Biosensor Based on Degree of Coherence of A Pair of Surface Plasma

Oct 9, 2012 - A novel “degree of coherence paired surface plasma wave biosensor” (DOC-PSPWB) is proposed wherein the principle of detection involv...
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Biosensor Based on Degree of Coherence of A Pair of Surface Plasma Waves Chien Chou,*,†,∥ Chien-Wa Ho,‡ Sheng-Yi Chang,§ Nai-Chuan Chen,† Ying-Feng Chang,⊥ Li-Chen Su,† and Cheng-Chung Lee# †

Graduate Institute of Electro-optical Engineering, ∥Center for Biomedical Engineering, and ⊥Molecular Medicine Research Center, Chang Gung University, Taoyuan 333, Taiwan ‡ Department of Electro-Optical Engineering, National United University, Maoli 360, Taiwan § Department of Mechanical Engineering and Graduate Institute of Electro-Mechanical Engineering, Ming Chi University of Technology, Taipei 243, Taiwan # Department of Optics and Photonics, National Central University, Taoyuan 320, Taiwan ABSTRACT: A novel “degree of coherence paired surface plasma wave biosensor” (DOC-PSPWB) is proposed wherein the principle of detection involves the degree of coherence of highly correlated surface plasma waves when these are excited on a metal/dielectric interface within the DOC-PSPWB. Using the sensor, the concentration of total PSA (t-PSA) in a phosphate-buffered saline solution was measured, and a detection limit of 0.015pg/mL was determined experimentally. Finally, the dynamic range of the DOC-PSPWB when used on samples with ultralow molecular concentrations is discussed.

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INTRODUCTION The use of surface plasmon resonance biosensors (SPRBs) is one of the most popular methods for measurement of the kinetics of biomolecular interactions.1−3 The advantages of the SPRB technique are that it is highly sensitive and label-free and results in the ability to monitor the interaction between biomolecules in real time. In addition, the localized electric field of the surface plasma wave (SPW) at the interface between a metal and a dielectric medium in a surface plasmon resonance (SPR) device allows us to sense the interactions of biomolecules in the vicinity of the metal/dielectric medium interface, without readings being influenced by any biomolecules located outside of the interaction region. However, the limit of detection (LOD) sensitivity of conventional SPRB related to change effective refractive index at Δneff ≈ 10−7 RIU (refractive index units)4 is due to the limitation in the angular resolution that is inherent in the tracking resonance angle method, or it is due to a limited resolution of the spectrometer used to measure the SPR wavelength. In addition, laser intensity fluctuations clearly play a critical role in the measurement of laser intensity at fixed incident angle. To address these issues, this paper describes work where an interferometric technique is integrated into a conventional SPRB to increase the detection sensitivity. Previously, a phasesensitive detection scheme was proposed and its effectiveness was demonstrated.5−7 However, a limited dynamic range of the phase detection method, which is defined by the range of detectable concentration of analyte such as protein in solution, has a consequence that only small biomolecules in low © 2012 American Chemical Society

concentration can be measured. In other publications, an amplitude-sensitive interferometric SPRB was proposed that is able to provide a wide dynamic range and also a high detection sensitivity.8−10 Recently, Chou et al.11 proposed a “paired surface plasma wave biosensor” (PSPWB), which uses a Zeeman He−Ne laser outputting a pair of linearly polarized two-frequency laser beam. To improve further the detection sensitivity in an amplitude-sensitive PSPWB, a real-time normalization of the detected heterodyne signal was accomplished using a reference heterodyne signal; this effectively reduces the excess noise from laser intensity fluctuations and improves the detection sensitivity significantly. The detection sensitivity of the PSPWB for changes in the refractive index was found to be Δneff ≈ 10−9 RIU, whereas a wide dynamic range of 105 was also obtained.12 Chou and coworkers also proposed a differential-phase SPRB that is able to convert the differencephase modulation into an amplitude modulation in a heterodyne signal, wherein the sensitivity of the scheme was greatly enhanced to an order of 10−10 RIU (for IgG/anti-IgG interaction in PBS buffer solution).13 However, a narrower dynamic range results restricts the utility of this phase detection method to observations of the interaction only small biomolecules in very low concentration. In this study, we propose a novel method wherein monitoring of the degree of coherence (DOC) between a pair of SPWs in the amplitudeReceived: June 2, 2012 Revised: August 27, 2012 Published: October 9, 2012 25022

