Photonic crystal fiber for layer-by-layer assembly and measurements of polyelectrolyte thin films.

The cladding air channels of an endlessly single-mode photonic crystal fiber (PCF) and the high-index sensitivity of its long-period gratings (LPG) inscribed by CO(2) laser have been exploited to deposit poly(vinyl pyrrolidone) (PVPON)/poly(methacrylic acid) (PMAA) polyelectrolyte thin films via layer-by-layer assembly (LbL) and to measure the deposition process. We show that LbL can be controllably carried out within the axially aligned air channels. PCF-LPG is highly sensitive to the LbL process as reflected by ~1.625 nm shift in the resonance wavelength per polyelectrolyte layer incorporated. PCF-LPG is also very robust for in situ monitoring of the release of PVPON from cross-linked polyelectrolytes, which results in the formation of pH-responsive PMAA hydrogel. PCF-LPG containing the hydrogel exhibits well-behaved response to changes in solution pH over 2 to 7.5. We demonstrate that PCF-LPG is 2 orders of magnitude more sensitive than its traditional all-solid counterpart through parallel investigation.