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DOI: 10.1039/C4NR05755D
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V. Romero, I. Costas-Mora, I. Lavilla and C. Bendicho* 5
Departamento de Química Analítica y Alimentaria, Área de Química Analítica; Facultad de Química, Universidad de Vigo, Campus As LagoasMarcosende s/n, 36310 Vigo, Spain. E-mail: [email protected]; Fax: +34-986-812556; Tel: +34-986-812281
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x 10
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In this work, a simple route for the synthesis of surfactant-free immobilized palladium nanoparticles (Pd NPs) and their use as effective nanocatalysts for metal hydride decomposition is described. A mixture of ethanol:water was used as reducing agent. Ethanol was added in a large excess to reduce the ionic Pd and stabilize the obtained Pd NPs. Ethanol is adsorbed on the surface of Pd allowing steric stabilization. Freshly prepared Pd NPs were immobilized onto quartz substrates modified with 3mercatpopropyltrimethoxysilane. Pd interacts with the thiol group of the alkoxysilane that is adsorbed on the surface of NPs without the dissociation of S-H bond. Different parameters affecting the synthesis of Pd NPs and their immobilization onto quartz substrates were evaluated. A comprehensive characterization of synthesized Pd NPs was carried out by transmission electron microscopy (TEM), whereas total reflection X-ray fluorescence (TXRF) spectrometry was applied in order to evaluate their catalytic activity for solid-gas reactions. Immobilized Pd NPs were applied as nanocatalysts for dissociative chemisorption of arsine at room temperature, yielding the formation of As-Pd bonds. Quartz substrates coated with nanosized Pd could be used as novel sensing platforms for total reflection X-ray fluorescence analysis. Arsenic can be detected in situ in natural waters with a limit of detection of 0.08 μg L-1
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In recent years, the use of metal NPs has received much attention for different applications, e.g., in electronic storage systems,1 biotechnology,2 separation and preconcentration of target analytes,3-5 and medicine for drug delivery.6 Metal NPs are considered as a suitable choice for catalysis.7 Usually, NPs smaller than 5 nm are employed for catalysis in order to achieve the highest reactivity.8-10 Depending on the catalytic reaction, different noble metal NPs can be used e.g., Au, Pt, Ag or Pd. For reactions involving the interaction between the catalyst and hydrogen, Pd NPs are the most suitable due to the high affinity of Pd toward hydrogen.11-14 Furthermore, NPs can be applied as solid substrates for chemical vapor deposition (CVD) of volatile species. CVD has much scientific and technological interest due to its broad number of applications, i.e., fabrication of microelectronics devices,15 production of fiber coatings 16 or design of sorbents for extraction of contaminants.1719 CVD could be performed at very low temperatures with high efficiency in short times, taking advantage of the catalytic behavior of NPs smaller than 5 nm. Conventional synthesis routes for Pd catalysts involve the deposition of Pd onto solid supports by impregnation20,21 and This journal is © The Royal Society of Chemistry [year]
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precipitation.22,23 Reduction is performed by high-temperature treatments under inert atmosphere involving reduction with H2 and calcination, or by chemical reduction using hydrazine, sodium borohydride, etc.24 However, conventional routes result in dispersions with broad size distribution without control over the nucleation and growth of particles. Colloidal synthesis routes constitute an interesting alternative to obtain small size and welldefined Pd NPs to be used as high activity catalysts. Metal precursor reduction by adding a reducing agent in solution, also so-called ‘bottom-up’ strategy, is the most widely used method for obtaining metal NPs. Several conventional reducing agents are available, e.g. sodium borohydride, hydroxylamine hydrochloride, sodium citrate, hydrazine monohydrate, etc. Those reagents are considered strong reducing agents, and thus stabilization of metal NPs should be considered to prevent particle agglomeration. Vinyl polymers, such as poly(vinylpyrrolidone) (PVP) and poly(vinylalcohol) (PVA) are usually employed for the preparation of stable Pd colloids.25,26 However, when polymer coated Pd NPs are intended for use as catalysts in gas-phase reactions, the coating agent must be removed in order to obtain direct interaction with Pd and achieve high catalytic activity. The coating removal could be performed by washing coated NPs with water, ethanol or acetone followed by centrifugation. The process is repeated until the coating is [journal], [year], [vol], 00–00 | 1
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Facile preparation of immobilized surfactant-free palladium nanocatalyst for metal hydride trapping: a novel sensing platform for TXRF analysis
Nanoscale
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Results and discussion In this work, different alcohols were tried for Pd NPs synthesis, i.e., ethanol, methanol, propanol, butanol and octanol. As can be seen in Fig. 1, Pd NPs size depends on the type of alcohol employed as reducing agent. The nucleation rate depends on the dielectric constant and viscosity of the reducing agent.38 A decrease in the dielectric constant and an increase in viscosity give rise to an increase in 2 | Journal Name, [year], [vol], 00–00
the nucleation rate. ‘Ostwald ripening’ occurs leading to large NPs with broad size distribution.
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Fig. 1 TEM images and size distribution histograms for the synthesis of Pd NPs with different alcohols.
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The largest nanoparticles are obtained with methanol, butanol and octanol. For long chain alcohols (e.g,, butanol and octanol), the dielectric constant is low and the viscosity is high, thus nucleation is faster resulting in the formation of large Pd NPs. In addition, although methanol has a high dielectric constant and low viscosity, large Pd NPs are obtained. Besides, since surfactants are not used in the reaction medium, Pd NPs growth is not suppressed and broad size distribution and agglomerates are obtained. Although methanol has higher dielectric constant and low This journal is © The Royal Society of Chemistry [year]
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totally removed. However, this process is time-consuming and NPs can agglomerate resulting in larger NPs that are unsuitable for catalytic applications.27-29 An alternative option is to use mild reducing agents to limit the nucleation rate and avoid coarsening of NPs. In this way, alcohols represent the best option because they can behave as solvents and reducing agents. The synthesis route using alcohols as reducing agents, also so-called ‘alcohol reduction process’, was first developed by Hirai et al.30 They developed a route for the synthesis of rhodium-coated PVA NPs, using methanol as reducing agent. The synthesis was performed under inert atmosphere for 4 h at 79°C. Although a mild reducing agent such as methanol was used, the presence of a stabilizer (PVA) was of paramount importance to control both the size and stability of the colloid. An alternative to stabilizers is the use of a co-solvent, which can stabilize NPs. In this way, Athilakshimi et al.31 tried different co-solvents using methanol as reducing agent for the synthesis of stable Pd colloids. Synthesis could be performed at room temperature but it took a few hours to accomplish. Burton et al.32 proposed a new procedure for the synthesis of Pd NPs within 30 min of reaction using dry methanol as reducing agent, and no co-solvent or stabilizer polymers were needed. However, rigorously dry conditions and inert atmosphere were needed in order to obtain a reproducible Pd colloid. Taking into account that alcohols can act as stabilizers of the colloid by adsorption on the surface of the NPs,33 the use of a large excess of alcohol in the reaction medium could serve two purposes, i.e., as reducing agent and stabilizer, hence avoiding the addition of stabilizing polymers or co-solvents. Thus, in this work different alcohols are tried as reducing agents and stabilizers for the synthesis of Pd NPs. In addition, with an water:alcohol mixture as reaction medium, well-controlled nucleation and growth rates are feasible. Until now, immobilization of Au and Ag nanoparticles onto solid substrates has been aimed at applying surface-enhanced Raman Spectroscopy (SERS).34-37 As far as we know, immobilization of Pd NPs onto solid substrates has not been reported. In this work, we describe the immobilization of Pd NPs onto quartz substrates by silanization strategy. Immobilized Pd NPs provide a suitable surface for its use as a nanocatalyst for solid-gas reactions. It could be a promising surface for removal highly toxic volatile compounds by chemisorption (i.e. arsine, hydrogen selenide, etc.) or amalgamation of mercury onto the nanocatalyst. In this work, the chemisorption of arsine at room temperature, aimed at achieving an effective trapping of arsenic, is demonstrated. Ultimately, immobilized Pd NPs onto quartz substrates can be applied as novel sensing platforms along with a portable total reflection X-ray fluorescence (TXRF) spectrometer for in situ detection of volatile hydride forming elements.
