2D hybrid networks of gold nanoparticles: mechanoresponsive optical humidity sensors

International audiencePlasmonic coupling is a fascinating phenomenon occurring between neighboring metal nanostructures. We report a straightforward approach to study such process macroscopically by fabricating 2D networks of gold nanoparticles, interconnected with responsive hygroscopic organic linkers. By controlling the humidity we tune the interparticle distance to reversibly trigger plasmo-nic coupling collectively over several millimeters. Colorimetric sensors are devices which undergo macroscopic changes in their UV-Vis absorbance or fluorescence characteristics upon interaction with the target analyte. Such changes typically fall within the visible light region and can be so obvious to be easily observed with the naked eye. 1 Until now, such kinds of devices, featuring three-dimensional structures, have been employed for the detection of various compounds in solution 2-4 and in the gaseous state. 5 Noble metal nano-particles (NPs), which can be synthesized in solution with precise control over their shape, size and optical properties, 6-10 represent an extremely versatile platform for the fabrication of such devices. 11 In solution, gold nanoparticles (AuNPs) are among the most exploited and best performing scaffolds for sensing because of their high chemical stability, which is achieved upon functionalization with various functional stabilizing shells, 12 and their optical properties, which are extremely sensitive to changes in the local environment. Isolated AuNPs exhibit distinct optical properties arising from their localized surface plasmon resonance (LSPR), 13 which depends on their size and shape, as well as on the refractive index of their environment. However, when the AuNPs are in close proximity to each other (i.e. with spacing of a few nm), a coupling of the LSPR takes place, dramatically modifying the optical properties and therefore altering their macroscopic colour. 14-18 Until now, this phenomenon has just been exploited for the detection of analytes in solution, where the AuNPs are dispersed or free to move and aggregate, 19 while sensing of gaseous compounds with dry NP films is usually achieved through modifications of the refractive index in close proximity to the particle's surface. 20 The latter approach suffers from poor sensitivity and can only be employed for sensing analytes which exhibit a very different refractive index compared to that of the shell used for AuNP stabilization. An interesting strategy to enhance the sensitivity in solid films could rely on the fabrication of dynamic structures in which the interaction with the target analyte affects, at the same time, the refractive index and interparticle distance. However, the fabrication of stable structures, providing a predictable and reproducible optical output, allowing the study and disentan-glement of the two contributions, still remains elusive. In contrast to previous studies reporting on unstable superlattices relying on evanescent non-covalent interactions, 21 here we report on an unprecedented strategy for the fabrication of humidity sensors based on 2D all-covalent networks of AuNPs connected by di-thiolated PEG molecules. The latter act as a receptor for atmospheric moisture. The peculiar nature of such a system allows water molecules not only to modify the local refractive index of the systems, but also to undergo reversible adsorption/desorption within the PEG backbone, determining an expansion/contraction of the latter macromolecule and dynamically modifying the interparticle separation, thus their LSPR coupling. The dynamic water physical adsorption-desorption and the use of di-thiolated linkers enable, for the first time, a fully reversible sensing process showing a response speed of 200 ms and stability over hundreds of cycles. The displayed shift of the LSPR band is as large as 35 nm in the visible range (from a dry state to complete immersion in water) with an unambiguous exponential trend. The mechanically robust 2D structure of such films should also enable the fabrication of flexible devices. † Electronic supplementary information (ESI) available. Se