Surface plasmon resonance prism coupler for enhanced circular dichroism sensing

Quoc-Hung Phan; Yu-Lung Lo; Chih-Ling Huang

doi:10.1364/OE.24.012812

1. Introduction

Circular dichroism (CD), i.e., the difference in absorption on excitation of the left- and right-hand circularly polarized components of the incident light, is an important optical property for characterizing the structures of proteins [1–5] and chiral metal nanoparticle assemblies [6]. CD measurement is a fast and straightforward analysis technique. However, the performance of conventional spectral analysis-based CD measurement methods is highly sensitive to instrumental noise, the optical path length, the protein concentration, and the instrument resolution [7]. Consequently, various alternative methods for CD measurement have been proposed. Liao and Lo [8] presented a hybrid model for decoupling the circular birefringence, depolarization and CD properties of ferrofluidic samples placed in an external magnetic field orientated perpendicularly to the illumination beam. Nehira et al. [9] used a fluorescence-based CD detection method to detect structural changes in the tertiary structure of metmyoglobin, which the conventional CD measurement could not do. Nagatomo et al. [10] investigated the potential for using the near-UV CD spectra and UV resonance Raman Spectra of L-tryptophan derivatives as structural markers for proteins. Various studies have used plasmonic CD techniques to analyze the structures of nanorod and protein assemblies [11,12]. Govorov [13] showed theoretically that the CD spectrum of chiral molecules is significantly enhanced by a plasmon excitation effect when close to metallic nano particles. Fernandez et al. [14] proposed a Mueller matrix transmission ellipsometry method for measuring the CD properties of chiral nematic films of cellulose nanocrystals loaded with plasmonic nano particles. However, the performance of the plasmonic CD measurement methods proposed in [13,14] is dependent on the type of host media, the type and size of the nano particles, and the density of the measured sample. Furthermore, the methods all require the use of a broadband visible wavelength and thus involve a complicated experimental setup and procedure.

In a recent series of studies, the current group presented a polarization scanning ellipsometry technique based on a rotated coordinate system for characterizing the effective parameters of thin-film materials [15], the optical properties of voltage-driven TNLC cells [16], and the effective parameters of turbid media [17]. The polarization scanning ellipsometry method provides a versatile and high accuracy technique for characterizing physical/optical properties of turbid media. The same group also proposed a highly-sensitive gas detection method based on the polarization scanning ellipsometry and a surface plasmon resonance (SPR) prism coupler [18]. It was shown that the coupler resulted in a detection sensitivity of 7.5 x 10⁴ deg/RIU (refractive index units) and a sensing resolution of 2x10⁻⁷ RIU for refractive indices in the range of 1.0~0.001.

In this study, a SPR prism coupler is combined with polarization scanning ellipsometry to obtain highly-sensitive measurements of the CD properties of scattering and non-scattering media. An analytical model is proposed for extracting the CD and degree of polarization (DOP) properties of the sample from the Stokes vectors obtained using a single visible wavelength. Simulations are performed to confirm the validity of the analytical model and to investigate the sensitivity of the CD measurements to changes in the refractive index of the sensed media under different polarization scanning angles. The real-world practicality of the proposed method is then investigated by comparing the experimental results obtained for the CD properties of chlorophyllin samples with concentrations ranging from 20 to 100 μg/ml with the simulation results. To the best of the authors’ knowledge, this study represents the first reported attempt in the literature to perform CD sensing using a SPR-enhanced polarization scanning ellipsometry technique.

