Surface plasmon resonance based multi-channel and multi-analyte fiber optic sensor

Surface plasmon resonance based multi-channel and multi-analyte fiber optic sensor

Roli Verma, Sachin K. Srivastava and Banshi D. Gupta*

Indian Institute of Technology Delhi, New Delhi, India - 110016

ABSTRACT

Surface plasmon resonance (SPR) based fiber optic sensor has been studied for multichannel and multianalyte sensing. Simulations have been carried out for a fiber optic sensor having two sensing regions coated with silver and gold for multichannel and multianalyte sensing. The simulated results have been obtained for different combinations of refractive indices of the samples around the probes. To support simulations optical fiber SPR probes with two sensing regions coated with silver and gold have been fabricated. SPR spectra for these sensors have been recorded for aqueous sucrose solutions of varying refractive indices. The refractive index of the liquid samples around both the gold and silver coated regions was kept the same to see the potential of SPR based fiber optic multichannel sensing, while it was kept different for studies related to multianalyte sensing. Both the theoretical and experimental results match qualitatively. The SPR resonance wavelengths for gold and silver being different, these sensors can be utilized for both multichannel and multianalyte sensing.

Keywords: Optical fiber, sensor, surface plasmon, multichannel, multianalyte

1. INTRODUCTION

Plasmonics has become one of the hottest research topics due to its potential applications in the field of wave guiding1, chemical and bio sensing2, imaging3, metamaterials4 etc. The branch of plasmonics is primarily concerned with the excitation of plasmon modes of either a metal dielectric interface or that of metal nanoparticles. The plasmon modes of a metal dielectric interface are termed as surface plasmons (SPs). When the wave vector of the light incident on this interface matches with that of surface plasmons for the structure, SPs get excited. This is called surface plasmon resonance (SPR). In general, the wave vector of surface plasmons is always larger than the light travelling in free space. Therefore the excitation of surface plasmons by light coming from free space is impossible. Some special techniques, such as Kretschmann configuration5 are employed for surface plasmon excitation. In Kretschmann configuration, the base of a high refractive index prism is coated with a thin metal layer. The metal layer is further surrounded by the dielectric under study. A beam of light incident on one side of the prism traverses it up to the base; gets total internally reflected and comes back from the other end. A part of incident power is absorbed in the metal film due to its imaginary part of dielectric constant. Therefore this is termed as attenuated total internal reflection (ATR). At a particular value of the angle of incidence, all the incident power gets transferred to the surface plasmons at the metal dielectric interface and a sharp dip is observed in the reflected light. This angle is called resonance angle. This resonance angle is highly dependent on the refractive index of the dielectric medium in the contact of the metal film, and changes with change in the refractive index of the dielectric medium. This property of surface plasmons is utilized in sensing. The configuration can be utilized other way also. Rather using a monochromatic beam and traversing a range of angles, a polychromatic source at a fixed incidence can be used for sensing. A dip corresponding to resonance wavelength is observed in the reflection spectrum. This is called resonance wavelength. Many researchers have used Kretschmann configuration in their studies. The prism based devices are quite bulky and require a number of mechanical parts. The use of optical fibers in place of prism is advantageous in terms of miniaturization, cost, remote sensing, online measurements etc. In a fiber optic SPR sensor, the prism is replaced by the core of an optical fiber. The metal layer is directly coated on the core of the optical fiber, which is further surrounded by the sensing medium. Light guided through the optical fiber excites surface plasmons in the sensing region. Power transmitted off the output face is collected by a spectrometer interfaced with a computer. Many studies utilizing fiber optic SPR based chemical and biochemical sensors have been reported in literature6, 7. Several attempts have also been made to increase the sensitivity of these sensors8. * [email protected]; phone+91-11-2659-1355; fax +91-11-2658-1114; iitd.ac.in

Third Asia Pacific Optical Sensors Conference, edited by John Canning, Gangding Peng, Proc. of SPIE Vol. 8351, 83512D · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.914781

