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.

Ionic liquid self-assembled monolayers (SAM) were designed and applied for binding streptavidin, promoting affinity biosensing and enzyme activity on gold surfaces of sensors. The synthesis of 1-((+)-biotin)pentanamido)propyl)-3-(12-mercaptododecyl)-imidazolium bromide, a biotinylated ionic liquid (IL-biotin), which self-assembles on gold film, afforded streptavidin sensing with surface plasmon resonance (SPR). The IL-biotin-SAM efficiently formed a full streptavidin monolayer. The synthesis of 1-(carboxymethyl)-3-(mercaptododecyl)-imidazoliumbromide, a carboxylated IL (IL-COOH), was used to immobilize anti-IgG to create an affinity biosensor. The IL-COOH demonstrated efficient detection of IgG in the nanomolar concentration range, similar to the alkylthiols SAM and PEG. In addition, the IL-COOH demonstrated low fouling in crude serum, to a level equivalent to PEG. The IL-COOH was further modified with N,N'-bis (carboxymethyl)-l-lysine hydrate to bind copper ions and then, chelate histidine-tagged biomolecules. Human dihydrofolate reductase (hDHFR) was chelated to the modified IL-COOH. By monitoring enzyme activity in situ on the SPR sensor, it was revealed that the IL-COOH SAM improved the activity of hDHFR by 24% in comparison to classical SAM. Thereby, IL-SAM has been synthesized and successfully applied to three important biosensing schemes, demonstrating the advantages of this new class of monolayers.

A simple anion metathesis in diluted aqueous carbonate at room temperature affords 1-(12-mercaptododecyl)-3-methyl-imidazolium carbonate (MDMI-HCO(3)) from MDMI salts self-assembled on gold films and nanoparticles. The properties of MDMI-SAM differ from MDMI in solution, for which the anion exchange reaction does not proceed.

The properties of a surface modified with an ionic liquid self-assembled monolayer (IL-SAM) can be tuned by simply changing the deposition temperature. Mid-IR, SERS, and molecular modelling demonstrated that 1-(12-mercaptododecyl)-3-methylimidazolium bromide (MDMIBr) exhibited a crystalline monolayer for deposition temperatures below 25 °C. Above 25 °C, the aliphatic chain collapsed into a disordered conformation. At 40 °C, another phase transition occurs due to the imidazolium group tilting parallel to the surface. Consequently, the wettability of IL-SAM was tuned over a broad range of contact angle (from 20° to nearly 40°) by varying the deposition temperature. Permeation of redox mediators to a Au electrode coated with MDMIBr strongly depends on the net charge of the redox mediator. Electron transfer was excellent for neutral and negatively charged redox mediators on electrodes coated with IL-SAM regardless of deposition temperature.