Amplification of Chiroptical Activity of Chiral Biomolecules by Surface

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Amplification of Chiroptical Activity of Chiral Biomolecules by Surface Plasmons Ben M. Maoz,† Yulia Chaikin,‡ Alexander B. Tesler,‡ Omri Bar Elli,† Zhiyuan Fan,§ Alexander O. Govorov,*,§ and Gil Markovich*,† †

School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel § Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, United States ‡

S Supporting Information *

ABSTRACT: Chiral molecules are shown to induce circular dichroism (CD) at surface plasmon resonances of gold nanostructures when in proximity to the metal surface without direct bonding to the metal. By changing the molecule-Au separation, we were able to learn about the mechanism of plasmonic CD induction for such nanostructures. It was found that even two monolayers of chiral molecules can induce observable plasmonic CD, while without the presence of the plasmonic nanostructures their own CD signal is unmeasurable. Hence, plasmonic arrays could offer a route to enhanced sensitivity for chirality detection.

KEYWORDS: Gold nanoparticles, surface plasmon resonance, chirality, circular dichroism, biosensing

S

chirality sensing tool.14−16 This preparation approach should be cheap, fast, and easy to implement. Moreover, it was shown that the basic interactions between chiral molecules to PNS’s can enhance the absorption and CD of chiral molecules17 and can be utilized for enhancement of CD of biological molecules for sensing applications.9 However, the nature of these interactions between chiral molecules and surface plasmons is yet to be resolved. One of the reasons for that is that there is no clear experimental evidence on the mechanism of the CD induction. A number of theoretical models16,18,19 proposed various possible mechanisms for plasmonic (or excitonic) CD induction, but lack of clear experimental evidence made it impossible to assign one particular mechanism. The following mechanisms were suggested: (a) coupling of electron wave functions of molecule and nanoparticle (orbital hybridization),20−23 (b) plasmon−plasmon interaction (in cases of chiral arrangement of nanoparticles),24−27 (c) molecule− plasmon Coulomb interaction,16,18,28,29 (dipolar and higher multipoles), and (d) long-range electromagnetic coupling in large enough nanostructures.14,19 While the first two mechanisms (orbital hybridization and plasmon−plasmon interaction) are less relevant to this case since the former relates to the case when covalent binding between the molecule and the PNS

ince Pasteur discovered the correlation between optical activity and chirality in tartaric acid,1 it was realized that the building blocks of life comprise chiral units such as amino acids and sugars. Therefore, biomacromolecules formed from these units also exhibit chirality on molecular and supramolecular scales. Chiroptical spectroscopic techniques such as circular dichroism (CD), optical rotatory dispersion (ORD), and Raman optical activity (ROA) are able to probe the molecular conformation but suffer from poor sensitivity.2,3 On the other hand, propagating- or localized surface plasmon resonances (SPR and LSPR, respectively)4−8 can sensitively detect minute changes in the plasmon excitation as a result of molecular adsorption to the plasmonic nanostructures (PNS’s), but they lack conformational information about the molecules, such as information about molecular chirality. The pioneering work by Hendry et al.9 used the interaction between chiral molecules to chiral PNS in order to use CD as a chirality detection technique. The main problem with such electron beam patterned plasmonic arrays is that they are hard to manufacture and expensive.10 In addition, such periodic patterns are also highly scattering due to their size and may cause a large linear dichroism contribution, which makes reliable CD measurements difficult.11 The present work is about using simple, achiral LSPR nanostructure films as a sensitive chirality sensing tool. This is done by taking achiral LSPR nanostructure films that are well-characterized,12,13 bringing chiral molecules in a quantitative manner to their close proximity, and using induced CD on the LSPR as a © 2013 American Chemical Society

Received: December 17, 2012 Revised: February 13, 2013 Published: February 14, 2013 1203

