S
huctures within airwaftjet turbines, nuclear reactors, and the human body are difficult to observe and analyze in situ. These hard-@ reach locations and hostile environments have motivated the development of a wide variety of industrial and medical endoscopes, optical aids for examining internal cavities that usualiy take the form of flexible or rigid tubes. In most cases, the heart of an endoscope is an imaging fiber made up of thousands of individual optical fibers melted and drawn together in a co. herent manner to carry and maintain an image from one end to the other. Although one picture can convey a thousand words, the visual information acquired from these endoscopesis only one part of a total analysis-chemical and physical characterizations are needed as well. Fiber-optic sensors can be used to monitor a wide range of physical and
Paul Pantano Davld R. Walt T u b University 0003-2700195/0367-481AlSW0010 (D 1995 American Cnemical Society
Optical imaging sewon can provide images and chemical informationfrom ha rd-to-reach locations and hostile environments chemical parameters, such as temperature, pressure, viscosity. humidity, and chemical concentrations. Remarkably, very few endoscopes can provide visual as well as chemical or physical data. R e c e n ~coherent , imaging fibers have been used to conshuct fiber-optic chemical sensors that can acquire both visual and chemical information and transmit it through the same imaging fiber. In this
Report, we will look at remote viewing instruments. review nromess . _ in the develop ment of imaging-fiber chemical sensors, and examine possible analytical uses in which the ability to combine visual and chemical information can be advantageous. Optical fiber configurations
An optical fiber is a filament made of lightguiding dielectric material that works by total internal reflection.It consists of an inner core of high r e W v e i n d e x material and an outer reaion of low refractiveindex material called the clad. This confguration enables light rays entering the core within a certain critical angle to be confined by total internal reflection at the cor-lad interface. An optical fiber's input and output behaviors are characterized by its numerical aperture, which depends on the differencein the miactive in. dices of the core and clad. The aperture defines the angle of the acceptancecone for light propagation along the fik. Optical fibers can be divided into various categories depending on the refracAnalyticai Chemistry, August I, 1995 481 A
tive index profile of the core and the number of modes the fiber can support. Single strands of transparent materials such as glass, plastic, or fused silica can propagate light over long distances with minimal attenuation. Because light undergoes thousands of reflections per meter as it proceeds down the length of the fiher, the position at which an individual ray exits the fiber will not correspond to the position at which it entered the fiber. As a re sult, a single optical fiber cannot be used to transmit an image (1-4). Individual optical fibers can be fused into a bundle. Both rigid and flexible fiberoptic bundles are available in many lengths, diametem, geomehies, and multifurcated combinations. Bundles can be housed in special jackets for use in unusual environments, and their relatively low cost makes them disposable. Fiber- optic bundles are commonly used to provide high-intensity, uniform illumination of W. visible, or IR radiation for equipment used in diagnos tic and surgical procedures such as headlight systems for surgeonsand illuminators for remote viewing instruments. In addition, laser delivety systems increasingly use fiber-opticbundles to carry high-output light energy to remote targets, for example to cure dental polymers and lixatives and for medical operative procedures (5). Most light-delivelybundles contain unaligned optical fibers and are used only to transmit energy. In contrast, imaging fibers are bundles in which the position of each optical fiber is the same at the input and output ends. In these bundles, each optical fiber acts like a pixel, and the size of the pixel determines the resolution of the transmitted image. A modem highresolution imaging fiber consists of 30W 100,000 individually clad optical fibers, each 3-10 pm in diameter Figure 1).(For information on fabricating optical fibers, see References 64.) One of the first demonstrations using a bundle of unclad fibers to transmit an image took place 40 years ago at Imperial College in London (9),and much of the pioneering work with clad imaging fibers is attributed to Will Hicks at American Micro-Optical.
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Remote viewing instruments
What makes fiber optics ideal for endos copy are the high numerical apertures that allow elevated levels of illumination kom
Figure I.A coherent fiber-optic bundle. (Len) An imaging fiber is compased of thousands of micrometer-sized optical fibers fused together in a single flexible-fiberformat. (Right) Image of a 350-pm-diameter imaging fiber with 6000 optical fibers. each with a diameter of 3-4 pm, taken by a CCD camera. Far-field viewing of 6-point text (“TUFTS) is made possible by a distal GRIN lens.
