Gold Nanostructures for Plasmonic Enhancement of Hyper Raman

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Gold Nanostructures for Plasmonic Enhancement of Hyper Raman Scattering Fani Madzharova, Zsuzsanna Heiner, Jan Simke, Soeren Selve, and Janina Kneipp J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10091 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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The Journal of Physical Chemistry

Gold Nanostructures for Plasmonic Enhancement of Hyper Raman Scattering Fani Madzharova†, Zsuzsanna Heiner†,§, Jan Simke‡, Sören Selve‡, and Janina Kneipp†,§* †

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany. §

School of Analytical Sciences Adlershof SALSA, Humboldt-Universität zu Berlin, Albert-Einstein-Str. 5-11, 12489 Berlin, Germany.

‡

Technical University Berlin, ZELMI, Strasse des 17. Juni 135, D-10623 Berlin, Germany.

E-mail: [email protected] Phone: +49 30 20 93 71 71

Abstract Surface enhanced hyper Raman scattering (SEHRS) is very useful for the vibrational characterization of organic and biological molecules and their interaction with noble metal nanostructures. Many potential applications should ideally make use of gold nanostructures in order to enhance both the excitation and the weak hyper Raman light, rather than of silver nanostructures. Here, we report high SEHRS enhancement from spherical gold nanoparticles with different particle diameters ranging from 30 nm to 70 nm and from gold nanorods. 1 ACS Paragon Plus Environment

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SEHRS data of the two molecules crystal violet and rhodamine 6G obtained at an excitation wavelength of 1064 nm, absorbance spectra, and FDTD simulations of the electromagnetic field enhancement provide evidence that the SEHRS enhancement relies on the formation of nanoaggregates, with higher SEHRS signals yielded with increasing size of the nanoparticles in the aggregates. Gold nanorods and their aggregates are shown to provide optical properties that are specifically suited to support enhancement of SEHRS. The reported results suggest that plasmon resonances at the excitation wavelength, as well as enhancement due to the lightning rod effect can contribute significantly to the total SEHRS enhancement. From the different concentration dependence of the signals of the different molecules as well as from comparison with salt induced aggregation it is concluded that the specific analyte induced aggregation determines the specific gold nanoaggregates geometry, arrangement and interparticle distances. Understanding the influence of the nanoaggregate properties therefore is crucial for exploiting gold SEHRS nanosensors in future applications. Introduction Surface enhanced hyper Raman scattering (SEHRS)1-2 is the nonlinear analogue of surface enhanced Raman scattering (SERS),3-5 and, as the latter, occurs for molecules residing in the highly confined local optical fields of plasmonic nanostructures.6-7 In SEHRS, the incoherent, spontaneous two-photon excited process of hyper Raman scattering (HRS),8-10 where the scattered radiation is shifted relative to the second harmonic of the excitation laser, provides information about the entire vibrational spectrum of the sample. Being dependent on a change in the hyperpolarizability of the molecule, SEHRS follows selection rules that are different from those acting in SERS,11 and hence SEHRS spectra can reveal complementary vibrational information, specifically from IR-active and silent vibrations. The effect benefits from the general advantages of near-infrared multiphoton excitation,12 including spatially more confined probe volumes,13 increased penetration depth and reduced photodamage for 2 ACS Paragon Plus Environment

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biological samples.14-15 In particular, a growing interest has emerged recently in exploiting SEHRS as a tool for probing of organic structures and materials,14, interactions20-23 as well as for microenvironmental sensing13,

16-19

24-25

molecule-surface

and spectroscopic

imaging.15, 26 As the cross sections of HRS are extremely low, the capabilities of SEHRS for sensitive vibrational probing strongly rely on plasmonic nanostructured materials providing high enhancement factors.27-28 In contrast to SERS, where the enhancement from numerous types of gold, silver, copper, aluminum and hybrid nanostructures with different morphologies in the liquid phase and on solid supports has been explored in great detail,29-31SEHRS experiments have been carried out mainly using traditional silver nanomaterials such as nanoparticles, nanoaggregates, and roughened electrodes.6, 32-34 Gold nanoparticles have been far less frequently used,14, 35 even though vibrational imaging and mapping of live cells with plasmonic nanoprobes and labels36 including SEHRS15, 37 would benefit very much from the use of gold nanostructures due to their high biocompatibility.38 Here, we investigate the SEHRS enhancement from spherical gold nanoparticles with different particle sizes and the same surface chemical properties, and from gold nanorods, which can provide plasmonic properties that are specifically suited to support enhancement of SEHRS. In particular, we discuss the effect of nanoaggregate formation on the electromagnetic field enhancement in SEHRS. As our spectra of the molecules crystal violet (CV) and rhodamine 6G (R6G) discussed here, as well as data on the plasmonic properties show, the type of interaction of the gold nanostructures with the molecules and with one another influences the enhancement, and therefore is crucial for exploiting gold SEHRS nanosensors in future applications.

