Novel porous iron phthalocyanine based metal–organic framework electrochemical sensor for sensitive vanillin detection

Vanillin is widely used as a flavor enhancer and is known to have numerous other interesting properties, including antidepressant, anticancer, anti-inflammatory, and antioxidant effects. However, as excess vanillin consumption can affect liver and kidney function, simple and rapid detection methods for vanillin are required. Herein, a novel electrochemical sensor for the sensitive determination of vanillin was fabricated using an iron phthalocyanine (FePc)-based metal–organic framework (MOF). Scanning electron microscopy and transmission electron microscopy showed that the FePc MOF has a hollow porous structure and a large surface area, which impart this material with high adsorption performance. A glassy carbon electrode modified with the FePc MOF exhibited good electrocatalytic performance for the detection of vanillin. In particular, this vanillin sensor had a wide linear range of 0.22–29.14 μM with a low detection limit of 0.05 μM (S/N = 3). Moreover, the proposed sensor was successfully applied to the determination of vanillin in real samples such as vanillin tablets and human serum.

A novel electrochemical sensor based on an iron phthalocyanine (FePc) MOF for the sensitive detection of vanillin.     

Two-dimensional conductive phthalocyanine-based metal–organic frameworks for electrochemical nitrite sensing

2D nickel phthalocyanine based MOFs (NiPc-MOFs) with excellent conductivity were synthesized through a solvothermal approach. Benefiting from excellent conductivity and a large surface area, 2D NiPc-MOF nanosheets present excellent electrocatalytic activity for nitrite sensing, with an ultra-wide linear concentration from 0.01 mM to 11 500 mM and a low detection limit of 2.3 μM, better than most reported electrochemical nitrite sensors. Significantly, this work reports the synthesis of 2D conductive NiPc-MOFs and develops them as electrochemical biosensors for non-enzymatic nitrite determination for the first time.

2D nickel phthalocyanine based MOFs (NiPc-MOFs) with excellent conductivity were synthesized through a solvothermal approach.

