Highly sensing and transducing materials for potentiometric ion sensors with versatile applicability

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Abstract

It remains great challenges to fabricate advanced potentiometric sensors based on ion-selective electrodes (ISEs) with ultrasensitivity, high selectivity, long-term durability, and even reproducible potential. Sensing and transducing materials are vital components for substantial performance optimization. This review systematically summarizes the progress in ionophore, formulation, preparation, nanostructure, and electrochemical properties of state-of-the-art sensing membranes and ion-to-electron transducing layers for comprehensively stabilizing potential and enhancing the sensing performance. This article elaborates the development of cutting-edge solid-contact (SC)-ISEs including wearable chip and needle electrodes to achieve versatile applications like health and environmental monitorings, medical diagnosis, food and plant analysis. As an indispensable element in SC-ISEs, the transducing materials particularly including conducting polymers, noble metals and carbon nanomaterials for stabilizing potential are thoroughly expounded. It is suggested that the highly transducing materials with practical applicability should possess two key features including high hydrophobicity and strong adherence onto the electrode substrates and ion-selective membranes besides high ion and electron conductivity, which could be used to guide the design and fabrication of next-generation ISEs based on multifeatured composites. The current startup status and future research direction of calibration-free ISEs with excellent potential reproducibility that would become a major hotspot in the field soon are highlighted.

Introduction

Sensor is a device, module, or subsystem that can sensitively, selectively and rapidly detect events or changes in its environment. Sensor can generally be classified as chemical sensor and physical sensor. Chemical sensor is a molecular structure that is used for sensing or monitoring the change of the concentration or activity of a chemical species (i.e., an analyte) in a given matrix such as solution, air, blood, tissue, waste effluent, and drinking water after a molecular interaction between sensing molecules and analytes. The sensor that measures physical quantities like temperature, humidity, pressure, acceleration, etc. is called a physical sensor. A potentiometric ion sensor or ion-selective electrode is a type of electrochemical sensor that may be used to determine the concentration of ionized analyte gas or solution by measuring the electrical potential of an electrode when no current is present [1], [2], [3]. Three important reviews by Pretsch et al. and Bobacka et al. published in 1997, 1998, 2008 comprehensively summarize the necessary knowledge on potentiometric ion-selective electrodes (ISEs), and are especially useful for novices. The first one is devoted to the basics of the ISEs and also optodes: ionophore-based sensors with optical signal [1]. The second gives the summary of ionophores known by the late 1990-s [2]. The third is focused on the advance in potentiometric ISEs since the beginning of this millennium and emphasizes the research progress between 2002 and 2006 [3]. Furthermore, the basics of the ISEs has been updated until 2012 and described in a technical book by Mikhelson [4]. Two brief and useful review papers were devoted specifically to problems and prospects of solid-contact ISEs (SC-ISEs) [5], [6]. More recent achievements of ISM materials and ionophore-based chemical sensors have been precisely reviewed by Mikhelson et al in 2015 [7], [8]. Just in 2019 and 2020, parts of four long review articles as very recent representatives are related to the potentiometric wearable ion sensors [9], smart potentiometric nitrate anion sensor [10], and paper-based electrodes and all-integrating platforms in potentiometric ion determination [11], and Ag/AgCl ISE for non-destructively potentiometric Cl- detection in concrete [12], respectively. This is a very brief retrospect of the previous review articles on potentiometric ISEs.

Living creatures like dogs possess bodies and five sense organs, which keep receiving changes in sound, light, pressure, temperature and humidity from the outside world, and have a sense of smell, taste, touch, and feel, as shown in Fig. 1. As the chemical sensors include multisensor systems proposed, a variety of electronic devices of simulating the functions of creatures have been developed [13], [14]. The successful case is gas sensors for explosives and chemical warfare agent developed by Swager et al. who create ultrasensitive sensors that can provide ultratrace detection of chemical vapors based on electronically active conjugated polymers and carbon nanotubes via amplification resulting from exciton migration [15], [16], [17]. The FidoXT hand-held explosive detectors produced by Nomadics Inc can detect 10–100 femtogram quantities of analytes, such as trinitrotoluene, and the sensing performance of the electronic device is comparable to dogs [18]. By contrast, ion sensor and ion sensor array (electronic tongue) do not seem to be so optimistic since there are still more challenges of responsive stability and sensitivity.

