Thin Films as Wavelength-Interrogated Waveguide Resonance

Jan 16, 2012 - Application of Porous TiO2 Thin Films as Wavelength-Interrogated. Waveguide Resonance Sensors for Bio/Chemical Detection. Zhe Zhang ...
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Application of Porous TiO2 Thin Films as Wavelength-Interrogated Waveguide Resonance Sensors for Bio/Chemical Detection Zhe Zhang, Dan-Feng Lu, 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 S Supporting Information *

ABSTRACT: Porous TiO2 thin-film waveguides with the gold clad were fabricated by colloidal deposition for real-time detection of small molecule substances based on interrogating the guided mode resonance wavelength (λR). By measuring the reflected light intensity spectrum, both λR and the sensor response were accurately obtained. The mode order corresponding to the detected λR and the porosity of the TiO2 film were determined by a combination of simulations and measurements. The sensor is responsive to nonspecific adsorptions of glutathione (GSH) molecules, lead(II) ions, and the block copolymer. The volume fraction ( f 3) of adsorbed GSH molecules was determined to be proportional to changes in λR (ΔλR). The saturation adsorption of GSH results in ΔλR = 8.1 nm, corresponding to f 3 = 0.085, that is 17.9% of the film porosity. The findings indicate that the adsorbed amount of GSH is equivalent to more than 10 complete monolayers with the same area as the porous TiO2 film used. A quasi-linear dependence of ΔλR on the logarithm of the lead(II) concentration with the detection limit of 1 ppm was obtained for the sensor. The best fit to the time course of ΔλR measured with the sensor during the block copolymer adsorption process was obtained based on the Langmuir kinetics. work published in 2007,11 we show the first example of a wavelength-interrogated PWMR sensor. A PWMR sensor with wavelength interrogation over a broad bandwidth allows to change the incident angle for flexible adjustment of sensitivity and consequently for extension of the measurement range of analyte concentrations.11−14 With a time-resolved CCD spectrometer, the PWMR sensor can be used for kinetic study of molecular adsorption inside the porous film.13 In this work, the PWMR sensor was prepared using the gold clad porous TiO2 thin-film waveguides, and the guided mode resonance wavelength of the sensor was determined directly from the reflected light intensity spectrum detected with the charge-coupled-device (CCD) spectrometer. The sensor’s responses to individual adsorptions of GSH molecules, lead(II) ions, and the block copolymer were in situ investigated. As a result, the interesting properties of the PWMR sensor were obtained, including the mode orders, the waveguide porosity, the volume fraction of adsorbed molecules, and its relationship with the induced shift in the resonance wavelength. These findings would be useful for a better understanding of the PWMR sensor.

1. INTRODUCTION Porous waveguide mode resonance (PWMR) sensors have recently attracted considerable attention because of their unique features such as electromagnetic immunity, ultrahigh sensitivity, molecular sieve function, and real-time and label-free detection ability, as well as good compatibility with the electrochemical technique.1−14 PWMR sensors generally have a similar configuration to conventional surface plasmon resonance (SPR) sensors but are much more sensitive than the latter for detection of molecular adsorption. The ultrahigh sensitivity of PWMR sensors results from the strong interaction between the guided mode and the analyte molecules adsorbed inside the nanoporous waveguide. For a conventional SPR sensor with a naked gold film, the interaction of the evanescent wave with the adsorbed target molecules occurs at the gold-layer/solution-sample interface, with an interaction depth as small as the size of analyte molecules, which leads to a limited sensitivity and consequently makes SPR sensors unable to directly detect small-molecule substances with molecular weight below 200 Da especially in the case of a low concentration.15 In practice, a large variety of toxic, harmful, and hazardous substances such as pesticides, cyanide, drugs, TNT, heavy metal ions, and volatile organic compounds are small molecules. They cannot be easily detected with SPR sensors. From the point of view of the trace detection of small molecules, PWMR sensors have the great advantage over SPR sensors,7 exhibiting a bright application prospect as an alternative to SPR sensors. So far, PWMR sensors have been studied almost exclusively in the angular interrogation mode.1−10 In our previous © 2012 American Chemical Society

2. PREPARATION OF THE PWMR SENSOR Slide glass substrates were obtained from the Matsunami Glass Ind., Ltd., and their RI dispersion is given as nS = 6.283 × 10−13λ4 − 1.898 × 10−9λ3 + 2.192 × 10−6λ2 − 1.172 × 10−3λ + 1.766 with Received: October 25, 2011 Revised: January 13, 2012 Published: January 16, 2012 3342

