Composite nanoparticles consisting of polymer gels with incorporated silver nanoparticles have been synthesized. The synthesis comprises two main stages. Initially, monodisperse hydrogel particles with a controlled diameter of approximately 500 nm are obtained by N-isopropylacrylamide polymerization. Then, silver ions are reduced on the surface of the polymer network. Variations in the concentration ratio between reductants and silver nitrate make it possible to produce silver nanoparticles with sizes in a range of 10-30 nm and different packing densities on the gel particle surface. The resultant nanocomposites have been studied by transmission electron microscopy, spectrophotometry, and dynamic light scattering. Depending on the size and packing density of the silver nanoparticles on the polymer particle surface, the plasmon resonance of the nanocomposites varies in a range of 420-750 nm, which determines variations in the color of the colloid from yellow, orange, and red to blue and blue-green. After the inclusion of silver nanoparticles, nanogels of poly(N-isopropylacrylamide) retain their capability for thermosensitive phase transition with a lower critical mixing temperature of 31 °C.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3509

Khlebtsov, B.N.

Panfilova, E.V.

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 2, 02PCN04(4pp) (2013)   2304-1862/2013/2(2)02PCN04(4) 02PCN04-1  2013 Sumy State University Synthesis and Study of PNIPAM Nanogels Incorporated with Colloidal Silver  B.N. Khlebtsov1,*, E.V. Panfilova1  Institute of Biochemistry and Physiology of Plants and Microorganisms, 13 Entuziastov Pros., 410049 Saratov, Russian Federation  (Received 08 May 2013; published online 30 August 2013)  Composite nanoparticles consisting of polymer gels with incorporated silver nanoparticles have been synthesized. The synthesis comprises two main stages. Initially, monodisperse hydrogel particles with a controlled diameter of approximately 500 nm are obtained by N-isopropylacrylamide polymerization. Then, silver ions are reduced on the surface of the polymer network. Variations in the concentration ratio between reductants and silver nitrate make it possible to produce silver nanoparticles with sizes in a range of 10-30 nm and different packing densities on the gel particle surface. The resultant nanocomposites have been studied by transmission electron microscopy, spectrophotometry, and dynamic light scattering. Depending on the size and packing density of the silver nanoparticles on the polymer particle surface, the plasmon resonance of the nanocomposites varies in a range of 420-750 nm, which determines variations in the color of the colloid from yellow, orange, and red to blue and blue-green. After the inclusion of silver nanoparticles, nanogels of poly(N-isopropylacrylamide) retain their capability for thermosensitive phase transition with a lower critical mixing temperature of 31 °C.  Keywords: Nanogels, Silver nanoparticles, Swelling   PACS numbers: 62.23.St, 42.70.Jk                                                           * bkhl@ibppm.sgu.ru  1. INTRODUCTION  Polymer hydrogels are porous materials that are swelling but insoluble in water [1]. When gel particles in colloids have micron or submicron sizes, the gels are commonly referred to as microgels [2] or nanogels [3]. At present, particular interest is focused on nanogels formed from so-called smart polymers [4, 5], which can change their swelling index with changes in ambient parameters, such as temperature, pH, or chemical composition [1, 5-7]. Such colloids are widely used as the platforms for target drug delivery [2, 8-10], as well as for the production of biosensors [11, 12] and composite nanomaterials [13, 14]. There are many polymers, primarily substituted derivatives of polyacrylamide, the lower critical temperature of which lies in a temperature range of 9-85 °С [15, 16, 17]. One of the most extensivelystudied thermosensitive polymers is poly(N-isopropylacrylamide) (PNIPAM), the phase transition temperature of which is 31-32 °С. Composite nanostructures representing metal nanoparticle containing polymer gels occupy a special place among modified nanogels. This nano techn-?logical approach enables one to combine the unique optical properties of metal based colloids and the physicochemical features of nanogels in the same structure. For example, in [18], silver nanoparticles were coated with a poly(N-isopropylacrylamide) shells containing incorporated insulin. The resonance scattering of silver nanoparticles served for determin ing glucose concentration, while conformational transformation of nanogel macromolecules was used for controlled release of insulin. Another significant group of studies deals with thephotothermal properties of metal nanoparticles that are used as ingredients of composites [20-25]. Usually, the studied composites consist of gold nanorods exhibiting plasmon resonance in the region in which biological tissues and thermosensitive nanogels are transparent. The combination of photothermal heating of gold nanorods and the thermosensitive phase transition of a nanogel makes such structures