Monolithic scaffolds for highly selective ion sensing/removal of Co( ii ), Cu( ii ), and Cd( ii ) ions in water

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DOI: 10.1039/x0xx00000x

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Monolithic Scaffolds for Highly Selective Ion

Sensing/Removal of Co(II), Cu(II), and Cd(II) Ions in

Water

M.A. Shenashen,a Sherif A. El-Safty,a,b,* E.A. Elshehy,a

High exposure to metals, such as Cobalt (Co), Copper (Cu), and Cadmium (Cd), potentially has adverse effects and can cause severe health problems leading to a number of specific diseases. This study primarily aims to monitor, detect, separate, and remove trace concentrations of Co(II), Cu(II), and Cd(II) ions in water, without a preconcentration process using aluminosilica optical sensor (ASOS) monoliths. These monolithic scaffolds with physical features (i.e., large surface area-to-volume ratios of scaffolds, active acid sites and the uniform mesocage cubic pores) can strongly induce H-bonding and dispersive interactions with organic chelating agent, resulting in the formation of stable ASOS. In this engineering process, ASOS offers a simple and one-step sensing/capture procedure for the quantification and visual detection of target elements from water, without requiring sophisticated instruments. The key result in our study is the ion selectivity exhibited by the designed ASOS toward the target Co(II), Cu(II), and Cd(II) ions in environmental and waste disposal samples, as well as its reproducibility over a number of analysis/regeneration cycles. These findings can be useful in the fabrication of ASOS can be tailored to suit various applications.

Introduction

As the global demand for rapid industrialization, urbanization, domestic activities, agricultural activities, fuel burning, and other geological and environmental changes become increasingly prevalent, detecting, removing, and recovering unfavorable or targeted metal ions to meet environmental quality are also becoming urgent. Contamination of freshwater systems has gathered considerable interest as a major environmental threat worldwide and is associated with key environmental problems being faced by humanity.1 Water and environmental pollution as a result of various organic, inorganic, and biological pollutants has increasingly become a serious issue in the present scenario, which leads to deteriorating daily the quality of water resources because of heavy metals that are notorious water and environmental pollutants.2-4 Heavy metal ions, such as Cd(II), Cu(II), and Co(II), are non-biodegradable pollutants and can accumulate in the environment and in living tissues, causing various diseases and disorders in living organisms even at trace levels include acute and chronic metabolic disorders, renal damage and toxicity, lung disease, hepatotoxicity and carcinogenicity, damage to the respiratory system, damage the liver and kidneys, stomach ache, hypertension, paralysis, mutation of living cells, and serious diseases such as Alzheimer’s disease.5-10 Cd(II), Cu(II), and Co(II) are a common heavy metal contaminant that is released into the atmosphere through both natural and anthropogenic sources that are commonly accessed by various

industries, resulting in these metals finding its way into natural bodies of water and the environment.11-15 The disposal of heavy metal ions from urban areas to the environment is a huge problem, given that the metal ions are vulnerable to leaching by exposure to groundwater; recent regulations have emphasized the importance of storm water treatment in protecting the quality of receiving water bodies.16-18 The permissible limit in drinking water containing Cd(II), Cu(II) and Co(II) are between 3 and 5 ppb, 2 ppm and 2 ppb, respectively.19-21 With the deleterious effects of heavy metals, they are considered potential environmental pollutants even at low concentrations; therefore, heavy metals should be detected and removed from the biological system and aquatic environment as part of the remedial process. 22,23 Several technologies have demonstrated a potential for this purpose, include atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, anodic stripping voltammetry, X-ray fluorescence spectrometry, electrochemical methods and adsorption.24 Despite their versatility and growing popularity, many of these technologies are limited by their poor aqueous solubility, low cross-sensitivity, matrix interference, complicated processing, and requirement for sophisticated instruments. In response to these shortcomings, recent research has focused on developing materials with high affinity, high detection capability, simple and rapid detection process, low cost, low detection limit, and high selectivity for target metals. The adsorption process is receiving increasing

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attention as an attractive and promising technology. Assessing the environmental impact of pollutants requires reliable analytical tools that can rapidly screen them with minimal sample handling.25-28 Recent research in the field of mesoporous materials has introduced numerous approaches for widespread applications in adsorption, separation, sensing, and electrochemical techniques for the detection of toxic metal ions because of their unique properties.29-31 The optical properties of mesoporous materials are closely related to the surface-induced changes in their electronic structure. These properties have been exploited in toxic ion detection systems, which are usually based on colorimetric techniques or fluorescence. Inorganic-organic hybrid materials have received considerable attention because of their tremendous potential in determining low concentrations of analytes in different matrices as a result of their enhanced properties that contribute to the implementation of reliable and sensitive strategies.32-36 To date, various types of silica particles have been used as adsorbents of heavy metals. These particles are highly capable of sequestering toxic ions from water samples because of their large surface areas, well-defined pore structures, and tunable surface properties.32-36 The incorporation of aluminum into the framework results in the formation of acidic active sites in the materials. Controlling the local aluminum structure, structural integrity, and thermal/hydrothermal stability of aluminosilica frameworks, particularly with low Si/Al ratio, remains a significant challenge.32-36

Designing an efficient sensor/captor system requires two most important measurable qualities are selectivity and sensitivity. In recent years, the use of optical sensors is a promising approach to the formulation of simple and cost-effective protocols with high sensitivity and rapid tracking of valuable and toxic metal ions in environmental samples/systems; this approach has a direct impact on the global water crisis and is affordable for majority of the world's population.

