Enantioselective silver nanoclusters: Preparation, characterization and photoluminescence spectroscopy

Materials Chemistry and Physics 180 (2016) 349e356

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Enantioselective silver nanoclusters: Preparation, characterization and photoluminescence spectroscopy Mostafa Farrag Chemistry Department, Faculty of Science, Assiut University, 71516, Assiut, Egypt

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


New wet chemistry method to prepare mirror image small silver clusters protected by penicillamine.
Preparation enantioselective catalysts by easy wet chemistry method.
The synthesized silver clusters have photoluminescence properties.
The synthesized silver clusters show high Anisotropy factors up to 3  104.
The adsorption isotherms of all synthesized clusters are mainly of type II of Brunaue’s classification.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 December 2015 Received in revised form 2 June 2016 Accepted 5 June 2016 Available online 11 June 2016

Herein, we report a new wet-synthesis method to separate some water-soluble chiral silver nanoclusters with high yield. The cluster material was obtained by the reduction of silver nitrate with NaBH4 in the presence of three ligands L-penicillamine (L-pen), D-penicillamine (D-pen) and racemic mixture of penicillamine (rac-pen), functioning as capping ligand. For characterizing all silver cluster samples, the particle size was assessed by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) and their average chemical formula was determined from thermogravimetric analysis (TGA) and elemental analysis (EA). The particles sizes of all three clusters are 2.1 ± 0.2 nm. The optical properties of the samples were studied by four different methods: UV-vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), photoluminescence spectroscopy (PL) and circular dichroism (CD) spectroscopy. The spectra are dominated by the typical and intense plasmon peak at 486 nm accompanied by a small shoulder at 540 nm. Infrared spectroscopy was measured for the free ligand and protected silver nanoclusters, where the disappearance of the S-H vibrational band (2535 e2570 cm1) in the silver nanoclusters confirmed anchoring of ligand to the cluster surface through the sulfur atom. PL studies yielded the fluorescent properties of the samples. The main focus of this work, however, lies in the chirality of the particles. For all silver clusters CD spectra were recorded. While for clusters capped with one of the two enantiomers (D- or L-form) typical CD spectra were observed, no significant signals were detected for a racemic ligand mixture. Furthermore, silver clusters show quite large asymmetry factors (up to 3  104) in comparison to most other ligand protected clusters. These large factors and bands in the visible range of the spectrum suggest a strong chiral induction from the ligand to the metal core. Textural features of the prepared silver nanoclusters were investigated using nitrogen adsorption-desorption at 196  C. Specific surface area SBET, pore volume and average pore diameter were calculated. © 2016 Elsevier B.V. All rights reserved.

Keywords: Enantioselective silver nanoclusters Chirality Photoluminescence spectroscopy Nitrogen adsorption-desorption isotherms L/D-penicillamine

E-mail address: [email protected]. http://dx.doi.org/10.1016/j.matchemphys.2016.06.017 0254-0584/© 2016 Elsevier B.V. All rights reserved.

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M. Farrag / Materials Chemistry and Physics 180 (2016) 349e356

1. Introduction Monolayer-protected nanoclusters (MPNCs) of noble metals (Au and Ag) have gained much attention in the last decade due to their physicochemical properties [1e7] and possible applicability in enantioselective catalysis [1], enantioseparation [2], material science, energy technology, biology, and medicine [3e5], as well as materials for liquid crystal displays [6], and optoelectronics [7]. Chiroptical properties in thiolate-protected metal clusters were first observed by Whetten and co-workers in 1998 by studying gold nanoclusters protected by L-glutathione (L-GSH) [8]. After purification of the clusters, CD spectra were recorded and Cotton effects were observed for transitions at higher wavelengths than that of the free glutathione ligand. The circular dichroism (CD) spectroscopy measures the difference between the absorbance for left- and right-handed light and a non-zero CD signal is detected if the sample contains achiral species. In the case of clusters, chiral structures exist and therefore CD signals are observed, as well [9,10]. Circular dichroism requires not only rotation of linearly polarized light but also the presence of a discrete electronic transition in the metal core and/or of the ligand molecule of monolayer protected metal clusters. The magnitude of the CD effect for an electronic transition is directly related to the product of the electric dipole and magnetic dipole transition moments and thus is also proportional to the optical absorption. In our previous work, small silver clusters protected by glutathione as ligand show strong chiroptical activity in the electronic transitions which are metal-based across the near-infrared, visible, and near-ultraviolet regions [11], however, large silver clusters just give very weak or not even any optical activity in this region [12]. Many researchers in the last decades tried to explore the origin of chiroptical effects of clusters and several models have been discussed in the literature. It was proposed that the clusters either have intrinsically chiral cores [13,14], or that the electrons feel a dissymmetric field that is created by chiral adsorbates, without a change in the geometric lattice of clusters [15]. While the latter is predicted by theory using a particle-in-a-box model, it is believed that intrinsically chiral structures of the core result from the interaction of the chiral ligand energetically favoring one specific chiral structure of the cluster. The optical absorption spectra of small silver Agn (n ¼ 10, 20, 35, 56, 84, 120) clusters was calculated by time-dependent density functional theory. It was found that Agn cluster spectra evolve from molecular-like to plasmon-like transitions with increasing (n). Furthermore, the plasmon width increases with decreasing cluster size for typical cluster shape distributions as predicted by Mie theory [16]. A similar size-dependent behaviour was also found experimentally: Larger silver nanoclusters in the 2 nm regime generally exhibit a broad plasmon resonance peak at around 450 nm in the visible region [11,12,17]. In contrast to clusters, such nanoparticles do not exhibit molecule-like properties and develop an optical absorption band originating from surface plasmon resonances. Smaller silver clusters with only several tens of atoms (up to about 1 nm), on the other hand, exhibit molecule-like optical transitions [11]. In this case more than one absorption maximum is visible in their spectra and a strong dependence of the spectral behaviour on the number of atoms in the cluster is observed [11,12,17]. Several quantum clusters (QCs) of silver with known chemical composition have been identified by mass spectrometry such as water soluble Ag7 [18], Ag7,8 [19] and Ag9 [20] as well as organic soluble Ag44 [21], ~Ag140 [22] and Ag152 [23]. These thiols protected silver clusters possess molecule-like behaviour in their optical properties but systematic changes in these properties were not

