Surface plasmon resonance (SPR) sensors are optical sensors that can detect minute changes in the refractive index near a metal surface. They have various applications in biomedical sensing, environmental monitoring, and more. SPR sensors can be classified as surface plasmon polariton-based or localized surface plasmon resonance-based. Sensitivity, detection limit, and dynamic range are important characteristics. SPR sensing can be performed through angular modulation, wavelength modulation, intensity modulation, or phase/polarization modulation. Diffraction gratings and prism couplers are common methods used to excite surface plasmons. Localized SPR sensors offer advantages like simpler instrumentation but lower sensitivity compared to SPR sensors.

1. Surface plasmon resonance sensors –A future sensing technology
Presented by
R.Gandhimathi

2. Sensors -convert one form energy into electrical energy
Optical sensors-convert light energy into electrical energy
Surface plasmon resonance (SPR) sensor - an optical sensor fabricated based on photonic excitation
Introduction to surface plasmon resonance sensor
Classification
▪ Surface Plasmon Polariton (SPP) based sensor
▪ Localized surface plasmon resonance (LSPR) based
sensors
Plasmonic sensors are fabricated using
▪ nanoparticles
▪ nanopatterned gratings
▪ Prism couplers
▪ Metal/Dielectric waveguide
Characteristics of sensors
▪ Sensitivity
▪ Detection limit
▪ Dynamic range performance
SPR sensor applications
▪ Biomedical
▪ Food science
▪ Environmental monitoring
▪ Toxic or chemical compound
detection
▪ Pharmacy and industry
▪ Medical diagnostics
SPR sensor is vey sensitive to variation in the refractive index of the medium located next to the metallic film

3. ▪ The incident light is directly coupled with SPs (tightly
confined optical field)
▪ Change in the refractive index of the analyte produces a
variation in the propagation constant of the surface plasmon
▪ It means a modification in one of the characteristics of the
optical wave interacting with the surface plasmon
▪ Binding between the analyte and the recognition molecule
caused changes in the refractive index of the dielectric and is
monitored as a shift in the resonance wavelength of the light
A strong EM field oscillation at the interface of metal/dielectric media with
p-polarized incident light resulting in a dark band profile in the light
reflectivity at a specific wavelength(res) and incident angle(I).
SPR Sensor Configuration Surface plasmon Polariton
SPR condition is sensitive to the environment variations and that can be utilized as sensors
Principle
Prism coupler-based SPR sensor
Prism coupler employing the attenuated total reflection method in
Kretschmann geometry is the widely used method in SPR biosensors
applications

4. At Resonance z SPPk k=
2
0 0 2
sin mr a
p
mr a
n
k n k
n



=
+
The expression for the sensitivity is obtained by
differentiating resonant condition equation with respect to
, , I,  and na
SPR sensor with
▪ Angular Modulation
▪ Wave length Modulation
▪ Intensity Modulation
▪ Phase or polarization modulation
0 sinz pk k n =Incident light
m mr =
2
d an =where
2
0 0 2
sin mr a
z p
mr a
n
k k n k
n



= =
+
𝑘0 − 𝐹𝑟𝑒𝑒 𝑠𝑝𝑎𝑐𝑒 𝑤𝑎𝑣𝑒 𝑛𝑢𝑚𝑏𝑒𝑟
𝜀 𝑚𝑟 − 𝑅𝑒𝑎𝑙 𝑝𝑎𝑟𝑡 𝑜𝑓 𝑑𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑜𝑓 𝑚𝑒𝑡𝑎𝑙𝑠
𝑛 𝑝 − 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 index of prism
𝑛 𝑎 − 𝑅𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 𝑖𝑛𝑑𝑒𝑥 𝑜𝑓 𝑎𝑛𝑎𝑙𝑦𝑡𝑒
Propagation
constant
The excitation of surface plasmons in the SPR sensor results in a change in one
of the characteristics of the light wave. Based on which characteristics of the
light wave is interacting with surface plasmon is measured and used as a sensor
output.
P
a
S
n



=I
a
I
S
n


=𝑆 𝜃 =
𝛿𝜃
𝛿𝑛 𝑎
𝑆𝜆 =
𝛿𝜆 𝑟𝑒𝑠
𝛿𝑛 𝑎
2 m d
SPP
m d
k
 
  
=
+
SPP
2
2
sin mr a
p
mr a
n
n
n



=
+
Resonance condition
Classification
Angular, Wavelength, Intensity and phase sensitivity

5. ▪ A monochromatic light wave is employed
to excite the surface plasmon
▪ The excited surface plasmon is observed at
multiple angles of incident light
▪ The strength of coupling between the
incident wave and the surface plasmon
depends upon the angles of incident light
▪ Angle of incidence yielding the strongest
coupling is measured and used as a sensor
output
▪ The sensor output is calibrated to refractive
index
deg
a
S
n RIU



= =
Angular sensitivity
  2 deg
10S
RIU
 =
- represents the change of resonance angle
-change in the refractive index
𝛿𝜃
𝛿𝑛 𝑎
At constant wavelength
The angle yielding the minimum light intensity on the SPR curve is
denoted as the resonance angle
Addition of diffractive grating and
temperature and noise stabilization are
the ways to increase angular
sensitivity
2 2 2 2 2
( ) ( )
mr mr
a mr a mr a p a p
S
n n n n n n

 
  