Nitrite (NO₂⁻) is a common environmental contaminant that appears in the water, soil and other environments, and also serves as a kind of preservative for the food industry.^1,2 Nitrite-rich contaminants cause terrible impacts on the ecological environment and public health due to the unreasonable utilization/treatment with nitrites in the field of farming, food industry, and environmental protection.³ Therefore, it is of great importance for the accurate determination of nitrite in the drinking water or pickled foods.⁴ Moreover, the World Health Organization (WHO) has established a maximum limit of nitrite dosage of 65.2 μM (3 mg L⁻¹) in drinking water. So, a determination strategy with a highly sensitive, selectively and rapid response toward nitrite is imperative. Capillary electrophoresis,⁵ spectrophotometry,⁶ and ion chromatography,⁷etc. are useful with a high sensitivity, but there is a time-cost and more operation skills are required toward these analytical methods.^8,9 Of these above approaches, electrochemical determination has been widely developed owing to its extra merits, including real-time, low-cost, and feasibility.^10–14Metal–organic frameworks (MOFs) were constructed by assembling transition metal ions and organic linkers through coordination reactions. MOFs were firstly utilized for gas adsorption and storage application due to their porous structure and large surface area.¹⁵ With the exploration of MOFs in the field of electrocatalysis, researchers found MOFs exposed more potential active sites on their larger surface, promoting easily the contact with target molecules, which further improved the electrocatalytic performance of MOFs,¹⁶ making MOFs be perfect candidates for sensing.¹⁷ However, great challenges remain for conventional MOFs due to their poor conductive/electronic properties, so the usage of MOFs in electrochemical applications is dramatically limited.^18–21To remove the above challenges, several strategies were put forward, such as (i) pyrolysis of MOFs, in which the carbonized MOFs possessed metal-doped or multi-atom-doped porous carbon, enhancing their electrocatalytic activity;^22,23 (ii) preparation of MOF-based hybrids, with conductive supports (carbon nanotube, graphene, metal foams, etc.) introduced for promoting their electrical conductivity;^24,25 (iii) synthesis of novel conductive MOFs, which can improve the electron transfer capacity directly without pre-treatments.^18,26 However, well-defined molecular active sites on MOFs are decomposed after the high-temperature process.²⁷ Also, the second method can promote their electrocatalytic activity to some extent, but it may reduce the inherent advantages of MOFs as well as decrease the surface area and reduce the accessible active sites. The third has more advantages over the other strategies, owing to the development of conductive MOFs, which can solve these challenges fundamentally and avoid the other approaches'' negative effects.²⁶Two-dimensional (2D) conductive MOFs represent an emerging class of nanomaterials, presenting their exceptional 2D characteristics, enhanced ability of electron transfer and high efficiency of the active sites, as well as the intrinsic merits of conventional MOFs.²⁸ Such 2D conductive MOFs offer a perfect platform for the study of the mechanism of electroanalysis, which is helpful for the enhanced sensing performance of MOFs.²⁹ Recently, 2D Ni₃HHTP₂ (HHTP₂, hexahydroxytriphenylene) was synthesized for neurochemical detection due to a favorable electron transfer and large surface area.³⁰ 2D Cu-TCPP (TCPP, tetrakis(4-carboxyphenyl)porphyrin) modified with gold nanoparticles and polyxanthurenic acid with an exceptional conductivity was demonstrated as an excellent electrochemical sensor towards dopamine with a low detection limit.³¹ 2D conductive materials also played an important role in gas analysis owing to their excellent conductivity.²⁹ Based on the above examples, 2D conductive MOFs present possibilities for achieving a superior electrocatalytic performance for electrochemical sensors.^31,32 However, the usage of 2D conductive MOFs in the electrochemical determination of small molecules has been rarely reported.In this work, nickel phthalocyanine (NiPc) was selected as an organic linker to assemble a 2D NiPc-MOF. Three main reasons arise from using this linker for synthesizing a 2D MOF: (i) metal active sites are atomically dispersed on metallophthalocyanines, theoretically; (ii) NiPc-MOFs extend in two-dimension with fully in-plane π delocalization and weak out-of-plane π–π stacking, further promoting electron transfer between electrocatalysts and analytes; (iii) the larger surface area of a 2D NiPc-MOF, the easier absorption on the electrode, keeping its electrochemical stability, and then achieving an excellent sensitivity. Herein, a 2D conductive NiPc-MOF was synthesized through the solvothermal method and used for the electrochemical determination of nitrite for the first time.The structural information of the as-prepared sample was explored by powder X-ray diffraction spectroscopy (PXRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). The XRD pattern of the sample in Fig. 1a exhibits peaks at 2θ = 4.15°, 9.85°, and 14.26°, indexed to the lattice planes of (200), (001), and (300), respectively. Long-range order within the ab-plane with a center-to-center (Ni⋯Ni) distance of 9.27 Å was further confirmed, which fits well with the simulation results of NiPc-MOF (Ni₃(C₃₂H₁₆N₁₆)_n) (Fig. S1^†) and no typical peaks of NiPc were found. XPS analysis indicates that the as-prepared sample is composed of Ni, C, N, and O. The Ni 2p spectrum exhibits the typical peaks for Ni 2p_3/2 and Ni 2p_1/2 at 854.6 eV peak 871.9 eV, respectively, which are the characteristic peaks of Ni(iii) species (Fig. S2^†).Open in a separate windowFig. 1(a) The PXRD pattern of NiPc-MOF and its predicted structure; (b) the XPS survey spectrum, (c) a TEM image, and (d) an AFM image of 2D NiPc-MOF nanosheets.The TEM image of the as-prepared sample exhibits it as an irregular shape with a nanosheet-like structure; several nanosheets are dispersed well in Fig. 1c. EDX spectrum of Fig. S3^† further confirms its chemical content, including carbon, nitrogen, oxygen and nickel. The AFM (Fig. 1d) image of the sample exhibits its rough surface, and the corresponding height profile (Fig. S4^†) reveals its thickness, ranging from 50 nm to 100 nm, indicating the as-prepared sample possesses a multi-layered structure. Furthermore, the FT-IR spectrum of the as-prepared sample (Fig. S5^†) presents typical vibration absorptions of the basic building unit (NiPc) that three peaks at ca. 1628 cm⁻¹, 1552 cm⁻¹ and 1114 cm⁻¹ assigned to C N stretching, C C stretching and C–H bending, respectively. Nitrogen adsorption–desorption isotherms of NiPc-MOF nanosheets were performed at 77 K (Fig. S6^†). The surface area of the NiPc-MOF nanosheets is 543 m² g⁻¹, which is a little smaller than we previously reported.²⁶ Combined with the above analysis, it is demonstrated that we have successfully synthesized NiPc-based MOF (NiPc-MOF) nanosheets with both 2D features and typical MOF characteristics.The electrocatalytic performance of the NiPc-MOF electrode towards nitrite was studied, as illustrated in Fig. 2. Fig. 3a shows cyclic voltammogram (CV) curves of the NiPc-MOF electrode in 0.1 M phosphate-buffered saline (PBS, pH 7.0) solution with/without 1.0 M nitrite. It can be found that the current response of the NiPc-MOF electrode increases sharply with the addition of nitrite compared to its response in the blank experiment. This means it is possible to achieve nitrite sensing on the surface of the NiPc-MOF electrode. To verify the NiPc-MOF electrode''s feasibility, a series of nitrite solutions with different concentrations (0.35–0.75 M) was added into the test system, as shown in Fig. 3b. The result exhibits that the relationship of the current response versus concentration is clearly linear with a c-efficient value of 0.9998 (Fig. 3c). It demonstrates that the NiPc-MOF electrode has the potential to realize nitrite determination.Open in a separate windowFig. 2A schematic diagram of the preparation of 2D NiPc-MOF and its use in electrochemical nitrite detection.Open in a separate windowFig. 