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an external light source as well as the flexibility of the bundles. The basic compe nents of a fiber-opticendoscope are a c e herent fiber for image transmission,anoncoherent fiber-optic bundle for illumination, a proxhnal objective, a proximal eyepiece andlor video camera, and a distal gradient index (GRIN)lens (2). GRIN lenses are short smalldiameter glass rods that can be mounted onto and optically aligned with the imaging fiber. The index of refraction within a GRIN lens is parabolic in that the light follows a sinusoidal path. The pitch (i.e., the length of rod required for one sinusoidal cycle) determines the imaging characteris tics of the lens so that a real image can be formed at the lens surface. Different
482 A Analytical Chemistry, August 1, 1995
GRIN lenses have different fractional pitches so that the distance for fwfield viewing can be preselected (e.g.+5-100 nun from the face of the lens). In addition to forward viewing, miniaturized mirrors and prisms can be coupled to the GRIN lens to allow observation from other perspectives. Endoscopes are classified by the optical apparatus used to provide the image and by the degree of flexibility. ?he most common rigid endoscopes are based on the Hopkins rod-lens system (10).These telescopic endoscopes do not use an imaging fiber; instead, light is transmitted by glass columns and rekacted through intervening air lenses. This system transmits images with the utmost resolution, clarity,
and brightness required for the most taxing of inspection needs. Although such endoscopes can be used to view regions through cavities as small as 1mm in diameter, their lack of flexibility limits their usefulness. Nonetheless, there are hundreds of a p plications in which rigid endoscopesare used, such as fuel lines, air ducts, sewer pipes, oil and gas lines, nuclear reactors, gun barrels, power station rotors, and jet and piston engines. As evidenced by this list, one of the major advantages of endoscopes is that they permit remote viewing of hostile environments up to 100 m irom the observer. They can be used under high and low temperatures and pressures, intense electric and magnetic fields, and in the presence of hazardous chemicals and ionizing radiation. Flexible endoscopes use the same dip tal lens and fiber-optic bundle illumination systems as rigid telescopic endoscopes. However, in flexible endoscopes image transmission is accomplished by a coherent imaging fiber or a distal charge-coupled device (CCD) (10).The resolution of flexible fiber-optic and electronicendoscopes depends on the size of the individual optical fibers or semiconductor pixels, respectively. In a modem flexible fiber-optic endoscope, up to 40.000 bundled fibers convey the image. As long as the fibers in the bundle are collimated, an image can be transmitted along the bundle regardless of twists and tums. Until recently, all flexible endoscopes used fiberaptic bundles for both illumination and image transmission. However, flexible endoscopes that use a CCD for image transmission have now been introduced. These electronic endoscopes have a miniature, high-resolution, color video camera and lens located at their distal tips. The choice between using a flexible fi. ber-optic or flexible electronic endoscope is based on its outer diameter, typically 0.7-8 mm for a fiher-optic endoscope and 6 1 6 mm for an electronic endoscope. In addition, the temperature of the sample's environment is a factor when using an electronic endoscope, because it affects the dark current of the distally sited CCD. In both cases, the ability to support a wide variety of surgical tools and manipulative instrumentshas enabled both
rigid and flexible endoscopes to be used in hundreds of different medical procedures. Imaging-fiber chemical sensors
Most fluorescence-based fiber-optic chemical sensors use an optical fiber as a light conduit to detect an analyte through a change in optical properties (e.g., intensity, wavelength, or lifetime). In one configuration, a light source is coupled to the proximal end of a fiber and excitation energy is propagated to the fiber's distal tip, where a fluorescentindicator is immobilized. The indicator-analyte interaction modulates the fluorescence intensity, and a portion of the isotropically emitted light retuming through the same fiber is directed to a suitable detector alter filtering out any residual excitation light (11).
