Modified Bimetallic Framework Nanosheets Remove Anionic Dye and Inhibit Bacteria

A team of materials chemists has synthesized two-dimensional copper and zinc-copper metal-organic framework nanosheets functionalized with silane-based amine groups, demonstrating dual functionality as recyclable dye adsorbents and antibacterial agents. Metal-organic frameworks, commonly abbreviated as MOFs, represent a class of porous crystalline coordination polymers assembled from metallic nodes and organic linkers that offer tunable chemical environments and large internal surface areas. While conventional bulk MOF structures frequently experience diffusion bottlenecks and incomplete pore accessibility during environmental remediation, exfoliating or directly growing these networks into ultrathin two-dimensional nanosheets exposes an abundant array of accessible coordination sites. In this newly reported laboratory study, investigators prepared monometallic copper-terephthalate nanosheets alongside bimetallic zinc-copper analogs via solvothermal crystallization before decorating their surfaces with 3-aminopropyltrimethoxysilane to introduce basic amine functional groups. Under bench-scale testing, the resulting hybrid nanosheets exhibited efficient uptake of the synthetic textile pollutant Direct Red 23 alongside measurable growth inhibition against the bacterium Staphylococcus aureus, establishing an integrated structural platform designed to remediate waterborne contamination.

Synthetic dyes released from manufacturing sectors such as textile finishing, leather tanning, and industrial paper processing constitute a pervasive class of waterborne contaminants that severely degrade aquatic habitats by obstructing solar radiation, elevating turbidity, and introducing potentially mutagenic organic compounds. Direct Red 23, an aromatic pollutant bearing anionic sulfonate functional groups, was selected as the representative target molecule to evaluate the removal performance of the synthesized framework nanosheets. Conventional wastewater treatments, including physical sedimentation, chemical oxidation, and biological degradation, are frequently hindered by elevated processing costs, incomplete target selectivity, and secondary sludge generation. While physical adsorption has long served as a flexible remediation strategy due to its operational simplicity, traditional porous adsorbents such as activated carbons and natural zeolites often exhibit restricted pore-size tunability and limited chemical selectivity toward complex industrial colorants. To overcome these morphological limitations, researchers have turned toward tailored two-dimensional architectures, where high surface-area-to-thickness ratios and minimized internal mass-transport resistance significantly accelerate the binding kinetics between host materials and dissolved organic guests.

The synthesis protocol developed by the research group relied on a controlled solvothermal route utilizing copper chloride dihydrate, zinc nitrate hexahydrate, and terephthalic acid dissolved in an equal-volume solvent mixture of N,N-dimethylformamide and methanol. Following initial ultrasonic dispersion to achieve complete precursor dissolution, the reaction mixtures were sealed and thermally treated at 120 degrees Celsius for 48 hours to promote two-dimensional crystal growth. After spontaneous cooling to room temperature, the collected solid products underwent centrifugal isolation, exhaustive ethanol washing cycles to purge unreacted reagents, and thermal activation at 100 degrees Celsius for six hours to evacuate guest solvent molecules trapped within the crystalline voids. Post-synthetic chemical modification was subsequently performed by dispersing the pristine nanosheets into a water-ethanol mixture and introducing 3-aminopropyltrimethoxysilane across specified framework-to-silane molar ratios of one-to-one, one-to-two, and one-to-four. Hydrolysis and surface condensation over a 24-hour reaction period allowed silane moieties to graft covalently onto surface hydroxyl groups, with subsequent characterization revealing that the one-to-two molar ratio yielded the most effective material for contaminant remediation.

Comprehensive microscopic and spectroscopic analyses verified the successful assembly and structural integrity of the parent frameworks and their amine-grafted derivatives. Scanning electron microscopy confirmed that both the monometallic Cu-BDC and the bimetallic Zn-Cu-BDC architectures adopted distinct two-dimensional sheet-like morphologies featuring an estimated average thickness of approximately 20 nanometers. The pristine copper nanosheets demonstrated lateral dimensions averaging approximately 1 micrometer, whereas the bimetallic nanosheets displayed a somewhat more compact average lateral width of roughly 500 nanometers. Transmission electron microscopy confirmed the thin, semitransparent nature of the functionalized nanosheets, indicating that surface silanization preserved the underlying two-dimensional geometry without inducing structural collapse or particle agglomeration. X-ray diffraction patterns for Cu-BDC revealed characteristic crystalline reflections at 10.14, 12.40, 13.80, 17.14, 20.38, 20.67, and 24.80 degrees two-theta, which remained intact in the bimetallic and amine-functionalized specimens alongside distinct low-angle peaks at 6.9 and 9.8 degrees corresponding to the (200) and (220) planes associated with zinc incorporation.