3(a) CV curves of the NiPc-MOF electrode in 1.0 M PBS buffer (pH 7.0) in the presence and absence of nitrite. (b) CV curves of the NiPc-MOF electrode at different nitrite concentrations (0.35–0.75 M), scan rate: 50 mV s⁻¹. (c) The linear calibration curve from the data in (b), concentration range: 0.35–0.75 M. (d) DPV curves of the 2D NiPc-MOF electrode during nitrite detection with successive additions (0.01–11 500 mM), DPV parameters: amplitude, 0.05 V; pulse width, 0.2 s; sampling width, 0.067 s; pulse period, 0.5 s. (e) The linear calibration curve from the data in (d) (concentration range: 0.01–2000 mM). (f) The linear calibration curve from the data in (d) (concentration range: 2500–11 500 mM).Under the optimal conditions (Fig. S7 and S8^†), the electrochemical sensing of the NiPc-MOF electrode for nitrite oxidation was carried out by a differential pulse voltammogram (DPV). Fig. 3d presents the DPV curves at the NiPc-MOF electrode by the successive adding of nitrite with various concentrations from 0.01 mM to 11 500 mM. Each DPV curve can be completed within 6 s, exhibiting a fast response toward nitrite sensing. Fig. 3e and f display clearly linear curves between the current response of NiPc-MOF electrode and nitrite concentration with the regression equations: (i) I/μA = 1.995 + 0.028 × c (R² = 0.999, inset of Fig. 2f), (ii) I/μA = 53.67 + 0.01 × c (R² = 0.9999, inset of Fig. 3f). The sensitivity of the NiPc-MOF electrode is calculated as 0.40 μA mM⁻¹ cm⁻² and 0.14 A mM⁻¹ cm⁻² at low (0.01–2000 mM) and high concentrations (2500–11 500 mM), respectively. Then, a limit of detection (LOD) is estimated as 2.3 μM at a signal to noise ratio of 3 (S/N = 3). Additionally, the sensitivity of the NiPc-MOF electrode is slightly greater in the low concentrations from 0.01 to 2000 mM. This phenomenon can be explained as (i) all nitrite ions absorbed on the surface of the NiPc-MOF electrode; (ii) enough active sites on the NiPc-MOF electrode can catalyze them efficiently at a low concentration region. However, the electrocatalytic process is influenced by the competitive effects, including nitrite adsorption and catalytic activation on the surface of the NiPc-MOF electrode as the concentration increased, finally decreasing in sensitivity. As well, it is noted that the oxidation peak current appears slightly unstable when the nitrite concentration is beyond 2000 mM. This may be attributed to the adsorption saturation of NO₂⁻ on the active sites of the NiPc-MOF electrode. Compared with recent literature (Table S1^†), the sensing performance of the NiPc-MOF electrode presents a very quick response, low LOD and ultra-wide linear range (0.01–11 500 mM).As shown in Fig. 4a, CV measurements of the NiPc-MOF electrode were studied in an electrochemical probe solution (5.0 mM ferricyanide and 0.1 M KCl); the NiPc electrode was employed for comparison. It was clearly found that the NiPc-MOF electrode has a larger closed curve area than that of the NiPc electrode. This phenomenon means NiPc-MOF nanosheets have a better electrical conductivity and faster electron transfer during electrochemical redox. EIS measurements provide valuable information about the interfacial properties of the modified electrode. Fig. 4b reveals the Nyquist plots of the NiPc and NiPc-MOF electrodes, which exhibit semicircles at the high frequency range corresponding to the electron-transfer-limited process and a straight line at the low frequency range corresponding to the diffusion-limited process.³³ Then, Randle''s equivalent circuit (Fig. S9^†) was employed to simulate the obtained Nyquist plots, and further understand the electrical properties of the modified electrodes. The electron charge transfer resistance (R_ct) could be obtained based on the semicircle diameter of the Nyquist plots. The value of R_ct (803.8 Ω) of the conductive NiPc-MOF electrode was much smaller than that of the NiPc electrode (1680 Ω), indicating that the NiPc-MOF electrode brought a better conductivity to the electrode surface. The acceleration of the electron transfer rate was ascribed to the excellent conductivity of 2D NiPc-MOF nanosheets.Open in a separate windowFig. 4(a) CV curves of the NiPc electrode and the 2D NiPc-MOF electrode in 1.0 mM ferricyanide with 0.1 M KCl, scan rate: 50 mV s⁻¹. (b) Nyquist plots of the NiPc electrode and the NiPc-MOF electrode in 1.0 mM ferricyanide containing 0.1 M KCl. (c) Electrochemical capacitance of the NiPc and NiPc-MOF electrodes. (d) The linear relationship between the oxidation peak currents and the square roots of the scan rates.Generally, the double-layer capacitance (C_dl) was utilized to evaluate the modified electrode''s active surface area. As shown in Fig. 4c, the effective active sites of the NiPc-MOF electrode for nitrite oxidation are obviously more than those of the NiPc electrode because the C_dl of that NiPc-MOF electrode is five times larger than that of the NiPc electrode (4.45 μF cm⁻²). Moreover, the electrochemical active surface area of the modified electrode also reveals a transferred electron on the surface of the electrode and determines the active sites for nitrite oxidation.^12,34 As presented in Fig. 4d and S10,^† there is a good linear relationship between the oxidation peak current (I_pa) and the square root of the scan rate (ν^1/2). Therefore, the electrochemical active surface area could be determined as stated by the Randles–Sevcik eqn (1)³⁵I_pa =(2.69 × 10⁵)n^3/2AD^1/2ν^1/2C₀1In eqn (1), n, A, C₀, D, and ν are the number of transferred electrons, the surface area of the working electrode, reactant concentration, diffusion coefficient, and scan rate, respectively. Based on the known information, the electrochemical active surface area for NiPc-MOF electrode is 5.31 times larger than that of NiPc electrode. Consequently, the NiPc-MOF electrode is favorable for nitrite sensing due to the more sensitive response, higher charge transfer efficiency, more catalytic sites and increased electrochemically active surface area.To verify the feasibility of the sensor, the fabricated sensor was utilized to monitor nitrite in real samples (tap water, and 0.1 M PBS were employed here for comparison) through a standard addition method. From the analysis results (Table S2^†), the recovery values of real samples by the electrochemical method in this work were between 93.6% and 101.6%, and the RSD was less than 5%. To confirm the accuracy of the proposed method, a 0.1 M standard PBS solution was selected to detect the spiked samples, and the results are consistent with those of our proposed method, suggesting the approach is reliable for nitrite sensing in an actual complex environment.An excellent selectivity is also an important standard for the prepared sensor, so the effect of interfering species, which possibly coexists with nitrite, on the response of the sensor was assessed. Fig. 5a shows the interference measurement at the applied potential of 0.9 V with continuous additions of 0.1 M nitrite, 0.1 M dopamine, 0.1 M ascorbic acid, 0.1 M uric acid, 0.1 M glucose, 0.1 M KNO₃, 0.1 M NaNO₃ and 0.3 M NaNO₂ in 0.1 M PBS (pH 7.5). Obviously, a clear i–t response increase is revealed by the injection of 0.1 M nitrite into the blank buffer solution. In comparison with the response of NO₂⁻, the i–t responses of other interfering species on the sensor were negligible. This phenomenon may be due to the specific electrocatalytic activity toward nitrite. In addition, as shown in Fig. 5b, the calibration curve of Fig. 5a exhibits a good linearity with a co-efficient value of 0.9998. The TEM image of the NiPc-MOF nanosheets after nitrite detection is presented in Fig. S11,^† keeping the original morphological structure. This result confirms that the NiPc-MOF electrode possesses a great selectivity for the electrochemical detection of nitrite in the presence of multi-interfering species.Open in a separate windowFig. 5(a) Amperometry curve of the NiPc-MOF electrode in 0.1 M PBS (pH 7.5) with successive additions of dopamine, ascorbic acid, uric acid, glucose, KNO₃, and NaNO₃ (all concentrations of the interfering species are 0.1 M) at an applied potential of 0.9 V. (b) The linear calibration curve of the data from (a) in the presence of interfering species.In addition, the repeatability of the as-prepared sensor was also investigated. Five individual electrodes were selected to study the repeatability of the NiPc-MOF electrode here. Five individual electrodes (#1–5) were prepared in the same conditions and applied to detect 0.1 M nitrite; the relative standard deviation (RSD) value was estimated to be 1.65%. As shown in Fig. 6a, the repeatability of the sensor was evaluated with 5 electrodes for detecting 0.1 M nitrite with a low RSD value (1.65%). As displayed in Fig. 6b, the peak current response of the NiPc-MOF electrodes has a similar behavior over stability tests, the NiPc-MOF electrode was stored in an ambient environment and monitored every week via the DPV method. The RSD value of 3.82% was obtained, suggesting the stability of the NiPc-MOF electrode is suitable for long-term nitrite detection. Therefore, all the results suggest the NiPc-MOF electrode is reliable for nitrite sensing due to its excellent stability, repeatability, and long-term repeatability.Open in a separate windowFig. 6(a) The repeatability of NiPc-MOF electrodes (#1–5) in 0.1 M PBS (pH 7.5) containing 0.1 M nitrite. (b) The stability of a NiPc-MOF electrode: peak current of DPV in 0.1 M PBS (pH 7.5) containing 0.1 M nitrite. DPV parameters: amplitude, 0.05 V; pulse width, 0.2 s; sampling width, 0.067 s; pulse period, 0.5 s.     

