Polymer Encapsulated Self-Assemblies of Ultrasmall Rhenium

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Polymer Encapsulated Self-assemblies of Ultra-small Rhenium Nanoparticles (NPs): Catalysis and SERS Applications Subrata Kundu, Lian Ma, Wei Dai, Yunyun Chen, Alexander M. Sinyukov, and Hong Liang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02175 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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ACS Sustainable Chemistry & Engineering

Polymer Encapsulated Self-assemblies of Ultra-small Rhenium Nanoparticles (NPs): Catalysis and SERS Applications Subrata Kundu,1,2* Lian Ma,3 Wei Dai,3 Yunyun Chen,1 Alexander M. Sinyukov4 and Hong Liang1,*

1

Department of Materials Science and Engineering, 201D-Doherty Building, Texas A&M University, College Station, Texas, TX-77843, USA.

2

Electrochemical Materials Science (ECMS) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006,Tamil Nadu, India. 3

Department of Mechanical Engineering, 3123, Engineering Physics (ENPH) Building, Texas A&M University, College Station, Texas, TX-77843, USA. 4

Department of Physics and Astronomy, Texas A&M University, College Station, Texas 77843, USA. * To whom correspondence should be addressed, E-mail: [email protected]; [email protected], [email protected], Fax: 979-845-3081; Phone: 979-985-9609.

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ABSTRACT Ultra-fast and facile formation of stable and self-assembled rhenium (Re) nanoparticles (NPs) using poly-allylamine hydrochloride (PAH) scaffold within 120 sec of wet-chemical reaction at room temperature in aqueous solution has been reported. The average diameters of the synthesized two different sets of Re NPs are ~ 0.7 ± 0.25 nm and ~ 1.7 ± 0.3 nm respectively which can be easily achieved by controlling the polymer to Re7+ molar ratio. The small size Re NPs are formed in the solution, self-assembled together to form the chain-like or necklace-like structure. The synthesized Re NPs were used in two different potential applications such as in catalysis and in surface enhanced Raman scattering (SERS) studies. Catalysis study was done for 4-nitro aniline (4-NA) reduction with excess NaBH4 taking two different sets of Re NPs as catalyst. The highest catalytic rate for nitro aromatics reduction ever reported of ~1.52 × 10-1 min-1 has been observed taking large size Re NPs as catalyst. In SERS, methylene blue (MB) was used as a Raman probe molecule. Strong SERS enhancements were observed with both sets of Re NPs due to their ultra-small size, narrow inter-particle gap and self-assembled structure in PAH scaffold. These closely tethered and self-assembled Re NPs can generate more number of surface active ‘hot spots’ that results good SERS enhancement. The present synthesis route is easy, cost-effective, fast and can generate stable Re NPs which could further be applied in other interdisciplinary fields other than catalysis and SERS in near future.

Keywords: Rhenium Nanoparticles; Self-assemblies; PAH; Chain-like; Catalysis; surface enhanced Raman scattering.

INTRODUCTION Research on nanomaterials focusing their preparation and characterization has been experiencing significant progress over the past two decades due to their unique physical and chemical properties. Due to their size and shape dependent unique physico-chemical properties, metallic nanostructures show a variety of unique optical,1 electronic,2 magnetic,3 catalytic4 properties and made themselves suitable for application in many interdisciplinary fields of research. Among the different metallic nanostructures, noble metal nanoparticles (NPs) are found to be more interesting due to their close lying valence and conduction bands which makes them


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as promising materials for nanoelectronics,2 catalysis,4-6 and surface enhanced Raman scattering (SERS)7-9 studies. As their properties varied with their morphology, tremendous efforts have been made in studying the synthesis of size and shape-controlled metal NPs now days. Among the different metal NPs, majorly focused metals are gold, silver, palladium and platinum. However, synthesis of zero-valent rhenium (Re0) has been passed over for a while. Re has the second highest melting point of 3185 °C among all metals except W which has a melting point of 3422 °C.10 This high melting point of Re makes it a very promising material of different practical applications which require high temperature withstanding ability such as jet engine components and to improve the life of and performance of several other electronic gadgets used in high temperature environments.11 Moreover, due to its low standard reduction potential of 0.3 V, it can be used for the preparation of alloy NPs by galvanic replacement reactions.10 Further, Re NPs can be used in medicinal fields for magnetically targeted radiotherapy,12 complex catalytic processes such as glycerol reforming, hydrocarbon transformation, hydrogenation of difficult functional groups,13 aqueous phase reforming of biomass materials,14 and also for different reduction reactions such as removal of perchlorate from water.15 Though there are many potential applications for Re NPs as mentioned above, the preparation of Re NPs is not much studied in literature.16-31 One of the major problems of preparing zero-valent Re NPs is its readiness for simultaneous oxidation in aqueous colloidal solution. So far, the wet chemical synthesis of stable Re NPs is really a challenging and hard task. Several synthesis methods had been reported to prepare metallic Re NPs in solution based routes such as, wet-chemical reduction,16-18,27-29 in reverse micelles,24,25 and alcohol assisted reduction.23 The reduction of Re salts in water generates many unknown products due to its oxidation in air or water and results in oxidized products rather than metallic Re NPs.16 Mucalo et al. synthesized Re NPs by the chemical reduction of K2ReCl6 salt in water but observed a rapid oxidation of the synthesized NPs.17 Ayvali et al. prepared Re NPs by reduction of [Re2(C3H5)4] under a H2 atmosphere at 3 bar and 120 °C for two days in anisole solvent in presence of hexadecylamine or polyvinyl pyrrolidone (PVP) as capping agent.27 Anantharaj et al. prepared Re NPs in DNA scaffold and studied their application in catalysis and SERS.28 Sakthikumar et al. prepared Re NPs in organic solvent utilizing a two phase extraction procedure in presence of a phase transfer catalyst followed by reduction with a strong reducing agent.29 There are few other reports where Re NPs has been synthesized using solid state methods such as



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thermal decomposition,19 impregnation followed by calcination under H2 gas,20 radiation,21 solid state thermolytic demixing, 22 by PLD (pulsed LASER deposition) assisted decomposition of Re salt21 and by thermal decomposition of Re2(CO)10.26 Most of the above reports described showed different drawbacks such as difficult synthetic protocols, polydispersity, lack of information on the oxidation state of resulting NPs, need for sophisticated instruments and in most of the cases generated particles were not morphologically unique. Hence, a wet-chemical based easy and faster synthesis route for water soluble, stable Re NPs still remains a bigger challenge to the scientific community. Nobel metal NPs viz., Au,7 Ag,32 Pd,33 Pt,34 Os,35 Rh36,37 have only been used widely in catalysis and SERS studies. As these metal NPs have large surface to volume ratio, they can readily act as excellent catalysts in various types of homogeneous and heterogeneous catalysis reactions. The chemical reduction of aromatic nitro compounds to their corresponding amino compound in presence of excess NaBH4 has been used as a model reaction to specifically check the catalytic properties of noble metal NPs.4-6,

32,35,36,37

Apart from catalysis, noble metal NPs

such as Au and Ag has been extensively used in SERS studies too as substrates.7,8 Nowadays, other than Au and Ag, other metal NPs such that Os,9,35 Rh,36,37 Pd,33 Pt,34 has also been tested as suitable SERS substrates. However, their SERS enhancement factors (EF) are poorer while compared to that of Au or Ag NPs. Kundu et al. reported the size-selective Rh NPs on DNA scaffold which acts as excellent material both in catalysis and in SERS studies.37 Recently, Veerakumar et al. reported the dispersion of Re NPs on carbon nanostructures which acts as an excellent catalyst for nitro compound reduction.38 SERS is a surface sensitive technique where Raman signal gets enhanced few orders of magnitude when a molecule is adsorbed on a rough metal surface. In SERS, there are two types of most accepted mechanisms till now, one is electromagnetic effect and the other one is the chemical effect. Electromagnetic effect deals with formation of localized surface plasmon where chemical effect deals with formation of chargetransfer complexes. For metal NPs, electromagnetic effect is the most predominant one where chemical effect contributes to a very less extent. In SERS, the plasmonic coupling effect at the nanometric gap junctions creates enormous electromagnetic field that generate strong SERS signals and efficient to detect single molecules of interest. Moreover, it is also highlighted that aggregation of NPs or self-assembling of NPs on suitable supports can generate more number of surface active ‘hot spots’ and subsequently leads to highly enhanced SERS signal.8,9 However,


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the SERS study taking Re NPs is very less. Up to now, there are only three reports on Re NPs for SERS study.22,28,29 To the best of our knowledge, there is no report for the uniform and fast formation of ultra-small Re NPs in aqueous solution using a polymer scaffold and its subsequent application in catalysis and SERS studies. With this we report the synthesis of ultra-small, self-assembled Re NPs in PAH scaffold for the first time. Due to very small size and its self-assembly over PAH scaffolds, it had shown advantageous results when applied to catalysis and SERS studies. The catalytic activity was examined for the reduction of aromatic nitro compounds i.e., 4-NA in presence of excess NaBH4. The SERS studies were done adapting MB as Raman probe. The self-assembled Re NPs generated significantly enhanced SERS signals at various positions of MB peaks. The synthesized Re NPs were stable for a significant period of time while kept in a sealed tube and stored in dark inside a refrigerator. The present synthesis process is easy, faster and costeffective.

