Fluorescence-based and absorption-based detection technologies have been at the heart of modern diagnostics, but when it comes to seeing single biomolecules, every photon (spectroscopically speaking) and every bit of contrast (microscopically speaking) counts. This is where Photonic Resonator Absorption Microscopy (PRAM) steps in, offering a way to make nanoparticles visible one by one. But there has been a long-standing challenge: bare nanoparticles don’t always give strong enough signals, leaving researchers straining to “see” them against the background.
In a new study published in Analytical and Bioanalytical Chemistry, researchers at the University of Illinois Urbana-Champaign have taken PRAM to the next level by designing metallic and magneto-plasmonic cryosoret nano-assemblies which are tiny clusters of gold and iron oxide nanoparticles that shine (or rather, absorb) far more brightly than single nanoparticles.
The Problem: Weak Signals from Lonely Nanoparticles
Imagine trying to spot a single person waving in a dark stadium, though it’s possible, but not easy. In PRAM, bare nanoparticles act as that lone person, scattering and absorbing light, but their signal is faint and sometimes lost in noise. The result? Lower detection contrast and difficulty in digitally counting biomolecular binding events.
The problem is partly physical: a single gold nanoparticle has a limited optical cross-section, meaning it can only absorb so much light. In a technique like PRAM, where the key signal is a tiny dip in transmitted light that limitation becomes a bottleneck for sensitivity of the platform itself.
The Cryosoret solution: Strength in numbers
To solve this, the team turned to cryosoret nanoengineering, where nanoparticles are cooled rapidly using liquid nitrogen, prompting them to self-assemble into tightly packed clusters. These “nano-assemblies” act like a group of people waving together and suddenly, the signal becomes unmissable.
But the researchers didn’t stop with gold alone. They introduced magneto-plasmonic nano-assemblies which are hybrid constructs made of gold plus iron oxide nanoparticles. This addition created not only electric-field hotspots but also magnetic-field hotspots within the nano-gaps of the assemblies. Think of these as “tiny light and magnetic amplifiers” that make every photon count more.
Using COMSOL simulations, the team showed that these assemblies concentrate both E-fields and H-fields far more effectively than single nanoparticles, leading to stronger PRAM signals.
Nano-engineering using cryosoret nano-assemblies and interface engineering using photonic crystals to enable better detection of target analytes in biosensing using photonic resonator absorption microscopy (PRAM)
The Photonic Crystal advantage: Guiding the Light
Of course, having a brighter absorber is only half the battle. To make PRAM work efficiently, light must be directed, focused, and coupled in just the right way. This is where photonic crystal (PC) substrates help. These engineered nanostructures act like traffic controllers for light, using guided-mode resonance to make the absorption signature of each nano-assembly stand out.
When the cryosorets sit on the photonic crystal, a beautiful synergy emerges: the localized plasmonic resonances of the assemblies, the magnetic dipole modes from the iron oxide, and the PC’s guided-mode resonance all couple together, producing a sharp, high-contrast absorption signal that is easy to detect and digitally count.
Real-World Impact: Counting Molecules, Detecting Disease
The team lead by Skye and Weinan put their system to the test with a digital microRNA assay, targeting miR-375-3p, a biomarker linked to cancer progression. They demonstrated single-particle digital counting with significantly improved image contrast, making detection events easier to recognize and classify.
Beyond cancer, the technology could be applied to a range of problems: environmental sensing, infectious disease diagnostics, or even fundamental studies of binding kinetics at the single-particle level. The addition of magnetic components could also allow magnetically guided capture and enrichment of rare analytes, paving the way for more versatile biosensors.
Bringing it all together
This research carried out by Prof. Brian Cunningham’s Nanosensors Group, represents a significant step forward in label-free digital biosensing. By combining cryosoret nano-assemblies, magneto-plasmonic enhancement, and photonic crystal substrates, they have shown that the whole is truly greater than the sum of its parts. While bare nanoparticles gave the team a starting point, the assemblies helped them see the biology with much better clarity enabling the team not only to measure, but to count, one molecule at a time.
Thanks to the immense support provided by Carl R. Woese Institute for Genomic Biology, Nick Holonyak Micro and Nanotechnology Laboratory, Materials Research Laboratory, Department of Electrical and Computer Engineering, University of Illinois Urbana-Champaign., the team is now exploring next-generation assays where light, magnetism, and nanotechnology converge to make biosensing faster, sharper, and more powerful.