Experimental Observation of Giant Chiroptical Amplification of Small

Article pubs.acs.org/JPCC

Experimental Observation of Giant Chiroptical Amplification of Small Chiral Molecules by Gold Nanosphere Clusters Rong-Yao Wang,*,† Peng Wang,† Yineng Liu,‡ Wenjing Zhao,† Dawei Zhai,† Xuhai Hong,† Yinglu Ji,§ Xiaochun Wu,§ Feng Wang,† Duan Zhang,† Wensheng Zhang,† Ruibin Liu,† and Xiangdong Zhang*,† †

School of Physics, Key Laboratory of Cluster Science of Ministry of Education, Beijing Institute of Technology, Beijing 100081, P. R. China ‡ Department of Physics, Beijing Normal University, Beijing 100875, P. R. China § CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: We have experimentally observed around 2 orders of magnitude circular dichroism (CD) enhancement in the visible region for cysteine molecules located in the hotspots of gold nanosphere clusters. The observed plasmon-induced CD responses show a significant correlation with the chiral nature of molecules at the hotspots. These results provide a concrete experimental demonstration on the predicted chiroptical transfer and amplification effect that arises from hotspot-mediated exciton−plasmon interactions in a strongly coupled metallic nanostructure, even though the exciton−plasmon coupling works at a far off-resonant regime. Our findings suggest here that plasmonic hotspot-based CD amplifier may provide a new strategy for ultrasensitive detection and quantification of molecular chiralitya key aspect for various bioscience and biomedicine applications.

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INTRODUCTION In resonant excitation of surface plasmon resonance (SPR) by external optical field allows metallic nanoparticles to concentrate electromagnetic field in subwavelength volumes, resulting in enormous field enhancement, especially at the so-called hotspots,1−3 e.g., nanogaps between adjacent particles. The intense local field enhancement promotes the light−matter interactions for the molecules located in the near field of metallic nanostructures, leading to various plasmon-enhanced spectroscopies, such as surface-enhanced Raman scattering (SERS),4,5 surface-enhanced fluorescence (SEF),6 surfaceenhanced Raman optical activity (SEROA),7 and Fano resonances in plasmon−exciton systems.8 Very recently, Govorov and co-workers9,10 predicted theoretically that the plasmonic hotspot effect can be used to realize circular dichroism (CD) spectroscopic enhancements for chiral molecules. More specifically, when a properly oriented chiral molecule situates in the nanogap of a nonchiral metallic particle dimer, upon both molecular dipolar field and external fields being greatly enhanced at the hotspot, Coulomb interactions between the molecule and particles would lead to an optical transfer of CD activity, from electronic transitions of the molecule to SPR absorptions of the metallic particles. As such, an intrinsically weak molecular CD response that is normally in 150−300 nm ultraviolet (UV) spectral region could be strongly amplified by presenting plasmon-induced optical activity in visible/near-infrared (vis/NIR) region. © 2014 American Chemical Society

This hotspot-mediated chiroptical transfer and amplification effect suggested a new strategy for ultrasensitive detection and characterization of chiral molecules in liquid phase. In such a chiral probe, the observed CD activity from nonchiral plasmonic nanostructures would be directly correlated to the molecular chirality of analytes. This induced plasmonic CD effect is substantially different from that in previous reports for chiroptical responses from chiral structures of metallic particles.11−20 In the latter case, intrinsic chirality of plasmonic nanostructures, such as gold particle helix,15 twisted arrangement of gold nanorods,13,14,17,18 is a prerequisite for achieving CD activity at the SPR frequencies, and the involved physical mechanism is associated mainly with interparticle plasmon− plasmon electromagnetic interactions,11,12 instead of the exciton−plasmon interactions. Recently, it has been reported that strong plasmonic CD response from chiral twisted geometry of gold nanorod or prolate ellipsoid dimers induced by molecular linkers can be used for ultrasensitive detection of DNA and other macromolecules.17−20 However, it appeared that such a generation of preferred handedness of chiral geometry and the resultant plasmonic CD activity may not be specific to the chirality of molecular linkerseven achiral molecules can trigger the formation of twisted gold nanorod Received: March 14, 2014 Revised: April 12, 2014 Published: April 14, 2014 9690

