The neutral nitrogen-vacancy (NV0) defects in diamond are photostable color centers, suitable for a wide range of applications in science and engineering. However, the photophysical properties of the centers have not yet been fully characterized. This work measured the stimulated emission cross sections of NV0 in a single-crystal diamond by two-photon excitation of its matrix at 344 nm. From the measured photoluminescence spectrum and the fluorescence lifetime of 20 +/- 1 ns, we determined a peak stimulated emission cross section of 1.43 +/- 0.07 x 10(-17) cm(2) at 650 nm for the NV0 centers. In addition, we have also examined the thermal shifts of the zem-phonon line of NV0 centers in nanoscale diamonds (similar to 100 nm in diameter) over the temperature range of 30-120 degrees C. A temperature measurement sensitivity of 0.2 K.Hz(-1/2) was achieved, which is about two-fold better than that of NV-, despite that the fluorescence intensity of NV0 is about six-fold lower than that of NV- in the same nanoparticles. The result is attributed to the smaller electron-phonon coupling strength of the neutral center, compared with its negatively charged counterpart.

Temperature sensing is a promising method of enhancing the detection sensitivity of lateral flow immunoassay (LFIA) for point-of-care testing. A temperature increase of more than 100 °C can be readily achieved by photoexcitation of reporters like gold nanoparticles (GNPs) or colored latex beads (CLBs) on LFIA strips with a laser power below 100 mW. Despite its promise, processes involved in the photothermal detection have not yet been well-characterized. Here, we provide a fundamental understanding of this thermometric assay using non-fluorescent CLBs as the reporters deposited on nitrocellulose membrane. From a measurement for the dependence of temperature rises on the number density of membrane-bound CLBs, we found a 1.3-fold (and 3.2-fold) enhancement of the light absorption by red (and black) latex beads at 520 nm. The enhancement was attributed to the multiple scattering of light in this highly porous medium, a mechanism that could make a significant impact on the sensitivity improvement of LFIA. The limit of detection was measured to be 1 × 105 particles/mm2. In line with previous studies using GNPs as the reporters, the CLB-based thermometric assay provides a 10× higher sensitivity than color visualization. We demonstrated a practical use of this thermometric immunoassay with rapid antigen tests for COVID-19.

Lateral flow immunoassay (LFIA) is a widely used tool for point-of-care testing (POCT). Although the method is fast and inexpensive, it provides only qualitative or semi-quantitative information, which limits the scope of its applications in POCT. Here, we report the development of a low-cost, portable LFIA reader based on photothermal detection of reporters (such as colloidal gold and colored latex beads) captured on LFIA strips using a low-power green laser as the heating source and a single-element infrared sensor as the detector. For 40-nm colloidal gold, we measured a detection limit of 3 x 10(5) particles/mm(2) by laser irradiation of the reporters at 532 nm with an intensity of 20 W/cm(2). The photothermal detection enables a 10-fold enhancement in sensitivity over color visualization with the naked eye. More importantly, with the use of the control line intensities as internal calibrants, the results so obtained are quantitative and useful to support critical decision-making in POCT, as demonstrated by gold-based assays for human chorionic gonadotropin and human immunodeficiency virus as well as latex-based assays for nucleocapsid protein of the SARS-CoV-2 virus.

Temperature sensing is a promising method of enhancing the detection sensitivity of lateral flow immunoassay for point-of-care testing. A temperature increase of more than 100 °C can be readily achieved by photoexcitation of reporters like gold nanoparticles (GNPs) or colored latex beads (CLBs) on the strips with a laser power below 100 mW. Despite its promise, processes involved in the photothermal detection have not yet been well-characterized. Here, we provide a fundamental understanding of this thermometric assay by combining experiments and simulations using non-fluorescent CLBs as the reporters deposited on nitrocellulose membrane. By measuring the dependence of temperature rises on the number density of membrane-bound CLBs, we determined a 1.5-fold enhancement of the light absorption at 520 nm by the beads (diameter of 0.4 μm). The enhancement, however, was compromised by a 5-fold reduction of the incident laser power due to multiple scattering of the light in this highly porous medium. The limit of detection was measured to be 1 × 10 5 particles/mm 2 . In line with previous studies using GNPs as the reporters, the CLB-based thermometric assay provides a 10× higher sensitivity than color visualization, as demonstrated with the immunoassay for nucleocapsid proteins of the SARS-CoV-2 virus.

Fluorescent nanodiamonds (FNDs) containing negatively charged nitrogen-vacancy (NV-) centers are applicable for nanoscale temperature sensing in frontier areas of science and engineering. As researchers take advantage of the unique chemical and physical properties of this nanomaterial, many new innovations are emerging. Here, we show that FNDs can be readily embedded in poly(2-hydroxyethyl methacrylate) (PHEMA) with a high density (1 X 10(11) particles in a film of 10 X 10 X 0.1 mm(3)) and a high uniformity (intensity variation of similar to 10% over a distance of 10 mu m). The FND-embedded polymer films can be fabricated in large scale and are useful as all -optical thermometers for practical applications. This work made a comparative study on the temperature dependence of the peak positions and heights of the zero phonon line (ZPL) of NV- centers in these films from 35 to 120 degrees C. A measurement sensitivity of 0.46-1.1 and 0.15-0.62 K Hz(-1/2) was achieved respectively for the ZPL shifts and height changes over this temperature range. The utility and versatility of this device were demonstrated with a study for the energy transfer kinetics of a resistively heated gold microwire embedded in the PHEMA film.