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(QWP); then, a polarizer selects p polarization component from circularly polarizations. In this manner, a pair of spatially and temporally highly correlated p-polarized (TM) light waves (p1 + p2) with temporal frequencies ω1 and ω2 is produced by the fact that a heterodyne signal of beat frequency at Δω = ω1 − ω2 is detected.12 As a result, when the laser beam is incident at the resonance angle θs, a pair of SPWs on the interface between the gold film and the dielectric medium is excited simultaneously under attenuated-total-reflection (ATR) arrangement.15 Under this condition, no contribution of linear momentum by surface modes of lattice vibration in the gold film is required to meet phase matching condition. Therefore, the highest DOC between a pair of SPWs is anticipated due to no random phase disturbance from longitudinal optical phonons in the gold film.16,17 However, the phase matching condition is required too when the incident angle is at an angle small deviated from the on resonance angle to excite SPW in SPR device. Hence, the longitudinal optical phonons are involved to contribute extra-linear momentum to satisfy phasematching condition for exciting SPW properly.16−20 Therefore, in DOC-PSPWB, each SPW excitation requests a different group of longitudinal optical phonons independently to meet the phase-matching condition under the arrangement at a small incident angle deviated from SPW resonance angle. As a result, the dephasing (or decorrelated) between paired SPWs becomes sensitive to the distribution of the number of densities of the longitudinal optical phonons in gold film at room temperature. The larger deviation of the incident angle from SPW resonance angle requires larger linear momentum from longitudinal optical phonons, and this results in a lower dephasing rate (or higher value of DOC) between paired SPWs. This is due to a smaller number available of the longitudinal optical phonons in gold film at room temperature and vice versa. A random phase is generated between paired SPWs, which in turn results in DOC degradation in the heterodyne signal. Qualitatively, DOC is proportional to the deviation of the incident angle from SPW resonance angle in SPR device. It then follows that the heterodyne efficiency of the detected heterodyne signal resulting from paired p-polarized laser beams becomes a critical parameter in regards to the ability to detect the DOC between paired SPWs in the DOC-PSPWB. In coherence theory, the DOC of paired SPWs is equal to the ratio between AC and DC components that make up the heterodyne signal under the condition of equal amplitude of two laser beams.21 In the setup described herein, both DC and AC components of the heterodyne signal are simultaneously measured, which means that the DOC of SPWs can be obtained in real time at different incidence angles. In addition, in this proposed method, the selfnormalization of the AC signal (the beat signal) to the DC component in the DOC measurement is conducted in realtime, enabling a reduction in the noise resulting from laser intensity fluctuations effectively. Shown in Figure 2 is the experimental setup of the proposed DOC-PSPWB, which uses a two-frequency Zeeman He−Ne laser. To generate a pair of p waves with high spatial and

sensitive version of a PSPWB is performed at different angles of incidence near on resonance. This facilitates high detection sensitivity and real-time monitoring of the change in the effective refractive index in the vicinity of a metal/dielectric interface.

