Metal-Clad Waveguide Resonance Sensor Using a Mesoporous TiO2

Aug 11, 2017 - The porosity of the film was derived from the best fitting of the simulation data to the measured resonance spectra. ...... Zourob , M...
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Metal-Clad Waveguide Resonance Sensor Using a Mesoporous TiO2 Thin Film as the Chemical Sensitive Core Layer Xiumei Wan,†,‡ Dan-feng Lu,† Ran Gao,† Jin Cheng,† and Zhi-mei Qi*,† †

State Key Laboratory of Transducer Technology, Institute of Electronics, Chinese Academy of Sciences, No. 19 Beisihuan West Road, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: A metal-clad optical waveguide sensor based on guidedmode resonance spectroscopy has been developed using a mesoporous TiO2 film as the chemical-sensitive core layer. The sol−gel block copolymer templated mesoporous TiO2 films are ∼275 nm thick, supporting both the transverse electric (TE) and transverse magnetic (TM) modes and thus providing the sensor with the TE- and TM-related dual sensitivities. Both the TE- and TM-mode resonance wavelengths (λR) at the same incident angle were obtained by broadband attenuated total reflection spectroscopy. The film porosity was determined to be ∼0.535 by using the combination of Fresnel formula and Bruggeman approximation to fit the measured values of λR. The TE- and TM-related sensitivities of the sensor to the two analytes of elaidic acid (EA) and Cr6+ were measured at the same incident angle. For each analyte, the TMrelated sensitivity is higher than that with the TE-related one. The two sensitivities can be calibrated with each other. Moreover, by adjusting the block copolymer concentration in the coating solution, the mesoporous TiO2 films with different porosities were prepared for investigating the porosity dependence of the sensor’s sensitivity. The findings indicate that a larger-porosity TiO2 film can lead to higher sensitivity. The work demonstrated that the metal-clad mesoporous TiO2 film-based guided-mode resonance sensor is advantageous over a conventional SPR sensor for in situ detection of small molecules.

1. INTRODUCTION A metal-clad optical waveguide sensor is an optical device enabling label-free and in situ detection of chemical and biological analytes with high sensitivity, and it has potential applications in diverse fields, including liquid refractometry, environmental monitoring, food safety testing, and point-ofcare diagnostic assay.1−5 The metal-clad optical waveguide sensor consists of a dielectric core layer overlaid on the gold film of a conventional SPR sensor, enabling to support both the transverse electric (TE) and transverse magnetic (TM) waveguiding modes.6,7 This polarization diversity offers the optical waveguide sensor dual sensitivities, allowing the sensor to be used for investigating the birefringence and optical dichroism of anisotropic materials.8−10 Compared with the surface plasmon mode, a waveguiding mode has a large penetration depth,11 making the metal-clad optical waveguide sensor suitable for label-free detection of macromolecules such as protein, bacteria, and cells on the sensor surface.12−16 Moreover, the metal-clad waveguide sensor with a core layer of a nanoporous dielectric film is able to sensitively detect smallmolecule targets. This is so because adsorption of target molecules on the pore wall of the film can lead to a change in refractive index of the film. In this case, the sensor is not an evanescent-wave sensor but a guided-wave sensor. An © XXXX American Chemical Society

evanescent wave based metal-clad waveguide resonance sensor is generally less sensitive than a conventional SPR sensor that is, however, less sensitive than a guided wave based counterpart. Owing to the extreme concentrating of optical energy in the subwavelength slab core layer along the vertical direction, even a subtle change in refractive index of the core layer can give rise to a considerable influence on the guided-mode resonance condition. Small-molecule analytes have light molecular weights, yet considerable refractive indexes, and they are not easily detected with a gravimetric sensor such as quartz crystal microbalance (QCM) but can be readily detected using an optical waveguide resonance sensor containing a nanoporous core layer.17−19 This work aims at developing high-performance nanoporous waveguide resonance sensors for rapid and high-sensitivity detection of hazardous substances that are generally small molecules. By using the sol−gel block copolymer templating method, the mesoporous TiO2 films with thicknesses of several hundred nanometers were easily prepared on the gold-layercovered glass substrates for application as metal-clad waveguide Received: May 9, 2017 Revised: August 11, 2017 Published: August 11, 2017 A

