Poster O2 - Effects of Geometry and Surface Modifications on the Sensitivity of Anisotropic Gold-Hydrogel Biosensors

Introduction: Biosensors are analytical devices that use a transducing component to emit a signal based on a molecular recognition event between a bound receptor identifying a biomarker/analyte. Of the various transducers that have been used for developing biosensors, gold nanoparticles (AuNPS) have been increasingly popularized in the last two decades. This popularity can be attributed to their optical properties including exhibition of the localized surface plasmon resonance (LSPR).
While there have been several studies investigating sensitivity of gold-based biosensors, there still lacks a comprehensive study on how plasmonic resonances are affected by the sensing environment and the binding dynamic biomolecules of anisotropic geometries. In this work, we investigated the influence of various factors on hybrid gold nanoparticle-hydrogel based biosensors in the pursuit of increasing their sensitivity towards detecting the proteins bovine serum albumin (BSA) and immunoglobulin G (IgG). The gold biosensors consisting of anisotropic AuNPs (e.g., bipyramids, nanorods, nanostars) were coupled to/embedded in charged poly(N-isopropylacrylamide) (PNIPAM)-based hydrogels through functionalization methods such as polyelectrolyte coatings (PEs). These surface modifications were explored to compare the effect each geometry will have on the localized surface plasmon resonance (LSPR) and their influence on the hybrid biosensor’s ability to detect surrounding proteins.

Materials and

Methods: Gold Nanoparticle Synthesis. All geometries of gold nanoparticles were synthesized in aqueous solutions using seed-mediated growth procedures. Once prepared the gold samples were washed and concentrated down to an optical density of 1 (OD=1) in fresh ultrapure water and characterized through scanning electron microscopy (SEM), UV-vis spectroscopy, and dynamic light scattering (DLS).
Functionalization. Before conjugation and encapsulation in the hydrogels, the AuNPs were functionalized with PEs. Coating the AuNPs with PEs were performed using a layer-by-layer deposition method involving alternating layers of the anionic poly (sodium 4-strenesulfonate) (PSS) and cationic poly(allylamine) solution (PAH). Following functionalization AuNPs were washed and resuspended in 10mM phosphate-buffered saline (PBS). Confirmation of functionalization was obtained through shifts in the UV-vis spectra and zeta potential measurements.
Hydrogel Synthesis. Poly (N-isopropylacrylamide-co-methacrylic acid) P(NIPAM-co-MAA) nanoparticles were fabricated using a free radical polymerization method. After characterization through SEM and DLS the P(NIPAM-co-MAA) nanoparticles were prepared by dispersing in 10mM PBS under sonication. This solution was then added to the previously functionalized AuNPs dropwise under vigorous stirring. This resulted in P(NIPAM-co-MAA) coated AuNPs that were then washed and redispersed in PBS.
Protein Sensing. Gold-hydrogel hybrid nanoparticles in PBS were mixed with varying concentrations of BSA and IgG ranging from 0.01 mg/mL to 1 mg/mL. After being mixed, the nanoparticles were then washed to remove any non-conjugated protein and redispersed in fresh PBS. Uv-vis spectra was then taken to identify any shifts in the LSPR indicating coupling of the proteins to the hybrid nanoparticles.

Results, Conclusions, and Discussions: At each stage of the development of these biosensors, the sensitivity of the sensing capabilities of the LSPR were compared and analyzed. It was found that the AuNPs alone were limited in their detection of the proteins and prone to aggregation. Once functionalized with the surface modifications we observed less issues with aggregation and shifts in the LSPRs were observed at lower protein concentrations in the AuNPs with plasmonic resonances closer to the near infrared. The addition of the hydrogel coating demonstrated greater shifts in the LSPRs. These results obtained facilitate an evolving study optimizing plasmonic-based sensing through AuNP geometry and functionalization.

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