Uniformly Mesocaged Cubic Fd3m Monoliths as Modal Carriers for

J. Phys. Chem. C 2008, 112, 4825-4835

4825

Uniformly Mesocaged Cubic Fd3m Monoliths as Modal Carriers for Optical Chemosensors Sherif A. El-Safty,* Adel A. Ismail, Hideyuki Matsunaga, Hiroshi Nanjo, and Fujio Mizukami Research Center for Compact Chemical Process, National Institute of AdVanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan ReceiVed: August 10, 2007; In Final Form: January 10, 2008

With recent advances in materials science and nanotechnology, development of optical chemosensors with uniformly shaped three-dimensional (3D) nanostructures applicable for large-scale sensing systems of toxic pollutants can forge new frontiers in materials. Here, highly ordered cubic Fd3m silica monoliths that had nanopore-like cages were fabricated, for the first time, by direct templating of cationic surfactant phases. This simple strategy offered significant control over the pore connectivity and structural regularity of the cubic Fd3m geometry. The potential functionalities of these uniformly sized cage cubic Fd3m materials show promise as the primary component in efficient sensing systems that can satisfy analytical needs as well, such as simplicity in fabrication design and sensing functionality in terms of selectivity and sensitivity with a fast response time of the recognition of pollutant cations. However, successful immobilization of chromophore probe molecules into the 3D network matrixes enabled manipulation of optically defined chemosensors into new shapes and functionality for visual detection of toxic analytes. Here, 3D cubic Fd3m chemosensors were developed and fabricated and successfully enabled highly revisable, selective and sensitive detection of Bi(III) target ions down to nanomolar concentrations (∼10-10 mol/dm3) with rapid response assessment (e25 s). Significantly, the HOM nanosensors not only worked under standardized conditions but also could be used for reliable sensing of the Bi(III) ion in a real-life sample such as wastewater.

Introduction The successful fabrication of materials at the nanoscale level to create regularly spaced pores and uniformly shaped dimensions has led to widespread advances in materials science, ranging from the development of optical, electronic, and photonic properties to the formation of materials with mimic, chiral, and semiconductor structures applicable to broad applications in catalysis, separation, and sensing technologies.1-3 Since the fabrication of the classic MS41 materials with pore sizes ranging from 2 to 10 nm by using cationic surfactants,4 significant effort has been expended to create new levels of hierarchical design of materials with strong control over the mesophase geometry and pore morphology by adjusting the surfactant templates and synthesis conditions.5-7 The flexibility in controlling the 3D geometrical structures with uniformly shaped cylindrical and cagelike pores is of interest for advanced adsorption and catalytic applications because the 3D pore functionality and connectivity should efficiently transport guest species to the network sites.1-3 Advances in strategies for fabricating ordered mesostructures led to recent development of the synthetically constructed face-centered cubic structure with Fd3m symmetry by using different amphiphiles in terms of character and structure over a wide range of synthesis conditions.8,9 According to the specific design strategies, spherical cages and cylindrical pores with bimodal sizes could be fabricated with cubic Fd3m structures.9 This 3D cubic Fd3m pore geometry might show particular promise for the design of optically defined chemosensors capable of sensitive determination of toxic metal ions. * To whom correspondence should be addressed. E-mail: [email protected].

Trace analysis of heavy metals is important in the chemical, environmental, and biomedical fields. Due to adverse effects of these toxic analytes on human health and the environment, both qualitative and quantitative recognitions of trace amounts of these analytes are crucial.10-15 Design of optical chemical sensors that are highly sensitive, selective, inexpensive, and capable of visual detection of toxic heavy metal ions is a rapidly developing field.11-13 Optical chemosensors with visual detection that do not require any special technique or sophisticated instruments are attractive, especially to developing countries.14 If such optical sensors can be developed within 3D cage nanostructures that have large-particle monoliths, high surface area, and uniform pores, then their application can be expanded broadly to control sensing of environmentally critical toxic species.16 In this respect, development of optical sensors for determination of toxic ions to the environment such as the Bi(III) ion target is particularly crucial. However, due to the common use of Bi(III) ions in industry, such as in semiconductors, cosmetic preparations, alloys, and metallurgical additives, and in the preparation and recycling of uranium nuclear fuels, direct determination of trace amounts of Bi(III) ions in seawater and biological treatments is urgently needed.17 In addition, bismuth compounds have been used orally for their astringent and antidiarrheal properties in a wide range of gastrointestinal tract disorders. Certain bismuth salts have antibacterial effects against Helicobacter pylori and are used particularly for ulceration and peptic gastritis. Bismuth has also been applied topically in skin disorders and anorectal disorders such as hemorrhoids.18 Although bismuth intestinal absorption is low, toxicity is possible with excessive overingestion and is related to neurotoxicity, encephalopathy, and kidney damage.18 Numerous techniques have been developed for bismuth determination such as