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Figure 1. (a) Schematic diagram of the waveguide resonance sensor containing a mesoporous TiO2 core layer. (b) Profiles for TE1 and TM1 modes confined in the metal-clad mesoporous TiO2 waveguide.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Titanium tetraisopropanolate (TTIP) was obtained from Aladdin Industrial Corporation. Block copolymer HO(CH2−CH2O)20(CH2CH3CHO)70(CH2CH2O)20H (P123) was received from Sigma-Aldrich Corporation. Hydrochloric acid (HCl), absolute ethanol (EtOH), and sucrose were purchased from Beijing Chemical Works. Sodium dichromate (Na2Cr2O7·2H2O) and elaidic acid (EA) were purchased from Johnson Matthey Company. All the above chemicals are analytical grade. 2.2. Preparation of the Sensor Chips. According to the synthetic method described in ref 24, the coating solution containing TTIP and P123 and HCl dissolved in absolute ethanol was first prepared in which the components have the mass ratio of m(P123)/m(TTIP)/m(HCl)/m(EtOH) = 1.0:5.23:3.2:12. The as-prepared coating solution was kept at room temperature for overnight before use. To fabricate the metal-clad waveguide resonance sensor chips, a 3 nm chromium film and a 40 nm gold film were successively sputtered on the glass substrate. The gold-covered glass substrate was then dip-coated with the coating solution. The substrate, after being sufficiently dried at 50 °C, was heated in air at 400 °C for 5 h for complete removal of P123 from the film. The resulting mesoporous TiO2 thin films were obtained, which were used as the gold-clad optical waveguide resonance sensor chip. To investigate the influence of the film porosity on the sensor’s performance, the total three mesoporous TiO2 films with different porosities (labeled as No. 1, No. 2, and No. 3) were prepared by adjusting the mass ratio of P123 to TIIP in the coating solutions. The mass ratio of P123 to TIIP is 0.5:5.23 for the No. 1 film, 1:5.23 for the No. 2 film, and 2.0:5.23 for the No. 3 film. 2.3. Construction of the Metal-Clad Waveguide Resonance Sensor. Figure 1a schematically shows the waveguide resonance sensor with Kretschmann configuration. The sensor operates at wavelength interrogation mode, mainly containing a homemade mesoporous TiO2 waveguide, a fluidic chamber, a glass prism (45°/45°/90°), a tungsten-halogen lamp, and a charge-coupled-device (CCD) spectrometer. The waveguide is tightly sandwiched between the prism and the chamber, and a close attachment of the glass substrate of the waveguide to the prism is realized with the high-index coupling liquid. A peristaltic pump is used for fluidic injection. Broadband light from the lamp transmits through a quartz fiber, a lens, and a linear polarizer to become a collimated and linearly polarized beam. The beam is incident upon the prism at the angle θ, undergoing the total internal reflection at the glass/ gold interface of the waveguide. The reflected light beam is

