Graphene Quantum Dots from Polycyclic Aromatic Hydrocarbon for Bioimaging and Sensing of Fe3+ and Hydrogen Peroxide

An easy approach for large‐scale and low‐cost synthesis of photoluminescent (PL) graphene quantum dots (GQDs) based on the carbonization of commercially available polycyclic aromatic hydrocarbon (PAH) precursors with strong acid and followed by hydrothermal reduction with hydrazine hydrate is reported. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) characterizations indicate that the size and height of GQDs are in the range of 5–10 nm and 0.5–2.5 nm, respectively. PAH, which has more benzene rings, generally forms GQDs with relatively larger size. The GQDs show high water solubility, tunable photoluminescence, low cytotoxicity, and good optical stability, which makes them promising fluorescent probes for cellular imaging. In addition, the fluorescence of GQDs shows a sensitive and selective quenching effect to Fe³⁺ with a detection limit of 5 × 10^?9m . By combination with the Fe²⁺/Fe³⁺ redox couple, the PL GQDs are able to detect oxidant, using H₂O₂ as an example. This study opens up new opportunities to make full use of GQDs because of their facile availability, cost‐effective productivity, and robust functionality.     

一步水热法合成的石墨烯量子点及其在锰离子探测中的应用

以还原氧化石墨烯为前驱体,采用一步水热法成功制备出了近似球状、分散性良好、尺寸均一的石墨烯量子点。通过傅立叶红外光谱(FTIR)、紫外-可见吸收光谱、荧光光谱等光学手段对样品的结构和光学性能进行了表征,结果显示制备的石墨烯量子点表面含有丰富的含氧官能团,在紫外区有很强的吸收,发射峰强而窄,表现出激发波长不依赖的荧光性能。研究结果表明石墨烯量子点可应用于Mn2+微量探测,石墨烯量子点的荧光强度会随着所加入的Mn2+浓度的增大而降低,在0~400μmol/L间的校准曲线呈线性相关。     

Hydrothermal Preparation of Photoluminescent Graphene Quantum Dots Characterized Excitation‐Independent Emission and its Application as a Bioimaging Reagent

Zero‐dimensional photoluminescent (PL) graphene quantum dots (GQDs) that can be used as the cell‐imaging reagent are prepared by a hydrothermal route using the graphene oxide (GO) as the carbon source. Under the optimized hydrothermal conditions, an initial hydrogen peroxide concentration of 0.5 mg mL^?1 at 180 °C for 120 min, the GO sheets can be cut into nanocrystals with lateral dimensions in the range of 1.5–5.5 nm and an average thickness of around 1.1 nm. The as‐prepared GQDs exhibit an abundance of hydrophilic hydroxy and carboxyl groups and emit bright blue luminescence with up‐conversion properties in a water solution at neutral pH. Most interestingly, they indicate excitation‐independent emission characteristics, and the surface state is demonstrated to have a key role in the PL properties. The fluorescence quantum yield of the GQDs is tested to be around 6.99% using quinine sulfate as a standard. In addition, the as‐prepared GQDs can enter into HeLa cells easily as a fluorescent imaging reagent without any further functionalization, indicating they are aqueous stability, biocompatibility, and promising for potential applications in biolabeling and solution state optoelectronics.     

碳纳米管制备蓝光石墨烯量子点及其在柔性有机存储器上的应用

通过化学裁剪法打开碳纳米管获得了粒径一致、性能稳定、具有蓝色荧光的石墨烯量子点。该方法属于化学溶液法，具有成本低廉、工艺简单、条件易控等优势。将该样品与半导体聚合物按一定比例混溶，通过旋涂技术形成基于石墨烯量子点掺杂的聚合物复合薄膜，进而制成柔性存储器。该柔性可弯曲存储器具有低驱动电压、接近10³的ON/OFF 比率、较好的循环次数，较好的重复性和稳定性。该研究结果为柔性有机存储器领域的研究展开了新的方向。     

