Fluorescence Based Turn-on Probe for the Determination of Caffeine Using Europium-Tetracycline as Energy Transfer Complex

The study describes a simple and sensitive fluorometric
sensor based on the enhancement of fluorescence
intensity of Europium ion (Eu3+) - tetracycline (TC) charge
transfer complex on addition of caffeine. The Eu3+-TC ternary
complex has a characteristic emission peak at 615 nm
(λex = 375 nm), the intensity of which increases with increase
in concentration of caffeine. The caffeine sensor assay was
found to be linear in the range of 0.0515 mM to 51.5 mM.
The limit of detection and quantification were found to be
0.0515 mM and 0.382 mM, respectively. A caffeine recovery
of 90 to 110 % in biological samples (serum and urine) indicated
minimal interference by commonly present excipients in
the samples. Rosenthal plots to calculate the binding capacity
of caffeine with the Eu3+- TC complex revealed an association
constant (K) of 238 x 103 L/mol and binding number (N) of
1.9. Bland-Altman plot comparing the developed assay and
HPLC showed good agreement between values obtained by
both the methods. The proposed fluorescent chemical sensor
is a rapid and convenient method to determine caffeine with
excellent recovery and low detection limit. The probable reaction
mechanism for the formation of the turn on fluorescent
probe enhancer is discussed.