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Detection of adulterated honey by surface plasmon resonance optical sensor
Optik 168 (2018) 134–139
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Detection of adulterated honey by surface plasmon resonance optical sensor Nurul Hida Zainuddin a , Yap Wing Fen b,∗ , Ali Abdulkhaleq Alwahib a , Mohd Hanif Yaacob a , Noriah Bidin c , Nur Alia Sheh Omar b , Mohd Adzir Mahdi a a
Wireless and Photonics Network Research Centre, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia Functional Devices Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia c Laser Research Centre, Universiti Teknologi Malaysia, 81300 UTM, Johor Bahru, Johor Darul Takzim, Malaysia b
a r t i c l e
i n f o
Article history: Received 5 July 2017 Accepted 10 April 2018 Keywords: Adulterated honey Kretschmann configuration Surface plasmon resonance Sensor
a b s t r a c t In this study, surface plasmon resonance based on Kretschmann configuration is employed as an alternative method to detect adulteration of pure honey. The adulterated honey was prepared by diluting three types of sugar adulterants (fructose, glucose and sucrose) into pure honey. The concentration of each adulterant is tested from 2% until 10% adulteration with 0% adulteration represents the reference for pure honey. All the resonance angles of adulterated honey demonstrated similar behavior by shifting to smaller angle than pure honey. The measured sensitivities are 0.1266◦ /%, 0.1065◦ /%, and 0.0988◦ /% for fructose, glucose and sucrose adulterants, respectively. The shift of resonance angle as a function of adulterants concentration in pure honey was plotted with linear regression greater than 0.95 for all samples. The outcome has disclosed a real-time, rapid and non-destructive sensor to be promoted as well-developed honey sensor. © 2018 Elsevier GmbH. All rights reserved.
1. Introduction Honey is a complex natural food with a lot of benefits to human. It made up of various chemical components, the main constitutes are carbohydrates including glucose, sucrose, fructose and maltose which constitute approximately 77% of the average honey [1]. The biochemical variation in the composition of pure honey is supposed to guarantee the originality taste and benefits. It is strictly said that the content of fructose and glucose together in honey should be not less than 60 g/100 g (60% in mass ratio) and sucrose content should be not more than 5% on mass basis even though amount of each composition is varied with several nature factors. Moreover, the Codex Standard for honey also states that honey sold as such shall not have added to it any food ingredient, including food additives, nor shall any other addition be made other than honey [2]. Therefore, any biophysical changes directly from the addition of foreign substances will forge the honey as a nonauthentic food that leads to the taste quality issue. This situation usually happened because of the limited stock availability and high demand market that triggers the pure honey towards adulteration [3]. The issue of product integrity among the honey supplier community should be highlighted. The kind of this valuable food has to inherent quality of containing all
∗ Corresponding author. E-mail address: [email protected] (Y.W. Fen). https://doi.org/10.1016/j.ijleo.2018.04.048 0030-4026/© 2018 Elsevier GmbH. All rights reserved.
