DOC PREVIEW
A Molecular bio-wire based multi-array biosensor with integrated potentiosta

This preview shows page 1 out of 4 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 4 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 4 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

29 1-4244-1525-X/07/$25.00 © 2007 IEEE 1A Molecular bio-wire based multi-array biosensorwith integrated potentiostatYang Liu∗,AmitGore∗, Shantanu Chakrabartty∗,Evangelyn Alocilja †[email protected], [email protected], [email protected], [email protected]∗Department of Electrical and Computer Engineering†Department of Biosystems Engineering Michigan State UniversityEast Lansing, MI 48824 USAAbstract— One of the important factors determining the sen-sitivity of any biosensing system is successful integration of bio-molecular transducers with peripheral signal processing circuitry.In this paper we present an architecture of a multi-array biosen-sor that integrates molecular bio-wires based immunosensor witha multi-channel potentiostat array. The biosensor operates byconverting binding events between antigen and antibody intoa measurable electrical signal using polyaniline nanowires asa transducer. The electrical signal is measured using a multi-channel potentiostat where each channel comprises of a semi-synchronous Σ∆ modulator. Measured results using a fabricatedpotentiostat array demonstrate sensitivity down to 50 femtoam-pere range which makes it ideal for detecting pathogens atlow concentration levels. Experiments using the biosensor arrayspecific to Bacillus Cereus bacterium validate the functionality ofthe platform in detecting the pathogen at different concentrationlevels.Index Terms— Potentiostat, biosensors, multi-channel con-verter, analog-to-digital converter, femtoampere current measure-ments.I. INTRODUCTIONBiosensors have emerged as important analytical tools forcontrolling disease outbreaks, wh ich according to the UnitedStates Department of Agriculture (USDA) cause 2.9−6.7billion worth o f losses every year [ 1]. The general functionof a biosensor is to convert binding events between biolog-ical receptors and target agents into a quantifiable electricalsignal [2], [3]. Current research in biosensor technology hasbeen towards developing better transducers that demonstratesuperior sensitivity, porta bility, accuracy and throughput. Inthis regard, immunosensors (biosensors that use antibodies asreceptors) are of great interest because of their applicability(any compound can be analyzed as long as specific antibod-ies are available), specificity (selectivity of antigen-antibodyreaction) an d high sensitivity. We had previously reported anantibody-based immunosensor [4] that used polyaniline basednano-wires to transduce binding events between pathogensand their target antibodies into change in conductance. Thesensor was successfully demonstrated for detecting E. Coli [4]and its sensitiv ity was reported to be 80 colony formingunits per milliliter (CFU/ml), with a r esponse time of lessthan 10 minutes. In [5] we have reported an electrical modelof the polyaniline immunosensor demonstrating a log-linearvariation of conductance with pathogen concentration. In thispaper we use this electrical model to motivate design ofFig. 1. Architecture of the model multi-array biosensor.ultra-sensitive potentiostats for detecting pathogens at lowconcentration levels. We reuse the design of our previouslyreported potentiostat architecture that used a semi-synchronousΣ∆ modulator [6]. The architecture combined asynchronoustime-encoding machine (TEM) [7] with a continuous time Σ∆(CTΣ∆) conversion [8]to facilitate measurement of ultra-smallcurrents.This paper is organized as follows: Section II we firstdescribe the operating principle of the immunosensor andpresent some sensitivity results based on its electrical model.Section III briefly describes the principle of operation of thesemi-synchronous Σ∆ modulator array. Section IV presentsmeasured response using a fabricated prototype of the poten-tiostat array and a multi-array biosensor. Section V providesconclusions with future directions.II. BIOSENSOR ARCHITECTUREThe architecture of the multi-array biosensor is shown inFigure 1. It consists of several biomolecular transistors whichare fabricated using strips of antibody layer patterned betweensilver electrodes (see Figure 1). The principle of operationof a single bio-molecular switch is illustrated in Figures 2(a)and 2(b), which shows a cross-sectional view of the antibodyAuthorized licensed use limited to: Michigan State University. Downloaded on August 4, 2009 at 13:51 from IEEE Xplore. Restrictions apply.30 2Fig. 2. Principle of operation of the model biosensorstrip. Before the sample is applied, the gap between th e elec-trodes in the capture pad is open (Figure 2(a)). Immediatelyafter sample is applied to the application pad, the solution con-taining the antigen flows to the conjugate pad, dissolves withthe polyaniline-labeled antibody (Ab-P) and forms an antigen-antibody-polyaniline complex. The complex is transportedusing capillary action into the capture pad containing theimmobilized antibodies. A second antibody-antigen reactionoccurs and forms a sandwich (Figur e 2(b)). Po lyaniline inthe sandwich then forms a molecular wire and bridges thetwo electrodes. The polymer structures extend out to bridgeadjacent cells and leads to impedance change between theelectrodes (Figure 2(b) and Figure 1). The impedance changeis determined by the number of antigen-antibody bindings,which is related to the antigen concentratio n in the sample.The unbound non-target organisms are subsequently separatedby capillary flow to the absorption membrane. The impedancechange is sensed as an electrical signal (current) across theelectrodes. Figures 2(a) and 2(b) also show SEMs of thecapture pad before and after the analyte with pathogen hasbeen applied.In [5], we presented a first-order electrical model for theconductance measured across the silver electrodes (Figure 1)as a function of the pathogen concentration in the sample. Thisis expressed as:G(X)=G0+ κ logXX0(1)where XBrepresents the concentration of the pathogen inCFU/ml, G0represents the ”control” transconductance, κrepresents sensitivity factor and X0is a detection constant.Note that equation (1) is valid only for X ≥ X0,whichisa reasonable assumption. It can be seen from equation (1),that the response of the biomolecular transistor is equivalentto a metal-oxide-semiconductor (MOS) transistor biased inweak-inversion [11]. Based on the equation (1), the changein conductance can be expressed as a function of change inconcentration δX with respect to a reference concentration XrasδG = κ


A Molecular bio-wire based multi-array biosensor with integrated potentiosta

Download A Molecular bio-wire based multi-array biosensor with integrated potentiosta
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view A Molecular bio-wire based multi-array biosensor with integrated potentiosta and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view A Molecular bio-wire based multi-array biosensor with integrated potentiosta 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?