In a medium saucepan, bring 1½ cups water, sugar, and zest to a boil over medium-high heat, stirring constantly. Reduce heat to low; simmer, stirring constantly, until sugar is dissolved, 3 to 4 minutes. Remove from heat, and let cool completely.

Strain mixture through a fine-mesh sieve into an airtight container. Refrigerate in an airtight container for up to 3 weeks.