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the composition parts that meets the agreed specifications [4]. Thus, a rapid venture to explore and develop a device with specialty to detect honey fraud is significantly useful to prevent persistent problems on this issue. When it comes on sample adulteration detection, optical method based on fiber optic is commonly utilized. From previous reports, this technique has been employed to determine; adulteration in a liquid sample [5], fuel adulteration [6], and adulteration traces in coconut oil [7]. An alternative method for honey adulteration detection, optical phenomenon of surface plasmon resonance (SPR) is proposed as it particularly pleading in food safety area since they may perceive analytes in complex matrices [8]. Moreover, this technique has sparked significant interests in food analysis lately [9,10]. SPR detection was able to perform on a mixture of several toxin serotypes in 20% honey solution [11] and also the chloramphenicol and chloramphenicol glucuronide residues in honey [12]. Thus, SPR is expected to be able to detect any foreign substances addition in pure honey. The SPR signal is induced from the interaction between incident light and a metal layer by generating surface waves that propagate along the interface between these two media. Normally, the metal layer is placed on a piece of prism based on the Kretschmann model [13]. The angle of incident light is controlled through a rotation stage and the reflected light from the prism is detected. At a specific angle, when the surface wave is generated, the reflected light is significantly attenuated owing to the energy transfer mechanism [14]. This measurement can be determined from the signal dip that corresponds to the refractive index of target material on the metal layer. This SPR signal is very sensitive to the change of refractive index on the metal layer [15]. Since SPR is a non-destructive method, it is suitable for various applications in biology and chemistry fields [16]. Therefore, the material composition is not influenced by this technique and it becomes one of the options for sensing valuable natural liquid such as honey. In this study, the purity of honey based on sugar content is investigated using the prism-based SPR technique. In order to create adulteration conditions, other sugar substitutes like fructose, glucose and sucrose are purposely added to pure honey (standard sample). It is based on its capability to detect bio molecular interaction whether biomolecules adsorption or desorption to the sensing surface [17]. The interaction leads to the mass density changes with high sensitivity on the sensing surface which in fact the refractive index is actually influenced by surface mass density too [18,19]. It is also believed that SPR is able to perform as it widely and successfully used to study and characterize various application fields in bio and chemical sensing [20]. Besides that, the advantage of SPR sensor as a non-destructive method makes the usage of the valuable and limited pure honey is able to minimize [21]. To the best of our knowledge, SPR is yet applied to detection of adulterated honey. Hence in this work, an attempt has been made to detect sugar (fructose, glucose and sucrose) adulterants in pure honey using SPR method. The footprints obtained will be able to provide useful information and parameters to develop a device that can foretell the sugar in adulterated honey or sugar contents in honey itself. 2. Theory In SPR, a charge-density wave is corresponding to an electromagnetic wave. The wave propagates along the interface between two media with dielectric constants of opposite sign such as a metal and a dielectric. The propagation is characterized by the wave vector of evanescent field (Kev ) and wave vector of a surface plasmon (Ksp ) as follows [22]; Ksp =
v0 c
ng sin
(1)
where v0 is the frequency of incident light, ng is the refractive index of the dense medium (glass), is the angle of incident light and c is the speed of light in a vacuum [23], and; Ksp =
v0 c
εm n2s εm + n2s
(2)
where εm is the permittivity of metallic film and ns is the refractive index of dielectric medium. The field of surface plasmon wave is the transverse magnetic (TM) polarized and when the field vectors reach their maxima at the interface at a specific angle of incident, it decay evanescently into both media. This situation happens whenever evanescent wave of the incoming light is able to couple with the free oscillating electrons (plasmons) in the metal film corresponding to when Ksp = Kev . Thus, the surface plasmon becomes resonantly excited in this situation. 3. Experimental setup 3.1. Materials and sample preparation The authentic honey was purchased from commercial supplier and used without any purification. The standard sugar used were fructose and sucrose manufactured by Systerm Sdn Bhd while glucose was manufactured by Qrëc (Asia) Sdn Bhd. The prism employed in SPR setup was purchased from WTS Photonics Co., Ltd. For the glass slide (10 mm × 10 mm), it was acquired from Deckglaser. This size can be used directly on the custom liquid sample holder and ply on the prism sensing surface. Gold (Au) layer with a thickness of about 50 nm was sputter-deposited on the glass slide using a conventional sputtering system (Emitech K575X Turbo). Prior to that, the glass slide was cleaned using a precision dust removal equipment. In this work, only Au layer was used as the sensing surface without any intermediate layer [24].
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Fig. 1. Schematic diagram of Kretschmann-configuration SPR sensor.
