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Graphene Oxide as an Optical Biosensing Platform Eden Morales-Narváez and Arben Merkoçi* (that converts a biorecognition event into a suitable signal). Biosensing influenced by nanotechnology is based on nanomaterials or nanostructures as transducer elements or reporters of biorecognition events. Since nanomaterials range in the same scale of the analytes, when linked to biorecognition probes (such as antibodies, DNA and enzymes), nanostructures allow the control, manipulation and detection of molecules with diagnostic interest through advantageous strategies. Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms ordered in a two-dimensional honeycomb lattice and is the basic building block for 0D, 1D and 3D carbon allotropes[1] (see Figure 1A). Graphene oxide (GO) distributes a similar atomically thin structural lattice, but possesses oxygen-containing functional groups (see Figure 1B). Since graphene, graphene oxide and reduced graphene oxide exhibit innovative mechanical, electrical, thermal and optical properties these two-dimensional materials are increasingly attracting attention and they are under active research.[2–8] Among these materials with lattice-like nanostructure, GO displays advantageous characteristics to be used in biosensing platforms owing to the excellent capabilities for direct wiring with biomolecules, heterogeneous chemical and electronic structure, the possibility to be processed in solution and the availability to be tuned as an insulator, semiconductor or semi-metal.[4,7,9,10] Here, we discuss different potentially exploitable properties of GO (such as 2D structure, photoluminescence, quenching capabilities and biomolecule interaction) and we present an overview of the current approaches with interest for optical biosensing applications. We also cover future perspectives and challenges.
Since graphene exhibits innovative mechanical, electrical, thermal, and optical properties, this 2D material is increasingly attracting attention and is under active research. Among the various graphene forms with lattice-like nanostructure, graphene oxide (GO) displays advantageous characteristics as a biosensing platform due to its excellent capabilities for direct wiring with biomolecules, a heterogeneous chemical and electronic structure, the possibility to be processed in solution and the ability to be tuned as insulator, semiconductor or semi-metal. Moreover, GO photoluminescences with energy transfer donor/acceptor molecules exposed in a planar surface and is even proposed as a universal highly efficient long-range quencher, which is opening the way to several unprecedented biosensing strategies. Here, the rationale behind the use of GO in optical biosensing applications is discussed by describing different potentially exploitable properties of GO, and an overview of the current approaches are presented along with future perspectives and challenges.
1. Introduction In the 21st century, nanotechnology has revolutionized many fields including medicine, biology, chemistry, physics, and electronics. In this regard, biosensors have been also benefited by nanotechnology, which is an emerging multidisciplinary field that entails the synthesis and use of materials or systems at the nanoscale. The rationale behind this technology is that nanomaterials possess optical, electronic, magnetic or structural properties that are unavailable to bulk materials. Generally, biosensors include biorecognition probes (responsible for the specific detection of the analytes) and a transducer element
Prof. A. Merkoçi Nanobioelectronics & Biosensors Group Catalan Institute of Nanotechnology Barcelona, 08193 Spain Website: http://www.nanobiosensors.org/group-leader/ E-mail:
[email protected] Prof. A. Merkoçi ICREA, Barcelona, 08010 Spain E. Morales-Narváez Nanobioelectronics & Biosensors Group Catalan Institute of Nanotechnology Barcelona, 08193 Spain E. Morales-Narváez ESAII Department Polytechnic University of Catalonia Barcelona, 08028 Spain
DOI: 10.1002/adma.201200373
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2. Potentially Exploitable Properties of GO in Optical Biosensing 2.1. Structural and Photoluminescence Properties GO is often produced by processes that entail oxidation of graphite in the presence of strong acids and oxidants. In these conditions, the exfoliation of graphite yields atomically thin GO sheets that can be dispersible by simple sonication in aqueous and organic solutions.[2,3] The intrinsic thickness of single layer GO sheets is ∼0.6 nm[4] and the lateral size of GO sheets can range in the order of tens of nanometers to tens of micrometers,
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. Schematic representations of graphenes and GO images. A) Graphene as the basic building block of carbon allotropes, from left to right: fullerene, nanotube and graphite. Reproduced with permission.[1] Copyright 2007, Nature Publishing Group. B) Sketch of GO. C) Sketch of (highly) reduced GO. Adapted with permission.[3] D) Atomic force microsopy (AFM) image of GO sheets. Adapted with permission.[44] E) Transmission electron microscopy (TEM) image of a GO flake, fewlayers. Adapted with permission.[35] Copyright 2011, American Chemical Society.
