Colorimetric humidity sensors based on block copolymer photonic gels

Abstract

The measurement of humidity is demanded in wide range of areas, including agriculture, horticulture, meteorological services, food processing, air-conditioning, and electronic processing. While electronic humidity sensors are commercialized, other types of humidity sensors that can be fabricated thin, light-weight and cheap are still required for many applications. Comparing with their conventional electronic counterparts, colorimetric humidity sensors based on photonic crystals offer specific advantages, such as small size and weight, cheap, and exemption of electromagnetic interference. Photonic crystals, artificial materials with a periodic refractive index, provide remarkable control of light and offer exciting prospects for next-generation optical sensors. Suitably designed photonic crystals respond to environmental changes with a visual color change and can act as a colorimetric sensor, requiring no power source and no read-out system other than the human eye. For example, colloidal crystal arrays (CCAs) have been functionalized with hydrogels for constructing tunable photonic crystals, and their unique responsive optical properties have been utilized for transducing chemical or biological signals directly into optical signals. While simple, this sensing approach is intuitively appealing and is explored as chemical indicators and biological assay techniques. Self-assembly of block copolymers have also been investigated for creating various photonic structures. Their abundant morphologies including lamellae, cylinders, gyroids and spheres were exploited to create 1D, 2D, and 3D photonic crystals. Since the band structures are highly dependent on both the refractive index and the periodicity of the dielectric layers, the photonic band gaps of block copolymer photonic crystals can be further modulated when a domain is swollen by selective solvents. Our group recently reported highly tunable photonic gels based on polystyrene-b-quaternized poly(2-vinyl pyridine) (PS-b-QP2VP). Selective swelling of PS-b-QP2VP block copolymer lamellae leads to increase both the domain spacing and the refractive index contrast between the layers, and which results in the shift of the photonic band position to longer wavelengths. The swollen photonic gels found many applications including as an active component of reflective displays, memory and biosensors. Herein, we report block copolymer photonic gels as colorimetric humidity sensor films where the measurement sensitivity and humidity range can be modulated by simple ion-exchanging or by varying molecular weight of block copolymers. The block copolymer photonic gel films were prepared as previously described. Briefly, in-plane oriented lamellar films were first prepared by spin casting PS57-b-P2VP57 or PS190-b-P2VP190 solution (6 wt% in propylene glycol monomethyl ether acetate, the subscript number represent the number-average molecular weight of each block, Mn×10) onto glass followed by annealing the films in chloroform vapor at 50 C for 2 days. The thickness of the film was controlled to ~1 μm. The P2VP blocks were then quaternized using iodomethane (5 vol% in hexane) at 50 C for 72 h. Finally, the counter anions pairing with pyridinium were exchanged with a variety of anions including iodide, bromide, chloride and acetate by immersing the films into aqueous salt solutions. The optical properties of PS-b-QP2VP block copolymer photonic gel films were measured on the UV-vis spectrometer by reflection mode while continuously varying humidity from 20% to 100%. Humidity was controlled by using the mixtures of ethylene glycol and water. For photonic stop bands (PSB) measurements, photonic gel films were placed in the chamber containing various compositions of ethylene glycol and water, and allowed to equilibrate for 10 min before measurement. Environmental scanning electron microscopy (ESEM) images were obtained a Philips XL30 ESEM. The relative humidity in the vicinity of the specimen was controlled by varying the vacuum level of the specimen chamber in ESEM.