石墨烯量子点荧光探针测定肾上腺色腙

通过高温裂解柠檬酸合成了水溶性石墨烯量子点(GQDs),并应用钝化剂PEG2000进行修饰,提高了GQDs的量子产率。应用荧光光谱、紫外-可见光谱、红外光谱对其发光特性进行了研究,测定了荧光寿命。实验发现,在p H=7.40的Tris-HCl缓冲液中,肾上腺色腙(CBZC)对GQDs荧光强度有明显的猝灭作用。基于此提出了以GQDs为探针测定肾上腺色腙的新方法。实验考察了缓冲溶液用量、缓冲溶液种类、量子点浓度、反应时间以及表面活性剂等多种因素对反应体系的影响。当量子点浓度为2.3×10-3mol/L时,肾上腺色腙浓度在4.0×10-7~1.2×10-5mol/L(0.995 6)范围内与荧光猝灭值ΔF呈良好的线性关系,方法检出限为1.5×10-7mol/L,相对标准偏差为0.15%(n=5,c=4.0×10-6mol/L)。该方法对于样品中肾上腺色腙含量测定的回收率为97.46%~101.6%。通过测定温度对猝灭常数的影响以及紫外-可见吸收光谱的变化确定了二者的猝灭过程和相互作用力类型。     

Synthesis of Blue‐, Green‐, Yellow‐, and Red‐Emitting Graphene‐Quantum‐Dot‐Based Nanomaterials with Excitation‐Independent Emission

A one‐pot method is described for the preparation of graphene quantum dots/graphene oxide (GQDs/GO) hybrid composites with emission in the visible region, through heteroatom doping and hydroxyl‐radical‐induced decomposition of GO. The NH₄OH‐ and thiourea‐mediated dissociation of H₂O₂ produces hydroxyl radicals. Treatment of GO with hydroxyl radicals results in the production of small‐sized GO sheets and GQDs, which self‐assemble to form GQDs/GO through strong π–π interactions. For example, the reaction of GO with a mixture of NH₄OH and H₂O₂ for 40, 120, and 270 min generates yellow‐emitting GQDs/GO (Y‐GQDs/GO), green‐emitting GQDs/GO, and blue‐emitting GQDs, while red‐emitting GQDs/GO (R‐GQDs/GO) are prepared by incubating GO with a mixture of thiourea and H₂O₂. From the analysis of these four GQD‐based nanomaterials by transmission electron microscopy, atomic force microscopy, and fluorescence lifetime spectroscopy, it is found that this tunable fluorescence wavelength results from the differences in particle size. All four GQD‐based nanomaterials exhibit moderate quantum yields (1–10%), nanosecond fluorescence lifetimes, and excitation‐independent emissions. Except for R‐GQDs/GO, the other three GQD‐based nanomaterials are stable in a high‐concentration salt solution (e.g., 1.6 m NaCl) and under high‐power irradiation, enabling the sensitive (high‐temperature resolution and large activation energy) and reversible detection of temperature change. It is further demonstrated that Y‐GQD/GO can be used to image HeLa cells.     

The pH Effect on Thermal Response of Fluorescence Spectroscopy of Graphene Quantum Dots for Nanoscale Thermal Characterization

Accurate and sensitive nanoscale thermal probing for thermophysical property characterization is appealing but still a challenge to date. Previous studies have revealed that graphene quantum dots (GQDs) are good temperature markers for their small dimension and superior fluorescence excitation. In this work, we show that the thermal response of fluorescence spectrum of GQDs is strongly pH-dependent. Significant decrease (about 56% to 30%) for temperatureinduced intensity reduction within a small range of 75°C under different excitation wavelengths of 370 nm, 390 nm, and 410 nm is observed as pH value increases from pH = 1 to pH = 13. The temperature coefficients of peak wavelength change from positive to negative with the increase of pH value, meaning that the blue shift happens as the condition is changed from acidity to alkalinity. Temperature dependence of peak width is also studied with the largest coefficient of 0.2255nm/°C, which is remarkable. These suggest that when using GQDs in nanoscale thermal probing, the pH value is an important factor that should be considered besides the excitation wavelength. Regarding the superior biocompatibility and low cytotoxicity, GQDs could play an important role in thermal probing or mapping in a complex biology system such as a cell, and help to develop novel treatments and diagnoses.     

