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6CCVD Research

Quantum sensing of paramagnetic analytes by nanodiamonds in levitated microdroplets and aqueous solutions

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-01-01
JournalChemical Science
AuthorsEmily K. Brown, Zachary R. Jones, Adrisha Sarkar, Brandon J. Wallace, Ashok Ajoy
InstitutionsUniversity of California, Berkeley, Lawrence Berkeley National Laboratory
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This analysis summarizes the quantum sensing of paramagnetic analytes (Gd+3) using Nitrogen-Vacancy (NV-) centers in carboxylated nanodiamonds (NDs), focusing on the engineering implications of environmental factors in bulk and microdroplet systems.

  • Core Mechanism Validation: The study confirms that efficient NV-ND detection of paramagnetic species requires a two-step process: diffusion of the analyte to, and subsequent adsorption onto, the ND surface.
  • Quantitative Modeling: A photophysical model incorporating a Langmuir adsorption isotherm successfully links the bulk concentration of Gd+3 to the optically detected magnetic resonance (ODMR) response, achieving a self-consistent description across varying conditions.
  • Adsorption Thermodynamics: The equilibrium constant (Keq) for Gd+3 adsorption to the carboxylated ND surface was determined to be (1 ± 0.5) x 105 M-1, corresponding to a favorable free energy of adsorption (ΔG° = -28 ± 1 kJ mol-1).
  • Environmental Sensitivity: ODMR response is highly dependent on pH (due to surface carboxylate protonation) and competitive ligands (e.g., acetate), which sequester free Gd+3 and prevent surface binding.
  • Shape and Size Scaling: ND shape (modeled as an ellipsoid) and size are critical, with sensitivity scaling as 1/r (inverse of the radius), confirming that NV centers closer to the surface provide enhanced detection.
  • Microdroplet Effects: Sensing in 17 ”m microdroplets showed a 10x reduction in sensitivity compared to bulk solutions, attributed to solution phase depletion of Gd+3 caused by the high ND surface area relative to the small compartment volume.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Nanodiamond Nominal Diameters70, 100, 140, 750nmCarboxylated (-COOH) NDs used.
NV Center Concentration~3ppmUsed in all ND samples.
ODMR Transition Frequency2.865GHzMeasured NV- transition frequency.
Bulk Laser Power (532 nm)0.2WCuvette measurements (spot size ~1 mm2).
Droplet Laser Power (532 nm)0.5WQuadrupole trap measurements (spot size ~1 mm2).
Microwave Power (Transmitted)~6WAverage power delivered to the antenna.
Rabi Frequency (Ω)9 x 106rad s-1Estimated for the NV center.
Intrinsic Spin Relaxation (T1, Bulk)2msAssigned value for NV center.
Dephasing Time (T2)1.5 x 10-4sEstimated T2 value.
Gd+3 Adsorption Keq(1 ± 0.5) x 105M-1Equilibrium constant for binding to carboxylated ND.
Adsorption Free Energy (ΔG°)-28 ± 1kJ mol-1Indicates spontaneous adsorption.
Maximum Site Density (Γ∞)8.5 x 1017m-2Estimated for fully deprotonated surface (pH 10-12).
ND Shape ApproximationEllipsoid (Aspect Ratio ~3)N/AUsed to accurately model NV-to-surface distance.
Microdroplet Radius17 ± 0.5”mLevitation stability maintained at 80% RH.
Bulk Detection Limit ([Gd+3])10nMLowest concentration measured in bulk solution.

Key Methodologies

Section titled “Key Methodologies”

The experimental approach combined quantum sensing (ODMR) with microfluidics (quadrupole trap) and rigorous photophysical modeling to quantify surface adsorption effects.

  1. Nanodiamond Preparation: Carboxylated NDs (70-750 nm) were used, suspended in a 45% glycerol/water mixture to match the 80% relative humidity (RH) required for droplet stability.
  2. Bulk ODMR Setup: Measurements were performed in a standard cuvette using a 532 nm laser and a microwave antenna to establish a baseline for adsorption thermodynamics (Keq).
  3. Microdroplet Levitation: Individual 17 ”m radius droplets were charged and levitated in a branched quadrupole trap, with RH controlled at 80% by flowing nitrogen.
  4. Self-Referential Droplet Collision: ODMR measurements were performed by colliding two oppositely charged droplets (one containing NDs, one containing GdCl3) to measure the change in contrast before and after analyte introduction.
  5. ODMR Signal Acquisition: The NV centers were excited by a 532 nm laser, and red fluorescence (>650 nm) was detected. Microwave frequency was modulated (1 kHz) near 2.865 GHz for lock-in detection of the ODMR contrast.
  6. Photophysical Model Integration: A comprehensive model was developed relating the total fluorescence intensity (I) to the NV spin relaxation rate (ks), which is dependent on the transverse magnetic field variance (B⊄2) generated by adsorbed Gd+3 spins.
  7. Adsorption Modeling: The surface spin density (σ) was determined using a modified Langmuir adsorption isotherm (Equation 27) that explicitly accounts for solution phase depletion of Gd+3 in the finite volume of the microdroplet.
  8. Shape Correction: ND geometry was modeled as an ellipsoid (aspect ratio 3) or an equivalent sphere (radius r/2.5) to accurately capture the R-6 distance dependence of the NV-to-surface spin interaction, correcting for inaccuracies arising from simple spherical approximations.

Commercial Applications

Section titled “Commercial Applications”

This research provides foundational knowledge for deploying NV-ND quantum sensors in complex, small-volume environments, relevant to several high-tech fields.

  • Quantum Sensing and Metrology:
    • Development of quantitative nanoscale Electron Paramagnetic Resonance (EPR) and Nuclear Magnetic Resonance (NMR) sensors for chemical analysis.
    • Creating robust, chemically selective quantum sensors capable of operating in complex matrices (e.g., buffers, high salt concentrations).
  • Bio-Sensing and Diagnostics:
    • In situ, non-destructive sensing of paramagnetic metal ions (e.g., Fe+3, Cu+2, Gd+3) and free radicals within single cells or biological microcompartments.
    • Designing functionalized NDs for targeted detection, where surface chemistry (pH, ligand competition) is precisely controlled for quantitative results.
  • Microfluidics and Lab-on-a-Chip:
    • Integration of ND-NV sensors into microfluidic platforms for high-throughput chemical kinetics studies in picoliter volumes.
    • Probing accelerated reaction rates and mechanisms in microdroplets and aerosols, relevant for chemical synthesis and atmospheric science.
  • Nanomaterial Manufacturing (NDs):
    • Informing the optimization of ND synthesis and surface functionalization (e.g., carboxylate density) to maximize sensor sensitivity and detection limits for specific analytes.
View Original Abstract

The role of the aqueous phase environment ( e.g. , pH and salt concentration) on the adsorption of gadolinium to the surface of nanodiamonds with nitrogen-vacancy (NV − ) centers is investigated in microdroplets and in bulk solution.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2023 - arXiv [Crossref]

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