Micrometer scale layers - waveguide mode of MP-SPR
Measure molecular interactions as well as layer properties of micrometer scale layers using MP-SPR technology.

Application Note #131

Langmuir-blodgett


"MP-SPR can work with thicker layers and wider range of materials than traditional SPR.

 While traditional SPR is typically limited to measurement on gold and ultrathin (layers up to a few microns thick. Thicker layers can be measured thanks to "deep SPR" or a waveguide-mode that forms multiple SPR peaks, while thin layers form only one. 

Moreover, MP-SPR is capable of working with ceramics, co-polymers, glass, alloys, metals and many other materials not accessible to traditional SPR. MP-SPR can measure molecular interactions on such layers as well as determine their thickness AND refractive index, surface coverage, etc."

Optical biosensor techniques based on evanescent field sensing, such as surface plasmon resonance (SPR) have typically an effective detection depth of a hundred nanometers. For many coating interaction studies, such small thickness can be difficult to achieve. Optical waveguides are phenomenon that can occur in structures able to capture and internally reflect light. A related optical phenomena, so-called SPR waveguide mode, might occur when between metal and the sample medium, there is a dielectric guiding layer with sufficiently high dielectric constant (ε) and thickness (d). The plasmon excitation by the visible light can exit at several angles which are dependent on the whole optical system properties, and an angular spectrum of reflected light intensity(SPR-angular curve) will show several minima (multiple SPR peaks). Also these minima will respond to optical changes such as thickness increase or protein absorption to the structure surface, allowing interaction measurements with much thicker layers than what is traditionally thought to be possible with SPR.

Polystyrene was dissolved in toluene (5 w/w) keeping mixture at room temperature for 48 hours. Polystyrene was spin-coated on top of a standard gold sensor surface with 1 minute spinning at 50 rps. The SPR Navi™ 200 was used to measure SPR-curves with two different wavelengths (670 and 785 nm) before and after sensor coating. Human serum albumin (HSA) interaction with polystyrene layer was measured with SPR Navi™ 200. Measured HSA concentrations were 1.6, 6.3, 25, 100, 400 and 1000 μg/ml. Trace Drawer™ 1.4 wasused to analyze the interaction steady-state affinity parameters.

Interaction of HSA-polystyrene was studied on a polystyrene of 660 nm. The thickness and refractive index of the polystyrene was determined from MP-SPR measurement at two wavelengths (670 nm and 785 nm) with 3% deviation between wavelengths analysed independently. The Trace Drawer™ analysis revealed that HSA interaction with polystyrene coating was 2 μM.

Recommended instrument for this application:

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Further reading

  • Do you want to see MP-SPR animation about thick layers? Click here.

  • If you are interested in reading more about thin film characterization, view AN#128 about thickness and refractive index and AN#149 about polymer characterization using MP-SPR.

  • Do you want to see how MP-SPR instruments work? Click here.

  • Do you want to see comparison of MP-SPR instruments? Click here.

  • Here are a few publications you can have a look at.

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