Abstract:
A sensor includes traps that are adjacent to a waveguide and capable of holding a contaminant for an interaction with an evanescent field surrounding the waveguide. When held in a trap, a particle of the contaminant, which may be an atom, a molecule, a virus, or a microbe, scatters light from the waveguide, and the scattered light can be measured to detect the presence or concentration of the contaminant. Holding of the particles permits sensing of the contaminant in a gas where movement of the particles might otherwise be too fast to permit measurement of the interaction with the evanescent field. The waveguide, a lighting system for the waveguide, a photosensor, and a communications interface can all be fabricated on a semiconductor die to permit fabrication of an autonomous nanosensor capable of suspension in the air or a gas being sensed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This patent document is related to a co-filed U.S. patent application entitled “Passive Evanescent Optical Nanosensor,” Ser. No. 11/127,869, which is hereby incorporated by reference in its entirety. 
   BACKGROUND 
   Detection of contaminants such as pollutants, toxins, poisons, and biological agents is critically important in many industrial, public, and private environments. Accordingly, a variety of environmental sensors have been developed. These environmental sensors are generally large enough to be handheld or mounted in the areas being monitored. Unfortunately, the size and need for separate mechanical and electrical components make these environmental sensors expensive when compared to the costs of integrated circuits. Sensors that could be manufactured using nanotechnology could potentially reduce sensing costs and permit new sensing capabilities, for example, for environments that are difficult to access or that have insufficient space to accommodate conventional sensors. 
   Fiber-optic evanescent fluorescence sensors, for example, are a known class of sensors used in biomedical applications. These sensors generally sense or measure the concentrations of target molecules that are known to absorb light having a first wavelength λ and to subsequently fluoresce by emitting light having a second wavelength λ′. Such sensors typically include an optical fiber that is inserted into a liquid containing the target molecules, while light having wavelength λ is directed through the optical fiber. The target molecules that are within the evanescent field surrounding the optical fiber can then absorb light of wavelength λ from the optical fiber and subsequently fluoresce to emit back into the optical fiber light having wavelength λ′. A detector coupled to the optical fiber measures the intensity of the light having frequency λ′, and that measurement indicates the presence or number of target molecules within the evanescent field of the optical fiber. 
   Current evanescent fluorescence sensors have a number of drawbacks. In particular, such sensors are relatively large and limited to sensing target molecules that have suitable fluorescent properties. Further, evanescent fluorescence sensors are typically limited to sensing target molecules in a liquid because contaminants in a gas at room temperature spend only a short time within the evanescent field, i.e., within a distance of about λ/4 of the optical fiber, and therefore generally move away from the fiber before fluorescing. 
   In view of the limitations of current environmental sensors, inexpensive sensors and sensing methods for detecting a variety of contaminant species in a gas or a liquid are needed. 
   SUMMARY 
   In accordance with an embodiment of the current invention, a sensor includes: a waveguide; a lighting system coupled to the waveguide; a trap adjacent to the waveguide; a photosensor; and a communications interface. The trap is capable of capturing and holding a target contaminant in an evanescent field of the waveguide, and the photosensor is positioned to detect light from the trap. The communications interface can be connected to the photosensor. 
   Another embodiment of the invention provides a method for detecting a target contaminant. The method includes capturing a particle of the target contaminant in a trap adjacent to a waveguide. When light is directed down the waveguide, the trap that has captured a particle holds the particle in an evanescent field caused by the light in the waveguide. Light that the target contaminant scatters from the waveguide can be measured. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically shows a nanosensor in accordance with an embodiment of the invention including a waveguide, a lighting system, traps, photosensors, and a communications interface. 
       FIG. 2  illustrates the change in an output signal as traps in the sensor of  FIG. 1  capture particles of a target contaminant. 
       FIG. 3  is a block diagram of a measurement system in accordance with an embodiment of the invention using multiple nanosensors and a central station. 
