MOF-based Sensor Research Articles

Lanthanide MOF-based luminescent sensor array for detection and identification of contaminants in water and biomarkers

In today's society, heavy metal ions and antibiotic contaminants have caused great harm to water systems and human health. In this study, six isostructural lanthanide metal-organic frameworks [Ln(H3imda)2(TPA)(H2O)2](Tb for CUST-881, Eu for CUST-882, Dy for CUST-883, Er for CUST-884, Nd for CUST-885, Sm for CUST-886) were constructed by selecting terephthalic acid (TPA) and 4,5-Imidazoledicarboxylic acid (H3imda) and lanthanide metal ions via solvethermal method. Among them, CUST-881 and CUST-882 can selectively detect Fe3+, Cr2O72−, CrO42, and ceftriaxone sodium (CRO) in water systems and uric acid in urine. CUST-881 shows very low detection limits for these five substances. Furthermore, Principal Component Analysis (PCA) was used to distinguish Fe3+, Cr2O72−, CrO42−, and CRO in water. To our knowledge, this is the first time that they have been able to be simultaneously distinguished. In addition, the possible sensing mechanism was studied through UV–visible spectroscopy, Infrared spectroscopy, and PXRD analysis. Furthermore, the probe also showed satisfactory repeatability and recovery when applied to UA samples that simulated urine. Based on the above results, lanthanide metal-organic frameworks have great potential for practical monitoring of contaminants in water environments.

In-situ grown MXene@PDA/MOF composites constructed electrochemical sensor for detecting trace l-cysteine

In this study, novel honeycomb-like morphology MXene@PDA/MOF composites were prepared in situ on the surface of MXene@PDA (polydopamine), combined with 2-methylimidazole and Zn2+. The characterizations of morphology and electrochemical properties show that MXene@PDA/MOF has large specific surface areas and outstanding electroconductibility. Furthermore, the MXene@PDA/MOF-based sensor was fabricated to detect l-cysteine (L-Cys). The response of differential pulse voltammetry (DPV) shows the current intensity increases as l-Cys concentration. Under the optimum conditions, DPV response shows a fine linear relationship with l-Cys concentration in a range of 0.01 – 5.0 μmol L−1 with the detection limit of 3.74 × 10−9 mol L−1. This sensor was used to detect l-Cys in real samples with recoveries of 100.34 % – 101.68 %. Thus, the MXene@PDA/MOF-based electrochemical sensor exhibits excellent selectivity, reproducibility, stability, and practicability.

A Luminescent MOF-Based Sensor for Monitoring of an Anticancer Drug and a Pyrethroid Fungicide Biomarker in Wastewater and Biological Fluids.

The extensive use of insecticides, such as pyrethroids, and pharmaceutical drugs, such as doxorubicin (DOX) has significantly increased to meet the growing demand for food production and disease treatment. Among them, 3-phenoxybenzoic acid (3-PBA), a metabolite of pyrethroid insecticides, poses various health and environmental risks. Similarly, DOX is a well-known anticancer drug and has been continuously used for many years. The high demand and unregulated disposal of these substances raise concerns for both humans and the environment. To address this issue, there is a pressing need to monitor the presence of these analytes in wastewater to protect our ecosystems. This challenge has inspired us to develop an MOF-based fluorometric dual sensor capable of rapid and selective detection of these analytes in aqueous solutions. This work represents the first MOF-based dual probe for detecting these targeted analytes. There was a 98% fluorescence quenching upon the introduction of DOX whereas about a 11-fold increment of the probe's fluorescence intensity took place in the presence of 3-PBA. The sensitivity of the probe is notably high as limits of detection (LOD) are 8.7 nM for DOX and 1.2 nM for 3-PBA. Our designed probe has the highest KSV value for DOX which is 3.37 × 106 M-1. The MOF demonstrated remarkable rapid response time of just 5 and 10 s for DOX and 3-PBA, respectively. The MOF exhibited outstanding selectivity in detecting DOX and 3-PBA, even when other interfering substances were present. We tested the probe's sensing abilities in various environments, such as serum, urine, wastewater, and different pH levels. These findings underscore the sensor's practicality and usefulness in real-world applications. The underlying mechanisms driving the sensing processes were thoroughly investigated by using various modern analytical methods.

