The development of electrochemiluminescent (ECL) luminophores with tuneable emission profiles is a key requirement in the development of multi-analyte ECL sensing platforms. This study presents the first reported use of cadmium selenide sulfide (CdSeS) quantum dots (QDs) as ECL emitters in which both the ECL emission profile and cathodic potential can be tailored by alteration of the Se/S ratio. CdSeS QDs were synthesised using an aqueous synthetic route thereby avoiding the use of organic reagents, high temperatures and inert gasses. The suitability of the CdSeS QDs to ECL sensing applications is demonstrated via the quantitative determination of TIMP-1 protein at clinically relevant concentrations. The developed cathodic ECL immunosensor exhibited a detectable linear range of 6¿60 ng/mL and a limit of detection (LOD) of 1.54 ng/mL. TIMP-1 protein plays a crucial role in pregnancy, wound healing, and cancer prognosis.