Substrate-Wrapped Single-Walled Carbon Nanotube-Based Photoluminescent Probes for Amylase Activity Measured in Circulating Flow Cells

Single-walled carbon nanotubes (SWCNT) can be tailored as photoluminescent probes toward enzyme activity through wrapping SWCNT with the target substrate of enzymatic reactions; the response of this kind of SWCNT sensor is usually measured in a static environment, such as microplates. Here, we develop SWCNT sensors toward amylases through wrapping SWCNT with sodium starch glycolate (SSG), a modified starch for improved solubility, and we measure the photoluminescence of the SWCNT sensor in a circulating flow cell system, which provides an agitated environment and is more suitable for kinetic analysis and industrial applications. With a modified initial rate method, a linear correlation between α-amylase concentration and SWCNT photoluminescence response can be obtained, which fits with the enzymatic reaction model. The sensors also show that α-amylase can enzymatically degrade the SSG wrapping of SWCNT and lead to quenching in photoluminescence, while glucoamylase can only affect the sensor and its photoluminescence with adsorption, which is in accordance with the results of liquid chromatography that less product is generated for glucoamylase reactions. By comparing the sensor responses at varied sensor concentrations, we conclude that the main source of change in photoluminescence of the sensors may not be the degradation of SSG wrapping of SWCNT. Rather, the likely main contributor is the collision and aggregation of unwrapped SWCNT afterward, which are reactions accelerated with agitation in the flow cells and thus cannot be properly observed in the static microplate. This research shows the importance of an agitated environment for kinetic measurements of substrate-wrapped SWCNT sensors and other kinds of sensors.