A Versatile Analytical Platform Based on Graphene-Enhanced Infrared

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A Versatile Analytical Platform Based on GrapheneEnhanced Infrared Attenuated Total Reflection Spectroscopy Yuan Hu, Angela Inmaculada López-Lorente, and Boris Mizaikoff ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00028 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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A Versatile Analytical Platform Based on Graphene-Enhanced Infrared Attenuated Total Reflection Spectroscopy Yuan Hu,† Ángela I. López-Lorente, ‡ Boris Mizaikoff *,† †

Institute of Analytical and Bioanalytical Chemistry, Ulm University, Ulm, Germany. Departamento de Química Analítica, Instituto Universitario de Investigación en Química Fina y Nanoquímica IUIQFN, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie Anexo, E-14071 Córdoba, España. ‡

ABSTRACT: Graphene with its unique properties including atomic thickness, atomic uniformity, and delocalized π bonds has been reported as a promising alternative material vs. noble metals for surface enhanced spectroscopies. Here, a simple and effective graphene-enhanced infrared absorption (GEIRA) strategy was developed based on infrared attenuated total reflection spectroscopy (IR-ATR). The IR signals of a broad range of molecules were significantly enhanced using graphene-decorated diamond ATR crystal surfaces vs. conventional ATR waveguides. Utilizing rhodamine 6G (R6G) as the main model molecule, the experimental conditions were optimized, and potential enhancement mechanisms are discussed. Aqueous sample solutions were directly analyzed utilizing graphene dispersions, which eliminates harsh experimental conditions, tedious sample pretreatment, and sophisticated fabrication/patterning routines at the ATR waveguide surface. The GEIRA approach presented here provides simple experimental procedures, convenient operation, and excellent reproducibility promoting a more widespread usage of graphene in surface enhanced infrared absorption spectroscopy.

KEYWORDS: graphene, surface enhanced infrared absorption spectroscopy, attenuated total reflection, rhodamine 6G.

Infrared (IR) absorption spectroscopy acquires molecular vibrational and rotational fingerprints associated with specific chemical, structural, and compositional information on molecular constituents by determining the absorption in the midinfrared spectral regime (2.5-20 µm).1 Hence, rapid, nondestructive, highly discriminatory, and label-free detection of analytes is enabled.2 Nevertheless, conventional IR techniques suffer from comparatively low sensitivities, which restrict practical applications in trace and ultra-trace analysis.3 Strategies for enhancing IR signals have received increasing attention since Hartstein et al. for the first time reported increased IR absorption features occurring at thin noble metal layers.4 To date, such enhancement effect has been observed on various nanostructured substrates5-6 including metallic nanoislands,7-8 nanoshells,9 nanogaps,10-11 nanoantennas,12-15 etc., which is commonly known as surface enhanced infrared absorption (SEIRA) effect in analogy to surface enhanced Raman scattering (SERS) and surface enhanced fluorescence (SEF).16 It is generally accepted that the enhancement of infrared absorption features at certain substrates may be attributed to at least two distinct mechanisms, i.e., electromagnetic (EM) enhancement - an increase of the local electric field at the substrate surface - and chemical enhancement (CE), which is derived from chemical interactions between the target molecule and the substrate surface.17-19 The majority of reported SEIRA substrates are based on noble metals (e.g., Au, Ag),20 as their plasmonic resonances can be extended to the infrared region via rational design of specific nanostructures.21 However, the plasmon frequencies of metals are fixed after structur-

ing, thereby limiting SEIRA detection of multiple IR bands across a broad spectral range.22-23 Additionally, the SEIRA effect is restricted to the immediate vicinity of the substrate surface,24 and greatly depends on the surface morphology.25 Hence, to provide sensitive, reproducible, and reliable SEIRA signals the reproducibility of nanostructure is essential, yet remains challenging.17 Consequently, more accessible, costefficient, environmentally friendly, and reproducible nonmetallic materials useful for broadband IR absorption enhancement is in demand. Graphene is a two dimensional (2D) semi-metallic material made of sp2-bonded carbon atoms arranged in a honeycomb pattern.26 Recently, it has been proposed as promising alternative material for enhancing infrared absorptions,27 as well as Raman scattering28-30 owing to its remarkable advantages such as the unique atomic thickness/uniformity, flexibility, chemical inertness, tight spatial plasmon confinement, extended plasmon propagation distances, exceptional electrical/optical tuning properties, and excellent ability to noncovalently bind to a wide variety of chemically and biologically relevant molecules. This clearly advantageously discriminates graphene from conventional metallic materials.31-32 Reports on graphene-enhanced IR absorption (GEIRA) remain rare to date, and were achieved mainly via graphene modified by chemical doping,22, 33 electrostatic gating,27 patterning into nanoribbons,34 nanodot,35 and combining with noble metals23, 36-37 . However, these methods usually require extended pretreatment procedures, sophisticated fabrication schemes, and rigorous control of the experimental conditions. Hence,