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λ in nm. The gold clad TiO2 nanoporous waveguides were fabricated by successive sputtering of a 3 nm Cr layer and a 45 nm gold layer on the glass substrate followed by dip-coating from the aqueous colloidal TiO2 solution mixed with a triblock copolymer (EO)20(PO)70(EO)20 (Pluronic P123). With tape, the back side of the substrate was protected from touching the solution during dipcoating. The substrates were dried at room temperature and then calcined in air at 400 °C to remove the P123 copolymer from the colloidal TiO2 layers. After calcination, the gold clad nanoporous TiO2 thin film waveguides were obtained. Figure 1a shows the wavelength-interrogated PWMR sensor system used in this work. It consists of a tungsten-halogen lamp

incoming light to the guided modes results in ATR dips that can be observed in the reflected light intensity spectrum. This means that the resonance wavelength (λR) and the induced changes in λR can be directly determined by use of the CCD to detect the reflected light intensity spectra. For understanding the PWMR sensor in the wavelengthinterrogation mode, the field profiles for the transverse magnetic (TM) modes of different orders and the corresponding ATR dips in the reflectance spectrum were also illustrated in Figure 1b. For all TM modes, the magnetic field Hx has a peak at the gold layer/TiO2 film interface (the field in the gold layer not shown). At a given angle of incidence, a higher-order mode has a smaller λR. The TM0 mode with the largest λR corresponds to the surface plasmon polariton (SPP).

3. RESULTS AND DISCUSSIONS 3.1. SEM Characterization of the Porous TiO2 Film. Figure 2 shows the scanning electron microscope (SEM)

Figure 1. (a) Photograph of the wavelength-interrogated PWMR sensor system used and (b) schematic of the prism waveguide chamber assembly (the field profiles for the TM modes of different orders and the corresponding resonance bands were also shown).

and a CCD spectrometer (both inside the bottom box), a pair of fiber collimators, a linear polarizer, a porous TiO2 waveguide, a fluidic chamber, and a high-RI glass prism (45°/45°/90°, np = − 5.56 × 10−10λ3 + 1.491 × 10−6λ2 − 1.368 × 10−3λ + 2.208). As shown in Figure 1b, with a high-RI liquid (CH2I2, n = 1.741) the prism is attached to the back of the waveguide that is then covered with the chamber. Broadband light from the lamp passes through a quartz fiber, the collimator, and the polarizer to produce a linearly polarized collimated beam. The beam is incident upon the prism at an angle θ (θ > 0 if the beam is between the glass substrate and the prism-surface normal, otherwise, θ < 0), undergoing attenuated total reflection (ATR) at the glass substrate/gold layer interface. ATR is accompanied by an evanescent field that penetrates through the gold layer to excite the guided modes in the porous TiO2 film at specific wavelengths. At these wavelengths, the energy transfer from the

Figure 2. SEM images of the gold clad porous TiO2 thin film waveguide (a, the exterior surface; b, the cross-section).

images of the surface and the cross-section of the gold clad porous TiO2 waveguide. The images reveal a random loose packing of TiO2 nanoparticle aggregates that offers the film the inter- and intra-aggregate pores and renders the film crack-free. Such morphology of the film makes its large internal surface area easily available for exterior molecules. Both the external and cross-sectioned surfaces of the porous TiO2 film are rather coarse, able to cause a large scattering of the guided mode to 3343

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indexes of the above three components. In the case of f 3 = 0, f 2 is the film porosity (P). Given n1 = − 4.676 × 10−9λ3 + 1.160 × 10−5λ2 − 9.854 × 10−3λ + 5.645 for TiO217 and n2 = − 1.4758 × 10−10λ3 + 3.4377 × 10−7λ2 − 2.836 × 10−4λ + 1.4112 for water,18 the average RI dispersion from 400 to 900 nm for the porous TiO2 film immersed in water was calculated as a function of porosity based on eq 1. Figure 4a displays the

consequently broaden its resonance band. The glass substrate/ gold layer and gold layer/TiO2 film interfaces are clearly seen. The thicknesses of the gold layer and the TiO2 nanoporous film were estimated as TAu = 42.2 nm and Tfilm = 310 nm, respectively. 3.2. Determination of Both the Mode Orders and the Film Porosity by a Combination of Simulations and Measurements. With the above porous TiO2 waveguide, both the mode orders and the film porosity were determined by a combination of simulations and measurements. Figure 3a displays

Figure 4. (a) Average RI dispersion of the water-filled porous TiO2 film calculated as a function of the film porosity without molecular adsorption and (b) that versus the volume fraction of adsorbed GSH molecules with the given porosity of P = 0.474.

