Letter pubs.acs.org/JPCL
Potential-Scanning Localized Plasmon Sensing with Single and Coupled Gold Nanorods Tokuhisa Kawawaki,†,‡,§ Heyou Zhang,‡ Hiroyasu Nishi,† Paul Mulvaney,*,‡ and Tetsu Tatsuma*,† †
Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, Japan School of Chemistry and Bio21 Institute, University of Melbourne, Parkville, Victoria 3010, Australia
‡
S Supporting Information *
ABSTRACT: Single plasmonic nanoparticles can potentially serve as optical sensors for detecting local refractive index changes. However, simultaneous monitoring of the scattering spectra from multiple nanoparticles is not practical. Here we perform potentialscanning localized surface plasmon resonance (LSPR) sensing. Gold nanorods are immobilized on an ITO electrode. Instead of collecting the full spectrum, as is done in conventional LSPR sensing, the electrode potential is scanned while the rod spectra are monitored at a single wavelength. We demonstrate that refractive index changes can be determined from single wavelength experiments and we further find that gold nanorod (NR) dimers exhibit higher refractive index sensitivities than single NRs in both potentialscanning and conventional wavelength-scanning based LSPR sensing.
P
measurements of single NPs, two-dimensional measurements are easily conducted with a widely used imaging device such as a CCD or CMOS image sensor, an inexpensive and small semiconductor laser, and an electronic circuit for potential scanning. Plasmonic NPs are immobilized on a transparent indium−tin oxide (ITO) electrode, and the electrode potential is scanned in the course of dark-field monitoring of the scattered light by the imaging device under illumination with monochromatic light. In this study, we measure plasmonic peak potentials of single Au nanorods (NRs) and coupled NRs as well as shifts of the peak potentials in response to refractive index changes. We demonstrate that higher refractive index sensitivities can be achieved by coupling the NRs. Prior to conducting “potential-scanning LSPR sensing”, we performed conventional wavelength-scanning LSPR sensing with single Au NRs in order to characterize the NRs. We immobilized Au NRs (length ∼ 100, width ∼ 40 nm) onto a transparent ITO electrode and their spectra were obtained by dark-field microspectroscopy (Figure 1a). The sample was immersed in 0.01 M KNO3 aqueous solution containing 0−20 wt % sucrose (refractive index n = 1.332−1.364). The electrode system was initially set up under “open-circuit” conditions and a potential was not applied to the ITO electrode. Figure 1b shows scattering spectra (λ-sct plots) of a single spot of scattering light, which is likely from a single Au NR (NR1), in solutions with different refractive indices. When the refractive index was 1.332, the scattering peak wavelength of the Au NR was 755 nm. The peak shifted almost linearly to longer
lasmonic nanoparticles (NPs) strongly absorb and scatter light due to localized surface plasmon resonances (LSPR). Their resonance wavelengths are red-shifted as the local refractive index increases. This response allows application of the plasmonic NPs as LSPR sensors for chemical analysis and bioanalysis.1−3 If a plasmonic NP is modified with receptor molecules such as an antibody, the local refractive index around the NP is increased and the resonance wavelength is red-shifted by binding of the receptor to the target analyte, such as an antigen. Two-dimensional mapping of refractive index changes and chemical composition is also possible, in principle, by using plasmonic NPs dispersed on a solid surface, because the scattered light from single NPs can be observed by dark-field microscopy.4,5 If the peak shifts from single NPs can be observed, then each NP can be employed as an independent LSPR sensor. This powerful tool is, however, not necessarily practical because it is not easy to acquire the single-NP spectra simultaneously, with sufficient signal quality and speed. An alternative to spectral mapping is potential scanning. It has been previously reported that the LSPR peak can be tuned by changing the carrier density of the NPs by electrochemical means.6−8 This enables the scattering spectra of single NPs to be tuned across 5−20 nm as a function of the applied potential.9 On the basis of these results,6−14 potential-scanning LSPR sensors have been developed.15 The electrode potential is scanned while the scattered light spectra are monitored at a single wavelength. This is vastly simpler than wavelength scanning, and the “plasmonic peak potential” thus determined is negatively shifted as the local refractive index increases. The refractive index changes can therefore be determined from the peak potential shift observed at a single wavelength. If this potential-scanning technique is applied to the concurrent © XXXX American Chemical Society
Received: June 24, 2017 Accepted: July 21, 2017 Published: July 21, 2017 3637
DOI: 10.1021/acs.jpclett.7b01620 J. Phys. Chem. Lett. 2017, 8, 3637−3641
Letter
The Journal of Physical Chemistry Letters
Figure 2. (a) Experimentally obtained and smoothed conventional scattering spectra for NR1 on ITO at different potentials in 0.01 M aqueous KNO3. The spectra are normalized to the LSPR peak. (b) Relationship between the applied potential E and the scattering peak shift (data obtained from (a)). Theoretical relationship between the charge decrease (−ΔN) and the resonance peak shift (Δλ) based on eq 1 is also plotted.
