IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM …nanophotonics.ece.umd.edu/publications_files/JSTQE...2 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS as a function of the surrounding

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 1

High Specificity Binding of Lectins toCarbohydrate-Functionalized Fiber Bragg Gratings:

A New Model for Biosensing ApplicationsGeunmin Ryu, Member, IEEE, Mario Dagenais, Senior Member, IEEE, Matthew T. Hurley, and Philip DeShong

Abstract—The functionalization of an etched fiber Bragg gratingwas realized using a carbohydrate-siloxane conjugate. No fluores-cent probes were used. Concanavalin A bound with high specificityto the glucose biosensor, but not to the lactose functionalized fiber.Conversely, peanut agglutinin bound to the lactose sensor with highspecificity over its glucose counterpart. Quasi-monolayer selectivebinding of the lectins to the fiber sensor was inferred based on atheoretical analysis of the observed changes in the refractive index.Our results open the way to the use of unlabeled carbohydrate-based sensors for the study of the human glycome.

Index Terms—Biosensor, Concanavalin A (ConA), carbohy-drates, evanescent field, fiber Bragg grating (FBG), monolayerdetection, peanut agglutinin (PNA), surface functionalization.

I. INTRODUCTION

I T IS NOW appreciated that the pathogenesis of severalwidespread and chronic diseases can be attributed to a change

in the glycome, in the absence of obvious changes in the genomeor proteome [1]. The human glycome might have several millioncarbohydrate structures. Several major diseases are associatedwith a change in the glycosylation pattern of a central proteinstructure. Cells and many proteins in nature are covered witha dense array of covalently bound sugar chains. Our ability tounderstand the factors that regulate normal glycosylation of pro-teins and lipids, resulting in normal structures, and those thatlead to disruption of normal sugar attachments will help us un-derstand diseases processes and control. Most of the diseasesthat affect mankind, including cancer, diabetes, heart diseases,infectious diseases, flu, Alzheimer, and rheumatoid arthritis,directly involve glycoconjugates. The science of glycobiologywill have a significant impact on our ability to understand howto stay healthy or how to manage diseases.

Manuscript received July 1, 2009; revised August 25, 2009. This work wassupported by the National Science Foundation Materials Research Science andEngineering Centers under Grant DMR 05-20471 for the shared experimen-tal facilities support for the X-ray Photoelectron Spectrometer. The work ofP. DeShong was supported by the National Science Foundation Nanoscale In-terdisciplinary Research Teams under Grant CHE 0511219478, the MarylandTechnology Development Corporation, and in part by the SD Nanosciences,Inc. The work of M. T. Hurley was supported by the Graduate Assistance inAreas of National Need Fellowship.

G. Ryu and M. Dagenais are with the Department of Electrical and ComputerEngineering, University of Maryland, College Park, MD 20742 USA (e-mail:[email protected]; [email protected]).

M. T. Hurley and P. DeShong are with the Department of Chemistry andBiochemistry, University of Maryland, College Park, MD 20742 USA (e-mail:[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2009.2032427

In this paper, we describe a fiber Bragg grating (FBG) sensorwith a sensitivity to change of the index of refraction as smallas 2–3 × 10−5 when the index of refraction of the surroundinganalyte is about 1.35. This is sufficient to detect the selectivebinding of a monolayer or less of proteins to a particular car-bohydrate. This sensor does not require the use of a fluorescentlabel. Our results demonstrate the potential of glycobiology forthe selective binding of proteins to carbohydrates. In particular,we demonstrate the binding of Concanavalin A (Con A) proteinto a monolayer of glucose covalently attached to the surfaceof a fiber. We also demonstrate that Con A does not bind to amonolayer of lactose covalently bound to the glass fiber. In a dif-ferent set of experiments, we demonstrate the selective bindingof the peanut agglutinin (PNA) protein to a monolayer lactosefilm but not to a monolayer of glucose on the surface of thesensor. This demonstrates the high selectivity of using a partic-ular sugar for detecting a particular protein and may ultimatelyhelp us understand how diseases evolve or can be controlled.Reviews of recent results on carbohydrate biosensors can befound in [2]–[5]. We have successfully modeled the attachmentof molecules to the sensor and can confidently predict whenapproximate monolayer coverage is obtained based on the in-dex of refraction and the dimension of the dry molecule. Themodel that we used is described in this paper. Strong tempera-ture dependence for the attachment of the proteins on our sensoris observed and can be understood by the fact that the proteincan denature at high temperatures and is less active at low tem-peratures. Our sensor is based on a Bragg grating written in aphotorefractive fiber. This fiber is etched down to a diameterof about 5 µm. The fiber is then functionalized with monolayerof carbohydrates by exposing the silica fiber to carbohydrateconjugates containing a terminal siloxane moiety that is used tocovalently anchor the conjugates to the surface of the fiber. Thefiber is then immersed in an analyte solution containing the pro-tein of interest. The evanescent wave in the fiber sensor sensesthe change of the index of refraction following the binding ofprotein molecules to the surface of the sensor. This change ofindex of refraction leads to a change of the Bragg wavelength,which is detected. Furthermore, we demonstrate very high se-lectivity by successfully preventing nonselective attachment ofmolecules to the sensor.

II. PREVIOUS WORK

Recently, we presented a simple theory to describe the shiftof the FBG resonance as a function of the fiber diameter and

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2 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS

as a function of the surrounding index of refraction [6], [7]. Aninstructive graphical solution was described. A good agreementfor the predicted shift of the FBG wavelength was observed be-tween theory and experiment as the fiber diameter and claddingindex of refraction were varied. A surrounding index sensitiv-ity as small as 7.2 × 10−7 was demonstrated [8], limited by awavelength resolution of .001 nm of our instrumentation. Thisindex sensitivity drops, as the probed analyte has an index ofrefraction closer to 1.35 rather than close to the index of max-imum sensitivity (n = 1.45). The change of wavelength of anetched core FBG sensor can be written as

∆λ = S∆n (1)

where S is defined as the sensitivity of the sensor and is measuredas the change in wavelength per unit change of index (RIU) ofthe surrounding medium and ∆n is the change of the index ofthe surrounding medium. The sensitivity S for the FBG sensoretched to a core size of 5 µm is 30 nm/RIU when the indexof the surrounding medium is close to that of water (1.325).The minimum refractive index change that can be measured bythe sensor is given by ∆λmin/S, where ∆λmin is the smallestwavelength change that can be measured. Therefore, in orderto increase the resolution of the measured index change, it isimportant to increase the sensitivity of the sensor and decreasethe minimum wavelength shift that can be measured in the sys-tem. For chemical and biological sensors, it is also importantto understand the minimum thickness of the adsorbed layer thatcan be measured by the sensor. The surface of the fiber is func-tionalized with the chemical or biological agent being detected.The index of the adsorbed layer is different from that of the sur-rounding medium (nmed ), causing a perturbation in the claddingindex and a shift in the wavelength. The index change that iscaused by the adsorbed layer is given by ∆n = neff − nmed ,where neff is the effective index of the fiber mode in the pres-ence of both the adsorbed layer and the surrounding analytesolution. Knowing the minimum ∆n that can be resolved bythe sensor, the minimum adsorbed thickness can be calculatedfor a given index of the adsorbed layer [7]. From equations (1)and from a beam propagation simulation, it is clear that it isimportant to increase the wavelength resolution and the sensi-tivity of the sensor in order to measure thin layers of adsorbedlayers.