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concentration of 2 mg L-1 of metal precursor was selected as optimal for obtaining Pd NPs.
viscosity, nanoparticles size is higher compared to ethanol and propanol. It should be noted that during the Pd NPs formation, alcohols are oxidized to the corresponding aldehyde and ionic palladium is reduced to Pd(0) as it is shown in Scheme1:
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Taking into account the hydrogenation enthalpies to the corresponding aldehydes, methanol oxidation is unfavorable compared to the other alcohols, which could worsen the reduction rate of Pd(II) to Pd(0) resulting in lower Pd nuclei formation. Therefore, the growth process leads to larger Pd NPs.39 In addition, Pd NPs agglomeration could also occur due to the poor steric stabilization. Methanol has the shortest carbon chain, and hence, repulsion forces between hydrocarbonated chains are not efficient. On the other hand, the smallest NPs are obtained with ethanol and propanol. Both nucleation and growth of NPs are slow, and small Pd NPs with narrower size distribution and high catalytic activity are achieved. In addition, a narrow size distribution is obtained with ethanol, thus yielding a well-dispersed colloid. Therefore, ethanol was found to be the best reducing agent in order to prepare colloidal Pd. Different mixtures of water:ethanol were tested varying the water:ethanol volume ratio with the aim of achieving monodispersed particles with small size. Results are shown in Fig. 2a. On increasing the volume of water, the formation of nuclei and growth of nanoparticles are slower and a narrower size distribution is achieved. Thus, an improved nanocatalyst activity, expressed as the amount of trapped arsenic onto Pd NPs (Fig 2b) is observed. However, for a 1:0.2 water:ethanol volume ratio, agglomeration occurs because the content of ethanol in the reaction medium is not sufficient for the reduction and stabilization of Pd NPs, hence resulting in larger NPs. Therefore, an water:ethanol volume ratio of 1:2 is selected as optimal. In addition, Pd NPs synthesized using a water:ethanol volume ratio of 1:2 were characterized by high-resolution TEM (HRTEM). From the selected area electron diffraction (SAED) pattern (Fig. 3a) and the HR-TEM image for a single nanoparticle (Fig. 3b), we can conclude that Pd NPs show high crystalline quality. Fig. 3b shows a spherical nanoparticle with continuously resolved lattice. Fringes correspond to Pd (1 1 1) lattice plane of the fcc structure, with an interplanar spacing around 0.77 Å. Besides, the NP growth is influenced by the metal precursor concentration. The concentration of Pd NPs in the obtained colloid as well as the concentration of Pd NPs immobilized on the quartz substrate is directly related to the initial concentration of metal precursor. The higher the concentration of Pd NPs on the quartz substrate the higher the catalytic activity and the amount of the arsenic retained onto Pd NPs. Different concentrations of Pd(II) (using Pd(NO3)2 as palladium source) from 0.2 mg L-1 to 4 mg L-1 were tried. The nanocatalyst activity as well as the amount of trapped arsenic increased up to a concentration of 2 mg L-1 of Pd(II) (Fig. 4a). For higher concentrations of metal precursor, Pd nuclei form aggregates rapidly giving rise to a black precipitate, which worsens the catalytic activity of Pd NPs. Therefore, a This journal is © The Royal Society of Chemistry [year]
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Scheme 1 Net chemical reaction for Pd NPs synthesis using ethanol as reducing agent.
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Fig.2 (a) TEM images for the different water:ethanol volume ratios and size distribution histograms. (b) Effect of the water:ethanol volume ratio in the amount of retained arsenic.
The control of the synthesis time is of paramount importance in order to ensure that the reaction is finished. For studying the synthesis time, the time-dependent UV-vis absorption spectra are recorded (Fig. 4b). At the beginning, the reaction medium colour is yellow and the absorption spectrum shows two absorption peaks, i.e., 240 nm and 370 nm. The peak at 370 nm disappears Journal Name, [year], [vol], 00–00 | 3
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Nanoscale Accepted Manuscript
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substrate. After standing at ambient temperature for 15 min, the quartz substrates were rinsed with water to remove the nonbonded Pd NPs and left at ambient temperature to dryness.
Fig. 3 HRTEM image (a) selected area electron diffraction (SAED) pattern (b) single Pd nanoparticle
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as the synthesis time increases and the base line uplifts due to plasmon scattering of Pd NPs. The plasmon scattering reached the maximun for a synthesis time of 10 min. The plasmon absorption kept constant indicating that the reaction finished completely at 10 min. Under optimal conditions (water:ethanol volume ratio of 1:2; Pd(NO3)2 concentration of 2 mg L-1; synthesis time of 10 min), spherical shaped Pd NPs with an average size of 3 nm are obtained. For silanization of the quartz substrates, mercaptopropylmethoxysilane (MPTMS) was used. MPTMS is a thiol-terminated alkoxysilane compound commonly used for immobilization of metal NPs by silanization.38-42 The efficiency of the Pd NPs immobilization depends on the concentration of MPTMS, which in turn, influences the catalytic efficiency of the solid phase immobilized on quartz substrates. Different concentrations of MPTMS were tried in this work (i.e., 5% v/v, 10 % v/v, 20% v/v and 30% v/v MPTMS in methanol). Results are shown in Fig. 4c. As can be noted, the amount of trapped arsenic increases on increasing MPTMS concentration. For MPTMS concentrations above 20% v/v, a plateau is reached meaning that suitable immobilization occurs. Thus, a concentration of 20% v/v MPTMS in methanol was selected as optimal for the silanization of the quartz substrates and immobilization of Pd NPs. Besides, the exchange of ethanol and MPTMS was evaluated comparing TEM images for Pd NPs synthesized under optimal conditions in the absence and presence of MPTMS (2 molar excess). Well dispersed Pd NPs were obtained when nanoparticles were synthesized in the presence of MPTMS, suggesting that the exchange between ethanol and MPTMS was efficient resulting in stable spherical Pd NPs (Fig. S1). In order to achieve the immobilization of Pd NPs, an aliquot of the freshly prepared colloid is deposited onto the silanized quartz 4 | Journal Name, [year], [vol], 00–00
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Fig. 4 (a) Effect of metal precursor initial concentration (b) UV-Vis spectra of Pd NPs for different synthesis time (c) Effect of the concentration of MPTMS (d) Effect of the volume of Pd NPs aliquot deposited onto quartz substrates 45
The volume of the deposited aliquot determines the diameter of the spot containing immobilized Pd NPs, and influences the amount of arsenic trapped. Different volumes were deposited on the quartz substrate to evaluate the influence of the diameter of This journal is © The Royal Society of Chemistry [year]
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temperature.50 Arsine was used as a volatile hydride model. Arsine was produced by means of a continuous-flow vapor generation system using sodium tetrahydroborate as reducing agent. The metal hydride was flushed onto the immobilized nanosized solid phase, thus achieving the dissociative chemisorption with the formation of As-Pd bonds at room temperature.51 Saturation of the nanosized Pd coating occurs after a trapping time of 10 min. A trapping efficiency of 60% was estimated under optimal conditions. Arsine trapping onto graphite
Fig. 5 1H NMR spectra of (a) MPTMS without nanoparticles (b) Pd NPs stabilized by MPTMS.
