Membrane-Based Electrochemical Detection of Uranium: A Review

Abstract

The determination of uranium in environmental samples has always been a crucial environmental issue due to its adverse impacts on human life. Electrochemical detection is one of the most suitable methods for directly determining uranium because of its portable instrument and quick response characteristics. The ion-selective membrane in the working electrodes is selectively responsible for uranium transport and separation. This mini-review provides a general overview of the membrane-based ion-selective electrodes in detecting uranium ions reported in the literature. The ion-selective membranes are classified according to their ionophore categories. Furthermore, the limits and outlook are also discussed to provide a reference for further developing membrane-based electrochemical uranium sensors.

1. Introduction

The determination of hazardous metals has always been a significant environmental issue due to their negative impacts on the ecological environment and human life. Uranium is one of the dangerous metal ions that have apparent chemical and radiation risks. Several industrial processes, including mining, ore processing, hydrometallurgy, and nuclear-related activities, may release uranium into the environment. Moreover, uranium also exists in natural water, due to the formation of soluble uranium species in the mineral dissolution reactions [1]. According to the Environmental Protection Agency, the maximum contamination level of uranium in the drinking water is 30 μg/L [2]. The intake of uranium-containing water has been associated with nephrotoxicity, genotoxicity and radiotoxicity.

Several analytical techniques were used to determine uranium, such as inductively coupled plasma mass spectrometry (ICP-MS), phosphorescence, and UV-visible spectroscopies. These techniques are based mostly on colorimetric or spectrometric instruments that cannot be adapted for field use, due to their complicated pretreatment and expensive instruments. There are also portable radiation-based uranium detectors, but their applicability is limited and can only be used to quantify radioactive uranium isotopes accurately.

The electroanalytical technique is one of the most appropriate methods for directly determining metal ions due to the portable instruments and low power requirement [3]. Currently, electroanalytical techniques have been used for the real-time detection of several metal ions, with the advantages of low cost, quick response time, and adaptable to small sample volumes. Furthermore, they could detect target ions with high accuracy and precision in the presence of other ions.

In electroanalytical techniques, the ion-selective membrane electrodes (ISMEs) are the critical component that transform metal ions’ concentrations into electrical signals. Since the late 1970s, ISMEs have been developed for monitoring uranium in environmental samples [4]. The selectivity of ISMEs is decided by the ionic permselective membranes, which are expected only to allow uranium species to diffuse into the electrode but hinder other interfering ions. Moreover, the stability and reproducibility of ISMEs are also significant for the reliable detection accuracy of the electrochemical analysis.

As far as we know, there is no review about the membrane-based electrodes for uranium detection. Accordingly, this paper reviews the scientific and technological development of various types of ISMEs for the detection of uranium ions.

2. Ion-Selective Membranes for Uranium Detection

As can be seen from Figure 1, the potentiometric cell for membrane-based electrochemical detection of uranium consists of an external reference electrode and an indicator electrode. The ISME as the indicator electrode typically consists of the internal reference electrode, internal reference solution and uranium ion-selective membrane [5]. The heart of the ISME is the permselective membrane capable of uranium species recognition, fixed on the top of the electrode tube. The essential analytical characteristics of membrane electrodes are their selectivity and stability.

The uranium ion-selective membranes usually consist of the polymer matrix, plasticizer, and ionophore (Figure 2). The polymer matrix is the skeleton part of the membrane, which is used to maintain mechanical stability. The plasticizer can improve the flexibility of the polymer and maintain the fluidity of membrane components. The ionophore can specifically recognize the target ion, which plays a decisive role in the detection performance of the membrane electrode. The ionophore is sensitive to a specific ion to be detected and represents less or even no sensitivity to the other ions. Therefore, the selectivity of the membrane electrode can be improved by adding appropriate ionophore into the membrane. In this paper, we have reviewed the ionophore in detail, and as shown in Figure 2, the ionophores are categorized into three groups: macrocyclic ligands, phosphorous-containing ligands, and nitrogen-containing ligands.

For the detection, the potential differences between the ISEs and the reference electrode were measured [6]. The response of the sensor for uranyl ions was examined by measuring electromotive force (EMF) of the following electrochemical cell, Ag|AgCl|sat. KCI|sample solution||PVC membrane||standard UO₂(NO₃)₂ in NaCI solution|AgCI|Ag.

For the ISE-based measuring system, batch equipment such as in Figure 1 are usually adopted due to their convenient sampling and simple instrumentation. Furthermore, ISEs in flow injection potentiometry (FIP, Figure 3) have also been developed with the advantages of high throughput, high precision, low detection limit, and low sample volume [7]. Moreover, the transient nature in FIP can weaken the signal of interfering ions if the electrode’s response to these ions is slower than that of uranium ions, and thus the selectivity could be enhanced.

