Mid-Infrared standoff spectroscopy using Quantum Cascade Lasers has been a focus of on-going research for many years. When attempting to detect trace analyte residues, the greatest challenge facing this technology is not in the lasers, but the difficulty in creating a spectroscopic background reference for an unknown surface. Such techniques as Differential Location Measurements fail when analyte concentrations are below 1 μg/cm2. To overcome this challenge of unknown surface backgrounds, we propose a technique to alter the IR absorption peaks of a target analyte by exposing the surface to a high intensity, alternating electric field in a standoff fashion. The high intensity electric field generates ozone radicals from the local air, oxidizing organic compounds on the surface. A spectrum of the surface before and after the ozone radicals is obtained. The ozone altered spectrum acts as the reference background and is compared against the un-altered spectrum, generating a differential signal used to identify the target analyte.
Chemical sensors based on micro/nanoelectromechanical systems (M/NEMS) offer many advantages. However, obtaining chemical selectivity in M/NEMS sensors using chemoselective interfaces has been a longstanding challenge. Despite their many advantages, M/NEMS devices relying on chemoselective interfaces do not have sufficient selectivity. Therefore, highly sensitive and selective detection and quantification of chemical molecules using real-time, miniature sensor platforms still remains as a crucial challenge. Incorporating photothermal/photoacoustic spectroscopic techniques with M/NEMS using quantum cascade lasers can provide the chemical selectivity without sacrificing the sensitivity of the miniaturized sensing system. Point sensing is defined as sensing that requires collection and delivery of the target molecules to the sensor for detection and analysis. For example, photothermal cantilever deflection spectroscopy, which combines the high thermomechanical sensitivity of a bimetallic microcantilever with high selectivity of the mid infrared (IR) spectroscopy, is capable of obtaining molecular signatures of extremely small quantities of adsorbed explosive molecules (tens of picogram). On the other hand, standoff sensing is defined as sensing where the sensor and the operator are at distance from the target samples. Therefore, the standoff sensing is a non-contact method of obtaining molecular signatures without sample collection and processing. The distance of detection depends on the power of IR source, the sensitivity of a detector, and the efficiency of the collecting optics. By employing broadly tunable, high power quantum cascade lasers and a boxcar averager, molecular recognition of trace explosive compounds (1 μg/cm2 of RDX) on a stainless steel surface has been achieved at a distance of five meters.
With the availability of tunable broadband coherent sources that emit mid-infrared radiation with well-defined beam characteristics, spectroscopies that were traditionally not practical for standoff detection1 or for development of miniaturized infrared detectors2, 3 have renewed interest. While obtaining compositional information for objects from a distance remains a major challenge in chemical and biological sensing, recently we demonstrated that capitalizing on mid-infrared excitation of target molecules by using quantum cascade lasers and invoking a pump probe scheme can provide spectral fingerprints of substances from a variable standoff distance.3 However, the standoff data is typically associated with random fluctuations that can corrupt the fine spectral features and useful data. To process the data from standoff experiments toward better recognition we consider and apply two types of denoising techniques, namely, spectral analysis and Karhunen-Loeve Transform (KLT). Using these techniques, infrared spectral data have been effectively improved. The result of the analysis illustrates that KLT can be adapted as a powerful data denoising tool for the presented pump-probe infrared standoff spectroscopy.