Polypyrrole decorated metal–organic frameworks for supercapacitor devices

Due to their large specific surface areas and porosity, metal–organic frameworks (MOFs) have found many applications in catalysis, gas separation, and gas storage. However, their use as electronic components such as supercapacitors is stunted due to their poor electrical conductivity. We report a remedy for this by combining the MOF structure with polypyrrole (PPy), a well-known conductive polymer. Three MOFs are studied for modification to this end: CPO-27-Ni and CPO-27-Co (M₂DOBDC, M = Ni²⁺, Co²⁺, DOBDC = 2,5-dihydroxy-1,4-benzenedicarboxylate) and HKUST-1 (Cu₃(BTC)₂, BTC = 1,3,5 benzenetricarboxylate). The gravimetric capacitance of pure MOFs is boosted several orders of magnitude after reinforcement of PPy (e.g., from 0.679 to 185 F g⁻¹ for HKUST-1 and PPy–HKUST-1, respectively), and is much higher than reported for pure PPy. In total, these PPy-d-MOFs exhibit specific capacitances up to 354 F g⁻¹, retaining 70% of this value even after 2500 cycles. Among them, the highest capacitance is found for PPy–CPO-27-Ni (354 F g⁻¹), followed by PPy–CPO-27-Co (263 F g⁻¹) and PPy–HKUST-1 (185 F g⁻¹). The maximum operating potential for these electrodes is 0.5 V, which is restricted by the contact of MOF with aqueous electrolyte and with extremely low PPy content. As a solution, higher PPy loading and rational adjustment of particle size and porosity of both MOF and PPy are recommended so that the MOF/electrolyte interface is limited, leading to more robust electrode. The work completed here describes a highly promising approach to tackling the electrically insulating nature of MOFs, paving the way for their use in electrochemical energy storage devices.

Great improvement of specific capacitance was achieved by reinforcing polypyrrole into the structure of CPO-27-Ni/Co and HKUST-1 metal–organic frameworks.     

