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Synthesis of Ag nanoparticles under a contact of water solution with silver(I)chloride biopolymer matrix
Journal of Molecular Liquids 291 (2019) 111354
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Synthesis of Ag nanoparticles under a contact of water solution with silver(I)chloride biopolymer matrix O.V. Mikhailov Kazan National Research Technological University, K. Marx Street 68, 420015 Kazan, Russia
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Article history: Received 6 October 2018 Received in revised form 27 June 2019 Accepted 10 July 2019 Available online xxxx Keywords: Ag nanoparticles BPIM Silver(I)chloride Re-precipitation
a b s t r a c t Nanoparticles of elemental silver (Ag-NP) obtaining as a result of the “re-precipitation” process of elemental silver according to scheme Ag → AgCl → Ag into biopolymer-immobilized matrices (BPIM) on the base of gelatin, has been found. On the first stage of this process, BPIM containing elemental silver, are processed with wateralkaline solutions containing KCl and K3[Fe(CN)6], on the second stage - with water-alkaline solutions containing tin(II) dichloride and organic or inorganic substance forming rather stable soluble coordination compounds with Ag(I). Some D = f(DAg) dependences where D is an optical density of “re-precipitated” silver in the matrix corresponding to initial density DAg have been presented. It has been found that a degree of influence of complexformed substance (D/DAg) is determined with its nature as well as its concentration in “reducting” solution; besides, the most considerable this influence is in the case of ethanediamine-1,2, the least considerable one, in the case of ammonia. Besides, silver formed in gelatin matrix consists of nanoparticles and distinguished by optical and XRD parameters from initial elemental silver. © 2019 Published by Elsevier B.V.
1. Introduction At the present time, a significant number of papers devoted to production of elemental silver nanoparticles (Ag-NP) using both purely chemical, and physical and biological methods for their synthesis, was published. The information concerning these publications can be found in review articles [1–11]. However, already 50 years ago in [12] it was mentioned about colloid element silver with rather small size of the particles, formed in gelatin layers at development of silver halide (AgHal) photographic materials. Silver halide photographic material actually is AgHal biopolymer-immobilized matrix (hereafter BPIM) where polymer mass is gelatin. According to [13–17], physical-chemical processes in this immobilized matrix system are often accompanied by the formation of such chemical compounds that cannot be obtained by carrying out similar processes in solution or solid phase. The reason
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for this is, on the one hand, the fact that these processes occur in the intermolecular cavities of gelatin and these cavities may be considered as peculiar nano-reactors; on the other hand, the fact that in this unusual reaction system, there is a preliminary reduction of entropy. Due to this, some processes that normally thermodynamic forbidden, can proceed in these specific conditions. In this connection, it may be expected that in such BPIM, specific redox processes, in particular, accompanied by the formation of nanoparticles of elemental metals, can occur. The present paper is devoted to consideration of redox-reactions connected with obtaining element silver nanoparticles. 2. Theoretical foundations Gelatin is known to be the poly-disperse mixture of relatively lowmolecular-mass polypeptides having general formula I
VM = (1/4)πD2h = (1/4) · 3.14. [285,000 · 10−10 cm). (1400 · 10−10 cm)2] = 4.38 · 10−19 cm3.
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O.V. Mikhailov / Journal of Molecular Liquids 291 (2019) 111354
Fig. 1. Three polypeptide α-chains form a triple-helix of gelatin.
In the case of maximal compact arrangement, these molecules occupy total volume equal to 4.38 · 10−19 cm3. (2.0–3.0) · 1015 = (8.76–13.15) · 10−4 cm3. In that way the volume of cavities indicated, is equal to total volume of polymer massif minus the volume occupied by gelatin molecules, namely (2.0 · 10−3–(8.76–13.15) · 10−4) cm3 that will be in the end (0.69–1.12) · 10−3 cm3. Then, the average volume of one intermolecular hollow can be found as a quotient from division of their total volume into the number of gelatin molecules and will be (0,69–1,12) · 10−3 cm3: (2,0–3,0) · 1015 = (2,3–5,6) · 10−19 cm3 = (2,3–5,6) · 102 nm3. The linear size of such an “average” cell assuming its spherical form is r = (6 V/π)1/3 = [6 · (2,3–5,6) · 10−19 cm3/3,14)]1/3 = (7,60–10,22) nm; for the cubic form it is a = V1/3 = [(2,3–5,6) · 10−19 cm3]1/3 = (6,13–8,24) nm. The values characterized these cavities as nano-sized. Therefore, only nano-particles of various substances can entry into these cavities and fix hardly in them. By entering into such cavities, nano-particles will be isolated from each other. Therefore, their aggregation with each other becomes rather difficult. Thus, the gelatin structure potentially is convenient for the formation of the BPIM containing namely nanoparticles. The gelatin structure does not allow any rigid crystal blocks to be implemented; but it has enough cells to take and fix molecules of the immobilized chemical compound. In addition, these cells, even being filled with such molecules, keep some freedom of motion in the space. In the latest 20 years, possibility of a wide variety of chemical reactions in the BPIM: nucleophilic substitution reaction, an electrophilic substitution ones (ionic exchange), and template synthesis was shown in large number of published works; more detail information on these specific reactions can be found in the reviews [13–17]. Based on publications data it may be expected that the chemical substances assortment consisting of nanoparticles, which can be obtained by using gelatin matrix, will be very significant. For example, nano-sized oxides, chalcogenides, silicates, phosphates of various p-, d- and f-elements, metal complexes, metalmacrocyclic compounds et al. can be obtained. And, elemental metal nanoparticles, too. Now we have already received by SEM direct experimental evidence that in the course of processes proceeding in the BPIM, the formation of nanoparticles with the most diverse nature, the sizes of which are in the range of 10–100 nm, takes place.
