Mid-IR sensing platform for trace analysis in aqueous solutions based on a germanium-on-silicon waveguide chip with a mesoporous silica coating for analyte enrichment

Abstract

A novel platform based on evanescent wave sensing in the 6.5 to 7.5 µm wavelength range is presented with the example of toluene detection in an aqueous solution. The overall sensing platform consists of a germanium-on-silicon waveguide with a functionalized mesoporous silica cladding and integrated microlenses for alignment-tolerant backside optical interfacing with a tunable laser spectrometer. Hydrophobic functionalization of the mesoporous cladding allows enrichment of apolar analyte molecules and prevents strong interaction of water with the evanescent wave. The sensing performance was evaluated for aqueous toluene standards resulting in a limit of detection of 7 ppm. Recorded adsorption/desorption profiles followed Freundlich adsorption isotherms with rapid equilibration and resulting sensor response times of a few seconds. This indicates that continuous monitoring of contaminants in water is possible. A significant increase in LOD can be expected by likely improvements to the spectrometer noise floor which, expressed as a relative standard deviation of 100% lines, is currently in the range of 10 −2 A.U.

A thermoelectrically stabilized aluminium acoustic trap combined with attenuated total reflection infrared spectroscopy for detection of Escherichia coli in water

“…hyperspectral imaging). In addition, the recently reported possibility of performing antibody labelling with the assistance of ultrasound particle manipulation on a chip, 25 combined with on-chip mid-IR sensing 51,52 points to possibly automating the overall sample handling scheme, including antibody labelling, in addition to indicating room for further miniaturization. Mid-IR sensing in general holds the potential for label-free sensing by either directly measuring bacteria or their intrinsic β-D-galactosidase or β-D glucuronidase enzyme activity.…”

Sol-Gel Thin Film Processing for Integrated Waveguide Sensors

“…Here, they followed the same coating protocol applied on ATR crystals for determination of benzonitrile and valeronitrile from a water matrix as in (Baumgartner et al, 2018) (Baumgartner et al, 2020b). An even more recent report described an hydrophobic mesoporous silica coating spin-coated on germanium waveguide structures for measuring toluene in water following the same idea proposed by Lopez and Brinker in 1996 on ATR crystals ( Figure 6B) (Benéitez et al, 2020).…”

Integrated Nanophotonic Waveguide-Based Devices for IR and Raman Gas Spectroscopy

“…Nevertheless, the presence of O-H groups on the large surface area of the pore network impairs transparency over time. This problem has been partially overcome by annealing and functionalization [177,189].…”

“…According to their calculations, the cladding can decrease the limit of detection by two orders of magnitude, compared to the waveguide without cladding, due to water background suppression and organic analyte diffusion into the cladding. In another work, germanium rib waveguides were coated with a mesoporous silica coating templated with cetyltrimethylammonium bromide (CTAB) and post-grafted with hexamethyldisilazane [177]. This sensor was used to determine the concentration of toluene in water in the 6.5-7.5 µm wavelength range.…”