Molecularly imprinted sol–gel nanoparticles for mass-sensitive engine oil degradation sensing

Abstract

Titanate sol–gel layers imprinted with midchain carbonic acids have proven highly useful for detecting engine oil degradation processes owing to selective incorporation of oxidised base oil components. Synthesising the material from TiCl₄ in CCl₄ and precipitating with water leads to imprinted TiO₂ nanoparticles with a diameter of 200–300 nm. Replacing the water by a 1 M ammonium hydroxide solution reduces the average particle size to 50–100 nm with retention of the interaction capabilities. Experiments with the latter solution revealed that the 100-nm particles take up substantially more analyte, indicating a size-dependent phenomenon. As the number of interaction sites within each material is the same, this cannot be a consequence of thermodynamics but must be one of accessibility. The sensor characteristic of water-precipitated particles towards engine oil degradation products shows substantially increased sensitivity and dynamic range compared with the corresponding thin films. Coating quartz crystal microbalances with such nanoparticle materials leads to engine oil degradation sensors owing to incorporation of acidic base oil oxidation products. Interaction studies over a large range of layer thicknesses revealed that both the absolute signal and the steepness of the correlation between the sensor signal and the layer height is 2 times higher for the particles.