2.1. Metamaterial-Enabling Gel Constituents

Because mechanically robust gels and films with, for example, high optical transmission coefficients and low scattering can be made from cellulose²⁰⁻²² and because well-characterized cellulosic gel chemistries exist in the literature,²³⁻²⁶ we have chosen to make strong and translucent gels using lyotropic CNCs, which are synthesized via a sulfuric acid-hydrolysis method.²⁷ The partial replacement of primary alcohols with negatively charged sulfate ester moieties forms a stable aqueous CNC colloidal dispersion with intrinsic molecular charge and electrostatic charge-screened interactions in acidic and basic conditions. Moreover, the average measured length of the CNCs (∼300 nm, Figure ​Figure11a) is approximately equivalent to their average persistence length, which is a function of their average diameter (∼50 nm, Figure ​Figure11a) and crystalline cellulose fraction and distribution.²⁸ Because of the CNCs’ stiffness and ability to form stable colloidal dispersions, monodomain alignment of lyotropic CNC dispersions can be achieved with relatively weak extrusion stresses, as we show in this work.

(a) Transmission electron microscopy (TEM) micrograph demonstrating CNCs’ anisotropy, which enables guest–host alignment in a gel. (b) Spectral characterization of GNR inclusions with transverse (∼530 nm) and longitudinal (∼630 nm) surface plasmon resonance (SPR) extinction peaks. An aqueous dispersion of GNRs as viewed with the background white light is shown in the inset (c). (d) A scanning electron microscopy (SEM) micrograph of the top surface of a gel with the GNRs’ longitudinal axis aligned with the local in-plane director N(r), as shown. Guest GNRs (white rods) are aligned by host CNCs (dark background) and constrained within the same gel. The gel with GNRs viewed in the plane of their transverse axes, as indicated by N(r), is shown in the micrograph (e). The imaged ends of the GNRs (white dots) are indicated by the white arrows. The inset (f) shows a TEM micrograph of GNRs drop-cast on a substrate.

CNCs can provide orientational ordering to composite gels. Even if no chemical reaction occurs with the CNCs’ surface functional groups, the CNCs can become mechanically constrained about their sol-phase ordering directions within the gel matrix. Additionally, spectral characterization of GNRs dispersed in deionized water reveals their anisotropy characterized by a transverse surface plasmon resonance (TSPR) extinction peak at ∼530 nm and a longitudinal surface plasmon resonance (LSPR) extinction peak at ∼630 nm. The extinction spectrum is shown in Figure ​Figure11b, while the GNRs dispersed in water are shown with white background illumination in Figure ​Figure11c. As shown in Figure ​Figure11d, the GNRs’ rodlike shape allows them to be aligned with the local in-plane director N(r) defined by the CNC alignment. Co-alignment of their longitudinal axes occurs via steric interactions within LC media such as lyotropic CNCs. Secondary-electron detection reveals a stronger signal from the GNRs than from the background CNCs, due to a larger cross-section of secondary-electron generation for the gold. The GNRs appear as white rods surrounded by a dark background in the SEM micrographs of Figure ​Figure11d. Meanwhile, a perspective orthogonal to the plane of Figure ​Figure11d is shown in Figure ​Figure11e. Therein, as indicated by N(r), the plane defined by the GNRs’ transverse axes is demonstrated by the ends of the rodlike GNRs. These hemispherical-like ends are indicated by the white arrows and appear as white dots. Both transverse and longitudinal profiles of the GNRs are shown in inset 1(f), which was captured using TEM. The GNRs appear dark, while the Formvar background appears bright because of the differences between the GNRs’ and CNCs’ electron transmission (see the Supporting Information for preparation and measurement details). The GNRs’ anisotropy demonstrated in inset Figure ​Figure11f is also visible with alignment shown by Figure ​Figure11 d,e, demonstrating that the GNRs are aligned along the local director field N(r) that is defined by the colloidal CNCs in the composite gel. The GNRs’ alignment and mechanical constraint enable the polarization-dependent excitement of each GNR’s two distinct plasmonic extinction modes within the bulk host medium.²⁹⁻³²