The growing interest in recent years in gold island films prepared by vapor deposition on transparent substrates is largely attributed to the prominent localized surface plasmon (SP) extinction associated with nanostructured metal films. In the present study, two types of evaporated Au island films were investigated: (i) Au films (2.5, 5.0, and 7.5 nm nominal thickness) evaporated on silanized glass and annealed 20 h at a temperature <250 °C; (ii) Au films (7.5 and 10 nm nominal thickness) evaporated on unmodified glass and annealed 10 h at 550 or 600 °C. The 3D morphology of the Au islands was analyzed using high-resolution scanning electron microscopy (HRSEM), crosssectional transmission electron microscopy (TEM), and atomic force microscopy (AFM) crosssectional profilometry. Annealing at high temperatures, close to the glass transition temperature of the substrate, results in wetting of the Au islands by the glass and partial island embedding. The mechanism of morphology evolution during annealing changes from island coalescence and coarsening (low nominal thicknesses) to dewetting of percolated films (higher nominal thicknesses). The aspect ratio of more than 90% of the islands in annealed films is <1.5; therefore, splitting of the SP band to transversal and longitudinal components is not observed. The bulk refractive index sensitivity (RIS), in terms of SP wavelength shift and plasmon intensity change (PIC) per refractive index unit (RIU) change of the medium, was determined by measuring UV-vis spectra of Au island films in a series of methanol/chloroform mixtures. The RIS values for SP wavelength shift (RIS λ ) and PIC (RIS ext ) are 66-153 nm/RIU and 0.2-0.81 abs.u./RIU, respectively. The RIS shows a strong dependence on the wavelength of the SP maximum extinction, i.e., a higher RIS is measured for Au island films exhibiting a SP band at longer wavelengths. Partial thermal embedding of the Au islands in the glass substrate stabilizes the systems but lowers the RIS. The results presented may be useful for tuning the morphology and optical response of Au island films.

Strongly bound and highly stable gold island films on glass are obtained by a simple, one‐step preparation procedure (see figure) based on high‐temperature annealing and partial embedding of gold nanostructures evaporated on glass, providing stable and effective localized plasmon transducers.

Gold island films displaying localized plasmon properties were prepared by evaporation of just-percolated Au films onto glass substrates followed by annealing at g550 °C. Annealing induces depercolation and formation of large, singlecrystalline, well-separated islands, partially embedded in the glass. Two dewetting mechanisms were identified, depending on the initial film morphology. The variability of island sizes and shapes provides effective means of tuning the position of the localized surface plasmon resonance (LSPR) band in a wide wavelength range. With an increase in the Au nominal thickness a transition occurs from transducers dominated by absorbance to ones dominated by scattering. Numerical simulations taking into account the shape and size distribution in actual island samples are in agreement with the experimental spectra. Refractive index sensitivity (RIS) measurements at a constant wavelength or at a constant extinction, tailored to the specific transducer, provide superior sensitivity to refractive index change, up to ca. 600 nm RIU À1 in wavelength shift.

Discontinuous, island-type gold films (typically < or = 10 nm nominal thickness) prepared by evaporation of the metal on transparent substrates show a localized surface plasmon resonance (LSPR) extinction in the visible-to-NIR range and can be used as optical transducers for monitoring local refractive index change. Such transducers, operated in the transmission configuration, provide an effective scheme for label-free biological sensing using basic spectrophotometric equipment. Optimization of the sensitivity of LPSR transducers requires consideration of the distance between the metal island surface and the bound analyte, strongly affecting the optical response due to the fast decay of the evanescent field of localized plasmons. In the present work Au island based LSPR transducers were used to monitor antibody-antigen interactions, demonstrating the effect of the biorecognition interface thickness. Evaporated Au island films derivatized with IgG or hCG antigens were used as biological recognition elements for selective sensing of antibody binding, distinguishing between specific and nonspecific interactions. The LSPR results are supported by XPS and ellipsometry data as well as by AFM and HRSEM imaging, the latter providing actual visualization of the two protein binding steps. Increase of the recognition interface thickness leads to a concomitant decrease in the extinction and wavelength sensitivity, generally conforming to a model of an exponentially decaying surface plasmon (SP) evanescent field.

Ultrathin (typically < or = 10-nm thick) gold island films evaporated on transparent substrates show a prominent localized surface plasmon (SP) extinction in the visible-to-NIR range. Changes in the dielectric properties of the contacting medium influence the SP absorption band, providing a scheme for optical sensing based on refractive index change. In the present work, the gas sensing capability of gold island based localized surface plasmon resonance (LSPR) transducers was explored using polymeric coatings as the active interface. LSPR transducers were fabricated by spin-coating of polystyrene (PS) or polystyrene sulfonic acid, sodium salt (PSS) onto 5-nm-thick (nominal thickness) gold island films evaporated on silanized glass and annealed. Detailed characterization of the transducers was carried out using high-resolution scanning electron microscopy, cross-sectional transmission electron microscopy, and in situ atomic force microscopy under controlled atmosphere. The hydrophobic PS film exhibits swelling and significant thickness increase upon exposure to chloroform vapor and little or no change in water vapor, whereas the hydrophilic PSS film shows the opposite behavior when exposed to the same vapors. Polymer swelling upon absorption of vapors of good solvents shows a net effect of lowering the effective refractive index in the vicinity of the gold islands, manifested as a characteristic decrease of the SP band intensity and a blue shift of the band maximum. The response, measured for four different vapors, is fast (approximately 15 s) and reversible. It is shown that gold island systems coated with polymeric films can be applied to vapor recognition in an array configuration.