Bimetallic Au-Ag alloy nanoislands for highly sensitive localized surface plasmon resonance biosensing

High performance label-free biosensors must possess beneficial features, including outstanding sensitivity, high durability and practical functionalization approaches for specific detection. Au and Ag are common plasmonic metals, but have drawbacks in plasmonic sensitivity and chemical stability respectively. Thus, proposing a new systematic framework for high performance biosensing is imperative. In this work, monodispersed bimetallic nanoislands (BMNIs) with various Au-Ag atomic ratios, synthesized by solid-state dewetting method, show not only improvement of localized surface plasmon resonance (LSPR) sensitivity from that of either Au or Ag, but have also provided chemical stability against spontaneous corrosion. Au-Ag alloying and BMNIs formation were achieved by a one-step self-assembly method. A large tunable plasmonic sensing range from 421 nm to 578 nm was found for all possible Au-Ag atomic ratios. By employing the common-path interferometric sensing system, the Au0.6Ag0.4 BMNIs showed the best refractive index sensitivity. The as-synthesized Au-Ag BMNIs sensing chips were functionalized with various protocols for biosensing applications, i.e. thiolate functionalization with 11-Mercaptoundecanoic acid and dielectric functionalization with aminosilanes. The dielectric functionalized Au-Ag BMNIs LSPR biosensors can detect sub-picomolar concentration of human IgG with wide dynamic range from 0.89 pM to 1000 pM, whereas thiolate functionalization actually degrades the sensing performance due to Ag degradation. Moreover, the dielectric functionalization layer also provided anti-degradation coverage to protect Au-Ag BMNIs from etchant agents, e.g. H2O2, which indicated the potential in wide biosensing applications. In virtue of the dielectric functionalization method, Au-Ag BMNIs can be utilized as a desirable alternative plasmonic medium in the biosensing community.