MULTIFUNCTIONAL SOLID STATE OPTICAL SENSORS
MULTIFUNCTIONAL SOLID STATE OPTICAL SENSORS
Optical chemical sensors used to measure the composition of solid, liquid, or gaseous substances and concentration of optically active species, can employ matched pairs of light emitting diodes (LEDs) and photodiodes (PDs) placed in an optically insulated chamber. The measurements of the active species’ unique properties are based on absorption, transmittance, reflection, scattering, or fluorescence of the LED light by the substance under the test and detection of the light changes caused by the interaction with the substance by the photodetector. Most advanced optical sensor systems can be assembled from LEDs and PDs integrated monolithically on a single layered substrate compatible with Micro Optical Electrical Mechanical Systems (MOEMS) used for in-situ, in-vivo, or in-line applications.
Experimental Simulation of Integrated Optoelectronic Sensors Based on III-Nitrides
Reliable, miniature, multifunctional, real-time optoelectronic sensors can be fabricated by using III nitride materials that have several advantages over the conventional semiconductors [1,2]. Recent advances in these materials allow integrated optoelectronic devices with tunable spectral characteristics. In addition, optically transparent sapphire substrates and commercially beneficial silicon wafers can be used for the device fabrication.
Two concepts of the integrated optoelectronic sensor development were presented in this project. These concepts were investigated by fabrication and testing simulators based on III nitride and Si commercial components.
In order to investigate these concepts as well as the applicability of III Nitride and Si-based components for multifunctional sensor applications, we have fabricated and tested multifunctional sensor prototypes (simulators) based on commercial III Nitride and Si-based components. For the first two simulators we used blue LEDs from Agilent Technologies with a maximum wavelength of 475 nm and Hamamatsu Si photodiodes mounted on a double side-polished sapphire substrate together with a long-pass optical filter from Oriel with a cut-off wavelength of 475 and 530 nm for Simulators 1 and 2, respectively. These simulators were designed to verify the first configuration concept by measurements of fluorescence from a single-component analyte with excitation and emission in a blue and a green band, respectively. Using the Simulator 2 we have measured fluorescence of a Fluorescein™ dye in methanol solutions at different pH values. Also, fluorescence was measured with Simulator 2 from chlorophyll extracted from three groups of green leaves. The detection limit in these measurements was increased by a background signal resulting from the direct illumination of the photodiode from the LED through the sapphire substrate and by light scattered by other components. Enhancement of the signal that was achieved by placing a metallic mirror in front of the sapphire substrate in the Simulator 1 with a higher background signal allowed us to perform measurements based on: a) optical absorption in potassium and cobalt salt; b) variation of the refractive index of commonly used organic solvents; c) scattering by alumina powders of various sizes. These results indicate that several sensor functions can be achieved using the same component setup.
Figure below shows schematics of the Simulator 1 and 2:
In order to investigate the second concept, with components configured at 90° on Si wafers, and explore other possible applications we fabricated Simulator 3.The UV (Nichia), blue, and green (Agilent Technologies) LEDs used in this simulator were set up at 90° to three Si photodiodes filtered by long-pass Roscolux optical filter films from Edmunds Industrial Optics (for the blue and green LEDs) and 6H-SiC wafer from Cree (for the UV LED) to separate the LED and the photodetector spectral bands, and at the same time to get a maximum photoresponse in a range next to the LED band. All spectral characteristics in this work have been measured by using an automated Triax 320 spectrometer system controlled by Windows compatible software. A broad-spectrum 150 W Xenon arc lamp was used as a reference source. Normalized spectral characteristics measured from Simulator 3 components determine the three narrow and three wide bands which allows us to perform fluorescence excitation, and measure fluorescence emission, respectively. Figure below represents the Simulator 3 setup:
Figure bellow illustrates various types of measurements performed using the Simulators 1 and 2 :
A- fluorescence from Fluorescein at various pH levels; B- fluorescence from chlorophyll;
C- absorption by metallic salts;
D- reflection by the interface with commonly used solvents having various refractive indexes; E- scattering by alumina powders of various size.
