DOI: 10.1002/cphc.201500215
Communications
Preparation of Cyclodextrin–Iron Species in Water by Laser Ablation: Secondary Ion Mass Spectrometry Sona Halaszova,*[a] Monika Jerigova,[a, b] Dusan Lorenc,[b] and Dusan Velic*[a, b] Supramolecular complexes between cyclodextrin and iron species are studied by using secondary ion mass spectrometry. The iron species are prepared by pulsed-laser ablation of bulk iron in water; this gives Fe + (56 m/z) and FexOy + (x, y = 1–7) species. Cyclodextrin is added to the water either before or after the laser ablation. When it is added before laser ablation, molecular fragments of cyclodextrin are detected as dehydrated glucopyranose units (C6H8O4 + ) associated with Fe + , FeO + , and Fe2O + species. The focus is to observe supramolecular host–guest complexes or adducts between intact molecules of cyclodextrin and iron species. When cyclodextrin is added after laser ablation, the relevant peak at 1210 m/z is observed and assigned as C42H67O35FeNa + , which corresponds to a cyclodextrin molecule minus three H atoms. Two possible explanations of this finding are the presence of the host–guest C42H67O35Na– Fe complex, in which Fe is in the cavity, or the presence of the adduct C42H67O34Na–FeO with FeO on the outer surface; the formation of these complexes are supported by the hydrophobicity of Fe and hydrophilicity of FeO, respectively. Due to the presence of 12 % of intact C42H70O35Na–Fe complex and an estimated Fe/FeO ratio of approximately 102, host–guest formation is assumed to be more significant.
Nanoparticles can be defined as particles that are sized between approximately 1 and 100 nm and they show properties that are not shared by non-nanoscale particles with the same chemical composition. They belong to currently the most studied branch of material science, and have a variety of applications in electronics, optics, diagnostics, and catalysis.[1–5] Methods for their preparation are emulsion–solvent evaporation, emulsion diffusion, and salting out or solvent displacement/ precipitation,[6] as well as a method that involves pulsed-laser ablation of a solid target in a liquid.[7] With the latter method, it is possible to prepare a wide range of novel materials, such as metallic nanoparticles (Au, Ag, Pt, Fe), semiconductors (Si, ZnO) and magnetic nanoparticles (Ni, Co, Fe2O3), by using different target materials and solutions and by varying the laser parameters. The preparation of pure iron nanoparticles by pulsed-laser ablation has been investigated by X-ray diffrac[a] S. Halaszova, Dr. M. Jerigova, Prof. Dr. D. Velic Department of Physical and Theoretical Chemistry Faculty of Natural Sciences, Comenius University Mlynska dolina, 842 15 Bratislava (Slovakia) E-mail: [email protected] [email protected] [b] Dr. M. Jerigova, Dr. D. Lorenc, Prof. Dr. D. Velic International Laser Centre Ilkovicova 3, 812 19 Bratislava (Slovakia)
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tion, transmission electron microscopy and X-ray photoelectron spectroscopy.[8] Iron oxide nanoparticles prepared by using pulsed-laser ablation have also studied.[9] The preparation of metallic nanoparticles in a cyclodextrin (CD) solution is one popular method to obtain stabilized nanoparticles.[10–12] CD molecules are cyclic oligosaccharides that consist of (a-1,4)linked a-d-glucopyranose units and have a hydrophobic cavity and hydrophilic outer surface. Owing to the cavity, CDs are widely used as host molecules in host–guest supramolecular chemistry. CDs can not only form host–guest complexes, but also adducts.[13] The CD cavity accommodates a guest molecule, which can be either organic, inorganic, metallic complexes, cations or anions.[14] These supramolecular host–guest complexes have been characterized by phase solubility, highperformance liquid chromatography, circular dichroism, nuclear magnetic resonance, X-ray powder diffraction, differential scanning calorimetry, thermogravimetric analysis, UV/Vis spectroscopy and Fourier transform infrared spectroscopy.[15, 16] A suitable technique for studying metallic species is secondary ion mass spectrometry (SIMS),[17–21] and here this method is used for the first time to study supramolecular complexes between CD and various iron species. As the CD, we used b-cyclodextrin (C42H70O35) and the iron species were prepared by pulsed-laser ablation of bulk iron in water. The ablation process generates iron species that cause colouration of the solution, owing to a collective oscillation of the electrons in the conduction band, which is known as surface plasmon oscillation. The colour of the metallic species is dependent on the shape and size of the particles and the dielectric constant of the solvent. The size distribution of the particles spans from a single iron species, through clusters of iron species, to ironspecies nanoparticles.