Detection of explosive residues using portable devices for locating landmine and terrorist weapons must sat- isfy the application criteria of high reproducibility, specificity, sensitivity and fast response time. Vibrational spectroscopies such as Raman and infrared spectroscopies have demonstrated their potential to distinguish the members of the chemical family of more than 30 explosive materials. The characteristic chemical fingerprints in the spectra of these explosives stem from the unique bond structure of each compound. However, these spectroscopies, developed in the early sixties, suffer from a poor sensitivity. On the contrary, MEMS-based chemical sensors have shown to have very high sensitivity lowering the detection limit down to less than 1 picogram, (namely 10 part per trillion) using sensor platforms based on microcantilevers, plasmonics, or surface acoustic waves. The minimum amount of molecules that can be detected depends actually on the transducer size. The selectivity in MEMS sensors is usually realized using chemical modification of the active surface. However, the lack of sufficiently selective receptors that can be immobilized on MEMS sensors remains one of the most critical issues. Microcantilever based sensors offer an excellent opportunity to combine both the infrared photothermal spectroscopy in their static mode and the unique mass sensitivity in their dynamic mode. Optical sensors based on localized plasmon resonance can also take up the challenge of addressing the selectivity by monitoring the Surface Enhanced Raman spectrum down to few molecules. The operating conditions of these promising localized spectroscopies will be discussed in terms of reliability, compactness, data analysis and potential for mass deployment.
The highly sensitive nanoporous cantilever beam without immobilized receptors was combined with highly selective mid-infrared (IR)
spectroscopy for molecular recognition of analytes using characteristic molecular vibrations. Unlike conventional IR spectroscopy, in
addition, the detection sensitivity and resolution are drastically enhanced by combining high power tunable quantum cascade laser
with a nanoporous cantilever having large surface area, low modulus, and nanowell structures. Further, analytes can be easily loaded
on the porous microcantilever without receptor due to nanowells. In addition, orthogonal signals, variations in the mass and IR
spectrum, provide more reliable and quantitative results including physical as well as chemical information of samples. We have used
this technique to rapidly identify single and double stranded DNA.
Standoff identification of explosive residues may offer early warnings to many hazards plaguing present and future
military operations. The greatest challenge is posed by the need for molecular recognition of trace explosive compounds
on real-world surfaces. Most techniques that offer eye-safe, long-range detection fail when unknown surfaces with no
prior knowledge of the surface spectral properties are interrogated. Inhomogeneity in the surface concentration and
optical absorption from background molecules can introduce significant reproducibility challenges for reliable detection
when surface residue concentrations are below tens of micrograms per square centimeter. Here we present a coupled
standoff technique that allows identification of explosive residues concentrations in the sub microgram per square
centimeter range on real-world surfaces. Our technique is a variation of standoff photoacoustic spectroscopy merged
with ultraviolet chemical photodecomposition for selective identification of explosives. We demonstrate the detection of
standard military grade explosives including RDX, PETN, and TNT along with a couple of common compounds such as
diesel and sugar. We obtain identification at several hundred nanograms per centimeter square at a distance of four
meters.
Here we report on a standoff spectroscopic technique for identifying chemical residues on surfaces. A hand-held infrared
camera was used in conjunction with a wavelength tunable mid-IR quantum cascade laser (QCL) to create hyperspectral
image arrays of a target with an explosive residue on its surface. Spectral signatures of the explosive residue (RDX)
were extracted from the hyperspectral image arrays and compared with a reference spectrum. Identification of RDX was
achieved for residue concentrations of 20 μg per cm2 at a distance of 1.5 m, and for 5 μg per cm2 at a distance of 15 cm.
A micro differential thermal analysis (DTA) system is used for detection of trace explosive particles. The DTA
system consists of two silicon micro chips with integrated heaters and temperature sensors. One chip is used for
reference and one for the measurement sample. The sensor is constructed as a small silicon nitride membrane
incorporating heater elements and a temperature measurement resistor. In this manuscript the DTA system
is described and tested by measuring calorimetric response of 3 different kinds of explosives (TNT, RDX and
PETN). This project is carried out under the framework of the Xsense project at the Technical University of
Denmark (DTU) which combines four independent sensing techniques, these micro DNT sensors will be included
in handheld explosives detectors with applications in homeland security and landmine clearance.
Realizing that no one sensing principle is perfect we set out to combine four fundamentally different sensing principles
into one device. The reasoning is that each sensor will complement the others and provide redundancy under various
environmental conditions. As each sensor can be fabricated using microfabrication the inherent advantages associated
with MEMS technologies such as low fabrication costs and small device size allows us to integrate the four sensors into
one portable device at a low cost.
