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A novel SERS sensor for adenine molecules is fabricated electrochemically using an ordered two-dimensional array of self-aligned silver nanoparticles encapsulated by alumina. Silver is electro-deposited on the interior surfaces at the bottom of nano-channels in a porous anodic aluminum oxide (AAO) film. After etching aluminum, the back-end alumina serves as a SERS substrate. SERS enhancement factor greater than 106 is measured by 532 nm illumination. It exhibits robust chemical stability and emits reproducible Raman signals from repetitive uses for eight weeks. The inexpensive mass production process makes this reliable, durable and sensitive plasmon based optical device promising for many applications.
©2011 Optical Society of America
1. Introduction
Surface-enhanced Raman scattering (SERS) has attracted considerable interest for its utility as an ultrasensitive optical tool for chemical and bioanalytical sensing and imaging [1–4]. For a SERS device to be practically useful, its environmental compatibility, long-term durability, reproducible signals from across multiple devices as well as repetitive uses of the same device, and low manufacturing cost are among desirable features besides its high sensitivity. SERS sensors of ultra-high sensitivity have previously been demonstrated in laboratories based on electromagnetic “hot-spots” [1], which are either vulnerable to harsh environments or occurring on the SERS substrate in a poorly controlled or unpredictable manner due to difficulties in manufacturing the nanostructures of SERS devices. SERS sensors meeting all the aforementioned criteria remain to be highly desirable yet not available. In this paper, a novel and low-cost manufacturing process is demonstrated for a SERS device which meets the criteria as an affordable practical optical sensor. The main strategy is the adoption of an inexpensive electrochemically self-aligned fabrication process allowing mass production of a large-area, 2-D array of ordered and closely spaced silver nanoparticles which are self-encapsulated by thin and high-density anodic alumina layers with controllable thickness of a few nanometers.
Noble metal nanoparticles, e.g. silver (Ag) and gold (Au), are the most commonly employed SERS substrates both in colloidal solutions and on solid surfaces [5,6]. These nanoparticles exhibit a characteristic surface plasmon resonance absorption that is significantly dependent on their size, shape, inter-particle distance, and ambient dielectrics [7–10]. With a proper excitation of this plasmon absorption, a strong local electromagnetic (EM) field enhancement in the vicinity of nanoparticles occurs. Such a local EM field enhancement is responsible for the enhancement in the Raman spectra of molecules near the surface of metal particles [11]. In addition, local plasmonic coupling between resonant nanoparticles, called as “hot spots,” can further enhance the EM field by several orders of magnitudes and thus can offer higher Raman enhancing power [12,13]. Moreover, it is possible to tune the SERS activity by means of adjusting the geometry of nanoparticles or nanostructured substrates with respect to the wavelength and orientation of the laser excitation [14].
To date, many kinds of colloidal nanoparticles, such as metal/alloyed/metal-dielectric spheres, rods, wires, prisms, cubes, stars, dendrites, and flower-like architectures, have been studied for producing extremely sensitive SERS active substrates [9,15–18]. However, the non-uniform dispersion or randomly distributed aggregations of nanoparticles result in randomly distributed “hot spots”, which are neither predictable on different parts of the surface of the same device nor reproducible from device to device. This deficiency together with complicated and expensive fabrication methods for producing high quality nanoparticles hinder their developments into stable, producible, reusable, highly sensitive, and large-scale nanostructure arrays for widely use as SERS active substrates. To overcome these obstacles, manufactured anodic aluminum oxide (AAO) films have been developed for obtaining homogenously nanostructured SERS active substrates [19]. Although these films based on the surface phase system of Ag or Au nanoparticles have yielded enhanced uniformity and repeatability of the SERS signal, the exposure of Ag or Au nanoparticles to harsh environments deprives them of long-term chemical stability which is essential for reproducible signals.