A collection of forcefield precursors for metal–organic frameworks

A host of important performance properties for metal–organic frameworks (MOFs) and other complex materials can be calculated by modeling statistical ensembles. The principle challenge is to develop accurate and computationally efficient interaction models for these simulations. Two major approaches are (i) ab initio molecular dynamics in which the interaction model is provided by an exchange–correlation theory (e.g., DFT + dispersion functional) and (ii) molecular mechanics in which the interaction model is a parameterized classical force field. The first approach requires further development to improve computational speed. The second approach requires further development to automate accurate forcefield parameterization. Because of the extreme chemical diversity across thousands of MOF structures, this problem is still mostly unsolved today. For example, here we show structures in the 2014 CoRE MOF database contain more than 8 thousand different atom types based on first and second neighbors. Our results showed that atom types based on both first and second neighbors adequately capture the chemical environment, but atom types based on only first neighbors do not. For 3056 MOFs, we used density functional theory (DFT) followed by DDEC6 atomic population analysis to extract a host of important forcefield precursors: partial atomic charges; atom-in-material (AIM) C₆, C₈, and C₁₀ dispersion coefficients; AIM dipole and quadrupole moments; various AIM polarizabilities; quantum Drude oscillator parameters; AIM electron cloud parameters; etc. Electrostatic parameters were validated through comparisons to the DFT-computed electrostatic potential. These forcefield precursors should find widespread applications to developing MOF force fields.

Atom-in-material (AIM) partial charges, dipoles and quadrupoles, dispersion coefficients (C₆, C₈, C₁₀), polarizabilities, electron cloud parameters, radial moments, and atom types were extracted from quantum chemistry calculations for >3000 MOFs.     

Determination of chloropropanol with an imprinted electrochemical sensor based on multi-walled carbon nanotubes/metal–organic framework composites

In this paper, a composite composed of carboxylated multi-wall carbon nanotubes (cMWCNT) incorporated in a metal–organic framework (MOF-199) has been synthesized using 1,3,5-benzoic acid as a ligand through a simple solvothermal method. The synthesized cMWCNT/MOF-199 composite was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffractometry (XRD). The cMWCNT/MOF-199 hybrids were modified on the surface of glassy carbon electrodes (GCE) to prepare a molecularly imprinted electrochemical sensor (MIECS) for specific recognition of 3-chloro-1,2-propanediol (3-MCPD). The electrodes were characterized by differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Under optimal conditions, the electrochemical sensor exhibited an excellent sensitivity and high selectivity with a good linear response range from 1.0 × 10⁻⁹ to 1.0 × 10⁻⁵ mol L⁻¹ and an estimated detection limit of 4.3 × 10⁻¹⁰ mol L⁻¹. Furthermore, this method has been successfully applied to the detection of 3-MCPD in soy sauce, and the recovery ranged from 96% to 108%, with RSD lower than 5.5% (n = 3), showing great potential for the selective analysis of 3-MCPD in foodstuffs.

In this study, cMWCNT/MOF-199 composites were used as the modified electrodes, and a MIECS having specific recognition of 3-MCPD was prepared by electrochemical polymerization for selective analysis of 3-MCPD in foodstuffs.     

Integration of fluorescent probes into metal–organic frameworks for improved performances

Recent years have witnessed a rapid development of fluorescent probes in both analytical sensing and optical imaging. Enormous efforts have been devoted to the regulation of fluorescent probes during their development, such as improving accuracy, sensitivity, selectivity, recyclability and overcoming the aggregation-caused quenching effect. Metal–organic frameworks (MOFs) as a new class of crystalline porous materials possess abundant host–guest chemistry, based on which they display a great application potential in regulating fluorescent probes. This review summarized the research works on the regulation of fluorescent probes using MOFs, with emphasis on the methods of integrating fluorescent probes into MOFs, the regulation effects of MOFs on fluorescent probes, the superiorities of MOFs in regulating fluorescent probes, and the outlook of this subject. It is desirably hoped that this review can provide a useful reference for the researchers interested in this field.

This review surveyed the research works for the regulation of fluorescent probes with metal–organic frameworks based on host–guest chemistry.     

Strategies for the application of metal–organic frameworks in catalytic reactions

Efficient catalysts play crucial roles in various organic reactions and polymerization. Metal–organic frameworks (MOFs) have the merits of ultrahigh porosity, large surface area, dispersed polymetallic sites and modifiable linkers, which make them promising candidates for catalyzation. This review primarily summarizes the recent research progress on diverse strategies for tailoring MOFs that are endowed with excellent catalytic behavior. These strategies include utilizing MOFs as nanosized reaction channels, metal nodes decorated as catalytic active sites and the modification of ligands or linkers. All these make them highly attractive to various applications, especially in catalyzing organic reactions or polymerizations and they have proven to be effective catalysts for a wide variety of reactions. MOFs are still an evolving field with tremendous prospects; therefore, through the research and development of more modification and regulation strategies, MOFs will realize their wider practical application in the future.

Metal–organic frameworks (MOFs) are promising candidates for catalyzation. This review primarily summarized the recent research progress in diverse strategies for tailoring MOFs which are endowed with more excellent catalytic behavior.     

Real-time drug release monitoring from pH-responsive CuS-encapsulated metal–organic frameworks

Real-time monitoring of drug release behaviors over extended periods of time is critical in understanding the dynamics of drug progression for personalized chemotherapeutic treatment. In this work, we report a metal–organic framework (MOF)-based nanotheranostic system encapsulated with photothermal agents (CuS) and therapeutic drug (DOX) to achieve the capabilities of real-time drug release monitoring and combined chemo-photothermal therapy. Meanwhile, folic acid-conjugated polyethylene glycol (FA-PEG) antennas were connected to the MOF through coordination interactions, endowing the MOF with an enhanced active targeting effect toward cancer cells. It is anticipated that such a theranostic agent, simultaneously possessing tumor-targeting, real-time drug monitoring and effective treatment, will potentially enhance the performance in cancer therapy.

A metal–organic framework-based nanotheranostic system was fabricated to achieve the capabilities of tumor-targeting, real-time monitoring of pH-responsive drug release and combined chemo-photothermal therapy.     

