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Abstract
In this study, the incorporation between gold modified-tyrosinase (Tyr) enzyme based graphene oxide (GO) thin film with surface plasmon resonance (SPR) technique has been developed for the detection of phenol. SPR signal for the thin film contacted with phenol solution was monitored using SPR technique. From the SPR curve, sensitivity, full width at half maximum (FWHM), detection accuracy (DA) and signal-to-noise ratio (SNR) have been analyzed. The sensor produces a linear response for phenol up to 100 µM with sensitivity of 0.00193° µM−1. Next, it can be observed that deionized water has the lowest FWHM, with a value of 1.87° and also the highest value of DA. Besides, the SNR of the SPR signal was proportional to the phenol concentrations. Furthermore, the surface morphology of the modified thin film after exposed with phenol solution observed using atomic force microscopy showed a lot of sharp peaks compared to the image before in contact with phenol proved the interaction between the thin film and phenol.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Phenolic compounds, which also can contributed as the pollutants, are widely distributed in the environment as by-products contaminant from industrial wastewater such as germicides, pesticides, textiles, petrochemical products and pharmaceutical industries [1–9]. Due to their inherent toxicity and persistence in the environment, it can be considered as a potential hazard to living organism including human if absorbed [10–14]. Some of these compounds and their derivatives can be extremely harmful to humans as it can cause damage to the nervous and respiratory systems, kidney and blood [15,16]. High levels of phenols have been demonstrated to be of detrimental effects on animal health and many phenols are listed as dangerous substances or priority pollutants by the European Commission (EC) and the United States Environmental Protection Agency (USEPA) [17,18]. Thus, the detection of the phenolic compounds has become an area of growing interest over the past decade [19].
Some methods such as gas chromatographic, liquid chromatographic (HPLC), and spectrophotometric methods have been introduced for the detection of phenolic compounds [20–23]. However, their disadvantages such as highly cost, time-consuming, demanding sample pretreatment, and require experienced technicians to operate greatly limit their practical applications [24]. Hence, electrochemical technique of redox reaction of different compounds in chemical reactions was introduced that have complemented the previous technique, offering high specificity, good sensitivity, easy to manufacture and low-cost assay [25]. However, the detection of phenolic compounds using electrochemical biosensor faces two challenges that might obstruct their use at industrial scales. Firstly, reaction by-products lead to fouling of electrodes, hence limiting their accuracy and lifespan [26]. Next, the similarity of co-existing phenol isomers’ chemical structures, for example, catechol, result in overlapping redox peaks that makes simultaneous detection of these species challenging [27].
These disadvantages have indicated for a need to more sophisticated analytical techniques. Surface plasmon resonance (SPR) is believed to be a valuable optical sensor in order to overcome the disadvantages from the previous methods [28]. As a rising detection technique, it also has its own advantages such as real-time detection, simple sample preparation, high sensitivity and label-free method [29–31]. SPR design has been introduced in some approaches including metal-insulator-metal waveguide [32], plasmonic perfect absorbers [33,34], noble metal nanoparticles [35], and also metal layer [36]. Metals were extensively used in some techniques including SPR such as silver (Ag) [37–39], gold (Au) [40], Au-Ag bi-metal [41,42], and copper (Cu) [43]. In this work, Au layer was used for the incorporation with SPR technique since it is chemically inert to solutions.
SPR is an optical process in which light satisfying a resonance condition excites a charge-density wave propagating along the interface between a metal and dielectric material by monochromatic and p-polarized light beam [44–47]. A sharp shadow (SPR phenomenon) can be observed by the reduction of the intensity of the reflected light at a certain incident angle due to the resonance energy occurs between the incident beam and surface plasmon wave [48–50]. In addition, when the incident light is totally reflected, the electromagnetic field component penetrates an evanescent wave into the dielectric medium [51]. The evanescent wave will propagate along the interface and decays exponentially with distance normal to the interface [28]. This wave is very sensitive to the changes in the refractive index of a medium adjacent to the metal or active layer [52–55]. The changes in refractive index may result in a shift of the resonance angle at a constant wavelength [56]. Hence, SPR has been utilized in a variety of biosensor application. However, SPR is not sensitive enough towards the low concentration of phenol because of the similarity of refractive index [57]. In order to improve the performance and response of the sensor, a thin layer of suitable material has to be deposited on the gold thin film [58].
