Detection of nitrophenols with a fluorescent Zr(IV) metal–organic framework functionalized with benzylamino groups

Abstract

Nitroaromatic compounds (NACs) are known explosives and environmental pollutants posing a risk for public health and national security. Thus, the development of efficient sensors for their rapid and efficient in-field detection is of high importance. Analytical methods based on fluorescence are gaining interest as current light detection technology allows the fabrication of miniaturized portable devices suitable for in-field use. Herein, we report the rational design and synthesis of Zr-1, a Zr(IV) based metal–organic framework (MOF) which is structurally analogous to UiO-66 with the assigned formula {Zr₆O₄(OH)₈(H₂O)₄(L-1)_4−x(NH₂bdc)_x}, (L-1 = 2-((benzyl)amino)-terephthalate; NH₂bdc²⁻ = 2-aminoterephthalate). Zr-1 incorporates a fluorescent ligand (L-1) with a pendant π-electron rich aromatic group and a basic secondary amine functionality, thereby targeting the selective detection of electron deficient and acidic NACs, 2,4,6-trinitrophenol (TNP) and 2,4-dinitrophenol (DNP). The stability of Zr-1 in an aqueous environment was confirmed by powder X-ray diffraction analysis on water treated samples. Fluorescence titration experiments on aqueous suspensions of acid activated Zr-1 (pZr-1) demonstrate that the material responds to small concentrations of TNP and DNP by displaying strong emission quenching, even in the presence of potentially competing compounds. The estimated limits of detection were found to be as low as 0.011 μM (2.5 ppb) for TNP and 0.026 μM (4.8 ppb) for DNP.

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Introduction

Nitroaromatic compounds (NACs) are among the most important groups of industrial chemicals as they are widely used precursors and/or intermediates in the preparation of a variety of chemical products including pesticides, explosives, and pharmaceuticals. They are potently toxic and/or carcinogenic compounds with a highly explosive nature and their extensive industrial use renders them widely distributed environmental pollutants presenting a considerable danger to human population. Furthermore, NACs constitute one of the main ingredients for the preparation of explosive devices, thereby posing a major human safety and national security risk. NACs are xenobiotics resistant to chemical and biological oxidation and hydrolysis due to the electron-withdrawing character of nitro groups. Consequently, they are persistent contaminants of soil, groundwater and wastewater, and their constant monitoring is indispensable for the protection and safeguarding of the environment.¹ Presently, determination of NACs is based mainly on traditional separation (i.e. GC, HPLC) and spectrometric (i.e. MS, IMS, IR) techniques, while their in-field detection is dominated by the use of trained canines. Despite their effectiveness, traditional analytical methods suffer from high cost, the requirement for highly trained personnel and lack of portability.²

In search of more convenient and cost-effective alternatives, great attention is directed to luminescence-based techniques that employ fluorescent materials as sensing elements. These sensors respond to their interaction with targeted analytes by producing detectable changes in their emission characteristics (i.e. emission intensity, wavelength of emission maxima, quantum yield). The advances in light generation and detection technologies allow detection limits at the single-molecule level to be easily attainable. Furthermore, device miniaturization is feasible, thereby allowing the potential development of small, portable, and user-friendly systems suitable for in-field use.³

Organic Conjugated Polymers (CPs) have been successfully employed in building optical sensors for nitro-explosives. Swager and Yang first reported the synthesis of luminescent polymeric films displaying high sensitivity towards TNT and DNT.⁴ Conjugated polymers provide a key advantage for optical sensing applications: the polymer chain, featuring multiple interconnected chromophores, provides a platform with high exciton mobility. The energy migration processes that occur in such systems amplify the material's response, as a single analyte binding event may affect a large number of chromophores thereby resulting in greatly enhanced detection signals.⁵ This is in contrast to sensors which are based on small molecules, where each analyte binding event essentially only affects one chromophore.⁶

