Surface enhanced Raman scattering of bacteria using capped and uncapped silver nanoparticles.

Surface enhanced Raman scattering (SERS) spectra of bacteria were obtained using citrate (capped) and borohydride (uncapped) generated silver nanoparticles (Ag NPs).The observed differences in SERS spectra are attributed to the manner in which these Ag NPs interact with bacteria. Capped Ag NPs are able to partition through the surface polysaccharides of the bacterial cell to bind to the inner and outer cell membranes, as well as the periplasmic space between them. The resultant spectra show contributions due to the components of the cell envelope and cellular secretions. Uncapped Ag NPs are unable to partition through the polysaccharide outer structures of the cells. Spectral features observed for these uncapped Ag NPs are secretions primarily due to the metabolites of purine degradation.

SERS substrates fabricated using ceramic filters for the detection of bacteria: Eliminating the citrate interference.

It was found that spectra obtained for bacteria on SERS substrates fabricated by filtering citrate-generated Ag nanoparticles (NPs) onto rigid, ceramic filters exhibited peaks due to citrate as well as the bacteria. In many cases the citrate spectrum overwhelmed that of the bacteria. Given the simplicity of the method to prepare these substrates, means of eliminating this citrate interference were explored. It was found that allowing a mixture of bacteria suspension and citrate-generated Ag NPs to incubate prior to filtering onto the ceramic filter eliminated this interference.

SERS substrates fabricated using ceramic filters for the detection of bacteria.

SERS substrates were fabricated by filtering either Ag or Au colloidal particles onto rigid, ceramic filters - onto which suspensions of bacteria were then filtered. SERS spectra of the bacteria were obtained using a Raman spectrometer that has an 'orbital raster scan' capability. It was shown that bacteria samples prepared in this manner were uniformly distributed onto the surface of the SERS substrate. The effect of common buffer systems on the SERS spectra was investigated and the utility of using the SERS technique for speciation of bacteria was explored.

Detection of perchlorate using Ag/DMAH(+) SERS-active capture matrices.

In this communication, the fabrication of SERS-active capture matrices for the detection of perchlorate is described. The amine groups of amine-modified magnetic microparticles were used to immobilize silver colloidal particles. Once immobilized, the silver was reacted with dimethylaminoethanethiol hydrochloride (DMAH(+)Cl(-)) to form a self-assembled monolayer (SAM). The DMAH(+) SAM exhibits reasonably good selectivity for perchlorate. It was shown that calibration curves could be generated by ratioing the perchlorate peak with a DMAH(+) peak that did not change upon interaction with the perchlorate ion. Flow experiments, using Ag/DMAH(+) capture matrices held in place by a magnet, showed instantaneous response to changes in perchlorate concentration. The use of solid phase extraction (SPE) to eliminate chloride ion interference was explored.

Detection of hexavalent chromium using gold/4-(2-mercaptoethyl) pyridinium surface enhanced Raman scattering-active capture matrices.

In this communication, the fabrication of SERS-active capture matrices for the detection of hexavalent chromium is described. The amine groups of amine-modified magnetic microparticles were used to immobilize gold colloidal particles. Once immobilized, the gold was reacted with 4-(2-mercaptoethyl) pyridinium (MEP) hydrochloride to form a self-assembled monolayer (SAM). The MEP SAM exhibits great selectivity for hexavalent chromium. It was shown that calibration curves could be generated by ratioing MEP peaks that increased in intensity upon complexation with chromate with a peak that did not change. Flow experiments, using Au/MEP capture matrices held in place by a magnet, showed instantaneous response to changes in chromate concentration.

Use of Raman spectroscopy to evaluate the selectivity of bifunctional anion exchange resins for perchlorate.

