Optical properties of hybrid T3Pyr/SiO2/3C-SiC nanowires
© Fabbri et al.; licensee Springer. 2012
Received: 7 November 2012
Accepted: 4 December 2012
Published: 17 December 2012
A new class of nanostructured hybrid materials is developed by direct grafting of a model thiophene-based organic dye on the surface of 3C-SiC/SiO2 core/shell nanowires. TEM-EDX analysis reveals that the carbon distribution is more spread than it would be, considering only the SiC core size, suggesting a main contribution from C of the oligothiophene framework. Further, the sulfur signal found along the treated wires is not detected in the as-grown samples. In addition, the fluorescent spectra are similar for the functionalized nanostructures and T3Pyr in solution, confirming homogeneous molecule grafting on the nanowire surface. Chemical and luminescence characterizations confirm a homogeneous functionalization of the nanowires. In particular, the fluorophore retains its optical properties after functionalization.
Organic functionalization is emerging as an important area in the development of new semiconductor-based materials and devices. Direct immobilization of molecules or other types of selective receptors onto a semiconductor surface allows the achievement of new physicochemical properties [1–3] that lead to novel sensing platforms based, for instance, on electrical or optical transduction of chemical reactions [4–6].
The sizes of the one-dimensional (1D) nanostructures are comparable to those of the biological and chemical species. Therefore, they represent the excellent transducers for producing signals from the species being sensed. Among the variety of 1D nanostructures that have received considerable attention, silicon nanowires (NWs) demonstrated superior properties for the development of chemi- and biosensor families, e.g., DNA sensing devices, virus detectors, or pH sensors [7–9].
In these fields, silicon carbide nanostructures are also promising due to their physicochemical properties in photocatalysis , superplasticity , biocompatibility, and resistance to photobleaching . In addition, recent works have demonstrated enhancement and possible tuning of the optical emission of silicon carbide NWs covered with native silicon dioxide amorphous shell [13, 14]. This effect could be employed to improve the performances of sensors, based on the transduction of chemical reaction in optical emission, and also in possible in vivo application.
In this work, we describe a new class of fluorescent hybrid nanomaterials consisting of SiC/SiO2 core/shell nanowires functionalized with a model thiophene-based organic dye as potential biosensing candidate. The employment of SiC/SiO2 core/shell NWs has been decided to exploit well-known chemical methods for the functionalization of silicon dioxide, i.e., reaction with the oxydrilic group of the silica surface [15–17]. The oligothiophene component was selected because of the outstanding structural versatility, fluorescence, and charge-transporting properties of thiophene-based compounds that are among the most used organic materials for applications ranging from plastic electronics to biosensing and biodiagnostic [18, 19]. Notably, oligothiophene compounds have been largely used for bio-imaging purposes and have shown high chemical and photochemical stability in biologic environment and non-toxicity for living cells [19, 20]. The fluorophore (T3Pyr) selected for this work is characterized by a pyridine antenna, capable of cation complexation, and a triethoxysilane end moiety for the covalent binding of oxydrilated surfaces [21, 22] such as the NW shell. This molecule has recently been used to realize fluorescent self-assembled monolayers with pH sensitive multicolor and bright fluorescence .
The synthesis of the 3C-SiC/SiO2/T3Pyr nanosystem and the morphological and optical characterization by combined scanning electron microscopy (SEM), transmission electron microscopy (TEM), and laser scanning confocal microscopy (LSCM) is herein described. The results show a homogeneous molecule grafting on the nanowire surface and the preservation, after the grafting, of the optical properties of the organic dye. This study can open new perspectives for applications of the novel nanostructured hybrid material as biosensing agent, combining the advantages of the surface-to-volume ratio of the SiC/SiO2 core/shell nanowires with the pH sensitive fluorescence properties of T3Pyr.
Before being placed into the reactor, the substrate is cleaned in organic solvents with an ultrasonic bath, dipped in a 0.01-M solution of Ni(NO3)2 in ethanol, and dried in air (Figure 1: bottom left, colored figure). For the NW growth, the substrate is placed inside an open tube in the center of a horizontal furnace previously purged with inert gas to remove air. The temperature is raised to 1,100°C, and after temperature stabilization carbon oxide is introduced into the tube. The growth time of NWs selected for functionalization experiments is 30 min.
