Colorimetric sensitivity of gold nanoparticles: minimizing interparticular repulsion as a general approach

GNPs (gold nanoparticles) as an eye-catching sensor rely on the high extinction coefficients and the shift of the surface plasmon band which signals the disperse-to-aggregate transformation. The selectivity of the sensors is dictated by the surface functionality whose density presumably has a positive correlation with the sensitivity toward the targeted analyte. To improve the analytical performance, most efforts in this research field focus on the design and synthesis of the sensing elements as well as on the increase in density on GNPs. Proposed here is an alternative rationale that the further improvement of the GNP sensitivity can be achieved by minimizing the electrostatic repulsion and hence the energy barrier for the recognition event to take place. Our model system begins with thioctic acid-stabilized GNPs which are subsequently modified with 15-crown-5 ether for the recognition toward K (+). For a given coverage of 15-crown-5 ether, the limits of detection (LODs) can be improved by more than 3 orders of magnitude via adjusting the solution pH and ionic strength which we suggest a general guideline for the optimization of a new GNP sensing scheme. Following this guideline, satisfactory performance with LODs at the micromolar level can be systematically and efficiently found for GNPs with a range of 15-crown-5 ether coverage.