[22] Secondary ion mass spectra were obtained and the following mass peaks were assigned. The presence of iron was confirmed by the mass at 56 m/z, which was assigned as Fe + . Iron oxides with the general formula of FexOy + (x, y = 1–7) were also detected (Table 1). These results show the presence of iron and a variety of iron oxides. The formation of iron oxides with such a high-energy laser is a result of interactions between iron and water in the excited plasma regime. Interestingly, pure iron ions were also detected by SIMS; in secondary ion mass spectra these ions could originate from iron oxides. Alternatively, the laser-excited plasma regime might also generate hydrogen radicals, which can reduce iron oxide into iron, with formation of an extra molecule of water. To investigate the origin of the pure iron ions, a separate SIMS analysis of the pure iron surface was performed. The pure iron surface resulted in mass peaks corresponding to Fe + and FeO + only; higher oxides (FexOy + ; x, y = 1–7) were not observed, which suggests that the higher iron oxides are generated in
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Communications Table 1. Peak assignment in the secondary ion mass spectra of the ablated iron target in water and in CD solution Iron target in water Fragment m/z
Iron target in CD solution Fragment m/z
Fe + FeO + Fe2O2 + Fe3O3 + Fe4O4 + Fe5O5 + Fe6O6 + Fe7O7 +
(C6H8O4)1Fe + (C6H8O4)2Fe + (C6H8O4)3Fe + (C6H8O4)4Fe + (C6H8O4)5Fe + (C6H8O4)6Fe + (C6H8O4)7Fe + (C6H8O4)1FeO + (C6H8O4)1Fe2O + (C6H8O4)2Fe2O + (C6H8O4)3Fe2O + (C6H8O4)4Fe2O + (C6H8O4)5Fe2O + (C6H8O4)6Fe2O + (C6H8O4)7Fe2O + (C6H8O4)1FeO + (C6H8O4)2FeO + (C6H8O4)3FeO +
56 72 144 216 288 360 432 504
+
200 344 488 632 776 920 1064 216 272 416 560 704 848 992 1136 216 360 504
+
Figure 1. Secondary ion mass spectra of cationized molecular ions of CD and cationized complexes between CD and iron species with the associated isotope distributions (triangles). The inset an expansion of the isotope distribution for C42H67O35FeNa + .
the laser plasma regime. The ratio of the Fe /FeO mass peak intensities for the pure-iron surface and the laser ablation species are approximately 105/103 and 104/102, respectively. Interestingly the difference between the mass peak intensities for the pure iron surface and the laser-ablation species is constant, at approximately 102. The secondary ion mass spectra of the pure iron surface always contain a FeO + mass peak, suggesting that approximately 1 % of the pure iron surface might contain iron oxide, as expected. For the laser-ablated iron species, the variety of iron oxides is larger, however, owing to the low intensities of these mass peaks, their estimated ratio to iron is approximately the same, 10¢2. Even if different secondary-ion yields of iron and iron oxides are assumed, one can conclude that the concentration of iron is higher than that of the iron oxides. Herein, two approaches to studying the interactions between the pulsed-laser ablated iron species with CD are presented. The focus is on the formation of supramolecular complexes based on CD and iron species. Both approaches are based on the pulsed-laser ablation of the bulk iron in water, but they differ in the order in which CD is added. In the first approach, CD is added to the solution of the iron species after the ablation process. The second approach, which is based on the ablation of bulk iron directly in an aqueous solution of CD, will be discussed later. The objective was to investigate the formation of a supramolecular complex between CD and iron species, ideally with an intact CD molecular ion. The spectral range of interest is that in which the molecular ions of intact CD molecules are observed. This region is shown in Figure 1, where indeed C42H70O35Na + and C42H70O35K + are detected, as ions of intact CD molecules with masses of 1157 and 1174 m/z, respectively (note, the corresponding isotope distributions are marked with triangles in Figure 1). This result is in agreement with the detection a supramolecular host–guest complex of CD–diphenylhexatriene by SIMS, together with its correspondChemPhysChem 2015, 16, 2110 – 2113
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ing Na- and K-cationized forms.[23] The next relevant mass peak in this region is at 1210 m/z and was assigned as C42H67O35FeNa + , presumably the Na-cationized form of C42H67O35Fe. This observation suggests the formation of a supramolecular CD–iron species complex, presumably with a 1:1 CD/iron species ratio. Note, the inset in Figure 1 shows the isotope distribution of C42H67O35FeNa + . A comparison between an intact CD molecule (C42H70O35Na + ) and C42H67O35FeNa + , shows that three atoms of hydrogen are missing, however, the number of oxygen atoms is unchanged. Notably, after iron ablation the solution contains both pure iron and iron oxides. One might then consider two options for the formation of a supramolecular complex: either with iron as C42H67O35Na–Fe or with iron oxide as C42H67O34Na–FeO. In the case of C42H67O35Na–Fe, three atoms of hydrogen are missing compared to an intact CD molecule. Before this observation can be rationalized, the isotope distribution in the