Since the last decade, nanohole arrays have emerged from an interesting optical phenomenon to the development of applications in photophysical studies, photovoltaics and as a sensing template for chemical and biological analyses. Numerous methodologies have been designed to manufacture nanohole arrays, including the use of focus ion beam milling, soft-imprint lithography, colloidal lithography and, more recently, modified nanosphere lithography (NSL). With NSL or colloidal lithography, the experimental conditions control the density of the nanosphere mask and, thus, the aspect of the nanohole arrays. Low surface coverage of the nanosphere mask produces disordered nanoholes. Ordered nanohole arrays are obtained with a densely packed nanosphere mask in combination with electrochemical deposition of the metal, glancing angle deposition (GLAD) or etching of the nanospheres prior to metal deposition. A review of these methodologies is presented here with an emphasis on the optical properties of nanoholes interesting in analytical chemistry. In particular, applications of these novel plasmonic materials will be demonstrated as substrates for a localized surface plasmon resonance (LSPR), Surface Plasmon Resonance (SPR), surface enhanced Raman spectroscopy (SERS), and in electrochemistry with nano-patterned electrodes.

A systematic study of the optical properties and analytical response is reported for gold and silver nanohole arrays with different hole diameters with a fixed periodicity of 450 nm. Nanosphere lithography in combination with oxygen plasma etching has been used to fabricate the nanohole arrays. The plasmonic response of nanohole arrays is characterized in transmission spectroscopy (λ = 500−1000 nm spectral region), which varied with the metal composition and diameter of the nanoholes. The sensitivity to bulk refractive index (in nm/RIU) and the full width at half-maximum (FWHM) were measured for each plasmonic mode to compare the biosensing potential of the various nanohole arrays. A sensitivity of nearly 400 nm/RIU was observed and was maximal with the plasmonic band at λ = 554 nm for Ag nanohole arrays with the smallest hole diameter of 120 nm. Generally, the ratio of the full height (transmission intensity) and FWHM is constant for various hole diameters with Au nanohole arrays, whereas it improves for Ag nanohole arrays with smaller hole diameters. Various bimetallic nanohole arrays composed of a Ag underlayer covered with Au were fabricated with a hole diameter of 254 ± 20 nm and a depth of 50 ± 12 nm. Sensitivity and FH/FWHM ratio are improved for Au on Ag nanohole arrays compared with nanohole arrays of pure metal.

The analytical and physical properties are reported for nanohole arrays prepared with glancing angle deposition (GLAD) or plasma treatment of a nanosphere lithography (NSL) mask prior to the deposition of a thin Au film. The nanohole arrays obtained with a 450 nm nanospheres mask are characterized using atomic force microscopy (AFM) to determine the depth and the width of the nanoholes, and the periodicity of the nanohole arrays. The analytical properties are reported in terms of the surface plasmon (SP) excitation wavelength (500 nm to 1000 nm), sensitivity to refractive index (27 nm RIU(-1) to 487 nm RIU(-1)), sensitivity to monolayer formation (shift of the SP band by approx. 1 nm), and refractive index resolution (10(-4) RIU). These simple techniques produce well-ordered nanohole arrays with tunable analytical and physical properties for the development of biosensors.

In this letter, we demonstrate that Au triangles between approximately 200 nm and 1.5 μm are transitioning from a predominantly localized surface plasmon resonance (LSPR) character to a regime where both LSPR and a short-range SPR mode are concomitantly present. The Au triangles are active in LSPR between approximately λ = 700 nm for the smaller triangles and λ = 2.5 μm for the largest Au triangles. Triangles with a base length of 200 and 300 nm do not exhibit an active short-range SPR mode, while triangles with 500 and 700 nm and 1.5 μm base lengths show a strong absorption peak in SPR at approximately λ = 800 nm, attributed to the short-range SPR mode. A low sensitivity of <50 nm/RIU is observed with this short-range SPR mode in triangles of near-micron size. Also, Au nanohole arrays exhibit features in both the submicron size, with gold “island” between neighboring nanoholes, and on the order of tens of microns, with gold “strips” along rows of nanoholes. The excitation of nanohole arrays in total internal reflection (TIR) results in the presence of both a short-range SPR mode similar to that of gold triangles and a propagating SPR mode similar to that of a thin Au film of macroscopic surface. For Au nanohole arrays of 820 nm periodicity, TIR experiments result in two overlapping absorption peaks at λ = 650 and 800 nm, corresponding respectively to propagating SPR in thin films (typically around λ = 615 nm) and the short-range SPR mode in Au triangles.