**4. Conclusions**

For effective placement of target analytes on sensor surfaces and their monitoring, we present a straightforward measurement method based on nanoplasmonic sensors of double-bent Au strip arrays, multiple dip-coating of analytes, self-alignment of analytes in the region of strong plasmonic fields, and spectrometric monitoring of LSPR peaks. Using this method, closely packed PS beads in the valleys of the DAS array and pH-dependent stability of the exosomes were successfully monitored in terms of shifts in the LSPR peaks. As a small amount of target analytes can accumulate in the plasmonic hot spot due to multiple dip-coating cycles, and the LSPR peak can be measured with a conventional UV-vis spectrometer under physiological conditions, it is expected that the proposed measurement platform will be useful for studying the stability of various drug delivery vesicles and their efficiencies.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/5/660/s1, Figure S1: (a) Measured absorbance curves of the DAS array (black) before and (red) after dip-coating thrice with exosomes and (blue) after immersion in a TRIzol reagen<sup>t</sup> for 1 h. (b) Magnified view of Figure S1a. Figure S2: (a) Measured absorbance curves of the DAS array (black) before and (red) after drying a 10 μL drop of an aqueous solution of exosome. (b) Table of LSPR peak shifts of the DAS arrays, measured after dipping in pH buffer solutions with pH 7.4, 5.0, and 4.0 for 18 hours.

**Author Contributions:** Conceptualization, J.-S.W.; methodology, J.-S.W.; validation, J.L. and J.-S.W.; formal analysis, J.L. and J.-S.W.; investigation, J.L. and J.-S.W.; resources, E.-A.Y. and D.W.H.; data curation, J.L. and J.-S.W.; writing—original draft preparation, J.-S.W.; writing—review and editing, S.K. and J.-S.W.; visualization, J.L.; supervision, S.K.; project administration, S.K. and J.-S.W.; funding acquisition, J.-S.W.

**Funding:** This study was supported by the Korea Research Institute of Standards and Science (Development of Platform Technology for Innovative Medical Measurements [KRISS-2019-GP2019-0013]) and by the National Research Foundation funded by the Ministry of Science and ICT of Korea (Bio and Medical Technology Development Program [2015M3A9D7029894], Global Frontier Project [HGUARD\_2013M3A6B2078962], and Nano Material Technology Development Program [2014M3A7B6020163, 2017M3A7B4041754]).

**Conflicts of Interest:** The authors declare no conflict of interest.