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Materials and Methods Gold(III) chloride trihydrate (HAuCl4.3H2O, 99.9% trace metals basis), silver nitrate (99.9999 % trace metals basis), Hexadecyltrimethylammonium bromide (CTAB) (99 %), and L-ascorbic acid were purchased from Sigma-Aldrich. Trisodium citrate dihydrate (99%) was obtained from Th. Geyer, sodium chloride, sodium borohydride, and crystal violet were purchased from J. T. Baker, and rhodamine 6G from Lambda Physik. All solutions were prepared using Milli-Q water (USF Elga Purelab Plus purification system). Gold nanoparticles with different sizes were produced by reduction of gold (III) chloride with sodium citrate. The synthesis of gold nanoparticles with mean diameters ranging from 43 to 72 nm, as estimated from transmission electron microscopy, was carried out according to the procedure described by Frens.39 Briefly, 50 mL of 0.3 mmol/L HAuCl4 solution was heated to boiling. Then, different aliquots (from 350 to 500 µL) of trisodium citrate solution (1% by weight) were added, and the reaction mixture was kept boiling for 30 min. Gold nanoparticles with a mean diameter of 32 nm were produced by the Lee and Meisel method.40 Citrate reduced silver nanoparticles were prepared as reported previously.17, 40 Gold nanorods were produced by the seed-mediated growth method as described previously.41 For all further experiments, 10-fold diluted solution of the nanorods was used in order to achieve similar concentration as in the solutions of spherical nanoparticles. UV-vis spectra were recorded from all samples on a UV-vis-NIR double-beam spectrophotometer (V-670, Jasco) in the wavelength range between 300 nm and 1200 nm in quartz cuvettes of 10 mm path length. Transmission electron micrographs were taken using a Tecnai G2 20 TWIN instrument operating at 200 kV. In SEHRS experiments, gold colloids were mixed with water or 1 mol/L sodium chloride solution (for the experiments with pre-aggregation), and finally crystal violet or rhodamine


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6G solution was added to obtain the desired concentration. The volume ratio was 8:1:1 (nanoparticles: water: dye solution). Each sample was measured directly after preparation. SEHRS spectra were measured with a 1064 nm mode-locked laser producing 7 ps pulses at 76 MHz repetition rate, and its second harmonic was used for one-photon excitation at 532 nm. The liquid samples were placed in microcontainers, the excitation light was focused onto the samples through a microscope objective (NA 0.3), and the Raman scattered light was detected by a liquid nitrogen cooled CCD detector. Spectral resolution was 3-6 cm−1, considering the full spectral range. SEHRS spectra were typically accumulated for 5 s with an excitation peak photon flux density of 2.7x1028 photons cm−2 s−1 (average power at the sample of 400 mW). HRS spectrum of 10−3 mol/L crystal violet solution (9:1 water/methanol) was collected for 7 min (not shown here). SERS spectra of crystal violet were measured using 532 nm excitation (1.2x1027 photons cm−2 s−1, 10 mW average power at the sample) and 1 s acquisition time, and for SERS spectra of rhodamine 6G a 785 nm cw laser (3.9x1024 photons cm−2 s−1, 37 mW average power) and 1 s accumulation time were used. All spectra were frequency calibrated using a spectrum of toluene, and are presented without any further pre-processing unless explicitly stated otherwise. The overall SEHRS enhancement factors (EF) were estimated using the equation:

=

# 1

and are the intensities of the crystal violet band at 1586 cm−1 in the SEHRS and HRS spectrum, respectively, and are the number of crystal violet molecules in the SEHRS and HRS experiment, respectively. Note that equation (1) underestimates the real EF because it assumes that all crystal violet molecules take part in the SEHRS process. Using TEM data, we estimated the surface area of the nanoparticles, and independent on the orientation of the molecules with respect to the surface, we expect full coverage of the 5 ACS Paragon Plus Environment


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nanoparticles’ surfaces (ranging from 70% to 1000%). This coverage amounts to ~103-104 molecules per gold nanoparticle, depending on the orientation of the molecules and the nanoparticle size. In our calculations, we assumed a perpendicular interaction that was recently discussed to be a favorable orientation on silver nanostructures.21 In this case, highest numbers of molecules per nanoparticle would be achieved (corresponding to surface coverages of 70-110%), leading to a conservative estimate of the enhancement. The electromagnetic SERS42 and SEHRS27 enhancement factors GSERS and GSEHRS, respectively, generated by the different gold nanostructures were estimated from the surface plasmon absorbance spectra using the following empirical relations:

