A glass coverslip (#1.5; thickness, 0.17 mm; HS159879H, Heathrow Scientific) was functionalized with APTES (Sigma-Aldrich) by soaking in 10% (v/v) APTES in anhydrous ethanol for 5 min, following a previously reported protocol (26). The coverslip was then rinsed with DI water and left to dry. The coverslip was then fixed onto an ibidi μ-Slide VI0.5 forming six microfluidic channels. Next, 100 μl of PBS was pipetted through a microfluidic channel. The channel was then filled with 50 μl of a nIRHT solution (10 mg/liter) and left to incubate at room temperature for 5 min. The channel was rinsed using three successive washes of 50-μl PBS. The surface-immobilized nIRHT in PBS was then imaged on an inverted light microscope with 721-nm excitation and a Ninox VIS-SWIR 640 camera (Raptor Photonics). Fluorescence signal was collected through a 900-nm longpass filter. One end of the flow channel was connected to a syringe pump (Harvard Apparatus) with Luer lock fittings. Before the start of image acquisition, the opposite flow reservoir was filled with PBS, and the pump was set to refill mode at a volumetric flow rate of 100 μl/min. Once the liquid in the reservoir was depleted, 50 μl of 100 μM 5-HT in PBS was added. The process was repeated using alternating additions of 200 μl of PBS and 50 μl of 5-HT solution.

To test the optical response of bare SWCNT immobilized onto a glass substrate, an SDS-coated SWCNT suspension was prepared: 1 mg of HiPCo SWCNT was added to 1 ml of 2 wt % SDS in PBS buffer. The resulting mixture was bath-sonicated for 2 min and tip-sonicated for 30 min at 5-W power in an ice bath. After sonication, the black suspension was centrifuged for 60 min at 16,100g to precipitate undissolved SWCNT, and the supernatant solution was collected. Fifty microliters of SDS-SWCNT (5 mg/liter) was incubated in a microfluidic slide as described above, and the SDS coating was gradually removed by introducing a continuous flow of PBS buffer into the channel for 1 hour at a flow rate of 50 μl/min. Following this surfactant stripping step, 50 μl of 10 μM 5-HT in PBS was added to the microfluidic channel.

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