It has been shown that Ag-based plasmonic devices can exhibit better intrinsic performance, e.g. longer plasmon propagation lengths and higher sensitivity for biosensing, than devices made from Au, Cu, or Al within the visible spectrum [20–22]. However, Ag has poor chemical stability, because it is easily oxidized in air or forming an Ag2S contaminating adlayer that is toxic to live cells. In many applications where interfacing with non-biological materials, for example, electrodes for plasmonic solar cells, silver has been applied due to its affordable cost, excellent optoelectronic properties, and well adhesion to glass substrates, though its stability remains an issue [23,24]. Therefore, we attempt to design and demonstrate a SERS active substrate that features the advantages of silver while at the same time avoiding its defects for biochemical uses. In this context, we present our study on the SERS detection power of well-arranged and closely spaced arrays of Ag nanoparticles encapsulated inside a highly ordered (1.58 cm2) matrix of AAO nanochannels, for which a simple and cost-effective electrochemical fabrication method is employed [25,26]. The Ag/AAO nanostructured film has a large area, high density, and uniform SERS enhancing 2-D Ag nanoparticle array which is isolated from ambient environments by an electrochemically anodized high density thin and inert alumina layer self-aligned with Ag nanoparticles. This study reveals a remarkable SERS reproducibility and a high enhancement factor of greater than 106 of the fabricated Ag/AAO film for probing natural DNA base adenine. Conformal coverage of silver nanoparticles, which are formed by a vacuum thin film process with a shadow mask, with alumina deposited by means of atomic layer deposition, has been reported to effectively protect silver nanoparticles from harsh environments [27]. Unlike the previously reported heated reactions requiring multiple breakage of vacuum to complete the fabrication of a device, the electrochemical process used for the fabrication of Ag/AAO films in this work does not involve vacuum process, heated reactions, and is inexpensive and suitable for mass production. It also has merits in manufacturing AAO films incorporated with various silver containing hybrid materials, desired particle sizes, and inter-particle distances for fine tuning the plasmon resonance peak of a substrate to be fit for a laser excitation or an observation window. Reliable, durable, reproducible, and highly sensitive SERS signals are demonstrated by the Ag/AAO substrate.
2. Experimental
2.1 Samples
All the reagents used were of analytical reagent (AR) grade. AAO films with self-ordered nanostructures and encapsulated arrays of silver nanoparticles were fabricated electrochemically [19,26,28]. High purity (99.99%) annealed aluminum (Al) foils were initially electro-polished in a mixture of perchloric acid and ethanol (1:5, volume ratio) for 5 min. These foils were anodized in 0.3 M oxalic acid under a constant applied DC voltage of 40 V at the temperature of 10°C for 2 h. The formed aluminum oxide layer was subsequently dissolved in an etchant solution consisting of 6% phosphoric acid and 1.8% chromic acid. These foils were again anodized under the same conditions for another 2.5 h, followed by a pore-widening process in 4% phosphoric acid at 20°C for 40 min. The average thickness of the bottom barrier layer (a compact alumina layer at the bottom of AAO nanochannels) was approximately 15 nm. Thereafter, Ag nanoparticles were electrochemically deposited at the bottom of nanochannels of AAO templates by applying an AC voltage of 16 V at a frequency of 200 Hz in an ethanol solution containing 0.05 M silver nitrate (99.9999%, Sigma-Aldrich) at 5°C. Then, the underlying aluminum was removed in a CuCl2/HCl (13.5g CuCl2 in 100 ml 35% HCl) solution to expose the back-end AAO barrier [29]. Finally, the back-end AAO barrier layer was partially etched in 5% phosphoric acid at 20°Cfor 10, 20, 30, and 40 min (hereafter, the etching time of this step is denoted as tetch), giving an average layer thickness of 11.3, 7.6, 3.8, and 0 nm, respectively.
