Seeing protein monolayers with naked eyethrough plasmonic Fano resonancesAhmet A. Yanika,b,1, Arif E. Cetina,b, Min Huanga,b, Alp Artara,b, S. Hossein Mousavic, Alexander Khanikaevc,John H. Connord, Gennady Shvetsc, and Hatice Altuga,b,1

aDepartment of Electrical and Computer Engineering, bThe Photonics Center, Boston University, Boston, MA 02215; cDepartment of Microbiology, BostonUniversity School of Medicine, Boston, MA 02218; and dDepartment of Physics, University of Texas at Austin, Austin, TX 78712

Edited* by Erich P. Ippen, Massachusetts Institute of Technology, Cambridge, MA, and approved June 2, 2011 (received for review February 4, 2011)

We introduce an ultrasensitive label-free detection techniquebased on asymmetric Fano resonances in plasmonic nanoholeswith far reaching implications for point-of-care diagnostics. Byexploiting extraordinary light transmission phenomena throughhigh-quality factor (Qsolution ∼ 200) subradiant dark modes, weexperimentally demonstrate record high figures of merits (FOMsas high as 162) for intrinsic detection limits surpassing that ofthe gold standard prism coupled surface-plasmon sensors (Kretsch-mann configuration). Our experimental record high sensitivitiesare attributed to the nearly complete suppression of the radiativelosses that are made possible by the high structural quality of thefabricated devices as well as the subradiant nature of the reso-nances. Steep dispersion of the plasmonic Fano resonance profilesin high-quality plasmonic sensors exhibit dramatic light intensitychanges to the slightest perturbations within their local environ-ment. As a spectacular demonstration of the extraordinary sensi-tivity and the quality of the fabricated biosensors, we showdirect detection of a single monolayer of biomolecules with nakedeye using these Fano resonances and the associated Wood’sanomalies. To fabricate high optical-quality sensors, we introducea high-throughput lift-off free evaporation fabrication techniquewith extremely uniform and precisely controlled nanofeatures overlarge areas, leading to resonance line-widths comparable to that ofthe ideally uniform structures as confirmed by our time-domainsimulations. The demonstrated label-free sensing platform offersunique opportunities for point-of-care diagnostics in resourcepoor settings by eliminating the need for fluorescent labeling andoptical detection instrumentation (camera, spectrometer, etc.) aswell as mechanical and light isolation.

surface plasmonics ∣ subradiant dark resonances ∣ biosensing ∣ label free ∣global health

Surface plasmonics, the science and engineering of electromag-netic waves trapped at the metal/dielectric interfaces, has

opened up a new realm of possibilities for a broad range of ap-plications ranging from biosensing to photovoltaics (1–5). Withinthe last decade, functional components of unparalleled opticaldevices creating, manipulating and concentrating light at metalsurfaces below the diffraction limit are shown (2, 6). Engineeringof these functionalities have led to the demonstration of revolu-tionary concepts such as superlensing (7) and optical cloaking (8),as well as groundbreaking observations in nonlinear photonics (9)and all-optical manipulation (10). By concentrating electromag-netic fields thousands of times smaller than the diffraction limitedvolume of light, extremely strong light-matter interactions lead-ing to orders of magnitude enhanced second harmonic genera-tion (9), fluorescence (11), surface enhanced Raman scattering(12), and surface enhanced infrared absorption spectroscopy(13–16) are shown. Developments within the last decade seemto hint at a bright future for plasmonic devices.

