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In-Situ Surface-Enhanced Infrared Absorption Spectroscopy of Aqueous Molecules with Facile-Prepared Large-Area rGO Island Film Fengjuan Cao, Lie Wu, Yudi Ruan, Jing Bai, and Xiue Jiang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05466 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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Analytical Chemistry

In-Situ Surface-Enhanced Infrared Absorption Spectroscopy of Aqueous Molecules with Facile-Prepared Large-Area rGO Island Film Fengjuan Cao,a, b Lie Wu,a Yudi Ruan,a, b Jing Bai,a and Xiue Jianga, b, c * a

State Key Lab of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China b University of Chinese Academy of Sciences, Beijing 100039, China c Department of Chemistry, University of Science and Technology of China, Anhui 230026, China ABSTRACT: Mid-infrared plasmons in patterned graphene could advance the development of surface-enhanced infrared absorption spectroscopy (SEIRAS). However, limitation in measuring the extinction spectra with transmission and external reflection configurations greatly restricts the analyses of aqueous samples. In addition, complicated, time- and cost-consuming preparation of patterned graphene also limits its progress. Here we demonstrate a facile-prepared large-scale reduced graphene oxide island film on a total internal reflection silicon prism, which not only shows a prominent enhancement effect in mid-infrared region, but also effectively eliminates the contribution of bulk solution by optical near-field effect. As a result, the entire vibrational fingerprints of methylene blue monolayer in aqueous solution can be acquired with high sensitivity in real time. Our work extends the application of graphene-based SEIRAS to aqueous environment, breaking through previously unattainable technology.

INTRODUCTION Infrared absorption of molecules adsorbed on rough surfaces of noble metals can be enhanced by several orders of magnitude.1,2 Taking advantage of near-field effect in the vicinity of nanostructure metal film, contributions of the bulk solution can be eliminated, achieving selective detection of signals from the adsorbed molecule even in aqueous solution. Therefore, surface-enhanced infrared absorption spectroscopy (SEIRAS) is a powerful technique that can intrinsically probe the subtle vibrational absorptions of various chemical bonds at the interface in a nondestructive label-free fashion,3-5 which has shown a prominent advantage in revealing the mechanism of surface catalysis, functional transformation of protein, and biological sensing6-10. Despite these successes, the low degree of electromagnetic confinement of metal and limitation in the method for molecular assembly at metal surfaces greatly restricts the extensive application of traditional plasmonic SEIRAS based on metal island film1,8,11 or periodic arrays12 prepared by wet-chemical method or electron beam lithography. Today, graphene (GN) has attracted great attention in the ongoing search for new and better plasmonic materials due to its long plasmon lifetime, high degree of light confinement,13,14 and tunability of the resonance frequency and magnitude,14-18 which dramatically enhance the light-matter interactions14,19 and enable selective detection of vibrational fingerprints20,21. To efficiently and selectively couple infrared light to localized plasmons, graphene is usually patterned into nanoribbons.14,21,22 However, the fabrication process often involves chemical vapour deposition on copper foil, electron-beam lithography and plasma etching procedures, which is complicated, costly, resource- and time-consuming, and often not

feasible in most laboratories.14,17,19 Therefore, a universal and facile one-step strategy for the construction of graphene plasmonic substrate is extremely attractive for various scientific studies and industrialized applications. In addition, to date, only dry samples can be measured with patterned graphene in transmission and external reflection configurations due to serious disturbance of strong absorption of water to sample spectrum in these configurations, which severely limits the application of graphene-based SEIRAS in aqueous environments. In this work, we report a reduced graphene oxide (rGO) island structure facilely prepared on a large scale by chemical reduction of GO on the surface of a single reflection silicon (Si) triangular prism. Plasmon resonances in the as-prepared rGO island film can be readily excited by infrared beam coupled into Si prism at the configuration of single total reflection, showing multiple resonance peaks and the maximal local field enhancements at the nanoscale intervals. To prove the applicability of the as-prepared graphene-based SEIRAS, we demonstrate in-situ label-free chemical-specific detection of methylene blue (MB) monolayer in aqueous environment in real time with high sensitivity. EXPERIMENTAL SECTION Preparation of rGO island film based substrate. The Si prism or wafer was polished completely with 1 µm Al2O3 slurry followed by washing thoroughly with water, and then immersed in a 40 wt % aqueous solution of NH4F for 3 min. An rGO island film was prepared on the treated flat surface of the Si prism by chemical reduction through exposing the treated surface to a 1:1:1 (vol/vol) mixture of 0.3 M Na2SO3 +0.1 M Na2S2O3 + 0.1 M NH4Cl, 3 % HF, and 1.6 mg/mL GO aque-

