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Optical Detection of Chemical Markers
Very recently, my colleague Julie Peller and myself have received a grant for using fluorescence methods to detect optical brighteners (OB-major component of household laundry wastewater) in recreational water so that we can correlate the presence of these brighteners with pathogenic bacteria such as E coli. While fluorescence is a powerful analytical method in its own right, in situ detection of OBs in recreational water is more problematic given that portable fluorimeters are in most cases (because of instrumental size limits) incapable of attaining the low detection limits that can be routinely attained in a laboratory setting. Consequently, alternative methods of detection are needed for in situ detection of OBs, in lakes, streams etc. This proposal seeks to examine one such alternate method of detection of these FWAs: surface-enhanced Raman scattering (SERS).
Raman scattering in simple terms is the inelastic scattering of incident light. Raman scattering is an inherently weak effect, but the invention of lasers in the early 70’s as powerful incident light sources made the scattered Raman photon eminently detectable and thereby gave Raman spectroscopy a major new lease of life. In recent years surface-enhanced Raman scattering (SERS) has become a even more valuable analytical tool in the detection of chemicals at very low concentrations. SERS is an enhancement (by several orders of magnitude) observed in the Raman scattering signal from a compound when it is adsorbed on a precious metal surface (e.g. gold or silver). It is now generally accepted that the enhancement arises from a phenomenon called surface plasmon resonance (SPR) - an enhancement in the local electric field resulting from the laser excitation of surface plasmon modes on the metal surface. The applicability of the technique has greatly expanded over the past 2 or 3 decades coinciding with the development of easy synthetic methods for metal nanoparticles The application of SERS requires the use of metal surfaces fulfilling the SPR condition in the region of the laser light employed for Raman excitation. This implies the use of mainly silver and gold nanoparticles.
Preliminary experiments carried out this summer show that SERS can be a powerful tool in the detection of OBs on silver colloidal nanoparticles. This project will examine the feasibility of using SERS on OBs adsorbed on silver colloids. We will examine the response of these systems as a function of laser wavelength and concentration of the target compound, with a view to develop in situ methods using portable Raman spectrometers. We will also examine the feasibility of direct detection of bacteria using SERS so that quick and cheap alternate methods can be developed to the current microbiological culture methods.
Development of TiO2 based nanotube arrays for photocatalysis.
The primary goal of this work is to develop a more effective photocatalyst for environmental remediation of gas-phase contaminants. Central to this goal is the development of a reliable, reproducible synthetic procedure and characterization methods for TiO2 nanotube arrays. . The TiO2 catalyst is cheap, nontoxic to most species and stable. The shortcoming associated with titanium dioxide is the need to immobilize the catalyst on a surface. Syntheses of TiO2 nanotubes are already well described in literature. My plan is to design an array of these tubes so that gases can flow though. Single component semiconductor nanoparticles exhibit relatively poor photocatalytic efficiency (<5%) since the majority of the photogenerated charge carriers undergo recombination. Almost a decade ago, in collaboration with Prof. Kamat at ND, I showed that much more efficient separation of photogenerated charge carriers can be achieved by combining TiO2 photocatalysts with SnO2. Composite semiconductor systems have also been shown to improve the photoconversion efficiency of dye sensitized photochemical solar cells and photocatalytic reactions. It is also well established that deposition of a noble metal such as platinum improves the efficiency of photocatalytic reactions. The metal acts as a sink for photogenerated electrons and improves charge separation efficiencies.
The research plan is:
1. to prepare TiO2 nanotube arrays using different synthetic procedures
2. evaluating the properties of these synthesized catalysts to determine if there is a correlation with the photocatalytic efficiency of these preparations.
3. synthesis of TiO2 nanotube composites with noble metals to improve photocatalytic efficiencies.
4. use of these catalysts in gas phase remediation
Sonochemical synthesis of bi and termetallic colloids from mixed precursors of the metal salts in aqueous and non-aqueous solution
In aqueous systems, sonochemical synthesis of the metal colloids is achieved by reaction of the solutes (e.g., Pt2+ and Ru3+ ions) with primary and secondary reducing radicals in the bulk solution phase. Sonochemistry occurs as a consequence of the violent collapse of microbubbles in a liquid subjected to an acoustic field. The high temperatures generated within the bubble core as a consequence of the collapse are sufficient to create radicals from the gaseous materials, such as water vapor (present within the bubbles) in aqueous solution or other gaseous material in non-aqueous systems. These reactive species/radicals may react within the bubble or react with solutes at the bubble/solution interface or escape the bubble completely and react with solutes in the bulk solution. For example, hydrogen atoms generated by the homolysis of water diffuse out of the cavitation bubbles and reduce metal ions in the bulk solution leading to the formation of metal nanoparticles. Earlier work has shown that the control of particle size and/or dispersion can be readily accomplished by varying the frequency or input power of the ultrasound field, since the particle size is inversely proportional to the rate of reduction.
The initial focus here will be on colloidal composites of Pt-Ru. The concentrations of the starting precursor solutions will be varied so as to create structures where the coverage of Ru on the Pt extends from core-shell structures to islands of Ru deposited on the Pt. While sonochemical reduction of metal ions has the advantage of relative simplicity, in some cases, a combination of methods might be appropriate. I will also examine other reduction methods such as polyol reduction with the goal of obtaining better control of the composition of the eventual structures.
It has been shown that ultrasonic irradiation of organic liquids containing dissolved organometallic complexes can produce metal colloids. The volatile complexes evaporate into the bubble during the expansion cycle. The high temperature conditions of the bubble during collapse result in the decomposition of the metal complex to produce metal ions. Metal colloids of Fe, Pd, Ni and Ni-Co alloys have been prepared by such methods. While my initial aqueous solution experiments cited above for Pt and Ru show pseudo core-shell geometries, I can achieve alloyed structures by using non aqueous solvents and suitable volatile precursors for Pt and Ru. As in the previous case, I will vary the amounts of precursors and extend the procedures to termetallic systems,
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