In this study, it is shown how the temperature range of luminescent thermometers can be widened in an unprecedented way by combining the intra‐ and interconfigurational transitions of the Pr3+ ion in a single material. Using Sr2GeO4:Pr3+ crystalline powders as an illustrative example, the implementation of luminescence thermometry is reported in the broadest temperature range up to now (17–600 K) with a remarkable performance: maximum relative sensitivity values of 9.0% · K−1 (at 22 K, cryogenic range), 0.6% · K−1 (at 300 K, physiological range), and 0.5% · K−1 (at 600 K, high‐temperature range) and minimum temperature uncertainty of 0.1 K.

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.

Having proven that the temperature range of luminescent thermometers can be greatly widened by combining the intra‐ and interconfigurational transitions of the Pr3+, the possibility to manage important thermometric parameters by bandgap engineering and variation of energy of excitation photons are examined. Partial replacement of Ge with Si to form Sr2(Ge,Si)O4:Pr is very useful to manage these luminescence thermometer properties. This allows control of the range of temperatures within which the 5d1→4f Pr3+ luminescence can be detected. Also, excitation energy appears to affect the thermometer's performance. These allow adjustment of the range of temperatures that can be measured with the highest accuracy, reaching the spectacular value of Sr = 9.2% K−1 at 65 K for a Sr2(Ge0.75,Si0.25)O4:0.05%Pr3+ thermometer upon 244 nm excitation. For the first time, it has been proven that excitation energy may significantly affect the performance of luminescence thermometers. In Sr2(Ge0.75,Si0.25)O4:0.05%Pr3+ the highest relative sensitivity shifts from 65 K upon 244 nm excitation (Sr = 9.2% K−1) to 191 K upon 253 nm excitation (Sr = 3.97% K−1). This occurs despite both excitation wavelengths fitting within the 4f→5d1 excitation band. This paper shows that bandgap management is useful to effectively design new luminescent thermometers.

Sensors play a key role on the Internet of Things (IoT), providing monitoring inside and outside the networks in a multitude of parameters. A fundamental parameter to sense is temperature, being essential to acquire knowledge on the best way to include thermal sensing into the communications networks. Despite that the temperature measurement in the optical domain is well known for its advantages compared with the electric one, its incorporation in the IoT is a challenge due to the lack of affordable strategies able to convert optical into an electronic signal in a cost‐effective way. The coupling of such optical sensors to smartphones appears as an exciting strategy for mobile optical (mOptical) sensing. Herein, advances in optical temperature sensors for mOptical sensing are reviewed and the chronological mechanistic context of waveguided and nonguided optical signal sensors is outlined. A new path for advances in photonics research is traced, established by the incorporation of smartphones as a tool in science and engineering that foresees new designs for mOptical temperature sensor toward IoT.