A metal–organic framework composite loaded with coumarin guests as a turn-on luminescent sensor for detection of cysteine in aqueous solution

In this work, a composite material (ACE@UiO-66-NH2) has been synthesized conveniently by loading a cysteine-responsive molecular probe Acrylic acid 3-acetyl-2-oxo-2H-chromen-7-yl ester (ACE) on a UiO-type zirconium-based metal–organic framework (UiO-66-NH2). Furthermore, the resulting ACE@UiO-66-NH2 demonstrated to be a fluorescent turn-on probe for selective recognition of Cys from other amino acids in aqueous HEPES buffer systems. Additionally, the proposed recognition mechanism has been confirmed by 1H NMR and TLC. The study provided a promising approach to rationally design MOF-based fluorescent sensor for Cys in aqueous system.

Eu(III)-functionalized metal–organic framework as a visual ratiometric fluorescence sensor for rapid discrimination and detection of multiple tetracyclines in agricultural products

The misuse of oxytetracycline (OTC), doxycycline (DOX), tetracycline (TC), and chlortetracycline (CTC) poses a serious threat to human health and it calls for rapid and visual methods. In this study, a novel ratio fluorescent sensor was constructed by simply combining Eu(III) with IRMOF-3. Spectral characterization and DFT theoretical calculations revealed different mechanism of interactions between the MOF-based sensor and different tetracyclines in detail. PCA analysis amplified these differences, enabling the simultaneous identification and differentiation of individual, binary or ternary mixtures of multiple tetracyclines. Subsequently, a rapid fluorescence sensing method (<5 min) for detecting OTC, DOX, TC, and CTC was established with low detection limits of 70, 75, 40, and 168 nM, respectively, which are all below the maximum residue limits by the European Union and the U.S. Food and Drug Administration regulations. A portable visual test strip was also conducted based smartphone with low detection limits of 1 μM, exhibiting an obvious color spectrum transition from blue to purple and finally to pink. Furthermore, the sensor was applied successful to pig feed, milk and veterinary drugs. The results showed good agreement with HPLC-MS/MS analysis.

Bimetallic MOF-based electrochemical sensor for determination of paracetamol in spiked human plasma

Metal-organic frameworks (MOFs) with their exceptional properties have the potential to revolutionize the field of electrochemistry and pave the way for new and exciting applications. MOFs is an excellent choice as an active electrocatalyst component in the fabrication of electrochemical sensors. Here, bimetallic NiCo-MOFs, monometallic Ni-MOFs, and Co-MOFs were fabricated to modify the carbon paste electrode. Moreover, the ratio between Co and Ni within the bimetallic MOFs was optimized. Our aim in this work is to synthesize different compositions from bimetallic MOFs and systematically compare their catalytic activity with mono-metallic MOFs on paracetamol. The structure and properties of the 2D NiCo-MOFs were characterized by scanning electron microscope, X-ray photoelectron spectroscopy, Fourier transform infrared, and electrochemical method. Bimetallic Ni0.75Co0.25-MOFs modified carbon paste sensor displayed the optimum sensing performance for the electrochemical detection of paracetamol. A linear response over the range 6.00 × 10− 7 to 1.00 × 10− 4 M with a detection limit of 2.10 × 10− 8 M was obtained. The proposed method was applied to detect paracetamol in spiked human plasma and to determine paracetamol in the presence of its major toxic impurity, p-aminophenol. These findings suggest the considerable potential use of the newly developed sensor as a point-of-care tool for detecting paracetamol and p-aminophenol in the future.

Reusable MOF-Coated Chitosan@Paper Strip Composite for Real-Time Monitoring of Pesticide Pendimethalin and Organoarsenic Feed Additive Roxarsone Levels in Environmental Water, Food, and Vegetable Samples.

An alarming increase in the use of pesticides and organoarsenic compounds and their toxic impacts on the environment have inspired us to develop a selective and highly sensitive sensor for the detection of these pollutants. Herein, a bio-friendly, low-cost Al-based luminescent metal-organic framework (1')-based fluorescent material is demonstrated that helps in sustaining water quality by rapid monitoring and quantification of a long-established pesticide (pendimethalin) and a widely employed organoarsenic feed additive (roxarsone). A pyridine-functionalized porous aluminum-based metal-organic framework (Al-MOF) was solvothermally synthesized. After activation, it was used for fast (<10 s) and selective turn-off detection of roxarsone and pendimethalin over other competitive analytes. This is the first MOF-based recyclable sensor for pendimethalin with a remarkably low limit of detection (LOD, 14.4 nM). Real-time effectiveness in detection of pendimethalin in various vegetable and food extracts was successfully verified. Moreover, the aqueous-phase recyclable detection of roxarsone with an ultralow detection limit (13.1 nM) makes it a potential candidate for real-time application. The detection limits for roxarsone and pendimethalin are lower than the existing luminescent material based sensors. Furthermore, the detection of roxarsone in different environmental water and a wide pH range with a good recovery percentage was demonstrated. In addition, a cheap and bio-friendly 1'@chitosan@paper strip composite was prepared and successfully employed for the hands-on detection of pendimethalin and roxarsone. The turn-off behavior of 1' in the presence of pendimethalin and roxarsone was examined systematically, and plausible mechanistic pathways were proposed with the help of multiple experimental evidences.