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Multichannel and multianalyte sensing has been a matter of curiosity for researchers due to miniaturization, cost and other issues, such as remote sensing of multiple parameters. The multichannel sensing is used for the studies in which a number of simultaneous sensing actions of the same parameter are performed by the same sensor at different places. However, if the same sensor performs sensing of a number of analytes, it is termed as multianalyte sensing. A number of studies have been performed for the multichannel and multianalyte sensing9, 10. Utilization of SPR for multichannel and multianalyte sensing has also been carried out on prism or grating based structures11. Optical fibers have shown the potential of multichannel and multianalyte sensing, as they can be used in bundles or different portions of a single fiber can be used for sensing of different parameters. A study of multianalyte sensing with fiber Bragg gratings coated with thin metal layers has also been reported in the literature12. But strictly speaking, such a mode cannot be called a pure surface plasmon mode, as Bragg gratings are written in a single mode optical fiber and it is only the fundamental mode of the fiber which gets slightly modified due to metal cladding. In a multimode optical fiber, polychromatic light is used for the excitation of surface plasmons and some higher order modes get coupled to excite pure surface plasmon modes. In the present study, we have first simulated and theoretically analyzed a fiber optic SPR sensor having two sensing regions, coated with silver and gold, in a single optical fiber and then carried out experiments to support our simulated results. The potentials of such a sensor have been studied for the multichannel and multianalyte sensing. Various numerical results and advantageous features of the sensor have been discussed first and then a fiber optic sensor having two sensing regions coated with silver and gold has been fabricated and characterized for multichannel and multianalyte sensing. A very good qualitative agreement has been found in both the studies. The present study may be used for making fiber optic SPR sensors for multichannel and multianalyte sensing.

2. THEORY

The theoretical modeling of our present study of multichannel SPR sensor is based on attenuated total internal reflection with Kretschmann configuration. We consider an optical fiber which has two unclad portions of 1 cm length in the middle. One of these unclad regions is coated with a thin film of gold and the other with silver. A schematic of the proposed sensor is shown in figure 1 (a). Sensing medium to be detected is placed around the sensing regions.

Figure 1. (a) A schematic of the proposed sensor, and (b) schematic of N-layer model

A p-polarized light beam from a polychromatic source is launched at one end of the fiber and collected at the other end. The transmitted light is then fed to a spectrometer to plot the transmitted power with wavelength for sensing media of



different refractive indices. Our modeling is based on wavelength interrogation so we have used the dispersive relation for fiber core, silver and gold to carry out the simulations13. We have used the Drude’s formula for dispersive nature of two metals gold and silver, which is given as

2

2( ) 1( )

cm

p c iλ λε λ

λ λ λ= −

+ (1)

where p candλ λ are the plasma and collision wavelengths respectively. The plasma and collision wavelengths of gold and silver have been taken from the literature13. We consider the planer waveguide approach and N-layer model as shown in figure 1(b) for calculating the normalized transmitted power13. In the present study we have considered N=3 layers as fiber core, metal layer and sensing medium. The arbitrary medium layer is defined by thickness dk, dielectric constant εk, permeability μk, and refractive index nk. The tangential field at the first interface 1 0= =Z Z and last interface 1−= NZ Z are related as

11

11

N

N

UUM

VV−

−

⎡ ⎤⎡ ⎤= ⎢ ⎥⎢ ⎥

⎣ ⎦ ⎣ ⎦ (2)

where 1U and 1V are defined as the components of electric and magnetic fields respectively at the interface of the first layer of structure, 1−NU and 1−NV are fields at the interface of thN layer and M is defined as the characteristic matrix of this structure given as

1

2

N

kk

M M−

=

=∏

where cos sin /

sin cosk k k

kk k k

i qM

iqβ ββ β

−⎛ ⎞= ⎜ ⎟−⎝ ⎠ ,

122 2( sin )k kq nε θ= − ,

122 2(2 / )( sin )k k kd nβ π λ ε θ= − and θ is the

angle of incidence of the p-polarized light from the normal to the interface. The amplitude of reflection coefficient ( pr ) for the p-polarized incident light is given by

11 12 1 21 22

11 12 1 21 22

( ) ( )( ) ( )

N Np

N N

M M q q M M qrM M q q M M q

+ − +=

+ + + (3)

In the present study we have considered two sensing regions coated with gold and silver on optical fiber core. We denote the amplitude of reflection coefficients for gold and silver as pgr and psr respectively. Therefore, the reflectance ( pR ) for the p-polarized light is given as

2= ×p pg psR r r

(4)

