Abstracts

The peroxyoxalate reaction, discovered in 1963 by Chandross, has been extensively investigated mechanistically since then and utilized in a great variety of analytical application. Mechanistic studies were mainly performed in anhydrous media, where the chemiexcitation efficiency is very high, however, analytical and bioanalytical application mostly necessitate aqueous media, where the efficiency drops drastically. Therefore, we studied the utilization of the cationic surfactants cetyltrimethyl-ammonium chloride (CTAC) and bromide (CTAB) and 1-cetyl-3-methylimidazolium chloride (C16MeImCl) in aqueous medium for the peroxyoxalate reaction. In previous studies conducted by our group, the reaction system could be defined, comprising bis(2-(methoxycarbonylphenyl)oxalate (2-MCPO), fluoresceine as chemiluminescence activator and 5.0 mmol L^-1 of phosphate buffer at pH 6.0. The critical micelle concentration (CMC) of the surfactants was measured by conductimetry in water and phosphate buffer (5.0 mmol L-1); the CMC of C16MeImCl decreased from 8.6 10^-4 mol L^-1 to 2.3 10^-4 mol L^-1, of CTAB from 9.0 10^-4 mol L^-1 to 4.6 10^-4 mol L^-1 and CTAC from 9.5 mol L^-1 to 0.55 mol L^-1, changing from water to phosphate buffer. Subsequently the effect of the surfactant concentration on the peroxyoxalate emission kinetics was studied in a range from 0.05 to 50 times the CMC. In all cases, the k_obs values show a considerable increase, followed by a gradual decrease, with increasing surfactant concentrations, although different profiles were observed with chloride and bromide as counterions. With chloride surfactants, the k_obs decreased from 4 × CMC to higher concentration, while for CTAB, the decrease in k_obs occurred only from 7 × CMC onwards. Additionally, for chloride surfactants, the chemiluminescence quantum yields (F_CL) reached their peak values at similar analytical concentrations but different multiples of CMC. Consequently, the impact of chlorine and bromine on the peroxyoxalate reaction in the absence of surfactant, aimed at avoiding micelle stabilization effects by anions and alterations in the reaction microenvironment, was investigated by varying the NaCl and NaBr concentrations in the reaction medium. Increasing chloride concentration led to a decrease in k_obs values, while the F_CL stay constant from 0 to 200 mmol L^-1, followed by a gradual decrease at higher concentrations. In contrast, for bromine, the k_obs increased from 0 to 5 mmol L^-1 and then remained virtually constant, while F_CL significantly decreased, indicating a quenching effect of this anion on peroxyoxalate chemiluminescence. It can be expected that around the micelle, the concentration of counterions is higher than in bulk solution; therefore, these effects could be observed at lower concentrations of bromine (due to high local concentrations), justifying the drastic decrease of F_CL in the variation of CTAB concentration. The higher quantity of this surfactant needed to decrease the k_obs could be attributed to the enhancement of k_obs by bromine in water.