Abstract
Time-temperature indicators (TTIs) are simple sensors that can tackle the global issue of food, vaccine, and medicine wastage. TTIs reveal true product quality by showing the cumulative effects of time and temperature. TTIs have not seen mainstream use due to drawbacks of cost, safety, adaptability, and reliability. A potential solution is to use polymeric colloidal crystals (CCs) that mimic natural opals. CCs contain arrays of polymer particles with diameters around half the wavelength of visible light, resulting in structural colour from the Bragg reflection of light. The polymeric basis of the CCs makes the structural colour highly temperature sensitive; thus, it is expected that CCs can act as novel TTIs, which has been investigated in this work.
Evaporation-driven self-assembly of a latex polymer was used to fabricate pristine CCs that were transparent when dried but bright white when hydrated. However, with 0.01 wt.% graphene inclusion, vibrant structural colours were achieved. The stopband midpoint was controlled by varying the particle diameters between 200 – 305 nm, resulting in CC colours across the visible spectrum. The novel application of ellipsometry to doped CCs found the stopband midpoint to be dependent on incident angle the midpoint blue shifted by ~ 125 nm as the incident angle changed from 15 to 85°.
An increase in CC thickness from 0.3 mm to 2 mm caused the stopband midpoint to be redshifted by ~ 20 nm. A novel approach to explain the shift is reported, which attributed the redshift to a 4% reduction in the average polymer packing fraction as the thickness increased. A unique in-house setup was built that recorded CCs during heating at different temperatures to quantify colour change over time. A refined setup based upon BS7901:1999 is now ready for large-scale testing. The activation energy EA was tuned, for the first time, by thickness and graphene content in the range of 36 – 55 kJ mol-1, which fell in the range of perishable products (8 – 210 kJ mol-1).
The first known mapping of self-assembled CC formation parameters used a modified Routh-Russell model. The model maps the dimensionless Péclet number (Pe) and control parameter λ ̅, which are the ratios of particle diffusion and deformation against time, respectively. The necessary conditions for CC formation were 1 ≤ Pe ≤ 10 and 105 ≤ λ ̅ ≤ 106. Brightly coloured iridescent thin film CCs were made by adding 10% butyl glycol; however, un-hydrated CCs had a preferable subtle colour. The first-ever production of CC-based TTI labels used CC thicknesses of 25 – 100 µm, and the labels had two EA separated by the Tg (~35 °C) and the main EA (68 – kJ mol-1) was driven by dry-sintering.
This TTI system can monitor one of the widest reported EA ranges, with potential for further extension. The CC-based TTI also combined colour change with a change in symbol, as demonstrated with a vanishing tick. The work is the first-ever demonstration of a CC-based TTI label made with scalable techniques that has a measurable, accurate, and tuneable time-temperature response. The technology is now ready for the next challenge of transitioning from lab to industrial scale.