Novel pH-responsive nanoplasmonic sensor: controlling polymer structural change to modulate localized surface plasmon resonance response

Abstract

The detection of chemical or biological analytes in physiological media remains a great challenge and current methods suffer from low sensitivity, reproducibility, and require expensive instruments. Here we report the design of a simple, pH-responsive nanoplasmonic sensor utilizing polymer structural changes to induce localized surface plasmon resonance (LSPR) shifts. The sensors were fabricated by chemical attachment of poly(allylamine) onto ∼28 nm gold nanoprisms bound to a silanized glass surface. The reversible change of polymer structure upon protonation and deprotonation of its amine groups alters the nanoprisms' LSPR properties. A spectral shift of the nanoprisms' dipole peak was observed because of changes in thickness of local dielectric environment, which are caused by shrinking and swelling of the pH-responsive polymer. The pH-induced shrinking and the swelling transition provided the opportunity to design ultrasensitive glucose sensors. The pH change from oxidation of β-glucose by glucose oxidase, resulted in up to a 17 nm LSPR peak shift because of the 3.7 nm change in polymer thickness measured by in situ atomic force microscopy. The lowest concentration of glucose that can be repeatedly detected in bovine plasma with this sensor was 25 μM. This nanoplasmonic sensor exhibited simplicity of operation and excellent reproducibility. The polymer-functionalized sensor provided a powerful avenue for simple, ultrasensitive, and cost effective detection of target analytes, which can be translated to clinical application.