Covalent organic frameworks (COFs) as carrier for improved drug delivery and biosensing applications

Porous organic frameworks (POFs) represent a significant subclass of nanoporous materials in the field of materials science, offering exceptional characteristics for advanced applications. Covalent organic frameworks (COFs), as a novel and intriguing type of porous material, have garnered considerable attention due to their unique design capabilities, diverse nature, and wide-ranging applications. The unique structural features of COFs, such as high surface area, tuneable pore size, and chemical stability, render them highly attractive for various applications, including targeted and controlled drug release, as well as improving the sensitivity and selectivity of electrochemical biosensors. Therefore, it is crucial to comprehend the methods employed in creating COFs with specific properties that can be effectively utilized in biomedical applications. To address this indispensable fact, this review paper commences with a concise summary of the different methods and classifications utilized in synthesizing COFs. Second, it highlights the recent advancements in COFs for drug delivery, including drug carriers as well as the classification of drug delivery systems and biosensing, encompassing drugs, biomacromolecules, small biomolecules and the detection of biomarkers. While exploring the potential of COFs in the biomedical field, it is important to acknowledge the limitations that researchers may encounter, which could impact the practicality of their applications. Third, this paper concludes with a thought-provoking discussion that thoroughly addresses the challenges and opportunities associated with leveraging COFs for biomedical applications. This review paper aims to contribute to the scientific community's understanding of the immense potential of COFs in improving drug delivery systems and enhancing the performance of biosensors in biomedical applications.

Simultaneous electrochemical sensing of heavy metal ions (Zn2+, Cd2+, Pb2+, and Hg2+) in food samples using a covalent organic framework/carbon black modified glassy carbon electrode

In this study, for the first time, a selective electrochemical sensor by glassy carbon electrode (GCE) modified with the covalent organic framework (COF) and carbon black (CB) was introduced and applied to simultaneous sensing of Zn2+, Cd2+, Pb2+, and Hg2+ via differential pulse anodic stripping voltammetry (DPASV). The COF is supplied through a condensation reaction between melamine and trimesic acid. The COF and CB, which are used to modify the GCE surface, increase electrochemical activity. The linearity to determine ions was achieved as Zn2+: 0.009-1100 nM, Cd2+: 0.005-1100 nM, Pb2+: 0.003-1100 nM, and Hg2+: 0.001-1100 nM. Besides, the detection limits for Zn2+, Cd2+, Pb2+, and Hg2+ have obtained 0.003, 0.002, 0.001 and 0.0003 nM, respectively. The CB-COF/GCE was applied to simultaneously measure the ions in food samples. For validation, atomic absorption spectrometry (AAS) was applied to measure the amount of target metal ions as a standard method in real samples.

Cost-effective removal of Cr(VI) ions from aqueous media using L-cysteine functionalized gold nanoparticles embedded in melamine-based covalent organic framework (Cys-AuNPs@COF)

Due to the growing concern about the environmental effects of heavy metals, researchers are developing materials that possess high absorption capacity in addition to selectivity and high absorption speed. Recently, covalent organic frameworks (COFs) have been considered as emerging and promising adsorbents for the removal of many types of pollutants. In this work, a novel and selective adsorbent (Cys-AuNPs@COF) was prepared by embedding gold nanoparticles functionalized with L-cysteine in melamine-based COF for the removal of Cr(VI) ions from wastewater. The synthesized Cys-AuNPs@COF were characterizedby Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction analysis (XRD), Field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDX), Thermo-gravimetric analysis (TGA), and elemental mapping (EMA) analysis. The removal of Cr(VI) ions was performed using a batch mode process by taking advantage of response surface methodology (RSM) based on a central composite design (CCD) model. The maximum adsorption capacity of Cys-AuNPs@COF was 151.5 mg g-1. The experimental results followed the Langmuir model and showed pseudo-second-order kinetics. A portable, low-cost, and highly sensitive device with a smartphone colorimeter platform was developed for in situ measurement of trace amounts of chromium (VI) ions. Due to its simplicity and versatility, this method has the potential to serve as an alternative to conventional field analysis methods.

Aptasensor based on high surface area covalent organic framework for simple and ultrasensitive detection of sarcosine in the diagnosis of prostate cancer

In this study, an electrochemical aptasensor was developed for the specific detection of sarcosine using a covalent organic framework (COF). The imine-based two-dimensional COF was synthesized through a solvothermal process using terephthaldehyde and melamine. This resulted in the formation of a structure that is highly porous and has a unique surface area of 908 m2/g. The produced biosensor demonstrated a significant linear relationship between charge transfer resistance (Rct) and various concentrations of sarcosine in blood serum samples. The aptasensor had two linear ranges, spanning from 0.5 fM to 700 fM and 10 pM to 0.12 nM, respectively, with a detection limit of 0.15 fM. The incorporation of high surface area COFs in the aptasensor design offers a promising combination of sensitivity, stability, and specificity. This combination creates a valuable device for diagnosing and monitoring of prostate cancer and potentially other diseases.

