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Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 25 — Dec. 11, 2006
  • pp: 12419–12431
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Nanowire-based enhancement of localized surface plasmon resonance for highly sensitive detection: a theoretical study

Kyujung Kim, Soon Joon Yoon, and Donghyun Kim  »View Author Affiliations


Optics Express, Vol. 14, Issue 25, pp. 12419-12431 (2006)
http://dx.doi.org/10.1364/OE.14.012419


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Abstract

A nanowire-based localized surface plasmon resonance (LSPR) biosensor has been investigated to evaluate the impact of design parameters of nanowires on the excitation of localized surface plasmons (LSPs) and the sensitivity enhancement of a LSPR biosensor. The results based on rigorous coupled wave analysis and finite difference time domain method indicate that significant sensitivity increase is associated with LSP excitation mediated by nanowires and that resonant coupling of LSPs through a nanogroove achieves larger field enhancement and sensitivity improvement than LSP excitation in a single nanowire. A specific optimization provided a nanowire-based structure with sensitivity increase by more than 23 times as well as good linear detection properties.

© 2006 Optical Society of America

1. Introduction

Sensitivity enhancement of a surface plasmon resonance (SPR) biosensor is extremely important to monitor tiny biomolecular interactions on a quantitative basis using a SPR-based biosensor. A large number of schemes have been proposed to enhance the sensitivity: for example, by amplifying local electromagnetic fields with antibody-conjugated nanoparticles and nanoshells [1–8

1. S. Kubitschko, J. Spinke, T. Brückner, S. Pohl, and N. Oranth, “Sensitivity enhancement of optical immunosensors with nanoparticles,” Anal. Biochem. 253, 112–122 (1997). [CrossRef] [PubMed]

], using magneto-optical effects [9

9. B. Sepúlveda, A. Calle, L. Lechuga, and G. Armelles, “Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor,” Opt. Lett. 31, 1085–1087 (2006). [CrossRef] [PubMed]

], combining with ellipsometric and phase measurements [10

10. S. Y. Wu, H. P. Ho, W. C. Law, C. Lin, and S. K. Kong, “Highly sensitive differential phase-sensitive surface plasmon resonance biosensor based on the Mach-Zehnder configuration,” Opt. Lett. 29, 2378–2380 (2004). [CrossRef] [PubMed]

,11

11. H. M. Cho, W. Chegal, Y. J. Cho, Y. Kim, and H. Kim, “Enhancement of biomolecular detection sensitivity by surface plasmon resonance ellipsometry,” in Nanosensing: Materials and Devices II, M. S. Islam and A. K. Dutta, eds., Proc. SPIE 6008, 293–298 (2005).

], or by means of nanowire-mediated resonance coupling of electromagnetic fields between periodic nanowires [12

12. K. M. Byun, S. J. Kim, and D. Kim, “Design study of highly sensitive nanowire-enhanced surface plasmon resonance biosensors using rigorous coupled wave analysis,” Opt. Express 13, 3737–3742 (2005). [CrossRef] [PubMed]

,13

13. W-C. Liu, “High sensitivity of surface plasmon of weakly-distorted metallic surfaces,” Opt. Express 13, 9766–9773 (2005). [CrossRef] [PubMed]

]. Many of these approaches are based on localized surface plasmon resonance (LSPR), which relies on the excitation of localized surface plasmons (LSPs) in a rough surface, as a sensitivity enhancement mechanism for a SPR based biosensor. The creation and maintenance of LSPs employ nanostructures [14

14. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424, 824–830 (2003). [CrossRef] [PubMed]

,15

15. D. Crouse and P. Keshavareddy, “Role of optical and surface plasmon modes in enhanced transmission and applications,” Opt. Express 13, 7760–7771 (2005). [CrossRef] [PubMed]

], usually metallic nanoparticles, since nanoparticles conjugated with antibodies can amplify a SPR signal when they are bound with target proteins, thus increasing the sensitivity of the detection. For improved control of LSP excitation over nanoparticles, nanowires have recently been investigated [12

12. K. M. Byun, S. J. Kim, and D. Kim, “Design study of highly sensitive nanowire-enhanced surface plasmon resonance biosensors using rigorous coupled wave analysis,” Opt. Express 13, 3737–3742 (2005). [CrossRef] [PubMed]

,16

16. K. M. Byun, D. Kim, and S. J. Kim, “Investigation of the profile effect on the sensitivity enhancement of nanowire-mediated localized surface plasmon resonance biosensors,” Sens. Actuators B 117, 401–407 (2006). [CrossRef]

]. The results report theoretically that the excitation of LSPs is highly sensitive to the nanowire structure and the nature of biomolecular interactions that may occur on the nanowire surface.

One of the issues in implementing a nanowire-based LSPR sensor is that despite the fairly obvious sensitivity improvement, typically by an order of magnitude, obtained LSPR characteristics are far from desired. The purpose of this study, therefore, is first to explore the structural dependence of LSPR on nanowire design parameters on a quantitative basis. In particular, the effect of the fill factor and the nanowire depth on the sensitivity characteristics of a LSPR-based biosensor is emphasized. Secondly, the implications of the structural dependence are investigated in conjunction with the sensitivity of an SPR biosensor to the change incurred by the presence of biomolecular interactions on nanowires. Earlier studies focused on the sensitivity enhancement of LSPR biosensors in the case of relatively weak LSPR despite potentially significant sensitivity enhancement with stronger LSPR, largely because the sensitivity enhancement in the event of strong LSPR is often accompanied by a broad SPR dip in the reflectance curve. The broadening in dispersion relation is associated with LSPR excitation that involves a large number of damping modes and is also explained as owing to the decreased effective mean free path of conduction electrons [17

17. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley-VCH, Weinheim, Germany, 1983) Chapter 12.

