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Mapping Infrared Enhancement Around Gold Nanoparticles Using Polyelectrolytes Harekrishna Ghosh, and Thomas Burgi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10613 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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The Journal of Physical Chemistry

Mapping Infrared Enhancement Around Gold Nanoparticles Using Polyelectrolytes Harekrishna Ghosh and Thomas Bürgi* Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland.

Corresponding author: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Enhancement of infrared signals from polyelectrolyte (PE) adsorbed on gold nanoparticles (GNPs) was studied in situ by attenuated total reflection infrared (ATR-IR) spectroscopy. Nanoparticles and polyelectrolytes were deposited using layer by layer (LBL) techniques and the IR signal was studied as a function of particles size, particles density and distance from particle surface. It was observed that enhancement is more pronounced for larger nanoparticles and it decreases with increasing distance from the particles surface. Furthermore at high GNPs coverage the signal from the first polyelectrolyte layer is particularly enhanced and the signal increases slowly with time in contrast to subsequent layers. We assign this to polyelectrolyte adsorption within narrow gaps between nanoparticles, where the electric field is enhanced. Furthermore enhanced absorption was observed in the gap between the GNPs and the germanium (Ge) internal element, which was confirmed by polarized measurements. This enhancement is more pronounced for silver particles and it represents a promising route for analysis of surfaces by infrared spectroscopy.

INTRODUCTION Infrared absorption of molecules can be enhanced, if they are adsorbed on metal surfaces such as gold, silver and copper. This effect is known as surface enhanced infrared absorption (SEIRA).1-4 Molecules on rough metal surfaces show typically infrared absorption that is 10– 1000 times enhanced with respect to measurements without the metal.5 The strength of the enhancement depends on the metal types, on surface morphology as well as on the nature of molecular binding with the metal.6 Particularly metal island films close to percolation have shown pronounced infrared enhancement. SEIRA has potential application for sensing and


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detection purposes where small amounts of molecular compounds need to be identified. Furthermore SEIRA can be used to study chemical reactions at metal surfaces.7 A major challenge represents the fabrication of suitable substrates in a reliable and reproducible way. Compared to ultra-high vacuum processes, wet chemical preparation methods have the advantage to be less complex, less time-consuming, and less expensive.8 An attractive alternative to metal films for infrared enhancement are metal nanoparticles. An advantage of metal nanoparticles over random island structures is the possible tuning of the particles’ optical properties by varying their size, shape and interparticle distance.9 Over the past decade a lot of experiments have been reported that measure the enhancement from nanoparticles of different size and shape by surface enhanced Raman scattering (SERS).10-12 Maximum values for electromagnetic enhancement for isolated single colloidal silver and gold spheroids are reported on the order of 105 to 107, whereas closely spaced interacting particles can provide extra field enhancement leading to even larger enhancement factors.10,

13

For anisotropic particles

enhancement can also be increased. For a single nanostar SERS signal enhancement was found to be up to 1010.14 Compared to SERS the enhancement factors found in SEIRA are weaker. Also, the physics of the SERS effect is much better understood. Compared to Raman scattering the cross sections for infrared absorption are intrinsically larger. However, in SERS the electromagnetic enhancement factor is proportional to the fourth power of the field incident on the molecule, whereas for SEIRA the enhancement factor depends on the square of the electromagnetic field. Apart from the enhanced electromagnetic field, which contributes most to the observed enhancement factor, the orientation of the analyte molecule with respect to the metal surface and a chemical enhancement were identified to play a role. For p-nitrobenzoic acid on silver island films an


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overall enhancement factor of 500-600 was found with contributions of factor 20-30 from electromagnetic enhancement, factor 3 from orientation and factor 10 from chemical enhancement due to chemical bonding between molecule and silver surface.15 We recently reported the preparation using the layer by layer (LBL) approach and optical properties of polyelectrolyte – metal nanoparticle composite films grown on glass slides and on Ge crystals used for attenuated total reflection infrared (ATR-IR) spectroscopy.16-20 LBL deposition of polyelectrolytes is a versatile technique21 with possible application in biomedicine22, solar cells23, drug delivery24 and light-emitting diodes.23 The environment friendly, aqueous solution based technique can be used to incorporate charged particles such as metal nanoparticles into multilayer systems.17 We recently showed that polyelectrolyte – metal nanoparticle composite films can be used as a sensor.18 Different sizes and types of citratestabilized nanoparticles were assembled in layers and the polyelectrolyte LBL technique allowed one to tune the distance between particles within different layers with nanometer precision.17 In this way enhancement from particles can be measured as a function of size and nature of the particles and the distance between absorber and particles surface. In the present work ATR-IR spectroscopy25-28 and scanning electron microscopy (SEM) were used to investigate enhancement of polyelectrolyte signal from nanoparticles. SEM was used to determine the number density of particles on the Ge surface and the structure of the amorphous film whereas in situ ATR-IR spectroscopy was used to measure the absorption due to polyelectrolyte in different sample geometries. In ATR-IR spectroscopy an evanescent field probes the volume close to the internal reflection element. The penetration depth of the field is several hundred nanometers, which is considerably larger than the typical thickness of the polyelectrolyte - nanoparticle composite films (tens of nanometers). It was found that