EXPERIMENTAL Preparation of Ultra-small Re NPs in PAH Scaffold The details about all the reagents used for the synthesis of Re NPs, for catalysis and SERS studies are given in Supporting Information (SI) section. Self-assembled Re NPs are synthesized on PAH scaffold by the reduction of NH4ReO4 with NaBH4 and continuous stirring for 120 sec. In a typical synthesis, 1 ml of 1% PAH solution was mixed with 5 ml of DI water and stirred well before adding 5 ml of 10-2 M NH4ReO4 solution and continued the stirring for few more minutes. Then, 2 ml of 0.1 M ice-cold freshly prepared NaBH4 solution was added at once upon stirring during which the transparent solution turned into light brown color initially and with the increasing time color changed to dark brown. The reaction was completed after 2 min. The completion of reaction was confirmed by the stability of the color and from UV-Vis absorption spectroscopy. The prepared solution exclusively contained self-assembled Re NPs having small size particles with diameter ~ 0.7 ± 0.25 nm as confirmed by TEM analysis. Another sets of Re NPs was prepared just by changing the metal salt to PAH molar ratio while keeping other reaction parameters intact. The reaction time, concentrations of reagents used, particles size and shape are given in Table 1. The step-wise formation of Re NPs by our present route was also shown as Scheme 1. The synthesized Re NPs of different average particle sizes



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were characterized using UV-Vis, TEM, EDS, XRD and XPS analyses and details of their sample preparation are given online SI section.

Preparation of Samples for Catalysis and SERS Studies The catalytic reduction of 4-NA was tested taking positively charged Re nanostructures. For a typical catalysis reaction, 4 mL of DI water was mixed with 600 µL of (10−3 M) stock 4NA solution and stirred for 5 min. Then, 600 µL of 0.1 M ice-cold NaBH4 solution was added and shaken well by hand. Then 100 µL of Re NPs solution was added and the reaction progress was monitored in-situ by an UV-Vis spectrophotometer. The corresponding absorption spectra were recorded at regular time intervals until completion of the reduction. The total time required for the reduction was ∼14 min for large size PAH capped Re NPs during which the light yellowish 4-NA solution became colorless due to the formation of the reduced product paraphenylenediamine (p-PDA). The completion of the reaction was confirmed by de-coloration of the reaction mixture from yellow to colorless and from the absorption spectra acquired using the UV-Vis spectrophotometer. Another set of prepared Re NPs was also tested for comparison purposes. MB dye was used as the probe for SERS studies. Samples for SERS studies were prepared as follows. Several standard solutions of MB with concentrations varying as 10−3 M, 10−4 M, 10−5 M, 10−6 M and 10−8 M were prepared in DI water. Then 200 µL of each of those MB stock solutions were separately mixed with 200 µL of the Re NPs solution and shaken well for ensuring homogeneous mixing. After ~ 10-15 min, about 20 µL of the solution mixture was placed over clean glass substrates and dried in air. After drying, the samples were ready for SERS analysis. During SERS analysis, the LASER beam was placed directly to the sample under darkness to establish an adsorption/desorption equilibrium, prior to get the Raman spectrums.

RESULTS AND DISCUSSION UV-Visible (UV-Vis) Spectroscopic Studies Self-assembled, ultra-small Re NPs were synthesized using PAH scaffold in presence of NaBH4 as reducing agent under continuous stirring for 120 sec. The ultra-small Re NPs were grown on PAH scaffold and generate self-assembled superstructures in nanoscale dimension. The UV-Vis spectra (Figure 1) of the different precursor solution used for the synthesis of Re NPs shows that the absorption spectra of aqueous PAH solution (curve a) has no specific peak


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but a small hump near 240 nm matches with earlier report.39 Curve b in Figure 1 is the absorption spectra of aqueous ammonium perrhenate solution which has a strong peak at 227 nm indicates ligand to metal charge transfer (LMCT) spectra. The LMCT band of ReO4- appeared due to the transition from O2- ion to Re+7 ion at the center of the complex. A similar type of LMCT band for Re salt was reported.28 Curve c in Figure 1 is the absorption feature of a mixture of PAH and ammonium perrhenate solutions which has no specific peak but a small hump near 242 nm due to either formation of complex between both or due to adsorption of ReO4- ions on PAH. Shifting of peak positions or appearance of new peak due to adsorption of metal ions with different scaffolds like cetyl-trimethyl ammonium bromide (CTAB) or de-oxyribonucleic acid (DNA) scaffold were discussed before.4,7,8,9 After the addition of NaBH4 solution, the perrhenate solution got reduced and color of the solution mixture changed to light brown initially and finally dark brown after 120 sec indicates the formation of Re NPs in the solution. Curve d, e and f show the absorption features of aqueous Re NPs solution in which the PAH solution concentration was varied as 0.5%, 0.75% and 1% respectively. From curves d, e and f, no peaks for perrhenate was observed which indicates all the perrhenate ions were reduced to Re (0) state. From curve d, e and f, we did not see any characteristic absorption maxima for Re NPs which might be due to its very low value of dielectric constant. Similar types of absorption features without any characteristic SPR maximum were observed for other metal NPs such as Pd,40 Os9,35 etc. From curve d, e and f, we can see that with the increase in PAH concentration the curve become narrower and shifted towards lower wavelength side which could probably be due to formation of small size Re NPs. Similar types of absorption features for Re NPs were reported.21,28,29 Inset of Figure 1 shows the camera image of three different sets of Re NPs solution corresponding to curve d, e and f.

Transmission Electron Microscopic (TEM) Analysis Figure 2 shows the TEM and HR-TEM micrographs of the synthesized Re NPs on PAH scaffold which revealed that 1 wt% PAH and 0.75 wt% PAH solutions led to the formation of Re NPs of almost similar size and morphology whereas 0.5 wt% PAH solution generated significantly different Re NPs assemblies with different particle size. Hence, we are showing the TEM and HR-TEM micrographs obtained for the Re NPs using 0.5 wt% and 1 wt% PAH solutions. Figure 2, A-D shows the low and high magnified TEM micrographs of the Re NPs



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obtained with 0.5 wt% PAH solution. From Figure 2, A and B, we can see that the long polymer chains are connected together to form ‘network-like’ structures and small Re NPs are embedded there. Figure 2, C shows the comparatively high magnified micrograph where the Re NPs are clearly visible and self-assembled on the PAH chains. Figure 2D shows the HR-TEM micrograph of the Re NPs where the corresponding lattice fringes are clearly visible. The average diameter of the Re NPs is ~ 1.7 ± 0.3 nm and the average PAH chain diameter is ~ 75 ± 20 nm. From Figure 2D, the spacing between two adjacent fringes is ~ 0.264 nm. The inset of Figure 2D shows the corresponding selected area electron diffraction (SAED) pattern which shows the ring patterns probably due to small size of Re particles which could not diffract the electron beam. Similar types of SAED patterns were observed earlier for small size Os metal NPs during TEM analysis.9,35 Figure 2, E-H show the TEM and HR-TEM micrographs of Re NPs with the 1 wt% PAH solution. Figure 2, E, F and G are the TEM micrographs at different magnifications and Figure 2H is the HR-TEM micrograph of the same sample. From these micrographs, it is clear that due to high concentration of PAH, the self-assembled polymer chains are denser, aggregated together and small size Re NPs were grown in the polymer chain. The average diameter of the individual Re NPs is ~ 0.7 ± 0.25 nm and the average chain diameter is ~ 30 ± 10 nm. Moreover, it also shown that the chains are interwoven with one another. Figure 2H is the corresponding HR-TEM micrograph and the average spacing between two individual lattice planes are ~ 0.204 nm. The inset of Figure 2H is the corresponding SAED pattern which says that the particles are nearly crystalline in nature. However, we did not get any separate diffraction spots rather we observed simple ring type diffuse patterns which might be due to very small size of the individual Re NPs and due to the presence of some amorphous materials such as the PAH on the surface of Re NPs. Hence, from the TEM and HR-TEM analyses, it is clear that small size Re NPs were formed in the solution which subsequently grown over the PAH chains, self-assembled and generate the chain-like morphologies.

