Review of potentiometric determination of cationic surfactants

3.1 Conventional ISEs

The conventional ISEs for CSs determination contain a solid electrode body with an inner electrolyte and a liquid membrane (Figure 4a). The liquid membrane means that membrane components are dissolved in liquid solvent, usually tetrahydrofuran (THF), but after solvent evaporation, a thin film is obtained. Then, the membranes of the desired size can be cut out of the film and mounted onto the electrode body. The liquid type of membrane usually consists of sensor material, polymer matrix, and plasticizer. The sensor material is usually an ion pair of CS and AS, but neutral carriers, such as crow ethers, can also be used. It has the greatest influence on characteristics of the electrode, and it determines its selectivity. The membrane usually contains 1% of sensor material. The polymer matrix ensures the mechanical stability of the membrane. It should be inert. Polyvinyl chloride (PVC) is the most often used polymer matrix in ISEs for CSs determination. The plasticizer is the quantitatively dominant component of the membrane. The weight ratio of polymer to plasticizer is usually 1:2. It reduces the viscosity of the membrane and ensures the mobility of the membrane components. Due to its high content in the membrane, the plasticizer also influences the response of ISE and affects its lifetime. The most commonly used plasticizers are o-nitrophenyl octyl ether (o-NPOE) and bis(2-ethylhexyl) phthalate (DOP). Consequently, variations in membrane composition (different components and/or different ratios of components) lead to the development of ISEs with improved analytical characteristics. Therefore, many authors optimized the membrane composition of their electrodes by varying the type of plasticizer and/or content of sensor material [34,35,36].

Figure 4

The comparison of the main electrode designs for ISEs for CSs determination: (a) the conventional ISE, (b) solid-contact ISE, and (c) all-solid-state ISE.

In 1970, Gavach and Seta [37] developed and described the first potentiometric sensor for CSs determination. It was an ISE with a liquid membrane. The sensor material in the membrane was an ion pair composed of quaternary ammonium cation and picrate or tetraphenylborate (TPB) anion. Using potentiometric titration with the described ISE as the sensor, it was possible to determine CSs at concentrations on the order of 10⁻⁶ M.

This was followed by the development of ISEs for CSs determination based on various sensing materials. These first liquid membranes often gave an unreproducible response and had a short lifetime due to the solubilization of the membrane components in an analyte solution. After Moody et al. [38] developed the calcium-selective electrode with a liquid ion exchanger in a PVC matrix, which led to a significantly prolonged lifetime of the electrode, the development of PVC-matrix electrodes began.

In 1977, Cutler et al. [39] developed electrodes for CS and AS determination with a plasticized PVC membrane based on the complex of CS and AS. The ion-exchange groups were chemically attached to the ends of the PVC chains, which led to a prolonged electrode lifetime due to reduced loss of sensor material by solubilization.

The development of ISEs with PVC membranes that were plasticized with different plasticizers followed. These electrodes showed improved stability. Also, the development of electrodes with different types of sensor materials began.

Khalil et al. [40] developed ISE for Hyamine determination with a PVC membrane, plasticized with diisooctyl phthalate. They used neutral carrier dibenzo-18-crown-6 as the new type of sensor material. The electrode with a membrane contained carrier had improved selectivity compared with the blank electrode, which contained only PVC and plasticizer.

Shoukry [41] applied a new approach for the potentiometric determination of CSs. He developed an accurate and precise potentiometric system with two ISEs, without a conventional reference electrode, for hexadecylpyridinium (HDP) bromide determination. Both ISEs contained plasticized PVC membranes with ion pairs. One was selective to the analyte (CS), and another to the titrant (AS). Instead of the usual sigmoidal titration curve, titration curves with sharp peaks or troughs were obtained. DOP was used as the plasticizer.

Baillarger et al. [42] developed organo-mineral ISE with a heterogeneous membrane for dodecyltrimethylammonium (DTA) ion determination. They used synthetic clay, laponite, as an anionic carrier of CS in PVC membrane plasticized with dinonyl phthalate.