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enhancement of infrared absorption is larger for larger nanoparticles. Furthermore, enhancement in gaps between nanoparticles is particularly pronounced. It is also demonstrated that adsorption of nanoparticles on a surface leads to enhancement of signals from molecules below the nanoparticles, an effect that is more pronounced for silver than for gold nanoparticles.

EXPERIMENTAL SECTION Methods and Materials. Hydrogen tetrachloroaurate (III) hydrate (Alfa Aesar, 99.999% metal basis), silver nitrate (Sigma-Aldrich, 99.9999% metal basis), sodium citrate (Sigma-Aldrich), poly(allylamine hydrochloride) (Alfa Aesar, average molecular weight of 120,000-200,000), poly(sodium 4-styrenesulfonate) (Sigma-Aldrich, average molecular weight of 70,000), tannic acid (Sigma-Aldrich), potassium carbonate (Sigma-Aldrich) were used as received. Commercial gold nanoparticles of 10, 20, 40, 60, 80 and 150 nm diameter, optical density (OD) 1, stabilized suspension in citrate buffer were also purchased from Sigma-Aldrich. All solutions were prepared using Milli-Q water (18.2 MΩ·cm).

Synthesis of spherical metal nanoparticles Preparation of 10 nm gold nanoparticles solution. For the preparation of 10 nm gold nanoparticles (GNPs) in diameter, several methods have been used in the past like the Turkevich method29 and the seed growth method.30 However, all these methods have limits for the synthesis of small GNPs because particles are not monodisperse and due to problems of stability and aggregation. For this purpose 10 nm gold nanoparticles solution was prepared in the following


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way (the procedure was adopted from ref

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). First 250 mL of 0.25 mM HAuCl4 was heated to

600 C in an oil bath. Separately in another oil bath 6.25 mL of 0.03 M sodium citrate solution was heated to 600 C and then 500 µl of 2.5 mM K2CO3 and 500 µl of tannic acid (1%) were mixed. After 5 min the whole mixture was transferred quickly to the HAuCl4 solution heated at 600 C. Heating at 60° C was continued for 15 min during which the color of the solution remained wine red. Then temperature was raised to 95°C and the solution was held at this temperature for 10 min. Finally the wine red colored solution was cooled down and its UV-Vis spectrum was checked (see TEM images in SI Figure S1). Using this method even smaller GNPs (2-5 nm) can be prepared. Preparation of 20 nm gold nanoparticles solution. Gold nanoparticles (GNP) solution was prepared according to the well-known Turkevich method.29,

32-33

In order to prepare spherical

GNPs with a diameter of about 20 nm, first 600 mL of a 0.25 mM solution of HAuCl4 under constant magnetic stirring was heated to 100°C in an oil bath. Then 15 mL of a 0.03 M sodium citrate solution was added to the HAuCl4 solution to reduce the gold ions. A series of color changes were observed up to 20 minutes. Finally, when the solution changed to a deep-red color the reaction vessel was removed from the oil bath and allowed to cool to room temperature. Preparation of 31 nm gold nanoparticles solution. Bigger gold nanoparticles were prepared by altering the ratio of gold salt to sodium citrate. The procedure is the same as described above for 20 nm GNPs. Here simply the amount of ligand was decreased to prepare bigger gold nanoparticles. However these larger particles come at the expense of decreased monodispersity of both size and shape. Specifically, to prepare 31 nm GNPs in diameter, the amount of the citrate ligand was decreased to half compared to the preparation of 20 nm GNPs. The particles