Figure 1. (a) Dark-field microscope with an electrochemical cell. Inset: A typical electron micrograph of Au NRs. (b) Experimentally obtained and smoothed conventional scattering spectra as a function of the solvent refractive index for a typical Au NR (NR1) on ITO in sucrose solutions (0−20 wt %) containing 0.01 M KNO3. (c) Relationship between the refractive index n of the solution and the scattering peak wavelength shift (data obtained from (b)). Simulated data are also plotted using FDTD.
wavelengths as the refractive index of the solution increased. The refractive index sensitivity in this conventional wavelengthscanning sensing mode (Sλ‑n) for NR1 was evaluated to be 324 nm RIU−1 from the slope of the plot in Figure 1c (red). We also measured two more scattering light spots emanating from single Au NRs (NR2 and NR3) and the average Sλ‑n value was found to be 325 ± 5 nm RIU−1. Scattering spectra were also obtained by simulation based on a finite-difference time-domain (FDTD) method for a NR (length = 100, width = 40 nm) on ITO. The calculated peak wavelength is plotted as a function of the refractive index of the surrounding medium in Figure 1c (blue). The Sλ‑n value obtained from the plot is 301 nm RIU−1, which is in good agreement with the experimental value. This sensitivity value is similar to those reported in previous reports. A typical sensitivity value for Au NR ensembles (aspect ratio = 2.2) on a glass substrate is 262 nm RIU−1.16
Figure 3. (a) Scattering-potential spectra at 755 nm for NR1 on ITO in the sucrose solutions (0−20 wt %) containing 0.01 M KNO3. (b) Relationship between the refractive index n and the scattering peak potential (data obtained from (a)).
The single particle measurements were performed for NR1 on ITO at different potentials (−1.24 to +1.17 V versus NHE) in 0.01 M KNO3 aqueous solution (n = 1.332). Figure 2a shows the results thus obtained. As the applied potential is shifted positively, the scattering peak shifted to longer wavelengths. The relationship between the scattering peak wavelength and the applied potential is shown in Figure 2b (red). The plot is almost linear, and the slope of this plot, namely, the potential 3638
DOI: 10.1021/acs.jpclett.7b01620 J. Phys. Chem. Lett. 2017, 8, 3637−3641
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The Journal of Physical Chemistry Letters
Figure 4. (a) Models of single and coupled Au NRs (spacing = 10 nm) used in the FDTD simulations and the calculated electric field distributions. (b) Simulated scattering spectra of the single and coupled Au NRs on ITO in a medium of n = 1.332 (interparticle spacing = 10 nm). (c) The relationships between the conventional refractive index sensitivity Sλ‑n and the peak wavelength at n = 1.332 (λstd). Data reported for NP ensembles15 and simulated results are also plotted. (d) Experimentally obtained scattering spectra collected from both single and coupled NRs (NR1 and NR21).
sensitivity of the peak wavelength (Sλ‑E) is 11.5 nm V−1. The average Sλ‑E value for the three NRs (NR1−3) is 12.4 ± 0.7 nm mV−1. This value is reasonable because the potential values for aspect ratios of ∼2 and ∼3 are ∼8 nm V−1 and ∼12 nm V−1, respectively.9,15 The resonant wavelength shift (Δλ) caused by a change in the electron density (ΔN) of a plasmonic NP follows:9 Δλ = −
λp =
⎛1 − L ⎞ ΔN ⎟ε λp ε∞ + ⎜ ⎝ L ⎠ m 2N
2πc = ωp
previously reported value of ∼6.1 × 104 electrons V−1 for Au NRs (length ∼66.7, width ∼31.2 nm).9 The data presented in Figure 2a show that the appropriate monitoring wavelength for potential-scanning based sensing is in the range 750−775 nm. Thus, the scattering intensity at 755 nm is plotted against the applied potential in Figure 3a (red plots, E-sct plot) for NR1 at refractive index of 1.332. Fitting of the plot with a Lorentzian function yields a scattering peak potential at −0.04 V versus NHE. Likewise, the E-sct plots at different refractive indices under 755 nm illumination are shown in Figure 3a. It is obvious that the peak potential shifts negatively as the refractive index of the solution increases. This trend is in line with the behavior of NP ensembles.15 The peak potential is plotted as a function of the refractive index in Figure 3b. The slope of the plot, 21 V RIU−1, is the refractive index sensitivity of the potential-scanning sensing (SE‑n) for NR1. The average SE‑n value for the three NRs (NR1−3), 22 ± 2 V RIU−1, is in line with the behavior of Au NR (aspect ratio ∼ 2.8).15 Thus far, measurements of local refractive index changes at a single wavelength by potential-scanning have only been possible using monodisperse NP ensembles. We have now demonstrated that it is possible with single NPs using dark-field electrochemical microscopy. The present method can be applied to two-dimensional mapping of refractive index changes. In addition, it allows assessment and screening of plasmonic NPs and NP oligomers in terms of the refractive index sensitivities Sλ‑n and SE‑n.
