Proceedings of the 2011 IEEEIICME International Conference on Complex Medical Engineering May 22 - 25, Harbin, China
Nano-plasmonic Biosensors: A Review Daryoush Mortazavi, Abbas Z. Kouzani, Akif Kaynak
Wei Duan
School of Engineering Deakin University Geelong, Victoria 3217, Australia
School of Medicine Deakin University Geelong, Victoria 3217, Australia
{dmortaza, Kouzani, akaynak}@deakin.edu.au
[email protected]
Abstract
-
II.
In this paper, first the fundamental concept of
nano-optical biosensing is studied. Since Raman scattered signal
A.
Raman Scattering The scattering of light is generally the redirection of light that happens when an electromagnetic (EM) wave strikes a scattering material (solid, liquid, or gas). Interaction of the EM wave with the matter periodically perturbs the electron orbits within the constituent. The oscillation of the electrons results in a periodic separation of charge within the molecules, which is called an induced dipole moment. The oscillating induced dipole moment is the main source of EM radiation resulting in scattered light. The majority of the scattered light has the same frequency (vo) of the incident light, a process which is known as elastic or Rayleigh scattering. However, another process referred to as inelastic Raman scattering causes additional light to be scattered at different frequencies (vo Vvib) referred to as Stokes scattering, and (vo + Vvib) which is
is very weak to be recognized by current measuring equipments, the signal must be amplified. SPR and LSPR are utilized to enhance the incident field of the target molecules, to improve the sensitivity of the sensor. The paper focuses on the use of LSPR to enhance Raman signal in SERS technology. Different structures of nano-particles in LSPR to improve enhancement of the SERS signal are reviewed and compared.
Index Termsenhancement.
Biosensor,
I.
nano-plasmonic,
Raman
signal
INTRODUCTION
A biosensor is an analytical device containing a biological recognition element immobilized on a solid surface and a transduction element which converts analyte binding events into a measurable signal [1]. There are various transducers including optical, magnetic, electrochemical, radioactive, piezoelectric, micromechanical, and mass spectrometric [2]. Optical transducers are highly sensitive to biomolecular targets, insensitive to electromagnetic interference, and present real time response to biomolecular interactions. Main optical methods employed in biosensors include fluorescence spectroscopy, interferometry, and surface plasmon resonance. The later one which works based on evanescent electromagnetic fields such as surface plasmon resonance (SPR), or localized surface plasmon resonance (LSPR) can monitor a wide range of analyte surface binding interactions such as absorption of small molecules, proteins, antibody antigen, DNA and RNA hybridization. Both SPR and LSPR methods are label free sensing methods and do not require labeling of the target molecules with different types of reagents, such as fluorescent dyes. In addition, surface enhanced Raman scattering (SERS) has been used as a signal transduction mechanism in biological and chemical sensing. Examples are trace analysis of pesticides anthrax [3], prostate-specific antigen [4], glucose [5-6], and nuclear waste [7]. SERS has also been implemented for identification of bacteria [8], genetic diagnostics [9], and immunoassay labelling [10-12]. A miniaturized and inexpensive SERS device can be used in clinics, field, and urban settings [13]. Various biomolecular interactions have been exploited in SPR and LSPR biosensors including antigen-antibody, receptor-ligand, hormone-receptor, streptatividin-biotin, protein-protein, protein-DNA [14], even detection of conformational changes in an immobilized protein [15].
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CONCEPTS
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called anti-Stokes scattering, where Vvib is the frequency of the vibrational mode of the target molecule (see Fig. 1). Raman scattering is always extremely smaller than Rayleigh scattering [16]. In addition, Raman scattering cross sections are typically 14 orders of magnitude smaller than those of fluorescence [17]. For this low cross section, large numbers of biomolecules are required to create a measurable signal. This problem can be solved by magnification of the Raman signal using SPR and LSPR.
Incident EM wave (vo)
wave
(vo)
Fig. I Light scattering by an induced dipole moment due to an incident EM wave [17].
B.