In view of the environmental mitigation and natural resources, we developed a simple optical sensor/captor technique (ASOS) in this study based on dense immobilization layers of chelating agent into mesoporous monolithic scaffolds. This technique, with its precision, simplicity, and pH dependence, can efficiently detect and remove ultra-trace concentrations of Co(II), Cu(II), and Cd(II) ions from environmental water samples within a short processing time. In general, the 3D cubic geometries with organized nanoscale cavity and pore entrances, large surface areas, active acid networks, and large, tunable, and open pores (6 nm to 10 nm) are crucial in constructed ASOS. Enrichment of the ASOS platform facilitated the selective uptake and visual detection of target ions, as well as reduced the time required to achieve the equilibrium binding state. The ASOS design reported in this study has advantages over other sensor design technologies in terms of selectivity for multi-target Co(II), Cu(II), and Cd(II) ions. This method offers not only selective recognition, but also the segregation or extraction of target metals at ultra-trace concentrations in aqueous water samples and ultra-fast detection or removal.

Materials and methods

Chemicals

All materials were used as produced without further purification. Triblock poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) [designated as EOmPOnEOm] copolymers, tetramethyl orthosilicate (TMOS), which was used as the silica source, aluminium nitrate enneahydrate (Al(NO3)3·9H2O), 4-hydrazinobenzoic acid and carbon disulfide (CS2) were obtained from Sigma–Aldrich Company Ltd. USA. The copolymer

surfactants were Pluronic F68 (EO80PO27EO80, Mav = 8400), and Pluronic F108 (EO141PO44EO141, Mav= 14.600). The solublizing agents of hydrocarbon alkanes with different alkyl chains, such as octane and nonane (CnH2n+2), 1,3,5-trimethylbenzene (C6H3(CH3)3, or TMB), and (dicarboxylate 1-(phenylamino)-3-phenylimino-thiourea, DCPPT) were obtained from Wako Company Ltd., Osaka, Japan. The Co(II), Cu(II), and Cd(II) ion-standard solutions were obtained from Wako Company Ltd. (Osaka, Japan). Buffer solutions of 0.2 M KCl–HCl, and CH3COOH–CH3–COONa were used to adjust the pH in the 1–6 range. A mixture of 2-(cyclohexylamino) ethane sulfonic acid (CHES), 3-morpholinopropane sulfonic acid (MOPS), and N-cyclohexyl-3-aminopropane sulfonic acid (CAPS) was used to adjust the pH in the 7–12 range by using 0.2 M NaOH. MOPS and CAPS were purchased from Dojindo Chemicals, Kumamoto, Japan. Chemicals and reagents like, KCl & CH3COONa and HCl & CH3COOH were purchased from Wako Pure Chemicals, Osaka, Japan.

Fabrication design of mesoporous cubic ASOS

A family of highly ordered cubic cage aluminosilica monoliths with Pm3n, Im3m and Ia3d symmetries was synthesized by using triblock copolymers (P123, F68, and F108) in the instantly direct templating liquid-crystal phase, as recently reported (Experimental section ESI†).37,38 Mesoporous cubic ASOS were fabricated through the direct immobilization/wrapping of chelating agent (DCPPT, ethanol/10 mg) into 0.5 g of cubic monolith scaffolds. The impregnation procedure was performed under vacuum at 25 °C until probe saturation was achieved. The ethanol was removed using a gentle vacuum connected to a rotary evaporator at room temperature, leading to the direct contact of the dye probe into the monoliths. The immobilization/wrapping step was repeated several times until the equilibrium adsorption capacity of the DCPPT molecules was detected spectrophotometrically as being saturated. Upon reaching saturation, the ASOS was thoroughly washed with deionized water until no elution of the probes was observed. The washed ASOS sensor was then dried at 65 °C for 2 h.39,40

Co(II), Cu(II), and Cd(II) ion-ASOS sensing systems

The optical recognition/removal of Co(II), Cu(II), and Cd(II) ions using the ASOS sensor was performed by adding a mixture that contains specific concentrations of target ions (Co(II), Cu(II), and Cd(II) ions ) adjusted at appropriate pH of 1 to 3.5 (0.2 M of KCl with HCl and KCl/HCl/H2SO4), 4 to 6 (0.2 M CH3COOHCH3COONa with HCl), 7.01 (MOPS with NaOH), 8 (using a mixture of CHES/MOPS), 9 to 10 (0.2 M CHES with NaOH), and 11.02 to 12.50 (0.2 M CAPS with NaOH) at constant volume (20 mL) and 10mg ASOS /captor with shaking in a temperature-controlled water bath with a mechanical shaker at 25 °C for 20 min at a constant agitation speed of 120 rpm to achieve good color separation. A blank solution was also prepared, following the same procedure for comparison. After equilibration time, the solid ASOS was collected by suction through a 25 mm diameter cellulose acetate filter paper (Sibata filter holder). The collected solid of target Co(II), Cu(II), and Cd(II) ion-ASOS was estimated qualitatively by the naked eye and quantitatively by UV–vis spectrometry. Metal ion concentration was analyzed using ICP-MS before and after recognition by the ASOS. Successive measurements were performed using wide-range concentrations of standard “well-known” solutions of metal ions to ensure both accuracy and precision of the metal ion sensing systems. The calculated standard deviation of all metal ions was in the range of 0.01–0.05%. The UV–vis absorption intensities of the [M−DCPPT]n+ complex formed using both the standard solutions and target samples were compared to estimate the Co(II), Cu(II), and

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Cd(II) ions. The stability constant (log Ks) of the formed [M-(DCPPT)2]

n+ complexes with addition of Co(II), Cu(II), and Cd(II) ions to ASOS at pH 7, 9.5 and 12.5, respectively, was estimated, according to the following equation:[40] log Ks = [ML]S /[L]S * [M]; where [M] refers to the concentration of Co(II), Cu(II), and Cd(II) ions in solution that have not reacted with the DCPPT, [L] represents not only the concentration of free DCPPT but also all concentrations of DCPPT not bound to the Co(II), Cu(II), and Cd(II) ions, and the subscript S refers to the total concentration and the Co(II), Cu(II), and Cd(II) ions in the solid phase of ASOS.