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ous solution for 3 min at 80 ºC. The obtained rGO substrate was washed thoroughly with deionized water to remove unstable or unreacted GO. The rGO-coated prism was mounted on a home-built poly(trifluorochloroethylene) cell with a Viton Oring. The mid-infrared beam from the interferometer of the spectrometer (IFS 66 V/S, Bruker, Ettlingen, Germany) was coupled to the Si prism at an incident angle of 60°, and the single total internal reflection infrared light intensity was recorded with a liquid nitrogen-cooled mercury cadmiumtelluride (MCT) detector in a nitrogen gas environment. Characterization of plasmons of rGO island film. To characterize the plasmons property of as-prepared rGO island film, the sample spectrum of rGO-coated Si prism was measured by taking a bare triangular Si prism as the reference spectrum. Infrared transmission measurements were performed using a FTIR spectrometer (VERTEX 70) with a DTGS detector at room atmosphere. The plasmonic extinction in the transmission, 1- T/T0 was measured on the silicon wafers with (T) and without (T0) rGO. Evaluation of the enhancement effect of the rGO island film. To evaluate the enhancement effect of the rGO island film, 200 µL of 10 µM MB aqueous solutions was deposited onto the bare Si and rGO island film-coated Si prism (2 nmol), respectively. Then, spectra were acquired until total evaporation of the liquid. ATR spectra were recorded by averaging 521 scans in the spectral range of 4000-800 cm-1 at a spectral resolution of 4 cm-1. Prior to measuring each set of samples, a bare Si or rGO island film-coated Si prism was measured as a background spectrum. For better comparison, water vapor subtraction and baseline correction were performed. The rGO island film sensor in aqueous solution. To demonstrate the sensor performance of rGO island film, a reference spectrum of aqueous solution was firstly recorded. Then, a series of sample spectra were acquired at intervals of 10, 60, and 300 s concomitant with replacing the background aqueous solution with 1 mL of MB aqueous solution at different concentrations. Simulation. All simulations were performed by using 3D finite-difference time-domain (FDTD) method on the basis of time-varying electromagnetic Maxwell's equations. Periodic boundary conditions and/or perfectly matched layer conditions were used to calculate extinction and near-ﬁeld enhancement data for rGO island film extracted from AFM image at an area of 1 µm ×1 µm. The incident light with a wavelength range from 2.5 um to 12.5 µm propagating along the negative z direction was normally illuminated on the surface. The optical permittivity parameters of Si, SiO2 and rGO used in simulation were extracted from the data of Palik and Jungseek Hwang.23,24 Plasmon nano-imaging. The scattering-type scanning nearfield optical microscopy (sSNOM, Molecular Vista) is basically an AFM operating in tapping-mode, where a gold-coated silicon AFM tip illuminated by a focused infrared light from a pulsed quantum cascade laser (QCL, Block Engineering) simultaneously maps the topography and the near-field scattered intensity of a sample. In order to isolate the genuine near-field contribution from the tip-sample region, the first harmonic of the scattered signal is used as input for a “Generalized Lock-In Amplifier (GLIA)” which calculates the amplitude and phase of the near-field component. Tapping amplitude and resonance frequency of the cantilever are 70 nm and 270 kHz, respec-