10.1021/jp0764283 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/11/2008

4826 J. Phys. Chem. C, Vol. 112, No. 13, 2008 inductively coupled plasma atomic emission spectroscopy and mass spectrometry (ICP-AES and ICP-MS, respectively), atomic absorption and fluorescence spectroscopy, potentiometric stripping analysis (PSA), and cathodic and anodic stripping voltammetry.19 Most of these methods require well-controlled conditions and equipment, thus leading to high operating costs.20 A major challenge, however, is the growing demand for determination of Bi(III) ions at a lower level of concentration by simple, inexpensive, and rapid assessment analyses. Here, an important practical functionality in developing efficient sensing systems of Bi(III) ions is designing the diphenylthiocarbazone (DZ) chromophore probe-doped nanoscale carrier modifications of cubic Fd3m network matrixes (HOM-11). The ordered Fd3m monolithic carriers were fabricated by a simple and rapid synthesis in which cationic surfactants were used for the first time as templates to control the design of such face-centered cubic (fcc) geometries. The sensing system of Bi(III) ions using the cubic Fd3m chemosensors was based on the binding of the DZ receptors to the Bi(III) ions, leading to color and signal changes of the receptorbased chemosensors. Although colorimetric recognition of Bi(III) ions in solution has been achieved by using the DZ or other molecular probes down to a detection limit of ∼10-8 mol/ dm3,18-21 our goals in the development of a sensing system included higher sensitivity and a simpler operating system. In addition, the cubic Fd3m chemosensors could be used as efficient preconcentrators for simultaneous detection by the naked eye for Bi(III) ions without the need for sophisticated instruments. On the basis of the results presented here, the mesocaged chemosensors showed evidence of stability, reversibility, selectivity, and sensitive detection of Bi(III) target ions down to nanomolar concentrations (∼10-10 mol/dm3) with rapid response assessment (e25 s). Experimental Procedures All materials were used as produced without further purification. DZ, tetramethylorthosilicate (TMOS), alkanes with different alkyl chain lengths (CnH2n+2), alkyltrimethylammonium bromide or chloride (CnTMA-B or -C, where n ) 14, 16, and 18, respectively) surfactants, and Bi(III) standard solution were obtained from Wako Company Ltd. (Osaka, Japan). Buffer solutions of either 0.01 M sulfuric acid or 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)ethanesulfonic acid (CHES), 3-morpholinopropanesulfonic acid (MOPS), and N-cyclohexyl-3-aminopropanesulfonic acid (CAPS) was used to adjust the pH in the 7-11 range by using 0.2 M NaOH. Synthesis of Cubic Fd3m Monolithic Carriers (HOM-11). Translucent cubic Fd3m cage monoliths were fabricated via an instant direct-templating method using a microemulsion system of cationic surfactants (CnTMA-B or -C, where n ) 14, 16, and 18) as the template. The addition of alkanes with a long alkyl chain (gC10-decane) to the hexagonal mesophase at either a CnTMA-B/TMOS or a CnTMA-C/TMOS mass ratio of 50 wt % led to the formation of cubic Fd3m phases. The use of the instant direct-templating method and the formation of the microemulsion system were previously reported.7 The typical conditions and procedure used in the synthesis of translucent cubic Fd3m monoliths at a specific CnTMA-B/TMOS ratio of 50 wt % in the microemulsion system were as follows. First, 1 g of cationic surfactants, 0.5 g of dodecane (C12-alkane), and 2 g of TMOS were mixed in a flask, yielding a milky solution. Then, 1 g of H2O/HCl (pH 1.3) was quickly added, thus forming a clear solution (i.e., homogeneous). The mass ratio of cationic

El-Safty et al. TABLE 1: Analytical Sensing Properties of the Chemosensors Based on DZ-Doped Cubic Fd3m (HOM-11)a for the Recognition of Bi(III) Target Ionsb mesocaged q, 1017D, 1010(LD), sensor mmol/g cm2/s mol/dm3 S1

0.026

3.17

S2

0.034

5.6

S3

0.033

7.75

S4(hex)c

0.024

2.9

DR, mol/dm3

109(LQ), Rt, mol/dm3 s

4.78 × 10-9

4.0

25

7.1

9.56 × 10-6 2.39 × 10-9

2.3

20

6.5

9.56 × 10-6 2.3 × 10-9

2.2

20

12

81

9.56 × 10-6 2.39 × 10-8

27

40

4.78 × 10-6 a

HOM-11 materials synthesized by using CnTMA-B or -C, where n ) 14, 16, and 18, respectively) were used to fabricate sensors S1S3. b Key: q, adsorption amount of the DZ probe molecules at saturation within the Fd3m structures; LD and LQ, limits of detection and quantification; DR, detection range; D, diffusion coefficient of Bi(III) ions; Rt, signal response time. c S4(hex) sensor based on hexagonal MCM-41 monoliths as carriers synthesized by using C18TMB as the template.

surfactant to dodecane to TMOS to H2O/HCl was 1/0.5/2/1. For all syntheses of cationic surfactants/silica cubic Fd3m mesophases, the microemulsion composition mixture domains were not aged (i.e., without static conditions while mixing). The methanol produced from the TMOS hydrolysis was removed by using a diaphragm vacuum pump connected to a rotary evaporator at 40-45 °C. Within ∼5 min, an optical gel-like solid was formed and acquired the shape and size of the reaction vessel. To obtain centimeter-sized, crack-free, and shapecontrolled silica translucent cubic Fd3m monoliths, the resultant translucent cationic surfactants/silica cubic Fd3m solid was gently dried at room temperature for 3 h and then allowed to stand in a sealed container at 40 °C for 10 h to complete the drying process (see Supporting Information S-I). The organic moieties were then removed by calcination at 450 °C for 7 h.7 Design of Mescaged Cubic Fd3m Chemosensors. Mesocaged cubic Fd3m sensors S1, S2, and S3 and hexagonal sensor S4 (Table 1) were fabricated by combinatorial immobilization of 10 mg of DZ into 0.5 g of HOM-11 mixed with 100 mL of ethanol. The ethanol was removed by a gentle vacuum connected to a rotary evaporator at room temperature. The immobilization process was repeated several times until the equilibrium adsorption capacity of DZ probe molecules reached saturation as determined via spectrophotometry. The resulting solid probe monoliths were thoroughly washed with deionized water until no elution of DZ was observed. The DZ-doped HOM-11 chemosensors were dried at 65 °C for 2 h and then ground to a fine powder (100 µm diameter particles) before being used for Bi(III) analyte detection. DZ probe with respect to adsorption time in this synthesis design was studied (see eq 1). The amount (q) of DZ probe adsorbed at saturation was calculated by eq 1,

qt ) (Co - Ct)V/m (mmol‚g-1)

(1)

where qt is the adsorbed amount at contact time t, V is the solution volume (L), m is the mass of HOM carriers (g), and Co and Ct are the initial concentration and the concentration at saturation time t, respectively (see Table 1). Recognition Procedure for the Bi(III) Ion-Sensing System. Different concentrations of Bi(III) solution ranging from 2.39 × 10-9 to 9.5 × 10-6 mol/dm3 and adjusted to pH 3.5 were

