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Al-Jeda, M.;  Mena-Morcillo, E.;  Chen, A. Applications of Micro-Sized pH Sensors. Encyclopedia. Available online: https://encyclopedia.pub/entry/39004 (accessed on 04 July 2024).
Al-Jeda M,  Mena-Morcillo E,  Chen A. Applications of Micro-Sized pH Sensors. Encyclopedia. Available at: https://encyclopedia.pub/entry/39004. Accessed July 04, 2024.
Al-Jeda, Muhanad, Emmanuel Mena-Morcillo, Aicheng Chen. "Applications of Micro-Sized pH Sensors" Encyclopedia, https://encyclopedia.pub/entry/39004 (accessed July 04, 2024).
Al-Jeda, M.,  Mena-Morcillo, E., & Chen, A. (2022, December 20). Applications of Micro-Sized pH Sensors. In Encyclopedia. https://encyclopedia.pub/entry/39004
Al-Jeda, Muhanad, et al. "Applications of Micro-Sized pH Sensors." Encyclopedia. Web. 20 December, 2022.
Applications of Micro-Sized pH Sensors
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This entry is adapted from the peer-reviewed paper 10.3390/mi13122143

Monitoring pH changes at the micro/nano scale is essential to gain a fundamental understanding of surface processes. Detection of local pH changes at the electrode/electrolyte interface can be achieved through the use of miniaturized pH sensors. When combined with scanning electrochemical microscopy (SECM), these sensors can provide measurements with high spatial resolution. This article explores the applications of miniaturized pH sensors in biological studies, corrosion science, in energy applications, and environmental research.

localized pH microelectrode probe microsensor nanosensor UME SECM scanning probe microscopy scanning electrochemical microscopy pH microscopy proton concentration

1. Biological Systems

pH measurements in live systems are important as even small changes at the intracellular and/or extracellular level can dramatically affect numerous biological functions. The pH can also influence the activity of microorganisms and the physiological response of the human body. The use of pH probes for localized pH measurements in biological systems has served as a powerful tool to study bacteria, human cells, and other microorganisms [1][2][3][4][5][6][7]. For example, Harris et al. [1] monitored urea hydrolysis and subsequent calcification processes induced by a Sporosarcina pasteurii biofilm in real time. It was found that the pH near the surface of the biofilm increased from 7.4 to 9.2 within 2 min, whereas the Ca2+ concentration decreased from 85 to 10 mM within 10 min. The CaCO3 precipitation caused an increase of 50 μm in biofilm height after 4 h. The authors concluded that bacterial enzymes promoted a fast urea hydrolysis process and that CaCO3 formation/precipitation was the rate-limiting step.
In another study, Joshi et al. [2] evaluated the microbial metabolic exchange between two bacterial species, Streptococcus gordonii and Streptococcus mutans. It was shown that H2O2 generated by S. gordonii was inhibited by the lactic acid produced by S. mutans. The authors applied pH sensors in 3D mapping, showing that the chemical environment was initially dominated by S. gordonii while the buffering capacity of the saliva was still valid (∼pH 6.0−7.2) and that eventually, S. mutans took control by decreasing the local pH (≤5.0). An investigation of pH gradients in the extracellular space of cancer cells and normal cells was carried out by Munteanu et al. [3]. The authors observed that cancer cells (HT-29) subjected to hypoxia displayed a more acidic (>0.4 pH units) extracellular environment than normal (HEK-293) or normoxic cancer cells. Work conducted by Liu et al. [4] evaluated the phycosphere pH of marine microorganisms subjected to different environmental conditions. The authors used a micropipette-based pH sensor to measure the phycosphere pH of single phytoplankton cells (~5 μm diameter) under consecutive light/dark cycles. It was demonstrated that the pH in the phycosphere was consistently different from that of bulk seawater, which challenged the previous assumption on both having the same pH. Zhang et al. [5] employed pH nanoprobes to measure dynamic changes in pH gradients in breast cancer MCF7 cells at the single-cell level. The authors achieved pH mapping with high spatiotemporal resolution, which revealed that the peri-cellular environments of melanoma and breast cancer cells display tumor heterogeneity.
Song et al. [6] and Xiong et al. [7] used a potentiometric dual-microelectrode to monitor the extracellular pH of MCF-7, HeLa, and HFF cells under electrical stimulation. Their studies showed that by increasing the stimulation potential, the extracellular pH decreased due to the cell membrane becoming more permeable, and that the extracellular pH of cancer cells was lower than that of normal cells. Aref et al. [8] applied a potentiometric pH nanosensor for intracellular measurements, revealing the pH gradient from the extracellular environment to the intracellular environment of a single PC12 cell, as well as variations in its intracellular pH after administration of the drug cariporide.

