Chemiluminescence of Luminol Induced by Reducing Agents and Co(II) and Its Analytical Applications
Date Issued
2009
Date
2009
Author(s)
Fang, Shiun
Abstract
Chemiluminescence (CL) is the emission of light from an electronic excited state of a species produced in a chemical reaction. In this study, the CL reaction of luminol with reducing agents and metal ions was studied by flow injection analysis and stopped-flow spectrometry. Upon addition of hydroxylamine to the luminol-Co(II) system, the CL intensity increased significantly. The use of hydrazine could further increase the CL intensity by five folds. The presence of Co(II) increased the CL intensity and reduced the CL duration. The effects of pH, concentration of reagents (luminol, Co(II), NH2OH, N2H4) and modes of reagent mixing on CL intensity were also investigated and optimized. The optimal conditions for maximum CL intensity are: flow rate = 5 mL/min, pH = 13.0, [luminol] = 30 μM, [Co(II)] = 100 μM, [NH2OH] = 200 μM and [N2H4] = 500 μM. The CL signal showed good reproducibility (RSD = 1.1% for n = 21). Deoxydenation of sample solutions by purging with nitrogen reduced the CL intensity by 20%, indicating that oxygen is involved in this CL reaction. Specific scavengers for ．O2- , 1O2 and ．OH decreased the CL intensity greatly, suggesting that these reactive oxygen species (ROS) played significant roles in the CL reaction. The CL detection system can be used to determine substance such as phenolic compounds that can effectively destruct ROS. For the determination of hydroquinone, catechol, and resorcinol, the dynamic ranges are 2~400 nM, 3 ~ 500 nM, and 0.3 ~ 10 μM, respectively, and the limits of diction are 1.05 nM, 1.35 nM and 0.10 μM, respectively. The proposed CL method is simple, rapid, convenient and sensitive. In this study, we have used the stopped-flow technique to study the effect of pH, reagent concentration, ROS scavengers on the CL intensity and duration. When increasing amount of NH2OH (0-50 μM) was added to the Co(II)-luminol solutions at pH 13.0, progressive increase in CL intensity and decrease in CL duration were observed. Interestingly, two distinct CL peaks appeared at higher concentrations of NH2OH (100~200 μM) and then a single peak was present at 250 μM NH2OH. The presence of increasing amount of Co(II) in luminol-NH2OH solutions enhanced the CL intensity and reduced the CL duration progressively, especially for the second peak. The CL intensity increased rapidly as the concentration of luminol increased, reached a maximum at 30 μM, and then dropped considerably at higher concentrations. Changing the pH has a profound effect on the CL emission. Strong and shortened CL emission was observed at high pH. These results indicate that the peak shape and duration of CL emission can be controlled by appropriate adjustments of reagent concentrations. It is found that CL peaks obtained at different conditions responded differently to the analytes in terms of selectivity and sensitivity. o gain a deeper insight into the mechanism of CL emission, we have investigated the effect of EDTA, ROS-scavenging, and deoxygenation on the CL emission. The addition of EDTA reduced the CL intensity and prolonged the CL emission dramatically. No CL emission was observed when the added EDTA exceeds the amount of Co(II), suggesting that free Co(II) is essential for CL. Deoxygenation of sample solutions reduced the CL intensity considerably. The extent of reduction in CL intensity is greater for the second peak than for the first peak. Thus oxygen is important in CL enhancement. ROS-scavenging studies revealed that ．O2- and ．OH are critical for the CL enhancement since the presence of scavengers for these two radicals decreased the CL intensity dramatically. It is postulated that complexation of Co(II) with suitable ligands such as NH2OH, OH-, O2, or luminol anion may lead to the generation of ROS (e.g., ．O2-, ．OH) or other reactive intermediate. Subsequent reactions of these reactive species with luminol results in the strong enhancement in CL emission.
Subjects
Chemiluminescence
reducing agent
Co(II)
phenolic compound
Type
thesis
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