π-stacking intercalation and reductant assisted stabilization of osmium organosol for catalysis and SERS applications

Abstract

Size-selective, mono-dispersed spherical osmium (Os) nanoparticles (NPs) have been synthesized for the first time in a two-phase (water-toluene) extraction procedure in organic medium (in toluene) under ambient conditions. A simple wet chemical synthesis route was employed to prepare the Os organosol from the precursor osmium tetroxide (OsO₄) and tetrabutylammonium borohydride (TBABH₄). Tetraoctylammonium bromide (TOAB) was used as a phase transfer catalyst (PTC) which quantitatively transferred Os precursors to the organic medium from the aqueous medium. Four different spherical Os NP organosols with varying sizes of 1 ± 0.2 nm, 10–30 nm, 22 ± 2 nm and 31 ± 3 nm were synthesized just by changing the concentration ratio of the metal precursor and the amount of reductant added. The role of all the precursor concentrations in the size-selectivity was examined in-detail. The synthesized osmium organosol were stabilized by the extensive π-stacking intercalation effect offered by toluene as well as the interaction of tetrabutylammonium ions (TBA⁺) presented in the organic medium. The synthesized spherical Os NP organosols were utilized in two different applications such as in catalysis and in Surface Enhanced Raman Scattering (SERS) studies. The catalytic activity of osmium organosol was tested for the reduction of hexavalent chromium (Cr⁶⁺) ions under UV light in the presence of sodium thiosulphate. The SERS activity was examined by taking methylene blue (MB) dye as a probe molecule. In the near future, the synthesized Os organosol might be utilized as a potential catalyst in organic catalysis reactions as well as in the field of fuel cells and sensors.

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Introduction

The recent evolution of nanoscience and nanotechnology at the end of the 20^th century had fashioned the world to depend on it, in almost all aspects. The syntheses of nanomaterials and their application in practice are also being widened now. Among the various kinds of nanomaterials, metal nanoparticles (NPs) always have a lead role over metal oxides and composites with different sizes and shapes. Due to the tremendous changes in the catalytic,¹ optical,² electronic³ and magnetic⁴ properties of metals on the nano scale, they have found a plethora of potential applications in practice. Hence, the syntheses of new nanomaterials and the modulation of syntheses routes of the existing materials are greatly desired. NPs of noble metals such as Ag,⁵ Au,⁶ Pt,⁷ Rh,⁸ Pd,⁹ Ir,⁸ Fe,¹⁰ Cu,¹¹ Ru¹² and Os¹² have been prepared and reported earlier. Among the different metal NPs studied, a very limited study has been carried out on osmium (Os) NPs. Os is an interesting metal among the different noble metals and has low compressibility but a high bulk modulus, which is comparable with diamonds. Os metal also has a high melting point.¹³ Due to all these different fascinating properties and applications the synthesis of mono-dispersed sized Os NPs is a challenging task to the researchers. Up to now various methods have been employed for the preparation of these metal NPs. Among them, are the well-known wet chemical methods, chemical vapor deposition (CVD), electro deposition and pulsed laser deposition (PLD) in recent years.^14–16 There are few reports for the fruitful preparation of Os NPs with different sizes and morphologies such as nano-wires,¹⁷ nano-clusters,¹⁸ wire-like aggregates in an organic–aqueous hybrid medium¹⁹ and thin films.¹⁵ Krämer et al. prepared iron, ruthenium, and osmium NPs using a metal carbonyl precursor as starting material.¹² Os thin film was fabricated from osmacene at higher temperature by atomic layer deposition technique.¹⁹ Small Os NPs with chain-like morphology was recently prepared using ascorbic acid reductant at 95 °C.¹⁴ Apart from these, our group very recently highlighted the synthesis of wire-like Os nanoclusters (NCs) using DNA scaffold,⁸ interconnected Os NCs in a surfactant media and wire-like aggregates of Os NPs in an organic–aqueous hybrid medium.^17–19 The applications of Os NPs are found in patent literatures in the area of catalysis, sensors, electronic devices and in fuel cells. Os metal NPs can be alloyed with other metal and applied to prepare fountain pen tips, electrical contacts and instrument pivots. Os doped SnO₂ was applied as a sensor material for sensing methane gas,²⁰ Os–Pd NPs embedded on CNTs was used as catalyst,²¹ Os metal NPs itself used as an anode material in direct borohydride fuel cell and as an electrocatalyst for the reduction of oxygen either in presence of or absence of methanol.¹⁶ Os wire-like interconnected chains and clusters were applied as an effective SERS substrate and used as a catalyst for the reduction of aromatic nitro compounds with sodium borohydride.¹⁷ Most of the above reports dealt with templates, stabilizing agents or scaffolds for the syntheses of Os metal NPs. But the utilization of supramolecular forces for the stabilization of metal and metal oxide NPs might open a new way for the synthesis of nanomaterials.

