Introduction

Optical properties of materials are an attractive object for researchers and describe the response of the material when it interacts with light. The measurements of the optical properties of materials require a high degree of accuracy and precision for the advancement of optical technology. They have made a change in the life of the whole world in the field of medicine, sensors, astronomy, manufacturing, communication, etc. Such measurements include reflectance, transmittance, emittance, absorptance, and index of refraction [1]. Any of these quantities depends on geometry and polarization. Refractive index is a fundamental optical property of the material [2,3,4]. Refractive index of the medium is dependent on its chemical composition, and significantly can be influenced by temperature [5, 6]. It plays a pivotal role in light propagation in the medium and its reflection at an interface. This dimensionless constant is directly related to measurable quantities such as reflectance and absorption and defined as the ratio of the light speed in a vacuum to light phase velocity in the material. Among all the methods that were used to measure the refractive index, surface plasmon resonance (SPR) technique emerged and proved its efficiency in determining the refractive index value and detecting its local small changes in real time [7,8,9,10,11,12,13,14,15,16]. This is because the working principle of the SPR technique relies on the variation of the refractive index in the evanescent field at the sensing medium [17,18,19,20,21,22,23]. SPR as a refractive index–based sensing technique has attracted extensive attention over the past years due to its advantages of simplicity, cost effectiveness, and real-time and label-free detection [24,25,26,27,28,29,30,31,32,33,34]. These important advantages of SPR technology make it desirable for medical applications [35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55], control and safety of food [56, 57], environmental protection [58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74], and other uses. The refractive index is affected by the accumulation of mass on the surface of the metallic thin layer. The adsorption of target molecules on the thin film induces the variation of the refractive index. The changes in the thin film optical properties shift the SPR dip, such that the SPR angular shifts and the refractive index variation could be measured. Also, the concentration changes could be detected and the binding affinity could be determined [75]. In this study, dopamine, the important neurotransmitter that controls the functions of the human body, will be the detected target and its refractive index will be determined. This will be conducted using the gold thin film only first. To ensure the adsorption efficiency of DA molecules and to improve the thin film sensitivity to the variations in refractive index, the gold thin film has to be modified using nanomaterials. In recent years, graphene quantum dots (GQDs) have captured the interest due to their distinctive photoluminescence properties, remarkable physicochemical properties, good photostability, biocompatibility, and low toxicity [76,77,78,79,80]. The incorporation of GQDs with chitosan (CS) the biopolymer with many amine groups will increase DA adsorption on the thin film [48]. To the best of our knowledge, the utilization of SPR technique to study CS-GQDs thin film optical properties and determine its thickness before and after interaction with DA has not been conducted yet.

Materials and Methods

Preparation of Chemicals

Dopamine hydrochloride, GQDs (1 mg/ml), CS and acetic acid (assay ≥ 99.7%) were purchased from Sigma-Aldrich. Firstly, 200 mg of chitosan was taken and dissolved in 25 ml of 1% acetic acid, stirred and left at room temperature overnight to obtain a homogeneous CS solution [29]. After that, 2 ml of pure GQDs was added into 10 ml of CS solution with stirring to form CS-GQDs solution. Then, deionized water (DW) was utilized to dilute 1 M solution of DA to get very low levels down to 1 fM using the dilution formula (M1V1 = M2V2).

Preparation of Thin Films

The gold thin films were deposited on clean glass substrates of dimensions (24 mm × 24 mm × 0.1 mm) using SC7640 Sputter Coater. Then, the surface of gold thin layer on the glass substrate was uniformly covered by 0.5 ml of CS-GQDs mixture. And CS-GQDs thin films were deposited using spin coating technique at 2000 rpm during 30 s.

