Similar to its popular older cousins the fullerene, the carbon nanotube, and graphene, the latest form of nanocarbon, the carbon nanodot, is inspiring intensive research efforts in its own right. These surface-passivated carbonaceous quantum dots, so-called C-dots, combine several favorable attributes of traditional semiconductor-based quantum dots (namely, size- and wavelength-dependent luminescence emission, resistance to photobleaching, ease of bioconjugation) without incurring the burden of intrinsic toxicity or elemental scarcity and without the need for stringent, intricate, tedious, costly, or inefficient preparation steps. C-dots can be produced inexpensively and on a large scale (frequently using a one-step pathway and potentially from biomass waste-derived sources) by many approaches, ranging from simple candle burning to in situ dehydration reactions to laser ablation methods. In this Review, we summarize recent advances in the synthesis and characterization of C-dots. We also speculate on their future and discuss potential developments for their use in energy conversion/storage, bioimaging, drug delivery, sensors, diagnostics, and composites.

Luminescent Carbon Nanodots: Emergent Nanolights

Carbon nanodots (C-dots) have generated enormous excitement because of their superiority in water solubility, chemical inertness, low toxicity, ease of functionalization and resistance to photobleaching. In this review, by introducing the synthesis and photo- and electron-properties of C-dots, we hope to provide further insight into their controversial emission origin (particularly the upconverted photoluminescence) and to stimulate further research into their potential applications, especially in photocatalysis, energy conversion, optoelectronics, and sensing.

Carbon nanodots: synthesis, properties and applications

Electrogenerated chemiluminescence (also called electrochemiluminescence and abbreviated ECL) is the process whereby species generated at electrodes undergo high-energy electron-transfer reactions to form excited states that emit light. The first detailed ECL studies were described by Hercules and Bard et al. in the mid-1960s, although reports concerning light emission during electrolysis date back to the 1920s by Harvey. After about 40 years study, ECL has now become a very powerful analytical technique and been widely used in the areas of, for example, immunoassay, food and water testing, and biowarfare agent detection. ECL has also been successfully exploited as a detector of flow injection analysis (FIA), high-performance liquid chromatography (HPLC), capillary electrophoresis, and micro total analysis (μTAS). Figure 1 illustrates a time line of various events in the development of ECL. A literature survey using SciFinder Scholar reveals that more than 2000 journal articles, book chapters, and patents on various topics of ECL have been published. The overall number of publications, as shown in Figure 2, has increased exponentially over the past 20 years, of which 40–50% were biorelated. Similar amounts of ECL papers could be also found from the Thomson ISI Web of Science as well as † Telephone (601) 266 4716; fax (601) 266 6075; e-mail wujian.miao@ usm.edu. Wujian Miao received his undergraduate diploma in chemistry from Nantong University (Nantong, China) in 1982, his M.Sc. degree in analytical chemistry from Zhongshan University (Guangzhou, China, with Jinyuan Mo) in 1991, and his Ph.D. degree in electrochemistry from Monash University (Melbourne, Australia, with Alan M. Bond) in 2000. He then served as a Research Scientist in CSIRO (Melbourne, Australia), followed by a postdoctoral fellowship at the University of Texas at Austin with Allen J. Bard in 2001. Since 2004 he has served as an assistant professor of chemistry at the University of Southern Mississippi.

Electrogenerated Chemiluminescence and Its Biorelated Applications

To obtain graphene-based fluorescent materials, one of the effective approaches is to convert one-dimensional (1D) graphene to 0D graphene quantum dots (GQDs), yielding an emerging nanolight with extraordinary properties due to their remarkable quantum confinement and edge effects. In this review, the state-of-the-art knowledge of GQDs is presented. The synthetic methods were summarized, with emphasis on the top-down routes which possess the advantages of abundant raw materials, large scale production and simple operation. Optical properties of GQDs are also systematically discussed ranging from the mechanism, the influencing factors to the optical tunability. The current applications are also reviewed, followed by an outlook on their future and potential development, involving the effective synthetic methods, systematic photoluminescent mechanism, bandgap engineering, in addition to the potential applications in bioimaging, sensors, etc.

