Cent. Eur. J. Chem. • 8(2) • 2010 • 448–452 DOI: 10.2478/s11532-009-0148-2 Central European Journal of Chemistry Novel modification of elemental silver formed in Ag4[Fe(CN)6]-gelatin-immobilized matrix Implants Research Article Oleg V. Mikhailov1*, Natal’ya I. Naumkina2 1 Kazan State Technological University, 420015 Kazan, Russia 2 Central Scientific-Research Institute of Geology Non-Ore Minerals, 420097 Kazan, Russia Received 30 September 2009; Accepted 10 December 2009 Abstract: A g4[Fe(CN)6]→Ag reduction process occurring in Ag4[Fe(CN)6]- gelatin-immobilized matrices, has been studied. In the course of reduction, these matrices are in a contact with water-alkaline solu-tion containing tin(II) dichloride along with an organic or inorganic compound capable of forming a stable soluble complex with Ag(I). It has been found that the substance formed in the gelatin matrix, is almost pure elemental silver, which consists two various phases which are distinguished by their optical and X-ray diffraction parameters. Keywords: Elemental silver • Silver(I)hexacyanoferrate(II) gelatin-immobilized matrix © Versita Sp. z o.o. 1. Introduction Over 40 years ago in [1] it was mentioned that small particles of colloidal elemental silver formed in gelatin layers during development of silver halide photographic emulsions. A number of works, in which there are indications of the existence of a separate phase of elemental silver consisting of nano-particles being generated as a result of photochemical reduction of Ag (I) salts, have appeared lately in the literature, in particular [2-13]. Before in [12,13] it had been noted that during development of gelatin layers of silver halide photographic emulsions by water-alkaline solutions containing tin(II) dichloride and some inorganic or organic substance forming rather stable coordination compounds with Ag(I), the gelatin layer is tinged brown or red. However, formation only of element silver occurs. Therefore, it should be noted that under standard development of using hydroquinone developers, gelatin layer is tinged black or grey. It is significant that with an increase of optical density of the gelatin layer containing such an elemental silver, a red tone in the coloring of the gelatin layer becomes more and more clearly expressed. A similar phenomenon takes place, too, when instead of silver halide (AgHal) in a gelatin matrix there are silver(I) compounds such as silver(I) hexacyanoferrate(II) Ag4[Fe(CN)6]. Whether this totality of particles constitutes a novel phase of element silver or is only a variety of previously known phases of the given simple substance has remained open till now. In the present work we have tried to decide it. 2. Experimental Procedure As the initial material to obtain silver-containing gelatinimmobilized matrix implants (hereafter GIM), X-ray film Structurix D-10 (Agfa-Gevaert, Belges) was used. Samples of the given film (which actually is nothing but AgHal-GIM) having format 20x30 cm2 were exposed to X-ray radiation with an irradiation dose the range 0.05-0.50 Röntgen. These exposed films were further * E-mail: [email protected] 448 Unauthenticated Download Date | 6/15/17 7:45 AM O. V. Mikhailov, N. I. Naumkina subjected to processing according to the following technology [12,13]: • Development in D-19 standard developer as indicated in [12,13], for 6 min at 20-25оC; • Washing with running water for 2 min at 20-25оC; • Fixing in 25% water solution of sodium trioxosulphidosulphate(VI) (Na2S2O3) for 10 min at 20-25°C; • Washing with running water for 15 min at 18-25оC. The first three stages of standard processing indicated (development, washing and fixing) were carried out in non-actinic green-yellow light, final washing – at natural light. The samples of GIM obtained which contained elemental silver (Ag-GIM), then were processed according to the following protocol: 1) Oxidation in water solution containing (g L–1) Potassium hexacyanoferrate(III) Potassium hexacyanoferrate(II) Potassium hydroxide Sodium trioxocarbonate(IV) (Na2CO3) Water 50.0 20.0 10.0 5.0 up to 1000 mL for 6 min at 20-25оC; 2) Washing with running water for 2 min at 20-25оC; 3) Reduction in water solution containing (g L–1) Tin(II) chloride Sodium N,N′-ethylenediaminetetraacetate Potassium hydroxide Reagent formed water-soluble complex with Ag(I) Water 50.0 35.0 50.0 1.0-100.0 up to 1000 mL for 1 min at 20-25оC; 4) Washing with running water for 15 min at 18-25оC; 5) Drying for 2-3 hours at 20-25оC. As reagents for forming water-soluble complexes with Ag(I) (hereafter CR from Complexation Reagent), ammonia NH3, potassium thiocyanate KSCN, sodium trioxosulphidosulphate(VI) Na2S2O3, ethanediamine-1,2 H2N–CH2–CH2–NH2, 2-aminoethanol H2N–CH2–CH2– OH and 3-(2-hydroxyethyl)-3-azapentanediol-1,5 N(CH2– CH2–OH)3, were