Various types of biosensor architecture for the detection of phenol based on oxidative enzymes including tyrosinase (Tyr) [59,60], laccase (Lac) [61,62], horseradish peroxidase (HRP) [63–65] have been reported. The ability of Tyr to undergo electron-transfer reaction without need of additional cofactors in oxidation of phenolic compounds in the presence of oxygen has drawn a lot of interest compared to other enzymes [66,67]. Tyr has the ability to catalyze two different enzymatic activities which is the hydroxylation of monophenols into diphenols and the oxidation of o-diphenols to o-quinones [68–73]. Tyr also has broad substrate specificity that can be an advantage in the determination of phenolic compounds such as phenol, catechol, cresol, dopamine and epinephrine in food, environment and clinical diagnosis [74]. However, general application of Tyr biosensor for the detection of phenolic pollutants experienced some drawbacks such as the low enzymatic catalysis activity, low enzyme stability and insufficient limit of detection [75–77]. Hence, the selection of suitable solid supports for the Tyr enzyme is the main key to overcome the drawbacks [78]. Therefore, graphene oxide (GO) was used as a support to Tyr that may contribute to the increase of the absorption properties of the thin film [79–81]. Additionally, GO also have large surface area and strong π‒π stacking interactions, making it the ideal candidate as coating materials for tracing aromatic pollutants in the environment [82–84].
To prior of our knowledge, the gold modified-tyrosinase enzyme based graphene oxide thin film for the sensing application of phenol using surface plasmon resonance are yet applied. Thus, in this article, gold modified-tyrosinase enzyme based graphene oxide thin film will be incorporated with surface plasmon resonance technique for the detection of phenol.
2. Materials and method
2.1 Reagents and materials
The graphene oxide (4 mg/mL) was purchased from Graphenea (Cambridge, MA, USA). Phenol (M = 94.11 g/mol) and tyrosinase from mushroom (EC 1.14.18.1, activity of 2687 units/mg, CAS 9002-10-2) were purchased from Sigma-Aldrich.
2.2 Preparation of chemical
Firstly, Tyr solution of 10 mg/mL concentration was firstly diluted into concentration of 5 mg/mL by adding phosphate buffered saline (PBS) by using M1V1 = M2V2 formula. Next, 1 mL of the Tyr solution was added to 5 mL of 4 mg/mL GO solution forming modified-Tyr based GO mixture with concentration of 4.167 mg/mL. Then, the mixture was stirred for 15 minutes.
For the preparation of phenol base solution, 9.411 mg of phenol was diluted with deionized water to form 1.0 M phenol solution. Next, the base solution was diluted with deionized water also using the dilution formula and the prepared solutions were 1, 5, 10, 20, 40, 60, 80 and 100 µM [85].
2.3 Preparation of thin film
Substrate glass with area of 24 mm × 24 mm × 0.1 mm were purchased from Menzel-Glaser. Firstly, the substrate was cleaned with acetone to remove fingerprint marks and dirt on the glass surface. Next, the glass slips were coated with gold by using SC7640 Sputter Coater for 67 seconds to produce a thin gold layer with thickness of 50 nm. To disperse the modified-Tyr based GO mixture uniformly on top of the gold layer, spin coating technique was used. About 0.5 mL of the mixture was placed on the gold layer surface of the glass slip and was spun at 3000 rpm at 30 seconds using spin coater P-6708D.
