Rotational motion of nanoobjects is an important feature that reveals and regulates their behaviors and functions, with implications in diverse fields ranging from biomechanics, nanomotors, enzymatic catalysis to rheology and fluid mechanics. Tracking the dynamic orientation and rotation at single nanoobject level has proven powerful as it overcomes the averaging effect in ensemble measurements. Light microscopy that relies on the orientation-dependent optical property has been a major choice toward this goal, owing to its excellent compatibility with application environments and sufficient temporal and spatial resolutions. For example, differential interference contrast microscopy, dark-field scattering microscopy and anisotropic fluorescence microscopy have been widely utilized for studying plasmonic and fluorescent nanomaterials, respectively. However, many important anisotropic nanoobjects, such as semi-conductors and biological particles (cell organelles, bacteria, and viruses), do not possess inherent fluorescent or plasmonic property and they can hardly be studied with the existing methods. An optical imaging technique that is suitable for all kinds of anisotropic nanoobjects, regardless of its inherent optical property, is thus highly desirable and it is yet to be demonstrated.
Wei Wang and his team (Chemical Imaging Lab, School of Chemistry and Chemical Engineering, State Key Lab of Analytical Chemistry for Life Science) recently developed surface plasmon resonance microscopy (SPRM), which is a wide-field imaging technique that measures the dielectric constant of single nanoparticles. Because dielectric constant is an inherent property of all kinds of materials, SPRM is capable of imaging broad types of nanomaterials including metal, semiconductor, polymer, and biological nanoparticles, as well as nanobubbles (Chem. Soc. Rev., 2018, 47, 2485). The team has developed advanced optical microscopy for imaging chemical processes of single nanoparticles. By achieving high-resolution electrochemical imaging without the need of recording current, they successfully measured the cyclic voltammograms of single electroactive nanoparticles (JACS, 2017, 139, 186) and the non-faradaic electrochemical impedance spectroscopy of single Au nanorods (Chem. Sci., 2018, 9, 4424). Based on the reduced optical signal of H2 molecules generated by photocatalyzed hydrogen evolution reactions, they interrogated the intermittent photocatalytic activity of single CdS nanocatalyst ( PNAS, 2017, 114, 10566; Chem. Sci., 2018, 9, 1448).
After investigating the PSF of SPRM systematically and quantitatively (Anal. Chem., 2018, 90, 9650), the team proposed an SPRM-based angular spectrum approach to determine the orientation of single CdS nanorods. It was found that two-dimensional Fourier transform of the asymmetrical wave-like SPRM image resulted in a peak in its angular spectrum in k space. Consistency between the peak angle and the geometrical orientation of the nanorod was validated by both in situ scanning electron microscope characterizations and theoretical calculations (Figure 1).
Figure 1. (A) In situ SEM image of a 2-μm-long CdS NR shows an intersection angle of 118° between NR and propagation direction of SPPs. Both experimental SPRM pattern (B) and the corresponding k-space image (C) exhibit asymmetric features. A bright spot in the k-space ring has an angle of circumference close to 118°. (D) The experimental (black curve) and theoretical (red curve) angular spectra of the particular CdS NR shown in A are in good agreement with each other. The theoretical angular spectrum is extracted from the theoretical SPRM pattern (E) and the corresponding k-space image (F) according to DCM calculation.
Real-time monitoring of the rotational dynamics of single CdS nanorods further revealed the accelerated rotation under appropriate reaction conditions for photocatalyzed hydrogen generation. The driving force was attributed to the asymmetric production of hydrogen molecules as a result of inhomogeneous distribution of reactive sites within the nanorod. The present work not only builds the experimental and theoretical connections between the orientation of anisotropic nanomaterials and its SPRM images; the general suitability of SPRM also sheds light on broad types of nonfluorescent and nonplasmonic anisotropic nanoobjects from semiconductors to bacteria and viruses.
The article was published online on March 14, 2019 in the Proceedings of the National Academy of Sciences of the United States of America with the title “Tracking the rotation of the single CdS nanorods photocatalysis with surface plasmon resonance microscopy” (DOI: 10.1073/pnas.182011416). The first author of the article is Yingyan Jiang and Professor Wei Wang is the corresponding author. Academician Hong-yuan Chen has given important guidance and support to this research. The National Natural Science Foundation of China, the Thousand Young Talents Program and the Shuang-Chuang Award, Jiangsu Province have provided financial support.