Photonic Crystal Improves the Absorptiometric Sensitivity of Colorimetric Membrane By Cavity-Induced Light-Matter Interaction
Authors: Kawasaki Daiki , Oishi Ryoutarou , Kobayashi Nao , Mizuta Tatsumi , Sueyoshi Kenji , Hisamoto Hideaki , Endo Tatsuro 
Published: May 2020

Introduction

Photonic crystal (PhC) makes it possible to spatially and temporally confine light based on photonic bandgap. A highly localized light with the strongly enhanced electric field generated by PhC extremely enhances light-matter interactions, which has been applied to the enhancement of spontaneous emission [1]. 2D-PhC concentrates light in the photonic band-edge frequency in its slab and re-emits it into free-space. We have achieved various applications of the polymer- or TiO2-based 2D-PhC to optical ion sensors [2].
On the other hand, very recently, we have developed a poly(vinyl chloride)-based membrane with a highly lipophilic ionic liquid (IL) consisting of extremely concentrated two functional chemical sensing molecules, an anionic calcium ionophore, and a cationic dye. We have applied this IL-based membrane to highly sensitive, rapid and simple absorptiometric sensing of calcium ions [3]. Its absorption sensitivity depends on two factors under the definite thickness of membrane, the concentration and the absorption efficiency of the dye molecules. The former almost reaches its limit while the latter can be little increased by the molecular design. Therefore, another approach is necessary to drastically increase the sensitivity of the IL-based membrane.
Here, a hybrid structure consisting of TiO2-based 2D-PhC and IL-based membrane (PhC/IL hybrid) was designed (Fig. 1). It was demonstrated by and the experimental results that the sensitivity was 78 times higher than that of hybrid structure without PhC (Plane/IL hybrid). The Finite-difference time-domain (FDTD) calculation revealed that highly localized light in PhC slab greatly enhanced the sensitivity enhancement factor.

Material and method

TiO2-based 2D-PhC was fabricated by using the liquid-phase deposition method. Colorimetrically calcium responsive ionic liquid, [KD-M13][OP2P], was prepared according to a previously reported method [3]. A mixture of PVC (10 wt%) and [KD-M13][OP2P] (90 wt%) was dissolved in mixture solutions of THF (80 wt%) and ethanol (20 wt%). Then, prepared solutions were spin-coated onto the surface of PhCs, and the hybrid structures with different thicknesses of IL-based membrane were prepared. The thickness (T) of the membrane was 50, 100, 230 or 300 nm respectively. A plane/IL hybrid structure with the same material composition (thickness T: 170 nm) of the PhC/IL hybrid structures above was fabricated as the control sample.
Reflection spectroscopy was carried out by using a home-built optical microscope. The reflection spectra of the hybrid structures in aqueous sample solutions (0.1 M H3PO4 aq., 0 or 10-6 to 10-1 M calcium ions-contained 50 mM Tris-HCl buffers (pH 7.0), or 0.1 M NaOH aq.) were obtained. The sensitivity enhancement factor of the PhC/IL hybrid structure was first calculated as the ratio of the decrement of reflection intensity (R) to the thickness (T); s = D
R / T [nm-1], where D
R =
R(Ca(-6)) – R(Ca(-1)), and then the ratio of s of the PhC to the plane structure, σ’: σ’= s

PhC
/ s

Plane
was calculated.
Purcell factor P
L (P
L = LDOScavity / LDOSfree-space, where LDOS is local density of state.) of the dipole at certain positions and L (L is the distance between the dipole and bottom of the hole) in the holes of PhC was calculated by using FDTD simulation, and then the enhancement of the absorption efficiency by the hybrid structure was theoretically modeled based on Purcell effect.

Results and discussion

It was observed from both the electromagnetic simulation and the experimental results that the highly localized light (l = 640 nm) was generated in the holes of the PhC, showing the typical peak in the reflection spectrum by the surface emission. When the hybrid was immersed in the acidic solution (H3PO4 aq.), the peak intensity was at its maximum, which was normalized as 1. It was decreased as the concentration of calcium ion was increased, finally being at its minimum when the hybrid was in the basic solution (NaOH aq.). The response curves for different solutions containing the 10-6 to 10-1 M concentrations of calcium ions were shown in Fig. 2. It was confirmed that the sensitivity of response curve became higher as the thickness of the IL-based membrane was decreased, and was 78.0 times higher than that of Plane/IL hybrid structure at T = 50 nm. This results suggested that the sensitivity enhancement factor of the IL-base membrane was proportional to the LDOScavity of dipoles in the PhC-holes because the absorption efficiency of the bare membrane was enough high to absorb well-localized light in holes. This absorption effifciency enhancement system was theoretically modeled based on purcall factor: PL
. Good agreement between the experimental results and the theoretical model was confirmed. Here, we have demonstrated the improvement of the absorption cross-section of the colorimetric ion responsive membrane, resulting in the drastic improvement of the absorptiometry sensitivity.
[1] J. Wolters, et al.,
Appl. Phys. Let

t., 2010, 97, 141108
[2] S. Aki, et al.,
Anal. Chem.
, 2014, 86, 11986-11991
[3] Y. Niwa, et al.,
Analyst
, 2019, DOI: 10.1039/C9AN01769K

Figure 1