Focusing on luminescent graphene quantum dots: current status and future perspectives

Microwave synthesis of fluorescent carbon nanoparticles with electrochemiluminescence properties

Reversible electrochemical injection of discrete numbers of electrons into sterically stabilized silicon nanocrystals (NCs) (∼2 to 4 nanometers in diameter) was observed by differential pulse voltammetry (DPV) in N , N ′-dimethylformamide and acetonitrile. The electrochemical gap between the onset of electron injection and hole injection—related to the highest occupied and lowest unoccupied molecular orbitals—grew with decreasing nanocrystal size, and the DPV peak potentials above the onset for electron injection roughly correspond to expected Coulomb blockade or quantized double-layer charging energies. Electron transfer reactions between positively and negatively charged nanocrystals (or between charged nanocrystals and molecular redox-active coreactants) occurred that led to electron and hole annihilation, producing visible light. The electrogenerated chemiluminescence spectra exhibited a peak maximum at 640 nanometers, a significant red shift from the photoluminescence maximum (420 nanometers) of the same silicon NC solution. These results demonstrate that the chemical stability of silicon NCs could enable their use as redox-active macromolecular species with the combined optical and charging properties of semiconductor quantum dots.

http://science.sciencemag.org/content/sci/296/5571/1293.full.pdf

Electrochemistry and Electrogenerated Chemiluminescence from Silicon Nanocrystal Quantum Dots

This communication reports novel electrochemistry of cadmium sulfide quantum dots (Q-CdS) in N,N′-dimethylformamide (DMF). During the past two decades, semiconductor nanoparticles or quantum dots (QDs), in particular those of the cadmium chalcogenides, have attracted considerable attention due to their tunable electronic properties as a function of sizesthe so-called quantum size effect. Recently, TOPO (tri-n-octylphosphine oxide)-capped Q-CdSe has been used for stimulated light emission where the laser frequency was tuned through simple variation of particle size.1 The photoelectrochemical2 and photophysical3 properties of these particles in their size-quantized states have been wellstudied. However, there have been relatively few reports on the electrochemical properties of semiconductor QDs.4,5 Using the “particle in a box” model, Brus6 has predicted a dependence of redox potential on particle size for Q-CdS. However, to our knowledge, this model has not been tested by actual electrochemical measurements of QDs in solution, largely because of the limited solvent window of many solvent/electrolyte systems and the instability of the particles. Here this is considered, and we show a direct correlation between the electrochemical band gap and the electronic spectra of CdS nanoparticles in DMF. Thioglycerol-capped, Q-CdS was chosen for study because stable monodisperse particles, which are readily soluble in DMF can be prepared relatively easily. We followed the method reported by Weller et al.7 and as-prepared particles were sizeselected8 to obtain monodisperse (<10%) fractions. The final four fractions (six to ten, denoted I-IV hereafter) were chosen for further study, and these were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis absorbance spectroscopy (Supporting Information). For the electrochemical measurements, CdS particle solutions were prepared in DMF with tetrahexylammonium perchlorate (THAP) as supporting electrolyte. All measurements were carried out in a helium-filled drybox, and the electrochemical properties of the dispersions were investigated by cyclic voltammetry (CV) at both Pt (A ) 0.062 cm2 ) and Au electrodes (A ) 0.3 cm2). A threeelectrode configuration was used where a Pt coil and a silver wire served as the counter and quasi-reference electrodes, respectively. The cell potential was normalized to NHE using the Fc/Fc+ couple (with the potential of this couple taken as 0.47V vs NHE). The solubility of the particles in DMF was found to be inversely proportional to their size, and the final fraction (IV) (2.4 ( 0.3 nm diameter) was the most soluble. A typical CV for IV at the Pt electrode is given in Figure 1 where it is compared with the response for the supporting electrolyte alone (Figure 1a). Increasing the amount of Q-CdS added resulted in increases in all peak currents without significant shifts in peak potentials. An identical CV was obtained with a Au electrode. Clear oxidation and reduction peaks are apparent at -2.15 V (A1) and 0.80 V (C1), respectively. The additional peaks only appear on scan reversal after traversing either A1 or C1 (Figure 1b). To show that the CV response is due to redox reactions of a solution species rather than an adsorbed film, the dependence of peak current and potential on scan rate (ν) from 10 to 500 mVs-1 was also investigated (Figure 2a). The linear fit of peak current versus ν1/2 for peaks A1 and C1 up to 100 mV s-1 indicates diffusion of solution Q-CdS to the electrode surface. The peak position shifted with increasing ν, indicating kinetic effects. The current response for the other peaks was neither clearly proportional to ν nor ν1/2. The response was stable on repetitive scanning and for several days with no evidence of fouling on either electrode surface The peak-to-peak separation between A1 and C1 is 2.96 V, a value comparable to the 3.23 eV (1s-1s transition) calculated from the electronic spectra (Supporting Information, Figure S1). Thus, these oxidation and reduction peaks can be correlated (1) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314. (2) (a) Hickey, S. G.; Riley, D. J. J. Phys. Chem. B. 1999, 103, 4599, (b) Hickey, S. J.; Riley, D. J.; Tull, E. J. J. Phys. Chem. B 2000, 104, 7623. (c) Torimoto, T.; Nagakubo, S.; Nishizawa, M.; Yoneyama, H. Langmuir 1998, 14, 7077. (d) Boxall, C.; John Albery, W. Phys. Chem. Chem. Phys. 2000, 2, 3651. (e) Boxall, C.; John Albery, W. Phys. Chem. Chem. Phys. 2000, 2, 3641. (3) (a) Burda, C.; Link, S.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 10775. (b) Burda, C.; Green, T. C.; Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 1783. (c) Logunov, S.; Green, T.; Marguet, S.; El-Sayed, M. A. J. Phys. Chem. A 1998, 102, 5652. (4) Hoyer, P.; Weller, H. Chem. Phys. Lett. 1994, 221, 379. (5) Chen, S.; Truax, L. A.; Sommers, J. M. Chem. Mater. 2000, 12, 3864. (6) Brus, L. E. J. Chem. Phys. 1983, 79, 5566. (7) (a) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmueller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (b) Chemseddine, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 636. (8) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. Figure 1. (a) CV response in the absence and presence of thioglycolcapped CdS Q-particles (1 mg/mL of fraction IV) at a Pt electrode. Sweep rate ) 50 mV s-1 and [THAP] ) 0.05 M. (b) Variation of the initial scan direction for IV illustrating that peaks A2, A3, and C3 are related to C1 and A1; sweep rate ) 10 mV s-1.

