Superior performance of copper based MOF and aminated graphite oxide composites as CO2 adsorbents at room temperature

CO2 Capture and Gas Storage Capacities Enhancement of HKUST-1 by Hybridization with Functionalized Graphene-like Materials

The role of graphene related material (GRM) functionalization on the structural and adsorption properties of MOF-based hybrids was deepened by exploring the use of three GRMs obtained from the chemical demolition of a nanostructured carbon black. Oxidized graphene-like (GL-ox), hydrazine reduced graphene-like (GL), and amine-grafted graphene-like (GL-NH₂) materials have been used for the preparation of Cu-HKUST-1 based hybrids. After a full structural characterization, the hybrid materials underwent many adsorption-desorption cycles to evaluate their capacities to capture CO₂ and store CH₄ at high pressure. All the MOF-based samples showed very high specific surface area (SSA) values and total pore volumes, but different pore size distributions attributed to the instauration of interactions between the MOF precursors and the specific functional groups on the GRM surface during MOF growth. All the samples showed a good affinity toward both gases (CO₂ and CH₄) and a comparable structural stability and integrity (possible aging was excluded). The trend of the maximum storage capacity values of the four MOF samples toward CO₂ and CH₄ was HKUST-1/GL-NH₂ > HKUST-1 > HKUST-1/GL-ox > HKUST-1/GL. Overall, the measured CO₂ and CH₄ uptakes were in line with or higher than those already reported in the open literature for Cu-HKUST-1 based hybrids evaluated in similar conditions.

Synergistic Effect of MIL-101/Reduced Graphene Oxide Nanocomposites on High-Pressure Ammonia Uptake

Ammonia has emerged as a potential working fluid in adsorption heat pumps (AHPs) for clean energy conversion. It would be necessary to develop an efficient adsorbent with high-density ammonia uptake under high gas pressures in the low-temperature range for waste heat. Herein, a porous nanocomposite with MIL-101(Cr)-NH₂ (MIL-A) and reduced graphene oxide (rGO) was developed to enhance the ammonia adsorption capacity over high ammonia pressures (3-5 bar) and low working temperatures (20-40 °C). A one-pot hydrothermal reaction could form a two-dimensional sheet-like nanocomposite where MIL-A nanoparticles were well deposited on the surface of rGO. The MIL-A nanoparticles were shown to grow on the rGO surface through chemical bonding between chromium metal centers in MIL-A and oxygen species in rGO. We demonstrated that the nanocomposite with 2% GO showed higher ammonia uptake capacity at 5 bar compared with pure MIL-A and rGO. Our strategy to incorporate rGO with MIL-A nanoparticles would further be generalizable to other metal-organic frameworks for improving the ammonia adsorption capacity in AHPs.

Application of Metal-Organic Framework-Based Composites for Gas Sensing and Effects of Synthesis Strategies on Gas-Sensitive Performance

Gas sensing materials, such as semiconducting metal oxides (SMOx), carbon-based materials, and polymers have been studied in recent years. Among of them, SMOx-based gas sensors have higher operating temperatures; sensors crafted from carbon-based materials have poor selectivity for gases and longer response times; and polymer gas sensors have poor stability and selectivity, so it is necessary to develop high-performance gas sensors. As a porous material constructed from inorganic nodes and multidentate organic bridging linkers, the metal-organic framework (MOF) shows viable applications in gas sensors due to its inherent large specific surface area and high porosity. Thus, compounding sensor materials with MOFs can create a synergistic effect. Many studies have been conducted on composite MOFs with three materials to control the synergistic effects to improve gas sensing performance. Therefore, this review summarizes the application of MOFs in sensor materials and emphasizes the synthesis progress of MOF composites. The challenges and development prospects of MOF-based composites are also discussed.