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WORKING PRINCIPLE OF DOC-PSPWB SPW is the state of the collective free electrons oscillation at the interface of the nanoscaled thin metal (gold or silver) film and dielectric medium under the resonance condition in attenuated total reflection (ATR) arrangement such as the Otto or Kretschmann configuration.14 To excite SPW, the chosen metal requires not only a large and negative dielectric constant but also TM wave (p-wave) of incident laser beam to satisfy not only the electromagnetic boundary condition but also phase matching condition for exciting surface electromagnetic wave propagating on metal/dielectric interface.15 Theoretically, the phase-matching condition between the excited SPW and the incident p wave propagating along the x direction on the gold/dielectric interface is satisfied for the condition kx = (ω/c)(ε1)1/2 sin θs = (ω/c)(ε2ε3/(ε2 + ε3))1/2, where ε1,ε2, and ε3 are the dielectric constants of the prism, gold film, and dielectric medium, respectively; kx is the component of the wave vector of the incident laser beam parallel to the gold/dielectric interface; and ω and θs are the temporal frequency and SPW on resonance angle of incidence of the laser beam, respectively. We consider a complex wave vector kx = k′x + iΓi, where Γi represents internal damping in the gold film. When the thickness d2 of the gold film is small enough for the condition of radiation damping to be produced via direct coupling between the SPW and the reflected laser beam in the prism, then a new resonance condition is given by i rad rad kx = (k′x + iΓi) + Δkrad x = (k′x + Re[Δkx ]) + i(Γ + Γ ), where the term Γrad = Im[Δkxrad] describes radiation damping. Therefore, the reflectance r of the laser beam from the SPR device can be written as15 r (θ ) = 1 −

4Γ iΓ rad [kx − (k′x + Re[Δkxrad])]2 + (Γ i + Γ rad)2 (1)

where θ is the incident angle. It follows that the SPW resonance angle θs can be expressed in terms of the dielectric constant of the medium and the thickness and dielectric constant of the gold film. In this study, we set up DOC-PSPWB in the Kretschmann configuration, in which a prism, a gold film, and a dielectric medium are arranged, as shown in Figure 1. The light source used is a Zeeman He−Ne laser, which outputs a pair of orthogonal linear polarized waves (p and s waves) with slightly different frequency. The laser beam subsequently converted into a pair of circularly polarizations via a quarter-wave plate

Figure 1. Kretschmann configuration of PSPW device.

Figure 2. Schematic of DOC-PSPWB. 25023

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temporal correlations, a half-wave plate (λ/2 plate) is combined with one-quarter-wave plate (λ/4 plate), and along with an analyzer, they are arranged so that two highly correlated p waves (p1 + p2) from the laser beam are produced with slightly different temporal frequencies Δω = ω1 −ω2. The amplitude and frequency of the two p waves are denoted by (Ap1,ω1) and (Ap2,ω2), respectively. Finally, the reflected and attenuated p waves (P1′ + P2′ ) are detected with the use of a photodetector, where the output heterodyne signal is expressed as ′ )2 + (A p2 ′ )2 + 2A p1 ′ A p2 ′ |ν0||ν| cos(Δωt + ϕ) I = (A p1 = IDC + IAC cos(Δωt + ϕ)

(2)

Figure 3. Response curves of DOC-PSPWB (blue line) and amplitude-sensitive PSPWB (green line).

Here IDC = (A′p1) + (A′p2) and IAC = 2A′p1A′p2|ν0||ν|. |ν0| = 1 is assumed for simplicity of the DOC of the Zeeman He−Ne laser combing with the DOC of the incident p1 and p2 waves. |ν| denotes the DOC of the SPWs and can be described by means of the temporal correlation of the random phase disturbance of paired SPWs according to partially coherence theory.22 |ν| is related to the dephasing between two SPWs caused by different groups of longitudinal optical phonons in gold film at the corresponding angle of incidence. ϕ is the phase in the coherence function of the heterodyne interference. Because Ap1 ′ = Ap2 ′ is assumed in this experiment with Zeeman He−Ne laser, the DOC is then equal to the visibility V of the heterodyne signal and can be determined using 2

V = |ν| = (IAC/IDC)

2

observations of protein−protein interactions at low molecular concentration. To summarize, the experimental results imply that a DOC-PSPWB is able to perform measurement with high detection sensitivity and a proper dynamic range, suitable for biomedical applications. Moreover, to calibrate the detection sensitivity of DOCPSPWB, the sucrose−water solution and glycerin −water solution at different concentrations (wt %) were tested, where a bare gold film (without surface treatment) was used. In this experiment, we used the gold film of 47 nm in thickness directly coated on BK-7 glass substrate as the sensor chip, which was purchased from Gentel Biosciences (Fitchburg, WI) for measurement. The quality of the gold film was checked by using AFM (Veeco Dimension 3100), as shown in Figure 4.