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size was found to be between 25-40 nm in diameter with an average of 31 nm (see TEM images in SI Figure 2). Preparation of silver nanoparticles solution. Silver nanoparticles (SNP, ~ 6 nm in diameter) were prepared in the following way.34 In a 500 mL round bottle flask, 196 mL of Milli-Q water was cooled down to 10 ºC. Then 2 mL of aqueous solutions of AgNO3 (25 mM) and sodium citrate (25 mM) were added under vigorous magnetic stirring. Then 600 µL of an aqueous icecooled NaBH4 (0.1 M) solution was added to that solution in a drop wise fashion. After two hours the mixture had a yellow color indicating completion of the reaction. The solution was kept in the fridge until used. Preparation of polyelectrolyte solution. 1 mg/mL of poly (allylamine hydrochloride) (PAH) and poly (sodium 4-styrenesulfonate) (PSS) were dissolved in a 0.1 M solution of sodium chloride in water to prepare solutions of positive and negative polyelectrolyte, respectively. Instruments. Scanning electron microscopy (SEM, JEOL JSM-7600F) was used to characterize the attached GNP on the surface of a functionalized Ge internal reflection element used for ATR spectroscopy. UV-vis spectra were recorded on a Cary Varian 50 Bio UV-Visible spectrometer. Transmission electron microscopy (TEM) was performed using a TEM Tecnai G2 to measure the size of small GNPs. Functionalization of Ge elements for ATR-IR spectroscopy. Ge internal reflection elements (IREs; 50 mm × 20 mm × 1 mm, Komlas) were used for ATR-IR experiments. The IREs were first polished with a 0.25-µm-grain size diamond paste (Buehler, Metadi II) and afterward rinsed copiously with Milli-Q water before the surface was plasma cleaned under a flow of air for 2 min (Harrick Plasma Instrument). Then the plasma cleaned negatively charged Ge ATR crystal was


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functionalized by adsorption of the positively charged polyelectrolyte PAH. That functionalized Ge surface was used for nanoparticles adsorption. The adsorption of the negatively charged nanoparticles is driven by the electrostatic interaction with the positively charged PAH on the Ge ATR crystal. Polyelectrolyte layers were deposited on top of the nanoparticles by first adsorbing positively charged PAH. Thicker multilayers were prepared by alternatingly adsorbing PAH and PSS. A washing step was done in between each step to grow the multilayers. ATR-IR measurements. ATR-IR spectra were measured with a Bruker VERTEX 80v Fourier transform infrared (FT-IR) spectrometer with a liquid nitrogen-cooled narrow-band mercury cadmium telluride (MCT) detector. Spectra were recorded at a resolution of 4 cm-1. For in situ ATR-IR experiments a dedicated flow-through cell was used made from a Teflon piece and a fused silica plate (64 mm × 30 mm× 5 mm) with holes for inlet and outlet (39 mm apart), and a Viton seal (1 mm). The volume of the used flow-through cell is 0.129 mL with a gap of 270 µm.35 The cell was mounted on an attachment for ATR measurements within the sample compartment of the Fourier transform infrared (FTIR) spectrometer. The solutions were passed through the cell and over the Ge crystal at a flow rate of 0.5 mL/min by means of a peristaltic pump (Ismatec, Reglo 100) located in front of the cell. All experiments were performed at room temperature and the spectrometer was evacuated to avoid contributions from gas-phase water and CO2. In some experiments a ZnSe wire grid polarizer was used.

RESULTS AND DISCUSSION Effect of particle size. We have shown before that molecules and polyelectrolytes in the vicinity of gold and silver nanoparticles show enhanced absorption of infrared light.17 For