Using this fiber sensor, we have previously demonstrated thatDNA hybridization can be measured [9]. The surface of theetched fiber was functionalized by attaching a 20 nucleotidesingle strand probe DNA (probe) to its surface and then sur-rounding the fiber with the matching 20 oligomer single strandtarget DNA (target). Recently, we have demonstrated the at-tachment of a glucose derivative on the fiber [10]. These exper-iments are a first step toward the development of carbohydrate-functionalized fibers as biosensors for the detection of lectins,carbohydrate binding proteins. A variety of biomolecules havebeen previously attached to silica and related surfaces to pro-vide recognition regimes for biochemical substances, includingsingle-stranded DNA, antibodies, enzymes, proteins, and cells.

Fig. 1. (a) Diagram of the JDSU fiber housing and positions of the etchantduring primary and secondary etches. (b) Fiber diameter profile after secondaryetch.

Until now, however, the functionalization of silica with carbohy-drates has received scant attention even though many cell–cellrecognition events are mediated by carbohydrate–protein mul-tidentate interactions [1].

Our fiber optics sensor [6]–[8] uses a commercially available(JDSU) single mode photosensitive fiber in which two FBGs,with a Bragg wavelength of 1533 and 1563.8 nm, are inscribedin the fiber core. The gratings are about 5 mm long and have apeak reflectivity of about 30 dB and extremely well-suppressedsidelobes. For the present experiment, we use only the λ = 1563nm grating. The sensor is temperature compensated. The etchedFBG sensor was chemically etched in a two-step process: firstto a diameter of 50 µm and then to a diameter of 5 µm (seeFig. 1). Fiber etching was conducted in 7:1 buffered oxide etchwith surfactant from J. T. Baker. Based on the MSDS data sheet,the solution is made up of 0.5–10% hydrogen fluoride, 40–70%water, 30–50% ammonium fluoride, and 0.5–10% surfactant.The low concentration of hydrogen fluoride and the additionof surfactant reduce the silica etch rate and enhance the sur-face smoothness, as compared to etch processes utilizing higherhydrogen fluoride concentrations.

For the biosensing experiment, we used a 5 µm diameteretched FBG sensor. We used the broadband amplified sponta-neous emission spectrum of an erbium-doped fiber amplifier asthe broadband source to probe the fiber Bragg reflection spec-trum and an optical spectrum analyzer to acquire the reflectionfrom the FBG. The experimental setup is shown in Fig. 2. Atypical reflection spectrum of an etched core FBG is shown inFig. 3. As observed, there exist several reflectivity peaks andminima. We use the minimum on the long wavelength side clos-est to the reflectivity peak to monitor the wavelength shift. Thisminimum is also the sharpest feature of the spectrum [11]. Thefeature is very reproducible and shifts by the same amount asthe peak wavelength. By using a least square fit, the wavelengthshift can be resolved down to 1 pm.

RYU et al.: HIGH SPECIFICITY BINDING OF LECTINS TO CARBOHYDRATE-FUNCTIONALIZED FIBER BRAGG GRATINGS 3

Fig. 2. Biosensing experimental setup.

Fig. 3. Spectrum of the etched core FBG sensor. The feature in the reflectionspectrum that is being monitored is also shvown.

III. DESCRIPTION OF CHEMICALS USED

A. HBS with Blocker Buffer

For the binding experiment, we used HEPES buffered saline(HBS) mixed with Tween-20 and bovine serum albumin (BSA)as the buffer solution. BSA works as a blocker because it isa protein that interacts nonspecifically to compete with othernonspecific interactions. Tween-20, on the other hand, is a de-tergent and is meant to prevent or break up nonspecific binding.Both chemicals were purchased from Sigma-Aldrich. HBS withblocker buffer is made with 100 mL of HBS buffer, 0.1 mL ofTween-20, and 5 g of BSA. To make 100 mL of HBS buffer,we first mix 2 ml of 0.5M HEPES, 1 mL of 0.1M CaCl2, and15 mL of 1.0 M NaCl solution. Second, we test the pH of thebuffer mixture that we made in the first step and then adjust thepH to 7.4 by adding the 0.2M NaOH solution. After we havemade the HBS buffer with a pH of 7.4, we add 1 mL of 0.1MMnCl2 and enough Millipore water to make 100 mL.

B. Sodium Ethoxide (NaOEt)

Sodium ethoxide is prepared by adding ∼1 g of freshly cutsodium metal to 25 mL of anhydrous ethanol under a nitrogenatmosphere at room temperature, followed by stirring at roomtemperature for 1 h, or until the solid metal has reacted com-pletely to provide a clear, colorless solution. The solution canbe stored under nitrogen atmosphere at room temperature for upto 7 days.

C. Synthesis of Glucose-Siloxane Conjugate 1

We have recently developed a general strategy for the prepa-ration of oligosaccharide conjugates that serve as the basis of themeans of attaching carbohydrate derivatives to silica surfaces.Although originally developed for the synthesis of complex gly-copeptide derivatives, the methodology has proven to be viablefor the synthesis of a wide variety of glycoconjugates, suchas glycolipids and glycosylated polyethylene glycocol (PEG)derivatives. For this experiment, glycosylamide derivatives 1(glucosyl) and 2 (lactosyl) were prepared and used to func-tionalize fibers using sol–gel methodologies as summarized inFigs. 4 and 5, respectively.

D. Synthesis of Lactose-Siloxane Conjugate 2

A similar approach was used to prepare covalently boundlactose to the fiber. In this instance, peracetylated lactopyranosylazide was converted into its siloxane conjugate(2) as outlined inFigs. 5 and 2 was used to functionalize etched fiber in a fashionanalogous to the glucose derivative shown in Fig. 4.