In general, it is desirable to have high-quality factor plasmonicresonances and enhanced near-fields for stronger light-matterinteractions. In practice though, both the radiative and nonra-diative (ohmic) losses exist in nanoplasmonic devices and can

severely shorten the life-times of the plasmonic excitations. Ad-vantages of plasmonic effects deteriorate with increasing losses inmany applications. For example, in biosensing applications basedon effective refractive index modulation, the detection limitsstrongly depend on both the refractive index sensitivity of theplasmonic resonances as well as the resonance line-widths (17).Narrower line-widths allow smaller resonance shifts (analyte con-centrations) to be detected. In applications critically depended onlight intensities, losses may limit the near-field enhancements.Hence, the realization of practical plasmonic devices strongly de-pends on our ability to control these decay channels. Nonradia-tive Drude (ohmic) damping processes are intrinsic to the metals.Radiative losses, on the other hand, can be effectively controlled(14, 18).

Surface roughness can cause stronger plasmon coupling to thecontinuum (scattering losses) leading to shorter plasmon propa-gation lengths and spectrally broader resonances. High-qualityfabrication methods can help minimizing contributions of theseradiative decay channels (19). Further enhancements of plasmo-nic lifetimes leading to narrower resonance line-widths can beachieved by engineering of light coupling mechanisms usingcollective excitations and tailored designs (14, 18). Subradiantdark modes are particularly promising for creation of high-qualityfactor plasmonic resonances due to their zero net-dipole momentcharacter forbidding direct light coupling to the continuum.Another fundamental physical phenomenon, Fano resonance,with its asymmetric line shape and strongly dispersive characterenables strong line-width narrowings (20). Plasmonic Fano reso-nances have been extensively studied in nanoparticle and meta-material systems by coupling broad line-width superradiant andnarrow line-width subradiant modes (4, 21–23). However, thereare very limited studies on Fano resonances created throughdirect coupling of the dipolar incident light to subradiant darkmodes (24).

In this letter, we experimentally demonstrate that stronglydispersive Fano phenomenon combined with subradiant darkmodes can be exploited for ultrasensitive label-free biosensing.Our approach is based on direct coupling of incident-light to mul-tipolar subradiant modes through retardation effects in plasmo-nic nanohole devices. By creating a distinct light coupling schemeallowing high numerical aperture (NA) imaging, we demonstrateextraordinary light transmission (EOT) resonances with remark-ably narrow line-widths (approximately 4.29 nm) that are solelylimited by the nonradiative Drude damping processes. By utiliz-

Author contributions: A.A.Y. and H.A. designed research; A.A.Y., M.H., and H.A. performedresearch; A.A.Y., A.E.C., M.H., A.A., S.H.M., A.K., J.H.C., G.S., and H.A. contributed newreagents/analytic tools; A.A.Y., A.E.C., G.S., and H.A. analyzed data; and A.A.Y. andH.A. wrote the paper.

The authors declare no conflict of interest.

*This Direct Submission article had a prearranged editor.

Freely available online through the PNAS open access option.1To whom correspondence may be addressed. E-mail: altug@bu.edu and yanik@bu.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1101910108/-/DCSupplemental.

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ing such spectrally sharp extraordinary light transmissions, weshow label-free biosensing with record high experimental figuresof merit (FOMs as high as 162) for detection limits surpassingthat of the gold standard prism coupled surface-plasmon sensors(25, 26). Extremely sharp plasmonic Fano resonances in high-quality plasmonic sensors also present dramatic intensity changesto slightest perturbations within their local environment. Thisintensity difference can be easily distinguished without using anyoptical detection instrument (camera and spectrometer, for ex-ample). By exploiting these Fano resonances and the associatedhigh contrast ratios of the on-resonance to the off-resonancetransmissions (Wood’s anomaly), we demonstrate direct detec-tion of a single monolayer of antibodies with naked eye. Suchsharp resonances could open door to a new generation of ultra-portable and ultrasensitive plasmonic biosensors for detection ofbiologically important molecules and pathogens (27, 28). Acrucial prerequisite for the record high sensitivities is the suppres-sion of the scattering losses caused by surface roughness andin-homogeneities. To achieve this, we introduce an exceptionallift-off free evaporation (LIFE) lithography technique for high-throughput fabrication of plasmonic devices with extremely uni-form and precisely controlled nano-features over large areas.