G =

G =

| | | | # 2 ̅ ̅

| | | | # 3 ̅ ̅

The relations consider the experimentally obtained absorbance Abs of the sample and the complex dielectric constant of the metal = ′ + #′′ )) at the incident laser ( ) and hyper Raman or Raman ( and ) frequencies, respectively; ̅ , ̅ and ̅ are the corresponding wavenumbers. Finite difference time domain (FDTD) simulations were carried out with Lumerical FDTD Solutions 8.6.3, using a mesh size of 0.5 nm for dimers and 1 nm for aggregates in the plotted area. For excitation, a plane wave was used, propagating along the negative z-axis direction. The frequency-dependent dielectric function of gold was taken from ref.

43

and the refractive

index of the surrounding medium was set to 1.33, as in the case of our SEHRS experiments. All output images were normalized to the intensity of the excitation source. For calculating the SEHRS enhancement, the electric field intensity enhancements at the laser (1064 nm) and Stokes HRS (580 nm) wavelengths were multiplied, that is, |Eex|4|EHRS|2. 6 ACS Paragon Plus Environment

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Results and Discussion SEHRS spectra from rhodamine 6G and crystal violet obtained with different gold nanostructures Gold nanoparticles were prepared by the citrate reduction method following the Frens,39 and Lee and Meisel40 protocols, resulting in spherical nanostructures with diameters ranging from 30 nm to 70 nm. The nanoparticle size distributions were determined from transmission electron microscopy (TEM), for TEM images of all gold particles see Supporting Information Figure S1. Figure 1A shows the UV-vis absorbance spectra of the colloidal gold nanoparticle solutions of the citrate stabilized, mostly spherical nanoparticles, displaying an increase of the absorbance maximum λmax with increased nanoparticle size (Figure 1B). This is in accord with previous findings.

39, 44-45

As in all syntheses, citrate serves as both the reducing and

capping agent, the chemical surface properties for all the spherical gold nanostructures are very similar. Figure 1C shows the absorbance spectrum of the CTAB-stabilized gold nanorods that were obtained according to ref.

41

, and that display a strong absorbance in the near

infrared due the longitudinal plasmon mode. As determined by TEM (see Supporting Information, Figure S1K- S1L), the nanorods have average dimensions of 80 nm x 17 nm. The SEHRS spectra of the two dyes crystal violet (CV) and rhodamine 6G (R6G) on all gold nanostructures were obtained by adding an aqueous solution of the respective molecule to the gold nanoparticle solution. Figure 2 shows typical SEHRS spectra yielded with spherical gold nanoparticles, Supporting Information Figure S2 contains several spectra obtained with the gold nanorods and for different concentrations of the two dyes. The SEHRS spectra of each of the molecules are qualitatively very similar, independent of the particle shape, size, and surface functionalization. The corresponding one-photon excited SERS spectra from the identical samples are shown in Supporting Information Figure S3. The spectra resemble those 7 ACS Paragon Plus Environment


previously reported in work using gold35 and silver nanostructures,2,

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26-27, 33-34

and also the

qualitative differences in relative band intensities between the one- and two-photon excited spectra (compare Figure 2 and Figure S3), determined by the different Raman and hyper Raman selection rules, are in good agreement with previous reports.2, 35 The second harmonic of the wavelength of 1064 nm that was used to excite the SEHRS spectra shown in Figure 2 and Figure S2 is close to the electronic transition of CV, as illustrated by the absorption spectra shown in Figure S4. Therefore, we observe a strong contribution of resonance enhancement19, 22, 46 of the SEHRS as well as of the 532 nm excited SERS. The relatively high similarity of the SEHRS and the SERS spectrum (compare top traces in Figure 2 and Figure S3) indicates that the same electronic transition in CV is responsible in both onephoton and two-photon excitation.47 As the absorption spectra of the two dyes illustrate (Figure S4), the excitation at 1064 nm allows to measure also a normal HRS spectrum of crystal violet, and thus to estimate an enhancement factor by comparing the intensity of the same bands in the HRS and SEHRS spectrum. It should be noted that it was not possible to use R6G for enhancement factor estimations due to very high two-photon excited fluorescence background in its HRS spectrum. By using the crystal violet band at 1586 cm−1, the enhancement factor for the gold nanoparticles was determined to range from 106 to 107 for the different spherical nanoparticles, and is 107 for the nanorods. We also estimated for comparison an enhancement factor for aggregates of citrate reduced silver nanoparticles to be on the order of 107 using the same experimental conditions regarding excitation and similar surface coverage of the silver nanoparticles with CV molecules as of the gold nanoparticles (spectra not shown here but previously in refs.