An electrochemical sensor based on copper-based metal–organic framework-reduced graphene oxide composites for determination of 2,4-dichlorophenol in water

In the present work, we reported the fabrication of a novel electrochemical sensing platform to detect 2,4-dichlorophenol (2,4-DCP) by using a copper benzene-1,3,5-tricarboxylate–graphene oxide (Cu–BTC/GO) composite. The sensor was prepared by drop-casting Cu–BTC/GO suspension onto the electrode surface followed by electrochemical reduction, leading to the generation of an electrochemically reduced graphene oxide network (ErGO). By combining the large specific area of the Cu–BTC matrix with the electrical percolation from the graphene network, the number of accessible reaction sites was strongly increased, which consequently improved the detection performance. The electrochemical characteristics of the composite were revealed by cyclic voltammetry and electrochemical impedance spectroscopy. For the detection of 2,4-DCP, differential pulse voltammetry was used to emphasize the faradaic reaction related to the oxidation of the analyte. The results displayed a low detection limit (83 × 10⁻⁹ M) and a linear range from 1.5 × 10⁻⁶ M to 24 × 10⁻⁶ M alongside high reproducibility (RSD = 2.5% for eight independent sensors) and good stability. Importantly, the prepared sensors were sufficiently selective against interference from other pollutants in the same electrochemical window. Notably, the presented sensors have already proven their ability in detecting 2,4-DCP in real field samples with high accuracy (recovery range = 97.17–104.15%).

In the present work, we reported the fabrication of a novel electrochemical sensing platform to detect 2,4-dichlorophenol (2,4-DCP) by using a copper benzene-1,3,5-tricarboxylate–graphene oxide (Cu–BTC/GO) composite.     

Synthesis and adsorption performance of La@ZIF-8 composite metal–organic frameworks

In this study, ZIF-8 with a rhombic dodecahedron structure was prepared by a hydrothermal method. Then La(OH)₃, was successfully loaded onto the ZIF-8 by an immersion deposition method, to form a lanthanide-based metal–organic framework (La@ZIF-8) composites. The structure and properties of La@ZIF-8 were verified by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and zeta potential measurements. The optimum process conditions are discussed within the materials and methods. The effects of initial phosphorus concentration, dosage, pH and contact reaction time on the phosphorus removal performance of the nanomaterial were investigated. The results indicated that La@ZIF-8 exhibited an excellent adsorption capacity (147.63 mg g⁻¹) and its phosphorus removal efficiency could reach as high as 99.7%. Experimental data were interpreted using different adsorption kinetic and isotherm models. The kinetic behavior conformed to the pseudo-second-order kinetic model, which indicated the chemisorption of phosphorus by La@ZIF-8. The adsorption behavior of phosphorus by La@ZIF-8 fitted well to the Langmuir isotherm model, suggesting a monolayer chemical adsorption process. The majority of the adsorbed phosphate could be desorbed by NaOH (2 mol L⁻¹), and the removal efficiency of the recycled La@ZIF-8 reached 90%, even after the fifth cycle. The obtained results demonstrate the great application potential of the prepared La@ZIF-8 as a fascinating adsorbent for the removal of phosphate.

In this study, La(OH)₃ was successfully loaded on ZIF-8 by immersion deposition method, to form lanthanide-based metal–organic frameworks (La@ZIF-8) composites.     

Screening of metal–organic frameworks for water adsorption heat transformation using structure–property relationships

It is of great importance to correlate the water adsorption performance of MOFs to their physicochemical features in order to design and prepare MOFs for applications in adsorption heat transformation. In this work, both data analysis from existing studies and Grand Canonical Monte Carlo molecular simulation investigations were carried out. The results indicated that the highest water adsorption capacity was determined by the pore volume of MOF adsorbents, while there was a linear correlation interrelationship between isosteric heats of adsorption and the water adsorption performance at a low relative pressure. More detailed analysis showed that the charge distribution framework and pore size of MOFs contributed together to the hydrophilicity. Electrostatic interaction between water molecules and the framework atoms played a key role at low relative water pressure. A quantitative structure–property relationship model that can correlate the hydrophilicity of MOFs to their pore size and atomic partial charge was established. Along with some qualitative considerations, the screening methodology is proposed and is used to screen proper MOFs in the CoRE database. Seven MOFs were detected, and four of them were synthesized to validate the screening principle. The results indicated that these four MOFs possessed outstanding water adsorption performance and could be considered as promising candidates in applications for adsorption heating and cooling.

Quantitative structure–property relationship models that correlate the water adsorption performance of MOFs to their physicochemical features have been established.     

DOX sensitized upconversion metal–organic frameworks for the pH responsive release and real-time detection of doxorubicin hydrochloride

Drug resistance is a major obstacle in cancer treatment, and designing a material that monitors real-time drug release remains a top priority. In this study, metal–organic frameworks doped with lanthanum and thulium were synthesized and then coated with aminated silica to form La/Tm-MOF@d-SiO₂ as a drug carrier. Doxorubicin hydrochloride (DOX) was selected as a drug model, and the drug loading and release were investigated. It was found that the release of DOX under acidic conditions reached an optimal level, indicating the pH-responsiveness of La/Tm-MOF@d-SiO₂. Under acidic conditions (pH = 5.8), upconversion fluorescence was generated after loading DOX on La/Tm-MOF@d-SiO₂. At pH = 5.8, the longer the drug released, the stronger the upconversion fluorescence. It was found that the upconversion fluorescence intensity is directly proportional to the amount of drug released; thus, the real-time monitoring of DOX release in tumor cells can be performed based on the upconversion fluorescence.

