Michael P. Rowe

An Integrated Micro-Analytical System for Complex Vapor Mixtures

A micro gas chromatograph (muGC) capable of quantitatively analyzing the components of complex vapor mixtures at trace concentrations is described. The muGC features a micro- preconcentrator/focuser (muPCF), dual-column pressure- and temperature-programmed separation module, and an integrated array of nanoparticle-coated chemiresistors. The latest design modifications and performance data are presented. Highlights include a 4-min separation of a 30-component mixture with a 3-m DRIE Si/glass microcolumn, a 14-sec separation of an 11-component mixture on a 25-cm microcolumn, a complete multi-vapor analysis from a hybrid microsystem that combines analytical, rf- wireless, and microcontroller modules, and a rapid analysis driven by a 4-stage peristaltic micropump.

1/f noise in gold nanoparticle chemosensors

ing of electrons between neighboring nanoparticles. 7 The reversible partitioning of the chemical species affects the tunnel coupling, and thus the resistance of the Au nanoparticle film typically changes in the presence of chemical species. Because the tunnel coupling is exponentially dependent on the properties of the medium in between the nanoparticles, they are highly sensitive sensors. Even a small amount of swelling associated with the absorption of chemical species into the thiolate coating can lead to a significant increase in film resistance. Such sensors are currently being developed as the detector elements of a microfabricated gas chromatography system which can analyze complex mixtures of gases for a wide range of applications, including monitoring of industrial emissions, detection of explosives, and medical diagnostics. 8,9

Baseline resistance cancellation circuit for high resolution thiolate-monolayer-protected gold nanoparticle vapor sensor arrays

Chemiresistive (CR) sensors and sensor arrays coated with thiolate-monolayer-protected gold nanoparticle (MPN) interfaces show great promise for high-sensitivity multi-vapor analysis but suffer from process variation and drift in baseline values. This paper describes a new readout circuit that cancels baseline resistance and compensates for baseline drift to achieve ppm resolution. Requiring only 5100 mum in a 0.5 mum CMOS process, the circuit is well suited for high density on-chip CR sensor arrays. The resulting CR array microsystem introduces a valuable tool for monitoring environmental hazards including explosive compounds.

Chemiresistor array with nanocluster interfaces as a micro-GC detector

An integrated array of chemiresistor (CR) microsensors employing a novel set of Au-thiolate monolayer-protected nanoclusters (MPN) as interface materials has been developed and tested as an ultra-low-dead-volume detector for a micro gas chromatograph (/spl mu/GC). Six MPNs 3-5 nm in mean core diameter each with a different type of organo-thiolate ligand were synthesized by a single-phase method, spray cast onto interdigital electrodes, and exposed to several organic vapors. Four of these MPNs were coated onto the same CR array and used as a detector for a conventional GC to analyze a mixture of 15 vapors. The array provides characteristic vapor response patterns, linear calibration curves, and good overall sensitivity, which increases at lower temperatures and detector-cell flow rates. On the basis of injected mass, GC responses indicate that detection limits in the low part-per-trillion range are achievable from preconcentrated sample volumes of <1 L. Sensitivity does not vary significantly among similarly coated CRs having electrode spacings ranging from 0.1-15 /spl mu/m consistent with a response model based on bulk-film swelling.

Electrical noise in gold nanoparticle chemiresistors

We studied electrical noise of gold nanoparticle chemical sensors with the intent of improving their limit of detection. All the sensors exhibit 1/f-type noise at low frequencies. The magnitude of the noise is found to be strongly dependent on the thickness of the films. The sensors containing a single monolayer of nanoparticles had the highest resistance and the worst noise performance. By making the films thicker, we were able to lower the 1/f noise by five orders of magnitude. The nanoparticle deposition of the thicker films was done with a micro-dispensing system, which resulted in highly non-uniform, coffee-stain patterned films. To get films of different thicknesses, we varied the number of drops deposited on each sensor. The noise prefactor extracted from different devices scaled linearly with the resistance of sensors. The effects of electron beam cross-linking were also studied and found to lower the noise of the sensors.

The role of interfacial metal silicates on the magnetism in FeCo/SiO2 and Fe49%Co49%V2%/SiO2 core/shell nanoparticles

We have investigated the role of spontaneously formed interfacial metal silicates on the magnetism of FeCo/SiO2 and Fe49%Co49%V2%/SiO2 core/shell nanoparticles. Element specific x-ray absorption and photoelectron spectroscopy experiments have identified the characteristic spectral features of metallic iron and cobalt from within the nanoparticle core. In addition, metal silicates of iron, cobalt, and vanadium were found to have formed spontaneously at the interface between the nanoparticle core and silica shell. X-ray magnetic circular dichroism experiments indicated that the elemental magnetism was a result of metallic iron and cobalt with small components from the iron, cobalt, and vanadium silicates. Magnetometry experiments have shown that there was no exchange bias loop shift in the FeCo nanoparticles; however, exchange bias from antiferromagnetic vanadium oxide was measured in the V-doped nanoparticles. These results showed clearly that the interfacial metal silicates played a significant role in th...

Structure and composition of iron nanoparticles synthesized using a novel anionic-element complex

We use a novel solution-based disassociation synthesis scheme of the ionic complex Fe(LiBH4)2 to form Fe nanoparticles. The complex was formed initially using a gentle mechanochemical process, and the Fe nanoparticles emerged after 4 h of ball milling in an air-free environment. Rietveld refinement of x-ray diffraction measurements in an air-free sample holder identified a Im 3¯m α-Fe phase. A room temperature Mossbauer spectrum of the sample presented a six-line spectrum unique to Fe0 metal, and the Fe nanoparticles were extremely well crystallized. Magnetometry results presented a reduced saturation magnetization (e.g., Ms∼ 85 emu/g at 50 K) that had a Bloch-like T2 temperature dependence, consistent with a gap in the magnon fluctuation spectrum due to finite-size effects. The Fe nanoparticles were magnetically soft, with a coercivity ranging from ∼10 to 20 mT with decreasing temperature from 350 K.

Nanoparticle-coated chemiresistors with CMOS baseline tracking and cancellation

Chemiresistors (CR) with thiolate-monolayer-protected gold nanoparticle (MPN) interfacial layers can provide sensitive detection of volatile organic compounds (VOCs). However, baseline resistances are high (~M-ohm) relative to the typical responses (~ohms), and baseline drifts can be large. These factors represent impediments to the implementation of CRs and CR arrays in microanalytical systems for multi-VOC monitoring. This paper presents a new baseline cancellation and tracking system for MPN-coated CRs. This system separates the sensor response and baseline in the analog domain. Concentrations down to hundreds of ppm were measured. The circuitry of the system demonstrated a sensor input range of over 45.8 dB. Most importantly, it accurately tracked and removed baseline drifts on the order of 2 mV/min and measured multiple sensors whose baselines differed by 27 MOmega.