|Abstract:||Silicon photonic microring resonators have emerged as a promising technology for the sensitive detection of biological macromolecules, including proteins and nucleic acids. These bulk property detectors rely on the changes in the effective refractive index of the sensing region, eliminating the need for chromophoric or fluorescently-tagged samples. This robust and versatile sensing platform has sensitivities down to 10-7 RIU and a linear dynamic range on the order of 1 RIU, eliciting interest in non-biological analytical challenges such as the detection of high molecular weight polymers within gradient separations (currently impossible with the linear dynamic range of differential RI detectors) and the detection of small, non-specifically binding organics, especially toxic and regulated species such as pesticides and carcinogens in real time and at low concentrations.
Functionalizing the surface of silicon photonic microring resonators with covalently bound organics is one detection strategy to both increase detector sensitivity (by localizing the analyte within the organic layer) and lend a degree of selectivity in the partitioning behavior of the analyte into the sensing region (by matching chemistries between the organic layer and the analyte). There are many chemistries compatible with silicon dioxide surfaces, but the two presented within, hydrogels and polymer brushes, lend the researcher unique control over the platform’s chemical and physical properties. Hydrogels, in particular poly(acrylamide) and poly(acrylic acid), have well-defined syntheses and modification routes as they are widely used in pharmaceuticals and agriculture. Patterned enthalpic gradients embedded in hydrogels, for example, have already been used to direct chemical agents across surfaces without the need for external energy input. This surface-directed transport can be used to separate and concentrate analytes directly to the sensor, a critical need as sensor area decreases to the nanoscale. Interfacing this technology with a microring resonator array would allow for the robust detection of such transported analytes, which are currently limited to those with fluorescent tags.
Surface-initiated polymerization has been used for many years to selectively alter the surface properties, and with the development of atom-transfer radical polymerization as a commonly-used and highly-controlled polymer brush growth method, the researcher has tremendous control over the surface functionalization, allowing for patterning and gradients in chemical and physical brush properties. This is ideal for preparing thin, well-defined organic coatings over the silicon resonators, allowing for rapid diffusion to the sensor surface and even partitioning, while also allowing the researcher to embed specificity in the brush-analyte interactions (Q-poly(2-methacryloyloxyethyltrimethyl ammonium fluoride brushes for detection of coumaphos degradation, for example).
Here a general method for modifying silicon microring resonator arrays with hydrogels and polymer brushes is presented, in addition to an overview of the fundamental processes which can be probed with such modifications. Tuning sensor selectivity and specificity by optimizing interactions between the agent(s) of interest and the polymer construct can lead to response enhancements in excess of 1000% percent, relative to non-functionalized sensors, an important advance in the detection of toxic species such as organophosphates. The combination of microring resonators with recent advances in the creation of precisely controlled gradients within polymeric surfaces might allow for the active and directed transport of concentrated analytes onto specific sensor elements, thereby integrating together the often disparate steps of separation, concentration, and detection to a single sensing device.