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example it was shown that upon adsorption of GNPs the water signals are increasing, which is due to the enhanced absorption of IR light by the water molecules in the vicinity of the GNPs. Here we now investigated the effect of particle size and particle density on enhancement. For this purpose citrate stabilized GNPs of around 10, 20 and 30 nm in diameter were synthesized. To study the infrared signal of PSS polyelectrolyte adsorbed on GNPs of different size the following experiments were performed: First a clean Ge ATR crystal was functionalized with PAH (positively charged) and then GNPs (negatively charged) were adsorbed up to 2 hours. After that, using the LBL technique, PAH and PSS were alternatingly adsorbed to form up to eight polyelectrolyte bilayers on top of the GNP array. The growth of successive polyelectrolyte layers was done in the ATR cell and followed in situ by ATR-IR spectroscopy (see Figure 1). The sample has therefore the following layer structure: Ge/PAH/GNP(PAH/PSS)n (n = 1…8). In Figure 2 we plot the incremental signal of one PSS polyelectrolyte layer as a function of the number of PSS layers n. Characteristic peaks of negatively charged PSS are assigned as follows: The peak at 1180 cm-1 is due to the asymmetric stretching vibration of SO3, the signal at 1125 cm-1 is due to an aromatic ring vibration, the one at 1038 cm-1 is due to the SO3 symmetric stretching vibration and the band at 1008 cm-1 is due to a C-H bending mode of the aromatic ring.36 We used the most prominent peak of PSS at 1176 cm−1 for the PSS signal measurement. To measure the incremental signal of one PSS layer the sample before PSS adsorption served as reference. An analogous experiment without GNPs was also performed and is reported for comparison in Figure 2. It emerges from Figure 2 that the incremental PSS signal is stronger in the presence of GNPs. We ascribe this effect to enhanced infrared absorption due to the GNPs. The incremental PSS signal decreases with increasing number of PSS layers, showing that the enhancement effect depends on the distance from the nanoparticle surface. For the 30 nm GNPs


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the enhancement is obvious up to the 8th bilayer (and probably beyond) whereas for the smaller (10 nm) GNPs the PSS signal is enhanced with respect to the reference sample (without GNPs) up to about the 5th polyelectrolyte bilayer (for SEM images see SI Figure S3). Similar behaviour was found for commercial 10 nm and 40 nm GNPs (for details see SI Figure S4). One polyelectrolyte double layer has a thickness of about 2.5 nm.37-38 This means, taking this reported value, that for 30 nm GNPs enhancement extends up to about 20 nm (and probably beyond) from the GNP surface whereas for the 10 nm GNPs enhancement is evidenced up to 10-12 nm from the GNP surface. The curve for the 20 nm GNPs shows unusually high signal for the first layer, which will be discussed below. The strength of the signal is more pronounced for larger particles. However, comparison of the signal strengths for different particle sizes has to be done with care, since the particle densities differ for the same adsorption time and also the surface area per particle is different (see SI, Figure S3). When comparing for example the 20 nm and the 30 nm particles (Figure 2 and SI Figure S3) for 2 hours of GNP adsorption on realizes that the particle density is about 950 GNPs / µm2 for the 20 nm GNPs whereas this value is only 300 GNPs / µm2 for the 30 nm GNPs as measured by SEM. Considering the surface area per particle (4πr2) the total gold surface area (surface area per particle x particle density) is about 40 % higher for the 20 nm GNPs. Still the ATR signal of the adsorbed PSS is larger for the 30 nm GNPs (Figure 2) showing that not only the accessible effective gold surface area is playing the role.


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0.04

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.02

1st (+) 1st (-) 2nd (+) 2nd (-) 3rd (+) 3rd (-) 4th (+) 4th (-)

0.00

1300

1200

1100 -1

1000

Wavenumber (cm )

Figure 1. ATR spectra of PAH (+) and PSS (-) during LBL growth inside ATR cell. As the reference served the clean Ge element in contact with water before polyelectrolyte adsorption.

Figure 2. Incremental PSS signal at 1176 cm-1 (as measured from peak height) as a function of PSS layer for different sizes of GNPs. As the reference served the sample before adsorbing the


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corresponding PSS layer. Curve (a) was obtained on blank Ge surface (without GNPs). Curves (b), (c) and (d) are for 10 nm, 20 nm, and 30 nm GNPs, respectively. In all cases GNP adsorption was performed for 120 minutes. The error bars represent standard deviations of three measurements.

Effect of particle coverage and interparticle separation. The experiments discussed above show that the GNPs enhance the infrared absorption of adsorbed PSS. Of course the measured PSS signal depends on the GNPs concentration on the surface (GNPs coverage). We therefore studied the effect of coverage in more detail for 20 nm GNPs. The coverage can be varied by controlling the time of GNP adsorption. Independently the GNPs coverage and the structure of the GNP layer can be determined by SEM. The incremental ATR-IR signals of the PSS band at 1176 cm−1 for such experiments are shown in Figure 3. The corresponding SEM images are given in Figure 4 and the particle coverage as determined by SEM is given in Table 1. It is obvious that higher GNPs coverage leads generally to stronger PSS signals but, interestingly, the behaviour of the incremental PSS signal is qualitatively changing for high coverage GNPs samples (120 min and 240 min adsorption time). Particularly the signal from the very first PSS layer is strongly enhanced. With increasing GNPs coverage the gaps between the particles become smaller. As is evident from the SEM images the GNPs are well separated in our case and no agglomeration is observed, which is ascribed to the repulsion between the negatively charged GNPs. No long-range order is observed, i.e. the samples are amorphous. Table 1 shows that the particle number density can be increased from 200 GNPs / µm2 to 1100 GNPs / µm2 when increasing the adsorption time from