]. The broadening, reported experimentally in many earlier studies [2

2. L. A. Lyon, D. J. Pena, and M. J. Natan, “Surface plasmon resonance of Au colloid-modified Au films: Particle size dependence,” J. Phys. Chem. B 103, 5826–5831 (1999). [CrossRef]

, 18–20

18. L. Genzel, T. P. Martin, and U. Kreibig, “Dielectric function and plasma resonances of small metal particles,” Z. Physik B21, 339–346 (1975).

], is generally undesirable for sensor operation in practical applications. In the current study, we see the changes of the LSPR biosensor behavior as LSPR is increasingly enhanced with changes in nanowire designs. This is an attempt to find an optimum structure that achieves maximal sensitivity enhancement at the expense of minimal broadening in the SPR characteristics. For clarification, LSPR is defined as being enhanced or strengthened when momentum matching between incident photons and excited LSP modes occurs at larger momentum. This definition of LSPR enhancement is approximately commensurate with using the quality factor to describe the nature of LSPR, which is typically defined as the ratio of the resonance position to the resonance width. Thirdly, the possibility of zero sensitivity that was suggested as a result of the phase retardation effect of a target binding layer is more thoroughly explored. The phase retardation shifts the resonance condition for target analytes. This consequently introduces a structure in which the presence of target analytes does not shift the resonance as angles or wavelengths in a SPR sensor are scanned. In other words, the resonance may not respond to the presence of a specific analyte. Moreover, it was computationally shown that a specific design may exhibit a negative shift depending on binding events, i.e., resonance shift to a smaller angle or shorter wavelength with target analytes. Such a negative shift has been confirmed experimentally in LSPR using nanoparticles [2

2. L. A. Lyon, D. J. Pena, and M. J. Natan, “Surface plasmon resonance of Au colloid-modified Au films: Particle size dependence,” J. Phys. Chem. B 103, 5826–5831 (1999). [CrossRef]

] and in waveguide-coupled SPR studies [21

21. Z. Salamon, G. Lindblom, L. Rilfors, K. Linde, and G. Tollin, “Interaction of phosphatidylserine synthase from E. coli with lipid bilayers: coupled plasmon-waveguide resonance spectroscopy studies,” Biophys. J. 78, 1400–1412 (2000). [CrossRef] [PubMed]

].

This paper is organized as follows. In Section 2, a numerical model for the rigorous coupled-wave analysis (RCWA) as well as finite difference time domain (FDTD) method is described. The impact of various design parameters on LSPR is presented in Section 3. Quantitative analysis of sensitivity enhancement to obtain an optimal nanowire structure is discussed in Section 4, which is followed by concluding remarks in Section 5.

2. Numerical model

A nanowire-based LSPR biosensing structure is studied with a model using well-established RCWA [22

22. M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of metallic surface-relief gratings,” J. Opt. Soc. Am. A 3, 1780–1787 (1986). [CrossRef]

], which has been used successfully to explain experimental results of nanostructures [23–25

23. Y. Kanamori, K. Hane, H. Sai, and H. Yugami, “100 nm period silicon antireflection structures fabricated using a porous alumina membrane mask,” Appl. Phys. 78, 142–143 (2001).

]. The applicability of such a classical approach to a metallic structure on a nanometer scale has been the topic of other studies [26

26. J. Lermé, “Introduction of quantum finite-size effects in the Mie’s theory for a multilayered metal sphere in the dipolar approximation: application to free and matrix-embedded noble metal clusters,” Eur. Phys. J. D 10, 265–277 (2000). [CrossRef]

, 27

27. E. Moreno, D. Erni, C. Hafner, and R. Vahldieck, “Multiple multipole method with automatic multipole setting applied to the simulation of surface plasmons in metallic nanostructures” J. Opt. Soc. Am. A 19, 101–111 (2002). [CrossRef]

]. To ensure the validity of RCWA, we have limited the nanowire dimension such that its minimum feature size exceeds 10λF, where λF denotes the Fermi wavelength and is approximately 0.5 nm for gold. Convergence in RCWA can be achieved by including a sufficient number of space harmonics for the calculation of metallic surface relief structures even with rapidly varying fields in space. For this study, our RCWA routine employed 20 space harmonic orders unless otherwise noted.

A nanowire-based SPR biosensor is represented as a one-dimensional array of rectangular gold nanowires on top of a gold thin film where bulk surface plasmon polaritons (SPPs) are excited, an attachment layer of chromium, and a BK7 glass substrate (see Fig. 1). The thickness of the gold and chromium films is fixed at 40 nm and 2 nm, respectively. Optical properties of BK7 glass, chromium, and gold were taken from Ref 28

28. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, Orlando, FL, U. S. A., 1985).

.

The spatial distribution of the electromagnetic field intensity has been visualized using FDTD. The FDTD calculation was performed with a software package EMXP™ [29

29. P. Liu, EM Explorer, http://www.emexplorer.net.

]. EMXP™ discretizes Maxwell’s equations for monochromatic plane-wave excitation using rectangular 3-D cells of finite volume and solves them based on a time marching algorithm with absorbing boundary conditions. FDTD results have been successfully validated with standard electromagnetic field calculation using Fresnel coefficients and also with RCWA for thin film and grating structures.