substrates coated with non-nanostructured Pd yielded a trapping efficiency of 55% at 300 ºC.52 Negligible trapping efficiency was reported when trapping experiments were performed at room temperature with non-nanostructured Pd, thereby demonstrating the enhanced performance of our Pd nanocatalyst. The trapping curve showing the maximum trapping capacity of the coating can be observed in the supplementary material (Fig. S2). Besides, J. McNally et al. demonstrated that spatially integrated atomic absorbance profiles obtained by electrothermal atomic absorption spectrometry (ETAAS) can provide information about the interaction between the analyte and the trapping surface.53 In order to evidence the specific interaction between As and Pd NPs, absorbance-time plots for various initial metal hydride concentrations were evaluated in the present work. By increasing the initial concentration of arsine, higher identical absorbance peaks were obtained, which can be ascribed to a firstorder release process indicative of a strong As-Pd interaction.53 Furthermore, profiles of As absorbance in the presence of Pd NPs were delayed and a higher atomization temperature is necessary as compared to the profile of arsenic absorbance in the absence of
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the spot on the catalytic decomposition of arsine. Volumes from 1 µL to 10 µL were assessed. Results are shown in Fig. 4d. The diameter of the spot containing the nanosized solid phase increases up to 5 mm, corresponding to an aliquot of 8 µL of colloidal Pd. For higher volumes, the diameter of the spot kept constant. Also, the larger the spot diameter, the larger the surface area and the catalytic activity. Thereby, a 8 µL volume of colloidal Pd was selected as optimal. The high affinity of organothiol, organodisulfide groups and amino groups toward noble metals is well-known.43,44 Depending on the functional group, molecular adsorption (for organothiol groups) or formation of methal-thiolate complexes (for organodisulfide groups) occurs on the surface of the noble metal. In the case of organothiol groups, several studies demonstrated that thiol-terminated compounds are adsorbed on the surface of the noble metal without the dissociation of the S-H bond.41-48 Hassan et al.45 provided evidence that the adsoption of an organothiol compound onto gold clusters occurs without cleavage of S-H bond, monitoring the obtained product by 1H nuclear magnetic resonance (1H-NMR). The presence of S-H triplet in the spectra indicates that hydrogen is present after adsorption. Besides, Lee et al.46 confirmed the nondissociative adsorption of an organothiol on a silver surface. Temperature-programmed desorption (TPD) studies after adsorption result in the ‘clean’ desorption of the organothiol compound without other desorption products, e.g. hydrogen sulfide, indicating that the S-H bond of the organothiol does not cleave on the noble metal surface during the adsorption process. Thiol groups also have high affinity toward Pd. Since 1H-NMR measurements can provide information about the interaction between the ligand and the nanoparticles,49 1H-NMR spectra for MPTMS and Pd NPs stabilized with MPTMS were performed in order to study the interaction between Pd and the organothiol compound. Results are shown in Fig. 5. In the spectrum for MPTMS (Fig 5a), a multiplet peak appears at about 3.4 ppm corresponding to the methoxy gropus (CH3-OR). In addition, two multiplet peaks appear for α-methylene group and β-methylene group bonded to the thiol group, at about 2.2 ppm and 1.6 ppm, respectively. Besides, a multiplet peak appears around 1.4 ppm, based on the chemical shift, this multiplet could be attributed to the proton of the thiol group (H-SR). As can be seen in the 1HNMR spectrum for Pd NPs stabilized with MPTMS (Fig 5b), multiplet peaks appear at the same position. Besides, it is observed that when MPTMS is bonded to Pd NPs, peaks for αmethylene and β-methylene groups are broader. It is also observed the appareance of a broad peak around 1.4 ppm, which could be attributed to the proton of the thiol group bonded to the Pd NPs. From these results we can conclude that the immobilization of Pd NPs on the quartz substrates occurs through adsorption of the thiol group of the MPTMS on the surface of Pd NPs without the dissociation of S-H bond. A general scheme for the preparation of the nanocatalyst and its application is shown in Scheme 2. Nanosized Pd immobilized onto a quartz substrate was applied as a heterogeneous catalyst for the decomposition of volatile hydrides. The increased number of active sites and enhanced catalytic activity of the nanostructured Pd film would facilitate the dissociative chemisorption of volatile hydrides at low
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Scheme 2 General procedure for the synthesis, immobilization and application of Pd NPs as nanocatalyst for arsine dissociative chemisorption
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Pd NPs (Fig. S3). This could be attributed to the formation of AsPd species that are less volatile.54 The nanostructured coating can be employed as a novel sensing platform for TXRF analysis. TXRF is increasingly employed for trace element analysis, yet direct analysis of low metal concentrations levels such as those found in natural water is troublesome without a prior preconcentration step.55 Besides, quartz substrates without any surface treatment and silanized quartz substrates without Pd NPs were also tried for comparison. Same conditions for arsine trapping were used. Obtained results show that no arsenic is retained on these quartz substrates, thus demonstrating that arsenic is only retained onto quartz substrates with immobilized Pd NPs on the surface. The limit of detection (LOD) obtained with our sensing platform by TXRF was 0.08 μg L-1 arsenic using a 10 min trapping time. This LOD is much lower than the maximum contamination level (MCL) established by the environmental protection agency (EPA) for arsenic in drinking water, i.e., 10 μg L-1. Besides, the trapped arsenic on the nanocatalyst is stable for at least 90 days without losses. Therefore, the immobilized Pd NPs could be applied for both on-site analysis in combination with a portable TXRF spectrometer and field sampling prior to analysis. A schematic diagram showing the novel sensing platform is shown in the supplementary material (Fig. S4).
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All chemicals were of analytical reagent grade and used as received. High-purity deionised water obtained from an Ultra Clear™ TWF EDI UV TM water system (Siemens, Barsbüttel, Germany) was used throughout.
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For immobilization of Pd NPs, quartz substrates were silanized by adding 5 µL of 20% (v/v) 3-mercaptopropyltrimethoxysilane (MPTMS) (Aldrich) in methanol. After that, quartz substrates were dried at 80°C for 30 min. After cooling at ambient temperature, 8 µL of freshly prepared Pd NPs colloid were deposited on the silanized quartz substrates. After standing at ambient temperature for 30 min, the quartz substrates were rinsed with water to remove the non-bonded Pd NPs and left at ambient temperature to dryness.
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Conditions for univariate optimizations
Experimental Materials
For Pd NPs synthesis, 1.5 mg of Pd(NO3)2 40% Pd basis (Aldrich) was accurately weighed using a MC5 Sartorius microbalance (Sartorious AG). The metal precursor was transferred into a glass tube and 300 µL of a water:ethanol mixture (1:2 v/v) were added. Then, reaction mixture was kept undisturbed for 10 min until reaction is finished and colloidal Pd is obtained.