Defective zirconium/titanium bimetallic metal-organic framework as a highly selective and sensitive electrochemical aptasensor for deoxynivalenol determination in foodstuffs

An electrochemical aptsensor for deoxynivalenol determination was successfully designed and constructed based on a defective bimetallic organic framework (denoted as ZrTi-MOF). The high porosity, large specific surface area, several structural defects, mixed metal clusters, and rich functionality of ZrTi-MOF markedly enhanced its electrochemical activity and facilitated the aptamer immobilization. As a result, the ZrTi-MOF-based aptasensor shows high sensitivity to detect deoxynivalenol via specific recognition between aptamer and deoxynivalenol, as well as the formation of aptamer-deoxynivalenol complex. On this basis, the developed ZrTi-MOF-based impedimetric aptasensor showed a low detection limit of 0.24 fg mL-1 for deoxynivalenol determination in the deoxynivalenol concentration range 1 fg mL-1- 1 ng mL-1 under optimized conditions, which also exhibited satisfactory selectivity, stability, reproducibility, and regenerability. Furthermore, determination of deoxynivalenol was achieved in bread and wheat flour samples via the developed ZrTi-MOF-based deoxynivalenol aptasensor. The result from this study showed that the ZrTi-MOF-based electrochemical aptasensor could become a promising strategy for detecting deoxynivalenol in foodstuffs in the future.

Recent Progress of Covalent Organic Frameworks Applied in Electrochemical Sensors

As an emerging porous crystalline organic material, the covalent organic frameworks (COFs) are given more and more attention in many fields, such as gas storage and separation, catalysis, energy storage and conversion, luminescent devices, drug delivery, pollutant adsorption and removal, analysis and detection due to their special advantages of high crystallinity, flexible designability, controllable porosities and topologies, intrinsic chemical and thermal stability. In recent years, the COFs are applied in analytical chemistry, for instance, chromatography, solid-phase microextraction, luminescent and colorimetric sensing, surface-enhanced Raman scattering and electroanalytical chemistry. The COFs decorated electrodes show high performance for detecting trace substances with remarkable selectivity and sensitivity, such as heavy metal ions, glucose, hydrogen peroxide, drugs, antibiotics, explosives, phenolic compounds, pesticides, disease metabolites and so on. This review mainly summarized the application of COF based electrochemical sensor according to different target analytes.

Size-controlling preparation of covalent organic framework nanospheres for electrochemical impedimetric aptasensing of oxytetracycline

Covalent organic framework (COF) nanospheres with controlled size of ∼500 and ∼1100 nm were successfully prepared by adjusting the HOAc amount in the synthetic system. The as-synthesized COFs have large conjugate aromatic skeleton, excellent stability, abundant pore, and uniform morphology. These advantages of COFs are benefit for immobilizing aptamers to fabricate the targeted electrochemical aptasensor. The commonly used oxytetracycline (OTC) is an analytic model to explore the sensing performance of the COF-based aptasensor, indicating that the smaller COF (∼500 nm) is more conducive to acquiring the sensitive sensor than that of the larger COF (∼1100 nm). Moreover, the limitation of detection of the COF (∼500 nm)-based aptasensor is calculated to be 7.4 fg mL^-1 using the response impedance signal. Additionally, the aptamer-based biosensor has fine reproducibility, good stability, excellent specificity, and available usability even in real samples.

Covalent organic framework: A state-of-the-art review of electrochemical sensing applications

Covalent organic framework (COF), a kind of porous polymer with crystalline properties, is a periodic porous framework material with precise regulation at atomic level, which can be formed by the orderly connection of pre-designed organic construction units through covalent bonds. Compared with metal-organic frameworks, COFs exhibit unique performance, including tailor-made functions, stronger load ability, structural diversity, ordered porosity, intrinsic stability and excellent adsorption features, are more conducive to the expansion of electrochemical sensing applications and the universality of applications. In addition, COFs can accurately integrate organic structural units with atomic precision into ordered structures, so that the structural diversity and application of COFs can be greatly enriched by designing new construction units and adopting reasonable functional strategies. In this review, we mainly summarized state-of-the-art recent advances of the classification and synthesis strategy of COFs, the design of functionalized COF for electrochemical sensors and COFs-based electrochemical sensing. Then, an overview of the considerable recent advances made in applying outstanding COFs to establish electrochemical sensing platform, including electrochemical sensor based on voltammetry, amperometry, electrochemical impedance spectroscopy, electrochemiluminescence, photoelectrochemical sensor and others. Finally, we discussed the positive outlooks, critical challenges and bright directions of COFs-based electrochemical sensing in the field of disease diagnosis, environmental monitoring, food safety, drug analysis, etc.

