• We are sorry, but NCBI web applications do not support your browser and may not function properly. More information
Logo of nanoreslettspringer open web sitethis articlemanuscript submissionregistrationjournal front pagespringer open web site
Nanoscale Res Lett. 2011; 6(1): 263.
Published online Mar 28, 2011. doi:  10.1186/1556-276X-6-263
PMCID: PMC3211326

Fabrication and evolution of multilayer silver nanofilms for surface-enhanced Raman scattering sensing of arsenate


Surface-enhanced Raman scattering (SERS) has recently been investigated extensively for chemical and biomolecular sensing. Multilayer silver (Ag) nanofilms deposited on glass slides by a simple electroless deposition process have been fabricated as active substrates (Ag/GL substrates) for arsenate SERS sensing. The nanostructures and layer characteristics of the multilayer Ag films could be tuned by varying the concentrations of reactants (AgNO3/BuNH2) and reaction time. A Ag nanoparticles (AgNPs) double-layer was formed by directly reducing Ag+ ions on the glass surfaces, while a top layer (3rd-layer) of Ag dendrites was deposited on the double-layer by self-assembling AgNPs or AgNPs aggregates which had already formed in the suspension. The SERS spectra of arsenate showed that characteristic SERS bands of arsenate appear at approximately 780 and 420 cm-1, and the former possesses higher SERS intensity. By comparing the peak heights of the approximately 780 cm-1 band of the SERS spectra, the optimal Ag/GL substrate has been obtained for the most sensitive SERS sensing of arsenate. Using this optimal substrate, the limit of detection (LOD) of arsenate was determined to be approximately 5 μg·l-1.


Since the discovery of surface-enhanced Raman scattering (SERS) in the late 1970s, SERS has been extensively studied as a sensitive analytical technique for fundamental studies of surface species [1-6]. The development of SERS substrates with high sensitivity and good reproducibility has been one of the most challenging tasks. Colloidal Ag or Au nanoparticles are the most widely used SERS substrates. The aggregation of the colloidal particles facilitating the formation of "hotspots" appears to be crucial for strong SERS enhancement [7-11]. However, the aggregation of colloidal particles is difficult to control, thus leading to poor reproducibility of both substrates and SERS signals [12,13]. Although the immobilization/assembly of colloidal nanoparticles onto solid supports could improve the controllable aggregation of the nanoparticles to some extent, the synthesis and fabrication processes for such assembled layers are usually laborious and time consuming, and usually require the use of organic molecules acting as reductants, stabilizing reagents, or coupling reagents.

In recent years, extensive efforts have been dedicated to developing stable nanostructured Ag or Au surfaces directly on solid substrates using various techniques including vacuum evaporation [14], sputtering [15], electrochemical deposition [16], thermal decomposition [17], and electroless plating [18-20]. The electroless plating of nanostructured metal films is attracting much attention due to its easy production, uniform coating, low cost, and no need for special and expensive equipments. A galvanic displacement reaction is a simple electroless plating process to prepare SERS-active Ag or Au films on metal and semiconductive substrates like copper, germanium, and silicon [19-22]. However, it cannot be applied to dielectric substrates like cheap glass slides. Although the well-known mirror reaction is suitable for the deposition of Ag nanofilms onto glass surfaces, this process includes multi-step reactions and requires complex reagents, resulting in difficulty in controlling the surface roughness of the resulting Ag films [20,23,24].

SERS-based techniques have been widely applied to chemical, biological, and medical sensing because SERS has been believed to be one of the most sensitive spectroscopic methods [1,2,5,10,25-27]. Most recently, SERS technique for environmental analysis and monitoring has been reviewed by Halvorson and Vikesland [25], and Alvarez-Puebla and Liz-Marzan [27], respectively. The SERS detections of some inorganic environmental pollutants such as perchlorate (ClO4-) [21,28,29], arsenate (AsO43-) [23], chromate (CrO42-) [30], uranyl (UO22+) [31,32], cyanide (CN-) [30], and thiocyanate (SCN-) [33] have been investigated. Arsenic (As) is one of the most toxic contaminants found in the environment, and long-term exposure to arsenic can cause various cancers and other serious diseases [34,35]. Based on the World Health Organization (WHO) guideline, many countries including the US have promulgated a more stringent drinking water standard for arsenic with a maximum contaminant level (MCL) of 10 μg·l-1 (ppb) [35,36]. There exists an urgent need for the development of methods for effective monitoring and measurement of arsenic in the field [37,38].

Currently, the commonly used laboratory methods such as atomic fluorescence spectroscopy (AFS), atomic absorption spectroscopy (AAS), inductively coupled plasma-atomic emission spectrometry or mass spectrometry (ICP-AES or ICP-MS) allow the detection of low arsenic concentration, but they are expensive, bulky, and usually involved in sophisticated and time-consuming preparations of the samples, making them infeasible for field assays. Moreover, these techniques cannot distinguish different arsenic species, such as arsenite (As(III)) and arsenate (As(V)), without sample pretreatments. In this case, the SERS technique, which can be used in conjunction with commercially available portable Raman systems, has emerged as a potentially promising solution in field assays due to its ability to provide ultrasensitive, reliable, non-invasive, nondestructive, fast, simple, and cost-effective measurements. It has been demonstrated that SERS technique is able to identify, detect, and screen single and multiple contaminants simultaneously in a small volume of sample [25,27]. More significantly, it is incomparable in speciation analysis including distinguishing among the arsenic species with no need for any complex sample preparation because it can provide a nice "fingerprint" of materials of interest [38]. The first SERS spectrum of arsenate at high concentrations (> 100 mg·l-1) was reported by Greaves and Griffith [39] using silver sols. Recently, Mulvihill et al. [38] fabricated Langmuir-Blodgett (LB) monolayers of polyhedral Ag nanocrystals for arsenate SERS detection in groundwater samples with low concentrations (< 10 μg·l-1). Most recently, we examined the effect of ions on the arsenate SERS sensing using Ag nanofilms prepared by modified mirror reaction [23].

