Osmium Organosol on DNA: Application in Catalytic Hydrogenation

Article pubs.acs.org/IECR

Osmium Organosol on DNA: Application in Catalytic Hydrogenation Reaction and in SERS Studies S. Anantharaj, U. Nithiyanantham, Sivasankara Rao Ede, and Subrata Kundu* Electrochemical Materials Science (ECMS) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006, Tamil Nadu India S Supporting Information *

ABSTRACT: Osmium (Os) organosol on DNA scaffold has been synthesized by utilizing a homogeneous reduction route. The synthesis was done by the reduction of OsO4 with tetrabutylammonium borohydride (TBABH4) in the presence of DNA in acetone within 10 min of stirring at room temperature. Different morphologies were synthesized by varying the DNA to OsO4 molar ratio and controlling the other reaction parameters. The eventual diameters of the individual Os particles in organosol were ∼1−3 nm, and the nominal lengths of the wires were ∼1−2 μm. The potentiality of the Os organosol was tested in two different applications: one is the catalytic hydrogenation of cyclohexene to cyclohexane and other is the surface enhanced Raman scattering (SERS) studies. The SERS study has been examined using MB as a Raman probe, and the EF value is found to be the highest in the case of Os organosol having aggregated wires (short size) compared to longer wires. The fast synthesis of Os organosol on DNA and their potential catalytic and SERS activity will be found to be very useful in the near future for the catalytic applications of various organic reactions and in the fields of sensors, electronic devices, and fuel cells.

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INTRODUCTION Nanostructured materials have attracted a great deal of attention because of their novel size and shape dependent electronic, magnetic, optical, and catalytic properties that differ drastically from their bulk materials. Materials at nanoscale regime, in particular, were expected to find useful applications in nanoelectronics, photonics, catalysis, and surface enhanced Raman scattering (SERS) studies.1−4 Moreover, the collective properties of nanoparticles (NPs) depend critically on their interparticle spacing and hierarchical organizations. For example, Ag NPs having interparticle spacing 1.5 to 2 nm shows a promising substrate in SERS compared to Ag NPs where interparticle spacing is larger.5 The top down approach is the most widely used method for arrangement of NPs at appropriate positions, but it is a tedious process. On the other hand, the bottom up approach was found more advantageous in terms of self-assembly although its application in controlling well-defined patterns on NPs assemblies is still limited and needs to be explored more. The synthesis of NPs through the bottom up approach via the wet chemical route in an aqueous environment is easy to manipulate due to a high dielectric constant of the medium which helps the ions and other molecules for easy solubilization although, apart from the great advantage in aqueous phase synthesis, there are other inherent difficulties such as removal of the excess stabilizer residue after synthesis, surface modification of synthesized particles, and increase in particles concentrations. Moreover, in aqueous solution, NPs always begin to agglomerate while increasing the concentration of particles. All of these problems can be easily overcome by synthesizing the particles using organic solvents. In the last couple of years the development of nanoscience and nanotechnology involves synthesis and stabilization of metal NPs in organic solvent.6−8 The synthesis of NPs in organic solvent has found certain © 2014 American Chemical Society

advantages. It shows a high degree of control over NPs size, monodispersity, and chemical nature of the NPs surface. There are a wide number of reactions that are carried out in organic media. There are mainly two approaches looked at in the literature for the synthesis of NPs in organic media. One is the transfer of metal ion from aqueous solution to organic solvent by using the phase transfer catalyst followed by reduction using a reducing agent. Another is the direct transfer of metal NPs to the organic media from aqueous media using the phase transfer catalyst, but, for both occasions, the major drawback is to remove the phase transfer catalyst from the synthesized NPs solution which contaminates the organosol while using different application purposes. Without the proper application of organosol systems, the study often fails to register the impact of the solvent on the organosol properties. Synthesis of organosol of different metals like Au,6 Ag,7 Pt,9 and Cu10 and metal oxides like MnO28 and Fe2O311 has been studied earlier6−11 although there is no study at all on Os metal NPs. While considering other noble metals, Os was found to have interesting characteristics due to its certain properties. Os metal has low compressibility but high bulk modulus which is comparable with diamonds and has a high melting point. As a result of various properties of Os and its oxides, Os metal is a demanding material to manufacture and work with. Different types of routes have been employed for the formation of Os NPs mostly in thin film form, for example, chemical vapor deposition (CVD),12 physical vapor deposition (PVD),13 electrodeposition,14 vacuum pyrolysis,15 and sol−gel16 techniques. Os metal generally alloys with other materials like Os Received: Revised: Accepted: Published: 19228

September 16, 2014 October 29, 2014 November 16, 2014 November 17, 2014 dx.doi.org/10.1021/ie503667y | Ind. Eng. Chem. Res. 2014, 53, 19228−19238

Industrial & Engineering Chemistry Research

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Table 1. Detailed Final Concentration of All the Reagents, Reaction Time, Average Particles Size, and Shapes set no.

concn of DNA in the final solution (M)

1 2

8.78 × 10−3 3.51 × 10−2

final concn of OsO4 (M)

amount of TBABH4 (mg)

total volume of the reaction mixture (mL)

total reaction time (min)

color of the Os organosol with λmax values (nm)

shape and average diameter of the particles (nm) and length of the chains

7.14 × 10−4 7.14 × 10−4

0.025 0.025

14 14

10 10

blackish violet (540) blackish violet (562)

wirelike (long), ∼2.6 ± 0.2 nm, 0.54 ± 0.03 micron aggregated wires (short), ∼1.2 ± 0.2 nm, 8−10 micron

