UNIVERSITY OF
KENTUCKY
Differentiating Surface and Bulk Interactions using
Localized Surface Plasmon Resonances of Gold Nanorods
1
Nehru,
1
Donev,
2
Yu,
2
Wei,
1
Hastings
Neha
Eugenii U.
Linliang
Yinan
and J. Todd
1Electrical and Computer Engineering and 2Department of Chemistry, University of Kentucky, Lexington,
Kentucky 40506, USA
Introduction
Localized Surface Plasmon Resonance (LSPR) is the result of
the collective oscillation of conduction electrons in a metal
nanostructure when illuminated with light which leads to
strong absorption and scattering at plasmon frequency. Metal
nanostructures supporting LSPR have found various
applications as biochemical sensors. Compared to traditional
propagating SPR sensors, it provides greater field
enhancement, sensing in dramatically reduced volume
solutions and extensive resonance wavelength tunability.
Despite the advantages, LSPR sensors suffer from the same
problem as propagating SPR sensors – interference from non
specific interactions (bulk and surface). Various groups [1-3]
have investigated propagating SPR sensor that support two
modes and can differentiate surface binding interactions from
bulk refractive index changes. Here we show that we can
achieve the same goal using nanostructures supporting LSPR.
Optical Characterization
Samples containing gold
nanostructures are
examined through an
inverted microscope
using a 20x/0.5NA dark
field objective for both
illumination and
collection. Scattered light
is routed through a
polarizer to a grating
spectrograph and
thermoelectric cooled
CCD camera. Schematic
of the experimental setup
[4] is shown in (a).
Scattering spectra of an
array of gold nanorods of
pitch 1um when (i) light is
unpolarized, (ii) when light
is polarized along the short
axis of the rods, and (iii)
when light is polarized
along the long axis of the
rods. Inset shows the SEM
image of nanorod array of
length 141nm and width
67nm on ITO coated glass
substrate.
(i)
1um
(iii)
(ii)
(a)
Background
The key concept behind this approach is the exploitation of the
multiple modes of a metallic nanostructure to distinguish
between specific and non-specific interactions. To differentiate
between bulk and surface changes we require nanostructures
that exhibit multiple plasmon resonances. Nanorods are the
simplest such structure and support at least two LSPRs.
Simple calculations using the electrostatic approximation for
coated ellipsoids suggest that the longitudinal and transverse
resonances respond differently to surface and bulk effects.
Optical characterization (cont.)
Sensing Measurements
(b)
(a)
(a)
300 nm
(c)
1 um
(d)
(b)
Calculated scattering cross
section for silver ellipsoids
of size 140x60x30nm in
water. The change in
scattering cross section is
due to (1) adsorption of a
3nm thick surface layer and
(2) change in the solution
index by 0.05. Inset shows
the schematic of a nanorod.
(c)
1 um
1 um
Shift in (a) transverse resonance
and (b) longitudinal resonance for
an array of nanorods of dimension
110x54nm with a change in
medium refractive index from air to
water. (c) Schematic [4] of flow cell
used for sensing experiments.
SEM images of array of nanorods of
various sizes on ITO coated glass
substrate used for measurement of
scattering spectra on the left.
(a) 95x50nm, (b) 100x56nm,
(c) 25x57.5nm, and (d) 136x58nm
Normalized unpolarized scattering spectra of
nanorod arrays of different sizes. The spectrum
exhibits a red shift with increasing size of the
nanorods.
Biosensing Experiment
Sample preparation
Linear model for resonance wavelengths-
Gold nanorod arrays are fabricated on an ITO-coated
glass substrate using electron beam lithography followed
by gold sputtering and lift off. Various steps involved in
nanorod fabrication are shown in the schematic below.
2
∆𝜆𝐿 = 𝑆𝑆−𝐿 Δ𝑡 + 𝑆𝐵−𝐿 Δ𝑛𝐵
2
∆𝜆 𝑇 = 𝑆𝑆−𝑇 Δ𝑡 + 𝑆𝐵−𝑇 Δ𝑛𝐵
Directly calculate solution index change (ΔnB)and
surface coverage(Δt)
∆𝜆 𝑇
∆𝜆𝐿
−
𝑆
𝑆𝐵−𝐿
Δ𝑡 = 𝐵−𝑇
𝑆𝑆−𝑇
𝑆
− 𝑆−𝐿
𝑆𝐵−𝑇
𝑆𝐵−𝐿
∆𝜆 𝑇
∆𝜆𝐿
−
𝑆
𝑆𝑆−𝐿
Δ𝑛𝐵 = 𝑆−𝑇
𝑆𝐵−𝑇
𝑆
− 𝐵−𝐿
𝑆𝑆−𝑇
𝑆𝑆−𝐿
1
1
3
1
1
1
3
1
3
1
(a)
1um
1
Sensor Characterization
2
• Sensitivity
ratio of change in plasmon resonance to change in index
solution (for SB) or change in layer thickness (for SS)
Bulk Sensitivity,
SB(nm/RIU)
Surface Sensitivity,
Ss (arb. units)
Longitudinal peak
261.8
3.1762
Transverse peak
161.3
2.3482
• Figure of merit
inversely proportional to the difference in the ratio of the
sensitivities.
𝑆𝑆−𝐿
𝑆𝑆−𝑇
1.3526
200 nm
𝑆𝐵−𝐿
𝑆𝐵−𝑇
1.6231
𝑆𝑆−𝐿
𝑆𝐵−𝐿
−
𝑆𝑆−𝑇
𝑆𝐵−𝑇
0.2705
Figure of merit, χ= 0.2705 lower in comparison to the dual
mode thin film propagating SPR sensor, χ = 1.4 [1].
1
2
1
1
3
1
(b)
Sensor response of biotin functionalized gold nanorod
array to streptavidin binding. (a) Resonance
wavelength versus time for transverse and
longitudinal plasmon modes. (b) Bulk refractive index
and relative surface layer thickness calculated from
(a). The solutions were introduced through the flow
cell in the following order: (1) 50mM Tris buffer, (2)
with 50% Glycerol, and (3) with streptavidin. (right)
SEM image of Au nanorod array of size 133by72.6nm
used for sensing measurements.
SEM images of Au nanorods fabricated on ITO-coated substrate of
sizes 182x69nm, 10x55nm and 112x63nm (top to bottom).
200nm
Acknowledgements and References
Conclusion and Future Goals
This material is based upon work supported by the National Science Foundation under Grant No. ECCS-0747810.
This work shows that nanostructures supporting dual plasmon modes can differentiate between surface
binding of target analyte and background refractive index changes. The figure of merit for the nanorod
array sensor is significantly lower when compared to a thin film SPR sensor. Future goals include-
[1] J. T. Hastings, et al., "Optimal short-referenced sensing using long- and short- range surface plasmons," Optical Society of
America, vol. 15, 2007.
[2] R. Slavík and et al., "Advanced biosensing using simultaneous excitation of short and long range surface plasmons,"
Measurement Science and Technology, vol. 17, p. 932, 2006.
[3] J. Guo, et al., "Dual-mode surface-plasmon-resonance sensors using angular interrogation," Opt. Lett., vol. 33, pp. 512514, 2008.
[4] www.zeiss.com, “Microscopy from the very beginning,” http://www.zeiss.com/industry/general_very_beginning.pdf.
• Design nanoparticle sensors with modes that better separate surface and bulk effects.
• Design nanoparticle sensors that support more than two modes to differentiate specific and
non-specific binding.