6, 13, 26

). Differing from this, Leng et al.34 reported an enhancement factor

obtained for silver nanoaggregates with CV on the order of 104 at 927 nm excitation, 8 ACS Paragon Plus Environment

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significantly lower than the enhancement observed for the gold nanostructures here. Lipscomb et al.35 discussed that the SEHRS enhancement factor for gold should be 1-2 orders of magnitude lower than for silver nanoparticles, without reporting absolute values. Here, our experimental results show that the plasmon enhancement on gold nanostructures acting for a SEHRS process at 1064 nm can be on the same order of magnitude as on silver nanoparticles. Similar to the observations made for an equally strong enhancement of one-photon excited SERS when gold and silver nanoparticles form nanoaggregates48-49 - in spite of different enhancement found for isolated nanoparticles - the similar enhancement in SEHRS for gold and silver nanostructures found here also strongly suggests the important role of nanoaggregate formation. In the SEHRS experiments, mixtures of the spherical gold nanoparticles of different size with the dye molecules yielded different overall signal intensities. In Figure 3 (colored dots) the SEHRS signals from both dyes are presented as a function of the mean size of the particles. To quantify the SEHRS signal, the band intensities at 1586 cm−1 and 1530 cm−1 for CV (Figure 3A) and R6G (Figure 3B), respectively, were used. For both molecules, the SEHRS signals increase with increasing size of the gold nanoparticles. As evidenced by the signals of other bands in the SEHRS spectra (see Supporting Information Figure S5), this trend is independent of the used band. Since in the case of the spherical, citrate stabilized gold nanoparticles we expect the same chemical contribution to the SEHRS enhancement from all types of nanoparticles, the difference in the SEHRS enhancement must come from differences in the electromagnetic enhancement for gold nanoparticles of different size. In order to analyze the plasmonic properties of the nanostructures in the different SEHRS experiments, UV-vis absorbance spectra of all samples were obtained (see Supporting Information Figure S6). The addition of dye molecules at 10−6 M concentration to the colloidal nanoparticle solutions results in the formation of gold nanoaggregates, which is 9 ACS Paragon Plus Environment


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indicated by the broadened plasmon band in the UV-vis spectra of the mixtures compared to the UV-vis spectra of the nanoparticles in the absence of the dye molecules (Figure S6), as well as from dynamic light scattering (DLS) data (Figure S7). Therefore, the observed increase in SEHRS enhancement with increasing particle size cannot be discussed in terms of electromagnetic enhancement from individual gold nanoparticles, as it has been done for SERS.50 The differences in the absorbance spectra of the different samples clearly show that the aggregates from nanoparticles of different size have very different plasmonic properties. For example, the broadening of the plasmon band for 70 nm gold nanoparticles is greater than that for the 30 nm gold particles (compare Figure S6A and S6J), explaining that the associated SEHRS enhancement at one particular excitation wavelength can vary for both types of nanoaggregates. Furthermore, dynamic light scattering experiments (Figure S7) indicate that also the size of the aggregates formed by the different gold nanoparticles differs. From the UV-vis absorbance spectra of the nanoparticle-dye mixtures (Figure S6), which contain information about the electromagnetic properties of the nanoaggregates, the electromagnetic SEHRS enhancement can be estimated empirically.

27, 42

Figure 3 compares

such empirical enhancement factors (black squares, scaled for clarity) calculated according to equation (3) with the actually measured SEHRS signal (colored dots). As can be seen from the data from crystal violet (Figure 3A), the estimated SEHRS enhancement for nanoaggregates, formed by particles with sizes from 30 nm to 60 nm, is very similar, although the measured SEHRS signal rises significantly. In the case of rhodamine 6G (Figure 3B) this is valid for gold aggregates from particles with sizes below 50 nm. These discrepancies support the assumption that most of the SEHRS signal originates from the “hottest” hot spots, i.e., from very few nanoaggregates, that are not observed in the far field UV-vis spectra, and whose contribution to the absorbance is ‘averaged out’ in the absorbance of the whole sample.