   

   Use of the same reference symbols in different figures indicates similar or identical items. 
   DETAILED DESCRIPTION 
   In accordance with an aspect of the invention, a sensor can employ traps that capture and hold target contaminants within the evanescent field surrounding a waveguide. The presence or number of particles of the target contaminants in the traps can be detected at anytime after capture by directing light of a desired wavelength through the waveguide and measuring light scattering from the contaminants. Such scattering can occur through many mechanisms including but not limited to linear and nonlinear resonance fluorescence or Raman scattering. The sensors can be made autonomous through integration of the waveguide, the traps, a lighting system, photosensors, and a communications interface into a single device. Further, sensors including the waveguides, the traps, photosensors, and communications interface can be integrated in or on a chip using nanotechnology. Accordingly, millions of inexpensive dust-sized nanosensors (each of which could have hundred or thousands of traps) can be dispersed within a gas and used to detect and map the concentration and/or distribution of the target contaminants. 
   Reference is now made in detail to specific embodiments, which illustrate the best mode presently contemplated by the inventors for practicing the invention. Alternative embodiments are also briefly described as applicable. 
     FIG. 1  shows a schematic illustration of a sensor  100  fabricated on a die  110 . In an exemplary embodiment of the invention, die  110  is a processed semiconductor chip, e.g., a silicon or GaAs chip, having an area of several square microns. On die  110 , sensor  100  includes a waveguide  120 , a collection of traps  130 , a lighting system  140 , photosensors  150 , and a communications interface  160 . 
   Waveguide  120  is fabricated in die  110  and has optical characteristics suitable for guiding light or other electromagnetic radiation having a wavelength that interacts with a target contaminant to be measured. In an exemplary embodiment of the invention, waveguide  120  is a channel of dielectric material such as silica, silicon dioxide, or lithium niobate that conventional processing techniques have deposited or grown on die  110 . Alternatively, waveguide  120  can be a line defect in a photonic crystal formed in die  110 . For a photonic crystal, holes or variations of the refractive index in die  110  can be formed in a pattern such that propagation of light having the desired wavelength is limited to a defect corresponding to waveguide  120 . Such defects are generally implemented as a variation in the pattern of the photonic crystal. 
   Traps  130  are designed to trap a target species or multiple target species of contaminant and are positioned to hold the target contaminant within a volume corresponding to an evanescent field of waveguide  120 . Accordingly, sensor  100  can sense contaminants in a gas because traps  130  confine the particles of the contaminant, which may otherwise be fast moving when free in the surrounding gas. Confinement of the particles may either be permanent or at least of sufficient duration for evanescent sensing. The holding of contaminants may be advantageous not only for sensing contaminants in a gas but also for sensing contaminants in liquid, particularly when fluorescence is sensed and the decay time for the contaminant to fluoresce is long. Traps  130  should further be such that traps  130  cause little or no scattering of the light from waveguide  120  when traps  130  are empty. In a typical application of sensor  100 , each trap  130  is a molecular trap that is designed to trap the same target molecule. Such molecular traps are well known in the art, and some specific examples of suitable traps are described further below. 
   Lighting system  140  directs light or other electromagnetic radiation for propagation through waveguide  120 . Lighting system  140  is preferably an active light source such as a laser or light emitting diode (LED) that is fabricated in and on die  110  and produces light having a wavelength that causes a target contaminant in the evanescent field of waveguide  120  to fluoresce or otherwise scatter the light. A power source (not shown), which may be, for example, a charged capacitor, an inductor or a receiver tuned to absorb power from an electromagnetic signal, or a photovoltaic cell can be provided on die  110  to power lighting system  140  and other circuit elements of sensor  100 . Alternatively, lighting system  140  can be a passive system that directs light from the surrounding ambient into waveguide  120 . The contaminants captured in traps  130  can then interact with the evanescent field that the light creates around waveguide  120 . 
