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Silver Molybdates with Intriguing Morphology and Peroxidase Mimic with High Sulfide Sensing Capacity Teresa Aditya, Jayasmita Jana, Ramkrishna Sahoo, Anindita Roy, Anjali Pal, and Tarasankar Pal Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01532 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016

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Crystal Growth & Design

Silver Molybdates with Intriguing Morphology and Peroxidase Mimic with High Sulfide Sensing Capacity

Teresa Aditya,a Jayasmita Jana,a Ramkrishna Sahoo,a Anindita Roy,a Anjali Palb and Tarasankar Pal.*a a

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India E-mail: [email protected]

b

Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, India

ABSTRACT This report entails the syntheses of morphologically different silver molybdate by simple manipulation of experimental conditions (concentration variation and stirred/unstirred condition) using stoppered glass vial under mild heat (~80 ˚C) treatment. We have elucidated for the first time the etching of robust hexagonal ammonium phosphomolybdate (APM) by silver nitrate (AgNO3) in aqueous solution. Here meticulous manipulation of the experimental condition brings about three different morphologies nanoflower (NF), nanowire (NW) and nanorod (NR) due to different recrystallization strategies and eventually identified an efficient catalyst. Among them the last two also exhibit high morphological stability. In identical

experimental

conditions,

while

a

lower

APM

concentration

always

(stirred/unstirred) yields NR, a higher APM concentration, owing to slow diffusion, produces NF from an unstirred solution and NW from a well stirred solution. The morphology being reliant on the concentration gradient of the Ag+ and the facile contact of the ions while etching APM. All the three different morphologies, once obtained, remain stable and are capable of acting as nanocatalyst. But the tuned NF morphology as expected, owing to its hierarchical

structure,

showed

highest

catalytic

efficiency

towards

3,3′,5,5′-

tetramethylbenzidine (TMB) oxidation. Explicit examination of the structures, stability, 1 ACS Paragon Plus Environment


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growth mechanism and catalytic activity of the morphologically different molybdates along with their comparative colorimetric properties towards TMB oxidation in presence of H2O2 is reported. Insightful investigation on the catalysis was executed with the representative NF which revealed efficient sulphide sensing capability in presence of other common interfering anions and the result is significant.

INTRODUCTION Polyoxometalate (POM) is a large group of fascinating inorganic clusters1 with generally anionic robust architecture of transition metal oxo anions which are linked by shared oxide ions. A throwaway POM of the undergraduate lab, ammonium phosphomolybdate (APM), with intriguing properties have gained significant momentum in research due to its fascinating properties and morphologies during syntheses. Nanomaterial POM2 with newly evolved morphologies3 by self-assembly or etching revealed interesting features.4 A variety of procedures are instrumental in producing silver molybdate viz. magnetron sputtering5, hydrothermal treatment6, chemical vapour deposition7 and laser annealing.8 We for the first time report a straight forward reproducible syntheses of three different composites with three varied morphology, Ag2MO2O7/Ag2Mo3O10.2H2O nanoflower (NF), Ag2Mo3O10.2H2O nanowire (NW) and Ag2Mo2O7/Ag6Mo10O3 nanorod (NR), from the same initial reagent by varying the proportion of APM and reaction condition. Selective etching of APM with AgNO3 is proved to be a straight forward method for the preparation of silver molybdate under our laboratory designed modified hydrothermal (MHT)9 reaction condition where heating is done under a 100 W lamp taking the reaction mixture in stoppered glass vials. Sulfides are well known harmful contaminants which need to be monitored and eradicated from large water bodies like river, sea, etc. The eradication of sulfides10 from contaminated environment is vital due to its toxic nature which is a lethal threat to human life11 and other organisms. Therefore, an efficient procedure of sulfide12 detection is indispensable for 2 ACS Paragon Plus Environment

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ecological and biological sustenance. Colorimetric13 procedure is actively pursued due to the efficiency, economic viability and executional convenience. In recent years, nanomaterial is evolving as a colorimetric detector because their extinction coefficients are high in magnitude compared to those of organic dyes14 and their syntheses is facile. For the first time, we have employed our as prepared composite nanocatalyst for highly efficient detection of sulphide in solution. Here the economically synthesized nanocomposite owing to its diverse morphology and composition at various conditions makes the detection possible following the standard 3,3′,5,5′-tetramethylbenzidine (TMB)15 oxidation reaction at room temperature. Preparation as well as sensing applications of a new POM with intriguing colorimetric property have been accounted in this report. To the best of our knowledge it is the first ever report of preparing colorimetric detector, in TMB oxidation reaction, exploiting silver molybdates having variable morphology which are synthesized from simple APM and silver nitrate solution in aqueous medium under modified hydrothermal treatment (MHT).