For nearly one hundred years, the electrochemical analysis with both macroelectrodes and microelectrodes including non-invasive scanning ion-selective electrode [19], scanning electrochemical microscope [20], light-addressable potentiometric sensors [21], [22], [23] has provided us with convenient testing methods for determination of a variety of ions [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40].

Natural sensor of living creatures and a variety of electrochemical techniques containing potentiometric ISEs-based ion sensors with various characteristics are illustrated in Fig. 1 [41], [42], [43]. Among them, potentiometry becomes more and more popular and reaches up to peak in 2009 and 2011 (Fig. 2) due to its convenience and minimal instrument cost input since it only needs a small digital millivoltmeter or mV meter of high-input impedance to realize the detection of electrical signals. Potentiometry with ISEs was booming in 1970-s. Nowadays, the progress is mostly focused on two issues: (i) the improvement of the detection limits [44], [45], [46], [47] and (ii) on solid-contact ISEs. The signal conversion element of this potentiometric sensor is an ion-selective membrane (ISM), through which two types of potentiometric sensors can be built: ion-selective electrode (ISE) and ion-sensitive field effect transistor (ISFET). Field effect transistor (FET)-based potentiometric sensors were introduced by Bergveld in 1970, when he removed the metal gate from a metal oxide semiconductor-based FET and exposed the silicon oxide gate insulator to a measured solution [48]. Having ion-sensitive membrane, like valinomycin impregnated plasticized poly(vinyl chloride) (PVC), placed over the gate region of the FET, results in a new device ISFET [49], [50]. Relative to ISFET, ISEs have relatively easier assembly procedure by simply covering ISM on substrate electrode without the need of encapsulation [51], [52]. In particular, the stability of ISEs is superior to ISFET due to less temperature drift, time drift and thus ISEs are robust and durable devices with longer lifetime. Therefore, as one of potentiometric ion sensors, ISEs that can be further divided into two kinds of configurations: liquid contact (LC)-ISE with three characteristics and solid-contact (SC)-ISE as seven sensor devices, have attracted considerable interests in ion determination via electrochemical techniques. The field of the potentiometric ion sensors is ripe for a review article but a systematical review on the new scientific accomplishment and technological development of highly sensing and transducing materials for the potentiometric ion sensors with versatile applicability has not been found so far to the best of our knowledge.

Potentiometric ISEs-based ion sensors are sensors that their potential response theoretically depends on the logarithm of ionic activity according to the Nernstian equation. With the linear relationship between the potential responses and ionic activities, the ion level from nM to M concentrations can be detected. ISEs have been traditionally electroanalytical techniques for about one hundred years. The most widely used ISE is the glass pH electrode, which utilizes a thin glass membrane that is responsive to changes in H+ activity. Haber, a Nobel prize winner in chemistry in 1918, was the first person to discover that the voltage of a glass membrane changed with the acidity of a solution early in 1901. In 1906, Cremer observed the pH dependence of measured potential across a thin glass membrane, and Cremer and Haber have established foundation for the glass pH electrodes that are continuously used till today [53], [54]. Another two ISEs, Ca2+-ISE containing liquid ion exchanger [55], [56] and F--ISE based on LaF3 membrane [57], [58], had been proposed by Ross in 1960s. Several years later, Ca2+-ISEs were applied to determination of ionized calcium in normal serum, ultrafiltrates and whole blood [59], [60]. Till 1970s, ISMs had received extensive and systematic research, and a large number of commercially available ISEs-based ion sensors for sensing K+, Na+, Ca2+, F-, Cl- and so on have been developed [61], [62], [63] with the development of electrically neutral lipophilic molecules [64]. Since then, almost all metal ions including serum Li+ [65], Cs+ in nuclear waste streams [66], and Pr3+ [67] shown in Fig. 3 have been analyzed by ISEs. Meanwhile, water hardness (total calcium and magnesium) has also been tried with divalent sensor systems [68], [69], [70]. Until now, the ISEs have received wide recognition and practical application owing to their remarkable advantages such as convenience to integrate potentiometric sensor array (electronic tongue) [71], [72], [73], possibility of real-time and continuous monitoring without sample pretreatment, portability for in-field monitoring, simple and effortless operation as well as fast and inexpensive assessment [74], [75]. The analysis of various ions including Cu2+ in food and food processing [76], [77], Ca2+ in reduced calcium milk protein concentrate [78], dye in beverages [79], dinotefuran insecticide [80], Pb2+ in environmental waters [81], [82], [83], heparin in undiluted whole-blood [84], Na+ in hyperlipidemic blood [85], Na+ in blood during haemolysis [86], total calcium in blood serum [87], [88], ionized Ca2+ in blood [89], [90], [91], tumor suppressor gene P53 in blood [92], telomerase activity [93], non-enzymatic glucose [94], gentamicin sulphate [95], cocaine [96], biperiden [97], amphetamine [98], and sulfide in oil refineries water [99], can be accomplished using ISEs for process control, environmental monitoring, and clinical diagnostics [100].

