Photon upconversion is the conversion of low-energy photons (long wavelengths) into high-energy photons (short wavelengths). Single-particle, single-cell bioimaging with high temporal-spatial resolution and biological background-free has already been accomplished via photon upconversion, utilizing the unique advantages of upconversion luminescence. Since then, long-wavelength photomodulation tools, such as photodynamic therapy, photocages, and optogenetics, have been developed in conjunction with photoactive molecules or proteins with short-wavelength absorption. Excitation at long wavelengths of upconverted luminescence and penetration into deep tissue permit a vast array of biological applications in vivo. Nowadays, photonic upconversion is impossible without upconverting luminescent compounds or materials; therefore, the development of novel structures that boost upconversion efficiency is a crucial objective. With this objective, numerous eminent chemists and material scientists have conducted extensive and substantial research, which has contributed to the rapid advancement of photonic upconversion in both fundamental and applied research.

In all facets of fundamental and applied analytical chemistry, the Journal of Analysis and Testing serves as an international academic forum for the publication of original research papers, rapid communications, and critical reviews. In order to further the advancement of photonic upconversion in analytical chemistry, a special column of this journal was compiled, comprising research articles and reviews. It's even more interesting that this special column has articles on both traditional inorganic upconversion nanomaterials and organic molecules-based triplet–triplet annihilation upconversion (TTA-UC) for bioimaging and sensing. This significantly broadens the applicability of TTA-UC. There are two reviews and three research papers in this special column.

Wenyue Lin, Ling Huang, et al. of the College of Chemistry at Nankai University have reviewed TTA-UC. This study presents the inaugural comprehensive overview of the bioimaging and sensing applications of TTA-UC. The main ideas of TTA-UC are explained at the beginning of the manuscript. These include the mechanism, upconversion efficiency, threshold power density, photosensitizers, and annihilators. Following this, the core design principles underlying bioimaging and sensing based on TTA-UC are presented. The subsequent section describes the primary applications of TTA-UC in the fields of bioimaging and sensing, such as mercury ion, temperature, oxygen, enzyme activity, and pressure analysis, in addition to upconversion imaging without biological background.

A cysteine assay based on TTA-UC was established by Changqing Ye and Shuoran Chen of Jiangsu University of Science and Technology. A cysteine recognition molecule was constructed using spirocyclic fluorescein with an acrylate modification. The absence of cysteine results in predominantly blue emission of PdTPBP/perylene; however, the presence of cysteine induces green fluorescence due to the opening of the spirocyclic ring of the fluorescein. At this time, the ratiometric determination of cysteine is achieved through the emission and reabsorption of energy transferred between PdTPBP/perylene and fluorescein.

A composite of lanthanide ion-doped upconversion nanoparticles (UCNPs) and organic functional dyes was developed by Xie Xiaoji et al. from Nanjing University of Technology in order to maximize the advantages of both substances. The optical characteristics of the two are modulated through modulation energy transfer and are extensively utilized in the fabrication of upconversion sensors. The applications of upconversion nanoparticle-organic dye composites for ion, reactive oxygen, reactive nitrogen, gas, and biomolecule sensing were discussed in the reviews. The paper highlights the design strategies, operating principles, advantages, and drawbacks of the sensors. Additionally, it discusses challenges and potential solutions in related fields.

The mechanism by which upconversion luminescence is prevented in water via particle excitation at distinct wavelengths (808 nm and 980 nm) was determined by a group of scientists led by Qian Liu of the Department of Chemistry at Fudan University. A comprehensive investigation was conducted to analyze the impact of water molecule absorption of excitation light on upconversion luminescence. This study utilized both spectroscopic and single-particle-level microscopic imaging techniques. Analyzing the upconversion luminescence behavior of UCNPs@DSPE under excitation at 980 nm and 808 nm, these results demonstrated that water molecule absorption at 980 nm significantly affects upconversion luminescence attenuation. This study presents a novel approach to investigating the luminescence characteristics of UCNPs in aqueous environments and imparts significant knowledge regarding their potential utilization and advancement in the field of biomedicine.

Using the principle of near-infrared light-excited afterglow developed by Zhanjun Li et al. from Guangzhou Medical University, afterglow imaging under low-dose irradiation conditions is promising but is constrained by the material's poor absorption of near-infrared light. The Nd-sensitized afterglow material ZnGa2O4:Sn0.1, Cr0.003, and Nd0.01 optimizes the light-excited “upconversion-like” afterglow of Cr at 700 nm and increases the absorption of 808 nm light. The afterglow was five times amplified after subjecting the material to X-ray irradiation and excitation with 808 nm light. In vitro experiments demonstrate that the material performs better for X-ray afterglow luminescence imaging due to its inherent self-luminescence.

An analytical chemistry special column containing photon upconversion only represents a limited selection of recent research. We aim to provide readers with a perspective that illustrates the present state of research in this particular field. Additionally, challenges and opportunities for future research can be identified through the reading of these publications. An ongoing challenge pertains to the enhancement of upconversion luminescence efficacy and its adaptability in the context of dynamic biological upconversion imaging.

The authors and reviewers are acknowledged for their contributions of high-quality papers to this column and for their assistance in evaluating these manuscripts, respectively. The editorial office's assistance throughout the process is also appreciated.