A Novel Strategy to Enhance the Performance of CO2 Adsorption Separation: Grafting Hyper-cross-linked Polyimide onto Composites of UiO-66-NH2 and GO

Graphene oxide (GO) is widely used to improve the pore structure, dispersion capacity, adsorption selectivity, resistance to acids and bases, and thermal stability of metal-organic frameworks (MOFs). However, it remains a daunting challenge to enhance selectivity simply by modifying the pore surface polarity and producing a suitable pore structure for CO₂ molecules through a combination of GO with MOFs. Herein, we demonstrate a novel porous hyper-cross-linked polyimide-UiO-graphene composite adsorbent for CO₂ capture via in situ chemical knitting and condensation reactions. Specifically, a network of polyimides rich in carbonyl and nitrogen atoms with amino terminations was synthesized via the reaction of 4,4'-oxydiphthalic anhydride (ODPA) and 2,4,6-trimethyl-1,3-phenylenediamine (DAM). The product plays a crucial role in the separation of CO₂ from N₂. As expected, the resulting composite (PI-UiO/GO-1) exhibited a 3-fold higher CO₂ capacity (8.24 vs 2.8 mmol·g^-1 at 298 K and 30 bar), 4.2 times higher CO₂/N₂ selectivity (64.71 vs 15.43), and significantly improved acid-base resistance stability compared with those values of pristine UiO-66-NH_2. Furthermore, breakthrough experiments verified that the porous composites can effectively separate CO₂ from simulated fuel gas (CO₂/N₂ = 15/85 vol %) with great potential in industrial applications. More importantly, this strategy can be extended to prepare other MOF-based composites. This clearly advances the development of MOF-polymer materials for gas capture.

Tale of Two Layered Semiconductor Catalysts toward Artificial Photosynthesis

The ever-increasing reliance on nonrenewable fossil fuels due to massive urbanization and industrialization created problems such as depletion of the primary feedstock and raised the atmospheric CO₂ levels causing global warming. A smart and promising approach is artificial photosynthesis that photocatalytically valorizes CO₂ into high-value chemicals. The inexpensive layered semiconductors like g-C₃N₄ and rGO or GO have the potential to make the process practically feasible for real applications. The suitable band positions with respect to the reduction potentials coupled with the typical surface properties of these layered semiconductors play a beneficial role in photoreduction of CO₂. Additionally, the creation of heterojunction interfaces to achieve the Z-scheme by anchoring g-C₃N₄ and rGO with another semiconductor with proper band alignment and dispersing plasmonic nano metals to obtain Schottky barriers on the layered surfaces also help retarding the electron-hole recombination and boost up the catalytic efficacy. Extensive exploration happened in recent years toward artificial photosynthesis over these materials, which needs a critical compendium. Surprisingly, in spite of the recent explosion of studies on photocatalytic reduction of CO₂ over metal-free semiconductors, there is not a single review on comparing the mechanistic aspects of photoreduction of CO₂ over the layered semiconductors g-C₃N₄ and rGO. This review stands out as a unique documentation, where the mechanism of photocatalytic reduction of CO₂ over this set of materials is critically examined in the context of band and surface modifications. An overall conclusion and outlook at the end indicates the need to develop prototypes for artificial photosynthesis with these well-studied semiconducting layered materials to yield solar fuels.

Graphene Oxide Protected Copper Benzene-1,3,5-Tricarboxylate for Clean Energy Gas Adsorption

Among microporous storage materials copper benzene-1,3,5-tricarboxylate (CuBTC MOF, Cu3(BTC)2 or HKUST-1) holds the greatest potential for clean energy gases. However, its usefulness is challenged by water vapor, either in the gas to be stored or in the environment. To determine the protection potential of graphene oxide (GO) HKUST1@GO composites containing 0-25% GO were synthesized and studied. In the highest concentration, GO was found to strongly affect HKUST-1 crystal growth in solvothermal conditions by increasing the pH of the reaction mixture. Otherwise, the GO content had practically no influence on the H2, CH4 and CO2 storage capacities, which were very similar to those from the findings of other groups. The water vapor resistance of a selected composite was compared to that of HKUST-1. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric (TG/DTG) and N2 adsorption techniques were used to monitor the changes in the crystal and pore structure. It was found that GO saves the copper-carboxyl coordination bonds by sacrificing the ester groups, formed during the solvothermal synthesis, between ethanol and the carboxyl groups on the GO sheets.

