Periodic plasmonic nanostructures are being widely studied, optimized, and developed to produce a new generation of low-cost and efficient chemical sensors and biosensors. The extensive variety of nanostructures, interrogation approaches, and setups makes a direct comparison of the reported performance from different sensing platforms a challenging exercise. In this feature Article, the most common parameters used for the evaluation of plasmonic nanostructures will be reviewed, with particular focus on the advances in periodic plasmonic nanostructures. Recent progress in the fabrication methods that allow for the high-volume production of periodic plasmonic sensors at low cost will be described, together with an assessment of the state of the art in terms of periodic structures employed for chemical sensing.

It is well known that the macroscopic physico-chemical properties of ionic liquids (ILs) are influenced by the presence of water that strongly interferes with the supramolecular organization of these fluids. However, little is known about the function of water traces within this confined space and restricted ionic environments, i.e. between cations and anions. Using specially designed ILs namely 1,2,3-trimethyl-1H-imidazol-3-ium imidazol-1-ide (MMMI·Im) and 3-n-butyl-1,2-dimethyl-1H-imidazol-3-ium imidazol-1-ide (BMMI·Im), the structure and function of water have been determined in condensed, solution and gas phases by X-ray diffraction studies, NMR, molecular dynamics simulations (MDS) and DFT calculations. In the solid state the water molecule is trapped inside the ionic network (constituted of contact ion pairs formed by π(+)-π(-) interaction) through strong H-bonds involving the water hydrogens and the nitrogens of two imidazolate anions forming a guest@host supramolecular structure. A similar structural arrangement was corroborated by DFT calculations and MDS. The presence of a guest@host species (H2O@ILpair) is maintained to a great extent even in solution as detected by (1)H-(1)H NOESY-experiments of the ILs dissolved in solvents with low and high dielectric constants. This confined water catalyses the H/D exchange with other substrates containing acidic-H such as chloroform.

Several fabrication techniques are recently used to produce a nanopattern for sensing, as focused ion beam milling (FIB), e-beam lithography (EBL), nanoimprinting, and soft lithography. Here, interference lithography is explored for the fabrication of large area nanohole arrays in metal films as an efficient, flexible, and scalable production method. The transmission spectra in air of the 1 cm2 substrate were evaluated to study the substrate behavior when hole-size, periodicity, and film thickness are varied, in order to elucidate the best sample for the most effective sensing performance. The efficiency of the nanohole array was tested for bulk sensing and compared with other platforms found in the literature. The sensitivity of ~1000 nm/RIU, achieved with an array periodicity in the visible range, exceeds near infrared (NIR) performances previously reported, and demonstrates that interference lithography is one of the best alternative to other expensive and time-consuming nanofabrication methods.

Plasmonic biosensors, particularly arrays of nanoholes on thin gold films, have been widely explored in recent years as possible platforms for fast medical diagnostic. In this work, we present a screening method for leukemia cancer markers that uses a plasmonic biosensor based on nanohole arrays fabricated on plastic substrates. The low-cost, scalable, and reproducible nanohole array structures were fabricated by UV nanoimprinting technique. The relative concentration of human immunoglobulin kappa and lambda light chains in blood serum was employed as a screening method. The kappa/lambda concentration ratio was used to determine an unbalance in the immunoglobulin production due to leukemia. The platform was tested using serum samples from patients with known leukemia diagnoses. The results indicated that this inexpensive and flexible plasmonic platform is a promising tool for routine screening in clinical settings.

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