基于石墨烯量子点的荧光探针应用于抗坏血酸检测的研究

基于石墨烯量子点(GQDs)的荧光性能建立了一种非标记荧光方法,用于灵敏和选择性测定抗坏血酸(AA)。GQDs溶液在紫外光激发下发出很强的蓝色荧光,当向溶液中加入AA后,GQDs溶液的荧光被猝灭。猝灭机理可能为在弱酸性介质中,AA与GQDs发生氧化还原反应,AA转移电子给GQDs。荧光猝灭强度与AA浓度在5.0×10~(-6)~7.5×10~(-5)mol/L范围内呈良好的线性关系,检出限低至1.0×10~(-6)mol/L。该体系成本低、操作简单,并且在多种可能干扰的物质存在下对AA表现出很高的选择性。本方法应用于生物样品中AA的检测,回收率在95.2%~115.3%之间。     

Graphene Quantum Dots

Graphene quantum dots (GQDs) are nanometer‐sized fragments of graphene that show unique properties, which makes them interesting candidates for a whole range of new applications. This review article gives an overview of the synthesis, properties and applications of GQDs. Synthesis methods discussed include top‐down and bottom‐up approaches. Properties such as luminescence up‐ and down‐conversion have been used in applications ranging from energy conversion to bio‐analytics. This article provides an overview of the state‐of‐the‐art and highlights promising findings as well as potential future directions of the research field.     

One-pot synthesis of highly fluorescent amino-functionalized graphene quantum dots for effective detection of copper ions

In this work, a green and simple one-pot route was developed for the synthesis of highly fluorescent amino-functionalized graphene quantum dots (a-GQDs) via hydrothermal process without any further modification or surface passivation. We synthesized the a-GQDs using glucose as the carbon source and ammonium as a functionalizing agent without the use of a strong acid, oxidant, or other toxic chemical reagent. The as-obtained a-GQDs have a uniform size of 3–4 nm, high contents of amino groups, and show a bright green emission with high quantum yield of 32.8%. Furthermore, the a-GQDs show effective fluorescence quenching for Cu²⁺ ions which can serve as effective fluorescent probe for the detection of Cu²⁺. The fluorescent probe using the obtained a-GQDs exhibits high sensitivity and selectivity toward Cu²⁺ with the limit of detection as low as 5.6 nM. The mechanism of the Cu²⁺ induced fluorescence quenching of a-GQDs can be attributed to the electron transfer by the formation of metal complex between Cu²⁺ and the amino groups on the surface of a-GQDs. These results suggest great potential for the simple and green synthesis of functionalized GQDs and a practical sensing platform for Cu²⁺ detection in environmental and biological applications.     

Effect of size variation on the cathodoluminescence characteristics of graphene quantum dots

Cathodoluminescence (CL) has been studied in graphene quantum dots (GQDs) by varying their average size (d) from 5 to 35 nm. The size dependence of CL peak wavelength is very analogous to that of photoluminescence (PL) peak wavelength unusually showing non-monotonic behaviors having a maximum at d = ∼17 nm. The CL behaviors can therefore be attributed to the novel feature of GQDs, i.e., the circular-to-polygonal-shape and corresponding edge-state variations of GQDs at d = ∼17 nm as d increases. However, the peak wavelengths of CL are especially much smaller than those of PL at both ends in the size range of GQDs, possibly resulting from the recombination of the electron-beam-excited e-h pairs at higher energy states before thermalization due to fast carrier-carrier scattering dominating over electron-phonon scattering in graphene.     