3. Experimental X-ray film Structurix D-10 (Agfa-Gevaert, Belges) was used as initial material to obtain silver-containing gelatin-immobilized matrix implants. Film samples having format 20 · 30 cm2 were exposed to Xray radiation with an irradiation dose at a range 0.05–0.50 Röntgen. These samples further were subjected to process according to [21]: • Development in D-19 standard developer, for 6 min at 20–25 °C; • Washing with running water for 2 min at 20–25 °C; • Treatment with 25% water solution of sodium trioxosulphidosulphate (VI) (Na2S2O3) for 10 min at 20–25 °C; • Washing with running water for 15 min at 18–25 °C. Three first stages (development, washing and fixing) were carried out at non-actinic green-yellow light, final washing – at natural light. The obtained samples of BPIM containing elemental silver (Ag-BPIM), were processed according to next technology: • Oxidation in water solution containing (g·L−1)
Potassium hexacyanoferrate(III) (K3[Fe(CN)6]) Potassium chloride (KCl) Sodium trioxocarbonate(IV) (Na2CO3) Water
50.0 50.0 5.0 up to 1000 mL
for 15 min at 20–25 °C; • Washing with running water for 2 min at 20–25 °; • Reduction in water solution containing (g·L−1)
Tin(II) chloride (SnCl2) 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 20–25 °C; • Washing with running water for 15 min at 18–25 °C; • Drying during 2–3 h at 20–25 °C.
Fig. 2. The fragment of gelatin structure containing intermolecular cavities (left); the general plane of intermolecular cavity in which can be nano-particles of various chemical compounds formed in chemical processes in the BPIM (right).
O.V. Mikhailov / Journal of Molecular Liquids 291 (2019) 111354
Ammonia NH3, potassium thiocyanate KSCN, sodium trioxosulphidosulphate(VI) Na2S2O3, ethanediamine-1,2 H2N–CH2– CH2–NH2, 2-aminoethanol H2N–CH2–CH2–OH and 3-(2hydroxyethyl)-3-azapenthanediol-1,5 N(CH2–CH2–OH)3, were used as complex-forming reagents that form water-soluble complexes with Ag(I). At the first stage of this process, conversion of initial Ag-BPIM into AgCl-BPIM was occurred, on the second stage, reduction of AgClBPIM with Sn(II) to elemental silver and formation of Ag-BPIM took place. And so, peculiar “re-precipitation” of elemental silver into gelatin matrix occurred. An isolation immobilized substances from BPIM was carried out by influence of some proteolytic enzymes water solutions (for example, trypsin) destroying the polymeric carrier of a BPIM (gelatin), and the subsequent separation of a solid phase from mother solution was done according to [21]. The powders with elemental silver isolated from the Ag-BPIM, were studied using scanning electron microscopy with high resolution (SEM). The study was conducted on a workstation AURIGA CrossBeam company CARL ZEISS, combined with ionic column COBRA mode detection of secondary electrons (Inlens detector). An accelerating voltage was 5 kV, working interval - 2-5 mm, for the optimal preservation of the sample from the electrons effects and the best contrast with this mode detection. The substances isolated from BPIM further were analysed by X-ray diffraction method using of spectrometer D8 Advance (Bruker, Germany). A scanning was carried out in an interval 2θ from 3 up to 65°, a step 2θ was 0,05. Calculation of reflexes intensities (I) and interplane distances (d) was done with standard software package EVA. Theoretical XRD spectra (X-ray patterns) were calculated by PowderCell program described in Refs. [22,23]. Ag-BPIM optical densities were measured by means of Macbeth TD504 photometer (Kodak, USA) in a range 0.1–5.0 units with accuracy of ±2% (rel.). 4. Results and discussion At visual observation over a process of transformation AgCl-BPIM into Ag-BPIM, following circumstance attracts its attention. Ag-BPIM received as a result of standard processing of exposed AgHal-BPIM, at rather small optical density (DAg), have grey colour, at big DAg, black colour. Colour of Ag-BPIM containing the “re-precipitated” element silver, varies from black-brown to red depending on the nature and quantity of complex-forming reagent contained in the solution used on the stage of reduction. SEM photo of element silver the particles contained in initial AgBPIM was shown in Fig. 3, the particles of “re-precipitated” element silver contained in the Ag-BPIM was shown in Fig. 4, the particles contained in powders isolated from these BPIM, was shown in Fig. 5. These photos demonstrated that, in the BPIM after the procedure “re-
Fig. 3. SEM elemental silver particles contained in the initial Ag-BPIM.
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precipitation” quite clearly visible silver particles with linear sizes vary in the range of 10–30 nm, in powders, in the range of 80–120 nm. At the same time, linear sizes of silver particles in initial Ag-BPIM are in average 150–200 nm (Fig. 3). Their form is more or less distinctly expressed that it is quite natural for such objects. It is interesting the fact that the aggregation of nanoparticles in this new phase of elemental silver is expressed not particularly strong, despite the high (and well known) general tendency of nanoparticles at all and silver in particular to the larger conglomerates formation. It is significant, that absorption spectra of both initial and the “reprecipitated” element silver in visible area do not contain any accurately expressed maxima. Besides, optical density Ag-BPIM with the “re-precipitated” silver (D), at the same volume concentration of element silver (CAgV) in BPIM, as a rule, is essentially more than DAg values and depends on nature and quantity of complex-forming reagent in solution contacting with BPIM. Examples of D = f(DAg) dependences for studied reagents were presented in Fig. 6. It is significant that (D/DAg) value, as a rule, N1.0, and in some cases, it reaches very high values (for potassium thiocyanate – nearly 5.0). It is important that the stronger the colour of the gelatinous layer with the “re-precipitated” elemental silver is different from the grey-black tones of the gelatinous layer initially Ag-gelatinimmobilized matrix, the greater is the (D/DAg) value. The maximal degree of amplification (D/DAg)max is also depends on the nature of the complexing agent (Table 1); the most profound effect on this parameter has ethanediamine-1,2 [(D/DAg)max = 5.95], the least severe – ammonia, the possibility degree of which, nevertheless, is also quite high [(D/DAg)max = 3.55]. For the ammonia, the growth (D/DAg) values is typical with increasing concentration of NH3 in solution to a relatively small its value (~5.0 g·L−1), after which the optical density D begin to fall (Fig. 6). It is noteworthy that red-brown colour of gelatinous layer attained at the indicated concentration, with a further increase in the concentration of ammonia does not change. The same situation in the case of the two other studied inorganic complexing agents – trioxosulfidosulfate(VI) and the thiocyanate anion was occured, with the only difference that maximum with the only difference that maximum amplification degree is achieved with more high concentration in comparison with NH3 (25.5 and ~70 g·L−1, respectively). In this regard, it was interesting to try the marked distinction with different sta− bility of 1:2 complexes formed by Ag(I) with NH3, S2O2− 3 and SCN (pK = 7.25, 13.32 and 8.39, respectively). However, in the presence of such correlation, the concentration value indicated for SCN− should be lower than for NH3, which is really not observed. In reality, for the three studied organic complex-forming substances, molar concentrations at which the maximum value (D/DAg) is reached, significantly greater than those for inorganic complex-forming substances (Table 1).