−
= =
+ − −
Angular modulation

6. ▪ Surface plasmon is excited by a collimated light wave containing multiple wavelengths.
▪ Angle at which the light wave is incident onto the metal film is kept constant.
▪ Coupling strength between the incident wave and SP is observed at multiple wavelengths and the wavelength yielding the strongest
coupling is measured and used as a sensor output
▪ Resonance wavelength is known to shift to the longer wavelength (red shift) as the refractive index at the sensor/dielectric medium is
increased
▪ wavelength Modulation based SPR sensors using prism couplers provide much better sensitivity than their grating-based counterparts
▪ Usage of Furie spectrometers, and multi-channel sensing help to improve sensitivity
  3 4
10 10
nm
S
RIU
 = −
The wavelength sensitivity of the SPR sensor is defined as the ratio between the
resonance wavelength shift to the variation of the refractive index of the surrounding
medium
Wavelength modulation
where Sλ is the SPR sensor sensitivity
is the shift in the SPR resonance wavelength
is the change in the refractive index
𝛿𝜆 𝑟𝑒𝑠
𝛿𝑛 𝑎
Wavelength sensitivity
2
3
2
( )
2
res mr
pa mr aa
mr a mr
p
S
nn d nn
n
n

 

 
 
= =
+ +

7. 𝛿𝑛 𝑎 = 𝑛2 − 𝑛1
▪ Excitation by single incidence angle and wavelength by changing the intensity of light
▪ P-polarized wave incident light is used and they are very sensitive to any intensity fluctuations of the light source
▪ Light source must be of high quality and stability
▪ Intensity is spatially modulated due to the excitation of surface plasmons and the changes are simultaneously measured in sensing
channel by means of a spatially sensitive detector such as two-dimensional charge coupled device
▪ Sensor output is defined as the difference of these two reflected intensities which is proportional to the reflectance
  3 4
1
%
10 10S
RIU
= −
I
a
I
S
n


=
Intensity modulation
The detection of small refractive index changes over a
relatively large volume is successful on sensors based on an
intensity modulation scheme down to a sensitivity of 10-6 RIU
Two light sources with different wavelength help to improve
the sensitivity with intensity modulation
Typical sensitivity- 15000%
𝑅𝐼𝑈

8. ▪ Surface plasmon excitation by shift in phase of the light wave at a incidence angle and wavelength
▪ Explicitly used for the coherent monochromatic light source in SPR instrumentation
▪ It needs phase shift equipment such as a lock in amplifier
where ∆ϕ is the differential phase changes corresponding to ∆n
The phase sensitivity which is defined as
𝛿𝑛 𝑎 = 𝑛2 − 𝑛1
Phase or polarization Modulation
P
a
S
n



=
Other than sensitivity the figure of merit (FOM) is another important parameter to characterize sensor performance
FWHM contains information on light absorption by the binding molecules
𝐹𝑂𝑀 =
𝑆
𝐹𝑊𝐻𝑀
Where S denotes Sensitivity

9. LSPR sensor SPR sensor
Resonance conditions are simpler The energy and momentum matching
conditions should be satisfied
Small size of plasma field (20-40nm)
Marginal bulk effect
Larger plasma field (200-1000nm)
Large Bulk effect
complexity resides in the surface of the
chip
complexity resides in the
instrumentation set up to excite SPR
and read it accurately.
Temperature independent More sensitive to thermal variation
Instrumentally simple Instrumentally complex
Localized surface plasmon resonance (LSPR) sensors
▪ A label-free and powerful surface sensing platform
with higher sensitivity, simple fabrication and
measurement equipment
▪ The extreme chemical sensitivity of metal
nanoparticles to minute changes in the local dielectric
environment, is revealed as a discrete change to their
optical response due to surface adsorption
▪ In LSPR sensor, light passes through the sample
solution are affected by absorption or scattering of the
sample
▪ Requires a simple optical configuration without a
prism
▪ Cost-effective and suitable for miniaturization

10. Analyte
Metal grating
Reflected light
P-polarized
Incident light
Grating period
SPR sensors using diffraction gratings
Incident light 2
sinz ak n



=
Diffracted wave vector
2 2
sinzm ak n m
 


 = +

At resonance
SPP zmk k=
2 2 2
sin m d
a
m d
n m
   

   
+ =
 +
After Simplification
sin m d
a
m d
n m
 

 
+ =
 +
At resonance condition
2
2
sin mr a
a
mr a
n
n m
n



+ = 
 +
2
22
3
3
2 22
a mr
mr amr a
a mra
mr mr a
nm
na nn
nmn
n




 
+
 ++
=
+

+
3
2
2
1
sin( )
cos( )
mr
a a mr an n n


  
   =  − 
 +  
Angular Modulation
Wave length Modulation
▪ The momentum mismatch is compensated by diffraction using a metallic diffraction grating
▪ The resonant transfer of optical energy into an SPP is observed as a dip in the angular or wavelength spectrum of reflected light
▪ Light propagates into the core through total internal
reflection and generates an evanescent field in the
vicinity of the waveguide boundary, which induces SPR
at the interface between the metal film and the sensing
medium
▪ Provides highly integrated, multichannel, and robust
sensing devices
The expression for the sensitivity is obtained by differentiating
resonant condition with respect to ,  and na
-grating period
Wave guide-based sensor
▪ Planar waveguide configuration - unable to
interrogate the incident angle scanning
▪ Wavelength interrogation is the only option for the
signal acquisition technique

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12. Thank you