(1)
4π 2c 2mε0 Ne 2
(2)
In these equations, N is the electron density in the uncharged NP, λp is the bulk plasma wavelength (= 131 nm for Au17), ε∞ is the high frequency contribution to the metal dielectric function (ε∞ = 12.2 for Au17), L is the NP shape factor, εm is the dielectric constant of the surrounding medium, c is the speed of light, ωp is the bulk plasma frequency, m is the effective mass of electrons, and ε0 is the vacuum permittivity. The L value is 1/3 for nanospheres and ∼0.08 for NRs with aspect ratio of 2.5.17 Thus, when the electrode potential is shifted positively and the electrode density is decreased (ΔN < 0), the resonant wavelength is red-shifted (Δλ > 0), as shown in Figure 2b (blue). This is qualitatively in line with the experimental results in Figure 2b (red). Charging ∼9.0 × 104 electrons when applying −1.0 V is roughly in line with the 3639
DOI: 10.1021/acs.jpclett.7b01620 J. Phys. Chem. Lett. 2017, 8, 3637−3641
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Finally, we have examined the behavior of coupled nanorods as potential-scanning LSPR sensors. If two Au NRs are close enough to interact electromagnetically with each other, their LSPR peaks red-shift from the single-NR peak position.18 This is known as surface plasmon coupling. We have examined the optical properties of different types of coupled Au NRs, namely, II-, T-, I-, and L-type dimers on ITO (Figure 4a) in media of different refractive indices by the FDTD method. Figure 4b shows the simulated spectra of each NR dimer for 10 nm interparticle spacings at n = 1.332. Since the L-type dimer exhibits two discrete peaks, we show the sum of the spectra with polarization angles of 0, 45, 90, and 135°. The refractive index sensitivity Sλ‑n of each dimer is plotted against its LSPR peak wavelength at n = 1.332 (λstd) in Figure 4c. The interparticle spacing is 5−30 nm for the I-type dimers and 10 nm for the other dimers. The plot shows a linear relationship regardless of the monomer/dimer type and the interparticle spacing. This trend has been known for plasmonic NPs of different shapes but the same composition.2,19,20 We experimentally examined three scattering spots with longer peak wavelengths (>800 nm) than those of NR1−3. We attribute those spots to coupled NRs (NR21−3), type L most likely, because the predicted peak wavelength (810 nm) matches the experimental peak wavelengths of those spots (810−830 nm), while the predicted spectral shape is also similar to those of the scattering spots, a strong peak accompanied by a shoulder at shorter wavelengths (Figure 4d). The experimentally obtained plots of Sλ‑n versus λstd for all the six scattering spots are in good agreement with those obtained by FDTD simulations and the data reported for ensembles of Au NRs (aspect ratio ∼2.8) and Au nanospheres (13 and 40 nm diameter; Figure 4c). Importantly, the average SE‑n value for the three coupled NRs (NR21−3) is 28 ± 3 V RIU−1, which is higher than that of single NRs (22 ± 2 V RIU−1). The plot of experimentally obtained SE‑n versus λstd for the single and coupled NRs (Figure 4c) shows a roughly linear relationship. The average Sλ‑E value for the three coupled NRs (NR21−3) is 12.3 ± 0.6 nm mV−1, which is in agreement with that obtained for the single NRs (12.4 ± 0.7 nm mV−1). Since the following relationship holds between SE‑n and Sλ‑n (eq 3),15 SE − n = Sλ − n/Sλ − E
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01620. Experimental methods for synthesis of gold nanorods, single particle spectroelectrochemical measurements, and FDTD simulations (PDF).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Paul Mulvaney: 0000-0002-8007-3247 Tetsu Tatsuma: 0000-0001-8738-9837 Present Address §
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611−0011, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by Grants-in-Aid for Scientific Research (No. JP16K14017 for TT) from the Japan Society for the Promotion of Science (JSPS). PM and HZ acknowledge support from the ARC Centre of Excellence in Exciton Science (CE170100026). T.K. thanks a JSPS Research Fellowship for Young Scientists.
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(3)
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