It is of great potential to apply nanomaterials in biomedical detections. Gold nanoparticles (AuNPs) are widely used due to good biocompatibility, conveniently modification and specific properties. In order to provide AuNPs with more bioactivities, AuNPs are functionalized with different active proteins. The effectiveness of protein functionalized AuNPs in the applications depends mostly on the structure and activity of the immobilized proteins. However, it is observed that the bioactivities of protein functionalized AuNPs is difficult to remain at their original level in most cases. The reason is that the proteins always lose the optimal structures and activities after immobilized on AuNPs either by covalent or nonspecific conjugations. The ability to maintain and modulate protein activity in these functionalized AuNPs becomes the key issue in this field.

To solve this problem, we systematically study the bioactivity of protein functionalized AuNPs by immobilizing the proteins on AuNPs by specific orientation of proteins, different surface densities of proteins, and the co-immobilization of proteins and stimuli-responsive polymers. Moreover, the activity of protein functionalized AuNPs is further modulated under different conditions in our work, including different environmental temperatures, pH change, and enzymatic digestion. Different orientations of proteins on AuNP are achieved by introducing the specific amino acid, cysteine, by mutation at fixed sites of the proteins for binding to gold with Au-S bond. The stimuli-responsive polymers with different molecular weights are also functionalized with thiol group to immobilize on AuNPs. Meanwhile, the surface density of proteins and polymer/protein ratio on AuNPs are investigated by the molar ratios of proteins, polymers and nanoparticles in the immobilization systems.

The results show that site-directed orientation and surface density of the proteins immobilized on AuNPs can both modulate the activity of protein functionalized AuNPs, and changing the surface density is not as effective as changing the protein orientation on AuNPs for modulating their activities. Moreover, both the molecular weight of stimuli-responsive polymers and polymer/protein molar ratio on AuNPs significantly affect the activity of protein functionalized AuNPs under different conditions. For example, pyrophosphatase (PPase) and poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) co-immobilized AuNPs can be almost used as an “on/off” switch for the modulation of bioactivity; furthermore, approximately 90% activity of the functionalized AuNPs is still remained after digested by trypin for 90 min. The fabrication of protein functionalized AuNPs with polymer co-modification provides a new way to produce the responsive nanohybrid with high sensitivity and tunable activity to the environmental stimuli. These results indicate that protein functionalized AuNPs have great potential in biomedical detection, targeted delivery, controllable biocatalysis, and molecular/cellular recognition.

The work was supported by National Natural Science Foundation of China (21374070 and 21474072), and the National Natural Science Foundation Fund for Distinguished Young Scholars (21125418).