The antibody-analyte binding is modeled with a 1-nm thick self-assembled monolayer (SAM) that is supported by gold nanowires and thin film. In this study, 1,6-hexanedithiol (HDT) is initially assumed for the dielectric SAM by approximating it as a homogeneous layer with n(HDT)=1.52643 for the HDT SAM [30

30. E. Hutter, S. Cha, J-F. Liu, J. Park, J. Yi, J. H. Fendler, and D. Roy, “Role of substrate metal in gold nanoparticle enhanced surface plasmon resonance imaging,” J. Phys. Chem. B 105, 8–12 (2001). [CrossRef]

]. The extinction coefficient of the HDT SAM, k(HDT), is responsible mainly for the reduction of reflectance, rather than the resonance angle shift. k(HDT) was ignored in this study, since the HDT SAM is extremely thin compared with the wavelength and the layer is essentially a lossless dielectric at the wavelength. Dielectric materials other than HDT, refractive indices of which range from 1.33 to 1.6, are considered in the later part of this paper. A SAM that is experimentally obtained is usually thicker than 1 nm. By assuming a 1-nm thick SAM for our study, we are taking the worst-case sensitivity data in the calculation of the sensitivity enhancement. The light source for the model is assumed to be a TM-polarized monochromatic plane wave at a fixed wavelength λ=632.8 nm as the incidence angle (θ) is scanned with an angular resolution of 0.01°.

Fig. 1. Schematics of a nanowire-based of SPR biosensor (df: gold film thickness fixed at 40 nm, dNW: gold nanowire thickness, Λ: nanowire period, light wavelength λ=632.8 nm)

As a quantitative measure of the sensor performance, we have employed the sensitivity enhancement represented by a sensitivity enhancement factor (SEF) and also the minimum reflectance at resonance (MRR) of the SPR dip characteristics. SEF measures the resonance angle shift due to target analyte binding on a nanowire-based biosensor with respect to that of a conventional SPR structure using a thin gold film with equal thickness df=40 nm, i.e., SEF=ΔθSP(nanowires)/ΔθSP(no nanowires). SEF can be negative if the resonance angle shifts lower as a result of surface modification by biomolecular interactions or structural changes in a LSPR based biosensor. On the other hand, MRR is a measure of the signal contrast of SPR and positively correlates with the angular width (AW) of SPR characteristics. While AW can be directly calculated in reference to the angle of total internal reflection, it may not properly evaluate the integrity of SPR characteristics and can merely be redundant with resonance angles, since the angle of total internal reflection depends only on the dielectric interface and its refractive index. Instead of AW, MRR has been employed as an indirect measure of the SPR characteristics. This is based on the fact that damping tends to be more evenly distributed among excitation modes as LSPR modes are excited, which in turn induces significant broadening.

3. Results

Figure 2 shows the impact of nanowire period (Λ=50, 100, 150, and 200 nm) and depth (dNW=10, 15, 20, and 25 nm) on the LSPR characteristics represented by the LSP momentum KSP=2πsinθSP/λ, where θSP is the LSPR angle, at various volume factors (VFs) in the absence and presence of target binding. Here, a VF is defined as the ratio of the volume occupied by gold in a nanograting. For one-dimensional nanowires, a VF is equivalent to a fill factor. The dotted line in Fig. 2 represents the resonance angle (θSP=45.13°) of a SPR biosensor at df=40 nm without nanowires.

Second, the LSPR is associated with two different modes of enhancement at a small or a large value of VF. For convenience, we define the VF at maximum LSPR enhancement as VFL and VFH, respectively for the enhancement associated with single-nanowire LSP excitation (low VF) and that associated with the resonant coupling of LSPs through a nanogroove (high VF). Note that the LSP excitation associated with a nanogroove tends to be noticeably stronger than that of a single nanowire. The data in Fig. 2 suggest that the two LSP excitation modes be coupled with effective LSPR enhancement. In other words, LSPR enhancement is reduced if the coupling between the two modes is weakened for a large nanowire period. At a very small period and large depth (see the case of Λ=50 nm and dNW=25 nm), it is apparent that the two modes of enhancement begin to merge and create significant coupling of LSPs excited in neighboring nanowires.

Third, in the presence of a SAM that models target binding, VFL becomes smaller and VFH larger. In Fig. 2, not only does the presence of a SAM shift the resonance condition, it also causes the resonance to occur more easily with a SAM, i.e.,

VFL(SAM)>VFL(noSAM)andVFH(SAM)<VFH(noSAM).
(1)
Fig. 2. Plasmon momentum KSP (y-axis in unit of µm-1) for various combinations of nanowire depths (dNW) and periods (Λ). x-axis is VF. Red and black solid lines represent the characteristics of a nanowire-based SPR biosensor with and without a HDT SAM as target analytes. For comparison, the dotted line shows KSPSP=45.13°) of a SPR biosensor at df=40 nm without nanowires.

