Infrared Attenuated Total Reflection Spectroscopy for the

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Infrared attenuated total reflection spectroscopy for the characterization of gold nanoparticles in solution Angela Inmaculada López-Lorente, Markus Sieger, Miguel Valcarcel, and Boris Mizaikoff Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 09 Dec 2013 Downloaded from http://pubs.acs.org on December 13, 2013

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Analytical Chemistry

1

Infrared attenuated total reflection spectroscopy for the

2

characterization of gold nanoparticles in solution

3

Ángela Inmaculada López-Lorente1, Markus Sieger2, Miguel Valcárcel1*,

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Boris Mizaikoff2*

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1

Department of Analytical Chemistry, University of Córdoba, E-14071 Córdoba, Spain

6 7

Phone/Fax +34 957 218616; E-mail: [email protected] 2

Institute of Analytical and Bioanalytical Chemistry, University of Ulm, Ulm, Germany

8

In-situ synthesis of bare gold nanoparticles mediated by stainless steel as reducing agent was

9

monitored via infrared attenuated total reflection (IR-ATR) spectroscopy. Gold nanoparticles

10

were directly synthesized within the liquid cell of the ATR unit taking immediate advantage of

11

the stainless steel walls of the ATR cell. As nanoparticles were formed, a layer of particles was

12

deposited at the SiO2 ATR waveguide surface. Incidentally, the absorption bands of water

13

increased resulting from surface enhanced infrared absorption (SEIRA) effects arising from the

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presence of the gold nanoparticles within the evanescent field. Next to the influence of the

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Au(III) precursor concentration and the temperature, the suitability of IR-ATR spectroscopy as

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an innovative tool for investigating changes of nanoparticles in solution including their

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aggregation promoted by an increase of the ionic strength or via a pH decrease, and for detailing

18

the sedimentation process of gold nanoparticles was confirmed.

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Keywords: attenuated total reflection, ATR, infrared spectroscopy, surface-enhanced infrared

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absorption, SEIRA, gold nanoparticles, nanoparticle synthesis, aggregation.

22

*Corresponding

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[email protected];

24

[email protected]

authors.

Boris Miguel

Mizaikoff. Valcárcel.

Tel.:

+49

Tel./Fax:

731 +34

1 ACS Paragon Plus Environment

50

22750;

E-mail:

957

218616;

E-mail:


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1. INTRODUCTION

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Metallic nanoparticles and especially gold nanoparticles (AuNPs) provide exceptional

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properties, which have promoted their usage in a diverse range of areas including biomedical

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imaging and diagnostic assays1, biological applications2, catalysis3, and for applications based

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on the enhanced optical properties of nanoparticles such as enhanced Rayleigh scattering or

30

surface-enhanced Raman scattering (SERS) of adsorbed molecules4.

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Furthermore, a diversity of synthetic procedures for the generation of AuNPs has emerged in

32

recent literature5. Consequently, advanced characterization tools for nanoparticles synthesized at

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a wide variety of conditions are increasingly demanded. Microscopic techniques are most

34

commonly utilized, although they suffer from several drawbacks, i.e., they are time-consuming

35

in order to ensure representative sampling, and imaging problems may arise from the

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aggregation of nanoparticles when studied in solution. Optical analysis techniques have proved

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to be useful for the characterization of nanoparticles, in particular when taking advantage of the

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pronounced surface plasmon resonances attributed to gold nanoparticles. Dynamic light

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scattering (DLS) is a common method for the size characterization of nanoparticles6. In

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addition, Raman spectroscopy has been applied for the characterization and quantification of

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gold nanoparticles previously extracted by ionic liquids7. In addition, separation techniques

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including electrophoresis8 have extensively been used for the characterization of synthesized

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gold nanoparticles. Finally, inductively coupled plasma mass spectrometry9 (ICP-MS) has

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emerged as a method of choice for quantitatively determining the presence of gold

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nanoparticles.

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In general, gold nanoparticles do not show pronounced mid-infrared (2.5-25 µm; MIR)

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absorptions, unless they are functionalized with organic ligands attached to the particle surface,

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which

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cetyltrimethylammonium bromide (CTAB) molecules adsorbed in ionic liquid media10 have

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been characterized by Fourier transformation infrared (FT-IR) spectroscopy. Furthermore, IR

gives

rise

to

characteristic

IR

spectra.