3. Polymeric Matrix

The PVC polymer has been used for all the reported uranium ion-selective membranes in the literature. The PVC polymeric matrix can support the ion-selective membranes with the advantages of low cost, appropriate hydrophobicity to incorporate organic ionophores, and excellent chemical, mechanical and thermal stability. PVC also has an outstanding film-forming ability, which is beneficial for preparing ion-selective membranes using various solvents. However, the migration of uranium ions in pure PVC membranes is negligible due to their dense and rigid structure [8]. As a result, plasticizers should be added to the membranes to permit the mobility of ionophores. Furthermore, in some cases, the PVC matrix maybe not compatible with the active component. Hence, other polymers, such as cellulose triacetate (CTA), could be used as the supporting matrix [9].

4. Plasticizer

The plasticizer in PVC could enhance its flexibility, workability, and extensibility. There is no chemical bond between plasticizer and PVC, and the interaction between them is intermolecular forces. As a result, the plasticizer can be migrated in the PVC membranes, helping transfer the ionophores across the membrane.

There are many common plasticizers, such as tributyl phosphate (TBP), dibutyl phosphate (DBP), 1-chloronaphthalene (1CN), diphenyl ether (DPE) and 2-nitrophenyl octylether (NPOE). An overview of the composition in uranyl selective membrane electrodes, including polymer matrixes and plasticizers, are shown in

Table 1. An overview of polymer matrixes, plasticizers and their weight percentage in uranyl ion selective membranes.

Polymeric Matrix	Plasticizer	Components (wt. %)		Ref.
		Polymeric Matrix	Plasticizer
PVC	Dioctylphtalate (DOP)	56	20	[16]
PVC	Tributyl phosphate (TBP)	52	35	[6]
PVC	Tris(2-ethylhexyl)phosphate (TEHP)	48.5	48.5	[17]
PVC	Benzyl acetate (BA)	33	57	[7]
PVC	Dimethylsebacate (DMS)	32.8	65.6	[18]
PVC	1-chloronaphthalene	32.7	66.7	[19]
PVC	Bis(2-ethylhexyl) sebacate (DOS)	32	66	[20]
PVC	2-nitrophenyl octylether (NPOE)	32	64	[21]
PVC	Dioctylphenyl phosphonate (DOPP)	31	67	[22]
PVC	Dibutylsebacate (DBS)	24.6	71.8	[23]
PVC	Dibutyl phthalate (DBP)	20	73	[24]

. In the ion-selective membranes, the contents of plasticizers are usually from 30% to 80%. It could be seen that the most common composition in these uranyl selective membrane is the 1:2 ratio of polymeric matrix to plasticizer, which is similar with those in other ion-selective membranes [10]. Interestingly, both of TBP [11,12,13] and DBP [14,15] can coordinate uranium ions even in a highly acidic environment (Figure 4), which can help the migration of uranium species in the membranes.

The migration rate of metal ions is of great concern for the fast response of detection. The diffusion coefficient of the metal ions in the PVC-based membrane can be used to describe their migration property. The ${UO}_2^{2+}$ diffusion coefficient in TBP/PVC liquid membranes is in the order of 10⁻⁸ cm² s⁻¹ [25], which resembles that of a viscous liquid.

The mobility of uranium species and ionophores will be restricted in the PVC-based membrane with less plasticizer. The diffusion coefficient of metal ions in a non-porous PVC film could be as high as 10⁻¹¹ cm² s⁻¹ [8]. However, membrane stability and ion migration rate have a trade-off effect. The higher plasticizer content will increase the risk of leakage of plasticizers into the environment [26]. The most used plasticizers in PVC are toxic to humans, and novel plasticizers should be developed. Furthermore, the ion-selective membranes with excessive plasticizers could exhibit worse adhesive quality, more anion interference, and less service life. Therefore, the control and optimization of plasticizer contents in uranium ion-selective membranes is of great concern to develop novel ISEs for uranium detection, and the synergistic effect of plasticizers on the coordination and migration of uranium species should also be addressed in the future.

5. Ionophores

In nature, hexavalent uranium is commonly associated with oxygen as the uranyl ion, ${UO}_2^{2+}$ [16]. Uranyl ions have a unique linear structure (O=U=O²⁺), and can coordinate with atoms in the equatorial plane perpendicular to their linear structure to form a quadrilateral pentagonal or hexagonal double pyramid coordination configuration [27]. Several ionophores embedded in membranes possess a high affinity for uranium and transport uranium selectively across the membranes. In the ion-selective membranes, ionophores’ contents are usually below 10%. In this part, the ionophores are categorized into three groups, macrocyclic ligands, phosphorous-containing ligands, and nitrogen-containing ligands.