inset of Figure 1, which does not correspond well, should be discussed. Therefore, the C42H67O35Na–Fe complex and similar complexes between an Fe atom and a CD molecule minus two hydrogen atoms, a CD molecule minus one hydrogen atom, and finally with an intact CD molecule are plotted as isotope distributions in Figure 2. These CD–iron species complexes, that is, C42H67O35Na–Fe, C42H68O35Na–Fe, C42H69O35Na–Fe and C42H70O35Na–Fe, with percentage partitions of 55, 25, 8 and 12 %, respectively, were used to fit the mass spectrum in Figure 2. The sum of isotope distributions corresponds well, thus suggesting the presence of complexes with 1–2 missing hydrogen atoms. More importantly, the presence of a complex of an intact CD molecule with Fe with a partition of 12 % is implied. The missing hydrogen atoms might result from a SIMS mechanism, and eventually they might play a role either in the reduction of iron species or in the stabilization of iron atoms within the complex. In the case of C42H67O34Na–FeO, three atoms of hydrogen and one atom of oxygen are missing and this could be rationalized in a similar way. However, more im-
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Figure 2. The C42H67O35Na–Fe, C42H68O35Na–Fe, C42H69O35Na–Fe and C42H70O35Na–Fe complexes are plotted as isotope distributions to fit the secondary ion mass spectrum.
portantly, a complex between an intact CD molecule and FeO was not observed. At this point, taking in account the hydrophobicity of the CD cavity and the hydrophilicity of the CD outer surface, the wettabilities of iron and iron oxide are compared. The wettabilities of iron and iron oxide are defined by the contact angle with values of approximately 70[24] and 108,[25] resulting in hydrophobicity and hydrophilicity, respectively. In other words, the iron species are most likely hydrophobic and the iron oxide species are most likely hydrophilic. These facts might support both cases of the supramolecular complexes. The ion of C42H67O35Na–Fe presumably represents a supramolecular host–guest complex in which the hydrophobic iron atom is positioned in the hydrophobic cavity of CD. The ion of C42H67O34Na–FeO presumably represents a supramolecular adduct in which hydrophilic iron oxide is positioned on the hydrophilic outer surface of CD cavity. The second approach is based on the direct use of a CD-saturated solution for the pulsed-laser ablation of bulk iron and again we are interested in the spectral region that contains the intact molecular ion of CD. As one might expect, in the high energy laser-ablation regime, the probability of a CD molecule remaining intact is low. Indeed, neither an intact molecular ion of CD nor the mass peak of 1210 m/z were observed. Along the rational of the high-energy ablation regime, the fragmentation of the CD molecules is assumed, where the glucopyranose units of C6H10O5 are presumably dehydrated into C6H8O4 + fragments. Moreover, these molecular fragments of CD were detected associated with iron and iron oxides and are summarized in Table 1. The association of the CD molecular fragments with iron species can be rationalized by considering the chemical composition of the glucopyranose unit. The glucopyranose unit has no steric hindrance and contains both hydrophilic and hydrophobic parts, therefore interactions with both iron and iron oxide are possible. Here, the aim was to investigate the preparation and formation of the supramolecular complexes between CD and iron species, ideally with an intact molecular ion of CD. b-CyclodexChemPhysChem 2015, 16, 2110 – 2113
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trin (C42H70O35) was used and the iron species were prepared by pulsed-laser ablation of bulk iron in water. A comparison of the SIMS ratio of the Fe/FeO mixture resulting from the laser ablation suggests that there is approximately 100 times more iron than iron oxides present, and even we assume that the ion yield for iron is 10 times higher than that for iron oxides, one can conclude an excess of iron compared with iron oxides. The solution of iron species was then mixed with CD and analysed with SIMS. The relevant mass peak was at 1210 m/z and was assigned as C42H67O35FeNa + , which can be interpreted as a supramolecular complex either with iron as C42H67O35Na–Fe or with iron oxide as C42H67O34Na–FeO. However, the isotope distribution was not in agreement, and therefore, it was fitted with similar CD–iron species complexes. Interestingly, the fit supported the presence of an intact CD molecule with Fe as C42H70O35Na–Fe with a partition of 12 %. In contrast, such a complex between an intact CD molecule and FeO was not observed. The hydrophobicity of iron and hydrophilicity of iron oxide might support formation of both supramolecular complexes as C42H67O35Na–Fe host–guest complexes and as C42H67O34Na–FeO adducts. However the excess of iron species and the presence of the C42H70O35Na–Fe ion in secondary ion mass spectra, suggest a preference for the formation of the host–guest complex.
