Photoluminescence of Graphene Oxide Infiltrated ... - ACS Publications

Nov 14, 2014 - Monica Terracciano , Vardan Galstyan , Ilaria Rea , Maurizio Casalino , Luca De Stefano , Giorgio Sbervegleri. Applied Surface Science ...
1 downloads 0 Views 5MB Size
Download PDF
Article pubs.acs.org/JPCC

Photoluminescence of Graphene Oxide Infiltrated into Mesoporous Silicon Ilaria Rea,*,† Lucia Sansone,‡ Monica Terracciano,†,§ Luca De Stefano,† Principia Dardano,† Michele Giordano,‡ Anna Borriello,‡ and Maurizio Casalino† †

Institute for Microelectronics and Microsystems, and ‡Institute for Polymers, Composites and Biomaterials, National Council of Research, Naples, Italy § Department of Pharmacy, University of Naples Federico II, Naples, Italy S Supporting Information *

ABSTRACT: Graphene oxide (GO) is a photoluminescent material whose application in optoelectronics has been strongly limited due to its poor emission intensity. In this work, a GO−porous silicon (GO−PSi) hybrid structure is realized in order to investigate the emission properties of GO infiltrated into a porous matrix. GO−PSi is characterized by Fourier transform infrared spectroscopy, spectroscopic reflectometry, and steady-state photoluminescence. A photoluminescence enhancement by a factor of 2.5 with respect to GO deposited on a flat silicon surface is demonstrated. Photoluminescence measurements also show a modulation of the emitted signal; this effect is attributed to the interference phenomena occurring inside the PSi monolayer.

1. INTRODUCTION Graphene oxide (GO) is composed of graphene sheets modified with oxygen functional groups in the form of epoxy and hydroxyl groups on both the basal plane and edges.1,2 In recent years, GO has received great interest because of its superior dispersion ability in water and a finite electronic band gap different with respect to graphene.3 Because of these properties, several applications ranging from electronic to biomedical field have been proposed.4−6 Innovative biosensors based on GO surface functionalization with small molecules or polymers through activation and amidation/esterification of either the carboxyls or hydroxyls have been demonstrated.7 In addition, new perspectives in optoelectronics have been opened by the discovery of the steady-state photoluminescence (PL) properties of GO. A broad PL emission from 500 to 800 nm has been reported.8 Unfortunately, GO PL is very weak due to the presence of oxygen functionalities (e.g., hydroxyl and epoxy groups) producing nonradiative recombination as a result of transfer of their electrons to the holes present in sp2 clusters.9 Many approaches based on oxidation or reduction treatments have been followed to get higher PL emission from GO.10,11 A new approach could be to infiltrate GO into large specific surface area substrates, and porous silicon (PSi), realized by electrochemical dissolution of crystalline silicon in a hydrofluoridric-based solution, is a perfect candidate for this task. PSi is characterized by a sponge-like morphology with a specific surface area up to hundreds of m2 cm−3. This property makes PSi an ideal support for sensing of different substances such as gases, liquids, and biological molecules.12−14 Changes of several © 2014 American Chemical Society

physical properties (e.g., dielectric constant and electrical conductivity) can be observed after infiltration of an analyte inside nanometric pores. Moreover, the PSi refractive index can be widely tuned between that of Si and air (approximately). This is achieved by varying the porosity of PSi during the electrochemical fabrication process. Owing to these characteristics, a lot of photonic structures, such as Fabry−Perot interferometers, Bragg mirrors , and microcavities, have been proposed.12,15,16 In this work, we investigated enhancement and wavelength modulation of photoluminescence signal of GO sheets infiltrated by spin-coating technique into silanized mesoporous silicon. GO-modified PSi layer was characterized by spectroscopic reflectometry, Fourier transform infrared spectroscopy, and steady-state photoluminescence. This hybrid structure showed an intense and wavelength-modulated photoluminescence signal on a broad range of optical frequencies.