The measurements of concentrations in mixtures of Escherichia coli bacteria expressing Green and Red Fluorescent Proteins (GFP and RFP) performed using Simulator 3 are shown in the figure below:
The fabricated devices exhibit multifunctionality expressed by the ability to perform measurements of optical absorption (metallic salts solutions), reflection (interface with commonly used solvents), scattering (alumina powder slurries), and fluorescence (chlorophyll, fluorescein, pyrene, anthracene, and Escherichia coli strains carrying plasmids which encode fluorescent proteins). These measurements indicate the applicability of the III nitride and Si-based components and their layout according to the described concepts for the development of integrated multifunctional optoelectronic sensors.
Instrumentation for Ultrafine Particles Characterization
Current biological agent detection systems are large, complex, expensive, and subject to false alarms. They can detect only a limited number of biological agents and only after exposure. Sensitivity, selectivity and durability of these detection technologies are not proven. Commercially available laser ablation aerosol spectrometers are based on particle velocity selection using pulsed laser light scattering to identify the speed of a size selected aerosol and to coordinate the ablation laser pulse timing as the particle enters the ion extraction region of a linear time of flight mass spectrometer. This work has been extended to include selection of only biogenic particles based monitoring the particle speed and its intrinsic peptide fluorescence signature using a complicated grating monochromator. The drawback of this elegant technique is that the sensitivity of the laser light scattering falls precipitously as the particle size decreases much below 1 micron. Therefore, the ultrafine fraction of the aerosol is inaccessible with current techniques.
The technology proposed in this project  can significantly reduce the current drawbacks of the existing aerosol characterization techniques by implementing a combination of several unique and powerful techniques in a single compact multifunctional device.
The objective of this project was to develop optoelectronic modules, capable of fluorescence, absorption, and scattering measurements, integrated with a Time-of-Flight (TOF) mass spectrometer for advanced analysis of ultrafine aerosol particles and MALDI experiments. In order to meet this objective the following goals should be reached:
- Fabrication and characterization of an ultrafine particle detector prototype based on scattering for determination of TOF starting point for individual particles and size identification.
- Fabrication and characterization of a fluorescence module capable of discrimination prior to mass spectrometry.
- Integration of the prototypes within an ORTOF mass spectrometer.
- Testing of the integrated setup and evaluation of its characteristics.
Several versions of the experimental setup have been designed, assembled, and tested during the project with a large variety of analytes in the aerosol and liquid form in order to investigate the feasibility of the proposed technology. The main ideas, concepts, and approaches investigated and exploited in this project include:
- Development of miniature multifunctional multi-wavelength optical sensors based on III nitrides suitable for integration with existing compact mass spectrometers. Optical characterization of the analyte prior to mass spectroscopy would result in getting of more detailed information about it also allowing selective analyte-specific mass spectrometry measurements.
- Employment of the newly developed (we were among the first users) III nitride-based UV 10 mW laser diode modules for detection and characterization of ultrafine particles. The shorter laser wavelength of 375 nm should allow detection of particles with sizes below 100 nm. At the same time such laser has been used as an efficient fluorescence excitation source.
- Development and investigation of principles for characterization of fluorescing aerosols including intrinsically fluorescing compounds dissolved in various liquids and available fluorescing polystyrene latex particles. Fluorescence is a very sensitive and specific technique, which, if combined with mass spectroscopy in an integrated instrument, would provide several benefits beyond those, offered by the two techniques used separately.
- Exploration of the idea for employment of the signal, resulting from detection of single aerosol particle in order to trigger the time-of-flight (TOF) mass spectrometry measurements. This would allow for selective and particle-specific TOF mass spectrometry measurements in order to avoid false alarms and data overload.