SPR Setup

To characterize the optical properties of the used solution and thin films, a custom-built SPR spectroscopy in Kretschmann configuration has been used. This homemade setup as shown in Fig. 1 contains a He–Ne laser at the wavelength (632.8 nm), a light chopper, a linear polarizer, a small pinhole, a prism (refractive index 1.77861), an optical rotating stage, a photodetector, and lock-in amplifier. SPR experiments were carried out for the gold thin film and CS-GQDs thin film that contact deionized water and DA with different concentrations. To record the reference signal, deionized water was inserted into the flow cell to be in contact with the gold thin film and then with the sensing layer one by one. After that, various levels of DA solution ranging from 1 to 1000 fM were inserted separately into the flow cell one after one to carry out the measurements by recording the intensity of the reflected laser light as a function of the incident angle.

Fig. 1
figure 1

SPR setup

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Fitting of Experimental Results to Theoretical

When plasmonic resonance occurs, and the partial transfer of the pumping light energy to the electron packages on the metal film takes place, surface plasmon wave with a transversally magnetic (TM) mode propagates freely along the metal thin film-dielectric interface. Its propagation depends strongly on the real and imaginary parts of the dielectric constants of these two media. The electric field component, E, is parallel to the incidence plane and perpendicular to the metal–dielectric interface while the magnetic field component, B, is perpendicular to incidence plane and lies in the plane of metal–dielectric interface [81]. Fresnel theory was used to investigate the interaction of the light and surface plasmons. In this work, the gold thin film and the sensor layer (CS-GQDs thin film) were sandwiched between the prism and the dielectric medium (DA solution) in Kretschmann configuration.

The magnitudes of the magnetic field and the electric field are related as shown in the following expression [82] :

$$B=\frac{E}{V}$$
(1)

where \(V\) is the wave speed and related to the speed of light in a vacuum \(c\) and the refractive index \(n\) by:

$$V=\frac{c }{n}$$
(2)

The constant \(c\) can be written as:

$$c=\frac{1}{\sqrt{{\varepsilon }_{0 }{\mu }_{0}}}$$
(3)

where \({\varepsilon }_{0}\) and \({\mu }_{0}\) are the permittivity and permeability of free space, respectively.

By combining Eqs. (1), (2), and (3), the magnitude of \(B\) can be written as:

$$B=\frac{E}{V}=\frac{n }{c}E=n\sqrt{{\varepsilon }_{0 }{\mu }_{0}}E$$
(4)

By using Eq. (4) and based on Fig. 2 where the boundary conditions are satisfied at the both interfaces, the magnetic and electric fields are related as follows:

$${B}_{a}={n}_{0}\sqrt{{\varepsilon }_{0 }{\mu }_{0}} \left({E}_{0}+{E}_{r1}\right)={n}_{1}\sqrt{{\varepsilon }_{0 }{\mu }_{0}} \left({E}_{t1}+{E}_{i1}\right)$$
(5)
$${B}_{b}={n}_{1}\sqrt{{\varepsilon }_{0 }{\mu }_{0}} \left({E}_{i2}+{E}_{r2}\right)={n}_{2}\sqrt{{\varepsilon }_{0 }{\mu }_{0}} {E}_{t1}$$
(6)
$${E}_{a}=\left({E}_{0}-{E}_{r1}\right) \cos{(\theta }_{0})=\left({E}_{t1}-{E}_{i1}\right)\cos{(\theta }_{t1})$$
(7)
$${E}_{b}=\left({E}_{i2}-{E}_{r2}\right) \cos{(\theta }_{t1})={E}_{t2}\cos{(\theta }_{t2})$$
(8)

where Er1 denotes the sum of all the multiple reflected beams from the thin film at the interface (a) as shown in Fig. 2, while Ei2 denoted the sum of all the multiple beams incident on the glass substrate at the interface (b), and so on.