resonance sensor chips. The prepared mesoporous TiO2 thin films are featured with high hydrothermal stability and largearea homogeneous surface morphology and uniform distribution of pores. Compared with the porous anodic alumina film earlier used for waveguide resonance sensing,17 the mesoporous TiO2 film is chemically more robust, making the waveguide resonance sensor more stable upon exposure to acidic and (or) basic solution samples. Moreover, the mesoporous TiO2 film is highly transparent in the spectral range from visible to nearinfrared, enabling the sensor to operate in a broad bandwidth with an inexpensive charge-coupled-device spectrometer. From this point of view, a nanoporous TiO2 waveguide sensor is advantageous over the porous silicon based counterpart reported recently.13 Refractive index (RI) of the mesoporous TiO2 thin films can be modulated in a wide range of 1 (air RI) < n ≤ 2.7 (RI of bulk TiO2), depending on its porosity. By using the homemade Kretschmann-type prism coupling platform, the polarized reflection spectra of the fabricated metal-clad optical waveguide chips were obtained, from which the narrow dips resulting from the resonance excitation of both the TE and TM modes in the mesoporous TiO2 film were observed. The corresponding resonance wavelengths were determined. The porosity of the film was derived from the best fitting of the simulation data to the measured resonance spectra. Moreover, by using the fabricated mesoporous TiO2 film as the chemical sensitive core layer, the metal-clad waveguide resonance sensor was employed for in situ nonspecific detection of hazardous substances, including elaidic acid (EA) and hexavalent chromium ions (Cr6+). EA widely exists in our food, and vast consumption of EA could cause serious diseases, including arteriosclerosis, loss of brain function, cancer, type II diabetes mellitus, and so forth.20,21 Cr6+ is a human respiratory carcinogen and has been identified as one of the highly dangerous toxic substances by EPA due to its persistent contamination to the environment.22,23 Therefore, the selection of EA and Cr6+ as the targets to be detected can well reflect the practical applicability of the sensor prepared in this work. The TE- and TM-related sensitivities of the sensor were obtained at the same incident angle. The mutual calibration of the dual sensitivities of the sensor was realized. Finally, to investigate the porosity dependence of the waveguide resonance sensor’s sensitivity to adsorbed small molecules, a series of mesoporous TiO2 films with different porosities were fabricated. The experimental conclusion was obtained that a larger porosity of the mesoporous TiO2 film can lead to a high sensitivity. The findings would be greatly helpful for understanding the sensor’s performance and for optimal design of the sensor. B

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uniform distribution of fine cracks on its surface. Considering the presence of the mesoporous structure, the TiO2 film possesses the micro−nano hierarchical structure. Such a hierarchical structure of the TiO2 film can enhance the availability of the pore walls for small molecules and allows for the rapid diffusion of small molecules into the film, consequently enabling to improve the sensor’s sensitivity and expedite the sensor’s response. From the cross-sectional image shown in Figure 2b, the thicknesses of the gold layer and the mesoporous TiO2 film were estimated as TAu = 40 nm and TTiO2 = 275 nm, respectively. 3.1.2. Determination of the Film Porosity. The TE- and TM-polarized reflection intensity spectra of the waveguide resonance sensor in air were first investigated in the wavelength range from 400 to 900 nm. Figure 3a shows the TM-polarized

detected with the CCD spectrometer. The reflection spectrum exhibits deep and sharp troughs resulting from the resonant evanescent coupling of the input power to the guided modes in the waveguide. Therefore, the guided-mode-resonance wavelengths (λR) can be determined from the measured reflection spectrum. As shown in Figure 1b, the guided modes excited in the mesoporous TiO2 core layer can be only TE or only TM or both. To investigate the refractive-index (RI) sensitivity of the mesoporous TiO2 waveguide resonance sensor, a series of aqueous sucrose solutions were prepared as standard RI samples. Using a commercial Abbe refractometer, the solution RI was measured to linearly increase from 1.3330 to 1.3388 with increasing the sucrose concentration from 0 to 4 wt %. To characterize the chemical sensing properties of the sensor, seven ethanolic EA solutions (0.05, 0.1, 1.0, 2.0, 3.0, 4.0, and 5.0 mmol/L) and five aqueous Cr6+ solutions (100, 200, 300, 400, and 500 μmol/L) were prepared. All the solution samples were kept for 2 h before use. During the measurements, the solution samples were step-by-step injected in the chamber according to the order from low to high concentration. Each concentration leads to a stable spectrum, from which λR is accurately determined. Finally, the concentration dependence of λR can be established.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Metal-Clad Mesoporous TiO2 Waveguide. 3.1.1. SEM Characterization of the Mesoporous TiO2 Film. The surface and cross-sectional morphologies of the gold-clad mesoporous TiO2 waveguide (No. 2) were investigated by using a scanning electron microscope (SEM). Figure 2 shows the SEM images. It is seen with Figure 2a that the TiO2 film is homogeneous with

