The selectivity of an optical sensor can be improved by combining optical detection with electrochemical oxidation or
reduction of the target analyte to change its spectral properties. The changing signal can distinguish the analyte from
interferences with similar spectral properties that would otherwise interfere. The analyte is detected by measuring the
intensity of the electrochemically modulated signal. In one form this spectroelectrochemical sensor consists of an
optically transparent electrode (OTE) coated with a film that preconcentrates the target analyte. The OTE functions as
an optical waveguide for attenuated total reflectance (ATR) spectroscopy, which detects the analyte by absorption.
Sensitivity relies in part on a large change in molar absorptivity between the two oxidation states used for
electrochemical modulation of the optical signal. A critical part of the sensor is the ion selective film. It should
preconcentrate the analyte and exclude some interferences. At the same time the film must not interfere with the
electrochemistry or the optical detection. Therefore, since the debut of the sensor’s concept one major focus of our group
has been developing appropriate films for different analytes. Here we report the development of a series of quaternized
poly(vinylpyridine)-co-styrene (QPVP-co-S) anion exchange films for use in spectroelectrochemical sensors to enable
sensitive detection of target anionic analytes in complex samples. The films were either 10% or 20% styrene and were
prepared with varying degrees of quaternized pyridine groups, up to 70%. Films were characterized with respect to
thickness with spectroscopic ellipsometry, degree of quaternization with FTIR, and electrochemically and
spectroelectrochemically using the anions ferrocyanide and pertechnetate.
Spectroelectrochemistry provides improved selectivity for sensors by electrochemically modulating the optical signal
associated with the analyte. The sensor consists of an optically transparent electrode (OTE) coated with a film that
preconcentrates the target analyte. The OTE functions as an optical waveguide for attenuated total reflectance (ATR)
spectroscopy, which detects the analyte by absorption. Alternatively, the OTE can serve as the excitation light for
fluorescence detection, which is generally more sensitive than absorption. The analyte partitions into the film, undergoes
an electrochemical redox reaction at the OTE surface, and absorbs or emits light in its oxidized or reduced state. The
change in the optical response associated with electrochemical oxidation or reduction at the OTE is used to quantify the
analyte. Absorption sensors for metal ion complexes such as [Fe(CN)6]4- and [Ru(bpy)3]2+ and fluorescence sensors for [Ru(bpy)3]2+ and the polycyclic aromatic hydrocarbon 1-hydroxypyrene have been developed. The sensor concept has been extended to binding assays for a protein using avidin–biotin and 17β-estradiol–anti-estradiol antibodies. The sensor has been demonstrated to measure metal complexes in complex samples such as nuclear waste and natural water. This sensor has qualities needed for security and defense applications that require a high level of selectivity and good detection limits for target analytes in complex samples. Quickly monitoring and designating intent of a nuclear program by measuring the Ru/Tc fission product ratio is such an application.
This work reports on continuing development of a lab-on-a-chip sensor for electrochemical detection of heavy metal zinc
in blood serum. The sensor consists of a three electrode system, including an environmentally-friendly bismuth working
electrode, a Ag/AgCl reference electrode, and a gold auxiliary electrode. By optimizing the electrodeposition of bismuth
film, better control of fabrication steps and improving interface between the sensor and potentiostat, repeatability and
sensitivity of the lab-on-a-chip sensor has been improved. Through optimization of electrolyte and stripping
voltammetry parameters, limits of detection were greatly improved. The optimized sensor was able to measure zinc in in
the physiological range of 65-95 μg/dL. Ultimately, with further development and integrated sample preparation sensor
system will permit rapid (min) measurements of zinc from a sub-mL sample (a few drops of blood) for bedside
monitoring.
