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Metamaterial based sucrose detection sensor using transmission spectroscopy
Optik - International Journal for Light and Electron Optics 205 (2020) 164276
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Short note
Metamaterial based sucrose detection sensor using transmission spectroscopy
T
Sajal Agarwala, Y.K. Prajapatib,* a b
Department of Electronics and Communication Engineering, Jaypee Institute of Information Technology, Noida, India Electronics and Communication Engineering Department, Motilal Nehru National Institute of Technology Allahabad, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Metamaterial Sensor Sensitivity Transmission spectrum Epsilon negative
In the present study, metamaterial based optical sensor is proposed and optimization of the proposed optical sensor for sucrose detection is done. In this paper single negative metamaterial surface is fabricated using E-beam lithography technique to be used as an optical sensor for broad wavelength range using UV–Vis–NIR spectrometer. The resultant sensitivity and detection accuracy of the proposed metamaterial based sensor is higher than simple thin film sensor, i.e. 1740.8 (nm/RIU) and 0.893 respectively. The center wavelength of the proposed optical sensor is 967 and 933 nm for gold and metamaterial based-sensor respectively, which ensures the larger bandwidth available in the case of metamaterial based-sensor. It is observed that high surface to volume ratio of metamaterial surface ensures the better sensitivity of the proposed sensor. Simple structure of presented metamaterial surface also makes sure reliable and precise fabrication of metamaterial surface.
1. Introduction In few decades, metamaterials attract much attention from all over the globe due to their extra ordinary properties [1,2]. Metamaterials are artificially structured materials used widely in many areas to manipulate light, sound, and other physical phenomenon [3–5]. Metamaterial has a periodic arrangement of nano-scale structure in x and y-directions. The single structure is known as unit cell and it works like an atom of metamaterial. The properties of metamaterial are acquired from both; its constituent material as well as the geometrical structure [3,6]. Metamaterials are gaining their popularity for optical devices mostly because of their compatibility with optical properties and small size [7]. There are a number of different applications of metamaterials proposed till now [8,9]. In this study, refractive index optical sensors is analyzed using fabricated metamaterial surface. Optical sensors measure the change in light properties to detect the change in analyte entity [10]. These entities could be anything such as, refractive index [11], temperature [12], strain [13], etc. But refractive index sensor is studied here because of its very inherent nature of high sensitivity and accuracy. Optical fiber grating [14], thin film, and interferometer [15], etc. are the most used optical sensors. Optical gratings are the in situ type sensors as the grating prepared with in the fiber using different fabrication techniques and then the grating is dipped in the analyte solution for the sensing. However, this type of set-up is assembled one and thus there is a probability of error in the results. Surface Plasmon resonance (SPR) is the most popular method of optical sensing, due to high sensitivity and accuracy. In SPR sensors, plasmons propagate in x and y-directions and decay evanescently in z-direction. Interaction between the electromagnetic (EM) wave and molecular surface leads to shift in resonance condition, and this can be read out by different modes. However, SPR based thin film sensors also have the probability of error because of the free space laser light ⁎
Corresponding author. E-mail addresses: [email protected] (S. Agarwal), [email protected] (Y.K. Prajapati).
https://doi.org/10.1016/j.ijleo.2020.164276 Received 18 October 2019; Received in revised form 20 January 2020; Accepted 20 January 2020 0030-4026/ © 2020 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 205 (2020) 164276
S. Agarwal and Y.K. Prajapati
Fig. 1. Schematic diagram of fabrication process flow.