Intercalation is one of the well-known phenomena as one of the supramolecular interactions. Many matrices had been employed as an intercalating material for metal NPs. The prominent intercalation techniques are polymer intercalation,²² graphene intercalation,²³ intercalation by kaolinite clay²⁴ where supramolecular forces play the major role. Among the various supramolecular forces, π-stacking is an interesting one, especially when the synthesis takes place in an organic medium. The π-stacking interactions were extensively studied by Hunter et al. by focusing on the π-stacking intercalation with metal ions by varying the substituents.²⁵ Hence, it is possible and necessary to find a new method in which the solvent itself can act as a stabilizer by utilizing its supramolecular interactions. So the synthesis of spherical Os NPs in a completely organic environment will be highly desired. The preparations of NPs in an organic medium fetch many advantages to the material such as long term stability which can be easily applied in various fields such as catalysis, especially when it is a heterogeneous catalysis reaction and taking place in organic medium.

As our recent finding, here we report the phase-transfer catalyst (PTC) assisted two-phase (water-toluene) extraction procedure for the rapid synthesis of spherical and monodispersed Os NPs stabilized in toluene. The Os organosol were synthesized from the precursor of osmium tetroxide (OsO₄) using the reducing agent tetrabutylammonium borohydride (TBABH₄). The tetraoctylammonium bromide (TOAB) acted as a phase transfer catalysts (PTC) which can quantitatively transfer OsO₄ precursors to the organic medium. The overall reaction was completed within 30 minutes and the synthesized Os organosol were used in catalysis study for the reduction of hexavalent chromium (Cr⁶⁺) by sodium thiosulphate and as a SERS substrate against the methylene blue probe molecule. The reduction of Cr⁶⁺ is an environmentally important aspect as it pollutes the environment severely and its being eluted from the tanneries and other related industries. The catalysis reaction was completed within a reasonable time scale and the catalytic rate was found to be superior compared to prior reports. Similarly, SERS experiment was carried out with MB as probe molecule in presence of Os organosol in toluene. The enhancement factor (EF) value was calculated ∼1.6 × 10⁵ which is the highest value reported so far for a metal organosol synthesized in toluene. The Os organosol was found to be stable for more than six months when stored in refrigerator under dark. The extreme stability of the Os NPs in toluene is attributed to the continuum of π-stacking intercalation offered by toluene as well as the presence of tetrabutylammonium ions (TBA⁺) surrounding the NPs. The overall synthesis process was simple, reliable, reproducible, less time consuming and cost-effective.

Experimental

Reagents and instruments

Osmium tetroxide (OsO₄), tetrabutylammonium borohydride (TBABH₄) and tetraoctylammonium bromide (TOAB) were purchased from Sigma-Aldrich, India and used without any further purification. Toluene was purchased from SRL. Methylene blue dye was purchased from Qualigens Fine Chemicals, Mumbai. Potassium dichromate (99%) and sodium thiosulphate anhydrous (99%) were purchased from Alfa Aesar and used without any further purification. Milli-Q water (18 MΩ cm⁻²) was used wherever required. The synthesized Os NPs were characterized using following techniques such as UV-Visible, TEM, XRD, EDS and FT-IR analyses. The UV-Visible (UV-Vis) absorption spectra were recorded in a Unico (model 4802) UV-Vis-NIR spectrophotometer equipped with a 1 cm quartz cuvette holder for liquid samples. The transmission electron microscopy (TEM) analysis was done with a Tecnai model TEM instrument (Tecnai™ G2 F20, FEI) with an accelerating voltage of 200 kV. The energy dispersive X-ray spectroscopy (EDS) analysis was done with the field emission scanning electron microscopy (FE-SEM) instrument (Zeiss ultra FE-SEM instruments) with a separate EDS detector (INCA) connected to that instrument. The X-ray diffraction (XRD) analysis was done using a PAN analytical advanced Bragg-Brentano X-ray powder diffractometer (XRD) with Cu K_α radiation (λ = 0.154 nm) with a scanning rate of 7° min⁻¹ in the 2θ range 10–90°. The FT-IR analysis was done with the model Nexus 670 (FTIR), Centaur ms 10× (Microscope) having spectral Range 4000 to 400 cm⁻¹ with a MCT-B detector. The LASER Raman measurements were carried out with Renishaw inVia Raman Microscope using an excitation wavelength of 632.8 nm (He–Ne laser). The excitation light intensity in front of the object was ∼10 mW with a spectral collection time of 1 s for Raman experiment. The integration time for our measurement was set to 10 s.

Synthesis of spherical Os NPs with various sizes

About 20 mL of osmium tetroxide stock solution (10⁻² M) was taken with 40 mL of toluene in a separating funnel of 60 mL volume. Then 0.25 mg of tetraoctylammonium bromide was then added and shaken well for 20 minutes to assist the transfer of OsO₄ from aqueous phase to toluene phase by the well-known phase transfer catalysis mechanism. The transfer of OsO₄ was witnessed by the color change in the organic layer from colorless to pale yellow. Then the organic layer was separated out. Now, 10 mL of toluene solution containing OsO₄ was taken separately in four different glass vials and solid TBABH₄ with different amounts such as 0.05 g, 0.025 g, 0.0125 g and 0.00625 g were added with constant stirring by mechanical means and marked as sample as 1, 2, 3 and 4 respectively. Initially, the color of the solution turned from pale-yellow to pale violet. After 5 min. the color of the solution became dark violet which indicated the completion of the reaction. The details of concentration of the reagents used are given in Table 1. The resulted Os NPs organosol were stored in a refrigerator and found that samples were extremely stable up to 6 months in dark.