Note that the LSPR enhancement is accompanied by the broadening in dispersion relation due to the decreased mean free path which incurs damping into a large number of LSP modes. The angular width of the dip in SPR characteristics ΔθSP is given by

ΔθSP=Im{KSP}nswccosθSP=(Γi+Γr)nswccosθSP,
(2)

with ns as the refractive index of a BK7 prism substrate [35

35. R. Bruns and H. Raether, “Plasma resonance radiation from non-radiative plasmons,” Z. Physik 237, 98–106 (1970). [CrossRef]

]. w and c are the angular frequency and the free-space speed of light. KSP is the plasmon momentum. Γi and Γr denote internal and radiative damping, respectively. Equation (2) indicates that the imaginary part of KSP becomes larger, which increases the damping and the width of the SPR characteristic. If ε1(=ε1′+iε1″) is the complex permittivity of a gold film and ε2,eff (=ε2,eff′+iε2,eff″) is the effective permittivity of the nanowire layer in combination with surrounding dielectric analytes, in the case of |ε1′|>>ε1″,

Im(KSP)wc(ε1ε2,effε1+ε2,eff)32ε12(ε1)2.
(3)

ε2,eff″ is assumed to be negligible [16

16. K. M. Byun, D. Kim, and S. J. Kim, “Investigation of the profile effect on the sensitivity enhancement of nanowire-mediated localized surface plasmon resonance biosensors,” Sens. Actuators B 117, 401–407 (2006). [CrossRef]

, 34

34. D. Kim, “Effect of resonant localized plasmon coupling on the sensitivity enhancement of nanowire-based surface plasmon resonance biosensors,” J. Opt. Soc. Am. A 23, 2307–2314 (2006). [CrossRef]

], which is consistent with the phenomenologically determined effective permittivity of a metallic grating [36

36. S. Moon and D. Kim, “Fitting-based determination of an effective medium of a metallic periodic structure and application to photonic crystals,” J. Opt. Soc. Am. A 23, 199–207 (2006). [CrossRef]

]. To the zeroth-order, Eq. (2) becomes at momentum matching

ΔθSP=2nscosθSP(ε1ε2,effε1+ε2,eff)32ε12(ε1)2,
(4)

which confirms that the enhancement of LSPR as |ε1′|~ε2,eff′ is accompanied by the broadening with cosθSP in the denominator of Eq. (4).

Comparison of the resonance at Λ=100 nm and dNW=20 nm with the one at Λ=50 nm and dNW=10 nm implies that the impact of depth (dNW) may be more significant than that of nanowire period (Λ), i.e., deeper nanowires yield larger LSPR enhancement at an identical ratio of Λ/dNW. This is supported by comparing the resonance of Λ=100 nm and dNW=10 nm versus Λ=200 nm and dNW=20 nm. This can be a basis of implementing a highly sensitive LSPR biosensor, since it hints at a possibility of sensitivity enhancement using relatively coarse nanowires so that they can be fabricated based on conventional lithography and simply adjusting the depth for sensitivity control, which is far easier to manipulate than the nanowire period.

Table 1. VFL+ VFH for various combinations of dNW and Λ (a) without a HDT-SAM and (b) with a HDT-SAM. N.A. stands for not available, i.e. neither of VFL and VFH exists at Λ=200 nm and dNW=10 nm.

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Λ=50 nmΛ=100 nmΛ=150 nmΛ=200 nm
dNW=10 nm1.0091.0091.002N.A.
dNW=15 nm0.9921.0001.0091.000
dNW=20 nm0.9730.9940.9951.008
dNW=25 nm0.9470.9820.9880.994
(b)

Details of the data presented in Fig. 2 are described in Fig. 3. Figure 3(a) presents VFL and VFH with dNW at various periods (Λ) in the absence of binding events. It corroborates eased LSPR enhancement with target binding and reveals quadratic dependence of VFL and VFH on dNW. It is interesting to find that LSPR enhancement occurs approximately at a fixed dimension of nanowires in the sense that VFL·Λ and VFH·Λ are almost constant for different Λ, as illustrated in Fig. 3(b), which in fact contrasts VFL·Λ and (1 - VFH)·Λ. In addition, LSP excitation in a nanowire and in a nanogroove is highly symmetric such that VFL+VFH≈1, as listed in Table 1(a) and (b), respectively for the absence and presence of target binding. While the symmetry suggests fundamental equivalence of the LSP excitation in a grating and in a groove on a nanometer scale, it is apparent that the symmetry starts to break with enhanced LSPR at small Λ and large dNW. This, in turn, indicates preferentially facilitated LSP excitation through a nanogroove. On the other hand, the impact of target binding on the symmetry is marginal, except that the variance is larger. Note also the variation of the LSPR angle θSP at VFL and VFH in Fig. 3(c). This shows that LSPR enhancement at small Λ and large dNW induces momentum matching of LSPs to occur at higher KSP and stronger LSP excitation associated with a nanogroove, i.e., KSP(VFH)>KSP(VFL). Overall implication of Fig. 3 and Table 1 is that nanowire structures that produce maximal LSPR enhancement based on single-wire LSP excitation and those of the resonant LSP coupling through a nanogroove are symmetric. Also, the matched LSP momentum is higher in the case of resonant LSP coupling than the case of single nanowire excitation.

Fig. 3. Variations of (a) VFL and VFH, (b) VFL·Λ and (1-VFH)·Λ, and (c) KSP(VF=VFL) and KSP(VF=VFH) with respect to dNW for various nanowire periods (Λ). No binding analytes have been assumed in the calculation.

