Abstract
This work summarizes the electrochemical response of a salicylic acid-based carbon electrode for use as a novel solid-state reference electrode in a redox-based pH sensor. This novel reference produces a pH insensitive response over a range of pH 3–10 in solutions with low buffer concentrations, different compositions, conductivities, and ionic strengths is produced. The pH of the local environment is shown to be determined by the chemistry and the electrochemical response of the redox active species on the surface of the electrode; the local pH can be controlled by the electropolymerized salicylic acid moieties due to the acid concentration on the surface, avoiding any perturbation in environmental pH and leading to a stable novel reference system. Sensitivities of −7.1 mV/pH unit, −2.4 mV/pH unit, −0.2 mV/pH unit, and 2.5 mV/pH units were obtained for different food medias, hydroponic solution, seawater, and cell-culture media, respectively, confirming its ability to control the local pH of the electrode. This reference system is paired with a new pH sensing element based on electropolymerized flavanone to provide a calibration free, pH sensitive sensor to effectively and accurately measure the pH of various media with high viscosity, low conductivity, low/high buffer concentration or cell-culture environment, presenting a maximum error of +/−0.03 pH units.
Highlights
Academic Editor: Paolo UgoThe monitoring of pH is extremely important in many industries, such as pharmaceutical, food, agricultural and environmental
This result is consistent with the literature data which shows that oxidation of the salicylic acid (SA) monomer in acidic conditions produces a polymeric species on the electrode surface
A novel solid-state reference electrode utilizing salicylic acid carbon composite-based electrodes has been successfully developed, which overcome the issues of standard reference electrodes
Summary
The monitoring of pH is extremely important in many industries, such as pharmaceutical, food, agricultural and environmental. Pioneering work by Hickman et al [14] demonstrated that conventional reference electrode limitations may be overcome by measuring the potential difference between the response of a pH sensitive and a pH insensitive (no proton transfer) redox compound Such systems produce a pH insensitive signal, it is often found that the redox potential of the species is dependent on the concentration of ions in solution. 20 -Hydroxyflavanone-based polymer is presented as a pH sensor [26], whereby Nafion® is used as a proton-exchange membrane to encourage the proton transfer between the media and the electropolymerized polymer This voltametric pH sensor is demonstrated in semi-solid food medias, such as soy sauce and raspberry syrup, in a hydroponic nutrient solution (Hoagland), sea water, and high buffered cell-culture media (DMEM), as well as standard buffer solutions. Hoagland solution provides every essential nutrient required by green plants and is appropriate for supporting growth of a large variety of plant species [27]
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