The adulterated honey samples were prepared by deliberately adding the sugar solution (fructose, glucose and sucrose) into the pure honey (standard sample) with several concentrations (2%, 4%, 6%, 8%, and 10%). All the sugar was prepared by diluting them using distilled water at a concentration of 50%. All the pure and adulterated of honey stocks were diluted with deionized water in 1:5 ratio. The standardized matrices were purposely formed to ensure their compatibility with the sensing configuration. The refractive index of all the adulterated honey solutions was measured using pocket refractometer (Atago, USA). 3.2. SPR setup The computer equipped SPR set up with Kretschmann configuration of sensor block was employed as depicted in Fig. 1. The setup consists of a light source, a chopper, a polarizer, a prism, a rotational stage with motor, and a custom sample holder [25]. The light source utilized is a monochromatic helium-neon (He-Ne) laser with a peak wavelength of 632.8 nm, a beam width of 1 mm2 and an emission power of 1 mW. The SPR sensor was coupled to a commercial glass prism (H-ZF62) with refractive index equal to n = 1.91413. The glass slide sputtered with Au layer was thumbed against the prism surface with an index matching oil was applied in between these substrates. This was to minimize the refractive index variations between substrates to support continuous proceeding of the incident light. This assembled optic was placed on a rotating stage to control the angle of incident light. The polarized of He-Ne laser irradiation was directed at the prism sensor surface for plasmon resonance activation. As the result, the reflection intensity of the radiation that reaches the prism-sample was detected, and its reflectivity was calculated where reflectivity is a ratio between the incident and reflected light intensity The system for the optical signal detection consists of a photo-detector, a lock-in amplifier, and a MatLab software was developed to record and store the measured SPR signal. The detected SPR was displayed on a control panel in real-time. 4. Results and discussion 4.1. SPR curve of pure and adulterated honey The main goal of this work is to focus on SPR sensing capability counter adulteration problem of pure honey. Firstly, the SPR experiment was carried out for pure honey. The SPR reflectivity as a function of incident angle was recorded. When a stable baseline of resonance angle for pure honey was obtained, the gold surface was then ready for binding tests with adulterated honey. All the samples were injected one after another from low to high concentration of adulteration towards the sensing surface. The SPR sensor response towards concentration variation of adulterated honey with three types of sugar adulterant, i.e. fructose, glucose and sucrose are shown in Fig. 2(a)–(c), respectively. The reflectance curves for all samples show their own narrow dips. The resonance dip is the result of surface plasmon (SP) excitation that involves the energy transfer from the incident photons energy [26]. The dips reveal their resonance angles as shown in Fig. 2. The resonance angle of pure honey was measured at 52.739◦ . The interaction of adulterated honey on the Au surface led to a decrease of the resonance angle, where the resonance angle shifted to the left. Based on these findings, the shift of minimum resonance angles of adulterated honey from its pure can be calculated. For fructose-adulterated honey (Fig. 2a), the resonance angles shift are 0.34031, 0.51017, 0.84931, 1.01858, and 1.18763◦ for 2, 4, 6, 8, and 10% adulteration percentage, respectively. On the other hand, the glucose-adulterated honey (Fig. 2b) revealed the shift of resonance angles for 2, 4, 6, 8, and 10% for adulteration percentage of 0.17025, 0.34031, 0.51017, 0.84931 and 1.01858◦ , respectively. While for sucrose-adulterated honey (Fig. 2c), the resonance angles shift are 0.17025, 0.34031, 0.51017, 0.84931, and 1.18763◦ for 2, 4, 6, 8, and 10% adulteration percentage, respectively. The sensor shows promising results by having several explicit angle differences by only physically testing on sugar adulterated honey. The shift between the resonance angles of adulterated honey implies the detection capability of addition sugar adulterants into pure honey.
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(a)
Fig. 2. SPR curves for (a) fructose (b) glucose, and (c) sucrose adulterated honey.
4.2. Effect of refractive index on SPR curve In fact, the sensing surface of the sensor is very sensitive towards refractive index of their surrounding medium. It is capable to monitor the changes of the surface properties. It is envisaged that the significant shift of the resonance angles for different samples is mainly due to the difference of refractive index [27]. The refractive index of medium has been
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Fig. 3. Refractive indices against adulterants concentration.
Fig. 4. The shift of resonance angle as a function of adulterant concentration in pure honey. The solid lines represent the linear fitted relationship.