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Riboflavin in PMMA Films. Figure 1 displays scanning electron microscope (SEM) and atomic force microscope

exists, the latter is suitable for cases where the plasmonic nanoparticles are arranged in chiral superstructures.20,21,23−26 The other two mechanisms are characterized by different length scales: near-field and long-range interactions. Long-Range Interactions. This is basically a far field electromagnetic coupling mechanism which scales as 1/d, hence decays over a wavelength distance.15,19 In this case, the CD induction comes from solving Maxwell’s equations written for a chiral medium interacting with a large enough (∼100 nm) plasmonic structure.15,19 The CD is proportional to the thickness of a chiral shell (when a thickness is ≤ λ) and therefore comes from a large volume of chiral molecules which are located at distances of the order of λ from the plasmonic object. Near-Field Interactions. Near-field interactions are basically a molecule−plasmon dipole−dipole or multipolar coupling. The near-field effect prevails when d, the molecule-tometal-surface distance, is very small, and the coupling strength decreases rapidly with 1/(R + d)3, or a higher power, where R is the metal particle radius.18,29 In this case the induced plasmonic CD comes mainly from the molecules located very close to the PNS, and therefore, the interaction extends only over several nanometers, depending on the metal nanoparticle size. In the total electric field inside a molecule−PNS complex there are competition and interference between three terms: the incident field, the plasmon fields of a PNS, and the field created by a molecular dipole. The CD induction originates likely from the plasmon−molecule Coulomb interaction mechanism.18,28 In this mechanism, the plasmonic CD signal is due to optical absorption coming from the chiral currents inside a PNS induced by a dipole of a chiral molecule. We believe that this mechanism gives the main contribution to the CD line at the plasmon frequency when a molecular band is off the plasmon resonance. In this work, a simple plasmonic system based on gold island films deposited on glass is used in conjugation with a natural chiral molecule such as riboflavin and polylysine and an achiral organic matrix (details on the methods appear in the Supporting Information). This scheme was used in order to study pure electromagnetic type interactions, the consequent induction of CD at the surface plasmon resonance of the gold islands, and its molecule−metal separation distance dependence. The theoretical modeling together with experimental CD data obtained for samples with an achiral spacer between the chiral molecules and the PNS’s proves that the chirality is transferred from the chiral molecule to the surface plasmon via Coulomb fields (near-field mechanism). It is also shown that this induced plasmonic CD could be used for enhancing the sensitivity of detection of chirality in molecular films. Gold PNS films were produced by evaporating a thin layer of gold on glass without an adhesion layer, followed by annealing of the substrate to promote dewetting of the metal. The thickness of evaporated gold film on the glass substrates combined with the film annealing temperature allowed control of the size of the gold islands.12,13,31 Experiments were performed with three different molecular systems: (1) (−) riboflavin molecules embedded in poly(methyl methacrylate) (PMMA) thin films spin-coated on the Au PNS film (Figure 2a), (2) achiral molecular spacer film of varying thickness separating between a charged riboflavin derivative and the Au PNS’s (Figure 4a), and (3) a chiral polylysine monolayer adsorbed directly on the PNS substrate.

Figure 1. High-resolution SEM images of (a) bare Au islands film of 5 nm nominal mass thickness annealed 10 h at 550 °C and (b) 5 nm nominal thickness Au islands coated with PMMA. (c, d) Au island film of 10 nm nominal mass thickness annealed 10 h at 550 °C and exposed to 8 bilayers of PAH-PSS. (c) The in-lens SE detector shows topographic map, while (d) the high efficiency Everhart−Thornley SE detector shows the core−shell structure with an organic shell of ∼19 nm, marked by the arrows. Samples in b−d were coated with Cr for the SEM images. AFM images of Au island films of (e) 5 nm and (f) 10 nm nominal mass thickness.30