Fd3m Monoliths as Carriers for Optical Chemosensors

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Figure 1. XRD patterns (A) and N2 adsorption/desorption isotherms at 77 K (B) of highly ordered cubic Fd3m (HOM-11) silica monoliths fabricated via the instant direct-templating method with microemulsion phases formed by addition of C12-alkane (dodecane) to hexagonal phase domains of cationic surfactants (CnTMA-B or -C, where n ) 14, 16, and 18) at a template/TMOS ratio of 50 wt %. The inset lists textural parameters of calcined HOM-11 materials.

added to 4 mg of solid HOM-11 chemosensors at constant volume (20 cm3) at room temperature. Regardless of the Bi(III) concentration, the reaction between the Bi(III) ions and the DZ probe-doped HOM-11 chemosensor was completed within ∼30 s. The solid chemosensor materials were collected by suction using 25 mm diameter cellulose acetate filter paper (Sibata filter holder). The color of the collected solid sample was estimated qualitatively using visual inspection and quantitatively using UV-vis spectroscopy. In general, the Bi(III) ion concentration was analyzed by using ICP-AES before and after detection by the chemosensor. Analyses. The metal ion concentration was determined by using a Seiko SPS-1500 ICP-AES instrument. The absorbance spectrum of the solid material was recorded using a UV-vis spectrometer (Shimadzu 3150, Japan). Small-angle powder X-ray diffraction (XRD) patterns were measured by using an MXP 18 diffractometer (Mac Science Co. Ltd.) with monochromated Cu KR radiation. The textural surface properties of the HOM-11 solid materials, including the specific surface area and pore structure, were determined by measuring the N2 adsorption-desorption isotherms at 77 K by using a BELSORP36 analyzer (JP. BEL Co. Ltd.). The specific surface area (SBET) was calculated using multipoint adsorption data from the linear segment of the N2 adsorption isotherms by using Brunauer-Emmett-Teller (BET) theory. The pore size distribution was then determined from the adsorption curve of the isotherms by using nonlocal density functional theory (NLDFT). Before the N2 isothermal analysis, both the HOM-11 monolith and the DZ-HOM-11 sensor were pretreated at 100-150 °C for 8 h under vacuum until the pressure was equilibrated to 10-3 Torr. Transmission electron microscopy (TEM) images were obtained by using a JEOL transmission electron microscope (JEM-2000EXII) operated at 200 kV with a side-mounted CCD camera (Mega View III from Soft Imaging System Co.). Three-dimensional TEM (3DTEM) surfaces were obtained by using the soft imaging program of the normally recorded TEM images with a side-mounted CCD camera. “Mega View III” (Soft Imaging System Co.). Thermogravimetric and differential thermal analyses (TG and DTA, respectively) were done using a Thermo Plus TG8120 (Rigaku, Japan). Energy-dispersive

X-ray microanalysis (EDS-130S) was used to determine the elemental compositions of the functionalized HOM carriers and sensors. Results and Discussion Fabrication of Mesocaged Cubic Fd3m Monoliths. The direct-templating strategy revealed new insight into actual control of the structural geometry, mesophase morphology, and mesopore organization.6,7 When the lyotropic liquid crystalline phases of cationic surfactants (CnTMA-B or -C) were used as templates, ordered hexagonal structure MCM-41-like monoliths that have large particle grains and are crack-free, centimetersized glass were successfully fabricated (see Supporting Information S-I).4 However, the addition of hydrocarbons with a long alkyl chain (gC10-alkane) to the hexagonal phase composition domains (at template/TMOS ratios e50 wt %) led to formation of the discontinuous face-centered cubic mesophase with Fd3m symmetry, for the first time, with shape- and size-controlled cage pores, as evidenced from XRD and TEM profiles (Figures 1 and 2). However, the addition of hydrocarbons significantly affected both the diameter and surface interfacial curvature of the hexagonal micelles. These changes likely led to the cubic phase with high surface curvature preferred by these cationic surfactants, as evidenced by the formation of cubic Fd3m phases. For all HOM-11 synthesis designs with microemulsion phases of cationic surfactants, the XRD patterns of these HOM-11 monoliths (Figure 1A) showed well-resolved, sharp Bragg diffraction peaks that allowed identification of cubic structures. These unique reflection planes were characteristic of highly ordered face-centered cubic Fd3m geometry with lattice constants in the 12.9-17.2 nm range. Such 3D mesopore connectivity and geometry broaden the possible applications of these monoliths to be used as modal carriers for nanosensor design (see below).1-3 The XRD profiles (Figure 1) revealed two significant insights. First, large-domain cubic Fd3m structures with different d311 values could be formed by varying the chain length of the CnTMA-B or -C surfactants. Second, the degree of solubilization of hydrocarbons (gC10-alkane) at hexagonal phase composition domains (at 50 wt % surfactant/TMOS)

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Figure 2. Representative TEM images of highly ordered face-centered cubic Fd3m monoliths (HOM-11) recorded along the (a) [311] and (b) [211] directions. The [311] direction changed to the [210] direction (i.e., short-term stability) with increasing intergrowth time of cubic Fd3m particle crystals. Intergrowth without faceted transition was clearly evident (dashed arrows) in TEM images viewed along the (c) [110] and [111] zone axes. In general, the face-centered cubic Fd3m structure neither changed the orientational pore geometries nor led to significant formation of other cubic symmetries, indicating retention of uniformly shaped cubic Fd3m cage pores, consistent with highquality diffraction patterns.

