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Evanescent field Sensors Based on Tantalum Pentoxide Waveguides – A Review

View Article: PubMed Central - PubMed

ABSTRACT

Evanescent field sensors based on waveguide surfaces play an important role where high sensitivity is required. Particularly tantalum pentoxide (Ta2O5) is a suitable material for thin-film waveguides due to its high refractive index and low attenuation. Many label-free biosensor systems such as grating couplers and interferometric sensors as well as fluorescence-based systems benefit from this waveguide material leading to extremely high sensitivity. Some biosensor systems based on Ta2O5 waveguides already took the step into commercialization. This report reviews the various detection systems in terms of limit of detection, the applications, and the suitable surface chemistry.

No MeSH data available.


Bidentate und monodentate phosphate coordination to tantalum ions. Adapted from [61].
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f14-sensors-08-00711: Bidentate und monodentate phosphate coordination to tantalum ions. Adapted from [61].

Mentions: Common to all publications cited above is the investigation of surface modifications by indirect methods, i.e. quality assessment based on the binding efficiency using DNA hybridization or immunoassays. Direct analyses of surface modifications on Ta2O5 by angle dependent X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle measurement or time-of-flight secondary ion mass spectrometry (ToF-SIMS) are published by the Laboratory of Surface Science and Technology, ETH Zurich [60, 61]. They obtained highly-oriented self-assembled monolayers (SAM) of octadecyl phosphoric acid on Ta2O5 surfaces. For the preparation of the waveguides 500 μM octadecyl phosphoric acid ester [C3H37OPO(OH)2] was dissolved in n-heptane/2-propanol in a 100/0.4 solvent mixture and filtered by a 0.2 μm cellulose nitrate filter. The SAM was formed by subsequent immersion in this solution for up to 48 h, rinsing with 2-propanol and blow-drying with He. AFM images of the produced layer show small areas with a hexagonal pattern with an average nearest-neighbour distance of 0.49 nm ± 0.01 nm. Nevertheless an ordered structure of the molecular layer is not clearly observable over larger areas. Calculations using published bond lengths [62] (0.16 nm for the O-P, 0.14 nm for the O-C, and 0.17 nm for the O-Ta) result in a density of the phosphate and hydrocarbon regions of 2.0 g/cm3 and 1.1 g/cm3, respectively. Taking these data into account for angle resolved XPS with an evaluation based on a three layer model for thin oxide films on metallic substrates (1st substrate Ta2O5, 2nd phosphate interfacial layer, 3rd hydrocarbon layer) a thickness of 2.2 nm ± 0.2 nm was found for the octadecylphosphate (ODP) monolayer at all emission angles. ToF-SIMS spectra showed high occurrence of tantalum oxide phosphate species and indicate a close packing of the ODP molecules. To some extent more than one phosphate head group is coordinated to one tantalum ion. The ToF-SIMS data are in very good accordance to the expected model of coordination of ODP on Ta2O5 involving the presence of both bidentate phosphate anchor groups bound to one tantalum ion and of monodentate coordination of two phosphate head groups to one tantalum ion (Figure 14). By near-edge X-ray absorption fine structure spectroscopy (NEXAFS) an average tilt angle of 30-35° was experimentally determined. With this value and the total length of the ODP molecule of 2.5 nm a theoretical thickness of 2.1 nm ± 0.05 nm was calculated showing good agreement with the theoretical value obtained in the experiment.


Evanescent field Sensors Based on Tantalum Pentoxide Waveguides – A Review
Bidentate und monodentate phosphate coordination to tantalum ions. Adapted from [61].
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3927514&req=5

f14-sensors-08-00711: Bidentate und monodentate phosphate coordination to tantalum ions. Adapted from [61].
Mentions: Common to all publications cited above is the investigation of surface modifications by indirect methods, i.e. quality assessment based on the binding efficiency using DNA hybridization or immunoassays. Direct analyses of surface modifications on Ta2O5 by angle dependent X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle measurement or time-of-flight secondary ion mass spectrometry (ToF-SIMS) are published by the Laboratory of Surface Science and Technology, ETH Zurich [60, 61]. They obtained highly-oriented self-assembled monolayers (SAM) of octadecyl phosphoric acid on Ta2O5 surfaces. For the preparation of the waveguides 500 μM octadecyl phosphoric acid ester [C3H37OPO(OH)2] was dissolved in n-heptane/2-propanol in a 100/0.4 solvent mixture and filtered by a 0.2 μm cellulose nitrate filter. The SAM was formed by subsequent immersion in this solution for up to 48 h, rinsing with 2-propanol and blow-drying with He. AFM images of the produced layer show small areas with a hexagonal pattern with an average nearest-neighbour distance of 0.49 nm ± 0.01 nm. Nevertheless an ordered structure of the molecular layer is not clearly observable over larger areas. Calculations using published bond lengths [62] (0.16 nm for the O-P, 0.14 nm for the O-C, and 0.17 nm for the O-Ta) result in a density of the phosphate and hydrocarbon regions of 2.0 g/cm3 and 1.1 g/cm3, respectively. Taking these data into account for angle resolved XPS with an evaluation based on a three layer model for thin oxide films on metallic substrates (1st substrate Ta2O5, 2nd phosphate interfacial layer, 3rd hydrocarbon layer) a thickness of 2.2 nm ± 0.2 nm was found for the octadecylphosphate (ODP) monolayer at all emission angles. ToF-SIMS spectra showed high occurrence of tantalum oxide phosphate species and indicate a close packing of the ODP molecules. To some extent more than one phosphate head group is coordinated to one tantalum ion. The ToF-SIMS data are in very good accordance to the expected model of coordination of ODP on Ta2O5 involving the presence of both bidentate phosphate anchor groups bound to one tantalum ion and of monodentate coordination of two phosphate head groups to one tantalum ion (Figure 14). By near-edge X-ray absorption fine structure spectroscopy (NEXAFS) an average tilt angle of 30-35° was experimentally determined. With this value and the total length of the ODP molecule of 2.5 nm a theoretical thickness of 2.1 nm ± 0.05 nm was calculated showing good agreement with the theoretical value obtained in the experiment.

View Article: PubMed Central - PubMed

ABSTRACT

Evanescent field sensors based on waveguide surfaces play an important role where high sensitivity is required. Particularly tantalum pentoxide (Ta2O5) is a suitable material for thin-film waveguides due to its high refractive index and low attenuation. Many label-free biosensor systems such as grating couplers and interferometric sensors as well as fluorescence-based systems benefit from this waveguide material leading to extremely high sensitivity. Some biosensor systems based on Ta2O5 waveguides already took the step into commercialization. This report reviews the various detection systems in terms of limit of detection, the applications, and the suitable surface chemistry.

No MeSH data available.