In fiber optic SPR sensor all the guided rays are launched in fiber core through the collimated source. The power distribution of all guided rays in the fiber at angle θ at the core and metal layer interface is given by14

1

2 21

2

2sin cos

(1 cos )ndP dn

θ θ θθ

∝−

(5)

And the total transmitted power at the output end of fiber is given by14

21

2 21

21

2 21

2

2

/2

/2

sin cos(1 cos )

sin cos(1 cos )

rNp

trc

c

nR dn

Pn dn

π

θπ

θ

θ θ θθ

θ θ θθ

−=

−

∫

∫ (6)



In Eq. (6), 12 1sin ( )θ −=c n n , is called critical angle and 1n and 2n are the refractive index of the optical fiber core and

cladding respectively. rN is the total number of reflections in the sensing region. If L be the length of the sensing region and D the diameter of fiber core then rN is given by

/ ( tan )rN L D θ= (7) In the present study we have considered the same length for the both sensing regions as 1= = =g sL L L cm. The thicknesses of gold layer (dg) and silver layer (ds) were taken to be 50nm and 40 nm respectively in our simulations. The transmitted power was calculated from Eq. (6) and SPR spectra were plotted between transmitted power and incident wavelength.

3. EXPERIMENTS

2.1 Fabrication of the sensor

A piece of 15 cm length of optical fiber was taken from a spool of optical fiber having 600 µm core diameter and 0.24 numerical aperture. Two sensing regions of 1cm length each were made in the probe by removing the cladding from the middle. The separation between the two sensing regions was 1 cm. A thin film of 50 nm thickness of gold was coated in one sensing region while that of 40 nm thickness of silver was coated on the other sensing region. These thicknesses of metal layers were chosen because they have been reported as optimized thicknesses for the best performance. The coating procedure was followed from our previous studies7, 8. Briefly, it is described as follows. A film of 1.6 nm of chromium (Cr) was coated before the coating of gold. Cr. Acts as an adhesive between fiber core and gold. We have used a vacuum chamber with thermal evaporation technique to coat the fibers. The fibers were first cleaned with acetone and then high tension arc plasma in vacuum chamber to remove any dust etc. from the core surface. The evaporation of metals in the chamber was performed by a low tension circuit. The metal plume evaporates and gets deposited on the substrate. The online monitoring of the thickness of the deposited film was performed by a quartz crystal thickness monitor fixed in the vacuum chamber. The vacuum in the chamber was kept at 5X10-6 mBar. The better the vacuum in the chamber, smoother is the deposited film. 2.2 Preparation of sample solutions

Aqueous samples of sucrose were prepared in Millipore® water. The refractive indices of these samples were varied from 1.335 to 1.370. The refractive indices of the samples were measured in white light by an Abbe’s refractometer having resolution of 0.001. 2.3 Experimental Setup

A schematic of the experimental setup used for the present study is shown in figure 2. After the fabrication of sensor, it was fixed in a glass flow cell of two inlets and two outlets. The partition of the flow cell was designed such that each sensing region lies in one flow cell. Polychromatic light from a tungsten halogen lamp was focused on the input end of the optical fiber probe with the help of a microscope objective. The light incident on the input end of fiber gets guided through the fiber due to total internal reflection at the core cladding interface, excites surface plasmons in both the sensing regions and collected at the output end. The light coming out of the fiber was then fed to a spectrometer interfaced with a computer. Liquid samples of same refractive indices were simultaneously filled in the flow cells to study the multichannel response of the sensor, while the samples of different refractive indices were filled for multichannel and multianalyte response both.



0450 550 650 750 850

Wavelength (nm)

Figure 2. A schematic of the experimental setup

4. RESULTS AND DISCUSSIONS

Simulated Transmittance spectra of the proposed multichannel - multianalyte sensor have been shown in figure 3, when the refractive index of the analyte in both the sensing regions is the same. These spectra are the characteristic of the surface plasmon resonances at the respective metal dielectric interfaces. It is observed that for the same value of the refractive index of the sensing medium around the sensing region of the probe, the SPR spectra show two resonance minima at different wavelengths. One of these resonance minima corresponds to silver and the other one to gold. This confirms that a fiber optic sensor with two different sensing regions coated with different metals can be employed for multichannel sensing even when the analyte is the same in both the sensing regions.