efficient for laser photo-?hermolysis [20-22] and photosensitive delivery of drugs [25] or nanoparticles themselves [23]. Finally, studies [26, 27] have been recently published in which such nanocomposites were used to capture targeted molecules in the course of their investigation by giant Raman scattering (amplification on gold nanoparticles inside a gel). There are three basic laboratory approaches to obtaining composite materials consisting of metal particles and PNIPAM. The first one is based on radical precipitation polymerization on the surface of plasmon resonant nanoparticles. A much simpler method of nanocomposite production was proposed in [20]. Since PNIPAM particles are charged negatively at pH 3-10, they can adsorb any positively charged nanoparticles, e.g., gold nanorods, due to electrostatic interactions. Finally, the third method entails chemical synthesis of metal nanoparticles in a preformed gel. At present, this method is, as a rule, used for the synthesis of bulky samples [21, 28], and there are few publications con_ cerning the production of nanocomposites containing gold nanoparticles in this way [29]. This study is devoted to the development of a methodology for synt?-esis of PNIPAM nanogels containing silver nanop-?rticles of different sizes and investigtion of the optical properties of such nanocomposites in relation to their composition and temperature.  2. MATERIALS AND METHODS  The following reagents and materials were used in this study: silver nitrate ( 99.9 %), isopropylacrylamide  B.N. KHLEBTSOV, E.V. PANFILOVA PROC. NAP  2, 02PCN04(2013)   02PCN04-2 (NIPAM), NN-methylenebisacrylamide (MBA), acrylic acid, isoascorbic acid, sodium borohydride, ammonium persulfate (all produced by Sigma-Aldrich and used as received), compressed argon (99.99 %), and MilliQ water. Poly(N-isopropylacrylamide) gel particles were synthesized as was described in detail in our recent publication [30]. In a typical synthesis protocol, water (50 mL) was placed into the flask and NIPAM (0.5 g), MBA (0.04 g), and acrylic acid (30 mkL) were added. The flask was thermostated at 70 C, while the reaction mixture was stirred (300 rpm) for 30 min. Then, an aqueous 0.074 M ammonium persulfate solution (2 mL) was added in order to initiate polymerization. The reaction was performed at 70 C under argon for 4 h, during which the colorless solution became milk-white, which evidenced the formation of colloidal particles of PNIPAM. In order to obtain nanogels containing silver particles of different sizes, a 10 mg/mL AgNO3 solution (0.05, 0.1, 0.2, 0.5, 1, or 2 mL) was added to the nanogel (4 mL). A reaction mixture was incubated for 30 min at room temperature; then, a 100 mM ascorbic acid solution (10 mkL) was added, and the mixture was incubated for 20 min until it acquired a yellowish tint. At the final stage of the synthesis, water (20 mL) and a 25 mM sodium borohydride solution (50 mkL) were added to the colloid under intense stirring. During subsequent 5-10 min, the colloid acquired a yellow, orange, red, blue, or blue-green color, depending on the amount of silver nitrate added to the system. The resultant composite nanogels were centrifuged at 4000 rpm for 5 min and redispersed in water (10 mL). This procedure was repeated five times to completely remove unbound particles. The samples were studied by transmission electron microscopy (TEM) using a Libra 120 microscope (Carl Zeiss) The geometrical parameters of the nanoc-?mposites and the temperature dependence of the diameter of swollen gel particles were determined using a Malvern Zetasizer Nano ZS system for dynamic light scattering under standard conditions of signal accumulation and processing. Absorbance spectra of the samples were measuredin 2-mm quartz cells using a Specord 250 spectrophotometer (Analytik Jena) in a wavelength range of 320-1000 nm.  3. RESULTS AND DISCUSSION  3.1 Nanomposite Sinthesis  The protocol of the synthesis of PNIPAM gel particles based on N-isopropylacrylamide polym-?rization is well known and has been described in many publications [10, 16, 31, 32]. According to [20], the concentrations of the monomers and polymerization initiator used in this study ensure the formation of a monodisperse colloid with a particle size of about 500 nm and their number con centration of about 7.5  108 mL – 1. Fig. 1 shows the TEM images of the synthesized PNIPAM nanogel. The analysis of the images has shown that the gel particle size is 360  20 nm. It should be noted that this size value refers to the sample dried in vacuum, and it can be significantly different from the gel particle size in an aqueous medium.   