2. SPR prism coupler

Figure 1(a) presents a schematic illustration of the SPR prism coupler used in the present study. As shown, the coupler comprises a half-ball lens, a Cr-Au isotropic thin-film layer, and aTa₂O₅ anisotropic layer. The half-ball lens couples the incident polarized light into the isotropic film and provides total internal reflection. Meanwhile, the isotropic and anisotropic layers enhance the sensing performance by manipulating the incident polarized light and inducing the SPR condition at the sensed interface. Notably, the Cr in the isotropic thin-film layer serves only to improve the bonding strength between the half-ball lens and the Au film. In other words, it plays no role in inducing SPR. In the present study, the half-ball lens was made of BK7 glass with a refractive index of n₀ = 1.517. In addition, the isotropic layer had a thickness of d₁ = 22 nm and a refractive index of n₁ = 0.36-2.9i, while the anisotropic layer had a thickness of d₂ = 27 nm and principal refractive indices of n₂₁ = 1.637, n₂₂ = 1.449 and n₂₃ = 1.589 [19]. (Note that the refractive indices of the Cr-Au and Ta₂O₅ layers were determined at a working wavelength of 632.8 nm.) It is also noted that the thickness of the anisotropic layer and isotropic layer were chosen to achieve a highest sensitivity of the SPR sensor based on the calculation shown in Section 3. It is found that the thickness of the anisotropic layer causes a little effect to the sensitivity of the SPR sensor, but the thickness of isotropic layer (metal layer) plays an important role. In addition, by adding the anisotropic layer in design, six ellipsometric parameters can be used for curve fitting, and it is better than by using only two ellipsometric parameters (a case of isotropic layer) for curve fitting. The three Euler angles of the Ta₂O₅ anisotropic layer were given as Φ_e = 0°, Ψ_e = 15°and θ_e = 90°, respectively [18]. The resonance angle of the prism coupler was found to be around 60° at a wavelength of 632.8 nm, as shown in Fig. 1(b). The minimum p-wave reflective coefficient r_pp was less than 0.3 at the incident angle of 60°. Note that r_pp was calculated by the Berreman’s 4x4 matrix method for a three-layer thin-film structure of isotropic/anisotropic/CD-sample [20].

Fig. 1 SPR optical sensor: (a) schematic illustration of sensor structure, (b) resonance angle determination.

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3. Analytical model for extracting CD properties

An optical system can be described by the matrix formulation S = MS′, where S is the Stokes vector of the output light, M is the 4x4 Mueller matrix of the system, and S′ is the Stokes vector of the input light. The general form of this relation is given as

[\begin{matrix} S_{1} \\ S_{2} \\ S_{3} \\ S_{4} \end{matrix}] = [\begin{matrix} M_{11} & M_{12} & M_{13} & M_{14} \\ M_{21} & M_{22} & M_{23} & M_{24} \\ M_{31} & M_{32} & M_{33} & M_{34} \\ M_{41} & M_{42} & M_{43} & M_{44} \end{matrix}] [\begin{matrix} {S^{'}}_{0} \\ {S^{'}}_{1} \\ {S^{'}}_{2} \\ {S^{'}}_{3} \end{matrix}]

The optical system shown in Fig. 1(a) can be modeled as

M = M_{C D} M_{R}

where M_CD is the Muller matrix of the CD sample and M_R is the Muller matrix of reflectance of the prism coupler. For a CD sample with circular dichroism R, M_CD has the form [17]

M_{C D} = [\begin{matrix} 1 + R^{2} & 0 & 0 & 2 R \\ 0 & 1 - R^{2} & 0 & 0 \\ 0 & 0 & 1 - R^{2} & 0 \\ 2 R & 0 & 0 & 1 + R^{2} \end{matrix}]

Note that R is equal to, where r_R and r_L are the absorptions of right-and left-hand circular polarization light, respectively. For the three-layer thin-film structure shown in Fig. 1(a) (i.e., isotropic layer/anisotropic layer/CD sample), the Mueller matrix of reflectance is determined by Berreman’s 4x4 matrix method and has the form [20]

M_{R} = [\begin{matrix} m_{11} & m_{12} & 0 & 0 \\ m_{12} & m_{11} & 0 & 0 \\ 0 & 0 & m_{33} & m_{34} \\ 0 & 0 & - m_{34} & m_{33} \end{matrix}]

It is noted that the elements in M_R are a function of the incident angle (θ_i), the refractive indices and thicknesses of the isotropic and anisotropic thin-film layers (n₀, n₁, n_21, n_22, n_23, d₁, d₂), and the refractive index of the CD sample (N).

Substituting Eqs. (3) and (4) into Eq. (2), the optical model of the system shown in Fig. 1(a) is obtained as

[\begin{matrix} S_{1} \\ S_{2} \\ S_{3} \\ S_{4} \end{matrix}] = M_{C D} M_{R} [\begin{matrix} {S^{'}}_{0} \\ {S^{'}}_{1} \\ {S^{'}}_{2} \\ {S^{'}}_{3} \end{matrix}] = [\begin{matrix} m_{11} (R^{2} + 1) & m_{12} (R^{2} + 1) & - 2 R m_{34} & 2 R m_{33} \\ - m_{12} (R^{2} - 1) & - m_{11} (R^{2} - 1) & 0 & 0 \\ 0 & 0 & - m_{33} (R^{2} - 1) & - m_{34} (R^{2} - 1) \\ 2 R m_{11} & 2 R m_{12} & - m_{34} (R^{2} + 1) & m_{33} (R^{2} + 1) \end{matrix}] [\begin{matrix} {S^{'}}_{0} \\ {S^{'}}_{1} \\ {S^{'}}_{2} \\ {S^{'}}_{3} \end{matrix}]

