Kevin Michael McPeak

In vivo plant imaging is crucial for understanding plant biology and the influence of external factors on plant health. While exogenous contrast agents are widely used in animal bioimaging to enhance contrast, track flows, and provide molecular specificity, their application in plant tissue remains challenging and underexplored. Herein, we highlight the shortcomings of contrast agents in optical coherence tomography of plant tissue while demonstrating the successful use of Au bipyramids (AuBPs) as effective NIR-II contrast agents in photoacoustic imaging, enabling spatiotemporal flow tracking in live Buttercrunch lettuce over 5 days. Furthermore, rigorous plant health studies showed no adverse effects of the AuBPs on the physiological properties of Buttercrunch lettuce 7 days postinfiltration. The use of AuBPs as contrast agents in plant imaging, combined with the versatility of their surface chemistry, opens the possibilities for studies with molecular specificity.

The self-assembly of nanoparticle colloids into large-area monolayers with long-range order is a grand challenge in nanotechnology. Using acoustic energy, i.e., acoustic annealing, to improve the crystal quality of self-assembled colloidal monolayers is a new solution to this challenge, but the characterization of the capillary waves driving the annealing process is lacking. We use a laser Doppler vibrometer and optical diffraction to uncover the frequency-dependent effects of capillary waves on the real-time self-assembly of submicrometer diameter polystyrene nanospheres at an air-water interface. Our study unambiguously demonstrates that low-frequency, e.g., sub-100 Hz, capillary waves are key to improving the long-range order of colloidal monolayers on an air-water interface. Furthermore, we demonstrate how a simple immersion transducer can generate capillary waves and how transducer placement and design affect vibrational spectra. Lastly, we show that frequency-shift keying of a high-frequency focused transducer provides a straightforward method of exciting low-frequency capillary waves that are effective at forming colloidal monolayers with excellent crystal quality, exhibited by grains over 3.5 cm².

Nanoparticles with high absorption cross sections will advance therapeutic and bioimaging nanomedicine technologies. While Au nanoshells have shown great promise in nanomedicine, state-of-the-art synthesis methods result in scattering-dominant particles, mitigating their efficacy in absorption-based techniques that leverage the photothermal effect, such as photoacoustic (PA) imaging. We introduce a highly reproducible synthesis route to monodisperse sub-100 nm Au nanoshells with an absorption-dominant optical response. Au nanoshells with 48 nm SiO₂ cores and 7 nm Au shells show a 14-fold increase in their volumetric absorption coefficient compared to commercial Au nanoshells with dimensions commonly used in nanomedicine. PA imaging with Au nanoshell contrast agents showed a 50% improvement in imaging depth for sub-100 nm Au nanoshells compared with the smallest commercially available nanoshells in a turbid phantom. Furthermore, the high PA signal at low fluences, enabled by sub-100 nm nanoshells, will aid the deployment of low-cost, low-fluence light-emitting diodes for PA imaging.

Photocatalytic advanced oxidation processes (AOPs) promise a chemical-free route to energy-efficient degradation of waterborne micropollutants if long-standing mass transfer and light management issues can be overcome. Herein, we developed a dual-porous photocatalytic system consisting of a mesoporous (i.e., 2-50 nm pores) TiO₂ (P25) photocatalyst supported on macroporous (i.e., >50 nm pores) fused quartz fibers (P25/QF). Our reusable photocatalytic AOP reduces chemical consumption and exhibits excellent energy efficiency, demonstrated by degrading various pharmaceutical compounds (acetaminophen, sulfamethoxazole, and carbamazepine) in natural waters with electrical energy per order (E_EO) values of 4.07, 0.96, and 1.35 kWh/m³, respectively. Compared to the conventional H₂O₂/UVC AOP, our photocatalytic AOP can treat water without chemical additives while reducing energy consumption by over 2800%. We examine these improvements based on mass transport and optical (UVA and UVC) transmittance and demonstrate that the enhancements scale with increasing flow rate.

Noble-transition metal alloys offer emergent optical and electronic properties for near-infrared (NIR) optoelectronic devices. We investigate the optical and electronic properties of CuxPd1−x alloy thin films and their ultrafast electron dynamics under NIR excitation. Ultraviolet photoelectron spectroscopy measurements supported by density functional theory calculations show strong d-band hybridization between the Cu 3d and Pd 4d bands. These hybridization effects result in emergent optical properties, most apparent in the dilute Pd case. Time-resolved terahertz spectroscopy with NIR (e.g., 1550 nm) excitation displays composition-tunable electron dynamics. We posit that the negative peak in the normalized increment of transmissivity (ΔT/T) below 2 ps from dilute Pd alloys is due to non-thermalized hot-carrier generation. On the other hand, Pd-rich alloys exhibit an increase in ΔT/T due to thermalization effects upon ultrafast NIR photoexcitation. CuxPd1−x alloys in the dilute Pd regime may be a promising material for future ultrafast NIR optoelectronic devices.

Resonant plasmonic-molecular chiral interactions are a promising route to enhanced biosensing. However, biomolecular optical activity primarily exists in the far-ultraviolet regime, posing significant challenges for spectral overlap with current nano-optical platforms. We demonstrate experimentally and computationally the enhanced chiral sensing of a resonant plasmonic-biomolecular system operating in the far-UV. We develop a full-wave model of biomolecular films on Al gammadion arrays using experimentally derived chirality parameters. Our calculations show that detectable enhancements in the chiroptical signals from small amounts of biomolecules are possible only when tight spectral overlap exists between the plasmonic and biomolecular chiral responses. We support this conclusion experimentally by using Al gammadion arrays to enantiomerically discriminate ultrathin (<10 nm thick) films of tyrosine. Notably, the chiroptical signals of the bare films were within instrumental noise. Our results demonstrate the importance of using far-UV active metasurfaces for enhancing natural optical activity.

Colloidal lithography (CL) is a promising method for large-area fabrication of nanohole and nanodot arrays with applications in optical biosensing, separations, and magnetic data storage. However, reducing the diameter of the polystyrene sphere mask by plasma etching unavoidably increases their coefficient of variation (CV) and deforms their shape, thereby limiting the pitch-to-hole-diameter ratio of the resulting nanohole array to less than 3:1 and the minimum hole size to 200 nm with a 10% or better CV. We show that tilt-rotate evaporation colloidal lithography (TRE-CL) breaks the trade-off between hole diameter and polydispersity by leveraging glancing angle evaporation, not plasma etching, to adjust the hole size. TRE-CL allows pitch-to-hole-diameter ratios as high as 7:1 and nanohole diameters down to 60 nm while maintaining a nearly constant CV below 10% and hole circularity above 91%. We transfer these hole arrays into ultrathin Si3N4 films to form nearly-monodisperse microsieves for separation applications. Furthermore, we extend TRE-CL to fabricate adhesion-layer-free plasmonic Au nanodot arrays down to 70 nm in diameter with 10% CV.