Characterization analyses

The metal ion concentration was determined using a PerkinElmer Elan-6000 Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-MS). The reflectance spectrum of the solid ASOS material was recorded using UV–visible spectrometer (Shimadzu 3150, Japan). The textural surface properties of the mesoporous ASOS including the specific surface area and the pore structure were determined by N2 adsorption–desorption isotherms N2 adsorption-desorption isotherms were measured using a BELSORP MIN-II analyzer (JP. BEL Co. Ltd) at 77 K. The pore size distribution (PSD) was determined from the adsorption branch model by using nonlocal density functional theory (NLDFT). The adsorption data were analyzed using a NLDFT model applicable to spherical pores with cubic micro/mesoporous aluminosilcia monoliths. Specific surface area (SBET) was calculated using multi-point adsorption data from linear segment of the N2 adsorption isotherms using Brunauer-Emmett-Teller (BET) theory. Before the N2 isothermal analysis, all membrane samples were pre-treated at 300 oC for 8 h under vacuum until the pressure was equilibrated to 10-3 Torr. Small angle powder X-ray diffraction (XRD) patterns were measured by using a 18 kW diffractometer (Bruker D8 Advance) with monochromated CuKα radiation and with scattering reflections recorded for 2θ angles between 0.1o and 6.5o corresponding to d-spacing between 88.2 and 1.35 nm. First, the powder samples were ground and spread on a sample holder. The samples were scanned in the range from 2θ = 0.1o – 6.5o with step size of 0.02o. To confirm the resolution of the diffraction peaks with standard reproducibility in 2-theta (± 0.005), the sample measurement was recorded by using both graphite monochromator and Göbel mirror detectors. Both detectors were used to generate focusing beam geometry and parallel primary beam. The sample measurement was repeated three times under rotating at various degrees (15o, 30o and 45o). Transmission electron microscopy (TEM) and three-dimensional TEM surfaces (3D TEM) were operated by applying an acceleration voltage of 200 kV to obtain a lattice resolution of 0.1 nm and the spherical aberration of 1.0 mm. The TEM images were recorded using a CCD camera. Fourier transform diffractograms (FTD) were recorded by a slow scan charge-coupled device (CCD) camera (Gatan Model 694). 29Si MAS NMR (29SiMagic-angle spinning nuclear magnetic spectroscopy) spectra were measured using a Bruker AMX-500 spectrometer. The samples were placed in a zirconia sample tube 7 mm in diameter. Fourier transform infrared spectra (FTIR) were recorded by using an FTIR Prestige-21 (Shimadzu, Japan). Thermogravimetric and differential thermal analyses (TG and DTA, respectively) were measured using a simultaneous DTA-TG Apparatus TG-60 (Shimadzu, Japan). 27Al Magic-angle spinning nuclear magnetic spectroscopy (27Al MAS NMR) was also recorded using a Bruker AMX-500 spectrometer. 27Al NMR spectra were measured at a frequency of 125.78 MHz with a 90° pulse length of 4.7 µs. For all samples, the repetition delay was 64 s with a rotor

spinning at 4 kHz for 27Al NMR. The chemical shift scale was externally adjusted to be zero for 27Al signal by using aqueous solution (1 N) of Al(NO3)3. To investigate the acidic properties of aluminosilica samples, NH3 temperature-programmed desorption (NH3-TPD) was measured by using a BEL-Japan TPD-1S system with a quadrupole mass spectrometer. To investigate the acidic properties of aluminosilica membranes, NH3 temperature-programmed desorption (NH3-TPD) was measured by using a BEL-Japan TPD-1S system with a quadrupole mass spectrometer.

Results and Discussion

Synthetically constructed ASOS structures

Mesoporous cubic scaffolds with the synthetic construction of 3D cubic geometries with organized nanoscale cavity and pore entrances, large surface areas, active acid networks, and large, tunable, and open pores (6 nm to 10 nm) can be used effectively to trap target molecules,41,42 or heavy metal ions,43 as shown in Scheme 1. Monolithic scaffolds with ultra- or micrometer-sized particle morphology and centmetric shape particles composed of 3D cage-shaped pores (Scheme 1A, 1B) can effectively be used in adsorption applications.42 The molecular engineering model of 3D cubic network clusters may be the key to the precise detection and removal of various hazardous ions (Schemes 1E to 1H). The decoration of nanostructured inorganic oxide materials with organic moieties has been reported for the directed and controlled detection, removal, and extraction of target ions.43-45 To expand the environmental applicability of mesoporous inorganic oxide materials, the use of functionalized mesoporous aluminosilica as an optical sensor for heavy metals in aqueous solutions was explored in this study, with a specific focus on the selectivity of heavy metal (Co(II), Cu(II), and Cd(II) ions) uptake (Scheme 1D). The synthetically constructed ASOS was fabricated throughout sufficiently physisorbed “short-range” interactions (i.e., van der Waals and H-bonding interactions) between the abundant hydroxyl groups of the active surface sites of aluminosilicas and the heteroatoms of DCPPT (Scheme 1). The successful design of ASOS was evidenced by the TG-DTA analyses, FTIR spectroscopies, 27Al NMR, and NH3–TPD studies (Table S1 and Figs. S1 and S2, ESI†). These findings revealed that the structural features, natural surfaces and the active acid sites of monoliths strongly induce H-bonding and dispersive interactions with the ligand receptors, leading to the formation of stable ASOS without receptor leaching during the sensing assays of the metal ions.45

Scheme 1 The results of 3D TEM stacking, 3D TEM surfaces, FTD micrographs, and SAXS and N2 isotherm profiling indicated that the ASOS design can retain a high degree of 3D ordering, micrometric-sized particle morphology, and good distribution of organic moieties into the mesopore surfaces of ASOS architectures (Figs. 1, 2 and 3). The 3D TEM images (Figs. 1A to 1F) of mesoporous cubic Pm3n, Ia3d, and Im3m ASOS were recorded along the [111], [321], and [100] directions, respectively. The images showed uniformly sized pores arranged with cubic lattice symmetries, which demonstrated high accessibility and retention of target metal ions during the sensing/capturing responses. The 3D TEM surface (Figs. 1B, 1D, and 1F) patterns showed uniformly ordered pore arrangement in large domains with ASOS nanometric-like particles. Furthermore, FTD patterns (insets) provided details on the Pm3n, Ia3d, and Im3m symmetries according to the abundance of lattice fringes. The arrangement of planes in these specific lattice fringes along the


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[111], [321], and [100] directions proved the formation of ordered cubic Pm3n, Ia3d, and Im3m lattice symmetries, respectively. 46-49

Fig. 1.