2. MATERIALS AND METHODS 2.1. Preparation of Graphene Oxide. The graphene oxide (GO) was purchased from Cheap Tubes, Inc. (USA). The GO was synthesized from purified natural graphite by the modified Hummers method.17−19 Ten milligrams of GO powder was suspended in 5 mL of distilled water (D.I. was purchased from Aldrich Chemical Co.) and sonicated by means Received: July 1, 2014 Revised: November 6, 2014 Published: November 14, 2014 27301

dx.doi.org/10.1021/jp506539n | J. Phys. Chem. C 2014, 118, 27301−27307

The Journal of Physical Chemistry C

Article

Figure 1. Size distribution of graphene oxide in water, analyzed by DLS (A). AFM image of graphene oxide deposited on silicon and corresponding height measurement (B).

80T) with a defined delay time, prior to spinning, necessary for GO infiltration. PSi substrate was completely covered with sufficient amount of the GO solution, allowed to stand for 60 s, after which the sample was allowed to spin at 600 rpm for 60 s to cause uniform spreading of the solution on the substrate, then at 800 rpm for 60 s to thin the solution layer, and finally at 1600 rpm for 60 s to dry the film. After spin coating, the sample was dried in desiccator overnight at room temperature. 2.4. Dynamic Light Scattering. Size distribution of GO sheets dispersed in water (pH = 7) was investigated by dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern Instruments, U.K.) equipped with a He−Ne laser (633 nm, fixed scattering angle of 173°, 25 °C). 2.5. Atomic Force Microscopy. A XE-100 AFM (Park Systems) was used for the imaging of GO sheets deposited on silicon substrate. Surface imaging was obtained in noncontact mode using silicon/aluminum coated cantilevers (PPP-NCHR 10M; Park Systems) 125 μm long with resonance frequency of 200 to 400 kHz and nominal force constant of 42 N/m. The scan frequency was typically 1 Hz per line. 2.6. Scanning Electron Microscopy. The morphology of GO infiltrated PSi layer was investigated by scanning electron microscopy (SEM) using a field emission instrument (ZeissSupra 35). Sample was mounted on a double-faced conductive adhesive tape. Images were acquired at 5 kV accelerating voltage and 30 μm wide aperture. 2.7. Fourier Transform Infrared Spectroscopy. Chemical composition of PSi layer before and after GO infiltration was analyzed by Fourier transform infrared (FTIR) spectroscopy. Spectra were recorded by a Thermo-Nicholet NEXUS Continuum XL (Thermo Scientific) equipped with a microscope, at 2 cm−1 resolution. 2.8. Spectroscopic Reflectometry. The reflectivity spectra of PSi samples were measured at normal incidence by means of a Y optical reflection probe (Avantes), connected to a white light source and to an optical spectrum analyzer (Ando,