In this article, a controllable one-step electroless plating process was applied to directly deposit multilayer Ag nanofilms on glass slides (Ag/GL substrates) for effective SERS sensing of arsenate. The Ag/GL substrates prepared under different conditions were characterized by SEM and UV-Vis spectra, and the formation mechanisms of the multilayer films were discussed. The SERS spectra of arsenate on Ag/GL substrates were analyzed. The relation between the preparation conditions, the resulting morphology of the Ag nanofilms, and the SERS sensitivity to arsenate was examined to optimize the Ag nanofilms for arsenate SERS sensing. Using optimized substrates, the limit of detection of arsenate was determined.



Sodium arsenate (Na3AsO4·7H2O) and silver nitrate (AgNO3) were purchased from Fisher Scientific (Fair Lawn, NJ, USA). Butylamine (BuNH2) was obtained from Sigma-Aldrich (Milwaukee, WI, USA). Anhydrous ethanol (Pharmco-AAPER, Brookfield, CT) was used as a plating solvent. All other chemicals were analytical grade and purchased from Sigma-Aldrich or Fisher Scientific and used as received. Deionized (DI) water with a resistivity of 18.2 MΩ·cm (Millipore Milli-Q System) was used throughout the experiments. Aqueous arsenate samples in the concentration range 0-300 μg·l-1 were prepared by diluting a stock solution of 106 μg·l-1 with DI water.

Preparation of Ag nanofilms

The Ag films deposited on glass slides were prepared by reduction of AgNO3 with BuNH2 in anhydrous ethanol using 6-well plates as reaction vessels, without stirring at room temperature (approximately 22°C). The glass slides were cut into 1 × 1 cm2, which were cleaned in piranha solution (a mixture of concentrated H2SO4 and H2O2 (30%) with a volume ratio of 70 to 30) at approximately 80°C for 1 h. After being washed with DI water, these glass slides were sonicated in 1 M NaOH solution for 30 min to obtain negatively charged surfaces for improved Ag+ adsorption on them during the AgNPs deposition followed by washing with DI water and ethanol in sequence, and then dried under a stream of compressed nitrogen gas. The cleaned glass slides were put into the wells of the 6-well plates (three slides in each well/batch) and a 14 ml fresh prepared ethanolic AgNO3/BuNH2 solution was added into each well within 10 min. The concentrations of AgNO3/BuNH2 in ethanol used in the experiments were 1/0.5 mM, 5/2.5 mM, and 10/5 mM (a constant ratio of 2:1). After a period of reaction time (0.5 to 40 h), one batch of Ag/GL substrates were rinsed with ethanol and dried under a stream of compressed nitrogen gas. Here, all the Ag/GL substrates are labeled as Ag/GL-c/c-t, where "c/c" and "t" represent the concentrations of the AgNO3/BuNH2 ethanolic solution in "mM" and the reaction time in "hours" or "h", respectively. For example, Ag/GL-5/2.5-18 means an Ag/GL substrate which is prepared in the ethanolic solution containing 5 mM AgNO3 and 2.5 mM BuNH2 with the reaction time of 18 h.

Instruments and methods

The morphology and microstructure of the Ag/GL substrates were examined using a field-emission scanning electron microscope (FESEM) (LEO 982, LEO Electron Microscopy Inc., Thornwood, NY, USA) operated at an accelerating voltage of 5 kV and a working distance (WD) of 6 mm. UV-Vis absorbance spectra were recorded with a Synergy™ HT Multi-Detection Microplate Reader (BioTek Instruments Inc., Winooski, VT, USA) with 2 nm resolution in the wavelength range between 350 and 750 nm, and a cleaned glass slide was used as a background. For SERS analysis, 10 μl of arsenate solution was dropped onto the Ag/GL substrate and a sample droplet with a diameter of approximately 3 mm was formed. After air-dried, the SERS spectra were collected in high resolution mode on a Thermo Nicolet Almega XR Dispersive Raman Spectrometer (Thermo Fisher Scientific Inc., Madison, WI, USA) equipped with a CCD detector, an optical microscope and a digital camera, and a 780 nm laser line with a laser source power of 30 mW (50% power was applied in the experiments). The Raman band of a silicon wafer at 520 cm-1 was used to calibrate the spectrometer. All the measurements were conducted in the backscattering geometry. A 10 × microscope objective was used, providing a laser spot size of 3.1 μm. The data acquisition time was 3 s per scan and five scans were used for each spectrum collection. For reliable and reproducible SERS measurements, a mapping method was employed and an averaged spectrum was obtained by averaging 25 spectra splitted from a mapping result with a scan area of 2 × 2 mm2, a step size of 0.5 × 0.5 mm2 (X × Y).