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Herein, we report a simple and efficient graphene-enhanced infrared absorption (GEIRA) approach taking advantages of graphene and IR-ATR spectroscopy avoiding harsh preparation conditions, toxic agents, and complex sampling operations. The experimental conditions were optimized using aqueous solutions of rhodamine 6G (R6G) as a model analyte. The generic applicability of the proposed GEIRA platform was verified via several additional analytes illustrating its potential and versatility. Besides, mechanistic studies were performed by investigating the evolutions of GEIRA spectra with time, and at a variety of analyte concentrations. Figure 1. Schematic illustration of the graphene-enhanced infrared absorption (GEIRA) platform (not scaled).

infrared absorption techniques remain of limited utility, if sophisticated surface nanostructures are required. Even though SEIRA can be implemented in transmission IR spectroscopy, using an attenuated total reflection (ATR) configuration appears advantageous, as the shallow penetration depth of IR light into the sample/media (i.e., few micrometers) allows the analysis of targets dissolved even in strongly IR absorbing media such as water,38 and ensures exquisite surface sensitivity.39 In addition, the surface of an ATR waveguide or crystal readily serves as supporting substrate for depositing signal-enhancing nanomaterials, which simplifies the experimental manipulation.24

RESULTS AND DISCUSSIONS A scheme of our the developed GEIRA platform is shown in Figure 1. Rhodamine 6G (R6G), used as a model analyte herein, is in fact a water contaminant, and toxic to human beings and animals. Figure 2A depicts the IR spectra of R6G solutions at different concentration levels recorded at the pristine diamond ATR waveguide. The IR absorbance of R6G remained quite weak, even at concentrations as high as 2.5 mg/mL, thereby indicating the rather low sensitivity of conventional IR-ATR measurements towards R6G. By contrast, in the presence of graphene the IR absorbances of R6G are

Figure 2. (A) IR spectral signatures of R6G solution at different concentrations recorded via a pristine diamond ATR waveguide; (B) Unprocessed IR spectra of 2.5 mg/mL R6G solution collected at the diamond waveguide with and without graphene-coating (2 µL of 0.05 mg/mL graphene was first deposited and dried at the diamond surface); (C) Effect of graphene concentration on the IR absorption of 2.5 mg/mL R6G; (D) Influence of different graphene addition methods on the GEIRA effect (R6G: 0.5 mg/mL).


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Table 1 Assignment and eEF of characteristic peaks of molecules analyzed via the developed GEIRA platform.a Molecule

Wavenumber (cm-1)

Assignmentb

eEFc

Auramine

1597

ν(ring), p-disubstituted aromatic ring

16.3

1378

ν(C–N), (C)3–N

21.5

Crystal violet

1588


12.74

1366

ν(C–N), (C)3–N

11.7

1176

ν(C–C), skeletal vibration

7.9

Fuchsine

Methylene blue

Neutral red

Rhodamine B

Rhodamine 6G

4-Hydroxybenzoic acid (pH adjusted to 1.9)

a

1588


14.3

1368

ν(C–N), (C)3–N

13.9

1169

δ(C–H)

7.3

1601

ν(ring), 1,2,4-trisubstituted ring

11.9

1395

ν(C–H), R–CH3

15.1

1357

ν(C–N), (C)3–N

13.8

1506

ν(ring), 1,2,4,5-trisubstituted ring

48.2

1330

ν(C–N), (C)3–N

146.3

1202

δ(C–H)

54.3

1589


18.5

1338

ν(C–N), Ph–N–(R)2

28.0

1181

δ(C–H)

24.7

1608

ν(C=N), C–N=C; ν(ring), 1,2,4-trisubstituted ring

44.9

1531


42.4

1503


47.6

1307

ν(C–N), Ph–NH–R

37.7

1706

ν(C=O), Ph–COOH

42.0

1376

ν(O–H), Ph–OH

64.9

1252

ν(C–O), Ph–COOH

26.4

1076

ν(C–O)

