Amplified nanoscale detection of labeled molecules via surface electrons on diamond
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2023-12-14 |
| Journal | Communications Physics |
| Authors | Ainitze Biteri-Uribarren, P. Alsina-Bolívar, Carlos Munuera-Javaloy, Ricardo Puebla, J. Casanova |
| Institutions | Universidad Carlos III de Madrid, University of the Basque Country |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research introduces an amplified quantum sensing protocol utilizing a hybrid diamond defect sensor for nanoscale molecular detection, significantly enhancing the measurement of inter-label coupling constants.
- Core Value Proposition: Achieving amplified nanoscale detection of labeled macromolecules (e.g., proteins) by measuring the dipolar coupling constant ($g$) between two electron-spin labels (L1, L2).
- Hybrid Sensor Design: The protocol leverages a hybrid sensor composed of a shallow Nitrogen-Vacancy (NV) center and a surface Dangling Bond (DB), where the DB acts as a signal mediator to bridge the coupling distance.
- Quantum Control Sequence: A multi-tone Dynamical Decoupling (DD) sequence is employed, specifically designed to encode the target coupling ($g$) into NV fluorescence oscillations while minimizing the effects of decoherence (T2).
- Performance Gain: Numerical simulations demonstrate that the hybrid NV-DB sensor achieves a 5 to 6 times higher Signal-to-Noise Ratio (SNR) compared to the standard NV-only sensing scenario.
- Efficiency Improvement: The hybrid protocol is significantly faster, requiring a duration of 10 µs, compared to 21 µs for the NV-only alternative, enabled by the stronger mediated interactions.
- Accuracy: The hybrid sensor provided a maximum-likelihood estimate of $g$ = 1.6(8) MHz, closely matching the true value of 1.734 MHz, demonstrating superior accuracy over the NV-only estimate (2(5) MHz).
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Zero-Field Splitting (D) | 2.87 | GHz | Intrinsic NV property |
| NV Gyromagnetic Ratio (B | γe | ) | 28 * 103 |
| NV Coherence Time (T2,NV) | 5 | µs | Used in master equation simulations |
| DB Coherence Time (T2,DB) | 1 | µs | Used in master equation simulations |
| Label Coherence Time (T2,L) | 1 | µs | Used in master equation simulations |
| NV Relaxation Time (T1,NV) | 20 | µs | Used in master equation simulations |
| DB Relaxation Time (T1,DB) | 29.4 | µs | Used in master equation simulations |
| Target Coupling Constant ($g$) | 1.734 | MHz | True dipolar coupling (AL1-L2) |
| Estimated Coupling ($g$) - Hybrid | 1.6(8) | MHz | Maximum-likelihood fit result |
| Estimated Coupling ($g$) - NV-only | 2(5) | MHz | Maximum-likelihood fit result |
| Protocol Duration (Hybrid NV-DB) | 10 | µs | Total sequence time for measurement |
| Protocol Duration (NV-only) | 21 | µs | Total sequence time for measurement |
| MW Driving Amplitude (Ω) | 10 | MHz | Applied Rabi frequency for pulses |
| π Pulse Duration | 50 | ns | Calculated from MW driving amplitude |
| Magnetic Field Gradient | ~30 | G nm-1 | Used for selective addressing of DB/Labels |
| Optimal NV-DB Distance (d) | 5.6 | nm | Configuration 1 (High SNR) |
| Optimal L1-L2 Distance (d12) | 3.8 | nm | Configuration 1 (High SNR) |
| SNR Enhancement Factor | 5-6 | Times | Hybrid sensor vs. NV-only sensor |
Key Methodologies
Section titled “Key Methodologies”The detection protocol relies on a hybrid quantum sensor and a specialized multi-tone microwave (MW) pulse sequence executed on a diamond surface.
- System Configuration: A shallow NV center is positioned near the diamond surface, coupled to a surface Dangling Bond (DB). Two electron-spin labels (L1, L2) are attached to the target macromolecule and placed in proximity to the DB.
- Initialization and Field Application: The NV center is initialized to its ground state using a green laser. A static external magnetic field (B) is applied along the NV axis (z-axis), along with a magnetic field gradient (~30 G nm-1) to ensure distinct Larmor frequencies for the DB and labels, enabling selective MW addressing.
- Multi-Tone Dynamical Decoupling (DD): A sequence of MW π and π/2 pulses is simultaneously applied across four distinct frequency channels corresponding to the NV, DB, L1, and L2. This sequence is structured as a series of spin echoes (castle-shaped).
- Signal Mediation and Encoding: The sequence utilizes the strong coupling between NV and DB, and DB and L1, to mediate the interaction. The target inter-label coupling constant ($g$) is encoded into the NV spin state during the free evolution periods (τ1, τ2, t).
- Decoherence Suppression: The DD sequence incorporates a Double Electron-Electron Resonance (DEER) structure during communication periods (τi) to cancel undesired interactions and minimize the impact of random dephasing (ηNV, ηDB), thereby enhancing the signal amplitude.
- Readout and Estimation: The final NV ground state population (P0) is measured via fluorescence. The coupling constant ($g$) is extracted by analyzing the oscillation frequency of P0 using Fourier transform and a maximum-likelihood estimation fit based on a phenomenological model.
- Simulation Environment: Numerical simulations account for decoherence channels (dephasing T2 and relaxation T1) using a Master equation in Lindblad form, and include cross-talk effects resulting from imperfect individual addressing during MW radiation.
Commercial Applications
Section titled “Commercial Applications”This amplified nanoscale detection technology has significant potential across several high-tech sectors, particularly those requiring ultra-high sensitivity and spatial resolution.
- Biophysics and Structural Biology:
- Tracking protein folding dynamics and conformational changes in real-time by measuring nanometer-scale distances between labeled sites.
- Investigating intermolecular associations and complex formation in biological systems.
- Nanoscale Magnetic Resonance:
- Enabling high-sensitivity Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR) experiments on microscopic or nanoscopic samples (e.g., single cells or femtomole volumes), overcoming the sensitivity limits of conventional bulk NMR.
- Quantum Sensing and Metrology:
- Development of next-generation solid-state quantum sensors utilizing hybrid defects (NV + DB) for enhanced sensitivity and Signal-to-Noise Ratio (SNR).
- Creating robust, room-temperature sensors for magnetic field and spin detection.
- Drug Discovery and Pharmaceutical Research:
- High-throughput screening and characterization of drug-target interactions at the single-molecule level.
- Analyzing the structural disposition of macromolecules relevant to disease (e.g., protein misfolding diseases).
- Advanced Materials Characterization:
- Probing local spin environments and defect structures on diamond surfaces with atomic-scale resolution.
View Original Abstract
Abstract The detection of individual molecules and their dynamics is a long-standing challenge in the field of nanotechnology. In this work, we present a method that utilizes a nitrogen vacancy (NV) center and a dangling bond on the diamond surface to measure the coupling between two electronic targets tagged on a macromolecule. To achieve this, we design a multi-tone dynamical decoupling sequence that leverages the strong interaction between the nitrogen vacancy center and the dangling bond. In addition, this sequence minimizes the impact of decoherence finally resulting in an increased signal-to-noise ratio. This proposal has the potential to open up avenues for fundamental research and technological innovation in distinct areas such as biophysics and biochemistry.