E. Binding of Lectins ConA and PNA to Glucose- and Lactose-Functionalized Fibers

For the binding studies using carbohydrate-functionalizedfibers, we used ConA and PNA. Both proteins were purchasedfrom Sigma–Aldrich. Proteins were taken from the freezer andwarmed to room temperature before they were mixed with theHBS buffer. After mixing the proteins with the HBS buffer, weleft them overnight at room temperature. On the following day,we filtered the protein solutions using a 0.2 µm disposable filterand measured the concentration. The concentrations of the pro-tein solutions were determined by using UV absorbance at 280nm (A280 = 1.37[mg/ml ConA], A280 = 0.96[mg/mL PNA]).The protein solutions were then diluted to 1 µM (based onConA tetramer of molar mass 104 kDa and PNA tetramer ofmolar mass 110 kDa).

IV. FUNCTIONALIZATION OF FIBER WITH GLUCOSE- AND

LACTOSE-SILOXANES

Glucose-functionalized fibers were prepared using the chem-istry summarized in Fig. 4. First a water reference was estab-lished by soaking the sensor in water. Then, a first reference toethanol was established by soaking the sensor in ethanol. Thisstep is required since the peracetylated glucose and lactose con-jugates 1 and 2, respectively, need to be dissolved in ethanol.A freshly etched fiber was treated with a 10 mM solution ofglucose-siloxane 1 dissolved in ethanol to yield a fiber whosesurface has been modified by the siloxane exchange reactionand results in the attachment of the glucose ligand. At the endof the experiment, an ethanol reference is measured and a waterreference is taken. Then, the sensor is put back in ethanol beforeimmersing it in a sodium ethoxide (NaOEt) solution at 50 ◦C for1.5 h to remove the acetate groups. Finally, an ethanol referencemeasurement and a water reference measurement are taken.

The attachment of the glucose and lactone-siloxanes to thefiber was monitored by measuring the wavelength shift of the


Fig. 4. Synthesis of glucose-siloxane conjugate and functionalization of a silica surface.

Fig. 5. Synthesis of lactose-siloxane conjugate 2.

Fig. 6. Time-dependent attachment of glucose-siloxane 1 to the fiber and finalshift with respect to water (average of five measurements). The error on eachmeasurement is given by ±2.1 pm and, for clarity, is only shown for t = 60 min.

fiber. The attachment of the glucose proceeds readily as shown inFig. 6. These results represent the average of five attachment ex-periments. The standard deviation σ on each point was extractedto be 2.1 pm. As can be seen, we first observe a decrease in theBragg grating wavelength that we attribute to the formation of alower index layer, a water layer, on the fiber, which is followedby the attachment of the glucose-siloxane layer, a higher indexmaterial. We first encountered this situation in our previous stud-ies of the fiber silanization process using APTES and APMDSand it was described in details in a previous publication [12]. Inthe present studies, after the initial shift downward, we observe agrowing shift of the Bragg wavelength as the attachment of glu-cose on the fiber progresses. This shift was measured when thesample was immersed in ethanol and was referenced to water.Most of the shift is due to the difference of index of refractionbetween ethanol and water. But an unmistakable measurableshift of the Bragg wavelength is observed, which is attributed tothe glucose attachment. We see that the attachment progressesmore rapidly at the beginning and then slows down. At the endof the attachment experiment, the sensor is first rinsed in ethanoland then in water. The inset in Fig. 6 shows the shift of the FBG

peak before and after glucose attachment with respect to water.This shift is in agreement with the shift observed following theformation of the lower index layer on the fiber. The dashed linein Fig. 6 is an extrapolation to t = 0 of the glucose attachmentprocess. A final shift of 24 pm (as shown in the inset of Fig. 6)is measured for the case of the glucosyl acetate 2, which cor-responds to a change of the surrounding index of 5.6 × 10−4 .If we assume that a solid glucose layer is formed on the fiberof index 1.543 [13], a beam propagation simulation, previouslydiscussed in [7] and [8], suggests that a monolayer of thicknessabout 1.3 nm is formed, which corresponds to a monolayer ofglucose-siloxane conjugate.

When the acetate groups are removed from glucose using a0.2 M solution of NaOEt, a glucose shift with respect to waterof 26 pm was registered. This shift is very similar to the shiftthat was measured for the acetylated glucose on the fiber andis compatible with our error bar of 2 pm. This indicates thatonly a small fraction of the acetate groups has been removed.We have previously reported on the time dependence of theattachment of APTES and APMDS on a fiber [12] and havedemonstrated the power of our technique for being able to reachmonolayer attachment in a controlled way, as compared to othertechniques that do not allow in situ real-time measurement ofthe functionalization process [14]. Here, we conclude that amonolayer of the glucose-siloxane conjugate is attached to thefiber by covalent bonding.

Lactose was attached covalently to the fiber using a lactose-siloxane conjugate 2 as shown in Fig. 5. As with glucose, thelactose is diluted in ethanol. As for the glucose case, the lactoseattachment first leads to a negative shift of the FBG wavelengthdue to the formation of a lower index of refraction layer on thefiber. Next, the Bragg wavelength shifts positively, reflectingthe attachment of lactose to the fiber. After extrapolating theBragg grating attachment shift to t = 0 (dashed line in Fig. 7),we evaluate a total shift of 22 pm to represent the attachmentof lactose to the fiber. A final shift of 23 pm (inset of Fig. 7)is measured by taking the difference between the initial (beforeattachment of lactose) and final (after the attachment of lac-tose) Bragg wavelength referred to water, in good agreementwith the time dependent shift measured between the end of themeasurement and the extrapolated shift to t = 0. This shift isabout the same as the one observed describing the attachmentof glucose to the fiber. This indicates that a lower density oflactose molecule attaches to the fiber as compared to the bulkdensity of lactose. We observe a negligible shift (< 2 pm) ofthe Bragg wavelength after the removal of the acetate groups,


Fig. 7. Time-dependent attachment of lactose-siloxane 2 to the fiber and finalshift with respect to water.

indicating that a small fraction of the accetate groups has beenremoved.

V. BINDING OF CON A AND PNA TO GLUCOSE- OR

LACTOSE-FUNCTIONALIZED FIBERS

Lectins, carbohydrate-binding proteins, Con A and PNA, areknown to bind with high specificity to glucose and galactose,respectively, in solution [15]–[20]. If the functionalized fiberswere to behave as true biosensors, we would have to observethe analogous high specificity binding of these lectins to theirrespective carbohydrates on the fiber surface. We began by tak-ing a reference measurement with the sensor in the HBS buffercontaining the Tween 20/BSA solution. The sensor equilibratesin this solution for 15 minutes to minimize nonselective attach-ment of the lectins. Typically, we observe only a 1–2 pm shiftduring this equilibration. Then the sensor is immersed in a 1µM solution of Con A (or PNA) at 34 ◦C for 1 h. The sen-sor is removed from the solution of lectin and immersed in aHBS/Tween 20/BSA buffer solution.