seen due to size and structural differences in cores (such as Ag64þ, 12þ Ag6þ 8 and Ag22 ) [24]. For example, while Ag14 is yellow emissive, Ag16 and Ag32 exhibit blue emission [24]. A toluene solution from Ag152(SCH2CH2Ph)60 silver clusters showed an absorption maximum at 460 nm [23]. These silver clusters exhibited photoluminescence at near-infrared (NIR) region with an emission wavelength of 800 nm, upon excitation at 375 nm. This NIR emission is attributed to the silver core [23]. The photoluminescence of metal (silver and gold) nanoparticles/ clusters has become a heavily investigated research field in chemistry due to the potential applicability of metal nanoparticles/ clusters as sensors [25], in biolabelling for drug delivery or for efficient energy transfer [25]. However, the origin of fluorescence is still discussed. For example, recently Wu and Jin [26] studied the fluorescence properties of Au25 clusters protected by several different, mostly hydrophobic, molecules to elucidate the role of the cluster’s charge state and of the ligand on the photoluminescence. From this study they suggested two possible mechanisms. According to their interpretation the fluorescence either may arise from the metal core or from the interaction of the metal core and the surface ligands. In the latter case, either charge transfer through the metal-sulphur bonds, or direct donation of delocalized electrons from electron-rich groups in the ligand molecules may be responsible for the interaction of the ligands with the cluster [26]. In this study, we report on a new method for the synthesis of silver clusters protected by chiral L- and D-penicillamine (Scheme 1), as well as their racemic mixture (rac-pen). This method is purely based on wet chemistry and has an advantage of producing a high yield of the cluster material. Silver clusters are directly produced with a very narrow size distribution (2.1 ± 0.2 nm) from the synthesis method without using any purification techniques like gel electrophoresis [27]. The optical properties of enantioselective silver clusters were studied by UV-vis, circular dichroism (CD) and photoluminescence spectroscopy (PL). The size and chemical composition of synthesized monolayer protected silver clusters were assessed by transmission electron microscopy (TEM), powder X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA) and elemental analysis. The complete isotherm of the synthesized clusters were measured using nitrogen adsorption-desorption at 196  C, specific surface area SBET, pore volume and average pore diameter were calculated.

2. Experimental In this work several cluster samples were isolated and studied. Three different types of silver clusters (1e3) were prepared by the same method, but with different enantiomeric forms of the ligand.

H

H2N

NH2

H3C

OH

H CH3

HO

SH HS

CH3 O

D-Penicillamine

O

H3C

L-Penicillamine

Scheme 1. L- and D-penicillamine (L- and D-pen) used as ligands for protecting the clusters.

M. Farrag / Materials Chemistry and Physics 180 (2016) 349e356

2.1. Chemicals Silver nitrate (AgNO3, 99% metals basis, Aldrich), Sodium borohydride (NaBH4, 96%, Aldrich), potassium hydroxide (KOH, 90%, Aldrich), L-penicillamine (L-pen, 99%, Aldrich), and D-penicillamine (D-pen, 99%, Sigma-Aldrich) were used in the synthesis of the ligand protected silver and gold nanoparticles. As solvents ethanol (HPLC grade, Aldrich) and methanol (HPLC grade, Aldrich) were taken. All chemicals were used as received. 2.2. Preparation of Agn(L-pen)m, Agn(D-pen)m and Agn(rac-pen)m nanoclusters (1e3) The 47 mg Silver nitrate (0.435 mmol) was dissolved in 5 ml 2nd distilled water, KOH (84 mg) was added to adjust the pH of the solution to around 10. Then 72.5 mg L-penicillamine (L-pen), Dpenicillamine (D-pen) or a racemic mixture of both forms (50% Lpen/50% D-pen, 0.485 mmol) were added and the solution was then vigorously stirred at room temperature for 30 min. The resulting solution was cooled to 0  C with an ice bath and a freshly prepared aqueous solution of NaBH4 (189 mg, dissolved in 10 ml ice-cold water) was added dropwise over a period of 20 min while stirring vigorously (~1100 rpm). During the reduction the solution colour became light yellow and then brown. The reaction was allowed to proceed under constant stirring for 1 h. Afterwards, 25 ml ethanol was added to precipitate the cluster. The resulting precipitate was then collected through centrifugal precipitation (5000 rpm, 10 min) and was repeatedly washed with ethanol to remove the unreacted material. The deep brown solid consisting of Agn(L-pen)m, Agn(D-pen)m or Agn(rac-pen)m clusters was finally dried under reduced pressure in a vacuum desiccator. The yield of protected silver clusters was ~62 mg. 2.3. Instrumentation and characterization To obtain the UV-vis absorption spectra of the three synthesized silver clusters and the bare ligand (L-penicillamine), aqueous solution of approximately 1e2 mg/mL was prepared. The spectra were recorded at ambient temperature from 200 to 900 nm with a double-beam spectrophotometer (Evolution 300). The CD spectra of all three types of clusters were measured with a Jasco J-710 spectropolarimeter, using a quartz cell of 1 cm path length and solutions with the same solvent and concentration as for UV-vis spectroscopy. Thermal gravimetric analysis (TGA, ~2 mg sample tested) was conducted in a N2 atmosphere (flow rate ~50 mL/min) with a ThermoStartTM TG/DAT (Pfeiffer Vacuum). All measurements were performed with a heating rate of 10  C/min, starting from room temperature and ramping up to 900  C. Analysis of C, S, N, and H content was performed by an elemental analyzer (Vario EL) that allowed controlled combustion of the samples with subsequent chromatographic separation and the detection of the asseparated species with a TCD detector. The amount of Ag was analyzed by a fast sequential atomic absorption spectrometer (ICAP 6200 ICP-OES analyzer e Thermo Scientific) using a silver lamp as light source. AgNCs were dissolved in aqua regia, and the solution was then evaporated completely. The sample was re-dissolved in 10% HCl. For calibration, silver solutions of different concentrations were prepared using the standard matrix. For TEM measurements, solutions with a concentration of 1e2 mg/mL were prepared by dissolving the cluster materials in 2nd distilled water. A droplet of these MPC solutions was casted onto carbon-coated copper grids. The solvent was then allowed to evaporate slowly. TEM images were obtained at a magnification of 150,000 for type 1e3 clusters, with a JEOL JEM 2010 with LaB6-cathode electron microscope operating at an acceleration voltage of 100 kV. The images were

