Rapid Equilibrated Colorimetric Detection of Protamine and Heparin: Recognition at the Nanoscale Liquid–Liquid Interface

Demand for rapid quantitation of polyions such as heparin and protamine are ever growing. Previous paper-based and polymeric optical and electrochemical sensing devices required more than several hours for signal stabilization. Therefore, signals were acquired with fixed sample exposure time modes, which was time-consuming and technically demanding. We present here for the first time the optical detection of protamine and heparin in equilibrium mode with emulsified nanospheres. The method significantly shortens the response time from hours to typically less than 10 s and offers tunable, sensitive, and colorimetric detection within the clinically relevant range (10 to 100 mg/L) for heparin. The improved characteristics are attributable to the small size of the nanospheres (ca. 50 nm in diameter) as well as the reversible recognition at the nanoscale liquid–liquid interface. Detection of the anticoagulant heparin was also successfully demonstrated in human blood serum background.