Gerlache et al. [43] developed ISE for CS determination with a PVC membrane plasticized with o-NPOE. The sensor material was an ion pair formed of 1,3-didecyl-2-methylimidazolium (DMI) and dodecyl sulfate (DDS). The proposed ISE could be used for CSs determination using potentiometric titration or for ASs determination using direct potentiometric measurements in an on-line flow system.

Matysik et al. [44] developed two types of electrodes with a polydimethylsiloxane (PDMS) membrane filled with zeolite. One electrode was a conventional electrode with an inner electrolyte, and another was an iridium disk electrode coated with a zeolite-PDMS membrane. The application of zeolites as sensor materials was based on the pores in their crystal lattice, in which guest molecules can penetrate. The results showed that for CSs determination using potentiometric titration, the conventional type of electrode was more convenient.

Saleh [45] developed ISE for CSs determination based on PVC membrane with macrocyclic diimine crown ether as sensor material, and plasticized with o-NPOE. He studied the influence of the concentration of the inner electrolyte on the ISE response.

In 2000, Abbas et al. [46] developed ISE for cetylpyridinium cation (CP) determination. The electrode contained a PVC membrane plasticized with DOP. CP iodomercurate (CP-IM) was used as a sensor material in the membrane. The slope of the ISE was sub-Nernstian which was explained by CP existence in a dimer-like structure. The electrode was successfully applied for CP determination in mouthwash and results were in good agreement with those obtained using the British Pharmacopoeia method. In the next year, the author from the same group [47] developed another ISE for CP determination in mouthwash. This time, he used CP-FTC as sensor material, which led to a reduced limit of detection (LOD) value and a near-Nernstian slope.

In 2004, Mahajan et al. [48] developed two ISEs for two CSs determination using electrodes with PVC membranes plasticized with DOP. The ion pairs of tetradecyltrimethylammonium (TTA) and HDP with DDS were used as sensor materials. The HDP-DDS based electrode exhibited a sub-Nernstian slope. The authors investigated the influence of various additives on CMC and concluded that additives increased the CMC value. It was explained by the formation of complexes between the surfactant and the additive.

In the same year, Masadome et al. [34] investigated the influence of various plasticizers on the response of ISE without sensor material (ion-exchanger), to CSs. As plasticizers, the authors used o-NPOE and its derivatives. Considering that the analytical performance of ISEs with sensor material can be affected by the choice of plasticizer, they assumed that the use of plasticizers with higher hydrophobicity leads to enhanced performance of the ISE. However, the investigated plasticizers reduced LOD, but did not enhance other electrode characteristics. As the best plasticizer, the authors chose o-nitrophenyl dodecyl ether (NPDOE).

In 2006, Mostafa [49] developed ISE for CPC determination, based on the ion pair between s-benzylthiuronium (s-BT) and TPB. He used DOP as the plasticizer. The obtained results, for CPC determination in mouthwash, were in good agreement with those obtained using the British pharmacopoeia method.

In 2008, Madunić-Čačić et al. [50] developed ISE, based on DMI-TPB as sensor material, in a liquid membrane mounted in a commercial Philips electrode body. They used the proposed ISE for CTAB, Hyamine, and CPC determination using potentiometric titration. The authors showed that NSs, which are common in commercial products based on CSs, had negligible influence on the potentiometric titration, using the proposed ISE as an end-point detector, if the mass ratio of NSs to CSs did not exceed 5. The new electrode was successfully applied for CSs determination in disinfectants for the food industry, disinfectants for hospital use, and mouthwashes. The results exhibited satisfactory agreement with those obtained using a commercial surfactant sensor and the standard two-phase titration method. In 2011, the same research group [51] developed the method for simultaneous potentiometric determination of CSs and NSs in commercial cleaning agents and disinfection products, using potentiometric titration. The one-step titration was based on the reaction of an anionic titrant with CS, followed by its reaction with pseudocationic complex, formed from a barium cation and NS. The authors used a DMI-TPB sensor, but instead of o-NPOE, they used DOP as the plasticizer. They investigated two-component mixtures of four CSs and three NSs and concluded that errors appeared when the consumption ratio of titrant for each component in the mixture surpassed 3:1. This problem was solved by the known addition of the surfactant with a lower consumption of the titrant.

