**1. Introduction**

Biosensors based on localized surface plasmon resonance (LSPR), the collective oscillation of electrons in nanostructured noble metals induced by resonant light, have been extensively investigated owing to their molecular-level sensitivity, rapid and label-free detection, and simple instrumentation [1–4]. Since LSPR sensors monitor the resonance wavelength shifts caused by analytes adsorbed on the metal surface, fabrication of uniform plasmonic nanostructures with sharp and clean resonance spectra is a prerequisite for achieving sensitive and reliable LSPR sensors [1–7]. Once well-defined plasmonic nanostructures are obtained, the delivery of analytes to plasmonic hot spots is essential to fully utilize the sensor performance. However, despite the availability of a number of reports on the fabrication of various metallic nanostructures and their application in plasmonic sensors, only a few studies have focused on the locating of analytes on sensing spots [8–10].

In this report, we present an efficient method for locating analytes in plasmonic hot spots. By a template-assisted self-assembly during the dip-coating process [11–14], target analytes spontaneously align in the valleys between neighboring Au strips, where electromagnetic fields are locally enhanced. During repeated dip-coating, analytes accumulate at the hot spots, and this phenomenon is monitored for changes in LSPR wavelength. Furthermore, we demonstrate that the proposed method can be used to monitor the stability of exosomes, extracellular vesicles (<100 nm in size), attracting much scientific

and engineering interest as biomarkers for diagnosing diseases and as delivery vehicles for bioresorbable drugs [15–18].