One of the approaches proposed was employment of modified to our specifications large (1” dia.) CCD chips for the development of a compact particle detector module. Such chips are currently produced by few CCD camera manufacturers for highly expensive and application-specific instruments, and cannot be sold separately. The sensitivity of the existing CCD chips was compared, and it was found that even highest sensitivity chips would not be capable of single particle detection without intensification. The CCD chip intensification is currently achieved by placing phosphor-coated micro-channel plates (MCP) in front of the CCD chip, which would make the proposed approach impossible. In addition, we have discovered that separation of the chips from the carrying addressing boards will increase the parasitic capacitance, which would result in a complete loss of the signal from the chip.
The results from measurements performed on water- and ethanol-based aerosol indicate that such parameters as aerosol flow and mean particle diameter can be characterized by using solid-state laser diodes. The employment of a UV laser allows characterization of aerosols with smaller particle mean diameters than it is possible with visible spectral range lasers.
Figure below shows measurements of the signal from the green laser light scattered by the particles of the water aerosol:
Characterization of fluorescing aerosols was performed in order to evaluate the capabilities of the III nitride-based optoelectronic components integrated within the ORTOF mass spectrometer. First, the capability of the UV laser diode for excitation of the fluorescing aerosols was investigated. Fluorescing aerosols produced from alcohol solutions of Fluorocsein, Pyrene, and Anthracene have been measured. All three intrinsically fluorescing substances can be excited with the UV laser diode light. In order to distinguish between the signals produced by fluorescence emission and UV laser scattered light on of two optical filters were used with the PMT:
- a) Long-pass 530nm glass filter for detection of fluorescence emission of the aerosol irradiated with the UV laser diode without sensing of the scattered by the aerosol particles UV light;
- b) UV band-pass filter (200-400nm) for detection only of the scattered UV laser light, without sensing the fluorescence emission from the aerosols.
The data obtained from measurements of fluorescing liquid aerosols indicates the III nitride based UV laser diode can be successfuly used not only as a light source for measurements of the scattered by the aerosol light, but also as an efficient optical source for excitation of various fluorescing compounds. Combination of the two methods provided by employment of a single compact device will result in achieving of more detailed information about the analyte prior to mass spectrometry. In the Phase II project we will investigate aerosols based on three intrinsically fluorescing proteins, such as: phenylalanine, tyrosine, and tryptophan. These important for biomedical applications compounds exhibit fluorescence emission at around 340 nm at excitation light wavelength around ~280 nm. Such measurements would be possible by employment of AlN/AlGaN based UV LEDs and GaN based solar-blind photodiodes planned to be developed during the second project year.
Table below represents detection limits and dynamic ranges for various chip combinations calculated after selected analyte measurements:
The graphs below show measurements of the fluorescence signal depending on the analyte concentration performed by using the multi-wavelength fluorescence module:
Fluorescence Based Integrated Optoelectronic Sensor System for Multivariable Analysis
The potential of optoelectronic sensors for bio-chemical applications utilizing various concepts based on, absorption, scattering, reflection, etc., was studied andthe issues expected during the sensor development were identified and analyzed. Sensitivity and selectivity must be compromised as a result of sensor miniaturization.
Most of the currently used sensors use statistical sampling and are capable ofdetecting only a very limited number of analytes (more often one). In addition, these sensors are designed either to detect or quantify the analyte, but not both. An important mi feature is the inability of using time-resolved measurements in a miniaturized, portable and inexpensive form. Several sensor prototypes were designed and fabricated in order to explore the basic concepts and steady-state capabilities of the proposed sensors. The prototypes made use of light-emitting diodes (LED) and photodetectors (PD) for excitation and detection. Some of the prototypes utilized LEDs for photo-detection and reasonable sensitivity was achieved. The prototypes were able to detect important analytes at concentrations as low as several parts per billion (ppb) and a dynamic range spanning more than six decades.
The process of collecting data using the sensor has been automated and neural networks was implemented to test the sensitivity of the device [4-7]. The use of Artificial Neural Networks (ANN) has improved the sensitivity lost during data acquisition, because of its capability to classify the signature-data collected by the multi-spectral sensor. Various ANN architectures were investigated and the optimum was selected for our application. In our case, classification involved both the identification of the sample analyte and its concentration. A miniaturized, hand-held, multi-spectral, filter-less optoelectronic sensor prototype developed in our lab using commercial LEDs only, was used as the sensing device.
