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).

A newly developed colorimetric immunosensor based on gold-coated magnetic nanoparticles (Fe3O4@Au) is presented, and its application for the detection of human immunoglobulin G (IgG) in water is demonstrated. By taking advantage of both the localized surface plasmon resonance (LSPR) phenomenon of gold nanoparticles and the magnetic property of the core, the Fe3O4@Au immunosensor provides a fast and effective method for detecting analytes. In a sandwich scheme, Fe3O4@Au nanoparticles are used to enhance the response of the nanostructured gold surface made of gold nanoparticles randomly placed onto a glass coverslip. Specifically, the detection of the target analyte (human IgG) occurs when Fe3O4@Au nanoparticles bind to the target from the top in the presence of a magnetic field, leading to a change in the absorption spectrum of the nanostructure. Preliminary results have shown that the colorimetric immunosensor can achieve a limit of detection of 1 ng/mL, with a measurement carried out in only 10 minutes. The use of gold-coated magnetic nanoparticles in conjunction with the plasmonic surface offers great potential for the sensitive and specific detection of analytes. This could pave the way for future applications of the immunosensor in rapid testing and mass screening