For


example,

AuNPs

with

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spectra of 2-(12-mercaptododecyloxy)methyl-15-crown-5-ether modified gold nanoparticles11,

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4-mercaptobenzoic acid modified AuNPs12, and AuNPs-metallocarbonyl conjugates13 have been

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published. Infrared attenuated total reflectance (IR-ATR) spectroscopy has been used to

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quantitatively determine competitive molecular adsorption of thiolated poly(ethylene glycol) at

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gold nanoparticle surfaces14. However, to date infrared spectroscopy has not been applied for

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the characterization of gold nanoparticles without any surface modification.

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Hartstein et al.15 were the first to report on so-called surface enhanced infrared absorption

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(SEIRA) effects occurring at corrugated noble metal surfaces. In general, two enhancement

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mechanisms – electromagnetic and chemical enhancement - are considered providing the main

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contributions to the enhancement of molecular absorption features in SEIRA spectroscopy16.

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The electromagnetic mechanism increases the local electric field at the surface, while chemical

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enhancement gives rise to an increase of the absorption due to chemical interactions between

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molecules and gold nanoparticles17. Among others, the SEIRA effect has been used for the

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determination of molecules such as p-nitrobenzoic acid18, carbon monoxide19, octadecanethiol20-

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22

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In the present study, the adsorption of water molecules at gold nanoparticles has been utilized

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for in-situ monitoring of the synthesis of bare gold nanoparticles via surface-enhanced infrared

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attenuated total reflection (SEIRA-ATR) spectroscopy. Gold nanoparticles were directly grown

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within the liquid cell of the ATR unit from an aqueous tetrachloroauric acid solution mediated

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by the stainless steel housing of the ATR cell following a mechanism previously discussed by

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the authors25,26. As nanoparticles were formed and deposited at the surface of the ATR

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waveguide, an enhancement of the water absorption features was observed, which correlated

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with the amount of AuNPs present at the surface. A similar effect has been observed in the case

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of gold nanoparticles synthesized in D2O with the absorption bands of deuterium oxide

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revealing similar enhancement during the formation of AuNPs. Moreover, the generic suitability

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of SEIRA-ATR spectroscopy for the investigation of gold nanoparticle solutions has been

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studied via the evaluation of effects such as the addition of salts or variation of the pH on the

, 4-pyridinethiol23, and p-mercaptoaniline24.



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aggregation state of gold nanoparticles in solution, as well as monitoring the involved

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sedimentation processes. Thus, in the present study IR-ATR spectroscopy is confirmed as an

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innovative analytical tool applicable for the characterization of bare AuNPs providing a

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straightforward and non-destructive characterization strategy.

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2. METHODS

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2.1. Materials and reagents

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HAuCl4 solution (200 mgdL-1 in deionized water)

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Steinheim, Germany) were used to synthesize the gold nanoparticles. Prior to synthesis, the

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material was washed with a mixture of nitric acid and hydrochloric acid (HNO3:HCl = 1:3

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mixture, aqua regia) (Panreac, Barcelona, Spain). A flat piece of 304-stainless steel was used as

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solid reducing agent of gold precursor in order to form gold nanoparticles. Sodium citrate

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dehydrate 99.5% (Sigma Aldrich, St. Louis, MO, USA) was used to synthesize citrate-coated

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gold nanoparticles. Silver nitrate, potassium nitrate, and sodium chloride purchased from

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Sigma-Aldrich (St. Louis, MO, USA) were used to investigate the aggregation of the

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nanoparticles. Cetyltrimethylamonium chloride (CTAC) was obtained from Fluka (Buchs,

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Switzerland). Deuterium oxide (D2O, 99.8%) was provided by Merck (Darmstadt, Germany).

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The stock solutions used herein were invariably prepared in Milli-Q water.

