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Sensor chips
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Introduction

In this section we discuss the sensors (sensor chips) which are commonly used in SPR systems. Since the size and the format (square, rectangular, with a protective sheet or without) differ between manufacturers, here we will focus on sensor surfaces.
Although the optical configuration (to measure the signal) and the fluidics (to deliver the sample) are important, the sensor can be seen as the heart of an SPR instrument. Despite being a disposable object, the sensor is still a high precision part and should be handled accordingly. For instance, al dust particles, finger prints and moisture on either side of the sensor surface will distort the SPR measurements. Therefore, let the sensor come to room temperature when stored cold and handle with care (forceps).

Sensor chip can be roughly divided in three types:

  • 2D planar-like surfaces
  • 3D-like surfaces
  • Inorganic surfaces
  • Metal coating

The sensors with 2D planar surfaces are virtually flat and the functionalisations are grafted directly on the gold layer. The 3D-like sensors have a matrix between the gold surface and the functionalisations which give more surface and capacity to bind compounds. Common matrices are dextran, alginate, agarose or poly-L-lysine (1), (2).

The dextran based sensor chips of Biacore vary the capacity by changing the percentage of carboxylation of the dextran. The capacity of the XanTec sensor chips is controlled by three parameters: i) spatial distance between the brush structured individual hydrogel chains, ii) hydrogel thickness via the chain length and iii) the charge density by the composition of the polycarbonates (1)

The density and thickness of the sensor surface should be optimized for the experiment. Use the info graphic below to make the best choice.

sensor surfaces thickness
Sensor surface thickness and densities - courtesy of Xantec

Both 2 and 3D can be functionalized with special inorganic binding groups such as carboxymethyl or NTA, or organic groups such as proteins or lipids.
Inorganic surfaces are overlaid over the gold plasmonic layer and can for instance be made of SiO2, TiO2, Al2O3, ITO, HfO2, ZnS, TiNO and are often used to test anti fouling systems.
When the metal itself is the surface it acts as the plasmonic of the sensor and can be used in studies of coating, corrosion, electrodeposition, electrode functionalization and others (3).

Overview of available sensor chips*
Modification Biacore BioNavis XanTec
plain gold surface Au Au Au
no matrix, low capacity C1 CMDP
short matrix, low capacity CMD-2D
short matrix, normal capacity CM3 CMD50, HC30
normal matrix, low capacity CM4 CMD-3DL HLCx
normal matrix, normal capacity CM5 CMD-3DM CMD200, 500, HC200
normal matrix, high capacity CM7 CMD-3DH CMD500D, HC1500M
poly HIS binding group NTA NID-2D, NID-3D NiHC1000M
hydrophobic surface HPA HPP
biotin capturing surface SA STR-2D, STR-3D SADxx, SAHCx
biotin capturing surface (regenerable) CAP-kit AVI-kit
biotin on surface BT-2D, BT-3D BDx
*) from manufacturers website

Keep in mind that the mentioned manufactures above all use their own technique to make the sensor chip and therefore it is difficult to compare the capacity of the sensor chip directly. On the site of Xantec more information is available about sensor chips.

Sensor surface applications
Sensor surfaceApplications
Long chain dextran/carboxymethyl dextran/alginateGeneral purpose; suitable for protein-protein interactions and small molecule analytes
Short chain dextran/carboxymethyl dextran/alginate or planar SAMsSuitable for protein-protein interactions and for large analyte molecules and particles
Immobilized (Strept)(Neu)(Extr)avidinCapture of biotinylated ligands
Immobilized NTACapture of poly-histidine tagged ligands
Immobilized Protein A, G or LCapture of IgG
Lipophilic modificationCapture of liposomes and supported lipid bilayers
Hydrophobic surfaceCapture of lipid monolayers
Plain gold surfaceSurface interaction studies and custom design of surface chemistry

References

(1)XanTec XanTec bioanalytics GmbH. (2020). Goto reference
(2)Bio-Rad ProteOn sensor chips - overview - bulletin 5404D. (2012).
(3)BioNavis BioNavis website. (2020). Goto reference