SERS detection of arsenic in water: A review.

Arsenic (As) is one of the most toxic contaminants found in the environment. Development of novel detection methods for As species in water with the potential for field use has been an urgent need in recent years. In past decades, surface-enhanced Raman scattering (SERS) has gained a reputation as one of the most sensitive spectroscopic methods for chemical and biomolecular sensing. The SERS technique has emerged as an extremely promising solution for in-situ detection of arsenic species in the field, particularly when coupled with portable/handheld Raman spectrometers. In this article, the recent advances in SERS analysis of arsenic species in water media are reviewed, and the potential of this technique for fast screening and field testing of arsenic-contaminated environmental water samples is discussed. The problems that remain in the field are also discussed and an outlook for the future is featured at the end of the article.

Surface-enhanced Raman scattering of perchlorate on cationic-modified silver nanofilms - Effect of inorganic anions.

Surface-enhanced Raman scattering (SERS) has emerged as one of the most sensitive spectroscopic analysis methods for the detection of environmental contaminants in water, including perchlorate (ClO4(-)). However, as with other commonly used analytical techniques, analysis of realistic environmental samples by SERS presents a challenge due to complex chemical components coexisting in the samples. In this work, we investigated the influence of inorganic anions (particularly oxyanions) on SERS spectra of ClO4(-) using a cationic thiol modified silver nanofilm substrate (Cys-Ag/rCu). The results show that the anions present in the samples did not shift the ClO4(-) characteristic band positions, but did decrease signal intensities due to their competitive binding with the -NH3(+) groups of cationic thiol molecules immobilized on the substrates. The pH changes caused by both the dissociation of H2PO4(-) and the hydrolysis of HCO3(-) may also play a non-negligible role. The selectivity of the Cys-Ag/rCu substrate towards these anions was determined to be in the following order: ClO4(-)>SO4(2-)>HCO3(-), NO3(-)>Cl(-)>H2PO4(-), indicating preferential adsorption of ClO4(-) ions. In the solutions with multiple anions present, the ClO4(-) SERS spectra were affected simultaneously by all the coexisting anions. Calibration curves with very good linear relationships were successfully obtained, demonstrating the great potential of quantitative detection of aqueous ClO4(-) in the matrix.

La3+-modified activated alumina for fluoride removal from water.

A La(3+)-modified activated alumina (La-AA) adsorbent was prepared for effective removal of fluoride from water. The surface properties of adsorbent were characterized with zeta potential analysis, SEM-EDS and EXAFS. Batch and column experiments were conducted to evaluate improvement of F(-) removal by the La-AA. SEM/EDS and EXAFS analyses determined the formation of La(OH)3 coating on the AA and strong bonding interactions between La(3+) and the Al atoms. The points of zero charge (pHPZC) of AA and La-AA were at pH 8.94 and 9.57, respectively. Batch experimental results indicated that the La-AA had much higher adsorption rate and capacity than the AA. The F(-) adsorption processes on La-AA and AA followed the pseudo-second-order kinetics and the Langmuir isotherm. Column filtration results shows that the La-AA and AA treated 270 and 170 bed volumes of the F(-)-spiked tap water, respectively, before F(-) breakthrough occurred. The results demonstrated that the La-AA was a promising adsorbent for effective removal of F(-) from water.

Hexavalent chromium removal mechanism using conducting polymers.

We report detoxification of Cr(VI) into Cr(III) using electrochemically synthesized polyaniline (PANI), polypyrrole (PPY), PANI nanowires (PANI-NW) and palladium-decorated PANI (PANI-Pd) thin films. Percent Cr(VI) reduction was found to be decreased with an increase in pH from 1.8 to 6.8 and with initial Cr(VI) concentration ranging from 2.5 to 10mg/L. Efficacy of PANI increased at higher temp of 37 °C as compared to 30 °C. PANI-Pd was found to be most effective for all three initial Cr(VI) concentrations at pH 1.8. However, efficacy of PANI-Pd was significantly reduced at higher pHs of 5 and 6.8. Efficacy of PANI and PANI-NW was found to nearly the same. However, there was a significant reduction in effectiveness of PANI-NW at 10mg/L of Cr(VI) at all the three pHs studied, which could be attributed to degradation of PANI-NW by higher initial Cr(VI) concentration. PPY and PANI-NW were found to be highly sensitive with respect to pH and Cr(VI) initial concentration. Chromium speciation on PANI film was carried out by total chromium analysis and XPS, which revealed Cr(III) formation and its subsequent adsorption on the polymer. PANI-Pd and PANI are recommended for future sensor applications for chromium detection at low pH.