Luminescent Metal Organic Frameworks (LMOFs) is another promising class of luminescent materials that produce signal gain upon interaction with analytes.⁷ MOFs are crystalline porous organic/inorganic hybrid materials that can be self-assembled from their corresponding metal ions/clusters and organic bridging ligands.⁸ The introduction of luminescent properties in MOFs can be achieved by incorporation of fluorescent bridging ligands and/or photoactive metal anions in their framework, that leads to infinite arrays of chromophores, analogous to what is observed for CPs.⁹ Moreover, MOFs exhibit well defined crystalline structures with permanent porosity, structural diversity and tunability, and surface functionality that distinguishes them from luminescent organic CP sensors. Luminescent MOFs can respond to the encapsulation of various guest species within their structure's pores and therefore, display great potential for use in next generation sensing devices for in-field detection.¹⁰ In recent years, they have been extensively studied as optical sensors, however, relatively few research works focus on the rational design of MOF's pores structural design with the employment of tailored functional ligands.¹¹

Herein, we study the rational design and synthesis of a hydrolytically stable Zr(IV) UiO-type¹² MOF with an average connectivity of 8 (Zr-1), based on a strongly fluorescent dicarboxylic ligand with a pendant π-electron rich benzylamino aromatic group (L-1). The incorporation of L-1 into the MOF structure yields a material with pores lined with π-electron rich units, which, in combination with the basic amino groups, form a favourable environment for electron-deficient and acidic nitroaromatic guests. Fluorescence titrations with suspensions of protonated Zr-1 (pZr-1) and 2,4-dinitrophenol (DNP) and 2,4,6-trinitrophenol (TNP),‡ have shown that our material can act as an efficient and selective optical sensor for the determination of these analytes in water displaying emission quenching with exceptionally low detection limits.

Results and discussion

Synthesis and characterization

We synthesized the porous Zr⁴⁺ MOF Zr-1 using the functionalized dicarboxylic ligand 2-benzylaminoterephthalic acid, L-1. The syntheses of L-1 and Zr-1 are summarized in Scheme 1.

Scheme 1 Schematic representation of the synthesis of ligand L-1 and MOF Zr-1.

L-1 was prepared in a two-step process previously reported by our group¹³ using the approach of reductive alkylation whereby the reaction of 2-aminorerephthalic acid with benzaldehyde affords an intermediate imine which is reduced in situ with sodium borohydride to the desired product. Zr-1 was prepared following well-established solvothermal synthetic methods^12a,14 by the reaction of L-1 with ZrCl₄ in dimethylformamide (DMF) in a closed vial at 120 °C using acetic acid as the reaction modulator. As will be discussed below, the incorporation of the pendant benzylamino groups of L-1 within the structure of Zr-1 ensures that the material's pores are lined with functional groups which possess both a π-electron-rich system and Brønsted basicity. To use this material as a fluorescent sensor for nitrophenols, we treated Zr-1 with 4.0 M HCl for 8 h to protonate the material's secondary amino groups to ArRNH₂⁺ Cl⁻, to produce a cationic and π-electron rich framework with exchangeable Cl⁻ anions (pZr-1) thus forming a favourable environment for hosting π-electron deficient and anionic nitroaromatic guests such as nitrophenol anions.

It has been observed by many researchers that ligands which are based on N-alkyl substituted 2-aminoterephthalic acid, often show various degrees of elimination of the alkyl substituent to form the parent ligand during MOF synthesis.¹⁵ Therefore, in order to examine the stability of L-1 once it is incorporated into the framework of Zr-1, we measured the ¹H-NMR spectra of digested MOF samples in D₂O/NaOD both before and after acid activation. As seen in Fig. S5 (ESI†), the spectrum of the digested as-synthesized MOF clearly shows the ¹H-NMR signals corresponding to L-1 along with weak signals due to 2-aminoterephthalate (NH₂bdc²⁻) indicating less than 10% elimination of the benzyl groups. However, ¹H-NMR spectra of digested samples of pZr-1, showed increased signals due to 2-aminoterephthalate. After conducting protonations of Zr-1 with different HCl concentrations and reaction times, we found that the use of 4.0 M HCl for 8 h produces highly active pZr-1 in terms of nitrophenol sensing, with ca. 30% benzyl group elimination (Fig. S7, ESI†). Thus, pZr-1 henceforth refers to the activated material produced under these conditions. In accordance with the ¹H-NMR and thermogravimetric (see next section) analysis of Zr-1 and pZr-1, the MOF formula is proposed to be {Zr₆O₄(OH)₈(H₂O)₄(L-1)_4−x(NH₂bdc)_x} (x = 0.4–1.2).