Fourier-transform infrared spectroscopy provided definitive chemical evidence tracking coordinate bond formation and post-synthetic grafting throughout the modification sequence. Characteristic metal-oxygen vibrational stretching bands corresponding to Cu-O and Zn-O coordination appeared at approximately 729 inverse centimeters across the spectra, demonstrating successful linkage between the metal cations and carboxylate linkers. Vibrational peaks observed at 1507, 1396, 1157, and 1013 inverse centimeters confirmed the preservation of the aromatic benzene ring structure of the terephthalate ligand, while a band near 1105 inverse centimeters reflected the C-O-Cu stretching vibration. Asymmetric and symmetric stretching vibrations of coordinated carboxylate groups appeared at 1570 and 1392 inverse centimeters, and residual coordinated dimethylformamide within framework pores generated a carbonyl peak at 1665 inverse centimeters. Following silanization, the spectra of the functionalized samples displayed new broad absorption envelopes between 3200 and 3600 inverse centimeters, marked by pronounced peaks at 3343 and 3570 inverse centimeters assigned to primary and secondary amine stretching, alongside a distinct C-N vibration near 1100 inverse centimeters confirming silane grafting.

Textural evaluation conducted through nitrogen gas sorption measurements at cryogenic temperatures revealed notable variations in surface area and pore architecture resulting from bimetallic integration. The pristine monometallic Cu-BDC nanosheets exhibited a Brunauer-Emmett-Teller specific surface area of 242.02 square meters per gram, a Langmuir surface area of 886.48 square meters per gram, an average pore diameter between 43.62 and 44.92 angstroms, and a total pore volume of 0.2683 cubic centimeters per gram. In contrast, the bimetallic Zn-Cu-BDC nanosheets displayed a reduced BET specific surface area of 157.09 square meters per gram, a Langmuir surface area of 722.43 square meters per gram, a total pore volume of 0.1945 cubic centimeters per gram, and a slightly larger average pore diameter ranging between 49.54 and 51.65 angstroms. Researchers attributed this relative decline in accessible internal surface area and pore volume to local framework distortions, decreased lattice uniformity, and partial pore blockage induced by the incorporation of zinc cations into the copper-dominated host structure.

Equilibrium adsorption experiments revealed that solution acidity exerted a decisive control over the uptake efficiency of Direct Red 23 from aqueous solutions. Evaluating dye removal across an adjusted pH range spanning from 2.1 to 10.0 demonstrated that both adsorbents achieved their maximum capacity under strongly acidic conditions before suffering substantial efficiency declines at near-neutral and basic pH levels. Specifically, the amine-modified copper framework achieved removal efficiencies of 92.1 percent at pH 2.1, 48.8 percent at pH 2.8, 45.6 percent at pH 6.0, 44.9 percent at pH 8.0, and 35.3 percent at pH 10.0. The modified zinc-copper bimetallic framework exhibited a comparable trend, recording removal values of 94.7 percent at pH 2.1, 49.6 percent at pH 2.8, 43.5 percent at pH 6.0, 42.1 percent at pH 8.0, and 33.6 percent at pH 10.0. This pronounced behavior stems from the surface protonation of grafted amine moieties into positively charged ammonium cations under acidic conditions, which fosters vigorous electrostatic attraction with the negatively charged sulfonate groups of Direct Red 23, whereas deprotonation at higher pH progressively suppresses these electrostatic interactions.

Systematic investigations examining the influence of adsorbent dosage and pollutant concentration established optimal operating regimes and kinetic characteristics under controlled benchtop conditions. Testing adsorbent quantities of 1, 2, 3, and 4 milligrams in 100 milliliters of a 10 milligram per liter dye solution revealed removal efficiencies of 58.4, 61.8, 92.1, and 95.6 percent for the modified copper framework, compared to 43.3, 50.8, 94.7, and 95.4 percent for the modified bimetallic analog. Because increasing adsorbent mass beyond 3 milligrams yielded only negligible improvements in removal percentage, 3 milligrams was identified as the practical threshold for saturated contaminant capture. Conversely, elevating initial dye concentrations from 100 to 250 milligrams per liter at fixed adsorbent mass and pH 2.1 caused removal efficiencies to decrease from 92.1 down to 29.7 percent for the copper framework, and from 94.7 down to 34.1 percent for the bimetallic framework, as available surface binding sites became progressively saturated by excess adsorbate molecules.

