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.
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.
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 reported rapid freezing of gold nanoparticles in liquid nitrogen to generate dense but nanogap-rich self-assemblies called "cryosorets."
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.
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.