Tandem MOF-Based Photonic Crystals for Enhanced Analyte-Specific Optical Detection

Owing to their structural variability, metal–organic frameworks (MOFs) lend themselves well as chemical sensing materials by providing both high sensitivity and selectivity. Here, we integrate different types of MOFs (ZIF-8, HKUST-1, CAU-1-NH₂) into photonic multilayers referred to as Bragg stacks (BSs), which report on adsorption events through changes in their effective refractive index (RI). The fabrication of photonic multilayers is accomplished by spin-coating colloidal suspensions of MOF nanoparticles and/or the high RI-material TiO₂. While their incorporation in BSs allows for the label-free readout of host–guest interactions, the choice of particular types of MOFs determines the sensing properties of the BS. Here, we present MOF-based BSs with enhanced specificity toward molecular analytes by combining two different MOFs in a single platform. The sensing performance of our BSs is demonstrated by a combined spectroscopic and principal component analysis of their vapor response. Time-dependent measurements reveal fast response times and good recoverability of the multilayers. Moreover, we demonstrate that combinatorial sensing is feasible by arranging different MOF BSs in a basic color pattern, which highlights the potential of MOF-based multilayers in arrayed sensor devices

Low-Cost Thermo-Optic Imaging Sensors: A Detection Principle Based on Tunable One-Dimensional Photonic Crystals

Infrared (IR) sensors employing optical readout represent a promising class of devices for the development of thermographic imagers. We demonstrate an infrared radiation detection principle based on thermally tunable one-dimensional (1D) photonic crystals acting as optical filters, integrated with organic and inorganic light emitting diodes (OLEDs and LEDs, respectively). The optical filters are composed of periodically assembled mesoporous TiO₂ and SiO₂ layers. Due to the thermal tunability of the transmission spectrum of the optical filter, the intensity of light passing through the filter is modulated by temperature. The tuned spectrum lies in the visible region and, therefore, can be directly detected by a visible-light photodetector. The thermal response of the luminance of the OLED-photonic crystal ensemble is 3.8 cd m^–2 K^–1. Furthermore, we demonstrate that the local temperature profile can be time and spatially resolved with a resolution of 530 by 530 pixel, thus enabling a potential application as an infrared imaging sensor featuring low power consumption and low fabrication costs

Humidity-Enhanced Thermally Tunable TiO₂/SiO₂ Bragg Stacks

Tunable, stimuli-responsive photonic crystals (PCs) have developed into a fast growing, interdisciplinary research field attracting attention from various scientific communities, such as photonics, sensing, and materials chemistry. Here, we propose a thermally tunable and environmentally responsive optical filter derived from nanoparticle-based TiO₂/SiO₂ one-dimensional photonic crystals, christened Bragg stacks (BSs). Photonic crystals with textural mesoporosity were obtained by bottom-up assembly based on sequential spin-coating suspensions of TiO₂ and SiO₂ nanoparticles on glass substrates. The mechanism of the BS thermal tunability is based on the thermo-optic effect, i.e., dependence of the refractive index on temperature. Notably, the optical response of the BS to temperature can be significantly enhanced by varying the relative humidity of the environment. Thus, the magnitude of the spectral shift increases more than fourfold from 4.4 to 21.9 nm with a change in relative humidity from 25% to 55% in the temperature range between 15 and 60 °C. Thus, humidity-enhanced thermal tuning causes shifts of the transmission spectra by up to −1.66 nm K^–1. The simulations of the wavelength shift based on the measurement of the effective thermo-optic coefficient of the individual TiO₂ and SiO₂ layers at ambient conditions closely correspond to the experimental values. Owing to their high inherent porosities and ease of fabrication, nanoparticle-based BSs offer a great potential for the development of sensitive, label-free photonic crystal temperature and humidity sensors