Carbon dots (CDs) are a versatile nanomaterial with attractive photoluminescent and photocatalytic properties. Here we show that these two functionalities can be easily tuned through a simple synthetic means, using a microwave irradiation, with citric acid and varying concentrations of nitrogen-containing branched polyethyleneimine (BPEI) as precursors. The amount of BPEI determines the degree of nitrogen incorporation and the different inclusion modes within the CDs. At intermediate levels of BPEI, domains grow containing mainly graphitic nitrogen, producing a high photoluminescence yield. For very high (and very low) BPEI content, the nitrogen atoms are located primarily at the edge sites of the aromatic domains. Accordingly, they attract photogenerated electrons, enabling efficient charge separation and enhanced photocatalytic hydrogen generation from water. The ensuing ability to switch between emissive and photocatalytic behavior of CDs is expected to bring substantial improvements on their efficiency for on-demand light emission or energy conversion applications.

DNA origami is a powerful approach for assembling plasmonic nanoparticle dimers and Raman dyes with high yields and excellent positioning control. Here we show how optothermal-induced shrinking of a DNA origami template can be employed to control the gap sizes between two 40 nm gold nanoparticles in a range from 1 to 2 nm. The high field confinement achieved with this optothermal approach was demonstrated by detection of surface-enhanced Raman spectroscopy (SERS) signals from single molecules that are precisely placed within the DNA origami template that spans the nanoparticle gap. By comparing the SERS intensity with respect to the field enhancement in the plasmonic hot-spot region, we found good agreement between measurement and theory. Our straightforward approach for the fabrication of addressable plasmonic nanosensors by DNA origami demonstrates a path toward future sensing applications with single-molecule resolution.