Powder X-ray diffraction (PXRD) patterns of as synthesized Zr-1, pZr-1 and the parent material UiO-66 can be seen in Fig. 1. All 2θ peaks of the diffraction pattern of Zr-1 are consistent with UiO-66, which demonstrates that the two materials are structurally equivalent. However, as will be discussed below, the average connectivity of Zr-1 is reduced to 8 compared to the 12-connectivity^12a of UiO-66.

Fig. 1 PXRD patterns of UiO-66 (black line), Zr-1 (red line), pZr-1 (blue line) and Zr-1 after water treatment (green line).

Chemical and thermal stability

Stability in water is a prerequisite for the potential application of a material in pollutant detection processes in aqueous environments. The stability of Zr-1 towards H₂O was studied by treating the material with H₂O for 24 h at room temperature.

After treatment, the solid was isolated via centrifugation and dried at 50 °C for 24 h before PXRD analysis was performed. The PXRD pattern of Zr-1 after water treatment is identical to the PXRD pattern of the as-synthesized material (Fig. 1). Evidently, the overall framework remains unaltered confirming that Zr-1 displays high stability in aqueous environment.

The thermogravimetric analysis (TGA) curve for Zr-1 after H₂O exchange is seen in Fig. 2. TGA analysis of the water treated sample under air reveals that Zr-1 loses weight in two distinct steps: the first is completed at ca. 200 °C, and is attributed to the loss of lattice and coordinated water molecules, while the second, which is completed at ca. 550 °C, corresponds to complete ligand loss and formation of ZrO₂^12b,16 (see ESI† for details). It is interesting to mention that Zr-1 exhibits a much lower weight loss due to the removal of water in comparison to what is observed for UiO-66-NH₂ (ca. 10% vs. 30%, Fig. S10, ESI†). This is attributed to the presence of benzyl side groups in Zr-1, which (i) decrease the free volume within the material's pores and (ii) render the material less hydrophilic. In contrast to Zr-1, HCl treated pZr-1 shows a more extensive initial weight loss due to (i) an increased water content because of the partial elimination of benzyl side groups (vide supra) and (ii) the possible additional elimination of HCl (Fig. S10, ESI†).

Fig. 2 TGA curve for Zr-1 after water treatment, under air.

Assuming 10% cleavage of the benzyl side group (as estimated by ¹H-NMR analysis on digested Zr-1), we would expect a 12-connected material to have the formula {Zr₆O₄(OH)₈(H₂O)₄(L-1)_5.4(NH₂-bdc)_0.6}, while an 8-connected material would have the formula {Zr₆O₄(OH)₈(H₂O)₄(L-1)_3.6(NH₂-bdc)_0.4}. Thus, the expected relative weight loss from 200 to 600 °C is expected to be 68.1% and 58.7% in the case of 12- and 8-connectivity, respectively. Based on the above, the experimentally determined 59.1% relative weight loss (Fig. 2) agrees with Zr-1 having an average connectivity of 8. The missing linker defects of our material can be attributed to the steric hindrance induced by the bulky side group of ligand L-1. The lower connectivity of Zr-1 possibly accounts for its lower decomposition temperature (∼390 °C) compared to that observed for UiO-66 (>500 °C).^12a The presence of missing-linker defects is a common feature of UiO-66(Zr)-type MOFs and defective materials reported in literature retain high crystallinity and thermal stability,¹⁷ as is also observed in our work.