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2.2. Synthesis of bare gold nanoparticles

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Gold nanoparticles were synthesized following a procedure previously described by the authors

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using stainless steel is used as solid reducing agent25,26. All glassware was cleaned with freshly

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prepared aqua regia (HNO3:HCl = 1:3 mixture) and then thoroughly rinsed with distilled H2O

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prior to use. A piece of 304-stainless steel (12.9 mm2 of total surface) was introduced into 100

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µL of a 0.2 mg mL-1 aqueous solution of HAuCl4. The reaction was carried out at room

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temperature; the stainless steel substrate was simultaneously used as stirrer during the reaction.

and gold(III) chloride (Sigma Aldrich,

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2.3. Synthesis of bare gold nanoparticles inside the ATR liquid cell

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Gold nanoparticles were also directly synthesized inside the liquid cell of the ATR unit by

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taking direct advantage of the stainless steel, which forms the walls of the ATR liquid cell. In

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this case, no stainless steel piece was added to the solution of tetrachloroauric acid, which was

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again used as a gold(III) precursor matrix. Once the tetrachloroauric acid solution was located

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within the ATR unit, the immediate onset of the formation of gold nanoparticles was observed.

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Thus, obtained AuNPs sedimented at the ATR crystal surface, thereby enabling direct

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monitoring via SEIRA-ATR spectroscopy. The reaction was performed at a variety of

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temperatures: room temperature (i.e., 22ºC), 40ºC, 50ºC, and 60ºC, as controlled by a thermostat

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unit.

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2.4. Synthesis of citrate-coated gold nanoparticles

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Gold nanoparticles obtained via HAuCl4 reduction mediated by sodium citrate were also

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prepared according to the method described by Turkevich et al.27 applying modifications

116

described elsewhere7. Firstly, all glassware were cleaned with freshly prepared aqua regia

117

(HNO3:HCl 1:3 mixture), and then thoroughly rinsed with distilled H2O prior to usage.

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Tetrachloroauric acid and sodium citrate solutions were filtered through a 0.45 µm nylon

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membrane prior to use. Once a 0.01% solution of HAuCl4 was boiling, 0.254 mL of a 1%

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sodium citrate solution were added. The system was then left to react while stirring for a period

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of 15 min. Afterwards, 5mL of 0.01% HAuCl4 solution were added to the system followed by

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0.254 mL of 1% sodium citrate solution, and again stirred while heating for 15 min. Then, the

123

heater was switched off, and the solution was stirred until cooling to room temperature. Thus

124

obtained AuNPs were stored at 4ºC in an amber bottle.

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2.5. Infrared spectroscopic analysis

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Infrared spectroscopic studies were performed using a Vertex 70 FT-IR spectrometer (Bruker

127

Optics, Ettlingen, Germany) equipped with a BioATR-II unit, and a liquid nitrogen cooled

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mercury-cadmium-telluride (MCT) detector (Bruker Optics, Ettlingen, Germany). Figure 1 5 ACS Paragon Plus Environment


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shows a schematic of the experimental setup. As evident although not to scale, the top ATR

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crystal interfacing with the sample is made from a circular section of a silicon wafer providing

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8-10 internal reflections. IR radiation is coupled into this silicon ATR plate via a secondary zinc

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selenide (ZnSe) ATR waveguide (which is not in contact with the sample) providing a

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hemispherical shape for efficient coupling of photons from the FT-IR spectrometer into the Si-

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ATR. The entire unit was connected to a circulating water bath ensuring a defined and constant

135

temperature during the IR-ATR analysis. Data acquisition and processing was performed using

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the OPUS 6.5 software package (Bruker Optics, Ettlingen, Germany). Spectra were acquired by

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averaging 300 spectra in the wavelength window of 4000 to 400 cm−1 at a spectral resolution of

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4 cm−1. Prior to analyzing each set of samples, a background spectrum of the tetrachloroauric

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acid solution was collected when monitoring the subsequent in-situ synthesis of AuNPs. If

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AuNPs were synthesized ex-situ, prior to a sample measurement a background spectrum of

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deposited AuNPs at time zero was recorded. Finally, measurements of the synthesis of gold

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nanoparticles in D2O media were acquired averaging 500 spectra within 4000 to 400 cm-1 at a

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spectral resolution of 4 cm-1.

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3. RESULTS AND DISCUSSION

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3.1. In-situ synthesis of gold nanoparticles within the IR-ATR liquid cell

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Gold nanoparticles were directly synthesized inside the ATR unit taking advantage of the

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stainless steel ring establishing the confining walls of the liquid cell, as previously demonstrated

148

by the authors25,26. While AuNPs are formed, they sediment at the ATR waveguide surface until

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Coulombic repulsion from already adsorbed AuNPs leads to an equilibrium.

