1.3 Applications of water stable MOFs 16
1.3.2 Chemical sensing 23
1.3.2.2 Sensing of targeted analytes in water 26
The application of MOFs for fluorescence sensing is a well-established field of research.
The lack of stability of many MOFs in aqueous medium restricts their applications in real field.
Water-stable MOFs break that restriction as they are hydrolytically stable. Among the various target analytes, the detection of nitroaromatic compounds (NACs) by stable MOFs has been well- studied.135-139 2,4,6-Trinitrophenol (TNP), commonly known as picric acid, is one of the most hazardous and water soluble explosive molecules, which acts as a strong fluorescence quencher.
Various research suggested different mechanisms for the picric acid induced quenching including photo-induced electron transfer (PET),140 fluorescence resonance energy transfer (FRET),141 molecular interactions (such as electrostatic interactions)142 and inner‐filter effect (IFE).143 A highly fluorescent amine-functionalized Zr(IV)-based MOF with UiO-68 topology was efficiently applied for the selective detection of picric acid in water.144 A MOF-coated paper test strip was also developed for in-field applications (Figure 1.16). Hou et al. reported a Tb-based water-stable MOF, which can detect the NACs even in vapor phase.145
Figure 1.16 (a) Paper strip based detection of explosives in the aqueous phase. (b) Response of MOF-coated paper strips towards various nitro analytes under UV light (A = 2,4,6-trinitrophenol (TNP), B = 2,4,6-trinitrotoluene (TNT), C = 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), D = 2,4-dinitrotoluene (2,4-DNT), E = 1,3-dinitrobenzene (DNB), F = 2,6-dinitrotoluene (2,6-DNT), G = nitrobenzene (NB), H = 2,3-dimethyl-2,3-dinitrobutane (DMNB)). Reproduced with permission from ref. 144. Copyright 2015 Royal Society of Chemistry.
A few stable MOFs were employed for the sensing of gas molecules. Ghosh et al.
developed a reaction-based detection methodology for the sensing of toxic hydrogen sulphide (H2S) gas dissolved in water.146,147 The presence of nitro (NO2) or azide (N3) functional group can act as a reaction site towards H2S gas. Based on this strategy, few stable MOFs were utilized as
fluorescence turn-on probes for the sensing of H2S gas.148-152 Gu et al. strategically developed a stable Zr(IV)-based MOF called PCN-224(Pt) containing platinum(II) porphyrin core, which can act as an oxygen-sensitive center.153 The fluorescent platinum(II) porphyrinic MOF showed a unique response towards dissolved oxygen. The water-dissolved SO2 gas readily formed a mixture of sulfite and bisulfite (SO32– and HSO3–) ions. The SO32– ion generated from SO2 gas could be selectively detected by stable MOF-5-NH2 material.154 The reusable nature of this MOF probe makes it a potential luminescent sensor to detect SO32– for practical applications.
Moreover, a large number of stable MOFs have been recently reported for the detection of cations and anions in water. A water-stable Eu(III)-functionalized Zr(IV)-MOF was constructed by encapsulating Eu3+ cations into the pores of Zr-UiO-66-(COOH)2.155 The Eu(III)-encapsulated MOF system showed a naked eye response for only toxic Cd2+ cation over other cations in water.
In another report, the Al-MIL-53 MOF was employed to detect the Fe3+ ion via metal ion exchange mechanism.156 Furthermore, several water-stable MOFs have been also prepared for the selective detection of anions in water. A stable Zr(IV)-based MOF constructed from C═C bridged di- isophthalate ligand showed high stability in water and it was employed for the sensitive detection of Cr2O72– anion.157 Cao et al. employed another water-stable Zr-MOF for the selective detection of Cr2O72− anion.158 A cationic dye namely 3,6-diaminoacridinium cation (DAAC) was loaded inside the porous channels of hydrolytically stable bio-MOF-1 by the conventional cation- exchange process by Ghosh et al. for the selective detection of lethal cyanide anion (CN−) in aqueous medium (Figure 1.17).159
In addition, a few water-stable MOFs materials have been reported for the selective detection of biologically active small molecules (e.g. ascorbic acid, dopamine, cysteine, glutathione, etc.).160-164 Post-synthetically modified UiO-66 MOF was also employed for pH sensing application.165 A few examples of water-stable MOFs are summarized in Table 1.4 with their applications for various sensing processes.
