Effect of Bonding Interactions between Arsenate and Silver Nanofilm on Surface-Enhanced Raman Scattering Sensitivity

Abstract

Arsenic is one of the most common inorganic contaminants in groundwater worldwide due to natural and anthropogenic sources.1 Although a number of laboratory instruments are used for accurate measurement of arsenic in environmental samples at microgram per liter concentrations, there is still a necessity for development of simple and rapid methods for field assays. A feasible promising route for field analysis of arsenic is to develop a highly active substrate for surface-enhanced Raman scattering (SERS) of arsenic that can be used in conjunction with portable Raman technology. Recently, Mulvihill et al. 3 and our group 4 used Ag nanoparticles for sensitive SERS analysis of arsenic species at the low microgram per liter level in solutions. In our study, Ag nanofilm was prepared on glass slides with a modified mirror reaction, and the SERS analysis was conducted by transferring 10 μL of arsenic solutions onto the nanofilm followed by recording the SERS spectra with a Raman spectrometer. 4 The results suggest that SERS offers great promise for the development of sensitive, rapid, and portable techniques for analysis of arsenate (As(V)) and arsenite (As(III)) species. However, we also observed that Ca 2+ , a common cation in natural waters, significantly reduced the sensitivity of SERS analysis.The extremely high sensitivity of SERS 5,6 is widely attributed to the electromagnetic enhancement (EM) effect and chemical enhancement (CE) or charge transfer effect. 7,8 While the EM effect is primarily caused by the local electric field around the adsorbed molecules, the CE effect requires direct proximity or chemical bonding between the molecules and the surface metal atoms. It has been reported that SERS sensitivities are related to the adsorption affinity and surface orientation of analytes for the metal nanosurface and to the distance between the analytes and the surface. 9À12 However, no bonding structures of analytes on the SERS active surface have been measured directly. The lack of this crucial information prevents accurate theoretical calculations of the CE factors. 13À17Extended X-ray absorption fine structure (EXAFS) is a wellestablished and powerful spectroscopic technique to determine local structural information for adsorbed species on mineral surfaces. 18À20 We have used the technique to determine the bonding structures of adsorbed arsenic species on iron and titanium oxides. 21 In this article, EXAFS was used to directly ABSTRACT: The effect of Ca 2+ on surface-enhanced Raman scattering (SERS) analysis of arsenate (As(V)) using Ag nanofilm was studied. Extended X-ray absorption fine structure (EXAFS) was used for the first time to investigate the bonding structures of adsorbed As(V) on the SERS substrate, as well as the mechanism of the Ca 2+ effect on the SERS analysis of As(V). As(V) was found to be adsorbed on Ag nanofilm through formation of bidentate binuclear surface complexes (AgO) 2 AsO 2 À with an average AsÀAg interatomic distance of 2.56 Å in the As(V) single adsorbate system, and through forma...

Rapid in situ identification of arsenic species using a portable Fe3O4@Ag SERS sensor

“…The various constituents in groundwater could have mixed effects on SERS detection. 10 Thus, the synergistic effect of coexisting ions in groundwater may minimize the matrix interference. Notably, rapid on-site As speciation analysis is of great importance because As(III) is readily oxidized to As(V).…”

“…[6][7][8][9] However, the Ag SERS substrate was susceptible to matrix effects, and could not even detect 10 mg L À1 As(V) in a spiked groundwater sample. 10 Therefore, either complicated pretreatment or a standard addition method is needed to compensate for the reduced SERS sensitivity in real environmental matrices. 6,10 The lack of reliable in situ methodology has prevented wide application of the SERS technique.…”

“…10 Therefore, either complicated pretreatment or a standard addition method is needed to compensate for the reduced SERS sensitivity in real environmental matrices. 6,10 The lack of reliable in situ methodology has prevented wide application of the SERS technique. Herein, we present a portable SERS sensor for onsite monitoring of arsenic speciation.…”

“…The appreciable shift in As-O bands (50-80 cm À1 , Table S1, ESI †) was due to the formation of a chemical bond between As and Fe 3 O 4 @Ag, as proven by previous studies. 8,10 The electromagnetic (EM) mechanism and short-range chemical (CT) effect simultaneously contribute to the overall SERS enhancement. 13 The former does not involve the formation of chemical bonds between the analyte and the substrate, and thus the Raman bands do not shift significantly.…”

Absorption of CO2with supported imidazolium-based ionic liquid membranes

“…The SERS spectrum was investigated to evaluate the interaction of CO 2 and SILM. The experiment was conducted on an EasyRaman‐I Raman spectrometer (Enwave Co, USA), with scanning time of 30 s. The AgNP was prepared as described previously . Briefly, 90 mL of 2% (wt) AgNO 3 solution and 100 µL of 3.2% (wt) KOH solution were mixed in a glass beaker, and the mixture was stirred with a magnetic mixer for several minutes to obtain fine brown precipitates of Ag 2 O.…”