Fig. 1 – TEM image of PNIPAM gels particles  The second stage of the composite nanogel synthesis is the targeted reduction of silver ions on the surface of the polymer network. The key moment of this pro cedure is the creation of the conditions for preferable silver reduction inside or on the surface of polymer gel particles rather than in the free bulk (dispersion medium). A similar problem is relevant to the synthe sis of metal nanoshells on dielectric cores [33, 34]. Positively charged silver ions are adsorbed due to their electrostatic interaction with the dissociated carboxyl groups of acrylic acid added as a comon-?merinto the reaction mixture in the course of nanogel synthesis. The yellowish tint, which the AgNO3 containing dispersion acquires in a time after the addition of ascorbic acid, is associated with the reduction of Ag + ions; the nucleation of metal nanoparticles starts already during the 20 min incubation of the dispersion containing gel particles, silver nitrate, and ascorbic acid. The subsequent addition of sodium borohydride leads to more complete and rapid reduction of silver ions observed as the formation of larger metal particles (as large as 30 nm). In addition, we have experimentally revealed that the system dilution before the addition of sodium borohydride yields colloids with different colors. The formation of silver particles on the surface of gel particles (confirmed by TEM analysis) evidences that silver ions do remain on the surface and, possibly, inside the gel particles for, at least, several seconds, which are necessary for the addition of the reductant. The subsequent washing of the composite nanogel by centrifugation at a low acceleration and redispersion in water almost completely removes free silver particles. According to the ratios between the optical den sities of supernatants and redispersed sediments of particles, the fraction of the precursor adsorbed by gel particles varies from 10 to 70 % for different samples. Fig. 2 depicts the survey and magnified TEM images of composite nanogels synthesized by the reduction of silver ions at AgNO3 / gel ratios of  (a) 0.125, (b) 0.5, and (c) 2.5 mg/mL. Obviously, silverparticles formed on the polymer network have different sizes and packing densities depending on the amount of silver nitrate added to the gel.  Obviously, silver particles formed on the polymer network have differ ent sizes and packing densities depending on the amount of silver nitrate added to the   SYNTHESIS AND STUDY OF PNIPAM NANOGELS INCORPORATED… PROC. NAP 2, 02PCN04 (2013)   02PCN04-3      Fig. 2 – Survey and (insets) magnified TEM images of composite nanogels obtained by the reduction of silver ions on the surface of PNIPAM nanogel at AgNO3 / nanogel ratios of (a) 0.125, (b) 0.5, and (c) 2.5 mg / mL  gel. In particular, at the lowest concentration of silver nitrate (Fig. 2a; 10 mg/mL AgNO3 solution (0.05 mL) and nanogel, (4 mL), the average particle size is 10 nm, while the interparticle distance is much larger. A densely packed array of silver particles with an average size of 30 nm is formed at the highest concentration of silver nitrate (Fig. 2c).   3.2 Extinction Spectra of the Composite  Nanogels  Fig. 3 illustrates the experimental extinction spectra of the composite nanogels containing different amounts of silver nanoparticles with different sizes. At the lowest amount of the reduced silver, the spectrum has a maximum at 420 nm (Fig. 3, curve 1). The shape of the spectrum and the half-width of the band with the maximum at 420 nm somewhat differ from those observed for typical silver colloids and correspond to the superposition of the absorbance spectra of small silver nanoparticles and submicron particles of the PNIPAM gel. The situation changes with an increase in the size and packing density of silver nanoparticles    Fig. 3 – Extinction spectra of composite nanogels obtained by the reduction of a 10 mg/mL AgNO3 solution ((1) 0.05, (2) 0.1, (3) 0.2, (4) 0.5, and (5) 1 mL) in the presence of PNIPAM nanogel (4 mL)  occurring on the polymer network (Fig. 3, curves 2-5). The growth of the amount of reduced silver nitrate leads to the appearance of a second maximum (or an inflection) in the composite gel spectrum in addition to the short wavelength maximum at approximately 420 nm, with the new maximum corresponding to the optically interfering particles and collective electron oscillation. The position of this maximum shifts from 520 to 750 nm with the growth of the packing density of silver nano particles.   3.3 Temperature Dependent Phase Transition in the Nanogels  Temperature dependence of the diameter of swollen gel particles was investigated by dynamic light scattering. Fig. 4 exhibits the temperature depen-?ences of the diameters of swollen particles for a PNIPAM gel and a composite nanogel in a range of 20-45 C. It can be seen (curve 1) that the average hydro-dynamic size of PNIPAM particles is 520 nm at 20-25 C and 270 nm at 35-45 C. These data confirm the above assumption that the size of the gel particles (about 400 nm) determined by TEM is “intermediate” or, to be more precise, somewhat underestimated, because dried particles have been examined with the microscope. The critical temperature of the polymer nanogel shrinkage is 31 C.    