The time-dependent binding of Con A and PNA to glucose-functionalized fiber are shown in Figs. 8 and 9, respectively.Con A binds strongly to the glucose-functionalized fiber, whilePNA shows no binding based on the change of refractive in-dex. Fig. 8 shows that the time-dependent binding of Con A toglucose-functionalized fiber resulted in a shift of 60 pm after 1h. When the binding of PNA to glucose-fiber was measured, ashift of about 0 ± 2 pm was obtained (Fig. 9). When the bindingof Con A to lactose-functionalized fiber was measured underidentical conditions, a shift of about 0 ± 2 pm was obtained(Fig. 10). Conversely, Fig. 11 shows that the time-dependentbinding of PNA to lactose-functionalized fiber gave a shift of40 pm. The results in Figs. 8–11 clearly demonstrate that eachof the lectins binds selectively to a particular carbohydrate-functionalized fiber and not to another one.

Theoretical simulation of the wavelength shift that wouldhave occurred assuming a monolayer of Con A of thickness 6.3nm and a Con A refractive index of 1.45 predicts a shift of 57pm. If the index of Con A is taken to be 1.5, then the shift ispredicted to be 80 pm. For PNA, simulations using a monolayerof PNA of thickness of 6.3 nm and a PNA index of refraction

Fig. 8. Time-dependent binding of Con A to a glucose-functionalized fiber at34 ◦C.

Fig. 9. Time-dependent binding of PNA to glucose-functionalized fiber at 34◦C; 2σ = 5 pm.

Fig. 10. Time-dependent binding of Con A to a lactose-functionalized fiberat 34 ◦C; 2σ = 5 pm.

of 1.45 predict a shift of 57 pm. Since the exact values for theindex of refraction of solid Con A and PNA are not known, wetake the experimental values of the shift to be in good agreementwith the theoretical values, after assuming a typical value of theindex for a protein of 1.45. Both of these shifts suggest that


Fig. 11. Time-dependent binding of PNA to a lactose-functionalized fiber at34 ◦C.

the respective lectins bind with high specificity to their cognateligands.

Taking our resolution limit of 1 pm and assuming a 60 pm shiftfor a monolayer formation of Con A, we estimate our detectionlimit in this experiment to be 1/60 of a monolayer of Con A.Since a molecule of Con A occupies a volume of 6.32 nm× 8.69nm × 8.92 nm and since Con A has a molar weight of 104 kDa,we estimate a detection limit of 2.14 × 108 molecules/mm2 or37 pg/mm2 . This calculation assumed a fiber radius of 2.5 µmand a grating length of 5 mm.

We have also performed a preliminary investigation of theoptimized temperature for lectin binding and have discovereda large effect of temperature on the binding efficiency. Bindingof Con A to a glucose-functionalized fiber led to a shift of 18pm at 22 ◦C, 60 pm at 34 C, and 16 pm at 38.5 C for a 1-hbinding experiment. This strong difference in binding affinitywith temperature indicates that a number of variables, such aspH, ionic strength, and temperature, have a key role to playin the optimization of the binding efficiency of this class ofsensors. Future papers will focus on the optimization of thesignal intensity of this biosensor.

VI. CONCLUSION

A new type of biosensor that measures the change in the re-fractive index of an FBG has been developed. Functionalizationof the fiber’s surface using carbohydrate-siloxane conjugatesyields a functionalized fiber that is exposed to physiologicallyrelevant concentrations of lectins. A high specificity of bindingfor the lectins Con A and PNA, respectively, with the cognateligand was observed. Quasi-monolayer selective binding of thelectins to the fiber occurred based on a theoretical analysis of theobserved changes in the refractive index. The high sensitivityobserved with this biosensor indicates that this general approachcan be utilized to measure a variety of biologically relevantprocesses including DNA–DNA or DNA–RNA hybridization,protein–protein interactions, and carbohydrate–protein interac-tions under physiological conditions.

ACKNOWLEDGMENT

The authors would like to thank Dr. V. Lee, Cell Biologyand Molecular Genetics Department, University of Maryland,College Park, for providing valuable insights into reducing non-selective attachments in the binding experiments..

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Geunmin Ryu (M’07) was born in Seoul, Korea. He received the B.S. degreewith Summa Cum Laude in electrical and computer engineering from The OhioState University, Columbus, OH, in 2006. Since 2006, he has been workingtoward the Ph.D. degree in electrophysics and microelectronics at the PhotonicsSwitching and Integrated Optoelectronics Laboratory, Department of Electricaland Computer Engineering, University of Maryland, College Park.

He is currently a Research Assistant in the Department of Electrical andComputer Engineering, University of Maryland. His research interests includechemical and biomedical sensor, biophotonics, nitride-based lighting, and in-terband cascade laser.

Mario Dagenais (A’84–SM’88) receivved the Ph.D. degree in physics from theUniversity of Rochester, Rochester, NY, in 1978.

After spending 2 year at Harvard University, Cambridge, MA, he joinedGTE Laboratories, Waltham, MA, where he was engaged for 7 years. Since1987, he has been a Professor of electrical and computer engineering in theDepartment of Electrical and Computer Engineering, University of Maryland,College Park. His research interests include photonic switching, photonic inte-grated circuits, biosensing, and optoelectronic packaging. In particular, he hasbeen actively involved in the development of Bragg grating biosensors, highpower semiconductor laser sources, tunable lasers, semiconductor optical am-plifiers, superluminescent light-emitting diodes, detectors, modulators, opticalswitches, and the integration of these components. He is the author of more than200 papers.

Dr. Dagenais is a Fellow of the Optical Society of America.

Matthew T. Hurley a native of Berkeley Springs, West Virginia, receivedthe B.S. degree in chemistry from West Virginia University, Morgantown, in2006, where he conducted undergraduate research in the laboratory of Dr. B.Soderberg. He is currently working toward the Ph.D. degree in chemistry in theDepartment of Chemistry and Biochemistry, University of Maryland, CollegePark, under the supervision of Dr. P. DeShong.

His research interests include the development of targeted, controlled releasesystems, and diagnostic tools using mesoporous silica nanoparticles.

Philip DeShong received the B.S. degreve from the University of Texas, Austin,in 1971, and the Sc.D. degree from the Massachusetts Institute of Technology(MIT), Cambridge, in 1976.

From 1976 to 1978, he was a Postdoctoral Fellow with the Swiss Federal In-stitute of Technology, Zurich, Switzerland, and with MIT for 1 year during 1979.He then joined the in the Department of Chemistry and Biochemistry, Universityof Maryland, College Park, where he is now a Full-Time Professor. His cur-rent research interests includes total synthesis of heterocyclic natural products,development of methodology for organic synthesis, organic–organometallic re-actions at high pressure, chemistry of carbohydrate derivatives, chemistry ofhypervalent silicon derivatives, and new methods for the synthesis of combina-torial libraries.