propagation till the analyte and the detector. Localized surface plasmon resonance (LSPR) also follows the same principle and enhance the EM field. In LSPR, light interact with much small particle and leads to oscillate the plasmon locally. LSPR is more sensitive than SPR sensors, but it also suffers from the same problem as SPR. Therefore, it is observed that open environment optical sensors may have some error in results. All the above mentioned sensors measure the change in the refractive index caused by sensing medium with waveguide or by adsorption of molecules on the surface and has limited performance. Thus to overcome these errors, here a new approach is used with the closed environment UV–Vis–NIR spectroscope. The main advantage of the approach presented, its high sensitivity than the other known optical sensing methods [11,14,16]. Moreover, measurement of wavelength change is much easier than the phase and angular interrogation [11,17]. The present study follows the Beer–Lambert Law for the absorption and transmission measurement and has practical considerations [18]. Intensity of light is measured for two samples; one is reference and other is actual sensor, ratio of these is known as transmittance [19]. In the present approach for sucrose sensing transmittance is observed; based on the change in wavelength of the transmittance dip sensitivity is calculated for the proposed sensor. 2. Structure and fabrication process Based on the study of metamaterial structures, it is decided to present a simple ‘U’ shaped resonator to get the metamaterial effect, same as fabricated in Ref. [20]. Dimensions used for the fabrication of metamaterial are same as previous work. It is observed that the proposed dimension cannot be realized using simple optical masking thus highly sensitive lithography unit is used for fabrication of metamaterial surface. Silicon (Si) is used as the substrate because of it stability in E-bean lithography moreover, use of Si substrate also ensures the working of proposed surface in near infrared region. Gold (Au) is used to make ‘U’ shaped resonators on substrate because of its stability in outer environment. Fig. 1 has the schematic diagram of process steps followed while fabrication. For the fabrication, p-type Si wafer (Sigma–Aldrich) is used with standard RCA cleaning process in class-1000 clean room using chemical hood. After the cleaning, substrate is dried using nitrogen N2. After that, photo resist; poly methyl methacrylate (PMMA) is coated using spin coater for 40 s at 6000 rpm. The approximate thickness of photo resist is 195 nm. Thereafter, pattern with required dimensions is sketched in the interfacing software of E-beam lithography in class 100 clean room. Than sample is loaded in lithography unit for pattern to write. After the writing, pattern is developed using methyl isobutyl ketone (MIBK): isopropyl alcohol (IPA) solution in 1:3 ratio and dried using N2, baked for 2 s at hot plate. Au is deposited in E-beam evaporation unit for 60 nm thickness. Deposition is done at room temperature at 3e −7 Torr pressure. After the deposition, lift off of remaining resist is done by dipping the sample in acetone for 1–2 min and again dried using N 2 . Then after, characterization is done. First, ellipsometer characterization is done to measure the permittivity of the fabricated surface. Refractive index (R.I.) of fabricated metamaterial surface is calculated through the permittivity and permeability ( μ = 1), given in Fig. 2. It is observed from the figure that R.I. varies for wide values of the considered wavelength ranges from 100 to 1700 nm. It is seen that real value of R.I. is small enough after 800 nm wavelength and thus provide good transmission for the targeted application. Scanning Electron Microscopy (SEM) is done to observe the surface topology. Fig. 3 has the SEM result. It is seen that ‘U’ shaped resonators are successfully fabricated on the substrate with few deviations in geometrical dimensions. After that the nature of fabricated metamaterial surface is decided by calculating the plasma frequency (ωp) using Eq. (1);
ωp =
Neff c 2 2πc 2 = 2 ϵ 0 meff a ln(a/ r )
(1)
where Neff is the effective refractive index, ϵ 0 is vacuum permittivity, meff is effective mass, e is electron charge, c is speed of light in free space, a is lattice constant, and r is the radius of the ‘U’ ring long leg. It is observed that ωp is 1.7687 × 1015 Hz. When plasma 2
Optik - International Journal for Light and Electron Optics 205 (2020) 164276
S. Agarwal and Y.K. Prajapati
Fig. 2. Measured refractive index of fabricated surface.
Fig. 3. SEM result with fabricated dimensions.
frequency of fabricated surface is compared with the Au, i.e. 2.183 × 1015 Hz, it is depicted that ωp of metamaterial is lower than its constituent metal and thus, the fabricated surface in epsilon negative metamaterial (ENG) [21].