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Thus, SEHRS can serve as much more sensitive indicator of the presence of a few small, SEHRS active aggregates than UV-vis absorbance. Together with the absorbance spectra, the SEHRS signals obtained here illustrate the importance of the nanoaggregates’ properties, such as the arrangement of the nanoparticles in the aggregates and interparticle distance that have been discussed to exert an important influence on the enhancement in SEHRS.49, 51-55 As discussed so far, if the dye molecules are directly added to the nanoparticles, we observe increasing SEHRS signal with increasing size of the particles (Figure 3). We also performed experiments, where first, the formation of nanoaggregates was induced by the addition of sodium chloride, and then the dye solutions were introduced into the colloidal solution. In that case, the SEHRS signals from all nanoparticle solutions are very similar, and no tendency of an increase in signal with increasing particle size can be found (see Supporting Information Figure S8 and compare with Figure 3). The signals obtained with these pre-aggregated nanostructures (Figure S8) are similar to those of the nanoparticles without the addition of NaCl in the medium size range (Figure 3), and the high intensities found especially in the larger nanoparticles (Figure 3) are not reached, if pre-aggregation by NaCl takes place. The observed different SEHRS intensities from aggregates of the same nanoparticle solution, produced with and without preaggregation with NaCl, e.g., the signals generated by the 70 nm nanoparticles in Figure 3 and Figure S8, suggest that the arrangement of the nanoparticles in the aggregates plays a more significant role in determining the SEHRS enhancement than the particle size itself. As an example, aggregates that differ regarding nanoparticle arrangement, resulting in different plasmonic properties, are formed by sodium chloride (data not shown) and by the two respective dyes (Figure S6). It is possible that nanoaggregates with different nanoparticle arrangement and/or interparticle distances are the result of the much larger sizes of the dye molecules compared to that of the sodium and chloride ions.56 11 ACS Paragon Plus Environment


Electromagnetic enhancement of SEHRS from FDTD To examine the impact of the gold nanoparticle size and shape on the electromagnetic enhancement in SEHRS of the nanoaggregates, we performed 3D finite-difference time domain (3D-FDTD) simulations on simplified model systems, using the sizes of the nanostructures from our experiments. The SEHRS intensity enhancement, |Eex|4|EHRS|2, with excitation at 1064 nm and hyper Raman scattering at 580 nm, the latter corresponding to the crystal violet band at 1586 cm−1, was calculated for an arrangement of two gold nanospheres with sizes of 32 nm, 44 nm and 60 nm. Figure 4 shows the results for an interparticle distance of 2 nm. The plots in Figure 4A, 4B and 4C indicate that the enhancement in the junction between the nanoparticles rises with their increasing size, which is in agreement with the experimental data from the aggregates that were formed by the addition of dye molecules shown in Figure 3, as well as with previous reports that have discussed the distribution of enhanced local fields in the case of SERS.57 The separate intensity distributions of the excitation and the hyper Raman fields are displayed in Figure S8 in the Supporting Information for the three dimers of Figure 4A, 4B and 4C at the excitation (Figure S9A) and HRS wavelengths (Figure S9B). The maximum field intensity in the junction between the two particles at 1064 nm is ~230, ~400, and ~860 for nanoparticles in the size of 32 nm, 44 nm, and 60 nm, respectively (Figure S9A), showing an increase with increasing nanoparticle size. It should be noted that Figure S9 shows the actual field intensity distributions, but that in SEHRS the enhancement depends quadratically on the excitation field intensity. Therefore, the small variations of the field intensity at 1064 nm cause significant differences in the total SEHRS enhancement between particles of different size. In contrast, at the HRS wavelength of 580 nm, higher field intensities are found, however, the enhancement at the visible


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wavelength is almost the same for the particles of varying size, especially for the case of the gold particles of 44 nm and 60 nm in diameter, ~13200 and 13100, respectively (Figure S9B). The total SEHRS enhancement for the nanorod dimers, shown in Figure 4D, is in the junction between the nanoparticles around one order of magnitude higher than for the spherical nanoparticles. This is in accord with our experiments, where we observe a slightly higher enhancement factor for the gold nanorods compared to all spherical nanoparticles. The field intensity distributions in Figure S10 show that in the case of the nanorods, the local fields at the excitation wavelength in the near IR present the main contribution to the total SEHRS enhancement, around 4 to 5 orders of magnitude, in contrast to the spherical nanostructures, where the fields are significantly enhanced at both, the excitation and scattering frequency (compare Figure S10 with Figure S9). The high field enhancements at the IR excitation wavelength can be attributed to plasmon resonances, yet they can also be the result of enhancement by the lightning rod effect.58-59 We also performed FDTD simulations using example nanostructures that we observed in the TEM micrographs, which include particles of varying sizes and also particles that are not ideally or not spherical particles. The SEHRS enhancement for two nanoparticle arrangements, taken from the samples displayed in Figure S1A and S1J, is shown in Figure 5. In agreement with the simulations for the dimers (Figure 4) and with the experiments, the arrangement of larger nanoparticles displays higher maximum of the SEHRS enhancement factor (109.5) than the arrangement of the smaller particles (109). However, this difference is smaller than the one observed for perfectly spherical particles. The intensities of the electric fields at the excitation (1064 nm) and Stokes hyper Raman (580 nm) wavelengths are displayed in Figure S11A and S11B, respectively. In contrast to the dimer simulations, the fields at the visible wavelength are two orders of magnitude less enhanced (compare Figure S11B with S9B). Meanwhile, the field at the IR excitation wavelength shows stronger 13 ACS Paragon Plus Environment


