Modification of Thermo-Responsive Smart Hydrogels by Embedding Prefabricated Gold and Silver Nanoparticles

Smart hydrogels are polymers that respond to an external stimulus (physical or chemical) by absorption or desorption of liquid. Therefore, they are interesting materials for sensing elements since the resulting volume change be related to the stimulus concentration or intensity [1,2]. The corresponding response is dependent on composition and chemical and mechanical properties of the material and can be tailored by adding metallic nanoparticles [3-5].<br/>Here we are presenting a study on how embedded gold and silver nanoparticles influence thermo-responsive Poly(N-Isopropylacrylamid) (PNIPAAm) hydrogels. Thereby, we aim at determining if and how adding of prefabricated metallic nanoparticles can alter properties of smart PNIPAAm hydrogels, specifically with regard to sensor applications.<br/>For sample fabrication, gold and silver nanoparticles of different diameters (15 nm and 50 nm) with polyvinylpyrrolidon (PVP) surface termination and dissolved in phosphate-buffered saline (PBS) solution (obtained from <i>nanoComposix Europe</i>) have been added to the pregel solution in varying concentrations. The hydrogels were subsequently polymerized at room temperature by free radical polymerization. Afterwards, samples were washed in deionized (DI) water for seven days (liquid exchange every 24 hours) to remove any chemical residues. Furthermore, plain PNIPAAm hydrogels of the same composition but without nanoparticles were fabricated as control samples.<br/>Sample volume change has been studied by time and temperature dependent weight measurements to investigate the influence of nanoparticle type, concentration and diameter on the thermo-responsiveness. For the time-dependent measurements, samples stored in 20°C DI water were directly placed into 37°C DI water and their weight obtained every 2 minutes for the duration of 90 minutes. The temperature dependent response was studied in the range of 20°C to 40°C (region of volume-phase transition) by placing a beaker containing DI water and the sample onto a hot plate. The temperature was increased by 2°C every 70 minutes and at the end of each time interval, sample weight was measured. Furthermore, we employed an optical detection setup [6] to directly track the dynamic sample response and determine swelling and shrinking time constants in 1x and 0.25x PBS solution.<br/>In these characterization measurements it is found that: (i) gold nanoparticles do neither influence the time- nor temperature-dependent hydrogel response, irrespectively of nanoparticles diameter and concentration; (ii) silver nanoparticles enhance the swelling response (faster and greater volume change) compared to plain hydrogels; (iii) polymerization time is strongly dependent on nanoparticle concentration in case of silver but independent for gold. These findings indicate that silver nanoparticles exert an influence already during the polymerization process leading to a change of the polymer network and resulting in the altered response. Further investigations are necessary to determine the exact causes and we will also study mechanical properties in stress-strain measurements to determine the nanoparticle influence.<br/> <br/>[1] A. Richter et al.: Review on hydrogel-based pH sensors and microsensors. <i>Sensors</i> 8(1):561, 2008<br/>[2] F. Ganji et al.: Theoretical description of hydrogel swelling: A review. I<i>ranian Polymer Journal </i>19(5):375, 2010<br/>[3] P. Thoniyot et al.: Nanoparticle–hydrogel composites: concept, design, and applications of these promising, multi-functional materials. Advanced Science 2(1-2):1400010, 2015<br/>[4] S. Rafieian et al.: A review on nanocomposite hydrogels and their biomedical applications. <i>Science and Engineering of Composite Materials</i> 26(1):154, 2019<br/>[5] C. Dannert et al.: Nanoparticle-hydrogel composites: from molecular interactions to macroscopic behavior. <i>Polymers</i> 11(2):275, 2019<br/>[6] K. Rückmann et al.: A facile real-time optical characterization method for dynamic properties of smart hydrogels. Under review in <i>Polymers</i>, Dec. 2021