Versatile Graphene Quantum Dots with Tunable Nitrogen Doping

This paper reports a facile fabrication of N‐doped graphene quantum dots (N‐GQDs) showing controllable chemical properties through a hydrothermal treatment. The N‐GQDs have a uniform size of 3.06 ± 0.78 nm and prefer the equilibrium shapes of circle and ellipse due to the minimization of edge free energy. The N/C atomic ratio in N‐GQDs can be precisely tailored in a range from 8.3 at% to 15.8 at% by simply controlling the concentration of N source (ammonium hydroxide). One order of magnitude quantum yield of 34.5% is achieved by N‐GQDs, compared with the N‐free GQDs, as the substitutional N has an essential role in more effective radiative emission. Excessive N dopants in N‐GQDs can lead to photoluminescence quenching, through nonradiative transition back to the ground state. The N‐GQDs are further found to be suitable as photocurrent conversion materials due to benign energy matching with anatase nanofibers, the ultrafast electron injection at their interface, and efficient electron transfer. This work provides an efficient and inspiring approach to engineering both chemical components and physical properties of N‐GQDs, and will therefore promote their basic research and applications in energy conversion.     

溶剂对石墨烯量子点荧光性质的影响

采用柠檬酸热解法制备了石墨烯量子点(GQDs),研究了非极性溶剂戊烷,极性溶剂乙醇、丙酮、乙二醇对GQDs荧光性质的影响。透射电子显微镜(TEM)和原子力显微镜(AFM)图像表明,制备的GQDs尺寸分布在2~12 nm(平均尺寸为4.9 nm),分散均匀,高度分布在0.5~2 nm。吸收光谱表明,GQDs具有明显的紫外吸收特性,吸收峰位于259 nm和274 nm。光致发光谱表明,GQDs的发光具有明显的溶剂依赖性。GQDs在极性溶剂乙醇、丙酮、乙二醇中,发光峰的位置依赖于激发波长,发射波长在可见光区。而在非极性溶剂戊烷中,GQDs表现出对激发波长不依赖的荧光性能,且发射波长在近紫外。     

Synthesis and characterization of graphene quantum dots and their size reduction using swift heavy ion beam

Graphene quantum dots (GQDs) are nanosized fragments of graphene displaying quantum confinement effect. They have shown to be prepared from various methods which include ion beam etching of graphene. However, recently the modification of the GQDs has garnered tremendous attention owing to its suitability for various applications. Here, we have studied the effect of swift ion beam irradiation on the properties of GQDs. The ion beam treatment on the GQDs exhibited the change in observed photoluminescence of GQDs as they exhibited a blue luminescence on excitation with longwave UV (≈365?nm) due to the reduction in size and removal of the ethoxy (–C–O–C–) groups present on the quantum dots. This was confirmed by transmission electron microscopy, particle size analysis, and Fourier transform infrared spectroscopy.     

Size‐Dependent Structural and Optical Characteristics of Glucose‐Derived Graphene Quantum Dots

It is of scientific importance to obtain graphene quantum dots (GQDs) with narrow‐size distribution in order to unveil their size‐dependent structural and optical properties, thereby further to explore the energy band diagram of GQDs. Here, a soft‐template microwave‐assisted hydrothermal method to prepare GQDs with diameters less than 5 nm ± 0.55 nm is reported. The size‐dependent photoluminescence (PL) quantum yield (QY) decay lifetime and electron energy loss spectroscopy (EELS) of the GQDs are studied systematically. The QY of the GQDs with an average diameter of 2 nm is the highest (15%) among all the samples investigated and the QY decreases with increasing diameter of the GQDs. The size‐dependence of the PL decay lifetime is also observed. The result suggests that spatial confinement effects related to radiative relaxation play an important role in the size‐dependent decay lifetime. A realistic energy band diagram of the GQDs is deduced from the experimental results.     

石墨烯量子点:制备及检测乙醇-水溶液组分

采用改进的非完全脱水碳化方法制备石墨烯量子点.产物尺寸约6 nm,激发峰355 nm,发射峰458 nm.基于乙醇对石墨烯锯齿边质子化的抑制效应,发现可通过石墨烯量子点荧光光强检测乙醇-水混合溶液组分.在0.01-100%范围内,乙醇体积百分比与发射峰光强增量呈线性关系.本研究为制备石墨烯量子点提供参考,提出了简易的较宽范围的乙醇水溶液含量检测方法 .     

