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We present an investigation on the use of low-index cavity layers to enhance the sensitivity of injection-molded guided-mode resonance (GMR) sensors. By adjusting the sputtering parameters, a low-index cavity layer is created at the interface between the waveguide layer and the substrate. Refractive index measurements show that a sensitivity enhancement of up to 220% is achieved with a cavity layer, in comparison to a reference GMR sensor without a cavity layer. Finite-element-method simulations were performed, and the results indicate that the cavities significantly redistribute the resonance mode profile and thus enhances the sensitivity. The present investigation demonstrates a new method for enhancing the sensitivity of injection-molded GMR sensors for high-sensitivity label-free biosensing.
© 2015 Optical Society of America
1. Introduction
Guided-mode-resonance (GMR) biosensors employing grating-waveguide couplers have attracted increasing attention for bio-medical and chemical detection [1–6]. Compared to some of the most commonly used optical biosensors, such as optical fiber biosensors [7–9], surface plasmon resonance (SPR) biosensors [10–12], and optical waveguide biosensors [13–15], GMR biosensors offers unique advantages including good sensitivity, label-free detection, outstanding integrability, and real-time detection, suitable for use in mass-produced portable instruments for practical applications. GMR biosensors are usually composed of a waveguide layer with periodic nanostructures that support guided modes, with an evanescence wave extending outside the waveguide. When GMR occurs, a light beam incident onto the structure can be coupled into the waveguide layer, and the GMR wavelength (λR) is strongly dependent on the refractive indices (RIs) of the constituent materials in the structure and the surrounding medium. As an analyte binds onto the surface of the waveguide layer, the RI at the surface of the biosensor is changed, resulting in a measurable shift in the GMR wavelength. Therefore, tracking the shifts of the GMR wavelength allows for very sensitive detection of small RI changes. The performance of a spectrally-resolved GMR biosensor can be characterized by its sensitivity, which is defined by the GMR wavelength shift (ΔλR) divided by the change in the RI of the analyte (Δna). To achieve more accurate detection of small molecules at low concentrations, and may even achieve single molecule detection [16–18], a high sensitivity is desired. In GMR biosensors, the sensitivity is highly related to the modal overlap between the analyte and the evanescent fields of the resonance modes. To enhance the sensitivity of GMR biosensors for practical applications, several approaches have been developed to modulate the evanescent wave distribution and optimize the modal overlap [19–22].
In addition to high sensitivity, another challenge for successful GMR biosensors is to develop low-cost, mass-manufacturable fabrication methods. To address this issue, we have developed plastic GMR biosensors fabricated using a combination of injection-molded and sputtering techniques, which offer unique advantages such as lower fabrication costs and reduced production time [23]. Moreover, by adjusting the sputtering parameters, cavities are created at the interface between the waveguide layer and the plastic substrate, leading to an enhancement of the RI resolution by ∼ 35% in comparison with a control biosensor without cavity structures. Meanwhile, the run-to-run and chip-to-chip reproducibility is excellent (standard deviation below 2.6%), as evaluated by RI measurements. These unique characteristics make the biosensors potentially attractive for the commercialization of practical applications. In this paper, we present a detailed investigation into the effect of cavities on the sensitivity of injection-molded GMR sensors. A series of GMR sensors was fabricated with different sputtering parameters. By increasing the oxygen content in the sputtering process, cavities with a controllable mean size is produced. We compare the performance of GMR sensors with and without cavities and observe significantly improved sensitivity. Finite-element-method (FEM) simulations are performed to analyze the effect of the cavities on the evanescent wave distribution and the sensitivity of the GMR sensors.
This paper is organized as follows. In section 2, the fabrication of the GMR sensors is described. In section 3, we present a characterization of the GMR sensors based on different experimental techniques. Scanning electron microscopy (SEM) experiments were conducted to probe the microstructures of the waveguide layers deposited with different sputtering parameters. Label-free biosensing based on RI changes observed from transmission measurements was carried out to determine the sensitivity of the different GMR sensors. Section 4 presents FEM simulation results for the field distribution and sensitivity of the GMR sensors to explain the enhancement in sensitivity of the GMR sensors. Conclusions are given in section 5.