Results and DiscussionLIFE Nanolithography. Technique is summarized in Fig. 1 for thealternative schemes based on electron-beam lithography (EBL)and interferometric (IL) lithography, respectively. This techniqueeliminates the need for any ion milling and lift-off processes andallows fabrication of high optical-quality plasmonic devices (29).The EBL-based scheme (Fig. 1A) consists of three consecutivesteps: (i) fabrication of the free standing membrane, (ii) pattern-ing on the membrane with EBL and reactive ion etching, and(iii) direct deposition of metallic plasmonic devices (SI Text).Unlike, previous studies based on EBL, our fabrication methoddoes not involve any lift-off processes, and uses well-establishedpositive electron-beam resists. Interferometric lithography can beconveniently adapted for massively parallel fabrication of plas-monic devices over large areas. The IL-based scheme (Fig. 1B)consists of four consecutive steps: (i) spin coating of antireflectionand photoresist layers, and wafer-scale patterning of plasmonicdevices, (ii) side deposition of a metallic mask layer, and reactiveion etching of the SiNx layer, (iii) fabrication of the free standingnanostructures using wet and reactive ion processes, and (iv)direct deposition of the metal layer (SI Text).

Experimentally measured resonance line-widths are a soundmeasure of the life-times of the plasmonic excitations that iscontrolled by the strength of the radiative and nonradiative(ohmic) decay channels. For a perfectly uniform device with

no structural in-homogeneity or surface roughness, the radiativedecay processes are expected to be minimized (19). Fig. 2A showsthe calculated (dashed line) and experimentally measured (solidline) transmission spectra of an e-beam fabricated device (with aperiodicity of 600 nm and a nanohole aperture with d ¼ 200 nm).Here, the Drude model parameters are obtained from previousexperimental studies (30). Spectrally narrow plasmonic reso-nances (as narrow as Δλ ¼ 4.3 nm) with Q-factors (as high asQair ¼ 195) are demonstrated in our measurements. Close agree-ment of the experimental measured spectra with that of the finitedifference time domain (FDTD) simulations demonstrates thatour fabrication scheme enables fabrication of highly uniformstructures with minimal surface roughness leading to resonanceline-widths comparable to ideally uniform structures.

Subradiant Dark Modes in Extraordinary Light Transmission Phenom-ena. In our experimental measurements (Fig. 2A), multiple extra-ordinary light transmission resonances are observed with thecreation of surface-plasmon polaritons (SPP) corresponding tothe different grating orders. For a square lattice, the momentummatching condition between the in-plane wave-vectors of the in-cident photons and the surface-plasmon polaritons is fulfilledwhen the Bragg coupling condition ~kSP ¼ ~kx � i ~Gx � j ~Gy is met(31). Here ~ksp is the surface-plasmon wavecetor, ~kx incident-lightwave vector in-x direction (parallel to the metal surface), θ is theangle of incidence to the surface normal, ði;jÞ is the grating orderfor reciprocal lattice vector ~Gx and ~Gy of the square lattice(j ~Gxj ¼ j ~Gyj ¼ 2π∕p, p is the periodicity). The wave-vector rela-tion for the SPPs, within a first approximation, can be given asjkspj ¼ ðω∕cÞ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

εdεm∕εd þ εmp

, where ω is the frequency of theincident light and εm∕εd is the dielectric constant of the metal/interface-medium. Fig. 2B shows the spectral dispersion of trans-mission minima as a function of the angle of incidence for differ-ent grating orders. This diagram also provides a qualitative guidefor the dispersive behavior of the resonant transmission maxima(31, 32). At normal incidence, the transmission resonancescorresponding to ð0;� 1Þ, ðþ1;0Þ and ð−1;0Þ grating orders atthe metal/air interface are nearly degenerate and polarizationindependent (Fig. 2B). The dispersion of the modes with the vary-ing angles of incidence is due to the changing horizontal projec-tion of the incident wavevector ( ~kx). As shown in Fig. 2B, theð0;� 1Þ modes have minimal dispersions for small angles ofincidence, because ~kx and ~Gy are perpendicular to each other.Dispersions of the ð�1;0Þ modes are quite large, because ~kxand ~Gx are parallel. Fig. 2C shows the FDTD calculated spectralcharacteristics of the transmitted signals for varying angles of in-