Fig. 2
figure 2

Light reflection from a single layer with thickness d [84]

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When different layers are used, after light passes through them the phase changes. Taking this into consideration gives:

$$\begin{aligned}&{E}_{i1}={E}_{r2}{e}^{-i\delta}, {E}_{i2}={E}_{t1}{e}^{-i\delta}\\&{B}_{i1}={B}_{r2}{e}^{-i\delta}, {B}_{i2}={B}_{t1}{e}^{-i\delta}\end{aligned}$$
(9)

The relationships between E1, B1 and E2, B2 can be obtained as follows using Euler identities:

$${E}_{a}=\cos(\delta ){E}_{b}- \frac{i\sin(\delta )}{{\gamma}_{1}}{B}_{b}$$
(10)
$${B}_{a}=-{\gamma }_{1} i\sin\left(\delta \right){E}_{b}+\cos(\delta ){B}_{b}$$
(11)
$$\mathrm{where}\ {\gamma }_{1}=\frac{{n}_{1}}{\cos{(\theta }_{t1})}\sqrt{{\varepsilon }_{0 }{\mu }_{0}}$$
(12)

Equations (10) and (11) can be combined in matrix form as:

$$\left[\begin{array}{c}{E}_{a}\\ {B}_{a}\end{array}\right]=\left[\begin{array}{cc}\cos(\delta )& - \frac{i\sin(\delta )}{{\gamma }_{1}}\\ -{\gamma }_{1} i\sin\left(\delta \right)& \cos(\delta )\end{array}\right]\left[\begin{array}{c}{E}_{b}\\ {B}_{b}\end{array}\right]$$
(13)

Thus, in the case of single layer with thickness d, the transfer matrix will be as follows:

$${M}_{1}=\left[\begin{array}{cc}\cos\left(\delta \right)& - \frac{i\sin\left(\delta \right)}{{\gamma }_{1}}\\ -{\gamma }_{1} i\sin\left(\delta \right)& \cos\left(\delta \right)\end{array}\right]$$
(14)

where \(\delta\) represents the phase shift when the light passes through multilayers:

$$\delta =\frac{2\pi }{\lambda }d\ n_{1}\cos{(\theta }_{t1})$$
(15)

When different layers are used, the glass substrate at boundary b will be replaced by the interface of the thin film added. In this case, Eq. (13) is still valid but second transfer matrix is needed to relate Eb and Bb to Ec and Bc at the rear boundary of the second thin film. Thus, for a multilayer film with N number of layers,

$$\begin{bmatrix}E_a\\B_a\end{bmatrix}={\textstyle\prod_{i=1}^N}M_N\begin{bmatrix}E_N\\B_N\end{bmatrix}$$
(16)

For the entire multilayer films, the inclusive transfer matrix MT can be represented by:

$${M}_{T}=\left[\begin{array}{cc}{m}_{11}& {m}_{21}\\ {m}_{12}& {m}_{22}\end{array}\right]$$
(17)

where m11, m12, m21, and m22 denote the elements of the transfer matrix.

Using Eqs. (5), (6), (7), (8), and (16), we get

$$\left[\begin{array}{c}\left({E}_{0}-{E}_{r1}\right) \cos{(\theta }_{0})\\ {n}_{0}\sqrt{{\varepsilon }_{0 }{\mu }_{0}} \left({E}_{0}+{E}_{r1}\right)\end{array}\right]=\left[\begin{array}{cc}{m}_{11}& {m}_{21}\\ {m}_{12}& {m}_{22}\end{array}\right]\left[\begin{array}{c}{E}_{t2}\cos{(\theta }_{t2})\\ {n}_{2}\sqrt{{\varepsilon }_{0 }{\mu }_{0}} {E}_{t1}\end{array}\right]$$
(18)

By simplifying the previous equations and using the reflection coefficient r that defined as

$$r=\frac{{E}_{r1}}{{E}_{0}}$$
(19)

the reflection coefficient will be written in the following formula

$$r=\frac{{m}_{21}+{m}_{22}{\gamma }_{2 }-{m}_{11}{\gamma }_{0 }-{m}_{12}{\gamma }_{2 }{\gamma }_{0 }}{{m}_{21}+{m}_{22}{\gamma }_{2 }+{m}_{11}{\gamma }_{0 }+{m}_{12}{\gamma }_{2 }{\gamma }_{0}}$$
(20)

whereby the reflectivity R is

$$R=r{r}^{*}$$
(21)

All reflectance curves obtained using the gold thin films were analyzed using a developed fitting program based on the equations explained above to evaluate the optical properties and the thickness of the gold thin film as well as the optical properties of DA solution. This information then was used for a subsequent mathematical processing and analyzing of the optical properties and thickness of CS-GQDs thin film before and after interaction with DA solution of different concentrations based on the mentioned matrix method [83,84,85,86,87].

