The pH-dependent adsorption isotherms for the charged chromophore crystal violet, CV(+), have been measured with three different bases by a free-running cavity implementation of evanescent wave cavity ring-down spectroscopy. The ratio of the maximal absorbance measurements at pH 5.10 and 9.05 is consistent with a Q2:Q3 silanol site ratio of 72.8:27.2. The adsorption isotherms have been interpreted in terms a cooperative binding adsorption allowing more than one ionic species to bind to each silanol group. The surface concentration is consistent with a silanol charge density of 1.92 +/- 0.55 nm(-2) and a total neutralized interface layer structure extending 9 nm from the surface. Binding constants and stoichiometric coefficients are derived for CV(+) to both the Q2 and Q3 sites. A variation of the adsorption isotherm with base is observed so that the isotherm at pH 9.05 adjusted with ammonium hydroxide sets up a competitive acid-base equilibrium with the SiOH groups with only 49% of the surface silanol sites dissociated. The implications for functionalized surfaces in chromatography are discussed.

Gold nanoparticles have been synthesized as colloids using a citrate and borohydride reduction of auric chloride producing 15 and 45 nm particles, respectively. The nanoparticle adsorption kinetics have been observed at the silica-water interface by evanescent wave cavity ring-down spectroscopy. AFM images have been used to determine the surface coverage from which the extinction coefficients of the particles in water have been determined at two interrogation wavelengths: 15 nm particles (635 nm) ) 6.4 ( 1.6 × 10 7 M -1 cm -1 , (830 nm) ) 9.8 ( 0.2 × 10 6 M -1 cm -1 ; and 45 nm particles (635 nm) ) 3.1 ( 1.8 × 10 9 M -1 cm -1 , (830 nm) ) 9.2 ( 1.5 × 10 8 M -1 cm -1 . These values are larger than the Mie scattering calculations would predict. Mono-and multilayer adsorption kinetics have been observed with monolayer binding constants K D ) 2.75 ( 0.55 nM for the 15 nm particles and 0.74 ( 0.47 nM for the 45 nm particles. An initial slope analysis of the binding kinetics shows the 15 nm particles undergo aggregation at the surface whereas the 45 nm particles do not. A multilayer co-operative sequential adsorption aggregation model is developed indicating that goldgold particle aggregation affinity is not as strong as the affinity of the gold to the surface. The refractive index sensitivity of the resulting particle plasmon surfaces has been measured, and the surfaces are sensitive to changes of typically 7 × 10 -4 but optimally 2.5 × 10 -6 .

Rapid adsorption kinetics have been observed for protein binding to a 800 nm aggregated nanoparticle, showing extreme sensitivity resulting from a non-linear particle plasmon response.

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We discuss a microwave planar evanescent waveguide sensor, based on a patented technology demonstrator (2016), for a range of potential UAV platform skin-embedded waveguide sensing applications such as: refractive index sensing, temperature sensing, or surface corrosion. Evanescent microwave sensors can probe the near surface region from millimetre to metric scales, with minimal direct radiation into the external environment, ideal for evading detection, and able of detecting various dielectric or metal materials adjacent to the surface. This paper examines evanescent planar sensor use as an environmental evanescent sensor to both detect and quantify metal or dielectrics without direct contact, measuring material loss from chosen waveguide materials, and waveguide surface cladding materials. We also present microwave transmission for a range of temperature sensor configurations, with good sensitivity across a broad range of temperature-related applications, including temperature hysteresis. [1].