BET surface area and porosity

To evaluate the porosity and specific surface area, we performed N₂ adsorption–desorption measurements (77 K) on both Zr-1 and pZr-1 (Fig. 3). Both isotherms can be classified as Type I physisorption isotherms, typical of microporous materials.¹⁸ The surface area was calculated by the Brunauer–Emmett–Teller (BET) model to 893 m² g⁻¹ for Zr-1 and 932 m² g⁻¹ after activation with 4.0 M HCl (pZr-1). These values fall within the range of surface area values calculated for functionalized UiO-66-type materials prepared with solvothermal reactions.¹⁹ The partial benzyl group cleavage that occurs during MOF activation possibly accounts for the slightly higher surface area determined for pZr-1. For comparison, we performed the same measurement on a sample of UiO-66-NH₂,²⁰ the 12-connected amino derivative of UiO-66, (Fig. S11, ESI†) and the BET surface area was determined to be 1274 m² g⁻¹, in good agreement with literature results^20b,21 The more open framework due to the 8-connectivity of Zr-1 partially compensates for the presence of the bulky benzyl side group thereby resulting in a BET surface area which is not dramatically lower than that of UiO-66-NH₂.

Fig. 3 Nitrogen sorption isotherms at 77 K of Zr-1 and pZr-1 after treatment with acetone and overnight evacuation (120 °C).

Photophysical properties

As described in the Experimental Part, Zr-1 was treated with 4.0 M HCl prior to its use in sensing studies. Acid activation is an essential step to achieve detection of nitrophenols. The pK_a values for TNP and DNP are 0.3 and 4.0, respectively, indicating that the analyte molecules are found in their deprotonated form in the aqueous environment of the MOF suspension (pH = 4.5). Treatment with HCl ensures that the MOF's secondary amine units are protonated and introduces exchangeable Cl⁻ counter-ions. Combined, these features yield a favourable environment inside the MOF's pores so that the material can function as an ion exchange sensor for the anionic nitrophenols.

The activated material pZr-1 readily forms a fine suspension in water and exhibits turquoise fluorescence upon selective ligand excitation at 400 nm. To examine the stability of the suspension we repeated multiple fluorescence measurements on a suspension of pZr-1 in H₂O (0.1 mg mL⁻¹) in the span of 1.5 h and as can be seen in Fig. S12 (ESI†), the emission spectrum remains unaltered. Our group has previously reported the capability of a similar protonated Zr⁴⁺-terephthalate MOF (MOR-2) to form fine and stable suspensions.¹³

The emission spectrum of pZr-1 consists of a broad band with maximum at 470 nm (Fig. 4), similar to the emission spectrum of ligand L-1 (Fig. S13, ESI†). The excitation spectrum of pZr-1 (Fig. 4) consists of a broad band with a maximum at 370 nm that shows good agreement with the UV-vis absorption band of L-1 (Fig. S13, ESI†) and with what is observed in UiO-66-NH₂(Zr/Hf)-type MOFs.^3b The emission quantum yields of aqueous suspensions of Zr-1 and pZr-1 were found to be ca. 1.2% (Fig. S14 and S15, ESI†), double compared to that of a suspension of UiO-66-NH₂ (ca. 0.56%, Fig. S16, ESI†). The emission quantum yields of MOFs are much lower than those of the free ligands in methanol solution (Φ_L-1 = 39%; Φ_NH2bdc = 62%, Fig. S17 and S18, ESI†). This can be attributed to self-quenching phenomena due to the relative proximity of the chromophores within the MOF matrix.^7a The latter assumption is further supported by the much higher emission intensity displayed by the 8-connected pZr-1 compared to the 12-connected UiO-66-NH₂, despite the free ligand NH₂bdc having a higher quantum yield than L-1. Despite its modest fluorescence quantum yield, pZr-1 (aqueous suspension 0.1 mg mL⁻¹) displays a clear fluorescence signal which can be recorded easily and accurately. This phenomenon has also been observed in MOFs that emit in the near IR region and has been attributed to the large number of luminophores per unit volume enabling the material to emit a large number of photons thereby leading to a clear and strong emission sigmal which is suitable for sensing and imaging studies.²²

Fig. 4 Excitation and emission spectra (λ_exc = 400 nm) of pZr-1 suspended in H₂O (0.1 mg mL⁻¹).

