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Bimetallic sliver/gold sensor chips are attractive since they combine the advantages of both silver and gold layers. Optical properties of the bimetallic sensor chips show significant aging effects. Surface plasmon resonance (SPR) curves were produced on an SPR device and the time dependence of aging on SPR curves was studied. The results show that resonance angle and full width at half maximum (FWHM) of response curves increase with the aging time after film deposition. The performance of the sensor chips in terms of intrinsic sensitivity (IS) degrades with aging time. The underlying mechanism of the aging effect is explained as the growth of a silver oxide layer between gold and silver during the aging process.
©2009 Optical Society of America
1. Introduction
Surface plasmons (SPs) are surface electromagnetic waves that propagate in a direction parallel to the interface of a metal and a dielectric material. A light beam is usually used to excite SPs based on attenuated total reflection method. SPR arises from the interaction of the incident light with SPs at the interface. SPR sensing has been widely applied in a diverse range of fields, including biology, chemistry and environmental studies [1–4]. The ability to detect ever-smaller quantities of biological or chemical species is critical in the fight against diseases, terrorism and environmental pollution. Therefore, a lot of methods have been developed to enhance sensitivity and resolution of SPR biosensors based on different SPR modes or detection methods [5–7]. Optical fibres, metal nanoparticles and various others have been very helpful in making SPR sensors much faster, more reliable, sensitive and accurate [8, 9].
Gold and silver are two main metals that are used for SPR sensor applications. Although silver films yield a more distinct SPR spectrum than gold, this metal tends to be unstable when exposed to chemicals and even oxygen. Hence a bimetallic film was proposed by using silver as an inner layer and gold as an outer layer [10]. Great progress has been made in the theory and application of the bimetallic film [11–14]. It is well known that the bimetallic film combines the advantages of gold and silver. Gold has larger resonance angle shift to the change of refractive index of the sensing layer and is chemically stable. Silver possesses sharper SPR response curve, which can minimize the error to determine the resonance angle. However, what is less well known is that the optical properties of such film can vary with time after film fabrication, i.e., the film may show physical aging effects. The aging effect makes it difficult to characterize the optical properties of the bimetallic films, particularly in an industrial production setting, where one needs to measure the optical properties within a short time after film fabrication for quality control purposes. Understanding this time-dependent behavior can predict the trend in optical properties for the long-term reliability of the sensor chips. The research was initiated to gain insight into the film aging behavior by studying the aging effect on SPR response curves of the bimetallic films. The aging effect on response curves is analyzed and discussed. The underlying mechanism is addressed finally.
2. Materials and methods
Glass slides with a refractive index of 1.61 and an area of 2 cm by 2 cm (Mivitec GmbH, Germany) were used for all of the experiments. Metal film deposition was performed on a diffusion-pumped evaporation system (BOC Edwards Auto 500) at a pressure of 2×10-6 mbar. The deposition rate and thickness of the films were measured and displayed by a film thickness monitor (FTM), which is based on crystal microbalance. The films were kept in the clean room with a relative humidity of 51% and a temperature of 21 °C. A commercial SPR device Biosuplar-3 from Analytical μ-Systems (Mivitec GmbH, Germany) was used to measure the response curves of the bimetallic sensor chips. The operating wavelength is 650 nm. The refractive index of sensing layer was changed by using water and ethanol as buffer solutions. The refractive index units of DI water and ethanol are 1.3321 and 1.3592 [15], respectively.
The performance of the SPR biosensors was evaluated in terms of two aspects. First, the shift in resonance angle for a given change in the sensing layer refractive index should be maximized. Second, the FWHM corresponding to the SPR curves should be minimized so that the error in determining the resonance angle is minimal. The accuracy of the detection of the SPR angle depends on the width of the response curve. To take both aspects into account, a performance parameter called intrinsic sensitivity (IS) is defined. It is directly proportional to angle shift and inversely proportional to the average FWHM of two SPR curves for a given change of refractive index of the sensing layer and the refractive index difference of the sensing layer [7]. The resonance angle (θSPR) closely depends on the refractive index of the sensing layer (ns). If the sensing layer refractive index is altered by δns, the resonance angle shifts by δθSPR. The IS of an SPR sensor with angle interrogation is defined as
Figure 1 shows simulation results of response curves for a 50 nm gold film and a bimetallic film of 43 nm silver and 7 nm gold in water and ethanol, respectively. A titanium film with a thickness of 1.0 nm is used as an adhesion layer in the simulation. The refractive indices of gold, silver and titanium are taken from Refs [16, 17]. The shift (δθSPR) in the SPR curve for Au film and that for the bimetallic film are 2.6791° and 2.5092°, respectively. On the other hand, the average FWHM of the SPR curve for Au and the bimetallic film, respectively, are 3.1736° and 1.9982°. The IS of the gold film is 31.15, while that of the bimetallic film is increased by 48.76%. Thus, a bimetallic film with a 43-nm-thick Ag layer and a 7 nm thick Au layer was used in experiments.
In order to improve the adhesion ability of metal to glass it is necessary to prepare a hydrophilic surface on the glass and add an adhesion layer. Glass slides were cleaned ultrasonically in isopropyl alcohol for 10 minutes to remove dust, and then rinsed in running deionised (DI) water for 1 minute. Next, they were dipped in a 1:3 mixture of concentrated sulphuric acid and hydrogen peroxide for 30 minutes at a temperature of 80 °C to remove organic contaminants, washed again in running DI water for 5 minutes. Afterwards, the glass slides were dried with nitrogen gun and baked in an oven for 3 hours at a temperature of 170 °C. A titanium film with a thickness of 1.0 nm was deposited on glass to improve the adhesion of silver to glass. A silver layer with a thickness of 43 nm and a gold layer with a thickness of 7 nm were deposited afterwards. The deposition rates for gold and silver were ~10 nm/min and 2~3 nm/min, respectively.