The common interfering ions in the samples for uranium detection include K⁺, Na⁺, Ag⁺, NH₄⁺, Co²⁺, Ca²⁺, Cu²⁺, Ba²⁺, Mg²⁺, Ni²⁺, Fe³⁺, Cr³⁺, Al³⁺, Th⁴⁺ and F⁻ [17,28]. The selectivity of the electrodes for uranium ions is quantified by the potentiometric selectivity coefficients ($K_{{UO}_2,interfering ion}^{Pot}$) according to the Eisenman equation [17,29,30,31]. Additionally, the selectivity coefficients below 10⁻² suggest that the disturbance by the interfering ions is negligible, while the interfering ions with selectivity coefficients more significant than 10⁻² have a non-negligible impact on uranium detection [7,32,33], which are listed in the tables.

5.1. Macrocyclic Ligand-Based Ionophores

Calixarenes and crown ethers are famous macrocyclic compounds, which have been employed as selective extractants for metal ions. They are the cyclic oligomer produced by the reaction of phenol-formaldehyde, with a large hole in the middle. Calixarenes and crown ethers have emerged as highly selective materials for use in the fabrication of uranium ion-selective electrodes. Their high-selective complexation ability makes them suitable for developing highly selective sensors.

For calixarenes, it has been proved that the pseudoplanar five or six coordination structures can be formed between uranium ions and ligands [34], and the recognition ability to uranyl varies with the hole size of calixarenes [35]. For example, as shown in Figure 5, calix [6]arene provides six coordination atoms, so it can form stable complexes with uranyl ions, forming the 1:1 complex of calix [6]arene with ${UO}_2^{2+}$ion [34].

For crown ethers, the most crucial property is that it can selectively bind metal ions with a specific size to form a host and guest complex with certain stability. The main factor affecting the complex stability is the size compatibility between the cavity and uranium ions. The 15-crown-5 has a suitable cave that matches the uranyl ions well, and several conformations could be formed, such as the inserted uranyl 15-crown-5 (Figure 6a) and sandwich uranyl di-15-crown-5 complex (Figure 6b) [36]. A membrane electrode based on the benzo-15-crown-5 as ionophore has been developed for the detection of uranium [30], and the electrode exhibits a quick response to uranium ions over a wide concentration range [16]. Moreover, 18-crown-6 (18C6) could also exhibit a synergistic effect when used in combination with calixarenes, which could facilitate the uranium transport across the liquid membrane [37].

Other macrocyclic compounds, such as the expanded porphyrin (Figure 6c) [38] and Schiff-base macrocycle (Figure 6d) [39], also have the ability to coordinate the uranyl ions. These macrocyclic compounds have also been used as the ionophores for uranium detection. For example, the benzo-substituted macrocyclic diamides have been used as ionophore in the PVC-based membrane electrode for uranium detection, and the electrodes exhibit an ultralow limit detection [7].

The macrocyclic ligands are usually neutral with multidentate O or N atoms that can coordinate with uranium ions. As a result, they can form the ligand-uranium complex with a ratio of 1:1. That is, they have a high uranium loading capacity. Hence, the macrocyclic ligand-based ionophores with a low content in the membrane can transfer uranium ions efficiently. Furthermore, the neutral macrocyclic ligands are not sensitive to pH values and can be used to detect uranium ions in wide pH ranges.

The performance of uranium detection by PVC-based membrane electrodes with macrocyclic ligand-based ionophores is summarized in

Table 2. An overview of uranium detection by PVC-based membrane electrodes with macrocyclic ligand-based ionophores.

Ionophore	pH Stability	Sensitivity [mV Per Decade]	Linear Range [mol L−1]	Selectivity Coefficients (>10−2)	Ref.
5,11,17,23-tetra-tert-butyl-25,27-bis(hydroxy)-26-(ethoxycarbonylmethoxy)-28-(diethylcarbamoyl-methoxy) calix [4]arene	5.5–8.5	36.4	-	Ca2+, Na+, ${SO}_4^{2-}$	[21]
4-tert-butylcalix [6]arene	2.2–3.2	29.1	1 × 10−1 to 3.9 × 10−5	Na+, Ag+, NH4+, Li+, K+	[6]
5,11,17,23-tetra-tertio butyl(25,27),-bis)2-)n-]2-hydroxy-5-dinitridphenilonitrilidine) amino etoxy-(26,28)-di hydroxy calix [4]arene (HAECA)	2.2–3.6	28.5 ± 0.3	1 × 10−4 to 1 × 10−10	No interference	[24]
Benzo-15-crown-5 (B15C5)	2.5–4.5	29.9 ± 0.4	1 × 10−1 to 1 × 10−7	No interference	[30]
Benzo-15-crown-5 (B15C5)	4–7	29.5 ± 2	1 × 10−1 to 1 × 10−4	No interference	[16]
4,13-didecyl-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane (Kryptofix 22DD)	1.5–4	29.6	1 × 10−1 to 1 × 10−4	K+, NH4+	[31]
1,18-diaza-3,4;15,16-dibenzo-5,8,11,14,21,24-hexaoxacyclohexaeicosane-2,17-dione	3–3.5	29.8 ± 0.4	8.2 × 10−3 to 3 × 10−6	No interference	[32]
1,11-bis(2-benzyloxy-5-formylphenoxy)-3,6,9-trioxaundecane	~3	22.7	-	Ba2+	[22]
5,7-dichloroquinoline-8-ol-4-vinyl pyridine (biomimetic)	6.0–8.0	15.0–29.0	1 × 10−2 to 2 × 10−8	No interference	[40]
6,7,9,10,12,13,15,16,23,24,25,26-dodecahydrodibenzo[n,v][1,4,7,10,13,17,20]pentaoxadiazacyclotricosine-22,27-dione	2.9–3.7	30.1	1 × 10−1 to 1 × 10−6	No interference	[7]