EXPERIMENTAL SECTION Chemicals. All the chemicals used were of AR grade. Details have been given in Supporting Information. Syntheses of nanoflower (NF), nanowire (NW) and nanorod (NR). In a typical preparation for NF and NW, 0.05 g of APM (syntheses provided in Supporting Information) was added in 4 ml water in two vials separately and sonicated for 15 min. After proper dispersion, 6 ml of 0.01 M AgNO3 was mixed thoroughly with the suspended APM dispersion in each vial. One of the vials was stirred at 500 r.p.m for 15 min after intervals of 1 h, while the other vial with the reaction mixture was kept undisturbed. Both the vials were heated in our laboratory designed modified hydrothermal (MHT) reactor illuminated by a 100 W lamp. NF morphology was obtained in 18 h from the unstirred reaction condition whereas stirred reaction mixture yielded NW morphology but in 12 h. The above reaction procedure was 3 ACS Paragon Plus Environment


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repeated with 0.01 mg APM which results in NR in 12 h, keeping all the conditions (unstirred and stirred) identical. All the precipitates were then collected separately and washed with water several times and then with ethanol. The products were dried in vacuum and used for characterisation. Detection of sulfide. In a typical reaction, in a 4 ml glass vial freshly prepared sulfide solutions were taken at pH 4.8, in variable concentrations of (0-83.33 µM) along with 25 µL 0.01 M TMB, 100 µL 1.5 M H2O2 and 0.5 mg of NF catalyst, due to its comparatively higher efficiency. The volume of the total mixture was kept 3 ml, at room temperature. For real samples, 50 µM S2- was spiked in the sample solutions collected from various environmental sources.

The

absorbance

of

the

blue

coloured

solutions

were

measured

spectrophotometrically in a UV-Visible spectrophotometer. This method also ensures the quantitative analysis of the sulfide concentration. Detection of sulfide is also evident by comparison of colour chart with naked eye. All the experiments were executed five times to assure reproducibility. RESULTS AND DISCUSSION XRD Analysis. The crystallinity of the as-synthesized composite was determined by X-ray diffraction (XRD) analysis. The diffraction peaks confirm the formation of a composite of Ag2MO2O7/Ag2Mo3O10.2H2O (JCPDS File No. 751505 and 390045) for the nanoflower (NF), Ag2Mo3O10.2H2O

(JCPDS

File

No.390045)

for

the

nanowire

(NW)

and

Ag2Mo2O7/Ag6Mo10O33 (JCPDS File No. 751505 and 721689) for the nanorod (NR). The patterns of NF and NR were both indexed to anorthic system, whereas NW were indexed as orthorhombic crystal structure. Figure 1 (a), (b), (c) shows the XRD patterns of the composite. The as-synthesized composites were highly stable.

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1500

Nanoflower

[501]

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[103]

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2000

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33

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[102]

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Figure 1. XRD analysis of (a) NF, (b) NW and (c) NR morphologies. XPS Analysis. X-Ray photoelectron spectroscopy (XPS), a quantitative technique for investigating the chemical state and surface atomic composition was employed to investigate the as-prepared nanomaterials. The comparative XPS analysis of Ag 3d is shown in Figure 2, and that of O 1s and Mo 3d is shown in Figure S1 in supporting information. It is evident that binding energy of 3d5/2 3d3/2 for Ag(I) at 368.7 eV and 374.3 eV was for one component and those

at

371.2

eV

and

376.8

eV

was

for

the

other

component

of

NF

(Ag2MO2O7/Ag2Mo3O10.2H2O). The peak of O 1s was obtained at 533.7 eV. In the Mo 5 ACS Paragon Plus Environment


spectrum two peaks were observed with binding energy 235.6 eV and 238.7 eV for 3d5/2 and 3d3/2, respectively. This shift was maybe due to the presence of electron withdrawing vicinity due to the high oxidation state of Mo (VI). For the NW (Ag2Mo3O10.2H2O) the peaks at 367 eV and 373 eV can be attributed to the Ag(I) 3d5/2 and 3d3/2, respectively. Peak corresponding to O 1s was observed at 532.3 eV and that of Mo 3d was at 231.3 eV and 234 eV. For NR of composition (Ag2Mo2O7/Ag6Mo10O33), however, the peak shifted slightly and appeared at 366.9 eV and 373.2 eV for one component, and 368.7 eV and 374.6 eV for the other component due to Ag(I) 3d5/2 3d3/2, respectively. The O 1s was observed at 532.1 eV while Mo 3d at 231.6 eV and 234.3 eV. The carbon standard for the analysis was 284.6 eV.