For a CD sample with scattering effects, Eq. (2) can be rewritten as follows:

M = M_{C D} M_{D} M_{R}

Where M_D is the depolarization Mueller matrix and has the form [21]

M_{D} = [\begin{matrix} 1 & p_{1} & p_{2} & p_{3} \\ 0 & e_{1} & 0 & 0 \\ 0 & 0 & e_{2} & 0 \\ 0 & 0 & 0 & e_{3} \end{matrix}]

As described in [21], the diagonal matrix elements e₁, e₂ and e₃ in Eq. (7) are the average lengths of the transformed axes due to depolarization. In addition, the top row elements p_1, p₂ and p₃ are asymmetric in the amounts of depolarization of the oppositely-directed input vectors. The degree of depolarization (DOP) is obtained as

Δ = 1 - \sqrt{\frac{e_{1}^{2} + e_{2}^{2} + e_{3}^{2}}{3}}, 0 \leq Δ \leq 1

Thus, Eq. (6) can be expressed as

[\begin{matrix} S_{0} \\ S_{1} \\ S_{2} \\ S_{3} \end{matrix}] = M_{C D} M_{D} M_{R} [\begin{matrix} {S^{'}}_{0} \\ {S^{'}}_{1} \\ {S^{'}}_{2} \\ {S^{'}}_{3} \end{matrix}] = [\begin{matrix} m_{11} (R^{2} + 1) + m_{12} p_{1} (R^{2} + 1) & m_{12} (R^{2} + 1) + m_{11} p_{1} (R^{2} + 1) & M_{13} & M_{14} \\ - e_{1} m_{12} (R^{2} - 1) & - e_{1} m_{11} (R^{2} - 1) & 0 & 0 \\ 0 & 0 & - e_{2} m_{33} (R^{2} - 1) & - e_{2} m_{34} (R^{2} - 1) \\ 2 R m_{11} + 2 R p_{1} m_{12} & 2 R m_{12} + 2 R p_{1} m_{11} & M_{34} & M_{44} \end{matrix}] [\begin{matrix} {S^{'}}_{0} \\ {S^{'}}_{1} \\ {S^{'}}_{2} \\ {S^{'}}_{3} \end{matrix}]

where

M_{13} = m_{33} p_{2} (R^{2} + 1) - m_{34} [2 {Re}_{3} + p_{3} (R^{2} + 1)]

M_{14} = m_{34} p_{2} (R^{2} + 1) + m_{33} [2 {Re}_{3} + p_{3} (R^{2} + 1)]

M_{34} = 2 R m_{33} p_{2} - m_{34} [2 R p_{3} + e_{3} (R^{2} + 1)]

M_{44} = 2 R m_{34} p_{2} + m_{33} [2 R p_{3} + e_{3} (R^{2} + 1)]

The use of four input lights (namely, three linear polarization lights (0°, 45°and 90°) and one right-hand circular polarization light) yields a sufficient number of equations to determine R from Eq. (5) and R and the depolarization matrix elements from Eq. (9). The Stokes vectors of the input lights are given as follows: S′_0° = [1,1,0,0]^T, S′_45° = [1,0,1,0]^T, S′_90° = [1,-1,0,0]^T and S′_R = [1,0,0,1]^T. Thus, R can be obtained from either Eq. (5) or Eq. (9) as

R = \frac{S_{0 °} (3)}{S_{0 °} (0)} \pm \sqrt{{[\frac{S_{0 °} (3)}{S_{0 °} (0)}]}^{2} - 1}, - 1 \leq R \leq 1

