Self-Assembly of Nanodiamonds and Plasmonic Nanoparticles for Nanoscopy

At a Glance

Metadata	Details
Publication Date	2022-02-28
Journal	Biosensors
Authors	Lukas Schmidheini, Raphael F. Tiefenauer, Volker Gatterdam, Andreas Frutiger, Takumi Sannomiya
Institutions	Uppsala University, ETH Zurich
Citations	11
Analysis	Full AI Review Included

Executive Summary

This research introduces a novel hybrid nanostructure composed of Nitrogen-Vacancy (NV) nanodiamonds (NDs) and Gold Nanoparticles (GNPs) coupled via DNA hybridization, designed for super-resolution microscopy (SRM).

Core Value Proposition: The system leverages the photostability of NDs and the strong, non-linear optical properties of GNPs to achieve stochastic blinking necessary for SRM, without relying on high-power, destructive linear excitation methods.
Coupling Mechanism: Precise nanoscale assembly is achieved through DNA hybridization, controlling the particle separation (down to 5 nm) and the ratio of coupled particles via target DNA concentration.
Excitation Principle: Multiphoton laser excitation (1020 nm) excites the GNPs, generating second harmonic emission (520 nm). This near-field harmonic emission acts as the localized excitation source for the NV centers in the coupled NDs.
Stochastic Blinking: The inherent instability and flickering of the GNP surface plasmon modes directly induce stochastic fluctuations (blinking) in the ND fluorescence emission.
Nanoscopy Achievement: A proof-of-principle super-resolved image was generated using the stochastic blinking data, demonstrating resolution of 187 nm, significantly below the diffraction limit (λ/2 ~ 325 nm).
Engineering Advantage: Harmonic generation offers attractive characteristics for bioimaging, including low power requirements, low background noise, and high tissue transparency compared to traditional linear excitation methods.

Technical Specifications

Parameter	Value	Unit	Context
GNP Diameter	50	nm	Chosen for LSP resonance matching NV excitation.
Nanodiamond Size (Simulated)	15	nm	Used in Multiple Multipole Program (MMP 6) modeling.
Particle Gap Distance	5	nm	Controlled by DNA hybridization length.
Excitation Wavelength (Multiphoton)	1020	nm	Used for two-photon absorption in GNPs.
GNP Emission Peak (SHG)	520	nm	Matches the NV center excitation wavelength.
ND Emission Peak (PL)	600-700	nm	Corresponds to NV^- center photoluminescence.
GNP Emission Power Dependence	1.97	Slope (log scale)	Confirms quadratic (two-photon) absorption mechanism.
Optimal Coupling Efficiency Power	~0.7	mW	Laser power yielding maximum I_ND/I_GNP ratio.
ND Oxidation Temperature	600	°C	Annealing temperature for surface functionalization.
Demonstrated Super-Resolution	187	nm	Achieved resolution, below the diffraction limit (λ/2 ~ 325 nm).

Key Methodologies

The hybrid system fabrication and analysis involved precise chemical functionalization, self-assembly, and advanced multiphoton microscopy:

Nanodiamond Surface Functionalization:
- Oxidation of nanodiamond surfaces via annealing at 600 °C in air.
- Two-step chemistry: Amine (-NH₂) functionalization of the oxidized surface.
- Attachment of DNA: Reaction of amine groups with Sulfosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate (SSMCC) crosslinker, followed by reaction with thiol-modified DNAs.
Gold Nanoparticle Functionalization:
- GNPs were functionalized separately with complementary DNA strands (DNA2).
DNA Hybridization Assembly:
- GNP-ND conjugates were formed via self-assembly using a target DNA strand containing sequences complementary to both GNP (DNA2) and ND (DNA1) tags.
- The concentration of the target DNA controlled the number and ratio of coupled particles.
Optical Characterization (Multiphoton Excitation):
- The assembled conjugates were coated on glass coverslips and analyzed using multiphoton microscopy (1020 nm excitation).
- Emission spectra confirmed two peaks: 520 nm (GNP harmonic emission) and 600-700 nm (ND fluorescence).
Coupling Efficiency Analysis:
- The ratio of ND emission intensity (I_ND) to GNP emission intensity (I_GNP) was measured as a function of laser power, revealing a non-linear dependence with a peak efficiency at ~0.7 mW.
Super-Resolution Imaging (STORM Principle):
- Time-series image stacks were acquired to capture the stochastic blinking of NDs induced by the flickering GNP plasmons.
- Image analysis utilized a localization algorithm and point spread function fitting to map emission centers, demonstrating sub-diffraction resolution.

Commercial Applications

This technology, leveraging plasmon-enhanced NV center properties and DNA-based assembly, is highly relevant to several high-tech sectors:

Intracellular Biosensing and Bioimaging: The DNA-based coupling mechanism allows for targeted assembly within complex biological environments, while the low-power, low-background harmonic generation is ideal for deep tissue or long-term live-cell imaging.
Super-Resolution Microscopy (SRM): Provides a novel, non-chemical method for inducing the fast, bright, and stochastic blinking required for techniques like STORM, utilizing the robust NV centers for long-term imaging stability.
Quantum Sensing Platforms: The ability to precisely couple NV centers to plasmonic structures enables enhanced control over NV decay rates and excitation, critical for developing highly sensitive nanoscale sensors for temperature, magnetic fields, or pH.
Nanomaterials Engineering: Offers a robust, scalable method for creating precisely spaced hybrid plasmonic-quantum emitter systems, useful for fundamental studies in quantum electrodynamics and light-matter interaction.
Diagnostics and Nanomedicine: DNA-functionalized nanoparticles are widely used in targeted drug delivery and molecular diagnostics, where the NV center provides a stable, traceable quantum label.

View Original Abstract

Nanodiamonds have emerged as promising agents for sensing and imaging due to their exceptional photostability and sensitivity to the local nanoscale environment. Here, we introduce a hybrid system composed of a nanodiamond containing nitrogen-vacancy center that is paired to a gold nanoparticle via DNA hybridization. Using multiphoton optical studies, we demonstrate that the harmonic mode emission generated in gold nanoparticles induces a coupled fluorescence emission in nanodiamonds. We show that the flickering of harmonic emission in gold nanoparticles directly influences the nanodiamonds’ emissions, resulting in stochastic blinking. By utilizing the stochastic emission fluctuations, we present a proof-of-principle experiment to demonstrate the potential application of the hybrid system for super-resolution microscopy. The introduced system may find applications in intracellular biosensing and bioimaging due to the DNA-based coupling mechanism and also the attractive characteristics of harmonic generation, such as low power, low background and tissue transparency.

Tech Support

Original Source

DOI: https://doi.org/10.3390/bios12030148

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