Figure 3. TG and DTA analyses of functionalized HOM-DZ nanosensors via the direct immobilization technique into the cubic Fd3m monolithic carriers.

might act as a driving force for significant constraint effects in the molecularly geometrical structures of cationic surfactants, leading to the cubic Fd3m phase formation (see Supporting Information S-I).9,22 The N2 isotherms for HOM-11 monoliths (Figure 1B) exhibited well-defined hysteresis loops of type H2 with sharp capillary condensation of isotherms with type IV, indicating uniformly large cagelike pores.23 These loops were particularly prominent in materials synthesized by using long-chain tails (C16 and C18) of surfactants. However, the steepness of the isotherms decreased in the HOM-11 monoliths synthesized by C14TMA-B or -C, indicating the decrease in the spherical cage cavity (see Figure 1B, inset). In this synthetic design, the HOM-11 monoliths exhibited appreciable textural parameters, namely, specific surface area and pore volume, but showed no evidence of microporosity (on the basis of Rs plots). TEM micrographs (Figure 2) revealed well-organized arrays and cagelike pores

El-Safty et al. over a large area of these cubic Fd3m lattices, as recently reported with AMS-8 materials.9 These micrographs (Figure 2a,b) revealed ordered pore networks with channels running along the [311] and [211] directions of the cubic geometry.7 The image along the [110] incidence (Figure 2c) revealed spherical uniform mesocaged pores of cubic F-type structures, confirming monodisperse spherical micellar ordering in Fd3m morphological structures as previously reported.8,9 The TEM micrograph (Figure 2d) viewed along the [111] direction shows well-defined pores with no defects. In general, such representative TEM images provide direct evidence of the formation of ordered cubic cagelike structure with Fd3m symmetry.9 Design of Mesocaged Cubic Fd3m Chemosensors. The chemosensors were successfully fabricated by using the direct grafting process in which the DZ probe was immobilized into HOM-11 monoliths without previous modification of their pore surfaces. In fact, the direct grafting of DZ chromophore molecules with high adsorption capacity (q) led to the design of optical molecular sensors for simple detection of Bi(III) ions (Table 1). The functional use of the mesoporous silica HOM monoliths with large particle morphology (size g150 µm) and 3D nanoscale cubic structures as modal carriers enhanced the potential sensing of Bi(III) ions (see Supporting Information S-II). Our results revealed that the direct immobilization of the DZ probe into HOM-11 mesocaged cubic Fd3m monoliths occurred mainly through the physical “short-range” interactions (i.e., van der Waals and H-bonding interactions) between the abundant hydroxyl groups of pore surface silicates and the threecentered heteroatoms of DZ (Scheme 1). Such interactions led to the stability of mesocaged chemosensors during the washing cycle and potential sensing detection of Bi(III) ions. The variation in the loading capacity (q) of the DZ probe for nanosensors S1-S4 (Table 1) was attributed to the 3D nanoscale pores and the cage characteristics. However, we observed that the large 3D nanoscale pores of S2 and S3 had higher adsorption capacity and accessibility of the DZ probe than the 2D nanoscale sensor S4 (Table 1). Compared with indirect immobilization of a probe in which commonly used silane or thiol coupling agents were used to tune the surface polarity of HOM, SBA, and MCM silicas,24,25 the direct immobilization enabled the design of uniformly mesocaged Fd3m chemosensors without pore blockage that commonly occurred with grafting of silane and thiol moieties.26a,b Furthermore, such a direct incorporation process not only led to a strongly bound DZ probe with a higher loading capacity on the pore surfaces, but also enhanced the accessibility of the DZ probe to Bi(III) ions without any increase in kinetic hindrance compared with that of an indirect grafting process.26c,27 In fact, the ability to achieve flexibility in the specific activity of the electron acceptor/donor strength of the DZ molecular probe might lead to easy generatation and transduction of an optical color signal as a response to the DZ-Bi(III) analyte binding events, as evidenced from the cubic Fd3m chemosensor responses (Table 1).27 The loading capacity of the DZ-immobilized HOM sensors was revealed on the basis of TG-DTA techniques (Figure 3). The TG profile shows the gradual decrease in the weight of the DZ-HOM sensor up to 17 wt % from 25 to 900 °C. The TG curve indicates three distinct stages of weight loss accompanied by a DTA exothermic peak in each of the three stages. First, the weight loss of 2.5 wt % before 200 °C might indicate the evaporation of physically adsorbed H2O and the remaining ethanol with the DZ-HOM sensor. Second, the weight decrease between 200 and 700 °C is related to organic group decomposition and silanol group condensation (dehydroxylation), which



SCHEME 1: Representative Scheme of the Building Design of Chemical Nanosensors via Direct Immobilization of the Hydrophobic DZ Chromophore with Possible Interactions in the Cubic Fd3m Pore Matrixes (a) and Signal Sensing Responses and Analytical Cycles of the Nanosensors for Bi(III) Ion during the Formation of the [Bi(DZ)3] Complex (b)

corresponds to the strong exothermic DTA peaks around 200650 °C. Thus, the weight loss of 12.5 wt % between 200 and 650 °C might be attributed to the decomposition of the DZ probe, coinciding with the adsorption amount (q) of the loaded organic moieties of these materials (Table 1). Due to the use of calcined monoliths (at 450 °C for 7 h) as carriers, the weight loss from the dehydroxylation reaction was negligible. Third, further weight loss after 650 °C might be assigned further condensation of the silica species. These results from the TG analyses were consistent with elemental content results from energy-dispersive X-ray microanalysis of C, H, N, and S. On the basis of elemental analyses, the composition of CHNS with DZ-modified sensors was 12.0 mass % (see Supporting Information S-III-A). The XRD patterns (Figure 4A) showed finely resolved Bragg diffraction peaks that are characteristic of ordered cubic Fd3m chemosensors. The 3D TEM and ED patterns (Figure 4B) exhibited regular and continuous pore matrixes of the HOM sensor without distortion, indicating intrinsic mobility of the Bi(III) analyte ions with high flux and homogeneous diffusion during interaction with DZ probe molecules. Such pore surfaces of the chemosensor enable a sensitive quantification and detection of the analyte without kinetic hindrance, as evidenced from the data in Table 1.16 N2 isotherms (see Figure 4C) indicated that large amounts of the immobilized DZ probe molecule became a rigid part in the inner and outer mesocaged pore surfaces of the monoliths. Our extensive study of HOM monoliths after impregnation with the DZ probe revealed two key features. First, the decrease in the pore structural parameters with the incorporation of DZ indicated that large amounts of this probe were embedded into the inner pores; however, a significant amount might be incorporated into the outer pore surfaces. Second, uniformly shaped pore geometries and textural properties of the 3D chemosensors were attained (Figure 4). Such retention in structural integrity led to a rational design of optical chemosensors in which the Bi(III) ions were detected with a fast response time (on the order of seconds) even at nanomolar concentrations. Bi(III) Ion-Sensing Systems Based on Cubic Fd3m-DZ Nanosensors S1-S3. The binding of Bi(III) ions with the DZ probe led to a color change of the nanosensors corresponding to the formation of the Bi(III)-chelate [Bi(DZ)3] complex (Scheme 1). The results (Table 1) indicated that chemical nanosensors 1-4 offer one-step and simple sensing procedures for both quantification and visual detection of Bi(III) ions without the need for sophisticated instruments (see Table 1).