confirmed which is in the range of 1.351 to 1.353. A plot of refractive index against adulterants concentration is illustrated in Fig. 3. The pure honey, which is represented by 0% of adulterant volume, shows the highest refractive index, i.e. 1.3533. The increment of adulterants percentage by 2% until 10% into the pure honey leads to the refractive index decrement. Refractive index differences can typically be clarified by differences in the structure of pure honey after inclusion with different type and concentration of sugar adulterants. Such adulteration activity triggers to the changes of pure honey nature. In can be concluded that higher refractive index of the pure honey shifts the spectrum curve towards bigger angle. Hence, by relating the Eq. (1), the reflectance curve is proved to behave based on the modification in the dielectric medium (honey sample). The adulterant is bound to Au sensing layer that shows the presence of bio-molecular interactions between Au ions and adulterant molecules. 4.3. Sensitivity of the sensor The experimental data were fitted by a linear curve in order to study the response characteristics of sensor as shown in Fig. 4. The sensitivity of the proposed sensor is unveiled from derivation of linear fitting expression for every sample variation. Based on the findings, the sensitivity for fructose, sucrose and glucose adulterants are 0.1266◦ /%, 0.1065◦ /%, and 0.0988◦ /%, respectively. From the result, each sugar adulterants give the difference sensitivity values. The correlation coefficients, R2 are 0.9782, 0.9583, and 0.9806 for fructose, glucose and sucrose adulterants, respectively [28]. 5. Conclusion A SPR sensor based on Kretschmann configuration was utilized as a device to detect honey adulteration through distinctive amount of adulterant substances. The 0% adulteration which is the pure honey exhibits the highest resonance angle. All the resonance angles of adulterated honey demonstrated similar behavior by shifting to smaller angle as compared to pure honey. The shift of resonance angle increases linearly with the increasing of adulterant concentration. It is proven as pure honey has the highest refractive index than adulterated honey. The sensor sensitivity obtained is 0.1266◦ /%, 0.1065◦ /%, and 0.0988◦ /% for fructose, sucrose and glucose adulterants, respectively. The good linear relation for shift or resonance angle as
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function of adulterants concentration in pure honey are found with more than 95% for all adulterants. All the information obtained can be used as notable parameter to develop a reliable sensor to differentiate the honey either it is pure or impure. Acknowledgments The authors like to express their thanks to Malaysian Ministry of Higher Education and Malaysian Ministry of Science, Technology and Innovation (MOSTI) through FRGS grant (FRGS/2/2014/TK03/UPM/01/1) and Science Fund (06-01-04SF2016/5450767) for the financial support in this project. References [1] R. Frew, K. McComba, L. Croudis, D. Clark, R. Van Hale, Modified sugar adulteration test applied to New Zealand honey, Food Chem. 141 (2013) 4127–4131. [2] Codex Alimentarius, Revised Codex Standard for Honey. (No. CODEX STAN 12-1981), Rev.2, 2001, pp. 1–7. [3] P.M.D. Silva, C. Gauche, L.V. Gonzaga, A.C.O. Costa, R. Fett, Honey: chemical composition, stability and authenticity, Food Chem. 196 (2016) 309–323. [4] L. Manning, J.M. 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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Detection of adulterated honey by surface plasmon resonance optical sensor Nurul Hida Zainuddin a , Yap Wing Fen b,∗ , Ali Abdulkhaleq Alwahib a , Mohd Hanif Yaacob a , Noriah Bidin c , Nur Alia Sheh Omar b , Mohd Adzir Mahdi a a
Wireless and Photonics Network Research Centre, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia Functional Devices Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia c Laser Research Centre, Universiti Teknologi Malaysia, 81300 UTM, Johor Bahru, Johor Darul Takzim, Malaysia b
a r t i c l e
i n f o
Article history: Received 5 July 2017 Accepted 10 April 2018 Keywords: Adulterated honey Kretschmann configuration Surface plasmon resonance Sensor
a b s t r a c t In this study, surface plasmon resonance based on Kretschmann configuration is employed as an alternative method to detect adulteration of pure honey. The adulterated honey was prepared by diluting three types of sugar adulterants (fructose, glucose and sucrose) into pure honey. The concentration of each adulterant is tested from 2% until 10% adulteration with 0% adulteration represents the reference for pure honey. All the resonance angles of adulterated honey demonstrated similar behavior by shifting to smaller angle than pure honey. The measured sensitivities are 0.1266◦ /%, 0.1065◦ /%, and 0.0988◦ /% for fructose, glucose and sucrose adulterants, respectively. The shift of resonance angle as a function of adulterants concentration in pure honey was plotted with linear regression greater than 0.95 for all samples. The outcome has disclosed a real-time, rapid and non-destructive sensor to be promoted as well-developed honey sensor. © 2018 Elsevier GmbH. All rights reserved.