Note that the +/− terms in Eq. (14) are used for the case of R<0 and R>0, respectively. In addition, the elements of the depolarization Mueller matrix are obtained as

e_{1} = \frac{1}{(R^{2} - 1)} x \frac{- [S_{0 °} (1) + S_{90 °} (1)]}{2 m_{12}}

e_{2} = \frac{1}{(R^{2} - 1)} x \frac{2 S_{45 °} (2) - [S_{0 °} (2) + S_{90 °} (2)]}{2 m_{33}}

e_{3} = \frac{4 R^{2} - {(R^{2} + 1)}^{2}}{2 R Y - X (R^{2} + 1)}

p_{1} = \frac{1}{2 R} x \frac{[S_{0 °} (3) + S_{90 °} (3)]}{m_{12}} - \frac{m_{11}}{m_{12}}

p_{2} = \frac{1}{2 R} (\frac{M_{43} + X m_{34}}{m_{33}})

p_{3} = \frac{X - e_{3} (R^{2} + 1)}{2 R}

where

X = \frac{{2 S_{R} (3) - [S_{0 °} (3) + S_{90 °} (3)]} m_{33} - m_{34} {2 S_{45 °} (3) - [S_{0 °} (3) + S_{90 °} (3)]}}{m_{33}^{2} + m_{34}^{2}}

Y = \frac{m_{33} {2 S_{R} (0) - [S_{0 °} (0) + S_{90 °} (0)]} - m_{34} {2 S_{45 °} (0) - [S_{0 °} (0) + S_{90 °} (0)]}}{m_{33}^{2} + m_{34}^{2}}

Having obtained e₁, e₂ and e₃ from Eqs. (15), (16) and (17), respectively, the DOP can be computed from Eq. (8).

4. Validity of analytical model

The validity of the analytical model derived above was investigated by comparing the value of R obtained from Eq. (14) with the known value (in the range of −1to1) inserted into the sample matrix M_CD in Eq. (3). In performing the simulation, the input R values were inserted into Eq. (3) and then M_CD was substituted into Eq. (5) or (9) in order to obtain the output Stokes vectors regarding to the input linear polarization light at 0°, 45°, 90°, and right circular polarization light. Consequently, the extracted R value by means of Eq. (14) was compared to the input value, and those data were illustrated in Fig. 2(a). Note that the refractive index of the CD sample was set as 1.33; the incident angle θ_i was set equal to the SPR angle of 60°; the thickness and the refractive index of isotropic and anisotropic layer of the SPR prism coupler were set as Section 3. These parameters provide an efficient means to determine all elements m₁₁-m₃₃ of M_R matrix in Eq. (4). The value of p₁, p₂, and p₃ in Eq. (7) was set randomly as 0.02, 0.13 and 0.06, respectively. Also, the value of e₁, e₂, and e₃ in Eq. (7) was set randomly as 0.68, 0.57, and 0.25, respectively, given the Δ value of 0.467. Figure 2(a) shows the extracted value of R for each input value. It is observed that a good agreement exists between the two values in every case. Figure 2(b) shows the error bar of extracted value of R with given input value R = 0.552 and it is conducted by given the accuracy of commercial Stokes polarimeter of ± 0.5% (PAX5710, Thorlabs Co.). As shown, the corresponding extracted value of R was found to deviate from the input value only by ± 5x10⁻⁴. The deviations of the extracted values from the input values were the same for every case of R values over the full range and equal to ± 5x10⁻⁴. In other words, the ability of the proposed model to extract R over the full range is confirmed.

Fig. 2 (a) Comparison of extracted value of R with known input value, (b) Error bar of extracted value of R with given input value 0.55.

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Further simulations were performed to evaluate the accuracy of the proposed analytical model in extracting the DOP property (Δ) of scattering CD samples. In performing the simulations, known values of Δ in the range of 0 to 1 were inserted into Eq. (6) and were then extracted using Eq. (8). As in the previous simulations, the refractive index of the sample was set as 1.33; the incident angle was 60°; R = 0.552; the thickness and the refractive index of isotropic and anisotropic layer of the SPR prism coupler were set as Section 3. These parameters provide an efficient means to determine all elements m₁₁-m₃₃ of M_R matrix in Eq. (4). The value of p₁, p₂, and p₃ in Eq. (7) was set randomly as 0.02, 0.13 and 0.06, respectively. As shown in Fig. 3(a), a good agreement was obtained between the extracted value and the input value of Δ in every case. Figure 3(b) shows the error bar of extracted value of Δ with given input value of 0.467 and it is also conducted by given the accuracy of commercial Stokes polarimeter of ± 0.5% (PAX5710, Thorlabs Co.). The corresponding extracted value of Δ was found to deviate from the input values only by ± 5x10⁻⁴. Note that the deviations of the extracted values from the input values were the same for every cased of Δ values over the full range and equal to ± 5x10⁴. Thus, the ability of the proposed model to extract the Δ of CD samples over the full range of 0 to 1 is confirmed.