dx.doi.org/10.1021/jp5025813 | J. Phys. Chem. C 2014, 118, 9690−9695

The Journal of Physical Chemistry C

Article

Spectroscopic Measurements. All extinction and CD spectra were measured by using a spectropolarimeter (BioLogic MOS-450) for the L-GNSs and D-GNSs samples solution in a cuvette. The optical path of 10 mm was used unless otherwise stated. The Raman spectra of the samples solution were collected using a Leica microscope equipped with a confocal Raman spectroscopic system (Renishaw InVia) and a 6 mW 785 nm laser excitation source. Microstructure Characterizations. For sample preparations, silicon substrates (with positive charges at the surfaces) were immersed in the sample solutions for 10 min. Then excess solutions were removed by filter papers. Microstructure analyses of the samples were conducted using a scanning electron microscope (SEM, JEOL, JSM-7500F) operated at 5 kV. To obtain the high-resolution electron images, sample films were also prepared by putting a drop of 4 μL of diluted solution on a copper grid, and then the dried samples were analyzed using a transmission electron microscope (TEM, JEM-2010) operated at 200 kV. Theoretical Calculations. We extended the original theoretical model,10 which was for a chiral molecule inserted into hotspot of a particle dimer, to the system consisting of arbitrary distributions of particles by means of multiple scattering of electromagnetic multipole fields.33 We calculated the extinction and CD spectra for linearly arranged GNSs dimer/trimer/tetramer with an interparticle distance of 0.7 nm. In these systems, the Cys linker at hotspot was idealized to be a single-point-like dipole located at the center of a nanogap with an orientation along the long axis of the anisotropic clusters. More details of the simulations are referred to the Supporting Information.

pairs and consequently showing strong plasmonic CD response.17 Therefore, plasmon-induced CD from nonchiral metallic nanostructures is more advantageous for chiral detections and characterizations due to its specificity to molecular chirality of analytes. In principle, the plasmon-induced CD response depends crucially on the optical rotatory dispersion (ORD) strength of the chiral molecule at SPR frequencies, which is proportional to 1/(ω0 − ωplasmon),10 where ω0 and ωplasmon are the absorption frequency of molecule and the SPR frequency of metallic nanostructure, respectively. Thus, a small value of ω0 − ωplasmon, i.e., resonant or near-resonant exciton−plasmon interaction, is advantageous for observing remarkable plasmon enhanced molecular CD effect. Indeed, by carefully selecting molecules and/or metallic nanoparticles such that resonant21 or nearresonant22−25 exciton−plasmon coupling is satisfied, strong plasmon enhanced CD effect has been experimentally observed in some molecule−NPs systems. For example, Lu and coworkers22 demonstrated a strong CD amplification from a particular exciton−plasmon system, which is composed of discrete Ag-based nanocrystals with a special shape (i.e., nanocubes), to blue-shift the SPR bands to around 350 nm such that they approach the absorption bands of the DNA macromolecules at near 300 nm and near-resonant exciton− plasmon coupling is satisfied. For ordinary systems composed of e.g. gold-based particles with SPR absorptions beyond 500 nm and chiral molecules with the absorption at UV region, the far off-resonant exciton−plasmon interactions produced only weak even nondetectable plasmonic CD signals.22,26−31 In this work, we demonstrate experimentally that the hotspot effect in a strongly coupled plasmonic linear structure of gold nanoparticles can be used to promote the far off-resonant exciton−plasmon interactions and consequently to result in around 2 orders of magnitude amplification for molecular CD.