2. Corrosion

Corrosion processes involve anodic metal dissolution often accompanied by a cathodic reaction, which produces a local increase in pH. Moreover, metal ions released during corrosion may cause hydrolysis reactions and acidify the surrounding environment [9]. Consequently, it is evident that analyzing localized pH changes can help gain a better understanding of corrosion mechanisms and help to develop corrosion mitigation strategies.
The initial application of pH microsensors in corrosion research was performed with galvanic couples of pure metals, such as Zn-Fe pairs, as model systems [9][10]. In another study, Filotás et al. employed double-barrel electrode assemblies to explore galvanic corrosion of Zn-Fe [11], Zn-Cu [12], and Cu-Fe couples [13]. Lowe et al. [14] modelled the cut edge corrosion of metallic coatings (Zn, 55% Al–Zn and solid solution Al–40% Zn) on steel. Their results showed that pH decrease caused by steel corrosion, along with the buffering effect of Al3+ and Zn2+ ions, impacted both the corrosion potential and cathodic current during electrochemical polarization. Etienne et al. [15] evaluated the local corrosion phenomena, the self-healing properties of an organic/metallic coating on steel, and tracked the sealing process of nanoporous alumina anodized layers [16]. In brief, the authors observed an increase in pH during metal dissolution, followed by the formation of a protective/sealing layer, and finally the consolidation of this layer indicated by the local pH value reaching the bulk pH.
Magnesium and its alloys have also been explored in biomedical applications through these pH sensors [17]. Jamali et al. studied the degradation of AZ31 [18] and AZNd coated with Pr(NO3)3 [19] in simulated body fluid. The authors found that in the case of AZ31, the interfacial pH was highly alkaline and significantly different from the bulk pH, even in a buffered solution. On the other hand, for the Pr(NO3)3 conversion coating on AZNd, they found that Pr+3 acted as an effective corrosion inhibitor with self-healing characteristics, taking advantage of its dynamic deposition at highly alkaline domains. Filotás et al. [20] explored the use of a multi-barrel electrode assembly to characterize the corrosion of AZ63 magnesium alloy. The authors achieved simultaneous detection of Mg2+ ions and pH changes, avoiding at the same time the effect of the electric field developed around local anodes and cathodes. Tefashe et al. [21] described the local degradation of PEDOT-coated AZ31B Mg alloy. They measured the pH changes of a coated AZ31B/bare AZ31B couple. The authors noticed that the PEDOT coating gradually lost its initial protective ability due to localized coverage of corrosion products. Gnedenkov et al. [22] identified differences in the corrosion process of MA8 Mg alloy in 0.83% NaCl solution and MEM at the microscale level. The authors found that the local pH of MEM was stabilized over time by the formation of a hydroxyapatite layer, which decelerated the corrosion process.
There are also other metallic materials that have been examined with pH probes. Zhu et al. explored the local pH and Fe2+ distribution over a 316 L stainless steel surface [23][24]. They detected high concentrations of Fe2+ at the local anode and an increase in pH values at the local cathode. These measurements allowed tracking the formation of a stable pit over the steel surface. Ramírez-Cano et al. [25] reported the pH distribution over Cu samples treated with an organic corrosion inhibitor. The authors found that the treated Cu surface displayed more alkaline pH values than the non-treated one. This behavior was attributed to the protective effect of the inhibitor, making the treated surface act as a cathode where oxygen reduction reaction took place. Asserghine et al. [26] monitored the local pH during the self-healing of the TiO2 passive layer on a Ti dental implant. They observed that during the formation of the passive layer, the pH decreased and that this process was not instantaneous, as previously assumed. Da Silva et al. [27] analyzed the distribution of reactive sites and local pH change during severe localized corrosion on 2098-T351 Al alloy. They observed that at the severe localized corrosion sites, there was lower pH, higher H2 evolution, and lower O2 consumption. More recently, additive manufacturing alloys have also been studied by means of tracking local pH changes [28][29]. 

3. Energy and Environment

Measuring pH in fields like green energy production and environmental remediation can help to understand and tune the electrochemical processes involved, thus making them more efficient. Chemical solubility and reaction kinetics are affected by a change in pH, and electrocatalytic reactions can also induce a shift in the local pH values. The first attempts to determine pH changes at the electrode/electrolyte interface were for hydrogen evolution and oxygen reduction reactions. The hydrogen evolution reaction (HER) at noble metals has been used as a model system to test both potentiometric [30] and voltammetric [31] pH sensors. During HER, water is reduced and the generated OH- ions increase the interfacial pH:
2H2O + 2e− → H2 + OH−
The oxygen reduction reaction (ORR) has also been used as a standard system for pH measurements [32]. As ORR occurs, the local environment becomes more alkaline due to the production of hydroxide anions:
O2 + 4e− + 2H2O → 4OH−
Regarding energy applications, Ben Jadi et al. [33] analyzed the permeability resistance and proton conductivity of Nafion membranes modified with polypyrrole [34] and polyaniline [33] for direct methanol fuel cells. The authors measured local pH changes at the membrane/solution interface caused by the diffusion of H+ through the membrane. Compared to commercial Nafion-112, the polypyrrole-modified membrane showed a decrease in proton conductivity, whereas the polyaniline-modified membrane displayed an increase in proton conductivity.
For electrocatalysis, Botz et al. [35] investigated the local activities of OH- and H2O in an operating oxygen depolarized cathode (ODC). It was demonstrated that the H2O/OH- activity ratio was double the value in bulk solution at 1 µm above the ODC surface during ORR. The authors suggested that this drastic change in the reaction environment resulted in the switch of the reaction inside the confined pores of the working ODC.
Monteiro et al. explored changes in pH at the diffusion layer during CO electro-oxidation [36] and CO2 electrochemical reduction [37][38]. In their first work, the authors noticed that the two distinct peaks from CO oxidation voltammetry were related to the diffusion limitation of CO and OH– species in the pH range between 7 and 11. Their studies showed a time-dependent decay in the interfacial pH after CO2RR was stopped at the electrode, and how the species at the interface recovered their initial concentration, similar to the bulk solution.
In summary, there has been great interest in probing the pH at the microscale. Significant progress has been achieved in the development of miniaturized, stable, and multifunctional pH probes for applications ranging from measuring pH changes in single cells to those in biofilms, from corrosion of biomedical alloys to novel 3D-printed metal parts, and from water splitting to CO2 reduction. When integrated with scanning probe microscopy methods, these pH sensors can provide information with high spatial resolution. Single measurements, diffusion/concentration profiles, line scans, and 3D maps are examples of the typical results that can be obtained with so-called pH microscopy.

References

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Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Muhanad Al-Jeda
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Emmanuel Mena-Morcillo
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Aicheng Chen
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Update Date: 20 Dec 2022
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