Experimental Section Chemicals CD as b-CD, C42H70O35 (Sigma–Aldrich, Bratislava, Slovakia) was used without further purification as a solution with a concentration of approximately 10¢2 mol.dm¢3. Distilled water was used as the solvent. The sample solution (70 mL) was dropped onto the silicon substrate (Goodfellow, Huntingdon, England) and the sample was dried in air at room temperature. Iron rods (99.9 %, Sigma–Aldrich, Bratislava, Slovakia) with diameter of 0.5 mm was used as the pure iron source.
Laser Ablation A pulsed-laser beam from an amplified femtosecond laser system (Coherent Legend Duo USP) with a central wavelength of 800 nm, a pulse energy of 3.3 mJ, a repetition rate of 3 kHz, and a pulse duration of 100 fs was passed through a f = 100 mm focusing lens (f15). It was focused such that the laser beam waist was located approximately 2 mm below the sample surface. The ablation process was limited to 100 seconds.
Mass Spectrometry Mass spectrometry measurements were performed by using a ToFSIMS IV (Ion-Tof Muenster, Germany) reflectron-type time-of-flight mass spectrometer equipped with a bismuth ion gun. Pulsed 25 keV Bi3 + were used as primary ions with ion current of 0.20 pA. Secondary ion mass spectra were measured by scanning over a selected 100 Õ 100 mm2 analysis area. Note that due to the spectral significance, only positive polarity spectra are shown.
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Communications Acknowledgements The authors acknowledge support for this research by ERDF OP R&D, Project meta-QUTE - Centrum excelentnosti kvantovy´ch technolûgi, APVV-0491–07, NanoNet2, and UK/461/2013. Keywords: cyclodextrins · iron · pulsed-laser ablation · SIMS · supramolecular chemistry [1] S. H. Ko, H. Pan, C. P. Grigoropoulos, C. K. Luscombe, J. M. J. Fr¦chet, D. Poulikakos, Nanotechnology 2007, 18, 345202. [2] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou, S. E. Hunyadi, T. Li, J. Phys. Chem. B 2005, 109, 13857 – 13870. [3] I. H. El-Sayed, X. Huang, M. A. El-Sayed, Nano Lett. 2005, 5, 829 – 834. [4] R. M. Crooks, M. Zhao, L. Sun, V. Chechik, L. K. Yeung, Acc. Chem. Res. 2001, 34, 181 – 190. [5] S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, Science 2000, 287, 1989 – 1992. [6] S. L. Pal, U. Jana, P. K. Manna, G. P. Mohanta, R. Manavalan, J. App. Pharm. Sci. 2011, 1, 228 – 234. [7] G. W. Yang, Prog. Mater. Sci. 2007, 52, 648 – 698. [8] D. Zhao, T. Liu, G. J. Park, M. Zhang, J. M. Chen, B. Wang, Microelectron. Eng. 2012, 96, 71 – 75. [9] T. Sasaki, S. Terauchi, N. Koshizaki, H. Umehara, Appl. Surf. Sci. 1998, 127, 398 – 402. [10] A. V. Kabashin, M. Meunier, Ch. Kingston, J. H. T. Luong, J. Phys. Chem. B 2003, 107, 4527 – 4531.