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For the optimization of the type of alcohol employed as reducing agent, 1 mg of Pd(NO3)2 (containing 0.4 mg of Pd(II)) were mixed with 300 μL of each alcohol. The reaction medium was kept undisturbed at room temperature for a time in the range of 5-15 min, depending on the alcohol used as reducing agent (methanol, ethanol and propanol: 5 min synthesis; butanol: 10 min synthesis; octanol: 15 min synthesis). Then, 5 μL aliquot of Pd NPs were deposited onto quartz substrates containing MPTMS 10% (v/v) for nanoparticle inmobilization. [journal], [year], [vol], 00–00 | 6
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Study of the As-Pd interaction
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Physical characterization Pd NPs were prepared for characterization by Transmission Electron Microscopy (TEM) using a microscope model JEOL JEM-1010 (JEOL). Freshly prepared colloidal Pd was deposited onto a substrate made of a copper mesh cover, with a film of carbon and a polymer (Carbon/Formvar). A 5 µL aliquot was deposited on the substrate and left at room temperature overnight to dryness. In addition, absorption spectra UV-vis were recorded using a Nanodrop® Model ND-1000 spectrophotometer (Thermo Scientific) (optical path length ∼1mm). Aliquots of 2 µL were taken from the reaction medium and were placed onto the pedestal of Nanodrop® spectrophotometer. The sample arm slightly compresses the droplet, and a liquid column is formed by surface tension.
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For ETAAS measurements, a Thermo Electron Corporation® series M5 Atomic Absorption Spectrometer (Cambridge, UK) equipped with deuterium background corrector was employed in combination with a Thermo GF95 graphite furnace. For the evaluation of the spatially integrated absorbance profiles, 20 µL of freshly prepared Pd NPs were deposited on a graphite tube and dried at 150°C for 40 s. Then, arsine was generated using the continuous flow system and flushed onto Pd NPs in the graphite tube for 10 min. Finally, ETAAS, measurements using an atomization temperature of 2250°C were performed. For measurements in the absence of Pd NPs, 20 μL of an As standard solution were injected into the graphite tube, and an atomization temperature of 2100°C was applied. X-ray fluorescence measurements
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A continuous-flow vapor generation system was used for the generation of arsine. A Perkin-Elmer gas-liquid separator (GLS) equipped with a polytetrafluoroethylene (PTFE) membrane (25 mm diameter), a flow-meter for measuring the argon flow-rate (50 mL min-1), a four-channel Gilson’s Minipuls 3 peristaltic pump (Gilson Inc., Middleton, USA) and Tygon® pump tubes (1.52 mm i.d. for sample tube, 1.14 mm i.d. reducing agent tube and 2.79 mm i.d. for waste tube) were used. Flow-rates in the system were: sample/carrier, 9 mL min-1; reducing agent, 6 mL min-1; inert gas (Argon), 50 mL min-1. The sample containing arsenic was mixed with the reducing agent (0.5% m/v NaBH4) in the GLS and arsine was produced. Then, the volatile hydride was separated and carried by an argon stream to the quartz substrate containing immobilized Pd NPs.
H-NMR measurements
X-ray fluorescence measurements were carried out by means of a portable total reflection X-ray fluorescence (TXRF) spectrometer model S2 Picofox™ (Bruker AXS Microanalysis GmbH), equipped with a silicon-drift detector (100 mm2 area) and an air cooled molybdenum lamp as X-ray source. After trapping arsenic onto Pd NPs, gallium (1 mg L-1) was added as internal standard. After that, the quartz substrate was placed over the sample changer of the S2 Picofox™. A measurement time of 500 s was used. For data evaluation software Spectra 6.1® was employed.
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H-NMR measurements were carried out by means of a NMR spectrometer model Bruker Advance DPX-400. Pd NPs were synthesized by reducing Pd(NO3)2 by ethanol-water mixture in the presence of MPTMS. 2-molar excess of MPTMS was used in order to ensure that the obtained Pd NPs were capped. After synthesis, Pd NPs purification were carried out by ultrafiltration using Amicon Ultra® filters 3kDa, in order to eliminate all the excess of reagents. Pd Finally, NPs were redissolved in perdeuterated benzene (3 mg of Pd NPs in 0.5 mL of solvent).
Conclusions
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In short, we have demonstrated the usefulness of a simple procedure to prepare well-dispersed surfactant-free Pd NPs using a mixture of ethanol:water and Pd(NO3)2 as metal precursor. The immobilization of Pd NPs onto quartz substrates using silanization strategy was successfully carried out, thus obtaining a nanosized coating with high catalytic activity. Moreover, we also showed the practical application of the nanosized phase as a novel sensing platform based on the dissociative chemisorption of a volatile covalent hydride (arsine). Application of the novel
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For the optimization of water:ethanol mixtures, 1 mg of Pd(NO3)2 (containing 0.4 mg of Pd(II)) were mixed with 300 μL of each mixture water:ethanol. The reaction medium was kept undisturbed at room temperature for 8 min. Then, 5 μL aliquot of freshly prepared Pd NPs were deposited onto a quartz substrate containing MPTMS 10% (v/v). For the optimization of the metal precursor concentration, 300 μL of a mixture water:ethanol 1:2 (v/v) was used as reducing agent. After addition of Pd(NO3)2, the reaction medium was kept undisturbed for 8 min. After that, 5 μL aliquot of Pd NPs was deposited onto a quartz substrate containing MPTMS 10% (v/v). For the optimization of the synthesis time, 300 μL of water:ethanol 1:2 (v/v) and 2 mg of Pd(NO3)2 were mixed. Then, 2 μL aliquots were withdrawn between 1-10 min of reaction and measured by UV-Vis spectrophotometry (see section Physical characterization). For 0 min measurement, 1.5 mg of Pd(NO3)2 were dissolved on 300 μL of water and a 2 μL aliquot of this solution was measured by UV-Vis spectrophotometry. For the optimization of MPTMS, 300 μL of water:ethanol 1:2 (v/v) were mixed with 1.5 mg of Pd(NO3)2. The reaction medium was kept undisturbed for 10 min. Then, a 5 μL aliquot of Pd NPs was deposited onto quartz substrates silanized with different concentrations of MPTMS. For the optimization of the volume of the deposited Pd NPs aliquot, 300 μL of water:ethanol 1:2 (v/v) were mixed with 1.5 mg of Pd(NO3)2. The reaction medium was kept undisturbed for 10 min. After that, different volumes of Pd NPs were deposited onto quartz substrates silanized with MPTMS 20% v/v.
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sensing platform along with a portable TXRF spectrometer yielded a LOD of 0.08 μg L-1 of arsenic, which allows the detection in situ of ultratrace amounts of arsenic in natural waters. The nanosized coating is also well-suited for sampling and storing volatile hydride forming elements prior to analysis.
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Acknowledgements
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The Spanish Ministry of Economy and Competitiveness (Project CTQ2012-32788) and the European Commission (FEDER) are gratefully acknowledged. The Spanish Ministry of Education, Culture and Sport is acknowledged for financial support through a FPU predoctoral grant to V. Romero. The authors thank to J. Benito and S. Escudero (CACTI, Vigo University) for their assistance with HR-TEM and 1H-NMR experiments.