. According to the table, these membrane electrodes exhibit a wide linear range. Several ligands listed in Table 2 have no interfering ions with the selectivity coefficients above 10⁻², suggesting the excellent selectivity of macrocyclic ligands for uranium ions.

5.2. Phosphorus-Containing Ligand-Based Ionophores

In nature, it is well-known that phosphates can complex with uranium in the phosphate rocks [41]. Furthermore, phosphorus-containing ligands have been proved for their efficacy in the extraction of uranium from aqueous solutions for several decades. It is also well known that TBP can extract uranium ions from the acidic solution [13,42,43]. Figure 7a shows the configuration of uranyl coordinated by phosphate groups of bis [2-(methacryloyloxy)ethyl] phosphate (B2MP) [44]. The phosphorus-containing ligand-based ionophore was bound to uranyl ion via proton exchange of P-OH, and P=O can also coordinate uranyl ions at the same time [45,46].

Membrane electrode sensors for uranium ions are commonly prepared using phosphorus-containing ligands, such as phosphates, phosphites, phosphine oxides, and diphosphine dioxides [23]. Three similar phosphinic acids, including bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272), bis(2,4,4-trimethylpentyl) monothiophosphinic acid (Cyanex 302), and bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex 301) (Figure 7b), have been used as ionophores for uranium detection [47]. The membrane electrode containing Cyanex 301 with the neutral carrier mechanism has superior detection performance relative to the other two.

Phosphorus-containing ionophores usually suffer complicated synthesis and purification processes and limited coordination sites for uranium ions. Hence, the commercially available and low-cost extractants with multidentate phosphorus-containing ligands have been used as the ionophore for uranium ions, such as the amino(trimethyl) phosphate in Figure 7c, which shows an excellent selectivity for uranium ions [48].

Usually, more than one phosphorus-containing ligand is needed to bind one uranium ion firmly. Hence, the complex size is large with a low diffusion rate, leading to a relatively long response time. Furthermore, the ionization and proton exchange of P-OH could be inhibited under a high acidic condition [49]. Moreover, it is confirmed that calixarenes attached with P=O ligands show a much stronger affinity to uranium ions than the simple P=O ligands, such as TBP and TOPO [34]. As a result, the combination of calixarenes and phosphorus-containing ligands could develop more efficient ionophores to transfer uranium ions.

Table 3. An overview of uranium detection by PVC-based membrane electrodes with phosphorus-containing ligand-based ionophores.