SEF and MRR for the nanowire structures under consideration have been calculated in Fig. 4. Obviously in Fig. 4(a), maximal sensitivity enhancement is achieved near VFL and VFH. A structure with strong LSPR enhancement tends to exhibit high SEF, although the correlation is not 100%. Highest peak SEF appears at Λ=50 nm and reaches 65.11. Also note that the change of LSPR characteristics in response to the target binding gives rise to SEF=0. In other words, a nanowire-based biosensor may not respond to the presence of specific biomolecules in the course of significant LSPR enhancement with VF. Across the zero-sensitivity condition, the sign of the shift reverses, i.e., θSP in the presence of target binding shifts to a smaller angle. As shown in Table 2, the VF at which SEF=0 changes negligibly for various target samples of a different refractive index. Namely, a nanowire that incurs SEF=0 is rather fixed regardless of binding events and can thus be easily avoided in the process of optimizing SEF.

Fig. 4. (a) SEF and (b) MRR in the y-axis for various nanowire periods with VF in the x-axis. Red and black solid lines represent the characteristics of a nanowire-based SPR biosensor with and without a HDT SAM as target analytes. For comparison, the dotted line in (b) shows MRR=2.68% of a SPR biosensor without nanowires at df=40 nm in the presence of a 1-nm thick HDT-SAM.

MRR shown in Fig. 4(b) measures the energy transfer between incident photons and LSPs through radiative and non-radiative damping processes. Large MRR is associated with the decrease of electron mean free path [17

17. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley-VCH, Weinheim, Germany, 1983) Chapter 12.

], which increases the imaginary part of metal permittivity [37

37. U. Kreibig, “Electronic properties of small silver particles: the optical constants and their temperature dependence,” J. Phys. F 4, 999–1014 (1974). [CrossRef]

]. This results in the increase of the radiation field back-scattered at the metal/air interface through enhanced radiative damping process, such that the incident wave is not compensated by destructive interference [38

38. H. Raether, Surface Plasmon on Smooth and Rough Surfaces and on Gratings (Springer-Verlag, Berlin, Germany, 1988) Chapter 2.

, 39

39. A. A. Maradudin, A. R. McGurn, and E. R. Mendez, “Surface plasmon polariton mechanism for enhanced backscattering of light from one-dimensional randomly rough metal surfaces,” J. Opt. Soc. Am. A 12, 2500–2506 (1995). [CrossRef]

]. The increase of MRR is the most notable near VF=VFL and VFH. The MRR patterns shown in Fig. 4(b) represent the degree of radiation damping and its interaction with an incident field by nanowires at various VFs. Total damping determines the width of SPR characteristics, which is inherently attributed to the excitation of and substantial energy redistribution into a large number of LSPR modes, as indicated by Eqs. (2)–(4). Note that LSPs excited in a sphere yields an infinite number of modes [38

38. H. Raether, Surface Plasmon on Smooth and Rough Surfaces and on Gratings (Springer-Verlag, Berlin, Germany, 1988) Chapter 2.

]. MRR is thus correlated with the width of SPR characteristics.

Table 2. VF(SEF=0) as a function of nanowire period Λ in the presence of a HDT-SAM. Nanowire depth dNW is fixed at 25 nm.

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For practical applications in biosensing, as long as MRR is maintained to be smaller than a reference threshold value, whether it is 0.1% or 0.01% does not make much difference. In Fig. 4(b), the reference is set to be the MRR of a conventional SPR biosensor of df=40 nm with a 1-nm thick HDT-SAM (≡MRRref), in which case MRRref=2.68% shown as a dotted line.

Fig. 5. Spatial distribution of the electric and magnetic field amplitude |Ex|, |Hy|, and |Ez| at resonance, calculated by FDTD (a) for a thin film without nanowires (df=40 nm), i.e. VF=0, and for a nanowire-based structure with dNW=25 nm and Λ=50 nm when (b) VF=VFL (=0.179), (c) VF=0.5, and (d) VF=VFH (=0.764). Maximum field amplitude for |Ex|, |Hy|, and |Ez| is respectively given by 35, 11, and 40.

4. Discussion

Optimization based on maximizing SEF/MRR gives a structure at VF=0.78 with dNW=20 nm and Λ=50 nm. Note that the determined VF is in line with the optimum VF (VF=0.8) in Ref. 34 based on a metric using the product of VF and resonance contrast, despite somewhat arbitrary choice of metrics in the optimization process. The consistency emphasizes more effective nanowire-based sensitivity enhancement in nanogroove structures.

Fig. 6. (a) θSP of a nanowire-based structure optimized for enhanced sensitivity (VF=0.78, dNW=20 nm, and Λ=50 nm) as the material index of a SAM (nSAM) is varied. The slope represents ∂θSP/∂nSAM. (b) SPR characteristics of the structure in comparison with a conventional SPR biosensor without nanowires for a HDT-SAM.

For a structure at VF=0.78, SEF=22.41 and MRR=3.38%. This clearly suggests that a nanowire-based LSPR biosensor achieve sensitivity enhancement by more than an order as the increase of MRR is controlled. The structural impact on the change of analyte refractive index (ΔnSAM) is shown in Fig. 6(a), which compares to that of a conventional SPR structure. Note the ratio of the slopes

θSPnSAM(nanowires)θSPnSAM(nonanowires)

is equivalent to a differential quantity of SEF. The result shows the enhancement of ∂θSP/∂nSAM by 23.64 times (=7.29/0.31), while it maintains reasonably good SPR characteristics as shown in Fig. 6(b). The structure also performs excellently in terms of linearity as indicated by the correlation coefficient R2=0.9996, which is better than that of a conventional SPR structure.