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3.5 min to 240 min. At the same time the typical interparticle distance (centre to centre) decreases from more than 70 nm to 33 nm. (For details regarding the estimation of the centre to centre distance see SI sketch 1 and sketch 2). When the separation between neighbouring GNPs becomes small the particles cannot be considered individual anymore because they start to influence each other. One consequence is that gaps are formed where locally the electromagnetic field is enhanced upon illumination (hot spots). For SERS Zihua et.al found that the critical centre to centre distance is about twice the nanoparticle diameter (2d) for generating noticeable electromagnetic coupling and the smaller the interparticle spacing, the larger the electromagnetic enhancement.11 The pronounced enhancement of PSS signals from the first layer for high density GNPs samples could be due to such an effect. It should be noted that the effect is well reproducible with the 20 nm GNPs. We furthermore tried similar experiments with larger particles and also some commercial GNPs. However, in none of these cases we could achieve high enough surface coverage to observe the pronounced enhancement of the first PSS layer (for more details see SI Figure S6).

0.06

d

-1

PSS peak absorbance at 1176 cm

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a) reference b) 40 min GNP c) 120 min GNP d) 240 min GNP e) GNP bilayers(40 min each)

0.05 0.04 c e

0.03 b

0.02

a 0.01

0

2

4

6

8

10

12

14

Number of PSS layers


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Figure 3. Incremental PSS signal at 1176 cm-1 as a function of PSS layer for different GNPs (20 nm) surface coverages. As the reference served the sample before adsorbing the corresponding PSS layer. Curve (a) was obtained on blank Ge surface (without GNPs). Curves (b), (c) and (d) are for 650 GNPs/µm2, 950 GNPs/µm2 and 1100 GNPs/µm2, respectively. Curve (e) is for a bilayer of GNPs separated by one PAH layer (1250 GNPs/µm2).


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Table 1. Coverage and related parameters as determined by microscopy (SEM) and absorbance signals as determined by ATR-IR for samples prepared by adsorbing 20 nm GNPs for different times on a PAH-functionalized Ge sample Experiments

No of GNPs/unit area

1) 3.5 min GNPs

200/µ2

PSS absorbance signal on top of

Surface coverage (%)

center

layer)

distance

0.017

6.28

More than 70 nm

350/µ2

0.021

10.99

adsorption 3) 40 min GNPs

center to

GNP (1st PSS

adsorption 2) 10 min GNPs

Typical

More than 55 nm

650/µ2

0.024

20.42

~ 40 nm

950/µ2

0.041

29.84

~ 35 nm

1100/µ2

0.058

33.84

~ 33 nm

1250/µ2

0.055

39.26

~ 30 nm

adsorption 4) 120 min GNPs adsorption 5) 240 min GNPs adsorption

6) 40 min+40 min GNPs double layer


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To test the hypothesis that the unusually strong signal of the first PSS layer originates from gaps between nanoparticles we preformed other experiments. First of all a double GNPs array was formed by adsorbing a first GNPs array (40 min) followed by PAH adsorption before a second GNPs array was adsorbed (40 min). According to Table 1 this leads to a coverage that exceeds the one of a single GNPs array adsorbed during 240 min. It is also evident from the SEM image of this sample (Figure 4) that the particles form larger aggregates thus leading to a large number of small gaps between neighbouring GNPs. As can be seen from Figure 3 the first PSS layer adsorbed on this sample shows strongly enhanced IR absorption similar to the one measured for a high density single GNPs array (240 min adsorption). For the second PSS layer the signal drops considerably similar to the observation made for the high coverage single GNPs array.

Figure 4. SEM images of some samples of GNPs (20 nm) on Ge. Different coverages were obtained by different adsorption times: a) 3.5 min, b) 40 min, c) 240 min. d) corresponds to a


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GNP double layer (40 min adsorption each) separated by one PAH layer (see also Table 1). Scale bar (white bar at the bottom (centre) of each figure) is 100 nm.