used. During the first stage of processing of Ag-GIM, conversion of Ag-GIM into Ag4[Fe(CN)6]-GIM occurred, while during the second stage, reduction of Ag4[Fe(CN)6]-GIM with Sn(II) to elemental silver took place. And so, peculiar “re-precipitation” of elemental silver into gelatin matrix occurred incidentally. Isolation of particulate substances from Ag-GIM was carried out by hydrolyzing the protein matrix with proteolytic enzymes (for example, trypsin) destroying the polymeric gelatin carrier of a GIM. Subsequently the solid phase was separated from the mother solution according to a technique described in [14]. The substances isolated thus from GIM were further analyzed by X-ray diffraction using a D8 Advance spectrometer (Bruker, Germany) scanning from 3 up to 65o 2q, with a step size of 0,05 2q. The calculation of intensities of reflexes (I) and interplane distances (d) was carried out using the standard software package EVA. Theoretical XRD spectra (X-ray patterns) were calculated with the PowderCell program described in [15]. Optical densities of the Ag-GIMs were measured by means of a Macbeth TD504 photometer (Kodak, USA) in a range 0.1-5.0 units with an accuracy of +2 % (rel.). The optical densities before and after exposure, DAg and DAg , respectively, were measured with a blue light-filter with a transmission maximum at 450 nm. 3. Results and Discussion Striking visual changes were observed over a course of processing during which Ag4[Fe(CN)6]-GIM is transformed into Ag-GIM. Ag-GIMs obtained as a result of standard processing of exposed AgHal-GIM, at rather small optical density (DAg) have grey colour, while at large DAg, black colour. The colouring of the Ag-GIM containing the “re-precipitated” element silver, varied from black-brown to brick-red depending on the nature and concentration of the CR. It is significant, however, that absorption spectra of both initial and the “re-precipitated” element silver in the visible area do not contain any accurately expressed maxima. The qualitative difference observed is that the optical density of the Ag-GIM with the “re-precipitated” silver (DAg), at the same volume concentration of element silver (CAgV) in GIM, is greater than the DAg values, although DAg depends on both the nature and quantity of the CR in solution contacting the GIM. Examples of DAg = f(CAgV) and DAg = f(CAgV) dependences for inorganic and organic CR are presented in Fig. 1 and Fig. 2, respectively. At the first stage of the given process, the reaction may be described by general Eq. 1 (the braces {….} indicate formulas of gelatin-immobilized chemical compounds): 4{Ag} + 4[Fe(CN)6]3– → {Ag4[Fe(CN)6]} + 3[Fe(CN)6]4– (1) Each of the CR under examination forms with Ag(I) a soluble complex having a metal ion: ligand ratio of 1:2. That is why, formation of silver(I) complexes with the corresponding CR will occur to some extent when Ag4[Fe(CN)6]-GIM is in contact with a solution containing any of the CR indicated. Gelatin-immobilized silver(I) hexacyanoferrate(II) as well as any of these soluble complexes, can be reduced with Sn(II). Consequently, two parallel processes Ag(I)→Ag(0) will take place during the contact of Ag4[Fe(CN)6]-GIM with solutions indicated above, containing Sn(II) and CR: 449 Unauthenticated Download Date | 6/15/17 7:45 AM Novel modification of elemental silver formed in Ag4[Fe(CN)6]-gelatin-immobilized matrix implants gelatin-immobilized silver(I) hexacyanoferrate(II) reduction proceeding in a polymer layer, • Ag(I) complex with CR reduction proceeding at the interface of phases a GIM/solution. In water solutions at pH=12-13, Sn(II) is mainly present as a hydroxo-complex [Sn(OH)3]–. General Eq. 2 represents the first of these processes: {Ag4[Fe(CN)6]}+2[Sn(OH)3]– + 6OH– → → 4{Ag} + 2[Sn(OH)6]2– + [Fe(CN)6]4– (2) while the second process is represented by general Eq. 3 with uncharged L: 2[AgL2]+ + [Sn(OH)3]– + 3OH– → 2Ag + 4L + [Sn(OH)6]2– (3) and more generally by Eq. 4: 2[AgL2](2z+1) + [Sn(OH)3]– + 3OH– → 2Ag + 4Lz + [Sn(OH)6]2– (4) where L represents a monodentate ligand with charge z). The particles of elemental silver formed as a result of Reactions 3 and 4, theoretically should have smaller sizes than the particles of elemental silver arising in the polymer layer of GIM. To be a part of a substance immobilized in GIM, these particles should place freely in intermolecular cavities of a gelatin layer. Only in this case, they may diffuse in GIM and may be immobilized in gelatin mass. According to data in [16], these cavities are nano-sized. Therefore, only nano-particles can gain entry into these cavities. By entering into such cavities, nano-particles of elemental silver are isolated from each other minimizing their aggregation with each other. Increasing the concentration of any CR, will