New class of cryosoret nanoassemblies advances biosensors

Each day, we are fortunate to enjoy the natural beauty of color. While many objects gain their appearance from pigments, butterfly wings, peacock feathers, certain woods, and chameleon skin are brilliant reflectors of specific wavelengths due to the presence of micro-nano-patterned structures. We have much to learn from nature, because similar nanostructures can be adapted to detect biomolecules.

Working in Professor Brian Cunningham’s NanoSensors group within the Center for Genomic Diagnostics (CGD) at the Carl R. Woese Institute for Genomic Biology (IGB), as an IGB Postdoctoral Fellow, I’m inspired by surface colors that emerge from “structure-property” interconnectedness.

By carefully observing nature’s ability to reflect light using nanopatterned surfaces, we can engineer them for specific purposes. We design and fabricate periodically repeating nanostructured surfaces, a.k.a. “photonic crystals,” to form optical resonances that are observed as narrow bands of wavelength, which are nearly perfectly reflecting but allow all other wavelengths to pass through. When resonant wavelengths are designed to occur within the visible part of the spectrum, photonic crystals reflect brilliantly, much like butterfly wings, without pigments or dyes. Directly on top of the photonic crystal, intense electromagnetic standing waves are formed, whose energy can be efficiently captured by nanometer-scale objects such as nanoparticles and biomolecules.

Cracking the fluorescence quenching problem

Chemical fluorescent dyes are used throughout biology to “tag” molecules (such as nucleic acids and proteins), which enables us to observe them. But because fluorophores are weak light emitters, metal nanostructures (such as gold nanoparticles) can enhance their output through electromagnetic “hotspots” at their surface.

In MRS Bulletin, Cunningham’s team recently reported¹ rapid freezing of gold nanoparticles in liquid nitrogen to generate dense but nanogap-rich self-assemblies called “cryosorets.”²

To make cryosorets, we cool a glass vial that contains a solution of ~20-nm-diameter nanoparticles with liquid nitrogen (-196°C) to create a steep internal temperature gradient that drives thermomigration toward the container’s outer wall. This “Soret Effect” drives particles from warmer to colder regions to yield dense, nanogap-rich assemblies, whose size and packing density can easily be tuned by controlling the cooling time.

The nanogaps within cryosorets provide a dense environment comprised of many hotspots that can efficiently gather energy from an external light source (such as a laser) when placed upon a photonic crystal. This photonic crystal not only efficiently excites the cryosoret hotspots, but also steers the fluorescence emission away from the surface at specific narrow angles for efficient collection by a detection system.

Our team developed a guided-mode resonance theoretical model that provides insightful inferences from simulations and experiments motivated by structure-property relationships found in nature.

Cryosorets are nanoparticle aggregates that range from 50 to 200 nm in size, which are comprised of many nanoparticles within the 20-nm size range that are held together by Van der Waals forces rather than by covalent bonds. An important challenge to address is to make the cryosorets structurally stable through all steps of using them as a light-emitting tag for biosensing, which can involve surface functionalization with biomarker-specific capturing molecules, centrifugation, and flow through microfluidic devices. Once synthesized, it is important to carefully manage the charge state of cryosorets (measured by their zeta potential) to prevent them from aggregating with each other.

Magneto-plasmonics: Bringing the magnetic field into the picture

While studying cryosorets, our group made new observations. In conventional optical biosensors, only the electric field component of the electromagnetic energy provided by a laser or light-emitting diode (LED) is used and the energy associated with the magnetic field isn’t exploited. Extensive simulations of cryosorets on photonic crystal surfaces reveal the presence of circulating displacement currents at the nanoscale emerging from the magnetic field component of illumination, which generate a new population of hotspots that are not present from electric field excitation alone.

A recent publication by our team in APL Materials demonstrates integration of magnetic and metallic hybrid cryosorets, in which the gold retains the signal enhancement with added magnetic tunability to render the biosensor platform open for external magnet-driven reconfiguration.³ Careful alignment of the photonic crystal’s forbidden energy band edge with the fluorophore emission wavelength resulted in highly directional extraction of a narrow band of fluorescence-emitting wavelengths.

Using nature’s playbook to develop new tools for understanding nature

Our team is currently focused on translating the electromagnetic principles of micro-nano-interfaces to detect “biomarkers” for human disease that include microRNAs, circulating tumor DNA, proteins, extracellular vesicles, and viruses.

My previous work demonstrated ferroplasmonic effects where ferromagnetic and plasmonic nanosystems “talk” to both electric and magnetic flux of light. Integrating novel magneto-plasmonic cryosorets with photonic crystals opens new avenues for biosensing. Our group is currently advancing beyond chemical fluorophores to explore cryosorets that incorporate semiconductor quantum dots, magnetic nanoparticles, nanodiamonds, and upconverting phosphors.

By taking a hike in the outdoors, the bright reflection from a butterfly wing or a chameleon leads us to think about the structures and physics principles used by nature. The group at the University of Illinois at Urbana-Champaign uses these reflections to guide the selection of materials and nanostructures that we can study using electromagnetic simulations and fabricate within the cleanroom to develop new tools for the life sciences. In a way, we’re using the biology of nature to develop better tools for understanding biology.

REFERENCES

1. S. Bhaskar, L. Liu, W. Liu, J. Tibbs, and B. T. Cunningham, MRS Bull., 50, 585–598 (2025); https://doi.org/10.1557/s43577-024-00850-2.
2. L. Liu, S. Bhaskar, and B. T. Cunningham, Appl. Phys. Lett., 124, 234101 (2024); https://doi.org/10.1063/5.0203701.
3. S. Bhaskar et al., APL Mater., 13, 041103 (2025); https://doi.org/10.1063/5.0251312.