A modified
jiber can detect and spatially resolve localized changes in the concentration of an analyte. Traditionally, fiber-optic chemical sensors have been fabricated using either a single fiber or a noncoherent fiber bundle. Bundles have the advantage of highlightgathering power and, as a result, the thickness of the sensing layer can be minimized to improve the temporal response of the sensor (3).Recently, we have used coherent imaging fibers to make fiberoptic chemical sensors. In one approach, imaging fibers are used to fabricate array sensors that can concurrently view a sample and detect a single analyte. In a second approach, sensors are made that have spatially discrete sensing sites for multianalyte determinations. Combined imaging and chemical sensing. Despite many innovations and developments in fiber-optic chemical sensors, fibers have not been used to new a sample and concurrently detect an ana-
lyte. Although chemical sensors using a single optical fiber or a noncoherent fiberoptic bundle have been applied to a wide variety of analytical determinations (12), they cannot be used for imaging. Similarly, imaging fibers and endoscopes in general are used only for their origindy intended purpose, image transmission. Combined imaging and chemical sensing requires that an analytesensitive indicator be immobilized in a thin layer on the distal surface of the fiber in such a way that it does not compromise the fiber's imaging capabilities. Polymeric methods of immobilization are especially suitable because a sufficiently thin (Le., < 5 pm), uniform, planar optically transparent sensing layer can be spin coated directly onto the distal surface of an imaging fiber, creating thousands of microsensors capable of simultaneously measuring chemical concentrations over tens of thousands of square micrometers. Because the imageanying capabilities of the fiber are preserved, the operator can position the sensor and correlate chemical measurements with visual information. A modified epifluorescence microscope has been used for both fluorescence measurements and imaging. The microscope is capable of making continuous ratiometric measurements using a CCD camera and computercontrolled filter wheels and shutters. By combining the distinct optical pathways of the imaging fiber with the spatial discrimination of a CCD, both visual and fluorescence measurements can be obtained with 4-pm spatial resolution (13)limited only by the size of the individual fibers comprising the bundle. When fluorescencemeasurements are taken, images are captured by the CCD camera with the filter wheels positioned at the dye's excitation and emission maxima During viewing, the sample is illuminated by an external source, and the images are caphlred by the same CCD camera witb the excitation shutter closed and the emission filter wheel moved to a neutral density filter. Alternatively, a beam splitter can be used with a second CCD camera so that simultaneousviewing and chemical sensing are possible. The iirst demonstration of this technique used a 350~-diameterimaging fiber coated with a pHsensitive or an acetylAnalytical Chemistry, August 1, 1995 483 A
is shown in the images in Figure 2. The excholine (ACh)-sensitive polymer layer. The pBsensitive polymer layer was immc- perimental setup shown in Figure 2a consists of an AChE-modified imaging fiber, a bilized by spin coating an N-fluorexeinylpicospritzer, and a pulled-glass capillary acrylamide/hydroxyethyl methacrylate polymer onto the distal face of an imaging tube filled with 1.0 mM ACh. The tip of the tube was submerged in buffer and posifiber. The ACh-sensitive layer was created by cc-immobilizing acetylcholinester- tioned normal to the distal surface of a vertically held AChE-modified imaging fiase (AChE) in a water-soluble functiondized prepolymer known as poly(acry1- ber. Figure 2b is an image of a glass pipette illuminated with white light, viewed amid~~~.acryloxysuccinimide) @'AN). through the AChE-modified imaging fiber The AChE-derivatizedPAN was spin and positioned withim the imaging fiber's coated onto an imaging fiber and allowed to react with fluorescein isothiocyanate field of view. Figures 2c-g show a sequence of flue (FITC) to incorporate the pH-sensitive rexence images from the AChE-modified dye into the polymer layer. AChE catalyzes the hydrolysis of ACh to choline and imaging fiber before and after a 2Wms pulse of ACh was delivered to the fiber. acetic acid; the dissociated protons from The ACh sensitivity of the AChE/FITC/ the enzyme-generated acetic acid react PAN layer is evidenced by the dark circuwith the immobilized F'ITC to provide a lar area that forms (as the FITC fluorespH-sensitive fluorescent signal proporcence is quenched by the enzymegenertional to the ACh concentration. These acetylcholinebiosensors have a detection ated acid) only in the region of the fiber where ACh was locally injected. This limit on the order of 35 pM and a redemonstrates that successivehuo-dimensponse time < 1s (13). sional profiles of a heterogeneous sample, The ability of a modified imaging fiber such as a single cell, can be acquired by a to detect and spatially resolve localized changes in the concentration of an analyte single modified-imagingfiber sensor.
