This work focuses on demonstrating proof-of-concept for a novel nanoparticle optical signal amplification scheme employing hybrid porous silicon (PSi) sensors. We are investigating the development of target responsive hydrogels integrated with PSi optical transducers. These hybrid-PSi sensors can be designed to provide a tunable material response to target concentration ranging from swelling to complete chain dissolution. The corresponding refractive index changes are significant and readily detected by the PSi transducer. However, to increase signal to noise, lower the limit of detection, and provide a visual read out capability, we are investigating the incorporation of high refractive index nanoparticles (NP) into the hydrogel for optical signal amplification. These NPs can be nonspecifically encapsulated, or functionalized with bioactive ligands to bind polymer chains or participate in cross linking. In this work, we demonstrate encapsulation of high refractive index QD nanoparticles into a 5wt% polyacrylamide hydrogel crosslinked with N,N'-methylenebisacrylamide (BIS) and N,N Bis-acryloyl cystamine (BAC). A QD loading (~0.29 wt%) produced a 2X larger optical shift compared to the control. Dissolution of disulphide crosslinks, using Tris[2-carboxyethyl] phosphine (TCEP) reducing agent, induced gel swelling and efficient QD release. We believe this hybrid sensor concept constitutes a versatile technology platform capable of detecting a wide range of bio/chemical targets provided target analogs can be linked to the polymer backbone and crosslinks can be achieved with target responsive multivalent receptors, such a antibodies. The optical signal amplification scheme will enable a lower limit of detection sensitivity not yet demonstrated with PSi technology and colorimetric readout visible to the naked eye.
This work focuses on the development of a proof-of-concept optical sensor design that incorporates an
amine-functionalized polyacrylamide hydrogel into a 1D porous silicon (PSi) photonic crystal. The PSi
acts as both a template and a transducer capable of detecting morphological and dielectric changes in the
incorporated hydrogel structure. Free radical copolymerization of acrylamide (AAm) and N-(3-
aminopropyl)-methacrylamide (NA) monomers was utilized to form copolymer chains with a controlled
concentration of nucleophilic amine moieties. These amine groups facilitated chemical cross-linking of the
copolymer chain to generate hydrogel networks. A molar fraction of >2 mol% of NA monomer was needed
to facilitate a visibly gelatinous hydrogel in a 5 wt% polymer solution. Addition of sodium formate (chain
transfer agent) during copolymer synthesis facilitated decreased copolymer chain length and improved
infiltration of the copolymer into the p-type PSi mesoporous sensor (pore diameters ~20-30 nm).
Controlled cross-linking of the copolymer chains was completed with using glutaraldehyde, as a model
system, to form a hydrogel network that could be optically monitored by the incorporated PSi sensor. These
results lay foundation for extending this versatile methodology towards the design of an affinity based
complimentary target-probe system to create a hybrid target-responsive hydrogel-PSi chemical sensor.