Highly aligned double wall carbon nanotubes (DWCNT) and multi-wall carbon nanotubes (MWCNT) were
synthesized in the shape of towers and embedded into microchannels for use as a biosensor. The towers were
fabricated on a substrate patterned in 1mm x 1mm blocks with 1 mm spacing between the blocks. Chemical vapor
deposition was used for the nanotube synthesis process. Patterned towers up to 8 mm high were grown and easily
peeled off the silicon substrate. A nanotube electrode was then soldered on printed circuit boards and epoxy was cast
into the tower under pressure. After curing, the top of the tower was polished. RF-plasma at 13.56 MHz was used to
enhance the electrocatalytic effect of the nanotube electrode by removing excess epoxy and exposing the ends of the
nanotubes. Au particles were electrodeposited on the plasma treated tower electrode. Cyclic voltammetry (CV) for
the reduction of 6 mM K3Fe(CN)6 (in a 1.0 M KNO3 supporting electrolyte) was performed to examine the redox
behavior of the nanotube tower electrode. Next, a master mold for polydimethylsiloxane (PDMS) was patterned
using SU-8 and then a Pt disk electrode was embedded into the PDMS. The final fluidic channel between the epoxy-nanotube
electrode and PDMS was sealed using a UV-curing adhesive. Impedance between the Pt and nanotube
electrodes was monitored while flowing different solutions and LNCaP prostate cells. The impedance changed in
proportion to the concentration of cells in the solution. A needle-type composite microelectrode was then fabricated
by injecting a carbon nanotube-epoxy solution into a pulled-glass tube. CV and differential pulse voltammetry
(DPV) to detect dopamine were showed a highly linear response with a sensitivity 100 nA/mM. Based on the
impedance results using the flowing cells and the CV and DPV results, carbon nanotube microelectrodes are a
promising candidate for cancer cell detection and neurotransmitter detection.
YeoHeung Yun, Adam Bange, Vesselin Shanov, William Heineman, H. Brian Halsall, Sarah Pixley, Michael Behbehani, Zhongyun Dong, Yi Tu, Sergey Yarmolenko, Sudhir Neralla, Mark Schulz
Highly aligned multi-wall carbon nanotube arrays up to 4 mm tall were synthesized on Si wafers using a chemical vapor deposition process with water delivery. Based on the long nanotube arrays, several prototype smart materials were developed including a biosensor, electrochemical actuator, and nanotube probes. The biosensor was formed by casting epoxy into a nanotube array and polishing the ends of the nanotubes. This electrode produced a near ideal sigmoidal cyclic voltammogram. Nanotube electrodes were then used to form a label-free immunosensor based on electrochemical impedance spectroscopy. The nanotube array immunosensor has good sensitivity, but decreasing the array size and improving the biofunctionalization is expected to dramatically increase the reproducibility and sensitivity. The electrochemical actuator was formed by bonding an electrode to a 1mm square by 4 mm long as-grown nanotube array post. The nanotube array actuator operated up to 10 Hz in a 2 M NaCl solution. With a driving voltage of 2 volts, the actuator produced 0.15% strain. Finally, nanotube bundles are being welded to tungsten tips and put inside glass needles for use as probes for biosensors and electrophysiology applications. All the smart materials applications discussed are recent, and further development is expected to yield improved performance and commodity level practical devices.
Fluorescently labeled beads may be utilized in transparent microfluidic devices to facilitate a variety of immunoassay based chemical measurements. We investigate the ability to visualize, quantitate, and reduce undesirable adsorption of beads within a polydimethylsiloxane (PDMS) microchannel device. These methods are prerequisites to the design of practical bead-based microfluidic sensing devices. The PDMS microchannels were shown to be transparent enough to make accurate quantitative optical measurements, although significant adsorption was observed. Epifluorescence microscopy was employed in an attempt to quantitatively evaluate microbead adsorption to PDMS microchannel walls and bulk surfaces after different agitation, solution, and surface treatments. This microscopy method provides reproducible imaging of individual beads and allows for characterization of adsorption to PDMS microchannel walls. Solution composition seemed to play a more important role in the ability to reduce the number of adsorbed beads to the PDMS surface than agitation. The most significant reduction in bead adsorption was seen in surface treatment. The most effective surface treatment examined in this study was Teflon AF.