1. Introduction Honey is a complex natural food with a lot of benefits to human. It made up of various chemical components, the main constitutes are carbohydrates including glucose, sucrose, fructose and maltose which constitute approximately 77% of the average honey [1]. The biochemical variation in the composition of pure honey is supposed to guarantee the originality taste and benefits. It is strictly said that the content of fructose and glucose together in honey should be not less than 60 g/100 g (60% in mass ratio) and sucrose content should be not more than 5% on mass basis even though amount of each composition is varied with several nature factors. Moreover, the Codex Standard for honey also states that honey sold as such shall not have added to it any food ingredient, including food additives, nor shall any other addition be made other than honey [2]. Therefore, any biophysical changes directly from the addition of foreign substances will forge the honey as a nonauthentic food that leads to the taste quality issue. This situation usually happened because of the limited stock availability and high demand market that triggers the pure honey towards adulteration [3]. The issue of product integrity among the honey supplier community should be highlighted. The kind of this valuable food has to inherent quality of containing all
∗ Corresponding author. E-mail address: [email protected] (Y.W. Fen). https://doi.org/10.1016/j.ijleo.2018.04.048 0030-4026/© 2018 Elsevier GmbH. All rights reserved.
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135
the composition parts that meets the agreed specifications [4]. Thus, a rapid venture to explore and develop a device with specialty to detect honey fraud is significantly useful to prevent persistent problems on this issue. When it comes on sample adulteration detection, optical method based on fiber optic is commonly utilized. From previous reports, this technique has been employed to determine; adulteration in a liquid sample [5], fuel adulteration [6], and adulteration traces in coconut oil [7]. An alternative method for honey adulteration detection, optical phenomenon of surface plasmon resonance (SPR) is proposed as it particularly pleading in food safety area since they may perceive analytes in complex matrices [8]. Moreover, this technique has sparked significant interests in food analysis lately [9,10]. SPR detection was able to perform on a mixture of several toxin serotypes in 20% honey solution [11] and also the chloramphenicol and chloramphenicol glucuronide residues in honey [12]. Thus, SPR is expected to be able to detect any foreign substances addition in pure honey. The SPR signal is induced from the interaction between incident light and a metal layer by generating surface waves that propagate along the interface between these two media. Normally, the metal layer is placed on a piece of prism based on the Kretschmann model [13]. The angle of incident light is controlled through a rotation stage and the reflected light from the prism is detected. At a specific angle, when the surface wave is generated, the reflected light is significantly attenuated owing to the energy transfer mechanism [14]. This measurement can be determined from the signal dip that corresponds to the refractive index of target material on the metal layer. This SPR signal is very sensitive to the change of refractive index on the metal layer [15]. Since SPR is a non-destructive method, it is suitable for various applications in biology and chemistry fields [16]. Therefore, the material composition is not influenced by this technique and it becomes one of the options for sensing valuable natural liquid such as honey. In this study, the purity of honey based on sugar content is investigated using the prism-based SPR technique. In order to create adulteration conditions, other sugar substitutes like fructose, glucose and sucrose are purposely added to pure honey (standard sample). It is based on its capability to detect bio molecular interaction whether biomolecules adsorption or desorption to the sensing surface [17]. The interaction leads to the mass density changes with high sensitivity on the sensing surface which in fact the refractive index is actually influenced by surface mass density too [18,19]. It is also believed that SPR is able to perform as it widely and successfully used to study and characterize various application fields in bio and chemical sensing [20]. Besides that, the advantage of SPR sensor as a non-destructive method makes the usage of the valuable and limited pure honey is able to minimize [21]. To the best of our knowledge, SPR is yet applied to detection of adulterated honey. Hence in this work, an attempt has been made to detect sugar (fructose, glucose and sucrose) adulterants in pure honey using SPR method. The footprints obtained will be able to provide useful information and parameters to develop a device that can foretell the sugar in adulterated honey or sugar contents in honey itself. 2. Theory In SPR, a charge-density wave is corresponding to an electromagnetic wave. The wave propagates along the interface between two media with dielectric constants of opposite sign such as a metal and a dielectric. The propagation is characterized by the wave vector of evanescent field (Kev ) and wave vector of a surface plasmon (Ksp ) as follows [22]; Ksp =