600

Ag 3d

a

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B in d in g E n e r g y ( e V )

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Ag 3d

500

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B in d in g E n e r g y ( e V )

Figure 2. XPS spectra of Ag 3d of (a) NF, (b) NW and (c) NR. 6 ACS Paragon Plus Environment

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FTIR Analysis. The FTIR analysis in Figure 3 shows peak below 1000 cm-1 and 800 cm-1 which were indexed as-symmetric vibrations of MoO4 of molybdate ion.16 Signature peaks of O-Mo-O stretching frequency were obtained between 800 cm-1 and 400 cm-1. The IR bands around 3320-3340 cm-1 and 1620-1640 cm-1 may be ascribed to –OH stretching vibration and bending vibration of surface adsorbed water molecules.

NF NW NR

% Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1638 688 900

500

3347

1000

1500

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2500

3000

3500

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Wavenumber cm-1

Figure 3. FTIR spectra of NF, NW and NR. FESEM, TEM and HRTEM Analysis. The as-synthesized material’s structure and morphology was analysed by FESEM as shown in Figure 4.



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Figure 4. Low and high resolution FESEM images of NF (a and a1), NW (b and b1) and NR (c and c1). The product obtained in 100 W lamp with AgNO3 at higher APM concentration (0.05 g) in unstirred condition for 18 h was hollow NF with prickly petals. However, the same reaction mixture when stirred for 15 min after intervals of 1 h, yielded NW after 12 h heating. When the concentration of APM was lowered (0.01 g), keeping all the other settings identical, under both stirring and unstirred conditions broomstick-like arrangement of NR was acquired. Figure 5 (a) and (a1) shows TEM images of NF. Each flower had diameter on an average of 3 µm with a hole at the centre and a hollow morphology. The images obtained from high 8 ACS Paragon Plus Environment

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magnification of the hollow NF revealed petals of width on an average of 50 nm. This structure makes the material highly porous and provides a large surface area for catalysis. Figure 5 (b) and (b1) shows TEM images of NW which was on an average of 20 µm and more in length and about 50 nm in width. The surface of all the morphologies were unevenly blistered. TEM image [Figure 5 (c) and (c1)] shows broomstick-like nature in NR which was 50 µm long and with a diameter of around 200-500 nm having hexagonal cross section.

Figure 5. TEM and HRTEM images of NF (a and a1), NW (b and b1) and NR (c and c1). Inset contains SAED patterns in each case.



EDX analysis (Figure S2 in supporting information) supported the presence of Ag, Mo and O elements in all the three cases. The absence of any peak of P confirmed the elimination of PO43- in the final products. TEM analysis was performed and matched well with the FESEM images obtained. The fringe spacing were matched with respective planes in each case. The fringe patterns showed 0.164 nm which matches with the (214) plane of Ag2Mo2O7 and 0.247 nm which matches with the (322) plane of Ag2Mo3O10.2H2O in the NF. For NW, the fringe spacing was 0.189 nm which matches with the (414) plane of Ag2Mo3O10.2H2O. For NR, fringe spacing observed was of 0.37 nm which matches with the (012) plane of Ag2Mo2O7, and 0.313 nm which matches with (022) plane of Ag6Mo10O33. SAED patterns were also well in agreement with the fringe spacing in each case. The growth mechanism in all the three cases were investigated as depicted in Figure 8. BET Analysis. The surface area of the materials with three different morphologies and their extent of porosity was evident from the FESEM and TEM images. Measurement obtained from nitrogen adsorption-desorption isotherm substantiates their surface area. Figure 6 (a), (b), and (c), represents the nitrogen adsorption-desorption isotherms of the NF, NW and NR, respectively, and the calculated Brunauer–Emmett–Teller (BET) surface areas obtained were 28.357, 25.547 and 25.005 m2 g-1, respectively. 0.005 35

NF

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a1

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0.003

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Figure 6. Nitrogen adsorption and desorption isotherm of all the three morphologies (a) NF, (b) NW and (c) NR. Pore size distribution plot of (a1) NF, (b1) NW and (c1) NR. The Barrett–Joyner–Halenda (BJH) method was adopted for pore volume/diameter distribution and were found to be 5.072e-2 cc gm-1, 7.848e-2 cc gm-1 and 4.911e-2 cc gm-1, pore diameter being 3.68 nm, 5.08 nm and 5.12 nm, respectively for NF, NW and NR. Hence, from the overall analysis the order of surface area was found to be NF > NW > NR, thus confirming that NF delivered the best surface area for catalytic performance with respect to NW and NR. TGA Analysis. In case of morphologically divergent silver molybdates we observed the decrease in % weight loss (Figure 7) due to the removal of physisorbed and chemically bound water molecules till ~450 °C.