A Ca-ISE-based aptasensor is applied to analyze Ca2+ ions released from the reaction between phenylboronic acid-functionalized CaCO3 nanospheres and carcinoembryonic antigen, realizing evaluation of target carcinoembryonic antigen-related glycoprotein and allowing sensitive detection of carcinoembryonic antigen of down to 7.3 pg mL−1 [101]. ISEs could be used to determine fluoride and arsenic concentrations in deep-well water and shallow water to compare corresponding rate of dental fluorosis in children [102]. Especially with the commercially available F--ISE, fluoride content can be easily measured to evaluate skeletal fluorosis in fluorosis mice [103] and fluoride release of restorative dental materials [104], fluoride concentration in medicinal plants [105], varnishes [106], brick tea [107], and silver diamine fluoride solution [108]. Recently, it is reported that F--ISE is useful to diagnosis of fluorosis by analyzing fluoride content in body fluids [109]. Considering that DNA methylation may be associated with fluorosis, F--ISE has obtained application for evaluating the dose effect of fluoride in drinking water on the level of 5-methylcytosine (in human and rat blood) as a very important modification in DNA methylation involved in human diseases [110]. A non-invasive methodology of bladder cancer screening through urine analysis with a potentiometric multisensor has been developed [111]. The multisensor consists of several types of ISMs made of various ionophores-containing plasticized PVC, polycrystalline Ag2S-AgCl chalcogenide glass and metallic Sb or Pt. It is discovered that the combination of mutual multisensing sensitivities to macro and trace elements and metabolites including carbonate ions, ammonium, and organic acids could be potentially useful for distinguishing urine samples between healthy people and patients with bladder cancer, which would be helpful for early diagnosis of bladder cancer and thus the reduction of mortality. A fully screen-printed potentiometric sensor with a hydrogel-based touchpad has been designed and fabricated for simple and non-invasive daily monitoring of human sweat Cl- anion [112]. A flexible potentiometric sensor based on membrane containing a mediator ([Fe(CN)6]3–/4–) on PET has been designed and fabricated to noninvasively analyze antioxidant activity of human skin for evaluating the effectiveness of phytocosmetic antioxidant products [113].

