Gupta group has pioneered the use of a continuous layer of a photo-functionalisable hydrogel to fabricate nano-/micro-structured sensors such as gratings. Thick hydrogel gratings with narrow pitch provide a good combination of angular separation between diffracted orders and measurement sensitivity, but are challenging to fabricate. We have addressed this challenge by devising a water soluble chemistry to incorporate high density of photofunctionalisable groups, while maintaining the optical clarity of the hydrogel.
We are now working to extend the approach to fabricate biosensors with three-dimensional patterns.
Benefits of using gratings: Gratings are internally-referenced and hence alleviate signal drift because of variations in temperature, bulk refractive index, emission wavelength of the light source and mechanical perturbations. Gratings are planar and can be completely made of hydrogels. Thus, gratings offer a significant potential to serve as wearable sensors for continuous monitoring of the concentration of biomolecules.
How gratings work: Our gratings comprise an array of alternate strips of recognition elements immobilised in a continuous hydrogel film. When light falls on this structure, it diffracts, where the angular separation between the diffracted orders depends on the wavelength of light and pitch of the grating.
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.
We can make amplitude and phase grating biosensors, which are based on imaginary and real refractive index contrast between alternate strips respectively. The imaginary refractive index is a measure of (absorption, scattering) losses of a material. The real refractive index on the other hand, is indicative of how fast last light travels in the material.
In case of amplitude grating biosensors, the absorption of the immobilised recognition elements change as a result of analyte binding. Amplitude gratings are well suited for measuring the concentration of small molecules such as metal ions present in low levels. In contrast, phase grating biosensors are based on a universal property of all species and hence can be adapted to measure a wide variety of analytes.