Optical Detection Techniques for Single Nanoparticles

Detecting and characterising individual nanoparticles through optical means has advanced rapidly over the past decade, driven by the need for label-free, high-sensitivity measurements in fields as diverse as biomedical diagnostics, environmental monitoring and materials science. Core approaches exploit elastic light scattering by nanoscale objects, with contrast determined by a particle’s scattering cross-section and polarizability. Interferometric scattering microscopy (iSCAT) and related modalities enhance weak signals by interfering the scattered light with a reference beam, enabling mass measurements and localisation at nanometre precision. Label-free mass photometry extends this principle to measure individual biomolecular masses in solution. Optical holography in non-common-path geometries further separates reference and scattered fields, yielding independent amplitude and phase information on single proteins. Dark-field and total internal reflection schemes serve to suppress background and improve detection limits, often combined with computational denoising and machine-learning algorithms to approach shot-noise-limited sensitivity. Together, these techniques provide real-time tracking, three-dimensional mapping and quantitative mass analysis of particles ranging from proteins and viruses to engineered nanomaterials, with applications spanning single-molecule biophysics, nanoparticle synthesis control and live-cell nanoscale imaging.

Research from Nature Portfolio

Recent studies have pushed the boundaries of label-free optical detection of biopolymers and proteins. A novel self-supervised machine-learning approach applied to interferometric scattering microscopy reduced technical noise and background speckle, enhancing sensitivity by a factor of four and enabling detection of proteins below 10 kDa in mass. In parallel, implementation of non-common-path optical holography using a dark-field scattering microscope achieved a five-order-of-magnitude boost in sensitivity over conventional holographic methods. By separating scattering and reference light into multiple phase-stable channels, this technique permits independent tuning of amplitude and phase, yielding direct measurements of single-protein polarizability and mass down to sub-100 kDa species.

Research from all publishers

Developments outside the portfolio have advanced both fundamental understanding and practical applications. Real-time interferometric scattering microscopy was integrated with spatially patterned photoreductant activation to monitor and control the growth of individual gold nanoparticles in solution, revealing heterogeneous nucleation and autocatalytic growth kinetics across hundreds of particles simultaneously. Foundational reviews of interferometric scattering microscopy have synthesised its theoretical underpinnings, experimental implementations and connections to bright-field and phase-contrast imaging, charting its performance limits and outlining prospects for ultrafast, high-resolution single-particle studies in complex environments.

Optical Detection Techniques for Single Nanoparticles publication trend

The graph below shows the total number of articles in optical detection techniques for single nanoparticles across all publications each year (not limited to Nature Index journals).

Technical terms

Interferometric scattering microscopy (iSCAT): A label-free imaging method that enhances nanoparticle scattering by interfering it with a coherent reference beam, enabling ultrahigh sensitivity and localisation precision.

Scattering cross-section: A measure of the probability that a particle will scatter incident light, directly related to its size, shape and refractive index contrast.

Polarizability: The extent to which an electric field induces a dipole moment in a particle, influencing its scattering strength and phase response.

Label-free detection: Techniques that do not rely on fluorescent or molecular tags, instead utilising intrinsic optical properties of the analyte.

Non-common-path holography: An interferometric arrangement in which reference and object beams travel separate optical paths, allowing independent control of phase and amplitude channels.

References

1. Self-supervised machine learning pushes the sensitivity limit in label-free detection of single proteins below 10 kDa. Nature Methods (2023).
2. Single-protein optical holography. Nature Photonics (2024).
3. Real-Time Monitoring and Control of Nanoparticle Formation. Journal of the American Chemical Society (2023).