Rhod-5N as a Fluorescent Molecular Sensor of Cadmium(II) Ion

Abstract

The photophysical and complexing properties of Rhod-5N (commercially available) in MOPS buffer are reported. This fluorescent molecular sensor consists of a BAPTA chelating moiety bound to a rhodamine fluorophore. Its fluorescence quantum yield is low and a drastic enhancement of fluorescence intensity upon cation binding was observed. Special attention was paid to the complexation with Cd2+, a well known toxic metal ion. Possible interference with other metal ions (Na+, K+, Mg2+, Ca2+, Zn2+, Pb2+) was examined. Rhod-5N was found to be highly selective of Cd2+ over those interfering cations except Pb2+. The limit of detection is 3.1 microg l(-1).

Selective fluorimetric detection of cadmium in a microfluidic device

“…1). This increase of fluorescence has already been observed during measurements in quartz cells [18] and the plateau value reached for a ratio [Ligand]/[Metal] near 1 is the result of a high complexation equilibrium constant for the complex of stoichiometry 1:1.…”

“…The blank was taken as the signal of Rhod-5N (10 − 6 mol L − 1 ) in MOPS at pH 7. Due to a better signal over noise ratio, the detection limit was found better than the one previously found by the classical method using quartz cuvettes [18] and is only twice larger than the maximal permissible concentration of 0.2 μg L − 1 than will be fixed in 2013 in the E U. In spite of its very good sensitivity towards cadmium ions, Rhod-5N suffers from at least one drawback: it is also sensitive to lead (Pb 2+ ) ions, as shown in Fig.…”

“…2. Indeed, the selectivity, defined as the ratio of the apparent stability constants of the 1:1 complexes formed with the metal ions K (Cd 2+ )/K (Pb 2+ ) is only 8 [18]. The selectivity of Rhod-5N was found good versus calcium and zinc which are generally interfering with cadmium.…”

“…An interference filter centred at 525 nm (Semrock FF01-525/ [15][16][17][18][19][20][21][22][23][24][25] was intercalated between the output of the LED and the entrance of the optical fibre in order to decrease the bandwidth of the LED and consequently to have a low baseline on the fluorescence signal. Fluorescent light was collected through a bundle of 7 optical fibres (400/440 μm) and focussed through a high pass filter (λ > 570 nm) on the entrance of a PM tube (Hamamatsu R928).…”

“…In this work, we propose a fluorimetric method to selectively determine the amount of cadmium ions in water in a microfluidic device. Our objective is to use a commercially available fluorescent sensor, Rhod-5N (see Scheme 1), that we already studied in a previous work as being very sensitive to cadmium and lead ions [18], and to find a procedure leading to the selective assay of cadmium ions.…”