In 2011, Najafi et al. [52] developed CSs selective electrode using single-walled carbon nanotubes (SWCNTs), for the first time. SWCNTs and CTAB were mixed and used as an ionophore. Due to the specific properties of SWCNT, the proposed electrode showed lower data scattering and better stability, compared to conventional PVC membrane electrode.

In 2012, Devi and Chattopadhyaya [53] developed ISE for CSs determination, using CP based Sn(iv) phosphate (CP-SnP) as sensor material. The authors determined CPC in mouthwash using direct potentiometry and potentiometric titration. Both results were in good agreement with those obtained using two-phase titration.

In 2014, Zorin et al. [54] demonstrated applicability of using polyelectrolyte-surfactant complex as sensor material in ISE for CSs determination. In developed electrodes, they used poly-2-acrylamido-2-methylpropanesulfonic acid (PAMPS) and different ammonium surfactants.

In 2021, Sakač et al. [55] synthesized a new long-chain quaternary alkyl ammonium salt, 1,3-dihexadecyl-1H-benzo[d]imidazole-3-ium bromide (DHBI-Br), and used its ion pair with TPB (DHBI-TPB), as sensor material in ISE for CSs determination. Also, they used a prepared electrode for simultaneous determination of CSs and NSs in solution containing barium cation, by potentiometric titration. Thus, the first inflexion point was for CS and the second was for NS. The authors showed that increased concentration of etoxylated (EO) NS, as well as the number of EO groups in the NS molecule, negatively affected the shape of the titration curve and the change in potential. The applicability of the sensor for CSs determination in personal care products and disinfectants was demonstrated. The same group of authors [35] investigated the influence of six different plasticizers on the potentiometric response characteristics of the DMI-TPB sensor. The best results were obtained using o-NPOE as the plasticizer. In 2022, they [56] synthesized one more quaternary alkyl ammonium salt, 1,3-dioctadecyl-1H-imidazol-3-ium bromide (DODI-Br). Again, they used its ion pair with TPB (DODI-TPB) as sensor material in ISE for CSs determination. The authors successfully used the proposed sensor for CSs determination in disinfectants and antiseptics. One year later, the same group of authors [57] demonstrated the use of their DODI-TPB sensor for the determination of technical grade quaternary ammonium compounds commonly contained in COVID-19 disinfectants and antiseptics: hyamine, N,N-didecyl-N,N-dimethylammonium chloride (DDAC), and N,N-dioctyl-N,N-dimethylammonium chloride (DOAC).

As it can be seen from above, many different conventional ISEs with liquid type of membrane, for CSs determination were developed. Their main drawback is leakage of the sensor material, which leads to a short lifetime of the ISE, due to changes in analytical performance and potentiometric response. Therefore, many authors tried to optimize the composition of their ISE membrane by using less water-soluble sensing materials. Some of them synthesized new CSs, used as part of sensing material. Some authors have even developed ISEs without sensor materials because gradual saturation of the plasticizer with the ion pair formed during the titration was expected. However, such electrodes usually had a sub-Nernstian slope and a slow response time. Also, it is shown that introducing nanomaterials into membrane composition leads to better electrode stability. Using conventional types of electrodes, CSs were commonly determined in mouthwashes, nose and eye drops, disinfectants, and antiseptics. A chronological overview of the conventional ISEs with liquid membranes for potentiometric determination of CSs is presented in Table 1.

3.2 Solid-state ISEs

Although conventional ISEs have been used for CSs determination for a long period of time, they have a problem of leaching sensor material, which causes an unstable potential response. The solid-state ISEs for CSs determination were developed in order to overcome this problem and to further simplify ISEs construction. The absence of an unwanted inner water layer led to enhanced ISEs performance. Also, solid-state ISEs can easily be miniaturized, which leads to lower reagents and samples consumption. A distinction should be made between solid-state ISEs with the liquid membrane (solid-contact ISEs) (Figure 4b) and all-solid-state ISEs (Figure 4c). The most commonly used solid-state concepts for CSs determination include coated-wire, graphite-based, and screen-printed electrodes. CSs determination using solid-state electrodes began in the 1970s.

