Inkjet printing, traditionally used in graphics, has been widely investigated as a valuable tool in the preparation of functional surfaces and devices. This review focuses on the use of inkjet printing technology for the manufacturing of different optical elements and photonic devices. The presented overview mainly surveys work done in the fabrication of micro-optical components such as microlenses, waveguides and integrated lasers; the manufacturing of large area light emitting diodes displays, liquid crystal displays and solar cells; as well as the preparation of liquid crystal and colloidal crystal based photonic devices working as lasers or optical sensors. Special emphasis is placed on reviewing the materials employed as well as in the relevance of inkjet in the manufacturing of the different devices showing in each of the revised technologies, main achievements, applications and challenges.

Optical planar waveguide sensors, able to detect and process information from the environment in a fast, cost-effective, and remote fashion, are of great interest currently in different application areas including security, metrology, automotive, aerospace, consumer electronics, energy, environment, or health. Integration of networks of these systems together with other optical elements, such as light sources, readout, or detection systems, in a planar waveguide geometry is greatly demanded towards more compact, portable, and versatile sensing platforms. Herein, we report an optical temperature sensor with a planar waveguide architecture integrating inkjet-printed luminescent light coupling-in and readout elements with matched emission and excitation. The first luminescent element, when illuminated with light in its absorption band, emits light that is partially coupled into the propagation modes of the planar waveguide. Remote excitation of this element can be performed without the need for special alignment of the light source. A thermoresponsive liquid crystal-based film regulates the amount of light coupled out from the planar waveguide at the sensing location. The second luminescent element partly absorbs the waveguided light that reaches its location and emits at longer wavelengths, serving as a temperature readout element through luminescence intensity measurements. Overall, the ability of inkjet technology to digitally print luminescent elements demonstrates great potential for the integration and miniaturization of light coupling-in and readout elements in optical planar waveguide sensing platforms.

Accurate positioning of luminescent materials at the microscale is essential for the further development of diverse application fields including optoelectronics, energy, biotechnology and anti-counterfeiting. In this respect, inkjet printing has recently attracted great interest due to its ability to precisely deposit with high throughput and no contact, functional materials on different types of substrates. Here, we present a novel photoacid catalysed organic-inorganic hybrid luminescent ink. The formulation, containing monomers bearing epoxy and silane functionalities, a photoacid generator and a small percentage of Rhodamine-B, shows good jetting properties and adequate wetting of the deposited droplets on the receiving substrates. Ultraviolet exposure of the deposited material triggers the cationic ring-opening polymerization reaction of the epoxy groups. Concomitantly, if atmospheric water is available, hydrolysis and condensation takes place, overall leading to a luminescent crosslinked hybrid organic-inorganic polymeric material obtained through a simple one-step curing process, without post baking steps. Advantageously, protection of the ink from actinic light delays the hydrolysis and condensation conferring long-term stability to the ink. Digital patterning leads to patterned emissive surfaces and elements with good adhesion to different substrates, mechanical and optical properties for the fabrication of optical and photonic elements and devices.

The present study investigates whether changes in headspace volatile organic compounds (VOCs) of refrigerated cooked meatballs (beef and pork) can predict the transition from freshness-to-spoilage driven by microbial growth. Hence, freshly prepared food samples were portioned into air-tight glass containers, stored at 6 °C, and analysed at regular intervals. Microbial growth was determined using culture-based techniques, while headspace VOCs were trapped on Tenax TA sorbent and analysed by thermal desorption heart-cut gas chromatography with flame ionisation and mass spectrometric detectors. Based on total viable counts (TVC) and the existing microbiological criteria, three food quality stages were defined: satisfactory (≤ 4 log CFU/g), acceptable (< 6 log CFU/g), and unacceptable (≥ 6 log CFU/g). All samples remained in a satisfactory stage up to 3 days, entered the acceptable region from days 4-5, and reached the unacceptable zone from day 6. TVC showed a significant positive correlation with all the microbial groups (r > 0.90, p < 0.0001). VOC profiles of fresh samples were dominated by aldehydes emitted by cooking, whereas spoiled samples contained microbially derived alcohols, acetals, esters, and sulfides. Particularly, 1,1-diethoxyethane was associated with TVC, LAB, psychrotrophs, yeasts and moulds, whereas 1-hexanol, 2,3-butanediol, sulfides and other VOCs were primarily associated with Pseudomonas. Ratios of aldehydes/1,1-diethoxyethane and aldehydes/2,3-butanediol changed dramatically from days 3-6 (Z = -22.43, p < 0.001; Z = 6.52, p < 0.001, respectively), thus emerging as potential markers indicating imminent spoilage. In contrast, hexanal/1-hexanol ratio differentiated spoilage progression from days 9-12 (Z = 52.72, p < 0.001).