The XRD pattern (Fig. 2) showed finely resolved Bragg diffraction peaks, which indicate highly ordered cubic structures of scaffolds and ASOS. Based on the assignment of the diffraction lines according to this XRD profile, we assigned the cubic structure to Ia3d symmetry (Fig. 2A). The lattice constant estimated from the TEM image agreed relatively well with those calculated from the XRD pattern (a = d211√6). This agreement proved that the mesostructure domains are characteristic of cubic Ia3d structures. However, the cubic Im3m monolith scaffold was widely synthesized when F108 (EO141PO44EO141) was used as a template in both the lyotropic and microemulsion systems (see ESI†). The XRD pattern (Fig. 2B) showed well-resolved diffraction lines in the region 0.9° ≤ 2θ ≤ 2.7°, indicating a high degree of cubic Im3m structural ordering with a large lattice constant of up to 9.7 nm. The lattice constant estimated from the TEM image agreed relatively well with those calculated from the XRD pattern (a = d110√2). Monoliths with primitive cubic Pm3n cage structures were fabricated based on the phase transition induced by the addition of TMB to the cubic Ia3d phase domains of F68. The XRD pattern revealed three poorly resolved high-intensity reflection peaks. Fig. (2C) illustrates the respective d spacing ratios of √4:√5:√6, which are indicative of primitive cubic phases with m3m or m3n point groups. The low resolution of these diffraction lines indicated that the XRD profile was not adequate to identify primitive cubic symmetry. However, the unique additional weak intensity peaks in the range 1° ≤ 2θ ≤ 3° were assigned to high-order cubic Pm3n structures. TEM images recorded along the [111] (Fig. 1) direction showed the presence of well-ordered pores connected to large regions of domains, which is consistent with the reported SBA-1 of cubic cage structures with the Pm3n space group.46 The interplanar distance of the [111] plane estimated from the TEM image agreed well with the d210 spacing of the cubic Pm3n diffraction pattern.

Fig. 2. Further evidence of the shape- and size-controlled cage mesostructured monoliths is the presence of N2 isotherms (Fig. 3). The N2 isotherms showed a typical type IV adsorption behavior, with a well-known sharp adsorption/desorption inflection and adsorption branches that significantly shifted toward a low relative pressure (P/P0). Moreover, the high surface area and large pore size (inset of Fig. 3) of mesostructured monoliths are considerable advantages in the fabrication of ASOS that recognize and capture ultra-trace amounts of target ions. The large type-H2 hysteresis loops and well-defined steepness of the isotherms indicate large, uniform cage structures for the scaffolds and ASOS.47-49 Both the isothermal shape and P/Po of 0.45 to 0.48 of the capillary evaporation were similar for all monolith scaffolds and ASOS, indicating that the sizes of the entrance pores were less than 4.0 nm. For ASOS cages, enlarged open-entrance pores were evident. However, the shift toward a high P/Po for the desorption isotherms indicated that other enlarged pore entrances of up to 7 nm to 9.7 nm in size were connected, without the loss of cage periodicity.48 Our results illustrated that the generation of large open-entrance pore systems was due to the expansion of copolymer micelles. This swelling action, in addition to the increase in size of both connecting pores and spherical interior cavities, resulted in a conformational change in the surface curvature of micelle to a curvature that is more compatible with the preferred mesophase structure formed according to the surfactant geometry.50-52 Furthermore, the decrease in surface area and pore volume upon immobilization of the organic moieties and metal uptake, followed by the reduction in the width of the

hysteresis loop, also proved the inclusion of the DCPPT probe and target ions in the inner core pores of the scaffolds and ASOS.

Fig. 3.

Optimization of ion-ASOS sensing system

Determining the ASOS behavior and environmental conditions that affect sensing efficiency is vital in completely understanding the sensing/removal/adsorption processes. The pH of a solution is known to affect the binding, selection, detection and removal of metal ions to the surface functional groups of ASOS. This parameter has been an emerging topic in analytical chemistry. The prominent color change and signal saturation in the reflectance spectra of captors were recorded after the response time (tR), which can be considered as a reference signal where practically no target ions exist in the original solution. The tR values of the Co(II)–, Cu(II)–, and Cd(II)–ions ASOS were sufficient to achieve a good color separation “signal” between the ASOS “blank” and the Co(II), Cu(II), or Cd(II) ion-sensing “sample”, even at low recognition or trace concentration levels of these metals. The effect of pore geometry and cage shape of monolithic scaffolds on the improved recognition of metal ions was examined by assessment the workability of the fabricated cubic Ia3d, Pm3n, and Im3m ASOS on signal responses to Co(II), Cu(II), and Cd(II) ions. In this regard, the metal-to-ligand binding kinetics with formation of the [M-DCPPT]n+ complexes was studied by continuously monitoring the UV/Vis reflectance spectra (Figure S3, ESI†) and the color change of the cubically organized Ia3d, Pm3n and Im3m cage ASOS after addition of metal ions as a function of time.53 Result showed that the time response (tR) signaling was significantly affected by the structural mesopore geometry and shape of the fabricated cubic ASOS, as clearly shown by the change in the tR value of 10-26 min. range. In solid-state ion-sensing systems, the ASOS is strongly sensitive in terms of its optical “color intensity” and signal responses toward the pH of the analyte solution. 51,54 The utilization of pH for the optical sensing and adsorption process is an emerging topic in these ion-ASOS sensing assays (Scheme 1 C- 1E).54 To address this issue, we explored the behavior of adsorption by using ASOS in aqueous solutions with different pH values (Fig. 4). The unique orbital contribution revealed high (DCPPT) activity in both central and terminal functional group binding sites (Scheme 1F). The electronegativity associated with the –SH group enabled the formation of bonds with divalent metal ions. Furthermore, the deprotonation/protonation of –NH groups had important functions in the formation of [M-DCPPT]n+ complexes. The sensitivity of the cubic ASOS in terms of its optical color intensities and signal responses to Co(II), Cu(II), and Cd(II) ions was revealed at pH levels of 7, 9.5, and 12.5, respectively. Our results showed that the pH value has a significant effect on the binding formation of metal ions to the chelating agent and on the efficiency of metal ion sensing using ASOS with remarkable ion-spectral response.