LIFE NANOLITHOGRAPHY

Lift-Off Free Nanofabrication

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B IL Scheme

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hνhν O2+CF4 RIE

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Fig. 1. Lift-off free evaporation (LIFE) nanolithography is summarized for schemes based on (A) electron-beam and (B) interference lithography. SEM imagesof the fabricated devices are shown for (A) p ¼ 600 nm, d ¼ 180 nm and (B) p ¼ 580 nm and d ¼ 230 nm, where p is the lattice periodicity and d is the nano-hole diameter.

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cidence. For perpendicularly incident light (θ ¼ 0°), ð0;� 1Þ andðþ1;0Þ modes are subradiant as shown in Fig. 2C (black curve);i.e., they are weakly coupled to continuum. The ð−1;0Þ mode, onthe other hand, is superradiant; i.e., it is strongly coupled to con-tinuum and radiative. When illuminated at a nonperpendicularincidence angle (θ ≠ 0°), the retardation effects (field gradientcreated along the surface) allows coupling of the incident lightto ð0;� 1Þ and ðþ1;0Þ dark modes (Fig. 2C). In our experimentalmeasurements (Fig. 2A), splitting of these dark modes becomeobservable at an angle of incidence θ ≥ 1° as confirmed by ourFDTD simulations (Fig. 2A, dashed line). In agreement withthe analytical predictions, ðþ1;0Þmodes show strong dependenceto the angular incidence, whereas the ð0;� 1Þ modes are nearlyindependent and degenerate (Fig. 2C). For angles of incidenceθ ≥ 2°, continuum coupling to the ðþ1;0Þmode becomes strongerleading to broader resonances and stronger spectral overlap withthe ð0;� 1Þ modes. For angles of incidence θ ≥ 6°, the ð0;� 1Þmodes becomes dark again. There exists a critical optical regime(angular incidence of θ ≈ 1°) where the spectral overlap of the

ðþ1;0Þ and ð0;� 1Þ dark modes is small, whereas the incidentlight coupling to them is strong enough to allow modest lighttransmissions. By maintaining such alignment requirements,one can directly couple incident light to the spectrally narrowsubradiant dark resonances. These experimentally observed fea-tures are explained in SI Text using leaky eigen-modes (Fig. S1 inSI Text).

Subradiant Plasmonic Resonances at the Drude Damping Limit. Sub-radiant nature of the ð0;� 1Þ and ðþ1;0Þ modes can be under-stood from the near-field characteristics of the surface plasmonsat the air/metal interface (33, 34). Hy field profile of the SPPð−1;0Þ mode for TM-polarized incident light (x-polarized) isshown in Fig. 3A. The standing field pattern (x-direction) in thebackground is due to the SPP excitations, whereas the enhancedhot spots around the rims of the nanoholes in the form a magneticdipole are due to excitation of the localized surface plasmons(LSPs) (35). Near-field intensities of these local excitations dic-tate the coupling efficiency of the incident light to the Bloch

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Fig. 2. (A) Experimentally (red curve) and numerically (blue curve) obtained EOT spectra are compared. The resonance peaks corresponding to the metal/airinterface are indicated. (B) Spectral dispersion of the transmission minima is presented as a function of the angle of incidence for different grating orders.(C) EOT spectra is shown for varying angles of incidence using FDTD analysis.