Table 1 Shows the details of concentration of the reagents used for the synthesis of Os organosol in toluene

Set number	OsO4 stock solution concentration (M)	Amount of reductant (TBABH4) added (g)	Time of stirring (min)	Color of the final solution with λmax values (nm)	Shape of the synthesized Os NPs	Average diameter of the particles (nm)
1	10^−2	5 × 10^−2	5	Dark violet with three peaks at 478, 598 and 647	Spherical	1 ± 0.2
2	10^−2	2.5 × 10^−2	5	Dark violet with three peaks at 478, 598 and 647	Spherical	10–30
3	10^−2	1.25 × 10^−2	5	Dark violet with three peaks at 478, 598 and 647	Spherical	22 ± 2
4	10^−2	0.625 × 10^−2	5	Dark violet with three peaks at 478, 598 and 647	Spherical	31 ± 3

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Catalytic reduction of hexavalent chromium (Cr⁶⁺) ion using Os organosol as catalyst

50 mL of 10⁻⁴ M dichromate solution was taken in a cleaned and dried beaker. 5 mL of Os organosol and 50 mL sodium thiosulphate solution (10⁻² M) were added one after another under continuous stirring in presence of UV light (365 nm). The successive reduction of Cr⁶⁺ was monitored as a function of time using an UV-Visible spectrophotometer. As the reaction time increases the color of the reaction mixture began to fade and became a white turbid solution at the end of the reaction. The completion of the reaction was monitored from the change in the color of the solution as well as the change in the λ_max values in the UV-Vis spectrophotometer. The more details about the catalysis study were discussed in the results and discussion section.

Preparation of samples for various characterizations

The as-synthesized Os organosol were diluted (as required), drop casted over carbon coated copper grids, dried in air and then analyzed with TEM instrument. For UV-Vis spectroscopic analysis the as-synthesized Os organosol was used directly. For FT-IR analysis, 10 μL of Os organosol was mixed with KBr and palletized then analyzed immediately with FT-IR instrument. For XRD, EDS and XPS analysis, a thin film of all the four samples were prepared by repeatedly pouring 100 μL of Os organosol over glass substrates and dried at room temperature. The process was repeated for more than 10 times. Samples for SERS studies were prepared as follows. Different MB dye solutions having concentration of 10⁻³ M, 10⁻⁴ M, 10⁻⁶ M and 10⁻⁸ M were prepared in acetone. Then 500 μL of each of those dye stock solutions were separately mixed with 500 μL of Os organosol solution and kept for 20 minutes after vigorous shaking to ensure the homogeneity throughout the solution. After that, 10 μL of each of those solutions were poured on a glass substrate and dried in air for SERS studies.

Results and discussions

Transmission electron microscopy (TEM) analysis

Fig. 1A–D shows the transmission electron microscope (TEM) images of synthesized spherical Os NPs organosol. Fig. 1A shows the TEM image of the sample 1 where the average size of the particles is 1 ± 0.2 nm which is the smallest compared to other previous reports on Os NPs.^14,17,18 Fig. 1B shows the TEM image from the sample 2. The average size of the particles is ∼10–20 nm. However, few other sizes (in minor proportions) were also observed. From this image it is clear that the particles are larger, spherical but order of uniformity in size was less. Fig. 1C is the TEM images of the sample 3 were we can see the particles are spherical in shape and the particles are highly size selective with an average size ∼22 ± 2 nm. Fig. 1D is the TEM image of the sample 4 where all the particles are spherical in shape and the average size of the particles is ∼31 ± 3 nm. The inset of Fig. 1A–D shows the corresponding histogram of the four different size Os NPs organosol. From the histogram it is clear that the particles are almost uniform in size. As discussed earlier, the four different samples were prepared by varying the amount of reducing agent. From this, it is clear that the change in the concentration of TBABH₄ played a major role in determining the size of the spherical particles. As the amount of the reductant was being reduced in 1 : 0.5 ratios from sample 1 to sample 4, the size and uniformity was also being tuned. For sample 1, we can see a cloud of particles with the size of 1 ± 0.2 nm. As the amount of reductant was decreased, the individual size of the particles was also increased. For sample 3 and sample 4 we can see the high size selectivity having average size ∼22 ± 2 nm and 31 ± 3 nm. The corresponding selected area electron diffraction patterns (SAED) of each sample are given in another inset of Fig. 1A–D respectively which implies that the particles are non-crystalline in nature which matches with other previous reports.^14,17,18

Fig. 1 (A–D) Transmission Electron Microscope (TEM) images of different sizes Os NPs organosol in toluene. (A) Corresponds to the smaller size Os NPs having size ∼1 ± 0.2 nm of sample 1. (B) Corresponds to the Os NPs having sizes from 10–30 nm of sample 2. (C) Corresponds to the Os NPs having sizes 22 ± 3 nm of sample 3. (D) Corresponds to the Os NPs of size 33 ± 2 nm of sample 4. The inset of each figure shows the corresponding selected area electron diffraction (SAED) pattern and particle size distribution histogram plots. In the particles size histogram, X-axis plotted as [particles size (nm)] and Y-axis plotted as [distribution].