351

then analyzed by using Image J software (version 1.44). Powder Xray diffraction (XRD) was performed on a philips X-ray powder diffractometer, model pw 2013/00. Ni-filtered Cu Ka with a wavelength of l ¼ 1.541838 Å was used as a constant source of radiation. The generator was operated at 35 KV and 20 mA, and diffractometer at 50 diverting and receiving slits and a scan rate of 20 mm/ min. Fine powder samples were loaded on a quartz plate holder by spreading the powders as a smooth thin layer on the plate. For all diffractograms, the following settings were used: scan range 4e90 (2 q ), scan step 0.06 . Photoexcitation and fluorescence studies were performed with a JASCo FP-6300 spectrofluorometer with a xenon lamp as excitation source. The band-pass for both, excitation and emission monochromators, was always kept at 5 nm. The system which investigates the textural studies of the samples is of the type NOVA 3000, version 6.10 high-speed gas sorption analyzer (Quantachrome corporation). Prior to analysis, the samples were outgases at 150  C for 2 h. Five points BET surface areas, total pore volumes, and pore size distribution (BJH method) were calculated from 24 points nitrogen adsorption-desorption isotherms. 3. Results and discussion After having successfully synthesized the monolayer-protected silver nanoclusters, the samples were further purified by several cycles of re-solvation in water and re-precipitation by adding ethanol. The as-prepared clusters were then characterized and studied by UV-vis spectroscopy, transmission electron microscopy (TEM), powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), elemental analysis (EA), circular dichroism spectroscopy (CD), photoluminescence spectroscopy and nitrogen sorption. 3.1. Chemical composition of protected silver nanoclusters (1e3) One of the most effective ways to obtain the ratio between organic fraction and metal content of the protected clusters are thermogravimetry and elemental analysis. With these techniques the metal-to-ligand ratio (M/L) and, hence, the average chemical formula of the MPCs can be derived. Penicillamine that was used as protecting ligands decomposes when heating up to 231  C in a single step, as shown in the thermogram in Fig. 1a. Fig. 1b shows the thermogram of Agn(L-pen)m clusters (1), the another two silver clusters (2,3) show the same thermogram. All clusters are found to be hygroscopic, which is indicated by the onset of the mass loss already at 40  C temperature. A mass loss at these low temperatures is attributed to the removal of adsorbed water molecules rather than to a decomposition of the ligand, since the pure ligand does not show any mass loss up to 200  C [11,12]. With the exception of the removal of water molecules, the other steps of the TGA curve correspond to the decomposition of the ligand protecting clusters. In the measurements of all species the decomposition of the ligand is completed before 900  C and the residue consists of metal atoms, only. From the relative weight of the residue with respect to the total weight loss of organic molecules, the average metal-to-ligand ratio can be calculated and thus the average molecular formula of the clusters can be derived [11,12,17]. The results together with all the thermogravimetric data are shown in Table S1. TGA analysis shows that the organic weight loss of the silver nanoclusters is 39.41% (wt.). Therefore, the silver content of Agn(Lpen)m clusters is calculated as 60.59% (Table S1). This composition of Agn(L-pen)m clusters is further supported by the elemental analysis. The organic content of this cluster consists of 11.89% C, 3.22% H, 2.77% N, 6.33% S and 15.24% O (sum. 39.45% wt.) (Table 1). In addition, by using atomic absorption spectroscopy, the percentage of Ag was directly obtained. It was found to be 60.55%

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synthesized silver nanoclusters Agn(L-pen)m can be estimated according to the broadening of the full width at half-maximum (fwhm) of the (111) diffraction peak by the DebyeScherrer equation [11]. The calculated particles size of Agn(L-pen)m clusters from XRD analysis is 2.1 ± 0.1 nm, which in good agreement with TEM analysis. According to the Bragg equation, this broad and intense peak indicates an average interplane spacing (d) of 2.37 ± 0.03 Å , which corresponds very well to the interplane spacing of bulk Ag (111) 2.37 Å [11].