b

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98.4 -8 98.2 50

100

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-10 400

Temperature (°C)

Figure 7. TGA DTA analysis of (a) NF, (b) NW and (c) NR. In case of NF and NW there was a continuous decrease of weight from temperature 25 °C to 450 °C. Presumably, in case of NF the physically adsorbed water molecules remain entrapped in the flowery architecture which was removed together with the chemically bonded water molecule of Ag2Mo3O10.2H2O, and the total weight loss was 4%. However, NW due to the presence of Ag2Mo3O10.2H2O species, contains chemically bonded water molecules in its backbone and also physisorbed water molecules in the interconnected wire like mesh. The entrapped water molecules lead to a loss of 20% in weight. In case of NR, the components present do not contain chemically bonded water molecule, however a small decrease in 12 ACS Paragon Plus Environment

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weight % about 1.8, in the temperature range of 25 °C to 360 °C, is probably due to the expulsion of negligibly adsorbed water molecule from its surface. This happens due to its lack of porosity or interconnected mesh like structures as evident from FESEM and TEM images, and BET analysis. The weight loss from NW and NF proves the presence of physically adsorbed water molecules and also two chemically bonded water molecules coming from Ag2Mo3O10.2H2O component and can be attributed to its removal, as the theoretical weight loss per water molecule is ~2.6%.17 The weight losses corresponding to endothermic effects are due to water which occur in very distinct temperature ranges, presenting clearly that one water molecule is more strongly linked than the other.17 Growth Mechanism. On perceiving the time dependent product formation we depicted the growth mechanism of the composites synthesized in aqueous medium. The most intriguing result is that, the high concentration of APM yielded NF as a composite from an unstirred reaction mixture and well defined morphology of NW from a stirred solution, while all the reagent concentrations remained same. On the other hand, low concentration of APM yielded NR composite from both stirred and unstirred solution. During the transformation of hexagonal APM to NF, NW and NR, NH4+ ion of APM was expelled and was confirmed by the formation of brown precipitate with Nessler reagent, when the supernatant liquid was subjected to the test. The supernatant liquid was also tested for the expelled phosphate ion and was confirmed by the formation of yellow precipitate of (NH4)3P(Mo12O40) on treatment with (NH4)2MoO4 in strongly acidic medium with respect to dilute nitric acid. All this along with EDX analysis confirms the expulsion of PO43- quantitatively by the etching of APM with Ag+ ion. After prolonged reaction time (~48 h) random depositions were observed in the morphology of NF, NW and NR. The XRD patterns remain the same as was with the initial product. Carrying out the reaction with other types of silver salts it was observed only silver nitrate exclusively evolved the specific morphology thereby confirming the anion effect on the growth process. Also different cations of nitrate was utilised but no significant 13 ACS Paragon Plus Environment


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morphology was evident. The TEM images showed that all the three structures had randomly formed blisters on their surface. Silver molybdates are insoluble in water and this is observed while silver ion from aqueous solution slowly etches robust APM from its water suspension forming hydrated silver molybdate membrane. Silver molybdate starts to crystallize under heat from the reaction mixture. Finally it forms either silver molybdate composites like NF and NR or well defined silver molybdates like NW by a double decomposition reaction (anion metathesis). Altogether, there occurs different but effective interaction of silver ion with the crystal faces of APM although the same ionic strength of silver nitrate is employed in all the cases. This difference in crystallization happens due to stirring and unstirring, observed for the first time. Mild heat (~80˚ C) treatment provides a favourable condition to precipitate out silver molybdates from the solution. The faces of APM act as semipermeable colloidal membrane and inside the membrane osmotic pressure is presumably higher than the silver nitrate solution in the bulk.18 The osmotic effect increases the pressure within the membrane which becomes pronounced with mild heat. This causes the membrane to tear, forming a hole [Figure 4 (a)]. The silver cations react with the molybdate anions at this scratch, forming new solid depending on the concentration of available Ag+. Interestingly, at higher concentrations of APM, stirred solution produces NW with distinct composition and unstirred solution due to inhomogeneity and produced concentration gradient reproducibly evolved NF morphology. While at lower concentration nanorod (NR) resulted from both stirred and unstirred condition. Propensity of reproducible composite formation presumably relies on the isomorphism and concentration gradient. The crystal formation is initiated from the sides of the hole, since the pressure inside is higher and APM comes out from within and react with silver ions. Hence, lower concentration of APM gives the same morphology as it is sufficiently in contact with Ag+ in solution in both stirred and unstirred condition. But for higher APM concentration the contact is insufficient and hence stirring produces NW and 14 ACS Paragon Plus Environment

