Pertinent parameters for simulation of nanofluid

## 1. Introduction

Several researchers have focused on biosensor for monitoring of biological interaction. Moreover, the detection of bimolecular is an extremely significant problem.Hence, the development of optical biosensors based on optical properties of noble metal nanoparticles using Surface Plasmon Resonancewas considered.

Surface plasmon resonance (SPR) is a powerful technique to retrieve information on optical properties ofbiomaterial and nanomaterials. Biosensor based on SPR is a versatiletechnique for biological analysis applications. Essentially, SPR depends on the optical properties of metal layer [1] and enviromental changes so it is related to charge density oscillation at the interface between them [2]. Hence, biomolecular possess an extreme sensitivity to plasmon resonance and they remove the requirement for extrinsic biomolecular labeling [3]. One advantage of SPR is, the light beam never passes through the dielectric medium of interest and hence the effect of absorption of the light in the analyte can be ignored. Hence, the main potential of surface plasmon resonance is characterization of medium after the metal layer.

Sometimes polymer layer was coated on gold later for detection and improvement the sensor performence. So, the properties of nanostructures gave rised to interest in biosensing. Nano particles andNano tube have improved the sensitivity, selectivity, and multiplexing capacity in biosensors. Different types of nanostructure-based biosensors including carbon nanotube,

Biomolecularinteractionare determined and predicted via angular modulation sensor; hence, the out put of SPR biosensor sensor is angle shift (_{spr}), associated with the point of minimum reflected light intensity and is very sensitive to the changes in the dielectric constant of the medium. So, the sensitivity of modern SPR sensing systems based on the Kretschmann configuration is such that they are capable of detecting refractive indexes as high as approximately 5 × 10^{−7} refractive index units, which corresponds to a 1 pg/mm^{2} surface coverage of biomolecules [4, 5].

Unfortunately, when sensing biomolecules of low molecular weight or in low concentration, the variation in SPR angle is very small and this causes difficulties in detecting the acquired signal [6]. Hence, several researchers have focused on enhancement of sensitivity of biomolecular sensor. Hence,the properties of metal nanoparticles (gold and silver) have contemplate as a result of the physical characteristics of surface-enhanced Raman scattering (SERS).Zhu at al., (1998) utilized colloidal gold nanoparticle films to enhance thethe Raman scattering of the underlying molecules [7]. On the other hand the colloidal gold nanoparticles were used as a immobilized element on the thin gold film[8, 9]. Moreover, to improve the roughness of the film, the inclusion of colloidal Au nanoparticles was added to the gold film. SPR biosensors using colloidal Au-modified gold films has the large angle shift and increase in minimum reflectivity suggesting that SPR spectrum depend on size and volume fraction of the Au nanoparticles [10]. To improve the detection of SPR biosensors, He et al., (2000)[11] and; Lyon et al., (1998)[10] reported using biomolecule-tagged colloidal Au nanoparticles. In this case, the SPW damped strongly because the intraction of the biomolecules -colloidal Au were occouured with theimmobilized bio-monolayer sensing.However, using this method to generate a biomolecular interaction signal involves complex procedures for the preparation of nano-sized colloidal Au particles and chemical immobilizations to tag the biomolecules. This process,eliminates the major advantage of SPR technology, namely that biomolecular labeling is not required [6]. On the methods for improving the the sensitivity of SPR biosensors was reported by Gestwicki et. al. [12].Theyenhanced the sensitivity and response of biosensor based on absorption spectroscopy techniques [13, 14]. They shown the resonance angle can be improved by caused by the conformational changes of proteins and enzymes when they bind with analytes of low molecular weights and the response of sensor can be increased by adopting Au nanoshells or nanoclusters.

The Au nanocluster-embedded dielectric film is fabricated through a simple co-sputtering method utilizing a multi-target sputtering system. In the proposed method, the resolution of the SPR biosensor is enhanced by precisely regulating the size and volume fraction of the Au nanoclusters in order to control the surface plasmon effect [6].