Phosphorus Containing Ligand-Based Ionophores for Ionophore	pH Stability	Sensitivity [mV Per Decade]	Linear Range [mol L−1]	Selectivity Coefficients (>10−2)	Ref.
Tris(2-ethylhexyl)phosphate (TEHP)	2.8–3.6	25.0 ± 0.2	1 × 10−1 to 2 × 10−5	Fe3+, Ca2+, V4+, F−	[17]
O-(1,2-dihydro-2-oxo-1-pyridyl)-N,N,N′,N′-bis(tetra-methylene)uranium hexafluorophosphate (TPTU)	2.5–3.5	27.5 ± 0.2	1 × 10−1 to 5 × 10−5	Fe3+, Th4+, F−	[17]
Di-[4-(n-octyl)phenyl]phosphoric acid	-	30	1 × 10−1 to 1 × 10−5	Sr2+	[50]
Di-[4-(l,1,3,3-tetramethylbutyl)phenyl]phosphoric acid	-	30	1 × 10−1 to 1 × 10−5	No interference	[50]
Bis(2,4,4-trimethylpentyl) phosphinic acid	2.1–3.7	29.4	1 × 10−1 to 5.3×10−4	Th4+	[47]
Bis(2,4,4-trimethylpentyl) monothiophosphinic acid	2.1–3.7	28.0	1 × 10−1 to 5.5×10−5	Fe3+, Th4+	[47]
Bis(2,4,4-trimethylpentyl) dithiophosphinic acid	2.2–3.7	29.3	1 × 10−1 to 5×10−6	Th4+	[47]
Tri-n-octylphosphine oxide (TOPO)	3.0	59	1 × 10−1 to 1×10−4	Ni2+	[51]
Tetraphenyl-o-xylyldiphosphine dioxide	2.75–3.25	26–29	1 × 10−1 to 1 × 10−4	No interference	[52]
Bis[di [4-(l,l,3,3-tetramethylbutyl)phenyl] phosphate]	3	29–31	1 × 10−2 to 1 × 10−4	No interference	[53]
Di(2-ethylhexyl)phosphoric acid	~3	25 ± 2	1 × 10−1 to 1 × 10−4	Fe3+	[54]
Tris(chloroethyl)phosphite	-	29	1 × 10−1 to 1 × 10−6	Ce4+, Cr3+	[55]
O-methyldihexyl phosphine oxide O′-hexyl-2-ethyl-phosphoric acid	-	70–83	-	Mn2+, Fe3+, Zn2+, Co2+, Ni2+, F−, Cu2+, H2PO42−	[20]
[O-methyldihexyl phosphine oxide O′-hexyl-2-ethyl-phosphoric acid (HL)]’s uranyl complex (UO2L2)	2.5–4	25–31	1.47 to 2.10 × 10−5	Fe3+, Mn2+, F−, H2PO42−	[20]
Tetraphenyl-o-xylylenediphosphine dioxide (o-PXDO)	-	28 ± 1.3	1 × 10−2 to 1 × 10−4	-	[56]
Tetratolyl-o-xylylenediphosphine dioxide (o-TXDO)	2.70 ± 0.05	30 ± 2.2	1 × 10−3 to 1 × 10−5	-	[56]
Bis [2-(methacryloyloxy)ethyl] phosphate (B2MP)	3	-	-	Zn2+, Ni2+, Cu2+, Co2+	[44]
Amino(trimethyl) phosphate (ATMP)	2–3.5	29.4	1 × 10−1 to 5.4 × 10−5	No interference	[48]

overviews phosphorus-containing ionophores for determining uranium by membrane electrodes. These membrane electrodes exhibit a linear range within 1 × 10⁻¹ to 5 × 10⁻⁶ M, and the range of pH stability is 2.0–4.0. The common interference ions are Fe³⁺, Th⁴⁺, Ni²⁺ and H₂PO₄²⁻.

5.3. Nitrogen-Containing Ligand-Based Ionophores

In addition to the above ionophores, nitrogen-containing ionophores are also used in membrane electrodes to determine uranium. The nitrogen-containing ligands, such as pyrrole, pyridine, amido, and Schiff-base ligands, could bind uranium via multidentate N-donor sites [38].

Nitrogen-based ionophores for uranium detection have been reported, including triethylenetetramine, 2,2′-[1,2-ethanediyl bis (nitriloethylidene)]bis(1-naphthalene), bis(2-hydroxyacetophenone)ethylenediimine, N,N′-bis[(11-ethoxycarbonyl)undecyl]-N,N′,4,5-tetramethyl-3,6-dioxaoctane diamide, 6,6-Dimethyl-4,8-dioxaundecanedioic nitrile, N,N′-Diheptyl-N,N′,6,6-tetramethyl-4,8-dioxaundecanediamide, 2-hydroxyacetophenoneoximethiourea-trioxane resin, N,N′-4,5-(ethylenedioxy)benzenebis(salicylideneimine) (SalphenH2), N,N′-(propylenedioxy)benzenebis(salicylideneimine), N,N′-4,5-(propylenedioxy)benzenebis(3,5-di-tert-butylsalicylideneimine), poly-(1-4)-2-amino-2-deoxy-β-D-glucan and 5,6,7,8-Tetrahydro-8-thioxopyrido [4′,3′,4,5]thieno [2,3-d]pyrimidine-4(3H)one. These multidentate nitrogen-donor ligands bind uranium with different strengths and donor atom numbers, resulting in different sensitivity and selectivity. Figure 8a shows the chemical structure of the complex from the nitrogen-based ionophore triethylenetetramine (TETA) and uranium ion [23]. The TETA-base membrane sensor could detect uranium ions in strong sulphate solutions, in which [UO₂(SO₄)₂]²⁻ anion is formed and extracted by the TETA ionophore. Figure 8b shows the 1:1 complexes between salphenH2 derivative and ${UO}_2^{2+}$ [57], and the membrane electrodes containing salphenH2 derivatives show high selectivity and low detection limit for uranium ions due to the stable and selective complexation.

