Bettina Baumgartner, Jakob Hayden and Bernhard Lendl

Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164 AC, 1060 Vienna, Austria

SYNTHESIS & CHARACTERISATIONOPTICAL SETUP

INTRODUCTION DESIGN OF ENRICHMENT LAYERŸ Mid-IR spectroscopy provides access to the information-rich

fingerprint region of the electromagnetic spectrum enabling identification and quantification of organic contaminants

Ÿ strong background absorption of water in the mid-IR region limits sensitivity

Ÿ sensitivity towards organic contamiants can be increased by coating attenuated total reflectance (ATR) crystals with

[1,2,3]polymer coatings

Ÿ Analytes are reversibly absorbed and thereby concentrated in the coatings in the region probed by the evanescent wave, while excluding spectral interferences of water

Ÿ limits of detection (LOD) for chlorinated and aromatic hydrocarbons in the mid-low ppb region have been reached

Ÿ polymer coatings used in literature rely on long enrichment due to the diffusion resistance of bulk polymers

Ÿ Diffusion and thereby response time can be enhanced by porous enrichment materials

Ÿ Soft templating of sol-gel materials provides access to mesoporous films that can be easily organically modified

Cooperative self-assambly: The precursor and the surfactant contribute both to the assembly. Usually, the surfactants are removed by calcination (> 300 °C) leading to highly ordered mesostructured materials. The variation of used surfactant and precursor/surfactant ratio leads to different mesophases, including 2D hexagonal, 3D hexagonal and 3D cubic.

CONCLUSIONOrganically modified silica with ordered porosity is the material of choice for ATR enrichtment coatings:Ÿ fast diffusion into enrichment layer (response time < 5 s) and fast regeneration Ÿ IR transparency in the spectral region of organic contaminantsŸ limit of detection of 30 ppmŸ precise pore design and pore functionalization, thereby allowing for selective sample enrichment

A Versatile Porous Enrichment Layer for Monitoring Organic Contaminants in Water via ATR Spectroscopy

Surfactant+ Precursor

surfactantmicelle

micellar rod hexagonal micellar arraysurrounded by silicates

mesoporous silica(2d hexagonal)

ΔT

This work is part of the AQUARIUS project, which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 731465. This project is an initiative of the Photonics Public Private Partnership. X-ray diffraction was performed at the interfaculty X-Ray Center of TU Vienna.

ENRICHTMENT OF BENZONITRILE

A novel flow cell system with easily exchangeable ATR crystals from low-cost silicon wafers allowing for fast screening of an extensive series of enrichment layers.

Ÿ Hydrophobic film repells water, which is largely eliminated from the probed region (compare absorption band at

-3400 cm ¹ for coated and uncoated silicon wafer)

Ÿ Uncoated wafers show no absorption of benzonitrile at -12230 cm (C-N mode)

Ÿ Limit of detection for benzonitrile is 30 ppm (3 x SEP)

Ÿ Response time: < 5 s was measured by displacing pure water in the flow cell with benzonitrile solutions while spectra were recorded. The sensor responds immediately and is just limited by the diffusion of the analyt into the sample chamber.

Ÿ time for regeneration: < 5 s

Institute of ChemicalTechnologies and Analytics

Synthesis of mesoporous silica by templating

Ordered periodic mesoporous materials with controlled pore size are synthesised by using surfactants as sacrificial template e.g. soft templates such as surfactants or

[5]amphiphilic block copolymers.

Covalent surface modification

Organosilanes (R'Si(OR) ) bearing functional groups R' can be either 3

introduced by co-condensation (pre-functionalisation) or grafted to silanol [6]groups at the surface of porous silica (post-functionalisation).

functionalised mesoporous silica

O

Functionalisation by co-condensation

Post-functionalisation by grafting

+ template

+ template+ R`-grafting group

HOO

O

O

O

O

R‘-Si-O-

R‘-Si-

R‘-Si-O-

Increasing hydrophobicity: Silica films with increased hydrophobicity are prepared by functionalisation with organosilanes with long aliphatic chains ((R'Si(OR) ), e.g. R' = 3

C8–C18), or trichloromethyl silane (R' = -CH₃) or hexamethyldisilazane (R'₃SiOR, R' = -CH₃).

