Facile Scalable Assembly of Uniform and Layered Plasmonic Hotspots by Force-Assisted Capillary-Mediated Colloidal Transfer

Three-dimensional (3D) assembly of colloidal metal nanoparticles for plasmonic detections has been prepared by Langmuir-Blodgett (LB) deposition. However, due to the small focal volume of an incident light, it suffers from laser spot-to-spot variations in near-field intensities, which undermines signal uniformity, thereby hindering its widespread use. Here, a facile and scalable method to fabricate focal volume-comparable is presented, uniform 3D architectures of plasmonic hotspots by devising the force-assisted, capillary-mediated stacking of colloidal metal nanoparticles. By deploying only a few pN per nanoparticle, it is shown that the method quickly produces multiple monolayers with lateral sizes in the submillimeter range, in which surface uniformity with a small variation of 4% is improved by ≈30% compared to the LB deposition. The simulation suggests that the volume fraction of electromagnetic field enhancement increases with increasing the number of layers, and its intensity can be maximized by more than 100% by simply stacking the monolayers. Additionally, tailoring the size of the void of the 3D assembly leads to a four orders of magnitude increase in the sensitivity of the Raman detection. It is indicated that the method can greatly improve the practical utility of colloidal metal nanoparticles for sensing applications from biomedicine to biomanufacturing.