Published: January 5, 2011

r 2011 American Chemical Society 701 dx.doi.org/10.1021/nl1038242 |Nano Lett. 2011, 11, 701–705

LETTER

pubs.acs.org/NanoLett

Plasmonic Circular Dichroism of Peptide-FunctionalizedGold NanoparticlesJoseph M. Slocik,† Alexander O. Govorov,‡ and Rajesh R. Naik*,†

†Air Force Research Laboratory, Materials & Manufacturing Directorate, Wright-Patterson Air Force Base, Ohio 45433, United States‡Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, United States

bS Supporting Information

ABSTRACT: Nature is remarkable at tailoring the chirality of different biomolecules tosuit specific functions. Chiral molecules can impart optical activity to achiral materials inthe form of the particle’s electronic transition frequency. Herein, we used peptides ofdiffering secondary structures (random coil and R-helix) to artificially create opticallyactive chiral gold nanoparticles through peptide-nanoparticle interactions as observedby circular dichroism (CD) spectroscopy. This interaction produces a CD signal at theplasmon resonance frequency (∼520 nm) of the chiral peptide-nanoparticle complex.Aggregation of the peptide-coated nanoparticles using metal ions results in a red-shiftedplasmonic CD response. Our results suggest that chiroptical properties of nanomaterialscan be engineered using peptides.

KEYWORDS: Circular dichroism, peptides, metamaterials, chirality, gold nanoparticles,surface plasmon

Many biologically active molecules are chiral and, amazingly,most of them exhibit the same handedness. For example,

DNAs usually form a right-handed helix, whereas typical naturallyoccurring proteins are left-handed. Consequently, this points outto an important role of handedness in nature.1 Handedness, orchirality, of a biomolecule is tested optically by using light pulsesof opposite circular polarizations. The difference in the absorbenciesof these two circularly polarized pulses is called circular dichro-ism (CD) and CD is routinely used to study chiral molecules.In nature, biomolecules occupy a wide range of structures andconformations with naturally strong CD signals in the UV(200-300 nm), but these are essentially absent in the visible region.2

Nanomaterials such as metallic nanoparticles exhibit strong absorp-tion in the visible wavelength range but are achiral with no inherentchiroptical properties. As a result when biomolecules are assembledwith nanomaterials, biomolecules can impart chirality to thenanomaterial and artificially create a plasmon-induced CD signalin the visible spectral region. This was previously shown by alimited number of chiral biomolecule-nanoparticle complexeswhich include DNA,3 peptide nanotubes decorated with goldnanoparticles,4 cysteine-modified gold and silver nanoparticles,5,6

and small glutathione-capped gold particles.7 In the current literature,there are examples of structures assembled with different methods,including an assortment of nanoparticles with chiral adsorbates.8-14

Consequently, optical chirality of nanosystems is an attractivearea of research that is receiving a lot of attention because of itspotential as optically active components in devices. Herein, wedemonstrate the chiroptical properties of peptide-functionalizedgold nanoparticles and observe the appearance of a moderatelystrong visible CD signal ∼530 nm, corresponding to the gold

nanoparticle’s surface plasmon resonance frequency. Addition-ally, we also compare the plasmonic CD responses of aggregatednanoparticle assemblies. Along with the experiments, we presentamodel of a chiral-molecule dipole interacting with ametal plasmon.The calculated plasmon-induced CD spectra are in qualitativeagreement with the experimental observations. A negative sign ofthe observed plasmonicCD response is consistent with the theoreticalprediction based on the dipolar interaction between a left-handedchiral molecule (peptide) and a plasmonic nanoparticle.

For nanoparticle functionalization with peptides, 10 nm Aunanoparticles (1.14�1013 particles/mL) was added to the FlgA3peptide (8.6�10-6 M peptide) or E5 coil peptide (5.0�10-6 M)to promote binding of the peptide to the nanoparticle surface.Peptide binding was confirmed by Fourier transform infrared(FT-IR) as shown in Supporting Information Figure S1. FT-IRshowed the presence of IR stretching frequencies for the amide Iand II bands shifted from that observed in the unbound peptide.The peptide functionalized nanoparticles were created by using twodifferent peptides. The first peptide is a helical peptide named E5(CGGEVSALEKEVSALEKEVSALEKEVSALEKE VSALEK).15,16