Figure 3. (a) TM- and (c) TE-polarized reflection intensity spectra of the mesoporous TiO 2 waveguide measured in air and the corresponding best-fit spectra (b,d).

spectra measured at different incident angles ranging from θ = 12° to θ = −12° (the interval is 4°). The TE-polarized spectra detected at the incident angles varying from 1° to −23° are shown in Figure 3b. Each spectrum contains a well-shaped dip, indicating the resonance excitation of a single guided mode in the waveguide at the present condition. A combination of Figure 3a,b reveals that both the TE and TM modes can be simultaneously excited in the mesoporous TiO2 core layer if setting the incident angle in the range of 1° to −12°. In addition, the experimental results indicate that both the TE and TM modes can also be excited after filling the chamber with water or ethanol. To determine the porosity of the mesoporous TiO2 film, the measured reflection intensity spectra were fitted based on the

Figure 2. (a) Surface and (b) cross-sectional morphologies of the gold-clad mesoporous TiO2 waveguide observed by SEM. C

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mesoporous TiO2 film based waveguide (No. 2) resonance sensor measured simultaneously with the prepared sucrose solutions for the TM (Figure 5a) and TE (Figure 5b) polarizations at an incident angle of θ = 8°. The resonant wavelength shifts for the TM (ΔλR‑TM) and TE (ΔλR‑TE) polarizations gradually increases with the increase of the sucrose concentrations. Figure 5c displays the linear dependences of ΔλR‑TM and ΔλR‑TE on RI values of sucrose solutions for both the TM and TE polarizations. It indicates that the RI sensitivity for the TM polarization is 2460 nm/RIU, which is about 2.8 times as high as that for the TE polarization. This is in accordance with the conclusion given in Figure 4. 3.2.2. In Situ Detection of EA. The same metal-clad optical waveguide resonance sensor chip as the one used above was employed in investigating the adsorption response to EA and Cr6+. Figure 6a,b displays, respectively, the reflected light intensity spectra measured simultaneously under the TM- and TE-polarized light at an incident angle of θ = 10° with different EA concentrations. The results shown in Figure 6a,b both indicate that the value of λR gradually increases with the concentration of EA solutions increasing from 0 to 5 mM. As shown in Figure 6c, binomial relationships exist between the resonant wavelength shifts (ΔλR) and the concentration of EA solutions for both the TM and TE modes. Besides, when the concentration of EA increased from 0 to 50 μM, ΔλR‑TM is estimated to be 0.8 nm. Due to the spectral resolution of the spectrometer (0.5 nm), the detection ability of the mesoporous TiO2 film-based metal-clad waveguide resonance sensor for EA detection is estimated to be at least 18 ppm. Based on the Fresnel theory and Bruggeman equation, the linear relationships between the calculated ΔλR and the volume fraction ( f 3) of adsorbed EA molecules (n3 = 1.4626) for the TM and TE modes were obtained, which are graphed in Figure 7. The initial resonance wavelengths for the TM and TE modes used in calculations are accordance with that given in Figure 6a,b. It can be seen from Figure 6a that ΔλR‑TM was measured to be 14.12 nm with the EA concentration varying from 0 to 5 mM. Then the value of f 3 was calculated to be about 0.155 according to the fitting result (ΔλR = 90.89706f 3 − 0.01581) for the TM mode shown in Figure 6. If all the adsorbed EA molecules are equivalent to a dense EA film with an area equal to the surface of the waveguide layer (TTiO2 = 275 nm), the thickness of the dense EA film was determined to be about 42.6 nm,19 which means that the amount of adsorbed EA molecules is much more than that of the EA monolayer film. Also, the same conclusion could be obtained by analyzing the experimental results of the TE mode. The findings above indicate that the large internal surface area of the mesoporous TiO2 film enables the metal-clad waveguide resonance sensor to give high performance in detecting small molecules. Furthermore, there are two experimental sensitivities obtained with the mesoporous TiO2 film-based waveguide resonance sensor for the TM and TE modes. The deviation between the experimental and calculated sensitivities for the TE mode could be acquired combining the measured ΔλR‑TM and the calculated results shown in Figure 7. The specific calculation results were shown in Table 1. First, ΔλR‑TM and ΔλR‑TE measured with different concentrations of EA solutions are listed in the second and the third columns. The values of f 3 (in the fourth column) were calculated with fit formula for the TM mode given in Figure 7. Then, the calculated values of ΔλR‑TE (in the fifth column) were obtained by substituting f 3 into the other fit formula for the TE mode (ΔλR = 45.71324f 3