v0 c
ng sin
(1)
where v0 is the frequency of incident light, ng is the refractive index of the dense medium (glass), is the angle of incident light and c is the speed of light in a vacuum [23], and; Ksp =
v0 c
εm n2s εm + n2s
(2)
where εm is the permittivity of metallic film and ns is the refractive index of dielectric medium. The field of surface plasmon wave is the transverse magnetic (TM) polarized and when the field vectors reach their maxima at the interface at a specific angle of incident, it decay evanescently into both media. This situation happens whenever evanescent wave of the incoming light is able to couple with the free oscillating electrons (plasmons) in the metal film corresponding to when Ksp = Kev . Thus, the surface plasmon becomes resonantly excited in this situation. 3. Experimental setup 3.1. Materials and sample preparation The authentic honey was purchased from commercial supplier and used without any purification. The standard sugar used were fructose and sucrose manufactured by Systerm Sdn Bhd while glucose was manufactured by Qrëc (Asia) Sdn Bhd. The prism employed in SPR setup was purchased from WTS Photonics Co., Ltd. For the glass slide (10 mm × 10 mm), it was acquired from Deckglaser. This size can be used directly on the custom liquid sample holder and ply on the prism sensing surface. Gold (Au) layer with a thickness of about 50 nm was sputter-deposited on the glass slide using a conventional sputtering system (Emitech K575X Turbo). Prior to that, the glass slide was cleaned using a precision dust removal equipment. In this work, only Au layer was used as the sensing surface without any intermediate layer [24].
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Fig. 1. Schematic diagram of Kretschmann-configuration SPR sensor.
The adulterated honey samples were prepared by deliberately adding the sugar solution (fructose, glucose and sucrose) into the pure honey (standard sample) with several concentrations (2%, 4%, 6%, 8%, and 10%). All the sugar was prepared by diluting them using distilled water at a concentration of 50%. All the pure and adulterated of honey stocks were diluted with deionized water in 1:5 ratio. The standardized matrices were purposely formed to ensure their compatibility with the sensing configuration. The refractive index of all the adulterated honey solutions was measured using pocket refractometer (Atago, USA). 3.2. SPR setup The computer equipped SPR set up with Kretschmann configuration of sensor block was employed as depicted in Fig. 1. The setup consists of a light source, a chopper, a polarizer, a prism, a rotational stage with motor, and a custom sample holder [25]. The light source utilized is a monochromatic helium-neon (He-Ne) laser with a peak wavelength of 632.8 nm, a beam width of 1 mm2 and an emission power of 1 mW. The SPR sensor was coupled to a commercial glass prism (H-ZF62) with refractive index equal to n = 1.91413. The glass slide sputtered with Au layer was thumbed against the prism surface with an index matching oil was applied in between these substrates. This was to minimize the refractive index variations between substrates to support continuous proceeding of the incident light. This assembled optic was placed on a rotating stage to control the angle of incident light. The polarized of He-Ne laser irradiation was directed at the prism sensor surface for plasmon resonance activation. As the result, the reflection intensity of the radiation that reaches the prism-sample was detected, and its reflectivity was calculated where reflectivity is a ratio between the incident and reflected light intensity The system for the optical signal detection consists of a photo-detector, a lock-in amplifier, and a MatLab software was developed to record and store the measured SPR signal. The detected SPR was displayed on a control panel in real-time. 4. Results and discussion 4.1. SPR curve of pure and adulterated honey The main goal of this work is to focus on SPR sensing capability counter adulteration problem of pure honey. Firstly, the SPR experiment was carried out for pure honey. The SPR reflectivity as a function of incident angle was recorded. When a stable baseline of resonance angle for pure honey was obtained, the gold surface was then ready for binding tests with adulterated honey. All the samples were injected one after another from low to high concentration of adulteration towards the sensing