Sensing of pH is often applied as a signal transduction way in biosensors, whereas chemical reactions catalyzed by some enzymes (e.g. glucose oxidase, urease, etc.) are accompanied with the change in pH of local environment [1, 2]. Moreover, pH sensing can be used to determine several physiological and pathological conditions for health care purpose, e.g. higher than normal pH of sweat can be related to the dehydration and/or cystic fibrosis [3]. Therefore, creation of reliable and accurate ways of pH sensing is important in the development of biosensors and other healthcare devices.
Prussian blue (PB) is an inorganic pigment widely applied in formation of sensors and biosensors [4]. Even though PB is most commonly used in (bio)sensors due to its selectivity to hydrogen peroxide [4, 5], PB also could be applied in sensing systems due to its optical [6, 7] and electrochemical [8, 9] sensitivity to the changes in pH of solution. Furthermore, optical sensitivity of PB to changes in pH have been successfully applied in construction of biosensor for urea [6].
In this study comparison between optical and electrochemical ways of pH sensing with PB layers is presented and differences between preparation of PB coatings for optical and electrochemical sensing of pH are discussed. The stability of PB layers during measurements of pH and the influence of composition of analysis solution on the performance of optical and electrochemical measurements of pH are also considered in this study.
The main difference between structures of PB, which are required for optical and electrochemical measurements of pH, is based on amount of Fe2+ ion in the crystal lattice of PB. In optical measurements of pH “insoluble” form of PB (Fe3+4[Fe2+(CN)6]3) is used [6, 7]. The optical sensitivity of “insoluble” PB to the change in pH is based on reversible hydrolysis of PB (Eq. (1)), which occurs due to increase in OH- ion concentration in solution [6, 7]. This reversible hydrolysis of pH causes a decrease in absorption of PB, thus, pH of solution can be determined from absorption spectrum of PB. This pH-induced change in absorption of PB layer can be possibly applied in construction of naked-eye sensing devices, which are very convenient for unqualified users. However, PB-based optical sensing of pH has some limitations – when pH value of solution is higher than 9 an “insoluble” form of PB over goes irreversible hydrolysis caused by destruction of Fe3+[Fe2+(CN)6]- ion (Eq. (2)). Therefore, optical PB-based sensors of pH can be utilized in analysis of sample with pH value up to 9. Nevertheless, during the PB-based optical analysis of pH high concentrations of K+ ion should be avoided, since K+ can induce spontaneous transformation of the “insoluble” PB into the “soluble” form.
$${\text{Fe}^{3+}_{4}[\text{Fe}^{2+}(\text{CN})_{6}]_{3} + 3\text{H}_{2}\text{O} = \text{Fe}(\text{OH})_{3} + 3\text{Fe}^{3+}[\text{Fe}^{2+}(\text{CN})_{6}] + 3\text{H}^{+}}\tag{1}$$ $$3\text{Fe}^{3+}[\text{Fe}^{2+}(\text{CN})_{6}]^{-} + 3\text{OH}^{-} \rightarrow \text{Fe}(\text{OH})_{3} + 3[\text{Fe}^{2+}(\text{CN})_{6}]^{4-}\tag{2}$$Even though PB has not been involved in formation of the electrochemical pH sensor yet, PB exhibits the dependency of charge transfer through PB film on pH [8, 9], which can be observed during the measurement of electrochemical impedance spectroscopy and cyclic voltammetry. This relation between pH and charge transfer resistance of PB is explained [8, 9] by the role of hydrogen ion in charge transport through the PB film – it is postulated that hydrogen ion is involved in electron-hopping between neighboring active positions in the PB [8, 9]. Therefore, the PB-coated electrode could be possibly used as an electrochemical pH sensor. Moreover, “soluble” form of PB (K+Fe3+[Fe2+(CN)6]) is suitable for observation of pH impact on charge transport through the PB [8, 9]. The ability to use “soluble” PB makes the electrochemical measurement of pH more accurate, since “soluble” PB exhibits better electrochemical stability and it is able to undergo multiple redox transitions [8, 9]. Moreover, “soluble” form of PB is considered to be more stable in alkaline conditions – during our recent investigation “soluble” form of PB appeared to be stable in pH values up to 11 [1]. Therefore, electrochemical sensing of pH with PB seems to be promising tool in the development of biosensors and other healthcare devices.