3.3. Optical properties of protected silver nanoclusters (1e3)

Fig. 1. Theromgravimetric analysis (TGA) of the bare penicillamine ligand (curve a) and Agn(L-pen)m (1) nanoclusters (curve b). Agn(L-pen)m clusters show four steps of mass loss upon heating from 40 to 900  C. There is an additional first step in TGA curve of the clusters, which results from the evaporation of water molecules. However, the ligand decomposes in one step at 231  C. For both samples, the clusters and the bare ligand, the destruction of the organic compound is complete before 900  C. Therefore, the residual mass of the clusters samples consists of metal atoms only.

(Table 1), thus confirming composition obtained from thermogravimetric analysis [12]. 3.2. Particles size of protected silver nanoclusters (1e3) For determination the average size of the metal clusters transmission electron microscopy (TEM) is one of the most often used techniques [11,12,17,22], as well as powder X-ray diffraction (XRD) [11]. Conventional TEM is particularly powerful when the particle size is larger than 1 nm, because large scattering cross sections of the metal atoms result in a strong contrast in the TEM image. Fig. 2 (IeIII) shows the TEM images of all three silver clusters protected by L-penicillamine (L-pen), D-penicillamine (D-pen) and a racemic mixture of penicillamine (rac-pen), respectively. In general, individual spherical nanoparticles can be seen for all the samples. The average particles sizes of all three clusters are 2.1 ± 0.2 nm. This is a clear indication that particles with a very narrow size distribution can be obtained with this synthesis method. Furthermore, as expected, the different enantiomers of the ligand do not affect the particles size distribution of the clusters. Powder X-ray diffraction (XRD) was used to compare the structures of the silver nanoclusters protected by penicillamine ligands with that of bulk silver. The powder X-ray diffraction pattern of Agn(L-pen)m nanoclusters together with the stick spectrum of pure bulk silver metal is shown in Fig. 3 [28]. While the silver clusters (1e3, 2.1 ± 0.2 nm) show a well-defined diffraction pattern with four different distinct peaks at 2 q of 38 ± 0.6 (indexed to the fcc (111) reflections of silver), around 44 (200), 65 (220), and 78 (311) (Fig. 3a). The stick pattern (Fig. 3b) is an indication of the diffraction peaks of the bulk Ag metal, which shows face-centered cubic (fcc) lattice at the same reflections. The particles size of the

Table 1 Elementals ratios of Agn(L-pen)m clusters were determined by elemental analysis (EA) and atomic absorption spectroscopy (AAS). Elements

C

H

N

S

O

Ag

Percentage

11.89

3.22

2.77

6.33

15.24

60.55

Plasmonic metal nanoparticles (Au and Ag) have a great potential for chemical and biological sensor applications, due to their sensitive spectral response to the local environment of the nanoparticles surface and the ease of monitoring the light signal due to their strong scattering or absorption [29]. In addition, the UV-vis spectra are very sensitive to changes in the metal core of the particles, in particular for clusters comprising of only a few tens of atoms. The optical response can thus be used to estimate and monitor the size of the particles. 2 nm regime silver and gold monolayer-protected clusters in general exhibit a broad plasmon resonance peak at around 450 nm and 550 nm in the visible region, respectively [11,17]. It was shown previously that for particles of this size the transition can be modelled by Mie theory in combination with the Drude model, which clearly demonstrates that such peaks originate from plasmonic transitions [12]. In accordance to such observations, silver nanoclusters protected by L-penicillamine (L-pen), L-penicillamine (D-pen) and rac-penicillamine (rac-pen) show a similar spectral behaviour (Fig. 4-I), also confirming the size of 2 nm obtained from the TEM and XRD analysis. The spectra are dominated by the typical and intense plasmon peak at 486 nm accompanied by a small shoulder at 540 nm. Note that the spectra of all samples are the same, which shows the expected behaviour, namely that different enantiomers of the same ligand do neither effect the peak position nor its broadness. The capping agent (L-penicillamine) shows one absorption peak at 216 nm (Fig. 4-II). The disappearance of ligand peak in the UV-vis spectrum of protected clusters, confirm the prepared clusters are completely pure from the unreacted ligand [30,31]. In addition, UV-vis spectroscopy can also be used for the investigation of the stability of the clusters [12]. The synthesized clusters are usually stored in a fridge and sealed to avoid exposure to air. The UV-vis spectra of the synthesized silver clusters were recorded every month. The peak positions and the intensity of the absorbance for all three types of silver clusters (1e3) do not change at all over six months. Moreover, the synthesized silver clusters are particularly stable, even when being exposed to air. There is no change in the intensity of the absorption peaks of the UV-vis spectra even after 24 h exposure to air (Fig. S1). This means the synthesized clusters are stable in presence and absence of air. Infra-red (IR) analysis elucidated the structural properties of the ligand binding to the metal nanoclusters. By comparing the IR absorption spectra of pure ligand with this of the protected metal nanoclusters, the disappearance of the SH vibrational band (25352564 cm1) in the protected nanoclusters confirms an anchoring of ligand to the cluster surface through the sulfur atom [12,30]. However, the rest of ligand bands appeared in the same position with the protected clusters, but with low intensity. This confirms the chemical structure of the capping ligands did not change in protected clusters. The prepared clusters are hygroscopic, therefore the absorption bands of water appear in the clusters spectra (Fig. S2).

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Fig. 2. Transmission electron microscopy (TEM) images of silver clusters of Agn(L-pen)m (I), Agn(D-pen)m (II), and Agn(rac-pen)m (III). All three images look very similar and show clusters of around 2 nm with a very narrow size distribution.