UV-Visible (UV-Vis) spectroscopic studies

UV-Vis spectroscopic studies were carried out with the as prepared Os NPs organosol samples by taking ethanol as a solvent. Fig. 2 represents the corresponding UV-Vis spectra at the different stages of the synthesis process. Curve a, b and c in Fig. 2 are the respective electronic spectra of only toluene, mixture of toluene with TOAB and OsO₄ transferred toluene. In both curve a and b the same peaks were observed which indicated that the addition of TOAB in toluene did not change the electronic absorption properties of toluene. Three distinct peaks at 220 nm, 261 nm and 270 nm were observed due to the presence aromatic benzene ring. In curve c, a small hump was observed at 294 nm which might be due to the interaction of Os⁸⁺ ion with toluene. But, this hump was disappeared when the solution was diluted to get other peaks of toluene. Curves d, e, f and g are the electronic spectra of spherical Os NPs organosol solutions for sample 1 to 4 respectively where the particles sizes are ∼1 ± 0.2 nm, ∼10–30 nm, ∼22 ± 2 nm and 31 ± 3 nm respectively. All these curves showed three surface plasmon resonance (SPR) bands at 479 nm, 598 nm and 647 nm. According to literature the absorption bands were shifted to either higher or lower wavelength side depending upon the size and shape of the particles²⁶ although in our present study for Os organosol, the absorption band almost remains nearly in the same positions but changes the absorption intensity. We assumed that the bigger particles are formed by the stacking of smaller ones so both the small and large particles absorb at the same region of the spectrum. Moreover, XRD analysis and SAED pattern also revealed the similar fact with the change in particle size.

Fig. 2 UV-Visible spectra of the reaction mixtures for the synthesis of Os organosol in toluene. Curve (a) is the electronic spectrum of toluene alone. Curve (b) represents the electronic spectrum of toluene after mixing with the PTC (TOAB). Curve (c) is the electronic spectrum of OsO₄ transferred to toluene. Curve (d), curve (e), curve (f) and curve (g) are respective SPR bands of synthesized Os NPs for four distinct samples.

Energy dispersive X-ray spectroscopic (EDS) and X-ray diffraction (XRD) studies

Fig. 3 shows the EDS spectrum of the synthesized spherical Os NPs organosol. From the image it was confirmed that all the elements presented in the precursors are preserved in the product after the reaction. All the expected peaks such as C, O, Na, Os, Si, Br and Ca were observed in the EDS spectrum. The O peak was observed due to the absorption of moisture from the environment. The C peak is due to presence of toluene, TBA⁺ and TOAB. The Br peak is due to the presence of phase transfer catalyst which is used to transfer the Os⁸⁺ ions to toluene layer. The peaks of Ca and Si came from the glass substrates used to fabricate the thin film of the material. The high intense Os peak came from the Os organosol. Fig. 4 shows the X-ray diffraction (XRD) pattern of the Os organosol. Very less intense peaks due to diffraction from 002, 101, 102, 110 and 103 planes are observed which were matched with the earlier reports where Os NPs were synthesized in aqueous environment.^17,18 The intensity of the peaks was low which was attributed to small size of the particles that not diffracted when X-ray fall on the samples and was consistent with the SAED analysis which was taken during TEM imaging. A big hump was observed between 20° and 30° as a consequence of glass substrate which was used to fabricate the thin film. From XRD and SAED analyses it was confirmed that the spherical Os NPs were either very small in size or non-crystalline in nature.

Fig. 3 Energy dispersive X-ray spectrum (EDS) of the synthesized Os organosol in toluene.

Fig. 4 The X-ray diffraction (XRD) pattern of the synthesized Os organosol in toluene.

FT-IR spectroscopic studies

In order to find out the interaction between the synthesized spherical Os NPs with the solvent and PTC, FT-IR spectroscopic studies were done. FT-IR spectroscopy is one of the versatile tools that can be employed to study the interactions of an organic compound with others. Fig. 5 shows the respective FT-IR spectra of only toluene (curve a), OsO₄ transferred toluene with the help of the TOAB (curve b) and the Os NPs organosol in toluene (curve c). Curve a has four sharp predominant peaks at 2920 cm⁻¹, 2851 cm⁻¹, 2200 cm⁻¹ and 1452 cm⁻¹ respectively. The peaks at 2920 cm⁻¹ and 2851 cm⁻¹ are due to the stretching vibrations of =C–H and –C–H bonds in toluene. The peak at 2200 cm⁻¹ stands for the asymmetric stretching frequency of C=C bonds in toluene. The peak at 1452 cm⁻¹ is the characteristic aromatic C=C deformational vibration which was expected in toluene. The peak observed at 1526 cm⁻¹ in both curve b and c was due to C–N stretching of tetrabutylammonium ion. The peak at 3674 cm⁻¹ was due to adsorption of moisture or water molecule during transfer of Os ions to toluene layer. Curve b represents the corresponding FT-IR spectrum of the OsO₄ transferred to toluene solution. From this, it was clearly understood that there were some interaction between the transferred osmium ions with toluene and TOAB. This was indicated by the shifting, broadening and appearances of new peaks in addition of the fundamental peaks of toluene. From curve c, it was witnessed that the interactions were still retained even after the reduction of OsO₄ to Os⁰ using TBABH₄. The changes in the peak positions and intensity were attributed to the interaction of Os particles with toluene. Similar types of peak shifting and change in FT-IR intensities were observed in our earlier reports where we used DNA and CTAB as stabilizing agent.^17,18 From the FT-IR study it was attributed that the osmium NPs were stabilized by the toluene molecules via π-stacking intercalation interactions as well as due to the presence of cationic tetrabutylammonium ions.