Fig. 4 – Temperature dependences of hydrodynamic particle diameters of (1) PNIPAM nanogel and (2) composite nanogel containing silver nanoparticles  Fig. 4 also shows a similar temperature dependence of the particle diameter of a composite nanogel containing silver nanoparticles with a diameter of 10 nm (TEM images of this sampleare depicted in Fig. 2a, while the spectral characteristic is shown in Fig. 3, curve 1). In this case, we also observe a  B.N. KHLEBTSOV, E.V. PANFILOVA PROC. NAP  2, 02PCN04(2013)   02PCN04-4 conformational transformation of the polymer macromolecule with temperature; however, the average particle diameter is somewhat larger, with the difference being more pronounced for the hydrophobic shrunk form. We assume that this difference is associated with the incorporated silver nanoparticles which hinder the free shrinkage of the polymer net work. It should be noted that, in the case of silver nanoparticles densely packed on the gel particle surface (TEM data are given in Fig. 2a and the spectralcharacteristic is shown in Fig. 3, curve 5), we have not revealed a significant shrinkage of the particles, and this fact additionally supports our assumption. In order to verify the stability of the dispersion composite particles upon the phase transition in thepolymer, we measured the hydrodynamic radii of the particles during 5 swelling / shrinkage cycles (Fig. 5).    Fig. 5 – Change in the hydrodynamic diameter of particles of (1) PNIPAM nanogel and (2) composite PNIPAM / Ag nanogel during five cycles of heating to 45°C followed by cooling to 20 °C  It can be seen that the five passages through the critical point upon the system heating and the five passages during its cooling do not lead to particle aggregation in either PNIPAM gel or composite nanogel containing silver nanoparticles.  4. CONCLUSIONS  In this study, composite particles consisting of polyisopropylacrylamide gel particles with a diameter of about 500 nm and incorporated colloidal silver have been synthesized. Variations in reagent concentrations make it possible to produce silver nanoparticles with different sizes and packing densities on the surface of the polymer particles. Concerning the optical properties of the composite nanogels, it should be noted that, when the silver nanoparticles are not densely packed on the surface, the colloid has one maximum of plasmon resonance near 420 nm, which is typical of silver nanoparticles. The optical interaction between more densely packed silver particles leads to a split of plasmon resonance into two bands and a shift of the collective plasmon resonance to the near IR region. Composite nanogels containing silver nanoparticles exhibit a thermosensitive phase transition with the critical temperature of polymer network shrinkage of ; their dispersions remain stable for at least five cycles of polymer network swelling / shrinkage.  AKNOWLEDGEMENTS  This study was supported by the Russian Founda tion for Basic Research (projects nos. 11_02_00128a and 12_02_00397a), Programs of Presidium of Russian Academy of Sciences ”Basic Sciences for Medicine” and “Basic Foundations of Technologies of Nanostructures andNanomaterials.”  REFERENCES  1. L.I. Valuev, T.A. Valueva, I.L. Valuev, N.A. Plat, Usp. Biol. Khim 43, 307 (2003). 2. T.T. Chastek, A. Wadajkar, K.T. Nguyen, S.D. Hudson, T.Q. Chastek, Colloid Polym. Sci. 288, 105 (2010). 3. W.H. Blackburn, L.A. Lyon, Colloid Polym. Sci. 286, 563 (2008). 4. Y. Sasaki, K. Akiyoshi, The Chemical Record 10, 366 (2010). 5. W. Wu, T. Zhou, A. Berliner, P. Banerjee, Sh. Zhou, Chem. Mater. 22, 1966 (2010). 6. Y. Chen, et.al., ACS Appl. Mater. Interfaces 2, 3532 (2010). 7. C. Chang, J. Biomater. Sci. 18, 1591 (2007). 8. K. Raemdonck, J. Demeester, S. de Smedt, Soft Matter. 5, 707 (2009). 9. J.K. Oh, et. al., J. Am. Chem. Soc. 129, 5939 (2007). 10. J.H. Ryu, et. al., J. Am. Chem. Soc. 132, 17227 (2010). 11. Ch. Pietschab, U.S. Schubert, R. Hoogenboom., Chem. Commun. 47, 8750 (2011). 12. W. Wu, T. Zhou, Sh. Zhou., Chem. Mater. 21, 2851 (2009). 13. Ch. Reese, A.V. Mikhonin, M. Kamenjicki, A. Tikhonov, S. Asher., J. Am. Chem. Soc, 126, 1493 (2004). 14. H. Gehan et. al., J. Phys. Chem. Lett. 2, 926 (2011). 15. K. Shouei, K. Kubota, I. Ando, J. Phys. Chem. 93, 3311 (1989). 16. R. Pelton, P. Chibante, Colloids Surf. 20, 247 (1986). 17. A.A Tager, Fiziko-khimiya polimerov (Moscow: Khimiya, 1978). [Russian] 18. W. Wu, N. Mitra, E. Yan, Sh. Zhou, ACS Nano 4, 4831 (2004). 19. Ch. Gota, K. Okabe, T. Funatsu, Y. Harada, S. Uchiyama, J. Am. Chem. Soc. 131, 2766 (2009). 20. I. Gorelikov, L.M. Field, E. Kumacheva, J. Am. Chem. Soc. 126, 15938 (2004). 21. A. Shiotani, et. al, Langmuir 23, 4012 (2007). 22. T. Nakamura, et. al., Nanoscale 2, 739 (2010). 23. A. Shiotani, et. al., Bioconjug. Chem. 21, 2049 (2010). 24. M. Karg, et. al., Langmuir 25, 3163 (2009). 25. M. Karg, et. al., ACS Nano 6, 5094 (2011). 26. R. Contreras-Caceres, et. al., Langmuir 27, 4520 (2011). 27. R.A. Alvarez-Puebla, L.M. Liz-Marzan, Chem. Soc. Rev. 41, 43 (2012). 28. X. Lian, J. Jin, J. Tian, H. Zhao, ACS Appl. Mater. Inter-faces 8, 2261 (2010). 29. J.H. Kim, T.R. Lee, Langmuir 23, 6504 (2007). 30. E.V. Panfilova, B.N. Khlebtsov, A.M. Burov, N.G. Khlebtsov, Kolloidn. Zh. 74, 104 (2012). 31. R. Pelton, Adv. Colloid Interface Sci. 85, 1 (2000). 32. I. Mejac, C.D. Tran, Anal. Chem. 83, 3520 (2011). 33. B.E. Brinson, J.B. Lassiter, C.S. Levin, R. Bardhan, N. Mirin, N.J. Halas, Langmuir 24, 14166 (2009). 34. O.V. Dement’eva, M.A. Filippenko, M.E. Kartseva, V.M. Rudoy, Kolloidn. Zh. 71, 569 (2009).  

