Figure 4.

Local readout of buffer concentration inside a nanochannel. (a) Simplified schematic depiction of the nanofluidic device used to manipulate the concentration, N, of TBE buffer inside the nanofluidic channel. The two microchannels were filled with 5× TBE (red) and water (blue), and independent pressures were applied to one inlet of each microchannel. The resulting pressure drop over the nanochannel causes a varying TBE concentration inside it, depending on the applied pressures. (b) The locally measured TBE concentration obtained by reading out the corresponding spectral shift, Δλ, of the LSPR of the individual gold nanoantenna sensor placed in the middle of the nanochannel using polychromatic illumination. The achieved detection limit is σ_N ≈ 0.3× TBE and can be improved to σ_N ≈ 0.08× TBE using the monochromatic readout introduced in Figure 3 (see Figure S10 in the SI). The used plasmonic nanoantenna was precalibrated in terms of Δλ(N) in order to derive the local concentration (see Figure S9 for details). The marked data point corresponds to the situation shown in (a), with pressures of 0.9 and 0.2 bar applied to the microchannels filled with water and 5× TBE, respectively.