Microfluidics-guided fluorescent nanodiamond assembly method for highly sensitive thermometry

Fluorescent nanodiamonds (FNDs) with nitrogen vacancy centers (NVCs) show promise for use as thermometers owing to their unique physicochemical and optical properties. FND-based thermometry has been steadily developing and is now transitioning into the practical application phase. Various approaches have been proposed for FND assembly, which is a key process for temperature mapping applications. However, the assembly of high-sensitivity FND thermometers remains challenging because it requires the fabrication of three-dimensionally aggregated FNDs across the target area to generate a strong fluorescence signal. To overcome this drawback, we aimed to enable the microfluidics-guided assembly of FNDs to enhance the sensitivity of FND-based thermometry. Specifically, a microfluidic template with through-holes was used to fabricate vertically aggregated FNDs (FND clusters) on a substrate, thereby improving the signal-to-noise ratio (SNR) by superimposing the individual fluorescence events of the FNDs. The FND clusters were arrayed over a millimeter-scale area, and sensitivity improvements were achieved through vertical aggregation. Furthermore, the fluorescence spectra of the FND clusters were investigated to test the increase in temperature-mapping accuracy. Our microfluidics-guided assembly method has potential as a practical approach for creating an array of high-sensitivity FND thermometers on scalable substrates, broadening their use in various fields such as chemistry, electronics, and biology. • A novel microfluidic method vertically assembles fluorescent nanodiamonds (FNDs). • 10 µm diameter FND clusters were arrayed at 20 µm pitch over a millimeter-scale area. • High signal-to-noise ratio (SNR) of the fluorescence achieved by vertical aggregation. • Vertical aggregation of FNDs was found to provide improved accuracy for temperature mapping. • Provision of a practical approach for the fabrication of sensitive FND thermometers.