SAXS Combined with UV-vis Spectroscopy and QELS: Accurate Characterization of Silver Sols Synthesized in Polymer Matrices
© Bulavin et al. 2016
Received: 30 October 2015
Accepted: 5 January 2016
Published: 27 January 2016
The present work demonstrates a validation of small-angle X-ray scattering (SAXS) combining with ultra violet and visible (UV-vis) spectroscopy and quasi-elastic light scattering (QELS) analysis for characterization of silver sols synthesized in polymer matrices. Polymer matrix internal structure and polymer chemical nature actually controlled the sol size characteristics. It was shown that for precise analysis of nanoparticle size distribution these techniques should be used simultaneously. All applied methods were in good agreement for the characterization of size distribution of small particles (less than 60 nm) in the sols. Some deviations of the theoretical curves from the experimental ones were observed. The most probable cause is that nanoparticles were not entirely spherical in form.
The size of metal nanoparticles determines optical, catalytic, or biomedical properties of nanosystems and defines limits for applications [1–4]. Despite the increasing interest in the applications of functional nanoparticles, a comprehensive understanding of the formation of nanosystems as well as their precise characterization is still a challenge.
Techniques to detect and characterize nanoparticles fall into two categories: direct, or “real space,” and indirect, or “reciprocal space.” Direct techniques include transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). These techniques can image nanoparticles, directly measure sizes, and infer shape information, but they are limited to studying only a few particles at a time. There are also significant issues surrounding the sample preparation for electron microscopy. In general, however, those techniques can be quite effective for obtaining basic information about a nanoparticle.
Indirect techniques for nanosystem characterization are absorption (ultra violet and visible (UV-vis) spectroscopy) and various scattering methods: quasi-elastic light scattering (QELS), X-rays, or neutron scattering. The techniques that become of greatest relevance to nanoscience are small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) [5, 6]. The advantage of those techniques is that they are able to characterize large numbers of nanoparticles and often do not require any particular sample preparation.
The main aim of the current research was to compare the data obtained by the complex of physical methods for evaluation of various indirect techniques for sol characterization.
The silver nanoparticles (AgNPs) were synthesized by reduction of the AgNO3 salt using sodium borohydride (NaBH4) as reductant. The synthesis of Ag sols was carried out in situ into an aqueous solution of nonionic polymer dextran-graft-polyacrylamide and its anionic derivative [7–9].
The synthesis of AgNPs was performed at the polymer concentration corresponding to dilute polymer solutions.
NaBH4 was purchased from “Pharma” (Ukraine).
AgNO3 (Sigma Aldrich) was used without additional purification.
Reduction of Ag salt was performed at T = 60 °C. Molar ratio of AA monomers to Ag+ cations was equal to 5. The syntheses were carried out in polymer solutions prepared using deionized water. The pH value of aqueous solutions of nonionic polymer was 5.5 that corresponds to the pH of deionized water. pH value of aqueous solutions of anionic polymers was around 7.33.
Two milliliters of a 0.1 mol L−1 AgNO3 aqueous solution was added to 5 mL of aqueous polymer solution (c = 1.10−3 g cm−3) and stirred for 20 min. Then, 2 mL of 0.1 mol L−1 aqueous solution of NaBH4 was added. The final aqueous solution was stirred for 30 min. It turned reddish brown; thus, the formation of AgNPs was indicated.
Size-Exclusion Chromatography (SEC)
Polymer characteristics determined by SEC and potentiometry
M w × 10−6, g mol−1
I = M w/M n
R g, Å
UV-visible absorption spectra of silver sols were recorded by Varian Cary 50 scan UV-visible spectrophotometer (Palo Alto, CA, USA).
Quasi-Elastic Light Scattering (QELS)
DLS measurements were carried out using Zetasizer Nano ZS90 (Malvern Instruments Ltd., UK). The apparatus contains a 4-mW He-Ne laser with a wavelength of 632.8 nm, and the scattered light is detected at an angle of 60°.
SAXS experiments were carried out on an instrument with a high-intensity microfocus rotating Cu anode X-ray generator in the Laboratory for Advanced Studies of Membrane Proteins (Moscow Institute of Physics and Technology, Dolgoprudniy, Russia), using a standard transmission configuration. An X-ray wavelength of λ = 1.54 Å was used, resulting in a momentum transfer Q in the range of 0.007–0.2 Å−1, where Q = (4π/λ) sin(θ/2) and θ is the scattering angle. The samples studied were placed in borosilicate capillaries of 1.5 mm diameter and 0.01 mm wall thickness (W. Muller, Berlin, Germany). Water was used as a buffer sample. Center of beam line and conversation channel to value of module q-vector was done using silver behenate .
Results and Discussion
The main characteristics of the polymers used as the matrices for in situ AgNP syntheses are drawn in Table 1, where I = M w/M n, the polydispersity index; R g, the radius of gyration; and A, the chemical charge fraction of polyelectrolytes obtained by alkaline hydrolysis of polyacrylamide.
SEC analysis indicates that polymer samples possess relatively low polydispersity index and display in aqueous solution rather large radii of gyration in agreement with their high average molecular weights. The peculiarities of the molecular structure of the copolymers dextran-graft-polyacrylamide (D-g-PAA) were discussed in [12–14]. These copolymers are star-like polymers, consisting of a compact dextran core and long polyacrylamide arms. As it was previously reported, the branched polymers, due to their more compact internal structure, have higher local concentration of functional groups with respect to their linear analogues [13, 14] that is why they are more efficient matrices for nanosystem fabrication .
