Optical Meets Mechanical: Use of Luminescence Spectroscopy To Assess Ageing in Biodegradable Films
Creators: name: Colaruotolo, Louis 
Publisher: name: University of Massachusetts Amherst
Year: 2019

description: With the growing concern of the accumulation of plastic-based food packaging waste, the search for bio-based biodegradable packages is on the rise. These materials differ from their petro-based counterparts in their degradation rates, which are much higher in the former. Not only do bio-based biodegradable materials degrade faster during post-usage processes but also they age faster during usage and storage, which affects their performance and functionality. The application of noninvasive testing methods with the capability to report on the matrix’s state could assist in the development of a more ubiquitous way to assess ageing in food packaging, particularly in biodegradable ones. To this end, the performance of a luminescence spectroscopy technique based on three luminescent probes, one intrinsic to the matrix and two added, was monitored and the sensitivity of the probes to report on ageing was analyzed.
Biodegradable films were made of 2% gelatin (type A) and 0.5% glycerol (plasticizer). Gelatin contains an intrinsic fluorophore, the aromatic amino acid tyrosine (Tyr), which can report on the molecular mobility of a matrix. Additionally, the films were doped with two extrinsic fluorophores, Fast Green FCF (FG) at 0.124 mM and pyranine (Pyr) at 0.05 mM, which can report on the physical state and available free water within a matrix, respectively. Films were casted onto plastic Petri dishes and stored at five relative humidities (RHs), namely 2.5, 25, 33, 53, and 75%, for five weeks with measurement collection every week. Films were tested using fluorescence spectroscopy at excitation and emission range wavelengths optimized depending on the assessed probe. Additional measurements to determine moisture content, changes in secondary protein structure using FTIR spectroscopy, and mechanical properties using a Universal Testing Machine in tensile mode aided in the evaluation of the sensitivity of the luminescent probes in sensing ageing.
Luminescent probes, intrinsic or added, have the capability to assess the physical state of the films <em>in situ</em> and can provide molecular level sensing of their local environment. Tyr emission showed a sharp increase in fluorescence intensity in films stored at low RH as a function of time. FG showed a similar pattern to that of Tyr but higher sensitivity to changes along the observed period. The two characteristics emission bands of Pyr provide information on the state of water within the matrix. Although the results on this probe hinted microstructural rearrangements within the films as a function of time, the sensitivity of this probe was not high enough at the conditions evaluated and provided limited information on films’ solvation. The sensitivity of the luminescent probes to changes during ageing were revealed through correlation of the photophysical properties of the two effective probes, Tyr and FG, and the mechanical properties of the films at different RH through storage. Both methods, mechanical and optical, were similarly sensitive to changes during ageing particularly after 3-week storage. However,, it can be speculated that because of the different scales at which optical and mechanical measurements report (local vs. bulk), the methods, they could also complement each other. These findings suggest that, in principle, a luminescence spectroscopy technique using intrinsic and extrinsic probes can replace mechanical testing to noninvasively monitor structural changes and stability of biodegradable packaging as a function of time.