Cl--ISE together with electron probe micro analysis has been used to monitor the efficiency of electrochemical chloride extraction in cementitious materials [114]. A paper-assisted potentiometric sensor has been conceived and developed as a low-cost, non-invasive and portable device for the corrosion diagnosis and pH monitoring of aging processes in reinforced concrete [115]. A solid-state ion-selective lab chip sensor was developed for on-site measurement of orthophosphate in small volumes of liquid [116]. A multi-electrode system has been designed for measurement of transmembrane ion-flux through living epithelial cells including Na+, K+, Cl-, HCO3 [117], illustrating the facility of the ISEs. Additionally, the selectivity of the ISEs toward specific ions has been focused on in particular for clinical laboratory and point-of-care analyzers in order to avoid incorrect clinical decisions and treatment [118]. Thus far, many potentiometric biosensors including multiparametric rigid and flexible platform have been developed for detection of glyphosate [119], urea [120], creatinine in urine [121], Na+, K+, pH in real urine [122] and in post-surgical intestinal tissue and liquid sample dip-stick measurements in vitro [123], and even for detection of H2S, a gasotransmitter produced at low levels in several tissues including the stomach [124]. Using a potentiometric Ag+-ISE, an immunoassay for the point-of-care detection of enterovirus 71 (EV71) was developed just by labeling the enterovirus with carboxylated dendrimer-doped AgCl nanospheres via a typical carbodiimide coupling method [125]. Using current-driven ion fluxes of ISE, enzymes and their inhibitors could be detected by potentiometric ISEs [126]. Potentiometric sensor is also useful for sensitive determination of some compounds in indirect way. For example, acetylcholine ISE has been well applied for indirect determination of trace level propoxur pesticide as an acetylcholinesterase through acetylcholinesterase inhibition in a propoxur concentration range from 10−25 M to 10−4 M, achieving an excellent detection limit of 2.89 × 10-17 M and a very extensive linear range from 5.5 × 10-15 M to 1.0 × 10-4 M [127]. Label-free potentiometric sensors based on recombinant human erythropoietin imprinted respective polydopamine or polysilicate membranes fabricated on PVC sensing membrane have been developed for determination of recombinant human erythropoietin by using tetra-butyl ammonium bromide as a marker ion without the need of immunoaffinity purification [128], [129]. The polydopamine imprinted polymer results in more sensitive determination of recombinant human erythropoietin over the range of 1.00–100.00 ng mL−1 with a superior limit of detection down to 0.33 ng mL−1, achieving a portable sensor for doping disclosure in sports cheating. Pb2+ concentration in coastal sediment pore water could be in-situ ultrasensitively determined by a potentiometric Pb2+-selective microelectrode based on a PEDOT/PSS modified Au wire with a diameter of 14 μm [130]. It is found that Ca2+- ISE as a reliable tool could be used to well monitor the adsorption and analyze adsorption mechanism of Ca2+ onto TiO2 and Fe2O3 [131].

Besides, the potentiometry with ISEs could be combined with fluorescence technique for detection of toxic cyanide CN– with a composable module-type cyanide microfluidic sensor [132]. The ISEs could also be designed as distributed sensor arrays for application in in-stream measurement of chloride concentration changes with time more recently [133]. And trace concentration profile and trace concentration versus time contours have been obtained with high spatial and temporal resolution (Fig. 4a,b). Compared to more expensive electrical conductivity meters, the potentiometric arrays are inexpensive in the order of a euro per sensor [133]. This is just a special case because the prices for individual commercially available ISEs are often of several hundred USD per sensor.

The operation of ISEs of multiple modalities like needles [134], glass capillaries [135], [136], and nanopipettes [137], working in harmony, together with the ability to integrate the measurements and turn them into coherent information, provides an opportunity for wider applications and improved performance. With the miniaturized electrode, a miniaturized electronic tongue with an integrated reference microelectrode could be used to recognize milk samples [138].

More interestingly, needle-type ion-selective microsensors had been constructed in plastic pipette for in situ determination of foliar uptake of Zn2+ in citrus plants (Fig. 4c) [134]. Similarly, ISEs have been already applied for measurement of net fluxes of selected ion from different regions of wetland plant root under realistic physiological conditions, including spatial aspects of heavy metal transport along roots, by ion flux measurement techniques with scanning ion-selective electrode and microelectrode [135].