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tively. The scattered light was detected by a liquid nitrogencooled Mercury Cadmium-Telluride (MCT) detector. RESULTS Synthesis and characterization of rGO plasmonic substrate. A thin rGO substrate was chemically deposited on the hydrophobic surface of Si prism that was prepared by polishing with 1 µm Al2O3 slurry and then incubation with a 40 wt% aqueous solution of NH4F for 3 min (Figure 1A). Subsequently, GO (well characterized as shown in Figure S1) was reduced in-situ on the hydrophobic surface of the Si prism with a mixture of 0.3 M Na2SO3 + 0.1 M Na2S2O3 + 0.1 M NH4Cl aqueous solution in the presence of 3 % HF at 80 ºC, resulting in bottom-up assembly of rGO into stacking layered structure at an area of 4 cm2 within 3 min (Figure 1A, photograph). Figure 1B shows the XPS high-resolution C1s spectra of GO and rGO fitted by five components including C-C/C=C (284.5 eV), C-OH (285.5 eV), C-O (286.6 eV), C=O (287.8 eV), and O-C=O (289 eV).25-27 The obvious decrease in the peak intensities of oxygenated C suggests that GO was substantially reduced by the chemical deposition process. The Raman spectra show that the relative intensity of D band to G was increased from 0.82 to 0.98 upon the reduction of GO (Figure 1C), indicating a decrease in the size of the in-plane sp2 domains, which could be understood that the newly formed graphitic domains in rGO are smaller in size to those present in GO, but the number is more numerous.27-29

Figure 1. (A) Schematic of chemical preparation of rGO island film on Si substrate, and corresponding photograph. (B) The C1s XPS of GO and rGO with experimental (black line) and fitted curves (red line) including deconvoluted components. (C) Raman spectra of GO and rGO. (D) SEM image and EDS spectrum (superimposed over the SEM image). (E) Typical two-dimensional AFM image of as-prepared rGO island film. (F) The corresponding height profile along a line scan between the two arrowheads in the image E. (F) 3D surface plot of the rGO film shown in E.

The low-magnification scanning electron microscopy (SEM) image indicates a large-area thin film with non-uniform size distribution and nanoscaled intervals of the rGO substrate on the Si wafer and X-ray energy dispersive spectroscopy (EDS) spectrum also clearly indicates the coexistence of C, O and Si elements. (Figure 1D). The height profile (Figure 1F) of the atomic force microscopy (AFM) image (Figure 1E) shows a varied thickness from 26~46 nm, suggesting a various stacking


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Analytical Chemistry layered structure from a single-layer GO sheet (Figure S1). The 3D image of the as-prepared rGO substrate (Figure 1G) further shows a wavy stacked film with islanded lamellar structures and nanoscaled intervals. Overall, the as-prepared rGO substrate shows non-uniform three-dimensional array islands with nanoscaled intervals, which could allow electromagnetic radiation to excite plasmons. Plasmons behavior of rGO layers in Mid-IR. The plasmonic response of as-prepared rGO island film and the interaction of mid-infrared light with the substrate were characterized by measuring transmission T and T0 through the Si substrate with and without the rGO island film (Figure 2A) with an infrared spectroscopy.14,16,30

Figure 2. (A) Schematic of extinction spectrum measurement with (T) and without (T0) graphene layer on silicon wafer and the corresponding extinction spectra (1-T/T0) from two individual experiments as shown in (B), up panel. (C) Schematic of absorption spectrum measurement with bare and rGO-coated Si prism as reference and sample, and the corresponding spectrum in (B), low panel. (D-E) Simulation extinction spectra of rGO island film with thickness (D) and size (E).

The two extinction spectra (Figure 2B, upper panel), 1-T/T0, measured on the rGO island film operated separately under the same procedure exhibit several continual absorption peaks over the whole fingerprint region although the frequency and intensity of some peaks are slightly different due to the random morphology and size of the rGO island film (Figure S2). The two maximal absorptions at ~1570 and 1200 cm-1 could originate from the coupling of rGO plasmons and intrinsic optical phonons (~1564 cm-1 of in-plane IR active E1u mode and ~1234 cm-1 of interlayer asymmetric vibrations A1’ mode in Figure S3), while the absorption at 1750 cm-1 comes from the plasmons of rGO as judged from the comparison between the aborsption of patterned rGO and the scraped rGO from the Si wafer (Figure S3). The interaction between midinfrared light and plasmons in rGO island film results in over 3.7% absorption. In comparison, the absorption resonances arising from plasmons in rGO island film were also investigated by measuring the reflection absorption spectrum (Figure 2B, low panel) of rGO island film with the bare Si prism as the reference and the rGO island film-coated Si prism as the sample (Figure 2C). In addition to rGO intrinsic plasmon-phonon resonance modes at ~1577 and ~1199 cm-1, we observed more complicate resonance modes over the entire fingerprint region. The evanescent waves produced by the