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unstirred solution results in NF. Finally all APM is consumed. The given ratio works best for NF, NW and NR formation which has been adjusted by trial experiment to obtain three neat morphologies. The FESEM images of the precursor material APM are hexagonal in shape19 with an average diameter of 2 to 4 µm. These shapes gradually etched and became hollow spheres. The corrosion on the APM particle results in 1D petals of the flower, and its number increased at the expense of the perishing hexagon. At one point both the etched APM structures and newly forming NF coexisted in solution. The small 1D petals of 30-50 nm juncture formed small 3D clusters of NF which bear the composition Ag2MO2O7/Ag2Mo3O10.2H2O. Eventually, we observe single unique NF morphology which forms after the hexagonal spherical structure scrapes out completely. Figure 8 (a) shows the details of growth mechanism. In depth study of the growth of wire morphology having Ag2Mo3O10.2H2O composition, proves its initial tendency to form aggregation of 1D flower-petal like particles. Hence there remains a mixture of hexagonal APM and flower-like morphology which eventually undergoes oriented attachment to give rise to augmented NW of more than 20 µm in length and 30-50 nm in diameter, under stirring condition. This transformation to wire is evident from Figure 8 (b) which shows the details of growth mechanism. A thorough study of the morphology in case of NR, of composition Ag2Mo2O7/Ag6Mo10O33, proves that there develops again an initial tendency of 1D flower-petal like aggregate formation after the etching of the hexagonal shapes of APM, followed by the development of complete single rod. Eventually, these rods show a splitting in the mid-section from where the daughter rods develop, which splits further to give broomstick-like structure. The size of the rod is primarily 3 µm and has a hexagonal cross section. In due course, each rod splits diagonally through the hexagon to form broomstick-like structure attached at the base and the diameter of each broom narrows down to about 500-100 nm. Some parts of the broom also 15 ACS Paragon Plus Environment


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exfoliates and forms a new root which splits diagonally through the hexagon yet again. Figure 8 (c) shows the details of growth mechanism. We can summarise that at both low and high concentrations of APM flower-like structure is the kinetically stable morphology, whereas at high concentration wire and at low concentration rod becomes the thermodynamically stable morphology under the mild heating condition. Therefore, the structures are the manifestation of etching of APM by AgNO3. Following which Ostwald ripening20, splitting, coalescence and orientated attachment21 build the evolved morphologies. In all the three cases, post the etching of APM, self-assembly3 and Ostwald ripening serves as an intermediary stage, a process in which the small particles coalesces to form larger ones. The concentration of the Ag+ solution, amount of the APM utilized, the application of mild heat and autogenic pressure in the MHT are vital for the formation of these structures. Excess AgNO3 promptly etches the APM thereby lacking in specificity of morphology. The same applies to very low concentration of AgNO3 solution where the etching is so slow that the final product lacks morphological identification. Hence the experimental conditions employed along with the concentrations are imperative for the hierarchical morphology. The growth was marked in the three cases with the formation of one dimensional petals, which etch from APM in 3D manner for NF in a 1D orientation for NW and NR. It is observed that silver molybdate has an inherent tendency to induce 1D growth22. This is facilitated when Ag+ ions are in the vicinity of the APM molecules and that easily happens with higher proportion of Ag+ as for NR. However, lower silver concentration can also exhibit 1D growth provided bulky molybdate ions are brought forward by mechanical stirring which evolve NW. Even at room temperature intriguing morphology evolution has been uniquely disclosed. Mild heating and intermittent microscopic and spectroscopic evidences helps the understanding of growth mechanism. Preliminary etching results in flower-like aggregation which finally evolves to NW and NR in steps. However, interconversion of the 16 ACS Paragon Plus Environment

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as-prepared morphologies were unattainable. A schematic representation of the growth mechanism has been provided in Figure 8 (d).



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d

Figure 8. Growth mechanism at various time intervals for NF (a, a1, a2, a3), NW (b, b1, b2, b3) and NR (c, c1, c2, c3). (d) Combined schematic representation of the growth mechanism.