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V. Romero, I. Costas-Mora, I. Lavilla and C. Bendicho* 5
Departamento de Química Analítica y Alimentaria, Área de Química Analítica; Facultad de Química, Universidad de Vigo, Campus As LagoasMarcosende s/n, 36310 Vigo, Spain. E-mail: [email protected]; Fax: +34-986-812556; Tel: +34-986-812281
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x 10
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In this work, a simple route for the synthesis of surfactant-free immobilized palladium nanoparticles (Pd NPs) and their use as effective nanocatalysts for metal hydride decomposition is described. A mixture of ethanol:water was used as reducing agent. Ethanol was added in a large excess to reduce the ionic Pd and stabilize the obtained Pd NPs. Ethanol is adsorbed on the surface of Pd allowing steric stabilization. Freshly prepared Pd NPs were immobilized onto quartz substrates modified with 3mercatpopropyltrimethoxysilane. Pd interacts with the thiol group of the alkoxysilane that is adsorbed on the surface of NPs without the dissociation of S-H bond. Different parameters affecting the synthesis of Pd NPs and their immobilization onto quartz substrates were evaluated. A comprehensive characterization of synthesized Pd NPs was carried out by transmission electron microscopy (TEM), whereas total reflection X-ray fluorescence (TXRF) spectrometry was applied in order to evaluate their catalytic activity for solid-gas reactions. Immobilized Pd NPs were applied as nanocatalysts for dissociative chemisorption of arsine at room temperature, yielding the formation of As-Pd bonds. Quartz substrates coated with nanosized Pd could be used as novel sensing platforms for total reflection X-ray fluorescence analysis. Arsenic can be detected in situ in natural waters with a limit of detection of 0.08 μg L-1
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In recent years, the use of metal NPs has received much attention for different applications, e.g., in electronic storage systems,1 biotechnology,2 separation and preconcentration of target analytes,3-5 and medicine for drug delivery.6 Metal NPs are considered as a suitable choice for catalysis.7 Usually, NPs smaller than 5 nm are employed for catalysis in order to achieve the highest reactivity.8-10 Depending on the catalytic reaction, different noble metal NPs can be used e.g., Au, Pt, Ag or Pd. For reactions involving the interaction between the catalyst and hydrogen, Pd NPs are the most suitable due to the high affinity of Pd toward hydrogen.11-14 Furthermore, NPs can be applied as solid substrates for chemical vapor deposition (CVD) of volatile species. CVD has much scientific and technological interest due to its broad number of applications, i.e., fabrication of microelectronics devices,15 production of fiber coatings 16 or design of sorbents for extraction of contaminants.1719 CVD could be performed at very low temperatures with high efficiency in short times, taking advantage of the catalytic behavior of NPs smaller than 5 nm. Conventional synthesis routes for Pd catalysts involve the deposition of Pd onto solid supports by impregnation20,21 and This journal is © The Royal Society of Chemistry [year]
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precipitation.22,23 Reduction is performed by high-temperature treatments under inert atmosphere involving reduction with H2 and calcination, or by chemical reduction using hydrazine, sodium borohydride, etc.24 However, conventional routes result in dispersions with broad size distribution without control over the nucleation and growth of particles. Colloidal synthesis routes constitute an interesting alternative to obtain small size and welldefined Pd NPs to be used as high activity catalysts. Metal precursor reduction by adding a reducing agent in solution, also so-called ‘bottom-up’ strategy, is the most widely used method for obtaining metal NPs. Several conventional reducing agents are available, e.g. sodium borohydride, hydroxylamine hydrochloride, sodium citrate, hydrazine monohydrate, etc. Those reagents are considered strong reducing agents, and thus stabilization of metal NPs should be considered to prevent particle agglomeration. Vinyl polymers, such as poly(vinylpyrrolidone) (PVP) and poly(vinylalcohol) (PVA) are usually employed for the preparation of stable Pd colloids.25,26 However, when polymer coated Pd NPs are intended for use as catalysts in gas-phase reactions, the coating agent must be removed in order to obtain direct interaction with Pd and achieve high catalytic activity. The coating removal could be performed by washing coated NPs with water, ethanol or acetone followed by centrifugation. The process is repeated until the coating is [journal], [year], [vol], 00–00 | 1
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Facile preparation of immobilized surfactant-free palladium nanocatalyst for metal hydride trapping: a novel sensing platform for TXRF analysis
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Results and discussion In this work, different alcohols were tried for Pd NPs synthesis, i.e., ethanol, methanol, propanol, butanol and octanol. As can be seen in Fig. 1, Pd NPs size depends on the type of alcohol employed as reducing agent. The nucleation rate depends on the dielectric constant and viscosity of the reducing agent.38 A decrease in the dielectric constant and an increase in viscosity give rise to an increase in 2 | Journal Name, [year], [vol], 00–00
the nucleation rate. ‘Ostwald ripening’ occurs leading to large NPs with broad size distribution.
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Fig. 1 TEM images and size distribution histograms for the synthesis of Pd NPs with different alcohols.
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The largest nanoparticles are obtained with methanol, butanol and octanol. For long chain alcohols (e.g,, butanol and octanol), the dielectric constant is low and the viscosity is high, thus nucleation is faster resulting in the formation of large Pd NPs. In addition, although methanol has a high dielectric constant and low viscosity, large Pd NPs are obtained. Besides, since surfactants are not used in the reaction medium, Pd NPs growth is not suppressed and broad size distribution and agglomerates are obtained. Although methanol has higher dielectric constant and low This journal is © The Royal Society of Chemistry [year]
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totally removed. However, this process is time-consuming and NPs can agglomerate resulting in larger NPs that are unsuitable for catalytic applications.27-29 An alternative option is to use mild reducing agents to limit the nucleation rate and avoid coarsening of NPs. In this way, alcohols represent the best option because they can behave as solvents and reducing agents. The synthesis route using alcohols as reducing agents, also so-called ‘alcohol reduction process’, was first developed by Hirai et al.30 They developed a route for the synthesis of rhodium-coated PVA NPs, using methanol as reducing agent. The synthesis was performed under inert atmosphere for 4 h at 79°C. Although a mild reducing agent such as methanol was used, the presence of a stabilizer (PVA) was of paramount importance to control both the size and stability of the colloid. An alternative to stabilizers is the use of a co-solvent, which can stabilize NPs. In this way, Athilakshimi et al.31 tried different co-solvents using methanol as reducing agent for the synthesis of stable Pd colloids. Synthesis could be performed at room temperature but it took a few hours to accomplish. Burton et al.32 proposed a new procedure for the synthesis of Pd NPs within 30 min of reaction using dry methanol as reducing agent, and no co-solvent or stabilizer polymers were needed. However, rigorously dry conditions and inert atmosphere were needed in order to obtain a reproducible Pd colloid. Taking into account that alcohols can act as stabilizers of the colloid by adsorption on the surface of the NPs,33 the use of a large excess of alcohol in the reaction medium could serve two purposes, i.e., as reducing agent and stabilizer, hence avoiding the addition of stabilizing polymers or co-solvents. Thus, in this work different alcohols are tried as reducing agents and stabilizers for the synthesis of Pd NPs. In addition, with an water:alcohol mixture as reaction medium, well-controlled nucleation and growth rates are feasible. Until now, immobilization of Au and Ag nanoparticles onto solid substrates has been aimed at applying surface-enhanced Raman Spectroscopy (SERS).34-37 As far as we know, immobilization of Pd NPs onto solid substrates has not been reported. In this work, we describe the immobilization of Pd NPs onto quartz substrates by silanization strategy. Immobilized Pd NPs provide a suitable surface for its use as a nanocatalyst for solid-gas reactions. It could be a promising surface for removal highly toxic volatile compounds by chemisorption (i.e. arsine, hydrogen selenide, etc.) or amalgamation of mercury onto the nanocatalyst. In this work, the chemisorption of arsine at room temperature, aimed at achieving an effective trapping of arsenic, is demonstrated. Ultimately, immobilized Pd NPs onto quartz substrates can be applied as novel sensing platforms along with a portable total reflection X-ray fluorescence (TXRF) spectrometer for in situ detection of volatile hydride forming elements.