:er
+
Detector
' I
Figure 2 Detecting and resolving localizedchanges in anaiyte concentration. (a) Pulled-glass capillary tube containing ACh, positioned normal to the distal surfaceof an AChE-modified imaging fiber Submerged in phosphate bufferat pH 7.0.(b) Pulled-glass capillary tube image acquired with a CCD camera viewed through a 350ym-diameter AChE-modified imaging fiber. (c) Fluorescence image at 530 nm taken with 490-nm excitation from an AChE-modified imaging fiber before and (d-g) after a 200-ms pulse of 1.O mM ACh was delivered onto the distal tip of the modified imaging fiber. All measurementswere acquired with a 50-ms CCD acquisition time at 200-msintervals. Red indicates high fluOreSCenCe
intensities. 484 A Anaiyficai Chemisfw, August 1, 1995
In this configuration, the thin analytesensitive polymer is coated directly on the distal face of the imaging fiber and objects are in focus only when they contact the face of the fiber. As described above, imaging fibers can also be fitted with a distally sited GRIN lens. In this configuration, the thin polymeric sensing layer can be coated directly onto the GRIN lens SUIface to permit k-field viewing of the sample and its environment, with the out-offocus fluorescence signal collected and correlated to analyte concentration (14). With this simple polymer-dye-enzyme chemistry, any commercially available endoscope can be modified to have both chemical sensing and imaging capabilities over small distances (without a GRIN lens) or over longer distances (with a distal GRIN lens). Multianalyte optical sensors. The ideal chemical sensor measures the in situ concentration of analyte continuously, reversibly, inexpensively, and rap idly. In most situations, it is importantto measure several parameters simultaneously. One of the benefits of using an imaging fiber to make a multianalyte fiberoptic chemical sensor is that the separate optical pathways of the imaging fiber can he used to carry signals simultaneously from multiple indicators immobilized at different locations on the fiber tip. We have developed a quick photoinitiated polymerization reaction whereby discrete sensing regions can be immobilized in precise locations on the distal face of a single optical imaging fiber (15,16). Photodeposition of an indicator-polymer matrix is performed using a UV source (e.g., a mercuy-xenon lamp). Light with the appropriate wavelength is directed through a pinhole and focused onto the proximal face of an imaging fiber using a microscope objective. The light travels through the imaging fiber to the distal face, which is submerged in a polymerization solution (Le.. monomer, initiator, and indicator). A small area (radius of 6-25 pm) is illuminated for a predetermined time during which polymer growth occurs only at the illuminated region on the distal face (Figure 3, top). After illumination. the residual monomer is removed by rinsing and the initiation light is focused onto a different region of the imaging fiber. The process may be repeated, or a
different dye-polymer combination can be used. Many distinct sensing regions can be photodewsited onto a single imaging fiber, with the size of the pinhole and the o b jective’s magnification controlling the size of each sensing region. Using a CCD, the fluorescence from each of the different sensing regions immobilized on the imaging fiber can be spatially resolved (Figure 3, bottom). The creation of spadiscrete sensing sites on a single optical sensor solves many of the prob lems associated with designing multiana. lyte optical sensors, such as spectral overlap of multiple indicators and the need to use individual optical fibers for each analyte. In addition, when these discrete sensing regions are immobilized around the periphery of the imaging fiber‘s distal face, the bare central region can be used to view the sample and its environment as described above. Simultaneous measurements are especially important in a number of clinical, biological, and environmental applications, especially when the dynamics of different analytes are closely interrelated (e.g., pH, pC0,. and pOJ. A multianalyte imaging fiber sensor that allows simultaneouspH, COS,and 0, sensing has recently been fabricated in our laboratory (17). The pHsensitive polymer matrix is created by covalently immobGng a pH indicator (acryloyl fluorescein) in a polymer hydrogel. The fluorescence intensity of the pH indicator is enhanced by deprotonation. The CO, measurement is made according to the principles of a Severinghaus eleftrode, in which a pH sensor (also acryloyl fluorescein in a polymer hydre gel) is used to detect H,CO, production from the reaction of CO, and H,O after the COStraverses a gas-permeable membrane (18).A pH-sensitive polymer matrix is overcoated with a gas-permeable photopolymerizable siloxane membrane that blocks the transport of hydrogen ions across the membrane but allows CO, to cross and react with water to form H,CO,. This reaction increases the acidity in the microenvironment of the polymer and thus decreases the fluorescence intensity of the pH indicator proportional to the COS concentration. Finally. the O+msitive polymer matrix is deposited by entrappinga transition
‘mused initiation light
I
Imaging fiber
t i a
mdivated area
I
Functionalized distal face of fiber
Capillary tube
\
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e. e Flgure 3. ImmobllWng sensing regions on an optlcal fIbe#. (Top) Setup of the photopolymerizationprocedure used to fabricate multicomponent fiber-optic chemical sensors. (Bottom) Fiuorescanm images 01 13 pH-sensitive matrices immobilized on the distal lace of a 500-ym-diameterimaging fiber in pH 8.0 (left) and pH 7.0 phosphate buffer (iight).All polymer-dye matrices aDDear as distinct fluorescentregions and respond to pH changes in a similar lasnion.