Practically, a SAM may not be well formed in the valleys between nanowires in the case of a small period and VF approaching 1. The result in this case is presented in Fig. 6(a). Although the sensitivity enhancement is reduced, ∂θSP/∂nSAM compared with that of a conventional SPR structure increases by 13.33 times (=4.11/0.31) with good linearity (R2=0.9924). In other words, even if valleys of nanowires do not participate in the detection process, sensitivity increment by 13.33 times can be obtained using the optimum structure.

5. Concluding remarks

Acknowledgments

References and links

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2.

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S. Y. Wu, H. P. Ho, W. C. Law, C. Lin, and S. K. Kong, “Highly sensitive differential phase-sensitive surface plasmon resonance biosensor based on the Mach-Zehnder configuration,” Opt. Lett. 29, 2378–2380 (2004). [CrossRef] [PubMed]

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H. M. Cho, W. Chegal, Y. J. Cho, Y. Kim, and H. Kim, “Enhancement of biomolecular detection sensitivity by surface plasmon resonance ellipsometry,” in Nanosensing: Materials and Devices II, M. S. Islam and A. K. Dutta, eds., Proc. SPIE 6008, 293–298 (2005).

12.

K. M. Byun, S. J. Kim, and D. Kim, “Design study of highly sensitive nanowire-enhanced surface plasmon resonance biosensors using rigorous coupled wave analysis,” Opt. Express 13, 3737–3742 (2005). [CrossRef] [PubMed]

13.

W-C. Liu, “High sensitivity of surface plasmon of weakly-distorted metallic surfaces,” Opt. Express 13, 9766–9773 (2005). [CrossRef] [PubMed]

14.

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424, 824–830 (2003). [CrossRef] [PubMed]

15.

D. Crouse and P. Keshavareddy, “Role of optical and surface plasmon modes in enhanced transmission and applications,” Opt. Express 13, 7760–7771 (2005). [CrossRef] [PubMed]

16.

K. M. Byun, D. Kim, and S. J. Kim, “Investigation of the profile effect on the sensitivity enhancement of nanowire-mediated localized surface plasmon resonance biosensors,” Sens. Actuators B 117, 401–407 (2006). [CrossRef]

17.

C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley-VCH, Weinheim, Germany, 1983) Chapter 12.

18.

L. Genzel, T. P. Martin, and U. Kreibig, “Dielectric function and plasma resonances of small metal particles,” Z. Physik B21, 339–346 (1975).

19.

L. A. Lyon, M. D. Musick, and M. J. Natan, “Colloidal Au-enhanced surface plasmon resonance immunosensing,” Anal. Chem. 70, 5177–5183 (1998). [CrossRef] [PubMed]

20.

S.-J. Chen, F. C. Chien, G. Y. Lin, and K. C. Lee, “Enhancement of the resolution of surface plasmon resonance biosensors by control of the size and distribution of nanoparticles,” Opt. Lett. 29, 1390–1392 (2004). [CrossRef] [PubMed]

21.

Z. Salamon, G. Lindblom, L. Rilfors, K. Linde, and G. Tollin, “Interaction of phosphatidylserine synthase from E. coli with lipid bilayers: coupled plasmon-waveguide resonance spectroscopy studies,” Biophys. J. 78, 1400–1412 (2000). [CrossRef] [PubMed]

22.

M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of metallic surface-relief gratings,” J. Opt. Soc. Am. A 3, 1780–1787 (1986). [CrossRef]

23.

Y. Kanamori, K. Hane, H. Sai, and H. Yugami, “100 nm period silicon antireflection structures fabricated using a porous alumina membrane mask,” Appl. Phys. 78, 142–143 (2001).

24.

T. R. Jensen, L. Kelley, A. Lazarides, and G. C. Schatz, “Electrodynamics of noble metal nanoparticles and nanoparticle clusters,” J. Cluster Sci. 10, 295–317 (1999). [CrossRef]

25.

J. Cesario, R. Quidant, G. Badenes, and S. Enoch, “Electromagnetic coupling between a metal nanoparticles grating and a metallic surface,” Opt. Lett. 30, 3404–3406 (2005). [CrossRef]

26.

J. Lermé, “Introduction of quantum finite-size effects in the Mie’s theory for a multilayered metal sphere in the dipolar approximation: application to free and matrix-embedded noble metal clusters,” Eur. Phys. J. D 10, 265–277 (2000). [CrossRef]

27.

E. Moreno, D. Erni, C. Hafner, and R. Vahldieck, “Multiple multipole method with automatic multipole setting applied to the simulation of surface plasmons in metallic nanostructures” J. Opt. Soc. Am. A 19, 101–111 (2002). [CrossRef]

28.

E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, Orlando, FL, U. S. A., 1985).

29.

P. Liu, EM Explorer, http://www.emexplorer.net.

30.

E. Hutter, S. Cha, J-F. Liu, J. Park, J. Yi, J. H. Fendler, and D. Roy, “Role of substrate metal in gold nanoparticle enhanced surface plasmon resonance imaging,” J. Phys. Chem. B 105, 8–12 (2001). [CrossRef]

31.

I. Pockrand and H. Raether, “Surface plasma oscillations in silver films with wavy surface profiles: a quantitative experimental study,” Opt. Commun. 18, 395–399 (1976). [CrossRef]

32.