Energy Dispersive X-ray Spectroscopic (EDS) and X-ray Diffraction (XRD) Analysis The energy dispersive X-ray spectroscopy (EDS) analysis was done to ascertain the elements present in our synthesized Re NPs solution. The EDS spectrum of the synthesized Re NPs is shown in Figure S1 in SI. As both the synthesized Re NPs have shown exactly similar spectral features, the EDS spectrum of Re NP synthesized using 0.5 wt% PAH solution only


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provided here. The EDS spectrum consists of different elements such as C, O, Cl, Na, N and Re. The C, Cl and N peak are arises due to the presence of PAH. The Na peak came as a result of the used reducing agent NaBH4 in our synthesis. The high intense Re peak came from the Re NPs and we observed Re peak in different places respective to the shells of varying energy. Other than the expected elements, we did not find any other peaks which confirmed the purity of our synthesized Re NPs. The X-ray diffraction (XRD) patterns of the synthesized Re NPs are shown in Figure S2 in SI. The ultra-small Re NPs were deposited on glass substrates and analyzed by XRD. Curve a of Figure S2 shows the XRD pattern obtained from small size Re NPs prepared with 1 wt% PAH solution whereas curve b of Figure S2 shows the XRD pattern of large size Re NPs prepared with 0.5 wt% PAH solution. Both the XRD patterns exhibited similar features and we did not observe any strong diffraction peaks. However, the expected peak positions are indicated in the corresponding patterns. These XRD patterns are in accordance with the SAED patterns as described before in Figure 2. Similar types of XRD patterns for ultra-small size Re NPs and Os NPs were reported.9,28,29,35 However, few other groups viz., Chong et al.21 and Yi et al.23 observed some diffraction peaks for Re NPs in their work due to larger size of the synthesized Re NPs as indexed with JCPDS card number of 00-087-0599.

X-ray Photoelectron Spectroscopic (XPS) Analysis Figure 3 shows the X-ray photoelectron spectroscopic (XPS) analysis of the synthesized Re NPs which used to identify the oxidation states of the corresponding elements present in the material. It is important to note here that as both the morphologies are chemically equivalent, we are showing the detailed XPS analysis of Re NPs are prepared using 0.5 wt% PAH solution. Figure 3A shows the survey spectrum which consists of different peaks corresponding elements Na 1s, C KLL, O KLL, O 1s, Na KLL, C 1s, Cl 2p, Re 4f respectively. From survey spectrum, the Na 1s peak appeared at a binding energy of 1072 eV, C KLL at 996 eV, O KLL at 746 eV, O 1s at 533 eV, Na KLL at 305 eV, C 1s at 286 eV, Cl 2s at 265 eV, Cl 2p at 201 eV and Re 4f at 46.8 eV respectively. The high resolution XPS spectrum of Re 4f, Re 4d and O 1s are shown in Figure 3, B-D respectively. From Figure 3B, the high resolution Re 4f peak was deconvoluted where Re 4f5/2 is appeared at a binding energy of 50.1 eV and Re 4f7/2 appeared at a binding energy of 47.7 eV. Similarly, Figure 3C shows the high resolution peak for Re 4d which was also



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deconvoluted where Re 4d3/2 and Re 4d5/2 peaks are appeared at 286.8 eV and 266.3 eV respectively. In addition of these two Re 4d peaks, another satellite peak was also appeared at a binding energy of 278.6 eV. Other than Re 4d and satellite peak, two other low intense peaks appeared at a binding energy of 271.1 eV and 291.04 eV respectively which are assigned to ReOx might appeared due to some degree of aerial oxidation of Re NPs during the entire synthesis process. It is well known that rhenium is a very oxidizable metal and it is expected to oxidize in air and featured the oxide peaks in XPS as we also observed in our present study. The formation of a thin oxide layer on surface of Re(0) is expected as reported before too.41 Figure 3D shows the high resolution XPS spectra for O 1s which appeared at a binding energy of 533.5 eV. The high resolution XPS spectrum of N 1s and C 1s are shown in Figure S3, A and B respectively in SI section which appeared at a binding energy of 401 eV and 286 eV respectively. All the peak positions observed are matching with NIIST XPS data file as expected. Similar types of XPS spectral results of Re NPs was reported by Kim et al. on their study of PtRe bimetallic catalyst,42 by Anantharaj et al.28 and Sakthikumar et al.29 for their study on Re NPs on DNA scaffolds.

Mechanism for the Formation of Self-assembled Re NPs on PAH Scaffold Self-assembled Re NPs has been synthesized by the reaction of NH4ReO4 salt with NaBH4 in presence of the polyelectrolyte PAH in aqueous solution within 120 sec of wetchemical reaction at room temperature. In present synthesis, we did couple of control experiments to check the importance of each reagent. In absence of PAH, while reducing NH4ReO4 with NaBH4, Re0 NPs were formed but immediately got precipitated due to the absence of any specific stabilizer in the medium. Similarly, the reaction did not generate any Re0 NPs without the addition of NaBH4 due to the absence of any reducing agent. Moreover we also varied the concentration of all these reagents. We have seen that at a very high concentration of NH4ReO4 salt (≥10-2 M) but keeping other reagents concentration same, the Re0 NPs are formed instantly but got precipitated within a shorter time. Similarly, at a low concentration of NH4ReO4 salt (≤ 10-4 M), the formation of Re0 NPs takes much longer time. We changed the concentration of PAH salt and have seen that stable Re0 NPs are formed while stock PAH concentration was between 1 to 0.5 wt%. When the PAH concentration was very less such as 0.1 wt% or 0.01 wt%, Re0 NPs are formed and precipitated during synthesis or within a short time after synthesis. At


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very high PAH concentrations such as 2.5 wt% or above, Re0 NPs are not formed in the experimental time scale. Similarly, stock NaBH4 concentration we fixed to 0.1 (M) although we have seen in other concentrations such as 0.01 (M) or 5 × 0.1 (M) also Re0 NPs formed but takes longer time compared to our experimental time scale.

Hence, proper concentrations and

combinations of both the reagents viz., NH4ReO4 salt, PAH and NaBH4 are essential for the successful generation of Re0 NPs in solution. All the details about reagents concentrations are given in Table 1. The Re salt we used was ammonium perrhenate which is an ammonium salt of perrhenic acid and readily soluble in water having molar mass 268.2 g/mol, density 3.97 gm/cm3, possess Scheelite crystal structure having melting point ~365 °C. The polyelectrolyte PAH is a cationic polymer prepared by the polymerization of allylamine and widely used for biomedical application, surface functionalization for layer by layer assembly of nanomaterials in device related applications. Before the synthesis and after the formation of Re NPs, we measured the pH of all the intermediate and final solution mixtures. The pH of PAH (1 wt% solution) was 3.73 and for PAH (0.5 wt% solution) was 3.79. The pH of NH4ReO4 slat solution (10-2 M) was 5.11, a mixture of PAH (1 wt% solution) with NH4ReO4 is 4.53, a mixture of PAH (0.5 wt% solution) with NH4ReO4 had a pH of 4.57. The pH of the Re0 NPs solution after formation was 9.01 with 1 wt% PAH solution and 9.18 with 0.5 wt% solution respectively. In our synthesis, we varied the concentration of PAH to Re salt and observed that only at a particular concentrations, stable Re0 NPs were formed. The formation mechanism of Re0 NPs is schematically shown in Scheme 2. Scheme 1 described before showed the step-wise formation of Re0 NPs within 120 sec of reaction. From Scheme 1, we can see that the solution mixture of PAH and NH4ReO4 were colorless. Then, once NaBH4 was added, within 10-20 sec, colorless solution turned to light brown color which indicates the nucleation and reduction of Re salt. With increasing time, the color of the solution became dark after 120 sec indicating the completion of the reaction. Scheme 2 proposes that Re NPs were initially formed in all the concentrations of PAH such as 1 wt%, 0.75 wt%, 0.5 wt%, 0.1 wt% and 0.01 wt%. However, the particles were stable only at high concentrations of PAH such as 1 wt%, 0.75 wt% and 0.5 wt%. In case of 0.1 wt% and 0.01 wt% PAH, the Re NPs were formed initially but got precipitated within a short time as depicted in Scheme 1 and in Table 1. In our synthesis, after the addition of NH4ReO4 with PAH solution, the negatively charged ReO4- ions might had been adsorbed first with positively charged polymer chain self-assemblies which was confirmed from the UV-Vis spectra where the peak of



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NH4ReO4 solution was disappeared and the nature of peak was also significantly altered and this is indicating that certain types of interaction had existed there between them. Then, as soon, NaBH4 was added to the solution mixture containing PAH and NH4ReO4, the light brown color started appear and the color got intensified with the increasing reaction time which is clearly indicating the formation of Re0 particles in the solution. We can expect the reduction of free ReO4- ions which were not adsorbed with PAH initially and with the increasing time the preformed Re0 particles might had acted as seed particles or as catalyst for the reduction of remaining ReO4- ions in solution or the ReO4- ions adsorbed on the PAH self-assemblies. Within a short time scale, all the ReO4- ions were reduced as indicated by the generation of dark brown Re0 NPs solution which simultaneously self-assembled on the PAH chains and finally formed self-assembled chain-like morphologies. The small size Re0 particles initially formed might have aggregated together and generated the large size Re0 NPs. From the Table 1 and Scheme 2, we can see that keeping other reaction parameters fixed but just altering the PAH concentration, we were able to generate different sizes Re NPs in the chain-like morphologies. When PAH concentration was high (1 wt%), smaller Re NPs having average diameter ~ 0.7 ± 0.25 nm while comparatively lower PAH concentration (0.5 wt%), larger size particles having diameter ~ 1.7 ± 0.3 nm were formed. However, at a very low PAH concentration of 0.1 wt% or 0.01 wt%, the particles were formed but precipitated soon after synthesis. The formation of different size particles was due to the presence of different concentrations of PAH in solution. At a lower PAH concentration (at 0.5 wt%), the Re0 particles those are formed had more free space during their growth and then aggregated to generate particles of larger diameter. While at a high PAH concentration (at 1 wt%), as the available space was comparatively lesser, once the particles were nucleated, they did not get sufficient space to grow and become agglomerated. Hence, generated Re NPs were smaller in size as evidenced from the TEM results in Figure 2. So using our present process we can easily tune the particles size although in all the cases morphologies are chain-like only with different chain diameter as shown in Table 1 and in Scheme 2. It is important to note here that in our previous study on PAH encapsulated Au NPs synthesis,39 we had seen that PAH acted as a reducing agent due to presence of amino group on its structure during synthesis and after the formation of Au NPs, they acted as stabilizers. However, in our present study, without the addition of NaBH4, reduction did not take place which means that PAH was not able to reduce ReO4- ions. However, during the growth of the particles and after