Fresnel equations combined with the Bruggeman approximation for an average RI (n) of porous material.19 The Bruggeman approximation can be expressed as eq 1. f1

n12 − n2 n12 + 2n2

+ f2

n2 2 − n2 n2 2 + 2n2

+ f3

n3 2 − n 2 n32 + 2n2

=0 (1)

where f1, f 2, and f 3 are volume fractions of the TiO2 and the pores and the analyte molecules adsorbed in the pores, respectively ( f1 + f 2 + f 3 = 1). n1, n2, and n3 are refractive indexes of the above three components. In the case of f 3 = 0, f 2 represents the film porosity (P = f 2 + f 3). Figure 3b,d display the best-fit spectra numerically calculated given TAu = 40 nm and nair = 1 and using the literature data of individual RI dispersions for glass and gold and TiO2. Considering the fact that the reflection spot moves with the incident angle and the thicknesses at different areas of the TiO2 film are slightly different (this can be judged visually from the color of the film), the best-fit spectra corresponding to all the measured spectra were obtained under the condition of setting the film porosity as P = 0.535 and modifying Tfilm within 15 nm around 275 nm. Therefore, P = 0.535 is the best value for the mesoporous TiO2 film (No. 2) under investigation. 3.1.3. Field-Enhancement Factor Distribution. The field enhancement factor (|Ey| for the TE mode and |Ex| for the TM mode) distributions in the gold-clad mesoporous TiO 2 waveguide with water superstrate were calculated based on the transfer matrix method25 and using the same parameters as those used for spectrum simulation. According to the reflection intensity spectra measured at θ = 8° with water superstrate, the resonance wavelengths of the TM and TE modes are 730 and 520 nm, respectively. Therefore, calculations were performed with λ = 730 nm for TM mode and λ = 520 nm for TE mode. Figure 4 shows the calculated |Ey(x)| for TE mode and |Ex(x)|

Figure 4. Calculated electric field distributions within the mesoporous TiO2 film based optical waveguide for the TM (λR0 = 730 nm) and TE modes (λR0 = 520 nm) with water atmosphere.

for TM mode. The two field profiles correspond to the TM1 and TE1 modes. The maximum of |Ey(x)| is in the mesoporous TiO2 core layer, and |Ex(x)| has a maximum at the TiO2 film/ water interface. The penetration depth of the evanescent field in the water superstrate for the TM1 mode is larger than that for the TE1 mode. Therefore, the RI sensitivity of the sensor with the TM1 mode should be higher than that with the TE1 mode at the same incident angle, which is demonstrated with the experimental results below. 3.2. In Situ Chemical Detection. 3.2.1. Refractive-Index Sensitivities. Figure 5 gives the bulk RI sensitivities of D

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Figure 5. Reflected light intensity spectra measured with different concentrations of aqueous sucrose solutions under the (a) TM- and (b) TEpolarized incident light. (c) Linear dependence of ΔλR on RI values of solutions under the TM- and TE-polarized incident light (θ = 8°).