   Photosensors  150  can be positioned along waveguide  120  or adjacent to traps  130  to detect light scattered or emitted by the contaminants captured in traps  130 . In an exemplary embodiment of the invention, photosensors  150  include two banks of photodiodes that are fabricated in die  110  adjacent to waveguide  120 . If desired, the surface of die  110  can be contoured to increase the efficiency with which photosensors  150  collect the light emitted or scattered from traps  130 , and baffles and/or optical filters (not shown) may be added to block stray light and to select a range of wavelengths for the light that photosensors  150  measure. Additional sensing circuitry such as a current-to-voltage current converter and/or an amplifier connected to photosensors  150  can be fabricated in die  110  and used to produce a measurement signal indicating the total intensity of light measured. In the exemplary embodiment of the invention, the measurement signal has an analog voltage or current level proportional to a total current output from the photodiodes in photosensors  150 . 
   Communications interface  160  receives the measurement signal from photosensors  150  and produces an output signal that can be externally received and interpreted. In an exemplary embodiment of the invention, communication circuit  160  includes a radio frequency (RF) transmitter or transceiver that broadcasts an output signal to a remote receiver (not shown). The output signal can simply be an RF signal having an analog intensity that is proportional to the output signal from photosensors  150 , so that a receiver can measure the intensity of the RF signal at the frequency used for signaling to determine the amount of contaminant captured in one or more nearby sensors  100 . Alternatively, any desired signaling protocol, including but not limited to digital signaling protocols, can convey measurement data from one or more sensors  100  to the central receiver. The central receiver can process the signals from the sensors  100  and estimate the contaminant concentration. 
   In another alternative embodiment, communications interface  160  implements an optical interface such as described in U.S. patent application Ser. No. 10/684,278, entitled “Photonic Interconnect System,” which is hereby incorporated by reference in its entirety. 
   Sensor  100  as described above uses traps  130  for capturing a target contaminant from a surrounding environment and for binding the captured contaminant within an evanescent field around waveguide  120 . In different embodiments of the invention, each captured particle of the target contaminant may be, for example, an atom, a molecule, a virus, or a microbe, and traps  130  generally have a chemistry or structure that is suitable and selected for capture of the target contaminant. Additionally, traps  130  must also firmly attach to the material of waveguide  120 , e.g., to silica, or to another material in sensor  100  adjacent to waveguide  120 . Particular molecular groups such as chlorine derivatives of silane, are well known to bind strongly to silicon dioxide and other materials. In an exemplary embodiment of the invention, waveguide  120  is formed from silicon dioxide that is uniformly coated with a molecular group of chlorine derivatives of silane, which in turn irreversibly binds traps  130  to waveguide  120 . 
   Some atomic species of environmental contaminants that often need to be monitored include toxins such as arsenic (As) or lead (Pb), fissionable materials such as uranium (U) or plutonium (Pu), and other radioactive materials such as certain isotopes of strontium (Sr). A range of “host-guest” chemistries have been developed for capture of either a specific type of atom or an atom from a specific chemical family such as the alkali metals or the rare earth metals. These host-guest chemistries often discriminate among various atomic or ionic species based on the diameter of the atom or ion. Molecule cages known as carcerands or hemicarcerands, for example, can trap an atom (or a small molecule) and permanently hold the trapped contaminant. In an embodiment of the invention that measures or detects contaminant atoms or small molecules, traps  130  can be implemented as carcerands and hemicarcerands creating a cage of the size required to trap a particle of the target contaminant. 
   Another type of trap  130  for atomic contaminants uses a chelating compound, such as the well-known bidentate molecule ethylenediamine or the hexadentate molecule EDTA (ethylenediaminetetraacetate), which can form complexes with a target atom. Such chelates can also be bound to waveguide  120  using a chlorosilane chemistry such as mentioned above. 