Adv. Mater. 2012, 24, 3298–3308
Eden Morales-Narváez received a degree in Bionics Engineering from the National Polytechnic Institute (IPN) of Mexico. In 2008 he was awarded a scholarship as a pre-doctoral researcher by the National Council of Science and Technology of Mexico (CONACyT). He then enrolled in a Biomedical Engineering PhD at the Polytechnic University of Catalonia in Spain. From 2008 to 2010, he worked with microarray technology at the Institute for Bioengineering of Catalonia. He is currently undertaking his PhD at the Catalan Institute of Nanotechnology, under Prof. Merkoçi. He is focusing his research on novel microarrays modified with nanomaterials towards diagnostic applications.
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see Table 1 and Figure 1D,E. Due to the oxidation process, GO bears the disruption a planar sheet of sp2-bonded carbon atoms ordered in a two-dimensional honeycomb lattice. As a consequence, GO possesses a recombination of electron–hole pairs localized within a small sp2 carbon domain embedded in a sp3 matrix,[11–13] displaying the photoluminescence property. The photoluminescence intensity has been reported to be correlated to the evolution of sp2 domains during reduction processes[14] and pH-dependent (Pan et al., 2010). The fluorescence emission spectra has been reported to depend upon the oxidation time.[16] Loh and co-workers have extensively covered the optical properties of GO reporting that “the manipulation of the size, shape and relative fraction of the sp2-hybridized domains of GO by reduction chemistry provides opportunities for tailoring its optoelectronic properties”.[4] Figure 2A displays an example of blue photoluminescence of GO.[14] The chemistry of GO has been widely discussed by Dreyer and colleagues.[17] Since GO bears carboxyl groups exposed on the edges and hydroxyls and epoxies groups on the basal plane, a wide variability in the type and coverage of the oxygen containing functional groups on GO can be observed, see Figure 1B. Due to this heterogeneous structure, GO can be fluorescent over a broad range of wavelengths, predominantly arising from differences in synthesis processes.[4] In fact, GO fluorescence from near infra-red region to ultraviolet emission has been reported. For example, graphene quantum dots emission derived from oxidized carbon fibers can be tuned by using different synthesized reaction temperatures displaying blue, green and yellow emission colors;[18] GO synthesized by microwave-assisted chemical method can show an emission peak at 750 nm by exciting at
Arben Merkoçi is ICREA Research Professor and head of the Nanobioelectronics & Biosensors Group at Catalan Institute of Nanotechnology in Barcelona. His research is focused on the integration of biological molecules and other species with microand nanostructures with an interest in the design of novel sensors and biosensors. He is author of more than 150 publications and an editor of books and special journals issues dedicated to the field of nanomaterials applications in biosensors. Prof. Merkoçi also lectures in various national and international scientific and divulgation meetings related to nanotechnology and biosensors.
500 nm[19] and GO obtained by modified Hummers method can exhibit an emission peak at 475 nm by exciting at 350 nm.[20] The ratio between the photons emitted to the photons absorbed, i.e. the quantum yield of photoluminescent GO has been reported to be enhanced by functionalization chemistry; specifically, surface amide formation and ring-opening amination of epoxide of an original GO by using various alkylamines.[21] Table 1 displays different examples of the photoluminescence from GO. Details on fabrication method, lateral size of the flakes, number of layers and quantum yield are included. 2.2. GO Quenching Capabilities Förster (or fluorescence) resonance energy transfer (FRET) is a phenomenon in which photo excitation energy is transferred
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www.MaterialsViews.com Table 1. Photoluminescence from GO.a) Exc/Em [nm] 380/∼560
Quantum yield
Fabrication method
Lateral Size
Layers
Reference
N/A
MHM
∼50 nm
Single to few layers
[70]
400/∼570
N/A
MHM
∼10–300 nm
Single-layer
[50]
500/∼750
N/A
Microwave-assisted chemical method
∼10 μm
Single layer
[19]
260/∼360
∼2.8–5.2%
Electrochemical exfoliation
∼500 nm
Single and bilayer
[71]
325/∼370
N/A
MHM
∼35 μm
Several monolayers
[14]
350/∼475 450/∼660
N/A
MHM
∼1 nm
Single layer
[20]
400/∼547
∼70%
MHM
∼0.5–2 μm
Single and bilayer
[16]