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EXPERIMENTAL SECTION Reagents and Instruments. Osmium tetraoxide (OsO4), double-stranded DNA from herring testes with average size ∼50K bp (base pair), cyclohexene (99%), and tetrabutylammonium borohydride [TBABH4, (CH3CH2CH2CH2) 4N(BH4)] were purchased from Sigma-Aldrich and used as received. The dye molecule, MB (C 16H 18N 3SCl), was purchased from Qualigens Fine Chemicals, Mumbai. Organic solvents like acetone (CH3COCH3), ethanol (C2H5OH), ethyl acetate (CH 3 −COO−CH 2 −CH 3 ), isopropyl alcohol (C3H7OH), cyclohexane (C6H10), and chloroform (CHCl3) were purchased from Sisco Research Laboratory (SRL) in India. Deionized (DI) water was used for the entire synthesis process. The Os organosol on DNA scaffold was characterized using several instrumental techniques, and the specifications of all of these tools are given in detail in the Supporting Information (SI). Synthesis of DNA Capped Os Organosol. Os NPs having long wires and aggregated short wires were synthesized by tuning the concentration of DNA to OsO4 and by changing other different reaction parameters. For a typical synthesis, 11 mL of acetone and 1 mL of stock DNA solution (0.6 mg/50 mL) were mixed with 1 mL of stock OsO4 solution (10−2 M, stock solution), and the solution was stirred well and was colorless at the initial state of stirring. Then 0.025 mg of TBABH4 was added at once while under stirring conditions. After the mixing of TBABH4, initially a faint violet color appeared, and with an increasing reaction time the solution became dark violet in color. The solution exclusively contained Os NPs having wirelike morphology (longer sizes). The other morphology was also prepared by changing the reaction parameters. The details of all reagents concentration, reaction time, particles size, shape, etc. are summarized in Table 1. Preparation of Samples for Other Characterizations. The synthesized DNA capped Os organosol was characterized using UV−vis, TEM, EDS, XRD, XPS, and FT-IR analysis studies. The DNA capped Os organosol was directly used for the measurement in UV−vis spectrophotometer. The same liquid solution was used for TEM sample preparation and other thin films preparation. The samples for TEM was prepared by placing a drop of the corresponding DNA capped Os organosol onto a carbon coated Cu grid followed by slow evaporation of solvent at ambient conditions. For EDS, XRD, XPS, and FT-IR analysis, glass slides were used as substrates for thin film preparation. The glass slides were cleaned with acetone and sonicated for about 30 min. The cleaned substrates were covered with the DNA capped Os organosol and then dried in air. After the first layer was deposited, subsequent layers were deposited by repeatedly adding more DNA capped Os organosol and drying. Final samples were obtained after 6−8 depositions and then analyzed using the above techniques. For the catalysis study, the reaction procedure is given in the discussion of the catalysis reaction section below. For the SERS study, a stock solution of MB of 0.01 (M) was made, and other

doped SnO2 used as a sensor and Os−Pd NPs used in catalytic reactions.15,16 Different types of biomolecules like amino acids, peptides, and protein have been used as a template and as a stabilizing agent during NPs synthesis. Deoxyribonucleic acid (DNA) is also used as a template or scaffold for the synthesis of inorganic nanostructures as it is used as an inexpensive, wellcharacterized, controllable, and easily adoptable material whose physical and chemical properties can be explored in an easy way.17−19 DNA has two binding sites: one is aromatic base molecules and the other is a negatively charged phosphate group which can bind with different metal cations and positively charged metal NPs with electrostatic interaction. DNA templated metal and metal oxide NPs in aqueous solution are reported.17−19 There are a couple of reports for the synthesis of Os NPs in aqueous solution via wet chemical routes.20,21 Recently, there are a few other reports for the synthesis of noble metal NPs in the presence of DNA as a scaffold in aqueous solution.22−24 To the best of our knowledge there is no report at all for the synthesis of organosol of osmium using DNA as a scaffold within a short reaction time. In this article, we report a facile and homogeneous reduction technique for the synthesis of self-assemble wirelike Os NPs in organic solvent. The synthesis was done by the reaction of OsO4 with tetrabutylammonium borohydride (TBABH4) in the presence of DNA in acetone. The synthesis was done within 10 min of reaction, and the synthesized particles were found to be extremely stable for more than 6 months while stored in the refrigerator under dark conditions. The size and morphology of the particles can be tuned by controlling the different reaction parameters and by controlling the concentration of the reagents. The eventual diameters of the particles are varying in the ∼1−3 nm range, and the lengths of the wires are varying in the ∼1−2 μm range. The synthesized Os organosol can be redispersed in aqueous solvent as well as several other organic solvents like ethanol, ethyl acetate, isopropyl alcohol, cyclohexane, and chloroform without any problem. The synthesized DNA-Os organosol has been used in two potential applications such as in catalytic hydrogenation and SERS studies. The catalytic activity has been tested for the hydrogenation of cyclohexene to cyclohexane using DNA-Os organosol as the catalyst. The SERS study has been carried out using methylene blue (MB) as a model SERS probe molecule. Both morphologies of Os organosol have found to be SERS active, and the highest enhancement factor (EF) was observed to be ∼105 for different peak positions. To the best of our knowledge, the formation of DNA capped Os organosol occurred within 10 min of the reaction time, and their pronounced catalytic activity for the hydrogenation reaction and SERS study has not been explored earlier. The overall process is simple, free from any phase transfer catalyst contamination, robust, and cost-effective. 19229