Figure 1(a) shows the typical scanning electron microscope (SEM, JEOL JSM-7001) photograph of a fully processed Ag/AAO film. The shown back-end (after removing the residual aluminum) barrier layer is constructed of hexagonal close-packed (hcp) convexes. The inset in Fig. 1(a) clearly exhibits the cross-sectional view of Ag nanoparticles self-aligned and electrodeposited on the bottom interior surfaces of alumina nanochannels. To expose silver nanoparticles inside the AAO channels for structural and material analyses, the back-end barrier layer was intentionally etched away in 5% phosphoric acid at room temperature. Figure 1(b) shows the corresponding EDS spectrum of the film, indicating the presence of Ag nanoparticles in the AAO film. The Al and O peaks come from the alumina template. The Pt peak comes from the coated thin conductive Pt layer on AAO for clearer observation of the film surface by SEM. As shown by the inset in Fig. 1(b), silver nanoparticles are arranged in a hexagonal array with the average channel diameter, inter-particle distance, and particle size of approximately 70, 30, and 70 nm, respectively. Figure 1(c) displays the far-field extinction spectra of an AAO film without embedded Ag and Ag/AAO films with the back-end barrier layer thickness of approximately 15.1, 11.3, 7.6, 3.8, and 0 nm as measured by a UV/VIS/NIR spectrometer (U-3010, HITACHI), respectively. The spectra for Ag/AAO films reveal extinction bands with the maximum located at the range of 390~440 nm, while that for the AAO film without embedded Ag nanoparticles is a featureless straight line. The comparison indicates that the extinction band originates from the embedded array of Ag nanoparticles, in consistence with their localized surface plasmon resonance (LSPR) wavelength. The extinction spectra of Ag/AAO films show a red-shift as compared with the calculated data by Creighton et al [30]. It is believed to be due to the fact that the AAO matrix surrounding Ag nanoparticles has a higher refractive index than that in air or vacuum. In addition, for these Ag/AAO films, as the thickness of alumina barrier layer reduces from 15.1 to 0 nm, the individual LSPR peak location reveals a slight blue-shift, from 435 nm to 398 nm in the spectra, which is consistent with the trend reported in a more detailed study on the nanosphere lithography (NSL) fabricated Ag nanoparticles covered with alumina layers [27]. However, the blue-shift of the fabricated Ag/AAO substrates is comparatively smaller than those of the NSL fabricated triangular Ag nanostructure substrates. These blue-shift differences are considered to be attributed to the distinct particle shapes, i.e. the NSL fabricated triangular Ag nanoparticles lead to a larger LSPR wavelength shift than the electrochemically deposited Ag nanoparticles as the thickness of the alumina layer covering their surfaces is reduced. Here, the laser with wavelength (λex) of 532 nm within the extinction spectral region was chosen for studying the SERS enhancement effect of Ag/AAO films with different thicknesses of the back-end alumina barrier layer for biomolecular detections.
Fig. 1 (a) SEM image of the back-end alumina layer (barrier layer) of an Ag/AAO film (scale bar: 300 nm). Inset: Cross-sectional SEM image of the Ag/AAO substrate where the back-end alumina layer has been chemically etched for 30 min to reduce the layer thickness to 3.8 nm. Silver nanoparticles were deposited inside the tube-like nanochannels (scale bar: 200 nm). (b) EDS spectrum of the Ag/AAO film. Inset: SEM image of the Ag/AAO substrate where the back-end alumina layer has been completely removed to expose the deposited silver nanoparticles inside the nanochannels (scale bar: 100 nm). (c) The extinction spectra of an AAO film without Ag and Ag/AAO films with different etching times (tetch) for thinning the back-end alumina layer.
