Quantum nanodiamonds for sensing of biological quantities: Angle, temperature, and thermal conductivity

Abstract

Measuring physical quantities in the nanometric region inside single cells is of great importance for understanding cellular activity. Thus, the development of biocompatible, sensitive, and reliable nanobiosensors is essential for progress in biological research. Diamond nanoparticles containing nitrogen-vacancy centers (NVCs), referred to as fluorescent nanodiamonds (FNDs), have recently emerged as the sensors that show great promise for ultrasensitive nanosensing of physical quantities. FNDs emit stable fluorescence without photobleaching. Additionally, their distinctive magneto-optical properties enable an optical readout of the quantum states of the electron spin in NVC under ambient conditions. These properties enable the quantitative sensing of physical parameters (temperature, magnetic field, electric field, pH, etc.) in the vicinity of an FND; hence, FNDs are often described as "quantum sensors". In this review, recent advancements in biosensing applications of FNDs are summarized. First, the principles of orientation and temperature sensing using FND quantum sensors are explained. Next, we introduce surface coating techniques indispensable for controlling the physicochemical properties of FNDs. The achievements of practical biological sensing using surface-coated FNDs, including orientation, temperature, and thermal conductivity, are then highlighted. Finally, the advantages, challenges, and perspectives of the quantum sensing of FND are discussed. This review article is an extended version of the Japanese article, In Situ Measurement of Intracellular Thermal Conductivity Using Diamond Nanoparticle, published in SEIBUTSU BUTSURI Vol. 62, p. 122-124 (2022).

Keywords: biosensing; cell; fluorescent nanodiamonds; nitrogen vacancy; quantum sensor.

Figure 1

(A) Structure of an NVC in diamond crystal. C: carbon atom, N: nitrogen atom, V: vacancy. (B) Energy level diagram of NV^–. D is zero-field splitting and 2γB is the Zeeman splitting, where γ is the NV gyromagnetic ratio. (C) Fluorescence and (D) ODMR spectra of FNDs excited at 532 nm. ZPL: Zero-phonon line.

Figure 2

Proof-of-principle experiment for orientation determination by ODMR. (A) The orthogonally aligned three-axis magnet system used to generate the arbitrary external magnetic field. (B) The four magnetic field directions used in this proof-of-principle experiments (blue arrows). (C) Typical ODMR spectra of a nanodiamond containing a single NV center acquired with each external magnetic field. The angles of the N–V axis relative to the applied magnetic field derived from the Zeeman split widths are indicated. Gray dots, raw data; downward black lines, best-fit curves obtained by fitting each spectrum independently; upward red lines, best-fit curves obtained by considering the geometrical limitation among the four angles. (D) Direction of the N–V axis in a 50 nm nanodiamond (inset; scale bar, 1 μm) calculated by considering the geometrical limitation. (E) Typical ODMR spectra of a 1 mm diamond cube with a (111) surface acquired with each external magnetic field. Gray dots, raw data; downward black lines, fit to a theoretical model; upward colored lines, signals attributed to the NV centers depicted with corresponding colors in (F). (F) The four directions of N–V axes in the diamond cube (colored arrows). The Tait–Bryan angles shown are relative to the default orientation explained in the main text. Modified with permission from Ref 21.

Figure 3

(A) Temperature-dependent shifts of an ODMR spectrum. (B) Frequency shifts of the ODMR peak center around 280–330 K. Modified with permission from Ref 35.

Figure 4

(A) Comparisons of the fluorescence spectra in the ZPL region of NV^– centers in PHEMA-embedded FNDs at 36.8–113.2°C. The spectra were normalized to the intensity at 638.161 nm. (B) Changes in ZPL position and height ratios over 35–120°C for three representative FND ensembles embedded in the PHEMA films. Curves represent the best fits of the experimental data using second-order polynomials. Modified with permission from Ref 38.

Figure 5

Surface coating of FND.

Figure 6

Membrane fluctuations of A431 cells correlate with density of the cytoskeleton. (A) Antigen-antibody association between a nanodiamond and an EGF receptor (schematic). (B, C) Bright-field (B) and selective imaging protocol (SIP). (C) images of nanodiamond-attached A431 cells. (D–F) Typical rotational motion of nanodiamonds on the membrane of untreated (D), EGF-treated (E), and latrunculin A-treated (F) A431 cells. Upper panels show the observed directions of the N–V axes. The directions of the N–V axes at each time point are indicated by the corresponding colored dots on the unit spheres. Lower panels show the time courses of the Tait–Bryan angles. (G–I) Histograms of the frequency of angular speeds of the rotational displacement of N–V axes in nanodiamonds on untreated (G), EGF-treated (H), and latrunculin A-treated (I) A431 cells at each measurement step. Each histogram shows the total data of three independent experiments conducted using different cells under the same conditions. Modified with permission from Ref 21.

Figure 7

(A and C) Merged photos of FNDs during FCCP stimulation (60 μM) and vehicle control experiments. Images in red and gray represent FND fluorescence and bright field, respectively. The numbers in the upper left-hand corner of each image correspond to specific times at which each picture captured during measurements shown in (B) and (D). Scale bars, 20 μm. (B and D) I_tot (top) and ΔT_NV (bottom) during FCCP stimulation and vehicle control experiments. The blue shaded regions represent periods when no temperature measurements were performed and in which the photographs in (A) and (C) were obtained. ΔT_NV was calculated using the equation dD/dT=–65.4 kHz C^–1 for both types of experiments. (E) Statistical plots of the maximum ΔT_NV for FCCP stimulation (red), vehicle control (blue), and static control (black, no solution added). n=10 for all data. Mean values±SE were 4.0±0.9, 0.9±0.5, and –0.1±0.3°C for the FCCP, vehicle control, and static control experiments, respectively. All measurements were performed at a constant temperature of 23°C with fluctuations of less than 0.25°C. Probed NDs were located within 20 μm of the antenna, with a mean distance of 8.9 μm. (F) The latency and durations of responses to increased temperature responses of ΔT_NV for the FCCP stimulus, whose means±SEs of 18.1±2.3 and 49.4±8.22 min, respectively. Modified with permission from Ref 75.

Figure 8

(A) Schematic illustration of dual functional PDA-coated FNDs (FND-PDA) prepared from FNDs. PDA-FNDs function as a luminescent nanothermometer, while PDA releases heat in a light-dependent manner. (B) TEM images of FND-PDA. Images are 751×534 nm. (C) Conceptual illustration showing how FND-PDA works as a sensor for thermal conductivity. (D) Plots and error bars indicating ΔT±standard deviation for each particle in different environments measured when increasing laser power from 7.3 to 25 mW. Modified with permission from Ref 22.