dx.doi.org/10.1021/ie503667y | Ind. Eng. Chem. Res. 2014, 53, 19228−19238


Article

nm, whereas for aggregated wires (short size) it appeared at ∼562 nm. The λmax value of the organosol was matched with early reports of Os NPs synthesized in aqueous solution.20,21 Although an earlier theoretical study by Creighton et al. showed that Os NPs having a spherical shape with monodispersed particles can have a SPR peak at 350 nm.25 In our present study, the shifting of λmax value is probably due to aggregation of the individual small particles to generate the clusters in the wirelike morphology. The inset of Figure 1 shows the camera images of Os NPs solution for two different morphologies, respectively. Transmission Electron Microscopy (TEM) Analysis. The transmission electron microscopic (TEM) images of the Os organosol on DNA are shown in Figure 2. Figure 2a-c shows the low and high magnified images of Os organosol on DNA formed long wirelike structure. Figure 2a denotes the low magnification image, and Figure 2b and c shows the comparatively high magnified image. From the image, it is clear that the small size individual Os particles are selfassembled on the DNA chain and generated the wirelike structure. The average diameter of the individual Os particles is ∼2.6 ± 0.2 nm, and the average lengths of the wires are ∼0.54 ± 0.03 μm. The average diameter of the DNA chain is ∼7 ± 0.3 nm. The inset of Figure 2c shows the corresponding selected area electron diffraction pattern (SAED) which speaks that the particles are either noncrystalline in nature or they are very small in size. Figure 2d-f shows the low and high magnified image of the aggregated wires (short size). Figure 2d and e shows the low magnified image where we are clearly able to see that Os organosol aggregated in wirelike fashion when DNA concentration is high compared to Figure 2a-c where DNA concentration is less. Figure 2f shows the high magnified image of a single wire (short size). The average diameter of the single Os particles is ∼1.2 ± 0.2 nm. The nominal lengths of the aggregated structures are ∼8−10 μm. Although the high magnification images in Figure 2c and 2f of long size wires and aggregated wires of Os organosol seem to be similar to each other, it is very clear from the low magnified images as given in Figure 2a,b (long size wires) and Figure 2d,e (aggregated wires), respectively. While comparing those TEM images, the difference in their morphology is clearly visible which is not possible to identify visually as the size difference is not much. The morphological changes of two different structures are discussed briefly in the reaction mechanism section. The inset of Figure 2f shows the corresponding SAED patterns which shows that the particles are noncrystalline in nature or being very small in size which does not diffract using the electron beam used in TEM. So TEM analysis clearly shows that Os organosol having different morphologies can be formed by varying the reagent concentration as given in Table 1. Energy Dispersive X-ray Spectroscopic (EDS) and Xray Diffraction (XRD) Analysis. Figures 3a and 3b show the energy dispersive X-ray spectroscopy (EDS) analysis and X-ray diffraction (XRD) pattern of the Os organosol on DNA, respectively. The EDS analysis shows the expected elements of C, N, O, P, Os, Ca, and Na. The Ca peak came from the glass substrate used to deposit the Os organosol for EDS analysis. The C and O peak came from the DNA or from the acetone. The N and P peak came from the DNA used as a scaffold for the stabilization of Os NPs. The high intense Os peak came from the Os organosol. The presence of N and P peak with Os peak clearly indicates the formation of Os particle on DNA scaffold. The X-ray diffraction pattern of the Os organosol is

different concentrations were prepared by appropriate dilution with D.I. water as necessary. A measured volume of dye solution at a specific concentration was mixed with the Os organosol and finally deposited over the glass slides and dried in air. The dried samples were used for SERS measurement.

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RESULTS AND DISCUSSION UV−Vis Spectroscopic Study. The UV−visible (UV−vis) spectra for the synthesis of DNA capped Os organosol at

Figure 1. UV−visible spectra for the synthesis of DNA capped Os organosol at different stages of reaction. (a) shows the UV−vis spectra of only DNA solution in acetone, (b) shows the absorption spectra of a mixture of DNA and OsO4. The inset in the figure shows another UV−vis spectrum where (a1) denotes the surface plasmon resonance (SPR) band of Os organosol (wirelike, longer size) and (b1) denotes one SPR band of Os organosol (aggregated wires, short size), respectively. The inset shows the camera images of two different Os NPs solution.

different stages of reaction are shown in Figure 1. Figure 1a shows the UV−vis spectra of only DNA solution in acetone that shows the λmax value at 264 nm. This band for DNA appeared due to absorption of aromatic base molecules on its skeleton. The original band of DNA in aqueous solution appeared at 258−260 nm which red-shifted 4−6 nm due to change in the solvent dielectric constant. Only the OsO4 solution shows an absorption band at 245 and 278 nm in UV−vis spectra (not shown here). A mixture of DNA and OsO4 shows two distinct absorption bands at 248 and 274 nm as seen in curve b, Figure 1. The original absorption peak of OsO4 has been shifted a few nm which might be due to the adsorption of Os ions on the phosphate backbone or due to the interaction with the aromatic base molecules. Now after addition of TBABH4 and stirring, initially a faint violet color appeared within 1 to 2 min of reaction, indicating the nucleation and formation of Os NPs in acetone. With increasing time, the faint violet color became dark and finally after 10 min of reaction, the solution is fully dark violet in color. The completion of the reaction is confirmed via the consistent color intensity and the absorption value in the UV−vis spectrum. The inset of Figure 1 shows another UV−vis spectrum where (a) denotes the surface plasmon resonance (SPR) band of Os organosol (wirelike, long size) and (b) denotes one SPR band of Os organosol (aggregated wires, short size), respectively. The λmax appeared for long wires at ∼540 19230