Amino-functionalized MOF-based fluorescent sensor for efficient detection of 3-nitrotyrosine in serum

Sensitive and rapid detection of biomarker is an important but challenging task for diagnosis, treatment and monitoring of diseases. Herein, this study reports an amino-functionalized MOF-based fluorescent sensor (FJU-40-NH2), which is selectively and significantly quenched by 3-nitrotyrosine (3-NT), a key biomarker of kidney disease, featuring ultra-fast detection time (<15 s), high quenching efficiency, superior anti-interference capability and low detection limit (0.55 µM). More importantly, this fluorescent sensor is successfully employed for the determination of 3-NT in both healthy and acute kidney injury mice serums with recoveries of 96.3–103.3 %. The fluorescence quenching mechanism is attributed to the synergistic effect of the photoinduced electron transfer and internal filtration effect, as confirmed by UV–vis spectra and theoretical calculations.

A Recyclable Luminescent MOF Sensor for On-Site Detection of Insecticide Dinotefuran and Anti-Parkinson's Drug Entacapone in Various Environmental and Biological Specimens.

The monitoring and precise determination of pesticides and pharmaceutical drugs and their residues have become increasingly important in the field of food safety and water contamination issues. Herein, a fluorescent aluminium MOF-based sensor (1) was developed for the selective recognition of neonicotinoid insecticide dinotefuran and anti-Parkinson's drug entacapone. Guest-free MOF 1' exhibited ultra-fast response (<5 s) and ultra-low detection limits of 2.3 and 7.6 nM for dinotefuran and entacapone, which are lower than the previously reported MOF-based sensors. In the presence of other competitive analytes, great selectivity was achieved towards both analytes. The probe was recyclable up to five cycles. The sensing ability was explored towards entacapone in human serum, urine and dinotefuran in real soil, rice, honey samples, different fruits, vegetables, real water specimens and a wide range of pH media. A low-cost, handy MOF-based polymer thin-film composite (1'@PVDF-PVP) was developed for the on-site detection of dinotefuran and entacapone. Mechanistic studies involving analytical techniques and theoretical calculations suggested that FRET and PET are the probable reasons for entacapone sensing whereas IFE is responsible for dinotefuran detection. The entire work presents a low cost, multi-use photoluminescent sensor of entacapone and dinotefuran to address the environmental pollution.

A europium MOF-based turn-off fluorescent sensor for Cr2O72−, l-tryptophan, Dimethylnidazole detection in water

Various pollutants in water are a serious threat to human health. It is very important to identify and detect harmful substances in water. In this paper, an unprecedented new coordination polymer of luminescent sensor, [[Eu(L)2] Cl·H2O] n, (Eu-MOF), with excellent fluorescence sensing abilities for anions, amino acid and antibiotics was developed. It was synthesized by N-heterocyclic carboxylic acid ligands 1, 3-bis (4-carboxylbenzyl) triazole chloride and lanthanide metal ion Eu3+ under solvothermal conditions. The fluorescence results indicate that Eu-MOF has excellent fluorescence sensing abilities for Cr2O72−, l-tryptophan, and Dimethylnidazole via fluorescence quenching. The limit of detection (LOD) is 8.98 × 10−6 mol·L−1, 4.94 × 10−5 mol·L−1 and 5.50 × 10−5 mol·L−1, respectively. Furthermore, the possible mechanisms of fluorescence quenching in the sensing process were systematically studied by UV–vis and DFT. It is revealed that the rational design and utilization of such one multifunctional Eu-MOF can offer guidance for fluorescence detection with wide applications.