3. Results and discussion Fabricated single negative metamaterial surface is used in this study for sucrose sensing through spectroscopy approach. UV–Vis–NIR spectrometer (Shimadzu UV-3600 plus) is used for sensing for metamaterial surface as well as thin gold film sensor. First, UV–Vis–NIR unit is calibrated with the bare silicon wafer to set the reference, due to; substrate used in the sensor chip is silicon. After setting the reference, sensor chip and the solution put in the cuvette together for the sensing. Since, spectrometer is the closed unit, it provide an optimized and error-free environment for sensing compared to the simple SPR sensor (Open environment) [22]. To study the sensing ability of the proposed sensor, different concentrations of sucrose are sensed in de-ionized (DI) water. Table 1 has the R.I. vs. concentration data of the prepared solutions for sensing. Fig. 4 has the transmission spectrum for wavelength interrogation using Si/Au thin film sensor. From Fig. 4 it is depicted that as the concentration of sucrose in DI water is increased the transmission dip shifts toward higher wavelength. The maximum deflection is 60.2 nm in wavelength for the R.I. change of 0.12. Shift in the transmission wavelength is experienced due to the LSPR resonance. Since, LSPR is sensitive to the outer environment thus as the concentration of the sucrose solution is changed the resonance wavelength is also shifted towards the higher wavelength. Fig. 5 has the transmission spectrum using metamaterial surface in place of gold sensor. Fig. 5 depicts that the proposed metamaterial surface also follows the same nature, i.e. increased wavelength for higher sensing R.I. (nc ). It is also observed that the maximum shift in wavelength is 208.9 nm. There is a large difference in the wavelength shift between simple gold film sensor and metamaterial sensor, i.e. 148.7 nm. The larger shift in the wavelength is because; metamaterial Table 1 Refractive index vs. sucrose concentration calibrated values. Sucrose concentration (%)
0
11
24
35
45
55
65
R.I.
1.33
1.35
1.37
1.39
1.41
1.43
1.45
3
Optik - International Journal for Light and Electron Optics 205 (2020) 164276
S. Agarwal and Y.K. Prajapati
Fig. 4. Transmission spectrum for different sucrose concentration using Si/Au sensor.
Fig. 5. Transmission spectrum for different sucrose concentration using metamaterial surface.
surface experiences high resonance intensity due to its small particle size. From the results, it is clearly seen that the metamaterial sensor provides better sensing and dramatically improve the sensor sensitivity. It should be noted that the surface to volume ratio of fabricated metamaterial surface is much larger than the simple thin film, which ensures the better interaction of analyte with the sensing surface and in turn increases the adsorption of molecules for higher shift in the LSPR resonance wavelength and effective sensing. Fig. 6 displays the change in transmission dip in correspond to the sensing R.I., it is observed that for the proposed metamaterial surface, change in the transmission dip is much larger than the uniform thin film of Au. This feature of the proposed metamaterial surface ensures the better sensing ability compared to conventional thin film sensor.
Fig. 6. Change in transmission dip with the sensing R.I. 4
Optik - International Journal for Light and Electron Optics 205 (2020) 164276
S. Agarwal and Y.K. Prajapati
Table 2 Comparative analysis of different sensors. Parameter
Sensitivity (nm/RIU) Detection accuracy
Sensor Conventional SPR reflective sensor
Si/Au transmission sensor
Proposed metamaterial transmission sensor
970 [23] 2 × 10−5 [23]
501.66 0.979
1740.8 0.893
Table 2 has the comparative analysis of the sensitivity (Δλ /Δnc ) and detection accuracy (Δλ /BW ) for different sensors. Where BW is the half power beam width of transmission/reflection curve. From Table 2 it is observed metamaterial surface displayed quite better performance than the other simple sensors in terms of sensitivity and has comparable detection accuracy. Thus, it is true that metamaterial surface improves the performance of sensor and implementation in commercial sensor will definitely help in low volume bio-molecular detection. 