The design is a layered piece of molecular engineering. At its core are multi-walled carbon nanotubes, the cylindrical rolls of graphitic carbon prized for their strength, conductivity, and enormous surface area. The researchers first deposited iron oxide (Fe₃O₄) nanoparticles onto the nanotube surfaces through in-situ co-precipitation, giving the material its magnetic personality. They then converted surface carboxylic acid groups into reactive acid chlorides using thionyl chloride, and coupled those with guanine—the same nitrogen-rich nucleobase found in DNA. Finally, copper(I) iodide was introduced, with the guanine’s nitrogen donor atoms gripping the Cu(I) centers in a stable coordination environment. The result, dubbed MWCNTs/MNPs-Gu-CuI, is a hybrid material combining a conductive carbon backbone, a magnetic core, a biomolecular linker, and catalytically active copper sites all in one nanostructure.

What makes the guanine layer more than decoration is its chemistry. Nitrogen-rich heterocycles like guanine are excellent ligands for transition metals, anchoring Cu(I) ions firmly enough to prevent leaching while keeping them accessible to substrates. According to the authors, this nucleobase framework stabilizes the copper centers and facilitates substrate activation, boosting both reaction rates and selectivity. It is an elegant example of borrowing a motif from biology—nature has been using nitrogen bases to coordinate metals for billions of years—and repurposing it for industrial-scale organic synthesis.

Characterization left little doubt that the assembly worked as designed. Fourier-transform infrared spectroscopy tracked each chemical transformation, from the appearance of Fe–O stretching bands around 580 to 620 wavenumbers after magnetite deposition to the emergence of N–H and amide carbonyl signals once guanine was grafted on, and finally to subtle shifts in nitrogen-containing vibrations upon copper coordination. X-ray diffraction confirmed the coexistence of the graphitic carbon peak near 26 degrees, the hallmark reflections of Fe₃O₄, and signals attributable to Cu(I) species and the organic ligand. Thermogravimetric analysis showed a total weight loss of roughly 29 percent up to 800 degrees Celsius, leaving about 71 percent inorganic residue—a sign of substantial, thermally stable content and a robust framework.

Microscopy and magnetometry filled in the physical picture. Scanning and transmission electron micrographs revealed nanotubes densely but uniformly decorated with small spherical nanoparticles, with no significant aggregation—a direct consequence of the guanine ligand stabilizing the particles and preventing clumping. Energy-dispersive X-ray spectroscopy confirmed the expected elemental inventory of carbon, nitrogen, oxygen, iron, copper, and iodine, while particle-size analysis showed an average diameter of approximately 71.84 nanometers. Inductively coupled plasma optical emission spectroscopy measured a copper loading of 1.29 × 10⁻³ moles per gram. Vibrating sample magnetometry demonstrated soft, near-superparamagnetic behavior with negligible coercivity, meaning the material responds strongly to a magnet yet demagnetizes instantly when the field is removed—ideal for rapid, clean separation.

With the catalyst in hand, the team optimized the reaction itself. The model transformation condenses a 2-bromobenzaldehyde, acetamide, and phenylacetylene in a one-pot, three-component annulation. Screening bases revealed that potassium acetate, a mild and non-nucleophilic choice, outperformed strong inorganic bases and organic amines alike; aggressive bases such as potassium tert-butoxide collapsed the yield to 15 percent, presumably through competing elimination and substrate degradation. Solvent screening proved equally decisive. Non-polar toluene managed only 29 percent yield, while the winning combination—PEG and water in a 2:1 ratio at 100 degrees Celsius—delivered 92 percent, later rising to 96 percent with the ideal 5 mole percent catalyst loading. Beyond that loading, yields plateaued, indicating the catalytic sites were saturated.

The substrate scope is where the method flexes its synthetic muscle. Across seventeen examples, yields ranged from 86 to 96 percent, encompassing electron-donating methyl and methoxy groups, electron-withdrawing chloro and bromo substituents, and demanding heteroaromatic rings including pyridyl, furyl, and thiophenyl units. That last category matters: coordinating heterocycles often poison copper catalysts, yet the guanine-anchored system kept working, a resilience the authors attribute to the strong immobilization of the metal centers. Even sterically hindered ortho-substituted substrates and multiply substituted dichloro compounds performed well, and the surviving halogen handles offer convenient hooks for later cross-coupling chemistry—a practical boon for medicinal chemists building molecular libraries.

Mechanistically, the reaction follows a copper-mediated sequence on the catalyst surface. The terminal alkyne first coordinates to Cu(I) through π-complexation, enhancing its nucleophilicity, while the aryl bromide undergoes oxidative addition to a neighboring copper center. Reductive elimination then forges the key carbon–carbon bond, after which nucleophilic attack by the amide component and intramolecular cyclization build the isoquinoline ring. A final aromatizing dehydrogenation releases the product and regenerates the catalyst, with water as the principal byproduct. A hot-filtration test provided the crucial control: removing the solid catalyst at the 90-minute mark froze conversion at roughly 48 percent, while the unfiltered reaction climbed to 96 percent. Catalysis, in other words, genuinely happens on the solid—no leached copper species are doing the work in solution.

The sustainability credentials are equally concrete. Compared with literature protocols, the new system outpaces a palladium method requiring 24 hours at 80 degrees Celsius, a ruthenium–copper system needing 12 hours in PEG-400, and a simple CuI/NaOH recipe that takes 48 hours in water. The PEG/water medium replaces volatile, toxic organic solvents with a low-toxicity, biodegradable mixture, and the magnetic separation eliminates filtration and centrifugation entirely. After seven consecutive runs, yields declined only from 96 to 86 percent, and post-use analysis showed the crystalline phases, functional groups, and magnetic response essentially intact—saturation magnetization fell only slightly, from 45.329 to 42.218 emu per gram, with copper content barely changed at 1.26 × 10⁻³ moles per gram.

The broader significance lies in the convergence of three ideas: nucleobase-functionalized nanomaterials as precision metal anchors, magnetic carbon nanohybrids as recoverable catalytic platforms, and green solvent systems as reaction media. By fusing them, the researchers have produced what they describe as the first guanine-anchored, MWCNT-supported Cu(I) magnetic nanocatalyst for multicomponent isoquinoline synthesis. If the approach generalizes to other heterocycles and metal-catalyzed transformations, it could shift how pharmaceutical intermediates are made—away from precious metals, harsh solvents, and wasteful workups, and toward catalysts that a magnet, a bottle of PEG, and a dash of water can keep in service for run after run.

Subject of Research: Development of a guanine-functionalized MWCNT-supported Cu(I) magnetic nanocatalyst for green, recyclable synthesis of isoquinoline derivatives in PEG/water

Article Title: Guanine-functionalized MWCNT-supported Cu(I) magnetic nanocatalyst for green synthesis of isoquinolines in PEG/H₂O

Article References: Daoud, E., Kareem, H. A.-D. H., Suleman, A. D., Raj, P. B., Ganesan, S., Surya, C. P., Chopra, L., & Mohammadi, R. (2026). Guanine-functionalized MWCNT-supported Cu(I) magnetic nanocatalyst for green synthesis of isoquinolines in PEG/H₂O. Journal of Saudi Chemical Society, 30(2), Article 22. https://doi.org/10.1007/s44442-026-00070-0

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00070-0

Keywords: nanocatalysis, copper catalysis, isoquinoline synthesis, carbon nanotubes, magnetic nanoparticles, green chemistry, PEG/water solvent, guanine functionalization, multicomponent reactions, heterogeneous catalysis, catalyst recyclability, Fe3O4 nanoparticles