Multi-Metals CaMgAl Metal-Organic Framework as CaO-based Sorbent to Achieve Highly CO2 Capture Capacity and Cyclic Performance

In this study, Ca-based multi-metals metal-organic framework (CaMgAl-MOF) has been designed as precursor material for carbon dioxide (CO₂) capture to enhance the CO₂ capture capacity and stability during multiple carbonation-calcination cycles. The CaMgAl-MOFs were constructed from self-assembly of metal ions and organic ligands through hydrothermal process to make metal ions uniformly distributed through the whole structure. Upon heat treatment at 600 °C, the Ca-based multi-metals CaMgAl-MOF would gradually transform to CaO and MgO nanoparticles along with the amorphous aluminum oxide distributed in the CaO matrix. XRD, Fourier transform infrared (FTIR), and SEM were used to identify the structure and characterize the morphology. The CO₂ capture capacity and multiple carbonation-calcination cyclic tests of calcined Ca-based metal-organic framework (MOF) (attached with O and indicated as Ca-MOF-O) were performed by thermal gravimetric analysis (TGA). The single metal component calcined Ca-MOF sorbent have the highest CO₂ capture capacity up to 72 wt.%, but a lower stability of 61% due to severe particle aggregation. In contrast, a higher Ca-rich MOF oxide sorbent with tailoring the Mg/Al ratios, Ca_0.97Mg_0.025Al_0.005-MOF-O, showed the best performance, not only having the high stability of ~97%, but also maintaining the highest capacity of 71 wt.%. The concept of using Ca-based MOF materials combined with mixed-metal ions for CO₂ capture showed a potential route for achieving efficient multiple carbonation-calcination CO₂ cycles.

Carbon capture and conversion using metal–organic frameworks and MOF-based materials

This review summarizes recent advances and highlights the structure–property relationship on metal–organic framework-based materials for carbon dioxide capture and conversion.

Copper benzene-1,3,5-tricarboxylate (Cu-BTC) metal-organic framework (MOF) and porous carbon composites as efficient carbon dioxide adsorbents

The development of novel porous materials for CO₂ capture and storage has received increasing attention due to the global warming problem. The aim of this work was to develop novel composites by merging Cu-BTC framework and porous carbon materials, including ordered mesoporous non-activated carbon (OMC), ordered mesoporous activated carbon (AC), and nitrogen-containing microporous carbon (NC) as efficient adsorbents for CO₂ capture. The morphology, porosity and surface area of the parent materials and composites were fully characterized. All resulting composites were identified as microporous materials with type I adsorption isotherm. During synthesis of these composites, additional micropores were formed in the interfacial region between heterogeneous phases, which greatly enhances both their specific surface area and porosity. As compared to the parent materials, namely carbons and Cu-BTC, the CO₂ uptake capability of the composites is greatly enhanced due to the presence of micropores at the interface. Specifically, NC-Cu-BTC composite exhibited the highest CO₂ capacity with ∼8.24 and ∼4.51 mmol/g under 1 bar at 0 and 25 °C, respectively. These novel porous carbon/MOF composites may have great potential for adsorption application including CO₂ capture.

Oriented growth of cross-linked metal-organic framework film on graphene surface for non-enzymatic electrochemical sensor of hydrogen peroxide in disinfectant

High-density and cross-linked copper-based metal-organic framework (Cu-MOF) sheets were successfully prepared via a simple oriented growth method on a carboxylated graphene-modified electrode surface. Hydrogen peroxide (H₂O₂) was selected as a model molecule to examine the performance of the thin film of Cu-MOF/graphene. The proposed sensor showed an extended linear detection range from 2.00 × 10^-7 to 1.85 × 10^-4 mol L^-1 (R = 0.998), a high sensitivity of 0.792 A (mol L^-1)^-1, and a low detection limit of 6.7 × 10^-8 mol L^-1, due to the synergistic catalysis from the porous structure and favorable electron transfer mediating function of the electroactive Cu-MOFs and the high conductive property of the graphene. The reduction peak current of H₂O₂ changed less than 3.7% in the presence of 57-fold high concentrations (2.0 × 10^-4 mol L^-1) of the potential interfering species. The good selectivity of the prepared modified electrode was acquired by the size exclusion (molecular sieving) for H₂O₂ because of the proper pore shape and pore size of Cu-MOFs. The feasibility of the assay was verified by test of H₂O₂ in disinfectant samples. The proposed strategy presents valuable information related to the construction of non-enzymatic electrochemical sensors.

Hierarchically porous metal–organic frameworks with single-crystal structures and their enhanced catalytic properties

Stable hierarchically porous metal–organic frameworks (HP-MOFs) have been successfully synthesized under hydrothermal conditions using a template strategy.

