Supporting information for the communication Label-Free Aptasensor. Based on Ultrathin-Linker-Mediated Hot-Spot Assembly to Induce

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1 Supporting information for the communication Label-Free Aptasensor Based on Ultrathin-Linker-Mediated Hot-Spot Assembly to Induce Strong Directional Fluorescence. Shuo-Hui Cao 1, Wei-Peng Cai 1, Qian Liu 1, Kai-Xin Xie 1, Yu-Hua Weng 1, Si-Xin Huo 1, Zhong-Qun Tian 2 and Yao-Qun Li 1 * 1 Department of Chemistry and the MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, , China. 2 State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, , China. *yaoqunli@xmu.edu.cn The quantity of PAH to compose the ultrathin linker was tested. The optimized situation should be that the biosensor does not respond to the control sample, but gives intense signal with responding to target sample. Figure S1a shows that when the concentration of PAH was below 1 μg/ml, no SPCE response was observed in the control experiment. The result means false positive signal was effectively suppressed. And Figure S1b shows that PAH with the concentration of 1 μg/ml displayed higher SPCE response in the sensing of thrombin, which indicates that PAH can expose charges efficiently to capture AgNPs after sensing. Therefore, the appropriate concentration of PAH was checked as 1 μg/ml in the experiment. S1

2 Figure S1. The effects of PAH concentration on the increase in sensing signal. The sensing of control sample (a) and 10 nm thrombin (b) was tested. The sample incubation time was tested. Figure S2 shows that the highest increase in signal was obtained after 30 min with the incubation of thrombin. The shorter time is not enough for the interaction with targets. On the contrary, the incubation time more than 30 min may cause some problems: the desorbed aptamers may interact with the surface again or the targets may lose the activity. As a result, 30 min was taken as the appropriate incubation time. S2

3 Figure S2. The effects of sample incubation time on the increase in sensing signal. The sample of 10 nm thrombin was tested. The signal of ultrathin-linker-mediated hot-spot SPCE observed through the prism was highly p polarized (Figure S3), demonstrating that the emission has the properties of both the fluorophore and the surface plasmon. Figure S3 Polarized emission spectra of the ultrathin-linker-mediated hot-spot SPCE. S3

4 The SPCE signal observed through the prism shows around 10-fold enhancement compared to the isotropic free space emission (FSE) observed from the air side of the gold film (Figure S4), demonstrating that SPCE is a more efficient way to utilize the hot-spot coupling. Figure S4. Spectra of SPCE observed through the prism at the defined angle and isotropic free space emission (FSE) observed from the air side of the gold film. The sensor surface after the incubation of thrombin was tested by AFM. A small section on the surface was scratched by AFM. And then the scratched area was used to determine the thickness of the linking layer on the surface through being compared to the undamaged area. As shown in Figure S5, the thickness of the layer was no greater than 2 nm, which served as the ultrathin linker between NPs and film to warrant the strong plasmonic coupling. S4

5 Figure S5. Height (a) and phase (b) AFM images of ultrathin linker modified gold surface after mechanical removal of a square section (~ 1 μm 1 μm). The extrusive edges are the deposition of scratched materials. (c) Depth analysis of the area shown in part (a) indicated by line. The electromagnetic response of metal nanostructures was simulated by the FDTD method with a commercial software package (Recom XFDTD). The 532 nm light was illuminated perpendicularly to the metal surface following the experimental configuration, with the polarization parallel to the metal surface. The mesh unit was nm 3. The dielectric constants of gold and silver were from John and Christy (Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, ). S5

6 Figure S6. FDTD simulation of electric field distribution (E 2 ) for the system of a silver nanoparticle on gold film with a gap of 2 nm. The diameters of silver nanoparticle are 100 nm (a), 50 nm (b), and 20 nm (c). Figure S7 FDTD simulation of electric field distribution (E 2 ) for the system of a 100 nm silver nanoparticle on gold film. The gaps are 2 nm (a), 10 nm (b), and 30 nm (c). S6