Fig. 5 The Fourier-transform infrared (FT-IR) spectrum of toluene, OsO₄ transferred toluene and the Os NPs in toluene. Spectral line (a) stands for toluene; (b) for toluene and PTC (TOAB) mixture and (c) for Os NPs in toluene.

Reaction mechanism for the formation of osmium organosol in toluene

Spherical Os NPs organosol with controllable sizes had been synthesized by exploiting two phase extraction procedure in toluene and details are given in experimental section. TOAB had been used as phase transfer catalyst (PTC) to transfer OsO₄ into toluene. We had seen that taking toluene twice the volume (40 mL) towards the volume of aqueous OsO₄ solution (20 mL), the maximum transfer was made possible. Once TBABH₄ was added to the toluene solution containing Os⁸⁺ ions, the initial pale yellow color of the solution turned into pale violet. After 5 min, the pale violet color became dark violet. This was the indication of completion of the reaction. As discussed earlier, TOAB acted as PTC and TBABH₄ used as reducing agent. We believe that both toluene and TBA⁺ (tetrabutylammonium ion) assist for the stabilization of the synthesized Os NPs.

In our reaction, we used OsO₄, TOAB, toluene and TBABH₄ as precursors and did few control experiments to check the role of individuals. Initially, keeping all the other reaction parameters fixed, we reduced the Os⁸⁺ ions in toluene using NaBH₄ instead of TBABH₄. The solution became dark violet after 5 min. of shaking but a certain % of particles were settled down after 1 h. The remaining solution retained some less intense violet color which indicated that toluene was able to stabilize them to a notable extent. In second control experiment, we used cyclohexane instead of toluene and keeping all other parameters fixed. OsO₄ was transferred to cyclohexane by TOAB and then the same quantity of the solution was reduced with TBABH₄. The resultant solution mixture turned into dark violet initially but the settling of particles was observed after 1 h there too. The remaining part of the above solution retains less intense violet color which indicated that the tetrabutylammonium ions could also be able to stabilize some of the Os NPs after their formation. For another control experiment, we conducted our reaction in an aqueous solution instead of organic solvent but reduced only with TBABH₄. We have seen that reduction took place in a faster rate compared to organic phase and the color of the reaction mixture was dark violet. The resultant Os NPs from the aqueous solution get precipitated after their formation (within 5–10 min.). The remaining part of the above solution was observed to be colorless which indicated that in a medium of high dielectric constant like H₂O, TBABH₄ was unable to stabilize the Os NPs. From all the control experiments discussed above, we believed that both TBA⁺ ions and toluene were playing the important role for the stabilization of the synthesized Os NPs organosol. Nevertheless, it should be emphasized here that the size selectivity was due to the variation made in the quantity of TBABH₄ added. So the major contribution for stabilization of particles arises from the tetrabutylammonium cation and the π-stacking intercalation effect from toluene was assisting the tetrabutylammonium cation in stabilizing the Os NPs to some more extent. At once TBABH₄ was added, it reduced the Os⁸⁺ ions to Os⁰. Once the small Os⁰ nuclei are formed they grow and initially small crystalline particles were formed. These small crystallites were assembled together to form small particles. These small particles grew further and formed bigger size particles. As the amounts of tetrabutylammonium cation added was reduced gradually from sample 1 to sample 4, the reduction of Os⁸⁺ ions also becomes slower. It is well known that slow reduction process can generate larger size particles compared to fast nucleation processes as we observed in our synthesis of sample 1. So the fast nucleation and fast reduction of sample 1 generated smaller size Os particles whereas slow nucleation and slow reduction generated larger size particles. Moreover, as the concentration of TBA⁺ ions reduced from sample 1 to sample 4, TBA⁺ cannot cap homogeneously onto the formed Os particles in case of sample 4 compared to sample 1. So the growth of the particles was not restricted in sample 4 and consequently generated larger size particles. The precursor OsO₄ is a 16 electron complex and acts as a Lewis acid. It is reducible with many reducing agents including molecular hydrogen generated in an in situ process. Since we used TBABH₄ as a reductant the solubility problem in organic solvent of metal borohydride reduction was overcome. The overall formation is depicted in Scheme 1 and the most probable chemical reaction taking place is given below. The size of the Os particles in all the four samples are 1 ± 0.2 nm, 10–30 nm 22 ± 2 nm and 31 ± 3 nm respectively. The synthesized spherical Os NPs organosol were applied in two different applications for the catalytic reduction of Cr⁶⁺ in presence of sodium thiosulphate and in SERS studies. The same is depicted in Scheme 2.

Scheme 1 The schematic representation for the formation mechanism of spherical Os NPs in toluene.

Scheme 2 Synthesis, catalytic and SERS applications of prepared Os NPs organosol in toluene.