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RESULTS AND DISCUSSION As demonstrated in previous studies,30−32 thiol-containing Cys molecules can be adsorbed at gold surfaces via forming strong Au−S bonds, and assembly of the GNSs in aqueous environment could be mediated by Cys-Cys zwitterionic electrostatic interactions between carboxylic and amine groups of Cys. In this way, we obtained the composite system that is composed of the simplest geometrical arrangement of GNSs, i.e., linear structures of small clusters, with Cys molecules at the hotspots, as illustrated in Figure 1a. Microstructure characterizations by scanning/transmission electron microscopy (SEM/ TEM) suggested a high yield of small linear clusters in both the L- and D-GNSs samples. Figure 1b shows representative example from the D-GNSs clusters. Based on the statistical analysis from more than 500 particles in the SEM micrographs (Figure S2), the percentage of small clusters (with particles number n = 2−4 per cluster) is in the range of 76−87%. Among them, the linear structures are the major constituents (Figure S3). TEM analysis (Figure 1c) indicated that the average gap distance between neighboring particles is around 1.1 ± 0.6 nm, which is approximately the length of Cys-Cys linkage via intermolecular electrostatic interactions.34 The SERS spectroscopy (Figure 1d) demonstrated that strongly enhanced local fields exist at the hotspots of the L- and D-GNSs clusters. The Au−S bond at ∼269 cm−1 and the C−S stretching bond at ∼660 cm−1 are two characteristic Raman signatures of the Cys linkers.35 These Raman signatures justified the location of Cys molecular linker at the hotspots. The peak intensity of C−S stretching mode was used to evaluate the ensembleaveraged SERS enhancement factor (EF). The derived EF value

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EXPERIMENTAL AND THEORETICAL METHODS Synthesis. Strongly coupled plasmonic linear structure of gold nanoparticles were prepared by a well-established molecularly mediated self-assembly method.32 Herein gold nanospheres (GNSs) capped with positive charged stabilizer, i.e., cetyltrimethylammonisum bromide (CTAB), were prepared by a seed-mediated method (see the Supporting Information). The mean diameter of the used GNSs is 33.2 ± 2.9 nm, and a stoichiometry ∼3947 cysteine (Cys) molecules per nanoparticle was used. The control of linear assembly of GNSs into small cluster of dimer/trimer/tetramer was achieved by carefully tuning the balance between the electrostatic CysCys attractive force and the interparticle repulsive force due to the presence of positively charged stabilizer. For sample preparation, GNS/CTAB (0.85 × 10−9 M) colloidal solution with a proper value of zeta potential (e.g., ∼40 mV) was mixed with L- or D-Cys aqueous solution (6.75 × 10−6 M) under a volume ratio of 2:1. After a short stirring (e.g., 30 s), formation of small clusters was indicated by the color change of the sample solution. To stabilize the cluster structures, negatively charged poly(styrene sulfonic acid) sodium salt (PSS, MW 70 000, 2 g/L, 6 × 10−3 M NaCl) was added in the sample solution and stirred for 1 h. Then the as-prepared clusters preserved by PSS encapsulation were subjected to spectroscopic and microstructural characterizations. For convenience, the GNSs clusters formed with enantiomeric L-Cys and D-Cys linkers are denoted as L-GNSs and D-GNSs, respectively. 9691



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responses in L- or D-GNSs clusters was strongly correlated with the presence of L- or D-Cys molecules at the hotspots. Notably, the observed CD signals show two spectral features: (1) a bisignate spectral line shape in the Vis/NIR region with the zero-crossing point located at the coupled plasmon peak at ∼750 nm and (2) a good mirror-image of the plasmonic CD responses corresponding to the opposite enantiomeric D- and LCys linkers at the hotspots. Note that no CD signal in the region of 200−300 nm was detectable because a tiny amount of Cys with the concentration of ∼2.3 × 10−6 M is far beyond the detection limit of the used CD instrument (Figure S9). However, the prominent plasmonic CD response with strongly enhanced intensity in vis/NIR spectral region allows us to identify the enantiospecific Cys molecules adsorbed in the small GNSs clusters. Note that the above CD responses occurred at the coupled plasmon absorptions of the GNSs clusters, which is identified as a collective longitudinal mode in the linear structures.36 This suggests that the observed CD amplification effect would depend on the structural features of the GNS clusters. To verify it, Cys-mediated partially and/or fully random aggregates of GNSs were prepared (see the Supporting Information for more experimental details). As shown in Figure 3, the disordered