Figure 6. (a) Reflected light intensity spectra measured with different concentrations of EA solutions under the (a) TM- and (b) TE-polarized incident light. (c) Shifts of λR versus the concentrations of EA solutions under the TM- and TE-polarized incident light (θ = 10°).

− 0.04449). Finally, the deviations between the experimental and the calculated values for the TE mode listed in the last column were obtained. By observing the values of the calculated deviation that are less than 18%, we can get that the sensing performance of the mesoporous TiO2 film-based waveguide resonance sensor is very excellent. Moreover, the two sensitivities obtained with the TM and TE modes can be calibrated with each other, which effectively improved the accuracy of the sensor’s performance. As mentioned above, the obtained two sensitivities for the TM and TE modes may also

be used to analyze the birefringence and optical dichroism of anisotropic materials, which is of significance for the applications of the metal-clad waveguide resonance sensor. 3.2.3. In Situ Detection of Cr6+. Correspondingly, Figure 8a,b gives the reflected light intensity spectra obtained with different concentrations of Cr6+ solutions with the TM and TE polarized light at an incident angle of θ = 8°. Similarly, a redshift of λR occurred with the concentration of EA solutions increasing from 0 to 500 μM. Figure 8c shows the linear relationship between ΔλR and the Cr6+ concentrations for the E

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500 μM, the positions of the reflected resonance bands are barely changed. Thus, it is effectively proven that the mesoporous TiO2 film-based waveguide resonance sensor’s responses to small molecules are much more sensitive than that of the conventional SPR sensor. 3.3. Sensitivity Comparison between Different Mesoporous TiO2 Film Based Waveguide Resonance Sensors. Since the porous thin film is used to amplify the adsorption ability to small molecule substances, here we made a sensitivity comparison using different mesoporous TiO2 film-based waveguide resonance sensors (No. 1−3). Figure 9d−f gives the corresponding SEM morphology of the prepared mesoporous TiO2 thin film (the insets are the zoomed-in SEMs of the original SEM images), from which we can see it easily that the porosity of the mesoporous TiO2 thin film increased as the mass ratio of P123 increased. Figure 9a−c shows the resonance curves measured with EA solutions whose concentrations are 0 and 5 mM. It indicates evidently that the value of the total shift of λR (ΔλRT) increased as the mass ratio of pore-forming reagent increased. Table 2 shows the comparison result of the adsorption sensitivity for Cr6+ and EA with TE mode. As the concentration of the Cr6+ solution changes from 0 to 500 μM and the concentration of the EA solution changes from 0 to 5 mM, the measured values of λR0 and ΔλRT changes and are given in the last four columns of Table 2. A conclusion was obtained that the larger the porosity, the higher the sensitivity. Moreover, for the metal-clad waveguide labeled by No. 1, the measured λR hardly shifts when the concentration of Cr6+ exceeds 200 μM. However, for the metal-clad waveguide labeled by No. 2 and No. 3, the measured λR persistently changes with the concentration of Cr6+ varied from 0 to 500 μM. This illustrates that it is easier for the mesoporous TiO2 film whose porosity is lower to reach the adsorption saturation, which limits the detection limit of the sensor. Based on the results obtained above, it was found that the greater the porosity of mesoporous TiO2 thin film, the higher the adsorption sensitivity to small molecules. It is easy to explain this phenomenon. The greater the porosity, the larger the specific surface area, and then more small molecules will be adsorbed within the mesoporous thin film. The interaction between the optical wave and the test samples will thus be stronger, and a higher adsorption sensitivity could be obtained. Thus, the mesoporous TiO2 thin film with larger porosity can offer the metal-clad waveguide resonance sensor an excellent capacity for small molecules detection.