4. Conclusion It is found from the current study that metamaterial surface exhibit more sensitivity than the simple thin film sensor because of the high surface to volume ratio. Moreover, it is observed that wavelength interrogating transmission sensor provided better sensitivity than the conventional surface plasmon resonance sensor. This study depicts that the proposed surface can be used for sucrose level detection efficiently through optical sensing method. Conflict of interest None declared. Acknowledgment The present work is partially supported by the Department of Science and Technology (DST), New Delhi, India under the fast track young scientist scheme no. SB/FTP/ETA-0478/2012. Authors are also grateful to the Center of Interdisciplinary Research (CIR), MNNIT Allahabad for the instrumental facility support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
A. Srivastava, Metamaterial properties of periodic laminates, J. Mech. Phys. Solids 96 (2016) 252–263. N.I. Landy, S. Sajuyigbe, J.J. Mock, D.R. Smith, W.J. Padilla, Perfect metamaterial absorber, Phys. Rev. Lett. 100 (20) (2008) 207402. F. Capolino, Theory and Phenomena of Metamaterials, CRC Press, 2017. V.M. Shalaev, Optical negative-index metamaterials, Nat. Photon. 1 (1) (2007) 41–48. M. Wegener, Metamaterials beyond optics, Science 342 (6161) (2013) 939–940. C.M. Watts, X. Liu, W.J. Padilla, Metamaterial electromagnetic wave absorbers, Adv. Mater. 24 (23) (2012) OP98–OP120. H.T. Chen, W.J. Padilla, J.M. Zide, A.C. Gossard, A.J. Taylor, R.D. Averitt, Active terahertz metamaterial devices, Nature 444 (7119) (2006) 597–600. W. Wang, F. Yan, S. Tan, H. Zhou, Y. Hou, Ultrasensitive terahertz metamaterial sensor based on vertical split ring resonators, Photon. Res. 5 (6) (2017) 571–577. J. Yang, C. Gong, L. Sun, P. Chen, L. Lin, W. Liu, Tunable reflecting terahertz filter based on chirped metamaterial structure, Sci. Rep. 6 (38732) (2016). Q. Liu, J.S. Kee, M.K. Park, A refractive index sensor design based on grating-assisted coupling between a strip waveguide and a slot waveguide, Opt. Express 21 (5) (2013) 5897–5909. S. Agarwal, P. Giri, Y.K. Prajapati, P. Chakrabarti, Effect of surface roughness on the performance of optical SPR sensor for sucrose detection: fabrication, characterization, and simulation study, IEEE Sens. J. 16 (24) (2016) 8865–8873. S.F. Leon-Luis, V. Monteseguro, U.R. Rodriguez-Mendoza, I.R. Martin, D. Alonso, J.M. Caceres, V. Lavin, 2CaO.Al2O3:Er3+ glass: an efficient optical temperature sensor, J. Lumin. 179 (2016) 272–279. E. Roh, B.U. Hwang, D. Kim, B.Y. Kim, N.E. Lee, Stretchable, transparent, ultrasensitive, and patchable strain sensor for human machine interfaces comprising a nanohybrid of carbon nanotubes and conductive elastomers, ACS Nano 9 (6) (2015) 6252–6261. J. Hromadka, B. Tokay, R. Correia, S.P. Morgan, S. Korposh, Highly sensitive volatile organic compounds vapour measurements using a long period grating optical fibre sensor coated with metal organic framework ZIF-8, Sens. Actuators B: Chem. 260 (2018) 685–692. J. Zhou, Y. Wang, C. Liao, B. Sun, J. He, G. Yin, S. Liu, Z. Li, G. Wang, X. Zhong, J. Zhao, Intensity modulated refractive index sensor based on optical fiber Michelson interferometer, Sens. Actuators B: Chem. 208 (2015) 315–319. H.E. Joe, H. Yun, S.H. Jo, M.B. Jun, B.K. Min, A review on optical fiber sensors for environmental monitoring, Int. J. Prec. Eng. Manuf. Green Technol. 5 (1) (2018) 173–191. X. Wu, B. Quan, X. Pan, X. Xu, X. Lu, X. Xia, J. Li, C. Gu, L. Wang, Sensing self-assembled alkanethiols by differential transmission interrogation with terahertz metamaterials, Appl. Opt. 52 (20) (2013) 4877–4883. A. Sassaroli, S. Fantini, Comment on the modified Beer–Lambert law for scattering media, Phys. Med. Biol. 49 (14) (2004) N255. A. Aitken, M.P. Learmonth, Protein determination by UV absorption, The Protein Protocols Handbook, Humana Press, Totowa, NJ, 2009. S. Agarwal, Y.K. Prajapati, Multifunctional metamaterial surface for absorbing and sensing applications, Opt. Commun. 439 (2019) 304–307. W. Cai, V. Shalaev, Optical Metamaterials: Fundamentals and Applications, Springer Science & Business Media, 2009. S. Agarwal, Y.K. Prajapati, J.B. Maurya, Effect of metallic adhesion layer thickness on surface roughness for sensing application, IEEE Photon. Technol. Lett. 28 (21) (2016) 2415–2418. J. Homola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors, Sens. Actuators B: Chem. 54 (1) (1999) 3–15.
