Keywords: nv color center, lock-in amplifier, quantum sensing
Note: This article uses the Sine Scientific Instruments OE1022D lock-in amplifier to make measurements.
[Overview]
Recently, Prof. Luo Yunhan and Prof. Chen Yaofei from School of Science and Technology, Jinan University published a paper titled "Nanodiamond-Based Optical-Fiber Quantum Probe for Magnetic Field and Biological Sensing" in ACS Sensors, an authoritative international chemistry journal. Biological Sensing" in the journal ACS Sensors, which presents a nanodiamond-based optical-fiber quantum probe for magnetic field and biological sensing with high sensitivity. In this work, the research team integrated the nanodiamond into the tapered fiber end face by chemical modification to prepare the probe, and found that the sensing performance of the probe can be regulated by optimizing the modification process; the magnetic resonance method of continuous-wave optical detection and lock-in amplification and the magnetic flux concentration enhancement technique were used to obtain a magnetic detection sensitivity of 0.57 nT/Hz1/2 @ 1Hz; the probe also demonstrated a high sensitive magnetic field and biosensing; the probe also exhibited a high sensitive magnetic field and biosensing. The probe also exhibits excellent paramagnetic detection capability, which provides conditions for the further development of high-performance biosensors.
[Samples & Tests]
The optical fiber was eroded into a conical shape, and the nanodiamond particles were modified on the surface of the conical region to complete the preparation of the probe. The tapered fiber end face was characterized by optical microscope, and the length of the tapered tip was about 270 μm; the nanodiamond particles were characterized by SEM, and the morphology and size of the nanoparticles were about 100 nm, and it was proved that they contained NV color centers and carboxylate groups on the surface by FTIR and spectroscopic tests; the prepared fiber quantum probe was tested by fluorescence microscope and spectroscopy successively, and the results showed that the fluorescence signal could be improved by optimizing the concentration of nanodiamond dispersion and the modification time during the modification process. The results showed that the fluorescence signal intensity could be improved by optimizing the concentration of nanodiamond dispersion and the modification time in the process.
The magnetic detection performance of the prepared fiber-optic quantum probes was tested. With the increase of the magnetic field strength applied to the probe, the ODMR spectrum (we call it LI-ODMR spectrum) was measured using a photodetector (Thorlabs APD410A/M) and a lock-in amplifier (LIA, Sine Scientific Instruments OE1022D), and the ODMR spectrum broadened to both sides centered on ν0 (~2.87 GHz which corresponds to the energy level splitting between the ground state |±1>) as the center and broadens to both sides. The magnetoelectric conversion factor and magnetic field detection sensitivity of the probe were enhanced to 1458.66 V/T and 0.57 nT/Hz1/2@1Hz, respectively, by using the magnetic flux concentration enhancement technique, which are nearly two orders of magnitude higher than those without magnetic flux enhancement, and this is the first time that the sensitivity of the nanodiamond-based fiber optic magnetic field sensor is enhanced to the order of PiTsla. In addition, the optimization of the fiber cone structure and the parameters of the magnetic flux aggregation device is expected to achieve a further increase in the sensitivity.
The prepared fiber optic quantum probes were tested for sensing of paramagnetic substances. Paramagnetic substances, such as free radicals and paramagnetic metalloproteins, are characterized by the presence of one or more unpaired electrons in their valence layers. More and more studies have shown that paramagnetic substances play crucial roles in various physiological processes, such as tumor growth, immune response, and metabolism, and endogenous paramagnetic substances have become biomarkers for many diseases. Therefore, the sensing of paramagnetic substances is very important. Gadolinium ion (Gd3+), one of the paramagnetic substances, is widely used as an NMR contrast agent and exhibits strong paramagnetism because it has seven unpaired electrons in the 4f sublayer. In the experiments, the magnetic spin noise generated by Gd3+ can extend to the GHz range, and its frequency component corresponds to the Larmor progression of the NV color centers. Therefore, the effect of Gd3+ on the NV color centers is presented in the form of lowering the polarization rate, which is ultimately manifested in the reduction of the fluorescence intensity.
A gradient of Gd3+ concentration was tested from 100 nM to 3 M. With the increase of Gd3+ concentration, the contrast of the ODMR spectra decreased significantly, and the resonance frequency remained unchanged, and the sensitivity of the detection of Gd3+ was about 26.8 nM/Hz1/2. As a control, the same conditions were applied to the testing of La3+ (which is chemically similar to that of Gd3+ but without any unpaired electrons), and the results showed that the ODMR spectra were almost unaffected by the concentration of La3+, which was non-paramagnetic. As a control, the same conditions were applied to La3+ (chemically similar to Gd3+, but without any unpaired electrons, i.e., non-paramagnetic), and the results showed that the ODMR spectra were almost unaffected by the concentration of La3+.
Shown in Fig. 1. (a) Schematic of the fiber optic quantum probe. (b) NV defects appearing in the diamond lattice. (c) Energy level structure. (d) Schematic of the ODMR spectrum. (e) Microscope image of a tapered fiber tip. (f-h) SEM images, FTIR spectra and photoluminescence spectra of NDs. (i) Schematic diagram of the probe preparation process. (j) Schematic diagram of the fluorescence spectroscopy testing device. (k) Schematic diagram of locked ODMR spectrum measurement.
Figure 2.(a,c) Fluorescence microscope images of the probes under different diamond concentration modifications. (b,d) Fluorescence microscope diagrams of the probe under different modification times Fluorescence microscope images of the probe and the corresponding fluorescence spectra.
Fig. 3 ODMR spectra at different magnetic field strengths
Figure 4.(a) Fiber optic probe with magnetic flux aggregation device applied (b) ODMR spectra at different magnetic field strengths
(d) Magnetic noise amplitude spectral density (e) Comparison of detection sensitivity of different probes with and without mfc
Fig. 5.(a) Schematic diagram of PS sensing using microfluidic tubes. (b) Schematic diagram of paramagnetic Gd3+ acting on nanodiamond NV color centers and non-paramagnetic La3+ with no interaction with NV color centers (c, d) LI-ODMR spectra measured at different concentrations of Gd3+ and La3+. (e) Dependence of the lock-in signal on the concentration at a fixed 2877 MHz frequency of the signal source.
[Summary].
The team proposed and demonstrated a new structure using diamond NV color centers as a quantum probe. The NV color-centered diamond nanoparticles are chemically covalently bonded to the tip of a tapered optical fiber, and are applied to the sensing of magnetic fields and paramagnetic substances based on the continuous-wave ODMR method and phase-locked amplification, achieving the highest sensitivities of 0.57 nT/Hz1/2@ 1/2@ 1 Hz and 26.8 nM/Hz1/2 in the experiments, respectively. The diamond NV color-center-based sensing method proposed in the research work lays the foundation for the development of a multifunctional fiber-optic quantum probe with high integration, small size and high sensitivity.
[Literature]