increase the concentration of coordination compounds formed by the CR with Ag(I). Correspondingly, the quantity of nano-particles of the elemental silver formed as a result of reduction of these coordination compounds by [Sn(OH)3]– complex, should increase, as well. We anticipate that when the concentration of these CR increases, the share of nano-particles contained in the “re-precipitated” elemental silver should increase correspondingly. Thus, at the same Ag concentration, nano-particles of elemental silver owing to their higher degree of dispersion should result in a higher degree of absorption of visible light (and, accordingly, higher optical density) relative to micro-particles in the polymeric layer GIM. The experimental data presented in Figs. 1 and 2, are in full conformity with the given prediction. The particles of elemental silver formed as a result of Reactions 3 and 4, are one- or two-nuclear. When their concentration is insufficient to aggregate at the interface [which occurs when the concentration of Ag(I) complexes at the interface of phases a GIM/solution is low enough], these particles, owing to their remoteness from each other, diffuse into the polymeric layer of GIM, where they become immobilized without a change of their size. With an increase in the concentration of CR [and, accordingly, of concentration of Ag(I) complex with • D CAgV, mole dm–3 Figure 1. Optical Density vs. [Ag]. The dependenc of DAg = f(CAgV) (-------) and DAg = f(CAgV) following reprecipitation by thereduction process, Ag4[Fe(CN)6]→Ag using sodium trioxosulphidosulphate(VI) as CR at varying concentrations: 2.0 g L–1 (curve 1), 4,0 g L–1 (2), 8,0 g L–1 (3), 24.0 g L–1 (4) and 40.0 g L–1 (5). The optical densities were measured with a blue light-filter with a transmission maximum at 450 nm. D CAgV, mole dm–3 Figure 2. Optical Density vs. [Ag]. The dependenc of DAg = f(CAgV) (-------) and DAg = f(CAgV) following reprecipitation by the reduction process Ag4[Fe(CN)6]→Ag using 1,2-diaminoethane as CR -at varying concentrations: 5.0 g L–1 (curve 1), 10,0 g L–1 (2), 20,0 g L–1 (3), 40.0 g L–1 (4) and 80.0 g L–1 (5). The optical densities were measured with blue lightfilter with a transmission maximum at 450 nm. given CR], the quantity of nano-particles indicated at the interface of phases the GIM/solution accrues. This leads to an increase in the number of such particles diffused into GIM. However, at some rather high concentration CR in a solution, the effect of aggregation of nanoparticles of elemental silver starts to affect. One- and twonuclear particles of element silver formed at reduction of corresponding Ag(I) complex, begin to unite to some extent with each other in larger particles. Polynuclear particles of elemental silver resulting from such an association, are not so mobile and, consequently, will 450 Unauthenticated Download Date | 6/15/17 7:45 AM O. V. Mikhailov, N. I. Naumkina Figure 3. XRD-pattern of elemental silver isolated from initial Ag-GIM. Figure 5. XRD-pattern of substance isolated from Ag-GIM containing «re-precipitated» elemental silver and obtained with using of solution containing ethanediamine-1,2 in concentration 20.0 g L–1. not diffuse into the polymeric layer of GIM. They will be precipitated near the interface GIM/solution (or even to form as a solid phase on the surface of the GIM). As a result, rates of an increment of number of one-and twonuclear particles of elemental silver with further growth of concentration CR begins to be slowed down. Thus, inevitably there should come the moment when the number of similar particles will reach some limiting value. That is why at certain “threshold” concentrations of CR in a solution, growth of DAg values must stop. Moreover, when present in excess of this “threshold” concentration, a decrease DAg should begin. The point is that the correspondence between the number of aggregated particles and number of one- and two-nuclear ones with growth of concentration of Ag(I) complex continuously grows and has no restrictions. These polynuclear particles are precipitated in the frontier zone GIM at minimal depth forming, the same kind of elemental silver as the microparticles of elemental silver formed as a result of reduction of gelatin-immobilized Ag4[Fe(CN)6] according to Reaction 2. That is why, DAg values with increasing CR concentration at first increase, reach a maximum and then decrease. Taking into consideration the foregoing, it may be assumed that “re-precipitated” gelatin-immobilized Figure 4. XRD-pattern of substance isolated from Ag-GIM containing «re-precipitated» elemental silver and obtained with using of solution containing Na2S2O3 in concentra-tion 20.0 g L–1. silver should contain, as a minimum, two phases of silver particles, one of which is formed by nanoparticles, and