5
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Short note
Metamaterial based sucrose detection sensor using transmission spectroscopy
T
Sajal Agarwala, Y.K. Prajapatib,* a b
Department of Electronics and Communication Engineering, Jaypee Institute of Information Technology, Noida, India Electronics and Communication Engineering Department, Motilal Nehru National Institute of Technology Allahabad, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Metamaterial Sensor Sensitivity Transmission spectrum Epsilon negative
In the present study, metamaterial based optical sensor is proposed and optimization of the proposed optical sensor for sucrose detection is done. In this paper single negative metamaterial surface is fabricated using E-beam lithography technique to be used as an optical sensor for broad wavelength range using UV–Vis–NIR spectrometer. The resultant sensitivity and detection accuracy of the proposed metamaterial based sensor is higher than simple thin film sensor, i.e. 1740.8 (nm/RIU) and 0.893 respectively. The center wavelength of the proposed optical sensor is 967 and 933 nm for gold and metamaterial based-sensor respectively, which ensures the larger bandwidth available in the case of metamaterial based-sensor. It is observed that high surface to volume ratio of metamaterial surface ensures the better sensitivity of the proposed sensor. Simple structure of presented metamaterial surface also makes sure reliable and precise fabrication of metamaterial surface.
1. Introduction In few decades, metamaterials attract much attention from all over the globe due to their extra ordinary properties [1,2]. Metamaterials are artificially structured materials used widely in many areas to manipulate light, sound, and other physical phenomenon [3–5]. Metamaterial has a periodic arrangement of nano-scale structure in x and y-directions. The single structure is known as unit cell and it works like an atom of metamaterial. The properties of metamaterial are acquired from both; its constituent material as well as the geometrical structure [3,6]. Metamaterials are gaining their popularity for optical devices mostly because of their compatibility with optical properties and small size [7]. There are a number of different applications of metamaterials proposed till now [8,9]. In this study, refractive index optical sensors is analyzed using fabricated metamaterial surface. Optical sensors measure the change in light properties to detect the change in analyte entity [10]. These entities could be anything such as, refractive index [11], temperature [12], strain [13], etc. But refractive index sensor is studied here because of its very inherent nature of high sensitivity and accuracy. Optical fiber grating [14], thin film, and interferometer [15], etc. are the most used optical sensors. Optical gratings are the in situ type sensors as the grating prepared with in the fiber using different fabrication techniques and then the grating is dipped in the analyte solution for the sensing. However, this type of set-up is assembled one and thus there is a probability of error in the results. Surface Plasmon resonance (SPR) is the most popular method of optical sensing, due to high sensitivity and accuracy. In SPR sensors, plasmons propagate in x and y-directions and decay evanescently in z-direction. Interaction between the electromagnetic (EM) wave and molecular surface leads to shift in resonance condition, and this can be read out by different modes. However, SPR based thin film sensors also have the probability of error because of the free space laser light ⁎
Corresponding author. E-mail addresses: [email protected] (S. Agarwal), [email protected] (Y.K. Prajapati).
https://doi.org/10.1016/j.ijleo.2020.164276 Received 18 October 2019; Received in revised form 20 January 2020; Accepted 20 January 2020 0030-4026/ © 2020 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 205 (2020) 164276
S. Agarwal and Y.K. Prajapati
Fig. 1. Schematic diagram of fabrication process flow.