SynthesisMesoporous silica coatings were synthesised by acidic condensation of tetraethoxysilane in ethanol with cetyl-trimethylammonium bromide by co-condensation with methyl-triethoxysilane (MTES) or grafting with hexamethyldisilazane (HMDS). The silica films were obtained by spin-coating on polished silicon wafers and subsequent calcination at 400 °C.

Characterisation-1Coatings are IR transparent in the information rich region at > 1300 cm : FTIR-ATR spectrum of a organically modified

-film shows absorptions of Si-O-Si stretching modes around 1070 cm ¹ and bands that can be assigned to -CH₃ -1 deformation modes at 1280 cm are visible.

Periodically ordered mesopores (2 - 50 nm) reduce diffusion resistance of the material. X-ray diffraction confirmes cubic and 2D hexagonal mesophase of functionalised films.

Left: FTIR spectra of mesoporous silica with R‘=CH . Right: X-ray diffraction pattern mesoporous silica with 2D hexagonal and cubic mesophase. 3

The film thickness was determined by scanning electron microscopy and with a profilometer and ranged from 250 – 400 nm, depending on the angular velocity of the spinner. The surface wettability was characterised by static contact angle measurements. The surface modification leads to increased conact angles from 27° for presitine silica to 88° for organically modified silica.

unmodified SiO₂ SiO₂ with TMES SiO₂ with HMDS

55°

e.g. R‘ = aliphatic chains

-1 2Left: Calibration curve for 100 - 1000 ppm of benzonitrile in water monitored at 2230 cm . R = 0.989. Right: Diffusion of benzonitrile into the sol-gel film for different concentrations.

FTIR spectra of water on an unmodified and a surface modified silicon ATR crystals.

Depth of penetration d of evanescent fieldp

For θ = 55° and n = 3.42 and n = 1.42:1 2-1 190 nm at 3400 cm (pathlength = 1140 nm)

-1 390 nm at 1600 cm (pathlength = 2340 nm)

Flowcell 100 µL

20 mm

55°

References[1] Flavin, K.; Hughes, H.; Dobbyn, V.; Kirwan, P.; Murphy, K.; Steiner, H.; Mizaikoff, B.; Mcloughlin, P. Int. J. Environ. Anal. Chem. 2006, 86 (6), 401–415.[2] Yang, J.; Cheng, M. L. Analyst 2001, 126 (6), 881–886.[3]Göbel, R.; Krska, R.; Kellner, R.; Seitz, R. W.; Tomellini, S. A. Appl. Spectrosc. 1994, 48 (6), 678–683.[4] Lu, Y.; Han, L.; Brinker, C. J.; Niemczyk, T. M.; Lopez, G. P. Sensors Actuators B Chem. 1996, 36 (1–3), 517–521.[5] Ciriminna, R.; Fidalgo, A.; Pandarus, V.; Beland, F.; Ilharco, L. M.; Pagliaro, M. Chemical Reviews 2013, 6592–6620.[6] Innocenzi, P.; Malfatti, L. Chem. Soc. Rev. 2013, 42 (42), 4198–4216.

520 µm

λ wavelengthn and n refractive index of ATR1 2

crystal and rare mediaθ angle of incidence ATR-Setup

Ÿ FTIR Spectrometer (Bruker Matrix)Ÿ LN-cooled MCT detectorŸ Silicon wafer (20 x 10 mm pieces) with 55° angled facettes and 6 active bounces Ÿ Flowcell (100 µL) connected to a peristalic pump

-Si-O-Si-

-Si-CH₃band

cubic (Pm3n)2d hexagonal