The first has been proposed by Pongracz et al. to explain the growth of nanocrystals  and directly involves silicon from the substrate. The latter has been more often reported in the literature [26–28] for the growth of nanowires and involves SiO obtained by intermediate reactions between the substrate and carbon oxides.
The morphological and optical properties of the nanowires were investigated before functionalization by SEM and cathodoluminescence (CL) spectroscopy in a SEM Cambridge 360 Stereoscan (Cambridge Instruments Ltd., Cambridge, England) equipped with a Gatan MONO-CL2 spectrometer (Gatan, Inc., Pleasanton, CA, USA). Their structural and chemical properties were studied by TEM in a field-emission microscope (JEOL 2200FS, JEOL Ltd., Akishima, Tokyo, Japan) working at 200 kV, equipped with in-column omega filter, high angle annular dark field (HAADF) detector for Z-contrast experiments, and energy-dispersive X-Ray (EDX) microanalysis.
Fluorescence imaging was performed on an inverted Nikon A1 (Nikon Co., Shinjuku, Tokyo, Japan) laser scanning confocal microscope equipped with a 405-nm pulsed/CW diode laser (PicoQuant GmbH, Berlin, Germany). Confocal fluorescence imaging was carried out on the samples at 20°C. The 1,024 × 1,024-pixel images were collected using a Nikon PLAN APO VC 60 oil immersion objective with NA 1.40. With this imaging configuration, spatial resolution is ca. 210 nm in the x and y direction. Z-stack imaging was performed on 2.5-μm sections for a total thickness of 50 μm. Fluorescence was collected in the 500- to 550-nm spectral window.
Fluorescence lifetime imaging was performed, applying the time-correlated single-photon counting (TCSPC) technique. A single-photon avalanche diode (SPAD) detector was used for this scope. A 460- to 500-nm band-pass filter was placed in front of the operating SPAD. The instrument is integrated with PicoHarp 300 electronics (PicoQuant GmbH) for TCSPC measurements. The repetition rate of the pulsed excitation at 405 nm was 40 MHz. FWHM of the laser pulse is ca. 120 ps. A tail fit has been performed on the histogram calculated for a region of interest of the sample image. Fitting of the experimental trace to tri-exponential decay function yielded satisfactory results with a good chi-square value.
Results and discussion
Characterization before surface functionalization
Characterization after surface functionalization
A new nanostructured hybrid material is developed by direct grafting of a model thiophene-based organic dye on 3C-SiC/SiO2 core/shell nanowire surface. A structural characterization carried out on the as-grown nanostructures reveals that the nanowires are a 3C-SiC core wrapped in a quasi-stoichiometric silicon dioxide. The cathodoluminescence spectroscopy presents two broad emission bands that resulted from three different components that peaked at 2.43 eV, related to the near-band-edge emission from the 3C-SiC core, and at 2.74 and 3.6 eV, involving the triplet and singlet states of ODC(II) in the silicon dioxide shell. TEM-EDX analysis of the functionalized nanowires reveals that the carbon distribution is more spread than it would be considering only the SiC core size, suggesting a main contribution from C of the oligothiophene framework. Further, a sulfur signal not detected in the as-grown samples is found along the treated wires. In addition, the confocal fluorescent spectroscopy shows similar line shapes of functionalized NWs and T3Pyr in solution, confirming homogeneous molecule grafting on the nanowire surface, without any influence of the luminescence properties of the inorganic component. Chemical and luminescence characterizations confirm a homogeneous functionalization of the nanowires; in particular, the organic dye maintains its optical properties after the grafting, thus opening a possible perspective of this new nanostructured hybrid material in the field of chemo-/biosensing applications.
The work was supported by Consorzio MIST-ER (project FESR-tecnopolo AMBIMAT), Ministero degli Affari Esteri (bilateral project ‘Nanoscale assEssMent of chEmical and phySical propertIes of advanced nanostructures (NEMESIS)’), and Fondazione Cariparma (project ‘Nanosistemi ibridi multifunzionali innovativi per applicazioni biomediche (BioNiMed)’. CARISBO foundation, Bologna, is acknowledged for its contribution in the acquisition of the Nikon A1 confocal microscope.
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