Gradient Colloidal Crystals via Infusion-Withdrawal Coating of Fluorescent Latex Particles

Colloidal crystals (CCs), composed of monodisperse polymer latex particles, find extensive use in various sensing applications. Their photonic properties in combination with an adjustable response to external triggers make them a versatile class of materials. However, in some cases, the information provided by one single type of CC is insufficient. We have found that iridescent thin films with a unidirectional gradient of a physical property can circumvent this problem. Localized optical characterization of such a gradient nanostructure allows for a much more diagnostically conclusive readout compared to homogeneous films. While most literature on gradient CCs focuses on a change in interparticle distance of non-close-packed particle assemblies, we have recently shown the fabrication of a different type of gradient material. Infusion-withdrawal coating enables the fabrication of ordered thin films consisting of two isometric particle types with different thermal properties arranged in a compositional gradient (https://doi.org/10.1002/adma.202101948). The characterization and calibration of these structures proceeded via the incorporation of trace amounts of fluorescent nanoparticles.<br/>Enhancing the fluorescence intensity and thereby the accuracy of this evaluation is difficult, as higher concentrations of tracer particles would disrupt the periodic mesostructure. Here, we improve upon this by utilizing post-synthesis fluorescent labeling of the latex particles themselves. A reversible swelling/deswelling procedure allows incorporation of a hydrophobic dye of choice while maintaining the monodispersity and capability of co-crystallization with the pristine building blocks. Not only does this allow a drastic enhancement of the signal intensity, but it also makes the incorporation of more than one different, tailor-made particle type feasible. The photonic properties, i.e., optical stop-band, are fully retained thereby. We prepare particles labeled with Nile red (red-fluorescent) as well as others with Coumarin 1 (blue-fluorescent). Self-assembled particle mixtures and fluorescence microspectroscopy thereof show that the intensity ratio of the two non-overlapping emission spectra provides a highly accurate readout of the local composition, independent of the sample thickness. This, in turn, allows us to examine more complex gradient profiles in addition to the simple linear case. Regarding infusion-withdrawal coating, the shape of this profile can theoretically be shown to depend directly on the relative pumping rates of the two syringes. Non-linear gradients can therefore be prepared in a controlled manner without sophisticated pumping programs. The improved calibration using the post-synthesis labeled particles provides sufficient accuracy to characterize these samples and correlate the experimental results with theoretical calculations. The simplicity of this coating method and the nevertheless complex gradient nanostructures open a wider range of applications for this emerging type of colloidal assembly.