Fig. 3 (a) Comparison of extracted value of Δ with known input value, (b) Error bar of extracted value of Δ with given input value of 0.467.

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5. Sensitivity of CD measurements to chlorophyllin concentration

Simulations were performed to investigate the sensitivity of the extracted CD value to changes in the concentration of chlorophyllin sodium copper salt samples (referred to hereafter simply as chlorophyllin samples) in aqueous solution. Chlorophyllin is a common photosynthetic pigment and can be obtained from spinach leaves or grass and the CD property of chlorophyllin was tested by Houssier in [22]. Chlorophyllin has the molecular structure shown in Fig. 4 (provided by Sigma-Aldrich Co. LLC). The macrocycles of chlorophyllin is made with four pyrroles and the symmetrical substituent placed at ring carbon C-7, C-8, and this molecular system can make a high absorbance and induce the CD properties [23]. In performing the simulations, changes in the concentration of the chlorophyllin sample were modeled by changing the value of the refractive index N. The CD value of each sample was then calculated using Eq. (14) for different scanning angles θ in the range of 0~180°.

Fig. 4 Molecular structure of chlorophyllin.

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As shown in Fig. 5(a), the extracted CD value is particularly sensitive to changes in the refractive index at scanning angles of 35° and 150°. Furthermore, Fig. 5(b) shows that the CD value varies approximately linearly over the refractive index range of 1.32 to 1.36 given a fixed scanning angle of 150°. Assuming the output Stokes vectors are obtained using a commercial Stokes polarimeter (PAX5710, Thorlabs Co.) with an accuracy of ± 0.5%, the estimated resolution of the extracted CD values is in the order of 10⁻⁵ RIU. (Note that the results presented in Fig. 5(a) show that the sensitivity of the extraction results is the same given a scanning angle of 35°or 150°). Thus, only the data obtained for a fixed scanning angle of 150° were considered in evaluating the results presented in Fig. 5(b).

Fig. 5 (a) Variation of extracted CD value with polarization scanning angle given refractive index of chlorophyllin sample in range of 1.32 to 1.36 (step size of 0.004). Note that the arrow shows the direction of increasing refractive index. (b) Variation of extracted CD value with refractive index given scanning angle of θ = 150°. Estimated resolution of CD measurement is equal to 10⁻⁵ RIU.

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Figure 6(a) shows the variation of the extracted CD value with the refractive index in the range of 1.3100 to 1.3118 given scanning angles of 0~180°. In this case, the maximum sensitivity is obtained at scanning angles of 45° and 135°. As shown in Fig. 6(b), the CD value varies approximately linearly over the refractive index range of 1.3100 to 1.3118. Assuming the output Stokes vector measurements to have an accuracy of ± 0.5%, the estimated resolution of the extracted CD value is in the order of 10⁻⁶ RIU. Note that the results presented in Fig. 6(b) relate to a scanning angle of 135° since the CD response is identical for both scanning angles.)

Fig. 6 (a) Variation of extracted CD value with polarization scanning angle given refractive index of chlorophyllin sample in range of 1.3100 to 1.3118 (step size of 0.0002). Note that the arrow shows the direction of increasing refractive index. (b) Variation of extracted CD value with refractive index given scanning angle of θ = 135°. Estimated resolution of CD measurement is equal to 10⁻⁶ RIU.

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Table 1 compares the estimated measurement resolution of the proposed SPR-based sensing method with that of existing plasmonic CD sensing mechanisms. Anker et al. [24] proposed a localized SPR (LSPR) sensor combining with surface-enhanced Raman spectroscopy technique for biological sensing. The proposed method measured the sensitivity of Reyleigh scattering spectrum of samples to the changes of the refractive index. It is seen that the resolution of the current study is one order higher than that of presented in [24]. Huang et al. [25] also used LSPR nanosensor based for biosensing by measuring the sensitivity of extinction spectra to the changes of the refractive index. Lee et al. [26] used plasmonic Au nanodisk based sensor for on-chip DNA detection to measure the extinction spectrum to the changes of the refractive index. Pineider et al. [27] and He et al. [28] used a magnetic field as a source of modulation for active plasmonics on colloidal gold nanoparticle and Au fan-shaped chiral plasmonic nanostructure for CD sensing, respectively. It is seen that the resolution of the current study is consistent with those proposed in [25–28].