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Figure 2. Transition Electron Microscopic (TEM) images of the Os organosol on DNA. (a-c) shows the low and high magnified images of Os organosol on DNA having long wirelike structure. The inset (c) shows the corresponding selected area electron diffraction pattern (SAED). (d-f) shows the low and high magnified image of the aggregated wires (short size). The inset of (f) shows the corresponding SAED patterns.

shown in Figure 3b. We did not find any strong or high intense peak rather having very low intense peaks are observed which assigned to the diffraction from (002), (101), (102), (110), and (103) planes of hexagonally close packed (hcp) Os particles having JCPDS card no. 06-0662.20,21 In Figure 3b, a1 and b1 show the diffraction pattern of two different types Os particles, respectively. The present XRD patterns are matches with the earlier report of Os while synthesis in aqueous solution or in the form of thin films via PVD routes. From XRD, we have not observed any sharp peaks as the size of the individual Os particles are very small which further conformed from the SAED pattern during TEM analysis as discussed earlier. At low 2θ values, a broad peak is observed in both the sample probably due to crystallization of unbound DNA molecules in the organosol. Similar types of XRD pattern were reported earlier by Mariam Al-Hinai et al. for their synthesis of palladium nanowire networks on DNA scaffold.26 So in our present study, EDS and XRD analysis shows that Os particles having a very small size are formed and stabilized by the DNA molecule. X-ray Photoelectron Spectroscopic (XPS) Analysis. Figure 4 shows the X-ray photoelectron spectroscopy (XPS) analysis of Os organosol on DNA. Figure 4a shows the XPS survey spectrum which consists of different elements at their specific binding energies. The peaks appeared at a binding energy of 1018 eV for Na 1s, 532.1 eV for O 1s, 474.4 eV for Os 4p, 290 eV for C 1s, 282.3 eV for Os 4d, 204.7 eV for Si 2s, and 63.7 eV for Os 4f, respectively. The Si 2s peak came from the glass substrate used to deposit Os organosol for XPS analysis. The C 1s peak came from the DNA. The high resolution Os 4d peak is shown in Figure 4b. The Os 4d5/2 peak appeared at a binding energy of 288.6 eV, while the Os 4d5/2 (satellite) peak appeared at a binding energy of 293.2 eV. The

O 1s and C 1s XPS spectrum are shown in Figures 4c and 4d, respectively. For O 1s, the peak appeared at a binding energy of 532.1 eV, and for C 1s it appeared at a binding energy of 290 eV which matches with the NIST XPS data file correctly. Our XPS result found similarity with other earlier reports of Os NPs.20 Fourier Transform Infrared Spectroscopic (FT-IR) Analysis. The FT-IR spectrum of the Os organosol on DNA is shown in Figure 5. Here we plot the FT-IR spectrum of wirelike morphology (long size). Other morphology was also tested which gave almost similar types of spectrum which was quite expected as both the morphologies are stabilized by DNA only. The spectra for only DNA are also plotted for comparison purposes. Curve a denotes the FT-IR spectrum of only DNA, and curve b denotes the FT-IR spectrum for Os organosol on DNA. For only DNA, three intense peaks at a lower wavenumber region at 502, 617, and 931 cm−1 appeared which are assigned to the deoxyribose region. All the peaks appeared due to the presence of the deoxyribose unit in the DNA structure. All these peaks have been shifted or appeared at different wavenumbers 562, 648, and 820 cm−1 in the case of Os organosol on DNA which confirms that the Os particles are attached with DNA. The peak for only DNA at 1132 cm−1 is due to the stretching vibration of C−O−C or C−C bonds. There are two other peaks appearing at 1236 and 1303 cm−1 which are due to asymmetric stretching of the PO3−2 group that specific bands are not visible in the case of Os organosol on DNA. The appearance of a peak at 1494 cm−1 is due to the bending mode of C−H bonds in the CH2 group which is shifted to 1500 cm−1 in the case of Os organosol on DNA. The peak near 1730 cm−1 is due to CO, CNH stretching and N−H bending vibration which shifted to 1765 cm−1 in the case 19231



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h) due to the absence of any stabilizing agents. While keeping other reagents the same and in the absence of TBABH4, no Os particles were formed in our experiment time scale. So the presence of all the reagents as given in the experiments mentioned in Table 1 is important for the formation of specific morphology of Os particles on DNA. In the present synthesis we directly synthesized Os particles in acetone which was found to be very stable after the synthesis, and there was no need of any phase transfer catalyst for the transfer of the ions from aqueous phase to organic phase. Moreover, the organosol in acetone can be easily dispersed in other organic solvents like EtOH, isopropyl alcohol, ethyl acetate, cyclohexane, chloroform, ethyl acetate, etc. Initially, after the addition of OsO4 on DNA, there is a shifting of adsorption bands of pure DNA as well as the original peak of OsO4 being also shifted as seen in Figure 1, in the UV−vis spectrum, which is due to adsorption of Os ions with the DNA molecule via electrostatic interaction due to the presence of oppositely charged ions. Now once the TBABH4 is added to the solution mixture, it reduces the OsO4 to generate Os0 particles, and the solution color changes from clear solution to faint violet which further turns to dark violet at the end. The possible chemical reaction that can take place is (CH3CH 2CH 2CH 2)4 N+BH4 − → (CH3CH 2CH 2CH 2)4 N+ + BH4 −