Figure 1. (a) Schematics showing Cys-mediated linear assembly of GNSs into a trimer. (b) SEM and (c) TEM images acquired from the D-GNSs clusters, respectively. (d) SERS spectra from the L-GNSs and D-GNSs clusters in solution phase.

is ∼105 for both the GNSs clusters, which is consistent with the previous reports on the similar systems.4 The chiroptical properties of the samples were detected by the CD spectroscopic technique. Figure 2a shows the typical

Figure 2. (a) Vis/NIR extinction spectra and (b) corresponding CD spectra from the isolated GNSs (blue solid line) without addition of Cys linkers and the L-GNSs (black solid line) and D-GNSs (red solid line) clusters formed with the addition of ∼2.3 × 10−6 M L- and D-Cys linkers, respectively.

Figure 3. SEM image (a) acquired from the disordered D-GNSs clusters; extinction (b), CD (c), and SERS spectra (d) of the disordered L-GNSs clusters (black line) and D-GNSs clusters (red line), respectively.

SPR spectral feature of the linear structures of GNSs clusters.36 Compared to the plasmon band at ∼520 nm from isolated particles (blue line in Figure 2a), a new resonant band appears at longer-wavelength side (black and red lines in Figure 2a). This red-shifted plasmon band is attributed to the strong nearfield plasmonic coupling between adjacent particles in the linear structures.36 Qualitatively, this new plasmon band with the peak position at ∼750 nm is in agreement with the above SEM observations in which small linear clusters of dimer/trimer/ tetramer are prevailed in the overall aggregations. Coincided exactly with this red-shifted plasmon resonance absorption, strong CD responses (black and red solid lines in Figure 2b) were observed from the GNSs clusters. Since no CD signal was detected from the GNSs colloidal solution without the addition of Cys linkers (blue solid line in Figure 2b), the strong CD

aggregates can still present strong enhancement of the local field at the hotspots, but show rather weak even nondetectable overall CD signal at the SPR frequencies. These results are consistent with that reported in previous studies.30,31 In these disordered aggregates of plasmonic particles, coexistence of opposite handedness of local chiral geometries was found to lead to a null of macroscopic CD effect,37 and random distribution of chiral molecules without a preferential orientation would reduce significantly the plasmon-induced CD signal.38,39 Compared to the disordered aggregates, the linear assembly of GNSs (Figure 1a) has two distinct structural features: (1) highly anisotropy of the spatial arrangement of the hotspots and (2) a preferential orientation of molecular linkers along the long axis of the anisotropic GNSs clusters due to the 9692



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Figure 4. (a) Mirror-image of the plasmonic CD responses in the probe of L-Cys (black solid line) and D-Cys (red solid line) molecules at the concentration of 9 × 10−7 M. (b) Linear correlation between the chiral purity of Cys molecules and the peak/valley intensities of the induced plasmonic CD bands measured at ∼700 nm (black scatters) and ∼790 nm (red scatters), respectively. These peak/valley intensities were derived from the corresponding full CD spectra shown in Supporting Information (Figure S10). The chiral purity is defined as the enantiomeric percentage (L-isomer:D-isomer) in the mixture of the Cys molecules with a fixed concentration at 9 × 10−6 M. Solid lines: the linear fittings with the Pearson coefficients equal to 0.997 and 0.986, respectively.