3.4. Chirality of the protected silver nanoclusters (1e3) The exploration of the origin of chirality in monolayer protected clusters protected by chiral ligands has been a very active research field in the last few decades [11,13,14]. The chirality of the clusters can be studied by CD spectroscopy. While large particles (>2 nm) usually give small chiroptical peaks, the bands of smaller clusters are more pronounced [11,32,33]. This observation of a decreasing optical activity with increasing particle size, can simply be explained by the increased configurational space for larger particles and thus an increased probability of multiple energy minima on the potential energy surface. An increasing number of conformers lead to a decreased optical activity as positive and negative bands of different conformers average out [34]. The CD spectra of particles covered with the two penicillamine enantiomers (L-pen and D-pen) show a mirror image relationship e a typical behaviour for enantiomers. With the exception of a small

Fig. 3. XRD pattern of silver clusters protected by L-penicillamine as ligand (Agn(Lpen)m), which shows a well-defined diffraction pattern (curve a) with four distinct diffraction peaks at 2 q of 38 (111), 44 (200), 65 (220), and 78 (311). The stick pattern (b) indicates the diffraction peaks of silver bulk metal, which shows face-centered cubic (fcc) lattice (111), (200), (220) and (311) reflections.

drift in the baseline, the CD spectra of the clusters protected by the two different enantiomeric forms of the ligand give perfect mirror images with several signals in the region from 290 to 600 nm (Fig. 5-I). In contrast to the spectra of the cluster that even exhibit bands in the visual region, the bare ligand molecule only show significant CD signals below 250 nm (Fig. S3). While bands below 300 nm in the spectra of the clusters may therefore be interpreted as red-shifted transitions of the ligands, peaks in a higher wavelength range (and particularly in the visual region) clearly indicate metal-based origins. In the CD spectra of the silver cluster five very well resolved transitions (at 339, 394, 445, 492 and 542 nm) are observed at the position of the plasmon peak (Fig. 5-I). This, however, is in stark contrast to the previously published work of Nishida et al. [27]. In their work no significant CD signals where found above 360 nm of all their cluster sizes (up to about 3.5 nm). This trend is further reflected by their UV-vis spectra, which significantly differ from the ones reported in here. We attribute this discrepancy to a different

Fig. 4. (I) UV-vis absorption spectra of Agn(L-pen)m (curve a), Agn(D-pen)m (curve b) and Agn(rac-pen)m (curve c) nanoclusters. All silver clusters exhibit a one intense peak at 486 nm with a small shoulder at 540 nm. (II) UV-vis absorption spectra of bare Lpenicillamine ligand, which shows one absorption peak at 216 nm.

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Fig. 5. CD spectra and anisotropy factors of Agn(L-pen)m (curve a), Agn(D-pen)m (curve b) and Agn(rac-pen)m (curve c) nanoclusters. (I) Agn(L-pen)m and Agn(D-pen)m nanoclusters show mirror images circular dichroism spectra from 290 to 600 nm. As expected the racemic mixture Agn(rac-pen)m do not show any CD signals. (II) The corresponding anisotropy factors of the silver clusters protected by opposite enantiomeric forms (1 and 2) of the ligand and the racemate calculated out of the CD and the absorption spectra. After this calculation anisotropy factors of up to 3  104 are found.

morphology or chemical composition (i.e. the number of protective ligand molecules) of the clusters due to the differences in synthesis and purification (i.e. recrystallization and polyacrylamide gel electrophoresis). For a better comparison, which is not dependent on the sample concentration, usually the anisotropy factor is calculated (Fig. 5-II). Anisotropy factors (g) are defined as the difference in absorption of left- and right-handed circularly polarized light (DA) divided by the absorption of the racemate (A).

g ¼ DA=A

(1)

The factor can be calculated from the measured ellipticity (q) and the absorption spectrum by using the following equation:

g ¼ q½mdeg=ð32980  AÞ

(2)

The result over the entire spectral range can be seen in the Fig. 5-II. It is found that the anisotropy factors are quite large for a cluster sample. In particular, for the silver clusters the anisotropies range up to 3  104. Notably, this value is not only one order of magnitude higher than that for the same silver clusters obtained via separation with gel electrophoresis [27], but even is one of the largest for silver clusters, in general.

the d-band. While the first path gives rise to a low energy PL band, the latter one results in a transition of higher photon energies [36]. Note that in a molecular picture the transitions are interpreted as fluorescence and phosphorescence, with the latter resulting from the emission of photons with lower energies [36]. The PL of small ligand protected silver clusters is by far not as thoroughly studied as that of gold clusters. Similarly, the excitationrelaxation pathway should be similar and proceed via excitation from the d-bands to the sp-bands. However, a detailed insight in the processes involved is still missing due to the lack of studies on the particles’ PL. First experimental data were obtained on silver clusters protected by tiopronin [25] and later PL spectra were recorded from Ag-particle growth on SiO2 glasses [37]. In both cases a luminescence peak was found in the visible region and significantly below 600 nm. However, more recently further work has been published on this topic. For example, Rao and Pradeep found luminescence in the visible for ligand protected Ag7 and Ag8 clusters [19]. Even earlier, Ras and co-worker reported electroluminescence as well as strong solvatochromic effects for the absorption and the luminescence of poly (methacrylic acid) stabilized Ag clusters of sizes smaller than 2 nm [38]. The maximum of the PL was found in a range between about 530 nm to over 630 nm when