2.4 Instrumental
An optical spectroscopy was designed in the laboratory to test the capability of the thin film to detect phenol based on surface plasmon resonance principle. Figure 1 shows the schematic diagram of the SPR instrument setup. The SPR measurement was carried out by measuring the reflected He-Ne laser beam (632.8 nm, 5 mW) as a function of incident angle. The SPR setup consists of a He-Ne laser, an optical stage driven by a stepper motor with a resolution of 0.001° (Newport MM 3000), a polarizer and an optical chopper (SR 540). The He-Ne laser beam that was p-polarized which emits transverse mode (TM) of the laser was incident on the prism (refractive index of 1.77861) [86–89]. The glass cover slip attached to one side of the prism using a refractive index matching liquid with a hollow cell was attached to the gold or gold modified Tyr-based GO thin film surface containing the phenol solution. The prism and the hollow cell were mounted on a rotating plate to control the angle of the incident light. The rotating plate was driven by a stepper motor with a resolution of 0.001° (Newport MM 3000). The incident light from He-Ne laser beam pass through the prism and hits the gold layer of the thin film to generate surface plasmon waves at the interface. When evanescent wave is generated due to the change in refractive index of the medium adjacent to the active layer at a specific angle of incident light, SPR response was induced. The reflected beam produced was then detected by a large area photodiode and then processed by the lock-in amplifier (SR 530) [90].
For atomic force microscopy (AFM) measurement, Q-scope 250, Quesant Instrument Corporation was used in Scan Asyst mode. The Scan Asyst mode is a Peak Force Tapping technique that enables to observe the surface morphology in the high-resolution images using single touch scanning of the thin film.
3. Result and discussion
3.1 SPR signal for phenol on gold single layer
Prior to the measurement, a preliminary SPR test was carried out for gold single layer in contact with deionized water. Approximately 1 mL of deionized water was injected into the cell to ensure that it was in contact with the gold layer thin film. The resonance angle obtained from the SPR curve of gold layer in contact with deionized water is 53.53°. The SPR experiment was then continued using phenol solution of different concentrations. Figure 2 shows the SPR curves for phenol solution ranging from 0 µM (deionized water) to 100 µM in contact with gold layer. Then, the resonance angles for all different concentrations of phenol solution were compared as shown in Fig. 3. From the figure, it can be observed that the resonance angle for all concentrations of phenol and deionized water are almost similar. It is probably due to the small amount of phenol exists in the low concentration solutions to be adsorbed to the gold surface and as the concentration of phenol increases from 1 to 100 µM, only a slight increment in the number of phenol that were attached to gold surface [92]. Furthermore, SPR is also very sensitive to the refractive index of materials that are adjacent to gold thin film [52]. However, the refractive index for low concentration of phenol is similar [93].
Fig. 2. SPR curves for gold layer in contact with different concentrations of phenol solution (0-100 µM).
3.2 SPR signal for phenol using gold modified-Tyr based GO thin film
The SPR experiment for the detection of phenol was then continued by replacing gold layer thin film with modified-Tyr based GO coated on the top of gold surface thin film. It was carried out using deionized water and phenol solution of different concentrations which are 1, 5, 10, 20, 40, 60, 80 and 100 µM as shown in Fig. 4. It was injected one after another into the cell. The resonance angles for all concentrations were determined from the SPR curves. The resonance angle for gold modified-Tyr based GO thin film in contact with deionized water is slightly higher than gold thin film with a value of 53.84°. This is probably due to the increasing of refractive index of the thin film when the modified-Tyr based GO was coated on top of the gold thin film. It was then used as the baseline signal to compare the results on different concentration of phenol solution.
Fig. 4. SPR curves for Au modified-Tyr based GO thin film in contact with different concentrations of phenol solution (0-100 µM).
Next, as the phenol solution increases from 1 µM to 100 µM, there is some further increment in the resonance angle. It can be observed that the lowest detectable value of the modified thin film is at 1 µM phenol solution as its resonance angle only has a slight shift from the resonance angle of deionized water. Any concentration below 1 µM produced the same resonance angle as compared to the resonance angle of deionized water. In other words, this sensor unable to detect phenol solution with concentration lower than 1 µM. In this work, quantitation limit is defined as the lowest concentration that can be distinguished by the sensor from its baseline signal. Taken together, this result suggests that the quantitation limit of the gold modified-Tyr based GO thin film is at 1 µM of phenol solution.