Results and discussion

Characterization and evolution process of Ag nanofilms

To investigate the evolution of the nanostructured Ag films and the relationship between the Ag film structures and the SERS effect, the surface morphologies of the Ag/GL substrates were characterized by SEM observation. Figure Figure11 presents typical SEM images of the Ag/GL substrates prepared in a 5/2.5 mM AgNO3/BuNH2 solution at different reaction times of 1.5-40 h. At the beginning stage of the reaction (≤ 2 h), a thin monolayer (1st-layer) consisting of small sphere-like AgNPs formed on the glass surfaces as shown in Figure 1a,b. Most of AgNPs in the monolayer were isolated from their nearest neighbors for the 1.5 h reaction time (Figure (Figure1a).1a). As the reaction time increased to 2 h, the AgNPs grew and some of them appeared to contact each other, and the AgNPs monolayer exhibited a close-packed structure.

Figure 1
SEM images of AgNPs multilayer films on glass slides prepared in a AgNO3/BuNH2 (5/2.5 mM) ethanolic solution with different reaction times: (a) 1.5 h, (b) 2 h, (c) 5 h, (d) 18 h, (e) 25 h, and (f) 40 h.

By continuously raising the reaction time to 5-40 h, newly reduced Ag atoms would grow on the top of the well-defined 1st-layer AgNPs to form second layer AgNPs (2nd-layer) as shown in Figure 1c,d,e,f. The 2nd-layer AgNPs were much larger than those in the 1st-layer and kept growing in particle size and coverage degree as the reaction time was prolonged. When a growing AgNP met another growing AgNP, coalescence of AgNPs occurred, which led to aggregated and agglomerated AgNPs congeries with irregular shapes and structures (Figure 1d,e,f). Figure 1e,f also indicate that the growth of the 2nd-layer AgNPs in the direction normal to the substrate surface was notable. It was also observed that the growth of the 1st-layer AgNPs was depressed when the reaction time was > 5 h probably due to the formation of the 2nd-layer AgNPs on its top and the competition between the two layers. It is also likely that the formation of the 2nd-layer AgNPs partially consumed the 1st-layer AgNPs through aggregation to lead to the depressed growth of the 1st-layer AgNPs. In addition, when the reaction time was > 9 h, some dendrites formed on the top of the double-layer with a statistically uniform distribution (figure not shown). The dendrite layer could be regarded as a 3rd-layer although its coverage degree was low. The mean particle sizes in the 1st-layer and 2nd-layer are listed in Table Table11.

Table 1
The mean sizes of the AgNPs in the double-layer Ag films prepared in 5/2.5 mM AgNO3/BuNH2 solution at different reaction times of 1.5-40 h

The time-dependent evolution of the microstructures of Ag/GL substrates discussed above was also observed when lower or higher AgNO3/BuNH2 concentrations were used. Note that almost no Ag dendrites were observed when the AgNO3/BuNH2 concentrations were 1/0.5 mM. The higher the AgNO3/BuNH2 concentrations were, the faster the Ag deposition occurred.

The Ag nanofilms were found to exhibit strong plasmon absorption. Figure Figure22 shows the UV-Vis absorption spectra of the Ag films prepared at different reaction times for three different concentrations of reactants. It was found that the bandwidth of the plasmon resonance peak varied with the reaction time, especially for the 1/0.5 mM AgNO3/BuNH2 reaction solution. The plots of maximum absorption wavelength (λmax) and the absorbance at λmax against the reaction time are shown in Figure 3A,B, respectively. Figure Figure3A3A shows that λmax of the Ag films increased (shifted to longer wavelength) with the reaction time at the initial reaction stage, indicating that the size of AgNPs was increasing during this period [24,40]. It is interesting that when a maximum (1st turning point) was reached within 2-5 h, the λmax started to descend till a valley occurred (2nd turning point) followed by another increase. By comparison with the SEM images, we noticed that the 1st turning point was right around the reaction time when the 2nd-layer AgNPs started to appear. The formation of the 2nd-layer AgNPs and their growth in the direction normal to the substrate surface might lead to decrease in their diameter-to-height (a/b) ratio, and consequently a decrease in λmax [41]. The second rise in λmax was probably related to the formation of the 3rd-layer Ag dendrites. Since only few Ag dendrites were observed on the film prepared in the 1/0.5 mM AgNO3/BuNH2 solution, the second increase in λmax for these samples appeared to be much slower than others.

Figure 2
UV-Vis absorption spectra of the AgNPs films prepared in AgNO3/BuNH2 ethanolic solutions at different reaction time. The reactants concentrations: (A) 1/0.5 mM, (B) 5/2.5 mM, and (C) 10/5 mM. The background spectra have been subtracted for all the UV-Vis ...
Figure 3
The plots of the maximum absorption wavelength (λmax) and the absorbance at λmax of UV-Vis absorption spectra of the AgNPs films against the reaction time. (A) λmax vs reaction time, and (B) absorbance at λmax vs reaction ...