of an ultrasonic processor (Misonix Incorporeted Ultrasonic liquid Processors, U.S.A.) for 4 h at room temperature so as to produce a stable yellow−brown colloidal suspension of GO sheets. Solution was then filtered with pore size of 0.22 μm (Millipore). 2.2. Porous Silicon Layer Fabrication. PSi was fabricated by electrochemical etching of p+ crystalline silicon (0.001 Ω cm resistivity, ⟨100⟩ oriented, 500 μm thick) in hydrofluoric acid (HF; 50% in weight)/ethanol = 1:1 solution, in dark and at room temperature. Before the anodization process, the silicon substrate was immersed in HF solution for 2 min to remove the native oxide layer. A current density of 200 mA/cm2 was applied for 15 s to produce a single layer of PSi with a porosity of 72% (n = 1.54) and a thickness of 2 μm. Porosity and thickness of PSi layer were estimated by spectroscopic ellipsometry (data not shown here). After the electrochemical process, pore dimension was increased to favor the infiltration of graphene oxide by rinsing the “as-etched” PSi layer in a KOH−ethanol solution (1.5 mM) for 15 min.20 PSi sample was immersed in Piranha solution, a 4:1 mixture of concentrated sulfuric acid (H2SO4, was purchased from Aldrich Chemical Co.) and 30% hydrogen peroxide (H2O2 30% w/w was purchased from Aldrich Chemical Co.), for 40 min in order to ensure the formation of Si−OH bonds on its surface. After Piranha treatment, sample was rinsed with deionized water and dried in a nitrogen stream. Afterward, the PSi surface was silanized (i.e., functionalized by aminosilane) by incubating the chip in a 5% v/v solution of 3-aminopropyltriethoxysilane (APTES was purchased from Aldrich Chemical Co.) and anhydrous toluene (anhydrous 99.8% was purchased from Aldrich Chemical Co.) for 30 min at room temperature.21−23 After the reaction time, we washed three times the sample in toluene so as to remove any physisorbed APTES and baked it at 100 °C for 10 min. 2.3. Graphene Oxide Infiltration into Porous Silicon Layer. GO solution was infiltrated into aminosilane-modified PSi layer by spin-coating processing (Karl Suss Microtec Delta 27302

dx.doi.org/10.1021/jp506539n | J. Phys. Chem. C 2014, 118, 27301−27307

The Journal of Physical Chemistry C

Article

Figure 2. SEM image of bare aminosilane-modified porous silicon and corresponding histogram of pores size distribution (A). SEM image of aminosilane-modified porous silicon after infiltration with graphene oxide (B). Cross-sectional SEM image of aminosilane-modified porous silicon after infiltration with graphene oxide (C). Scale bar corresponds to 200 nm.

investigated by DLS analysis. Results, reported in Figure 1A, highlight the presence of two peaks corresponding to 30 ± 5 and 110 ± 40 nm size populations (poly dispersion index; PdI = 0.211). It is worth noting that the heights of two peaks shown in Figure 1A are related to the scattering intensities and not to the amount of small and big GO particles. The low value of PdI indicates that GO sample is constituted by only two populations, but we cannot say which is the most abundant. AFM image of GO deposited on flat crystalline Si and corresponding height profile of GO sheets are reported in Figure 1B. Nanosheets with different sizes ranging from few tens of nanometers to hundreds of nanometers can be observed. Height measurement demonstrates the presence of sheets up to 4 nm thick. Since a single-layered sheet is known to be as high as 1 nm,24 the measured thickness is compatible with a multilayered morphology. The chemical nature of graphene oxide was confirmed by Raman spectroscopy. Raman spectrum of GO deposited on

AQ6315B). The spectra were collected over the range 600− 1200 nm with a resolution of 0.2 nm. 2.9. Steady-State Photoluminescence. Steady-state photoluminescence (PL) spectra were excited by a continuous wave He−Cd laser at 442 nm (KIMMON Laser System). PL was collected at normal incidence to the surface of samples through a fiber, dispersed in a spectrometer (Princeton Instruments, SpectraPro 300i), and detected using a Peltier cooled charge coupled device (CCD) camera (PIXIS 100F). A long pass filter with a nominal cut-on wavelength of 458 nm was used to remove the laser line at monochromator inlet.

3. RESULTS AND DISCUSSION Even if GO sheets are nanometric in size, their infiltration in a porous material, such as a PSi layer, requires a careful characterization of both the GO size distribution and the PSi pores average dimension. Size distribution of GO nanosheets dispersed in water (pH = 7) at a concentration of 2 mg/mL was 27303