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Gold nanoparticles are characterized by a strong plasmon resonance in the UV/Vis region as a

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consequence of the collective oscillation of electrons within the conduction band. However,

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bare gold nanoparticles do not inherently show a pronounced absorption in the MIR. Despite the

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IR-inactivity of AuNPs, it was found that their formation could still be monitored via IR

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spectroscopy by analyzing the increase of the water absorption features during the synthesis 6 ACS Paragon Plus Environment

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process, which arises from a SEIRA effect, as discussed below. Figure 2 shows a series of in-

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situ IR-ATR spectra recorded during gold nanoparticles being synthesized within the ATR unit.

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According to literature28, the vibrational modes of water in the infrared region are the H2O

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bending mode at 1644 cm-1, the combination of H2O bending and libration mode at 2128 cm-1,

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and a pronounced vibration around 3404 cm-1, which is composed of the overtone of the

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bending mode at 3250 cm-1, the symmetric OH stretch at 3450 cm-1, and the antisymmetric OH

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stretch at 3600 cm-1 22. As gold nanoparticles are formed and deposited at the ATR waveguide

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surface, an increase of the H2O bending and the OH stretching bands is observed despite the

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effective decrease of water molecules present within the evanescent field being displaced by

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gold nanoparticles. This increase is attributed to an enhancement of the water absorptions within

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the evanescent field resulting from the presence of AuNPs. While the magnitude of the

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enhancement certainly decreases with increasing the distance from the waveguide surface (i.e.,

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the nanoparticulate Au film), it compensates for the decreasing number of water molecules

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steadily displaced by AuNPs. The hypothesis of enhancing the IR signature of water via the

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formation of AuNPs is further corroborated by the observed increase of the water absorption

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signal vs. the background spectrum.

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For monitoring the kinetics of the synthesis and deposition of AuNPs at the ATR waveguide

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surface, the peak height of the water bending mode (δ(H2O)) was analyzed in detail. Figure 2a

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shows the peak height of the H2O bending mode vs. time with t=0 indicating the initiation of the

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in-situ synthesis of gold nanoparticles from tetrachloroauric acid solution. The obtained data

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readily fit to a Langmuir model, which describes a random adsorption process within the first

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monolayer (Figure 2c). The obtained goodness-of-the-fit was r2=0.997.

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Likewise, the absorption band corresponding to the OH stretching vibration revealed an increase

178

during the synthesis of gold nanoparticles (figure 2b). Plotting the peak height vs. time again fits

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a Langmuir isotherm with a goodness-of-the-fit of r2=0.999. Table 1 summarizes the main

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parameters obtained when fitting both vibrational water bands to a Langmuir isotherm.



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In order to further study the in-situ formation of gold nanoparticles within the ATR cell, a

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stainless steel O-ring of similar dimensions was glued onto a piece of silicon wafer, in order to

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simulate the scenario inside the ATR liquid cell (i.e., mock-up ATR cell; MU-ATR) enabling

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access to subsequent scanning electron microscopy (SEM) studies of the obtained AuNPs

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without additional manipulation. Similar to the in-situ synthesis monitored by IR-ATR

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spectroscopy, a small amount of HAuCl4 (200 mg dL-1) was added to the MU-ATR in order to

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synthesize AuNPs. The reaction was stopped after 20 min and 100 min, respectively, by

188

removing the aqueous solution followed by rinsing with dried air.

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After 20 min, a sub-monolayer coverage with few agglomerates of AuNPs was determined at

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the Si wafer surface (Figure 3 left), as expected from the in-situ IR-ATR studies. After 100 min,

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the AuNPs in solution show significant agglomeration with already adsorbed gold

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nanoparticles, as evident in Figure 3 (right). During the process of deposition of AuNPs at the

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ATR crystal surface, the distribution, size, and shape of the NPs as well as their interaction

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change. This morphological development results in changes of the electromagnetic field

195

properties near the ATR waveguide surface. Firstly, a monolayer of AuNPs is formed at the

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surface (Figure 3 left), which leads to a strong water bending and OH stretching signal

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enhancement in the first few minutes. The SEIRA activity results from an enhanced near-field

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around the NPs, which decays with increasing distance from the particle surface. As previously

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explained, the near-field enhancement around AuNPs overcompensates the reduced number of

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water molecules present within the evanescent field, as they are increasingly replaced by AuNPs

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during their adsorption on the waveguide surface. Once gold nanoparticles are deposited on the

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surface, aggregates or islands of nanoparticles are observed, as evident within the SEM images

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(Figure 3, right), along with no further increase of the water absorption features. At such

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islands, the electromagnetic near-field properties change, as accordingly the absorption of IR

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radiation21.

