Light-microgel interaction in resonant nanostructures

Abstract

Combination of responsive microgels and photonic resonant nanostructures represents an intriguing technological tool for realizing tunable and reconfigurable platforms, especially useful for biochemical sensing applications. Interaction of light with microgel particles during their swelling/shrinking dynamics is not trivial because of the inverse relationships between their size and refractive index. In this work, we propose a reliable analytical model describing the optical properties of closed-packed assembly of surface-attached microgels, as a function of the external stimulus applied. The relationships between the refractive index and thickness of the equivalent microgel slab are derived from experimental observations based on conventional morphological analysis. The model is first validated in the case of temperature responsive microgels integrated on a plasmonic lab-on-fiber optrode, and also implemented in the same case study for an optical responsivity optimization problem. Overall, our model can be extended to other photonic platforms and different kind of microgels, independently from the nature of the stimulus inducing their swelling.

A Time-Efficient Dip Coating Technique for the Deposition of Microgels onto the Optical Fiber Tip

“…The morphological analysis has been carried out only outside of the fiber core area, because the presence of the nanostructure makes difficult to evaluate the CF. However, as already demonstrated in our previous studies [8,11,12] the pattern does not alter the distribution of the MGs, which are still conformally deposited onto the gold nanostructure. The AFM image shown in Figure 2b confirms the creation of a uniform and compact MGs film (with an average height lower than 40 nm, compatible with the formation of a single layer [11]), characterized by a CF higher than 90%.…”

“…In particular, by setting low operating pH (3), low temperature (10 • C) and high concentration (5%) of particle dispersions, we achieved high coverage factor (CF) films. We have demonstrated that CFs larger than 90% warranted the maximum degree of light-MGs interactions onto the fiber tip, and thus the maximum responsivity to MGs swelling/collapsing induced by the specific external stimulus of interest [11,12].…”

“…In this framework, we have recently proposed a LOF optrode integrated with microgels (MGs), i.e., 'smart' or responsive polymers, that change their size following environment changes [8][9][10][11][12]. Specifically, MGs are colloidal microsized hydrogel particles, which are synthesized to be selectively sensitive to physical (temperature) or chemical (i.e., pH variations, molecular binding events) parameters of interest when immersed in a liquid environment [13,14].…”

“…Before the MGs deposition, the reflection spectrum of the LOF probe in air (dashed black curve) presents a dip centered at 1363 nm with a FWHM of 85 nm. The reflection dip is due to the excitation of a LSPR[8][9][10][11][12]. Characterization of the MGs film deposited following the previous procedure onto the goad coated optical fiber tip.…”

Lab-On-Fiber Technology: A Roadmap toward Multifunctional Plug and Play Platforms

“…Further sensitivity improvements were recently achieved in the case of label-free “lab-on-tip” nanobioprobes, for controlled integration on the nanostructured tip of single-mode optical fibers in multiresponsive and functionalized microgels [ 58 ].…”