Sensing of Copper(II) by Immobilized Ligands: Comparison of Electrochemical and Surface Plasmon Resonance Transduction

Abstract

Two sensors for the detection of the metal ion Cu(II), based on the same sensing layer, are compared. They rely on different transduction methods, i.e., electrochemistry and surface plasmon resonance (SPR). D,L-penicillamine was used as the specific receptor since it is a strong ligand for copper(II). Moreover it is easily immobilized on a gold layer by a self assembling procedure by contacting the gold layer overnight with water/EtOH (80/20) containing D,L-penicillamine, taking advantage of the spontaneous interaction of thiols with gold surfaces. Both the electrochemical and SPR platforms were derivatized in the same way. In the first case a gold disc was used as working electrode, and in the second one a thin gold layer (60 nm thick) was deposited by sputtering over the exposed core of a plastic optical fiber (POF), after removing the cladding along half circumference.

An Optical Fiber Chemical Sensor for the Detection of Copper(II) in Drinking Water

“…These results indicate that the selectivity of the proposed sensor in water is very good, considering that the natural sample examined has a high mineral content, for example a calcium concentration as high as 2 × 10 −2 M (see Table 3). This confirms some previously reported preliminary results, showing that the resonance wavelength of the SPR-POF sensor with d , l -penicillamine SAM is not affected by low concentrations of metal ions as Ca(II) or Fe(III) [22].…”

“…The Δλ max value, here obtained (3.56 nm), is lower than that obtained in the preliminary investigation [22], which was about 6 nm. This is ascribable mainly to the fact that a different photoresist was here used, as the buffer layer, to build the sensor.…”

“…By increasing the concentration of the analyte, the resonance wavelength is shifted to higher values, indicating that the metal ion binds to d , l -penicillamine linked to the gold surface even at this very low acidity, causing an increase of the refractive index of the receptor layer. The shape of the spectrum is similar to that obtained in the preliminary investigation [22] while the resonance wavelength (606 nm) is slightly higher than 604 nm [22], obtained however with a different sensor and at a different pH.…”

“…This is ascribable mainly to the fact that a different photoresist was here used, as the buffer layer, to build the sensor. It has been shown [25] that the S1813 photoresist here used produces an optical sensitivity about one half than that of the photoresist used in the preliminary investigation [22], at the considered refractive index/resonance wavelength.…”

“…Moreover, it can be easily linked as a self-assembled monolayer to a gold surface taking advantage of the –SH group present in d , l -penicillamine molecule, as demonstrated in the case of a previously proposed electrochemical sensor [21]. Some preliminary results [22] were very promising, so that a systematic investigation on the possibility of applying the proposed SPR sensor in OF to the determination of Cu(II) in real drinking water samples was investigated in the present research.…”