surface. The SPR sensor response towards concentration variation of adulterated honey with three types of sugar adulterant, i.e. fructose, glucose and sucrose are shown in Fig. 2(a)–(c), respectively. The reflectance curves for all samples show their own narrow dips. The resonance dip is the result of surface plasmon (SP) excitation that involves the energy transfer from the incident photons energy [26]. The dips reveal their resonance angles as shown in Fig. 2. The resonance angle of pure honey was measured at 52.739◦ . The interaction of adulterated honey on the Au surface led to a decrease of the resonance angle, where the resonance angle shifted to the left. Based on these findings, the shift of minimum resonance angles of adulterated honey from its pure can be calculated. For fructose-adulterated honey (Fig. 2a), the resonance angles shift are 0.34031, 0.51017, 0.84931, 1.01858, and 1.18763◦ for 2, 4, 6, 8, and 10% adulteration percentage, respectively. On the other hand, the glucose-adulterated honey (Fig. 2b) revealed the shift of resonance angles for 2, 4, 6, 8, and 10% for adulteration percentage of 0.17025, 0.34031, 0.51017, 0.84931 and 1.01858◦ , respectively. While for sucrose-adulterated honey (Fig. 2c), the resonance angles shift are 0.17025, 0.34031, 0.51017, 0.84931, and 1.18763◦ for 2, 4, 6, 8, and 10% adulteration percentage, respectively. The sensor shows promising results by having several explicit angle differences by only physically testing on sugar adulterated honey. The shift between the resonance angles of adulterated honey implies the detection capability of addition sugar adulterants into pure honey.
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(a)
Fig. 2. SPR curves for (a) fructose (b) glucose, and (c) sucrose adulterated honey.
4.2. Effect of refractive index on SPR curve In fact, the sensing surface of the sensor is very sensitive towards refractive index of their surrounding medium. It is capable to monitor the changes of the surface properties. It is envisaged that the significant shift of the resonance angles for different samples is mainly due to the difference of refractive index [27]. The refractive index of medium has been
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Fig. 3. Refractive indices against adulterants concentration.
Fig. 4. The shift of resonance angle as a function of adulterant concentration in pure honey. The solid lines represent the linear fitted relationship.
confirmed which is in the range of 1.351 to 1.353. A plot of refractive index against adulterants concentration is illustrated in Fig. 3. The pure honey, which is represented by 0% of adulterant volume, shows the highest refractive index, i.e. 1.3533. The increment of adulterants percentage by 2% until 10% into the pure honey leads to the refractive index decrement. Refractive index differences can typically be clarified by differences in the structure of pure honey after inclusion with different type and concentration of sugar adulterants. Such adulteration activity triggers to the changes of pure honey nature. In can be concluded that higher refractive index of the pure honey shifts the spectrum curve towards bigger angle. Hence, by relating the Eq. (1), the reflectance curve is proved to behave based on the modification in the dielectric medium (honey sample). The adulterant is bound to Au sensing layer that shows the presence of bio-molecular interactions between Au ions and adulterant molecules. 4.3. Sensitivity of the sensor The experimental data were fitted by a linear curve in order to study the response characteristics of sensor as shown in Fig. 4. The sensitivity of the proposed sensor is unveiled from derivation of linear fitting expression for every sample variation. Based on the findings, the sensitivity for fructose, sucrose and glucose adulterants are 0.1266◦ /%, 0.1065◦ /%, and 0.0988◦ /%, respectively. From the result, each sugar adulterants give the difference sensitivity values. The correlation coefficients, R2 are 0.9782, 0.9583, and 0.9806 for fructose, glucose and sucrose adulterants, respectively [28]. 5. Conclusion A SPR sensor based on Kretschmann configuration was utilized as a device to detect honey adulteration through distinctive amount of adulterant substances. The 0% adulteration which is the pure honey exhibits the highest resonance angle. All the resonance angles of adulterated honey demonstrated similar behavior by shifting to smaller angle as compared to pure honey. The shift of resonance angle increases linearly with the increasing of adulterant concentration. It is proven as pure honey has the highest refractive index than adulterated honey. The sensor sensitivity obtained is 0.1266◦ /%, 0.1065◦ /%, and 0.0988◦ /% for fructose, sucrose and glucose adulterants, respectively. The good linear relation for shift or resonance angle as
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