3. Results and discussion
3.1 Experimental
Figure 2 shows SPR curves for a bimetallic sensor chip in water and ethanol on the 15th day and the 180th day after fabrication, respectively. The resonance angle in water on the 15th day is 62.079°, and shifts to 64.458° in ethanol, indicating a corresponding angle shift of 2.379°. FWHM of the response curves are 2.525° and 2.622° degrees in water and ethanol, respectively. The response curves of the bimetallic sensor chip on the 180th day are much wider. The IS of the bimetallic film on the 15th day is 34.11, while that on the 180th day is decreased to 24.88. It is observed that aging process has obvious impact on response curves of the bimetallic film. In order to gain insight of aging effect on optical properties, the detailed dependence of resonance angle, FWHM and IS on aging time was examined.
The resonance angles of the SPR curves for the bimetallic film in water and ethanol at different aging time are shown in Fig. 3. For increasing aging time, an increase of the resonance angle in both buffer solutions is observed. For instance, the resonance angle of 62.02° for the film in water on the 8th day shifts to 62.692° on the 169th day, while that of 64.848 ° in ethanol shifts to 65.749° during the same period.
Aging process also plays an important part in FWHM, which is shown in Fig. 4. The FWHM of the SPR curve in water and ethanol, respectively, are 2.525° and 2.622° on the 15th day, while these values shift to 3.66° and 4.66° on the 169th day. An increasing tendency with aging time is clearly shown, which indicates that the error in determining the resonance angle is becoming bigger during the aging process.
To examine the effect of aging time on the sensor chip performance, Fig. 5 shows the variation of IS with aging time for the bimetallic film. The IS of the sample on the 15th day is 34.11, while this value shifts to 24.88 on the 180th day. It was calculated that the performance parameter of the SPR sensor on the 180th day was decreased by 27.06% compared with that on the 15th day.
The same bimetallic chip was used to examine the aging effect in all the experiments. Thus it is necessary to wash the chip after each experiment for next time. The sensor chip in all the experiments was washed in isopropyl alcohol for 10 times altogether. The washing process might introduce roughness of the bimetallic film surface, which could induce broadening of the SPR curves. Another bimetallic sensor chip was used to analyze the effect of washing process on SPR curves on the same day (see Fig. 6). It is clearly shown that the SPR curves for the chip washed once are nearly the same compared with that washed for 21 times, indicating that the effect of washing process on SPR curves can be ignored.
3.2 Simulation
Two possible causes may contribute to the degradation of the sensor performance. The first is interpenetration between silver and gold during the aging process, forming a new layer between silver and gold. The other is oxidation of the silver film during the aging process and gives rise to a thin layer of silver oxide, thereby decreasing the IS.
In the first mechanism, the interpenetrating layer is a mixture of gold and silver, whose effective relative permittivity can be derived following Maxwell-Garnett [18, 19] and Genzel and Martin [20]. The average dielectric function of this layer is assumed to be the average of the two components, which is a good approximation for particles homogeneously distributed within the matrix. Volume fraction was chosen as 0.5, indicating homogeneous distribution of two materials.
Here E Au and EAg are the fields inside gold particles and silver particles, respectively. The average polarization is given by
Where ϵ Au, ϵ Ag and ϵ av are the relative permittivity of gold, silver and the composite, respectively.
An important assumption is that the electric displacement field is consistent inside two materials.
Thus,
The average dielectric function is obtained by combining Eqs. (2)–(5).
This is a more useful form of the MG dielectric function for a composite material. The ϵ av is -14.6659+1.1247i calculated by using Eq. (6). The real part and imaginary part of the refractive index are 0.1467 and 3.8324, which lie between that of gold and silver.
Figure 7 shows the effect of composite layer thickness on resonance angle and FWHM of the SPR curves in water. The simulation values are much smaller than experimental values shown in Figs. 3 and 4, which indicate that simulation results do not agree with experimental results. It also implies that interpenetration between gold and silver is not responsible for the aging effect.
Figure 8 shows the effect of oxide layer thickness on resonance angle and FWHM based on the second possible mechanism. The real part and imaginary part of the refractive index of silver oxide were chosen as 2.5 and 1, respectively, which were taken from Ref [21]. The resonance angle and FWHM of response curves increase with the thickness of the oxide layer due to the aging process, which are consistent with the experimental results shown in Fig. 3 and Fig. 4. Furthermore, the values also agree well with experiments. For instance, the resonance angle of SPR curve for the bimetallic film without oxide layer in water is 61.8393°, which shifts to 62.9184° for the bimetallic film with an oxide layer thickness of 3 nm. The experimental values on the 15th day and on the 180th day are 62.079° and 62.851°, respectively, which agree very well with simulation results. Simulation results on FWHM also agree well with experimental results. Figure 9 shows the variation of IS for the bimetallic film with an oxide layer. It is observed that IS degrades with the oxide layer, which is consistent with the experimental results shown in Fig. 5. Therefore, the oxidation is the cause of aging effect, which agrees well with experimental results.
4. Conclusions
Effect of aging time on SPR curves for bimetallic films was studied. The results show that SPR curves for the bimetallic film strongly depend on the aging time. It is observed that resonance angle and FWHM of response curves increase with the aging time after film fabrication. The sensitivity performance degrades with aging time, which will help predict the response curve reliability of the bimetallic film during the aging process. The cause of the aging effect is explained as the result of the oxidation of the silver layer, which agrees well with experimental results.
Acknowledgment
This work was funded by European Union (FP6 RaSP).
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