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Electrochemistry of CdS Nanoparticles: A Correlation between Optical and Electrochemical Band Gaps

Room-temperature ionic liquids (ILs) have been proposed as alternative solvents for organic synthesis, separations, and electrochemical applications. Here, we report studies that probe the electrochemical and solvation properties of a tetraalkylammonium (methyltributylammonium bis(trifluoromethylsulfon)imide, M3BNIm) and an imidazolium (1-butyl-3-methylimidazolium hexafluorophosphate, BMIPF6) based ionic liquid. It is demonstrated that despite impurities, the cathodic limit at a Pt electrode is enhanced for the tetraalkylammonium-based IL. Electrogenerated chemiluminescence of tris(2,2‘-bipyrindinyl)ruthenium (Ru(bpy)32+) was observed in both ionic liquids, and differences in the response were interpreted in terms of the solvent reactivity and polarity. As ILs have been proposed as alternatives to organic solvents in extraction processes, an understanding of the relative lipophilicity of the IL ions and the equilibrium potential difference established across the IL/water interface is of fundamental releva...

Novel Electrochemical Studies of Ionic Liquids

We report the novel use of the scanning electrochemical microscope (SECM) to follow the compression of a monolayer of monodisperse silver thiol-capped nanoparticles on a Langmuir trough. SECM allowed the direct electrochemical visualization of the lateral connectivity of the particles in the film at the air/water interface through monitoring the transition from negative feedback (insulating substrate) to positive feedback (conducting substrate) at the Pt SECM tip, with ferrocene methanol (FcMeOH) as the redox mediator, as the interparticle separation decreased by compressing the film. The transition from an insulating to conductive behavior with compression could thus be followed.

/pdf/electrochemical-observation-of-a-metal-insulator-transition-1h9efcrt3v.pdf

Bernadette M. Quinn

Papers

Electrochemistry and Electrogenerated Chemiluminescence from Silicon Nanocrystal Quantum Dots

Electrochemistry of CdS Nanoparticles: A Correlation between Optical and Electrochemical Band Gaps

Novel Electrochemical Studies of Ionic Liquids

Electrochemical observation of a metal/insulator transition by scanning electrochemical microscopy