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Fig. 3. Magnetic field profiles of (A) ð−1;0Þ bright mode, (B) Wood’s anomaly, (C) ð0;� 1Þ dark mode and (D) ðþ1;0Þ dark mode are shown. Electric field profilesreveal the (E) dipolar, (G) quadrupolar and (H) monopolar character of the ð−1;0Þ, ð0;� 1Þ and ðþ1;0Þmodes, respectively. (F) At Wood’s anomaly excitation ofthe dipolar field mode is minimal however, finite.

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mode of the nanohole openings, thus the strength of the resonanttransmission (35). For the ð−1;0Þ mode, these local excitationshave a dipolar mode character (Fig. 3E), which allows strong cou-pling of the incident light to the LSPs and leads to enhanced lighttransmissions in agreement with the experimental observations(Fig. 2). As shown in Fig. 3B, resonance transmission minimumresponsible from the spectral Fano line-shape is due to the nearlynull-field profile of the SPP excitation at the nanohole openings.This null pattern relates two fundamental observations: (i) weakcoupling of the incident light to the Bloch mode of the nanoholeopenings leads to transmission minimum (Fig. 3F); (ii) SPP dis-persion relations are minimally affected by the lower air-holeconductivities at the resonance minimum due to the negligibleoverlap of the SPP with the holes (36). Hence, the resonanceminima are observed when the Bragg condition correspondingto the excitation of SPPs on a flat unperturbated surface is met(36). As shown in Fig. 2C (black curve), ð0;� 1Þ and ðþ1;0Þmodes are subradiant for the perpendicularly incident light;i.e., radiative couplings are minimal. However, for incident lightimpinging with a small angle of incidence to the normal (θ ≈ 1°),the introduced field gradient (retardation effects) enables cou-pling of the dipolar incident field to the dark modes that haveno net-dipole moments (Fig. 3 C and D). Field profiles of theð0;� 1Þ and ðþ1;0Þ dark modes confirming their quadrupoleand monopole character are given in Fig. 3G andH, respectively.As a result, additional transmission peaks appear for nonperpen-dicularly incident light as confirmed by the experimental mea-surements (Fig. 2C). Unlike previous studies based on nano-particles and metamaterials, the spectral overlap of the dipolarsuperradiant ð−1;0Þ mode to monopolar/multipolar subradiantðþ1;0Þ∕ð0;� 1Þ modes is minimal (Fig. 2A). In other words,in-direct coupling of the incident light to the subradiant modesthrough the superradiant mode is negligible (Fig. 3 G and H).This leads high-quality factor (Qsub

ð0;�1Þ ≈ 100 andQsubðþ1;0Þ ≈ 200) re-

sonances. Close agreement with the experimental measurementsand the numerical simulations is observed (Fig. 2A) without em-ploying any enhanced damping constants for the thin gold films.

Ultrasensitive Plasmonic Nanosensors. Intrinsic detection limits ofthe resonance based biosensors are dictated by the spectral line-widths of the resonances and the refractive index sensitivities ofthe resonance wavelengths (17). In Fig. 4, refractive index sensi-tivities are obtained using NaCl solutions (Sigma–Aldrich 99.5%)at varying concentrations (5%, 10%, 15%, 20%) in deionized(DI) water with the corresponding refractive indices (1,3418,1.3505, 1.3594, 1.3684) obtained at room temperature. As shownin Fig. 4A, Inset, the plasmonic resonances are fitted to an ana-lytical Fano interference model (32) given as

TðωÞ ¼ T0 þ A��

1þ∑ν

qνΔεν

�2

∕�1þ

�∑ν

qνΔεν

�2��

; [1]

where Δεν ¼ 2ℏðω − ωνÞ∕Γν, T0 is the transmission offset, A isthe continuum-discrete coupling constant, ων is the resonantfrequency, Γν is the full width at half maximum (FWHM) ofthe resonance and qν is the Breit–Wigner–Fano parameter deter-mining asymmetry of the resonance profile for the νth resonancestate. Experimentally obtained sensor parameters are summar-ized in Table S1 (SI Text). Interferometric LIFE-lithography ofplasmonic nanostructures is shown to yield wafer-scale fabrica-tion of nanohole sensors with the similar optical qualities ofthe EBL fabricated ones (Fig. S2 in SI Text). Refractive index sen-sitivities as high as 717 nm are obtained (Fig. 4C). Remarkablynarrow plasmonic resonances with 4.29 nm (Qsub

