθ dR2 + dG2 dθ

= 255 sin 2θ (MR 2 + MG 2)

(2)

where MR and MG for the AuNR in Figure 5 are ∼0.67 and ∼0.21, respectively, and γNR is 179.0 shades/rad. Note that for simplicity and consistency of analysis, we approximate the discrete color shades as continuous. In the most simple linear approximation where the R and G depend linearly on θ (expressions in S.I. III), the sensitivity γNRL equals 114.0 shades/rad. Thus, the simple linear approximation of interchannel dependence yields a maximum achievable sensitivity approximately 36% less than the ideal cosine squared dependence. Overall, the sensitivity can be significantly higher if the structure had two resonances of similar strength. Also, further sensitivity increases could be gained using the third channel (B), if an alternative registry motif is employed. Two or three comparative resonances are therefore required for R, G, and B colors to maximize the sensitivity of the colorimetric orientation sensing. This can be achieved with an anisotropic nanoparticle, such as a nanoprism44,45 with three unequal dimensions, or a complex assemblage of nanoparticles that have comparatively strong resonances at different orientations, such as a nanostructure consisting of two parallel AuNRs capped at one end by a third perpendicularly orientated AuNR (well-known Dolmen structure).46,47 Such a tilted Dolmen structure is shown in Figure 6a (inset), and have been synthesized via a bottom up chemical approach based on a method described in ref 48. The tilted Dolmen structure (dimensions provided in S.I. III) shows two peaks with comparative intensities at 600 and 750 nm. The higher energy peak arises from the antibonding bright mode where the dipoles in the two AuNRs in the dimer are oriented in the same direction. This is verified by the finite difference time domain (FDTD) modeling results displayed in Figure 6b. The lower energy peak at 750 nm appears to be due to the bonding mode where the dipoles in the AuNRs dimer are oriented in opposite directions. The appearance of this dark mode may occur due to coupling of the near field of the cap rod to the dark mode of the tilted AuNR,46,47 or from a net nonzero dipole moment of the antiparallel resonance due to the reduced symmetry of the tilted dimer. Polarized dark-field scattering images and intensities at different polarization angles are displayed in Figure 6c. It is obvious that the color change is more prominent in all directions than a single AuNR (Figure 5). The modulation depths for R at 750 nm and G at 600 nm are 0.67 and 0.92, respectively. For a complicated nanostructure like the Dolmen, the electromagnetic analysis becomes complex, and an accurate analysis would require a detailed experimental analysis of the angular dependence of the resonances with theoretical validation (which will be presented in a forthcoming publication). Therefore in the first order linear approximation discussed above, the net color sensitivity γDolmelL is 183.3 shades/rad. The colorimetric sensitivity thus improves by a factor of 1.6 compared to a single AuNR. Thus the change

⎛ G − C2 ⎞ R = MR 255⎜1 − ⎟ + C1 MG255 ⎠ ⎝

where MR and MG are the modulation depths of R and G channels and, C1 and C0 are constants related to the minimum 2572

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orientation of multiple AuNRs in a single set of measurements without involving data analysis provides a simple, visual, rapid and convenient platform for high throughput imaging and sensing applications. The false color representation of AuNRs with the three color channels R, G, and B assigned to the resonance wavelengths of the nanostructure has been shown to improve the quantitative orientation sensing by 200% over true color settings. Finally the sensitivity further improves by a factor of 2 with a reduced symmetry AuNR Dolmen structure, which has two spectrally distinct resonances of comparative intensities. Further improvements can be achieved with a complex nanoparticle (e.g., nanoprism with three unequal dimensions) or a hierarchical nanostructure, which has three spectrally distinct resonances of competing intensities. Thus high-accuracy and high-sensitivity visual colorimetric orientation sensing, in lieu of the use of detector optics with a spectrophotometer and hyperspectral data reduction, will provide an efficient, simple, and low-cost tool for highthroughput structure and dynamics studies in biology and material science.

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ASSOCIATED CONTENT

* Supporting Information S

SI includes experimental details on materials, AuNR synthesis, general characterization, optical characterization. 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]; Phone number: 937-7859209; Fax number: 937- 656-6327. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ruth Pachter and Jinsong Duan (Air Force Research Laboratory) for their FDTD modeling input on the Dolmen structure, and the National Research Council Associateship for supporting this research. We also thank Materials and Manufacturing Directorate, Air Force Research Laboratory, and Air Force Office of Scientific Research for providing the resources in this research project.

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Figure 6. (a) Scattering spectrum of tilted Dolmen (dimensions in S.I. III) shown in the SEM image (inset) with dark-field image; experimental (red) and FDTD modeled (black) with polarized light with direction along the cap AuNR. (b) Charge distributions at 600 nm (left) and 750 nm (right) showing the antibonding and bonding modes with dipoles aligned in the same and opposite directions, respectively. (c) Scattered colors at different polarizations (top panel); R, G, B intensities plotted at polarizations where R→600 nm, G→700 nm, and B→450.

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