Furthermore, to develop suitable Bi(III) ion-sensing systems by the chemical nanosensors based on HOM monoliths fabricated here, we used HOM-11 with three different pore sizes and 2D hexagonal MCM-41 monoliths as DZ probe carriers. Due to the 3D functionality and connectivity of the cubic Fd3m (HOM11) carriers, the results revealed that the 3D nanosensors S1S3 exhibited high accessibility and adsorption amount (q). These featured properties led to high-affinity DZ receptor-Bi(III) ion binding events, as clearly evidenced by the significant sensing ability of 3D nanosensors S2 and S3 for Bi(III) ions, particularly at a trace level of concentrations (Table 1). Thus, we focused here on the ion-sensing utility of the 3D cubic Fd3m nanosensors S1-S3 that exhibited an excellent sensing system in their detection range (DR) and limit (LD) with fast kinetic assessment (Rt); see below. Control Bi(III) Ion-Sensing Analysis. Control sensing experiments of the chemical nanosensors S1-S3 for the Bi(III) ion were studied as a function of the HOM nanosensor amount, temperature, solution pH, and contact time, “signal response time” (Figure 5). In this study, we carried out a series of experiments to systematically define and evaluate the relative importance of these factors in a HOM-DZ chemosensor for Bi(III) ion detection. In general, the extent of the bismuth chelation with the DZ receptor was quantitatively monitored after equilibration at real-time response (i.e., gRt) in which the prominent color change and signal saturation of the complex equilibrium of DZ-Bi(III) binding were achieved (see Figure 5C). In such a quantification procedure, the response time (Rt) can be considered as a reference signal with practically no Bi(III) analyte ion remaining (see Table 1).28 Furthermore, the pH response was studied by continuously monitoring the signalsensing response of the nanosensors S1-S3 for 2.39 × 10-6 mol/dm3 Bi(III) ion at different solution pH values (from 1 to 11) and at 25 °C. The results showed that the color of the [Bi(DZ)3] complex changed when the pH was increased from 3 to 5; however, the optimum color intensity was observed at pH 3.5 (Figure 5A). In fact, the amount of adsorbed DZ probe significantly affects the ion-sensing system. Our results showed that the color of the [Bi(DZ)3] complex also depended on the amount of solid HOM-DZ nanosensor used; however, the DZ probe concentration significantly affects the formation of the [Bi(DZ)3] complex (Figure 5B). Our findings revealed that 4 mg of HOM-DZ was sufficient to achieve good color separation between the blank and the Bi(III) ion sample even at low Bi(III) concentration.


Figure 4. XRD patterns (A), 3D surfaces of the TEM micrograph and electron diffraction (ED) pattern (B, a, b) recorded along the [110] direction, and N2 isotherms (C) of mesocaged cubic Fd3m HOM-11DZ chemosensors 1-3. Despite incorporation of the large-sized DZ probe molecules (∼14 Å), structural ordering of the optical HOMDZ chemosensors was attained, consistent with the appearance of a unique diffraction plane (111) with this densely packed immobilization process of chemosensors 2 and 3 (c, d). The inset lists (A, C) give the unit lattice constant (aFd3m ) d311x11), mesopore size (P), volume (Vp), and surface area (SBET).

One-Step and Simple Ion-Sensing Procedures. To elucidate the high efficiency of the cubic Fd3m nanosensors, the determination analysis of the Bi(III) concentration was first studied in aqueous solution by using the DZ receptor as an indicator dye (see Supporting Information S-III). However, both quantitative and visual detection of Bi3+ ions was carried out at specific conditions such as a pH of 3.5, a signal response time Rt g 45 s, and a constant temperature of 25 °C. The DZ receptor shows evidence of signal-sensing responses of Bi(III) ions that were induced by the DZ-Bi(III) ion binding events.21 The binding events transduce color signaling responses (see Sup-