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[11] J. P. Sylvestre, S. Poulin, A. V. Kabashin, E. Sacher, M. Meunier, J. H. T. Luong, J. Phys. Chem. B 2004, 108, 16864 – 16869. [12] J. P. Sylvestre, S. Poulin, A. V. Kabashin, E. Sacher, M. Meunier, J. H. T. Luong, J. Am. Chem. Soc. 2004, 126, 7176 – 7177. [13] F. Hapiot, S. Tilloy, E. Monflier, Chem. Rev. 2006, 106, 767 – 781. [14] J. N. Spencer, Q. He, X. Ke, Z. Wu, E. Fetter, J. Solution Chem. 1998, 27, 1009 – 1019. [15] H. Wang, N. Shao, S. Qiao, Y. Cheng, J. Phys. Chem. B 2012, 116, 11217 – 11224. [16] Ch. Yuan, Z. Jin, X. Xu, Carbohydr. Polym. 2012, 89, 492 – 496. [17] K. Ghule, A. V. Ghule, B. J. Chen, Y. Ch. Ling, Green Chem. 2006, 8, 1034 – 1041. [18] Y. P. Kim, E. Oh, M. Y. Hong, D. Lee, M. K. Han, H. K. Shon, D. W. Moon, H. S. Kim, T. G. Lee, Anal. Chem. 2006, 78, 1913 – 1920. [19] S. Rajagopalachary, S. V. Verkhoturov, E. A. Schweikert, Anal. Chem. 2009, 81, 1089 – 1094. [20] L. Yang, Z. Zhu, X. Y. Yu, E. Rodek, L. Saraf, T. Thevuthasan, J. P. Cowin, Surf. Interface Anal. 2014, 46, 224 – 228. [21] A. S. Mohammadi, J. S. Fletcher, P. Malmberg, A. G. Ewing, Surf. Interface Anal. Published online 2014. [22] T. Iwamoto, T. Ishigaki, J. Phys. 2013, 441, 1 – 5. [23] L. Rabara, M. Aranyosiova, D. Velic, Appl. Surf. Sci. 2006, 252, 7000 – 7002. [24] R. Wang, L. Cong, M. Kido, Appl. Surf. Sci. 2002, 191, 74 – 84. [25] P. M. Kulal, D. P. Dubal, C. D. Lokhande, V. J. Fulari, J. Alloys Compd. 2011, 509, 2567 – 2571. Received: March 13, 2015 Published online on April 27, 2015
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Preparation of Cyclodextrin–Iron Species in Water by Laser Ablation: Secondary Ion Mass Spectrometry Sona Halaszova,*[a] Monika Jerigova,[a, b] Dusan Lorenc,[b] and Dusan Velic*[a, b] Supramolecular complexes between cyclodextrin and iron species are studied by using secondary ion mass spectrometry. The iron species are prepared by pulsed-laser ablation of bulk iron in water; this gives Fe + (56 m/z) and FexOy + (x, y = 1–7) species. Cyclodextrin is added to the water either before or after the laser ablation. When it is added before laser ablation, molecular fragments of cyclodextrin are detected as dehydrated glucopyranose units (C6H8O4 + ) associated with Fe + , FeO + , and Fe2O + species. The focus is to observe supramolecular host–guest complexes or adducts between intact molecules of cyclodextrin and iron species. When cyclodextrin is added after laser ablation, the relevant peak at 1210 m/z is observed and assigned as C42H67O35FeNa + , which corresponds to a cyclodextrin molecule minus three H atoms. Two possible explanations of this finding are the presence of the host–guest C42H67O35Na– Fe complex, in which Fe is in the cavity, or the presence of the adduct C42H67O34Na–FeO with FeO on the outer surface; the formation of these complexes are supported by the hydrophobicity of Fe and hydrophilicity of FeO, respectively. Due to the presence of 12 % of intact C42H70O35Na–Fe complex and an estimated Fe/FeO ratio of approximately 102, host–guest formation is assumed to be more significant.
Nanoparticles can be defined as particles that are sized between approximately 1 and 100 nm and they show properties that are not shared by non-nanoscale particles with the same chemical composition. They belong to currently the most studied branch of material science, and have a variety of applications in electronics, optics, diagnostics, and catalysis.[1–5] Methods for their preparation are emulsion–solvent evaporation, emulsion diffusion, and salting out or solvent displacement/ precipitation,[6] as well as a method that involves pulsed-laser ablation of a solid target in a liquid.[7] With the latter method, it is possible to prepare a wide range of novel materials, such as metallic nanoparticles (Au, Ag, Pt, Fe), semiconductors (Si, ZnO) and magnetic nanoparticles (Ni, Co, Fe2O3), by using different target materials and solutions and by varying the laser parameters. The preparation of pure iron nanoparticles by pulsed-laser ablation has been investigated by X-ray diffrac[a] S. Halaszova, Dr. M. Jerigova, Prof. Dr. D. Velic Department of Physical and Theoretical Chemistry Faculty of Natural Sciences, Comenius University Mlynska dolina, 842 15 Bratislava (Slovakia) E-mail: [email protected] [email protected] [b] Dr. M. Jerigova, Dr. D. Lorenc, Prof. Dr. D. Velic International Laser Centre Ilkovicova 3, 812 19 Bratislava (Slovakia)