Unlike the undulatory variation of λmax, the absorbance at λmax of each UV-Vis spectrum increased consistently with reaction time as shown in Figure Figure3B.3B. This means that the density of AgNPs or the thickness of the Ag films kept increasing throughout the reaction [42]. This increase exhibited a rapid kinetics at the initial reaction stage, followed by a relatively slow one. In short, the factors determining λmax, absorbance and bandwidth of the plasmon resonance are multifaceted, including the size, shape, and density of the metal particles, dispersion of particle sizes and their aggregation on the substrates [40-43]. Table Table22 lists the maximum λmax, valley λmax, and the corresponding reaction time needed to reach them for the three reaction systems.

Table 2
The maximum λmax, valley λmax, maximum peak height of 780 cm-1 band and the corresponding reaction time needed them to reach for the three reaction systems.

There may be two mechanisms in the formation process of the multilayer Ag nanofilms: (1) Ag+ was reduced to AgNPs/nanostructures by BuNH2 directly on the glass surfaces; (2) AgNPs/nanostructures were formed in the ethanol and then assembled on the glass surfaces. When the 1/0.5 mM AgNO3/BuNH2 were used, the reaction solution did not change in color and remained transparent throughout the reaction. The UV-Vis spectra of the reaction solution were recorded at different reaction times and did not indicate a significant difference among them (data not shown). This could suggest that the AgNP double-layer film was formed directly on the substrates following the first mechanism: Ag+ was first anchored on the negatively charged glass substrates (NaOH treated) followed by BuNH2 reduction to form AgNP seeds. Then more Ag+ ions were reduced onto the seeds resulting in the AgNPs growth and the formation of the Ag double-layer film.

For the reaction solution of 5/2.5 mM AgNO3/BuNH2, the Ag double-layer film had been formed at approximately 5 h (see Figure Figure1c);1c); while the color of the reaction mixture was found to start to become yellow at approximately 7 h. As the reaction time was prolonged, the color became darker and darker, and the transparency decreased gradually, suggesting that a bulk reduction took place and the AgNPs were formed in the reaction mixture. As time went on, the AgNPs aggregated and precipitated on the bottom of the reaction vessel and the surface of the Ag/GL substrates. A similar phenomenon was also observed for the 10/5 mM AgNO3/BuNH2 solution. Compared with the 1/0.5 mM AgNO3/BuNH2 solution in which almost no Ag dendrites were observed on the Ag double-layer film, the above-mentioned appearance was an indication that the formation of the 3rd-layer Ag dendrites possibly followed the second mechanism.

SERS spectra of arsenate on Ag nanofilms

Figure Figure4A4A shows typical background spectra (curves (a) and (c)) and SERS spectra of arsenate (curves (b) and (d)) on the Ag/GL-1/0.5-25 and Ag/GL-5/2.5-18 substrates, respectively. In the background spectrum (curve (a) in Figure Figure4A)4A) of the Ag/GL-1/0.5-25 prepared in lower concentrations of AgNO3/BuNH2, there existed some Raman bands in the range of 300 to 1200 cm-1. Their intensities varied depending on the preparation conditions of the Ag films. When the higher concentrations of AgNO3/BuNH2 were used, the resulting Ag/GL-5/2.5-18 substrate had a simpler background spectrum (curve (c) in Figure Figure4A).4A). Compared with the background spectrum of the Ag/GL-1/0.5-25, the intensities of two Raman bands centered at 1046 ± 8 cm-1 and 688 ± 3 cm-1 are much higher, while the other bands diminished or even disappeared.

Figure 4
Typical SERS spectra of arsenate using various Ag/GL substrates as active substrates. (A) SERS spectra of arsenate: (a) 0 μg·l-1 (background) and (b) 300 μg·l-1 on Ag/GL-1/0.5-25 substrate; and (c) 0 μg·l ...

In the SERS spectra of arsenate obtained on these two Ag films as shown in Figure Figure4A4A (curves (b) and (d)), most of the background bands could still be discerned but the peak intensities were suppressed. The two characteristic Raman bands of arsenate occurred at 780 ± 2 cm-1 and 420 ± 10 cm-1 due to the υ1 (A1) symmetric As-O stretch and a superposition of υ2 (A1) and υ5 (E) stretching modes of the arsenate molecule, respectively. Our recent study demonstrated that the υ1 (A1) symmetric As-O stretch resulted in a similar SERS band at approximately 780 cm-1 using Ag nanofilms made using the mirror reaction [23]. Compared with the result obtained from the LB monolayers of AgNPs [38], this SERS band had a shift of approximately 20 cm-1 to lower frequency. Since the 780 cm-1 SERS band happened to be more intense than the 420 cm-1 SERS band, and there were no serious interference bands around the 780 cm-1 position in the background spectra, its intensity will be used to evaluate the SERS effects of arsenate in the subsequent studies. However, the arsenate SERS detection at low concentration using Ag/GL substrates may suffer from the interference of the nearest background band at approximately 815 cm-1.

Optimization of Ag nanofilms for arsenate SERS sensing

Figure Figure4B4B shows the SERS spectra of arsenate (200 μg·l-1) on the Ag/GL substrates prepared in 5/2.5 mM AgNO3/BuNH2 solution at different reaction times of 2, 3.5, 5, 9, 18, 25, 31, and 40 h, respectively. It was of particular interest that the Ag/GL-5/2.5-18 as a SERS substrate gave the most intense arsenate peak among all eight Ag/GL substrates, while the peaks from Ag/GL-5/2.5-2 and Ag/GL-5/2.5-40 substrates were extremely weak. This clearly reflects the effect of the preparation conditions on the SERS sensitivity of the substrates. In order to more distinctly illustrate the results mentioned above, the peak heights of the arsenate Raman bands were plotted against the reaction time to yield a histogram (Figure (Figure5B).5B). From the figure, we can see that as the reaction time was prolonged from 2 to 40 h, the peak height increased gradually until a maximum appeared, and then decreased. The maximum peak height was obtained from the Ag/GL-5/2.5-18 substrate.

