are the main features exhibited by metal nanostructures thereby opening up the possibility for manipulating light at nanometric scale, well below the diffraction limit. [1] So far, the tremendous potentialities of the plasmon-related effects have been already represented a breakthrough in many application fields such as cancer treatment, [2] ultrasensitive molecule detection, [3] integrated circuitry, [4] quantum optics, [5] optoelectronics, [6] photovoltaics. [7] Several types of plasmonic nanostructures are being conceived these days aiming at improving the performance of plasmon-based devices. [8,9] At the basis of the nonpropagating plasmon phenomena there is the localized surface plasmon resonance (LSPR), namely, the collective oscillation of the conduction electron cloud against the metal core. [10] The plasmonic properties of metal nanomaterials strongly depend on the nanostructure geometry, arrangement, and environment. For instance, exotic nanostructures such as nanocages, nanoscaffolds, and bow-tie nanoantennas exhibit higher field enhancement than conventional nanoparticles with smooth surfaces thereby representing a considerable advantage in applications relying on signal amplification such as surface-enhanced Raman spectroscopy (SERS), [11,12] surface-enhanced infrared absorption (SEIRA), [13,14] and plasmon-enhanced fluorescence (PEF). [15,16] In addition, when nanostructures are ordered in periodic arrays, new modes can arise as a result of the near-or far-field coupling among the localized plasmons so as to activate hybrid effects such as coupled LSPR (c-LSPR) [17,18] and surface lattice resonance (SLR), [19][20][21] respectively. Besides, plasmonic properties offered by metamaterials were recently investigated and sparked considerable interest since they demonstrated to offer significantly better performance as compared to metal-based nanostructures in many fields of applications such as biosensing, [22] photonics, [23] photovoltaics, [24] and optoelectronics. [25] Nevertheless, the actual implications are still far-reaching for many scientific and engineering fields due to their complexity and low awareness. [25] Therefore, the possibility to tune the optical response of a nanostructure by tailoring the material, shape, and size, as well as the pattern architecture, is spurring the researchers to explore new approaches, in terms of both nanofabrication and nanoapplications, in order to go beyond the current limits of many techniques.The aim of the present work is to provide a comprehension of this growing field of research and to convey the main features of the nanostructured surfaces to biosensing applications.Conventional laboratory techniques exhibit impressive sensing performance and still constitute an irreplaceable tool in bioanalytics. Nevertheless, high costs, time consumption, and need for well-equipped laboratories and skilled personnel make highly desirable to explore novel strategies to carry out biochemical analyses. In this regard, biosensor-based methods represent a promising appr...

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The optical response of different configurations of functionalized gold nanoparticles (f-AuNPs) and SARS-CoV-2 virions is simulated in order to explore the behavior of a colloidal solution containing 105–1013 virions/ml. The analysis herein reported is carried out for three concentration regimes: (i) low (≲108 virions/ml), (ii) intermediate (∼109–1010 virions/ml), and (iii) high (≳1011 virions/ml). Given the high binding effectiveness of f-AuNPs to virions, three different configurations are expected to arise: (i) virions completely surrounded by f-AuNPs, (ii) aggregates (dimers or trimers) of virions linked by f-AuNPs, and (iii) single f-AuNP surrounded by virions. It is demonstrated that 20 nm diameter gold nanoparticles functionalized against all three kinds of SARS-CoV-2 proteins (membrane, envelope, and spike) allow one to reach a limit of detection (LOD) of ∼106 virions/ml, whereas the use of only one kind of f-AuNP entails a ten-fold worsening of the LOD. It is also shown that the close proximity (∼5 nm) of the f-AuNP to the virions assumed throughout this analysis is essential to avoid the hook effect, thereby pointing out the importance of realizing an apt functionalization procedure that keeps thin the dielectric layer (e.g., proteins or aptamers) surrounding the gold nanoparticles.

Metal nanostructures exhibit tremendous potentialities in biosensing applications relying on fluorescence, Raman spectroscopy, infrared absorption, and colorimetry thanks to unique plasmonic effects. In particular, nanostructured surfaces can be smartly employed for realizing chip-based bioassays with remarkable sensing performance. In article number 2101133, Antonio Minopoli, Adriano Acunzo, Bartolomeo Della Ventura, and Raffaele Velotta review the recent developments in plasmonic biosensors.

In the last few decades, plasmonic colorimetric biosensors raised increasing interest in bioanalytics thanks to their cost-effectiveness, responsiveness, and simplicity as compared to conventional laboratory techniques. Potential high-throughput screening and easy-to-use assay procedures make them also suitable for realizing point of care devices. Nevertheless, several challenges such as fabrication complexity, laborious biofunctionalization, and poor sensitivity compromise their technological transfer from research laboratories to industry and, hence, still hamper their adoption on large-scale. However, newly-developing plasmonic colorimetric biosensors boast impressive sensing performance in terms of sensitivity, dynamic range, limit of detection, reliability, and specificity thereby continuously encouraging further researches. In this review, recently reported plasmonic colorimetric biosensors are discussed with a focus on the following categories: (i) on-platform-based (localized surface plasmon resonance, coupled plasmon resonance and surface lattice resonance); (ii) colloid aggregation-based (label-based and label free); (iii) colloid non-aggregation-based (nanozyme, etching-based and growth-based).