A micro differential thermal analysis (DTA) system is used for detection of trace explosive particles. The DTA
system consists of two silicon micro chips with integrated heaters and temperature sensors. One chip is used
for reference and one for the measurement sample. The sensor is constructed as a small silicon nitride bridge
incorporating heater elements and a temperature measurement resistor. In this manuscript the DTA system is
described and tested by measuring calorimetric response of DNT (2,4-Dinitrotoluene). The design of the senor is
described and the temperature uniformity investigated using finite element modelings and Raman temperature
measurements. The functionality is tested using two different kinds of explosive deposition techniques and
calorimetric responses are obtained. Under the framework of the Xsense project at the Technical University of
Denmark (DTU) which combines four independent sensing techniques, these micro DNT sensors will be included
in handheld explosives detectors with applications in homeland security and landmine clearance.
In an effort to produce a handheld explosives sensor the Xsense project has been initiated at the Technical University of
Denmark in collaboration with a number of partners. Using micro- and nano technological approaches it will be
attempted to integrate four detection principles into a single device. At the end of the project, the consortium aims at
having delivered a sensor platform consisting of four independent detector principles capable of detecting concentrations
of TNT at sub parts-per-billion (ppb) concentrations and with a false positive rate less than 1 parts-per-thousand. The
specificity, sensitivity and reliability are ensured by the use of clever data processing , surface functionalisation and
nanostructured sensors and sensor surfaces.
Nanomechanical response of molecular adsorption has been demonstrated as the basis for a number of extremely
sensitive sensors. Molecular adsorption on microcantilevers results in nanomechanical motion due to adsorption-induced
surface stress variation. Chemical selectivity in nanomechanical sensors is achieved by immobilizing receptors
on the cantilever surface. Although receptor-based detection has high selectivity for biomolecular detection, it fails
when applied to small molecule detection. Nanomechanics, however, offer new possibilities for achieving chemical
selectivity that do not use any receptors. For example, small thermal mass or high temperature sensitivity of a cantilever
beam could be used for detecting molecular adsorption using photothermal effects and physical property variation due
to temperature. Here we describe two such techniques for achieving chemical selectivity without using any receptor
molecules.
Recent terrorists events have shown that an urgent and widespread need exists for development of novel sensors for
chemical and biowarfare agents. The advent of inexpensive, mass-produced microcantilever sensors, promises to bring
about a revolution in detection of terrorists threats. Extremely sensitive chem/biosensors can be developed using
microcantilever platform. Both frequency and bending of microcantilevers can be used to detect the chemical and
biological species in air or solution. The specificity is achieved by immobilizing chemically-specific receptors the
cantilever. This short report will give an overview of chemical/biological warfare agents sensor recently developed
based on microcantilevers.
Charles Britton, R. Warmack, S. Smith, Patrick Oden, Gilbert Brown, W. Bryan, Lloyd Clonts, Michael Duncan, Mike Emery, M. Ericson, Z. Hu, Robert Jones, Michael Moore, J. Moore, Jim Rochelle, Timothy Threatt, Thomas Thundat, Gary Turner, Alan Wintenberg
Selectively coated cantilevers are being developed at ORNL for chemical and biological sensing. The sensitivity can exceed that of other electro-mechanical devices as parts- per-trillion detection can be demonstrated for certain species. We are now proceeding to develop systems that employ electrically readable microcantilevers in a standard MEMS process and standard CMOS processes. One of our primary areas of interest is chemical sensing for environmental applications. Towards this end, we are presently developing electronic readout of a mercury-sensitive coated cantilever. In order to field arrays of distributed sensors, a wireless network for data reporting is needed. For this, we are developing on-chip spread-spectrum encoding and modulation circuitry to improve the robustness and security of sensor data in typical interference- and multipath-impaired environments. We have also provided for a selection of distinct spreading codes to serve groups of sensors in a common environment by the application of code-division multiple-access techniques. Most of the RF circuity we have designed and fabricated in 0.5 micrometers CMOS has been tested and verified operational to above 1 GHz. Our initial intended operation is for use in the 915 MHz Industrial, Scientific, and Medical band. This paper presents measured data on the microcantilever-based mercury detector. We will also present design data and measurements of the RF telemetry chip.