   Chemistries that have been developed to complex many of the environmental pathogens or chemical agent molecules can also be used for traps  130  in sensor  100 . For example, various bioactive pathogens attack particular molecular structures such as a protein or DNA in cells. For these pathogens, the specific protein or DNA strand may be used as “bait” trapping for the pathogen. Carcerands and other related systems have also been developed for capture of specific molecules (or a specific family of molecules) and could be used as traps  130  that bind a molecular species. 
   For a larger contaminant such as a virus, e.g. the polio or ebola virus, an antibody for the virus can be bound to waveguide  120  as traps  130  because in many cases the antibody contains a protein that binds specifically to the external coating of the virus particle. Alternatively, traps  130  could include a suitable protein from the antibody or any other type of molecule designed to recognize and bind to a particular virus or include a type of bait that resembles the lipid layer of a cell that attracts the virus. 
   Finally, sensor  100  could successfully detect microbes using entities such as anthrax spores or other bacterial agents attached to waveguide  120  as traps  130 . Alternatively, the cell wall or coatings of microbe contaminants can be recognized or bound using specific proteins or sugars. 
   In order to achieve a high level of certainty in detection, sensor  100  may to use several different types of traps  130  to recognize and bind the same contaminant on either the same or different waveguides  120 . With separate waveguides  120  using different traps  130 , a coincidence of separate measurement signals could provide confirmation of trapping of the target contaminant and minimize spurious signals or false positives. 
     FIG. 2  illustrates the typical behavior of the measurement signal for sensor  100  as a function of time. Ideally, traps  130  are protected from contaminants until a test start time (t=0) or are activated chemically or electromagnetically at the test start time. At a subsequent time T 1 , a first of the traps  130  captures a target contaminant at which point the scattered light measured by photosensors  150  jumps. At subsequent times T 2 , T 3 , T 4 , T 5 , and T 6 , other traps  130  capture additional particles of the target contaminant, causing discrete increases in the scattered light, until all of the traps  130  are filled. The jumps in the measurement signal identify when the target contaminant is detected. Further, the mean rate at which the measurement signal increases indicates the concentration of the target contaminant since higher contaminant concentrations will cause higher rates of capture. Accordingly, with appropriate calibration or analysis, the rate of signal increases or the capture rate can be used to measure the concentration of the target contaminant. 
     FIG. 3  illustrates a system  300  for using sensors  100  to measure the presence, the concentration, and/or the distribution of a target contaminant. For system  300 , a collection of sensors  100 , which are nanosensors that are sufficiently small to float in air, are released into an environment to be tested for a target contaminant. Sensors  100  may, for example, be released into a ventilation system in a building. The number of sensors  100  used will generally depend on the expected density of the target contaminant, the size of the environment tested, and how sensitive the environment is to the dust that discarded sensors  100  create when testing is complete. Traps in each sensor  100  distributed into the environment capture particles of the contaminant as sensors  100  move through the environment. 
   A central station  310  can be moved to any location where sensors  100  are present. In the illustrated embodiment, central station  310  includes a transmitter that transmits a signal to activate one or more sensors  100 . The transmitted activation signal may, for example, be a radio or microwave signal that a communications interface  160  receives and converts to power that operates lighting system  140  and other circuit elements in sensors  100 . If desired, the activation signal can be directional or limited in range so that only sensors  100  in a particular area are activated. 
   The activated sensors  100  transmit back measurement signals indicating the number of captured contaminants in the activated sensors  100 . Central station  310  can continue monitoring or polling of the measurement signals from sensors  100  over a period of time sufficient to determine rates of increase of the measurement signals and from the rates of increase determine the concentrations of the contaminant in the specific areas containing the activated sensors. Information concerning the presence, the concentration, and the distribution of the target contaminant or contaminants can thus be obtained. 
   Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. For example, although the above embodiments often describe use of light and light sources, it should be understood that embodiments of the invention are not limited to use of visible light but more generally can employ other wavelengths of electromagnetic radiation that provide an evanescent field suitable for detection of target contaminants. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.