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the formation, PAH played a major role to stabilize the colloidal solution of Re NPs. The amino group of the PAH might have coordinated with the ReO4- ions and stabilized the Re0 NPs after their formation and finally generate the self-assembled chain-like structures. Similar types of NPs assembly on a polyelectrolyte scaffold had been highlighted earlier by Minko et al.43 Then, taking the two sets of different Re NPs prepared with 0.5 wt% and 1 wt% PAH solutions, we studied their applicability in catalysis for the reduction of aromatic nitro compound and as a substrate for SERS studies as described below in the following sections.

Catalytic Reduction of 4-nitro Aniline (4-NA) Using PAH stabilized Re NPs as Catalyst in Presence of NaBH4 The synthesized self-assembled Re NPs were tested for the catalytic reduction of nitro aromatic compound i.e., 4-NA in presence of excess NaBH4 at room temperature under ambient condition. As discussed before in the introduction part that Re NPs has been used extensively different types of catalysis reaction.13-17 Other than mentioned before, Re NPs can also used in other catalysis reactions such that hydrogenation of succinic acid,44 isomerization of fatty alcohol and alcohol dehydrogenation.21 It is important to note here that different types of metal NPs such as Au,4,5 Ag,32 Pd,45 Pt,46 Os,9,35 Cu47 etc. had been used exclusively for testing the catalytic reduction of aromatic nitro compounds earlier due to its industrial importance. However, there are only one report by Anantharaj et al. for the catalytic reduction of nitro compounds using DNA encapsulated Re NPs as catalyst.28 Moreover, this catalysis reaction will also confirm the formation of Re0 NPs in our synthesis. The reduction of 4-NA with only NaBH4 solution is very slow and only 6.02 % reduction took place after keeping the reaction mixture for 5 days as seen from the UV-Vis spectrum in Figure S4 in online SI section. From Figure S4, curve a is for only aqueous 4-NA solution and curve b is for the mixture of 4-NA and NaBH4 solution after keeping them for 5 days. The inset shows two camera image of 4-NA solution corresponding to curve a and curve b. Hence, this experiment proved that the 4-NA reduction was not taken place significantly in absence of a catalyst which might be due to presence of some kinetic barriers that prevent the electron transfer from BH4- to the nitro compound. We also conducted two more control experiment in our catalysis reaction. We tested the catalysis reaction with PAH alone and Re0 NPs solution without the addition of NaBH4. The catalysis reaction did not take place at all after waiting for more than



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2-3 days in either cases and signifies that a proper mixture of all the three reagents such as 4-NA, Re0 NPs and NaBH4 were important for the catalysis reduction to take place. The reduced product of the 4-NA is para-phenylenediamine (p-PDA) is an important material used extensively in dyeing industry for making different azo and fur dyes. In our present catalysis reaction, 4 ml of DI water was mixed with 600 µl of 10-2 M 4-NA solution and then ice-cold freshly prepared 600 µl of NaBH4 was added. Finally, 100 µl of Re0 NPs solution was added and the reaction was monitored using UV-Vis spectrophotometer. We measured the pH of the 4-NA solution (10-3 M) which was 5.64, pH of NaBH4 solution (0.1 M) was 10.66 and pH of the reduced solution mixture was 10.01. The successive reduction of 4-NA peak can be easily monitored in-situ using UV-Vis spectrophotometer with change in reaction time. The 380 nm peak for 4-NA gradually died down with time and a new peak at 238 nm had begun to appear due to formation of p-PDA in solution. Figure 4A and 4B show the UV-Vis spectra for the successive reduction of 4-NA taking large size and small size Re0 NPs respectively. From Figure 4A we can see that the reaction was completed after 14 min and from Figure 4B, the same reaction was completed after 25 min. The insets of both figures show the yellowish color 4-NA solution before reduction and colorless p-PDA after reduction. Figure 4C and 4D show the corresponding ln (Abs) vs time (T, min) plots for the catalysis reaction taking large and small Re NPs as catalysts respectively. The ln (Abs) vs T (time, min) plots show linear relationship with the rate constant values of 1.52 × 10-1 min-1 and 7.58 × 10-2 min-1 respectively for large and small size Re NPs. Table S1 (online SI section) shows the observed rate constant values with % of yield and product selectivity for the catalysis reaction. The catalytic rate observed in this reaction was found to be faster than reported ones for Re0 NPs. Previous report by Anantharaj et al. showed a catalytic rate of 6 × 10-2 min-1 which is much lower than our present results taking selfassembled Re NPs in PAH scaffold as scaffolds.28 Although, it is interesting to note that the average diameter of the individual particles was ~1.1 ± 0.1 nm and 0.7 ± 0.1 nm in the DNA scaffold having aggregated chain-like morphology which is comparable to the Re NPs size in our present study. The observed catalytic rate in our present study is higher compared to reported value is probably due to specific morphology, ultra-small particles size in the PAH scaffolds. Highly stable Rhenium organosol on DNA Scaffold was reported by Sakthikumar et al. and they studied their application for catalytic reduction of hexavalent chromium and in SERS study.29 As there is only one report in nitro compound reduction taking Re NPs as catalyst, we compared our


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present catalysis results with other metal NPs such as Au,4,5,

48-51

Ag,32 Rh,36 Os9,35 NPs and

observed that our results are better than many of them. All the other catalysts used, their catalytic rate, name of the capping agent used for the synthesis are tabulated in Table S2 (online SI section). From Table S2, it is also confirmed that Re NPs showed highest rate constant value when compared to others. The formation of the reduced product was further confirmed by 1H NMR and HPLC studies (results are not shown here). Apart from this, we checked the formation of reduced product by comparing the Raman spectra of 4-NA with the reduced product as seen in Figure S5 in SI. In Figure S5, curve a is the Raman spectra for 4-NA and curve b is the Raman spectra for p-PDA solution where a clear difference between the two spectra with new additional peaks are observed that confirmed the conversion of 4-NA to p-PDA. To check the versatility of our reaction, we tested the same catalysis reaction taking few other nitro compounds such as 2 nitro phenol (2-NP), 4-nitro phenol (4-NP) etc. and observed that the catalysis reaction took place and completed within a short time scale. In our present catalysis reaction, the reducing agent BH4- transferred electron via the self-assembled Re NPs in PAH scaffold to the nitro compounds and nitro compounds were reduced. Scheme S1 in online SI section shows the electron transfer pathways for the catalysis reaction taking two different sizes Re NPs as catalyst. From Table S1 (online SI section), we can see that the catalytic reaction rate was faster in case of larger size Re NPs compared to smaller Re NPs. For any homogeneous catalysis reaction, the catalytic rate depends upon two important parameters, one is the number of catalyst particles in the solution and other is the availability of active surface area. In our present study, we tried to fix the number of catalyst particles by adding different volumes of catalyst solution. As the quantity of particles or the loading of particles is approximately same, the observed difference in catalytic rate was mainly due to the difference in their surface area. As in both cases the NPs are spherical in morphology and amounts of particles are same, the available active surface area of NPs having small diameter will be higher compared to the larger diameter particles and the catalytic rate will also expected to be higher in case of smaller diameter Re NPs. But, in our present study we observed opposite trend and the order of the catalytic rate is: larger diameter Re NPs > smaller diameter Re NPs. Although, as the particles size difference for two morphology is very less, the reason for difference in catalytic rate and the exact mechanism of electron transfer process during catalysis needs further insight study to get the clear picture at the nanoscale catalytic interfaces. Finally, it is important to mention that as we used all the reagents in the same



