Abstract
An easy-to-prepare pH sensor based on electrochemically obtained polypyrrole doped with methyl orange ions is described. It enables the determination of a pH value in the 3–13 range for volumes below 1 µL. In a wide pH range, resonance and pre-resonance methyl orange Raman spectra, excited with the 514.5 nm line of an Ar+ laser, changed noticeably in function of H+ concentration. Two types of measurements were performed. In the first case, Raman spectra of the analyzed solutions were collected for samples placed on the sensor surface using a confocal microscope equipped with a 10x objective. Next, measurements were conducted for the same samples without the sensor. On the basis of these spectra, partial least-squares models were elaborated and validated. Relative standard errors of prediction for calibration, validation, and test samples were found to be in the 3.7%–3.9% range. An analogous model build using spectra registered without the sensor was characterized by slightly worse parameters.
Highlights
One of the most intensively explored areas of analytical chemistry is the design and preparation of optical sensors, which can be used to monitor chemical reactions in different environments.A variety of spectroscopic techniques are used for this purpose
We describe a pH sensor composed of electrochemically prepared fibres of polypyrrole doped with methyl orange, which enables the determination of a pH value in the 3–13 range for volumes below 1 μL, based on resonance and pre-resonance MO Raman spectra
Partialleast-squares least-squares (PLS) model was built using spectra registered without a polypyrrole–methyl orange (PPy–MO) sensor
Summary
One of the most intensively explored areas of analytical chemistry is the design and preparation of optical sensors, which can be used to monitor chemical reactions in different environments.A variety of spectroscopic techniques are used for this purpose. Fluorescence [1,2,3], ultraviolet–visible spectroscopy [4], and surface-enhanced Raman scattering [5,6] are the most popular techniques, while spontaneous and resonance Raman methods are rarely applied [7,8]. The electropolymerization of pyrrole in the presence of an anionic dye, a one-step synthetic procedure, results in the deposition of a conducting polypyrrole–methyl orange (PPy–MO) composite on an electrode surface, which is similar to the reported cases of chemical polymerization of pyrrole [9,10]. Modifications of the dye molecule reflecting changes of the analyte concentration can be monitored using Raman spectroscopy, which is not a very sensitive method. Similar information can be obtained more from resonance or pre-resonance Raman spectra due to significant enhancement of their intensity [11]
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