In situ synthesis of poly(N-isopropylacrylamide) decorated with silver nanoparticles using pulsed electrical discharge in contact with water interface

Abstract Currently, the combination between metal nanoparticles and polymers have gathered some attention, especially in the case of a polymer like poly(N-Isopropylacrylamide) (PNIPAM) which is a temperature-responsive polymer that changes from hydrophilic to hydrophobic at around 32 °C. This property together with the innate properties of silver nanoparticles (e.g. thermal and antimicrobial activity) poses an interesting option for a nanocomposite that can be used as drug carrier, tissue scaffold, and many other biomedical applications. In-situ synthesis performed by pulsed discharges has barely been reported nor has been fully researched. In this work, in-situ synthesis was performed by variating the ratio of NIPAM and silver nitrate solution, resulting in square shaped nanoparticles with sizes in the range of 50 to 500 nm distributed in the polymer matrix and sphere-shaped nanoparticles entangled in the polymer matrix. These results demonstrate an interesting and novel synthesis process for nanomaterial composites with future biomedical applications. Graphical Abstract


Introduction
Different methods for the synthesis of nanocomposites based on polymers and metal nanoparticles have been investigated. Particularly, thermo-sensitive nanomaterials stand out for their ability to change their behavior depending on the temperature, poly(N-isopropylacrylamide) (PNIPAM) being one of the most researched polymers that belong to this category. This polymer has been used in the biomedical area for diagnostic devices, imaging tests enhancers, and drug-delivery. 1 These applications combined with the intrinsic antimicrobial activity of silver nanoparticles, 2,3 offers a broader list of biomedical applications including wound dressing and tissue engineering. [4][5][6][7][8][9] However, the dispersion of nanoparticles homogeneously into the polymer matrix or to be covered by it still represents a challenge due to the behavior of the nanoparticles to gather with each other which leads to the increase in size. 10 In order to address this concern, polymers have been used for stabilization and capping agent for the nanoparticles adding surface functionalization. Different methods for the synthesis of nanocomposites between polymers and metal nanoparticles have been investigated and overall, these methods can be classified in two routes: In situ and the ex situ route.
Nanocomposites synthesized by the in situ route consist of either the polymerization of monomers in the presence of dispersed nanoparticles in the monomeric solution, the metal ion reduction into the polymer matrix or the metal ion reduction and polymerization simultaneously. Hangxun Xu et al. were successful in the in situ synthesis of silver nanoparticles dispersed into PNIPAM by carrying the reduction of silver nitrate inside an aqueous solution of PNIPAM by sodium borohydride under rigorous stirring, followed by a purification process 11 ; a similar process was used by the group of Morones et al., they performed the reduction of the silver salt inside the polymer matrix. However, instead of immediately going to the purification process, they precipitated the Ag-PNIPAM by performing a centrifugation step after the solution was heated to 40 C therefore obtaining silver nanoparticles capped with a PNIPAM shell. 12 Another report stated that hydrogels discs were prepared through the free radical aqueous polymerization of NIPAM in the presence of the N, N'-methylene bisacrylamide (MB) cross-linker with potassium persulfate (KPS) as initiator, and were subsequently used for the deposition of silver ions from the solution of silver nitrate followed by the addition of sodium borohydride, finally obtaining a silver nanoparticles loaded PNIPAM gel. 13 There was a report in the synthesis of silver nanoparticles embedded shell cross-linked hybrid PNIPAM nanocapsules by a two-step grafting process starting with the crosslinking of PNIPAM and finishing with the in-situ synthesis of silver nanoparticles in the nanocapsules. 14 A different method, electrical discharges, have played an important role in the synthesis of nanomaterials, there are many reports in the synthesis of nanoparticles and polymers by continuous electrical discharges under different gas atmospheres. [15][16][17] Nonetheless, throughout the years, it has been disclosed that continuous wave discharges often leads to overheating and low energy efficiency. Therefore, by using pulsed power excitation high instantaneous power density and reduced electric field can be provided and moreover enhancing the power efficiency and allowing better control of the reactions produced from the plasma. 18,19 An example of a metal nanoparticles-polymer composite by the in situ synthesis route by using room temperature atmospheric pressure plasma was presented by Zhang et al. where all the precursors where first mixed before applying a continuous microplasma (voltage: 0.8-2 kV, current: 5 mA) in the presence of helium gas for different discharge times: 2, 5, 10, and 20 min. 20 The majority of these reports are a clear example of the in situ route in where the metal ion reduction takes place inside the already polymerized matrix.
In our previous work, the synthesis of PNIPAM by pulsed arc discharge was proven successful, therefore, in this work, it was hypothesized that by using the species generated from the synthesis of the PNIPAM and an azo amidine initiator as a reducing agent for silver nitrate, could provide a PNIPAM decorated with silver nanoparticles while adjusting the ratio and discharge conditions, the morphology could be changed. 19