Drug resistance is a major obstacle in cancer treatment, and designing a material that monitors real-time drug release remains a top priority.     

Highly sensitive humidity-driven actuators based on metal–organic frameworks incorporating thermoplastic polyurethane with gradient polymer distribution

Ambient humidity plays an important role in the fields of industrial and agricultural production, food and drug storage, climate monitoring, and maintenance of precision instruments. To sense and control humidity, humidity-responsive actuators that mimick humidity responsive behavior existing in nature, have attracted intense attention. The most common and important class of humidity actuators is active bilayer structures. However, such bilayer structures generally show weak interfacial adhesion, tending to delaminate during frequent bending and restoration cycles. In this work, to address this problem, a novel monolayer humidity-driven actuator with no adhesive issue is developed by integrating the swellable metal–organic frameworks (MIL-88A) into thermoplastic polyurethane films. The proposed actuators display excellent humidity response that under the conditions of relative humidity simulated with saturated salt solution, the MIL-88A/polyurethane composite films show good self-folding response and stability for recycling use. In addition, a deep insight into the self-folding of the composite films is also provided and a new response mechanism is proposed. In this case, the results show that both the preparation method and response properties of the humidity actuators are improved. Therefore, it suggests a new promising way to develop and design flexible humidity actuators.

By combining MIL-88A and thermoplastic polyurethane, a novel humidity-driven actuator was fabricated. The composite films curl from the bottom up, attributed to the uneven vertical gradient distribution of TPU phase. The method promises a new route to humidity actuators.     

Mesoporous Cu–Cu2O@TiO2 heterojunction photocatalysts derived from metal–organic frameworks

This study reveals a unique Cu–Cu₂O@TiO₂ heterojunction photocatalyst obtained with metal–organic framework as the precursor, which can be utilized in dye photodegradation under visible light irradiation. The composition, structure, morphology, porosity, optical properties and photocatalytic performance of the obtained catalysts were all investigated in detail. The Cu–Cu₂O@TiO₂ nanocomposite is composed of lamellar Cu–Cu₂O microspheres embedded by numerous TiO₂ nanoparticles. Methylene blue, methyl orange and 4-nitrophenol were used as model pollutants to evaluate the photocatalytic activity of the Cu–Cu₂O@TiO₂ nanocomposite for dye degradation under visible light irradiation. Nearly 95% decolourisation efficiency of Methylene blue was achieved by the Cu–Cu₂O@TiO₂ photocatalyst within 3 h, which is much higher than that of TiO₂ or Cu₂O catalysts. The excellent photocatalytic activity was primarily attributed to the unique MOF-based mesoporous structure, the enlarged photo-adsorption range and the efficient separation of the charge carriers in the Cu–Cu₂O@TiO₂ heterojunction.

Cu–Cu₂O@TiO₂ heterojunction photocatalyst derived from a metal–organic framework shows high photocatalytic activity for dye degradation under visible light irradiation.     

Oriented self-assembly of metal–organic frameworks driven by photoinitiated monomer polymerization

The self-assembly of metal–organic frameworks (MOFs) is crucial for the functional design of materials, including energy storage materials, catalysts, selective separation materials and optical crystals. However, oriented self-assembly of MOFs is still a challenge. Herein, we propose a novel strategy to drive oriented self-assembly of MOF polyhedral particles at the water–liquid interface by photoinitiated monomer polymerization. The MOF polyhedral particles self-assemble into ordered close-packed structures with obvious orientation in the polymer film, and the orientation is determined by the casting solvent on the water surface. The prepared large-area MOF polymer films show a Janus structure, containing a MOF monolayer and a polymer layer, and can be easily transferred to a variety of substrates. In addition, mixed MOF particles with different sizes and morphologies can also be assembled by this method. This novel method can be foreseen to provide a powerful driving force for the development of MOF self-assembly and to create more possibilities for utilizing the anisotropic properties of MOFs.

The self-assembly of metal–organic frameworks (MOFs) is crucial for the functional design of materials, including energy storage materials, catalysts, selective separation materials and optical crystals.     

Chitosan modified metal–organic frameworks as a promising carrier for oral drug delivery

Metal–organic frameworks (MOFs) are composed of both organic linkers and metallic ions, which have emerged as excellent drug delivery agents for the treatment of cancer and other diseases. Currently, MOF studies are mainly focused on intravenous administration, while studies dedicated to oral administration are relatively scarce. In this study, five MOFs, namely UiO-66, UiO-66-NH₂, UiO-66-COOH, UiO-67 and Zr-NDC, were synthesized, of which Zr-NDC had the largest drug loading capacity for 5-FU. Next, a chitosan (CS) modified Zr-NDC was developed to provide a strong impetus for the oral administration of 5-FU. In vitro release experiments of fluorescein isothiocyanate (FITC)-labeled chitosan demonstrated that the cumulative release rates of FITC-labeled chitosan in artificial gastric juice and artificial intestinal fluid were about 20% and 90%, respectively. The in vitro drug release profiles showed that under the protection of CS-MOF, the release of 5-FU into an acidic environment was only 20%, but the release in artificial intestinal fluid reached 70%. Pharmacokinetic analysis revealed that the coating of chitosan on the surface of MOFs exerted a controlled drug release effect, and further improved the oral bioavailability of 5-FU. These findings suggest that CS coating can break through the limitation of MOF intolerance to acid. It is expected that CS-MOF@5-FU can serve as a potential drug delivery system for the oral administration of 5-FU.