Figure 3. (a) Measured light intensity spectrum and calculated reflectance spectrum at θ = 15° with the air clad; (b) the calculated effective RI dispersions for the TM1 and TM2 modes with the water and air clads; (c) the measured and calculated spectra at θ = 6° with the water clad.

calculated data. The n vs λ curves at P = 0 and P = 1 represent the RI dispersions of TiO2 and water, respectively. The average RI dispersion versus porosity was also calculated by setting n2 = 1 for the porous TiO2 film exposed to air (air RI independent of wavelength, the result not shown). These calculated data are useful for the numerical calculations based on the Fresnel equations. The numerical calculations were fulfilled using TAu = 42.2 nm and Tfilm = 310 nm and given f 3 = 0. As shown in Figure 3a, the best fit to the measured resonance band was obtained with the fitting parameter f 2 = 0.474. The obvious broadening of the measured band in contrast to the simulated band is mainly attributed to light scattering of the porous TiO2 film. To determine the order of the guided mode observed at λR = 671 nm, the effective RI (N) versus wavelength (λ) for the TM modes in the porous TiO2 waveguide were calculated by solving the eigenvalue equation for a three-layer planar

a TM-polarized light intensity spectrum detected at θ = 15° with the empty chamber. The spectrum includes a single ATR dip at λR = 671 nm. The best fit to the measured data was carried out by use of the Fresnel equations combined with the Bruggeman approximation for the average RI (n) of a porous material that can be expressed as eq 1.16

f1

n12 − n2 n12 + 2n2

+ f2

n 2 2 − n2 n2 2 + 2n2

+ f3

n32 − n2 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 3344

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waveguide that can be written as eq 2.

2π TTiO2 n2 − N2 λ ⎛ 2 ⎞ n N2 − ε ⎟ = mπ + arctan⎜⎜ ⎟ ⎝ ε n2 − N 2 ⎠ ⎛ 2 ⎞ n N2 − nc 2 ⎟ ⎜ + arctan ⎜ 2 2 2⎟ ⎝ nc n − N ⎠

(2)

where m represents the mode order, ε is the real part of gold’s dielectric constant, nc is the clad RI. Figure 3b shows the effective RI dispersion curves calculated with the water and air clads, implying that the porous TiO2 waveguide is able to support the TM1 and TM2 modes in the wavelength range 400 to 900 nm. According to the phase-matching condition, at the resonance wavelength the propagation constant of the guided mode is equal to the z axis component of the wave vector of the incoming light. Namely, the following equation should be satisfied at the resonance wavelengths.

⎡ ⎛ sin θ ⎞⎤ ⎟⎟⎥ N = n p sin⎢45° + arcsin⎜⎜ ⎢⎣ n ⎝ p ⎠⎥⎦

(3)

where np is the prism RI. By calculating the right side of eq 3 with θ = 15° and 6°, the two curves were obtained as shown in pink in Figure 3b. The curve with θ = 15° and the effective RI dispersion curve with air for the TM1 mode intersects at exactly λ = 671 nm, indicating that the ATR dip observed in Figure 3a corresponds to the TM1 mode. The curve with θ = 6° and the effective RI dispersion curve with water for the TM2 mode intersects at λ = 558 nm. A resonance band appears at the same wavelength in the Fresnel reflectance spectrum calculated with θ = 6°, f 2 = 0.474, TAu = 42.2 nm, and TTiO2 = 310 nm (see Figure 3c). Moreover, the reflected light intensity spectrum measured at θ = 6° with water in the chamber exhibits an ATR dip at λR = 562 nm, very close to λ = 558 nm. The good agreement between the measured and calculated results suggests that the best-fitting parameter f 2 = 0.474 is approximately equal to the real porosity of the TiO2 film used (i.e., P = 0.474). In addition, the findings reveal that the ATR dips detected with the air and water clads correspond to the TM1 and TM2 modes, respectively. 3.3. Study of the GSH-Sensing Behavior of the PWMR Sensor. The same waveguide as used above was employed in investigating the response of the PWMR sensor to small molecule adsorption. To do this, a series of aqueous GSH solutions of different concentrations were prepared. GSH is the principal intracellular low molecular weight (307.3 Da) thiol playing a critical role in the cellular defense against oxidative and nitrosative stress in mammalian cells. Detection of GSH levels is important to study of cell apoptosis. Figure 5a shows the reflected light intensity spectrum measured at θ = 5° with the water clad and that recorded at the equilibrium of GSH adsorption from the 0.1 mM solution. The two spectra were normalized to the individual peak intensities for a better comparison. λR is 595.4 nm with the water clad. Adsorption of GSH molecules within the porous TiO2 film leads to a redshift of ΔλR = 3 nm. According to dn/dC = 0.188 mL/g,19 the RI difference between water and the solution of 0.1 mM GSH in water is Δn = 5.53 × 10−6, much smaller than the RI detection