metal complex, such as Rui.€’h,phen)$+, in a photopolymerizable siloxane membrane (19).This transductionmechanism is based upon 0, collisional quenching of the ruthenium complex fluorescence. When the distinct optical pathways of the modified imaging fiber are combined with the spatial discrimination of a CCD, the fluorescenceintensity of each region can be monitored simultaneously, as shown in Figure 4. CO,, O,,and pH sensors can be converted into biosensors by coupling indica tor dyes with enzyme reactions in which transducing species such as protons, CO,, or oxygen are liberated or consumed (20).However, the transducing species must he measured independently or kept constant. To account for nonenzymatic fluctuations of pH, CO,, and 0, in the sample matrix,these biosensors typically use a second sensor as a reference. The site-selective photopolymerization method allows separate enzymecontainingmahi ces and pH, CO,, and 0, sensitive matrices to be immobilized in proximity to each other on the distal face of a single imaging fiber for simultaneous and independent measurements of each entity.
We recently described the fabrication and performance of a biosensor that per, mits the simultaneous measurement of penicillin and pH (21).This biosensor is based on the enzyme penicillinase and was fabricated by siteselective photodeposition of penicillin- and pH-sensitive polymer matrices on a 35OWiameter imaging fiber. The penicillin-sensitive matrices were fabricated by entrapping the lyophilized enzyme as micrometer-sized particles in a polymer hydrogel containing the covalently bound pH indicator acryloyl fluorescein. Dissociated protons from enzyme-generated penicilloic acid decrease the fluorescenceintensity of the pH indicator in proportion to the penicillin concentration. This imaging fiber sensor detects penicillin in the range 0.25-10.0 mM (m the pH range 6.2-7.5) with a penicillin detection limit of 1M)pM. The ability to monitor the pH of the medium in conjunction with the penicillin concentration should allow this sensor to monitor penicillin production in complex sample matrices such as fermentation broths, where dramatic pH changes can occur during the fermentation process (22). Analytical Chemistv, August I, 1995 48!! A
figura 4.-bimuitanaous Iiuonrcence maasummentsoi O,, CO,, and pH USing a 350-wm-diametermuitianalyte imaging fiber sensor. All measurements were made in 0 1 M phosphate buners (pH 5 0 or 7 2) that were air saturated (003% COJ21% OJ, oxygen saturated (100%4) or. bubbled with a 15% COJ05% N, gas mixture The sensor was Dlaced in the SDeoIied buner lor 60 5 and fluorescencemanes were
taken every 10 s
The same differential measurement strategy has been applied to oxygendependent biosensors. Members of our group have fabricated a fiber-optic sensor for the continuous and simultaneous determination of both glucose and oxygen with separate glucose and oxygen sensing sites photoimmobilized in distinct positions on the distal faceof a 3 5 G d i a m e t e r imaging fiber. The oxygen-sensingsite is composed of an oxygen-sensitive ruthenium complex entrapped in a hydrophobic gas permeable copolymer. The glucosesensing site is made by coating an oxygen-sensing site with a second polymeric layer containing glucose oxidase. Glucose is quantitated via the enzymatic consumption of oxygen with a response time of 30 s. This fiberoptic biosensor should be ideal for in situ glucose measurements where oxygen tensions fluctuate. Applications Combining viewing and chemical sensing with imaging fiber chemical sensors offersa significant advance in studying fundamental properties on a cellular level and should open the door for a variety of sci4ES A Ana/ytica/ Chemistw, August 1, 1995
entificaUy important experimentsand clinical measurements. Preliminary work in our laboratory to image biological samples with planar, polymer-modified imaging fibers has included sea urchin eggs (SO-lW pm diameter) (14)and mouse fibroblast cells (10-20 vm diameter) (13). Potential applications include measuring cell surface chemistries to correlate positional and morphological information with chemical distributions (e.g., monitoring in situ neurochemical dynamics). Modified imaging fibers may allow chemical concentrationsat s h e s to be measured on scales smaller than a single cell and ultimately may provide information such as cellular surface concenhation gradients. In some cases, extracellular measurements may provide an alternative to intracellular measurements when changes in inhacellularchemical concentrations appear as complementary extracellular changes via membrane transport pumps. The physical flexibility of imaging fibers may also allow them to be used for diagnosis in which cell-bycell histological examination can be combined with surface chemical determinations.