(3)

Experimentally, the visibility of the heterodyne signal was measurable in real time by using an amplitude demodulator to obtain the DC and AC components of the heterodyne signal precisely and simultaneously.

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EXPERIMENTAL RESULTS To characterize the decorrelation of paired SPWs versus incident angle, we measured the DOC response versus the incident angle of the DOC-PSPWB in air in this experiment. The wavelength of Zeeman He−Ne laser (Agilent HP5517A, Santa Clara, CA) is 632.8 nm, and its beat frequency is 1.9 MHz. A BK-7 prism, coated with a gold film (53 nm in thickness) on its base directly, was positioned as shown in Figure 2. First, the DOC of SPWs on a gold/air interface was measured at a fixed angle of incidence in real time, where the AC and DC components of the detected heterodyne signal were measured with a HP 54600B oscilloscope (Agilent, Santa Clara, CA). Then, the dependence of the DOC (visibility) of the heterodyne signal on the angle of incidence was recorded by scanning the incident angle of laser beam, as shown in Figure 3 (blue line). In Figure 3, the measurement of the response of an amplitude-sensitive PSPWB of our previous studies was conducted (green line) separately for comparison,11,12 where the AC component was amplitude demodulated using the conventional envelope detection technique (Agilent 54600B, Santa Clara, CA), in conjunction with a narrow bandwidth filter. This enables the efficient reduction of the noise power, and thus the detection sensitivity of the device is significantly enhanced.23 A larger slope and smaller dynamic range are apparent in the DOC response curve, as compared with the amplitude-sensitive PSPWB, as shown in Figure 3. This indicates an improvement in detection sensitivity for the DOC-PSPWB compared with what is achievable in the amplitude-sensitive PSPWB.12 In the mean time, the dynamic range of the DOC-PSPWB found in Figure 3 is capable of

Figure 4. Picture of AFM of the gold film.

During the measurement, the glass substrate was optically contacted to the prism by using an index-matching oil (n = 1.52). A reaction chamber with dimensions of 2 cm × 1 cm × 1 cm was designed, in which sucrose−water solution of different concentrations adhering to the gold film was placed, and the sucrose concentration was obtained by properly measuring the DOC. In these measurements, AC and DC components of the heterodyne signal were measured simultaneously by using HP 54600B oscilloscope. Before the sucrose−water solution was tested, the stability of DOC of the laser beam was measured in 1 min, as shown in Figure 5 by using tridistilled water as the medium of dielectric. Experimentally, the random phase noise from p1 and p2 waves caused by He−Ne laser itself will not cancel out automatically, which results in the DOC of the laser beam at 0.60489. 25024

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water solutions as a function of wt % concentration,13,26,27 the variation of the refractive index in the range of concentrations close to 10−5 wt % of the sucrose solution relative to tridistilled water (0 wt % concentration) is Δn = 1.4 × 10−8 RIU. Following the linear relationship between the refractive index and concentration in wt %, as shown in Figure 6b, the detection sensitivity of DOC-PSPWB at concentration of 5 × 10−7 wt % of sucrose−water solution on the order of Δn ≈ 10−10 RIU. These data are then used to calculate the LOD of DOCPSPWB based on the relation of δ(Δn) = Δn/SNR, where SNR (signal-to-noise ratio) is defined by SNR = ΔV/δ(ΔV). The standard deviation of the detected signal is δ(ΔV), which is equivalent to the noise level, whereas the signal itself is ΔV. In Figure 6b, SNR ≅ 6 was obtained, and thus the LOD of DOCPSPWB on sucrose−water solution detection is on the order of 10−10 RIU. Additional testing was performed using a glycerin−water solution at concentrations in the range of 10−5 to 10−3 wt %. Figure 7 shows the experimental results, where a linear

Figure 5. Stability of DOC of incident laser beam using tridistilled water as the dielectric medium.