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3.2. Influence of the gold(III) precursor concentration

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The influence of the concentration of the gold(III) precursor (i.e., tetrachloroauric acid) was

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studied. While the kinetics of the reaction follow a different rate depending on the

210

concentration, in all cases the obtained rates may be fitted with a Langmuir isotherm.

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If the concentration of gold precursor within the ATR unit is high (i.e., 200 mgdL-1), the

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increase of the absorption features related to the water spectrum is rapidly observed (i.e., within

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1 min). At lower concentrations (i.e., 66 mgdL-1), the formation of gold nanoparticles is

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correspondingly slower, which is reflected in the IR spectrum (Figure S1, Supporting

215

Information). Figure S2 (Supporting Information) shows the H2O bending region at different

216

times of reaction. Initially, AuNPs deposited at the surface appear isolated with only few

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particles agglomerated. With increasing reaction time, packed island of connected nanoparticles

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are evident. This behavior agrees with findings previously reported in literature22 for the water

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OH stretching band during the adsorption of a submonolayer of AuNPs at a substrate initially

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treated with (aminopropyl)triethoxysilane (APTES).

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3.3. Influence of the temperature

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If AuNPs are synthesized outside the ATR unit by inserting a piece of stainless steel piece into

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the solution25, an increase of the rate of production of gold nanoparticles is observed with

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increasing temperature. The in-situ synthesis was performed at different temperatures (23, 40,

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50, and 60ºC) for spectroscopically analyzing this behavior. With increasing temperature, the

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enhancement of water bands more rapidly appears, which in turn confirms that AuNPs are more

227

rapidly formed.

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3.4. Reproducibility

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A study of the reproducibility of the in-situ AuNPs synthesis and the observed SEIRA-ATR

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water signal was performed by repeating the analysis four times during different days for the

231

synthesis procedure based on a 200 mg dL-1 HAuCl4 solution at 60ºC. As reported in Table 1,



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the relative standard deviation calculated from the H2O bending peak height was 10.55%, while

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the relative standard deviation obtained from the OH stretching band was 8.16%.

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3.5. In-situ synthesis of gold nanoparticles in D2O

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The synthesis of the gold nanoparticles mediated by stainless steel inside the ATR unit was

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finally also performed in D2O, using AuCl3. The aim of this study was to potentially observe the

237

SEIRA effect on the associated D2O bands in a different spectral regime for confirmation of the

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observations initially made for H2O. No significant difference was noted concerning the

239

formation of AuNPs during the chemical reaction compared to the studies in H2O. Similar to the

240

observations in water, as gold nanoparticles were emerging, the absorption signals of the υ(OD)

241

and the δ(D2O) band appear with enhanced intensity. Again, thus obtained data were fitted using

242

a Langmuir model with a goodness-of-the-fit of r2=0.994 for the δ(D2O) band (see Figure S3,

243

Supporting Information), and of r2=0.998 when evaluating the OD stretching band (Table 1).

244

The same behavior was observed when performing the synthesis in a 1:1 mixture of H2O and

245

D2O resulting in the corresponding simultaneous enhancement of water and deuterium oxide

246

infrared absorption features, respectively.

247

3.6. Influence of the ionic strength

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As IR spectra are also sensitive to variations of the chemical environmental, next to non-

249

destructively monitoring the synthesis of gold nanoparticles one may furthermore analyze

250

changes in aggregation or stability of the obtained AuNP dispersions. The aggregation of

251

AuNPs causes coupling of their plasmonic modes, thereby resulting in a red shift and

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broadening of the longitudinal plasmon resonance band in the optical spectrum, as evident in the

253

UV/Vis absorption spectra shown in Figure 4 (top). It is known that variation of the ionic

254

strength is a particle-independent strategy to affect the repulsion distance between charged

255

particles. Increasing the ionic strength decreases the Debye length, thus also decreasing the

256

averaged distance between particles29. Two hypotheses have been proposed to explain the

257

observed change in the color of such solutions when increasing the ionic strength: (i) gold 10 ACS Paragon Plus Environment

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nanoparticles physically coalesce to form larger particles, and (ii) the mean interaction distance

259

is reduced such that their surface plasmons may interact29.