It has been demonstrated, through electron paramagnetic resonance, time-resolved absorption spectroscopy and theoretical calculations, that the fluorescent excited state in Zr⁴⁺ MOFs with 2-aminoterephtahlate as bridging ligand predominantly involves shift of electron density from an orbital which is largely localized on the amino group to an unoccupied antibonding orbital of the aromatic ring without an appreciable contribution from a ligand-to-metal-charge-transfer (LMCT) component.²³ This type of intraligand charge transfer (ILCT) transitions show sensitivity to the local environment around the chromophore, such as the presence of hydrogen bond donors and/or electron accepting/donating moieties, and are thus well suited for optical sensing applications.^3b,9a,24

Fluorescence titrations

Benefitting from the stability in water and its strong emission signal, we investigated the ability of pZr-1 to sense trace amounts of nitrophenols in aqueous media. To evaluate the sensing ability of the material, we performed fluorescence titration experiments with gradual addition of aqueous solutions (5.0 × 10⁻⁵ M) of TNP and DNP to a suspension of pZr-1 in H₂O (0.1 mg mL⁻¹) at pH 4.5. It should be noted that we applied Inner Filter Effect (IFE) correction to all our experimental data (see ESI† for details). As can be seen in Fig. 5, fluorescence measurements clearly demonstrate that TNP and DNP have a strong quenching effect on the initial emission intensity of pZr-1, up to 92% and 86% at the end of the titration, respectively. For both titration experiments, the calibration curves show good linearity with respect to analyte concentration up to ca. 250 ppb. Through analysis of the linear fitting, we determined the Limits of Detection (LOD) and Quantification (LOQ) (see ESI† for details). LOD and LOQ values were calculated to be 0.011 and 0.037 μM (2.5 and 8.5 ppb) for TNP and 0.026 and 0.086 μM (4.8 and 16.0 ppb) for DNP detection, respectively. These values demonstrate that pZr-1 exhibits effective sensing behaviour towards TNP and DNP in aqueous solutions.

Fig. 5 Fluorescence titrations (λ_exc = 400 nm) of pZr-1 suspended in H₂O (2 mL, 0.1 mg mL⁻¹) at pH 4.5 upon gradual addition of aqueous solution (5 × 10⁻⁵ M) of (a) TNP (b) DNP (c) Corresponding Stern–Volmer plots for both titrations, showing linearity at low analyte concentrations.

The pores of pZr-1 are lined with π-electron rich units which are potentially suitable for donor–acceptor association with the electron-deficient nitroaromatic compounds. Furthermore, we theorize that entry of analytes in the pores could be further assisted by H bonding interactions and electrostatic interactions between the positively charged protonated secondary amine units and the anionic nitroaromatic guests. An identical titration experiment with TNP carried out using Zr-1, the as synthesized material that is not fully protonated, produced non consistent results with higher LOD and LOQ values (0.030 μM and 0.139 μM, respectively). Additionally, the same experiment with a deprotonated MOF sample, treated with triethylamine/MeOH solution, resulted in much weaker fluorescence quenching. (Fig. S23, ESI†) These observations support our proposition that the MOF-analyte interaction has a strong electrostatic component.

To further examine the quenching efficiency, we analyzed the data with the Stern–Volmer equation, eqn (1)