BH4 − + 4H 2O → H3BO3 + OH− + 4H 2 Os8 + + 4O2 − + 4H 2 → Os0 + 4H 2O

So OsO4 is reduced by TBABH4, and Os0 particles are formed. The small Os (0) nuclei are aggregating together to form small crystalline Os particles. After that small Os (0) particles grow together and form Os NPs, having definite size. Sampath et al. reported earlier that Os3+ ions get reduced by ascorbic acid to form small Os clusters, and these clusters attract each other to form small size NPs and finally those particles reach an equilibrium size and self-assemble together to form a nanochain structure.20 Pal et al. described the mechanism of formation of MnO2 organosol in toluene.10 They described that the larger size particles are formed by the growth of small particles via Ostwald ripening process. The growth of crystal and dissolution of nanocluster are taking place simultaneously, so the formation of uniform particles can be accounted as a consequence of the balance between solubilization and crystal growth in a low dielectric solvent. Moreover the dissociation process is easier and faster, while the growth process is comparatively slower because small particles often grow more rapidly than macrocrystals. So finally a uniform size distribution takes place which is a general characteristic of organosol unlike hydrosol where unidirectional growth becomes an usual path.28 In our present study, we also believed that initially small Os (0) nuclei are formed, and finally they grow to uniform bigger particles as suggested by Pal et al. earlier.10 In our study, DNA acts as a stabilizing agent, and it also helps during the growth process. The stabilization action of DNA was reported earlier.17−19 Another important thing that needs to be noted is that the present organosol formation is tested with NaBH4 also, but we observed that Os particles are formed but the Os particles becomes agglomerated within a short time as NaBH4 is not fully dissolved in organic solvents also. As discussed in Table 1, Os organosol having two different morphologies were synthesized by controlling the reaction parameters. It was seen

Figure 3. (a) Shows the energy dispersive X-ray spectroscopy (EDS) analysis of the Os organosol on DNA. (b) Shows the X-ray diffraction patterns of the Os organosol where a1 and b1 denote the diffraction pattern of two different morphologies of Os organosol.

of Os organosol on DNA clearly indicates the interaction among them. The peak at 2891 and 2987 cm−1 in both spectrums is due to the symmetric stretching vibration of C−H bonds in the −CH2 groups from the DNA molecule. The broad peak at 3625 cm−1 in the case of DNA is shifted to 3616 cm−1 in the case of DNA bound Os organosol. This phenomenon is due to the stretching vibration of the hydroxyl (−OH) group that probably appears from DNA or by adsorbing atmospheric moisture. From the FT-IR analysis on Os organosol on DNA it is clear that the parent DNA peaks have been shifted or are not visible after binding with Os organosol. The shifting of parent peaks after binding with DNA was observed earlier by others.17−19 Original FT-IR bands for DNA27 and the bands observed by our experiments taking herring testes DNA and their corresponding band assignments are elaborated in Table T-1 (SI). Mechanism of Synthesis of Os Organosol on DNA. Os organosol having wirelike (long size) and aggregated wires (short size) morphologies has been synthesized by the reaction of OsO4 with TBABH4 in the presence of DNA in acetone for 10 min of continuous stirring. We have conducted a few control experiments to check the importance of DNA and other reaction parameters in our synthesis. We have seen that by keeping all parameters fixed but in the absence of DNA, Os particles were formed but precipitated within a short time (1−2 19232



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Figure 4. X-ray photoelectron spectroscopy (XPS) analysis of Os organosol on DNA. (a) shows the XPS survey spectrum; (b) shows the high resolution Os 4d peak; (c) shows the high resolution O 1s peak; and (d) shows the high resolution C 1s peak.

that at low DNA concentration, the process generates wirelike morphology (long size), while with comparatively high DNA concentration, the process generates aggregated wires (short size). While DNA concentration is less, the Os particles are grown only on the DNA chain homogeneously and generate the wirelike morphology. While DNA concentration increases, the particles did not get enough space to grow and the DNA molecules themselves cross-linked together to form aggregated structures. There are few wires (short size) separately also found from the TEM analysis which is due to the breakdown of longer wires during the growth process. Now with these Os organosol, we examine two possible applications: one is the catalytic hydrogenation of cyclohexene to cyclohexane and the other is the SERS studies as discussed below. Catalytic Hydrogenation of Cyclohexene to Cyclohexane Using Os Organosol As a Catalyst. Transition metals and their ions have revolutionized the catalysis field in inorganic as well as in organic chemistry. Many transition metal catalyzed organic reactions have been industrialized already such as Ziegler−Natta polymerization, the Wacker process, the