Fig. 4. SEM elemental silver particles contained in the Ag-BPIM after the procedure “reprecipitation”.
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O.V. Mikhailov / Journal of Molecular Liquids 291 (2019) 111354 Table 1 The maximal (D/DAg) and pKs values of Ag(I) complexes for various complex-forming reagents. Complex-forming reagent (D/DAg)max
Complex-forming reagent concentration at which reaches (D/DAg)max, g·L−1
pKs of Ag(I) complex having 1:2 composition
NH3 Na2S2O3 KSCN HO–(CH2) 2–NH2 H2N–(CH2)2–NH2 N(CH2–CH2–OH)3
4.9 25.5 70,5 110.0 81.0 101.5
7.25 13.32 8.39 6.62 7.84 3.64
3,55 4.19 4.95 5.45 5.95 3,90
of chemical compounds contained in the BPIM, have been indicated): 3− 4− − þ Cl →fAgClg þ FeðCNÞ6 fAgg þ FeðCNÞ6 Fig. 5. SEM elemental silver particles contained in the powder extracted from Ag-BPIM after the procedure “re-precipitation”.
Stability the of silver(I) complexes with each of these ligands is lower than with NH3, and the correlation between these concentrations and the stability of coordination compounds of Ag(I) with given ligands is still visible. As it may be easily noticed when comparing the data of Table 1, the maximum amplification degree decreases in the direction of ethanediamine-1,2 N 2-aminoethanol N SCN− N S2O2− N NH3 N 33 (2-hydroxyethyl)-3-azapenthanediol-1,5. However, the stability of complexes formed by these ligands with silver(I), decreases in the di− rection S2O2− 3 N SCN N ethanediamine-1,2 N NH3 N 2-aminoethanol N 3-(2-hydroxyethyl)-3-azapenthanediol-1,5. Thus, the complexing is though important, but not the single determinant for the influence degree of complex-forming agents on the redox process considered here. At the first stage of the given process, reaction which can be described by general equation (1), takes place (in the braces {…}, formulas
Each of complex-forming agents under examination forms with Ag (I) soluble complex having a metal ion: ligand ratio of 1:2. That is why, formation of silver(I) complex with corresponding complex-forming agent will occur to some extent when AgCl-BPIM is at the contact with the solution containing any of indicated complex-forming reagent. Both gelatin-immobilized silver(I) chloride and any of these soluble silver(I) complexes, can participate in process of reduction with Sn(II). In this connection, two parallel processes Ag(I) → Ag(0) will take place at contact of AgCl-BPIM with solution containing SnCl2 and complexing reagent: • gelatin-immobilized silver(I) bromide reduction proceeding in a polymer layer, • Ag(I) complex with complex-forming agent reduction proceeding on interface of phases a BPIM/solution.
In water solutions at pH = 12–13, Sn(II) is mainly in a form of hydroxo-complex [Sn(OH)3]−. So general Eq. (2) may be offered for the first of these processes. − 2− − 2fAgClg þ SnðOHÞ3 þ 3OH− →2fAgg þ SnðOHÞ6 þ 2Cl
D
ð1Þ
ð2Þ
For the second of these processes, general equation (3). − 2− 2½AgL2 þ þ SnðOHÞ3 þ 3OH− →2Ag þ 4 L þ SnðOHÞ6
ð3Þ
in the case of non-charged ligands and general equation (4). − 2− 2½AgL2 ð2z−1Þ– þ SnðOHÞ3 þ 3OH− →2Ag þ 4Lz– þ SnðOHÞ6
DAg Fig. 6. Dependences of D = f(DAg) of reduction process AgCl → Ag with using of NH3 at concentration 5.0 g·L−1 (curve 1), 12.0 g·L−1 (2), 25.0 g·L−1 (3), 50.0 g·L−1 (4). Optical densities DAg and D were measured with blue light-filter with a transmission maximum at 450 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ð4Þ
in the case of negative charged “acid”-ligands may be ascribed [L – symbol of ligand (actually complex-forming agent), z – its charge]. The particles of element silver formed during reactions (3) and (4) theoretically should have smaller sizes than the particles of element silver arising in polymer layer of the BPIM. To be a part of substance immobilized in the BPIM, these particles should place freely in intermolecular cavities of a gelatin layer. Only in this case, they may diffuse in the BPIM and be immobilized in gelatinous mass. With concentration growth of any complexation reagent mentioned above, concentration of coordination compounds formed given complexation reagent with Ag(I) must increase. Correspondingly, the quantity of the element silver nanoparticles formed as a result of reduction of these coordination compounds by [Sn(OH)3]− complex, should increase, too. Thus, when concentrations of these complexation reagents in a solution increase, the share of nanoparticles contained in the “reprecipitated” elemental silver, should increase gradually. At the same concentration of element silver nanoparticles owing to their higher
O.V. Mikhailov / Journal of Molecular Liquids 291 (2019) 111354
dispersion degree in comparison with micro-particles should provide higher degree of absorption of visible light (and, accordingly, higher optical density) polymeric layer of the BPIM. The experimental data are in full conformity with the given prediction. The particles of element silver formed during the reactions (3) and (4), are one- or two-nuclear. While it is not enough of them it occurs, when concentration of Ag(I) complexes on interface of phases a BPIM/ solution is low enough], these particles owing to their remoteness from each other have not time to be aggregated. They diffuse in polymeric layer of BPIM, where are immobilized without change of their sizes. With increase of complex-forming reagent concentration [and, accordingly, of concentration of Ag(I) complex with given reagent], quantity of nanoparticles indicated on interface of phases the BPIM/solution accrues. It leads to increase a number of such particles diffused into BPIM. However, at some rather high concentration complex-forming reagent in a solution, the aggregation effect of element silver nanoparticles starts to affect. One- and two-nuclear particles of element silver formed at reduction of corresponding Ag(I) complex, begin to unite with each other in larger particles. Poly-nuclear particles of element silver resulting in such an association, are not so mobile and, consequently, will be not diffuse into polymeric layer of BPIM. They will be precipitated in it near to BPIM/solution interface (or even to escape as a solid phase in the solution contacting with BPIM). As a result, rates of number increment of one- and two-nuclear particles of elemental silver with further growth of concentration complex-forming reagent begin to be slowed down. Therefore, inevitably