Carbon nanotubes (CNTs)[15] have been used as both electrode and transducer components in biosensors becuse of their interesting electrochemical and electrical properties including thier flexibility for varying the electrical property (from metallic to semiconductor) and their, small diameters, high mechanical strength, and easy fabrication/integration. Wang [16] and Balasubramanian and Burghard [17], presented the design methodologies, performance characteristics, and potential applications of CNT biosensors, as well as the factors limiting their practical use. It is obvious that many challenges still do remain, especially regarding the sensitivity of the nanotube response to environmental effects, and also the chosen type of raw CNT material, and its functionalization.

The other nanostructured sensors is based on nano ceramic such as

Fundamentally, the base of the biosensor is the coupling of a ligand-receptor binding reaction [1] to a signal transducer. Moreover, angular, phase and polarization modulations are the various methods applicable for distinguishing the interaction of bimolecular with the receptor. Essentially, SPR is a quantum electromagnetic phenomenon that appears at the interface of the dielectric and the metal. Under certain conditions, the energy of the light beam is absorbed by collective excitation of the free electrons called surface Plasmon (SP), which lies between the interface of the dielectric and the metal. On the other hand, when the momentum of the photon matches that of the Plasmon, the resonance appears as an interface of two two media with dielectric constants of opposite signs, and the SP wave propagates along the interface. In accordance with the SP wave properties, the SPR is classified as propagating the SPR, the long-range SPR and the localized SPR.

In the case of propagating SPR (PSPR) [20], the amplitude of the SP wave is decaying exponentially normal to the interface, which the Plasmon propagates, for a distance of about one to ten microns. The interactions between the surface and molecules in the analyte lead to the SPR signal shift which can be observed in shift of the angle of resonance and the wavelength. A second type of Plasmon’s wave is Long-range SPR (LRSPR) that occurs in thin metal films with low attenuation; hence, the traveling distance of the Plasmon wave is about a millimeter in the visible range.

The localized SPR (LSPR) [20, 21] is exited on a nano-scale. On the other hand, the LSPR appears in the metal structure as a lateral size that is less than half the wavelength of the excitation photon. Hence, the light beam will interacts with the nanoparticles, so as to leads to a locally Plasmon oscillation occurring around the particle with a specific frequency similar to LSPR. However, the LSPR is more sensitive to changes in the local dielectric analyte [20]. Typically, researchers will measure the changes in the local dielectric environment or analyte via making shifts in the LSPR wavelength measurements. Moreover a variant of angle-resolved sensing that is possible for LSPR is also possible, and LSPR can provide real-time kinetic data for the binding processes. In addition, the PSPR signal is more sensitive to changes in the bulk refractive index than are the LSPR signals, so when the LSPR and PSPR signals are examined the interaction with the molecular adsorption layer and, the response of the two methods becomes comparable in the short-range[20]. Consequently, both sensitivity and flexibility are the capability of the LSPR. Moreover, the decay length is shorter than PSPR; hence, the LSPR are offered for the sensor.

## 2. Optical properties of metal

Assume that the material such as metal consist of charges which can be set in motion by an oscillating electric field of light that is polarized in one direction.In accordence with Newton’s second law of motion, displacement of the electron and damping factor which is a function ofmass of electron and relaxation time,the polarization density (_{c}) of the conduction electrons is given as

where

Wher _{b}is the background polarization, and the susceptibility (* ɛ*) is

By using the equations (1-3) the dielectric function can be written as [2, 21],

or

In the case of high frequency of

Then by using equations (4) and (6),

Two frequency regimes of interest in equation (7) are to be considered for studying the material response [2].

Low frequency regime

For the case of the frequency * α*) is

High frequency regime

Since the dielectric function is real in the high frequency regime, the imaginary part of Eq. (7) can be ignored.In this case the frequency

or,

Where, the plasma frequency (_{p}) is defined as

As can be seen from equation (10) the dielectric function vanishes at the plasma frequency.However, for * plasmon*which can be excited by X ray and light or by an inelastic electron scattering [22].

