Enhancement of Chemiluminescence of the Luminol-KIO4 System by Cr(III) and Its Applications

We have studied the enhancement in CL for luminol-KIO4 system caused by chromium(III) ion using flow injection analysis. For the KIO4/ luminol CL systm﹐about 100-fold increase in CL intensity was observed upon addition of 150 μM Cr(III) at pH 13. The effect of concentration of reagents (chromium(III)﹐luminol﹐potassium periodate)﹐pH﹐flow rate﹐and modes of reagent mixing on CL emission were also investigated and optimized.
In emission spectrum, the CL maximum occurred at 429 nm. So the CL is caused by luminol in Cr(III)/ KIO4/ luminol system. When the dissolved oxygen was removed from the solution by purging with nitrogen﹐the CL decreased by 20 %. This might indicate that superoxide ion played a minor role in this CL reaction. In order to know the mechanism of the CL of Cr(III)/ KIO4/ luminol system, we tested a series of free radical scanvengers for their effect on CL emission. The CL intensity decreased greatly in the presence of these scavengers. These results showed that in addition to •O2－, 1O2 and •OH also participated in CL reaction. The CL-enhancement may result from the reaction of KIO4 and dissolved oxygen in strong alkaline solution to produce reactive oxygen species (ROS) in the prescence of catalytic Cr3+. However, the major reason for CL-enhancement would be the formation of LuH-Cr-IO4 complex that increases the oxidation of luminol.
By using the CL system, we tested 44 compounds for their effect on the CL emission, including amino acids, polyphenol, catecholamine, nitrogenous bases, vitamins, etc. The results showed that the system has high selectivity for tryptophan.
Furthermore, we found that common antioxidants such as benzenediols and catecholamines showed inhibiting effect on our CL system because these compounds might eliminate •O2－. On the contrary, the CL emission increased at low concentration of benzenetriol because of the generation of more •O2－ upon oxidation of benzenetriol .Nevertheless, the CL emission decreased at high concentration.
Finally we did a series of optimization for the best inhibitory effect of hydroquinone and catechol, to achieve low detection limits of 1.02 nM and 0.96 nM respectively.