Biosensors and Bioelectronics 21 (2005) 212–216 Short communication Encapsulation of glucose oxidase and an oxygen-quenched fluorophore in polyelectrolyte-coated calcium alginate microspheres as optical glucose sensor systems J. Quincy Brown, Rohit Srivastava, Michael J. McShane ∗ Biomedical Engineering Program and the Institute for Micromanufacturing, Louisiana Tech University, 911 Hergot Ave., P.O. Box 10137, Ruston, LA 71272, USA Received 6 June 2004; received in revised form 6 August 2004; accepted 11 August 2004 Available online 17 September 2004 Abstract Microspheres coated with polyelectrolyte multilayers (PEM’s) are being investigated for potential use as implantable biosensors—so-called “smart tattoos.” In this work, the feasibility of this approach for glucose sensors was demonstrated by glucose oxidase encapsulated within calcium alginate microspheres, followed by entrapment of an oxygen-quenched ruthenium compound in the same microstructure. A novel feature of these microdevices is the formation of multilayer nanofilms on the surface of the microspheres, used to stabilize enzyme entrapment and control substrate diffusion. Confocal microscopy was used to confirm the stable encapsulation of sensor chemistry. The reversible response of sensors to step changes in glucose was observed, and preliminary experimental data were compared to theoretical predictions produced by a computational model. These findings demonstrate the promise of the described nanoengineering approach for production of functional implantable glucose sensor materials. © 2004 Elsevier B.V. All rights reserved. Keywords: Polyelectrolyte multilayers; Microspheres; Optical glucose sensors 1. Introduction Methods for glucose monitoring in the management of di- abetes mellitus that would replace the gold-standard “finger- prick” method have been intensely investigated over the past three decades. These efforts stem from the need for a non- invasive way for diabetics to measure their blood glucose in order to encourage more frequent testing events and, thus, more effective disease management. These alternative sens- ing methods include both spot and continuous monitoring, and can be grouped into two broad categories: (1) truly non- invasive optical sensors, and (2) minimally invasive sen- sors, employing either electrochemical or optical detection (Heinemann and Koschinsky, 2001). ∗ Corresponding author. Tel.: +1 318 257 5100; fax: +1 318 257 5104. E-mail address: mcshane@latech.edu (M.J. McShane). Minimally invasive implantable optical glucose sensors based on fluorescence spectroscopy are one method being investigated, including an elegant approach aimed at intrader- mal implantation of fluorescent microsphere glucose sensor systems, with transdermal interrogation of the sensors using light—the so-called “smart tattoo” (McShane et al., 2000a,b). Properly calibrated, these sensors could provide accurate spot blood glucose measurements, replacing conventional methods, as well as providing the potential for continuous monitoring, with all of the attendant advantages. One partic- ular example that has been studied in this area is the use of a competitive-binding fluorescent assay for glucose encap- sulated within poly(ethylene glycol) microspheres (Russell et al., 1999). Although this previous work demonstrated the feasibility of the approach, particularly the ability to monitor fluorescence of sensors implanted in the dermis (McShane et al., 2000a,b), the devices still are not useful for clini- cal monitoring, due to poor reversibility and stability of the 0956-5663/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2004.08.020