Surface modification of silver nanofilms for improved perchlorate detection by surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS), as one of the most sensitive spectroscopic analysis methods, has been investigated extensively for the detection of environmental contaminants in recent years. In this work, we reported the new development of robust SERS substrates for rapid and sensitive sensing of aqueous perchlorate, a widespread environmental contaminant. The fabrication of the substrates consisted of two simple steps: (a) formation of Ag nanofilms on Cu and surface-roughened Cu foils (Ag/Cu and Ag/rCu nanofilms) using a controllable and inexpensive one-step electroless plating process, and (b) surface modification of the Ag nanofilms with cysteamine (Cys) self-assembly monolayer (SAM) (Cys-Ag/Cu and Cys-Ag/rCu substrates). Due to the strong affinity of -NH(3)(+) groups of the Cys molecules for perchlorate ions, the rapid SERS detection of perchlorate has been realized with a limit of detection (LOD) down to 5 µg L(-1) (ppb) for aqueous samples without need for drying. Various calibration curves with good linear relationships were obtained, indicating the quantification potential of SERS analysis of perchlorate using these new substrates. It was found that the neutral pH yielded the maximum SERS signals, and 85% of original sensitivity was remained in 5 days of storage time in the air, indicating the substrates are fairly stable. Within 10 regeneration-reuse cycles, the SERS signals of perchlorate kept in the range of 85-105% of the original value, verifying its reusability.

Surface-enhanced Raman scattering spectroscopy of explosive 2,4-dinitroanisole using modified silver nanoparticles.

2,4-Dinitroanisole (DNAN) is being used as a replacement for 2,4,6-trinitrotoluene (TNT) as a less-sensitive melt-cast medium explosive than TNT. In this paper, we studied the surface-enhanced Raman spectroscopy (SERS) analysis of DNAN using Ag nanoparticles (AgNPs) modified by L-cysteine methyl ester hydrochloride. Due to the formation of a Meisenheimer complex between DNAN and the modifier, the modified AgNPs can detect 20 µg/L (0.2 ng) and 0.1 mg/L (1 ng) DNAN in deionized water and aged tap water, respectively. Three other chemicals (L-cysteine, N-acetyl-L-cysteine, and L-cysteine ethyl ester hydrochloride) were used as AgNPs modifiers to study the mechanism of the SERS of DNAN. It was confirmed that the amino group of L-cysteine methyl ester hydrochloride was the active group and that the methyl ester group significantly contributed to the high SERS sensitivity of DNAN. In order to further test the mechanism of Meisenheimer complex formation, the effect of anions and cations present in natural water on the SERS of DNAN was studied. It was found that CO(3)(2-), Cl(-), and K(+) at 100 mg/L did not negatively affect the SERS of 10 mg/L DNAN, while SO(4)(2-), Na(+), Mg(2+), and Ca(2+) at 100 mg/L significantly quenched the SERS of 10 mg/L DNAN. The negative effect of the bivalent cations could be offset by SO(4)(2-).

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.

Surface-enhanced Raman scattering for arsenate detection on multilayer silver nanofilms.