there should come the moment when the number of similar particles will reach some limiting value. That is why, since certain “threshold” concentration complexforming reagent in a solution, growth of (D/DAg) values must stop. Moreover, at excess of this “threshold” concentration, certain decrease (D/DAg) should begin. The point is that an alignment between number of the aggregated particles and number one- and two-nuclear ones with growth of concentration of Ag(I) complex continuously grows and has no restrictions. These poly-nuclear particles are precipitated in frontier zone BPIM on rather small depth and form the same kind of elemental silver, as the elemental silver micro-particles formed as a result of reduction of gelatin-immobilized AgCl according to reaction (2). That is why, (D/DAg) values with increase of complex-forming reagent concentration at first increase, reach a maximum and then decrease. It can be assumed that “re-precipitated” gelatin-immobilized silver should contain, as a minimum, two phases of the silver particles, one of which is formed by nanoparticles, and other, by micro-particles. To confirm this fact, we carried out the analysis of elemental silver isolated from initial Ag-BPIM and elemental silver isolated from Ag-BPIM after end of “re-precipitation” process by X-ray powder diffraction method. It should be noted especially that XRD-pattern of initial elemental silver with grey-black colour of gelatin layer and XRD-patterns of “re-precipitated” elemental silver, essentially differ from each other. So, in XRDpatterns of “re-precipitated” elemental silver obtained with availability of any studied complex-forming agent in a solution contacting with BPIM, there are accurate reflexes having d values equal to 333.6, 288.5, 166.7 and 129.1 pm that are absent in 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 in them. These reflexes are characteristic for the known phase of elemental silver isolated from initial AgBPIM. There are all reasons to believe that the “re-precipitated” elemental silver, obtained with using solution containing any of complexforming reagent indicated above, contains at least two structural modifications of elemental silver. It is interesting that reflexes with d = 333.6, 288.5, 204.2, 166.7 and 129.1 pm are rather close to 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 crystal system, Fm3m group of symmetry according to the international classification). It may be assumed that the structure of the novel phase contained in
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“re-precipitated” elemental silver resembles structure AgBr and its crystal lattice is similar to a lattice of silver(I) bromide where positions of atoms Br occupy atoms of silver. To answer on the question, whether the reflexes indicated can belong to elemental silver with such a space structure in principle, theoretical XRD-patterns of assumed element silver structure by program PowderCell described in works [22,23], have been constructed. Calculated theoretical d values (333.6, 288.7, 204.2, 174.1, 166.7, 144.4, 132.5, 129.1, 117.9 pm) for specified above structure with an elementary cell parameter a = 288.72 pm and Pm3m symmetry group, practically coincide with d values experimentally observed in XRD-pattern of the “re-precipitated” elemental silver (d = 333.6, 288.5, 166.7 and 129.1 pm). It should be noted that d values calculated theoretically for the elemental silver isolated from initial Ag-BPIM (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 and Fm3m symmetry group, inter-plane distances in which are 235.7, 204.1, 144.4, 123.1 and 117.9 pm. Thus, we suppose that formation of a novel phase of elemental silver which really consisting of nanoparticles, occurs here indeed. 5. Conclusion In conclusion, BPIM is usable medium for obtaining of elemental silver nanoparticles by using the process of “re-precipitation” of elemental silver according to scheme Ag → AgCl →Ag. Herewith, it is very important, that formation of new phase of elemental silver, which has a higher dispersion and optical density in the visible region of the spectrum in comparison with the initial one, occurs. Acknowledgement This work was supported in the framework of draft No 4.5784.2017/ 8.9 to the competitive part of the state task of the Russian Federation on the 2017–2019 years. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.111354. References [1] Y.A. Krutyakov, A.A. Kudrinskiy, A.Y. Olenin, G.V. Lisichkin, Synthesis and properties of silver nanoparticles: advances and prospects, Russ. Chem. Revs. 77 (2008) 233–257. [2] A.Y. Olenin, G.V. Lisichkin, Metal nanoparticles in condensed media: preparation and the bulk and surface structural dynamics, Russ. Chem. Revs. 80 (2011) 605–630. [3] O.V. Mikhailov, Progress in the synthesis of ag nanoparticles having manifold geometric forms, Revs. Inorg. Chem. 38 (2018) 21–42. [4] C.J. Murphy, A.M. Gole, S.E. Hunyadi, C.J. Orendorff, One-dimensional colloidal gold and silver nanostructures, Inorg. Chem. 45 (2006) 7544–7554. [5] X.K. Meng, S.C. Tang, S.A. Vongehr, A review on diverse silver nanostructures, J. Mater. Sci. Technol. 26 (2010) 487–522. [6] B. Wiley, Y.G. Sun, B. Mayers, Y.N. Xia, Shape-controlled synthesis of metal nanostructures: the case of silver, Chem. Eur. J. 11 (2005) 454–463. [7] M. Ovais, A. Raza, S. Naz, N.U. Islam, A.T. Khalil, S. Ali, M.A. Khan, Z.K. Shinwari, Current state and prospects of the phytosynthesized colloidal gold nanoparticles and their applications in cancer theranostics, Appl. Microbiol. Biotechnol. 101 (2017) 3551–3565. [8] D. Aherne, D.M. Ledwith, M. Gara, J.M. Kelly, Optical properties and growth aspects of silver nanoprisms produced by a highly reproducible and rapid synthesis at room temperature, Adv. Funct. Mater. 18 (2008) 2005–2016. [9] M. Rai, S. Birla, A.P. Ingle, I. Gupta, A. Gade, K. Abd-Elsalam, P.D. Marcato, N. Duran, Nanosilver: an inorganic nanoparticle with myriad potential applications, Nanotechnol. Revs. 3 (2014) 281–310. [10] K.B. Narayanan, N. Sakthivel, Green synthesis of biogenic metal nanoparticles by terrestrial and aquatic phototrophic and heterotrophic eukaryotes and biocompatible agents, Adv. Colloid and Interface Sci. 169 (2011) 59–79. [11] Y. Xia, Y. Xiong, B. Lim, S.E. Skrabalak, Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics? Angew. Chem. Int. Ed. 48 (2009) 60–103.
