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The photothermal effect in nanomaterials, resulting from resonant optical absorption, finds wide applications in biomedicine, cancer therapy, and microscopy. Despite its prevalence, the photothermal effect in light-absorbing nanoparticles has typically been assessed using bulk measurements, neglecting near-field effects. Beyond standard imaging and therapeutic uses, nanosecond-transient photothermal effects have been harnessed for bacterial inactivation, neural stimulation, drug delivery, and chemical synthesis. While scanning probe microscopy and electron microscopy offer single-particle imaging of photothermal fields, their slow speed limits observations to milliseconds or seconds, preventing nanoscale dynamic investigations. Here, we introduce decoupled optical force nanoscopy (Dofn), enabling nanometer-scale mapping of photothermal forces by exploiting unique phase responses to temporal modulation. We employ the photothermal effect’s back-action to distinguish various time frames within a modulation period. This allows us to capture the dynamic photothermal process of a single gold nanorod in the nanosecond range, providing insights into non-stationary thermal diffusion at the nanoscale.

 

Gold nanorods are well-known surface-enhanced Raman scattering substrates. Under longitudinal plasmonic excitation, the ends of the nanorods experience larger local electric fields compared to the sides of the rods, suggesting that Raman-active molecules would be best detected if the molecules could preferentially bind to the ends of the nanorods. Coating the tips of gold nanorods with anionic mesoporous silica caps enabled surface-enhanced Raman scattering (SERS) detection of the cationic dye methylene blue at lower concentrations than observed for the corresponding silica coating of the entire rod. By analyzing the intensity ratio of two Raman active modes of methylene blue and the surface plasmon resonance peak shift of the gold nanorod composites, it can be inferred that at a low concentration of methylene blue, molecules adsorb to the tips of the tip coated silica gold nanorods. Functionalization of the anionic silica endcaps with cationic groups eliminates the SERS enhancement for the cationic methylene blue, demonstrating the electrostatic nature of the adsorption process in this case. These results show that anisotropic silica coatings can concentrate analytes at the tips of gold nanorods for improvements in chemical sensing and diagnostics. 

Surface patterning of inorganic nanoparticles through site-selective functionalization with mixed-ligand shells or additional inorganic material is an intriguing approach to developing tailored nanomaterials with potentially novel and/or multifunctional properties. The unique physicochemical properties of such nanoparticles are likely to impact their behavior and functionality in biological environments, catalytic systems, and electronics applications, making it vital to understand how we can achieve and characterize such regioselective surface functionalization. This Feature Article will review methods by which chemists have selectively modified the surface of colloidal nanoparticles to obtain both two-sided Janus particles and nanoparticles with patchy or stripey mixed-ligand shells, as well as to achieve directed growth of mesoporous oxide materials and metals onto existing nanoparticle templates in a spatially and compositionally controlled manner. The advantages and drawbacks of various techniques used to characterize the regiospecificity of anisotropic surface coatings are discussed, as well as areas for improvement, and future directions for this field.