In 1974, Fogg et al. [58] described the solid-state silicone-rubber membrane ISE for CSs determination, based on the ion pair of CS and AS. The membrane contained hexadecyltrimethylammonium (CTA) DDS, was attached to the lead glass tube. The proposed ISE responded only to CSs, not to ASs.

Coated-wire electrodes are ISEs formed by coating of thin polymer films, containing sensor material, onto a metallic conductor [59]. In order to obtain the desired film thickness, metal wire can be immersed in a membrane cocktail a few times, with the electrode drying period between every immersion. In addition to the fact that the construction of these electrodes is very simple, it is also low cost, and allows for a highly miniaturized design. Also, by changing the content of the coating material or wire construction, they can easily be modified and adopted for various applications. The first coated-wire ISE was developed in 1971 for Ca²⁺ determination [60]. The optimization of coatings, using different sensor materials and plasticizers, followed.

Vytřas et al. [61] developed an aluminum coated-wire electrode without an ion-exchanger, for the determination of CS and AS using potentiometric titration. The determination was based on the formation of an ion-pair between the analyte and the titrant. They used PVC and poly(vinyl butyral) (PVB) as plastic matrix and obtained better results using PVC. Six different plasticizers were used for membrane optimization, and the best results were obtained using 2-nitrophenyl 2-ethylhexyl ether. They used the same type of electrode for CS determination using indirect titration [62] where the excess sodium TPB (NaTPB) used as a titrant was back-titrated with thallium(i) nitrate solution.

Pinzauti et al. [63] developed a robust silver-silver sulfide solid-state electrode for CSs, sodium halides, and thiols determination using potentiometric titration. CSs were analyzed in antiseptic formulations and contact lens solution.

Dowle et al. [64] developed a graphite electrode with a PVC membrane plasticized with tritolyl phosphate, based on the tetrabutylammonium dodecylsulfate ion pair. Compared to the blank electrode, the electrode with the ion pair had a faster response time and a better slope. Also, the electrode showed good performance in mixed systems containing water and ethanol if the ethanol content was lower than 20%. It was successfully applied for CSs determination in washing-up liquids and washing powders using potentiometric titration. Using the same membrane mixture, Dowle et al. [65] also developed flow-through electrode for CSs determination in routine control analyses. The membrane solution was applied to the inner surface of a graphite tube. The electrode showed a limited lifetime due to the leaching of the sensor material.

In 2000, Matešić-Puač et al. [66] developed and compared two electrodes for CSs determination. The electrodes contained a PVC membrane plasticized with o-NPOE, and the Ca²⁺ carrier N,N,N′N′-tetracyclohexyl-3-oxapentanediamide (NTO) as the sensing material. The potassium tetrakis (4-chlorophenyl) borate was used as a lipophilic anionic site in order to reduce the coextraction of anions in the membrane. Both electrodes were obtained by coating of membrane mixture on the body of the electrode (graphite-polytetrafluoroethylene (PTFE) universal Ružička selectrode and platinum metallic electrode). Although both electrodes were successfully applied for CSs determination in commercial formulations, better results were obtained using graphite-PTFE electrode. The same group of authors [67] developed and compared four solid-contact ISEs for CSs determination. All ISEs had teflonized graphite as the conducting substrate, but every ISE had a different ion-pair complex as the sensor material. Only the electrode with dodecyldimethylbenzylammonium dodecylbenzenesulfonate exhibited a Nernstian response for both CSs investigated.

Various attempts to improve the selectivity of the electrodes followed. PVC molecular sieves with tailored pore size proved to be suitable for this purpose [68]. Also, zeolites with their unique molecular sieving properties, based on the size and shape of the molecules, can be successfully applied for ISE design [69].