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Synthesis of Ag nanoparticles under a contact of water solution with silver(I)chloride biopolymer matrix O.V. Mikhailov Kazan National Research Technological University, K. Marx Street 68, 420015 Kazan, Russia
a r t i c l e
i n f o
Article history: Received 6 October 2018 Received in revised form 27 June 2019 Accepted 10 July 2019 Available online xxxx Keywords: Ag nanoparticles BPIM Silver(I)chloride Re-precipitation
a b s t r a c t Nanoparticles of elemental silver (Ag-NP) obtaining as a result of the “re-precipitation” process of elemental silver according to scheme Ag → AgCl → Ag into biopolymer-immobilized matrices (BPIM) on the base of gelatin, has been found. On the first stage of this process, BPIM containing elemental silver, are processed with wateralkaline solutions containing KCl and K3[Fe(CN)6], on the second stage - with water-alkaline solutions containing tin(II) dichloride and organic or inorganic substance forming rather stable soluble coordination compounds with Ag(I). Some D = f(DAg) dependences where D is an optical density of “re-precipitated” silver in the matrix corresponding to initial density DAg have been presented. It has been found that a degree of influence of complexformed substance (D/DAg) is determined with its nature as well as its concentration in “reducting” solution; besides, the most considerable this influence is in the case of ethanediamine-1,2, the least considerable one, in the case of ammonia. Besides, silver formed in gelatin matrix consists of nanoparticles and distinguished by optical and XRD parameters from initial elemental silver. © 2019 Published by Elsevier B.V.
1. Introduction At the present time, a significant number of papers devoted to production of elemental silver nanoparticles (Ag-NP) using both purely chemical, and physical and biological methods for their synthesis, was published. The information concerning these publications can be found in review articles [1–11]. However, already 50 years ago in [12] it was mentioned about colloid element silver with rather small size of the particles, formed in gelatin layers at development of silver halide (AgHal) photographic materials. Silver halide photographic material actually is AgHal biopolymer-immobilized matrix (hereafter BPIM) where polymer mass is gelatin. According to [13–17], physical-chemical processes in this immobilized matrix system are often accompanied by the formation of such chemical compounds that cannot be obtained by carrying out similar processes in solution or solid phase. The reason
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for this is, on the one hand, the fact that these processes occur in the intermolecular cavities of gelatin and these cavities may be considered as peculiar nano-reactors; on the other hand, the fact that in this unusual reaction system, there is a preliminary reduction of entropy. Due to this, some processes that normally thermodynamic forbidden, can proceed in these specific conditions. In this connection, it may be expected that in such BPIM, specific redox processes, in particular, accompanied by the formation of nanoparticles of elemental metals, can occur. The present paper is devoted to consideration of redox-reactions connected with obtaining element silver nanoparticles. 2. Theoretical foundations Gelatin is known to be the poly-disperse mixture of relatively lowmolecular-mass polypeptides having general formula I
VM = (1/4)πD2h = (1/4) · 3.14. [285,000 · 10−10 cm). (1400 · 10−10 cm)2] = 4.38 · 10−19 cm3.
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Fig. 1. Three polypeptide α-chains form a triple-helix of gelatin.
In the case of maximal compact arrangement, these molecules occupy total volume equal to 4.38 · 10−19 cm3. (2.0–3.0) · 1015 = (8.76–13.15) · 10−4 cm3. In that way the volume of cavities indicated, is equal to total volume of polymer massif minus the volume occupied by gelatin molecules, namely (2.0 · 10−3–(8.76–13.15) · 10−4) cm3 that will be in the end (0.69–1.12) · 10−3 cm3. Then, the average volume of one intermolecular hollow can be found as a quotient from division of their total volume into the number of gelatin molecules and will be (0,69–1,12) · 10−3 cm3: (2,0–3,0) · 1015 = (2,3–5,6) · 10−19 cm3 = (2,3–5,6) · 102 nm3. The linear size of such an “average” cell assuming its spherical form is r = (6 V/π)1/3 = [6 · (2,3–5,6) · 10−19 cm3/3,14)]1/3 = (7,60–10,22) nm; for the cubic form it is a = V1/3 = [(2,3–5,6) · 10−19 cm3]1/3 = (6,13–8,24) nm. The values characterized these cavities as nano-sized. Therefore, only nano-particles of various substances can entry into these cavities and fix hardly in them. By entering into such cavities, nano-particles will be isolated from each other. Therefore, their aggregation with each other becomes rather difficult. Thus, the gelatin structure potentially is convenient for the formation of the BPIM containing namely nanoparticles. The gelatin structure does not allow any rigid crystal blocks to be implemented; but it has enough cells to take and fix molecules of the immobilized chemical compound. In addition, these cells, even being filled with such molecules, keep some freedom of motion in the space. In the latest 20 years, possibility of a wide variety of chemical reactions in the BPIM: nucleophilic substitution reaction, an electrophilic substitution ones (ionic exchange), and template synthesis was shown in large number of published works; more detail information on these specific reactions can be found in the reviews [13–17]. Based on publications data it may be expected that the chemical substances assortment consisting of nanoparticles, which can be obtained by using gelatin matrix, will be very significant. For example, nano-sized oxides, chalcogenides, silicates, phosphates of various p-, d- and f-elements, metal complexes, metalmacrocyclic compounds et al. can be obtained. And, elemental metal nanoparticles, too. Now we have already received by SEM direct experimental evidence that in the course of processes proceeding in the BPIM, the formation of nanoparticles with the most diverse nature, the sizes of which are in the range of 10–100 nm, takes place.
