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 , sputtering , electrochemical deposition , thermal decomposition , 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 , and Alvarez-Puebla and Liz-Marzan , respectively. The SERS detections of some inorganic environmental pollutants such as perchlorate (ClO4
-) [21, 28, 29], arsenate (AsO4
3-) , chromate (CrO4
2-) , uranyl (UO2
2+) [31, 32], cyanide (CN-) , and thiocyanate (SCN-)  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 . The first SERS spectrum of arsenate at high concentrations (> 100 mg·l-1) was reported by Greaves and Griffith  using silver sols. Recently, Mulvihill et al.  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 .
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.