(AFM) images of two Au PNS films with: (1) an average island diameter of 32.6 nm with an average interparticle distance of 41.2 nm (Figure 1a) and (2) an average diameter of 103 nm with an average interparticle distance of 169 nm (Figure 1b). The interparticle distances are of the same order of magnitude or even larger than gold island sizes; therefore, in this system, the coupling of plasmonic fields between nanoparticles does not occur.32 As can be seen in Figure 1a,b, spin coating a PMMA film with dissolved riboflavin resulted in a uniform organic film coating the gold islands that is 19 ± 0.5 nm thick. The high efficiency Everhart−Thornley secondary electron detector in a high resolution SEM can image the organic film by tilting the sample, and then, its thickness can be estimated due to different contrast between metal core and organic shell. It should be noted that different island sizes were produced in this work and all showed the same results. As expected, the randomly oriented and shaped Au islands coated with PMMA are not chiral and do not exhibit any CD signal (Figure 2c). On dissolving the riboflavin in the PMMA solution and then coating the Au islands, a new CD signal appears at the plasmon resonance wavelength range (Figure 1204

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Figure 3. (a) Dipolar model of interaction between a spherical gold particle and a chiral molecular dipole (marked by arrow) used in the calculations. (b) Calculated CD spectra according to this model for the riboflavin/PMMA only and on top of the gold particle at an average separation of 9.5 nm, which is half of the PMMA thickness; particle radius is rnp = 23 nm, which is the average radius of the islands in the experiment. Figure 2. (a) A scheme of the experiment, bare gold islands (lateral dimensions: 75 nm × 64 nm, average height ∼20 nm) are covered with a 19 nm thick PMMA film which includes embedded riboflavin molecules. (b) Absorption spectra of bare Au islands, Au + PMMA, Au, and riboflavin embedded in PMMA and riboflavin/PMMA deposited on bare glass. Also shown is the absorption spectrum of Au islands after the PMMA and riboflavin were removed by acetone dipping. (c) CD spectra of the same samples.

depend on the medium surrounding the molecules. As the chiral center is located on the ribose sugar moiety and the chromophore is at two sigma bonds distance away from this center, the CD lines corresponding to the chromophore (isoalloxazine), appearing around 260, 360, and 450 nm, depend in intensity and sign on the relative conformation of these two parts of the molecule. If the ribose folds to interact with the chromophore π orbitals, or if the molecules aggregate together to form a chiral stack, where the isoalloxazine units interact with each other, than a stronger CD response is expected at those wavelengths. We believe that within the PMMA the relatively concentrated riboflavin is indeed aggregated and the latter mechanism is the cause of the significant CD signal at the three above-mentioned bands (only the 360 and 450 nm bands are visible through the glass substrate). The clear plasmonic CD signal obtained from such a small amount of riboflavin molecules (about 700 molecules per island) motivated us to push further the experiments to try to measure CD from much thinner layers of riboflavin and to obtain more information on the mechanism of the CD induction. Riboflavin Derivative on Top of an Achiral Spacer Layer. To study the distance dependence of the CD induction mechanism we used a layer-by-layer (LBL) deposition of achiral spacers between the PNSs and a riboflavin derivative bilayer. The LBL method was first introduced by Decher and coworkers.33,34 Kedem et al. used this method in order to characterize the plasmon decay distance from the gold surface.31 Adsorption of the polyelectrolyte layers on gold islands replaces air near the gold surface, produces shifts of the surface plasmon resonance band, and increases the extinction amplitude with the organic film thickness as:35,36

2c). When the PMMA layer with chiral molecules was washed away using acetone, the CD signal completely disappeared (Figure 2). As control samples bare gold islands, gold with PMMA only, and gold with PMMA and an achiral dye (Rhodamine b) were used. All of these control samples showed no plasmonic CD induction (see also Supporting Information, Figure S1). These interesting results reveal important new aspects of the CD induction phenomenon. Hence, the conclusion from the results shown above is that the CD mechanism in the present case does not involve orbital hybridization, as there is no direct binding between the riboflavin molecules and the gold. According to the simple dipolar model the strong CD line of the (aggregated) molecules at the 450 nm band should play a significant role in the formation of the plasmonic CD signal at 570 nm, as the molecule-induced plasmonic signal should scale as 1/Δω, where Δω is the frequency separation of the chiral molecular CD bands and the surface plasmon resonance.18,29 Figure 3 shows the calculated CD spectra according to the simple dipolar interaction model (see Supporting Information for details of the model, and ref 29), which consists of a molecular dipole separated from the surface of a spherical plasmonic gold particle by a distance d. This simple calculation reproduces qualitatively the experimental data. It is based on the molecular CD response of the riboflavin/PMMA without the gold. An important issue that should be taken into account is the nature of the CD spectrum of the riboflavin. This spectrum is sensitive to the conformation of the molecule and would thus