Figure 5 shows the absorbance of the first PSS layer as a function of the number density of GNPs on the surface, which was varied from 200 GNPs/µm2 to 1100 GNPs/µm2 by changing the adsorption time (see corresponding SEM images in SI, Figure S7). For individual (noninteracting) GNPs one would expect a linear relation between GNP coverage and PSS signal. The samples with low particle coverage (up to about 20% corresponding to about 600 GNPs / µm2) follow a trend that is in agreement with such behaviour. However, for the denser GNP layers the signal is larger than expected assuming a linear relationship. For the denser GNP layers the fraction of particles in dimer or larger aggregates is increasing. For example the fraction of aggregates is 45% for 350 GNPs / µm2, 57% for 950 GNPs / µm2 and 75% for GNPs / µm2, as estimated from the corresponding SEM images (Supporting Information Figure S7). The stronger increase at higher GNP coverage is in agreement with a stronger enhancement from nanoparticle gaps.


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Figure 5. Absorbance signal of the first PSS layer adsorbed on top of a GNPs layer as a function of GNPs coverage. A reference without GNP is also shown. At high GNPs coverage the gap between GNPs becomes narrower, which is also schematically shown by the sketches (red lines: PAH; blue lines: PSS).

The different behaviour of the first and of the subsequent PSS layers is also reflected in the different time evolution of the measured PSS signal upon adsorption. Figure 6a compares the signal as a function of time for the first, second and third PSS layer for a sample that was prepared from 20 nm particles adsorbed for 120 min. Whereas the signal of the second and third PSS layer stabilizes after about 5 min of adsorption the signal of the first PSS layer continues to grow for a much longer time. Similarly, different time evolution for the adsorption of the first PSS layer on different GNP arrays can be observed. For low coverage GNP arrays, which do not show the pronounced enhancement of the first layer, the PSS signal saturates after a short time (see Figure 6b). In contrast, for the high coverage GNP arrays the signal continues to grow even after one hour. Both observations highlight the unique behaviour of the first PSS layer on high density GNP arrays. A consistent interpretation of these observations is that a large part of the PSS signal for the high coverage GNP arrays is generated in narrow gaps between GNPs. The accessibility of these is hindered and that is why the kinetics of PSS adsorption in the gaps is slow. A consequence of this view would be that only one polyelectrolyte bilayer fits in GNP gaps between nanoparticles, because the second PSS layer shows considerably lower signal. This leads to an estimate of the upper limit of gap size for efficient infrared enhancement, which amounts to about 5 nm, corresponding to one polyelectrolyte double layer (PAH/PSS) on each of the two GNPs forming the gap. It should be noted that the optical properties of gold nanoparticle


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dimers is different from the one of isolated nanoparticles. For example a second plasmon peak appears at higher wavelength, a phenomenon that has been called cross-talk.39 A shift to longer wavelengths of the plasmon band means that the frequency difference between the plasmon resonance and the excitation, in our case infrared light, is reduced. This effect could contribute to a larger enhancement from dimers. Interestingly, it was found that the coating with organics let the cross-talk start at larger particle separations, which could again contribute to the observed enhancement. An alternative interpretation is that upon adsorption of the first PSS layer the GNPs rearrange, which affects the enhancement more for the high density nanoparticles arrays. There is no clear indication for such a rearrangement from SEM and therefore we prefer the interpretation based on the filling of narrow gaps given above.


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0.032

0.024

0.016

120 min GNP+1st PSS layer 120 min GNP+2nd PSS layer 120 min GNP+3rd PSS layer

0.008

0

5

10

15

Time/min

-1

0.06


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1


0.05

0.04

0.03 120 min GNP 40 min GNP 240 min GNP

0.02

0.01 0

20

40

60

80

100

Time/min

Figure 6. (top) PSS signal at 1176 cm-1 as a function of time for the first, second and third PSS layer. GNPs (20 nm) were adsorbed for 120 minutes. (bottom) Signal at 1176 cm-1 as a function of time for adsorption of the first PSS layer on GNPs (20 nm) layers of different coverage. The coverage was varied by different GNPs adsorption times.

