Designing plasmonic-based colloidal pH microsensors

Many natural processes start with an alteration of their immediate environment at the microscopic level. Therefore there is a need to develop pH microsensors that can easily reveal local changes in pH in any heterogeneous system. In this presentation, we show that with a careful design of colloids made of closely packed gold nanoparticles embedded in a thin pH-responsive polymeric shells, we can synthesize sensors at the micron scales that can be dispersed into heterogeneous systems and from which we can read sensitive information on local pH. We found that at low pH, close-packed jammed gold nanoparticles experience strong coupling in their plasmonic resonance, changing the plasmonic resonance to the red part of the spectrum, and giving a strong blue color to the microparticles. As pH is increased, the polymeric crust of the particles gets charged and swells, increasing the distance between the nanoparticles, thus suppressing the plasmonic correlations and inducing a strong color change with an absorption in the blue part of the spectrum, changing the microcapsules into red. Our results demonstrate that these colloidal capsules provide us with an easy way to unambiguously detect local pH changes at the microscale level in heterogenous systems.