Raman spectroscopy is used to evaluate the selectivity of two bifunctional anion exchange resins, Purolite A-530 and Amberlite PWA-2. It was found that the adsorption of anions on the resins is described by a Frumkin isotherm, which is determined by the ion pair constant, K, and the Frumkin parameter, g. The ion pair constant, K, is a measure of the strength of interaction between the resin and the anion and the Frumkin parameter, g, takes into account interactions between adsorbed anions. Although both resins have a polystyrene backbone and trihexylammonium and triethylammonium functional groups, the Purolite A-530 resin exhibits greater selectivity for perchlorate. The only discernable differences between the two resins is that the Amberlite PWA-2 has a higher trihexylamine content and the Purolite A-530 resin exhibits greater cross-linking. How the trihexylamine/triethylamine content and the degree of cross-linking affects the performance of these resins is discussed.

Optical properties of surface-enhanced Raman-active capture matrices.

Silver and gold colloidal particles can be immobilized on amine-derivatized magnetic microparticles. Once immobilized, the colloidal particles can be reacted with thiols to form a self-assembled monolayer (SAM). Earlier it was shown that the resultant derivatized magnetic microparticles, i.e., capture matrices, are surface-enhanced Raman (SERS) active and that they can be used to extract trace amounts of analyte from an aqueous sample. In this investigation, the optical properties of the capture matrices are examined. Imaging of these magnetic microparticles shows that the immobilized silver/gold colloidal particles exhibit blinking behavior. An increase in the SERS signals due to the SAM and the continuum is observed with constant laser illumination of these magnetic microparticles. Such an increase can be used to improve sensitivity. This increase in signal is attributed to the electromagnetic enhancement mechanism (EEM).

Surface-enhanced Raman spectroscopy substrate composed of chemically modified gold colloid particles immobilized on magnetic microparticles.

In this paper, immobilization of gold colloidal particles onto amine-modified magnetic microparticles is demonstrated. Once immobilized, the gold was then reacted with pentachlorothiophenol (PCTP) to form a self-assembled monolayer. The PCTP-gold colloid on magnetic microparticles was then used to extract naphthalene from aqueous samples. A magnet was used to concentrate the microparticles onto the side of the sample vial, allowing detection of naphthalene by surface-enhanced Raman spectroscopy. Using the PCTP-gold colloid on magnetic microparticles the limit of detection for naphthalene achieved was 0.3 microg mL(-1). Multiple extractions can be done with the PCTP-gold colloid on magnetic microparticles to further lower the detection limit.

The role of hydrogen bonding in the selectivity of L-cysteine methyl ester (CYSM) and L-cysteine ethyl ester (CYSE) for chloride ion.

The interaction of cysteamine (CY), L-cysteine methyl ester (CYSM), and L-cysteine ethyl ester (CYSE) with nitrate, sulfate, perchlorate, dihydrogen phosphate, and chloride ions was investigated using surface enhanced Raman spectroscopy (SERS). CYSM and CYSE are chemical derivatives of CY. These thiols have a quaternary ammonium group to attract the anions to the SERS surface. Dihydrogen phosphate did not interact with these cationic thiols. The CY interaction with perchlorate, nitrate, and sulfate is stronger than the interaction with chloride. However, replacing a hydrogen on the carbon adjacent to the quaternary ammonium group with either a methyl or ethyl ester group results in stronger complexation with chloride ion than with either sulfate or nitrate ion. In the case of CYSM, the chloride interaction is five times stronger than the interaction with perchlorate. Molecular modeling indicates that the high selectivity of CYSM/CYSE for chloride is due to hydrogen bonding between the chloride ion and the hydrogen of the CH3 moeities of adjacent ester groups.

Detection of lead derived from automotive scrap residue using a direct push fiber-optic laser-induced breakdown spectroscopy metal sensor.

At Naval Base Point Loma in San Diego, California, a canyon had been filled with construction debris and automotive scrap residue (ASR), the latter of which included lead acid batteries. A magnetic survey and induced potential (IP)/DC resistivity survey showed the presence of anomalies at the western end of the site where historic records indicated that the ASR had been placed. Lead concentration depth profiles were obtained in situ and in real time at the site using a direct push fiber-optic laser-induced breakdown spectroscopy (FO-LIBS) sensor probe. Lead, along with strontium and titanium, was detected at depths of 7 to 8 m bgs. These results provided confirmation that the magnetic/IP anomalies at the site are due to ASR.