Monsanto acetic acid process, and the hydroformylation reaction. Other than these some interesting C−C coupling reactions like the Suzuki reaction, the McMurry reaction, Takai alkenylation, the Negishi reaction, and the Stille reaction are using transition metal as the catalyst. Remarkably almost 90% of these reactions are carried out using group VIII metals and metal ions. As one of the metals from the eighth group of the periodic table osmium also gives some interesting catalytic activities. Its compounds are used as reagents in organic chemistry. For example osmium tetroxide is the well-known species used as the reagent in syn dihydroxylation of alkenes and successive oxidative cleavage reaction to produce carbonyl compounds. The synthesized Os organosol on DNA has found to be miscible with almost all other organic solvents. As described earlier, there is no report of Os organosol and their applications in catalysis. The hydrogenation of alkenes in the presence of eight group metals such as Pt, Pd, Ni, Au, Ru, Ir, Rh, and even Fe has been reported in the literature29−36 although there is rare data available in the case of Os which says that osmium is very poor in hydrogenating alkenes. Odebunmi 19233



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Scheme 1. Chemical Structure of the Oxidized Form of MB (Upper Structure) and Three Different Color Solutions Containing Only MB (Bottom Left), Only Os Organosol (Bottom Middle), and a Mixture of MB and Os Organosol (Bottom Right) As Indicated by a, b, and c, Respectively

Figure 5. Fourier Transform Infrared Spectroscopy (FT-IR) analysis of Os organosol on DNA. Curve a denotes the FT-IR spectrum of only DNA, and curve b denotes the FT-IR spectrum for Os organosol on DNA.

Figure 7. (A) shows the normal Raman spectra of 10−3 M MB solution. (B) shows the normal Raman spectra of DNA (curve a) and Raman spectra of DNA capped Os organosol (curve b). Figure 6. (a) The UV−vis absorption spectra of only cyclohexene (curve a1) and curve b1 shows the absorption spectra of the mixture of cyclohexene and Os-DNA organosol. Curve c1 shows the spectra of the product cyclohexane. (b) shows the 1H NMR spectra of the product cyclohexane in ethanol using CDCl3 as solvent.

hydrogenation ability of Pt, Pd, Rh, Ir, Ru, and Os toward 3thiolene-1,1-dioxide,38 but, in most of the reports, the Os catalyst is used as bulk material or it was synthesized in PVD routes in the form of thin films. We expected better activity toward hydrogenation of alkenes using our Os organosol as the catalyst due to its ultrasmall size with large surface to volume ratio. We have examined the hydrogenation reaction of

et al. reported the alumina supported Os metal catalyst for the hydrogenation of CO.37 Kulishkin et al. reported the relative 19234



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Table 2. Original MB Bands (Reported Value) and Experimentally Observed Bands in Our Studies with the Corresponding Band Assignmentsa reported Raman bands for MB39 449 502 612 670

1030 1184 1301 1396 1442

1513 1617

observed Raman bands for MB in water

MB − Os organosol (SERS)

448 502 597 671 771 826 852 953 1038 1186 1222 1328 1396 1447

448 502

assignment of various peaks39 δ(C−N−C) δ(C−N−C) δ(C−S−C) γ(C−H)

771

948 1038 1186 1302 1396 1447

1473

1472

1502 1625

1502 1625

β(C−H) ν(C−N)

α(C−H) νasym(C−N) ring νasym(C−C) ring ν(C−C) ring ν(C−C) ring

a Abbreviations: ν, stretching; α, in-plane ring deformation; β, in-plane bending; γ, out-of-plane bending; and δ, skeletal deformation. Reference 39 is given in the reference list.

Scheme 2. Schematic Representation of the SERS Experiment Figure 8. (A) shows the concentration dependent SERS spectra at various dye concentration where a shows the normal Raman spectra of MB and curves b, c, d, and e show the SERS spectra with Os organosol at different dye concentrations of 10−3 M, 10−4 M, 10−6 M, and 10−8 M, respectively. (B) shows the morphology dependent SERS spectra taking two different types of Os organosol where curve a is for aggregate wires and curve b is for long wires.

hydrogen gas was passed. Sedimentation of Os-DNA particles was observed after the passage of hydrogen gas for about 10 min, even though hydrogen gas was continuously passed for about 15 h. The characteristic sharp odor of cyclohexene disappeared. Nevertheless it became difficult to analyze the product as sedimentation took place in the beginning, and it also restricted the possibility of checking the recyclability of the catalyst. So we chose the heterogeneous hydrogenation to overcome this pitfall. In the heterogeneous catalysis reaction, a thin film of DNAOs particles was prepared on a glass substrate by depositing 100 μL of Os-DNA organosol each time for more than ten times, and then it was immersed into the ethanolic solution of cyclohexene followed by passing hydrogen gas. The ethanolic solution of cyclohexene was prepared by mixing both cyclohexene and ethanol in the ratio of 1:1. An ethanolic cyclohexene solution (5 mL) was taken into a 15 mL glass vial, and then the thin film glass plate was immersed into the solution. A well cleaned and dried glass tube with 0.3 mm diameter was inserted into the vial for the purpose of passing hydrogen gas through one neck of the double necked stopcock to prevent the interference of other gases from the atmosphere. Another glass tube was also inserted into another neck to allow

cyclohexene in the presence of DNA-Os organosol as the catalyst using both homogeneous and heterogeneous hydrogenation reactions. We have conducted a couple of control experiments to check the influence of ethanol, DNA, and the catalyst in the hydrogenation reaction. We have carried out the reaction in the absence of NPs but by passing H2 gas to the ethanolic solution of cyclohexene. We also checked our reaction by adding only DNA solution but without any Os catalyst. In both the cases, the catalysis reaction did not take place at all in our experimental time scale which confirms that the presence of all the reagents such as ethanol, the DNA-Os catalyst, cyclohexene, and hydrogen gas are extremely important to carry out the reaction. In the homogeneous hydrogenation reaction, DNA-Os organosol solution was directly added to the ethanolic solution of cyclohexene, and 19235