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phase, it was not possible to remove the catalyst after reduction from the reduced product. Nevertheles, as we used very less concentration of catalysts, there was no impurity observed in the reaction product. Moreover, we also checked the recyclability of our catalyst by adding more 4-NA solution in the same reduced solution with the necessary addition of NaBH4 solution again and observed that the catalysis reaction takes place significantly up to 4-5 consecutive cycles. Beyond 7-8 cycles, the catalytic rate was reduced which might be due to surface poisoning of the catalyst. Taking two sets of different Re NPs, we tested their application in SERS study as discussed below. Surface Enhanced Raman Spectroscopic (SERS) Studies with PAH Capped Re0 NPs as Substrate with MB as Raman Probe After the discovery of SERS in 1974 by Feildsmann, a tremendous effort has been made to get SERS active materials of high EF.52 Till date, the high EF values are mostly observed with Au and Ag metal NPs as their SPR bands fall in the same region with an excitation wavelength near or in the visible region.53-55 The sensitivity of SERS depends upon several factors such as specific size and shape of the NPs, presence of rough surface, aggregation of metal NPs, selfassembled metal NPs having a specific gap between two particles which can generate more surface active ‘hot spots’ and enormous SERS signal.56-59 Mazumdar et al. reported that aggregated Ag NPs having interparticles distance below 2 nm gave highly enhanced SERS signal compared to monodispersed Ag NPs with no aggregation.8 Nithiyanantham et al. reported selfassembled Os nanoclusters (NCs) gave better SERS enhancement.9 Kundu reported the shapeselective SERS activity of Au NPs with different shapes before where he observed that Au nanoprisms gave better SERS signal compared to Au nanorods or Au nanospheres.7 Other than Au and Ag, metal NPs such as Os,9,35 Pd,33 Pt,34 Re,22,28,29 do not have any specific surface plasmon resonance bands in visible region to generate highly enhanced SERS signal. Hence, they always result in low EF values. Valenzuela et al. reported high index facet SERS active Re NPs by considering phase demixing.22 In this present study, we prepared ultra-small Re NPs selfassembled on PAH scaffold expected to give good SERS activity due to their small size and selfassembled structure. In our present study, we checked the SERS activity taking two different Re NPs we prepared in PAH scaffold. We used MB as a SERS probe molecule as it has large molecular


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structure, charged functional group and well-studied before as SERS probe molecule. The specific preparation of SERS sample is elaborated in experimental section. Figure 5A shows the chemical structure of the Raman probe molecule MB at the middle and left side optical image of MB solution and right side optical image is of the mixture of MB and Re NPs solution. Figure 5B shows the UV-Vis absorption spectra of the dye MB, the Re NPs solution and the mixture of MB and Re NPs solution together. Figure 5B, curve a indicates the absorption spectra of MB solution which has a prominent band at 664 nm and two other small bands at 611 nm and 291 nm respectively. Curve b, Figure 5B indicates the absorption band of only Re NPs solution which has no significant bands in the UV-Vis region. Curve c, Figure 5B shows the absorption bands of a mixture of MB and Re NPs solution where a decrease in absorption intensity of the peak at 664 nm but an increase in the absorption intensity of the peak at 291 nm were observed. This change in absorption intensity is indicating that there is some physical interaction or adsorption of MB dye on Re NPs surface took place. The Raman spectra of only aqueous MB (10-3 M) has been taken as seen in Figure S6 in SI. From Figure S6, we can see several high and low intense Raman bands for MB at a different Raman shift values of 1624 cm-1, 1430 cm-1, 1392 cm-1, 1328 cm-1, 1147 cm-1, 1073 cm-1, 954 cm-1, 895 cm-1, 800 cm-1, 660 cm-1, 594 cm-1, 495 cm-1, 478 cm-1 and 444 cm-1. These peaks of MB are assigned to different stretching and bending vibration modes of different bonds present in MB.60 All the observed peaks of MB in our present study, reported in literature, the SERS bands observed after mixing MB with Re NPs and the corresponding band assignments are elaborated in Table S3 in SI section. Before doing the SERS experiment, we tested the Raman spectra of the polymer PAH used to synthesized Re NPs and the Re NPs in PAH as seen in Figure S7 in SI. In Figure S7, curve a indicates the Raman spectra of PAH while curve b indicates the Raman spectra of PAH-Re NPs. Most of the peaks are matching with one another as PAH is present in both samples. However, for PAH-Re NPs sample, few additional peaks were appeared at 1606 cm-1, 1529 cm-1, 1188 cm-1 and 963 cm-1 respectively. Figure 6 shows the comparison SERS spectra taking two different size Re NPs. In the same picture we included the Raman spectra of only MB (10-3 M) also to make a clear comparison between the Raman spectra and SERS spectra. In Figure 6, curve a is the Raman spectrum MB (10-3 M), curve b and c are the SERS spectra of the same on small and large size Re NPs as substrates respectively. From Figure 6, we can see that better SERS enhancement taking large size Re NPs compared to small size Re NPs. The enhancement factor (EF) value for



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SERS is calculated according to a specific equation as given in literature.7,8 The EF value was calculated by choosing three different intense bands at 1623 cm-1, 1391 cm-1 and 447 cm-1. All the EF values are tabulated in Table S4 in online SI section. From Table S4, we can see that EF values are almost comparable between two morphologies. However, the EF value is high for large size Re NPs at a peak position of 1623 cm-1 and 1391 cm-1 whereas for small size Re NPs EF value is high at a peak position of 443 cm-1. The difference of EF values is less just an order of one or two. Moreover, in both cases for small and large size Re NPs, the interparticles gap between two individual particles are low below 2 nm which can generate more number of surface active ‘hot spots’ that intern gives better and enhanced SERS signal. The highest EF value is observed for small size Re NPs as 5.18 × 102. The observed EF value is less while compared to other metal NPs like Au7 or Ag8. However, it is comparable with several other less studied metal NPs like Os,9,35 Re,22,

28, 29

Rh36,37 etc. In our present study, we tested the

concentration dependent SERS taking two different size Re NPs into consideration as shown in Figure 7. Figure 7A shows the concentration dependent SERS spectra taking large size Re NPs while Figure 7B for small size Re NPs as substrate. In both cases, we plotted the Raman spectra of only MB for comparison purposes. From Figure 7 we can see that we were able to detect dye concentration up to 10-5 M level. We prepared few other samples having dye concentration lower than that such as 10-6 M, 10-8 M etc. However, we were unable to detect using our present experiment taking Re NPs as substrates. Considering Figure 6, Figure 7 and Table S4, we can see that we got good SERS EF and the EF values are varied for two different size Re NPs. From TEM images in Figure 2, we can see that ultra-small size Re NPs were self-assembled together having their individual particles size below 2 nm and the inter-particle gap was also below 2 nm. This small size of the NPs and less inter-particle gap generated more surface active ‘hot spots’ that results better SERS signal. Similar types of phenomenon were described before for selfassembled Os and Rh NPs on DNA scaffolds.35-37 The Schematic presentation of our SERS experiment taking MB dye and Re NPs as substrate are shown in Scheme S2 in online SI section. This strong EF values and good SERS signal observed taking Re NPs in PAH scaffold as substrate might find potential applications in future for trace detection of biologically important molecules and various other bio-medical application.

CONCLUSION


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In conclusion, we have highlighted a easy, ultra-fast and facile route for the formation of stable Re NPs using the polyelectrolyte PAH scaffold within 120 sec of reaction at room temperature in aqueous solution. The average diameters of the Re NPs were 0.7 ± 0.25 nm and 1.7 ± 0.3 nm respectively for two different morphologies. The size of the Re NPs was tuned just by controlling the polymer to metal salt molar ratio. We had seen that the synthesized Re NPs were extremely stable while prepared in PAH having concentration of 1 wt%, 0.75 wt% and 0.5 wt%. However, at lower PAH concentrations viz., 0.1% or 0.01%, the formed Re NPs were precipitated within a short time after synthesis. The potentiality of the synthesized Re NPs was assessed in two different applications such as catalytic reduction of aromatic nitro aromatics and in SERS studies. Catalysis study was done for the reduction of 4-NA in presence of excess NaBH4 taking two different sizes Re NPs as catalyst. The highest catalytic rate ever reported of 1.52 × 10-1 min-1 was observed taking large size Re NPs as catalyst. The order of the catalytic rate was: large size Re NPs > small size Re NPs. SERS study was done taking MB as Raman probe molecule and strong SERS signal was observed with both sizes Re NPs due to their ultrasmall size, narrow inter-particle gap and close packing in PAH scaffold. These close packed Re NPs are self-assembled together in PAH chain and generated more surface active ‘hot spots’ and ultimately resulted in good SERS signal. Overall, our present synthesis route is easy, costeffective, ultra-fast which completed within 120 sec and generated stable Re NPs which can be further applicable in other interdisciplinary research fields in near future other than catalysis and SERS.

ASSOCIATED CONTENT Supporting Information (SI) Available Information on reagents used for synthesis, catalysis and SERS studies, instrument used for different characterizations, preparation of samples for other characterizations, Figures related to EDS, XRD, XPS, UV-Vis spectra related to catalysis study and Raman spectra related to control experiments, Tables related to catalysis and SERS and Schemes related to Catalysis and SERS are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS



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Financial support from Bhaskara Advanced Solar Energy Fellowship (BASE) program in the year 2016 from Department of Science and Technology (DST) and Indo US Science and Technology Forum (IUSSTF) are thankfully acknowledged. S. Kundu wishes to thank Dr. Choongho Yu (Mechanical Engineering Department, TAMU) for discussion and Mr. S. R Ede and Mr. S. Anantharaj (SRF, CSIR-CECRI) for helping to draw some illustration and proof reading the MS. S. Kundu also acknowledges Dr. Vijayamohanan K Pillai, Director, CSIRCECRI for his continuous support and encouragement.