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Herein, the effect of the addition of silver(I) nitrate and potassium nitrate was investigated. As

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gold nanoparticles are synthesized and deposited within the evanescent field at the ATR

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waveguide surface, an enhancement of the water absorption features is observed. If the ionic

263

strength of the solution is altered by addition of salts, a sudden increase of the absorption of the

264

water bands is observed, without any further change with time.

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While AuNPs synthesized in-situ using the stainless steel assisted method do not contain citrate

266

ligands bound to the nanoparticle surface, chloride ions are present in abundance and may shield

267

the obtained nanoparticles. Such a chloride shell may prevent nanoparticle agglomeration, and

268

may also facilitate the stabilization of the solution due to the achieved spacing between

269

individual NPs. When adding silver nitrate to the solution, which increases the ionic strength,

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the enhancement of the water bands terminates (see Figure 4, bottom). Apparently, silver ions

271

interact with chloride ions, and thus, AuNPs will be destabilized and rapidly deposit at the

272

waveguide surface. Once deposited as islands on the surface, the increase of the water

273

absorption bands terminates in analogy to the observations during the deposition of AuNPs at

274

in-situ synthesis conditions at extended periods of time.

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A similar behavior was observed when adding potassium nitrate; once nanoparticles aggregate,

276

an increase in water signal is observed followed by no further changes of the IR absorption

277

band.

278

3.7. Influence of the pH value

279

The influence of a decrease in pH during the synthesis of gold nanoparticles within the ATR

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unit was investigated by adding HNO3 during the synthesis. 10 µL of a 10% (v/v) HNO3

281

solution were added to 40 µL of 200 mg dL-1 HAuCl4 solution previously placed into the ATR

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liquid cell. It has previously been described that low pH values induce the aggregation of gold

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nanoparticles30. Again, the suitability of IR-ATR spectroscopy to determine changes in 11 ACS Paragon Plus Environment


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aggregation state of the synthesizing gold nanoparticles was demonstrated observing a behavior

285

similar to that when increasing the ionic strength.

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3.8. Investigating the sedimentation behavior of ex-situ synthesized unmodified and citrate

287

coated gold nanoparticles via IR-ATR spectroscopy

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The sedimentation behavior of gold nanoparticles has been studied via UV/Vis spectroscopy

289

under normal gravitational force31. The optical detection during sedimentation of particles in

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solution enables the accurate determination of the particle size distribution in a polydisperse

291

nanoparticle sample31. Herein, IR-ATR spectroscopy is proposed as an alternative strategy for

292

monitoring gravitational sedimentation of gold nanoparticles. Hence, the sedimentation rate of

293

both unmodified gold nanoparticles obtained via stainless steel assisted reduction outside the

294

ATR liquid cell, and citrate-capped gold nanoparticles was compared. In order to achieve

295

acceptable sedimentation rates, CTAC was added, as previously reported in the literature32.

296

Cationic surfactant causes aggregation by neutralizing the negative charge at the nanoparticle

297

surface.

298

Experiments were carried out by placing a mixture of 20 µL of stainless steel assisted bare gold

299

nanoparticles synthesized outside the cell with 20 µL of 0.1 mol L-1 CTAC solution into the

300

ATR unit. An increase of water absorption features is again observed with time with an

301

accelerated enhancement in the presence of CTAC in the solution. A similar experiment was

302

conducted by placing a mixture of 20 µL of citrate-coated gold nanoparticles with 20 µL of 0.1

303

mol L-1 CTAC solution into the ATR unit. Figure 5 shows a comparison of the behavior of both

304

types of nanoparticles during sedimentation at the ATR surface.

