The core insight is that UCNPs do not respond to light the way ordinary fluorescent dyes do. Conventional organic fluorophores are essentially linear emitters: when you modulate the excitation intensity, the fluorescence follows the same waveform, containing exactly the same frequency components as the input. Upconversion nanoparticles, by contrast, rely on a sequential, multi-step energy transfer process. A sensitizer ion, typically ytterbium, absorbs an infrared photon and hands its energy to an activator ion such as thulium or erbium, which must receive a second energy packet before it can emit. This inherent nonlinearity means the emission does not simply mirror the excitation. When the team ran numerical simulations based on rate equations for a two-photon energy-transfer system, they found that sinusoidally modulated excitation at a frequency f produced upconversion emission containing a second-harmonic component at 2f, a frequency entirely absent from the excitation light itself.

The experiments confirmed the predictions in striking fashion. Using NaYF4 nanoparticles doped with 20 percent ytterbium and 0.5 percent thulium and coated with an inert shell, the researchers applied square-wave excitation at 980 nanometers with a 50 percent duty cycle. The Fourier transform of the excitation trace showed only odd harmonics of the 20 hertz base frequency, as expected for a square wave. But the Fourier spectrum of the resulting upconversion luminescence, collected between 430 and 850 nanometers, revealed prominent even harmonics at 40, 80, and 120 hertz and beyond. These components were generated by the nanoparticles themselves, not by any instrumental imperfection, since the laser light monitored throughout the experiments never displayed comparable even-harmonic content. The strength of this second-harmonic signal, quantified as a weight relative to neighboring frequency components, depended on excitation intensity and on the base modulation frequency, exactly as the nonlinear photophysics would predict.

Second harmonics, however, come with a practical catch. Because they sit at twice the modulation frequency, they can be too fast for ordinary scientific cameras, which are limited in frame rate by the Nyquist theorem. The researchers realized that the same nonlinear physics could be pushed further. If the nanoparticles are excited by two laser beams modulated at two different base frequencies, f1 and f2, the nonlinear response mixes these frequencies, generating a beat-frequency signal at the difference, f2 minus f1, as well as a sum-frequency component. In effect, the nanoparticles behave as optical frequency mixers, analogous to the mixers used in radio engineering. When the team superimposed two 980-nanometer beams modulated at 21 and 23 hertz, the upconversion emission spectrum clearly contained a beat-frequency signal at 2 hertz, alongside the base frequencies, their second harmonics at 42 and 46 hertz, and a sum-frequency peak at 44 hertz. Crucially, none of these extra components existed in the excitation light.

The beauty of the beat-frequency channel is that it can be placed at an arbitrarily low frequency, well within the reach of a slow camera, while remaining completely free of residual excitation light, which carries only the base frequencies and their harmonics. The researchers then showed that the strength of this signal could be tuned through nanoparticle engineering. By inserting an intermediate shell doped with ytterbium between the core and the outer inert shell, creating a core-multishell architecture, they substantially boosted the beat-frequency weight of the 800-nanometer thulium emission. Erbium-activated particles with neodymium-containing outer shells, excitable at both 980 and 808 nanometers, also produced robust beat-frequency signals. The team was careful to note that the relationship between architecture and performance is not simply that more complex means better; rather, it is the way specific design elements modify energy transfer rates, saturation behavior, and ion kinetics that governs how efficiently dual-frequency excitation couples into the emission.

The imaging demonstrations are where the method reveals its full power. In a surface-imaging experiment, the team imaged a fluted cross-shaped pattern, a screw cap coated with engineered nanoparticles, under dual-frequency excitation at 21 and 23 hertz. The time-averaged image was dominated by reflected laser light that leaked past a short-pass filter, obscuring the pattern. But when each pixel of the image sequence was filtered at the 2-hertz beat frequency, the residual excitation light vanished almost entirely while the nanoparticle pattern remained crisp. Filtering at the base frequency of 21 hertz, the conventional lock-in approach, failed to achieve the same result, because the leaked excitation light is modulated at exactly that frequency. The researchers then flooded the scene with strong, nonuniform ambient light that completely washed out both the signal and the reflection in the averaged image. The beat-frequency filter still recovered the pattern cleanly, demonstrating immunity to ambient interference as long as the detector is not saturated.