ΔP = mΔn[1 − exp( −d /l)]

where ΔP is the surface plasmon wavelength shift or peak extinction amplitude change, m is the refractive index sensitivity 1205

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(RIS), which accounts for the plasmonic island properties, Δn is the change in the refractive index of the surrounding medium between air and the adsorbate layers (polyelectrolytes), d is the dielectric (adsorbate) layer thickness, and l is the plasmon effective decay length. Figure 4a shows a scheme of the LBL spacing method in our experimental setup. We used achiral polyelectrolytes to build the LBL spacer, and the last two layers were alternating negatively charged riboflavin 5′-monophosphate bound to the positively charged polyelectrolyte. By changing the number of spacer layers we were able to obtain riboflavin 5′-monophosphate at separation distances from the gold surface (d)

increasing by 2.1 nm for every bilayer (positively and negatively charged polyelectrolyte layers). Figure 4b,c shows the absorbance and the CD spectra from a bilayer of riboflavin 5′-monophosphate with different separation distances from the gold surface (Figure 1c,d). An important result of this experiment was that the induced plasmonic CD signal was strong enough to be measured from a bilayer of riboflavin 5′monophosphate at sub 10 nm separation distances. Due to the low concentration and the way the riboflavin layers are adsorbed on the polycation layer, we believe that no riboflavin aggregates were formed and no visible or near UV molecular CD could be measured for these films. Consequently, it is assumed that the CD induction in this case is directly caused by the CD response of the sugar groups around 200 nm. It should be noted that direct measurement of the molecular CD from a riboflavin bilayer is not possible due to the low absorbance, which yields a weak and noisy CD signal that cannot be distinguished from the baseline of the spectrum. Consequently, the surface plasmon excitation of the gold nanostructures could be used for sensing of chiral molecules in thin films at surface coverage of about 90 molecules per island. Although it was shown that chiral PNS’s prepared by electron beam lithography can be used for biosensing,9 the fabrication of such chiral PNSs is complicated and expensive.10 The method presented here is cheap, simple, and sensitive. As well-known among experts on chiroptical spectroscopy, the Achilles heel of the CD is its poor sensitivity, and artifacts occurring when solid samples are measured, due to scattering and mixing of linear dichoism in the CD measurements. To avoid this situation, several control experiments were performed: (1) CD measurements with achiral molecules (Rhodamine) showing no induced plasmonic CD, and (2) totating the sample slides by 90° and 180° around the surface normal to make sure that the CD signal does not change (unlike linear dichroism). There is a number of interesting points regarding the enhanced plasmonic CD sensitivity: 1. In the framework of the dipolar model, the induced plasmonic CD magnitude and polarity are sensitive to the orientation of the molecular dipole relative to the metal surface.29 Thus, to obtain a substantial induced plasmonic CD the riboflavin molecules should be in a preferred orientation with respect to the metal surface; otherwise the signal might average to zero.19 One advantage of the plasmonic islands is that they already experience an asymmetric adsorption due to the presence of the substrate. In addition, the LBL deposition method may create a preferred molecular orientation around the gold islands.37,38 2. The measurable plasmonic CD signal (Δε) is observed due to the large plasmonic extinction coefficient (ε), where even a relatively low dissymmetry factor (Δε/ε) of the induced CD would lead to measurable Δε. The relatively large lateral size of the gold islands (>30 nm) helps in this respect. The interaction between the surface plasmons and the molecules may also produce enhanced CD and absorption at the molecular transitions.17 Another important issue is the sensitivity of the induced plasmonic CD response to the handedness of the probed chiral molecules. Previous experiments have shown that opposite enantiomers would produce opposite induced plasmonic CD responses,9,39 and the dipolar interaction model supports this effect. Figure 5 shows the spacer-thickness dependence of the plasmon peak wavelength and amplitude shift. The separation