Synthesis via pulsed arc discharge
A consistent amount of initiator concentration was used (1 wt%) to 100 mL solution of NIPAM monomer (0.89 M) and silver nitrate (1.0 mM). Three different ratios of silver nitrate to monomer were tried: 50/50, 60/40, and 75/25, hereon referred as to v1, v2, and v3, respectively, increasing the silver nitrate and reducing the monomer ratio. Afterwards, 10 mL of the mixed solution were introduced in the reactor for applying the electrical discharge at 1, 40, and 50 pulses per second (pps) for 40, 10, and 1 s respectively. Details of the equipment and configuration are presented elsewhere. 19

Characterization
Presence of silver nanoparticles was confirmed by ultraviolet-visible absorption spectroscopy (GENESYS 10S UV-Vis Spectrometer, Thermo Fisher Scientific, USA) in disposable cuvettes in a range from 280 to 800 nm. Morphology and size of the composite was analyzed with transmission electron microscope (TEM, JEM-1400 plus), analysis of the particle size of the particles was performed using the graphic design program ImageJ dedicated to analyze images; the samples were prepared under atmospheric temperature and pressure. The static contact angle data was collected using the h/2 method by depositing a drop of the sample over a square glass substrate as in the previous report. 19 After the mentioned analyses were performed, samples were left in an oven to dry overnight at 40 C in petri dishes followed by Fourier-transform infrared spectroscopy (FTIR, Frontier: PerkinElmer) in ATR (Universal) mode in the range of 640 to 400 cm À1 .

Results and discussion
In Figure 1, the results of UV-vis and FTIR analysis are presented. In the literature, it is well known that the characteristic peak of silver nanoparticles in the Uv-vis spectra appears from 380 to 420 nm. 21,22 The same peak at 360 nm appeared in all samples  indicating the presence of silver nanoparticles with similar morphology. Furthermore, in order to confirm this, silver nitrate too was analyzed presenting a peak at around 300 nm. Following, FTIR analysis was conducted to confirm the synthesis of PNIPAM. It is thought that since the condition of 1 pps for 40 s uses slower pulsation rate, one of the two processes (synthesis of silver nanoparticles and polymer) occurs at a faster rate which is more likely to be the formation of silver nanoparticles by increasing its ratio. In other conditions, additional peaks to those that are characteristic to PNIPAM appeared at around 2970 cm À1 which may indicate the presence of aliphatic compounds formed during the growth of the silver nanoparticles and the propagation step of the polymerization. Since the ratio of NIPAM monomer was reduced, it was also important to confirm that the polymer still had its characteristic hydrophobic behavior. Figure 2 shows all the samples at room temperature and at a temperature above their lower critical solution temperature (LCST) presenting a white portion confirming their hydrophobic behavior.
Finally, TEM analysis was conducted to study the morphology of the synthesized composite, the result can be seen in Figure 3. It is interesting to notice that at 50/50 and 75/25 the morphology of the silver nanoparticles was more inclined to a square like morphology rather than a sphere like one.
Moreover, in Figure 3g, because of its color, morphology, and size it is thought to be an agglomeration of silver rather than an agglomeration of nanoparticles. As it was expected, higher pulsation rate results in smaller average particle size (downleft rectangle in the TEM images) even when the ratio was variated. On the other hand, maintaining the discharge condition at 4 pps for 10 s, which is not high neither low, it shows that silver nanoparticles are entangled in a PNIPAM net, are well disperse and similar in size. In comparison, silver nanoparticles with smaller or almost uniform size can be obtained with 50 pps, however, their agglomeration is more evident and it seems to favor square like morphology.
There is an interest in the utilization of this composite as a wound dressing material, and for this, an important property is wettability, and the conventional way to measure this is by calculating the contact angle. The average of the results can be seen below in Table 1.
In the literature, it has been suggested that for a compound to be known to have good wettability it should present a contact angle below 90 . Therefore, by looking at the results, the synthesized sample counts as having good wettability. In other works, the common value of contact angle for PNIPAM goes from 40 to 80 degrees, and it was also noted that the contact angle presented here was slightly smaller than the one calculated in our previous work, meaning that the silver nanoparticles may positively affect the overall wettability of the composite. 19,23,24

Conclusions
In this work, it is confirmed that size and morphology can be decided upon ratio and discharge conditions. To our knowledge, this is the first time that the in situ synthesis of PNIPAM/silver nanoparticles composites has been successful at such short times and with the possibility to control size and morphology of the overall nanocomposite. It is thought that by further changing the ratio and discharge conditions it is possible to further improve the size for its utilization in the biomedical area as it also demonstrated good wettability.

Acknowledgment
We would like to thank the members of the Research unit in Nano-materials processing for medical, cosmetic, and environmental applications from the International Research Organization for Advanced Science and Technology (IROAST) and their insight in our research.

Disclosure statement
No potential conflict of interest was reported by the authors. Rodolfo Morales Ibarra, proud father of a beautiful little girl and a loving husband of a beautiful woman. Also, a scientist and associate professor with tenure at Universidad Aut onoma de Nuevo Le on, Mexico and designated associate professor in Nagoya University, Japan. He is part of the National System of Researchers of the Mexican Council for Science and Technology (SNI Candidato, CONACYT). He has directed several government and private projects of technological development with industry through the PEI Program of CONACYT. His interest in research lies on material science, focused on the use of supercritical fluids for processing materials, synthesis of graphene and other nanomaterials.