SERS Study Of The Carcinogenic Azo Dyes

Azo dyes are one of the most important classes of dyes and are widely applied inmany areas.Recently, the ues of some azo dyes have been banned due to their adversereactions with skin.These azo dyes are potentially toxic and carcinogenic. Manycountries have legislation to prohibit the production or importation of consumer goodscontaining banned azo dyes.Surface enhanced Raman scattering (SERS) has shown advantages in the analysisof environmental and relevant compounds due to its tremendous enhancement ofRaman signals and the large amount of structural information provided by thetechnique. Therefore, we can detect trace banned azo dyes intermediates molecules inwater by SERS.Noble metal colloids, in particular, gold and silver colloids, have been widelyused as the SERS substrates in SERS detection during the past two decades. In thisdissertation, the use of SERS to detect trace banned azo dyes intermediates moleculeswas investigated, where gold and silver colloid nanoparticles were used as the SERSsubstrates, and the corresponding excitations are633nm and514nm, respectively.The detection condications were optimized for different azo dyes intermediatesmolecules and the oritation of azo dyes absorbed on silver nanoparticles were studied.The main contents are listed as follows: (1) We have compared the SERS signals of benzidine molecules on silver colloidnanoparticles, assembled silver nanoparticles films and nitric acid etched silver foils.The enhancement on the silver colloid nanoparticles is significantly larger than that onthe other two. SERS active silver colloid nanoparticles were thus used as substrate todetect the trace benzidine and its SERS peaks were assigned. The influence ofaggregating agent (MgCl2) on the SERS signals was also investigated. The resultsindicated that SERS intensity firstly increased and then decreased with increase in theconcentration of MgCl2. There was an optimized concentration of MgCl2. The limit ofdetection for benzidine was around10-8M under the optimized condition.(2) We have compared SERS signals obtained on the gold colloids of differentparticle sizes, and that obtained on gold colloids and gold nanoparticle films. Thestrongest enhancement was obtained on the30nm gold nanoparticles, which werethus used as the SERS substrates for the following detection of different targetmolecules. The detection limits of different target molecules were obtained and thepeaks were also assigned. The effect of the aggregating agent (MgCl2) on thedetection limit was also investigated.(3) We have prepared SERS active gold and silver colloids as SERS substrates todetect intermediates of banned azo dyes under acidic and alkaline conditions. In thecase of silver nanoparticles as substrates, the relative intensities of SERS peaks ofbenzidine molecules at1145cm-1and1189cm-1changed under different pHconditions. When using gold nanoparticles as substrates, the variation of SERSspectra under different pH conditions showed dependence on the type of targetmolecules. For the intermediates with only one amino group that could interact withgold nanopaticles, the SERS signals became weak in acidic solutions because of theprotonation of amino group which weaken the interaction between intermediates andgold nanoparticles. For the intermediates containing nitro and amino groups, thevariation of SERS spectra under different pH condition was different, which might bedue to the possible interaction between nitro group and gold nanoparticles. For the intermediates containing two amino groups, the SERS spectra obtained from pH3to11just exhibited the decrease in the peak intensities, indicating that only one aminogroup was was used to take interaction with gold nanoparticles and the protonation ofthe other showed no influence on the one interacted with gold nanoparticles.(4) We have prepared SERS active silver nanoparticles and used them as SERSsubstrate to detect the time-dependent SERS spectra of benzidine. Our results showedthat, at low concentration of benzidine (10-5M), the positions and the intensities ofsome peaks changed with time and finally turned out to be similar with that obtainedat higher concentration (10-4M). Also, the SERS spectra of10-5M benzidine weredifferent on different silver nanoparticles. These facts should be attributed to theorientation of benzidine adsorbed on silver nanoparticles changed with time at lowconcentrations. The changes in SERS spectra when using o-tolidine as targetmolecules was not observed because the changes in orientation was hindered by sterichindrance effect from Methyl.