total internal reflection of infrared beam at the Si prism interface could more effectively excite the plasmons in rGO island film and the surface optical (SO) phonons in the Si substrate at 806 and 1168 cm-1 (SO phonons of SiO217,22) than the transmission light31 because the mismatch of the wavevector between SPPs and incident light having the same frequency can be compensated by the high refractive index of prism31 and thus may result in more plasmon and phonon resonance peaks. The interaction of rGO plasmons with rGO intrinsic optical phonons and/or SO phonons in the underlying SiO2 substrate could result in absorption resonances at ~1450 and ~905 cm-1, respectively (discuss in detail in Figure S3). The strong plasmon-phonon interaction in the vicinity of their crossing energies15,17,18,22,24 and the hybrization of the plasmons among the rGO island with different layer number and sizes could significantly modify mid-infrared plasmon dispersion and damping,15,17,22,30 and thus resulting in multiple resonances (Figure S3). It is worthy to note that at a large area (4 cm2), the stacked rGO would include all kinds of different layer numbers and sizes, which could produce complex hybridization of the plasmons among the rGO layer in the stack, and thus always resulting in multiple resonances over the entire fingerprint region even though the frequency and amplitude could be different from sample to sample. To qualitatively understand this, the frequency and magnitude variation are described computationally as thickness and size, respectively with electrodynamic Maxwell simulations based on finite-difference time-domain method.18,19 Here, the simulated rGO plasmonic structure were imported from 1×1 µm2 area of AFM images of the as-prepared rGO film and the thickness and size were varied based on the same structure. The incident light with a wavelength range from 2.5 to 12.5 µm propagating along the negative z direction was normally illuminated on the surface. When we studied the effect of the thickness, the area was fixed at 0.78×0.78 µm2 and the thickness of rGO flakes was changed from 16 nm to 26, 36, and 46 nm (Figure 2D). When we studied the effect of size, the thickness was fixed at 36 nm and the size of rGO was changed from 0.38 µm to 0.58, 0.78, and 0.98 µm. The simulated extinction spectra mainly show two maximal peaks with similar characteristics to those of experimental spectra (Figure 2B, upper panel). Both the resonance frequency and the intensity of the plasmons in rGO island film increase gradually with graphene layer number (Figure 2D), while the frequency downshifts and the intensity increases with increasing size of rGO island (Figure 2E). The slight differences in the plasmonic resonance frequency and magnitude between the experimental and simulated results may come from the approximation of parameters of material and dimension in the simulations, and the presence of other non-idealities in the experiments.19,32 Enhancement effect of the rGO plasmonic layers. The broad and multiple resonances covering the whole fingerprint region could be favorable for efficiently detecting various vibrational modes of a molecular species.18 Firstly, as a proof of concept, MB was used as probe molecule to demonstrate the enhancement effect of the rGO island film (different concentrations of MB were detected as shown in Figure S4). Obviously, the vibrational absorption of MB (2 nmol) that was dropped and dried on the substrate is weak on the Si prism but is significantly enhanced on the rGO island film (Figure 3A) due to the enhancement of the light-matter interactions.21



Figure 3. (A) A comparison of the enhanced IR spectra for 10 nmol MB with and without the rGO island film on Si prism, and the transmission spectrum of MB in a KBr pellet. (B) Schematic of vibrational positions of MB and the adsorption orientation of MB molecule on the rGO surface. (C) The average enhancement factor of the main vibrational modes of MB adsorbed on the rGO island film. Data are shown as Mean ± SD (n=3 for each group). (D) FDTD simulations of the near-field enhancement distribution |E/E0| of the same rGO island film shown in E at λ = 5.7 µm. (E) The AFM topography shows some nano-gaps in the rGO film (image size 1×1 µm2). (F) Infrared nearfield amplitude image of the area shown in A, measured at λ = 5.7 µm. (G) Merged image of E and F.