The drug delivery system of CS-MOF@5-FU was developed to achieve oral administration of 5-FU.     

Enhanced electrochemical properties of cerium metal–organic framework based composite electrodes for high-performance supercapacitor application

Cerium metal–organic framework based composites (Ce-MOF/GO and Ce-MOF/CNT) were synthesized by a wet chemical route and characterized with different techniques to characterize their crystal nature, morphology, functional groups, and porosity. The obtained Ce-MOF in the composites exhibit a nanorod structure with a size of ∼150 nm. The electrochemical performance of the composites was investigated in 3 M KOH and 3 M KOH + 0.2 M K₃Fe(CN)₆ electrolytes. Enhanced electrochemical behavior was obtained for the Ce-MOF/GO composite in both electrolytes and exhibited a maximum specific capacitance of 2221.2 F g⁻¹ with an energy density of 111.05 W h kg⁻¹ at a current density of 1 A g⁻¹. The large mesoporous structure and the presence of oxygen functional groups in Ce-MOF/GO could facilitate ion transport in the electrode/electrolyte interface, and the results suggested that the Ce-MOF/GO composite could be used as a high-performance supercapacitor electrode material.

The presence of oxygen functional groups in GO enhances the charge storage behavior of Ce-MOF/GO composites for use as supercapacitor electrode materials.     

Enhanced transformation of CO2 over microporous Ce-doped Zr metal–organic frameworks

Metal–organic frameworks (MOF) have been studied extensively for the adsorption and catalytic conversion of CO₂. However, previous studies mainly focused on the adsorption capabilities of partially or totally Ce substituted UiO-66, there are few studies focusing on transformation of the structure and catalytic activity of these materials. In this work, a series of Zr/Ce-based MOFs with UiO-66 architecture catalysts were prepared for the conversion of CO₂ into value-added dimethyl carbonate (DMC). Owing to the different addition order of the two metals, significantly varied shapes and sizes were observed. Accordingly, the catalytic activity is greatly varied by adding a second metal. The different catalytic activities may arise from the different acid–base properties after Ce doping as well as the morphology and shape changes. Besides, the formation of terminal methoxy (t-OCH₃) was found to be the rate limiting step. Finally, the reaction mechanism of CO₂ transformation in the presence of a dehydrating agent was proposed.

Different doping order of Ce/Zr have a significant effect on the morphologies, acid properties as well as on the activities for CO₂ conversion of the MOF materials.     

Modelling drug adsorption in metal–organic frameworks: the role of solvent

Solvent plays a key role in biological functions, catalysis, and drug delivery. Metal–organic frameworks (MOFs) due to their tunable functionalities, porosities and surface areas have been recently used as drug delivery vehicles. To investigate the effect of solvent on drug adsorption in MOFs, we have performed integrated computational and experimental studies in selected biocompatible MOFs, specifically, UiO-AZB, HKUST-1 (or CuBTC) and NH₂-MIL-53(Al). The adsorption of three drugs, namely, 5-fluorouracil (5-FU), ibuprofen (IBU), and hydroxyurea (HU) were performed in the presence and absence of the ethanol. Our computational predictions, at 1 atmospheric pressure, showed a reasonable agreement with experimental studies performed in the presence of ethanol. We find that in the presence of ethanol the drug molecules were adsorbed at the interface of solvent and MOFs. Moreover, the computationally calculated adsorption isotherms suggested that the drug adsorption was driven by electrostatic interactions at lower pressures (<10⁻⁴ Pa). Our computational predictions in the absence of ethanol were higher compared to those in the presence of ethanol. The MOF–adsorbate interaction (U_HA) energy decreased with decrease in the size of a drug molecule in all three MOFs at all simulated pressures. At high pressure the interaction energy increases with increase in the MOFs pore size as the number of molecules adsorbed increases. Thus, our research shows the important role played by solvent in drug adsorption and suggests that it is critical to consider solvent while performing computational studies.

Solvent plays a key role in drug loading in metal–organic frameworks.     

Tailoring biocompatibility of composite scaffolds of collagen/guar gum with metal–organic frameworks

Metal–organic frameworks (MOFs) are microporous materials with high potential for biomedical applications. They are useful as drug delivery systems, antibacterials, and biosensors. Recently, composite materials comprised of polymer matrixes and MOFs have gained relevance in the biomedical field due to their high potential as materials to accelerate wound healing. In this work, we studied the potential applications of composite hydrogels containing MgMOF74, CaMOF74, and Zn(Atz)(Py). The composite hydrogels are biodegradable, being completely degraded after 15 days by the action of collagenase and papain. The composites showed high biocompatibility reaching cell viabilities up to 165.3 ± 8.6% and 112.3 ± 12.8% for porcine fibroblasts and human monocytes, respectively. The composites did not show hemolytic character and they showed antibacterial activity against Escherichia coli reaching up to 84 ± 5% of inhibition compared with amoxicillin (20 ppm). Further, the immunological assays revealed that the composites produce a favorable cell signaling stimulating the secretion of the TGF-β and MCP-1 cytokines and maintaining the secretion of TNF-α in normal levels. Finally, the composites showed potential to be used as controlled drug delivery systems reaching a release efficiency of 30.5 ± 2.5% for ketorolac. Finally, results revealed that ColGG-Zn(Atz)(Py) was the best formulation evaluated.

MOF Zn(Atz)(Py) tailored the biocompatibility of collagen/guar gum hydrogels stimulating the cell metabolism and the secretion of TGF-β and MCP-1. Further, Zn(Atz)(Py) increased the antibacterial activity and improved the drug release performance.