Fig. 4.

Co(II), Cu(II), and Cd(II) ion-selective ASOS

To investigate the selectivity of ASOS in the presence of interfering multi-components that may coexist with Co(II), Cu(II), or Cd(II) ions in wastewater and in the environment, the selectivity of ASOS was evaluated and the effect of adding various anions and cations with high concentration ranges on the recognition and removal of the sensor/captor systems was individually examined prior to the addition of 2 ppm concentrations of Co(II), Cu(II), or Cd(II) ions.

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This examination was conducted at the following ion-sensing conditions: pH, of 7, 9.5, and 12.5 and ASOS with the amount at 20 mg, volume at 20 mL, and temperature at 25 °C (Fig. 5). The effect of the interfering ions on the target ion-selective sensor based on ASOS was determined in two steps. The first step was initially adding the interfering cations and anions with higher tolerance concentrations than those of the target to test the effect of each interfering metal ion with and without target ions. The second step was studying the effect of the ions as a binary or a group. Considering the various coexisting multi-metal ions and target metal ions at different concentrations in waters and industrial wastewater, we examined the sorption selectivity of a given ASOS toward target metals in presence of anions (Table 1, Fig. 5 and Fig. S4, ESI†). Results obtained from the selectivity profiles of the ASOS showed no interference from [200 ppm] alkali and alkaline–earth metal. The addition of an equivalent amount of transition metals of Pb(II), Ni(II), Fe(III), Pd(II), Hg(II), Bi(III), and Zn(II) ions into ion–ASOS systems changed the color map, signal intensity, and disturbance (±3% to 5%) in the quantitative determination and removal of Co(II), Cu(II), and Cd(II) target ions, particularly at pH 7, 9.5, and 12.5, respectively (Figs. 5A to 5C). However, the addition of high doses of these actively interfering transition metal ions (i.e., 2.0 ppm) caused a high disturbance (±12%) in the removal of the target. Therefore, a 0.15 mmol/L mixture of citrate, thiosulfate, and tartrate were added to the ion-sensing/extraction assays to enhance the matrix tolerance concentrations by up to a 10-fold excess over Co(II), Cu(II), and Cd(II) target ions. Results of the effect of interfering metal ions and their effect as groups (Fig. 5 D- F) showed no significant changes in the visible color patterns and in the reflectance spectra of the ASOS at ion-selective conditions for each metal ion. The metal ions of the lanthanide series were absolutely non-competent for the ASOS. Other heavy metal ions, such as Mo(III) and Sb(III), did not interfere with the ASOS, even with a tolerance of up to a 10-fold excess over the target ions. The target ion-selectivity indicated the high capacity and affinity of target ions to bind ASOS, despite the addition of target ions in single, binary and mixture (group) systems containing interfering ions or anions. The Co(II), Cu(II) and Cd(II) ion-selectivity may be due to the high stability binding constant of target ions to sulpher and nitrogen hetero-atoms of the chelating agent and the fast response M-to-ligand binding during the removal and recognition system of ASOS, among all competitive metal ions. This finding was approved through the determination of binding constant (logKs), page 6. We found that the binding constants (log Ks) of the complexes formed with addition of Co(II), Cu(II) and Cd(II) ions were ≈ 1.5, 9.2, and 1.9 at pH 7, 9.5, and 12.5, respectively, indicating that the nitrogen (N)- and sulphur (S)-containing DCPPT ligand typically bind tightly to M2+ ions onto mesoporous cubic ASOS pores (see Scheme 1).

Fig. 5 Table 1

Concentration dependence of visual detection of ion-

ASOS

The signaling change in the reflectance spectra of the ASOS was monitored during the formation of the [M−DCPPT]n+ complexes (Fig. 6) into the ASOS platforms (Scheme 1). The reflectance spectra of the [M−DCPPT]n+ complexes were observed at a λmax at 409, 451, and 519 nm for Co(II), Cu(II), and Cd(II) ions, respectively, as a result of the uptake of Co(II), Cu(II), and Cd(II) target ions with the DCPPT ligand (Figs. 6A to 6C). The charge-

transfer reflection band of the complex was based on the ligand-binding affinity with the central metal ions during recognition and on the nature of the complex formation under specific sensing conditions.40,53,55 Moreover, the color change provided a simple procedure for the sensitivity determination and visual detection of Co(II), Cu(II), and Cd(II), without requiring sophisticated instruments (Fig. 6).56 The data revealed that the ASOS was strongly sensitive to signal responses during the optical detection of target ions at low concentrations. The rapid click sensing and the flexible Mn+-to-DCPPT binding events with the formed complexes on the organized mesopore ASOS resulted in the separation of ultra-trace concentrations from Co(II), Cu(II), and Cd(II) ions. The adsorption isotherms and the linear form of the Langmuir plot of Co(II), Cu(II), and Cd(II) target ions onto ASOS (10 g/L) at pH 7, 9.5, and 12.5, respectively, with a response time of 20 min and temperature of 25 °C are shown in Fig. S5, ESI† .57