Mathematical modeling of the experimental sorption isotherms and time-dependent uptake profiles elucidated the governing physical mechanisms underpinning contaminant sequestering. Across equilibrium evaluations, experimental data demonstrated superior statistical correlation with the Langmuir isotherm model compared to Freundlich and Temkin equations, yielding high correlation coefficients of 0.9674 for the modified copper nanosheets and 0.9965 for the modified zinc-copper nanosheets, with comparative capacity listings reaching 3654 milligrams per gram for the bimetallic composite. This conformity indicates that Direct Red 23 molecules bind primarily through localized monolayer coverage across energetically homogeneous surface sites without lateral intermolecular interactions. Kinetic profiling over a fixed 75-minute duration demonstrated rapid initial uptake within early contact intervals that conformed closely to a pseudo-second-order kinetic model. This kinetic alignment supports the premise that chemisorptive rate-limiting processes, specifically electrostatic attraction and hydrogen bonding driven by surface amine functionalities, govern pollutant capture onto the two-dimensional framework surfaces.

The intrinsic antibacterial efficacy of the synthesized coordination frameworks was evaluated against the Gram-positive bacterium Staphylococcus aureus using standardized agar disk diffusion assays. Inoculated nutrient agar plates treated with 2-milligram framework deposits produced distinct, measurable zones of clearance after incubation, demonstrating that both the monometallic and bimetallic nanosheets actively suppressed bacterial proliferation. The pristine copper framework produced an average inhibition zone diameter of 8.4 millimeters, while the bimetallic zinc-copper framework generated an inhibition zone of 8.9 millimeters, operating closely behind a silver-containing reference material that produced a 9.3-millimeter zone of clearance. The antibacterial action is attributed to the controlled release and biointerfacial activity of divalent copper and zinc ions, which coordinate to hydroxyl groups on bacterial cell walls, compromise membrane integrity, and generate intracellular reactive oxygen species that induce fatal oxidative stress. The minor numerical difference between the copper and bimetallic samples indicated that co-incorporating zinc did not yield synergistic bactericidal enhancement.

Reusability and structural endurance assessments highlighted the practical potential and present boundaries of the functionalized bimetallic nanosheets for continuous water purification. Across five consecutive adsorption and desorption cycles, the amine-modified zinc-copper composite retained robust dye-removal capability, recording successive efficiency ratings of 94.7, 92.0, 90.0, 88.0, and 85.0 percent. Post-treatment X-ray diffraction patterns matched the crystallographic reflections of the freshly synthesized material, confirming that repetitive chemical regeneration did not compromise framework crystallinity. Nonetheless, the authors emphasized critical limitations inherent to the experimental scope, noting that tests relied exclusively on pure laboratory solutions under unvalidated, highly acidic conditions. Because practical industrial effluents contain dissolved organic foulants, fluctuating pH levels, and diverse competing ions that can disrupt electrostatic binding and suppress adsorption capacity, the reported performance metrics require independent external validation and pilot-scale trials across realistic, complex wastewater matrices before real-world engineering deployment can be realized.

Subject of Research: Bimetallic metal-organic framework nanosheets for dye adsorption and antibacterial water treatment

Article Title: Engineering bimetallic MOF nanosheets: post-modification strategy, antibacterial activity, and pollutant adsorption efficiency

Article References: Mahmoodi, B., Mahmoodi, N. M., Hayati, B., & Rabeie, B. (2026). Engineering bimetallic MOF nanosheets: post-modification strategy, antibacterial activity, and pollutant adsorption efficiency. Journal of Saudi Chemical Society, 30(6), Article 83. https://doi.org/10.1007/s44442-026-00142-1

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00142-1

Keywords: metal-organic frameworks, bimetallic MOFs, two-dimensional nanosheets, adsorption, Direct Red 23, water remediation, antibacterial activity, Staphylococcus aureus, solvothermal synthesis, post-synthetic modification, APTMS, Langmuir isotherm, pseudo-second-order kinetics, dye removal, copper-based MOFs, zinc-copper BDC, surface functionalization