enhancement for the aggregates with many nanoparticles (compare Figure S11A with S9A), which could result from plasmon resonances but also from an enhancement due to the lightning rod effect. It should be noted here that the spatial coordinates with maximum field enhancement at 1064 nm are not the same as for 580 nm. The interaction with analyte molecule determines the SEHRS enhancement As indicated by our spectra, the availability of nanoaggregates must play a key role for the observation of the strong enhancement of the HRS signals. At low crystal violet and rhodamine 6G concentration (10−7 M), no SEHRS spectra were obtained with spherical gold nanoparticles, even though spectra down to concentrations of 10−8 M were measured when gold nanorods were used as plasmonic structures (Figure S2). Also, as discussed above, when aggregation of the gold nanoparticles was induced by addition of sodium chloride, it was possible to collect SEHRS spectra at these concentrations (see Figure 6). The possibility to obtain SEHRS spectra with aggregates of the spherical nanoparticles (Figure 6) suggests that an R6G concentration of 10−7 M is not sufficient for the formation of aggregates with properties that result in high enough SEHRS enhancement with these nanoparticles. The SEHRS signal from crystal violet is lower than from rhodamine 6G, indicating that crystal violet and rhodamine 6G probably interact in a different way with the gold particles. This is also supported by the different absorbance of the aggregates in the presence of crystal violet and rhodamine 6G (Figure S6, compare the red and purple spectra in each panel), providing evidence that nanoaggregates with different properties must form. Similarly to the spherical nanoparticles, also the nanorods form aggregates in the presence of crystal violet and rhodamine 6G. The UV-vis spectra of gold nanorod solutions with crystal violet at different concentrations are displayed in Figure 7A, and indicate the presence of different types of aggregates when different amounts of the dye are added to the nanorod solution. Rapid changes in the optical properties of nanorod aggregates are observed for the 14 ACS Paragon Plus Environment

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concentration range between 1x10−7 and 1x10−6 M crystal violet, and are also associated with a strong increase of the CV SEHRS signals (see Figure 7B). From the fact that spectra with the nanorods can be obtained easily at relatively low dye concentration of 10−8 M (Figure S2), we conclude that already small amounts of CV and R6G can induce the formation of nanorod aggregates. This suggests a stronger interaction between the gold nanorods and the dye molecules than between the spherical nanoparticles. The formation of this type of nanorod aggregates has been discussed to be very beneficial for SERS as well.60-61

Figure 8A shows SEHRS signals at different dye concentrations for one of the solutions of spherical gold nanoparticles (71 nm). The SEHRS signal from both molecules depends on their concentration (Figure 8A, purple points for CV and red points for R6G). There exists a very small concentration range, for which the SEHRS signal rises very fast, and which differs in CV and R6G: For rhodamine 6G it is possible to acquire SEHRS spectra at lower concentrations, and the saturation of the signal is reached at 5x10−7 M, while the crystal violet signal does not rise further only at a concentration of 15x10−7 M. The saturation of the signals can be interpreted by a maximum possible surface coverage, or a maximum coverage of hot spots, being reached at this particular concentration. Excess molecules cannot participate in the SEHRS process, nevertheless, they can lead to further aggregation of the nanoparticles and to a decrease of the signal due to the diminished surface area when aggregates form. A similar dependence is observed also for the one-photon excited SERS signal, as plotted for crystal violet in the Supporting Information, Figure S12, or discussed in previous work.50 The absorbance spectra, shown in Figure 8B indicate that, the concentration range, in which we observe a strong increase in the SEHRS signal, is associated with significant changes in the optical properties of the gold nanoaggregates. The nanoaggregates, formed by different crystal violet concentrations, provide different enhancement. For analytical applications of SEHRS, 15 ACS Paragon Plus Environment


