UC Riverside

Stimuli-responsive nanomaterials are engineered nanostructures that change their properties in response to external stimuli, such as mechanical stress, electric and magnetic fields, temperature, and light. They have potential applications in sensors, actuators, smart windows, optical devices, and drug delivery. Introducing responsive features into nanomaterials requires advanced synthetic and post-synthetic techniques such as structural engineering, surface functionalization, chemical vapor deposition, and self-assembly. These methods are usually time-consuming, materials-specific, and lacking the controllability and rational-design of specific responsive-functionality. This dissertation discusses different strategies of responsiveness-engineering unrestricted to the nature of the nanomaterials comprising rationally-designed colloidal synthetic approaches.

The dissertation starts with introducing the development of a rapid, reversible, and stable light-responsive system based on nitrogen-doped titanium dioxide (N-TiO2) nanocrystals. The synthetic method involves TiO2 structural engineering through nitrogen doping in a hydrolysis reaction and in-situ surface functionalization with organic glycols acting as sacrificial electron donors (SEDs). The photoreversible system showed excellent color-switching from white to black under light irradiation, corresponding to the surface self-reduction of Ti(IV) to Ti(III). Nitrogen doping and surface functionalization accelerated and stabilized the system responsive-coloration. This system was implemented as a rewritable paper showing a reversible color-switching that could retain writing for hours in ambient conditions without the need for redox dyes or chromic polymers. This functionalization strategy possesses vital advantages in its low cost, rapid response, and excellent reversibility and stability, that could be applied to several photo-responsive systems.

In the second part, a rational-design strategy of synthesizing mechanically-responsive Au@Ag@void@TiO2 nanoparticles to fabricate a colorimetric stress sensor is introduced. The sensor changes color from yellow to dark orange/red under a mechanical force based on the deformation of the nanoparticles. The color-changing mechanism consists of shifting the plasmonic absorption of Au@Ag@void@TiO2 by changing the surrounding environment of Au@Ag nanostructures from air to TiO2 after deforming the outer particle shell by a mechanical force. The pressure sensor could be applied to static and impact forces, showing potentials as instantaneous impact detectors in sports and bicycle helmets, sports mouth guards, and automobile crash dummies. Increasing the thickness of the TiO2 outer shell enhanced the mechanical strength of the materials, thereby withstanding different pressure thresholds directly correlated with shell thickness. This pre-designed synthetic approach could be employed in the future developments of smart materials.

Finally, the development of a universal magnetizing method of nanomaterials utilizing amorphous molybdenum sulfide (a-MoSx) as a transitional layer to anchor magnetic iron oxide particles is presented. The underlying mechanism relies on the strong bonding between molybdenum sulfide and iron oxide nanoparticles, which is found to occur through (Mo-O) interaction. The applicability and versatility of the magnetizing technique are endowed by developing a colloidal coating method of a-MoSx on silicon dioxide (SiO2) that could be induced on various nanostructured materials. The colloidal coating method consists of SiO2 surface modification with (3-aminopropyl)triethoxysilane (APTES), followed by the acidification of (NH4)2MoS4 to produce APTES-SiO2@a-MoSx nanostructures. The instantaneous attachment of iron oxide nanoparticles on APTES-SiO2@a-MoSx could be achieved by mixing both materials in polar solvents. The practicality and advantage of this technique were demonstrated by magnetizing Au microplates to fabricate a magnetically-tunable micromirror. The versatility, practicality, rapid functionalization, and low toxicity are crucial advantages in introducing magnetic-responsiveness in nanostructured materials, especially for biomedical applications.