In 2000, Kulapin and Arinushkina [70] developed and compared few solid-contact ISEs. They used graphite and silver leads. CP dodecylsulfate and CP-TPB were used as sensor materials, and dibutyl phthalate (DBP) as a plasticizer. They used the developed ISEs for separate determination of ASs, CSs, and NSs, and for simultaneous determination of CSs and NSs. In 2003, Kulapin et al. [71] applied a new approach for the potentiometric determination of CSs. They developed graphite solid-contact ISE with membrane modified with PVC molecular sieves. Such a concept significantly improved electrode selectivity. Molecular sieves were inert PVC matrices, plasticized with DBP, with different pore diameters. As membrane sensor material, they used CP-TPB. Using such a modified electrode, it was possible to separately determine alkylpyridinium chlorides in binary and ternary mixtures. In 2005, the same group of authors [72] made additional improvements. Using ISEs that contained molecular sieves with predetermined pore sizes in their membranes, they improved selectivity of the sensors. Furthermore, using inselective sensors they developed a multisensor system, like an “electronic tongue” for the separate determination of surfactant homologues in multicomponent solutions. Artificial neural networks were utilized to process analytical signals. They were trained on pure multicomponent solutions. It allowed the determination of composition and concentrations of surfactants in a five-component solution.

In 2002, Matysik et al. [73] developed an all-solid-state disposable planar ISE with a zeolite-PDMS membrane casted on gold leaf, which was fixed to the copper surface of the electrode. Although the electrode had a disposable character due to its low cost and simple preparation procedure, it had good stability. This was a consequence of the structure of zeolite-PDMS membrane, which prevented the leaching of the membrane components. The electrode was successfully used for CSs and ASs determination using potentiometric titration.

In 2006, Plesha et al. [74] developed and compared three coated-wire ISEs with different sensor materials, for CSs determination in cleaning solutions. All ISEs proposed, exhibited a sub-Nernstian response. Also, they investigated the effect of CSs on other ISEs with PVC membranes, and concluded that surfactant concentrations higher than 1 × 10⁻⁵ M significantly decreased the slope of the ISE.

At the end of the 2000s, researchers realized that there was a need to develop miniaturized electrodes, that would be suitable for in situ monitoring using portable analyzers. Therefore, they began to develop screen-printed electrodes for CSs determination. Screen-printed electrodes are ISEs produced by printing ink, containing sensor material, on plastic or ceramic substrates [75]. These electrodes are robust, disposable, low cost, and offer versatile electrode designs, including working and the reference electrode on the same substrate, or multiplexed working electrode designs. Furthermore, it is easy to prepare and modify these electrodes, using home-made carbon ink. One of the main advantages of screen-printed electrodes is the possibility of performing measurements in small volumes, which leads to a significant reduction in sample and reagent volumes.

In 2008, Khaled et al. [76] developed the first disposable screen-printed carbon paste electrodes for CSs determination, using home-made printing ink. The electrodes contained both, working and reference electrodes in the same strip. Effects of plasticizer type and content, binding material, carbon content, and solvent were investigated in order to get an optimized printing ink formulation. The optimized screen-printed electrodes were applied for CSs determination in pharmaceutical preparations. The results were in good agreement with those obtained using commercial surfactant electrode and the official method. Also, the analytical performances of the new electrode were compared with those of coated-wire, coated-graphite, carbon paste, and conventional electrodes. The proposed electrodes enabled significant reductions in reagent and analyte volumes. In 2010, Mohamed et al. [77] performed the analytical characterization of their electrodes. They also determined the CTAB concentration in pharmaceutical formulations and spiked sea water. In the same year, Mohamed et al. [78] modified their electrode using tricresylphosphate (TCP) as a plasticizer. The electrode was used for CPC determination. One year later, Mohamed et al. [79] made another modification of their screen-printed carbon paste electrode. They added an ion pair, made of 1-(ethoxycarbonyl)pentadecyltrimethylammonium bromide (Septonex) and TPB, in the carbon ink used for electrode preparation. They used the new electrode for Septonex determination by potentiometric titration and direct potentiometry. In 2014, Ali et al. [80] developed and optimized a carbon paste electrode for dodecyl(2-hydroxyethyl)dimethylammonium (DHDA) determination based on potassium tetrakis(4-chlorophenyl)borate (KTCPB) as the sensor material. They varied the content of the sensor material and the type of the plasticizer, and concluded that the sensor with 7.5% of the sensor material and DOP as the plasticizer, exhibited the best results. The applicability of the sensor for DHDA determination in water samples was demonstrated. In 2015, Ali et al. [81] prepared carbon paste electrode for 1-dodecyl-5-methyl-1H-benzo[d][1,2,3]triazol-1-ium bromide (DMTB) determination. This time, the sensor material was 1,4-bis-(8-mercaptooctyloxy)-benzene (MPB). Again, they varied the content of the sensor material and the type of the plasticizer, but this time, the best results were obtained using 10% of the sensor material and o-NPOE as the plasticizer. In 2017, Khaled et al. [82] applied a completely new approach and optimized their disposable screen-printed electrodes, thus developing enzymatic ISEs for CSs determination. The electrodes were prepared using ion-sensing printing ink printed on graphite/PVC conducting track. The ink cocktail contained α-cyclodextrin (α-CD) as the sensing ionophore, KTCPB as the anionic site, 2-fluorophenyl 2-nitrophenyl ether (f-PNPE) as the plasticizer, THF as the solvent, PVC, and multi-walled carbon nanotubes (MWCNTs) as the ion-to-electron transducer for improving the stability of the electrode potential. The working principle of the proposed biosensor was inhibition of the enzyme acetylcholinesterase (AChE) by surfactants followed by measuring the residual activity of the enzyme using the proposed biosensor. The inhibition of the enzyme was dependent on the nature of the surfactants. The developed biosensor was successfully applied for CSs determination in mouthwash and commercial detergent.

