Extraordinary sensitivity enhancement of Ag-Au alloy nanohole arrays for label-free detection of Escherichia Coli.

Alloy nanostructures unveil extraordinary plasmonic phenomena that supersede the mono-metallic counterparts. Here we report silver-gold (Ag-Au) alloy nanohole arrays (α-NHA) for ultra-sensitive plasmonic label-free detection of Escherichia Coli (E. coli). Large-area α-NHA were fabricated by using nanoimprint lithography and concurrent thermal evaporation of Ag and Au. The completely miscible Ag-Au alloy exhibits an entirely different dielectric function in the near infra-red wavelength range compared to mono-metallic Ag or Au. The α-NHA demonstrate substantially enhanced refractive index sensitivity of 387 nm/RIU, surpassing those of Ag or Au mono-metallic nanohole arrays by approximately 40%. Moreover, the α-NHA provide highly durable material stability to corrosion and oxidation during over one-month observation. The ultra-sensitive α-NHA allow the label-free detection of E. coli in various concentration levels ranging from 103 to 108 cfu/ml with a calculated limit of detection of 59 cfu/ml. This novel alloy plasmonic material provides a new outlook for widely applicable biosensing and bio-medical applications.

Ag/Au Alloyed Nanoislands for Wafer-Level Plasmonic Color Filter Arrays.

Alloyed metals in nanoscale exhibit some intriguing features that are absent in mono-metallic nanostructures. Here we report silver and gold alloyed nanoislands with high tunability of localized surface plasmon resonance (LSPR) wavelength in the visible range for wafer-level plasmonic color filter arrays. The nanofabrication includes two simple steps of concurrent thermal evaporation of Ag and Au grains and solid-state dewetting of the as-deposited nanocomposite thin film. The alloy ratio during the evaporation precisely tunes the LSPR wavelengths within 415-609 nm spectrum range. The elemental composition map reveals that alloyed nanoislands are completely miscible while preserving uniform size, regardless of the alloy ratio. Besides, the multiple lift-off processes and thermal dewetting of Ag/Au nanocomposite thin films successfully demonstrate the wafer-level nanofabrication of plasmonic color filter mosaic. Each plasmonic color pixel comprises different alloy ratio and efficiently transmits colors ranging from cyan, yellow, and magenta. The transmission spectra transposed onto a CIE 1931 color map show comparable color diversity to the plasmonic color filters fabricated by conventional e-beam lithographic techniques. This novel method provides a new direction for large-scale and visible plasmonic color filter arrays in advanced display or imaging applications.

Nanoislands as plasmonic materials.

Subwavelength metal nanoislands thermally dewetted from a thin film emerge as a powerful and cost-effective photonic material, due to the formation of substantially strong nano-gap-based plasmonic hot spots and their simple large-area nanofabrication. Unlike conventional nanostructures, nanoislands dewetted from thin metal films can be formed on a large scale at the wafer level and show substrate-dependent plasmonic phenomena across a broad spectral range from ultraviolet to infrared. Substrate-selective dewetting methods for metal nanoislands enable diverse nanophotonic and optoelectronic technologies, underlining mechanical, structural, and material properties of a substrate. Emerging bioplasmonic technology using metal nanoislands also serves as a high-throughput and surface-sensitive analytical technique with wide-ranging application in rapid, real-time, and point-of-care medical diagnostics. This review introduces an assortment of dewetting fabrication methods for metal nanoislands on distinct substrates from glass to cellulose fibers and provides novel findings for metal nanoislands on a substrate by three-dimensional numerical modeling. Furthermore, the plasmonic properties of metal nanoislands and recent examples for their photonic applications, in particular, biological sensing, are technically summarized and discussed.