Dr. DeShong is a Fellow of the American Association for the Advancementof Science.

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 1

High Specificity Binding of Lectins toCarbohydrate-Functionalized Fiber Bragg Gratings:

A New Model for Biosensing ApplicationsGeunmin Ryu, Member, IEEE, Mario Dagenais, Senior Member, IEEE, Matthew T. Hurley, and Philip DeShong

Abstract—The functionalization of an etched fiber Bragg gratingwas realized using a carbohydrate-siloxane conjugate. No fluores-cent probes were used. Concanavalin A bound with high specificityto the glucose biosensor, but not to the lactose functionalized fiber.Conversely, peanut agglutinin bound to the lactose sensor with highspecificity over its glucose counterpart. Quasi-monolayer selectivebinding of the lectins to the fiber sensor was inferred based on atheoretical analysis of the observed changes in the refractive index.Our results open the way to the use of unlabeled carbohydrate-based sensors for the study of the human glycome.

Index Terms—Biosensor, Concanavalin A (ConA), carbohy-drates, evanescent field, fiber Bragg grating (FBG), monolayerdetection, peanut agglutinin (PNA), surface functionalization.

I. INTRODUCTION

I T IS NOW appreciated that the pathogenesis of severalwidespread and chronic diseases can be attributed to a change

in the glycome, in the absence of obvious changes in the genomeor proteome [1]. The human glycome might have several millioncarbohydrate structures. Several major diseases are associatedwith a change in the glycosylation pattern of a central proteinstructure. Cells and many proteins in nature are covered witha dense array of covalently bound sugar chains. Our ability tounderstand the factors that regulate normal glycosylation of pro-teins and lipids, resulting in normal structures, and those thatlead to disruption of normal sugar attachments will help us un-derstand diseases processes and control. Most of the diseasesthat affect mankind, including cancer, diabetes, heart diseases,infectious diseases, flu, Alzheimer, and rheumatoid arthritis,directly involve glycoconjugates. The science of glycobiologywill have a significant impact on our ability to understand howto stay healthy or how to manage diseases.

Manuscript received July 1, 2009; revised August 25, 2009. This work wassupported by the National Science Foundation Materials Research Science andEngineering Centers under Grant DMR 05-20471 for the shared experimen-tal facilities support for the X-ray Photoelectron Spectrometer. The work ofP. DeShong was supported by the National Science Foundation Nanoscale In-terdisciplinary Research Teams under Grant CHE 0511219478, the MarylandTechnology Development Corporation, and in part by the SD Nanosciences,Inc. The work of M. T. Hurley was supported by the Graduate Assistance inAreas of National Need Fellowship.

G. Ryu and M. Dagenais are with the Department of Electrical and ComputerEngineering, University of Maryland, College Park, MD 20742 USA (e-mail:[email protected]; [email protected]).

M. T. Hurley and P. DeShong are with the Department of Chemistry andBiochemistry, University of Maryland, College Park, MD 20742 USA (e-mail:[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2009.2032427

In this paper, we describe a fiber Bragg grating (FBG) sensorwith a sensitivity to change of the index of refraction as smallas 2–3 × 10−5 when the index of refraction of the surroundinganalyte is about 1.35. This is sufficient to detect the selectivebinding of a monolayer or less of proteins to a particular car-bohydrate. This sensor does not require the use of a fluorescentlabel. Our results demonstrate the potential of glycobiology forthe selective binding of proteins to carbohydrates. In particular,we demonstrate the binding of Concanavalin A (Con A) proteinto a monolayer of glucose covalently attached to the surfaceof a fiber. We also demonstrate that Con A does not bind to amonolayer of lactose covalently bound to the glass fiber. In a dif-ferent set of experiments, we demonstrate the selective bindingof the peanut agglutinin (PNA) protein to a monolayer lactosefilm but not to a monolayer of glucose on the surface of thesensor. This demonstrates the high selectivity of using a partic-ular sugar for detecting a particular protein and may ultimatelyhelp us understand how diseases evolve or can be controlled.Reviews of recent results on carbohydrate biosensors can befound in [2]–[5]. We have successfully modeled the attachmentof molecules to the sensor and can confidently predict whenapproximate monolayer coverage is obtained based on the in-dex of refraction and the dimension of the dry molecule. Themodel that we used is described in this paper. Strong tempera-ture dependence for the attachment of the proteins on our sensoris observed and can be understood by the fact that the proteincan denature at high temperatures and is less active at low tem-peratures. Our sensor is based on a Bragg grating written in aphotorefractive fiber. This fiber is etched down to a diameterof about 5 µm. The fiber is then functionalized with monolayerof carbohydrates by exposing the silica fiber to carbohydrateconjugates containing a terminal siloxane moiety that is used tocovalently anchor the conjugates to the surface of the fiber. Thefiber is then immersed in an analyte solution containing the pro-tein of interest. The evanescent wave in the fiber sensor sensesthe change of the index of refraction following the binding ofprotein molecules to the surface of the sensor. This change ofindex of refraction leads to a change of the Bragg wavelength,which is detected. Furthermore, we demonstrate very high se-lectivity by successfully preventing nonselective attachment ofmolecules to the sensor.

II. PREVIOUS WORK

Recently, we presented a simple theory to describe the shiftof the FBG resonance as a function of the fiber diameter and

1077-260X/$26.00 © 2009 IEEE


as a function of the surrounding index of refraction [6], [7]. Aninstructive graphical solution was described. A good agreementfor the predicted shift of the FBG wavelength was observed be-tween theory and experiment as the fiber diameter and claddingindex of refraction were varied. A surrounding index sensitiv-ity as small as 7.2 × 10−7 was demonstrated [8], limited by awavelength resolution of .001 nm of our instrumentation. Thisindex sensitivity drops, as the probed analyte has an index ofrefraction closer to 1.35 rather than close to the index of max-imum sensitivity (n = 1.45). The change of wavelength of anetched core FBG sensor can be written as

∆λ = S∆n (1)

where S is defined as the sensitivity of the sensor and is measuredas the change in wavelength per unit change of index (RIU) ofthe surrounding medium and ∆n is the change of the index ofthe surrounding medium. The sensitivity S for the FBG sensoretched to a core size of 5 µm is 30 nm/RIU when the indexof the surrounding medium is close to that of water (1.325).The minimum refractive index change that can be measured bythe sensor is given by ∆λmin/S, where ∆λmin is the smallestwavelength change that can be measured. Therefore, in orderto increase the resolution of the measured index change, it isimportant to increase the sensitivity of the sensor and decreasethe minimum wavelength shift that can be measured in the sys-tem. For chemical and biological sensors, it is also importantto understand the minimum thickness of the adsorbed layer thatcan be measured by the sensor. The surface of the fiber is func-tionalized with the chemical or biological agent being detected.The index of the adsorbed layer is different from that of the sur-rounding medium (nmed ), causing a perturbation in the claddingindex and a shift in the wavelength. The index change that iscaused by the adsorbed layer is given by ∆n = neff − nmed ,where neff is the effective index of the fiber mode in the pres-ence of both the adsorbed layer and the surrounding analytesolution. Knowing the minimum ∆n that can be resolved bythe sensor, the minimum adsorbed thickness can be calculatedfor a given index of the adsorbed layer [7]. From equations (1)and from a beam propagation simulation, it is clear that it isimportant to increase the wavelength resolution and the sensi-tivity of the sensor in order to measure thin layers of adsorbedlayers.