In 2010, Price and Wan [83] tested the Metrohm ionic surfactant electrode for quaternary ammonium compounds determination in eight antimicrobial products, using the potentiometric titration method. They emphasized that the electrode was very sensitive to ethanol and methanol, and that organic solvents caused deterioration of the membrane which significantly reduced the electrode lifetime. It should be taken into consideration due to the presence of ethanol in many antimicrobial products.

In 2012, Gaber et al. [84] developed and optimized an all-solid-state sensor for Hyamine determination. As sensor materials, they tested ion pairs of Hyamine and four heteropoly acids. The best results were obtained using electrode with ion pair composed of Hyamine and phosphomolybdic acid (Hyamine-PM). Also, the authors varied plasticizers. They used five plasticizers, and concluded that the electrodes plasticized by a mixture of plasticizers (DBP and DOP) gave better results, compared to electrodes plasticized by a single plasticizer. The optimized carbon paste electrode was successfully applied for Hyamine determination in eye and ear drops.

In 2016, Issa et al. [85] developed and optimized carbon paste electrode for Hyamine determination based on Hyamine-TPB ion pair as sensor material. In the optimization process, the authors varied the content of the sensor material and the type of the plasticizer. The best results were obtained using the electrode with 5% of sensor material, and tricresyl phosphate (TCP) as the plasticizer. The applicability of the sensor was shown for Hyamine determination in antibacterial spray. The authors also prepared and optimized conventional membrane-sensor, but it exibited worse results than those obtained using carbon paste electrode.

Till the end of the 2010s, it was well known that carbon nanomaterials have unique properties such as a high surface-to-volume ratio, the existence of mobile electrons on the nanotubes surfaces, high hydrophobicity, and the ability to facilitate electron transfer [86]. These properties can lead to the development of stable and reliable ISEs with prolonged lifetime and improved response characteristics [87]. Therefore, the introduction of nanomaterials in ISE design was a promising research area. They can be implemented in ISE design as transducers, as mentioned above, or as part of sensor materials [88].

In 2017, Hajduković et al. [89] applied a new approach for developing potentiometric sensor for CSs determination. They used a hybrid sensor material that consisted of MWCNTs functionalized with a sulfate group and CP cation (MWCNT-OSO₃ ⁻CP⁺). The chemical modification of MWCNTs led to improved membrane stability and signal drift, as well as a prolonged life time of the sensor due to the reduced leaching of the sensor material from the membrane. Also, immobile MWCNT-OSO₃ ⁻ ion in the membrane formed repulsive electrostatic barrier for the anions. To demonstrate the applicability of the sensor, the authors successfully determined CSs in commercial products and mixtures of two CSs.

In 2023, Frag et al. [36] developed a carbon paste electrode for nonyltrimethylammonium bromide (NTAB) determination in formation water. For the carbon paste optimization, they used five plasticizers and different amounts of four ion pairing agents. The best results were obtained using NaTPB, as an ion pairing agent, and TCP as the plasticizer.