153.3

Auramine: 0.2 mg/mL; Others: 0.5 mg/mL. b ν = stretching, δ = bending. c experimental enhancement factor (eEF)

significantly enhanced with excellent repeatability during 5 parallel tests, as shown in Figure 2B. After optimizing the amount of pre-deposited graphene (Figure 2C), the enhancement effect was increasing with increasing concentration of graphene (from 0.01 to 0.05 mg/mL) at the same initial volume of 2 µL. Herein, 2 µL was determined as the minimum volume of graphene dispersion required to entirely coat the diamond surface. Moreover, the enhancement of the IR absorptions associated with R6G was not further enhanced when increasing the graphene volume from 2 to 4 µL (Figure S1). Accordingly, 2 µL of 0.05 mg/mL graphene was selected for the subsequent experiments to ensure a maximized GEIRA effect. The effectiveness of the developed graphene-coating method resulting in dried graphene flakes at the diamond ATR surface was further confirmed by comparing the spectral response of R6G with alternative procedures for depositing graphene: (i) the same amount of graphene dispersion was directly deposited at an identical diamond waveguide surface, yet without drying, and (ii) graphene grown via chemical

vapor deposition on copper foil (i.e., CVD-graphene) was transferred onto a pristine diamond waveguide (for details see supporting information). The IR spectrum of 0.5 mg/mL R6G solution revealed a sizeable GEIRA effect when graphene was applied via the three different procedures, among which driedgraphene-decorated diamond prepared by the proposed dropcoat method revealed maximum signal enhancement (Figure 2D). To meet the need of various measurement scenarios, the generic applicability of an analytical method is vital. Bearing this in mind, the developed GEIRA platform was tested for alternative molecules in aqueous solution at the same experimental condition. The concentration of the analyte solutions was fixed at 0.5 mg/mL (except 0.2 mg/mL for Auramine due to its low solubility in water) enabling comparative studies. As shown in Figure 3, without the assistance of graphene all the tested molecules provide only weak IR signatures. In contrast, the IR spectra of all tested analytes were prominently enhanced using the dried-graphene-decorated diamond ATR waveguide confirming the generic performance of the GEIRA platform. The IR fingerprint patterns in Figure 3 were further normalized


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Figure 3．Comparison of IR fingerprint patterns of (A) auramine, (B) crystal violet, (C) fuchsine, (D) methylene blue, (E) neutral red, and (F) rhodamine B recorded on dried-graphene-decorated and bare diamond waveguide surface. The insets reveal the chemical structure of the molecules.

such that the characteristics of the spectra are comparable at the same scale as depicted in Figure S2 (supporting information). From the normalized spectra of a given analyte recorded with and without graphene, it is evident that the presence of graphene does not induce noticeable changes of the characteristic peaks (i.e., band position or shape), yet, proportionally magnifies the absorption bands. The reliability of the proposed GEIRA platform is thereby verified. For a more straight-forward comparison of the variation of peak intensity, an experimental enhancement factor (eEF) was calculated:

eEF

the same core structure, however, may be discriminated by their characteristic IR fingerprint. R6G provides higher eEFs, which is attributed to the non-optimized measurement conditions of RB, as detailed below. It should be noted that the experimental conditions were not particularly optimized for either specific molecule. Therefore, the calculated eEF may be further increased via appropriate optimization strategies. As an exemplary parameter, the pH

(1)

whereby IGEIRA and I0 is the enhanced and non-enhanced IR signal intensity (i.e., peak height) of the considered band of the analyte analyzed via the GEIRA platform and at the pristine diamond ATR crystal, respectively. In other words, the eEF reports a multiplication (i.e., enhancement) factor of the pristine absorption signal.36, 40-41 In this study, only bands that were also evident in the pristine IR spectra (i.e., recorded in absence of graphene) were considered. The calculated eEFs of thus considered bands and the corresponding assignments are listed in Table 1. Evidently, the eEF values of different peaks of one specific molecule are of the same order of magnitude. Taking auramine as an example, its characteristic bands around 1597 cm-1 and 1378 cm-1 are attributed to the stretching vibration of the p-disubstituted aromatic ring, and C–N in (C)3–N, of which the eEF is 16.3 and 21.5, respectively. The molecular structures of crystal violet (CV) and fuchsine are similar, with very close eEF values at each peak position. Although methylene blue (MB) and neutral red (NR) have a lot in common, their spectra appear very different, and neither are their eEF values alike. Rhodamine B (RB) and R6G share

Figure 4. pH-dependence of the IR absorption spectra of (A) 0.5 mg/mL 4-HBA, and (B) 2.5 mg/mL R6G solution analyzed at the (dried-graphene-coated) diamond ATR. The corresponding GEIRA eEF of the considered bands of (C) 4-HBA and (D) R6G at the respective pH values.