Figure 5
Histograms indicating the change in peak heights of the 780 cm-1 band with reaction time. The peak heights were measured from the SERS spectra of 200 μg·l-1 arsenate on the Ag/GL substrates prepared in (A) 1/0.5 mM, (B) 5/2.5 mM, and ...

It is noticed that Ag/GL substrates covered by the AgNPs monolayer film resulted in much lower SERS enhancements than those covered by the AgNPs double-layer film (see Figures Figures1,1, ,5B).5B). When 2nd-layer AgNPs grew to 80 ± 20 nm in diameter (Ag/GL-5/2.5-18), the maximum SERS enhancement was observed for arsenate. The AgNPs with different sizes and shapes can have very different enhancement effects. Ag particles (or aggregates) with size of approximately 90 nm are reported to yield the highest SERS enhancement [39]. Moreover, as stated before, the aggregated and agglomerated AgNPs congeries with irregular shapes and structures had been formed in the Ag/GL-5/2.5-18. In this case, more "hotspots" or "active sites" could exist between two adjacent AgNPs or at the corners/edges of the irregular AgNPs in the double-layer film. The arsenate ions adsorbed on "hotspots" or "active sites" might produce extremely strong enhancements. It is possible that these two factors (suitable size of AgNPs and more "hotspots" or "active sites") contribute simultaneously to the remarkable SERS enhancement.

Similar reaction time-dependent profiles of SERS were observed for the Ag/GL substrates prepared in both lower (1/0.5 mM) and higher (10/5 mM) concentrations of AgNO3/BuNH2 solutions (Figure 5A,C). In summary, for a given concentration of AgNO3/BuNH2 concentration with constant molar ratio of 2:1, there was an optimum reaction time yielding a substrate with the maximum SERS enhancement. The higher the concentrations of the reactants were, the shorter the optimum reaction time was. In our experiments, the optimal Ag/GL substrate (i.e., Ag/GL-5/2.5-18) could be made in 5/2.5 mM AgNO3/BuNH2 ethanolic solution with a reaction time of 18 h at room temperature. The maximum peak height of the 780 cm-1 band and the optimum reaction time for each set of AgNO3/BuNH2 concentrations are also listed in Table Table22.

Comparing the results in Table Table2,2, it is found that each optimum reaction time is equal to or near to the time needed to reach each valley λmax (2nd turning point in Figure Figure3A).3A). It has been demonstrated that at the 2nd turning points, the 3rd-layer Ag dendrites had formed but still at the early stages for the two high concentrations of AgNO3/BuNH2 solutions. These observations imply that the 3rd-layer Ag dendrites may play a significant role in the arsenate SERS enhancements. Figure Figure66 presents the SEM images of the three optimized Ag/GL substrates (Ag/GL-1/0.5-25, Ag/GL-5/2.5-18, and Ag/GL-10/5-6) in both low magnification (large area) and high magnification (high resolution). The large area SEM images show that some Ag dendrites with 1-5 μm size appeared; larger area images (data not shown) show the Ag dendrites were distributed uniformly on the double-layer films of Ag/GL-5/2.5-18 and Ag/GL-10/5-6 substrates, while only few Ag dendrites were observed on the double-layer film of the Ag/GL-1/0.5-25 substrate. The high-resolution SEM images indicate that there is not much difference in AgNPs properties and microstructures among the three double-layer films except that the AgNPs look a little more irregular in the Ag/GL-5/2.5-18 substrate. The Ag dendrites consist of small AgNPs of 20-50 nm. The previous study also indicated that Ag dendrites are very SERS-active [19,22,44,45]. Therefore, the two Ag/GL substrates with Ag dendrites on them exhibited much higher SERS effects than that without Ag dendrites. For a given concentration of reactants, after the optimum reaction time, some AgNPs in the Ag dendrites agglomerated together to form big solid Ag masses (images not shown), leading to the decreasing SERS magnitude.

Figure 6
SEM images of AgNPs multilayer films of the substrates: (a) Ag/GL-1/0.5-25, (b) Ag/GL-5/2.5-18, and (c) Ag/GL-10/5-6. The top panels present the low magnification (large area) images, and the bottom panels are high magnification (high resolution) images ...

Limit of detection of arsenate

The SERS spectra of arsenate on every optimized substrate were measured in the concentration range 0-200 μg·l-1. Figure Figure77 shows the SERS spectra as a function of arsenate concentration recorded on Ag/GL-5/2.5-18 substrate. It is clear that a steady decrease in SERS intensity or peak height of the arsenate Raman band is observed with decreasing arsenate concentration. When the concentration is lower than 5 μg·l-1, the arsenate SERS band at approximately 780 cm-1 appeared to be a shoulder of the 815 cm-1 band, which is not easily discerned. Therefore, the LOD of Ag/GL-5/2.5-18 substrate for arsenate was determined to be approximately 5 μg·l-1. SERS sensing of low concentration arsenate may suffer from the interference of the 815 cm-1 background Raman band. LODs of Ag/GL-1/0.5-25 substrate and Ag/GL-10/5-6 substrate were also determined, and the relatively high values are obtained, i.e., approximately 50 and 20 μg·l-1, respectively.

