Rapid detection of tetracyclines by polyethyleneimine and Mg2+-based fluorescence enhancement
As a class of broad-spectrum antibacterial drugs,tetracycline antibiotics(TCs)are widely used to treat hu-man and animal diseases with remarkable effects.However,the abuse of TCs may result in their accumulation in food,posing significant risk to both human health and the conservation of natural environments.Therefore,it is nece-ssary to establish a rapid and sensitive analytical method for the detection of TCs residues to improve safety of animal-derived foods.At present,there have been limited studies found on fluorescence detection of TCs in acidic conditions.In this study,based on the synergistic fluorescence sensitization effect of polyethyleneimine(PEI)and magnesium ion(Mg2+)on TCs,a novel,rapid,simple,and highly sensitive fluorescence analytical method for the detection of TCs was developed under weakly acidic conditions.Normally,TCs typically manifest as a weakly fluorescent substance in acidic conditions.The green fluorescence intensity of TCs at 525 nm is increased when combined with the cationic sur-factant PEI upon an excitation wavelength at 370 nm.Furthermore,the fluorescence intensity of TCs was significantly enhanced at 525 nm in the coexistence of PEI and divalent metal ion(Mg2+),resulting in strongest green fluorescence emission under identical condition.The deprotonation of hydroxyl and carboxyl groups of TCs was facilitated by the amine-rich cationic surfactant PEI through the operation of electrostatic forces.Concurrently,the oxygen atoms of ke-tone and hydroxyl groups of TCs chelated with Mg2+to form a stable complex.Consequently,under acidic conditions,the complex of TCs,PEI,and Mg2+might be more stable,leading to a significant increase in fluorescence.With the optimal experimental conditions,the results showed that the analytical method had a good linear relationship with the concentration of TCs within the range of 5 nmol/L to 2 µmol/L,and the correlation coefficients(R2)of metacycline(MTC),tetracycline(TC),and oxytetracycline(OTC)were 0.939,0.997 and 0.999,respectively.The detection limits(LOD)of MTC,TC,and OTC were determined as 3.6,2.7 and 4.8 nmol/L,respectively,which were far lower than those of the maximum residue limits(MRLs)in milk of China and the EU.Furthermore,the analytical method showed high specificity for MTC,TC,and OTC,and interacted in a non-interfering manner with other com-mon antibiotics.In addition,the reliability of the analytical method on TCs was substantiated by spiking with different concentrations of the MTC into the three distinct milk samples.As a result,the recovery rates of MTC in the real sam-ples were ranged from 91.64％to 104.05％,accompanied by the relative standard deviations(RSD)of 1.20％to 10.62％.This analytical method offers a novel approach for identifying TCs in food and water and could be applied to analyze TCs residues in acidic systems.
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