3. Experimental X-ray film Structurix D-10 (Agfa-Gevaert, Belges) was used as initial material to obtain silver-containing gelatin-immobilized matrix implants. Film samples having format 20 · 30 cm2 were exposed to Xray radiation with an irradiation dose at a range 0.05–0.50 Röntgen. These samples further were subjected to process according to [21]: • Development in D-19 standard developer, for 6 min at 20–25 °C; • Washing with running water for 2 min at 20–25 °C; • Treatment with 25% water solution of sodium trioxosulphidosulphate (VI) (Na2S2O3) for 10 min at 20–25 °C; • Washing with running water for 15 min at 18–25 °C. Three first stages (development, washing and fixing) were carried out at non-actinic green-yellow light, final washing – at natural light. The obtained samples of BPIM containing elemental silver (Ag-BPIM), were processed according to next technology: • Oxidation in water solution containing (g·L−1)
Potassium hexacyanoferrate(III) (K3[Fe(CN)6]) Potassium chloride (KCl) Sodium trioxocarbonate(IV) (Na2CO3) Water
50.0 50.0 5.0 up to 1000 mL
for 15 min at 20–25 °C; • Washing with running water for 2 min at 20–25 °; • Reduction in water solution containing (g·L−1)
Tin(II) chloride (SnCl2) 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 20–25 °C; • Washing with running water for 15 min at 18–25 °C; • Drying during 2–3 h at 20–25 °C.
Fig. 2. The fragment of gelatin structure containing intermolecular cavities (left); the general plane of intermolecular cavity in which can be nano-particles of various chemical compounds formed in chemical processes in the BPIM (right).
O.V. Mikhailov / Journal of Molecular Liquids 291 (2019) 111354
Ammonia NH3, potassium thiocyanate KSCN, sodium trioxosulphidosulphate(VI) Na2S2O3, ethanediamine-1,2 H2N–CH2– CH2–NH2, 2-aminoethanol H2N–CH2–CH2–OH and 3-(2hydroxyethyl)-3-azapenthanediol-1,5 N(CH2–CH2–OH)3, were used as complex-forming reagents that form water-soluble complexes with Ag(I). At the first stage of this process, conversion of initial Ag-BPIM into AgCl-BPIM was occurred, on the second stage, reduction of AgClBPIM with Sn(II) to elemental silver and formation of Ag-BPIM took place. And so, peculiar “re-precipitation” of elemental silver into gelatin matrix occurred. An isolation immobilized substances from BPIM was carried out by influence of some proteolytic enzymes water solutions (for example, trypsin) destroying the polymeric carrier of a BPIM (gelatin), and the subsequent separation of a solid phase from mother solution was done according to [21]. The powders with elemental silver isolated from the Ag-BPIM, were studied using scanning electron microscopy with high resolution (SEM). The study was conducted on a workstation AURIGA CrossBeam company CARL ZEISS, combined with ionic column COBRA mode detection of secondary electrons (Inlens detector). An accelerating voltage was 5 kV, working interval - 2-5 mm, for the optimal preservation of the sample from the electrons effects and the best contrast with this mode detection. The substances isolated from BPIM further were analysed by X-ray diffraction method using of spectrometer D8 Advance (Bruker, Germany). A scanning was carried out in an interval 2θ from 3 up to 65°, a step 2θ was 0,05. Calculation of reflexes intensities (I) and interplane distances (d) was done with standard software package EVA. Theoretical XRD spectra (X-ray patterns) were calculated by PowderCell program described in Refs. [22,23]. Ag-BPIM optical densities were measured by means of Macbeth TD504 photometer (Kodak, USA) in a range 0.1–5.0 units with accuracy of ±2% (rel.). 4. Results and discussion At visual observation over a process of transformation AgCl-BPIM into Ag-BPIM, following circumstance attracts its attention. Ag-BPIM received as a result of standard processing of exposed AgHal-BPIM, at rather small optical density (DAg), have grey colour, at big DAg, black colour. Colour of Ag-BPIM containing the “re-precipitated” element silver, varies from black-brown to red depending on the nature and quantity of complex-forming reagent contained in the solution used on the stage of reduction. SEM photo of element silver the particles contained in initial AgBPIM was shown in Fig. 3, the particles of “re-precipitated” element silver contained in the Ag-BPIM was shown in Fig. 4, the particles contained in powders isolated from these BPIM, was shown in Fig. 5. These photos demonstrated that, in the BPIM after the procedure “re-
Fig. 3. SEM elemental silver particles contained in the initial Ag-BPIM.
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precipitation” quite clearly visible silver particles with linear sizes vary in the range of 10–30 nm, in powders, in the range of 80–120 nm. At the same time, linear sizes of silver particles in initial Ag-BPIM are in average 150–200 nm (Fig. 3). Their form is more or less distinctly expressed that it is quite natural for such objects. It is interesting the fact that the aggregation of nanoparticles in this new phase of elemental silver is expressed not particularly strong, despite the high (and well known) general tendency of nanoparticles at all and silver in particular to the larger conglomerates formation. It is significant, that absorption spectra of both initial and the “reprecipitated” element silver in visible area do not contain any accurately expressed maxima. Besides, optical density Ag-BPIM with the “re-precipitated” silver (D), at the same volume concentration of element silver (CAgV) in BPIM, as a rule, is essentially more than DAg values and depends on nature and quantity of complex-forming reagent in solution contacting with BPIM. Examples of D = f(DAg) dependences for studied reagents were presented in Fig. 6. It is significant that (D/DAg) value, as a rule, N1.0, and in some cases, it reaches very high values (for potassium thiocyanate – nearly 5.0). It is important that the stronger the colour of the gelatinous layer with the “re-precipitated” elemental silver is different from the grey-black tones of the gelatinous layer initially Ag-gelatinimmobilized matrix, the greater is the (D/DAg) value. The maximal degree of amplification (D/DAg)max is also depends on the nature of the complexing agent (Table 1); the most profound effect on this parameter has ethanediamine-1,2 [(D/DAg)max = 5.95], the least severe – ammonia, the possibility degree of which, nevertheless, is also quite high [(D/DAg)max = 3.55]. For the ammonia, the growth (D/DAg) values is typical with increasing concentration of NH3 in solution to a relatively small its value (~5.0 g·L−1), after which the optical density D begin to fall (Fig. 6). It is noteworthy that red-brown colour of gelatinous layer attained at the indicated concentration, with a further increase in the concentration of ammonia does not change. The same situation in the case of the two other studied inorganic complexing agents – trioxosulfidosulfate(VI) and the thiocyanate anion was occured, with the only difference that maximum with the only difference that maximum amplification degree is achieved with more high concentration in comparison with NH3 (25.5 and ~70 g·L−1, respectively). In this regard, it was interesting to try the marked distinction with different sta− bility of 1:2 complexes formed by Ag(I) with NH3, S2O2− 3 and SCN (pK = 7.25, 13.32 and 8.39, respectively). However, in the presence of such correlation, the concentration value indicated for SCN− should be lower than for NH3, which is really not observed. In reality, for the three studied organic complex-forming substances, molar concentrations at which the maximum value (D/DAg) is reached, significantly greater than those for inorganic complex-forming substances (Table 1).