another, by micro-particles. In order to corroborate this conclusion, we analyzed the elemental silver isolated from initial Ag-GIM and elemental silver isolated from Ag-GIM following the “re-precipitation” process by X-ray powder diffraction. The X-ray powder diffraction patterns (XRD-patterns) of samples obtained are presented in Figs. 3-5. As may be seen, the XRDpattern of initial elemental silver with grey-black color of the gelatin layer (Fig. 3) and XRD-patterns of “reprecipitated” elemental silver (Figs. 4-5) essentially differ from each other.In XRD-patterns of “re-precipitated” elemental silver obtained from all of the studied CR in a solution in contact with GIM, there are accurate reflexes having d = 333.6, 288.5, 166.7 and 129.1 pm that are absent in the XRD-pattern of initial elemental silver. At the same time, reflexes with d = 235.7, 204.1, 144.4, 123.1 and 117.9 pm are observed. These reflexes are characteristic for the known phase of elemental silver isolated from initial Ag-GIM. These XRD data are consistent with the “re-precipitated” elemental silver obtained with any of the CR indicated above, containing at least two structural modifications of elemental silver. Curiously, the reflexes with d = 333.6, 288.5, 204.2, 166.7 and 129.1 pm approximate the d values of reflexes of silver(I) bromide AgBr (number of card PDF 06-0438, parameter of an elementary cell a0 = 577.45 pm, face-centered lattice, cubic syngonia, Fm3m group of symmetry according to the international classification [17]). In this connection, it may be assumed that the structure of the novel phase contained in “re-precipitated” elemental silver, at least in outline, resembles the structure of AgBr and its crystal lattice is similar to a lattice of silver(I) bromide where the Br atom positions are occupied by atoms of silver. To determine whether the reflexes indicated can belong to elemental silver with such a space structure in 451 Unauthenticated Download Date | 6/15/17 7:45 AM Novel modification of elemental silver formed in Ag4[Fe(CN)6]-gelatin-immobilized matrix implants inter-plane distances in which are 235.7, 204.1, 144.4, 123.1 and 117.9 pm. Thus, formation of a novel phase of elemental silver which, probably, has not been described in the literature up to now, takes place here indeed. 4. Conclusion Figure 6. Theoretical XRD-patterns of elemental silver contained in initial Ag-GIM (a) and of elemental silver formed in GIM as a result of “re-precipitation” process (b). principle, theoretical XRD-patterns of assumed structure of element silver were generated using PowderCell as described in work [15]. These XRD-patterns are presented in Fig. 6. As may be seen, the theoretical d values calculated (333.6, 288.7, 204.2, 174.1, 166.7, 144.4, 132.5, 129.1, 117.9 pm) for the structure described above with an elementary cell parameter a = 288.72 pm and Pm3m symmetry group, practically coincide with d values experimentally observed in the XRD-pattern of the “re-precipitated” elemental silver (d = 333.6, 288.5, 166.7 and 129.1 pm). It should be noted in this connection that d values calculated theoretically for the elemental silver isolated from initial Ag-GIM (235.4, 204.3, 144.5, 123.2 and 118.0 pm), correspond to compact-packed crystal structure having an elementary cell parameter a = 408.62 pm belonging to the Fm3m symmetry group, Formation of novel phase of elemental silver occurs as a result of redox-processes proceeding into AgHalgelatin-immobilized matrices during their contact with water-alkaline solutions containing Sn(II) and NH3, SCN–, S2O32–, ethanediamine-1,2, 2-aminoethanol or 3-(2-hydroxyethyl)-3-azapenthanediol-1,5. XRD data show that this novel phase consists of nano-particles of elemental silver. The symmetry group of this phase is Pm3m with elementary cell parameter a = 288.72 pm whereas symmetry group of elemental silver is Fm3m with elementary cell parameter a = 408.62 pm.Besides, nano-particles of the new phase are formed in intermolecular cavities of gelatin mass and give red color to gelatin layer. Acknowledgements The Russian Foundation of Basic Researches (RFBR) is acknowledged for the financial support of given work (grant No. 09-03-97001). References [1] D.G. Skillman, C.R. Berry, J. Chem. Phys. 48, 3297 (1968) [2] T. Linnert, P. Mulvaney, A. Henglein, H. Weller, J. Am. Chem. Soc. 112, 4657 (1990) [3] S. Fedrigo, W. Harbich, J. Butter, Phys. Rev. 47, 10706 (1993) [4] A. Henglein, Israel J. Chem. 33, 77 (1993) [5] N. Satoh, H. Hasegawa, K. Tsujii, K. Kimura, J. Phys. Chem. 98, 2143 (1994) [6] B.G. Ershov, G.V. Ionova, A.A. Kiseleva, Zh. Phis. Khim. 69, 260 (1995) (in Russian) [7] T. Sato et al., Chem. Phys. Lett. 242, 310 (1995) [8] A.H.R. Al-Obaidi et al., J. Phys. 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