Using this fiber sensor, we have previously demonstrated thatDNA hybridization can be measured [9]. The surface of theetched fiber was functionalized by attaching a 20 nucleotidesingle strand probe DNA (probe) to its surface and then sur-rounding the fiber with the matching 20 oligomer single strandtarget DNA (target). Recently, we have demonstrated the at-tachment of a glucose derivative on the fiber [10]. These exper-iments are a first step toward the development of carbohydrate-functionalized fibers as biosensors for the detection of lectins,carbohydrate binding proteins. A variety of biomolecules havebeen previously attached to silica and related surfaces to pro-vide recognition regimes for biochemical substances, includingsingle-stranded DNA, antibodies, enzymes, proteins, and cells.

Fig. 1. (a) Diagram of the JDSU fiber housing and positions of the etchantduring primary and secondary etches. (b) Fiber diameter profile after secondaryetch.

Until now, however, the functionalization of silica with carbohy-drates has received scant attention even though many cell–cellrecognition events are mediated by carbohydrate–protein mul-tidentate interactions [1].

Our fiber optics sensor [6]–[8] uses a commercially available(JDSU) single mode photosensitive fiber in which two FBGs,with a Bragg wavelength of 1533 and 1563.8 nm, are inscribedin the fiber core. The gratings are about 5 mm long and have apeak reflectivity of about 30 dB and extremely well-suppressedsidelobes. For the present experiment, we use only the λ = 1563nm grating. The sensor is temperature compensated. The etchedFBG sensor was chemically etched in a two-step process: firstto a diameter of 50 µm and then to a diameter of 5 µm (seeFig. 1). Fiber etching was conducted in 7:1 buffered oxide etchwith surfactant from J. T. Baker. Based on the MSDS data sheet,the solution is made up of 0.5–10% hydrogen fluoride, 40–70%water, 30–50% ammonium fluoride, and 0.5–10% surfactant.The low concentration of hydrogen fluoride and the additionof surfactant reduce the silica etch rate and enhance the sur-face smoothness, as compared to etch processes utilizing higherhydrogen fluoride concentrations.

For the biosensing experiment, we used a 5 µm diameteretched FBG sensor. We used the broadband amplified sponta-neous emission spectrum of an erbium-doped fiber amplifier asthe broadband source to probe the fiber Bragg reflection spec-trum and an optical spectrum analyzer to acquire the reflectionfrom the FBG. The experimental setup is shown in Fig. 2. Atypical reflection spectrum of an etched core FBG is shown inFig. 3. As observed, there exist several reflectivity peaks andminima. We use the minimum on the long wavelength side clos-est to the reflectivity peak to monitor the wavelength shift. Thisminimum is also the sharpest feature of the spectrum [11]. Thefeature is very reproducible and shifts by the same amount asthe peak wavelength. By using a least square fit, the wavelengthshift can be resolved down to 1 pm.


Fig. 2. Biosensing experimental setup.

Fig. 3. Spectrum of the etched core FBG sensor. The feature in the reflectionspectrum that is being monitored is also shvown.

III. DESCRIPTION OF CHEMICALS USED

A. HBS with Blocker Buffer

For the binding experiment, we used HEPES buffered saline(HBS) mixed with Tween-20 and bovine serum albumin (BSA)as the buffer solution. BSA works as a blocker because it isa protein that interacts nonspecifically to compete with othernonspecific interactions. Tween-20, on the other hand, is a de-tergent and is meant to prevent or break up nonspecific binding.Both chemicals were purchased from Sigma-Aldrich. HBS withblocker buffer is made with 100 mL of HBS buffer, 0.1 mL ofTween-20, and 5 g of BSA. To make 100 mL of HBS buffer,we first mix 2 ml of 0.5M HEPES, 1 mL of 0.1M CaCl2, and15 mL of 1.0 M NaCl solution. Second, we test the pH of thebuffer mixture that we made in the first step and then adjust thepH to 7.4 by adding the 0.2M NaOH solution. After we havemade the HBS buffer with a pH of 7.4, we add 1 mL of 0.1MMnCl2 and enough Millipore water to make 100 mL.

B. Sodium Ethoxide (NaOEt)

Sodium ethoxide is prepared by adding ∼1 g of freshly cutsodium metal to 25 mL of anhydrous ethanol under a nitrogenatmosphere at room temperature, followed by stirring at roomtemperature for 1 h, or until the solid metal has reacted com-pletely to provide a clear, colorless solution. The solution canbe stored under nitrogen atmosphere at room temperature for upto 7 days.

C. Synthesis of Glucose-Siloxane Conjugate 1

We have recently developed a general strategy for the prepa-ration of oligosaccharide conjugates that serve as the basis of themeans of attaching carbohydrate derivatives to silica surfaces.Although originally developed for the synthesis of complex gly-copeptide derivatives, the methodology has proven to be viablefor the synthesis of a wide variety of glycoconjugates, suchas glycolipids and glycosylated polyethylene glycocol (PEG)derivatives. For this experiment, glycosylamide derivatives 1(glucosyl) and 2 (lactosyl) were prepared and used to func-tionalize fibers using sol–gel methodologies as summarized inFigs. 4 and 5, respectively.

D. Synthesis of Lactose-Siloxane Conjugate 2

A similar approach was used to prepare covalently boundlactose to the fiber. In this instance, peracetylated lactopyranosylazide was converted into its siloxane conjugate(2) as outlined inFigs. 5 and 2 was used to functionalize etched fiber in a fashionanalogous to the glucose derivative shown in Fig. 4.

E. Binding of Lectins ConA and PNA to Glucose- and Lactose-Functionalized Fibers

For the binding studies using carbohydrate-functionalizedfibers, we used ConA and PNA. Both proteins were purchasedfrom Sigma–Aldrich. Proteins were taken from the freezer andwarmed to room temperature before they were mixed with theHBS buffer. After mixing the proteins with the HBS buffer, weleft them overnight at room temperature. On the following day,we filtered the protein solutions using a 0.2 µm disposable filterand measured the concentration. The concentrations of the pro-tein solutions were determined by using UV absorbance at 280nm (A280 = 1.37[mg/ml ConA], A280 = 0.96[mg/mL PNA]).The protein solutions were then diluted to 1 µM (based onConA tetramer of molar mass 104 kDa and PNA tetramer ofmolar mass 110 kDa).

