Abstract
Quick Response (QR) codes are a gateway to the Internet of things (IoT) due to the growing use of smartphones/mobile devices and its properties like fast and easy reading, capacity to store more information than that found in conventional codes, and versatility associated to the rapid and simplified access to information. Challenges encompass the enhancement of storage capacity limits and the evolution to a smart label for mobile devices decryption applications. Organic–inorganic hybrids with europium (Eu3+) and terbium (Tb3+) ions are processed as luminescent QR codes that are able to simultaneously double the storage capacity and sense temperature in real time using a photo taken with the charge‐coupled device of a smartphone. The methodology based on the intensity of the red and green pixels of the photo yields a maximum relative sensitivity and minimum temperature uncertainty of the QR code sensor (293 K) of 5.14% · K−1 and 0.194 K, respectively. As an added benefit, the intriguing performance results from energy transfer involving the thermal coupling between the Tb3+‐excited level (5D4) and the low‐lying triplet states of organic ligands, being the first example of an intramolecular primary thermometer. A mobile app is developed to materialize the concept of temperature reading through luminescent QR codes.
Highlights
Luminescent Quick Response (QR) codes were fabricated with Eu3+/Tb3-doped organic–inorganic hybrid materials
These figures of merit result from the thermal coupling between the 5D4 Tb3+-excited level and the low-lying triplet states of the organic ligands and are among the best ones known for luminescent thermometers
The ratio between the intensity of the green and red pixels of the photos are the basis for the temperature sensing through a unique intramolecular primary thermometer opening the possibility for the implementation of QR codes in mobile Internet of Things (IoT) without the need of any technological adaptation of current smartphones
Summary
To the well-known relation between the luminescence intensity of a given transition and the lifetime of the upper transition level[27] (Equation S4 in Supporting Information), the thermal dependence of the 5D0 → 7F2 (IEu) and 5D4 → 7F5 (ITb) transitions (Figure 2c) should be analogous to that found for τ(T) of the 5D0 and 5D4 states (Figure 2b) This is observed in the 280–317 K range. We want to emphasize that rewriting the thermometric parameter as ΔN the system follows the same functional dependence previously reported by some of us for luminescent thermometers described by two thermally coupled emitting levels ruled by the Boltzmann law.[28] To validate the use of Equation (3) to calculate the absolute temperature, we first use the well-established analysis of the emission spectra. The app is free and available for download at https://tinyurl. com/qr-luminescent
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