A lithium-modified zirconium-based metal organic framework (UiO-66) for efficient CO2 adsorption

The adsorption mechanism of carbon dioxide (CO₂) on Li/UiO-66 was studied by an in situ DRIFTS study.

A novel luminescent hybrid material based on zinc complexes and graphene oxides for detection of Fe3+ in aqueous media

A novel GOS-modified zinc complex probe with water dispersibility is designed to detect Fe³⁺ in aqueous media (Stern–Volmer constant K_SV = 4 × 10⁶ L mol⁻¹).

Fluorescent zinc(ii)-based metal–organic frameworks for nitroaromatics sensing

A new structural metal–organic framework with fluorescence properties was synthesized and applied for sensitive and selective sensing of nitroaromatics.

Nitrogen doped RGO-Co3O4 nanograin cookies: highly porous and robust catalyst for removing nitrophenol from waste water

The fabrication of nanograins with a uniform morphology wrapped with reduced graphene oxide (RGO) in a designed manner is critical for obtaining a large surface, high porosity and efficient catalytic ability at mild conditions. Hybrid structures of metal oxides decorated on two-dimensional (2D) RGO lacked an interface and channels between the individual grains and RGO. The present work focuses on the synthesis of RGO-wrapped Co₃O₄ nanograin architecture in micron-sized polyhedrons and the ability to reduce aromatic nitro compounds. Doping N in the designed microstructure polyhedrons resulted in very large surface area (1085.6 m² g^-1) and pore density (0.47 m³ g^-1) microcages. Binding energies from x-ray photoelectron spectroscopy (XPS) and Raman intensities confirmed the presence of doped N and RGO-wrapped around Co₃O₄ nanograins. However, the morphology and microstructure was supported by FESEM and HRTEM images revealing the fabrication of high integrity RGO-Co₃O₄ microstructure hybrids composed of a 10 nm grain size with narrower grain size distribution. Ammonia treatment produced interconnected channels and dumbbell pores that facilitated ion exchange between the catalyst surface and the liquid medium at the grain boundary interfaces, and offered less mass transport resistance providing fast adsorption of reactants and desorption of the product causing surface renewal. Prepared N-RGO-Co₃O₄ shows the largest percentage reduction (96%) of p-nitrophenol (p-NP) at room temperature as compared to pure Co₃O₄ and RGO-Co₃O₄ nanograin microstructures over 10 min. Fabricated architectures can be applied effectively for fast and facile treatment of industrial waste streams with complex organic molecules.

Synthesis of Hierarchically Structured Hybrid Materials by Controlled Self-Assembly of Metal-Organic Framework with Mesoporous Silica for CO2 Adsorption

The HKUST-1@SBA-15 composites with hierarchical pore structure were constructed by in situ self-assembly of metal-organic framework (MOF) with mesoporous silica. The structure directing role of SBA-15 had an obvious impact on the growth of MOF crystals, which in turn affected the morphologies and structural properties of the composites. The pristine HKUST-1 and the composites with different content of SBA-15 were characterized by XRD, N₂ adsorption-desorption, SEM, TEM, FT-IR, TG, XPS, and CO₂-TPD techniques. It was found that the composites were assembled by oriented growth of MOF nanocrystals on the surfaces of SBA-15 matrix. The interactions between surface silanol groups and metal centers induced structural changes and resulted in the increases in surface areas as well as micropore volumes of hybrid materials. Besides, the additional constraints from SBA-15 also restrained the expansion of HKUST-1, contributing to their smaller crystal sizes in the composites. The adsorption isotherms of CO₂ on the materials were measured and applied to calculate the isosteric heats of adsorption. The HS-1 composite exhibited an increase of 15.9% in CO₂ uptake capacity compared with that of HKUST-1. Moreover, its higher isosteric heats of CO₂ adsorption indicated the stronger interactions between the surfaces and CO₂ molecules. The adsorption rate of the composite was also improved due to the introduction of mesopores. Ten cycles of CO₂ adsorption-desorption experiments implied that the HS-1 had excellent reversibility of CO₂ adsorption. This study was intended to provide the possibility of assembling new composites with tailored properties based on MOF and mesoporous silica to satisfy the requirements of various applications.