In this review, we summarize some of the most recent advances in potentiometric ion sensors, focusing on highly sensing membranes and transducing materials to improve sensitivity, potential stability, and durability for further expanding their applicability. The main goals of the review are to guide the design and development of the next generation ion sensors for rapid and accurate detection of variety ions including those from toxicant in real-world samples like tap water, river/lake water, and seawater, as well as human biofluids such as blood, urine, sweat, and saliva. And reproducible E° of ISEs and calibration-free ISEs are both highlighted in this review article. Its scope and limitations are mainly focused on LC-ISEs and SC-ISEs with advanced performance and versatile applicability. For the former, the lower detection limit and lifetime have been focalized with the purpose of acquiring extremely sensitive and durable LC-ISEs. For the latter, the potential stabilization and electrode miniaturization of advanced SC-ISEs are major topics through optimizing transducing materials. Future research directions such as development of practical calibration-free ISEs and wearable integrated sensors for in situ measurement have been proposed.

Section snippets

Preparation of ion-selective sensing membranes and ion-to-electron transducing layers

Ion-selective sensing membranes and ion-to-electron transducing layers are one of the most critical elements in the potentiometric sensors because the chemical components, supramolecular structure, morphology, specific interaction to analytes, and no ionophore leakage of the membranes and the layers play a decisive role in ensuring excellent sensing performance including low membrane resistance, highly selective and fast response, stable responsive potential, strong anti-interference, and long

Ionophores for K+ and Na+ ions

Sensitively and selectively sensing components in ISMs, ionophores in particular, have been one of the dominant research activities in ISEs-based potentiometric ion sensors. Still widely used ionophores in sensing membrane materials for macroanalysis of routine electrolytes of biofluids include typical valinomycin ionophore for K+ ion [142], and 4-tert-butylcalix[4]arene tetraacetic acid tetraethyl ester as one of the best Na+ ionophores [2], [143]. Meanwhile, more effective ionophores like

New progress and application in liquid-contact potentiometric ion sensors

There are two contact styles on ISEs: liquid contact and solid contact modes, and thus we have liquid contact ISE (LC-ISE) and solid contact ISE (SC-ISE). Both have ionophore-doped membranes as sensing elements, and the former contains inner solution as a conjunction between the membrane and the internal reference electrode (e.g. Ag/AgCl, as ion-to-electron transducer, like the respective layer in solid-contact ISEs), while the latter does not need inner filling solution (IFS) anymore but

Solid-contact potentiometric sensors toward in vivo analysis of bioelectrolytes

SC-ISEs have an asymmetrical configuration, where the membrane on one side is in contact with a solid contact, and on the other side the membrane is in contact with a sample solution. The new configurations in solid contact enable them to work in any position and operate in high-pressure environments. Moreover, when properly optimized solid-contact materials were applied, the SC-ISEs fabricated thus could have a capability of achieving even more superior detection limit than traditional

Self-assembled monolayers by chemisorption

The early developed solid-contacts have been confined to some electron–ion-exchangers (resins) since 1983 [494] when some redox-active species possessing both the oxidized and reduced forms of redox couples were introduced. Self-assembled monolayers (SAMs) [481], [495], [496] and mixed monolayers [496] by chemisorption from the solution of lipophilic redox-active compounds were proposed by Pretsch group for fabricating SC-ISEs. The incorporated redox species, e.g. thiol-derivatives [481],

Relationship between sensing/transducing materials and performance of the potentiometric sensors

The sensing properties of the potentiometric ion sensors strongly depend on the sensing and transducing materials used inside. Here, we try to summarize the semi-quantitative relationship between ISM components/transducing layers and sensing properties of the potentiometric sensors in Table 21. Ionophore and ion exchanger respectively exhibit major and minor impacts on the limit of detection. Because the plasticizer-free matrices generally have much higher membrane resistance than plasticized

Conclusions and perspectives

ISEs have received wide applications in the field of various ion analyses thus far. Indeed, there may not be a specific sensor technology that is suitable for all applications. Different sensor configuration has its own characteristics and application areas. LC-ISEs have instinctive potential stability if regularly refilled with a fresh portion of the internal solution. LC-ISEs are favorable for developing ultrasensitive sensors with extremely low detection limit and high durability at the same

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the National Natural Science Foundation of China (No.52173011). We are also grateful to Mr. Fei-Ran Li for his important help and support.