2. Fabrication of injection-molded GMR sensors
The GMR sensors used in this study were fabricated using a combination of injection-molded and sputtering technologies [23]. The fabrication flow is schematically displayed in Fig. 1. First, an injection-molded technique is adapted to prepare cyclic olefin copolymer (COC) chips with a grating structure having a period of Λ = 416 nm and a depth of d = 100 nm, determined by atomic force microscopy (AFM) [23]. The external dimensions of the COC grating chip are 46 mm (Length) × 22 mm (Width) × 2 mm (Height), while the grating pattern located at the center of the COC grating chip has an area of 20 × 20 mm2. Subsequently, to create a grating-waveguide coupler structure, a 90-nm-thick TiO2 layer was deposited on the COC grating chips by a direct current magnetron sputter from a titanium target with pure Ar and O2 acting as the sputtering and reactive gases, respectively. To investigate the effect of the O2/Ar gas flow mixture in the sputtering process, five samples were fabricated with varying O2/Ar flow ratio. The O2/Ar gas flow ratio for each sample is given in Table 1. To realize simple and easy handling of liquid samples, the GMR sensors are integrated with a fluidic module. A COC fluidic module with a fluidic channel of 32 mm (Length)× 3 mm(Width)× 0.2 mm(Height) as the sensing region was fabricated using the injection-molded technique, which was connected to two flexible tubes on the top of the fluidic channel to act as the sample inlet and outlet. The TiO2-coated COC chips and the COC fluidic module were then bonded to obtain a low-cost, durable, stable, label-free biosensing platform.
Fig. 1 Schematic fabrication flow of the injection-molded GMR sensors. The inset shows an optical image of a fabricated GMR sensor.
Table 1. Summary of sputtering deposition conditions and GMR wavelengths recorded in DI water for the samples.
3. Characterization of GMR sensors
Figure 2 shows SEM images of the microstructure of TiO2 gratings deposited under different sputtering conditions. For sample A, which was deposited with the lowest O2 content in the sputtering process, a TiO2 layer is uniformly formed on the COC grating surface due to the excellent step coverage of the sputtering, resulting in a double-sided grating waveguide (DSGW) structure. For the other samples deposited with higher O2 oxygen contents in the sputtering process (sample B to sample E), a TiO2 DSGW with good uniformity and periodicity is also visible. Meanwhile, many cavities are created at the TiO2/COC interface in the vicinity of the valleys of the COC grating structure below the TiO2 DSGW structure. Moreover, the mean size of the cavities increases with the O2 content in the sputtering process. The formation of cavities is attributed to the etching of the COC surface by the O2 plasma [24], and the etch rate increases with the O2 content, resulting in larger cavities. By introducing the cavities into the structure, the RI of the medium beneath the waveguide layer is effectively reduced, thereby impacting the evanescence wave distribution and sensitivity of the sensors, which will be discussed later. Compared to previous approaches for creating porous or cavity structures in substrates, which usually require additional fabrication steps [13, 25, 26], the approach used in this study can simultaneously realize the deposition of waveguide layers and the creation of cavities in one step, thereby reducing the fabrication complexity of the GMR sensors.
Fig. 2 SEM cross-section images of TiO2 DSGW structures deposited at different O2/Ar gas flow ratio: (a) sample A (1/6), (b) sample B (1/5), (c) sample C (1/4), (d) sample D (1/3), and (e) sample E (1/2). As the O2 content is increased in the sputtering process, many cavity structures are created beneath the TiO2 waveguide layer, as indicated by arrows.
To evaluate the sensitivities of the different GMR sensors, label-free optical sensing was performed based on RI measurements obtained via transmission experiments under normal incidence conditions. The setup of the transmission experiments is schematically shown in Fig. 3. A highly stable halogen lamp operated at a wavelength range of 400–2400 nm was used as the light source, and the emitted light was coupled to an optical fiber. A lens was used to collimate the light beam, followed by an iris to adjust the light beam size to be smaller than the sensing area of the GMR sensor. Subsequently, a linear polarizer with an extinction ratio > 1000 was adapted to control the polarization of the light beam, which was then used to normally irradiate COC substrate of the GMR sensor. Because the optical response of GMR sensors is highly polarization-dependent, here we focus on the TM modes, which give higher GMR wavelength shifts in response to RI changes in comparison to the TE modes [19]. The transmitted light was collected with a silicon spectrometer with a spectral resolution of ∼0.35 nm to analyze the transmission spectra. For label-free optical sensing, analyte solutions (different concentrations of sucrose solutions in DI water) with different RIs ranging from na = 1.333 to 1.373 were prepared for the RI experiments. After the analyte solution was injected into the sensor and fully filled the sensing region, the transmission spectra were recorded for different analyte RIs. The transmittance of the GMR sensors were then determined by T(λ) = I(λ)/I0(λ), where I(λ) and I0(λ) are the detected light intensities with and without a GMR sensor in the transmission experiments. Samples of the measured TM transmittance spectra for the fabricated GMR sensors are displayed in Figs. 4(a)–4(c), which display two characteristics. First, for sample A, which