propagation till the analyte and the detector. Localized surface plasmon resonance (LSPR) also follows the same principle and enhance the EM field. In LSPR, light interact with much small particle and leads to oscillate the plasmon locally. LSPR is more sensitive than SPR sensors, but it also suffers from the same problem as SPR. Therefore, it is observed that open environment optical sensors may have some error in results. All the above mentioned sensors measure the change in the refractive index caused by sensing medium with waveguide or by adsorption of molecules on the surface and has limited performance. Thus to overcome these errors, here a new approach is used with the closed environment UV–Vis–NIR spectroscope. The main advantage of the approach presented, its high sensitivity than the other known optical sensing methods [11,14,16]. Moreover, measurement of wavelength change is much easier than the phase and angular interrogation [11,17]. The present study follows the Beer–Lambert Law for the absorption and transmission measurement and has practical considerations [18]. Intensity of light is measured for two samples; one is reference and other is actual sensor, ratio of these is known as transmittance [19]. In the present approach for sucrose sensing transmittance is observed; based on the change in wavelength of the transmittance dip sensitivity is calculated for the proposed sensor. 2. Structure and fabrication process Based on the study of metamaterial structures, it is decided to present a simple ‘U’ shaped resonator to get the metamaterial effect, same as fabricated in Ref. [20]. Dimensions used for the fabrication of metamaterial are same as previous work. It is observed that the proposed dimension cannot be realized using simple optical masking thus highly sensitive lithography unit is used for fabrication of metamaterial surface. Silicon (Si) is used as the substrate because of it stability in E-bean lithography moreover, use of Si substrate also ensures the working of proposed surface in near infrared region. Gold (Au) is used to make ‘U’ shaped resonators on substrate because of its stability in outer environment. Fig. 1 has the schematic diagram of process steps followed while fabrication. For the fabrication, p-type Si wafer (Sigma–Aldrich) is used with standard RCA cleaning process in class-1000 clean room using chemical hood. After the cleaning, substrate is dried using nitrogen N2. After that, photo resist; poly methyl methacrylate (PMMA) is coated using spin coater for 40 s at 6000 rpm. The approximate thickness of photo resist is 195 nm. Thereafter, pattern with required dimensions is sketched in the interfacing software of E-beam lithography in class 100 clean room. Than sample is loaded in lithography unit for pattern to write. After the writing, pattern is developed using methyl isobutyl ketone (MIBK): isopropyl alcohol (IPA) solution in 1:3 ratio and dried using N2, baked for 2 s at hot plate. Au is deposited in E-beam evaporation unit for 60 nm thickness. Deposition is done at room temperature at 3e −7 Torr pressure. After the deposition, lift off of remaining resist is done by dipping the sample in acetone for 1–2 min and again dried using N 2 . Then after, characterization is done. First, ellipsometer characterization is done to measure the permittivity of the fabricated surface. Refractive index (R.I.) of fabricated metamaterial surface is calculated through the permittivity and permeability ( μ = 1), given in Fig. 2. It is observed from the figure that R.I. varies for wide values of the considered wavelength ranges from 100 to 1700 nm. It is seen that real value of R.I. is small enough after 800 nm wavelength and thus provide good transmission for the targeted application. Scanning Electron Microscopy (SEM) is done to observe the surface topology. Fig. 3 has the SEM result. It is seen that ‘U’ shaped resonators are successfully fabricated on the substrate with few deviations in geometrical dimensions. After that the nature of fabricated metamaterial surface is decided by calculating the plasma frequency (ωp) using Eq. (1);
ωp =
Neff c 2 2πc 2 = 2 ϵ 0 meff a ln(a/ r )
(1)
where Neff is the effective refractive index, ϵ 0 is vacuum permittivity, meff is effective mass, e is electron charge, c is speed of light in free space, a is lattice constant, and r is the radius of the ‘U’ ring long leg. It is observed that ωp is 1.7687 × 1015 Hz. When plasma 2
Optik - International Journal for Light and Electron Optics 205 (2020) 164276
S. Agarwal and Y.K. Prajapati
Fig. 2. Measured refractive index of fabricated surface.
Fig. 3. SEM result with fabricated dimensions.
frequency of fabricated surface is compared with the Au, i.e. 2.183 × 1015 Hz, it is depicted that ωp of metamaterial is lower than its constituent metal and thus, the fabricated surface in epsilon negative metamaterial (ENG) [21].