To obtain the conditions of excitation to result in resonance, let’s consider the electrical field component in the * x*direction,

where

By using the transverse wave equation (

The plot of this equation is shown in Figure 1a. In the figure the wave number of the surface plasmon wave is always larger than the photon wave number and hence the photon would not be able to excite surface plasmon. The dispersion curve of the photon and surface plasmon wave must cross each other as shown in Figure 1.b [2, 20] for surface plasmon resonance to occur when the momentum of incoming light is equal to momentum of the plasmons. This can be done by using prism coupling method as in the present work.

## 3. Optical properties of the metal nano particles

The nanoparticles of interest are typically large enough to accurately apply classical electromagnetic theory to describe their interaction with light [23, 24]. However, they are also small enough to allow observation strong variations in the optical properties depending on the particle size, shape, and local environment. Because of the complexity of the systems being studied, efficient computational methods are the capable of treating large size materials are essential. In the past few years, several numerical methods have been developed to determine the optical properties of small particles, including the discrete dipole approximation (DDA) [23], T-matrix, and spectral representation methods (SR) [25]. We employed the DDA, which is a computational procedure suitable for studying the scattering and absorption of electromagnetic radiation by particles with sizes in the order or less of the wavelength than the incident light. DDA has been applied to a broad range of problems, including interstellar and interplanetary dust grains, ice crystals in the atmosphere, human blood cells, surface features of semiconductors, metal nanoparticles and their aggregates, and more. The DDA was first introduced by Purcell and Pennypacker [26] and has been subjected to several improvements, in particular those made by Draineet. al [27]. Below, we briefly describe the main characteristics of DDA and its numerical implementation, namely, the DDSCAT cod

Discrete Dipole Scattering (DDSCAT) is a Fortran code for simulation and calculation of scattering and absorption of light by particles.

. For a full description of DDA and DDSCAT [23], the reader can consult refs [24-28].The main idea behind DDA is to approximate a scatter, in our case the nanoparticle, by large enough array of polarizable point dipoles. Once the location and polarizability of each dipole are specified, the calculation of the scattering and absorption efficiencies by the dipole array can be determined, depending on the accuracy of the mathematical algorithms and the capabilities of the computational hardware. Although the calculation of the radiated fields in DDA is, in theory and principle, also possible, that calculation is actually beyond the computational capabilities of any of the current systems.

Let us assume an array of * N*polarizable point dipoles located at

*, each one characterized by a polarizability*N

*,*t

*and*c

where* i*th element, and

*units. Once we solve the 3*cgs

*-coupled complex linear equations given by the relation*N

and determined each dipole moment

where

Certain arbitrariness in the construction of the array of dipole points represents a solid target of a given geometry. For example, the geometry of the grid where the dipoles have to be located is not uniquely determined, and a cubic grid is thus usually chosen. Also, it is not obvious how many dipoles are actually required to adequately approximate the target or which choice is the best for the dipole polarization.If one chooses the separation between dipoles

where

In 2003, Sosa et. al. [23] defined the extinction, absorption and scattering efficiencies or coefficients,

Where

In Figure 2 [23], we show* (<20 nm*) the light absorption process dominates the extinction spectrum, whereas for very large nanoparticles

*), the light-scattering process dominates. As we show in this paper, to elucidate the optical properties of medium-size nanoparticles, it is indispensable to undertake an in-depth study of the scattering and absorption efficiencies, and not only of the extinction one, because both phenomena, namely, scattering and absorption, are the same order of magnitude. Figure 2shows the optical efficiencies for a sphere with a radius of*(>100 nm

*. In the spectra, we can observe that at about*50 nm

*all the efficiencies have a local minimum that corresponds to the wavelength at which the dielectric functions of silver. Therefore, this feature of the spectra is inherent to the material properties and, as we observe below, that feature independent of the particle geometry. Below*320 nm

*, the absorption of light is mainly due to the intra-band electronic transitions of silver; therefore, this feature of the spectra should also be quite independent of the shape and size of the particles, as it is actually corroborated in all of the graphs shown below that corresponding to a silver particle. At about 350 nm, the spectrum of*320 nm