A LabView based circuit interface was built to perform the experiments and collect the data for ANN training and testing. We were able to attain a classification accuracy of above 89% from 4 different analytes at various concentrations totally 19 different categories. The results achieved by using light intensity-based measurements indicate that implementation of time-resolved measurements approach would result in significant improvement of the most important sensor parameters, such as sensitivity, selectivity, and classification accuracy. This work can result in the capability to more accurately analyze a much larger variety of important variables by using the same miniature device. Moreover, the use of solid-state optoelectronic devices in time-resolved spectroscopy could be viewed as a new beginning for compact and cost-effective spectroscopy. The different modules of time-resolved measurement system were analyzed and preliminary stages were fabricated and tested.
In this project the following goals have been reached:
- Designed, fabricated Steady-state sensor and ANN
- Results from Steady state measurements were encouraging (96% accuracy for 38 categories)
- Simulated improvement in accuracy by inclusion of Time-resolved data
- Analyzed, designed, implemented Time Resolved Modules individually. Feasibility of TRM is demonstrated
- Resolved issues with integration of TRM modules – PD Amplifier Circuit, ADC interfacing
 D. Starikov, C. Boney, N. Medelci, J-W. Um, A. Bensaoula , M. Larios Sanz, and G. E. Fox, “Experimental simulation of integrated optoelectronic sensors based on III Nitrides”. J. Vac. Sci. Tech. B: 20(5), 1815-1820 (2002). http://avs.scitation.org/doi/abs/10.1116/1.1498276
 D. Starikov, F. Benkabou, Nasr, Medelci, and A. Bensaoula. “Integrated Multi-Wavelength Fluorescence Sensors”. ISA/IEEE Sensors for Industry (SIcon/02) Conference proceedings,15-18, (2002).
 Summary of NIH SBIR Innovations in Biomedical Information Science and Technology Awards for Program Announcement PA-00-118 Newly Funded in FY 2003. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0ahUKEwio9J21sabRAhVDuBoKHXGnC4EQFggvMAM&url=https%3A%2F%2Fwww.bisti.nih.gov%2FShared%2520Documents%2FFundedProjects%2FYear2003%2FSBIRFY03.pdf&usg=AFQjCNFELBXQTw7R7uHRdBMVeqpFfnbkuQ&bvm=bv.142059868,d.d2s&cad=rja
 Boukadoum, M., Tabari, K., Bensaoula, A. & Starikov, D. & Aboulhamid, E.M. (2005) FPGA implementation of a CDMA source coding and modulation subsystem for a multiband fluorometer with pattern recognition capabilities, proc. International Symposium on Circuits and Systems (ISCAS 2005), Kobe (Japan), May 2005, pp. 4767-4770. http://ieeexplore.ieee.org/document/1465698/
 Starikov, D., Clement, J., Bensaoula, A. & Boukadoum, M. (2005) Chip-based integrated filterless multi-wavelength optoelectronic biochemical sensors, proc. IEEE Sensors for Industry Conference (SICON 2005), Houston, February 2005, Conference Proceedings pp. 129-132.
 Tabari, K., Boukadoum, M., Bensaoula, A. & Starikov, D. “Neural Network Processor for a FPGA-based Multiband Fluorometer Device,” proc. IEEE Computer Architectures for Machine Perception and Sensors (CAMPS’06), pp. 197-201, Montreal (Canada), (2006).
 Clement Joseph, Hanae Naoum, Mounir Boukadoum, David Starikov, Earl J. Charlson and Abdelhak Bensaoula, “Integrated Solid-State Optoelectronic Sensor System for Biochemical Detection and Quantification”, Solid State Electronics Journal, 2008.
 Clement Joseph, Mounir Boukadoum, Joe Charlson, David Starikov and Abdelhak Bensaoula. High-speed front end for LED-Photodiode based fluorescence lifetime measurement system. Proceedings of IEEE Symposium Circuits and Systems Society (ISCAS 2007), New Orleans, 27 -30 May, 2007.