A highly efficient luminescent MOF-based TNP sensor fabricated by new AIE ligand

Designing and synthesizing functional metal–organic frameworks (MOFs) remains great challenges. In this study, a new aggregation-induced-emission (AIE) molecule 4,4′-(((9H-fluoren-9-ylidene)methylene)bis(4,1-phenylene))dipyridine (L) was developed as functional ligand to construct a unique MOF [Cu(I)2Cu(II)L2(SCN)4]n (1) with mixed-valence Cu centers. 1 is fabricated by 4-connected Cu(II) and Cu(I) building units, and “V” shaped L and linear SCN− bridges, and forms a three-dimensional (3-D) binodal (4, 4)-connected framework with rare non-interpenetrated (64·82)(66)2 network. 1 demonstrates excellent water, pH and thermal stabilities, and can efficiently detect TNP in aqueous medium. Its quenching constant and detection limit for sensing TNP reach 2.16 × 105 M−1 and 1.05 × 10−5 mM, respectively, which are better than those of the most MOF-based TNP sensors. Moreover, 1 exhibits notable TNP sensing selectivity and recyclability. The luminescence quenching sensing mechanism of 1 toward TNP is in-depth investigated.

Thymine-Hg2+-Thymine strategy in MOF-based electrochemical aptamer sensor for PAEs detection

Phthalate acid esters (PAEs), known food contaminants, can lead to reproductive problems, respiratory diseases, childhood obesity, and neuropsychological disorders. This work aims to develop an electrochemical aptamer sensor for the detection of DAP and its analogs. To achieve specific recognition ability for PAEs, we have introduced the thymidine-Hg2+-thymidine strategy for the first time in an electrochemical aptamer sensor, incorporating metal–organic frameworks (MOFs) as the carrier for the aptamer. Additionally, nano signal amplification technology was applied by immobilizing MXene on the electrode to intensify the sensor’s current signal. In the presence of PAEs, these molecules specifically bound to the aptamers, resulting in competitive release of Hg2+ and aptamers connected to Hg2+, subsequently reducing current signaling. Under optimal experimental conditions, the designed sensor achieved a linear range from 1.65 × 10−5 mg/mL to 3 × 10−3 mg/mL and an extremely low detection limit of 8.94 × 10−6 mg/mL. Furthermore, it exhibited good reliability and practicality in detecting actual samples.

Bimetallic MOF-Based Sensor for Highly Sensitive Detection of Ammonia Gases.

The demand for the detection of ultralow concentrations of ammonia gas is growing. A bimetallic metal-organic framework (MOF) comprising Prussian blue analogs (PBAs) was used to achieve highly sensitive and stable detection of ammonia gas at room temperature in this study. First, PB was enriched by using ammonia for improved gas sensing properties. Second, a sensitive membrane with more vacancies was formed by partially replacing Fe3+ with Cu2+ through a cation-exchange strategy. Finally, a capacitive sensor was developed for ultralow-concentration ammonia detection using a Cu-Fe PBA sensitive membrane and interdigitated electrodes (IDEs). To investigate the adsorption efficiency of the designed composite sensitive film for ammonia, PBAs nanoparticles were deposited on a quartz microcrystal balance (QCM) via cyclic voltammetry and a hydrothermal method. Approximately 10 ppm of ammonia was adsorbed under 1 atm by the Cu-Fe PBA film prepared using a reaction time of 36 h, and the adsorption efficiency was measured to be 2.2 mmol g-1 using the QCM frequency response. The Cu-Fe PBAs were also tested using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Brunauer-Emmett-Teller theory. The introduction of Cu2+ ions significantly increased the specific surface area of Cu-Fe PBAs MOF, and the number of adsorption sites for ammonia also increased; however, its skeleton structure remained similar to that of PB. Then, the capacitive sensor based on Cu-Fe PBA sensitive membrane and IDE was fabricated and the gas sensing detection device was established for ammonia detection. Overall, the developed capacitive sensor exhibits a linear response of 75-1000 ppb and a detection limit of 3.8 ppb for ultralow ammonia concentrations, which makes it superior to traditional detection methods and thus allows excellent application prospects.

Functional Group-Assisted Fluorescence Sensing Platform for Nanomolar-Level Detection of an Antineoplastic Drug and a Neurotransmitter from Environmental Water and Human Biofluids.