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Table 3. SERS Enhancement Factor (EF) Values Using Two Different Os Organosol Structures enhancement factor (EF) values at a peak position nanoclusters (NCs) used

dye used

concn of the dye used (M)

448 cm‑1

1396 cm‑1

1625 cm‑1

Os organosol (long wires) Os organosol (aggregated short wires)

MB MB MB

10−4 10−4 10−8

2.48 × 102 2.71 × 102 5.22 × 105

3.26 × 102 4.29 × 102 4.41 × 105

3.35 × 102 4.37 × 102 1.62 × 105

where Raman signals get enhanced few orders of magnitude when a molecule is adsorbed on a rough metal surface. In SERS, mainly two predominant factors contribute: one is called the chemical effect and the other is called the electromagnetic effect. In the case of metal NPs the electromagnetic effect contributes mainly where the contribution of the chemical effect is much less. In SERS, it is well understood that the plasmonic coupling effect of the nanometric gap junctions between particles induces an enormous electromagnetic enhancement that allows the SERS signal to detect with single molecular sensitivity.39 It is also well established that when metal NPs aggregate or come to close proximity with a narrow interparticle gap results in high enhancement compared to monodispersed spherical particles.5 Apart from size, the specific shape of metal NPs also generates a high SERS signal due to generation of greater numbers of surface active “hot spots”.40 It is given in the literature that when the SERS enhancement factor (EF) value is ≥107, the process might be able to detect single molecules.39,41 Nowadays, in SERS, the major drawback is the poor reproducibility of “hot” SERS active nanostructures and generation of narrow distribution with high EF values. The majority of the SERS experiment is done where the metal NPs are synthesized in aqueous solution or in the form of thin films by PVD route. The utilization of the SERS effect using metal organosol is very rare, and the EF value is also poor and approximately ∼102 as reported earlier.6,42 In this present study, we checked the SERS effect by taking DNA capped Os organosol for the first time in the presence of MB as a SERS probe molecule. The SERS study with Os NPs prepared in aqueous solution is reported recently. The stock dye solution we prepared was 10−2 (M), and, by diluting, several other solutions were also prepared. Scheme 1 shows the chemical structure of the oxidized form of MB (blue in color) (upper structure) and three different color solutions containing only MB (bottom left), only Os organosol (bottom middle), and a mixture of MB and Os organosol (bottom right) as indicated by a, b, and c, respectively. Figure 7A shows the normal Raman spectra of 10−3 M MB solution. The characteristic MB bands appeared at 1625 cm−1, 1502 cm−1, 1473 cm−1, 1447 cm−1, 1396 cm−1, 1328 cm−1, 1222 cm−1, 1186 cm−1, 1153 cm−1, 1038 cm−1, 953 cm−1, 852 cm−1, 826 cm−1, 771 cm−1, 671 cm−1, 597 cm−1, 502 cm−1, and 448 cm−1, respectively. The literature reported the original bands of MB,43 and the experimentally observed bands in our present study and the corresponding band assignments are listed in Table 2. Figure 7B shows the normal Raman spectra of DNA (curve a) and Raman spectra of DNA capped Os organosol (curve b). For only the DNA sample (curve a) the characteristic peaks appear at 1657 cm−1, 1470 cm−1, 1445 cm−1, 1328 cm−1, 961 cm−1, 788 cm−1, 738 cm−1, and 678 cm−1, respectively. The Raman band for DNA-Os organosol does not show any sharp peaks rather a broad spectrum as shown in curve b, Figure 7B. Schematic representation of the SERS experiment is shown in Scheme 2. Figure 8A shows the concentration dependent SERS spectra at the various dye concentrations. The normal Raman

the hydrogen gas to go out. The whole vial was covered with aluminum foil to avoid auto-oxidation of cyclohexene to peroxides in the presence of light and air. Hydrogen gas was passed from the cylinder with a constant flow rate of 0.3 mL per second. After 15 h, it was observed that the characteristic sharp odor of the ethanolic solution of cyclohexene disappeared which indicated the completion of the reaction which further confirmed using a UV−vis spectroscopy study. The probable catalytic reaction takes place as