2.2 Reagents and Raman spectroscopy measurements
In order to evaluate the Raman-enhancing capability of Ag/AAO films, 10−6 M adenine (≥99%, Sigma-Aldrich) solution was dispersed on the as-prepared substrates and incubated at room temperature for 2 hours. These substrates were subsequently rinsed with DI water to remove multilayer of adenine and dried by nitrogen gas. Raman spectra were measured by using Renishaw's InVia micro-Raman system, equipped with a laser excitation wavelength (λex) of 532 nm and a Peltier cooled (−70°C) CCD detector. A 40 × (NA = 0.8) objective lens was used to focus the laser beam onto the sample surface. The incident laser power (Iext) was set to be 1 mW. The data acquisition time (t) was 5 s. All acquisitions were repeated at nine locations on the sample surface and averaged to account for any variation in molecular concentration across the surface.
3. Results and discussion
The correlation between the etching duration (tetch) and the remaining thickness of back-end alumina barrier layer was investigated. Figure 2(a) (the abscissa to the left ordinate) depicts the estimated average thickness of back-end alumina layer on the Ag/AAO substrates as a function of etching time according to ellipsometry (SpecEL-2000-VIS, Mikropack) measurements and individual SEM images. The relative standard deviation of data from eight Ag/AAO samples is indicated by the error bars. For the fitted curve, each data point represents the average thickness of eight Ag/AAO samples. It is obvious that the thickness of the barrier layer decreases linearly with increasing etching duration. Under our experimental conditions, the back-end alumina barrier layer of Ag/AAO film is completely removed and the deposited Ag nanoparticles are exposed after 40 min etching time. From this analysis, we conclude that at a constant temperature of 20°C, the dissolution of the alumina barrier layer can be assumed to start when Ag/AAO samples are immersed in the solution of phosphoric acid and the dissolution proceeds with an almost constant rate of 0.38 nm/min until the back-end barrier layer is completely dissolved.
Fig. 2 (a) Left side: The thickness of the back-end alumina barrier layer can be adjusted by controlling the etching duration in phosphoric acid. Right side: Intensity comparison of the SERS peak at 734 cm−1 (I 734) among Ag/AAO films with increased etching time corresponding to the reduced average thickness of back-end alumina layer of 15.1, 11.3, 7.6, 3.8, and 0 nm, respectively. Error bars indicate relative standard deviations from measurements of eight samples. (b) SERS spectra of adenine nucleobase measured on different substrates. λex = 532 nm; Iext = 1 mW; t = 5 s.
The variation in SERS efficiency of fabricated Ag/AAO substrates with different etching times (tetch) for thinning the back-end alumina layer was demonstrated. The SERS response of the substrates was investigated by using the natural DNA base adenine as the probe molecule. Adenine was chosen because its purine and pyrimidine bases absorb at wavelengths shorter than 280 nm and therefore it is an optically non-resonant species at the excitation wavelength in this study, avoiding the fluorescence interference [31]. Figure 2(b) exhibits the collected SERS spectra of adenine nucleobases (10−6 M) adsorbed on the AAO film without embedded Ag and Ag/AAO films with etching durations (tetch) of 0, 10, 20, 30, and 40 min, respectively. Several typical Raman peaks of adenine, 734, 974, 1228, 1319, and 1462 cm−1, are indicated by arrows, including two prominent peaks at 734 and 1319 cm−1, corresponding to the purine ring breathing mode and the CN stretching mode, respectively [32]. A notable increase of the spectral signal strength can be observed as the etching time (tetch) of Ag/AAO barrier layers increases from 0 to 40 min, leading to the reduction in thickness of the back-end alumina layer from 15.1 to 0 nm. No obvious Raman signals are detected for the AAO film without depositing Ag nanoparticles.