A fast, sensitive, selective, and biocompatible dual sensor of an antineoplastic medication (methotrexate) and a neurotransmitter (adrenaline) is still being searched by present-day scientists. To overcome this issue, we have designed a functionalized, robust, bio-friendly luminescent MOF for the sensitive, selective, and rapid monitoring of methotrexate and adrenaline. This probe is the first ever reported MOF-based fluorescence sensor of methotrexate and second only for adrenaline. This fluorescence probe has a very low limit of detection (LOD) of 0.34 and 11.2 nM for adrenaline and methotrexate, respectively. The sensor can detect both the targeted analytes rapidly within 5 s. It can also detect adrenaline and methotrexate from human blood serum and urine accurately and precisely. This reusable sensor is equally efficient in detecting methotrexate from environmental water specimens. Biocompatible, user-friendly, and inexpensive chitosan@MOF@cotton composites were fabricated for the detection of adrenaline and methotrexate from the nanomolar to the micromolar range by the naked eye under a fluorescence lamp. This probe displayed high reproducibility, precision, and accuracy in sensing methotrexate and adrenaline. Fluorescence resonance energy transfer (FRET) and the inner filter effect (IFE) are the possible mechanisms for adrenaline and methotrexate sensing, respectively. The possible mechanism was supported by using required instrumental techniques and theoretical simulations.

Eu MOF-enhanced FeNCD nanozymes for fluorescence and highly sensitive colorimetric detection of tetracycline.

The constrained enzymatic activity and aggregation challenges encountered by small-sized nanozymes pose obstacles to their practical utility, necessitating a strategy to mitigate aggregation and boost enzymatic catalytic efficiency. In this work, a negatively charged Eu MOF was utilized as the encapsulation matrix, encapsulating the small-sized nanozymes FeNCDs into the Eu MOF to synthesize an FeNCDs@Eu MOF. The dispersibility of the encapsulated FeNCDs was increased, and owing to the negative charge of the FeNCDs@Eu MOF, electrostatic pre-concentration of the positively charged target molecule tetracycline (TC) was facilitated, thereby amplifying the enzymatic catalytic efficiency of the FeNCDs. The response of the FeNCDs to TC increased by nearly 6 times upon encapsulation. The TC detection limit (LOD) of the FeNCDs@Eu MOF-based sensor is as low as 11.63 nM. The incorporation of fluorescence detection expanded the linear range of the sensor, rendering it more suitable for practical sample detection.

Lanthanide MOF-based luminescent sensor arrays for the detection of castration-resistant prostate cancer curing drugs and biomarkers.

In recent years, castration-resistant prostate cancer (CRPC) has profoundly impacted the lives of many men, and early diagnosis of medication and illness is crucial. Therefore, a highly efficient detection method for CRPC biomarkers and curing drugs is required. However, the complex and diverse structures of CRPC drugs pose significant challenges for their detection and differentiation. Lanthanide metal-organic frameworks (Ln-MOFs) show great potential for sensing applications due to their intense and characteristic luminescence. In this work, a series of new bimetallic Ln-MOFs (EuxTb1-x-MOF) based luminescent sensor arrays have been developed to identify CRPC drugs, including in mixtures, via principal component analysis (PCA) and hierarchical cluster analysis (HCA) methods. These Ln-MOFs are built with a highly conjugated H2L linker (H2L = 5-(4-(triazole-1-yl)phenyl)isophthalic acid) and exhibit robust strong luminescence emissions (mainly located at 543 and 614 nm) and high energy transfer efficiencies. More specifically, Eu0.096Tb0.904-MOF (MOF 3) has demonstrated good sensing performances for CRPC curing drugs in real human serum samples. Furthermore, the curing drug hydroxyflutamide has been combined with MOF 3, to construct a robust composite sensing platform MOF 3@hydroxyflutamide for highly efficient detection of CRPC biomarkers such as the androgen receptor (AR) and prostate-specific antigen (PSA). Finally, luminescence lifetime measurements, zeta potential measurements, and density functional theory (DFT) calculations were performed to gain insights into the sensing mechanism.

An amino and triazole-containing metal-organic framework: cellobiose sensing and its catalytic conversion under mild conditions.

A novel metal-organic framework functionalized with amino and triazole moieties, namely Zn-TDA, has been synthesized. It represents the first example of a MOF-based sensor for highly selective sensing of cellobiose over other saccharides. Furthermore, Zn-TDA was used as a catalyst for the conversion of cellobiose to 5-hydroxymethylfurfural (5-HMF), achieving an excellent yield of 68.7% within 90 min under mild conditions.

Comprehensive Qualitative and Quantitative Colorimetric Sensing of Volatile Organic Compounds Using Monolayered Metal-Organic Framework Films.

