Addition of Silica-Coated Ag Nanoparticles to Enhance Luminescence Intensity of Pressure-Sensitive Paints

Enhancement of the luminescent output of pressure-sensitive paint (PSP) has been achieved by adding silver nanostructures into a standard formulation by taking advantage of a metal-enhanced fluorescence effect. Traditionally, metal-enhanced fluorescence has been used in microscopy or thin-film sensing. However, in PSP applications, the luminescent film is applied onto large surfaces by using conventional paint application techniques, and imaging is acquired from longer distances. As such, many of the techniques deployed for maximum enhancement (such as careful spacing of the metal centers) is not easily achievable. Initial trials consisted of adding silver nanostructures into a PSP formulation. The luminescent enhancement was highly dependent on the concentration of the oxygen-sensitive dye. At very low concentrations of the dye (<10 μM), enhancements more than a factor of 3 could be achieved. However, as the concentration of the dye increased (∼800 μM), the enhancement effect reduced and eventually diminished the signal. This was due to the dark color of the silver and the decrease in the distance between the dye and the silver nanostructures, leading to quenching of luminescence. To alleviate this problem, silver nanoparticles were coated with a silica shell, which acted as a spacer keeping the dye and silver nanoparticles at a suitable distance from each other to maximize the luminescence enhancement without quenching being observed regardless of the nanoparticle concentration. This resulted in an enhancement factor of more than 1.5. Excited-state lifetime measurements were also acquired to verify that the enhancement was due to metal-enhanced fluorescence.