However, the standard deviation of DOC is 0.00067 in Figure 5. This introduces SNR ≈ 1000 in the measurement. The high stability of DOC implies the assumption of |ν0| ≅ 1 is acceptable in DOC-PSPWB. This implies that it is suitable for ultra-high sensitivity of detection in protein−protein interaction. Figure 6a shows the results of testing sucrose−water solution of concentration in the range of 0 to ∼5 × 10−5 wt %, where a

Figure 7. DOC versus concentration of glycerin−water solutions in a range of 0 to 10−3 wt %. The error bar is small enough and marked by a single dot.

response between the measured DOC versus concentration is clearly evident and where a dynamic range of 102 is also demonstrated. According to published data, the difference in the refractive index between 10−5 wt % of glycerin−water solution relative to triple-distilled water (0 wt %) is Δn = 3.0 × 10−9 RIU.13 On the basis of this result and SNR ≅ 9 from Figure 7, δ(Δn) = 3.4 × 10−10 RIU is calculated. Additionally, total PSA (t-PSA) is a biomarker used for prostate cancer diagnosis in serum for male only. In our recent study, however, t-PSA might also be a potential biomarker useful for breast cancer diagnosis.28 To test further the measuring sensitivity of the DOC-PSPWB, we measured tPSA at various concentrations in an experiment. Figure 8 shows the sensogram of the immobilization of t-PSA antibody versus AC signal of the DOC-PSPWB based on our homemade sensor chip of 47 nm of gold film coated on BK-7 glass plate. First, we sprayed methyl alcohol on the gold surface of a BK-7 glass plate, which was then washed with tridistilled water and dried using N2 gas. The glass plate was immersed in 1 mmol/L 11-mercapto-undecanoic acid for 20 h at 4 °C dark cold storage to continue the surface modification. After this, we took out the glass plate and sprayed it again with methyl alcohol, washed it with tridistilled water, and dried it once more. We used a mixture of EDC and NHS solutions to reactivate the gold film for 10 min, immersed it in a pH 5.0 acetate buffer for 5 min, and then put it into a t-PSA antibody acetate solution for 30 min, which acted as the capture

Figure 6. DOC versus concentration of sucrose−water solution at concentration (a) in a range of 0 to 5 × 10−5 wt % and (b) in a range of 0 to 10−5 wt %. The error bar is small enough and marked by a single dot.

larger concentration increment was chosen. Similarly, the measurement in the range of 0 to 10−5 wt % with a smaller concentration increment is shown in Figure 6b, where a linear relationship between the concentration and DOC in a range of 5 × 10−7 to ∼10−5 wt % is clearly seen. However, it appears exponentially increasing in the range of 0 to ∼ 10−7 wt % at ultralow concentration. In Figure 6a,b, the uncertainties of the DOC measurement are much smaller in comparison with the detected signals, and they are marked by a single dot. These experimental results are consistent with the measurements by Endo’s group24 and Corn’s group25 at ultralow concentration. According to published data on the refractive index of sucrose− 25025