3.5. Photoluminescence of the protected silver nanoclusters (1e3) The exact mechanism of the photoluminescence (PL) of monolayer protected noble metal cluster is still under debate. Due to the non-fluorescent properties of the ligand (in this work penicillamine), the fluorescence must originate from the metal core. However, the ligand can strongly influence PL and, in particular, largely increase its intensity as indicated by the investigation of several Au25 clusters [26]. In general, among ligand protected clusters, gold clusters have been studied the most widely by PL. Already in 1998 an influence of size of spherical gold clusters on PL was reported [35]. With liquid chromatography two fractions were separated; one of small clusters (smaller than 5 or 2.5 nm), which exhibited very strong PL but gave no well-resolved plasmon peak and another one with clusters of about 15 nm, where a distinct plasmon peak but no PL was observed [35]. Furthermore, the asymmetry of the band indicates that possibly two luminescence pathways contribute to the spectra. It was found [36] that small gold clusters are excited from the d-band into the sp-band and then radiatively decay via an intraband transition into lower levels of the sp-band or in a second path back to the levels of

Fig. 6. Photoemission spectra of Aun(L-pen)m (curve a) and Aun(D-pen)m (curve b) nanoclusters. The emissions of both silver clusters show one emission maximum at 800 nm by an excitation with 486 nm. The PL-spectra are independent of the enantiomeric form of the protective ligand.

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Fig. 7. (I) Nitrogen adsorption-desorption isotherms (aec) of all synthesized silver nanoclusters protected by L-penicillamie (L-pen), D-penicillamie (D-pen), and racemicpenicillamine (rac-pen), respectively. All the three isotherms show mainly type II of Brunaue’s classification and type H4 hysteresis loops according to IUPAC classification of hysteresis loops. The pore volume distribution curves (II) and Vaet plots (III) of all silver nanoclusters (1e3). All the three Vaet plots were exhibited a downward deviation and the three pore volume distribution curves show one maximum sharp peak at ~23 Å, which confirms the protected clusters have micro porous.

Table 2 Texture data obtained from the analysis of nitrogen sorption isotherms of synthesized silver nanoclusters. No.

Clusters

SBET (m2 g1)

St (m2 g1)

Total pore volume (104 cc g1)

Average pore diameter (Å)

1 2 3

Agn(L-pen)m Agn(D-pen)m Agn(rac-pen)m

5.02 4.99 5.04

5.02 5.00 5.05

86.7 86.0 87.1

91.50 91.25 90.43

dissolved in water or methanol, respectively. Most recently, the optical properties and synthesis of Ag10 clusters in reverse micelles and by induction of visible light has been reported [39]. For such clusters PL was found to occur at 680 nm for different excitation wavelength. While most studies report a fluorescence of such small silver clusters in the visible regime, the Ag clusters protected by pen in this work show strong luminescence in the NIR at 800 nm (Fig. 6) upon excitation with photons of 486 nm. In addition, the peak is very symmetric and indicates that it is due to one transition, only. While it may be speculated that for these clusters only a relaxation via an intra-sp-band transition takes place, the exact reason for the occurrence of this peak still remains unclear. Note, however, that all PL spectra are independent of the enantiomeric form of the protective ligand, as it would have been expected. Near-infrared fluorescent (NIRF) materials are promising labeling reagents for sensitive determination and imaging of biological targets. Meanwhile, near-infrared radiation can penetrate into sample matrices deeply due to low light scattering. Thus, in vivo and in vitro imaging of biological samples can be achieved by employing the NIRF probes [40]. 3.6. Texture analysis of the protected silver nanoclusters (1e3) The important application of protected metal clusters is using as catalysts. Therefore, the surface properties and texture of protected cluster should know. In particular, gas adsorption has become one of the most widely used procedures for determining the surface area and pore size distribution of a diverse range of powders and porous materials. In this work we used nitrogen to measure the adsorptionedesorption isotherms of the three synthesized silver nanoclusters (Fig. 7-I). Inspection of this figure reveals that the adsorption isotherms of all clusters are mainly of type II of Brunaue’s classification indicating multilayer adsorption process [41]. The hysteresis loops of all synthesized silver clusters nearly belong to type H4 according to IUPAC classification of hysteresis loops [42]. All the hysteresis loops are closed at low relative pressure values, which means the clusters have micropores, as shown in the pore

volume distribution curves (Fig. 7-II), which agree with the t-plots analysis (Fig. 7-III). The Vaet plots of all synthesized clusters exhibited a downward deviation (Fig. 7-III), this clearly suggests the presence of micropores nature in these clusters [43,44]. Table 2 gives the textural data obtained through the analysis of N2 sorption data of the synthesized silver clusters. The values of surface area which calculated by BET equation (SBET) and t-method (St) are close to each other for all investigated clusters (1e3). This confirms the correct choice of standard t-curves for pore analysis and indicate the absence of ultra-micropores in this adsorbents [44e46]. All clusters (1e3) show one maximum sharp peak at ~ 23 Å in pore volume distribution curves (Fig. 7-II), which confirms the protected clusters have micro porous structure. Table 2 exhibits the surface area (SBET and St), total pore volume and average pore diameter of all silver clusters, it is clear there is no effect of the different enantiomers on the textural data. 4. Conclusion In this work we present the synthesis, the characterization and the optical properties of silver nanoparticles protected by the two penicillamine enantiomers and the racemic mixture. By the synthetic route presented, Ag cluster of 2 nm can be obtained with a narrow size distribution, which measured by TEM and XRD analysis. The clusters show the typical plasmon peak in the UV-vis spectrum and exhibit very well resolved transitions in their CD spectra. Moreover, the spectrum of the clusters with the other enantiomeric form of the ligand shows mirror-image behaviour. The clusters additionally exhibit photoluminescence at 800 nm, when the plasmon transition at 486 nm is excited. The complete isotherms of the synthesized silver clusters were measured using nitrogen adsorption-desorption at 196  C, specific surface area SBET, pore volume and average pore diameter were calculated. While new insights on the optical properties of Ag clusters protected by penicillamine are obtained in this work, the synthetic route chosen results in high cluster yield in comparison to methods using chromatography for size separation. This significantly higher yield of sample has allowed for a more thorough and better