dx.doi.org/10.1021/jp506539n | J. Phys. Chem. C 2014, 118, 27301−27307

The Journal of Physical Chemistry C

Article

of aminosilane-modified PSi (A), GO deposited on flat crystalline silicon support (B), and GO infiltrated in aminosilane-modified PSi (C). FTIR spectrum of bare silanized PSi is characterized by the peak at 1100 cm−1, corresponding to Si− O−Si bond, and the peak at 955 cm−1, corresponding to Si− OH group, both due to Piranha treatment in strong acid. Surface aminosilane modification is demonstrated by the presence of peaks at 1475 and 785 cm−1, indicating amine and SiOCH2CH3 groups, respectively.27 In the spectrum of GO deposited on crystalline silicon, a peak at 1740 cm −1 corresponding to CO carbonyl stretching of −COOH can be observed; other peaks are present at 1640 cm−1 for CC and at 1460 cm−1 for CH2 deformation vibrations. The broad band located at about 970 cm−1 can be assigned to epoxy group.28 FTIR spectrum of GO infiltrated aminosilanemodified PSi shows peaks characteristic of both silanized PSi and GO, demonstrating the formation of the hybrid structure. The optical spectrum of a PSi layer consists of a fringe pattern due to interference occurring at air/PSi and PSi/bulk crystalline silicon interfaces. In other words, the PSi layer optically acts as a Fabry−Perot interferometer whose reflectance can be written as29

crystalline silicon is reported in Figure S1, Supporting Information: the broad G and D peaks and the low intensity 2D and D + G bands characteristic of GO2,25 are clearly visible. In our experience, GO cannot be simply deposited in the PSi sponge-like matrix by drop casting and spin coating: without a proper chemical modification of the PSi surface, we did not obtain any infiltration of GO into the PSi. On the contrary, GO strongly interacts with a silanized surface. For this reason, GO solution was spinned on PSi layer modified by APTES. Silanization procedure is crucial to guarantee interaction between GO and PSi surface; carboxyl and epoxy groups of GO can interact with positively charged amine group of APTES forming a stable GO−PSi hybrid structure.26 Figure 2 shows SEM top view images of an aminosilane-modified PSi layer before (Figure 2A) and after (Figure 2B) GO infiltration. In the case of bare silanized PSi, histogram of pores size distribution, reported in the inset of Figure 2A, reveals dimensions included between 20 and 80 nm with an average value of 40 nm. This mean size allows infiltration of smaller GO nanosheets (size ≈ 30 nm), while larger particles (size ≈ 110 nm) are accumulated on PSi surface, as it can be observed in SEM image of Figure 2B. The result is also evident from cross-sectional SEM image of GO−PSi structure, reported in Figure 2C. The image highlights GO infiltrated along nanometric PSi pores as well as GO layer deposited on the chip surface. The thickness of this GO layer is just of few nanometers, much smaller than the overall PSi thickness (approximately 2 μm). Chemical composition of the hybrid structure was investigated by FTIR analysis. Figure 3 shows FTIR spectra

⎛ 4πnPSiL ⎞ ⎟ R(λ) = (r1 + r2) + 2r1r2 cos⎜ ⎝ λ ⎠

(1)

where r1 = (nAir − nPSi)/(nAir + nPSi) is the Fresnel reflection coefficient at air/PSi interface, r2 = (nPSi − nSi)/(nPSi + nSi) is the Fresnel reflection coefficient at PSi/crystalline silicon interface, L is the thickness of the PSi layer, λ is the wavelength, and nAir, nPSi, and nSi are the refractive indices of air, porous silicon layer, and bulk silicon, respectively. The optical path (nPSiL) of the PSi layer can be calculated from the reflectivity spectrum just counting fringe maxima, which satisfy the relationship mλ = 2nPSiL, where m is an integer and λ is the wavelength of the incident light.30 A faster method for calculating the optical path is based on the fast Fourier transform (FFT) of PSi reflectivity spectrum. FFT of the spectrum displays a peak whose position along the x-axis corresponds to two times the optical path (2nPSiL) of the layer.30 In Figure 4A, normal incidence reflectivity spectra are shown for aminosilane-modified PSi layer before and after GO infiltration. The corresponding Fourier transforms, reported in Figure 4B, show two peaks centered at 5800 and 6000 nm, before and after GO infiltration, respectively. Since the thickness L of the layer is fixed, the peak shift of about 200 nm is ascribed to the increase of the PSi layer average refractive index. The GO solution penetrates into the porous silicon matrix, and some nanosheets bind the silanized porous silicon surface, partially filling the pores. Since air is displaced by GO into the pores, the layer density increases, and as consequence, the average refractive index increases, too. Considering a PSi thickness of 2000 nm, a refractive index increase of 0.05 after GO infiltration can be calculated. Since SEM characterization showed the presence of a thin GO layer on the top of PSi surface that could clog pores, we investigated their accessibility exposing GO−PSi hybrid device to ethanol vapors. It is well-known that vapors of volatile substances can condensate into nanometric pores by capillary condensation inducing an increase of the average refractive index of the PSi layer and a consequent red shift of its optical spectrum. Figure 5 reports the reflectivity spectra (A) of bare

