## 3 Results and Discussion
The extent of aggregation of the particles was observed to increase on increasing the amount of polymer grafting on the surface of particles, thus, further confirming the above mentioned findings. It was also necessary to confirm that the particles also have environmentally reversible behavior after the grafting of environmentally responsive PNIPAAM brushes from their surface. Therefore, the microscope grids were prepared at high temperature (37 \({}^{\circ}\)C) as at temperatures higher than the lower critical solution temperature, the PNIPAAM chains are hydrophobic in nature and collapse on the surface of the particles. Figure 4 shows the TEM micrograph of the particles adsorbed on the microscope grids at 37 \({}^{\circ}\)C. The brushes observed in the case of Figures 1 and 2 were observed to collapse completely on the surface of the particles owing to their hydrophobic nature. The collapsed bushes on the surface were partially visible in the voids between the particles. The brushes were observed to gain extended morphology when the particles were adsorbed at 10 \({}^{\circ}\)C confirming the translation of environmentally responsive behavior to the particles also. Figure 5 also represents this phenomenon schematically. At temperature below the lower critical solution temperature (LCST) of PNIPAAM, the nature of the particles is hydrophilic owing to extended morphology of the polymer brushes and hydrogen bonding  
Figure 3: (a) Cryo-SEM micrograph of the polymer particles after PNIPAAM grafting and (b) TEM micrograph demonstrating the pulling of two grafted particles apart. formed between the polymer chains and water molecules. When the temperature is higher than the lower critical solution temperature of PNIPAAM, the nature of the particles reverses to hydrophobic as in this case the polymer chains tend to form hydrogen bonds between CO and NH groups in the chains and thus coil or collapse on the surface of the particles. This reversible process is also quite fast and thus the functional surfaces can be used for different applications like temperature controlled adsorption and desorption of proteins, viruses _etc._, thermal filters, responsive textiles _etc._  
To demonstrate the application of the PNIPAAM grafted polymer particles for the temperature controlled adsorption and desorption of proteins and viruses, IgG protein supported on gold particles and tobacco mosaic virus were chosen. The particles mixed with either protein or virus at 10 \({}^{\circ}\)C (lower than the LCST), were observed to have extended morphology of the brushes on the particles and no adsorption of protein or virus molecules as shown in Figure 6a. However, when the mixture were equilibrated at 37 \({}^{\circ}\)C (higher than LCST), the brushes collapse and the nature of the surface becomes suitable for the adsorption of the biological entities. As observed in Figures 6b,c, the particles tend to aggregate owing to hydrophobic interactions and the protein supported gold particles (observed as tiny dots around the particles in the micrograph) or virus (seen as long tubes in the micrograph) molecules  
Figure 4: TEM micrograph of the PNIPAAM grafted particles, when the grids were prepared at 37 \({}^{\circ}\)C.  
Figure 5: Representation of reversible hydrophilic and hydrophobic nature of the polymer particles grafted with PNIPAAM brushes. adsorb on the surface of the particles. The same mixture when re-cooled to 10 \({}^{\circ}\)C showed desorption of the protein or virus molecules and the brush morphology was again extended as shown in Figure 6a. Similar behavior has also been quantitatively shown for Human Serum Albumin (HSA) protein [60]. This behavior confirms the potential of these polymer materials in advanced applications. As mentioned above, not only spherical but also flat macro-surfaces are of tremendous interest for different applications.