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tion, transmission electron microscopy and X-ray photoelectron spectroscopy.[8] Iron oxide nanoparticles prepared by using pulsed-laser ablation have also studied.[9] The preparation of metallic nanoparticles in a cyclodextrin (CD) solution is one popular method to obtain stabilized nanoparticles.[10–12] CD molecules are cyclic oligosaccharides that consist of (a-1,4)linked a-d-glucopyranose units and have a hydrophobic cavity and hydrophilic outer surface. Owing to the cavity, CDs are widely used as host molecules in host–guest supramolecular chemistry. CDs can not only form host–guest complexes, but also adducts.[13] The CD cavity accommodates a guest molecule, which can be either organic, inorganic, metallic complexes, cations or anions.[14] These supramolecular host–guest complexes have been characterized by phase solubility, highperformance liquid chromatography, circular dichroism, nuclear magnetic resonance, X-ray powder diffraction, differential scanning calorimetry, thermogravimetric analysis, UV/Vis spectroscopy and Fourier transform infrared spectroscopy.[15, 16] A suitable technique for studying metallic species is secondary ion mass spectrometry (SIMS),[17–21] and here this method is used for the first time to study supramolecular complexes between CD and various iron species. As the CD, we used b-cyclodextrin (C42H70O35) and the iron species were prepared by pulsed-laser ablation of bulk iron in water. The ablation process generates iron species that cause colouration of the solution, owing to a collective oscillation of the electrons in the conduction band, which is known as surface plasmon oscillation. The colour of the metallic species is dependent on the shape and size of the particles and the dielectric constant of the solvent. The size distribution of the particles spans from a single iron species, through clusters of iron species, to ironspecies nanoparticles.[22] Secondary ion mass spectra were obtained and the following mass peaks were assigned. The presence of iron was confirmed by the mass at 56 m/z, which was assigned as Fe + . Iron oxides with the general formula of FexOy + (x, y = 1–7) were also detected (Table 1). These results show the presence of iron and a variety of iron oxides. The formation of iron oxides with such a high-energy laser is a result of interactions between iron and water in the excited plasma regime. Interestingly, pure iron ions were also detected by SIMS; in secondary ion mass spectra these ions could originate from iron oxides. Alternatively, the laser-excited plasma regime might also generate hydrogen radicals, which can reduce iron oxide into iron, with formation of an extra molecule of water. To investigate the origin of the pure iron ions, a separate SIMS analysis of the pure iron surface was performed. The pure iron surface resulted in mass peaks corresponding to Fe + and FeO + only; higher oxides (FexOy + ; x, y = 1–7) were not observed, which suggests that the higher iron oxides are generated in
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Communications Table 1. Peak assignment in the secondary ion mass spectra of the ablated iron target in water and in CD solution Iron target in water Fragment m/z
Iron target in CD solution Fragment m/z
Fe + FeO + Fe2O2 + Fe3O3 + Fe4O4 + Fe5O5 + Fe6O6 + Fe7O7 +
(C6H8O4)1Fe + (C6H8O4)2Fe + (C6H8O4)3Fe + (C6H8O4)4Fe + (C6H8O4)5Fe + (C6H8O4)6Fe + (C6H8O4)7Fe + (C6H8O4)1FeO + (C6H8O4)1Fe2O + (C6H8O4)2Fe2O + (C6H8O4)3Fe2O + (C6H8O4)4Fe2O + (C6H8O4)5Fe2O + (C6H8O4)6Fe2O + (C6H8O4)7Fe2O + (C6H8O4)1FeO + (C6H8O4)2FeO + (C6H8O4)3FeO +
56 72 144 216 288 360 432 504
+
200 344 488 632 776 920 1064 216 272 416 560 704 848 992 1136 216 360 504
+
Figure 1. Secondary ion mass spectra of cationized molecular ions of CD and cationized complexes between CD and iron species with the associated isotope distributions (triangles). The inset an expansion of the isotope distribution for C42H67O35FeNa + .