Enhancement of PSS signal from bottom of GNP layers. The experiments above clearly show that the GNPs can enhance infrared absorption in general but particularly in gaps between GNPs. We furthermore wondered if enhanced absorption can be found in the gap that is formed


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between GNPs and the Ge internal reflection element. This is appealing because it would allow one to detect molecules on surfaces simply through the adsorption of nanoparticles, similarly as was recently shown for Raman spectroscopy. Shell-isolated nanoparticles (SHINERS) were deposited on surfaces to enhance the Raman signal of adsorbed molecules or of the substrate. This technique has potential for sensing applications.40 A similar approach is particularly appealing for IR spectroscopy because in modern single beam FTIR spectrometers one needs to acquire a reference spectrum. The reference spectrum can then just be the (unknown) surface before adsorption of the nanoparticles. To verify the feasibility of such an approach we first prepared a PAH/PSS/PAH polyelectrolyte multilayer on the Ge ATR element. After thorough washing with Milli-Q water a background was recorded. Then the GNPs were adsorbed by flowing a GNPs solution over the sample and ATR-IR spectra were recorded in situ. Clearly PSS signals appeared during GNPs adsorption (see Figure 7, black spectrum). Note that for this experiment the reference was taken before adsorbing the gold nanoparticles. It is important to note that the amount of polyelectrolyte did not change between reference and sample measurement. The spectrum arises only due to the enhanced polyelectrolyte signal at the bottom of the nanoparticles. Several control experiments were performed to exclude that the signals observed were due to adsorption of additional PSS (for example from the tubing) on the surface. For example we never observed an increase of the PSS signals during washing steps. Also, after GNP adsorption the PSS signal stopped growing during the washing step. Finally, we also used virgin tubing for the experiment and the results were unchanged. Therefore, the observed PSS signals are indeed due to the enhanced electric field in the gap between GNPs and the Ge surface. As expected the increase of the PSS signal


21


and the one of the water show the same time behaviour because both depend on the number of adsorbed GNPs (for details see SI Figure S8).

0.020

[PSS+GNPs] - PSS PSS [PSS+SNPs] - PSS

0.015

Absorbance

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0.010

0.005

0.000

1300

1250

1200

1150

1100

Wavenumber/cm

1050

1000

-1

Figure 7. ATR-IR spectra of PSS layers. The red spectrum corresponds to one adsorbed PSS layer without nanoparticles. As the reference served the Ge/PAH sample before PSS adsorption. The black and blue spectra correspond to the signals measured upon adsorption of GNPs and SNPs respectively, onto a Ge/PAH/PSS/PAH sample. As reference served the Ge/PAH/PSS/PAH sample before nanoparticle adsorption. The signal at high wavenumbers in the blue spectrum is due to a baseline drift.


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The same experiment was repeated with silver nanoparticles (SNPs) instead of GNPs (Figure 7). In the case of silver the signals from the PSS layer were much stronger amounting to 1.2 % absorbance for the band at 1035 cm-1, assigned to the symmetric stretching vibration of the SO3 group, after one hour of SNP adsorption. This corresponds to an increase by about one order of magnitude compared to the experiment with GNPs. It should be noted that silver nanoparticles are prone to oxidation and it has been shown for SERS that even little oxidation of the SNP surface leads to a strong loss of enhancement.41 In our conditions, ambient air, it is likely that oxidation plays a role. Therefore, preventing oxidation could lead to even larger enhancement in our case as well. The experiments described above are a proof of principle for the detection of surface species using enhanced IR spectroscopy by adsorption of GNPs and SNPs. The signal observed for GNPs are relatively small, however this signal is generated only in the gap between GNPs and the Ge surface. Compared to the total surface area of the Ge sample this gap area is very small, which indicates that the enhancement in this gap is considerable.

Experiment with polarized light. The experiments described above show that enhancement at different locations around the particles can be probed (gap between GNPs and Ge surface, gap between particles) depending on the experiment. The absorber (PSS in our case) feels the near field around the nanoparticles, which is induced by the evanescent field close to the internal reflection element in the ATR experiment. The latter field is polarized in all three directions of space and therefore, by choosing specific polarization of the incoming beam, it might be possible to selectively induce a near field at certain positions around a nanoparticle. In such a case one


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can expect a polarization dependence of the measured IR signals. To investigate if this is the case we measured spectra with both parallel and perpendicular polarized light. Note that the polarization direction refers to the plane of incidence. A parallel polarized incident beam creates an evanescent field polarized in the z (perpendicular to the Ge surface) and x direction (parallel to the Ge surface), whereas a perpendicular polarized incident field gives a y polarized evanescent field (parallel to the Ge surface). Table 2 gives the dichroic ratio R of the bands at 1008 cm-1 and 1035 cm-1, which is defined as