Table 1. Biosensing performance of existing plasmonic-based methods.

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6. Experimental setup and results

Figure 7 presents a schematic illustration of the proposed PSR-based scanning polarization ellipsometry system. As shown, the major items of equipment include a He-Ne laser source (SL 02/2, SIOS Co., central wavelength 632.8nm), a polarizer (GTH5M, Thorlab Co.) to produce linear polarized light, a quarter-wave plate (QWP0-633-04-4-R10, CVI Co.) to convert the linear polarized light into circular polarized light, a second polarizer (GTH5M, Thorlab Co.) set to a scanning angle in the range of θ = 0~180°, and a neutral density filter (NDC-100C-2, Oneset Co.) and power detector (8842A, OPHIR Co.) to calibrate the intensity of the input polarization light. Following the calibration process, the power detector was removed from the experimental setup, and the light emerging from the neutral density filter was reflected on the SPR sensor and detected by a commercial Stokes polarimeter (PAX5710, Thorlabs Co.). The linear scanning polarization lights were produced by removing the first polarizer and quarter-wave plate from the experimental configuration. The right-hand circular polarization light (R-) was produced by removing the second polarizer from the system. To achieve a precise alignment of the optical components in the experimental setup, a pin hole was placed in front of the polarizers and quarter-wave plate and the components were then adjusted such that the reflected laser beam passed though the pin hole in turn.

Fig. 7 Schematic illustration of experimental setup.

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To enhance the SPR effect at the sensing interface, the Stokes polarimeter was placed on a mechanical stage (SGSP-60YAW, Sigma Koki Co.) controlled by a step motor (Mark 204-MS, Sigma Koki Co.) and rotated through an angle of 60° such that the incident light was incident on the prism coupler at the resonance angle of 60° (see Fig. 7). In performing the experiments, the scanning angle of the second polarizer was increased incrementally from 0~180° in steps of 15° using a mechanical stage (SGSP-60YAW, Sigma Koki Co.). In addition, the chlorophyllin samples were stored in plastic cuvettes with dimensions of 10 x 10 x1 mm. The prism coupler was attached to the cuvettes using industrial glue and a layer of silicon at the border edge. Prior to mounting the coupler, the cuvette was drilled with a small hole (diameter 6mm) such that the sample made direct contact with the half-ball lens (thereby avoiding measurement interference by the cuvette material). The experiments were performed using 2.5 ml chlorophyllin samples with concentrations ranging from 20~100 μg/ml in 20 μg/ml increments. (Note that the samples were prepared by diluting 500 μg/ml stock chlorophyllin (C6003, Sigma-Aldrich Co. LLC) with DI water as required.)

In experiment, the R value of sample with scanning polarization angle from 0°-180° is measured. The results were used to extract the refractive index sample by curve fitting technique on Fig. 5(a). Based on the extracted refractive indices, the measured R value at 150° was compared to the simulated value (corresponding to the extracted refractive index of the sample). By the curve fitting technique, the refractive index of the chlorophyllin sample with concentration of 20 μg/ml, 40 μg/ml, 60 μg/ml, 80 μg/ml and 100 μg/ml was 1.347, 1.3471, 1.3472, 1.3473 and 1.3474, respectively. The comparison of the experimental and simulated CD values of the chlorophyllin sample given a scanning angle of 150° in every case are shown in Fig. 8. It is seen that the CD value increases linearly with an increasing chlorophyllin concentration over the considered range of 20~100 μg/ml. Moreover, for all of the samples, a good qualitative agreement exists between the experimental and simulated results confirmed the validity of the proposed approach. The slight discrepancy between the two sets of results is most likely due to alignment errors in the optical system or imperfections in the optical components themselves. The deviations in the experimental CD values obtained in five repeated tests for chlorophyllin sample with concentration of 20 μg/ml, 40 μg/ml, 60 μg/ml, 80 μg/ml, and 100 μg/ml were found to be ± 6.49x10⁻⁴, ± 6.54x10⁻⁴, ± 5.47x10⁻⁴, ± 6.68x10⁻⁴, and ± 5.92x10⁻⁴, respectively. It is found that the experimental resolution is around 1x10⁻⁴ RIU and it is one order lower than the theoretical resolution calculated from Fig. 5(b). The difference between the experimental and theoretical resolutions is mostly due to the noise floor from variations in environments and electrical components.

