title: Layer-by-layer assembled semipermeable membrane for amperometric glucose sensors.
abstract: BACKGROUND
  The performance of implantable glucose sensors is closely related to the behavior of the outer membrane. Such membranes govern the diffusion characteristics of glucose and, correspondingly, the sensitivity of the sensors. This manuscript discusses the selection of various membrane materials and their effect on the device response.
  
  
  METHODS
  Sensors were fabricated utilizing a 50-microm platinum wire followed by immobilization of the glucose oxidase (GO(X)) enzyme. Sequential adsorption of various ionic species via a layer-by-layer process created devices coated with bilayers of humic acids/ferric cations (HAs/Fe(3+)), humic acids/poly(diallyldimethylammonium chloride) (HAs/PDDA), and poly(styrene sulfonate)/poly(diallyldimethylammonium chloride) (PSS/PDDA). The in vitro amperometric response of the sensors was determined at 0.7 V vs an Ag/AgCl reference electrode in phosphate-buffered saline (37 degrees C) for various glucose concentrations. The diffusion coefficients of glucose and hydrogen peroxide (H(2)O(2)) through these membranes were calculated and analyzed.
  
  
  RESULTS
  Outer membranes based on the sequential deposition of bilayers of HAs/Fe(3+), HAs/PDDA, and PSS/PDDA were grown successfully on immobilized layers of GO(X). The amperometric response and reversibility upon changing the in vitro concentration of glucose were investigated.
  
  
  CONCLUSIONS
  Through alteration of the number of bilayers of the outer membrane, it was possible to modulate the diffusion of glucose toward the sensor as a result of its flux-limiting characteristics. Semipermeable membranes based on five HAs/Fe(3+) bilayers exhibited a superior behavior with a minimum hysterisis response to glucose cycling and a lesser current saturation at hyperglycemic glucose concentrations because of a more balanced inward diffusion of glucose and outward diffusion of H(2)O(2).
authors.affiliation: Nanomaterials Optoelectronics Laboratory, Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, USA. papadim@mail.ims.uconn.edu
authors.forename: Ritesh
authors.initials: R
authors.lastname: Tipnis
authors.forename: Santhisagar
authors.initials: S
authors.lastname: Vaddiraju
authors.forename: Faquir
authors.initials: F
authors.lastname: Jain
authors.forename: Diane J
authors.initials: DJ
authors.lastname: Burgess
authors.forename: Fotios
authors.initials: F
authors.lastname: Papadimitrakopoulos