ܴ=

஺೛ೌೝೌ ஺೛೐ೝ೛

Where Apara is the absorbance measured with parallel polarized incident light and Aperp is the absorbance measured for perpendicular polarized incident light. It is evident from Table 2 that the dichroic ratio is significantly different for the experiments where the GNPs were deposited on top of the polyelectrolyte layer and for the polyelectrolyte adsorbed on top of the GNPs. Specifically the dichroic ratio is much larger for the former case. We ascribe this effect to the larger electric field in the gap between GNPs and Ge surface when excited with parallel polarized light due to the z-component of the evanescent field. Thus the polarized measurements confirm the experiments described above for the enhancement from the gap between GNP and Ge surface. In addition, it is also remarkable that the two bands at 1008 cm-1 and 1035 cm-1 show different intensity ratios for the two geometries (see Figure 8). For the experiment where the GNPs were deposited onto the polyelectrolyte the two bands have almost equal intensity, whereas this is not


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the case if the polyelectrolyte is deposited onto the GNPs. This may be due to a different conformation or orientation of the absorber within the GNP – Ge gap compared to the absorber on top of the GNPs.

GNPs on top of PSS at para GNPs on top of PSS at perp PSS on top of GNPs at para PSS on top of GNPs at perp

0.03

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.02 *5 0.01

*5

0.00 1250

1200

1150

1100

1050

Wavenumber/cm

1000

-1

Figure 8. Polarized ATR-IR spectra of PSS for two different configurations: GNPs on top of PSS and PSS on top of GNPs. For GNPs on top of PSS (red and blue spectra), the sample Ge/PAH/PSS/PAH before adsorbing the GNPs served as the reference, measured with the corresponding polarization. For PSS on top of GNPs (pink and gray spectra) the sample with the adsorbed GNPs served as the reference, measured with the corresponding polarization.

Table 2. Dichroic ratios for the PSS bands at 1035 cm-1 and 1008 cm-1 for PSS adsorbed on top of a GNP layer and for the experiment where the GNPs were


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adsorbed on a Ge/PAH/PSS/PAH sample

Experiments

Dichroic ratio (R) at 1035 cm-1

Dichroic ratio (R) at 1008 cm-1

GNPs on top of PSS

1.77

1.86

PSS on top of GNPs

1.34

1.32

CONCLUSIONS ATR-IR spectroscopy is a very useful tool to investigate enhanced IR absorption from polyelectrolyte-nanoparticles composite films. LBL technique is used to tune the distance between absorber and nanoparticles surface for GNPs layers of different coverage. The signal strength is more pronounced for bigger particles and decreases with distance from particles surface. At lower surface coverage the enhancement follows a linear behaviour with particles concentration but at higher particles coverage the polyelectrolyte signal increases strongly, particularly when the centre to centre distance becomes less than twice a particles diameter. In this regime, where coupling effects between particles play an important role, the enhancement behaviour changes qualitatively, especially for the first polyelectrolyte layer. It is proposed that this strong PE enhancement at high GNPs coverage is generated from narrow gaps between GNPs where the electromagnetic field is enhanced upon illumination. The upper limit for strong enhancement from this nanogap is about 5 nm, which corresponds to one polyelectrolyte double layer. Enhancement from the bottom of nanoparticles, i.e. from the gap between GNPs and Ge


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surface, is also observed and confirmed by polarized measurement. This enhancement is more pronounced for silver nanoparticles and is promising for analysis of surfaces by IR spectroscopy.

ASSOCIATED CONTENT Supporting information TEM images of 10 nm and 30 nm GNPs, SEM images of 10 nm, 20 nm and 30 nm GNPs (synthesized), experiment with commercial GNPs and free ligands, estimation of typical interparticles distance and ATR-IR signals as a function of time during adsorption of GNPs on a Ge/PAH/PSS/PAH sample. This material is available free of charge via internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources The work was supported by the University of Geneva. ACKNOWLEDGMENT The authors would like to thank Dr Gérard Klein for the help during synthesis of tannic acid reduced 10 nm GNPs. The work was supported by the University of Geneva.

ABBREVIATIONS PE, polyelectrolyte; GNP, gold nanoparticle; ATR-IR, attenuated total reflection infrared; LBL, layer by layer; PAH, poly (allyl amine hydrochloride); PSS, poly (sodium 4-styrenesulfonate); SEIRA, surface enhanced infrared absorption; SERS, surface enhanced Raman scattering; SEM, scanning electron microscopy; TEM, transmission electron microscopy; SNP, silver nanoparticle.

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Mapping Infrared Enhancement around Gold ... - ACS Publications

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