Figure 5. (a) Reflected light intensity spectrum measured at θ = 5° with the water clad and that recorded at the adsorption equilibrium of GSH from the 0.1 mM aqueous solution; (b) the resonancewavelength shifts versus the GSH concentrations; (c) the calculated relationship between the resonance-wavelength shift and the volume fraction of adsorbed GSH molecules.

limit of Δn ≈ 10−4 for the sensor (see Supporting Information). Figure 5b displays ΔλR versus the GSH concentration. As the concentration increases from C = 0 to C = 1 mM, ΔλR initially rises fast and then tends to stabilize. With the 1 mM solution, GSH adsorption apparently reaches saturation, leading to the maximum redshift of ΔλR = 8.1 nm. The best fit to the measured data with the formula ΔλR = 49C/(1 + 5C) was also shown in Figure 5b. Fixing the film porosity at 0.474 and given RI = 1.571 for GSH (the RI dispersion not available),20 the average RI dispersion of the porous TiO2 film was calculated versus the volume fraction (f 3) of adsorbed GSH molecules. The calculated data were shown in Figure 4b, which were used to determine the relationship between f 3 and ΔλR based on the Fresnel equations. As shown in Figure 5c, f 3 is proportional to ΔλR with ∂f 3/∂(ΔλR) = 0.0102 nm. The maximum shift of ΔλR = 8.1 nm corresponds to f 3 = 0.085. It suggests that even in the case of saturation adsorption, the adsorbed GSH 3345

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molecules only occupy 17.9% of the total pore volume (P = 0.474). According to its definition, f 3 can be expressed as eq 4.

f3 =

V3 S T T = film 3 = 3 Vfilm SfilmTfilm Tfilm

(4)

where Vfilm and Sfilm are volume and area of the porous TiO2 film exposed inside the chamber, V3 is the volume occupied by the adsorbed GSH molecules, T3 is the thickness of the dense GSH film with the same area of Sfilm and containing the same amount as that adsorbed inside the porous TiO2 film. By substituting f 3 = 0.085 and Tfilm = 310 nm into eq 4, T3 is determined to be 26.35 nm. Considering that GSH is a long chain molecule with a length of about 2.5 nm,21 a 26.35 nm thick dense GSH film corresponds to more than 10 complete monolayers of GSH. The findings indicate that the large internal surface area of the porous TiO2 film enables the adsorbed amount of GSH to increase more than 10 times relative to that adsorbed on a dense film surface. 3.4. Detection of Lead(II) Ions from Water by the PWMR Sensor. A new porous TiO2 thin film waveguide was used to investigate the response of the PWMR sensor to heavy metal ion adsorption. A series of aqueous Pb(NO3)2 solutions with different concentrations ranging from 0.1 ppm (0.3 μM) to 1000 ppm (3 mM) were prepared as the samples to be analyzed. By fixing the incident angle at θ = 6°, the reflected light intensity spectrum was first detected with water in the chamber. From the raw spectrum shown in Figure 6, the value

Figure 7. (a) Reflected light intensity spectra detected at different time points after injection of the P123 solution and (b) the best-fitting curve to the time course of ΔλR.