Deep-tissue imaging posed an even sterner test, and here the safety constraints of real biomedicine come into play. A capillary filled with thulium-doped core-shell nanoparticles was buried behind a five-millimeter slab of chicken breast tissue, and the excitation intensity was kept at roughly 72 milliwatts per square centimeter, nearly an order of magnitude below the ANSI safety limit for continuous-wave 980-nanometer skin exposure. Two coaxial beams modulated at 22 and 23 hertz illuminated the tissue at an oblique angle, and a scientific CMOS camera recorded sequences with 400-millisecond exposures. Individual frames and even frame averages were murky, contaminated by excitation leakage that persisted despite a combined optical density of 18 in the detection filters. Spectral measurements confirmed that the leakage was concentrated at the excitation wavelength. Yet Fourier analysis of the integrated image intensity revealed the 1-hertz beat-frequency component, and pixel-wise filtering at that frequency suppressed the leakage and other background contributions, improving the signal-to-background ratio by nearly an order of magnitude.

The method proved equally resilient under dynamically varying ambient light and at drastically reduced nanoparticle concentrations. When the researchers diluted the nanoparticle suspension a hundredfold, to 100 micrograms per milliliter, they simply shifted the beat frequency down to 0.1 hertz by modulating the lasers at 22 and 22.1 hertz and lengthened the camera exposure. Even through two millimeters of chicken tissue, the inclusion remained clearly detectable with a high signal-to-background ratio. The authors emphasize that PP-LID improves contrast and background rejection but does not circumvent the fundamental spatial resolution limits imposed by diffusive light transport in tissue, which remain on the millimeter scale in wide-field luminescence diffuse optical tomography geometries. What it does provide is a cleaner signal that enables less aggressive regularization and better discrimination during image reconstruction.

What makes PP-LID particularly attractive for practical deployment is its modest technical demands. Unlike conventional lock-in detection, it requires no precise synchronization between the excitation source and the detector, tolerates ambient light provided detector saturation is avoided, and imposes no stringent requirements on detector time resolution. It is also fundamentally distinct from previously reported frequency-encoding techniques for UCNPs, which modulated and detected at the same frequency and therefore remained vulnerable to excitation leakage. The approach can be combined with established background-suppression methods such as time-gated detection, and it does not even require two separate laser beams; a single beam carrying a superposition of two modulation waveforms would suffice. The authors point to several directions for future work, including quantitative models that explicitly account for ion distributions and interionic interactions, strategies to boost nanoparticle brightness, and solutions to the temporal fluctuations introduced by physiological motion such as breathing and blood flow in living subjects.

If the promise holds, the implications could reach well beyond the laboratory. Background-free imaging at safe excitation powers, with inexpensive cameras and minimal synchronization hardware, could sharpen small-animal studies, deepen the reach of optical biosensing, and bring upconversion-based diagnostics closer to clinical reality. In a field where progress has often meant building ever more elaborate filters and detectors, the KTH-led team has taken a different path: they let the physics of the nanoparticles do the filtering. By coaxing lanthanide-doped nanocrystals to act as tiny frequency mixers, they have opened a spectral channel that no stray laser light or ambient glare can follow, and in doing so they may have rewritten the rules of what low-light bioimaging can achieve.

Subject of Research: Photophysical lock-in detection of upconversion nanoparticles for background-free bioimaging

Article Title: Photophysical lock-in detection enables background-free upconversion emission imaging

Article References: Bagheri, N., Wang, C., Guo, D., Lakshmanan, A., Zhu, Q., Chen, X., Ghazyani, N., Zhan, Q., Sotiriou, G. A., Liu, H., & Widengren, J. (2026). Photophysical lock-in detection enables background-free upconversion emission imaging. Light: Science & Applications, 15(1), Article 371. https://doi.org/10.1038/s41377-026-02414-2

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02414-2

Keywords: upconversion nanoparticles, lock-in detection, bioimaging, lanthanide, frequency mixing, beat frequency, deep-tissue imaging, signal-to-background ratio, nanoparticle engineering, fluorescence, optical filters, sCMOS camera

In a pilot study published in Holistic Integrative Oncology, researchers from Liaoning Cancer Hospital and Dalian University of Technology evaluated a sprayable fluorescent probe that targets gamma-glutamyl transpeptidase, an enzyme abbreviated γ-GGT or GGT2. The enzyme sits on the membranes of mammalian cells and is normally abundant in secretory organs such as the liver, kidney, and pancreas, but its expression and activity rise sharply in many malignancies, including liver, lung, colorectal, breast, and ovarian cancers. Previous work has suggested that γ-GGT helps tumors progress, invade, and resist chemotherapy by reshaping intracellular redox metabolism, and elevated pretreatment serum levels of the enzyme have been linked to advanced stage and poor prognosis in epithelial ovarian cancer. That combination of high tumor expression and clinical relevance made γ-GGT an attractive molecular bullseye for intraoperative imaging.