Figure 3. FTIR spectra of aminosilane-modified porous silicon (A), graphene oxide on silicon (B), and graphene oxide infiltrated in aminosilane-modified porous silicon (C). 27304

dx.doi.org/10.1021/jp506539n | J. Phys. Chem. C 2014, 118, 27301−27307

The Journal of Physical Chemistry C

Article

Figure 4. Normal incidence reflectivity spectra of aminosilane-modified porous silicon layer before (black line) and after (blue line) GO infiltration (A). Corresponding Fourier transforms (B).

Figure 5. Normal incidence reflectivity spectra of bare PSi structure in air (black line) and on exposure to ethanol vapors (blue line) (A). Corresponding Fourier transforms (B). Normal incidence reflectivity spectra of GO−PSi hybrid structure in air (black line) and on exposure to ethanol vapors (blue line) (C). Corresponding Fourier transforms (D).

PSi in air and on exposure to ethanol vapors together with corresponding FFTs (B). We observed a FFT shift of 50 nm due to ethanol condensation inside pores of the PSi device.29 The experiment was repeated in the case of GO−PSi device. Spectra are shown in Figure 5C, together with corresponding FFTs (Figure 5D): a FFT peak shift of 100 nm was measured. The larger shift is due to the presence of GO nanosheets that, coating the pores wall of the hybrid device, reduce their diameter making the structure more sensitive, according to the Clausius−Kelvin equation, which rules the condensation.31

This experimental result demonstrates that, even if GO nanosheets are deposited onto and inside the PSi, the pores are still accessible to organic vapors, probably due to both the presence of voids between the GO sheets and the partial permeability of these extremely thin (just few nanometers) layers, whereas thicker and more compact GO coatings have been found to be quite impermeable.32 The PL signal emitted from GO nanosheets infiltrated in PSi was investigated at an excitation wavelength of 442 nm. Results are reported in Figure 6A together with PL emission of bare 27305

dx.doi.org/10.1021/jp506539n | J. Phys. Chem. C 2014, 118, 27301−27307

The Journal of Physical Chemistry C

Article

Figure 6. Photoluminescence spectra of aminosilane-modified PSi (solid line), GO on silanized Si (dashed line), and GO infiltrated in silanized porous Si layer (short dashed line) at an excitation wavelength of 442 nm (A). Comparison between photoluminescence spectrum (upper graph) and reflectivity spectrum (lower graph) of GO-infiltrated PSi layer (B).

after GO deposition. Spectra, reported in Figure S2, do not show any difference since few nanometers of GO cannot contribute to a measurable increase of the optical path. Reflectivity and photoluminescence spectra of GO−aSi were compared in Figure S3. As in the case of GO deposited on flat crystalline silicon, a very low and noisy signal was recorded. From this measurement, we can deduce that the presence of a homogeneous interferometer under the GO layer slightly modulates its photoluminescence, but not as much for the PSi layer. We also investigated where the luminescence of GO sheets deposited on PSi was occurring by confocal microscopy analysis on the hybrid structure. Figure S1A,B clearly shows a luminescence signal coming from both surface and inside the chip.