Uncooled infrared sensors are significant in a number of scientific and technological applications. A new approach to uncooled infrared detectors has been developed using piezoresistive microcantilevers coated with thermal energy absorbing material(s). Infrared radiation absorbed by the microcantilever detector can be sensitively detected as changes in the electrical resistance as a function of microcantilever bending. These devices have demonstrated sensitivities comparable to existing uncooled thermal detector technologies. The dynamic range of these devices is extremely large due to measurable resistance change obtained with only nanometer level cantilever displacement. Optimization of geometrical properties for selected commercially available cantilevers is presented. Additionally, we present results obtained from a modeling analysis of the thermal properties of several different microcantilever detector architectures.
The feasibility of micromechanical optical and infrared (IR) detection using microcantilevers is demonstrated. Microcantilevers provide a simple means for developing single- and multi-element sensors for visible and infrared radiation that are smaller, more sensitive and lower in cost than quantum or thermal detectors. Microcantilevers coated with a heat absorbing layer undergo bending due to the differential stress originating from the bimetallic effect. Bending is proportional to the amount of heat absorbed and can be detected using optical or electrical methods such as resistance changes in piezoresistive cantilevers. The microcantilever sensors exhibit two distinct thermal responses: a fast one ((tau) 1thermal less than ms) and a slower one ((tau) 2thermal approximately 10 ms). A noise equivalent temperature difference, NEDT equals 90 mK was measured. When uncoated microcantilevers were irradiated by a low-power diode laser ((lambda) equals 786 nm) the noise equivalent power, NEP, was found to be 3.5 nW/(root)Hz which corresponds to a specific detectivity, D*, of 3.6 multiplied by 107 cm (DOT) (root)Hz/W at a modulation frequency of 20 Hz.
Physical mapping of DNA can be accomplished by direct AFM imaging of site specific proteins bound to DNA molecules. Using Gln-111, a mutant of EcoRI endonuclease with a specific affinity for EcoRI sites 1000 times greater than wild type enzyme but with cleavage rate constants reduced by a factor of 104, we demonstrate site-specific mapping by direct AFM imaging. Images are presented showing specific-site binding of Gln-111 to plasmids having either one (pBS+) or two (pMP32) EcoRI sites. Identification of the Gln-111/DNA complex is greatly enhanced by biotinylation of the complex followed by reaction with streptavidin gold prior to imaging. Image enhancement coupled with improvements in our preparation techniques for imaging large DNA molecules, such as lambda DNA (47 kb), has the potential to contribute to direct AFM restriction mapping of cosmid-sized genomic DNAs.
Alkanethiols self-assemble into monolayers on gold surfaces. It has been shown that gold surfaces derivatized with two-carbon, bifunctional alkanethiols differentially adsorb DNA. Gold surfaces modified with either 2-(N,N-dimethylamino)ethanethiol or 2-aminoethanethiol immobilized DNA at solution pH's where the amino end groups are protonated. The cationic layer holds the DNA in place by ion-pairing with the negatively-charged phosphate groups on the DNA backbone. This ion-pairing is sufficiently strong to resist changes in the DNA's location and conformation induced by the scanning tunneling microscope (STM) tip. With these chemically modified surfaces, the reliable and reproducible imaging of DNA is possible. When the length of the alkane spacer is increased to eleven carbons, the observed affinities for radiolabeled DNA are comparable to that observed for the two carbon spacer. However, clearly resolved STM images of DNA immobilized on 11-(N,N'-dimethylamino)- undecanethiol-modified gold have not been obtainable. We hypothesize that images of immobilized DNA are not observed because of the interaction of the scanning probe with the self-assembled alkanethiol monolayer.
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