Synthesis and Study of PNIPAM Nanogels Incorporated with Colloidal Silver

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 2, 02PCN04(4pp) (2013) 
 
 
2304-1862/2013/2(2)02PCN04(4) 02PCN04-1  2013 Sumy State University 
Synthesis and Study of PNIPAM Nanogels Incorporated with Colloidal Silver 
 
B.N. Khlebtsov1,*, E.V. Panfilova1 
 
Institute of Biochemistry and Physiology of Plants and Microorganisms, 13 Entuziastov Pros., 410049 Saratov, 
Russian Federation 
 
(Received 08 May 2013; published online 30 August 2013) 
 
Composite nanoparticles consisting of polymer gels with incorporated silver nanoparticles have been 
synthesized. The synthesis comprises two main stages. Initially, monodisperse hydrogel particles with a 
controlled diameter of approximately 500 nm are obtained by N-isopropylacrylamide polymerization. Then, 
silver ions are reduced on the surface of the polymer network. Variations in the concentration ratio 
between reductants and silver nitrate make it possible to produce silver nanoparticles with sizes in a range 
of 10-30 nm and different packing densities on the gel particle surface. The resultant nanocomposites have 
been studied by transmission electron microscopy, spectrophotometry, and dynamic light scattering. 
Depending on the size and packing density of the silver nanoparticles on the polymer particle surface, the 
plasmon resonance of the nanocomposites varies in a range of 420-750 nm, which determines variations in 
the color of the colloid from yellow, orange, and red to blue and blue-green. After the inclusion of silver 
nanoparticles, nanogels of poly(N-isopropylacrylamide) retain their capability for thermosensitive phase 
transition with a lower critical mixing temperature of 31 °C. 
 
Keywords: Nanogels, Silver nanoparticles, Swelling 
 
 PACS numbers: 62.23.St, 42.70.Jk 
 
 
                                                        
* bkhl@ibppm.sgu.ru  
1. INTRODUCTION 
 
Polymer hydrogels are porous materials that are 
swelling but insoluble in water [1]. When gel particles 
in colloids have micron or submicron sizes, the gels are 
commonly referred to as microgels [2] or nanogels [3]. 
At present, particular interest is focused on nanogels 
formed from so-called smart polymers [4, 5], which can 
change their swelling index with changes in ambient 
parameters, such as temperature, pH, or chemical 
composition [1, 5-7]. Such colloids are widely used as 
the platforms for target drug delivery [2, 8-10], as well 
as for the production of biosensors [11, 12] and 
composite nanomaterials [13, 14]. There are many 
polymers, primarily substituted derivatives of 
polyacrylamide, the lower critical temperature of which 
lies in a temperature range of 9-85 °С [15, 16, 17]. One 
of the most extensivelystudied thermosensitive 
polymers is poly(N-isopropylacrylamide) (PNIPAM), 
the phase transition temperature of which is 31-32 °С. 
Composite nanostructures representing metal 
nanoparticle containing polymer gels occupy a special 
place among modified nanogels. This nano techn-
?logical approach enables one to combine the unique 
optical properties of metal based colloids and the 
physicochemical features of nanogels in the same 
structure. For example, in [18], silver nanoparticles 
were coated with a poly(N-isopropylacrylamide) shells 
containing incorporated insulin. The resonance 
scattering of silver nanoparticles served for determin 
ing glucose concentration, while conformational 
transformation of nanogel macromolecules was used for 
controlled release of insulin. Another significant group 
of studies deals with thephotothermal properties of 
metal nanoparticles that are used as ingredients of 
composites [20-25]. Usually, the studied composites 
consist of gold nanorods exhibiting plasmon resonance 
in the region in which biological tissues and 
thermosensitive nanogels are transparent. The 
combination of photothermal heating of gold nanorods 
and the thermosensitive phase transition of a nanogel 
makes such structures efficient for laser photo-
?hermolysis [20-22] and photosensitive delivery of 
drugs [25] or nanoparticles themselves [23]. Finally, 
studies [26, 27] have been recently published in which 
such nanocomposites were used to capture targeted 
molecules in the course of their investigation by giant 
Raman scattering (amplification on gold nanoparticles 
inside a gel). 
There are three basic laboratory approaches to 
obtaining composite materials consisting of metal 
particles and PNIPAM. The first one is based on radical 
precipitation polymerization on the surface of plasmon 
resonant nanoparticles. A much simpler method of 
nanocomposite production was proposed in [20]. Since 
PNIPAM particles are charged negatively at pH 3-10, 
they can adsorb any positively charged nanoparticles, 
e.g., gold nanorods, due to electrostatic interactions. 
Finally, the third method entails chemical synthesis of 
metal nanoparticles in a preformed gel. At present, this 
method is, as a rule, used for the synthesis of bulky 
samples [21, 28], and there are few publications con_ 
cerning the production of nanocomposites containing 
gold nanoparticles in this way [29]. This study is 
devoted to the development of a methodology for synt?-
esis of PNIPAM nanogels containing silver nanop-
?rticles of different sizes and investigtion of the optical 
properties of such nanocomposites in relation to their 
composition and temperature. 
 