Figure 4. (a) A scheme of the LBL experiment, where d is the total thickness of the polyelectrolyte spacer. Here the gold island average lateral dimensions were 34 nm × 31 nm and average height ∼12 nm. (b) Absorbance spectra of the Au islands as a function of spacer thickness. (c) CD spectra from Riboflavin derivative with increasing separation from the gold surface, measured at distances of d = 0.8, 2.9, 5.0, 7.1, and 20.2 nm, together with a bare gold CD spectrum. The CD spectra were smoothed for clarity. (d) Calculated CD spectra for different d values for a model system containing chiral molecules with a CD line at 200 nm. 1206

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than the dipolar model for spherical particles, because of large multipole contributions. In other words, the experimental CD may decay faster than the dipolar CD (∼1/(d + rnp)3) because the involved multipole fields decay as ∼1/(d + rnp)n with n > 3. Moreover, due to the large error bars the shape of the experimental decay curve may be somewhat inaccurate. In principle, the plasmonic island films could be tailored to have shorter interisland separations. Then, the elongated geometry of the islands together with the relatively narrow gaps between the particles would create strongly enhanced plasmonic fields which may further increase the plasmonic CD signals as predicted recently in theory.40 In general, two electromagnetic mechanisms (dipolar and electrodynamic) would play a role in the interaction of metal nanostructures with molecules around them. The relative contributions of the two mechanisms depend on the size of the metal islands. When the PNSs are substantially smaller than the wavelength, as in the present case, the local field effects would dominate the interaction with the molecules and the dipolar (or multipolar) mechanism would be the relevant one.18 The longrange electrodynamic mechanism of CD induction becomes important when the metal nanostructures become larger than 100 nm, and the molecular film thickness is also on a similar scale.19 Poly-L-lysine Layer on the Gold Island Film. Another issue that should be addressed is the importance of resonance between the molecular transitions and the surface plasmon excitations of the metal nanostructures (as in the riboflavin case). A good sensing device needs to be sensitive to a wide range of materials. Most of the protein, amino acids, and sugars exhibit absorption and CD in the UV range only,2 that is, far from the Au or Ag surface plasmon resonances. To obtain information on this aspect, poly-L-lysine was adsorbed on the gold island films, again by the LBL technique. The CD and absorbance spectra of these samples are presented in Figure 6a,b. It can be seen that 3 monolayers of polylysine induce a measurable CD at the plasmon resonance of the gold, while the UV CD (∼200 nm) of the same quantity of polylysine deposited on bare fused silica is hardly detectable. Although the polylysine absorbs in the far UV and it is off-resonance with respect to the surface plasmons, it interacts with the surface plasmons of the Au and induces a CD signal. This is not surprising since Hendry et al. have shown that similar biomolecules may interact with PNS’s and affect their chiroptical response.9 The stronger induced CD in the case of the riboflavin is probably due to the proximity of the flavin CD peak wavelength to the surface plasmon resonance of the gold. According to the dipolar model the induced CD should scale as 1/Δω, where Δω is the detuning between the plasmon resonance frequency and the molecular transition.18,28,29 Nevertheless, as mentioned above, the orientations of the molecules could also have a strong affect on the magnitude of the plasmonic CD induction in this case. In conclusion, we were able to obtain plasmonic CD induction in gold islands from a bilayer of riboflavin 5′monophosphate and from polylysine molecules. This induction decays within about 10 nm from the gold island surfaces. The sensitivity of the method to a small quantity of chiral molecules makes it an interesting candidate for general sensing of chiral biological molecules. Moreover, the chiral molecule does not have to be in resonance with the Au plasmon making the CD induction sensing a general method for biological molecules, although the sensitivity of the technique improves when the