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Figure 5. Evolution of the GEIRA signals from (A) 0.05 mg/mL R6G, (B) 0.5 mg/mL R6G, (C) 2.5 mg/mL R6G, (D) 0.02 mg/mL auramine, and (E) 0.5 mg/mL neutral red with increasing time after adding the sample solution onto the dried-graphene-decorated diamond ATR crystal. (F) GEIRA signal intensities of selected peaks of the corresponding analytes versus time after the sample addition.

value of the sample solution was investigated in more detail, as it potentially affects the interaction between graphene and the analyte. For instance, 4-hydroxybenzoic acid (4-HBA, 0.5 mg/mL, pH 3.4) displayed a moderate enhancement effect on the dried-graphene-modified ATR waveguide (Figure 4A). If the pH value of the sample solution was appropriately adjusted using diluted HCl or NaOH solution, the highest band intensities were in fact achieved at pH = 1.9. In respect of 2.5 mg/mL R6G solution, a noticeable GEIRA effect was only evident at the initially selected pH = 3.6 without improved enhancement after pH adjustment (Figure 4B). Figure 4 C and D exhibit the gain in eEFs of these exemplary molecules at optimized pH values. The possible reason of the variation of enhancement effect caused by sample solution pH value will be discussed later. In addition, it was observed that the peak intensity of the GEIRA spectrum was not constant with time. Figure 5A–E show how the GEIRA spectra for a selection of molecules evolved with increasing time after adding the sample solution onto the dried-graphene-decorated ATR crystal, and the corresponding intensities of their selected peaks as a function of time were plotted in Figure 5F. For molecules at low concentration, e.g., R6G at 0.05 mg/mL (Figure 5A), and auramine at 0.2 mg/mL (Figure 5D) the IR absorbance increased only slowly with time, and almost reached an equilibrium value after approx. 4 min. If the sample concentration was increased to 0.5 mg/mL (i.e., R6G and neutral red, Figure 5 B and E, respectively), the IR peak intensity kept increasing for the entire duration of the measurement (i.e., 10 min). As for R6G at a high concentration of 2.5 mg/mL (Figure 5C), the IR signal increased sharply until approx. 4 min, and then appeared to have reached saturation. For practical reasons, samples without and with graphene were both investigated for periods of 10 min, thereby ensuring a suitable compromise

between maximizing the signal enhancement and maintaining a reasonable analysis time. As previously mentioned, electromagnetic enhancement and chemical enhancement are two mechanisms that may contribute to the total enhancement of the IR signal. The electromagnetic fields of graphene IR plasmons exhibit substantial subdiffractional confinement of IR radiation, i.e., reportedly up to two orders of magnitude higher compared to metals.27 Nevertheless, by virtue of the relatively high momentum mismatch between the surface plasmons and the IR photons,42 and the scarcity of materials with sufficiently high refractive index and low optical losses for near- and mid-IR prisms43, excitation of surface plasmons in graphene flakes remains challenging. Hence, adequate modification of graphene is obligatory in order to overcome such mismatch.33-35, 44 However, apart from the electromagnetic contribution graphene could also act as a molecular enrichment matrix, as reported in graphene-based SERS system contributing to the chemical enhancement via π– π interactions etc.28, 32, 41, 45 Note that the graphene used in this study has not been specifically structured in size or doped, and the GEIRA effect has been observed within a broad range of IR frequencies. Taking into account these factors and the obtained experimental results, it is thus hypothesized that the observed GEIRA effect predominantly results from a chemical mechanism (CM), which in turn relates to adsorption processes playing a dominating role. More specifically, the tested molecules all comprise aromatic moieties, which benefit adsorption onto the graphene matrix via π-π stacking interactions.46-51 Consequently, sample molecules are enriched at the graphene surface.30, 32, 52-53 In general, the penetration depth of the evanescent field in this wavelength regime is typically on the order of magnitude of few micrometers.2 The diamond ATR crystal covered by dried graphene via the drop-coat method revealed much more pronounced GEIRA effects vs. CVD-graphene transferred to


