Graphene-Based Materials for Biosensors: A Review <s> Graphene-Based Quantum Dots <s> Arc-synthesized single-walled carbon nanotubes have been purified through preparative electrophoresis in agarose gel and glass bead matrixes. Two major impurities were isolated: fluorescent carbon and short tubular carbon. Analysis of these two classes of impurities was done. The methods described may be readily extended to the separation of other water-soluble nanoparticles. The separated fluorescent carbon and short tubule carbon species promise to be interesting nanomaterials in their own right. <s> BIB001 </s> Graphene-Based Materials for Biosensors: A Review <s> Graphene-Based Quantum Dots <s> Oxidization of carbon nanotubes by a mixed acid has been utilized as a standard method to functionalize carbon nanomaterials for years. Here, the products obtained from carbon nanotubes and graphite after a mixed-acid treatment are carefully studied. Nearly identical carbon dot (Cdot) products with diameters of 3–4 nm are produced using this approach from a variety of carbon starting materials, including single-walled carbon nanotubes, multiwalled carbon nanotubes, and graphite. These Cdots exhibit strong yellow fluorescence under UV irradiation and shifted emission peaks as the excitation wavelength is changed. In vivo fluorescence imaging with Cdots is then demonstrated in mouse experiments, by using varied excitation wavelengths including some in the near-infrared (NIR) region. Furthermore, in vivo biodistribution and toxicology of those Cdots in mice over different periods of time are studied; no noticeable signs of toxicity for Cdots to the treated animals are discovered. This work provides a facile method to synthesize Cdots as safe non-heavy-metal-containing fluorescent nanoprobes, promising for applications in biomedical imaging. <s> BIB002 </s> Graphene-Based Materials for Biosensors: A Review <s> Graphene-Based Quantum Dots <s> Left-handed Z-DNA has been identified as a transient structure occurred during transcription. DNA B-Z transition has attracted much attention because of not only Z-DNA biological importance but also their relation to disease and DNA nanotechnology. Recently, photoluminescent carbon dots, especially highly luminescent nitrogen-doped carbon dots, have attracted much attention on their applications to bioimaging and gene/drug delivery because of carbon dots with low toxicity, highly stable photoluminescence and controllable surface function. However, it is still unknown whether carbon dots can influence DNA conformation or structural transition, such as B-Z transition. Herein, based on our previous series work on DNA interactions with carbon nanotubes, we report the first example that photoluminescent carbon dots can induce right-handed B-DNA to left-handed Z-DNA under physiological salt conditions with sequence and conformation selectivity. Further studies indicate that carbon dots would bind to DNA major groove with GC preference. Inspired by carbon dots lighting up Z-DNA and DNA nanotechnology, several types of DNA logic gates have been designed and constructed based on fluorescence resonance energy transfer between photoluminescent carbon dots and DNA intercalators. <s> BIB003 </s> Graphene-Based Materials for Biosensors: A Review <s> Graphene-Based Quantum Dots <s> High fluorescence quantum yield graphene quantum dots (GQDs) have showed up as a new generation for bioimaging. In this work, luminescent GQDs were prepared by an ameliorative photo-Fenton reaction and a subsequent hydrothermal process using graphene oxide sheets as the precursor. The as-prepared GQDs were nanomaterials with size ranging from 2.3 to 6.4 nm and emitted intense green luminescence in water. The fluorescence quantum yield was as high as 24.6% (excited at 340 nm) and the fluorescence was strongest at pH 7. Moreover, the influences of low-concentration (12.5, 25 μg/mL) GQDs on the morphology, viability, membrane integrity, internal cellular reactive oxygen species level and mortality of HeLa cells were relatively weak, and the in vitro imaging demonstrated GQDs were mainly in the cytoplasm region. More strikingly, zebrafish embryos were co-cultured with GQDs for in vivo imaging, and the results of heart rate test showed the intake of small amounts of GQDs brought little harm to the cardiovascular of zebrafish. GQDs with high quantum yield and strong photoluminescence show good biocompatibility, thus they show good promising for cell imaging, biolabeling and other biomedical applications. <s> BIB004 </s> Graphene-Based Materials for Biosensors: A Review <s> Graphene-Based Quantum Dots <s> At present, the actual mechanism of the photoluminescence (PL) of fluorescent carbon dots (CDs) is still an open debate among researchers. Because of the variety of CDs, it is highly important to summarize the PL mechanism for these kinds of carbon materials; doing so can guide the development of effective synthesis routes and novel applications. This review will focus on the PL mechanism of CDs. Three types of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs), and polymer dots (PDs). Four reasonable PL mechanisms have been confirmed: the quantum confinement effect or conjugated π-domains, which are determined by the carbon core; the surface state, which is determined by hybridization of the carbon backbone and the connected chemical groups; the molecule state, which is determined solely by the fluorescent molecules connected on the surface or interior of the CDs; and the crosslink-enhanced emission (CEE) effect. To give a thorough summary, the category and synthesis routes, as well as the chemical/physical properties for the CDs, are briefly introduced in advance. <s> BIB005 </s> Graphene-Based Materials for Biosensors: A Review <s> Graphene-Based Quantum Dots <s> Abstract Carbon-based photoluminescent nanodot has currently been one of the promising materials for various