this may present a limitation that can be addressed or even an advantage. As indicated by the high standard deviation of the SEHRS signal of crystal violet (Figure 7) in the concentration range where SEHRS signals rise, some of the nanoaggregates at this concentration provide sufficient enhancement, and some do not. The data presented here indicate that observing the SEHRS signal at different concentrations of potential analyte molecules can reveal information about the interaction and adsorption of the molecule on the metal surface. Our results from the comparison of crystal violet and rhodamine 6G illustrate that the affinity of the rhodamine dye to the gold surface is higher than of crystal violet, and that this affects the sensitivity of the experiment. This is of great importance if SEHRS approaches are developed in order to address real analytical applications, such as studies on the interaction of biomolecules. Similar aspects have been discussed for SERS experiments, and their discussions led to very practical developments, e.g., the arrangement of silver substrates for the detection of proteins,62 or the immobilization of nanostructures in order to maintain defined enhancement for quantitative experiments.63-65 The observation that rhodamine 6G has a different affinity to the gold surface than crystal violet, reflected by changes in SEHRS enhancement, suggests the possibility to use SEHRS as a tool for investigating adsorption and interaction processes.

Conclusions We have demonstrated that gold nanostructures of different size, shape and also surface functionalization provide efficient plasmonic enhancement of hyper Raman scattering. By using HRS signals of the dye crystal violet, we estimate enhancement factors for aggregates of spherical gold nanoparticles and gold nanorods in the same order of magnitude as enhancement factors of silver nanostructures (106-107), which have been most frequently used as SEHRS substrates so far. This strongly suggests the important role of nanoaggregate 16 ACS Paragon Plus Environment

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formation, independent of the nanoparticle material. The high enhancement obtained with the gold nanostructures opens new possibilities for exploiting such particles as SEHRS nanosensors, e.g., in future analytical applications involving biological samples. As our experiments and theoretical simulations of the electromagnetic enhancements show, the SEHRS signals depend on the particle size and shape. SEHRS spectra of crystal violet and rhodamine 6G, obtained with spherical citrate reduced gold nanoparticles with sizes ranging from 30 nm to 70 nm, indicate that nanoaggregates, formed by these dye molecules, yield higher SEHRS intensities with increasing size of the particles in the aggregates. This is supported by FDTD simulations of the SEHRS enhancement in junctions between gold nanoparticles of varying size, which revealed that the field enhancement at the near IR excitation frequency was slightly more sensitive to variations of the particle size than the field enhancement of the hyper Raman scattered light in the visible. The square dependence of the SEHRS signal on the intensity of the excitation light leads to a strong influence of these small changes in nanoparticle size in the gold nanoaggregates. Furthermore, we observe higher SEHRS enhancement factors for aggregates of gold nanorods than of spherical gold nanostructures. Here, our data indicate that plasmon resonances at the excitation wavelength, as well as enhancement due to the lightning rod effect may contribute significantly to the total SEHRS enhancement in the local environment of the rod-like nanostructures. Experiments at dye molecule concentrations where no enhancement occurs, as well as SEHRS spectra obtained after pre-aggregation of the nanoparticles using sodium chloride indicated that the interaction of the gold particles with one another is required for the formation of nanoaggregate structures. Strong plasmonic enhancement of the HRS signals is provided when the analyte molecules induce the formation of the nanoaggregates. As indicated by our data, the same nanoparticle solution can form aggregates with different optical properties and SEHRS enhancement, depending on the analyte molecule, and its concentration. The 17 ACS Paragon Plus Environment


geometrical properties of these aggregates, specifically the arrangement of the particles in the aggregates and the inter-particle distances, might be the main factor that determines the total SEHRS enhancement, in addition to the size and shape of individual particles. As we have demonstrated using the examples of crystal violet and rhodamine 6G, both spherical and rodlike gold nanoparticles can be used for experiments with SEHRS that investigate interactions between different molecules and nanoparticles. Understanding the influence of potential analyte molecules on the enhancement in SEHRS will help us to control it, and to optimize it for potential applications in analytical chemistry and biophysics. Supporting Information Available Transmission electron micrographs of gold nanostructures, SERS and absorption spectra of crystal violet and rhodamine 6G, UV-vis absorbance spectra of nanoaggregates from gold nanostructures mixed with dye molecules, dynamic light scattering of gold nanoaggregates, FDTD simulations of the electric field distribution near gold nanostructures, dependence of SERS and SEHRS signal of different gold nanostructures on the dye concentration and nanoparticle size. This material is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgements We thank Dr Harald Kneipp for valuable discussions and support in setting up experiments. We also thank Vesna Zivanovic for useful discussions. We acknowledge Dr Franziska Emmerling and Dr Ralf Bienert for giving us the opportunity to perform dynamic light scattering experiments at Bundesanstalt für Materialforschung und -prüfung (BAM). Funding by ERC Starting grant No. 259432 MULTIBIOPHOT to J.K., DFG GSC 1013 SALSA to Z.H., and a Chemiefonds Fellowship (FCI) to F.M. is gratefully acknowledged.