The second peptide used is a previously identified unstructuredgold-binding FlgA3 peptide (DYKDDDDKPAYSSGAPPMPPF).17

E5 is a 38 amino acid helical peptide that forms a thiol linkage tothe 10 nm gold nanoparticle via the N-termini cysteine residueand extends from the surface similar to other cysteine-modifiedpeptides functionalized onto gold,18 while the FlgA3 is a 21 amino

Received: October 30, 2010Revised: December 20, 2010

702 dx.doi.org/10.1021/nl1038242 |Nano Lett. 2011, 11, 701–705

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acid peptide that noncovalently interacts through a flat-on two-dimensional (2D) conformation with the gold nanoparticlesurface via a multidentate fashion of multiple residues interactingwith the gold surface. These two peptides were selected becausewe were interested in investigating the two surface binding modesand whether the peptide-functionalized nanoparticles exhibitchiroptical properties. In terms of gold binding, both peptidesbind strongly to gold (Supporting Information Figure S2).

The E5 peptide was covalently attached to the gold nanopar-ticles and purified. The UV-vis spectrum of the E5-gold nano-particle complex shows the characteristic UV absorption of theE5 peptide and the surface plasmon resonance of the nanoparticle inthe visible range (∼520 nm). Using CD spectroscopy, the E5peptide displayed characteristic CD responses in the UV rangewith negative ellipticity peaks at ∼208 and 222 nm indicative ofan R-helical structure (Figure 1B), and the absence of CDresponses in the visible range. The bare achiral gold nanoparticlesshowed a noisy spectrum and the lack of a UV-vis CD response(190-700 nm). We attribute the noise due to the interaction ofincoming circularly polarized light with the electromagneticfields generated from the gold (see Figure 2B). However thegold nanoparticles do not exhibit any chiroptical features and areconsidered achiral, which is consistent with other unfunctionalizedmetallic nanoparticles.3,4,6 When the E5 peptide was covalentlyattached to the gold nanoparticle surface, we observed a moderateand reproducible CD response in the visible region of the spectrum.The negative ellipticity of the E5-gold nanoparticle complex inthe visible range correspond to the surface resonance of the goldnanoparticle (∼520 nm) that is presumably induced by thedipole of the peptide interacting with the plasmon resonance ofthe gold nanoparticle. The chiral E5 peptide interacts with thegold nanoparticle due to electronic interaction between the chiralpeptide moiety and metal electrons. Also observed were minorchanges in the UVCD spectra with the E5 peptide functionalizednanoparticles (Figure 1C). This can be due to conformationalchange from the peptide native state in solution upon binding tothe nanoparticle surface, and/or energy transfer from the peptideto the metal nanoparticle. It is likely that the second possibilitymight be the dominant mechanism since the E5 peptide stillretained its main helical features based on analysis of the CDspectra using CDPro software. Similar wavelength shifts has alsobeen observed upon protein adsorption to metallic surfaces.19

Another example is the observations with the FlgA3 peptidegold nanoparticle complex. As mentioned earlier, the FlgA3 peptideis an unstructured random coil peptide that binds via noncova-lent interactions to gold nanoparticles. We have previouslydemonstrated the interaction between a gold nanoparticle andthe FlgA3 peptide.17 The FlgA3 peptide interacts intimately withthe gold nanoparticle surface due to multidentate interactionsvia multiple amino acid side groups. As shown in Figure 2B,even the noncovalently interacting FlgA3 peptide-gold nano-particle complex induced a moderate and reproducible CDresponse at ∼520 nm, again due to the due to electronicinteraction between the chiral peptide and metal electrons. Aswith the E5-gold nanoparticle complex, we also observedchanges in the UV region of the CD spectrum that is attributeto changes in the peptide. Both a decrease in the intensity and redshift in the negative ellipticity were observed (Figure 2B,C).These changes in FlgA3 peptide structure can be attributed tochanges in the secondary structure upon interacting with thenanoparticle and energy transfer from the peptide to the metalnanoparticle.