El-Safty et al. porting Information S-III). The UV-vis absorption of the DZ receptor in aqueous solution showed a strong Bi(III)-to-ligand charge-transfer band at 515 nm by the addition of Bi(III) target ions in the narrow quantification range of 4.78 × 10-8 to 9.56 × 10-6 mol/dm3 (see Supporting Information S-III). Although the absorbance signal of the [Bi(DZ)3] complex might indicate the color development of the DZ sensing system, the visual color transition profile of the DZ probe was slightly distinctive with addition of a Bi(III) ion concentration up to 9.5 × 10-6 mol/ dm3 (see Supporting Information S-III). Key to our developed nanosensor is that the 3D nanoscale solid materials immobilized by the chelating agent DZ could be used as highly efficient preconcentrators and chemosensors for simultaneously visual inspection and simple detection of Bi(III) ions even at trace levels (∼10-10 mol/dm3) over a wide, adjustable range.29 No elution of the probe molecules was evident with the addition of Bi(III) analyte ions during the detection process. In addition, the color change corresponding to the formation of the [Bi(DZ)3] complex (Scheme 1) provided a simple procedure for sensitive, selective detection of Bi(III) ions by the naked eye without the need for sophisticated instruments (Figure 6A).30,31 The color intensity and homogeneity increased with increasing Bi(III) ion concentration. Figure 6A shows a stable visualization “map” of the color change for a wide, adjustable Bi(III) ion concentration range between 2.39 × 10-9 and 9.5 × 10-6 mol/dm3. Colorimetric changes in the Bi(III)-sensing systems were quantitatively monitored using UV-vis spectroscopy. The reflectance spectra of cubic Fd3m-DZ chemosensors at 604 nm exhibited a blue shift to 485 nm by addition of Bi(III) ions, indicating formation of the charge-transfer [Bi(DZ)3] complex (Figure 6B).32 Significantly, colorimetric determination using UV-vis spectroscopy quantitatively validated the broad detection range (DR) of Bi(III) ions (2.39 × 10-9 to 9.5 × 10-6 mol/dm3) (Table 1). This finding shows that a high level of Bi(III) ion transport was achieved over a wide range of detection (Table 1). In turn, the large mesocaged chemosensors (S2 and S3) exhibited significant sensing ability, particularly at low concentrations of Bi(III) ions (Table 1). In view of the quantification and detection results of Bi(III) ions in both bulk solution and the nanoscale solid using the DZ receptor, different ion-sensing utility in terms of the color development and sensitivity was observed from both sensing systems. Clearly evident was that the optical properties of the DZ receptor in the nanoscale solid may change with respect to that of an aqueous solution due to the physical “short-range” interactions with the silica pore surfaces (Scheme 1). The influence of the accommodation of the DZ probe in the HOM solid was evidenced from the shift in the wavelength of the signal spectra of the DZ probe and the [Bi(DZ)3] complex in both systems (Figure 6; see also Supporting Information S-III). The high surface area, nanoscale cage cavity, and 3D pore geometry of the cubic Fd3m carriers have great influence to offer structural arrangement and organization of the entrapped DZ probe to move or reorient inside the pores. At this point, it is important to mention that the DZ probe molecular interaction with Bi(III) ions in the nanoscale solid enhanced the ion-sensing utility of the nanosensors that are in conspicuous divergence from solution chemistry (for comparison, see Figure 6 and Supporting Information S-III). Calibration Graphs and Analytical Parameters of Nanosensors. The reflectance band of the [Bi(DZ)3] complex at 485 nm was recorded after correction of the baseline of the reflection spectra between the nanosensor signal of the blank and the



Figure 5. Study of the optimal sensing conditions of the chemical nanosensor S3 for Bi(III) ion (500 ppb) by monitoring the signal spectra of the [Bi(DZ)3] complex at λ ) 485 nm as a function of the solution pH, HOM-DZ nanosensor amounts, and signal response time for both S1 and S3 at a constant temperature of 25 °C. Note that the nanosensors S1 and S2 show similar optimal pH and amount values during the study of the recognition of Bi(III) ion at a batch equilibrated time (Rt g 20 s) and at 25 °C.

Figure 6. Representative concentration-dependent changes of mesocaged cubic Fd3m chemosensor S2 in the color map (A) and in UV-vis reflection spectra at λ ) 485 nm (B) with addition of various concentrations of Bi(III) ions. The signal responses of chemosensor S2 were monitored at specific sensing conditions (pH 3.5, 4.0 mg, 20 s, and 25 °C).

concentration-dependent signal for Bi(III) ions. A linear calibration in the Bi(III) ion concentration range from 2.39 × 10-9 to 4.44 × 10-7 mol/dm3 with a correlation coefficient of 0.998 was characteristic of the calibration curve for the cubic Fd3m chemosensors S1-S3 (Figure 7). Due to saturation effects, however, a nonlinear correlation at the inflection point was observed with high Bi(III) ion concentration (g4.44 × 10-7 mol/dm3).24 The nonlinear curves indicated that the Bi(III) analyte can be detected with the highest sensitivity at low concentrations. In fact, the quality of the calibration methods is necessary to ensure both accuracy and precision of the Bi(III) ion-sensing system. Several quantification measurements

(g10 times) were carried out using wide-range concentrations (2.39 × 10-9 to 10-5 mol/dm3) of the standard “well-known” solutions of Bi(III) ions at the specific sensing conditions (Figure 7A,b,B,b). The calculated standard deviation for the analysis of Bi(III) ions using all nanosensors S1-S3 was in the range of 0.1-0.3%, as evidenced for the fitting plot of the calibration graphs. In addition, the total adsorption capacity of the DZ probe (Table 1), for example, q ) 0.33 mmol/g with S3, indicated that the adsorbed amount of DZ was 13.2 × 10-7 mol/4 mg of HOM material used. Therefore, derived from the deviation from linearity at the inflection point in this calibration curve (Figure


El-Safty et al.

Figure 7. Calibration curves of the chemosensors S3 (A) and S1 (B) for the Bi(III) ion. The plots represent Ac - Ab vs [Bi(III)], where Ac is the absorbance of the [Bi(DZ)3] complex at a specific Bi(III) concentration and Ab is the absorbance of the chemosensor S3 and S1 “blank” at λ ) 485 nm. The fitting of the calibration plots of the chemosensors S3 (A, b) and S1 (B, b) was depicted from the mean of 10 successive measurements for a wide range (2.3 × 10-10 to 9.5 × 10-6 mol/dm3) of Bi(III) ion concentration at specific conditions of pH 3.5, 4.0 mg, 30 s, and 25 °C.

7), the stoichiometry of the [Bi-DZ]n+ complex was 1/3 (Bi(III)/DZ) for chemosensors S1-S4. Further evidence of the stoichiometric Bi(III)-DZ reaction was revealed from Job’s plot (data not shown) in which changes in the absorbance of the [Bi(DZ)3] complex formation in solution under our experimental conditions were monitored. The results indicated a 3/1 binding for DZ with the Bi(III) ion (Scheme 1).21 The stability constant (log Ks) of the formed [Bi(DZ)3] complex in the nanosensors at a specific pH was estimated as 14.0 according to the following equation:

log Ks ) [ML]S/[L]S[M]

(2)

where [M] refers to the concentration of Bi(III) ions in solution that have not reacted with the chelating agent, [L] represents not only the concentration of free DZ ligand but also the concentration of all ligands not bound to the metal, and the subscript S refers to the total concentration of the species in the solid phase.24 The results from the binding constant of the [Bi(DZ)3] complex indicated that two nitrogen-chelating groups of the DZ ligand typically bind tightly to Bi(III) ions in the octahedral [Bi(DZ)3] complex at a solution pH of 3.5. In general, the high Bi(III)-to-receptor binding affinity was due to the intrinsic mobility of the DZ-based nanosensor solid to efficiently bind the Bi(III) ion (Scheme 1). The detection (LD) and quantification (LQ) limits of Bi(III) ions by using the mesocaged Fd3m chemosensors were estimated from the linear part of the calibration plot (Figure 7),24,33 according to eqs 3 and 4,