Surface-enhanced Raman scattering (SERS) has recently emerged as a promising method for chemical and biomolecular sensing. SERS quantification analysis of arsenate (As(V)) was investigated using multilayer Ag nanofilms deposited on glass slides as SERS-active substrates (Ag/GL substrates) by an electroless deposition process. The As(V) limit of detection (LOD) was determined to be ∼5 µg L(-1) or lower with or without coexisting multiple background electrolytes (Na(+), K(+), Ca(2+), Mg(2+), Cl(-), NO(3)(-), SO(4)(2-) and H(2)PO(4)(-)). The presence of the background electrolytes at low concentrations was observed to enhance the SERS sensitivity of the substrate towards As(V) more than twofold. Standard calibration curves were prepared in the absence and presence of the background electrolytes. Excellent linear relationships between the peak heights of the As(V) SERS band at ∼780 cm(-1) and the As(V) concentrations were obtained in a concentration range of 0-250 µg L(-1). The selectivity of the Ag nanofilm towards oxyanions was examined to be in the order of As(V)≫phosphate≫nitrate, sulphate. A low sample-to-sample relative standard deviation (RSD) of 5.2% was also determined, suggesting the Ag/GL substrate was uniform and highly reproducible. Experimental results indicated that the SERS method could be used for quantitative analysis of As(V) in groundwater samples.

Surface-enhanced Raman spectroscopy of arsenate and arsenite using Ag nanofilm prepared by modified mirror reaction.

A modified mirror reaction was developed to prepare a sensitive and reproducible Ag nanofilm substrate for the surface-enhanced Raman scattering (SERS) analysis of arsenate (As(V)) and arsenite (As(III)). A good linear relationship between the SERS intensity of As(V) and As(III) and their concentrations in the range from 10 to 500 microg-As/L was achieved using the SERS substrate. As(V) and As(III) appear to be adsorbed on the Ag nanofilm through formation of surface complexes with Ag, based on comparisons of the Raman spectra of the arsenic species in solutions, on the SERS substrate, and in silver arsenate and arsenite solids. As(V) and As(III) species on the SERS substrate and in the solids had the same Raman band positions at 780 and 721 cm(-1), respectively. The effect of eight ions in natural waters on the SERS analysis of As(V) was studied. K(+), Na(+), SO(4)(2-), CO(3)(2-), and NO(3)(-) in the range of 0.1-100 mg/L did not interfere with the SERS detection of As(V) for a As(V) concentration greater than 100 microg-As/L. While Cl(-) (50 mg/L), Mg(2+) (10 mg/L), and Ca(2+) (1 mg/L) were found to quench the SERS intensity of 100 microg/L As(V). Cl(-) (at concentrations >10 mg/L) formed silver chloride with the adsorbed Ag(+) and decreased the SERS detection limits for arsenic species. The mechanism of the Ca(2+) effect on the SERS analysis of As(V) was through the formation of surface complexes with As(V) in competition with Ag. When the Ca(2+) concentration increased from 0 to 100 mg/L, the amount of As(V) adsorbed in Ag nanoparticles was reduced from 38.9 to 11.0 microg/mg-Ag. When the Ca(2+) concentration increased to values higher than 1 mg/L in the As(V) solution, the As(V) peak height was decreased in the corresponding SERS spectra and the peak position shifted from 780 to 800 cm(-1). The fundamental findings obtained in this research are especially valuable for the development of sensitive and reliable SERS methods for rapid analysis of arsenic in contaminated water.

Modulated conjugation as a means of improving the intrinsic hyperpolarizability.

A new strategy for optimizing the first hyperpolarizability based on the concept of a modulated conjugated path in linear molecules is investigated. A series of seven novel chromophores with different types of conjugated paths were synthesized and characterized. Hyper-Rayleigh scattering experiments confirmed that modulated conjugation paths that include benzene, thiophene, and/or thiazole rings in combination with azo and/or ethenyl linkages between dihydroxyethylamino donor groups and various acceptor groups result in enhanced intrinsic hyperpolarizabilities that exceed the long-standing apparent limit for two of the chromophores. The experimental results are analyzed and interpreted in the context of quantum limits, which show that conjugation modulation of the bridge in donor/acceptor molecules simultaneously optimizes the transition moments and the energy-level spacing.

Preparation and evaluation of thiol-functionalized activated alumina for arsenite removal from water.