Fig. 6

Calibration graphs and analytical parameters of ion-

ASOS

The calibration curves of the click sensing method employed by the Co(II), Cu(II), and Cd(II) ion-ASOS are shown in Fig. (7). The calibration plots generally showed a linear correlation at low concentration ranges of Co(II), Cu(II), and Cd(II) analyte ions (insets of Fig. 7). Several quantification measurements (≥10 times) were performed using a wide range of concentrations of the standard “well-known” solutions of metal ions under specific sensing conditions. The limit of detection (LOD) and the detection range (DR) of the target ions were estimated from the linear part of the calibration plot using the ASOS (Fig. 7) and according to the equation LOD = kSb/m, where k = 3, Sb is the standard deviation for the control and m is the slope of the calibration graph in the linear range.58 The DR signified the precise correlation of our experimental sensing procedure of target ion-sensing data obtained from the fabricated ASOS. The linear correlation values ranged from 8.48 × 10−3 to 0.17 µM, from 7.87 × 10−3 to 0.157 µM, and from 4.45 × 10−3 to 0.089 µM for Co(II), Cu(II), and Cd(II) ions, respectively. This result indicated that highly efficient removal and sensitivity can be achieved at low concentrations of Co(II), Cu(II), and Cd(II) ions. The capability of our ASOS assays, which were the first to achieve the sensitive, optical removal of nanomolar concentrations of Co(II), Cu(II), and Cd(II) ions, was determined by the LOD values (2.847 × 10−9, 3.12 × 10−9, and 2.464 × 10−9 mol/dm3 for Co(II), Cu(II), and Cd(II) ions, respectively), illustrating that the ASOS performed better recognition of Mn+ ions.56, 59

Fig. 7

Elution/recovery and multi-reuse studies of ion-ASOS

The use of ASOS is cost-effective sensor materials because it can be reused in multiple cycles without compromising its functionality for sensing and separation after the regeneration of the ion-ASOS. The recovery of Co(II), Cu(II) and Cd(II) ions from ASOS pore surfaces was studied using different eluting solutions or stripping agents, such as different concentrations of H2SO4, HCl, and HClO4 solutions. The adsorption of Co(II), Cu(II) and Cd(II) ions on the ASOS at the low pH region was negligible, suggesting that the high elution/release efficiency at low pH value is contributed by sufficiently high hydrogen ion concentration that leads to a strong competitive sorption. The reversibility and reusability of the ASOS was also evaluated after the elution/regeneration process for six cycles (Fig. 8). The results revealed that 0.30 M H2SO4, 0.3 M HCl,


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and 0.1 M HClO4 solutions were sufficient to extract/release the adsorbed Co(II), Cu(II), and Cd(II) ions, respectively, from the ASOS adsorbent. After elution, the adsorbent was regenerated into its initial form after rinsing with water. The adsorption capacity of target ions slightly decreased by 5.8% after four cycle operations. In addition, the adsorption capacities of Co(II), Cu(II), and Cd(II) ions were 213.96, 193.95, and 218.84 mg/g, respectively, in the first cycle. These values decreased to 196.84, 175.33, and 196.5 mg/g after six cycles (Fig. 8).

Fig. 8

Real applicability of ASOS in environmental samples

To validate the practicality of our proposed ASOS sensing approach in analyzing different target ions in environmental samples, we used our sensor/captor to detect and remove the Co(II), Cu(II), and Cd(II) ions in 50 ml samples of tap water, lake water, and mineral water at ion-selective conditions. The accuracy of the proposed procedure was investigated by conducting recovery experiments of the water samples and by adding a known quantity of the analyte to the analyzed matrices. The analyses were performed in triplicate, and the results are summarized in Table 2. Neither an ICP-MS analysis nor our sensing approach can detect the presence of any target ions in the water samples after the removal process. This result indicated that our proposed approach can be used with high efficiency for the practical sensing and removal of target ions from environmental water samples.

Table 2 In addition, the applicability of ASOS as an optical adsorbent of 0.2 ppm Cd(II) was tested in simulated environmental samples (Table 3). Metal ions, namely, Ca(II), Na(I), K(I), Mg(II), Cu(II), Zn(II), Mn(II), Fe(II),(III), and Al(III) were found in environmental sample (i.e. hospital waste water) containing biological and radioactive species, thus they were used as interference ions. The Cd(II) solution of containing the corresponding interfering ions was prepared and studied according to the ion-selective procedure (i.e. pH 12.5, 15 min, 25 oC) . Cd(II) ions were detected/removed successfully (Table 3). Potential interferences from the major matrix elements and biological species elicited minimal effects on the removal of Cd(II). The sensing/removal process indicated a detectable amount of 0.193 ppm (97.8%±1.5%). Therefore, the ASOS ensemble sensor/adsorbent is a potential candidate for the simultaneous monitoring and sorption of Cd(II) ions from the hospital waste water.

Table 3 Scheme 1. Schematic design for the mesoporous cubic ASOS fabrication, (A) ordered mesocage aluminosilica monoliths, (B) Geometrical model of mesocage aluminosilica with pore structure, (C) three-dimensional (3D) molecular structure of DCPPT probe molecule with the atomic charge distribution of each atom, (D) geometrical model of mesocage cubic ASOS after the DCPPT decoration of the inner cavity and pore surface matrix (E). (F, G and H) the formation of tetrahedral and octahedral [M−DCPPTn]n+ complexes after addition of the Co(II), Cu(II) and Cd(II) target ions to ASOS at specific pH of 7, 9.5, and 12.5, respectively. The molecular orientation of the complexes was occurred into the interior cavity and entrance pore surfaces of the mesocage ASOS, leading to significant colour changes of ASOS.