Beyond single peptide functionalized nanoparticles, we haveexplored the plasmon-induced CD of peptide-assembled nano-structures by the addition of metal ions. The interactions of metalions with peptide-functionalized gold nanoparticles induces nano-particle aggregation and alters peptide secondary structure.20-22

Using the FlgA3 peptide and its propensity to coordinate metalions, we introducedmetal ions into the FlgA3 peptide-functionalizedgold nanoparticle solution via the addition of Pd4þ or Agþmetalsalt solution (200 μM). The FlgA3 peptide contains amino acidfunctional groups that can interact with metal ions. Charged,aromatic and hydroxyl-containing amino acids are known to

Figure 1. Optical characterization of gold nanoparticles functionalizedwith the E5 peptide (5.0 μM) with 1.14� 1013 particles/mL. (A) UV-visspectra of E5 and E5-gold nanoparticles. (Inset) Illustration of goldnanoparticle surface covalent linked to the E5 peptide via the thiollinkage (red circle). (B) CD spectra and (C) CD spectra of UV region ofE5 peptide gold nanoparticles with or without E5 peptide.

703 dx.doi.org/10.1021/nl1038242 |Nano Lett. 2011, 11, 701–705

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interact with metal ions. As shown in Figure 3A, the addition ofPd4þ or Agþ ions resulted in a rapid color change of the peptide-nanoparticle solution (Figure 3A) and a red shift of the plasmonpeak in the absorption spectra of the nanoparticle-metal ioncomplex. The broadening and shifting of the plasmon peak is dueto the aggregation of the nanoparticles induced by the metal ionas previously observed (Supporting Information Figure S4).22 ByCD, the addition of Pd4þ or Agþ metal ions to FlgA3 peptide-functionalized gold caused the plasmon-induced CD peak to redshift similar in magnitude to that observed by UV-vis spectros-copy due to aggregation of gold particles (Figure 3B). Thisshift and enhancement in the plasmon resonance peak reflectedin both the absorption and CD spectrum can be attributed tothe aggregation and plasmonic coupling of the close-packed

nanoparticle clusters. Notably, the negative UV ellipticity of thepeptide changed and increased in intensity upon metal ionaddition. Structurally, the FlgA3 undergoes a transition fromunordered to a helical-like structure upon addition of metal ions(Supporting Information Figure S5).

Using theoretical modeling to explain our experimental ob-servations, we considered a generic model incorporating a dipoleof a chiral molecule and a nonchiral plasmonically active metalnanoparticle. The plasmon-induced CD response can originatefrom the dipolar interactions between chiral peptides and nonchiralgold nanoparticles. According to the theory of optical activity,23

the CD signal of a chiral molecule is given by CDmolecule0 �

Im[μB12 mB21], where μB12 and mB21 are the electric and magneticdipole moments of a molecule, and indexes 1 and 2 refer to theground and excited states of a molecule. However, in thepresence of metal nanoparticle, this equation should be changedsince the electromagnetic field at the molecular dipole becomesmodified. Furthermore, according to the theory,24 an additionaland leading term in the CD signal, CDNP, should appear due todissipative currents in a metal nanoparticle (NP) induced by themolecular dipole. The total CD signal of molecule-nanoparticlecomplex is now given by CDmolecule-NP = CDmolecule þ CDNP,where CDmolecule � Im[(P̂ 3 μB12)mB21] is a modified CD signalfrom the molecule that includes now an important plasmon-enhancement factor P̂ (see Supporting Information). Importantly,our plasmon-dipole system is strongly off resonance, λdipole ,λplasmon, where λdipole∼ 200 nm is the absorption wavelength ofa biomolecule and λplasmon∼ 520 nm is the plasmon wavelength.

Figure 2. Optical characterization of gold nanoparticles functionalizedwith theFlgA3peptide (8.6μM)with 1.14� 1013 particles/mL. (A) UV-visspectra of FlgA3 and FlgA3-gold nanoparticles. (Inset) Illustration ofgold nanoparticle surface linked to the FlgA3 peptide via the noncova-lent interactions. (B) CD spectra and (C) CD spectra of UV region.

Figure 3. Optical characterization of FlgA3 peptide-coated gold nano-particles upon addition of Pd4þ or Agþ ions (200 μM). (A) UV-visspectra of FlgA3 gold nanoparticles and of metal-complexed FlgA3nanoparticles. (Inset) Optical micrograph of FlgA3-gold nanoparticlesand after addition of metal ions. (B) CD spectra of FlgA3 gold nano-particles and with Pd4þ or Agþ ions.