does not have cavities in the structure, the spectrum exhibits a dip at 650.6 nm for na = 1.333, corresponding to the GMR wavelength. In contrast, for the GMR sensors with cavities, the GMR wavelength shifts to lower wavelengths, as summarized in Table 1. From the GMR wavelengths obtained under normal incidence conditions, the effective RI of the guided mode neff can be extracted by neff = λR/Λ [6]. The results for na = 1.333 are depicted in Fig. 5(a). For sample A, which has no cavities in the structure, neff = 1.564 is obtained. For the other samples with cavities, neff decreases as the mean size of the cavities increases. The reduced neff provides evidence that the RI of the medium beneath the waveguide layer is effectively reduced by the cavities, and its value becomes smaller with increasing mean size of the cavities. Furthermore, from the transmittance spectra, we extract the full-width-at-half-maximum (FWHM) values of the transmittance dips for the GMR sensors, and the results are depicted in Fig. 5(b). The FWHM value does not significantly increase as the mean size of the cavities increases, indicating that the incident light experiences little light scattering in the cavity regions. Second, as na increases, the GMR wavelength continuously shifts to longer wavelengths for all samples. Correspondingly, the GMR wavelength shifts for different samples as a function of RI change are depicted in Fig. 5(c). These data were fit to a linear function, and the slope ΔλR/Δna represents the sensitivity of the GMR sensor. The extracted sensitivities for all samples are depicted in Fig. 5(d). For sample A, without cavities, a sensitivity of 82.5 nm/RIU is obtained. In comparison, the other samples exhibit higher sensitivities, mainly due to the introduction of cavities in the structure, which reduces the RI of the medium underlying the waveguide layer. In particular, a high sensitivity of 181.9 nm/RIU is achieved for sample E. Compared with the reference GMR sensor (sample A), the sensitivity for sample E is enhanced by 220%.
Fig. 4 TM transmission spectra for (a) sample A, (b) sample C, and (c) sample E at different analyte RIs. In the RI range of na = 1.333–1.373, the GMR occurs in the wavelength range of 625–665 nm. As the analyte RI increases, the GMR wavelength shifts to longer wavelengths.
Fig. 5 (a) Extracted effective RI of the guided mode for the GMR sensors at na = 1.333. (b) FWHM of the guided mode for the samples at na = 1.333. (c) GMR wavelength shifts as a function of analyte RI for the GMR sensors. (d) Experimental sensitivities of the fabricated GMR sensors. Five measurements are performed, and the mean values and error bars (determined from the standard deviations) are depicted.
4. Numerical simulations and discussion
To understand the origin of the enhancement in sensitivity obtained by the cavity structures, FEM simulations were performed for GMR sensors with and without cavities. The GMR sensor is simulated as a 2D unit cell with periodic boundary conditions applied along the x-direction to obtain the field distributions and transmission spectra for different analyte RIs. Perfectly matched layers (PMLs) were used at the top and bottom of the simulation cell to fully absorb the outward waves. For excitation, we use a plane wave light source that is located at the lower part of the simulated unit cell, propagating along the y-direction and impinging normally on the structure from the bottom of the COC substrate. A power monitor layer located at the upper part of the unit cell was employed to record the light intensity transmitted through the DSGW structure. In the modeling, we focus on the TM modes that give higher GMR shifts caused by RI changes. The wavelength-dependent RIs of the COC and TiO2 layers used in the FEM simulations are n(TiO2) = 4.74 − 9.85λ +13.64λ2 − 6.44λ3 and n(COC) = 1.68 − 0.58λ + 0.75λ2 − 0.34λ3, respectively, which were experimentally obtained with a VASE ellipsometer (J.A. Woollam Co., Inc.). Note that λ is the free-space wavelength of the light in units of μm. The cavity layer underlying the TiO2 layer is modeled as a continuous layer with an effective refractive index of np. Because the cavities are filled with air, the RI of the cavity layer should be in the range of n(COC) ≤ np ≤1 depending on the mean size of the cavities. Figures 6(a) and 6(b) show the simulated energy distribution for the GMR sensors without and with a cavity layer at their resonance wavelengths, respectively. For the GMR sensors without cavities, the energy distribution exhibits a fundamental mode in the structure, as shown in Fig 6(a). The energy distribution of the GMR sensor with cavities also exhibits a similar fundamental mode in the grating structure, as shown in Fig. 6(b). However, the resonance mode is shifted towards the analyte area due to the presence of the low-index cavity layer. Correspondingly, Fig. 6(c) compares the normalized energy distribution for the two cases across the y direction at the valley of the TiO2 DSGW structure. Without the cavity layer, the energy distribution is highly asymmetric and extends more into the COC substrate than in the desired analyte region, due to the much larger RI of the COC substrate (n(COC) = 1.53) in comparison to that of the analyte (na = 1.333). In contract, with the cavity layer, the shifted resonance mode significantly increases the evanescent wave in the analyte region and suppresses leakage into the COC substrate. As a result, the interaction between the analyte and the electromagnetic field is enhanced, thereby improving the sensitivity of the GMR sensors. These results explain the enhanced sensitivity for the GMR sensors with cavities, as shown in Fig. 5(d).
