As the cost of 802.11 hardware continues to fall, the appeal of inserting unauthorized wireless access into enterprise networks grows. These rogue access points (APs) expose the enterprise network to a barrage of security vulnerabilities in that they are typically connected to a network port behind the firewall. Most of the current approaches to detecting rogue APs are rudimentary and are easily evaded by hackers. We propose the use of temporal traffic characteristics to detect rogue APs at a central location. This detection is independent of the wireless technology (802.11a, 802.11b, or 802.11g), is scalable, does not posses the inefficiencies of the current solutions, and is independent of the signal range of the rogue APs. Globecom 2004
We report on a CMOS-based microsensor array, featuring 576 electrodes and 24 channels, for label-free electrochemical detection of DNA hybridization. Each channel comprises a potentiostatic circuit that keeps the electrode at a defined potential and measures the respective electrical current. The readout channels rely on a firstorder delta-sigma architecture, where the electrode-electrolyte capacitance is used as the integrator. This results in a very compact recording circuit inclusive of A/D conversion; the circuit consists of only the measurement electrode, a comparator, a feedback capacitance and a few switches. Biological experiments with short DNA samples and DNA extracted from human immunodeficiency virus (HIV) are presented.There is an urgent need for rapid on-the-spot diagnosis of infectious diseases like HIV. A very common diagnostic method includes the use of fluorescence-based DNA analysis on micro-arrays after preceding DNA amplification (PCR) [1]. Here we describe a label-free exclusively electronic method to detect hybridization events that obviates the need for optical setups. The measurement principle (Fig. 8.4.1) relies on cyclic voltammetry (CV). In a CV experiment, the potential on a working electrode (WE) is periodically varied with respect to a reference electrode, while the respective current is measured. This results in I-V curves, which depend on the electrochemistry at the electrode surface. The sensor relies on an electropolymer (polypyrrole), which is oxidized and reduced. The movement of the chloride counter ions is affected by the negative charges of the phosphate groups on the DNA strands, the number of which increases upon hybridization. The kinetics of the ionic travel and, consequently, the shapes of the CV curves are altered through hybridization.A common approach to measuring electro-chemical currents includes the use of a current-conveyor to decouple the large electrolyte capacitance from the rest of the circuit [2]. The current can then be digitized using an integrating ADC. A more compact approach is to integrate the charge on the measurement electrode directly [3]. Delta-sigma converters are also suitable, since the electrochemical signals are slow, and a first-order structure provides sufficient resolution [4].Here we use the electrode-electrolyte interface as the integrator in a delta-sigma structure (Fig. 8.4.3). In a first approximation, this interface behaves like an ideal capacitor. However, the nonlinearity of the capacitor must be considered [5] and so must the charge-transfer resistance, R CT , that exists in parallel with the capacitance and causes leakage. Leaky integrators can lead to dead zones in delta-sigma architectures.Each sensor of the 24×24 array consists of a platinum (Pt) electrode, functionalized with a polymer bilayer ( Fig. 8.4.1) and an immobilized DNA probe molecule. The electrode diameter ranges between 10 and 40μm, and the electrode pitch is 100μm. A schematic of the chip architecture is shown in Fig. 8.4.2. Each row is multiplexed to one readou...
The utility and performance of label-free, oligonucleotide probes for reagentless detection of dilute target analytes was examined using a voltammetric transduction principle in an array format. Multistep, solid-state fabrication yielded preproduction arrays of 16 individually addressable, 30 μm diameter microelectrodes in a 30 mm × 6.5 mm × 0.5 mm dipstick disposable device. The specificity of 16 nucleotide (nt) 2'-O-methylribonucleic acid and 22 nt DNA backbone probes bound through Mg(2+)-phosphonate bridges to polypyrrole films on the microelectrodes were studied using microbial target RNAs of various lengths. Probe-specific interactions with Escherichia coli O157 H7 23S rRNA (2907 nt) and Candida albicans 18S rRNA (1788 nt) were detected at 65 and 58 fmol/mL, respectively, in volumes as low as 0.5 mL. Specificity studies showed that, for a given probe, "nontarget" transcripts can contribute to changes in the voltammetric detection signal, though with responses that never exceed 70% of the detection signal acquired for specifically designed complementary targets. These results statistically validate the use of the voltammetric microelectrode array for obtaining a "yes-no" answer on complementary specific binding. The study also identifies challenges and pitfalls for the selection strategies of oligonucleotide probes.
A new technique for measurement of magnetic properties of materials is demonstrated. It can be used for the measurement of thin magnetic films during their chemical modification. The resonance frequency of a quartz crystal microbalance (QCM) with conducting polymer (polyaniline) suspension in poly(ethylene glycol) was observed to increase with increasing the externally applied uniform dc magnetic field. Slowly sweeping the magnetic field between 0 and 3.1 T results in a frequency-field response curve. Chemical doping was done by exposing the polyaniline-emeraldine base film to HCl vapor. The change in population of free spins is reflected in increased frequency-field curve magnitude after HCl doping. Two working hypotheses explaining this observation are offered to explain how frequency of QCM with deposited magnetic film shifts with increasing intensity of the magnetic field.
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