(I₀/I) = 1 + K_SV × [A] (1)

where I₀ and I are the integrated fluorescence intensities of pZr-1 aqueous suspension before and after the addition of the analyte, respectively, K_SV is the quenching constant (M⁻¹) and [A] is the molar concentration of the analyte.²⁵ The Stern–Volmer (SV) plots for both analytes (Fig. 5c) are nearly linear at low concentrations and show an upward curvature at higher concentrations. The non-linear nature of the Stern–Volmer plots is a characteristic feature observed when both static and dynamic quenching occurs for the same fluorophore. Therefore, it is reasonable we assume that quenching of Zr-1 fluorescence is due to simultaneous presence of static and dynamic photoinduced electron transfer between Zr-1 and the analytes. The static portion of the observed quenching can be attributed to stacking interactions between nitrophenols and the benzylamino groups inside the pores of Zr-1. Linear fitting of the SV plots at lower concentration range (0–2 μM) offer a quenching constant of 7.2 × 10⁵ M⁻¹ (R² = 0.9995) for TNP and 2.9 × 10⁵ M⁻¹ (R² = 0.9998) for DNP.

To further test our MOF's sensing performance, we performed detection experiments using a sample of acid activated UiO-66-NH₂, (Fig. S24 and S25, ESI†) and the results are shown in Table 1. Comparison of the fluorescence measurements for the two functionalized UiO-66-type MOFs demonstrates that pZr-1 shows superior quenching response and sensitivity. The increased sensing ability of our MOF is the result of the strategical design that incorporates a π-electron-rich system inside the MOF's pores, thus enhancing the interaction with electron deficient nitrophenolic analytes.

Table 1 LOD, LOQ and K_SV values calculated for fluorescence titrations (λ_exc = 380 nm) of acid activated UiO-66-NH₂ and Zr-1 suspended in H₂O (0.1 mg mL⁻¹) at pH 4.5 upon gradual addition of aqueous solution of TNP and DNP. (IFE correction was applied to all experimental data)

	Analyte	LOD (μM)	LOQ (μM)	K SV (M^−1)	Maximum quenching %
UiO-66-NH2	TNP	0.040	0.136	1.6 × 10^5	69
	DNP	0.042	0.140	1.7 × 10^5	64
pZr-1	TNP	0.011	0.037	7.2 × 10^5	92
	DNP	0.026	0.086	2.9 × 10^5	86

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Detection of nitrophenols by luminescent MOF sensors has been reported in numerous studies over the last years.^10e In Table 2 we show LOD and LOQ values and K_sv values of some of the most efficient sensors reported. (A more extensive collection of representative recent examples can be found in Table S1, ESI†). Comparison of the data confirms that our material is among the best performing sensors and shows extremely high sensitivity. The LOD value for TNP (0.011 μM) is comparable with the lowest detection limit so far reported for TNP detection in H₂O, by Joarder et al. (12.9 nM).²⁷ The standout feature of pZr-1 is that the material is hydrolytically stable and forms fine and stable suspension in water and, thus, can be used in analysis of aqueous samples. Furthermore, the λ_exc (400 nm) used in titration experiments lies in lower energy than the absorption maximum region of nitrophenols and the emission maximum of pZr-1 does not overlap with the analytes’ absorption bands, ensuring that the inner filter effect on the emission measurements is kept at a minimum.

Table 2 Representative examples of some of the most efficient luminescent MOF sensors for TNP and DNP

Luminescent MOF	Analyte	Solvent	LOD (μM)	K SV (M^−1)	λ exc (nm)	λ em,max (nm)	Ref.
[Zn2(TCPE)(tta)2]·2DMF·4H2O·2Me2NH2^+	TNP	DMF	0.036	3.06 × 10^4	392	461	26
	DNP		0.029	3.75 × 10^4
[Zn8(ad)4(BPDC)6O·2Me2NH2]·G	TNP	H2O	0.012	4.6 × 10^4	340	405	27
Zr6O4(OH)8(H2O)4(TTNA)8/3	TNP	H2O	0.043	5.1 × 10^5	324	399	28
{Zr6O4(OH)8(H2O)4(L-1)4−x(NH2bdc)x}	TNP	H2O	0.011	7.2 × 10^5	400	470	This work
	DNP	H2O	0.026	2.9 × 10^5