Results and Discussion

Characterization the Optical Properties of Gold Thin Film

At the first stage, the prefatory SPR experiment was conducted for gold thin films in contact with DW in order to determine the optical properties of the gold thin layer (the real and imaginary parts of refractive index, n and k respectively; in addition to the thickness of the thin film, d). The optical properties of the three gold layers used were obtained by fitting the reflectance curve as shown in Fig. 3. According to the fitted SPR signal, the refractive index values, n and k, for the first gold thin layer were (0.2164 ± 0.0001) and (3.6867 ± 0.0001) respectively, where the thickness, d, was determined as (53.67 ± 0.01) nm. The refractive index for DW water in room temperature is 1.3333 [88]. The optical properties of the other gold thin films used are listed in Table 1.

Fig. 3
figure 3

Fitted reflectance curves of three gold thin films exposed to DW

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Table 1 Refractive index and thickness of the gold thin films exposed to DA
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Characterization of DA Optical Properties

The SPR experiments were also conducted for all concentrations (ranging from 1 to 1000 fM) of DA solutions contacting the second gold thin film to determine the refractive index of the solutions. The experimental SPR curves were not shifted from the reference curve with increasing the DA level as shown in Fig. 4. These experimental curves were fitted with theoretical data for gold thin film in contact with DA solutions as shown in Fig. 5. Using the obtained thickness and refractive index of the second gold layer d (63.26) nm, n (0.2758), and k (3.8798), the fitting yielded the real part, n, and imaginary part, k, of DA solutions which were the same as the refractive index of DW as illustrated in Table 2. Also, the n, k, and d values of the bare gold film were not changed with increasing DA levels. These findings (Δθ = 0, and Δn = 0) confirmed that the adsorption of DA molecules on the film surface did not take place, and demonstrated the insensitivity of Au-based sensor towards DA. The dependence of the complex index of refraction on the concentration at room temperature was not obvious for the used concentrations of DA transparent solution. Here, because the analyte concentrations are sufficiently low (1 fM to 1000 fM), the refractive index remained constant [89].

Fig. 4
figure 4

The experimental SPR curves of the gold thin film exposed to different levels of DA

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Fig. 5
figure 5

Experimental and fitted reflectance curves related to the gold thin film exposed to DA solution for a 1 fM, b 10 fM, c 100 fM, and d 1000fM

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Table 2 Refractive index of both the gold film and DA solution after contact
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Characterization of CS-GQDs Optical Properties