The 734 cm−1 (I 734) SERS intensity versus the thickness of the back-end alumina layers of Ag/AAO substrates are revealed in Fig. 2(a) (the right side in blue color). Each data point represents the average I 734 from eight Ag/AAO samples, with relative standard deviations shown by error bars. As seen, the SERS intensity tends to increase exponentially as the alumina layer thickness gradually decreases from 15.1 to 0 nm. This result requires special consideration, given that the SERS intensity significantly increases with the thinning of alumina encapsulation layer between the Raman scatterers and the metal surface of nanoparticles. While the reduction in thickness of alumina layer can blue shift the LSPR peak of Ag/AAO substrates (herein, the blue shift of LSPR for a total of 15.1 nm reduction in alumina layer is close to 37 nm), this is unlikely the cause of the enhanced SERS intensity seen here. The SERS intensity is essentially dependent on both the EM field and the number of adsorbed molecules. The effects due to possible variation in molecular concentration across the film surface have been excluded by averaging the repeated on-site measurements in our experiments. Therefore, the increase in SERS intensity is due to the spatial EM field enhancement when the adsorbed molecules further approach the surface of Ag nanoparticles with reducing thickness of the back-end alumina layer. Considering the resonance peak of 405 nm for an alumina encapsulation layer of 3.8 nm in thickness shown in Fig. 1(c) as well as its extinction around the excitation wavelength of 532 nm, the change in resonance alone is not enough to cause a nearly 12.8-fold (1280%) enhancement in Raman intensity from that for an encapsulation layer of 15.1 nm in thickness. The difference in SERS intensities among Ag/AAO films examined can be attributed to the difference in the thickness of back-end alumina layers, resulting in the distinct strength of localized EM field, originating from encapsulated Ag nanoparticles, being coupled into exciting analyte molecules for Raman scattering. Thus, we can conclude that as the thickness of alumina layer decreases, the intensity of SERS spectrum increases due to the further enhancement in localized EM field at a reduced distance from the silver nanoparticle surface. The distance dependent SERS data shown in Fig. 2(a) can be fit by
where is the SERS intensity of a given Raman mode, a is the average radius of curvature of the field-enhancing features on the nanoparticle surface, and r is the distance from the surface of a silver nanoparticle to an adsorbate [33,34]. This distance dependence is a result of the decay of the field enhancement away from the silver surface, the 4th power dependence of on the field enhancement, and the scaling with of the effective surface area of the hemi-spherically shaped alumina barrier layer encapsulating each silver nanoparticle. The best fit line to Eq. (1) is shown as the blue curve in Fig. 2(a), which yields a value of 34.8 nm in close consistence with the average size of silver nanoparticles as shown by the SEM image in Fig. 1(b).Because the Ag/AAO film with 30 min etching time provides stronger SERS intensity as compared with other Ag/AAO films with alumina encapsulation layer shown in Figs. 2(a) and 2(b), it is chosen to quantify the relationship between SERS intensities and adenine concentrations. The collected SERS spectra of adenine concentration of 10−9~10−5 M measured on the Ag/AAO films, are displayed by spectra (1)~(5) in Fig. 3(a) . It is obvious that the SERS spectra have these same spectral features except that the spectral intensity increases with the increment of adenine concentration. As described previously, because the alumina encapsulation layer separates the Ag nanoparticle arrays from the target molecules, chemical reactions between adenine nucleobases and Ag nanoparticles are prevented. Molecules on the back-end alumina layer of Ag/AAO substrate are physically adsorbed on the film surface. Since all samples are treated with adenine containing solution by identical procedures, the surface coverage of the adsorbed adenine nucleobases on Ag/AAO films is likely proportional to the concentration of the adenine solutions. Therefore, the increase in overall SERS intensity seen in spectra (1)~(5) in Fig. 3(a) comes from an increase in the number of scattering adenine molecules on each of the Ag/AAO films.
Fig. 3 (a) Curve (0): Reference Raman spectrum acquired from an AAO film without embedded Ag treated by 10−3 M adenine nucleobases . λex = 532 nm; Iext = 20 mW; t = 20 s. Curves (1)~(5): SERS spectra acquired from 10−9~10−5 M adenine nucleobases measured on the Ag/AAO films (the back-end alumina layer has been chemically etched for 30 min to reduce the layer thickness to 3.8 nm). λex = 532 nm; Iext = 1 mW; t = 5 s. The spectra have been background corrected for clarity. Spectra (0), (1), (2), and (3) have been multiplied by a factor of 5, 5, 2.5, and 2, respectively, to make them visible on the scale. (b) I 734 taken from SERS spectra as a function of the molar concentration of adenine on a logarithmic scale. Each data point represents the average value from eight Ag/AAO films with identical conditions. Error bars indicate relative standard deviations from independent measurements of eight samples.