silanized PSi and of GO spin-coated on silanized crystalline silicon, for comparison. As it is well-known, any PL signal cannot be detected in the case of p+ PSi. A weak and broad photoluminescence was measured in the case of GO spinned on silanized Si in the wavelength range between 500 and 800 nm. This very low signal is due to the presence of a thin GO layer, with an average height of about 3 nm (see AFM characterization reported in Figure 1). After infiltration in PSi, a strong enhancement of the PL emitted from GO by a factor of almost 2.5 with respect to GO on crystalline silicon was experimentally measured. This strong enhancement in PL emission was attributed to the high GO concentration inside the sponge-like PSi structure. Moreover, a modulation of the photoluminescence signal can be observed, too. PL intensity modulation can be explained by considering the theory of the Fabry−Perot interferometer.33 Among all the wavelengths, λem, emitted by GO infiltrated in PSi, only those fullfilling the relationship L = m(λem/2nPSi), with L thickness of the PSi layer and m integer, can constructively interfere forming maxima in the photoluminescence spectrum of hybrid structure. As it can be seen in Figure 6B, the distance between two consecutive photoluminescence maxima is about 76 nm, exactly corresponding to the free spectral range (i.e., distance between two consecutive maxima/minima in the reflectivity/ transmittivity spectrum of an interferometer) of the GO−PSi hybrid structure. In order to exclude the possibility that the interference fringes present in photoluminescence spectrum of GO−PSi device could be due to the thin GO layer deposited on the PSi interferometer surface and not to the material infiltrated inside pores, GO was spinned on a layer of aminosilane-modified amorphous silicon (aSi), 800 nm thick. The homogeneous layer (not porous) of aSi acts as an interferometer with a free spectral range of about 55 nm. The fabrication procedure of GO-deposited aSi (GO−aSi) is described in the Supporting Information. We monitored reflectivity spectra of aminosilane-modified aSi before and

4. CONCLUSIONS This work demonstrates the enhancement and the modulation of the photoluminescence signal emitted by GO nanosheets infiltrated in silanized mesoporous silicon matrix. The formation of GO−PSi hybrid structure is confirmed by several spectroscopic techniques, such as FTIR spectroscopic reflectometry and steady-state photoluminescence. FFT analysis reveals a refractive index increase, due to the infiltration of GO inside the PSi layer, of 0.05. In addition, a strong enhancement of the PL emitted from GO by a factor of almost 2.5 with respect to GO on crystalline silicon has been experimentally measured. Wavelength modulation of GO photoluminescence emission is very attractive and opens new perspectives for GO exploitation in innovative optoelectronic devices and high sensitive fluorescent sensors.

■

ASSOCIATED CONTENT

S Supporting Information *

Raman analysis, aSi optical characterization, and confocal microscopy imaging. This material is available free of charge via the Internet at http://pubs.acs.org. 27306