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Peroxidase-like Activity. Artificial enzymes in bio catalysis is a topic of infinite interest in contemporary research. The catalytic efficiency of the as-synthesized enzyme-like nanocatalyst Ag2MO2O7/Ag2Mo3O10.2H2O was investigated by oxidation of the familiar peroxidase substrate, TMB, in the presence of H2O2 at room temperature to blue coloured product which can be evidenced by the absorbance maxima at 652 nm after 15 min. Preferential efficiency of NF as the catalyst, as shown in Figure 9 (a), over the other morphologies was because of greater surface area of NF, as obtained from BET results, thereby triggering a more facile reaction condition. A catalyst dose of 500 µL triggered the reaction most effectively as shown in Figure 9 (b), which was calculated to be 0.5 mg. Time dependent UV-vis spectra was used to monitor the kinetics of the reaction employing the nanocatalyst, as shown in Figure 9 (c). The reaction was monitored under different pH and the best absorbance was obtained at pH 4.8 as shown in Figure 9 (d). Hence 15 min time and 0.01 M acetate buffer with a pH of 4.8 was taken as the optimal condition for the best catalytic activity showing an absorbance at 652 nm.



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a

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b

d

c

Figure 9. Optimisation of reaction conditions. (a) Comparison of catalyst efficiency, (b) catalyst amount (c) time for the reaction (d) pH condition. To test the superiority of our material a comparative absorbance graph was collected for all the three types of nanocatalyst which were methodically added to three different sets of reactions each containing same volume of TMB, H2O2 at pH 4.8. The study shows that the NF gave the best performance under identical reaction conditions and hence further experimentation was performed with NF catalyst. The sequence of efficiency of the catalysts prepared were NF > NW > NR. The values can be explained in terms of surface area of the


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catalyst. The flower has the largest surface area whereas the wire has lesser surface area and the rod has the least. Akin to other natural enzymes TMB oxidation is dependent on the pH and the concentration of H2O2. Keeping the pH at 4.8 and with a fixed amount of TMB variable concentrations of H2O2 (0-50 mM) was taken to demonstrate the effect of H2O2 in the oxidation process. This could also be a potential method for colorimetric detection of H2O2 using absorption spectrum at 652 nm as explained in Figure S3 in Supporting Information. The graph shows the calibration curve with 0-50 mM range of concentration of H2O2, and the limit of detection (LOD) calculated to be 24 mM the RSD being ~3% taking 40 mM H2O2, with three parallel measurements. Steady-state kinetic analysis of TMB oxidation. The peroxidase-like activity of the asprepared NF was evidenced by spectrophotometric investigation of the oxidised product of TMB, at a definite pH, on addition of TMB and H2O2 into the reaction medium. The sharp increase in absorbance at 652 nm due to the blue colouration of the product was noted to monitor the progress of the reaction. The steady state kinetics of the reaction was examined by changing the variable substrate, once with H2O2 and once with TMB, keeping the concentrations of all the other components unaltered. The absorbance value obtained in each case were converted to the concentration terms using ε = 39000 M-1 cm-1 of the blue product after oxidation. Over a certain range of concentration the typical Michaelis-Menten Curve was recorded for both H2O2 and TMB as shown in Figure10 (a) and (b), respectively.



a1

a

0.89

1.0

Km = 5.2 mM

Km = 5.2 mM

-1 Velocity (V0) (10-8) (M s )

0.88

0.8

1/V0 (10 ) (M s)

0.87 0.86

-1

0.6

8

0.4

0.2

0.85 0.84 0.83 0.82 0.81

0.0

0.80

0.00

0.01

0.02

0.03

0.020

0.04

0.025

0.030

0.035

0.040

-1

H2O2 concentration (S0) (mM)

1/S0 (mM )

b

b1 1.0

3.0 2.8

-1 Velocity (V0) (10-8) (M s )

Km = 0.0467 mM

Km = 0.0467 mM

2.6

0.9

2.4 -1

) (M s)

0.8

8

0.7

1/V0 (10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.5

2.2 2.0 1.8 1.6 1.4 1.2

0.4

1.0

0.3 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

5

10

15

20

25

30

35

-1

TMB concentration (S0) (mM)

1/S0 (mM )

Figure 10. Steady-state kinetic analyses using Michaelis-Menten (a and b) and LinewaverBurk model (a1 and b1) by varying concentration of H2O2 keeping TMB concentration constant (a and a1) and varying concentration of TMB keeping H2O2 concentration constant (b and b1). The vital parameter to examine the binding affinity between substrate and enzyme is the Michaelis constant (Km) which can be evaluated by plotting the following graphs. The data recorded from the kinetic study are applied in the Michaelis-Menten equation