It is also believed that the modified-Tyr based GO coated on the top of gold surface plays an important role in the detection of phenol. The observation can be attributed by the interaction between phenol and gold modified-Tyr based GO thin film leading to the formation of enzyme-substrate complex when phenol and tyrosinase undergo enzymatic reactions. The reaction products arising from the breakdown of the enzyme-substrate complex will release free enzyme that will change the refractive index of the sensing layer [94]. On the other hand, graphene oxide plays an important role in increasing the optical absorption in both the visible and infrared ranges [93]. In summary, these results show that the sensitivity is enhanced in the presence of the modified-Tyr based GO thin film as compared to the only gold layer thin film.
3.3 Sensitivity of gold modified-Tyr based GO thin film
One of the significant findings from this study is that the modified-Tyr based GO coated on top of gold thin film has a great ability for the detection of phenol. The shift of resonance angle (Δθ) has been introduced as a parameter to measure the sensitivity of sensor [95]. Sensitivity can be defined as the ratio between the Δθ and measured target concentration (C). Δθ has been calculated by finding the difference between the resonance angle of different concentration of phenol solution and deionized water (0 µM represents deionized water). Table 1 shows the resonance angle and shift of resonance angle for gold modified-Tyr based GO thin film in contact with different concentrations of phenol solution.
Table 1. The resonance angle and shift of resonance angle for Au modified-Tyr based GO thin film in contact with different concentrations of phenol solution
There are no changes in the resonance angle (Δθ) for phenol concentrations in contact with gold thin film as shown in Fig. 3. As the modified-Tyr based GO was coated on the top of gold thin film, the resonance angle increases. The values of shift of resonance angle have been calculated and recorded. Figure 5 shows the comparison between the shifts of resonance angle for gold thin film and gold modified-Tyr based GO thin film in contact with different concentration of phenol solution in the range from 1 µM to 100 µM, respectively.
Fig. 5. Resonance angle shift of Au and Au modified-Tyr based GO thin film in contact with different concentrations of phenol solution.
The figure clearly shows the sensitivity enhancement of the gold modified-Tyr based GO thin film towards phenol in comparison with the gold single layer. The sensitivity of the modified thin film can be determined from the linear progression analysis in Fig. 5, that produces a slope of 0.00193 with a correlation coefficient of 0.99581. Hence, this result indicates that the optical sensor using gold modified-tyrosinase enzyme based graphene oxide thin film for the detection of phenol has a sensitivity of 0.00193° µM−1 and also has a quantitation limit of 1 µM.
3.4 Full width at half maximum of SPR
Full width at half maximum (FWHM) can be defined as the angular width of the SPR curve for the half value of the maximum reflectance [96,97]. Theoretically, the FWHM of the SPR curves should be as small as possible in order to minimize the error in determining the reflectance [98]. FWHM can be determined by measuring the width of the reflectivity curve corresponding to the half value of the maximum reflectance as shown in Fig. 6. The FWHM of the SPR curve were calculated for all concentrations of phenol. The FWHM value for gold modified-Tyr based GO thin film is the lowest for deionized water with 1.87°. The value then increased to 1.98°, 2.08° and 2.13° for 1 µM, 5 µM and 10 µM phenol solution, respectively. The highest FWHM value is at 2.21° for both 20 µM and 40 µM concentration. It might be due to the intensified internal loss due to the increase in total thickness of sensor [99]. The FWHM values are lower from 60 to 100 µM compared with 20 µM and 40 µM concentration that might be contributed by the functional groups in modified-Tyr based GO layer which will reduced the scattering of free electrons [100].
3.5 Detection accuracy and signal-to-noise ratio
Detection accuracy (DA) is inversely proportional to the FWHM. The DA depends on the width of SPR curve. Hence, SPR curve with smaller FWHM will produced higher DA. The DA of the SPR sensor for phenol sensing using gold modified-Tyr based GO thin film was shown in Fig. 7. The deionized water records the highest DA for gold modified-Tyr based GO SPR sensor as the FWHM value is the lowest.