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concentration of 2 mg L-1 of metal precursor was selected as optimal for obtaining Pd NPs.
viscosity, nanoparticles size is higher compared to ethanol and propanol. It should be noted that during the Pd NPs formation, alcohols are oxidized to the corresponding aldehyde and ionic palladium is reduced to Pd(0) as it is shown in Scheme1:
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Taking into account the hydrogenation enthalpies to the corresponding aldehydes, methanol oxidation is unfavorable compared to the other alcohols, which could worsen the reduction rate of Pd(II) to Pd(0) resulting in lower Pd nuclei formation. Therefore, the growth process leads to larger Pd NPs.39 In addition, Pd NPs agglomeration could also occur due to the poor steric stabilization. Methanol has the shortest carbon chain, and hence, repulsion forces between hydrocarbonated chains are not efficient. On the other hand, the smallest NPs are obtained with ethanol and propanol. Both nucleation and growth of NPs are slow, and small Pd NPs with narrower size distribution and high catalytic activity are achieved. In addition, a narrow size distribution is obtained with ethanol, thus yielding a well-dispersed colloid. Therefore, ethanol was found to be the best reducing agent in order to prepare colloidal Pd. Different mixtures of water:ethanol were tested varying the water:ethanol volume ratio with the aim of achieving monodispersed particles with small size. Results are shown in Fig. 2a. On increasing the volume of water, the formation of nuclei and growth of nanoparticles are slower and a narrower size distribution is achieved. Thus, an improved nanocatalyst activity, expressed as the amount of trapped arsenic onto Pd NPs (Fig 2b) is observed. However, for a 1:0.2 water:ethanol volume ratio, agglomeration occurs because the content of ethanol in the reaction medium is not sufficient for the reduction and stabilization of Pd NPs, hence resulting in larger NPs. Therefore, an water:ethanol volume ratio of 1:2 is selected as optimal. In addition, Pd NPs synthesized using a water:ethanol volume ratio of 1:2 were characterized by high-resolution TEM (HRTEM). From the selected area electron diffraction (SAED) pattern (Fig. 3a) and the HR-TEM image for a single nanoparticle (Fig. 3b), we can conclude that Pd NPs show high crystalline quality. Fig. 3b shows a spherical nanoparticle with continuously resolved lattice. Fringes correspond to Pd (1 1 1) lattice plane of the fcc structure, with an interplanar spacing around 0.77 Å. Besides, the NP growth is influenced by the metal precursor concentration. The concentration of Pd NPs in the obtained colloid as well as the concentration of Pd NPs immobilized on the quartz substrate is directly related to the initial concentration of metal precursor. The higher the concentration of Pd NPs on the quartz substrate the higher the catalytic activity and the amount of the arsenic retained onto Pd NPs. Different concentrations of Pd(II) (using Pd(NO3)2 as palladium source) from 0.2 mg L-1 to 4 mg L-1 were tried. The nanocatalyst activity as well as the amount of trapped arsenic increased up to a concentration of 2 mg L-1 of Pd(II) (Fig. 4a). For higher concentrations of metal precursor, Pd nuclei form aggregates rapidly giving rise to a black precipitate, which worsens the catalytic activity of Pd NPs. Therefore, a This journal is © The Royal Society of Chemistry [year]
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Scheme 1 Net chemical reaction for Pd NPs synthesis using ethanol as reducing agent.
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Fig.2 (a) TEM images for the different water:ethanol volume ratios and size distribution histograms. (b) Effect of the water:ethanol volume ratio in the amount of retained arsenic.
The control of the synthesis time is of paramount importance in order to ensure that the reaction is finished. For studying the synthesis time, the time-dependent UV-vis absorption spectra are recorded (Fig. 4b). At the beginning, the reaction medium colour is yellow and the absorption spectrum shows two absorption peaks, i.e., 240 nm and 370 nm. The peak at 370 nm disappears Journal Name, [year], [vol], 00–00 | 3
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substrate. After standing at ambient temperature for 15 min, the quartz substrates were rinsed with water to remove the nonbonded Pd NPs and left at ambient temperature to dryness.
Fig. 3 HRTEM image (a) selected area electron diffraction (SAED) pattern (b) single Pd nanoparticle
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as the synthesis time increases and the base line uplifts due to plasmon scattering of Pd NPs. The plasmon scattering reached the maximun for a synthesis time of 10 min. The plasmon absorption kept constant indicating that the reaction finished completely at 10 min. Under optimal conditions (water:ethanol volume ratio of 1:2; Pd(NO3)2 concentration of 2 mg L-1; synthesis time of 10 min), spherical shaped Pd NPs with an average size of 3 nm are obtained. For silanization of the quartz substrates, mercaptopropylmethoxysilane (MPTMS) was used. MPTMS is a thiol-terminated alkoxysilane compound commonly used for immobilization of metal NPs by silanization.38-42 The efficiency of the Pd NPs immobilization depends on the concentration of MPTMS, which in turn, influences the catalytic efficiency of the solid phase immobilized on quartz substrates. Different concentrations of MPTMS were tried in this work (i.e., 5% v/v, 10 % v/v, 20% v/v and 30% v/v MPTMS in methanol). Results are shown in Fig. 4c. As can be noted, the amount of trapped arsenic increases on increasing MPTMS concentration. For MPTMS concentrations above 20% v/v, a plateau is reached meaning that suitable immobilization occurs. Thus, a concentration of 20% v/v MPTMS in methanol was selected as optimal for the silanization of the quartz substrates and immobilization of Pd NPs. Besides, the exchange of ethanol and MPTMS was evaluated comparing TEM images for Pd NPs synthesized under optimal conditions in the absence and presence of MPTMS (2 molar excess). Well dispersed Pd NPs were obtained when nanoparticles were synthesized in the presence of MPTMS, suggesting that the exchange between ethanol and MPTMS was efficient resulting in stable spherical Pd NPs (Fig. S1). In order to achieve the immobilization of Pd NPs, an aliquot of the freshly prepared colloid is deposited onto the silanized quartz 4 | Journal Name, [year], [vol], 00–00
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Fig. 4 (a) Effect of metal precursor initial concentration (b) UV-Vis spectra of Pd NPs for different synthesis time (c) Effect of the concentration of MPTMS (d) Effect of the volume of Pd NPs aliquot deposited onto quartz substrates 45
The volume of the deposited aliquot determines the diameter of the spot containing immobilized Pd NPs, and influences the amount of arsenic trapped. Different volumes were deposited on the quartz substrate to evaluate the influence of the diameter of This journal is © The Royal Society of Chemistry [year]
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temperature.50 Arsine was used as a volatile hydride model. Arsine was produced by means of a continuous-flow vapor generation system using sodium tetrahydroborate as reducing agent. The metal hydride was flushed onto the immobilized nanosized solid phase, thus achieving the dissociative chemisorption with the formation of As-Pd bonds at room temperature.51 Saturation of the nanosized Pd coating occurs after a trapping time of 10 min. A trapping efficiency of 60% was estimated under optimal conditions. Arsine trapping onto graphite
Fig. 5 1H NMR spectra of (a) MPTMS without nanoparticles (b) Pd NPs stabilized by MPTMS.