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characterization of the samples and will enables further studies on the applicability of these systems as sensors or asymmetric catalysts. Acknowledgements This work is supported by Assiut University, Egypt. I kindly acknowledge Prof. Dr. U. Heiz and Dr. M. Tschurl for carrying out circular dichroism measurement and for helpful discussions, TUM. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.06.017. References [1] A. Baiker, Progress in asymmetric heterogeneous catalysis: design of novel chirally modified platinum metal catalysts, J. Mol. Catal. A Chem. 115 (1997) 473. [2] M. Bieri, C. Gautier, T. Bürgi, Probing chiral interfaces by infrared spectroscopic methods, Chem. Phys. Phys. Chem. 9 (2007) 671. [3] R.W. Murray, Nanoelectrochemistry: metal ,nanoparticles, nanoelectrodes, and nanopores, Chem. Rev. 108 (2008) 2688. [4] G. Han, P. Ghosh, V.M. Rotello, Functionalized gold nanoparticles for drug delivery, Nanomedicine Lond. U. K. 2 (2007) 113. [5] M. Farrag, M. Tschurl, A. Dass, U. Heiz, Infra-red spectroscopy of size selected Au25, Au38 and Au144 ligand protected gold clusters, Phys. Chem. Chem. Phys. 15 (2013) 12539. [6] H. Qi, T. Hegmann, Formation of periodic stripe patterns in nematic liquid crystals doped with functionalized gold nanoparticles, J. Mater. Chem. 16 (2006) 4197. [7] J.J. Wei, C. Schafmeister, G. Bird, A. Paul, R. Naaman, D.H. Waldeck, Molecular chirality and charge transfer through self-assembled scaffold monolayers, J. Phys. Chem. B 110 (2006) 1301. [8] T.G. Schaaff, G. Knight, M.N. Shafigullin, R.F. Borkman, R.L. Whetten, Isolation and selected properties of a 10.4 kDa gold:glutathione cluster compound, J. Phys. Chem. B 102 (1998) 10643. [9] G.D. Fasman, Circular Dichroism and the Conformational Analysis of Biomolecules, Plenum, New York, 1996. [10] N. Berova, K. Nakanishi, R.W. Woody, Circular Dichroism: Principles and Applications, John Wiley & Sons, Inc., New York, 2000. [11] M. Farrag, M. Tschurl, U. Heiz, Chiral gold and silver nanoclusters: preparation, size selection, and chiroptical properties, Chem. Mater 25 (2013) 862. €mer, M. Tschurl, T. Bürgi, U. Heiz, Preparation and spectro[12] M. Farrag, M. Tha scopic Properties of monolayer-protected silver nanoclusters, J. Phys. Chem. C 116 (2012) 8034. n, J.A. Reyes-Nava, J.I. Rodríguez-Hern n, [13] I.L. Garzo andez, I. Sigal, M.R. Beltra K. Michaelian, Chirality in bare and passivated gold nanoclusters, Phys. Rev. B 66 (2002) 073403. n, M.R. Beltr lez, I. Gutíerrez-Gonza lez, K. Michaelian, [14] I.L. Garzo an, G. Gonza ndez, Chirality, defects, and disorder in J.A. Reyes-Nava, J.I. Rodríguez-Herna gold clusters, Eur. Phys. J. D. 24 (2003) 105. [15] M.R. Goldsmith, C.B. George, G. Zuber, R. Naaman, D.H. Waldeck, P. Wipf, D.N. Beratan, The chiroptical signature of achiral metal clusters induced by dissymmetric adsorbates, Phys. Chem. Chem. Phys. 8 (2006) 63. [16] C.M. Aikens, S. Li, G.C. Schatz, From discrete electronic states to plasmons: TDDFT optical absorption properties of Agn (n ¼ 10, 20, 35, 56, 84, 120) Tetrahedral Clusters, J. Phys. Chem. C 112 (2008) 11272. [17] A. Taleb, C. Petit, M.P. Pileni, Synthesis of highly monodisperse silver nanoparticles from AOT reverse micelles: A way to 2D and 3D self-organization, Chem. Mater 9 (1997) 950. [18] Z. Wu, E. Lanni, W. Chen, M.E. Bier, D. Ly, R. Jin, High yield, large scale synthesis of thiolate-protected Ag7 clusters, J. Am. Chem. Soc. 131 (2009) 16672. [19] T.U. Bhaskara Rao, T. Pradeep, Luminescent Ag7 and Ag8 clusters by interfacial synthesis, Angew. Chem. Int. Ed. 49 (2010) 3925. [20] T. Udayabhaskararao, B. Nataraju, T. Pradeep, Ag9 quantum cluster through a