H. Raether, “Dispersion relation of surface plasmons on gold- and silver gratings,” Opt. Commun. 42, 217–222 (1982). [CrossRef]

33.

K. M. Byun, S. J. Kim, and D. Kim, “Profile effect on the feasibility of extinction-based localized surface plasmon resonance biosensors with metallic nanowires,” Appl. Opt. 45, 3382–3389 (2006). [CrossRef] [PubMed]

34.

D. Kim, “Effect of resonant localized plasmon coupling on the sensitivity enhancement of nanowire-based surface plasmon resonance biosensors,” J. Opt. Soc. Am. A 23, 2307–2314 (2006). [CrossRef]

35.

R. Bruns and H. Raether, “Plasma resonance radiation from non-radiative plasmons,” Z. Physik 237, 98–106 (1970). [CrossRef]

36.

S. Moon and D. Kim, “Fitting-based determination of an effective medium of a metallic periodic structure and application to photonic crystals,” J. Opt. Soc. Am. A 23, 199–207 (2006). [CrossRef]

37.

U. Kreibig, “Electronic properties of small silver particles: the optical constants and their temperature dependence,” J. Phys. F 4, 999–1014 (1974). [CrossRef]

38.

H. Raether, Surface Plasmon on Smooth and Rough Surfaces and on Gratings (Springer-Verlag, Berlin, Germany, 1988) Chapter 2.

39.

A. A. Maradudin, A. R. McGurn, and E. R. Mendez, “Surface plasmon polariton mechanism for enhanced backscattering of light from one-dimensional randomly rough metal surfaces,” J. Opt. Soc. Am. A 12, 2500–2506 (1995). [CrossRef]

40.

T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature 391, 667–669 (1998). [CrossRef]

OCIS Codes
(050.2770) Diffraction and gratings : Gratings
(130.6010) Integrated optics : Sensors
(240.0310) Optics at surfaces : Thin films
(240.6680) Optics at surfaces : Surface plasmons

ToC Category:
Optics at Surfaces

History
Original Manuscript: August 14, 2006
Revised Manuscript: October 30, 2006
Manuscript Accepted: November 28, 2006
Published: December 11, 2006

Virtual Issues
Vol. 2, Iss. 1 Virtual Journal for Biomedical Optics

Citation
Kyujung Kim, Soon J. Yoon, and Donghyun Kim, "Nanowire-based enhancement of localized surface plasmon resonance for highly sensitive detection: a theoretical study," Opt. Express 14, 12419-12431 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-25-12419