Dissociation of TBABH₄:

(n-Bu)₄NBH₄ → (n-Bu)₄N⁺ + [BH₄]⁻

Reduction of OsO₄ by [BH₄]⁻:

OsO₄ + [BH₄]⁻ → Os⁰ + [BO₂]⁻ + 2H₂O

Overall reaction:

(n-Bu)₄NBH₄ + OsO₄ → Os⁰ + [(n-Bu)₄N]⁺[BO₂]⁻ + 2H₂O

Catalytic activity of spherical Os NPs for the reduction of hexavalent chromium (Cr⁶⁺) ions

Hexavalent chromium and its compounds are the most significant threats to the environment because of their extensive toxicity. The eluent disposed by tanneries and other related industries are requiring more sophisticated but less expensive routes in order to ensure the minimal hexavalent chromium content. Though some other eco-friendly methods are available for tanning such as vegetable tanning and oil tanning, chrome tanning is the most preferable one many industries. Because, they are cheap, less time consuming and ease of application. Beyond these advantages the toxicity posed a great barrier towards the use of dichromate for industrial large scale tanning processes. Up to now, a number of methods are already reported for the reduction of Cr⁶⁺. Reduction assisted by microbes and their metabolism and photocatalytic activities of conducting transparent metal oxides with dye sensitizers are the predominant methods and very well-known in literature. Rajkumar et al. reported the reduction of Cr⁶⁺ by pseudomonas sp.²⁷ Reduction of chromate (CrO₄)²⁻ by an enrichment consortium and an isolate of marine sulphate-reducing bacteria was reported by Cheung et al.²⁸ Gao et al. reported the removal of Cr⁶⁺ ions from the waste water by combined electro coagulation-electro floatation methods without any filter.²⁹ The toxicity of Cr⁶⁺ on human DNA was extensively studied and reported by Flores and his co-workers.³⁰ In order to overcome this pitfall of using Cr⁶⁺ in tanning it is advised to treat the eluent before the disposal by a method which can be fast, eco-friendly and economically affordable. The most studied oxides for the reduction of Cr⁶⁺ are TiO₂ and ZnO. Yang et al. studied the visible light reduction of Cr⁶⁺ with ZnO NPs by taking the concentration change as a function of time at different pH values.³¹ Aarthi et al. reported the reduction of Cu²⁺ and Cr⁶⁺ with the combustion synthesized TiO₂ NPs and a dye sensitizer.³² Chen et al. reported the Mn²⁺ catalyzed Cr⁶⁺ reduction with citrate.³³ Interestingly, Bi nanowires had also been investigated and found to be successful towards the reduction of hexavalent chromium (Cr⁶⁺).³⁴ All the above methods required more reaction time which was ranging from 5–17 h which makes the industries to spend more time on the reduction of Cr⁶⁺ rather than their core processes. Some of the above methods were too costly. Recently, Yang et al. reported the reduction of Cr⁶⁺ with ZnO mesocrystals under UV irradiation within 3 h and 20 min of reaction time.³¹ By referring all these previous reports, the catalytic efficiency of our material was also studied for the reduction of Cr⁶⁺ in a less expensive and a less time consuming way. The details of the catalysis reaction are given in experimental section.

The successive reduction of Cr⁶⁺ ions by thiosulphate in presence of Os organosol as catalyst was monitored by UV-Vis spectrophotometer. In our study, we did some control experiment to check the role of our catalyst. We checked the reaction without catalyst but keeping other things as such, where we observed no reduction. We also checked our reaction keeping all to be the same but in the absence of UV light and there was no reaction took place in our experimental time scale. We also checked the reaction in visible light keeping other reaction parameters fixed, there too we observed no reduction. Finally keeping all other reaction parameters fixed, we conducted the reaction in absence of thiosulphate. There also we found that reduction of Cr⁶⁺ did not take place in our experimental time scale. So a proper concentration of all the reagents was extremely necessary to carry out the reaction. In the catalysis reaction, it was observed that initially the color of the solution was pale yellow. As the reaction proceeded it began to fade and at the end it became a white turbid solution. During the reaction, in every 4 minutes, 3 mL of the reaction mixture (only the aqueous layer) was drawn and the absorption spectrum was recorded using UV-Vis spectrophotometer. Fig. 6A shows the representative UV-Vis spectra of the whole reduction process. From Fig. 6A, it is understandable that the characteristic peaks of Cr⁶⁺ is observed at 195 nm, 244 nm and 342 nm which were gradually decreasing in their intensity as the reaction time increases. This indicated the reduction Cr⁶⁺ concentration with time. Incredibly all the dichromate that we had taken was reduced within 1 h and 40 min. time. The inset of Fig. 6A shows the yellowish orange color hexavalent chromium ions and white turbid final product. We had seen that reaction followed first order reaction kinetics with respect to dichromate concentration. From the ln(conc.) versus Time (min) plot given as Fig. 6B, the rate constant value was found to be 1.3 × 10⁻² min⁻¹. The corresponding standard deviation and correlation coefficient values are 0.103 and 0.96 respectively. We had seen that our reaction is faster than the earlier reports.^28,31,33,35 In our catalysis reaction thiosulphate acted as a reducing agent and electron transfer happened from thiosulphate to Cr⁶⁺ via the Os NPs and Cr⁶⁺ got reduced to its lower valence state. From this, it is clear that Os organosol acted as a catalyst which helped during the electron transfer process for the reduction. As the catalyst used was in organic solvent, the separation of catalyst from the reaction mixture and its recyclability became much easier compared to homogeneous catalysis methods. In our earlier reports^17,18 we had seen that Os NPs synthesized in aqueous solution can be used as a catalyst for the reduction of aromatic nitro compounds, however, the separation of catalyst particles was much difficult or rather impossible as both reactants were in the same phase. The recyclability of the catalyst was also examined and found that the catalytic efficiency was not lost even after 7–10 cycles for the same reaction. The overall catalysis study was done with the sample 3 solution where the average Os particles size was 22 ± 2 nm. However, other samples were also tested for preliminary study and observed that all four samples could be used as catalyst for this reaction. Moreover, the same catalysis reaction was carried out again after 6 months taking the same Os organosol where we observed almost similar catalytic activity. Although a minute reduction of catalytic rate was noticed might be due to surface passivation. A more detailed study on the size effect on the same catalysis reaction will be conducted and discussed in near future.