dx.doi.org/10.1021/jp506539n | J. Phys. Chem. C 2014, 118, 27301−27307

The Journal of Physical Chemistry C

■

Article

(17) Hummers, W.; Offeman, R. J. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (18) Chen, W. F.; Yan, L. F.; Bangal, P. R. Preparation of Graphene by the Rapid and Mild Thermal Reduction of Graphene Oxide Induced by Microwaves. Carbon 2010, 48, 1146−1152. (19) Donner, S.; Li, H. W.; Yeung, E. S.; Porter, M. D. Fabrication of Optically Transparent Carbon Electrodes by the Pyrolysis of Photoresist Films: Approach to Single-Molecule Spectroelectrochemistry. Anal. Chem. 2006, 78, 2816−2822. (20) DeLouise, L. A.; Meng Kou, P.; Miller, B. L. Cross Correlation of Optical Microcavity Biosensor Response with Immobilized Enzyme Activity. Insights into Biosensor Sensitivity. Anal. Chem. 2005, 77, 3222−3230. (21) Kim, J.; Seidler, P.; Wan, L. S.; Fill, C. Formation, Structure, and Reactivity of Amino-Terminated Organic Films on Silicon Substrates. J. Colloid Interface Sci. 2009, 329, 114−119. (22) De Stefano, L.; Oliviero, G.; Amato, J.; Borbone, N.; Piccialli, G.; Mayol, L.; Rendina, I.; Terracciano, M.; Rea, I. Aminosilane Functionalizations of Mesoporous Oxidized Silicon for Oligonucleotides Synthesis and Detection. J. R. Soc. Interface 2013, 10, 20130160. (23) Terracciano, M.; Rea, I.; Politi, J.; De Stefano, L. Optical Characterization of Aminosilane-Modified Silicon Dioxide Surface for Biosensing. J. Eur. Opt. Soc. Rapid Publ. 2013, 8, 13075−6. (24) Gomez-Navarro, C.; Thomas Weitz, R.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. Nano Lett. 2007, 7, 3499−3503. (25) Dimiev, A. M.; Tour, J. M. Mechanism of Graphene Oxide Formation. ACS Nano 2014, 8, 3060−3068. (26) He, Q.; Sudibya, H. G.; Yin, Z.; Wu, S.; Li, H.; Boey, F.; Huang, W.; Chen, P.; Zhang, H. Centimeter-Long and Large-Scale Micropatterns of Reduced Graphene Oxide Films: Fabrication and Sensing Applications. ACS Nano 2010, 6, 3201−3208. (27) Vandenberg, E. T.; Bertilsson, L.; Liedberg, B.; Uvdal, K.; Erlandsson, R.; Elwing, H.; Lundström, I. Structure of 3-Aminopropyl Triethoxy Silane on Silicon Oxide. J. Colloid Interface Sci. 1991, 147, 103−118. (28) Hontoria-Lucas, C.; López-Peinado, A. J.; López-González, J.; de, D.; Rojas-Cervantes, M. L.; Martín-Aranda, R. M. Study of Oxygen-Containing Groups in a Series of Graphite Oxides: Physical and Chemical Characterization. Carbon 1995, 33, 1585−1592. (29) Pacholski, C.; Sartor, M.; Sailor, M. J.; Cunin, F.; Miskelly, G. M. Biosensing Using Porous Silicon Double-Layer Interferometers: Reflective Interferometric Fourier Transform Spectroscopy. J. Am. Chem. Soc. 2005, 127, 11636−11645. (30) Wu, J.; Sailor, M. J. Chitosan Hydrogel-Capped Porous SiO2 as a pH-Responsive Nano-Valve for Triggered Release of Insulin. Adv. Funct. Mater. 2009, 19, 733−741. (31) Ouyang, H.; Striemer, C. C.; Fauchet, P. M. Quantitative analysis of the sensitivity of porous silicon optical biosensors. Appl. Phys. Lett. 2006, 88, 163108. (32) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R. Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes. Science 2014, 343, 752−754. (33) Casalino, M.; Coppola, G.; Iodice, M.; Rendina, I.; Sirleto, L. Critically Coupled Silicon Fabry−Perot Photodetectors Based on the Internal Photoemission Effect at 1550 nm. Opt. Express 2012, 20, 12599−12609.

AUTHOR INFORMATION

Corresponding Author

*Phone: +390816132594. Fax: +390816232598. E-mail: ilaria. [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. M. Pirozzi of IBP-CNR for assistance with confocal microscopy acquisition, and Dr. P. Musto and Dr. P. Lamanna of IPCB-CNR for Raman measurements.