Fig. 1 Three-dimensional (3D) stacking (A, C, and E), 3D surface (B, D, and F) of TEM, and FTD of mesoporous cubic ASOS with Pm3n, Ia3d and Im3m space groups recorded along the (A,B) [111], (C,D) [321], and (E, F) [100] directions, respectively. The micrograph patterns revealed the formation of wide-range domains of 3D ordered pores along the cubic Pm3n, Ia3d and Im3m crystal lattices.

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Fig. 2 XRD of cage cubic Ia3d (A), Im3m (B), and Pm3n (C) aluminosilica monolithic scaffolds, ASOS fabricated throughout the dense immobilization of DCPPT into cage cavity and window, and the collected solid Co(II)-Pm3n-,Cu(II)- Ia3d- and Cd(II)-Im3m-ASOS after applying in the ion-sensing/capture assays .

Fig. 3 N2 isotherms of cage cubic Ia3d (A), Im3m (B), and Pm3n (C) aluminosilica monoliths, sensor/captor ASOS fabricated throughout the dense decoration of DCPPT probe into cage cavity and window, and the collected solid Co(II)-Pm3n-,Cu(II)- Ia3d- and Cd(II)-Im3m-ASOS after applying in the ion-sensing/capture assays . Inserts (A, B, and C) are BET surface area (SBET), NLDFT mesopore size (D), and pore volume (Vp).


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Fig. 4 pH-dependent response curves of ASOS during the measurement of reflectance spectra of the [M−DCPPT]n+ complexes during the detection/removal of [0.5 & 2 ppm] Co(II) (A), [0.5 & 1 ppm] Cu(II) (B), and [0.5 & 2 ppm] Cd(II) (C) at λmax of 409, 451 and 519 nm, and pH 7, 9.5 and 12.5, respectively and at a response time of 15.0 min and 25 °C. (D-F) specific analysis of the signaling response efficiency (E%, the percentage reflectance ratio of R at different pH to maximum R at significant pH signaling at 7, 9.5, and 12.5 with addition of [2ppm] Co(II) (D), [1 ppm] Cu(II) (E), and [2 ppm] Cd(II) (F) at λmax of 409, 451 and 519 nm, respectively).

Figure 5 Selectivity of the ASOS toward 2 ppm of Co(II), Cu(II), and Cd(II) ions at ion-sensing conditions: pH, of 7, 9.5, and 12.5, respectively, and ASOS with the amount at 20 mg, volume at 20 mL, and temperature at 25 °C. The Co(II), Cu(II), and Cd(II) ion-selectivity profiles were studied with addition of three additive ion-systems containing single (A-a, B-a, and C-a), binary (A-b, B-b and C-b,) and a mixture of cationic group (D-F) of additive ions. The effect of addition of competitive cations in single (i.e., addition of target or ions to ASOS, separately) and binary (i.e., addition of both target+ ion to ASOS) sensing assays on the sequential reflectance spectra signaling at λmax = 409, 451, and 519 nm of target species of Co(II)-, Cu(II)-, and Cd(II)-ion, respectively (R), in comparing with the ASOS sensor (blank, i.e., metal-free assay) (Ro) was quantitatively determined (A-C) . (D-F) The change in the reflectance spectra signaling of ion-sensing system was recorded with addition of a mixture of competitive ions to ASOS (i.e., G1-G5) compared with addition of [2ppm] target ions to ASOS at ion-sensing conditions. The group of interfering cations are listed as G1 (Na+, Mg2+, and Ca2+ ions), G2 (Se4+, Cr6+, and Sn2+), G3 (Sb3+, Al3+, and Pb2+), G4 (Fe3+, Ni2+, and Zn2+) and G5 (Hg2+, Pd2+, Zn2+and the other target ions).

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Fig. 6 Observed concentration-proportionate changes in terms of color transition profiles and reflectance spectra of cubic Ia3d-ASOS with addition of different concentrations of Co(II), Cu(II), and Cd(II) ions at pH values of 7, 9.5, and 12.5, respectively, with a solution volume of 20 mL shaken at 25 °C for 15 min.

Fig. 7 Calibration curves of the reflectance spectra of the ASOS during the determination and detection of different concentrations of Co(II), Cu(II), and Cd(II) target ions at pH 7, 9.5, and 12.5, respectively, with a response time of 20 min and temperature of 25 °C. The relationship between R−Ro and metal ion concentration, obtained by measuring the relative reflectance of the formed [M–(DCPPT)2] complex (R) relatively to a blank solid ASOS (Ro). The inset in the graph shows the low-limit colorimetric responses of Co(II), Cu(II), and Cd(II) ions with a linear fit line in the linear concentration range before saturating the calibration curve.


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Fig. 8 Reusability studies of ASOS in terms of the reflectance and efficiency changes (inserts) for the recognition and capturing properties of [2 ppm] Co(II), Cu(II), and Cd(II) target ions after six regeneration/reuse cycles. The efficiency (E%) of the ASOS was calculated from the %(Cc/Co) ratio of the concentration of Co(II), Cu(II), and Cd(II) ions (Cc/µM) per reuse cycle (No.) and the initial target ion concentration (Co/µM). The experiments were performed at 25 °C. Note: the E% was determined by ICP-MS; however, the reflectance spectra indicated the color change of the ASOS after capturing of target ions.