Fig. 8 Experimental and simulation results for variation of CD value with chlorophyllin concentration. Note that the scanning angle is θ = 150° and the incident angle is 60° in every case.

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To evaluate the performance of the proposed measurement method for samples with scattering properties, polystyrene microsphere particles (Duke Standard^TM) with dimensions of 2 μm, 5 μm and 9 μm, respectively, were added to the original chlorophyllin samples. Figure 9 shows the experimental results obtained for the variation of the DOP with the chlorophyllin concentration given the addition of particles with different sizes. For a constant sample concentration, the DOP increases with a decreasing particle size since a smaller particle dimension increases the particle density in the aqueous sample and thus enhances the scattering effect. However, for a given particle size, the DOP remains constant with an increasing sample concentration since the volume of particles added to each sample is the same in every case. For each of the considered samples, the average deviation of the DOP measurement was found to be just ± 1.0x10⁻⁴ over four repeated tests.

Fig. 9 Variation of DOP with chlorophyllin concentration as function of scattering effect. Note that the scanning angle is θ = 150° and the incident angle is 60° in every case.

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Figure 10 shows the variation of the measured CD value with the chlorophyllin concentration given the addition of particles with different sizes. It is seen that the CD value increases linearly with the chlorophyllin concentration both with and without scattering effects. However, the CD value decreases with a decreasing particle size due to the corresponding increase in the depolarization effect. Finally, as for the DOP property, the average deviation of the CD measurements over four repeated tests was found to be ± 1.0x10⁻⁴.

Fig. 10 Variation of CD value with chlorophyllin concentration as function of scattering effect. Note that the scanning angle is θ = 150° and the incident angle is 60° in every case.

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7. Conclusions

This study has presented a novel CD measurement technique based on a SPR prism coupler and polarization scanning ellipsometry. The validity of the proposed method has been demonstrated both numerically and experimentally. The simulation results have shown that the proposed measurement method has a resolution of 10⁻⁵~10⁻⁶ RIU over the refractive index range of 1.32 to 1.36 and 1.31-1.3118. In addition, the experimental results have shown that the measured CD value increases linearly with an increasing chlorophyllin concentration over the range of 20~100μg/ml for both scattering and non-scattering samples. Furthermore, the average deviation of the CD and DOP measurements over four repeated tests is equal to just ± 1.0x10⁻⁴. Thus, the feasibility of the enhanced SPR-based polarization scanning ellipsometry technique proposed in this study for practical CD sensing applications is confirmed.

Acknowledgments

The authors gratefully acknowledge the financial support provided to this study by the Ministry of Science and Technology of Taiwan (MOST) under Grant Nos.104-2221-E-006-125-MY2, 104-3113-E-006-002 and 105-3113-E-006-002. The research was also supported in part by the Ministry of Education, Taiwan, under the “Aim for Top University Project” of National Cheng Kung University (NCKU), Taiwan.

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Sensing Medium	Mechanism	Enhanced Method	Wavelength (nm)	Resolution (RIU)	Ref.
Nitrogen,methanol, propan, chloroform, benzene	Resonant Rayleigh scattering spectrum	Plasmonic	450-700	10⁻⁴ - 10⁻⁵ (Not for CD sensing)	[24]
DNA	Extinction spectrum	Plasmonic	860-940	10⁻⁵ - 10⁻⁶ (Not for CD sensing)	[25]
Prostate-specific antigen	Extinction spectrum	Plasmonic	327	10⁻⁵ - 10⁻⁶ (Not for CD sensing)	[26]
Collodial gold nanosphere	Magnetic CD spectrum	Plasmonic	400-800	10⁻⁵ - 10⁻⁶	[27]
Chiral meta-materials	CD spectrum	Plasmonic	666-780	10⁻⁵ - 10⁻⁶	[28]
Chlorophyllin	Single wavelength	SPR	633	10⁻⁵ - 10⁻⁶	Current study

Surface plasmon resonance prism coupler for enhanced circular dichroism sensing

Abstract

1. Introduction

2. SPR prism coupler

3. Analytical model for extracting CD properties

4. Validity of analytical model

5. Sensitivity of CD measurements to chlorophyllin concentration

6. Experimental setup and results

7. Conclusions

Acknowledgments

References and links

Cited By

Figures (10)

Tables (1)

Equations (22)

Optics Express