the laser plasma regime. The ratio of the Fe /FeO mass peak intensities for the pure-iron surface and the laser ablation species are approximately 105/103 and 104/102, respectively. Interestingly the difference between the mass peak intensities for the pure iron surface and the laser-ablation species is constant, at approximately 102. The secondary ion mass spectra of the pure iron surface always contain a FeO + mass peak, suggesting that approximately 1 % of the pure iron surface might contain iron oxide, as expected. For the laser-ablated iron species, the variety of iron oxides is larger, however, owing to the low intensities of these mass peaks, their estimated ratio to iron is approximately the same, 10¢2. Even if different secondary-ion yields of iron and iron oxides are assumed, one can conclude that the concentration of iron is higher than that of the iron oxides. Herein, two approaches to studying the interactions between the pulsed-laser ablated iron species with CD are presented. The focus is on the formation of supramolecular complexes based on CD and iron species. Both approaches are based on the pulsed-laser ablation of the bulk iron in water, but they differ in the order in which CD is added. In the first approach, CD is added to the solution of the iron species after the ablation process. The second approach, which is based on the ablation of bulk iron directly in an aqueous solution of CD, will be discussed later. The objective was to investigate the formation of a supramolecular complex between CD and iron species, ideally with an intact CD molecular ion. The spectral range of interest is that in which the molecular ions of intact CD molecules are observed. This region is shown in Figure 1, where indeed C42H70O35Na + and C42H70O35K + are detected, as ions of intact CD molecules with masses of 1157 and 1174 m/z, respectively (note, the corresponding isotope distributions are marked with triangles in Figure 1). This result is in agreement with the detection a supramolecular host–guest complex of CD–diphenylhexatriene by SIMS, together with its correspondChemPhysChem 2015, 16, 2110 – 2113
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ing Na- and K-cationized forms.[23] The next relevant mass peak in this region is at 1210 m/z and was assigned as C42H67O35FeNa + , presumably the Na-cationized form of C42H67O35Fe. This observation suggests the formation of a supramolecular CD–iron species complex, presumably with a 1:1 CD/iron species ratio. Note, the inset in Figure 1 shows the isotope distribution of C42H67O35FeNa + . A comparison between an intact CD molecule (C42H70O35Na + ) and C42H67O35FeNa + , shows that three atoms of hydrogen are missing, however, the number of oxygen atoms is unchanged. Notably, after iron ablation the solution contains both pure iron and iron oxides. One might then consider two options for the formation of a supramolecular complex: either with iron as C42H67O35Na–Fe or with iron oxide as C42H67O34Na–FeO. In the case of C42H67O35Na–Fe, three atoms of hydrogen are missing compared to an intact CD molecule. Before this observation can be rationalized, the isotope distribution in the inset of Figure 1, which does not correspond well, should be discussed. Therefore, the C42H67O35Na–Fe complex and similar complexes between an Fe atom and a CD molecule minus two hydrogen atoms, a CD molecule minus one hydrogen atom, and finally with an intact CD molecule are plotted as isotope distributions in Figure 2. These CD–iron species complexes, that is, C42H67O35Na–Fe, C42H68O35Na–Fe, C42H69O35Na–Fe and C42H70O35Na–Fe, with percentage partitions of 55, 25, 8 and 12 %, respectively, were used to fit the mass spectrum in Figure 2. The sum of isotope distributions corresponds well, thus suggesting the presence of complexes with 1–2 missing hydrogen atoms. More importantly, the presence of a complex of an intact CD molecule with Fe with a partition of 12 % is implied. The missing hydrogen atoms might result from a SIMS mechanism, and eventually they might play a role either in the reduction of iron species or in the stabilization of iron atoms within the complex. In the case of C42H67O34Na–FeO, three atoms of hydrogen and one atom of oxygen are missing and this could be rationalized in a similar way. However, more im-
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Communications
Figure 2. The C42H67O35Na–Fe, C42H68O35Na–Fe, C42H69O35Na–Fe and C42H70O35Na–Fe complexes are plotted as isotope distributions to fit the secondary ion mass spectrum.
portantly, a complex between an intact CD molecule and FeO was not observed. At this point, taking in account the hydrophobicity of the CD cavity and the hydrophilicity of the CD outer surface, the wettabilities of iron and iron oxide are compared. The wettabilities of iron and iron oxide are defined by the contact angle with values of approximately 70[24] and 108,[25] resulting in hydrophobicity and hydrophilicity, respectively. In other words, the iron species are most likely hydrophobic and the iron oxide species are most likely hydrophilic. These facts might support both cases of the supramolecular complexes. The ion of C42H67O35Na–Fe presumably represents a supramolecular host–guest complex in which the hydrophobic iron atom is positioned in the hydrophobic cavity of CD. The ion of C42H67O34Na–FeO presumably represents a supramolecular adduct in which hydrophilic iron oxide is positioned on the hydrophilic outer surface of CD