We　herein　report　a　DNA　hairpin　structure　containing　a　polycytosine　loop　region,　and　this　hairpin　can　operate　like　a　nanomachine　allowing　independently　controlled　stability　of　the　i-motif　loop　and　duplex　stem　region.　This　was　made　possible　by　the　opposite　salt-dependent　stability　of　DNA　duplex　and　hairpin,　thus　providing　a　new　method　for　designing　molecular　devices　or　switches　design.　A　singly-labeled　fluorescent　method　was　used　to　measure　the　stability　of　an　i-motif　DNA　in　the　presence　of　various　metal　ions.　Salt　in　general　destabilizes　the　i-motif　but　stabilizes　duplex　DNA,　allowing　us　to　engineer　an　i-motif　containing　hairpin　for　modulating　the　stability　of　each　secondary　structure　independently.