Surface Plasmon Resonance ( SPR) The collective excitation of the electron gas of a conductor is called a plasmon. If the excitation is confined to the near surface region, it is called a surface plasmon. Surface plasmons can either be propagating e.g. on the surface of a grating, or localized e.g. on the surface of a spherical particle, which are called surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR), respectively. Accordingly, the study of electromagnetic response of metals to optical waves is called plasmonics, or nano-Plasmonics in
31
nano-scale. When a light beam propagating in a medium of higher refractive index meets an interface at a medium of lower refractive index at an angle of incidence above a critical angle (9), the light is totally reflected at the interface and propagates back into the high refractive index medium, a phenomena which is called total internal reflection (TIR) [18]. Although the fully reflected beam does not lose any net energy across the TIR interface, the light beam leaks an electrical field intensity called an evanescent field wave (E) into the low refractive index medium. The amplitude of this evanescent field wave decreases exponentially with distance from the interface, with penetration into the optically thinner medium ofn2• Thus, if the TIR-interface is coated with a layer of a suitable conducting material, such as a metal of a suitable thickness, the p-polarized component of the evanescent field wave may penetrate the metal layer and excite electromagnetic surface plasmon waves propagating within the conductor surface that is in contact with the low refractive index medium. For a non-magnetic metal like gold, this surface plasmon wave will also be p-polarized and, due to its electromagnetic and surface propagating nature, will create an enhanced evanescent wave compared to the intensity of the incident electromagnetic field. This is used to detect mass changes of the metal film and dielectric, thus, to measure binding coated on the surface. The propagation constant of the surface plasma wave propagating at the interface between a semi-infinite dielectric and metal is given by the following dispersion relation:
kSP = k 0 where A =
Em E d
w
Localized Surface Plasmon Resonance ( LSPR) The development of large scale biosensor array comprising highly miniaturized signal transducer elements is a great milestone in producing biosensors. In this array format, it is tried to minimize the number of analyte molecules per sensor element, to decrease the measurement time, increase accuracy, and decrease the volume of required sample, but SPR has problem in these issues as the transducer element size is limited to few Ilm2 depending on the excitation wavelength. Also, wavelength shift detection methods are very difficult in very large arrays due to the optical complexity of the instrumentation. The third problem is that real time sensing or kinetic measurement using SPR is highly mass transport limited, e.g. by the order of 103 to 104 for a bulk concentration of analyte of the order less than 10-6 to 10-7 M. To overcome these problems, noble metal nano particles are employed, which guides us towards nano plasmonics theory [23]. In LSPR, light interacts with particles much smaller than the incident wavelength. This leads to a plasmon that oscillates locally around the nano-particle with a frequency known as the LSPR [19]. Like plasmonic devices, the basic components of nano-plasmonic devices are noble metals such as gold, silver, and copper in nano-scale coated on a substrate. Moreover, in SPR, light is in contact with the surface of the metal film using a prism, while in LSPR plasmon is excited by direct illumination. Theorictical Calculations of LSPR and SERS We consider the simplest model of a nano-particle consisting of a single metal sphere, small compared to the wavelength of light, which is irradiated by a laser field. Raman scattering arises from molecules that are adsorbed on the surface of this sphere. Therefore, Maxwell's equations may be approximated by electrostatic Laplace's equations to determine the field both inside and outside the sphere. The resulting field outside the sphere, Eout, can then be written as [21, 24-25]:
E.
(1)
Em + E d
2m
D.
is the incident light wavelength,
w
is the
incident light wavelength, c is the light speed, Em the dielectric constant of the metal, Ed the dielectric constant of the 2 = denotes the free dielectric (e.g. prism), and ko =
; �
space wave-vector [19]. According to (1), the surface plasmon wave (SPW) may be supported if the metal used possesses a negative real and small positive imaginary dielectric constant, such as gold and silver [20].
Eout(x, y, z) = Eoz - aEo
[:3 !: (xx + -y,y + ZZ) ] -
(2)
where Eo is the amplitude of the incident electromagnetic wave, the first term is the applied field, and the second one is the induced dipole that results from polarization of the sphere electron density; also, x, -y" and z are the usual Cartesian coordinates; r is the radial distance; x, y, and Z are the Cartesian unit vectors; and a is the metal polarizability expressed as: a = ga3 D) where a is the radius of the sphere and g is defined as: g = (Em - Ed)/(Em + 2 Ed) (4) It can be seen that the maximum enhancement occurs when the denominator of g approaches zero, i.e. (Em � -2 Ed ). Also, examining (2) reveals that the field enhancement decays with r-3, implying the existence of a finite sensing volume around the nano-particle. Assume e is the angle between the applied field direction and the vector r that locates positions on the sphere surface. Note that if Igl is large, then E;ut = E51g12(1 + 3cos29). This indicates that
Surface Enhanced Raman Scattering ( SERS) Electromagnetic coupling between the adsorbate and surface at optical frequencies, which is the basis of plasmonic or nano-plasmonic biosensors, arises from the dipole moment in the adsorbed [21]. Accordingly, the magnitude of the Raman scattering signal can be greatly enhanced when the scatterer (e.g. biomolecular target) is placed on or near a roughened noble metal substrate. This enhanced scattering process which is called electromagnetic surface enhanced Raman scattering (SERS), is used for biological and chemical sensing [22]. When the Raman scatterer is subjected to these intensified electromagnetic fields, the magnitude of the induced dipole increases, which results in SERS [13]. Experiments show large improvement in the SERS cross section per molecule in the order of 10-16 cm2 which is comparable to Raman cross section of 10-30 cm 2• This shows a high magnification of about 14 orders in the scattered light. C.