IV. FUNCTIONALIZATION OF FIBER WITH GLUCOSE- AND

LACTOSE-SILOXANES

Glucose-functionalized fibers were prepared using the chem-istry summarized in Fig. 4. First a water reference was estab-lished by soaking the sensor in water. Then, a first reference toethanol was established by soaking the sensor in ethanol. Thisstep is required since the peracetylated glucose and lactose con-jugates 1 and 2, respectively, need to be dissolved in ethanol.A freshly etched fiber was treated with a 10 mM solution ofglucose-siloxane 1 dissolved in ethanol to yield a fiber whosesurface has been modified by the siloxane exchange reactionand results in the attachment of the glucose ligand. At the endof the experiment, an ethanol reference is measured and a waterreference is taken. Then, the sensor is put back in ethanol beforeimmersing it in a sodium ethoxide (NaOEt) solution at 50 ◦C for1.5 h to remove the acetate groups. Finally, an ethanol referencemeasurement and a water reference measurement are taken.

The attachment of the glucose and lactone-siloxanes to thefiber was monitored by measuring the wavelength shift of the


Fig. 4. Synthesis of glucose-siloxane conjugate and functionalization of a silica surface.

Fig. 5. Synthesis of lactose-siloxane conjugate 2.

Fig. 6. Time-dependent attachment of glucose-siloxane 1 to the fiber and finalshift with respect to water (average of five measurements). The error on eachmeasurement is given by ±2.1 pm and, for clarity, is only shown for t = 60 min.

fiber. The attachment of the glucose proceeds readily as shown inFig. 6. These results represent the average of five attachment ex-periments. The standard deviation σ on each point was extractedto be 2.1 pm. As can be seen, we first observe a decrease in theBragg grating wavelength that we attribute to the formation of alower index layer, a water layer, on the fiber, which is followedby the attachment of the glucose-siloxane layer, a higher indexmaterial. We first encountered this situation in our previous stud-ies of the fiber silanization process using APTES and APMDSand it was described in details in a previous publication [12]. Inthe present studies, after the initial shift downward, we observe agrowing shift of the Bragg wavelength as the attachment of glu-cose on the fiber progresses. This shift was measured when thesample was immersed in ethanol and was referenced to water.Most of the shift is due to the difference of index of refractionbetween ethanol and water. But an unmistakable measurableshift of the Bragg wavelength is observed, which is attributed tothe glucose attachment. We see that the attachment progressesmore rapidly at the beginning and then slows down. At the endof the attachment experiment, the sensor is first rinsed in ethanoland then in water. The inset in Fig. 6 shows the shift of the FBG

peak before and after glucose attachment with respect to water.This shift is in agreement with the shift observed following theformation of the lower index layer on the fiber. The dashed linein Fig. 6 is an extrapolation to t = 0 of the glucose attachmentprocess. A final shift of 24 pm (as shown in the inset of Fig. 6)is measured for the case of the glucosyl acetate 2, which cor-responds to a change of the surrounding index of 5.6 × 10−4 .If we assume that a solid glucose layer is formed on the fiberof index 1.543 [13], a beam propagation simulation, previouslydiscussed in [7] and [8], suggests that a monolayer of thicknessabout 1.3 nm is formed, which corresponds to a monolayer ofglucose-siloxane conjugate.

When the acetate groups are removed from glucose using a0.2 M solution of NaOEt, a glucose shift with respect to waterof 26 pm was registered. This shift is very similar to the shiftthat was measured for the acetylated glucose on the fiber andis compatible with our error bar of 2 pm. This indicates thatonly a small fraction of the acetate groups has been removed.We have previously reported on the time dependence of theattachment of APTES and APMDS on a fiber [12] and havedemonstrated the power of our technique for being able to reachmonolayer attachment in a controlled way, as compared to othertechniques that do not allow in situ real-time measurement ofthe functionalization process [14]. Here, we conclude that amonolayer of the glucose-siloxane conjugate is attached to thefiber by covalent bonding.

Lactose was attached covalently to the fiber using a lactose-siloxane conjugate 2 as shown in Fig. 5. As with glucose, thelactose is diluted in ethanol. As for the glucose case, the lactoseattachment first leads to a negative shift of the FBG wavelengthdue to the formation of a lower index of refraction layer on thefiber. Next, the Bragg wavelength shifts positively, reflectingthe attachment of lactose to the fiber. After extrapolating theBragg grating attachment shift to t = 0 (dashed line in Fig. 7),we evaluate a total shift of 22 pm to represent the attachmentof lactose to the fiber. A final shift of 23 pm (inset of Fig. 7)is measured by taking the difference between the initial (beforeattachment of lactose) and final (after the attachment of lac-tose) Bragg wavelength referred to water, in good agreementwith the time dependent shift measured between the end of themeasurement and the extrapolated shift to t = 0. This shift isabout the same as the one observed describing the attachmentof glucose to the fiber. This indicates that a lower density oflactose molecule attaches to the fiber as compared to the bulkdensity of lactose. We observe a negligible shift (< 2 pm) ofthe Bragg wavelength after the removal of the acetate groups,


Fig. 7. Time-dependent attachment of lactose-siloxane 2 to the fiber and finalshift with respect to water.

indicating that a small fraction of the accetate groups has beenremoved.

V. BINDING OF CON A AND PNA TO GLUCOSE- OR

LACTOSE-FUNCTIONALIZED FIBERS

Lectins, carbohydrate-binding proteins, Con A and PNA, areknown to bind with high specificity to glucose and galactose,respectively, in solution [15]–[20]. If the functionalized fiberswere to behave as true biosensors, we would have to observethe analogous high specificity binding of these lectins to theirrespective carbohydrates on the fiber surface. We began by tak-ing a reference measurement with the sensor in the HBS buffercontaining the Tween 20/BSA solution. The sensor equilibratesin this solution for 15 minutes to minimize nonselective attach-ment of the lectins. Typically, we observe only a 1–2 pm shiftduring this equilibration. Then the sensor is immersed in a 1µM solution of Con A (or PNA) at 34 ◦C for 1 h. The sen-sor is removed from the solution of lectin and immersed in aHBS/Tween 20/BSA buffer solution.

