Joseph Cole’s research on plasmonic nanoparticles

Joseph R. Cole (PhD 2009, Rice Univ.) was a member of Naomi Halas’s Lab, where he co‐authored several papers on plasmonic nanoparticles – metal nanostructures (especially gold) whose free electrons resonate with light. These resonances enable strong absorption/scattering in the near-infrared (NIR) and can convert light into heat or modify nearby fluorescence. In particular, Cole’s work addressed 

1. photothermal conversion in gold nanoshells and nanorods (for cancer therapy), 
2. fluorescence enhancement near gold nanostructures (for bioimaging), and
3. designing nanoparticle mixtures to harvest sunlight (for solar energy).

Photothermal efficiency: Nanoshells vs. nanorods (Cole et al. 2009)

Cole et al. (2009) measured and modeled photothermal transduction efficiencies (light-to-heat conversion) of three plasmonic nanoparticle types: silica-core/gold nanoshells, Au₂S-core/gold nanoshells, and gold nanorods. This work was motivated by nanoparticle-mediated photothermal cancer therapy: systemically delivered gold nanoparticles accumulate in tumors and, when illuminated by an NIR laser, produce localized heating to kill cancer cells. Although nanoshells and nanorods both have strong NIR absorption, their detailed absorption and scattering differ due to geometry.

In the study, Cole et al. used continuous-wave NIR lasers and phantoms to measure the temperature rise of nanoparticle suspensions at known laser power, and they compared this to theoretical absorption predictions (using Mie and dipole theory) for each particle type. They assumed a random orientation distribution of nanorods (as in blood vessels) when averaging theory. The key finding was that all three particle types had broadly similar photothermal efficiency – their measured heat-conversion efficiencies differed by “a factor of 3 or less”. In other words, none of the shapes was dramatically better than the others in converting NIR light to heat. Instead, particle size was dominant: larger particles (regardless of shape) showed higher efficiencies for both absorption and scattering. Thus, the study concluded that to maximize photothermal heating per particle, one should use larger gold nanoparticles (within practical size limits). This result matches later findings: for example, Pattani & Tunnell (2013) also reported that, at the same optical density, gold nanorods had higher per-particle photothermal efficiency than nanoshells, whereas larger nanoshells produced more heat per particle due to their greater cross section.

Overall, Cole et al.’s 2009 JPCC paper shows that both nanoshells and nanorods can serve as effective photothermal agents, and that design emphasis should be on maximizing absorption/scattering cross-section (e.g. via size or material) rather than on choosing one particular shape. These insights help guide the design of nanoparticle-based therapies and imaging agents. For example, later work showed that smaller (~100 nm) core–shell “nanomatryoshkas” with very high absorption cross-section doubled tumor clearance and survival in mice compared to larger (150 nm) nanoshells, consistent with Cole’s finding that higher absorption yields better photothermal performance.

Fluorescence enhancement by nanoshells and nanorods (Bardhan et al. 2009)

In another 2009 study, Rizia Bardhan, Nathaniel Grady, Joseph Cole et al. investigated how gold nanoshells and nanorods modify nearby fluorescence. Metallic nanostructures can dramatically enhance or quench the fluorescence of adjacent dye molecules by altering their radiative decay rates. Bardhan et al. placed a near-IR fluorescent dye (IRDye 800CW, “IR800”) at a fixed distance from each nanoparticle type using a protein spacer (human serum albumin), and they directly measured the dye’s quantum yield and lifetime.

They found a huge enhancement: IR800’s quantum yield jumped from ≈7% (free in solution) to 86% when bound to a nanoshell complex, and to 74% with a nanorod complex. In other words, the dye became ~10–12 times brighter when near these particles. The authors attribute this to a combination of increased local excitation field and an increased radiative decay rate (due to the plasmonic resonance of the metal shell). This “fluorescence enhancement” implies much stronger signals in fluorescence imaging. Bardhan et al. noted that “this dramatic increase in fluorescence shows tremendous potential for contrast enhancement in fluorescence-based bioimaging”.

Importantly, the study also compared nanoshells vs. nanorods: the nanoshells produced a somewhat higher yield (86% vs. 74%), suggesting shell geometry provided slightly stronger enhancement under their conditions. Both cases represent orders-of-magnitude improvement in brightness over the isolated dye. This work demonstrates how Cole and colleagues helped quantify metal-enhanced fluorescence in bio-relevant assemblies, guiding the design of brighter imaging probes.

Solar spectrum harvesting (Cole & Halas 2006)

Earlier (2006), Joseph Cole and Naomi Halas studied plasmonic nanoparticles for solar energy harvesting. They asked: how should one design a “mixed” layer of metal nanoparticles to absorb or scatter sunlight across the solar spectrum? Using theoretical modeling (Mie theory), they optimized the size distributions of spherical metal nanostructures (both solid spheres and core–shell nanoshells) so that their combined plasmon resonances would match the AM1.5 solar spectrum. In other words, they computed the ideal mix of particle sizes and types that would provide broad, high cross-section coverage of sunlight.

The result was a prescription for “ideal” distributions in a sub-monolayer of particles: one optimized for absorption and another for scattering. Both distributions were tuned to maximize interaction with the full solar spectrum and were then compared to conventional broad-spectrum absorbing or scattering media. This work showed the potential of plasmonic nanoparticle coatings or nanofluids to improve solar collectors: by mixing, say, 70–100 nm spheres with ~100–130 nm nanoshells (for absorption) or somewhat larger sizes (for scattering), one could outperform a uniform absorber. In essence, Cole & Halas (2006) pioneered the idea of engineering multicolored plasmonic ensembles to efficiently harvest sunlight – a concept now explored in plasmonic photovoltaic and photothermal solar technologies.

Context and significance

Together, these publications illustrate Cole’s focus on how nanoparticle geometry and composition govern optical responses for practical applications. He worked in Naomi Halas’s plasmonics group at Rice University, where gold nanoshells were first developed. (A gold-silica nanoshell consists of a silica core covered by a thin Au layer; by tuning the core size and shell thickness, its plasmon resonance can be placed in the NIR (700–1300 nm).) These structures – including Halas’s commercial “AuroShell” – are already used in clinical trials for cancer photothermal therapy. Cole’s 2009 JPCC and ACS Nano papers help explain why such particles work and how to make them work better: they quantify heat conversion and fluorescence boost as functions of shape and size. Likewise, his 2006 APL paper laid groundwork for using nanoparticles in solar energy, matching the sun’s spectrum at the nanoscale.

In summary, Joseph Cole co-authored multiple influential studies on plasmonic nanoshells and nanorods. His work showed that, for photothermal therapy, all common particle types have comparable light-to-heat efficiency (within a few‐fold) and that larger particles are generally better absorbers. He also demonstrated enormous fluorescence enhancements from nanostructures, and devised optimal nanoparticle mixtures for solar harvesting. These contributions are widely cited in nanophotonics and nanomedicine, reflecting a deep understanding of how nanoscale design controls optical and thermal behavior.

Sources: The above is based on Cole’s publications and related literature (JPCC 2009, ACS Nano 2009, Appl. Phys. Lett. 2006, etc.), as well as reviews and follow-on studies that discuss gold nanoshells and photothermal therapy.