Figure 7
SERS spectra of arsenate at the different concentrations using Ag/GL-5/2.5-18 as an active substrate: (a) 0, (b) 5, (c) 25, (d) 75, and (e) 150 μg·L-1. The spectra were shifted vertically for clarity but the relative intensity was kept ...


A simple one-step electroless deposition process has been applied to prepare Ag/GL substrates for arsenate SERS sensing. Monolayer, double-layer, and multilayer AgNPs films with different nanostructural characteristics could be controllably deposited on glass by varying the reactant concentrations and deposition times. Two formation mechanisms have been proposed to lead to the multilayer Ag films. The SERS spectra of arsenate show that characteristic SERS bands of arsenate appear at approximately 780 and 420 cm-1, and the former possesses higher SERS intensity consistently regardless of the film nanostructures and the arsenate concentrations. By comparing the peak heights of the approximately 780 cm-1 band of the SERS spectra, the most sensitive Ag/GL substrate for arsenate SERS sensing has been obtained in 5/2.5 mM AgNO3/BuNH2 solution with a deposition time of 18 h. This substrate is covered by a multilayer film consisting of one double-layer of AgNPs and one layer of Ag dendrites distributing uniformly over the double-layer. The lowest arsenate LOD was determined to be approximately 5 μg·l-1 on this substrate, indicating its high SERS activity to arsenate.

The reproducibility, effects of coexisting electrolytes, and quantitative analyses of arsenate in spiked water samples and real groundwater samples using the optimal multilayer Ag nanofilm as the SERS-active substrate have been studied and the results will be reported in another article [46].


AAS: atomic absorption spectroscopy; AFS: atomic fluorescence spectroscopy; AgNPs: Ag nanoparticles; DI: deionized; FESEM: field-emission scanning electron microscope; ICP-AES or ICP-MS: inductively coupled plasma-atomic emission spectrometry or mass spectrometry; LB: Langmuir-Blodgett; LOD: limit of detection; MCL: maximum contaminant level; ppb: part per billion; SERS: surface-enhanced Raman scattering; WD: working distance; WHO: World Health Organization.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

JH and MH conceived of the study, carried out the preparation of multilayer Ag nanofilms, UV-Vis spectra measurements and SERS spectra collections, and drafted the manuscript. ZX participated in the SERS spectra analysis and discussion. JL participated in the SEM measurements. XM is the PI of the project participating in the design of the study and revised the manuscript, and conducted coordination. All authors read and approved the final manuscript.


We would like to thank Dr. Tsan-Liang Su from the Center for Environmental Systems (CES) and Dr. Tsengming Chou from the Laboratory for Multiscale Imaging (LMSI) for their technical support in the facilities. We also thank Dr. Hongjun Wang and his Ph.D. student Xiaochuan Yang from Department of Chemistry, Chemical Biology & Biomedical Engineering (CCBBME) for their assistance in UV-Vis spectra measurements.


