High-throughput nitrogen-vacancy center imaging for nanodiamond photophysical characterization and pH nanosensing
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-01-01 |
| Journal | Nanoscale |
| Authors | Maabur Sow, Horst Steuer, Sanmi Adekanye, Laia Ginés, Soumen Mandal |
| Institutions | University of Oxford, Cardiff University |
| Citations | 32 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research introduces a high-throughput, ratiometric wide-field imaging technique for the comprehensive photophysical characterization of Nitrogen-Vacancy (NV) centers in nanodiamonds (NDs).
- High-Throughput Characterization: The method allows for the parallel detection and analysis of hundreds of single NV centers in seconds, providing statistical insight into ND heterogeneity and NV center content (single NV/ND fraction).
- Ratiometric Charge State Detection: By measuring the ratio of Red (655-750 nm) to Green (550-620 nm) fluorescence intensity (R/G ratio), the technique reliably distinguishes between the two photoactive charge states, NV- and NV°, based on their distinct emission spectra.
- Dynamic Analysis and Lifetimes: Coupled with Hidden Markov Modeling (HMM), the approach captures dynamic charge-state transitions and calculates the lifetimes of NV- and NV° states in NDs, a study previously limited to single particles or bulk diamond.
- pH Nanosensing Capability: The charge state of NV centers in 10 nm NDs was found to be directly and reversibly manipulated by pH, shifting the R/G ratio linearly across the pH 4 to 10 range.
- Performance: The pH sensing capability in 10 nm NDs achieved an estimated accuracy of ~0.4 pH units, demonstrating robustness even in the presence of biological molecules or divalent ions.
- Material Optimization: This technique serves as a crucial screening tool for optimizing ND manufacturing processes to maximize the yield of small, bright NDs containing stable single NV centers for quantum and biological applications.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| ND Diameter Range Studied | 5 to 200 | nm | Commercial HPHT and detonation NDs |
| Excitation Wavelength | 532 | nm | CW Laser |
| Excitation Intensity | 7.8 | kW cm-2 | Maximum intensity used for imaging |
| Standard Exposure Time | 100 | ms | Used for most experiments |
| NV- R/G Ratio (50 nm NDs) | ~0.9 | Dimensionless | High R/G ratio indicates NV- state |
| NV° R/G Ratio (50 nm NDs) | ~0.6 | Dimensionless | Low R/G ratio indicates NV° state |
| NV- Lifetime (40 nm NDs) | ~38 ± 1.5 | s | HMM analysis, single exponential fit |
| NV° Lifetime (40 nm NDs, short component) | ~3 ± 0.3 | s | HMM analysis, double exponential fit |
| NV° Lifetime (40 nm NDs, long component) | ~53 ± 7.7 | s | HMM analysis, double exponential fit |
| pH Sensing Range (10 nm NDs) | 4 to 10 | pH units | Range showing linear R/G ratio response |
| pH Measurement Accuracy | ~0.4 | pH units | Estimated average error using R/G ratio mode |
| Field of View (FOV) | 50 x 80 | ”m | Wide-field imaging area |
| Objective Numerical Aperture (NA) | 1.4 | Dimensionless | Oil-immersion objective |
Key Methodologies
Section titled âKey Methodologiesâ- Nanodiamond Immobilization: ND suspensions (5 nm to 200 nm, doped and undoped HPHT) were acid-cleaned and spin-coated onto glass microscope slides at extremely low density (down to 1 fluorescent ND per 40 ”m2) to ensure single-particle observation.
- Wide-Field Epifluorescence Setup: A single-molecule desktop wide-field microscope (Nanoimager S) was used with a 532 nm CW laser. Illumination was set near the critical angle (~50°) to achieve total internal reflection fluorescence (TIRF) conditions, minimizing out-of-focus background.
- Two-Channel Spectral Splitting: Emitted fluorescence was split using dichroic mirrors (DM1: 545/650 nm; DM2: 650 nm long-pass) into two distinct channels:
- Green Channel: 550-620 nm (captures NV° emission peak).
- Red Channel: 655-750 nm (captures NV- emission peak).
- High-Throughput Data Acquisition: Time-traces (96 to 589 per sample) were collected using a sCMOS camera with 100 ms exposure time, allowing for rapid collection of photon count distributions and R/G ratios across large ensembles of NDs.
- Ratiometric Analysis: Custom software (GapViewer) performed local background subtraction and calculated the R/G ratio for each ND spot in every frame, providing spectral information used to monitor the NV charge state (NV- or NV°).
- Dynamic Trace Modeling: Dynamic time-traces exhibiting charge state transitions were analyzed using Hidden Markov Modeling (HMM, specifically ebFRET) to identify discrete states, determine transition frequencies, and calculate the characteristic dwell-times (lifetimes) of the NV charge states.
- pH Sensing Experiment: 10 nm doped NDs were sequentially immersed in buffer solutions (pH 1.2 to 12.9). The mode of the R/G ratio distribution was measured for each pH condition to establish a linear sensing curve and confirm the reversibility of the charge state conversion.
Commercial Applications
Section titled âCommercial Applicationsâ- Quantum Computing and Information: Used for rapid quality control and screening of ND batches to identify those with high fractions of stable single NV centers, which are essential building blocks for solid-state quantum registers and single-photon sources.
- Nanoscale Biosensing (pH): The 10 nm NDs serve as non-photobleachable, highly stable probes for pH nanosensing. This is critical for monitoring localized pH changes in microfluidic systems, complex chemical reactions, or within living biological environments (e.g., mapping pH inside organelles like lysosomes).
- Single-Molecule Bioimaging: Provides a robust method for long-term (hours) intracellular tracking and imaging, especially using smaller NDs (10 nm) which offer better diffusion and lower toxicity compared to larger particles or photobleaching organic dyes.
- Material Science and Nanocrystal Engineering: The high-throughput methodology allows material scientists to quickly assess how different manufacturing parameters (size, nitrogen content, surface chemistry) affect the photophysical stability and charge state of NV centers, accelerating the development of optimized nanocrystals.
- Advanced Microscopy Development: The technique validates the use of simple, wide-field ratiometric imaging setups for complex spectral analysis, potentially simplifying instrumentation required for NV-based sensing compared to complex confocal or photon-correlation systems.
View Original Abstract
A method to observe individual fluorescent crystal defects in nanodiamonds is reported and opens new nanosensing avenues ( e.g. pH nanosensing).