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Pehr E. Pehrsson Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5000

Wei Zhaoa) Department of Chemistry, University of Arkansas, Little Rock, Arkansas 72204 (Received 29 March 2006; accepted 25 May 2006)

We recently observed that surfactant sodium dodecyl sulfate (SDS)-encased HiPco single-walled carbon nanotubes (SWNTs) respond optically to hydrogen peroxide (H2O2) in the near-infrared region. In this report, we demonstrate that SDS-encased SWNTs immobilized with glucose oxidase (GOx) can be used to optically detect an enzymatic reaction of glucose based on their H2O2 sensitivity as well as pH sensitivity. Only the enzymatic product H2O2 induces the SWNT near-infrared spectral changes in buffer solutions (pH ⳱ 6.0), but both H2O2 and gluconic acid products do this in unbuffered solutions. The SWNT optical response to glucose possesses sensitivity and selectivity similar to an electrochemical method using carbon nanotube nanoelectrode arrays. Our results suggest possible carbon nanotube-based optical tools for molecular recognition applications.

I. INTRODUCTION

There is intense interest in the solution chemistry of single-walled carbon nanotubes (SWNTs), especially since the advent of soluble SWNTs.1–3 An understanding of the solution chemistry may lead to separation of SWNTs with different chiralities,4–9 development of nanosensors,10–14 and other potential applications such as drug delivery and gene delivery vehicles.15 Surface modified water-soluble single-walled carbon nanotubes (SWNTs) are optically sensitive to pH changes in the near-infrared (IR) region.16–20 Very recently, we found that nanotubes encased in the surfactant sodium dodecyl sulfate (SDS) respond optically to hydrogen peroxide with a detection limit of 11 ␮M.21 As a byproduct of biological reactions, hydrogen peroxide is a major reactive oxygen species in living organisms and has been implicated in cancer, aging, and several chronic neurodegenerative diseases.22 The observation of SWNT optical sensitivity to H2O2 may enable new nanotube-based optical tools for studying its physiological and pathological roles. Furthermore, because numerous enzymecatalyzed reactions produce hydrogen peroxide,23 the SWNT sensitivity to H2O2 suggests an optical sensing method for important biological molecules such as glua)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0343 J. Mater. Res., Vol. 21, No. 11, Nov 2006

http://journals.cambridge.org

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cose for diabetes diagnostics.24–31 The glucose enzymatic reaction catalyzed by the enzyme glucose oxidase (GOx) produces hydrogen peroxide and gluconic acid (Scheme 1),23,24 so GOx-modified SWNTs should respond optically to glucose. Glucose Oxidase H2O2 ␤ − D − Glucose + O2 + H2O → + D − Gluconic Acid SCHEME 1 SWNTs may offer several advantages as optical probes for sensor applications: (i) their near-IR features are in an important spectral window (800–1500 nm) that could be covered by the semiconducting laser wavelength