Arsenic species such as arsenite [As(III)] and arsenate [As(V)] are known human carcinogens. Though lots of metal oxide adsorbents have been developed for removal of As(V), they are much less effective for As(III) adsorption. In this study, various inorganic-organic hybrid adsorbents bearing thiol groups have been prepared by modifying activated alumina (AA) with mercaptopropyl-functionalized silica under different experiment conditions. Raman spectra demonstrated the successful functionalization of AA and verified the formation of As-S complexes after As(III) adsorption. Batch experiments were applied to evaluate the As(III) adsorption performance of the hybrid adsorbents. Compare with AA, the hybrid adsorbents exhibited enhanced adsorption abilities for As(III) due to the introduction of thiol groups, and as the thiol loading increased, the uptake of As(III) increased. Experimental results indicated that the hybrid adsorbents still maintained the merit of the AA for As(V) adsorption. Based on the results, one hybrid adsorbent referred to as BL(AA)(30)(MPTS)(3.3) has been selected by consideration of not only the adsorption capacity but also its environmentally friendly and cost-effective production. The investigation has indicated that the hybrid adsorbents are very promising for As(III) removal from water.

Improved stability and selectivity of lytic peptides through self-assembly.

Widespread clinical applications of peptide drugs have been hindered by their low stability and selectivity. Peptides can be easily digested by various enzymes in the blood and thus show a short life-span. Meanwhile, peptide drugs can cause severe normal tissue damage due to their low selectivity. Therefore, for effective therapy, a high dosage of peptide is required which is usually in excess of the clinically and economically acceptable level. In this study, we have tried to design new lytic peptides which can self-assemble into peptide fibrils with defined nanostructures as observed under atomic force microscopy. Lytic peptides in self-assembled peptide fibrils will lose their cell lysis activity but become resistant to enzyme degradation. Such lytic peptide self-assembly has proven to be a reversible process which is controlled by surrounded environments. A concentration controlled sustained release of free and active lytic peptide from self-assembled peptide fibrils has been achieved. Self-assembled lytic peptides with enzyme resistance, sustained release, and prodrug feature may have great clinical application potentials.

Direct evidence of arsenic(III)-carbonate complexes obtained using electrochemical scanning tunneling microscopy.

Electrochemical scanning tunneling microscopy (ECSTM), ion chromatography (IC), and electrospray ionization-mass spectrometry/mass spectrometry were applied to investigate the interactions between arsenite [As(III)] and carbonate and arsenate [As(V)] and carbonate. The chemical species in the single and binary component solutions of As(III), As(V), and carbonate were attached to a Au(111) surface and then imaged in a 0.1 M NaClO4 solution at the molecular level by ECSTM. The molecules formed highly ordered adlayers on the Au(111) surface. High-resolution STM images revealed the orientation and packing arrangement of the molecular adlayers. Matching the STM images with the molecular models constructed using the Hyperchem software package indicated that As(III) formed two types of complexes with carbonate, including As(OH)2CO3- and As(OH)3(HCO3-)2. No complexes were formed between As(V) and carbonate. IC chromatograms of the solutions revealed the emergence of the new peak only in the aged As(III)-carbonate solution. MS spectra showed the presence of a new peak at m/z 187 in the aged As(III)-carbonate solution. The results obtained with the three independent methods confirmed the formation of As(OH)2CO3-. The results also indicated that As(OH)3 could be associated with HCO3- through a hydrogen bond. The knowledge of the formation of the As(III) and carbonate complexes will improve the understanding of As(III) mobility in the environment and removal of As(III) in water treatment systems.

Absolute configuration of monodentate phosphine ligand enantiomers on Cu(111).

Scanning tunneling microscopy (STM) has been employed to investigate the chirality of monophosphine compounds that are highly efficient chiral ligands in transition-metal-catalyzed organic transformations. The absolute configuration of 1-(2-diphenyphosphino-1-naphthyl)isoquinoline enantiomers with axial chirality was discriminated directly by the "marker" group, PPh(2) substitutes. Although the two enantiomer molecules adsorb on a Cu(111) surface and form well-defined (4 x 4) structures, the positions of PPh(2) substitutes in the molecular adlayers are different. The mirror symmetry between two adlayers is demonstrated. On the basis of STM results, structural models are proposed to interpret the chiral adsorption. The results presented here supply a straightforward method for axial chirality analysis in adsorbed adlayers by STM.