Other combinations of imaging and spectroscopic determinations could be used for diagnosis. Feld showed that near-IR Raman spectra can be used to differentiate diseased and normal regions in a human aorta (23)and McCreery used a fiber-optic probe to obtain near-IR Raman spectra from breast tissue in a clinical setting (24).The spectral or chemical sensing and morphological examination of cellular-scale features could be acquired concurrently via optical imaging fibers with the ultimate goal of creating a system for rapid, minimally invasive diagnostic techniques, eliminating the need to take a specimen from the patient by biopsy. The union of fluorescenceimaging and endoscopy also shows great promise for the diagnosisof medical conditions. The localization and early identification of diseased tissue can be accomplished either by tissue antofluorescence or by the characteristic fluorescencesignature o b tained when an administered porphyrin deriMtive is seleaively retained by ma. lignant tissue. Such information Can be used to direct laser irradiation or surgical instruments to only those tissue regions requiring treatment Lam has exploited the difference in autofluorescencebetween diseased and normal tissue to develop an imaging system based on a CCD camera attached to the eyepiece of a conventional flexible fiber-optic endoscope (25).The endoscope's noncoherent fiber bundle delivers 442 nm exdtation, and the resulting tissue autofluorescenceis collected through the coherent imaging fiber. A computercontrolled filter wheel permits the sequential acquisition of green (480-520nm) and red (> 630 nm) fluorescenceimages at 115ms intervals. Preliminary results have shown s i i c a n t improvement over conventional whitelight endoscopy for detecting dysplasia and carcinoma in lung lesions. These conditions are d%cnlt to detect in situ because the lesions are only a few layers thick and a few millimeters wide. Recently, a multispectral fluorescence imaging system was reported by A n d m son-Engelsin which fluorescenceimages are acquired simultaneously from three different spectral regions and displayed on a CCD camera (26).Fluorescenceimages from tumors on the hind legs and in
the brains of rats injected with a porphyrin derivative were clearly improved by multicolor processing. Work is in progress to adapt the system for transmitting fluorexence images through an imaging fiber endoscope. Improvements in the specificity of autofluorescence measurements have been made by Feld, who developed a diag nostic algorithm based on excitationemission matrices (27).Using an in vitro model, fluorescence emission spectra from 370 nm excitation differentiates h e tween normal and diseased colon tissues in 96%of 26 cases tested. Similar results were also obtained when the emission spectra were collected in real time from patients by using an optical fiber introduced through an endoscopic channel (28).
Combining viewing and chemical sensing offersa sign@cant aduance. In all of these applications, the advantages of delivering and collecting light from the endoscope's coherent imaging fiber are numerous and significant. Reducing the number of operative channels means that endoscopes will have a smaller diameter, thereby decreasing the sue of the incision or puncture needed andfor increasing the number of the internal regions that can he accessed by endoscopic examination.