Litchi-like Fe3O4@Fe-MOF capped with HAp gatekeepers for pH-triggered drug release and anticancer effect

A novel litchi-like porous composite composed of a magnetic core, a tunable metal–organic framework (MOF) shell and a pH-sensitive hydroxyapatite (HAp) gatekeeper was successfully fabricated in this work.

Unveiling anomalous CO2-to-N2 selectivity of graphene oxide

Graphene oxide (GO) exhibits anomalous increase in CO₂-to-N₂ selectivity with temperature rise utilizing CO₂-philic functional groups and large macropores.

Highly efficient adsorbent design using a Cu-BTC/CuO/carbon fiber paper composite for high CH4/N2 selectivity

Cu-BTC/CuO/CFP, which was obtained via atomic layer deposition, has higher selectivity for CH₄/N₂, temperature uniformity, and lower pressure drop compared to Cu-BTC.

A Novel CuxO Nanoparticles@ZIF-8 Composite Derived from Core-Shell Metal-Organic Frameworks for Highly Selective Electrochemical Sensing of Hydrogen Peroxide

A novel core-shell heterostructure of CuxO nanoparticles@zeolitic imidazolate framework (CuxO NPs@ZIF-8) was successfully prepared through facile pyrolysis of a nanocrystalline copper-based metal-organic framework [nHKUST-1, i.e., Cu3(BTC)2 (BTC = 1,3,5-benzene-tricarboxylate)]@ZIF-8, based on the different thermal stability of the two metal-organic frameworks (MOFs). The small CuxO NPs derived from nHKUST-1 were uniformly dispersed inside the host material and provided active sites, while ZIF-8 kept the original structure as the molecular sieving shell. Owing to the proper pore shape and pore size of ZIF-8, H2O2 could diffuse through the shell, but bigger molecules could not pass. Thus, the composite material exhibited high selectivity when it was used to construct a H2O2 sensor. In addition, the sensor showed an extended linear detection range (from 1.5 to 21442 μM), low detection limit (0.15 μM), and high sensitivity, due to the good electrocatalysis of CuxO NPs and the synergistic effect of the core-shell structure.

Increased Thermal Conductivity in Metal-Organic Heat Carrier Nanofluids

Metal-organic heat carriers (MOHCs) are recently developed nanofluids containing metal-organic framework (MOF) nanoparticles dispersed in various base fluids including refrigerants (R245Fa) and methanol. Here, we report the synthesis and characterization of MOHCs containing nanoMIL-101(Cr) and graphene oxide (GO) in an effort to improve the thermo-physical properties of various base fluids. MOHC/GO nanocomposites showed enhanced surface area, porosity, and nitrogen adsorption compared with the intrinsic nanoMIL-101(Cr) and the properties depended on the amount of GO added. MIL-101(Cr)/GO in methanol exhibited a significant increase in the thermal conductivity (by approximately 50%) relative to that of the intrinsic nanoMIL-101(Cr) in methanol. The thermal conductivity of the base fluid (methanol) was increased by about 20%. The increase in the thermal conductivity of nanoMIL-101(Cr) MOHCs due to GO functionalization is explained using a classical Maxwell model.

Fabrication of Palladium Nanoparticles on Porous Aromatic Frameworks as a Sensing Platform to Detect Vanillin

Here, we report the fabrication of palladium nanoparticles on porous aromatic frameworks (Pd/PAF-6) using a facile chemical approach, which was characterized by various spectro- and electrochemical techniques. The differential pulse voltammetry (DPV) response of Pd/PAF-6 toward the vanillin (VA) sensor shows a linear relationship over concentrations (10-820 pM) and a low detection limit (2 pM). Pd/PAF-6 also exhibited good anti-interference performance toward 2-fold excess of ascorbic acid, nitrophenol, glutathione, glucose, uric acid, dopamine, ascorbic acid, 4-nitrophenol, glutathione, glucose, uric acid, dopamine, and 100-fold excess of Na(+), Mg(2+), and K(+) during the detection of VA. The developed electrochemical sensor based on Pd/PAF-6 had good reproducibility, as well as high selectivity and stability. The established sensor revealed that Pd/PAF-6 could be used to detect VA in biscuit and ice cream samples with satisfactory results.

Chemiluminescence determination of ascorbic acid using graphene oxide@copper-based metal–organic frameworks as a catalyst

In this work, composites with different amounts of graphene oxide (GO) and the copper-based metal–organic frameworks (HKUST-1) were synthesized.

