Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
Analytical Chemistry Chemical Cytometry Quantitates Superoxide
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GOx<br />
glucose + O298 gluconic acid +<br />
0.7 V<br />
H2O298 H 2 O 2 (GOx: glucose oxidase) (1)<br />
2H + + O 2 + 2e -<br />
The reaction may be carried out by mixing the reagents in solution<br />
to produce H2O2 which is then subsequently detected by<br />
integrated electrodes, 7,8 or by immobilizing GOx directly onto<br />
the detection electrode surface to react with glucose in solution. 9<br />
The electrochemical detectors in these microchips displayed fast<br />
response times and low detection limits. Immobilizing GOx<br />
certainly reduces its consumption significantly compared to<br />
devices in which GOx is added in solution. However, the major<br />
disadvantage of this approach is that the enzyme layer is prone<br />
to biofouling in in vivo applications, which might cause a decrease<br />
in activity over time and poor performance as a result. 10,11<br />
Over the past decade, a number of glucose sensor designs<br />
have been developed and become available for continuous<br />
monitoring purposes. 12-16 Mastrototaro et al. 14 reported an in<br />
vivo needle-type continuous glucose monitoring system (CGMS),<br />
which is currently being used by consumers as a kit (MiniMed).<br />
The detector in this system is based on the reaction of immobilized<br />
glucose oxidase on the electrode with glucose. Schoemaker et<br />
al. 15 and Wientjes et al. 17 have both reported a subcutaneous<br />
continuous glucose monitoring system (SCGM) based on a<br />
microdialysis probe and using the solution-based enzymatic<br />
reaction with glucose. They reported reliable measurements in<br />
diabetes patients over periods of four days 18 up to two weeks. 19,20<br />
However, due to their bulky size and the need for connecting<br />
tubes between system components, these systems exhibited<br />
relatively long physical lag times of 30 min or more. Pickup et<br />
al. 13 reviewed the clinical use of glucose monitoring systems and<br />
introduced new technology for noninvasive glucose sensing, based<br />
on near-infrared spectroscopy and fluorescence. Wentholt et al. 21<br />
(10) Gerritsen, M.; Jansen, J. A.; Lutterman, J. A. Neth. J. Med. 1999, 54, 167–<br />
179.<br />
(11) Wisniewski, N.; Moussy, F.; Reichert, W. M. Fresenius’ J. Anal. Chem. 2000,<br />
366, 611–621.<br />
(12) Maran, A.; Crepaldi, C.; Tiengo, A.; Grassi, G.; Vitali, E.; Pagano, G.; Bistoni,<br />
S.; Calabrese, G.; Santeusanio, F.; Leonetti, F.; Ribaudo, M.; Di Mario, U.;<br />
Annuzzi, G.; Genovese, S.; Riccardi, G.; Previti, M.; Cucinotta, D.; Giorgino,<br />
F.; Bellomo, A.; Giorgino, R.; Poscia, A.; Varalli, M. Diabetes Care 2002,<br />
25, 347–352.<br />
(13) Pickup, J. C.; Hussain, F.; Evans, N. D.; Sachedina, N. Biosens. Bioelectron.<br />
2005, 20, 1897–1902.<br />
(14) Mastrototaro, J. J. Diabetes Technol. Ther. 2000, 2, 13–18.<br />
(15) Schoemaker, M.; Andreis, E.; Roper, J.; Kotulla, R.; Lodwig, V.; Obermaier,<br />
K.; Stephan, P.; Reuschling, W.; Rutschmann, M.; Schwaninger, R.;<br />
Wittmann, U.; Rinne, H.; Kontschieder, H.; Strohmeier, W. Diabetes Technol.<br />
Ther. 2003, 5, 599–608.<br />
(16) Suzuki, H.; Tokuda, T.; Miyagishi, T.; Yoshida, H.; Honda, N. Sens. Actuators,<br />
B 2004, 97, 90–97.<br />
(17) Wientjes, K. J. C.; Grob, U.; Hattemer, A.; Hoogenberg, K.; Jungheim, K.;<br />
Kapitza, C.; Schoonen, A. J. M. Diabetes Technol. Ther. 2003, 5, 615–620.<br />
(18) Kapitza, C.; Lodwig, V.; Obermaier, K.; Wientjes, K. J. C.; Hoogenberg, K.;<br />
Jungheim, K.; Heinemann, L. Diabetes Technol. Ther. 2003, 5, 609–614.<br />
(19) Wientjes, K. J.; Vonk, P.; Vonk-van Klei, Y.; Schoonen, A. J.; Kossen, N. W.<br />
Diabetes Care 1998, 21, 1481–1488.<br />
(20) Schoonen, A. J. M.; Schmidt, F. J.; Hasper, H.; Verbrugge, D. A.; Tiessen,<br />
R. G.; Lerk, C. F. Biosens. Bioelectron. 1990, 5, 37–46.<br />
(21) Wentholt, I. M. E.; Hoekstra, J. B. L.; DeVries, J. H. Diabetes Technol. Ther.<br />
2007, 9, 399–409.<br />
(2)<br />