substrates coated with non-nanostructured Pd yielded a trapping efficiency of 55% at 300 ºC.52 Negligible trapping efficiency was reported when trapping experiments were performed at room temperature with non-nanostructured Pd, thereby demonstrating the enhanced performance of our Pd nanocatalyst. The trapping curve showing the maximum trapping capacity of the coating can be observed in the supplementary material (Fig. S2). Besides, J. McNally et al. demonstrated that spatially integrated atomic absorbance profiles obtained by electrothermal atomic absorption spectrometry (ETAAS) can provide information about the interaction between the analyte and the trapping surface.53 In order to evidence the specific interaction between As and Pd NPs, absorbance-time plots for various initial metal hydride concentrations were evaluated in the present work. By increasing the initial concentration of arsine, higher identical absorbance peaks were obtained, which can be ascribed to a firstorder release process indicative of a strong As-Pd interaction.53 Furthermore, profiles of As absorbance in the presence of Pd NPs were delayed and a higher atomization temperature is necessary as compared to the profile of arsenic absorbance in the absence of
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the spot on the catalytic decomposition of arsine. Volumes from 1 µL to 10 µL were assessed. Results are shown in Fig. 4d. The diameter of the spot containing the nanosized solid phase increases up to 5 mm, corresponding to an aliquot of 8 µL of colloidal Pd. For higher volumes, the diameter of the spot kept constant. Also, the larger the spot diameter, the larger the surface area and the catalytic activity. Thereby, a 8 µL volume of colloidal Pd was selected as optimal. The high affinity of organothiol, organodisulfide groups and amino groups toward noble metals is well-known.43,44 Depending on the functional group, molecular adsorption (for organothiol groups) or formation of methal-thiolate complexes (for organodisulfide groups) occurs on the surface of the noble metal. In the case of organothiol groups, several studies demonstrated that thiol-terminated compounds are adsorbed on the surface of the noble metal without the dissociation of the S-H bond.41-48 Hassan et al.45 provided evidence that the adsoption of an organothiol compound onto gold clusters occurs without cleavage of S-H bond, monitoring the obtained product by 1H nuclear magnetic resonance (1H-NMR). The presence of S-H triplet in the spectra indicates that hydrogen is present after adsorption. Besides, Lee et al.46 confirmed the nondissociative adsorption of an organothiol on a silver surface. Temperature-programmed desorption (TPD) studies after adsorption result in the ‘clean’ desorption of the organothiol compound without other desorption products, e.g. hydrogen sulfide, indicating that the S-H bond of the organothiol does not cleave on the noble metal surface during the adsorption process. Thiol groups also have high affinity toward Pd. Since 1H-NMR measurements can provide information about the interaction between the ligand and the nanoparticles,49 1H-NMR spectra for MPTMS and Pd NPs stabilized with MPTMS were performed in order to study the interaction between Pd and the organothiol compound. Results are shown in Fig. 5. In the spectrum for MPTMS (Fig 5a), a multiplet peak appears at about 3.4 ppm corresponding to the methoxy gropus (CH3-OR). In addition, two multiplet peaks appear for α-methylene group and β-methylene group bonded to the thiol group, at about 2.2 ppm and 1.6 ppm, respectively. Besides, a multiplet peak appears around 1.4 ppm, based on the chemical shift, this multiplet could be attributed to the proton of the thiol group (H-SR). As can be seen in the 1HNMR spectrum for Pd NPs stabilized with MPTMS (Fig 5b), multiplet peaks appear at the same position. Besides, it is observed that when MPTMS is bonded to Pd NPs, peaks for αmethylene and β-methylene groups are broader. It is also observed the appareance of a broad peak around 1.4 ppm, which could be attributed to the proton of the thiol group bonded to the Pd NPs. From these results we can conclude that the immobilization of Pd NPs on the quartz substrates occurs through adsorption of the thiol group of the MPTMS on the surface of Pd NPs without the dissociation of S-H bond. A general scheme for the preparation of the nanocatalyst and its application is shown in Scheme 2. Nanosized Pd immobilized onto a quartz substrate was applied as a heterogeneous catalyst for the decomposition of volatile hydrides. The increased number of active sites and enhanced catalytic activity of the nanostructured Pd film would facilitate the dissociative chemisorption of volatile hydrides at low
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Scheme 2 General procedure for the synthesis, immobilization and application of Pd NPs as nanocatalyst for arsine dissociative chemisorption
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Pd NPs (Fig. S3). This could be attributed to the formation of AsPd species that are less volatile.54 The nanostructured coating can be employed as a novel sensing platform for TXRF analysis. TXRF is increasingly employed for trace element analysis, yet direct analysis of low metal concentrations levels such as those found in natural water is troublesome without a prior preconcentration step.55 Besides, quartz substrates without any surface treatment and silanized quartz substrates without Pd NPs were also tried for comparison. Same conditions for arsine trapping were used. Obtained results show that no arsenic is retained on these quartz substrates, thus demonstrating that arsenic is only retained onto quartz substrates with immobilized Pd NPs on the surface. The limit of detection (LOD) obtained with our sensing platform by TXRF was 0.08 μg L-1 arsenic using a 10 min trapping time. This LOD is much lower than the maximum contamination level (MCL) established by the environmental protection agency (EPA) for arsenic in drinking water, i.e., 10 μg L-1. Besides, the trapped arsenic on the nanocatalyst is stable for at least 90 days without losses. Therefore, the immobilized Pd NPs could be applied for both on-site analysis in combination with a portable TXRF spectrometer and field sampling prior to analysis. A schematic diagram showing the novel sensing platform is shown in the supplementary material (Fig. S4).
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All chemicals were of analytical reagent grade and used as received. High-purity deionised water obtained from an Ultra Clear™ TWF EDI UV TM water system (Siemens, Barsbüttel, Germany) was used throughout.
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For immobilization of Pd NPs, quartz substrates were silanized by adding 5 µL of 20% (v/v) 3-mercaptopropyltrimethoxysilane (MPTMS) (Aldrich) in methanol. After that, quartz substrates were dried at 80°C for 30 min. After cooling at ambient temperature, 8 µL of freshly prepared Pd NPs colloid were deposited on the silanized quartz substrates. After standing at ambient temperature for 30 min, the quartz substrates were rinsed with water to remove the non-bonded Pd NPs and left at ambient temperature to dryness.
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Conditions for univariate optimizations
Experimental Materials
For Pd NPs synthesis, 1.5 mg of Pd(NO3)2 40% Pd basis (Aldrich) was accurately weighed using a MC5 Sartorius microbalance (Sartorious AG). The metal precursor was transferred into a glass tube and 300 µL of a water:ethanol mixture (1:2 v/v) were added. Then, reaction mixture was kept undisturbed for 10 min until reaction is finished and colloidal Pd is obtained.