2. MATERIALS AND METHODS 
 
The following reagents and materials were used in 
this study: silver nitrate ( 99.9 %), isopropylacrylamide 
 B.N. KHLEBTSOV, E.V. PANFILOVA PROC. NAP  2, 02PCN04(2013) 
 
 
02PCN04-2 
(NIPAM), NN-methylenebisacrylamide (MBA), acrylic 
acid, isoascorbic acid, sodium borohydride, ammonium 
persulfate (all produced by Sigma-Aldrich and used as 
received), compressed argon (99.99 %), and MilliQ 
water. Poly(N-isopropylacrylamide) gel particles were 
synthesized as was described in detail in our recent 
publication [30]. In a typical synthesis protocol, water 
(50 mL) was placed into the flask and NIPAM (0.5 g), 
MBA (0.04 g), and acrylic acid (30 mkL) were added. 
The flask was thermostated at 70 C, while the reaction 
mixture was stirred (300 rpm) for 30 min. Then, an 
aqueous 0.074 M ammonium persulfate solution (2 mL) 
was added in order to initiate polymerization. The 
reaction was performed at 70 C under argon for 4 h, 
during which the colorless solution became milk-white, 
which evidenced the formation of colloidal particles of 
PNIPAM. In order to obtain nanogels containing silver 
particles of different sizes, a 10 mg/mL AgNO3 solution 
(0.05, 0.1, 0.2, 0.5, 1, or 2 mL) was added to the nanogel 
(4 mL). A reaction mixture was incubated for 30 min at 
room temperature; then, a 100 mM ascorbic acid 
solution (10 mkL) was added, and the mixture was 
incubated for 20 min until it acquired a yellowish tint. 
At the final stage of the synthesis, water (20 mL) 
and a 25 mM sodium borohydride solution (50 mkL) 
were added to the colloid under intense stirring. During 
subsequent 5-10 min, the colloid acquired a yellow, 
orange, red, blue, or blue-green color, depending on the 
amount of silver nitrate added to the system. The 
resultant composite nanogels were centrifuged at 
4000 rpm for 5 min and redispersed in water (10 mL). 
This procedure was repeated five times to completely 
remove unbound particles. 
The samples were studied by transmission electron 
microscopy (TEM) using a Libra 120 microscope (Carl 
Zeiss) The geometrical parameters of the nanoc-
?mposites and the temperature dependence of the 
diameter of swollen gel particles were determined 
using a Malvern Zetasizer Nano ZS system for dynamic 
light scattering under standard conditions of signal 
accumulation and processing. Absorbance spectra of 
the samples were measuredin 2-mm quartz cells using 
a Specord 250 spectrophotometer (Analytik Jena) in a 
wavelength range of 320-1000 nm. 
 
3. RESULTS AND DISCUSSION 
 
3.1 Nanomposite Sinthesis 
 
The protocol of the synthesis of PNIPAM gel 
particles based on N-isopropylacrylamide polym-
?rization is well known and has been described in many 
publications [10, 16, 31, 32]. According to [20], the 
concentrations of the monomers and polymerization 
initiator used in this study ensure the formation of a 
monodisperse colloid with a particle size of about 
500 nm and their number con centration of about 
7.5  108 mL – 1. Fig. 1 shows the TEM images of the 
synthesized PNIPAM nanogel. The analysis of the 
images has shown that the gel particle size is 
360  20 nm. It should be noted that this size value 
refers to the sample dried in vacuum, and it can be 
significantly different from the gel particle size in an 
aqueous medium. 
 
 
Fig. 1 – TEM image of PNIPAM gels particles 
 
The second stage of the composite nanogel synthesis 
is the targeted reduction of silver ions on the surface of 
the polymer network. The key moment of this pro 
cedure is the creation of the conditions for preferable 
silver reduction inside or on the surface of polymer gel 
particles rather than in the free bulk (dispersion 
medium). A similar problem is relevant to the synthe 
sis of metal nanoshells on dielectric cores [33, 34]. 
Positively charged silver ions are adsorbed due to 
their electrostatic interaction with the dissociated 
carboxyl groups of acrylic acid added as a comon-
?merinto the reaction mixture in the course of nanogel 
synthesis. The yellowish tint, which the AgNO3 
containing dispersion acquires in a time after the 
addition of ascorbic acid, is associated with the 
reduction of Ag + ions; the nucleation of metal 
nanoparticles starts already during the 20 min 
incubation of the dispersion containing gel particles, 
silver nitrate, and ascorbic acid. The subsequent 
addition of sodium borohydride leads to more complete 
and rapid reduction of silver ions observed as the 
formation of larger metal particles (as large as 30 nm). 
In addition, we have experimentally revealed that the 
system dilution before the addition of sodium 
borohydride yields colloids with different colors. 
The formation of silver particles on the surface of 
gel particles (confirmed by TEM analysis) evidences 
that silver ions do remain on the surface and, possibly, 
inside the gel particles for, at least, several seconds, 
which are necessary for the addition of the reductant. 
The subsequent washing of the composite nanogel 
by centrifugation at a low acceleration and redispersion 
in water almost completely removes free silver 
particles. According to the ratios between the optical 
den sities of supernatants and redispersed sediments of 
particles, the fraction of the precursor adsorbed by gel 
particles varies from 10 to 70 % for different samples. 
Fig. 2 depicts the survey and magnified TEM 
images of composite nanogels synthesized by the 
reduction of silver ions at AgNO3 / gel ratios of  
(a) 0.125, (b) 0.5, and (c) 2.5 mg/mL. Obviously, 
silverparticles formed on the polymer network have 
different sizes and packing densities depending on the 
amount of silver nitrate added to the gel.  
Obviously, silver particles formed on the polymer 
network have differ ent sizes and packing densities 
depending on the amount of silver nitrate added to the  
 SYNTHESIS AND STUDY OF PNIPAM NANOGELS INCORPORATED… PROC. NAP 2, 02PCN04 (2013) 
 
 
02PCN04-3 
    
 
Fig. 2 – Survey and (insets) magnified TEM images of composite nanogels obtained by the reduction of silver ions on the surface 
of PNIPAM nanogel at AgNO3 / nanogel ratios of (a) 0.125, (b) 0.5, and (c) 2.5 mg / mL 
 
gel. In particular, at the lowest concentration of silver 
nitrate (Fig. 2a; 10 mg/mL AgNO3 solution (0.05 mL) 
and nanogel, (4 mL), the average particle size is 10 nm, 
while the interparticle distance is much larger. A 
densely packed array of silver particles with an average 
size of 30 nm is formed at the highest concentration of 
silver nitrate (Fig. 2c).  
 
