NANOCOATINGS FOR BIOSENSORS

MAKING SENSE OF BIOSENSORS

A biosensor is a device that detects the presence or concentration of specific biomolecules, microorganisms or other biological analytes. Detection takes place in three steps:

  • binding of the analyte to a specific recognition element (bioreceptor)
  • translation of the biological binding event into a physicochemical signal (transduction)
  • processing of the signal into useful information (e.g. concentration)


Many different types of transducers are available, all translating a biological signal into a measurable quantity, which can be optical (e.g. refractive index, light intensity), electrical (e.g. potential, current) or mechanical (mass). Once the signal is transduced, it can be processed (e.g. amplification, filtering) and converted into relevant (bio)chemical information such as concentration.

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SURFACE MODIFICATION FOR BIOSENSORS

The key requirement for any biosensor is that it only detects a very specific analyte, which may be present in a very low concentration and surrounded by many other components. To ensure that only the analyte of interest is detected, a specific bioreceptor needs to be immobilized on the surface of the transducer element. Bioreceptors can be proteins (antibodies, enzymes), nucleic acids (DNA, RNA), or other biological or biomimetic structures. Thus, surface modification is a key step in the fabrication of biosensors. Beside specific binding of the analyte to the bioreceptor, undesired non-specific binding of the analyte or other components to the surface may also occur through Van der Waals or electrostatic interactions. This can be prevented by application of a protein-repellent antifouling nanocoating.


Surfix’s nanocoatings have been extensively used for the immobilization of bioreceptors on sensor surfaces. Moreover, patterned nanocoatings offer the possibility to concentrate the analyte on a specific area of the sensor surface, while at the same time reducing non-specific adsorption on the rest of the sensor.

Depending on the application, 2D or 3D nanocoatings may be required for optimal performance of the biosensor. Also, each analyte requires a specific bioreceptor to be immobilized on the sensor surface. Therefore, Surfix develops a custom nanocoating for each biosensing application to obtain the highest possible sensitivity and selectivity. Some examples are elaborated in the R&D projects BioCDx and BioMeander.




EXAMPLE: MATERIAL-SELECTIVE NANOCOATING FOR BIOSENSORS

Many biosensor surfaces consist of more than one material. Examples include semiconductor nanowires or metal electrodes on a dielectric substrate, or optical sensors based on Si3N4 waveguides embedded in SiO2. In all these examples, detection of the analyte can only take place on one of the materials present at the surface (the ‘sensing material’, i.e. the semiconductor nanowire, the metal electrode and the exposed area of the Si3N4 waveguide). Therefore, it makes sense to use a patterned nanocoating to immobilize the bioreceptor only on the sensing material, ensuring that specific binding of the analyte to the bioreceptor only occurs where it can be detected. To prevent non-specific adsorption, an antifouling nanocoating can be applied to the non-sensing material in a second surface modification step.




In the case of a Si3N4/SiO2 waveguide biosensor, the sensing material (Si3N4) typically makes up less than 1% of the sensor surface. When using a uniform nanocoating for biofunctionalization, this results in the bioreceptor being immobilized on the whole sensor surface, over 99% of which consists of the non-sensing SiO2. Therefore, a significant improvement is expected when a material-selective nanocoating is used to immobilize the bioreceptor only on the Si3N4, and an antifouling nanocoating is applied on the SiO2.

A direct comparison between sensors with a uniform and a material-selective nanocoating indeed shows important benefits of the material-selective nanocoating:

  • increased sensitivity (especially at low concentrations)
  • reduced non-specific adsorption