Cross-responsive chemical sensors are in high demand owing to their ability to distinguish a broad range of analytes. In this study, a vapochromic sensor array based on metal-organic frameworks (MOFs), which exhibits distinct patterns when exposed to volatile organic compounds (VOCs) and humidity, is developed. Conventional sensor arrays consist of various receptors that produce different responses. The vapochromic MOF-based sensor comprises dicopper paddlewheel clusters and dimethylamine azobenzene as binary colorimetric sensing moieties. Upon exposure to VOCs, the constructed sensor encompasses a broad spectrum of colors, ranging from green to red. Furthermore, the color of the MOF is influenced by the solvent used during the pretreatment. Consequently, monolayered MOF thin films can be adapted to multicomponent array systems by immersing the MOF in different solvents. This system provides both qualitative and quantitative sensing, generating unique color patterns corresponding to specific VOC types. Notably, the sensor successfully discriminates each of 14 common VOCs and water and accurately categorizes unknown samples. Moreover, the system undergoes reversible color changes in response to humidity, obviating the need for high-temperature regeneration steps. This novel approach offers insights into the versatile applications of MOFs by creating a colorimetric sensor array capable of detecting various analytes.

A Rational Strategy to Assemble Porphyrins and Phthalocyanins into Crystalline Arrays: The SURMOF Approach

Realizing molecular “Designer Solids” by programmed assembly of building units taken from libraries is a very appealing objective. Recently, metal-organic frameworks (MOFs), or porous coordination polymers (PCPs) have attracted a huge interest in this context. Here, we will focus on MOF-based electrochemical, photoelectro-chemical, photovoltaic, and sensor devices built using porphyrin- and phthalocyanine-containing linkers. Internal interfaces in MOF heterostructures are also of interest with regard to photon-upconversion and the fabrication of diodes.Since the fabrication of reliable and reproducible contacts to MOF-materials represent a major challenge, we have developed a layer-by-layer (lbl) deposition method to produce well-defined, highly oriented and monolithic MOF thin films on appropriately functionalized substrates. The resulting films are referred to as SURMOFs [1,2] and have very appealing properties in particular with regard to optical applications [3]. The fabrication of hetero-multilayers is rather straightforward with this lbl method. In this talk, we will describe the principles of SURMOF fabrication as well as the results of systematic investigations of electrical and photophysical properties exhibited by empty MOFs and after loading their pores with functional guests. Band structure effects occurring in crystalline porphyrin arrays assembled using the SURMOF approach will also be discussed. [5] We will close with discussing further applications [4] realized by loading MOFs with nanoparticles or quantum dots and by creating molecular solids lacking inversion symmetry for second harmonic generation (SHG).[5]In the last part of the presentation we will point out that SURMOFs are very well suited for unattended experimentation approaches. Using robot systems controlled by machine learning (ML) algorithms allows to efficiently optimize thin film properties (orientation, crystallinity, roughness). We will discuss this point in particular in connection with the fabrication of SURMOF-based sensor systems.

MOF-composite sensors to eliminate the QCM positive frequency shift

In a quartz crystal microbalance (QCM) Sauerbrey equation can usually be applied in the ideal scenario and when the QCM frequency shift goes to the negative direction. In some cases, however, the requirements to achieve the ideal condition are not fulfilled and thus leading to QCM positive frequency shift phenomenon. Herein we have also observed that a porphyrin-based metal organic frameworks (MOF), namely MOF-525(Zn), QCM sensor also exhibits the positive frequency shift phenomenon when exposed to xylene isomers, which could be caused by the loose attachment of the MOF nanoparticles on the QCM surface. Therefore, in the presence of the analytes, the bonding between the MOF and the QCM surface is disturbed and higher analyte concentration also leads to the increase of the sensitive layer stiffness, resulting in higher positive frequency shift. However, such a phenomenon can be completely eliminated by simply adding polymer to MOF nanoparticle sensors to strengthen the bonding of MOF nanoparticles to the QCM surface. Through this study we have shown that adjusting the amount of polymer is very crucial to completely reverse the positive frequency response. Otherwise, it appears a critical analyte concentration of gas analytes where the QCM frequency shift changes its direction from negative to positive response. Lastly, the same strategy has also been used to other MOF-based QCM sensor systems where their sensitive layer is built from other MOFs, namely MOF-525 and NU-902, and ethanol is used as the analyte to prove the generalizability of this simple approach.