Glucose Oxidase Micropumps: Multi-Faceted Effects of Chemical Activity on Tracer Particles Near the Solid–Liquid Interface

Raluca-Elena Munteanu,Szilveszter Gáspár,Mihail N Popescu

doi:10.3390/condmat4030073

Raluca-Elena Munteanu, Szilveszter Gáspár + Show 1 more

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https://doi.org/10.3390/condmat4030073

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Journal: Condensed Matter	Publication Date: Jul 25, 2019
Citations: 5	License type: CC BY 4.0

Affiliation: International Centre of Biodynamics

Abstract

We report the development of glucose oxidase pumps characterized by small lateral dimensions (≈200 μ m). We studied the effects of the activity of the enzyme pump on silica particles (“tracers”) sedimented around the enzyme pump/patch. Once the activity of the pump was turned on (i.e., the glucose substrate was added to the solution), in-plane motion of the tracers away from the enzyme patch, as well as the emergence of an in-plane region around the patch which was depleted by tracers, was observed. The lateral extent of this depletion zone increased in time at a rate dependent both on the glucose concentration and on the areal density of the enzyme in the patch. We argue that, when the tracers were very near the wall, their motion and the emergence of the depletion zone were most likely the result of diffusiophoresis and drag by osmotic flows induced at the wall, rather than that of drag by a solutal buoyancy driven convective flow. We infer that, for the glucose oxidase enzymatic pumps, bulk (solutal buoyancy), as previously reported, as well as surface (osmotic) driven flows coexist and have to be explicitly accounted for. It seems plausible to assume that this is the case in general for enzyme pumps, and these complementary effects should be considered in the design of applications, e.g., stirring or sensing inside microfluidic systems, based on such pumps.

Highlights

Immobilized enzymes have already found numerous applications [1], only recently it was reported that patches of surface anchored enzymes can stir/pump the surrounding solution, when this contains species that the anchored enzyme is catalytically converting [2]
With a radius R 110 μm, these patches were some of the smallest enzyme micropumps investigated to date
This is correlated with the emergence of a region depleted of tracers around the enzyme patch; the linear size of this region increases slowly, but systematically, as a function of time. If this is to be attributed to a buoyancy-driven convective flow, as usually done in the literature on enzyme pumps [2,4,5,8,10,11], the convective flow must be “outward” at the bottom wall of the cell, on which the enzyme patch is located. This direction would be in agreement with that of Zhang et al [5], who argued that the gluconolactone/gluconic acid products of the catalytic reaction promoted by glucose oxidase (GOX) increase the density of the solution compared to that of the bulk glucose solution [18,19], but it would contradict the reports in [2,8] of “inwards” flows, for seemingly the same enzyme–substrate type of pump as the one studied here

Summary

Introduction

Immobilized enzymes have already found numerous applications (including industrial ones) [1], only recently it was reported that patches of surface anchored enzymes can stir/pump the surrounding solution, when this contains species that the anchored enzyme is catalytically converting [2]. The magnitude of these flows increases with increasing substrate concentrations (see, e.g., [2,3,4]), given that this is below the saturation regime of the Michaelis–Menten kinetics of the reaction catalyzed by the enzyme. This dependence, in combination with the geometry of the experimental cell containing the enzyme patch, provides means for tuning, to a certain extent, the pattern of flow within the cell (see, e.g., [5]). A recent review of the developments in this area was provided by Zhao et al [7]

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