Anionic metal–organic framework hybrids: functionalization with lanthanide ions or cationic dyes and fluorescence sensing of small molecules

An anionic metal–organic framework, [HDMA]₂[Zn₂(BDC)₃(DMA)]·6DMF is modified by lanthanides and they exhibit selective adsorption ability to cationic dyes. RhB@1 has a rapidest response and realizes sensing acetone and aniline.

Adsorption of CO2 on MIL-53(Al): FTIR evidence of the formation of dimeric CO2 species

FTIR spectra of ¹²CO₂ and ¹²CO₂ + ¹³CO₂ mixtures adsorbed on MIL-53(Al) reveal the formation of highly symmetric dimeric (CO₂)₂ species connected to two structural OH groups.

Lanthanide metal–organic frameworks as selective microporous materials for adsorption of heavy metal ions

Four microporous lanthanide metal–organic frameworks (MOFs), namely Ln(BTC)(H₂O)(DMF)_1.1 (Ln = Tb, Dy, Er and Yb, DMF = dimethylformamide, H₃BTC = benzene-1,3,5-tricarboxylic acid), have been used for selective adsorption of Pb(ii) and Cu(ii).

Interfacial Growth of Metal Organic Framework/Graphite Oxide Composites through Pickering Emulsion and Their CO₂ Capture Performance in the Presence of Humidity

We proposed an in situ interfacial growth method induced by the Pickering emulsion strategy to produce metal organic framework (MOF)/graphite oxide (GO) composites of Cu3(BTC)2/GO, in which GO was demonstrated to be a promising stabilizer for producing the Pickering emulsion and provided a large interfacial area for the in situ growth of Cu3(BTC)2 nanoparticles. When Cu3(BTC)2/GO composites were used as adsorbents for CO2 capture from the simulated humid flue gas, they showed both significantly improved thermodynamic and dynamic properties. Because most of the H2O molecules were adsorbed on the highly exfoliated GO sheets in Cu3(BTC)2/GO-m, CO2 uptake reached 3.30 mmol/g after exposure to the simulated flue gas for 60 min and remained unchanged for up to 120 min. This highlighted its potential application for real CO2 capture. More importantly, the in situ interfacial growth of nanoparticles induced by Pickering emulsions would be a promising strategy for designing and fabricating nanocomposites.

Metal-organic framework composites: from fundamentals to applications

Metal-organic frameworks (MOFs) are a class of crystallized porous polymeric materials consisting of metal ions or clusters linked together by organic bridging ligands. Due to their permanent porosity, rich surface chemistry and tuneable pore sizes, MOFs have emerged as one type of important porous solid and have attracted intensive interests in catalysis, gas adsorption, separation and storage over the past two decades. When compared with pure MOFs, the combination of MOFs with functional species or matrix materials not only shows enhanced properties, but also broadens the applications of MOFs in new fields, such as bio-imaging, drug delivery and electrical catalysis, owing to the interactions of the functional species/matrix with the MOF structures. Although the synthesis, chemical modification and potential applications of MOFs have been reviewed previously, there is an increasing awareness on the synthesis and applications of their composites, which have rarely been reviewed. This review aims to fill this gap and discuss the fabrication, properties, and applications of MOF composites. The remaining challenges and future opportunities in this field, in terms of processing techniques, maximizing composite properties, and prospects for applications, have also been indicated.

Metal-organic frameworks constructed from d-camphor acid: bifunctional properties related to luminescence sensing and liquid-phase separation

Three metal-organic frameworks (MOFs) [M2(d-cam)2(bimb)2]n · 3.5nH2O (M = Mn for 1, Co for 2) and [Cd8(d-cam)8(bimb)4]n (3) (d-H2cam = d-camphor acid, bimb = 4,4'-bis(1-imidazolyl)biphenyl), solvothermally synthesized, exhibit structural diversity. The charming aspect of these frameworks is that compound 3 is the very first MOF-based sensor for quantitatively detecting three different types of analytes (metal ions, aromatic molecules, and pesticides). And also, both compounds 2 and 3 show rapid uptake and ready regeneration for methyl orange (MO) and can selectively bind MO over methylene blue (MB) with high MO/MB separation ratio.