*to about*380 nm

*that corresponds to plasmon excitations due to higher multi-polar charge distributions [23]. If we look only at the*500 nm

*to*320 nm

*, with a maximum being about*750 nm

*, three times more intense than the maximum of*400 nm

## 4. Theory

### 4.1.Surface. plasmon resonance with prism coupling

We study SPR in the classical Kretschmann’s configuration shown in Figure 3, where one side of the glass prism is coated thin gold or silver film in contact with the probed sample. Surface plasmons, which are collective oscillations of free electrons, are propagating along the interface between the sample and the metal film, and are thus sensitive to the optical properties of both the sample and the metal film. These plasmons can only be excited with TM-polarized light, which has an electric field component normal to the surface that generates the required surface charge distribution for the excitation [34].

The incident light is totally internally reflected at the interface and its intensity decreases with incident angle which is also called as coupling angle.By using the momentum and energy conservation laws [1, 35] the condition of resonance is obtained as follows:

where

To excite the SPW the electrical and magnatisam fields are parallel and perpendicular in the incident plane which is defined by light propagation vector and the normal to the interface. Thus, in accordance with Figure 4 and the boundary conditions, the relationship between electrical and magnetism fields can be written as follows [2, 22]:

The relationship between

as

is

Consequently, the layers cause the phase change of light and the matrix layers obtain from the coefficient of equations (30) and (31) as follows:

where * M*is a matrix consist of

where

where _{th} layer as follows [2, 36, 37]

Substituting,

and the characteristic matrix [ 38] is

If one layer is deposited on the prism, the reflection and transmission coefficients are defined respectively as

and

and from equations (25, 26, 27, 28 and 36), it can be obtained that

By substituting equations (37 and 38) in equation (39) and after simplification, the reflection coefficient is as follows:

whereby the reflectivity is

If there are three media of prism,

According to equations (37, 40) and (41), the reflection coefficient is

Substituting

and the phase change [39] is

### 4.2. Theory of localized SPR

In a general and simple case, we can assume that the nanoparticle is as metallic sphere with the radius R. It is embedded in a liquid medium. So, the SP wave just can propagate less than the 100 nm or the size of particle; hence, becomes localized.

The electrical field outside the sphere is [20]:

where* z*direction. The free electron clouds are dislocate by the electromagnetic wave field and produces uncompensated charges near the particle surface which produces corresponding opposing forces (as in Figure 5 [20]).

The optical resonance related to these oscillations is called the localized SPR (LSPR) [20]. The origin of the term “surface” comes from the knowledge that the oscillations are caused by the polarization of the particle surface, and because the generated electric field is larger near the particle surface and decays farther away far from the surface, which makes that field similar to the evanescent field at a flat interface in the propagating SPR case. In the general case, the frequency of the collective oscillations does not coincide with the applied wave frequency and is determined by many factors, electron concentration, electron effective mass, the particle shape and size, interaction between the particles, and the influence of the environment. However, for an elementary description of the nanoparticles plasmon resonance it is sufficient to use the usual dipole approximation, and the Drude model. The electrostatic polarizability of particle

Moreover, static polarizability is [20]

Where

is the medium wave number. The fundamental assumption in the theoretical model is that of a spherical particle shape, which allows us to utilize extended the Mie theory. Furthermore, we can assume that the particles are distributed in a triangular two-dimensional array instead of in a disordered layer. This assumption allow us to perform efficient estimates of the inter particle coupling effects. The calculated quantity is the extinction cross-section, which is formally obtained from the integral [41]:

Here, (

In the static case the extinction cross-section will be [43] explained by Eq (50). One can see from the latter expression that the extinction spectrum has a strong resonance when:

The permittivity of bulky metal is explained by the Drude model; hence, we utilize it to obtain the resonance frequency:

where

The resonance frequency and resonance wavelength are given by [20]

where

are the resonance frequency, resonance wavelength and plasma wavelength respectively, demonstrated as [44, 45] that is near the resonance, and sometimes for comparison between the metallic bulk permittivity and the metallic nano particle permittivity, the absorption spectrum can be approximately reduced to a Lorenzian profile.

