Measuring changes in shape and size of nanobiological objects and layers in real time is essential to optimize their performance. Optical sensing involves tagless operation and has high throughput and sensitivity. However, state-of-the-art techniques require the measurement of several parameters to unravel conformational or thickness changes in biomolecular layers.
Study: Quantification of time-resolved thickness and shape change using a dual-band nanoplasmonic ruler with sub-nanometer resolution. Image credit: ACS Nano (2022), Nugroho, FAA et al. (2022)
A paper published in ACS Nano presented dual-band nanoplasmonic rulers composed of plasmonic nanoparticles in mixed arrays with spectrally separated resonance peaks. Based on model experiments and electrodynamic simulations, these nanorules allowed simultaneous measurements of the variation of the refractive index and the thickness of heterogeneous layers with a uniform surface and sub-nanometer resolution.
Additionally, changes in the shape of the nanostructure can be tracked by quantifying the degree of deformation in the lipid vesicles at a stage prior to rupture. Thus, the study presents a practical route for multimodal nanoplasmonic optical sensing.
Characterization of biological nanosystems using nanoplasmonic rules
Nanotechnology and nanoscience largely rely on the precise measurement of shape and size, which determine the functionality and properties of nanomaterials in the life sciences. Moreover, as nanoscopic biological species and their functionality are at the origin of many diseases, a precise characterization of these entities allows the development of advanced therapeutic systems and tools.
Although various analytical techniques have been established to characterize nanoscopic biological entities, most of them have limitations. In this context, plasmon-based sensors are excellent tools for the label-free detection of small biomolecules. An interesting group of these sensors are nanoplasmonic rulers, which rely on the hybridization of plasmons as they change their morphology to detect distances at the nanoscale.
Many optical sensing techniques have been explored over the past decades to measure the presence or changes of the biomolecular layer on the surface of a sensor via changes in the refractive index between the interfacial layers. Thus, it provides label-free, sensitive and real-time detection with simple operation and high throughput.
Sensors based on localized surface plasmon resonance (LSPR) can serve as plasmonic nanorulers, enabling measurement of the thickness of biological nanoparticles
Plasmonic nanorulers have some distinct advantages over the fluorescence resonance energy transfer (FRET) technique that is traditionally used for nanoscale distance measurement. However, a major prerequisite for the application of plasmonic nanorules is the systematic calibration and standardization of the spectral shift as a function of interparticle separation.
Double band nanoplasmonic rule and its application
In the present work, plasmonic nanorules were introduced to accurately determine the layer thickness of a biological entity with sub-nanometer resolution. The dual-band plasmonic nanorulers were composed of plasmonic nanoantennas with two mixed populations in size, resulting in two independent LSPR peaks in the extinction spectrum.
The resonant antennas created short-range evanescence, which allowed accurate real-time refractive index and thickness determination in the 10-nanometer layer thickness range.
A customized version of hole-mask colloidal lithography was used to demonstrate the potential of nanofabricated dual-band plasmonic nanorulers to measure and resolve thickness changes in systems, including atomic layer deposition of a film of alumina (Al2O3) in air, adsorption of silica (SiO2) nanospheres on a supported lipid bilayer (SLB) and planar SLB on SiO2 liquid. Thus, plasmonic nanorulers are distinguished from other optical nanorulers by their label-free, real-time, and high-throughput characteristics.
Additionally, the nanorules in the present work were used to quantify shape changes in adsorbed lipid vesicles during SLB formation. Thus, their direct information on the degree of deformation of the lipid vesicles was obtained before their rupture.
Conclusion
In summary, dual-band plasmonic nanorules have been developed to determine changes in refractive index and thickness of arbitrary multilayers and shape of deposited nanoparticles with sub-nanometer resolution.
Here, the surfaces of the plasmonic nanosensors were composed of two differently sized nanoantennas that probe the adlayers independently and jointly to reveal the sensor reading from the change in adlayers thickness, refractive index, and shape.
The accuracy of plasmonic nanorulers has been substantiated by measuring thickness changes in systems in air and liquid environments. The nanorulers presented in this work are advantageous over other optical nanorulers due to their real-time, label-free, and high-throughput characteristics. Thus, plasmonic nanorules address concerns related to the confirmation and size of nanoscopic biological entities.
Reference
Nugroho, FAA et al. (2022). Quantification of time-resolved thickness and shape change using a dual-band nanoplasmonic ruler with sub-nanometer resolution. ACS Nano. https://pubs.acs.org/doi/10.1021/acsnano.2c04948