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According to SPW excitation theory, when light incident angle is not exactly on resonance, this indicates that a specific group of lattice vibrations of optical phonons in a gold film contributes to the phase-matching condition for SPW excitation. Therefore, the dephasing or decorrelation between paired SPWs in an SPR device occurs, and measurement of DOC of paired SPWs is proposed. This results in a steeper slope of the DOC linear response curve than what is produced in conventional SPRB via intensity detection.23 Moreover, DOC is highly sensitive to the surface quality of the gold film too so that it becomes critical to the performance of DOCPSPWB as well.29 Whereas a smaller dynamic range is produced in DOC-PSPWB when compared with conventional SPRB, the dynamic range of DOC-PSPWB is nevertheless suitable for protein−protein interaction studies at low concentration. In addition to that, the common-path configuration of DOC-PSPWB can effectively reduce common background noise such as temperature variation in reaction chamber by integrating with heterodyne detection to become temperature insensitivity. In the mean time, a self-normalization of AC/DC signal components in DOC measurements can also reduce the excess noise too arising from laser intensity fluctuations.30 In our experiments, tests with sucrose−water and glycerin−water solutions implied LOD of DOC-PSPWB on the order of 10−10 RIU. Additionally, dilute concentrations of t-PSA in a buffer solution were measured as well. The measurement results demonstrated LOD of DOC-PSPWB on t-PSA in buffer solution at 0.015pg/mL and the dynamic range at 107. This implies that DOC-PSPWB is capable of measuring protein−protein interactions at ultra-low concentrations.31 Finally, the features of a DOC-PSPWB are summarized as follows: (1) The DOC probe between paired SPWs can be a highly sensitive method that allows the monitoring of biomolecular interactions in real time. (2) It involves high stability of DOC between the localized electric field of SPWs. (3) A linear relationship exists between the DOC and the angle of incidence. (4) It achieves a high SNR in the detected heterodyne signal via synchronized detection in association with a narrow bandwidth filter detection scheme. (5) The excess noise resulting from laser beam fluctuations can be reduced significantly via a self-normalization algorithm in DOC measurement. (6) The insensitivities of temperature and environmental disturbances occur due to common-path configuration and heterodyne detection. (7) High detection sensitivity and suitable dynamic range for studies of protein− protein interactions at low concentrations exist. (8) It involves a simple optical setup. All of these advantages result in the degree of coherence probe of paired SPW probe in PSPWB, a new sensing mechanism in SPR biosensing technologies at an ultra-high sensitivity.

Figure 8. Sensogram of the sensor chip.

antibody, with a concentration of 40 mg/mL at room temperature. After blocking with an acetate buffer immersion for 5 min, we measured 400 μL aliquots of t-PSA into a phosphate-buffered saline solution, producing various concentrations ranging from 0.001 to 10 000 pg/mL. We chose Thorlabs PDA36A as a photo detector (Newton, NJ). The DC component of the heterodyne signal was measured by using Agilent 34401A digital multimeter, whereas SR844 lock-in amplifier (Stanford Research Systems, Sunnyvale, CA) was adopted for AC component of the heterodyne signal simultaneously. The experimental result is shown in Figure 9.

Figure 9. DOC versus concentration of t-PSA in PBS solutions in a range from 0.001 to 10 000 pg/mL.

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The fitting curves are then analyzed by the GraphPad Prism software to fit a nonlinear regression using a four-parameter dose−response curve (variable slope model). The equation can be described by Y = (2.22 × 10−6) + (0.00263 + 2.22 × 10−6)/ (1 + 10(−0.1965−X)×0.5323) with R2 = 0.9810, where Y is the DOC (visibility) and X is the t-PSA concentration. In this experiment, LOD was calculated by putting three times standard deviation of blank measurement (without the t-PSA) into fitting curves. It was calculated that the LOD of DOC-PSPWB is 0.015 pg/mL.

AUTHOR INFORMATION

Corresponding Author

*Phone: 886-3-2118800-3677. Fax: 886-3-2118507. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by research grant NSC 98-2221-E182-063-MY3 and NSC 98-2221-E-182-064-MY3 from National Science Council in Taiwan.

CONCLUSIONS AND DISCUSSION In the present study, the DOC between a pair of SPWs in a novel proposed DOC-PSPWB was measured in real time. 25026

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