As compared with the transmission spectrum of MB in a KBr pellet, nearly all characteristic absorptions corresponding to various stretching and deformation vibrational modes at the vibrational fingerprint region can be enhanced due to the electric fields of graphene plasmons following along both the x and z direction, and thus enhancing both in-plane and out-ofplane vibrational modes.21 The distinct difference is that the bands at 1395 and 1337 cm-1 assigned to the multiple ring stretching (Figure 3B) and the CAr-N stretching33,34 are significantly red-shifted to 1387 and 1328 cm-1, respectively on rGO island film, suggesting a strong conjugation effect between rGO and MB ring due to their strong π-π conjugation and hydrophobic interactions, which could induce a “flat-on’’ orientation of MB molecules on rGO film (Figure 3B). In addition, the strong conjugation effect between adsorbed MB and rGO would induce a chemical effect in the enhancement of the substrate because of electron transfer and the mixing of molecular orbitals32. The average enhancement factor of rGO film was calculated as ~8.9 times by comparing the peak intensity of different modes of adsorbed MB (2 nmol) with those measured on Si prism (Figure 3C). Although it is weaker than the enhancement effect of patterned graphene, where the strongest peak of adsorbed polyethylene oxide could be enhanced by more than 10 times21, the much easier and cheaper preparation of the rGO island film by chemical reduction than the patterned graphene by nanolithographic methods still make the as-prepared nanostructure an attractive substrate for SEIRAS. Considering that it is the average enhancement for all detection area, the enhancement effect at the nanoscaled interval edges should be stronger,14,21 which is further understood by calculating /E/E0/ factor with the finite difference time domain (FDTD) method on the basis of time-varying electromagnetic Maxwell's equations. The spatial electric field distribution (Figure 3D) shows various hotspots at the nanoscaled intervals

of rGO layers with a large field enhancement effect over a 1 µm x 1 µm area of AFM image (Figure 3E) at the wavelength of 5.7 µm (1750 cm-1). As a comparison, the scattering-type scanning near-field optical microscopy (sSNOM) was used to simultaneously map the topography and the near-field scattered intensity at an incident wavelength of 5.7 µm.35 A typical topography image of the as-prepared rGO film shows multiple nanoscaled intervals (Figure 3E). The simultaneously recorded scattering amplitude image (Figure 3F) shows strong electronic fields which are just located in the gaps between flakes (Figure 3G) as the calculated near-field plasmonic enhancements by FDTD simulation (Figure 3D). The structure of rGO plasmonic substrate with nanogaps and close-packed islanded lamellas are favorable for coupling between rGO plasmons in neighboring nanoisland, resulting in significant enhancement. To better understand this phenomenon, an rGO substrate with wide nanoscaled intervals and less continuous rGO layers were prepared, which only shows a low near-field enhancement (E/E0) around the layered rGO (Figure S5). Quantitative detection of self-assembled MB layer by in situ real time ATR-SEIRAS. The ability of rGO island film to sense in aqueous solution in situ is further evaluated by measuring the time-resolved SEIRA spectra of MB molecules adsorbed on the surface of rGO island film at the concentration of 50 µM with aqueous solution-incubated rGO island film as the reference (Figure 4A-B). The absorption kinetics was tracked following the absorbance of the characteristic peak at 1598 cm-1 versus absorption time (Figure 4C), indicating that the adsorption nearly attains to saturation within 30 min. The kinetic data were further analyzed by plotting T/A against T (Figure 4C, inset), which could be well described by pseudo-second-order kinetic model (linear form: T/A=1/kAe2+1/Ae×T, where A and Ae represent IR absorbance of MB at any time T and at equilibrium, respectively, k is rate constant36) with residual square correlation (R2) of 0.999, suggesting that the chemical adsorption is the rate determining