The wavelength modulation is the common method used for LSPR sensor. So, it is the shift in the maximum (or minimum) of the spectrum curve that is monitored as a function of changes in the local dielectric environment that is caused by analyte absorption. The mechanism of this method is described in Eq. (48), and it has been demonstrated for variation of refractive index or the length of a molecular which was adsorbed to receptor. In this equation, the sensitivity to the bulk refractive index (

Where* R*was introduced by assuming that the particle size is smaller than the mean free path of the conduction electrons. The damping frequency

When

One can see from the above equations that the metal particle’s shape dictates the spectral signature of its plasmon resonance; the ability to change this parameter and study the effect on LSPR response is a very important experimental challenge. The development of increasingly sophisticated lithographic and chemical methods now allows for the routine production of a wide variety of complex NPs and their assemblies [50].

### 4.3. Metal cluster

The intraction of light with the metal cluster is the atractive aspect in nanotechnology. The electrons between metal atoms in small clusters are localized, and the plasmon resonance is a size dependent phenomenon.

The absorption and scattering cross section

Where

A multidipolar contribution, quadrupole extinction and quadrupole scattering are conquered in the scattering cross section in this region. Consequantly, the position and shape of the plasmon resonance will depend on the size, the dielectric functions of the metal clusters, the dielectric function of the surrounding medium andthe shape of the particles, its aggregations and, interactions with the substrate etc..

### 4.4. The nanoparticle in liquid matrix

Today, any investigation of nano-fluid is considerable. Hence, in this instance, the complex refractive index of liquids containing isotropic nanoparticles and optical the response of medium are investigated. The diameter of the particles is much smaller than the wavelength of the incident light. The response of a medium to an external electromagnetic field cannot be completely described using macroscopic fields as local field effects play an important role. This pont was first demonstrated by Lorentz who showed an impressive accuracy when predicting the dielectric function of the medium [49]. The external field induces a collection of dipole moments [50] and the local field factor relates the macroscopic fields to the local ones. These local field factors are important in the derivation of various effective medium theories (EMTs) considered next here. Regardless of the composition of the nanocomposite, the effective dielectric function can always be estimated by Wiener [51]

where the summation is more than the number of constituents * N*, and their corresponding volume fractions

*the Wiener bounds give upper and lower limits for the effective complex dielectric function for a certain volume fraction irrespective of the geometry of the nanostructure. Even tighter limits, such as Hashin–Shtrikman and Bergman–Milton, can be utilized in the estimation of the effective dielectric function for two-phase nanocomposites with known volume fractions of the constituents [51–54]. For a more detailed description of the different models see,*i.e.

*, a review article by Hale [55].*e.g.

The Maxwell Garnet (MG) EMT can be used to describe the effective dielectric function of a solution containing a small number of spherical nanoparticles. This dilute limit must be assumed as the MG model itself does not include interaction between nanoparticles or agglomeration. In such a case the effective dielectric function of a medium is given by

where

Mathematically this low volume fraction limit of the MG formalism arises from the asymmetric equation for the effective dielectric function since the effective dielectric function would become dependent on the choice of the host material for a two-phase MG system with comparable volume fractions. The Bruggeman effective medium theory (BMT) [56] is based on the assumption that the inclusions are embedded in the effective medium itself, which is invariant if the constituents are replaced by each other. The Bruggeman formalism has been extended to inclusions with non-spherical shape by introducing a geometric (or depolarization) factor * g*[57], which depends on the shape of the inclusions. This generalized BMT is

For example, the classical Bruggeman formula is obtained with a value of g = 1/3 which corresponds to spherical inclusions whereas for two-dimensional circular inclusions the value of g = 1/2. Thus by extracting the effective dielectric function of a two-phase nanocomposite with known volume fractions and dielectric functions of the constituents, we can solve 10 for * g*and then obtain the geometrical shape of the investigated particles.