3.2 Extinction Spectra of the Composite  
Nanogels 
 
Fig. 3 illustrates the experimental extinction 
spectra of the composite nanogels containing different 
amounts of silver nanoparticles with different sizes. At 
the lowest amount of the reduced silver, the spectrum 
has a maximum at 420 nm (Fig. 3, curve 1). The shape 
of the spectrum and the half-width of the band with the 
maximum at 420 nm somewhat differ from those 
observed for typical silver colloids and correspond to 
the superposition of the absorbance spectra of small 
silver nanoparticles and submicron particles of the 
PNIPAM gel. The situation changes with an increase 
in the size and packing density of silver nanoparticles 
 
 
 
Fig. 3 – Extinction spectra of composite nanogels obtained by 
the reduction of a 10 mg/mL AgNO3 solution ((1) 0.05, (2) 0.1, 
(3) 0.2, (4) 0.5, and (5) 1 mL) in the presence of PNIPAM 
nanogel (4 mL) 
 
occurring on the polymer network (Fig. 3, curves 2-5). 
The growth of the amount of reduced silver nitrate 
leads to the appearance of a second maximum (or an 
inflection) in the composite gel spectrum in addition to 
the short wavelength maximum at approximately 
420 nm, with the new maximum corresponding to the 
optically interfering particles and collective electron 
oscillation. The position of this maximum shifts from 
520 to 750 nm with the growth of the packing density 
of silver nano particles.  
 
3.3 Temperature Dependent Phase Transition 
in the Nanogels 
 
Temperature dependence of the diameter of swollen 
gel particles was investigated by dynamic light 
scattering. Fig. 4 exhibits the temperature depen-
?ences of the diameters of swollen particles for a 
PNIPAM gel and a composite nanogel in a range of 20-
45 C. It can be seen (curve 1) that the average hydro-
dynamic size of PNIPAM particles is 520 nm at 20-
25 C and 270 nm at 35-45 C. These data confirm the 
above assumption that the size of the gel particles 
(about 400 nm) determined by TEM is “intermediate” 
or, to be more precise, somewhat underestimated, 
because dried particles have been examined with the 
microscope. The critical temperature of the polymer 
nanogel shrinkage is 31 C. 
 
 
 
Fig. 4 – Temperature dependences of hydrodynamic particle 
diameters of (1) PNIPAM nanogel and (2) composite nanogel 
containing silver nanoparticles 
 
Fig. 4 also shows a similar temperature dependence 
of the particle diameter of a composite nanogel 
containing silver nanoparticles with a diameter of 
10 nm (TEM images of this sampleare depicted in 
Fig. 2a, while the spectral characteristic is shown in 
Fig. 3, curve 1). In this case, we also observe a 
 B.N. KHLEBTSOV, E.V. PANFILOVA PROC. NAP  2, 02PCN04(2013) 
 
 
02PCN04-4 
conformational transformation of the polymer 
macromolecule with temperature; however, the average 
particle diameter is somewhat larger, with the 
difference being more pronounced for the hydrophobic 
shrunk form. We assume that this difference is 
associated with the incorporated silver nanoparticles 
which hinder the free shrinkage of the polymer net 
work. It should be noted that, in the case of silver 
nanoparticles densely packed on the gel particle 
surface (TEM data are given in Fig. 2a and the 
spectralcharacteristic is shown in Fig. 3, curve 5), we 
have not revealed a significant shrinkage of the 
particles, and this fact additionally supports our 
assumption. In order to verify the stability of the 
dispersion composite particles upon the phase 
transition in thepolymer, we measured the 
hydrodynamic radii of the particles during 
5 swelling / shrinkage cycles (Fig. 5). 
 
 
 
Fig. 5 – Change in the hydrodynamic diameter of particles of 
(1) PNIPAM nanogel and (2) composite PNIPAM / Ag nanogel 
during five cycles of heating to 45°C followed by cooling to 
20 °C 
 
It can be seen that the five passages through the 
critical point upon the system heating and the five 
passages during its cooling do not lead to particle 
aggregation in either PNIPAM gel or composite 
nanogel containing silver nanoparticles. 
 
4. CONCLUSIONS 
 
In this study, composite particles consisting of 
polyisopropylacrylamide gel particles with a diameter 
of about 500 nm and incorporated colloidal silver have 
been synthesized. Variations in reagent concentrations 
make it possible to produce silver nanoparticles with 
different sizes and packing densities on the surface of 
the polymer particles. Concerning the optical 
properties of the composite nanogels, it should be noted 
that, when the silver nanoparticles are not densely 
packed on the surface, the colloid has one maximum of 
plasmon resonance near 420 nm, which is typical of 
silver nanoparticles. The optical interaction between 
more densely packed silver particles leads to a split of 
plasmon resonance into two bands and a shift of the 
collective plasmon resonance to the near IR region. 
Composite nanogels containing silver nanoparticles 
exhibit a thermosensitive phase transition with the 
critical temperature of polymer network shrinkage of ; 
their dispersions remain stable for at least five cycles of 
polymer network swelling / shrinkage. 
 