To investigate and determine the optical properties of the CS-GQDs thin film, the prepared thin film was placed on one of the sides of the prism in the SPR spectroscopy. Above all, the SPR measurement was conducted by injecting deionized water into the flow cell to contact the CS-GQDs thin film. After recording the reflected beam, the results showed that when this sensing layer was used and contacted the DW, the plasmonic resonance occurred at higher angle compared with the case of the gold thin film, and the SPR dip was shifted significantly to the right. This is due to the difference in refractive index value and the thickness of the CS-GQDs thin film compared to the third gold thin film used. The refractive index values for both gold thin film and DA solution as well as the thickness of the gold thin film obtained were used to analyze the optical properties of the CS-GQDs thin film. After fitting the experimental reflectance curve using the obtained refractive index and thickness of the third gold layer n (0.1205), k (3.6920), and d (59.4) nm as shown in Fig. 6, the values of refractive index, n and k, for the sensing layer were (1.6990 ± 0.0001) and (0.1302 ± 0.0001) respectively, where the thickness, d, was determined (6.36 ± 0.01) nm. After that, the SPR experiment was continued with DA solutions. The inserted sample of DA into the system with concentration of 1 fM led to increase the resonance angle and shifted the SPR dip to the right remarkably as shown in Fig. 7. Continuing to gradually increase the concentration of DA up to 1000 fM gave the opportunity for more DA molecules to attach to the surface of CS-GQDS thin film and change its optical properties, and all this, in turn, led to more angular shift of the SPR signals to the right. The fitting showed that the real part n of the refractive index of CS-GQDs has increased from 1.6990 to 1.6999 and the thickness became 7.26 nm. The results showed that the values of n of the sensing layer CS-GQDs increased from 1.6990 to 1.7820 as the level of DA solutions increased from 0 to 1000 fM, while the k values decreased from 0.1302 to 0.1140. The thickness of the proposed active layer increased from 6.36 to 8.64 nm using this range of DA concentrations as shown in Table 3. These changes in the values of the refractive index and the enhancement of the sensor sensitivity towards DA might be due to the electrostatic interactions, hydrogen bonding, and strong π-π interaction of DA with the functional groups of the GQDs, and what reinforced this interaction is the electrostatic attraction between CS and DA [77, 90]. During these interactions, more molecules of DA were captured on the CS-GQDs thin layer and led to changes in its refractive index and thickness. This, in turn, shifted the SPR dips significantly to the right because the SPR signal is very sensitive towards any change in the surrounding thin films.

Fig. 6
figure 6

The experimental and fitted reflectance curves of the CS-GQDs thin layer exposed to DA solution for a 0 fM, b 1 fM, c 10 fM, d 100 fM, and e 1000 fM

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Fig. 7
figure 7

The experimental SPR curves of the CS-GQDs thin film exposed to different levels of DA

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Table 3 The real and imaginary parts of refractive index and the thickness of CS-GQDs thin film, Δθ, and Δn, for different levels of DA solutions
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Refractive Index Sensitivity of the SPR/CS-GQDs System

It is very important to evaluate the sensitivity of the utilized SPR technique. The refractive index sensitivity is defined as the ratio between the change of resonance angle, Δθ, and the variation of the real part refractive index, Δn [91,92,93,94,95] :

$$S=\Delta \theta /\Delta n$$
(22)

The ∆θ was calculated as the difference between the resonance angle of different concentrations of DA solution contacted CS-GQDs thin layer and the resonance angle of DW as shown in Table 3. The variation of real part refractive index, Δn, was calculated for CS-GQDs thin film that was exposed to all concentrations of DA. It is clear from Fig. 8(a) that both Δθ and Δn Increased gradually as a level of DA solution increased. This is because the binding of DA with different levels (0 to 1000 fM) to the surface of CS-GQDs thin film changed the refractive index of the film and this caused shifting of the SPR signals. As shown from Fig. 8(b), slope sensitivity of 10.186°/RIU (refractive index unit) was observed with a correlation coefficient, R2 of 0.973. These results demonstrated the high potential and efficiency of SPR technique to monitor the changes in refractive index of CS-GQDs thin film when it contacts 1 fM of DA.

Fig. 8
figure 8

a The variation of Δθ and Δn with DA concentrations, and b the refractive index sensitivity of the CS-GQDs thin film for DA sensing

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Conclusion

In the present study, SPR technique has been successfully developed and used to characterize the optical properties of DA solutions, gold thin films, and CS-GQDs thin films by theoretical fitting of the experimental SPR signals. The real part of the refractive index, n, and the imaginary part of the refractive index, k, for all gold thin films used were good as in agreement with the previous studies. The n and k values of DA solution for all concentrations were the same as deionized water. The values of n of the sensing layer CS-GQDs increased from 1.6990 to 1.7820 as the concentration of DA solutions increased, while the k values decreased from 0.1302 to 0.1140. The thickness of the proposed sensing layer increased from 6.36 nm to 8.64 nm. The results showed that the CS-GQDs thin layer has improved the sensitivity of the SPR sensor towards DA and the achieved sensitivity was 10.186°/RIU.