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Encouraged by these results we investigated the sensing selectivity of pZr-1 towards DNP and TNP. We performed fluorescence measurements to examine the quenching efficiency towards several competing non-acidic compounds, including nitrobenzene (NB) 1,3-dinitrobenzene (1,3-DNB), 2-nitrotoluene (2-NT), 4-nitrotoluene (4-NT), 2,4-dinitrotoluene (DNT), 2,6-dinitrotoluene (2,6-DNT), 2-nitrophenol (2-NP), 3-nitrophenol (3-NP), 4-nitrophenol (4-NP), aniline (NA) and 3-nitroaniline (3-NA). The bar graphs in Fig. 6, illustrate the fluorescence response suspensions of activated pZr-1 in H₂O (pH = 4.5) in the presence of competing compounds. These non-acidic compounds have a negligible quenching effect compared to the strong quenching observed upon subsequent addition of equal amounts of TNP or DNP (IFE correction was applied to all experimental data).

Fig. 6 Normalized fluorescence response I/I₀ (λ_exc = 400 nm) of Zr-1 suspended in H₂O (2 mL, 0.1 mg mL⁻¹) at pH 4.5 in the presence of various analytes (12 μM) The dark bars represent the response in the presence of competing analytes and the light bars represent the response upon addition of (a) TNP (10 μM) and (b) DNP (12 μM). I₀ corresponds to emission intensity before the addition of analytes.

To further investigate our MOF's selectivity, we carried out fluorescence titration experiments with the above compounds (Fig. S26, ESI†). The S–V plots of these titrations are shown in Fig. 7 along with those for TNP and DNP. In addition to the expected low quenching effect, the competing compounds produce strictly linear S–V plots within the studied concentration range. This series of fluorescence measurements with a range of different analytes clearly demonstrates that pZr-1 offers a remarkable combination of sensitivity and selectivity towards TNP and DNP. The lack of quenching response in the presence of mono-nitrophenols can be explained by considering that the pK_a values of these molecules are in the range 7.0–8.3, meaning that the hydroxylic groups are not deprotonated in the aqueous MOF suspension.

Fig. 7 Comparison of Stern–Volmer plots of fluorescence titrations (λ_exc = 400 nm) of pZr-1 suspended in H₂O (2 mL, 0.1 mg mL⁻¹) at pH 4.5 with different nitroaromatic analytes.

The treatment of pZr-1 with saturated aqueous solutions of TNP and DNP (ca. 10⁻³ M) results in an instant color change from yellow to orange. After washing with MeOH to remove any loosely attached DNP or TNP, we digested the samples in 40% NaOD in D₂O, to perform ¹H-NMR analysis. The ¹H-NMR spectra confirmed the presence of the analytes within Zr-1 (Fig. S28 and S30, ESI†) with peak area analysis showing guest-to-bridging ligand molar ratios of ca. 0.14 for TNP and ca. 0.22 for DNP. The higher molar ratio that we observed for the DNP guest could be attributed to a size effect. When we performed an analogous experiment in the concurrent presence of both analytes (ca. 10⁻³ M), subsequent ¹H-NMR analysis (Fig. S32, ESI†) showed that the material was loaded with equal amounts of TNP and DNP, thus reflecting their solution ratio. (The low solubility of DNP prevented us from performing further experiments at higher concentrations.)

Comparison of the PXRD pattens and IR spectra of pZr-1 before and after being loaded with DNP and TNP confirm the material's structural stability upon its interaction with the analytes (Fig. S27, S29, S31 and S34, ESI†). It is worth noting that the IR spectrum of pZr-1 shows the characteristic broad signal in the 3000–3800 cm⁻¹ region due to absorbed water. In line with water molecules being displaced by the guest analyte, the water signal is greatly decreased in DNP-loaded pZr-1 revealing the characteristic peak at 3382 cm⁻¹ due to the NH stretch of the secondary amino groups (Fig. S33, ESI†).