Figure 3: Simulated SPR spectra of fiber optic sensor for the same analyte in both the sensing regions



(.1

' 450 550 650 750 850Wavelength (nm)

Figure 4. SPR spectra for the (a) same refractive index in silver coated region and varying refractive index in gold coated region, and (b) same refractive index in gold coated region and varying refractive index in silver coated region

To study it more elaborately, we have plotted the simulated SPR spectra in figure 4 (a) for the same value of refractive index of analyte in the silver coated region and varying refractive indices in gold coated sensing region and in figure 4 (b) for the varying refractive indices of sensing medium around silver coated region and at a fixed value of the refractive index of the sensing medium around gold coated sensing region. From figures 3 and 4 it is evident that the resonance minimum at smaller wavelength corresponds to silver while that at large wavelength is due to gold. It is also observed in figure 3 that the resonance minima of both gold and silver show red shift for varying refractive indices from 1.335 to 1.365. The red shift in the resonance wavelengths corresponding to the silver coated region is lesser than that for gold coated region of the same sensing length. This implies that the sensitivity of gold coated sensing region is greater than that of silver. This is in accordance with previous reports13. It is found from figures 4(a) and 4(b) that for the fix value of refractive index at one channel and varying refractive index at the other channel, the shift in resonance wavelength in both the sensing channels does not affect each other, but it shows a red shift in both the channels independently.

Figure 5. SPR spectra for varying different refractive indices in both the sensing regions



Furthermore we have plotted SPR curve in figure 5 for different refractive indices of sensing medium in both the channels to study the potentials of such a sensor for multianalyte sensing. The resonance minima of the SPR spectra obtained in the two sensing regions for different ranges of refractive indices of sensing medium are far apart from each other. This shows a clear cut possibility of use of such sensor for multianalyte testing. This theoretical study was further supported by experiments. The fiber optic SPR sensors were fabricated with two sensing regions coated with silver and gold and the transmitted spectra were recorded for aqueous sample solutions of sucrose around the sensing regions. In figure 6, we have plotted the experimental normalized transmittance spectra of the fabricated sensor with wavelength for a number of aqueous sample solutions of sucrose of varying refractive indices around the sensing region. Figure 6 (a) shows the variation of normalized transmittance with wavelength for a change in the refractive index of the sensing medium, when the analyte in both the sensing regions is the same, while figure 6 (b) shows the SPR spectra for different analytes around both the sensing regions. The experimental SPR spectra in figure 6 (a) match qualitatively with the simulated curves of figure 3 while that of figure 6 (b) match with the figure 5. Thus a fairly good agreement has been found between experimental and simulation results. The kind of variations in figures 6 (a) and 6 (b) can be explained with the same arguments as given for figures 3 and 5. A quantitative mismatch between the simulated and experimental results may be due to non inclusion of skew rays and cylindrical geometry in simulations and certain fabrication errors occurred in the experiments. However, we have repeated the experiments three times on different probes and different days. A very good repeatability of experimental results has been found. Thus a fiber optic SPR sensor with different metal coatings in different sensing regions shows the potentials of its use for multichannel and multianalyte sensing.

Figure 6. Experimental SPR spectra for samples of (a) same refractive indices, and (b) different refractive indices in both sensing regions.

5. CONCLUSIONS

We have studied both theoretically and experimentally the performance of a multianalyte and multichannel fiber optic SPR sensor. Multichannel sensor is useful to detect the same sample with different sensitivities in both sensing regions, while the multianalyte sensor is useful to sense different sample simultaneously in two sensing regions. It is found that multianalyte and multichannel sensors are very important tools for simultaneous sensing of more than one parameter. The use of different metals provides an opportunity of utilization of different regions of visible spectral range for simultaneous sensing of various analytes. For example, a fiber optic sensor can be fabricated for simultaneous monitoring of blood glucose, cholesterol, hemoglobin etc. in a small amount of blood. Various advantageous features of the present sensor have been discussed. The present study finds use in applications of SPR based fiber optic multianalyte sensors.



ACKNOWLEDGEMENTS

Roli Verma and Sachin K. Srivastava are thankful to the Council of Scientific and Industrial Research (CSIR), India for providing research fellowships. This work was partially supported by the Department of Science and Technology (DST), India.

REFERENCES

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Surface plasmon resonance based multi-channel and multi-analyte fiber optic sensor

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