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Figure 6. (A) Concentration-dependence of GEIRA spectra of R6G, obtained by measuring R6G solution with the developed GEIRA platform. (B) The corresponding peak intensities at 1608 cm-1, 1531 cm-1, 1503 cm-1, and 1307 cm-1 as a function of the R6G concentration, the inset is the best-fit Langmuir-Freundlich isotherm of data at 1607 cm-1. The error bars indicate the standard deviation in GEIRA intensity for three individual experiments.

the surface, and graphene dispersion mixtures (Figure 2D), respectively, as apparently more graphene flakes remain at or close to the ATR waveguide surface. Hence, more analyte molecules are concentrated via chemical interaction with graphene within the penetration depth of the evanescent field. Furthermore, the increase of the GEIRA signals with time after addition of the sample solution (Figure 5) may indeed result from the absorption process to reach equilibrium. A shift in solution pH apparently does not influence the characteristics of the IR finger patterns, as the normalized spectra of 4-HBA and R6G collected at different pH values demonstrate in Figure S3. Nonetheless, pH changes may still affect molecules such that the adsorption performance of graphene is altered, thereby inducing changes in the GEIRA effect (Figure 4). To be specific, 4-HBA as a weak acid with a pKa value of 4.54 would partially dissociate in water. At a solution pH of 1.9 (Figure 4A), H+ release from 4-HBA was greatly suppressed. Conversely, dissociated 4-HBA (i.e., as negatively charged ion) dominated as the pH increased to 6.6 or 8.4. The accumulation of dissociated 4-HBA at the graphene surface was thereby restrained via electrostatic repulsion between the negatively charged groups. The case of R6G appears more complex. As a cationic dye, R6G may dissociate in the aqueous solution with pKa of 18.72 (i.e., strongest acidic) and 2.34 (i.e., strongest base). If the solution pH was set at 1.7 (Figure 4B), R6G appeared mostly protonated (i.e., positively charged), while it appears deprotonated (i.e., negatively charged) beyond a pH of 5.2, thus giving rise to strong electrostatic repulsion between charged R6G species adverse to their adsorption onto graphene.54 As a result, the most prominent SEIRA effect was apparent, if the majority of analytes were present as electroneutral molecules at the appropriate individual pH conditions (i.e., pH = 3.6 in this case). Additionally, owing to the π-π interactions between graphene and the aromatic moieties the distance between graphene and the molecules is small, and charge transfer may be facilitated.55 This likely contributes to the chemical enhancement.25, 29, 41, 56 As evident in Figure 2A, the band intensity (I) – here, of R6G – analyzed using conventional ATR varies with the sample concentration (C). Figure 6A displays the concentration-

dependent GEIRA spectra of R6G. The peak intensities of four characteristic bands at 1608 cm-1, 1531 cm-1, 1503 cm-1, and 1307 cm-1 obtained from the GEIRA spectra, as well as the IRATR spectra recorded without graphene are plotted against the R6G concentration in Figure 6B and Figure S4, respectively. At each concentration, the peak intensities of the four nonenhanced bands were comparable, and increased with increasing concentration. Consequently, linear calibration functions were fitted to the experimental data (Figure S4), indicating that the signal intensity of the non-enhanced IR spectra was proportional to the concentration of R6G. Evaluating the GEIRA spectra, the peak intensity was not linearly dependent on the R6G concentration (Figure 6B), instead, the experimental data were fitted using a quasiLangmuir-Freundlich equation, which is generally applied for modeling multilayer adsorption processes:

(2)

where I is the IR peak intensity, Q refers to the ratio of IR peak intensity versus adsorption amount, C is the equilibrium concentration of adsorbate in solution, K and n are constants for a given adsorbate and adsorbent at certain temperature. Exemplarily, the values of the band at 1608 cm-1 along with the fitted curve are shown in the inset of Figure 6B with K and n at 1.61 and 3.03, respectively, and a correlation coefficient R2 of 0.9983. Here the value (I/Q) is used to replace the amount of adsorbate in the general Langmuir-Freundlich isotherm equation. Q of the fitting isotherm is 0.969, which indicates that the per unit amount of R6G contributes approx. 0.969 (a.u.) to the peak intensity in the GEIRA spectra. The remarkably good fit provided by the quasi-LangmuirFreundlich model implies that R6G molecules are indeed adsorbed to graphene at or adjacent to the ATR surface via a multilayer adsorption process, which in turn confirms that the adsorption of analyte at graphene is the main contribution to the observed GEIRA effect. In conclusion, this study presents a simple yet effective method based on graphene-enhanced infrared absorption