305

4. CONCLUSIONS

306

In the present study, we confirm he utility of IR-ATR spectroscopy as an analytical strategy for

307

studying gold nanoparticles in solution. IR-ATR was used to monitor the in-situ synthesis of

308

gold nanoparticles within the ATR unit mediated by the stainless steel enclosure of the ATR

309

liquid cell. As gold nanoparticles were synthesized, they sedimented within the evanescent field 12 ACS Paragon Plus Environment

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310

at the ATR waveguide surface, thereby displacing the water molecules. Despite the fact that

311

bare gold nanoparticles are inherently not IR-active, the deposition of gold nanoparticles at the

312

ATR waveguide surface may indirectly be observed via the surface enhanced infrared

313

absorption (SEIRA) features of water molecules simultaneously present within the evanescent

314

field. The observed SEIRA effect overcompensates the steadily reduced number of water

315

molecules present within the evanescent field as they are increasingly replaced by AuNPs, and

316

therefore facilitates the determination of the presence of gold nanoparticles at the waveguide

317

surface. A particular benefit of the described procedure is the fact that the obtained gold

318

nanoparticles are not covalently attached to the ATR waveguide surface, thereby providing the

319

potential for future SEIRA-assay applications base upon the in-situ generation and subsequent

320

removal/renewal of the SEIRA-active layer at the waveguide surface. Moreover, IR-ATR not

321

only enables monitoring the particle sedimentation/density at the ATR waveguide surface, but

322

also allows studying chemical changes in solution that may affect AuNPs. Finally, it has been

323

confirmed that IR-ATR spectroscopy is a useful strategy for the physico-chemical

324

characterization of the sedimentation process of gold nanoparticles at a waveguide surface.

325

Consequently, future studies will further investigate the properties of such in-situ synthesized

326

bare gold nanoparticles deposited at the ATR waveguide surface for SEIRA studies of

327

additional (bio)molecules present in solution, thereby evaluating the potential of SEIRA-

328

bioassay applications.

329

ACKNOWLEDGMENTS

330

A.I. López-Lorente and M. Valcárcel wish to thank Spain’s Ministry of Innovation and Science

331

for funding Project CTQ2011-23790 and Junta de Andalucia for Project FQM4801. A.I. López-

332

Lorente also wishes to thank the Ministry for the award of a Research Training Fellowship

333

(Grant AP2008-02939), and gratefully acknowledges funding of her stay in Germany at the

334

University of Ulm to conduct the research reported in the present study. M. Sieger and B.

335

Mizaikoff gratefully acknowledge support of this study by the Kompetenznetz Funktionelle

336

Nanostrukturen Baden Wuerttemberg, Germany. Finally, the authors acknowledge the Focused 13 ACS Paragon Plus Environment


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Ion Beam Center UUlm supported by FEI Company (Eindhoven, Netherlands), the German

338

Science Foundation (INST40/385-F1UG), and the Struktur- und Innovationsfonds Baden-

339

Württemberg for support during nanoparticle characterization.

340

The authors declare no competing financial interest.

341

REFERENCES

342

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(4) López-Lorente, A.I.; Simonet, B.M.; Valcárcel, M. Comprehensive Analytical Chemistry

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2012, 59, 33-89.

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Takahashi, K.; Nakamura, A.; Kinugasa, S. Toxicol. In Vitro 2009, 23, 927-934.

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(7) López-Lorente, A.I.; Simonet, B.M.; Valcárcel, M. Analyst 2012, 137, 3528-3534.

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(8) López-Lorente, A.I.; Simonet, B.M.; Valcárcel, M. TrAC, Trends Anal. Chem. 2011, 30, 58-

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71.

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(9) Allabashi, R.; Stach, W.; de la Escosura-Muñiz, A.; Liste-Calleja, L.; Merkoçi, A. J.

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Nanopart. Res. 2009, 11, 2003-2011.

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(10) Wang, B.; Wang, X.; Lou, W.; Hao, J. J. Coll. Interf. Sci. 2011, 362, 5-14.

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Anal. Bioanal. Chem. 2011, 401, 1631–1639.

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Surfaces A: Physicochem. Eng. Aspects 2011, 385, 241– 248.


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(14) Tsai, D.-H.; Davila-Morris, M.; DelRio, F.W.; Guha, S.; Zachariah, M.R.; Hackley, V.A.

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Langmuir 2011, 27, 9302–9313.

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(15) Harstein, A.; Kirtley, J.R.; Tsang, J.C. Phys. Rev.. Lett. 1980, 45, 201-204.