Glossary

AuNP
Au nanoparticles
3DOM
Three-dimensionally ordered microporous
BGO
Boron doped graphene oxide
BP
Black phosphorus
BPIM:DFC
1-butyl-3-propargyl imidazolium bromide:diclofenac
CB
Carbon black
CD
Cyclodextrin
CFG
Carboxyl-functionalized graphene derivative
CIM
Colloid-imprinted mesoporous
CNT
Carbon nanotube
CNF
Carbon nanofiber
CPE
Carbon paste electrode
CRGO
Chemically reduced graphene oxide
CWE
Coated wire electrode
DBP
Dibutyl phthalate
DFP
Diisopropyl fluorophosphate
DMeF
1,1-dimethylferrocene
DOP
Dioctyl phthalate
DOS
Dioctyl

Mei-Rong Huang was appointed as a full professor of Tongji University in 2003. She received her Bachelor and Master in chemical fiber at China Textile University (Currently: Donghua University) in 1985 and 1988, respectively. She has been a visiting scholar/professor of Univ. of Oxford, Univ. of California at Los Angeles, Massachusetts Institute of Technol., and Kyoto Univ. She focusses molecular design and synthesis of electrical conducting polymers with versatile applicability in functional

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    Mei-Rong Huang was appointed as a full professor of Tongji University in 2003. She received her Bachelor and Master in chemical fiber at China Textile University (Currently: Donghua University) in 1985 and 1988, respectively. She has been a visiting scholar/professor of Univ. of Oxford, Univ. of California at Los Angeles, Massachusetts Institute of Technol., and Kyoto Univ. She focusses molecular design and synthesis of electrical conducting polymers with versatile applicability in functional membranes and nanocomposites as sensing and transducing materials in ion-selective electrodes-based sensors. She is coauthor of 150 articles, 1 book, and 1 book chapter that have received ca. 7000 citations with an H-index of 45 (WOS). She holds 72 Chinese patents, 9 US patents, 1 Eur. Patent, and 1 Japanese patent. Prof. Huang was awarded the Citation Classic Award (ISI), 4 Natural Science Prizes of Shanghai China, and Natural Science Prize of Ministry of Education China.

    Xin-Gui Li was appointed as a full professor in 1993 and Chang-Jiang Scholar of China in 1999. He received his Bachelor/Master/PhD in chemical fiber at China Textile University (Donghua University) in 1982/1986/1989. He has been a visiting scholar/professor of Technol. Univ. of Berlin, Harvard Univ., Univ. of Oxford, Univ. of Cambridge, Univ. of California at Los Angeles, Massachusetts Institute of Technol., and Kyoto Univ. During the past 35 years he has been involved exclusively with thermotropic liquid-crystalline polyesters, cholesteric liquid-crystalline cellulose derivatives, separation membrane, conducting polymers, environmentally friendly polyesters and polyurethane. Particularly, since 1999, his field of expertise has become molecular design, synthesis, chemistry, doping, electrochemistry, fluorescence emission and its amplified quenching, and processing of electrical conducting polyaniline, polypyrrole, polythiophene, polyfluoranthene, oligofluoranthene, oligotriphenylene, and their derivatives/copolymers and graphene with versatile applicability in functional membranes, environmentally friendly materials, powerful adsorbents toward heavy metal ions, and nanocomposites as sensing and transducing materials in ion-selective electrodes-and fluorescence-based sensors. He has published 1 book, and 1 book chapter and over 200 articles in international leading journals including Chem. Rev., Nano Lett., ACS Nano, and Adv. Funct. Mater., which have received ca. 10000 citations with an H-index of 53 (WOS). He holds over 70 Chinese patents, 9 US patents, 1 Eur. Patent, and 1 Japanese patent. Prof. Li was awarded the Citation Classic Award (ISI), the scholarship from the Royal Society UK, the JSPS Professor Fellowship Japan, ACS Membership Award, and 5 Natural Science Prizes of Shanghai and Ministry of Education, China.

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