Figure 5. (a) Plot of the plasmon extinction peak wavelength shift (relative to bare gold) as a function of spacer thickness. (b) Change in the amplitude of the extinction peak with respect to the peak of the bare gold islands as a function of spacer thickness. (c) The magnitude of the induced plasmonic CD peak as a function of the spacer thickness, showing both experimental data and dipolar model calculation.

dependence of the induced plasmonic CD is displayed in Figure 5c. It can be seen that the increase of the absorbance (extinction) as a function of spacer thickness reproduces the results of Kedem at el.31 As the number of layers increases the incremental increase in the intensity is reduced, and in parallel, the surface plasmon band wavelength is red-shifted, until both saturate at a thickness of about 20 nm. Figure 4c shows that the CD peak is red-shifted the same way as the absorbance peak. We note that in the model we do not have the red-shift effect since it involves only a single molecule that cannot noticeably influence the plasmon resonance. As seen in Figure 5c, the experimental decay length of the induced plasmonic CD is about 10−15 nm, which is of the same order as the absorbance change decay length of ∼20 nm. Thus, both effects correlate with the plasmon decay distance into the dielectric medium surrounding the gold islands. The calculated induced CD seems to decay with d somewhat slower than the experimental CD (Figure 4c). The reason can be the morphology of the experimental islands having elongated shapes with sharp corners and the local fields decaying faster 1207

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ASSOCIATED CONTENT

S Supporting Information *

Experimental methods and details of model calculation. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Israel Science Foundation grant no. 172/10, the James Frank program on light-matter interaction, and NSF (USA). The authors are grateful to Ofer Kedem for fruitful discussions and Alexander Vaskevich and Israel Rubinstein for their support of this work. B.M.M. was supported by The Tel Aviv University Center for Nanoscience and Nanotechnology.

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REFERENCES

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Figure 6. (a) Absorbance of bare Au islands, poly-L-lysine on glass and Au islands with poly-L-lysine. The gold island average lateral dimensions were 34 nm × 31 nm and average height ∼12 nm. (b) CD spectra of the same samples. It can be seen that a new CD peak appears at the plasmon resonance frequency in the gold-polylysine sample, while the UV CD line of polylysine around 200 nm is unmeasurable. Note that the spectra of the Au island samples are measurable down to ∼350 nm only due to the large absorption of the glass and Au substrates below this wavelength.

molecular absorption wavelengths get close to the plasmon resonance wavelength. The CD decay made it possible to learn about the mechanism of the CD induction. It was shown that in this case the dominant mechanism is likely a near-field (dipolar/multipolar) mechanism. The near-field induction of CD is expected to decay over a characteristic size of the nanocrystal.18,28 Indeed, the observed plasmonic CD decays within ∼10−15 nm, which is similar to the width of a gold islands. Importantly, the CD induction occurs over a relatively wide separation range (1−16 nm) that completely excludes the possibility of orbital-hybridization as a mechanism of CD induction. Thus, in this work we were able to provide a clearer experimental proof for a long-range mechanism of the CD induction and believe that it will help promoting the understanding of basic interactions between molecules and PNS’s in general. The advantage of these plasmonic substrates is that they are well-characterized, having tunable surface plasmon bands that spread from visible to near IR regions.13 They are reproducible with a long shelf lifetime, cheap, and easy to produce. In addition, the gold island films are extremely stable against immersion in various organic solvents as well as physiological buffers.41 On washing the polymer/molecules from the surface, the CD signal disappears, and the system is ready for the next experiment. Hence, this system offers a new concept for general sensing of molecular chirality, which may be useful in determining whether tested materials originate in an organism or not. 1208

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