As it can be seen from above, solid-state electrodes were predominant for CSs determination. The coated-wire ISEs are simple, but their performance may show deterioration over time. Disposable strip screen-printed electrodes are easy to mass produce and open the ability for field measurements. Compared to conventional electrodes and coated-wire ISEs, carbon paste electrodes have very low Ohmic resistance, a short response time, and a long lifetime [80]. Also, their preparation is very simple, handmade paste is easy to modify, and the developed electrode can be used immediately after assembly without additional time for solvent evaporation. Many authors made an effort to improve the selectivity of their ISEs by introducing new sensor materials and molecular sieves, or to improve analytical characteristics by optimizing the sensor material and plasticizer content and introducing nanomaterials into their electrode designs. Solid-state ISEs were mainly used for CSs determination in disinfectants, hair rinses, mouthwashes, eye and ear drops, fabric softeners, and cleaning solutions. A chronological overview of the solid-state ISEs for potentiometric determination of CSs is presented in Table 2.

3.3 Ion-sensitive field-effect transistors

An ion-sensitive field-effect transistor (ISFET) is a potentiometric device obtained by coupling ISE and a field-effect transistor (FET). It is based on a change in current through the transistor as a consequence of a change in ion concentration in the analyte solution. ISFETs have fast response and good reproducibility [90]. They are robust and easy to handle. Compared to conventional ISEs, they have a low impedance output signal, are easier to miniaturize, and, due to their small size, require much lower volumes of sample and reagents. Their main advantage compared to conventional ISEs is their compatibility with complementary metal-oxide semiconductor (CMOS) technology, which enables integration of electronics and the sensor on the same chip and leads to low-cost mass fabrication of multi-parametric sensors with low power consumption [90]. Furthermore, using those principles, wearable multianalyte sensing platforms with real-time monitoring can be developed [91]. Also, ISFETs are widely used in continuous flow systems (flow injection analysis and sequential injection analysis), providing automatic sampling and calibration [92,93,94,95]. Considering the above, ISFETs found their application in different areas of analytical chemistry, so it is surprising that only a few authors have developed ISFETs for CSs determination.

Campanella et al. [96] developed three ISFETs for CSs determination based on PVC membranes plasticized with bis(2-ethylhexy1)sebacate. As sensor materials, they used benzyldimethyl-hexadecylammonium reineckate, DTA reineckate and HDP phosphotungstate, respectively. The sensor with the best analytical characteristics was applied for CSs determination in environmental water samples.

Masadome et al. [97] used ISFET to investigate the contribution of the plasticizer to the response of ISE for ionic surfactants determination. They concluded that the plasticizer itself does not respond to surfactants, but a plasticized PVC membrane with no sensor material exhibited a Nernstian response. In 2006, Masadome et al. [98] developed and optimized ISFET for CSs determination. They varied the sensor materials and showed that the ISFET, based on an ion pair formed between the benzyldimethyltetradecylammonium (Zephiramine) and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), exhibited the best analytical performance. o-NPOE was used as the plasticizer. Also, they developed a system for CSs determination, composed of a microfluidic polymer-based microchip embedded with a developed ISFET. Both developed systems exhibited similar results. The obtained slope was 58.5 ± 1.7 mV/decade of activity in the range from 1.00 × 10⁻⁶ to 1.00 × 10⁻³ M. The LOD was 2.90 × 10⁻⁷ M. The ISFET could be used in pH range from 3 to 9. Furthermore, the microchip integrated with the developed ISFET, was successfully applied for CPC determination in dental rinses. The accuracies obtained were in the range between 94.70% and 103.30%.

Although ISFETs can be used as fast and reliable devices for CSs determination, they are not widely used in practice. The reason for that could be their long-term drift and hysteresis which lead to limited accuracy for long-term measurements [99]. Furthermore, temperature variations cause thermal drift, which causes the nonlinear behavior of ISFET device [100]. In samples with low analyte concentrations, achieving a good signal-to-noise ratio can be challenging. Also, the complexity of fabrication, compared to conventional and solid-state ISEs could be a problem.