solid state route, J. Am. Chem. Soc. 132 (2010) 16304. [21] I. Chakraborty, W. Kurashige, K. Kanehira, L. Gell, H. Ha€kkinen, Y. Negishi, T. Pradeep, Ag44(SeR)30: a Hollow cage silver cluster with selenolate protection, J. Phys. Chem. Lett. 4 (2013) 3351. [22] M.R. Branham, A.D. Douglas, A.J. Mills, J.B. Tracy, P.S. White, R.W. Murray, Arylthiolate-protected silver quantum dots, Langmuir 22 (2006) 11376. [23] I. Chakraborty, A. Govindarajan, J. Erusappan, A. Ghosh, T. Pradeep, B. Yoon, R.L. Whetten, U. Landman, The superstable 25 kDa monolayer protected silver nanoparticle: measurements and interpretation as an icosahedral Ag152(SCH2CH2Ph)60 cluster, Nano Lett. 12 (2012) 5861. [24] T. Udayabhaskararao, M.S. Bootharaju, T. Pradeep, Thiolate-protected Ag32 clusters: mass spectral studies of composition and insights into the Agethiolate structure from NMR, Nanoscale 5 (2013) 9404. [25] Y. Negishi, Y. Takasugi, S. Sato, H. Yao, K. Kimura, T. Tsukuda, Magic-numbered Aun clusters protected by glutathione monolayers (n ¼ 18, 21, 25, 28, 32, 39): isolation and spectroscopic characterization, J. Am. Chem. Soc. 126 (2004) 6518. [26] Z. Wu, R. Jin, On the ligand’s role in the fluorescence of gold nanoclusters, Nano Lett. 10 (2010) 2568. [27] N. Nishida, H. Yao, T. Ueda, A. Sasaki, K. Kimura, Synthesis and chiroptical study of D/L-penicillamine-capped silver nanoclusters, Chem. Mater 19 (2007) 2831. [28] H. Yao, M. Saeki, K. Kimura, Induced Optical Activity in Boronic-acid-protected silver nanoclusters by complexation with chiral fructose, J. Phys. Chem. C 114 (2010) 15909e15915. [29] R. Jin, Quantum sized, thiolate-protected gold nanoclusters, Nanoscale 2 (2010) 343. [30] M. Farrag, Preparation, characterization and photocatalytic activity of size selected platinum nanoclusters, J. Photochem. Photobiol. A Chem. 318 (2016) 42. [31] M. Farrag, Monodisperse and polydisperse platinum nanoclusters supported over TiO2 anatase as catalysts for catalytic oxidation of styrene, J. Mol. Catal. A Chem. 413 (2016) 67. [32] Z. Wu, J. Suhan, R. Jin, One-pot synthesis of atomically monodisperse, thiolfunctionalized Au25 nanoclusters, J. Mater. Chem. 19 (2009) 622. [33] T.G. Schaaff, R.L. Whetten, Giant goldglutathione cluster compounds: Intense optical activity in metal-Based transitions, J. Phys. Chem. B 104 (2000) 2630. [34] C. Gautier, T. Bürgi, Chiral N-Isobutyryl-cysteine protected gold nanoparticles: preparation, size selection, and optical activity in the UV-vis and infrared, J. Am. Chem. Soc. 128 (2006) 11079. [35] J.P. Wilcoxon, J.E. Martin, F. Parsapour, B. Wiedenman, D.F. Kelley, Photoluminescence from nanosize gold clusters, J. Chem. Phys. 108 (1998) 9137. [36] S. Link, A. Beeby, S. Fitz Gerald, M.A. El-Sayed, T.G. Schaaff, R.L. Whetten, Visible to infrared luminescence from a 28-Atom gold cluster, J. Phys. Chem. B 106 (2002) 3410. € ßer, [37] M. Eichelbaum, K. Rademann, A. Hoell, D.M. Tatchev, W. Weigel, R. Sto G. Pacchioni, Photoluminescence of atomic gold and silver particles in sodalime silicate glasses, Nanotechnology 19 (2008) 135701. [38] I. Díez, M. Pusa, S. Kulmala, H. Jiang, A. Walther, A.S. Goldmann, H. E. Axel Müller, O. Ikkala, R.H.A. Ras, Color tunability and electrochemiluminescence of silver nanoclusters, Angew. Chem. Int. Ed. 48 (2009) 2122. [39] A. Mohanty, A. Mondal, Visible light induced synthesis of fluorescent silver clusters in reverse micelles, Nanoscale 5 (2013) 7238. [40] C.L. Amiot, S. Xu, S. Liang, L. Pan, J.X. Zhao, Near-infrared fluorescent materials for sensing of biological targets, Sensors 8 (2008) 3082e3105. [41] S.L. Brunauer, L.S. Deming, W.S. Deming, E. Teller, On a theory of the van der Waals adsorption of gases, J. Am. Chem. Soc. 62 (1940) 1723. [42] IUPAC recommendations for the characterization of porous solids, Pure Appl. Chem. 66 (1994) 1739. [43] G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Surface area and pore texture of catalysts, Catal. Today 41 (1998) 207e219. [44] M.Th. Makhlouf, B.M. Abu-Zied, T.H. Mansoure, Effect of calcination temperature on the H2O2 decomposition activity of nano-crystalline Co3O4 prepared by combustion method, Appl. Surf. Sci. 274 (2013) 45e52. [45] W.M. Shaheen, A.A. Zahran, G.A. El-Shobaky, Surface and catalytic properties of NiO/MgO system doped with Fe2O3 Colloids and Surfaces A, Physicochem. Eng. Aspects 231 (2003) 51e65. [46] M. Farrag, Microwave-assisted synthesis of ultra small bare gold clusters supported over Al2O3 and TiO2 as catalysts in reduction of 4-nitrophenol to 4aminophenol, Micropor. Mesopor. Mat. (Accepted 2016).