A critical component when doing a Surface Plasmon Resonance experiment is to choose the proper sensor chip. Included below is a description of seven common sensor chip types. This is not meant to be an exhaustive list but is meant to highlight common types of sensor chip surfaces that are useful when running SPR experiments. In terms of experiment type, the first two types of sensor chips are used for covalent coupling experiments. The next four are used for capture experiments. Lastly, the plain gold sensor chip sees its greatest utility for experiments involving user specific coating chemistries, adsorption studies or for redox experiments (SPR-Electrochemistry):

Dextran Sensor Chip - By far the most common sensor chip in use with SPR. Used for direct coupling, the target or ligand must be dissolved in a lower pH, lower ionic strength buffer. Hydrogel surfaces, particularly carboxymethyl dextran hydrogels, offer many advantages when used as a SPR sensor chip surface. Carboxymethyl dextran surfaces are very stable and resistant to non-specific binding of biomolecules. The dextran layer is in the form of a highly flexible non-cross-linked brush-like structure extending 100 to 200 nanometers from the surface. The flexible nature of the dextran contributes to the accessibility of binding sites on an immobilized ligand. Biomolecules are easily coupled to the surface using a variety of techniques similar to those used in affinity chromatography. Large amounts of protein, up to 50 ng/mm3, can be immobilized on carboxymethyl hydrogel surfaces due to the 3-dimensional nature of the hydrogel layer. This is important for experiments where a researcher is interested in studying how a low molecular weight analyte binds to a surface immobilized protein. Other hydrogels such as linear polycarboxylate offer a similar high capacity surface but with different selectivity and different non-specific binding properties. These chips are used for all types of biomolecular interactions but are particularly critical to use when there is a large difference in molecular weight between the protein coupled to the surface and the analyte flowing over the surface. These surfaces can be derivatized either off-line or on-line – for example with Ni-NTA or Streptavidin to create high capacity capture surfaces.

Planar Sensor Chip - There are several types of planar or low capacity slides that can be used for studying primarily protein-protein interactions. One version is to use a shorter chain dextran matrix which shares many of the benefits of longer chain dextran surfaces. Another version is a surface that consists of a mixed, self-assembled monolayer of alkanethiolates generated from the combination of polyethylene glycol-terminated alkanethiol (90%) and COOH-terminated alkanethiol (10%). The terminal polyethylene glycol chains minimize non-specific binding while the COOH groups provide a functional attachment site for immobilizing/capturing a molecule of interest. As was described above for a dextran chip, direct coupling involves dissolving the target or ligand in a lower pH, lower ionic strength buffer. Amine coupling, utilizing EDC/NHS chemistry, is the most common ligand immobilization approach but thiol, aldehyde, and maleimide coupling are also possible with this surface using the appropriate cross-linking chemistry. In addition, this surface can be converted to a capture surface such as a Ni-NTA or Protein A surface. It can also be modified with streptavidin or biotin for capturing biotinylated compounds or derivatized to create a hydrophobic surface.

Ni-NTA Sensor Chip
– is used to capture histidine-tagged molecules such as recombinant proteins. The captured molecule is dissolved in running buffer and injected directly. The ligand or target is oriented well on the surface since it is captured directly at the histidine tag via Ni²+/NTA chelation. The surface is easily regenerated with an injection of imidazole or EDTA to remove the metal ions and the captured ligand. Non-specifically bound material can be removed with 50 mM sodium hydroxide. This type of capture experiment can result in a decaying surface – this problem can sometimes be overcome by capturing a minimal amount of target or waiting only a short time before injecting the analyte. Or, if studying an interaction where a large amount of target needs to be captured in order to detect a small molecule flowed over it, the decay can be effectively eliminated by running a capture-couple experiment where the captured target becomes permanently attached to the chip and does not get removed after each analyte injection/ regeneration cycle.

Streptavidin or NeutrAvidin Sensor Chip – is used for the high affinity capture of biotinylated molecules such as proteins, peptides, and nucleic acids. The capture molecule is usually dissolved in running buffer, but for nucleic acids 1 M NaCl is also added to the solution. The binding of NeutrAvidin or Streptavidin to biotin is one of the strongest non-covalent interactions known so the surface can be regenerated without having to recapture the target or ligand after each regeneration step. Minimal biotinylation of the ligand and subsequent capture on a NeutrAvidin or streptavidin chip results in a more oriented arrangement of ligand molecules on the surface as compared to the more random arrangement from chemical immobilization such as amine coupling. This surface tends to lead to a less dense but more active

Hydrophobic Sensor Chip – is used to study lipid binding. Usually a long chain alkanethiol self-assembled monolayer (SAM) is formed on the surface of a gold chip. This results in a hydrophobic surface that is amenable to formation of lipid monolayers. This chip is used to study binding directly to the lipid captured on the surface, or to a protein that is first bound to the lipid. It is easily regenerated with an injection of a detergent (for example CHAPS).

Protein A Sensor Chip - is used for capturing certain antibodies. Protein A contains four high affinity binding sites capable of interacting with the Fc region from IgG of several species including human and rabbit. Optimal binding occurs at pH 8.2, although binding is also effective at neutral or physiological conditions (pH 7.0 to 7.6).

Plain Gold Sensor Chip – allows user to study surface formation and adsorption in real-time on bare Au and allows researchers to coat the chip with user-defined chemistries. In addition, a plain gold chip is useful for study redox reactions if an electrochemical accessory is available for the SPR instrument.