Luminescent Carbon Dots: Characteristics and Applications .Luminescent Carbon Dots: Characteristics

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Transcript of Luminescent Carbon Dots: Characteristics and Applications .Luminescent Carbon Dots: Characteristics

Luminescent Carbon Dots: Characteristics and Applications

Submitted bySaurabh Sonia

Project SupervisorProf. Dr. Maria A. Loib

Top Master Program in NanoscienceZernike Institute of Advanced Materials

University of Groningen(14th February, 2016)

as2848481, Top Master Nanoscience studentUniversity of Groningen

email: s.soni.1@student.rug.nlbProfessor of Photophysics and OptoElectronics

FMNS, Zernike Research Institute of Advanced MaterialsUniversity of Groningen

email: m.a.loi@student.rug.nl

Carbon Quantum Dots, or just Carbon Dots (C-Dots) are a recently discoverednew class of fluorescent materials from the nano-carbon family. These C-Dots havecome forth as potential competitors for inorganic quantum dots (QDs) and othertoxic, heavy-metal based materials, because of their characteristics like low toxicity,elemental abundance and biocompatibilty. In the last decade, numerous C-Dots syn-thesis routes have been developed, and it has been observed that different synthesismethods lead to different carbogenic core and surface structure, along with differentcharacteristic properties related to their composition, luminescence, functionalization,bio-compatibility, surface passivation, etc. In this review, it is discussed how thesedifferent synthesis routes and properties of C-Dots can be advantageous in their use inapplications in the fields of bioimaging and medicine, optoelectronic devices, chemicaland bio-sensing, etc. The report mainly focuses on the luminescence behaviour of theseC-dots, discussing their tunable optical behaviour and excitation dependent emission,good electron donor and acceptor tendencies, appreciable quantum yield (QY), elec-tron and charge transfer property, etc., and how can these be used efficiently over theexisting materials and techniques for the aforementioned applications.

CONTENTS

I. Introduction 1

II. Synthesis Methods 2A. Physical Methods 2

1. Laser Ablation Method 22. Arc Discharge Method 33. Plasma Treatment 3

B. Chemical Methods 31. Electrochemical Synthesis 32. Combustion / Hydrothermal /

Solvothermal Synthesis 43. Microwave / Induction-heater assisted

Synthesis 64. Support assisted Synthesis 6

III. Optical Properties 6A. Absorbance 6B. Photo luminescence 7C. Up-Conversion Photo luminescence 9D. Electrochemical Luminescence 10

IV. Applications 10A. Optoelectronic Devices 10B. Bioimaging 13

1. Fluorescence Imaging 132. Multiphoton Bioimaging 143. Cytotoxicity Analysis 14

C. Chemical and Bio-Sensing 15

V. Outlook 15

VI. Conclusion 17

VII. Acknowledgments 18

References 18

I. INTRODUCTION

Several class of materials have shown to exhibit lumi-nescence in different forms, and this has been studiedextensively since long by researchers for various appli-cations in optoelectronics, fluorescence imaging, sensing,photocatalysis, etc.[15] However, there has always been aconstant need to look for materials which lead to low costfor synthesis/fabrication, and which are non-toxic, eco-friendly, as well as biocompatible. Carbon dots (C-Dots)are one such highly fluorescent carbogenic nanoparticlesor nano-dots, with average size of roughly 1 - 10 nm,which have shown some of these properties to an appre-ciable extent[68].

Since their early accidental discovery by Xu, X. et al.using arc discharge-electrophoresis separation method[9]

in 20004, and synthesis by Sun, Y. P. et al. using laserablation method[10] in 2006, C-Dots have gained atten-tion as new class of highly luminescent quantum dotswith their characteristic advantages like non toxicity, bio-compatibility, elemental abundance, low cost, etc. Theseproperties of C-Dots give them an added advantage overinorganic QDs, which have been used extensively for op-toelectronic and imaging applications[1], though the prac-tical usage of QDs have been limited due to their toxicnature and use of costly heavy metals. Apart from theirstrong and tunable Photoluminescence, C-Dots are alsosuperior in terms of their solubility, functionalizability,photostability and chemical inertness.

Here, we discuss some of the various synthesis meth-ods that have been developed for C-Dots. C-Dots syn-thesis methods can be classified broadly into two routes:

1

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a) physical synthesis methods, like laser ablation[1012],arc discharge[9], and plasma induced[13], and b)the chemical synthesis methods, like electrochemicalsynthesis[1417], hydrothermal[1820], solvothermal[21, 22],pyrolysis[2325], microwave-assisted[2628], and support-assisted[29, 30]. Different synthesis methods using dif-ferent precursors result in different percentage of Car-bon, Oxygen and Nitrogen, with different functionalgroups, and therefore different surface and other func-tional properties. Synthesized C-Dots using differentmethods are found to comprise of amorphous, to nano-crystalline graphitic or turbostatic carbon (sp2 Car-bon), to diamond like core structure (sp3 Carbon).Some synthesis routes also involve surface passivation,via passivating agents like poly(ethylene glycol) (PEG),poly(propionylethylenimine-co-ethylenimine) (PPEI-EI),etc., which holds importance in the fluorescence enhance-ment, functionalization, solubility, etc.

