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Solid-state nanofluidic channels are promising tools for selective sensing of analytes. However, delayed interaction of recognition moieties with analytes hinders their application in rapid detection devices. In an article recently published in the journal Analytical Chemistry, the researchers fabricated two-dimensional covalent organic frameworks (2D COFs) to integrate them into solid-state nanofluidic channels and achieve selective, sensitive, and rapid recognition units that can detect the contaminants in sample analytes.

Study:  Integrating Ordered Two-Dimensional Covalent Organic Frameworks to Solid-State Nanofluidic Channels for Ultrafast and Sensitive Detection of Mercury.  Image Credit: BeataGFX/Shutterstock.com

Human health-threatening contaminants, including heavy ions, biotoxins, veterinary drugs, foodborne pathogens, and pesticides present in food and the environment call for robust analytical platforms for rapid detection of these contaminants.

Although high-performing liquid or gas chromatography-equipped mass spectrometry, electrochemical detection, and fluorescence sensing serve the purpose of analysis in real samples, more sensitive, rapid, selective, and portable analysis equipment is highly desirable.

Solid-state nanofluidic channels functionalized with a specific recognition moiety are sensitive and selective towards sensing specific analytes. To this end, nanofluidic sensors are sensitive to even a single molecule causing steady-state current changes attributed to the nanofluidic channel’s chemical change, wettability, surface charge, and diameter.

Introducing specific recognition moieties, including phenylboronic acid, aptamer, antibodies, and metal-organic frameworks (MOFs) into nanofluidic channels can improve target-specific selectivity. However, the time-delayed interaction between recognition moieties and targets limits the use of nanofluidic channel-based sensors as rapid detectors.

Thus, it is critical to improve the interaction kinetics between the recognition moieties and analytes on the nanofluidic channels. The crystalline COFs have ordered structure, permanent pores, and large surface area and are known to have enhanced kinetics for interaction with analytes, suggesting that the 2D COFs can be promising nanofluidic channels.

In the present study, the researchers designed a nanofluidic sensor based on 2D COF functionalized anodic aluminum oxide (AAO) for precise and rapid detection of contaminants. 2D thiourea-linked COF (JNU-3)-functionalized AAO ([email protected]) was fabricated by decorating the AAO’s outer surface with JNU-3 via covalent interaction.

Further, the sensitivity of the prepared nanofluidic sensor was confirmed by selecting Hg (II) as the analyte. The affinity of JNU-3 for Hg (II) promoted selectivity and kinetics in [email protected] sensors for Hg (II) recognition. These sensors were ideal for practical applications in detecting Hg contamination in water and rice samples.

Scanning electron microscope (SEM) images showed that the AAO reaction with (3-aminopropyl)-triethoxysilane (APTES) resulted in a smoother surface than the original AAO confirming the formation of AAO-amine (NH2). Subsequent reaction of AAO-NH2 with COF monomer resulted in a color change from white to dark orange indicating growth of JNU-3.

Controlling the concentration of COF monomer tuned the amount of bonded JNU-3. Increasing 1,3,5-triformylphloroglucinol (Tp) concentration from 0.05 to 0.1 mole per liter resulted in a darker color of nanochannels. SEM images revealed that with the increased concentration of Tp to 0.075 moles per liter, the obtained JNU-3 covered all the channels, and an extended increase to 0.1 moles per liter resulted in a reaction tube filled with COF that decreased the reproducibility of [email protected]

The spectra of AAO-NH2 obtained from X-ray photoelectron spectroscopy (XPS) showed peaks of nitrogen (N) 1s and silicon (Si) 2p, which depicted the introduction of the -NH2 group from APTES. Similarly, the XPS spectra of [email protected] showed a peak of sulfur (S) 2p, indicating the growth of JNU-3 on AAO-NH2.

The change in contact angle and zeta (ζ-) potential also proved the formation of [email protected] On introducing the -NH2 with a positive charge, the ζ-potential changes from −43.3 ± 1.8 to −11.8 ± 1.3 electronvolts. The in situ reaction of [email protected] with Tp and 1,4-phenylenebis-(thiourea) (Pa-S) resulted in a further drop in ζ-potential to −42.1 ± 3.1 electronvolts, and this drop is attributed to the negative charge on JNU-3.

The contact angle of [email protected] increased from 17.2 ± 1.4 to 35.1 ± 0.6 degrees due to the alkyl chain of APTES. An additional increase in contact angle to 49.5 ± 1.9 degrees was observed after the reaction of [email protected] with Tp and Pa-S, due to the lower hydrophilicity of JNU-3.

Powder X-ray diffraction (PXRD) patterns of AAO-NH2 showed no PXRD peaks. However, [email protected] revealed the presence of two characteristic peaks of JNU-3 at 3.50 and 5.99 degrees, indicating the JNU-3 crystallinity on AAO-NH2.

In summary, the researchers designed and fabricated thiourea-linked COF-functionalized AAO as a nanofluidic sensor to detect Hg (II). A [email protected] sensor was  developed in situ to recognize Hg (II). JNU-3 consists of ordered channels that greatly promote the kinetics of interaction between [email protected] and Hg (II) - this allows ultrafast detection of Hg (II) in real samples.

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Ran, X. Q., Qian, H. L., and Yan, X. P. (2022). Integrating Ordered Two-Dimensional Covalent Organic Frameworks to Solid-State Nanofluidic Channels for Ultrafast and Sensitive Detection of Mercury. Analytical Chemistry. https://pubs.acs.org/doi/abs/10.1021/acs.analchem.2c01595

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Bhavna Kaveti is a science writer based in Hyderabad, India. She has a Masters in Pharmaceutical Chemistry from Vellore Institute of Technology, India, and a Ph.D. in Organic and Medicinal Chemistry from Universidad de Guanajuato, Mexico. Her research work involved designing and synthesizing heterocycle-based bioactive molecules, where she had exposure to both multistep and multicomponent synthesis. During her doctoral studies, she worked on synthesizing various linked and fused heterocycle-based peptidomimetic molecules that are anticipated to have a bioactive potential for further functionalization. While working on her thesis and research papers, she explored her passion for scientific writing and communications.

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