Table of Contents (TOC)

Optical sensor mesostructures enabled selective recognition, removal, and extraction of multiple metal ions from water samples with unique sensing properties

Conclusions

Diverse mesoporous surface functionality can be designed to adjust the sensitivity and selectivity of a sensor (ASOS) to detect, recover, and recycle target metal ions. The ASOS design can be fabricated throughout dense immobilization layers of chelating agent into mesoporous aluminosilica monolith scaffolds, leading to visual detection and effective removal of multi-target metal ions of Co(II), Cu(II), and Cd(II)] from aqueous solutions. In solid-state ASOS systems, physical features (i.e., large surface area-to-volume ratios of scaffolds, active acid sites and the uniform mesocage pores) of the ASOS enabled the synthesis of multi-particulate processing systems for the optical monitoring/detection/removal of Co(II), Cu(II), and Cd(II) ions even at trace concentrations in drinking water. The designed cage ASOS can be used to develop selective sensing/removal systems for the target ions. Thus, hindrance from actively interfering components, particularly competitive element ions, can be prevented. This finding indicated that the ion-ASOS model allows the development of a simple and effective technique for water treatment and management. Moreover, the capture-and-release process of the ASOS for target ions has an efficiency of up to 97% in controlled waste management. Notes and references a National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba-shi,

Ibaraki-ken, 05-0047, Japan. b Graduate School for Advanced Science and Engineering, Waseda

University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan.

Tel. No.: +81-298592135 Fax No.: +81-298592025

E-mail address: [email protected]; [email protected]

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† Electronic Supplementary Information (ESI) available: [details of any

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Table of Contents (TOC)

Optical sensor mesostructures enabled selective recognition, removal, and extraction of multiple

metal ions from water samples with unique sensing properties


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Table 1: Tolerance concentration for interfering matrix species during the recognition of [2.0 ppm]

Co (II), Cu(II), and Cd(II) ions by using ASOS under sensing experimental conditions of pH, 7, 9.5, and

12.5, respectively, ASOS amount of 20 mg, 20 mL volume, response time 15 min, and 25 °C

* Note: Ion-sensing system with addition of masking agents of: (a) 0.1 M mixture of citrate, thiosulfate, and

tartrate, (b) 0.1 M sodium citrate, and (b) 0.1 M sodium tartrate were used to obtain the selectivity by ASOS

sensor within the addition of the competitive interfering foreign ions to each analyte ions under the normal

sensing conditions.

Tolerance limit for foreign cations (ppm)

Target

Na(I)

Ca(II)

Mg(II)

Sn(II)

Cr(VI)

Se(IV)

Fe(III)

Al(III)

Sb(III)

Zn(II)

Ni(II)

Pb(II)

Hg(II)

Pd(II)

Cd(II)

Cu(II)

Co(II)

Co(II)

15 30 15 2.

4 7 4 3.5 9 4 3.5 5 2 1.7 5 4 5 2

Cu(II)

15 25 20 3 5.5 4 4 9 4 2.4 6 2 2 2 5 2 4

Cd(II)

17 30 22 1 10 2.4 5 11 5 5 7.5 1.6 1.5 6 2 4 4

ICP-AES analysis data for the removal of different target ions

Co(II)

15.6 29.4 15.1 2.39 7.03 3.96 3.65 9.2 4 3.46 5 1.19 1.71 4.95 3.88 3.9 0.06

Cu(II)

15.3 25 20.2 2.92 5.47 4.03 4.05 9.06 4 2.3 6 1.89 1.96 1.99 4.92 0.05

6 3.88

Cd(II)

16.5 29.7 21.8 1.02 10 2.39 5.1 11 5 4.9 7.5 1.52 1.48 5.97 0.04 3.94 3.95

ICP-AES analysis data for the recovery of different target ions

Target

Ions

Uptake

E%

Recovery

Error Target

Ions

Uptake

E%

Recovery

Error Target

Ions

Uptake

E%

Recovery

Error

Co(II)

1.94

97%

1.92

97±1

Cu(II)

1.944

97.2%

1.935

96.7±0.5

Cd(II)

1.96

98%

1.93

96.5±1.5


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Table (2): Removal of Co(II), Cu(II), and Cd(II) added to aqueous samples by 20 mg ASOS

sensor in 50 ml volume, and at pH, 7, 9.5, and 12.5, respectively, and 25 °C Aqueous matrix

Amount (ppb)

Co(II) Cu(II) Cd(II)

Recovery

E%

Error%

Recovery.

E%

Error%

Recovery.

E%

Error%

Tab water

100

250

500

97.6

248

483

97.6

99.2

96.6

-2.4

0.8

-3.4

98.66

256

501

98.66

102.4

100.2

1.34

-2.4

-0.2

99.5

246.4

488.6

99.5

98.56

97.72

-0.5

-1.44

-2.28

Mineral water

100

250

500

98.3

244.2

493

98.3

97.67

98.6

-1.7

-2.43

-1.4

98.86

246.7

494.8

98.86

98.68

98.96

-1.14

-1.32

-1.04

99.7

248.4

489.6

99.7

99.36

97.92

-0.03

-0.64

-2.08

Lake water

100

250

500

95.22

227.8

467.3

95.22

91.12

93.64

-4.78

-8.88

-6.36

101

243.5

482.3

101

97.4

96.46

1.0

-2.6

-3.54

98.3

237.7

479.2

98.3

95.08

95.84

-1.7

-4.92

-4.16


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Table (3): Adsorption of Cd(II) ions from simultaneous waste water sample by 20 mg ASOS

sensor in 50 mL solution volume, and at pH, 7, 9.5, and 12.5, respectively, and 25 °C Sam

ple

Foreign ions/(ppm)

[Cd(II)]/(ppm)

Spiked sam

ple

[Cd(II)]/(ppm)

Adsorbed

[Cd(II)]/(ppm)

Inadmissible

Adsorption E%

Sim

ultaneous

waste water

sample Ca(II)1200; Na(I) and K(I) were 4500; Mg(II) 1200;

Cu(II), and Zn(II) were 50; Mn(II) 15;

Fe(II), Fe(III), and Al(III) were 40.

0.20

0.1956 ±0.003

0.0044±0.003

97.8±1.5%


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Documents

Monolithic scaffolds for highly selective ion sensing/removal of Co( ii ), Cu( ii ), and Cd( ii ) ions in water