LD ) k1Sb/m

(3)

LQ ) k2Sb/m

(4)

where the constants k1 and k2 are equal to 3 and 10, respectively, Sb is the standard deviation for the blank, and m is the slope of the calibration graph in the linear range (Figure 7). The calculated relative standard deviation of LD was 0.046%. The LD value (Table 1) indicated that the cubic Fd3m chemosensor enabled an effective detection of Bi(III) target ions up to a

concentration of ∼6 × 10-10 mol/dm3 by using this simple sensing system. Furthermore, LQ signifies the precise correlation of our experimental sensing procedure of Bi(III) ion-sensing data obtained from the fabricated nanosensors (see Table 1). Utility of the Nanosensor for Ion-Sensing Systems. The response time (Rt) of the metal ion-sensing system is a practical parameter to determine the quality of the mesocaged Fd3m chemosensor designs. The response time of the ion-sensing systems was an indicator of the reaction kinetics of the metalto-ligand [Bi(DZ)3] complex formation and the diffusion of the Bi(III) ion into the 3D nanoscale pores of the nanosensors. Here, the reaction kinetics of the [Bi(DZ)3] complex formation with nanosensors S1 and S3 was studied by monitoring the reflectance spectra at λ ) 485 nm (as shown in Figure 5C). The result showed that the charge transfer between the Bi(III) ions and the DZ probe was accomplished within a short time (Rt e 25). Despite the low or even high concentration of Bi(III) ions used, the real-time response remained constant (Table 1), indicating the time-independent absorption enhancement with the Bi(III) analyte concentration.34 This result is inconsistent with the accumulative effect of the analyte concentration in any dosimetric system in which the metal-to-ligand chemical reactions were induced by the target analyte (time-dependent in most cases).35 Thus, the chemical nanosensors were characteristics of the Bi(III) ion-sensing systems. Furthermore, the diffusion coefficient (D) of the intraparticle transport of the Bi(III) ion into the spherical cavity of the cage nanosensors was calculated (Table 1) using eq 5,36 where R is the radius of the nanosensors

D ) 0.03R2/t1/2

(5)

and t1/2 is the time for half a signal response (see Figure 5C). The results indicated that the open, uniform pore-cage architectures allowed efficient binding and diffusion of Bi(III) ions to the DZ probe. The high metal flux, namely, ion transport, and the affinity of the Bi(III)-DZ binding were significantly affected by the 3D pore geometry and shape, as evidenced from the D value of the nanosensors S1-S4 (Table 1). To check the reversibility of the chemosensor design-based mesocaged cubic Fd3m carriers with multiple regeneration/reuse



TABLE 2: Textural Parameters and Sensing Features of Optical Nanosensors S1-S3 after Several Regeneration/ Reuse Cycles and during the Recognition of the Bi(III) Ion Target mesocaged sensor S1 S2 S3

cycle no.

SBET, m2/g

V p, cm3/g

P, nm

Rt, s

E,a %

1 2 3 1 2 3 1 2 3

970 930 900 970 950 930 700 690 690

0.48 0.45 0.45 0.79 0.76 0.77 0.7 0.67 0.65

2.3 2.3 2.3 2.8 2.8 2.9 3.2 3.2 3.3

30 32 35 25 30 32 25 27 32

98 96 94 98 97 96 99 97 96

a The efficiency of the Fd3m sensors within the regeneration/reuse cycles was estimated on the basis of sensor sensitivity during detection of Bi(III) ions compared with the original data in Table 1.

cycles of Bi(III) ions, we used the stripping agent ClO4- anions at 0.1 mol/L concentration to effectively remove the Bi(III) ions (i.e., decomplexation) after a complete detection process.34,37 After the liquid-exchange process was repeated several times, the cubic Fd3m chemosensor was collected and washed by deionized H2O. The UV-vis reflectance spectra of cubic Fd3m-DZ chemosensors at λ ) 604 nm were measured and revealed no change in the signal intensity. The regenerated sensors exhibited interesting sensitive behavior as follows (Table 2). The effective quantification of the Bi(III) ion after 3-4 regeneration/reuse cycles slightly decreased (∼6%), indicating reversible sensing systems (Scheme 1). The retention of the nanosensor efficiency, to high extent, with the severe liquidexchange process indicated that the chemosensor described the recognition of the Bi(III) ion-sensing system. To understand the differences in the chemosensor efficiency depending on the cycle, textural parameters were examined for sensors after the reuse cycles (Table 2). The results revealed that all of the chemosensors experienced a slight reduction in surface area and pore volume, in particular. As a consequence of the regeneration/ reuse cycle process, the effective binding of the Bi(III) target ions to the DZ probe functional sites might become slightly restricted due to the substantial influence of the stripping agents upon cycling and therefore might affect the utility of the chemosensor in further detection. Elemental analyses (data not shown) of the regenerated chemosensors (>3 times) revealed no significant decrease in the probe adsorption amount, indicating no elution of the immobilized DZ probe by the stripping agent or by binding of the Bi(III) ion. This result indicated the potential stability of the chemosensor during the cycle use, as evidenced from the color and absorbance changes with addition of Bi(III) ions. However, the stripping agent mainly affected the deactivation “degradation” behavior of the specific activity of the probe functional group, as evidenced from the slight increase of the signal response time (Rt) with increasing number of regeneration/reuse cycles (Table 2). Although the response time of such regenerated sensors was generally influenced by the extensive cycling process, the binding and signaling remained relatively fast, on the order of seconds, and fully revisable. Selectivity of Nanosensors for the Bi(III) Ion. A major advantage of the mesocaged cubic Fd3m chemosensors is the ability to create selective sensing systems, thus preventing hindrance from actively interfering components such as anions and cations (Figures 8 and 9) that otherwise might make differentiation of the Bi(III) target ions difficult.30-34,37 To investigate the selectivity of chemosensors in the presence of interfering multicomponents which might coexist with the Bi-