reported an overview of the current applications and clinically<br />
relevant aspects of continuous glucose monitors (CGMs), with<br />
emphasis on the calibration procedure, interpretation of continuous<br />
glucose data, and some important limitations. Overall, these<br />
authors concluded that improved accuracy, reliability for longer<br />
periods of time, miniaturization, and cost-effectiveness are the<br />
main issues which need to be considered in the further development<br />
of continuous glucose monitoring systems. 21<br />
Chip-based microfluidic technologies are a good alternative to<br />
improve on conventional monitoring approaches such as the<br />
SCGM described above. Microfluidic systems, also termed<br />
“miniaturized total analysis systems (µTAS)” 22 or “lab-on-a-chip”,<br />
are now widely used in analytical chemistry and biological<br />
applications. Advantages of these systems include dramatically<br />
reduced consumption of chemical reagents, faster reaction times<br />
and cost-effectiveness. Micro SCGM systems can thus be envisaged<br />
which exploit the solution-based reaction of GOx with<br />
glucose in nL volumes in micrometer-sized channels for glucose<br />
sensing. The speed of this analysis is determined by the efficiency<br />
of mixing reagents in the microfluidic channels. Mixing at the<br />
micrometer scale is a challenge, as flows are generally extremely<br />
well-defined and laminar. Mixing of two solution streams in a<br />
straight microchannel is possible only through means of diffusion,<br />
a passive molecular transport process which is very slow. There<br />
has therefore been an enormous amount of research done in the<br />
past decade or so on how to implement efficient mixing at the nL<br />
scale. So-called passive micromixers are generally preferred for<br />
many micro analytical flow systems, since these elements do not<br />
require the application of an external force to achieve mixing. A<br />
large number of passive micromixers have been reported, including<br />
a planar laminar flow mixer, 23 a cross-shaped micromixer 24<br />
and a droplet mixer. 25 The approach chosen for our work is one<br />
based on chaotic mixing, first described by Stroock et al. 26,27<br />
Mixing is achieved through the incorporation of an array of<br />
microgrooves into a microchannel. Flow over the groove array<br />
assumes a helical or corkscrew pattern, in which the contact area<br />
between two adjacent solutions is increased dramatically to<br />
facilitate mixing by diffusion.<br />
The long-term goal of the present project is to realize an<br />
autonomous, portable sensing system for continuous in vivo<br />
glucose monitoring, based on the reaction in solution of GOx with<br />
glucose to produce H 2O2. To accomplish this, we have designed<br />
a miniaturized glucose sensing system based on microdialysis<br />
sampling and lab-on-a-chip technology. In this system, nL<br />
amounts of sample and enzyme rapidly mix and react. As highrecovery<br />
microdialysis requires flow rates in tissue less than 1<br />
µL/min, 17 we have adapted the microreactor dimensions<br />
according to these conditions. In this paper, we describe a new<br />
application for chaotic mixing, that is, the efficient and fast<br />
mixing of GOx and glucose for reaction. To that end, either<br />
(22) Reyes, D. R.; Iossifidis, D.; Auroux, P.-A.; Manz, A. Anal. Chem. 2002, 74,<br />
2623–2636.<br />
(23) Melin, J.; Gimenez, G.; Roxhed, N.; van der Wijngaart, W.; Stemme, G.<br />
Lab Chip 2004, 4, 214–219.<br />
(24) Wong, S. H.; Bryant, P.; Ward, M.; Wharton, C. Sens. Actuators, B 2003,<br />
95, 414–424.<br />
(25) Paik, P.; Pamula, V. K.; Fair, R. B. Lab Chip 2003, 3, 253–259.<br />
(26) Stroock, A. D.; Dertinger, S. K. W.; Ajdari, A.; Mezic, I.; Stone, H. A.;<br />
Whitesides, G. M. Science 2002, 295, 647–651.<br />
(27) Stroock, A. D.; Dertinger, S. K.; Whitesides, G. M.; Ajdari, A. Anal. Chem.<br />
2002, 74, 5306–5312.<br />
<strong>Analytical</strong> <strong>Chemistry</strong>, Vol. 82, No. 16, August 15, 2010<br />
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