Joseph R. Cole’s Research in Plasmonic Nanoparticles

Joseph R. Cole, a researcher at Rice University (in Naomi Halas’s group), co-authored several influential papers on plasmonic nanoparticles – primarily gold nanoshells and nanorods – for biomedical and solar-energy applications. His 2006–2009 publications explored how these nanostructures absorb light and convert it to heat or enhanced fluorescence, with implications for cancer therapy, bio‐imaging, and solar harvesting. For example, Cole et al. noted that “with clinical trials for photothermal tumor ablation… underway, increasing understanding of the efficacy of plasmonic nanoparticle-based photothermal heating [was] urgent”. He therefore undertook detailed experimental and theoretical studies of gold nanoshells and nanorods to quantify their light-to-heat and light-to-fluorescence conversion efficiency.

Photothermal Nanoparticles for Cancer Therapy

Cole’s 2009 J. Phys. Chem. C paper systematically measured the photothermal transduction efficiency of various gold nanoparticles under near-infrared (808 nm) light. Key findings include:

• Particle types studied: Silica-core Au nanoshells, Au₂S-core Au nanoshells, and solid Au nanorods – all with plasmon resonances tuned to NIR. The work compared experimental heating (“photothermal transduction”) with theoretical absorption for each type.

• Efficiency differences: The efficiencies among the three particle types differed by at most a factor of ~3. Importantly, particle size dominated the efficiency: larger particles showed much stronger absorption and scattering.

• Nanorods vs. nanoshells: Under identical optical conditions, gold nanorods were about twice as efficient at converting absorbed light into heat as silica–Au nanoshells. In practice, Cole et al. report photothermal efficiencies of roughly 25% for nanoshells and ~50% for nanorods (i.e. η≈0.25 vs. 0.50), meaning nanorods delivered about 2× the heat per absorbed photon. (The authors note that roughly 36× more nanorods by number would be needed to match the heat of a given number of nanoshells, reflecting their much smaller mass per particle.)

• Implications: These results inform nanoparticle design for cancer hyperthermia. For example, nanorods’ higher efficiency (per gold mass) might enable more compact therapies, while nanoshells’ larger size could aid simultaneous optical imaging. In all cases, the work provides a quantitative baseline for comparing nanorod and nanoshell heating in biomedical contexts.

Plasmon-Enhanced Fluorescence Imaging

In a 2009 ACS Nano paper, Cole (with Bardhan, Grady, Joshi, Halas, et al.) investigated how Au nanostructures boost the fluorescence of nearby dye molecules. They sandwiched a near-infrared dye (IR800) a few nanometers from either an Au nanoshell or an Au nanorod (using a protein spacer) and measured the dye’s emission. Key results:

• Quantum yield enhancement: The intrinsic quantum yield of IR800 was only ~7% in solution, but when placed near an Au nanoshell it jumped to ~86%, and near a Au nanorod it rose to ~74%. In other words, nanoshells increased the dye’s fluorescence output by ≈12× (from 7%→86%), and nanorods by ≈10.6× (7%→74%).

• Mechanism: The authors attribute this to the strong local field enhancement of the plasmonic particles and increased radiative decay rate for the dye. Their experiments used human serum albumin as a precise spacer layer, confirming that the metal–fluorophore distance was controlled.

• Bioimaging potential: Such dramatic fluorescence boosts (“tremendous potential for contrast enhancement in fluorescence-based bioimaging”) suggest that plasmonic nanoparticles could serve as effective optical contrast agents or sensors. This work quantifies exactly how much emission gain can be achieved with nanoshell vs. nanorod antennas.

Plasmonic Solar-Spectrum Harvesting

Cole also co-authored theoretical studies on using plasmonic nanoparticles to absorb sunlight efficiently. An Applied Physics Letters (2006) paper (Cole & Halas) and a 2007 Air Force report (Task Order) used optimization algorithms to design nanoparticle ensembles that match the solar spectrum. Major points:

• Mixed nanoparticle distributions: The 2006 APL paper optimized mixtures of spherical metal particles (both solid nanospheres and nanoshells) so that their combined plasmon resonances cover the full AM1.5 solar spectrum. Using Mie theory, the study computed the ideal size distributions for maximum broadband absorption or scattering. The authors conclude that carefully chosen distributions of different-sized shells/spheres can outperform uniform materials in solar harvesting.

• Optimal nanoshell sizes: In a related 2007 study (DFARS Task Order 0035), Cole and Halas applied these ideas specifically to photovoltaic surfaces. They showed that a bimodal nanoshell solution – one shell species (core radius 47 nm with shell radius 58 nm) and another (core 28 nm with shell 42 nm) in a 6:5 volume ratio – would optimally absorb AM1.5 sunlight when coated on a silicon surface. (These correspond to ~116 nm and ~84 nm outer diameters, respectively.) This mixture maximizes absorption of solar photons. In contrast, for scattering-dominated applications they found that a single shell size sufficed.

• Applications: These findings provide design rules for plasmonic solar cells and photothermal panels. For example, integrating a mixed-layer of metal nanoshells with those dimensions into a solar cell could enhance light coupling into the silicon absorber. Cole’s work thus bridges fundamental plasmonic physics and practical solar-energy engineering.

Sources: Cole’s key findings are documented in peer-reviewed publications (cited above) and related technical reports. All quoted results come from those sources. These works collectively highlight Joseph R. Cole’s contributions to understanding and engineering plasmonic nanoparticles for photothermal therapy, bioimaging, and solar energy.