Electrochemical (Bio)Sensors Based on Covalent Organic Frameworks (COFs)

Covalent organic frameworks (COFs) are defined as crystalline organic polymers with programmable topological architectures using properly predesigned building blocks precursors. Since the development of the first COF in 2005, many works are emerging using this kind of material for different applications, such as the development of electrochemical sensors and biosensors. COF shows superb characteristics, such as tuneable pore size and structure, permanent porosity, high surface area, thermal stability, and low density. Apart from these special properties, COF’s electrochemical behaviour can be modulated using electroactive building blocks. Furthermore, the great variety of functional groups that can be inserted in their structures makes them interesting materials to be conjugated with biological recognition elements, such as antibodies, enzymes, DNA probe, aptamer, etc. Moreover, the possibility of linking them with other special nanomaterials opens a wide range of possibilities to develop new electrochemical sensors and biosensors.

Impedimetric aptasensor based on porphyrin-based covalent-organic framework for determination of diethylstilbestrol

An impedimetric sensing strategy was developed for sensitively determining diethylstilbestrol (DES) based on a platform of porphyrin-containing covalent-organic framework (p-COF). The p-COF was synthesized using 5,10,15,20-tetra (4-aminophenyl) porphyrin (TAPP) and 1,3,6,8-tetrakis(4-formylphenyl) pyrene (TFPy) as building blocks via condensation reaction, for which p-COF was named as TAPP-TFPy-COF. Considering the large specific surface area (302.9 m² g^-1), high porosity, rich nitrogen functionality, superior electrochemical activity, and strong bioaffinity toward DNA strands, the TAPP-TFPy-COF-based platform exhibited enhanced, non-label, and amplified electrochemical signal, large number of immobilized DES-targeted aptamer strands, and fast-response toward the analyte. Electrochemical results reveal that the TAPP-TFPy-COF-based aptasensor promoted the sensing performance for the detection of DES, resulting in an extremely low limit of detection of 0.42 fg mL^-1 within a DES concentration ranging from 1 fg mL^-1 to 0.1 pg mL^-1, which was substantially lower than those of most reported DES sensors. Furthermore, the TAPP-TFPy-COF-based aptasensor possessed outperformed stability, high selectivity, ascendant reproducibility, and acceptable applicability in diverse environments. The recovery values for DES detection in milk, tap water, and frozen shrimp were in the range 91.80-118.50% with low relative standard deviation of 0.11-4.26%. This work provides a new sensing electrochemical approach based on COF network for DES detection and shows a deep insight into the construction of COF-based biosensors, which can be extended to be used for other target compounds.

Recent advances in metal/covalent organic framework-based electrochemical aptasensors for biosensing applications

The booming development of novel porous materials, metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) has been attracting a lot of attention due to their designabilities, diversities, and extensive applications. MOFs and COFs provide a new potential opportunity and platform to fabricate electrochemical aptasensors for biosensing applications. Compared to other traditional materials, MOF/COF-based electrochemical biosensors can appreciably amplify the electrochemical response signals to improve the sensing performance. Herein, we provide a comprehensive overview of MOF/COF-based electrochemical aptasensors for monitoring different ultra-trace analytes (e.g. antibiotics, pesticides, and cancer markers). This review systematically discusses the classification of electrochemical aptasensors based on various functional materials, including pure MOFs, MOF/conductive composites, metal nanoparticle/MOF composites, pure COFs, COFs/conductive composites, and other hybrid materials. Furthermore, some typical MOF/COF-based electrochemical aptasensors in the recognition of specific targets are described in detail to improve and guide further research for biosensing applications.

A nanoscale genosensor for early detection of COVID-19 by voltammetric determination of RNA-dependent RNA polymerase (RdRP) sequence of SARS-CoV-2 virus

A voltammetric genosensor has been developed for the early diagnosis of COVID-19 by determination of RNA-dependent RNA polymerase (RdRP) sequence as a specific target of novel coronavirus. The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) uses an RdRP for the replication of its genome and the transcription of its genes. Here, the silver ions (Ag⁺) in the hexathia-18-crown-6 (HT18C6) were used for the first time as a redox probe. Then, the HT18C6(Ag) incorporated carbon paste electrode (CPE) was further modified with chitosan and PAMAM dendrimer-coated silicon quantum dots (SiQDs@PAMAM) for immobilization of probe sequences (aminated oligonucleotides). The current intensity of differential pulse voltammetry using the redox probe was found to decrease with increasing the concentration of target sequence. Based on such signal-off trend, the proposed genosensor exhibited a good linear response to SARS-CoV-2 RdRP in the concentration range 1.0 pM–8.0 nM with a regression equation I (μA) = − 6.555 log [RdRP sequence] (pM) + 32.676 (R² = 0.995) and a limit of detection (LOD) of 0.3 pM. The standard addition method with different spike concentrations of RdRP sequence in human sputum samples showed a good recovery for real sample analysis (> 95%). Therefore, the developed voltammetric genosensor can be used to determine SARS-CoV-2 RdRP sequence in sputum samples.