Figure 7. Calculated shifts of λR versus the volume fraction ( f 3) of adsorbed EA molecules with the TM- and TE-polarized incident light.

TM and TE polarizations. Among the measurement results, the linear slope for Cr6+ detection with the TM polarization was Δλ calculated as ΔCR = 10.657 nm/mM , which means the detection limit for Cr6+ of the mesoporous TiO2 film-based optical waveguide resonance sensor is about 2.4 ppm with the consideration of spectral resolution of the spectrometer. As the same with EA detection, the sensitivity for Cr6+ detection with the TM mode is higher than that obtained simultaneously with the TE mode. This is in accordance with the calculated results on the electric field distribution among the multilayer metalclad waveguide shown in Figure 4. Significantly, as the refractive indexes of EA and Cr6+ solutions are almost the same with the solvent (blank solution), the variation of λR in the test is certainly caused by the molecule adsorption rather than the bulk refractive index of solutions. It is proved convincingly that the mesoporous TiO2 film-based metal-clad waveguide is a great structure for small molecules in biosensing. In addition, we can see the difference between the relationships presented in Figure 6c (a binomial relationship between ΔλR and analyte concentrations for EA detection) and Figure 8c (a linear relationship between ΔλR and analyte concentrations for Cr6+ detection). This resulted from the difference between the analyte concentrations. The maximum concentration for EA detection is 5 mM, while the maximum concentration for Cr6+ detection is 0.5 mM. This makes it simple to understand that, for the same waveguide resonance sensor chip, it is easier for the former to reach the adsorption saturation due to the higher concentration of test samples, and a linear relationship exists between ΔλR and concentration of Cr6+ solutions due to the lower concentration of test samples. As a comparison, we have studied the adsorption responses to EA and Cr6+ based on the conventional SPR sensor decorated by naked-gold thin film. The results show that whether the concentration of EA solution changes from 0 to 5 mM or the concentration of Cr6+ solution changes from 0 to

4. CONCLUSIONS A gold-clad waveguide resonance sensor using a mesoporous TiO2 thin film as the chemical sensitive core layer was prepared with a sol−gel template method in this article. Combined with

Table 1. Deviations between the Measured ΔλR‑TE (exptl) and the Calculated ΔλR‑TE (calcd) for the TE Mode C (mM)

ΔλR‑TM (nm, exptl)

ΔλR‑TE (nm, exptl)

f3

ΔλR‑TE (nm, calcd)

0 1 2 3 4 5

0 4.42 7.95 9.71 12.8 14.12

0 2.27 4.55 5.91 7.73 8.19

0 0.04880 0.08764 0.10700 0.14100 0.1556

0 2.19 3.96 4.85 6.4 7.07

F

|ΔλR ‐ TE(calcd) − ΔλR ‐ TE(exptl)| ΔλR ‐ TE(exptl)

(%)

0 3.524 12.967 17.935 17.206 13.675 DOI: 10.1021/acs.jpcc.7b04408 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 8. Reflected light intensity spectra measured with different concentrations of Cr6+ solutions under the (a) TM- and (b) TE-polarized incident light. (c) Shifts of λR versus the concentration of Cr6+ solutions under the TM- and TE-polarized incident light (θ = 8°).

Figure 9. Measured resonance curves with ethanol (blank solution) and EA solution (5 mM) using different metal-clad waveguide resonance sensors: (a) No. 1, (b) No. 2, and (c) No. 3. Corresponding SEM images of the surface morphology of the mesoporous TiO2 thin film: (d) No. 1, (e) No. 2, and (f) No. 3.