Fig. 6 Simulated normalized energy distribution of GMR sensors (a) without and (b) with the cavity layer at their resonance wavelengths. The RIs of the analyte and cavity layer are na = 1.333 and np = 1.4, respectively. (b) Comparison of normalized energy distributions for GMR sensors with and without a cavity layer. The dashed lines represent the top and bottom interfaces of the TiO2 waveguide layer.
The sensitivity of GMR sensors is highly related to modal overlap between the analyte region and the field distribution of the resonance mode [19, 27]. Having shown the resonance mode distributions for the GMR sensors with and without cavities, we then investigate the sensitivity of the GMR sensors in terms of the cavity layer RI. Figures 7(a) and 7(b) shows the calculated penetration depth of the evanescent wave in the analyte region, and the fractions of resonance energy distributing in the analyte region and COC substrate as a function of the cavity layer RI, respectively. (The penetration depth is defined by the distance where in the analyte region the intensity falls to 1/e of the intensity at the TiO2/analyte interface.) Three distinct regions are observed. In region 1 (np > na), the resonance field is strongly asymmetric with a significant distribution in the COC substrate. As a result, the penetration depth in the analyte region is only ∼ 140 nm, giving a small resonance energy of ∼ 32% distributed in the analyte region. In contrast, ∼ 40% of the total resonance energy is distributed in the COC substrate, but it cannot interact with the analyte region for biosensing, thus limiting the response to RI changes in the analyte region. As np is significantly lowered (region 2 and region 3), the shifted resonance mode simultaneously increases the resonance energy in the analyte region and suppresses the resonance energy leakage into the COC substrate. Accordingly, more than 50% of the total evanescence energy can distribute in the analyte region for biosensing, thereby greatly improving the sensitivity of the GMR sensors. Correspondingly, Fig. 7(c) shows the calculated sensitivity as a function of np for the GMR sensors. (In calculating the sensitivity of the GMR sensors for a given np, FEM simulations were performed to obtain the transmission spectra for different analyte RIs ranging from na = 1.333 to 1.373, from which the GMR wavelengths were obtained. The sensitivity was then determined by ΔλR/Δna.) As np is decreased, the sensitivity can be continuously enhanced due to the redistributed resonance energy in the analyte region. These results match our experimental results as shown in Fig. 5(d), confirming that the sensitivity enhancement is attributed to the cavity structures that lower the RI of the medium beneath the waveguide layer. Ultimately, if the medium beneath the waveguide layer is completely removed, forming a suspended structure [28, 29], np = 1. In this case, our calculations predict that the sensitivity of the GMR sensors can reach a remarkable value of 245 nm/RIU. Further improvement in sensitivity is also possible by combined with other approaches, such as optimizing the grating geometries [30], to enable high-sensitivity label-free biosensing.
Fig. 7 (a) Calculated penetration depth as a function of the RI of the cavity layer of the GMR sensor. (b) Calculated fraction of evanescent energy for the analyte region of the sensors as a function of the RI of the cavity layer of the GMR sensor. (c) Calculated sensitivity of the GMR sensors as a function of the RI of the cavity layer of the GMR sensor.
5. Conclusion
In conclusion, we have demonstrated high-sensitivity GMR sensors fabricated by injection-molding and sputtering techniques. Through the incorporation of a low-index cavity layer at the interface between the waveguide layer and the substrate, obtained by adjusting the sputtering parameters, a sensitivity of up to 181.9 nm/RIU is achieved, which is 2.2-fold higher than that of the sensors without a cavity layer. Analysis of the resonance mode profiles shows that the introduction of the cavity layer significantly shifts the evanescent wave into the analyte region, thereby enhancing the sensitivity. With the benefits of reduced cost and enhanced sensitivity, these injection-molded GMR sensors are promising for practical bio-sensing applications with more sensitive detection of smaller biomolecules at lower concentrations.
Acknowledgments
This work at CCU was supported by the Ministry of Science and Technology of Taiwan under Grant Nos. MOST 102-2221-E-194-053-MY3, MOST 103-2221-E-194-016, and MOST 103-2120-M-194-004-CC2.
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