  • Kneipp K, Wang Y, Kneipp H, Perelman LT, Itzkan I, Dasari RR, Feld MS. Single molecule detection using surface-enhanced Raman scattering (SERS) Phys Rev Lett. 1997;78:1667. doi: 10.1103/PhysRevLett.78.1667. [Cross Ref]
  • Kneipp K, Kneipp H, Itzkan I, Dasari RR, Feld MS. Ultrasensitive chemical analysis by Raman spectroscopy. Chem Rev. 1999;99:2957. doi: 10.1021/cr980133r. [PubMed] [Cross Ref]
  • Braun G, Lee SJ, Dante M, Nguyen TQ, Moskovits M, Reich N. Surface-enhanced Raman spectroscopy for DNA detection by nanoparticle assembly onto smooth metal films. J Am Chem Soc. 2007;129:6378. doi: 10.1021/ja070514z. [PubMed] [Cross Ref]
  • Schatz GC. Theoretical studies of surface enhanced Raman scattering. Acc Chem Res. 1984;17:370. doi: 10.1021/ar00106a005. [Cross Ref]
  • Dieringer JA, Wustholz KL, Masiello DJ, Camden JP, Kleinman SL, Schatz GC, Van Duyne RP. Surface-enhanced Raman excitation spectroscopy of a single rhodamine 6G molecule. J Am Chem Soc. 2008;131:849. doi: 10.1021/ja8080154. [PubMed] [Cross Ref]
  • Maher RC, Maier SA, Cohen LF, Koh L, Laromaine A, Dick JAG, Stevens MM. Exploiting SERS hot spots for disease-specific enzyme detection. J Phys Chem C. 2009;114:7231. doi: 10.1021/jp905493u. [Cross Ref]
  • Chen MC, Tsai SD, Chen MR, Ou SY, Li WH, Lee KC. Effect of silver-nanoparticle aggregation on surface-enhanced Raman scattering from benzoic acid. Phys Rev B. 1995;51:4507. doi: 10.1103/PhysRevB.51.4507. [PubMed] [Cross Ref]
  • Dou X, Jung YM, Cao Z-Q, Ozaki Y. Surface-enhanced Raman scattering of biological molecules on metal colloid II: effects of aggregation of gold colloid and comparison of effects of pH of glycine solutions between gold and silver colloids. Appl Spectrosc. 1999;53:1440. doi: 10.1366/0003702991945803. [Cross Ref]
  • Zou X, Dong S. Surface-enhanced Raman scattering studies on aggregated silver nanoplates in aqueous solution. J Phys Chem B. 2006;110:21545. doi: 10.1021/jp063630h. [PubMed] [Cross Ref]
  • Nie S, Emory SR. Probing single molecules and single nanoparticles by surface-enhanced Raman scattering. Science. 1997;275:1102. doi: 10.1126/science.275.5303.1102. [PubMed] [Cross Ref]
  • Tian Z-Q, Ren B, Wu D-Y. Surface-enhanced Raman scattering: from noble to transition metals and from rough surfaces to ordered nanostructures. J Phys Chem B. 2002;106:9463. doi: 10.1021/jp0257449. [Cross Ref]
  • Hu J, Zhao B, Xu W, Fan Y, Li B, Ozaki Y. Simple method for preparing controllably aggregated silver particle films used as surface-enhanced Raman scattering active substrates. Langmuir. 2002;18:6839. doi: 10.1021/la020151a. [Cross Ref]
  • Sackmann M, Materny A. Surface enhanced Raman scattering (SERS) - a quantitative analytical tool? J Raman Spectrosc. 2006;37:305. doi: 10.1002/jrs.1443. [Cross Ref]
  • Pan Z, Zavalin A, Ueda A, Guo M, Groza M, Burger A, Mu R, Morgan SH. Surface-enhanced Raman spectroscopy using silver-coated porous glass-ceramic substrates. Appl Spectrosc. 2005;59:782. doi: 10.1366/0003702054280658. [PubMed] [Cross Ref]
  • Merlen A, Gadenne V, Romann J, Chevallier V, Patrone L, Valmalette JC. Surface enhanced Raman spectroscopy of organic molecules deposited on gold sputtered substrates. Nanotechnology. 2009;20:215705. doi: 10.1088/0957-4484/20/21/215705. [PubMed] [Cross Ref]
  • Lee SJ, Morrill AR, Moskovits M. Hot spots in silver nanowire bundles for surface-enhanced Raman spectroscopy. J Am Chem Soc. 2006;128:2200. doi: 10.1021/ja0578350. [PubMed] [Cross Ref]
  • Hou X, Fang Y. Investigation of p-hydroxybenzoic acid from a new surface-enhanced Raman scattering system. J Colloid Interface Sci. 2007;316:19. doi: 10.1016/j.jcis.2007.07.073. [PubMed] [Cross Ref]
  • Sun Y, Pelton M. Laser-driven growth of silver nanoplates on p-type GaAs substrates and their surface-enhanced Raman scattering activity. J Phys Chem C. 2009;113:6061. doi: 10.1021/jp900638m. [Cross Ref]
  • Gutes A, Laboriante I, Carraro C, Maboudian R. Silver nanostructures on silicon based on galvanic displacement process. J Phys Chem C. 2009;113:16939. doi: 10.1021/jp9055297. [Cross Ref]
  • Saito Y, Wang JJ, Batchelder DN, Smith DA. Simple chemical method for forming silver surfaces with controlled grain sizes for surface plasmon experiments. Langmuir. 2003;19:6857. doi: 10.1021/la0301240. [Cross Ref]
  • Hao J, Xu Z, Han M-J, Xu S, Meng X. Surface-enhanced Raman scattering analysis of perchlorate using silver nanofilms deposited on copper foils. Colloids Surf A. 2010;366:163. doi: 10.1016/j.colsurfa.2010.06.010. [Cross Ref]
  • Song W, Cheng Y, Jia H, Xu W, Zhao B. Surface enhanced Raman scattering based on silver dendrites substrate. J Colloid Interface Sci. 2006;298:765. doi: 10.1016/j.jcis.2006.01.037. [PubMed] [Cross Ref]
  • Xu Z, Hao J, Li F, Meng X. Surface-enhanced Raman spectroscopy of arsenate and arsenite using Ag nanofilm prepared by modified mirror reaction. J Colloid Interface Sci. 2010;347:90. doi: 10.1016/j.jcis.2010.03.028. [PubMed] [Cross Ref]
  • Park HK, Yoon JK, Kim K. Novel fabrication of Ag thin film on glass for efficient surface-enhanced Raman scattering. Langmuir. 2006;22:1626. doi: 10.1021/la052559o. [PubMed] [Cross Ref]
