Sensing with near-infrared laser trapped fluorescent nanodiamonds

TL;DR

The study probes how a 1064 nm NIR trapping beam affects NV-center–bearing fluorescent nanodiamonds used for nanoscale biosensing. By integrating PL, ODMR, and fluorescence relaxometry with a two-field setup and a rate-equation model plus a temperature heat-transfer analysis, it decouples photothermal heating from charge-state dynamics. In air, NIR heating causes sizable ODMR shifts and shortened $T_{relax}$, whereas in water heating is largely suppressed and charge-state dynamics dominate, altering the NV$^-$ to NV$^0$ balance and the PL signature. The work demonstrates reliable sensing of pH, paramagnetic species (Gd$^{3+}$), and temperature with trapped FNDs and provides practical guidelines for mitigating NIR artefacts, highlighting the platform's potential for intracellular nanoscale biosensing with optimized trap parameters and media.

Abstract

Biosensing based on optically trapped fluorescent nanodiamonds is an intriguing research direction potentially allowing to resolve biochemical processes inside living cells. Towards this goal, we investigate infrared near (NIR) laser irradiation at 1064 nm on fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV) centers. By conducting comprehensive experiments, we aim to understand how NIR exposure influences the fluorescence and sensing properties of FNDs and to determine the potential implications for the use of FNDs in various sensing applications. The experimental setup involved exposing FNDs to varying intensities of NIR laser light and analyzing the resultant changes in their optical and physical properties. Key measurements included T1 relaxation times, optical spectroscopy, and optically detected magnetic resonance (ODMR) spectra. The findings reveal how increased NIR laser power correlates with alterations in ODMR central frequency but also that charge state dynamics under NIR irradiation of NV centers plays a role. We suggest protocols with NIR and green light that mitigate the effect of NIR, and demonstrate that FND biosensing works well with such a protocol.

Figures (12)

• Figure 1: Graphical abstract illustrating the sensing modalities explored to investigate effects of NIR laser on NV center for sensing. Temperature, abundance of paramagnetic species, and pH, are altered while optical spectroscopy, ODMR spectra, and fluorescence polarization relaxometry is measured from optically trapped nanodiamonds.
• Figure 2: a) Illustration of the optical setup with the most important lasers, optical lenses, filters and equipment. Details of the components are specified in Table \ref{['tab:setupcomponents']} in the appendix. b) Laser sequence for fluorescence relaxometry measurement. In green the 532 nm laser and in red the 1064 nm laser. Pulse-widths are $t_{\rm trap} = 20 \mu s$, $t_{ini} = 35 \mu s$, $t_{RO} = 15 \mu s$ while the variable waiting time $\tau$ allows to record the relaxometry at times spanning from $7.2 \mu s$ to $720 \mu s$.
• Figure 3: Effect of CW NIR laser on dry FND emission spectrum. Optical emission spectrum of one FND, dry on glass, under CW NIR laser exposure at 0 mW (green), 30 mW (orange) and 60 mW (red) of 1064 nm laser power.
• Figure 4: Effect of CW NIR laser power on ODMR spectrum of dry FND. ODMR spectra of one FND under CW NIR laser exposure at 0 mW (green) and 60 mW (red) of 1064 nm laser power. The resonance frequency shift and decrease in contrast with increasing CW NIR power is observed. The power of the 532 nm laser was 0.7 mW, measured before the objective lens, and the microwave amplifier was set to -2dB.
• Figure 5: Effect of NIR laser power on $T_{\rm relax}$ relaxation times of dry FNDs.$T_{\rm relax}$ relaxometry measurements for one FND dried on glass under NIR laser powers of 0 mW (green) and 60 mW (red) at 1064 nm wavelength. Pulsing as in Fig \ref{['fig:setup']}b).