Spectroscopic studies of the interaction between crown ethers and organic nitriles.

The interaction of 18-crown-6 (18C6), diaza-18-crown-6 (DA18C6), and dithia-18-crown-6 (DT18C6) with acetonitrile and malononitrile was investigated using infrared spectroscopy, 1H and 13C NMR spectroscopy, and molecular modeling. The interaction between 18C6 and the nitriles involves hydrogen bonding between the CH3/CH2 protons of the nitriles and the ether oxygens of the crown. In contrast, the interaction between the organic nitriles and DA18C6 involves the nitrogen of the nitrile group and the N-H groups of the crown. Due to the bulkiness of the sulfur atoms of DT18C6, no interaction was observed to occur between acetonitrile and the crown.

Detection of anions by normal Raman spectroscopy and surface-enhanced Raman spectroscopy of cationic-coated substrates.

The use of normal Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) of cationic-coated silver and gold substrates to detect polyatomic anions in aqueous environments is examined. For normal Raman spectroscopy, using near-infrared excitation, linear concentration responses were observed. Detection limits varied from 84 ppm for perchlorate to 2600 ppm for phosphate. In general, detection limits in the ppb to ppm concentration range for the polyatomic anions were achieved using cationic-coated SERS substrates. Adsorption of the polyatomic anions on the cationic-coated SERS substrates was described by a Frumkin isotherm. The SERS technique could not be used to detect dichromate, as this anion reacted with the coatings to form thiol esters. A competitive complexation method was used to evaluate the interaction of chloride ion with the cationic coatings. Hydrogen bonding and pi-pi interactions play significant roles in the selectivity of the cationic coatings.

Use of fluorescently labeled phage in the detection and identification of bacterial species.

Phages are viruses whose hosts are bacterial cells. They identify their hosts by specific receptor molecules on the outside of the host cell. Once the phages find their specific receptors, they bind to the bacterial cell and inject their nucleic acid inside the cell. The binding between phage and host can be so specific that only certain strains of a single species can be infected. In this communication, the specificity of phage P22 for Salmonella typhimurium LT2 is exploited to allow the detection of Salmonella in the presence of other bacterial species. In particular, the dsDNA of P22 is bound to SYBR gold, a highly sensitive, fluorescent nucleic acid stain. When multiple phages infect the same cell, the fluorescence emissions of the phage DNA inside the cell allow it to be imaged using an epifluorescence microscope. The advantages of using phages as the bacterial recognition element in a sensor over antibodies are discussed.

Field demonstrations of a direct push FO-LIBS metal sensor.

A direct push, FO-LIBS sensor probe for real-time, in-situ measurement of metals in soils has been built and its capabilities demonstrated at three sites. The response of the FO-LIBS sensor is affected by the soil matrix conditions of grain size, composition, and water content. For each site, uncontaminated soil was collected and spiked with known quantities of the metal under investigation. These calibration standards were used to generate a site-specific calibration curve, which compensates for variations in soil matrix conditions of grain size, composition, porosity, etc. To compensate for moisture content, extra laser pulses are used to volatilize the water, higher power densities and/or shortened acquisition delay times are used. The LIBS experiment inherently exhibits poor precision since it is a point measurement. When multiple spectra are taken of a single, homogenized soil sample, there is a significant amount of variability in the peak intensities. There is no internal standard available to correct for this variability. In this communication, it is shown that normalizing the peak areas under the spectral curves to one effectively corrects for variation due to differences in the plasma volume. Because the LIBS technique is essentially a point measurement the FO-LIBS sensor is used as a semiquantitative field screening tool and not as a quantitative analytical method. In the field demonstrations reported in this communication, remarkably good agreement between the in-situ FO-LIBS results and ICP lab results were obtained.