G

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Table 2. Sensor Sensitivity to EA and Cr6+ Adsorption Measured with Different Mesoporous TiO2 Film-Based Optical Waveguides for the TE Mode m(P123)/m(TTIP)

λR0

ΔλRT

λR0

ΔλRT

0.5:5.23 (No. 1) 1.0:5.23 (No. 2) 2.0:5.23 (No. 3)

536.1 516.1 566.48

5.4 8.1 10.4

532.9 511.54 555.61

2.27 4.44 6.34

the Kretschmann configuration, the guided mode propagating in the mesoporous TiO2 thin-film-based waveguide can be excited by both the TM- and TE-polarized incident light. Operated in the wavelength interrogation, deep and narrow waveguide coupling dips were acquired using the fabricated waveguide. Both λR and the sensor’s responses can be accurately obtained by measuring the reflected light intensity spectra. Combining the measurement results and simulations, the porosity of the mesoporous TiO2 thin film was determined to be 0.535. The waveguide resonance sensor is responsive to nonspecific adsorptions of EA and Cr6+. There are binomial relationships between ΔλR and the concentrations of EA solutions, and the in situ detection ability for EA is estimated to be 18 ppm. Additionally, a quasi-linear dependence of ΔλR on the Cr6+concentrations was obtained for the sensor from which an in situ detection limit of 2.4 ppm for Cr6+ was achieved. The two sensitivities obtained with the TM and TE modes can be calibrated with each other, which significantly improved the accuracy of the sensor’s performance. It has been illustrated that the sensor responses to EA and Cr6+ adsorption are much superior to those of the SPR sensors. The findings also demonstrated that the greater the porosity of mesoporous TiO2 thin film, the higher the adsorption sensitivity to small molecules. The mesoporous TiO2 thin-film-based waveguide resonance sensor operating in the wavelength interrogation mode exhibits potential application for practical detection of biochemical small molecules.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04408. Theoretical simulations on sensitivities of the wavelength-interrogated waveguide resonance sensor (PDF)

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REFERENCES

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Cr6+ (nm)

EA (nm)

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiumei Wan: 0000-0001-7401-7727 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (973) (2015CB352100), National Natural Science Foundation of China (61377064, 61675203), and Research Equipment Development Project of Chinese Academy of Sciences (YZ201508). H

DOI: 10.1021/acs.jpcc.7b04408 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (20) Park, K. H.; Kim, J. M.; Cho, K. H. Elaidic Acid (EA) Generates Dysfunctional High-Density Lipoproteins and Consumption of EA Exacerbates Hyperlipidemia and Fatty liver Change in Zebrafish. Mol. Nutr. Food Res. 2014, 58, 1537−1545. (21) Salimon, J.; Omar, T. A.; Salih, N. An Accurate and Reliable Method for Identification and Quantification of Fatty Acids and Trans Fatty Acids in Food Fats Samples Using Gas Chromatography. Arabian J. Chem. 2017, 10, S1875−S1882. (22) Fierro, S.; Watanabe, T.; Akai, K.; Einaga, Y. Highly Sensitive Detection of Cr6+ on Boron Doped Diamond Electrodes. Electrochim. Acta 2012, 82, 9−11. (23) Hassan, S. H. A.; Van Ginkel, S. W.; Oh, S. E. Detection of Cr6+ by the Sulfur Oxidizing Bacteria Biosensor: Effect of Different Physical Factors. Environ. Sci. Technol. 2012, 46, 7844−7848. (24) Alberius, P. C. A.; Frindell, K. L.; Hayward, R. C.; Kramer, E. J.; Stucky, G. D.; Chmelka, B. F. General Predictive Syntheses of Cubic, Hexagonal, and Lamellar Silica and Titania Mesostructured Thin Films. Chem. Mater. 2002, 14, 3284−3294. (25) Kaur, D.; Sharma, V. K.; Kapoor, A. High Sensitivity Lossy Mode Resonance Sensors. Sens. Actuators, B 2014, 198, 366−376. (26) Elaidic acid. https://pubchem.ncbi.nlm.nih.gov/compound/ Elaidic_acid#section=Spectral-Properties.

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DOI: 10.1021/acs.jpcc.7b04408 J. Phys. Chem. C XXXX, XXX, XXX−XXX