Immobilisation of nanosheets onto the SPR Sensor Surface. Planar polyethylene glycol/carboxyl coated Au chips, purchased from Reichert Technologies (Buffalo, USA) were installed onto a Reichert 2 SPR following the manufacturer's instructions. The sensor surface was then preconditioned by running buffer PBST (PBS pH 7.4 and 0.01 % Tween 20) at 10 µL min -1 until a stable baseline was obtained. The flow rate of 10 µL was maintained throughout the immobilisation process. In order to activate carboxyl groups on the surface of the sensor chip, a freshly prepared aqueous solution (1 mL) of EDC (40 mg) and NHS (10 mg) was injected onto the sensor chip surface for 6 minutes. To the activated surface, 300 µg of the Hf-BTB-NH2 nanosheets dissolved in 1 mL of the running buffer (PBST) and 10 mM sodium acetate, was injected only to the left channel of the surface for 1 minute. Finally, quenching solution (1 M ethanolamine, pH 8.5) was injected for 8 minutes to deactivate carboxyl groups and wash away the unbound Hf-BTB-NH2 nanosheets. A continuous flow of running buffer (PBST) at 10 µL min -1 was maintained after Hf-BTB-NH2 nanosheet immobilisation. SPR assays were carried out after a stable baseline was achieved. The left channel was the working channel, and the right channel was the reference.