Figure 8. Study of the ion selectivity of the cubic Fd3m nanosensor S3 for the Bi(III) ion by illustration of the effect of addition of cations and anions and surfactants as interference ions (1-14) on the reflectance spectra of the [Bi(DZ)3] complex formed during recognition of Bi(III) ions at 2.39 × 10-6 mol/dm3 at λ ) 485 nm. Note that the interference ions were added to the sensing systems prior to the addition of the Bi(III) ion at specific sensing conditions (pH 3.5, 20 s, and 25 °C). The additive cations from left to right (1-14) were Cd2+ and Ni2+ (200 ppm), Cr3+ and Al3+ (225 ppm), Ca2+ and Mg2+ (300 ppm), Sb2+, Co2+, and Zn2+ (150 ppm), Pb2+ and Hg2+ (100 ppm), Fe3+ and Cu2+ (40 ppm), and Sn2+ (50 ppm), respectively. The additive anion and surfactant species from left to right (1-14) were PO43-, SO42-, and SO32- (100 ppm), NO3- and CO32- (175 ppm), C6H5O73-, C2O42-, and C8H4O42- (125 ppm), Cl- and CH3COO- (250 ppm), NO2- (25 ppm), Triton X100 (25 ppm), SDS (50 ppm), and CTAB (300 ppm), respectively.

Figure 9. Illustration of the ion selectivity of the cubic Fd3m nanosensor S2 for the Bi(III) ion by studying the effect of the interfering species of cations (A) and anions and surfactants (B) on the color density of the [Bi(DZ)3] complex during the naked-eye detection of Bi(III) at a concentration of 2.39 × 10-6 mol/dm3.

(III) ion in wastewater and the environment, we quantitatively (Figure 8) and qualitatively (Figure 9) studied the effect of the addition of high concentrations of various anions, surfactants, and cations prior to the addition of Bi(III) ions of 2.39 × 10-6 mol/dm3 concentration in the recognition sensing systems at a specific pH condition of 3.5. The signal response and visual detection of the nanosenors for 2.39 × 10-6 mol/dm3 Bi(III) ion in the presence of the extraneous ions was monitored at a batch equilibrated time of 40 s and at an ambient temperature of 25 °C (Figures 8 and 9). The results showed slight changes in either the color map or the signal intensity of the [Bi(DZ)3] complex during the addition of various types of anions and cations (Figures 8 and 9) as effective disturbance species at concentrations up to 600 and 800 times higher, respectively, than that of the Bi(III) ions, confirming the selective sensing

4834 J. Phys. Chem. C, Vol. 112, No. 13, 2008 systems of Bi(III) analyte ions. Although the slight effects of some extraneous ions such as Fe(III) and Cu(II) cations, Triton X100 surfactant, and NO2- anion were found with a permissible tolerance limit of (5%, the nanosensors were selective for Bi(III) target ions even at nanomolar-level recognition. Applicability of the Nanosensor for Real-Sample Analysis. To assess the applicability of the ion-sensing procedures of nanosensors for the Bi(III) ion, reliable measurements of the trace bismuth ions in real-life samples such as wastewater were examined. For this study, a simulated multicomponent solution containing Cd2+ and Ni2+ (50 mg/L), Cu2+ and Fe3+ (10 mg/ L), Sb3+, Co2+, and Zn2+ (50 mg/L), and Ca2+ and Mg2+ (100 mg/L), along with PO43-, SO42-, and SO32- (100 mg/L), CH3COO- and Cl- (100 mg/L), and NO3- and CO32- (100 mg/L) samples were spiked to a standard solution of Bi(III) of 2.39 × 10-6 mol/dm3 concentration. Despite the addition of these effective disturbance species, particularly when these species exist in a multicomponent system at a high concentration, the analytical data reveal no significant effects of these spiked multicomponents in the selectivity of the developed nanosensors for the Bi(III) ion. The recognition of Bi(III) was visualized by the naked eye and quantitatively determined by UV spectra at λ ) 485 nm. The quantification data of the Bi(III) ion sample examined five times were fitted to the calibration plot (Figure 7). The calibrated concentration of the Bi(III) ion in these multicomponent samples was 2.395 × 10-6 ( 0.005 mol/dm3 with a confidence level of 99% and with a relative standard deviation of 2%. In addition, the analytical results of the spiked samples by the nanosensors were in good agreement with the results obtained by the ICP measurements. Such evidence of high selectivity and performance of nanosensor Bi(III) ion-sensing systems led to a durable signal as a response to the Bi(III)-DZ binding event in major toxic environment and wastewater samples. Conclusion Ordered cage cubic Fd3m silica monoliths that were fabricated by using cationic surfactants were used as modal materials for optical chemosensors. The design-made nanosensors through the direct immobilization of a DZ probe onto 3D cage carriers enabled the creation of high sensing responses with revisable, selective, and sensitive detection of Bi(III) ions. Such a 3D pore geometry of the nanosensors significantly enabled facile diffusion and accessibility of Bi(III) target ions toward the probe molecules without kinetic hindrance, consistent with the short response time (on the order of seconds) even at a low concentration of Bi(III) analyte (∼10-10 mol/dm3) in such sensing systems. The ability of the optically defined chemosensors to control recognition and signaling of toxic analyte ions via a simple, rapid assessment technique is expected to expand the applications of chemical sensing systems. The HOM nanosensors not only worked under standardized conditions but could also be used for reliable sensing of the Bi(III) ion in a real-life sample such as wastewater. Supporting Information Available: Formation of largesized glassy monoliths with cubic Fd3m and MCM-41 structures, functionality of the cage Fd3m carriers in terms of accessibility and adsorption of the DZ probe, energy-dispersive X-ray microanalysis of the elemental compositions of the designmade DZ-HOM-11 sensors, and color map and UV-vis spectroscopic sensing responses for the DZ receptor in an aqueous solution. This information is available free of charge via the Internet at http://pubs.acs.org.

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