ðþ1;0Þ ¼ 196.9) areshown in solution (DI water with 10% NaCl). Overall perfor-mances of the biosensors are often quantified by a general figureof merit FOME ¼ Si;jðeVRIU−1Þ∕ΓνðeVÞ in energy units, whichis the refractive index sensitivity Si;jðeVRIU−1Þ of the resonancefrequency divided by the resonance width ΓνðeVÞ of the plasmo-nic resonance (17). This quantity is widely accepted as a propermeasure for the performance of plasmonic biosensors based onlocalized and extended surface plasmons (17, 21, 37, 38). Inaccordance with the observed spectrally narrow resonances, ex-perimental FOMEs as high as 144.28 are obtained for the subra-diant ðþ1;0Þmode (see Table S1), a record high figure showing anorder of magnitude improvement over the reported values forplasmonic sensor based on nanoparticles and metamaterials(21, 37, 39). Due to high optical quality of the fabricated devicesenabled by LIFE lithography, even the superradiant ð−1;0Þmodes are shown to have record high experimental figures ofmerit (FOME ≈ 43) (38).

Surpassing the Detection Limits of Gold Standard SPR Sensors. Prismcoupled surface plasmon resonance (SPR) sensing is the gold stan-dard technique for real-time, label-free measurement of biomole-cular interactions, and plays an important role in drug discoveryand biomedical research (25). Any alternative modality must beadvantageous in its intrinsic detection limits. Nanohole basedplasmonic sensors have recently taken much interest due to theirultrasmall interrogation volumes and (near) collinear illuminationsettings. Normal excitation of SPPs by grating-coupling in nano-hole arrays holds the promise for massive multiplexing. However,plasmonic nanohole sensors have not been widely adapted as aresult of broader resonance line-widths and lower spectral resolu-tions due to larger radiative losses. The widely adapted metricfor the intrinsic resolving power of the prism coupled SPR sensorsis the figure of merit FOM ¼ Si;jðnmRIU−1Þ∕ΓνðnmÞ in wave-lengths units, defined as the refractive index sensitivitySi;jðnmRIU−1Þ of the resonance frequency divided by the spectralwidth ΓνðnmÞ of the resonance. As listed in Table S1, the obtainedfigure of merit (FOM ¼ 162 in wavelength units) for the ðþ1;0Þsubradiant dark mode is higher than the theoretically estimatedupper limits (FOM ¼ 108) of the gold standard SPR sensors

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(25, 26) as well as nearly an order of magnitude larger than thepreviously reported record high FOMs in nanohole sensors usingperpendicularly incident light (38).