The signal-to-noise ratio (SNR) is another important parameter that can be obtained from SPR sensor apart from the sensitivity. The SNR can be known as the basic figure-of-merit (FOM) of SPR and was calculated by multiplying the shift of resonance angle, Δθ with DA [101]. The SNR of the SPR sensor for phenol sensing using gold modified-Tyr based GO thin film was shown in Fig. 8. As the phenol concentration increases, the SNR of the SPR sensor also increases although the DA variation is uncertain.
Furthermore, it can be observed that the SNR plotted in Fig. 8 is almost similar to the plotting of resonance angle shifts of gold modified-Tyr based GO thin film in Fig. 5. This proved that the Δθ has bigger effect in determining the value of SNR compared to the DA [102]. Table 2 show the summary of shift of resonance angle, FWHM, DA, and SNR for SPR sensor of gold modified-Tyr based GO thin film for the detection of phenol concentration ranging from 0 µM to 100 µM.
Table 2. Resonance angle shift, FWHM, DA, and SNR data for gold modified-Tyr based GO SPR sensor for different concentrations of phenol solution
3.6 Surface morphology
The microscopic characteristic of gold modified-Tyr based GO thin film was identified using atomic force microscopy (AFM). AFM images of gold modified-Tyr based GO thin film before and after in contact with phenol solution were shown in Figs. 9(a) and 9(b), respectively.
Fig. 9. AFM images of gold modified-Tyr based GO thin film (a) before and (b) after in contact with phenol solution.
Figure 9(a) shows the image of irregular edges with an average height of 4.1 nm. Meanwhile, Fig. 9(b) shows a lot of sharp peaks with an average height 3.1 nm. The numbers of nanoneedles also increase after the gold modified-Tyr based GO thin film was treated with phenol. This is probably due to the phenol has already covered the surface of the thin film. This AFM results confirmed the interaction of phenol with the gold modified-Tyr based GO thin film that already proven by the previous SPR results. The AFM results also revealed that the thickness of the gold modified-Tyr based GO layer to be approximately 60 nm.
4. Conclusion
In this work, gold modified-tyrosinase (Tyr) enzyme based graphene oxide (GO) thin film has been successfully developed and incorporated with surface plasmon resonance (SPR) technique for sensing application of phenol. The binding interactions of different concentrations of phenol ranging from 1 until 100 µM with bare gold film and the modified-Tyr based GO coated on gold surface were monitored using SPR system. The sensitivity of the SPR system was enhanced in the presence of modified-Tyr based GO film as compared to the bare gold film. The shifts of resonance angle shows a linear relationship for gold modified-Tyr based GO thin film in contact with phenol solution with a sensitivity of 0.00193° µM−1, and it can detect phenol solution as low as 1 µM. Other parameters that were calculated from the SPR curve are full width at half maximum (FWHM), detection accuracy (DA) and signal-to-noise ratio (SNR). It can be observed that deionized water has the lowest value of FWHM, which is 1.87° and the DA value of it was also the highest. As the phenol concentrations increase, the SNR of the SPR signal were also increase. Besides, atomic force microscopy (AFM) also confirmed the interaction between gold modified-Tyr based GO and phenol as the AFM image of gold modified-Tyr based GO thin film after in contact with phenol solution showed a lot of sharp peaks compared to the image before in contact with phenol. Thus, these results confirmed that the gold modified-Tyr enzyme based GO thin film has high potential sensing for phenol using surface plasmon resonance technique.
Funding
Ministry of Higher Education, Malaysia through the Fundamental Research Grant Scheme (FRGS) (FRGS/1/2019/STG02/UPM/02/1); Putra Grant Universiti Putra Malaysia.
Acknowledgments
The laboratory facilities provided by the Institute of Advanced Technology, Department of Physics, Department of Chemistry, Universiti Putra Malaysia, are acknowledged.
Disclosures
The authors declare no conflicts of interest.
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