Fig. 6 (A) The UV-Visible spectra for the successive catalytic reduction of hexavalent chromium ions by thiosulphate in presence of Os organosol in toluene as catalyst. Inset shows the yellowish orange color hexavalent chromium ions and white turbid product. (B) ln(conc.) versus Time (T, min) plot for the same catalytic reaction.

Surface Enhanced Raman Scattering (SERS) studies using Os NPs organosol

Surface Enhanced Raman Spectroscopy (SERS) is a surface phenomenon of metal NPs which have surface plasmon resonance (SPR) at localized positions on its surface. These positions are called ‘hot spots’. Any molecule with Raman shift values comparable to that surface plasmon resonance bands are suitable to be a probe molecule. The clear-cut theory behind the so called SERS phenomenon of a coinage metal surface is not well-known until now. Nevertheless, there are two widely accepted theories, namely ‘Electromagnetic field theory’ (EMFT) and ‘Chemical Theory’ (CT). Each theory has its own leads and drawbacks. In the case of metal NPs, the EMFT is the more appropriate one than the CT. According to EMFT, the molecule of which signal is to be enhanced should get adsorbed to the ‘hot spots’ of the metal surface physically whereas with CT it is demanded that the molecule should coordinate chemically to the surface. Moreover, according to the EMFT, the orientation of the molecule should be perpendicular to the direction of excitation of plasmonic band electrons. This reveals why the surface needs to be coarse for better enhancement. The SERS phenomenon was first observed by Fleischmann et al.³⁶ and later by Blatchford et al.³⁷ Among the different metals checked for SERS, the reports using coinage metals Ag³⁸ and Au³⁹ are rich in literature. Beyond these, other platinum group metals were also studied intensively either as a single metal¹⁸ or their alloys/core–shells.⁴⁰ There are many reports on SERS studies with various metal NPs with various sizes and morphologies although the reports in case of Os is very less and those are only in aqueous medium.^14,17,18 Beyond this silicon nanowires with SPR also reported to be an efficient substrate for SERS application.⁴¹ The SERS application of metal NPs in an organic solvent is also the desired one. The synthesized Os NPs organosol have SPR bands at 479 nm, 598 nm and 647 nm. Recently, we reported the DNA mediated wire-like Au and Ag clusters with different size and shape can influence the SERS signal.^38,42 We also tested that DNA encapsulated Os NCs can be used as potential SERS substrate in aqueous medium.¹⁷ Here, first time we are going to highlight the SERS effect by taking spherical and monodispersed Os organosol as a substrate in presence of MB as SERS probe molecule.