Seeing Biomolecular Monolayers with Naked Eye. State-of-art detec-tion platforms require mechanical or light isolation as well asexpensive detection instrumentation for signal transduction. Inour sensing platform, the superradiant modes ð−1;0Þ have strongcoupling to the dipolar incident light leading to extraordinarylight transmission. This strong signal could be easily detectedby eye and discernible in ordinary laboratory conditions withoutany mechanical/light isolation. An experimental demonstrationof this with end-point measurements is summarized in Fig. 5.Initially the surfaces are activated and functionalized using ascheme summarized in an earlier study (27). Protein A/G (Pierce)is chosen as the capturing molecule due to its high affinity to IgGimmunoglobulins. Blue curve in Fig. 5B shows the transmissionspectra of a functionalized biosensor acquired from a nanoholearray of 90 μm × 90 μm in dimensions. A notch filter (FWHM≈10 nm) spectrally tuned to the plasmonic resonances peak is usedto filter the light outside resonant transmission peak of the brightð−1;0Þ mode (black curve in Fig. 4B). CCD images of the trans-mitted light are shown in Fig. 5A, in addition to images obtainedfrom an unfunctionalized control sensor. As shown in Fig. 5B,capturing of a single monolayer of mouse IgG antibody (Sigma–Aldrich) causes 22 nm red shifting of the plasmonic resonance.This resonance shift is large enough to cause spectral overlappingof the transmission minima (Wood’s anomaly) of the nanoholearray with the transmission window of the notch filter (Fig. 5C).Accordingly, a dramatic reduction in transmitted light intensitiesis observed after the capturing of a monolayer of the antibody.This intensity change is strong enough to discern with naked eyein ordinary laboratory settings (Fig. 5A). For the control sensors(Fig. 5C), spectral shift is minimal due to nonspecific bindingscausing negligible change in the transmitted light intensities (27).This detection scheme, not requiring dark-environment measure-ments, utilizes broad-band light sources allowing direct detectionwith human eye without any safety concerns. The transmittedlight spectrum through the superradiant modes is dependent onthe angle of incidence (Fig. 2C). As a result, the proposed detec-tion scheme could be implemented by utilizing nearly collimatedlight obtained from an integrated light emitting diode for user

independent measurements. Sensing surfaces employing goldare inert to the environment. No degradation of device charac-teristics (Q-factors and FOMs) is observed over extended periodsof time. Antibodies functionalized on the device surfaces are ob-served to be stable without any sign of significant denaturalizationup to 10 days at ordinary storage conditions for biomolecules.

ConclusionsWe introduced an ultrasensitive label-free biodetection techni-que based on Fano resonances in plasmonic nanohole devices.By utilizing spectrally sharp extraordinary light transmissionsthrough subradiant dark modes, we showed record high experi-mental figures of merit (FOMs as high as 162) for detectionlimits surpassing that of the gold standard prism coupled surfaceplasmon sensors. To achieve this, we developed a high-through-put LIFE nanolithography technique leading to spectrally nar-row subradiant dark resonances (FWHM of ≤4.26 nm) withnearly complete suppression of the radiative losses and high-quality factors (Qsub

ðþ1;0Þ ≈ 200). We also demonstrated direct de-tection of a monolayer of antibodies with naked eye by exploitingstrongly dispersive plasmonic Fano resonances and the asso-ciated Wood’s anomalies. The demonstrated label-free sensingplatform offers unique opportunities for detection of biologicallyimportant molecules and pathogens in resource poor settingsby eliminating the need for labeling and optical detection instru-ments as well as mechanical/light isolation.

Materials and MethodsLIFE Lithography. Nanohole devices were fabricated in a lift-off free manneron suspended SiNx membranes by using a single layer deposition process asdescribed in SI Text.

Spectral Measurements. All spectral data were obtained using a Nicon Eclipse-Ti microscope coupled to a SpectraPro 500i spectrometer and a PrincetonInstruments Acton CCD camera.

ACKNOWLEDGMENTS. This work is supported in part by the National ScienceFoundation (NSF) CAREER Award (ECCS-0954790), the Office of NavalResearch (ONR) Young Investigator Award, the Massachusetts Life ScienceCenter New Investigator Award, the NSF Engineering Research Center onSmart Lighting (EEC-0812056), Boston University Photonics Center, and theArmy Research Laboratory.

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Fig. 5. Extremely sharp plasmonic Fano resonances in high-quality nanohole sensors enable seeing single biomolecular monolayers with naked eye. (A) CCDimages of the transmitted light obtained from detection and control sensors are compared. Capturing of the antibody causes a dramatic reduction of thetransmitted light intensities through the detection sensors. (B) Transmission spectra are shown before (blue curve) and after (red curve) the capturing of theantibody. Spectral characteristic of the notch filter is also given (green curve). (C) Transmitted light intensities in the presence of the notch filter is given before(blue curve) and after (red curve) the capturing of the antibody.

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