704 dx.doi.org/10.1021/nl1038242 |Nano Lett. 2011, 11, 701–705

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Therefore, a theory tells that CDmolecule,CDNP at λ∼ λplasmon.This happens since CDmolecule�Δλ-2, whereas CDNP�Δλ-1,where Δλ = λplasmon - λdipole is the wavelength detuning in ourpair. The leading contribution to the CD signal at the plasmonfrequency

CDNP ¼ A

�����3ε0

εNP þ 2ε0

�����2a3NPffiffiffiffiε0

pR3

ω Im½εNP�

� Imμ12xm21x þ μ12ym21y - 2μ12zm21z

pω- pω0 þ iΓ12 -Gω

� �� �ð1Þ

where εNP and ε0 are the dielectric functions for Au25 and water,

respectively, and k and ω are the wave vector and frequency,respectively; ω0 = 2πc/λdipole; R and aNP are the nanoparticle-molecule center-to-center distance and the nanoparticle radius,respectively. We observed that the contribution CDNP rapidlydecreases with the distance and can be important only for systemswith a relatively small nanoparticle-molecule separation. In Figure 4,we show the calculated function CDmolecule-NP for a dipolemimicking a peptide dipole. The chosen distances R = 5.3 and6 nm and the nanoparticle radius aNP = 5 nm. The correspondingdipole-to-surface distances are Δ = 0.3 and 1 nm. The distanceΔ = 0.3 nm corresponds to the separation between the center of apeptide lying flat on the surface and the surface of the goldnanoparticle. This case mimics the FlgA3 peptide as a diameter ofa peptide molecule is∼0.6 nm. Details of modeling can be foundin Supporting Information and in ref 24. The theory predicts theappearance of a plasmon band at λplasmon ∼ 520 nm for a dipoleof chiral molecule attached to a nanoparticle surface (Figure 4).And this is what we have observed in our experiments with E5and FlgA3 peptides (Figures 1B and 2B). Another feature in thetheoretical curves is a decrease of the CD at 200 nm for theshortest distance (Figure 4). In our model, it comes from energytransfer to the metal nanoparticle. The experimental CD curvesalso exhibit this behavior in the UV region that is attributed to thepeptide (Figures 1B and 2B). We should stress that our theory isable to provide only qualitative description for the followingreasons. First, an available analytical theory24 is developed for apointlike dipole, whereas a peptide is an extended object. Second,the theory based on theCoulomb interaction (dipole andmultipole)does not include orbital hybridization effects that may occur atshort distances. An alternative explanation of the observedplasmon-induced CD is a modification of surface states of metal

nanoparticle via a contact interaction with a chiral adsorbate(peptides).8,9 However, the dipolar theory predicts substantialCD signals at the plasmon wavelength and, therefore, it is a rea-sonable explanation for the observed plasmonic CD. Furtherexperimental and theoretical investigations are underway toresolve this issue.

To conclude, we demonstrated a strong induced opticalchirality in the visible range of spectrum using chiral moleculesand nonchiral plasmonic nanoparticles. Ultimately, the abilityto induce chiroptical properties within nanosystems throughthe use of biomolecules may have tremendous implications formetamaterials, recognition/separation of chiral molecules, anddesign of nanostructured assemblies.

’ASSOCIATED CONTENT

bS Supporting Information. Nanoparticle functionalization,nanoparticle assembly, characterization of peptide-nanoparticleinteraction, theoretical details, and additional figures. This material isavailable free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: Rajesh.Naik@WPAFB.AF.MIL.

’ACKNOWLEDGMENT

We acknowledged the support of this work by the Air ForceOffice of Scientific Research and the Materials and Manufactur-ing Directorate (AFRL/RX). A.O.G. acknowledges support ofthis work by the National Science Foundation.

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Figure 4. The calculated CD spectra for a dipole of chiral molecule(black curve for R =¥) and for a dipole-nanoparticle complex with twoseparations (R = 5.3 and 6 nm); the absorption wavelength of the dipoleλdipole = 200 nm. Inset shows the model used.

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