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For the optimization of the type of alcohol employed as reducing agent, 1 mg of Pd(NO3)2 (containing 0.4 mg of Pd(II)) were mixed with 300 μL of each alcohol. The reaction medium was kept undisturbed at room temperature for a time in the range of 5-15 min, depending on the alcohol used as reducing agent (methanol, ethanol and propanol: 5 min synthesis; butanol: 10 min synthesis; octanol: 15 min synthesis). Then, 5 μL aliquot of Pd NPs were deposited onto quartz substrates containing MPTMS 10% (v/v) for nanoparticle inmobilization. [journal], [year], [vol], 00–00 | 6
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Study of the As-Pd interaction
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Physical characterization Pd NPs were prepared for characterization by Transmission Electron Microscopy (TEM) using a microscope model JEOL JEM-1010 (JEOL). Freshly prepared colloidal Pd was deposited onto a substrate made of a copper mesh cover, with a film of carbon and a polymer (Carbon/Formvar). A 5 µL aliquot was deposited on the substrate and left at room temperature overnight to dryness. In addition, absorption spectra UV-vis were recorded using a Nanodrop® Model ND-1000 spectrophotometer (Thermo Scientific) (optical path length ∼1mm). Aliquots of 2 µL were taken from the reaction medium and were placed onto the pedestal of Nanodrop® spectrophotometer. The sample arm slightly compresses the droplet, and a liquid column is formed by surface tension.
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For ETAAS measurements, a Thermo Electron Corporation® series M5 Atomic Absorption Spectrometer (Cambridge, UK) equipped with deuterium background corrector was employed in combination with a Thermo GF95 graphite furnace. For the evaluation of the spatially integrated absorbance profiles, 20 µL of freshly prepared Pd NPs were deposited on a graphite tube and dried at 150°C for 40 s. Then, arsine was generated using the continuous flow system and flushed onto Pd NPs in the graphite tube for 10 min. Finally, ETAAS, measurements using an atomization temperature of 2250°C were performed. For measurements in the absence of Pd NPs, 20 μL of an As standard solution were injected into the graphite tube, and an atomization temperature of 2100°C was applied. X-ray fluorescence measurements
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A continuous-flow vapor generation system was used for the generation of arsine. A Perkin-Elmer gas-liquid separator (GLS) equipped with a polytetrafluoroethylene (PTFE) membrane (25 mm diameter), a flow-meter for measuring the argon flow-rate (50 mL min-1), a four-channel Gilson’s Minipuls 3 peristaltic pump (Gilson Inc., Middleton, USA) and Tygon® pump tubes (1.52 mm i.d. for sample tube, 1.14 mm i.d. reducing agent tube and 2.79 mm i.d. for waste tube) were used. Flow-rates in the system were: sample/carrier, 9 mL min-1; reducing agent, 6 mL min-1; inert gas (Argon), 50 mL min-1. The sample containing arsenic was mixed with the reducing agent (0.5% m/v NaBH4) in the GLS and arsine was produced. Then, the volatile hydride was separated and carried by an argon stream to the quartz substrate containing immobilized Pd NPs.
H-NMR measurements
X-ray fluorescence measurements were carried out by means of a portable total reflection X-ray fluorescence (TXRF) spectrometer model S2 Picofox™ (Bruker AXS Microanalysis GmbH), equipped with a silicon-drift detector (100 mm2 area) and an air cooled molybdenum lamp as X-ray source. After trapping arsenic onto Pd NPs, gallium (1 mg L-1) was added as internal standard. After that, the quartz substrate was placed over the sample changer of the S2 Picofox™. A measurement time of 500 s was used. For data evaluation software Spectra 6.1® was employed.
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H-NMR measurements were carried out by means of a NMR spectrometer model Bruker Advance DPX-400. Pd NPs were synthesized by reducing Pd(NO3)2 by ethanol-water mixture in the presence of MPTMS. 2-molar excess of MPTMS was used in order to ensure that the obtained Pd NPs were capped. After synthesis, Pd NPs purification were carried out by ultrafiltration using Amicon Ultra® filters 3kDa, in order to eliminate all the excess of reagents. Pd Finally, NPs were redissolved in perdeuterated benzene (3 mg of Pd NPs in 0.5 mL of solvent).
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In short, we have demonstrated the usefulness of a simple procedure to prepare well-dispersed surfactant-free Pd NPs using a mixture of ethanol:water and Pd(NO3)2 as metal precursor. The immobilization of Pd NPs onto quartz substrates using silanization strategy was successfully carried out, thus obtaining a nanosized coating with high catalytic activity. Moreover, we also showed the practical application of the nanosized phase as a novel sensing platform based on the dissociative chemisorption of a volatile covalent hydride (arsine). Application of the novel
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For the optimization of water:ethanol mixtures, 1 mg of Pd(NO3)2 (containing 0.4 mg of Pd(II)) were mixed with 300 μL of each mixture water:ethanol. The reaction medium was kept undisturbed at room temperature for 8 min. Then, 5 μL aliquot of freshly prepared Pd NPs were deposited onto a quartz substrate containing MPTMS 10% (v/v). For the optimization of the metal precursor concentration, 300 μL of a mixture water:ethanol 1:2 (v/v) was used as reducing agent. After addition of Pd(NO3)2, the reaction medium was kept undisturbed for 8 min. After that, 5 μL aliquot of Pd NPs was deposited onto a quartz substrate containing MPTMS 10% (v/v). For the optimization of the synthesis time, 300 μL of water:ethanol 1:2 (v/v) and 2 mg of Pd(NO3)2 were mixed. Then, 2 μL aliquots were withdrawn between 1-10 min of reaction and measured by UV-Vis spectrophotometry (see section Physical characterization). For 0 min measurement, 1.5 mg of Pd(NO3)2 were dissolved on 300 μL of water and a 2 μL aliquot of this solution was measured by UV-Vis spectrophotometry. For the optimization of MPTMS, 300 μL of water:ethanol 1:2 (v/v) were mixed with 1.5 mg of Pd(NO3)2. The reaction medium was kept undisturbed for 10 min. Then, a 5 μL aliquot of Pd NPs was deposited onto quartz substrates silanized with different concentrations of MPTMS. For the optimization of the volume of the deposited Pd NPs aliquot, 300 μL of water:ethanol 1:2 (v/v) were mixed with 1.5 mg of Pd(NO3)2. The reaction medium was kept undisturbed for 10 min. After that, different volumes of Pd NPs were deposited onto quartz substrates silanized with MPTMS 20% v/v.
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sensing platform along with a portable TXRF spectrometer yielded a LOD of 0.08 μg L-1 of arsenic, which allows the detection in situ of ultratrace amounts of arsenic in natural waters. The nanosized coating is also well-suited for sampling and storing volatile hydride forming elements prior to analysis.
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The Spanish Ministry of Economy and Competitiveness (Project CTQ2012-32788) and the European Commission (FEDER) are gratefully acknowledged. The Spanish Ministry of Education, Culture and Sport is acknowledged for financial support through a FPU predoctoral grant to V. Romero. The authors thank to J. Benito and S. Escudero (CACTI, Vigo University) for their assistance with HR-TEM and 1H-NMR experiments.
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