To quantify the kinetic response of pZr-1 towards TNP we performed a kinetic fluorescence scan (Fig. 8) where TNP was added to a suspension of pZr-1. The data could be satisfactorily fitted to two pseudo-first order processes²⁹ with time constants of 5.17(2) and 48.2(3) s, with the faster process having a far greater contribution (Fig. S36, ESI†). Thus, the material's fluorescence response to the presence of TNP is dominated by fast kinetics, as is desirable for an effective chemosensor.³⁰

Fig. 8 Time-dependent fluorescence response (λ_exc = 400 nm, λ_em = 470 nm) of pZr-1 suspended in H₂O (2 mL, 0.1 mg mL⁻¹) at pH 4.5 upon addition of 25 μL an aqueous solution of TNP (10⁻³ M). The concentration of TNP corresponds to that at the end of a typical titration experiment (8 μM). Black circles represent experimental data, the red curve represents the best fit based on a biexponential decay.

To evaluate the structural integrity and reusability of Zr-1, we examined the sensing ability of an activated MOF sample that was treated with TNP aqueous solution and re-activated. Fig. S34 (ESI†) displays the PXRD patterns of pZr-1 after TNP sorption and reactivation confirming that the material retains good crystallinity apart from some minor. As shown in Fig. S35 (ESI†), fluorescence titration with gradual addition of TNP (5.0 × 10⁻⁵ M) on a suspension of the recycled sample in H₂O (2 mL, 0.1 mg mL⁻¹) resulted in quenching efficiency up to 89% with a quenching constant of 4.5 × 10⁻⁵ M⁻¹. Thus, we observe that after one cycle of analyte sorption and re-activation, the quenching constant value of pZr-1 is slightly reduced, albeit the regenerated material still exhibits strong quenching in the presence of TNP.

Conclusions

We have presented the synthesis and study of a porous Zr(IV) MOF (Zr-1) incorporating a bridging ligand (L-1) with a pendant benzylamino group, thereby creating a π-electron rich and basic environment within the material's pores, which is favourable for the hosting of acidic nitroaromatic guest species. Powder X-ray diffraction and thermogravimetric analysis showed that Zr-1 possesses a UiO-66 type structure with a lower average connectivity of 8, compared to the 12-connectivity of UiO-66. This lower connectivity may be attributed to the bulky side groups on L-1, which prevent a closer approach between the bridging units. The N₂ adsorption isotherm of Zr-1 corresponds to a BET surface area of 893 m² g⁻¹, indicating that the lower average connectivity combined with the partial cleavage (ca. 10%) of the benzyl side groups has a beneficial effect on the material's porosity. The material retains its structural integrity after protonation with 4.0 M HCl to yield pZr-1, which has a BET surface area of 932 m² g⁻¹, a small increase compared to as synthesized Zr-1, partly attributed to the increased cleavage of the benzyl side groups (reaching ca. 30% after protonation). When pZr-1 is treated with aqueous solutions of TNP and DNP (10⁻³ M), we observe a darkening of the material's colour and the presence of analytes within the material is confirmed through ¹H-NMR analysis. Fluorescence titrations of aqueous suspensions of pZr-1 with TNP and DNP lead to a strong quenching effect (92% and 86% by the end of the titration, respectively) with good signal linearity for analyte concentrations of up to ca. 250 ppb. We determined limits of detection and quantification of 0.011 and 0.037 μM for TNP, and 0.026 and 0.086 μM for DNP, respectively. Comparison with literature data places our material among the most efficient fluorescence sensors for the above analytes. It is important to emphasize that the excitation wavelength used in our studies (400 nm) is offset form the region of maximum absorption of DNP and TNP, thereby minimizing inner filter effects. We have confirmed this by observing insignificant alterations to the initial titration data after applying inner-filter corrections. Research on fluorescent metal–organic frameworks for the detection of nitroaromatic compounds is ongoing in our group.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research project was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the “1st Call for H.F.R.I. Research Projects to support Faculty members and Researchers and the procurement of high-cost research equipment” (Project Number: 3371).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tc02494b

‡ Caution: This work involves the study of nitroaromatic compounds which are toxic and potentially explosive. To avoid the risk of explosion, nitroaromatics should only be handled in small quantities and in aqueous solution. Gloves and appropriate eyes and face protection should be worn at all times.

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