cavity. The second approach is based on the direct use of a CD-saturated solution for the pulsed-laser ablation of bulk iron and again we are interested in the spectral region that contains the intact molecular ion of CD. As one might expect, in the high energy laser-ablation regime, the probability of a CD molecule remaining intact is low. Indeed, neither an intact molecular ion of CD nor the mass peak of 1210 m/z were observed. Along the rational of the high-energy ablation regime, the fragmentation of the CD molecules is assumed, where the glucopyranose units of C6H10O5 are presumably dehydrated into C6H8O4 + fragments. Moreover, these molecular fragments of CD were detected associated with iron and iron oxides and are summarized in Table 1. The association of the CD molecular fragments with iron species can be rationalized by considering the chemical composition of the glucopyranose unit. The glucopyranose unit has no steric hindrance and contains both hydrophilic and hydrophobic parts, therefore interactions with both iron and iron oxide are possible. Here, the aim was to investigate the preparation and formation of the supramolecular complexes between CD and iron species, ideally with an intact molecular ion of CD. b-CyclodexChemPhysChem 2015, 16, 2110 – 2113
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trin (C42H70O35) was used and the iron species were prepared by pulsed-laser ablation of bulk iron in water. A comparison of the SIMS ratio of the Fe/FeO mixture resulting from the laser ablation suggests that there is approximately 100 times more iron than iron oxides present, and even we assume that the ion yield for iron is 10 times higher than that for iron oxides, one can conclude an excess of iron compared with iron oxides. The solution of iron species was then mixed with CD and analysed with SIMS. The relevant mass peak was at 1210 m/z and was assigned as C42H67O35FeNa + , which can be interpreted as a supramolecular complex either with iron as C42H67O35Na–Fe or with iron oxide as C42H67O34Na–FeO. However, the isotope distribution was not in agreement, and therefore, it was fitted with similar CD–iron species complexes. Interestingly, the fit supported the presence of an intact CD molecule with Fe as C42H70O35Na–Fe with a partition of 12 %. In contrast, such a complex between an intact CD molecule and FeO was not observed. The hydrophobicity of iron and hydrophilicity of iron oxide might support formation of both supramolecular complexes as C42H67O35Na–Fe host–guest complexes and as C42H67O34Na–FeO adducts. However the excess of iron species and the presence of the C42H70O35Na–Fe ion in secondary ion mass spectra, suggest a preference for the formation of the host–guest complex.
Experimental Section Chemicals CD as b-CD, C42H70O35 (Sigma–Aldrich, Bratislava, Slovakia) was used without further purification as a solution with a concentration of approximately 10¢2 mol.dm¢3. Distilled water was used as the solvent. The sample solution (70 mL) was dropped onto the silicon substrate (Goodfellow, Huntingdon, England) and the sample was dried in air at room temperature. Iron rods (99.9 %, Sigma–Aldrich, Bratislava, Slovakia) with diameter of 0.5 mm was used as the pure iron source.
Laser Ablation A pulsed-laser beam from an amplified femtosecond laser system (Coherent Legend Duo USP) with a central wavelength of 800 nm, a pulse energy of 3.3 mJ, a repetition rate of 3 kHz, and a pulse duration of 100 fs was passed through a f = 100 mm focusing lens (f15). It was focused such that the laser beam waist was located approximately 2 mm below the sample surface. The ablation process was limited to 100 seconds.
Mass Spectrometry Mass spectrometry measurements were performed by using a ToFSIMS IV (Ion-Tof Muenster, Germany) reflectron-type time-of-flight mass spectrometer equipped with a bismuth ion gun. Pulsed 25 keV Bi3 + were used as primary ions with ion current of 0.20 pA. Secondary ion mass spectra were measured by scanning over a selected 100 Õ 100 mm2 analysis area. Note that due to the spectral significance, only positive polarity spectra are shown.
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Communications Acknowledgements The authors acknowledge support for this research by ERDF OP R&D, Project meta-QUTE - Centrum excelentnosti kvantovy´ch technolûgi, APVV-0491–07, NanoNet2, and UK/461/2013. Keywords: cyclodextrins · iron · pulsed-laser ablation · SIMS · supramolecular chemistry [1] S. H. Ko, H. Pan, C. P. Grigoropoulos, C. K. Luscombe, J. M. J. Fr¦chet, D. Poulikakos, Nanotechnology 2007, 18, 345202. [2] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou, S. E. Hunyadi, T. Li, J. Phys. Chem. B 2005, 109, 13857 – 13870. [3] I. H. El-Sayed, X. Huang, M. A. El-Sayed, Nano Lett. 2005, 5, 829 – 834. [4] R. M. Crooks, M. Zhao, L. Sun, V. Chechik, L. K. Yeung, Acc. Chem. Res. 2001, 34, 181 – 190. [5] S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, Science 2000, 287, 1989 – 1992. [6] S. L. Pal, U. Jana, P. K. Manna, G. P. Mohanta, R. Manavalan, J. App. Pharm. Sci. 2011, 1, 228 – 234. [7] G. W. Yang, Prog. Mater. Sci. 2007, 52, 648 – 698. [8] D. Zhao, T. Liu, G. J. Park, M. Zhang, J. M. Chen, B. Wang, Microelectron. Eng. 2012, 96, 71 – 75. [9] T. Sasaki, S. Terauchi, N. Koshizaki, H. Umehara, Appl. Surf. Sci. 1998, 127, 398 – 402. [10] A. V. Kabashin, M. Meunier, Ch. Kingston, J. H. T. Luong, J. Phys. Chem. B 2003, 107, 4527 – 4531.
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