References. (1) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B, 2003, 107, 668-677. DOI: 10.1021/jp026731y (2) Kundu, S.; Liang, H. Nanowires

on

Photochemical Synthesis of Electrically Conductive CdS

DNA

Scaffolds.

Adv.

Mater.,

2008,

20,

826-831.

DOI:

10.1002/adma.200702162 (3) Wu, W..; He, Q.; Jiang, C. Magnetic Iron Oxide Nanoparticles: Synthesis and surface Functionalization

Strategies.

Nanoscale

Res.

Lett.

2008,

3,

397-415.

doi:

10.1007/s11671-008-9174-9 (4) Kundu, S.; Lau, S.; Liang, H. Shape-Controlled Catalysis by Cetyltrimethylammonium Bromide Terminated Gold Nanospheres, Nanorods, and Nanoprisms. J. Phys. Chem. C, 2009, 113, 5150-5156. DOI: 10.1021/jp811331z (5) Kundu, S.; Wang, K.; Liang, H. Size-Controlled Synthesis and Self-Assembly of Silver Nanoparticles within a Minute Using Microwave Irradiation. J. Phys. Chem. C, 2009, 113, 134–141. DOI: 10.1021/jp808292s (6) Saha, S.; Pal, A.; Kundu, S.; Basu, S.; Pal, T. Photochemical Green Synthesis of Calcium-Alginate-Stabilized Ag and Au Nanoparticles and Their Catalytic Application to 4-Nitrophenol Reduction. Langmuir, 2010, 26, 2885-2893. DOI: 10.1021/la902950x (7) Kundu, S. A New Route for the Formation of Au Nanowires and Application of Shapeselective Au Nanoparticles in SERS Studies. J. Mater. Chem. C 2013, 1, 831-842. DOI: 10.1039/C2TC00315E


Page 21 of 40

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


(8) Majumdar, D.; Singha, A.; Mondal, P. K.; Kundu, S. DNA-mediated Wirelike Custers of Silver Nanoparticles: An Ultrasensitive SERS Substrate. ACS Appl. Mater. Interfaces, 2013, 5, 7798-7807. DOI: 10.1021/am402448j (9) Nithiyanantham, U.; Ede, S. R.; Kundu, S. Self-assembled Wire-like and Honeycomblike Osmium Nanoclusters (NCs) in DNA with Pronounced Catalytic and SERS Activities. J. Mater. Chem. C, 2014, 2, 3782-3794. DOI: 10.1039/c4tc00049h (10)

Haynes, W.M. CRC Handbook of Chemistry and Physics, 91st ed., CRC

Press/Taylor and Francis, Boca Raton, FL, 2011. (11)

Naumov, A. Rhythms of Rhenium. Russ. J. Non-Ferrous Metals, 2007, 48, 418-

423. DOI:10.3103/S1067821207060089 (12)

Cao, J.; Wang, Y. ; Yu, J.; Xia, J.; Zhang, C.; Yin, D.; Häfeli, U.O. Preparation

and Radiolabeling of Surface-modified Magnetic Nanoparticles with Rhenium-188 for Magnetic targeted Radiotherapy. J. Magn. Magn. Mater., 2004, 277, 165-174. DOI: 10.1016/j.jmmm.2003.10.022 (13)

Kessler, V. G.; Seisenbaeva, G. A. Rhenium Nanochemistry for Catalyst

Preparation. Minerals, 2012, 2, 244-257. DOI:10.3390/min2030244 (14)

Kirilin, A.V.; Tokarev, A.V.; Manyar, H.; Hardacre, C.; Salmi, T.; Mikkola, J.

P.; Yu Murzin, D. Aqueous Phase Reforming of Xylitol over Pt-Re Bimetallic Catalyst: Effect

of

the

Re

Addition.

Catal.

Today,

2014,

223,

97-107.

DOI:

10.1016/j.cattod.2013.09.020 (15)

Hurley, K. D.; Zhang, Y. Z.; Shapley, J. R. Ligand-enhanced Reduction of

Perchlorate in Water with Heterogeneous Re-Pd/C Catalysts. J. Am. Chem. Soc., 2009, 131, 14172-14173. DOI: 10.1021/ja905446t (16)

Pacer, R.A. Borohydride Reduction of the Perrhenate ion. J. Inorg. Nucl. Chem.,

1973, 35, 1375-1377. https://doi.org/10.1016/0022-1902(73)80213-1 (17)

Mucalo, M. R.; Bullen, R. Rhenium-based Hydrosols: Preparation and Properties.

J. Colloid Interface Sci., 2001, 239, 71-77. https://doi.org/10.1006/jcis.2001.7539 (18)

Babu, K. M.; Mucalo, M. R.

XPS Studies of Freshly Prepared Rhenium

Nanoparticle Dispersions from Hydrazinium Hydrate and Borohydride Reduction of Hexachlororhenate Solutions. J. Mater. Sci. Lett., 2003, 22, 1755-1757.



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

(19)

Page 22 of 40

Yurkov, G. Y.; Kozinkin, A. V.; Koksharov, Y. A.; Fionov, A. S.; Taratanov, N.

A.; Vlasenko, V. G.; Pirog, I. V.; Shishilov, O. N.; Popkov, O. V. Synthesis and Properties of Rhenium–polyethylene Nanocomposite. Composites, Part B, 2012, 43, 3192-3197. DOI:10.1016/j.compositesb.2012.04.019 (20)

Bare, S. R.; Kelly, S. D.; Vila, F. D.; Boldingh, E.; Karapetrova, E.; Kas, J.;

Mickelson, G. E.; Modica, F. S.; Yang, N.; Rehr, J. J. Experimental (XAS, STEM, TPR, and XPS) and Theoretical (DFT) Characterization of Supported Rhenium Catalysts. J. Phys. Chem. C, 2011, 115, 5740-5755. DOI: 10.1021/jp1105218 (21)

Chong, Y. Y.; Chow, W. Y.; Fan, W. Y. Preparation of Rhenium Nanoparticles via

Pulsed-laser Decomposition and Catalytic Studies. J. Colloid Interface Sci., 2012, 369, 164-169. https://doi.org/10.1016/j.jcis.2011.12.015 (22)

Valenzuela, C. D.; Valenzuela, M. L.; Caceres, S.; O’Dwyer, Solution and

Surfactant-Free Growth of Supported High Index Facet SERS Active Nanoparticles of Rhenium by Phase Demixing, C. J. Mater. Chem. A, 2013, 1, 1566-1572. DOI: 10.1039/C2TA00262K (23)

Yi, J.; Miller, J. T.; Zemlyanov, D. Y.; Zhang, R.; Dietrich, P. J.; Ribeiro, F. H.;

Suslov, S.; Abu-Omar, M. M.

A Reusable Unsupported Rhenium Nanocrystalline

Catalyst for Acceptorless Dehydrogenation of Alcohols through γ-C–H Activation, Angew. Chem., Int. Ed., 2013, 53, 833-836. DOI: 10.1002/anie.201307665 (24)

Gyger, F.; Bockstaller, P.; Gerthsen, D.; Feldmann, C. Ammonia-in-Oil-

Microemulsions and Their Application, Angew. Chem., 2013, 52, 12443-12447. DOI: 10.1002/anie.201305289 (25)

Revina, A.; Kuznetsova, M.A.; Chekmareva, A.M. Physicochemical Properties of

Rhenium Nanoparticles Obtained in Reverse Micelles, Doklady Phys. Chem., 2013, 450, 119-121. DOI: 10.1134/S0012501613050059 (26)

Taratanov, N.A.; Yurkov, G.Y.; Koksharov, Y.A.; Bouznik, V.M. Preparation and

Properties of Composite Materials Based on Rhenium-Containing Nanoparticles and Micrograins of Polytetrafluoroethylene, Inorg. Mater. Appl. Res., 2011, 2, 118-124. DOI:10.1134/S2075113311020201


Page 23 of 40

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(27)

Ayvalı, T.; Lecante, P.; Fazzini, P.-F.; Gillet, A.; Philippot, K.; Chaudret, B. Facile

Synthesis of Ultra-Small Rhenium Nanoparticles Chem. Commun. 2014, 33, 1080910811. DOI: 10.1039/C4CC04816D (28)

Anantharaj, S.; Sakthikumar, K.; Ayyappan, E.; Ravi, G.; Karthik, T.; Kundu, Ultra-

Small Rhenium Nanoparticles Immobilized on DNA Scaffolds: An Excellent Material for Surface Enhanced Raman Scattering and Catalysis Studies, S. J. Colloid Inter. Sci., 2016, 483, 360-373. DOI: 10.1016/j.jcis.2016.08.046 (29)

Sakthikumar, K.; Anantharaj, S.; Ede, S. R.; Karthick, K.; Kundu, S. A Highly Stable

Rhenium Organosol on a DNA Scaffold for Catalytic and SERS Applications, J. Mater. Chem. C, 2016, 4, 6309-6320. DOI: 10.1039/C6TC01250G (30)

Yadgarov, L.; Stroppa, D.G.; Rosentsveig, R.; Ron, R.; Enyashin, A.N.; Houben, L.;

Tenne, R. Investigation of Rhenium-Doped MoS2 Nanoparticles with Fullerene-Like Structure. Zeitschrift Fur Anorg. Und Allg. Chem., 2012, 638, 2610-2616. (31)

Sun, Q. C.; Mazumdar, D.; Yadgarov, L.; Rosentsveig, R.; Tenne, R.; Musfeldt, J. L.