In order to quantitatively estimate the Raman signal improvement owing to the increase in the number of scattering molecules, the Raman peak intensity at 734 cm−1 (I 734) of each collected spectrum is plotted against the performed adenine concentration, as demonstrated in Fig. 3(b). Each data point represents the average I 734 intensity measured from eight Ag/AAO films fabricated under identical conditions (tetch = 30 min). The relative standard deviation for each measured data point is shown by the error bar. It is seen that the peak intensity increases linearly with increasing adenine concentration and the linear dynamic range includes the measured concentration from 10−9 to 10−5 M. The uniformity of SERS signals on the same Ag/AAO substrate was examined by measuring the Raman signals at nine randomly selected spots on the surface of the same sample under identical experimental conditions. The results showed acceptable uniformity with the relative standard deviation of SERS intensity of adenine being approximately ± 5.13% from the average value. This low value indicates good uniformity of embedded Ag nanoparticles and the surface concentration of adsorbates, and the reliability of large-area Ag/AAO films (tetch = 30 min) for SERS measurements.
To quantitatively evaluate the magnitude of the enhancement factor (EF) for the Ag/AAO substrate (tetch = 30 min), we compare the measured SERS intensity of the 734 cm−1 (I 734) peak for 10−6 M adenine solution acquired from the Ag/AAO film (λex = 532 nm; Iext = 1 mW; t = 5 s) to the peak intensity of non-enhanced Raman spectrum for 10−3 M adenine solution acquired from an AAO reference film without embedded Ag (λex = 532 nm; Iext = 20 mW; t = 20 s) according to
where ISERS is the Raman peak intensity obtained from the Ag/AAO substrate, IRaman is the Raman peak intensity obtained from the reference AAO substrate without embedded Ag nanoparticles, Iext is the excitation intensity, t is the integration time, MSERS and MRaman are the concentrations of adenine solution used to incubate the Ag/AAO film and the reference AAO film without Ag, respectively [18,35–37]. It should be mentioned that the exact number of molecules irradiated by the laser beam depends on how well the laser beam is focused on the SERS substrate surface and the used optics. However, because the Ag nanoparticles are uniformly distributed and encapsulated in the AAO film, and the surface packing density of adsorbed molecules on the barrier layer surface is assumed to be proportional to the concentration of adenine solutions used for treating the films, we will roughly estimate the EF by directly adopting the concentration of adenine solutions for comparison. The calculated EF for adenine on the Ag/AAO film (tetch = 30 min) is approximately 6.8×106. The EF is several orders of magnitude lower than EM “hot-spot” based intrinsic Raman enhancement factors on the order of 1014 to 1015 [1] but is high enough for many practical SERS measurements. The reduced EF is an acceptable compromise with the excellent SERS uniformity based on large-area 2-D ordered and closely spaced silver nanoparticles and the desirable protection from environmental degradation achieved by electrochemically anodized alumina of nanometers thick which encapsulates individual silver nanoparticles.For practical applications, long-term stability without needing frequent maintenance is among important criteria for a good SERS substrate. In this study, the temporal stability of the fabricated Ag/AAO films (tetch = 30 min) was investigated over a period of 2 months. Four identical Ag/AAO films (tetch = 30 min) and four identical AAO films without Ag (totally 8 samples with the AAO films without Ag being used as the reference) were individually stored in 50 ml centrifuge tubes (polypropylene, PP) in the dark prior to use. The SERS spectra of 10−6 M adenine applied on these films were recorded once every 7 days. Figure 4(a) exhibits a typical SERS spectrum (I) of fresh adenine measured on one Ag/AAO film (tetch = 30 min) that has been stored for 8 weeks. The spectrum keeps almost the original intensity as compared with that measured on the first day.