Due to their high oxygen content, many times C-Dotsare also referred to as Carbogenic quantum dots[18, 23, 29],and sometimes C-Dots with graphitic sp2 core struc-ture or 1-10 stacked graphene layers in the core,are also referred distinctively as graphene quantumdots[13, 17, 31, 32]. However, there still is no clear consentamongst researchers regarding this distinction, and theseterminologies have sometimes been used interchangeably.Therefore, for the sake of simplicity, in this review we willaddress them all as C-Dots.

Further, we discuss the interesting luminescence be-haviour of C-Dots and the different explanations thathave been put forward in different publications for theobserved luminescence. Briefly, the luminescence be-haviour of C-Dots is usually believed to be originatingdue to radiative recombination of excitons in core andsurface electronic states, surface functional groups andsurface energy traps, the quantum size confinement ef-fect, etc.[14, 1719, 23, 24, 26, 30, 3235] We also discuss thevarious other optical phenomenon that have been ob-served in different C-Dots, like Up Conversion Photo Lu-minescence and Near Infra Red Emission[11, 22, 36], sizeand excitation wavelength dependent emission, Electro-chemical Luminescence[16, 26], etc., and some other ob-served properties of C-Dots like their electron acceptorand donor behaviour[37] and photo-induced charge trans-fer.

These interesting properties of C-Dots have at-tracted researchers to study this new material andlook for the applications where they can be uti-lized as non-toxic and inexpensive alternatives to ex-isting materials. We, therefore, in the next sec-tion discuss the potential use of C-Dots as materialsin optoelectronic devices like solar cells[38] and LEDsfor multicolor and white light emission[24, 27, 3941]; innanomedicine for single and multiphoton bioimaging, andbiosensing[22, 34, 36, 42, 43]; and in chemical sensing of sen-sitive metal ions and other molecules by fluorescencequenching mechanism[19, 25, 28]. The reported quantumyields of various C-Dots have also been tabulated in theoutlook section.

C-Dots have by now been reviewed many times by re-searchers now[68, 31, 44, 45], and these reviews with thisreport aims at encouraging further studies on this newclass of interesting material. However, C-Dots are a rela-tively new class of materials, and some of their character-istics like reason behind strong fluorescence, multiphotonemission, etc. are yet to be understood completely. Un-ambiguity in synthesis methods also result in lower con-trol on size spread and structural non-uniformity, whichin turn affects the luminescence of C-Dots. Thus furtherresearch is needed for studying the mechanisms respon-sible behind different luminescent features of C-Dots,which will also then help in better understanding andutilization of this material for applications in optoelec-tronics, bioimaging, chemical and bio-sensors, etc.

II. SYNTHESIS METHODS

A. Physical Methods

1. Laser Ablation Method

Laser Ablation synthesis of C-Dots was one of the ear-liest synthesis methods used. Sun, Y.-P. et al. used aQ-switched Nd:YAG laser on a Carbon target, preparedusing graphite powder and cement, in the presence ofwater vapor and argon gas[10]. The obtained Carbonnanoparticles exhibited a size spread with no detectablePhotoluminescence (PL). However, when refluxed withHNO3 for 12h, and passivated using PEG1500N - whichare diamine terminated oligomeric poly-(ethylene glycol)

2

FIG. 1. Schematic of the one-step laser ablation synthesis ofC-Dots using graphite powder in PEG200N. (Ref [12])

molecules - the obtained C-Dots exhibited bright PL inboth solution-like suspension state and solid state. Otherorganic molecules like poly(propionylethyleneimine-co-ethyleneimine) (PPEI-EI) could also be used as pas-sivating agents. They also demonstrated synthesis ofZnS- and ZnO-doped C-Dots (CZnS-Dots and CZnS-Dots)[11], using the carbon nanoparticles obtained fromlaser ablation[10]. After refluxing with HNO3, dia-lyzing and centrifugating, Zn(CH3COO)2 was addedthrough hydrolysis with Na2S and NaOH for CZnS-Dotsand CZnO-Dots, respectively. The later was annealedto convert Zn(OH)2 to ZnO. Both C-Dots were mixedwith sodium dodecyl sulfate via sonication, were fil-tered, washed and dried, and then mixed thoroughly withPEG1500N. The mixture was heated, stirred and thencooled down followed by centrifugation to obtain CZnS-Dots and CZnO-Dots.

A single step route to create fluorescen