The quadratic Eq. (61) has two solutions for the complex effective dielectric function but only the positive branch of the square root is physically reasonable and is given by

Where

The frequency-dependent effective refractive index

We show in Figure 4.4 the effective dielectric functions for MG and BR type curves with geometric factors g = 1/3 and g = 1/2, which are inside the Wiener bounds as expected.

Further, at low volume fractions these three curves are almost overlap, whereas there is a rather larger difference for larger volume fractions. Hence it is important to know how to model the medium with nanoparticles to gain an accurate retrieval of the complex refractive index of an individual nanoparticle. Before closing this section we want to mention that the MG model can be generalized to hold for ellipsoids [58] by using a geometric factor such as that for BR [59]. The calculation of this geometric factor * g*for ellipsoids can be found from [60].

In the Rayleigh limit

dependence in order to define an effectiverefractive index of the medium surrounding the metal core according to:

## 5.SPR sensor parameters

The main performance parameters of SPR sensors for nano application are sensitivity, linearity, resolution, accuracy, reproducibility and limitation of detection.

Sensor sensitivity is the ratio of the change in the sensor output to the change in the refractive index [1].

Sensor linearity is related to linear relationship of output sensor to quantity which should be measured.

The resolution of a sensor is the smallest change in the measured quantity which produces a detectable change in the sensor output. The resolution of SPR sensor usually refers to a bulk refractive index resolution. In SPR biosensor, an equivalent of this term is the limit of detection. Sensor accuracy describes the closeness of agreement between the measured quantity and the true value of the quantity such as refractive index or concentration of the analyte. It is usually expressed in absolute terms or as a percentage of the error/output ratio.

Reproducibility is the ability of a sensor to provide the same output when measuring the same value of the quantity measurement such as concentration or refractive index under the same operating condition over a period of time. It is expressed as the percentage of full range. The limitation of detection is an important parameter for evaluation of sensor. It is related to minimum detection of concentration of analyte. The minimum variation of quantity which measured with the sensor is related to minimum concentration value which is expressed the limitation detection.

## 6. Simulation of SPR Curve for various refractive index

Figure 6 shows the simulation of SPR curve for variation of the refractive index of analyte. The refractive index of prism and gold layer are 1.83 and.235+3.31i respectively. The wavelength of incident light was 632.8 nm (He-Ne Laser). When the incident angle was changed from 30º to 90º the refractive index of analyte shifted from 1.1 up to 1.6. Figure 7 shows the resonance angle shifted from 39.76º to 80.24º. Moreover, in accordance with Figures 8 and 9, the changes in refractive index are 0.01 and 0.001 respectively. As a result, the resonance angle shifts for small variation of refractive index of analyte are 10.944 and 0.08 respectively [61].

Figure 10 shows the variation of reflectivity with refractive index of analyte. Consequently, the reflectivity increased with increasing refractive index of analyte.

If the nano-fluid is considered, the refractive index of analyte has an imaginary part which is changed from 0.001 to 0.2, and the resonance angle and reflectivity are shifted from 52.34ºto 54.628ºand.0252to 0.6587 respectively, that are depicted in figure 11.

The pertinent parameters were sorted in Table 1. Figure 12a and 12b show the variation of resonance angle and reflectivity versus the imaginary part.

Refractive index | Resonance angle | Reflectance |

1.331+0.001i | 52.34 | 0.0252 |

1.331+0.005i | 52.364 | 0.0663 |

1.331+0.01i | 52.405 | 0.1221 |

1.331+0.05i | 52.864 | 0.4140 |

1.331+0.1i | 53.523 | 0.5546 |

1.331+0.2i | 54.628 | 0.6587 |

## 7. Application of SPR nano-metal

One of the most attractive aspects of collective excitations of Plasmon is their use to concentrate light in sub wavelength structures and enhance transmission through periodic arrays of sub wavelength holes in optically thick metal films. SPs are tightly bound to metal–dielectric interfaces penetrating around 10 nm into the metal (the so-called skin depth) and typically more than 100 nm into the dielectric (depending on the wavelength). SPs at the optical wavelengths concentrate light in a region that is considerably smaller than their wavelength, a feature that suggests the possibility of using surface – plasmonpolaritons for the fabrication of nanoscale photonic circuits operating at optical frequencies. In addition the possibility of manufacturing such nano beds in planar form makes them remarkably important for biosensing towards biochip application.