  • Halvorson RA, Vikesland PJ. Surface-enhanced Raman spectroscopy (SERS) for environmental analyses. Environ Sci Technol. 2010;44:7749. doi: 10.1021/es101228z. [PubMed] [Cross Ref]
  • Chursanova MV, Dzhagan VM, Yukhymchuk VO, Lytvyn OS, Valakh MY, Khodasevich IA, Lehmann D, Zahn DR, Waurisch C, Hickey SG. Nanostructured silver substrates with stable and universal SERS properties: application to organic molecules and semiconductor nanoparticles. Nanoscale Res Lett. 2009;5:403. doi: 10.1007/s11671-009-9496-2. [PMC free article] [PubMed] [Cross Ref]
  • Alvarez-Puebla RA, Liz-Marzan LM. Environmental applications of plasmon assisted Raman scattering. Energy Environ Sci. 2010;3:1011. doi: 10.1039/c002437f. [Cross Ref]
  • Gu B, Ruan C, Wang W. Perchlorate detection at nanomolar concentrations by surface-enhanced Raman scattering. Appl Spectrosc. 2009;63:98. doi: 10.1366/000370209787169894. [PubMed] [Cross Ref]
  • Ruan C, Wang W, Gu B. Surface-enhanced Raman scattering for perchlorate detection using cystamine-modified gold nanoparticles. Anal Chim Acta. 2006;567:114. doi: 10.1016/j.aca.2006.01.097. [PubMed] [Cross Ref]
  • Mosier-Boss PA, Lieberman SH. Detection of anions by normal Raman spectroscopy and surface-enhanced Raman spectroscopy of cationic-coated substrates. Appl Spectrosc. 2003;57:1129. doi: 10.1366/00037020360695991. [PubMed] [Cross Ref]
  • Ruan C, Luo W, Wang W, Gu B. Surface-enhanced Raman spectroscopy for uranium detection and analysis in environmental samples. Anal Chim Acta. 2007;605:80. [PubMed]
  • Bhandari D, Wells SM, Retterer ST, Sepaniak MJ. Characterization and detection of uranyl ion sorption on silver surfaces using surface enhanced Raman spectroscopy. Anal Chem. 2009;81:8061. doi: 10.1021/ac901266f. [PubMed] [Cross Ref]
  • Tan S, Erol M, Sukhishvili S, Du H. Substrates with discretely immobilized silver nanoparticles for ultrasensitive detection of anions in water using surface-enhanced Raman scattering. Langmuir. 2008;24:4765. doi: 10.1021/la703831q. [PubMed] [Cross Ref]
  • Hao J, Han MJ, Meng X. Preparation and evaluation of thiol-functionalized activated alumina for arsenite removal from water. J Hazard Mater. 2009;167:1215. doi: 10.1016/j.jhazmat.2009.01.124. [PubMed] [Cross Ref]
  • Mohan D, Pittman CU Jr. Arsenic removal from water/wastewater using adsorbents--A critical review. J Hazard Mater. 2007;142:1. doi: 10.1016/j.jhazmat.2007.01.006. [PubMed] [Cross Ref]
  • Hao J, Han M-J, Wang C, Meng X. Enhanced removal of arsenite from water by a mesoporous hybrid material - Thiol-functionalized silica coated activated alumina. Microporous Mesoporous Mater. 2009;124:1. doi: 10.1016/j.micromeso.2009.03.021. [Cross Ref]
  • Melamed D. Monitoring arsenic in the environment: a review of science and technologies with the potential for field measurements. Anal Chim Acta. 2005;532:1. doi: 10.1016/j.aca.2004.10.047. [Cross Ref]
  • Mulvihill M, Tao A, Benjauthrit K, Arnold J, Yang P. Surface-enhanced Raman spectroscopy for trace arsenic detection in contaminated water. Angew Chem Int Ed. 2008;47:6456. doi: 10.1002/anie.200800776. [PubMed] [Cross Ref]
  • Greaves SJ, Griffith WP. Surface-enhanced Raman scattering (SERS) from silver colloids of vanadate, phosphate and arsenate. J Raman Spectrosc. 1988;19:503. doi: 10.1002/jrs.1250190803. [Cross Ref]
  • Kelly KL, Coronado E, Zhao LL, Schatz GC. The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment. J Phys Chem B. 2002;107:668. doi: 10.1021/jp026731y. [Cross Ref]
  • Schaefers S, Rast L, Stanishevsky A. Electroless silver plating on spin-coated silver nanoparticle seed layers. Mater Lett. 2006;60:706. doi: 10.1016/j.matlet.2005.05.085. [Cross Ref]
  • Jia H, Zeng J, Song W, An J, Zhao B. Preparation of silver nanoparticles by photo-reduction for surface-enhanced Raman scattering. Thin Solid Films. 2006;496:281. doi: 10.1016/j.tsf.2005.08.359. [Cross Ref]
  • Grabar KC, Smith PC, Musick MD, Davis JA, Walter DG, Jackson MA, Guthrie AP, Natan MJ. Kinetic control of interparticle spacing in Au colloid-based surfaces: rational nanometer-scale architecture. J Am Chem Soc. 1996;118:1148. doi: 10.1021/ja952233+. [Cross Ref]
  • Brejna PR, Griffiths PR, Yang J. Nanostructural silver and gold substrates for surface-enhanced Raman spectroscopy measurements prepared by galvanic displacement on germanium disks. Appl Spectrosc. 2009;63:396. doi: 10.1366/000370209787944253. [PubMed] [Cross Ref]
  • Guteés A, Carraro C, Maboudian R. Silver dendrites from galvanic displacement on commercial aluminum foil as an effective SERS substrate. J Am Chem Soc. 2010;132:1476. [PubMed]
  • Han M-J, Hao J, Xu Z, Meng X. Surface-enhanced Raman scattering for arsenate detection on multilayer silver nanofilms. Anal Chim Acta. 2011. [PubMed]

Articles from Nanoscale Research Letters are provided here courtesy of Springer


Related citations in PubMed

See reviews...See all...

Cited by other articles in PMC

See all...


Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...