32
the largest field intensities are obtained for angles e equal to zero or 180.°, i.e. along the polarization direction. In this case, the overall enhancement arising from incident and scattered fields is approximately: , 2 w 2 CR = \Eout( )\ \Eout(W )\ ex 161g121g'12 (5) \EO(W)\4 where the primed symbols refer to fields evaluated at the scattered frequency. For small Stokes shifts, Igl and Ig'l are maximize at approximately the same wavelength, this is commonly referred to as E4 enhancement or the fourth power of field enhancement at the nano-particle surface. The Drude model can be used for wavelength above 600 run, to fmd the metal permittivity where the Plasmon resonance occurs: w2 (6) Em = 1---Pw(w+iy) where wp is the plasmon frequency and y is the plasmon width. However, at wavelengths below 600 nm, the Drude model is replaced with a Lorentz oscillator model. The Drude parameters for gold were taken from ref. [26]. The exact analytical solution to the electrodynamics of spheroidal particles is very complex. If we consider a spheroid whose major axis is of length 2b and minor axis 2a, with a constant field Eo applied along the major axis, then an explicit expression for the Raman enhancement factor for molecules that are randomly distributed on the spheroid surface (i.e. averaged over the surface) has been given by Zeman and Schatz [27] as follows: g = Em-Ed (7)
the RI sensItIVIty on spectral peak posItIon in air for Au nanodisks at 750 nm wavelength, directly on glass is 175 RIU-l, when supported on 20 nm Si02 pillars is 250 RlU-l, and when supported on 80 nm Si02 pillars is 325 RIU-1 [29]. For isolated particles, with particle sizes and shapes that are commonly studied, the enhancement factors of CR = 108 is suggested. However, larger values of CR = 1010 -1011 can be obtained for dimers of silver nano-particles. These values, which are associated with the gap between the two nano particles, are still below the required estimates of single molecule SERS (SMSERS) enhancement factors (1014) or larger by a factor of 103 or more. In analyzing these enhancement-factor predictions, it is important to note that the key parameter that controls the size of the enhancement factor for a dimer of nanoparticles is the size of the gap between the particles. It is only for gaps on the order of 1 nm to 2 nm that one can obtain exceptionally large values such as CR = 1011.
--
Em+XEd
where parameter X is the shape factor. In fact the parameter X equals 2 for a sphere, but for prolate spheroids (i.e. those with b > a ), X is larger than 2, and for oblate spheroids (i.e. b < a) it is less than 2. When X is greater than 2, the plasmon resonance condition, Re(Em + X Ed)= 0, is satisfied for a wavelength that is to the red of that for a sphere. III.
ENHANCEMENT
TECHNIQUES
A.
Morphology Based Enhancement Because the shape and size of a metallic nano-particle dictate the spectral signature of its plasmon resonance, the ability to change these two parameters and study the effect on the LSPR is an important experimental challenge [19]. The most popular nano-particle shapes are spheroids, triangular prisms, rods, and cubes which are shown in Fig. 2. The enhancement is directly dependant on the aspect ratio of the nano-particle, e.g. the enhancement for a nano-rod with an aspect ratio of 4, is around 26 times as much as that of a sphere nano-particle [28]. The other alternative to double the refactive index (RI) sensitivity of LSPR, is to lift the metal nano-particles above the surface by a dielectric nano-pillar to decrease the spatial overlap between substrate and the enhanced fields generated at plasmon resonance. For instance, to double the refractive index sensitivity of LSPR, the metal nano-particles are lifted above the surface by a dielectric nano-pillar to decrease the spatial overlap between substrate and the enhanced fields generated at plasmon resonance. For example, it is shown that
(c) (d) Fig. 2 Bright field TEM images: (a) Gold nano-rods, (b) Gold colloids, (c) Silver triangular prisms, and (d) Silver nano-cubes [28].
An array of dimers (see Fig. 3) of spheres with optimized spacing for producing the highest possible field enhancement from near-field and long-range effects gives CR = 109, but other structures using an array of dimers of truncated tetrahedrons, leads to SMSERS enhancement of CR = 1013. Two-dimensional arrays with different spacing in each direction presents remarkably large electromagnetic enhancement factors of CR = 1013-14 [30].