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(19) Miyake, H.; Ye, S.; Osawa, M. Electrochem. Commun. 2002, 4, 973-977.

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Status Solidi B 2010, 247, 2071-2074.

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788, 122-128.

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Mizaikoff, M. Talanta 2014, 118, 321-327.

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(31) Alexander, C.M.; Dabrowiak, J.C.; Goodisman, J. J. Colloid Interf. Sci. 2013, 396, 53-62.

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389 15 ACS Paragon Plus Environment


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FIGURES

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Figure 1

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402 403

Figure 2

404 405 406 407 408



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409 410

Figure 3

411 412 413 414 415 416 417 418 419 420 421 422 423 424 18 ACS Paragon Plus Environment

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(a)

1,6

(b) 0,09

1,4

0,08

1 0,8 AuNPs 0,6

AuNPs+AgNO3

0,4

υ(H2 O) band height

1,2

absorbance (a.u.)

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


0,07 0,06 0,05 0,04 0,03 0,02 0,01

0,2

0

0 400

500

600

700

0

800

20

Wavelength (nm)

425 426

40 Time (min)

Figure 4

427 428 429 430 431 432 433 434


60

80


0.0025 δ(H2O) band height

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Page 20 of 23

0.002 0.0015 stainless steel

0.001

citrate 0.0005 0 0

20

40

60

80

100

Time (min)

435 436

Figure 5

437 438 439 440 441 442 443 444 445 446 447 448


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449

FIGURE CAPTIONS

450

Figure 1. Schematic of the ATR configuration used during the present studies (not to scale; for

451

details see Bruker BioATR-II unit, Ettlingen, Germany). As evident, the walls of the liquid cell

452

are composed of stainless steel, which acts as reducing agent leading to the in-situ formation of

453

gold nanoparticles from tetrachloroauric acid solution.

454

Figure 2. In-situ SEIRA-ATR spectra obtained during the synthesis of gold nanoparticles inside

455

the ATR unit in an aqueous environment (from a 200 mg dL-1 HAuCl4 solution). (a) H2O

456

bending mode, (b) OH stretching mode. In order to evaluate the SEIRA effect of gold

457

nanoparticles enhancing the water absorption bands, a spectrum of tetrachloroauric acid solution

458

in water was recorded prior to initiating the synthesis, and was used as the background

459

(reference) spectrum. Thus, the water features shown in the spectra herein reflect the observed

460

enhancement as gold nanoparticles sediment at the surface vs. the absorption of pure water in

461

absence of AuNPs. (c) Height of the δ(H2O) bending vibration band at 1644 cm-1 with fitted

462

Langmuir isotherm describing the evolvement of band height vs. time (r2 =0.997). At t=0, the

463

HAuCl4 solution was injected into the ATR liquid cell.

464

Figure 3. Scanning electron microscopy (SEM) images of in-situ synthesized gold

465

nanoparticles: (left) after 20 min, (right) after 100 min of sedimentation.

466

Figure4. (a) UV-Vis spectra of the gold nanoparticle solution prior and after the addition of

467

NaCl for increasing the ionic strength, which leads to the aggregation of AuNPs. (b) Plot of the

468

height of the water OH stretching band vs. time. At minute 11 (red arrow), 10 µL of 1M AgNO3

469

solution were added.

470

Figure 5. Plot of the height of the H2O bending mode vs. time during the sedimentation of

471

citrate-coated and bare gold nanoparticles in CTAC containing aqueous solution. The SEIRA

472

effect is evidenced by recording the spectrum of the gold nanoparticle solution at t=0 as

473

background (reference) spectrum. Thus, the water absorption features shown in the spectra

474

correspond to the obtained SEIRA-effect as gold nanoparticles deposit at the waveguide surface. 21 ACS Paragon Plus Environment


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475

Table 1. Parameters obtained for fitting a Langmuir isotherm to the water and deuterium oxide

476

absorption features vs. time in aqueous media.

477

Langmuir fit (r2)

Reproducibility* (%)

H2O bending

0.997

10.55

OH stretching

0.999

8.16

D2O bending

0.994

-

OD stretching

0.998

-

*Study performed in aqueous media

478 479 480 481 482 483 484 485 486 487 488 489


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490

For TOC only:

491

492 493


Infrared Attenuated Total Reflection Spectroscopy for the

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