Press: Oxford, 1987; pp. 638-54. (4) Seitz, W. R CRC Cnt. Rev. Anal. Chem. 1988,19.13>73. (5) Anderson-Engels,S.; Johansson. 1.;Svmberg. S.; Svanberg,K Anal. Chem. 1989, 61, 1367 A-1373 A and 1990,62,19A27 A (6) Tsumanuma,T., et al. SPIEPrm. 1988,
. .
References (1) Wobeis, 0. S. In MolecularLuminescence Spectrosco&: Methods and Applications; Schulman, S . G., Ed.; John Wiley and Sons: NewYork, 1988: pp. 129-281. (2) Hopkins, H. H. In Endoscopx Berci, G., Ed.; Appleton-CentutyCrofts:New York, 1976; pp. 27-63. (3) Schulb,J. S. In Biosenson: Fundamentals
and Applications: Turner, AP.F.; Karube,
e
erative'Manual afEndoscopic Surgery;' Cuschieri, A; Buess, G.; Perissat,I.. Eds.;
Springer-Verlag Berlin, 1992; pp. 14-36. (11) Arnold, M. A. Anal Chem. 1992, 64, 1015A-25A (12) Biosenson with Fiberoptics;Wise, D. L.;
Wingard. L B., Jr., Ed.; Humana: Clion, NJ, 1991. (13) Walt, D. R.; Bronk, K. S.; Pantano. P.; Michael, K L SPIE Proc. 1995,2388, 554-57. (14) Walt, D. R; Bronk, K S. U.S. Patent 5 298 741,1994. (15) Bamard, S. M.; Walt, D. R Nature 1991, 353,338-40. (16) Bronk, K. S.; Walt, D. R. Anal. Chem. 1994,66,35IWO. (17) Healey, B. G.; Walt, D. R. SPIE Proc. 1995,2388,519-23. (18) Severinghaus,J. W.; Bradley, A F.]. Appl. Physiol. 1958,13,51>20. (19) Bacon, 1. R.; Demas. J. N. Anal. Chem. l987,59,278M. (20) Arnold, M. A; MeyerhofZM. E. CRC Cnt. Rev. Anal. Chem. 1988,ZO, 149-96. (21) Healey, B. G.; Walt, D. R. SPIE Pruc. 1995,2388, 568-73.
Hamamatsu Hollow Cathode Lamps am now available from maim lab suppliers.
Hamamatsu single and multi(22) Agayn, V. 1.;Walt, D. R Biw'Technolugv 1993,11,72629. element Hollow Cathode Lamps (23) Rava, R P.; Baraga, J. J.; Feld. M. S. Specoffer superior stability, spectral trochinz.Acta 1991,47A, 509-12. purityandoutput intensity, even for (24) Frank,C.J.;Redd.D.C.B.:Gansler.T.S.; such elements as afsenic and McCreety, R L Anal. Chem. 1994,615, ?lW?fi _._ ._
(25)
Lam,S., et a1.J l7tomc. Cardiouasc. Surg.
iss3,io5,in3~1 (26) Andersson-Engels,S.; Johansson.I.; Svmberg, S. Appl. Opt. 1994,33,8022-29. (27) Feld, M. S., et a]. Photuchem. Photobiol. 1991,53,77746. (28) Feld, M. S., et al. Gasfrointest.Endosc. 1990,36,10>11.
We gratefully acknowledge the National Instibtes of Health for Bnancial support, and our co- Paul Pantano is a postdoctoral scientist workers, especially Brian G. Healey, Lin Li, whose research interests include developand Suneet Chadha,for their scientific contribu- ment ofenzyme-modified microelectrodes tions.
b
I.: Wilson, G. S., Eds.; Oxford University
selenium. They are compatible with most commercial spectrophotometers, including Beckman, Z e i s and Perkin-Elmer. And best of all. they're available from your local lab supplier.
For Application Information, Call 1-800-524-0504 Ext. 100
andfiberaptic chemical sensorsfor biochemical analyses. David R. Walt is professor and chair of the Department ofChemistry, His research interests include optical senson, biomaterials, neurochemistry, and microfubricafion techniques. Address come spondence to Walt a t T u b Univenity, Max Tishler Laboratory for Organic Chemistv, Department ofchemistvy, 62 Talbot Auenue, Me4ford. MA 02155. alyfical Chemistry, Augusf 1, 1995 487 A