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(GEIRA) spectroscopy for analyzing a variety of molecules containing aromatic moieties based on attenuated total reflection. Pronounced IR signal enhancement effects were observed for several characteristic bands across the entire IR-ATR spectrum with excellent reproducibility utilizing aqueous graphene dispersions. Given that chemical enhancement is the dominating contribution to the observed signal increase, the dependence of the GEIRA effect on the pH value of the sample solution was investigated and confirmed. Furthermore, using ATR spectroscopy facilitated the direct analysis of aqueous sample solutions providing additional insight on the temporal evolution of the GEIRA signal. Last but not least, these studies enabled fundamental understanding on the chemical enhancement mechanism in GEIRA spectroscopy. METHODS Reagents and materials. All chemicals were at least of analytical grade and applied without further purification. Ultrapure water was obtained from a water purification system (conductivity 18.0 MΩ cm-1; Elga Labwater, VWS Deutschland GmbH, Celle, Germany) and employed throughout this work. The sample solutions were prepared directly by dissolving the specific chemical in ultrapure water. The commercially available graphene dispersion (0.05 mg/mL, with 2% w/v of sodium cholate as surfactant) was purchased from Nanointergris Inc. (Skokie, USA, Product name: PureSheets MONO). The graphene solution contains 27% single layer graphene, 48% double layer graphene and 20% triple layer graphene with ~10000 nm2 average flake area. Procedure. Infrared spectroscopic studies were carried out using a Bruker Vertex 70 FT-IR spectrometer (Bruker Optik, Ettlingen, Germany), equipped with a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector (Bruker Optics, Ettlingen, Germany) and an attenuated total reflection configuration (ConcentratIR2; Harrick Scientific Products, New York, USA) consisting of a diamond ATR (10 reflections) waveguide attached to a ZnSe optical coupling element. To investigate the GEIRA effect, the diamond crystal was decorated using drop-coating: 2 µL of graphene dispersion (0.05 mg/mL) was slowly dropped and distributed evenly across the surface of the diamond waveguide mounted in the ATR assembly, and completely dried on site in open air under ambient conditions. Then, 20 µL of sample solution was added on top of the graphene-coated diamond ATR, spectra were acquired by averaging 300 sample scans within the spectral range of 4000–600 cm-1 at a spectral resolution of 2 cm-1, and repeated at least 3 times. Prior to sample measurements, background spectra of water (blank) were recorded and subsequently subtracted from the sample spectra. Between each measurement, the ATR cell was thoroughly cleaned using cotton swabs along with ethanol and water. For comparison, samples were also measured at the same conditions at the identical diamond crystal, however, without graphene decoration. Data was collected and processed using the OPUS 6.5 software package (Bruker Optik, Ettlingen, Germany).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Figure S1, effect of graphene dispersion volume on the IR absorption of R6G. Detailed descriptions of transfer of CVD-graphene on diamond ATR. Figure S2 and S3, normalized spectra of different analytes measured on bare and dried-graphene-decorated diamond waveguide (at different pH values). Figure S4, IR peak intensity dependence on the concentration of R6G solution (PDF).

AUTHOR INFORMATION Corresponding Author * E-Mail: [email protected]. Tel: +49 731 50-22750.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Yuan Hu gratefully acknowledges the Chinese Scholarship Council (CSC) for financial support. A.I. López-Lorente thanks the Alexander von Humboldt Foundation for the award of a Postdoctoral Fellowship at the Institute of Analytical and Bioanalytical Chemistry (Ulm University, Germany).

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Manuscript title: Versatile Analytical Platform Based on Graphene-Enhanced Infrared Attenuated Total Reflection Spectroscopy Authors: Yuan Hu, Ángela I. López-Lorente, Boris Mizaikoff Graphene-decorated attenuated total reflection (ATR) waveguide significantly enhanced the characteristic IR bands absorption of a variety of aromatic molecules. The enhancement effect has been observed across the entire IR-ATR spectrum with excellent reproducibility via direct analysis of aqueous sample solution. This work enabled fundamental understanding on the chemical enhancement mechanism in graphene-enhanced infrared absorption (GEIRA) spectroscopy.


A Versatile Analytical Platform Based on Graphene-Enhanced Infrared

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