molecularly induced CD effect. For example, pronounced plasmonic CD response was observed from the side-by-side assembly of gold nanorod bridged by the long strand of DNA18 due to the fact that left- or right-handedness of chiral twisted geometry of gold nanorod pairs was produced by DNA linkers, but the end-to-end assembled collinear chains of gold nanorod with same DNA linkers exhibited nondetectable plasmonic CD signal. In the latter case, the nondetectable CD signal was attributed to the preferred nonchiral structure geometry and negligible CD induction effect due to a weak local field enhancement at hotspot caused by a relative large size of DNA linkers.18 In our case, the GNSs in the linear chains linked by small Cys molecules are closely connected and strongly coupled to provide intense electric fields at the nanogaps, thus facilitating plasmon-induced CD effect. Although the presence of chiral twisted geometry between the assembled nonspherical particle pairs cannot be excluded completely due to the distribution of different shapes and sizes of the particles, this chiral structure-based plasmonic CD would have negligible contribution to the overall CD signal, as demonstrated by the previous studies.18,31 In addition, it is noteworthy that chiral geometry-based plasmonic CD activity displayed less dependence on the chiral nature of molecular linkers.17 In contrast, we found that the plasmon-induced CD from nonchiral linear GNSs clusters possessed a strong correlation with the chiral nature of molecular linkers. For Cys molecules with the fixed concentration of 9 × 10−6 M, when the chiral purity of molecules was tuned by regulating stepwise the enantiomeric percentage (L-Cys:D-Cys) in the mixture, the sign and magnitude of plasmonic CD signal changed accordingly (Figure S10). For the racemic mixture, i.e. L-Cys:D-Cys = 50%:50%, a null CD response at plasmonic frequencies was shown, probably due to the average out of opposite sign of CD signals. Impressively, strong linear correlation (Figure 4b) exists between the chiral purity of Cys molecules and the peak/valley intensities of the CD bands (measured at ∼700 and ∼790 nm, respectively). This suggests that, similar to the conventional electronic CD technique, plasmon-induced CD spectroscopy is able to quantify the enantiomeric purity of the analytes, which would have important applications, for example, for enantiomer excess determination in asymmetric syntheses.40 Moreover, such a significant linear correlation provides an experimental

directional Cys-Cys electrostatic interactions at hotspots. These structural features could play a key role in promoting the hotspot-mediated exciton−plasmon interactions, consequently resulting in a strong induced CD amplification effect. We evaluated the chiroptical amplification for the plasmonic CD spectra of the linear GNSs clusters. As in previous study,10 the amplification factor (AF) was defined to be the ratio of transferred CD signal at plasmon frequencies to the native molecular CD in the UV region. Without precise knowledge of the number of molecular linkers located at hotspots, we first calculated a low-bound AF value by supposing that all the Cys molecules added in solution would make contributions to the observed plasmonic CD signals. Based on five independent measurements, the low-bound AF value are ∼84 ± 8 for the LGNSs clusters and 82 ± 8 for the D-GNSs clusters. As demonstrated above, the disordered aggregates of GNSs would have negligible contribution to the overall CD signal; therefore, it is rationalized that Cys molecules participating in the formation of small clusters should make a major contribution to the amplified CD signal. Based on the percentage distribution of small clusters (n = 2−4) (Figure S3), the AF values were calculated by supposing that all the Cys molecules could participate in the formation of GNSs clusters. We obtained the AF values as high as 99−119 for the L-GNSs clusters and 90− 109 for the D-GNSs clusters (see the Supporting Information for the details of calculations). Such an ultrahigh CD amplification effect from linear GNSs clusters allows us to do chiral analyses for Cys molecules in solution phase by using the plasmonic CD spectroscopic technique. In our experiment, a small quantity of L- or D-Cys molecules as low as ∼9 × 10−7 M can be detected, showing clearly a mirror-image of CD response at plasmon frequencies (Figure 4a). The conventional electronic CD measurement technique has not been able to detect and characterize enantiomer-specific Cys molecules with similar levels of sensitivity. We noticed that strong plasmonic CD activity presented in molecules−nanoparticles composites can also be attributed to the chiral architectures of metallic particles mediated/templated by chiral molecules.13−19 In this case, CD activity at the SPR absorption bands originates mainly from interparticle plasmon−plasmon electromagnetic interactions.11 Such a plasmonic CD effect based on intrinsic chirality of nanostructure is generally over 1 order of magnitude larger than that of 9693