Fig. 1.17 Schematic representation of chemodosimetric approach in bio-MOF-1⊃DAAC prompted by CN− inducing a turn-on fluorescence response. Reproduced with permission from ref. 159. Copyright 2017 Royal Society of Chemistry.
Table 1.4 Summary of reported MOFs with porous features and water adsorption capacities measured at 298 K.
Type of Targeted Analyte
MOF Analyte Limit of
Detection
Ref.
nitro- aromatics
BUT-12 TNP 23 ppb 136
BUT-13 TNP 10 ppb 136
{[Zn2(L)2(bpy)]·(DMF)·(H2O)2}n TNP 2.62 ppm 138
UiO-68 TNP 1.6 × 10−6 M 141
UiO-68-NH2 TNP - 144
{Cd(INA)(pytpy)(OH)·2H2O}n TNP 9.1 × 10−3 mM 166
bio‐MOF‐1 TNP 2.9 ppb 167
{[Zn(2, 5‐tdc)(3‐abit)]⋅H2O}n TNP 3.3 × 10−3 mM 168
biomolecules UiO-66-DNS cysteine 9.8 μM 163
UiO-66-PSM bilirubin 0.59 pmol/L 169
MN-ZIF-90 amino acids - 170
Tb3+@1 dipicolinic acid 3.6 nM. 171
Eu@MIL-121 hippuric acid 9 μg mL−1 172
Fe-MIL-88 H2O2 46 nm 173
Cu2+@MIL-91(Al:Eu) uric Acid 1.6 μmol L–1 174
cations MIL-53-L Cu2+ - 175
UiO-66-NH2-Eu Cd2+ 0.22 μM 155
[In2(OH)2(H2TTHA)(H2O)2]n Ru3+ 0.26 ppm 176 [In2(OH)2(H2TTHA)(H2O)2]n UO22+ 0.42 ppm 176 {Cd(INA)(pytpy)(OH)·2H2O}n Cu2+ 3.9 × 10−3 mM 166
[Zn(dbp)]n Mn2+ 1.80 × 10−4 M 177
[Cd(dbp)(H2O)·2H2O·CH3CN]n Mn2+ 5.08 × 10−6 M 177
NNU-1 Fe3+ 0.20× 10−3 M 178
[(CH3)2NH2][In(TNB)4/3]
·(2DMF)(3H2O) ⊃DSM
Hg2+ 1.75 ppb 179
[Eu2L(1,3-bdc)3]·5H2O Fe3+ 0.023 mM 180
anions EuCPBDA MnO4− - 181
bio-MOF-1⊃DAAC CN− 5.2 ppb 159
Pt(II)TMPyP/rho-ZMOF S2− 27 nM 182
{Eu2L3(DMF)}·2DMF PO43− 6.62 μM 183
UiO-66-NH2 PO43− 1.25 μM 184
[Eu7(mtb)5(H2O)16]+ Cr2O72− 0.56 ppb 185 [(CH3)2NH2][In(TNB)4/3]
·(2DMF)(3H2O) ⊃DSM
Cr2O72− 0.079 μM (23 ppb)
179
{[Zn(2, 5‐tdc)(3‐abit)]⋅H2O}n Cr2O72−/CrO42− - 168 Linkers are abbreviated as: pytpy = 4′-(4-Pyridinyl)-2,2′:6′,2′′-terpyridine; INA = isonicotinic acid;
2,5‐tdc = 2,5‐thiophenedicarboxylic acid; 3‐abit = 4‐amino‐3,5‐bis(imidazol‐1‐ylmethyl)‐1,2,4‐
triazole; H2TTHA = 1,3,5-triazine-2,4,6-triamine hexaacetic acid; H2dbp = 4′-(4-(3,5- dicarboxylphenoxy)phenyl)-4,2′:6′,4′′-terpyridine; TNB = 4,4′,4′′-nitrilotribenzoicacid; CPBDA = 3-(6-carboxypyridin-3-yl)benzene-1,4-dioic acid; mtb = 4-[tris(4-carboxyphenyl)methyl]benzoic acid.