A plasmon-assisted optofluidic (PAOF) system for measuring the photothermal conversion efficiencies of gold nanostructures and controlling an electrical switch

Abstract

An optofluidic system was constructed from a diode laser as the energy source, an aqueous suspension of plasmonic nanostructures as the photothermal transducer, and a glass capillary for measuring the volumetric expansion of the suspension. The suspension could be driven to move up the capillary by more than 30 mm and be used to control the operation of an electrical switch.

Figure 1

a) A schematic of the plasmon-assisted optofluidic system. The arrow indicates the end of the plastic tube where the medium was pumped into the microfluidic device. b) A schematic drawing of the microfluidic device. 1: PDMS block used to seal the reservoir. 2: PDMS block cast with a reservoir to host the medium. 3: Plastic tube serving as a channel for pumping the medium. 4: Glass capillary used for measuring the volumetric expansion. c) Photograph of an assembled optofluidic system.

Figure 2

TEM images of the three types of Au plasmonic nanostructures used for converting light into heat: a) nanocages with an outer edge length of 45 nm and a wall thickness of ca. 5 nm, b) nanorods with an average diameter of 17 nm and an aspect ratio of ca. 3.3, and c) hexapods with an average distance of ca. 60 nm between the ends of two opposite vertices. d) UV-vis-NIR extinction spectra of the three different types of Au nanostructures.

Figure 3

a) Plot of the rise in height (Δh) and increase in temperature (ΔT) upon irradiation as a function of time when suspensions of Au nanocages of four different concentrations were used: 1.0×10⁹, 2.5×10⁹, 5.0×10⁹, and 1.0×10¹⁰ particles/mL (from bottom to top). Each cycle of irradiation lasted for 40 min (on for 10 min and then off for 30 min) and the cycle was repeated three times. b) Plot of the energy conversion efficiency (η) as a function of particle concentration. For all the irradiation processes, the laser density was set to 0.4 W/cm² and the illuminated area was 1.13 cm².

Figure 4

a) Schematic illustration of a photo-responsive electrical switch. A 5-mL glass vial (1) was capped with a PDMS block (2) inserted with a 1.0-mm capillary tube (3). A standard copper wire (4) was used to connect the switch to three 1.5 V AA batteries (5) connected in series to a standard LED light bulb (6). A suspension of Au nanocages (1.0×10¹⁰ particles/mL) saturated with NaCl was loaded into the vial until it reached the top (*) of the lower copper wire but did not reach the bottom (**) of the upper wire. When the vial was irradiated with a diode laser (7), the solution expanded, filling tube 3 and bridging the 5-mm gap between the two copper wires, turning on the LED bulb. b) Demonstration of the on-off switching of a LED over four cycles.