Spectroscopic

Determination

of

Phonon

Lifetimes

in

Rhenium-Doped

MoS2

Nanoparticles, Nano Lett., 2013, 13, 2803-2808. DOI: 10.1021/nl401066e (32)

Kundu, S. Formation of Self-Assembled Ag Nanoparticles on DNA Chains with

Enhanced Catalytic Activity, Phys. Chem. Chem. Phys., 2013, 15, 14107-14119. DOI: 10.1039/C3CP51890F (33)

Chen, H.; Wei, G.; Ispas, A.; Hickey, S. G.; Eychmüller, A. Synthesis of Palladium

Nanoparticles and Their Applications for Surface-Enhanced Raman Scattering and Electrocatalysis, J. Phys. Chem. C, 2010, 114, 21976-21981. DOI: 10.1021/jp106623y (34)

Kim, K.; Kim, K. L.; Lee, H. B.; Shin, K. S. Surface-Enhanced Raman Scattering

on Aggregates of Platinum Nanoparticles with Definite Size, J. Phys. Chem. C, 2010, 114, 18679-18685. DOI: 10.1021/jp1078532 (35)

Ede, S. R.; Nithiyanantham, U.; Kundu, S. Enhanced Catalytic and SERS Activities

of CTAB Stabilized Interconnected Osmium Nanoclusters, Phys. Chem. Chem. Phys., 2014, 16, 22723-22734. DOI: 10.1039/C4CP03068K (36)

Kundu, S.; Wang, K.; Liang, H. Photochemical Generation of Catalytically Active

Shape Selective Rhodium Nanocubes, J. Phys. Chem. C 2009, 113, 18570-18577. DOI: 10.1021/jp906745z



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

(37)

Kundu, S.;

Chen, Y.;

Dai, W.;

Page 24 of 40

Ma, L.; Sinyukov, A. M.; Liang, H. Enhanced

Catalytic and SERS Activities of Size-selective Rh NPs on DNA Scaffold, J. Mater. Chem. C, 2017, 5, 2577-2590. DOI: 10.1039/C6TC05529J (38)

Veerakumar, P.; Thanasekaran, P.; Lin, K. –C.; Liu, S. –B. Well-Dispersed Rhenium

Nanoparticles on Three-Dimensional Carbon Nanostructures: Efficient Catalysts for the Reduction of Aromatic Nitro Compounds, J. Colloid Interface Sci., 2017, 506, 271-282. DOI: 10.1016/j.jcis.2017.07.065 (39)

Kundu, S.; Liang, H. Polyelectrolyte-Mediated Non-Micellar Synthesis of

Monodispersed ‘Aggregates’ of Gold Nanoparticles Using a Microwave Approach, Colloids

Surf.

A:

Physicochem.

Eng.

Aspects,

2008,

330,

143-150.

https://doi.org/10.1016/j.colsurfa.2008.07.043 (40)

Kundu, S.; Wang, K.; Huitink, D.; Liang, H. Photoinduced formation of electrically

conductive thin palladium nanowires on DNA scaffolds, Langmuir, 2009, 25, 1014610152. DOI: 10.1021/la900939c (41) 3

(42)

Okal, J.; Tylus, W.; Kȩpiński, L. XPS study of oxidation of rhenium metal on γ-Al2O support, J. of Catalysis 2004, 225, 498-509. https://doi.org/10.1016/j.jcat.2004.05.004 Kim, H. –D.; Park, H. J.; Kim, T. –W.; Jeong, K. –E.; Chae, H. –J.; Jeong, S. –Y.;

Lee, C. –H.; Kim, C. –U. The Effect of Support and Reaction Conditions on Aqueous Phase Reforming of Polyol over Supported Pt–Re Bimetallic Catalysts, Catal. Today, 2012, 185, 73-80. https://doi.org/10.1016/j.cattod.2011.08.012 (43)

Minko, S.; Kiriy, A.; Gorodyska, G.; Stamm, M. Mineralization of Single Flexible

Polyelectrolyte Molecules, J. Am. Chem. Soc., 2002, 124, 10192-10197. DOI: 10.1021/ja026784t (44)

Hong, U.G.; Park, H.W.; Lee, J.; Hwang, S.; Yi, J.; Song, I.K. Hydrogenation of

Succinic Acid to Tetrahydrofuran (THF) Over Rhenium Catalyst Supported on H2SO4Treated Mesoporous Carbon, Appl. Catal. A Gen., 2012, 415-416, 141-148. https://doi.org/10.1016/j.apcata.2011.12.022 (45)

Göksu, H. Recyclable Aluminium Oxy-Hydroxide Supported Pd Nanoparticles for

Selective Hydrogenation of Nitro Compounds via Sodium Borohydride Hydrolysis, New J. Chem., 2015, 39, 8498-8504. DOI: 10.1039/C5NJ01492A


Page 25 of 40

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


(46)

Lee, J. H.; Hong, S. K.; Ko, W. B. Reduction of 4-Nitrophenol Catalyzed by Platinum

Nanoparticles Embedded into Carbon Nanocolloids, Asian J. Chem., 2011, 23, 23472350. (47)

Sasmal, A. K.; Dutta, S.; Pal, T. A ternary Cu2O–Cu–CuO nanocomposite: a catalyst

with

intriguing

activity,

Dalton

Trans.,

2016,

45,

3139-3150.

DOI:

10.1039/C5DT03859F (48)

Shin, K. S.; Kim, J. H.; Kim, I. H.; Kim, K. Poly(ethylenimine)-Stabilized Hollow

Gold-Silver Bimetallic Nanoparticles:Fabrication and Catalytic Application, Bull. Korean Chem. Soc., 2012, 33, 906-910. (49)

Vadakkekara, R.; Chakraborty, M.; Parikh, P. A. Reduction of aromatic nitro

compounds on colloidal hollow silver nanospheres, Colloids Surf. A, 2012, 399, 11-17. https://doi.org/10.1016/j.colsurfa.2012.02.016 (50)

J. Wu, N. Zhao, X. Zhang and J. Xu, Cellulose/silver nanoparticles composite

microspheres: eco-friendly synthesis and catalytic application, Cellulose, 2012, 19, 1239-1249. DOI: 10.1007/s10570-012-9731-3 (51)

Lee, K. Y.; Hwang, J.; Lee, Y. W.; Kim, J.; Han, S. W. One-Step Synthesis of

Gold Nanoparticles Using Azacryptand and Their Applications in SERS and Catalysis, J. Colloid

Interface

Sci.,

2007,

316,

476-481.

DOI:

https://doi.org/10.1016/j.jcis.2007.07.076 (52)

Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman Spectra of Pyridine

Adsorbed at A Silver Electrode, Chem. Phys. Lett., 1974, 26, 163-166. DOI: https://doi.org/10.1016/0009-2614(74)85388-1 (53)

Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Plasma Resonance- Enhanced

Raman Scattering by Absorbates on Gold Colloids: The Effects of Aggregation, Surf. Sci., 1982, 120, 435-455. DOI: 10.1016/0039-6028(82)90161-3 (54)

Hao, E.; Schatz, G. C. Electromagnetic Fields Around Silver Nanoparticles and

Dimers, J. Chem. Phys., 2004, 120, 357-366. doi: http://dx.doi.org/10.1063/1.1629280 (55)

Kerker, M.; Siiman, O.; Wang, D. S. Effect of Aggregates on Extinction and Surface-

Enhanced Raman Scattering Spectra of Colloidal Silver, J. Phys. Chem., 1984, 88, 31683170. DOI: 10.1021/j150659a003



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

(56)

Page 26 of 40

Zhu, Z.; Zhu, T.; Liu, Z. Raman Scattering Enhancement Contributed from Individual

Gold Nanoparticles and Interparticle Coupling, Nanotechnology, 2004, 15, 357-364. DOI: 10.1088/0957-4484/15/3/022 (57)

Wu, H.-L.; Tsai, H.-R.; Hung, Y.-T.; Lao, K.-U.; Liao, C.-W.; Chung, P.-J.; Huang,

J.-S.; Chen, I.-C.; Huang, M. H. A Comparative Study of Gold Nanocubes, Octahedra, and Rhombic Dodecahedra as Highly Sensitive SERS Substrates, Inorg. Chem., 2011, 50, 8106-8111. DOI: 10.1021/ic200504n (58)

Wang, T.; Hu, X.; Dong, S. Surfactantless Synthesis of Multiple Shapes of Gold

Nanostructures and Their Shape-Dependent SERS Spectroscopy, J. Phys. Chem. B, 2006, 110, 16930-16936. DOI: 10.1021/jp062486x (59)

M. Li, S. K. Cushing, J. Zhang, J. Lankford, Z. P. Aguilar, D. Ma and N. Wu, Shape-

Dependent

Surface-Enhanced

Raman

Scattering

in

Gold–Raman-Probe–Silica

Sandwiched Nanoparticles for Biocompatible Applications, Nanotechnology, 2012, 23, 115501(1-10). DOI: 10.1088/0957-4484/23/11/115501 (60)

Xiao, G. N.; Man, S. Q. Surface-Enhanced Raman Scattering of Methylene Blue

Adsorbed on Cap-Shaped Silver Nanoparticles, Chem. Phys. Lett., 2007, 447, 305-309. DOI: 10.1016/j.cplett.2007.09.045

Figure captions Figure 1. Shows the UV-Vis spectra of the different solution mixtures for the formation of selfassembled Re NPs in PAH scaffold. Curve a is the absorption spectra of only aqueous PAH solution; curve b is the absorption spectra of colorless ammonium perrhenate salt solution solution; curve c is the absorption band of a mixture of PAH and ammonium perrhenate solution; curve d, e and f shows the absorption band of aqueous Re NPs solution where the stock PAH solution concentration was 0.5%, 0.75% and 1% respectively. Inset shows the three different Re NPs solution corresponding to curve d, e and f.