Fig. 4 (a) SERS spectra of (I) adenine (10−6 M) obtained on an Ag/AAO film (tetch = 30 min), (II) after adenine was removed and the surface was cleaned, and (III) adenine (10−6 M) obtained on the same cleaned Ag/AAO film after adenine was applied again. λex = 532 nm; Iext = 1 mW; t = 5 s. (b) Time course of the SERS intensity (I 734) variation. The used Ag/AAO films have been chemically etched for 30 min to reduce the average alumina layer thickness to 3.8 nm. The adenine concentration of 10−6 M was applied for repeated SERS measurements.
In order to test the reproducibility of SERS signals from repetitive uses of the same sample, the adsorbed adenine molecules were intentionally washed away by using 1% hydrogen peroxide, DI water, and drying with nitrogen gas. As shown by spectrum (II), no significant obvious Raman signals can be detected from the cleaned sample. The same cleaned Ag/AAO film was treated with the same adenine solution under the same conditions again. The SERS spectrum of spectrum (I) is restored both in the peak positions and the intensities as shown by the spectrum (III). Figure 4(b) displays the time variation of the SERS intensity (I 734) of those four Ag/AAO films (tetch = 30 min) under examination. Each data point on the curve represents the average I 734 from these four Ag/AAO samples, with the relative standard deviation among four samples shown by an error bar. The relative standard deviation of SERS signal intensity of adenine is less than 13.8% of the average peak intensity. The results suggest that the Ag/AAO film (tetch = 30 min) can retain its SERS activity performance during the period of 2 months (temporal stability experiments are being continued) and can be used repeatedly. The prominent advantage in terms of the long-term stability, incorporated with the inexpensive and large-area fabrication method, provides a promising feasibility for the demonstrated Ag/AAO film to be extensively applied for SERS detections and many applications based on plasmon induced by ordered and closely spaced 2-D silver nanoparticles which are encapsulated by thin and reliable anodic alumina.
Selection of a wavelength for excitation closer to the peak of the extinction spectrum for the silver nanoparticle array is expected to provide better Raman signal intensity. Excitation wavelength of 532 nm is within the range of strong plasmon enhanced absorption by the silver nanoparticle array. This wavelength is used because it is the wavelength, which is closest to the peak of the extinction spectrum available to us at the time of this work. Excitation by light of shorter wavelengths in or near the UV range might have adverse effects on some bio-molecules. In addition, optimization of the device performance by fine tuning the size, spacing, and shape of silver nanoparticles, and comparison among wavelengths of excitation are being undertaken and will be reported in subsequent publications.
4. Conclusions
We have demonstrated a novel SERS sensor for adenine molecules fabricated electrochemically using an ordered two-dimensional array of self-aligned silver nanoparticles encapsulated by alumina in a porous AAO film. Close and periodical packing of silver nanoparticles is controllable by electrochemical process parameters to provide high and uniform SERS enhancement factors greater than 106 while the self-aligned back-end alumina helps encapsulate silver nanoparticles to protect them from harsh environments. Reproducible SERS signals from different spots of the same SERS substrate and those from the same spot of a SERS substrate after it is cleaned and re-treated by adenine solution under the same conditions have been demonstrated. The high-performance Ag/AAO film is chemically stable and emits reproducible SERS signals during the test period of eight weeks. The inexpensive electrochemical process without needing vacuum processing equipment is desirable and suitable for mass production of inexpensive SERS substrates for a wide variety of practical applications.
Acknowledgments
We gratefully acknowledge the financial support by the Ministry of Education and National Science Council in Taiwan via grants 99-2120-M-006-004 and 99-2911-I-006-504.
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