Table 4. An overview of uranium detection by PVC-based membrane electrodes with nitrogen-containing ligand-based ionophores.

Ionophore	pH Stability	Sensitivity [mV Per Decade]	Linear Range [mol L−1]	Selectivity Coefficients (>10−2)	Ref.
Triethylenetetramine	2.5–3.8	−26.5	10−1 to 10−5	Fe3+, Th4+	[23]
2,2′-[1,2-ethanediyl bis (nitriloethylidene)]bis(1-naphthalene)	3–4	28.5	10−1 to 10−7	Mg2+, Ca2+, Ag+, Fe3+, Zn2+, Na+	[33]
Bis(2-hydroxyacetophenone)ethylenediimine	3.0–4.5	29.3	10−2 to 10−6	K+, Ag+, Fe3+, Cu2+, Zn2+	[18]
N,N′-bis[(11-ethoxycarbonyl)undecyl]-N,N′,4,5-tetramethyl-3,6-dioxaoctane diamide	-	21.6	-	-	[22]
6,6-Dimethyl-4,8-dioxaundecanedioic nitrile	~3	-	10−2 to 10−4	H+	[19]
N,N′-Diheptyl-N,N′,6,6-tetramethyl-4,8-dioxaundecanediamide	2–4.3	45–55	10−2 to 10−5	H+	[58]
2-hydroxyacetophenoneoximethiourea-trioxane resin	2.5–3.5	39.0	10−1 to 10−5	Na+, K+, Tl+, Li+, Ag+, NH4+, Ca2+, Ba2+, Sr2+, Mn2+, Pb2+, Zn2+, Ni2+, Cu2+, Mg2+, Hg2+, Al3+, La3+, Fe3+	[59]
N,N′-4,5-(ethylenedioxy)benzenebis(salicylideneimine) (SalphenH2)	1.5–4.0	28.0–30.9	10−2 to 10−6	Cu2+, Pb2+	[60]
N,N′-(propylenedioxy)benzenebis(salicylideneimine)	1.0–5.0	27.0	10−2 to 10−6	No interference	[57]
N,N′-4,5-(propylenedioxy)benzenebis(3,5-di-tert-butylsalicylideneimine)	1.0–5.0	28.8	10−2 to 10−6	No interference	[57]
Poly-(1-4)-2-amino-2-deoxy-β-D-glucan	3–8	28.2–29.7	10−1 to 10−5	Ce3+, Cu2+, Fe2+, NH4+, Na+, K+	[61]
5,6,7,8-Tetrahydro-8-thioxopyrido [4′,3′,4,5]thieno [2,3-d]pyrimidine-4(3H)one	2.0–3.5	30	10−1 to 10−5	No interference	[62]

presents the uranium detection performance by PVC-based membrane electrodes with nitrogen-containing ligand-based ionophores. These membrane electrodes exhibit a linear range within 1 × 10⁻¹ to 1 × 10⁻⁷ M, and the range of pH stability is 1–5. There are lots of interference cations for uranium detection by nitrogen-containing ligands in Table 4. The common protons (H⁺) and alkali-metal ions (Na⁺, K⁺) also can interfere with uranium detection.

6. Response Times and Lifetime

6.1. Response Times

The response time is one of the most significant evaluation indicators for the performance of ion-selective electrodes [16,63]. It is defined as the average time it takes for the electrode to reach a stable potential reading [6,63]. According to the recommendation of IUPAC, the practical response time required for the detection of ${UO}_2^{2+}$ is measured in different analyte solutions with a 10-fold difference in concentration, and the final equilibrium potential value can be stabilized with the fluctuation of ±1.0 mV [6,64]. There are several influence factors that affect the response time, such as the membrane thickness, ionic concentrations, and solution types [7].

6.1.1. Membrane Thickness

Usually, membranes with less thickness could shorten the diffusion path and enhance the response speed. In

Table 5. An overview of the response time and lifetime of uranium ion-selective membrane electrodes.