Aslanet. al., reported on Nanogold-plasmon-resonance-based glucose sensing in 2004 [62]. Their sensor based on the aggregation and disassociation of 20-nm gold particles. The plasmon absorption was shifted due to interaction of nanoparticle and glucose and the response range is

Application of Au nanocluster-embedded film was reported by Hu, et. al. in 2004 [6]. The used the mentioned dielectric film (SiO_{2}) in biosensor and prepared a novel ultrahigh-resolution surface plasmon resonance biosensor. The limitation of sensor was reported 0.1 pg/mm^{2} and this sensor can detect the interaction of small molecules in low concentrations. The sensitivity of sensor depend on the size and volume fraction of the embedded Au nanoclusters[6].

Another application of gold nanoparticle in SPR sensor was reported by Chau et al in 2006 [63]. A novel localized surface plasmon resonance based on fiber-optic was used for sensingNi_{2}^{+} ion and label-free detection of streptavidin and staphylococcal enterotoxin B at the picomolar level usingself-assembled gold nanoparticles [63].

In order to develop the fully integrated portable surface plasmon resonance (SPR) system for detection of explosives, the amplification strategy of SPR signal was investigated. Indirect competitive inhibition method allowed the middle-sized SPR sensor to detect trinitrotoluene (TNT) at ppt level [64]. However, this enhanced SPR signal was not high enough to detect TNT at ppt level by a miniaturized SPR sensor. Therefore, localized surface plasmon resonance (LSPR) effect using Au nanoparticle as further signal amplification approach was used. The amplification method of indirect competitive inhibition and LSPR were combined together for fabrication of the immunosurface using Au nanoparticle. TNT detectable range of this immune surface was from 10 ppt (10 pg/ml) to 100 ppb (100 ng/ml), which was almost comparable to that without Au nanoparticle. The observed resonance angle change due to binding monoclonal TNT antibody (M-TNT Ab) with the immune surface modified with Au nanoparticlewas amplified to four timeshigher than that in absence of Au nanoparticle [64].

LSPR sensors and silver nanoparticle were applied with various thiolate self-assembled monolayers (SAM) to provide chemical selectivity for detection of volatile organic compounds (VOCs) [65]. LSPR spectrums of silver nanoparticles were shifted due to binding and for detection should be measured as the response signal.The limitation of sensor detection was as low as 18–30ppm for heptanone, depending on the surface modification of Ag nanoparticles [65].

Moreover for colorimetric hydrogen peroxide, the LSPR of poly(vinylalcohol) capped silver nanoparticles was used [66]. The silver nanoparticles are directly synthesized in the PVA matrix treatment in aqueous medium. No other reagent is used.. The silver nanoparticles have the catalytic ability for the decomposition of hydrogen peroxide; then the decomposition of hydrogen peroxide induces the degradation of silver nanoparticles. Hence, a remarkable change in the localized surface plasmon resonance absorbance strength could be observed. As a result, the yellow color of the silver nanoparticle–polymer solution was gradually changed to transparent colour. Furthermore, when this transparent solution was subjected to thermal treatment, it became again yellow and the UV–vis spectroscopy confirmed that nanoparticles were again formed, suggesting the renewability of this sensor. The determination of reactive oxygen species such as hydrogen peroxide has possibilities for applying to medical and environmental applications [66].

Salmonella was detected using LSPR and Au nanoparticles [67]. In this case, the small contact area between the nanoparticle and the bacteria was the cause of the short range interaction of the local electric field and bacteria. Hence, the SPR signals were shifted to higher value that can be explainedby the Mie theory and effective medium theory. So, this is alternative methods for detection of Salmonella [67].

## Notes

- Discrete Dipole Scattering (DDSCAT) is a Fortran code for simulation and calculation of scattering and absorption of light by particles.