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Research Article

Optical visualization and quantification of enzyme activity using dynamic droplet lenses

Lauren D. Zarzar, Julia A. Kalow, Xinping He, Joseph J. Walish, and Timothy M. Swager
PNAS April 11, 2017 114 (15) 3821-3825; first published March 27, 2017; https://doi.org/10.1073/pnas.1618807114
Lauren D. Zarzar
aDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139;
bInstitute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, MA 02139
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Julia A. Kalow
aDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139;
bInstitute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, MA 02139
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Xinping He
aDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139;
bInstitute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, MA 02139
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Joseph J. Walish
aDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139;
bInstitute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, MA 02139
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Timothy M. Swager
aDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139;
bInstitute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, MA 02139
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  • For correspondence: tswager@mit.edu
  1. Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved March 7, 2017 (received for review November 14, 2016)

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    Fig. 1.

    Reconfigurable droplets act as tunable lenses and the optical transmission of an emulsion film depends on the droplet morphology. (A) Schematic ray diagrams of the complex droplets composed of hydrocarbon (e.g., hexane, heptane) and fluorocarbon (e.g., perfluorohexane, FC770) within a continuous phase of aqueous solution containing the enzyme and substrate/surfactant. Approximate refractive indices are given. Depending on the surfactant conditions and the interfacial tensions at the fluorocarbon–water (γF) and hydrocarbon–water (γH) interfaces, these droplets dynamically change their morphology and can adopt three general configurations: H/F/W, Janus, or F/H/W. We note that depending on the specific refractive indices of liquids used, the most transmissive Janus drop may not be the precise morphology as pictured (where γF = γH) but may be a Janus drop with slight curvature at the fluorocarbon–hydrocarbon interface. Because fluorinated liquids have a greater density, the droplets orient with gravity when sitting on a surface, which is important to achieving the desired optical response. (B) Aligned beneath the droplet schematics are corresponding photographs of polydisperse emulsions in a Petri dish placed over an image of a happy face to demonstrate changes in the optical transmission. (Scale, 1 cm.) Pictured below are optical micrographs of representative droplets. (Scale, 100 µm.)

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    Fig. 2.

    Measurement of α-amylase activity using variations in transmission of responsive droplet lenses. (A) When Triton X-100 surfactant is complexed with γ-CD it is no longer an effective surfactant, and droplets in the presence of this Triton X-100/ γ-CD complex along with Zonyl fluorosurfactant exhibit H/F/W morphology. When the γ-CD is hydrolyzed by α-amylase, the Triton X-100 becomes freely available, thereby reducing γH relative to yF and pushing the droplets closer toward the F/H/W state. (B) Schematic of the experimental setup; a stabilized light source was used for illumination and a USB spectrometer was used to measure the intensity of transmitted 650-nm light. (C) Exemplary transmission versus time data collected for a sample with 20 FAU/L α-amylase activity. Droplets began in the opaque H/F/W state, transitioned through the transmissive Janus state, and ended in an F/H/W configuration. We define t* to equal the time difference between when maximum and half-maximum transmission is reached. (D) Exemplary transmission versus time plots for varying concentrations of amylase all using the same reaction scheme shown in B. Data were collected at 37 °C. (E) The time parameter t* was determined for 22 independent samples across a range of relevant amylase activities (Left). The rate of the reaction, characterized by 1/t*, is plotted versus amylase activity and yields a linear correlation with R2 = 0.989 (Right).

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    Fig. 3.

    Measurement of lipase and sulfatase activity via direct enzymatic degradation of surfactants. (A) Droplets in the presence of an ester-linked surfactant (1) and Zonyl initially exhibit a Janus-type morphology. When the ester-linked surfactant is hydrolyzed by lipase, γH increases relative to yF and the droplets transition toward the H/F/W state. (B) Exemplary transmission versus time plot for emulsions sensitized by the lipase-responsive surfactants shown in A at 70 LU/L. Although the droplets follow the reverse trajectory as demonstrated in Fig. 2, we can still apply the same definition of t*. (C) Transmission versus time plots for samples containing varying concentrations of lipase and using the reaction scheme shown in A. Data were collected at 21 °C. (D) The time parameter t* was determined for 22 independent samples across a range of relevant lipase activities (Left). The rate of the reaction, characterized by 1/t*, is plotted versus lipase activity and yields a linear correlation with R2 = 0.99 (Right). (E and F) Measurement of sulfatase activity. (E) Droplets in the presence of sulfate surfactant (2) and Zonyl initially exhibit a Janus-type morphology. When the sulfate surfactant is hydrolyzed by sulfatase, γH increases relative to yF and the droplets transition toward the H/F/W state. (F) The time parameter t* was determined for 18 independent samples across a range of sulfatase activities (Left). The rate of the reaction, characterized by 1/t*, is plotted versus sulfatase activity and yields a linear correlation with R2 = 0.977 (Right). Data were collected at 37 °C.

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    Fig. 4.

    Portable electronic devices such as smartphones and tablets have light sensors that can be used along with ambient lighting conditions to collect enzyme activity data similar to that achieved using a spectrometer and a computer. The data shown at right were collected for 200 U/L lipase at 21 °C using a Samsung tablet.

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Enzyme activity using dynamic droplet lenses
Lauren D. Zarzar, Julia A. Kalow, Xinping He, Joseph J. Walish, Timothy M. Swager
Proceedings of the National Academy of Sciences Apr 2017, 114 (15) 3821-3825; DOI: 10.1073/pnas.1618807114

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Enzyme activity using dynamic droplet lenses
Lauren D. Zarzar, Julia A. Kalow, Xinping He, Joseph J. Walish, Timothy M. Swager
Proceedings of the National Academy of Sciences Apr 2017, 114 (15) 3821-3825; DOI: 10.1073/pnas.1618807114
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