The Lineweaver-Burk plot of 1/V0 vs. 1/S0 is used to evaluate the Km values from the following equation 1 1


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In both equations V0 is the initial velocity, S0 is the concentration of the substrate considered, Vmax is the maximum velocity of the reaction and Km is the Michaelis constant. After the completion of the kinetic study of the steady state, it was observed that the TMB oxidation follows Michaelis –Menten model with the as prepared catalyst for both TMB and H2O2 as substrates. The Michaelis constant (Km) value for NF towards TMB as substrate is 0.0467 mM (R2 = 0.97), which is lower than the value for horseradish peroxidase (HRP) enzyme (0.062 mM), as observed from Figure 10 (b1). The low value of Km ensures better binding of the nanocatalyst when compared to HRP enzyme. Table 1 shows a comparative study of Km values of TMB with HRP enzyme and other reported materials which proves the superiority of our as-synthesized silver molybdate.

Nanomaterial used

Km (mM)

References

ZnFe2O4 nanoparticle

0.85

23

TiO2 nanoparticles

0.127

24

Fe3O4 nanoparticles

0.098

25

HRP

0.062

14

Our nanoparticle Ag2MO2O7/Ag2Mo3O10.2H2O

0.0467

Present work

Table 1: Comparative assay of Km value for TMB using different nanomaterial The Km value for NF towards H2O2 as substrate is 5.2 mM (R2 = 0.97), as observed from Figure 10 (a1), which is comparable to the Km value for HRP (3.7 mM). From the observations it is evident that our catalyst silver molybdate NF has a high efficiency in binding with TMB and can be used as a substitute for HRP enzyme. Sulfide Detection. Sulfides are toxic contaminants and pose various health issues if injected into biological system. Sulfides have very lethal effects on human body beginning with loss 23 ACS Paragon Plus Environment


of consciousness, brain damage to even death due to neurotoxic effects. Hence, the need of a competent, sensitive, and inexpensive method is indispensable for the convenient detection of trace concentrations of sulfide in the eco system. Herein, we devised a technique where TMB oxidation reaction was employed for sulfide ion detection selectively and quantitatively with silver molybdate. We studied the sensitivity of quenching of the blue colouration of TMB oxidation reaction in the presence of sulfide anion. The presence of S2- in the solution inhibits the activity of H2O2 on TMB in the presence of catalyst. On increasing the S2- dose in the reaction, diminishing blue colour of the reaction mixture was observed. This motivated us to carry out a more detailed study of the reaction under optimal conditions using UV-vis spectrophotometer for noting the changes in absorption maxima at 652 nm. Varying concentrations of S2- ions were incorporated into the TMB oxidation reaction and their absorbance was noted, as illustrated in Figure 11 (a). From the graph in Figure 11 (a1) we observe a linear relationship of the concentration of the S2- ion (0-100µM) with respect to absorbance.

a

0.4

0.3

0.2

0.1

0.50 0.45

b

0.40

Absorbance (a.u)

0 µΜ 3.33 µΜ 8.33 µΜ 16.66 µΜ 25 µΜ 33.33 µΜ

0.5

Absorbance (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.35 0.30 0.25 0.20 0.15 0.10

0.0

0.05 -5

-0.1 550

600

650

700

0

5

10

15

20

25

30

35

S ulfide concentration ( µ M )

W avelength (nm)

Figure 11. (a) Absorbance of TMB oxidation in presence of various concentrations of sulfide ion. (b) Linear calibration plot of sulfide sensing employing silver molybdate as the artificial enzyme. The limit of detection was calculated to be 1.4 µM, RSD being ~2-3%, from the regression analysis as examined in five parallel measurements, which is superior compared to other reported materials as shown in Table 2. 24 ACS Paragon Plus Environment

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Technique/material used

LOD (µM)

References

Colorimetry/ Au nanoparticles

24

26

Colorimetry/Cu nanoparticles

8.1

27

Colorimetry/Au nanoparticles

3.0

28

Colorimetry/our nanoparticles

1.4

Present work

Table 2: Comparative assay of sulfide sensing using different technique of detection The sensing protocol with trace amount of S2- makes the silver molybdates NF a superior catalyst with a wide application in real samples as given in Table 3 with a high recovery percentage. Concentration

Measured

of spiked S2- concentration

Sample of water

Recovery Relative of

S2- error

(µM)

(avg.) of S2- (µM)

(%)

(%)

50

49.8 ± 0.1

99.6

0.4

48.7 ± 0.07

97.4

2.6

53.8 ± 0.14

107.6

7.6

Tap water (collected from laboratory tap, IIT Kharagpur) Drinking

water

(collected

from reservoir of drinking 50 water, IIT Kharagpur) Drain water ( collected from local drains, IIT Kharagpur)