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5b) exhibits a satisfactory agreement with the experimental results, particularly, a highly similarity in the bisignate CD line shape.

evidence to support the theoretical model that the induced plasmonic CD activity originates from hotspot-mediated exciton−plasmon interactions.9,10 For a deeper insight into the physical mechanism responsible for the above plasmon-induced CD phenomenon in the linear GNSs clusters, simulations based on multiple scattering of electromagnetic multipole fields were conducted. The calculated CD spectra for the dimer/trimer/tetramer system are plotted in Figures 5a, 5b, and 5c, respectively. The

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CONCLUSION We have demonstrated a simple strongly coupled plasmonic system composed of linearly arranged gold nanospheres dimer/ trimer/tetramer that can serve as a giant CD amplifier for ultrasensitive detection and characterization of molecular chirality. The experimental observation of the induced plasmonic CD spectral response upon the presence of small chiral molecules at the hotspots of the linear clusters is qualitatively supported by theoretical simulations that are based on the hotspot-mediated exciton−plasmon electromagnetic interactions operating at a far off-resonant regime. Moreover, this work provides a further verification of the universality and versatility of plasmonic hotspot effect on the plasmon-enhanced spectroscopy for molecular chirality sensing beyond the limitation of resonant/near-resonant exciton−plasmon coupling. We believe that this hotpot-based CD nanosensor would have a tremendous value in various biomedicine and pharmaceutics applications.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information for sample preparation; characterizations by electron microscopy, Raman/circular dichroism (CD)/ extinction spectroscopy; calculations and simulations. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 5. Calculated CD spectra for a GNSs cluster: (a) dimer, (b) trimer, and (c) tetramer. Insets show the position and orientation of a Cys molecular dipole at a plasmonic hotspot. μ⃗ and m⃗ denote respectively the electric and magnetic dipolar moments of a chiral molecule. Black and red lines correspond to the cases with L-Cys and D-Cys at a hotspot, respectively. I, II, and III represent the peak/valley and zero-crossing point positions in the CD signal of a trimer. (d) The corresponding extinction spectra for the GNSs dimer (black line), trimer (red line) and tetramer (green line).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel +86 010 68912632 (R.-Y.W.). *E-mail [email protected]; Tel +86 010 68912632 (X.Z.). Notes

corresponding extinctions are given in Figure 5d. One can see that, from dimer to tetramer, the induced plasmonic CD signals at the coupled plasmon modes show a gradual red-shift of the spectral position, which is in accordance with the redshifted longitudinal plasmon absorptions. Compared to the experimental results, the simulated extinction and CD spectra show deviations at bandwidths and peaks positions. Additionally, the AF values in our calculations are 11 for the dimer, 20 for the trimer, and 14 for the tetramer, which are nearly 1 order of magnitude smaller than the experimental results. These could be attributed to a much complicated situation in the real experimental system, for example, coexistence of different structures and compositions of GNSs clusters and various molecular orientations at hotspots. It is noteworthy that our theoretical model deals with a very simple scenario, i.e., only a single molecular dipole located at one of the nanogaps of a particles cluster. In a real system, there are two nanogaps in a trimer and three in a tetramer, and each nanogap includes many chiral molecular dipoles rather than a single molecular dipole. Moreover, collective effect of exciton−plasmon interactions in the linear arranged multiple hotspots would be expected to lead to a much higher CD amplification than individual hotspot units. These issues are worthy of further theoretical studies. Despite of the above limitations in our simulations, the simple two-/three-/four-particle models with a molecule at a nanogap can still reproduce the main features of the experimental CD spectra. Notably, the simulated CD spectra for a trimer (Figure

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. H. Zhang from the Ohio University for his kind help in our theoretical simulations. This work was supported by the National Natural Science Foundation of China (11174033, 91127013) and the National Key Basic Research Special Foundation of China under Grant 2013CB632704.

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Experimental Observation of Giant Chiroptical Amplification of Small

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