Microbead immunoassay with electrochemical detection has been developed as a sensitive and selective technique for rapid and small volume analyses. In this assay, applications of paramagnetic microbeads in a microfluidic system have aided the automation of all assay steps to enable near-continuous monitoring. These mobile microbeads can be transported through microchannels, captured and held at specific points by a magnet. Hence, by performing immunoassay on microbeads, they can be dispersed throughout a small sample of water, where they provide a large surface area to sample volume ratio that enhances the capture of the target antigen by minimizing diffusional distances. They can then be collected magnetically and manipulated to accomplish all the assay steps to determine if any target was captured. In addition, the microbeads can be accommodated in small volumes, which reduces the dilution of the enzyme product in the detection step thus maximizing sensitivity. Further, electrochemical detection coupled with enzyme-labeled immunoassay has led to the development of a sensitive analytical technique. In this area, interdigitated array electrodes are particularly suited to microfluidics. Improved sensitivity is obtained by redox cycling of the species being detected. In this work, the microbead immunoassays is demonstrated for the virus MS2 bacteriophage.
The detailed kinetics and associated optical waveforms of the spectroelectrochemical sensor in competitive binary mixtures of analytes are presented. The extension of the sensor concept to planar waveguide structures is described. Two different planar waveguide designs have been made and evaluated. One design was an asymmetric slab waveguide with the waveguide layer sandwiched between a silica substrate and a chemically-selective film. The other design was a channel waveguide, which consisted of an ion-exchanged channel between two gold electrodes in a 'bus bar' configuration. These new designs are described in detail and evaluation of them with respect to operation of the sensor in the near ultraviolet region is discussed. Results with the devices in demonstration of the novel spectroelectrochemical concept with three modes of selectivity are given. Instrumentation developed specifically for operation of the sensors and acquisition of sensor data is described.
A new type of spectroelectrochemical sensor that embodies tow modes of instrumental selectivity in addition to selective partitioning through an applied film barrier is described. The sensor consists of a planar optical substrate/electrode coated with a chemically-selective film. SEnsing is based on the change in the attenuation of light passing through the guided wave substrate which accompanies a chemical reaction of an analyte induced by electromodulation. Threefold selectivity for a chosen analyte relative to other environmental components is obtained by the choice of coating material, the electrolysis potential, and the wavelength for optical monitoring. The sensor concept is demonstrated with an indium tin oxide coated glass guided wave device that has been over-coated with a sol-gel derive charge-selective thin film. One such selective coating used was a charge-selective sol-gel processed Nafion-SiO2 composite film. Prototype analytes have been used to demonstrate that the change in the transmittance of the waveguide resulting from electrochemical oxidation/reduction can be used to quantify an analyte.
Two new series of chemically-selective optical materials have been made and tested on chemical sensor. One new series of materials was based on polymer blending in a host of glutaraldehyde cross-linked poly(vinyl alcohol). Chemically- selective dopants in this host demonstrate property- selective separations of chemicals from mixtures. We have optimized the composition, optical properties and the coating procedures for several specific blends for optical sensing. These blends have clear UV and visible spectral regions for direct spectroscopic sensing and they are excellent absorber of many inorganic and organic charged species from aqueous environments. A second new series consists of polyelectrolyte containing silica composites prepared by sol-gel processing. The thickness of spin-coated films of these materials on glass can be varied from 0.1 micrometers to 4 micrometers . These materials are ion exchangeable and less brittle than the parent silica substrate due to the incorporation of the organic polyelectrolyte. These new composites retain the nano-scale porosity and optical transparency into the UV of the parent silica sol-gel processed glasses making them attractive host matrices for the immobilization of a variety of chemical reagents. Results obtained with film-clad sensors from both new series of materials are presented.
Radiopharmaceuticals containing 99mTc, a gamma emitting radionuclide, are commonly used in nuclear medicine to image specific organs for the purpose of diagnosis. Following injection of a chemical complex of this isotope and concentration of the activity within the organ of interest, scanning with a gamma ray camera provides an image from which diagnostic information can be obtained. Although, in most instances the composition of the injected radiopharmaceutical is known, the chemical form of the agent that is actually responsible for the image has not been identified and may represent an altered form due to in vivo reaction. Sensors that could be implanted in specific organs in order to specify the chemical form of the radioactive complex that is imaging an organ would be especially useful. In order to accomplish this goal, sensors for in vivo monitoring of imaging agents that are used in nuclear medicine are being developed. Such sensors must be multiwavelength since chemical information is contained in the spectrum of agents. A brief outline of the results of our efforts to make highly efficient catheter-sized absorbance sensors is presented.
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