Shabnam Siddiqui

Wafer-scale fabrication of patterned carbon nanofiber nanoelectrode arrays: a route for development of multiplexed, ultrasensitive disposable biosensors.

One of the major limitations in the development of ultrasensitive electrochemical biosensors based on one-dimensional nanostructures is the difficulty involved with reliably fabricating nanoelectrode arrays (NEAs). In this work, we describe a simple, robust and scalable wafer-scale fabrication method to produce multiplexed biosensors. Each sensor chip consists of nine individually addressable arrays that uses electron beam patterned vertically aligned carbon nanofibers (VACNFs) as the sensing element. To ensure nanoelectrode behavior with higher sensitivity, VACNFs were precisely grown on 100 nm Ni dots with 1 microm spacing on each micro pad. Pretreatments by the combination of soaking in 1.0 M HNO(3) and electrochemical etching in 1.0M NaOH dramatically improved the electrode performance, indicated by the decrease of redox peak separation in cyclic voltammogram (DeltaE(p)) to approximately 100 mV and an approximately 200% increase in steady-state currents. The electrochemical detection of the hybridization of DNA targets from E. coli O157:H7 onto oligonucleotide probes were successfully demonstrated. The 9 arrays within the chip were divided into three groups with triplicate sensors for positive control, negative control and specific hybridization. The proposed method has the potential to be scaled up to NxN arrays with N up to 10, which is ideal for detecting a myriad of organisms. In addition, such sensors can be used as a generic platform for many electroanalysis applications.

Characterization of carbon nanofiber electrode arrays using electrochemical impedance spectroscopy: effect of scaling down electrode size.

We report here how the electrochemical impedance spectra change as (i) electrode size is reduced to nanometer scale and (ii) spacing between vertically aligned carbon nanofiber (VACNF) electrodes is varied. To study this, we used three types of electrodes: standard microdisks (100 microm Pt, 10 microm Au, and 7 microm glassy carbon), randomly grown (RG) VACNFs where spacing between electrodes is not fixed, and electron beam patterned VACNF nanoelectrode arrays (pNEAs) where electrode spacing is fixed at 1 microm. As the size of the microdisk electrode is reduced, the spectrum changed from a straight line to a semicircle accompanied by huge noise. Although a semicircle spectrum can directly indicate the electron transfer resistance (R(ct)) and thus is useful for biosensing applications, the noise from electrodes, particularly from those with diameters < or =10 microm, limits sensitivity. In the case of VACNFs, the electrode spacing controls the type of spectrum, that is, a straight line for RG VACNFs and a semicircle for pNEAs. In contrast to microdisks, pNEAs showed almost insignificant noise even at small perturbations (10 mV). Second, only pNEAs showed linearity as the amplitude of the sinusoidal signal was increased from 10 to 100 mV. The ability to apply large amplitudes reduces the stochastic errors, provides high stability, and improves signal-to-noise (S/N) ratio. This new class of nanoelectrochemical system using carbon pNEAs offers unique properties such as semicircle spectra that fit into simple circuits, high S/N ratio, linearity, and tailor-made spectra for specific applications by controlling electrode size, spacing, and array size.

A quantitative study of detection mechanism of a label-free impedance biosensor using ultrananocrystalline diamond microelectrode array.

It is well recognized that label-free biosensors are the only class of sensors that can rapidly detect antigens in real-time and provide remote environmental monitoring and point-of-care diagnosis that is low-cost, specific, and sensitive. Electrical impedance spectroscopy (EIS) based label-free biosensors have been used to detect a wide variety of antigens including bacteria, viruses, DNA, and proteins due to the simplicity of their detection technique. However, their commercial development has been hindered due to difficulty in interpreting the change in impedance upon antigen binding and poor signal reproducibility as a result of surface fouling and non-specific binding. In this study, we develop a circuit model to adequately describe the physical changes at bio functionalized surface and provide an understanding of the detection mechanism based on electron exchange between electrolyte and surface through pores surrounding antibody-antigen. The model was successfully applied to extract quantitative information about the bio surface at different stages of surface functionalization. Further, we demonstrate boron-doped ultrananocrystalline diamond (UNCD) microelectrode array (3 × 3 format, 200 μm diameter) improves signal reproducibility significantly and increases sensitivity by four orders of magnitude. This study marks the first demonstration of UNCD array based biosensor that can reliably detect a model Escherichia coli K12 bacterium using EIS, positioning this technology for rapid adoption in point-of-use applications.

Characterization of ultrananocrystalline diamond microsensors for in vivo dopamine detection

We show the technical feasibility of coating and micro patterning boron-doped ultrananocrystalline diamond (UNCD®) on metal microwires and of applying them as microsensors for the detection of dopamine in vivo using fast-scan cyclic voltammetry. UNCD electrode surface consistently generated electrochemical signals with high signal-to-noise ratio of >800 using potassium ferrocyanide-ferricyanide redox couple. Parylene patterned UNCD microelectrodes were effectively applied to detect dopamine reliably in vitro using flow injection analysis with a detection limit of 27 nM and in the striatum of the anesthetized rat during electrical stimulation of dopamine neurons.

Low temperature boron doped diamond

Low temperature boron doped diamond (LT-BDD) film deposited under 600 °C (460 °C minimum) has been reported. Study reveals that the deposition temperature and boron dopant cause nanocrystalline diamond (NCD) instead of ultrananocrystalline diamond (UNCD®). Unlike conventional NCD, LT-BDD has faster renucleation rate, which ensures a low surface roughness (approximately 10 nm at 0.6 μm thickness). The overall characteristics of LT-BDD are mixed with the characteristics of conventional NCD and UNCD. Raman spectrum and electrochemical characterization prove that the quality of LT-BDD is similar to those grown under 650-900 °C. LT-BDD enables diamond applications on microelectromechanical systems, bio- and optical technologies.