D-g-PAA copolymer was transformed into polyelectrolyte, referred as D-g-PAA(PE) by alkaline hydrolysis. The process of D-g-PAA hydrolysis was not attended by irrelevant processes (breaking or cross-linking of the macromolecules) .
It is evident that saponified polymers contain two types of functional groups: carbamide and carboxylate ones. The pH value of the solutions was equal to 7.33 after the D-g-PAA(PE) sample dissolved in bi-distilled water. Thus, carboxylate groups of polymer were partially hydrolyzed in such conditions. Obviously, the nucleation process occurring just after reductant addition differs for silver ions interacting with carbamide or carboxylate moiety. That could lead to a different size distribution for nanoparticles synthesized in branched nonionic and polyelectrolyte polymer matrices.
In situ syntheses of AgNPs into dilute aqueous solutions of both uncharged (nonionic) and polyelectrolyte branched polymer matrices resulted in rather stable colloids. Our previous attempts to synthesize the stable colloid in anionic linear PAA matrices were not successful; some precipitation has been observed .
The sols were studied using indirect technique for nanosystem characterization, namely UV-vis spectroscopy and two scattering methods: QELS and SAXS.
Size characteristics of sols calculated using Mie theory
Some deviations of the theoretical curves from the experimental ones were observed (Fig. 2). The most probable cause is that nanoparticles were not entirely spherical in form, as described in the theoretical model. But the average diameters of AgNPs estimated from the theoretical curves proved to be very close to the ones evaluated from TEM images in our previous work .
Statistical analysis of size distribution curves obtained by QELS data analysis
R h, Å
(at peak maximum)
St. error, Å
The peaks in the range of 200–1000 Å can correspond to the aggregates of AgNPs as well as to the macromolecules of polymer matrices (Table 1). QELS results are in good agreement with the UV-vis results excluding the peak of AgNP aggregates and macromolecules.
SAXS Data Analysis
For the analysis of experimental data, the following methods were used: Guinier plot, size distribution function plot, and fitting of the obtained scattering curves. For Guinier plot and for the size distribution function plot, the PRIMUS program from the software package ATSAS was applied [18, 19]. Experimental curve fitting was provided using SASVIEW program .
The radii of gyration (R g) of AgNPs from Guinier plot
R g, Å
qR g limits
70 ± 2
134 ± 33
This function depends both on the particle’s geometry, expressing numerically the set of distances joining the volume elements within a particle, and on a particle’s inner inhomogeneity distribution.
The gyration radius and maximum particle size in the sols (from size distribution functions)
R g, Å
R max, Å
Here, R max is the largest distance between the volume elements within a particle.
Summary results of SAXS analysis
R g, Å
R g, Å
70 ± 2
134 ± 33
Some fitting inaccuracies occurred on Fig. 6a, b in the range of small q values. It can be caused by the interaction between scattering particles in the aggregates. The fit model does not take it into account. However, the dimensions obtained after the fitting of SAXS results are in good agreement with the size characteristics derived by other methods. This fact indicates the correction of the fit model.
Table 6 joins all parameters obtained from SAXS analysis.
SAXS analysis demonstrates monomodal scatterer size distribution in both sols in contrast to QELS and UV-vis spectroscopy, where multimodal particle size distribution is observed. Such contradiction may be caused by the ability of QELS and UV-vis to register large particles or aggregates within the range 300–600 Å. SAXS data analysis is accurate in the limited q-range value, 0.02 Å−1 < q <0.4 Å−1. Thus, large particles and aggregates are “invisible” for the q values we used.
Size parameters of the nanoparticles estimated correctly by UV-vis, QELS, and SAXS have been marked by italic font within Tables 2, 3, 4, and 5. These values appeared to be close for all techniques. Three different indirect methods also reveal the similar difference in size distribution in nanosystems synthesized in nonionic and anionic branched polymer matrices. The reason for such distinction is the various chemical nature of the polymer template affecting on the nucleation process in the process of nanoparticle formation.
The TEM investigation of silver sols has shown that most AgNPs synthesized in the solution of nonionic branched polymer matrices D-g-PAA had sizes in the range of 8–15 nm. The small number of aggregates was observed too. Silver sols synthesized in branched anionic polymer matrices D-g-PAA(PE) along with NPs have a size of 10–15 nm, i.e., the same as in the sols synthesized in the nonionic polymer matrix. Nanoparticles with a size of 2–5 nm and some large aggregates were observed.
Thus, the present work confirmed the validation of UV-vis spectroscopy and scattering methods for accurate investigation of sols. But UV-vis and SASX are limited for characterization of polydispersed nanosystems and should be used in combination with QELS or TEM.
The present study proved the efficiency of using branched nonionic and anionic polymers as matrices for the stable silver sols preparation. It was demonstrated that the chemical nature of polymer matrix (uncharged or charged) and the polymer internal structure affect the nanoparticles’ actual control on the sol size characteristics and nanoparticle size distribution in the nanosystems. The analysis of the silver sols was performed using UV-vis spectroscopy, QELS, and SAXS. All methods used were in good agreement for the characterization of size distribution of small particles (less than 60 nm) in the sols. The polydispersity estimated by various methods was comparable. It was shown that for precise analysis of sols synthesized in polymer matrices all these techniques should be used simultaneously. It should be noted that nanoparticle aggregates and macromolecules of the polymer matrix can be characterized only by QELS.
We acknowledge support of this work by the MIPT “5Top100” program of the Ministry of Education and Science of the Russian Federation. This work benefitted from SasView software, originally developed by the DANSE project under NSF award DMR-0520547.
The authors are grateful to Dr. Michel Rawiso from Institute Charles Sadron (Strasbourg, France) for polymer sample characterization.
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