reflected light intensity spectrum was monitored until it became stable. Figure 6 shows the raw spectra recorded at adsorption equilibrium of lead(II) ions from each solution sample. The resonance band gradually moves to longer wavelengths with increasing the lead(II) concentration from 1 ppm. The spectrum obtained with the 0.1 ppm Pb(NO3)2 solution is almost identical to that with water, indicating that in the present case, the lead(II) detection limit of the sensor is 1 ppm. As shown in the inset in Figure 6, plotting ΔλR against the logarithm of the lead(II) concentration produces a nearly straight line with the slope of d(ΔλR)/d(log C) = 3.37 nm. After the above measurement, the waveguide was cleaned by circulating deionized water through the chamber. The cleaning could not make the resonance band completely return to the position before the lead(II) adsorption, revealing that a small part of adsorbed lead(II) ions are retained in the porous TiO2 film. 3.5. Investigation of the P123 Adsorption Process Using the PWMR Sensor. Since P123 was used as a poreforming reagent during the film preparation, the resultant porous TiO2 waveguides could easily accommodate a large number of P123 molecules inside it. To investigate the adsorption behavior of P123 molecules within the porous TiO2 film by the PWMR method, a solution sample was prepared by dissolving 0.01 g of P123 in 14.13 mL of deionized water (CP123 = 0.122 mM), and the porous TiO2 waveguide cleaned above was used once again as the sensor chip. After filling the chamber with deionized water, the incident angle was adjusted to be slightly larger than 6° in order to produce an optimal resonance band (note that an increase in the frequency of use can impair the porous TiO2 film and consequently the sensor spectrum). Prior to injecting the sample in the chamber, the reflected light intensity spectrum was recorded in a time

Figure 6. Reflected light intensity spectrum with the water clad and those with the aqueous Pb(NO3)2 solutions of different concentrations (the spectra were recorded at the adsorption equilibrium of lead(II), the inset shows a linear dependence of the resonance-wavelength shift on the logarithm of the lead(II) concentration).

of λR with water was determined to be 621.2 nm. Considering that the two porous TiO2 waveguides were prepared from the same coating solution at different time points, their porosities should be identical with each other. Therefore, given that porosity = 0.474, the numerical simulation was carried out in order to estimate the thickness of the new waveguide. As a result, the waveguide thickness was determined to be 344.3 nm, larger than that of the first waveguide used (TTiO2 = 310 nm). To investigate the response of the PWMR sensor to lead(II) adsorption, the water was removed out of the chamber that was then filled with a solution sample. The successive injections of five samples were performed according to the order from lower concentration to higher concentration. After each injection, the 3346

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interval of 1 s in order to observe the initial adsorption behavior of P123 in the porous TiO2 film. Figure 7a shows the raw spectra detected at different time points after the sample injection. The resonance band moves from λR = 620.1 nm at t = 0 s to 627.8 nm at 86 s due to adsorption of P123 molecules within the porous TiO2 film. After 86 s, the resonance band almost stabilized, indicating the establishment of adsorption equilibrium. Figure 7b displays the measured time course of ΔλR and the best-fitting curve obtained with the timedependent Langmuir equation.22,23 The findings suggest that the adsorption process of P123 molecules within the porous TiO2 film approximately obeys the Langmuir kinetics.

ASSOCIATED CONTENT

S Supporting Information *

The reflected light intensity spectra and the refractive-index sensitivities of the PWMR sensor at different incident angles were measured using a new porous TiO2 thin-film waveguide. The experimental condition and results are described in detail. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSIONS The wavelength-interrogated PWMR sensor was prepared using the gold clad porous TiO2 thin film waveguides. A single resonance band, corresponding to the TM1 mode with the air clad and TM2 mode with the water clad, was observed in the reflected light intensity spectra from which their resonance wavelengths were accurately determined. The sensor was demonstrated to be responsive to individual adsorptions of GSH molecules, lead(II) ions, and P123 copolymer. An approach to determine either the film porosity or its thickness with one of them given was obtained by a combination of the theoretical and experimental analyses of the PWMR sensor. The volume fraction of GSH molecules adsorbed within the porous TiO2 film was determined to be proportional to the induced shift in the resonance wavelength. The maximum shift of ΔλR = 8.1 nm and the corresponding volume fraction of 0.085 was obtained with the saturation adsorption of GSH, revealing a more than 10-fold enhancement in the adsorption capacity of the porous TiO2 film relative to that of a dense film. The present PWMR sensor showed a quasi-linear dependence of ΔλR on the logarithm of the lead(II) concentration with the detection limit of 1 ppm. The best fit to the time course of ΔλR measured with the sensor during the P123 adsorption process was obtained based on the Langmuir kinetics. The PWMR sensor operating in the wavelength interrogation mode exhibits the interesting performance and potential application for real time detection of chemical and biochemical substances.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 60978042 and No. 61078039) and the National Basic Research Programme of China (No. 2009CB320300). 3347

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