The probe itself, designated NIR-SN-GGT, is a product of a medical-engineering collaboration between the cancer hospital and the State Key Laboratory of Fine Chemicals in Dalian. Chemically, it couples a dicyanoisophorone-based near-infrared fluorophore to a γ-glutamyl recognition unit through an amide bond. In its intact state the molecule is essentially dark: the γ-glutamyl group acts as an electron-withdrawing quencher that suppresses intramolecular charge transfer, the process responsible for fluorescence. When the probe encounters γ-GGT on the surface of a cancer cell, the enzyme cleaves the γ-glutamyl bond, releasing a free amine that restores charge transfer and triggers a rapid turn-on fluorescence signal in the near-infrared region, with an emission maximum around 650 nanometers, excitation near 445 nanometers, and a large Stokes shift of roughly 205 nanometers. Because the reaction occurs only where the enzyme is abundant, background fluorescence in healthy tissue stays negligible, which is precisely what a surgeon needs to distinguish tumor from normal tissue in real time.

Crucially, the team did not stop at cell cultures or mouse models, the point at which most targeted fluorescent probes have historically stalled. Instead, they built a three-tier validation pipeline that moved from bioinformatics to organoids, then to frozen tissue sections, and finally to fresh human specimens removed during surgery. Analysis of The Cancer Genome Atlas confirmed elevated GGT2 mRNA in ovarian tumors compared with normal ovary, and immunohistochemistry on archival paraffin blocks from 36 high-grade serous ovarian cancer cases showed robust γ-GGT protein on cancer cell membranes. The researchers then grew patient-derived ovarian cancer organoids in three-dimensional matrix cultures and observed distinct probe fluorescence within these miniature tumors, confirming that the probe can label γ-GGT-positive cells in a system that mimics the tumor microenvironment far better than flat cell monolayers.

The next tier involved frozen sections cut at 100-micrometer thickness from freshly resected cancer and pericancerous tissue. After spraying the sections with a 10 micromolar probe solution and waiting just one minute, confocal microscopy revealed a fluorescence pattern that colocalized almost perfectly with DAB immunostaining for γ-GGT, with markedly stronger signal in cancerous regions than in adjacent non-cancerous tissue. This spatial agreement between enzyme activity imaging and classical pathology provided the bridge from in vitro validation to real human tissue, setting the stage for the ex vivo experiments that form the heart of the study.

Between February and November 2023, the team collected fresh tissue from 16 patients aged 46 to 77 who underwent primary, interval, or secondary cytoreductive surgery for epithelial ovarian cancer, most of them at advanced FIGO stage III or IV. From these patients they harvested 42 confirmed cancerous lesions, including 18 primary ovarian lesions, 15 omental metastases, and 9 peritoneal metastases, spanning diameters of 0.5 centimeters, 0.3 centimeters, and smaller than 0.3 centimeters. Each specimen was sprayed with probe solution at 1, 5, or 10 micromolar within 30 to 40 minutes of resection, then imaged every minute for half an hour and again at one hour and five hours using a simple setup: an iPhone 12 mini camera fitted with an emission filter, working at a distance of 15 to 20 centimeters under dark conditions. Signal-to-background ratios, calculated as average lesion fluorescence divided by average fluorescence of surrounding normal tissue, served as the primary metric, with values above 1.2 considered visible to the naked eye.