applications. The remaining challenges are the carbon sources and the simple synthetic processes that enhance the quantum yield, photostability and biocompatibility of the nanodots. In this work, the synthesis of blue photoluminescent carbon nanodots from limeade via a single-step hydrothermal carbonization process is presented. Lime carbon nanodot (L-CnD), whose the quantum yield exceeding 50% for the 490 nm emission in gram-scale amounts, has the structure of graphene core functionalized with the oxygen functional groups. The micron-sized flake of the as-prepared L-CnD powder exhibits multicolor emission depending on an excitation wavelength. The L-CnDs are demonstrated for rapidly ferric-ion (Fe3 +) detection in water compared to Fe2 +, Cu2 +, Co2 +, Zn2 +, Mn2 + and Ni2 + ions. The photoluminescence quenching of L-CnD solution under UV light is used to distinguish the Fe3 + ions from others by naked eyes as low concentration as 100 μM. Additionally, L-CnDs provide exceptional photostability and biocompatibility for imaging yeast cell morphology. Changes in morphology of living yeast cells, i.e. cell shape variation, and budding, can be observed in a minute-period until more than an hour without the photoluminescent intensity loss. <s> BIB006 </s> Graphene-Based Materials for Biosensors: A Review <s> Graphene-Based Quantum Dots <s> The photoluminescence of graphene quantum dots (GQDs) is rigorously investigated due to their potential applications. However, GQDs from graphene oxide are inherently embedded with non-negligible defects and oxygen grafted onto the edge and basal plane, which induce a change of innate electronic structure in the GQDs. Thus, graphene oxide based GQDs can misrepresent the characteristic properties of primitive GQDs. Here we report the size and pH dependent photophysical properties of GQDs that minimize the content of oxygen and defects. From auger electron spectroscopy, the oxygen content of the GQDs with two different lateral sizes (∼2 nm and ∼18 nm) was probed and found to be ∼5% and ∼8%, respectively. Two common photoluminescence (PL) peaks were observed at 436 nm (the intrinsic bandgap) and 487 nm (the extrinsic bandgap) for both GQDs. The characteristic PL properties in extrinsic and intrinsic bandgaps examined by optical spectroscopic methods show that the emission peak was red-shifted and that the peak width was widened as the size increased. Moreover, the PL lifetime and intensity were not only reversibly changed by pH but also depended on the excitation wavelength. This is in line with our previous report and is ascribed to the size variation of sp2 subdomains and edge functionalization. <s> BIB007 </s> Graphene-Based Materials for Biosensors: A Review <s> Graphene-Based Quantum Dots <s> Fluorescent nanodots have become increasingly prevalent in a wide variety of applications with special interest in analytical and biomedical fields. The present overview focuses on three main aspects: (i) a systematic description and reasonable classification of the most relevant types of fluorescent nanodots according to their nature, quantum confinement and crystalline structure is provided, starting with a clear distinction between semiconductor and carbon-based dots (graphene quantum dots, carbon quantum dots and carbon nanodots). A new set of abbreviations and definitions for them to avoid contradictions found in literature is also proposed; (ii) a rational classification allows the establishment of clear-cut differences and similarities among them. From a basic point of view, the origins of the photoluminescence of the different nanodots are also established, which is a relevant contribution of this overview. Additionally, the most outstanding similarities and differences in a great variety of criteria (i.e. year of discovery, synthesis, the physico-chemical characteristics like structure, nature, shape, size, quantum confinement, toxicity and solubility, the optical characteristics including the quantum yield and lifetime, limitations, applications as well as the evolution of publications) are thoroughly outlined; and (iii) finally, the promising future of fluorescent nanodots in both analytical and biomedical fields is discussed using selected examples of relevant applications. <s> BIB008
This new material is called "carbon-based dots". The discovery of carbon-based dots happened accidentally during purification of single-walled carbon nanotubes (SWNTs) from arc-discharge soot. The fluorescent carbon obtained had different color emissions under UV light when several sizes and concentrations of chopped SWNT dispersions were irradiated. These carbon particles were subsequently called carbon quantum dots (CQDs) BIB001 . For consistency, the word "dot" involving "quantum" is distinguished from the spherical semiconductors. The dots are obtained due to the size reduction of basal-plane carbon structures to a few nanometers approaching that of the dots with quantum phenomena. More importantly, the new materials are less toxic and are compatible for cellular internalization as verified by many groups BIB003 BIB006 BIB002 BIB007 BIB004 . Figure 5 shows the photoluminescent carbon-based dots categorized into GQD, CQD, and CND as differentiated to the SQD BIB005 . Reed BIB008 has defined the term of the SQD of sizes in the quantum-confined regime which is less than the Bohr exciton radius, typically within a few nanometers. The fine SQDs display resonant tunneling with discrete density of states. As explained, the carbon-based dots are based on structures with sp 2 -sp 3 hybridized domains of carbon materials in which the structure of graphene exists as a core. Moreover, the term quantum dot is also reserved for the carbon-based dots that satisfy the nanometer-size and functionalized structures with the quantum confinement. based dots that satisfy the nanometer-size and functionalized structures with the quantum confinement.