The time-dependent binding of Con A and PNA to glucose-functionalized fiber are shown in Figs. 8 and 9, respectively.Con A binds strongly to the glucose-functionalized fiber, whilePNA shows no binding based on the change of refractive in-dex. Fig. 8 shows that the time-dependent binding of Con A toglucose-functionalized fiber resulted in a shift of 60 pm after 1h. When the binding of PNA to glucose-fiber was measured, ashift of about 0 ± 2 pm was obtained (Fig. 9). When the bindingof Con A to lactose-functionalized fiber was measured underidentical conditions, a shift of about 0 ± 2 pm was obtained(Fig. 10). Conversely, Fig. 11 shows that the time-dependentbinding of PNA to lactose-functionalized fiber gave a shift of40 pm. The results in Figs. 8–11 clearly demonstrate that eachof the lectins binds selectively to a particular carbohydrate-functionalized fiber and not to another one.

Theoretical simulation of the wavelength shift that wouldhave occurred assuming a monolayer of Con A of thickness 6.3nm and a Con A refractive index of 1.45 predicts a shift of 57pm. If the index of Con A is taken to be 1.5, then the shift ispredicted to be 80 pm. For PNA, simulations using a monolayerof PNA of thickness of 6.3 nm and a PNA index of refraction

Fig. 8. Time-dependent binding of Con A to a glucose-functionalized fiber at34 ◦C.

Fig. 9. Time-dependent binding of PNA to glucose-functionalized fiber at 34◦C; 2σ = 5 pm.

Fig. 10. Time-dependent binding of Con A to a lactose-functionalized fiberat 34 ◦C; 2σ = 5 pm.

of 1.45 predict a shift of 57 pm. Since the exact values for theindex of refraction of solid Con A and PNA are not known, wetake the experimental values of the shift to be in good agreementwith the theoretical values, after assuming a typical value of theindex for a protein of 1.45. Both of these shifts suggest that


Fig. 11. Time-dependent binding of PNA to a lactose-functionalized fiber at34 ◦C.

the respective lectins bind with high specificity to their cognateligands.

Taking our resolution limit of 1 pm and assuming a 60 pm shiftfor a monolayer formation of Con A, we estimate our detectionlimit in this experiment to be 1/60 of a monolayer of Con A.Since a molecule of Con A occupies a volume of 6.32 nm× 8.69nm × 8.92 nm and since Con A has a molar weight of 104 kDa,we estimate a detection limit of 2.14 × 108 molecules/mm2 or37 pg/mm2 . This calculation assumed a fiber radius of 2.5 µmand a grating length of 5 mm.

We have also performed a preliminary investigation of theoptimized temperature for lectin binding and have discovereda large effect of temperature on the binding efficiency. Bindingof Con A to a glucose-functionalized fiber led to a shift of 18pm at 22 ◦C, 60 pm at 34 C, and 16 pm at 38.5 C for a 1-hbinding experiment. This strong difference in binding affinitywith temperature indicates that a number of variables, such aspH, ionic strength, and temperature, have a key role to playin the optimization of the binding efficiency of this class ofsensors. Future papers will focus on the optimization of thesignal intensity of this biosensor.

VI. CONCLUSION

A new type of biosensor that measures the change in the re-fractive index of an FBG has been developed. Functionalizationof the fiber’s surface using carbohydrate-siloxane conjugatesyields a functionalized fiber that is exposed to physiologicallyrelevant concentrations of lectins. A high specificity of bindingfor the lectins Con A and PNA, respectively, with the cognateligand was observed. Quasi-monolayer selective binding of thelectins to the fiber occurred based on a theoretical analysis of theobserved changes in the refractive index. The high sensitivityobserved with this biosensor indicates that this general approachcan be utilized to measure a variety of biologically relevantprocesses including DNA–DNA or DNA–RNA hybridization,protein–protein interactions, and carbohydrate–protein interac-tions under physiological conditions.

ACKNOWLEDGMENT

The authors would like to thank Dr. V. Lee, Cell Biologyand Molecular Genetics Department, University of Maryland,College Park, for providing valuable insights into reducing non-selective attachments in the binding experiments..

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Geunmin Ryu (M’07) was born in Seoul, Korea. He received the B.S. degreewith Summa Cum Laude in electrical and computer engineering from The OhioState University, Columbus, OH, in 2006. Since 2006, he has been workingtoward the Ph.D. degree in electrophysics and microelectronics at the PhotonicsSwitching and Integrated Optoelectronics Laboratory, Department of Electricaland Computer Engineering, University of Maryland, College Park.

He is currently a Research Assistant in the Department of Electrical andComputer Engineering, University of Maryland. His research interests includechemical and biomedical sensor, biophotonics, nitride-based lighting, and in-terband cascade laser.

Mario Dagenais (A’84–SM’88) receivved the Ph.D. degree in physics from theUniversity of Rochester, Rochester, NY, in 1978.

After spending 2 year at Harvard University, Cambridge, MA, he joinedGTE Laboratories, Waltham, MA, where he was engaged for 7 years. Since1987, he has been a Professor of electrical and computer engineering in theDepartment of Electrical and Computer Engineering, University of Maryland,College Park. His research interests include photonic switching, photonic inte-grated circuits, biosensing, and optoelectronic packaging. In particular, he hasbeen actively involved in the development of Bragg grating biosensors, highpower semiconductor laser sources, tunable lasers, semiconductor optical am-plifiers, superluminescent light-emitting diodes, detectors, modulators, opticalswitches, and the integration of these components. He is the author of more than200 papers.

Dr. Dagenais is a Fellow of the Optical Society of America.

Matthew T. Hurley a native of Berkeley Springs, West Virginia, receivedthe B.S. degree in chemistry from West Virginia University, Morgantown, in2006, where he conducted undergraduate research in the laboratory of Dr. B.Soderberg. He is currently working toward the Ph.D. degree in chemistry in theDepartment of Chemistry and Biochemistry, University of Maryland, CollegePark, under the supervision of Dr. P. DeShong.

His research interests include the development of targeted, controlled releasesystems, and diagnostic tools using mesoporous silica nanoparticles.

Philip DeShong received the B.S. degreve from the University of Texas, Austin,in 1971, and the Sc.D. degree from the Massachusetts Institute of Technology(MIT), Cambridge, in 1976.

From 1976 to 1978, he was a Postdoctoral Fellow with the Swiss Federal In-stitute of Technology, Zurich, Switzerland, and with MIT for 1 year during 1979.He then joined the in the Department of Chemistry and Biochemistry, Universityof Maryland, College Park, where he is now a Full-Time Professor. His cur-rent research interests includes total synthesis of heterocyclic natural products,development of methodology for organic synthesis, organic–organometallic re-actions at high pressure, chemistry of carbohydrate derivatives, chemistry ofhypervalent silicon derivatives, and new methods for the synthesis of combina-torial libraries.

Dr. DeShong is a Fellow of the American Association for the Advancementof Science.

Documents

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM …nanophotonics.ece.umd.edu/publications_files/JSTQE...2 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS as a function of the surrounding