Synopsis

Plasmonic nanostructures

Surface plasmon-polariton resonances (SPR) on smooth surfaces were recognized in the 1970s and 80s as a powerful means for sensing monolayers of material, and indeed commercial devices became available based on this technique with one-dimensional spatial sensitivity. Advances in nanotechnology have paved the way to extend the general idea of SPR sensors to metallic nanostructures. Such systems offer better sensitivity and spatial resolution owing to a localized plasmon-polariton resonance (LPR). An LPR leads to an enhanced coupling between the nanostructure and light, as is the case for an antenna. As a result, the absorption, emission, and Raman scattering of molecules in the near field of a nano-antenna can be very strongly modified. Indeed, the past five years have witnessed an explosion of activities on the synthesis, fabrication, measurements, and applications of metallic nanostructures. One of the challenges ahead is to translate simulated optimal structures into efficient and economically feasible devices.

Functionalization

Another important concern for the optimal design of high-sensitivity sensors is to achieve a response specific to the analyte in question. For interface-based label-free sensing this requires functionalization of the sensor surface in order to capture the analyte of interest while suppressing interactions of all other molecules. While the task of achieving high specificity for biosensing is remarkably demanding, the requirements for accurate single-molecule detection are even higher because no other molecule can be allowed to interact and produce a signal. Under real world conditions, this requires highly optimized thin film coatings. The most successful solution is the creation of molecular monolayers that even for thicknesses less than 10 nm can provide a steric repulsive barrier to adsorption. However, these properties have not been demonstrated yet for nanoparticles, where curvature effects on binding and packing are important and where the sensor sensitivity to defects is much higher. There is also a challenge in making these coatings thin enough to not consume too much of the highly localized sensitivity of the LPR, which is of the same magnitude as the present typical coating thickness.

Device Integration

Tasks like automatic and continuous monitoring of industrial (bio-)chemical processes, ambient and remote sensing, medical diagnostic for global health and the desire for smaller sample volumes, require sensitive, compact and easy-to-operate sensors. Moreover, due to the large fragmentation of the sensor market and the relatively small number of specific sensors that are generally needed, one has to focus on developing sensor platforms that can be tailored to a large variety of applications with minor modifications. The sensing principles based on LPRs can be implemented in an integrated device. Moreover, microfluidic platforms that have been developed for lab-on-a-chip applications allow one to manipulate and transport very small amounts of analytes to the sensing area. The advantages of such systems are precise analyte delivery, control of aqueous environment, small volume consumption, possible automation of characterization, and cheap reusable sensors.