AKNOWLEDGEMENTS 
 
This study was supported by the Russian Founda 
tion for Basic Research (projects nos. 11_02_00128a 
and 12_02_00397a), Programs of Presidium of Russian 
Academy of Sciences ”Basic Sciences for Medicine” and 
“Basic Foundations of Technologies of Nanostructures 
andNanomaterials.”
 
 
REFERENCES 
 
1. L.I. Valuev, T.A. Valueva, I.L. Valuev, N.A. Plat, Usp. 
Biol. Khim 43, 307 (2003). 
2. T.T. Chastek, A. Wadajkar, K.T. Nguyen, S.D. Hudson, 
T.Q. Chastek, Colloid Polym. Sci. 288, 105 (2010). 
3. W.H. Blackburn, L.A. Lyon, Colloid Polym. Sci. 286, 563 
(2008). 
4. Y. Sasaki, K. Akiyoshi, The Chemical Record 10, 366 
(2010). 
5. W. Wu, T. Zhou, A. Berliner, P. Banerjee, Sh. Zhou, Chem. 
Mater. 22, 1966 (2010). 
6. Y. Chen, et.al., ACS Appl. Mater. Interfaces 2, 3532 (2010). 
7. C. Chang, J. Biomater. Sci. 18, 1591 (2007). 
8. K. Raemdonck, J. Demeester, S. de Smedt, Soft Matter. 5, 
707 (2009). 
9. J.K. Oh, et. al., J. Am. Chem. Soc. 129, 5939 (2007). 
10. J.H. Ryu, et. al., J. Am. Chem. Soc. 132, 17227 (2010). 
11. Ch. Pietschab, U.S. Schubert, R. Hoogenboom., Chem. 
Commun. 47, 8750 (2011). 
12. W. Wu, T. Zhou, Sh. Zhou., Chem. Mater. 21, 2851 (2009). 
13. Ch. Reese, A.V. Mikhonin, M. Kamenjicki, A. Tikhonov, 
S. Asher., J. Am. Chem. Soc, 126, 1493 (2004). 
14. H. Gehan et. al., J. Phys. Chem. Lett. 2, 926 (2011). 
15. K. Shouei, K. Kubota, I. Ando, J. Phys. Chem. 93, 3311 
(1989). 
16. R. Pelton, P. Chibante, Colloids Surf. 20, 247 (1986). 
17. A.A Tager, Fiziko-khimiya polimerov (Moscow: Khimiya, 
1978). [Russian] 
18. W. Wu, N. Mitra, E. Yan, Sh. Zhou, ACS Nano 4, 4831 
(2004). 
19. Ch. Gota, K. Okabe, T. Funatsu, Y. Harada, S. Uchiyama, 
J. Am. Chem. Soc. 131, 2766 (2009). 
20. I. Gorelikov, L.M. Field, E. Kumacheva, J. Am. Chem. Soc. 
126, 15938 (2004). 
21. A. Shiotani, et. al, Langmuir 23, 4012 (2007). 
22. T. Nakamura, et. al., Nanoscale 2, 739 (2010). 
23. A. Shiotani, et. al., Bioconjug. Chem. 21, 2049 (2010). 
24. M. Karg, et. al., Langmuir 25, 3163 (2009). 
25. M. Karg, et. al., ACS Nano 6, 5094 (2011). 
26. R. Contreras-Caceres, et. al., Langmuir 27, 4520 (2011). 
27. R.A. Alvarez-Puebla, L.M. Liz-Marzan, Chem. Soc. Rev. 
41, 43 (2012). 
28. X. Lian, J. Jin, J. Tian, H. Zhao, ACS Appl. Mater. Inter-
faces 8, 2261 (2010). 
29. J.H. Kim, T.R. Lee, Langmuir 23, 6504 (2007). 
30. E.V. Panfilova, B.N. Khlebtsov, A.M. Burov, 
N.G. Khlebtsov, Kolloidn. Zh. 74, 104 (2012). 
31. R. Pelton, Adv. Colloid Interface Sci. 85, 1 (2000). 
32. I. Mejac, C.D. Tran, Anal. Chem. 83, 3520 (2011). 
33. B.E. Brinson, J.B. Lassiter, C.S. Levin, R. Bardhan, 
N. Mirin, N.J. Halas, Langmuir 24, 14166 (2009). 
34. O.V. Dement’eva, M.A. Filippenko, M.E. Kartseva, 
V.M. Rudoy, Kolloidn. Zh. 71, 569 (2009). 
 


English

https://essuir.sumdu.edu.ua/bitstream/123456789/35090/1/princon_2013_2_2_6.pdf

Synthesis and Study of PNIPAM Nanogels Incorporated with Colloidal Silver

Abstract

Similar works

Full text

Available Versions

Electronic Sumy State University Institutional Repository

SocioEconomic Challenges Journal (Sumy State University)