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Analytical Chemistry step36 to control MB adsorption corresponding to the strong π-π conjugation. Due to the strong interaction of MB with rGO island film, the intensity of SEIRA spectrum at 1598 cm-1 is double time higher than that of MB adsorbed on traditional Au film at the identical conditions (Figure S6) even the Au film shows about 20 times enhancement toward 4-aminothiophenol that was adsorbed through a favorable interaction between Au and sulfur group (Figure S7), further suggesting the significance of extending plasmonic substrate for breaking the limitation of sensor at metal surfaces. The equilibrium adsorption of the characteristic peak at 1598 cm-1 against the concentration of MB shows a sharp increase with the MB concentration varying from 5×10-10 to 5×10-6 M, and then a slower growth mode from 1×10-5 to 20×10-5 M till reaching equilibrium, which can be fitted well with the Langmuir adsorption isotherm (R2 = 0.99),5 suggesting a monolayer coverage of MB on rGO island film due to the chemical adsorption. The inset of Figure 4E is the corresponding calibration plot between IR response and MB concentration, which shows a linear range of 5×10-10 M to 5×10-7 M (R2 = 0.98) with the estimated limit of detection (LOD) of 8.23×10-11 M based on signal to noise ratio of 3 and the sensitivity was calculated as 1064 OD.M-1, indicating a high sensitive detection based on rGO island film-based SEIRAS. These results indicate that the proposed rGO plasmonic substrate with a good enhancement effect in mid-IR regime is a simple and sensitive sensor, and is favorable for a wide range of applications including detection of protein, DNA and especially molecules with aromatic structure14,19.

internal reflection configurations. The rGO plasmons can be excited by evanescent waves at the total reflection interface of prism to overcome the momentum mismatch between the surface plasmon and free-space photons, resulting in multiple resonance peaks and local field enhancements at the nanoscale intervals, which significantly improve weak light-matter interaction between µmwavelength infrared light and nano-size molecules. As a result, the cost effective rGO island film can sensitively detect MB in aqueous media through directly probing vibrational characteristics in the entire molecular fingerprint region with a detection limit of 10 pM. These remarkable features may open new avenues to explore in situ SEIRAS checking system in aqueous environment with enhanced optical field resulted from graphene like substrates.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental Procedures (Chemicals and reagents; Apparatus; Synthesis of graphene oxide (GO); Preparation of rGO island film; Au Film Preparation; Characterization of plasmons of rGO island film and sensor in aqueous solution; Simulation; Plasmon nanoimaging) Supplementary Figures (Figure S1: Synthesis and characterization of GO; S2: Reproducibility of rGO film on silicon wafers; S3: Intrinsic optical phonons and plasmons of rGO; S4: Characterization of rGO film with wide nanoscale intervals and less stacked layer; S5: In situ sensors of MB molecules in aqueous solution with rGO and Au island film based SEIRAS; S6: Evaluation of the enhancement factor of Au film).

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Figure 4. (A) Schematic of in situ sensor of MB molecules in aqueous solution with rGO island film-based SEIRAS in real time. (B) Time-dependent SEIRA spectra of MB adsorbed on rGO island film in aqueous solution at the concentration of 50 µM. (C) IR peak intensity at 1598 cm-1 was plotted with time (pink diamonds), and the fitting curve (black line). Inset: linear fitting with pseudo-second-order kinetic model. (D) SEIRA spectra of MB adsorbed on rGO island film in aqueous solution at 90 min with various concentrations: from bottom to top: 5×10-10, 5×10-9, 5×108 , 5×10-7, 1×10-6, 5×10-6, 1×10-5, 5×10-5, 10×10-5, 20×10-5 (mol/L). (E) IR absorbance at 1598 cm-1 was plotted as concentration, and corresponding Langmuir fitting curve (black line). Inset: plot of linear response ranged from 5×10-10 to 5×10-7 (mol/L).

CONCLUSIONS In this work, we demonstrate a new kind of rGO island film with nanoscale intervals that can be prepared by a facile chemical reduction of GO on Si prism at a large scale for in-situ SEIRAS in

This work was financially supported by the National Natural Science Foundation of China (21675149), Joint Sino-German Research Projects (21761132028), the Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-SLH019), and the Science and Technology Development Program of Jilin Province (20150519014JH, 20170414037GH, 20170520133JH).

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Analytical Chemistry

Table of Contents

A large-scale reduced graphene oxide (rGO) island film with nanoscaled intervals has been facilely prepared on a single total internal reflection silicon prism by chemical reduction of GO, and its plasmons can be readily excited by infrared evanescent waves, yielding a prominent enhancement effect in mid-infrared region, which, as a result, can acquire the entire vibrational fingerprints of methylene blue monolayer in aqueous solution with high sensitivity in real time.


In-Situ Surface-Enhanced Infrared Absorption Spectroscopy of

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