ELSEVIER Journal of Electroanalytical Chemistry 434 (1997) 185-189 Tailoring the macroporosity and performance of sol-gel derived carbon composite glucose sensors Joseph Wang *, Deog Su Park, Prasad V.A. Pamidi Department af Chemistry and Biochemist~:v,Ne.' Mexico ~tote Unl~,ersity, Los Cruces, NM 8,~fi~L~. USA Received 18 Deceml'a'r 1996; revised 25 March 1997 Allslraet Ctmiml of tl~e pl~paralioll eol|dtlions is used to enhance lhe performance of sol-.gel derived carbon coml~)sile enzyme el¢ctr~les, In particular, changes in the water content during the acidocatalyzed Iiydrolysis strongly afl~cl |he macrol~)n~sity of the re~uliing hiogel network, Hence. while tbr the low°water process the response is controlled by the enzyme kinetic~, high°waler preparation~ lead to mass~traasl~tt restrictions. Such diffusional limitations wilhin the electrode interior resull in an extended linear dy,)amic range (wilhout the need for external membranes). Scanning electron microscopy provides images of biogel microstructure:~,accrued from the use of different water contents. The ceramic-carbon biosensors offer improved retention of artificial electn)n accepters compared to coaventioRal carbon composite bioelectrodes. The silica-containing surface also displays an intrinsic electrocatalytic hydrogen peroxid ~ponse,and hence offers a low-potential monitoring of the glucose substrate. © 1997 Elsevier Science S.A. l~¢'v~atd.~': S~l-~gel; Biosensurs; Carhtm ~:ompt~site;Glucose; Thick lihn electrodes; Electn~catalysis I. Introduction Considerable efforts have been devoted to the develop- ment of effective schemes for immobilizing enzymes onto physical mmsdueers in connection with the design of biocatalytic sensors [I,2]. A number of immobilization techniques have been devised, often dependent upon the specific application or requirements, and upon the trims- ducer material or dimension. Recent studies have demon- strated that the low.temperature sol-gel processing of inorganic ceramics [3,4] holds great promise for the immo- bilization of biomoleeules onto physical transducers [5,6]. Such sol-gel immobilization technology is simple, verna- tile and offers good retention of the biochemical and recognition properties. While most activity in the area of sol-gel derived biosensors has focused on the develop- ment of optical devices, amperometric sensors can also benefit from the attractive features of the sol-gel imn,obi- lization strategy. In particular, Lev's group [6-8] illus- trated the utility of composite ceramic-carbon biosensors, based on the dispersion of oxidoreductase enzymes and graphite powder in a silica xerogel matrix. The rigidity of such bioelectrodes made surface renewal by polishing " Corresponding author. Fax: + 1-505-6466033. 0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. Pil S0022.0728(97)00256-8 possible. We have combined tile sol-gel and screen,oprint° ing technologies for addressing enzyme deacliv,~tion probe lems during the thermal curittg of thick-filnl biosensors [9]. In this paper we demonstrate that the tunable porosity of sol-gel carbon composite cleetr~es can be used for imparting diffusional constraints, and hence tbr extending the linear dynamic range of these amperometric biosen- sors. Such control of the overall rate of the bioeatalytic reaction through control of the sol-gel preparation condi= tions obviates the needs for an external (mass-transport limiting) membrane. This greatly simplifies the fabrication of thick.film biosensor,; or the operation of renewable (polishable) biocomposite electrodes. We also report on the low-potential detection of ~he glucose substrate accrued from the intrinsic electroeatalytic action of the silica to= wards hydrogen peroxide, and on the improved retention of ferroeene derivative mediators. The study also provides many useful insights into the experimental factor, ~, affecl.- ing the behavior of sobogel carbon bi~x:omposites, 2. Experimental 2, l. Apparatus Voltammetrie and amperometric mea.~urements were carried out with a Bioanaly6cal System (BAS) vol~ammel°