The product obtained after completion of the reaction was separated from the reaction mixture and was analyzed with a UV−vis and a 1H NMR study. The UV−vis absorption spectral study was carried out for the product obtained in the homogeneous hydrogenation reaction. Both studies confirmed the hydrogenation of cyclohexene to cyclohexane in the presence of DNA-Os particles as the catalyst. Figure 6a, curve a1 shows the UV−vis absorption spectra of only cyclohexene, and curve b1 shows the absorption spectra of the mixture of cyclohexene and Os-DNA NPs solution. Curve c1 shows the absorption spectra of cyclohexane. Curve a1 shows a peak nearly 240 nm which is characteristic to cyclohexene. Curve b1 shows an additional peak nearly at 260 nm along with the characteristic cyclohexene peak at 240 nm is due to the presence of DNA in the DNA-Os NPs solution. Curve c1 shows only one peak nearly at 260 nm is again due to DNA. Here the peak of cyclohexene has disappeared which indicates that cyclohexene was successfully hydrogenated. Figure 6b shows the NMR spectra of the ethanol and cyclohexane mixture in the CDCl3 solvent. The triplet at 1.25 ppm is due to the CH3 group of ethanol. The quartet at 3.68 ppm is due to the ethanolic methylene group. A small broad peak at 2.1 ppm is due to the exchangeable OH proton. The intense singlet at 1.45 ppm is due to the 12 unique protons of cyclohexane. There was no peak observed for alkenyl protons which indicated that the whole cyclohexene was successfully hydrogenated. Both NMR and UV−vis absorption studies confirmed the hydrogenation takes place in the presence of DNA-Os organosol as the catalyst. This is the first report where DNA stabilized Os organosol can be used as a catalyst for the hydrogenation reaction of cyclohexene to cyclohexane within 15 h of reaction. We also did some postreaction studies to check the recyclibilty of the catalyst and conducted the same reaction again and again for 7 consecutive times. It was found that up to 3−4 cycles, the effectiveness of the catalyst was not reduced much, but, after that, the time taken by the catalyst gradually increased cycle to cycle which might be due to the significant loss of the catalyst by leaching. Surface Enhanced Raman Scattering (SERS) Study Using DNA Encapsulated Os Organosol. Surface enhanced Raman spectroscopy (SERS) is a surface sensitive technique 19236



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spectrum of MB (curve a, Figure 8A) is also included for comparison purposes. Curves b, c, d, and e show the SERS spectra with Os organosol at different dye concentrations of 10−3 M, 10−4 M, 10−6 M, and 10−8 M, respectively. We are able to detect the dye concentration up to 10−8 M, and below that concentration we are unable to detect with Os organosol. The Raman intensity of MB was found to be highly enhanced when mixed with Os organosol, and we have calculated the EF values of three specific peak positions at 1625 cm−1, 1396 cm−1, and 448 cm−1. The EF values at different concentrations and using two different Os organosol structures are summarized in Table 3. From Table 3, we can see the highest EF observed with Os organosol having aggregated wires (short size) compared to Os organosol wires (longer size). The highest EF value is calculated as 5.22 × 105 in our present experiment. The EF value is calculated according to specific equations as given in the literature.5,40 Figure 8B shows the morphology dependent SERS spectra taking two different types of Os organosol. Curve a is for Os organosol having aggregated wires where curve b is the Os organosol with long wires. The EF value is found to be comparatively high in the case of aggregated wires compared to each other. The high EF in the case of aggregated wires is probably due to the creation of a greater number of “hot spots” as the smaller Os particles are in close proximity in the aggregates compared to long wires. The EF value we observed is found to be the highest in the case of Os organosol, and as there is no report on Os organosol, we compare the EF value with Au and Ag organosol only.6,42 The EF value observed in the present study was found to be comparable or slightly less while compared with Os NPs synthesized in aqueous solution which is quite reasonable and understandable. A more detailed study by dispersing the Os organosol in other organic solvents and the specific effect of particles size and morphology will be discussed in the near future.

Article

ASSOCIATED CONTENT

S Supporting Information *

Discussions related to instrument specification and a table related to FT-IR data are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91 4565-241487. Fax: +91 4565-227651. E-mail: [email protected] and [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S. Anantharaj and S. R. Ede wish to acknowledge Council of Scientific and Industrial Research (CSIR) for a JRF fellowship, and U. Nithiyanantham wishes to thank CSIR-CECRI for a research internship fellowship. We also wish to acknowledge Mr. E. Ayyappan, B.Tech (Final year, project student, Chemical & Electrochemical engineering, CSIR-CECRI) for his timely help during characterizations and proof reading. The support from the Central Instrumental Facility (CIF) and help from Mr. A. Rathishkumar (TEM in-charge, CIF), Mr. S. Radhakrishnan (NMR in-charge, CIF) CSIR-CECRI, Karaikudi are greatly appreciated. The authors wish to thank Dr. Vijayamohanan K. Pillai, Director and Dr. M. Jayachandran, HOD-ECMS Division, CSIR-CECRI for their time, support, and encouragement.

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REFERENCES

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CONCLUSION In summary, we reported a homogeneous reduction route for the synthesis of Os organosol on DNA scaffold for the first time. The synthesis was done by the reduction of OsO4 with TBABH4 in the presence of DNA in acetone within 10 min of stirring at room temperature. We synthesized different morphologies by varying the DNA to OsO4 molar concentration and by controlling the other reaction parameters. The eventual diameters of the individual Os particles in the organosol are ∼1−3 nm, and the nominal lengths of the wires are ∼1−2 μm. The potentiality of the Os organosol was tested for the first time in two different applications: one is the catalytic hydrogenation of cyclohexene to cyclohexane and the other is the SERS studies. The SERS experiment was done taking MB as Raman probe and the EF value is found to be highest in the case of Os organosol having an aggregated wirelike structure compared to other morphologies. The Os organosol in acetone can be easily redispersed in aqueous and several other organic solvents. The synthesized DNA-Os organosol was found to be very stable for more than 6 months while stored in the refrigerator. The fast synthesis of Os organosol in DNA and their potential catalytic and SERS activity will be found to be very useful in the near future for the catalytic applications of various organic reactions and in the field of sensors, electronic devices, and fuel cells. 19237



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Osmium Organosol on DNA: Application in Catalytic Hydrogenation

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