Thick hydrogel gratings with narrow pitch provide a good combination of angular separation between diffracted orders and measurement sensitivity, but are challenging to fabricate via current methods i.e., soft lithography and photolithography. We have addressed this challenge by creating gratings in continuous layer of photo-functionalisable hydrogels. We have devised water soluble chemistries to obtain optically clear hydrogels with functional groups (e.g., amines as shown in the figure) that are blocked by photoremovable groups. By selective exposure to light, the functional groups can be freed in alternate strips and then reacted with recognition elements to obtain hydrogel grating biosensors.
We are using photofunctionalisable hydrogels to create amplitude and phase hydrogel grating biosensors.
How gratings work: When light falls on gratings, it diffracts. The angular separation between the diffracted orders depends on the wavelength of light and pitch of the grating (Equation 1). You can learn about the effect of the pitch of the grating on the diffraction pattern by watching this video. Equally, the diffraction efficiency (I±1/I0) is a function of the difference in optical transmission and real refractive index of alternate strips in case of amplitude and phase gratings, respectively (Equation 2).
Sensing principle: As analyte binds to the recognition elements, the refractive index contrast between alternating strips changes. Thus, the ratio of intensity of light in the first to zero diffracted order (i.e. diffraction efficiency) changes proportionally to the concentration of analyte in the sample, allowing quantitative measurements.
Benefits of using gratings: Gratings are internally-referenced and hence their output is independent of variations in ambient temperature, sample composition, wavelength of the light source and mechanical perturbations. Gratings are planar and can be completely made of hydrogels. Thus, hydrogel gratings can serve as wearable sensors for continuous monitoring of the concentration of biomolecules.