50

Table 3. Analysis of S2- (spiked) present in various environmental water samples using our colorimetric protocol. 25 ACS Paragon Plus Environment


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Chemistry of Sulfide Detection. Sulfides have very severe effects on human body starting with loss of consciousness, brain damage to even death due to neurotoxic effects. Hence, the need of an efficient, sensitive, and low cost method is essential for the convenient detection of trace concentrations of sulfide in the ecosystem. In our work, a trace amount of S2- ion causes a quenching of the blue colouration of the TMB solution in presence of H2O2. It was observed that S2- ion whether added before or after the addition of H2O2, either way, hinders the oxidation of TMB. As denoted in Table 4, the reduction potential of H2O2 is the highest, 1.76 V, thus making it a potentially strong oxidising agent. However, the reduction potential of TMB is between 0.2-0.7 V and that of S2- is 0.14 V in acidic medium.29 E0 (vs. SHE)

1.76 V

0.22-0.7 V

TMBOX

TMB

0.14 V (in acidic medium)

Table 4: Standard reduction potentials of H2O2, TMBox and Sulfur. H2O2 and TMB forms the redox system in absence of S2-, but owing to the greater difference in E0 between H2O2 and S2- they form the thermodynamically preferred active redox system. Hereafter, thermodynamic parameters drives H2O2 to oxidise the sulfide to sulfur thus inhibiting the oxidation of TMB solution, resulting into a discoloured solution. This 26 ACS Paragon Plus Environment

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quenching of blue colour is dependent on concentration of the sulfide ion in the solution which is manifested in the absorbance spectra. With the colorimetric assay the selectivity of the sulfide ion detection, without much interference from other anions, by catalytic oxidation of TMB was evidenced from Figure 12.

Figure 12. Comparison of selectivity of silver molybdate catalyst towards various anions and inset corresponds to the digital image of the same. The ions taken were salts of sulfide, fluoride, chloride, bromide, sulphite, nitrite, nitrate, pyrosulphate, thiosulphate, sulphate and phosphate. However, it was observed that none of the anions showed significant hindrance in the TMB oxidation. Consequently, it was established that the adopted method was highly selective for S2- which is vulnerable to oxidation in presence of H2O2 and our nanocomposite. The difference in colouration resulting from quenching can be distinguished with naked eye for sulfide. High concentrations of the other anions were also employed to check, but in all the cases no significant difference of the discolouration was observed w.r.t. the control solution. Hence, this protocol can be applied to any type of real sample from water bodies to confirm the sulfur levels in it, as established by means of investigation of various real samples in our locality. CONCLUSION



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In a nutshell, variable morphologically evolved silver molybdates with NF, NW and NR structures have been synthesized from throwaway material APM, together with low concentration of sliver nitrate in a straight forward manner. It has been concluded that, morphologically the NW and NR are the most stable form of silver molybdate. This becomes a deliverable but obtained from complicated etching processes conducted by silver nitrate on APM. The morphology of the different silver molybdate composites from different sets of reaction mixture proves the dependence of the structures on the concentration gradient of the Ag+ ions and the facile contact of these ions while etching hexagonal APM particles. After accomplishment of etching we have acquired several porous materials for efficient catalysis of oxidation reaction. The meticulously obtained as-synthesized NF structured silver molybdate has been employed to gauge its efficiency for peroxidase like activity, as compared to HRP enzyme for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in presence of H2O2 as the oxidising agent. The HRP enzyme being difficult to manipulate makes it inconvenient for practical application. On account of the Michaelis-Menten equation NF catalyst with improved binding property attested by the low Km values have been successfully synthesized. For the first time, the oxidation of TMB in presence of silver molybdates nanocatalyst and H2O2 has been efficaciously employed for sensing S2- with high selectivity exploiting a simple UV-vis spectrophotometer. Therefore, making it a convenient sensing protocol with a wide range of detection and a low sensing limit for monitoring S2- contaminant in real samples. This inexpensive as-synthesized nanocomposite with an efficient catalytic property provides a scaffold for multifaceted application in the expanse of catalysis. SUPPORTING INFORMATION Chemicals used, experimental section, XPS spectra, EDX analysis and linear calibration plot. ACKNOWLEDGEMENT


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TOC

Syntheses of silver molybdates with different morphology and their application in peroxidase mimic along with sulfide sensing ability.


Silver Molybdates with Intriguing Morphology and ... - ACS Publications

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