Fig. 4. SEM elemental silver particles contained in the Ag-BPIM after the procedure “reprecipitation”.
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O.V. Mikhailov / Journal of Molecular Liquids 291 (2019) 111354 Table 1 The maximal (D/DAg) and pKs values of Ag(I) complexes for various complex-forming reagents. Complex-forming reagent (D/DAg)max
Complex-forming reagent concentration at which reaches (D/DAg)max, g·L−1
pKs of Ag(I) complex having 1:2 composition
NH3 Na2S2O3 KSCN HO–(CH2) 2–NH2 H2N–(CH2)2–NH2 N(CH2–CH2–OH)3
4.9 25.5 70,5 110.0 81.0 101.5
7.25 13.32 8.39 6.62 7.84 3.64
3,55 4.19 4.95 5.45 5.95 3,90
of chemical compounds contained in the BPIM, have been indicated): 3− 4− − þ Cl →fAgClg þ FeðCNÞ6 fAgg þ FeðCNÞ6 Fig. 5. SEM elemental silver particles contained in the powder extracted from Ag-BPIM after the procedure “re-precipitation”.
Stability the of silver(I) complexes with each of these ligands is lower than with NH3, and the correlation between these concentrations and the stability of coordination compounds of Ag(I) with given ligands is still visible. As it may be easily noticed when comparing the data of Table 1, the maximum amplification degree decreases in the direction of ethanediamine-1,2 N 2-aminoethanol N SCN− N S2O2− N NH3 N 33 (2-hydroxyethyl)-3-azapenthanediol-1,5. However, the stability of complexes formed by these ligands with silver(I), decreases in the di− rection S2O2− 3 N SCN N ethanediamine-1,2 N NH3 N 2-aminoethanol N 3-(2-hydroxyethyl)-3-azapenthanediol-1,5. Thus, the complexing is though important, but not the single determinant for the influence degree of complex-forming agents on the redox process considered here. At the first stage of the given process, reaction which can be described by general equation (1), takes place (in the braces {…}, formulas
Each of complex-forming agents under examination forms with Ag (I) soluble complex having a metal ion: ligand ratio of 1:2. That is why, formation of silver(I) complex with corresponding complex-forming agent will occur to some extent when AgCl-BPIM is at the contact with the solution containing any of indicated complex-forming reagent. Both gelatin-immobilized silver(I) chloride and any of these soluble silver(I) complexes, can participate in process of reduction with Sn(II). In this connection, two parallel processes Ag(I) → Ag(0) will take place at contact of AgCl-BPIM with solution containing SnCl2 and complexing reagent: • gelatin-immobilized silver(I) bromide reduction proceeding in a polymer layer, • Ag(I) complex with complex-forming agent reduction proceeding on interface of phases a BPIM/solution.
In water solutions at pH = 12–13, Sn(II) is mainly in a form of hydroxo-complex [Sn(OH)3]−. So general Eq. (2) may be offered for the first of these processes. − 2− − 2fAgClg þ SnðOHÞ3 þ 3OH− →2fAgg þ SnðOHÞ6 þ 2Cl
D
ð1Þ
ð2Þ
For the second of these processes, general equation (3). − 2− 2½AgL2 þ þ SnðOHÞ3 þ 3OH− →2Ag þ 4 L þ SnðOHÞ6
ð3Þ
in the case of non-charged ligands and general equation (4). − 2− 2½AgL2 ð2z−1Þ– þ SnðOHÞ3 þ 3OH− →2Ag þ 4Lz– þ SnðOHÞ6
DAg Fig. 6. Dependences of D = f(DAg) of reduction process AgCl → Ag with using of NH3 at concentration 5.0 g·L−1 (curve 1), 12.0 g·L−1 (2), 25.0 g·L−1 (3), 50.0 g·L−1 (4). Optical densities DAg and D were measured with blue light-filter with a transmission maximum at 450 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ð4Þ
in the case of negative charged “acid”-ligands may be ascribed [L – symbol of ligand (actually complex-forming agent), z – its charge]. The particles of element silver formed during reactions (3) and (4) theoretically should have smaller sizes than the particles of element silver arising in polymer layer of the BPIM. To be a part of substance immobilized in the BPIM, these particles should place freely in intermolecular cavities of a gelatin layer. Only in this case, they may diffuse in the BPIM and be immobilized in gelatinous mass. With concentration growth of any complexation reagent mentioned above, concentration of coordination compounds formed given complexation reagent with Ag(I) must increase. Correspondingly, the quantity of the element silver nanoparticles formed as a result of reduction of these coordination compounds by [Sn(OH)3]− complex, should increase, too. Thus, when concentrations of these complexation reagents in a solution increase, the share of nanoparticles contained in the “reprecipitated” elemental silver, should increase gradually. At the same concentration of element silver nanoparticles owing to their higher
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dispersion degree in comparison with micro-particles should provide higher degree of absorption of visible light (and, accordingly, higher optical density) polymeric layer of the BPIM. The experimental data are in full conformity with the given prediction. The particles of element silver formed during the reactions (3) and (4), are one- or two-nuclear. While it is not enough of them it occurs, when concentration of Ag(I) complexes on interface of phases a BPIM/ solution is low enough], these particles owing to their remoteness from each other have not time to be aggregated. They diffuse in polymeric layer of BPIM, where are immobilized without change of their sizes. With increase of complex-forming reagent concentration [and, accordingly, of concentration of Ag(I) complex with given reagent], quantity of nanoparticles indicated on interface of phases the BPIM/solution accrues. It leads to increase a number of such particles diffused into BPIM. However, at some rather high concentration complex-forming reagent in a solution, the aggregation effect of element silver nanoparticles starts to affect. One- and two-nuclear particles of element silver formed at reduction of corresponding Ag(I) complex, begin to unite with each other in larger particles. Poly-nuclear particles of element silver resulting in such an association, are not so mobile and, consequently, will be not diffuse into polymeric layer of BPIM. They will be precipitated in it near to BPIM/solution interface (or even to escape as a solid phase in the solution contacting with BPIM). As a result, rates of number increment of one- and two-nuclear particles of elemental silver with further growth of concentration complex-forming reagent begin to be slowed down. Therefore, inevitably there should come the moment