■

REFERENCES

(1) Cai, W. W.; Piner, R. D.; Stadermann, F. J.; Park, S.; Shaibat, M. A.; Ishii, Y.; Yang, D. X.; Velamakanni, A.; An, S. J.; Stoller, M.; et al. Synthesis and Solid-State NMR Structural Characterization of 13CLabeled Graphite Oxide. Science 2008, 321, 1815−1817. (2) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906−3924. (3) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. Processable Aqueous Dispersions of Graphene Nanosheets. Nat. Nanotechnol. 2008, 3, 101−105. (4) Wang, Y.; Li, Z.; Hu, D.; Lin, C.-T.; Li, J.; Lin, Y. Aptamer/ Graphene Oxide Nanocomplex for in Situ Molecular Probing in Living Cells. J. Am. Chem. Soc. 2010, 132, 9274−9276. (5) Lu, C.-H.; Yang, H.-H.; Zhu, C.-L.; Chen, X.; Chen, G.-N. A Graphene Platform for Sensing Biomolecules. Angew. Chem. 2009, 121, 4879−4881. (6) Wang, Y.; Li, Z.; Wang, J.; Li, J.; Lin, Y. Graphene and Graphene Oxide: Biofunctionalization and Applications in Biotechnology. Trends Biotechnol. 2011, 29, 205−212. (7) Chen, D.; Feng, H.; Li, J. Graphene Oxide: Preparation, Functionalization, and Electrochemical Applications. Chem. Rev. 2012, 112, 6027−6053. (8) Chien, C.-T.; Li, S.-S.; Lai, W.-J.; Yeh, Y.-C.; Chen, H.-A.; Chen, I.-S.; Chen, L.-C.; Chen, K.-H.; Nemoto, T.; et al. Tunable Photoluminescence from Graphene Oxide. Angew. Chem. 2012, 51, 6662−6666. (9) Gupta, A.; Shaw, B. K.; Saha, K. Bright Green Photoluminescence in Aminoazobenzene-Functionalized Graphene Oxide. J. Phys. Chem. C 2014, 118, 6972−6979. (10) Eda, G.; Lin, Y.; Mattevi, C.; Yamaguchi, H.; Chen, H.; Chen, I.; Chen, C.; Chhowalla, M. Blue Photoluminescence from Chemically Derived Graphene Oxide. Adv. Mater. 2010, 22, 505−509. (11) Gokus, T.; Nair, R. R.; Bonetti, A.; Bohmler, M.; Lombardo, A.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Hartschuh, A. Making Graphene Luminescent by Oxygen Plasma Treatment. ACS Nano 2009, 3, 3963−3968. (12) De Stefano, L.; Rendina, I.; Moretti, L.; Rossi, A. M. Optical Sensing of Flammable Substances Using Porous Silicon Microcavities. Mater. Sci. Eng., B 2003, 100, 271−274. (13) De Stefano, L.; Malecki, K.; Della Corte, F. G.; Moretti, L.; Rea, I.; Rotiroti, L.; Rendina, I. A Microsystem Based on Porous SiliconGlass Anodic Bonding for Gas and Liquid Optical Sensing. Sensors 2006, 6, 680−687. (14) Rea, I.; Lamberti, A.; Rendina, I.; Coppola, G.; Gioffrè, M.; Iodice, M.; Casalino, M.; De Tommasi, E.; De Stefano, L. Fabrication and Characterization of a Porous Silicon Based Microarray for LabelFree Optical Monitoring of Biomolecular Interactions. J. Appl. Phys. 2010, 107, 014513. (15) Dancil, K. P. S.; Greiner, D. P.; Sailor, M. J. A Porous Silicon Optical Biosensor: Detection of Reversible Binding of IgG to a Protein A-Modified Surface. J. Am. Chem. Soc. 1999, 121, 7925−7930. (16) Snow, P. A.; Squire, E. K.; Russell, P. S. J.; Canham, L. T. Vapor Sensing Using the Optical Properties of Porous Silicon Bragg Mirrors. J. Appl. Phys. 1999, 86, 1781−1784. 27307

dx.doi.org/10.1021/jp506539n | J. Phys. Chem. C 2014, 118, 27301−27307