Page 27 of 40

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


Figure 2. Transmission electron microscopic (TEM) and high resolution TEM micrographs of the self-assembled Re NPs on PAH scaffold. (A-D) shows the low and high magnified TEM image of the Re NPs obtained while stock PAH solution concentration of 0.5 wt%. (E-H) shows the low and high magnified TEM images of Re NPs while the stock PAH solution concentration 1 wt%. The inset of Figure 2D and 2H shows their corresponding SAED pattern respectively. Figure 3. X-ray photoelectron spectroscopic (XPS) images of the synthesized self-assembled Re NPs on PAH scaffold. (A) is the survey spectrum; (B) is the high resolution spectrum of Re 4f; (C) is the high resolution spectrum of Re 4d; (D) is the high resolution spectra for O (1s). Figure 4. (A) and (B) shows the UV-Vis spectra for the successive reduction of 4-NA taking large size and small size Re0 NPs respectively. (C) and (D) shows the corresponding ln (Abs) vs time (T, min) plot taking large and small size Re NPs as catalyst respectively. Figure 5. (A) chemical structure of MB is shown in middle while left image for only MB dye and the right image indicate mixture of MB dye and Re NPs solution together; (B) shows the UV-Vis absorption spectra where curve a is for only MB solution; curve b is the absorption band of only Re NPs and curve c is the absorption band of the mixture of MB and Re NPs together. Figure 6. Raman spectra of only MB (10-3 M) (curve a) and SERS spectra taking small size Re NPs (curve b) and large size Re NPs (curve c) respectively. Figure 7. The concentration dependent SERS spectra taking large size Re NPs (A) and small size Re NPs (B) in PAH scaffold as substrate. Scheme 1: Schematic presentation of the synthesis of self-assembled Re NPs on PAH scaffold Scheme 2: Schematic presentation for the formation mechanism of self-assembled Re NPs on PAH scaffold.

Table 1: Final concentration of all the reagents, the resultant particle size and shapes for the synthesis of self-assembled Re NPs on PAH scaffold.



a = only PAH solution b = only Re salt solution c = PAH + Re salt mixture d = Re NPs (0.5% PAH) e = Re NPs (0.75% PAH) f = Re NPs (1% PAH)

1.5

1.2

Absorbance

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

Page 28 of 40

227

0.9 0.5%

0.75%

1%

0.6

e 0.3

0.0

c

b

a 225

300

d f 375

450

525

Wavelength (nm)

Figure 1


600

675

Page 29 of 40

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


A

B Re NPs

Chain-like Re NPs

DNAchain PAH Chain-like Re NPs

D

0.7 ± 0.1 nm

0.264 nm

C Figure 2, A-D


1.7 ± 0.3 nm


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

Page 30 of 40

E

F PAH chain Re NPs

PAH chain

H PAH chains crosslinked

0.7 ± 0.25 nm 0.204 nm

G Figure 2, E-G


Page 31 of 40

A

B

C 1s

3200

Re 4f

4f7/2

3000

2600

cps

cps

60000

20000

2400 2200

Re 4f

Cl 2p

40000

4f5/2

2800

Cl 2s

Na KLL

O 1s

O KLL

C KLL

80000

Na 1s

100000

2000

Re NPs Survey Scan

1800

0

1600

1000

800

600

400

200

0

52

50

Binding Energy (eV)

C Re 4d

48

46

Binding Energy (eV)

14000

D

13000

O1s

4d3/2

O1s 12000

12000

4d5/2

11000

cps

10000

cps

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


8000

ReOx

Re

sat

6000

10000 9000

ReOx Re

8000

4000 295

290

285

280

275

270

265

7000 538

260

536

534

532

Binding Energy (eV)

Binding Energy (eV)

Figure 3 ACS Paragon Plus Environment

530

528


A 1.5 4-NA catalysis using Re NPs (large size)

Absorbance

1.2

4-NA p-PDA

379 nm 00 min 02 min 04 min 06 min 09 min 12 min 14 min

0.9

0.6

0.3 238 nm 0.0 250

300

350

400

450

500

550

Wavelength (nm)

B 1.6

4-NA catalysis using Re NPs (small size) 4-NA

1.2

Absorbance

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

Page 32 of 40

379 nm

p-PDA

a = 01 min b = 05 min c = 10 min d = 15 min e = 20 min f = 25 min

0.8

0.4

238 nm 0.0 250

300

350

400

Wavelength (nm) Figure A-B ACS Paragon Plus 4, Environment

450

500

550

Page 33 of 40

C

0.5

ln (Abs) vs Time (T, min) plot Linear fit 0.0

ln (Abs)

-0.5

-1.0

Rate constant (k) = 1.52 x 10

-1.5

-1

min

-1

-2.0 0

2

4

6

8

10

12

Time (min)

D 0.5

ln (Abs) vs Time (T, min) plot Linear fit 0.0

ln (Abs)

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


-0.5

-1.0

-1.5 Rate constant (k) = 7.58 x 10

-2

min

-1

-2.0 0

5

10

15

Time (min)

Figure 4, C-D


20

25


A

B 1.5

a = Only MB b = Only Re NPs c = Mix. of MB + Re NPs

1.2

Absorbance (a. u.)

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

Page 34 of 40

664

0.9

0.6

611

b

a

291

0.3

c 0.0 300

400

500

600

Wavelength (nm)

Figure 5


700

800

Page 35 of 40

35000 SERS spectra of MB with different sizes Re NPs 1623 30000

Raman Intensity (a.u.)

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


-3 a = Only MB (10 M) b = SERS, Re NPs (small size, 1%PAH) c = SERS, Re NPs (large size, 0.5%PAH)

25000 20000 15000 447

10000 5000

1391 857

497 769 668 590

1478

1353 1436

1068 1036 1153 1303

c

948 1213

b

0 400

600

800

1000

a 1200

1400

Raman Shift (cm-1)

Figure 6


1600

1800


A

Conc. dependent SERS spectra # Re NPs (large size) 447

1623

1391

d -5 SERS-MB (10 M)


60000

c -4 SERS-MB (10 M)

40000

-3 SERS-MB (10 M)

20000

b a 0

-3 Only MB (10 M)

600

900

1200

1500

1800

Raman Shift (cm-1)

B

Conc. dependent SERS spectra # Re NPs (small size) 1623

447

1391

30000

d -5 SERS-MB (10 M)


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

Page 36 of 40

c

20000 -4 SERS-MB (10 M) -3 SERS-MB (10 M)

10000

b a

-3 Only MB (10 M)

0 600

900

1200

Raman Shift (cm-1)

Figure 7 ACS Paragon Plus Environment

1500

1800

Page 37 of 40

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


Table 1

Set number

1

2

3

4

5

Conc. of stock PAH (weight %)

1

0.75

0.5

0.1

0.01

Final conc. of PAH (weight %)

Final conc. of NH4ReO4 solution (M)

3.84 × 10-3

0.076

3.84 × 10-3

0.057

3.84 × 10-3

0.038

3.84 × 10-3

0.0076

0.00076

3.84 × 10-3

Final conc. of NaBH4 (M)

1.53 × 10-2

1.53 × 10-2

1.53 × 10-2

1.53 × 10-2

1.53 × 10-2


Stirring time by magnetic Stirrer (sec)

Average size (nm), morphology and Particles distribution

120

0.7 ± 0.25, Chainlike, spherical, 100%

120


120


120

NPs formed but precipitated with an hour after synthesis

120

NPs formed but precipitated with 20 min of synthesis


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

Scheme 1


Page 38 of 40

Page 39 of 40

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


Scheme 2



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

For Table of Contents Use Only

……………………………………………………………………………………………………… Abstract Ultra-small, self-assembled rhenium (Re) nanoparticles (NPs) having diameter < 2 nm are synthesized in PAH scaffold within 120 seconds of wet-chemical reaction and the NPs were screened as an excellent sustainable materials for catalysis and SERS studies. ………………………………………………………………………………………………………


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Polymer Encapsulated Self-Assemblies of Ultrasmall Rhenium

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