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Figure 1: A) UV-vis spectra of the colloidal gold nanoparticle solutions used in the SEHRS experiments. B) Corresponding mean particle diameters as determined from transmission electron micrographs (see Supporting Information, Figure S1), the error bars represent the standard deviation. To determine the size, ~700 particles from each type were analyzed. C) UV-vis absorbance spectrum of the gold nanorods.


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Figure 2: SEHRS spectra of the dye molecules crystal violet (top trace) and rhodamine 6G (bottom trace) obtained with gold nanoparticles. Excitation: 1064 nm, acquisition time: 5 s, photon flux density: 2.7x1028 photons cm−2 s−1, dye concentration: 1x10−6 mol/L.



Figure 3: SEHRS signal (in counts per second, cps) of (A) the crystal violet band at 1586 cm−1 (purple dots) and (B) the rhodamine 6G band at 1530 cm−1 (red dots) as a function of the gold nanoparticle size. Each value is the mean of 30 spectra, the error bars represent the standard deviation. All SEHRS spectra were obtained under the experimental conditions described in Figure 2. The black squares in A and B represent the scaled estimated electromagnetic enhancement factors determined from UV-vis absorbance spectra of the SEHRS samples (see Supporting Information, Figure S5 for the original data) based on the empirical equation (3).


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Figure 4: SEHRS enhancement as result of 3D finite difference time domain (FDTD) simulations in the xy-plane for two gold nanostructures. The SEHRS enhancement is the product of the normalized electric field intensity distributions at the excitation (1064 nm) and Stokes HRS (580 nm) wavelengths, shown for gold nanoparticles (AuNPs) in the size of (A) 32 nm, (B) 44 nm, and (C) 60 nm, and (D) for gold nanorods. The legend in D applies also to panels A-C. The shortest distance between the two respective nanostructures is 2 nm in all cases. The polarization of the incident wave plane was in x direction. The monitor was placed in the equatorial plane. The maximum field enhancement in the junction between the two nanostructures is (A) 108.6 for the 32 nm spherical particles, (B) 109.3 for the 44 nm spherical particles, (C) 1010 for the 60 nm spherical particles, and (D) 1011.4 for the nanorods.



Figure 5: SEHRS enhancement as result of 3D finite difference time domain (FDTD) simulations in the xy-plane for two experimentally obtained arrangements of gold nanoparticles from transmission electron microscopy. The nanoparticle arrangement in A corresponds to the marked with red square part of the TEM image in Figure S1-A (mean particle size 32 nm), and in B to the marked with red square part of the TEM image in Figure S1-J (mean particle size 72 nm). The SEHRS enhancement is the product of the normalized electric field intensity distributions at the excitation (1064 nm) and Stokes HRS (580 nm) wavelengths, shown in Figure S11. The polarization of the incident wave plane was in x direction. The maximum SEHRS enhancement is (A) 109 for the arrangement of 32 nm particles, and (B) 109.5 for the arrangement of 72 nm particles.


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Figure 6: SEHRS spectra of the dye molecules crystal violet (top trace) and rhodamine 6G (bottom trace) obtained with gold nanoaggregates that were produced by pre-aggregation of gold nanoparticles with sodium chloride. Excitation: 1064 nm, acquisition time: 30 s, photon flux density: 2.7x1028 photons cm−2 s−1, dye concentration: 1x10−7 mol/L. The crystal violet spectrum is the mean of 30 spectra.



Figure 7: (A) UV-vis absorbance spectra of gold nanorods in the presence of crystal violet at different concentrations (B) SEHRS signal of the crystal violet band at 1586 cm−1 as a function of crystal violet concentration. Excitation wavelength: 1064 nm, acquisition time: 5 s, photon flux density: 2.1x1028 photons cm−2 s−1. The samples in (A) are identical to the samples in (B).


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Figure 8: (A) SEHRS signal of crystal violet at 1586 cm−1 (purple dots) and of rhodamine 6G at 1530 cm−1(red dots) obtained with gold nanoparticles with a diameter of 71 nm as a function of the dye concentration. The values were determined from 90 spectra, respectively, and the error bars represent the standard deviation. Excitation parameters for obtaining the SEHRS spectra are the same as described in Figure 2. Corresponding SERS data obtained with 532 nm excitation for crystal violet are shown in the Supporting Information, Figure S10. (B) UV-vis absorbance spectra of gold nanoparticles (71 nm) in the presence of crystal violet at different concentrations. The samples in (B) are identical to the crystal violet samples in (A).



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Gold Nanostructures for Plasmonic Enhancement of Hyper Raman

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