Ionophores	Membrane Composition [wt. %]	Thickness of Membrane	Uranyl Solution	Response Time [s]	Lifetime	Ref.
4-(1-((1,5-Dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl) imino) ethyl)-1-dodecylpyridin-1-ium bromide	PVC (32.50) : ionophore (2.00) : o-NPOE (65.00) : KTpClPB (0.50)	5 mm	Uranyl nitrate solution	9	8 weeks	[63]
4-tert-Butylcalix[6]arene	PVC (52.00) : ionophore (7.00) : TBP (35.00) : NaTPB (6.00)	-	Uranyl nitrate solution	10	5 months	[6]
5,11,17,23-tetra-tertio butyl(25,27),-bis)2-)n-]2-hydroxy-5-dinitridphenilonitrilidine) amino etoxy-(26,28)-di hydroxy calix [4]arene (HAECA)	PVC (20.00) : ionophore (5.00) : dibutylphthalate (73.00) : NaTPB (2.00)	-	Uranyl nitrate solution	20–25	6 weeks	[24]
Benzo 15-crown-5	PVC (56.00) : ionophore (4.00) : dioctylphtalate (20.00) : carbon powder (20.00)	-	Uranyl nitrate solution	15	6 weeks	[16]
5,7-dichloroquinoline-8-ol-4-vinyl pyridine (biomimetic)	-	0.6 mm	Uranyl nitrate solution	120	3 months	[40]
Benzo 15-crown-5	PVC (33.00) : ionophore (2.50) : MWNT (2.00): o-NPOE (60.50) : NaTPB (2.00)	-	Uranyl nitrate solution	4	6 months	[30]
4,13-didecyl-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane (Kryptofix 22DD)	PVC (30.00) : ionophore (3.60) : dibutylphetalate (60.00) : graphite powder (6.40)	-	Uranyl nitrate solution	10	12 weeks	[31]
6,7,9,10,12,13,15,16,23,24,25,26-dodecahydrodibenzo[n,v][1,4,7,10,13,17,20]pentaoxadiazacyclotricosine-22,27-dione	PVC (33.00) : ionophore (7.00) : NPOE (57.00) : STPB (3.00)	0.3 mm	Uranyl nitrate solution	5	13 weeks	[7]
Bis(2,4,4-trimethylpentyl) phosphinic acid	-	0.2 mm	Uranyl nitrate solution	26	10 weeks	[47]
Bis(2,4,4-trimethylpentyl) monothiophosphinic acid	-	0.2 mm	Uranyl nitrate solution	35	10 weeks	[47]
Bis(2,4,4-trimethylpentyl) dithiophosphinic acid	-	0.2 mm	Uranyl nitrate solution	20	10 weeks	[47]
Tri-n-octylphosphine oxide (TOPO)	Ionophore (18.18) : TBP (72.73) : NaTPB (9.09)	0.3 mm	Uranyl chloride and sulfate solution	-	1 months	[51]
Tetraphenyl-o-xylyldiphosphine dioxide	PVC (33.00) : ionophore (1.00) : o-NPOE (65.50) : KTpClPB (0.50)	0.2 mm	Uranyl chloride and nitrate solution	120	4 months	[52]
Bis[di [4-(l,l,3,3-tetramethylbutyl)phenyl] phosphate]	-	-	Uranyl nitrate solution	20	10 months	[53]
Di(2-ethylhexyl)phosphoric acid	-	-	Uranyl chloride solution	a few minutes	4 to 8 weeks	[54]
Tris(chloroethyl)phosphite	PVC (77.95) : ionophore (2.01) : bis(2-ethylhexyl) (2-ethylhexyl)phosphonate (20.04)	0.3–1.2 mm	Uranyl chloride solution	10	3 months	[55]
[O-methyldihexyl phosphine oxide O′-hexyl-2-ethyl-phosphoric acid (HL)]’s uranyl complex (UO2L2)	PVC (32.00) : ionophore (2.00) : bis(2-ethylhexyl) sebacate (66.00)	0.3 mm	Uranyl nitrate solution	10–30	20 days	[20]
Amino(trimethyl) phosphate (ATMP)	-	0.2 mm	Uranyl nitrate solution	10–20	7 weeks	[48]
Tris(2-ethylhexyl)phosphate (TEHP)	PVC (48.50) : ionophore (48.5) : NaTPB (3.00)	0.3 mm	Uranyl nitrate solution	50	4 weeks	[17]
O-(1,2-dihydro-2-oxo-1-pyridyl)-N,N,N′,N′-bis(tetra-methylene)uranium hexafluorophosphate (TPTU)	-	0.3 mm	Uranyl nitrate solution	30	6 weeks	[17]
Triethylenetetramine	PVC (24.60) : ionophore (3.60) : o-NPOE (71.80)	0.3 mm	Uranyl sulphate solution	30	12 weeks	[23]
2,2′-[1,2-ethanediyl bis (nitriloethylidene)]bis(1-naphthalene)	PVC (30.50) : ionophore (4.00) : DOP (63.50) : NaTPB (2.00)	0.3 mm	Uranyl nitrate solution	20	2 months	[33]
Bis(2-hydroxyacetophenone)ethylenediimine	PVC (32.80) : ionophore (1.00) : DOP (65.60) : MTOACl (0.60)	-	Uranyl nitrate solution	5	2 months	[18]
6,6-Dimethyl-4,8-dioxaundecanedioic nitrile	PVC (32.70) : ionophore (0.60) : 1-chloronaphthalene (66.70)	-	Uranyl nitrate solution	20	-	[19]
2-hydroxyacetophenoneoximethiourea-trioxane resin	PVC (40.00) : ionophore (30.00) : DBP (30.00)	0.5 mm	Uranyl nitrate solution	30	4 months	[59]
N,N′-4,5-(ethylenedioxy)benzenebis(salicylideneimine) (SalphenH2)	PVC (32.66) : ionophore (0.99) : NPOE (65.91) : KTpClPB (0.44)	0.2 mm	Uranyl nitrate solution	60	-	[60]
N,N′-(propylenedioxy)benzenebis(salicylideneimine)	PVC (33.02) : ionophore (1.00) : TEHP (65.98)	0.2 mm	Uranyl nitrate solution	20	10 months	[57]
N,N′-4,5-(propylenedioxy)benzenebis(3,5-di-tert-butylsalicylideneimine)	PVC (33.13) : ionophore (1.00) : TEHP (65.87)	0.2 mm	Uranyl nitrate solution	20	10 months	[57]
Poly-(1-4)-2-amino-2-deoxy-β-D-glucan	PVC (50.86) : ionophore (0.31) : DBP (18.31) : carbon powder (30.52)	-	Uranyl nitrate solution	3	115 days	[61]
5,6,7,8-Tetrahydro-8-thioxopyrido [4′,3′,4,5]thieno [2,3-d]pyrimidine-4(3H)one	PVC (32.52) : ionophore (1.63) : o-NPOE (65.04) : KTClPB (0.81)	0.2 mm	Uranyl nitrate solution	40	2 months	[62]
Uranyl-carboxybenzotriazol	-	0.5 mm	Uranyl nitrate solution	60	2 months	[28]