Stock solutions of MB dye were prepared in acetone with varying concentrations as follows 10⁻³ M, 10⁻⁴ M, 10⁻⁶ M and 10⁻⁸ M. About 500 μL of each of those dye stock solutions were separately mixed with 500 μL of Os NPs organosol and kept for 20 min. after vigorous shaking to ensure the homogeneity throughout the solution. After that, 10 μL of each of those solutions were poured on a clean dry glass substrate and dried in air at RT which were used for SERS studies. Raman spectrum for each of these samples were taken and plotted. It is important to note here that we choose acetone instead of toluene for mixing with MB is due to better miscibility and solvating ability of acetone compared to toluene towards MB dye. Both in toluene and in acetone, MB gives similar types of absorption feature. Fig. 7A is the chemical structure of MB dye. Fig. 7B is the camera images of the reaction mixture before the SERS experiment, where a is of only MB dye solution, b is of only Os NPs solution and c for the mixture of MB dye and Os NPs solution. Fig. 7C shows the UV-Vis spectra of the solutions mixtures as given in the camera image in Fig. 7B. In Fig. 7C, curve ‘a’ denotes the MB dye solution in acetone, curve ‘b’ denotes the Os NPs solution and curve ‘c’ denotes the mixture of both. In curve ‘a’ we can observe three peaks at 207 nm, 270 nm and 654 nm where the peaks at 207 nm and 270 nm are due to π–π* and n–π* transitions of acetone respectively. The peak at 654 nm is due to the MB dye. Curve ‘b’ is the UV-Vis spectrum of Os NPs solution in which three SPR bands of Os are observed at 479 nm, 598 nm and 647 nm respectively which is already discussed in the UV-Vis study in Fig. 2 earlier. Curve ‘c’ is the UV-Vis spectrum of the mixture of MB dye and Os NPs solution. The two SPR bands at 479 nm and 598 nm were not shifted but the intensities were enhanced whereas the SPR band at 647 nm was shifted to 654 nm and the intensity was also increased which indicated the interaction between the dye molecule and the Os NPs. Fig. 8 shows the Raman spectra of MB solution and surface enhanced Raman spectra (SERS) of the MB dye molecules (at various dye concentrations) with Os NPs. Curve ‘a’ stands for only MB dye solution and curves ‘b’, ‘c’, ‘d’ and ‘e’ are the SERS spectra of dye solutions with 10⁻³ M, 10⁻⁴ M, 10⁻⁶ M and 10⁻⁸ M. From these the enhancement of Raman signals is clearly seen. The enhancement factor (EF) value was calculated using the formula used in our earlier reports.^17,18 The result and the calculation of the EF value are discussed below. The main reasons for taking MB as a probe molecule were it can give sharp distinct peaks within the range of 400 to 2000 cm⁻¹ and it is a big molecule having charge states, less expensive, soluble in water as well as in other organic solvents. The maximum EF value was calculated to be 1.6 × 10⁵ for the dye concentration of 10⁻⁸ M. Though the EF value is less compared to the value that we observed for Os NCs prepared in aqueous medium,^17,18 it is a worthy work to find the effect of change of solvent from polar to non-polar. As far as our knowledge, this is the highest EF value ever reported for metal NPs in an organic solvent like toluene. Moreover, the SERS application of Os organosol may be utilized further in sensors where the aqueous Os metal NPs are not applicable. For example, in fine chemical industries, large scale organic synthesis reactors, electro-organic synthesis bath where the presence of water will lead to unwanted side reaction and narrow down the application of the synthesized materials. The whole study by taking spherical Os NPs organosol in SERS is schematically depicted in Scheme 3. We checked the SERS activity by ageing the sample for 6 months in ambient condition and observed almost similar EF values what we observed with the as-synthesized samples. This again proves the high stability of our synthesized Os organosol. To understand the effect of change in polarity of the solvent in SERS activity and to investigate further about the stabilization of NPs by secondary forces such as H-bonding, π–π stacking, π-metal stacking and other supramolecular forces, detailed experimental studies and theoretical simulations will be conducted in near future.

Fig. 7 (A) Molecular structure of the methylene blue (MB) dye in its oxidized form. (B) The optical image of MB dye solution (left), Os NPs solution in toluene (middle) and the mixture of MB and Os NPs solution. (C) UV-Visible spectra of the reaction mixture for SERS study. Curve ‘a’ is the absorption spectrum of MB in acetone; curve ‘b’ is the SPR band of Os NPs in toluene; curve ‘c’ is the absorption band of the mixture of MB dye and the Os NPs.

Fig. 8 Raman spectra of MB dye (curve a) and SERS spectra of MB with Os NPs at a MB concentration of 10⁻³ M (curve b), 10⁻⁴ M (curve c), 10⁻⁶ M (curve d) and 10⁻⁸ M (curve e) respectively.

Scheme 3 The schematic representation of the SERS studies carried out using MB dye and the synthesized Os NPs organosol in toluene.

Conclusion

In summary, spherical Os NPs with varying and controllable sizes (1 ± 0.2 nm, 10–30 nm, 22 ± 2 nm and 31 ± 3 nm) were prepared by two phase extraction (water-toluene) procedure for the first time. Os organosol in toluene with varying sizes were prepared just by changing the concentration of TBABH₄ in toluene medium by employing phase transfer catalysis mechanism. It was found that at higher reductant concentration the particles are either not stable or not giving particles with uniform size whereas at lower concentrations size-selectivity was good, controllable and reproducible. The synthesized Os organosol had been utilized in catalysis and in SERS studies. The catalytic activity of the synthesized spherical Os NPs was examined for the reduction of hexavalent chromium (Cr⁶⁺) ions with thiosulphate in presence of UV light and found that the reduction was fastest ever reported and completed within a short time having a rate constant value of 1.3 × 10⁻² min⁻¹. The recyclability of the catalyst was also checked and the activity was retained up to 7–10 cycle without any significant loss in catalytic activity. The SERS activities of the Os NPs organosol were studied by taking MB as SERS probe molecule and the maximum EF value found was 1.6 × 10⁵ at a dye concentration of 10⁻⁸ M which is the highest value so far reported for a metal organosol in toluene. Overall, we reported a new route for the synthesis of spherical Os NPs organosol in toluene which showed enhanced SERS and catalytic application. In futuristic, we will try to understand the effect of solvent polarity on SERS activity and the theoretical simulation to find the ability of π-stacking more precisely in stabilizing the nanomaterials.

Acknowledgements

S. Anantharaj and S. R. Ede would like to acknowledge Council of Scientific and Industrial Research (CSIR) for JRF fellowship. U. Nithiyanantham wants to thank CSIR-CECRI for research internship fellowship. The support from the Central Instrumental Facility (CIF), CSIR-CECRI, Karaikudi are greatly appreciated. Authors are willing to thank Dr Vijayamohanan K. Pillai, Director, CSIR-CECRI for his all-time support and encouragement.

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