Colloidal photonic crystals for sensor applications

Abstract

Colloidal crystals can be formed by self-assembly in which structural reflective color can be highly sensitive to interparticle distance or reflective index mismatch between particle and matrix. Therefore, if either matrix or particles are made out of stimuli-responsive materials, various environmental parameters can be sensed such as pH, temperature, pressure, mechanical deformation and so on. Also, by introducing chemical probes, various biomolecules (DNA) or toxic chemicals including gas and volatiles can be detected, which would be further extended to biomedical sensor platform. Compared with other sensor device, colloidal crystals are mechanically and chemical stable and their color change is reversible and could be operated without external (electrical or photon) energy. In this chapter, basic design principle of colloidal photonic crystals and sensors are described and representative examples of sensors are provided for various stimulus. Monodisperse colloidal particles can spontaneously organize into highly ordered arrays, known as colloidal crystals, when they are concentrated or possess repulsive interparticle potentials. The colloidal crystals provide periodic modulation of refractive index, which gives rise to selective diffraction of light through constructive interference. In particular, when the size of colloids is comparable to half the wavelength of visible light, the crystals exhibit iridescent reflection colors. The resonance wavelength is determined by periodicity and refractive index of the lattice structure. Therefore, if the periodicity or refractive index are changed under specific conditions, the wavelength or equivalently reflection color is also altered. This enables us to make colloidal crystal-based sensors. The wavelength shift or color change is quantitatively analyzable with reflectance or transmittance spectra. The design principles of the sensors rely on stimuli-responsive change of periodicity or refractive index. Several distinct structures of colloidal crystals and their derivatives have been prepared with various sets of stimuli-responsive materials to provide such sensors and provided new opportunity in various sensing applications. This chapter covers basic design principle of colloidal photonic crystals and sensors. In subsequent four sections, we discuss (1) the basic mechanisms of colloidal crystallization, (2) optical and mechanical properties of colloidal crystals and their derivatives, (3) stimuli-responsive materials, and (4) three distinct formats of colloidal structures. In fifth section, we classify sensors with respect of stimulus and provide a short review of representative examples of sensors for each stimulus.