, the thickness for most of the reported uranium ion-selective membranes is in the range of 0.2 mm to 0.5 mm. However, the ultrathin membranes will increase the risk of component leakage.

6.1.2. Concentrations

In general, the solution with a high ionic concentration has a large uranium concentration gradient for the transfer of uranium species [17]. Hence, the response rate could be quicker when proceeding from dilute to concentrated tested solutions. If the transfer of uranium species in the membrane is quick enough and the diffusion process is not the rate-determining step in a range of concentrations, the response time will keep unaltered [65,66].

6.1.3. Types of Uranium Solution and Competing Ions

The anions may participate in the uranium coordination process and transfer with uranium at the same time. Therefore, the membrane for uranium detection can behave diversely in different types of uranium solutions [52]. For uranium detection, the nitrate and perchlorate species can be dissolved in the membrane phase easily, and the cell potential will become very low when the concentration of nitrate and perchlorate anions is more than 0.001 mol/L [54,67]. Besides, the sulfate and phosphate anions can also reduce the activity of uranium species and thus lower the cell potential [54,68]. The competing cations can consume the ligands and transfer at different rates in membranes. As a result, the response speed and sensitivity will reduce during the detection [54].

6.2. Lifetime

In PVC-based ion-selective membranes, there are no chemical bonds between different components, and the stability of the membranes is limited. As a result, the loss of components, such as the plasticizer and carrier, is inevitable. In Table 5, the average lifetime for the reported uranium ion-selective membranes is in the range of 4 weeks to 10 months. With the increase in operating time, the slope of the sensor will reduce, and the limit of detection will also increase [24].

7. Limitation and Outlook

This review summarizes comprehensive literature regarding ion-selective membranes for electrochemical detection of uranium ions, focusing on the ionophores that could coordinate uranium ions selectively. The conventional PVC-based membranes have been used to transport uranium ions for quantitative analysis for several decades due to their accessible manufacturing technologies and long-term reliable records. In particular, the membrane electrodes with macrocyclic ligand-based ionophores exhibit a wide linear range and little interfering ions for uranium detection.

However, there are also some limitations for the present membrane electrodes. For one thing, the leakage of ionophores and plasticizers is inevitable for these polymer membranes, especially in the solutions with high acidity. For another, because of the high atomic weight of uranium and the complicated complex structures, the transfer rate of uranium species in the membranes is usually not high, which limits the response rates. Furthermore, the transfer mechanism of uranium-loaded ionophores is still unclear.

Future improvements could be addressed in the following aspects: (1) The diffusion mechanism of the uranium complexes in the membranes should be studied for a better understanding of uranium species migration. (2) Efforts are still required to seek novel materials for fabricating advanced thin membranes to shorten the diffusion path and realize the ultra-fast uranium detection. (3) Membrane components, such as plasticizers, could be more environmentally friendly and do not cause a potential threat to the ecosystem and human health.