Ashish Kar

Controlling the organization of plasmonic nanoparticles with optical forces is essential for designing reconfigurable light‐responsive materials. However, the role of particle shape in determining optical binding geometries remains unresolved. Here, it is demonstrated that the interplay between gold nanoparticle (Au NP) morphology and optical scattering governs distinct near‐field and far‐field configurations under optical trapping at a water‐glass interface. Au spheres, rods, plates, and decahedra exhibit characteristic orientations and binding behaviors that directly correlate with their shape‐dependent scattering responses to linearly polarized near‐infrared lasers. By tuning the trapping wavelength, transitions in interparticle spacing, orientation, and collective arrangement are induced across two‐, three‐, and five‐particle systems. These results establish NP shape as a versatile design parameter for programming optical matter, offering new opportunities for dynamic nanoscale assembly, tunable plasmonic interactions, and light‐driven metamaterials.

Single-molecule measurements are crucial for studyingthe interactionsbetween G-quadruplex (GQ) DNA and ligands, as they provide higherresolution and sensitivity compared to those of bulk measurements.In this study, we employed plasmon-enhanced fluorescence to investigatethe real-time interaction between the cationic porphyrin ligand TmPyP4and different topologies of telomeric GQ DNA at the single-moleculelevel. By analyzing the time traces of the fluorescence bursts, weextracted dwell times for the ligand. For parallel telomeric GQ DNA,the dwell time distribution followed a biexponential fit, yieldingmean dwell times of 5.6 and 18.6 ms. For the antiparallel topologyof human telomeric GQ DNA, plasmon-enhanced fluorescence of TmPyP4was observed, with dwell time distributions following a single-exponentialfit and a mean dwell time of 5.9 ms. Our approach allows the nuancesof GQ-ligand interactions to be captured and holds promisefor studying weakly emitting GQ ligands at the single-molecule level.

End-to-end dimers of gold nanorods are predicted to be excellent substrates for surface-enhanced spectroscopy. However, synthesis of solution-stable end-to-end dimers remains challenging. Here we exploit the pH-dependent configurational change of polyelectrolytes to initiate and terminate the gold nanorod assembly formation to produce end-to-end linked dimers at high yield. The gold nanorods are first overcoated with a polyelectrolyte and the end-to-end attachment is initiated by adding a thiol linker in acidic medium. The assembly formation is then terminated at the dimer stage by changing the pH of the medium by the addition of appropriate amount of 1,4-diazabicyclo[2.2.2]octane (DABCO).The nanorod dimers synthesized here are stable in solution for a week without any additional surface encapsulation.

End-to-end assemblies of anisotropic plasmonic nanostructures with small nanogaps are of great interest as they create strong hot spots for enhancing weak fluorescence and/or scattering of molecules. Here we report the growth of dithiol-linked end-to-end assemblies of gold nanorods from dimers to large networks containing thousands of individual nanorods, directed by in situ tuning of nanorod's surface charge. Surface charge was lowered to initiate the aggregation process but was subsequently increased to achieve slow tip-specific growth over seven days to form end-to-end networks of nanorods, which were stable in solution for over one month. Furthermore, we showed that these assemblies contained strong plasmonic hot spots which enhanced the fluorescence signal of a weak emitter by 10⁴-fold. This enhancement is approximately 10-fold larger than that obtained using a single gold nanorod and is comparable to the largest enhancement obtained using more expensive lithographically made in-plane antenna arrays.

We show that many complex gold nanostructures such as the water chestnut, dog bone, nanobar, and octahedron, which are not easily accessible via a direct seed-growth synthesis approach, can be prepared via overgrowth of the same gold nanorods by varying pH and Ag concentrations in the growth solution. Overgrown nanostructures’ shapes were determined by the rate of gold atom deposition, which is faster at higher pH. In the presence of AgNO₃, codeposition of gold and silver atoms affects the shapes of overgrown nanostructures, particularly at high pH.