when the number of similar particles will reach some limiting value. That is why, since certain “threshold” concentration complexforming reagent in a solution, growth of (D/DAg) values must stop. Moreover, at excess of this “threshold” concentration, certain decrease (D/DAg) should begin. The point is that an alignment between number of the aggregated particles and number one- and two-nuclear ones with growth of concentration of Ag(I) complex continuously grows and has no restrictions. These poly-nuclear particles are precipitated in frontier zone BPIM on rather small depth and form the same kind of elemental silver, as the elemental silver micro-particles formed as a result of reduction of gelatin-immobilized AgCl according to reaction (2). That is why, (D/DAg) values with increase of complex-forming reagent concentration at first increase, reach a maximum and then decrease. It can be assumed that “re-precipitated” gelatin-immobilized silver should contain, as a minimum, two phases of the silver particles, one of which is formed by nanoparticles, and other, by micro-particles. To confirm this fact, we carried out the analysis of elemental silver isolated from initial Ag-BPIM and elemental silver isolated from Ag-BPIM after end of “re-precipitation” process by X-ray powder diffraction method. It should be noted especially that XRD-pattern of initial elemental silver with grey-black colour of gelatin layer and XRD-patterns of “re-precipitated” elemental silver, essentially differ from each other. So, in XRDpatterns of “re-precipitated” elemental silver obtained with availability of any studied complex-forming agent in a solution contacting with BPIM, there are accurate reflexes having d values equal to 333.6, 288.5, 166.7 and 129.1 pm that are absent in 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 in them. These reflexes are characteristic for the known phase of elemental silver isolated from initial AgBPIM. There are all reasons to believe that the “re-precipitated” elemental silver, obtained with using solution containing any of complexforming reagent indicated above, contains at least two structural modifications of elemental silver. It is interesting that reflexes with d = 333.6, 288.5, 204.2, 166.7 and 129.1 pm are rather close to 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 crystal system, Fm3m group of symmetry according to the international classification). It may be assumed that the structure of the novel phase contained in
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“re-precipitated” elemental silver resembles structure AgBr and its crystal lattice is similar to a lattice of silver(I) bromide where positions of atoms Br occupy atoms of silver. To answer on the question, whether the reflexes indicated can belong to elemental silver with such a space structure in principle, theoretical XRD-patterns of assumed element silver structure by program PowderCell described in works [22,23], have been constructed. Calculated theoretical d values (333.6, 288.7, 204.2, 174.1, 166.7, 144.4, 132.5, 129.1, 117.9 pm) for specified above structure with an elementary cell parameter a = 288.72 pm and Pm3m symmetry group, practically coincide with d values experimentally observed in XRD-pattern of the “re-precipitated” elemental silver (d = 333.6, 288.5, 166.7 and 129.1 pm). It should be noted that d values calculated theoretically for the elemental silver isolated from initial Ag-BPIM (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 and Fm3m symmetry group, inter-plane distances in which are 235.7, 204.1, 144.4, 123.1 and 117.9 pm. Thus, we suppose that formation of a novel phase of elemental silver which really consisting of nanoparticles, occurs here indeed. 5. Conclusion In conclusion, BPIM is usable medium for obtaining of elemental silver nanoparticles by using the process of “re-precipitation” of elemental silver according to scheme Ag → AgCl →Ag. Herewith, it is very important, that formation of new phase of elemental silver, which has a higher dispersion and optical density in the visible region of the spectrum in comparison with the initial one, occurs. Acknowledgement This work was supported in the framework of draft No 4.5784.2017/ 8.9 to the competitive part of the state task of the Russian Federation on the 2017–2019 years. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.111354. References [1] Y.A. Krutyakov, A.A. Kudrinskiy, A.Y. Olenin, G.V. Lisichkin, Synthesis and properties of silver nanoparticles: advances and prospects, Russ. Chem. Revs. 77 (2008) 233–257. [2] A.Y. Olenin, G.V. Lisichkin, Metal nanoparticles in condensed media: preparation and the bulk and surface structural dynamics, Russ. Chem. Revs. 80 (2011) 605–630. [3] O.V. Mikhailov, Progress in the synthesis of ag nanoparticles having manifold geometric forms, Revs. Inorg. Chem. 38 (2018) 21–42. [4] C.J. Murphy, A.M. Gole, S.E. Hunyadi, C.J. Orendorff, One-dimensional colloidal gold and silver nanostructures, Inorg. Chem. 45 (2006) 7544–7554. [5] X.K. Meng, S.C. Tang, S.A. Vongehr, A review on diverse silver nanostructures, J. Mater. Sci. Technol. 26 (2010) 487–522. [6] B. Wiley, Y.G. Sun, B. Mayers, Y.N. Xia, Shape-controlled synthesis of metal nanostructures: the case of silver, Chem. Eur. J. 11 (2005) 454–463. [7] M. Ovais, A. Raza, S. Naz, N.U. Islam, A.T. Khalil, S. Ali, M.A. Khan, Z.K. Shinwari, Current state and prospects of the phytosynthesized colloidal gold nanoparticles and their applications in cancer theranostics, Appl. Microbiol. Biotechnol. 101 (2017) 3551–3565. [8] D. Aherne, D.M. Ledwith, M. Gara, J.M. Kelly, Optical properties and growth aspects of silver nanoprisms produced by a highly reproducible and rapid synthesis at room temperature, Adv. Funct. Mater. 18 (2008) 2005–2016. [9] M. Rai, S. Birla, A.P. Ingle, I. Gupta, A. Gade, K. Abd-Elsalam, P.D. Marcato, N. Duran, Nanosilver: an inorganic nanoparticle with myriad potential applications, Nanotechnol. Revs. 3 (2014) 281–310. [10] K.B. Narayanan, N. Sakthivel, Green synthesis of biogenic metal nanoparticles by terrestrial and aquatic phototrophic and heterotrophic eukaryotes and biocompatible agents, Adv. Colloid and Interface Sci. 169 (2011) 59–79. [11] Y. Xia, Y. Xiong, B. Lim, S.E. Skrabalak, Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics? Angew. Chem. Int. Ed. 48 (2009) 60–103.
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