3. Results and discussion Fabricated single negative metamaterial surface is used in this study for sucrose sensing through spectroscopy approach. UV–Vis–NIR spectrometer (Shimadzu UV-3600 plus) is used for sensing for metamaterial surface as well as thin gold film sensor. First, UV–Vis–NIR unit is calibrated with the bare silicon wafer to set the reference, due to; substrate used in the sensor chip is silicon. After setting the reference, sensor chip and the solution put in the cuvette together for the sensing. Since, spectrometer is the closed unit, it provide an optimized and error-free environment for sensing compared to the simple SPR sensor (Open environment) [22]. To study the sensing ability of the proposed sensor, different concentrations of sucrose are sensed in de-ionized (DI) water. Table 1 has the R.I. vs. concentration data of the prepared solutions for sensing. Fig. 4 has the transmission spectrum for wavelength interrogation using Si/Au thin film sensor. From Fig. 4 it is depicted that as the concentration of sucrose in DI water is increased the transmission dip shifts toward higher wavelength. The maximum deflection is 60.2 nm in wavelength for the R.I. change of 0.12. Shift in the transmission wavelength is experienced due to the LSPR resonance. Since, LSPR is sensitive to the outer environment thus as the concentration of the sucrose solution is changed the resonance wavelength is also shifted towards the higher wavelength. Fig. 5 has the transmission spectrum using metamaterial surface in place of gold sensor. Fig. 5 depicts that the proposed metamaterial surface also follows the same nature, i.e. increased wavelength for higher sensing R.I. (nc ). It is also observed that the maximum shift in wavelength is 208.9 nm. There is a large difference in the wavelength shift between simple gold film sensor and metamaterial sensor, i.e. 148.7 nm. The larger shift in the wavelength is because; metamaterial Table 1 Refractive index vs. sucrose concentration calibrated values. Sucrose concentration (%)
0
11
24
35
45
55
65
R.I.
1.33
1.35
1.37
1.39
1.41
1.43
1.45
3
Optik - International Journal for Light and Electron Optics 205 (2020) 164276
S. Agarwal and Y.K. Prajapati
Fig. 4. Transmission spectrum for different sucrose concentration using Si/Au sensor.
Fig. 5. Transmission spectrum for different sucrose concentration using metamaterial surface.
surface experiences high resonance intensity due to its small particle size. From the results, it is clearly seen that the metamaterial sensor provides better sensing and dramatically improve the sensor sensitivity. It should be noted that the surface to volume ratio of fabricated metamaterial surface is much larger than the simple thin film, which ensures the better interaction of analyte with the sensing surface and in turn increases the adsorption of molecules for higher shift in the LSPR resonance wavelength and effective sensing. Fig. 6 displays the change in transmission dip in correspond to the sensing R.I., it is observed that for the proposed metamaterial surface, change in the transmission dip is much larger than the uniform thin film of Au. This feature of the proposed metamaterial surface ensures the better sensing ability compared to conventional thin film sensor.
Fig. 6. Change in transmission dip with the sensing R.I. 4
Optik - International Journal for Light and Electron Optics 205 (2020) 164276
S. Agarwal and Y.K. Prajapati
Table 2 Comparative analysis of different sensors. Parameter
Sensitivity (nm/RIU) Detection accuracy
Sensor Conventional SPR reflective sensor
Si/Au transmission sensor
Proposed metamaterial transmission sensor
970 [23] 2 × 10−5 [23]
501.66 0.979
1740.8 0.893
Table 2 has the comparative analysis of the sensitivity (Δλ /Δnc ) and detection accuracy (Δλ /BW ) for different sensors. Where BW is the half power beam width of transmission/reflection curve. From Table 2 it is observed metamaterial surface displayed quite better performance than the other simple sensors in terms of sensitivity and has comparable detection accuracy. Thus, it is true that metamaterial surface improves the performance of sensor and implementation in commercial sensor will definitely help in low volume bio-molecular detection. 4. Conclusion It is found from the current study that metamaterial surface exhibit more sensitivity than the simple thin film sensor because of the high surface to volume ratio. Moreover, it is observed that wavelength interrogating transmission sensor provided better sensitivity than the conventional surface plasmon resonance sensor. This study depicts that the proposed surface can be used for sucrose level detection efficiently through optical sensing method. Conflict of interest None declared. Acknowledgment The present work is partially supported by the Department of Science and Technology (DST), New Delhi, India under the fast track young scientist scheme no. SB/FTP/ETA-0478/2012. Authors are also grateful to the Center of Interdisciplinary Research (CIR), MNNIT Allahabad for the instrumental facility support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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