Recent progresses of quantum confinement in graphene quantum dots

Graphene quantum dots (GQDs) not only have potential applications on spin qubit, but also serve as essential platforms to study the fundamental properties of Dirac fermions, such as Klein tunneling and Berry phase. By now, the study of quantum confinement in GQDs still attract much attention in condensed matter physics. In this article, we review the experimental progresses on quantum confinement in GQDs mainly by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Here, the GQDs are divided into Klein GQDs, bound-state GQDs and edge-terminated GQDs according to their different confinement strength. Based on the realization of quasi-bound states in Klein GQDs, external perpendicular magnetic field is utilized as a manipulation approach to trigger and control the novel properties by tuning Berry phase and electron–electron (e–e) interaction. The tip-induced edge-free GQDs can serve as an intuitive mean to explore the broken symmetry states at nanoscale and single-electron accuracy, which are expected to be used in studying physical properties of different two-dimensional materials. Moreover, high-spin magnetic ground states are successfully introduced in edge-terminated GQDs by designing and synthesizing triangulene zigzag nanographenes.     

煤液化沥青烯的荧光光谱表征及缔合结构研究

通过分子荧光光谱和紫外可见吸收光谱表征、研究了煤液化沥青烯结构及缔合行为。结果发现：煤液化沥青烯属于萘环结构为主的芳香化合物混合体系,具有强的荧光效应;溶剂与沥青烯分子能够形成激态复合物,导致沥青烯荧光峰红移和荧光猝灭;沥青烯分子间存在非共价键缔合作用,高温液化沥青烯芳香度高,缔合作用强;沥青烯分子间缔合属于逐步缔合过程,不存在“临界缔合浓度”,其表观荧光强度随浓度变化的临界浓度受激发波长影响。     

Quantum-confined electronic states in atomically well-defined graphene nanostructures

Despite the enormous interest in the properties of graphene and the potential of graphene nanostructures in electronic applications, the study of quantum-confined states in atomically well-defined graphene nanostructures remains an experimental challenge. Here, we study graphene quantum dots (GQDs) with well-defined edges in the zigzag direction, grown by chemical vapor deposition on an Ir(111) substrate by low-temperature scanning tunneling microscopy and spectroscopy. We measure the atomic structure and local density of states of individual GQDs as a function of their size and shape in the range from a couple of nanometers up to ca. 20 nm. The results can be quantitatively modeled by a relativistic wave equation and atomistic tight-binding calculations. The observed states are analogous to the solutions of the textbook "particle-in-a-box" problem applied to relativistic massless fermions.     

Highly sensitive and selective detection of glutathione using ultrasonic aided synthesis of graphene quantum dots embedded over amine-functionalized silica nanoparticles

Glutathione (GSH) is the most abundant antioxidant in the majority of cells and tissues; and its use as a biomarker has been known for decades. In this study, a facile electrochemical method was developed for glutathione sensing using voltammetry and amperometry analyses. In this study, a novel glassy carbon electrode composed of graphene quantum dots (GQDs) embedded on amine-functionalized silica nanoparticles (SiNPs) was synthesized. GQDs embedded on amine-functionalized SiNPs were physical-chemically characterized by different techniques that included high resolution-transmission electron microscopy (HR-TEM), X-ray diffraction spectroscopy (XRD), UV–visible spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy. The newly developed electrode exhibits a good response to glutathione with a wide linear range (0.5–7 µM) and a low detection limit (0.5 µM) with high sensitivity(2.64 µA µM⁻¹). The fabricated GQDs-SiNPs/GC electrode shows highly attractive electrocatalytic activity towards glutathione detection in the neutral media at low potential due to a synergistic surface effect caused by the incorporation of GQDs over SiNPs. It leads to higher surface area and conductivity, improving electron transfer and promoting redox reactions. Besides, it provides outstanding selectivity, reproducibility, long-term stability, and can be used in the presence of interferences typically found in real sample analysis.