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References

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  2. L. A. Lyon, D. J. Pena, and M. J. Natan, "Surface plasmon resonance of Au colloid-modified Au films: Particle size dependence," J. Phys. Chem. B 103, 5826-5831 (1999). [CrossRef]
  3. L. He, M. D. Musick, S. R. Nicewarner, F. G. Salinas, S. J. Benkovic, M. J. Natan, and C. D. Keating, "Colloidal Au-enhanced surface plasmon resonance for ultrasensitive detection of DNA hybridization," J. Am. Chem. Soc. 122, 9071-9077 (2000). [CrossRef]
  4. M. D. Malinsky, K. L. Kelly, G. C. Schatz, and R. P. Van Duyne, "Chain length dependence and sensing capabilities of the localized surface plasmon resonance of silver nanoparticles chemically modified with alkanethiol self-assembled monolayers," J. Am. Chem. Soc. 123, 1471-1482 (2001). [CrossRef]
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  7. E. Hutter and J. H. Fendler, "Exploitation of localized surface plasmon resonance," Adv. Mater. 16, 1685-1706 (2004). [CrossRef]
  8. K. Aslan, J. R. Lakowicz, and G. D. Geddes, "Plasmon light scattering in biology and medicine: new sensing approaches, vision and perspectives," Curr. Opin. Chem. Biol. 9, 538-544 (2005). [CrossRef] [PubMed]
  9. B. Sepúlveda, A. Calle, L. Lechuga, and G. Armelles, "Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor," Opt. Lett. 31, 1085-1087 (2006). [CrossRef] [PubMed]
  10. S. Y. Wu, H. P. Ho, W. C. Law, C. Lin, and S. K. Kong, "Highly sensitive differential phase-sensitive surface plasmon resonance biosensor based on the Mach-Zehnder configuration," Opt. Lett. 29, 2378-2380 (2004). [CrossRef] [PubMed]
  11. H. M. Cho, W. Chegal, Y. J. Cho, Y. Kim, and H. Kim, "Enhancement of biomolecular detection sensitivity by surface plasmon resonance ellipsometry," Proc. SPIE 6008, 293-298 (2005).
  12. K. M. Byun, S. J. Kim, and D. Kim, "Design study of highly sensitive nanowire-enhanced surface plasmon resonance biosensors using rigorous coupled wave analysis," Opt. Express 13, 3737-3742 (2005). [CrossRef] [PubMed]
  13. W-C. Liu, "High sensitivity of surface plasmon of weakly-distorted metallic surfaces," Opt. Express 13, 9766-9773 (2005). [CrossRef] [PubMed]
  14. W. L. Barnes, A. Dereux, and T. W. Ebbesen, "Surface plasmon subwavelength optics," Nature 424, 824-830 (2003). [CrossRef] [PubMed]
  15. D. Crouse and P. Keshavareddy, "Role of optical and surface plasmon modes in enhanced transmission and applications," Opt. Express 13, 7760-7771 (2005). [CrossRef] [PubMed]
  16. K. M. Byun, D. Kim, and S. J. Kim, "Investigation of the profile effect on the sensitivity enhancement of nanowire-mediated localized surface plasmon resonance biosensors," Sens. Actuators B 117, 401-407 (2006). [CrossRef]
  17. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley-VCH, Weinheim, Germany, 1983) Chap. 12.
  18. L. Genzel, T. P. Martin, and U. Kreibig, "Dielectric function and plasma resonances of small metal particles," Z. Physik B21, 339-346 (1975).
  19. L. A. Lyon, M. D. Musick, and M. J. Natan, "Colloidal Au-enhanced surface plasmon resonance immunosensing," Anal. Chem. 70, 5177-5183 (1998). [CrossRef] [PubMed]
  20. S.-J. Chen, F. C. Chien, G. Y. Lin, and K. C. Lee, "Enhancement of the resolution of surface plasmon resonance biosensors by control of the size and distribution of nanoparticles," Opt. Lett. 29, 1390-1392 (2004). [CrossRef] [PubMed]
  21. Z. Salamon, G. Lindblom, L. Rilfors, K. Linde, and G. Tollin, "Interaction of phosphatidylserine synthase from E. coli with lipid bilayers: coupled plasmon-waveguide resonance spectroscopy studies," Biophys. J. 78, 1400-1412 (2000). [CrossRef] [PubMed]
  22. M. G. Moharam and T. K. Gaylord, "Rigorous coupled-wave analysis of metallic surface-relief gratings," J. Opt. Soc. Am. A 3, 1780-1787 (1986). [CrossRef]
  23. Y. Kanamori, K. Hane, H. Sai, and H. Yugami, "100 nm period silicon antireflection structures fabricated using a porous alumina membrane mask," Appl. Phys. 78, 142-143 (2001).
  24. T. R. Jensen, L. Kelley, A. Lazarides, and G. C. Schatz, "Electrodynamics of noble metal nanoparticles and nanoparticle clusters," J. Cluster Sci. 10, 295-317 (1999). [CrossRef]
  25. J. Cesario, R. Quidant, G. Badenes, and S. Enoch, "Electromagnetic coupling between a metal nanoparticles grating and a metallic surface," Opt. Lett. 30, 3404-3406 (2005). [CrossRef]
  26. J. Lermé, "Introduction of quantum finite-size effects in the Mie’s theory for a multilayered metal sphere in the dipolar approximation: application to free and matrix-embedded noble metal clusters," Eur. Phys. J. D 10, 265-277 (2000). [CrossRef]
  27. E. Moreno, D. Erni, C. Hafner, and R. Vahldieck, "Multiple multipole method with automatic multipole setting applied to the simulation of surface plasmons in metallic nanostructures" J. Opt. Soc. Am. A 19, 101-111 (2002). [CrossRef]
  28. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, Orlando, FL, U. S. A., 1985).
  29. P. Liu, EM Explorer, http://www.emexplorer.net.
  30. E. Hutter, S. Cha, J-F. Liu, J. Park, J. Yi, J. H. Fendler, and D. Roy, "Role of substrate metal in gold nanoparticle enhanced surface plasmon resonance imaging," J. Phys. Chem. B 105, 8-12 (2001). [CrossRef]
  31. I. Pockrand and H. Raether, "Surface plasma oscillations in silver films with wavy surface profiles: a quantitative experimental study," Opt. Commun. 18, 395-399 (1976). [CrossRef]
  32. H. Raether, "Dispersion relation of surface plasmons on gold- and silver gratings," Opt. Commun. 42, 217-222 (1982). [CrossRef]
  33. K. M. Byun, S. J. Kim, and D. Kim, "Profile effect on the feasibility of extinction-based localized surface plasmon resonance biosensors with metallic nanowires," Appl. Opt. 45, 3382-3389 (2006). [CrossRef] [PubMed]
  34. D. Kim, "Effect of resonant localized plasmon coupling on the sensitivity enhancement of nanowire-based surface plasmon resonance biosensors," J. Opt. Soc. Am. A 23, 2307-2314 (2006). [CrossRef]
  35. R. Bruns and H. Raether, "Plasma resonance radiation from non-radiative plasmons," Z. Physik 237, 98-106 (1970). [CrossRef]
  36. S. Moon and D. Kim, "Fitting-based determination of an effective medium of a metallic periodic structure and application to photonic crystals," J. Opt. Soc. Am. A 23, 199-207 (2006). [CrossRef]
  37. U. Kreibig, "Electronic properties of small silver particles: the optical constants and their temperature dependence," J. Phys. F Met. Phys.  4, 999-1014 (1974). [CrossRef]
  38. H. Raether, Surface Plasmon on Smooth and Rough Surfaces and on Gratings (Springer-Verlag, Berlin, Germany, 1988) Chap. 2.
  39. A. A. Maradudin, A. R. McGurn, and E. R. Mendez, "Surface plasmon polariton mechanism for enhanced backscattering of light from one-dimensional randomly rough metal surfaces," J. Opt. Soc. Am. A 12, 2500-2506 (1995). [CrossRef]
  40. T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, "Extraordinary optical transmission through sub-wavelength hole arrays," Nature 391, 667-669 (1998). [CrossRef]

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