The optimization experiments delivered clear answers. Across all 42 lesions, every sample produced a signal-to-background ratio above 1, yielding 100 percent fluorescence detection, and 95.2 percent were visible without any image processing. The 10 micromolar concentration proved to be the minimum effective dose, achieving signal-to-background ratios of 1.40 plus or minus 0.17 for primary lesions and 1.32 plus or minus 0.12 for omental metastases, significantly better than the 1.17 and 1.27 values seen at lower concentrations. Just as importantly, the imaging window proved remarkably forgiving: fluorescence appeared within one minute of spraying and remained stable from one minute to one hour, with ratios hovering between 1.26 and 1.29, before fading by five hours. Lesion size made no difference, with sub-0.3-centimeter implants imaged just as brightly as larger ones, a finding with obvious implications for catching the millimeter-scale seedings that surgeons most often miss.

The clinical validation phase then put the optimized protocol to the test on 27 sub-0.3-centimeter lesions from 8 patients, along with adjacent normal tissues and a negative control of normal peritoneum. Fluorescence imaging identified 26 of the 27 cancerous lesions, a true-positive rate of 96.3 percent, and 23 lesions were visible to the naked eye, a macroscopic detection rate of 81.5 percent. Only one false positive appeared, in a sample of normal peritoneal tissue that pathology confirmed lacked γ-GGT expression; the spurious signal was attributed to residual red blood cells on the tissue surface, whose protoporphyrin content autofluoresces in the same spectral region. Immunohistochemistry confirmed membrane γ-GGT expression in 100 percent of cancerous lesions in both cohorts while normal tissues stained negative, underscoring the target’s specificity. The authors note that meticulous intraoperative hemostasis and thorough saline rinsing of tissue surfaces before spraying are essential precautions to keep blood-related false positives in check.

Statistical analysis of the clinical variables added a note of caution for future use. Univariate analysis found that preoperative serum γ-GGT levels and treatment modality both influenced imaging quality, with patients whose serum levels reached at least 20 units per liter showing higher signal-to-background ratios. In multivariate analysis, only the preoperative treatment approach remained independently predictive, most likely because neoadjuvant chemotherapy can reduce γ-GGT expression or activity in tumor tissue and thereby weaken probe binding. Encouragingly, clinical stage, lesion location, and pathological type did not alter probe performance, suggesting the technique is robust across the diverse clinical presentations of ovarian cancer.

The authors are candid about the study’s limits. The cohort of 16 patients is too small for firm statistical conclusions, and larger trials will be needed to stratify effects of specific chemotherapy regimens and timing of interval surgery. The single false negative occurred in a γ-GGT-positive lesion buried more than three millimeters beneath the tissue surface, a reminder that near-infrared light at 650 nanometers penetrates tissue only shallowly; deeper lesions may require surgical exposure or next-generation probes emitting in the second near-infrared window above 1000 nanometers. Standardized fluorescence evaluation criteria will also be needed to reduce observer variability. Even so, the head-to-head comparison is striking: the FDA-approved folate receptor-alpha probe pafolacianine detects ovarian cancer lesions with 83.0 percent sensitivity and a 32.7 percent false-positive rate, whereas the sprayed γ-GGT probe achieved a 98.6 percent true-positive rate with only a 1.4 percent false-positive rate in the ex vivo experiments, while topical delivery slashes the dose required compared with intravenous agents and reduces the risk of systemic toxicity. If larger clinical trials confirm these results, a bottle of enzyme-activated dye and a filtered smartphone camera could one day become standard equipment in the fight against one of medicine’s most elusive cancers.

Subject of Research: Sprayable γ-GGT-activated fluorescent probe for intraoperative visualization of epithelial ovarian cancer

Article Title: Exploration of the application of the sprayed γ-GGT fluorescent probe for visual imaging of epithelial ovarian cancer: a pilot study

Article References: Liu, Z., Zhang, M., Li, H., Liu, L., Chen, F., Li, M., Gao, Y., Wang, D., & Yang, Z. (2026). Exploration of the application of the sprayed γ-GGT fluorescent probe for visual imaging of epithelial ovarian cancer: a pilot study. Holistic Integrative Oncology, 5(1), Article 46. https://doi.org/10.1007/s44178-026-00268-7

Image Credits: AI Generated

DOI: 10.1007/s44178-026-00268-7

Keywords: ovarian cancer, fluorescent probe, gamma-glutamyl transpeptidase, fluorescence imaging, intraoperative imaging, cytoreductive surgery, organoids, signal-to-background ratio, near-infrared fluorescence, tumor imaging, pafolacianine, precision oncology